EP2231924A1

EP2231924A1 - Method for dealing with faults occurring during the manufacture of a material web

Info

Publication number: EP2231924A1
Application number: EP08863004A
Authority: EP
Inventors: Arto Ikonen; Eero Suomi; Pekka Vantola
Original assignee: Metso Paper Oy
Current assignee: Valmet Technologies Oy
Priority date: 2007-12-14
Filing date: 2008-12-12
Publication date: 2010-09-29
Anticipated expiration: 2028-12-12
Also published as: FI20070984A; FI20070984A0; CN101903594A; CN101903594B; EP2231924B1; ATE511574T1; WO2009077650A1; FI120317B

Abstract

The invention relates to a method for dealing with faults occurring during the calendering of a material web, said method comprising at least the following steps of: -identifying the location of a fault (H) on the surface of a roll (5) or in a material web and estimating or calculating an arrival time of the fault (H) at a roll nip (N) and a dwell time for the fault in the roll nip, -transmitting from a control system (4) to at least one digital valve unit (100), used for controlling a nip pressure of the roll nip (N), a feed-forward adjustment instruction (F) for dealing with a fault in the roll nip (N) of a calender, -reducing as per adjustment instruction (F), through the intermediary of at least one digital valve (1), a nip pressure of the roll nip (N) at the latest when the fault (H) arrives at the roll nip (N) and increasing the nip pressure as the fault exits the roll nip.

Description

Method for dealing with faults occurring during the manufacture of a material web

The invention relates to a method as set forth in the preamble of claim 1 for dealing with faults occurring during the manufacture of a material web.

The invention relates also to an apparatus for implementing the method set forth in the preamble of claim 1.

The invention involves the use of digital valve units for controlling the action of fiber machine sections operated by hydraulic or pneumatic actuators. >

The fiber machine sections operated by hydraulic or pneumatic actuators refer to machine sections whose operation can be controlled by adjusting the flow rate and/or pressure of a fluid supplied to said sections. The fluid, in turn, refers to a liquid, such as oil or water, or to a gas, such as air.

The digital valve unit refers to a valve unit, including at least one digital valve group which in turn comprises a number of digital valves connected in parallel with respect to a fluid flow passing through the digital valve group. The volume flow of a fluid passing through a digital valve group is determined by an aperture of the digital valve group, i.e. a total valve opening area of the digital valves presently in an open position in a digital valve group. On the other hand, the digital valve refers to a valve, which is intended for adjusting the volume flow of a fluid and which has 2 - N different stepwise discrete adjustment positions, preferably 2 different discrete adjustment positions (open/shut), and the control signal delivered thereto from a control system being preferably digitalized. The control of a volume flow proceeding through a digital valve group occurs on the basis of a pressure difference between the fluid arriving at the digital valve group and that discharging from the digital valve group, which difference in turn depends on an aperture established by digital valves presently in an open position. Knowing the pressure difference of a fluid across a digital valve and the aperture of a digital valve group enables anticipating precisely the fluid volume flow proceeding through the digital valve group, which can be utilized in various high speed feed-forward adjustments. In the concept of feed-forward adjustments, the connection combinations of fluid- passing digital valves are selected in such a way that the desired action takes place in a pneumatic or hydraulic actuator coupled with the digital valve group.

On the other hand, the analog valve refers in this application to a valve, in which the rate of volume flow through the valve (throttling, valve opening degree) depends on the magnitude of a control signal. Typical analog valves include servo and proportional valves.

The fiber web refers to a web containing a fibrous material of at least partially natural origin, such as wood fibers. The employed fibrous material may also consist of straw, bagasse, grass, etc., among others.

Adjusting the pressure and/or flow rate of a fluid arriving in a hydraulic actuator by adjusting the slide position of an analog valve proportional to the value of a control signal involves several drawbacks:

-Each slide valve must be specifically designed for a particular flow channel (the nominal size of a valve, i.e. the maximum volume flow through a valve, as well as the size of a valve's various flow ports within the size range, and the like details of a valve). This increases considerably the designing work for systems involving analog valves.

-Analog valves are largely manufactured in individual pieces or in small series, which increases their manufacturing costs.

-Analog valves exhibit fluctuation in adjustment, especially if there are major discrepancies between the initial adjustment position and the final adjustment position regarding the volume flow of a fluid passing through the valve. The fluctuation of adjustment leads to a slower control and a higher energy consumption. Another control slowing factor is the internal feedback of a slide valve, which is used for adjusting the position of a slide valve in the process of driving the valve from volume flow A to volume flow B.

More specifically, the invention relates to a method as set forth in claim 1 and to an apparatus capable of implementing said method. Replacing an analog regulation valve with a group of digital valves provides a considerable benefit in the control of hydraulic actuators used in paper machines and enables discarding the foregoing prior art problems:

-Similar digital valves and digital valve groups can be used for a variety of applications, i.e. digital valves are multi-function valves. The operation and usability of a digital valve group depend on its control, because there is no disparity between digital valve groups in terms of the design of digital valves included therein. -Digital valves are inexpensive mass production articles, being needed in just a few sizes irrespective of the intended use, i.e. the design and operation of a hydraulic actuator controlled by digital valves.

-A single digital valve is simple in design, having a good repeatability of operation, no hysteresis.

-A digital valve group is fault tolerant in operation, as when one of the digital valves develops a fault, other valves are able to make up for its operation, i.e. the fault is "circumvented".

-Requirements regarding the filtration of a fluid are less stringent. The replacement of a particular analog valve in a paper machine with a digital valve group results in a considerably simplified design process as each operation enables the use of digital valve groups with standardized flow characteristics instead of application-specifically designed analog valves. This represents often a major saving in costs both in the valve designing process and also in the valve operating costs, because considerable energy savings are also often achieved by the use of digital valves. The savings are due to the fact that a digital valve group is more precise and high-speed in operation; driving an analog valve between two adjustment positions of highly unequal volume flows is very likely to cause fluctuation of adjustment as a result of the adjustment feedback. With a digital valve group, on the other hand, the same procedure can be performed in a feedforward mode of adjustment rapidly and accurately without delay and fluctuation, because the adjustment is applied to a volume flow passing through a plurality of smaller side-by-side arranged digital valves, the rate of a total volume flow proceeding therethrough (and also a pressure difference across the valve) being well predictable for each combination of currently open digital valves.

The adjustment instruction (control signal) arriving at a digital valve group is digital, such as binary, in character. According to the adjustment instruction, an adjustment is made on a volume flow discharging from the digital valve group and on a flow-inflicted pressure by opening a particular valve combination in the digital valve group in view of achieving a desired aperture and volume flow rate of fluid for the digital valve group. As opposed to the analog valve, each parallel- connected digital valve can only have a limited number of adjustment positions, i.e. the digital valve only has certain discrete flow positions. In one form thereof, the digital valve has three positions: open / shut / high-speed opening. Preferably, however, the positions of each digital valve are simply just on/off; the valve allows in its open position a certain volume flow to pass through, in its shut position it totally blocks the passage of a fluid flow therethrough.

In most subsequently described embodiments of the invention, the digital valve group consists of digital valves with two positions (on/off). Thus, in two digital valves following each other in terms of nominal volume flow rate, the volume flow proceeding through the valve with a higher nominal volume flow in its open position is always twice as high as the volume flow of the valve with a lower nominal volume flow. Hence, such a digital valve group can be supplied with a binarized control signal, in which the control signal has its magnitude converted into a binary number.

As an exemplary comparison, regarding differences of adjustment position between an analog valve and a hydraulic valve group, the following can be presented: should an analog valve be adjusted by means of a control signal (adjustment instruction) with a magnitude of 12 units, the stem of said analog valve travels a distance proportional to the 12^th control unit, whereby the valve opening allows therethrough a respectively magnified volume flow. On the other hand, when a digital valve group, established by five on/off digital valves parallel- connected in an inlet flow, is controlled by means of a 12-unit control signal (adjustment instruction) of the same magnitude, wherein the sizes of volume flows admitted by digital valves 1 , 2, 3, 4, 5 are respectively 1 , 2, 4, 8 and 16 units, the control signal is binarized into a control signal 01100 (0 x 2⁴ + 1 x 2³ + 1 x 2² + 0 x 2¹ + 0 x 2°= 12) (corresponding to valves 5, 4, 3, 2,1) and valves 3 and 4 are opened. The above-described operation of a digital valve group with respect to an analog valve is further illustrated in figs. 1A and 1B. Figs. 1A and 1B show the effect of the number of digital valves included in a digital valve group on the attainable number of volume flows and on the accuracy of adjustment.

Figs. 1A and 1B show the effect of the number of on/off digital valves included in digital valve groups on the attainable accuracy of adjustment as the digital valve group has 4 (fig. 1A) and 6 (fig. 1 B) digital valves in parallel connection. The graph shows a proportional volume flow for a digital valve group as a function of control, when the volume flow proceeding through the larger one of two valves with successive nominal volume flow rates among the group's digital valves is typically twice as high as that of the preceding valve (it is also possible to organize the volume flows of two valves with consecutive nominal volume flows in ratios other than multiples of two). In the figures, the volume flow 0 represents a condition in which the digital valve group has an opening =0, i.e. there is no flow proceeding therethrough, and 1 represents a condition in which the digital valve group has an aperture which is maximal, i.e. all of its digital valves are in an open position; when N=4, the maximal total volume flow Vmax through a digital valve group is Vmax=1V + 2V + 4V + 8V= 15V, and when N=6, the maximal volume flow is Vmax=1V + 2V + 4V + 8V + 16V + 32V= 61V. As can be seen from figs. 1A and 1B₁ the digital valve group has its control response approaching rapidly the response attainable by an analog slide valve as the number of valves is increased in the digital valve group, since each digital valve added to the group enables the number of possible opening combinations to be approximately doubled.

The following description deals more closely with applying the invention to the adjustment of various sections and functions of a paper machine and with advantages gained by the invention therein. The presented examples include several references for applying the invention to the adjustment of a given parameter in a roll nip. In this application, the roll nip refers to a roll nip between two rolls, or also to a roll nip present between a roll and a belt, unless otherwise indicated. The belt can be e.g. a metallic, polymeric, felt type or wire type belt.

Hence, the invention shall be illustrated by the following figures.

Figs. 1 A and 1 B show the effect of the number of digital valves included in a digital valve group on the attainable number of volume flows and on the accuracy of adjustment.

Fig. 2A shows schematically a multi-roll calender in a view directly to the calender's end face.

Fig. 2B shows a pressure load adjustment for a supporting lever present at the end of two intermediate rolls by means of hydraulic actuators controlled by a digital hydraulic unit.

Fig. 2C shows diagrammatically data and fluid flows for the control of another hydraulic actuator according to fig. 2B.

Fig. 2D shows a synchronized control by means of two separate digital valve units for a hydraulic actuator coupled with supporting levers present at the ends of two intermediate rolls. Fig. 3A shows schematically the formation of a relief and reset pulse for a roll present in a multi-roll calender's roll nip by using a digital valve unit as the roll nip is about to receive an fault of fig. 3B; the figure shows the flows of a hydraulic fluid in the hydraulic actuator, as well as a pressure load for the roll.

Fig. 3B shows, in a view directly to the end face, a roll nip between two rolls, which is about to receive a splice between two fiber webs.

Fig. 3C shows a steady-state condition control for the supporting lever of an intermediate roll by means of a digital valve unit prior to the formation of a relief and reset pulse of fig. 3A.

Fig. 3D shows a control for the supporting lever of an intermediate roll by means of a digital valve unit during a relief pulse of fig. 3A.

Fig. 3E shows a control for the supporting lever of an intermediate roll by means of a digital valve unit during a reset pulse of fig. 3A.

Fig. 3F shows, in a view directly to the end face, a roll nip between two rolls, which is about to receive a web break present in a fiber web.

Fig. 3G shows the flows of a hydraulic fluid in a hydraulic actuator, existing in the high-speed opening of a multi-calender's roll nip established by digital hydraulic units, as well as a respective roll position in the roll nip being opened.

Fig. 3H shows pressure and flow conditions existing in the high-speed opening process according to fig. 3A of a roll nip between a multi-calender's two intermediate rolls, in the control by a hydraulic cylinder for the pressure load of a supporting lever, on the piston head side and piston rod side of the cylinder.

Fig. 4A shows schematically a long-nip calender in a view directly to the calender's end face. Fig. 4B shows in an enlarged scale a long-nip zone for the calender of fig. 4A.

Fig. 5 shows schematically a control process for an oil-water heat exchanger by means of a digital hydraulic unit.

Fig. 6 shows schematically a pressure load adjustment mode for one loading element based on a closed control system according to the prior art. Fig. 7 shows schematically a pressure load adjustment mode for an active roll loading element based on an open control system according to the invention.

Calendering

The adjustment of sections containing hydraulic actuators, such as the hydraulic cylinders of a calender intended for the calendering of paper or board, is presently performed by using analog slide valves, in which the slide's position (flow, pressure) is proportional to the value of an adjustment instruction (control signal). Typical operations for adjusting the hydraulic actuators of a calender by means of analog slide valves include: -pressurization of hydraulic cylinders coupled with the bottom and/or top roll of a multi-roll calender's roll set, which are used for adjusting pressure in the roll set's roll nips, as well as the opening process of roll nips in faults such as web breaks;

-load adjustment for a multi-roll calender's roll nips by changing the pressure load of hydraulic actuators on supporting levers acting on the ends of intermediate rolls; -adjustment of the total pressure in a multi-roll calender's roll nip and the CD- directed pressure profile of a roll nip by changing the pressure load of intra-roll loading elements (hydraulic actuators applying pressure to the roll shell from inside a roll). A typical roll like this is the Applicant's so-called Sym-roll;

-high-speed opening of a multi-roll calender's roll nips in faults such as, for example, web breaks, by reducing rapidly the pressure load of respective loading cylinders on supporting levers present at the ends of intermediate rolls and on the calender's top and/or bottom rolls;

-reduction of the nip load in a multi-roll calender's roll set in the process of replacing a fiber web to be run on the calender (web feeding). In this case, the multi-roll calender can be lever-loaded, i.e. the roll nips are loaded by means of hydraulic cylinders coupled with supporting levers present at the ends of intermediate rolls or by means of loading elements operated by intra-roll hydraulic actuators.

Controlling the operation of hydraulic actuators performing various calender functions (changes of the loading profile and load of roll nips in a roll set, the opening/closing of roll nips, and web feeding) by means of servo or proportional valves (slide valves), in which the slide's position is proportional to the value of a control signal, involves several drawbacks:

Each slide valve must be designed specifically for a particular flow channel (the nominal size of a valve and additionally the size of a valve's flow ports within the size range), which increases considerably the designing work for systems involving hydraulic components;

The controls of hydraulic actuators executed by slide valves are fault-sensitive and, in addition, the electronics of control components for slide valves is susceptible to heat aging and faults caused by high temperatures; Slide valves are designed in individual pieces with the result that their spare parts will probably be expensive;

The efficiency of slide valves is relatively poor as a result of adjustment fluctuation;

The implementations effected by means of slide valves necessitate doubling of the sensor system because of adjustment feedback. In the invention, hydraulic actuators are controlled by the digital valve groups of a digital valve unit, all or some of the previously used slide valves being replaced thereby. The digital valves included in digital valve groups are structurally identical, the only difference between the parallel-connected digital valves lying in a volumetric flow rate allowed through thereby as the valves are in an open position. In addition, as long as the volume flows allowed by the digital valves of a group to pass through are planned correctly, it is possible, in practice, to compensate for a failure of one valve by changing appropriately the volume flows proceeding through the other valves. There is hardly any control electronics contained in digital valves themselves, but the control thereof is performed by a separate apparatus-specific control system.

For the above reasons, several advantages are achieved by substituting one or more digital valve groups for slide valves used for controlling the pressure load of a hydraulic actuator in a calendar:

-Control of a hydraulic actuator with a digital valve group provides considerably more fault tolerance than hydraulic actuator control effected by a slide valve, because the failure of a single valve is not enough to significantly impair the operation of a digital valve group; -A digital valve group contains hardly any control electronics, whereby the resistance of its electronic components to heat aging, and to temperature as well as to vibration, shall not become a problem;

-Using digital valve groups with a high-speed adjustment response for controlling the operation of hydraulic actuators enables often the use of so-called feedforward type adjustment strategies with the result that sensor systems become simpler than in feedback adjustment strategies used in connection with slide valves, which necessitate a continuous sensing of adjustment parameters and changing of control on the basis thereof;

-Replacing a slide valve with one or more digital valve groups provides in many applications an efficiency improvement as high as 30 to 50%;

-In digital valve groups, all digital valves are standard components identical in terms of their technical configuration, the replacement of a malfunctioned component with another being significantly less expensive than in the case of a slide valve.

-Synchronization of the control for hydraulic actuators present at various ends of a roll is considerably easier by using digital valve units than by traditional means. When carried out in a traditional way, the control of two hydraulic actuators present at the opposite ends of a roll has relied on flow distributors, on arranging the actuators in series, and/or, in the case of actuator-specific control, on controlling each parallel-connected actuator with a feedback adjustment by means of servo/proportional valves. The use of precise control-action digital valve units enables the parallel-connected actuators to be controlled in parallel actuator- specifically by means of two separate digital valve units or by means of a single digital valve unit with the use of flow distributors.

Changing the load of a calender's supporting levers

Next follows a more detail description of one embodiment of the invention, i.e. the control by digital valve groups for hydraulic actuators working on the pressure of a calender's roll nip, as well as the control by analog valves for the same hydraulic actuators in the adjustment of a roll set's nip load, in the high-speed opening of roll nips, and in the rapid relief of roll nips. The examples describe applying the inventive embodiment to a multi-roll calender, but the invention can also be applied to single-nip calenders. Prior art

Fig. 2A shows schematically a multi-roll calender 500 in a view directly to the calender's end face.

The multi-roll calender 500 of a per se known design, shown in fig. 2A, comprises alternating heatable thermo rolls and polymer-coated rolls. The number of such thermo rolls and polymer-coated rolls in a multi-roll calender is typically 6 to 16 examples, e.g. the so-called Optiload calender used by the Applicant features typically 6 to 12 rolls, 2 to 5 of which are thermo rolls and 4 to 7 are polymer- coated rolls. The multi-roll calender's 500 roll set 50, shown by way of example in the figure, comprises 6 rolls, said rolls being provided with internal loading elements for changing the line pressure profile zone by zone in a roll nip N between two rolls. At least a top roll 5; 5c and a bottom roll 5; 5b of the roll set 50 are heatable thermo rolls, having also hydraulic actuators (hydraulic cylinders) 2; 20; 202, 201 coupled therewith for pressurizing the roll set in vertical direction. Between the top roll 5; 5c and the bottom roll 5; 5b are set four deflection compensated intermediate rolls 5; 5a, at the ends of which are supporting levers 3 linked to a calender body 55. The lifting force (=pressure load) applied by the supporting levers 3 to the intermediate rolls 5a, and at the same time a nip load of the roll nip N between two rolls with respect to a nip load of other roll nips (adjustment of the nip pressure profile in a roll set), can be regulated by means of the hydraulic actuators 2; 20 connected to the supporting levers. In addition, the supporting levers 3 are used for adjusting a deflection of the intermediate rolls 5a, which is caused by the own weight of these rolls 5a as well as by bearing loads existing at the ends of the rolls.

When the pressure load, applied for example on the supporting lever 3 of one of the intermediate rolls 5a in fig. 2A, included for example in the multi-roll calender 500 of fig. 2A and established by means of the hydraulic cylinder 20, is controlled by an analog slide valve regulating the flow and/or pressure on either side of the hydraulic cylinder's piston (both on the piston rod side and the piston head side), the adjustment strategy must be based on a feedback adjustment of the volume flow (and hence the pressure) on the cylinder's piston side. This causes problems especially in high-speed adjustment actions such as in opening processes of the roll nip N and in rapid pressure load changes of the roll nip N, particularly in the event that the pressure load of the roll nip N established by hydraulic cylinders must be changed to a considerable degree. Because of the internal feedback of a valve, the slide valve has a considerable internal delay and, furthermore, the major changes of a volume flow inflict fluctuation of the volume flow (and pressure) discharging from the valve. This reduces the speed and effectiveness of the slide valve adjustment, as well as undermines efficiency. The volume flow into the pressure and working side of a hydraulic cylinder is impossible to change rapidly by means of a slide valve, and thus, in practice, the adjustment of a back pressure on the rod side of a hydraulic cylinder must be limited to a passive changing of the volume flow as a function of the fluid flow delivered into the piston side and the pressure caused thereby. Therefore, neither a high-speed opening of the roll nip N in malfunction situations nor a high-speed relief of the roll nip load can be performed with a slide valve.

Description of a preferred embodiment of the invention

The method according to the invention, by using a digital valve group, provides a capability of regulating precisely and rapidly both the fluid pressure existing on the hydraulic cylinder's 2; 20 pressure side (piston head side) and the volume flow of a hydraulic fluid, as well as also the back pressure of a hydraulic fluid existing on the hydraulic cylinder's working side (piston rod side) and the volume flow. This is achieved by virtue of the digital valve group having a high-speed adjustment response to a digital adjustment instruction and by means of a non-feedback adjustment of the volume flow proceeding through the digital valve groups included in a digital valve unit. The latter benefit is a result of the fact that the digital valve group comprises a plurality of small digital valves, the volume flow proceeding therethrough being always constant in an open position, whereby the pressure difference between a fluid arriving at the valve and a fluid discharging from the valve is highly predictable for each aperture of the digital valve group. In one embodiment of the invention, the pressure of a hydraulic cylinder working on the supporting lever of a calender's intermediate roll is adjusted by means of at least one digital valve group connected to both the pressure and working sides of a hydraulic cylinder, regulating both the pressure side for its volume flow and pressure and the working side for its volume flow and pressure. In one preferred configuration of the invention, the hydraulic cylinders applying load on each supporting levers of one roll are controlled by their own digital valve units, which are synchronized functionally at a control system level. In another preferred configuration of the invention, the operation of a hydraulic cylinder applying load on the supporting lever of a roll is regulated by means of two digital valve groups coupled with the cylinder's pressure side and by means of two digital valve groups coupled with the cylinder's working side. Furthermore, in another preferred embodiment, the digital valve groups coupled with the pressure side of a hydraulic cylinder are used for performing a high-speed opening of the hydraulic cylinder.

In yet another embodiment of the invention, the pressurization of a multi-roll calender's roll nips is changed rapidly by means of digital valve groups coupled with the pressure and working sides of a hydraulic cylinder by changing the ratio of fluid pressures existing on the pressure and working sides of said hydraulic cylinders.

A digital-valve controlled adjustment according to the invention for regulating the pressure load for a calender's supporting levers provides multiple benefits over the above-described prior art:

-A digital valve group provides a capability of controlling rapidly the fluid pressure and the volume flow both on the pressure and the working side of a hydraulic cylinder, which enables an active and high-speed adjustment of the pressure existing on a first side and the back pressure existing on a second side of the cylinder's piston. The position of a hydraulic cylinder's piston and hence the pressure load applied by a hydraulic cylinder on supporting levers at the ends of a roll can be rapidly adjusted as desired, because the ratio of volume flow on various sides of the piston lends itself to a rapid adjustment. This results in a precise position adjustment (pressure load) of the intermediate roll's supporting lever and a possibility of executing a non-fluctuating and high-speed adjustment of the pressurization of a supporting lever even over narrow pressurization ranges. This, in turn, enables a high-speed and precise adjustment of the pressure load profile between various roll nips in a calender and the cd-directed pressure profile of the same roll nip.

-Because the pressure load adjustment for the supporting lever of an intermediate roll, carried out according to the invention, provides a means of controlling the pressure and working side fluid flow (and thereby also the fluid pressure) in a hydraulic actuator working actively on the loading of a lever, the resulting control process will be high-speed and good in terms of its efficiency and energy consumption both in changes of the pressure load at the start and end of a calendering process and in the high-speed opening of a roll nip, as well as in a steady-state condition during calendering, in which the objective is to maintain a certain equilibrium pressure state in the hydraulic actuator. When compared to the adjustment of a supporting lever loading carried out by analog technology, the hydraulic actuator adjustment mode according to the invention, based on digital hydraulic valve units, is capable of achieving an energy saving of about 30 to 50%.

-The adjustment mode of the invention is practically non-fluctuating, because it involves controlling the fluid flow and the fluid pressure on both sides of a hydraulic cylinder by means of digital valve groups with a high-speed opening capability and by means of a feed-forward mode of adjustment.

With regard to further benefits attainable by the invention, it should be noted that the pressure load adjustment for a roll supporting lever implemented by means of digital valve groups receiving a digital adjustment instruction is more versatile than the respective adjustments implemented by means of slide valves subjected to analog control. This is due to the fact that a digital valve group provides a capability of operating effectively over a considerably more extensive operating range than what is possible by means of a slide valve; one and the same digital valve group makes it possible to operate within the range of both a minor and a major volume flow delivered into a hydraulic actuator. This is due to the structural design of a digital valve group; each digital valve group consists of 2...n examples of separate parallel-connected digital valves, the rate of volume flow passing therethrough in an open position being known exactly and providing a possibility of freely selecting those of said valves that have a fluid flow passing through. Thus, with a certain number of digital valves, it is possible to establish various volume flows by opening appropriate digital valve combinations. On the other hand, in the process of expanding the nominal flow and pressure difference of a slide valve, there is a likelihood of encountering flow engineering type restrictions (laminar vs turbulent flow), delays caused by intra-valve feedback, technical restrictions in terms of valve materials, and increasing investment costs. In addition, the adjustment of hydraulic actuators, carried out by digital valve groups, is practically non-fluctuating, which is why it is considerably more stable and quicker than that carried out by means of slide valves of the prior art.

The calender's supporting lever loading process according to the invention, implemented with digital valve groups, will now be described in more detail by way of example.

Fig. 2B shows schematically a control of hydraulic actuators 3 used for loading the supporting levers 3 of one intermediate roll in a multi-roll calender, performed by a single digital valve unit 100. Fig. 2C shows schematically data and fluid flows for the control of one of the hydraulic actuators of fig. 2B.

Fig. 2D shows likewise schematically a control of hydraulic actuators 2 used for loading supporting levers 3 present at the ends of one intermediate roll in a multi- roll calender, performed by specified digital valve groups 100; 100' and 100; 100", and a synchronization of operation for the thus controlled hydraulic actuators 2.

Fig. 2B visualizes the adjustment of pressurization for supporting levers 3 present at each end of an intermediate roll (not shown in the figure) by means of a digital hydraulic unit 100 containing four digital valve groups 10. The adjustment of pressure load for the supporting lever 3 present at each end of an intermediate roll by means of a hydraulic actuator 2; 20; 20' and 2; 20; 20" coupled therewith is performed with identical systems, which is why the following discussion relates more closely to the pressure load adjustment of only the left-hand side supporting lever 3. The inlet flow proceeding to a pressure side 20b of the hydraulic actuator 2; 20 passes by way of a valve unit 30, which includes high-speed opening and safety valves; these can be implemented either by traditional slide valve engineering or alternatively by one or more digital valve groups.

Visualized down below in fig. 2B is a digital valve unit 100, which is coupled with the hydraulic cylinder 20 functioning as a hydraulic actuator 2 used for loading the supporting lever 3, and which replaces a commonly employed 4/2-way valve (slide valve). The presented digital valve unit 100 enables a concurrent control of two flow channels 6; 61 , 62, which extend to pressure and working sides 20b, 20a of the hydraulic cylinder 20, respectively. In this case, the digital valve unit 100 comprises 20 on/off digital valves divided into four digital valve groups 10; 10a, 10b, 10c, 10d, comprising two digital valve groups 10c, 10d regulating actively the pressure side (piston head side) 20b of the hydraulic cylinder and two digital valve groups 10, 10b regulating actively the working side (piston rod side) 20a of the hydraulic cylinder. The digital valve groups 10b and 10c regulate an inlet flow v_s of pressurized hydraulic fluid from a supply line 7; 71 respectively to the working side 20a of the cylinder 20 as well as to the cylinder's pressure side 20b along respective flow lines 6; 61 and 6; 62. The digital valve groups 10d and 10a regulate respectively a hydraulic fluid outlet flow from the cylinder's pressure side 20b and working side 20a to a tank line 7; 72. The fluid flow, pressurized to a certain known pressure, arrives by way of the same supply line 7; 71 across a throttle valve at the digital valve groups 10c, 10b which control the inlet flow v_s to be delivered both to the pressure and the working side 2Ob₁ 20a of the hydraulic cylinder 20. The pressure of inlet flow along the supply line 7; 71 is established for example by a pump (not shown in the figure). The hydraulic fluid discharges from the respective digital valve groups 10d and 10a, which regulate the fluid flow coming out of the pressure and working sides of the hydraulic cylinder, proceeding into the tank line 7; 72 which carries the fluid into a storage tank (not shown in the figure) by way of a counter-valve. Each digital valve group 10 of the digital valve unit 100 includes 5 on/off digital valves 1 , the flow rates passing therethrough in an open position being proportioned in such a way that the first valve has a flow rate of 1 unit, the second has 2 units, the third has 4 units, the fourth has 8 units, and the fifth valve has 16 units, whereby the digital valves 1 of each digital valve group have a capability of providing 31 different valve combinations 1_A of open-state digital valves, corresponding to 31 flow rate combinations or different opening degrees for the digital valve group 10. The pressure and working sides 20b, 20a of the hydraulic cylinder 20 have their respective flow lines 6; 61 and 6; 62 connected to respective pressure measuring devices M; M", M', each of which comprises a bellows type equalizer as well as a pressure gauge. The pressure measuring devices M enable measuring a fluid pressure existing at a particular time in the flow lines 6; 61 and 6; 62 of the hydraulic cylinder's 20 pressure and working sides 20b, 20a in an inlet or outlet flow arriving at or discharging from the digital valve group 10. Based on the measured pressure and on an inlet pressure p_s of the inlet flow v_s coming to the digital valve groups 10 from the supply line 71 , it is possible to determine an appropriate aperture for a digital valve group (= a total area presented by the openings of open-state digital valves in the group) for each desired pressure load of the hydraulic cylinder 20 as described hereinafter. In this context, a pressure load P_k of the hydraulic cylinder 20 refers to the pressure applied by the hydraulic cylinder 20 (piston 22) on the supporting lever 3.

The use of such a digital valve unit 100 makes it possible that a volume flow V; V_2Oa, V₂Ob to be directed onto either side 20; 20a, 20b of the hydraulic cylinder's 20 piston 22 be adjusted actively, faster, and accurately over a more extensive flow range than what is achieved by most commercially available slide valves, which control two different flow channels simultaneously. Such an active adjustment, carried out by the digital valve groups 10 and regarding volumetric flow rates V₂₀; V₂Ob, V₂Oa and fluid pressures P₂o; P₂Ob, P20a of the hydraulic cylinder's 20 pressure and working sides 20b and 20a, proceeds substantially faster than with a slide valve, in which the adjustment of the hydraulic cylinder's 20 working side 20a cannot be effected quickly and actively because of delays caused by the internal feedback of a slide valve and because of the slowness of a feedback adjustment to be carried out by a slide valve. As opposed to this, the adjustment according to fig. 2B, implemented by means of the digital valve unit 100, enables the use of a feed-forward, anticipating mode of adjustment for controlling a back pressure (= working side pressure) P_20a in the hydraulic cylinder 2; 20, because the flow rate proceeding through a valve combination 1_A of the digital valves 1 currently in an open position within the digital valve group 10, and thereby also the hydraulic fluid pressure difference attainable by such a combination between an inlet pressure p_s of the fluid arriving at the digital valve group 10 and a pressure of the fluid discharging from the digital valve, i.e. a flow line pressure P₆ ; P₆i or P₆; P₆2 , can be anticipated at a high accuracy.

In the exemplary circuit depicted in figs. 2B and 2C, the fluid flow arrives under the certain pressure p_s at the digital valve unit 100 by way of the supply line 7; 71 closable by a throttle valve and proceeds to the cylinder's pressure side 20; 20b or working side 20; 20a through the fluid-flow controlling, respective digital valve group 10; 10c or 10; 10b. In the supply side digital valve group 10; 10c or 10; 10b there has been opened a digital valve combination 1_A; 120b tai 1_A; i20a. which provides a desired volume flow V₆; V₆₂ in the flow line 6; 62 extending from the digital valve group 10; 10c to the cylinder's 20 pressure side 20b or a volume flow V; V₆i in the flow line 6; 61 extending from the digital valve group 10b to the cylinder's working side 20a. The volume flow V₆ discharging from each digital valve group 10; 10b, 10c to the hydraulic cylinder 2; 20 along the flow line 6 and the volume flow V₂o; V₂oa_> V₂Ob established on the various sides 20a, 20b of the cylinder's piston 22, respectively, can be anticipated accurately from a pressure difference dp existing between an inlet pressure p_s of the fluid arriving at the digital valve group (10b or 10c) along the supply line 71 and a pressure P₆; Pβ2, Pβi existing in the respective flow line 6; 62, 61 extending downstream of the digital valve group 10 to the cylinder's 20 pressure or working side (the pressure Pe of the flow line 6 is P₆i in the flow line 6; 61 extending to the cylinder's working side 20a or P₆₂ in the flow line 6; 62 extending to the pressure side 20b), as well as from a current aperture 1_A of the digital valve group 10 (for example 1₂o_a in the digital valve group 10b controlling a flow proceeding to the hydraulic cylinder's working side 20a or 1₂o_b in the digital valve group 10b controlling a flow proceeding to the hydraulic cylinder's pressure side 20b). The pressure difference dp between a flow V₆, which has proceeded through any digital valve group 10 into the flow line 6, and an inlet flow v_s, which has arrived at this particular digital valve group 10, depends in turn on the aperture 1_A of this digital valve group 10.

When a change of pressure load in the roll nip N is desired, a change is made in the ratio of respective volume flows V₂Ob, V₂Oa streaming into the pressure side 20b and on the working side 20a of the cylinder 20, resulting in a respective change in the ratio of fluid pressures P₂Ob, P2₀a existing respectively on the cylinder's pressure and working sides 20b, 20a. This guides a pressure load P_k, applied by the cylinder 20 to the supporting lever 3 of an intermediate roll, in a desired direction. The volumetric flow rates V₂₀b ja V₂o_a arriving in the hydraulic actuator's 20 pressure side 20b and working side 20a are changed by modifying an aperture 120_b or 1_20a of the respective digital valve groups 10c or 10b controlling the inlet side flow of a hydraulic fluid. The volume flow V₆; V₆₂ or V₆; V₆i discharging from the inlet side digital valve group 10; 10c and/or 10; 10b into the flow line 6; 62 or 6; 61 and/or the respective fluid pressure P₆; P₆₂ or P₆; P₆₁ existing in said flow lines result in a certain volume flow and fluid pressure on various sides 20a and 20b of the hydraulic cylinder's 20 piston. Therefore, a modification of the aperture 1A ;i20b or 1A ; 1₂₀a for the digital valve group 10c or 10b can be effected on the basis of a new volumetric flow rate level V₂o; V_20b and/or V₂₀; V_20a and/or a new respective pressure and back pressure level P₂₀; P2ob and/or P₂₀; P₂Oa desired for the hydraulic cylinder's 20 pressure side 20b and/or working side 20a, when said volume flow or pressure resulting in the cylinder's 20 various sides 20a or 20b is known by calculation or empirically on the basis of a volume flow V₆; V₆i or V₆; V₆₂ of the flow line 6; 61 or 6; 62 and/or on the basis of a fluid pressure P₆; P₆i or P₆; P₆₂ existing in the flow line. When the inlet side digital valve group 10b or 10c, leading to the hydraulic actuator's 20 pressure or working side 20b or 20a, has one or more of its digital valves in an open position, the respective outlet side digital valve group 10a or 10d, controlling the outlet flow of the pressure or working side, has all of its digital valves in a closed position (the aperture 1A of the group 10a or 10d is 0). When the desired loading pressure P_k for a supporting lever 3 is reached, the control proceeds to a steady-state condition (equilibrium) by bringing the equilibrium pressures existing on the hydraulic cylinder's 20 pressure and working sides 20b and 20a to equality P₂obτ=P2θaτ- When knowing the fluid pressure P₂oτ and the volume flow V₂₀τ , i.e. the volume flow V₂Ob=V₂ObT ja V_2Oa=V₂oaτ to be supplied into the hydraulic cylinder's 20 pressure and working sides 20b and 20a during a calendering process (steady-state condition), as well as the level of the equilibrium pressure P20b=P20bτ and the back pressure P₂Oa =P22₀aτ existing on the hydraulic cylinder's pressure and working sides 20b and 20a during a calendering process, for example on the basis of a pressure load P_k required from the hydraulic cylinder 20 on the supporting lever 3, this knowledge can be used for calculating or estimating the valve combination 1A for the digital valves 1 to maintained open each time in the digital valve group 10, which combination is able to establish the fluid pressure P20T and the volume flow V2₀T for an equilibrium T of said hydraulic cylinder. The pressure downstream of a digital valve group 10 can also be monitored by a pressure measuring device M and adjustment operations can be checked on that basis as necessary.

On the other hand, if there is a desire to quickly reduce the volume flow V₂ob/V2oa and the fluid pressure P2_0b/P_20a on the hydraulic cylinder's 20 pressure/working side 20b/20a, the pressure displayed each time by a pressure gauge M coupled with the respective flow line 62/61 of the pressure/working side 20b/20a and the new pressure desired for the hydraulic cylinder's pressure/working side are used as a basis for selecting the appropriate digital valves to be opened in the outlet- flow controlling digital valve group 10a or 10d. The respective digital valves of the inlet side digital valve group 10b or 10c are closed for the purpose that the pressurized fluid flow coming from the supply line 71 be prevented from migrating into the flow line 6; 62 or 6; 61 leading to the cylinder's 20 pressure or working side 20b or 20a. As it is, the fluid flow proceeds from the hydraulic cylinder's 20 pressure or working side 20b or 20a along the respective pressure- or working- side flow line 6; 62 or 6; 61 to the discharging flow (outlet flow) controlling digital valve groups 10d or 10a. The appropriately opened digital valves 1 of these digital valve groups 10a and 10d enable adjusting a flow rate V_6; V₆i or V₆;V₆₂ proceeding from the discussed digital valve groups to a tank line 7; 72 along the flow line 61 or 62, and thereby adjusting the rate of outlet flow and the magnitude and rate of pressure fall on the cylinder's 20 pressure or working side 20b or 20a.

Fig. 2C shows schematically data flows traveling between a control system 4 and a digital valve unit 100 in the process of changing and maintaining a pressure load

P_k for loading levers 3; 3' and 3; 3"^" of a multi-roll calender's one intermediate roll, and on the basis of the data flows, also fluid flows to a pair of hydraulic cylinders

20; 20', 20" applying a load on the supporting levers 3; 3' and 3; 3" present at the ends of an intermediate roll. The figure illustrates one of the hydraulic cylinders in the pair of hydraulic cylinders 20, because the hydraulic cylinders are structurally identical. A control unit 42 included in the control system 4 receives information continuously or at specific intervals from pressure gauges M; M' and M; M" regarding fluid pressures P₆; P₆₂, PΘ; Pβi existing at a particular instant in the flow lines 6; 62 and 6; 61 leading from the digital valve unit 100 to the piston head side (pressure side) 20b and the piston rod side (working side) 20a of the hydraulic cylinders 20. On the basis of this pressure data P₆ measured by measuring gauges and on the basis of a pressure load P_k ; P_k- as well as P_k; PK to be applied on the supporting levers 3; 3' and 3; 3", the control system 4 is able to determine an appropriate anticipating adjustment strategy for changing processes of a roll nip load or for holding a roll nip at a steady-state load. In the case of a changing process, the control systems 4 decides, on the basis of a previously programmed anticipating adjustment strategy, which way to change the ratio of fluid pressures P2₀; P∑oa and P₂₀; P_20b on the hydraulic cylinders' 20 working and pressure sides, to which extent and over which time period, such that a nip pressure for the roll nip, and at the same time pressure loads P_k ; P_k ¹, P_k acting on the hydraulic cylinders' nip pressure, are established as desired. On the basis of these pressure changing parameters, the control system's 4 control unit 42 works out volume flows V₂o; V_20a and V₂o; V_2Ob of fluid desired at a particular instant for each hydraulic cylinder's 20; 20' or 20; 20" piston rod side (working side) 20a and piston head side (pressure side) 20b, and possibly also respective fluid pressures P₂₀; P₂Oa and P₂₀; P₂Ob- The volume flows V₂₀; V_20a and V₂₀; V_20b of each cylinder's working side and pressure side are matched by certain volume flows V₆;V₆i and V₆; V₆₂ as well as pressures PβiPβi ja P₆; P₆₂ in flow lines 6; 61 and 6; 62 extending to the hydraulic cylinder's 20 working and pressure sides 20a and 20b downstream of the digital valve groups. The control system's 4 control unit 42 supplies a calculator unit 41 with information about these new flow rates of the cylinder's 20 working and pressure sides, the calculator unit 41 working out as to which aperture 1A of each digital valve group 10 is needed in order to reach desired volumetric flow rates and transmitting a respective adjustment instruction to each digital valve group. The adjustment instruction transmitted to each digital valve group 10; 10a, 10b, 10c, 10d is a binary-mode adjustment instruction, which comprises a volume flow adjustment function F(V) for hydraulic fluid or a position adjustment function F(X) for a hydraulic cylinder's piston and contains information regarding at least which valves 1 in each digital valve group 10; 10a, 10b, 10c, 10d will be open and which ones will be closed (opening of a digital valve group) and for how long.

The above-described modification of fluid flows and pressure for the hydraulic cylinder's 20 pressure and working sides concerns primarily a starting/finishing process in the calendering of a particular paper/board grade, in which the changes of a pressure load are quite substantial. As the calendering continues in a steady- state condition, the objective is to maintain the pressure-side pressure and the working-side counter pressure of each hydraulic cylinder 20; 20' as well as 20; 20" equal to each other and thereby the objective is that the pressure load P_k , which is applied from the hydraulic cylinder to the loading lever of an intermediate roll, be maintained at a certain constant level. Because each digital valve group 10 has an ability to provide a large number of unequal discrete volume flows in the flow lines 6; 61 and 6; 62, resulting in an equally large number of volume flow/pressure states for the hydraulic cylinders' 20 pressure and working sides, it is possible to use one and the same digital valve group to implement both gradual volume flow and pressure modifications taking place in the steady-state condition and also major pressure and volume flow changes taking place at the starting and finishing stage of a calendering process. When using the above-described mode of adjustment, there is previous knowledge regarding a volume flow that can be established for flow lines 6 by a particular aperture 1_A of each digital valve group 10, i.e. by a certain combination of open-state valves, and hence the adjustment of pressure for the hydraulic cylinder's working or pressure side need not be performed in a feedback manner. Thus, the demand of measuring signals, needed in the system 4 controlling a fluid flow and its pressure for the pressure and working sides of each hydraulic cylinder 20, will be simplified and a doubled sensor system, needed for a return cycle in connection with analog valves, is no longer necessary.

Synchronization of hydraulic actuators

The synchronization of two or more hydraulic actuators 2 has been traditionally implemented in papermaking industry either by using flow distributors, by connecting the actuators in series or by controlling each actuator independently with servo/proportional valves featuring a positional or flow-related feedback.

In the event that synchronization is carried out by means of flow dividers, the accuracy of said synchronization is dependent on manufacturing tolerances in the components of said flow distributors. On the other hand, if the hydraulic actuators

2 are set in series, there will be a problem of malfunction; if going out of synchronism, the actuators must be subjected to a maintenance work which generally requires external actions. A problem with actuator-specific control, effected by means of control circuits containing servo/proportional control valves, is the high cost of such circuits. In addition, a drawback with such control valves is a substantial pressure loss, and in order to implement synchronization, there is needed a special position adjustment control as well as a feedback of adjustment.

Such synchronization for the operation of two hydraulic actuators is possible to carry out in a traditional manner by means of digital valve units 100, both by using a series connection of the actuators 2 and by using flow distributors, whereby a volume flow discharging from one and the same digital valve group is distributed for various hydraulic actuators. The system depicted in figs. 2B and 2C uses the same digital valve groups 10 of the digital valve unit 100 to control synchronically two hydraulic actuators 2; 20' and 2; 20" present at each end of a roll. The flows emerging from each digital valve group are branched at an appropriate point for various hydraulic actuators, as displayed in fig. 2B.

However, synchronization of the hydraulic actuators 2 can be performed preferably by digital valve engineering by using actuator-specifically one or more digital valve units 100: each actuator is adjusted separately by means of its own digital valve unit and the operation of these digital valve units is synchronized at a control system level. Each digital valve group of the digital valve unit 100 is supplied as an adjustment instruction with a time-linked flow instruction F (V) or a position adjustment function F (X) (see fig. 2C), and this is followed by the digital valve group adjusting accurately, without delay, a volume flow bound for the actuator / arriving from the actuator. Thus, the digital valve group regulates accurately the speed of a hydraulic actuator. The accuracy of adjustment carried out by the digital valve unit 100 results a) from the fact that the operation of each digital valve group 10 of a digital valve unit can be controlled accurately in a feed-forward mode of adjustment, the adjustment taking place without feedback and without time delay, and b) from the fact that the adjustment accuracy of a digital valve unit is directly proportional to the number of digital valve units contained in a system and to the nominal volume flow of each valve, as indicated above in relation to the description of figs. 1A and 1B. Even a very minor increase in the number of on/off digital valves results in a remarkable improvement in the accuracy of adjustment.

The embodiment shown schematically in fig. 2D illustrates a control for the operation of two parallel-connected identical hydraulic actuators 2, said control being implemented by means of two digital valve groups 100; 100' and 100; 100" which are separate, yet functionally interconnected by means of a control system. The hydraulic actuators 2 are hydraulic cylinders 20, which are used for controlling the position of relief levers present, for example, at the ends of a common roll the same way as presented in figs. 2B and 2C. Each of both hydraulic cylinders 20; 20' and 20; 20" is controlled by its own specific digital valve group 100; 100' and 100; 100". Each digital valve unit 100' or 100" contains four digital valve groups 10; 10a, 10b, 10c, 10d. The digital valve units 100' and 100", as well as the digital valve groups 10 contained therein, are structurally and operationally identical to each other, which is why like structural components of said digital valve units are designated with like reference numerals. The digital valve groups 10a, 10b, 10c, 10d contained in each digital valve unit 100; 100', 100" are used for adjusting a flow arriving in the pressure or working side of whichever hydraulic actuator 20' or 20" or discharging therefrom. Each digital valve group 10 comprises n pieces of digital valves, fig. 2D only displaying the first and last digital valves 1 in each digital valve group 10. The digital valve groups, designated by reference numerals 10a and 10b, are used for adjusting the inlet and outlet flows for each hydraulic actuator's pressure side (piston head side) 20b by way of a flow line 6; 62. The digital valve groups, designated by reference numerals 10c and 10d, are in turn used for adjusting the inlet and outlet flows for each hydraulic actuator's 20 working side (piston rod side) 20a by way of a flow line 6; 61. Thus, A pressurized hydraulic fluid flow v_s ; v_S' tai v_s; v_s-, arriving from an supply line 7; 71 , proceeds by way of a valve to the digital valve unit 100; 100' or 100; 100" and further through the digital valve groups 10b or 10c of each digital valve unit 100' or 100" into the relevant hydraulic cylinder's 20 pressure or working side 20b or 20a (in this order). The hydraulic fluid flow v_t; v_t- or v_t; v_t- discharges from each hydraulic cylinder's 20; 20' or 2; 20" pressure or working side 20b or 20a by way of the flow line 62 or 61 to the digital valve groups 10a or 10d and further to a tank line 7; 72. The establishment of a volume flow of desired magnitude for each hydraulic cylinder's 20; 20' or 20; 20" pressure or working side 20b or 20a is performed in a manner analogous with the system shown in figs.2B and 2C, yet bearing in mind that each hydraulic cylinder 20; 20' or 20; 20" has its own digital valve unit 100; 100' or 100; 100" controlling the same. The inlet flows v_s; v_s- ja v_s; v_s- arriving at the hydraulic cylinders20* and 20" from the supply line 7; 71 or the outlet flows v_t; v_r ja v_t;v_t- discharging from the hydraulic cylinders 20' and 20" to the tank line 7; 72 are in no contact whatsoever with each other before reaching the respectively common supply line 7; 71 or tank line 7; 72. Indeed, the mutual synchronization for the operation of the digital valve units 100' and 100", and at the same time that of the hydraulic actuators 20, is handled in terms of control engineering through the intermediary of a control system (not shown in the figure) issuing adjustment instructions thereto. Thus, the control of each hydraulic actuator's 20' and 20" volume flow with a certain delay time is conducted by a feed-forward mode of adjustment by supplying each digital valve unit 100' or 100" with a desired volume flow instruction F(V)' and F(V)" at a specific time interval, said volume flow instructions F(V)' and F(V)" being identical. The digital valve units could just as well be only supplied with a position adjustment instruction F(X) regarding the hydraulic actuator's 20 piston as a result of knowing the pressures existing at the position of the hydraulic actuator's 20 piston and on the hydraulic actuator's working and pressure sides. This is due to the fact that the pressures existing at a specific time on the hydraulic actuator's 20 working and pressure sides 20a, 20b are directly proportional to fluid pressures existing in the flow lines 61 and 62 and to a volume flow proceeding through the digital valve groups 10 of the digital valve unit 100 (to the aperture of digital valve groups), as explained in connection with the description of a system visualized by figs. 2B and 2C. The digital valve units 100; 100', 100" do not involve major delay times, but, instead, the implementation of a certain volume or position adjustment instruction F(V), F(x) is precise, high- speed, and excellent in terms of its repeatability, whereby the respective hydraulic actuators 20', 20" coupled with these digital valve units 100', 100" operate in synchronism.

The operational synchronization of two different hydraulic actuators' working and pressure sides is not possible in practice as a result of differences deriving from the manufacturing tolerances of valves, a high price, and adjustment problems at small openings of valves. On the other hand, by using digital valve units, such a 4- way coupling of two hydraulic actuators, in which flows arriving in and discharging from both the working and pressure sides 20a and 20b of a hydraulic cylinder are controlled by means of their own, independently controlled hydraulic group 10a, 10b, 10c, 10d, can be established without problems.

High-speed opening of a calender's roll nip and instant changing of a roll nip load

In the process of changing board/paper grades or linking a board/paper web to another board/paper web, it would be desirable to relieve a calender's roll nips only when the splice between two successive fiber webs passes through the calender's roll nips. Such a demand has developed in calenders located downstream of on-machine coaters, as well as in off-machine flying splice calenders.

On the other hand, in the event that a calender's roll nip must be opened completely, for example in a web break situation, it would be desirable if the driving moment of rolls (rotating speed of rolls) did not need changing in the process of opening and again closing the roll nip.

At present, servo and proportional valves (slide valves) are used for adjusting the pressure of hydraulic actuators acting on supporting levers at the ends of intermediate rolls or from inside a roll on the roll shell and/or for adjusting a loading cylinder which lifts or lowers the top or bottom roll of a multi-roll calender. However, slide valves only work effectively over a narrow volume flow/pressure range and have a relatively long delay in the performance of adjustment; if the pressure load of a hydraulic cylinder must be changed quickly and at the same time the volume flow into the hydraulic cylinder's working and pressure side undergoes a considerable change, there will be problems because of a delay resulting from the feedback adjustment of a slide valve's stem and because of a fluctuation of adjustment resulting from the feedback adjustment strategy, even if the hydraulic actuator itself were to operate instantly and accurately. In practice, the slide valves are capable of performing neither the high-speed relief of a calender's roll nip load nor the quick instant opening and closing of a roll nip while maintaining the calender's running speed.

It is an object of the invention to eliminate the drawbacks existing in the foregoing prior art. Accordingly, an objective of the invention is to provide an apparatus and method, which enable the nip load of a roll nip to be instantly relieved and to reset the original nip load in such a way that the loading-relieving-reloading cycle of a roll nip becomes as rapid as possible, yet at the same sufficiently precise in terms of the roll nip's pressure load variation. Another objective of the invention is to provide the high-speed opening and re- closing of a roll nip while the running speed of a calender remains unchanged.

The apparatus embodied according to the invention, as well as the roll nip relieving and opening method applied therein, shall provide a capability of attaining the above-described objectives. This embodiment of the invention is based on the fact that the hydraulic actuators, such as hydraulic cylinders 20, which act on levers 3 present at the ends of intermediate rolls 5 or from inside a roll on the roll shell, and/or on a loading cylinder which applies pressure directly on the top or bottom roll of a calender, are controlled by means of a digital valve unit 100 for creating a relief pulse for the pressure load of a roll nip N and thereafter a reset pulse for the pressure load of the roll nip N. The position of a joint included in a fiber web arriving at a multi-roll calender is identified and the time of its arrival at the calender's each roll nip and its passage therethrough is estimated or calculated. The successive roll nips of a multi-roll calender 500 are subjected to a roll nip relief pulse and reset pulse in an appropriate synchronism with each other for conveying a joint H; W₈ between two fiber webs in a controlled manner through the multi-roll calender's roll nips.

In order to produce a relief pulse, the pressure load in the roll nip N is reduced by using the digital valves of an appropriate digital valve group 10 for instantly cutting back the volume flow into the pressure side (piston side) of a hydraulic cylinder 20 with respect to the volumetric fluid flow existing on the hydraulic cylinder's 20 working side in the state of equilibrium. Once the pressure load applied by the cylinder 20 on a loading lever 3 has been adequately reduced, the ratio of pressures existing on the hydraulic cylinder's 20 working and pressure sides is returned to equality. By using the digital valves of an appropriate digital valve group 10 (a loading pulse) for instantly increasing, after a certain period of time, the fluid flow to be supplied into the pressure side of a hydraulic actuator with respect to the volumetric fluid flow existing on the working side in the state of equilibrium, the pressure load applied by the hydraulic actuator to a roll nip shall be returned to a level existing prior to the relief pulse. Once the pressure load has been returned to the pre-change level, the pressures of a hydraulic actuator, such as a hydraulic cylinder 20, shall be equalized with each other.

Another embodiment of the invention is in turn based on subjecting roll-loading elements, which are coupled with hydraulic actuators (e.g. hydraulic cylinders), to a stepwise, accelerated opening pulse by means of digital valve units linked to the hydraulic actuators. The reduction of load for the roll-loading elements takes place in a feed-forward based adjustment mode by modifying a volume flow arriving at the hydraulic actuators (and at the same time a pressure existing therein) according to a certain, previously determined volume flow modification profile. This embodiment of the invention is viable for example during a fiber web breaking incident occurring in the process of calendering; the fiber web break point on a fiber web arriving at the calender is identified and the time of its arrival at the calender's roll nip is estimated and calculated. When the fiber web break point arrives at a roll nip, said roll nip will be instantly opened by a method of the invention and then closed in a conventional manner. These embodiments of the invention are specified hereinafter by exemplary working examples and by figures 3A and 3B relevant thereto.

Fig. 3A shows schematically the provision of a relief and reset pulse for a roll nip in a multi-roll calender as said roll nip receives an fault present on a fiber web.

Fig. 3B shows one roll nip in a multi-roll calender, which is about to receive an fault present on a fiber web.

Figs. 3C-3E illustrate the changes of volume flow and fluid pressure taking place during the relief and rest pulse of fig. 3A in a hydraulic cylinder loading the supporting lever of a roll. Fig. 3F shows one roll nip in a multi-roll calender, which is about to receive a web break.

Fig. 3G shows in turn the instant opening of a multi-roll calender's roll nip by a method of the invention as said roll nip is about to receive a web break present on a fiber web. Fig. 3H illustrates the changes of volume flow and fluid pressure taking place during the roll nip opening of fig. 3E in a hydraulic cylinder loading the supporting lever of a roll.

The provision of a relief and reset pulse for a roll nip shown in figs. 3A-3E, as well as the instant opening of a roll nip shown in figs. 3F-3H, will be illustrated by way of example as applied to the multi-roll calender 500 of fig. 2A, whose control system 4 for the supporting lever 3 of an intermediate roll 5; 5a and whose digital valve unit 100 controlling the supporting lever are consistent for example with those of fig. 2D. Likewise, the control of loading cylinders used for lifting the bottom roll 5; 5b and pressing the top roll 5; 5b of the calender's 500 roll set can be implemented with an at least partially similar digital valve unit 100 and its control system 4.

Figs. 3A-3E illustrate the provision of a relief and reset pulse, controlled by a digital valve unit 100, for a hydraulic cylinder 20 acting on a supporting lever present at the end of a multi-roll calender's intermediate roll as the roll nip is traversed by an fault H, such as a joint H; W₈ between two fiber webs as shown in fig. 3B. The relief and rest pulse provides a means for changing a pressure load K_p applied by the hydraulic cylinder's 20 piston 22 to the supporting lever while the fault passes through the roll nip. The figures only illustrate the changing of a pressure load applied to just one of the supporting levers at the end of an intermediate roll by means of a relief and reset pulse, because of the fact that the supporting supporting lever at the other end of an intermediate roll is subjected to an identical, synchronized relief and reset pulse with a control similar to that shown in figs. 2B-2D.

Fig. 3B visualizes two superimposed rolls 5, leaving a roll nip N therebetween. The rolls comprise for example two rolls of fig. 2A, included in a multi-roll calender. The roll nip is traversed by a fiber web W to be calendered. The figure shows a condition, in which the roll nip N is just about to receive a joint W₈ between two different fiber webs W; W1 and W; W2.

In the top part of fig. 3A is shown a roll nip relief and reset pulse produced as the fiber web's W joint H; W₅ of fig. 3B arrives at a roll nip. The figure visualizes the changes during a roll nip relief and reset pulse in a volume flow V20b of hydraulic fluid into the hydraulic cylinder's 20 pressure side (piston head side) 20b over a certain time t, and the bottom part of the figure shows the changes that have taken place over the same time t in a pressure load K_p applied on the supporting lever. This embodiment of the invention has its basis on a feed-forward control of the pressure load K_p implemented by a digital valve unit 100 both during the roll nip's N pressure load relief pulse and during the roll nip's N pressure load reset pulse.

Figs. 3C-3F illustrate the actions of a hydraulic cylinder's piston 22, which take place during a roll nip relief and reset pulse and which are used for controlling the changes of a pressure load K_p for the supporting lever of a roll located at the roll nip. In addition, figs. 3B-3D visualize hydraulic fluid flows V_2Oa and V₂Ob delivered from a digital valve unit 100 into various sides 20a and 20b of the hydraulic cylinder's 20 piston 22, as well as a fluid pressure P2₀a.P2₀b established by the hydraulic fluid flow. Figs. 3C-3F illustrate the digital valve unit 100 schematically and it can be for example similar to what is shown in fig. 2D, comprising four digital valve groups 10a, 10b, 10c, 10d, two of which are used for controlling a fluid flow into the piston head side 20b of a hydraulic cylinder and two of which do the same for the piston rod side 20a.

Fig. 3F visualizes two superimposed rolls 5, leaving a roll nip N therebetween. The rolls comprise for example two rolls of fig. 2A, included in a multi-roll calender. The roll nip is traversed by a fiber web W to be calendered. The figure shows a condition, in which the roll nip N is just about to receive a web break W_k present on the fiber web W.

Fig. 3G, on the other hand, shows the instant opening of a roll nip N by a method of the invention as a web break H; W_k shown in fig. 3F is arriving at the roll nip. The top view shows the change over a certain time period t in a volumetric fluid flow V₂Oa to be delivered into the hydraulic cylinder's 20 pressure side 20b, and the bottom view shows schematically the positional change of a roll 5 in vertical direction (e.g. in a roll nip between two rolls, the positional change of the bottom roll's highest point in vertical direction) over the same time. As fig. 3E shows the positional change of a roll 5 in a calender's roll nip N, the change in the relative position of a piston 22 loading the roll by way of a supporting lever 3 with a pressure load K_p will be similar in a longitudinal direction of the hydraulic cylinder 20. The changes of the piston's 22 position in the opening of a roll nip and those of a relevant roll supporting lever's pressure load K_p are shown in more detail in fig. 3H.

When a volume flow V2₀ and a pressure P2₀ of the fluid to be supplied into a hydraulic cylinder 20 are controlled by a digital valve unit 100 containing a plurality of digital valve groups 10, as specified in connection with the description regarding the control of a roll supporting lever 3 in figs. 2B-2D, the volume flow passing through a particular digital valve group 10 in the digital valve unit 100 will be known beforehand within all the volume flow ranges of a hydraulic cylinder 20. The volume flow is known from the aperture 1A of each digital valve group 10, as well as from a pressure difference between a fluid pressure p_s existing in the supply line arriving at a digital valve group or a fluid pressure p_t existing in the tank line 7; 72 departing from this digital valve group and a fluid pressure P₆ running in the flow line 6 extending to the hydraulic cylinder 20.

When a digital valve group 10b of the digital valve unit 100 shown for example in figs. 3C-3F is supplied with an inlet flow V₈ along the supply line 7; 71 , the average pressure applied thereby to the walls of the supply line being p_s, the rate of a volume flow V₆₂ departing along a flow line 6; 62 from this particular digital valve group 10b to the pressure side 20b of a hydraulic cylinder 20 will be determined on the basis of a fluid pressure P₆₂ existing in the flow line 62 leading to the loading cylinder 20 and a current opening of the digital valve group 10b. The fluid pressure P₆₂ is precisely predictable on the basis of the digital valve group's 10b opening, i.e. a total flow port established by currently open-state digital valves, i.e. on the basis of a throttling degree provided by the digital valve group. Because the volume flow V₆₂ departing from the digital valve unit's 100 digital valve group 10b into the flow line 6; 62 (and further into the hydraulic cylinder's pressure side 20b), and the pressure P₆₂ existing in the flow line 62, are precisely predictable, a fluid pressure P_2Ob and a fluid flow V_2Ob developing on the hydraulic cylinder's 20 piston head side 20b are predictable even without a feedback adjustment, resulting in a high-speed and accurate adjustment. Reliably predictable the same way are also a fluid pressure P_2Oa and a fluid flow V_20a existing on the piston rod side 20a and the rate of a volume flow Vβi and a fluid pressure Pβi in the flow line 6; 61 as a result of knowing the opening 1A of a digital valve group 10c, which controls the flow into the flow line 6; 61 from the supply line 7; 71 (see also the description of fig. 2D).

Now, when it is desirable to produce in the digital valve unit 100, by means of a hydraulic cylinder 20 coupled through the intermediary of flow lines 6, a highspeed roll-nip load relief and reset pulse for the pressure load K_p of an intermediate roll's supporting lever, the volume flow V₂₀b of a fluid flowing into the hydraulic cylinder's 20 piston head side 20b is momentarily reduced with respect to the volume flow V₂o_a flowing into the hydraulic cylinder's 20 rod side 20a in order to produce a relief pulse.

Preferably, the delivery of a relief and reset pulse is performed by reducing first a volume flow into the hydraulic cylinder's piston head side 20b to a certain extent and by restoring thereafter the volume flow into the piston head side 20b to its former level after a certain time period t. After this, the load reset pulse is delivered by increasing first the volume flow into the hydraulic cylinder's piston head side 20b to a certain extent and by restoring thereafter the volume flow into the piston head side 20b to its former level after a certain time period t. As a result of this operation, the roll nip pressure load K_p applied by the hydraulic cylinder's piston 22 on a supporting lever is reduced, as displayed in the bottom view of fig. 3A, from a pressure level A to a pressure level B over a time period t1.

The volume flow into the piston head side 20b of a hydraulic cylinder is reduced by means of that/those digital valve group/groups of a digital valve unit 100, which is/are used for regulating a volume flow V_{6; 62} and thereby also a fluid pressure P_6; Q₂ in the flow line 6; 62 extending to the hydraulic cylinder's 20 piston head side. The adjustment of a volume flow V₆₂ is performed, as already previously explained in connection with figs. 2B and 2D, by selecting an appropriate opening for the digital valve group 10b for providing the reduced volume flow V₆₂. This is followed by restoring the digital valve group's 10b opening to what it was before the opening was changed. By this procedure, there is first reduced the volume flow of a fluid supplied into the loading cylinder's piston head side 20b by way of the flow line 6; 62 and then, after a short time period (measured from the start of changing the flow), there is restored the volume flow supplied into the piston side to its former level (pulse 1 in fig. 3A) for the equalization of pressures on the hydraulic cylinder's pressure and working sides 20; 20b, 20a. The effect of a relief pulse on the position of a hydraulic cylinder's piston 22 and on the pressure load Kp applied by the piston on a supporting lever is illustrated in a manner of example by figs. 3D and 3E. Presented in fig. 3C is a condition just before the delivery of a relief pulse, for example in the situation of fig. 3B, during the course of fiber web calendering in a steady state of equilibrium. In the state of equilibrium, a digital group is supplied by the control system with an equilibrium-sustaining adjustment instruction F(V); F(VT), according to which the digital valve group 10b presents an opening 1_A; I2ot_>τ and the digital valve group 10c presents an aperture 1_A; 12OaT- With these apertures of the digital valve groups 10b and 10c, the hydraulic cylinder 20 has both its piston head side 20b and its piston rod side 20a supplied from the digital valve group 100 through the intermediary of the digital valve groups 10b and 10c with a certain equilibrium-state volume flow V₂ot_>τ and V₂oaτ. which establishes a corresponding equilibrium-state fluid pressure P₂ObT and P_20aτ in the hydraulic cylinder 20 both on its piston head side 20b and on its piston rod side 20a. This is followed by supplying the digital group 100 with a relief pulse (pulse 1) adjustment instruction F(V); F(Vi). According to the adjustment instruction, volume flow - and pressure - of the piston rod side is maintained the same, i.e. at V₂o_aτ, P20aτ. but the volume flow of the piston head side 20b is first decreased from the equilibrium- state volume flow V_20bτ to V₂₀bi, which is matched by a reduced fluid pressure P₂o_bi. and then, after the time period t1 , is restored to the equilibrium-state volume flow and pressure V_2ObT. P2_0bτ- As a result of the relief pulse, the piston 22 travels in a way to reduce the pressure load Kp applied thereby to a supporting lever.

After the lapse of an appropriate time t2, measured from the inactivation of a relief pulse (pulse 1), the digital valve unit is supplied as an adjustment instruction (for the load K_p) 20 with a reset pulse (pulse 2) F(V); F(V₂) by momentarily increasing a volume flow supplied into the hydraulic cylinder's 20 piston side 20b (pressure side) with respect to a volume flow supplied into the cylinder's working side 20a (piston rod side). This is preferably implemented by modifying first in an appropriate manner the aperture of a digital valve group 10b controlling the flow supplied into the piston side 20b from an aperture 1₂o_bτ to an aperture 1₂ob2. whereby the volume flow by way of a flow line 62 into the hydraulic cylinder's pressure side (piston head side) increases from an equilibrium-state flow V_2ObT to a flow V₂ob2, and then the aperture of the digital valve group 10b is modified, after a time period t3 from the activation of a reset pulse, back to what it was, i.e. from a higher volume-flow allowing aperture I2θb2 back to an equilibrium-state aperture 12ObT, this operation also serving to restore the fluid volume flow to its former level, i.e. from an increased rate V20b2 to an equilibrium-state volume flow V_2ObT (pulse 2 in fig. 3A) for the equalization of fluid pressures existing on the cylinder's pressure and working sides. During a reset pulse, the volume flow and fluid pressure for the working side 20a of a hydraulic cylinder are generally maintained constant V_2Oa_T. P₂oaτ- As a result of this procedure, the pressure load K_p applied by the cylinder's piston on a supporting lever and further on a roll nip increases, as shown in the bottom view of fig. 3A, from the pressure level B to the pressure level A over a time period t3 and the roll nip pressure load is restored to a level existing before the delivery of a relief and reset pulse.

In case the calender is a multi-roll calender 500 with a certain number of roll nips N (e.g. a calender shown in fig. 2A), the load relief and rest pulses of consecutive roll nips are phased in such a manner that the joint between two fiber webs proceeds through all roll nips N involved in a calendering process without changing the calendering speed.

The exact effect and effect delay of a load relief and reset pulse issued by the digital valve unit 100 on hydraulic cylinders 20 acting on supporting levers 3 present at the ends of each intermediate roll may fluctuate slightly, depending on pressure losses in tube and pipe systems, as well as on structural differences in the hydraulic cylinder 20 and in the supporting levers 3 coupled therewith and other such roll-specific factors (regarding the bottom roll and the top roll, the cylinder 20 is a loading cylinder with a direct effect on the vertical position of the roll). In this case, the phasing of load relief and reset pulses with respect to the calender's 500 each running speed can be performed for example in such a way that the effect and delay of relief and reset pulses issued by the digital valve unit

100 on the pressure load of supporting levers and further on the pressure load of rolls are measured and, if necessary, the timing of pulses and the flow rate are tuned in a roll-specific manner.

On the other hand, when it is desirable to use the hydraulic cylinder 20, controlled by a digital unit, for instantly opening a roll nip N between two rolls, for example when a web break H; W_k of fig. 3F passes through the roll nip, the high-speed relief of a nip pressure in the roll nip is performed by modifying the ratio between volume flows V₂₀b, V_20a arriving in the pressure and working sides 20b, 20a of hydraulic cylinders 20 applying pressure on the supporting levers of a roll present at the roll nip. Fig. 3G visualizes a volumetric flow profile of the fluid being supplied into the piston head side of one hydraulic cylinder applying pressure on a supporting lever, while the volume flow for the hydraulic cylinder's working side remains more or less unchanged. This results in an instant reduction of the loading effect applied on a roll nip or on a calender's top or bottom roll by an element coupled with the hydraulic cylinder and working on the position of a roll in a roll nip N (the element is for example the piston head of a hydraulic cylinder raising the bottom roll, the shell of an intermediate roll, which is pressurized by a loading cylinder, or the supporting levers, which are present at the ends of an intermediate roll and pressurized by the pistons of loading cylinders). When an fault Wk (a web break) arrives at a roll nip, the roll's vertical position is first changed with a quick initial acceleration (step 1 in fig. 3G) by rapidly reducing the fluid pressure existing on the piston head side of a hydraulic cylinder with respect to the fluid pressure existing on the piston rod side. In order to change the ratio of fluid pressures, the volume flow (and fluid pressure) proceeding through a digital valve group 10b supplying fluid into the cylinder's 20 pressure side 20b is reduced, according to fig. 3G, from an equilibrium-state volume flow V₂ot_>τ and from a matching fluid pressure P_20bτ down to a certain predetermined volume flow V₂obi and a matching fluid pressure P₂o_bi , thus achieving an appropriate reduction of the hydraulic-cylinder produced pressure load K_p to a value K_pi. The volume flow is reduced by limiting the size of a flow port 1A established by the digital valve group 10b, by selecting an appropriate aperture 1A; I₂ot_>i of the relevant digital valve group 10c for providing said lower volume flow rate V₂ot>i- At the same time, the volume flow V₂Oa to be supplied into the piston rod side 20a of a cylinder 20 can be momentarily increased from an equilibrium-state volume flow V_2OaT to a new higher volume flow rate V₂o_ai in order to increase a back pressure P₂o_a existing on the piston rod side with respect to a fluid pressure P₂ot.τ existing on the cylinder's pressure side (piston side) 20b in the state of equilibrium (P_20ai >P2₀bi)- Thus, the loading effect of a piston 22 on an element (in this case, an intermediate roll supporting lever) pressurizing the roll present in a roll nip N is reduced and the roll changes its vertical position, as depicted in fig. 3G, from a position D to a position E. After this initial acceleration, the difference between pressures P₂Ob and P₂o_a is reduced by increasing in a stepwise manner, as shown in the top view of fig. 3G, the volume flow of a hydraulic fluid supplied into the piston side 20b and possibly by simultaneously reducing slightly the volume flow V_2Oa supplied into the rod side 20a. As a result, the loading effect K_p of a hydraulic cylinder 20 on a roll- pressuring element and on the roll itself will be reduced further, but at a slower rate and in a stepwise manner, and at the same the vertical position of the roll changes in a stepwise fashion as depicted in the bottom view of fig. 3E. The pressure difference between fluid pressures P20b and P₂₀a existing in the hydraulic cylinder's 20 piston head side 20b and piston rod side 20a is equalized gradually by increasing the volume flow V_2Ob supplied into the piston head side 20b, and the stepwise control of the roll's vertical position and at the same time the opening of the roll nip N are stopped prior to roll end blocks by adapting the fluid pressures on the hydraulic cylinder's 20 piston side 20b and rod side 20a to become equal, whereby also the volume flows for the piston side and the rod side have a ratio which is the same as before the opening of the roll nip. In a final state of equilibrium, the fluid pressures P2ot_>τn and P₂OaTn are lower than the fluid pressures P2₀bτ and P_20aτ of the state of equilibrium existing at the start of the roll nip N opening process.

The control of a roll nip's N high-speed opening process is effected as a direct control of the volume flow proceeding through various digital valve groups 10 of a digital valve unit according to a pre-established volume flow profile and the tuning of a high-speed opening is effected on each roll nip of a calender as the roll/calender is used for the first time. If necessary, profile changes for volume flows supplied into various sides of a loading cylinder can be performed on the basis of verifying measurements.

The above-described changes of the pressure load K_p and the position of a hydraulic cylinder's piston 22 occurring in an instant opening process are further illustrated in more detail in fig. 3H. Depicted schematically in fig. 3H is another supporting-lever pressurizing hydraulic cylinder 20, which is present at the end of a multi-roll calender's intermediate roll and the control of which is effected by a digital valve controlled system similar to that illustrated earlier in figs. 2D as well as 3B-3D. In the state of equilibrium (a steady-state condition), prevailing prior to the high-speed opening, during the course of continuous calendering while the pressure load is constant, there exists an equilibrium-state pressure load K_pτ. On the hydraulic cylinder's piston head side exists a fluid pressure P20aτ, which is established by a volume flow V₂OaT- Respectively, on the hydraulic cylinder's pressure side, i.e. on the piston rod side 20b in the figure, exists an equilibrium- state fluid pressure P2₀bτ, which is established by a volume flow V_2ObT- In the state of equilibrium, the digital valve unit is controlled by an adjustment instruction F(V); F(VT), which may be of a feedback type. When the vertical position of a roll is changed in a high-speed initial acceleration (step 1 in fig. 3E), the digital valve unit is supplied with an adjustment instruction F(V); F(\Λ), on the basis of which the volume flow (and fluid pressure) proceeding through a digital valve group 10b delivering fluid into the cylinder's 20 pressure side 20b is reduced by the digital valve unit from an equilibrium-state volume flow V₂o_t>τ and a matching fluid pressure P₂ObT down to a predetermined V₂ObI and a matching fluid pressure P₂obi. resulting in a quick reduction of the hydraulic-cylinder established pressure load K_p from an equilibrium-state pressure load K_pT to a lower pressure load K_pi. At the same time, the volume flow supplied into the cylinder's 20 piston rod side 20a is increased from an equilibrium-state volume flow V_2OaT to a new higher-rate volume flow V₂oai for increasing a counter pressure P₂o_a existing on the piston rod side with respect to a fluid pressure P_20bτ existing on the cylinder's pressure side (piston side) 20b in the state of equilibrium. This is followed by a stepwise equalization of the pressure difference between fluid pressures P20b and P20a existing on the hydraulic cylinder's 20 piston head side 20b and piston rod side 20a as shown in fig. 3E. In order to implement this, the digital valve unit is given a string of adjustment instructions (F(V); F(V₂), F(V₃)... F(V_n), which are used for increasing a volume flow V_20b supplied into the piston head side 20b and possibly for simultaneously reducing a hydraulic fluid volume flow V_20a supplied into the piston rod side 20a as the pressure load K_p is falling in a stepwise manner Kp2, Kp3..K_pn. The equilibrium-state pressure load, existing at the end of the roll nip N opening process, is K_pn, which is substantially lower than the pressure load K_pt existing prior to the opening process.

In one variant of the invention, the high-speed opening of a roll nip is executed by a so-called hybrid control, wherein the major changes in a loading cylinder concerning a volumetric fluid flow in the initial acceleration stage of a roll opening process (e.g. step A in fig. 3E) are implemented quickly by means of a digital valve group with a feed-forward adjustment strategy. A slower stepwise continued opening of the roll, wherein the changes of a volume flow on various sides of the loading cylinder are less dramatic, can be implemented thereafter with traditional slide valves by using a feedback mode of adjustment.

Suppression of vibrations

In response to moving machine parts, the paper machine develops resonance vibrations in several components, which may damage paper machine equipment and reduce the running speed of a paper machine. At certain rotational frequencies of rolls, the multi-roll calenders may experience a so-called barring effect with the successive rolls of a calender developing resonance vibration. The barring effect is often a result of md-directed faults present on the fiber web. The barring effect is detrimental to the coating of polymer-coated rolls.

Film transfer technique is currently one of the most popular coating, surface-sizing, and pigmenting methods for paper and board. The film transfer technique comprises forming a film on a roll with an application device and transferring the film onto the surface of a fiber web in a roll nip between the roll and its counter-roll. In film transfer technique, especially at high fiber web running speeds, the faults of a roll surface may develop resonance vibrations in the roll nip between the roll and the application bar, which cause clouding of the coating or surface size and/or uneven spreading of the film on the roll and thereby on the fiber web.

Today, the process of winding a paper web on a storage reel generally involves the use of a winding unit, wherein the paper web proceeds onto a storage reel by way of a roll nip between the storage reel and a breast roll and at the same time the storage reel is supported from below by means of a metal belt driven between two rolls. In the event that malfunctions occur in feeding a paper web from a breast roll onto a storage reel, the paper wound up on the storage reel develops faults, which are likely to cause further disturbances in paper feeding as the breast roll and/or the storage-roll supporting endless metal belt begins to resonate with the fault present in a roll of paper carried by the storage reel.

In all these cases, resonance vibrations can be suppressed by means of an apparatus of the invention and a method used therein. Hence, the method according to the invention is based on the procedure that the location of an fault H causing vibrations be identified on the surface of a roll or on a fiber web about to arrive at a roll nip, for example by means of a pressure measurement linked with the roll nip, and the arrival moment of an fault at a roll nip and the dwell time of an fault in a roll nip be estimated or calculated. After this, as the trouble spot arrives at a roll nip, the pressure in the nip is reduced momentarily by means of an anticipating feed-forward adjustment. The pressure reduction is effected by diminishing the bearing/pressurization action of a roll and/or its counter-roll bearing/pressurizing element through the intermediary of a hydraulic actuator, which is coupled with one or more digital valve groups used for controlling a volume flow to the hydraulic actuator. The momentary relief of a load applied on a supporting element 3 by a hydraulic actuator 2 coupled with a digital valve group 100 and the reset of said load in a roll nip take place for example in a manner similar to what has been earlier illustrated in figs. 3A-3E when describing the establishment of a relief pulse and a rest pulse for the pressure load of a roll nip in a multi-roll calender.

Compensation for belt deformations in a long-nip calender

One type of calender used today for the soft calendering of a fiber web is a so- called shoe calender, wherein the fiber web to be calendered is conveyed to a long nip established between a hard-surfaced counter roll (usually a heatable thermo roll) as well as a shoe roll opposite thereto and provided with an endless belt. In the event that the endless belt extending around a shoe roll used in a shoe calender or the surface of a counter roll opposite to the shoe roll becomes in certain areas thinner than the rest of the structure as a result of wearing, the material web W may become calendered in a long nip to a lesser thickness every time the thinner spot of the endless belt or the counter roll arrives at the roll nip. The part of a shoe calender commonly subjected to wearing is the endless belt (e.g. a fabric-reinforced polyurethane belt) rotating on top of the shoe element of a shoe roll, not so much the counter roll.

It is an object of the invention to eliminate the foregoing drawbacks discovered in relation to a long-nip calender.

The uneven calendering of the material web W can be precluded by a method of the invention and by an apparatus used therein. In this method, the hydraulic actuators, such as hydraulic cylinders applying a load on the shoe element of a shoe calender, are coupled with one or more digital valve groups. The method is based on the procedure that every time a thinner spot present in the endless belt of a shoe calender or in its counter roll rotates into the long nip, the load of hydraulic cylinders pressurizing the shoe element will be relieved and, after the thinner spot has passed the long nip, the load of hydraulic cylinders pressurizing the shoe element will be reset to its former level. The invention will be described in more detail with reference to figs. 4A and 4B. Fig. 4A shows schematically a shoe calender in view directly to an end face.

Fig. 4B shows the long-nip zone of fig. 4A in an enlarged scale. Depicted schematically in fig. 4A is a typical shoe calender 800 without its lubrication system. Fig. 4B₁ in turn, shows an endless belt 8a on a shoe roll 8 in a long-nip zone N₁ on top of which lies a reduced-thickness spot of the fiber web W, such as a paper web.

The shoe roll 8 consists of a loadable shoe element 8b, hydraulic actuators 2 applying a load on the shoe element in a roll nip N between the shoe roll 8 and a counter roll 80 opposite thereto, an endless belt 8a rotating on top of the shoe element, as well as a lubrication system (not shown in the figures) provided between the shoe element 8b and the belt 8a. The hydraulic actuators 2 visualized in the figures consist of two side-by-side rows of hydraulic cylinders 200; 200' and 200; 200", the rows extending from one end of the shoe element to the other in a direction (CD-direction) perpendicular to a machine direction (MD-direction). Each hydraulic cylinder row 200; 200' and 200; 200" is controlled by its own digital valve unit 100; 100' or 100; 100". Operation of the digital valve units 100 is synchronized by a control system 4. The design of the digital valve units 100' and 100', the operation control of each hydraulic cylinder row 200' and 200" by these digital valve units, and the synchronization of said digital valve units for mutual operation by the control system 4 can be analogous with respect to the previously described working example according to fig. 2D. In the shoe calender 800, the shoe roll 8 has its counter roll 80 comprising a heated thermo roll 80 used in the soft calendering of a material web, whereby the shoe calender 800 has its long nip N established between the shoe element 8b, as well as the endless belt 8a running on top of the shoe element, and the thermo roll 80, the fiber web W to be calendered being conveyed into said nip. Monitoring deformations in the endless belt 8a can be conducted by measuring continuously, on-line, a pressure load P1 established by the hydraulic cylinders 200 loading the shoe element 8b, as well as a surface pressure P2 of the endless belt in the long nip N. Measuring the surface pressure P2 of an endless belt in a long nip can be conducted by using for example the method described in the patent document FI-20055020. Working out a differential pressure dp = P1-P2 between the roll nip pressure load P1 and the endless belt surface pressure P2 enables a thickness-reducing process of the endless belt 8a to be detected and a reduced-thickness spot to be localized. This is followed by having the control system 4 work out an appropriate adjustment instruction F for relieving a nip pressure through the intermediary of a digital valve unit 100 whenever a reduced- thickness spot H; H_t of the endless belt 8a is calculated to arrive at the roll nip N. Hence, in the process of calculating the duration and commencing moment of a relief pulse, the control system 4 takes into consideration at least a rotating speed of the endless belt 8a, a length of the long nip N in machine direction, and a thickness and surface area of the endless belt's reduced-thickness spot H_t. The nip pressure is relieved and reset again by supplying the hydraulic actuators 2, such as the hydraulic cylinders 200, loading the shoe element 8b and coupled with the digital valve group/groups 100, with an appropriately timed and proper-duration load relief pulse and a load reset pulse, as explained in more detail in connection with the description of figs. 3A-3D. The relief pulse and the load reset pulse enable first the pressure load of the hydraulic cylinders 200 on the shoe element 8b and thereby on the roll nip N to be reduced and, after an appropriate time period has lapsed from the commencement of the relief pulse, the nip loading pressure is reset to its original value by means of the reset pulse.

As a result of delays occurring in the hydraulic actuators 200 (see e.g. the bottom view in fig. 3A), the actions aimed at relieving the nip pressure are commenced with the digital valve groups slightly before the reduced-thickness spot H; H_t rotates into the long nip's N zone The relief pulse for the hydraulic actuators 200 is given by changing (reducing) the volume flow of a fluid passed into the hydraulic actuators' piston side from the digital valve groups 100; 100' or 100; 100" with respect to the volume flow passed into their rod side from the digital valve unit, as depicted in figs. 3A-3E, and by momentarily resetting the volume flows to the level existing before the change by closing and opening appropriate valves in a digital valve group of the digital valve unit 100. The flow relief pulse issued by the valves of a digital valve group leads to a reduction of the pressure applied by the hydraulic cylinders' 200 piston on the shoe element 8b and further on the shoe element's endless belt 8a having rotated into the long nip's N zone; since the endless belt 8a rotated into the range of the long nip N has its part H_t reduced in thickness, the pressure load applied by the endless belt 8a on the fiber web W nevertheless remains in the long nip N at its previous level. Slightly before the trailing edge of the endless belt's 8a reduced-thickness spot H_t leaves the long nip's N zone, the actions aimed at resetting the pressure load of the shoe element 8b are commenced with the digital valve units 100. For this purpose, the fluid flow passed into the hydraulic actuators' 200 piston side is increased with respect to the flow passed into their rod side and momentarily the ratio of flows is reset to its original value by means of an appropriate aperture of a digital group in the digital valve unit, as described in connection with the specification of fig. 3A, (and, at the same time, the fluid pressures existing on various sides of each hydraulic cylinder's piston are reset to equal fluid pressures existing in a steady-state condition). By virtue of the load reset pulse given by the digital valve units 100, the pressure load of the hydraulic actuators 200 on the shoe element 8b present within the long nip's N zone will be reset to the level of a steady-state condition existing prior to the relief pulse.

For the above-described relief of a load and load reset on the endless belt 8a rotating in the long nip N (or actually the relief of a load and load reset applied on the shoe element 8b present under the endless belt 8a), the control system 4 of the digital valve units 100 is supplied with data regarding at least a running speed of the fiber web W and/or a rotation speed of the endless belt 8a, a length of the long nip N in machine direction, and a length of the endless belt's 8a reduced- thickness spot H_t in machine direction. In addition, the control system 4 is supplied with data regarding a pressure load applied by the hydraulic actuators 200 on the shoe element and changes of the endless belt's 8a load profile in the long nip N. Of these parameters, the long nip's N length, the fiber web's W running speed, and the endless belt's 8a rotation speed can be obtained by measuring and/or are otherwise previously known. The length of the endless belt's 8a reduced-thickness spot H_t and the thickness of said reduced-thickness spot with respect to the other thickness of the endless belt, as well as a length of the reduced-thickness spot in a lengthwise direction of the long nip N (in machine direction), are obtained on the basis of measurements as described earlier. The degree of relieving the compression force P1 applied by the hydraulic actuators' 200 pistons on the shoe element 8b, required in a method of the invention, depends on how much the endless belt has thinned with respect to the non-reduced thickness of the rest of the endless belt. Because changes in the flow rates of a hydraulic fluid into the hydraulic actuators' 200 pressure and working sides result in a certain change in the compression force applied by the hydraulic actuators' 200 pistons on the shoe element 8b and thereby on the endless belt 8a, the required changes of compression force can be used as a basis for finding out empirically and/or by calculation and/or by table lookup the parameters needed by the adjustment program for changing the volumetric hydraulic fluid flow in the digital valve units 100 coupled with the hydraulic actuators 200. As already pointed out above, the flow rate passing through each digital valve group depends on the aperture 1A of a digital valve group, i.e. on the total area of open-state digital valves' ports and on the pressure difference between a fluid arriving at a particular digital valve group and a fluid having passed through the same (see fig. 2D and related specification). The pressure difference across the aperture 1A of a particular digital valve group is obtained by measurements or it is otherwise readily predictable. As a result, the control system 4 enables working out a rapid feed-forward adjustment instruction for a particular time period according to a particular adjustment profile (see fig. 3A) for relieving the loading of the endless belt 8a and resetting it after the fault H; H_t has passed through the long nip N, whereby the control system 4 selects, on the basis of flow rates to be conducted on the pressure and/or working side of desired hydraulic cylinders 200, those hydraulic elements' volume-flow adjusting digital valve groups of the digital valve units 100; 100' or 100; 100" which enable producing the load relief and reset pulses consistent with the adjustment instruction, and thereafter, on the basis of a desired degree of change, selects from among these digital valve groups those digital valves which are open at specific times. As mentioned earlier, the time lapse between relief and reset pulses for the load of the shoe element 8b and thereby the endless belt 8a depends on a length of the endless belt's reduced-thickness spot (H; H_t) and a length of the long nip N.

Lubrication

One embodiment of the invention is based on a lubricating oil circulation for bearings implemented by means of one or more digital valve groups.

In modern paper machines, lubrication is performed by having oil circulate in the bearing systems of rolls both in wet, dryer, and finishing sections. Other bearing systems of paper machines' equipment and motors, such as the bearing systems of fans, barking drums, refiners, mixers, coaters, winders and slitters, can be provided with lubrication by circulating oil. The lubrication by circulating oil is used in paper machines principally for extending the longevity of bearing systems, because it is the degree of purity of lubricating oil which is most critical for the longevity of a bearing.

Examples of paper machine rolls currently provided with circulating oil lubrication:

-suction rolls, which are located in a wire section of the wet end and have a rotating roll shell and which are driven by means of a planetary gear. The most common bearing used both in suction and driving sides for suction rolls comprises roller ball bearings and oil is generally conducted to the center of bearings.

-in a press section, the fiber web travels, while supported by felts, in roll nips between press rolls and most of the water still contained in the web is removed in compression. In most cases, the press rolls are provided with roller ball bearings on both service and driving sides. In this case, as well, the oil is typically conducted to the center of a bearing and removed from either side of the bearing into an oil-collecting chamber and then out of a bearing housing by way of an outlet formed in the bearing housing. The dimensioning of oil circulation is most of all influenced by temperature planned for the bearing system and by the grade of lubricating oil. The bearing system temperature, on the other hand, is influenced by the diameter, rotation speed and weight of the roll.

-in a calendering process, the surface quality of a fiber web is upgraded for printing. The multi-roll calenders used for the soft calendering of a fiber web include several superimposed deflection compensated polymer-coated rolls, as well as heated thermo rolls, while the roll nips used for fiber web calendering are made up by a pair of rolls, including a thermo roll and opposite thereto a polymer- coated roll. The actual calendering process is influenced, among other factors, by a nip load existing in each roll nip, temperature of the thermo roll, and moisture of the fiber web. Because the roll nips present in the roll set of a multi-roll calender may have unequal existing nip loads, it is necessary, when designing bearing systems for the rolls, to consider the position of a roll in the roll set and the nip load existing in the roll nip. Because the roll set of even one and the same multi- roll calender often has its rolls in unequal operating conditions, it may also be required that the flow rate of a circulating oil lubrication brought to each bearing and removed from the bearing housing be dimensioned in a roll- and bearing- specific manner, whereby the bearing system for a multi-roll calender's roll set is likely to become laborious in terms of its design work. Typically, the quantitative adjustment of lubricating oil for each bearing must indeed be implemented as a feedback through the intermediary of an oval wheel or a turbine measurement. In multi-roll calenders as well, the most common type of bearing comprises roller ball bearings.

The circulation of lubricating oil is currently implemented with analog regulating valves in several parts of a paper machine, such in the bearing systems of press and calender rolls, but also in other bearing systems of a paper machine's equipment and motors. The rolls of paper machines are generally provided with spherical roller ball bearings, while typical rolls include the deflection compensated rolls of wet end calenders. In circulating oil lubrication implemented with analog valves, it is required that the feedback adjustment of oil supply for each bearing be dimensioned and planned separately, whereby the bearing system and its lubrication are likely to become complicated and expensive. Controlling the lubricating oil circulation with one or more digital valve groups of a digital valve unit results in a substantial advantage over the circulation control effected by means of analog valves. Hence, the lubricating oil is dispensed by way of a digital valve unit, composed of one or more digital valve groups, to bearings housed in an apparatus such as a multi-roll calender. Preferably, each bearing has a specific valve unit assigned thereto for dispensing the amount of lubricating oil required by the bearing at a particular time. The volumetric circulation flow of lubricating oil needed by various bearing systems in paper machines and the extent of change in the volumetric flow are such that the digital valve group of a digital valve unit serving each bearing has conveniently 3 to 6 digital valves in parallel connection. The dosage of lubricating oil to a bearing is suitably conducted by changing the aperture of a digital valve group supplying oil to the bearing. In the event that the lubricating oil circulation of an apparatus such as a multi-roll calender is implemented by means of bearing-specific digital valve units, the supply of oil to the digital valve units can be conducted in a manner equivalent to fig. 2D, i.e. by way of the inlet line 7; 71 common to the digital valve units.

The digital valve unit itself does not include a dosage control for lubricating oil, but it only functions as a dosage dispenser. If the control is applied for example to a lubricating oil circulation for the rolls of a multi-roll calender, the amount of oil needed by all bearings of the calender can be controlled in a centralized manner with a separate control system coupled with the digital valve units by using a feedforward based adjustment strategy. The control system is used for calculating the amount of lubricating oil required for each bearing at a particular instant e.g. on the basis of acquired bearing-related measuring data and for opening appropriate digital valves included in a digital valve group of the digital valve unit supplying oil to the bearing in order to provide a desired volumetric lubricating oil flow to the bearing system. By virtue of a feed-forward adjustment, the control system is quick. Accordingly, the control system can be apparatus-specific, such as calender-specific, because there is no need to design a specific lubricating oil circulation for each bearing. As the lubricating oil circulation control system is apparatus-specific instead of being bearing-specific, the control system becomes simpler and more attractive in terms of its costs. If desired, the lubricating oil circulation implemented by means of digital valve units can also have a feedback coupled therewith, for example by the intermediary of presently used flow measurements, but the lubricating oil circulation is very well viable even without feedback. Effecting the dosage of circulating oil lubrication through the intermediary of a digital valve unit provides a considerably simplified control of oil feeding, even with variation in the amounts of lubricating oil delivered to the bearings. The control of oil feeding, implemented as described, is precise and fault tolerant.

Adjustment of compressed air blasting output

In one aspect, the invention relates to the adjustment of the blasting output of compressed air, especially in web feeding, when exchanging a paper grade to be processed in a paper machine. Web feeding, as regards a fiber web, is particularly needed in the process of conveying the end of a fiber web across unsupported spaces in calendering, coating, pressing, and reeling. When a paper grade is exchanged, it may be necessary to provide the paper machine with dozens of compressed air jets with various outputs. At present, adjusting the output of an air blast is conducted by using resistance valves, which are practically fan-specific in the sense of only enabling the output of blasting air to be adjusted over a specific narrow range of volumetric flow at a time. In a resistance valve, the air proceeds through a short throttle passage. The amount of flow passed through a valve port depends on a pressure difference on either side of the valve port, as well as on a surface area of the port. The amount of flow and at the same time the air pressure existing on either side of the port are regulated in a resistance valve by adjusting the size of the throttle passage. Adjustment of a high-output air blast with such an analog resistance valve is inaccurate, wasteful in terms of energy, and expensive.

The embodiment of the invention intended for blasting compressed air is aimed at eliminating the drawbacks appearing in the foregoing prior art.

The invention has an objective of providing a method and apparatus for adjusting the output of a compressed air blast, especially in the process of fiber web feeding.

The method and apparatus according to the invention enable achieving the foregoing objectives.

The method according to this embodiment of the invention relies on conveying the compressed air through a digital valve group, which is present in a compressed air flow channel and which comprises 2 to 8, generally 3 to 6 digital on/off valves side by side. The sizes of valves included in a digital valve group are preferably selected in such a way that the amount of air per unit time passing through the larger one of two open-state valves with consecutive flow rates is twice as much as that having passed through the smaller valve. The size of an aperture established by a digital valve group is adjusted by opening and closing appropriate on/off digital valves in the digital valve group. The area of this aperture determines a pressure difference between the air flow supplied to a digital valve group and the air flow having passed through the digital valve group. The volume flow of compressed air having passed through a digital valve group is in turn determined on the basis of an aperture area and the above-mentioned pressure difference. Provided that the digital valve apertures with consecutive diameters have the size thereof appropriately selected, the digital valve group will be capable of providing the effective adjustment of a compressed air flow in fiber web feeding and, at the same time, a valve unit to be fitted in the air flow channel enables adjusting the compressed air flow over an extensive flow range. The adjustment of a compressed air flow output, implemented as described, reduces considerably the number of necessary compressed air flow regulating valve units. In addition, the adjustment of air output with a digital valve group is remarkably more precise than with throttle valves, the energy saving being as much as 30 to 50% in the web feeding process of a large-scale paper machine.

Digital valve controlled heat exchanger

Heat exchangers are necessary in various parts of a paper machine. One of the required services thereof involves the cooling of lubricating oil arriving from the process.

The heat exchangers used for various services in a paper machine are often oil- water heat exchangers with oil moving on the primary side and cooling water on the secondary side. At present, the regulation of a cooling water circulating rate is implemented by means of a pneumatic throttle valve supplied by the manufacturer, which is nevertheless expensive in terms of its purchase costs. Especially in heat exchangers of the lowest price range, the cooling water regulating valve makes up an excessive portion of the total costs of a heat exchanger, necessitating the use of valves adapted to this particular heat transfer device. However, when the regulation of cooling water circulation is implemented by adapted valves not originally designed for this particular heat exchanger, such valves are often inferior in terms of energy and water efficiency to those specifically designed for the discussed actuator. Such valves are often inaccurate in terms of adjustment properties, especially with low rates of primary side fluid flow occurring when cold starting the equipment. The invention is aimed at eliminating the drawbacks appearing in the prior art. Thus, it is an objective of the invention to provide a fluid-fluid heat exchanger, in which the fluid circulation regulating system for a secondary circulation is as attractive as possible in terms of its purchase costs and structurally simple. In addition, the heat exchanger should provide an exact adjustment over the entire adjustment range for the amount of fluid traveling on the secondary side.

With a heat exchanger according to the invention, the drawbacks appearing in the prior art can be eliminated.

In a heat transfer device of the invention, the heat transfer fluid, such as water, to be supplied into the secondary side of a heat exchanger is delivered by way of a digital valve group. The digital valve group has 2 to 8, preferably 3 to 6 on/off digital valves connected in parallel, depending on a volume flow required for the secondary side.

The use of a digital valve group on the supply side of a heat exchanger enables providing the heat exchanger with a regulation which is accurate at both low and high flow rates. The on/off digital valves are externally identical and only differ from each other in terms of the diameter of their fluid passage apertures, whereby the investment costs of a digital valve group are substantially lower than those incurred by formerly used actuator-specific regulating valves. With regard to additional benefits gained by a heat exchanger of the invention, it should be noted that one and the same heat exchanger can be used for a variety of projects, because the heat exchanger has its secondary side flow adjustable within an extensive range.

The heat exchanger according to the invention is described more closely with reference to fig. 5.

Fig. 5 shows schematically an oil-water heat exchanger of the invention.

Visible in fig. 5 is an oil-water heat exchanger 9, wherein the oil circulating on a primary side 92 is for example oil flowing in a lubricating oil circulation. The cooling water supply on a secondary side 9; 91 of the heat exchanger is integrated with a digital valve unit 100, comprising one digital valve group 10 which includes 6 parallel-connected on/off digital valves 1.

The oil circulating on the primary side 92 is cooled with water, the cooling capacity (volumetric flow) of which must be dimensioned in such a way that one and the same water circulation of the secondary side 91 enables both the cooling of hot oil, with a temperature of about 200⁰C, and the slight heating of oil in connection with cold starting the apparatus lubricated by the primary side. Therefore, the water circulation 92 must have a very extensive volumetric flow range. The number of digital valves 1 contained in a digital valve group 10 and the volume flow passing therethrough are adapted to match the required cooling capacity. In the heat exchanger 9, depicted in the figure, the supplied cooling water is regulated by a digital valve group 10, which includes 6 pieces of on/off digital valves 1 disposed in a parallel relationship in the cooling water supply flow. The volume flow passing through the digital valves 1 , with the latter in an open position, is selected such that, in two valves of consecutive volume flows, the valve with a larger flow port has a volume flow which 2 x that of the smaller valve. Such a digital valve group 10, containing six valves, provides an ability to establish 31 different volume flows of cooling water, the potential volume flow commencing from vary small flows (volume flow V = 1) extending up to a flow of cooling water several dozen times larger (maximally 1V + 2V + 4V + 8V + 16 V + 32V = 31V).

Adjustment of pressure load for a loading element pressurizing a roll member

Open or partially open adjustment systems based on digital valve units are viable for replacing current closed feedback type adjustment systems, which are based on analog adjustment valves for example in the process of calendering or spreading a fiber web with so-called active rolls. The active rolls refer here to rolls, which are provided with roll-engaged internal or external loading elements enabling a surface profile to be modified in a longitudinal direction of the roll

Fig. 7 visualizes the pressure adjustment with a digital valve unit 100 for a loading element, which is applying load on a roll and coupled with the piston rod of a hydraulic cylinder 20. The roll is for example a so-called Sym-roll provided with several intra-roll loading elements, or a roll with whose stub shafts is coupled a hydraulic cylinder having a loading element on its piston head. For the sake of comparison, fig. 6 illustrates a corresponding traditional system for adjusting a pressure Kp for a loading element applying load on a roll surface from inside. The adjustment system comprises the use of a closed (feedback type) adjustment system for controlling the pressure load Kp for a loading element coupled with a hydraulic cylinder's 20 piston 22, wherein changing of the pressure load is performed by using a prior art analog adjustment valve and a pressure regulator. Therein, the pressure regulator is given a target pressure value Pref, for example by way of a potential message. A pressure P₆ existing in a line 6 leading to the hydraulic cylinder 20 is measured continuously or at specific intervals, for example by means of an electronic sensor or optionally in a hydromechanical manner, by conducting the pressure to an end face of the valve's slide. Thus, the pressure regulator receives continuously or at specific intervals information about a difference between the target pressure value Pref and a pressure Ptot measured from the line 6 and applies on that basis an adjustment instruction Pinstruction for correcting all the time the adjustment valve's slide to such a position at which the measured pressure Ptot is as close as possible to the target pressure Pref. The adjustment of the slide's position can be executed either by an electronic adjuster and actuator or hydromechanically, for example by means of springs. A problem with such a traditional adjustment system is a risk of its instability. The question is about a so-called closed adjustment system, the stability of which depends, among other things, on a pressure regulator and its tuning parameters, as well as on the dynamic behavior of a loading element, a pipe system, and an adjustment valve. The instability manifests itself as a fluctuation of the loading element's pressure load Kp, a vibration which deteriorates for example the quality of paper surface as paper is calendered in a roll nip provided with a roll whose surface is pressurized from inside with such a loading element. The adjustment system may also respond unnecessarily to an intermittent impulse resulting from the rotation of a roll.

Fig. 7 shows an adjustment method by a digital valve unit 100 for a pressure load Kp applied on a roll surface by one loading element included in a roll member, such as a roll provided with internal loading elements. The digital valve unit 100 comprises two digital valve groups 10. The flow supplied by way of a flow line 6 to a hydraulic cylinder's 20 pressure side 20b has its flow rate V₆ and flow pressure P₆ regulated by a digital valve group 10; lOpressure. On the other hand, the flow proceeding from the hydraulic cylinder's pressure side 20b to a tank line 7; 72 has its flow rate and pressure regulated by a digital valve group 10; lOreturn. Both the digital valve group 10; lOpressure, regulating pressure and flow rate in the flow line 6, and the digital valve group 10; lOreturn, regulating the pressure P₆ and the flow rate V₆ of a fluid conveyed into the tank line, include N examples of parallel- connected on/off digital valves with an unequal flow-through in the on-position. Each digital valve can be either totally open or totally closed. The number N of digital valve can be unequal in the digital valve group 10; lOreturn, controlling a flow from the flow line 6 to the tank line 7; 72, and in the digital valve group 10; lOpressure, controlling a flow from the supply line 7; 71 to the flow line 6. In the digital valve group 10; lOpressure, the number of on/off digital valves with a flow- through unequal relative to each other is N. The aperture 1_A of a digital valve group 10 is a total sum of digital valves controlled at specific times in the digital valve group to an open position and may only attain specific discrete values. When the digital valve group has N examples of unequal on/off digital valves, the aperture 1_A; lApressure can achieve 2^N unequal opening combinations and discrete apertures. Hence,, the flows proceeding through the digital valve group 10; lOpressure attain 2^N unequal discrete conditions, depending on the aperture V. lApressure- Because each digital valve in a digital valve group can be either totally open or totally closed, each aperture 1A can achieved at high accuracy. The advantage achieved thereby is that a digital valve group enables eliminating the uncertainties, such as hysteresis and zero creep, associated with analog adjustment valves.

The flow rate of a fluid passing at a particular time by way of a digital valve group into the line 6 depends on the supply pressure Ps of a fluid arriving at the digital valve group by way of the supply line 7; 71 and on the digital group's aperture 1 ; lApressure at a particular time. A Specific aperture lApressure , iApressurei , iApressure2

••l_ApressureN is then matched by a specific pressure load Kp; Kp1 , Kp2..KpN of the loading element, because the pressure load Kp sets at such a pressure that the volume flow that has passed through the digital valve group 10; lOpressure is equal to the volume flow proceeding through the head of a piston 22 coupled with the loading element. These pressure loads can be worked out by two optional models: -a mathematical model, in which a mathematical model is developed between a digital valve group and a loading element and the model is used for working out the loading element's pressure loads Kp matching various apertures 1A; lApressure of the digital valve group 10; lOpressure, or

-an empirical model, which involves measuring a loading element's pressure load Kp matching each aperture 1_A; Upressure of the digital valve group 10; lOpressure. After this so-called calibration measurement, there is no longer need for a pressure measurement from the line 6 leading to a hydraulic cylinder.

Both the mathematical model and the empirical model actually provide a model describing an adjustment system made up by a hydraulic cylinder and a digital valve unit 100, enabling the definition of a pressure load Kp obtained with various apertures 1_A; lApressure ; lApressurei . iApaine2 • • 1 ApressureN of the digital valve group 10; IOpressure controlling a fluid flow into a hydraulic cylinder's pressure side 20b. Thereafter, by means of these models, the pressure load Kp can be adjusted by selecting such an aperture 1A; lApressure by which the aperture-defined pressure load Kp is close to a target pressure. The pressure load adjustment can be conducted without the feedback of pressure, i.e. without measuring for example a pressure V₆ in the line 6 leading to a hydraulic cylinder and checking the aperture 1A on the basis of this measured pressure. What is essential is that each aperture 1A of a digital valve group is attainable in such a repeatable manner that a pressure load provided by the model matches sufficiently well the real pressure load. In the selection of a correct aperture 1_A; ^pressure . it is also possible to utilize a technique based on penalty function, which enables making compromises for example between the number of digital valve connections and the accuracy of adjustment.

The above-described adjustment system does not consider variations of a supply pressure P₈ in the inlet flow V₆, P₆, which pressure variations have an effect on the rate of volume flow passing through each aperture of a digital valve group as the volume flow depends on the supply pressure P_s and on the aperture 1_A; ^pressure- Variation of the supply pressure P₈ can be compensated for by measuring a supply pressure and presuming, for example, that the ratio of a loading element's pressure load Kp to a supply pressure remains constant.

The pressure adjustment system for a roll member's loading element established by means of a digital valve unit can also be used as part of a closed or feedback type adjustment system. In this case, the target value for a loading element's pressure load Kp is first used as a basis for selecting an appropriate aperture 1A; l_Apr_es_sure for a digital valve group, as described above. Now, however, a measurement is conducted on the pressure load and, based on the difference between a measured pressure load and a target pressure load, the target value of a pressure load Kp is changed by means of a closed adjustment system which comprises an analog adjustment valve in coupling with a pressure regulator. An advantage gained by such a hybrid system over a traditional adjustment system of the type presented for example in fig. 6 is the fact that the open model-based adjustment system implemented with a digital valve unit handles most of the work and the closed adjustment system only handles fine tuning. This is a solution more stable than the prior known traditional solution in which the entire adjustment operation is handled by a closed system. Below is still described in a method mode the operation of the above-discussed adjustment systems used for controlling the pressure of a loading element.

A. A basic system without consideration for pressure variations in an inlet flow arriving at an adjustment system established by a hydraulic cylinder, a loading element, and a digital valve unit.

1. Supplying the adjustment system with a target value for the pressure load Kp or the pressure of a hydraulic cylinder's working side.

2. Determining on the basis of an empirical or mathematical model, describing the adjustment system established by a hydraulic cylinder, a loading element, and a digital valve unit, the load element's pressure loads Kp or the hydraulic cylinder's working side pressures matching the apertures 1A of a digital valve group passing a fluid into the hydraulic cylinder's pressure side.

3. Selecting an optimal digital valve group aperture 1A; lAopt. which corresponds as well as possible to a target value set for the loading element's pressure load Kp or for the hydraulic cylinder's working side pressure.

4. Controlled opening of those digital valves in a digital valve group which provide an optimal aperture 1_Aopt for the digital valve group.

5. The optimal aperture l_Aop_t of a digital valve group establishing, in a line leading to the hydraulic cylinder, a specific volume flow and pressure matched by a desired pressure load Kp or a pressure of the hydraulic cylinder's working side.

B. In the event that variations of a supply pressure P₈ are also taken into consideration in the adjustment system, the operation of an adjustment system described above in paragraphs 1 to 5 shall be modified as follows:

1. Supplying the adjustment system with a target value for the pressure load Kp or the pressure of a hydraulic cylinder's working side, as well with a measured value of the supply pressure P₅.

2. Determining on the basis of an empirical or mathematical model describing the adjustment system established by a hydraulic cylinder, a loading element, and a digital valve unit, which model contains a correction term for the supply pressure P_s, the load element's pressure loads Kp or the hydraulic cylinder's working side pressures matching the apertures 1A of a digital valve group passing a fluid into the hydraulic cylinder's pressure side. 3. Selecting an optimal digital valve group aperture 1_A; U_opt, which corresponds as well as possible to a target value set for the loading element's pressure load Kp or for the hydraulic cylinder's working side pressure.

4. Controlled opening of those digital valves in a digital valve group which provide an optimal aperture 1 _Aop_t for the digital valve group.

5. The optimal aperture "Uopt of a digital valve group establishing, in a line leading to the hydraulic cylinder, a specific volume flow and pressure matched by a desired pressure load Kp or a pressure of the hydraulic cylinder's working side.

C. On the other hand, if the adjustment system is a hybrid adjustment system, which contains a closed adjustment system and an open one and which involves changing the pressure of a loading element's pressure load Kp or that of that of the hydraulic cylinder's working side with a closed adjustment system, including for example an analog slide valve, and thereafter adjusting the flow with a digital valve unit to match new target values set for the pressure load Kp or for the hydraulic cylinder's working side pressure, the operation of such an adjustment system proceeds as follows:

1. On the basis of a deviation between the target value of a pressure load Kp or the target pressure of a hydraulic cylinder's working side and the measured pressure load or the working side pressure, there is determined, by means of a closed adjustment system contained in an analog slide valve, a new target value for the pressure load or for the hydraulic cylinder's working side pressure, which target value is fed into an open adjustment system which includes at least a hydraulic cylinder, a loading element, and a digital valve unit.

2. Determining on the basis of an empirical or mathematical model describing the open system established by a hydraulic cylinder 20, a loading element, and a digital valve unit, which model possibly contains a correction term for the supply pressure P_s, the load element's pressure loads Kp or the hydraulic cylinder's working side pressures matching the apertures 1_A of a digital valve group passing a fluid into the hydraulic cylinder's pressure side. 3. Selecting an optimal digital valve group aperture V, l_Aop_t, which corresponds as well as possible to a new target value set for the loading element's pressure load Kp or for the hydraulic cylinder's working side pressure. 4. Controlled opening of those digital valves in a digital valve group which provide an optimal aperture 1_AoPt for the digital valve group.

5. The optimal aperture iA_Op_t of a digital valve group establishing, in a line leading to the hydraulic cylinder, a specific volume flow and pressure matched by a desired new pressure load Kp or a pressure of the hydraulic cylinder's working side.

The above-described adjustment systems contained in a hydraulic cylinder can also be implemented by using equivalent pneumatic cylinders, provided that the adjustment is applied to equivalent pneumatically operated roll members.

Claims

1. A method for dealing with faults occurring during the manufacture of a material web, said method comprising at least the following steps of:

-identifying the location of a fault (H) on the surface of a roll (5) or in a material web and estimating or calculating an arrival time of the fault (H) at a roll nip (N) and a dwell time for the fault in the roll nip,

-transmitting from a control system (4) to at least one digital valve unit (100), used for controlling a nip pressure of the roll nip (N), a feed-forward adjustment instruction (F) for dealing with a fault in the roll nip (N) of a calender, -reducing as per adjustment instruction (F), through the intermediary of at least one digital valve (1 ), a nip pressure of the roll nip (N) at the latest when the fault (H) arrives at the roll nip (N) and increasing the nip pressure as the fault exits the roll nip.

2. A method as set forth in claim 1 , characterized in that the adjustment instruction (F) is in a digital, such as binary mode and/or the roll nip is a roll nip present in a calender, a press, a dryer, a coater or a winder.

3. A method as set forth in claim 1 or 2, characterized in that determined in the adjustment instruction (F) is the volume flow of a fluid passing through the digital valve unit (100) and/or the location of an adjustment element's (2) piston over the adjustment period.

4. A method as set forth in any of the preceding claims, characterized in that the volumetric fluid flow proceeding through a digital valve group (10) of the digital valve unit (100) is changed according to the adjustment instruction (F) by changing an aperture (1_A) of this particular digital valve group, i.e. by opening and closing the digital valve group's desired digital valves (1) which are connected in parallel with respect to the fluid flow passing therethrough.

5. A method as set forth in any of preceding claims 1-4, characterized in that the method comprises at least the following steps of:

-identifying, counter-clockwise in a material web traveling direction relative to the roll nip (N), the location of a joint (H; W_s) present in the material web arriving at the roll nip (N), as well as the extent of the joint (H; W₃) of two interlinked material webs, such as a fiber web (W), -estimating or calculating an arrival time of the joint (W₅) at the roll nip (N), as well as a passage time for said joint through said roll nip (N)₁

-transmitting from the control system (4) to at least one digital valve unit (100), used for controlling a nip pressure of the roll nip (N), a feed-forward adjustment instruction (F) for dealing with a fault inflicted by the joint (W_s) in said roll nip (N),

-reducing a nip pressure of the roll nip (N), as per adjustment instruction (F), through the intermediary of at least one digital valve (1), at the latest when the joint (W₈) arrives at the roll nip (N) and increasing the nip pressure as this joint exits the roll nip (N).

6. A method as set forth in claim 5, characterized in that

-the adjustment instruction (F) comprises a relief pulse for the nip pressure of a roll nip, wherein the nip of a specific roll nip is lowered by reducing a pressure load (K_p) applied by a hydraulic actuator (2) on a roll supporting lever (3), as well as a load reset pulse, wherein the pressure load (K_p) applied by the hydraulic actuator (2) on the very same supporting lever (3) is reset to its former level,

-the relief pulse for a nip pressure (N_p) and the nip pressure reset pulse are developed by using digital valves included in digital valve groups (10) of a digital valve unit (100) for changing the mutual ratio between volumetric fluid flows (V20a_> V_20b) arriving on various sides (20a, 20b) of the hydraulic actuator's (2) piston, and for changing thereby fluid pressures (P_2Oa, P2ot_>) existing on the various sides (20a, 20b) of the hydraulic actuator's (2) piston with respect to volumetric fluid flows (V₂oaτ, V₂o_bτ) arriving on various sides of the piston and thereby with respect to fluid pressures (P_2OaT, P_20bτ) existing on various sides of the piston in the state of equilibrium.

7. A method as set forth in claim 6, characterized in that

-the relief pulse as per adjustment instruction (F) is used for reducing a volumetric fluid flow streaming into the pressure side of the hydraulic actuator's (2), such as a hydraulic cylinder's (20) piston, and at the same time a fluid pressure existing on the hydraulic cylinder's pressure side from an equilibrium-state fluid pressure (P₂obτ) and volume flow (V_20bτ) of said pressure side to a lowered fluid pressure (P20bi) and a reduced volume flow (V₂o_bτ) for a specific time period with respect to an equilibrium-state volumetric fluid flow (V₂o_ba) existing on the working side of the same hydraulic actuator's (20) piston in the state of equilibrium and with respect to an equilibrium-state fluid pressure (P₂₀aτ) established by this volumetric flow, said hydraulic actuator (2) having effect on the loading of a supporting lever (3), and this is followed by resetting a reduced volume flow (V₂ot_>i) and a lowered fluid pressure (P20ai) of the first side to a volume flow (V₂obτ) and a fluid pressure (P₂o_t>τ) existing in the previous state of equilibrium.

-the reset pulse of the adjustment instruction is used for increasing a volumetric fluid flow streaming into the pressure side of the hydraulic actuator's (2), such as the hydraulic cylinder's (20) piston, and at the same time a fluid pressure existing on the hydraulic cylinder's pressure side, from an equilibrium-state pressure level (P2ot_>τ) and volume flow (V₂ot_>τ) of the first side to an increased fluid pressure (P2_0b2) and increased volume flow (V20b2) for a specific time period with respect to an equilibrium-state volumetric fluid flow (V₂₀aτ) existing on the working side of the same hydraulic actuator's (20) piston in the state of equilibrium and with respect to an equilibrium-state fluid pressure (P2₀aτ) established by this volumetric flow, and this is followed by resetting an increased volume flow (V₂ob2) and pressure level (P_20b2) of the pressure side to a volume flow (V₂obτ) and pressure level (P20bτ) existing in the state of equilibrium,

8. A method as set forth in claim 7, characterized in that

-in order to develop a relief pulse as per adjustment instruction (F), the volume flow passing through an aperture (1A) of a specific digital valve group (10) in a digital valve unit (100) is first reduced from an original volumetric flow, which exists in a state of equilibrium and which is established by a combination (1₂obτ) of digital valves (1) open in the state of equilibrium, to a new, lower volumetric flow rate (V_20bi) by selecting an appropriate combination (i20_b-ι) of open-state digital valves (1) in the same digital valve group, the volume flow passing therethrough matching a desired reduced volumetric flow rate (V_20bi). and thereafter, following a short time period from the start of a relief pulse developing process, the volume flow passing through the combination of open-state digital valves (1 ) in the digital valve group (10) is reset to the original equilibrium-state volumetric flow by selecting the combination of open-state digital valves to be the same as the original equilibrium- state combination (1₂obτ),

-in order to develop a load reset pulse as per adjustment instruction, an equilibrium-state volume flow (V), passing through the equilibrium-state valve combination (i_20bτ) of the open-state digital valves (1 ) in a digital valve group (10), is first increased to a new, higher volumetric flow rate (V) by selecting an appropriate valve combination (I2_θb2) of open-state digital valves 1 in the same digital valve group (10), the volume flow passing therethrough matching a desired higher volumetric flow rate (V₂o_b2) into the piston's pressure side 20b, and thereafter, following a short time period from the start of a reset pulse developing process, the volume flow passing through the valve combination (1_A) of open-state digital valves (1) in the digital valve group (10) is reset to the equilibrium-state volumetric flow by selecting the combination of open-state digital valves to be the same as the original equilibrium-state combination (i2ot_>τ) capable of establishing the former equilibrium-state volume flow (V2ot_>τ) on the piston's pressure side (20b).

9. A method as set forth in claim 8, characterized in that a nip pressure of the roll nip (N) prior to the delivery of a relief pulse is equal to what it is after the delivery of a reset pulse.

10. A method as set forth in any of claims 5-9, characterized in that the relief pulse and the reset pulse for a nip pressure of the roll nip (N) are developed in an apparatus containing several consecutive roll nips in such a way that the relief pulse and the reset pulse for consecutive roll nips take place in consecutive roll nips in a manner phased according to a speed of the apparatus at a specific time.

11. A method as set forth in claim 10, characterized in that the timing of a relief pulse and a reset pulse for the successive roll nips (N) is influenced not only by the arrival time of a joint (H; W₈) at said roll nip (N) of a calender as well as the passage time of the joint (H; W₅) through the roll nip, but also by structural aspects of a roll set (50) in the calender as well as pressure losses of the employed hydraulic system.

12. A method as set forth in any of claims 1-4, characterized in that the method comprises at least the following steps of

-identifying, counter-clockwise in a material web traveling direction relative to the roll nip (N), the location of a web break (H; W_k) present in the material web arriving at the roll nip (N),

-estimating or calculating an arrival time of the web break (H; Wk)) at the roll nip (N) of a calender, as well as its passage time through said roll nip (N),

-transmitting from the control system (4) to at least one digital valve unit (100), used for controlling a nip pressure of the roll nip (N), a feed-forward adjustment instruction (F) for dealing with a fault inflicted by said web break in said roll nip (N) of a calender,

-opening the roll nip (N), as per adjustment instruction (F), through the intermediary of at least one digital valve unit (100), at the latest when the web break arrives at said roll nip.

13. A method as set forth in claim 12, characterized in that the roll nip (N) is opened by reducing a load applied on at least one or rolls making up the roll nip (N) through the intermediary of a hydraulic actuator (2), such that the load applied on said roll by an element (3) acting on the roll is reduced by changing, through the intermediary of at least one digital valve unit (100) and according to the adjustment instruction (F) for an aperture (1A) transmitted to said digital valve unit(s), the ratio between volume flows (V_20b, V2₀a) arriving on a pressure side (20b) and a working side (20a) of said hydraulic actuator (2).

14. A method as set forth in claim 13, characterized in that the adjustment instruction (F) for the aperture (1A) comprises reducing in a stepwise manner a pressure load (P_k) of the element (3) acting on the pressurization of a roll present in the roll nip (N) between two rolls, such that, at the start of opening the roll nip (N)₁ the pressure load of a hydraulic actuator (2) on said pressurization element (3) is reduced rapidly down to a specific pressure load (P_k) (step 1), and thereafter the pressure load (P_k) of the hydraulic actuator (2) on said pressurization element is reduced at a slower rate, in a stepwise manner (step 2), until a desired aperture of the roll nip (N) is attained.

15. A method as set forth in claim 14, characterized in that a part of the roll-nip (N) opening process, as directed in the adjustment instruction (F), is implemented through the intermediary of a digital valve unit (100) in a feed-forward adjustment mode and a part of it by means of a slide valve(s) in a feedback adjustment mode.

16. A method as set forth in claim 15, characterized in that the initial acceleration stage of a roll-nip opening process (step 1) is implemented through the intermediary of a digital valve unit (100) and a slower stepwise continued opening of the roll nip (step 2) is implemented by means of a slide valve(s).

17. A method as set forth in any of claims 13-16, characterized in that, in the acceleration stage (step 1 ) of the roll-nip (N) opening process, ratio of volume flows (V₂Ob. V₂Oa) arriving on the hydraulic actuator's (2) pressure side (20b) and working side (20a) is changed by changing the aperture of a first digital valve group (10) in the at least one digital valve unit (100) in such a way that an equilibrium-state volumetric fluid flow (V₂obτ), proceeding through said digital valve group into the hydraulic actuator's (2) pressure side (20b) in the state of equilibrium prior to opening the roll nip, is reduced down to a new, lower volumetric flow (V₂ot>i), and possibly by removing fluid at the same time from the hydraulic actuator's (2) pressure side (20b) by way of a digital valve group (10) leading to a fluid outlet line (7; 72), whereby the fluid pressure existing on the hydraulic actuator's pressure side falls from an original equilibrium-state fluid pressure (P₂ObT) to a lower fluid pressure (P₂obi)-

18. A method as set forth in claim 17, characterized in that, as the aperture (1A) of the first digital valve group (10) is downsized in such a way that the volume flow (V_2Ob). proceeding into the hydraulic actuator's (2) pressure side (20b), decreases from the equilibrium-state fluid flow rate (V_20bτ) down to a specific lower flow rate (V₂o_bi ). said reduced volumetric fluid flow (V₂₀bi) being matched by a specific fluid pressure (P₂o_b-ι) of the hydraulic actuator's (2) pressure side, which has fallen from an equilibrium-state fluid pressure (P₂o_bτ), the rate of a volumetric flow, proceeding into the hydraulic actuator's (2) working side (20a), is simultaneously increased from an equilibrium-state volume flow (V_20aτ), which is matched by a specific equilibrium-state fluid pressure (P₂oaτ)_> up to a specific increased volume flow (V_20ai) of the working side (20a), said increased volumetric fluid flow being matched by a specific increased fluid pressure (P₂oai) of the hydraulic actuator's working side, by upsizing the aperture (1A) of a second digital valve group (10).

19. A method as set forth in claim 17 or 18, characterized in that, in the acceleration stage of an opening process (fig. 3B, step 1 ), an aperture (1₂ob) of a digital valve group (10) in at least one digital valve unit (100), controlling a fluid flow into the pressure side (20b) of a hydraulic actuator, is downsized from an original equilibrium-state aperture (1₂obτ), which is matched by a specific equilibrium-state volumetric fluid flow into the pressure side (V2_20bτ), to a new downsized aperture (1A) of the digital valve group (10) in the same digital valve unit (100) for attaining a desired reduced volumetric fluid flow (V₂o_bi) into the hydraulic actuator's pressure side (20b), by selecting a new valve combination (i20_bi) of open-state valves (1 ) in the digital valve group (10) and possibly by removing hydraulic fluid from the hydraulic actuator's pressure side (20b) by way of a digital valve group (10) leading to a fluid outlet line (7; 72).

20. A method as set forth in claim 18, characterized in that the rate of a volumetric flow proceeding into the hydraulic actuator's (2) working side (20a) is increased from an equilibrium-state volumetric flow (V₂oaτ), which is matched by a specific equilibrium-state fluid pressure (P2o_aτ) and a specific valve combination (i20aτ) of the valves (1) open in the state of equilibrium in a digital valve group (10) of the digital valve unit (100), up to a specific increased volumetric flow (V₂o_ai)_> which increased volumetric fluid flow (V₂oai) is matched by a specific increased fluid pressure (P₂OaO of ^tne hydraulic actuator's working side, by upsizing the aperture of a second digital valve group (10) for selecting such a new valve combination (1₂o_ai) of the open-state valves (1 ) in the digital valve group (10) of the digital valve unit (100) controlling the volumetric flow rate that said increased volumetric flow (V_20ai) is attained.

21. A method as set forth in any of claims 13-16, characterized in that the stepwise, slower continued opening of the roll nip (N) (step 2), subsequent to the accelerated roll-nip opening stage (step 1 ), is implemented through the intermediary of at least one digital valve unit (100).

22. A method as set forth in claim 21 , characterized in that the stepwise continued opening of the roll nip (N) is performed by reducing in a stepwise manner, through the intermediary of at least two discrete digital valve groups (10), the difference between fluid pressures (P_2Ot_>, P2₀a) existing on the hydraulic actuator's (2) piston head side (20b) and piston rod side (20a).

23. A method as set forth in claim 22, characterized in that a volume flow (V₂ot>) to be supplied into the hydraulic actuator's (2) pressure side (20b) in the stepwise continued opening (step 2) of the roll nip (N), and hence also a fluid pressure (P_20b) existing on the pressure side (20b), is increased in a stepwise manner and a back pressure (P_2Oa) existing on the piston's working side (20a) is possibly slightly reduced at the same time in a stepwise manner, such that the fluid pressures existing on the hydraulic actuator's (2) pressure side and working side are in equilibrium at the end of the stepwise opening process, whereby the piston's working side (20a) has a new equilibrium-state fluid pressure (P₂oan ) existing therein and the piston's pressure side (20b) has a new equilibrium-state fluid pressure (P₂ot>n) existing therein and these fluid pressures (P₂oan, P2ot_>n) of a new state of equilibrium are both lower than the fluid pressures of a state of equilibrium (P2₀aτ. P20bτ) existing both on the working side (20a) and on the pressure side (20b) of the piston before the roll nip is opened.

24. A method as set forth in claim 23, characterized in that a volume flow to be supplied into the hydraulic actuator's (2) pressure side (20b) is increased in a stepwise manner through the intermediary of a digital valve group (10) in at least one digital valve unit (100) by selecting apertures (12O_bZ, I2θb₃-- -i20bn) for the digital valve group (10) which establish consecutive increasing volumetric fluid flows (V₂ob2, V₂ob3-V2Obn) for the pressure side (20b).

25. A method as set forth in claim 23 or 24, characterized in that a back pressure (P₂₀) existing on the hydraulic actuator's working side (20a) is reduced in a stepwise manner the intermediary of a digital valve group (10) in at least one digital valve unit (100) by selecting apertures (12OaZ, I2θa3-- i20an) for the digital valve group which establish consecutive decreasing volumetric fluid flows (V₂oa2, V₂oa3- -V₂Oa_n) for the pressure side (20b).

26. A method as set forth in any of claims 1-4, characterized in that

-the location of a fault (H)₁ such as a surface roughness, present on the surface of a fiber web (W) arriving in a multi-roll calender (500), is identified counterclockwise before the roll nip N in the traveling direction of the fiber web, and the arrival time of the fault in each roll nip of the multi-roll calender, as well its passage time through each roll nip (N), is estimated or calculated,

-to at least one digital valve unit (100), used for controlling the nip pressure of each roll nip (N), is transmitted from a control system a feed-forward adjustment instruction (F) for dealing with the fault spot (H) in the roll nip (N) of the multi-roll calender (500),

-according to the adjustment instruction (F), the nip pressure of the roll nip (N) is reduced through the intermediary of at least one digital valve at the latest when the fault (H) arrives in the roll nip (N) and the nip pressure is increased when the fault exits said roll nip.

27. A method as set forth in claim 26, characterized in that

-the adjustment instruction comprises a relief pulse for a nip pressure of the roll nip (N), wherein a nip pressure (P) at a specific roll nip (N) of the multi-roll calender (500) is lowered by reducing a pressure load (P_k) applied by a hydraulic cylinder (20) on a roll supporting lever (3), as well as a load reset pulse, wherein the pressure load (P_k) applied by the hydraulic cylinder (20) on the same supporting lever (3) is reset to what it was, -the relief pulse for a nip pressure (N_p) and the reset pulse for the nip pressure (Np) are developed by changing the aperture of digital valve groups (10) in a digital valve unit (100) in such a way that the mutual ratio of volumetric fluid flows (V_20a, V_2Ob) arriving from the digital valve groups (10) in various sides (20a, 20b) of a hydraulic cylinder's (20) piston (22), and thereby also the ratio of fluid pressures existing on the various sides (20a, 20b) of the hydraulic cylinder's piston, is changed with respect to volumetric fluid flows (V_2OaT, V₂ObT) arriving in various sides of the piston and fluid pressures (P₂oaτ,P2_θbτ) existing on various sides of the piston in a state of equilibrium.

28. A method as set forth in any of claims 1-4, characterized in that

-the location of a fault spot (H) present on the surface of an application roll or its counter roll in a coater or surface sizer for a fiber web (W), such as a surface roughness on an application roll or its counter roll, is identified,

-the arrival time of the fault spot (H), such as a surface roughness on an application roll or its counter roll, in a roll nip between the application roll and its counter roll, as well as its passage time through said roll nip, are estimated or calculated,

-to at least one digital valve unit (100), used for controlling the nip pressure of a roll nip (N), is transmitted from a control system (4) a feed-forward adjustment instruction (F) for dealing with a fault spot in the roll nip (N) between the application roll and its counter roll,

-according to the adjustment instruction (F), the nip pressure of a roll nip is reduced through the intermediary of at least one digital valve at the latest when a fault spot arrives in the roll nip, and the nip pressure is increased as the fault spot exits the roll nip (N).

29. A method as set forth in any of claims 1-4, characterized in that

-the location of fault spot (H) in a paper web (W) rewound on a storage reel, such as a surface roughness of rewound paper, is identified counter-clockwise in the paper web traveling direction upstream of a roll nip (N) between the storage reel and its counter roll, such as a backing roll and/or a breast roll, and the arrival of the fault spot (H) in the roll nip (N) between the storage reel and its counter roll, as well as its passage time through said roll nip/roll nips, are estimated or calculated, -to at least one digital valve unit (100), used for controlling the nip pressure of said roll nip (N), is transmitted from a control system (4) a feed-forward adjustment instruction (F) for dealing with the fault spot (H) in the roll nip (N) between the storage reel and its backing roll and/or breast roll,

- according to the adjustment instruction (F), the nip pressure of a roll nip is reduced through the intermediary of at least one digital valve unit (100) at the latest when the fault spot (H) arrives in the roll nip between the storage reel and its backing roll and/or breast roll, and the nip pressure is increased as the fault spot exits said roll nip/roll nips.

30. A method as set forth in claim 28 or 29, characterized in that

-the adjustment instruction comprises a relief pulse for the nip pressure of a roll nip, wherein a nip pressure (N_p) at a roll nip between a roll and its counter roll is lowered by reducing a pressure load applied by a hydraulic actuator (2) on an element (3) supporting/pressurizing the roll and/or its counter roll, as well as a load reset pulse, wherein the pressure load applied by the hydraulic cylinder (2) on such an element (3) is reset to what it was,

-the relief pulse and the reset pulse for the nip pressure (N_p) are developed by using digital valves included in the digital valve groups (10) of a digital valve unit (100) for changing a relative ratio (V_2Oa, V₂ot_>) of volumetric fluid flows arriving in various sides (20a, 20b) of a hydraulic actuator's (3) piston and thereby fluid pressures (P_2Oa. P2ot_>) existing on the various sides (20a, 20b) of the hydraulic actuator's piston with respect to volumetric fluid flows (V₂OaT, V₂obτ) arriving in various sides of the piston and hence fluid pressures (P₂oaτ,P2ot_>τ) existing on various sides of the piston in a state of equilibrium.

31. A method as set forth in any of claims 1-4, characterized in that the method comprises at least the following steps of

-identifying, in a counter-clockwise manner relative to the rotating direction of an endless belt with respect to a long nip (N), the location of a spot (H; H_t), thinner than the rest of the belt, on an endless belt (8a) rotating into the long nip (N) in a shoe calender (800), and the extent of the thinner spot (H; H_t), said long nip (N) being established between the endless belt (8a), rotating on top of a shoe roll's (8) shoe element (8b), and the shoe roll's counter roll (80), -estimating or calculating an arrival time for the spot (H; H_t) of the endless belt (8a), thinner than the rest of the endless belt, in the long nip (N), as well as a passage time for the spot (H; H_t) of the endless belt (8a), thinner than the rest of the endless belt, through said long nip (N),

-transmitting from a control system (4) to at least one digital valve unit (100), used for controlling a nip pressure in the long nip (N), a feed-forward adjustment instruction for dealing with the spot (H; H_t) of the endless belt (8a), thinner than the rest of the endless belt, in the long nip (N),

-reducing, as per adjustment instruction, through the intermediary of at least one digital valve unit (100), a nip pressure of the long nip (N) at the latest when the spot (H; H₄) of the endless belt (8a), thinner than the rest of the endless belt, arrives in the long nip, and increasing the nip pressure as this reduced-thickness spot exits the long nip (N).

32. A method as set forth in claim 31 , characterized in that -the adjustment instruction (F) comprises a relief pulse for a nip pressure (P1) of the long nip (N), wherein the nip pressure (P1 ) at the long nip (N) is lowered by reducing a pressure load (P1) applied by hydraulic cylinders (200) on the shoe element (8b,) as well as a nip-pressure reset pulse, wherein the pressure load (P1 ) applied by the hydraulic actuators (200) on the same shoe element (8b) is reset to what it was,

-the nip-pressure relief pulse and the nip-pressure reset pulse are developed by changing the aperture of digital valve groups in a digital valve unit (100) in such a way that the relative ratio of volumetric fluid flows arriving in various sides of the hydraulic cylinders' (200) piston, and thereby the fluid pressures existing on various sides of the hydraulic actuators' (200) piston, are changed with respect to volumetric fluid flows arriving in various sides of the piston and hence fluid pressures existing on various sides of the piston in a state of equilibrium.

33. A method as set forth in claim 31 or 32, characterized in that the location of a spot (H; H_t) of the endless belt (8a), thinner than the rest of the endless belt, is identified on the basis of measuring a surface pressure (P2) of the endless belt in the long nip (N), as well as on the basis of measuring in the long nip (N) a pressure load (P1) of the shoe element (8b) established by the hydraulic cylinders (200), and on the basis of establishing a differential pressure between these pressures (P1 , P2).

34. A method as set forth in any of claims 31-33, characterized in that taken into consideration in the relief pulse of the adjustment instruction (F) are a rotation speed of the endless belt (8a), a length of the long nip (N) in machine direction, as well as a surface area and a thickness reduction degree of the endless belt's reduced-thickness spot (H; H_t).

35. A method as set forth in any of claims 31-34, characterized in that

-in the adjustment instruction's (F) relief pulse, a volumetric fluid flow streaming into the first sides of the pistons of hydraulic cylinders (200) pressurizing the shoe element (8b) and a fluid pressure existing on the first sides of the hydraulic cylinders (200) are reduced from the equilibrium-state fluid pressure and volumetric flow of the first sides to a lowered fluid pressure and a reduced volumetric flow for a specific time period, and thereafter the reduced volumetric flow and fluid pressure existing on the first sides of the hydraulic cylinders' (200) pistons are reset to a volumetric flow and fluid pressure existing in the equilibrium state of the first side of said hydraulic cylinder's piston, whereby, in said state of equilibrium, the fluid pressures existing on the first sides of the hydraulic cylinders (200) are equal to the equilibrium-state fluid pressures (back pressures) existing on the second sides of the pistons of the hydraulic cylinders (200),

-the adjustment instruction's reset pulse is used for increasing a volumetric fluid flow streaming into the first sides of the hydraulic cylinders' (200) pistons and at the same time a fluid pressure existing on the first sides of the hydraulic cylinders' (200) pistons from the equilibrium-state pressure level and volumetric flow of the hydraulic cylinders' (200) first sides to an increased fluid pressure and to an increased volumetric flow, and thereafter the increased volumetric flow and fluid pressure of the first sides of the hydraulic cylinders' (200) pistons are reset to a volumetric flow and fluid pressure existing in the state of equilibrium, whereby, in said state of equilibrium, the fluid pressure existing on the hydraulic cylinders' (200) first side is equal to the equilibrium-state fluid pressures (back pressures) on the second sides of the hydraulic cylinders' (200) pistons.

36. A method as set forth in claim 35, characterized in that

-in order to develop a relief pulse as per adjustment instruction (F), a volume flow (V) passing through a combination (1_A) of open-state digital valves (1) in a specific digital valve group (10) of the digital valve unit (100) is first reduced from a volumetric flow, which exists in a state of equilibrium and which is established by a specific combination of digital valves (1) open in the digital valve group (10) in the state of equilibrium, to a new, lower volumetric flow rate by selecting an appropriate combination of open-state digital valves (1 ) in the same digital valve group (10), the volume flow passing therethrough matching a desired reduced volumetric flow rate, and thereafter, following a short time period from the start of a relief pulse developing process, the volume flow (V) passing through the combination of open-state digital valves (1) in the digital valve group (10) is reset to the original equilibrium-state volumetric flow by selecting the combination of open-state digital valves (1) in the digital valve group to be the same as the original equilibrium-state combination,

-in order to develop a load reset pulse as per adjustment instruction (F), a volume flow (V), passing through a specific combination (1A) of the open-state digital valves (1) in a specific digital valve group (10) of the digital valve unit (100), is first increased from an original volume flow, which exists in the state of equilibrium and which is established by a specific equilibrium-state combination of digital valves 1 open in the digital valve group (10) in the state of equilibrium, to a new, higher volumetric flow rate by selecting an appropriate combination of open-state digital valves (1 ) in the same digital valve group (10), the volume flow passing therethrough matching a desired increased volumetric flow rate, and thereafter, following a short time period from the start of a reset pulse developing process, the volume flow (V) passing through the combination of open-state digital valves (1 ) in the digital valve group (10) is reset to the original equilibrium-state volumetric flow by selecting the combination of open-state digital valves (1 ) in the digital valve group to be the same as the original equilibrium-state combination.

37. An apparatus for implementing a method as set forth in claim 1 in a calender with at least one roll set, which is mounted on a calender body and which comprises at least two rolls (5) between which remains a roll nip (N), enabling a material web to be calendered therebetween, characterized in that the apparatus includes -a control system (4), which comprises means for identifying the location of a fault (H) in the material web or on the calender roll (5), means for estimating or calculating the arrival time for a fault approaching a roll nip (N) between two rolls, means for estimating or calculating the dwell time for a fault in the same roll nip (N), as well as means for developing a feed-forward adjustment instruction (F), said adjustment instruction enabling the fault (H) arriving in the roll nip (N) of a calender to be dealt with, -an adjustment system, including at least one digital valve unit (100) which enables the nip pressure of a roll nip (N) to be adjusted according to an adjustment instruction received from the control system by a feed-forward adjustment mode through the intermediary of at least one hydraulic actuator (2) in such a way that the nip pressure of a roll nip can be reduced according to the adjustment instruction (F) through the intermediary of at least one valve (1) in the digital valve unit (100) at the latest when the fault (H) arrives in the roll nip and the nip pressure can be increased when the fault exits the roll nip (N).

38. An apparatus as set forth in claim 37, characterized in that the hydraulic actuator (2) is a hydraulic cylinder (20).

39. An apparatus as set forth in claim 37 or 38, characterized in that the control system (4) comprises one digital valve unit (100) per each hydraulic actuator (2).

40. An apparatus as set forth in any of claims 37-39, characterized in that the digital valves present in a digital valve group (10) of the digital valve unit (100) have two positions; open and shut.

41. An apparatus as set forth in claim 40, characterized in that, in digital valves, which are consecutive in terms of their nominal volumetric flow and included in a digital valve group (10), the volumetric flow proceeding through a valve with a higher nominal flow capacity is twice as high as the volumetric flow proceeding through a valve with a lower nominal flow capacity.

42. An apparatus as set forth in any of claims 37-41 , characterized in that the apparatus is a multi-roll calender (500), wherein between a bottom roll (5; 5b) and a top roll (5; 5b) of the roll set remain a number of intermediate rolls (5; 5a), having at each end thereof roll supporting levers (3), each of which has coupled therewith a hydraulic actuator (2), the loading effect of which on a roll nip (N) between two rolls can be changed by means of an adjustment system comprising at least one digital valve unit (100).

43. An apparatus as set forth in claim 42, characterized in that, per each hydraulic actuator (2), the adjustment system has one digital valve unit (100), including four digital valve groups (10) which can be used for adjusting a fluid pressure (P20a. P20b) existing on a pressure side (20b) and on a working side (20a) of each hydraulic actuator (2).

44. An apparatus as set forth in claim 43, characterized in that each intermediate roll (5; 5a) of the roll set (50) carries a supporting lever (3) whose operation is controlled by a hydraulic actuator (2), and the operation of each hydraulic actuator (2) is adjusted independently by its specific digital valve unit (100), whereby these digital valve units are functionally synchronized by way of the control system (4) without a direct hydraulic fluid flow communication therebetween, whereby each digital valve unit (100) has its own inlet for a pressurized hydraulic fluid from a hydraulic fluid supply line (7; 71) and its own outlet for a hydraulic fluid to a tank line (7; 72).

45. An apparatus as set forth in claim 44, characterized in that the adjustment of a hydraulic actuator (2), which is present at each end of each intermediate roll (5; 5a) and which pressurizes the supporting lever (3), is performed by means of digital valve units (100; 100', 100") structurally identical with each other, the adjustment instructions (F) delivered from the control system to the control circuits of said valve units being identical over a specific time span.

46. An apparatus as set forth in claim 45, characterized in that the adjustment instruction (F) is an adjustment instruction F(V) for a volumetric flow (V) arriving in or discharging from a hydraulic actuator (2) or an adjustment instruction F(X) for the position of a hydraulic actuator's (2) piston, and that the control of a hydraulic actuator (2), as per adjustment instruction, takes place without feedback, by a feed-forward mode of adjustment.

47. An apparatus as set forth in any of claims 37-41 , characterized in that the calender is a shoe calender (800), comprising a shoe roll (8) which consists of a shoe element (8b) with a rotatable endless belt (8a) disposed thereon, whereby said shoe element (8b) can be pressurized from below with a specific pressure (P1 ) by means of a number of hydraulic cylinders (200) bracing themselves against a stationary calender body (85), such that the endless belt (8b) rotating on top of such element presses against a counter roll (80) of the shoe roll by a specific pressure (P2) in a long nip (N), which long nip (N) remains between the counter roll (80) and the endless belt (8b) and into which long nip (N) is guided a material web (W) to be calendered, whereby the pressure established by the hydraulic cylinders (200) can be adjusted by at least one digital valve unit (100) which can be supplied with an adjustment instruction (F) by means of the control system (4).