AT515024A2 - Web tension control in pilgrim step method - Google Patents

Abstract

The invention relates to a method for web tension control in a web processing machine for processing a web (12), wherein the web (12) is processed by means of wenigs least one processing unit (20), wherein the web (12) by means of transport works (20 a) in a first Operating phase of a pilgrim step method in a first direction and in a second operating phase of the pilgrim step method in a second, opposite direction is moved, wherein a web tension (F2, F4) in one of a first and a second transport mechanism of the transport works (20a) limited first web section ( A1, A2) and is controlled by a web tension control to a web tension setpoint by a rotational speed of the second trans- port is influenced by a controller manipulated variable, wherein the rotational speed of the second conveyor in the first operating phase of the pilgrim step method with a positive control sense and in the second operating phase of the pilgrim step method is influenced with a negative control sense.

Description

Bahnzuqkraftrilelung Pilqerschrittverfahren

description

The present invention relates to a method for web tension control in a Bahnbe¬ processing machine for processing a web.

Although the invention will be described below essentially with reference to printing machines, it is not limited to such an application, but rather applicable to all types of web processing machines in which a tensile force of a material web is to be predetermined. The web can be made of paper, cloth, cardboard, plastic, metal, rubber, in foil form, etc.

State of the art

In web processing machines, in particular printing machines, a web is conveyed along driven rollers (web transport rollers), such as e.g. Pull rolls or feed rolls, and non-driven rolls, such as e.g. Deflection, guide, drying or cooling rollers, moved. The web is simultaneously processed by means of usually likewise powered processing units, for example, printed, stamped, cut, folded, etc.

The tensile force or web tension (as long as no change in cross-section occurs is proportional to tensile force and tension, but usually the tensile force is measured) of the web is essentially influenced by so-called nip points which pinch the web shape or non-positively. These are usually driven transport or processing plants. In a printing press, a nip is usually formed by a transport or tensioning unit in which a frictional connection exists between a driven tension roller, a pressure roller or impression cylinder and the material web. The web is divided into web sections, wherein a Warenbahn¬ section is limited by two terminal points. Within a web section kön¬nen further driven and / or non-driven rollers can be arranged. Often the entire web is subdivided into several web sections, sometimes with different tensile force command values. To maintain the setpoints, a so-called web tension control (web tension control) is usually used. The adjustment or regulation of the web tensile force usually takes place via an elongation as a manipulated variable, by influencing the rotational speed of the rollers of the clamping points. This is also referred to below as " adjusting the nip " designated.

The adjustment or regulation of the web tension of a web section can be done by different methods. Downstream means that the clamping section delimiting the web section downstream is adjusted in order to influence the web tension in the web section. Upstream means that the clamping section delimiting the web section upstream is adjusted in order to influence the web tension in the web section. In this simple embodiment, however, the web tension infeeding and / or subsequent web sections is not decoupled from the adjusting movement. Rather, the change in web tension following the course of the web is transported by the machine and must be corrected in all subsequent sections. In addition to this indirect disturbance due to the transport of the web, a direct disturbance occurs in the web section adjoining the displaced nip due to the control engagement , However, it is possible to decouple the remaining web sections by means of differently configured pre-controls, i. to leave the prevailing web tension unaffected by the control action. An overview of numerous methods for adjusting the web tension with and without decoupling can be found in DE10 2011 014 074 A1. Possibilities for changing the web tension in one of the remaining web sections are described in DE 10 2011 105 448 A1.

All of these prior art solutions are based on the premise that the web is moved only in the forward direction. In addition, however, there are also so-called. Pilgerschrittverfah¬ren in which occur during operation and movements in the reverse direction. The forward direction and the backward direction are referred to herein as different operating phases.

For example, reference is made here to DE 34 30 333 C2, in which a pilgrim step method is used in order to be able to specify a print format length independently of a printing cylinder circumference. In pressure-free areas, i. when the printing plate does not touch the paper, the web is moved back to achieve the largest possible Materialausnut¬zung.

In today's widely distributed individually driven pressure cylinders Pilgerschritt¬ method is usually generated by electronic cams. In this case, a mostly virtual (i.e., mathematically generated) guide axis (usually angular position as a function of time) predetermines a mean feed speed of the web, which follows the printing units, wherein the movement of the web conveyances is derived from the guide axis by means of the cam function. The cam function describes a substantially freely definable relationship (for example, in a table) between the master axis position (master axis position) as an input variable or x value and a follower axis position (follow-up angular position) as the output variable or y value.

Apart from the fact that the previously described methods for web tension adjustment can not be applied to a backward movement, there are no suitable engagement possibilities for the pilgrim step method generated by cam disks in order to accept the speed-influencing manipulated variables of these methods. As explained, the cam discs are position-based.

On this basis, it is desirable to be able to use the established methods for web tension adjustment also in pilgrim step methods.

Disclosure of the invention

According to the invention, a method for web tension control in a web processing machine for processing a web with the features of patent claim 1 is proposed. Advantageous embodiments are the subject of the dependent claims as well as the following description.

Advantages of the invention

One aspect of the invention is that the instantaneous direction of movement of the web (as the operating phase of the pilgrim step method) is taken into account in web tension control. In particular, depending on the instantaneous direction of movement, the control sense (i.e., an increase in the controller control variable leads to an increase (control sense positive) or reduction (control sense negative) of the web tensile force) and also the decoupling regulation. In this case, the conveyor belt is moved in a first direction by means of transport mechanisms in a first operating phase of the pilgrim step method, and in a second, opposite direction in a second operating phase of the pilgrim step method. The web is thereby processed by at least one processing plant. A web tension in a first web section delimited by a first and a second conveyor is determined, in particular measured, and regulated by a web tension control to a web tension setpoint by influencing a rotational speed of the second conveyor by a controller manipulated variable. In this case, the rotational speed of the second transport unit in the first operating phase of the pilgrim step method is influenced with a positive control sense and in the second operating phase of the pilgrim step method with a negative control sense. This can e.g. be implemented simply by the fact that in the second operating phase, the negative controller control variable is used. If the instantaneous direction of movement were not taken into account when adjusting the control sense, then the control loop would become unstable during the backward movement, with the backward movement having a speed increase of a certain clamping point which has a different effect on the web tension than in the forward movement.

In order to decouple a second product web section from an adjusting movement in the first product web section, a pre-control variable v is calculated from the controller manipulated variable in accordance with a decoupling instruction and used to control the second web section. The instantaneous direction of movement generally determines which of the web sections is the second web section.

A particularly preferred decoupling instruction comprises a differential component and preferably also a deceleration component (in a simple embodiment, therefore, a DT1 member). The transfer function of a DT1 link is:

KD: differentiation factor (= differentiation time Το)

Ti. Delay time

These controller parameters can be calculated in a simple manner, as described in detail in the introductory and in turn referenced publications, from the web section length and the web speed (m / s). If the instantaneous direction of movement were not taken into account in the determination of the decoupling instruction, no decoupling would be realized during the backward movement.

According to a first embodiment, each transport mechanism is assigned its own cam disk function, which derives the movement setpoint values for the transport mechanism from the leading axle. The cam function is usually calculated directly in the drive controller in this case.

According to a second embodiment, a plurality of transfer mechanisms is assigned a common cam disk function which derives the desired movement values for the plurality of transport units from the lead axle. In other words, a desired rotational angle position for a second transport mechanism is at the same time a desired rotational angular position for a first and / or at least one further transport mechanism. This common cam function is expediently calculated in a higher-level control and fed from there to the individual transport works. In this case, the Kurvenschei¬benausgang is expediently also used differentiated as a speed setpoint on which the respective control manipulated variable acts. The differentiation can be found especially in the drive controller. It is now only a single cam can be expected, which transforms the continuous movement of the virtual master axis in a pilgrim step of the virtual Folge¬ axis. If all real following axes are now operated synchronously with this one additional virtual following axis, it is possible by means of standard web tension control to specify a multiplicative velocity offset as controller manipulated variable, since this structure of the virtual following axis contains an option for interpolating a multiplicative velocity offset.

On the other hand, the motion setpoints of the at least one machining unit without the cam function are derived from the master axis. The at least one machining unit follows the leading axis, if necessary under the influence of a so-called register control, which, for example, provides an angular position difference between the desired angular position for the at least one machining unit and the desired angular position of the master axis.

Web is set angular position for the transport works are preferably by means of a cam function from a desired angular position of a leading axis gerech¬net.

Conveyors preferably follow a cam function. Machining, however, will be without cam function

Another aspect of the invention is the provision of an intervention possibility for a Bahn¬zugkraftregelung in position-dependent cam functions. A preferred possibility for this is the application of the y-value (position or rotation angle desired value) of a cam disk function with the controller actuating variable of the web tension control, in particular a multiplicative calculation. In this case, it can be ensured that, despite a feed (x value) which can be influenced separately, the following axis can be operated synchronously with the master axis. If, instead, one would instead stretch or compress the x-axis of the cam disks via a multiplicative percentage speed offset, the follower axis and the master axis would execute an asynchronous pilgrim step on the basis of a regulator interengagement.

According to another embodiment, the y-value of the cam function is next differentiated to obtain a speed setpoint, and the obtained speed setpoint is then offset with the controller manipulated variable. Preferred possibilities here are the calculation as a so-called transmission fine adjustment, as a multiplicative Geschwin¬digkeitsoffset or as an additive speed offset. This configuration allows the use of conventional web tension regulators in which the regulator manipulated variable is designed accordingly.

An arithmetic unit according to the invention, e.g. a control device of a web-processing machine is, in particular programmatically, adapted to carry out a method according to the invention.

The implementation of the invention in the form of software is also advantageous, since this allows particularly low costs, in particular if an executing arithmetic unit is still used for further tasks and therefore already exists. Suitable media for providing the computer program are, in particular, floppy disks, hard disks, flash memories, EEPROMs, CD-ROMs, DVDs and the like. It is also possible to download a program via computer networks (Internet, Intranet, etc.).

Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the combination given, but also in other combinations or alone, without departing from the scope of the present invention.

The invention is illustrated schematically by means of exemplary embodiments in the drawing and will be described in detail below with reference to the drawing.

figure description

FIG. 1 schematically shows a mechanical part of a web processing machine designed as a printing press.

Figure 2 shows schematically a drive controller for controlling the rotation of a Druck¬walze a printing unit.

FIG. 2a schematically shows a (simpler) drive controller for controlling the rotation of a draw roller of a transport mechanism.

Figure 3 shows schematically a controller structure underlying the processing machine.

Figure 4a shows an exemplary cam function as a relationship between an x value as an input and a y value as an output.

FIG. 4b shows a time profile of the y-value of a cam function according to FIG. 4a.

FIG. 5 shows an exemplary web tension control loop.

Figure 6a shows a control and decoupling structure of a web processing machine for moving the web of material to the right.

Figure 6b shows the control and decoupling structure of the web processing machine for moving the web to the left.

FIG. 7 schematically shows the decoupling structure with the associated input and output variables.

Figure 8 shows schematically an electrical part of the web processing machine. Detailed description of the drawing

FIG. 1 schematically shows a mechanical part 10 of a web-forming machine designed as a printing press, in which a web 12, for example paper, is transported through the machine and processed. The machine 10 shown in FIG. 1 has three transport mechanisms 20a (index = 1; 3; 5) and two printing units 20 (index = 2; 4). Each of the plants 20a, 20 has a separately driven roller 21a, 21 (eg pull roller or pressure roller) and a respectively associated pressure roller 22. For a Regis¬terregelung the printing units 20, a register mark sensor 25 is provided, which is located behind the last printing unit and all Registered register marks. However, register regulation should not be deepened here. In essence, the register control provides an angular position difference as a correction value (shown in FIG. 3 for index n as Δφη) between the

Target rotational position for the at least one machining unit and the desired angular position of the leading axis ready.

The transport mechanisms 20a are clamping points, wherein the material web 12 is clamped between the driven roller 21a and the pressure roller 22, so that movement of the material web 12 is caused by the movement of the driven roller 21a. The pressure rollers 21 of the printing units 20 have printing plates 23. Depending on the length of a single printed image, a printing plate 23 extends only over a part of the circumference of the driven printing roller 21. As a result, a printing unit 20 clamps the material web 12 only when the printing plate 23 touches the material web 12. In the pressure-free Umfangs¬ range no clamping is present. Therefore, the web 12 can be moved in the pilgrim step at this time counter to the normal transport direction. The printing units continue to rotate unchanged. Because of the interruption of the clamping, the printing units 20 can also be disregarded in the course of the web tension control. The web 12 is therefore subdivided into web sections A1 - A2, wherein one web section is bounded by two adjacent conveyers 20a. Between the transport sections 20a delimiting the respective web sections A1, A2, the web 12 passes through measuring devices 24 for measuring the respective tensile force F2, F4 in the respective web section A1, A2.

The first one of the transport mechanisms 20a (index n = 1) is a reference nip whose driven roller 21a rotates in accordance with a cam function 64 (FIG. 3) which outputs a cam rotation position as the initial value (< py in FIG The leading axis (< px in FIG. 3) is determined. The roller 21a is driven by an electric drive 30 (for example a synchronous motor), the movement of which is in turn controlled by a drive controller 32a (see Figure 2a), which in particular controls the actual rotational speed ωη to a desired rotational speed ω1. For this purpose, the electric drive 30 has a rotary encoder 31, which measures a rotational angle position φη and transmits it to the drive controller 32a. The electric drive 30 receives from the drive controller 32a a manipulated variable ln, which in the present example predetermines a motor current or motor torque.

In a drive controller 32 (see FIG. 2) for the printing units 20, in the present example a speed control is subordinated to a position control. The rotational angle position <pn is supplied on the one hand to a differentiating element 40 for determining the rotational speed or rotational speed ωη and on the other hand to a reference junction 44 for comparison with a nominal rotational angular position cpwn. From the reference junction 44, a control deviation of the position control is determined and fed to the position controller 42. This is designed in particular as a linear, continuous controller, preferably as a PI controller. The position controller manipulated variable is the setpoint speed uw. This is compared at a reference junction 43 with the rotational speed ωη. From the reference junction 43, a control deviation of the speed is determined and fed to a speed controller or speed controller 41. This is in particular likewise designed as a linear, continuous controller, preferably as a PI controller. The speed controller manipulated variable is ln.

In a drive controller 32a for the transport mechanisms 20a, the position control can be dispensed with in the present example (see FIG. 2a), as it does not depend on a precise relative position between printing units and transfer mechanisms.

In Figure 8, an electrical part 11 of the printing machine is shown schematically. In addition to the electric drives 30, rotary encoders 31 and drive controllers 32, 32a, this also includes a communication connection designed as data bus 34 (for example ethernet-based real-time fieldbus such as Sercos III) and a control device 33 (for example PLC) in which many of the control and regulating steps described here take place.

As mentioned, the first of the transport mechanisms 20a (index n = 1) is a reference clamping point. The following transport mechanisms 20a (index n = 3, 5, ...) are adjusted for web traction control with respect to the first transport mechanism, i. their desired speeds u) wn be adapted. It should be noted here that a different nip than the first transport can be selected as reference nips. Thus, the illustration in Figure 1 is only one embodiment.

An exemplary web tension control loop is shown schematically in FIG. The measured actual web tension values Fk (index k = 2, 4) and the respectively associated web tension setpoint values Fwk are each fed to a comparison element 51, from which a respective control deviation is calculated and fed to a web tension controller 50, which derives from the control deviation in accordance with a regulation (eg PI) calculates the controller manipulated variable uk + 1b. This is in particular also designed as a linear, continuous controller, preferably as a PI controller with a proportional coefficient KP and a readjustment time TN. The regulator manipulated variable uk + i and a pilot control variable vk + 1 are calculated.

In this case, the pilot control quantity vk + 1 is calculated from the control manipulated variable uk + 1 in accordance with a decoupling instruction 52, which in FIG. 5 is designed, for example, as a DT1 element with a differentiation coefficient KD and a delay time T *. For each of the following transport mechanisms 20a (index n = 3; 5;...) There is a web tension control loop according to FIG. 5, so that two regulator manipulated variables uk and two pilot control variables vk (index k = 3; 5; ) be determined. These are fed to a combining element 63 (FIG. 7), which calculates therefrom modified control manipulated variables uk (index k = 3; 5;...) In accordance with a combination rule. The combination rule is a matrix calculation u '= u + Mv.

The concrete form of the combination matrix M only depends on the instantaneous direction of movement of the configuration of the web processing machine for a given number of inputs for controller manipulated variables (and possibly pilot control variables), i. specifically, from the position N (here N = 1) of the reference nip. The instantaneous direction of movement can be easily deduced from the sign of the cam disc speed toy, which is thus likewise supplied to the combination element 63.

The combination member 63 may be programmably equipped with a number of decoupling matrices, covering in particular both directions of movement. Thus, only the information about the direction of movement is required for the selection of the combination matrix to be used.

By means of a conversion element 61, the respective desired rotational speed cown is formed from a modified controller manipulated variable uk 'and from the differentiated cam output coy (see FIG. 3), which in turn is supplied to a drive controller 32 according to FIG. 2a. For example, the conversion of the modified regulator actuating value to a desired rotational speed co'wn takes place according to one of the following formulas: co'wn = u) yx (1 + u'n) - so-called geared fine adjustment (preferred) co'wn = oüy * u'n - multiplicative velocity offset co'wn = coy + u'n - additive velocity offset

In Figure 3, the underlying controller structure is shown schematically. The virtual guide axis is given as a rotating angular position &lt; px over time, in particular by the controller 33. This transmits the rotating angular position &lt; px over the data bus 34 in a fixed time frame. For example, a current angular position φχ is then transmitted at each transmission time.

The printing units (index n = 2; 4) are operated in accordance with this angular position &lt; px, whereby a necessary register control corrects the angular position &phis; &phis; by a correction value (shown in Figure 3 for index n as Δφη). In parallel with this, from the angular position <px according to the cam disk function 64, the cam angle position <py is calculated. In the example shown here, an embodiment is shown in which only a common cam function is calculated for all transport mechanisms 20a, whereby a virtual following axis cpy is generated as a target rotational position for all transport mechanisms 20a.

An exemplary cam function is shown in FIG. 4a. Without taking into account any corrections, the angular position ≦ px corresponds to the angular position of a printing roller of a printing unit, the cam disk angular position φγ to the angular position of a drawing roller of a conveyor mechanism. As a function of present (i.e., several revolutions), this results in a movement of the tension roller, as shown in Figure 4b.

Referring again to FIG. 3, the cam angle position &lt; py is differentiated by a differentiator 60 to form a cam speed coy, thus producing an engagement point for traction control. Since, as mentioned, the first of the transport works represents the reference nip, the associated target speed is generated ωηιunkorrigiert. The further target rotational speeds ωη3 and target rotational speed ton5 are generated as shown in FIG.

The elements 40 to 44 are expediently calculated in the drive controllers 32, whereas the elements 50 to 64 in the control unit 33 are calculated. Accordingly, the data cpwn or u) wn transmitted via the data bus 34 need not be interpolated within the above-mentioned time frame.

Preferred decoupling matrices are illustrated graphically in FIGS. 6a and 6b. Four transport mechanisms (index n = 1; 3; 5; 7) are shown. Web sections with corresponding tensile forces Fk (index k = 2, 4, 6) are formed between the conveyors. Each web section is assigned a web tension controller 50 which calculates a controller manipulated variable un (index n = 3, 5, 7) according to FIG. As mentioned, the first transport mechanism is the reference nip. Therefore cow1 is uncorrected.

Figure 6a shows the case that the web 12 moves to the right (indicated by the arrow above). Shown here is a downstream control with downstream decoupling. To increase the tractive force F2, raise cow3 and decrease caw3 to reduce the tractive force F2. The sense of control is therefore positive. cow3 is charged with the positiveu3. Via a DT1 element 52, which in the figure is represented by the equivalent representation &quot; 1-PT1 &quot; is shown, the pilot control quantity v3 is calculated from u3. The pilot control quantity is used, as explained above, for decoupling the subsequent transfer mechanisms.

The above applies mutatis mutandis to the tensile forces F4, F6 and the associated control manipulated variables u5, u7 and pilot control variable v5. An upstream decoupling is not necessary. Respective links 53 are therefore &quot; 0 &quot;. If the direction of movement now changes to the left in the pilgrim step method, the control and decoupling scheme must be changed accordingly so that the functionality remains. Figure 6b shows the case that the web 12 moves to the left.

In particular, in the context of the invention, the direction of movement in the control sense is considered. In the case of a movement to the left, in order to increase the tensile force F2, it is necessary to reduce töw3 and to increase the tensile force F2 <mw3. The sense of control is negative. cow3 is compared to the addition inverted (ie the negative) u3, i. -u3, charged. It should be noted that in Figures 6a and 6b, only the corresponding decoupling are shown, which is not shown in addition to a direction reversal necessary inversion of the control sense. Namely, the inversion of the control sense occurs here, for example, in the calculation of u) 'wn in the conversion unit 61 according to: U>' wn = (OyX (1 -U'n) CU'wn = U) yX -U'nW Wn = Wy - U n

Alternatively, the inversion of the control sense can also already be achieved in the combination element 63. This can be set up programmatically in accordance with the selected configuration (reference terminal N) to invert all necessary control and pilot control signals.

Furthermore, in the context of the invention, the direction of movement during decoupling is taken into account. The original downstream scheme with downstream decoupling now becomes an upstream scheme with upstream decoupling. This can be done by an addition. Subtraction of PT 1 can be performed on each limb. An adaptation of the decoupling to the web running direction is possible by all the Vorsteuglieder of the various controllers by addition or subtraction are modified so that the desired uncontrolled axis (here on the left) is controlled by any controller. Attention is that the uncontrolled axis in the concrete example according to the web running direction in the first phase of operation, the incoming nip and in the second Be¬triebsphase the expiring nip is.

Claims (18)

Claims 1. A method for web tension control in a web processing machine for processing a web (12), wherein the web (12) by means of at least one processing unit (20) is machined, wherein the web (12) by means of transport mechanisms (20a) in a in a first direction and in a second operating phase of the pilgrim step method, in a second, opposite direction, wherein a web tension (F2, F4) is limited in one of a first and a second conveyor of the transfer mechanisms (20a) first web section (A1, A2) and is controlled by a web tension control (50) to a web tension setpoint (Fwk) by a rotational speed (caw3) of the second conveyor (20a) by a controller manipulated variable (uk) is influenced, the rotational speed (ww3 ) of the second conveyor (20a) in the first operating phase of the pilgrim step method with a po in the second operational phase of the pilgrim step method with a negative control sense. 2. The method of claim 1, wherein a desired angular position ((pw2, cpw4) for the at least one processing unit (20) from a desired angular position (<px) of a, in particular virtual, leading axis is calculated. 3. The method of claim 2, wherein the target angular position (<pw2, q> w4) for the at least one processing unit (20) is influenced by a register control. 4. The method of claim 2 or 3, wherein the desired angular position (cpw2, cpw4) for the at least one processing unit (20) is not influenced by the web tension control (50). 5. The method according to any one of claims 2 to 4, wherein the target rotation angle position (<pw2, cpw4) for the at least one machining unit (20) from the target angular position (cpx) of the guide axis and a correction value (Δφη) is calculated. 6. The method according to any one of claims 2 to 5, wherein a desired angular position (rpy, q> wn) for the second transport of the transport works (20a) by means of a Kurvenscheiben¬ function (64) from the desired angular position (cpx) of the leading axis is calculated. 7. The method of claim 6, wherein the target angular position (<pw2, q> w4) for the at least one machining unit (20) without the cam function (64) from the desired angular position (φχ) of the master axis is calculated. 8. The method according to claim 6 or 7, wherein the target angular position (<py, (pwn) for the second conveyor of the conveyors (20a) simultaneously a desired angular position (<py, <pwn) for the first and / or at least one further transport mechanism of the transport works (20a) is. 9. The method according to any one of claims 6 to 8, wherein the target rotational position (tpy, cpwn) for the second transport mechanism of the transport mechanisms (20a) is multiplicatively applied to the controller control variable (uk). 10. The method according to any one of claims 6 to 9, wherein the target angular position (cpy, cpwn) for the second transport mechanism of the transport works (20a) is differentiated by a target rotational speed (cown) for the second transport mechanism of the transport works (20a) to receive. 11. The method according to claim 10, wherein the setpoint rotational speed (ojwn) for the second transport mechanism of the transport mechanisms (20a) is applied multiplicatively and / or additively to the controller manipulated variable (uk). 12. The method of claim 10 or 11, wherein the target rotational speed (cown) for the second transport mechanism of the transport mechanisms (20a) is applied multiplicatively with a sum of a number and the controller control variable (uk). 13. The method according to any one of the preceding claims, wherein the determined Bahnzug¬ force (F2, F4) in the first web section (A1, A2) with a target value (Fwk) is compared to calculate a control deviation, wherein the controller control variable (uk) off the control deviation is calculated in accordance with a regulation rule. 14. The method according to any one of the preceding claims, wherein from the controller manipulated variable (uk) in accordance with a Entkoppelvorschrift a pilot control quantity (vk) is calculated, wherein the rotational speed (cow5) of a third conveyor (20a), which limits a second web section, by the pilot control quantity (vk ) being affected. 15. The method of claim 14, wherein the Entkoppelvorschrift is given in dependence on the operating phase of the pilgrim step method. A computer unit adapted to perform a method according to any one of the preceding claims. A computer program that causes a computing unit to perform a method according to any one of claims 1 to 15 when executed on the computing unit, in particular according to claim 16. 18. A machine-readable storage medium having a computer program stored thereon according to claim 17.