WO1991004108A1

WO1991004108A1 - Rolling mill with controllable deflection roll

Info

Publication number: WO1991004108A1
Application number: PCT/GB1990/001378
Authority: WO
Inventors: Christopher David Collinson; David Michael James
Original assignee: Davy Mckee (Poole) Limited
Priority date: 1989-09-13
Filing date: 1990-09-06
Publication date: 1991-04-04
Also published as: GB8920763D0

Abstract

A rolling mill has a pair of roll assemblies, at least one of which includes a controllable deflective roll. A first control circuit arranges for the support elements (5) of the roll to position the roll shell (2) in a desired position relative to the beam (1). A second control circuit arranges for the support elements (5) to apply a desired distribution of pressure along the length of the shell (2). The responses of the two circuits are independent and non-interactive.

Description

ROLLING MILL WITH CONTROLLABLE DEFLECTION ROLL

This invention relates to a rolling mill having at least one roll which is referred to hereinafter as a roll of the type set forth, and comprises a fixed elongate beam, a cylindrical roll shell rotatable around the beam and supported thereon by a plurality of fluid-operable support elements spaced apart along the length of the beam and arranged to apply bearing forces to the roll shell.

When such a roll is used in a mill for rolling strip material, it is an object to actively control the gap between the rolls of the mill and hence the thickness of the material being rolled. In addition, and simultaneously, it is required to maintain a controlled force distribution across the width of the material in order to control the flatness of the material. This force distribution may be varied automatically in response to meas'irec* r-.Lεtnes.-**** ~^y *~^r-

Various control means have been described whereby groups of pressures may be raised or lowered to achieve the demanded position, but such systems are inherently unsuitable for maintaining simultaneous control of both roll pressure distribution and effective roll position. This is because they do not take into account the problem of interaction between the position control and the pressure distribution control.

According to the invention, a rolling mill comprises a pair of roll assemblies between which metal strip can be rolled and where at least one of the assemblies includes a roll comprising a fixed elongate beam, a cylindrical roll shell rotatable around the beam and supported thereon by a plurality of fluid operable support elements spaced apart along the length of the beam and arranged to apply bearing forces to the roll shell; means for determining the actual position of the shell relative to the beam; a first control circuit arranged to receive data from the determining means and a command signal representing the desired position of the shell relative to the beam and to respond by controlling the individual controllers so that the support elements position the shell in the desired position relative co the beam; means for determining the fluid pressure of each support *3leme>nt: and a second control circuit arranged to receive data from the fluid pressure determining means and command signals representing the desired distribution of pressure applied to the shell along its length and to respond independently of, and in a non-interactive manner with, the response of the first control circuit by controlling the individual controllers so that the pressure applied to the support elements represents the desired distribution of pressure along the length of the shell.

When a roll of the type set forth is employed in a rolling mill, the roll force is generated by the supporting elements along the length of the roll shell. In order to achieve a uniform change in thickness across the width of the strip being rolled, all the supporting elements must move in unison, in response to a position command. Such movement will cause the shell to be displaced relative to the elongate beam and, consequently, to cause a change in the roll gap. It is also possible for the roll pressure distribution to be responsive to load distribution commands without affecting roll position. The action resulting from the operation of the first and second control circuits is that the control actions are designed to take place independently, concurrently and without interaction.

The roll is provided with means for measuring the position of the shell relative ~ the bαam. This position is compared with the desired position and is used to generate a position error signal which is simultaneously applied to each of the supporting elements. Means are available to vary the proportion of the position error which is applied to each supporting element so that the element will correctly contribute to an overall displacement of the roll shell without disturbing the roll pressure distribution. This is accomplished, for example, by causing the supporting elements at the centre of the roll to move further than the supporting elements towards the roll ends. This means any deflection of the internal support beam is compensated such that correct pressure distribution is maintained along the length of the roll both during, and as a result of, position changes and such that the flatness of the strip being rolled is maintained.

In order that the invention may be more readily understood, it will now be described, by way of example only, with reference to Figure 1 of the accompanying drawing which diagrammatically shows the control circuits for a roll of the type set forth.

A controllable deflection roll, which has previously been referred to as a roll of the type set forth, comprises a fixed support beam 1, a cylindrical roll shell 2 rotatable around the beam 1 and supported thereon at its ends by bearings 3.

A plurality of support elements in the form of hydrostatic pads 5 are spaced apart along the length of the beam 1 and pistons supporting the pads are located in cylinders formed in the beam 1. Position transducers 6A positioned at opposite ends of the beam indicate the position of the roll shell with respect to the beam. Signals from the transducers are taken along control lines 7 to a comparator 8 to which a signal representing the desired position of the shell with respect to the beam is supplied.

Each cylinder is supplied with hydraulic fluid from a source 7A by way of an individual servo- valve 9. The output signal from the comparator 8 is supplied in parallel to controllers 10A, 10B ... ION which are arranged to control respective servo-valves 9. Thus, in use, the control circuit receives signals at the comparator 8 representing the desired and actual positions of the shell with respect to the beam and the position error, if any, causes an error signal to be applied to each of the controllers 10A, ... ION. The gain of the individual controllers 10A, ... ION can be varied so that the outputs of the servo-valves controlled by these controllers vary the proportion of the position error which is applied to each support element so that the movement of the shell will correctly contribute to an overall displacement of the shell without disturbing the roll pressure distribution. This is accomplished, for example, by causing the pistons at the centre of the roll to move further than the pistons towards the roll ends. Any deflection of the beam 1 is compensated such that correct pressure distribution is maintained along the length of the roll both during, and as a result of, position changes and, in this way, the flatness of the strip is maintained. The correct proportion of position error to be applied to each support element may be determined experimentally or by mathematical modelling of the deflection characteristics of the roll shell and the beam.

To overcome any errors in pressure distribution, and to provide means for varying the pressure distribution in response to flatness errors, a second control circuit is provided. The pressure in each of the pistons is monitored and compared in a comparator 11 with a series of command values which represent the desired pressure distribution along the length of the roll. The command values will normally be expressed in terms of a desired pressure ratio for each support element. Similarly, the pressures in the cylinders are converted to values corresponding to the ratio of each individual element as a proportion of the force generated by all the elements and any errors between the desired pressures in the cylinders and the measured pressure in these cylinders is used to generate a corrective offset signal supplied from controllers 12A, 12B, ... 12N to which the error signals are supplied. The pressure distribution correction signals are relatively slow in effect in comparison with the position correction signals and this enables the desired roll position to be maintained by means of the high speed closed loop position system. The desired pressure ratios are arranged such that a change in desired pressure distribution does not cause a change in overall force and thus will not disturb the roll position.

Figure 2 of the accompanying drawings illustrates how the pressure distribution is controlled. Assuming that there are five support elements on the roll, the pressure in the cylinder of each element is represented by Pi - P5, respectively. The total load developed by the roll is measured from the sum -P of the individual cylinder pressures PI - P5. Let the desired pressure distribution be represented by TI, T2, T5 whose sum is 1.00 by definition, so TI - T5 represent the proportions of the total load required for the cylinders 1 - 5, respectively. For each zone represented by a support element, a pressure reference is calculated, this being the product of T≤LP. The actual pressure in the cylinder of the support element of that zone is compared with the corresponding T £? to produce a series of error signals which are applied to the controllers 12 to generate a corrective signal which is added to the position command error signals applied to the controllers 10.

The first and second control circuits described above may be incorporated into automatic strip thickness and flatness control systems in a rolling mill. The thickness and flatness systems operate independently and non-interactively. Measured thickness errors or measured factors affecting strip thickness accuracy are combined by the respective controller 10A ... ION which generates a roll position command. Similarly, measured flatness errors or measured factors affecting flatness accuracy are analysed and combined in controllers 12A ... 12N which generate roll force distribution commands corresponding to the individual pressures of the support elements.

Such control systems may also embody other actuators, such as external roll bending cylinders, axial roll movement, coolant distribution, roll heating means or inflatable rolls. The roll system described may be used with any combination of the above and with any combination of solid rolls and similar controllable deflection rolls in a roll stand.

Referring now to Figure 3 of the accompanying drawings, a roll of the type set forth is arranged as the upper back-up roll of a four-high rolling _r.ill having work rolls 20, 21. Metal strip 22 passes between the work rolls and, downstream of the mill, its thickness is measured by an X-ray gauge 23 and the shape of the metal strip across its width is determined by a shapemeter 24. The strip is then coiled at a coiler 25. The support elements on the roll 2 are supplied with hydraulic fluid and each element is controlled by an individual servo-valve 9. A position controller 30 receives signals from an AGC computer 31 representing the desired position of the shell with respect to the beam 1 and signals representing the actual position of the shell with respect to the beam from the transducers 6A are also supplied to the controller. The controller controls the individual servo-valves 9. The computer 31 receives signals corresponding to mill inputs, such as speed, load tension, etc., and also a signal from the X-ray gauge 23. The computer also receives a signal from a thickness reference device 32 and, in the computer, the actual thickness of the gauge of the strip material is compared with the thickness reference and the difference is used to supply a desired position signal to the position controller 30. In this controller the desired position is compared with the measured position and error signals, if required, are supplied to the servo-valves 9 in order to adjust the position of the shell with respect the beam 1 to thereby vary the gap between the rolls 20, 21.

A second control circuit for controlling the shape of the strip as it is rolled includes pressure controllers 12 which receive signals corresponding to the measured pressures in each of the support elements on line 33 and signals representing desired pressures on the support elements on line 34. Signals from the shapemeter 24 and from a flatness reference 35 are supplied to a flatness error analysis matrix 36. In the matrix the desired pressure distribution is analysed to produce an equation having terms of at least the first four orders. Signals representing each of the orders of the equation are supplied separately to a pressure distribution synthesis matrix 37 from where the individual pressure signals are supplied to the pressure controllers 12.

In use, the required changes to the existing roll gap are computed from a variety of mill input signals in the circuit 31. The precise algorithm for automatic thickness control will vary according to the sensors which are used and will depend upon the mill duty. In every case the AGC system computes a roll gap correction signal which is applied to the servo-valves 9 and which is designed to reduce exit thickness errors to zero. The roll gap correction signal is proportioned between each of the servo-valves 9 so as to cause each of the load support elements to move in unison and thereby change the roll gap. Any disturbance in the pressure distribution is detected and used to correct the individual pressures in accordance with the pressure distribution required by the operator or in accordance with a pressure distribution command signal generated by the automatic flatness control system. Departures from desired strip flatness are determined by comparing measured flatness with desired flatness. This difference is referred to as the flatness error. It is a multi-channel vector which will have polarity and amplitude corresponding to each measurement zone across the strip width which is measured by the shapemeter 24. In the matrix 36 the flatness error is analysed into a number of component errors each corresponding to flatness influences which can be achieved using the roll pressure distribution either in isolation or in conjunction with other available flatness actuators, such as roll bending systems or variable crown rolls. There are numerous ways in which the flatness error can be analysed, the general principle being to establish non-interactive responses between the various flatness components, which, at the same time, generates flatness correction signals within the mechanical capabilities of the actuators.

By way of example, the flatness error may be resolved using Fourrier or polynomial techniques by which the 1st Order (Tilt), 2nd Order (Parabolic), 3rd Order (Asymmetric), and 4th Order (Quartic) components of flatness error are quantified. Typically, this analysis will be achieved by using a "least square" method in which product of the flatness error vector and the various Fourrier or polynomial terms is minimised by appropriate selection of coefficients.

Alternatively, the flatness error may be correlated with functions other than conventional Fourrier or polynomial functions, sometimes referred to as •Characteristic Functions' . These express the characteristic elastic deflections achievable in a particular roll stack assembly by means of various roll force actuators, thereby making corrective actions more efficiently.

For each Fourrier component, there is a corresponding matrix of zone pressure changes which will most exactly compensate the measured error. For a tilt error, for instance, the pressure in each zone would be raised or lowered asymmetrically, about the strip centre line, in a ramp pattern of the correct magnitude to produce a flatness change in the strip, exactly equal to the measured tilt error. Similarly, there will be a matrix of pressure changes for optimum correction of 2nd Order (Parabolic) error, and so forth. The maximum number of orders correctible will, in practical terms, depend upon the number of control zones available. In general, it is not possible to correct flatness errors of a higher order than the number of independent actuators.

The shapemeter 24 may be of the type sold under the trade mark VIDIMON by Davy McKee (Poole) Limited, Wallisdown Road, Poole, Dorset BH12 5AG.

Claims

Claims :

1. A rolling mill having a pair of roll assemblies between which metal strip can be rolled and where at least one of the assemblies includes a roll comprising a fixed elongate beam, a cylindrical roll shell rotatable around the beam and supported thereon by a plurality of fluid operable support elements spaced apart along the length of the beam and arranged to apply bearing forces to the roll shell; means for determining the actual position of the shell relative to the beam; a first control circuit arranged to receive data from the determining means and a command signal representing the desired position of the shell relative to the beam and to respond by controlling the individual controllers so that the support elements position the shell in the desired position relative to the beam; means for determining the fluid pressure of each support element; and a second control circuit arranged to receive data from the fluid pressure f?f»t.eriϊi n.ng means and command signals representing the desired distribution of pressure applied to the shell along its length and to respond independently of, and in a non-interactive manner with, the response of the first control circuit by controlling the individual controllers so that the pressure applied to the support elements represents the desired distribution of pressure along the length of the shell.

2. The rolling mill of claim 1, wherein the second control circuit includes a comparator to which the data from the fluid pressure determining means and the command data are applied and which provides error signals which are supplied to separate controllers which control the servo-valves by way of the first- mentioned controllers.

3. A rolling mill as claimed in claim 1 or 2, wherein the command signal supplied to the first control circuit is obtained from an automatic gauge control circuit of the mill.

4. A rolling mill as claimed in any preceding claim, wherein the command signals representing the desired distribution of pressure which is applied to the second control circuit are obtained from an automatic flatness control circuit of the mill.

5. A rolling mill as claimed in claim 4, wherein the automatic flatness control circuit includes a shapemeter for determining the shape of strip material rolled between the roll assemblies.