During cold rolling of metal, a continuous strip of metal is passed through a mill stand so as to reduce the gauge (thickness) of the metal strip. Fundamentally, the mill stand consists of a pair of rolls that, when in use, are separated by a distance slightly less than the desired exit gauge of the metal strip which passes between the rolls. Where the desired gauge of the metal strip is small, closed gap rolling is performed where the surfaces of the rolls are touching outside of the width of the strip and the metal strip is able to pass between the rolls as a result of a slight deflection of the surface of the rolls. Commonly, the mill stand comprises a set of rolls consisting of a pair of small work rolls each of which is in turn in contact with one or more much larger backup rolls. The components of a mill stand have their own natural frequencies of vibration; however, resonant vibrations of the mill stand can develop that negatively affect the quality of the sheet metal being rolled. For example, typically a 10% variation in exit gauge can be caused by gauge chatter that usually develops very quickly and can only be stopped by reducing the speed of the sheet metal through the mill stand. Gauge chatter is a self-excited phenomenon and, once it has developed, it is subject to a feedback mechanism that can result in the vibration building up to unacceptable levels very quickly.

This feedback mechanism is inherent in every rolling mill stand and is illustrated in Figure 1 schematically. Figure 1 shows the passage of a strip 2 of metal through two stands, n and n+1, of a multi-stand rolling mill.

Each stand comprises smaller diameter work rolls 10, between which the strip 2 passes, and each of which is supported by a respective larger diameter backing roll 11. At each stand n, n+1, a change in the rolling load (for example from a change in the hardness of the material being rolled or any other disturbance) produces a change in exit strip thickness,

5 (exit gauge). The change in the thickness of the exit strip produces a change of the entry strip speed, 8 (entry speed), due to a continuity of mass flow through the mill (mass in=mass out). In turn, the change in entry strip speed causes a consequential change in the tension in the entry strip, b (entry tension). This arises because if the speed of metal increases at one end relative to the speed at the other end, then the strip will be forced to stretch like a spring and will have a higher tension. Finally, to complete the feedback loop, the change in entry strip tension produces a change in rolling load, 8 (rolling load).

This problem becomes more complicated when, as illustrated, there is more than one mill stand in series because the mill vibration can interact via the metal strip joining them. A change in entry tension in stand n+1 is equivalent to a change in exit tension in stand n which will produce a change in rolling load in stand n. A change in exit gauge in stand n will travel with the strip into stand n+1 and produce a change in rolling load in stand n+1.

Currently, the only solution to avoiding gauge chatter or restricting it once the vibration starts to develop, is to restrict the speed with which the metal can be passed through the mill. W096/27454 provides a mathematical analysis of gauge chatter and proposes the use of one or more additional rolls, close to the roll bite, that roll with the metal strip, thereby inducing a phase change in the mechanical system consisting of the metal strip and the mill stand. The interposition of the additional roll (s) is said to introduce a phase advance into the feedback mechanism associated with the roll stand, thus stabilising the loop and preventing vibration. This phase advance is achieved by placing the additional roll (s) close to the roll bite, which has the effect of making the stiffness of the strip between the entry roll and the additional roll (s) much smaller than that between the additional roll (s) and the roll bite. In addition, it is stated that the inertia of the additional roll (s) must be such as to introduce the required phase advance. The practical effect of this is to reduce the magnitude of the change in strip entry tension, b (entry tension), thereby reducing the

gain of the total feedback loop and increasing the stability of the mill, allowing higher rolling speeds to be achieved without vibration.

JP08-238511 describes a vibration control apparatus for use in a rolling mill comprising two spaced roll stands. In between the two roll stands the material being rolled is passed over a control roll which is provided with a tension governor to vary the moment of inertia of the roll.

The tension governor is realised by a weight which rotates with the roll and whose radius of rotation can be varied. A further roll detects the tension in the material being rolled and the result of this is used to vary the moment of inertia of the control roll.

JP61-18658 describes a vibration absorbing device for use in a strip processing line. The device comprises a vibration-absorbing roll which is resiliently pressed against the strip. Lateral vibration of the strip is inhibited by a controlled lateral movement of the roll.

International patent application WO 02/49782 describes a mill vibration control apparatus which acts on a vibration control roll positioned upstream or downstream of the mill stand, and which comprises an inertia element in the form of a flywheel which is connected to the roll by a stiffness element in the form of a compliant shaft. The components of the vibration control apparatus form a mechanical system which can be tuned in such a way as to introduce a speed fluctuation at the vibration control roll which matches that of the strip material as it enters the roll bite. In this condition, the velocity change of the vibration control roll due to mill vibration is in phase with and of equal magnitude to the velocity change of the strip at entry to the roll bite. Under these circumstances, the length of the strip between the roll bite and vibration control roll does not change and so the mill vibration does not produce any change in entry strip tension.

The result is that the feedback mechanism is destroyed and mill vibration is no longer self-exciting.

The present invention seeks to provide an apparatus and a method that reduces vibration in a mill, particularly a metal rolling mill, so as to

control gauge chatter and enable metal to be rolled at higher speeds without undesirable vibration building up.

Although described herein exclusively in relation to metal rolling, it is envisaged that the principles of the invention may be applied to other rolling processes that suffer vibration.

In a first aspect the present invention therefore provides mill apparatus for rolling strip material, said apparatus comprising at least one mill stand having a set of rolls between which the strip material is passed, and vibration control apparatus comprising a vibration control roll positioned so that the strip material passes over it at a position upstream or downstream of the set of rolls, said apparatus being characterised in that said vibration control apparatus further comprises means for varying the speed of said vibration control roll, means for monitoring one or more parameters of the operation of the mill stand, and feedback means for controlling said varying means in dependence upon the value of said parameter or parameters.

In a second aspect, the present invention provides a method of restricting vibration in a rolling mill comprising at least one mill stand having a set of rolls between which a strip material is passed, said method comprising passing the strip material over a vibration control roll positioned upstream or downstream of the set of rolls, monitoring one or more parameters of the operation of the mill stand, and controlling the speed of rotation of said vibration control roll in dependence upon the value of said parameter or parameters.

The vibration control roll may be positioned upstream or downstream of the set of rolls. In the preferred embodiment it is mounted upstream and will thus be referred to hereinafter as an entry roll.

During normal operation of the mill, the entry roll is driven to rotate by the passage over it of the strip material. The means for varying the speed of the roll may thus be such as to tend to reduce the speed of rotation, for example a brake, or such as to increase or reduce the speed of rotation, for example an electromagnetic device such as an electric motor.

In all cases the speed varying means is controlled by a control signal which is generated by a control circuit which receives one or more input signals containing information as to the operational parameters of the mill stand.

The input signal or signals are themselves derived from one or more sensors which measure the operational parameters of the mill stand and, in particular, its vibration. This sensor or sensors comprise said monitoring means. Examples of parameters which could be used are entry or exit strip tension, mill stand load, mill stand hydraulic pressure, exit gauge of strip material, speed of the strip material at the entry to or exit from the roll bite, chock motion for example using an accelerometer or proximity probe, or roll motion for example using a proximity or non-contact laser vibrometer. Any one or more of these parameters can be used to create a suitable control signal, depending upon the circumstances.

It has already been mentioned that the movement of the strip material over the entry roll causes the roll to rotate. The vibration feedback mechanism results in small cyclic changes in the entry speed, 8 (entry speed), of the strip material entering the set of rolls. These small changes of speed are transmitted back through the strip material entering the mill and set up corresponding variations in the speed of rotation of the entry roll. It is advantageous if the entry roll is able to follow these variations reasonably accurately and, for this purpose, it is desirable that the strip material is firmly engaged with the entry roll so that changes of speed are accurately transmitted through to the entry roll. Various means can be used to reduce slippage to a minimum. For example the arc of contact (wrap angle) between the strip material and the roll can be increased, and/or the surface of the roll can be roughened and/or a pinch roll could be used to press the strip more firmly against the entry roll.

Keeping the moment of inertia of the entry roll as low as possible also helps the entry roll to follow rapid variations in strip speed.

The vibration control apparatus varies the entry roll speed and hence the speed of the strip material as it passes over the entry roll. By this means, it is possible to remove, or at least considerably reduce, the

mill vibration. This is achieved by monitoring the vibration conditions at the mill stand, as represented by one or more of the parameters listed above, and providing a suitable feedback control signal to the speed varying means to correct the vibration. By optimising the feedback it is possible to achieve the stability condition in which variations in the velocity of the strip material at the roll bite are in phase with and of equal amplitude to corresponding variations in the strip material as it passes over the entry roll. Vibration control is thus achieved by means of control of the rotation of the entry roll ; the entry roll does not execute any translational movement in a direction lateral to the strip and, indeed, the entry roll may be mounted for rotation about a fixed axis.

One advantage of the present invention is that it will often be possible to retro-fit the vibration control apparatus to an existing mill. This is because many mills already have one or more entry or exit rolls positioned upstream or downstream of the set of rolls. These entry rolls are used for various purposes, the most common being to guide the strip material to the roll bite. Thus an existing entry or exit roll can be adapted as described above by the addition of a speed varying means. If, as is often the case, there are multiple entry/exit rolls, it is preferred that the adaption be carried out on the entry/exit roll closest to the roll bite, since it is desirable that the strip material has a free run between the modified entry/exit roll and the roll bite. However, if the vibration control elements are fitted to an entry/exit roll that is not the nearest to the bite roll, they should still work but perhaps not in so effective a manner, and the measured parameters may have to be adapted to the different circumstances. If the entry/exit roll, and its associated vibration control elements, are to be fitted as a unit to an existing machine (i. e. without using an existing entry/exit roll), it will be understood from the above that it is desirable that the new entry or exit roll be fitted such that it is the closest roll to the roll bite.

It has already been mentioned that it is advantageous for the entry roll used for vibration control to have a reasonably low moment of inertia so

that it can follow the variations in the velocity of the strip material as vibration occurs. To this end, it is preferred that the roll is constructed so as to have a low moment of inertia and, in particular is as low in mass as reasonably possible. For example, as an alternative to steel (the normal material), the roll could be wholly or partly made from lightweight plastics material, or a light metal such as aluminium, or a lightweight reinforced material such as a reinforced resin, or carbon fibre. Clearly, considerations of strength would be important when using such materials, particularly considering the high forces involved in a metal rolling mill, and the generally unfavourable environment.

In order that the invention may be better understood, an embodiment thereof will now be described by way of example only and with reference to the accompanying drawings in which:- Figure 1 is a schematic diagram of two conventional mill stands illustrating the self-exciting feedback mechanism that arises with gauge chatter; Figures 2,3 and 4 are schematic perspective views, each illustrating a mill stand incorporating a respective embodiment of a vibration control apparatus according to the present invention; and Figures 5 and 6 are schematic views in end and side elevation respectively of a mill stand showing the location of various sensors suitable for monitoring the operational parameters of the mill stand.

Referring to Figure 2, it will be seen that a metallic strip material 2 is guided to the roll bite 3 between the work rolls 10 via an upstream entry roll 4. The entry roll bears against the moving strip material 2 and thus moves, or at least attempts to move, therewith.

Fitted to the entry roll 4 is a vibration control apparatus according to the present invention, this comprising a speed varying means in the form of an electric motor 6. The electric motor is mounted so as to be coaxial with the entry roll 4, and is mechanically connected to the entry roll by a shaft 7.

However, the motor 6 could be connected by belts or gearing to the entry roll 4, in which case the motor could be mounted at any suitable location.

The gearing could be 1: 1, or step up or step down. The motor is controlled by an electric control signal received from a control circuit 8 which in turn receives its input via input lines 9 from one or more sensors arranged to monitor the vibration of the mill stand. These sensors will be discussed in more detail below.

As will be clear, the passage over the entry roll 4 of the strip material 2 causes the entry roll to rotate; any variations in the speed of the strip material over the roll will cause corresponding variations in the rotational velocity of the roll. As has already been discussed in some detail, vibration in the mill stand can set up a vibration feedback loop which causes variations in the speed of the strip. As the gauge of the exit strip varies, so the variation of entry speed AV2 of strip material entering the work rolls 10 changes in sympathy in a cyclic manner. The change in speed is transmitted upstream to the entry roll 4 where a corresponding variation of velocity AVr occurs. This variation is in turn transmitted to the entry roll 4 itself due to the rolling contact between the strip material 2 and the entry roll 4. The lower the moment of inertia of the entry roll 4, the more perfectly will the variation of velocity of the strip material be transmitted to the entry roll 4.

The purpose of the motor 6 is to counteract these variations in the speed of the strip material entering the work rolls by controlling the rotational velocity of the entry roll by either increasing or reducing its rotational velocity. The action which the motor takes is itself controlled by the control signal which is in turn dependent upon the feedback information received from the vibration sensor or sensors. The rotational velocity of the entry roll is controlled in such a way as to achieve the stability condition in which the variation of velocity AV2 of the strip material 2 at the roll bite 3 moves in phase with and with equal amplitude to, the variation of velocity AV, of the strip material at the entry roll 4. Under this condition, no stress due to vibration is applied to the portion of the strip material extending from the entry roll 4 to the roll bite 3.

Figure 3 shows a second embodiment of the invention in which the

speed varying means takes the form of a mechanical brake comprising a brake disc 20 rotatably connected to the entry roll 4 by means of a connecting shaft 7. Although shown mounted coaxially with the entry roll, the brake disc could be mounted in some other orientation, and joined to the entry roll via a belt or geared drive.

Positioned against the brake disc is a brake pad 21 which is controlled by a control signal from the control circuit 8. The brake pad 21 is operable to bear against the perimeter of the brake disc to reduce its velocity of rotation, and hence also that of the entry roll. The pad 21 may also act on one or both sides of the drum, as is well known from vehicle technology. Likewise, a drum brake, operated under the control of the control circuit 8 could be used in place of the illustrated arrangement.

Figure 4 shows a third embodiment of the invention in which the speed varying means takes the form of a non-contact inductive force actuator 22. This is shown acting directly on the entry roll 4, but it could act on a separate member (not shown) mounted for rotation with the entry roll. The operation of the actuator 22 is controlled by a signal from the control circuit 8, as in previous embodiments. The actuator 22 is able to both accelerate and decelerate the entry roll by application of a suitable electromagnetic inductive force to the roll, somewhat in the manner of a linear motor.

The control signal for controlling the above described speed varying means is derived from one or more measured signals indicative of the vibration of the mill stand. The or each measured signal is taken from a respective sensor associated with the mill stand, and is input to the control circuit 8. Although it has been assumed herein that the feedback circuit is essentially electrical in operation, in fact all or part of it could be realised by hydraulic circuitry.

Figures 5 and 6 show two views of the mill stand showing the type and positioning of possible sensors for use in the feedback circuit. In Figure 5, the roll neck for each of the rolls is illustrated diagrammatically : for the work rolls 10, the roll neck at each end is illustrated by reference 30;

for the backing rolls 11, the roll neck is illustrated by reference 31. The bearings in which the backing rolls rotate are also illustrated under reference 32.

Examples of suitable sensors include : 1) non-contact proximity probes 33 or laser 34 which are positioned to detect vibrational movement of the rolls in a direction, indicated by the double-headed arrow, at right angles to the plane of the strip material.

2) contact motion detectors 35, for example an accelerometer, which are positioned to detect vibrational movement of the rolls in the direction of the arrow.

3) a load cell 36 and associated hydraulic pressure cylinder 37 which are positioned to detect vibrational movement of the rolls in the direction of the arrow.

Figure 6 shows examples of further suitable sensors as follows : 4) non-contact entry speed sensor 40, or exit speed sensor 41.

Examples of suitable sensors include lasers. These sensors detect the entry and exit speeds respectively of the strip material 2 as it passes through the mill stand.

5) an exit gauge sensor 42 for measuring the gauge of the strip material 2 at the exit to the mill stand.

6) Entry and exit tension sensors 43,44 associated with the entry and exit rolls 4,45 respectively. An example of a suitable sensor is a load cell. The sensors 43,44 measure the variation in tension in the strip 2 by measuring the change of the force, or at least a component thereof, which the strip applies to the respective entry or exit roll. The sensor does not itself change the roll speed, but will generate appropriate signals which can be fed back, through a control circuit as described above, to change the speed of the respective roll.

The signal or signals from the or each sensor are passed to the control circuit which essentially acts as an interface device to translate the measured signal or signals to a control signal which is appropriate for controlling the particular speed varying means in use in such a way as to

achieve the desired stability condition and thereby reduce or eliminate vibration in the mill stand.

One advantage of the invention described herein is that it can be readily applied to an existing mill without necessarily having to modify the position or moment of inertia of the entry roll or rolls. To this end, the vibration control apparatus can be added to an existing entry roll, preferably that closest to the roll bite, without the need for extensive modification. Alternatively, a new entry roll, together with the vibration control elements can be fitted to a new or existing mill stand. With a new installation, attention can be paid to the entry roll to keep its moment of inertia as low as practicable. Also, the new entry roll can be positioned such that, between itself and the roll bite, the strip material is substantially free of any components such as deflector rolls, bridle rolls, or colour rolls which contact the strip material. This ensures that the full effect of the vibration control apparatus is felt.