WO 00/78476 _1 - PCT/IBOO/00788 "METHOD TO CONTROL THE VIBRATIONS IN A ROLLING STAND AND RELATIVE DEVICE" * * * * * FIELD OF THE INVENTION 5 This invention concerns a method to control the vibrations in a rolling stand, and the device which achieves the method, as set forth in the respective main claims. The invention is applied in rolling processes for plane products such as strip, sheet and large plate in which 10 rolling stands are used and in which the working rolls receive motion by means of transmission elements connected to the motor mean by a kinematic chain. BACKGROUND OF THE INVENTION In rolling trains for plane products the state of the art 15 covers the use of rolling stands arranged in sequence which achieve the progressive reduction in thickness of the product passing through. In the roughing and pre-finishing passes, every rolling stand normally reduces the thickness by a value of between 20 30% and 60% with respect to the measurement at inlet; the limit of reduction is defined by the maximum value of the angle at which the rolled stock enters the pass, the maximum rolling torque applicable and the maximum rolling force. The final thickness of the rolled stock is defined either 25 in a reversible finishing rolling mill for sheet or strip (for example of the steckel type), or in a finishing train with tandem stands, in which the reduction percentages in the individual stands can be between 65% and 10% with respect to the measurement at inlet. 30 In four-high rolling stands there are working rolls which act on the product to be rolled, and back-up rolls, with a greater diameter and co-operating with a relative working roll, the function of which is to support the rolling loads, WO 00/78476 - 2 _ PCT/IBOO/00788 preventing bending and deformations of the relative working rolls. Motion is supplied to the working rolls of every rolling stand, particularly in the finishing stands but also in the 5 roughing mills, by transmission elements, known as spindles, which in trains for hot rolled strip are moved by a single drive mean through reduction units and motion-splitting units, or by two independent motors, one for each roll. In modern hot-rolling mills, where there is a tendency to 10 push the reduction in thickness for every rolling pass to high values, the phenomenon of vibrations in the stand is often encountered during the passage of the rolled stock; these vibrations negatively influence the quality of the final product obtained. 15 This is a particular problem in the first stands of a hot rolling train for strip, particularly in cases where the product has a high thickness value at inlet and a low thickness value at outlet. It has been found that there are ranges of values of the 20 main working parameters (rolling speed, rolling force, torque supplied by the motor, ratio between the length of contact of the rolls and strip, and the average thickness in the clamping zone) which facilitate the onset of strong vibrations in the stand. 25 Such vibrations are prevalently of the torsional type and are caused by the fact that the main kinematic chain which transmits motion from the motor to the rolls oscillates torsionally at a frequency which is generally near its own. The actual frequency of vibration of the stand is a 30 function of the characteristics of the material being rolled in the segment clamped between the two working rolls. The mechanical characteristics of the material being worked, in transit in the rolling gap, therefore generate a WO 00/78476 PCT/IBOO/00788 change in the frequency of oscillation with respect to the theoretical frequency of the kinematic chain; this change is neither constant nor predictable. The vibrations are started more easily if the kinematic 5 chain does not transmit the speed uniformly, for example due to pitch errors in the gears (reduction gear and splitter gear) , or defects in the spindle joints, errors in the alignment of the working rolls and other defects of a mechanical origin. Defects deriving from the grinding 10 operations of the working rolls also lead to instantaneous variations in speed. When vibrations of a torsional type begin, the two working rolls, with the relative spindles, vibrate reciprocally in counter-phase, causing a vibration to start between the 15 rolls and the box of the pinions connected to the spindles. This vibration, in the case of rolls fed by a single motor with a motion-splitting system, substantially closes in a ring on the last part of the kinematic chain, without involving, or at least only involving to a minimum degree, 20 the part upstream comprising the motor and the reduction unit. To solve the problem of vibrations in the stand, various solutions have been proposed which have however proved unsatisfactory. 25 A first proposal was to increase the damping of the mechanical/electrical system by acting on the kinematic chain and/or directly on the control of the motor. To this end, the state of the art includes the use of damping joints or control systems applied to the motor which, as the speed 30 of the motor and/or the torque transmitted are detected from moment to moment, introduce a damping action by acting in counter-phase on the said motor. One example of this solution is described in JP 54-044714, WO 00/78476 PCT/IB00/00788 -4 which provides a system to control the torque output from the motor and a feedback ring to correct any possible discrepancies with respect to the pre-set torque values. This solution, however, is not satisfactory because, when 5 the vibrations begin, they are kept "alive" by the dynamic variation in the coefficient of friction between the working rolls and the product being rolled, and therefore the process becomes unstable. In such critical conditions for the generation of 10 vibrations, a reduction in the rolling torque corresponds to an increase in the speed of the working rolls, and therefore the counter-phase control of the motor is not able alone to cancel the vibrations in the stand. As already explained, the vibrations occur almost 15 completely in the circle of the pinion box/working rolls, while they are only transmitted to a minimum extent onto the upstream organs, such as the motor. For this reason, in the classic configuration of the kinematic chain, the damping systems of an electric 20 electronic type which are based exclusively on the control of the working parameters of the motor are not very efficacious. In fact, in such systems the motor is controlled by detecting the currents which go through the coils, the speed and the angular position of the motor 25 itself, but these sizes are influenced to a minimum extent by the vibrations because the vibrations tend to be restricted substantially downstream of the motor. On the contrary, in the case where every roll is connected to its own kinematic chain equipped with a respective motor, 30 the vibrations propagate along the respective chains and are detected on the motor too; in this case it may therefore be possible-, by detecting the torque output from every motor, to intervene individually and independently, with a feedback WO 00/78476 PCT/IBOO/00788 -5 ring, on every kinematic chain to stop such vibrations from starting. This teaching is described, for example, in the article "Compensation of a digitally controlled static power 5 converter ... " by David H. E. Butler et al., IEEE, vol. Meeting 25, 1990, pages 583-588, and in US-A-5,263,113, in the name of some of the same people who wrote the article. These documents do in fact teach to correct the outlet speed of a motor which drives a load, in the event that 10 there are discrepancies between the set speed value and the speed value which was actually detected, due to vibrations detected on the shaft which connects the motor to the load, in a single-line design. However, this teaching cannot be applied in the case 15 where, by means of a motion-splitting system, a single motor drives two loads vibrating in counter-phase, as happens in the case of a pair of rolling rolls. In this case, as we have said, the vibrations of the rolls, in counter-phase and of substantially the same 20 entity, close in a ring between the rolls and the motion splitting system, and therefore they are not "seen" and cannot be detected on the motors, which are therefore insensible to and not affected by the vibrations. There has also been a proposal to introduce mechanical 25 dampers applied directly on the transmission organs connected to the rolls, but this solution has also proved impracticable especially because of the size of the dampers, which are extremely cumbersome if compared to the size of the spindles. Using such dampers in another position on the 30 kinematic chain, outside the ring inside which the vibrations are enclosed, does not allow to obtain good results for the reasons explained above. A further problem is that in current rolling mills AC WO 00/78476 - 6 - PCT/IBOO/00788 motors are used, normally controlled by cycle-converters or other systems to reconstruct the controlled frequency sinusoidal waves. Compared with traditional DC current motors, the 5 vibrations in these motors are reduced by a very limited amount, if at all. JP 60-148616 describes a control method which does not refer to the cancellation of torsional vibrations in a rolling stand, but to controlling the elastic energy 10 accumulated on the spindles and on the transmission chain when a rolling mill is braked. In this case, after the mill has stopped, the rolls tend to rotate in the opposite direction due to the release of the elastic energy accumulated on the kinematic chain. 15 The teaching of JP 60-148616 proposes precisely to create a controlled braking, suitable to prevent the rolls from moving in the opposite direction when they reach zero speed. The present Applicant has devised and embodied this invention to overcome the shortcomings of the state of the 20 art, which businessmen in this field have long complained of, and to obtain further advantages as will be explained hereafter. SUMMARY OF THE INVENTION The invention is set forth and characterised in the 25 respective main claims, while the dependent claims describe other characteristics of the main embodiment. The purpose of the invention is to achieve a method to control the vibrations in a rolling stand which will reduce to a minimum and cancel the vibrations of the stand during 30 the rolling passes. The invention is applied on rolling stands where the rolls are commanded by a single motor by means of at least a motion-splitting system.
WO 00/78476 7 PCT/IBOO/00788 The invention is based on the fact that for the most part the vibrations in the stand start and finish in the ring which comprises the splitter pinions, the spindles and the rolling rolls, and provides: 5 - to detect the working conditions, such as in particular the torque and/or speed of rotation and/or angular position, of at least some of the elements which constitute the kinematic ring, that is to say, the working rolls, spindles, and splitter pinions; 10 - to send the signals detected in feedback to a unit which controls the motor drive; - to process the signals in order to verify the existence of conditions which generate the vibrations in the stand; - and then to supply the motor with a damper torque which, 15 applied to the kinematic chain, and particularly to the part directly connected to the working rolls, contributes to damping said vibrations. In other words, the invention provides to detect the existence of vibrations in the points of the kinematic chain 20 where the vibrations are actually evident and can be detected, that is to say, in the ring which closes between the splitter system and the rolls. Upstream of the ring, and particularly on the motor, the vibrations are minimal due to the fact that the rolls and 25 the spindles vibrate in counter-phase and with substantially identical values, and therefore the variations of torque caused by vibrations on the motor and on the possible reduction gear are compensated and therefore practically zero. 30 In one embodiment, the invention provides to detect the torque transmitted to at least one of the spindles connected to the working rolls, advantageously to both, and/or the speed of at least one of the pinions which supply motion to WO 00/78476 PCT/IBOO/00788 the spindles. According to a variant, instead of measuring the torque of the spindles, the actual speed of rotation of the spindles is detected. 5 According to a further variant, the actual speed of rotation of the working rolls and/or the torque transmitted to the working rolls is detected, or also detected. The signals are then sent to the motor control unit and processed to generate a damper torque of a frequency 10 correlated to the frequency of the vibrations. When the damper torque, which is added to the torque supplied by the motor under normal working conditions, is delivered to the components of the kinematic chain which are most subject to vibrations (pinions, spindles, rolls), it 15 supplies a damper contribution which tends to attenuate the oscillations and to eliminate them. BRIEF DESCRIPTION OF THE DRAWINGS These and other characteristics of the invention will be made clear by the description of the preferential embodiment 20 of the invention, with reference to the attached drawings wherein: Fig. 1 is a schematic view given as an example of a rolling train in which the invention is adopted; Fig. 2 is the diagram showing the transmission of motion to 25 the working rolls of a four-high rolling stand of the type with a single motor; Fig. 3 is a block diagram of how the invention functions. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT The rolling train 10 shown partly and schematically in 30 Fig. 1 comprises four rolling stands 11, in this case four high stands lla, 11b, 11c and 11d, arranged in sequence to perform progressive reductions in the thickness of a strip or plate 12 passing through.
WO 00/78476 - 9 - PCT/IB00/00788 The invention is applied in the same way to roughing trains with 1 or 2 reversible or non-reversible stands, to pre-finishing trains with 1 or 2 non-reversible stands, to finishing trains with any number of stands from 3 to 8, to 5 reversible finishing trains of the single type, or tandems of the type known as steckel mill, included in any rolling line for plane products. The invention is also applied in the same way to two-high stands comprising only the working rolls, or to six-high 10 stands comprising working rolls, back-up rolls and intermediate rolls, or also to stands with a more complex structure. Each stand 11 comprises, in this case, a pair of working rolls 13 and a mating pair of back-up rolls 14. 15 Motion is supplied to the working rolls 13, in this case (Fig. 2), by a single motor mean 26 which, by means of a reduction unit 15, transmits motion to a pair of splitter pinions 20 for each of the working rolls 13. The reduction unit 15 may be replaced by a multiplier unit, or there may 20 be a direct motor-splitter connection without a reduction unit. Respective spindles 16, directly associated with the rotation shafts 17 of the relative rolls 13, are connected with the outlet pinion 20. 25 According to the invention, in a preferential embodiment, associated with the spindles 16 there are sensors 18 suitable to measure the actual torque delivered by the motor 26 to the shafts 17 of the rolls 13. The sensors 18 may be of the electric type (strain gauges), of the optical type, 30 for example laser, or of any other conventional type. According to a variant, the sensors 18 are speed sensors and are suitable to measure the momentary speed at which the spindles 16 rotate.
WO 00/78476 PCT/IBO0/00788 - 10 Moreover, in another preferential embodiment, associated with the pinions 20 there are sensors 21 suitable to measure the momentary speed of rotation thereof. According to a variant, the sensors 21 are suitable to 5 measure, or also measure, the angular position of the relative pinions 20. According to a further variant, there are sensors 29 associated with the shafts 17 of the working rolls 13 and suitable to measure the momentary speed and/or angular 10 position and/or the torque transmitted directly to the working rolls 13. The signals detected by one and/or the other of the sensors 18, 21 and 29 along the kinematic chain in the zone which is most subject to vibrations during the rolling 15 passes are sent in feedback to a control unit 19 which processes them according to appropriate algorithms, possibly comparing them with pre-memorised tabular reference values. The signals detected by the sensors 18, 21 or 29 allow to detect the torsional vibrations which start in the closed 20 ring which comprises the lower pinion 20, the lower spindle 16, the lower roll 13, the material being rolled 12, the upper roll 13, the upper spindle 16 and the upper pinion 20. In this way, the vibrations are detected where they are evident and can be measured, and not outside the ring, where 25 they are substantially cancelled, or are substantially reduced, due to the fact that the rolls and the relative spindles vibrate in counter phase to each other. Based on this processing, the control unit 19 generates a correction signal indicated in the Figures by the reference 30 number 22, which is sent to the control system of the motor 26, and interacts with the electrical sizes fed to the motor 26 by the relative speed transducer device 25. In Fig. 3 the damper control device is indicated in its WO 00/78476 PCT/IBOO/00788 - 11 entirety by the reference number 23, and comprises the afore-cited control unit 19, the sensors 18 associated with the spindles 16, the sensors 21 associated with the pinions 20 and the sensors 29 associated with the shafts 17 of the 5 rolls 13. The electric signal 22 generated by the control unit 19 according to the signals of torque and/or speed and/or position detected by the respective sensors 18, 21 and 29 is added to the electric signal supplied to the motor 26 by the 10 speed regulator 24. The speed regulator 24 controls the drive of the motor 26 in a conventional manner, with a feedback ring, according to the data detected by the relative speed transducer 25. Fig. 3 also shows the current control ring, indicated in 15 its entirety by the reference number 27, and the torque control circuit, indicated by the reference number 28. Apart from the signals arriving from the sensors 18, 21 and 29, the signals relating to the speed of the motor 26 and to the current supplied to the motor 26 detected by the 20 feedback ring controlled by the regulator 24 are also sent to the control unit 19. The invention therefore allows to correct the vibrations and the oscillations which start in a rolling stand 11, analysing the working conditions, in terms of torque and/or 25 speed, of the motion-transmission elements nearest the rolls 13 and hence more subjected to and influenced by said vibrations. The feedback control occurs by introducing a correction, in terms of amplitude and/or phase, to the electrical sizes, 30 particularly the current, feeding the motor 26; the correction is continued until the values detected by the sensors 18 and 21 show that the vibrations have been minimised or eliminated.
WO 00/78476 PCT/IBOO/00788 - 12 The damper intervention is purely electrical, it does not involve mechanical dampers which may have negative effects on the correct transmission of motion and consequently on the efficient functioning of the working rolls 13.