US3600920A - Screwdown offset system and method for improved gauge control - Google Patents

Screwdown offset system and method for improved gauge control Download PDF

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Publication number
US3600920A
US3600920A US872461A US3600920DA US3600920A US 3600920 A US3600920 A US 3600920A US 872461 A US872461 A US 872461A US 3600920D A US3600920D A US 3600920DA US 3600920 A US3600920 A US 3600920A
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rolling
gauge
rolling stand
stand
screwdown
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Andrew W Smith Jr
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Westinghouse Electric Corp
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Westinghouse Electric Corp
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B37/00—Control devices or methods specially adapted for metal-rolling mills or the work produced thereby
    • B21B37/16—Control of thickness, width, diameter or other transverse dimensions

Definitions

  • a screwdown recalibration system is provided for use in a computer-controlled rolling mill.
  • the computer uses the difference between gauge directly calculated by mass flow concepts and gauge calculated from a measured screwdown unloaded roll opening and roll force at each stand to recalibrate the screwdowns for that stand.
  • the system is adaptable to either providing infonnation to an automatic roll force gauge control system according to a predetermined schedule or to function as an online recalibration scheme which is an integral part of a more comprehensive r011 force gauge control system.
  • the present invention relates to metal-rolling mills and more particularly to calibration of the rolling mill in cooperation with roll force gauge control systems and methods which are used to operate such mills.
  • both the unloaded roll opening and the speed for each tandem mill stand or for each reversing mill pass are set up either by an operator or by a computer to produce successive workpiece (strip or plate) reductions resulting in an on-gauge finished work product. It may be assumed that the loaded roll opening at a stand equals the stand delivery gauge since there is little or no elastic workpiece recovery.
  • a stand gauge control system must be em ployed to closely control the stand delivery gauge.
  • a stand gauge control system is normally used for a reversing mill stand and for predetermined stands in tandem-rolling mills.
  • the roll force gauge control system uses Hooke's law in controlling the screwdown position at a rolling stand, i.e., the loaded roll opening under workpiece rolling conditions equals the unloaded roll opening (screwdown position) plus the mill spring stretch caused by separating force applied to the rolls by the workpiece.
  • a load cell or other force detector measures the roll separating force.
  • the screwdown position is then controlled to balance roll force changes from a reference or set point value and thereby hold the loaded roll opening at a substantially constant value.
  • the following formula expresses the basic relationship:
  • the roll force gauge control system is an analog configuration including analog comparison and amplification circuitry which responds to roll force and screw position signals to control the screwdown position and hold the following equality:
  • AF measured change in roll force from a force reference AS controlled change in screwdown position from a screwdown reference.
  • screwdown offset it is herein meant to refer to the change in screwdown position made to correct a gauge error which is caused or expected to be caused by a single mill variable or by a combination of mill variables, and which is uncorrectable or inadequately correctable by internal roll force gauge control operations alone.
  • steady state gauge error it is meant to refer to an error which is correctable by screwdown offset.
  • a second roll force control system uses an absolute roll force setup reference wherein the stand is roll force controlled to operate from the setup roll force.
  • cor rect steady state gauge may be achieved since the initial screwdown position is changed to correct for any initial roll force error.
  • use of an absolute roll force setup reference may result in erroneous steady state stand delivery gauge which requires screwdown offset, particularly if the initial screwdown position calibration has drifted or if the mill spring constant has a value different from the value assumed in the setup calculation at the gauge being rolled. Since screwdown calibration drift and/or a changed mill spring constant will directly affect steady state gauge, a screwdown offset may be required if a change occurs in either or both of these variables during workpiece rolling following a correct setup or to provide a better setup from an incorrect setup.
  • the initial screwdown position calibration is a direct electromechanical measurement which traditionally has only been made at the beginning of the work roll life and, if desired, new initial" calibrations are made at various subsequent time points in the work roll life.
  • One means of recalibration of the work rolls was accomplished by driving the rolls together until the load cell detected a force which was thought to be on the linear portion of the mill spring curve. Since the slope of the mill spring curve equals the negative of the fraction of force divided by distance, it was possible to determine a new recalibration or "zero" point by withdrawing the screws a distance defined by the fraction of force divided by the slope of the mill spring curve. Unfortunately, each recalibration required a time interval which delayed the rolling of the next workpiece and valuable production time was lost. Moreover, no allowance was made for the change in calibration which may occur during the rolling of a workpiece or for that matter between scheduled recalibrations. Any such change would necessarily require a screwdown offset for correction of the roll force gauge control operation.
  • the actual loaded roll opening i.e., the actual gauge differs from the expected value calculated by an amount equal to the erroneous calibration.
  • This difference represents a gauge error condition which is correctable by a screwdown offset or more specifically, a screwdown recalibration.
  • the actual loaded roll opening differs from the expected loaded roll opening based on mill stretch which is a function of the erroneous mill spring constant. This gauge error condition is similarly correctable by the determination of a screwdown offset.
  • steady state gauge In the case of mill acceleration or deceleration, steady state gauge apparently changes or tends to change as a result of transient changes in stand entry and exit workpiece tension values which are reflected in rapid roll force changes that cannot be tracked by the roll force control. Moreover, it is possible that rate of workpiece speed change could cause transient calibration changes or transient mill spring constant changes or otherwise have an effect on gauge independent of roll force. In any event, screwdown offset is required for steady state correction of gauge error effects produced by a sustained workpiece speed change rate.
  • the well known monitor gauge control system is usually employed to product screwdown offset for the roll force controls.
  • an X-ray or other radiation gauge is placed at one or more predetermined process points and usually at a single process point following the delivery end of the mill in order to sense actual delivery gauge after a workpiece transport delay from the point in time at which the actual delivery gauge is produced at the preceding stand or stands.
  • the monitor system compares the actual delivery gauge with the desired delivery gauge and develops an analog feedback control signal to adjust the operation of the reversing mill stand roll force gauge control system or one or more predetermined tandem mill roll force gauge control systems to supply desired steady state mill delivery gauge.
  • the conventional monitor system provides for correction of steady state gauge errors which are caused or which are tending to be caused by a single mill variable or by a combination of mill variables.
  • the mill spring curve has typically and usually justifiably been treated as having a constant and uniform linear slope and is more commonly referred to as the mill spring line. Actually, at lower force levels which typically have been encountered infrequently in mill use, the spring curve is nonlinear and the spring constant accordingly varies over the nonlinear portion of the curve.
  • the spring constant corresponding to the slope of the linear part of the spring curve is subject to change during mill operation.
  • the slope of the linear portion of the spring curve can be changed (perhaps as much as +l0 percent) both by individual changes or combinations or changes in certain mill parameters including the backup roll diameter and the workpiece width and by a steady state gauge error.
  • any change in the slope of the mill spring curve and resultant steady state gauge error can be resolved by a recalibration system which determines gauge as a function of unloaded roll opening and mill stretch. That is, by moving the unloaded roll opening and thereby changing the relative posi tion of an assumed mill spring curve for which the assumed slope is in error, a gauge can be determined under a particular roll force condition which would exactly match the gauge at another unloaded roll, opening for which a correctly assumed mill spring slope might otherwise be used.
  • a further object of the present invention is to provide a new and improved recalibration system for at least one rolling stand in a rolling mill operating under a roll force gauge control system wherein any sustained calibration error will be removed after the rolling of several workpieces.
  • Another object of the present invention is to provide a new and improved rolling mill operative under a roll force gauge control system wherein recalibration information is a result of rolling a workpiece as opposed to a separate duty cycle.
  • a further object of the present invention is to provide a new and improved rolling mill operative under a roll force gauge control system wherein the recalibration information from the rolling of one workpiece is used to provide an improved setup for the rolling of the next workpiece.
  • An additional object of the present invention is to provide a new and improved recalibration system for a rolling mill operative under a roll force gauge control system wherein the recalibration may be independent of any change in measured roll force.
  • a still further object of the present invention is to provide a new and improved recalibration system for a rolling mill operative under a roll force gauge control system which compensates for nonlinearity in the mill spring curve.
  • Still another object of this invention is to provide a new and improved recalibration system for a rolling mill operative under a roll force gauge control system wherein each stand of the rolling mill is individually recalibrated on a predetermined time cycle.
  • a further object of the present invention is to provide a new and improved recalibration system for a rolling mill which cooperates with a roll force gauge control system by providing an offset factor representing the online relative change in calibration over a predetermined time period.
  • the delivery gauge for the last stand is measured by an X-ray gauge and a comparison is made between this measured gauge and the gauge calculated using the screwdown unloaded roll opening and the measured roll force. Since the mass flow for the rolling mill is substantially constant, precise measurement of stand speed can be used to calculate the delivery gauge from any of the other stands by using the following equation:
  • FIG. I is a diagrammatic representation of a rolling mill showing the necessary inputs and outputs in a system for screwdown recalibration based on the principles of the present invention.
  • FIG. 2 is a diagrammatic representation of the logic flow required to provide for recalibration of the screwdowns.
  • FIG. 3 is a diagrammatic representation of an online recalibration system in cooperation with a roll force gauge control system
  • FIG. 4 is a graph of roll force versus workpiece thickness roll opening showing the mill spring curve and the metal plastic deformation curve for a particular rolling stand.
  • FIG. 1 a diagrammatic representation of a tandem rolling mill is shown in connection with the inputs and outputs relative to a preferred embodiment of the present invention and is designated by the numeral 10. It should be noted that the principles of this invention may be applied as well to a single stand reversing mill having comparable inputs and outputs.
  • a multistand rolling mill is shown comprised of a plurality of stands 5,, S S,,.
  • An unfinished workpiece I4 is fed into the first stand S of the mill and necessarily reduced in gauge stand by stand to a desired finished gauge final product which is then coiled by a coiler 16.
  • Each stand comprises a set of cooperating rolls R R R, respectively which effects a certain amount of thickness reduction in the workpiece I4 as it passes therebetween.
  • the rolls R of stand S are driven by a direct current motor M, which is connected to drive the rolls R through suitable mechanical coupling means.
  • the other stands 8,, S are similarly provided with individual drive motors M M, and mechanical couplings to operate the rolls R,, R, respectively.
  • Each of the drive motors are provided with an individual speed controller SC SC,, SC, which is responsive to a control signal from a computer control system I8 to be described later.
  • Mechanically connected to each of the drive motors M,, M,, M, is a respective tachometer T T T, which provides signals responsive to the speed of its respective drive motors to the computer control system I8.
  • load cell As the workpiece is reduced in thickness at a rolling stand, the force required to effect the reduction is transmitted through the work rolls and is measured by a load cell.
  • load cells are designated by LC LC LC, and provide respective input signals to the computer control system 18.
  • the delivery gauge from the last stand S, is measured by an X- ray gauge 20 which provides an input signal proportional thereto to the computer control system I8.
  • Recalibration of the rolling stand in accordance with principles of the present invention may occur either as required and determined by an operator or computer or as an integral part of an online computer gauge control system.
  • recalibration occurs at necessary and convenient times such as for the startup of a mill, following a wreck, or alter a roll change.
  • the most opportune time would be immediately after mill is full, i.e., metal is being rolled in all rolling stands.
  • recalibration is done on a continuing basis and provides a necessary correction or offset factor to a roll force gauge control system which may be utilized both during the rolling of a workpiece and for the setup for the rolling of a subsequent workpiece.
  • offset factor is herein defined as a correction factor which represent the amount of screwdown offset necessary because of a change in calibration.
  • the flow diagram as shown in FIG. 2 sets forth the basic principles of a programming system compatible with a com puter control system 18 and the rolling mill I0 of FIG. 1.
  • the programming system provides some degree of calibration control with the X-ray gauge 20 off, it is desirable for maximum effectiveness that both the X-ray gauge 20 be operating and that the workpiece 14 be present at all stands.
  • Generally calibration control is achieved by comparing the calculated exit gauge at each stand as determined from measured roll force and screwdown unloaded roll opening with the actual gauge at each stand and causing or permitting a recalibration of the screws based on all or a portion of this difference.
  • That desired gauge may be different from gauge as determined by X-ray is of no avail as much as this discrepancy is reconciled and the screwdowns adjust by an absolute roll force gauge control system.
  • the function of this invention to cause the actual and calculated gauge to coincide and the function of a roll force gauge control system and if necessary a monitor system to cause the actual, the calculated gauge and the desired gauges to coincide.
  • the combined result of both systems is thus to force the actual or Xray gauge to that of the desired gauge and thereby provide a more perfect and on-gauge end product.
  • Block 210 provides initialization of certain counters and accu mulators required in recalibration; the sum of the mass flow (EMF) is set to zero, the stand number counter (i) is set to one and as a second stand counter NP is set to zero. That portion of H6. 2 comprised of blocks 220, 230, 240, and 245 is used to accumulate mass flow and calculate a gauge based on roll force. Block 220 checks to see if stand i or initially if stand l is rolling metal. If the stand is rolling metal, the gauge is then calculated in block 230 according to the following formula:
  • AMF MFINP whereby the average is based on the number of stands rolling metal.
  • Hc, AMF/V,, where V, stand speed from tachometer T,,.
  • the stand speed as measured from a tachometer may not provide an accurate indication of the linear speed of the workpiece as it passes through that stand.
  • the stand speed V. may need to be modified by a forward slip factor to provide a proper measure of the true linear speed.
  • the calculated last stand exit gauge He is then in block 280 compared to the actual exit gauge H1, as measured by the X-ray gauge to determine if the measured gauge is within :5 percent of the actual gauge. If so, in block 290, the mass flow MF as calculated from the last stand is determined by:
  • the mass flow MF is then set equal to the average mass flow AMF as determined at block 250. It should be noted that if the X-ray gauge is not operating as determined at block 260, the mass flow MP is similarly set to the average mass flow AMF in block 295.
  • the stand counter i is incremented by one in block 380 and the offset computation cycle is then repeated for the next stand. It should be noted that when a stand is not rolling metal as determined by the block 310 or the measured gauge He, does not fall within tlO per cent of the actual gauge Hx no offset calculation is made and the next stand is interrogated via blocks 3'10, 380 and 310.
  • the screw position required for rolling accurate gauge is equal to a predetermined unloaded roll opening plus a predetermined mill stretch as previously indicated. Further, the offset factor is employed by the roll force gauge control 470 in reflecting the true value of the unloaded roll opening as screw position changes are made to compensate for roll force error.
  • the online roll force gauge control can itself be embodied in computer form or in some cases it may possibly be a more or less self-contained conventional analog system. In any case, online screwdown recalibration enables more accurate stand gauge to be achieved more accurately.
  • the mill spring curve ML and the plasticity curve are shown on a graph of roll force versus thickness to illustrate a condition wherein a recalibration is required.
  • the mill spring curve ML is shown for a particular stand in the mill. This line is linear and has a constant slope at higher forces and nonlinear with a varying slope at lower forces.
  • a plasticity curve P relative to the material hardness intersects the mill spring curve ML at a point T which, when projected to the X axis, represents the calculated exit stand gauge He.
  • the relative position of the mill spring curve ML on the X axis is determined by the sum of the unloaded screwdown opening S0 and any offset value US as a result of previous recalibrations. Because most metal rolling is done at higher force levels, the aforementioned sum is equal to the intersection of the slope of the linear portion of the mill spring curve ML with the X axis as shown in FIG. 4.
  • a recalibration or a change in offset is determined by the difference between the calculated gauge He and the actual gauge Hx. Any offset factor previously used is stored in memory of the computer control system 18 to FIG. I.
  • a measured force Fx is received as input from the load cell associated with the stand.
  • the intersection of this force P1: with the mill spring curve ML at point T then determines the calculated gauge Hc. Since the mill spring curve ML is a function of measured force Fx, by substituting the force measured E: into the equation for the mill spring curve the calculated or measured gauge is determined.
  • the equation for determining the calculated gauge He is given by the following formula:
  • the unloaded roll opening plus the previous offset (S,,+0S') is given by the intersection of the linear slope of the mill spring line ML with the X axis.
  • the product of the inverse of the slope of the mill spring curve at the point of intersection T and the measured force F, provide the additional data required to calculate a measured gauge H,. If the point of intersection T is on the nonlinear portion of the mill spring curve ML, a factor A! equal to the difference it intercepts with the X-axis of the slope K and the slope of the linear portion of the mill spring curve ML is subtracted from the previous sum to compensate for the nonlinearity. Suppose that the X-ray gauge determines that the actual gauge H, is heavier than calculated.
  • the term 08 then represents the screwdown offset necessary to bring the screws into calibration either on the next workpiece or on the same workpiece if an online roll force gauge control system is being used.
  • An apparatus for controlling a rolling mill having at least a first rolling stand followed by a second rolling stand and being operative to reduce the thickness of a workpiece, said rolling mill having a screwdown system for at least said first rolling stand, the combination of means responsive to the roll force of said first rolling stand and the unloaded roll opening of said first rolling stand for determining a calculated roll force delivery gauge for said first rolling stand,
  • the apparatus of claim 1 including computer means operative with said first rolling stand for providing mill spring curve information to be utilized in determining said calculated roll force delivery gauge for said first rolling stand.
  • 3. The apparatus of claim 2, with said screwdown system icing operative to adjust the roll opening of the rolling stand issociated with said screwdown system, and with said re- :alibration offset being determined after the rolling of a given vorkpiece to provide unproved setup conditions for the 'olling of a subsequent workpiece.
  • the apparatus of claim 8 including means for determin ing the average mass flow delivery gauge for at least one of said stands in relation to the number of said rolling stands that are operative to reduce the thickness of said workpiece.
  • a screwdown recalibration system comprising the combination of:
  • said determining and effecting means include computer means operative to determine the calculated stand gauge from a stored mill spring curve and from the aforementioned roll force and position output signals,
  • said computer means includes a programming system which provides for using the initial offset factor in its entirety upon a change in mill setup to provide a screwdown offset and thereafter weighing further offset factors on the same setup according to a predetermined ratio with the previous offset factor to provide an updated screwdown offset.
  • the method ofclaim 13 including the step of establishing the roll opening of at least said one earlier rolling stand in accordance with said offset factor for that same rolling stand, with said offset factor being utilized in its entirety upon a change in the setup of that same rolling stand and thereafter, weighing further established offset factors for that same rolling stand in accordance with a predetermined relationship with the previous offset factor to provide an updated offset factor.

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US872461A 1967-10-23 1969-11-24 Screwdown offset system and method for improved gauge control Expired - Lifetime US3600920A (en)

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Cited By (4)

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Publication number Priority date Publication date Assignee Title
US3851509A (en) * 1972-11-06 1974-12-03 Westinghouse Electric Corp Rolling mill gauge control method and apparatus including speed correction
US3892112A (en) * 1974-03-27 1975-07-01 Westinghouse Electric Corp Rolling mill gauge control
US4240147A (en) * 1976-03-26 1980-12-16 Hitachi, Ltd. Gauge control method and system for rolling mill
US4292825A (en) * 1979-02-23 1981-10-06 Hitachi, Ltd. Gauge and tension control system for tandem rolling mill

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3568637A (en) * 1968-05-13 1971-03-09 Westinghouse Electric Corp Tandem mill force feed forward adaptive system
BE793757A (fr) * 1972-01-06 1973-07-09 Westinghouse Electric Corp Procede et appareil de commande de calibre comprenant la determination de la plasricite de la piece laminee, pour laminoir de metaux

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US3328987A (en) * 1964-05-14 1967-07-04 Crucible Steel Co America Gage-control apparatus
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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3851509A (en) * 1972-11-06 1974-12-03 Westinghouse Electric Corp Rolling mill gauge control method and apparatus including speed correction
US3892112A (en) * 1974-03-27 1975-07-01 Westinghouse Electric Corp Rolling mill gauge control
US4240147A (en) * 1976-03-26 1980-12-16 Hitachi, Ltd. Gauge control method and system for rolling mill
US4292825A (en) * 1979-02-23 1981-10-06 Hitachi, Ltd. Gauge and tension control system for tandem rolling mill

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