US3869891A

US3869891A - Speed optimizing system for a rolling mill

Info

Publication number: US3869891A
Application number: US458840A
Authority: US
Inventors: John D Higham
Original assignee: Measurex Corp
Current assignee: Honeywell Measurex Corp
Priority date: 1974-04-08
Filing date: 1974-04-08
Publication date: 1975-03-11
Anticipated expiration: 1992-03-11

Abstract

Rolling mill control system for optimizing operating speed and thereby maximizing output while maintaining gauge. This is accomplished by making adjustments to variables other than speed on a mill such as payoff tension, interstand tension or rewind tension, hydraulic roll pressure, screwdown position, etc., in such a way that they will tend to make the gauge go heavy. Simultaneously, a compensatory increase in speed is made to maintain the gauge at target thickness.

Description

1451 Mar. 11, 1.975

[ SPEED OPTIMIZING SYSTEM FOR A ROLLING MILL Primary Examiner-Milton S. Mehr Attorney, Agent, or Firm-Owen, Wickersham &

[75] Inventor: John D. Higham, Menlo Park, Calif. Erickson [73] Assignee: Measurex Corporation, Cupertino,

57 ABSTRACT [22] Filed: P 1974 Rolling mill control system for optimizing operating [21] APPL 458,340 speed and thereby maximizing output while maintaining gauge. This is accomplished by making adjustments to variables other than speed on a mill such as U-S- 1 t payoff tension interstand tension or rewind tensinn hydraulic roll pressure screwdown position etc in Fleld of Search 1 such a way that tend to make thg gauge g heavy. Simultaneously, a compensatory increase in References cued speed is made to maintain the gauge at target thick- UNITED STATES PATENTS ness.

1096,671 7/1963 Vossberg 72/11 X 3,194,036 7/1965 Canfor et al 72/11 16 Draw'ng F'gures MILL ROLL FORCE OR PRESSURE 30 CONTROL 14 34 1 I 1 TH1cKNEss GAUGE I8 U x PAYOFF MILL TENSION /-5O SPEED ,42

CONTROL 38 CONTROL I 54 PAYOFF PAYOFF MILL MILL MEASURED TENS'ON 4s TENSION SPEED SPEED GAUGE ACTUATOR ACTUATOR COMPUTER AND INTERFACE ELECTRONICS SPEED OPTIMIZING SYSTEM FOR A ROLLING MILL BACKGROUND OF THE INVENTION This invention relates to control systems for rolling mills, and more particularly to a system for enabling a mill to operate at maximum speed while producing an output of strip metal at a desired thickness.

In the rolling of metal a number of controllable factors affect the final gauge of the metal. Amongst these are the roll or screwdown force, the mill speed, the entry tension, exit tension and, on a multistand mill, the interstand tensions.

On a mill stand, with a given roll force, given entry and exit tensions, there will only be one speed at which the mill will run to give a desired exit gauge given that the input gauge is constant. The scheduling" of the mill is arranged so that with typical metal hardness that speed will be an acceptable speed when the operator sets his tensions and roll forces at normal" working values.

In many cases, for example, if the metal is hard or the reduction is a little greater than normal, the normal operator settings of tension and pressure result in a nonoptimum speed. The present invention solves this problem with a system utilizing a digital computer, for adjusting tensions, pressures and speeds in order to maximize mill speed while maintaining the desired finish gauge.

SUMMARY OF THE INVENTION It is therefore a primary object of the present invention to provide a control system for a rolling mill that will enable the mill to operate at maximum speed while maintaining the desired gauge of the output strip.

Another object of the invention is to provide a speed optimization system for a rolling mill that utilizes a commercially available digital computer and peripheral electronic support equipment and provides the necessary input data to the computer without requiring extensive or complicated modification of the mill components.

Other objects of my invention are to provide a speed optimization system for a rolling mill that is easy to operate and maintain without highly skilled personnel and which will perform within narrow specified accuracy limits for long periods.

In general, the system operates by making adjustments to other variables on a rolling mill such as payoff tension, interstand tension or rewind tension, hydraulic or screwdown roll pressure and roll gap spacing in such a way that they will tend to make the gauge go heavy, allowing a compensatory increase in speed to maintain gauge constant. On the mill the strip metal stored on a payoff reel passes through the pressure rolls, through an output gauge that measures the thickness of the rolled metal and onto a takeup reel. A speed control means, a means for controlling payoff tension, and a means for controlling roll pressure on the strip are provided as part of the mill in the conventional manner. The speed, tension and pressure means are controlled by signals from the computer.

When the mill is first placed in operation, after threading of the strip material through it, the system is placed in its automatic control mode. The speed is gradually increased at a constant rate, heading for a predetermined initial run speed chosen for the particular alloy and thickness and stored in the computer. If, during this process, the gauge as measured by the output gauge, should go light by more than a predetermined amount set in the computer, the speed stops increasing. When the gauge comes back in range, the speed increase continues until the predetermined speed has been reached. Once this occurs, the computer will attempt to maximize the speed of the mill within constraints. In one embodiment of the invention, the computer samples the existing tension value ofthe strip and if this lies above the tension maximize value, a signal will be provided to increase speed while at the same time reducing tension in order to achieve constant gauge. Since increasing tension and increasing speed both reduce gauge, one variable can be traded for another.

When the tension goes below the tension maximize value speed maximization will stop and the speed will be held steady. If the tension should subsequently go above the tension maximize value then the speed will continue to be maximized. If during the running of the coil the output gauge drops below a lower limit, speed maximization will stop until the gauge is back in range. If the tension at this time is above the tension maximize value, speed maximization will continue. The speed is limited by a maximum speed set in the computer, and when this maximum speed is reached and even though the tension may still be above the tension maximize value, no further speed maximization occurs.

If the tension should drop below the lower operating tension value the speed is reduced until the tension is above the lower operating tension value. As long as tension lies between the tension maximize value and the lower operating tension value, the speed will stay constant. If the tension should persistently stay below the lower operating tension value, speed will continuously be reduced until either the tension goes back above the lower operating tension or the speed reaches the minimum speed limit.

The values of the tension limits, the speed limits, and the gauge or thickness limits are stored in the computer and can be individually determined on an alloy, thickness, width and hardness basis. In addition, the rate of increase during acceleration optimization and deceleration can be chosen in the same way.

In another version of the invention, roll pressure varied by a screwdown or a hydraulic control system on the mill rolls is used as the control parameter instead of the strip tension to optimize the mill speed in accordance with the invention.

Other objects, advantages and features of the present invention will become apparent from the following detailed description which is presented in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a block diagram showing a rolling mill with a speed optimization control system in accordance with the principles of the present invention;

FIG. 2 is a block diagram showing the elements of a speed maximization system utilizing tension control;

FIGS. 3 5 are a series of three traces showing characteristic variations of tension, mill speed and gauge deviation during a typical operating period ofa mill utilizing my speed optimization system.

DETAILED DESCRIPTION OF EMBODIMENTS With reference to the drawing, FIG. 1 shows schematically a rolling mill utilizing a speed optimization system according to the present invention. The mill comprises a payoff reel 12 supporting a coil of metal strip material 14 that extends between a pair of pressure or work rolls l6 and 18 to a rewind reel 20. As in a typical mill arrangement the work rolls are backed up by back-up

rolls

22 and 24 normally carried in

chocks

26 and 28. One chock 26 is movable and connected to a screw-down or roll force mechanism 30 for varying the spacing between the work rolls l6 and 18 and establishing the rolling force on the strip material 14 passing through the mill. In a mill of this type both the payoff and rewind reels and the rolls are all driven by separate electrical motors (not shown). The main drive of the strip material and hence its speed control is provided by the work rolls which are mechanically supported by the back-up rolls. The payoff motor is used as a brake applying tension to the strip entering the mill. The rewind motor is operated so that tension is applied to the strip leaving the mill.

A computerand electronics interface elements for my speed optimization system, as designated by the numeral 32, includes a digital computer, such as a 16 bit word minicomputer. The interface elements include power supplies, A-D converters, digital input-output systems. In addition, there will be an operators station with suitable display or readout devices. Since all of these interface elements are used with and connected to the computer in a conventional manner, they are not shown or described in detail.

In the embodiment shown in FIG. 1, five control inputs to the computer are provided to produce speed optimization control of the mill in accordance with the invention. A thickness gauge 34 located between the rolls and the takeup reel provides signals to the computer via a lead 36 corresponding to the actual thickness of the rolled strip material. Power for the main drive that controls mill speed is typically supplied from a suitable power source to both work rolls l8 and 16, and signals representing actual mill speed are furnished from the main drive to the computer via a lead 38. A feedback signal is supplied via a lead 40 from the computer to a mill speed control unit 42 connected to the driving rolls [6 and 18. Entry tension of the strip material is supplied to the computer via lead 46 either from the current drawn by motor 44 on the payoff reel, or from a tensiometer roll in the conventional manner. A tension control signal is supplied from the computer via a lead 48 to a payoff tension control unit 50 connected to the payoff reel 12.

In an alternate form of my invention the tension control input may be replaced with a similar input indicating the roll force or pressure on the work rolls l6 and 18 via a lead 52. A control lead 54 is connected between the computer and the roll force mechanism for the work rolls.

In another alternate form of my invention both the

tension control

48 and 46, and the

roll force control

52 and 54 are used.

Thus, as will become apparent, speed optimization can be accomplished according to my invention by controlling either strip tension or roll pressure, and under certain circumstances a combination of the two parameters may be utilized.

The schematic diagram of FIG. 2 depicts the functions implemented in the computer 32 of FIG. I which are used to accomplish optimized speed control in accordance with the invention. Starting from the top of this diagram, it is seen that the signal from the output thickness gauge 34 is supplied via the lead 36 to section 60 of the computer 32 that provides a subtraction function. This function allows the mathematical subtraction of the gauge target H, on lead 62 from the measured gauge H on lead 36 to produce an output error signal X.

The Subtraction Function can be expressed mathematically as where Xis the output of 60.

The signal X is fed to another computer section 64 of the Limit Function type which" provides a gauge error check and produces an output Y that is the value of the incoming signal X restricted to lie in a range of values X, to X The section 64 also includes a logic switch so that if the signal X lies outside the X, to X, range, a logic switch Z is set for disabling any speed increase on low gauge. Mathematically, the Limit Function of block 64 may be represented as follows:

where X is the lower limit on the incoming signal, in section 64 a lower limit on gauge error, and X, is an upper limit on the incoming signal, in section 64 an upper limit on gauge error.

The output of section 64 is furnished to a feedback control function in section 66 of the computer. This control function will determine what action on tension to request via summing function 68 as a result of the gauge error computed in the limit function 64. The exact form of the control function 66 is not critical to the invention so long as it achieves feedback control. Typically, the function of the block 66 will be linear and can be expressed in Z transform notation as:

where Hz) is the signal leaving the function and X(z) is the signal entering. The specific values of the coefficients of this equation for feedback control are chosen as a function of the dynamic characteristics of the mill. A typical method of determining these coefficients is given in an article published by applicant in Control" magazine for February, 1968. In many cases, the equation can be as simple as:

which is the digital equivalent of integral only feedback control commonly used in process industries. 0r

which is the digital equivalent of proportional-plus- T,"- A T," AT

where the superscript n denotes the sample or calculation period.

The output T of block 68 passes through a limit function block 72 similar to block 64 described in equation (2). This block 72 restricts the value of the tension set point to lie between the values T,, the Tension High Limit and T the Tension Low Limit.

This limited value of tension setpoint out of block 72 then goes to a subtraction function 74 where the actual tension value on lead 46 is subtracted from it. The tension error signal out of function 74 then passes through the function block 76 which is the tension feedback controller. in general, this block 76 has the form (Fz) of equation (3) but in practice will probably be as simple as equations (4) or (5).

The signal out of block 76, which denotes a required change in tension actuator 48 in FIG. 1, is then converted in the computer interface into an appropriate plant signal to cause an increase or decrease in the ten sion actuator value.

The actual mill tension signal in lead 46 is also provided as an input to a special function block 78 that provides a limit test to determine whether to increase or decrease speed or do nothing. This special function network operates so that:

then y 0 where y is the output of block 78 and x is the input (7) lfX T.,, then y 8,,

If T X, then y =S where 8,, is a speed increment, and S is a speed decrement.

The output of the limit test function block 78 is furnished to the logic switch 80 in a lead 82 which is controlled by the limit function block 64 to disable any speed increase when the gauge of rolled material falls below a predetermined minimum X (speed decreases are allowed).

When the switch 80 is closed the signal from block 78 through a switch 80 is supplied through a lead 82 to the feedforward block 70. The signal from block 70 through switch 71 is added to the tension setpoint in summing function 68, as previously described. The signal from block 78 is also supplied by a lead 84 to a summing function 86 where it is added to a value which represents the setpoint for speed. The feedforward block 70 is a function of the general form F(z) described in equation (3). The coefficients of the equation are chosen so that the signal in lead 85 which will cause a decrease (or increase) in tension setpoint, will cause a gauge change exactly equal and opposite in magnitude to the gauge change caused by the increase (or decrease) in mill speed setpoint via lead 84.

Typically the response of gauge to a tension change into function 68 can be approximated by a first order equation Similarly the response of gauge to a speed change into function 86 can be approximated by a first order equation [f (8) and (9) are true then the equation for function is This is the digital equivalent of what is commonly referred to in the literature as a lead-lag" network. This is a simple, commonly used form of equation (3) in a feedfoward application.

The summing function 86 also receives inputs from a limit function block 88. When speed optimization control is enabled and the mill is initially at thread speed a switch 110 is enabled and a switch ll2 disabled. The initial desired run speed S is compared to the actual mill speed target via a lead 114 in a subtraction function 106, a function of the form of equation (1). The size of the signal out of function 106 is limited in block 88, a function of the form ofequation (2). This signal which represents a change in speed is then added to the current speed setpoint in summing function 86. The effect of this is to cause the speed setpoint in 86 to increase in steps. As soon as the desired run speed S is reached, switch 110 is disabled and remains disabled until the speed optimization system is restarted by pushing a start button (not shown).

A limit function block operates in a similar manner to the block 88 at the end of a coil being rolled to reduce speed. When the operator pushes a thread speed button (not shown) or when an automatic coil end detection system detects the roll end approaching, the switch 112 is enabled. From the subtraction function 108 and the limit function 90 the speed setpoint in summing function 86 is reduced in steps until the required thread speed 8., is reached. Switch 112 is then opened and is not enabled until the thread speed button is again pushed, or a coil end condition automatically detected.

The summing network 86 produces an output in a lead 94 which is furnished through a logic switch 98 to a limit function block 96 or via a lead 98 to a subtraction function 102. The limit function 96 provides a maximum/minimum speed test wherein speed setpoint is compared with a predetermined maximum speed limit S, and minimum speed limit S This is the same type function as equation (2). The block 96 controls the logic switch 71 to disable the tension changes generated by the feedforward block 70 whenever the speed hits the preset maximum or minimum values S or 5;.

If the mill is below the minimum speed S;,, because

switches

110 or 112 are enabled at the beginning or end of a coil, then switch sends the speed setpoint signal from summing function 86 via leas 98 to subtraction function 102. If

switches

110 and 112 are disabled then the switch 100 sends the speed setpoint signal from function 86 via block 96 to function 102.

In the subtraction function 102, the mill speed via lead 38 is subtracted from the speed setpoint via leads 95 or 98, to give a speed error signal into a function block 104.

The block 104 is functionally equivalent to the block 76 and provides a speed feedback control output to the mill speed actuator or control device 40 (in FIG. 1).

The operation of the speed optimization system according to the invention may be best described relative to FIGS. 3 S which shows three typical traces of the various parameters involved in a rolling mill operation with respect to time. In this example, variation of roll force to optimize speed has not been shown). All three traces start when the metal strip is assumed to be at thread speed (a low mill speed about 300 fpm) after being threaded from the payoff reel through the gauges and roll bite to the rewind reel. FIG. 3 is a trace of the payoff tension, FIG. 4 is a trace of mill speed and FIG. 5 shows gauge deviation from a preselected target gauge.

During the threading of the mill, it is customary for the operator to not use automatic control since there are many problems with physical placement of the strip which only he can make judgements about. Up to points Al, A2 and A3, therefore, the mill is in what is generally referred to as thread mode. As shown in FIG. 5, during this time, the outgoing thickness of the strip is usually heavy (A3) and is certainly rarely on target.

At point (Al) in FIG. 3, the mill is placed on automatic speed optimization control. The tension control attempts to correct for the heavy gauge (A3) by increasing tension (Al-AAl) via

blocks

66 and 76 in FIG. 2. During this time, the speed is being increased at a constant rate (AZ-B2) via block 88, lead 98 and block 104 in FIG. 2, heading for a predetermined initial run speed S2 chosen for the particular alloy and thickness and stored in the computer. But, increasing speed also reduces gauge and after a period of time the gauge may drop below target, as at point AA3. Now, the tension control, blocks 66, 72 and 76 in FIG. 2, attempts to counteract this trend by reducing tension (AA- l-Bl). If during this process, the gauge should go light (B3), detected by block 64 in FIG. 2, by more than a predetermined amount X set in the computer, the speed ramp will be disabled by switch 116 in FIG. 2 (B2-C2).

When the gauge comes back in range (C3), switch 116 is enabled again and the speed ramp continues (C2) to the point (D2), at which time the predetermined speed S2 is reached and in FIG. 2 switch 110 is disabled and switch 100 is set to send signals through block 96. Once the'predetermined initial speed S has been reached, the computer now maximizes the speed of the mill within constraints.

Via block 78 the computer inspects the tension value and if this lies above the tension maximize value T the computer will increase speed (DZ-E2) via lead 84 while at the same time reducing tension (DI-E1) via block 70 in order to achieve constant gauge (D3-E3). Since increasing tension and increasing speed both reduce gauge, one variable can be traded for another. When the tension goes below the tension maximize value T (El), speed maximization from block 78 will stop and the speed will be held steady (E2-F2) since there are no speed changes coming into function 86. If the tension should subsequently go above the tension maximize value T (Fl), then the speed will continue to be maximized from block 78 (F2-G2). If, during the running of the coil the gauge drops below a limit X (G3), speed maximization will stop (G2-H2) until the gauge is back in range (H3). This is controlled by switch 80 in FIG. 2. If the tensionat this time is above the tension maximize value T (HI), speed maximization will continue (H2I2). The speed is limited by a maximum speed S set in the computer in block 96. At point I2 on FIG. 4, this maximum speed is reached and even though the tension may still be above the tension maximize value T and block 78 is requesting speed increases via lead 84 no further speed maximization occurs. No feedforward tension changes are made during this time because block 96 disables switch 71 when S, is reached. As a practical matter when block 96 is limiting the speed setpoint, the value in function 86 is reset to this limiting value. This prevents what is commonly referred to as integral wind-up.

If the tension should drop below the lower operating tension value T (J1) block 78 outputs decreases in speed via lead 84 (124(2) and increases in tension via block 71 (Jl-Kl) until the tension is above the lower operating tension value T (Kl). So long as tension lies between the two values, T and T the output of block 78 in FIG. 2 is zero and the speed will stay constant (K2-L2). If the tension should persistently stay below the lower operating tension value T (L1) speed will continuously be reduced by block 78 via lead 84 (L2-M2) and tension increased via block 70, until either the tension goes back above the lower operating tension T or the speed hits the minimum speed limit S as shown at the point (M2).

The only point in time when the speed is allowed to drop below the minimum speed limit S is when the end of the coil has been reached (N2). At this time the operator may put the automatic system into manual and takes the speed of the mill down manually. Alternatively in situations where an automatic coil end detection system is in operation, or where a thread speed button is supplied, the reduction of the mill speed to thread speed at point N2 may be undertaken by the computer using the block 90 in FIG. 2.

Whereas, the invention has been described in the embodiment of FIGS. 1 and 2 with respect to a system utilizing payoff tension to optimize the mill speed, the invention could also be applied in a similar manner by replacing payoff tension with some alternate variable that controls gauge such as roll pressure, roll force, screwdown or other variable for adjusting the roll gap; rewind tension; or interstand tension on a multistand mill. In such an alternate arrangement the blocks in FIG. 2 labled tension should be denoted with the alternate variable and with the connections being made to the appropriate actuator.

Tothose skilled in the art to which this invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the spirit and scope of the invention. The disclosures and the description herein are purely illustrative and are not intended to be in any sense limiting.

I claim:

1. In a method for optimizing the speed of strip material issuing from a rolling mill during its normal operation, the steps of:

a. preselecting and setting a maximize value and a lower operating value of a control variable of the mill which affects gauge, preselecting and setting a minimum speed, a maximum speed, an initial run speed, a gauge high limit, a gauge low limit and a rate of speed increase, said preselected values being selected as a function of the material being rolled;

b. increasing speed of the strip material in the mill at a constant rate toward a preselected initial speed;

c. generating a signal as a function of the difference between a control variable of the mill that affects gauge and a corresponding preselected maximize value of the control variable and as a function of the difference between the said control variable and a corresponding preselected lower operating value of the control variable; and

d. changing the speed in response to said signal when the control variable lies outside the region bounded by said preselected maximize value and the preselected lower operating value in such a direction that the change in the control variable required to maintain constant gauge will be in a direction such that the control variable will move toward the region bounded by the preset maximize value and the lower operating value.

2. The method as described in claim 1 including the step of simultaneously generating signals to change the control variable at the same time above speed changes are made in such a manner that the output gauge from the mill stays constant.

3. The method as described in claim 1 including the step of suspending both changes in speed and changes in control variable of output gauge drops below a preset limit.

4. The method as described in claim 1 including the step of suspending increases in speed and corresponding changes in control variable if speed reaches a preset maximum value.

5. The method as described in claim 1 wherein said control variable is payoff tension.

6. The method of claim 1 wherein said control variable is roll force.

7. A method for optimizing the speed of strip material through a roll mill having a payoff tension control means and a mill speed control means comprising the steps of:

a. preselecting and setting a maximize value and a lower operating value of a control variable of the mill which affects guage, preselecting and setting a minimum speed, a maximum speed, an initial run speed, a gauge high limit, a gauge low limit and a rate of speed increase, said preselected values being selected as a function of the material being rolled;

b. providing a gauge feedback control means that generates a signal that varies a control variable of the mill in such a manner as to maintain the measured gauge at a constant target value;

c. generating a speed change signal as a function of the value of payoff tension;

d. combining said speed change signal with a speed setpoint to produce a new speed setpoint for the system actuated controller.

8. The method as described in claim 7 including the steps of generating a gauge error signal proportional to the difference between the gauge of step material leaving the mill with a target gauge;

supplying said gauge error signal to said feedback controller means to produce a feedback output signal;

generating a feedforward tension compensating signal in response to the feedforward change system;

combining said feedback output signal with a feed foward tension feed compensating signal to provide a change in tension setpoint for a tension actuator feedback controller.

9. The method of claim 7 wherein the speed setpoint is limited between a preselected minimum and a maximum value.

10. The method of claim 9 wherein the feedforward tension compensation is suspended when the speed setpoint is being limited at a minimum or maximum value.

11. The method of claim 7 wherein the speed increases are suspended if said gauge error signal drops below a'preselected value.

12. The method of claim 7 wherein mill speed is increased at a constant rate when it is below a preselected minimum speed immediately after the mill has been threaded.

13. The method of claim 7 wherein mill speed is decreased at a constant rate below the minimum speed when an end of coil signal is received.

14. An apparatus for a rolling mill having a mill variable control means and a mill speed control means for optimizing the speed of strip material through the mill, said apparatus comprising:

a. gauge feedback control means for generating a signal supplied to the mill variable control means so as to maintain the measured gauge at a constant target value;

b. means for generating a speed change signal as a function of an output of said variable control means; and

c. means for combining said speed change signal with a speed setpoint to produce a new speed setpoint for said mill speed control means.

15. The apparatus as described in claim 12 wherein said variable control means comprises a payoff tension controller.

16. The apparatus as described in claim 15 including a feedforward tension compensating means for providing a signal to change payoff tension to compensate for the effects of speed changes on gauge.

Claims

1. In a method for optimizing the speed of strip material issuing from a rolling mill during its normal operation, the steps of: a. presElecting and setting a maximize value and a lower operating value of a control variable of the mill which affects gauge, preselecting and setting a minimum speed, a maximum speed, an initial run speed, a gauge high limit, a gauge low limit and a rate of speed increase, said preselected values being selected as a function of the material being rolled; b. increasing speed of the strip material in the mill at a constant rate toward a preselected initial speed; c. generating a signal as a function of the difference between a control variable of the mill that affects gauge and a corresponding preselected maximize value of the control variable and as a function of the difference between the said control variable and a corresponding preselected lower operating value of the control variable; and d. changing the speed in response to said signal when the control variable lies outside the region bounded by said preselected maximize value and the preselected lower operating value in such a direction that the change in the control variable required to maintain constant gauge will be in a direction such that the control variable will move toward the region bounded by the preset maximize value and the lower operating value.

6. The method of claim 1 wherein said control variable is roll force.

7. A method for optimizing the speed of strip material through a roll mill having a payoff tension control means and a mill speed control means comprising the steps of: a. preselecting and setting a maximize value and a lower operating value of a control variable of the mill which affects gauge, preselecting and setting a minimum speed, a maximum speed, an initial run speed, a gauge high limit, a gauge low limit and a rate of speed increase, said preselected values being selected as a function of the material being rolled; b. providing a gauge feedback control means that generates a signal that varies a control variable of the mill in such a manner as to maintain the measured gauge at a constant target value; c. generating a speed change signal as a function of the value of payoff tension; d. combining said speed change signal with a speed setpoint to produce a new speed setpoint for the system actuated controller.

8. The method as described in claim 7 including the steps of generating a gauge error signal proportional to the difference between the gauge of step material leaving the mill with a target gauge; supplying said gauge error signal to said feedback controller means to produce a feedback output signal; generating a feedforward tension compensating signal in response to the feedforward change system; combining said feedback output signal with a feed foward tension feed compensating signal to provide a change in tension setpoint for a tension actuator feedback controller.

11. The method of claim 7 wherein the speed increases are suspended if said gauge error signal drops below a preselected value.

14. An apparatus for a rolling mill having a mill variable control means and a mill speed control means for optimizing the speed of strip material through the mill, said apparatus comprising: a. gauge feedback control means for generating a signal supplied to the mill variable control means so as to maintain the measured gauge at a constant target value; b. means for generating a speed change signal as a function of an output of said variable control means; and c. means for combining said speed change signal with a speed setpoint to produce a new speed setpoint for said mill speed control means.