CA1180422A - Method and apparatus for speed compensation due to abrupt changes in load in a metal rolling mill - Google Patents

Abstract

METHOD AND APPARATUS FOR SPEED
COMPENSATION DUE TO ABRUPT CHANGES
IN LOAD IN A METAL ROLLING MILL

Abstract of the Disclosure A speed control system for a motor drive employed to drive a work roll of a metal rolling mill in which a metal work piece is passed between a pair o work rolls to deform the work piece includes first and second feedback paths providing signals respectively proportional to roll speed and motor current which are combined to form a basic speed control of the motor. Speed changes due to abrupt changes in loading of the rolls occasioned by inconsistencies in the work piece entering between the rolls are compensated for through the generation of a compensating signal which is combined with the basic speed signal to develop a final control signal for the motor. The compensating signal is generated as a function of the force acting to separate the work rolls due to the presence of the work piece between the rolls.

Description

2 1-D~S~256 1

3.
METHOD AN~ APPARATUS ~OPL SPE:E D
CO~PENS~4T ION DIJE TO ABE~UPT C~AN~ES
IN LC~AD IN A METAL R :)LLING A~ILL
Back round of the Invent ion The present i~vention relates generally to the speed control of the work rolls of a metal rolliIlg mill and more parti~ularly, to a method and apparatus :Eor com~
5 pensating ~or speed changes OI the work rolls whis:~h changes are occasioned by ~brupt i~consisterlcies in t;he work piece between the rolls.
It has been commo~ practice in a metal rolling mill to regulate speed by pro~iding two con~rol loops. The 10 first or outer loop controls speed directly as a function o:e the roll speed, as, Ior example, by way of a tachometer providing a direct feedback proportional to roll speed which is compa~ed to a re~'ere:~ce v~lu~. ~ second o:r in~er loop co~trols the curren~ o~ the rnotor driving the work 15 roll alld acts as a ~aster response loop to provide a vernier ef~ecl; to th~ outer colltrol loop. The objective o~ controlling current is to control the torque furnished to the rolls~ Since, howev~r, direct sensi~g o~ ~orque is not today prac~ical ~rom a cost or ~echnology sta~dpoi~lt 20 (the lat ter due largely to torque oscillat:Lons ~ound in mécha~ical sy~tems ) the synthesis OI torque using current is the ge~erally accep-ted method~
O~e common know~ problem in this dual loop sp~ed con trol is that which a~risPs when -there ar~

changes, often abrupt, associated with the work piece. This is sometimes known as impact loading. The most common occurrence of impact loading is when a workpiece enters a rol]in~ mlll. The reasons for these abrupt changes are, however, varied, and, for example, in a strip mill may be the result of a region within the s-trip being rolled which is harder or softer than the rest o~ the strip because of me~allurgical dif*erences. Another example of an abrupt change occurs when s-trips are welded together. The presence of the weld ~etween the work rolls will cause such a variation in the loading of the rolls. Other wxamples are changes in width or the thickness of the work piece as it enters the rolls. In a rod or bar mill it is common practice to roll more than one rod or bar a-t the same time. Since the rods or bars do not necessarily enter or leave the work rolls sim-ultaneously, it is readily seen that when a single rod, in a mill rolling multiple rods simultaneously, leaves the rolls, there will be an abrupt chanqe in the loading of those rolls. This general problem is discussed at some lenqth in an article, "Electric Drive Systems for ~od and Bar Mills" by D.J. Fapiano and R.M. Sills which was published in the December, 1973 issue of I _n & Steel Enqineer.
The basic problem, as is discussed in the above-cited article, is tha-~ wit'h an abrupt change in load the speed control, particularly the current loop, tends to react in the opposite direction to that desired. For example, assuming a hard spot in the strip reaches the rolls, the increased loading on the rolls tends to increase the motor current. The current feedback control loop will interpret the increased current as an increase in motor speed; whereas in actuality the increase in roll load will result in a decrease in roll speed. The initial reaction of the control system to the increase in current is to ,~
, J

2~

- 3 ~ 21-DSS-2561 reduce the terminal voltage of the motor which will, in turn, fur~her reduce motor speed. This, as was indicated, is a correction in the wrong direc-tion. The direct speed feedback loop operatiny rom, for example, the tachometer earlier mentioned, will eventually correct this situation but during the interim perlod, un~il this correction is made, the rolling speed is incorrect which generally results in an inferior product. Of course, if the rolls experience a lesser loading, the reverse situation will happen.
One prior solution to this problem, as is discussed in the above-cited article, and which is illus-trated graphically in Figure 2 to be discussed hereinafter, is to build control regulators which have a faster response time. This is not always practical for a number of con-siderations, including the expense of such faster response regula~ors, plus the fact that these faster response regulators tend to be more critical and require additional maintenance and frequent tuning due to the aging of components, variations due to temperature changes, etc.
Summary bf the Invention It is, therefore, an object of the present invention to provide method and apparatus for improving speed control in a metal rolling mill.
It is a further object to provide method and apparatus for compensation for speed changes in the wo~k rolls of a metal rolling mill occasioned by changes in the loading of the rolls.
Still another object is to provide compensation for speed changes due to changes in the load in a metal rolling mill.
~ n additional object is to provide compensation for speed change~ in a metal rolling mill which are occa-sioned by changes in the loading of the mill work rolls through the use and as a function o~ the separation force exerted by the work piece on the rolls.

Still an additional object is to provide compen~
sation for speed changes in a metal rolling mill ~hich are ocGasioned by~*~ changes in the loading o~ the mill work rolls through the use and as a ~u~ction of the separa-tion force e~erted by the ~vor~ piece on the work rolls inconju.nction ~ith the extant motor ~lux.
The ~oregoing and other objects are achieved in accordance with the method and apparatus of the present invention by providing, in a me~al rolling ~ill o~ the type having work rolls driven by electric motor and employing a speed control loo~ and a motor current control loop to effect basic speed control o~ the motor, a compensation scheme which employs the sensing of the roll separation force occasioned by the presence o~ the work piece between the rolls to provide a ~orce signal proportional to that force. This force signal is modified as a ~u~ction of motor operating parameters ~o develop a speed correction or compensation signal, which latter ~ignal is then combined into the basic control loops to provide subs~antially in-sta~ eous correction of the roll speed error caused by thechan~e~ i~ the loading. I~ the pre~erred embodiment, the ~orce signal is combined with a signal proportional to -the motor flux, and in the general case there is further included the width of the work piece in the developme~t o~ the cor ~ection or compensatirlg signal.
Brief Desc o~ o~ the Drawi~gs While the present invention is described in par-ticularity in the claims an~exad to and forming a part of this speci~icatio~, a ~etter understa~ding of the invention can be had by refere~ce to the following description taken in conjunction with the accompanyi~g drawings in which:
Figure 1 is a schematic representation OI a typica:L metal rolling mili illustra~i~g Iu~ction and demo~-strating signals normally directly or i:ndirectly available 35 with respect to the mill and use:ful in u~ders~anding the problem the present invention corrects;

Figure 2 is a graph illustrati.ng the problem -to wnich the prese~t i~ven-tion is directed, and ~urther illus-tratiug one prior art method of compensation;
~igure 3 is a trarls~er ~unction block diagram o~
a -typical metal rolling mill speed control ~ystem which ~urther illustrates, in block ~orm, the inclusion o~ the compensation ~eature o:E the present invention;
Figuxe 4 is a schematic diagram illustrating one possible method of implementing the compe~sation ~eature of ~he presen~ invention in the ge~eral ~orm;
Figure 5 is a schematic diagram illustrating a special case im~lementation of the compensation feature of the present inventio~ when the work piece width is assumed to be constan-t;
Figures 6 and 7 illustrate possible impleme~tations ~or developing the ~1~Y signal which is utilized in the im~
plementa~ion o~ the present inven-tion in its preferred embod-im~nt.
Detailed Description Re~erenci~g now Figure 1, there is shown in diagramatical ~orm a typical stand of a rolling mil.l and a portion o~ the control circuitry and signals generated wi~h respect to the rolling of a metal work piece. As illustrated, a work piece 10 ~a strip) is passed between a pair of work rolls 12 and 14 to de~orm the work piece 10. ~s illustrated, the de~ormation per~ormed is that of thickness reductio~, th~ work piece bei~g reduGed from an initial thick~ess o~ h1 to a ~inal or exit -thickness o~ h2.
The work rolls 12 aud 14 are spaced apart by a distanc~
and held in position by suitable screw mechanisms (not shown). Work roll 12 .is driv~n by a mo~or 16, whil~ work roll 14 is dri~n by a motor, shown generally at 18, having an armature winding 20 and a ~ield winding 22. As is common and well ~nown in the art~ the ~orce occasioned by the work piece 10 betweeu the work rolls ~2 and 14 is se~se~
by a suitable load c2Il ~4, associa~ed wi-th one of the work , . .

~ ~ 8 ~ L~

rolls, which develops a signal (F) proportio~al -to the roll separation force. An additional signal proportional -to -the work piece width~ signal W, is also provided in -the illus-trated embodiment. This latter signal is shown as heing derived Erom a suitable width gage 26, although as is also well known in the art, the width signal may be the function of an operator input where the width of the worX piece is known. It is, thereEore, to be expressly understood that the width gage 26 included in Figure 1 is only for illustrative purposes.
A further signal propor~ional to the speed of the roll, signal N, is shown as being derived from the suita~le tachometer 28, which is operatively connected to the motor 18 as indicated by the dashed line. A further ~S signal (~ ) generated or utilized within this system is that proportional to the motor flux. For illustrative purposes, the flux signal is shown derived from a pair of f7ux coils 30 associated with the armature which provides an output to a suitable scaling circuit 32 having a transfer function K, all in a manner well known in the art. As will be further understood as this description proceeds, the generation of flux signal can be achieved in a number of ways, and once again, this particular mode of generation of the flux signal is Eor illustrative purposes only~ More specifically, as this description proceeds and as the invention is described in its preferred embodiment, the flux signal is preferably derived from motor operating parameters as opposed to the use of flux coils.
As indicated by the Figure 1 depiction, motor 18 is a shunt-wound dc motor. The field coil 22 is excited by way of a field exciter 34 in response to a reference signal lIF' Reference) which is applied to the field exciter 34 by way of a line 36. The armature winding 20 receives power from a suitable power source 38 which supplles power to the armature in response to a control signal applied to the source by way of line 40. Typically, the power source 1 2 .~

38 will be a solid state phase con-trolled bridge which out-puts a voltage (VT) to the mo-tor armature. A signal (IA) proportional to the magni-tude of the motor current is, for illustrative purposes, illustra~ed as being derived Erom a suitable shunt ~2 wh:ich is located in one of the lines connecting source 38 to the armature 20. Motor 16 would normally be controlled in a manner similar to that as shown wi-th respec-t to motor 18.
As will be more fully understood, particularly with respect to the description of the transfer block diagram of Figure 3, the motors 16 and 18 are speed controlled to provide the operation or the speed of the two rolls. Before leaving Figure 1, however, one additional showing needs explanation.
A shaded area ~4 within the work piece 10 is representative of a discontinuity of some type ~hich will result in a change in the loading of the work rolls. As earlier indicated, this discontinuity, for example, may constitute a hard or soft spot in the ~ork piece itself, due to metal-lurgical properties of the material, or may represent a weld or a change in width or thickness of the work piece.
In the case of rod and bar mill, the discontinuity may constitute the entry or the exit of a rod or bar to or from the work rolls. In any case, the discontinuity and its associated loading of the rolls will, as is understood, 2~ effect a change in the control signals. An increase in the work roll loading will produce an increase in the motor current and its related feedback signal. Conversely, a reduction in the roll loading will result in a reduc-tion in the curren-t feedback signal.
Figure 2, which is essentially Figure 1 of the aforementioned article by D.J. Fapiano and R.M. Sills, illustrates the problems associated with the slow response regulators of the prior art and also the prior art method of compensation through the use of Easter response time regulators. In Figure 2, the motor terminal voltage ~VT) is plotted as the ordinate vs. time from impact as the ) 4 ~ ~

abscissa. Time of impact, in the present description, is that time the discontinuity reaches the work rolls. Curve 46 illustrates the typical slow response oE a system in which there is a speed feedback loop d:irectly proportional to speed and a c~lrrent Eeedback loop proportional to arma-ture current. Wi-th -~his regulation system it is seen tha-t, for a sudden increase in roll loading immedia-tely after impace (time zero), the terminal voltage will decrease in response to the increase in the current feedb~ck signal. Subsequently, as shown by curve 46, the direct speed control loop will take over and the terminal voltage will be increased to the appropriate value. As shown by curve 48 of Eigure 2, with a faster response tume regulation the decrease in terminal voltage does not occur, but in fact, the terminal vo]tage is increased to compensate for the increase in the roll loading force.
Figure 3 is a transfer :Eunction hlock diagram of a speed control system of the type here being discussed with that portion shown within the dashed line block 50, being a simplified depiction of the showing of Figure 3 of the aforementioned article. An impact speed change compensation circuit shown at 100 constitutes the ~ of the present invent:ion when considered in conjunction with the basic speed con-trol system as shown within block 50. Referring first to tha-t portion of the circuitry within the dash line block 50, and recognizing that the circu~t will be described in its transfer function form, it is seen that a speed reference signal is deri~ed from a suitable source 52 which is, for illustrative purposes, shown as an operator controlled device. The speed reference signal from source 52 is applied to a summing junction 54 which further receives a signal (N) proportional to speed by way of line 57. The two signals applied to the summing junction 54 are of relatively opposite polarities such -that there appears, at the output of the summing junction 54, a signal (NE) proportional to the error in speed. This speed error signal is applied to a suitable scaling amplifier 56, the ou-tput of which is provided to a limi-~ cireuit 5~ which limits the maximum error whieh can be passed instan-taneously.
The output of the limit circui-t 58 serves as one input to another summlng function 60 which also receives a signal from a current loop feedback 62 via line 63. The feedbaek signal on line 63 is repre~enta-tive of motor armature current.
Summing junction 60 also receives a third input (IA) applied in the opposite sense to the signal via line 63 from the impact speed change compensation circuit 100. The signal IA is the curren-t correction signal which, in effeet, is an estimated or calculated value representing eurrent. The output of sum~
ming junction 60 is a current error signal (IE) which is passed through a suitable sealing amplifier 64 and a seeond limit cireuit 66 to a summing junction 68 which receives, by way of a current rate loop feedbaek 70 and line 72, an additional siynal applied in the opposite sense. Current rate loop feedbaek 70 serves merely to limit the rate at which the current will be permit~ed to rise. The OUtpllt of junetion 68 is applied via a suitable sealing amplifier 7A to a power source or converter 76 and serves as a control signal to that converter to control the motor terminal voltage (VT).
The VT signal is combined in summing junction 78 with a signal on line 80 proportional to the eounter eleetromotive foree (CE~F) of the motor. Provided at the motor armature 82 output is a signal ~IA) proportional to the armature eurrent whieh serves as input to the eurrent rate loop feedbaek and the current loop feedbaek earlier described.
The IA signal is also applied to a suitable transfer funetion block 84 having a transfer function of KT, whieh funetion is expressed in the terms of torque per ampere such that the ou-tput of that bloek as seen at summing junetion 86 is a unitized or per-unit value in units of foot pounds. Summing junetion 86 also reeeives a feedbaek signal via line 96 from a speed loop feedbaek 94, adjusting for tachometer torque, and a further signal proportional to load torque.

1 ~80~

The output o~ the summing ~unction 86 is applied to a suitable transfer ~unction block 88 providing the func-tion l wherein:
pJ
p = ~d , and J = inertia of motor and mill.
The output o~ the transfer fl~nction block 88 at node 90 will, there~ore, be representative of the mo~or speed which serves as an input to the speed loop ~eedback 94 earlier mentioned and which is also applied to a trans~er ~unctio~
block 92. Block 92 has a transfer ~unction sta1;ed in terms o~ volts per unit of rotational speed to thereb~J represent the CEIU~ signal earlier describedO It should be specifically noted that the description o~ the block 50, as was earlier indicated, is a trans~er :eunction description and not neces-sarily directly corresponding to a physical imp:Lementation.
In the actual physical implementation of the system o~ the present invention, ~or example, the speed sig~na:l would probably be derived from a tachometer such as indicated in Figure 1 and the armature current a~d termina.l voltage sig-nals would also be achieved in a more direct ma~ner such as is also there shown.
The description thus ~ar and that within the block 50 is that known in the art an~ well unde:rs~ood.
Assumlng that the response loops o~ current and speed were of a relati~ely slow. nature, the overall response would be similar to that shown by cur~e 46 in ~igure 2'. The inclu-sion of the impact speed change compensation circuit 100 which is combined with ~he result o~ ~he current and speed feedback loops constitutes the e~sence o~ the present in-vention and, as was earlier indicated, serves to provide a substantially instantaneous compensation for work roll speed changes due to inconsistencies i~ the applied work piece.

.2 ~ 21-DSS-2561 As has been i.ndica-ted, the compensatlon Eea-ture of the present invention is accompllshed as a :Eunction of -the roll separation Eorce oecasioned by the presenee of the work piece between the work rolls (signal F in Figure 1). ~mis, of course, impl.i.es that there is a clirect rela-tionship be-tween force (F) and the motor current (I~). As sueh, pri.or to a description of an actual implemen-ta-tlon of the circuit 100 and its method of operation, it ls believed helpful to define this relationship.
Since there is an ex-tremely large number of variahles in a metal rolling mill and since the circuitry employed in the l'normal" control ls of limited accuracy, the transfer func-tion from force to current can be made relatively simple if approximate values are used. It has been found through computer simulation that the use of approximate values p.rovides significant improvement over prior art control sys-tems not having the features of the present invention. As such, as a first approximation using terms associated with the mill system:
1) Torque = Force Contact arc ~ Kl 2) Current (I~) ~ Torque/Field Current = ~Torque/Flux) K3 3) Force = Contact are Yield stress K2 Width

4) ... Estimated Current (I ) = rce K - 3 A Flux 1 Yleld stress K2~ Wldth Force K4 Flux Wi.dth Yield stress wherein, Kl = Torque arm multiplier of the rolls K2 = Friction constant associated with the roll contact with the work piece K3 = Motor operating parameter constant K~ = 1 3 , and Contact arc is the arc of contact of roll with work piece, and yield stress is that o the material of -the work piece.

2.~

For a restricted xange oE rolling conditions such as exist in a given mill, yield stress is reasonably constant. ~s such:
Force K5 K4

5) E ti ated Current = 1 Width ; wherein, 5 Yielcl Stress Figure 4 illustra-tes one way in which equation t5) can he implemented. In Figure 4, the limit circuit 58 and the summing junction 68 of Figure 1 have been included as points of reference.
Shown within the dash line block labeled 60, 64 and 66 is one possible implementa-tion of the summing junction 60, amplifier 64 and the limit circuit 66. The impact speed change compensation block 100 of Figure 3 is shown in Figure 4 and employs three multipliers 102, 104 and 106, which may be standard analog multipliers such as are well known in the art. Each of the multipliers includes an X and a Y input. The X input to multiplier 102 receives the W signal proportional to work piece width while the Y input of that multiplier receives the ~ signal proportional to motor flux.
The output of the multiplier 102 is, therefore, the product ~f these two inputs and serves as the Y input to multiplier 104.
The roll separation force signal F is applied to both and X and Y
inputs of multiplier 106 such that its output is proportional to the square of the force (F ). The F term is applied, via a suitable input resistor 108, to a summing amplifier 110. The output of amplifier 110 forms the X input of multiplier 104, the output of which is also applied to the summing amplifier 110 by way of an input resistor 111. The output of summing amplifier 110 is, therefore, the estimated current signal IA or compensation signal and is equal to approximately the quantity -F . (In actuality, the output of F3 ~ ~
amplifier 110 is equal to 1 W~~ ~ but it is recogni~ed that `'1"
is very small compared to the W ~ term and thus the approximation indicated is appropriate.) The output of summing amplifier 110 is applied by way of resistor 112 to the input of a summing amplifier 114 having the feedback resistor 116 connected between its output and its input. The signal from limit circuit 58 is also applied through resistor 118 to the input of amplifier 114. The current limi-t function indicated by circuit 66 (Figure 1) is achieved hy the use of antiparallel disposed diodes 120 and 122 connected between the output and -the input of amplifier 114 by way of voltage divider consisting oE resistors 124 and 126 connected between potentials -~V and -V. IE desired, so as to change ,he limits of circuit, the diode connections to -the two resis-tors 124 and 126 may be variable. The output of the circuit 60, 64 and 66 is applied to the summing junction 68 of Figure 3 as indicated. Thus, it i9 seen tha-t, by selecting the circuit components of Figure 4 to provide the multiplication factor K5, the desired relationship expressed by e~uation 5 to provide the compensation indicated is achieved through the circuitry such that the compensation for sudden changes in the current due to inconsistencies in the work piece applied between the work rolls is compensated for through the sensing of the force.
In many instances the width of the material is approximately constant and thus need not be considered as a variable in the actual circuitry, but may be included in the overall gain of the amplifiers, etc., involved in the circuitry.
E'igure 5 illustrates an implementation in which width is considered constant. As shown in Figure 5, the force signal F is again applied to the X and Y inputs of a suitable analog multiplier 130, the output of which is the signal F which forms the X input to a suitable analog divider 132. rrhe ~ signa] is applied as the Y input to this divider such that the output IA is equal to which signal is applied to the summing junction 60 (Figure l);
i.e., that shown in Figure 4 within the dash line block 60, 64 and 66. This simplified version of the present invention can, of course, only be used in constant width situations.
The ~ signal proportional to motor flux can be derived in a number of ways. In Figure 1, the ~
signal was shown as being derived from flux coils. Figures

6 and 7 illustrate other possible ways of deriving the O ~

flux signal (~ ) using motor operating parameters. The Figure 6 depic-tion is one which recogn1zes the fact -that below saturation of the fi.eld, the field current is proportional to flux. As such, and as shown in Fiyure 1, a field reference signal (IF Referellce) is derived :Erom cl suitable input device such as a control station 1~0. This signal is applied to a transfer function block 142 which compensates for saturation effects of the Eield winding to thus provide a signal ~IF' ~eference) at the output thereof which is applied to the field exciter 34 (Figure l.). This signal controls the excitation of the field throughout the motor operating range. Since block 142 compensates for field saturation effects, the IF Reference signal may be used to represent motor field flux (~ ).
Other methods of deriving the flux signal can also be employed. It is known from standard motor theory that the motor flux is proportional to the quotient of the terminal voltage (VT) divided by the motor speed (N). Thus, the Figure 7 implementation for deriving the flux signal shows the signals N and VT being applied to a suitable analog divider 14~ the output of which, when properly scaled by an amplifier 148, serves as a flux signal suitable for use in either of the implementations of Figures 4 and 5.
Thus, there have been shown and described a relatively simple method. and apparatus for providing for speed change compensation in a metal rolling mill which speed changes are caused by material inconsistencies which are reflected in the separating force of the work rolls.
While there have been shown and described what are at presently considered to be the preferred em~odiments of the present invention, modifica-tions thereto will readily occur to those skilled in the art. As indicated, various methods of deriving the width and flux signals could be employed as could other means -for deriving the roll separation force. For examplel digital equivalents of the 21~DSS-2561 analog circuits shown could be employed with equal ~acility.
In addition, it wi-ll be remembered ~rom the early discussion in this specification that it was stated that -the object of controlling current was to control roll torque. As such, the present invention contemplates the use of an actual torque measurement in the compensation scheme. In this situation, the ~lux signal would not be employed as it is in the preierred embodiment since ~lux is there used to mai~tain the proportionality between torque and current.
Thlls, in the situation where an acceptable torque signal can be obtained, the impact speed compensation circuit 100 (~igure 3) would develop a compensation signal proportional to the quotient of the square of the force divided by the width, or in the case o~ substantially constant width, simply proportional to the square of the ~orce. It is not desired, there~ore, that.the invention be limited to the specific circuits and methods sho~ and described and it is intended to cover in the appended claims all such modiiications as ~all within the true spirit and scope of the invention.

Claims (12)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a metal rolling mill employing an electric motor for driving at least one of a pair of work rolls utilized to deform a metal workpiece and subject to impact loading of the rolls, a speed control system including anticipatory means for correcting for anticipated motor current changes due to the impact loading comprising:
a) means to provide a reference signal indicative of the desired rotational speed of the motor;
b) a first feedback loop for providing a speed signal proportional to the actual motor speed;
c) means to combine said reference signal and said speed signal to generate a speed error signal;
d) a second feedback loop for providing a current signal proportional to the motor armature current;
e) means to generate a force signal proportional to the roll separation force occasioned by the presence of the metal workpiece between said work rolls;
f) compensation means responsive to said force signal to generate an estimated current signal for providing a current correction and representing anticipated motor current changes due to changes in the load on said rolls;
g) means to combine said speed error signal, said actual current signal and said estimated current signal to thereby develop a current control signal; and, h) means responsive to said current control signal to control the current to and thus the speed of said electric motor.

2. The invention in accordance with claim 1 further including means to develop a flux signal proportional to motor flux and wherein said compensation means is responsive to said flux signal as well as said force signal to generate said estimated current correction signal.

3. The invention in accordance with claim 2 wherein said means to develop the flux signal includes flux coils associated with the motor.

4. The invention in accordance with claim 2 wherein said means to develop the flux signal includes:
a) means to develop a field reference signal designating desired motor field current; and, b) circuit means responsive to said field reference signal to generate said flux signal.

5. The invention in accordance with claim 2 wherein said means to develop the flux signal includes:
a) means to derive a voltage signal proportional to motor terminal voltage; and, b) divider means to provide said flux signal as a function of said voltage signal divided by said speed signal.

6. The invention in accordance with claim 2 wherein said compensation means responsive to the flux and force signals comprises circuit means including an analog multiplier and an analog divider.

7. The invention in accordance with claim 1 further including:
a) means to provide a width signal proportional to the width of the workpiece;
b) means to provide a flux signal proportional to motor flux; and, c) wherein said compensation means is responsive to said width signal, said flux signal and said force signal to generate said estimated current signal.

8. The invention in accordance with claim 7 wherein said compensation means generates said estimated current signal in accordance with the relationship generally defined by the expression:

wherein, IA = estimated current signal, F = roll separating force, W = workpiece width, ? = motor field flux, ? = workpiece yield stress in plane strain, K1 = torque arm multiplier, K2 = factor to account for friction in roll bite, and K3 = motor parameter, current per-unit of torque.

9. The invention in accordance with claim 7 wherein said compensation means generates said estimated current signal in accordance with the relationship generally defined by the expression:

wherein, IA = estimated current signal, F = roll separation force, ? = motor field flux, W = workpiece width.

10. The invention in accordance with claim 7 wherein the relationship between roll separation force and the estimated motor current (?A) is approximated by the relationship:

wherein, F = roll separation force, W = workpiece width, ? = motor field flux, and K6 = an empirically chosen proportionality constant.

11. A method of providing anticipatory impact loading speed compensation in a metal rolling mill drive of the type having a pair of motor driven opposed work rolls between which is passed a metal workpiece to be deformed and in which normal speed control is achieved through the use of an outer speed feedback path and an inner motor current feedback path combining to provide a basic control signal for a motor driving a work roll comprising the steps:
a) generating a force signal proportional to the force resulting from passing the metal workpiece between the rolls;
b) generating a flux signal proportional to the flux of a motor controlled by the basic control signal;
c) generating an anticipatory motor current compensation signal as a function of said force signal and said flux signal; and, d) combining said current compensation signal with the basic control signal to develop a final control signal for controlling the motor speed.

12. The invention in accordance with claim 11 further including the step of developing a width signal proportional to the width of the metal workpiece and wherein said current compensation signal is generated as a function of said width signal in addition to said force and flux signals.