Tension control in a metal rolling mill

A metal rolling mill for reducing the thickness of a metal workpiece passed between a pair of opposed work rolls is controlled to maintain substantially constant tension in the workpiece on the entry side of the work rolls to thereby improve the uniformity of the workpiece gate at the mill output. Working in conjunction with a standard control for delivering the workpiece is a scheme for compensating for thickness variations in the workpiece at the entry side of the work rolls which variations would result in tension changes which would adversely affect output gage consistency.

BACKGROUND OF THE INVENTION 
The present invention relates generally to metal rolling mills and more 
particularly to a method of compensating for tension changes in a 
workpiece at the entry side of a pair of opposed roller elements (work 
rolls) which tension changes are occasioned by thickness variations in the 
workpiece as it enters between the work rolls to be reduced in thickness. 
In a metal rolling mill, one of the prime objectives is to produce a 
product having uniform thickness or gage. In a typical rolling mill, the 
workpiece is supplied in the form of a coil (payoff coil) from which it is 
unreeled and supplied, in certain instances by way of powered puller 
rolls, to one or more mill stands of opposed pairs of work rolls to be 
reduced in thickness. After reduction, the workpiece may be wound on 
another coil called the takeup coil. As is understood in the discipline, 
workpiece thickness, or gage, is essentially a function of the roll 
separation force and/or the tension within the workpiece. With respect to 
tension, that on the entry side of the work rolls is of the greater 
importance. 
Since the amount of material delivered from payoff coil must equal the 
amount of material which is placed upon the takeup coil, the volume flow 
over a period of time is constant. As such, since in a metal rolling mill 
the workpiece width and the work roll speed are each essentially constant, 
it is apparent that the velocity of the workpiece at the entry side of a 
mill stand multiplied by its thickness must be equal to the product of the 
velocity and the thickness of the workpiece at the exit side of the stand. 
From the above discussion, if it is now assumed that the exit thickness and 
the velocity are set, essentially, by the rolling force and work roll 
speed, it is intuitively apparent and mathematically true that any change 
in the thickness at the entry side of the stand will inversely affect the 
entry side velocity. This change in velocity has, in prior art controls, 
resulted in a change in tension at the entry side of the work rolls which 
in turn resulted in a change in delivery thickness and hence an "off-gage" 
product. 
Prior art controls have provided power to the payoff coil to adjust the 
output torque thereof to provide the desired level of tension and to 
maintain tension constant as the torque arm changes when the coil diameter 
changes. During planned mill accelerations and decelerations of the 
workpiece, torque changes are made to prevent the inertia of the payoff 
coil system from affecting tension. 
When powered puller rolls are present, the roll diameters of these drives 
remain constant so it is not necessary to account for changing torque arms 
or changing inertia. As with the payoff coil drive, torque is adjusted for 
obtaining the desired level of tension and for planned mill accelerations 
and decelerations to prevent the fixed inertia from affecting tension. 
These systems do not, however, compensate for velocity changes within the 
workpiece, due to changes in the entry thickness, which result in the 
change in tension and hence gage. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide an 
improvement in the control of workpiece gage in a metal rolling mill. 
It is a further object to provide an improvement in the control of output 
workpiece gage of a metal rolling mill through the improved control of 
workpiece tension. 
It is still another object to provide an improved method of controlling 
workpiece tension in a metal rolling mill by compensating for workpiece 
thickness variations at the entry side of a mill stand. 
It is a still further object to provide an improved method of controlling 
the output gage of a metal rolling mill through the more precise control 
of tension in a workpiece by compensating for thickness variations at the 
entry side of a stand and through the modification of the standard mill 
control. 
The foregoing and other objects are achieved in accordance with the present 
invention by providing, in a metal rolling mill which includes a stand 
having a pair of opposed roller elements for reducing the thickness of a 
workpiece passed therebetween, a system for compensating for tension 
changes in the workpiece at the entry side of the mill stand which are 
occasioned by the variations in the workpiece thickness. In accordance 
with this method, sequential workpiece segments of a prescribed length are 
defined and any change in thickness within a segment is determined. Also 
determined is the time at which a workpiece segment enters between the 
roller elements. Based upon any change in thickness, the anticipated rate 
of change in the velocity of the workpiece at the entry side of the roll 
mill stand is determined and in response to this anticipated change, the 
workpiece tension at the entry side of the work rolls is controlled to a 
substantially constant value.

DETAILED DESCRIPTION 
Reference is now made to FIG. 1 which shows, in schematic form, a typical 
single stand mill in association with which the present invention may be 
practiced; although, it will be apparent to those skilled in the art that 
the present invention is not limited to single stand mills but could be 
applied to tandem mills as well. 
Referencing now FIG. 1, it is seen that provided are two opposed work rolls 
10 and 12 between which a workpiece 14 is passed for the purpose of being 
deformed as by a reduction in thickness. It is well understood by those 
skilled in the art that the work rolls 10 and 12 are driven by suitable 
motors and are maintained within an appropriate housing having means such 
as screw or hydraulic mechanisms for adjusting the gap or opening between 
the two rolls and for maintaining a desired rolling force. For sake of 
simplicity, these items which are well understood within the art have not 
been included in the FIG. 1 showing. Workpiece 14 is supplied to the work 
rolls from a payoff coil 16. The workpiece 14 passes from that coil over a 
suitable deflector roll 18 and by a device 20 for measuring workpiece 
thickness (e.g., an X-ray gage). At the output or exit side of the mill, 
the workpiece 14 passes over a second deflector roll 19 and from there to 
a suitable takeup coil 22. Rolls 18 and 19 may be driven by suitable 
motors which have not been shown; although frequently these rolls are 
undriven. 
Both the payoff coil 16 and the takeup coil 22 are driven by suitable motor 
systems. The motor for a takeup coil 22 is not shown here since it is not 
involved in the present invention. Payoff coil 16 is shown powered by a 
motor 24 the output torque of which is varied, as is known in the prior 
art, in proportion to the radius of coil 16 and, as will be described, in 
accordance with the present invention. In the illustrated example, motor 
24 is supplied with variable power from a power supply 28 which is under 
the functional control of a suitable control 30 responsive to input 
signals 32. As a typical example of such control and such as might be 
employed in association with the present invention, reference is made to 
"Selection of Electrical Equipment for Temper and Skin Pass Mills" by J. 
E. Peebles et al., Iron and Steel Engineering, September 1958, pages 
115-128. 
An additional factor normally necessary in a rolling mill and which is 
necessary for the operation of the present invention is some means of 
obtaining the velocity of the workpiece. In FIG. 1, two possible sources 
are illustrated. The first of these consists of a tachometer 34 associated 
with the deflector roll 18. Tachometer 34 may be connected directly to the 
roll or to a motor powering the roll and could be of either the digital or 
analog type. Most commonly, in today's mill which employs computers, the 
digital type tachometer would be employed. The output of tachometer 34, 
signal V' on line 36, is proportional to the rotational speed of roll 18 
and is, therefore, assuming no slippage between the roll and the 
workpiece, directly proportional to the linear velocity of the workpiece 
by the factor of the roll diameter. An alternate method of obtaining 
workpiece entry velocity would be to provide a similar type of tachometer 
shown at 38, associated with a one of the work rolls 10 or 12 to provide 
an output signal V" on a line 40. This velocity signal should be increased 
by a forward slip factor with an approximate value of 1.05 and reduced by 
the ratio of measured delivery to entry thickness to obtain a value which 
once again could be used to represent linear entry speed of the workpiece. 
Other means such as surface inspection devices, for example laser 
velocimeters, could also be used. The important thing insofar as the 
present invention is concerned is that there be at least a close 
approximation of the entry linear speed of the workpiece. 
The thickness measuring device 20 provides an output signal H on line 21 
which is proportional to the extant gage (thickness) of the workpiece at 
the device. As was indicated, device 20 typically is an X-ray gage but 
other forms of gages, such as isotope or mechanical contacting gages, 
could be used with equal facility. 
Control 30 responds to input control signals on lines 32 and is, inter 
alia, designed to compensate for coil diameter changes in the payoff coil 
and to adjust the payoff coil drive torque during mill speed changes to 
prevent tension changes being caused by the need to supply or extract 
energy from the payoff coil drive system. Control 30 also permits the 
selection of the magnitude of torque (e.g., by the varying motor current) 
to provide the desired level of entry tension. 
The standard control 30 is not, per se, a part of the present invention and 
if further information with respect thereto is desired, reference is again 
to be made to the aforementioned Peebles et al. article. As earlier 
stated, the present invention in the embodiment being described is 
designed to work into or in cooperation with such a control. 
FIG. 2 demonstrates the generation of certain signals which are employed in 
the present invention and further demonstrates various relationships with 
respect to the mill. In FIG. 2, the work rolls 10 and 12 are again shown 
as is the workpiece 14. In the implementation of the method of the present 
invention, the workpiece is considered as sequential segments S.sub.1 to 
S.sub.n of equal length L. The segments are illustrated in FIG. 2 to the 
left of the work rolls 10 and 12 and are each shown to have a length L as 
measured between successive "h" measurements. The exact length of the 
individual segments is somewhat arbitrary and generally, the shorter the 
segment, the more accurate the system. The minimum segment size will, of 
course, be limited by practical considerations such as computational time, 
computer capabilities, etc. 
The thickness variations of the segments are determined in order to effect 
a signal which is representative of the rate of change of the velocity 
entering the work rolls due to thickness changes. In FIG. 2, workpiece 
thickness variations are shown greatly exaggerated and it is seen that two 
thickness measurements are taken with respect to each segment--one at the 
leading edge and one at the trailing edge. Thus, the workpiece length 
between indicators h.sub.1 and h.sub.2 is one workpiece segment (S.sub.1), 
that between h.sub.2 and h.sub.3 the next segment (S.sub.2) and so forth. 
The h.sub.1, h.sub.2 . . . h.sub.n designations are intended to represent 
the thickness measurements taken at their respective points and it is 
apparent that after the initial measurement the trailing edge measurement 
of one segment also serves at the leading edge measurement for the next 
segment. 
In FIG. 2, deflector roll 18 with its associated tachometer 34 provides a 
signal suitable for gating and for velocity measurement purposes. For 
purposes of this illustration, tachometer 34 may be considered a digital 
tachometer providing a string of output pulses on line 36 at a rate 
proportional to the speed of the deflector roll and hence, the velocity of 
the workpiece. These pulses may be utilized for several purposes. First of 
all, the pulses may be applied to a pulse to velocity conversion circuit 
40 which, as is known in the art, counts the number of pulses in a given 
interval of time to provide an output signal V on line 42 which will be 
representative of workpiece velocity. The pulses on line 36 may also be 
supplied to a suitable device such as a ring counter 44 which will provide 
an output signal on line 46 each time the counter reaches a given count 
which represents a given length of the workpiece (segment length L in this 
illustration). 
In FIG. 2, thickness measuring device 20 is shown providing signals on its 
output lines 21 which are representative of the extant thickness of the 
workpiece at the device. The signals on lines 21 are supplied to a gate 48 
which further receives the signal on line 46. Each time a signal appears 
on line 46, gate 48 is enabled and the signals from the measuring device 
20 are furnished to the input end of a suitable queue store such as a 
shift register 52. This same signal on line 46 may be employed to 
sequentially shift subsequent readings from the measuring device through 
the shift register such that by properly correlating the number of 
readings in the register to the distance between the measuring device 20 
and the work rolls 10 and 12, there will appear in the two most right hand 
portions of the register (here indicated as h.sub.1 and h.sub.2) 
quantities representing the leading and trailing edge thickness 
measurements of that workpiece segment which is immediately to enter 
between the work rolls. 
Reference is now made to FIG. 3 which illustrates, in functional form, one 
implementation of the method of the present invention. It will be apparent 
that the present invention may be implemented by either analog or digital 
means. In today's industrial world the more likely form of implementation 
would be by digital means and attached hereto, as Appendix A forming a 
part of this specification, is a computer listing written in Control Block 
language capable of being run on a Digital Equipment PDP 11/44 computer 
which listing constitutes such a digital implementation. 
Specifically referencing now FIG. 3, it will be recalled that it was 
earlier stated that the basic mill control 30 compensates for mill speed 
changes. This is normally achieved through the application of a signal 
representative of the rate of mill acceleration/deceleration in accordance 
with the previously referenced article by Peebles et al. 
It is the function of the present invention to compensate for anticipated 
workpiece velocity changes caused by thickness variations by providing a 
signal which is proportional to the rate of acceleration or deceleration 
of the workpiece resulting from such a change in thickness; i.e., a change 
in strip velocity as a function of time (dv/dt). In accordance with the 
preferred embodiment of the present invention, this dv/dt signal is 
defined by the relationship 
##EQU1## 
and is combined with the prior art rate of mill acceleration/deceleration 
to effect overall control of the payoff reel. In this equation, the 
various terms are defined as: 
h.sub.a =determined workpiece thickness at the leading edge of workpiece 
segment; 
h.sub.b =determined workpiece thickness at the trailing edge of workpiece 
segment; 
V=linear velocity of workpiece entering between work rolls; 
L=length of workpiece segment. 
As such the term 
##EQU2## 
is the per unit (percentage) thickness change in the segment over the 
length thereof while the V.sup.2 /L term is the acceleration factor. 
The functional implementation of this formula shown in FIG. 3 provides that 
the h.sub.a and h.sub.b signals (the thickness signals at the ending and 
trailing edges of a segment) are applied to a first summing function 60 to 
provide a summed output h.sub.a +h.sub.b. These same two signals are also 
provided in a subtractive mode to a summing function 62 which provide an 
output h.sub.a -h.sub.b. The output of function 60 is applied as one input 
to a multiplying function 64, the other input of which is a signal 
representing 0.5 such that the output of that function is 0.5 (h.sub.a 
+h.sub.b). This latter output forms one input to an additional multiplying 
function 66, the other input to which is a signal L proportional to the 
length of the segment. (The L signal proportional to segment length is, in 
any given situation, an operator settable constant and will, of course, 
correspond to the full count of counter 44 in FIG. 2.) Thus, the output of 
function 66 is the denominator of the above equation and serves as one 
input to a division function 68. The output of the function 62, the 
(h.sub.a -h.sub.b) signal forms one input to a multiplication function 70, 
the other input of which is a signal representing the square of the 
velocity (V.sup.2) derived from a multiplication function 72 which has, 
applied to both inputs thereof, a signal V proportional to the velocity, 
such as the signal derived on line 42 in FIG. 2. The output of function 70 
is, therefore, a signal representing the numerator, (h.sub.a 
-h.sub.b)V.sup.2, of the above equation and this signal serves as the 
second input to the division function 68, the output of which, in 
accordance with the above equation, will be the dv/dt signal for the 
workpiece. The dv/dt signal is developed for each of the sequential 
segments at the time the respective segment reaches the work rolls and is 
applied to a summing function 76, the other input of which is, in 
accordance with the known art, a signal proportional to the rate of mill 
acceleration or deceleration as normally implemented. The output of this 
summing function 76 is applied to control 30 which will now respond to an 
acceleration/deceleration signal which has been modified in accordance 
with the present invention. 
Control 30 may, of course, be provided with other control signals such as 
the tension reference signal and a velocity signal. These other parameters 
do not, however, form a part of the present invention and their general 
inclusion in FIG. 3 is only for purposes of illustration completeness. 
FIG. 4 illustrates a second possible application of the present invention 
in a rolling mill employing powered puller rolls located intermediate the 
payoff coil and the work rolls. As was the situation described with 
respect to FIG. 1, a workpiece 14 is furnished to work rolls 10 and 12 
from a payoff coil 16. Coil 16 is powered by motor 24 supplied with 
electrical power from a source 28 in response to the output of a control 
30. Also as earlier described, a suitable thickness gage provides 
thickness output signals 21 and the workpiece velocity may be determined 
from the output V" of a tachometer 38 associated with work roll 12. In a 
manner similar to that described in FIG. 1 with respect to tachometer 34, 
an alternate method of obtaining a workpiece velocity signal is through 
the employment of a tachometer 94 associated with one of the puller rolls 
(e.g. roll 86) to provide an output V'" on line 96. 
In FIG. 4, two powered puller rolls 80 and 82 are positioned intermediate 
coil 16 and work rolls 10 and 12. The output torques of the rolls 80 and 
82 result from the operation of respective motors 84 and 86 which, in this 
example, are connected in parallel to a power source 88, the output of 
which is controlled by a control 90 in response to inputs 92. The function 
of the puller rolls is to control tension in the workpiece as is described 
in the forementioned Peebles et al. article. The control 90 is similar to 
control 30 as described in that article excepting that compensation for 
reel inertia and torque arm changes due to radius change is not necessary 
and is, therefore, not present. 
Insofar as the present invention is concerned, FIG. 3 is applicable if 
control 90 is substituted for control 30 as indicated by the parenthetical 
representation in that Figure. In all other aspects the application is the 
same. That is, the dv/dt signal is combined with the prior art 
acceleration/deceleration signal to provide the required compensation to 
both controls 30 and 90 simultaneously. 
Thus, it is seen that there has been provided a system which accurately and 
efficiently compensates for variations in thickness of the workpiece which 
would, in turn, modify the tension resulting in gage changes to the 
detriment of the overall product quality. 
While there has been shown and described what are at present considered to 
be the preferred embodiments of the present invention, modifications 
thereto will readily occur to those skilled in the art. For example, the 
previous description describes the present invention applied to the drive 
of the payoff coil or to both the drives of the payoff and puller rolls. 
It is apparent that the present invention could be utilized with other 
entry drive configurations. Also, the specific relationship for 
determining the dv/dt value earlier specified is one designed for accuracy 
while maintaining simplicity. Using the previously defined designations, 
other relationships such as: 
##EQU3## 
could be employed and still retain the basic benefits of the invention. It 
is not desired, therefore, that the invention be limited to the specific 
embodiment shown and described and it is intended to cover in the appended 
claims all such modifications as fall within the true spirit and scope of 
the invention. 
APPENDIX A 
LOGIC FOR ENTRY THICKNESS DEVIATION, ENTRY LENGTH 
BLOCK AICB,400,PCDEVR,A:@INP22E,P:0.0000611,R:1 
BLOCK AICB,410,PCDEVR,A:@INP22CP:0.0000611,R:1 
BLOCK AICB,420,$A420,A:@INP229,P:1.0,R:3 
BLOCK AMPY,425,$A425,A:$ARW0,B;@LBRIPP 
BLOCK AICB,430,$A430,A:@INP22A,P:;1.0,R:3 
BLOCK AMPY,435,$A435,A:$A430,B:@RBRIPP 
BLOCK ANSW,440,HESET,A:@XRSETR,B:@XRSETL,C:PCB0,D:&LF 
BLOCK ANSW,450,HDSET,A:@XRSETL,B:@XRSETR,C:PCB0,D:&LF 
BLOCK ANSW,460,PCDLHE,A:@PCDEVR,B:@PCDEVL,C:PCB0,D:&LF 
BLOCK ANSW,470,PCDLHD,A:@PCDEVL,B:@PCDEVR,C:PCB0,D:&LF 
BLOCK ANSW,480,@DELLNG,Y:@LNGINC,A:$A425,B:$A435,C:PCB0,D:&LF 
BLOCK ANSW,490,$A490,A:HESET,B:PCDLHE 
BLOCK AMPY,500,DHDMEA,A:HDSET,B:PCDLHD 
BLOCK ASUM,510,$A510,A:HESET,B:-HDSET 
BLOCK ASUM,520,HEMEAS,A:HESET,B:$A490 
BLOCK ASUM,530,HDMEAS,A:HDSET,B:DHDMEA 
BLOCK ACMP,540,EXLTDX,A:$A510,B:&MILO,P:&lt; 
BLOCK LSHT,550,$P550,A:EXLTDX 
BLOCK LORI,551,@LARXRS,Y:$P550,A:&LT,B:&LT 
BLOCK LAND,560,$L560,A:PCB0,B:GTPAS1 
BLOCK ANSW,570,ENTLNG,A:&ELENHR,B:&ELENHL,C:$L56,D:&LF 
CALCULATE THE INERTIA COMPENSATION REFERENCE 
BLOCK ADVD,700,$A700,Y:FFCALC,A:FFREFF,B:MFREFF 
BLOCK AMPY,705,$A705,Y:FFCALC,A:VEFPM,B:0.2 
BLOCK AMPY,710,$A710,Y:FFCALC,A:$A705,B:$A705 
BLOCK AMPY,720,$A720,Y:FFCALC,A:$A700,B:$A710 
BLOCK AMPY,730,$A730,Y:FFCALC,A:$A720,B:&REAFIV 
BLOCK AMPY,740,$A740,Y:FFCALC,A:$A730,B:&KFIVE 
BLOCK AMPY,750,FFIC,Y:FFCALC,A:$A740,B:ENTLNG