Method and system for controlling an interstand tension in a continuous rolling mill

In a method and a system for controlling an interstand tension in a continuous rolling mill having a looper provided between a pair of successive rolling stands and driven by a looper drive motor, a correction to the speed target value for the looper drive motor is determined in accordance with the deviation of a detected looper operating angle from an angle target value, and the looper drive motor is controlled in accordance with the speed of corrected speed target value.

BACKGROUND OF THE INVENTION 
The present invention relates to method and a system for controlling an 
interstand tension in a continuous rolling mill. 
Most important factors for evaluating the quality of products rolled in the 
continuous rolling mills are thickness and width of sheet; the amount of 
crowning and flatness. These factors are greatly affected by an interstand 
tension i.e., a tension exerted on the workpiece passing between two 
successive rolling stands. The variation in the interstand tension must 
therefore be kept as small as possible. For this reason, hot continuous 
rolling mills are provided with a looper between successive rolling stands 
to minimize variation in the tension. In addition, the speed of the 
rolling stand adjacent the looper is controlled to minimize the range of 
angle over which the looper swings. 
FIG. 1 shows an example of a conventional tension control system. A 
workpiece 1 to be rolled passes through a rolling stand 2 having a pair of 
working rolls 2a and 2b, engages with a looper 4 and then passes through 
another rolling stand 3 having a pair of working rolls 3a and 3b. The 
looper 4 is of a type with which the looper torque given by a looper drive 
motor 8 is in balance with the tension on the workpiece 1. The operating 
angle .theta., i.e., the angle between the axis of the arm 4a of the 
looper 4 and an imaginary horizontal line 4b is detected by a looper angle 
sensor 5. The detected angle .theta. is applied to an operation unit 6 
which computes, in accordance with the detected angle .theta., the torque 
to maintain the tension at a desired value. More particularly, the 
detected angle .theta. is used for the calculation by the looper operation 
unit 6 to determine the target value of the electric current of the looper 
drive motor 8, and the target value is supplied to a current control 
device 7 which drives the looper drive motor 8. 
The output from the looper angle sensor 5 is also fed to an operation unit 
9 which computes a target value of the speed of a roll drive motor 11 to 
return the angle .theta. to a desired value. The speed target value is 
applied to a speed control device 10 which controls the speed of the roll 
drive motor 11. 
The control system described above, however, has the following drawbacks. 
First, since a current control device 7 is used to control the looper 
drive motor 8, a circuit for compensation for stabilization is required, 
and it is necessary to compute the initial current target value. 
Secondly, the change of the speed of the roll drive motor 11 results in 
variation of the length of the workpiece 1 between the successive rolling 
stands 1 and 2, following which the looper angle control is made. As a 
result, considerable variations occur in the tension on the workpiece. 
Thirdly, making the looper angle control to reduce the variation in tension 
necessitates lowering the response of control. This degrades the 
capability of the system to follow rapidly changing disturbance. 
SUMMARY OF THE INVENTION 
An object of the invention is to eliminate the drawbacks of the 
conventional control system. 
Another object of the invention is to provide an interstand tension control 
method which can be implemented by a system of a simpler construction. 
Another object of the invention is to provide an interstand tension control 
system of a simpler construction. 
Another object of the invention is to provide a method and a system for 
interstand tension control having a quick response. 
Another object of the invention is to provide a method and a system for 
interstand tension control with which the variation in the tension can be 
reduced. 
According to one aspect of the invention, there is provided a method for 
controlling an interstand tension in a continuous rolling mill having a 
looper provided between a pair of successive rolling stands and driven by 
a looper drive motor, said method comprising: 
detecting the operating angle of the looper, and 
determining, in accordance with the deviation of the detected angle from an 
angle target value, a correction to the speed target value for the looper 
drive motor, 
the looper drive motor being controlled in accordance with the speed target 
value. 
According to another aspect of the invention, there is provided a system 
for controlling an interstand tension in a continuous rolling mill having 
a looper provided between a pair of successive rolling stands and driven 
by a looper drive motor, said system comprising: 
means for detecting the operating angle, 
means for determining a deviation of the detected angle from an angle 
target value, 
means responsive to the deviation of the detected angle for determining a 
correction to a speed target value of the looper drive motor, and 
a looper drive motor speed control device responsive to the correction for 
controlling the speed of the looper drive motor.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 2 shows a preferred embodiment of a looper control system according to 
the present invention, in which the same reference numerals as in FIG. 1 
designate similar elements. In place of the operation units 6 and 9 in 
FIG. 1, a motor speed control unit 10 and operation units 14 and 15 and a 
noninterference operation unit 16 are provided. In addition, a tension 
sensor 13 is provided to detect the interstand tension, i.e., the tension 
on the workpiece 1 between the successive rolling stands 2 and 3. 
A comparator 15A determines the deviation of the detected tension T from 
the tension sensor 13 with reference to a tension target value T.sub.0. 
The deviation is applied to the operation unit 15 which performs P 
(proportional) and I (integral) control operation to determine a speed 
compensation .DELTA.V.sub.R *. A comparator 14A determines the deviation 
of the detected angle .theta. with reference to an angle target value 
.theta..sub.0. The deviation is applied to the operation unit 14 which 
also performs PI control operation to determine a speed compensation 
.DELTA.V.sub.Q *. 
The noninterference operation unit 16 comprises operation units 17, 18, 19 
and 20, and adders 21 and 22. The speed compensation .DELTA.V.sub.R * is 
applied to the operation units 17 and 18, while the speed compensation 
.DELTA.V.sub..theta. * is applied to the operation units 19 and 20. The 
adder 21 determines the sum of the outputs from the operation units 17 and 
19, while the adder 22 determines the sum of the outputs from the 
operation units 18 and 20. The output .DELTA.V.sub.R from the adder is 
applied, as a speed correction, via an adder 10A to the speed control 
device 10 while the output .DELTA.V.sub..theta. is applied, as a speed 
correction, via an adder 12A to the speed control device 12. 
The purpose of providing the noninterference operation unit 16 is as 
follows. The relationship between, on one hand, the speed correction 
.DELTA.V.sub.R actually fed to the speed control device 10 for the roll 
drive motor 11, and the speed correction .DELTA.V.sub..theta. actually fed 
to the speed control device 12 for the looper drive motor 8, and, on the 
other hand, the variation .DELTA.T in tension and the variation 
.DELTA..theta. in looper angle may be expressed by the following 
transfer-function matrtix: 
##EQU1## 
where s is the Laplace transform variable. 
The Eq. (1) shows that the speed correction .DELTA.V.sub.R intended for 
correction of the tension variation .DELTA.T also affects the looper angle 
variation .DELTA..theta., while the speed correction .DELTA.V.sub..theta. 
intended for correction of the angle variation .DELTA..theta. also affects 
the tension variations .DELTA.T. 
It follows that if the noninterference operation unit 16 were not provided 
and the speed compensation .DELTA.V.sub..theta. * were used as the signal 
.DELTA.V.sub..theta. to be applied to the control device 12 and the speed 
compensation .DELTA.V.sub.R * were used as the signal .DELTA.V.sub.R to be 
applied to the control device 10, mutual interference would occur. This is 
the reason why the present invention provides the noninterference 
operation unit 16 which receives the speed compensations .DELTA.V.sub.R * 
and .DELTA.V.sub..theta. * and generates the speed corrections 
.DELTA.V.sub.R and .DELTA.V.sub..theta. to avoid the interference. 
The operation units 17-20 are so formed as to have transfer functions with 
the following relationships between them. The operation unit 19 has such a 
transfer function as produces an output of a value contributing to a speed 
variation of the roll drive motor 11 for cancelling the interstand tension 
variation due to the looper angle variation due to the output of the 
operation unit 20. In other words, the output of the operation unit 19 is 
determined to counteract the attendant effect on the interstand tension of 
the output of the operation unit 20. The operation unit 18 has such a 
transfer function as produces an output of a value contributing to a speed 
variation of the looper drive motor 8 for cancelling the looper angle 
variation due to the interstand tension variation due to the output of the 
operation unit 17. In other words, the output of the operation unit 18 is 
determined to counteract the attendant effect on the looper angle of the 
output of the operation unit 17. 
The transfer functions that satisfy the above-mentioned requirements can be 
determined based upon the following consideration. Let the transfer 
functions of the operation units 17-20 be designated by G.sub.11 (s), 
G.sub.12 (s), G.sub.21 (s) and G.sub.22 (2), respectively, then we have 
the following relationship: 
##STR1## 
If the transfer functions have the following relationships: 
##EQU2## 
then Eqs. (1), (2) and (3) can be rewritten as follows: 
##EQU3## 
Eq. (4) shows a situation wherein the compensation .DELTA.V.sub.R * 
affects only the tension variation .DELTA.T while the compensation 
.DELTA.V.sub..theta. * affects only the looper angle variation 
.DELTA..theta.. In other words, if the transfer functions of the units 
17-20 satisfy the relationships given by Eq. (3), the above-described 
mutual interference can be eliminated. 
To prevent the transfer functions, respectively, of the main feedback loops 
(for controlling the looper drive motor 8 in response to the detected 
looper angle .theta. and for controlling the roll drive motor 10 in 
response to the detected tension) from becoming complicated, it is 
preferable that G.sub.11 and G.sub.22 are simple constants, e.g., 
EQU G.sub.11 =-1 
EQU G.sub.22 =+1 
The speed correction .DELTA.V.sub.R, which is the output from the 
noninterference operation unit 16, is added at an adder 10A to an initial 
speed set value V.sub.R0 and the sum is fed to the speed control device 
10, and is used to correct the speed of the roll drive motor 11. The speed 
correction .DELTA.V.sub..theta., which is another output from the 
noninterference operation unit 16, is fed to the speed control device 12, 
and is used to correct the speed of the looper drive motor 8. 
If the mutual interference is eliminated in the manner described above, the 
variations in the interstand tension can be substantially reduced. In 
addition, quick response in control is achieved. 
Where it is difficult to form operation units that exactly satisfy Eq. (3), 
that is, if it is difficult to form operation units with the transfer 
functions G.sub.11 (s), G.sub.12 (s), G.sub.21 (s) and G.sub.22 (s) 
satisfying Eq. (3), the arrangement may be alternatively such that the 
relations of Eq. (3) are approximately satisfied with a limited frequency 
range. Such an arrangement can result in similar effects. 
In the embodiment described above, the units 14 and 15 perform PI control 
operation. They may however be ones performing P, I, D (differential) 
control operation. In the embodiment described, the roll drive motor 11 
for the stand 2 positioned upstream of the looper is controlled, but the 
arrangement may alternatively be such that a roll drive motor for the 
stand 3 downstream of the looper is controlled. 
In the above embodiment, in which not only the looper drive motor but also 
the roll drive motor is controlled, has the additional advantage of 
eliminating the mutual interference. It should however be noted that the 
control of the roll drive motor as explained is not an essential element. 
The use of the tension deviation for the looper drive motor control is not 
an essential element, either. The essence of the invention resides in the 
control of speed rather than current of the looper drive motor and the 
primary merit thereof is simplification of the control system. That is, if 
the current control device 7 is used as shown in FIG. 1 to control the 
looper drive motor 8, there are disadvantages in that a circuit for 
compensation is needed for stabilizing the control system and the 
computation of an initial current set value is needed. In contrast, the 
present invention adopts the speed control of the looper, and is free from 
these disadvantages. In addition, since the speed of the looper drive 
motor 8 is zero when the looper 4 is stopped, the initial speed set value 
can be determined at 0.