Method and system for controlling a tandem rolling mill

Claim 2

In a control system for a tandem rolling mill comprising upstream and downstream rolling stands, said stands having separate roll opening controls and being operative, successively, to reduce the thickness of a strip of material passing through the mill, the combination comprising: means for producing strip speed signals V1, V2 and V3 variable, respectively, in accordance with the speed of the strip entering the upstream stand, the speed of the strip between the upstream and downstream stands, and the speed of the strip leaving the downstream stand, means for producing an entry gage signal G1a variable in accordance with the entry gage of the strip entering the upstream stand, first computer means receiving as input signals said strip speed signals V1 and V2 and said entry gage signal G1a for computing a predicted exit gage signal G1b for said upstream stand, means for controlling the roll opening of the upstream stand in accordance with said predicted exit gage signal G1b, second computer means receiving as inputs said strip speed signals V2 and V3 and an entry gage signal G2a for computing a predicted exit gage signal G2b for said downstream stand, said signal G2a being derived from said calculated exit gage signal G1b of said upstream stand, and means for controlling the roll opening of the downstream stand in accordance with said predicted exit gage signal G2b.

Claim 3

However, to eliminate the transport lag effect the signal G1b is routed to the computer unit 41 by leads 42 and 43 through an additional buffer storage unit 44. This unit delays the entry gage signal G2a for an interval corresponding to the time required for the strip increment measured by X-ray gage 27 to pass from the roll bite of stand 1 to the roll bite of stand 2 and in this manner eliminates the transport lag which would otherwise occur. To enable the buffer storage unit 44 to provide the desired time delay function automatically it receives a timing signal in lead 36a from the tachometer generator 36. The input quantity V2 is also provided by the tachometer generator 36 via lead 45 and the remaining input quantity V3 is provided by an additional tachometer generator 46 via lead 47. To enable the tachometer generator 46 to provide an output signal proportional to the quantity V3 it is driven by a roller 48 which engages and is rotated by the strip downstream with respect to the roll bite of stand 2. The output signal G2b from computer unit 41 is fed to a comparator unit 49 via lead 50 where it is compared with a desired exit gage signal from a gage set 51 which is manually set by an operator. Any difference between the predicted exit gage signal G2b from stand 2 and the desired exit gage signal is fed as an error signal on lead 20 to the screw control unit 19 of stand 2 to cause adjustment of the roll opening of stand 2 in a proper direction to reduce the error signal to zero. Thus, in a manner similar to the control of stand 1, the AGC system for stand 2 has a separate control loop into which a feedback signal is introduced which varies in accordance with the predicted gage of strip 10 as it leaves the roll bite of stand 2. In the case of the two stand tandem mill illustrated this is also the predicted exit gage of the strip leaving the mill. For various reasons, the calculations of the predicted strip exit gages from stands 1 and 2 by the computer units 39 and 41 may not always be 100 percent accurate. For example, the diameters of the rolls 34, 37 and 48 which drive the speed signal tachometers 33, 36 and 46 by not be accurately known or they may change due to wear. To correct for small system control errors of this nature there is provided a system for monitoring the actual measured gage of the strip leaving the mill and making any required corrections by a vernier type of adjustment to the roll opening control of stand 2. For this purpose there is provided a second X-ray gage 52 having an upper portion 52a producing X-rays and a lower portion 52b comprising an X-ray sensor between which the strip 10 passes after leaving the roll bite of stand 2. The X-ray gage 52 produces an exit gage signal proportional to the thickness or gage of a measured increment of the strip as it leaves the roll bite of stand 2. These measurement signals are transmitted to a monitor unit 53 via lead 54. By a known process of integration the monitor averages the exit gage thickness signal and transmits it to the comparator unit 49 via lead 55. If there is any deviation between the actual exit gage and the desired exit gage as determined by the setting of gage set unit 51, the set point of the screw control unit 19 of stand 2 is automatically adjusted in a direction to correct for the exit gage error. Because of the averaging action of the monitor unit 53 the effect of any transport lag between the roll bite of stand 2 and the X-ray gage 52 is eliminated. Also, it should be understood that because of the relatively slow action of the monitor system the time constant of the feedback signal on lead 55 is long as compared with the time constant of the predicted feedback gage signal on lead 50 so that it does not affect the stability of the AGC system controlling the roll opening of stand 2. From the foregoing, it will now be apparent that there is provided, by this invention, a control system for a tandem rolling mill that eliminates the effect of transport lag without direct coupling of the AGC systems controlling the roll openings of adjacent upstream and downstream stands. This is accomplished by providing separate AGC systems for each stand with each system utilizing as its primary feedback control signal the predicted gage of the strip leaving the stand. To implement this concept calculation of the downstream stand exit gage utilizes, as an input, the calculated exit gage of the upstream stand with buffer storage delay to eliminate the effect of transport lag caused by the time required for a measured strip increment to pass between the roll bites of the adjacent stands. Because the adjacent stands are not directly coupled by the AGC controls the tension regulator 25 can function in a normal manner to control interstand strip tension without any instability caused by interaction between the strip tension and gage controls. While the invenTion has been illustrated as applied to a tandem rolling mill having two stands, the control principle is applicable to mills having a greater number of stands. Thus, for example, if the mill has three stands there will be provided an additional AGC control system for stand 3 similar to that illustrated for stand 2 and the X-ray gage 52 will be located downstream with respect to stand 3 with the gage correction signal on lead 55 applied to the screw control of stand 3 rather than stand 2. Also, the system may be used with either analog or digital type of control. For clarity of illustration the computer units 28 and 41 and the buffer storage units 30 and 44 have been shown as separate components. In actual practice they can be combined with and form a part of the process control computer which is programed to adjust the mill controls automatically. While there has been shown what is presently considered to be a preferred embodiment of the invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the invention. What is claimed as new and desired to be secured by Letters Patent of the United States is:

Claim 4

The control system of claim 1 wherein the calculation of the signal G1b is in accordance with the equation G1b G1a V1/V2 and the calculation of the signal G2b is in accordance with the equation G2b Glb V2/V3

Claim 5

The control system of claim 1 including delay means for receiving the exit gage signal G1b and providing, as an output signal, the entry gage signal G2a which is proportional, but delayed in time, with respect thereto, said delay means being controlled by the strip speed signal V2 to determine the amount of time delay.

Claim 6

The control system of claim 3 including means for measuring the gage of the strip delivered from the downstream stand and producing an average correction signal variable with deviation of the exit gage from a predetermined value and means controlled by said correction signal for modifying the control of the roll opening of the downstream stand to correct for measured errors in the gage of the strip delivered from the downstream stand.

Claim 7

The control system of claim 4 including means for regulating the tension of the strip between the uPstream and downstream stands.