US3765203A

US3765203A - Automatic gauge control by tension for tandem rolling mills

Info

Publication number: US3765203A
Application number: US00230299A
Authority: US
Inventors: R Peterson
Original assignee: Westinghouse Electric Corp
Current assignee: AEG Westinghouse Industrial Automation Corp
Priority date: 1972-02-29
Filing date: 1972-02-29
Publication date: 1973-10-16
Anticipated expiration: 1990-10-16
Also published as: BE796055A; JPS5021429B2; JPS48100361A; CA968872A

Abstract

Method and apparatus for varying the gain of an automatic gauge control loop for rolling mills wherein gauge is controlled by varying tension in the rolled strip between the last two stands of a tandem rolling mill. The gain of the loop is varied as a function of transport time between the bite of the rolls in the last stand and a thickness gauge positioned beyond the last stand. At low mill speeds (i.e., long transport times), the gain of the loop is maintained low and varied as a function of the cross-sectional area of the strip between the last two stands. At high mill speeds (i.e., short transport times), the gain of the loop is increased and varied as a function of both crosssectional area and the speed of the last stand.

Description

United States Patent .[1'91 7 Peterson Oct. 16, 1973 1 AUTOMATIC GAUGE CONTROL BY TENSION FOR TANDEM ROLLING MILLS [75] inventor: Robert S. Peterson, Williamsville,

[22] Filed: Feb. 29, 1972 [21] Appl. N0.: 230,299

[52] US. Cl. 72/9, 72/12 [51] int. Cl. B21b 37/02 [58] Field of Search 72/8, 9, 10, 11, 72/16, 12

[56] References Cited UNITED STATES PATENTS 3,049,036 8/1962 Wallace et al. 72/9 7/1962 Wallace et a1. 72/9 3,492,844 2/1970 Silva ..72/8 3,158,049 11/1964 Huntley ..72/8

Primary Examiner--Milton S. Mehr Attorney-F. H. Henson et al.

[5 7] ABSTRACT Method and apparatus for varying the gain of an automatic gauge control loop for rolling mills wherein gauge is controlled by varying tension in the rolled strip between the last two stands of a tandem rolling mill. Thegain of the loop is varied as a function of transport time between the bite of the rolls in the last stand and a thickness gauge positioned beyond the last stand. At low mill speeds (i.e., long transport times), the gain of the loop is maintained low and varied as a function of the cross-sectional area of the strip between the last two stands. At high mill speeds (i.e., short transport times), the gain of the loop is increased and varied as a function of both crosssectional area and the speed of the last stand.

8 Claims, 4 Drawing Figures FIG] SPEED REGULATOR TENSION REGULATOR 22 AUTO MATIC v GAUGE CONTROL TENSION REFERENCE GAUGE REFERENCE PATENTEDUBT 1 sum 'SHEEI 10F 2 AUTOMATIC GAUGE CONTROL BY TENSION FOR TANDEM ROLLING MILLS CROSS REFERENCES TO RELATED APPLICATIONS Application Ser. No. 230,298 and application Ser. No. 230,300 both filed concurrently herewith and assigned to the assignee of the present application.

BACKGROUND OF THE INVENTION As is known, the gauge of the strip material passing through a tandem rolling mill can be varied by varying the tension in the strip between the last two stands in the mill. Actual tension between the last two stands is compared with desired tension for a specified gauge;

and if the two are not the same, the speed of the last stand is varied. This changes the mill stretch of the last stand which changes the roll gap between the last stand work rolls until the strip is at the desired gauge.

In a tension gauge control loop of this type, the strip material, after passing through the bite of the rolls in the last stand, must travel to a thickness gauge positioned, for example, about five feet beyond the bite of the rolls. Thus, a deviation in gauge from desired gauge is not detected until the strip material has traveled five feet to the thickness gauge which then develops an error signal used to take corrective action. This gives rise to what is known as transport time required for the strip to travel between the bite of the rolls and the thickness gauge. At low speeds and long transport times, the response time or gain of the control loop should be low. Otherwise, because of the long time delay between detection of a gauge error and correction, a high gain system would become unstable (i.e., oscillate). At high speeds, on the other hand, the response time or gain of the control loop can and should be increased to achieve better gauge control.

In the past, automatic gauge control loops of this type did not provide for a change in gain with changes in the product being rolled or the operating speed of the mill. The automatic gauge control loop was adjusted for the worst possible condition which is for large strip crosssectional area and low mill speed operation, on the order of about 500 feet per minute. Below this mill speed of 500 feet per minute, the automatic gauge control loop could not be utilized; and at high mill speeds and/or small strip cross-sectional areas, the response of the automatic gauge control loop was very slow.

SUMMARY OF THE INVENTION In accordance with the present invention, a method and apparatus are provided for controlling gauge at the output of a tandem rolling mill by varying the tension between the lasttwo stands in the mill, and wherein the gain of an automatic gauge control loop for varying the last stand speed is varied as a function of strip crosssectional area at low speeds and as a functionof strip cross-sectional area and of the speed of the mill at high speeds. In this manner, the gain of the loop gradually varies upwardly as the speed of the mill increases, thereby insuring optimized gain characteristics under all operating conditions.

Specifically, the invention involves l) measuring the gauge of strip material issuing from the last stand of a tandem rolling mill at a point removed from the last stand and producing a signal proportional to the actual measured gauge, (2) comparing the actual gauge signal with a desired gauge signal as determined by an operator to derive a gauge deviation signal for varying the tension between the last two stands by varying the speed of the last stand, (3) multiplying the gauge deviation signal by the cross-sectional area of the strip material being rolled to derive a first error signal, (4) multiplying the first error signal by the speed of the last stand to derive a second error signal, and (5) summing said first and second error signals and utilizing the sum to control the speed of the last stand and, hence, the tension between the last two stands.

At low strip speeds, the first error signal is effective to control the last stand speed and, hence, tension between the last two stands. On the other hand, at low speeds, the second error signal is negligible since it is the product of the last stand speed (which is small and attenuates the error signal) and the gauge deviation signal. At high mill speeds, the second error signal which is the product of speed and gauge deviation is greater than the first error signal and controls the gain of the loop at these high speeds.

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification, and in which:

FIG. 1 is a schematic diagram of a tandem rolling mill operation incorporating the automatic gauge control system of the present invention in block form;

FIG. 2 is a Bode plot of rolling speed versus gain, showing the manner in which the gain increases as a function of speed in accordance with the system of the invention;

FIG. 3 is a detailed block diagram of the automatic gauge control system of the invention; and

FIG. 4 illustrates in greater detail the gauge control system of the invention.

With reference now to the drawings, and particularly to FIG. 1, a five-stand tandem rolling mill is shown including five stands S1, S2, S3, S4 and S5. The strip inaterial 10 to be rolled passes between the rolls of the successive stands 31-55 and is progressively reduced in gauge while the speed of the strip material increases at the output of each stand. The rolls for each of the stands are provided with drive motors, only the motors M4 and M5 for stands S4 and S5 being shown in FIG. 1. Motors M4 and M5 are controlled by speed regulators SR4 and SR5, respectively, which receive a master speed reference signal on lead 12 from a master mill speed controller, not shown. Suitable screwdown mechanisms and controls therefor are provided for each of the stands 51-85. In accordance with the present invention, the spacings between the work rolls of the last stand S5 are not varied by the screwdown control during a rolling operation, the final output gauge being controlled by varying stand S5 speed to vary the tension between the last two stands as explained above.

The gauge of the strip material 10 issuing from the last stand S5 is measured by an X-ray gauge 14 or the like which produces a signal on lead 16 proportional to actual gauge. The signal from X-ray gauge 14 is compared at summing point 18 with a gauge reference signal on lead 20 determined by the operator of the mill, or possibly by a computer; this gauge reference signal being proportional to the desired output gauge. If the desired gauge signal on lead 20 is not equal to the actual gauge signal on lead 16, an error signal is developed which is then converted to a percent error signal (volts per percent) based on the desired delivery gauge, and is then applied to an automatic gauge control circuit 22, hereinafter described in detail.

Also applied to the automatic gauge control circuit 22 is a signal derived from a tachometer or pulse generator 23. This signal is proportional to the circumferential speed of the rolls in the last stand S5 and, hence, the speed of the strip material issuing from the mill. The output signal from the automatic gauge control circuit 22 is then summed at summing point 24 with a tension reference signal on lead 26 and with an actual tension signal on lead 28 derived from a tensiometer 30 in engagement with the strip material between the last stands S4 and S5. The signal from the gauge control circuit 22 and the tension reference signal 26 are summed and compared in subtractive relationship at point 24 with the actual tension signal from tensiometer 30. The resulting signal is then applied as an error signal to a tension regulator 32, the details of which may be had by reference to copending application Ser. No. 230,300, filed concurrently herewith. Note that the tension reference signal is also applied to the automatic gauge control circuit 22 for a purpose which will hereinafter be described in greater detail.

As was explained above, it is necessary in accordance with the present invention to control the gain of the automatic gauge control loop at low speeds as a function of the cross-sectional area of the strip material being rolled. Accordingly, means must be provided for determining the cross-sectional area of the strip material 10 between the last two stands. To this end, the gauge of the strip material between stands S1 and S2 is measured by X-ray gauge 34 and applied to circuit 36 along with signals from tachometer generators or

pulse generators

38 and 40. Tachometer generator 38 is connected to the rolls of the stand S1 and, hence, produces an output signal proportional to the speed of stand S1; whereas tachometer generator 40 is connected to the rolls of stand S4 and produces an output signal proportional to the speed of stand S4. X-ray gauge 34, of course, produces a signal proportional to the thickness of the strip material between the first and second stands.

It is known that the width of strip material being rolled does not vary materially in passing from one stand to the next. Furthermore, it is known in accordance with the constant volume principle that the volume of material entering the bite of the rolls ofa rolling mill is equal to the volume of material leaving. That is,

where:

G, and G the gauges of the strip material entering and leaving the second stand S2, for example; V, and V the velocities of the strip material entering and leaving the second stand S2; and W the width of the strip material. Consequently, by knowing the gauge of the strip material between the first and second stands, the speed of the first stand, and the speed of the fourth stand, the area, A4, of the strip material between the fourth and fifth stands S4 and 85 can be determined from the equation:

Circuit 36, therefore, performs this computation and derives a signal on lead 42 proportional to A4, the area of the strip meterial between stands S4 and S5. This signal on lead 42 is applied to the automatic gauge control circuit 22 as well as the tension regulator 32.

The details of the automatic gauge control circuit are shown in FIGS. 3 and 4. The gauge deviation signal from summing point 18 is applied to error compensation amplifier 44 which produces a linear output signal variable above and below the zero axis, depending upon the polarity of the deviation signal, and is limited at points above and below the zero axis as shown by the transfer characteristics on the block 44 of FIG. 3. The output signal from block 44, comprising a signal on lead 46 proportional to deviation from desired gauge, is then multiplied in multiplier 48 with the signal on lead 42 proportional to the area A, between the last stands S4 and S5. The output of the multiplier 48 is then applied to a second multiplier 50 where it is multiplied with a signal V,, on lead 52 from the tachometer generator 23.

The signal at the output of multiplier 48 is applied through a potentiometer K2 to a correction amplifier 54; While that at the output of multiplier 50 is applied through potentiometer Kl to the correction amplifier 54. Note that the amplifier 54 has a tension reference signal applied thereto via lead 56 for a purpose hereinafter described.

The output of the amplifier 54, comprising the original gauge deviation signal as modified by

multipliers

48 and 50, is applied to the summing point 24. Assuming that the tension signal from tensiometer 30 matches the tension reference signal on lead 26 or that the gauge of the issuing strip material has deviated from desired gauge, the summation of the signals at point 24 will produce an error signal which is applied to tension regulator 32 to either increase or decrease the speed of the last stand S5 via speed controller SR5. It should be understood, however, that the error signal can be applied directly to the speed controller without an intervening tension regulator, if the advantages of the tension regulator, described in the aforesaid copending application Ser. No. 230,300 can be sacrificed. In effect, an error signal from the automatic gauge control circuit 22 varies the tension reference signal as applied to summing point 24. This, in turn, changes the strip tension which causes a change in the roll force of the last stand which brings about a change in the mill stretch of the last stand. By changing the mill stretch in the last stand S5, the roll gap is changed to bring the strip on gauge. At the same time, the change in delivery stand speed to provide the required tension between the last two stands to bring the strip on gauge will give the correct speed relationship between the delivery stand S5 and the other stands to maintain the strip on gauge.

The details of the

amplifiers

44 and 54 and the potentiometers K1 and K2 are shown in FIG. 4. Amplifier 44 comprises an operational amplifier 58 having two feedback paths including, respectively, a resistor 60 and limiter 62 which limits the maximum output of the amplifier above and below the zero reference. One input to the operational amplifier 58 is connected through

resistors

64 and 66 to the summing point 18; while the other input to the amplifier 58 is connected through resistor 68 to ground. The opposite ends of the resistor 64 are connected through capacitor 70 and resistor 72 to ground as shown.

Each of the potentiometers K1 and K2 is provided with a movable tap connected through

resistors

74 and 76, respectively, to a summing point 78. Point 78 is connected through

resistors

80 and 82 to ground. Point 78 is also connected to one input of operational amplifier 84; while the other input to the operational amplifier 84 is connected through resistor 86 to ground. The amplifier 84 is provided with a feedback path including resistor 88 and capacitor 90. Also connectedv between the input and output of amplifier 84 is a variable limiter 94 which, in response to a tension reference signal on lead 96, limits the maximum output of the circuit 54 as applied to the tension regulator 32 so as not to exceed a maximum tension value and possibly cause a break in the strip. In shunt with multiplier 48 is a potentiometer K3, the purpose of which will hereinafter be described.

In the operation of the invention, the automatic gauge control loop response at low threading speeds is determined by the potentiometer gain setting K2, which is a signal proportional to the gauge deviation signal multiplied by the cross-sectional area of the strip between stands S4 and S5. As will be appreciated, the signals from potentiometers K1 and K2 are summed at point 78 before being applied to circuit 54. However, at low speeds, the signal from potentiometer K1 is negligible since the signal at the output of multiplier 50 is derived by multiplication by the speed V, of stand S5 (which is small at low speeds and attenuates this control signal).

Since total tension in pounds is being controlled and not pounds per square inch, which is the variable which directly affects strip gauge and not total tension, the fixed plant of the automatic gauge control loop has a gain which varies inversely with the cross-sectional area between the last two stands. As'will be appreciated, this gain variation in the fixed plant is compensated for by multiplexing the gain inthe automatic gauge control circuit 22 by the cross-sectional area between stands S4 and S5. Since the variation in strip cross-sectional area can be as high as it is important that this gain compensation be included in a tension-type automatic gauge control loop. This latter feature adapts the loop to strip product variation.

At higher speeds (i.e., 500 feet per minute and above), the signal at the output of the multiplier 50 becomes significant and in conjunction with the output of multiplier 48 controls the amplifier 54. Hence, at low speeds, the gain of the gauge control loop is controlled by the product of the gauge deviation signal and strip cross-sectional area; whereas at very high speeds, above 2000 feet per minute, the gain of the loop is controlled primarily by the product of gauge deviation times cross sectional area, times speed of the strip.

At very light gauges, it is desirable to increase the gain of the loop over that which would be derived from the output of multiplier 48. Hence, a third potentiometer K3 is connected in shunt with the multiplier 48 and is adjusted, when rolling light gauges, to bypass the signal around multiplier 48.

The remaining strip variable of the fixed plant which can vary is the strip hardness gain function K Since very little reduction is taken on the last stand of a tandem rolling mill (approximately 5 percent), and since the strip hardness has probably reached its maximum value before the strip enters stand S5, this gain factor K, can be neglected in many cases. However, if it is desired to introduce the gain factor K,, it can be introcontributed to the control loop by the speed regulator on the last stand. Hence, at high speeds, a plot of loop gain versus speed is that shown by curve 100 in FIG. 2. At low speeds, the transport time delay between the delivery gauge 14 and stand S5, which is approximately 7 2 seconds, limits the crossover frequency of the automatic gauge control loop to approximately 0.5 radian per second, resulting in curve 102 as shown in FIG. 2.

The present invention thus provides a means for controlling the gain of an automatic gauge control loop for tandem rolling mills utilizing gauge control by tension wherein the gain of the loop is varied as a function of the cross-sectional area of the strip entering the last stand at low speeds, and by the speed of the last stand in addition to the cross-sectional area of the strip entering the last stand at high speeds. Although the invention has been shown in connection with a certain specific embodiment, it will be readily apparent to those skilled in the art that .various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention.

I claim as my invention:

1. In the method for controlling final output gauge of strip material passing through a tandem rolling mill by varying the tension in the strip material between the last two stands in said tandem rolling mill, the steps of:

measuring the gauge of strip material issuing from said last stand at a point removed from the last stand and producing a signal proportional to the actual measured gauge,

comparing said actual gauge signal with a desired gauge signal to derive a percent gauge deviation signal for varying the tension between the last two stands by varying the speed of said last stand, electrically calculating the cross-sectional area of the strip material between said last two stands and modifying said gauge percent deviation signal as a function of the cross-sectional area of the strip material thus calculated to derive a first error signal,

modifying said first error signal as a function of the speed of said last stand to derive a second error signal, and

combining said first and second error signals and controlling the speed of said last stand as a function of the combined error signals.

2. The method of claim 1 wherein said combined error signals are added to a tension reference signal and compared with a signal proportional to actual tension in the strip between the last two stands to derive a signal for controlling the speed of said last stand.

3. The method of claim 1 wherein said gauge deviation signal is multiplied by the cross-sectional area of the strip material between said last two stands to derive said first error signal and said first error signal is multiplied by the speed of said last stand to derive the second error signal.

4. The method of claim 1 wherein said gauge percent deviation signal is summed with said first error signal to produce a third error signal, and including the steps multiplying said third error signal by the speed of said last stand to derive said second error signal, and running said second and third error signals and using the sum to control the speed of said last stand.

5. The method of claim 1 including the step of producing a signal proportional to the cross-sectional area of the strip material between said last two stands by multiplying the gauge of the strip material between the first two stands in said tandem rolling mill by the speed of strip material as the output of the first stand in the rolling mill, and dividing the product by the speed of the fourth stand in the tandem rolling mill.

6. Apparatus for controlling the final output gauge of strip material passing through a tandem rolling mill by varying the tension of the strip material between the last two stands in said mill, comprising an automatic gauge control loop including means for measuring the gauge of strip material issuing from said last stand at a point removed from the last stand and for producing a signal proportional to the actual measured gauge, means for comparing said actual gauge signal with a desired gauge signal to derive a gauge percent deviation signal based on desired delivery gauge, means for varying the speed of said last stand to thereby vary the tension between the last two stands of the tandem rolling mill, means for generating a cross-sectional area signal which varies as the cross-sectional area of the strip material between the last two stands of the mill varies, means for generating a signal which varies as a function of the speed of the last stand in the mill, means for modifying said gauge percent deviation signal as a function of variations in said cross-sectional area signal and said speed signal, and means for applying said modified gauge percent deviation signal to said means for varying speed whereby the speed of the last stand will be modified in response to changes in both strip crosssectional area and strip speed.

7. The apparatus of claim 6 wherein said means for modifying comprises means for multiplying said gauge percent deviation signal by the cross-sectional area of the strip material between said last two stands to derive a first error signal, means for multiplying said first error signal by the speed of said last stand to derive a second error signal, and means for combining said first and second error signals and for applying the combined error signals to said means for varying speed.

8. The apparatus of claim 6 including a speed regulator for said last stand, tension regulating circuit means having its output connected to said speed regulator, and means for applying the modified gauge percent deviation signal to said tension regulating circuit means. a

Claims

1. In the method for controlling final output gauge of strip material passing through a tandem rolling mill by varying the tension in the strip material between the last two stands in said tandem rolling mill, the steps of: measuring the gauge of strip material issuing from said last stand at a point removed from the last stand and producing a signal proportional to the actual measured gauge, comparing said actual gauge signal with a desired gauge signal to derive a percent gauge deviation signal for varying the tension between the last two stands by varying the speed of said last stand, electrically calculating the cross-sectional area of the strip material between said last two stands and modifying said gauge percent deviation signal as a function of the cross-sectional area of the strip material thus calculated to derive a first error signal, modifying said first error signal as a function of the speed of said last stand to derive a second error signal, and combining said first and second error signals and controlling the speed of said last stand as a function of the combined error signals.

6. Apparatus for controlling the final output gauge of strip material passing through a tandem rolling mill by varying the tension of the strip material between the last two stands in said mill, comprising an automatic gauge control loop including means for measuring the gauge of strip material issuing from said last stand at a point removed from the last stand and for producing a signal proportional to the actual measured gauge, means for comparing said actual gauge signal with a desired gauge signal to derive a gauge percent deviation signal based on desired delivery gauge, means for varying the speed of said last stand to thereby vary the tension between the last two stands of the tandem rolling mill, means for generating a cross-sectional area signal which varies as the cross-sectional area of the strip material between the last two stands of the mill varies, means for generating a signal which varies as a function of the speeD of the last stand in the mill, means for modifying said gauge percent deviation signal as a function of variations in said cross-sectional area signal and said speed signal, and means for applying said modified gauge percent deviation signal to said means for varying speed whereby the speed of the last stand will be modified in response to changes in both strip cross-sectional area and strip speed.

8. The apparatus of claim 6 including a speed regulator for said last stand, tension regulating circuit means having its output connected to said speed regulator, and means for applying the modified gauge percent deviation signal to said tension regulating circuit means.