US3550414A - Apparatus for stabilizing gagemeter control of rolling mills - Google Patents

Description

United States Patent 3,550,414 APPARATUS FOR STABILIZING GAGEMETER CONTROL OF ROLLING MILLS Harold A. List, 38 W. University Ave., Bethlehem, Pa. 18015 Filed Nov. 18, 1968, Ser. No. 776,599 Int. Cl. B211) 37/02, 37/08 US. Cl. 72-8 3 Claims ABSTRACT OF THE DISCLOSURE In order to stabilize a gagemeter control system for rolling mills the roll force deviation signal is partially delayed for a finite period sufiicient to allow reaction of the loopers to reestablish correct tension in strip in the mill after a change in screwdown has been initiated.

BACKGROUND OF THE INVENTION So-called gagemeter gage control systems such as basically disclosed in Pat. No. 2,680,978 to Hessenberg are widely used in the control of rolling mill stands. The performance of these gagemeter systems deteriorates as the strip thickness decreases and the strip width increases. As strip material is reduced in gage the load cell signals which are used to calculate the spring of the mill stands become more and more similar to the screw position signals required to compensate for the spring until with very thin material, and particularly with thin and wide material, the load cell signal changes by almost exactly the same amount with a change in screwdown as the screw position signal changes. When this somewhat ambiguous condition is reached sudden surges of feedback, such as occur when large corrections of gage are attempted, may, in combination with slight errors in adjusting the relative magnitude of the roll force and the screw position signals cause the screwdowns to run away, either in the up or the down direction. To minimize the tendency of a gagemeter stand to run away when adjusted to control gage closely for a thin and wide product it has been customary to adjust the ratio of the roll force and screw position signals to give substantially less screw movement than is desired to compensate for mill spring. The gage control performance is, however, thereby compromised since the strip gage rolled by the use of a gagemeter compensated in this manner is for example always slightly heavier than desired When a downward screw motion is required. In addition even with such a reduction of compensation, on many hot strip mills the gagemeter control must be turned off on the last one or two stands on light gage orders to avoid cobbles and wrecks.

I have discovered that a great deal of the instability of previous gagemeter control systems has arisen because of the relatively slow reaction of the associated looper control systems. Because the loopers comprise fairly massive mechanical systems the tension of the strip regulated by the looper control cannot be adjusted instantaneously. In a standard gagemeter system, the screwdown of the rolling stand will move down in response to disturbances such as local cold or other hard spots in the strip. In a tandem mill when the screws move down there is a temporary reduction in incoming tension. This leads to an unmediate increase in roll force in the rolling stand for two reasons: (1) there is a substantial increase due to the increased reduction and (2) there is a substantial increase due to the reduction in strip tension because the tension stress in the roll bite no longer aids the stand to make a reduction in strip thickness. The latter increment of increased roll force increases the screw movement beyond that required by the gagemeter control to remove all excess gage. This increment of roll force, however, is only 3,559,414 Patented Dec. 29, 1970 ice temporary until the looper has time to react and restore proper tension to the strip. Meanwhile, however, the additional screw movement and roll force incident to the looper delay may have thrown the system into an unstable condition and caused the screwdown to run away, or, if the increase has not been suflicient to initiate a runaway screwdown, it will cause over-correction of the screwdown, and probable hunting of the control system. As mentioned supra this tendency to instability and hunting of the system may be alleviated by adjusting the ratio of screwdown movement to indicated gage deviation so that the screw movement is sufiicient only to remove a portion, such as for example of the excess gage of the strip. This, however, is undesirable as it in effect means that much off-gage strip is produced.

SUMMARY OF THE INVENTION I have discovered that the foregoing disadvantages of a gagemeter system can be overcome by the employment of a time delay circuit to delay a portion of the roll force signal change for a time period sufiicient to allow the strip tension to be substantially restored to its desired regulated value after a change in screwdown has been initiated by the gagemeter system.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view of a gagemeter control system for the last stand of a tandem rolling mill according to the present invention.

FIG. 2 is a diagrammatic illustration of a suitable time delay circuit for the gagemeter control arrangement shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 a strip 11 is shown passing through the last two rolling stands 13 and 15 of a tandem rolling mill. Each of these two rolling stands comprises an upper work roll 17 and a lower work roll 19. A screwdown mechanism 21 is provided on each stand to move the upper rolls 17 to determine the opening between the rolls 17 and 19. A looper 23 is located between the rolling stands 13 and 15 to take up slack in the strip 11 as shown and to provide a looper position signal to a speed matching controller 25 through a summing point 27 which may comprise any suitable algebraic computer amplifier element. A predetermined reference signal representing the desired position of the looper is also directed from a reference signal generating means 26 to summing point 27 for comparison with the actual position signal. The speed matching controller 25 regulates the speed of main drive motor 29 which drives the rolls of rolling stand 15. The speed of rolls 17 and 19 of stand 15 is increased through the action of the looper control system if looper 23 detects excess slack between stands 13 and 15 and is decreased if looper 23 detects less slack and consequently more tension than is desired between stands 13 and 15.

A gagemeter control system 31 is provided for stand 15 comprising a load cell 33 positioned under the lower work roll 19 which load cell 33 directs a signal proportion to the separating force between the rolls to a single function control element 35 where it is converted to a mill deflection signal through the application of the standard mill deflection formula based on Hooks Law. The mill deflection signal is then passed into a time delay control element 37 where a portion of the signal is delayed a predetermined period sufiicient to allow reaction of the looper system to maintain proper tension before the signal is passed on as a compensated mill deflection signal to summing point 39 where the compensated mill deflection signal is algebracially added to a screw position signal derived from any suitable screw position detector and signal generator associated with the screwdown mechanism 21 and stand 13. The combined signals are also compared at the summing point 39 with a reference signal from a reference signal generating means 40 representing the desired gage of strip leaving the mill. Summing point 39 may comprise any suitable type of computer element operative to add the three signals together algebraically or otherwise suitably combine them to provide a gage error signal representative of the departure of the opening between the work rolls-and consequently the strip passing through the rolls-from the desired gage of the strip as it leaves the stand. The gage error signal is then directed to screw motor and control 41 which controls and operates the screwdown mechanism 21 to adjust the opening between the rolls 17 and 19 of stand to provide correct strip gage.

The mill deflection signal derived from single function controller is also directed before reaching time delay control element 37 to summing point 43 where it is combined with a portion of the screw position signal derived from screwdown mechanism 21 to provide a combined signal proportional to the actual opening between the rolls 17 and 19 of the rolling mill. The combined signal may be applied to any suitable gage or meter to represent to the operator the actual opening between the rolls of the stand and consequently the actual strip gage being rolled at any given time.

One suitable form of time delay control element 37 is shown in FIG. 2. The mill deflection signal from single function controller 35 shown in FIG. 1 is directed to a variable percent compensation potentiometer 45 which is adjusted to provide a signal compensated as to the amount of roll movement which will be made in response to a load cell signal of any given amount. The movement of the rolls may thus be controlled to provide any desired percentage of the total movement which is normally called for by any given load cell signal. This signal is then split between two variable signal distribution potentiometers 47 and 49 on legs 51 and 53 respectively of the time delay circuit. A variable capacitor 55 is connected to leg 53 of the time delay circuit and ground 57. Both legs of the circuit then lead to a summing amplifier 59 where the signals from legs 51 and 53 are combined, amplified and directed from time delay control element 37 to summing point 39 shown in FIG. 1.

When a mill deflection signal is received in time delay element 37 from single function computer element 35 the signal is brought to the desired strength by prior adjustment of the setting of variable potentiometer 45 and is then split between legs 51 and 53 of the time delay circuit. The proportion of the signal passing through each leg will be determined by the settings of distribution potentiometers 47 and 49. The signals then pass to summing amplifier 59 where they are recombined and passed on as a single amplified and compensated mill deflection signal to summing point 39. The portion of the mill deflection signal passing through leg 51 of the time delay circuit passes through the control without any change in its time dependent signal characteristics but the portion of the mill deflection signal passing through leg 53 of the time delay circuit is partially deflected into variable capacitor 55 until the capacitor 55 is fully charged. The length of time necessary to build up a full charge on capacitor 55 may be varied by varying the capacitance and is selected to be substantially the time necessary for the looper control to restore full tension after a change in screwdown has occurred. Since the buildup in the charge on capacitor 55 is grandual the increase in the signal passing through leg 53 of the time delay circuit will also be gradual. The total signal through both legs thus will quickly reach an initial value as the signal passes freely through leg 51 and will more gradually reach a final value along a decreasing curve as the remainder of the signal passes through leg 53 and the charge in capacitor 55 builds up. Preferably the variable potentiometers are adjusted so that the proportion of the signals which pass throu;gh the two legs of the time delay circuit are proportional to the amount of the total mill deflection signal which derives from the original error in strip gage and the amount of the total mill deflection signal which derives from the temporary increase in load in the mill as a result of temporary loss of strip tension before the looper mechanism can react to restore proper tension. While such a relation of the signals will provide the most desirable results, excellent results will also be had if the entire mill deflection signal is partially delay. This maybe done using the same simple time delay circuit shown in FIG. 2 merely by adjusting variable potentiometer 47 so that no, or substantially no, signal passes therethrough. This in effect provides a single rather than a split time delay circuit and the entire mill deflection signal passes through leg 53 of the time delay circuit so that substantially the entire signal is subjected to the effects of capacitor 55. This provides a single continuously increasing compensated mill deflection signal which increases exponentially at a decreasing rate until the capacitor 55 is fully charged. Such a simplified circuit may be desirable for special purposes. Other suitable time delay circuits and other possible locations of the time delay in the gagemeter control loop will occur to one skilled in the art but I have found that the illustrated arrangement provides the most desirable results.

The invention has been illustrated with respect to use on the last stand of a tandem rolling mill and will be found to be most useful on the last two stands of tandem rolling mills as these are normally the most unstable stands due to the thinner gage strip passing through them. The invention, however, may also be used advantageously on the other stands of a tandem mill. If the invention is used on one of the stands of the mill having a looper located on both sides of the stand rather than on the last stand of the mill the time delay of part of the mill deflection signal will compensate not only for the effects of the looper immediately preceding the mill stand but also for the effects of the looper following the mill stand and the time delay will be adjusted to have a period which will compensate for the delay of both loopers. Ordinarily, of course, this delay will be similar for both loopers and will occur simultaneously. The looper preceding the mill stand ordinarily has the greatest effect on the gage of the strip passing through the mill stand due to the fact that the strip passes directly to the bite of the roll on this side.

While the invention has been described with respect to operation during the downward movement of the screws of a rolling mill-since this is a more crucial operation it is also effective during upward movement of the screws of a rolling mill to delay the decay of the roll force signal until the loopers have an opportunity to react and decrease excess tension between the rolling stands resulting from the increase in gage of the strip. Too great an increase in tension between the rolling stands could result in necking down of the strip in a hot mill or the tearing of strip in a cold mill.

I claim:

1. An automatic gage control system for a rolling stand of a tandem rolling mill having a looper speed control means comprising:

(a) load cell means to provide a first signal proportional to the separating force between the rolls of said stand,

(b) means to provide a second signal nominally proportional to the separation between the rolls of the said stand,

(c) reference signal means to indicate the desired sep aration between the rolls of the stand by a third signal,

(d) means to convert said first signal into a fourth signal proportional to the increased separation between the rolls of the first rolling stand due to the yield of the mill under the separating force,

(e) means to combine said second signal with said fourth signal and balance against said third signal portion at least partially delayed proportional to the proto provide a fifth signal proportional to the error portion of the signal deriving from increased gage rebetween said desired roll position and said actual sultant upon loss of Strip tension between the rolling roll position; stands as the screwdown is operated to remove the original (f) means responsive to said fifth signal to operate the 5 error signal.

screwdown of the said rolling stand, and

(g) means to delay the full effects of the first signal References Cited from reaching said means (e) for substantially the UNITED STATES PATENTS time it takes the looper speed control system associated With said mill stand to react to restore tension 10 3,062,078 11/ 1962 Hulls 72205X lost between said rolling stand and any adjacent roll- 3,084,314 4/ 1963 Zlffer 318-28 ing stand when said screwdown is operated by means 3,170,101 2/ 1965 Stringer 6t 31 7216X (f). 3,357,217 12/1967 Wallace et .al. 728 2. The automatic gage control system according to 3,443,600 6/1969 Co e a1. 7 -8 claim 1 wherein the effects of said first signal are delayed 15 3,492,844 2/1970 Silva 728 by delaying said fourth signal by a delay circuit means.

3. The automatic gage control system according to MILTON MEHR: Primary Examiner claim 2 wherein said delay circuit means includes a split circuit in which the fourth signal is split into a completely undelayed porton substantially proportional to the amount 20 72 205, 16 of the signal derived from an original error signal and a UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3,550,414 D t d December 29, 1970 Invent r( Harold A. List It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading to the printed specification, after line 5, insert assignor to Bethlehem Steel Corporation, a corporation of Delaware Signed and sealed this 25th day of May 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents

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