US 3478551 A
Description (OCR text may contain errors)
Nov. 18. 1969 c. F. ALSOP 3,478,551
CONTROL SYSTEMS Filed May 4, 1967 2 Sheets-Sheet 1 N Q mulllllillllllllllilllllllllllfl Q2655 o @zEmw "Em d8 mm mm Q mm A W 5&6 2mm 4 I I. N l. 3: MW ,mfiwfiz :2: $3 m: mm X mw mm R Qm & Q 8 Fl i I I i l l I I l l I I I I i i I I I l l i ll C. F. ALSOP CONTROL SYSTEMS Nov. 18, 1969 Filed May 4, 1967 2 Sheets-Sheet 2 M Q 7 l I l I l l I I l l l I I Q2353 $255 m new :8 9 u .8 m \\\Nk \I\! P 535% qfiz 1 QZRWDEQQ mm XII .w IWQ m5 :3 m M m Xv a t \g x QM a r I l I l I I l I I I I I l l I I I l l I i I I I I United States Patent 3,478,551 CONTROL SYSTEMS Charles Francis Alsop, Baslow, Bakewell, England, assignor to Davy and United Instruments Limited, Sheffield, Yorkshire, England Filed May 4, 1967, Ser. No. 636,109 Claims priority, application Great Britain, May 6, 1966, 20,138/ 66 Int. Cl. B21b 37/12 US. Cl. 728 8 Claims ABSTRACT OF THE DISCLOSURE This invention relates to control systems for controlling rolling mills. It is particularly concerned with such a control system, in which the roll gap setting is controlled by signals generated in dependence on the rolling load and the roll gap setting, in order to maintain the gauge of the material leaving the rolling mill substantially at a constant value. By roll gap setting is meant the roll gap under zero rolling load conditions, the roll gap setting being measured in a conventional mill stand by an indicator coupled to the screws or other device supplied for controlling the roll gap.
As described for example in the paper by R. B. Sim-s and P. R. A. Briggs in Sheet Metal Industries March 1954, the rolling load signal (F) and the roll gap setting (So) are combined according to the equation M being the spring coefficient of the mill and h the gauge required of the strip leaving the mill stand. The error signal is applied to control the device adjusting the roll gap setting.
Errors in the thickness of the work leaving the rolling mill stand may arise from two sources. The first is the non-uniformity of the work entering the mill; the thickness, hardness and temperature may vary. The second is the mill stand itself, which, because of the eccentricity of the rolls, the non-circularity of the rolls and the nature of the oil film bearings for the rolls, may cause cyclic variations in the roll gap dimension at the frequency of revolution of the rolls, or a harmonic thereof.
The control system referred to above is designed to compensate for thickness variations arising from the work itself (referred to hereinafter as work variations) and operates to produce, on the detection of a roll load change in a given direction, a further change in rolling load in the same direction; for example, if the thickness of the strip entering the mill increases, the fact is detected by a rise in the rolling load and causes a compensating decrease in the roll setting which in turn causes a further increase in the rolling load, the sum of the aggregate change of rolling load divided by the spring coefiicient (M) and the aggregate change of roll setting being zero for constant outgoing thickness. The rolling load feed back-loop through the material being rolled the rolling load detector, the computing system and the roll gap setting adjusting device is thus a positive feed backloop.
If now this control system is faced with thickness variations arising from the mill (hereinafter called cyclic variations, the system operates to accentuate the thickness errors imprinted in the outgoing strip. For example, if due to eccentricity, the roll gap decreases with the effect of reducing the material thickness, a rise in rolling load is detected and the control system responds to the detected change by decreasing the roll gap and thus further reducing the material thickness. If the response ice of the control system is sufficiently high therefore, the system will not only not compensate for cyclic variations but will accentuate their effect on the thickness of the strip leaving the mill. The higher the response of the control system is made, the greater are the thickness variations produced in the strip by cyclic variations, and it is this factor that has imposed a limit on the system response in the past and hence the efiiciency with which the system can compensate for work variations.
In a conventional mill stand, the roll gap adjusting rolling mill stand, in which signals are generated in accordance with the rolling load and the roll gap setting and these signals are used jointly to control means for adjusting the roll gap setting, has a path for the rolling load signal to the roll gap adjusting means such that a negative feed-back control loop is constituted for rolling load variations at frequencies of the same order as the cyclic variations, but a positive feed-back control loop is constituted for rolling load variations at lower frequencies.
In a conventional zmill stand, the roll gap adjusting means are the screwdowns which act between the housings and the roll ends. In other mill stands, however, the roll gap adjusting means may take other forms; for example they may be constituted by wedges between the housings and rolls; or, in the case of prestressed mills, =by hydraulic cylinders which apply an external force to put the stand parts into compression or tension, and which are adjustable to vary the elongation of the stand and thus the roll gap; or, by position regulated jacks acting on the rolls in place of the conventional screwdowns. The roll gap adjusting means usually consist of two devices, one acting on each side of the mill, and actuating mechanisms for those devices and it is desirable to crosscouple the two devices, or their actuating mechanisms, to ensure that the two devices move together uniformly; thus a signal, representing the position and/ or velocity of the device, may be taken from each device or actuating mechanism and fed into the complementary device or mechanism so that both devices are maintained in close synchronism both dynamically and statically.
In a preferred form of the invention, the "automatic control system comprises means for generating a signal in accordance with the rolling load of the stand, means for generating a signal in accordance with the roll setting, a first path for applying the rolling load signal to control the roll gap adjusting means in a direction such that any change in rolling load gives rise to a change in roll setting causing a change of the rolling load in the same direction, a second path in parallel with the first path for applying the rolling load signal to the adjusting means in a direction such that a change in rolling load causes a change in rolling load in the opposite direction, means for applying the roll setting signal to the adjusting means in conjunction with the rolling load signal of the path, and frequency sensitive filter means to render the first path substantially unetfective at frequencies of the same order as the cyclic variations, the arrangement being such that, for variations at frequencies of the same order as the cyclic variations, the adjusting means is controlled to maintain the rolling load substantially constant, while, at lower frequencies, the adjusting means is controlled jointly by the roll setting signal and rolling load signal to compensate substantially for work variations.
By the invention, it is possible simultaneously to have a high system response and to reduce the gauge variations resulting from eccentricity.
The invention will be more readily understood by way of example from the following description of a rolling mill gauge control system in accordance therewith reference being made to the drawing accompanying the pro companying drawing (FIGURE 3), in which:
FIGURE 1 schematically illustrates a rolling mill for strip metal,
FIGURE 2 schematically illustrates the control svstem. and
FIGURE 3 shows a modification of FIGURE 2.
.In FIGURE 1, the rolling mill 12 is schematically illustrated by a pair of rolls 13, and screws 14 driven by a motor 15 to adjust the gap between the rolls and hence the reduction performed on the strip 16 passing between the rolls. The roll gap setting is measured by a potentiometer 16 or other position transducer, coupled to the screws, while load cells 17 or other force transducers interposed between the screws 14 and the roll chocks measure the rolling load. The roll gap setting is the gap between the rolls for zero rolling load and can be conveniently measured in a conventional mill by the potentiometer 16; in a prestressed mill, in which certain mill components are subject to a force other than the rolling load, the potentiometer signal may require to be adjusted to take account of the change in the roll gap due to that other force.
While, for simplicity, screws are shown for adjusting the roll gap setting, other adjusting mechanisms such as wedges, may be used, a position transducer being coupled to the mechanism in the manner of the potentiometer 16. In the conventional mill illustrated, the two screws 14 are coupled together so as to be equally operated by motor 15. In a prestressed mill, the roll gap adjustment may be effected by more than two screws or like devices, which may or may not be commonly driven or by variation of the prestress pressure. In all cases, however, the roll gap adjusting means at the two sides of the mill stand must be connected together so as to move uniformly, by actuating them from a common drive, by connecting together the drives when independent, by having a common supply of liquid under pressure in the case of a prestressed mill or by electrically cross-connecting the drives, specially when hydraulic drives are employed. By cross-connecting the drives is meant that a signal, representing position and/ or velocity of the adjusting device, is taken from each device and is fed into the control for the complementary device, in order that both devices may be maintained in close synchronism both dynamically and statically.
Turning to FIGURE 2, the signals from the load cells 17 are summed to give an average value for the rolling load F and are applied in parallel to two paths. The first path contains a circuit 20 which modifies the load signal by a factor K. The modified signal is applied through an adding circuit 21 where the roll gap setting signal is added in, a further adding circuit 22, a limiting circuit 23 and an integrating circuit 24, to a further adding circuit 25, the output from which is applied through a switch 26 to control the screw motor 15. A feed-back path is supplied by the line 27 from the output of integrator 24 to the adding circuit 22.
The second path consists only of a circuit 28 which modifies the load signal by an adjustable factor K The output of circuit 28 is applied to adding circuit 25.
Adding circuit has three further inputs. A first is supplied from an X-ray gauge 30 which as shown in FIG- URE 1 is located downstream of the mill and which generates a signal representing the error between the actual gauge rolled and the required gauge. A second input is provided by a potentiometer 31 which is set by the operator to the initial roll gap setting. The third input is derived from a potentiometer 32 driven by a servo-motor 33 which can be connected through switch 26 to the output of the adding circuit 25.
Disregarding for the moment the limiting circuit 23, the integrator 24 and the second path constituted by circuit 28, the control circuit illustrated in FIGURE 2 is the well-known Gaugemeter circuit. The factor K is chosen to be equal to the compliance of the mill (l/M), so that the sum of the signals from circuit 20 and the potentiometer 16 represents theoutgoing gauge of the strip. This is compared in circuit 25 with the signal set on potentiometer 31 by the operator and the resulting error signal is applied to control the screws 14 to reduce that error signal towards zero. If the resulting strip gauge differs from that required, the X-ray gauge 30 detects an error and applies an added reference signal to circuit 25, tending to bring the gauge nearer the required value. If now the operator changes over the switch 26, the residual error is applied to servo-motor 33 which drives potentiometer 32 to reduce that residual error to zero. Switch 26 is then changed back to the position shown, when the system continues to control to the gauge set on the X-ray gauge.
For the reasons explained above, the control system so far described, however high its response, i unable to compensate for roll and roll bearing eccentricity and noncircularity, and it is for this purpose that the elements 23, 24 and 28 are introduced. Whereas the rolling load path through the circuit 20' has a positive feed-back characteristic, the path through circuit 28 has a negative characteristic, K Taking the two paths together, the positive feed-back gain is (K K and the values are selected so that (K -K is equal to the mill compliance (l/M).
The circuit 23 has a limiting characteristic having a positive output of constant value for all inputs exceeding a set value and a negative output of constant value for all negative inputs exceeding a set value. When a large input signal is applied to limiting circuit 23, the integrator gives a ramp output having a maximum rate limited by the set value. Under this circumstance, the integrators output increases at uniform rate so long as a positive signal is applied to circuit 23 and then decreases at uniform rate so long as a negative signal is applied. The feed-back path 27 ensures that under steady state conditions the output of integrator 24 will equal the input to circuit 23. The eifect of the circuits 23, 24 is to cause the output of integrator 24 to follow the input to circuit 23 with accuracy when the rate of change of the input is equal to or less than the maximum ramp rate of circuits 23, 24, but to attenuate input signals, the rate of change of which is greater than the maximum ramp rate. The constants of the circuits are selected so that signals at the frequencies of the cyclic variations are substantially attenuated, while those at the smaller frequencies of possible variations in the incoming strip sufier hardly any attenuation.
It will be appreciated that the circuits 22, 24, 27 thus act as a low pass filter, and in fact any circuit which has the described differential attenuating function may be used in their place.
When the load cells 17 detects a variation in rolling load at a low rate of change, due to changes in the characteristics of the ingoing strip, both paths are eifective and the system operates with a positive feed-back gain of (K +K )=1/M in the manner described. On the other hand, cyclic variations give rise to load cell signal variations which pass unattenuated only through the path containing the circuit 28; the rolling load loop then acts as a negative feed-back loop of gain K and by appropriate selection of the value of K may operate to compensate and largely remove variations in output gauge due to this cause. For this purpose, the roll gap adjusting mechanism must be fast acting in relation to the strip speed and, for high strip speeds, should be devoid of inertial lags; for this reason hydraulic actuating adjusting mechanisms are preferred.
As will be appreciated the filter constituted by the elements 22, 23, 24 is effective, at the frequency of the cyclic variations, on both the rolling load signal from load cells 17 and the roll setting signal from indicator 26. In the modification of FIGURE 3, that filter is replaced by two low pass filters 34, 35, each of which may be constituted by the elements 22, 23, 24, 27 as described, and
one 34 of which is connected between the modifying circuit 20 and the adding circuit-21 so as to be effective only on the rolling load signal, and-the other 35 of which is connected between the indicator 16 and the adding circuit 21, to be effective only on the roll setting signal. The function of the two separate filters 34, 35 is identical with that described for the single filter 22, 23, 24, 27 of FIGURE 2, but the selectivities of the two filtersmay be chosen to have different values. Then, if the rolling load feed-back loop through load cells 17 and the roll setting feed-back loop through indicator 16 are found to have different stabilities, the selectivity of one filter can be made higher than that of the other to give maximum overall sensitivity without danger of instability.
An alternative system to attain similar results may be to permit the first AGC positive feedback loop through circuit 20 to be operative at all useful frequencies and to introduce a frequency selective circuit into the negative loop, through circuit 28, this frequency selective loop .to comprise a high pass filter. In this case, the gain of circuit 20, K is equal to 1AM, while the loop gain at eccentricity frequencies is (Kg-K1).
Advantageously, the feed-back loop gain (K -K in the first case and K in the second case may be adjusted, preferably automatically to compensate for roll flattening, certain speed effects and for changes in the width of the strip-rolled. In addition, it is preferable to adjust automatically the pass band of the frequency selective circuit in accordance with a function of the speed of the strip 16, so that compensation is made for the variation with speed in the frequencies of the cyclic variations.
1. In a rolling mill comprising a pair of work rolls defining therebetween a roll gap, and signal-responsive means for adjusting the setting of said roll gap;
a rolling mill control system comprising means for generating a signal which varies with the rolling load applied to said rolls by the work passing therebetween, and
transmission means for transmitting said signal to said adjusting means in order to control said adjusting means, I
said transmission means comprising means for modifying said signal, when any variations in said signal are generated at frequencies of the same order as the cyclic variations aIiSing from the mill, to produce a change in the transmitted signal in a direction opposite to that of the change, but for producing a change in said transmitted signal in the same direction as the change when such variations are generated at lower frequencies.
2. In a rolling mill stand comprising a pair of work rolls defining therebetween a roll gap for the work to be rolled, and means for adjusting the setting of said roll p;
an automatic control system comprising means for generating a rolling load signal in accordance with the rolling load generated in said stand by the work,
means for generating a signal in accordance with said roll gap setting,
and signal transmitting means comprising a first conductive path for applying said rolling load sig-. nal to the roll gap adjusting means in afdirection such that any change in rolling load gives rise to a change in roll setting causing a change of the rolling load in the same direction as the change, 1
a second conductive path in parallel with the first path for applying said rolling load signal to said adjusting means in a direction such that a change in rolling load causes a change in rolling load in the opposite direction,
means for applying said roll gap setting signal to said adjusting means in conjunction with the rolling load signal transmitted by said first path, and frequency sensitive filter means to render said first path substantially ineffective at frequencies of the same order as cyclic variations in said rolling load signal arising from the stand, where- 'by for variations at frequencies of the same order as the cyclic variations, the adjusting means is controlled to maintain the rolling load substantially constant, while, at lower frequencies, the adjusting meansis controlled jointly by the roll setting signal and rolling load Signal to compensate substantially for work variations.
3. An automatic control system for a rolling mill stand according to claim 2 in which only said first path includes said filter means, and the roll setting signal applying means. are eifective to apply that signal also to the said filter means. 7
4. Anautomatic control system for a rolling mill stand according to claim 2 in which the filter means comprise a first filter element in the first path and effective on the rolling load signal only and a second filter element in the roll setting signal applying means.
5. An automatic control system for a rolling mill stand according to claim 4 in which the two filter elements have dilfering selectivities.
6. An automatic control system for a rolling mill stand according to claim 3 in which each of the paths contains a circuit element altering the characteristic of the rolling load signal whereby, at frequencies below the frequencies of the cyclic variations, the effect of the two paths is to modify the rollingload signal by a factor approximating to the compliance of the mill.
7. An automatic control system for a rolling mill stand according to claim 2, in which the roll gap adjusting means comprise separate devices effective on adjustment of the roll setting at the respective roll ends, and those devices are cross-connected to ensure equality of movement.
8. In a rolling mill stand comprising a pair of work rolls defining therebetween a roll gap for the work to be rolled, and means for adjusting the setting of said roll p;
an automatic control system comprising means for generating a rolling load signal in accordance with the rolling load generated in said stand by the work,
means for generating a signal in accordance with said roll gap setting, and
signal transmitting means comprising a first conductive path for applying said rolling load signal to control the roll gap adjusting means in a direction such that any change in rolling load gives rise to a change in roll setting causing a change of the rolling load in the same direction as the change,
a second conductive path in parallel with the first path. for applying said rolling load signal to said adjusting means in a direction such that a change in rolling load causes a change in rolling load in the opposite direction,
means for applying said roll setting signal to said adjusting means in conjunction with the rolling load signal passed by said first path,
and frequency sensitive filter means to render said second path substantially ineffective at frequencies of a lower order than those of cyclic variations arising from the stand, whereby for variations at frequencies of the same order as the cyclic variations, the adjusting means is controlled to maintain the rolling load substantially constant, while,
7 8 at lower frequencies, the adjusting means 3,177,346 4/1965 Green 728 is' controlled jointly by the roll setting sig- 3,186,200 6/ 1965 MaxWel1 728 rial and rolling load signel to compensate 3,287,946 11/1966" Perraultet a1. 728 I substannally for work vanetlons. OR I PATENTS References Cited 125,389 1 /1966 Canada.
UNITED STATES PATENTS MILTON Primary Examiner 8/1966 Wright 728 A a