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Automatic computer control systems for rolling mills
Automatic Computer Control Systems for Rolling Mills A. B. CHELUSTKIN Introduction The application of computing techniques to automatic control systems is valuable for predicting how the controIled process will behave under the action of an input disturbance: one may thus determine how long the control system wiIl take to compensate for the effect of the disturbance. Thus computing techniques significantly extend the effectiveness of the so-ca lIed systems in regulating disturbances. However, to determine the control action upon the process requires its accurate mathematical description-a condition which can seldom be fulfiIled, because there may be inadequate knowledge of the process and insufficient power to change its dynamic response to uncontroIled disturbances. Even when the system of regulation against disturbance is as simple as the compounded field of ad.c. generator, the external characteristics of the generator are affected by the variation of the temperature of the power circuits, by the replacement of worn brushes or by changes in the condition of the commutator. Automatic Control of Screw-down One of the simplest examples of the use of such systems is an automaticaIly programmed screw-down of a reversing hot rolling mill. This system consists of a programming device which sets the position of the top roIl, and a servo system executing the necessary adjustment of it. With the aim of minimizing the adjustment time, the servo system is designed as optimal, ensuring automatic maintenance of speed and torque of the screw-down motor during the periods of acceleration and braking. Switching of the screwdown drive into the braking regime is carried out abruptly at the moment when the distance of misalignment is equal to the braking distance calculated with the aid of the computer. Since the electric drive system ensures a constant braking torque the braking distance is proportional to the available kinetic energy, i.e. to the square of the velocity at the instant of starting the braking. ] t follows that for the definition of the instant when the screw-down motor should go into the braking regime, the computer must carry out the operations of squaring the speed, multiplying it by some coefficient of proportionality and subtracting the result from the value of the misalignment distance. Obviously, the accuracy of calculation of the braking distance depends on the constancy of the value of the braking torque, which may vary within some limits with the random variation of friction (for instance, because of change or warming up of the lubricant). To find automatically the required coefficient of proportionality between the value of the square of the velocity of the screw-down drive and the value of the braking distance a second computer is used which, on the basis of the value and the sign of the error which has arisen, calculates and determines in the first computer such coefficient of proportionality that the error wiIl be a minimum. Since the braking distance of the drive equipped with an optimal control system is proportional to the distance during the period of
acceleration of the motor, instead of calculating the value of the braking distance one may remember the value of the accelerating distance and subject it to the same operations as the square of the velocity of the drive. Figure 1 shows a schematic explanation of the principle of designing a digital servo system in which the braking distance is calculated as a quantity proportional to the accelerating distance. Programme code of the number A, defining the required gap of the rolls, is introduced through the contacts of the programme setting relays PI to Po, and goes simultaneously into all the digits of the reversible 'electronic counter (at first zeroed). From shaft position digilizer
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n buses
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Figure 1.
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Q uenchi ng CIrCUIt
Block diagram of optimal digital servo system
The output of each stage of the counter is connected to the unit determining the sign of the misalignment SE, which is a combined coincidence circuit. To the unit SE are connected also the circuits of the brushes of all digits of contact paths. If the code of the number A fed by the programming device is larger than the code of the number B, given by the digitizer, then at the output I of the unit SE appears a voltage, but at output 1I there is none. For B> A, the reverse occurs-a voltage appears at output 1I. With exact equality of both codes, at neither output is there voltage . Both outputs of the unit SE are switched to command devices determining the direction of motion of the screw-down motor. Apart from the code of the number A, fed into the counter its first stage receives pulses through the trigger T R from the frequency modulator FM which converts the voltage of the tachogenerator TG of the screw-down motor. Depending on the sign of the misalignment the pulses fed to the input of the counter are either added to or subtracted from the code fed into the counter through the contacts of the programme setting relays. The type of operation is determined by applying voltages to either the addition or subtraction bus. At the moment when the drive achieves a steady speed the contacts of the full-speed relay SR break the circuit feeding the pulses to the counter input.
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Thus the number A, stored in the counter, will be increased or decreased by a number proportional to the screw-down distance traversed in the process of accelerating, and consequently the unit SE will initiate the braking of the screw-down motor with a lead equal to the accelerating distance. For a check of the correspondence of the actual screw-down position to that desired, check of the code numbers of A and B is made at the end of the process of braking, when the speed of the drive is sufficiently reduced. Such a check is effected by zeroing the counter and again feeding the code of the number A, with momentary closing and breaking of the contacts of relay K (not shown in Figure 1). If desired screw-down motion is not executed sufficiently accurately, the unit SE gives a command to switch the drive in the corrective direction; however, the reduction of this error will be carried out at a low speed, excluding the possibility of overshooting. At the same time a correcting computer determines the sign and value of the error that has accumulated in the screw-down motion and automatically adjusts the frequency modulator which sets the correct instant for starting the braking during the following pass. In a simple case the correcting computer is itself a logical circuit which drives the required sign of the change of the coefficient of proportionality and changes the conversion coefficient of the frequency modulator. Automatic Regulation of the Discharge Speed The most complicated unit of an automatic reversing hotrolling mill is the main drive. Excessive speed of the rolls at the instant of grabbing will cause slipping, increasing the time of the rolling cycle. With automatic operating of the mill, speed of grabbing is determined by the distance of discharge of the metal from the rolls. Too high a velocity of the rolls at the instant of discharge throws the ingot too far out so that, when it returns, the rolls have had time to reach too high a velocity. Thus the main factor determining the time of the rolling cycle is the speed of discharge. To control the speed of discharge use is made of a computer, calculating the length of the unrolled part of the ingot and the braking distance of the drive at a given velocity of rotation. The schematic of the computer is shown in Figure 2. The length of the unrolled part of the ingot at a distance Lo from the rolls of the mill is determined by a photo-relay PhR. At the instant when the tail of the metal is leaving the field of view of the photo-relay the integrator I begins working, producing a voltage proportional to the integral of the rotational velocity of the rolls. Thus, the length of the unrolled part is represented by the difference between the voltage Vo (proportional to the distance Lo) and the voltage of the integrating circuit VI = Sndt: thus VN R = Vo - S ndt. The voltage VN R is compared to the voltage of the function generator FG, proportional to the running distance of the rolls at a given velocity. When the difference reaches the value VF , proportional to the required speed of discharge, a voltage appears at the zero voltage relay NR and initiates braking through the brake command relay BCR in the control circuit of the main drive. The speed of entry of metal into the rollers URX is proportional to the speed of the rolls U R and the amount of reduction of the ingot during the pass URX
=
URS(l- H/h)
where H is the thickness of the strip at the input, h is the thickness at the output, and S is the amount of lead. Thus, for the integrating device to evaluate correctly the
length of the unrolled part of the ingot, it is necessary that the voltage of the tachogenerator should depend on the ratio H/h (S assumed constant). This is achieved by having twin exciter windings on the tachogenerator which is driven from the shaft of the main drive motor. One winding creates in the TG armature the voltage UR!j, and the other, URS(H/h).
TG
BCR Figure 2.
Schematic diagram of discharge speed cOl1lrol
To obtain excitation proportional to H/h the second winding of the tachogenerator is fed from the computer carrying out the operation of dividing H/h. The value of H is supplied to the computer from the memory unit, where the value of the gap of the rolls at the time of the preceding pass is stored. The value of h is given directly by the screw-down position transducer. The required value of the discharge speed VF is supplied to the control system from the programming device. If the actual speed of discharge differs from that required for any reason (for instance, due to a variation in the dimensions of the roll caused by their wear or temperature deformation), a special correcting circuit adjusts the parameters of the computer to obtain in the subsequent passes conformity between the actual and the required speeds of discharge. The programme discharge speed cannot always be optimal, because the condition of the roller bed and rolls, and the coefficient of friction of the ingot thereon, may vary within some limits. To shorten the time of the pass, the control system automatically corrects the programme of discharge speeds. This adjustment is effected by a separate computer, which receives statistical data obtained on the time of pass. It then performs an automatic search for the optimum values of grabbing speed and discharge speed. The optimal values fed into the programming device are effective for the entire period of rolling a given batch of-ingots under the given programme. Control of a Heating Furnace The quality of the rolled articles depends to a considerable degree on the temperature stability of the incoming billets. If billets are insufficiently heated there is increased stress on the roll of the stand, causing increased deformation of the stand, so that the metal is not rolled to size. Accurate rolling is now demanded and regulation of the temperature in the working zones of furnaces becomes imperative. Modern furnaces have temperature regulators which maintain constant temperature in temperature-controlled zones. The temperature of the ingot and of the zone will differ by an amount depending on the time spent by the'ingot in the zone. Thus, any change in the speed of travel through the furnace will
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affect the temperature of the ingot. As the heated ingot subsequently passes a series of zones, its final heat content depends on the time spent in each zone. This is determined not only by the speed of travel at a given moment but also by the character of all the preceding changes of this speed. The regulation of the heating zones should ensure constant heating of the ingot despite changing speeds of rolling. There fore, the settings of the temperature regulators should not be constant but should respond to changing discharge speeds and allow for all the previous changes of this speed. Figure 3 shows a system ensuring automatic change of settings of temperature regulators in zones. This change is achieved by means of a computer which calculates the heat content of the billets as a function of the distribution of the temperature field in each zone and the speed of travel of the billets through these zones. The computer then finds a distribution of the temperature field in the furnace such that the billets have the stipulated heat content at the time of discharge from the furnace. The requisite temperature field corresponds to definite values of zone temperatures which are set at temperature regulators.
An optical pyrometer, measuring the deviation of the temperature of the rolled metal from the nominal value, causes through a proportioning unit a corresponding variation of the
Ah
TM
Figure 4. PM
Figure 3. TC RT P BVM MH
eT PhP LH CD
thermocouple
Since the heat content of the ingots may be measured only approximately, errors inevitably arise in the choice of temperature regulator settings, and this results in fluctuations in the temperature of the billets discharged from the furnace. To correct the parameters of the computer a self-adjusting system is applied, in which the calculated value of the temperature of the billets is compared to the actual value measured on the mill. Gauge Control on Hot Strip Mills Stabilization of the temperature of heating of billets does not yet ensure a uniform temperature of the billet during rolling. Because of cooling, the tail end of the billet is rolled in the stand at a lower temperature than the front end. The consequent extra deformation of the stand causes nonuniformity of thickness along the length of the billet, thickening towards the tail end. Compensation may be applied according to the diagram shown in Figure 4.
pyrometer
DU
delay unit
PB
proportioning block
MC TM CC
control system of motor M thickness meter
correcting computer
references the servo system of the screw-down motor. However, the proportionality between the amount of deviation of the temperature and the position of the upper roll is affected by the properties of the rolled metal and its absolute thickness. A computer is therefore introduced to determine the requisite relationship. This computer measures the variations of the temperature and of the thickness of the strip. Since the latter may be caused not only by temperature but also by other reasons, it is necessary to segregate those thickness variations which are due only to temperature. For this the computer evaluates the correctness of the choice of the coefficient of proportionality from the value of the cross-correlation relationship
Diagram of automatic control of temperature regulator settings for zones of heating furnaces
zone te:nperature regulator pusher meter measuring billet speed through furnace modulator, determining heat absorption of billets computer, determining required temperature distribution for furnace zones photopyrometer, measuring temperature of billets during rolling device evaluating heat loss of billets before rolling correcting device. adjusting the parameters of M H and eT by comparison of actual and calculated temperature
Schematic of compensation for the influence of temperature on thickness
q,
=
J:
C!.T(t)C!.h(t)dt
(assuming that the measurements of temperature and thickness are made at the same point of the strip). With a correctly chosen coefficient of proportionality a change of tJ.t does not involve any change of :3.h; and the integral has a definite value. When rolling strip in a continuous mill, thickness is affected also by variations of the tension. Rolling of the front and tail end takes place without tension, so that there are extra deforming forces on the stand and a significant increase of the thickness at the edges of the strip. However, the change of thickness along the strip is not random so that a programming system may be applied to adjust the setting of the rolls. The wide variety of strip rolled in the mill calls for a large number of programmes, which makes such a system extremely cumbersome. Instead of the introduction of constant programmes, at the Institute of Automation and Telemechanics of the Academy of Science of the U.S.S.R. a system has been worked out which is adaptive. That is to say, it is self-programming, in accordance with the actual known distribution of the thickness of the strip. The programmes are worked out by a computer, statistically analysing data from the measurements of a sufficiently large quantity of rolled strip. Such analysis permits the location of the deviation of the strip from nominal along
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its length, and, in the same way, the finding of programmes for setting the stand screw-down (Figure 5).
rotating knives of the flying shears with the front end of the moving metal. In the system shown in Figure 6 the problem is solved by the aid of a computer, determining the amount of misalignment between the position of the cutters and the position of the front end of the strip. The computer also calculates the speed diagram necessary to establish the chosen misalignment of the cutters at a given moment.
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8
Knives
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Figure 5.
Diagram of automatic gauge control an thin strip hot-rolling mill
LC load cell SP screw-down sensor SD summing device STY set thickness value of strip MC system for control of motor M SS serve system TM thickness meter [
integrator
CM! . CMO
input and output commu-
of memory storage MS commutator control and
tatoTs
CC!. CC2
determination of lengths of rolle d sections of strip
TO PhR
Figure 6.
Block diagram of automatic control of /lying shear
tachogenerator photo-relay
The computer is made up of an integrator and several memory circuits corresponding to the number of sections into which the whole length of the strip is broken. Integration of deviations is carried out separately for each section of the strip, which permits the evaluation of the mean deviations of the thickness for all the sections. With the help of a step uniselector the mean value of the deviation is fed to the memory device. Signals from the memory devices are handled by another stepped uniselector; when fed to servo systems of the stand screw-downs, they call for the variation of the amount of reduction by rolling corresponding to the section of the strip. In order that random deviations of the length of the strip should not unduly change the programme, it is worked 'out in the process of rolling not one but several strips. This achieves a choice of value of the coefficient of proportionality of the integrating link of less than unity. To accelerate the working out of the programme with the contained system, the signals of the memory devices not equal to zero, and their aggregate, constitute a preliminary programme. This is set manually and then made precise in the process of further rolling. Instead of adjusting the rolls, the thickening of the strip at the ends may be prevented by modifying the tensioning programme for the drawing of the strip. Before the rear end of the strip has come out (or if the front end is accessible) tension is altered by changing the set values of the motor speed regulator of the stand. The change may either follow a previously chosen programme, or a programme worked out by a computer analogous to that shown in Figure 5. Control of Flying Shears Metal rolled in a continuous mill is cut to measured lengths by a flying shear, whose speed is automatically adjusted to match the rolling velocity. To reduce the waste of metal it is advisable to cut off only a short length of the front end. For this a system is applied automatically to align the continuously
The velocity of the cutters and of the last stand of the mill is adjusted by a tachometer regulator, incorporating tachogenerators of the stand TGS and of the knives TGN, and an electronic amplifier A exciting a rotary amplifier RA of the motor-generator system driving the knives. A limit switch LS is fixed on the shaft of the knives and closes every time they close. Thus contact LS permits the fixing of one point in the position of the knives. Continuous control of their position with the existence of one controlled and fixed point for each turn (or for several turns with rotation of the top and bottom knives at different speeds) is exercised by the saw tooth voltage generator, starting at the instant of operation of LS. The positioning of the front end of the billet relative to the knives is carried out with the aid of the photo-relay PhRI and the electronic time relay ETR, whose holding time depends on the speed of rolling (change of holding time of ETR is equivalent to space displacement of the photo-relay PhRI along the roller table between the mill and the shears). At the instant of operation of ETR, which occurs when the front end is at a set distance from the knives, a voltage is applied to the electronic commutator EC; also, at the input of the amplifier A there is, apart from the voltage proportional to the error of the tachometric system b.n, an a.c. voltage. This voltage has sufficient amplitude to cause a change of polarity of the amplifier A, reversal of the magnetic field of the rotary amplifier and intensive braking of the drive of the shears. At the same time the rising of the generator voltage is checked when ETR operates, which is fixed. The fixed voltage is remembered as the initial error. The voltage proportional to the amount of error is equated to the voltage of the integrator of the computer CT S b.n dt. When the voltage proportional to Sb.n dt becomes equal to half the voltage proportional to the error, the electronic commutator changes the phase of the a.c. voltage applied to A by 180°. This causes intensive acceleration of the drive of the knives, during which they take up the remaining half of the distance of error (the .
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This scheme, in accordance with the resulting sign, changes the polarity of the voltage supplied to the integrator and proportional to the modulus of the measured deviation of the thickness of rolled strip. In continuous cold-rolling mills measurement of thickness before the strip enters the first stand is prevented by corrugations in the strip. Instead, gauge control is based on the readings of a gauge meter behind the first stand. In view of the existence of a lag, use is made of a pulse regulator, eliminating only the low-frequency components of deviation of thickness. It follows that the non-uniformity of thickness of strip behind the first stand has fundamentally only the character of momentary deviations. Their reduction requires an accurate regulating system, in which the readings of the thickness deviation meter enter a compensating computer, which supplies the necessary derivative component of the regulating action in the speed control loop of the last stand of the mill. The compensating computer takes account of the time the strip takes to travel from the meter to the penultimate stand of the mill and of the basic relations characterizing the process of continuous rolling. Allowance is also made for such factors as the correlation between the speeds of the stands and the tension, between the tension and the forced deformation of the stands and the transfer function of the electric drive of the stands, etc. The complicated character of these relationships necessitates a special electronic model of the mill, consisting of a series of units representing the stands of the mill. As a result of the model study a simplified structure of a compensating computer, most accurately representing the rolling process in a multi stand mill, has been developed. From the basic information on the non-uniformity of the thickness of the strip, and these correlations, the compensating computer calculates the changes in relative velocities of the stands to ensure that the thickness of the strip at the output of the mill will have the minimum deviation from nominal. As the programme of the compensating computer is approximate and does not completely correspond to the rolling process in a given example, the possibility of errors is not excluded. Consequently the thickness of the output strip will necessarily have a deviation. An accurate system of regulation will therefore include another computer-a correcting one. It establishes fluctuations in the thickness at the input and the output of a stand of the mill, and carries out automatic adjustment, optimizing the parameters of the compensating computer
drive of the knives ensures the equality of acceleration and deceleration). Thus, at the instant of closing of the knives the front end of the billet comes between the knives and is cut off. In the scheme considered the cut off length of the front end is determined by the setting of the time relay ETR. It is desired to hold this relay in accordance with the speed of the mill, but the initial choice of holding time may be achieved inaccurately. Moreover, in the process of mill operation the correlation between the speed of rotation of the rolls and the speed of motion of the rolled billet may be changed; therefore in the system ETR is automatically adjusted by comparing the actual length of the front end with that specified. The control of the actual lengths cut off the front end is achieved with the aid of the photo-relay PhR2, located at some distance from the knives. Automatic adjustment of the holding time is achieved by a reversing uniselector, whose contacts switch the setting resistance of ETR until the time of cutting coincides with the time of illumination of PhR2. A trial of the arrangement under consideration has shown that spread in the time of actuating the electric drive does not exceed 0·05 sec, which, at a rolling speed of 10 m/s, ensures the accuracy of cutting within the limit of 50 mm. Strip Gauge Control on Cold-rolling Mills A most urgent problem appears to be the regulation of the strip gauge in the cold-rolling process. Investigation has shown that non-uniform thickness of strip rolled at steady speeds originates mainly in the process of hot-rolling. In connection with this, of great interest are systems whose control action corresponds to the readings of a gauge meter, mounted at the entry rather than at the delivery side of the stands. In such a system the time-lag of the mill controllers may to a significant degree be compensated for by detecting and measuring the disturbance in advance. Also, as for a hot-rolling mill, in this system the coefficient of proportionality between the amount of deviation and the amount of displacement is determined by a computer comparing the non-uniformity of the thickness at the entry and delivery side of the mill. Computation is simplified by adopting a logical scheme of multiplication of the sign of the deviation of the thickness before and after rolling, which determines the required sign of the change of the coefficient of proportionality (Figure 7).
(Figure 8).
TG
Figure 7.
Block diagram of automatic regulation of thickness in a reversing hot-rolling mill
TM 1, TM2 TG TCI
thickness meters stand tachogenerator computer with self-adjusting parameters for the thickness-regulating system (by readings of TM I) computer of pulse system of thickness regulation (by readings of TM2) screw-down motor
TC2
M
Investigation of the regulating system on an electronic model of the mill has shown that the correcting computer must carry out automatic adjustment of two parameters of the compensating computer: the gain and the time lag 7. In optimizing these parameters it is usually convenient to make a step-by-step search by the gradient method. To decrease the deviation of the thickness in the periods of adjustment of the parameters of the compensating computer, caused, for example, by the temperature deformation of the rolls of the stand or the varying speed of rolling, the thickness meter, located behind the last stand of the mill, reveals any deviation of thickness and adjusts the speed regulator of the stand quite apart from the compensating computer. Furthermore, in high-speed cold-rolling mills a change in the rolling speed significantly affects the thickness of the rolled strip. Hence with varying rolling speed there arises the need for automatic control of the amount of reduction in the stand. Such control may be achieved by setting a programme of screwdown displacement and of the degree of tension on the strip at varying rolling speeds. The magnitudes of these displacements
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vary for different assortments of rolled strips. Therefore these programmes may be adjusted by a method similar to that used to adjust the programmes for regulating the thickness of thin I TMl
8D
receiving table and the pit from which the ingot is being drawn. Thus the resulting voltage is proportional to the actual distance between the crane and the receiving table at each particular moment of time. The pass transfer car also has a similar system of control of its position relative to the receiving roller bed.
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-BU RHO I C)RH11
: dH(I-T) L RH1_'0-_...!I---.:::.:....:..:..:..-...:..t:1:~---lC
AH (I-T)
CC2
Figure 8.
Block diagram of thickness regulation in continuous cold-rolling mill
TM I, TM2 DU RHO, RH!, RH2 CCl ee2 VR
Figure 9.
thickness meters variable delay unit recording and reproducing heads compensating computer correcting computer (for optimizing the parameter of eCI) speed regulator for stand motor SM
CD ID 11
le
x cc ' Xci VD
sheet in hot-rolling mills. The whole range of working speeds is divided into several sections. Then by statistical assessment of the readings of the thickness meter the required screw-down displacement for each section is calculated and stored.
BD Se. Si
6.S AB CBe, CBI
Adjusting Control of Pass Transfer Cars and Grab Cranes of the Soaking Pits The above examples of the application of computers are directly concerned with the technology of rolling processes. Below is considered an example of the use of computers for combined automatic control of several transport mechanisms. For blooming mills operating at full, special significance attaches to the control of the pass transfer car and the grab cranes which draw the heated ingots from the soaking pits. To reduce the time of these operations the practice is to arrange for the crane moving the ingot to advance towards a moving pass transfer car. Obviously, both moving mechanisms must stop at the same time and at the same place so that, without any additional shift of either, the ingot may be deposited in the cradle of the car. To ensure coordinated stopping of the car and the crane, the latter must be equipped with a servo system controlling the position of both mechanisms in the pit furnace bay. Figure 9 shows one version of a system for combined control of the pass transfer car and the grab crane, in which digital computing elements are applied. Such a system, consisting of a pulse transducer for the motion of the bridge and an electronic integrator for the storage of pulses, will position the bridge of the crane relative to the receiving roller bed of the blooming mill. The pulse transducer is on the bridge of the crane and generates a pulse when the bridge shifts by some given small distance (the transducer may be of the brush type, making contact with bars insulated from earth and located on rails. under the crane). When the crane moves with the ingot towards the blooming mill, the output voltage of the electronic integrator is proportional to the number of pulses fed from the transducer, i.e. to the shift of the bridge of the crane. This voltage is subtracted from the voltage proportional to the distance between the
Block diagram of combined control of pass tramler cars and grab crane pulse movement transducer same for ingot·carriage integrator for accumulating pulses unit showing initial coordinates of grab crane bridge current value of coordinates of grab crane and pass transfer car unit determining displacement velocity (speed of change of coordinates with time) braking distance calculator calculated values of braking distance for grab crane and pass transfer car current value of distance between crane and pass transfer car unit for advanced calculation of beginning of braking command devices for braking crane and pass transfer car
Voltages proportional to the positions of both mechanisms relative to the receiving roller bed are fed into the computer, which calculates the braking distance of these mechanisms and initiates their braking with such positioning of the bridge of the crane and the car that their complete stop will be achieved at one and the same point. Thus the operator of the grab crane controls the mechanisms of the crane only at the moment of grabbing the ingot from the pit and lifting it. All the other operations are carried out automatically, down to stopping of the grab crane over the pass transfer car, after which the operator visually checks the relative position of the car and the crane and controls the lowering of the ingot into the cradle of the car. At the instant of opening of the jaws and the beginning of raising them, a command is automatically given to move the car to the receiving roller-bed. The system for automatic control of the pass transfer car brakes it as it approaches the receiving roller bed and transfers the ingot thereto. After the ingot is loaded on to the table, which is also carried out automatically with the help of a photo-relay placed by the side of the receiving roller bed, the car starts its motion in the direction of the group of pits from which the next ingot will be drawn. Conclusions Along with the use of computers in closed loop systems for automating complicated technological processes of great importance is the application of computers to separate problems of automatic control, permitting considerable improvements of the quality of control. In the majority of cases even for comparatively simple problems, systems of automatic control should include self-adjusting elements.
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DISCUSSION H. KRUGER (German Democratic Republic) What instrument is used for measuring the temperature of the hot metal in the furnace? A. B. CHELUSTKIN, in reply. No instrument is provided for measuring the temperature directly in the furnace. The temperature of the metal is measured after rolling by the first stand. Question from the floor What elements are used in the automatic control of the mill screws, and how much apparatus is required? A. B. CHELUSTKIN, in reply. The system can be built up from many types of elements; we use ferro-transistor elements. The number of elements depends on the desired accuracy; this is determined by the number of discharges of the counter and the logical circuits. Question from the floor What accuracy is achieved in transforming the tachogenerator voltage into a frequency? A. B. CHELUSTKIN, in reply. Transformation of a tachogenerator voltage into a frequency is difficult. However, if the engine runs at a low speed the accuracy is higher and reaches I per cent. Question from the floor What is used as a 'full speed' relay? A. B. CHELUSTKIN, in reply. An ordinary electronic relay is used as a full speed relay 'PC'. V. A. IVANOV (U.S.S.R.) I should like to deal with the application of computing equipment in a control system based on the disturbance. A distinctive feature of such a control system is its high speed of response and it ensures the possibility of complete elimination of deviations in the parameter to be controlled. However, these systems also have a considerable drawback: in changing the parameters of the plant the compensation of the disturbance
may not be complete and this may lead to considerable deviations of the controlled quantity at the output. Application of computing devices for setting the parameters in disturbance actuated control systems is a new and very promising trend in the correction of systems of automatic control which will make it possible to obtain optimum operating parameters. Therefore, the computing equipment which is used for setting the parameters, i.e. correcting computer apparatus (which controls the value of a certain function of the inputs and outputs of the plant), will automatically set the parameters of the compensators if the parameters of the plant change, hence the disturbances will be compensated in the best way. Thus, thanks to the application of correcting computer equipment, it is possible to retain the advantages of open-loop control, for example, fast response, and at the same time, to eliminate the main drawback, i.e. incomplete compensation of disturbances when the parameters and the characteristics of the plant change. A control system similar to the one described in the paper for controlling the thickness of the metal in hot rolling was developed by the Institute of Automation and Telemechanics, Academy of Sciences, U.S.S.R., for controlling the welding conditions on an electric tube stand. Setting the parameters of the compensator was also carried out on the basis of the correlation dependence between the input disturbancedeviation in the thickness of the tube wall, which is equivalent to the thickness of the strip from which the tube is made; the output parameter was the temperature of the weld seam which must be maintained constant irrespective of thickness variations. Due to high frequency disturbances at the input end, current control systems based on deviations do not permit the required control quality to be obtained. Therefore it is advisable in this case to apply the above-mentioned principle of designing a control system based on disturbances, with setting by means of correcting computing equipment. The system produced enables a high quality of control of the temperature to be obtained and, consequently, also a high quality of the weld seam which is directly related to the temperature. Compared with manual control, the deviation of temperature from the optimum value has been reduced by a factor of 6 to 8.
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acceleration of the motor, instead of calculating the value of the braking distance one may remember the value of the accelerating distance and subject it to the same operations as the square of the velocity of the drive. Figure 1 shows a schematic explanation of the principle of designing a digital servo system in which the braking distance is calculated as a quantity proportional to the accelerating distance. Programme code of the number A, defining the required gap of the rolls, is introduced through the contacts of the programme setting relays PI to Po, and goes simultaneously into all the digits of the reversible 'electronic counter (at first zeroed). From shaft position digilizer
--.
.- I
n buses
_-I-"--....J
Figure 1.
L
Q uenchi ng CIrCUIt
Block diagram of optimal digital servo system
The output of each stage of the counter is connected to the unit determining the sign of the misalignment SE, which is a combined coincidence circuit. To the unit SE are connected also the circuits of the brushes of all digits of contact paths. If the code of the number A fed by the programming device is larger than the code of the number B, given by the digitizer, then at the output I of the unit SE appears a voltage, but at output 1I there is none. For B> A, the reverse occurs-a voltage appears at output 1I. With exact equality of both codes, at neither output is there voltage . Both outputs of the unit SE are switched to command devices determining the direction of motion of the screw-down motor. Apart from the code of the number A, fed into the counter its first stage receives pulses through the trigger T R from the frequency modulator FM which converts the voltage of the tachogenerator TG of the screw-down motor. Depending on the sign of the misalignment the pulses fed to the input of the counter are either added to or subtracted from the code fed into the counter through the contacts of the programme setting relays. The type of operation is determined by applying voltages to either the addition or subtraction bus. At the moment when the drive achieves a steady speed the contacts of the full-speed relay SR break the circuit feeding the pulses to the counter input.
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A. B. CHELUSTKIN
Thus the number A, stored in the counter, will be increased or decreased by a number proportional to the screw-down distance traversed in the process of accelerating, and consequently the unit SE will initiate the braking of the screw-down motor with a lead equal to the accelerating distance. For a check of the correspondence of the actual screw-down position to that desired, check of the code numbers of A and B is made at the end of the process of braking, when the speed of the drive is sufficiently reduced. Such a check is effected by zeroing the counter and again feeding the code of the number A, with momentary closing and breaking of the contacts of relay K (not shown in Figure 1). If desired screw-down motion is not executed sufficiently accurately, the unit SE gives a command to switch the drive in the corrective direction; however, the reduction of this error will be carried out at a low speed, excluding the possibility of overshooting. At the same time a correcting computer determines the sign and value of the error that has accumulated in the screw-down motion and automatically adjusts the frequency modulator which sets the correct instant for starting the braking during the following pass. In a simple case the correcting computer is itself a logical circuit which drives the required sign of the change of the coefficient of proportionality and changes the conversion coefficient of the frequency modulator. Automatic Regulation of the Discharge Speed The most complicated unit of an automatic reversing hotrolling mill is the main drive. Excessive speed of the rolls at the instant of grabbing will cause slipping, increasing the time of the rolling cycle. With automatic operating of the mill, speed of grabbing is determined by the distance of discharge of the metal from the rolls. Too high a velocity of the rolls at the instant of discharge throws the ingot too far out so that, when it returns, the rolls have had time to reach too high a velocity. Thus the main factor determining the time of the rolling cycle is the speed of discharge. To control the speed of discharge use is made of a computer, calculating the length of the unrolled part of the ingot and the braking distance of the drive at a given velocity of rotation. The schematic of the computer is shown in Figure 2. The length of the unrolled part of the ingot at a distance Lo from the rolls of the mill is determined by a photo-relay PhR. At the instant when the tail of the metal is leaving the field of view of the photo-relay the integrator I begins working, producing a voltage proportional to the integral of the rotational velocity of the rolls. Thus, the length of the unrolled part is represented by the difference between the voltage Vo (proportional to the distance Lo) and the voltage of the integrating circuit VI = Sndt: thus VN R = Vo - S ndt. The voltage VN R is compared to the voltage of the function generator FG, proportional to the running distance of the rolls at a given velocity. When the difference reaches the value VF , proportional to the required speed of discharge, a voltage appears at the zero voltage relay NR and initiates braking through the brake command relay BCR in the control circuit of the main drive. The speed of entry of metal into the rollers URX is proportional to the speed of the rolls U R and the amount of reduction of the ingot during the pass URX
=
URS(l- H/h)
where H is the thickness of the strip at the input, h is the thickness at the output, and S is the amount of lead. Thus, for the integrating device to evaluate correctly the
length of the unrolled part of the ingot, it is necessary that the voltage of the tachogenerator should depend on the ratio H/h (S assumed constant). This is achieved by having twin exciter windings on the tachogenerator which is driven from the shaft of the main drive motor. One winding creates in the TG armature the voltage UR!j, and the other, URS(H/h).
TG
BCR Figure 2.
Schematic diagram of discharge speed cOl1lrol
To obtain excitation proportional to H/h the second winding of the tachogenerator is fed from the computer carrying out the operation of dividing H/h. The value of H is supplied to the computer from the memory unit, where the value of the gap of the rolls at the time of the preceding pass is stored. The value of h is given directly by the screw-down position transducer. The required value of the discharge speed VF is supplied to the control system from the programming device. If the actual speed of discharge differs from that required for any reason (for instance, due to a variation in the dimensions of the roll caused by their wear or temperature deformation), a special correcting circuit adjusts the parameters of the computer to obtain in the subsequent passes conformity between the actual and the required speeds of discharge. The programme discharge speed cannot always be optimal, because the condition of the roller bed and rolls, and the coefficient of friction of the ingot thereon, may vary within some limits. To shorten the time of the pass, the control system automatically corrects the programme of discharge speeds. This adjustment is effected by a separate computer, which receives statistical data obtained on the time of pass. It then performs an automatic search for the optimum values of grabbing speed and discharge speed. The optimal values fed into the programming device are effective for the entire period of rolling a given batch of-ingots under the given programme. Control of a Heating Furnace The quality of the rolled articles depends to a considerable degree on the temperature stability of the incoming billets. If billets are insufficiently heated there is increased stress on the roll of the stand, causing increased deformation of the stand, so that the metal is not rolled to size. Accurate rolling is now demanded and regulation of the temperature in the working zones of furnaces becomes imperative. Modern furnaces have temperature regulators which maintain constant temperature in temperature-controlled zones. The temperature of the ingot and of the zone will differ by an amount depending on the time spent by the'ingot in the zone. Thus, any change in the speed of travel through the furnace will
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AUTOMATIC COMPUTER CONTROL SYSTEMS FOR ROLLING MILLS
affect the temperature of the ingot. As the heated ingot subsequently passes a series of zones, its final heat content depends on the time spent in each zone. This is determined not only by the speed of travel at a given moment but also by the character of all the preceding changes of this speed. The regulation of the heating zones should ensure constant heating of the ingot despite changing speeds of rolling. There fore, the settings of the temperature regulators should not be constant but should respond to changing discharge speeds and allow for all the previous changes of this speed. Figure 3 shows a system ensuring automatic change of settings of temperature regulators in zones. This change is achieved by means of a computer which calculates the heat content of the billets as a function of the distribution of the temperature field in each zone and the speed of travel of the billets through these zones. The computer then finds a distribution of the temperature field in the furnace such that the billets have the stipulated heat content at the time of discharge from the furnace. The requisite temperature field corresponds to definite values of zone temperatures which are set at temperature regulators.
An optical pyrometer, measuring the deviation of the temperature of the rolled metal from the nominal value, causes through a proportioning unit a corresponding variation of the
Ah
TM
Figure 4. PM
Figure 3. TC RT P BVM MH
eT PhP LH CD
thermocouple
Since the heat content of the ingots may be measured only approximately, errors inevitably arise in the choice of temperature regulator settings, and this results in fluctuations in the temperature of the billets discharged from the furnace. To correct the parameters of the computer a self-adjusting system is applied, in which the calculated value of the temperature of the billets is compared to the actual value measured on the mill. Gauge Control on Hot Strip Mills Stabilization of the temperature of heating of billets does not yet ensure a uniform temperature of the billet during rolling. Because of cooling, the tail end of the billet is rolled in the stand at a lower temperature than the front end. The consequent extra deformation of the stand causes nonuniformity of thickness along the length of the billet, thickening towards the tail end. Compensation may be applied according to the diagram shown in Figure 4.
pyrometer
DU
delay unit
PB
proportioning block
MC TM CC
control system of motor M thickness meter
correcting computer
references the servo system of the screw-down motor. However, the proportionality between the amount of deviation of the temperature and the position of the upper roll is affected by the properties of the rolled metal and its absolute thickness. A computer is therefore introduced to determine the requisite relationship. This computer measures the variations of the temperature and of the thickness of the strip. Since the latter may be caused not only by temperature but also by other reasons, it is necessary to segregate those thickness variations which are due only to temperature. For this the computer evaluates the correctness of the choice of the coefficient of proportionality from the value of the cross-correlation relationship
Diagram of automatic control of temperature regulator settings for zones of heating furnaces
zone te:nperature regulator pusher meter measuring billet speed through furnace modulator, determining heat absorption of billets computer, determining required temperature distribution for furnace zones photopyrometer, measuring temperature of billets during rolling device evaluating heat loss of billets before rolling correcting device. adjusting the parameters of M H and eT by comparison of actual and calculated temperature
Schematic of compensation for the influence of temperature on thickness
q,
=
J:
C!.T(t)C!.h(t)dt
(assuming that the measurements of temperature and thickness are made at the same point of the strip). With a correctly chosen coefficient of proportionality a change of tJ.t does not involve any change of :3.h; and the integral has a definite value. When rolling strip in a continuous mill, thickness is affected also by variations of the tension. Rolling of the front and tail end takes place without tension, so that there are extra deforming forces on the stand and a significant increase of the thickness at the edges of the strip. However, the change of thickness along the strip is not random so that a programming system may be applied to adjust the setting of the rolls. The wide variety of strip rolled in the mill calls for a large number of programmes, which makes such a system extremely cumbersome. Instead of the introduction of constant programmes, at the Institute of Automation and Telemechanics of the Academy of Science of the U.S.S.R. a system has been worked out which is adaptive. That is to say, it is self-programming, in accordance with the actual known distribution of the thickness of the strip. The programmes are worked out by a computer, statistically analysing data from the measurements of a sufficiently large quantity of rolled strip. Such analysis permits the location of the deviation of the strip from nominal along
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2016
A. B. CHELUSTKIN
its length, and, in the same way, the finding of programmes for setting the stand screw-down (Figure 5).
rotating knives of the flying shears with the front end of the moving metal. In the system shown in Figure 6 the problem is solved by the aid of a computer, determining the amount of misalignment between the position of the cutters and the position of the front end of the strip. The computer also calculates the speed diagram necessary to establish the chosen misalignment of the cutters at a given moment.
e-
Stands
8
Knives
' -'-
'-
'-
0>
~-
-"0>" I
PhR2
GS
&n
Figure 5.
Diagram of automatic gauge control an thin strip hot-rolling mill
LC load cell SP screw-down sensor SD summing device STY set thickness value of strip MC system for control of motor M SS serve system TM thickness meter [
integrator
CM! . CMO
input and output commu-
of memory storage MS commutator control and
tatoTs
CC!. CC2
determination of lengths of rolle d sections of strip
TO PhR
Figure 6.
Block diagram of automatic control of /lying shear
tachogenerator photo-relay
The computer is made up of an integrator and several memory circuits corresponding to the number of sections into which the whole length of the strip is broken. Integration of deviations is carried out separately for each section of the strip, which permits the evaluation of the mean deviations of the thickness for all the sections. With the help of a step uniselector the mean value of the deviation is fed to the memory device. Signals from the memory devices are handled by another stepped uniselector; when fed to servo systems of the stand screw-downs, they call for the variation of the amount of reduction by rolling corresponding to the section of the strip. In order that random deviations of the length of the strip should not unduly change the programme, it is worked 'out in the process of rolling not one but several strips. This achieves a choice of value of the coefficient of proportionality of the integrating link of less than unity. To accelerate the working out of the programme with the contained system, the signals of the memory devices not equal to zero, and their aggregate, constitute a preliminary programme. This is set manually and then made precise in the process of further rolling. Instead of adjusting the rolls, the thickening of the strip at the ends may be prevented by modifying the tensioning programme for the drawing of the strip. Before the rear end of the strip has come out (or if the front end is accessible) tension is altered by changing the set values of the motor speed regulator of the stand. The change may either follow a previously chosen programme, or a programme worked out by a computer analogous to that shown in Figure 5. Control of Flying Shears Metal rolled in a continuous mill is cut to measured lengths by a flying shear, whose speed is automatically adjusted to match the rolling velocity. To reduce the waste of metal it is advisable to cut off only a short length of the front end. For this a system is applied automatically to align the continuously
The velocity of the cutters and of the last stand of the mill is adjusted by a tachometer regulator, incorporating tachogenerators of the stand TGS and of the knives TGN, and an electronic amplifier A exciting a rotary amplifier RA of the motor-generator system driving the knives. A limit switch LS is fixed on the shaft of the knives and closes every time they close. Thus contact LS permits the fixing of one point in the position of the knives. Continuous control of their position with the existence of one controlled and fixed point for each turn (or for several turns with rotation of the top and bottom knives at different speeds) is exercised by the saw tooth voltage generator, starting at the instant of operation of LS. The positioning of the front end of the billet relative to the knives is carried out with the aid of the photo-relay PhRI and the electronic time relay ETR, whose holding time depends on the speed of rolling (change of holding time of ETR is equivalent to space displacement of the photo-relay PhRI along the roller table between the mill and the shears). At the instant of operation of ETR, which occurs when the front end is at a set distance from the knives, a voltage is applied to the electronic commutator EC; also, at the input of the amplifier A there is, apart from the voltage proportional to the error of the tachometric system b.n, an a.c. voltage. This voltage has sufficient amplitude to cause a change of polarity of the amplifier A, reversal of the magnetic field of the rotary amplifier and intensive braking of the drive of the shears. At the same time the rising of the generator voltage is checked when ETR operates, which is fixed. The fixed voltage is remembered as the initial error. The voltage proportional to the amount of error is equated to the voltage of the integrator of the computer CT S b.n dt. When the voltage proportional to Sb.n dt becomes equal to half the voltage proportional to the error, the electronic commutator changes the phase of the a.c. voltage applied to A by 180°. This causes intensive acceleration of the drive of the knives, during which they take up the remaining half of the distance of error (the .
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AUTOMATIC COMPUTER CONTROL SYSTEMS FOR ROLLING MILLS
This scheme, in accordance with the resulting sign, changes the polarity of the voltage supplied to the integrator and proportional to the modulus of the measured deviation of the thickness of rolled strip. In continuous cold-rolling mills measurement of thickness before the strip enters the first stand is prevented by corrugations in the strip. Instead, gauge control is based on the readings of a gauge meter behind the first stand. In view of the existence of a lag, use is made of a pulse regulator, eliminating only the low-frequency components of deviation of thickness. It follows that the non-uniformity of thickness of strip behind the first stand has fundamentally only the character of momentary deviations. Their reduction requires an accurate regulating system, in which the readings of the thickness deviation meter enter a compensating computer, which supplies the necessary derivative component of the regulating action in the speed control loop of the last stand of the mill. The compensating computer takes account of the time the strip takes to travel from the meter to the penultimate stand of the mill and of the basic relations characterizing the process of continuous rolling. Allowance is also made for such factors as the correlation between the speeds of the stands and the tension, between the tension and the forced deformation of the stands and the transfer function of the electric drive of the stands, etc. The complicated character of these relationships necessitates a special electronic model of the mill, consisting of a series of units representing the stands of the mill. As a result of the model study a simplified structure of a compensating computer, most accurately representing the rolling process in a multi stand mill, has been developed. From the basic information on the non-uniformity of the thickness of the strip, and these correlations, the compensating computer calculates the changes in relative velocities of the stands to ensure that the thickness of the strip at the output of the mill will have the minimum deviation from nominal. As the programme of the compensating computer is approximate and does not completely correspond to the rolling process in a given example, the possibility of errors is not excluded. Consequently the thickness of the output strip will necessarily have a deviation. An accurate system of regulation will therefore include another computer-a correcting one. It establishes fluctuations in the thickness at the input and the output of a stand of the mill, and carries out automatic adjustment, optimizing the parameters of the compensating computer
drive of the knives ensures the equality of acceleration and deceleration). Thus, at the instant of closing of the knives the front end of the billet comes between the knives and is cut off. In the scheme considered the cut off length of the front end is determined by the setting of the time relay ETR. It is desired to hold this relay in accordance with the speed of the mill, but the initial choice of holding time may be achieved inaccurately. Moreover, in the process of mill operation the correlation between the speed of rotation of the rolls and the speed of motion of the rolled billet may be changed; therefore in the system ETR is automatically adjusted by comparing the actual length of the front end with that specified. The control of the actual lengths cut off the front end is achieved with the aid of the photo-relay PhR2, located at some distance from the knives. Automatic adjustment of the holding time is achieved by a reversing uniselector, whose contacts switch the setting resistance of ETR until the time of cutting coincides with the time of illumination of PhR2. A trial of the arrangement under consideration has shown that spread in the time of actuating the electric drive does not exceed 0·05 sec, which, at a rolling speed of 10 m/s, ensures the accuracy of cutting within the limit of 50 mm. Strip Gauge Control on Cold-rolling Mills A most urgent problem appears to be the regulation of the strip gauge in the cold-rolling process. Investigation has shown that non-uniform thickness of strip rolled at steady speeds originates mainly in the process of hot-rolling. In connection with this, of great interest are systems whose control action corresponds to the readings of a gauge meter, mounted at the entry rather than at the delivery side of the stands. In such a system the time-lag of the mill controllers may to a significant degree be compensated for by detecting and measuring the disturbance in advance. Also, as for a hot-rolling mill, in this system the coefficient of proportionality between the amount of deviation and the amount of displacement is determined by a computer comparing the non-uniformity of the thickness at the entry and delivery side of the mill. Computation is simplified by adopting a logical scheme of multiplication of the sign of the deviation of the thickness before and after rolling, which determines the required sign of the change of the coefficient of proportionality (Figure 7).
(Figure 8).
TG
Figure 7.
Block diagram of automatic regulation of thickness in a reversing hot-rolling mill
TM 1, TM2 TG TCI
thickness meters stand tachogenerator computer with self-adjusting parameters for the thickness-regulating system (by readings of TM I) computer of pulse system of thickness regulation (by readings of TM2) screw-down motor
TC2
M
Investigation of the regulating system on an electronic model of the mill has shown that the correcting computer must carry out automatic adjustment of two parameters of the compensating computer: the gain and the time lag 7. In optimizing these parameters it is usually convenient to make a step-by-step search by the gradient method. To decrease the deviation of the thickness in the periods of adjustment of the parameters of the compensating computer, caused, for example, by the temperature deformation of the rolls of the stand or the varying speed of rolling, the thickness meter, located behind the last stand of the mill, reveals any deviation of thickness and adjusts the speed regulator of the stand quite apart from the compensating computer. Furthermore, in high-speed cold-rolling mills a change in the rolling speed significantly affects the thickness of the rolled strip. Hence with varying rolling speed there arises the need for automatic control of the amount of reduction in the stand. Such control may be achieved by setting a programme of screwdown displacement and of the degree of tension on the strip at varying rolling speeds. The magnitudes of these displacements
505
2018
A. B. CHELUSTKIN
vary for different assortments of rolled strips. Therefore these programmes may be adjusted by a method similar to that used to adjust the programmes for regulating the thickness of thin I TMl
8D
receiving table and the pit from which the ingot is being drawn. Thus the resulting voltage is proportional to the actual distance between the crane and the receiving table at each particular moment of time. The pass transfer car also has a similar system of control of its position relative to the receiving roller bed.
1I
8 ·
: IdH :-1-
I ~
-BU RHO I C)RH11
: dH(I-T) L RH1_'0-_...!I---.:::.:....:..:..:..-...:..t:1:~---lC
AH (I-T)
CC2
Figure 8.
Block diagram of thickness regulation in continuous cold-rolling mill
TM I, TM2 DU RHO, RH!, RH2 CCl ee2 VR
Figure 9.
thickness meters variable delay unit recording and reproducing heads compensating computer correcting computer (for optimizing the parameter of eCI) speed regulator for stand motor SM
CD ID 11
le
x cc ' Xci VD
sheet in hot-rolling mills. The whole range of working speeds is divided into several sections. Then by statistical assessment of the readings of the thickness meter the required screw-down displacement for each section is calculated and stored.
BD Se. Si
6.S AB CBe, CBI
Adjusting Control of Pass Transfer Cars and Grab Cranes of the Soaking Pits The above examples of the application of computers are directly concerned with the technology of rolling processes. Below is considered an example of the use of computers for combined automatic control of several transport mechanisms. For blooming mills operating at full, special significance attaches to the control of the pass transfer car and the grab cranes which draw the heated ingots from the soaking pits. To reduce the time of these operations the practice is to arrange for the crane moving the ingot to advance towards a moving pass transfer car. Obviously, both moving mechanisms must stop at the same time and at the same place so that, without any additional shift of either, the ingot may be deposited in the cradle of the car. To ensure coordinated stopping of the car and the crane, the latter must be equipped with a servo system controlling the position of both mechanisms in the pit furnace bay. Figure 9 shows one version of a system for combined control of the pass transfer car and the grab crane, in which digital computing elements are applied. Such a system, consisting of a pulse transducer for the motion of the bridge and an electronic integrator for the storage of pulses, will position the bridge of the crane relative to the receiving roller bed of the blooming mill. The pulse transducer is on the bridge of the crane and generates a pulse when the bridge shifts by some given small distance (the transducer may be of the brush type, making contact with bars insulated from earth and located on rails. under the crane). When the crane moves with the ingot towards the blooming mill, the output voltage of the electronic integrator is proportional to the number of pulses fed from the transducer, i.e. to the shift of the bridge of the crane. This voltage is subtracted from the voltage proportional to the distance between the
Block diagram of combined control of pass tramler cars and grab crane pulse movement transducer same for ingot·carriage integrator for accumulating pulses unit showing initial coordinates of grab crane bridge current value of coordinates of grab crane and pass transfer car unit determining displacement velocity (speed of change of coordinates with time) braking distance calculator calculated values of braking distance for grab crane and pass transfer car current value of distance between crane and pass transfer car unit for advanced calculation of beginning of braking command devices for braking crane and pass transfer car
Voltages proportional to the positions of both mechanisms relative to the receiving roller bed are fed into the computer, which calculates the braking distance of these mechanisms and initiates their braking with such positioning of the bridge of the crane and the car that their complete stop will be achieved at one and the same point. Thus the operator of the grab crane controls the mechanisms of the crane only at the moment of grabbing the ingot from the pit and lifting it. All the other operations are carried out automatically, down to stopping of the grab crane over the pass transfer car, after which the operator visually checks the relative position of the car and the crane and controls the lowering of the ingot into the cradle of the car. At the instant of opening of the jaws and the beginning of raising them, a command is automatically given to move the car to the receiving roller-bed. The system for automatic control of the pass transfer car brakes it as it approaches the receiving roller bed and transfers the ingot thereto. After the ingot is loaded on to the table, which is also carried out automatically with the help of a photo-relay placed by the side of the receiving roller bed, the car starts its motion in the direction of the group of pits from which the next ingot will be drawn. Conclusions Along with the use of computers in closed loop systems for automating complicated technological processes of great importance is the application of computers to separate problems of automatic control, permitting considerable improvements of the quality of control. In the majority of cases even for comparatively simple problems, systems of automatic control should include self-adjusting elements.
506
2019
AUTOMATIC COMPUTER CONTROL SYSTEMS FOR ROLLING MILLS
DISCUSSION H. KRUGER (German Democratic Republic) What instrument is used for measuring the temperature of the hot metal in the furnace? A. B. CHELUSTKIN, in reply. No instrument is provided for measuring the temperature directly in the furnace. The temperature of the metal is measured after rolling by the first stand. Question from the floor What elements are used in the automatic control of the mill screws, and how much apparatus is required? A. B. CHELUSTKIN, in reply. The system can be built up from many types of elements; we use ferro-transistor elements. The number of elements depends on the desired accuracy; this is determined by the number of discharges of the counter and the logical circuits. Question from the floor What accuracy is achieved in transforming the tachogenerator voltage into a frequency? A. B. CHELUSTKIN, in reply. Transformation of a tachogenerator voltage into a frequency is difficult. However, if the engine runs at a low speed the accuracy is higher and reaches I per cent. Question from the floor What is used as a 'full speed' relay? A. B. CHELUSTKIN, in reply. An ordinary electronic relay is used as a full speed relay 'PC'. V. A. IVANOV (U.S.S.R.) I should like to deal with the application of computing equipment in a control system based on the disturbance. A distinctive feature of such a control system is its high speed of response and it ensures the possibility of complete elimination of deviations in the parameter to be controlled. However, these systems also have a considerable drawback: in changing the parameters of the plant the compensation of the disturbance
may not be complete and this may lead to considerable deviations of the controlled quantity at the output. Application of computing devices for setting the parameters in disturbance actuated control systems is a new and very promising trend in the correction of systems of automatic control which will make it possible to obtain optimum operating parameters. Therefore, the computing equipment which is used for setting the parameters, i.e. correcting computer apparatus (which controls the value of a certain function of the inputs and outputs of the plant), will automatically set the parameters of the compensators if the parameters of the plant change, hence the disturbances will be compensated in the best way. Thus, thanks to the application of correcting computer equipment, it is possible to retain the advantages of open-loop control, for example, fast response, and at the same time, to eliminate the main drawback, i.e. incomplete compensation of disturbances when the parameters and the characteristics of the plant change. A control system similar to the one described in the paper for controlling the thickness of the metal in hot rolling was developed by the Institute of Automation and Telemechanics, Academy of Sciences, U.S.S.R., for controlling the welding conditions on an electric tube stand. Setting the parameters of the compensator was also carried out on the basis of the correlation dependence between the input disturbancedeviation in the thickness of the tube wall, which is equivalent to the thickness of the strip from which the tube is made; the output parameter was the temperature of the weld seam which must be maintained constant irrespective of thickness variations. Due to high frequency disturbances at the input end, current control systems based on deviations do not permit the required control quality to be obtained. Therefore it is advisable in this case to apply the above-mentioned principle of designing a control system based on disturbances, with setting by means of correcting computing equipment. The system produced enables a high quality of control of the temperature to be obtained and, consequently, also a high quality of the weld seam which is directly related to the temperature. Compared with manual control, the deviation of temperature from the optimum value has been reduced by a factor of 6 to 8.
507
2020