US3695075A - Correction system for continuous rolling mill - Google Patents

Abstract

In a tandem mill a roll current for a first mill stand is measured after a workpiece has entered only the first stand and after it has also entered a second stand. A difference between the measured currents predicts a speed correction factor for the second stand by which the speed of the second stand is modified. Then the roll current for the first stand is again measured and the speed of the second stand is adjusted so as to put a difference between the first and third measured current within predetermined limits. Otherwise, the above process is repeated until a difference between the first and last measured currents is put in the limits. The last speed correction factor determined is stored in a computer.

Description

United States Patent 1151 3,695,075 Kubota 1 Oct. 3, 1972 [54] CORRECTION SYSTEM FOR 3,540,248 11/1970 Hostetter et al. ..72/8

CONTINUOUS ROLLING MILL Primary Examiner-Milton S. Mehr [72] Inventor Nobuo Kubota Kobe Japan Attomey-Robert E. Burns and Emmanuel J. Lobato [73] Assignee: Mitsubishi Denki Kabushiki Kaisha,

Tokyo, Japan [57] ABSTRACT [22] Filed: June 9, 1971 In a tandem mill :1 roll current for a first mill stand is measured after a workpiece has entered only the first [21] Appl 15l442 stand and after it has also entered a second stand. A difference between the measured currents predicts a [30] Foreign Application Priority Data speed correction factor for the second stand by which the speed of the second stand is modified. Then the June 11, 1970 Japan ..45/50439 roll current for the first stand is again measured and the speed of the second stand is adjusted so as to put a [52] US. ((51 difference between the first and third measured cup [51] Int.Id 72/8 17 6 rent within predetermined mim Otherwise, the above [581 Fla 0 process is repeated until a difference between the first R f C1 d and last measured currents is put in the limits. The last [56] e erences I e speed correction factor determined is stored in a com- UNITED STATES PATENTS P 3,457,747 7/1969 Yeomans ..72/19 2 Claims, 11 Drawing Figures #2: l M 5 K rukmn Z rn/vm. f

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PATENIEHUCIB I972 SHEET 8 0F 8 CORRECTION SYSTEM FOR CONTINUOUS ROLLING MILL BACKGROUND OF THE INVENTION This invention relates to a workpiece thickness control system for a continuous rolling mill and more particularly to a workpiece thickness control system for a multiple stand tandem mill by which workpieces can be continuously rolled in the absence of a tension or a pressure exerted upon that portions of the workpieces traveling between each pair of adjacent mill stands.

Upon continuously rolling a workpiece by a multiple stand tandem rolling mill, the workpiece is rolled while it bridges rolling mill stands. In order that the rolled products are particularly increased in accuracy of dimension, it is required to roll workpieces while each pair of adjacent mill stands do not exert any tension or pressure upon that portion of the workpiece traveling therebetween.

This purpose has been previously accomplished by the following two processes: One of the processes has been to roll a workpiece while a loop is formed along the workpiece traveling path between each pair of adjacent mill stands as in hot strip mills, wire mills, etc., while that process is possible to be effected with workpieces small in dimension, it is difficult to form such loops with large workpieces because they increase in V bending stress with an increase in dimension thereof.

Also the loop may have preferably a slack as small as possible but this measure has encountered a difficulty in stably controlling the speeds of rolling mill stands.

The other process has been to detect a tension or a pressure exerted on that portion of a workpiece traveling between each pair of adjacent mill stands to control the speeds of the mill stands. This detection of the tension or pressure is generally accomplished by measuring the roll current. However the roll current greatly depends upon various factors affecting the system operation, rendering the control of the speeds of the mill stand difficult.

SUMMARY OF THE INVENTION Accordingly it is an object of the invention to provide a new and improved workpiece thickness control system for a multiple stand tandem rolling mill capable of continuously rolling workpieces with no tension nor a pressure exerted on the workpiece traveling between rolling mill stands in the rolling mill and in which the disadvantages of the prior art practice as above described are eliminated.

The invention accomplishes this object by the provision of a workpiece thickness control system for a multiple stand tandem rolling mill comprising at least two rolling mill stands, wherein there are provided means for measuring a roll current for a first rolling mill stand, means for determining a variation in the roll current means for predicting a compensation component of a speed of a second rolling mill stand from the variation in the roll current, to modify the speed of the second rolling mill stand and means for controlling the speed of the second mill stand so as to put a variation in a roll current again measured for the first rolling mill stand within predetermined limits.

Preferably the workpiece thickness control system may comprise means for storing roll current for a first rolling mill stand, means for detecting a current difference between the stored roll current for the first rolling mill stand and a roll current for a second rolling mill stand obtained after a workpiece enters the second rolling mill stand and means for controlling an oscillator forming a sampling switch so as to put the current difference within predetermined limits thereby to adjust the speed of the second rolling mill stand.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 a schematic view of a detection mechanism for use with the conventional type of workpiece thickness control systems for continuous metal strip rolling mills to detect a tension or a pressure on a metal strip traveling between a pair of adjacent rolling mill stands;

FIG. 2a is a schematic view of a plurality of rolling mill stands of a tandem stand roughing wire mill;

FIGS. 2b and c are graphs illustrating rolling currents for the respective rolling mill stands shown in FIG. 2a under the control of a conventional tension-less control system;

FIG. 3 is a combined block and circuit diagram of a speed control system of the conventional type operatively coupled to a rolling mill stand of a blooming mill with parts illustrated in perspective;

FIGS. 4 and 5 are views similar to FIG. 3 but illustrating modifications of the arrangement shown in FIG. 3;

FIG. 6 is a block diagram of a workpiece thickness control system for use with multiple stand tandem rolling mills constructed in accordance with the principles of the invention;

FIG. 7 is a block diagram of a modification of the invention;

FIG. 8a is a schematic view of the rolling mill stands shown in FIG. 7;

FIGS. 8b and c are graphs illustrating roll currents for the respective rolling mill stands plotted against time;

FIG. 8d is a curve plotting a stand speed against time; and

FIG. 9a, b and c are logic flow charts explaining the operation of the arrangement shown in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 of the drawings, there are illustrated a first and a second rolling mill stand SI and SH respectively each including a pair of work rolls 10' and 12 between which a workpiece 14 is to be reduced in area. The rolling mill stands SI and SI] are operatively coupled to individual press-ductors 16 used to detect a tension on that portion of the workpiece 14 extending between both mill stands. Then controls not shown are used to control the speeds of the rolling mill stands so as to exert a null tension or pressure upon the workpiece. The arrangement of FIG. 1 is generally used with the conventional workpiece thickness control systems for multiple stand tandem rolling mills.

However, in small-sized and moderate sized rolling mills, rolling mill stands are generally exchanged for new ones after some time intervals of service leading to a disadvantage in view of the stand point of the maintenance of press-ductors disposed on the mill stands.

Therefore the exchange of rolling mill stands is of no utility also it has been already proposed to measure a roll reaction on a rolling mill stand to give a measure of a roll force exerted by the mill stand. However there have not yet been developed the equations describing the relationship between the roll reaction and roll force on the rolling mill stand with a sufficiently high accuracy.

Referring now to FIG. 2a a workpiece 14 has been already rolled by opposed work rolls l and 12 on a first and a second rolling mill stands SI and SH and reached somewhere short of a third rolling mill stand SIII of a roughing wire mill. Upon entering the workpiece 14 into the first mill stand SI a current flows through a driving motor (not shown) operatively coupled to that stand as shown at curve I in FIG. 2b. Then the workpiece 14 enters the second mill stand SII whereupon another driving motor (not shown) operatively coupled to that stand has flowing therethrough a roll current I, as shown in FIG. 2c. At the same time the roll current I is temporarity decreased then the mill stand SII is finely adjusted in speed to control the roll current I for the first mill stand SI to its original steady-state magnitude whereby the tension is removed from that portion of the workpiece l4 traveling between the mill-stands SI and SII. The process as above described is repeated with the succeeding mill stands such as a stand SIII to effect the tension-less workpiece thickness control.

Referring now to FIG. 3, there is illustrated a speed control system of conventional construction for use with blooming mills for one rolling mill stand thereof. The arrangement illustrated comprises a pair of vertical work rolls and 22, a drive motor 24 connected to drive the vertical rolls 20 and 22, and a pair of horizontal work rolls l0 and 12 driven by another drive motor (not shown) having a speed controlled in accordance with a speed compensated for screwdown by the vertical rolls 20 and 22. A load cell 26 is disposed below the horizontal work rolls l0 and 12 to sense the roll separation force between the rolls 20 and 22. That is, the load cell 26 is responsive to a workpiece 14 being entered between the horizontal rolls 20 and 22 to supply a corresponding electric signal to a metal-in-stand circuit 28 connected to ground through an operating winding of a transfer relay 30.

The drive motor 24 is connected across a source of electric power 32 through a current transformer 34. The source is controlled by thyristors to control the speed of the motor 24 and the current transformer 34 is operative to detect a current flowing through the drive motor 24 and to supply an output to a current detector 36. The transfer relay 30 includes two sets of normally open contacts 30a and b and two sets of normally closed contacts 30c and d. The current detector 36 is coupled through the normally closed relay contacts 30d to an operational amplifier 38 having connected between its output and input a capacitor and a series combination of a resistor and the normally closed contacts 300. The operational amplifier 38 is connected at the output to another operational amplifier 40 through the normally open relay contacts 30a and a resistor while the current detector 36 is also connected directly to the operational amplifier 40 through the normally open contacts 30b and a resistor. The amplifier 40 has a feedback resistor and is connected to a current control circuit 42 subsequently connected to a voltage control circuit 44. Then the voltage control circuit 44 is connected to the source 32.

The arrangement thus far described is operated as follows: When the workpiece enters between the vertical rolls 20 and 22, a current flows through a circuit with the roll motor 24 in accordance with the particular roll loading. That current is detected by the current detector 36 through the current transformer 34 and the detected current is applied to the operational amplifier 38. The operational amplifier 36 is pre-set to provide an output voltage equal to that from the current detector 36.

Then the workpiece 14 will be entered between the horizontal work rolls l0 and 12. At that time the load cell 26 supplies an output to the metal-in-stand circuit 28 indicating that the workpiece has been entered between the horizontal rolls l0 and 12. This causes the energization of the relay winding 30w to open the contacts 30c and d whereupon the operational amplifier 38 changes the operation from the time delay mode to the integration mode. This means that the operational amplifier 38 holds that magnitude of the output current developed at the instant the relay 30 has been just energized. That magnitude of the output current is applied through the now closed contacts 30a to the operational amplifier 40, also having applied thereto the actual magnitude of the current flowing the motor 24 through the now closed contacts 30b.

Therefore the operational amplifier 40 provides an output current equal to a difference between the roll current for the vertical work rolls 20 and 22 stored by the operational amplifier 38 and the current detected by the current detector 36 and applied to the amplifier 40. This output current or current difference is delivered to the current control circuit 42. The current control circuit 42 is operative to control the source 32 through the voltage control circuit 44 and by means of the thyristors disposed in the source 32 until the current difference becomes null. In other words, the control is effected to equal the current controlling the vertical rolls 20 and 22 to the current stored by the operational amplifier 38.

The process as above described is effected on the basis of the current control in which the speed of one set of work rolls is rendered equal to that of the other set of work rolls through the intermediary of the corresponding currents. Therefore the arrangement is disadvantageous in that any acceleration or deceleration directly reflects a variation in force exerted on a workpiece between the adjacent roll stands and that one must take into account of influences in the system operation resulting from a decrease in temperature of a workpiece being rolled, a change in loading such as skied marks on the workpiece and so on.

FIG. 4 wherein like reference numerals designate the components identical or similar to those shown in FIG.

3 illustrates a modification of the arrangement as shown in FIG. 3. FIG. 4 shows a pair of succeeding rolling mill stands SI and SII of a multiple stand tandem blooming mill. One drive motor 24-l or -II for each mill stand is adapted to drive a pair of horizontal work rolls NH and l2I or 10 and l2II rather than the vertical work rolls such as shown in FIG. 3 and operatively coupled to an individual pilot generator 461 or 4611 connected to the respective thyristor controlled source 32- I or -lI through an amplifier and also to a variable resistor 481 or 48H for setting up the speed of the associated mill stand SI or SII. Then the variable resistor 48-1 and -II are connected to a positive buss B+.

For each stand, the metal-in-stand circuit 281 or 28II is connected to ground through an individual relay 30I or 3011 including two sets of normally open contacts 30Ia and b or 3011a and b. By omitting the current detector 36 as shown in FIG. 3, the current transformer 32 is directly coupled to the operational amplifier 38 through the relay contacts 301!) and also to the operational amplifier 40 through the relay contacts 30IIb with the relay contacts 301a and 30Ila substituted for the relay contacts 300 and 30a shown in FIG. 3. The operational amplifier 40 is connected to the junction of the pilot generator 46II and the setting up resistor 48H. In other respects the arrangement is identical to that shown in FIG. 3.

After the setting up of the starting conditions including the magnitude of the resistors 48I and 48II, both the motors 241 and 24Il are energized to start the respective mill stands SI and SII. The workpiece l4 enters the first mill stand S] to permit the load cell 261 to energize the relay 301 through the metal-in-stand circuit 281. When energized, the relay 30I closes its contacts 301a and b to cause a current flowing through the motor 24I to be stored by the operational amplifier 38 through the current transformer 34.

Similarly, the workpiece entering the second mill stand SII causes the energization of the associated relay 30Il. Therefore its contact 30Ila and b are closed to deliver to the operational amplifier 40 both the roll current for the first mill stand SI stored by the operational amplifier 38 and the actual roll current for the same mill stand. A current difference between these roll current provided by the operational amplifier 40 is applied to the junction of the pilot motor 4611 and the resistor 48Il until it is corrected.

This results in the workpiece thickness control being continuously effected under non-tensioned state.

After the workpiece 14 has passed through the arrangement as shown in FIG. 3, the speed setting up resistor 4811 can be set to the command magnitude corresponding to the corrected speed obtained with that workpiece. This measure permits head end gauge of or gauges of the succeeding similar workpiece or workpieces to increase in accuracy because the mill stands have now their speed more proper than those set for the previous workpiece.

With a third mill stand (not shown) following the second mill, it is to be noted that the speed correction on the second mill stand should have been acomplished before the workpiece will enter the third mill stand.

In the process as above described, the tensions or pressures due to the first and second mill stands serve to effect the closed loop control through both the roll current for the first mill stand and the speed of the second mill stand. However the roll current greatly depends upon the material, temperature and cross sectional area of a workpiece being rolled, percent rolling reduction, the magnitude of the motor's field excitation, etc. This results in great changes in factors affecting the system operation. Among them that factor affecting the roll current is particularly large in change as will be readily understood from the equation where I is a current flowing through a rolling motor for a mill stand, A! is a variation in the current I, A is a control voltage for correcting the speed of the succeeding stand, AA is a'variation in the voltage A and K is a constant. Therefore it has been difficult to form a stable control system of the arrangement as shown in FIG. 4.

Therefore it can be presumed that the relationship between a variation in roll current and a speed of a rolling mill stand may be preferable to be somewhat loose. To this end, a difference signal for a roll current for the first mill stand may be used to drive a servooperated potentiometer thereby to control the speed of the second mill stand. Alternatively an on-off switch may be operated to control sampling while at the same time a sensitivity switch is used to compensate for the system gain.

FIG. 5 show a tension-less workpiece thickness control system including a servo-operated potentiometer such as above described. In FIG. 5 where like reference numerals designate the components identical to those shown in FIG. 4, the operational amplifier 40 is connected to a servo-motor 49 for controlling the speed setting up resistor 48H but not connected to the junction of that resistor and the pilot generator 46. In other respect, the arrangement is identical to that shown in FIG. 4. The arrangement is disadvantageous in that the system response is slow. Assuming that the law of conservation of mass flow is held, it is required only to maintain a constant value of a speed ratio between the successive mill stands.

Referring now to FIG. 6, there is schematically illustrated a workpiece thickness control system for a multiple stand tandem rolling mill constructed in accordance with the principles of the invention and particularly operative in the'sampling mode. The arrangement illustrated comprises an input terminal 50, leading to a current detector for roll current such as labelled 36 in FIG. 3, a first operational amplifier 52 acting as an integrator and a second operational amplifier 54 connected in the similar manner as above described in conjunction with the operational amplifiers 38 and 40 as shown in FIG. 3. More specifically, the input terminal 50 is coupled through a set of normal closed contacts Rlb to the operational amplifier 52 including a capacitor, a resistor and a set of normal closed contacts Rla in its feedback circuit and serving to store a roll current under non-tensioned state for the associated mill stand, for example, a first mill stand of a multiple stand tandem rolling mill (not shown), as previously described in conjunction with FIG. 3. The operational amplifier 52 is coupled through a set of normally open contacts R2d to the operational amplifier 54 including a capacitor, a resistor and a set of normally open contacts Me. The operational amplifier 54 is then connected to a tapped resistor 56 including a plurality of switching taps, in this case, 10 taps 1 through 10 selectively coupled to an operational amplifier 58 through a set of normally closed contacts R2b. The operational amplifier 58 includes a resistor, a capacitor serially interconnected and connected across a set of normally closed contacts R3a in its feedback circuit and forms a proportion plus ingration circuit having a proportional constant and an integration constant sufficient to limit an overshooting of a roll current for the succeeding or second mill stand due to the rising portion of the output therefrom, to a magnitude as low aspossible. The operational amplifier 58 is connected to an output terminal 60 leading to an input to a speed control limiter (not shown).

The input terminal 50 is further coupled through a set of normally open contacts R2e to the operational amplifier 54 and through a set of normally open contacts Rld to a comparison amplifier 62 including a feedback resistor with the integrator 52 coupled to the comparison amplifier 62 through a set of normally open contacts Rlc. The comparison amplifier 62 is operative to compare the stored magnitude of roll current in the integrator 52 with the actual magnitude of roll current for the same mill stand and provide an amplified current difference therebetween. The amplifier 62 is coupled to amplifiers 64 and 66 for energizing the respective relays R4 and R5. The amplifiers 64 and 66 have setting up elements 68 and 70 connected respectively to their inputs.

The arrangement further comprises a sampling switch formed of an operational amplifier 72, a relay R6 and their associated components. Specifically, a setting up element 74 including a variable capacitor and a potentiometer serially interconnected is coupled to the operational amplifier 72 including a capacitor and a set of normally closed contacts R20, another setting up element 76 including a capacitor and a parallel combination of a relay R7 and a potentiometer is also coupled to the operational amplifier 72 which is, in turn, connected to an amplifier 76 for energizing the relay R6. An additional setting up element 80 including a variable capacitor and a resistor is connected to the amplifier 78 at its input.

The relays each includes their contacts designated by the reference characters denoting that relay and suffixed with a reference character 0, b, c For example, the relay R1 includes two sets of normally closed contacts R and b and three sets of normally open contacts R10, 0 and e.

On the lower portion of FIG. 6, there are shown straight line circuits for relays. As shown, the relay Rl winding is connected between a positive buss 8+ and a negative buss B through a set of normally open contacts 82 and a set of normally closed contacts 84. The contacts 82 are adapted to be closed when a roll current for that mill stand operatively coupled to the arrangement of FIG. 6, in this case, a first stand is to be stored while the contacts 84 are adapted to be opened when a roll current for the next but one stand or a third mill stand is to be stored. The relay R1 winding has connected thereacross a series combination of a set of normally closed contacts R80 and the relay R3 winding and also a winding of a slow operating relay R9 having a time delay for example, about 0.2 second within which a roll current for the associated mill stand is restored to its steady-state magnitude after the impact drop thereof. The relay R2 winding is connected between the busses 8+ and B through normally open contacts R90, normally closed contact R60 and normally closed contacts R8b while a winding of a relay R8 is connected between both busses B+ and B through normally open contact Rle, normally open contacts R40, and normally closed contacts R50 with a set of normally open contacts R8c connected across the contacts R40 and R50;

In operation a workpiece (not shown) enters the second rolling mill stand (not shown) to close the contacts 82 to energize the relays R1, R3 and R9. When energized, the relay R9 closes its contact R90 thereby to energize the relay R2. The energization of the relay R2 causes the closure of its contacts R2c, d and e permitting the operational amplifier 54 to compare the roll current for the first mill stand stored by the operational amplifier 52 with the actual roll current for the same stand to produce an amplifiered current difierence between both current as will readily be understood from the previous description of the operational amplifiers 38 and 40 as shown in FIG. 3.

Now the relay R6 is put in its energized stated in the sampling switch to open its R60 to deenergize the relay R2. Therefore the contacts of the relay R2 are returned back to the original positions and the operational amplifier 54 holds the current difference between the stored and actual roll currents therein.

On the other hand, if the output from the operational amplifier 62 is within a, percent for example, 3 percent of a predetermined value, then the relay R4 is energized and the relay R5 is deenergized. When energized, the relay R4 closes its contacts R40 resulting in the energization of the relay R8. The energized relay R8 closes its contacts R to form a self-holding circuit therefor and also opens its contacts R8a to deenergize the relay R3 to render the output from the operational amplifier 58 null. The output from the operational amplifier 58 is applied to the output terminal 60 leading to a speed control limiter (not shown). Also the deenergization of the relay R2 causes the operational amplifier 72 to produce a null output thereby to deenergize the relay R6. After the deenergization of the relay R6, a predetermined time interval lapses until the relay R2 is again energized. Since the operational amplifier 72 is of the integration type, the output therefrom energizes the relay R6 after a predetermined time interval. In this way, the relay R6 is repeatedly energized at predetermined time intervals sampling switch in the oscillation mode. Therefore the sampling switch operates as an oscillator.

Then the workpiece enters the next but one mill stand or the third stand (not shown) to open the contacts 84 whereupon the relays R1, R3 and R9 are deenergized resulting in the termination of the correction of the speed of the succeeding or second mill stand in the sampling mode of operation. At the same time, the process as above described in initiated and repeated with the next but one mill stand or the third stand by using an arrangement identical to that illustrated in FIG. 6and so on.

As computer controls have been lately developed, the direct digital control (DDC) system is increasingly substituded for the conventional analog control system.

For example, position controllers of the conventional construction have been operatively coupled to the small-sized computer operative in the time sharing mode. Furthermore the small sized computer has been in corporated into hot strip mills for purpose of effecting the automatic strip thickness control of the DDC type.

In FIG. 7 wherein like reference numerals designate the components identical or similar to those shown in FIG. 4, there is illustrated a predicted adaptable workpiece thickness control system incorporated into a multiple stand tandem rolling mill in accordance with the principles of the invention. In FIG. 7 four rolling mill stands SI, SH, SIII and SIV each are shown as having a pair of horizontal work rolls and 12 and having the comments identical to those operatively coupled to the first mill stand SI as shown in FIG. 4 except for the omission of the metal-in-stand circuit. Instead the load cell 26 for each mill stand is directly connected to a small-sized DDC computer 86 as does the current transformer 34, also all the speed setting up resistors 48 are controlled by the computer 86. Each component for one mill stand is designated by the same reference numeral denoting the corresponding component shown in FIG. 4 and suffixed with the Roman reference numeral for that stand. For example, the drive motor for the first stand SI is designated by the reference numeral 34] and the resistor for the third stand SIII is designated by the reference numeral 48III.

The operation of the arrangement shown in FIG. 7 will now be described with reference to logic flow charts as shown in FIGS. 9a, b and c. As shown in FIG. 9a, the control operation is initiated at the start block 100 in which a workpiece enters the nth stand for example the first stand SI as determined by the associated load cell 261. Then block 101 interrogates to see if that workpiece is a first one to be processed in accordance with the particular rolling reduction schedule. If so, the control program goes to block 102 in which a speed correction factor Kp is obtained from a table stored in the process computer 86. If the workpiece has been determined not to be a first one, block 103 is reached where the speed correction factor is given as a K value updated for the previous workpiece. Thereafter a short time delay is initiated in block 104 until a roll current for the driving motor 241 escapes from its impact drop and has a steady-state magnitude while the associated mechanical stand system has been attenuated. The time delay is usually of about 0.2 second. After the time delay 104, the process proceeds to block 105 where the roll current is measured repeatedly generally three times at different time points to determine their average 1 which is, in turn, stored in the computer. While smoothing filter means are usually used to eliminate the effects of a ripple current originating from the associated pilot generator 46, a ripple voltage from the source resulting from the associated rectifier, etc., upon the roll current, it is required to measure the roll current repeatedly at several different time points and to average the measure magnitudes of roll current. In this case, three magnitudes of roll current have been averaged to the roll current 1 After the measurement of the roll current, the program for the stand (n) or the first stand terminates at block 106.

Then the workpiece enters the succeeding stand (n l), in this case, the second stand as shown at block 200 in FIG. 9b. After the workpiece has entered the second stand a predetermined time delay is initiated in block 201 until the roll current for the preceding or first stand has its steady-state magnitude. Thereafter, the roll current for the first stand is again measured three times and averaged to I in block 202. The control program then proceeds to block 203 in which Al -=1 I is calculated and then to block 204 in which the succeeding or second stand is corrected in speed according to the equation AVm/ V =K,AI, where Kp for the first stand, this Kp may be what is stored in the table or in the form of a prediction equation within the process computer. Alternatively it may be delivered to the DDC computer after it has been computed by another computer. After the speed correction thus calculated has been supplied to the driving system for the second stand, a time delay 205 is initiated until that driving system has responded to the speed correction. After that time delay the roll current for the first stand is also measured three times and the measured values of the current is averaged to I, in block 206. Then in block 207 a difference between the roll currents previously and now measured for the first stand or A! I I, is calculated and block 207 interrogates to see whether the absolute value of the A! or IAI I is greater than a predetermined fraction, for example, 3 percent of the previous current I If the l l is not greater than 3 percent of the I then the control program goes to block 209. This means that the operation of correcting the speed of the second stand has been completed and therefore that no tension is applied to that portion of the workpiece traveling between the first and second stands. In block 209 the roll current for the succeeding stand (n l), in this case, the second stand is measured three times and their average I is stored in the computer. Thus the roll current in non-tensioned state for the next stand has been determined. The program goes to block 210 in which the speed correction factor Kp used to correct the speed of the second stand in block 204 or updated as will be described is stored in the computer. Then the program terminates at block 21 1.

On the other hand, if the l AI 1 is greater than 3 percent as determined by block 208 then the program is executed through a subroutine as shown in FIG. 9c. In that event the first speed correction has been insufficient to compensate for the particular change in roll current so that the Kpvalue first used is considered to have been erroneous. Therefore, as shown in FIG. 90, a second speed correction is effected in block 301. More specifically, the first value of speed correction and the value of current correction are used to calculater a second value of speed correction according to the equation IIl/ II AI where V is a voltage across the motor for the stand (n l), in this case, the second stand and AV, and AV are their changes. After a time delay 302 the program is executed through blocks 303, 304 and 305 similar in function to blocks 206, 207 and 208 as shown in FIG. 9b excepting that instead of the I, and AI "L2 and A! are calculated as shown within blocks 303 and 304 in FIG. 9c.

If the absolute value of the AI or 1 Al I is not greater than 3 percent of the I m as determined by block 305 then a new speed correction factor Kp' is calculated by using the equation m/ VII in block 306. Then, in block 307 a statistic technique is used to update the speed correction factor Kp according to Kp =updated wKp BKp where a+ B 1. After a time delay 308, the program is returned back to block 209 and terminates at block 211 as above described in conjunction with FIG. 9b.

If the block 305 answers in the affirmative then another subroutine as shown in FIG. 90 is executed. Specifically, this subroutine passes from block 401, through 402, 403 and 405 and to block 404. These blocks 401, through 404 are similar in function to blocks 301 through 305 respectively excepting that the speed for the stand (n l) or second stand is corrected according to the equation where AV is a change in voltage across the motor for the stand (n l) or the second stand while A1 is calculated according to AI 1 I, where I is an average of measured roll current for the first stand, as shown within blocks 401 and 404 in FIG. 9c.

If block 405 gives a negative answer with respect to the ratio A1,; I I then a new speed correction factor Kp" is calculated by dividing (AV +AV V by (A1, AI in block 406 after which in the Kp is updated according to Kp updated =aKp BKp' 'yKp" where a+B+y= 1 in block 407. Then the program proceeds to block 308 until it terminates at block 211 as previously described.

m/ rr= 12 On the contrary if the absolute value l A1 I is greater than 3 percent of the I as determined by block 405 then block 408 interrogates to see if time is still left for updating the stand (n 1) speed. This is because the execution of blocks 406, 407 and 209 must be completed before the head end of the workpiece enters the stand (n 2), in this case, the third stand. If block 408 answers in the negative the program proceeds to block 209 with a time delay 308. Otherwise the program is returned back to block 302 and proceeds through the successive blocks 303, 304

In summary, the roll current for the stand (n) wherein may be equal to l is stored in the computer and a variation in roll current for the stand (n) is measured after the workpiece has entered the stand (n I). Then this variation in roll current for the stand (n) is utilized to determine a force exerted on that portion of the workpiece traveling between the stand (n) and the stand (n 1). That is, the speed correction for the stand (n l is predicted from the variation in roll current for the stand (n) thereby to update the speed of the stand (n l). Thereafter, a variation in stand (n) current is again measured thereby to correct the prediction equation for speed correction and to update the stand (n l) speed. If the variation in roll current is not within predetermined limits the routine as above described is repeated to further update the stand (n l) speed until the variation in stand (n) current is put within the predetermined limits. Thereafter the roll current for the stand (n l) is measured and stored in the computer as a reference value utilized when the workpiece will have entered a stand (n 2) or a third stand. Also the last speed correction factor updated is stored in the computer and utilized to process the succeeding piece. The roll currents for the respective stands change as the functions of time as shown in FIG.

8. In FIG. 8, the axis of abscissas represent time and the axis of ordinates represents a roll current 1 for the stand (n) or the first stand SI in FIG. 8b and a roll current I, for the stand (n l) or the second stand 81! in FIG. 8cand aspeedN ofthe stand (n+ l) in FIG. 8d. As shown in FIG. 8b the roll current for the stand SI is rised in response to the entering of a workpiece into that stand and has a steady-state magnitude at and after a time point to. At that time point, the roll current I, is stored in the computer and provides a roll current I stored in non-tensioned state at a time point t after the workpiece has entered the second stand SII the roll current I, is measured. The measured current I t is less than the tension-free current I, stored by a magnitude of Al indicating that a tension is exerted on that portion of the workpiece traveling between both stands SI and 811. Therefore the speed N of the second stand SII is caused to decrease as shown in FIG. 8d. This causes a decrease in that tension permit the roll current I, for the first stand SI to approach the steady-state magnitude 1, stored. Similarly, the speed of the second stand SH are updated at time points t,, i to progressively decrease difference between the steadystate current I, stored and the measured currents for the first stand as shown at Al Al in FIG. 8b so that the roll current I, approximates the steady-state current I, stored. Eventually, the roll current I is equal to the steady-state current I, stored whereupon the non-tensioned state is established between the first and second states SI and SI] with the speed correction factor Kp provided as a function of the magnetic flux in the associated driving motor. As well known, parameters of rolling status such as a roll reaction P, a torque G, a forward rate f, and a backward rate e have certain mathematical relationship with workpiece parameters such as an entry workpiece thickness H, a delivery workpiece thickness h, an entry workpiece width W, a delivery workpiece width W, a background tension t, a forward tension t,, a temperature T, a coefficient of friction p. etc. Therefore their models may be used to compute the speed correction factor Kp.

From the foregoing it will be appreciated that the invention has provided a workpiece thickness control system for a multiple stand tandem rolling mill capable of continuously rolling workpieces while each pair of adjacent rolling mill stands do not exert any tension or any pressure on that portion of the workpiece traveling therebetween.

What is claimed is:

1. In a workpiece thickness control system for use with a multiple stand tandem rolling mill comprising at least two rolling mill stands, the combination of means for measuring a roll current for a first one of the rolling mill stands, means for determining a variation in roll current for the first rolling mill stand, means for predicting a speed correction for a second rolling mill stand from the variation in roll current for the first rolling mill stand to modify the speed of the second rolling mill stand, and means for controlling the speed .of the second rolling mill stand so as to put a variation in roll current again measured for the first rolling mill stand within predetermined limits.

2. In a workpiece thickness control system for use with a multiple stand tandem rolling mill comprising at least two rolling mill stands, the combination of means for controlling said oscillator forming said sampling switch to put said current difference within predetermined limits, thereby to adjust the speed of the second rolling mill stand.

Claims (2)

1. In a workpiece thickness control system for use with a multiple stand tandem rolling mill comprising at least two rolling mill stands, the combination of means for measuring a roll current for a first one of the rolling mill stands, means for determining a variation in roll current for the first rolling mill stand, means for predicting a speed correction for a second rolling mill stand from the variation in roll current for the first rolling mill stand to modify the speed of the second rolling mill stand, and means for controlling the speed of the second rolling mill stand so as to put a variation in roll current again measured for the first rolling mill stand within predetermined limits.

2. In a workpiece thickness control system for use with a multiple stand tandem rolling mill comprising at least two rolling mill stands, the combination of means for storing a roll current for a first rolling mill stand, means for determining a current differencE between the stored roll current for the first rolling mill stand and a roll current for a second rolling mill stand measured after a workpiece enters the second rolling mill stand, a sampling switch comprising an oscillator, and means for controlling said oscillator forming said sampling switch to put said current difference within predetermined limits, thereby to adjust the speed of the second rolling mill stand.

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