US2251493A - Control system - Google Patents

Description

1941' T. B. MONTGOMERY ETAL 2,251,493

CONTROL SYSTEM Filed Dec. 18, 1937 Patented Aug. 5, 1941 CONTROL SYSTEM Terryl B. Montgomery, Wauwatosa, and Edward H. Stivender, West Allis, Wia, assignors to Allis-Chalmers Manufacturing Company, Milwaukee, Wia, a corporation of Delaware Application December 18, 1937, Serial No. 180,520

- 13 Claims.

This invention relates in general to electrical control systems and particularly to electrical systems for controlling a strip of material being wound upon and unwound from reels or similar devices while an operation is being performed on the strip and for controlling and indicating the tension applied to such strip during such operation.

In rolling mills where steel strip is passed between rolls to change its dimensions, outer surface or internal structure, it has been found desirable to maintain the strip under tension during such rolling and to maintain this tension as nearly constant as practicable. Tensioning may be provided by connecting dynamo electric machines to the reels or to rolls at various points along the strip and by controlling the power of such machines. The power (input if a motor, output is a generator) of any tensioning machine must be maintained constant for a constant tension at a given constant strip speed. If the strip speed is varied, the tensioning power must be varied to keep the tension constant. The loss or gain in tensioning power due to acceleration and deceleration of the strip must be accounted for to maintain a constant tension. The tension may be indicated by an electrical instrument reading the tensioning machine power orsome factor thereof suitably compensated.

It is therefore an object of this invention to provide an improved control system for a strip rolling mill wherein a more nearly constant tension can be simply, reliablyand automatically maintained.

It is a further object of this invention to provide in a strip tensioning system a control providing substantially constant tension on the strip It is a further object of this invention to provide a steel strip rolling mill control system in which the tension control is energized from a source of energy having a relatively great difference of potential thereacross but a relatively low voltage therefrom to ground.

Objects and advantages other than those above set forth will be apparent from the following description when read in connection with the ac-' companying drawing in which the single figure is a diagrammatic representation of a strip rolling mill and control system therefor embodying the present invention.

A strip of material I is unwound from a reel 6, passed through the mill rolls l3 and wound upon the reel 8 in the arrow direction shown. The mill rolls it have large backup rolls [4 and are driven by the main mill motor l5 energized from a generator it operated by a suitable source of power such as a synchronous motor H. In the system described, the strip 5 is tensioned at three points by the dynamo electric machine 1 connected mechanically to reel 6, the dynamo elec-- tric machine 8 connected mechanically to the reel I and the dynamo electric machine I: connected mechanically to the strip 5 by one or both of the rolls Ii. The speed of the mill, and hence the speed of the strip through the rolls I3, is controlled by controlling the field current in the fields II and it of the generator I6 and the motor I5.

during acceleration and deceleration of the strip.

It is a further object of this invention to pro vide in a strip tensioning system a control providing substantially constant tension on the strip regardless of change in strip speed.

It is a further objectof this invention to provide in a strip tensioning system a control providing substantially constant tension on the strip by an improved compensation for inertia effect of the mill during changes in strip speed.

It is a further object of this invention to pro,- vide in a control system utilizing any or all of the above features to maintain constant strip ten-.

sion, a simple and accurate means of indicating the strip tension.

It is a further object of this invention to provide in a strip tensioning system of the above type, tension indicating means compensated for increased'length of strip due to reduction of the thickness thereof.

It is a further object of this invention to provide in a strip tensioning system a means for adiusting the strip tension which will be accurate at very low or zero tensions.

The speed control is effected by a motor operated rheostat shown generally at 43 having a motor 52 provided with windings 53 and 54 whereby it is rotatable in either direction to move the contact bar 49 to the left or right as viewed in the drawing. Movement of the contact bar 9 to the left decreases and to the right increases the speed of the mill. In the lower speed range, the speed change is eii'ected by decreasing the amount of resistance 59 in circuit with the generator field it. In the upper speed range, the speed change is effected by decreasing the amount of resistance in circuit with the motor field it. This permits starting the mill with full armature current and full torque on the motor ii.

The three tensioning machines I, 9 and [2 are I regulated to maintain the tension on the strip substantially constant by means of regulators 25, 28 and 21 respectively. The pull transferred by any of these machines to the strip 5 is equal to the power (input, if a motor and output, if a generator) divided by the speed of the strip. Hence for a given strip speed, which is substantially constant for a given pass, the power of the tensioning machines must be maintained constant in order to maintain a constant tension.

The machies 'I and 9 are connected in series loop with a booster generator 28 which has a reg ulator 32, responsive to the current in interpole winding 33, for controlling the current in the field 29 so that a constant current flows in the loop circuit. A rheostat 81 is provided for adjusting the value at which the current is maintained constant by the regulator 32. With a constant current flowing in the armatures of machines 1 and 9, the voltage across each machine is maintained constant in order to maintain a constant power (input or output) and therefore a constant tension supplied to the strip. The voltage across the machine 9, for example, is maintained constant by the regulator 26 which controls the current through the field 24 in response to energization of coil 45 responsive to the voltage across machine 9. Thus the power supplied motor 9 remains constant regardless of speed changes due to the building up of material of the reel 8. The core upon which the coil 45 acts is biased downwardly in any suitable manner, for example, as by gravity, and the coil 45 tends to pull upwardly on the core. For a given strip speed and tension the current in coil 45 is constant and the regulator is balanced.

If the speed of the strip 5 is maintained constant and a constant tension thereon is maintained, the speed of the machine 9 must necessarily decrease as the material builds up on the reel 8. To consider the above change, let us note the eiiect of a small change in build up of material on the reel.

First, a small increas of material, increases proportionally the lever arm through which the motor torque acts on the strip. Assuming the strip speed constant, it would be necessary to increase the power supplied to th motor 9 in proportion to the lever arm increase in order to maintain the motor speed constant. However, with constant strip speed, as no change in power supplied the motor has yet been effected, the motor speed decreases proportionally to the lever arm increase. The decrease in speed of motor 9 decreases the generated voltage thereof, thereby decreasing silghtly the power supplied to the motor and consequently slightly decreasing the tension in strip 5. To return the power (and thereby the strip tension) to normal, the decrease in voltage across the motor 9 decreases the current flowing in the coil 45 and allows the core to move downwardly'due to the bias thereon.

The downward core movement decreases the regulator resistance, thereby increasing the current through the field 24 which in turn increases the power supplied to the motor to its normal value and the regulator 26 is again balanced and the tension is again nomal. The regulator regulates the generator 7 in a similar manner by controlling the field 23.

In order to hold a constant tension on the tensioning motor l2, it is necessary to supply a constant power to this motor for a given speed of strip 5. This power must be increased or decreased directly as the strip speed increases or decreases to maintain tension constant. Therefore it is evident that for a given strip speed and tension, the voltage across motor I2 is constant and if a constant current is maintained in the armature thereof, the tensioning mot'or power and therefore the pull on the strip will be constant. The main coil 55 of the regulator 27 is therefore connected across the interpole winding 34 of the motor (2 and operates the regulator to vary the current in the field 22 of the motor l2 so that the current through the interpole winding 34 (and therefore motor current) is maintained constant.

The above provisions for maintaining a constant tension on the strip apply only for a given constant strip speed. When the speed of the mill is changed, the tensioning control means must be compensated therefor in order to maintain the tension constant. This compensation is provided by resistances 63, 62 and 64 in series with the coils 45, 35 and 55 respectively. Increments of these resistances are added to their respective circuits upon movement of the bar 49 to the right, as shown in the drawing, which increases the speed of the mill.

For example, increasing the resistance 53 in series with the coil of regulator 26 causes an increase in the voltage across the motor 9 in order to balance the regulator. The resistance 53 is calibrated so that the increase in motor voltage (and hence motor power as the motor current is constant) is proportional to the strip speed change. The tension is therefore automatically maintained constant regardless of change of speed of the strip 5.- The changes in resistance 62 similarly compensate the control of generator 1.

As previously stated, the voltage of the generator I6 is constant only over the upper speed range of the mill During the lower range of mill speed the increase in speed of the mill, and therefore the speed of the strip, is directly proportional to the generator voltage. The generator voltage is also the voltage across the machine l2. Therefore, during the lower range of speed, the tension is maintained constant by maintaining a constant current through the motor l2 and by increasing the voltage across the motor l2 directly proportional to the speed of the strip.

During the upper speed range the voltage of generator 16 and hence voltage of motor I2 is maintained constant. Upon raising speed in this range, added increments of resistance 64 are included in series with coil of regulator 21, necessitating a greater voltage drop across the interpoles 34 to balance the regulator. The resistance 64 is calibrated so that the regulator 21 increases the power supplied motor l2 directly as the speed changes and the tension is therefore maintained constant.

In order that the regulator 25 be responsive to that part of the terminal voltage of the machine 9 available for power transmitted to the reel 8, the current in coil 45 is compensated by a voltage drop through a resistor 72. This resistor l2 is-energized from an outside source and the voltage drop there across is made equal to the IR drop through the armature of motor 9. In the mill shown resistor 12 could be suitably connected across the excitation circuits 85, 86. however, if a similar compensation is to be also provided for the machine 1, a separate outside source such as the rectifier and alternating current source shown must be used in order that the circuits may be separate.

This IR. drop compensation can generally be omitted from the drag generator regulator 25, as the entry tension is seldom sufficiently low in value to make such compensation necessary. The eiTect of the 1R drop becomes negligible at high field strengths. The delivery reel motor 9 may frequently, in practice, run at nearly zero torque.

Provision is made for adjusting the tension in a new and improved manner. Separate adjusting coils 31, 41 and 51 are provided on the regulators 25, 26 and 2'! and the current in these adjusting coils may be varied manually by the rheostats 38, 48 and 58. The utilization of separate adjusting coils provides definite advantages. The regulators can operate effectively and accurately at low and even zero current in the main coils. Indication of tension can be effected in an improved manner.

The regulators 25, and 21 each always balances against the biasing force at a fixed number of ampere turns in the solenoid. These ampere turns may be supplied by one or more of the regulator coils, but the total ampere turns supplied is constant. The ampere turns of the adjusting coils 31, 41 and 51 add to those of coils 35, 45 and 55.

For example, suppose the rheostat 48 is in its extreme left position so that all the resistance is in circuit with coil 41, and that in such position the coil 45 takes two amperes to balance the regulator. It is then desired to decrease the tension and the rheostat 48 is moved to the right cutting out half of its resistance and thereby adding ampere turns to coil 41 in such amount that only one-half the ampere turns in the main coil 45 are now required to balance the regulator. This means that only one ampere is required in coil 45. As the resistance of main coil circuit does not change, one ampere has a voltage drop one-half that of two amperes. Thus by moving the rheostat 48 one-halt its distance, the voltage of motor 8 decreases to one-half its value and as the motor current is constant, the power and therefore tension decreases to onehalf its value. The fact that the change in regulator coil current is directly proportional to 1 tension change is utilized in an improved tension indicating means.

In prior art control, the tension is adjusted by change of a resistance in series with the main coil of a vibratory regulator, the tension being decreased by decreasing the amount of resistance in series. In such systems, the tension could not be decreased to zero or low tensions due to the fact that the regulator coil itself had resistance which could not be cutout. The regulators utilized in the prior art were of the vibrating type which at zero and low current would become inoperative.

In the case of the tension regulator 21 this type of adjusting coil provides further advantages. This rheostat is manually operated to change the resistance in series with the main regulator coil and is handled by the mill operator. The prior art rheostats are energized from the relatively high voltage circuit of the main power circuit. The coil 51 is energized from the relatively low voltage excitation circuit 85, 85 and hence there is less danger to a person operating a rheostat 58. In the prior art control the adlusting resistance operates on a relatively low difference of potential or voltage drop across such resistances. Coil 51 operates on a relatively high difference of potential or voltage drop thereacross and therefore a more accurate, smootheradjustment can be obtained in that a greater number of steps with a greater difference in voltage for each step is provided. Coarse and Vernier rheostats are not required. Also with the high drop rheostat 58, the leads and rheostat contacts are effected less easily by grease and dirt. This is important in steel mill control systems where the manually operated rheostats such as 58 must necessarily be near to the mill and therefore sub- The use of the high jected to the dirt thereof.

drop rheostat permits it to be made smaller in size.

Other important advantages are derived from connecting the coil 55 of the balance regulator 21 across the interpole winding 34 of the tensioning machine. The interpole winding 34 serves as an inductive shunt in contradistinction to the non-inductive shunts utilized in the prior art to energize a vibratory type of regulator. By using the balanced type of regulator, a low voltage drop such as is obtained from the interpole winding 34 may be used. The vibratory type of regulator needs a greater voltage drop for energization thereof. In the inductive shunt as shown, the regulator is not only operated by the change in current through the shunt, but also receives the inductive kick due to the voltage induced by the current change in the shunt. Another feature of the use of this shunt is that it is made entirely of copper and therefore as the regulator coil is entirely of copper there is no change in the relative balance due to change in the temperature. In the prior art system where a vibratory type of regulator is used it is necessary to use iron grids to obtain sufiicient voltage drop to operate the regulator. Temperature changes of the iron cause a change in current and therefore destroy the accuracy of the regulator operation.

Upon acceleration or deceleration of the mill, the power necessary for the acceleration or deceleration must be added or subtracted as the case may be. This is accomplished by the coils 35, 45 and 55 of the regulator which are connected in series circuit and are energized with current of one polarity or the other through contacts 51 or 59. Upon acceleration power must be added to tensioning machines on the delivery end of the strip. When the speed change switch 58 is closed to raise the speed of the mill, contact 58 closes a circuit from the positive lead through the coils 55, 45 and 35 through the resistor 84 to the negative lead 55. Coil 55 is therefore energized to subtract ampere turns from the regulator so that more power will be added to the motor I! to balance the regulator 21.

On the entry end of the mill, machine 1 being a generator, upon acceleration of the mill, coil 35 will add ampere turns to the regulator 25 so that less power in the machine 1 will balance the regulator. A similar action in a reverse direction takes place when the switch 55 is closed to lower the speed of the mill. The contactor 51 closes a circuit from the negative conductor 85 through the coils 55, 46 and 35, through resistor 83 to the positive conductor 85. This reverses the energization of these coils relative to the energization upon acceleration thereby taking away energy from the machine l2 or 9 on the delivery end of the mill and adding energy to the machine 1 on the entry end of the mill.

This inertia (WR compensation, utilizing separate coils on the regulator solenoid actuated by the speed change initiating means, is an improvement over prior art systems utilizing means responsive to the rate of speed change of the mill. The prior art systems could not supply the power change quickly enough to keep the tension in the strip. First, the acceleration compensation was initiated only after the speed changed and then was further belated due to the time constant of the motor field to be compensated The present system of WR compensation starts the compenastion control prior to the speed change, so that the compensation and speed change are simultaneous and the compensation therefore efiective to maintain constant tension. Due to the fact that the rheostat motor 52 runs at a constant speed the rate of acceleration or deceleration is always the same and the need for added apparatus to provide WE, compensation in accordance with the rate of speed change is avoided.

The present invention also includes an improved means for indicating the tension applied to the strip. In order that such indication may be accurate, compensation must be effected for changes of speed of the mill. Tension indicators 16 and 18 are provided for the machines I and 8, respectively, and these indicators each have a coil energized from a shunt 82 in series with the machines I and 9. These indicators are of the wattmeter type and each have a potential coil connected across a source of current proportional to the current in the regulator main coil.

As previously stated the current in the main regulator coil 56 is proportional to the tension supplied by motor 22 and the current in the main regulator coils 35 and 45 are proportional to the tension of machines 1. and 8 if the current in the loop circuit is maintained constant. The potential coil of indicator i8 is connected across a resistance '19 in the circuit of coil 45 and therefore may be calibrated directly to read pounds tension.

The potential coil of indicator '18 is energized from a source artificially made to represent current in regulator coil 35. This is done by conmeeting the indicator coil across a potentiometer rheostat Ti energized from the constant potential source 8t 36. The rheostat Ti is in tandem with the tension adjusting rheostat 3B, the movement oi which is proportional to the current in regulator coil 35 as previously explained. The current in the potential coils of the indicators is therefore always proportional to the regulator coil current.

cases where tension on the loop circuit machines and 9 is desired above that provided by maximum field strength, this may be supplied by increasing the loop circuit current by changing the setting or" regulator 32 of the booster generator 28. The changed current through the current coils of the indicators connected to shunt 82 provides that these indicators properly indicate the tension.

The use of the rheostat 77 on the entry end of the mill permits an improved compensation of the tension adjustment and indication due to reduction of the strip (increase in length) after passing through the rolls. This compensation is provided by the rheostat 39, connected in parallel with rheostat 38, and provided with a scale graduated in percent reduction. This rheostat is manually set for the reduction of a given mill pass and shunts rheostat 38 to a given degree so that its position and that of rheostat l"! are proper for the tension.

For example, if the reduction is fifty percent, machine i must operate at only one-half the speed of machine 9 to supply the same tension. By means of rheostat 39, rheostat 38 provides ampere turns in coil 3? for one-half voltage while rheostat 38 is in full voltage position, thereby properly operating rheostat T! to indicate truly and accurately the tension. If the mill is a reversing mill, the compensation of the entry tension indicator must be provided at both ends. If the mill is of the type in which little or no reduction is taken, such as a temper mill through which the steel strip is passing merely to obtain better grain structure or surface, indicators of the type of 18 may be used on both ends of the mill.

The ammeter H is connected so as to indicate directly the tension provided by the tensioning motor l2. This indicator reads the current in the regulator coil 55 and as previously explained this current is an accurate indication of tension.

This is an improvement over prior art systems of control which need a wattmeter type instrument to indicate tension.

Although but one embodiment of the present invention has been illustrated and described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:

1. In a control system for winding and reeling a strip of material, a dynamo-electric ma.- chine mechanically connected to said strip for tensioning the same, means for maintaining said tension substantially constant comprising an electroresponsive device provided with a first winding operatively responsive to a change in a characteristic of said dynamo-electric machine. and means for adjusting said tension to a changed substantially constant value comprising a second winding on said device operative to maintain the current in said first winding proportional to said tension.

2. In a control system for winding and reeling a strip of material, a dynamo-electric machine mechanically connected to said strip for tensioning the same, means for maintaining said tension substantially constant comprising an electroresponsive device provided with a first winding operatively responsive to a change in a characteristic of said dynamo-electric machine, means for performing an operation on said strip, means for varying the speed of said operation performing means, means connected in circuit with said first winding for indicating the tension on said strip, and means responsive to operation of said speed varying means for varying the energization of said first winding and said indicating means.

3. In a control system for Winding and reeling a strip of material, a dynamo-electric machine mechanically connected to said strip for tensioning the same, means for maintaining said tension substantially constant comprising an electroresponsive device provided with a first winding operatively responsive to a change in a characteristic of said dynamo-electric machine variable with a change in said tension, means for varying said tension, means connected in circuit with said Winding for indicating said tension, and means operatively responsive to operation of said tension varying means for compensating said-tension indicating means, so as to correctly indicate the changed tension.

4. In a control system for winding and reeling a strip of material, a dynamo-electric machine mechanically connected to said strip for tensioning the same, means for maintaining said tension substantially constant comprising an electroresponsive device provided with a first winding operatively responsive to a change in a character istic of said dynamo-electric machine variable with a change in said tension, means for varying said tension, means connected in circuit with 2,251,4iil8 said winding for indicating said tension, means for performing a reducing operation on said strip, means operatively responsive to operation of said tension varying means for compensating said tension indicating means for said tension change, and means for modifying said tension varying means to compensate for reduction of said strip.

5. In a control system for winding and reeling a strip of material, a dynamo-electric machine mechanically connected to said strip for tensioning the same, means for maintaining said ten sion substantially constant comprising an electroresponsive device provided with a first winding operatively responsive to a change in a characteristic of said dynamo-electric machine, means for performing an operation on said strip, means for varying the speed of said operation performing means, and means responsive to initiation of operation of said speed varying means for compensating said first winding for said speed change,

6. In a control system for winding and reeling a strip of material, a dynamo-electric machine mechanically connected to said strip for tensioning the same, means for maintaining said tension substantially constant comprising an eiectroresponsive device provided with a first wind.-

ing operatively responsive to a change in a chai acteristic of said dynamo-electric machine, means for adjusting said tension to a changed substantially constant value comprising a second winding on said device operatively independ ent of said changes in said characteristic of said dynamo-electric machine, and tension indicating means energized proportionally to the current in.

said first winding.

7. In a control system for winding and reeling a strip of material, a dynamo-electric machine mechanically connected to said strip for tensioning the same, means for maintaining said tension substantiallyconstant comprising an electrore" sponsive device provided with a first winding operatively responsive to a change in a oharac teristic of said dynamo-electric machine, means for performing an operation on said strip, a motor mechanically connected to said operation performing means and electrically connected in circuit with said dynamo-electric machine, means for adjusting said tension to a changed substan tially constant value comprising a second winding on said device, and means for energizing second winding independently of said motor cir" cuit.

8. In combination, a dynamo-electric machine, means including an electroresponsive device provided with a first winding operatively responsive to a change in a' controlling characteristic of said dynamo-electric machine for maintaining a corn trolled characteristic thereof substantially con stant, and means for adjusting said controlled characteristic to a changed substantially constant value comprising a second winding on said device operative to maintain the current in said first,

winding proportional to said controlled characteristio.

9. In a tension controlling system for winding and reeling a strip of material, a dynamo-electric machine mechanically connected to said strip, a balanced regulator for controlling the power of dynamo-electric machine comprising an eiectroresponsive device provided with a first winding connected across the terminals of said dynamo-electric machine, and means for adjusting said tension comprising a second winding on said device energized independently of said armature circuit,

iii. in tension controlling system for winding and reeling a strip of material, a dynamoelectric machine mechanically connected to said strip, a balanced regulator for controlling the power of said dynamo electric machine comprising an electroresponsive device provided with a first winding operatively responsive to the voltage across said dynamo-electric machine, a secand winding on said electroresponsive device, and means energized from a constant voltage source for adding or subtracting ampere turns to said second winding.

11., In a tension controlling system for win ing and reeling a strip oi: material, a dynamm electric machine mechanically connected to said strip, a balanced regulator for controlling the power of said dynamo-electric machine compris ing an electr: esponsive device provided with a first winding connected across the terminals of. said dynamo-electric machine, and means for compensating the energization of said first winch lug whereby it is responsive to the generated voltage of said dynamo-electric machine.

12. In a control system for a strip of material adapted to be unwound from a first reel and wound upon second reel, dynamo-electric machine connected to said first reel, 9, second dynamo-electric machine connected to said sec-- ond reel, means connecting the armatures of said dynamo-electric machines series, means for performing an operation on said strip during said unwinding and windin motor i'or driving said operation peiiorining i cans, means for varying the speed oi said motor, a second motor meohanically connected. to said strip between said operation performing means and one or said dynan1oelectric mac lies, means connecting the armaturcs oi" said motors in parallel, means for regulating the power supplied said second motor comprising iirst electroresponsive device having a va'inding, means for regulating the power of one of said dynammelectric machines comprising a second electroresponsive device having a wind" tension indicator comprisii responsive the current in inding oi said first electroresponsive device, and a tension indicator con lsing wattmeter provided with t h'ao ,sive to current proportional to the current .n said winding of said second clcctroresponsive device.

13. control system for Winding and reeling a strip of r erisl, a dynammelectric machine connected to said reel, means for regal" 1 power oi said dynamo-electric machine coinprising an elcctroresponsive device provided with Winding, and means opei'ativcly responsive to the armature current of said dynamo-"electric mact'ne and to a current proportional to the out .t in said winding for indicating the tensi 1 supplied said strip by said dynamo-electric machine.

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