US2846632A - Control system - Google Patents

Description

Aug. 5, 1958 H. J. DUDENHAUSEN' 2,846,632

CONTROL SYSTEM Filed Jan. 27, 1954'.

INVENTOR. Hons Jurgen Dudenhousen ATTORNEY United States Patent 0 C DNllil-Cl. SYSTEM Hans Jurgen Dudenhausen, Stuttgart, Germany, assignor to Intavex, Inc, New York, N. Y., a corporation of New York Application January 27, 1954, Serial No. 406,485 Claims priority, application Germany July 1.7, 1953 4 Claims. (Cl. 313 257) erate, to a far-reaching extent, with a setting speed which is independent of the load and proportional to the control amount.

There are known circuits of relay-controlled electric motors which fulfill the above requirement to a farreaching extent in the manner that the armature voltage and the armature current of the electric motor, which is generally developed as a D. C. shunt-wound motor, are led back or fed back respectively to additional windings of the polarized control relay.- (See Uhlands lngenieur- Kalender, special vol. 1, published by Verlag Kroner of Stuttgart, page 175, Fig. 391 and Regelungstechnik, published by Verlag Oldenburg of Munich, No. 1, page 14.)

inasmuch as in control circuits there is generally demanded a high sensitivity of response of the control relay, the above solution has the drawback that a large part of its winding space is used for the voltage or current coil, due to which the winding space of the coil traversed by the pilot current is reduced and in this way the sensitivity to response of the relay is also impaired. The currents flowing in the coils (pilot coil, voltage return coil and current feedback coil) result in a high thermal load of the relay. The development of polarized relays, particularly of moving coil relays with multiple windings is difiicult and expensive.

The subject matter of the present invention is a relaycontrolled servomotor (shunt-wound electric motor) with reversible, load-independent adjusting speed which is, to a far-reaching extent, proportional to the control magnitude. The invention is based on the known arrangement of relay-controlled electric motors in connection with which the armature voltage and the armature current are led back and fed back respectively to additional windings of polarized control relay. In accordance with the invention, the return of the armature voltage and the feedback of the armature current are effected via the same resistance ohmically to a single coil of the polarized control relay, which coil is traversed by the control current. The invention also otters the possibility, due to the fact that the armature voltage and the armature current are led back and fed back respectively ohmically over the same resistance, of effecting in the simplest manner this return and feedback respectively to ohmic resistances traversed by the control current, as is necessary, for instance, in connection with the grid control of D. C. vacuum tube preamplifiers.

My invention will be further described by reference to the accompanying drawings of which:

Fig. l shows the basic electric wiring diagram of the novel relay control of electric motors.

Fig. 2 illustrates application of the control device in a position-transmission system.

-response current of the relay 4.

Fig. 3 illustrates an application of the control system in relay control of electric motors utilizing electronic preamplification of the control current.

Fig. 4 illustrates means for effecting differentiation of a control current.

The slide contact 1 of a controller developed in this case as a potentiometer 2 and connected to a D. C. source is connected with one end of the coil 3 of a polarized relay 4, the armature of which can be restrained in the central position shown in the drawing in nonexcited condition. The other end of the coil 3 is connected to the electrical centerpoint of the same D. C. source formed by the resistances 5 and 6. If the slide contact 1 is shifted for instance by the angle B from its central position into the position shown in dotted line, the coil 3 is traversed by a current in the direction indicated by the dotted line and the armature 7 of the polarized relay 4 comes, in the position shown in dotted line, onthe relay contact connected via the resistance 8 with the plus current bar of the said D. C. source. The D. C. shunt-wound motor 9, one brush of which is connected with the relay armature 7 and the other brush of which is connected with an electric centerpoint, formed by the resistances 10 and 11, of the same D. C. source, is thus placed under voltage and starts up, for instance, in clockwise direction. At the same time, a current now flows in the direction shown in the drawing through the resistance 12 via the resistance 5 and the coil 3 of the relay 4 to the minus terminal of the common current source. The ohmic value of the resistance 12 can be so regulated that the current in the coil 3 which is supplied by the potentiometer 2, for instance corresponding to its angular deflection B, is now compensated to such an extent that the armature 7 just remains in the position shown in dotted line. The electric motor will, therefore, again run with its full speed of rotation in clockwise direction. The current in the coil 3 which is necessary in order to move the armature from its restrained central position into the position shown in dotted line, i. e. the response current of the polarized relay 4, is, let us assume, fifty times less than the current supplied by the control potentiometer 2, corresponding to its angular deflection B to the coil 3. If the angle of deflection B of the slide contact 1 is now reduced to such an extent that for instance the current flowing through the coil 3 has only half the value of the current which corresponds to the angle of deflection B, the balancing current flowing through the resistance 12 predominates and the armature 7 is returned to the central position. The electric motor is without current and its speed decreases so that the armature 7 is only again drawn into the position shown in dotted line by the current supplied by the control potentiometer 2 when the speed of revolution of the motor and thus its delivered generator voltage has dropped to a value which corresponds to a balancing current, the value of which is equal to the control current, less the If, therefore, the speed of rotation of the motor has dropped to such an extent, the armature 7 will be moved back into the position shown in dotted line, due to which the electric motor 9 will again be under voltage and its speed of rotation will increase. At the same time, therefore, the balancing current flowing through the resistance 12 will become fully effective and the armature 7 will again be released. This regulation cycle is repeated with such a frequency and contact closure times that the generator voltage of the motor 9 is in equilibrium with the voltage supplied by the control potentiometer 2. The generator voltage of a D. C. shunt-wound motor is U =EI -R,, in which E is the electromotive force proportional to its speed, I is the current flowing through the armature and R is the internal resistance of the armature. The speed of revolution of the electric motor 9 is, accordingly, therefore, only proportional to the voltage supplied by the control potentiometer 2 when either the internal resistance of the armature or the current flowing through the armature is negligibly small. Both, however, are not applicable to the D. C. shunt-wound electric motor of small power, such as can be controlled by relay circuits of the type indicated here so that its generator voltage and thus its speed of revolution would decrease greatly with increasing external load in collaboration with the relay control circuit indicated up to the present time, inasmuch as a drop of the generator voltage of the motor 9 under an external load results in an increase of the balancing current flowing through the resistance 12 and thus in a reduction of the mean voltage of the motor armature. This undesired increase of the balancing current will, however, be prevented by the resistance 8, inasmuch as in the case of load on the motor 9 and its increased current adsorption resulting therefrom, there takes place a voltage drop on the resistance 8 so that the balancing current flowing through the resistance 12 is reduced and thus the decrease in speed produced by the loading is extensively again counteracted.

The resistance 813 furthermore prevents on the one hand a possible short circuit between the two stationary contacts of the relay 4, as might otherwise occur by ionization with the formation of an arc and furthermore it limits the charging current of the capacitor 14, which reduces the disconnecting voltage of the motor 9 to the amount permissible for the contacts of the relay 4.

If the slide contact 1 of the control potentiometer 2 is moved out of its central position towards the right, the motor will run in counter-clockwise direction with a speed of rotation proportional to the control order (slider deflection), as can be deduced from the above statements. Inasmuch as the maximum balancing current flowing through the coil 3 of the relay 4, as a result of the selection of the ohmic value of the resistance 12 is in this case 50 times as great as the current required for the response of the relay 4, the motor can be regulated in a speed control ratio or 1/50 with this relay arrangement. However, by changing the ohmic value of the resistance 12, it is possible to obtain both a smaller speed control ratio and also a considerably larger one than the one obtained above, inasmuch as the thermal load of the coil 3 of the relay 4 is always very small. For all practical purposes, there flows in it, within the range of the control characteristic, only a difference current which is only unsubstantially greater than the current which must fiow through the coil 3 in order to move the armature 7 from its central position to one of the two current contacts.

Fig. 2 shows a position-transmission-system, in connection with which the novel speed-regulating device described herein is applied. The similar windings of two ring potentiometers 15 and 16 are each tapped at 90 and connected conductively with each other in the manner shown in the drawing. On each of the windings of the two ring poteniometers 15 and 16 there slide two separate slide contacts 17, 18 and 19, 2t displaced by 180 from each other. The slide contacts 19, 20 are applied to the busbars of a voltage source, which also supply the relay-controlled motor 9, in accordance with Fig. l.

The winding 3 of relay 4 is connected to the slide contacts 17, 13 and is without current, corresponding to the position of the slide contacts 17, 18 and 19, 20, shown in the drawing. If, however, the pair of slide contacts 19, 20 is turned for instance by the angle B, a current will flow through the coil 3 in the direction shown in dotted line in Fig. l and the electric motor 9 will turn with a speed of rotation which is proportional to this current, a corresponding balancing current, as stated above, flowing through the resistance 12. The axis of the electric motor 9 is coupled with the axis of the ring potentiometer on which the two slide contacts 17, 18 are fastened, insulated from each other, by means of the mechanical shaft 21.

The two slide contacts 17, 18 are thus turned in the direction shown by the arrow in the drawing. Due to this, the potential constantly decreases between the two slide contacts 17, 13, so that the speed of operation of the electric motor 9 also decreases proportionally.

In this manner, there takes place an approximately non-oscillatory movement of the slider 17, 18 into a new position at which the potential between the slide contacts 17, 18 is approximately equal to zero. Inasmuch as the windings of the two ring potentiometers are made similar to each other, the sliders 17, 18 are turned by the same angular path by the motor 9, as the sliders 19, were previously turned and therefore in the present case by the angle B shown in the drawing. As results from the known potential distribution of the four-terminal system shown, any desired angular path can be rotatably transmitted with this arrangement, doing so with a very high following speed with very accurate position transmission, inasmuch as the following speed, due to this novel relay control, is proportional to the voltage potential of the bridge imbalance of this four-terminal system.

The accuracy of the position transmission by means of such a system depends on the response sensitivity of the relay 4. Frequently, however, the response sensitivity even of the most sensitive polarized relay known up to the present time is not sufficient to achieve a desired accuracy of adjustment, particularly when the control current transmitter can give off only a very low power, as for instance in the case of very small friction in efficient accurate potentiometers of known construction. Fig. 3 shows the fundamental electric construction of a relay control of electric motors, the manner of operation of which, in the same manner as in Fig. 1, also depends on the fact that the voltage and the current of the electric motor 9 are led back and fed back respectively ohmically via a common resistance to the input circuit of a vacuum tube preamplifier which is traversed by the control current.

The input amplifier consisting in this case of a double triode having a common cathode 23 and series-connected filaments 24 in a balanced circuit makes it possible, by adjusting the resistance 25, by means of the adjusting clamp 26, to balance the plate currents of the two triode systems in such a manner that no current flows in the coil 3 of the relay 4 when the voltage potential on the two slide contacts 27 and 28 of the control potentiometers 29 and 30 is zero, as for instance in the manner shown in the drawing. By means of an iron-hydrogen resistance 31 connected in series with the filament 24, or other known means, this condition is retained in force when the D. C. source which is common to the entire system shows voltage variations on its busbars 32 and 33. If now, for instance, the slide contact 27 of the control potentiometer 29 is turned by the angle B, a minus potential is connected via the resistance 34 to the grid which is blocked by the resistance 35 and a difference current flows in the direction shown in the drawing in the coil 3. This difference current is proportional to the angle of deflection B of the slider 27 of the control potentiometer 29 or the voltage difierence between the sliders 27 and 28 of the voltage potentiometers 29 and 30, which can be brought about by a correct selection of the operating point of the tube characteristics by means of the cathode resistance 36.

The grid- blocking resistances 35 and 37, the gridleak resistances 38 and 39 and the resistance 34 and 40 have normally ohmic values which are equal for each pair. The capacitors 41 and 42 effect a differentiation of the control current supplied by the control potentiometers 29 and 30, which differentiation is necessary in order to dampen the control process, particularly when this arrangement is used in control circuits operating in accordance with the velocity method.

If, therefore, as already stated, the slide contact 27 is deflected for instance by the angle B shown in the drawing, a difference current flows in the direction shown in the drawing in the coil 3 of the polarized relay 4 and moves the armature 7 into the position shown in dotted lines so that the winding 43 of the relay 44 is placed un der voltage and the contact springs 45 and 46 are shifted to their counter-contacts. The contact springs are, in this case, adjusted mechanically in such a manner that, first of all, the contact spring 45 comes against its counter contact, due to which the armature of the D. C. shunt- Wound motor is placed under voltage via a resistance 47 having a high positive temperature coefficient, for instance an iron-hydrogen resistance, and via a second resistance 48. Immediately thereafter, the contact spring 46 comes against its counter-contact and short-circuits the resistance 48. In this way, it results that the motor is placed under voltage, first of all, via a series resistance 48, which is necessary in order to protect the contact of the relay 44. On the other hand, the armature of the motor 9 is short-circuited via the resistances 48 and 49 via the resistance 47 when neither the relay 44 nor the relay 50 is excited. In this way, it results that the running motor 9 is electrically braked very rapidly when the relays 44 and 50 are without current.

The armature of the motor 9 with its series resistance 47 and the resistances 51 and 52 forms an electric bridge. The bridge voltage is connected ohmically via the two similar resistances 12 with negative feedback to the input circuit of the preamplifier, which circuit is formed by the two resistances 38, 39 and is under the control potential of the two potentiometers 29, 30, so that the armature of the motor 9 receives current pulses of such a frequency and pulse-length that its speed of rotation is proportional to the electric potential difference between the slide contacts 27 and 28. As has been explained on the basis of Fig. 1, it is a prerequisite in this connection, however, that due to the internal resistance of the armature which cannot be neglected in this case, the current flowing through the armature must also be fed back to the input. In other words, the negative feedback of the armature voltage must be weaker when the current absorbed by the motor 9 increases, for instance under an increasing external load, in order to eliminate the drop in speed and thus the drop of the generator voltage of the motor 9 under the influence of the internal resistance of the armature. Let us assume, for instance, that the resistance 47 has an ohmic value equal to the internal resistance of the armature while the ohmic value of the resistance 52 is greater than that of the resistance 51. Upon an increasing load on the motor 9, its impedance however will become smaller and in this way the bridge voltage present on the resistances 12 will also become smaller so that there are now fed to the motor 9 current pulses of longer duration and the drop in speed will again be compensated. The proportion of the effective current feedback as compared with the voltage return is determined by the resistance ratio of the resistances 51 and 52. As has already been explained on basis of Fig. 1, the speed-control ratio, for instances 1:50, of the motor 9 can be adjusted by selection of the ohmic value of the resistance 12. In order that the motor 9 starts even with very small control voltages of the potentiometers 29 and 30, i. e. at very strong but very short armature current pulses, its time constants must be as small as possible, in other words the D. C. shunt-wound motor 9 used in this case must have, in addition to a small moment of inertia of its armature a very strong excitation and armature field, due to which its thermal load and particularly its thermal armature load becomes great. The resistance 47, however, protects the armature of the motor 9 from this thermal overload without impairing its small time constant, which is necessary for the starting, for when the integral of the current pulses flowing through the armature 9 and thus also through the resistance 47 is small, as is the case for instance for small control voltages of the potentiometers 29 and 30, the ohmic value of the resistance 47, which was selected with a strong positive temperature coeflicient, is also small.

As already mentioned, the ratio of the current feedback with respect to the voltage return of the motor 9 to the input circuit of the vacuum tube preamplifier is determined by the ratio of resistance 51 to resistance 52. This ratio cannot be made as large as desired, due to the time constants of the individual members of this control arrangement (motor 9, relay 44, 50, relay 4, preamplifier 22), inasmuch as the control arrangement would otherwise, in case of excessively strong superimposition of the current feedback, enter into natural oscillation. However, it is desirable to increase the pro-portion of the current feedback with increasing load on the motor 9 in order to obtain the smallest possible decrease of the speed of the motor. This job is also taken over by the resistance 47, inasmuch as with increasing external load on the motor 9, the current flowing through its armature 9 and the resistance 47 also increases and the bridge voltage present on the resistances 12 is additionally reduced by the now increasing ohmic value of the resistance 47, which corresponds to a desired increase in the proportion of the current feedback. The same effect can be obtained, to a still greater degree, if the resistance 51 also has a strong positive temperature coeflicient. If a motor 9, having a large time constant, is to be controlled with this arrangement, it is necessary to delay the action of the voltage return so that the motor starts even with small control voltages of the potentiometers 29, 30. This is effected by the capacitors 53 and 54 by means of which a reversal, which is limited in time, of the voltage return into voltage feedback on the input circuit of the vacuum tube amplifier is obtained and thus an increase of the contact closure time of the relay 4 and of the relays 44, 5'0.

As already mentioned, it is frequently desirable to differentiate the control currents supplied by the potentiometers 29 and 30, which is done by the capacitors 41 and 42. The quality of the differentiation depends, as is known, on the time constant of the RC member used and in this case, therefore, on the capacity of the capacitors 41, 42 and the ohmic value of the resistances 34, 40. inasmuch as the response sensitivity of this arrangement decreases in an undesirable manner with increasing omhic value of the resistances 34 and 40, the possibility of useful differentiation with such I an RC member is limited.

Fig. 4 shows how, the electric structure according to Fig. 3 otherwise remaining'unchanged, this difierentiation of the control current supplied by the potentiometers 29, 30 can be effected by means of a transformer 55. The control potential of the slide contact 27, 28 is on the one hand conducted directly, as in Fig. 3, via the resistances 34, 40 to the corresponding grid of the preamplifier tube 22. The change of the voltage potential between the slide contacts 27, 28 produces a change in the current flow in the primary winding 56 of the transformer and thus causes a voltage at the ends of the secondary winding 57 or at the grids of the tubes 22 which leads in phase the voltage change on the slide contacts 27, 28. The quality of this transformer differentiation device is superior to diiferentiation by means of an RC member, in accordance with Fig. 3, particularly when, as in this special case, potentiometers 29, 3t} (current transmitters) are used as control transmitters.

I claim:

1. A servo mechanism comprising a direct current electric motor, a potentiometer, and a polarized relay having its armature connected in the armature circuit of said motor, the actuating coil of said relay being connected in series between the movable tap of said potentiometer 7 and the armature of said relay, whereby upon energization of said motor via said relay, the back E. M. F. of said motor causes current flow in said actuating coil in opposition to that caused by said potentiometer.

2. A servo mechanism comprising a resistance connectible between the terminals of a source of potential difference, a movable tap engaging said resistance between the ends thereof, a polarized relay including an actuating coil, an armature and two fixed contacts connectible to the terminals of said source of potential difference, a voltage divider connected across said source of potential difi rence, and a reversible direct current motor having a first armature terminal connected to said relay armature and a second armature terminal connected to a point on said voltage divider, said actuating coil being connected between said tap and said first motor.

armature terminal.

3. A servo mechanism comprising a polarized relay, a reversible direct current motor having its armature circuit connected in series With the armature of said relay, a vacuum tube amplifier having the actuating coil of said relay in its plate circuit, and means to apply a portion of the back E. M. F. of said motor to the grid circuit of said amplifier in phase opposition to the signal in said grid circuit causing energization of said relay.

i ieferences Cited in the file of this patent UNITED STATES PATENTS 2,356,828 Crane et al. Aug. 29, 1944 2,429,257 Bond Oct. 21, 1947 2,454,134 Kliever Nov. 16, 1948 2,528,688 Chin et al Nov. 7, 1950 2,534,801 Siltimaki Dec. 19, 1950 2,654,859 Drnek Oct.'6, 1953 2,674,707 DeMott Apr. 6, 1954 FOREIGN PATENTS 684,473 France June 26, 1936

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