US2437313A

US2437313A - Electrical servo system

Info

Publication number: US2437313A
Application number: US570624A
Authority: US
Inventors: Alda V Bedford
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1944-12-30
Filing date: 1944-12-30
Publication date: 1948-03-09
Anticipated expiration: 1965-03-09

Description

A. v. BEDFORD 2,437,313

ELECTRICAL s'mvo SYSTEM Filed Dec. 30, 1944 2 Sheets-Sheet 1 INVENTOR.

mivie March 9,1948. fllflkwrlszomm -2,437,313

f .-ELECiRICAL SERVO SYSTEM 7 2 Sheets-Sheet 2 Filed Dec. '30, 1944 INVENTOR.

fective to overcome the efi'ects of friction.

meansfor producing auxiliary derivatives sig-' Patented Mar. 9, 1948 ELECTRICAL SERVO srs'rEM ,Alda Redford," Princeton, N. J.,.assignor to Radio or Delaware 13 Claims.

This invention relates to electrical servo systems, and more particularly to improvements in the art of stabilizing electrical servo systems of the type employing A.-C. control signals.

An electrical servo system, as the term is used herein, is defined as comprising an output shaft, an electric motor coupled to said shaft, means providing a displacement signal related in some characteristic, such as its amplitude, to the difierence between the actual position of the output shaft and the position to which it is to be driven, and means for energizing the motor in response to the displacement signal, the motor tends to drive the shaft to reduce the efiect ofthe displacement signal, and hence the motor energization, to zero. It is well known to those skilled in the art that such systems tend to be inaccurate and sluggish if the displacement signal produces too little efiect on the motor, and

tend to overrun the correct position and fhunt if the displacement signal is made sufilciently ef- It is common practice to combat these difiiculties' by adding to the displacement signal further signals which are in efifect time derivatives of the displacement signal. If the displacement signal is a D.-C. voltage of variable magnitude, the proper auxiliary signals are also D.-C. voltages, having magnitudes which vary as the first and higher order time derivatives of the displacement signal. .If the displacement signal variable amplitude, the auxiliary signals must also be A.-C. voltages, of the same frequency as the displacement signal, varying in amplitude in accordancewith the time derivatives of the amplitude of the displacement signal. It is important to note that the auxiliary voltage waves are not time derivatives of the displacement voltage wave.

The principal object of the present invention is to provide improved servo systems of the type wherein a variable amplitude A.-C. voltage is employed as the displacement signal, including nals;

Another object is to provide improved methods of and means for deriving from a variable amplitude A.-C. voltage, further A.C. voltages varying in their amplitudes in accordance with the time derivatives of the amplitude of said first voltage.

'A' further object is to provide an improved servo system adapted for automatically positioning directive radio antennas in response to radio signals picked up thereby.

in such manner that is an A.-C, voltage of Corporation oi America, a corporation r Application December 30, 1944, Serial No. 570,624

" I (Cl. 318-28) 7 2 These and other objects will become apparent to those skilled in the art upon consideration of the following description, with reference to the i accompanying drawings, of which Figure 1 is a schematic diagram of a radio 1ocator system embodying the instant invention,

Figure 2 is a graph illustrating a variation of the position of the output shaft of the system of Figure 1 with respect to the correct position of said shaft, j i T Figure 3 is a graph illustrating variations with time of the amplitude of a radio signal received in the operation of the system of Figure 1,-

Figure 4 is a graph illustrating the output of the radio receiver of the system of Figure 1 under the conditions represented by Figures 1-3,

Figure 5 is a graph illustrating the voltage of Figure 4 after being delayed.

Figure 6 is a graph of the difference between the voltages of Figures 4 and 5,

Figure '7 is a schematic diagram of a modification of one of the subcombinations of the system of Figure 1,

Figure 8 is a further modification or one of the subcombinations of Fig. 1,

Figure 9 is a graph illustrating the voltage of Figure 4 after being delayed by a different amount than that corresponding to Figure 5, and

Figure 10 is a graph illustrating the sum of the voltages of Figures 4 and 9.

Referring to Figure 1, only those elements of a radio locator system which are necessary to an explanation of the present invention are shown.

way switch device 3 to a pair of radiators 5 and 'I. The radiators 5 and 'l are directive, and are positioned upon a supporting member 9 in such manner that their directive patterns overlap with the maximum directivity 0f the radiator 5 lying in a line to the right of the equisignal axis, and that of the radiator I lying to the left. The supporting member 9 is rotatable by means of a shaft I I, which is coupled to a motor [3.

The switch 3 is coupled to a synchronous motor I 5, arranged to be energized from an A.-C. source, not shown. The motor l5 may be arranged to drive the switch 3 at a constant speed of 3600 R. P. M., for example, connecting the transmitter *that any known type of reversible A.-C. motor may be used. The field winding of the motor I:

is connected through a phase shifter I! to the a well known type of phase invertor.

" reference numerals, primed.

A.C. supply. The armature of the motor I3 is connected to the output circuit of an amplifier IS.

'A radio receiver 2|, provided with an antenna 23, is tuned to the frequency of operation of the transmitter I. The output circuit of the receiver 2| is coupled to the control grid of an electron discharge tube 25. The tube 25 is provided with two

load resistors

21 and 29, connected in the anode and cathode circuits respectively. A grid leak 3| is also provided, with its lower end connected to a D.-C. source of biasing potential. The tube 25, with its associated resistors, constitutes It will be apparent that any other known type of phase invertor may be substituted. The phase invertor is designated generally by the reference numeral 33 in Figure 1.

The anode of the tube 25 is coupled through a blocking capacitor 35 directly to the control grid of a tube 31, and through a resistor 39.to the control grid of a tube 4|. The cathode of the tube 25 is coupled through a blocking capacitor 43,

i a. delay network 45, and a resistor 41 to the con trol grid of the tube 4|. The output end of the delay network 45 is also coupled through an amplifier 42 to the input circuit of a phase invertor 33', similar to the phase invertor 33.

The phase. invertor 33' is coupled to the control grid of a tube 49 through resistors and a delay network, in exactly the same manner as the phase invertor 33 is coupled to the tube 4|. The elements in the connections from the phase invertor 33' which are similar to those associated'with the phase invertor 33 are denoted by corresponding The tubes 31, 4| and 49 are provided with a common load resistor 5|, which is coupled through a capacitor 53 to the input circuit of the amplifier l9.

The time'delay networks 45 and 45' illustrated in Figure 1 are of the same general construction as low pass filters, and in fact are-low pass filter circuits. They are designed to pass atleast the fundamental frequency of the output signal'of the receiver 2|, and are terminated in such manner and provided with the proper number of sec- -tions to introduce a delay of substantially one cycle,-i. e. if the signal is 60 cycles per second, the networks 45 and 45' each cause a delay of ,4 second. If desired, the networks can be designed to pass higher frequencies as well, subject only to the condition that the required ,4 second decomprising series inductors and shunt capacitors. It willbe understood by those skilled in the art that series resistors may be used instead of series inductors, and other types of networks may be substituted for. those shown in Figure 1.

The operation of the system of Figure 1 is as follows: The transmitter provides radio frequency output which is applied alternately to the radiators and I through the switch 3. Although the transmitter I may be modulated, it is assumed for the sake of simplicity of description that it merely provides a continuous wave. The operation of the servo system is substantially the same whether or not the transmitter is modulated. Signal is radiated alternately by the. radiators 5 and If a reflecting target lies on a line midway between the directive axes of the radiators, the strength of the reflected signal is the same regardless of which radiator is energized. However, if the target is to the left of the-equisignal line, the reflected signal is stronger while the radiator is energized, and weaker when the radiator 5 is energized.

' lay is provided. The networks are illustrated as Figure 2 shows a typical variation of the devia# tion of the equisignal line from the line of sight to the target, such as would be caused by motion of the target toward the left. Referring to Figure 3, the amplitude of the reflected signal varies accordingly, the pulses ,L2,' L3 etc. representing energy transmitted from the antenna 1 becoming larger, and the right pulses R2, R3 etc. becoming smaller. It should be understood that each of the pulses L and R of Figure 3 represents only the amplitude of the reflected wave. The pulse frequency is 60 cycles per second, i. e. 60 L pulses and 60 R pulses occur each second, but the frequency of the signal itself may be several hundred megacycles per second. 1

The reflected signals are pickedup by the antenna 23, and amplified and detected by the receiver 2|, providing an output voltage represented by the graph of Figure 4. This is a 60cycle wave,

is applied to the control grid'of the tube 25, causing corresponding varia-' tions of the anode current thereof, and hence similar variations in the voltage drops across the

resistors

21 and 29. Upon increase of the anode current of the tube 25, the voltageat the anode becomes less positive, referred to ground potential, and that at the cathode becomes more positive; The blocking

capacitors

35 and 43 pass only the A.C. components of these voltages;

thus the voltage at the control grid of the tube .31 is similar in form to that-applied to the input of the delay network 45, but out of phase with it. The voltage input'to the delay network.

is in phase with the receiver output. The output of the delay network 45 is similar to the input,

but delayed one cycle. This voltageis represented by the graph of Figure 5.

The current through the resistor 39 is propor- I tional to the A.C. component of the anode voltage'of the tube 25. The current through the resistor 41 is proportional to the output voltage of the network 45. Both of these currents flow throughthe resistor 40. The voltage drop across the resistor '40 is thus substantially proportional to the sum of the A.C. anode voltage of the tube 25 and the output voltage of the delay network.

The'voltage across the resistor 40 is represented by the graph of Figure 6. Since the voltage appliedthrough the resistor 39 is identical with that represented by Figure 4, but reversed in phase, the wave of Figure 6 is actually the difference between those of Figure 4 and Figure 5. Therefore, the magnitude of each pulse of the wave of Figure 6 is proportional to the difference between successive pulses of the wave of Figure 4. The difference between successive pulses is proportional to the time rate of change of amplitude. Thus, the amplitude of the wave of Figure 6 is proportional to the rate of change of amplitude of the wave of Figure 4. When the wave of Figure 4 is increasing in amplitude, the derivative wave of Figure 6 is in phase with it. When the wave of Figure 4 is decreasing in amplitude, that of Figure 6 is out of phasewith it.

The phaseinvertor 33' and the delay network 45' operate upon the differential signal appearing across the resistor 40 to provide at the control grid of the tube 49 a wave corresponding in amplitude to the rate of change of amplitude of the a difierential signal. This voltage, appearing across the resistor 40', is proportional to the second derivative of the displacement. It will be apparent that further derivative signals may be produced by adding further networks similar to those illustrated.

The displacement voltage is amplified by the placement component and the derivative components may be adjusted by varying the values of the

resistors

39, 391,, 40 and 40'.

The composite voltage across the resistor 5| is amplified by the amplifier l9 and applied to the motor, I 3. The motor 13 is energized thereby to' rotate the shaft I I and direct the antennas 5 and 1 toward the target. Initially, while the displacement is increasing, the derivative component aids the displacement signal, providin increased motor torque to overcome friction. The second derivative component also aids the displacement signal while the rate of change of displacement is increasing, to overcome inertia during acceleration of the motor l3, and bucks the displacement signal while the rate of change. ofdisplacement is decreasing. As the dis-v placement signal decreases, the first derivative component bucks it, tending to deenergize the motor more rapidly. so that the system will coast to a stop without overshooting. The second derivative signal also bucks the displacement signal while the rate of change of displacement is decreasing, tending to overcome the momentum of the moving parts. Thus, by properly proportioning the resistors to control the amplification of the derivative signals in accordance-with the friction and mass of the motor l3 and its mechanical load, the system may be made to operate smoothly and accurately, without lag or hunting:

In the operation of the system of Figure 1, the voltage appearing at the amplifier I9 comprises ,three components: an undelayed displacement I signal, a signal similar to the displacement signal but delayed by one cycle, and-a signalsimilar to the displacement signal but delayed by two cycles. Considering the operation of the system from this viewpoint, rather than that of successive derivatives," it becomes apparent that some of the elements of the system of Figure 1 may be omitted without altering the mode of operation of the overall system. Referring to-Flgure '7, a single phase invertor tube 33" is connected like the phase invertor 33 of Figure 1 to anode and cathode load resistors 21" and 29" respectively. 7 The cathode load resistor 29" is coupled through a blocking capacitor 33" to a delay network 45" which, like the delay networks of Figure 1 is designed to provide a delay of one cycle. The output of the network 45" is connected to a second identical delay network 45". The anode of the tube 33" is coupled through a blocking ca-'. pacitor 35" and a resistor to the input circuit L through the capacitor and the resistor 10 to.

the amplifier I9. It is transmitted in reverse phase through the network which introduces a delay of one cycle, and then through the resistor H to the amplifier Ill. The third component, delayed by two cycles, travels through both of the networks '45" and 45" and the resistor 12 to the amplifier l9. The relative values of the resistors 10, II and 12 may be adjusted to provide the required proportionality between the three components. Thus the composite voltage applied to the amplifier I9 is identical with that applied to the amplifier IS in the system of Figure 1, although the derivative voltages are not produced separately at any point in the circuit. Refer to Figure 8. The circuit including the phase invertor 33 and delay network 45 of the systemof Figure 1 may be replaced by a, delay v network 6 l, bridged by a resistor 63, The network 6| is similar in construction to the network 45, but designed to provide a delay of only one-half cycle. The delayed signal is applied to a resistor 65. The original signal is also applied to the resistor through the resistors 63 and 61. The delayed signal is represented by the graph of Figure 9. This is added in the resistor 65 to the original signal, represented by the graph of Figure 4. Owing to the half cycle delay of the network GI, each pulse of the resultant voltage is proportional in magnitude to the difference be- .tween successive left and right pulses of the original signal. The first pulse has a magnitude L2-R1, which, in the illustrated case, is merely L2. The secondpulse has a magnitude R2L2, etc, Thus, the amplitude of the A.-C. component of the wave of Figure 10 is at all times proportional to the rate of change of amplitude of the wave, of Figure 4. The low frequency component 'of the wave of Figure 10 is of no effect, since it is removed by the blocking capacitors in the power amplifier.

Although the invention has been described in connection with an electrical servo system associated with a radio locator, it will be understood that it is equally applicable to any A.-C. servo system, and may be employed as well for any other purpose which requires difierentiation of the envelope of an A.-C wave. The various graphs in the drawing illustrate rectangular waves, However, voltages of sinusoidal or other wave forms may be used, without altering the design or operation as described.

I claim as my invention: i

1. In an. electrical servo system including an output shaft, a motor coupled to said shaft, means for producing a A.-C. displacement signal of frequency .f cyc es per second and amplitude substantially proportional-to the difference between the actual angular, position of said shaft and the position to which said shaft is to be driven, and means for applying, said displacement signal to said motor, anti-hunt means ineluding a time delay network designed to provide a delay of length r 2f seconds, wherein 11. is an integer, means for applying said displacement signal to said delay network to produce a delayed displacement sig nal, ,and means for applying said delayed disa placement signal to said motor, in addition to said original displacement signal. 2. In an electrical servo system including motor means adaptedto be energized-by an alternating current, means for producing an A.-C. displacement signal, anti-hunt means comprising a time delay network designed to provide a delay of current to said motor means. 1

3. In an electrical servo system including motor means adapted to be energized by an alternating current, means for producing an .A.-C. displacement signal, anti-hunt. means comprising a time delay network arranged to provide a delay of seconds, wherein f is the frequency of said displacement signal, means for applying said displacement signal to said network to produce a delayed Ae-C. displacement signal, means for combining said delayed signal with said original displacement signal in phase opposition thereto to produce a resultant alternating current, and means for applying said resultant current to said motor means.

4. In an electrical servo system including mo tor means adapted to be energized by an alternating current, means for producing and utilizing in known manner an A.-C. displacement signal, anti-hunt means comprising a time delay net work arranged to provide a delay of a seconds, wherein J is the fundamental frequency of said displacement signal, means for applying said displacement signal to said network to progize said motor, phase inverter means, means for applying said displacement signal to said phase invertor means to produce two outputs, both similar to said displacement signal but 180 out of phase with each other, means for applying one of said outputs substantially without delay to said amplifier, and means for applying the other of said outputs to said amplifier with a delay of seconds, wherein f is the fundamental frequency of said displacement signal;

6. In an electrical servo system including means for producing an A.-C. displacement signal, a motor, and an amplifier connected to energize said motor, means for applying said dis-- placement signal substantially without delay to said amplifier, and further means for applying said displacement signal to said amplifier wit a delay of seconds, wherein j is the fundamental frequency of saiddisplacement signal,

7.. In an electrical servo system including means for producing an A. C. displacement signal, a motor, an amplifier connected to energize said motor and means for applying said signal to said amplifier, comprising a phase inverter including two output circuits, means for applying said displacement signal to said phase invertor, a voltage combining network connected to one of saidoutput circuits, a time delay network connected between the other of said output circuits and said combining network, and means for applying the output of vsaidcombining network to'said amplifier.

8. In an electrical servo system including means for producing an A.-C. displacement signal, a motor, an amplifier connected to energize said motor and means for applying said signal to said amplifier, comprising a' phase inverter including two output circuits, means for applying said displacement signal to said phase inverter, a voltage combining network connected to one of said output circuits, a time delay network connected between the other of said output circuits and said combining network, means for applying the output of said combining network to said amplifier, a second phase inverter including two further output circuits, a second voltage combining network connected to one of said further output circuits, a second time delay network connected between the other of said further output circuits and said second combining network, and means for applying the output of said second combining net-- work to said amplifier,

9. The invention as set forth in claim 8 wherein said delay network is designed to provide a delay of seconds, wherein j is the fundamental frequency of said displacement signal.

10. The invention as set forth in claim 7 wherein said delay networks are each designed to provide a delay of seconds, wherein f is the fundamental'frequency of said displacement signal.

11. The invention as set forth in claim 13 wherein said delay network is designed to provide a delay of seconds, wherein j is the fundamental frequency of said displacement signal.

12. The invention as set forth in claim 13 wherein said delay networks are each designed to provide a delay of seconds, wherein f is the fundamental frequency of said displacement signal, and n is an integer.

13. In an electrical servo system including means for producing an A.-C. displacement signal, a motor, an amplifier connected to energize i said motor and means for applying said signal to said amplifier, comprioing a voltage combining circuit in the input circuit of said amplifier. means for applying said signal directly to said combin.

deriving a. difierence voltage whose amplitude is proportional to the difl'erence between the ampiitude of each cycie oi. said displacementsignel and the next succeeding cycle, means for applying 10 nnnnmcns man The following references are of record in the ing circuit, means including a. delay network for 5 me P n UNITED STATES PATENTS Number said diflerence voltage to said combining circuit, m and means for appl the output of said combining circuit touid amplifier.

. AIDA V. BEDFORD.

Name

Date Scott Oct. 6, 194-2 Hull Mar. 4, 1941 Aiexandereon Sept. 22, 1925 Retail. Jnn,9,140