US3664234A

US3664234A - Digital electrohydraulic servo actuator

Info

Publication number: US3664234A
Application number: US31905A
Authority: US
Inventors: Gerald F Simons; Frederick W Lynch; Gary C Mckown
Original assignee: Sperry Rand Corp
Current assignee: Honeywell Inc; SP Commercial Flight Inc
Priority date: 1970-04-27
Filing date: 1970-04-27
Publication date: 1972-05-23
Anticipated expiration: 1989-05-23
Also published as: FR2086381A1; GB1343681A; CA948294A; DE2120661A1

Abstract

A hydraulic boost actuator having variable displacement and rate of displacement commands and controlled by a digital electronic system driving a stepper motor. The actuator may be operated either in a series or a parallel mode.

Description

Simons et al.

ited States Patent [151 3,664,234

[541 DIGITAL ELECTROHYDRAULIC [56] References Cited SERVO ACTUATOR UNITED STATES PATENTS 1 Inventors: Gerald E Simons; Frederick y a 3,079,899 3/1963 Inaba et al. ..91/37s Gary MCKOWII, all of Phoemx, Arlz- 3,216,331 1 1/1965 Kreuter ...91/3s2 73 Assignee; Sperry d Corporation Great Neck, 3,222,996 12/ 1965 Thieme et al. ...9l/382 NY. 3,266,378 8/1966 Shaw ..9l/363 [221 Med: 1970 Primary Examiner-Jan] E. Maslousky [211 Appl. No.: 31,905 Attorney-S. C. Yeaton 52 us. c1. ..91/363, 91/367, 91/378 [571 ABSTRACT I [5 l I llll. Cl ...Fl5b 9/03, F151) 9/09, F 1 5b 9/ 10 A hydraulic boost actuator h i i bl di l t d [58] Field of Search "91/382, 375, 363, 361,367, rate of displacement commands and controlled by a di i 91/378 electroni system driving a stepper motor. The actuator may be operated either in a series or a parallel mode.

6 Claim, 5 Drawing fi ures P U L S E G E N E RA T0 R 1 IN T E6 RATO R 1 2 I s 3 11 13 M E T ERIN c j S 12 S S HuT T L E THRESHOLD PULSE SIGNAL MOTOR GEAR SOURCE SHAPER (Zoe's/wk SE IS L J EI QEER TRAN T H R l: 5; H0 L D r f I FROM [\C l UA I'UR HAM xhii ZERO POSITION PULSE, I M E MO R Y E LE M e m [451 May 23, 1972 Patented MOTOR COMMAND PULSES May 23, 1972 STEPPER MOTOR 3 Sheets-Sheet 5 p- 2 m 235 2 cu o l-og W E m a m I Q N Q N E @E Mm r- I o M w LL LUI x0 I/Vl/E/VTORS GERALD E. S/MO/VS FREDERICK W. LYNCH GARY 6. MC KOW/V BY /6e 2:4y77

ATTORNEY DIGITAL ELECTROI-IYDRAULIC SERVO ACTUATOR CROSS-REFERENCES TO RELATED APPLICATIONS This application is a modification of the electronic circuitry described in the copending application entitled Digital Automatic Control System with Pseudo Position Feedback and Monitor, Ser. No. 31,986, the latter being filed concurrently with the instant application and assigned to the same assignee as the present invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention The hydraulic boost actuator modification described is applicable to and compatible with almost all existing hydraulic actuating systems. The use of digital techniques and a stepper motor permits accurate incremental variations of displace ment and rate of displacement of the boost actuator and hence the hydraulic system itself.

2. Description of the Prior Art There are two basic types of electrohydraulic servo control mechanisms. The first of these, and by far the most common is the flapper type of valve. The second is the jet pipe type. Both of these valve types require a second stage of hydraulics to provide feedback and power amplification to a magnitude sufficient to control the hydraulic ram. In the flapper type actuator, the flow of hydraulic fluid through a pair of opposed orifices onto a flapper is controlled. Pressure differentials which result from the flow control cause a second stage spool to move and port fluid to the ram. Since the second stage spool is connected to the flapper mechanically, the spool motion provides the necessary feedback. In the jet pipe type, the fluid is controlled through fluid impingement on one or the other of the two receiver orifices. Flow through each orifice causes the second stage spool to move. This motion ports fluid to the ram, and also as in the flapper type, provides the necessary feedback. Both of these control mechanisms are generally satisfactory but the flapper-nozzle has a failure characteristic which leads to hard-over actuation, the jet pipe is relatively expensive, and neither is adaptable to an existing boost actuator for converting the latter to a servo boost actuator without extensive modifications.

SUMMARY OF THE INVENTION The application describes a typical hydraulic boost actuator whose metering shuttle control arm has been modified by the addition of a stepper motor actuator to thereby convert it to a servo boost actuator. Prior to the modification, the metering shuttle control arm was a simple bar which carried the human pilot input to the metering shuttle by pivoting the arm and stroking the metering shuttle. This action ports fluid to the ram, which causes the actuator or shuttle valve housing to move to restore the metering shuttle to its neutral position.

In this invention, a stepper motor is introduced as an actuating mechanism. When the automatic control system is off, the stepper motor is in a locked position and operation of the boost actuator is the same as a normal boosted manual system.

When the automatic control system is active and the input control demands a motion from the actuator, the stepper motor receives a prescribed number of pulses in one direction which incrementally opens the metering shuttle, causing the ram to move. Immediately following the removal of the input command, the stepper motor rotation is reversed by'a spring centering mechanism or by electronic means whereby the same number of pulses of an opposite sense are fed to the stepper motor. Other means for returning the stepper motor to its initial position are also anticipated. By whatever means are selected, the return of the stepper motor to its original position will close the metering shuttle. This will cause the ram to stop its motion, but it will be in a new position gained as a result of the motion during the time the metering shuttle was open. If the demand of the control system is still not satisfied, another train of pulses is fed to the stepper motor to again open the metering shuttle, etc. until the control system is at a closed loop null.

The described system is admirably suited to airborne manual boost control systems, for example in helicopters, where precise controlled movement is paramount without extensive complex feedback arrangements. The invention may be easily incorporated into existing hydraulic boost systems at only a nominal weight increase. The input signal may be analog or digital from the receiver of a remote control system or it may be the input from a flight stabilization or computer controlled system.

A primary advantage of the instant invention is to provide a low cost, low weight digital system for controlling a hydraulic boost actuator.

Another advantage of the invention is to provide a system requiring only the addition of a stepper motor to an existing hydraulic boost actuator to convert it to a servo boost actuacontrol system for a hydraulic boost actuator that can be im- I plemented by analog input signals.

Another advantage of the invention is to provide a hydraulic boost actuator that can be simply implemented to operate in either a parallel or series mode.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a basic electronic circuit for driving the stepper motor;

FIG. la illustrates the pulse count control circuitry;

FIG. 2 illustrates a means for attaching thestepper motor and gear train to a hydraulic boost actuator; and

FIG. 2a and FIG. 2b illustrate modifications affording a selfcentering stepper motor.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates an electronic control circuit that may be used to command and control the hydraulic boost actuator. A detailed description of the circuit is incorporated in patent application entitled Digital Automatic Control System with Pseudo Position Feedback and Monitor," Ser. No. 31,986, filed Apr. 27, 1970, and assigned to the instant assignee. In general, the circuit functions in the following manner.-

An input signal represented by the signal source 1 may be an analog signal indicative of a desired change or an error signal. This initial command signal is shaped in shaper 2 to render it compatible with the circuit and appears at the summing junction 4. A takeofi from the shaper output is fed through integrator 3 to isolate and determine long term errors, and appears at summing junction 4-. The algebraic sum of the signals from summing junction 4 is fed to a threshold circuit 5 having positive and negative levels. The initial signal as modified by the summing junction 4, if of sufficient absolute magnitude will cause the threshold circuit 5 to enable either flip-flop 6 or flipfl0p 7 (See FIG. la). When either flip-flops are enabled, the output of pulse generator 9 is fed via the pulse count control 8 to the motor pulse sequencer 11 which in turn provides the intelligence to drive the stepper motor 12 the desired number of steps inthe desired rotational direction. By an appropriate gear train 13, the output arm 17 is displaced in proportion to the amount of motor movement and controls the hydraulically operated portion of the actuator 14.

The response of the actuator 14 in FIG. 2 is conditioned upon two independently variable parameters, the amount of displacement and the rate of displacement. In the instant invention these two parameters are easily and simply controlled by electronic means and thereby allows the establishment of optimum hydraulic parameters which need not have variation capabilities and their attendant complexity. The amount of displacement is controlled by the time during which the pulses are fed to the stepper motor 12 and the time required to return the stepper motor 12 to its initial position. Thus, pulse count control 8 may control the displacement by determination of a preselected, but variable pulse train time period. This time period may be determinable by the magnitude of the input to the threshold circuits, contingent upon some other condition, preselected and fixed, or manually variable. Thus for any given pulse repetition rate, the angular displacement and return or full cycle of the stepper motor 12 and the linear displacement and return of the control arm 17 is dependent upon the time period specified.

Two alternatives for re-centering the control arm 17 that are economical and reliable, are presented in FIGS. 2a and 2b. This recentering feature is required for parallel operation of the actuator, as described below, but will not be required for series operation, also described below. FIG. 2a illustrates a simple helical spring or coil 31 attached to both the stepper motor shaft 30 and its casing. In operation in the parallel mode with brake 23 engaged, the stepper motor 12, under command of the circuit in FIG. 1 will rotate the specified amount and wind coil 31. Rotation of stepper motor 12 will translate shuttle valve 16, say to the right, porting pressure oil to the left side of actuator ram 26 thereby moving output arm 27 and valve housing 14 to the right, return oil being ported from the right side of ram 26 to the sump through the fluid relief. Since the stepper motor 12 cannot translate relative to housing 14 in the parallel mode, it must be returned to its normal zero position when the ram has moved the commanded amount. This is accomplished by the return springs of FIG. 2a or 2b. After completion of rotation and the input command satisfied there will not be any electromotive force holding the stepper motor 12 at the new position and the force of the wound coil 31 will return the stepper motor shaft 30 to its initial position arresting the flow of oil to ram 26 and stopping movement of the control surface. Thus the surface is deflected an amount proportional to the command signal. If desired, the pulse count control unit 8 can be made to generate a steady signal when a commanded pulse train is not present which corresponds to the initial position of the stepper motor 12 and thus insure an accurate return to and hold at the initial position. Some hysteresis may be present in the coil 31 and the generated initial position steady signal will obviate problems stemming therefrom. In situations where a powerful stepper motor is used so that it need not rotate 360 to cause an appreciable displacement of the output arm 17, a simple centering spring 36, shown in FIG. 212, may be employed. The spring 36 is mounted on post 37 attached to the motor casing with its

legs

34, 35 extending to either side of post 32 attached to motor shaft 30. Two stop posts 33, 33 attached to the casing established the positional center for each of

spring legs

34, 35. In operation, rotation of the motor will cause post 32 to be angularly displaced normal to the axis of rotation. Movement of the post 32 will displace one or the other of

legs

34, 35, the other leg being forced against and held by its respective post 33 or 33 and wind the spring 36. At the completion of the commanded stepper motor movement, the force of the spring 36 will center the motor shaft 30. As in the previously described modification, the pulse count control unit 8 can be made to generate a signal to electrically center the stepper motor 12 at its initial position. I

In a third alternative system for re-centering the stepper motor 12 and control 17, the pulse count control unit 8 may be designed to generate a train of pulses equal in number and rate to the commanded train of pulses but of opposite polarity. This second train of pulses would then command the motor pulse sequencer 11 to rotate the stepper motor 12 in the reverse direction and return the stepper motor 12 and its output arm 17 to the initial starting position. Should an error signal enter the system such that the return movement would be incomplete, the shuttle 16 would fail to finally close the hydraulic ports and resulting effect would be a hard-over condition. To obviate this result an auxiliary spring could be attached to the motor shaft 30 as described to insure a full return to the starting position.

The rate of displacement of the hydraulic actuator 14 is dependent upon the quantity of fluid which is metered to the actuator during a given time period, and is proportional to the .rate at which the port is opened. The rate of port opening is controlled by the stepping rate of the stepper motor 12, or the pulse repetition rate of the pulse generator 9. The displacement is normally controlled by the absolute magnitude of the input signal source, but may also be limited to a definite number of steps. The displacement rate may be contingent upon any condition existing within the circuit, fixed or preselected or manually variable and implemented by changing the repetition rate of the pulse generator 9. In this manner, both the amount and rate of displacement of the actuator may be individually controlled.

The above-discussed system lends itself to a simple but effective feedback and synchronizing system. Attached to the output portion of the pulse count control 8 is a parallel feed to the pulse to analog memory element 10. The pulses corresponding to the commanded stepper motor movement and hence the movement of ram 27 are fed to the memory element 1 1. The analog equivalent thereof is algebraically summed with the input signal in summing junction 4, thereby providing a modified input to the threshold circuits 5. Synchronization may be obtained by placing a pick-off 29 and pick-off responsive device 28 on the hydraulic ram 27 which is responsive to its zero position. The output of the device 28 may then be used to clear the memory element 11 as its output should be zero when the ram 27 is at the zero position. The effect accomplished is that of clearing the memory element 1 l of any spurious pulses that have entered the system without having moved the stepper motor 12. If desired, the synchronizing feature may operate at a point other than that of the zero position of the hydraulic ram 27 by simple modification of the pick-off device 29 and the memory element 1 1.

Referring to FIG. 1a, the pulse count control circuit 8 may be described in more detail as follows. If an error exists at the output of the summing junction 4 demanding movement of the actuator ram 27, the appropriate control flip-flop 6 or 7, is set on." This does two things. First, it assures that the other control flip flop cannot be turned on until a complete cycle is accomplished. Second, the activated flip-flop supplies an input to the full pulse circuit 40 which will then gate out the next series of'full pulses from the pulse generator 9. The output from the full pulse circuit 40 is ANDed with the

appropriate buffer

41 or 42 to drive the corresponding clockwise or counterclockwise motor drive in the motor pulse sequencer 11. In addition, the full pulse circuit 40 drives a power amplifier 44 through a buffer 43 so that only a reduced voltage is applied to the stepper motor 12 between pulses and between cycles. The full pulse circuit 40 also drives a differentiator 45 through a buffer and into a modulo 4 counter 46. The modulo 4 counter 46 along with the count 3 decode 47 and end of count 3 reset 48 insure that the counter 46 will proceed through a single cycle of 4 counts each time that the threshold 5 is exceeded or, it will produce a series of 4 count cycles if the threshold 5 remains exceeded during the reset time.

The count 3 decode 47 feeds a power amplifier 49 so that the power is withdrawn from the stepper motor 12 during the three count so that the spring on the stepper motor 12 can begin to center the stepper motor 12. After the third count, reduced voltage is applied to hold the stepper motor 12 at the start or null position.

Referring to FIG. 2, there is shown the mechanical connection between the stepper motor 12, the direct pilot input 19 and a boost actuator 14. The stepper motor 12 and gear train 13 may be built as a unit for simplicity having an output arm 17 which extends and retracts. If desired, a pivoting angular motion may be incorporated without departing from the scope of the invention. Attached to the output arm 17 is the metering shuttle arm 15. The operation of the metering shuttle arm is simply that of moving the shuttle 16 to control the input of hydraulic fluid to the hydraulic ram system 26 in a well known manner. In more detail, the stepper motor 12 and gear train 13 unit is pivotally attached to the actuator housing at point 24 through the

rigid arm

18, 25. A rigid arm 21 and brake device 23 is attached to the actuator housing 14. The rigid arm 21 is connected to the control arm 18 through a lockable sloppy link connection 20. In other words, the stepper motor 12 and gear train 13 form an integral assembly with the normal

valve actuator arm

18, 25 of the otherwise conveontional boost actuator.

The actuator may operate as either a parallel or a series ac tuator. In operation, as a parallel actuator, the brake 23 activated by a manually controlled switch 30 locks the sloppy link connection and

units

21, 18, 13 and become a rigid or integral unit. Any movement of the stepper motor 12 will be reflected in a lateral movement of the output shaft 17 and metering shuttle arm 15 such that the shuttle or pilot valve 16 is moved. Movement of the shuttle 16 will operate the hydraulic system 26, causing the ram 27 to move. For example, if motor 12 moves shuttle 16 to the right, pressure oil will be ported to the left side of ram 26 and return oil from the right side of ram 26 will be ported to the sump. Output 27 will thus move to the right carrying housing 14 with it. The rigid unit described above and boost actuator housing 14 is effectively attached to the ram 27 and and therefore moves with the ram. The ram movement will thus be reflected at the pilot's control stick through arm 19 as is characteristic of a parallel actuator. When the command is satisfied, i.e., the control member operated by member 27 has reached its commanded position, the signal input to motor 12 goes to zero and the shuttle 16 returned to its zero position, as by return spring described above. It will of course be understood that should the pilot desire to command the actuator through arm 19 he only needs to operate the brake switch to release the link or

lever

18, 25.

Should the pilot wish to control the movement of the ram 27, he must disengage the brake system 23. A movement of the control stick will reflect through arm 19 a movement of the control arm 18 within the confines of the sloppy link 20. As the

control arm

18, 25 is pivotally attached to unit 14, the latter will pivot about point 24. The gear train within unit 13 maybe reversible, and as such, movement of unit 13 may or may not be reflected at output arm 17 depending on the amount of friction present. The gear train could be made nonreversible but would then have to be much stronger and larger to withstand loads in the non-reversible direction. The presently used small size gear train 13 may be used provided that the movement of the output arm 17 be limited so that a force exerted on the unit 13 will be reflected through movement of the output arm 17 and acting through the limit positions. The degree of freedom between the limit positions must be less than half of the relative total movement of sloppy link 20, otherwise a movement of control arm 18 would be absorbed by the relative movement between the output arm 17 and unit 13 resulting in no movement of the shuttle 16. Some slop would be present under the means of manual control described above, but manual control would be maintained even if the stepper motor 12 or its control system failed. During a non-failure condition when the pilot wished to provide full time manual control, the motor could be locked electronically by simple switching means and thus the above-described slop would not be present.

In operation, as a series actuator, the sloppy link 20 plays a critical role. The brake is disengaged by switch 30 to allow movement of the pin 22 within the sloppy link 20. If, by means of a commanded control signal, the output shaft 17 and metering shuttle arm 15 were displaced to the right, the movement could be absorbed in two areas, depending on which one offered a greater frictional resistance to motion. Either the shuttle 16 would move within the actuator housing 14 causing a flow of hydraulic fluid, or the

units

25, 13 and 18 would pivot about point 24 causing the control stick arm 19 to move. The

first alternative is the only desirable result, as the hydraulic ram 27 would then reflect the desired change. Under the second alternative, movement of the ram 27 could still be effected by the pilot resisting the movement of the control stick arm 19, but this places an undesirable burden on the pilot. Thus, the control stick arm 19 should be configured to have more friction or spring load than the shuttle mechanism.

An example of the operation of the actuator of the present invention in the series mode is as follows. The brake 23 is released allowing the lever arm, comprising

members

18, 25, and 13 to pivot about pivot 24 relative to housing 14. Normally, the frictional load in the control linkage connecting the pilots controller with arm 19 is much greater than the force required to move shuttle 16. Thus, assume that an electrical command is supplied to motor 12 to move the shuttle, say to the right. The motor housing gear train 13,

links

18 and 25 are essentially rigid being held in position by the load at arm 19 and the motor 12 therefore moves shuttle to the right porting pressure oil to the left side of ram 26 and porting returning oil from the right side of ram 26 to the sump. The ram moves to the right carrying housing 14 with it. Since arm 19 is fixed by its frictional load, the lever arm including motor 12 is forced to pivot clockwise about pivot 24 (within the confines of slot 20) thereby moving shuttle 16 to the left relative to housing 14 shutting off the supply of pressure oil. Movement of ram 26 then stops and the control surface is positioned in accordance with the electrical input signal. This action is accomplished without reflection at the pilots control column as is characteristic of a series actuator. In operation, the movement of the shuttle 16 will also cause the housing 14 to reposition itself as discussed. This will, in turn, cause the sloppy link 20 to move. If the movement is less than the full distance of the slot within the sloppy link 20, the pin 22 will not be moved by the sloppy link 20 and may be accomplished by suitable geometry and lever arm lengths. The movement of the housing 14 will cause movement of the control arm 18, through

units

13 and 25 and pivot 24. Because of the aforementioned frictional resistance of the control stick arm 19, the control arm 18 will tend to pivot about the point of attachment of the control stick arm 19 and control arm 18. Thus, for relatively small correctional control movements, the motion of the housing 14 and ram 27 will not be reflected at the control stick. If the commanded controlsignal requires a movement greater than the restriction, preferably 50 percent of available movement, the excess will be reflected at the pilot's control stick. Unless this restriction is imposed, there will not be an incremental distance within which the control stick can be moved by the pilot if he wishes to supplement a commanded signal. The geometry of the pin 21 against one side or the other of the sloppy link 20 would then prevent a movement of the control arm 18 relative to the housing 14 to effect a movement of the output arm 17, metering shuttle arm 15 and shuttle 16. Similarly, if the pilot wishes to oppose the commanded signal, he could do so as the pin 21 would not be adjacent to the sloppy link 20 on the side of the desired movement either.

While the invention has been described in its preferred embodiment, it is to be understood that the words which have been used are words of description rather than limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

What is claimed is:

l. A control system for comprising, a hydraulic boost actuator of the type having valve means displaced in one direction or the other from a zero position to control the flow of hydraulic fluid to move said actuator a source of error signal in the form of discrete pulses,

a stepper motor energized by said pulses,

means coupled with said valve means and responsive to saidstepper motor for incrementally positioning said valve means in accordance with said pulse error signal, and means responsive to said error signal going to zero for returning said stepper motor to its original position whereby to return said valve means to its zero position and thereby arrest further motion of said actuator. 2. A control of the character set forth in claim 10 wherein said source of error signal comprises,

electronic circuitry means including threshold detector means responsive to a command signal and a feedback signal and adapted to supply discrete pulses when the difference between said command and feedback signals exceeds a predetermined value, and said circuitry further including means responsive to said pulses for providing said feedback signal whereby said pulse error signal is representative of the control system error. 3. A control system as described in claim 2 in combination with synchronizing means wherein the synchronizing means comprises a position sensing device responsive to the movement of said actuator through a predetermined position, and

means modifying said feedback signal in response to said position sensing device, whereby spurious signals causing lack of coincidence between said pulse signal and the actual position of said actuator are removed.

4. A control system as set forth in claim 1 including further means responsive to said error signal going to zero for impressing upon said stepper motor a holding signal coincident with the stepper motor zero position.

5. A hydraulic boost actuator system for operating an output member from a manual input member, the combination comprising a hydraulic actuator having a fixed part and a movable part coupled with said output member,

a control valve housing coupled with the movable part of said actuator and including valve means for controlling the flow of fluid to said actuator,

a valve means actuating arm pivoted on said housing and coupled with said manual input member for moving said valve means upon movement of said manual member whereby movement of said arm by said manual means ports fluid to said actuator to move said output member and simultaneously to return said valve means to its unactuated position,

brake means between said valve actuating arm and housing for clamping said arm relative to said housing, whereby said actuator is operated as a parallel actuator,

a stepper motor mounted on said valve actuating arm and adapted upon energization to move said valve means independently of said arm,

electronic control circuitry coupled with said stepper motor and responsive to an electrical input command for displacing said valve means and moving said output member, and

further means responsive to removal of said command signal for returning said stepper motor and valve means to its unactuated position whereby to arrest movement of said output member.

6. The combination as set forth in claim 5 further comprising a sloppy link connection between said arm and said valve housing, and

means for rendering said brake means ineffective and said sloppy link connection effective whereby said servo boost actuator may be operated as a series actuator.

Claims

1. A control system for comprising, a hydraulic boost actuator of the type having valve means displaced in one direction or the other from a zero position to control the flow of hydraulic fluid to move said actuator a source of error signal in the form of discrete pulses, a stepper motor energized by said pulses, means coupled with said valve means and responsive to said stepper motor for incrementally positioning said valve means in accordance with said pulse error signal, and means responsive to said error signal going to zero for returning said stepper motor to its original position whereby to return said valve means to its zero position and thereby arrest further motion of said actuator.

2. A control of the character set forth in claim 10 wherein said source of error signal comprises, electronic circuitry means including threshold detector means responsive to a command signal and a feedback signal and adapted to supply discrete pulses when the difference between said command and feedback signals exceeds a predetermined value, and said circuitry further including means responsive to said pulses for providing said feedback signal whereby said pulse error signal is representative of the control system error.

3. A control system as described in claim 2 in combination with synchronizing means wherein the synchronizing means comprises a position sensing device responsive to the movement of said actuator through a predetermined position, and means modifying said feedback signal in response to said position sensing device, whereby spurious signals causing lack of coincidence between said pulse signal and the actual position of said actuator are removed.

5. A hydraulic boost actuator system for operating an output member from a manual input member, the combination comprising a hydraulic actuator having a fixed part and a movable part coupled with said output member, a control valve housing coupled with the movable part of said actuator and including valve means for controlling the flow of fluid to said actuator, a valve means actuating arm pivoted on said housing and coupled with said manual input member for moving said valve means upon movement of said manual member whereby movement of said arm by said manual means ports fluid to said actuator to move said output member and simultaneously to return said valve means to its unactuated position, brake means between said valve actuating arm and housing for clamping said arm relative to said housing, whereby said actuator is operated as a parallel actuator, a stepper motor mounted on said valve actuating arm and adapted upon energization to move said valve means independently of said arm, electronic control circuitry coupled with said stepper motor and responsive to an electrical input command for displacing said valve means and moving said output member, and further means responsive to removal of said command signal for returning said stepper motor and valve means to its unactuated position whereby to arrest movement of said output member.

6. The combination as set forth in claim 5 further comprising a sloppy link connection between said arm and said valve housing, and means for rendering said brake means ineffective and said sloppy link connection effective whereby said servo boost actuator may be operated as a series actuator.