US3561326A

US3561326A - Pulse phase modulated servoactuator

Info

Publication number: US3561326A
Application number: US710916A
Authority: US
Inventors: Ernest W Cassaday
Original assignee: Parker Hannifin Corp
Current assignee: Parker Hannifin Corp
Priority date: 1968-03-06
Filing date: 1968-03-06
Publication date: 1971-02-09
Anticipated expiration: 1988-02-09

Abstract

Pulse phase modulated servoactuator for use as in flight control of missiles and the like characterized in that the servoactuators which turn the respective missile fins to control the pitch and the yaw of the missile are under the control of valves which are operated in or out of phase with a main control valve.

Description

United States Patent F151) 9/09,F15b 15/17 SYNC.

SIGNAL SUMMING JUNCTION COMMAND ERROR l l5 5 GNA L 55 HZ. SQUARE WAVE VOLTAGE SOURCE [50] Field of Search 91/448, (cards), 368, 362(Cursory), 446(Cursory), 417(Cursory), 459(Cursory). 361,(Cursory) [56] References Cited UNITED STATES PATENTS 2,782,798 2/1957 Ericson 91/448 2,900,960 8/1959 Grotzmuller 91/448 3,295,421 II! 967 McCormick 91/363 Primary Examiner-P2111] E. Maslousky Attorney-Oberlin, Maky, Donnelly & Renner ABSTRACT: Pulse phase modulated servoactuator for use as in flight control of missiles and the like characterized in that the servoactuators which turn the respective missile fins to control the pitch and the yaw of the missile are under the control of valves which are operated in or out of phase with a main control valve.

PHASE SHIFT NET.

AMPLIFIER /|o 19 30 r +W MULTIVIBRATOR sol-5N0") omven FEEDBACK COMPENSATION I u omiml 27 3 34 3 SUPPLY DRN 45 1 miminraasmn 3,561,326

SYNC. l8 1 SIGNAL PHASE SHIFT NET.

r summus JUNCTION AMPLIFIER IO COMMAND l9 2o 5 I HAL I 4 MULTIVIBRATOR SOLENO'D DRIVER 55 Hz. SQUARE WAVE VOLTAGE R E FEEDBACK COMPENSATION n DRAIN 27 SUPPLY 28 5? 32 55 a9, s9 7 "26 5o 37 5| 7; 38 2 1 I I 52 v v 5 4s l 45 f v 5 A r '-49 46''" 6 1 A 'f as 40 40 v f a 5 6 5 s |||-ii||ii||: T L35/ PRESSURE OSCILLATOR I L I l l 35 46 1 a I A i I i i 56 35 46 I 1 l l I w'ulARs: 5.15mi INVENTOR l l I ERNEST m 6.435404) W (LARGE PHASE SHIFT) ATTORNEYS 1 PULSE PHASE MODULATED SERVOACTUATOR BACKGROUND OF THE INVENTION Heretofore, it has been known to employ electrohydraulic servovalves for controlling velocity, acceleration or position of a hydraulic actuator by use of an acceleration or position of a hydraulic actuator by use of an electrical signal to control hydraulic output. Such servovalves may be used to direct and meter flow to and from the hydraulic actuator. As known, electrohydraulic servovalves are generally of either the single stage or the twostage type wherein the valve element is actuated by an electromechanical transducer in response to an input signal (single stage type) or wherein the electromechanical transducer actuates a pilot valve member which, in turn, directs fluid to shift the servovalve element.

When the servovalve is a three-way valve one side of the hydraulic actuator is pressurized by system pressure and the pressure on the other side is controlled by the three-way servovalve and in such case, the area of the hydraulic actuator which is exposed to the valve-controlled side thereof, is usually made approximately twice that of the side exposed to system pressure.

In known actuators, provision is often made for feedback so that in the case of a two-stage servovalve or pilot operated servovalve, the position of the main stage relative to command is signalled to the pilot stage so that any error can be corrected.

Commonly used techniques for feedback include mechanical, hydraulic and electrical, the latter embodying electrical apparatus for position pickup and for receiving the electrical signal. The electrical feedback ratio can be changed by adjusting the gain of the amplifier which compares the input signal to the signal produced by the servovalve element displacement.

SUMMARY OF THE INVENTION In the illustrative embodiment of the invention herein disclosed, the fluid servoactuator is operated by phase modulated electrical pulses which operate control valves for the servoactuator to determine the direction and speed of turning of the missile fins.

One object of this invention is to provide a pulse phase modulated servoactuator which operates from a carrier signal and a synchronized signal that is phase shift modulated with respect to the carrier signal in response to a system error signal.

It is another object of this invention to provide a pulse phase modulated servoactuator in which the available output torque remains constant.

It is another object of this invention to provide a pulse phase modulated servoactuator which affords unique control e.g. for small command signals of slow time varying functions, the system offers small low flow rate fluid pulses of constant pressure; and for large command signals of rapid time varying functions the system offers large flow rate fluid pulses of constant pressure. This provides for fine and coarse resolution in connection with small and large command signals respectively, whereby the missile fins may be rapidly actuated through large angles (coarse resolution) but as the command signal becomes smaller, the fins may be made to approach final desired positions with fine resolution.

It is another object of this invention to provide a pulse phase modulated servoactuator which is a position servo rather than a pressure servo capable of supplying constant pressure to the load regardless of fin angle, the servo valves being oversized so that the timing controls the flow and not fluid pressure drop.

It is another object of this invention to provide a servoactuator of the character indicated which is provided with a lock for the output shaft which retains the latter in neutral position until unlocked by fluid pressure.

Other objects and advantages of the present invention will become apparent as the following description proceeds.

kit

To the accomplishment of the foregoing and related ends. the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail a certain illustrative embodiment of the invention, this being indicative, however, of but one of the various ways in which the principle of the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic wiring and piping diagram of a preferred embodiment of the invention as used in actuating the fins of a missile or the like; and

FIG. 2 is a flow diagram illustrating the servoactuator operation with electrical pulses in and out of phase with carrier pulses.

DESCRIPTION OF THE INVENTION Referring to FIG. 1, the missile or the like with which the present invention is adapted to be used is provided with vertical yaw and horizontal pitch fins 2 which are adapted to be turned about their respective vertical and horizontal axes to control the missile path. The servocontrol unit 3 is secured to the missile housing and comprises, for example, four actuators 5 each having a shaft 6 to which a missile fin 2 is connected for rotation therewith.

It can be seen that the pulse phase modulated servoactuator 3 herein comprises:

1. The Electronic Module 10;

II. The Pressure Oscillator Module 11; and

III. The Servoactuator Modules 5. I. The Electronic Module In the embodiment illustrated in FIG. 1 the electronic module 10 comprises a 55 Hz square wave voltage source 14 for operating the pressure oscillator 11 and for providing a sync signal to each of the four channels of the servoactuator modules 5, only one of such sync signal channels being illustrated in FIG. 1. Included in each actuator module 5 channel is a summing junction 15 which receives the command signal and a position feedback signal from the feedback potentiometer 16 of the associated servoactuator 5 via a position feedback compensation unit 17. The resulting error signal, if any, is connected to an electrical phase shift network amplifier 18 together with the sync signal, and to an astable multivibrator 19. The electrical phase shift network amplifier 18 and astable multivibrator 19 have outputs +w and w connected to a solid state solenoid driver 20 and the output of the latter is a 55 Hz. square wave voltage source which is pulse phase modulated with respect to the square wave output of the voltage source 14. The pulse phase modulated source of each channel is connected to the associated servoactuator 5 in a manner as will hereinafter appear such that the fin actuator shaft 6 will turn in the desired direction and at desired speed in accordance with the magnitude of the command signal and the magnitude of the error signal as determined by a comparison of the command signal of the command signal with the position feedback signal.

In essence, the function of the electronic module 10 is to provide voltage pulses to the pressure oscillator module 11 and to provide pulse phase modulated pulses of the same frequency to the respective servoactuator modules 5, the modulation being such that a comparison of the command signal and a position feedback signal produces an error signal to determine the direction and speed of actuation of the respective servoactuators 5.

II. The Pressure Oscillator Module 11 The pressure oscillator module 11 is common to the four servoactuators 5 and herein is illustrated as a three-way closed center type valve having a piston or spool 25 reciprocable in housing 26 under the control of a three-way ball pilot solenoid valve 27, the latter being preloaded by spring 28 and balanced against system supply pressure in conduit 29 to attain consistent pull-in response. The pilot valve 27 is preferably of the loose ball poppet type because the ball 30 thereof is self-aligning with its seats and IS essentially frictionless and, furthermore. is less expensive to manufacture than a slide valve and also can crush any contaminant without jamming or sticking.

With reference to the solenoid 31, a pull solenoid can be more simply spring force balanced against inlet pressure than a push type solenoid and the resulting tower loading permits reducing solenoid size and voltage requirements. Furthermore, a pull solenoid 31 permits using a solenoid case 32 that is vented to return pressure in conduit 34 thus reducing weight.

The spool 25 is shown as being of the differential area type (preferably 2:1) having its large end communicated with the pressure source 29 (solenoid 31 energized) or with the drain conduit 34 (solenoid 31 deenergized) and its small end communicated with the pressure source 29 via conduit 35.

The solenoid pilot valve 27 operates the three-way valve spool 25 in a manner such that when the solenoid 31 is energized the armature 36 thereof is pulled to the right whereby the ball 30 is unseated by system pressure in conduit 29 from its left seat and engages its right seat to close off the drain or return conduit 34, whereby fluid pressure acting on the large end of the spool 25 overcomes the fluid pressure acting on the small end thereof so that the spool 25 is moved downwardly to communicate the conduit or control part 37 with the drain conduit or vent port 38. As will be seen this is the actuator vent cycle whereby the shaft 6 thereof will be turned in a clockwise (+w) direction if, during the communication of conduits 37 an 38 as aforesaid, a two-way solenoid operated valve 39 in actuator 5 is open during the solenoid 31 energization pulses 40 (see FIG. 2).

On the other hand, when the solenoid 31 is deenergized, the spring 28 will cause reengagement of the ball 30 with its left seat thereby venting the large end of the spool 25 to the drain port conduit 34, whereupon system fluid pressure in conduit 35 acting on the small end of the spool 25 will shift the same upwardly to communicate system pressure in conduit or inlet part 41 with the conduit 37. This is the actuator 5 pressure cycle whereby the shaft 6 thereof will be turned in a counterclockwise (-w) direction if, during the communication of

conduits

37 and 41, the aforesaid two-way valve 39 is open during the solenoid 31 deenergization pulses 42 (see FIG. 2).

Accordingly, to obtain positive and negative torques on the shafts 6 at maximum speed (coarse resolution), the signal for each two-way control valve 39 will be modulated to be in phase with the pressure pulses 40 or 42 (FIG. 2). In FIG. 2, the pulses 56 and 57 represent the control signal pulses (at small error signals) that energize solenoid 50 to open valve 39, these pulses being phase shifted to large degree with respect to

pulses

40 and 42 respectively so that each time valve 39 is opened, the spool 25 a short time thereafter either closes the vent connection 37 to 38 to permit only slight upward movement of piston 45 with consequent fine resolution, or closes the pressure connection 41 to 37 to permit only slight downward movement of piston 45 with consequent fine resolution.

The actuator position, i.e. shaft 6 and fin 2, achieved as a result of the applied torque is sensed by the feedback potentiometer 16 which feeds the summing junction of a servoamplifier 18 through a compensation network 17 to close the loop,

For small command signals, the phase modulation angle between the

solenoids

31 and 50 is nearly maximum as shown in FIG. 2 so that minute flow occurs during either the pressure or vent cycle of actuator 5, and for large command signals, the phase modulation angle is nearly zero so that maximum flow can occur during either the pressure or vent cycle of actuator 5.

IIIv The Servoactuator Modules 5 Each Servoactuator module 5 comprises a differential piston 45 (preferably 2:l area ratio) reciprocable in housing 46, said.

evident that when the piston 45 as viewed in FIG. 1 is moved.

upwardly, the fin 2 and shaft 6 will be turned in a clockwise (+w) direction, and when the piston 45 moves downwardly,

the fin 2 and shaft 6 will be turned in a counterclockwise (w).

direction. The shaft 6 also has mounted thereon a feedback potentiometer 16 which cooperates with a slider 49 having an.

electrical connection with the position feedback compensation unit 17.

Fluid pressure in

conduits

29 and 35 is conducted into housing 46 to act on the small end of the piston 45. System pressure from the same source 29 is also conducted to the large end of the piston 45 by way of conduit 41, three-way valve element 25, conduit 37, and two-way valve 39. In FIG. 1 one servoactuator 5 has been shown in cross section and it is to be noted that the other three servoactuators are connected to the pressure supply in the same manner, i.e., system pressure in conduit 29 is at all times acting on the small ends of the respective double area pistons 45 and the conduit 37 is connected to the respective two-way valves 39.

The solenoid operated two-way control valve 39 associated with each Servoactuator 5 is preferably a DC solenoid operated poppet type valve, the solenoid 50 preferably being of the pull type having a coaxial plunger or armature 51. The valve member 52 is pressure balanced for full flow are pressure reversals and is urged by the spring 53 against its seat to achieve consistent pull-in response conditions.

It the solenoid 50 of the two-way valve 39 is energized to open said valve 39 at a time when conduit 37 is communicated with the drain conduit 38, the system pressure in conduit 35 acting on the small end of the double area piston 45 will cause said piston 45 to be moved upwardly to turn the fin drive shaft 6 in a clockwise (w) direction as viewed in FIG. 1, since at that time, the large end of the piston 45 will be communicated with the drain conduit 38 via the conduit 37 and the downwardly shifted spool 25 of the pressure oscillator 11. On the other hand, if the solenoid 50 of the two-way valve 39 is energized to open said valve 39 at a time when the conduit 37 is communicated with the pressure conduit 41, the system pressure in conduit 37 acting on the large end of the piston 45 will cause said piston 45 to be moved downwardly to turn the shaft 6 in a counterclockwise (w) direction, since the downward force on the large end will exceed upward force on the small end.

Another feature of the present invention is that each actuator 5 is hydraulically locked and provides no torque output on shaft 6 when the two-way control valve 39 is closed during communication of conduit 37 with

conduit

38 or 41. Furthermore a ball detent lock or the like may be provided to lock the piston 45, and thus the shaft 6 and fin 2, in neutral position as shown in FIG. 1 until a detent release plunger or the like is hydraulically actuated to compress a spring.

From the foregoing it can be seen that the present invention provides a simple and efficient Servoactuator which for small command signals of slow time-varying functions automatically offers small low flow rate fluid pulses of constant pressure and conversely, for large command signals of rapid time-varying functions automatically offers large/high flow rate fluid pulses of constant pressure. The electric pulses for operating the

solenoids

31 and 50 are preferably of square wave form having the same frequency and the same pulse width whereby actuation direction and speed is determined by comparing a feedback signal with the command signal, the resulting error signal being effective through the electronic module 10 to operate the two-way valve solenoid 50 in desired phase relation with the three-way valve pilot solenoid 31.

Other modes of applying the principle of the invention may be employed, change being made as rev" "is tl detail:

described, provided the features stated in any of the following claims, or the equivalent of such, be employed.

I therefore particularly point out and distinctly claim as my invention:

I claim:

1. A servoactuator comprising a fluid motor having a member movable therein under the influence of fluid pressure; oscillating value means alternately opening and closing fluid communication between a fluid pressure source and said motor thus to move said member in stepwise manner; said valve means comprising a pair of valves in series between such fluid pressure source and said motor, whereby both of said valves must be open at the same time to permit fluid flow from such fluid pressure source to said motor, said valves being operated at substantially the same frequency; and phase shifting means operatively connected to at least one of said valves thus to control the phase relation between said valves for controlling the rate of movement of said member.

2. The servoactuator of claim 1 wherein said valves are solenoid operated valves; and wherein electronic signal means including said phase shifting means are operatively connected to said valves to supply electrical pulses thereto in desired phase relation.

3. The servoactuator of claim 2 wherein said servoactuator has position feedback signal means operated by said member to actuate said signal means to close one of said valves thus to arrest movement of said member.

4. A servoactuator comprising a fluid motor having a movable member therein biased in one direction by fluid under pressure acting on one end thereof and movable in such one direction in stepwise manner by intermittent venting of fluid from the other end of said member; oscillating valve means alternately permitting and preventing venting of fluid from said other end of said member; said valve means comprising valves in series between such vent and said motor, whereby both of said valves must be open at the same time to permit venting of fluid from said other end of said member, said valves being operated at substantially the same frequency; and phase shifting means operatively connected to at least one of said valves thus to control the phase relation between said valves for controlling the rate of movement of said member.

5. The servoactuator of claim 4 wherein said valves are solenoid operated valves; and wherein electronic signal means in cluding said phase shifting means are operatively connected to said valves to supply electrical pulses thereto in desired phase relation.

6. The servoactuator of claim 5 wherein said servoactuator has position feedback signal means operated by said member to actuate said signal means to close one of said valves thus to arrest movement of said member.

7. A servoactuator comprising a fluid motor having a differential area member movable therein in one direction by fluid under pressure constantly acting on the smaller area end thereof upon venting of the larger area end thereof and in the opposite direction by admitting such fluid under pressure to act on such larger area end; oscillating valve means selectively operative either to alternately permit and prevent venting of fluid from said larger end for stepwise movement of said member in such one direction under the influence of fluid pressure acting on said smaller end or to alternately open and close fluid communication between a fluid pressure source and said larger end for stepwise movement of said member in such other direction; said valve means comprising a pair of valves in series between said motor and such fluid pressure source and vent, one of said valves being operative alternately to connect the other of said valves to such fluid pressure source and such vent, whereby the phase relation of said valves will determine the direction of flow of fluid to and from said larger end of said member, said valves being operated at substantially the same frequency; and phase shifting means of movement of said member in the selected direction.

8. The servoactuator of claim 7 wherein said valves are solenoid operated valves; and wherein electronic signal means including said phase shifting means are operatively connected to said valves to supply electrical pulses thereto in desired phase relation.

9. The servoactuator of claim 7 wherein said valves are solenoid operated valves; wherein electronic signal means including said phase shifting means are operatively connected to said valves to supply electrical pulses thereto in desired phase relation; and wherein said servoactuator has position feedback signal means operated by said member to actuate said signal means to close one of said valves thus to arrest movement of said member.

10. A servoactuator comprising a fluid motor having a differential area member movable therein in one direction by fluid under pressure constantly acting on the smaller area end thereof upon venting of the larger area end thereof and in the opposite direction by admitting such fluid under pressure to act on such larger area end; oscillating valve means selectively operative either to alternately permit and prevent venting of fluid from said larger end for stepwise movement of said member in such one direction under the influence of fluid pressure acting on said smaller end or to alternately open and close fluid communication between a fluid pressure source and said larger end for stepwise movement of said member in such other direction; said valve means comprising valves in series operated at substantially the same frequency; and phase shifting means operatively connected to at least one of said valves thus to vary the rate of movement of said member in the selected direction, one of said valves comprising a two-way valve; and another of said valves comprising a three-way valve which, when said two-way valve is open, either vents fluid form said larger end or opens fluid communication between such fluid pressure source and said larger end.

11. The servoactuator of claim 10 wherein said two-way and three-way valves are solenoid operated valves which are arranged so that when the solenoids thereof are energized said two-way valve is opened and said three-way valve is a position either to vent said large end or to supply fluid under pressure to said large end thus to move said member in desired direction.

12. The servoactuator of claim 11 wherein electronic signal means including said phase shifting means are operatively connected to said valves to supply electrical pulses thereto in desired phase relation.

13. The servoactuator of claim 12 wherein said servoactuator has position feedback signal means operated by said member to actuate said signal means to deenergize and thus close said two-way valve to discontinue movement of said member.

14. A servoactuator comprising a fluid pressure oscillator having a fluid pressure inlet port, a vent port, and a control port; a three-way valve member reciprocable in said oscillator alternately to communicate said control port with said inlet and vent ports; said valve member having a small end which is constantly exposed to fluid under pressure and a large end which is adapted to be alternately vented and exposed to fluid under pressure thus to effect reciprocation of said valve member; an oscillating pilot valve for alternately venting said large end and admitting fluid under pressure to act on said large end; a fluid motor having a differential ar'ea piston member movable therein in one direction by fluid under pressure constantly acting on the smaller area end thereof upon venting of the larger area end thereof through said control port and vent port and in the opposite direction by admitting fluid under pressure through said inlet port and control port to act on such larger area end; an oscillating two-way valve in said control port between said oscillator and the larger area end of said piston member to open and close said control port; and oscillating means for oscillating said pilot valve and twoway valve at the same frequency and in a phase relation to move said piston member in stepwise manner in desired direction.

15. The servoactuator of claim 14 wherein sald oscillating means includes phase shifting means to vary the rate of movement of said piston member.

16. The servoactuator of claim 14 wherein said piston member has position feedback means associated therewith to close said two-way valve to arrest movement of said piston member.

17. The servoactuator of claim 14 wherein said pilot valve and said two-way valve are solenoid operated; and wherein said oscillating means comprises an electrical signal source to supply electrical pulses for operating said pilot valve and said two-way valve in a phase relation to move said piston member in desired direction.

18. The servoactuator of claim 22 wherein said electrical source also include phase shifting means to vary the rate of movement of said piston member.

19. A servoactuator comprising a fluid pressure oscillator having a fluid pressure inlet port, a vent port, and a control port, and first oscillating valve means for alternately communicating said control port with said fluid pressure inlet port and said vent port; an actuator operatively connected to said control port and having a member movable therein in one direction when said control port is in communication with said inlet port and means for moving said member in the opposite direction when said control port is in communication with said vent port; and second oscillating valve means in said control port selectively operated to open said control port to said member during either the pressure pulses or venting pulses of oscillator thus to move said member in either direction in stepwise manner; and means for oscillating said first and second oscillating valve means at substantially the same frequency in a desired phase relation to efiect movement of said member in the desired direction.

20. The servoactuator of claim 19 wherein said means for oscillating said first and second oscillating valve means includes an oscillating signal source having a phase shifting means to vary the rate of movement of said member in the selected direction.

21. The servoactuator of claim 20 wherein said oscillating signal source effects oscillation of said oscillator and said valve means at substantially the same frequency and pulse width in such desired phase relation to effect movement of said member in the desired direction as aforesaid.

Claims

5. The servoactuator of claim 4 wherein said valves are solenoid operated valves; and wherein electronic signal means including said phase shifting means are operatively connected to said valves to supply electrical pulses thereto in desired phase relation.

7. A servoactuator comprising a fluid motor having a differential area member movable therein in one direction by fluid under pressure constantly acting on the smaller area end thereof upon venting of the larger area end thereof and in the opposite direction by admitting such fluid under pressure to act on such larger area end; oscillating valve means selectively operative either to alternately permit and prevent venting of fluid from said larger end for stepwise movement of said member in such one dirEction under the influence of fluid pressure acting on said smaller end or to alternately open and close fluid communication between a fluid pressure source and said larger end for stepwise movement of said member in such other direction; said valve means comprising a pair of valves in series between said motor and such fluid pressure source and vent, one of said valves being operative alternately to connect the other of said valves to such fluid pressure source and such vent, whereby the phase relation of said valves will determine the direction of flow of fluid to and from said larger end of said member, said valves being operated at substantially the same frequency; and phase shifting means operatively connected to at least one of said valves to control the phase relation between said valves for controlling the rate of movement of said member in the selected direction.

14. A servoactuator comprising a fluid pressure oscillator having a fluid pressure inlet port, a vent port, and a control port; a three-way valve member reciprocable in said oscillator alternately to communicate said control port with said inlet and vent ports; said valve member having a small end which is constantly exposed to fluid under pressure and a large eNd which is adapted to be alternately vented and exposed to fluid under pressure thus to effect reciprocation of said valve member; an oscillating pilot valve for alternately venting said large end and admitting fluid under pressure to act on said large end; a fluid motor having a differential area piston member movable therein in one direction by fluid under pressure constantly acting on the smaller area end thereof upon venting of the larger area end thereof through said control port and vent port and in the opposite direction by admitting fluid under pressure through said inlet port and control port to act on such larger area end; an oscillating two-way valve in said control port between said oscillator and the larger area end of said piston member to open and close said control port; and oscillating means for oscillating said pilot valve and two-way valve at the same frequency and in a phase relation to move said piston member in stepwise manner in desired direction.

15. The servoactuator of claim 14 wherein said oscillating means includes phase shifting means to vary the rate of movement of said piston member.

19. A servoactuator comprising a fluid pressure oscillator having a fluid pressure inlet port, a vent port, and a control port, and first oscillating valve means for alternately communicating said control port with said fluid pressure inlet port and said vent port; an actuator operatively connected to said control port and having a member movable therein in one direction when said control port is in communication with said inlet port and means for moving said member in the opposite direction when said control port is in communication with said vent port; and second oscillating valve means in said control port selectively operated to open said control port to said member during either the pressure pulses or venting pulses of oscillator thus to move said member in either direction in stepwise manner; and means for oscillating said first and second oscillating valve means at substantially the same frequency in a desired phase relation to effect movement of said member in the desired direction.