CA1285835C - Pneumatic stepping actuator positioner - Google Patents

Abstract

PNEUMATIC STEPPING ACTUATOR POSITIONER
Abstract A pneumatic control system for a reversible stepping actuator has an analog pneumatic control signal and an analog signal indicative of actual position of the actuator. Imbalance in these signals opens one or the other of a pair of comparator valves having inlets connected to a source of pneumatic pressure. Opening of either of the comparator valves enables a pulse timer which converts the pneumatic pressure from either of the comparator valves to pulses of pneumatic pressure applied to one or the other of the operators of the reversible stepping actuator. An oscillating pneumatic timer may be used to produce the pressure pulses, or an electric timer can be used to "chop" one of the pneumatic pressures into periodic pulses.

Description

~3S~3~15 PNEUMATIC STEPPING ACTUATOR POSITIONER
-Field of the Invention This invention relates to a control 6ystem which provides pulses of pneumatic pressura for operating a reversible stepping actuator such as used for setting a valve position.

Backqround of the Invention In some oil field applications it is desirable to employ a fluid powered reversible stepping actuator for setting a valve position. Such an actuator can be quite useful where the valve is in a remote location. For example, such a valve and actuator may be located at a sea floor wellhead with control provided from a platform or on-shore location.
A stepping actuator i5 desirable in many such applications because of the precise control easily obtainable. Such a stepping actuator may be used, for example, for rotating a valve stem just a few degrees for each step. The rotation of the stepping actuator may be only three degrees per pulse of pressure and if desired this can be geared down to rotate a valve stem. Thus, it is easy and straightforward to provide precise control of valve position.

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1 An Qxemplary fluid power~d reversible stepping actuator is described in U.S. Patent No. 4,403,523. In such an ectuator each pulse of pressure applied to the actuator rot~tes the valve 6tem through a known angle. So long as the pulse duration is above 60me minimum, the stepping actuator will move one increment and only one increment ~or each pulse, even though the pulse may be of long duration. Thus, excessive rotation of the ~tepping actuator i5 easily avoided.
It is desirable to provide a control system for such a reversible stepping actuator whiGh provides pulses of pneumatic pressure with the pulse length and time interval between pulses being independently controllable~ This assures that the actuator will execute each step regardless of delays in the lines between the control system and actuator. The control system should provide pulses that will drive the actuator in either direction as required to reach a desired setting.
It is often desirable that such a c~ntrol system be entirely pneumatic. This can avsid u~e of electrical sensors and control devices where such us~ge is inconvenient or hazardous. It can often be desirable to have such a system operate only when a change in valve setting is desired, since in many applications a valve 2S position may be set and remain unchanged for days or weeks. It is desirable in such a situation to have the control system stand by without power consumption.
It is desirable that such a system operate automatically in response to a control setting. That is, one should be able to provide a control signal indicative of a desired valve position and have the system automatically operate the stepping actuator until the desired position is reached. This eliminates need for manual adjustment of the valve and provides the option of : .
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35~;~5 1 either manually or automatically setting the desired valve po6itio~.

Summary of the Invention There i5 there~ore provided in practice of this invention according to presently preferred embodiments, a pneumatic 6tepping actuator control system ~or a fluid operated stepping actuator having one operator ~or opening it and a second ~perator for closing it. A pulse timer is used for generating alternately an ON ~ignal of controlled duration and an O~F signal of controlled duration. A
normally closed pressure comparator valve having an inlet connected to a source of pneumatic pre~sure and an outlet connected to the pulse timer compares a pneumatic signal indicative of actual actuator position with a pneumatic control signal indicative of a desired actuator positior..
Similarly, a normally open pressure compensator valve having an inlet connected to the source of pneumatic pressure and outlet connected to the pulse timer compares such ~ignals. The normally closed valve opens when the actual position signal is greater than the control signal and the normally open valve is kept closed under those conditions. The opposite occurs when the control signal is greater than the actual position signal. Pulses of pneumatic pressure corresponding to the ON signals are applied to the first operator when the normally closed valve is opened, thereby tending to move the actuator toward an open position. Likewise, pulses of pneumatic pressure corresponding to the ON signals are applied to the second operator when the normally open valve is open due to the control signal being greater than the actual pressure signal. Either a pneumatic timer subsystem or an electrical timer subsystem may be used.
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1 Brief Description of the Draw~lnqs These and other features and advantages of the present invention will be appreciated as the 6ame becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
FIG. 1 iB a pneumatic circuit diagram of a ~tepping actuator control system employing a pneumatic timer, and FIG. 2 is a pneumatic circuit diagram of a ~tepping actuator control system employing an electrical timer.
The drawinqs employ conventional graphic symbols for fluid power diagrams as specified in ~merican National Standards/ANS Y32.10.

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583~i l _etailed Uescription The pneumatic control system provided in practice of this invention operates a conventional reversible stepping actuator such as, for example, as described in U.S. Patent No. 4,403,523. A typical 6tepping actuator has a cylinder and piston operator ll for moving the actuator toward an open position. A similar pneumatic operator 12 moves the actuator toward a closed position. When pneumatic pr~ssure is applied to one of the operators it causes the actuator to move one increment toward the opened or closed position. The actuator does not move an additional increment until the pressure is relieved and pressure is again applied to the operator.
~ n actuator position sensor 13 is mechanically connected to the actuator. In this embodiment the position sensor i5 a pneumatic pressure regulator having 100 PSIG air inlet. The output of the pressur~ regulator is an analog pneumatic signal in the range of from 6 to 30 PSIG. The lower pressure of this range indicates that the actuator is at its closed position and the higher pressure indicates that the actuator is at its open position. This signal indicative of the actual position of the actuator is compared with a pneumatic ~ignal indicative of a desired actuator position. For example, an electrical control signal which can be generated manually or automatically is applied to a conventional transducer 14 which converts the electrical signal to an analog pneumatic signal in the range of 6 to 30 PSIG. Supply air at lO0 PSIG is provided to the transducer to power the signal indicative of a desired actuator position. There is ample compressed air at oil field sites and such air can be used in the control system without special conditioning.
The supply air is also applied to the inlet of a : :

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1 normally closed pressure comparator valve 16 and ~he inlet of a normally open comparator valve 17~ The control signal from the ~ransducer 14 is applied to thç B operator of the normally closed comparator valve and to the operator of the normally open comparator valve. The signal indicative of actual actuator position from the regulator 13 i~ applied to the A operators of both the normally closed and normally open comparator valves 16 and 17.
The normally closed comparator valve is ~djusta~ly spring biasad to a closed position. When the pressure of the actual position ~ignal on the A operator is greater than the pressure of the control sig~al on the B operator, the normally closed comparator valve opens and remains open as long as the actual position signal is greater than the control signal. As will become apparent, opening of the normally closed comparator valv~ serves to apply pulses of pressure to the closing operator 12 of the stepping actuator. Thus, if the signal from the regulator 13 indicates that the actuator is open beyond the desired set point, pulses are applied toward closing the actuator.
The normally open comparator valve 17 is adjustably spring biased toward the open position. When the actual position signal on the A operator of this valve is greater than the control signal on the B operator, the valve is kept closed. If the actual position of the actuator is further closed than the desired set point, the control signal is greater than the actual position signal on the A
operator. This permits the valve to open and, as will become apparent, causes pressure pulses to be applied to the opening operator 11 of the actuator.
When the actual position signal equals the control ~ignal, indicating that the actuator is in its desired position, the normally closed comparator valve is closed !: :

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1 and the normally open comparator valve i6 also closed.
Thus, no changes occur ln the actuator position.
The 6priny bias that maintain~ the normally closed valve in it~ closed position i6 adjustable 60 that the ~agnitude of the di~ference ln pre6sure that causes the valve to open can be selected. By making a similar adjustment on the normally open comparator valve the "deadband" of the control system can be set. Thus, if $he spring biases ~re adjusted 50 that it takes a pressure difference of 0.3 PSI to change a comparator valve, the error in the actual position of the actuator is larger than when the comparator valves are adiusted to change when the pressure difference is only 0.1 PSI, for example.
~he output of the normally closed comparator valve is connected to a first operator of a three position, pilot-controlled, spring centered, direction control valve 18.
Similarly the outlet of the normally open comparator valve 17 is applied to the other operator of the three position direction control valve. When the comparator valves are in their closed positions, the respecti~e operators of the direction control valve are vented, and the direction control valve remains centered and closed. The outlets of both comparator valves are applied to an oscillating pulse timer subsystem 19 by way of check valves 21. Thus when either of the comparator valves i6 open, the 100 PSI
supply air is applied to the pulse timer. The pulse timer provides periodic pulses of pneumatic pressure with control of both the duration of the pressure pulse and the duration of the interval between successive pressure pulses.
The pneumatic pulse timer comprises an ON timer valve 22, an OFF timer valve 23 and a bi-stable timer relay valve 24. The ON timer valve and OFF timer valve are identical. Each timer valve is a spring biased, ' ,. . ' . - ~ :
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1 pneumatically operated three-way valve. Supply air from the checX valves 21 i6 applied to an inlet of each timer valve. The other "inl~t" of each timer valve $s a vent.
The timer valves are ~pring biased 60 that they are normally cl~sed, with the valve outlet connected to the vent. Each timer valve has an adjustable t~med pneumatic op~rator 23, 27. When pneumatic pressure is applied to one of the timed operators, a timer le ~tarted. At the end of a 6elected interv~l the respective operator causes the ti~er valve to switch to its open position. When the valve has been opened for a 6hort interval (typically a fraction of a second) it again closes.
The outlet of the ON timer valve 22 is connected to the A operator of the timer relay valve 24. Similarly the outlet of the O~F timer valve 23 is connected to the B
operator of the timer relay valve. Supply air from the check valves is also connected ~o the inlet of the timer relay valve. One outlet of the timer relay valve is con-nected to the vperator 26 of the ON timer valve and the other outlet is connected to the operator 27 of the OFF
timer valve. One outlet of the relay, in this case the one to the OFF timer operator, is connected to the inlet of the direction control valve 18 to provide periodic pressure pulses. It will become apparent that periodic pressure pulses are available at either outlet from the timer relay valve because of the symmetry of the pneumatic timer system.
Assuming that the pulse timer system 19 is in the state illustrated when one of the comparator valves is opened, air pressure is appli~d by way of the timer relay valve to the operator 26 of the ON timer valve. This starts the timer of the ON valve and after its set interval, the ON timer valve opens. Opening of the ON
timer valve applies supply air pressure to the A operator .
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1 of the timer relay valve. This causes the timer relay valve to ~huttle to its other position.
In this second position of the relay valve, pressure i6 shut off from the operator of the ON timer val~e and in a fraction of a 6econd the ON timer valve again closes.
Air pressure is instead applied to the operator 27 of the O~F timer valve. This start~ the timed cycle of the OPF
timer and at the end of it~ preset interval, the OFF timer valve opens. Opening of the OFF timer valvP ~pplies pneumatic pressure to the B operator of the timer relay, ~hifting it back towards its initial state. (The A
operator of the timer relay valve i~ vented through the ON
timer valve, which i~ then in its closed position, to permit free shuttling of the relay valve). Shuttling the timer relay valve back to its initial position cuts off air pressure to the OFF timer operator. Pressure in the connecting lines drops off quickly through the OFF timer operator and the OFF timer valve reverts to its closed position.
The shuttling of the timer relay valve to its initial state also applies pneumatic pressure to the ON timer operator 26 to restart the cycle. The free running timer system continues to repeat the cycle as long a~ air pressure is applied through the check valves, thereby providing periodic pneumatic pressure pulses to the inlet of the direction control valve.
The length of the pressure pulse in the illustrated embodiment is determined by the time setting of the OFF
timer operator. In effect the ON timer valve turns on the pulse and the OFF timer valve turns off the pulse.
Similarly, the interval between pulses is determined by the time period set at the ON timer operator. In an exemplary embodiment, the OFF timer operakor might be set at two seconds or more to assure adequate time for the .:
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l pressure pul~e to build up in the operator of the actuatorand cause the actuator to step one interval. If the pneumatic line to the ~ctuator i6 long, a longer duration of pulse may be used. The interval between pulses eet on the ON timer operator may be two or more 6econds to assure that pressure is relieved on the respective operator and the position ~ensor signal has had time to change before the next pulse.
The pulse timer operates only when one of the comparator valve~ is open. Otherwise there is no air pressure applied to it to cause it to operate. When the pulse timer is running, pulses of pneumatic pressure are applied to the inlet of the direction control valve. This valve is normally closed; however, when air pressure is applied to the pulse timer, the valve is also open in one position or the other.
In the event the normally closed comparator valve is open, air pressure is applied to the A operator of the direction control valve. This opens the control valve so that pulses of pneumatic pressure are applied to the closing operator 12 of the actuator. When the valve is in this position, the opening operator ll of the actuator is vented through the control valve. Thus, as long as the normally closed comparator valve is open, pulses of pneumatic pressure cause incremental movement of the actuator towards its closed position. When the position sensor indicates that the actual position of the actuator corresponds to the desired position as indicated by the control signal, the normally closed valve closes, which both closes the direction control valve 18 and shuts down the pulse timer.
Likewise, when the normally open comparator valve 17 is open, air pressure is applied to the B operator of the direction control valve and the pulse timer. This causes .-,. . , , . ~ .
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1 the direction control valve to open 60 that pulses of pneumatic pressure are applied to the opening operator of the actuator and the closing operator i5 vented.
It is desirable that the pulse timer operate only when pneumatic pulses are needed for operating the stepping actuator. In most applications of such equipment, the actuator is operated only occasionally and long periods may elapse between changes of valve position.
During ~uch periods there is no reason to run tha pulse timer, with concomitant wear and ~light ralease of compressed air.
FIG. 2 illustrates ~chematically another embodiment of control system for a reversible 6tepping actuator constructed according to principles of this invention.
Portions of this system are identical to the embodiment illustrated in FIG. 1 and like components have been identified with references numbers 100 larger than the corresponding reference numbers in FIG. 1. Thus, ~or example, the actuator position ~ensor 13 in ~IG.
corresponds exactly to the position sensor 113 in ~IG. 2.
Since the actuator operators 111 and 112, the signal transducer 114 and t~e comparator valves 116 and 117 operate identically to the corresponding components described with respect to FIG. 1, a redundant description of their operation is omitted.
The pulse timer 119 comprises a conventional adjustable electrical timer 31 which in this embodiment operates from a source 32 of 24 volt direct current.
Clearly other power supplies could be used but the low voltage is desirable for safety reasons. The electrical timer 31 is enabled by a pneumatically operated single throw, double contact switch 33. When either of the comparator valves 116 or 117 is open, pneumatic pressure is applied through the check valves 121 for closing the : .
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~285;83~i 1 normally open enabling switch 33. Closing the switch applies current to the timer for enabling lts operation.
Such a timer is adjustable to provide an ON slgnal of a desirPd duration and an interval (or OFF ~ignal) of a desir~d duration between ON signals. When the timer is on it ~pplies current to the solenoids 34 o~ a pair of normally closed solenoid valves 36, 37. Thus, when the electrical timer is in the ON portion of its cycle, the solenoid valves are both open, and when the timer is off both solenoid valves are closed.
The inlet to one of the solenoid valves 36 comes from the normally closed comparator valve 116. The outlet of `that solenoid valve is connected to the opening operator 111 of the actuator. The inlet to ~he other solenoid valve 37 comes from he normally open comparator valve 117 and the outlet of the solenoid valve is connected to the closing operator 112 of the actuator. When the solenoid valves are closed the actuator operators are vented. When the solenoid valves are open, the operators are connected directly to the respective comparator valves.
When the actuation position sensor shows that the actual position differs from the desired position, one of the comparator valves is opened as hereinabove described.
The resultant air pressure enables the pulse timer, which causes the two 601enoid valves to periodically open and close for intervals as provide.d by the adjustable timer.
Pneumatic pressure from the comparator valve that is open is thereby applied to the corresponding actuator operator.
The other of the actuator operators is vented through the solenoid valve and other of the comparator valves.
For example, if the normally closed comparator valve 116 is open, pneumatic pressure is applied to the opening operator 111 of the actuator each time the solenoid valves open. The closing operator 112 is continually vented, .
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1 either directly through the vent of it6 solenoid valve 37 or through the vent of the normally open comparator valve 117.
In both embodiments of the control ~ystem, the reversible actuator i6 operated directly by pneumatic pressure. In the event a hydraulic actuator i8 desired, the pneumatic pressure pulse6 otherwise applied directly to the actuator operators are applied to pneumatically operated hydraulic valves which in turn operate the actuator.
In the illustrated embodiments the actuator position sensor comprises a pressure regulator mechanically coupled to the actuator to provide an analog pressure signal indicative of actual position of the actuator. If desired an electrical sensor can be coupled to the actuator with its signal applied to an electrical pneumatic tran ducar (6i~ilar to the transducer 14) to produce a pneumatic signal for application to the comparator valves.
Specifie pneumatic devices are illustrated for imple-menting this invention. It will be apparent thatsubstitutions can be made without departing from principles of this invention. For example, in the embodiment illustrated in FIG. 1, instead of a single three-position direction control valve, separate valves could be connected to each source of pneumatic pressure from ~he respective comparator valves. Similarly in the embodiment illustrated in FIG. 2, instead of using separate solenoid valves 36 and 37, a single four-way valve can be used.
It will be apparent that many other modifications and variations can be made in practice of this invention. It is therefore to be understood that within the scope of the appended claims, this invention may be practiced otherwise than as specifically described.

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Claims (18)

1. A pneumatic control system for a reversible stepping actuator comprising:
means for generating an analog pneumatic signal indicative of actual actuator position;
means for generating an analog pneumatic control signal corresponding to a desired actuator position;
comparator valve means having a pair of inlets connected to a source of pneumatic pressure and connected to the pneumatic signals for remaining closed when the signals are equal and opening one of the valve means when the signals are unequal;
means for converting the pneumatic pressure from either of the open valve means to periodic pulses of pneumatic pressure; and means for applying such pressure pulses to the operators of a reversible stepping actuator for stepping the actuator.

2. A control system as recited in claim 1 wherein the means for converting comprises an oscillating pneu-matic timer connected to the pneumatic pressure from either of the valve means.

3. A control system as recited in claim 1 wherein the means for converting and the means for applying comprise:
an oscillating electric timer for producing an ON
signal of controlled duration and an OFF signal having a controlled interval between successive ON signals; and solenoid valve means connected to the timer to open in response to an ON signal and close in response to an OFF signal, and interconnecting the comparator valve means and the actuator.

4. A control system as recited in claim 1, 2 or 3 comprising a control valve for applying the pressure pulses to the actuator and means for opening the control valve in one direction when the control signal is greater than the actual position signal and for opening the con-trol valve in the other direction when the actual position signal is greater than the control signal.

5. A pneumatic reversible stepping actuator control system comprising:
means for generating an actual actuator position signal;
means for generating a desired actuator position signal;
means for comparing the actual position signal and the desired position signal and producing a first pneumatic pressure when the desired position signal is greater, and a second pneumatic pressure when the actual position signal is greater;
an oscillating pneumatic timer;
means for connecting the first pneumatic pressure to the timer and means for connecting the second pneumatic pressure to the timer for converting either the first pneumatic pressure or the second pneumatic pressure to periodic pulses of pneumatic pressure;
means for applying the pulses of pneumatic pres-sure to a first operator of such a reversible stepping actuator when the desired position signal is greater; and means for applying the pulses of pneumatic pres-sure to a second operator of such a reversible stepping actuator when the actual position signal is greater.

6. A control system as recited in claim 5 wherein the means for applying the pulses of pneumatic pressure to the first operator comprises:
a first normally closed valve means connecting the timer output to the first operator and means for opening the first valve means in response to the first pneumatic pressure; and the means for applying the pulses of pneumatic pressure to the second operator comprises a second normal-ly open valve means connecting the timer output to the second operator and means for opening the second valve means in response to the second pneumatic pressure.

7. A control system as recited in claim 5 wherein the timer comprises:
a normally closed ON valve;
ON timer means for temporarily opening the ON
valve at the end of a first selected timed interval;
a normally closed OFF valve;
OFF timer means for temporarily opening the OFF
valve at the end of a second selected timed interval;
a bi-stable relay valve;
means for connecting either the first or second pneumatic pressure to the inlets of the ON valve, the OFF
valve and the relay valve;
means for connecting the outlet of the OFF valve to a first operator of the relay valve for shifting the relay valve to a first state;
means for connecting the outlet of the ON valve to a second operator of the relay valve for shifting the relay valve to a second state;
a first outlet from the relay valve connected to the OFF timer means, the first state of the relay valve connecting the inlet and the first outlet for operating the OFF timer means; and a second outlet from the relay valve connected to the ON timer means, the second state of the relay valve connecting the inlet and the second outlet for operating the ON timer means, so that at least one of the outlets of the relay valve provides pulses of pneumatic pressure.

8. A control system as recited in claim 5, 6 or 7 wherein the means for generating an actual actuator pressure signal comprises a pneumatic pressure regulator mechanically coupled to the actuator for converting actual position to an analog pneumatic signal.

9. A control system as recited in claim 5 wherein the position signals each comprise a pneumatic pressure signal and the means for comparing comprises:
first pneumatic comparator valve means having an inlet connected to a source of pneumatic pressure;
second pneumatic comparator valve means having an inlet connected to a source of pneumatic pressure; and means for connecting the desired position signal and the actual position signal to both comparator valve means for selectively opening the first comparator valve means when the desired position signal is greater or opening the second comparator valve means when the actual position signal is greater.

10. A control system as recited in claim 9 wherein the means for applying the pulses of pneumatic pressure to the operators comprises:
direction control valve means having an inlet connected to the means for converting pneumatic pressure to pressure pulses and having first and second outlets connected to the first and second operators, respectively;
and means for shifting the direction control valve means to the first outlet when the desired position signal is greater and shifting the direction control valve means to the second outlet when the actual position signal is greater.

11. A control system as recited in claim 5 wherein the means for generating periodic pulses of pneumatic pressure and applying such pulses to the operators comprise first solenoid valve means connecting the first pneumatic pressure to the first operator;
second solenoid valve means connecting the second pneumatic pressure to the second operator; and an electric timer for alternately opening and closing both solenoid valve means.

12. A control system as recited in claim 11 wherein the timer is enabled only in response to either the first or second pneumatic pressure.

13. A pneumatic reversible stepping actuator control system comprising:
means for generating an analog pneumatic signal corresponding to actual actuator position;
means for generating an analog control signal corresponding to a desired actuator position signal;
a normally closed comparator valve having an inlet connected to a source of pneumatic pressure;
a normally open comparator valve having an inlet connected to a source of pneumatic pressure;
means for connecting the control signal and the actual position signal to the normally closed valve for keeping the valve closed when the two signals are equal and for opening the valve when the actual position signal is greater than the control signal;
means for connecting the actual position signal and the control signal to the normally open comparator valve for keeping the valve closed when the signals are equal and opening the valve when the control signal is greater than the actual position signal;
an oscillating pneumatic timer for generating periodic pulses of pneumatic pressure;

means for connecting the inlet of the pulse timer to the outlets of the first and second comparator valves;
a first operator for stepping a reversible actuator in one direction;
a second operator for stepping the actuator in the opposite direction;
a direction control valve having an inlet connec-ted to pneumatic pressure pulses from the pulse timer, a first outlet connected to the first operator and a second outlet connected to the second operator;
means for connecting the outlet of the normally closed comparator valve to the direction control valve for opening its inlet to the second outlet and venting the first outlet when the normally closed comparator valve is open; and means for connecting the outlet of the normally open comparator valve to the direction control valve for opening its inlet to the first outlet and venting the second outlet when the normally open comparator valve is open.

14. A control system as recited in claim 13 wherein the timer comprises:
a normally closed ON valve;
ON timer means for temporarily opening the ON
valve at the end of a first selected timed interval;
a normally closed OFF valve;
OFF timer means for temporarily opening the OFF
valve at the end of a second selected timed interval;
a bi-stable relay valve;
means for connecting either the first or second pneumatic pressure to the inlets of the ON valve, the OFF
valve and the relay valve;
means for connecting the outlet of the OFF valve to a first operator of the relay valve for shifting the relay valve to a first state;

means for connecting the outlet of the ON valve to a second operator of the relay valve for shifting the relay valve to a second state;
a first outlet from the relay valve connected to the OFF timer means, the first state of the relay valve connecting the inlet and the first outlet for operating the OFF timer means; and a second outlet from the relay valve connected to the ON timer means, the second state of the relay valve connecting the inlet and the second outlet for operating the ON timer means, so that at least one of the outlets of the relay valve provides pulses of pneumatic pressure.

15. A control system as recited in claim 13 or 14 wherein the means for generating an actual actuator pres-sure signal comprises a pneumatic pressure regulator mechanically coupled to the actuator for converting actual position to an analog pneumatic signal.

16. A pneumatic reversible stepping actuator control system comprising:
means for generating an analog pneumatic signal corresponding to actual actuator position;
means for generating an analog control signal corresponding to a desired actuator position signal;
a normally closed comparator valve having an inlet connected to a source of pneumatic pressure;
a normally open comparator valve having an inlet connected to a source of pneumatic pressure;
means for connecting the control signal and the actual position signal to the normally closed valve for keeping the valve closed when the two signals are equal and for opening the valve when the actual position signal is greater than the control signal;
means for connecting the actual position signal and the control signal to the normally open comparator valve for keeping the valve closed when the signals are equal and opening the valve when the control signal is greater than the actual position signal;
an electric timer for generating an ON signal of controlled duration and an OFF signal of controlled duration;
first solenoid means interconnecting the outlet of the normally closed comparator valve and a first opera-tor of a reversible stepping actuator for stepping the actuator in one direction;
second solenoid means interconnecting the outlet of the normally open comparator valve with a second opera-tor of the actuator for stepping the actuator in the oppo-site direction;
means for connecting the outlets of both the com-parator valves to the timer for enabling the timer; and means for connecting the timer to both solenoid valve means for opening both solenoid valves in response to the ON signal and closing both solenoid valves in res-ponse to the OFF signal.

17. A control system as recited in claim 16 wherein the means for generating an actual actuator pressure sig-nal comprises a pneumatic pressure regulator mechanically coupled to the actuator for converting actual position to an analog pneumatic signal.

18. A pneumatic reversible stepping actuator control system comprising:
means for generating an actual actuator position signal;
means for generating a desired actuator position signal;
means for comparing the actual position signal and the desired position signal and producing a first pneumatic pressure when the desired position signal is greater, and a second pneumatic pressure when the actual position signal is greater;
first solenoid valve means for connecting the first pneumatic pressure to the first operator of such a reversible stepping actuator when the desired position signal is greater;
second solenoid valve means for connecting the second pneumatic pressure to the second operator of such a reversible stepping actuator when the actual position signal is greater;
an electric timer for alternately opening and closing both solenoid valve means; and means for enabling the electric timer only in response to either the first pneumatic pressure or the second pneumatic pressure for applying pulses of one of said pneumatic pressures to the respective operator.