GB1559463A - Valve control system - Google Patents

Description

(54) VALVE CONTROL SYSTEM (71) We, E-SYSTEMS INC., a corporarion organised under the laws of the State of Delaware, United States of America, and of 1600 Pacific Avenue, Dallas, Texas 75201, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement :- The present invention relates to a valve control system in which the position of a valve member is adjusted to control a fluid flou rate.

In most process control systems, valve positioning is accomplished through the use of pneumatic actuators. The pneumatic actuators require compressed, clean, dry air for operation. The air transport requires long runs of metal tubing, filters, and continuously operating compressors. Further, the pneumatic system requires two types of motivational forces, electricity to run the compressor and compressed air.

Some fully elecuical valve positioning devices have been provided but are sull subject to several disadvantages. Heretofore, synchronous motors, although desirable, have not been applicable to most systems since they require long bursts of AC voltage for their operation. One of the contributing factors which made it necessary have long long A (, bursts to operate the synchronous motor was the inherent friction of the actuator device itself. Such bursts were required to overcome that friction and initiate movement of the actuator which is operatively coupled to the valve device. Because of the nature of the long bursts, it has heretofore been impossible to ascertain the precise moment and duration of the motor shaft movement. Thus, position feedback arrangements have been required to monitor valve position. Such feedback systems are complicated and add an unnecessary dimen sion to the control system and an additional source of error. A means is required to lock the motor rotor when it is not being driven so that when a power failure occurs a preloaded spring can drive the valve open or closed.

Some prior art devices have included stepper motors with a DC power signal being selectively applied to the field windings of the motor. Such systems however require large and expensive DC power supplies for generating the DC power.

Further, most prior art systems have been at least partially analog in their configuration and the simplicity and precision of a digital valve control system has not heretofore been provided.

Some prior art valve drive systems have included a reversible motor but have been subject to inaccuracies when the motor direction is reversed. Many systems in which reversible operation is required include a motor which is electrically driven in only one direction and returned by means of a mechanical spring device. The mechanical return device introduces a new source of error and imprecision vis a vis an all electric, digital system.

The present invention provides a valve control system comprising a valve having an adjustable valve member, terminals for connection to an AC supply, controlled rectifying means connected to said terminals, timing signal generating means connected to said terminals for deriving timing pulses in synchronism with the pulsations of the AC supply when connected to said terminals, a reversible synchronous motor having field coils and a rotor, phase shifting means connected to the field coils and to said controlled rectifying means, an actuator coupled to said rotor and to said valve member to control the rate of fluid flow through the valve, means to detect a variation in said flow rate to provide a measurand, control means for comparing the measurand with a reference to derive directional, locking and drive command signals to provide the control required with regard to said variation and said reference, and a logic circuit connected to the timing signal generating means and to the controlled rectifying means, said logic circuit being responsive to said directional and drive command signals and to said timing signals to cause said controlled rectifying means to be switched to supply bipolar pulses to the phase shifting means and to said field coils to drive the rotor, in predetermined incremental amounts in one direction or the other, and being responsive to said locking command signal to supply said field coils with unipolar pulses to arrest said rotor whilst it is not driven by said bipolar pulses.

A valve control system according to a preferred embodiment of the invention, is entirely electrically driven thus avoiding the use of compressed air. It is substantially digitally controlled and operated. No valve position feedback arrangement is necessary and a separate DC power supply is not required.

The system also allows fail-open or close operations to occur upon a loss of primary power.

The referred embodiment of the invention will now be described, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of an exemplary flow control system in which the preferred embodiment is utilized; Figure 2 is a schematic diagram of a portion of the circuit shown in Figure 1 ; Figure 3 is a detailed schematic diagram of a logic circuit shown in Figure 2; Figure 4 is a cross-sectional view of a valve actuator; Figure 5 is a cross-sectional view of another valve actuator; and Figure 6 is a timing chart useful in explaining several operational features of the preferred embodiment.

In Figure 1, a flow meter 1 monitors the flow within a conduit 3. The conduit 3 passes through a valve member 5. In the exemplary system of Figure 1, the controlled variable is the flow of the fluid within the conduit 3 and the signal representative of the sensed flow at the flow meter 1 is applied to a process computer-controller 7. The process computercontroller 7 compares the detected flow signal with a set point or dialed-in flow magnitude and provides a series of signals to a control circuit 9. The control circuit 9 in turn applies several control signals to a motor 11 which operates an actuator 13. The actuator 13 is operatively coupled to the valve 5 to control the movement of a flow control element of the valve 5.

Fig. 2 shows a schematic diagram of the control circuit 9 and its connections to the terminals of the motor 11. The control circuit 9 in the exemplary embodiment receives a 60 Hz AC power at terminals 15 and 17 of the control circuit 9. The controller also provides a Direction signal Dl, an External Lock Signal EL and a Drive signal DR at terminals 19,21 and 23 of the control circuit 9. The Direction signal Dl is provided to indicate in which of two directions the regulated element is to be moved. For example, if there is a logic 1 on the line designated D1, the controlled element within the valve 5 is to move in the direction D1 or, for example, in a downward direction. Similarly, if there is a logic 0 level on the line D1, the regulated element within the valve 5 is to move, for example, in an upward or open direction. D1 may represent either upward or downward movement but for purposes of illustration, a logic"l"on D1 represents an downward movement in the exemplary embodiment. The rotor is locked at all times when a drive command is not applied. The External Lock over- rides all direction and drive commands. The External Lock signal EL commands that the rotor should be locked in place notwithstanding any other signal. The Drive signal DR represents a command from the computercontroller 7 to effect a rotation of the rotor in accordance with the Direction signal D1.

In the present example, a logic 1 level is indicative of a true statement as represented by approximately a + 12 volt DC signal while a logic 0 or false level is represented by a 0 voltage level. Thus, if it is desired to move the regulated element upwardly, D1 will be at a logic 0 level, DR will be at a logic 1 level, and EL will be at a logic 1 level (indicating that the statement that there is not an external lock condition is true).

A zero voltage crossing switch or timing signal generating means 25 receives the 60 Hz power signal and provides the Timing signal T to a logic circuit 27. The logic circuit 27 also receives the Direction signal D1, External Lock signal EL and Drive signal DR.

The zero crossing switch 25 includes a bistable switching element 29 and a one-shot multivibrator 31. Timing signal T in the present example, is a positive going pulse generated each time the power signal crosses the 0 voltage level, i. e. 120 times per second.

The logic circuit 27 provides first, second and third operational signals, viz. an Open Signal O, a Close signal C and a Lock signal L for application to a switching device generally indicated at 33. The motor 11 has three terminals 35,37 and 39 for connection to external circuits. The terminal 35 in the present example is directly connected to the power line applied to the input terminal 15 of the control circuit 9. The motor comprises a rotor 41 and first and second field windings 43 and 45, respective (y. The winding 43 is connected between terminals 35 and 37 and the winding 45 is connected between terminals 35 and 39. The terminals 37 and 39 are connected through a phase-shifting network, comprising capacitor 47 and resistor 49, to the switching device 33. The switching device 33 is arranged to selectively couple the terminals 37 and 39 to the other side of the power line connected to terminal 17 in response to the control signals O, C and L.

More specifically, the motor terminal 37 is connected to a first common point 51. The first common point 51 is connected to the anode terminal of a diode 53 the cathode terminal of which is connected through a resistor 55 to one switching terminal of a triac 57. The other switching terminal of the triac 57 is connected to a second common point 59 which is in turn connected to the second power lead from the input terminal 17 of the control circuit 9. Another triac 61 is also connected in parallel between the common termina ! 51 and the common terminal 59. The triac 61 has a gate terminal connected to the logic circuit 27 for receiving the Open control signal O. The motor terminal 39 is connected to a third common terminal 63 of the switching device 33. Common terminal 63 is connected through a triac 65 to the second common terminal 59 and has its control terminal connected also to the logic circuit 27 for receiving the Close control signal C. The common terminal 63 is connected to the cathode terminal of a diode 67 and the terminal of which is connected through a resistor 69 to one switching terminal of a triac 71. The other switching terminal of the triac 71 is connected to the common point 59. The gate terminals of the triac 57 and 71 are arranged to receive the Lock signal L.

The signals D1, EL and DR which are applied to the control circuit 9 from a process computer-controller 7 may be readily derived from many process computer-control lers on the market. Their only requirements are as follows : D1 remains at a logic 1 level so long as the regulated element is to move in a first direction and changes to a logic 0 level when the direction of travel is to be reversed; EL remains high when there is no external condition requiring that the rotor be locked in place and changes to a logic 0 level when such an external condition appears; and the DR signal is a train of pulses with each pulse representing a desired rotor step (i. e. wherein the rotor is driven bv a predetermined incremental amount).

Fig. 3 shows the details of the logic circuit 27. The input terminal 19 is connected to both input terminals of a NAND gate 73.

Input terminal 19 is also connected to one input terminal of a four-input NAND gate 75.

The output terminal of the gate 73 is connected to one input terminal of another fourinput NAND gate 77. The gate 73 output is also connected through a resistor 79 to one input terminal of an exclusive OR gate 81.

That input terminal of the gate 81 is also connected through a capacitor 83 to a common reference potential. The output terminal of the gate 73 is also connected directly to a second input terminal of the gate 81. The output terminal of the gate 81 is connected to a one-shot multivibrator 85 which provides a Reverse Lock output signal RL. As hereinafter explained in connection with the operation of the present invention, the signal RL is generated, i. e. assumes a logic 0 level whenever the Direction command D1 changes states.

The RL signal is applied to one input terminal of gates 75 and 77. The RL signal is also applied to one input of a two-input NAND gate 87.

The input terminal 21 of the logic circuit 27 is connected to ene input terminal of a two-input NAND gate 89. The output of the gate 89 is applied to an inverting NAND gate 91 which provides a Conditioned Synchronized Drive signal CSDR which is applied to the other input of the gate 87 and also to one input of each of gates 75 and 77. The other input of the NAND gate 89 is provided by a synchronization circuit to which the input terminal 23 is connected. The input terminal 23 provides the Drive signal DR which is applied to both input terminals of a NAND gate 93. The output of the gate 93 is applied to both input terminals of another NAND gate 95 the output terminal of which is connected to a one-shot multivibrator 97.

The one-shot multivibrator 97 provides an output which is connected to one input terminal of a NAND gate 99 the other input terminal of which is connected to the output terminal of the gate 93. The output terminal of the gate 99 is connected to a bi-stable switching device 101 which also receives the Timing signal T from the zero voltage switch 25 as shown in Fig. 2. The device 101 has two output terminals one of which is connected to one terminal of a two-input NAND gate 103. The other input termina) of the gate 103 receives the Timing signal T. The reset input of the device 101 is connected back to the reset of the multivibrator 97. The output of the gate 103 is applied to both input terminals of a NAND gate 105, the output of which is connected to another oneshot multivibrator 107. The output of the one-shot multivibrator 107 is connected back to the reset of the one-shot multivibrator 97 and also provides the Synchronized Drive signal SDR which is applied to the other input terminal of the gate 89.

The Timing signal T from the zero voltage switch 25 is applied to one input terminal of each of gates 75 and 77 and also to one input terminal of a NAND gate 109. The other input terminal of the gate 109 is connected to the output terminal of the gate 87.

The output terminal of the four-input NAND gate 77 is connected to both input terminals of an inverting NAND gate 111, the output terminal of which is connected to the input of a current drive circuit 113. The current drive circuit 113 provides the logic output Open signal O which is applied to the gate terminal of the triac 61 as shown in Fig. 2.

Similarly, the output terminal of the four-input NAND gate 75 is connected to both input terminals of an inverting NAND gate 115 and the output of the NAND gate 115 is connected to the input of another current drive circuit 117. The output of the current drive circuit 117 provides the Close signal C which is applied to the gate terminal of the triac 65 in the switching device of the control circuit 9. The output terminal of the gate 109 is connected to both input terminals of an inverting NAND gate 119. The output terminal of the gate 119 is connected to a current drive circuit 121 which, in turn, provides the Locking signal L. As shown in Fig.

2 the Locking signal L is applied to both gate or control terminals of triacs 57 and 71 shown in Fig. 2.

Figs. 4 and 5 shown different arrangements for the actuator 13 which has the motor 11 mounted thereon. In Fig. 4, the motor 11 drives a threaded ballscrew, and a ballscrew nut, which is connected to the regulated element, is free to traverse the threaded ballscrew as the ballscrew rotates in response to the rotation of the motor shaft. In Fig. 5 the motor 11 is connected to a ballscrew nut and a threaded ballscrew, which is coupled to the regulated element of the valve, is free to traverse up and down within the ballscrew nut in accordance with the rotation of the nut thereby controlling the movement of the regulated element. In both Figs. 4 and 5 like numerals designate like parts.

Specifically, in Fig. 4 the motor 11 is shown mounted on top of a yoke 123 which houses the actuator 13 as shown in Fig. 1. A handwheel 125 may be included to selectively engage the rear motor shaft for manual operation. The motor is held down with four bolts 127 through the motor flange into the yoke 123. The actuator assembly is inserted through a hole in the yoke and held in place by the motor. A cabinet 129 may be mounted alongside the actuator and motor to house the control circuit 9 as shown in Fig. 1. The shaft 131 of the motor 11 extends from the motor housing into the actuator assembly and is keyed to a coupler 133 with a keying device 135. The coupler 133 is connected to a ballscrew 137 by means of a locking pin 139 so that rotation of the motor shaft 131 effects the rotation of the ball screw 137. Angular contact load bearings are provided between the wall of the actuator 13 and the coupler 133. A ballscrew nut 141 engages the ballscrew 137 and is prevented from being rotated so that the rotation of the ballscrew 137 effects the linear movement of the ballscrew nut 141 up and down along the ballscrew. To prevent rotation of the ballscrew nut 141, the nut may be, for example, a Saginaw square ballscrew nut sliding up and down in a slot. The ballscrew nut is attached to an actuator stem 143 which is arranged to be connected to a regulated element of a valve through an opening at the bottom of the yoke 123. A spring 145 is mounted between an actuator stem base plate 147 and the upper shoulders of the yoke 123. The spring 145 is arranged to drive the actuator stem closed in the absence of a positive holding signal or driving signal applied to the motor 11. The spring 145 may also be arranged to fail-safe drive the actuator stem in the opposite direction.

In Fig. 5, another embodiment of the actuator assembly 13 is shown. The design of Fig. 5 and the design of Fig. 4 are operationally similar except that the role of the screw and nut in the translation of rotary to linear motion is reversed. The motor shaft 131 is keyed to a coupler 151 by means of a keying device 153. The coupler 151 is connected to a ballscrew nut 155 in such a manner that a rotation of the motor shaft 131 effects rotation of the ballscrew nut 155. A ball screw 157 is coupled to the ballscrew nut 155 and is free to traverse up and down within the area 159 as the ballscrew nut 155 is rotated. The ballscrew 157 is variously coupled to an actuator stem 161 which is arranged for connection to a regulated element of the valve 5 (not shown) through an opening 163 of the actuator assembly 13 of Fig. 5. A plurality of tabs 165 provide an anti-rotation device for restraining the ballscrew from rotation. The tabs 165 run in slots along an extension 167 of the actuator housing and yoke 169. As in Fig. 4, a spring 171 is provided to fail-safe close the actuator whenever the motor 11 does not provide holding torque or is not driven in one direction or the other.

The ballscrew and ballscrew nuts of the actuator shown in Figs. 4 and 5 provide sub stantially reduced friction to enable more accurate control and longer life of the regulated element of the valve 5 through the operation of the synchronous motor 11 as hereinafter noted.

In operation, a 120 AC 60 Hz power signal is applied to terminals 15 and 17 of the control circuit 9. That voltage is applied to the zero voltage switch 25 which provides the Timing signal T. The one-shot multivibrator 31 actually delays its output signal by approximately 20 degrees so that it is synchronized with the voltage applied to the field coils since there is a 20 degree inherent lag due to the inductive loading of the coils. However, for purposes of explanation, the 60 Hz power signal and the Timing signal T are shown in synchronism beginning at TO on the abscissa of the timing chart shown in Fig. 6. Basically, a Direction signal D1 and a Drive signal DR are furnished by the computer-controller 7.

In the present example, an external lock signal EL is also applied to the control circuit 9 although it is not considered an indispensable feature of the present invention. In Fig. 6 the Direction signal D1 is shown initially in the logic 1 or high state. In the present example, when the Direction signal D1 is at a logic 1 level, it will be assumed that the regulated element within the valve 5 is to be dnven in the downward or closed direction.

Conversely, when the direction signal D2, which is the inverse of D 1, goes high, the regulated element within the valve 5 is to be driven in the upward or open direction.

When all of the inputs to the NAND gate 77 are at a logic 1 level, its output will go to a logic 0 level. Therefore, the output of gate 111 will go to a logic 1 level. That signal is applied to a current drive circuit 113 to provide a positive going Open signal O. Similarly, when all of the inputs to the gate 75 are at a logic 1 level, the Close signal C will go to a logic 1 level to trigger the triac 65. It is noted that either the open gate 77 or the close gate 75 will be partially enabled at any one time since each receives as one of the inputs thereto inversely related direction signals. Thus, if the signal D2 is at a logic 1 level the open gate 77 may be enabled but the close gate 75 cannot be enabled since, necessarily, the Direction signal D1 will be at a logic 0 level thus precluding a low going output signal from the closed gate 75.

Upon a change of states of the Direction signal, a Reverse Lock signal RL is generated by the one-shot multivibrator 85. The purpose of the RL signal is to lock the rotor for a predetermined period thereby enabling a smoother and more accurate transition in changing directions. As shown in Fig. 6, at a time Tl for example, the Direction signal D1 reverses and goes from a high logic level to a low logic level. That reversal precipitates the generation of a Reverse Lock signal RL in the form of a negative going pulse as shown in Fig. 6. In the present example, the duration of the Reverse Lock signal RL is approximately five milliseconds. Since the Reverse Lock signal RL is applied to both open and closed enable gates 77 and 75, neither an Open O or a Close C signal can be generated during the continuance of the RL signal, i. e. while the RL signal is at a 0 logic level.

The Drive signal DR is applied to terminal 23 of the control circuit 27 from the com puter-controller 7. The drive signal in the present example has a frequency of 60 Hz and is a positive going signal which may vary in duration from ten microseconds to five milliseconds. A filtering circuit (not shown) may be inserted between the input terminal 23 and the gate 93 to ground out noise signals have a duration of less than ten microseconds.

In Fig. 6, three Drive signal pulses are generated beginning at time T2. The Drive pulses DR are inverted at gate 93 and again at gate 95 and applied to the one-shot multivibrator 97. The output of the multivibrator 97 is shown in Fig. 6 as the"97"signal. Whenever the drive signal DR goes high, the multivibrator 97 will provide a high going pulse of a duration of approximately eight milliseconds. The output of the one-shot multivibrator 97 is applied with the inverse of the Drive signal DR to the gate 99. The output of the gate 99 is shown in Fig. 6, and the Drive signals DR are synchronized with the Timing signal T through the flip-flop circuit 101 and the NAND gate 103 and inverted at the NAND gate 105. That signal is applied to a one-shot multivibrator 107 provides a Synchronized Drive signal SDR which is a positive going pulse having a duration of approximately sixteen milliseconds in the present example. As shown in Fig. 6, the Synchronized Drive signal SDR will remain high so long as sequential Drive signals are being received from the computer-controller 7.

The Synchronized Drive signal is applied to an NAND gate 89 along with the External Lock signal EL. The External Lock signal EL will normally be at a logic 1 level as shown in Fig. 6 unless a condition external to the control circuit 27 requires that the regulated element in the valve 5 be stopped in which case the EL signal will drop to a logic 0 level.

If no external conditions exist which would require the motor to be locked, EL will be at a high logic level. Therefore, while the Synchronized Drive signal SDR is at a logic 1 level the Conditioned Synchronized Drive signal CSDR will also be at a logic 1 level thereby selectively allowing either gate 75 or gate 77 to be enabled depending upon which direction signal is present. Since, in the present example, at time T2 the Direction signal D2 is at a logic 1 level, the CSDR signal may effect the opening of the regulated element in the valve 5 but not the closing thereof.

At the time the Conditioned Synchronized Drive signal CSDR is generated in the present example, the D2 signal as well as the RL signal are both at a logic 1 level and there fore the output of the open gate 77 will vary with the Timing signal T so long as all of the other inputs to the Open gate 77 remain at a logic 1 or high logic level. Thus, the Open signal 0 will vary as shown in Fig. 6 applying a series of positive going pulses to the control terminal of the triac 61.

In the absence of Drive signals DR, and assuming there is no external lock condition, the Conditioned Synchronized Drive signal CSDR applied to the gate 87 will remain at a low logic level. Therefore, regardless of the condition of the signal applied to its other input terminal, the output of the gate 87 will remain at a high logic level. The gate 109 will therefore be enabled to provide an inverted Timing signal T which is once again inverted at gate 119 and applied to a current drive circuit 121. The circuit 121 provides the Lock signal L in substantially concurrent timed relation with the Timing signal T whenever there are no Drive signals DR applied to the terminal 23.

When the Lock signal L goes high as shown in Fig. 6, either triac 57 or triac 71 will be enabled to pass current therei ; ough. At time TO when the Lock signal L goes high the power signal applied to the motor coils is beginning a positive half-cycle. Therefore, the triac 57 will be enabled and current will flow through the diode 53, resistor 55 and triac 57 completing a circuit to the motor coils. The next positive going Lock signal pulse appears when the power signal is beginning a negative half cycle and therefore triac 71 will be enabled and current will flow through the diode 67, resistor 69 and triac 71. Diodes 53 and 67 thus act as a rectifying device operable to rectify the AC voltage applied to the field windings of the motor 11 thereby providing for the application of a DC voltage level to the field coils whenever the Locking signal L is generated. Thus, the Locking signal L effects the locking of the motor shaft in place thereby establishing the holding torque sufficient to overcome the force of the spring or biasing device 145 or 171 in Figs. 4 and 5 and holding the regulated element within the valve 5 at the position to which it was last moved.

However, upon the generation of a subsequent Drive pulse, and again assuming no external lock condition exists, the CSDR signal will go to a logic 1 level. Assuming that there has been no change in the direction signal, the output of the gate 87 will go to a logic 0 level thus disabling the gate 109 and holding the Lock signal L at a logic 0 level as shown at time T2 in Fig. 6. The Open signal O will go to a logic 1 level thereafter as shown in Fig. 6 thus enabling conduction through the triac 61. At that time the power signal is beginning a positive half-cycle and the triac 61 will begin conducting. When the power signal next crosses the zero axis, the Open signal again goes positive thereby enabling the maintenance of conducti

Claims (8)

WHAT WE CLAIM IS :-,.'

1. A valve control system comprising a valve having an adjustable valve member, terminals for connection to an AC supply, controlled rectifying means connected to said terminals, timing signal generating means connected to said terminals for deriving timing pulses in synchronism with the pulsations of the AC supply when connected to said terminals, a reversible synchronous motor having field coils and a rotor, phase shifting means connected to the field coils and to said controlled rectifying means, an actuator coupled to said rotor and to said valve member to control the rate of fluid flow through the valve, means to detect a variation in said flow rate to provide a measurand, control means for comparing the measurand with a reference to derive directional, locking and drive command signals to provide the control required with regard to said variation and said reference, and a logic circuit connected to the timing signal generating means and to the controlled rectifying means, said logic circuit being responsive to said directional and drive command signals and to said timing signals to cause said controlled rectifying means to be switched to supply bipolar pulses to the phase shifting means and to said field coils to drive the rotor, in predetermined incremental amounts in one direction or the other, and being responsive to said locking command signal to supply said field coils with unipolar pulses to arrest said rotor whilst it is not driven by said bipolar pulses.

2. A valve control system according to claim 1 wherein said motor is provided with a pair of field coils which-are each commonly connected, at one end, to one of the AC supply terminals and connected, at the other end, to respective and oppositely poled current paths in said rectifying means, each of the latter paths being bypassed by a bidirectional current path connected to the other one of the AC supply terminals, said logic circuit having first and second outputs for respec tively providing first and second gating pulses, each of said current paths including a gate controlled bidirectional rectifier, said oppositely poled current paths including respective diodes of opposite polarity, the gates of said rectifiers in said bypass current paths being connected to the first and second outputs respectively of the logic circuit, and the phase shifting means being connected between said other ends of the field coils whereby said first or second gating pulses cause the bipolar pulses to be supplied to the phase shifting means and to said field coils to drive the rotor in respective directions.

3. A valve control system according to claim 2 wherein said logic circuit includes a third output for producing third gating pulses, and wherein said rectifiers in the oppositely poled current paths have their gates commonly connected to the third output of the logic circuit whereby said unipolar pulses are supplied to said field coils.

4. A valve control system according to any one of the preceding claims wherein said actuator comprises a ball screw device.

5. A valve control system according to claim 4 wherein the valve member is operable between first and second stops, said actuator including biasing means, operable in the absence of rotor torque, to bias the valve member towards one of the stops.

6. A valve control system according to claim 5 wherein the means to detect a variation in said flow rate is a flow meter.

7. A valve control system according to any one of the preceding claims wherein the logic circuit is substantially as herein described with reference to Figs. 2 and 3 of the accompanying drawings.

8. A valve control system substantially as herein described wherein said flow rate is controlled by the means substantially as herein described with reference to either Fig. 4 or Fig. 5 or the accompanying drawings.