AU1945883A - Valve converter/positioner with remote feedback and memory - Google Patents

Description

Valve converter/positioner with remote feedback and memory

BACKGROUND OF THE INVENTION The present invention relates in general to appara- tus for effecting accurate positioning of valves, and in particular to such apparatus which provide, remotely available indication of true valve position.

In virtually all process control systems, some types of valves ar.e used to regulate the flow of pro- cess fluids. Proper operation of these valves is an important factor in achieving the formulation of a product within specifications, as well as in an effi¬ cient manner. Conceivably in potentially explosive processes, it also could mean the difference between a safe and an unsafe operating condition.

To facilitate accurate operation, a variety of valve positioning mechanisms have been used extensively in the past. Their basic operating principle is to maintain an energizing signal into a valve actuator until feedback provided from the valve itself, indica¬ ting that it has attained the desired position, termi¬ nates the energizing signal. Valve positioning mechanisms are of two basic types: the positioner, in which the feedback is provided by a mechanical linkage to the valve stem, and the converter, in which the feedback information is conveyed by either the pneuma¬ tic pressure or the electric current present at the valve actuator, depending on the type of actuator used. Both of these mechanisms are intended to ensure that the valve reaches the appropriate setting, despite friction in the actuator or in the valve stem packing.

Nevertheless there are certain shortcomings in the previously known types of positioners and converters. Specifically, the communication between a remotely- located controller and the valve positioning mechanism is typically one-way. That is, although the controller emits a command signal to the positioning mechanism, there is no confirmation at the controller that the valve has assumed the desired setting. Although local feedback exists between the valve and its actuator, as describex_L__above, there has not been available an effi¬ cient means for providing feedback to the remσtely located controller. However, in certain processes, the human operator may need positive indication of actual valve position to reliably and safely regulate the process. Often, the only way the operator is made aware of an improperly set valve is by the response of the process itself. For example, if a cooling water valve must be opened more fully to reduce the tempera¬ ture of a particular reaction, the only confirmation that the valve has opened adequately may be the even¬ tual reduction in temperature. However, because of the typically long lag time in a temperature loop, an indication of failure may not become evident until after an unacceptably long period of time.

It is known to use certain auxiliary devices for indicating valve position. One such device is a po- tentiometer, in which the wiper arm is attached to the valve stem. A constant voltage input is maintained across the total resistance of the potentiometer, while the movements of the wiper arm change the output signal in proportion to the valve position. However, such an arrangement involves three extra signal lines and additional circuitry.

In particularly critical applications, operators have been known to install direct observation schemes,

OMPI such as television monitors, for viewing the position of the valve.

There also- exists a need for the valve positioning mechanism to have a memory, i.e., to be able to hold a valve in a preset position in the event of loss of power to the positioning mechanism, and yet do so at sufficiently low energy levels to avoid ignition of

_potentially explosive atmospheres. Memory schemes operating within the framework of a conventional elec- trically-based positioning mechanism typically exceed intrinsically safe operating limits and therefore are unusable in hazardous environments.

Therefore, in view of the above, it is an object of the^' present invention to achieve a valve positioning mechanism having an integral remote-feedback system to provide an affirmative indication, at a remotely located control room, of the actual response of the valve to a control signal emanating from that control room. It is another object of the present invention to provide the remote feedback along the same transmission lines on which the control signal is transmitted from the control room to the valve positioning mechanism.-

It is a further object of the invention to achieve the above result with a system that is intrinsically safe for use in explosive atmospheres.

SUMMARY OF THE INVENTION The present invention operates in the context of a valve positioning mechanism of the type in which a valve actuator located within a process environment moves the valve stem in accordance with a command signal generated by a process controller within a remotely-located control station. Such a mechanism

OMPI generally has a local feedback scheme for terminating the operation of said actuator when the valve attains a final position as dictated by the command signal.

In accordance with a specific embodiment of the present invention a remote feedback system generates an output signal for transmission to the remotely-located control station, the frequency of the output signal being indicative of—the status of th& valve-. The output signal is communicated to the control station via a two-wire transmission line, commonly used for process control instrumentation communications, which also serves to transmit the command signal from the control station to the valve actuator.

The feedback system includes an oscillator whose output frequency changes in accordance with the induc¬ tance value of a variable inductor responsive to changes in the status of the valve.

Also in this embodiment, the command signal initi¬ ates a pneumatic control signal to the actuator by means of a low-power, electropneumatic switch, which operates at voltage and power levels well within the intrinsically safe limits established for hazardous environments.

The configuration of the electropneumatic switches achieves a memory function, in that a preexisting valve position will be maintained even in the case of an interruption of the command signal from the controller, for example, as in the case of a power failure.

In an alternate embodiment, the invention functions in a fail-safe mode in case of power failure. Upon loss of signal the memory is disengaged and the valve is automatically set to a fail-safe or other predeter¬ mined condition. Optionally the return to a fail-safe -5- setting can be delayed for a preset period of time after the signal interruption.

DESCRIPTION OF THE DRAWINGS The novel aspects and distinct advantages of the present invention will be made clear by the following detailed description, in conjunction with the accom¬ panying drawings, in which:

FIG. 1 is a schematic diagram of a first embodiment ^~" f a^~valve converter mechanism in accordance with the- present invention;

FIG. 2 is a partial schematic of an embodiment of a valve positioner mechanism in accordance with the present invention;

FIGS. 3A through 3E are detailed schematics of the circuitry of the INTERFACE CARD portion of FIG. 1;

FIG. 4 is a detailed schematic of the circuitry of the FIELD ELECTRONICS portion of FIG. 1;

FIGS. 5A and 5B are detailed views of the variable inductor of the pneumatic transducer of FIG. 1; FIG. 6 is a graph depicting the relationship be¬ tween oscillator frequency and variable inductor arma¬ ture position; and

FIG. 7 is a schematic . diagram of a second embodi¬ ment of a valve converter mechanism in accordance with the present invention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS Throughout the following description and the accom¬ panying drawings, the same reference numerals are used to indicate like components.

Referring now to FIG. 1, there is depicted a process control system 10 employing a novel scheme for adjusting the setting of a valve 11, to vary the rate of passage of process fluids therethrough. A conven-

OMPI tional controller mechanism 13, a device whose con¬ struction and operation are well known to those skilled in the process control arts, is the source of command signals which initiate valve^" action. In a known manner, the controller compares an incoming measurement signal indicating the value of the particular process variable being controlled, with a predetermined set¬ point.-signal re_p esenting the desired value of that variable, and generates an appropriate command—signal —. over a line 15. This command signal is intended to effect a change in the setting of the valve, so as to drive the measured value of the process toward the desired level.

The valve as shown in FIG. 1 is pneumatically- actuated, ^'although the teachings of the present inveπ- tion are equally applicable to a valve operated by an electrical or other conventional mechanism. A pneuma¬ tic signal supplied via an air line 17 is applied to any one of a variety of pneumatically-powered drive mechanisms, represented generally by reference numeral 1 , which is coupled to a stem 21 of the valve. In the illustrated embodiment, raising of the valve stem further opens the valve, while lowering the stem closes the valve, although oppositely functioning configura¬ tions are possible. The command signal from the controller 13, in this case in the form of a d.c. voltage in the range of 0 to 10 volts, is supplied to an electronics interface card 22, whose structure and function will be described more fully hereinafter. The interface card and the controller are located at a control station remote from the controlled process, and communicate via a two-wire transmission line 23 to a field electronics assembly 24, located in the general vicinity of the valve. Such a typical two-wire transmission line, which is intended to carry two-way transmission of power and information signals, has found widespread application for long distance communication in the process control industry. A typical use of such a transmission line is described in U.S. Patent No. 4,118,977. This patent has the same assignee as the presentsinvention, and its contents are hereby incorporated by reference. "^' The circuitry of the field electronics assembly 24, also to be described hereinafter, accepts the electri¬ cal signal, usually in the form of a current, from the two-wire transmission line 23, and directs the signal to either of a pair of electropneumatic switches 25, 26. These switches initiate * the flow of pneumatic signals for control purposes. In this embodiment the top switch 25 effects a. decrease in the pneumatic pressure ultimately supplied to the valve actuator 19, while the bottom switch 26 increases the pressure. Each of the switches 25, 26 includes an outer housing 27 in which there are an upper air chamber 29 and a lower air chamber 33. The bottom of the upper chamber is defined by a flexible diaphragm 35. A nozzle 36 permits communication of air between the upper chamber and the exterior of the housing through a line 37. A flapper 38 normally rests against the nozzle opening, sealing off the nozzle against passage of air and permitting pressurization of the upper chamber. An outlet port 39 is in fluid communication with the lower chamber 33.

Disposed beneath the diaphragm 35 is a plate 42 supported upon and pivotable about a second flexible diaphragm 43. A pedestal 44, fastened to the underside

OMPI -8- of this plate, receives an upward bias from a helical spring 45 seated within the lower air chamber 31, so as to maintain the plate in intimate contact with the diaphragm 35. Instrument air from a supply 46, typically at 20 psi, is introduced into the upper chamber 29 via a channel 47 and a restrictor 48. As long as the flapper ^~3S is sealed against—the—n&zzle 36, the pressure de¬ veloped within the upper chamber forces the diaphragm 35 downwardly, causing a gasket 49 located in the underside of the pedestal 44 to seat firmly against a second nozzle 50, despite the upward bias provided by the spring 45. This condition occurs with the electro- pneumatic switch in the "off" position.

A solenoid assembly 51 is located atop each elec- tropneumatic switch, adjacent the flapper 38. In the absence of an actuating signal to the solenoid, the spring force of the flapper keeps the flapper tightly pressed against the nozzle 36. However, when a current signal is applied to the solenoid, an electromagnet 53 attracts the flapper away from contact with the nozzle. The pressure in the upper chamber 29 decreases, al¬ lowing the diaphragm 35 to rise, in turn lessening the force applied against the plate 42. The spring 45 lifts the pedestal and gasket 49 away from the second nozzle 50, allowing the flow of air between the outer port 39 and an inlet line 55, via the lower chamber 33 and the nozzle 50. This represents the "on" condition. In the case of the top switch 25, the outlet port 39 is vented to the atmosphere, so that excess pressure coupled to the inlet line 55 from elsewhere in the pneumatic network can be released. Thus this switch acts to decrease the pressure within the pneumatic lines leading ultimately to the valve actuator. For the bottom switch 26, the outlet port is connected to the 20 psi air supply 46. When this switch is turned on, 20 psi air is supplied to the pneumatic control lines, in effect increasing the pressure. As will be described later, the interface card 22 and field elec¬ tronics assembly 24 acting in concert selectively actuate either switch 25 or switch 26, depending on whether the process conditions call for line pressure- to the valve to be decreased or increased.

It should be pointed out that although the electro- pneumatic switches, which usually are located within the process environments, are electrically triggerable, they have a very low power consumption. In fact, since the considerable motive power which effects the move¬ ment of the valve is supplied by the pneumatic pres¬ sure, only minimal electrical energy is required to activate the switches, to appropriately route the flow of air. Thus the valve control system according to the present invention can operate effectively at current and voltage levels which are within the intrinsically safe limits typically required for safe operation within hazardous or explosive atmospheres. Such levels are defined in the publication, "Intrinsically Safe Apparatus for Use in Division 1 Hazardous Locations," published by The National Fire Protection Association, the contents of which are hereby incorporated by reference. The SPEC 200 family of electronic control¬ lers manufactured by The Foxboro Company, Foxboro, Massachusetts generates electrical control signals within these intrinsically safe operating limits, and is compatible with the operating requirements of the present invention. Since the electropneumatic switches

OV.PI utilize a solenoid, which is an inductive, energy- storing device, a shunt protective component of some sort must be coupled to the solenoid to suppress transient voltages or currents above the safe limits. However, such components, also discussed in the above- referenced publication, are well known to those skilled in the field of intrinsic safety, and will not be discussed further herein.

The pneumatic signal, whether from ^{^}switch 25 or switch 26, is supplied over a line 56 to a pneumatic transmitter 57. The transmitter, together with a standard pneumatic relay 59 (such as the Model 40 Relay also manufactured by The Foxboro Company) comprise a pneumatic transducer assembly 61. Within the pneumatic transmitter 57, an expandable receiver bellows 63 is attached at its lower end to a molinting base 65, and at its upoer end to one end of a lever arm 67. The lever arm is able to pivot about a fulcrum 69 on a balance bar 71, to cause repositioning of a flapper 73 relative to a nozzle 75. The nozzle is connected via a pneumatic line 77 to the standard relay 59 and to the air supply 46. In a manner well known to those skilled in the pneumatic arts, the spacing of the flapper relative to the nozzle deter- mines the magnitude of an amplified output signal pro¬ duced by the relay on the output line 17, and in turn supplied to the valve actuator 19. When air entering from the line 56 causes the bellows to expand, the flapper is pivoted closer to the nozzle, resulting in an increase in the output pressure of the relay. When the bellows contracts, the flapper is withdrawn from the nozzle, decreasing the relay output.

Whether the bellows 63 expands or contracts depends

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OMPI

^ ° on which of the electropneumatic switches 25, 26 is activated at any given time. If the "decrease" switch 25 is activated, then a flow path is created from the interior of the bellows to the outside atmosphere, allowing excess pressure within the bellows to escape, and the bellows to contract. On the other hand if the "increase" switch 26 is activated, supply pressure is applied to the bellews-, causing-it to expand.

While the position of the flapper 73 is altered by operation of the receiver bellows 63, the position of the nozzle 75 itself can be adjusted as well. In the embodiment of FIG. 1, a feedback bellows 81 is inter¬ posed between the base 65 and the balance bar 71. A spring 83 provides a downward bias on the left-hand end of the bar. Thus this balance bar similarly can be pivoted about a flexure 84 to relocate the nozzle relative to the flapper.

The pneumatic signal into the feedback bellows 81 comes from the same output signal of the relay 59 as is supplied to the valve actuator 19. Thus, changes in the actuation pressure to the valve are reflected by an increase or decrease in the internal pressure of the feedback bellows 81, which causes the bellows to expand or contract accordingly. The movement of the bellows repositions the nozzle 75 relative to the flapper 73 until a new equilibrium position is reestab¬ lished, at which point no further changes in the relay output occur. Thus, in short, the action of the receiver bellows prompts a pneumatic drive signal to the valve actuator until it is counterbalanced by the corresponding action of the feedback bellows.

Due to the fact that the feedback between the valve actuator 19 and the pneumatic transmitter 57 is accom- pushed by means of a pressure signal to the feedback bellows 81, the type of valve positioning mechanism depicted in FIG. 1 is known as a valve converter. However, as shown in FIG. 2, the feedback also can be provided by a direct mechanical linkage 85 between the valve stem 21 and the balance bar 71. In this arrange¬ ment, the valve positioning mechanism is more accurate¬ ly known- as a valve pαsitioner. _.

Referring again to FIG^". 1, there is attached to the top surface of the balance bar 71 an armature 87 in the shape of a truncated triangular wedge, which forms part of a variable inductor assembly 89. The inductor is a component within an oscillator circuit encompassed within the circuitry of the field electronics assembly 24 (see FIG. 4). The oscillator can be any conven¬ tional circuit whose output frequency is dependent on the value of the variable inductor, and as such its details will not be further discussed herein. The upward and downward movement of the balance bar, in response to feedback signals supplied to the pneumatic transmitter, repositions the armature within the air gap 91 of a magnetic assembly 93 included within the variable in ductor, as seen more clearly in FIGS. 5A and 58. The specially tapered geometry of the armature and its manner of movement within the gap cause the inductance to change in such a way that the output frequency of the oscillator varies in a linear fashion with respect to armature position. FIG. 6 demonstrates the linear relationship between oscillator frequency and armature position over the normal operating range of the oscillator.

Since each position of the armature 87 can be equated to a corresponding unique position of the valve

OMPI 11, the oscillator output frequency itself is uniquely related to the true valve position, in the case of a valve positioner, or to the valve position actuator pressure, in the case of a valve converter. This frequency signal is fed back along the same two-way transmission line 23 to the interface card 22, to be processed in a manner described below.

With refer-ence_ now-_t_Q___FIGS. 3A thrgujgJ" 3E,„ a more detailed description of the^" circuitry of^~th^'e interface-' card 22 and its operation can be given. The feedback signal from the oscillator within the field electronics assembly 24, entering via the two-wire transmission line 23, is processed through a band-pass amplifier 97 and an opto-isolator circuit 99 into a conventional frequeπcy-to-voltage converter 101. The converter senses the frequency of the oscillating feedback signal and transforms it to a corresponding voltage signal whose magnitude is proportional to the frequency. Therefore, this feedback voltage is now indicative of the valve position. Such frequency-to-voltage conver¬ ter circuits are well known to those skilled in the electronics arts. After passing through a two-pole filter stage 103, the feedback voltage signal is pro¬ cessed by a sample track and hold circuit 105. This circuit counteracts the effects of severe line tran¬ sients produced whenever the electropneumatic switches 25, 26 are activated. Finally, the feedback voltage signal is fed into a deviation amplifier 106.

The command signal from the process controller 13, in the form of a 0-to-lOV d.c. electrical signal, also is provided to the deviation amplifier 106. The devi¬ ation amplifier amplifies the difference between the controller command signal and the feedback voltage signal, and inputs the resulting error voltage into both a deadband comparator stage 107 and a deviation band comparatα-r stage 109. Depending on the magnitude of the difference between the controller signal and the feedback signal, one or the other of these stages supplies a current trigger signal to the appropriate electropneumatic switch 25, 26 so as to properly repo¬ sition the valve 13. If the error voltage is less than a previously selected deadband threshold voltage VR1 which is supplied from an adjustable external voltage source (not shown), neither stage is activated and no current signal will be provided to either of the elec¬ tropneumatic current switches. In the absence of an activated comparator stage, a current source 111 generates only a quiescent current level for powering the - ariable frequency oscillator in the field elec¬ tronics assembly 24. If the amplified error voltage signal is greater than the deadband threshold voltage VR1 but less than a deviation band threshold voltage VR2 (also supplied from an external source), the dead¬ band comparator 107 is activated. This in turn causes the switched current source 111 to operate in a pulsed mode, the pulse width and duty cycle of the pulse train having been previously determined to yield efficient repositioning of the valve. The pulsed mode in effect offers a fine-tuning type adjustment. The polarity of the error voltage determines the sense of the output current delivered to the field electronics assembly 24.

Referring now to FIG. 4, the field electronics assembly 24 receives from the interface card 22 either the +1 or -I current signal, and actuates either "increase" switch 26 or "decrease" switch 25 respec¬ tively. If the magnitude of the error voltage is consider¬ ably greater, and in fact exceeds the deviation band threshold, the deviation band comparator stage 109 takes over. This comparator drives the ^• switched current source 111 in a "full-on" mode. Again, the polarity of the error voltage determines the current sense, and therefore which electropneumatic switch is actuated.

In summary, the^{^} switched current source ^"til^"acting under the control of either the deadband comparator stage 107 or the deviation comparator stage 109 (de¬ pending on the magnitude of the error voltage) contin¬ ues to supply current signals to either of the two electropneumatic switches 25, 26 until the error volt- age signal is reduced below the deadband threshold voltage. At this point in time, the current sources will supply- only a quiescent current level to the oscillator, and both electropneumatic switches will be in the "off" position. it should be noted that there is a built-in memory feature in the particular embodiment described above. In the absence of any command signals to actuate the electropneumatic switches 25, 26, each of the switches remains in the "off" condition. Thus, the status quo with regard to the position of the valve is maintained. In the case of an electrical power failure, or other interruption of the current signals from the interface card 22, the electropneumatic switches merely remain disabled, and the valve stays in its previous position. Once electrical power is resumed, an initialization circuit (not shown) senses the feedback voltage from the frequency-to-voltage converter 101 (see FIG. 3B) which indicates valve position, and resets the con- troller so as to achieve a "bumpless" transfer.

Referring now to FIG. 7, an alternate embodiment of a valve converter in accordance with the present invention is achieved by substituting a slightly modi- fied electropneumatic switch 113 for the top switch 25 (see FIG. 1). This modified switch functions in basically the same manner as the electropneumatic switches 25 and 26 discussed_, in detail above. The only difference^" is that^"" the nozzle 36' is located on the opposite side of the flapper 38, so that in the "off" condition, i.e., with the solenoid 51 deactivated, the flapper is not in contact with the nozzle. So, whereas the switches 25, 26 are "normally closed" in the ab¬ sence of a control signal to the solenoid, switch 113 is "normally open," to achieve the memory function described above with ^• reference to the embodiment of FIG. 1. Clearly, appropriate modifications must be made to the electronic circuitry of the field elec¬ tronics assembly 24 and/or the interface card 22. whereas in the previous embodiment the absence of electrical power to both of the switches 25, 26 would maintain the status quo, now power must be maintained to the modified switch 113 to reach the same result.

As long as electrical power above a predetermined threshold value is maintained to the electropneumatic switch 113 along a line 125 from the field electronics assembly 24, the switch remains off. However, once the electrical power drops below the predetermined level, the switch 113 turns on and permits venting of any excess pressure within the receiver bellows 63 of the pneumatic transducer 61. This in turn causes the valve to go to a fail-safe position, as determined by the characteristics of the process being controlled. It is also possible to incorporate within the pneumatic system a conventionally known pneumatic delay device, to forestall the movement of the valve to the failsafe position until after passage of a predetermined amount of time. If, prior to the expiration of the delay period, power is restored to the switch 113, a bump- less resumption of control again can be reestablished. Although the presents invention has been described in terms of^'lihe illustrated tffFfbodirnent^7^~certain- modi-- fications may become apparent to those skilled in the art. For example, alternate constructions of an elec¬ trically triggerable, yet pneumatically powered switch may be envisioned, which will operate within the context of the present invention in an intrinsically safe manner. Nevertheless, it is intended that such modifications be included within the scope of the following claims.

Claims (17)

1. WHAT IS CLrA ^'MED IS:

   ^""""" 1. In a valve positioning mechanism of the type in which a valve actuator located within a process environment moves the valve stem in accordance with a command signal generated by a process controller within a remotely-located control station, apparatus compris¬ ing: remote feedback means for generating an output_ signal for ^" transmission to said remotely-located control station, the frequency of said output signal being indicative of the status of said valve; and a two-wire transmission line linking said remotely- located control station and said process environment, for transmitting both said command signal to said valve actuator and said output signal to _*>said rεmotely- located control station.
2. 2. Apparatus as set forth in. claim 1, further comprising interface means intermediate said process controller and said valve actuator, including: means for generating a control signal corresponding to said command signal and for transmitting said control signal along said two-wire transmission line to initiate operation of said actuator; and means responsive to said remote feedback means, for discontinuing said control signal when the status of said valve is as dictated by said command signal.
3. 3. Apparatus as set forth in claim 2, further comprising memory means for maintaining said valve in a preexisting position upon said discontinuance of said input control signal.
4. 4. Apparatus as set forth in claim 3, wherein said memory means comprises: a plurality of switches selectively triggerable in
5. OMPI response to said control signal from an OFF condition to an ON condition, said valve actuator being actuated only when at least one of said switches is in said ON condition; whereby upon said discontinuance of said control signal said valve remains in the status dictated by said command signal until a subsequent command signal is generated.

   [^{' "}3. Apparatuses^"se ^'Υortb^"~iτr claiπr4^" wherein said control signal is an electrical signal and said valve actuator operates by pneumatic pressure, and wherein each of said switches comprises: means for alternating from said OFF condition to said ON condition in response to said electrical signal; and means for varying the pneumatic pressure to said valve actuator when in—said ON condition.
6. 6. Apparatus as set forth in claim 5, wherein said means for alternating is responsive to an elec¬ trical signal having current and voltage levels within intrinsically safe limits.
7. 7. Apparatus as set forth in claim 1, wherein said remote feedback means comprises: oscillating means including a variable inductance means whose inductance value varies in accordance with tns status of said valve, the output signal of said oscillating means having a frequency dependent on said inductance value.
8. 8. Apparatus as set forth in claim 2, wherein said remote feedback means comprises: oscillating means including a variable inductance means whose inductance value varies in accordance with the status of said valve, the output signal of said

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   -20- oscillating means having a frequency dependent on said inductance value.
9. 9. Apparatus as set forth in claim 8, wherein said remote feedback means further comprises: frequency-to-voltage conversion means for trans¬ forming said output signal of said oscillating means into a voltage signal whose magnitude corresponds to said frequencyj and wherein said means for discontinuing said control signal further comprises: means for generating an error signal corres¬ ponding to the difference in the magnitude of said voltage signal and said command signal; and means, responsive to said error signal, for triggering said control signal generating means only when said error signal exceeds a predetermined thresh¬ old .
10. 10. Apparatus as set forth in claim 9, wherein said control signal generating means is capable of assuming a pulsed mode of operation and a continuous mode of operation, and wherein said triggering means further comprises: a first comparator circuit for causing said control signal generating means to operate in said pulsed mode when said error signal exceeds said threshold by less than a designated amount; and a second comparator circuit for causing said control signal generating means to operate in said continuous mode when said error signal exceeds said threshold by more than said designated amount.
11. 11. In a valve positioning mechanism of the type in which a pneumatically operable valve actuator located within a process environment moves the valve stem in accordance with a command signal generated by a process controller within a remotely-located control station, apparatus comprising: a two-wire transmission line linking said remotely located control station and said process environment; oscillating means including a variable inductance means whose inductance value varies in accordance with the status of said valve, the output signal of said oscillating means having a frequency dependent on said inductance value and being transmitted along said two- wire transmission line to said remotely located control station; frequency-to-voltage conversion means for trans¬ forming said output signal of said oscillating means into a voltage signal whose magnitude corresponds to said frequency; interface means intermediate said process control¬ ler and said valve actuator including means for gener¬ ating an electrical control signal corresponding to said command signal and for transmitting said electri¬ cal control signal along said two-wire transmission line for initiating operation of said actuator, and means responsive to said voltage signal for discontin¬ uing said electrical control signal when the status of said valve is as dictated by said command signal.
12. 12. Apparatus as set forth in claim 11, further comprising: a plurality of switches selectively triggerable in response to said electrical control signal from an OFF condition to an ON condition, each of said switches including means for varying the pneumatic pressure to said valve actuator only when said switch is in said ON condition, whereby upon said discontinuance of said

    OMPI sflr ^WIPO 1 electrical control signal said valve remains in the status dictated by said command signal until a subse¬ quent command signal is generated.
13. 13. Apparatus as set forth in claim 11, further 5 comprising a plurality of electropneumatic switches selectively triggerable in response to said electrical control signal from an OFF condition to an ON condi¬ tion, each of said switches including:

    — -- solenoid means responsive to said electrical

    10 control signal; pressure inlet means in fluid communication with a source of pneumatic pressure; pressure outlet means in fluid communication with said valve actuator;

    15 a flaoper-nozzle combination, intermediate said pressure inlet means and said pressure outlet means , switchable between a first and a second operating condition by said solenoid means, said first operating condition permitting fluid communication between said 0 pressure inlet means and said pressure outlet means, said second operating condition terminating fluid communication between said pressure inlet means and said pressure outlet means, said valve actuator being actuated only when the flapper-nozzle combination in 5 at least one of said switches is in said first opera¬ ting condition.
14. 14. Apparatus as set forth in claim 13, wherein said solenoid means is responsive to an electrical control signal having current and voltage levels with- 0 in intrinsically safe limits.
15. 15. In a valve positioning mechanism for use in an explosive process environment, of the type in which a pneumatically operated valve actuator located within

    OMPI the process environment moves the valve stem in accor¬ dance with an electrical control signal from a remotely-located control station, apparatus comprising: memory means for maintaining said valve in a pre- existing condition upon interruption of said command signal, said memory means being operable by an elec¬ trical control signal having current and voltage levels within itriπsically safe limits for said explosive process environment.
16. 16. Apparatus as set forth in claim 15, wherein said memory means comprises: a plurality of switches selectively triggerable by said electrical control signal from an OFF condition to an ON condition, each of said switches including means for varying the pneumatic pressure to said waive actuator only when said switch is in said ON condition.
17. 17. Apparatus as set forth in claim 15,^" wherein said memory means comprises a plurality of electropneu¬ matic switches selectively triggerable by said elec- trical control signal from an OFF condition to an ON condition, each of said switches including solenoid means responsive to said electrical con¬ trol signal; pressure inlet means in fluid communication with a source of pneumatic pressure; pressure outlet means in fluid communication with said valve actuator; a flapper-nozzle combination, intermediate said pressure inlet means and said pressure outlet means, switchable between a first and a second operating con¬ dition by said solenoid means, said first operating condition permitting fluid communication between said pressure inlet means and said pressure outlet means,

    OMPI_. said second operating condition terminating fluid com¬ munication between said pressure inlet means and said pressure outlet means, said valve actuator being actu¬ ated only when the flapper-nozzle combination in at least one of said switches is in said first operating condition.

    OMPI