CA1200590A - Valve converter/positioner with remote feedback and memory - Google Patents

Abstract

Abstract:
Valve converter/positioner with remote feedback and memory A valve positioning mechanism, configurable as either a positioner or a converter, acts in response to a control signal supplied over a two-wire transmis-sion line by a remotely-located controller, and pro-vides a feedback signal over the same two-wire line to the controller. Depending on the configuration used, this feedback signal verifies either that the valve has assumed the position directed by the controller or that the actuating pressure to the valve is as directed by the controller. The control signal triggers an elec-tropneumatic switch which initiates the flow of air to operate a pneumatic mechanism driving the valve stem.
A variable inductor, its inductance value being depen-dent upon the valve position/actuating pressure, forms part of an oscillator whose output frequency varies with the inductance value. The oscillator in turn provides a measurement signal to the controller over the two-wire transmission line, the frequency of the signal being proportional to the valve position/
actuating pressure. The controller terminates adjust-ment of the valve when the measurement signal equals the controller setpoint. Upon loss of power, the valve can be maintained at a pre-existing position, or, in an optional arrangement, be returned to a fail-safe condition, with or without a predetermined delay.

Description

1 Valve converter/positioner with remote feedback and memory BACKGROUND OF THE I NVENTION
The present invention relates in general to appara-tus for effecting accurate positioning of valves, andin particular to such apparatus which provide remotely available indication of true valve position.
In virtually all process control systems, some types of valves are used to regulate the flow of pro-10 cess fluids. Proper operation of these valves is animportant factor in achieving the formulation of a product within specifications, as well as in an ef~i-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 20 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 25 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 usedO
Both of these mechanisms are intended to ensure that 30 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.

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1 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 described above, there has not been available an effi-cient means for providing feedback to the remotely 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 thevalve 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 ar~angement involves three extra signal lines and additional circuitry.
In particularly critical applications, operators have been known to install direct observation schemes, , ;

1~3l~5~3 l 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 5 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-10 trically~based positioning mechanism typically exceedintrinsically 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 15 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 25 the above result with a system that is intrinsically safe for use in explosive atmospheres.
SUMMARY OF THE INVENTlON
The present invention operates in the context of a valve positioning mechanism of the type in which a 30 valve actuator located within a process environment moYes the val~e stem in accordance with a command signal generated by a process controller within a remotely-located control station. Such a mechanism 5~

1 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 5 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 the valve. The output signal is communicated to the control station via a 10 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 15 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 20 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 25 achieves a memory function, in that a pree~isting valve position will be maintained even in the case of an interruption of the command signal from the controller, for ~xample, as in the case of a power failure.
In an alternate embodiment, the invention functions 30 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 :

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setting can be delayed for a preset period of time after the signal interruption.
DESCRIPTION OF THB DRAWINGS
The novel aspects and distinct advantages of the present invention will be made clear bv the following detailed descrip-tion, in conjunction with the accompanying drawings, in which:
Figure 1 is a schematic diagram of a first embodiment of a valve converter mechanism in accordance with the present invention;
Figure 2 is a partial schematic of an embodiment of a valve positioner mechanism in~accordance with the present inven-tion;
Figures 3A through 3E are detailed schematics of the circuitry of the INTERFACE CARD portion of Figure l;
Figure 4 is a detailed schematic of the circuitry of the FIELD ELECTRONICS portion of Figure l;
Figures 5A and 5B are detailed views of the variable inductor of the pneumatic transducer of Figure l;
Figure 6, appearing in the same sheet as Figure 2, is a graph depicting the relationship between oscillator frequency and variable inductor armature position; and Figure 7 is a schematic diagram of a second embodiment of a valve converter mechanism in accordance with the present invention.
DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
Throughout the following description and the accompanying drawings, the same reference numerals are used to indicate like components.

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Referring now to Figure 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-~3~5~

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1 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 5 signal indicating the value of the particular process variable being controlled, with a predetermined set-point signal representing the desired value of that variable, and generates an appropriate command signal over a line 15. This command signal is intended to 10 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 inven-15 tion are equally applicable to a valve operated by anelectrical 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 20 19, 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 30 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 ~20~5`9~

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 wides~
pread application for long distance communication in the process control industry. A typical use of such a transmission line is described in United States patent No. 4,118,977, which was issued on October 13, 1976 to The Foxboro Company.
The circuitry of the field electronics assembly 24, also to be described hereinafter, accepts the electrical 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 flowof 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 permit-ting pressurization of the upper chamber. An outlet port 39 is in fluid communication with the lower chamber 33.
Disposed beneath the diaphram 35 is a plate 42 supported upon and pivotable about a second flexible diaphragm 43. A
pedestal 44, fastened to the underside s~

1 of this plate, receives an upward bias from a helical spring 45 seated within the lower air chamber 33, 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 38 is sealed against the nozzle 36, the pressure de-veloped within the upper chamber forces the diaphragm 10 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 20 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 lessenîng 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 s~

_9_ lines leading ultimately to the valve actuator. For the bottom switch 26/ the outlet port is connected to the 20 psi air supply ~6. 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 electropneu-matic switches, which usually are located within the processenvironments, are electrically triggerable, they have a very low power consumption. In fact, since the considerable motive power which effects the movement of the valve is supplied by the pneu-matic pressure, 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 1978," published by The National Fire Protection Association. The SPEC 200 family o-f electronic controllers manufactured by The Foxboro Company, Foxboro, Massachusetts generates electrical control signals within these intrinsically safe operating limits, and is compa-tible with the operating requirements of the present invention. Since the electropneumatic switches .~

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1 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 mounting base 65, and at its upper 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 i supplied to 'he valve actuator 19. Whe~ air entering from the line 56 causes the bellows~ to expand, the flapper is pivoted closer to the nozzl~, resulting in ~ an increase ~n 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 ~V~5~

1 on which of the electropneumatic switches 25, 26 is activated at any given time. If the "decrease" switch 25 is activated, then a fl~ow path is created from the interior of the bellowsb to the outside ~tmosphere, allowing excess ~ressure within the bellows~to escape, and the bellows~to contract. On the other hand if the "increase" switch 26 i3 activated, supply pressure is applied to the bellows~, causing it to expand.
While the position of the flapper 73 is altered by 10 operation of the receiver bellows 63, the position of the noz~le 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. ~hus, 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 25 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. T~us, in short, the action of the receiver bellows prompts a pneumatic drive signal to 30 the valve actuator until it is counterbalanced by the corIesponding action of the feedback bellows.
Due to the fact that the feedback between the valve actuator 19 and the pneumatic transmitter 57 is accom-_12-1 plished by means of a pressure signal to the feedback bellows Bl, 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 positioner.
Referring again to FIG. 1, there is attached to the top sur~ace 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 bar9 in response to feedback signals supplied to the pneumatic transmitter, repositions the armature within the air ~ gap 91 of ~ magne~ic assembly 93 included within the - ~ variable in ductor, as seen more clearly in FIGS. 5A
and 5B. 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 ~7 can be equated to a corresponding unique position of the valve s~

1 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 reference now to FIGS. 3A through 3E, a more detailed description of the circuitry of the 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 frequency-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 ~eedback voltage signal is fed into a deviation amplifier 106.
The command signal from the process controller 13, in the form of a O-to-lOV d.c~ electrical signal, also is provided to the deviation amplifier 106. The devi-ation amplifier amplifies the difference between the cor7troller command signal and the feedback voltage 5~0 1 signal, and inputs the resulting error voltage into both a deadband comparator stage 107 and a deviation band comparator stage 109. Depending on the magnitude of the difference between the controller signal and the 5 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 VRl 10 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 15 generates only a quiescent current level for powering the variable frequency oscillator in the field elec-tronics assembly 24. If the amplified error voltage signal is greater than the deadband threshold voltage VRl but less than a deviation band threshold voltage 20 VR2 (also supplied rrom 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 25 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 30 assembly 24 receives from the interface card 22 either the +I or -I current signal, and actuates either "increase" switch 26 or ~decrease" switch 25 respec-tively.

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1 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 5 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 lll acting 10 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~
15 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 25 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.
30 Once electrical power is resumed, an initialization circuit (not shown) senses the feedback voltage from the frequency-to-voltage converter 101 (see FIG. 3B) ~hich indicates valve position, and resets the con-lX(~S~

1 troller so as to achieve a ~bumpless" transfer.
Referring now to FIG. 7, an alternate embodimentof a valve converter in accordance with the present invention is achieved by substituting a slightly modi-5 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 10 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.
20 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 25 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 pn~umatic transducer 61. ~his in turn causes the valve to go to a fail-safe position, as determined by the characteristics of the process being controlled. It 5~3~

1 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 5 of timeO 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 present invention has been described in terms of the illustrated embodiments, certain modi-fications may become apparent to those skilled in theart. For example, alternate constructions of an elec-trically triggerable, yet pneumatîcally powered switch may be envisioned, which will operate within the context of the present invention in an intrinsically 15 safe manner. Nevertheless, it is intended that such modifications be included within the scope of the following claims.

Claims (26)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a valve positioning mechanism in which a valve actua-tor located in a process environment actuates a valve in accor-dance with a command signal generated by a process controller located in a remote control station, apparatus comprising:
A) remote feedback means for generating a status signal whose frequency is indicative of the valve status; and B) a two-wire transmission line linking said remotely located control station and said process environ-ment carrying said command signal to said valve actuator and carrying said status signal from said feedback means to said remotely located control station.

2. Apparatus as set forth in claim 1, further comprising interface means connected between said process controller and said valve actuator, including:
A) 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 B) means responsive to said remote feedback means, for dis-continuing said control signal when the status of said valve is as dictated by said command signal.

3. Apparatus as set forth in claim 2, further comprising:
memory means responsive to said control signal and operatively linked to said valve actuator for maintaining said valve in a pre-existing position upon said discontinuance of said control signal.

4. Apparatus as set forth in claim 3, wherein said memory means has:
a plurality of switches selectively triggerable in response to said control signal from a default OFF condition to an ON con-dition, said switches effecting a change in said valve actuator only when at least one of said switches is in said ON condition;
whereby upon said discontinuance of said control signal, said switches default to said OFF condition and said valve remains in the status resulting from the last said control signal triggering an ON condition.

5. Apparatus as set forth in claim 4 wherein:
A) said control signal is an electrical signal;
B) said valve actuator operates by pneumatic pressure;
C) each of said switches has means for alternating from said OFF condition to said ON condition in response to said electrical control signal; and D) each of said switches has means for varying the pneumatic pressure to said valve actuator when in said ON condi-tion.

6. Apparatus as set forth in claim 5, wherein said means for alternating is responsive to an electrical control signal having current and voltage levels within intrinsically safe limits.

7, Apparatus as set forth in claim 1, wherein:
said remote feedback means further has oscillating means including a variable inductance means for generating an inductance value that varies in accordance with the status of said valve, the output status signal of said oscillating means having a frequency dependent on said inductance value.

8. Apparatus as set forth in claim 2, wherein:
said remote feedback means further has oscillating means including a variable inductance means for generating an inductance value that varies in accordance with the status of said valve, the output status signal of said oscillating means having a frequency dependent on said inductance value.

9 Apparatus as set forth in claim 8, wherein said remote feedback means further has:
A) frequency-to-voltage conversion means for transforming said output status signal of said oscillating means into a voltage signal whose magnitude corresponds to said frequency;
B) said means for discontinuing said control signal further has means for generating an error signal corresponding to the difference in the magnitude of said voltage signal and said command signal; and C) means, responsive to said error signal, for triggering said control signal generating means only when said error signal exceeds a predetermined threshold.

10. Apparatus as set forth in claim 9, wherein:
A) said control signal generating means has means for operating in quiescent, pulsed, and continuous modes;
B) said triggering means has 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 C) said triggering means has 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. In a valve positioning mechanism in which a pneumatically operable valve actuator located in a process environment actuates a valve in accordance with a command signal generated by a process controller located in a remote control station, apparatus compri-sing:
A) a two-wire transmission line linking said remotely loca-ted control station and said process environment;
B) oscillating means including a variable inductance means for generating an inductance value varying in accordance with the status of said valve, the output status 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;
C) frequency-to-voltage conversion means connected between the oscillating means and the control station, for trans-forming said output status signal of said oscillating means into a voltage signal whose magnitude corresponds to said frequency; and D) interface means intermediate said process controller and said valve actuator, including i) means for generating an electrical control signal corresponding to said command signal, and for trans-mitting said electrical control signal along said two-wire transmission line for initiating operation of said actuator, and ii) means responsive to said voltage signal, for dis-continuing said electrical control signal when the status of said valve is as dictated by said command signal.

12. Apparatus as set forth in claim 11, further comprising:
a plurality of switches selectively triggerable in response to said electrical control signal from a default 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 electrical control signal, said valve remains in the status resulting from the last said command signal triggering an ON condition.

13. Apparatus as set forth in claim 11 wherein said plural-ity of switches are electropneumatic switches including:
A) solenoid means responsive to said electrical control signal;
B) pressure inlet means in fluid communication with a source of pneumatic pressure;
C) pressure outlet means in fluid communication with said valve actuator; and D) a flapper-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 pressure inlet means and said pressure outlet means, said second operating condition stopping fluid communication between said pressure inlet means and said pressure outlet means, said valve actuator being actuated only when the flapper-nozzle combination in at least one of said switches is in said first operating condition.

14. Apparatus as set forth in claim 13, wherein said solenoid means and said electrical control signal are operated within intrinsically safe levels of current and voltage.

15. In a valve positioning mechanism for use in an explosive process environment, in which a pneumatically operated valve actuator located in the process environment actuates a valve in accordance with an electrical control signal from a remotely-located control station, apparatus comprising:
memory means operatively connecting to said valve actua-tor for maintaining said valve in a preexisting condition upon interruption of said control signal, said memory means being operable by the electrical control signal having current and voltage levels within intrinsically safe limits for said explosive process environment.

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 condi-tion, each of said switches including means for varying the pneu-matic pressure to said valve actuator only when said switch is in said ON condition.

17. Apparatus as set forth in claim 15, wherein said memory means comprises a plurality of electropneumatic switches selec-tively triggerable by said electrical control signal from an OFF
condition to an ON condition, each of said switches including:

A) solenoid means responsive to said electrical control signal;
B) pressure inlet means in fluid communication with a source of pneumatic pressure;
C) pressure outlet means in fluid communication with said valve actuator; and D) a flapper-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 pressure inlet means and said pressure outlet means, said second operating con-dition stopping fluid communication between said pressure inlet means and said pressure outlet means, said valve actuator being actuated only when the flapper-nozzle combination in at least one of said switches is in said first operating condition.

18. A method for generating a pneumatic signal comprising the steps of:
A) generating a command signal;
B) generating a status signal by a feedback means for pro-viding information as to the pneumatic signal;
C) calculating from the command signal and the status signal a control signal having one of at least a first and second state;
D) communicating the control signal to at least first and second switches of pneumatic pressure;
E) switching the first switch from a default OFF condition to an ON condition upon receipt of a signal of the first state to cause an increase in the pneumatic signal; and F) switching the second switch from a default OFF condition to an ON condition upon receipt of a signal of the second state to cause a decrease in the pneumatic signal.

19. The method in claim 18, further including the step of discontinuing the control signal when the pneumatic pressure signal is as dictated by the command signal.

20. The method in claim 18, wherein the step of calculating includes generating an error signal.

21. The method in claim 18, further including the step of triggering the communication of the control signal only when the error signal exceeds a predetermined threshold.

22. The method in claim 18, wherein the switches are electro-pneumatic switches.

23. The method in claim 22, wherein the control signal is an electric signal within intrinsically safe levels.

24. The method in claim 18, wherein generating the status signal includes sensing the status of the pneumatic signal by variable inductive means.

25. The method in claim 18, wherein the status signal gener-ated is a frequency signal.

26. A method for generating a pneumatic signal comprising the steps of:
A) generating a command signal;
B) generating a feedback status signal by sensing the status of the pneumatic signal by variable inductive means; and generating a frequency indicative of the pneumatic sig-nal;
C) calculating from the command signal and the status signal an error signal;
D) triggering, when the error signal exceeds a predetermined threshold, an electric control signal having one of at least a first and second state, and being within intrin-sically safe levels;
E) discontinuing the control signal when the error signal is below the predetermined threshold;
F) communicating the control signal, if any, to at least first and second electropneumatic switches of pneumatic pressure;

G) switching the first switch from a default OFF
condition to an ON condition upon receipt of a signal of the first state to cause an increase in the pneumatic signal; and H) switching the second switch from a default OFF
condition to an ON condition upon receipt of a signal of the second state to cause a decrease in the pneumatic signal.