Abstract:
A valve positioner system with zero bleed at steady state is disclosed. The system has a pilot valve, the operation of which is controlled by an electronic circuit powered from a signaling and power connection of a positioner device. A plurality of pneumatic valves are activated and deactivated by the pilot valve to control a valve actuator. With varying configurations and arrangements of normally open or normally closed pilot valves and pneumatic valves, fail freeze and fail safe operations are contemplated. The activation and deactivation of the pilot valve is controlled by an electronic circuit that monitors a valve position signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to U.S. Provisional Patent Application Ser. No. 61/581,833 entitled VALVE POSITIONING SYSTEM WITH BLEED PREVENTION filed Dec. 30, 2011. 
    
    
     STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to control systems for industrial processes, and more particularly, to pneumatic valve positioning systems and electrical circuits thereof with steady state gas bleed prevention. 
     2. Description of the Related Art 
     Many industrial processes involve the movement of fluid such as gas, steam, water, and chemical compounds. The flow of the fluid is regulated by a control valve that has a passageway that is selectively opened and closed with a movable obstruction or valve element connected to a stem. An actuator, in turn, is connected to the stem, and provides the motive force to open and close the valve element. Pneumatic, hydraulic, electrical, or mechanical energy is converted by the actuator to linear or rotational motion, depending on the configuration of the control valve. 
     A conventional pneumatic actuator is comprised of a piston sealed within a cylinder, and the piston includes a connecting rod that is mechanically coupled to the valve element. Compressed gas is forced into and out of the cylinder to move the connecting rod, which is mechanically coupled to the stem of the control valve. In a single-acting actuator, the compressed gas is taken in and exhausted from one end of the cylinder and is opposed by a range spring, while in a double-acting actuator, air is taken in one end of the cylinder while simultaneously exhausting it out of the opposing end. 
     Precise and accurate control of the valve actuator, and hence the valve element, can be achieved with a positioner device coupled thereto. Pneumatic valve positioners, which can cooperate with aforementioned pneumatic actuators, are well known in the art. The proportional movement of the actuator is accomplished by the movement of compressed gas into and out of the actuator piston. More particularly, valve positioners incorporate a spool (or other devices) that either rotates or slides axially in a housing to port the flow of compressed gas to the actuator or to one or more exhaust ports. 
     An electrical control circuit provides a variable current signal to the positioner device that proportionally corresponds to particular states of the actuator and hence a particular position of the control valve. The electrical control circuit and the electrical current signals generated thereby may be part of a broader process managed by a distributed control system (DCS). Generally, the electrical current varies between 4 milliamps (mA) and 20 mA according to industry-wide standards; at 4 mA the valve positioner may fully open the valve element, while at 20 mA the valve positioner may fully close the valve element. The positioner compares the received electrical signal to the current position of the actuator, and if there is a difference, the actuator is moved accordingly until the correct position is reached. 
     One previous solution involves an external component that monitors the variable current signal for the electrical control circuit and the actual position of the valve element, and responds by driving a lower power solenoid valve coupled to the valve actuator. These additional external components tend to be costly, and require a safe external power source with associated connectivity components. Along these lines, additional wiring and a separate junction box is required. In general, there are additional complications and costs, particularly for deployment in hazardous environments. 
     Another previous solution utilizes a valve positioner having normally closed on/off valves. However, the flow coefficient (Cv) of such valves is low, and oftentimes necessitate boosters for meeting stroking time requirements of most typical deployments. Furthermore, such boosters are also understood to exhibit some degree of leakage, largely negating the advantages of normally closed valves. In any case, the position of the valve actuator must be restored. 
     Accordingly, there is a need in the art for an improved valve positioner having zero bleed in a steady state position, and having such capability regardless of the flow coefficient of the positioner. It would be desirable for the valve positioner to be powered by the electrical current signal loop and not an external source, and therefore intrinsically safe. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, a valve positioner system with zero bleed at steady state is contemplated. There is a very low power pilot valve, the operation of which is controlled by an electronic circuit that is powered from a signaling and power connection of a positioner device. The circuit may monitor a valve position signal from the signaling and power connection in order to make control decisions to activate and deactivate the pilot valve. The valve positioner system may also include a plurality of pneumatic valves that are actuated in turn by the pilot valve. With varying configurations of normally open or normally closed pilot valves and pneumatic valves, fail freeze and fail safe operations may be possible while having zero bleed in steady state conditions. The present invention will be best understood by reference to the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the various embodiments disclosed herein will be better understood with respect to the following description and drawings, in which: 
         FIG. 1  is a block diagram illustrating the various components of a valve positioning apparatus in accordance with a first embodiment of the present disclosure including a normally closed pilot valve and normally closed on/off pneumatic valves operating with a double-acting actuator and configured for fail freeze operation; 
         FIG. 2  is a block diagram illustrating the components of the valve positioning apparatus in accordance with a second embodiment including a normally closed pilot valve and normally closed on/off pneumatic valves operating with a single-acting actuator configured for fail freeze operation; 
         FIG. 3  is a flowchart showing the steps of a method for regulating a process while preventing bleed in connection with fail freeze operation utilizing the first and second embodiments of the valve positioning apparatus; 
         FIG. 4  is a block diagram showing a third embodiment of the valve positioning apparatus including a normally open pilot valve and normally closed on/off pneumatic valves operating with a double-acting actuator and configured for fail safe operation; 
         FIG. 5  is a block diagram showing the fourth embodiment of the valve positioning apparatus including a normally open pilot valve and normally closed on/off pneumatic valves operating with a single-acting actuator and configured for fail safe operation; 
         FIG. 6  is a flowchart showing the steps of a method for regulating a process while preventing bleed in connection with fail safe operation utilizing the third and fourth and of the valve positioning apparatus; 
         FIG. 7  is a block diagram illustrating the components of a fifth embodiment of the valve positioning apparatus including a normally closed pilot valve and normally open on/off pneumatic valves operating with the double-acting actuator and configured for fail safe operation; 
         FIG. 8  is a block diagram illustrating the components of a fourth embodiment of the valve positioning apparatus including the normally closed pilot valve and normally open on/off pneumatic valves operating with the single-acting actuator and configured for fail safe operation; and 
         FIG. 9  is a flowchart showing the steps of a method for regulating a process while preventing bleed in connection with fail safe operation. 
     
    
    
     Common reference numerals are used throughout the drawings and the detailed description to indicate the same elements. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of certain embodiments of an electro-pneumatic valve positioner having steady-state zero bleed capabilities and is not intended to represent the only forms that may be developed or utilized. The description sets forth the various functions in connection with the illustrated embodiments, but it is to be understood, however, that the same or equivalent functions may be accomplished by different embodiments that are also intended to be encompassed within the scope of the present disclosure. It is further understood that the use of relational terms such as first and second, and the like are used solely to distinguish one entity from another without necessarily requiring or implying any actual such relationship or order between such entities. 
     With reference to the block diagram of  FIG. 1 , a first embodiment of the valve positioner system  10   a  with steady-state zero bleed includes a positioner device  12  that controls a valve actuator  14 . The valve actuator  14 , in turn, is linked to and modifies the position of a control valve (not shown) that regulates a part of a fluid flow process. In further detail, the valve actuator  14  includes a cylinder body  16  defining an interior chamber  18 . A piston  20  reciprocates within the cylinder body  16  as gas is supplied and/or exhausted therefrom. The piston  20  is attached or otherwise mechanically coupled to a connecting rod  22 , which is linked to the control valve. The foregoing description of the valve actuator  14  is presented by way of example only, and any other type of actuator, such as a rotary type or a diaphragm type may be substituted without departing from the scope of the present disclosure. 
     In the first embodiment of the valve positioner system  10  shown in  FIG. 1 , the valve actuator  14  is of a double-acting type ( 14   a ) including a first fluid flow passageway  24  and a second fluid flow passageway  26 . Moving the piston  20  and the connecting rod  22  to its maximum extended state is achieved by supplying gas through the first fluid flow passageway  24 , while simultaneously exhausting air through the second fluid flow passageway  26 . On the other hand, moving the piston  20  and the connecting rod  22  to its maximum retracted state is achieved by exhausting gas through the first fluid flow passageway  24 , while simultaneously supplying gas to the second fluid flow passageway  26 . Other embodiments of the valve positioner system  10  may include a single-acting type valve actuator  14   b , the details of which will be described further below in relation to those embodiments that utilize such type of a valve actuator. 
     The components of the valve positioner  10  are variously described herein as being driven by compressed air, but it will be recognized that any other inert gases may be utilized. Along these lines, other power systems such as hydraulics may be substituted, though pneumatics offer several advantages with respect to safety while operating in hazardous environments. Those having ordinary skill in the art will appreciate the modifications to the other components of the valve positioner system  10  described herein that are attendant to the use of such alternative power systems. 
     The supplying and exhausting of the compressed gas to and from the valve actuator  14  is governed by the positioner device  12 , which may also be referred to as a valve positioner controller or a servomechanism. Again, the porting of gas to the valve actuator  14  is understood to provide a motive force thereto such that the position of the control valve can be adjusted. The ultimate source of such compressed gas is a pressure line  28 , and depending on the various states of a pilot valve  30  and on/off two-way pneumatic valves  32 , the compressed gas is selectively ported to the first fluid flow passageway  24  and the second fluid flow passageway  26 . The state of the pilot valve  30  (and by logical extension, pneumatic valves  32 ) is set by a pilot valve control module  34  that executes control logic methods in accordance with several embodiments of the present disclosure. 
     In its basic configuration, the positioner device  12  may include an electrical input port  36 , a pressure line intake port  38 , a first output port  40 , and a second output port  42 . Generally, the signal received on the electrical input port  36  and the actuator actual position defines the degree to which the compressed gas from the pressure line intake port  38  is directed or exhausted to the first output port  40  and the second output port  42 . The electrical input port  36  is connectable to a two-wire connection  44  delivering an analog electrical current ranging between 4 mA and 20 mA in the form of a valve position signal  46 . The two-wire connection  44  is linked to a central regulator station that transmits the valve position signal  46  to the positioner device  12 , among possibly other positioner devices. Although the basic operation of the valve positioner system  10  does not require it, the valve position signal  46  can carry a digital signal utilized by the positioner device  12  for additional functionality such as diagnostics, configuration, and so forth. The digital signal, as well as the related hardware interfaces, may be HART (Highway Addressable Remote Transducer) compliant. It is understood that the valve position signal  46  also provides electrical power to the positioner device  12  and other associated components. 
     The valve position signal  46  can be quantified as a percentage of the fully open or fully closed position of the valve actuator  14  and hence the control valve, and more specifically, as the pressure of the compressed gas that is ported from the pressure line  28  to the first and second output ports  40 ,  42  for achieving that position. For example, upon proper calibration, a 0% (4 mA) input signal may be defined as the fully closed position, while a 100% signal (20 mA) input signal may be defined as the fully open position. A 12 mA signal may represent a 50% position. 
     A separate positioner control module  37  may be independent of the pilot valve monitoring module may govern such functionality. In order to ensure the correct positioning of the valve actuator  14 , a feedback sensor may be incorporated therein that reports to the controller module its actual position. The valve position signal  46  includes a set point or reference value, to which the value of the actual position signal is compared. The positioner control module  37 , via an electro-pneumatic transducer  39 , adjusts to supply more or less compressed gas to the valve actuator  14  to position the same to the designated set point. A variety of different methods may be used to effect a change in the flow rate of compressed gas to the valve actuator  14 . 
     The positioner device  12  may be suitable for hazardous environments where flammable gasses in the environment have the potential to ignite from sparks typical in regular circuits and constituent components thereof. In this regard, the positioner device  12  may be intrinsically safe, in that, among other things, the electrical components and any others devices utilized therein operate on low voltages. 
     As shown in  FIG. 1 , the pressure line intake port  38  is not in direct fluid communication with the pressure line  28 . Instead, the pilot valve  30  selectively connects the pressure line  28  to the pressure line intake port  38 . Thus, the porting or directing of the compressed gas from the pressure line  28  to the first and second output ports  40 ,  42  is further proscribed by the control logic methods executed by the pilot valve control module  34 . 
     The pilot valve  30  is understood to be a conventional normally closed three/two way valve with spring return. In this regard, there is an electrical pilot  47  that is connected to the pilot valve control module  34 . Applying an electrical signal to the electrical pilot  47  switches from a normally closed or deactivated first position, to an open or activated second position. Power consumption is understood to be approximately 6 milliwatts (mW), and while having a very low fluid flow rate (CV), further work may be performed with its output. Such low power devices are known and may be intrinsically safe and suitable for use in hazardous environments. Such low power can be taken from the 4-20 mA loop to maintain the full operability of the positioner device  12 . 
     In further detail, the pilot valve  30  has a pressure line intake port  48  coupled to the pressure line  28 , a primary output port  50 , and a secondary output port  52 . In its normally closed or deactivated first position, the pressure line intake port  48  is not in fluid communication with either the primary output port  50  or the secondary output port  52 . Instead, the primary output port  50  is in fluid communication with the secondary output port  52  that is being exhausted. In the activated, second position of the pilot valve  30 , the pressure line intake port  48  is in fluid communication with the primary output port  50 . In this state, the compressed gas from the pressure line  28  flows through and other work is performed therewith. 
     The primary output port  50  of the pilot valve  30  is coupled to a first on/off two-way pneumatic valve  32   a , a second on/off two-way pneumatic valve  32   b , and a third on/off two-way pneumatic valve  32   c . In accordance with one embodiment of the present disclosure, the pneumatic valves  32  are understood to be standard 2/2 valves with pneumatic control. In this regard, each of the first, second, and third pneumatic valves  32  includes a corresponding pneumatic pilot  54   a ,  54   b , and  54   c , respectively. Compressed gas flowing from the pressure line  28  through the pilot valve  30  is ported to the pneumatic pilots  54 . The pressure line  28  is in fluid communication with an input port  56   a  of the first pneumatic valve  32   a . An output port  58   a  is in fluid communication with the pressure line intake port  38 . Regarding this segment of the valve positioner system  10 , with the pilot valve  30  being activated as a result of an electrical signal to the electrical pilot  47 , the compressed gas from the pressure line  28  is ported through the first pneumatic valve  32   a  to the pressure line intake port  38 , as the valve itself is activated in response to the activated first pneumatic pilot  54   a  thereof. 
     With the second pneumatic valve  32   b , its input port  56   b  is in fluid communication with the first output port  40  of the positioner device  12 , while its output port  58   b  is in fluid communication with the first fluid flow passageway  24  of the valve actuator  14 . Similarly, with the third pneumatic valve  32   c , its input port  56   c  is in fluid communication with the second output port  42  of the positioning device  12 , while its output port  58   c  is in fluid communication with the second fluid flow passageway  26  of the valve actuator  14 . As indicated above, the positioner device  12  may have a controller module  37  independent of the pilot valve monitoring control  34 . The positioner device  12  ports or directs the compressed gas from the pressure line intake port  38  to the first output port  40  and the second output port  42  based proportionally upon the valve position signal  46  received on the electrical input port  36 . 
     In accordance with various embodiments of the present disclosure, the pilot valve control module  34  is powered by the electrical current of the valve position signal  46 . The pilot valve control module  34 , being placed in series with the two-wire connection  44 , also monitors the valve position signal  46  and utilizes it as input in making control decisions in which the pilot valve  30  and the pneumatic valve  32  are manipulated. The current draw of the pilot valve control module  34  is understood to be minimal and have little to no effect on the remainder of the positioner device  12 , including the aforementioned separate controller module. Without the pilot valve control module  34 , input voltage to the positioner device  12  is understood to be within the range of 12 V to 30 V. With a series addition of the pilot valve control module  34 , the minimum input voltage increases while the current remains constant. The pilot valve control module  34  may be integrated into the positioner device  12 , though various other embodiments contemplate the pilot valve module  34  being independent of the positioner control module  37 . 
     The valve positioner system  10  may also be adapted for a single-acting valve actuator  14   b . Referring to  FIG. 2 , again there is a positioner device  12  drawing power and receiving the valve position signal  46  through the two-wire connection  44  connected to the electrical input port  36 . The pilot valve control module  34  is connected in series with the two-wire connection  44 , and executes various control logic that manipulates the pilot valve  30 . Like the first embodiment  10   a , the pilot valve  30  of the second embodiment  10   b  is a conventional normally closed three/two way valve with spring return including the electrical pilot  47  connected to the pilot valve control module  34 , the pressure line intake port  48 , the primary output port  50 , and the secondary output port  52 . The application of an electrical signal to the electrical pilot  47  from the pilot valve control module  34  switches the pilot valve  30  from a normally closed position to the open position, thereby porting the compressed gas from the pressure line  28  through the pilot valve  30  to the primary output port  50  thereof. 
     The primary output port  50  of the pilot valve  30  is in fluid communication with the first pneumatic valve  32   a  and the second pneumatic valve  32   b , and specifically to the pneumatic pilots  54   a ,  54   b , respectively, thereof. Additionally, the pressure line  28  is in fluid communication with the input port  56   a . Upon being pneumatically activated via the pneumatic pilot  54   a , the compressed gas is ported through the input port  56   a  to the output port  58   a  and to the pressure line intake port  38  of the positioner device  12 . As indicated above, the electro-pneumatic transducer  39  selectively ports the input compressed gas on the pressure line intake port  38  to the second output port  42  based upon the control methods executed by the positioner control module  37 . The second output port  42  is in fluid communication with the input port  56   b  of the second pneumatic valve  32   b , which is activated and set to an open position (from a normally closed position) via the pneumatic pilot  54   b . The compressed gas that is ported to the output port  58   b  is then passed to the single-acting valve actuator  14   b  through the second fluid flow passageway  26  thereof. 
     In the first and second embodiments of the valve positioner system  10   a ,  10   b , there is contemplated a “fail-freeze” function. This refers to a function where the position of the valve actuator  14  is held to the most recent prior to a failure. These failures include loss of power due to the two wire connection  44  being disconnected from the signal source, a loss of pressure in the pressure line  28 , loss of the actuator position feedback signal, and so forth. Other failure conditions besides those enumerated above may also trigger the fail-freeze function, and it is understood that the valve positioner system  10  may be adapted thereto. 
     Now, with additional reference to the flowchart of  FIG. 3 , the details of the aforementioned control method executed by the pilot valve control module  34  will be described. The method begins with a step  200  of receiving the valve position signal  46 . The method continues with a step  202  of executing the control methods by the positioner device  12 , and specifically the positioner control module  37 . Once one cycle of the control method is executed in step  202 , there is a decision branch  204  of determining whether the current (amperage) value as specified in the valve position signal  46  is less than a predetermined failure value. If it is, the method continues with a step  206  of deactivating or closing the pilot valve  30 . In response, the pneumatic valves  32   a ,  32   b , and  32   c  are also moved to the closed position per step  208 . Otherwise, the method continues with a step  210  of determining if the actual position of the valve actuator  14  is within a predetermined tolerance band. 
     Following the deactivation of the pilot valve  30 , and the closing of the pneumatic valves  32 , the method continues with a decision branch  212  of again determining whether the current (amperage) value as specified in the valve position signal  46  is less than the predetermined failure value. This decision branch is repeated until evaluated true, at which point the loop is exited and continues with a delay step  214 . Thereafter, the pilot valve  30  is again activated in step  216 , and continues back to executing the control methods in step  202 . 
     Per decision branch  210 , if the actual position of the valve actuator  14  is not within the predetermined tolerance band, the method returns to the decision branch  202  of determining whether the current (amperage) value specified in the valve position signal  46  is less than the predetermined failure value. If within the predetermined time spent, the method continues with a step  218  of deactivating the pilot valve  30 , followed by the attendant movement of the pneumatic valve  32  to the closed position per step  220 . Next, in decision branch  222 , if the actual position of the valve actuator  14  is within the predetermined tolerance band, the method returns to the decision branch  204  of determining whether the current value specified in the valve position signal  46  is less than the predetermined failure value. If not, however, the method proceeds to the step  216  of activating the pilot valve  30 . 
     A third embodiment of the valve positioner system  10   c  shown in  FIG. 4  contemplates an alternative including a fail-safe function where the position of the valve actuator  14  is transitioned to a “safe” stroke end upon failure. In further detail, this embodiment utilizes the double-acting valve actuator  14   a  that has the first fluid flow passageway  24  and the second fluid flow passageway  26 . As with the first and second embodiments of the valve positioner system  10   a ,  10   b , the third embodiment includes the positioner device  12  drawing power and receiving the valve position signal  46  through the two-wire connection  44  that are connected to the electrical input port  36 . The pilot valve control module  34  is connected in series with the two-wire connection  44  and executes control logic that manipulates a normally open electro-pneumatic pilot valve  60 . 
     The pilot valve  60  is a conventional three/two way valve with spring return including an electrical pilot  62  connected to the pilot valve control module  34 . Additionally, the pilot valve  60  includes a pressure line intake port  64  connected to the pressure line  28 , a primary output port  66 , and a secondary output port  68 . In its deactivated state, the normally open pilot valve  60  is porting the compressed gas from the pressure line  28  to the primary output port  66 . When activated by the pilot valve control module  34 , the pilot valve  60  switches from the open position to the closed position thereby restricting the compressed gas to the normally closed first pneumatic valve  32   a , second pneumatic valve  32   b , and third pneumatic valve  32   c . In other words, the pneumatic valves  32  are deactivated when the pilot valve  60  is activated, and vice versa. 
     Each of the first, second, and third pneumatic valve  32   a - c  include the respective pneumatic pilots  54   a - 54   c  that are in fluid communication with the primary output port  66  of the pilot valve  60 . The first pneumatic valve  32   a  has the input port  56   a  that is also in fluid communication with the pressure line  28 . The output port  58   a  is in fluid communication with the pressure line intake port  38  of the positioner device  12 . Upon being activated via the pneumatic pilot  54   a , the compressed gas is ported through the input port  56   a  to the output port  58   a  and to the pressure line intake port  38  of the positioner device  12 . The electro-pneumatic transducer  39  selectively ports the compressed gas on the pressure line intake port  38  to the first output port  40  and the second output port  42  based upon the control methods executed by the positioner control module  37 . The first output port  40  is in fluid communication with the input port  56   b  of the second pneumatic valve  32   b , which is activated and set to an open position (from a normally closed position) via the pneumatic pilot  54   b . Along these lines, the second output port  42  is in fluid communication with the input port  56   c  of the third pneumatic valve  32   c . The compressed gas that is ported to the output ports  58   b ,  58   c  of the respective second pneumatic valve  32   b  and third pneumatic valve  32   c  are then passed to or exhausted from the double-acting valve actuator  14   a  through the first and second fluid flow passageways  24 ,  26  thereof. 
     The third embodiment of the valve positioner system  10   c  utilizes the double-acting valve actuator  14   a  and is configured for fail safe operation. A fourth embodiment of the valve positioner system  10   d  shown in  FIG. 5  is likewise configured for failsafe operation, but instead utilizes the single acting valve actuator  14   b . The configuration of the valve positioner system  10   d  is identical in all respects to the third embodiment  10   c , except for the elimination of the third pneumatic valve  32   c  since there is only the first fluid flow passageway  24 , and not the second fluid flow passageway  26 . 
     The valve positioner system  10   d  includes the positioner device  12  drawing power and receiving the valve position signal  46  through the two-wire connection  44  that are connected to the electrical input port  36 . The pilot valve control module  34  is connected in series with the two-wire connection  44  and executes control logic that manipulates the normally open electro-pneumatic pilot valve  60 . 
     In its deactivated state, the normally open pilot valve  60  is porting the compressed gas from the pressure line  28  to the primary output port  66 . When activated by the pilot valve control module  34 , the pilot valve  60  switches from the open position to the closed position thereby restricting the compressed gas to the normally closed first pneumatic valve  32   a  and the second pneumatic valve  32   b . Each of the first and second pneumatic valve  32   a - b  includes the respective pneumatic pilots  54   a - 54   b  that are in fluid communication with the primary output port  66  of the pilot valve  60 . The first pneumatic valve  32   a  has the input port  56   a  that is also in fluid communication with the pressure line  28 . The output port  58   a  is in fluid communication with the pressure line intake port  38  of the positioner device  12 . Upon being activated via the pneumatic pilot  54   a , the compressed gas is ported through the input port  56   a  to the output port  58   a  and to the pressure line intake port  38  of the positioner device  12 . The electro-pneumatic transducer  39  selectively ports the compressed gas on the pressure line intake port  38  to the second output port  42  based upon the control methods executed by the positioner control module  37 . The second output port  42  is in fluid communication with the input port  56   b  of the second pneumatic valve  32   b . The compressed gas that is ported to the output port  58   b  of the second pneumatic valve  32   b  is then passed to or exhausted from the single-acting valve actuator  14   b  through the second fluid flow passageway  26  thereof. 
     The flowchart of  FIG. 6  details one embodiment of the control method executed by the pilot valve control module  34  in connection with the normally open pilot valve  60  and the normally closed pneumatic valves  32  to implement fail-safe operation of the valve actuator  14 . The method begins with a step  250  of receiving the valve position signal  46  followed by a decision branch  251  of determining whether the current (amperage) value as specified in the valve position signal  46  is less than a predefined failure value. If it is not, the method continues with a step  252  of the activating the pilot valve  60  to an open state. As a result, pneumatic valves  32  are moved to the open position according to step  254 . In this condition, per step  256 , the method involves executing the control methods by the positioner device  12 , i.e., the positioner control module  37 . If the current (amperage) value is less than the predefined failure value, the pilot valve  60  is naturally de-energized per step  258 , and the pneumatic valves  32  move or have moved to the open position according to step  260 . As a result, the positioner device  12  drives the valve actuator  14  to the failed position in step  262 , and returns to the decision branch  250 . 
     After the control methods are executed in step  256 , the method continues with a decision branch  264  of determining whether the actual position of the valve actuator  14  is within the predetermined tolerance band. If not, the method returns to the decision branch  250 . If it is, however, execution proceeds to activating the pilot valve  60  in step  266 , which results in the pneumatic valve  60  being moved to the closed position according to step  268 . This is followed by the decision branch  270  in which it is again determined whether the actual position of the valve actuator  14  is within the predetermined tolerance band. The method loops until this condition is false, at which point execution returns to the decision branch  250 . 
     A fifth embodiment of the valve positioner system  10   e  shown in  FIG. 7  contemplates yet another an alternative including a fail-safe function where the position of the valve actuator  14  is transitioned to a “safe” stroke end upon failure. In further detail, this embodiment utilizes the double-acting valve actuator  14   a  that has the first fluid flow passageway  24  and the second fluid flow passageway  26 . As with the forgoing first, second, third and fourth embodiments  10   a - 10   d , the positioner device  12  draws power and receives the valve position signal  46  through the two-wire connection  44  that is connected to the electrical input port  36 . The pilot valve control module  34  is connected in series with the two-wire connection  44  and executes control logic that manipulates the normally closed electro-pneumatic pilot valve  30 , which is a conventional three/two way valve with spring return. 
     The pilot valve  30  includes the pressure line intake port  48  connected to the pressure line  28 , the primary output port  50 , and a secondary output port  52 . In its activated state, the normally closed pilot valve  30  is porting the compressed gas from the pressure line  28  to the primary output port  50 . When deactivated by the pilot valve control module  34 , the pilot valve  30  switches from the closed position to the open position thereby restricting the compressed gas to a normally open first pneumatic valve  70   a , a second pneumatic valve  70   b , and a third pneumatic valve  70   c . The pneumatic valves  70  are activated when the pilot valve  30  is activated. 
     Each of the first, second, and third pneumatic valves  70   a - c  include the respective pneumatic pilots  72   a - 72   c  that are in fluid communication with the primary output port  50  of the pilot valve  30 . The first pneumatic valve  70   a  has the input port  74   a  that is also in fluid communication with the pressure line  28 . The output port  76   a  is in fluid communication with the pressure line intake port  38  of the positioner device  12 . Upon being activated via the pneumatic pilot  72   a , the compressed gas is ported through the input port  74   a  to the output port  76   a  and to the pressure line intake port  38  of the positioner device  12 . 
     The electro-pneumatic transducer  39  selectively ports the compressed gas on the pressure line intake port  38  to the first output port  40  and the second output port  42  based upon the control methods executed by the positioner control module  37 . The first output port  40  is in fluid communication with the input port  74   b  of the second pneumatic valve  70   b , which is activated and set to an closed position (from a normally open position) via the pneumatic pilot  72   b . Along these lines, the second output port  42  is in fluid communication with the input port  74   c  of the third pneumatic valve  70   c . With the pneumatic valves  70  being deactivated, that is, when the pilot valve  30  is deactivated, the compressed gas that is ported to the output ports  76   b ,  76   c  of the respective second pneumatic valve  70   b  and third pneumatic valve  70   c  are then passed to or exhausted from the double-acting valve actuator  14   a  through the first and second fluid flow passageways  24 ,  26  thereof. With the pilot valve  30  activated, the pneumatic valves  70  are activated, the stopping the flow of compressed gas from the input ports  74  to the output ports  76 . 
     As mentioned above, this embodiment contemplates the use of the double-acting valve actuator  14   a . A sixth embodiment of the valve positioner system  10   f  shown in  FIG. 8  is also configured for failsafe operation, but instead utilizes the single acting valve actuator  14   b . The configuration of the valve positioner system  10   f  is identical in all respects to the fifth embodiment  10   e , except for the eliminated third pneumatic valve  70   c  since there is only the first fluid flow passageway  26 , and not the second fluid flow passageway  24 . 
     The valve positioner system  10  includes the positioner device  12  drawing power and receiving the valve position signal  46  through the two-wire connection  44  that are connected to the electrical input port  36 . The pilot valve control module  34  is connected in series with the two-wire connection  44  and executes control logic that manipulates the normally open electro-pneumatic pilot valve  30 . 
     In an activated state, the normally closed pilot valve  30  is porting the compressed gas from the pressure line  28  to the primary output port  50 . When activated by the pilot valve control module  34 , the pilot valve  30  switches from the closed position to the open position, porting the compressed gas to the normally open first pneumatic valve  70   a  and the second pneumatic valve  70   b . Each of the first and second pneumatic valve  70   a - 70   b  includes the respective pneumatic pilots  72   a - 72   b  that are in fluid communication with the primary output port  50  of the pilot valve  30 . The first pneumatic valve  70   a  has the input port  74   a  that is also in fluid communication with the pressure line  28 . The output port  76   a  is in fluid communication with the pressure line intake port  38  of the positioner device  12 . When deactivated, the compressed gas is ported through the input port  74   a  to the output port  76   a  and to the pressure line intake port  38  of the positioner device  12 . When activated, the first pneumatic valve  70   a  is closed. 
     The electro-pneumatic transducer  39  selectively ports the compressed gas on the pressure line intake port  38  to the second output port  42  based upon the control methods executed by the positioner control module  37 . The second output port  42  is in fluid communication with the input port  74   b  of the second pneumatic valve  70   b . The compressed gas that is ported to the output port  76   b  of the second pneumatic valve  70   b  is then passed to or exhausted from the single-acting valve actuator  14   b  through the second fluid flow passageway  26  thereof. 
     The flowchart of  FIG. 9  describes another embodiment of the control method executed by the pilot valve control module  34  in connection with the normally closed pilot valve  30  and the normally open pneumatic valves  70  to implement fail-safe operation of the valve actuator  14 . The method begins with a step  300  of receiving the valve position signal  46 , followed by a decision branch  301  of determining whether the current (amperage) value as specified in the valve position signal  46  is less than a predefined failure value. If it is not, the method continues with a step  302  of the activating the pilot valve  60  to a closed state. As a result, pneumatic valves  70  are moved to the open position according to step  304 . In this condition, per step  306 , the method involves executing the control methods by the positioner device  12 , i.e., the positioner control module  37 . If the current (amperage) value is less than the predefined failure value, the pilot valve  30  is naturally de-energized per step  308 , and the pneumatic valves  70  move or have moved to the open position according to step  310 . As a result, the positioner device  12  drives the valve actuator  14  to the failed position in step  312 , and returns to the decision branch  300 . 
     After the control methods are executed in step  306 , the method continues with a decision branch  318  of determining whether the actual position of the valve actuator  14  is within the predetermined tolerance band. If not, the method returns to the decision branch  300 . If it is, however, execution proceeds to activating the pilot valve  30  in step  320 , which results in the pneumatic valve  70  being moved to the closed position according to step  322 . This is followed by the decision branch  324  in which it is again determined whether the actual position of the valve actuator  14  is within the predetermined tolerance band. The method loops until this condition is false, at which point execution returns to the decision branch  300 . 
     The particulars shown herein are by way of example only for purposes of illustrative discussion, and are not presented in the cause of providing what is believed to be most useful and readily understood description of the principles and conceptual aspects of the various embodiments a fourth of the present disclosure. In this regard, no attempt is made to show any more detail than is necessary for a fundamental understanding of the different features of the various embodiments, the description taken with the drawings making apparent to those skilled in the art how these may be implemented in practice.