Abstract:
A method of monitoring a fluid process control system having a control loop for controlling the flow of a material through a path in the fluid process control system. The control loop a control valve, a valve controller, and a fluid control line. The control valve is disposed in the path and is movable between an open position and a closed position. The valve controller is for controlling movement of the control valve. And, the fluid control line couples the valve controller to the control valve. The method detects a first pressure in the control loop with a first pressure sensor. Then, a second pressure can be detected at a location that is external to the control loop with a second pressure sensor. Finally, the method can include determining a characteristic of the control loop based on the first pressure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a divisional of U.S. patent application Ser. No. 12/582,126, filed Oct. 20, 2009, which is a divisional of U.S. patent application Ser. No. 11/426,109, filed Jun. 23, 2006, which is based on and claims priority to U.S. Provisional Patent Application No. 60/760,665, filed Jan. 20, 2006, and is a continuation-in-part of U.S. patent application Ser. No. 10/117,007, filed Apr. 5, 2002, which is based on and claims priority to U.S. Provisional Patent Application Ser. No. 60/281,852, filed Apr. 5, 2001. The entire disclosure of each of the foregoing documents is hereby expressly incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This patent generally relates to emergency shutdown systems used in process control environments and more particularly to a versatile controller for use in the testing and diagnostics of emergency shutdown devices and supporting equipment used in a process control environment. 
       BACKGROUND 
       [0003]    Safety instrument systems typically incorporate emergency shutdown valves which are normally in a fully opened or a fully closed state and are controlled by a logic solver, a Programmable Logic Controller (PLC), or an emergency shutdown controller of some type to change states in the event of an emergency situation. To ensure that these valves can function properly, process control system operators typically periodically test the emergency shutdown valves by running these valves through a stroke test, which partially or completely opens or closes the valve. Because these tests are typically performed while the process is operating on-line or is operational, it is important to perform any test reliably and then return the valve to its normal state as quickly as possible. In this context, the term “normal state” refers to the position or state of the emergency shutdown valve when there is no emergency and the emergency shutdown valve is not being tested, i.e., when the process is operating normally. 
         [0004]    In many cases, the emergency shutdown tests are performed at predetermined intervals by remotely located controllers. For example, emergency shutdown tests may be performed only a few times each year due to cumbersome test procedures and issues related to manpower. Also, during emergency shutdown tests, the emergency shutdown valve, or other emergency shutdown device being tested, is not available for use if an actual emergency event were to arise. However, limited, periodic testing is not an efficient way of verifying the operability of an emergency shutdown test system. As a result, digital valve controllers have been, in some cases, programmed to assist in the operation of the valve test to make the testing more automatic, user friendly and reliable. 
         [0005]    Additionally, it is typically important that any emergency shutdown system be able to activate an emergency shutdown device (an emergency shutdown valve, for example) to its safe condition even when commanded by the emergency shutdown controller to do so in the unlikely but possible situation where an emergency event occurs during an emergency shutdown device test. In this context, the term “safe condition” refers to the position of the emergency shutdown device that makes the process plant or portion of the process plant “safe.” Typically, this safe position is associated with a position of the shutdown device that shuts down or halts some portion of the process plant. 
         [0006]    While there are many systems that test the ultimate emergency shutdown device, such as an emergency shutdown valve, itself, in many cases there is supporting equipment associated with the emergency shutdown device that should also be tested to assure the complete operability of the emergency shutdown capabilities at any particular plant location. For example, in some pneumatic valve configurations, a solenoid valve is connected between a pneumatic valve actuator of an emergency shutdown valve and an emergency shutdown controller to redundantly control the operation of the valve actuator in response to signals from the emergency shutdown controller. While the emergency shutdown valve may be functional, it is possible for the solenoid device to become defective and therefore not operate properly as a redundant method of actuating the emergency shutdown valve. In some cases, an improperly operating solenoid device may even prevent the emergency shutdown valve from actuating properly when the emergency shutdown controller sends a shut-down signal to the valve controller for the emergency shutdown valve. 
         [0007]    While it is possible to develop and provide specialized equipment at each emergency shutdown location within a plant to perform testing of each different emergency shutdown device and its supporting equipment, it is more desirable to provide a universal or generic set of equipment that may be used in many different situations to test different types of emergency shutdown devices and the supporting equipment associated therewith or to perform other functions in the plant. For example, it is desirable if such versatile equipment is able to control and test different types of emergency shutdown valves and solenoid valve configurations while simultaneously or alternatively operating as part of a closed loop distributed process control system. 
       SUMMARY 
       [0008]    A multi-functional or versatile emergency shutdown device controller, such as an emergency shutdown valve controller, may be used in various different emergency shutdown configurations to enable the control and testing of different types and configurations of emergency shutdown devices and the supporting equipment associated therewith while also being able to be used in other plant configurations, such as in closed loop process control configurations. In one example, a digital valve controller for use with an emergency shutdown valve includes two pressure sensors and is adapted to be connected to a pneumatic valve actuator and to a solenoid valve device to assist in the on-line testing of the valve actuator as well as the on-line testing of the solenoid valve. 
         [0009]    To perform testing of the solenoid device, the valve controller may measure the pressure at different ports of the solenoid valve as the solenoid valve is actuated for a very short period of time. The valve controller may determine whether the solenoid device is fully functional or operational based on the derivative of the difference between the measured pressure signals, i.e., based on the rate of change of the difference between the measured sensor signals over time. In this case, the digital valve controller, or an emergency shutdown test system connected to the digital valve controller, may determine that the solenoid is in acceptable operational condition if the absolute value of the determined derivative is greater than a predetermined threshold and may determine that a problem exists with the solenoid valve if the absolute value of the determined derivative is less than the same or a different predetermined threshold. 
         [0010]    In one case, the digital valve controller may be used as a pressure transducer to control a valve based on measurements of the pressure supplied to the valve actuator which may be, for example, a spring and diaphragm type of valve actuator. In this case, the digital valve controller may use both of the pressure sensors, one to perform control of the valve and the other to perform testing of the solenoid. Alternatively, the digital valve controller may use one of the pressure sensors to perform pressure based control, i.e., within the servo control loop of the valve, and may use the other pressure sensor, not to test the solenoid valve, but to measure some other pressure signal within the process plant. This other pressure signal need not be associated with the control or testing of the emergency shutdown device or its associated equipment. In another case, the digital valve controller can use one of the pressure sensors to control or limit an amount of force used to test the valve. So configured, the digital valve controller can minimize inadvertent effects on the process by overmodulating the valve position during the test. 
         [0011]    In another case, the digital valve controller may be used as a positioner and control movement of the valve based on position measurements provided to the digital valve controller by position sensors. In this case, the digital valve controller may use one of the pressure sensors to perform testing of the solenoid or other equipment associated with the emergency shutdown device and may use the second pressure sensor to sense a further pressure signal not needed within the servo control loop of the emergency shutdown device or for the testing of the emergency shutdown device. In this case, for example, the second pressure sensor of the digital valve controller may be connected to another location within the process plant, such as to a fluid line output from the emergency shutdown valve, to provide a process variable signal to the emergency shutdown controller or even to a process controller associated with normal control of the process. 
         [0012]    Still further, the same digital valve controller may be used outside of an emergency shutdown device configuration and may control a valve using either pressure control (i.e., as a pressure transducer) or position control (i.e., as a positioner). In the former case, one of the sensors may be used to measure the pressure in the pneumatic loop of the valve for control purposes, i.e., as a pressure feedback, while the other of the sensors may be used to measure a pressure not associated with the valve or needed for controlling or testing the valve. In the latter case, both of the sensors may be used to measure pressures not associated with the valve or needed for controlling or testing the valve 
         [0013]    The emergency shutdown device controller may include a processor, a memory coupled to the processor, and a communication input coupled to the processor that is adapted to receive a test activation signal from, for example, an emergency shutdown controller, a user, etc. One or more first test routines are stored in the memory and each is adapted to be executed on the processor to cause an emergency shutdown test of some kind to be performed in response to the receipt of an appropriate test initiation signal from, for example, the emergency shutdown controller. These test routines may be, for example, partial or full stroke test routines for the valve, test routines for the solenoid valve, etc. One or more second routines are stored in the memory and are adapted to be executed on the processor during the emergency shutdown test of, for example, a solenoid valve, to cause the one or more sensor outputs to be stored in the memory for subsequent retrieval and/or to be processed to determine the operational functionality of one or more devices, such as the solenoid valve, associated with the emergency shutdown device. 
         [0014]    As noted above, the emergency shutdown device controller may include a communication unit, wherein the communication unit is coupled to the processor and communicates with a diagnostic device or a controller via a communication network or line using an open communication protocol, such as the HART protocol, the FOUNDATION® Fieldbus communication protocol or any other desired proprietary or non-proprietary communication protocol. The communication unit may, in some configurations, send one or more of the collected sensor signals to a further device within the process control system via the communication network or communication line. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a schematic diagram of several components of an example emergency shutdown system including a pneumatic emergency shutdown valve, a valve actuator, a digital valve controller and a solenoid valve configured to perform emergency shutdown operations and tests; 
           [0016]      FIG. 2  is a block diagram of a digital valve controller associated with the emergency shutdown system of  FIG. 1 ; 
           [0017]      FIG. 3  is a schematic diagram of an example emergency shutdown system including the digital valve controller of  FIGS. 1 and 2  configured to operate within the emergency shutdown system as well as to collect a pressure signal not used by or associated with the emergency shutdown system; and 
           [0018]      FIG. 4  is a schematic diagram of a typical valve configuration including the digital valve controller of  FIGS. 1 and 2  configured to operate to perform valve control within a distributed control system of a process plant to thereby perform closed loop control of a valve as well as to collect one or more auxiliary pressure signals not used for the closed loop control of the valve. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    In a multitude of industries, valves and other mechanical devices are used in process control systems to bring a variety of processes quickly into a safe state if an emergency situation arises. It is important to periodically test these valves and associated electro/mechanical devices to ensure that they are in proper functioning condition. For example, to verify the performance of an emergency shutdown valve, mechanical movement of the valve needs to be verified in a reliable and secure manner without unduly affecting the process. Additionally, if the valve has supporting equipment, such as attendant solenoids, etc., it is desirable to be able to test this supporting equipment in a safe and reliable manner while the process is operating on line, but in a manner that does not unduly upset the process. 
         [0020]      FIG. 1  illustrates an example emergency shutdown system  10  that may be used to test the operation of an emergency shutdown valve  12  connected within a process plant. It will be appreciated by those skilled in the art that, while an emergency shutdown valve system is illustrated in the embodiment of  FIG. 1 , the emergency shutdown system  10  may include or be used to control other types of emergency shutdown devices, including other types of control devices, other types of valve devices, etc. 
         [0021]    As illustrated in  FIG. 1 , the emergency shutdown valve  12  may be disposed within a fluid line in a process plant, such as in a pipeline  13  having a portion that supplies fluid to an inlet  12   a  of the emergency shutdown valve  12  and having a portion that receives fluid from an outlet  12   b  of the emergency shutdown valve  12 . The emergency shutdown valve  12 , which is actuated by a valve actuator  14 , may be located normally in one of two positions, i.e., in a fully open position which permits fluid to flow freely between the inlet  12   a  and the outlet  12   b,  or in a fully closed position which prevents fluid from flowing between the inlet  12   a  and the outlet  12   b . To ensure that the emergency shutdown valve  12  will properly function in a true emergency shutdown condition, the emergency shutdown valve  12  may be periodically tested by causing the valve actuator  14  to partially open or close the emergency shutdown valve  12 , which is referred to as a partial stroke test. Of course, other types of tests may be performed to test the operational capabilities of the valve  12 . 
         [0022]    In the example system of  FIG. 1 , the emergency shutdown system  10  includes the valve actuator  14 , illustrated as a pneumatically controlled actuator, and further includes a digital valve controller (DVC)  16  and a solenoid valve  18  which are pneumatically connected to the valve actuator  14  to control the operation of the valve actuator  14 . Additionally, the DVC  16  and the solenoid valve  18  are communicatively connected to an emergency shutdown controller  20  via communication lines and/or power lines  22  and  24 . In one embodiment, the DVC  16  may be the DVC6000 valve controller sold by Fisher Controls International LLC. In the embodiment of  FIG. 1 , the solenoid valve  18  has a solenoid S that is energized via a 24 volt DC power signal sent from the emergency shutdown controller  20  on the lines  22 , while the DVC  16  communicates with the emergency shutdown controller  20  via a 4-20 milliamp communication line  24 , which may be for example, a traditional 4-20 ma control line, a HART protocol line, etc. Of course, if desired, the DVC  16  could be communicatively connected to the emergency shutdown controller  20  via any other desired proprietary or non-proprietary communication network, such as a FOUNDATION® Fieldbus network, a Profibus communication network, or any other known or later developed communication network. Likewise, the solenoid S of the solenoid valve  18  may be connected to and receive control signals from the emergency shutdown controller  20  using any other desired communication or power signals provided on any desired or suitable communication or power lines. 
         [0023]    The valve actuator  14  of  FIG. 1  is illustrated as a spring and diaphragm type actuator which is configured to receive a pneumatic signal on one side (referred to herein as the top side) of a spring biased diaphragm (not shown), to cause movement of a valve stem  28  of the valve  12 . If desired, however, the valve actuator  14  could be a one-sided or a two-sided piston type actuator or could be any other type of known pneumatic valve actuator. To control the actuator  14 , the DVC  16  receives a pneumatic supply pressure signal from a supply line  30  and provides a pneumatic signal via a pneumatic line  34 , a valve portion of the solenoid valve  18  and a pneumatic line  36  to the top side of the valve actuator  14 . As will be understood, the DVC  16  controls movement of the valve actuator  14  by controlling the pressure provided to the top side of the actuator  14  to thereby control movement of the valve stem  28 . Of course, the DVC  16  may cause movement of the diaphragm of the valve actuator  14  in response to control signals sent to the DVC  16  by the emergency shutdown controller  20  via the communication lines  24 . 
         [0024]    The DVC  16  may include a memory which stores one or more stroke tests, such as partial stroke tests or full stroke tests, for testing the valve  12 , and the DVC  16  may initiate these tests in response to one or more test signals sent by the emergency shutdown controller  20 , input by a user or an operator at the DVC  16  itself or provided to the DVC  16  in any other desired manner. Of course, the DVC  16  may be used to perform any known or desired test(s) on the valve  12  and the valve actuator  14  to assure the operability of these devices. 
         [0025]    In safety instrumented systems that employ air-operated valve actuators, such as that illustrated in  FIG. 1 , the pneumatic solenoid valve  18  is often used as a redundant means of assuring that all air is evacuated from the actuator  14  when an emergency demand occurs, to thereby cause the valve/actuator combination to be forced to the emergency seat, i.e., into the safe state. Under normal, non-emergency conditions, the valve actuator  14  is pressurized to force the valve  12  against the normal or non-emergency seat, and the solenoid valve  18  is positioned to maintain pneumatic pressure in the actuator  14 , and to allow the DVC  16  to adjust that pressure via the pneumatic line  34 . In particular, in the embodiment of  FIG. 1 , during the normal operation of the emergency shutdown valve  12  (i.e., the normal, non-safe or non-shutdown state), the solenoid valve  18  connects a port A thereof, as shown in  FIG. 1 , to a port B to enable the DVC  16  to control the pressure in the line  36  and thereby control the pressure at the associated input of the valve actuator  14 . However, during an emergency shutdown operation, the solenoid valve  18  actuates (usually based on the removal of the 24 volt DC power signal from the lines  22 ) to connect the port A to a port C of the solenoid valve  18  while simultaneously disconnecting the line  34  from the line  36 . It will be understood that the port C is vented to the atmosphere. When this action occurs, the pressure supplied to the valve actuator  14  via the line  36  is vented to the atmosphere, causing the spring biased diaphragm and associated linkage within the valve actuator  14  to move the valve stem  28  and the valve plug from the normal seat to the emergency seat. 
         [0026]    Thus, in normal operation, power is applied to and maintained at the input of the solenoid valve  18  to actuate the solenoid valve  18 , allowing air, or other gas, to freely pass between solenoid ports A and B, which allows the DVC  16  to exchange air with the actuator  14  and thereby control the internal pressure at the top side of the valve actuator  14 . When an emergency shutdown occurs, power is removed from the solenoid S of the solenoid valve  18 , allowing a healthy solenoid valve  18  to move to the opposite position. This action closes off port B, and connects port A to port C, thereby allowing air within the valve actuator  14  to escape to the atmosphere. This operation can occur in conjunction with or as a redundant operation to the DVC  16  removing pressure from the line  34  (such as by venting this pressure to the atmosphere) which would also cause the valve actuator  14  to move the valve  12  to the emergency seat in the absence of movement of the solenoid valve  18 . 
         [0027]    As noted above, it is desirable to periodically test the solenoid valve  18  during normal operation of the plant to assure that, in the event of an actual emergency, the solenoid valve  18  will actuate as expected to actually disconnect the DVC  16  from the valve actuator  14  and to allow all or most gas/air to escape from the top side of the valve actuator  14 , thus moving the valve  12  to the emergency seat position. 
         [0028]    To assist in this testing procedure, the DVC  16  is provided with two pressure sensors  40  and  42  which are positioned to monitor the flow of air or other gas through the solenoid valve  18 . In particular, the pressure sensor  40  monitors the valve controller output pressure provided at the solenoid valve port B, i.e., in the line  34 , while the sensor  42  is fluidly connected to and monitors the valve actuator pressure at the solenoid valve port A. As illustrated in  FIG. 1 , the sensor  42  is fluidly connected to the port A of the solenoid valve  18  via a line  45 . Additionally, the DVC  16  may be provided with a testing routine that may collect, store and process the measurements made by the sensors  40  and  42  to determine the operational capabilities of the solenoid valve  18  based on the measured pressure signals, as discussed in more detail below. 
         [0029]    Generally speaking, during a test of the solenoid valve  18 , the emergency shutdown controller  20  may remove power from the solenoid S of the solenoid valve  18  for a short period of time, thereby causing a healthy solenoid valve to actuate. At this time, the controller output pressure measured by the sensor  40  should remain nominally constant (because the DVC  16  will not vent the pressure in the line  34  to the atmosphere), while the pressure at port A measured by the sensor  42  will fall rapidly as the valve actuator  14  evacuates. Generally speaking, the mechanical health of the solenoid valve  18  may be estimated by inferring the rate and extent of travel as the solenoid valve  18  transitions from one position to the other. This inference may be made by continuously monitoring or determining the absolute value of the difference between pressures measured by the sensors  40  and  42  as a function of time. 
         [0030]    More particularly, if the solenoid valve  18  only partially actuates, it will not fully open or close the ports A, B and/or C. Such attenuated solenoid travel will reduce the rate of the evacuation of the valve actuator  14 , causing a slower rate of change in pressure at port A than would occur with a healthy or normally operating solenoid valve  18 . Depending on solenoid valve constructions, such partial actuation may also partially open the port B to the atmosphere, causing the port B pressure, as measured by the senor  40 , to drop as well (instead of staying the same). Either of these phenomena reduces the rate of change in the pressure difference between ports A and B. Likewise, if the solenoid valve  18  actuates more slowly due to friction caused by a degraded physical condition, the solenoid valve  18  will also open and close the ports A, B and/or C thereof more slowly, which will also affect the rate of change with respect to time of the pressure difference between ports A and B. 
         [0031]    As a result, during the test of the solenoid valve  18  (i.e., when power is removed from the solenoid S of the solenoid valve  18 ), the DVC  16  may collect and store pressure measurements made by the sensors  40  and  42 . During or after the test, the DVC  16  may process these measurements to determine the operational condition of the solenoid valve  18 . In particular, the DVC  16  may implement a discrete time domain, digital algorithm as generally defined by equation (1) below to determine the health of the solenoid valve  18 . 
         [0000]      DP= abs ((S1−S2) dt )   (1)
 
         [0000]    where: DP=the derivative of the differential pressure with respect to time; 
         [0032]    S 1 =the measurement of the pressure sensor  40 ; and 
         [0033]    S 2 =the measurement of the pressure sensor  42 . 
         [0000]    It will be understood, however that other implementations of the same basic calculation or equation are possible and may be used instead. 
         [0034]    Equation (1) above may be performed periodically during the solenoid valve test or at separate times associated with the solenoid valve test, to calculate the absolute value of the derivative with respect to time of the differential pressure between the two ports A and B of the solenoid valve  18 . The output of this equation reflects the rate of change of the pressure drop of port A with respect to port B of the solenoid valve  18 . As will be understood, the value DP will be larger when the pressure difference changes more rapidly, meaning that the solenoid valve operated more quickly in response to the removal of the power from the lines  22 . Comparing the quantity DP to an expected threshold, MinDP, provides if this pressure transition is sufficient to constitute a healthy solenoid condition. In other words, solenoid valves which are operating properly and which are free of obstructions, or other binding friction, will rapidly “snap” to the new position, producing a sharp, rapid transition in pressure, resulting in a larger value for DP. Solenoid valves which are clogged, slow to travel, or which do not fully actuate, will produce more sluggish, rounded pressure waveforms, or attenuated pressure differences, thus producing a time-based derivative (DP) which is smaller in amplitude. Solenoid valves which produce a DP valve less than MinDP may be determined to be at risk of failing to perform as expected when required during an actual emergency, and thus may be determined to be faulty or in need of repair or replacement. 
         [0035]    In practice, i.e., during an actual test, an external system such as the emergency shutdown controller  20  may command the DVC  16  to initiate a solenoid valve test, which begins by collecting sensor measurements from the sensors  40  and  42  and watching for a pressure pulse at the input of one or more of the sensors  40  and  42 . The receipt of this pressure pulse may start the periodic evaluation of equation (1) above. After sending the test signal to the DVC  16 , the emergency shutdown controller  20  may then interrupt the solenoid power on the lines  22  for a brief instant. The actual time of the power interruption will depend on the dynamics of the system, but may typically be on the order of tens or hundreds of milliseconds. The time should be long enough to cause full travel of a healthy solenoid at normal operating pressure, but not long enough to cause significant actual movement of the valve  12 , thus preventing the introduction of a significant disturbance within the process being controlled. In particular, the sensors  40  and  42 , as well as the pneumatic lines connecting these sensors to the ports A and B of the solenoid valve  18  are configured to determine a drop or change in pressure at these ports, but the solenoid valve  18  is not actuated long enough to allow the valve actuator  14  to move very much or to actually move the valve  12  a significant amount. That is, the solenoid valve  18  may be de-energized an amount of time less than or on the order of the dead-time associated with the operation of the solenoid valve, valve actuator, and valve stem configuration, so that by the time the valve  12  actually begins to move, the solenoid valve  18  is re-energized and returned to its normal, non-emergency, condition or state. Of course, this operation assumes that the solenoid valve  18  operates much faster (e.g., orders of magnitude faster) than the valve  12 , which is typically the case. 
         [0036]    In any event, after power is restored to the solenoid valve  18 , the DVC  16  may be polled by, for example, the emergency shutdown controller  20  via the communication network  24  to determine if the signal DP was ever large enough to exceed the expected criterion MinDP. If so, the solenoid valve  18  may be deemed to be healthy. Of course, the calculations of equation (1) above may be made while the solenoid valve is moving from one position to another in response to de-energization of the solenoid S, when the solenoid valve  18  is sitting in the emergency position (i.e., has connected port A to port C) and/or when the solenoid valve  18  is moving from one position to another in response to a re-energization of the solenoid S. 
         [0037]    Generally speaking, it will be understood that the value MinDP may be user adjustable or selectable based on the solenoid type, the pressures involved, and the dynamics of the system and may be determined in any desired manner, such as by experimental testing. Still further, the description provided herein is provided in the context of solenoids that are normally powered, and actuators that are normally pressurized. However, the technique described herein can also be applied in systems where the solenoid is normally unpowered, with power being applied only during an emergency demand condition, and/or where the valve actuator is normally unpressurized, with pressure being applied only during an emergency, or any combination thereof. Still further, while the pressure calculations are described as being performed by the DVC  16  during the test, the pressure calculations may be made based on collected (i.e., stored) pressure signals after the solenoid valve  18  has actuated, i.e., after the test, and/or may be made by any other device, such as by the emergency shutdown controller  20 . In this case, the DVC will provide pressure signals from the sensors  40  and  42  either in real time or as stored pressure signals to the emergency shutdown controller  20 . Still further, any means of performing the derivative calculation in equation (1) may be performed, including, for example, using periodic digital sampling and digital calculations, using mechanical devices or using analog electronic circuitry. 
         [0038]    Referring now to  FIG. 2 , a block diagram of the DVC  16  is illustrated to show some of the internal components associated with the DVC  16 . In particular, in addition to the pressure sensors  40  and  42  illustrated in  FIG. 1 , the DVC  16  includes a processor  50 , a memory  52 , one or more analog-to-digital (A/D) converters  54 , one or more digital-to-analog (D/A) converters  56 , and a current-to-pressure converter  58 . The memory  52  is utilized to store instructions or scripts, including tests  60  for testing the valve  12  and the valve actuator  14 , and tests  62  for testing the solenoid valve  18  and any other associated devices. The memory  52  may also store collected sensor signals and diagnostic data. The A/D converters  54  convert analog sensor inputs, such as signals from the sensors  40  and  42 , into digital signals which the processor  50  may process directly and/or store in the memory  52 . Other examples of sensor inputs that may be acquired and stored by the DVC  16  include valve stem travel or position signals (or valve plug travel or position signals), output line pressure signals, loop current signals, etc. 
         [0039]    The D/A converters  56  may convert a plurality of digital outputs from the processor  50  into analog signals which, in some cases, may be used by the current to pressure converter  58  to control a pressure or pneumatic switch  64 . The pneumatic switch  64  couples the pressure supply line  30  (of  FIG. 1 ) to one or more output lines, such as the line  34  of  FIG. 1 . Of course, the pneumatic switch  64  may also or in some cases, connect the line  34  to an atmospheric line  65  to vent pressurized gas to the atmosphere. Alternatively, the current-to-pressure converter  58  may receive digital signals directly from the processor  50 , or may receive analog current signals, such as 4-20 ma current signals from a communication unit  70 , to perform pressure switching and controlling functions. 
         [0040]    The communication unit  70  serves as an interface to the communication network  24  of  FIG. 1 . The communication unit  70  may be or include any desired type of communication stack or software/hardware combination associated with any desired communication protocol. As is known, the communication unit  70  may serve to enable signals received by the processor  50  to be communicated to the emergency shutdown controller  20  or any other device connected to the communication network  24 , such as a process controller responsible for controlling one or more portions of the process not associated with the valve  12 , a user interface or any other device. In particular, the processor  50  may receive and process the pressure signals from the sensors  40  and  42  and may provide one or more of these signals as digital data to be sent via the communication unit  70  and the communication network  24  to other devices. In this manner, one or more of the sensors  40  and  42  may be used to perform measurement activities within the process plant that are not needed for the control and/or testing of the emergency shutdown system  10  of  FIG. 1 . This feature makes the DVC  16  more versatile and useful in processes or emergency shutdown devices that do not need both of the sensors  40  and  42  for control and/or testing of the components within the emergency shutdown device. 
         [0041]    As illustrated in  FIG. 2 , the DVC  16  may also include a clock  72  and an auxiliary input interface  74  which may be used by the processor  50  to monitor or receive auxiliary inputs such as inputs from electrical switches or other devices connected directly to the DVC  16  via the auxiliary interface  74 . Additionally, if desired, the DVC  16  may include a housing  76  which may be an explosion proof housing used to prevent sparks from reaching explosive gasses in a plant, and thus reduce the likelihood that the emergency shutdown system  10  will cause an explosion. 
         [0042]    While the DVC  16  has been described as storing and performing stroke tests and integrity tests on the valve  12 , the valve actuator  14  and the solenoid valve  18  of  FIG. 1 , it will be understood that the DVC  16  can also store and implement any other types of or any additional tests that are based on or that use other diagnostic data collected by the DVC  16  in addition to or alternatively to the data collected by the sensors  40  and  42 . Sensor or diagnostic data collected during, for example, an emergency shutdown test may be collected by other types of sensors not shown in  FIG. 2  and/or may be retrieved using a handheld computing device that may communicate with the DVC  16  via the auxiliary interface  74  or via the communication unit  70 . Many possible tests are described in United States Patent Application Publication No. US 2002-0145515 A1, which is hereby expressly incorporated by reference herein. Additionally or alternatively, if desired, the DVC  16  may send collected data back to a main control room via, for example, the emergency shutdown controller  20 , for processing by other devices. 
         [0043]      FIG. 3  illustrates a different configuration of an emergency shutdown system  100  that uses the DVC  16  of  FIGS. 1 and 2  in a slightly different manner. This configuration and that of  FIG. 4  described below are provided to indicate only a couple of examples of the many ways in which the DVC  16  described herein is versatile enough to be used in different process plant configurations without being significantly altered. The emergency shutdown system  100  of  FIG. 3  is similar to the system  10  of  FIG. 1 , with like elements having the same reference numbers. In the system  100  of  FIG. 3 , however, the DVC  16  is not set up to perform the solenoid valve test discussed above, but is instead configured to use the output of the sensor  40  to perform closed loop pressure control of the valve actuator  14  to cause the valve actuator  14  to actuate in any desired manner in response to the receipt of, for example, an emergency shutdown signal at the DVC  16  or to perform testing, such as partial stroke testing, of the valve actuator  14 . In this case, the DVC  16  uses the sensor  40  to operate as a pressure transducer for the valve actuator  14  and may provide any type of control of the valve  12 . 
         [0044]    However, as illustrated in  FIG. 3 , the second sensor  42  of the DVC  16  may be connected to any desired fluid line within the process to acquire process variable measurements not needed for the control and/or testing of the valve  12 , the valve actuator  14  or the solenoid valve  18 . While the sensor  42  is illustrated in  FIG. 3  as being connected to the outlet  12   b  of the valve  12 , it could instead be connected to any other fluid line or pressure take-off associated with any other process control device or equipment. This other process equipment may, but need not be, associated with the emergency shutdown device  100 . Additionally, it will be understood that the output of the sensor  42 , which is a process variable, may be stored in and sent by the DVC  16  to other devices, such as to the emergency shutdown controller  20  via the communication network  24 , to a handheld device via the main communications controller or the auxiliary interface  74  ( FIG. 2 ), to a distributed controller or a user interface not associated with the emergency shutdown system  100  via the communication network  24  or the auxiliary interface  74 , etc. Thus, collection and use of the sensor data from the sensor  42  is not limited to use in an emergency shutdown device or system in which the DVC  16  is located. This feature makes the DVC  16 , when used as part of an emergency shutdown system, more versatile because it enables the DVC  16  to provide an auxiliary pressure input to a distributed process control system or to a maintenance system associated with a process plant. 
         [0045]    Alternatively, the emergency shutdown device  100  can be configured as shown in  FIG. 3 , wherein the DVC  16  operates to perform position control of the valve  12 , i.e., wherein the DVC  16  operates as a positioner. In this case, however, the sensor  40  may be connected as shown in  FIG. 3  to perform pressure control as a fallback control method if a problem occurs with the position control servo loop, such as if a position sensor associated with this loop fails. This fallback control method is described in more detail in U.S. patent application Ser. No. 11/195,281, entitled “System and Method for Transfer of Feedback Control for a Process Control Device,” which was filed Aug. 2, 2005, the disclosure of which is hereby expressly incorporated by reference herein. In this case, however, the additional sensor  42  may still be used to measure a pressure signal external to the valve  12  and valve actuator  14  configuration. While the sensor identified by reference numeral  40  has just been described as performing the pressure control in  FIG. 3 , an alternate embodiment can include either the sensor identified by reference numeral  40  or the sensor identified by reference numeral  42  performing the pressure control as a fallback control method. Additionally, the sensor identified by reference numeral  40  may alternately be used as a pressure sensor for measuring a pressure external to the valve  12  and actuator  14  configuration. Thus, it should be appreciated that either sensor may perform any of the above-described functions, as necessary for a desired application. 
         [0046]    Likewise,  FIG. 4  illustrates the DVC  16  of  FIGS. 1 and 2  being used in a closed loop valve control configuration  110  to control a valve  12  and a valve actuator  14  combination which are not part of an emergency shutdown device. The closed loop valve control configuration  110  includes elements that are the same as or similar to the system  10  of  FIG. 1 , with like elements having the same reference numbers. In the configuration  110  of  FIG. 4  however, the DVC  16  is illustrated as being connected to a process controller  200  and is used to provide standard servo control of the valve  12  which, in this case, may be a process valve not associated with an emergency shutdown device or system. In this configuration, the sensor  40  of the DVC  16  may be connected to the input of the valve actuator  14  (e.g., may be connected to the pneumatic line  34 ) to perform pressure control of the valve actuator  14 . Here, the DVC  16  operates as a traditional pressure transducer for controlling the valve  12 . However, as shown in  FIG. 4 , the pressure sensor  42  is connected outside of the valve and valve actuator configuration to acquire an external pressure measurement not needed for controlling the valve  12 . Thus, in a manner similar to the earlier described configurations, the DVC  16  may allow the process controller  200  to use the sensor  42  as another pressure transmitter disposed within the process plant. 
         [0047]    Alternatively, the valve control configuration  110  can be configured as shown in  FIG. 4 , wherein the DVC  16  operates to perform position control of the valve  12 , i.e., so that the DVC  16  operates as a positioner within the process control scheme. In this case, however, the sensor  40  may still be connected as shown in  FIG. 4  to perform pressure control as a fallback control method if a problem occurs with the position control loop, such as if a position sensor fails. This fallback control method is described in more detail in U.S. patent application Ser. No. 11/195,281 as noted above. 
         [0048]    Still further, while not shown in  FIG. 4 , if the DVC  16  is used to perform position control of the valve  12 , i.e., if the DVC  16  operates as a positioner within the process control scheme, both of the sensors  40  and  42  may be used as external or auxiliary pressure transmitters to measure any desired pressure signals associated with or present within the process plant, including pressure signals not associated with the controlling or testing the valve  12  or the valve actuator  14 . Of course, the outputs of the sensors  40  and  42  may be stored in the DVC  16  and/or may be sent to other devices, such as to the process controller  200 , to user interface devices (not shown), etc. via for example, the communication network  24 . 
         [0049]    It will be understood from the description provided above that the DVC  16  may be used in many different process plant configurations and scenarios to provide different pressure measurements for different uses, and that the DVC  16  may be used as part of an emergency shutdown device or as part of a distributed process control device when performing these pressure measurements. While the DVC  16  has been described and illustrated as including two pressure sensors  40  and  42 , it will be understood that the DVC  16  is not limited to the use of two pressure sensors, but instead that additional pressure sensors could be provided on the DVC  16  to perform other pressure measurements within the process plant. 
         [0050]    While the present invention has been described with reference to specific examples, which are intended to be illustrative only and not to be limiting of the invention, it will be apparent to those of ordinary skill in the art that changes, additions and/or deletions may be made to the disclosed embodiments without departing from the spirit and scope of the invention.