Patent Description:
In certain industries, such as the petroleum industry, partial stroke testing of emergency shutdown valves (ESVs) is increasingly required by regulatory bodies. However, ESVs and/or other valve assemblies that are part of "Safety Instrumented Systems" (SISs) are generally designed for on/off operation. Connections between valve stems and actuators are not tight resulting in significant lost motion. Further, ESVs are typically characterized by high seal friction and prominent stick-slip dynamics. All of these factors contribute to poor throttling control and complicate partial stoke testing.

Also, ESVs and/or other components of SISs are typically high gain devices. For example, SIS actuators are often single action pistons with a spring return. A very small change in pressure within a chamber of an actuator can cause a large movement of the piston. As a result, when coupling SISs actuators, or other SIS components, to process control devices (e.g., to perform PSTs or other tests), biases of the process control devices, such as I/P (current to pressure) biases, can have a dramatic impact on the calibration of the SIS components. If a calibration of the SIS components is off by a significant amount, results from tests on the SIS components, such as partial stroke tests, will be meaningless. Example of valve positioner system is disclosed in document <CIT>, in which the valve positioner system includes one or more control methods and devices, including various routines to facilitate continuous maintenance, calibration, and adjustment requirements of a control valve.

The present invention is directed to a method of calibrating a positioner, as defined by claim <NUM>.

The present invention is also directed to a process control system as defined by claim <NUM>.

The present invention is also directed to a computer device as defined by claim <NUM>.

Further embodiments of the invention are defined in the dependent claims.

The following describes calibrating positioners or servo controllers, such as valve positioners, using pressure control techniques. The positioners or servo controllers can perform tests with pressure control techniques.

Embodiments and examples in the following description which are not covered by the appended claims are considered as being not part of the present invention, and are merely provided for the purpose of understanding.

In particular, embodiments implementing measuring of pressure supplied to the actuator, are not part of the claimed invention.

The following describes a method and apparatus to: (i) determine a bias of a positioner by controlling a pressure within, or supplied to, an actuator at constant volume, during end-point pressure control, or at a middle pressure value in a range of controlled pressures (not part of the claimed invention), or at another convenient time or state of the actuator (not part of the claimed invention), and (ii) perform tests (e.g., partial stroke tests) by controlling a pressure within or supplied to an actuator, rather than controlling a travel or position of the actuator. For ease of discussion, specific types of positioners, such as valve positioners coupled to emergency shutdown valves, will be referred to throughout this description. Generally, however, the described method and apparatus may calibrate any suitable components of control valve assemblies and utilize those components to perform tests with pressure control techniques.

By utilizing pressure control (i.e., as opposed to travel control) to calibrate positioners and perform tests, the described techniques may alleviate certain difficulties resulting from the loose connections, significant lost motion, high seal friction, and significant stick-slip dynamics that characterize many emergency shutdown valves (ESVs). Specifically, a valve positioner controlling an ESV may generate an estimate of current to pressure (I/P) bias for the valve positioner, which estimate of I/P bias is free of inconsistencies associated with lost motion and valve friction. The positioner may also perform partial stroke testing of an ESV while maintaining control of a pressure within an actuator of the ESV, even in the event that the ESV is stuck.

However, the described techniques may generally facilitate the calibration and testing of any suitable positioners other than positioners coupled to ESVs, such as positioners coupled to and controlling compressor antisurge valves, vent valves, etc. For example, a controller may cause a positioner coupled to a compressor antisurge valve to perform calibrations and tests using pressure control techniques as described herein, where the compressor antisurge valve is configured to prevent surges that occur when a compressor outlet pressure is too high in relation to the flow through the compressor. An example controller may also cause a positioner coupled to a throttling valve/actuator assembly to perform calibrations and tests a using pressure control techniques. By utilizing pressure control (i.e., as opposed to travel control) in these scenarios, a controller may, for example, calibrate positioners while the positioners are in service, without disrupting a corresponding process.

Referring now to <FIG>, a process control system <NUM> is depicted incorporating one or more field devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> in communication with a process controller <NUM>. The process controller <NUM> may cause one or more of the field devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> to perform calibrations and/or tests using pressure control techniques, as discussed further below. The process controller <NUM> is also in communication with a data historian <NUM> and one or more user workstations <NUM>, each having a display screen <NUM>. So configured, the controller <NUM> delivers signals to and receives signals from the field devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> and the workstations <NUM> to control the process control system.

In additional detail, the process controller <NUM> of the process control system <NUM> of the version depicted in <FIG> is connected via hardwired communication connections to field devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> via input/output (I/O) cards <NUM> and <NUM>. The data historian <NUM> may be any desired type of data collection unit having any desired type of memory and any desired or known software, hardware or firmware for storing data. Moreover, while the data historian <NUM> is illustrated as a separate device in <FIG>, it may instead or in addition be part of one of the workstations <NUM> or another computer device, such as a server. The controller <NUM>, which may be, by way of example, a DeltaV™ controller sold by Emerson Process Management, is communicatively connected to the workstations <NUM> and to the data historian <NUM> via a communication network <NUM> which may be, for example, an Ethernet connection.

As mentioned, the controller <NUM> is illustrated as being communicatively connected to the field devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> using a hardwired communication scheme which may include the use of any desired hardware, software, and/or firmware to implement hardwired communications. The hardwired communications may include, for example, standard <NUM>-<NUM> mA communications, and/or any communications using any smart communication protocol such as the FOUNDATION® Fieldbus communication protocol, the HART® communication protocol, etc. The field devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> may be any types of devices, such as positioners, servo controllers, sensors, pressure regulators, control valve assemblies, etc., while the I/O cards <NUM> and <NUM> may be any types of I/O devices conforming to any desired communication or controller protocol. In the embodiment illustrated in <FIG>, the field devices <NUM>, <NUM>, <NUM>, and <NUM> are standard <NUM>-<NUM> mA devices that communicate over analog lines to the I/O card <NUM>, while the digital field devices <NUM>, <NUM>, <NUM>, and <NUM> can be smart devices, such as HART® communicating devices and Fieldbus field devices, that communicate over a digital bus to the I/O card <NUM> using Fieldbus protocol communications. Of course, the field devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> may conform to any other desired standard(s) or protocols, including any standards or protocols developed in the future.

In addition, the process control system <NUM> depicted in <FIG> includes a number of wireless field devices <NUM> and <NUM> and a number of other field devices <NUM>, <NUM>, <NUM>, and <NUM> communicatively connected to a wireless router or other module <NUM>. The field devices <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> are depicted as transmitters (e.g., process variable sensors) while the field device <NUM> is depicted as a control valve assembly including, for example, a control valve and an actuator. Wireless communications may be established between the controller <NUM> and the field devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> using any desired wireless communication equipment, including hardware, software, firmware, or any combination thereof now known or later developed. In the version illustrated in <FIG>, an antenna <NUM> is coupled to and is dedicated to perform wireless communications for the transmitter <NUM>, while the wireless router or other module <NUM> having an antenna <NUM> is coupled to collectively handle wireless communications for the transmitters <NUM>, <NUM>, <NUM>, and <NUM>. Likewise, an antenna <NUM> is coupled to the control valve assembly <NUM> to perform wireless communications for the control valve assembly <NUM>. The field devices or associated hardware <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> may implement protocol stack operations used by an appropriate wireless communication protocol to receive, decode, route, encode, and send wireless signals via the antennas <NUM>, <NUM>, and <NUM> to implement wireless communications between the process controller <NUM> and the transmitters <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> and the control valve assembly <NUM>.

The process controller <NUM> is coupled to one or more I/O devices <NUM> and <NUM>, each connected to a respective antenna <NUM> and <NUM>, and these I/O devices and antennas <NUM>, <NUM>, <NUM>, and <NUM> operate as transmitters/receivers to perform wireless communications with the wireless field devices <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> via one or more wireless communication networks. The wireless communications between the field devices (e.g., the transmitters <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>, and the control valve assembly <NUM>) may be performed using one or more known wireless communication protocols, such as the WirelessHARTO protocol, the Ember protocol, a WiFi protocol, an IEEE wireless standard, etc. Still further, the I/O devices <NUM> and <NUM> may implement protocol stack operations used by these communication protocols to receive, decode, route, encode, and send wireless signals via the antennas <NUM> and <NUM> to implement wireless communications between the controller <NUM> and the transmitters <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>, and the control valve assembly <NUM>.

As illustrated in <FIG>, the controller <NUM> conventionally includes a processor <NUM> that implements or oversees one or more process control routines (or any module, block, or sub-routine thereof) stored in a memory <NUM>. The process control routines stored in the memory <NUM> may include or be associated with control loops being implemented within the process plant. Generally speaking, and as is generally known, the process controller <NUM> executes one or more control routines and communicates with the field devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>, the user workstations <NUM> and the data historian <NUM> to control a process in any desired manner(s).

Any one of the field devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> illustrated in <FIG>, such as control valve assemblies or valve positioners, and/or other suitable types of field devices utilized by a process plant, may be calibrated using pressure control techniques and/or perform tests, such as Partial Stroke Tests (PSTs), with pressure control techniques, as described herein. The controller <NUM> and/or a valve positioner coupled to the respective field device <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> may control a pressure supplied to an actuator and measure a pressure within the actuator to determine an I/P bias and/or other suitable bias of the valve controller. The controller <NUM> and/or a valve positioner may also ramp a pressure within the actuator up or down to a pressure limit to test a travel of the actuator (e.g., a travel of a piston of the actuator). <FIG> illustrates such a controller and positioner in further detail.

In some implementations, one or more of the field devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> illustrated in <FIG>, may be valve positioners coupled to ESVs or other valve/actuator assemblies associated with a safety instrument system. In such cases, the ESVs or other valve/actuator assemblies may primarily be on/off devices characterized by loose connections, significant lost motion, high seal friction, and stick-slip dynamics. Such characteristics are further illustrated in <FIG>, and <FIG>.

Specifically, <FIG> is a plot of relative travel vs. time for a one hundred second scan of a pneumatic valve actuator during which the travel of the pneumatic valve actuator is controlled. As can be seen in <FIG>, the travel of the pneumatic valve actuator is not a smooth curve or line. Rather, as a function of time, the travel of the pneumatic valve actuator includes various moments of sticking, represented by the flat line segments in the travel curve of <FIG>, followed by moments of slipping of the pneumatic valve, represented by vertical line segments or steps in the travel curve of <FIG>.

<FIG> illustrates a corresponding plot of actual pressure (within the pneumatic valve actuator) vs. time for the same one hundred second scan that is described with reference to <FIG>. At each of the moments of slipping of the pneumatic valve actuator, the pressure within the actuator dramatically shifts up or down due to a sudden change in volume within the actuator. These stick and slip dynamics are further illustrated in <FIG> which includes a parametric plot of the travels and pressures illustrated in <FIG> (e.g., relative travel vs. actual pressure).

In some implementations, one or more of the field devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> illustrated in <FIG>, may include positioners or servo controllers coupled to and controlling valve/actuator assemblies other than ESVs. These other valve/actuator assemblies, such as compressor antisurge valves or vent valves, may primarily be configured for precision operations, such as throttling and control, in contrast to ESVs primarily configured as on/off devices.

Specifically, one or more of the field devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> illustrated in <FIG> may be positioners including spool valves. These spool valves included in the positioners may be characterized by a balanced design allowing the spool valves to move under extreme conditions, such as very high pressures. As such, positioners including spool valves may operate similarly at many different pressures, and positioners including spool valves may be calibrated and/or may perform tests at many different pressures or within a range of pressures.

In some cases, positioners including spool valves may utilize end-point pressure control techniques. In particular, when a controlled valve is seated at an end-point (e.g., fully open or fully closed), this type of positioner controls a pressure within or supplied to the controlled valve ("end-point pressure control") such that the controlled pressure is below or above a maximum or minimum pressure, respectively, that can be supplied to the controlled valve. In this manner, the positioner may more quickly unseat or move the controlled valve from the end-point as compared to scenarios in which the pressure is at the maximum or minimum pressure. Positioners described below may, in these cases, perform calibrations during end-point pressure control scenarios such that the positioners and corresponding controlled valves are calibrated while the controlled valves are in service (e.g., without disrupting a process).

One or more of the field devices <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> illustrated in <FIG> may also be positioners including a pneumatic relay, or poppet valve driven by a diaphragm assembly. This type of positioner may be characterized by an unbalanced design such that a bias of the positioner is dependent on the pressure supplied to or within a controlled valve. Positioners including pneumatic relays may perform calibrations (e.g., of an I/P bias) at a middle pressure value within a range of pressure values, such as a range of pressure values defined by a bench set, in an implementation.

Turning now to <FIG>, an example positioner <NUM> may control an actuator/valve assembly <NUM>, such as an actuator/valve assembly exhibiting behavior as illustrated in <FIG>, and <FIG> or another suitable actuator/valve assembly (a compressor antisurge valve, a vent vale, etc.). In some cases, the positioner <NUM> may be configured to perform partial stroke tests, or other tests, on the actuator/valve assembly <NUM>. To this end, the positioner <NUM> may be pneumatically and/or electrically coupled to the actuator/valve assembly <NUM> via a coupling <NUM> and communicatively coupled to a controller <NUM>.

In particular, the positioner <NUM> controls a pressure within or supplied to the actuator/valve assembly <NUM> based on signals (e.g., analog or digital) received from the controller <NUM> and/or based on control logic <NUM>. For example, the controller <NUM> may generate various signals (e.g., <NUM>-20mA signals) indicative of set point values or requests to perform calibrations and/or tests. Triggered by these signals from the controller <NUM>, the positioner <NUM> may generate a pneumatic output to control the actuator/valve assembly <NUM> based on the control logic <NUM> stored on one or more non-transitory memories <NUM> of the positioner <NUM>. The control logic <NUM> may implement at least a portion of one or more control loops and may be executed by one or more processors <NUM> of the positioner <NUM>. The positioner <NUM> may, in some implementations, generate an internal current signal based on a current signal received from the controller <NUM> (e.g., a <NUM>-20mA signal). This internal current may be supplied to a current to pressure converter (I/P) and spool valve/relay component <NUM> of the positioner <NUM>. Based on the internal current signal, the I/P and spool valve/relay component <NUM> may generate the output pressure supplied to the actuator/valve assembly <NUM> via the pneumatic coupling <NUM>.

Control loops implemented by the control logic <NUM> may receive feedback pressure and/or travel values from one or more sensors <NUM> in the positioner <NUM> and/or any number of other sensors coupled to the positioner <NUM> and/or to the actuator/valve assembly <NUM>. These sensors <NUM> may provide pressure values and/or travel values to the control logic <NUM>. Further details of example control loops that may, at least partially, be implemented by the example positioner <NUM> are described with reference to <FIG>.

The control logic <NUM> may include a calibration routine <NUM>. When executed by the one or more processors <NUM>, the calibration routine <NUM> may cause the positioner <NUM> to control a pressure in or supplied to the actuator/valve assembly <NUM>. In some cases, the calibration routine <NUM> may cause the positioner <NUM> to control a pressure in or supplied to the actuator/valve assembly <NUM> at constant volume, during end-point pressure control (in accordance with the claimed invention), or at a middle pressure value in a range of pressure value, or at any other suitable time or state of the actuator/valve assembly <NUM>. In particular, the calibration routine <NUM> may operate in conjunction with the control logic <NUM> (as depicted in <FIG>) or as a standalone routine to provide control signals to the I/P and spool valve/relay component <NUM>. These signals may cause the I/P and spool valve/relay component <NUM> to control a pressure in or supplied to the actuator/valve assembly <NUM> while the calibration routine <NUM> adjusts an I/P bias or other suitable bias of the positioner <NUM>. The calibration routine <NUM> may adjust the I/P bias by replacing a nominal bias with an adjusted bias such that the difference between the adjusted bias and the nominal bias is accounted for in future control of the actuator/valve assembly <NUM>. Further details of an example method for calibrating a positioner with pressure control, which example method may be at least partially implemented by the calibration routine <NUM>, are discussed with reference to <FIG>.

In some implementations, the I/P and spool valve/relay component <NUM> of the positioner <NUM> may feedback a position of a spool valve and/or pneumatic relay of the I/P and spool valve/relay component <NUM> to the control logic <NUM>. The control logic <NUM> may utilize such a feedback in a damping term of a control loop, for example. To utilize this feedback, the calibration routine <NUM> may determine and/or adjust a null state of the spool valve or pneumatic relay, or a "minor loop feedback bias," in addition to or instead of an I/P bias. For example, a travel of a pneumatic relay may be between <NUM>,<NUM> counts and <NUM>,<NUM> counts with a nominal operating point of <NUM>,<NUM> counts. The feedback to the control logic <NUM> in this example may be a normalized value dependent on the measured travel (e.g., in counts) of the pneumatic relay minus the nominal operating point. Such a feedback signal is zero around a null state of the pneumatic relay and goes positive or negative depending on the travel of the pneumatic relay, where the null state may be adjusted by the calibration routine <NUM>.

The control logic <NUM> may also include a partial stroke test routine <NUM>. When executed by the one or more processors <NUM>, the partial stroke test routine <NUM> may cause the actuator/valve assembly <NUM> to undergo a partial stroke test to test the operation of the actuator/valve assembly <NUM>. For example, the partial stroke test routine <NUM> may operate in conjunction with the control logic <NUM> (as depicted in <FIG>) or as a standalone routine to provide control signals to the I/P and spool valve/relay component <NUM>. These signals may cause the I/P and spool valve/relay component <NUM> to ramp a pressure within or supplied to the actuator/valve assembly <NUM> to cause a travel of the actuator/valve assembly <NUM>. Further details of an example method for performing a partial stroke test, which example method may be at least partially implemented by the partial stroke test routine <NUM>, are discussed with reference to <FIG>.

As discussed above, the controller <NUM> may trigger or otherwise cause the positioner <NUM> to initiate calibrations and/or to test (e.g., perform a PST), or the positioner <NUM> itself may initiate such calibrations or tests at periodic or otherwise determined times. Additionally, in some implementations, the positioner <NUM> or a separate device, module, or component operatively coupled to the positioner <NUM> may include one or more buttons, switches, control panels, touchscreens, or other interfaces allowing a human operator to manually initiate calibrations or test at the positioner <NUM> (e.g., by the pushing of buttons, entering of codes, etc.). In some cases, a human operator may also override previously initiated calibrations or PSTs (e.g., initiated by the controller <NUM>) so as to stop, cancel, or otherwise modify calibrations or PSTs in certain situations, such as emergency, testing, maintenance, or other situations.

Although <FIG> illustrates the processors <NUM>, the memories <NUM>, the control logic <NUM>, the calibration routine <NUM>, and the partial stroke test routine <NUM> as components of the positioner <NUM>, the controller <NUM> may alternatively, or additionally, include at least some components substantially similar to the processors <NUM>, the memories <NUM>, the control logic <NUM>, the calibration routine <NUM>, and the partial stroke test routine <NUM>. In fact, in some implementations, the controller <NUM> may implement all or most of the calibration and testing functionality discussed with reference to <FIG> and <FIG> to control pressures within the actuator/valve assembly <NUM> and/or to perform partial stroke tests on the actuator/valve assembly <NUM>. Generally, the functionality associated with controlling a pressure within the actuator/valve assembly <NUM> and/or performing partial stroke tests on the actuator/valve assembly <NUM> may be distributed in any suitable manner between the controller <NUM> and the positioner <NUM>.

<FIG> is a flow diagram of an example method <NUM> for calibrating a positioner, such as the positioner <NUM>, using pressure control techniques. Specifically, the example method <NUM> may be utilized to determine a bias, such as a current to pressure (I/P) bias or minor loop feedback bias, of the positioner <NUM>. For ease of discussion, the components of the example positioner <NUM> may be referenced in the description of the method <NUM>, but, gene, the method <NUM> may be utilized to calibrate any suitable device coupled to an actuator/valve assembly and may be implemented by any suitable combination of a controller and the device coupled to the actuator/valve assembly.

The positioner <NUM> may determine pressures corresponding to a particular state of the actuator/valve assembly <NUM>, such as one or more hard stops or endpoints (as in the claimed invention), travel stops, stationary positions, middle points of a range of pressures (e.g., defined in a bench set), etc. (block <NUM>). In some cases, a bench set of an actuator may define a pressure range (e.g., three psig to fifteen psig) that corresponds to <NUM>% to <NUM>% travel of the actuator. In such cases, the positioner <NUM> may determine a pressure just below a low end of the pressure range or just above a high end of the pressure range. For example, for a bench set of three psig to fifteen psig, the positioner <NUM> may determine a pressure between zero and three psig to maintain a fixed volume at the low pressure end of the bench set of a pressure between fifteen and twenty psig to maintain a fixed volume at the high end of the bench set. In other cases when a bench set is not known, the positioner <NUM> may determine a pressure based on pre-determined or approximated value. For example, the positioner <NUM> may determine a pressure of <NUM>+<NUM> = <NUM> psig to maintain a fixed volume at an estimated low end of actuator travel or a pressure of <NUM>-<NUM> = <NUM> psig to maintain a fixed volume at an estimated high end of actuator travel.

The positioner <NUM> may determine such pressure during a scenario in which end-point pressure control techniques are being utilized. For example, when the positioner <NUM> performs end-point pressure control as in the claimed invention to prevent a pressure within the actuator/valve assembly <NUM> from reaching a maximum or minimum possible value of the pressure. The positioner <NUM> may determine a pressure value slightly below a maximum pressure value or slightly above a minimum pressure value while the actuator/valve assembly <NUM> is seated at an end-point (e.g., fully open or fully closed).

In still other cases not part of the claimed invention, the positioner <NUM> may determine a pressure value at near the middle or at another relative position within a range of pressure values. For example, when the positioner <NUM> includes a pneumatic relay, the positioner <NUM> may determine a particular pressure somewhere in between pressure limits (e.g., defined by a bench set). The determined pressure may be a pressure value in the middle of the range (e.g., having the same absolute value of pressure difference between the middle value and both a high pressure limit and a low pressure limit). However, the positioner <NUM> may determine a pressure at any suitable position in the range of pressures, such as a ten percent relative pressure, twenty percent relative pressure, etc. The positioner <NUM> may even determine multiple pressure values in a range of pressures so as to determine multiple different bias values for a pneumatic relay.

Returning to <FIG>, the positioner <NUM> may control a pressure in the actuator/valve assembly <NUM> while maintaining the actuator/valve assembly <NUM> at the determined particular state of the actuator/valve assembly <NUM> (block <NUM>). That is, the positioner <NUM> may maintain a constant volume within the actuator/valve assembly <NUM> while a pressure within the actuator/valve assembly <NUM> is controlled, maintain a pressure within the actuator/valve assembly <NUM> near an end-point of the actuator/valve assembly <NUM> (e.g., during end-point pressure control), or maintain a pressure within the actuator/valve assembly <NUM> at a particular pressure value in a range of pressure values. For example, the positioner <NUM> may control a pressure at a constant volume by controlling the pressure within the actuator/valve assembly <NUM> according to a set point pressure value that is above or below the determined upper or lower pressure limit, respectively, determined at block <NUM>. Alternatively or additionally, the positioner <NUM> may control a pressure within a range of pressures or at a pressure value utilized during end-point control (e.g., while the actuator/valve assembly <NUM> is near an end-point, such as fully open) by controlling the pressure within the actuator/valve assembly <NUM> according to a set point pressure value that is in the pressure range (e.g., in the middle of the range) or below/above a maximum or minimum pressure, respectively, and determined at block <NUM>.

While the pressure is controlled at the particular state of the actuator/valve assembly <NUM>, the positioner <NUM> may adjust a bias of the positioner <NUM> based on the set point, a feedback of an actual pressure within the actuator/valve assembly <NUM>, and/or a nominal or default bias of the positioner <NUM> (block <NUM>). In some implementations, the positioner <NUM> may adjust a default or nominal bias (e.g., existing values stored in the positioner <NUM> or default values provided to the positioner <NUM>) until a measure of error in the pressure over time satisfies a convergence criterion (e.g., is at or below a threshold for a certain period of time). The measure of error may be at least partially based on a difference between the feedback of an actual pressure within the actuator/valve assembly <NUM> and the set point. The measure of error may, at least in some implementations, correspond to an integral term in a proportional-integral-derivative (PID) controller integrated into the control logic <NUM>.

The positioner <NUM> may update a bias of the positioner <NUM> (e.g., the nominal or default bias) according to the adjustments at block <NUM> (block <NUM>), or the positioner <NUM> may replace a default bias with an adjusted bias based on the adjustments at block <NUM>. This update and/or replacement may ensure that subsequent control of the actuator/valve assembly <NUM> accounts for a most recently adjusted bias of the positioner <NUM>. For example, before placing the positioner <NUM> in service, an operator or the control logic <NUM> may configure the positioner <NUM> with a default or nominal bias (e.g., by setting the travel set point to <NUM>% and turning on a travel integrator). Then, the positioner <NUM> may determine an adjusted measure of bias of the positioner <NUM> as described above, and the positioner <NUM> may update the default or nominal bias according to the adjusted measure of bias. Thus, the positioner <NUM> may refine a default bias or other currently used bias at suitable times and/or over time to compensate for changes in the bias due to temperature, wear, aging of components, etc..

<FIG> illustrates an example control loop <NUM> utilizing a measure of I/P bias, which measure of I/P bias may be generated according to the method <NUM>. The controller <NUM> and/or the positioner <NUM> may implement at least a portion of the control loop <NUM>, for example. Specifically, the positioner <NUM> may implement a portion <NUM> of the control loop <NUM>. In other implementations, the functionality of the portion <NUM> of the control loop <NUM> may be divided in any suitable manner between the positioner <NUM> and the controller <NUM>.

The positioner <NUM> receives pressure feedback values indicative of pressures within (not illustrated in <FIG>) or supplied to an actuator <NUM>. The positioner <NUM> may also generate a control signal (e.g., a <NUM>-<NUM>. 42mA control signal) indicative a pressure based on the pressure feedback values, a pressure set point (or "SP"), and various terms of the control loop <NUM> scheme. At least some of these various terms ("Ki/s," "K," etc.) may be added or otherwise combined with the pressure set point to generate the control signal, and, in particular, a measure of I/P bias may be added to a default bias to account for a bias of the positioner <NUM>.

Upon receiving the control signal, an I/P component <NUM> and a relay/spool valve component <NUM> of the positioner <NUM> may cause a pressure to be supplied to the actuator <NUM> to produce a travel. Because the positioner <NUM> accounts for the I/P bias of the positioner <NUM>, the positioner <NUM> may precisely control a pressure supplied to the actuator <NUM>, at least within pre-defined tolerances. Such precision may be of importance when controlling high gain actuator/valve assemblies, because small changes in pressure may result in large travels of the high gain actuator/valve assemblies, for example. This precision may also be of importance in other types of actuator/valve assemblies to calibrate actuator/valve assemblies, or devices such as positioners coupled to actuator/valve assemblies, while the actuator/valve assemblies are in service. Further, by adjusting a bias when a valve is at a hard stop (e.g., during end point pressure control scenarios), some implementations of positioners may adjust biases to account for temperature changes, wear, and aging of components without having to disturb a process (e.g., without having to shut down a particular line).

In some implementations, controllers, such as the controller <NUM> or the controller <NUM>, may trigger positioners, such as the positioner <NUM>, to test actuator/valve assemblies. These tests may ensure that the actuator/valve assemblies are able to function (e.g., that an actuator or piston is able to travel). Certain regulatory bodies may require such testing, for example. In particular, positioners, such as valve positioners coupled to ESVs, may perform partial stroke testing of actuator/valve assemblies to test an operation to open or close a valve without fully opening or closing the valve, so as to not disrupt a process.

When performing partial stroke testing, positioners and/or controllers may utilize pressure control techniques, as opposed to travel or position control techniques. In this manner, difficulties arising from loose connections, significant lost motion, high seal friction, and stick-slip dynamics may be substantially minimized (e.g., by reducing errors below a tolerance), for example, and/or tests may be performed even when travel control functionality of device is not operational or is malfunctioning. To illustrate these points and contrast the current pressure control techniques for partial stroke tests (PSTs), <FIG>, and <FIG> illustrate a partial stroke test of a pneumatic valve actuator using travel control techniques. Although a specific pneumatic actuator exhibiting certain characteristics is discussed with reference to <FIG>, and <FIG>, positioners may utilize pressure control to test any suitable types of valves, such as ESVs, compressor antisurge valves, vent valves, etc..

In particular, <FIG> illustrates a plot of relative travel vs. time for a twenty second scan of the pneumatic valve actuator, or a travel set point ramp of <NUM>%/second to <NUM>% displacement. The plot illustrates an initial transition of the pneumatic valve actuator off of a hard stop, where, during this initial transition, is unloaded from a supplied pressure to an high end pressure of a bench set (or upper bench set). These dynamics are further illustrated in the pressure vs. time graph illustrated in <FIG> (corresponding to the same twenty second scan). The pressure exhibits dramatic shifts during the initial transition of the pneumatic valve actuator off of the hard stop.

Upon examining a parametric plot of the pressure vs. relative travel for the twenty second scan, as illustrated in <FIG>, one can see a clear non-symmetric (e.g., varying in time) behavior of the pneumatic valve actuator during the PST. Bernoulli and/or choked flow effect around the pressure sensor may cause this example behavior. That is, high velocities within the actuator may distort reading of the pressure sensor such that the reading to not accurately reflect actual pressures within the actuator. Because certain alerts (e.g., alerts corresponding to stuck valves) may be triggered off of a pressure threshold, distortions in pressure reading may result in false alerts. Generally, this type of behavior and/or other types of behavior occurring during travel control (e.g., resulting from stick and slip dynamics) may complicate the PST and may cause the pressure within the pneumatic valve actuator to go open-loop (e.g., out of the control of the controller) in the event of a stuck valve.

In contrast to PSTs facilitated by travel control, the current techniques may utilize pressure control to perform PSTs. In particular, a positioner may cause a pressure in a actuator/valve assembly to ramp from an initial pressure towards a minimum or maximum pressure value. When the minimum or maximum pressure is reached or when a travel of the actuator/valve assembly is detected, the positioner may cause the pressure to ramp back towards the initial pressure. In this manner, the pressure within the actuator/valve assembly is always under control.

<FIG>, and <FIG> illustrate a scenario in which a PST is performed using pressure control techniques. <FIG> illustrate plots of relative travel vs. time and pressure (and pressure set point) vs. time, respectively, for a <NUM>%/second ramping of a pressure within a pneumatic valve actuator similar to the pneumatic valve actuator tested in <FIG>, and <FIG>. As can be seen in <FIG>, the relative travel of the pneumatic valve actuator remains nearly constant while the pressure in the pneumatic valve actuator is ramped towards a pressure limit (e.g., twenty pounds per square inch gauge (psig), as illustrated by the dotted line in <FIG>).

At a certain time (around sixty-five seconds), the pneumatic valve actuator travels, and, at this time, the ramping of the pressure may be reversed back towards the initial pressure (before reaching the pressure limit, in this scenario). During the PST, even at times when the travel of the pneumatic valve actuator remained near constant, the pressure within the pneumatic valve actuator is under control, as further illustrated in <FIG> by a symmetric and smooth pressure and travel response of the pneumatic valve actuator.

<FIG> is a flow diagram of an example method <NUM> for testing an actuator/valve assembly with pressure control techniques. The method <NUM> may be implemented by a suitable combination of the controllers <NUM> and <NUM> and the positioner <NUM>, for example. For ease of discussion, the components of the example positioner <NUM>, such as the partial stroke test routine <NUM>, may be referenced in the description of the method <NUM>, but, generally, the method <NUM> may be utilized by any suitable controller or positioner to test any suitable actuator/valve assembly.

The controller <NUM> and/or positioner <NUM> may execute the partial stroke test routine <NUM> to determine a pressure limit and/or travel limit (block <NUM>). The partial stroke test routine <NUM> may utilize the pressure and travel limits during a controlled ramping of a pressure within the actuator/valve assembly <NUM>. In some implementations, the partial stroke test routine <NUM> determines the pressure limit to be a pre-configured pressure value programmed, or otherwise configured, in the partial stroke test routine <NUM>. In other implementations, the partial stroke test routine <NUM> may retrieve the pressure limit from a data storage device (e.g., database) operatively connected to the controller <NUM>, or the partial stroke test routine <NUM> may even determine the pressure limit in near real-time (e.g., when executing to perform a partial stroke test) based on user input into the controller <NUM>, current or historical pressure and/or travel feedback values, etc..

The pressure limit (e.g., programmed as a parameter in the partial stroke test routine <NUM>) may define a pressure such that the actuator/valve assembly <NUM> is expected to move (e.g., based on prior bench tests) as the partial stroke test routine <NUM> ramps a pressure within the actuator/valve assembly <NUM> to the pressure limit. In some cases, the pressure limit defines a pressure such that the actuator/valve assembly <NUM> does not move past a maximum travel or relative travel (e.g., <NUM>%) when a pressure within the actuator/valve assembly <NUM> is ramped to the pressure limit. In this manner, the partial stroke test routine <NUM> may test the operation of the actuator/valve assembly <NUM> while preventing disruption of a process, which disruption may occur when the actuator/valve assembly <NUM> travels past the maximum travel.

The pressure limit may be an upper pressure limit or a lower pressure limit depending on the configuration of the actuator/valve assembly <NUM>. For example, if the actuator/valve assembly <NUM> is a normally open ESV, the positioner <NUM> may utilize a lower pressure limit, whereas the positioner <NUM> may utilize an upper pressure limit for a normally closed ESV.

The partial stroke test routine <NUM> ramps a pressure within the actuator/valve assembly <NUM> from an initial pressure within the actuator/valve assembly <NUM> towards the pressure limit (block <NUM>). For example, the partial stroke test routine <NUM> and/or other components of the control logic <NUM> may implement at least portions of a pressure control loop, such as one of the pressure control loops discussed with reference to <FIG> and <FIG>, to control the pressure to ramp towards the pressure limit.

The partial stroke test routine <NUM> may then determine if the actuator/valve assembly <NUM> has reached the travel limit (block <NUM>). For example, one or more sensors sensing travel of the actuator/valve assembly <NUM> may feedback data indicative of a travel or relative travel (e.g., percentage of total travel) to the positioner <NUM>. In some implementations, the positioner <NUM> may continue to ramp the pressure until a certain percentage of total travel of the actuator/valve assembly <NUM> is detected (e.g., a <NUM>% relative travel limit), whereas, in other implementations, the positioner <NUM> may continue to ramp the pressure until any amount (e.g., any finite amount) of travel of the actuator/valve assembly <NUM> is detected.

If the travel limit of the actuator/valve assembly <NUM> is reached, the flow may continue to block <NUM>, where the partial stroke test routine <NUM> may reverse the ramping of the pressure such that the pressure is ramped back towards the initial pressure. However, if no travel, or a relative travel less than the travel limit, is detected, the flow may continue to block <NUM>. At block <NUM>, the partial stroke test routine <NUM> may determine if the pressure limit has been reached. If the pressure limit has been reached, the flow may continue to block <NUM>, but, if the pressure limit has not been reached, the flow may revert to block <NUM> where the ramping of the pressure continues towards the pressure limit.

In some implementations, instead of simply reversing the ramping of the pressure upon a detection of travel or a detection of a relative amount of travel, the positioner <NUM> (e.g., the partial stroke test routine <NUM>) may control the pressure in the actuator/valve assembly <NUM> to: (i) step back towards the initial pressure by a finite amount, and (ii) then continue ramp back towards the initial pressure. That is, the positioner <NUM> may near instantaneously reinitialize the pressure before ramping the pressure back towards the initial pressure. In this manner, the positioner <NUM> may minimize further drifting of the actuator/valve assembly <NUM> past the detected travel or amount of relative travel.

<FIG>, and <FIG> include plots similar to those of <FIG> illustrating pressure (and pressure set point) vs. time, relative travel vs. time, and pressure vs. relative travel for a ramping of a pressure within a pneumatic valve actuator at a rate of <NUM>%/second. However, instead of simply reversing the ramping of pressure as illustrated in <FIG>, <FIG> illustrates a stepping of pressure upon detecting travel of the pneumatic valve actuator (at approximately sixty-five seconds) and a subsequent ramping of the pressure back to the initial pressure. By employing this stepping of the pressure, a positioner may prevent drifting of a pneumatic valve actuator past a maximum desired travel (illustrated by the dotted line in <FIG>).

Although <FIG>, and <FIG> include curves illustrating a stepping of a pressure upon detecting travel of a pneumatic valve actuator, controllers and/or positioner may step pressures at any suitable times during a partial stroke test. For example, the partial stroke test routine <NUM> may: (i) step a pressure at the beginning of a partial stroke test from an initial pressure to a pre-defined pressure, and (ii) then ramp from the pre-defined pressure towards a pressure limit. Such a procedure may allow more time efficient partial stroke tests. Generally, a stepping of pressure may occur at the beginning, towards the end, upon a detection of travel, and/or at any other point during a partial stroke test.

Moreover, a positioner may further reduce a drifting of a pneumatic valve actuator past a maximum travel or relative travel by employing dynamic ramp rates, as illustrated in <FIG>, and <FIG>. <FIG>, and <FIG> include plots similar to those of <FIG>, <FIG>, and <FIG> illustrating pressure (and pressure set point) vs. time, relative travel vs. time, and pressure vs. relative travel for a ramping of a pressure within a pneumatic valve actuator at a rate of <NUM>%/second. However, in contrast to <FIG>, <FIG>, and <FIG>, two ramp rates are utilized in the ramping of the pressure toward the pressure limit. For example, a ramping of the pressure may be slowed as the pressure approaches the pressure limit at one or more thresholds of pressure. As can be seen in <FIG>, such a dynamic ramping further prevent drifting a pneumatic valve actuator past a maximum desired travel or relative travel (illustrated by the dotted line in <FIG>). Although two ramp rates are utilized in the test depicted in <FIG>, and <FIG>, it is understood that any number of ramp rates may be utilized in ramping a pressure towards a pressure limit and/or in reversing the ramping of the pressure back towards an initial pressure.

<FIG> illustrates an example control loop <NUM> which may be utilized (e.g., by the positioner <NUM>) to perform partial stroke or other tests with pressure control techniques, as described further with reference to <FIG>. The controller <NUM> and/or the positioner <NUM> may implement at least a portion of the control loop <NUM>, for example. Specifically, the example positioner 200may implement a portion <NUM> of the control loop <NUM>. As with the control loop <NUM>, some implementations of the control loop <NUM> may include components of the control loop <NUM> (e.g., of the portion <NUM>) distributed in any suitable manner between a positioner and a controller, such as the controller <NUM>.

In the control loop <NUM>, the positioner <NUM> may receive pressure feedback values from an actuator <NUM>. However, in the control loop <NUM>, the positioner <NUM> may also receive travel feedback values from the actuator <NUM>. The positioner <NUM> may generate a control signal (e.g., 1mA nominal signal plus or minus <NUM>. 4mA) indicative a pressure based on an internal pressure control portion <NUM> of the control loop <NUM> nested within an outer travel control portion <NUM> of the control loop <NUM>.

Claim 1:
A method of calibrating a positioner (<NUM>), the method comprising:
determining a pressure value corresponding to a particular state of an actuator (<NUM>),
wherein the actuator (<NUM>) is controlled by the positioner (<NUM>), and wherein the particular state of the actuator (<NUM>) is an end-point corresponding to an end-position of the actuator;
controlling, with the positioner (<NUM>), a pressure within the actuator (<NUM>) according to a set point pressure, wherein the set point pressure is based on the pressure value such that the particular state of the actuator (<NUM>) is maintained;
receiving an actual pressure value measured within the actuator (<NUM>); and
determining a bias of the positioner (<NUM>) based on the measured pressure value and the set point pressure, wherein the bias of the positioner (<NUM>) is determined by controlling the pressure within the actuator (<NUM>) at constant volume during end-point pressure control.