Chapter 6 Control Using Wireless Throttling Valves

Future Vision – WirelessHART Throttling Valves in Closed Loop Control  Based on the broad acceptance of wireless transmitters, manufacturers have developed and introduced wireless actuators for on/off valves. These devices are being used to implement closed loop discrete control.  In the future, it is envisioned that manufacturers will introduce wireless throttling valves that may be used with a wireless transmitter to implement closed loop control.

PIDPlus for Control with Wireless Transmitter and Valve  The PIDPlus features may be combined with the modifications for control using a wireless valve to address these different combinations of wired and wireless field devices.  The changes in the PIDPlus for use with a wireless valve and wireless transmitter are illustrated.  The use of the “implied or actual response indication ” as the input to a positive feedback filter enables the reset contribution to automatically compensate for delays in the positioner response

PIDPlus Structured to Minimize Changes in Target Valve Position  The PID may be modified to minimize the number of changes made in the target valve position when control is implemented using a wireless valve and a wireless transmitter.  To minimize the power consumed by the valve positioner, calculated PID output is transmitted to the wireless valve only if the criteria determined by non-periodic control communications have been met.  The PID is typically scheduled to execute much faster than the minimum period at which the target valve position may be communicated to the wireless valve.

Wireless Communication to the Valve  The key to applying non-periodic control communications is understanding that the PID reset calculation is implemented using a positive feedback network based on the implied valve position, which is communicated to the PID with minimal delay as the feedback in response to a change in the target position.  Ideally, this feedback of implied valve position (i.e., the target position that the valve accepted and is working to achieve) would be communicated by the wireless valve back to the wireless gateway in the response to the target position write request.

Example Control Implementation Using Wireless Valve  A new WirelessHART command has been proposed that supports the inclusion of a “time to apply” field with the output value communicated to a wireless valve.  This added field specifies a time in the future when the output value takes effect. The time to apply value is selected to ensure that the valve receives the output communication before this future time.  Thus, it is possible to calculate the implied valve position based on the target position communicated to the valve and the specified time when the valve takes action on the new target position.

 The command sent to the valve will contain the new target value and the time that the valve is to act on the new target position.  This time value will be based on the time at which the new target value was accepted plus the delay time configured by the user or set by the manufacturer.  The sequence in which the AO output is processed, communicated to the valve and then acted on by the valve is shown in this example

Control Structure Used in Field Trial  Field trails were conducted to evaluate a wireless positioner for a throttling valve  In these field trails the functionality associated with determining the target to minimize valve movement and calculation of the external reset input used in the PID was done in the control module.

Test Module – Control Using a Wireless Valve  As part of the applied research into control using a wireless valve positioner that was conducted by Emerson Process Management in the spring of 2014, a module was created that allows control using a wireless valve to be tested in a simulation environment.  Composites within the module are used to simulate the Controller Output Processing (using the new HART command), the communications and delay associated with the wireless gateway, and the wireless valve. In addition, a composite is provided to simulate the dynamic process.

 In the first two tests, the control performance was evaluated using a wired transmitter and a wireless valve vs a wired transmitter and a wired valve. Identical changes in setpoint and unmeasured disturbances were introduced into both control loops during the tests.  In the first test, the wireless valve communication to the valve was set to 3 seconds and the delay in the PID seeing the valve response was set to 3 seconds in the simulation of wireless communication.  In the second test, the delay to the valve and the valve response were set in the simulation to 6 seconds.

Performance Using a Wireless Valve with a Wired Transmitter  During the test, statistics on the number of communications, valve movement and IAE were captured in the module by the PERFORMANCE composite.  Stable control was observed for changes in setpoint and load disturbances using the wireless valve. Through the use of valve minimization the number of changes in valve target was reduced by a factor of 23.

PID Control Using Wired Valve and Transmitter vs Wireless Valve and Transmitter  The tests were repeated using a wireless transmitter with the wireless valve.  The transmitter used window communications mode where the period was 6 seconds, default report time was 12 seconds and deadband in reporting was 3%.

Performance Using a Wireless Valve with a Wireless Transmitter  The results achieved for wireless control using PIDPlus with the modifications for the wireless transmitter and valve vs a wired transmitter and valve using PID are summarized in this table.  Stable control was observed for changes in setpoint and load disturbances using the wireless transmitter and valve. Through the use of minimization of valve movement, the number of changes in valve target was reduced by a factor of 23.

Flow Lab Where Wireless Control Was Tested  A prototype wireless valve was tested in one of Fisher Controls’ flow labs located in Marshalltown, Iowa using a DeltaV control system and its embedded PIDPlus algorithm.  In these tests, closed loop flow control was evaluated using both wireless and wired flow measurement.  Communications with the wireless valve used a new HART command that allows a time to apply to be specified.  The PIDPlus external reset input was modified to allow delay to be used optionally to compensate for the time to apply. In addition, a new technique for minimizing valve movement was evaluated using a wired and wireless input to the valve.

Field Trial Summary The test results can be summarized as follows:  PID tuning was set strictly based on the process gain and dynamics. The fact that the tuning was never changed throughout the wireless test illustrates that the PIDPlus tuning is not impacted by transmitter and valve update rate and delay introduced by communications. Good control was achieved in all wireless valve and wireless transmitter tests using this tuning.  Using a wired transmitter and valve and then applying valve minimization reduced the number of changes in valve position by a factor of 70 for 0.1 second loop execution and cut total valve travel by over 50%. Introduction of valve minimization had no impact on loop stability and had minimal impact on control performance – less than 50% increase in IAE.  The wireless transmitter update rate was set to 8 seconds for most of the tests and introduced 4–10 seconds variable delay in the flow measurement used in control. However, this had no impact on the stability of PIDPlus control and had minimal impact on control performance  When a wireless transmitter was used with PIDPlus, the number of changes in valve position was reduced by a factor of 47 since the output of the PIDPlus only changes when a new measurement is received or the setpoint is changed.  Changing the wireless transmitter update rate from 8 seconds to 16 seconds had minimal impact on control performance – increasing IAE approximately 60% for setpoint changes.

Field Test of Wireless Control

Control Module for Wireless Field Trial  Modules created for the field test allow the selection of a wireless valve and/or wireless transmitter in a test run and the selection of a modified DeltaV wireless interface to the Rosemount 1420 or the standard output cards to be used in control.  The apply delay may be optionally selected to compensate for the delay in the time the target valve position is acted on when control uses wireless communication to the device.

Time to Apply Arrival During Test  The time to apply was set to 8 seconds in all wireless valve tests. The chart in Figure 6-15 shows a log of normalized Time to Apply as published by the modified 4320 over the course of testing.  It shows when the command was received by the device in relation to when the new setpoint would be applied via the Time to Apply variable.  Most commands were received before the Time to Apply but a few arrived late. It shows that the command had a range of unpredictable arrival times.

Setpoint Change Response for Wired Transmitter and Valve  The response to setpoint changes using a wired transmitter and wired valve in control is shown.

Response to Unmeasured Disturbance for Wired Transmitter and Valve  The response to an unmeasured disturbance using a wired transmitter and wired valve in control is shown.

Setpoint Change, Valve Movement Minimized, Wired Transmitter and Wired Valve  The response to setpoint changes when valve movement is minimized using a wired transmitter and wired valve in control is shown.

Disturbance Response, Valve Movement Minimized, Wired Transmitter and Wired Valve  The response to an unmeasured disturbance when valve movement is minimized using a wired transmitter and wired valve in control is shown

Setpoint Change, Wired Transmitter and Wireless Valve  The response to setpoint changes using a wired transmitter and wireless valve in control is shown.

Disturbance Response, Wired Transmitter and Wireless Valve  The response to an unmeasured disturbance using a wired transmitter and wireless valve in control is shown

Setpoint Change Response, Wireless Transmitter and Wired Valve  The response to setpoint changes using a wireless transmitter and wired valve in control is shown

Disturbance Change Response, Wireless Transmitter and Wired Valve  The response to an unmeasured disturbance using a wireless transmitter and wired valve in control is shown

Setpoint Change Response, Wireless Transmitter and Wireless Valve  The response to setpoint changes using a wireless transmitter and wireless valve in control is shown

Disturbance Change Response, Wireless Transmitter and Wireless Valve  The response to an unmeasured disturbance using a wireless transmitter and wireless valve in control is shown

Response to Setpoint Change  The response to setpoint changes using a wireless transmitter and wireless valve in control with minimization of valve movement enabled is shown

Response to Unmeasured Process Disturbance  The response to an unmeasured disturbance using a wireless transmitter and wireless valve in control with minimization of valve movement enabled is shown

Setpoint Change Response  The response to setpoint changes using a wireless transmitter with a reporting rate of 16 seconds and a wired valve in control is shown

Setpoint Change Response, Wireless Transmitter at 16 sec, Wireless Valve, Two-Hop Network  The response to setpoint changes using a wireless transmitter with a reporting rate of 16 seconds and a wireless valve in control is shown

Exercise: Control Using Wireless Throttling Valves This workshop provides several exercises that can be used to further explore the control using a wireless measurement and wireless valve.  Open the module that will be used in this workshop and observe the control and simulated processes.  Step 2: Initialize the Performance Index (IAE) and then change the SP parameter of both control loops by 10%. Observe the control response using a plot of the setpoint, control measurements and output.  Step 3: Note the IAE and the number of communications for the wireless and wired control. A significant difference should be seen in the number of communications for wired vs wireless control that were required to respond to the setpoint change.  Step 4: Initialize the Performance Index and change the Disturbance input from zero to 10. Observe the response of the PID and PIDPlus to this unmeasured process disturbance.  Step 5: Note the IAE and the number of communications for the wireless and wired control. A significant difference should be observed in the number of communications for wired vs wireless control that were required to respond to the unmeasured process disturbance.

Process: Control Using Wireless Throttling Valves A simulation of two identical flow processes is used to compare the control performance of PIDPlus using a wireless transmitter and wireless valve to PID using a wired transmitter and wired valve.