PID Control for Loops with Wireless Measurement and Final Control Elements

The ever-prolific Greg McMillan is at it again with an article, Wireless – Overcoming challenges of PID control & analyzer applications in the July/August edition of Intech. His article concludes:

Wireless and composition measurements offer a significant opportunity for optimizing process operation. A smart PID can dramatically improve the stability, reliability, and speed of response for wireless measurements and analyzers. The result is tighter control of the true process variables and longer battery and valve packing life.

He authored a companion in depth whitepaper to the article, DeltaV v11 PID Enhancements for Wireless. I’ll highlight some of the points Greg shares. The goal of the whitepaper is to describe:

…how enhancements to the PID block for wireless loops in DeltaV v11 improve performance, simplify tuning, and inherently protect against failures. The benefits are also applicable to valves with stick-slip and backlash, and analyzers with cycle times and sample systems used for closed loop control.

Greg first describes the operation of IEC 62591 WirelessHART communications:

The wireless update time termed “default update rate” (refresh time) is the time interval for periodic reporting. The wireless update sensitivity termed “trigger level” is the minimum change in measurement value for exception reporting. To save power, the transmitter goes to sleep and wakes up periodically to check if the change in sensor value from the last value transmitted is larger than the trigger level. When the change exceeds the trigger level or the time since the last communication exceeds the default update rate, then the value is transmitted. The time interval between periodic checks of the sensor is the “triggered update rate” (wakeup time). If only periodic reporting is available, this time interval is the default update rate. Increases in the default update rate and trigger level settings reduce the number of transmissions, which increases battery life.

Wireless communications are fast enough to perform PID control for slower processes with process response times in minutes to hours, such as composition, level and temperature control. Greg explains the problem with traditional PID algorithms with wireless measurements:

…the integral and derivative modes are computed each execution of the PID block. The PID algorithm uses the execution time in the integral and derivative mode calculations. Thus, the reset and derivative contribution calculated by traditional PID may not be appropriate when used with a wireless measurement where the default update rate is significant compared to the process response time. In such cases, the traditional PID will ramp the controller output through continual integral action even though the actual measurement has not changed. The PID is acting on old information. The detrimental effect of integral mode acting on old information is greatest if the process response has largely responded before the next update, which occurs when the process response is faster than the update time or the measurement value has not been updated because of device or communication failure.

He describes the changes made to the PID algorithm to accommodate the wireless inputs:

The PIDPlus computes the integral and derivative mode contributions to the controller output when there is a measurement update and uses the elapsed time between updates in its calculations. Thus, the PIDPlus only acts on new information and considers the observed change in the measurement to have occurred not in just the last PID execution time but over the elapsed time.

The use of a positive feedback network for the DeltaV PID reset calculation enables a further enhancement. If the reset time is set equal to the process time constant, which is the tuning method predominantly used within Emerson, the filter provides a model of the process’s exponential response for the integral mode response. The inclusion of the knowledge of the process response eliminates the need to detune the PID to suppress extensive oscillations when the update is much slower than the process response. For these cases, the controller gain can be set equal to the inverse of the controller gain to provide considerably tighter control.

Another consideration in the changes to the PID algorithm is the case of a temporary loss of communications from a wireless measurement or final control device. In this condition, the PID controller provides no additional reset action. Greg notes:

The PIDPlus waits for new information whereas the traditional PID output ramps to an output limit. When communication is restored, the PIDPlus acts on the new information only. The traditional PID sees the effect of the ramp to the output limit that occurred during the loss of communication or loss of valve travel.

Greg shows a number of tests comparing the control loop performance of the traditional PID and PIDPlus. These test cases include flow, static mixer pH, vessel pressure, extruder temperature, vessel pressure, and polymer line analyzer. He sums up his overall findings:

The DeltaV v11 PID enhancement PIDPlus can reduce the process variability from measurement sensitivity limits, valve stick-slip and backlash, and failures for all loops. For loops where the update time is much larger than the process response time, the PIDPlus can dramatically improve the setpoint response (reduce the overshoot, rise time, and settling time) and the load response (reduce the integrated absolute error). For these loops, the PIDPlus eliminates the need to retune the controller to eliminate process oscillations. For integrating processes, such as vessel pressure, the PIDPlus can significantly improve the load response (reduce the peak error and integrated absolute error) if the update time is not too large.

After our July hiatus, Greg and I will be resuming his demo/seminar (a.k.a. deminar) series next week. In these sessions, he demonstrates this control performance in action. We hope to see you at the next one:

PID Control of True Integrating Processes (How to Reduce the Batch Cycle Time for Temperature and pH Loops by 25%) – Aug 11, 10:00 am CDT / 15:00 UTC.

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Update: I’ve updated the post with the link to the whitepaper in the DeltaV whitepaper section of the Emerson Process Management website.

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