Wireless Device-Based Control Loops

ControlGlobal.com: Addressing Wireless Control ApplicationsControlGlobal.com has a great article, Addressing Control Applications Using Wireless Devices—How Wired and Wireless Transmitter Installations Differ in Closed-Loop Control Applications and What Changes Are Required in the PID, by Emerson’s Terry Blevins, Mark Nixon, and Marty Zielinski.

In the article, they describe how the IEC 62591 WirelessHART standard provides communications modes to support wireless devices used in control loops. They also highlight the changes required in a PID [proportional-integral-derivative] control algorithm to accommodate the non-periodic measurement updates. Conserving power is absolutely critical in battery-powered wireless devices, so these devices are designed to “sleep” and wakeup periodically to measure and transmit the measurements.

The communication modes in WirelessHART devices are defined in the HART 7 standard. The ones best suited for wireless control are Continuous and Window. The authors define the preferred Window mode communications:

…device wakes up at a configured update period, senses the measurement and then communicates the measurement if the specified trigger value is exceeded.

Window communication is preferred:

…since for the same update period window communications will always require less power than continuous communications. When window communication is selected, a new value will be communicated only if:

  • the magnitude of the difference between the new measurement value and the last communicated measurement value is greater than a specified trigger value;
  • or if the time since the last communication exceeds a maximum update period.

The standard PID algorithm was designed:

…to over-sample the measurement by a factor of 2x to 10x to avoid the restrictions of synchronizing the measurement value with the control.

They also note that the rule of thumb that feedback control should be 4 to 10 times faster than the process response time. The PID algorithm also assumes a new measurement is available each execution and control is performed on a periodic basis.

Since these assumptions are not the case with wireless devices, a modified PID algorithm, PIDPlus in the DeltaV system, was developed. For the integral portion of the PID algorithm, the time since a value was last communicated is part of the equation. The authors note:

PIDPlus tuning is based on the process dynamic (e.g. RESET= process time constant plus dead time); PIDPlus reset automatically compensates for variations in the measurement update rate. No change in PID tuning is required for varying update rate.

They highlight the change required in the derivative portion of the PID algorithm:

The derivative contribution is only updated when a new measurement becomes available. Also, the calculation is based on the elapsed time since a new value was communicated.

Equations and block diagrams of the changes in the PID algorithm are included in the article. Also, the authors provide a comparison of standard PID versus this wireless device-modified PID and show overlaid trends.

Give the article a read to see this comparison, some of the installations where the technology was applied, and conclusions based upon the field experience applying closed-loop control with wireless devices.

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