The Story behind Wireless Device Closed PID Loop Control

Emerson’s Terry Blevins has a series of ModelingAndControl.com posts going on the subject of control with IEC 62591 wireless field devices. Posts to date include:

In the initial Part 1 post, Terry notes how the WirelessHART standard was designed for wireless devices to be used in control applications:

From the very beginning, the WirelessHART design included the features required for both monitoring and control applications. When WirelessHART field devices are used in closed loop control, the PID must be modified to utilize the slow periodic or non-periodic exception reporting of measurement values. The PID in DeltaV v11 (released last year), includes the PIDPlus option that may be selected when using a WirelessHART transmitter in closed loop control.

Terry relayed the interest of the NAMUR process manufacturing automation user community in wireless control:

Earlier this year, the Namur Working Group 4.15 [Wireless Automation] invited Emerson and four other major manufactures of distributed control systems and wireless transmitters to present their activities and R&D work concerning closed loop real time control applications based on wireless technology. In this meeting with Working Group 4.15, each vendor was given one hour for their presentation and questions and answer. During each vendor’s presentation, only members of Working Group 4.15 were present. Namur requested that the presentations avoid general information and to concentrate on wireless control R+D activities and examples.

In the post Control Using Wireless Devices – The Challenge, Terry notes the differences in the way wired and wireless transmitters report values. Terry note:

…most multi-loop controller used in DCS systems today are designed to over-sample the measurement by a factor of 2-10X to avoid the restrictions of synchronizing the measurement value with the control. Also, to minimize control variation, the typical rule of thumb is that feedback control should be executed 4X to 10X times faster than the process response time which we will define as the process time constant plus process delay. Also, the conventional PID design utilized in DCS controllers assumes that a new measurement value is available each execution and that control is executed on a periodic basis. The measurement update and control execution that are typically assumed in a traditional control application using wired transmitters is illustrated below.

He concludes:

The key to understanding how the PID [for wireless device-based control] must be modified is to realize that in the when the PID reset is implemented using a positive feedback network… that the filter time constant is a direct reflection of the process dynamic response. For example, when the Lambda PID tuning rules are used, then the reset (filter time constant) is set equal to the process time constant plus the process deadtime.

In the next Solution Part 1 post, Terry shares how the controller PID algorithm is modified:

Control execution is set much faster than measurement update. This permits immediate action on setpoint change and update in faceplate. PIDPlus tuning is based on the process dynamic (e.g. RESET= process time constant plus deadtime); PIDPlus reset automatically compensates for variations in the measurement update rate. No change in PID tuning is required for varying update rate.

In the Solution Part 2 post, Terry notes others changes made to the PID control algorithm:

The derivative component of the PIDPlus is also modified to account for the fact that a new measurement value is not available each execution of the PID… The derivative contribution is only updated when a new measurement become available. Also, the calculation is based on the elapse time since a new value was communicated.

If you’d like a deeper understanding of how the IEC 62591 WirelessHART technology is designed for close loop control and how PID controllers can work with the differences in communication with wire devices, visit Terry’s series of posts.

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