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Mar
05

Control Using Wireless Devices – The Challenge

There are significant difference in the frequency and manner in which anew measurement value is updated by a wired transmitter vs. a wireless transmitter. Thus, it is natural to question what impact this has whena wireless transmitter is used in closed loop control. Since most wireless transmitters are battery powered, it is desirable to minimize how often a measurement value is sensed and communicated to reduce transmitter power consumption. However, 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 DSC 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.

The conventional PID design (based on difference equation, z-transform) utilized in DSC controllers assumes that a new measurement value is available each execution and that control is executed on a periodic basis. When the measurement is not updated on a periodic basis, the calculated reset action may not be appropriate. If control is only executed when a new measurement is communicated, this could result in a delayed control response to setpoint changes and feedforward action on measured disturbances that occur between measurement updates. Also, as the PID execution period is increased the basic assumptions made in the PID design of the reset and derivative calculation may no longer be valid. For example, two common ways of implementing PI control are shown below.

In many cases the reset contribution of the PID is realized using a positive feedback network (top implementation) in which the time constant of the filter in this network defines the reset time in seconds per repeat. This approach is often taken since it supports the implementation of external reset for use in cascade and override application. When the reset is implemented as an integrator (bottom approach), then logic is used to avoid reset windup. However, using either approach, as the period of execution becomes significant compared to the process response time then the approximations in the reset implementation often breaks down and negatively impact control performance.
It may at first appear that there is no technical solution that minimizes how often a measurement is communicated without compromising control performance. In fact both requirements can be met using a combination of the communication capability of WirelessHART devices and a modification in the way the PID is implemented. The key to understanding how the PID must be modified is to realize that in the when the PID reset is implemented using a positive feedback network (top implementation above) 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.