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Mar
19

Control Using Wireless Devices – The Solution Part 2

In Solution Part 1 we examined that can be made in the PID reset to accommodate slow and/or non-periodic measurement update. To further enhance the response for continuous changes in setpoint the implementation of the PIDPlus may be modified as shown below.

This PIDPlus implementation is standard in DeltaV v12 and allows the reset calculation to automatically compensate for setpoint change and measurement update rate. When either PIDPlus implantation is used there is no need to modify tuning as sample rate changes i.e. reset is based strictly on the process response dynamics.
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 changes in the derivative calculation are shown below.

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.