Many digital controllers used in the process industry today allow the end user to specify the control loop period of execution. However, books on process control often provide little or no information on how execution period impacts control performance. Also, no mention is made of the fact that the amount of filtering needed to avoid measurement aliasing is directly impacted by the execution period. Thus, I put together a workshop for Emerson Exchange 2010 on Guidelines for Setting Filtering and Module Execution Rate. In this workshop four simple rules are presented for setting module execution rate and filtering.
A false indication of a measurement is seen in the control loop if the frequencies making up the measurement signal are higher than ½ the control execution frequency. This phenomenon, known as aliasing, is illustrated in the example below.
When a measurement contains significant process noise then aliasing may be avoided by setting the transmitter damping based on the control execution frequency. Alternatively, in the DeltaV system, oftware filtering is provided at the IO card level that may be configured to remove frequencies above ½ the module execution frequency.
If you are interested in learning more about this topic, then the link below may be used to access the workshop presentation given on Tuesday, October 5. This presentation is around 45 minutes in length.