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Apr
22

Capturing Process Dynamics

When the on-demand tuning application resides in a DCS workstation, communications between the controller and the workstation can introduce variable delay and jitter in the observed process response. As addressed in Chapter 4 of Advanced Control Foundation – Tools, Techniques, and Applications, the process may be more accurately identified by distributing the on-demand tuning functionality between the workstation and the controller as illustrated below.

Capturing process dynamics in the field (controller or device) instead of using test data transferred to the workstation eliminates any variation introduced by communications and thus allows better process identification, particularly for the fastest loops. For example, this is the way on-demand and adaptive tuning capability are structured in the DeltaV control system. When the process response is captured in this manner, the tuning rules calculation may reside in the workstation. Capturing the process response in a controller or field device requires only simple mathematical operations and is no more CPU resource-consuming than the control algorithm. Thus, switching over to the identification algorithm does not result in an overall load increase in the controller or field device.

Most on-demand tuning applications are designed to provide feedback indicating when process testing is in progress. Also, safety features are common and provide quick response to process conditions, such as too large a deviation of the controlled parameter from setpoint, which can indicate abnormal operating conditions, limit conditions, or significant process disturbance. These conditions will cause process identification to abort and normal control to resume.