To ensure proper process operating conditions, it may be necessary to maintain some measurable parameters such as vessel temperature, pressure, and level within certain operating limits (constraint limits). The implementation of override control is often the most effective way to maintain the process within its operating constraint limits. Tools are provided in most control systems that allow override control to be implemented. Override control may be applied to a process that is characterized by having one manipulated parameter, one controlled parameter, and one or more constraint outputs. The basis for override control involving one constraint output is illustrated below.
During normal process operating conditions, the controlled parameter is maintained at setpoint and the constraint variable is far from the constraint limit. The output of the PID associated with the controlled parameter is selected and passed to the analog output block. However, as
processing conditions change, the constraint parameter value may start to approach the constraint limit and at some point the output of the override PID will be selected.
To avoid windup in an override control strategy, the integral calculation of the PID blocks must take into account which PID output was selected by the selector block. The back calculation input provided by the control selector indicates when a PID was not selected. Also, the selected output value is reflected in the back calculation. This information is used to modify the PID integral calculation to avoid windup when the PID output is not selected. If the PID blocks support dynamic reset, the best process response may be achieved by enabling dynamic reset in the PID blocks used in an override control strategy. When dynamic reset is enabled, then the value of the selector output provided by the back calculation input is used in the integral
An override strategy involving one constraint variable can be implemented using a control selector block, two PID blocks and associated analog input and output blocks as illustrated below.
The control selector block defined by Foundation Fieldbus supports upstream and downstream back calculation connections. Numbered pairs of input and back calculation outputs of the control selector should be connected to the same PID. The control selector may be configured as a high or low selection.
Several override control application examples are detailed in chapter 13 of Control Loop Foundation – Batch and Continuous Processes. Also, the override control workshop included in that chapter provides several exercises that may be used to further explore override control. By accessing the book’s web site, you may complete this override control workshop using your web browser. The viewer below may be used to see the solution to this exercise.