Glossary · Control theory and process control
Override control
Also known as: Constraint control, Limiting control
German: Begrenzungsregelung
In process control, override control is a structure in which a second controller takes over the manipulated variable from the normal controller when a constraint variable, such as a pressure or temperature limit, approaches its limit.
- Control theory
- Process industry
In one sentence
Override control lets a limiting controller take over the actuator from the normal controller when a process constraint nears its limit.
Example
A compressor flow controller normally drives the speed, but when discharge pressure nears its limit, a pressure controller overrides it through a low selector.
How it applies
- Engineering: Override control is usually built with min or max selectors between controller outputs. The inactive controller must track the selected output to avoid windup and allow smooth takeover.
- Operation: Operators must be able to see which controller is currently in charge; otherwise the process seems not to respond to setpoint changes.
- Safety: Override control keeps a process within operating limits. It is a basic process control function, not a Safety function. Protection against hazardous conditions needs independent safety instrumented functions or other protective measures.
- Documentation: Describe the normal and override controllers, the selector logic and the HMI indication. Explain in the operating manual why the normal setpoint may not be reached while an override is active.
Override control vs. safety shutdown
Override control limits and keeps running. A safety shutdown brings the process to a safe state. Keep the terms separate in all documents.