Glossary · Control theory and process control
Single-loop control
Also known as: Single-loop controller, SISO control
German: Einkreisregelung
In control engineering, single-loop control is a control structure with one measured variable, one controller and one manipulated variable, without nested or coordinated loops.
- Control theory
In one sentence
Single-loop control uses one measurement, one controller and one actuator; it is the simplest and most common control structure.
Example
A tank heater with one temperature sensor, one PI controller and one thyristor power controller is a single-loop control.
How it applies
- Engineering: Single-loop control is sufficient when disturbances are slow or small and when there's little interaction with other loops. Compact single-loop controllers are also sold as standalone devices with display and keys.
- Commissioning: Tuning is straightforward and can use standard rules or auto-tuning.
- Documentation: Single loops are documented in loop lists with tag, measured variable, manipulated variable, controller type and parameters. For standalone controllers, include the device's parameter list and keypad operation in the operating manual or reference the manufacturer's manual.
Single-loop control vs. cascade control
Cascade control adds an inner loop for faster disturbance rejection. If a single loop can't correct disturbances in time, cascade or feedforward is the next step before Multivariable control.