Glossary · Control theory and process control
Cascade control
Also known as: Cascade loop
German: Kaskadenregelung
In control engineering, cascade control is a structure of two or more nested control loops in which the output of the outer (primary) controller is the setpoint of the inner (secondary) controller, so disturbances in the inner loop are corrected before they affect the primary variable.
- Control theory
- Process industry
In one sentence
Cascade control nests loops: the outer controller sets the setpoint of a faster inner loop that corrects disturbances early.
Example
A reactor temperature controller sets the setpoint of a jacket temperature controller, which in turn drives the steam valve.
How it applies
- Engineering: The inner loop must be significantly faster than the outer loop. Drives use a classic cascade of current, speed and position loops. Tune from the inside out: inner loop first, then the outer loop with the inner loop in automatic.
- Operation: Operators need to see both loops and their modes. If the inner loop is in manual, the outer loop is effectively open.
- Documentation: Show the cascade structure in the control narrative or a block diagram, name which loop is primary and secondary, and describe the switching order for startup and shutdown. Consistent names for setpoints (local vs. remote setpoint) avoid misunderstandings in operator training.
Cascade control vs. single-loop control
Single-loop control measures only the primary variable and reacts once the disturbance has affected it. Cascade control adds a secondary measurement closer to the disturbance, at the cost of an extra sensor and more tuning effort.