Cascade control
Two controllers in series, where the outer loop sets the setpoint of a faster inner loop.
What it means
In a cascade the outer (master, primary) controller does not drive the valve directly. It sets the setpoint of an inner (slave, secondary) controller, which does. The point is that the inner loop absorbs disturbances before they ever reach the outer one.
A cascade only works when the inner loop is substantially faster than the outer — several times, not marginally. If the inner loop is the slower of the two, the outer keeps moving a setpoint the inner has not reached yet and the two trade corrections. That produces a slow hunt visible in both loops which survives re-tuning either one in isolation.
Related
Where this comes up
Last reviewed 2026-08-01.