Cascade Control
Cascade control nests a fast inner loop inside a slower outer loop so the inner loop rejects disturbances before they reach the primary variable.
Loops within loops
Cascade control uses two nested feedback loops. The outer, primary loop controls the variable that ultimately matters and its controller output becomes the setpoint for the inner, secondary loop. The inner loop controls an intermediate variable that responds quickly and sits between the disturbance and the primary output. A classic example is a temperature loop, slow, whose controller commands the setpoint of a faster flow loop that manipulates the valve.
Why it helps
The benefit is disturbance rejection. Disturbances that enter the inner loop, a supply pressure fluctuation, say, are sensed and corrected by the fast inner controller before they can propagate to the slow primary variable. Without the inner loop, such a disturbance would only be caught after it had already moved the primary output, far too late. The inner loop also linearizes and speeds up the effective process the outer loop sees, easing outer-loop tuning.
Design rules
For cascade to work the inner loop must be substantially faster than the outer, a factor of three to five in bandwidth is a common guideline, so the outer loop sees the inner loop as effectively instantaneous. Tuning proceeds inner loop first, with the outer loop open or in manual, then the outer loop. The inner measurement must lie on the path between a significant disturbance and the primary variable, otherwise the extra loop adds no value.
- Outer controller output sets the inner controller's setpoint
- Inner loop rejects disturbances before they reach the output
- Inner loop must be several times faster than the outer
- Tune inner loop first, then the outer
Both loops need anti-windup care, since the inner loop can saturate and the outer integrator can wind up against it. Bumpless transfer between manual and automatic keeps commissioning smooth.
For a design-stage plant with a slow primary variable subject to fast upstream disturbances, cascade control would catch those disturbances early, evaluated in simulation. It is one of the most effective and widely used process-control structures, often paired with ratio control.