In cascade control, the primary loop controls the setpoint of which loop?

• A. The feedforward loop
• B. The secondary loop
• C. The primary loop
• D. The external loop

Answer: (B)

In cascade control, you have two loops working in series: the outer (primary) loop and the inner (secondary) loop. The essential setup is that the controller in the outer loop outputs a setpoint for the inner loop. The inner loop then uses its own feedback to drive its manipulated variable so that its process variable tracks that setpoint. This arrangement lets the fast, inner loop quickly counter disturbances, while the slower outer loop handles broader changes.

So, the primary loop controls the setpoint of the inner loop. This is what makes cascade control effective: the outer loop shapes the target for the inner loop, and the inner loop keeps its variable tightly around that target with rapid response.