In cascade control, how are the two controllers arranged?

• A. A primary controller and a secondary controller where the secondary responds to the primary
• B. Two independent controllers
• C. A single controller with internal cascade
• D. A triple nested loop

Answer: (A)

Cascade control uses two controllers in series: the outer (primary) controller sets the overall goal and the inner (secondary) controller responds to that goal. The primary computes a setpoint for the inner loop based on the difference between the desired process outcome and what’s actually happening. The secondary then acts on the plant to drive the inner variable toward that setpoint, handling fast dynamics and disturbances quickly. This arrangement lets the outer loop manage slow, large-scale changes while the inner loop stabilizes rapid fluctuations, giving better disturbance rejection and overall stability. The other options describe different setups—two independent controllers, a single controller with internal cascade, or a triple nested loop—none of which match the standard cascade arrangement where the secondary responds directly to the primary’s output.