Name a common anti-windup technique used with PID controllers and describe its basic principle.

• A. Integrator clipping clamps the integral term at the actuator limit.
• B. Derivative kick uses the derivative to reduce windup.
• C. Proportional action alone is enough to prevent windup.
• D. Back-calculation uses the actuator limit to feed a correction to the integrator when saturation occurs.

Answer: (D)

Windup happens when the actuator hits its limit and the integral term keeps accumulating error, which can cause a large overshoot when the actuator comes out of saturation. A common anti-windup method is back-calculation: when the actuator is saturated, the actual output is limited, and the difference between the commanded control signal and the saturated actuator output is fed back into the integrator as a correction signal. This keeps the integrator from continuing to accumulate during saturation, so the integral term is effectively reduced or halted while the actuator cannot respond, leading to a quicker, smoother recovery once saturation ends.