Integral Windup / Anti-windup

An actuator pinned at its limit while the integral term keeps accumulating, causing overshoot; anti-windup keeps that term in check during saturation.

Integral windup occurs in controllers with an integral term (PI, PID). Actuators have physical limits — motor torque and current can't be infinite. When the error is large, say from a sudden setpoint change or a joint blocked by an obstacle, the controller wants an output beyond that limit, but the actual output is capped, so the error shrinks slowly while the integral term K_i·∫e dt keeps accumulating. Once the error flips sign, the integral term takes a long time to unwind, producing large overshoot and prolonged oscillation. Common anti-windup fixes: clamping, capping the integral term within bounds; conditional integration, pausing accumulation while the output is saturated; and back-calculation, feeding the difference between the saturated and unsaturated output back into the integrator with a gain, so it tracks what's actually happening. Both the velocity and current loops of a robot joint need to handle this.

ExampleAn arm's joint is held by a person's hand; the velocity loop's output sits pinned at the torque limit while the integral term keeps building up, and the joint lurches violently past its target the instant it's released. Adding clamping or back-calculation anti-windup to the integrator lets the joint settle smoothly back onto its commanded trajectory instead.

Also called

Integrator Windup, Reset Windup

Related

Proportional-Integral-Derivative Control · Torque Limiting (Saturation) · Step Response Metrics (Overshoot / Settling Time / Steady-State Error) · Cascade Control · Active Disturbance Rejection Control

Sources

Wikipedia: Integral windup