Tuning a PID Temperature Control Loop

… more less A heater loop reads a temperature sensor and drives a heater, often with a PWM output. Such loops are slow, with real dead time while heat soaks through the parts. They are one-way, since the heater can add heat but only the room takes it away. So the loop heats fast and cools slowly, and gains that look right going up can crawl coming down. Start with a step test at the working temperature, not from cold. Heat loss grows as the part gets hotter, so the process gain and time constant change with temperature. Use the dead time to time constant ratio to judge how hard the loop is, then apply the tuning method you trust. On warm-up the output sits at 100 percent for a long time. Without anti-windup the integral piles up, and the reading sails past setpoint before it comes back. That big first overshoot is windup, not a gain set too high. Turn on anti-windup, and use setpoint weighting or a setpoint ramp if a step still overshoots. Small derivative can help, but filter it, since a noisy sensor makes D chatter. A slow, steady swing can mean too much integral, a sensor placed far from the heater, or a drafty room. Keep a hard over-temperature cutoff in place before any test. After this Concept you can step test a heater at its working point, tune it with anti-windup on, and tell warm-up windup from a gain problem.