J
jeremy
7 minutes
Output Deadband and Hysteresis
A deadband is a small zone around setpoint where the controller leaves the output alone. If the error is smaller than the band, the PID treats it as zero. Hysteresis is the on/off version. A heater turns on when the reading falls a little below setpoint and stays on until it rises a little above it. The gap between the two switch points is the hysteresis. Both stop chatter, which is fast back-and-forth action on the actuator. Chatter wears out relays, contactors and valves, and you can often hear it. The cost is that the loop no longer holds setpoint exactly. It can rest anywhere inside the band, and an on/off loop will swing across the whole gap. A wider band means less wear and a looser hold. Before you add a band, ask why the output chatters. Noise on the sensor and too much derivative both cause chatter, and a filter fixes those better. A cycle with a square or saw-tooth shape may be a sticky valve, which a band can hide but not repair. Use a band when the chatter is from small, real errors the process does not care about. Set the band at or just above the size of the noise you can live with. After this Concept you can set a deadband or hysteresis band, read the offset it allows on a trend, and tell when it fixes chatter and when it covers a fault.