Electrical Engineering Essentials
4 minutes
Gain Scheduling Across Operating Points
Gain scheduling means keeping more than one set of gains and switching between them as conditions change. You need it when one tuning cannot cover the full range. A heater loop may be calm at low temperature and cycle near the top. A valve may be far more touchy near closed than near open. First, test the process at several operating points, such as low, middle and high load. Tune a set of gains at each one. Then choose a scheduling variable, a measured value that tells the controller where it is. Load, setpoint, speed, or valve position are common. Store the gains in a table. The controller picks the row that matches, or blends between two rows. Blending is smoother. Watch the boundaries. A sudden gain change can bump the output, so switch smoothly. If the scheduling variable sits near a boundary, it can flip back and forth. So add a little hysteresis, a gap between the switch-up and switch-down points. Pick a scheduling variable that changes slowly next to the loop. Check each region with a disturbance test, not just a setpoint step. A loop that cycles in only one region may also have a sticky valve there. If the process changes only a little, one robust tuning is simpler. After this Concept you can build a small gain table for a loop that behaves differently across its range.