J
jeremy
6 minutes
Direct Acting vs Reverse Acting Controller
Every controller must know which way to push. That choice is called its action. A reverse acting controller lowers its output when the measured value rises above setpoint. A heater loop is reverse acting: too hot means less heat. A direct acting controller raises its output when the measured value rises. A cooling fan or a chiller is direct acting: too hot means more cooling. Some controllers name this in other words, or flip the sign of the error instead. Every controller shows this differently; check its manual. If the action is wrong, the loop pushes the wrong way. The measured value runs away from setpoint. The output slams to a limit and stays there. That can look like a dead sensor, a broken actuator, or a gain set far too high, so check the sign first. To test it, put the loop in manual. Move the output up by a small step and watch which way the measured value moves. If it rises, the loop needs reverse action. If it falls, it needs direct action. On a live process, set alarms and limits before the test. After this Concept you can test which way your process responds and set the controller action so the loop pulls toward setpoint.