EXCEL CARE CONTROL ICONS ALPHABETIC REFERENCE197 74-5577–33 (US)EN2B-0184 GE51 R0518 (Europe)PID OperationClosed Control Loops A PID is generally part of a closed control loop. Control is possible only in a closedcontrol loop because the controller requires feedback from the controlled sectionwith respect to changes in the positioning signal. The following diagram illustratesclosed control loop operation:“Disturbance” is the changeable quantity that influences the controller, for example,an outdoor air temperature.“Controlled section” refers to the part of the system where the controlled variableshould be kept constant (for example, a heating circuit).“Actuator” is the device in the system that the positioning signal repositions tomaintain the setpoint. An example of such a device is a mixing damper.Continuous Controllers The PID in CARE applies to continuous controllers only. Continuous controllers canassume any desired intermediate value between a minimum (yMIN) and a maximum(yMAX) for its positioning signal.Reverse vs Direct Acting Controllers can also be classified as direct-acting and reverse-acting.The output of a direct-acting controller goes lower as the sensed value becomessmaller.The output of a reverse-acting controller goes higher when the sensed value goessmaller.The CARE PID operator is reverse-acting.
