Process Controller

ON/OFF CONTROLLER IS THE SIMPLEST FORM OF TEMPERATURE CONTROL DEVICE. THE OUTPUT FROM THE DEVICE IS EITHER ON OR OFF, WITH NO MIDDLE STATE. AN ON-OFF CONTROLLER WILL SWITCH THE OUTPUT ONLY WHEN THE TEMPERATURE CROSSES THE SET POINT. FOR HEATING CONTROL, THE OUTPUT IS ON WHEN THE TEMPERATURE IS BELOW THE SET POINT, AND OFF ABOVE SET POINT. SINCE THE TEMPERATURE CROSSES THE SET POINT TO CHANGE THE OUTPUT STAGE, THE PROCESS TEMPERATURE WILL BE CYCLING CONTINUALLY, GOING FROM BELOW SET POINT TO ABOVE, AND BACK BELOW. IN CASES WHERE THIS CYCLING OCCURS RAPIDLY, AND TO PREVENT CONTACTORS AND VALVES FROM GETTING DAMAGED, AN ON-OFF DIFFERENTIAL, OR “HYSTERESIS,” IS ADDED TO THE CONTROLLER OPERATIONS. ON-OFF HYSTERESIS PREVENTS THE OUTPUT FROM “CHATTERING” OR MAKING FAST, CONTINUAL SWITCHES IF THE CYCLING ABOVE AND BELOW THE SET POINT OCCURS VERY RAPIDLY.