How Do Limit Controllers Work?

Unlike a proportional-integral-derivative controller (PID), limit controllers do not compare and regulate process variables, including temperature, level, flow, position, and current. Instead, these devices are considered on/off controllers that stop operations when process or machine temperatures are out of range. Watlow’s limit controllers can be used as a backup to other controllers when safety concerns may be present, including temperatures that are too high or too low.  For machinery, manufacturer’s typically preset the upper and lower temperature setpoint values. For process control, the programmable logic controller (PLC) programmer will input the upper and limits into the human-machine interface (HMI) or PLC. Whenever a preset condition is violated, the limit controller forces a system shutdown or can trigger another process, such as a cooling phase.