A temperature controller is a device used to hold a desired temperature at a specified value. There are three basic types of controllers: on-off, proportional and PID. Depending upon the system to be controlled, the operator will be able to use one type or another to control the process.
An 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. On-off control is usually used where a precise control is not necessary in system.
Proportional controls are designed to eliminate the cycling associated with on-off control. A proportional controller decreases the average power being supplied to the heater as the temperature approaches set point. The proportioning action occurs within a “proportional band” around the set point temperature. Outside this band, the controller functions as an on-off unit, with the output either fully on (below the band) or fully off (above the band).
PID (Proportional with Integral and Derivative)
The third controller type provides proportional with integral and derivative control, or PID. This controller combines proportional control with two additional adjustments, which helps the unit automatically compensate for changes in the system. These adjustments, integral and derivative, are expressed in time-based units; they are also referred to by their reciprocals, RESET and RATE, respectively.