Temperature controllers

What is a temperature controller?

Temperature controllers ensure that a process reaches the desired temperature and is then maintained at that temperature. Generally, these are used for closed-loop control where the temperature controller takes an input from a temperature probe (thermocouple, resistance thermometer, temperature transmitter) and compares that actual temperature with the programmed set point. It then adjusts its output signal, as required, to the relevant control element (electrical heater, cooling circuit, steam control valve, etc.). It can be a simple ON/OFF control (also known as 2-point, binary or bang-bang control), acting like a thermostat, or a variable output, where the output signal to the process is between 0 and 100%.

How does a temperature controller work?

For ON/OFF control, the heating circuit is switched on when the temperature is below the set point, and off above it. There could also be a cooling circuit, switched on above the set point and off below it. For variable control, it is often carried out by a PID controller (3-term controller). This controller uses a refined algorithm on the error (the difference between the set point and the measured value) which produces an output designed to achieve and maintain the set point with minimum overshoot and hold it as stable as possible.

What is a PID controller?

3-term or PID controllers (proportional-integral-derivative) can be used for proportional only (P), PI or PID control, depending on the process requirements. Proportional control regulates the output in proportion to the deviation from the set point. A proportional band is defined below and/or above the set point. Outside this band, the heating (below) or cooling (above) output is 100%. Inside the band it reduces linearly to 0% at the set point. As this can lead to a slow approach to the set point, the Integral term is then able to further adjust the output based on the rate-of-change of the error (achieving the set point quicker). This can lead to overshooting the set point, so the Derivative term estimates the future error and adjusts the output before overshoot occurs. It also works to quickly eliminate any deviations from the steady state. The three terms (PID) can be individually tuned (or even auto-tuned) to match the varied nature of different processes.

What are the most common applications of industrial temperature controllers?

Digital temperature controllers are widely used for displaying, controlling and monitoring temperatures. They are used in general industrial applications, in plastics technology, in ventilation and air-conditioning technology, in process engineering and machine building. PID controllers are recommended for control systems with frequent load changes and where close, automatic control is required, for example when the set point is constantly changing (e.g. for a programmed temperature profile). Common areas of application for PID temperature controllers are plant and industrial furnace construction, process engineering, plastics machinery and ventilation and air conditioning technology.