In this article will be explain an overview of commonly used industrial types of control function, it is temperature control. For more specific this article will present three basic types of temperature mode control. Temperature control may be used to maintain a specified temperature within a process or ti protect against overtemperature conditions.
A temperature controller is used to accurately control process temperature without extensive operator involvement. The controller accepts a temperature sensor such as a thermocouple or RTD as input and compares the actual temperature to the desired control temperature, or set point, and provides an output to control element.
Thermocouple is the junction of two dissmilar metals and has a voltage output proportional to the difference in temperature between the hot junction and the lead wires (cold junction).
There are three basic types of temperature mode control, namely :
- ON/OFF Temperature Control
With ON/OFF Temperature Control, the output turns on when the temperature falls below the set point and turns off when the temperature reaches the set point. Control is simple, but overshoot and cycling about the set point can be disadvantageous in some processes. ON/OFF Temperature Control is usually used where a precise control is not necessary, in systems which cannot handle the energy being turned on and off frequently, where the mass of the system is great that temperatures change extremely slowly, or for temperature alarm.
- Proportional Temperature Control
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. This has the effect of slowing down the heater, so that it will not overshoot the set point but will approach the set point and maintain a stable temperature. Depending upon the process and the precision required, either a simple proportional control or one with PID may be required.
- PID Temperature Control
A PID, or three-mode controller, combines the proportional, integral (reset) and derivative (rate) actions, and is ussualy required for tight control of temperature-sensitive applications. The outputs turn on and off in proportion to temperature deviation from the set point. The rate function (derivative action) shortens the time it takes the temperature to stabilaze near the set point. The reset function (integral action) eliminates any offsets from the temperature set point.
– Petruzella, Frank D. Industrial Electronics. 1996. McGraw-Hill International Editions. pg. 283-284.
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