What is On/Off Control

 5th Feb 2014

Q: What is On/Off Control

On off control is like operating a switch. This type of temperature controller will turn on the heat when the process variable is below the set point and turn it off when the process variable is above the set point. These controllers normally include a delay, hysterisis and or a cycle time to reduce the cycling or "hunting" when the process variable is close to the set point. In truth, they are closer to thermostats than controllers, because they do not balance heat loss with heat gain at the desired value.

On/ Off control is simple to understand, but forces a compromise between good control, long warm up times and increased cost. This compromise happens when you select the power rating of your heating  element.

If it has too much power. an on/off  temperature controller will struggle to provide tight temperature control because the heating element will store and deliver too much heat before the temperature reached the "off" point. As a result the temperature will overshoot. 

If the heating element is specified with enough heat without too much overshoot, then warm up times from cold will be lengthened, leading to longer batch times and less production.

In both cases manufacturing costs are increased.

Tighter temperature control and cost savings can be achieved by using PID or Fuzzy Logic control.

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