Kinda OT : Heating and Cooling strategy

Hi
I was modifying some code and saw a strategy for heating and cooling that had me wondering.

BAsically the PID output (0-100.0%) is compared to a cyclic (10 seconds)

So, if the heating is called 20% at the PID control block, the heating will be called for 2 seconds out of 10 seconds. Like wise cooling will be called when the PID goes negative. There is a deadband of maybe +/-3%

Does anyone know the name for this type of control, and why anyone would use this rather than the PLC PID function directly

Sounds like...

http://en.wikipedia.org/wiki/Pulse-width_modulation

It's called a time proportioning output. Some also call it pulse width modulation, but usually on quicker time cycles.

It is used where your heating or cooling device is an on/off system, not a full modulating system. For instance, we have gas fired furnaces at our shop. The burners are setup to operate at full flow, no modulation. In order to control the heat in the furnace, the PID loop calculates an output that is fed to a 120 second time proportioned digital output. The thermal mass in the furnace keeps the temperature steady. This setup is often used where full modulating control would be more expensive than a simple on/off system and provide little or no benefit.

Edit: Looks like Mickey beat me to the punch.

Brian

I use this method for seal bar heating when temperature controllers are not called for in the project. I have had pretty good luck with it.

Heat/Cool is also called duplex control. It's used in food, chemical and pharma processing for jacketed vessels - heat supplied by steam or hot water, cooling by chilled water.

Time proportional provides straight line control rather than the inherent saw tooth response of on-off control.

Time proportional is also used extensively for electric heat temp control on things like sealing bars or extruder zone control.

Dan

thanks for the repies, interesting