Proportional Control Vs. On/Off Control

gerard · Jan 31, 2012

Hi,

I have a pizza press at work with heating elements going through the large metal platens that form the top and bottom press. The heating elements maintain the temp of the platens constant using on/off control. As a result of this the temp fluctuates quite a lot. So, if they are set for 160DegC, they can go +/- 8DegC. As a result of temp change we sometimes get sticking and ruining of product. Would proportional control tighten the tolerances and has anyone ever used PID control on inductive heating elements. Temp is monitored and controlled by Omron E5EK controller.

Thanks

netblindpaul · Jan 31, 2012

You can get simple adaptive PID loop Omron 1/4 DIN temperature controllers, replaced one old model for a new type & reprogrammed it to mimic the old one to the machine control recently.

gerard · Feb 1, 2012

Do I not have to get some sort of switching devices the will only allow a portion of the power to the heating elements? Some sort of chopper/pulse modulated control unit. At the moment the are turned on/off with large contractors - full current or nothing?

netblindpaul · Feb 1, 2012

You can use full current or nothing as long as you have feedback and you tune the on/off controller PID loop to the "heat sink" characteristics of the load.
Or, you can go full hog and switch over to a full closed loop PWM or other type controller.

Remember however, that if you mod a machine then you become responsible for it.

Wazzajnr · Mar 22, 2012

What i would suggest is changing to SSR (Solid state relays) Or rather adding the SSR's in series with the contactors. Then you can switch the SSR's very frequently without damage and the have a "policeman" thermocouple controlling the contactor for overtemp.

PM me if you need any advice, been working on furnaces for almost 10 years now!!

vaughant · Mar 26, 2012

We use something similar where I work to seal packing film on paper reams,it's called a "ladypack",French thing.It's all done thermostatically as you describe above through a little "pactuer" control unit and never fluctuates by a degree or more.Works well.