temperature controlled stove

antiwong · Jul 22, 2005

I are working on a project using a plc to control a electric heater for boiling purposes (boiling a pot of water). i am new to plcs and have a question to ask.
all off the shelf temperature controller has PID incorporated. Since we are using a PLC, can i use the temperature transmitter (which gives us the temperature of the liquid in a pot) and software PID (programmed in the PLC) to control the electric heater (analog output).

Thanks,
Anthony wong

RheinhardtP · Jul 22, 2005

What PLC

Yes, you can. What PLC are u using??

jstolaruk · Jul 22, 2005

RheinhardtP said:
Yes, you can. What PLC are u using??

And if that is its only function, wouldn't it be overkill to use a PLC?

RheinhardtP · Jul 22, 2005

Since we are using a PLC

Maybe automate the rest off the house in the future <<JOKE>>

No jstolaruk is right if this is the only function rather use the PID functionality on the controller.

JohnW · Jul 22, 2005

You can use a PLC, the minmum you need is a temperature input (either thermocouple or RTD) and a digital output. (For heating you don't need an analog output as you can use a pulse output to turn an electric heater on and off via a solid state relay).

Unregistered · Jul 22, 2005

Just think about it a minute , what temperature is the water in the pot , and can it get higher ?
If you don't need it , you can do away with the analogue altogether and just use a simple rising/falling temperature switch .

Since you can only do on off control ( realistically with your heating medium , you don't really need a PLC with a PID controller , you can make your own , particularly as know pretty much the heat requirement for the system will always be .

Andybr · Jul 22, 2005

I think that Unregistered was trying to point out that once the liquid in the pot is boiling all of the available energy is used as latent heat so the process is massively non linear and PID control would not be effective.

TIC · Jul 22, 2005

You got it - you can pour as much energy as you like in there , once it has boiled , it doesn't matter . Now to keep it simmering , that requires an adaptive PID loop , something that is capable of learning .

I have had the same task with steam jacket mixers , with all sorts of fancy valves etc , all to no avail , you can use simple on or off control and get it to work just as well as a PID in this case , with very little variation in temperature , for $1000 less.

antiwong · Jul 25, 2005

thanks to all for the replies. i am rather new at this and hence did not see the replies until today.

we are monitoring the temperature in the pot. not always boiling to 100 degrees C. the temperature gradient affects our process and hence we do need a feedback loop. the temperature we need is around 75 degrees C.

regarding the on/off control. i did think about using that but just wanted to know when pid is possible.

looking around for a cheap hardware for temperature measurement. asked around about temperature transmitters.. not too cheap. same price as the temperature controllers with on/off control.. still looking around..

once again.. thanks a lot for all your help!! much appreciated.

antiwong · Jul 25, 2005

missed out a few questions..

whether we are using this for just heating?

nope.. motion, weight monitoring, flow control, etc..

PLC brand?
omron..

BobB · Jul 25, 2005

If you are using the CJ1 or CS1 they have a 2 degree of freedom auto tune PID function. This is the same algorithm Omron use in their temperature controllers (advanced PID).

You just turn on a bit, the function auto tunes the PID loop, the bit turns off and all is well.

JohnW · Jul 25, 2005

A cheap but effective temperature controller is the WEST 2300. you need to use an RTD for temperature control at this level. You will get +/- 0.5 deg C accuracy as opposed to +/- 2 deg C with a thermocouple.

You can add an SSR module to west controller for your heating element.

Look here http://www.westinstruments.com

antiwong · Jul 29, 2005

i am actually using the cj series plc. we are now looking at the cost of the standalone controllers also. but we need the temperature reading and ability to adjust the temperature from the user interface. hence we need to interface with the standalone controllers. will check out the leads that were given. west is not commonly used here in singapore. will find out more.

temperature controlled stove

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