Heater control

[WonderY]

Guys, as the topic suggests, how can I control 2 230VAC 250W heater within a range of 150 degrees to 230 degrees?

Regards

[support]

You need a sensor that can output between 0 to 5V that correspond to the range of temperature you want to measure and connect that to the T100MD+ or T100MX+ ADC input, then the PLC can periodically check the ADC to see if the current temperature has deviated from the set point value and turn ON/OFF the heater. This is the simplest scheme.

For more precise temperature control with , you may have to control the power output to the heater by using PWM output of the PLC to control a solid state relay so that you can vary the amount of power to be applied to the heater. You may also need to use PID control to obtain smoother temperature rise without too much overshoot.

[WonderY]

Hi. Read from example that feedback of signal is done by thermocouple with signal conditional. What is a signal conditional? How to determine the P, I and D value as stated in the example? I'm really confused as I'm new to the automation industry. Please help. Thanks.

[WonderY]

If a J thermocouple has two wire, one will go to A/D and where will the other go to? Analog output? If heater is AC and has two wire, it must be controlled using a solid state relay?

[evanh]

SSR is optional but is recommended if you decide to use PID control.  Relays and Contactors are also used for slower threshold control.

Thermocouple signals are measured in micro-Volts.  So, one function of a signal conditioner is as a voltage amplifier.  A second function of a signal conditioner is to perform differential to single ended signal conversion (Two wires IN, one wire OUT).  A third function of a signal conditioner is as a filter to smooth the ripples away and make it act similar to the changes in temperature of the body being measured.  A fourth function is electrical isolation which can be vital and certainly makes for a more rugged design.
A fifth, and very thermocouple specific, function is the "cold-junction compensation" - because the themocouple voltage is only the temperature difference between the two ends, one has to add on the temperature at the instrument end to find the absolute temperature at the measured end.

In short, you'd be advised to search for and purchase a thermocouple signal conditioner that produces a 0-5 volt or 0-10 volt analogue output.  This would suitable for connecting to any PLC.


Evan

[evanh]

PID isn't that hard to roughly tune by hand, can get very tedious though.

The Proportional component should be a primary component and set first.  Keep increasing the strength of this until it causes oscillation then back off a whole third.

The Intergral component becomes important for temperature control to achieve zero error with a steady state setpoint.  This is ment to be a time factor of the oscillation period but problem is the heating and cooling is not usually even so this might take many days or even months of fine tune.  Normally only adjust this after Prop has been set.  In some PID functions this will drastically modify the Prop action.    Sorry I'm not much help in defining this more clearly.

Forget about the Derivative component for temperature control.


Evan

[WonderY]

Thanks evanh. seems like quite a tough task to use the PLC to control temperature. Maybe will get some Fuji Electric controller and just hook up the thermocouple, SSR and heater. More direct right?

[WonderY]

BTW, is there anyway to use the PLC to supply a constant current to the heater to heat up? In other words, is it possible to set up an open system where the amount of current to the heater can be varied?