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Heater Control trough solid state relay using PWM

Hello,

At our university lab we somebody needs to build a gas phase chemoreactor. for this purpose he bought a cFP-2120 with a Relay module, a thermocouple module and last but nog least a DO-403 sinking output module.

One of the most important things the cFP needs to do is control and the temperature of 5 separate saturation units; These are basically just 1 litre metal vessels filled with compounds such as ethanol, hexane and various other solvents. Temperature of these saturators needs to be fairly precise becaus this is what determines compound concentrations in the reactor feed.

Previously we used dedicated PID controllers like the ones from eurotherm or watlow for control of these saturators.

Now I am planning on using the labview PID addon for handling the temperature control and this is where my problem starts; The heating device is a standard resistor wire in a metal casing, connected to the 220V AC mains. This heater can be switched on or of using a solid state relay which takes 24V DC input. Because of this I wil need my PID output to control a pulse width modulation type process in order to achieve proper control of my system.

I noticed that many examples regarding PWM (like the one attached) seem to be based on built in counter support on various DAQ devices sold by NI. NI also sels dedicated PWM modules or counters for FieldPoint. However, we currently do not have any more funds left to actually buy one of these.

I am therefore looking for a way to achieve the same result using only our DO-403 module to drive the solid state relay. The only thing I cannot figure out is how I can reliably generate some sort of pulse train to achieve my PWM. Is this at all possible or does an application like this really need a dedicated hardware component to generate this sort of signal? (Is there a risk of the fieldpoint falling over itself if I use something like the signal generation VI's?)

Any help would be greatly appreciated!!
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With the risk of sounding a bit impatient but; Is there anybody who could help me out? I need a solution quite urgently...


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Hey KrisJa,

It's possible to attain a similar output like the PWM modules. You will need to calculate how long you want your digital output to be high based on the % output of the PID.
Now, downside is that this timing is software timed and thus by far not as accurate as hardware timing.
That will not really be an issue if you keep the following in mind.

You are opening and closing solid state relays. Those have limited lifespan of opening/closing.
PWM steering is really not an option when speeds are high. In this case the relay will not be able to keep up the opening/closing signals of your digital output and the relay will be broken in no time. Lifespan and opening/closing times of the relay can be found in your hardware specs. Whether it's beneficial for you to use this type of controlling will depend on the response time of the system that you wish to control.

Kind regards,
Wouter Van Hoof
National Instruments Belgium
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Thank you for the reply.

I currently finished a first version of a possible implementation for my heater control VI.

Regarding response and liftime of the relays;

http://www.crydom.com/userResources/productFamilies/24/crydom_S1.pdf

This is the type of relay we plan to use. As I understand it is safe to assume the response time for switching to be less then 50 ms worst case in any scenario. This is probably more than adequate for my system.

Am I also correct if I assume (seeing that SSR's don't have moving parts) that this type of relay is also not susceptible to limited lifetime in the case of very frequent switching (like was mentioned in your reply)?
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Hello,

I am trying to do something very similar with a wire heater and a solid state relay.  I was wondering if you wouldn't mind sharing your results as to how you ended up programming this in labview.  I know very little about programming and was given sole responsibility for coming up with programming for this.  Unlike you, however, I have NI data acquisition hardware.  I appreciate your input!  Thank you.

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