Non-contact infrared temperature control.... 1

[davek0974]

Hi all, first post

I have a question reagrding interface methods.

We have a printing press in the shop, it has an infrared dryer built in, it's basically 8kw of halogen lamps controlled by a thyristor stack power controller and temperature is user adjusted by a potentiometer. The printed sheets pass under this lamp array for a split second on their way out of the press to the delivery stack. For maximum effect, the printed paper stack should be maintained at a temperature of 30-35c, any less and it has little or no effect, much more and there is a risk of the stack catching fire due to the ink solvents on the sheets etc.

This temperature is under user control but measurement is also done manually, I use a laser/infrared thermometer at regular intervals and set the knob where I think it should be. This is not perfect as the stack temperature is machine speed dependant and paper type dependant - thicker paper warms up slower etc.

I am working on a way of making it closed-loop, i.e. building in some sort of IR thermostat to give a variable output based on a set-point?

I have sourced a suitable non-contact pyrocouple and a process controller but here we hit a snag, i need to integrate without altering the existing machine in any severe way.

I know the potentiometer is used in a two-wire config, and not as i had hoped as a three-wire potential divider whereas i could simply output a suitable voltage to control the power.

So i am looking for ways to control without alteration. Is there such a thing as an electronic variable resistance that could be wired in series with the existing pot?? It could be controlled by a 0-10v or 4-20mA signal?

Anyone got any interface ideas that might work here? I had even thought of a pwm controller and a simple radio-control servo which could connect physically to the existing knob. It only uses a small portion of the 270deg span of the pot, maybe 90deg total.

Any thoughts would be very much appreciated.

Yours

Dave Kearley

 

[Compositepro]

I think your question is far to complicated to be answered here. You are trying to automate the simplest part of your control. Start-up and shutdown is where you will be starting fires.

 

[davek0974]

Thanks,

I thought it might be a tough one, maybe I'd be better on an electronics forum?

Startup and shutdown is already handled by the base machine, all I need to do is find a way of altering the position of a control knob based on a sensed temperature, scaling, limits and so on are all handled by the process controller.

Dave

 

[BrianE22]

You could put a transistor in series with the variable resistor and control the transistor with your controller.

 

[davek0974]

Thanks Brian,

I can see that being possible but would need some resistor padding etc to get the 0-10v signal to control the transistor properly, only trouble with that is - I am useless with discrete components, I'm ok with PLC's, Process controllers, pneumatics and maybe even valves but never done any design with transistors.

I can likely find the voltage i am dealing with easily and being a standard film pot, the power wont be >1W so its a small load.

Its just the scaling 0-10v = off to on gradually thing i cant deal with.

 

[davek0974]

Just found that a J-FET will do this but again, beyond my knowledge.

Any ideas??

Dave

 

[davek0974]

Ok, finally had a chat with the manufacturer. He sent me a diagram of an add-on board that was used for 4-20mA control loop operation of the unit, i have sketched it out and attached it.

VR1 is the existing set point control, this must remain in circuit.

I have no values for the other components.

It seems this is exactly what is needed so i just need to fill in the blanks i guess.

 

[BrianE22]

I think you're mixing up the "setpoint" to the controller (your variable pot) with the "feedback" that the controller uses to monitor temperature.

Correct me if I'm wrong, but I think you want to use the pot to set a temperature and use the ouput of the pyro in the control loop, so, for instance, if you command the pot for a temperature of 200 degrees and the pyro says you're only at 190 degrees then the heaters would turn on. Is this correct?

 

[davek0974]

Ok, the only reason that the existing pot is left is because the circuit required it. I would prefer to use the process controller to do all the work. I have since noticed that anything that varies the resistance will likely do ok.

The vactrol mentioned or even simpler an LDR and lamp driven from the 0-10v output of the controller should work, as far as I understand it, a lower resistance = more power.

I could wire an LDR in series with the existing pot which would then give me an adjustable max limit, the answer is now within the realms of a few experiments I think.

Thanks for all the advice

Dave