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