interpolating ribbon temperature by resistance

[rossfree]

Testing Testing...

[rossfree]

Ok...

It worked.

I've tried twice now to submit a reply to this thread from an email link over the last couple of days and both times lost all of what I had written. Bother.

Thank you all for getting back. All good stuff.

I am familiar of course with the temperature controllers out there. They do work great... no question. I guess I just want to re-invent the wheel. There... I said it! :-)

There are a lot of off the shelf modules out there. Every one of them will do MOST of what I want... but never quite all of what I want. And then to do what I want requires that the operator be able to go through multiple layers of menus. Not good for the business I am in... heatsealing high temperature fluoropolymers. The only controller that would appear to come the closest is the ROPEX controller and it's expensive... about $2k.

What makes my heat control dream somewhat unique is that I want it to be easily configured to different material heater ribbons, of different shapes, of different thickness and lengths. Then I want the controller to control the current to the heater ribbon, bringing it's temperature from room to 700 degrees F (or any temperature in between) in 8 to 10 seconds and hold it there for an operator chosen dwell time. When the cycle is over, the current is shut off, the temperature drops and the mechanical jaws open at 150 degrees F to allow the sealed materials to be removed from the machine.

All this control, in a small package, built on the cheap (say under $200 US) with no more than two-layer-deep operator configurable control over heat time and pressure. And of course... I only need two or three of them. HA HA HA HA HA!

Ahhh... but those small problems don't quench my insatiable desire to stick my neck out and say "LET HER GO!"

The ribbon I am currently designing around is 303 stainless... .005" thick x .75" wide x 17" long. That works out to about .12 ohms of resistance at room temperature. I honestly haven't worked out what the resistance is at 700 degrees F. I keep burning my fingers (kidding!).

My current controller is using two pic processors. One is dedicated to doing only one thing... waiting for a zero-cross pulse, checking an eight bit port and then outputing a pulse to turn on an alternistor after a pauseus (based on the port data input. It repeats this every half cycle at 60hz (120 times a second). And it is rock solid. No complaints there.

The other processor polls a max 6675 for a thermocouple reading. Based on this reading, it raises or lowers the pauseus data for the second controller. But the 6675 is slow at 220ms between conversions. So I can poll it about 4 times a second.

I want it to be faster! :-)

I'm banging a lot of amps (75 to 100) or so at 1-2 volts ac for microseconds through this ribbon. It heats up quick and cools down in roughly the same time.

That's the story... I'll fill in more details as you guys thrash me behind the dumpster.

Thanks for the great input. So... how would you read the ribbon resistance between current pulses? Any thoughts??????

Ross

[rossfree]

This is just to correct an oops in an earlier post. I didn't even realize the post went through. Ah well...

Anyway... I posted that I polled the thermocouple device every 21 seconds. What I meant was every .210 miliseconds (aprx). It looked stupid the way I wrote it... needed to correct that. :-)

Thanks again for all the comments!

Ross

[Archangel]

Hi rossfree,
I was just wondering if you had tried sandwitching 2 of these SS strips together with some heat sink compound and (mica ?) insulators so you could heat one with current and measure the hot resistance of the other one ? It seems to me you could then allow adequate time for heat saturation to get an accurate idea of the hot resistance of the strip. Other methods might be to insert strip into a heat treat oven while wired to an ohmmeter. If you do not have one, I'll bet your local junior college does and pizza seems to be their official currency .
JS

[Acetronics2]

Hi, Ross

Depending on the voltage applied, the trick could be simple:

for DC supply, just apply Heating power by PWM ... and allow a continuous current flowing into your resistor.

So simple ... just measure the voltage across your heating element during the "0v" part of the PWM ...

For AC supply, Low voltage ... the PWM will be replaced by burst mode ...and constant DC current only applied when no AC provided.

AC, High voltage ... a probe really seems the simplest way !!!

Alain

[rossfree]

Thank you for the feedback...

Right now I am pulsing a 2KVA 48 Volt CT transformer using 110VAC line and an alternistor. One pic does the timing and sends the pulse to the alternistor to turn it on at the right time in the half cycle. The other pic runs the program, picking up the temperature and passing timing information to the first pic.

Depending on the ribbon size and length (it changes all the time) I may require 50 or so amps of current pulsed 120 times / second for very short duration to achieve the ramp and dwell time I require to bring the temperature of the ribbon up to the required temperature and sit there soaking heat into the film to melt it.

The voltage across the ribbon would never exceed 48VAC.

I like the idea of applying a constant current across the ribbon in series with an external resistor. Then reading the voltage across the resistor and interpolating the ribbon temperature with a look-up table or something like that. But I'm not to good at figuring out how to decouple the 48VAC transformer such that it does not affect the results. And forgive my ignorance... but I also don't know how the pulses would affect the constant current circuit. Boy... I wish I had a mind for this stuff!

My electronics buddy suggested injecting a 100khz signal across the ribbon and external series resistor and using a capacitor to decouple from the 48vac transformer. But he said that would likely get into some parts and require some tweaking.

And lastly... I have again revisited using a thermocouple... but instead of the MAX6675 Thermocouple to Digital converter (I am currently using)... I would use a thermocouple amplifier/conditioner like the AD597 which ouputs 10mv/degree C analog in real time. I could feed that into the A/D pin of the pic and convert it to Farenheit for display. This would be much faster than the MAX6675.

So I have choices... but still... I would really like to know the simplest... most precise way of reading the ribbon resistance. That would be the cats meow!

Do you smell it???

That's my brain.

Ross