interpolating ribbon temperature by resistance

[paul borgmeier]

I have maybe a four millisecond window at most to read this resistance and act on the A/D value. Timing is everything.

As Steve noted, more details are needed...but as a rule of thumb, a 10 bit ADC acquisition takes about 20uS or less to complete (provided you do not violate the input impedance specification) Further, the lower the input impedance, the lower the acquisition time. If you do not need full 10 bit accuracy, you can cut the acquisition short to further save time - see the Microchip MidRange Manual for details

An ADC example using the registers directly (and not the ADCIN command) is here
http://www.melabs.com/resources/samples/pbp/a2d10.bas
(if you use this latter approach, drop the Pause 5 in the loop)

[rossfree]

Hi guys, thank you for your reply.

I'm sorry I didn't get back to you quicker... I did write a reply a couple of days ago.. when I hit the send button it asked me to log in. When I did... something messed up and I lost the message I was going to send. Bother.

So this will be a quick one.

The ribbon is stainless steel, .005 inches thick by 3/4" wide by 17" long. I did the math at work and the resistance at room temperature is something less than one ohm. (.34?) I don't remember. Sigh**

The ribbon is mashed between two parallel bars with silicone and mica and a few other materials in between. I want to pulse the ribbon with enough current to get it up to about 700 degrees F in 10 seconds or less. Then hold it there for a few dwell seconds, and then turn off the power and allow it to cool back to 150 degrees F.

Of course I want to control this sealer for all other temperatures as well. The typical maximum is 700 degrees F. Fluropolymers such as Teflon FEP, PFA and Modified PTFE seal between 550 and 650 degrees F.

The sealer I built currently has a Dallas thermocouple device that reads a thermocouple input and interpolates it. The pic polls the device and retrieves temperature serially every 21(?) seconds or so. This is pretty good but not nearly as fast as reading the ADC channel directly. So if I could tap in to that speed, I could more precisely control current and thus temperature.

I know this can be done because it is being done in a Ropex controller. I believe they are using a current sensor to measure the current during an on cycle. They are also either reading the voltage drop across the bar during an on cycle or the resistance inbetween cycles.

Well I have to run for now.

Thank you for your feedback! Any other comments, criticisims, wise cracks or suggestions are eagerly sought!

Ross

[Acetronics2]

Hi, Ross

I do not exactly know the 'professionnal level" you want to build your control, but, what exists :

1) the programmable temp regulator ... I used it to control Cast iron heat treatments. costs some $$$ ... but most reliable !!!

2) The "el chapo" device ... have a look to Parallax "industrial applications" booklet ... there's how to realize a simple PID temp regulator ( w/soft offered !!! ) . an AD595 or MAX 6675 will be a good interfacing to Thermocouple.

With Darrel's instant interrupts, il wil be easy to drive the system clock , now.

On this basis, I've realized a vent regulator to control with a R/C servo the cowling apertures for a model plane engine ...

Here, I output the command signal @ 40 Hz ... so it might be much enough for your application !!!

Alain

[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