interpolating ribbon temperature by resistance


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  1. #1
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    Default interpolating ribbon temperature by resistance

    I am currently building a small high temperature heatsealer for fluoropolymer plastics like Teflon. I have used off the shelf controllers with thermocouple inputs and have even designed one (with the help of an electronics friend/genius) that is in use today in our business (using an 877 and picbasic pro).

    I would like to do away with reading a thermocouple (too slow) and read the resistance of the ribbon directly with an A/D pin, go to a lookup table and display the temperature based on imperical data taken during the initial setup.

    The ribbon is pulsed with current from an alternistor driven transformer. The transformer is only turned on a few milliseconds every eight miliseconds. While the current is off, I want to read the resistance of the ribbon.

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

    I do not have a good handle on how long it takes to convert analog to digital in a pic running at 20mhz using PicbasicPro. I've also seen mention of reading the pic directly for faster access.

    Can someone tell me how fast the A/D channel works, and what issues you see in how I'd like to do this? It has been done before in a similar controller (Ropex) but they measure by current and voltage during an on-cycle. I just want to know what others might have tried and find out if this can be done.

    Don't beat me up too badly... I'm mostly made up of crazy ideas... not a genious in the electronics department.

    Many thanks!

    Ross
    Never enough knowledge to be called intelligent but just enough knowledge to be considered dangerous!

    I like that! :-)

  2. #2
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    unless we have a little bit more details it will be hard to answer this. But look in the datasheet about acquisition time, it will depend at least of the external impedance.

    Read/Write to the A/D register instead of ADCIN will also increase speed.

    You may find some good infos and trick bellow
    http://www.sfcompiler.co.uk/wiki/pmw...User.MinADtime
    Steve

    It's not a bug, it's a random feature.
    There's no problem, only learning opportunities.

  3. #3
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    Smile

    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)
    Paul Borgmeier
    Salt Lake City, UT
    USA
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  4. #4
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    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
    Never enough knowledge to be called intelligent but just enough knowledge to be considered dangerous!

    I like that! :-)

  5. #5
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    Lightbulb Basic regulator design

    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
    ************************************************** ***********************
    Why insist on using 32 Bits when you're not even able to deal with the first 8 ones ??? ehhhhhh ...
    ************************************************** ***********************
    IF there is the word "Problem" in your question ...
    certainly the answer is " RTFM " or " RTFDataSheet " !!!
    *****************************************

  6. #6
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    Testing Testing...
    Never enough knowledge to be called intelligent but just enough knowledge to be considered dangerous!

    I like that! :-)

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