Torch Heigh Control for DIY CNC plasma cutter table project

Discussion in 'The Projects Forum' started by jlindst, Apr 2, 2011.

  1. jlindst

    Thread Starter New Member

    Mar 31, 2011
    3
    0
    Hello Forum.
    I'm hoping some folks can help me with a project I've been hacking at for some time. I have a CNC table that uses a plasma torch to cut various designs out of various metals.
    As the torch cuts the metal it introduces heat into the material thus warping it upwards which decreases the distance to the torch head. This fluctuating distance results in undesirable cut quality.

    The premise:
    The plasma is a constant current source and as the material warps it decreases the voltage between the work piece and torch (electrode).
    I'm trying to sample that voltage and adjust the torch height accordingly.

    My setup as of now:

    • tapped leads fed into a 50 amp bridge rectifier
    • simple voltage divider (50:1) feed into lowpass rc filter
    • 0-5vdc feed into 12-bit MCP 3201 ADC
    • ADC values feed to Atmega328 MCU over SPI
    • ADC values used in if/else control function calculations which are encapsulated via modbus and sent to CNC controller
    Here's my problem:

    The converted values from my ADC jitter a by couple volts (+/-) 5-8 dc out of a 50-200dc output scale. It's not horrible but does limit the speed at which I can cut.
    As a beginner my understanding of the analog side of things is extremely limited.
    I've read that plasma output is very noisy and has high frequency signals that I assume are making there way into my ADC.


    I've been reading about two stage Pi LC low-pass filter because that's what commercial units use with success.
    I just purchased an oscilloscope and could acquire some data but not exact sure what I'd be looking for just yet not to mention leery about hooking up to the plasma leads. I remember reading a warning about ground potential differences.

    My Plea:
    Does anyone have experience with two stage Pi LC filters and willing to share some knowledge?

    I've been studying these commercial circuits and trying to visualize what is transpiring underneath but I'm in over my head.

    Here's a pic of the circuit on the and what I think may be happening in the underneath.....

    [​IMG]
    [​IMG]
     
  2. Rbeckett

    Member

    Sep 3, 2010
    205
    32
    Been doing the same thing as you for a while. PM me offline and I will be glad to share a ton of info as well as design and coding if you would like.
    Bob
     
  3. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Bob,
    How about posting that info here. That's the concept of the forum. ;)
    Ken
     
  4. Rbeckett

    Member

    Sep 3, 2010
    205
    32
    Ken, I would love to, but the distance I have traveled from the beginning of this project to where I am is dismal on the device front but huge in terms of data and design. It is such a small niche for this type of device that it would be a waste of bandwidth to include all of the info for the benefit of one or perhaps two interested parties. Additionally, since it does encompass high volts and high amps and could pose a threat to most inexperienced users safety is the reason I suggested a PM. Sorry.
    Bob
     
  5. jlindst

    Thread Starter New Member

    Mar 31, 2011
    3
    0
    Bob. Not sure if you got my email but I'd love to see what you got.
    Especially anything related to the plasma output waveform/shape.
    Ive got most of this nipped except for this ADC jitter which I'm pretty certain is RF/HF noise that I can hopefully squish with a well designed filter.
     
    Last edited: Apr 3, 2011
  6. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    jlindst,

    Edit your post and remove your email address. Spy-bots will find it and spam you. Use the Forum's private mail (PM) for off-thread communications.

    Ken
     
  7. jlindst

    Thread Starter New Member

    Mar 31, 2011
    3
    0
    Because of my newness I dont have the ability to pm as of yet.
     
  8. gerrymoore

    New Member

    Mar 3, 2012
    4
    0
    Hi everyone....I know this is an old post but lots like this on DIY THC seem to just 'stop'!! I would love to build my own THC but just do not have the knowledge - or courage - to play with -300 volts!! Has anyone got any further with their own designs that would like to share? regards Gerry
     
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