Electrical discharge machining Electronics

Discussion in 'The Projects Forum' started by ronv, Jun 9, 2015.

  1. ronv

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    I'm not sure if this is the right place to post this, but hey.
    Background:
    I have been working with Shortbus on some electronics for his EDM mechanics. We started out trying to make it as fast as possible and with all the bells and whistles of a big boy machine. Needless to say this one got pretty complicated pretty fast so where I ended up was a simpler version that will run (I hope) at about 1000 arcs per second. There are a few constraints that seem to limit this. The first is his transformer is rated at 75 volts RMS at 15 amps - yes a brute. This gave us about 95 volts after rectification and filtering. Since it drives a 40,000 ufd cap I thought it best to follow the recommendations to derate it by 40% so 9 amps or so. The targeted arc current is around 35 amps so this limits the duty cycle.
    The second is the use of a stepper motor to drive the ram up and down. The stepper runs in 1/10 microstep mode so my guess is maybe it can do 1000 steps/second from a dead stop. I would appreciate input on this. The first design only delayed the arc when the motor needed to move, but again got complicated, so I decided to let the motor set the duty cycle as it will be safe for the transformer.
    If I was convinced that the motor would never have to move two or three times in succession it might be possible to speed this up as well. But if the motor fails to move it would be bad news.
    The third bell and whistle that bit the dust was a provision for a delayed arc. That's a case where for whatever reason the arc doesn't start right away. This meant another timer to decide how long to wait for an arc so this case will now just have a shorter burn time.
    We left in the option of starting the arc with a higher voltage since he already had the stuff.

    How it works. Logic attached.
    There is a timer that is adjustable from about 25 usec to 125 us for the time the arc is active and for the other half of the cycle allows the discharge capacitors to recharge. (U13) A flip flop to get it to start with a charge cycle and turn on the discharge of the 130 volts to start the arc. Once the arc starts that flip flop is reset and a second one is turned on to discharge the 95 volt capacitors into the probe. After a short delay there is a sample of a window comparator that indicated if the voltage of the arc is above the window (to far away from the work piece) or below the window (to close to the work piece) or in the window - just right.. This is stored in the latches U10 and 18 to be sent to the step motor controller. If the window voltage is to low the arc is turned off so as not to make a short. If ther is no arc the motor is moved in and the process starts again. The rest of the logic is just resets to keep it looping under the various conditions.

    Power section attached:
    On the left it a FET driver for the 130 volt supply turned on by the discharge 130 from the logic. This signal remains high until an arc is detected by the comparator U8 at the bottom right. It senses the voltage drop across two power diodes. There are other ways to do this but we kind of liked this one.
    On the right is another driver to turn on a FET to charge the caps C1. C30 & C31. These are selectable by the operator to set a power level.
    In the center is the discharge circuit. It discharges both the 130 volts and the caps into the gap. These supplies are or'ed thru D1 and D2. The gap and electrodes are represented by the neon (U4). This turns out to be an easy way to simulate it because you can change the strike voltage and it's on resistance. I used little isolated dc to dc converters instead of separate charge pumps to keep it simple.
    I've seen some warnings about using supplies to do this due to inter-winding capacitance, but these seem to be made for it as the capacitance is only pico farads. Anyone used them for this?
    Finally the window comparator. It is operator adjustable with a window about 6 volts wide. Shortbus and I have a disagreement on this. I think it needs to be sampled out in the wave form somewhere (picture attached) and he thinks not. I see a lot of circuits on the net showing the window, but I don't know how it could work without a sample. Maybe there is an EDM guy that could splain it to me.
    Also attached is the simulation of the whole thing except for the bulk supplies. I have schematics for them and separate simulations if anyone is interested.
    I would really appreciate your input and insights. There are a lot of poor circuits running around so a good one might be something nice to have.
     
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  2. shortbus

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    Wow Ron, even you get the same response to EDM circuits that I do. :) Or maybe it was the mention of shortbus?
     
  3. cmartinez

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    Hi guys... I'm back... not sure if I can be of much help here, but I'm going to read and re-read your post and study the sims carefully.
    I'll try to help if I can.
     
  4. #12

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    I only wish you would spell the title out. I keep seeing it as, "Electronics Design Magazine" because that phrase is more familiar to me. People that might search for this subject in the future will probably spell out the search terms, too. ;)
     
  5. cmartinez

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    Good point... a better title would be Electrical discharge machining electronics, or EDM wire-cutting electronics... or SOS on EDM ... :D
     
  6. ronv

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    Maybe the moderators could change it. I never realized EDM stood for so many things. Guess I worked on this to long. Anyway after Googling it I off on Electronic Dance Music.:cool::D

    @DickCappels
    @ScottWang
     
  7. cmartinez

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    Not'n wrong with that....

    This new version of the forum allows the TS to change the title, I think...
     
  8. bertus

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    Hello,

    I changed the EDM to Electrical discharge machining for you.

    Bertus
     
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  9. cmartinez

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    Oh..... the HORROR!!! :eek::eek::eek::eek::eek::eek:
    :D:D:D:D:D I'm laughing my rear out ..... ha ha ha ....
     
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  10. bertus

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    Hello,

    I noticed the other meaning later, so I adapted the title again.

    Bertus

    PS @cmartinez , you are fast.
     
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  11. cmartinez

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    This was probably the funniest thing I've seen happen in this place...
    Cheers, Bertus!
     
  12. ronv

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    I missed it! :(:(
    Thanks!
     
  13. shortbus

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    With many thanks to Ron for his help, but shouldn't my simpler circuit also be in this post? For a fair comparison? With the other tread going so many pages it may not have been seen. But don't want to hijack your thread Ron.
     
  14. ronv

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    Maybe a fresh thread starting with the circuit & we can debug it?
     
  15. cmartinez

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    I really see no need for that. Why don't you just post the last version here and we'll take it from there?
     
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  16. ronv

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    Sure, why not. There doesn't seem to be any input. So here is Shortbus' circuit.
    http://forum.allaboutcircuits.com/attachments/edm091-jpg.86756/
    I simulated the set/reset circuit and came up with a couple of things.
    1- I added a pull down on the switch so the line wouldn't be floating.
    2- Added clamp diodes to the pulse circuit so it wouldn't drive the inputs below ground. Don't think the IC's like that.
     
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  17. shortbus

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    Thank you Ron! The things your showing in your add on's to my circuit are the things that no one ever shows. Most of the circuits in a book or ones posted online never show the nuances like you did. Every where I looked the things I put in them are what was put in my circuit. I'm using stuff that was shown elsewhere. This may be why a lot of circuits shown don't work when built. The designers leave out the little stuff thinking the builder should know it's needed. Thank you so much for sharing your knowledge.

    In response to your point #1, I thought by using a SPST toggle the line would not be floating but stay at supply (12V) level. I was under the assumption that the S/RS would only cycle once and then stay at the level that results from that.
     
    Last edited: Jun 12, 2015
  18. shortbus

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    Thanks C. Be aware that my schematic doesn't show all the voltage dividers and other assorted things needed. It is just a rough outline proof of concept that still needs work.
     
  19. ronv

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    [QUOTE="shortbus, post: 869976, member: 6184
    @shortbus
    In response to your point #1, I thought by using a SPST toggle the line would not be floating but stay at supply (12V) level. I was under the assumption that the S/RS would only cycle once and then stay at the level that results from that.[/QUOTE]
    That's true when the switch is at reset, but when it is in start (as shown) it is not at 12 volts or ground, but what I call floating. A little bit of noise would make it look like another reset / start pulse.
     
    Last edited: Jun 12, 2015
  20. ronv

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    Speaking of dividers......
    So if we are ok on the first 2 changes I have a question about U13.1 & .2. I think you want the thresholds to be 95 and 20 when the timer is not fired????. Then when there is an arc the timer fires and you want the thresholds to change. What do you want them to change to???
     
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