Mk2/4 tool sensor input not working, damaged buffer chip?

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Thread: Mk2/4 tool sensor input not working, damaged buffer chip?

  1. #1

    Default Mk2/4 tool sensor input not working, damaged buffer chip?

    I upgraded one of my CNC machines with a VFD controlled, 0.8kW brushless spindle and seems that has caused damage to the control board from stray currents from the spindle.
    The tool input is always on and when I measure the resistance between the IN5 and GND it shows about 700 Ohms, which is different from other working inputs (limit switch inputs) that go through a different buffer chip, IN1 also is always on so I guess the buffer chip for those inputs is damaged.
    It's an old board so I don't think it would be worth sending it for repair, and I can replace SMD chips myself anyway, a 74HC14D is the correct part, yes?

    The stray current problem will still remain, since it's inevitable to have stray current when using an EMI filter like they one I have in the VFD so if I want to use the tool sensor I'll have to figure out a way to do it without damaging the board again, or my other board a newish Mk3 9 axis, and luckily I didn't try out the new spindle with that one.

    I suppose I should use some sort of isolation circuit to protect the Mk2/4 inputs, any suggestions on the best way to do that?

    My idea would be to use an optocoupler powered by the power supply in the VFD control box and have the Mk2/4 IN5 on the other side, as in this schematic.

    Mk2/4 tool sensor input not working, damaged buffer chip?-toolsensorisolation-jpg

    Does it look correct?

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    Attached Thumbnails Attached Thumbnails Mk2/4 tool sensor input not working, damaged buffer chip?-toolsensorisolation-jpg  


  2. #2

    Default Re: Mk2/4 tool sensor input not working, damaged buffer chip?

    Hmmm, looking back at that I think there's a big issue, the GND in the power supply is not actually connected to Ground (Earth Ground), and I don't think it would be a good idea to connect them together. So in that circuit current would be flowing from the +5V to Earth Ground, I'm not very knowledgeable in grounding techniques (it probably shows!) but that seems like a no-no to me.

    So the issue then is, if the spindle is grounded (Earth Ground), also the machine frame and the work piece also is grounded, how would a tool sensor work?



  3. #3
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    Default Re: Mk2/4 tool sensor input not working, damaged buffer chip?

    I have a circuit board as tool sensor, the spindle and all the frame is grounded to earth and it`s just work, but I have nothing between input board and tool sensor, the board is this https://planet-cnc.com/product/limit-input-board/ and is powered 12V independent. The IN terminal is always connected to the frame and 12V goes to circuit board aka tool sensor.
    Hope this help.



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    Default Re: Mk2/4 tool sensor input not working, damaged buffer chip?

    Quote Originally Posted by CaptainObvious View Post
    Hmmm, looking back at that I think there's a big issue, the GND in the power supply is not actually connected to Ground (Earth Ground), and I don't think it would be a good idea to connect them together. So in that circuit current would be flowing from the +5V to Earth Ground, I'm not very knowledgeable in grounding techniques (it probably shows!) but that seems like a no-no to me.

    So the issue then is, if the spindle is grounded (Earth Ground), also the machine frame and the work piece also is grounded, how would a tool sensor work?
    What type of tool setter are you using, Tool to a touch plate, you can get Rotor voltage that can destroy your board inputs no matter how much Grounding you have when using a VFD Drive the spindle motor Rotor Voltage can build up, so when you do a Tool Touch off the Rotor will Ground it's self when it touch's the Touch off Plate, so will release the voltage out through what ever the Tool touches

    Apart from adding a mechanical Brush to Ground the Rotor Shaft, you can also just use a electro static piece of plastic or brush down at the front of the spindle, this is what is used in the industry for this problem so from looking at this you will see what you have to do if you want to use the same type of Touch Plate, https://www.est-aegis.com/

    If your spindle motor has Ceramic Bearings then this is even worse as there is no path to Ground for the spindle Rotor Ideal you have to add a brush to the Spindles Rotor this will then Ground the Rotor and you will have no more problems

    This is the best type of Tool setter when you have these problems, this will isolate the Board and not cause any of these problems

    Attached Thumbnails Attached Thumbnails Mk2/4 tool sensor input not working, damaged buffer chip?-tool-setter-png  
    Mactec54


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Mk2/4 tool sensor input not working, damaged buffer chip?

Mk2/4 tool sensor input not working, damaged buffer chip?

Mk2/4 tool sensor input not working, damaged buffer chip?