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Black Box Probe show closed loop before touching plate

Discussion in 'Controller Boards' started by dexus, Nov 4, 2019.

  1. dexus

    dexus New
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    Hi!

    I have a problem with the black box probe function. I have a cnc with a vfd driven 2.2kw router, but when connecting my probe it will light up as soon as i connect the clip to the drill bit, without touching the plate.

    The thing is, it did work for a couple of weeks, and was brilliant to use, but suddenly I get this problem out of the blue. I have measured out my probe cables, there are no short. The probe show nothing until I touch the clip to the bit, or the router itself. The router is grounded.

    Any ideas? Could this be a problem with the Black Box or with the vfd or router?

    Any help appreciated.
     
  2. David the swarfer

    David the swarfer OpenBuilds Team
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    did you try the other clip on the bit? in other words, swap the clip wire for the plate wire.
     
  3. dexus

    dexus New
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    Well what do you know.. That is really strange.. It actually worked! I didn't try that, as it was working before, and i also tried reversing the polarity of the probe in the grbl settings without any result.
    Why this is working now, is beyond me.. I thought it was only a closed loop, and it didn't matter what cable was where..

    Thank you.
     
  4. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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    That changes the logic (react to logic high or logic low on the signal pin, but still same pin) - wiring swop changes the actual polarity.

    Confusing though, you want the Spindle body EARTHED (mains earth, to trip GFI) not Grounded (V-/GND/0vDC), they sound almost the same but are very different things. If your spindle body is tied to logic GND, all the EMI from the motor is getting dumped into GND on the controller!
     
    #4 Peter Van Der Walt, Nov 5, 2019
    Last edited: Nov 5, 2019
  5. dexus

    dexus New
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    Yes.. That makes sense.

    The spindle is actually earthed. It is a Chinese spindle, and based on a tip from one of the users here, I actually opened the spindle up to check if the earth pin on the cable plug was actually connected to the spindle body. It was not. So I cabled it myself, so now it is.

    Other than that the spindle is great. low noise, very powerful and the vfd is controllable from the black box, although I haven't hooked it up yet.
     
  6. David the swarfer

    David the swarfer OpenBuilds Team
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    that is exactly the issue, your spindle (clip on the bit) is now controller GND so clipping the sense wire to it will always close the loop. by clipping the GND wire to the bit it can now only sense when it hits the plate which is now the actual sense pin connection.
     
  7. dexus

    dexus New
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    Yes. I got that.. The strange part is that it was working before.. I haven't done any change to the configuration. I did have a power outage, but I haven't noticed anything wrong with either the vfd, spindle or the blackbox other than the probe started lighting up when I clipped the bit before probing.

    But it is working now, and after measuring, the spindle is earthed.. so.. I guess I'm fine. :D
     
  8. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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    measure if Earth is connected to GND (Bad!)
     
  9. dexus

    dexus New
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    I will.. Where is the easiest point to measure gnd on the controller?
     
  10. Alex Chambers

    Alex Chambers Master
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    Limit switches have a ground connection, and you probably have one close to the spindle.
    Alex.
     
  11. dexus

    dexus New
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    Now.. this is where it gets.. interesting..

    I measured from the spindle which is earthed, to the ground on the limit switch, and what do you know.. they were connected!

    I wondered how could this happen? I mean the blackbox is not earthed.. so where does it get earth? The Spindle is not hooked up to the blackbox, just to the vfd which in turn is connected to the mains.. and is earthed.
    I started measuring about, and low and behold measuring from the usb gnd to the spindle made a beep.. So.. usb gnd was earthed.. but from where? I pulled the power cord on the laptop, and it was still connected. The only other thing connected to earth was the external screen. Pulled that one, and the connection was gone.

    Am I missing something here?

    So earth goes from the external monitor through either vga or dvi (tried both) to the laptop via the usb plug into the blackbox and then to gnd on limit switches.. is this normal?

    Tried another monitor.. same thing.. Unplugged monitor and ran on the laptop screen, and that fixed it.. so it's definitely the external screen. Any ideas?
     
  12. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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    It is weird for sure, normally PC components dont have GND connected to earth! Very old monitor? (the last 5-6 I've owned all had DC power bricks, havent seen a monitor with onboard AC in like a decade or more)
     
  13. dexus

    dexus New
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    Well.. It's not new.. It's an old HP monitor, and it has an internal transformer.. but still. I did try it out on a LG Tv as well (which also has an internal powerbrick) with the same results.. Most european monitors and tv's use the euro-cable standard 3 prong wire to connect to ac, and has internal power bricks. But it can't be a coincidence that both a relatively new LG tv and an old HP monitor (19inch vga/dvi) would have the same error, which leads me to think it's that way with all monitors..

    that said.. Who needs earth on the monitor? It's all plastic on the outside, and I never touch it anyway.. and it's not near any water source that's for sure.. (air cooled spindle).. So I cut the ground wire from ac to the monitor, and now there is no earth connection on the ground pins on the black box..
    Not a perfect solution, but it will work until I find another one.
     

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