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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.
     
  14. David the swarfer

    David the swarfer OpenBuilds Team
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    This issue has been teasing my mind for a bit. I really don't want to connect a microcontroller pin to the spindle nor to a random bit of metal that could end up anywhere on my desk, including contacting other bits of metal which may or may not be connected to 'something'.
    Then there is a ground loop thing we all want to avoid.
    And then there is the knowledge that optocouplers can vastly reduce noise problems on limit switches, so why not use on on the probe as well?

    So I grabbed a couple of paper sensors harvested from an old photocopier. These have an optocoupler with a gap that is occluded by a plastic lever.
    The lever makes the sense distance too long for a probe, we want the actual probe thickness to be minimal because most of our machines have limited Z travel.
    I cut off the lever and modified the rest of it to have a short lever. This ended up 12.8mm thick and worked but was sensitive to bit position and size. Not what we want.

    I started again and end up with this: I removed the sensor from its plastic base and discarded the lever. Glued it to a plastic card and then built a new lever.
    But using a long flat lever like this we can minimize offset errors caused by the diameter of the tool. this triggers within 1mm of this resting position, thus minimizing the offsets even more.
    aim the tool at the black dot on the icecream stick and probe...

    The lever detail, the lead weights are to keep it where I put it otherwise the wires will move it at the worst possible moment. It is important that it is held flat.

    The experimental wiring... I am using a bare Arduino Uno for my controller and this is a plug in 'screw shield' that gives me quick access for this experiment. I will make a more permanent connector in due course.
    20191221_095346-wires.jpg

    I figured out the wiring using sites like this one Making endstops from printer photo interrupters. ending up with a 150 ohm resistor (the blue one in the photo) from power to the LED, and a 10k from power to the output line.

    Some optosensors have a very narrow gap, I used a wider one to make it easy to build my lever though a narrow gap is theoretically more accurate.

    NOTE:
    the output from this sensor is always on (acting as a NC switch) so you have to invert the probe sense. $6=1

    Once you have it working you need to figure out how high it is. I set the offset to 0 then probed, removed the sensor, then jogged down slowly until my favourite bit of paper was just touched by the tool.
    At this point my z was at -9.7mm

    My paper is 0.1mm thick so I could add that to the 9.7 to get 9.8. When I put that into the probe wizard the tool ends up at the right spot for Z=0 at the surface of the work.
    I may refine this value after some experience with actual work.
     
    Mark Carew likes this.
  15. dexus

    dexus New
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    Cool! I actually used an octocoupler for a x-axis endstop, but had to do away with it, replacing it with a small quality microswitch. The problem was, that once in a while, when milling something where I didn't want to use the dust shoe (because I need to examine what happens with feeds/speeds etc.) A chip would acutally jump in between the octocoupler sensor, tripping the x-axis endstop, making my whole cut ruined.. (well. at least before I got used to actually using the endstops properly.. :-D).

    After watching one of my favourite 3d-printing youtubers, Thomas Sanladerer doing a real life test on the accuracy of sensors, I found that microswitches are ok.. check it out here:

    That said, I just love using parts from discarded tech in my projects.. so I find this really interesting.
     
  16. juansierra

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    My question now that I got the XYZ probe working is. Can you do an XYZ probe while using a tapered BN bit? When using an end mill I see no problem. Another problem that I see is entering bit diameter size is in mm only, so if you're using an imperial size I have to convert it to mm, It would be nice to be able to select mm or inches.
     
  17. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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    Giarc and Alex Chambers like this.

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