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Recurring false 'Hard limit triggered.' Alarm

Discussion in 'Control Software' started by roses, Mar 23, 2020.

  1. roses

    roses New
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    I am getting intermittent and recurring 'Hard limit triggered' events when running a gcode that is nowhere near the limit switches. Is there a list of potential causes for the 'false' alarm that I can work through?

    Thanks!
     
  2. Alex Chambers

    Alex Chambers Master
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    Depends on what sort of limit switches you use. Normally open switches are particularly susceptible to EMI (interference). Normally closed switches can give false triggers if there is a dodgy wiring connection. Keep mains and spindle cables away from low voltage stuff, check your wiring. If you think it might be EMI (and you have your mains cables well away from other wiring) ferrite cores - especially near the controller - on switch wiring and usb cables will help.
    Alex.
     
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  3. roses

    roses New
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    Thanks, I am using an Acro 1510 with the OpenBuilds Xtension Limit Switch (normally open). Interestingly, the low voltage lines to come within a few inches of the power cable to the power supply so I've separated those and will see if I experience the problem again. Will post here how it goes.
     
  4. Alex Chambers

    Alex Chambers Master
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    Do let us know if that solves the problem - the Xtension Limit Switches are quite well protected but nothing is completely immune.
    Alex.
     
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  5. Alex Chambers

    Alex Chambers Master
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    Might be worth trying a different (better quality?) USB lead as well.
    Alex.
     
  6. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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    What tool head is on the acro? (laser, pen, etc?) is it something that does make any static or EMI? What's the exact setup, details count. Dust extractor hoses make awesome static electricity generators for example, while easy to overlook. Tell us what's around it and we'll help identify causes. Xtension switches are very resilient against EMI. Hf plasma or VFD might push it over the edge
     
  7. roses

    roses New
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    The tool head is a Nema 17 linear actuator with lead screw controlling the Z axis. The lead screw is about 15cm long, and the gantry holds an acrylic pen. The Acro is mounted to an IKEA table (laminated wood table top supported by metal legs). It's powered on the standard EU 220 voltage and I have been using it without incident for over a month at least.

    I recently upped the speeds on the X and Z axes which may have been a contributing factor.

    The alarm was still triggered after moving the power cord further away from the low voltage cables and also after switching to a different USB cable. Running the same gcode file does not result in the alarm being generated at the same point.

    The alarm was not triggered after disconnecting the Xtension limit switches from the BlackBox Controller.

    My gcode files have about 40,000 to 60,000 instructions in them, so static electricity seems like it could be a cause, if there is a way to discharge that from the system I will try that out. I suppose I can also reconnect one limit switch at a time to see if it is associated with a specific one.
     
  8. Alex Chambers

    Alex Chambers Master
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    Your last suggestion (connect 1 limit switch at a time) would be a good move.
    Alex.
     
  9. phil from seattle

    phil from seattle Well-Known
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    I suspect you need to run shielded wiring to your switches. If you do, make sure the ground/shield connection is at the controller. Honestly, I'd like to see OB come out with NC style limits switches rather than their current NO ones. I know there is disagreement on this point but NO switches are more susceptible to noise than NC switches. It also helps if there is filtering or optoisolation on the inputs at the controller. Not sure what BB has.
     
  10. Alex Chambers

    Alex Chambers Master
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    phil from seattle likes this.
  11. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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    Xtension switches has RC filters right onboard too (so doubled up with a Blackbox but even with lesser controllers Xtension Limits got you covered)
    I am doubting this IS EMI at all here. No big EMI sources nearby. Could be a faulty switch (mechanical side) that triggers from vibration.

    Xtension Limits run fine with unshielded wire next to Cut40 Hf plasma inverters (;
    We did a lot of testing and by adding active filtering, we solved the EMI issues, and still able to keep No wiring for all the advantages it lends (easier wiring, default config values, etc)
     
  12. roses

    roses New
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    I encountered the same hard limit alarm error again today (multiple times) despite having the limit switches disconnected from the BlackBox. I'm mentioning this in case it narrows the search space of potential causes. It isn't really negatively impacting me, aside from being an annoyance. I may test out a different set of gcode files tomorrow to see if I hit the same issue.
     
  13. Alex Chambers

    Alex Chambers Master
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    What are your $20, $21 and $22 settings at the moment? Try setting them all to 0 and see if that helps. (disables homing, soft and hard limits). Also see if you can find any faults on the limit switches - can you get a connection between signal and ground with a tiny movement?
    Alex.
     
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  14. phil from seattle

    phil from seattle Well-Known
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    Yes, agree that's worth doing.

    Did you disconnect them at the switches or the BB? I'd also take a close look at both sides of the BB circuit board to see if there is anything obvious like metal debris or loose wires or ...
     
  15. roses

    roses New
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    Switches were disconnected at both the BB side and the switch side, $20 was set to 0, $21 and $22 were set to 1 (when I was getting the alarms)

    I have set them all to 0 and haven't encountered the hard limit alarm since then. :)

    If there is anything else I should check just let me know. Thanks!
     
  16. Alex Chambers

    Alex Chambers Master
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    As @phil from seattle said, check both sides of the black box circuit board for any obvious faults - they are quite rigorously checked before despatch, but still worth checking.
     
  17. Alex Chambers

    Alex Chambers Master
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    Although that gets you running, it doesn't find the problem. Have you checked your switches to see if one of them is faulty? Use a multimeter to check for continuity between sig and gnd and see if a very small movement of the lever gives continuity. Also double check your wiring for any stray "whiskers" of wire that might be making intermittent contact.
    Alex.
     
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