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  1. #1

    Join Date
    Sep 2018
    Posts
    13

    Limit Switch Noise Fixed!

    Thought I would share my limit switch noise solution. I searched here looking for answers a couple years ago when my PM25 CNC conversion kept getting false triggers on the limit switches. I wired in some resistors which helped, but never totally fixed it. I tried shielded wires too but that didn't work.

    I kept getting "Limit Switch Triggered" in Mach 3 while cutting, which would freeze the machine. I have a 48V system two large Nema 23's and a Nema 34, all running at 5A's.

    Anyways, I researched it and decided to make a better solution.

    My limit switch filter board puts a load across the limit switches to reduce voltage spikes, an RC circuit to filter that signal, then a Schmitt trigger to remove jitter and add hysteresis, a level shifter to convert the signal to 5V, and current limiting resistors to protect the output wires from short circuits.

    It's been on the machine for the last couple months, and has finally fixed my noise problem!

    Pics of the final board and a screenshot of the signals!

  2. #2
    Member
    Join Date
    Jan 2005
    Posts
    11275

    Re: Limit Switch Noise Fixed!

    Quote Originally Posted by patrick31 View Post
    Thought I would share my limit switch noise solution. I searched here looking for answers a couple years ago when my PM25 CNC conversion kept getting false triggers on the limit switches. I wired in some resistors which helped, but never totally fixed it. I tried shielded wires too but that didn't work.

    I kept getting "Limit Switch Triggered" in Mach 3 while cutting, which would freeze the machine. I have a 48V system two large Nema 23's and a Nema 34, all running at 5A's.

    Anyways, I researched it and decided to make a better solution.

    My limit switch filter board puts a load across the limit switches to reduce voltage spikes, an RC circuit to filter that signal, then a Schmitt trigger to remove jitter and add hysteresis, a level shifter to convert the signal to 5V, and current limiting resistors to protect the output wires from short circuits.

    It's been on the machine for the last couple months, and has finally fixed my noise problem!

    Pics of the final board and a screenshot of the signals!
    No it fixed your limit switch problem, the noise is still there are you using any EMI Main Power Filters as this normally will solve most EMI noise
    Mactec54

  3. #3

    Join Date
    Sep 2018
    Posts
    13

    Re: Limit Switch Noise Fixed!

    Quote Originally Posted by mactec54 View Post
    No it fixed your limit switch problem, the noise is still there are you using any EMI Main Power Filters as this normally will solve most EMI noise
    My limit switches are fine, the problem was noise while it was machining parts causing it to freeze up in mach 3. The machine would think a limit switch triggered, but the machine was not touching any limit switches while machining. I'm using a filter, the one I built!

  4. #4
    Community Moderator Al_The_Man's Avatar
    Join Date
    Dec 2003
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    23376

    Re: Limit Switch Noise Fixed!

    The only thing with filters, they may cure the problem but the cause is still there, I have lost track of the amount of post That have had apparent spurious L.S triggering where I have advised on earth bonding all supply commons and possible equipotential earth bonding of the machine and it has cured the problem.
    Unfortunately many of these legacy posts are no longer there to quote as they disappear after a certain age due to forum 'cleaning'
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  5. #5

    Join Date
    Sep 2018
    Posts
    13

    Re: Limit Switch Noise Fixed!

    Quote Originally Posted by Al_The_Man View Post
    The only thing with filters, they may cure the problem but the cause is still there, I have lost track of the amount of post That have had apparent spurious L.S triggering where I have advised on earth bonding all supply commons and possible equipotential earth bonding of the machine and it has cured the problem.
    Unfortunately many of these legacy posts are no longer there to quote as they disappear after a certain age due to forum 'cleaning'
    Al.
    I never did the earth bonding. I just hooked all the ground wires together. But yeah I'm glad this cured the problem, even if the grounding is still not perfect, my issue is fixed so I'm happy and making parts.

  6. #6
    Member
    Join Date
    Jan 2005
    Posts
    11275

    Re: Limit Switch Noise Fixed!

    Quote Originally Posted by patrick31 View Post
    My limit switches are fine, the problem was noise while it was machining parts causing it to freeze up in mach 3. The machine would think a limit switch triggered, but the machine was not touching any limit switches while machining. I'm using a filter, the one I built!
    The reason you had noise that caused false triggers is because of bad Grounding somewhere in your system and no Mains Power Filter the filter you made is just a band-aid, the noise is still there which will affect other parts of your electronics over time

    This is one of the most common problems with Hobby builds and easy to fix without the filter you made, this type of filter you made normally will slow down the reaction time of the limit switches

    Are you using a VFD Drive for your Spindle control / Drive if so this is where most of the EMI noise comes from
    Mactec54

  7. #7

    Join Date
    Sep 2018
    Posts
    13

    Re: Limit Switch Noise Fixed!

    I appreciate your willingness to diagnose possible underlying problems with my machine. The one thing you are correct about, is that the filter board slows down the reaction time. That is an engineered value. You can see that in action in my first post with the logic analyzer screenshot.

    I do have a VFD. I've also tested your theory as that was mentioned on the forums here. It ran the program without the spindle/without cutting, it would still kick out due to limit switch errors.

    I scoped the switches, there was high frequency noise coming from the steppers under high acceleration moves (my machine runs a heavy vice, heavy spindle/headstock) and those high current moves were inducing noise into the limit switch wires.

    My motion control board sends the weak 12v signal from the switches through a resistor, then straight to the CPU pin. Bad design, but I didn't design that. The board should have had it's own engineered filter for the inputs. My filter board solved that problem. You can call it band-aid 20 more times, but I have had zero issues since installing this board, and am very happy to be able to make parts without issue.

  8. #8
    Member
    Join Date
    Jan 2005
    Posts
    11275

    Re: Limit Switch Noise Fixed!

    Quote Originally Posted by patrick31 View Post
    I appreciate your willingness to diagnose possible underlying problems with my machine. The one thing you are correct about, is that the filter board slows down the reaction time. That is an engineered value. You can see that in action in my first post with the logic analyzer screenshot.

    I do have a VFD. I've also tested your theory as that was mentioned on the forums here. It ran the program without the spindle/without cutting, it would still kick out due to limit switch errors.

    I scoped the switches, there was high frequency noise coming from the steppers under high acceleration moves (my machine runs a heavy vice, heavy spindle/headstock) and those high current moves were inducing noise into the limit switch wires.

    My motion control board sends the weak 12v signal from the switches through a resistor, then straight to the CPU pin. Bad design, but I didn't design that. The board should have had it's own engineered filter for the inputs. My filter board solved that problem. You can call it band-aid 20 more times, but I have had zero issues since installing this board, and am very happy to be able to make parts without issue.
    So you new where the noise was coming from so why did you not fix that source, before you made the board, there is nothing wrong with you making the filter board, just not needed if you had fixed the problem

    Sounds like your motion control board is not great as well which one did you use is good to post about control boards that could cause problems, so others don't make the same buy
    Mactec54

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