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  1. #1
    Join Date
    Jan 2010
    Posts
    76

    Stall Sensor for Steppers?

    I have run into a few instances with my machine where i had a motor stall that could have been disasterous if i had not been right by the machine (caused the router to plunge collet deep into the work).

    Is anyone familiar with a way to monitor the motor current of a stepper (unipolar or bipolar) to cut the machine off in case of a detected stall?

    I was wondreing about using a low amperage circuit breaker in series with the motor windings (assumption: current spikes when motor stall?) tied into a relay that feeds back into the limit switch connections?

  2. #2
    Join Date
    Nov 2003
    Posts
    244
    encoders
    ...He who makes no mistakes makes nothing! ...
    Tom

  3. #3
    Join Date
    Apr 2007
    Posts
    8082
    You mean something like this?

    Been there, done that....


    CarveOne
    Attached Thumbnails Attached Thumbnails DSCN0016.jpg  
    CarveOne
    http://www.carveonecncwoodcraft.com

  4. #4
    Join Date
    Mar 2003
    Posts
    35538
    http://rogersmachine.net/encoderinterface.html

    But, it's cheaper and easier to just run them within their capabilities. They only stall if you try to go faster than they're capable of.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  5. #5
    Join Date
    Jun 2007
    Posts
    3757

    Limit the speed

    Why do want to push the speed too close to the limits?
    Use bigger motors or back off.
    Cutting too much air at high speed? Optimize tool cutting paths.
    Run within the proper limits steppers NEVER stall.
    For reliability I have reduced 2500mm/min to 1500, and with expensive tooling and material have never had a problem in over 1000 hours, but might have saved 10 hours and lost $$$$$s of time and expensive materials. 80000++ parts and no stuffups, all on a syil SX3.
    If you want a rocket ship use a servo based system and pay the money.
    Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.

  6. #6
    Join Date
    Jan 2010
    Posts
    76
    I don't want to push the steppers faster than they are capable of, but it would be nice to have the extra peice of mind of stall protection, as you never know when something might go wrong. Something mechanically could fail (timing belt break, screw vibrate loose, etc) and then you could stall the machine, even if you are not running the steppers faster than they are normally capable of. I don't want to overengineer, but I like safeguards just in case. I had thought about encoders but was hoping for something a little less pricy. Does anyone know if my assumption about power spiking when steppers stall is wrong?

  7. #7
    Join Date
    May 2005
    Posts
    1662
    Quote Originally Posted by basskitcase View Post
    Does anyone know if my assumption about power spiking when steppers stall is wrong?
    Don't know, interesting idea.

    Digikey has some nice rotary encoders for ~$30, maybe more than you want to spend. A resourceful electronics guy can probably salvage something functional for $0. Being neither resourceful or electron literate I paid Digikey. An encoder reading directly off the screw (not the motor) would cover +90% of the common failures. That said, if there's a cheaper way than encoders let's hear it

    edit/ well +90% of the mechanical failures. The nut behind the wheel is usually the weak link
    Anyone who says "It only goes together one way" has no imagination.

  8. #8
    Join Date
    Jun 2007
    Posts
    3757

    Don't rely on electroinics to protect you.

    Do the protection mechanically. eg belt.
    Sense failure, by belt tension or similar
    It is emergency stop dropping out a safety relay (you should already have).
    Keep the protection SIMPLE. Never rely on anything electronic.
    Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.

  9. #9
    Join Date
    Jan 2010
    Posts
    76
    Quote Originally Posted by cyclestart View Post
    Don't know, interesting idea.

    Digikey has some nice rotary encoders for ~$30, maybe more than you want to spend. A resourceful electronics guy can probably salvage something functional for $0. Being neither resourceful or electron literate I paid Digikey. An encoder reading directly off the screw (not the motor) would cover +90% of the common failures. That said, if there's a cheaper way than encoders let's hear it

    edit/ well +90% of the mechanical failures. The nut behind the wheel is usually the weak link
    So the encoders themselves seem reasonable, but don't i need an additional interface board, or can I tie them directly into Mach3 somehow?

  10. #10
    Join Date
    May 2005
    Posts
    1662
    Quote Originally Posted by basskitcase View Post
    So the encoders themselves seem reasonable, but don't i need an additional interface board, or can I tie them directly into Mach3 somehow?
    Sounds like a question for Mach support ?

    Here's something from the guy at candcnc
    http://www.cnczone.com/forums/showpo...41&postcount=4
    He suggests it's possible(?) although you might want use a relatively low count encoder if counting in software. The CUI amt encoders can be set within a range of counts/rev.
    Anyone who says "It only goes together one way" has no imagination.

  11. #11
    Join Date
    Mar 2003
    Posts
    35538
    You can wire them to a parallel port. But, if you want them to stop your machine, you'll need to write a plugin or macropump to do it.

    The encoder interface I linked comes with a plugin to stop the machine when it loses position.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  12. #12
    Join Date
    Feb 2009
    Posts
    1290
    This is more what I was looking at when I asked about encoders on steppers a while ago. I to would just like to stop my machine if a certain (x) resolution threshold has been reached or compromised. I’m not looking at having Mach3 control the steppers. I also was thinking about having some sort of sensor on the work piece to tell me if it has moved. I do a lot of PCB/Plastic routing where I hold the material down with double sided tape so it would be nice to know if the work piece has moved even slightly.

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