Free 173 Best Homemade Tools eBook:  
Results 1 to 9 of 9

Thread: Adaptive Backlash Reduction (well... almost)

  1. #1
    Supporting Member rgsparber's Avatar
    Join Date
    Nov 2012
    Location
    Phoenix, AZ
    Posts
    653
    Thanks
    224
    Thanked 1,226 Times in 361 Posts

    rgsparber's Tools

    Adaptive Backlash Reduction (well... almost)

    This article presents a partially worked out means of automatically reducing variable backlash in a CNC system. All was going just fine until I ran right into a bit of reality. However, others will hopefully see a good solution to the problem that blocked me and the scheme can be reduced to practice.

    If you are interested, please see

    http://rick.sparber.org/ABR.pdf

    Your comments are welcome. All of us are smarter than any one of us.

  2. The Following 3 Users Say Thank You to rgsparber For This Useful Post:

    Jon (09-16-2015), kbalch (09-15-2015), PJs (09-16-2015)

  3. #2
    Andy from Workshopshed
    Supporting Member
    Workshopshed's Avatar
    Join Date
    Mar 2013
    Location
    North London, UK
    Posts
    202
    Thanks
    126
    Thanked 131 Times in 85 Posts

    Workshopshed's Tools
    Hi Rick,
    an interesting idea and definitely worth investigating. I'd be concerned that dirt and other containments on the screw might affect the reliability.

    I've seen variations on this using a digital readout to get the absolute position of the moving parts. You might also be able to look at the current flowing through the motor, with a low load that could be less than when the motor is actually moving the parts.

    Also perhaps a load sensor in the thrust bearings of the lead screw might work?

    Good luck with your experiments.

    Andy
    Andy from Workshopshed
    "Making and repairing things in a shed at the bottom of the garden"
    workshopshed.com

  4. The Following User Says Thank You to Workshopshed For This Useful Post:

    C-Bag (09-16-2015)

  5. #3
    Supporting Member C-Bag's Avatar
    Join Date
    Aug 2015
    Location
    California, central coast
    Posts
    720
    Thanks
    689
    Thanked 845 Times in 470 Posts

    C-Bag's Tools
    Quote Originally Posted by Workshopshed View Post
    Hi Rick,
    an interesting idea and definitely worth investigating. I'd be concerned that dirt and other containments on the screw might affect the reliability.

    I've seen variations on this using a digital readout to get the absolute position of the moving parts. You might also be able to look at the current flowing through the motor, with a low load that could be less than when the motor is actually moving the parts.

    Also perhaps a load sensor in the thrust bearings of the lead screw might work?

    Good luck with your experiments.

    Andy
    I'm glad you jumped in here Andy. This like so many of Rick's journey's into realms I know little about I don't feel qualified to respond to. But my take on it was similar. I've contemplated how to get exact coordinates in the x and y axis with the sloppy lead screws on my mill and it all comes down to a DRO. Now how that can be interfaced with a program like Mach3 is beyond my pay grade. That would seem to be the only way to truly know exactly where you are irrelevant to the slop in the leadscrews. Does anybody use DRO output instead of motor pulses?

    But all this is why I backed off on getting a CNC plasma table. When you started getting into the servo vs stepper etc etc my brain started to swim. Also looking into people converting existing mills to CNC and the expense of ball screws, adapting them and then the motors and interfaces makes me salute them for their tenacity.

  6. #4
    Andy from Workshopshed
    Supporting Member
    Workshopshed's Avatar
    Join Date
    Mar 2013
    Location
    North London, UK
    Posts
    202
    Thanks
    126
    Thanked 131 Times in 85 Posts

    Workshopshed's Tools
    I found this comment about using a DRO with Mach3, it's only used for display not for feedback

    can DRO be used to make closed loop system?

    This is seconded by another respondent

    http://www.model-engineer.co.uk/foru...s.asp?th=96662
    Andy from Workshopshed
    "Making and repairing things in a shed at the bottom of the garden"
    workshopshed.com

  7. The Following User Says Thank You to Workshopshed For This Useful Post:

    Paul Jones (09-16-2015)

  8. #5
    Supporting Member C-Bag's Avatar
    Join Date
    Aug 2015
    Location
    California, central coast
    Posts
    720
    Thanks
    689
    Thanked 845 Times in 470 Posts

    C-Bag's Tools
    that really explained a lot. The two systems are really speaking different language essentially. The Mach3 generates everything internally and outputs the #'s to the steppers. And that's why there can't be any play and why ball screws are used. But it's also why there can be so much problem with precision because this stuff runs in a hostile environment.

    I can also see why at the speeds CNC wants to run outside loop as in a DRO would be slower. But it makes you wonder at the speeds the outputs on the DRO and the inputs to the software are running now how slow it would be in reality?

  9. The Following 2 Users Say Thank You to C-Bag For This Useful Post:

    Paul Jones (09-16-2015), PJs (09-17-2015)

  10. #6
    PJs
    PJs is offline
    Supporting Member PJs's Avatar
    Join Date
    Sep 2014
    Location
    Northern CA
    Posts
    1,840
    Thanks
    8,317
    Thanked 1,042 Times in 684 Posts

    PJs's Tools
    Hi Rick, I've followed your great work () for some time now and this post has inspired me to chime in my 2 cents worth. The idea you have for the adaptive system is quite good but has its limitations as you point out. The other thing that came to mind is if you needed to perform an operation at a float point then your system would automatically push or pull to the no backlash point although by your measuring seems to rest in the bottom half. Also steppers are kind of a pain in that they tend to fall off position under load, which can lead to other issues, so I tend to lean toward servos. Using some kind of quadrature add-on system might give some advantage but adds to the complexity and interface although I believe M3 has a quadrature interface option. The real issue to me is all the variables in the screw, nut, end play, concentricity, bow, etc. and it occurred to me that maybe approaching this more from the mechanical aspects (as you have been) would give you more leverage on the tolerance stack.

    If you go to the trouble of making a 2 material split nut, why not build or buy a ball screw nut that would fit the pitch of the screw. They still have an inherent slop but much less than the standard nut. If the balls could match the pitch of the screw then possibly it could be split horizontally (instead of vertically) and spring load the top half (not sure if that's possible in your nut mounting) to be more like half nuts that force the balls into the pitch of the screw. I've seen split ball nuts somewhere in my travels but slept since then...and have CRS sometimes. ~¿@ This I think would give you constant contact with the balls to the pitch over the entire run, except for excessive bow slightly larger than the width of the nut. Also not sure how it might be affected by high travel speeds. Hemingway, my 2cents. Great post and got me thinking too!! Thanks, ~PJ
    ‘‘Always do right. This will gratify some people and astonish the rest.’’
    Mark Twain

  11. The Following 2 Users Say Thank You to PJs For This Useful Post:

    C-Bag (09-16-2015), Paul Jones (09-16-2015)

  12. #7
    Jon
    Jon is offline Jon has agreed the Seller's Terms of Service
    Administrator
    Supporting Member
    Jon's Avatar
    Join Date
    Jan 2012
    Location
    Colorado, USA
    Posts
    16,804
    Thanks
    4,022
    Thanked 17,090 Times in 5,027 Posts
    Thanks Rick! I've added your Adaptive Backlash Reduction Method to our CNC category, as well as to your builder page: Rick Sparber's Homemade Tools. Your receipt:

    Homemade Tools Supporters Forum
    Tool plans, build guides, ebooks, and more.

    Click here to become a supporter

  13. #8
    Content Editor
    Supporting Member
    DIYer's Avatar
    Join Date
    Aug 2013
    Posts
    3,091
    Thanks
    460
    Thanked 981 Times in 904 Posts
    An interesting idea, Rick. I'm waiting to see if you can come up with a definitive solution for your problem, or adopt a different method.

  14. #9
    Supporting Member rgsparber's Avatar
    Join Date
    Nov 2012
    Location
    Phoenix, AZ
    Posts
    653
    Thanks
    224
    Thanked 1,226 Times in 361 Posts

    rgsparber's Tools
    The method I am using right now is to precisely align my X and Y lead screws and use modified bronze take up nuts. It gets me a backlash of 2 thou on each axis with no binding. I'm releasing articles describing this work as time permits. The Y axis is out now. Will release the X axis nut mod tomorrow morning and then only have the X axis leadscrew alignment description to do.


    Post your reply!
    Join 41,949 of us and get our 173 Must Read Homemade Tools eBook free.



    173 Must Read Homemade Tools

  15. The Following User Says Thank You to rgsparber For This Useful Post:

    PJs (09-18-2015)

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •