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Could use opinions on motion and positioning options — Parallax Forums

Could use opinions on motion and positioning options

T ChapT Chap Posts: 4,223
edited 2008-09-11 06:06 in General Discussion
What is the best method for accurately homing a linear motion project on power up? I have broken several fixed switches on CNN machines, so I tend to want to avoid physical switches that are in the 'path'. I am looking for a tolerance of 1/32" or less accuracy on every power up when the machine homes automatically. Reed switches? Hall sensors? LED? It is not a CNC machine, so extreme precision is not required.

I have one pin available on the Prop with a pullup, so that grounding a switch (switches) on each end of travel sets the limits. An encoder tracks things from there.

Any suggestions welcome.

Comments

  • FranklinFranklin Posts: 4,747
    edited 2008-09-11 03:29
    An optical slot switch should work but the main question is why are you breaking the switches? If properly set up the motors should stop before the parts get to the point of breaking anything. Perhaps you should look at that first.

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    - Stephen
  • T ChapT Chap Posts: 4,223
    edited 2008-09-11 04:02
    The main point being that I will not be the one setting up the systems, others will be in most cases. If someone slams it into the stops, the switches are

    The object being moved weighs over 100 lbs, so there is very little forgiveness on the switches if they are in the path.

    In a perfect world, the motor would stop at the point of contact, but in reality the object will be slid around often even when power is off, and the motors and controller are not even on, rubber stops are required to prevent to hard an impact.

    For these reasons, I want something non destructive, so that should the object slam against the stops, the switches survive. The optical slot sounds interesting, I will google it up.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2008-09-11 05:54
    The reason that you're breaking switches may be that you're using manual switches and not limit switches. Limit switches are designed to accommodate a certain amount of overtravel, whereas manual switches are not. However, you still have to match the switch to the task at hand. Many Microswitch-style lmit switches are just too flimsy to endure many cycles on heavy machinery. You may need something that looks

    like these l_334.jpg rather than these 180px-Microswitches.jpg.

    -Phil

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    'Still some PropSTICK Kit bare PCBs left!
  • T ChapT Chap Posts: 4,223
    edited 2008-09-11 06:06
    Good point. I will find those tomorrow.
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