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Improving accuracy and precision

Discussion in 'General Talk' started by joe5, Aug 4, 2016.

  1. joe5

    joe5 New
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    Dear Readers,

    The open builds actuator testing data sheet reports a repeatability of 0.078 mm and a accuracy of 0.091 mm for the C-Beam™ Linear Actuator Bundle Pack. How could I improve the repeatability and accuracy to reach 0.02 mm or better? Would changing the lead screw from 8mm/turn to 5 mm/turn help?

    The torque and load of the part which will be transported are negligible and the travelling speed is of minor importance.

    Thank you for your help and with my best regards.
     
  2. Jonathon Duerig

    Jonathon Duerig Veteran
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    When looking at the overall accuracy of a machine, I think that the odds of individual linear actuators being the limiting factor is small. Almost any minor flaw in the machine can be the weak link that prevents you from getting 0.02mm accuracy. The flex in the (plastic!) wheels, even the flex in an aluminum cross beam. Having two separate steppers that might be slightly out of sync. A very slightly non-square machine. I am not entirely sure how it would even be possible to get that kind of accuracy in a DIY rig without expensive and specialized tools even if the materials were theoretically capable of it.

    OTOH, if you figure out how to do it, I'd love to know more about how you managed it. The precision I can get with my CNC router makes me happy out of proportion to the actual usefulness of said precision in my projects. :)

    -D
     
    GrayUK likes this.
  3. David the swarfer

    David the swarfer OpenBuilds Team
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    Theoretically we can calculate the needed thread or belt reduction. However, we must remember that microstepping is not accurate so we are going to have step to step variations which depend on load and other factors no matter how much resolution we build in.
    The only way around this is to use servos or steppers with feedback, like these....


    Anyhow, back to calculating:

    For our usual M8x8 leadscrew, 200 step/rev motor, at various microstepping settings:
    8 mm per rev / (200 * microsteps) = mm per step: this is our theoretical resolution
    8 / (200 * 1) = 0.04 mm/step full steps or 1x microstepping
    8 / (200*2) = 0.02mm/step 2x microstepping
    8 / (200 * 4) = 0.01mm/step 4x microstepping
    8 / (200 * 8) = 0.005mm/step 8x microstepping

    Looks nice hey!? Just up the microstepping to 8x and we should be able to get hit 0.02mm every 4 steps.
    or, 4x will give us more power, and 2 steps should give you your .02mm travel.
    more microstepping gives less power though, and less power gives more error per step.
    BUT....
    there are many sources of error that will work against us here...
    • the leadscrew, how was is made? ground, machined, rolled? I have a rolled one from Banggood, it sucks.
    • backlash, play in the nut to leadscrew fit.
    • bent? this will give periodic binding.
    • motor: accuracy of the windings, a few extra turns on one winding will make a difference
    • motor: accuracy of the stator and magnet assembly
    • driver: there are various control methods, each suited to different motors types and voltage ranges etc.
    • mechanical resonance
    • mechanical resistance.
    So, while achieving 0.02mm repeatability and accuracy (not the same thing) is possible, the components are going to be expensive, and you will need to set it up quite carefully.

    You will need a really good motor and driver combo, and at least a ground ballscrew to get near to 0.02mm.

    That does make it easier.... heavy loads throw the microstepping off.
     
    Joe Santarsiero likes this.

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