Generic Kflop glass scale closed loop question


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    Default Generic Kflop glass scale closed loop question

    Kindly please help me to figure out the following generic situation:
    a simple router with steppers, Kflop and closed loop with glass scales. For
    simplicity, I will use for length measurement microsteps.
    Lets presume that the glass scale have 5 microsteps resolution, the mechanical
    backlash is 50 microsteps and the movement is made in 1 microsteps increments.
    The desired level of accurate positioning is 10 microsteps.
    What I am aware is that I left out as input infos the inertia and the cutting forces.
    Based on real life examples, within a range of weights, speeds and forces involved,
    what do you predict in terms of reducing the effect of backlash (if possible) and also please comment the relation between backlash , desired accuracy and scale resolution
    Thanks,
    Adrian

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    Default Re: Generic Kflop glass scale closed loop question

    Hi Adrian,

    Sorry, but I don't think there is an easy way to predict the accuracy of such a system without an extensive analysis that takes into consideration all the mechanical dynamics, masses, compliances, cutting forces, friction, resonances, etc.

    With a backlash of 50 microsteps there is basically no way to guarantee errors to be less than 50 microsteps. This is because any sudden force could instantly move the axis 50 microsteps. The closed loop feedback would then attempt to correct the error. How fast the error could be corrected will depend on the system dynamics.

    So in cases where loads and cutting forces occur gradually and the system is low mass and stiff then backlash effects may be reduced somewhat, but otherwise possibly not significantly reduced at all.

    As a rule of thumb to achieve a certain precision/accuracy of X it is best to have a measurement and movement resolution of at least 10X better. So attempting an accuracy of 10 with measurement of 5 is unlikely to work. The main need for higher measurement resolution it to know velocity. In that case a motion of 5 microsteps is required to have any information on whether the axis is moving. And then there will be virtually no information on its velocity.

    HTH

    Regards
    TK http://dynomotion.com


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    Default Re: Generic Kflop glass scale closed loop question

    Thanks for your explanation , now I have a better understanding of the closed loop with glass scales, which at first glance looks so appealing. I was not aware of the obvious fact that an axis can not have a better positioning accuracy than the mechanical backlash.
    Best Regards ,
    Adrian



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Generic Kflop glass scale closed loop question

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Generic Kflop glass scale closed loop question