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Accuracy

Discussion in 'General Talk' started by Praveen Jain, Jan 24, 2018.

  1. Praveen Jain

    Builder

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    So my main question was about the accuracy of controlling the trolley on the pulley driven linear actuator designed by Mr. Mark carew
     
  2. David the swarfer

    David the swarfer OpenBuilds Team
    Staff Member Moderator Builder

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    The basics.....
    (pulley teeth * belt pitch) / (motor steps per rev * microsteps) = mm per step

    This gives you a theoretical distance per step (resolution).
    what makes it vary from the theory?
    • belt stretch
    • microstepping
    • forces on the belt
    belt stretch. belts stretch, they always do. how much depends on tension and age and size and temperature.
    this means the pitch is not exactly the number printed on the side, but slightly longer, which gives longer travel peer tooth.

    microstepping. microstepping is great, but it is not 100% accurate. the distance from step to step varies depending on motor construction, driver construction, and load.

    forces on the belt = load. if there is a constant weight on the belt then the motor shaft will be rotated by the load by an amount that can be measured and compensated for. if the load varies then the 'out of position' rotation amount will also vary. how much and where this will occur will depend on the motor, drivers, power supply voltage and more.

    an example....
    my Nguni has 2mm pitch belts and 20 tooth pulleys. 200 step per rev motors and 8x microstepping
    this gives 0.025mm per step theoretical resolution. the accuracy will be more than that, maybe as much as 3 to 5 times as much depending on the varying loads. I don't bother to jog less than 0.1mm.
     
    MaryD likes this.

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