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uneven servo movement

Discussion in 'DrawBot' started by mccoyspace, Jan 15, 2020 at 3:58 PM.

  1. mccoyspace

    Builder

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    I've got a new Arco 1015+BB plotter build running in a 2 stepper + servo configuration. I've made a z-axis using a drawer slide driven by the servo. I've gotten everything working and have made some initial drawings. Very cool, but I'm noticing an issue.

    The servo doesn't change position very smoothly. For example, in the servo setting wizard, when using the sliders to set the UP and DOWN values, changing the value from say 167 to 165 may not move the motor. I have to go from, for example 167 to 161 to get it to change position and then when it does it jumps. The effect is that the positioning is jumpy and quantized. This is a real pain when you are trying to set a pen down position where a millimeter or two really matters. The steppers on X & Y are smooth as silk, but the servo (a Futaba S3003) not as much. Are there suggestions for how to improve this behavior?

    (note that it does this even if I disconnect it from my z-axis, so I know its not an issue of mechanical binding)
     
  2. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
    Staff Member Moderator Resident Builder Project Maker Builder

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    Did you remember to set $30=255?
    Steppers are far superior to servos (only 255 steps on a servo, where a stepper motor at 1/8 Microstep, has 1600 steps per rotation) so keep that in mind.
    Depending on the servo itself it may also interpolate some steps

    Pen down should be "floating" not only to cope with this, but also with uneven surfaces below the pen, servo "drops out of the way" and gravity/rubber band/spring etc presses pen to paper. Servo arm swings in and lifts pen up against gravity (or whatever other mechanism you used to provide downward force). We have a design that will be released sometime in the near future but the preview in the wizard shows how it works (look closely at the picture on the down-calibration step, the arm doesnt PUSH down - it just moves out of the way so the carriage can drop down )
     
  3. mccoyspace

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    yes to $30=255

    and thanks for the mechanical suggestions. the solution probably lies that way. I'll need a better design.
    and I'll look more closely at the drawing on the wizard screen for ideas.
    thanks!
     
  4. mccoyspace

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    follow up question on the "floating". It isn't possible to turn the servo off. sending 0 *pushes* and holds it at its lowest position under tension. so how does it ever float? I'm guessing design-wise, the servo's lower position would have to be below the lowest float position that the pen holder would have? gets tricky....
     

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