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Precision Control of a Robot Encoders or Steppers? — Parallax Forums

Precision Control of a Robot Encoders or Steppers?

172heavy172heavy Posts: 55
edited 2011-11-01 18:53 in Robotics
What is the most precise method to control a robot? I noticed that the S2 incorporated the use of Encoders, would it be more precise to use Stepper Motors wihout feedback? I am just trying to get an idea of the best way to repeatably predict the exact location of the robot for a given set of commands.

Comments

  • Martin_HMartin_H Posts: 4,051
    edited 2011-11-01 16:24
    I've only used motors with encoders. This works well, but wheel slip is the biggest source of error. This happens when you don't ramp up to speed or stop abruptly. Also if the robot's wheels are free wheeling because it is stuck you will be completely lost.

    I imagine the same is true for stepper, but rather than count pulses in, you count pulses out.
  • ercoerco Posts: 20,256
    edited 2011-11-01 17:04
    If your stepper stalls, all position count is lost. I prefer optical encoders.

    Preferably made of wood. I'm searching for Zebra wood with the perfect natural stripe spacing. :)
  • PublisonPublison Posts: 12,366
    edited 2011-11-01 17:08
    172heavy wrote: »
    What is the most precise method to control a robot? I noticed that the S2 incorporated the use of Encoders, would it be more precise to use Stepper Motors wihout feedback? I am just trying to get an idea of the best way to repeatably predict the exact location of the robot for a given set of commands.

    The Holy Grail of Robots, "exact location".

    It doesn't matter whether you use a DC Motor, Servo, or Stepper with and encoder, they will all report revolutions. They problem is, if there is any slippage on the driving tires, you calculations on your final distance are not going to be valid.

    You need to have a have have a homing, (reference) beacon to reference to keep a robot on course. This could be GPS, (not very accurate), or IR, (line of sight only).
  • TtailspinTtailspin Posts: 1,326
    edited 2011-11-01 18:18
    Preferably made of wood. I'm searching for Zebra wood with the perfect natural stripe spacing. :smile:
    Oh no erco, you did not just go there... Hrmmm.. Ebony, and Iron wood or Hemloc, should contrast nicely against each other.
    I just happen to have a need for an encoder set-up, Now where did I put that circle glue-up jig?...


    -Tommy
  • Tom CTom C Posts: 461
    edited 2011-11-01 18:53
    I am presently using two of the Parallax Position Controller kits (http://www.parallax.com/Store/Robots/RoboticAccessories/tabid/145/CategoryID/22/List/0/SortField/0/catpageindex/2/Level/a/ProductID/665/Default.aspx) on my six wheeled robot rover. The Position Controllers are driven by the middle wheel on each side of the rover chassis while the two outer wheels on each side of the chassis are powered by geared electric motors. The center wheels are attached to the chassis slightly below the centerline of the dirve wheel attachement points which ensures that they make good contact with ground. So far they have provided good control of the rover speed, but I have yet to try them in an atonomous mode where the rover drives a programmed distance based on the position controller feedback. However, Steve Norris has successfully used the position encoders on his Nomad HD Rover (http://www.botmag.com/issue25/) for moving a programmed distance.
    Regards,
    TCIII
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