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New programmer feeling my way around. I set the most simple "steering block" to 25, the power to 50, and the number of motor rotations to 40 or 50.

Then I let it run expecting the bot to go in a circle. Instead it circled around somewhat erratically with roughly the right radius, but without coming anywhere close to following the same track.

Hard to tell exactly what it was doing but maybe the outside wheel on the turn was moving steadily forward and the inside wheel (the slower one) was progressing unevenly.

What do I need to do to get nice clean circles?

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It is quite common for the tires to slip and cause this type of behavior. For example, here is a similar question. The answer here is the same.

  • Make sure your center of gravity puts most of the weight of your bot on the drive wheels.
  • Make sure the tires and the surface your are driving on are free of dirt/dust.
  • Use sensors to help compensate for any slippage of the wheels.

If you only want your robot to go in a perfect circle and nothing else, you could fix a beam to the floor on one end an your robot on the other so that it cannot deviate from the circle.

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  • Thanks again David.A little weight on the wheels did the job. Like magic.. Motivation was to be able to program exact courses using arcs and line segments.
    – Bill Myers
    Jun 26 '15 at 0:44
  • What is the best solution? would a fabric surface like a pool table work better? What about using sandpaper or a wire brush on the wheels?
    – Bill Myers
    Jun 27 '15 at 17:33
  • I have heard second hand that using sandpaper to rough up the wheels does increase traction. It should be noted though that for anyone participating in FIRST LEGO League or another competition that prohibits the modification of LEGO parts, this is against the rules. "Best" is a subjective term, so you will have to experiment and come to your own conclusion.
    – David Lechner
    Jun 28 '15 at 0:45

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