Minibot wheel slip

[Gary Dillard]

Originally Posted by vamfun

As others have pointed out, even if you increase your traction, you must have the normal force necessary to utilize it. You can calculate the normal force required for non slip pretty easily. Typically if you are using two motors (14v battery) with a 2:1 gearing you will need about 7.2 lbs of normal force.

Make sure you test this by pulling the minibot off the pole with a with a scale reading the normal force(easy to do with magnet minibots not so easy with others).

There is a trade off between normal force that won't allow slippage and the drag force that occurs from the normal force on the bearings. I have done some simulation trade offs and it seems that you want the Normal force to be about 2x the weight. Any extra normal force will only hurt due to the extra drag.

Good calcs. I assume when you measure the drag coefficient with the motor "disconnected" you mean mechanically and not just electrically, otherwise the significant gearbox drag is in the equation. Did you measure the static friction coeffient between the pole and the wheels? 1.2 seems awfully high with that slick pole, regardless of what you do to the wheels.

[vamfun]

Originally Posted by Gary Dillard

Good calcs. I assume when you measure the drag coefficient with the motor "disconnected" you mean mechanically and not just electrically, otherwise the significant gearbox drag is in the equation. Did you measure the static friction coeffient between the pole and the wheels? 1.2 seems awfully high with that slick pole, regardless of what you do to the wheels.

Yes, mechanically disconnected. (updated post to reflect this)

The static friction coefficient was not measured. I just picked a value one can strive toward. You have to use what your design dictates.... maybe more in the neighborhood of .8 if nothing special is done to increase it. So you might need more normal force than I showed.

I ran the acceleration test to really determine the magnetic drag. I suspected that it was limiting the magnetic minibots and putting them at a disadvantage. I was unable to detect any significant acceleration that was a function of speed, but the data set was fairly noisy. It would be interesting to see what the distribution of speeds are for the magnetic minibots vs the clamp on's.