Minibot wheel slip

[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.

[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.

[IKE]

Originally Posted by vamfun

... 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.

Also, the stall torque is approximately 2X the peak power torque for the electric motors. Because of this, you want at around 2x the traction to insure minimal wheel slip (assuming you designed around peak power).

We had a design that had too much friction, and it would do a bizarre "powerhop" maneuver. It is actually pretty neat to see a little 20W robot powerhopping on the pole.

We have since gone in a different design direction (well, we have gone in several directions since then, but who is counting).

[J.Warsoff]

hey guys, thank you all for the advice you have given me. i switched the gearing around to increase the torque, so i will test it today and see how it works

[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.

[scooperman]

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

Vamfun, great tool, very helpful, thank you.
Just wondering, did you really intend to use a 6" wheel diameter (0.25 ft radius) as your baseline case for this study, or is that a typo?

[boomergeek]

From a physics POV, doesn't the formula need a component for the desired acceleration rate up the pole?

"Peeling out" would be reduced by greater normal force.
Faster accelerating minibots benefit from greater normal force. There is a balance needed to minimize normal force for gearing friction and maximizing normal force to minimize peeling out in the drag race. Maximizing coefficient of friction is always a good target.

[vamfun]

Originally Posted by boomergeek

From a physics POV, doesn't the formula need a component for the desired acceleration rate up the pole?

"Peeling out" would be reduced by greater normal force.
Faster accelerating minibots benefit from greater normal force. There is a balance needed to minimize normal force for gearing friction and maximizing normal force to minimize peeling out in the drag race. Maximizing coefficient of friction is always a good target.

I agree, you could set the normal force, N, solely based upon acceleration requirement rather than no_slip.

My simulations have shown that the no_slip condition is not the optimum. After the initial acceleration to steady state speed, the extra drag caused by the high no_slip N causes an unwanted power loss. There is an optimum N that balances the acceleration and steady climb phase ... and for our robots it lies between 1.8 and 2.0 * weight.