Minibot wheel slip
 

hi everyone. so after much redesigning, my minibot is nearly finished, i just need a locking mechanism, which is being worked on. so i was testing my minibot on the pole, keeping it on with zipties. it certainly has enough grip on the pole, but even so, the single drive wheel still slips, and the bot does not climb. is there some kind of modification i could do to the wheel/tire to increase traction? someone suggested i use lighter fluid to irritate the rubber and give it a little more traction, but i want to know what you guys think. any suggestions will be appreciated.

Re: Minibot wheel slip
 

We're still getting some slippage (the minibot falls down the pole a little but keeps going) with 2 motors and 4 wheels and enough magnets to almost pull the pole out of the base (exaggeration, but you get what I mean). It's the wheels itself, and that is definitely on our radar.

As for suggestions, try putting two motors on it, more torque > more pushing power, if you can afford the weight/space. As for wheel modifications, I don't know anything for sure.

Re: Minibot wheel slip
 

Try grinding the rubber to match the diameter of the pole. We found spinning it up on a lathe and using a dremel tool with a grinding wheel spinning opposite the wheel rotation works well, followed by using some 80 grit sandpaper to "adjust" the diameter.

Re: Minibot wheel slip
 

we actually just turned the tires inside out, and it matches the diameter. even with that and the fact that the wheel it right up against the pole, it still slips

Re: Minibot wheel slip
 

im using one motor, and it has more than enough power. that's not my issue though. plus, i do not believe i can afford the space/weight.

i jus want to get the tires to have more traction

Re: Minibot wheel slip
 

try wrapping rubber bands or surgical tubing around the wheels

Re: Minibot wheel slip
 

Move the battery away from the pole, slightly -- definitely do not put it right up against the pole. Put it level with or just slightly below the traction wheel in terms of vertical positioning. This change in center of mass creates a pivot point at the traction wheel. That pivot point will then create a moment at whatever contact is on the opposite side of the pole. If that contact point is higher than the traction wheel, it then acts as a pivot to create a moment on the traction wheel itself, thereby increasing the wheel's force to the pole (i.e. traction). Note that this will also increase the friction on 'zip ties' or whatever other locking mechanism you use, so design accordingly.

The very first prototype one of my students made was exactly like this, yet didn't lose traction. It was a 4-4.5 second bot due to the excess friction though.

Re: Minibot wheel slip
 

Wow... I thought that we weren't doing all that well but now I feel differently. I have a video if you guys want to see it and make your minibots similar!

Re: Minibot wheel slip
 

We noticed that the wheels picked up dust from the shop real easy. A wipe down with a moist paper towel before each run took care of the issue for us. We couldn't break 3 seconds without that little bit of maintenance each time.

Is anyone planning to wipe down the poles to make sure they are clean before their matches? I wonder if field reset will do this automatically as part of their routine. I imagine the poles could get a bit dirty after several minibots make their way up and down.

Re: Minibot wheel slip
 

Hmmmmmmmm... Slippage occurs because the tangential force due to the applied torque exceeds the friction force, which is normal force times coefficient of friction. Adding a second motor doubles the tangential torque force, so if Ft > N(mu) for slippage, I would bet that 2*Ft > N(mu) as well.

Your choices are 1) increase the normal force and 2) increase the coefficient of friction

Re: Minibot wheel slip
 

Check: you can increase the normal force with a very ingenuous way using the motors themselves...


sandpaper anyone :D

Re: Minibot wheel slip
 

i already got my positioning figured out, so that's solved. my battery does not touch the pole. only the pvc guide and drive wheel do. the pvc is getting good grip, and the wheel has plenty of power and torque thanks to the gear ratio. even still, the wheel slips against the metal pole.

would irritating the rubber tires with some kind of fluid to give it some traction? i think my tires are the major factor in my problem

Re: Minibot wheel slip
 

How much of your normal force is going to that PVC guide instead of your wheel?

Re: Minibot wheel slip
 

If avoidable, you wouldn't want any pvc 'grip' at all. Excessive drag forces could attribute to some of the wheel slip. Perhaps you could figure out a way to make the pvc guides into rollers so there's less resistance.

The placement tweaking of the battery is a manipulation of the c.g. in order to acquire more normal force to the pole for your traction wheel. It works in the same manner that the mechanism at 1:27 in this video works for creating normal force to the pole. The c.g. relative to the pole was out from and below the driving wheel, while the idler wheel was above it. If you've already put it where you can based upon the sizing constraints, then that's about all you can do for that.

Re: Minibot wheel slip
 

[quote=would irritating the rubber tires with some kind of fluid to give it some traction? i think my tires are the major factor in my problem[/QUOTE]

Are you using the stock FTC 3 or 4 inch wheel?

All we did was to radius them to match the radius of the pole (see above post) and use the sand paper trick and we are getting 2.5 second climb rates consistently with 2 motors powering one wheel and magnets holding the whole thing on the pole.

Before using any type of fluid, I would check the legality of such use in the Manual first.

Re: Minibot wheel slip
 

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.

Re: Minibot wheel slip
 

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.

Re: Minibot wheel slip
 

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).

Re: Minibot wheel slip
 

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

Re: Minibot wheel slip
 

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.

Re: Minibot wheel slip
 

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?

Re: Minibot wheel slip
 

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.

Re: Minibot wheel slip
 

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.