Coefficient of Friction Testing

[DonRotolo]

Data? No. That's lost to me.

The first question we wanted to answer was if 12 wheels were any better than 6. One test was to have a 6-wheel robot and a 12-wheel robot (both same wheelbase and weight) push against each other, to see who would win. At first, the loser was universally the one which spun its wheels first. With extremely careful and skilled control, the 12 could win but the 6 could not. Repeatable with difficulty, but repeated enough times that we went with 12.

The second question was related to whether traction control - to prevent wheel spin - would provide an advantage.

We first needed to find the amount of wheel slippage that would be acceptable, and for Lunacy, IIRC we found that grip vs wheel velocity relative to the surface was somewhat linear - higher speeds giving less grip. We did this by powering up the wheels with a PID loop to a certain speed, and measuring the resulting pull with a scale (think fish scale, but of a higher quality, up to about 40 Lbs).

We then needed to measure the effect of a slip limiter. Using a 5th wheel for odometry, we used a PID loop to limit PWM input to Jaguars such that the slip was some percentage of the robot speed. (I think we settled on 12%, but I can't say why). We then went drag racing, timing the robot over a fixed distance, the driver simply giving full throttle and letting the system throttle it back.

Our final result was that a simple rate-of-change limiter in software was nearly as effective, avoiding a lot of hardware, and that's what we competed with.

[Chris Fultz]

we did a test with a kit robot and various wheel widths and diameters.
when we got done, we could not make sense of the data.

we made a test robot and used a pull scale. The robot was set in a frame, with carpet on the floor. We set the control system to ramp up the motor power at a predetermined rate. we ramped up until the wheels broke free and recorded the pulling force at that break point.

I think this test could easily be repeated with various types of wheel treads, using the bridge material instead of carpet.

[R.C.]

Originally Posted by Chris Fultz

we did a test with a kit robot and various wheel widths and diameters.
when we got done, we could not make sense of the data.

we made a test robot and used a pull scale. The robot was set in a frame, with carpet on the floor. We set the control system to ramp up the motor power at a predetermined rate. we ramped up until the wheels broke free and recorded the pulling force at that break point.

I think this test could easily be repeated with various types of wheel treads, using the bridge material instead of carpet.

Chris,

By any chance, would you mind putting up the results?

Thanks,

-RC

[Siri]

Originally Posted by JVN

Do you have any results you can share?

-John

Generally the published results were fairly reflective of our results, but we ended up with more noise than I was expecting, for which we couldn't find any real trend or explanation. We also found that the blue nitrile wore better and handled dust, etc better than the roughtop. (No numbers--a reflection that's reminded me to belatedly add wear testing to our summer procedure.) Colsons are somewhat better on the bridge, but by the time we did the switch we were using our real robot (CG around ~15") and we always tipped before falling* -- at well over the bridge's maximum angle. We haven't done much wedgetop testing yet--on the summer list.

*EDIT: Do'h. Absolutely need to point out that this test was conducted with our pivots at 45deg angles. Wow. My bad.
/...\
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[IKE]

Why do you want to know the COF of your robot?*


*Are you sure you are asking the right question?

Very cool post. I have used a similar method for estimating COF of material on material.
The tilt test is also a pretty good way of testing CG location, but it can get tricky.

[AdamHeard]

This is something 973 was planning on testing internally during the fall on a couple of our projects. We're very experienced with roughtop/wedgetop style tread and like their performance, but are interested in how colsons and the popular variants of pneumatic wheels compare.

We plan to do the incline plane test with carpet and polycarb sometime in November. Ideally we'll publish results.

[Lil' Lavery]

Originally Posted by IKE

Why do you want to know the COF of your robot?*


*Are you sure you are asking the right question?

I'm going to echo this question. Just for further iterative designs to objectively improve your CoF? Otherwise I'm struggling to see why having CoF data that late in the design process (ie, when you already have your drive base built) would be useful.

[JVN]

Originally Posted by Lil' Lavery

I'm going to echo this question. Just for further iterative designs to objectively improve your CoF? Otherwise I'm struggling to see why having CoF data that late in the design process (ie, when you already have your drive base built) would be useful.

One would test drivetrain configurations (size wheels, location of wheels, qty of wheels, robot weight, CoG location) so one has better understanding of those particular configurations, and their CoF, to better understand their potential pushing force.

You don't need to complete your robot to get this data. You can get it early in the design process... or even in pre-season.

-John

[JVN]

Originally Posted by IKE

*Are you sure you are asking the right question?

I was pretty sure... until your post. Am I missing something?

-John

[kramarczyk]

The last time I did qualitative traction testing was in 2009. See this post.

Since then I have not needed to build a traction limited drivetrain, so the actual number was not essential. We did some qualitative testing for the polycarbonate on the bridge this year. We were looking for the best material that we had access to, so A>B testing was adquate.

Admittidly, I would like to have a library of exactly the information the OP is asking about, but it is less important than foundation work that needs to be done with/for the team.

[JamesCH95]

The more I read up on various stories the more I believe that most FRC wheel act like car tires. It matters what temperature they're at and what contact pressure they see. Therefore it is important to test the drivetrain as a whole unit, not just individual wheels. I

f some wheels in the setup are more heavily weighted than others they will have a lower CoF than identical wheels loaded less, i.e. a drop-center 6wd, where the center two wheels generally bear 90%+ of the robot's weight. In a configuration like this I believe that it would be most efficient to use wide wheels in the middle and narrower wheels on the outside corners.

Additionally, if one were doing testing, it would be interesting to heat up the wheels by spinning them for a few moments, then re-trying the static friction test.

@DonRotolo- your slippage value of 12% is very similar to high performance traction control systems in race cars; IIRC typically 8-10% slippage resulted in maximum straight-line acceleration. This may be because the surface of the wheel/tire is getting 'ripped up' a little bit, causing it to engage the driving surface positively, but I am no tribologist.

[DonRotolo]

Originally Posted by JamesCH95

@DonRotolo- your slippage value of 12% is very similar to high performance traction control systems in race cars;

Despite finding our values empirically, they correlate well with the automotive values found in the Bosch Handbook. (I work for a car company)

Originally Posted by JamesCH95

but I am no tribologist.

Yeah, I don't know much about tribbles either.

[IKE]

Originally Posted by JVN

I was pretty sure... until your post. Am I missing something?

-John

Let me prefice by saying this is a really good test, and a pretty accurrate method, but as this thread is starting to expose, like any acquired taste, there are various levels of CoF snobbery.

************************************************** ************
I am only slightly messing with you. As Don and James have described, there really isn't a CoF number, but more of a CoF curve that depends on a lot of parameters. As having the most accurrate CoF is generally not an award at a Robotics competitoin (though it would make a great science fair project), one must assume that you are trying to get a CoF to do a calculation. If you are trying to do a calculation, the inherent question you need to ask yourself is how accurrate do I need to be? 1%, 2%, 5%, 10%? As the CoFs I have seen for non-lunacy wheels tend to range from 0.8 to 1.3 with most being in the 0.9 to 1.1 range, I would assume that a test with +/-10% accuracy is probably not sufficient.

Back to how accurrate does it need to be, within the measuring technique you showcase, If your angle accuracy is accurate to within +/-1 degree, at 45 degrees, you will come up with a CoF of 0.966 to 1.036. Or a total error band of about 7%. +/-2 degrees will get you 0.93 to 1.07. In terms of FRC wheels, that would be pneumatics fully inflated (according to AM wbsite) to fresh wedgetop. My phone has an inclinometer app on it that measures within 0.1 degrees, but on a flat surface, with me holding it steady, it varies +/-0.7 degrees.

As you said in your post, you don't believe a patch, or a single wheel will give you represenative data. This tells me that you believe that the weight and weight distribution must have an effect. If this is true, then a tilt table will shift the normal force distribution on those wheels and thus the tilt table will give you a different result than flat ground pushing. this is especially true when the board reaches high angles which just so happen to correspond to high tractions. Also, small flex in the board can throw off the angles as you will get a different contact patch than you were expecting.

Couple this information with the Slip % variation Don and James are talking with (on street tires it is often around 0.95 peak at 8-12% slip, and then dropping down to 0.8-ish past 20% slip), and you will find a lot of neat variation that adds up.

We used a similar method on the polycarbonate seeing which wheels slid first. We didn't need to know the exact number, just which had more traction.

****************************************

I may be a bit sensitive on this subject as I am coming off of a 24 hour race with a bunch of CoF super snobs. At around the 12 hour mark, there was a 1% difference in lap count between 5th and 14th place. There was only about 3% difference between 1st and 20th.

[kramarczyk]

Originally Posted by IKE

Back to how accurrate does it need to be, within the measuring technique you showcase, If your angle accuracy is accurate to within +/-1 degree, at 45 degrees, you will come up with a CoF of 0.966 to 1.036. Or a total error band of about 7%. +/-2 degrees will get you 0.93 to 1.07. In terms of FRC wheels, that would be pneumatics fully inflated (according to AM wbsite) to fresh wedgetop. My phone has an inclinometer app on it that measures within 0.1 degrees, but on a flat surface, with me holding it steady, it varies +/-0.7 degrees.

Do you guys actually measure the angle and then do the trig? Wouldn't measuring the rise over run with a carpenters square provide better resolution. Rise / run = tan theta = CoF

An 1/8" difference in rise is pretty easy to measure and works out to ~ 1% change in CoF.
12/18 = 0.66667, 12.125/18 = 0.67361, 1.03% difference
17/12 = 1.41667, 17.125/12 = 1.42708, 0.73% difference

[JamesCH95]

Originally Posted by kramarczyk

Do you guys actually measure the angle and then do the trig? Wouldn't measuring the rise over run with a carpenters square provide better resolution. Rise / run = tan theta = CoF

An 1/8" difference in rise is pretty easy to measure and works out to ~ 1% change in CoF.
12/18 = 0.66667, 12.125/18 = 0.67361, 1.03% difference
17/12 = 1.41667, 17.125/12 = 1.42708, 0.73% difference

How accurately can you measure the rise and run with respect to gravity? Using a carpenter's square is good, but you have to ensure that it is square with gravity also.