Coefficient of Friction Testing

[Ether]

Originally Posted by JamesCH95

I would use a digital angle finder.

For purposes of this specific dialog, can you see that Step1 is analogous to setting up your ramp on a level surface?

[DonRotolo]

Originally Posted by JamesTerm

is there some cutoff point where the value changes from one state to another or is it a blend.

Mu versus relative velocity is an asymptotic curve approaching zero as velocity increases, but there's a small bump up just after you start to slip (very much depends on the surface and wheel material, so not always).

In general, static friction is higher than kinetic friction. Usually.


As for experimental error: Yes, by having the gravity vector point through something other than the bottom(ish) of the robot, you're changing things and that is usually significant.

A better method if you need actual numbers (instead of comparative effects) would be a calibrated pulling device that drags the robot across the test plate.

[KrazyCarl92]

I had been toying with the idea of making a simple test bed for finding coefficients of firction earlier this summer while doing some drivetrain CAD.
I was thinking a piece of plywood with carpet, polycarbonate, and HDPE covers that would tilt with a window motor and some sort of sensor to detect angle (pot, encoder, gyro, accelerometer, etc.). Thought it might be a good preseason project with basic motor control, sensor use, and automation.

Then I ran into that dilemma about testing a single wheel/piece of tread vs. testing an entire robot. I think the main problem for most teams (including ours) in testing their actual robot drivetrain configuration is that by the time there is a robot with that particular drive configuration to test, it's already well past the point in the season to make use of that information for design purposes.

Maybe the drivetrain skeleton is finished by week two or so...would you really have the time or resources to restart from scratch with your gearbox based on that information at that point in the season? In some cases it might be as simple as a sprocket size change, but this isn't a one size fits all approach. I can see where a team that uses the same or similar drive configurations from year to year could easily make use of this kind of test bed prior to a point of no return in their design. Even then, the exact CoG, wheel position, weight, etc. might not match those of this year's robot.

Does anyone have thoughts on how a test bed like this could be used in season to produce useful results prior to the drive train design phase of the season (like gathering information in days 3-5 of build)? My question deals more with the utility and practicality of such an apparatus in the actual season rather than the theory, design, or production of the apparatus.

[Chris is me]

Originally Posted by KrazyCarl92

Does anyone have thoughts on how a test bed like this could be used in season to produce useful results prior to the drive train design phase of the season (like gathering information in days 3-5 of build)? My question deals more with the utility and practicality of such an apparatus in the actual season rather than the theory, design, or production of the apparatus.

I think there are a few options:

1. Previous robots - if you do a particular style of drivetrain twice in a row, take an old robot, maybe remove some parts from it, redistribute weight with bricks / batteries to approximate various CGs.

2. Kitbot - This test might not be as functional since the Kitbot frame may be less rigid than a welded tube frame, but for many teams testing with a kitbot is a good approximation.

It's important to note for both of these tests, you don't even need to have gearing established yet, since the wheels will be locked. The key things to simulate in these tests from what I can gather are weight, CG, frame rigidity, wheel type, and your contact polygon, so aim to prototype with those in mind.

I'll go ahead and throw this on the "list of things I wish we did". We've approximated our way into a great drivetrain the past 3 years.