Coefficient of Friction Testing

[Ether]

Originally Posted by kramarczyk

John,

Just for reference, what is the resolution on that inclinometer?

From the picture, there are markings every 2.5 degrees. I suppose a very careful observer could eyeball it to 1 degree.

A 40" long ramp at a angle of 45 degrees changes height by approx 1/2" for 1 degree change in angle.

My tape measure is marked in 1/32" intervals.

[Ether]

Originally Posted by EricVanWyk

A plumb-bob and a protractor should be a good start.

It would be fun to see a YouTube video of students making that measurement to the required accuracy for useful results.

[JamesCH95]

Originally Posted by Ether

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

That particular angle gauge isn't exactly appropriate then. There are digital angle gauges with a "level" function that does not require calibration on a particular surface.

[Jared Russell]

If you want to know the CoF of your robot, it's generally to answer one or both of the following questions:

1) How well can my robot push/resist pushing?
2) How well will my robot turn/handle?

#2 seems decidedly more complicated than #1 to me. Then again, #2 depends on a lot more factors than just CoF. To measure #1, wouldn't a swath of carpet and a stationary scale/load cell to push against be sufficient?

It would be swell if a team brought such a rig to the Championship some year. If they actually give the Hall of Fame teams more than a cocktail table next year, maybe that would be a good use of the space...

[Joe Ross]

Originally Posted by Jared341

It would be swell if a team brought such a rig to the Championship some year. If they actually give the Hall of Fame teams more than a cocktail table next year, maybe that would be a good use of the space...

10 years ago, 494 brought a dyno to championships.

[Clinton Bolinger]

Originally Posted by Joe Ross

10 years ago, 494 brought a dyno to championships.

I am pretty sure they still have the Robot Dyno in their facility.

-Clinton-

[Ether]

Originally Posted by JamesCH95

That particular angle gauge isn't exactly appropriate then. There are digital angle gauges with a "level" function that does not require calibration on a particular surface.

They do not require calibration by the user because they are calibrated at the factory. That is a source of error, analogous to using a bubble level to locate a level surface for test set-up.

In most modern buildings, it is easy to find a flat level location using a bubble level. Once that location is selected for the test setup, multiple "tilt" tests can be conducted without the need to measure the gravity reference each time.

Whether or not the factory calibration of a device with a built-in level is inferior or superior to the use of a mechanical bubble level is an open question at the moment.

The digital readout of an angle sensor is certainly more convenient¹, but that's not the point of contention here.


¹It's also more expensive if you don't already happen to have one on hand

[JamesCH95]

Originally Posted by Ether

They do not require calibration by the user because they are calibrated at the factory. That is a source of error, analogous to using a bubble level to locate a level surface for test set-up.

In most modern buildings, it is easy to find a flat level location using a bubble level. Once that location is selected for the test setup, multiple "tilt" tests can be conducted without the need to measure the gravity reference each time.

Whether or not the factory calibration of a device with a built-in level is inferior or superior to the use of a mechanical bubble level is an open question at the moment.

The digital readout of an angle sensor is certainly more convenient¹, but that's not the point of contention here.


¹It's also more expensive if you don't already happen to have one on hand

The electronic angle finder/level that I've used extensively, and that I really like, was calibrated from the factory to +/- 0.2deg of level and +/- 0.1deg angle change from level. McMaster PN 21465A81. Just turn it on and directly measure...

Your average carpenter level is only good to about +/- 1.5deg of level. More precise machinist bubble levels are available of course

You're right, a digital angle finder/level might be considered expensive, but it's the sort of tool that will last for many years of reliable service. It is substantially easier to use and more accurate than a non-digital angle finder. It is especially useful when making complex mill setups or doing complex tube notching and cutting.

[EricVanWyk]

Good news everyone! I have obtained a magic inclinometer with infinite precision and perfect repeatability! It is pretty sweet.

What is the accuracy of the inclined plane test now? What sources of error still exist?

[Ether]

Originally Posted by EricVanWyk

Good news everyone! I have obtained a magic inclinometer with infinite precision and perfect repeatability! It is pretty sweet.

What is the accuracy of the inclined plane test now? What sources of error still exist?

Curvature of the Earth

[EricVanWyk]

Originally Posted by Ether

Curvature of the Earth

Maybe curvature of the gravitational field, but at that point we are back to complaining that the weight of the people moving in the stands are throwing off the accelerometer measurements.

I'm just trying to understand out how much of the spread is due to mismeasuring the angle, and how much is from other sources. As soon as it starts moving, it goes into kinetic friction. So, if you jostle the setup it will start moving before it 'should'. My untested hypothesis is that this thread is focusing too much on the angle, and not enough on things like how quickly you raise the ramp, or how steady your hand is.

So - is your money better spent on a better inclinometer, or keeping the freshmen away from the red bull?

[Ether]

Originally Posted by EricVanWyk

Maybe curvature of the gravitational field, but at that point we are back to complaining that the weight of the people moving in the stands are throwing off the accelerometer measurements.

I'm just trying to understand out how much of the spread is due to mismeasuring the angle, and how much is from other sources. As soon as it starts moving, it goes into kinetic friction. So, if you jostle the setup it will start moving before it 'should'. My untested hypothesis is that this thread is focusing too much on the angle, and not enough on things like how quickly you raise the ramp, or how steady your hand is.

I think even under laboratory conditions with expensive test equipment and experienced technicians you would still see a lot of variation regardless of the measurement methods used. Friction is finicky.

[Richard Wallace]

Originally Posted by EricVanWyk

So - is your money better spent on a better inclinometer, or keeping the freshmen away from the red bull?

Hand being drawn toward the spotlight button ...

[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.