Coefficient of Friction Testing

[Ether]

Rise/Run requires 2 measurements.

Do it with just one: measure the height "h" of the ramp.

Then CoF=h/sqrt(L²-h²) ... where "L" is the (fixed) length of the ramp.

[Ether]

Originally Posted by JamesCH95

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.

Just do it on a level surface.

[JamesCH95]

Originally Posted by Ether

Just do it on a level surface.

One still needs to measure the level surface to see that it is, in fact, level.

[Ether]

Originally Posted by JamesCH95

One still needs to measure the level surface to see that it is, in fact, level.

I don't see the point you are trying to make, in the context in which you originally posted.

You'd have to do that anyway, if you were measuring the angle between the ramp and the floor.

[JamesTerm]

Originally Posted by Ian Curtis

Fun question: Is dynamic friction a constant with respect to velocity? I know a Physics 101 textbook will tell you it is, but I wonder if you ask a tribology (isn't that a fun word) expert about the dynamics of tread on carpet Iwhat the answer is. Or if anyone has ever tested it?

That is, do you have the same coefficient of friction with a tire sliding at 1 ft/s as you do with a tire sliding at 10 ft/s.

I find this interesting... I've never heard it called dynamic friction (just static or kinetic)... so I googled around... it appears kinetic and dynamic are synonymous, but now I wonder if dynamic means in terms of velocity. Kinetic friction is very clear... its either static or kinetic values depending on if object is moving, but is there some cutoff point where the value changes from one state to another or is it a blend. This is one reason why I find the test methods interesting. In code I measure the value 0 velocity against a theshold of around 1E-05.

[Ether]

Originally Posted by JamesTerm

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

search for "Stribeck effect" or "Stribeck friction"

[JamesCH95]

Originally Posted by Ether

I don't see the point you are trying to make, in the context in which you originally posted.

You'd have to do that anyway, if you were measuring the angle between the ramp and the floor.

In my original post my point is that unless you measure actual rise (movement parallel to the force of gravity) and actual run (movement perpendicular to the force of gravity) the angle you calculate will be inaccurate by however off the reference surface is.

Furthermore, one has to measure either the angle of the reference surface (the surface that the carpenter's square is placed on) and then measure rise/run, or just measure the slope directly. The former contains three sources of error, the latter has only one source of error. I would prefer to measure just one thing, the angle of inclination, instead of the angle of a reference surface and two distances.

Determining if these errors are significant is the responsibility of the experimenter.

[Ether]

Originally Posted by JamesCH95

just measure the slope directly [with] only one source of error.

How do you propose to "just measure the slope directly" with only one source of error?

[EricVanWyk]

Originally Posted by Ether

How do you propose to "just measure the slope directly" with only one source of error?

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

[JamesCH95]

Originally Posted by Ether

How do you propose to "just measure the slope directly" with only one source of error?

I would use a digital angle finder.

[JVN]

Originally Posted by EricVanWyk

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

We have a device called an inclinometer which is these two things in one package.

[kramarczyk]

John,

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

[JVN]

Originally Posted by kramarczyk

John,

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

Not very good. I think ours is +/- 1-degree.

-John

[Tom Line]

Originally Posted by JVN

I just published a post which highlights my method for calculating the coefficient of friction for robotic drivetrain systems.



See full post here.

So this brings up my next question...
Who actually bothers to do this?

Does anyone? Is it just me?
Does your team have their own method?
Does your team just trust the manufacturer's specs?
Do you even care, or do you just say "this is grippy as heck" and not bother?

Please share.

Originally posted here.

John, I believe you MUST check as a full robot, and with varying weights on the wheels. The tread we use does not act as an ideal surface - it has sub-surface interaction and interlocks with the carpet pile. In addition, on a six wheel drive the center of gravity is rarely in the center. So how, exactly, are you riding on your wheels?

A number of people have created spreadsheets detailing 'can my robot turn'. I don't believe those tell you what you actually want to know. You've seen robots on the field that can turn. Then you've seen robots like 254 and 1114 that can FLY. I think everyone knows the difference I'm talking about.

Something that I want to do later this year (during Beta Test) is to get our precision scales from the academy's physics class. Attach carpet to them, and put them on one of our marble measuring tables, then set the robot on them. I'd like to get the normal force each wheel exerts.

Between 1718's 3 nearly fully assembled robots, it should be possible to create a 'model' of drivetrain geometry and weight distribution that produces a robot that turns well rather than one that 'just turns'.

I've got 'turning' stuck in my head because we've fought with it on a couple robots and had to go back and change wheel types and treads to get acceptable performance. Frankly, I have never ONE seen one of our 4 wheel, 6 wheel, or 8 wheel drive robots lose traction, except when getting pushed completed sideways by a robot with treads and a low gear. I think how the friction affects robot turning is the more important application of the data you folks are talkign about. (Completely apart from the fact that I believe if you're in a pushing matches often, you're likely playing the game wrong).

[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?