Re: Coefficient of Friction Testing
 

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.

Re: Coefficient of Friction Testing
 

Just do it on a level surface.

Re: Coefficient of Friction Testing
 

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

Re: Coefficient of Friction Testing
 

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.

Re: Coefficient of Friction Testing
 

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.

Re: Coefficient of Friction Testing
 

search for "Stribeck effect" or "Stribeck friction"

Re: Coefficient of Friction Testing
 

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.

Re: Coefficient of Friction Testing
 

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

Re: Coefficient of Friction Testing
 

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

Re: Coefficient of Friction Testing
 

I would use a digital angle finder.

Re: Coefficient of Friction Testing
 

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

Re: Coefficient of Friction Testing
 

John,

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

Re: Coefficient of Friction Testing
 

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

-John

Re: Coefficient of Friction Testing
 

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).

Re: Coefficient of Friction Testing
 

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