Quote:
Originally Posted by writchie
We get repeatable results with both the incline and pull force methods.
The conditions are new undamaged wheels and new undamaged Glassliner FRP from home depot.
We see static mu of 0.12 +- 0.02 and transverse of 0.13 +- 0.02. All measurements taken with 4 wheels.
At this point, I for one do not believe static mu of 0.05 without some form of lubrication on the floor. But those are the FRC numbers and they are sure sticking to them.
As a further aside, I cannot think of any way to traverse 48 ft of floor in 6 seconds in a standard kit bot frame without mu > 0.083, but then again I could be missing something.
In any case, we are preparing for static mu to be in the range from 0.06 to 0.15 and for the dynamic/static ratio to range from 1.0 - 2.5. As long as you can work under the full range of conditions you should be OK.
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Equations:
F = m*a
F = u*N
m*a = u*N
Plugging in:
(mtrailer + mrobot) * a = u * mrobot * g
a = (u * mrobot * g)/(mtrailer + mrobot)
Let mrobot = 120lbs + 18lbs (bumpers) + 7lbs (battery) + 5 lbs (trailer tongue weight) = 150 lbs = 68.0 kg
Let mtrailer = 35lbs (not supported by the robot) = 15.9 kg
a = u*68.0*9.8/(68.0+15.9)
a = u*7.94 m/s^2
We know that:
x = .5*a*t^2
If x = 48 ft = 14.6 m and t = 6s, then
14.6 = .5*a*36
a = 0.811 m/s^2
Plugging back into our first equation...
0.811 m/s^2 = u*7.94 m/s^2
u = .102 (at least)
Possible Conclusion 1: If we are to believe Dave's claims about traversing the field forward and back in about 12 seconds, then the published FIRST figures must be wrong.
Possible Conclusion 2: FIRST measured their CoF on a worn floor, while Dave drove on a new floor.
FWIW: Our new wheels on new regolith came out to about u=0.10