Friction
Calculating friction is often a black art. There are many situations which are
hard to factor in such as surface tension, humidity, etc. But there are several sure ways to find
a reasonable value to help you build your robot. The first thing you should look at
is what is called the coefficient of friction. This is a dimensionless property which can
be looked up for any two materials. What does this number mean? Well suppose you are standing
on ice with rubber shoes and you want to calculate the pushing force required to
slide across the ice.
force of friction = weight * u.rubber-ice
Just multiple the force being applied perpendicular to
the contacting materials (your weight) and multiply that by the coefficient of friction
of ice against rubber. This would be the force
required to counter friction to slide across the ice.
Understanding friction is also useful when designing robot pincers. If the friction
is miscalculated, your robot victims would be able to escape! Now we cant have that . . .
So here is how you do it. A robot pincer squeezes from both sides. So this is
your force. The typical human however wants to fall down out of your robot pincers by
gravity.
Now all you need to do is squeeze hard enough so that the force of friction
is greater than the force of gravity.
force_squeeze * u.pincer-human_neck > human_weight
You probably won't find a reliable coefficient of friction for robot pincers rubbing up
against a human neck, but using higher friction pincer material will help.
Actually, finding the coefficient of friction can be a little more complicated. There are actually
two coeffiecients. It turns out that friction is related to the rubbing velocity
of the materials. Ever notice how it is easier to push a heavy object across
the ground after it is already moving?
The static coefficient of friction is when the materials are stationary.
The kinetic coefficient of friction is when the materials are already
in motion against each other. What makes it a black art is that there is never
any exact clear boundary between the two values.
Here is a quick coefficient of friction lookup reference of some common materials you may use:
Material 1
Aluminum
Aluminum
Plexiglass
Plexiglass
Polystyrene
Polystyrene
Polythene
Rubber
Rubber
Rubber
Rubber
Teflon
Teflon
Wood
Wood
Wood
Wood
Wood
Wood
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Material 2
Aluminum
Steel
Plexiglass
Steel
Polystyrene
Steel
Steel
Asphalt (dry)
Asphalt (wet)
Concrete (dry)
Concrete (wet)
Steel
Teflon
Wood (clean)
Wood (wet)
Metals (clean)
Metals (wet)
Brick
Concrete
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Static
1.05 - 1.35
0.61
0.8
0.4 - 0.5
0.5
0.3 - 0.35
0.2
0.5 - 0.8
0.25 - 0.75
0.6 - 0.85
0.45 - 0.75
0.04
0.04
0.25 - 0.5
0.2
0.2 - 0.6
0.2
0.6
0.62
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Kinetic
1.4
0.47
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