Building Robots at School

June 24, 2009

VEX Workshop: Traction

Filed under: Uncategorized — dtengineering @ 12:33 am

inline transverseFor most wheels on a robot we typically think of traction as being “in line” with the wheel.  The inline traction is a measure of how much pushing or pulling force the wheel can generate in its direction of rotation.  Equally important, however, is the transverse traction, as this determines the wheels resistance to skidding sideways.  When you are building a skid steer robot, you need to balance your inline traction with your transverse traction in order to turn smoothly… or at all!

 The traction of any given wheel can be estimated by using the standard “coefficient of friction” formula taught in Physics 11. 

Ff = μ FN

In other words, the force due to friction equals the coefficient of friction (u) multiplied by the normal force.  In our case, the normal force is the amount of weight being supported by any given wheel.  As shown in the preceeding post, however, the amount of weight over any given axle can change… and thus the amount of traction (both inline and transverse) over any given wheel can also change.

Now before going too much further with this formula, it is important to address the idea that because the “friction formula” doesn’t include surface area, or “contact area”, that the amount of material distributing the force to the surface is irrelevant.  This isn’t exactly true… but it doesn’t mean that the friction formula is “wrong” either.  It simply means that the friction formula makes a few assumptions that might not always hold true under the stress of competitive robotics.  The first assumption is that the materials making contact with each other have sufficient shear strength so that they don’t shred apart under the forces acting upon their surfaces.  The second assumption is that neither surface deforms significantly due to the forces involved.  This isn’t always the case in VEX, as the foam playing surface can deform slightly under the weight of a heavy robot, particularly when that force is concentrated in one place.  How does this affect your coefficient of friction?  That’s up for you to learn through experimentation… but don’t assume that contact area doesn’t matter just because your physics teacher told you it doesn’t.  You’re being taught a simplified (and very useful) model for friction… there just isn’t a simple way to calculate friction without making some simplifying assumptions.

The important thing to focus on here, however, is that to maximize your robot’s traction you want to have 100% of your normal force (your robot’s weight) transferred to the ground through driven wheels (wheels that you are actively turning, not ones that just drag along).  This is why all wheel drive systems are so popular in robotics… they allow you to use 100% of your weight to create pushing force.


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