Four wheel differential robots have one major problem which people often forget to take
care of while designing there robot. While robot takes turn with zero turning radius its
wheels skids. It reduces accuracy of motion and can damage both robot’s motors and
wheels.
Above figure shows foot print of Dexter 2 robot. Dexter 2 is 5 feet tall and weights
almost 125 Kg. Consider this robot wants to turn clockwise. Robot will move its left pair
of wheels forward and right pair of wheels backward.
• Dotted circle indicates path on which wheels will move it passes through
center point of wheels.
• Blue arrow indicates wheels instantaneous trajectory it is tangent to the
dotted circle passing through wheels center.
• Red arrow indicates wheels motion direction.
Observation: Notice that there is a small angle of 28.43 degrees between red arrow and
blue arrow. Here red arrow shows hat wheel is actually trying to move forward or
backward. But wheels are moving in circular direction i.e. they are literally skidding
along the circular direction.
That’s why in this robot base is designed as wide as possible and distance between the
front and back wheels is kept as small as possible without compromising stability of therobot. If distance between robot’s front and back wheel is further increased then this
angle will increase and will cause large frictional losses. If this angle is increased above
45 degrees it is very likely that robot will not be able to rotate with zero turning radius
because motors will not be able to overcome friction due to skidding.
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