Two Wheels Connected by an Unactuated Variable Length Axle on Uneven Ground: Kinematic Modeling and Experiments

Abstract

It has been shown in previous work that when two wheels independently joined by a fixed-length axle are placed on an uneven surface, such that the line joining the wheel contact points is not coplanar with the axle, rolling without slip is possible only if the wheels are driven at the same speed. Otherwise, the wheels must slip against the terrain due to kinematic constraints. Based on analytical kinematics, it is shown in this article that a method to prevent slip on uneven terrain is to use a variable length axle. This research explores the instantaneous and gross motion of two wheels joined by a variable length axle on uneven terrain. Next, the overall range of the axle length change is investigated through numerical simulations. Based on the results from the simulation, a novel variable length axle that possesses no prismatic joints, hence has low friction, and has high stiffness has been designed. Experimental results are obtained for the axle length change as a three wheeled laboratory prototype that includes the variable length axle travels on a smooth uneven surface. These experiments verify the kinematic model. Limitations of using an unactuated variable length axle and related future research topics are also discussed.