This paper deals with the kinematic analysis of a wheeled mobile robot (WMR) moving on uneven terrain. It is known in literature that a wheeled mobile robot, with a fixed length axle and wheels modeled as thin disk, will undergo slip when it negotiates an uneven terrain. To overcome slip, variable length axle (VLA) has been proposed in literature. In this paper, we model the wheels as a torus and propose the use of a passive joint allowing a lateral degree of freedom. Furthermore, we model the mobile robot, instantaneously, as a hybrid-parallel mechanism with the wheel-ground contact described by differential equations which take into account the geometry of the wheel, the ground and the non-holonomic constraints of no slip. Simulation results show that a three-wheeled WMR can negotiate uneven terrain without slipping. Our proposed approach presents an alternative to variable length axle approach.
