Intelligent Mobility Platform with Active Spoke System (IMPASS) is a unique wheel-leg hybrid robot that can walk in unstructured environments by stretching in or out three independently actuated spokes of each wheel. The latest prototype of IMPASS has two actuated spoke wheels and one passive tail. In order to maintain its stability, the tail of the robot is designed as a rigid shell with a geometrically convex surface touching the ground. IMPASS is considered as a mechanism with variable topologies (MVTs) due to its metamorphic configurations. Its motions on the ground, such as steering, straight-line walking and other combinations, can be uniformly interpreted as a series of configuration transformations. Among all cases of its topologies, the cases with two spokes and the tail in contact with the ground possess two d.o.f and contribute the most to its ground motion. To fully understand the characteristics of such topologies, the forward and inverse displacement analysis is developed for these cases, with the polynomial equations derived. Numerical solutions from simulation are present to validate their formulation. These results lay the kinematics foundation for the motion monitoring and planning of IMPASS. It also contributes to the design optimization of the tail’s surface geometry to improve its adaptability on uneven terrains.

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