This paper describes a hybrid mobility system that combines the advantages of both legged and wheeled locomotion. The legs of the hybrid mobility system permit it to surmount obstacles and navigate difficult terrain, while the wheels allow efficient locomotion on prepared surfaces and provide a reliable passive mechanism for supporting the weight of the vehicle. We address the modeling, analysis and control of such hybrid mobility systems using the specific example of a wheelchair with two powered rear wheels, two passive front casters, and two articulated, two-degree-of-freedom legs. We exploit the redundancy in actuation to actively control and optimize the contact forces at the feet and the wheels. Our scheme for active traction optimization redistributes the contact forces so as to minimize the largest normalized ratio of tangential to normal forces among all the contacts. Simulation and experimental results for the prototype are presented to demonstrate and evaluate the approach.

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