Active compliance is often used in the control of the legs of a walking machine to allow a vehicle to adapt to terrain irregularity. This technique balances force and position errors in Cartesian space to achieve the operation of a damped spring. It is shown that, when decomposed into individual actuator systems, Cartesian compliance may require positive feedback of the equivalent joint torque error to a certain joint. Thus, such a joint can be locally unstable if the environment which the leg system contacts is not passively compliant enough. An inherent stability problem associated with this Cartesian-space compliance control is investigated, and an alternative structure of compliance control that eliminates the inherent source of instability and still permits adaptability to terrain irregularity is introduced. The new algorithm was implemented on the Ohio State University Hexapod vehicle and experimentally verified as giving better performance in system stability than the existing Cartesian compliance algorithm.>

Force is often used in the control of the legs of a walking machine to allow a vehicle to adapt to terrain irregularity. However, interactions in force among the legs have the capability of causing control system instabilities if not properly treated. Different criteria for allocating forces to the legs of a walking machine are considered, properties of force-induced instability modes are studied, and the plan of hybrid control allocated by legs as a means of avoiding these force interaction modes without requiring an excessively high control frequency is introduced.