An inherent stability problem in Cartesian compliance and an alternative structure of compliance control

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