The dexterity and singularities of an underactuated robot

Underactuated robots are robotic systems with more joints than actuators. A robot may be underactuated by design as in the case of a hyper-redundant robot with passive joints or may become underactuated as a result of an actuator failure. Ln this article, we examine the dexterity of underactuated robots whose passive joints operate in either a locked or free-swinging mode. The ability to an analyze the dexterity of an underactuated robot has important applications especially for the control of passive joints with brakes and for the fault tolerance analysis of an otherwise fully actuated kinematically redundant robot. The approach applied here is to use kinematics and dynamics-based formulations of manipulator dexterity. We then characterize passive-joint singularities, i.e., configurations where full end-effector control is lost because one or more joints are passive instead of active. Lastly, we introduce a new characterization of joint-limit singularities, which are configurations where full end-effector control cannot be achieved because one or more joints are at their joint limits. (C) 2001 John Wiley & Sons, Inc.