Underactuated mechanisms will provide low-cost automation, easily overcome actuator failures, and be particularly useful for space applications because of their reduced mass and lower power consumption. In space underactuation can be effectively introduced in robot manipulators. Such mechanisms will however be difficult to control because of the fewer number of actuators in the system. It is shown that when the actuated joints do not have brakes, it is possible to bring the system to a complete rest and converge the actuated joints to their desired values, provided the system maintains zero momentum and none of the unactuated joints are cyclic coordinates. To converge both the actuated and the unactuated joints to their desired set of values, it is assumed that the number of actuated joints is more than the number of unactuated joints, and the unactuated joints have brakes. It is shown that if there exists sufficient dynamical coupling between the set of actuated and unactuated joints it is possible to converge all the manipulator joints to their desired values.
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