One of the key features of upper limb exoskeletons is their ability to take advantage of the human arm kinematic redundancy in order to modify the subject’s joint dynamics without affecting his/her hand motion. This is of particular interest in the field of neurorehabilitation, when an exoskeleton is used to interact with a patient who suffers from joint motions desynchronization, resulting e.g. from brain damage following a stroke. In this paper, we investigate this problem from the robot control point of view. A first general controller is derived which uses viscous force fields in order to generate joint torques counteracting any velocities that are perpendicular to a given set of constraints. In order to minimize energy dissipation, a second controller is proposed that still uses viscous force fields, but in a way that the mechanical power dissipated by the viscous control is null at any time. This controller does not impose any trajectory to the hand and the robot only moves in response to the forces generated by the patient. This approach is experimented on a 4-DOF exoskeleton with a healthy subject. Results exhibit the basic properties of the controller and show its capacity to impose an arbitrary chosen joint constrain for 3-DOF pointing tasks without constraining the hand motion.

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