This paper describes the design of a gravity balanced spatial orthosis. Previous works introduced planar mechanisms only and neglected friction. They relied on counterweight or auxiliary parallelograms. The main contributions of this paper are: (i) we succeed in designing a spatial (3D) mechanism and (ii) we analyzed the effects of friction. We designed a very simple system made with two pulleys and two springs in order to ensure gravity compensation. The system is in static equilibrium whatever its posture. It has three passive degrees of freedom at the shoulder and two at the elbow. A prototype of this orthosis is presented at the end of the paper.

Many underactuated grippers with fingers have been developed these last years. Their drawback is that they only ensure conditionally grasp stability. This paper presents a study of the grasp stability of an isotropic underactuated finger, which is made by two phalanxes and uses cams and tendon for actuation. The paper presents also a study of the internal forces developed in the transmission chains. The proposed model serves for the gripper using as for part dimensioning.