Towards a multi-DOF passive balancing mechanism for upper limbs

Gravity balancing has great significance in many rehabilitation applications and one example is in minimizing the resistance of lifting a patient's upper limb due to its own weight. As compared to commonly used active method in exoskeleton, passive weight compensation offers many other advantages such as elimination of motors which reduces the overall load acting on the patient. However, the torque about each joint due to the weight of the upper and fore arm is dependent on the orientation and location of each other, which makes it a challenge to compensate. Hence, this paper proposes and develop a passive method to compensate the torques on each joint. The proposed design includes a decoupling mechanism for isolating the torsional effect between various linkages and a realizable torsional compliant beam which can provide a specific torsional stiffness function to compensate the torque due to the arm's weight. In place of the usual expensive zero-length spring as the solution, an algorithm that generates the appropriate compliant beam design is also presented in this paper. The end result of the manipulator arm is a reduced required joint torque for gravity compensation using only passive means. A two-DOF prototype, with easy possible extension of additional DOF, has been built, and experiments are performed to illustrate, verify and quantify the effectiveness of the proposed method.