Development of Multi-DOF Forearm Prosthesis

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The first step of this research was to develop a new actuator named AMA (Antagonistic Muscle-like Actuator) which has fundamentary identical performance to human articulation with antagonistic muscles structure. Theoretical study of controlling AMA revealed that AMA is able to control the stiffness of joints in the wide range when non-linear springs are combined into it. The structure of the AMA is summarized as follows : A pair of ball-screws controls one degree of freedom. They are driven by only one DC-motor while some driving modes are altered by the gears which are driven by oen stepping motor There are three typical driving modes. (1) Single Driving : One ball-screw is rotated. (2) Ipsilateral Driving : A pair of ball-screws are simultaneously rotated along the same direction. This mode are mainly used to adjust the stiffness of the joint. (3) Contralateral Driving : A pair of ball-screws are simultaneously rotated along the opposite direction. This mode are mainly used to rotate the joint. The ball-screw nuts and the axis of joints are connected by the wire via non-linear springs. The most effective advantage of this mechanism is that one AMA can control some DOFs and this application of the AMA into active prosthesis whch has severe volume and weight limitation. Then it follows the developmen of multi-DOF forearm prosthesis as an application of the AMA.The multi-DOF forearm prosthiesis developed has six DOFs total ; two DOFs for the wrist and four for the two fingers. Only one DC-motor and one stepping motor drive and control the overall DOFs. The non-linear springs enable the adjustment of the stiffniess from almost all free to completly rigid. A finger has compound four bar linkages with planet gears system, which enables three joints of a finger (PIP DIP and MP joints) automatically adjust their flexion angle according to shape of an object to be grasped. The control experiments revealed the developed system performed well.