This study proposes a motion-assist arm that can accurately support the positioning of a human upper limb. The motion-assist arm is a three-degree-of-freedom (DOF) planer under-actuated robotic arm with a 1-DOF passive joint that can be driven by an human. A control method for the robot arm is as follows. First, when the human moves an output point of the arm manually, the passive joint is rotated with the movement of the output point. Then, for accurate positioning of the output point on a target path, the actuated joints are controlled according to the displacement of the passive joint. Based on the above method, the human can adjust the velocity of the output point deliberately while its position is accurately corrected by the actuated joints. To confirm its effectiveness, the authors conducted tests to assist the human’s upper limb movement along straight target paths, a square path, and free curves paths such as italic letters with the proposed robot arm prototype. From the results of the tests, the authors confirmed that the proposed robot arm can accurately position the upper limb of the human on the target paths while the human intentionally moves the upper limb. It is expected that the proposed arm will be used for rehabilitation because it can aid patients to move their arms correctly. In addition, the proposed arm will enable any human to achieve complex work easily.

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