The Development of a Direct Drive Master Arm
Tetsuo Kotoku, Erhard J. Hüsler, Kazuo Tanie and Akio Fujikawa
Robotics Department, Mechanical Engineering Laboratory, Namiki Tsukuba-shi, Ibaraki 305, Japan
This paper deals with the development of a 4 degree of freedom direct drive master manipulator system which has the ability of virtually adjusting its mechanical impedance. In the field of bilateral master-slave manipulation, there are some important points to be considered when a master manipulator is designed. One point is related to how to measure the human operator’s arm motion with high accuracy (which the operator produces to teach the trajectory of tasks). Another point is how to design an effective force/torque generator to make an operator feel the constraint forces the slave arm will receive from the environments during tasks. To satisfy these requirements, a manipulator with a variable mechanical impedance structure and joints equipped with high resolution angular displacement sensors is expected to be developed. The use of the variable impedance structure provides a capability of reflecting the constraint forces to an operator not only statically, but also dynamically. This is effective to present the constraints to an operator with high fidelity. In the research, joint mechanisms and joint sensors suitable for the impedance control were discussed, while the mechanical structure of the master manipulator which is effective to construct a simple impedance control law was investigated. With these considerations in mind, a master manipulator was designed and its impedance control law was formulated. The features of the manufactured manipulator are summarized as follows; (1) each joint is driven by a direct drive torque motor with less friction and has a high resolution angular encoder attached to a processor which can provide joint angular displacement, angular velocity and angular acceleration. (2) the manipulator has the decoupled and configuration-invariant inertia structure in part which is effective to simplify impedance control law. In the paper, the design concept of the master manipulator is first discussed. Secondly, the details of the manufactured manipulator structure are explained. Finally, the results of the evaluation experiments are described and the fundamental characteristics of the system are confirmed.