Design of Direct-Drive Mechanical Arms

This paper describes the design concept of a new robot based on the direct-drive method using rare-earth d-c torque motors. Because these motors have high torque, light weight and compact size, we can construct robots with far better performance than those presently available. For example, we can eliminate all the transmission mechanisms, such as reducers and chain belts, between the motors and their loads, and construct a simple mechanism (direct-drive) where the arm links are directly coupled to the motor rotors. This elimination can lead to excellent performance: no backlash, low friction, low inertia, low compliance and high reliability, all of which are suited for high-speed, high-precision robots. First we propose a basic configuration of direct-drive robots. Second a general procedure for designing direct-drive robots is shown, and the feasibility of direct drive for robot actuation is discussed in terms of weights and torques of joints. One of the difficulties in designing direct-drive robots is that motors to drive wrist joints are loads for motors to drive elbow joints, and they are loads for motors at shoulders. To reduce this increasing series of loads is an essential issue for designing practical robots. We analyze the joint mass system for simplified kinematic model of the direct-drive robots, and show how the loads are reduced significantly by using rare-earth motors with light-weight and high torque. We also discuss optimum kinematic structures with minimum arm weight. Finally, we describe the direct-drive robotic manipulator (CMU arm) developed at Carnegie-Mellon University, and verify the design theory.