Analysis and Design of Semi-Direct-Drive Robot Arms

Direct drive is an ideal drive method which has excellent features including no backlash, low friction, and high mechanical stiffness. Applying the direct drive method to robot arms, however, leads to an impractical design because of the following two drawbacks. One is that a direct-drive arm tends to be heavier than a conventional arm with transmission mechanisms; because the motors of direct-drive arm are located at each joint of the arm the weight of each motor is a load to the previous motors down the serial linkage of the arm. Another problem is insufficient drive torques due to the elimination of gears which amplify the motor torques. This paper describes a lightweight arm mechanism for the direct-drive robots. The major improvements are: (1) the serial link mechanism is replaced by a four-bar-link mechanism in order to avoid the mounting of heavy motors at the serial links, (2) high performance brushless motors have been developed so that large torques can be obtained with relatively small power dissipation. Throughout the basic analysis and experiments, the arm which has been developed is evaluated in terms of efficiency, load capacity and dynamic response.