Development of CMU Direct-Drive Arm II

The CMU Direct Drive Arm II (DD II) is the second direct-drive arm designed and constructed at the Robotics Institute, Carnegie Mellon University. It is an electric 6 degree-of-freedom robot, in which all of the joints are of direct drive construction. Featuring high performance Samarium Cobalt magnet brushless DC motors and light weight aluminum construction, the robot has been designed to have a minimum payload of 2.5 Kg with a maximum transit time of 1 second (corresponding to tip speeds of 4 m/sec). High resolution pancake resolvers are mounted directly to the joint shafts for very accurate feedback. Static accuracy is ±0.1 mm. Taking advantage of the dynamic simplicity inherent in direct drive design, the controller is capable of dynamic force compensation in real-time. Such a controller can accurately follow a trajectory at very high speeds. In this paper we discuss the design of this new arm, particularly our solutions to the difficulties of practical implementation of direct drive.