compact drive system for geared robotic joints and actuation mechanisms

• Online Access: http://hdl.handle.net/2047/D20236861

Robotic manipulators require compact Joint Drive Systems (JDS) that typically comprise an actuator, a transmission and a joint structure that can deliver high torques through stiff mechanical ports. Today's conventional drive systems are made from off-the-shelf actuators and multi-stage planetary transmissions or harmonic drives. This current practice has certain benefits such as short development time due to the availability of mechanical components, however it lacks a system-level integration that accounts for the actuator structure and size, output force, transmission structure, gear ratio and strength, and the robot's joint structure, and often leads to long and bulky assemblies with large number of parts. This research is aimed at developing a new robotic hardware that simplifies the complexity of the robot drive system into one component which is optimized for its size and maximum torque density. This is accomplished by designing the robotic joint around a special transmission which, when numerically optimized, can produce unlimited gear-ratios from only two stages. The design is computerized to obtain all the valid solutions that satisfy its kinematic and constitutive relationships. The theoretical results demonstrates the potential of an example device for which a proof-of-concept prototype was designed and fabricated that could deliver more than 200 Nm of torque in a package as small as a human elbow joint. Compared to conventional precision drive systems such as harmonic drives, the proposed actuator offer similar advantages such as high-torque in a compact assembly but with the potential of better stiffness characteristics and lower transmission friction and more predictable output speed response; and when compared to existing space-flight actuators, the proposed design leads to shorter assemblies with significantly lower number of parts for the same output torque. The behavior of one prototype was experimentally characterized where simple but accurate models for the transmission friction, stiffness and kinematic error are obtained. A dynamic model of sufficient complexity is proposed that captures the open-loop velocity dynamics with good accuracy. An analytical stiffness model was also developed for the transmission mechanism using the bending flexure of gear-teeth and transmission geometry, and finally the dynamic impact of using shorter drive systems in manipulation is studied. The proposed technology combines precision with size and torque and as such could have immediate technological implications in many robotic and motion control applications.