Space robots require compact joint drive systems (JDSs), typically comprising of actuator, transmission, joint elements that can deliver high torques through stiff mechanical ports. Today's conventional space drive systems are made from off-the-shelf actuators and multistage transmissions that generally involve three to six stages. 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, size and output force, transmission structure, gear-ratio, and strength, and often leads to long and bulky assemblies with large number of parts. This paper presents a new robotic hardware that integrates the robot's JDS into one compact device that is optimized for its size and maximum torque density. This is done by designing the robotic joint using a special transmission which, when numerically optimized, can produce unlimited gear-ratios using only two stages. The design is computerized to obtain all the solutions that satisfy its kinematic relationships within a given actuator diameter. Compared to existing robotic actuators, the proposed design could lead to shorter assemblies with significantly lower number of parts for the same output torque. The theoretical results demonstrates the potential of an example device, for which a proof-of-concept plastic mockup was fabricated, that could deliver more than 200 N·m of torque in a package as small as a human elbow joint. The proposed technology could have strong technological implications in other industries such as powered prosthetics and rehabilitation equipment.
Advanced robotics requires a new generation of actuators able to exhibit a number of desirable characteristics ranging from high power density and high efficiency, high positioning resolution, high torque capacity and torsional stiffness, lightweight designs and low-cost packages. In this paper, we present the development and the experimental evaluation of a new actuator, aimed at improving the torque density and mechanical efficiency of actuated robotic joints, and enhancing the portability and effectiveness of robotic systems engaged in biomechanical applications such as rehabilitation robots and wearable exoskeletons. The new actuator, called the Gear Bearing Drive (GBD), consists of a two-stage planetary gear arrangement coupled through the planets and driven by an external rotor brushless motor that is inscribed within the input stage sun gear. This planetary configuration enables for incredible high-speed reductions and allows for embedding the motor directly within the gearbox saving significant space on the actuator length. Our initial experimental prototypes have demonstrated impressive performance with the potential to deliver more than 30Nm of continuous torque with 85% mechanical efficiency and 0.0005 degree of backlash, and up to 200 rpm maximum output speed in a highly compact and robust package.
The Gear Bearing Drive (GBD) is a recently developed high gear-ratio transmission concept based on NASA’s high-reduction gear bearing technology and brushless outrunner motor technology. In this paper, we describe the experimental setup and characterization of the GBD transmission’s stiffness, friction, backlash, hysteresis as well as its kinematic error. Compared to conventional precision drive systems such as harmonic drives, the GBD can offer similar advantages such as high gear-ratio in a compact assembly but with the potential of better stiffness characteristics and a more predictable output speed response. The models derived in this paper were then fed into a dynamic model that can accurately simulate the velocity open-loop response of the transmission under various input current functions.
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