Mechanical Design and Thermal Management of a High-Performance Robotic Actuator

New robotic applications in unstructured environments require fast motions in high torque operations, along with the ability to manage dynamic physical interactions. Classic robotic actuators with high reduction gearing cannot attain this level of performance. This dissertation proposes a Quasi Direct Drive actuator designed expressly for use in our novel hybrid legged-wheeled robot, which takes advantage of minimal reduction gearing to deliver over 26,Nm continuous torque while operating at speeds exceeding 37 rad/s. As a result of these qualities, the designed actuation unit has a high dynamic performance and can respond quickly to disturbances caused by unanticipated physical contact. The design and selection of gearing components were carried out to employ commercial items with minimum customization while resulting in a fairly uniform shape. A single-level planetary gearbox is devised for the reduction unit to ensure high back-drivability and transparency of the actuator, thereby making the actuator safe against external impacts while allowing for accurate torque control using motor current measurement. Given the radial space required for the gearbox dealing with the torque requirements, the actuator motor was chosen to be small in height (pancake type), which ensures high torque density within smaller dimensions at high-speed operation. The mechanical design of the actuator is detailed in this thesis, and the actuator specifications are compared with similar state-of-the-art actuators. Furthermore, the thesis describes the actuator's mechanical model and impedance replication in order to support its use in robotic joints. As the second contribution of this work, we propose a thermal control scheme to enhance the motor capability in operating at peak torque while ensuring that the thermal limits of the robotic actuators are respected. This permits extending the duration of high torque operating points while still respecting the thermal constraints. We propound regulating the maximum current flow based on the motor temperature. We also evaluate the effect of forced cooling of the stator on the actuator's performance in terms of steady-state current values and time for peak current operation. Finally, we assess the use of this actuator in a newly proposed hybrid legged-wheeled robot that we designed for high payload and high-speed applications on the ground and uneven terrains. We go into the intricacies of both the actuator and robot designs, and we show how we can design a robot that can use the stated actuator to meet the requirements of both high-speed wheel joints and high torque leg joints with minimal customization, complication, or articulation.