Nikola Georgiev- Ph. D. Thesis Defense
Abstract: The main objective of this thesis is to enable development of high performance actuation for legged, limbed and mobile robots. Such robots need to support their own weight, therefore, their actuators need to be light weight, compact, and efficient. In addition, these actuators need to exhibit significant shock tolerance and backdrivability due to the robots physical contact with the environment. A dynamics analysis also shows that the actuators' design may have significant impact on a robot's dynamics sensitivity. These consideration motivate improvements in all actuator design aspects compared to current approaches.
First, the application-specific design of outer rotor motors with concentrated windings is considered for three main categories: electric vehicles, drones and robotic joints. It is shown that an intrinsic design trade-off exists between a motor's copper loss, core loss and mass, which allows development of motors with superior performance for each application. In particular, it is shown that outstanding torque density may be reached with high pole count outer rotor motors and the design and optimization of such motors is outlined in terms of robotic applications. Analytic motor design scaling modes are also derived to highlight implementation challenges of high torque motors in robotics.
Next, the design of gearboxes for robotic actuation is discussed. A novel type of high reduction Bearingless Planetary Gearbox is introduced that allows a large range of reduction ratios to be achieved in a compound planetary stage. In the concept, all gear components float in an unconstrained manner as the planet carrier is substituted with a secondary sun gear. This is achieved by introducing an additional kinematic constraint that allows the planets to be uniform. The advantages of the Bearingless Planetary Gearbox over current approaches in terms of improved robustness, load distribution, manufacturability, and assembly are outlined.
Finally, analysis, design, and prototyping of rotary planar springs for rotary series elastic actuators is described. A model based on curved beam theory that allows rapid iteration and comparison between design parameters of rotary springs is developed. Mass reduction techniques based on composite arm structures are introduced and internal arm contact modeling is presented. Motivated by strain energy density analysis, an optimization based spring design approach is developed that allows significant increase in the torque and torque density.