Mechanical and Civil Engineering Seminar
Creating ARCHER: A 3D Hopping Robot with Flywheels for Attitude Control
Ph.D. Thesis Defense
Abstract: The field of robotic hopping began over 40 years ago, when it was first shown that robust hopping could be achieved on real hardware. In the years since then, it's become clear that hopping requires high performance and precision from its actuation and planning, due to its extreme interactions with the environment occurring over periodic, yet very short durations of time. Despite being of lower dimensionality than many other legged robots, hoppers are very underactuated which only adds to the difficulty of planning motions quickly, for real-time needs.
The studies into robotic hopping presented in this thesis start with a look into two different actuation styles for creating vertical periodic motion: a compress-release mechanism and a moving-mass mechanism. The dynamics of each were examined from the perspective of stability and robustness to uncertainties in the model and measurements. The compress-release hopper (CRH) was found to be very stable, simple to control, and robust to all uncertainties, but inherently had some inefficiencies due to the requirement of holding compression during portions of the aerial phase. The moving-mass hopper (MMH) required optimization to generate the proper cyclic motions as well as closed-loop control to make them stable. Furthermore, the original configuration the MMH was also less energetically efficient and robust to uncertainty than the CRH.
In an effort to improve the efficiency of the MMH, a second-generation robot was designed using the principle of parallel elasticity. This involved placing a second spring in parallel to the actuator which would naturally guide the motion of the moving-mass into an optimal path, eliminating a significant portion of actuation effort and improving the overall efficiency. An added benefit of this change was that the robot no longer required closed loop control to create stable hopping. This new robot was built and tested in the lab showing a dramatic improvement over the previous design. The principle of controlling the compliance in the actuator for efficient motion was then taken one step further by creating custom, nonlinear stiffness springs which would provide a more ideal trajectory of motion. This process utilized a design-in-the-loop optimization strategy that would both design these springs as well as the motions of the moving-mass to yield better actuation efficiency. A set of these springs was created and attached to the second-gen MMH, replacing the lower spring, and tested in the lab. These springs did slightly improve the efficiency of the robot, but were restricted by the material selection of the springs due to manufacturing limitations.
Moving into the realm of 3-Dimensional hopping, a final robot was designed and built: ARCHER. Unlike traditional hopping robots which use a torso with very large inertia to control the leg motion and balance, ARCHER uses a set of three flywheels. The goal of this robot was twofold: to study the feasibility of using flywheels alone to control attitude, and to take advantage of the principle of decoupled systems. By using strategically placed flywheels, the dynamics of the leg and the attitude subsystems were decoupled, meaning their actuation did not have a direct influence on each other. This allows for simpler motion planning and control. The culmination of this thesis was running experiments with this robot, showing its initial performance and ability to hop with separate controllers for each subsystem.