Title:  Whole-body Control of Wheeled Inverted Pendulum Humanoids

Committee: 

Dr. Christensen, Advisor       

Dr. Romberg, Chair

Dr. Theodorou

Abstract:

The objective of the proposed research is to develop a framework for controlling a Wheeled Inverted Pendulum (WIP) Humanoid to perform useful interactions with the environment as it balances itself dynamically on two wheels. As humanoid platforms are characterized by several degrees of freedom, they have the ability to perform several tasks simultaneously, such as balancing themselves, maintaining a specific body pose, controlling the gaze, lifting a load or carrying a tray of cups filled to the brim. These tasks are all performed simultaneously, and the whole body participates in achieving each objective, with priorities assigned to each. The control also has to operate within constraints of angle and torque limits on each joint, as well as safety constraints of avoiding self-collision and collision with obstacles. This problem is referred as “Whole-Body Control” in the wider humanoid literature, and several successful solutions have recently been demonstrated for bipedal humanoid platforms. Little has been done to solve the same problem for WIP humanoids. WIP systems feature under-actuated, nonlinear and highly unstable dynamics and it is critical to address these issues for a given solution to succeed. The proposed approach is hierarchical with a low level controller responsible for controlling the manipulator/body and a high-level controller that defines center of mass (CoM) targets for the low-level controller to control zero dynamics of the system driving the wheels. The low-level controller plans for shorter horizons while considering more complete dynamics of the system, while the high-level controller plans for longer horizon based on an approximate model of the robot for computational efficiency. Our core contributions are: 1. Showing how to isolate the dynamics of the manipulator from those of the wheels such that the resulting model is amenable to existing whole-body control techniques, such as operational space control and quadratic programs (QP) 2. Using differential dynamic programming (DDP) to generate optimal trajectories for center of mass to control wheel motion.