Omnidirectional Tilt-Rotor Flying Robots for Aerial Physical Interaction: Modelling, Control, Design and Experiments

Abstract

This doctoral thesis addresses the study of omnidirectional tilt-rotor aerial robots, and their application to aerial physical interaction tasks. Through modelling, control, prototype design, and experimental evaluation, this work carves a new direction in aerial robotics research, and seeks to inspire a future of versatile and autonomous aerial manipulators. Recent developments in the field of fully actuated aerial robots have demonstrated the exceptional advantages of these systems for physical interaction. Characterised by their decoupled translational and rotational system dynamics, these systems not only outperform their underactuated counterparts, but extend their capabilities. Through the dynamic re-orientation of actuated thrust vectors, we now have access to a great expanse of possible morphologies, dynamic system capabilities, and new applications. Extending these novel tilt-rotor systems with an active manipulator further demonstrates enhanced end effector performance for manipulation tasks. The concept of macro-micro manipulation -- using a highly dynamic end effector mounted to a powerful base -- overcomes dynamic limitations that currently restrict the efficacy of aerial manipulators. In pursuit of versatile and high performance systems for aerial physical interaction, the present work combines these concepts to advance the state-of-the-art in aerial manipulation. The design space of a tilt-rotor aerial robot is selected by optimizing a general model around desired performance metrics and system parameters. The resulting system, chosen for a balance of omnidirectional and efficient flight capabilities, is compared against other state-of-the-art fully actuated systems. Aerial interaction models are developed for fixed and active manipulators, and a geometric optimization is performed to determine the design of a parallel manipulator in the context of an omnidirectional flying base. The control problem divides the system conceptually into tracking control of a pure wrench generating base, and a subsequent actuator allocation problem to achieve a six degrees of freedom wrench with 18 individual actuator commands. The nonlinear and highly dimensional actuator space is addressed with instantaneous and differential allocation methods, the latter of which incorporates secondary tasks, such as the unwinding of tilt-arm cables, in the actuation null space. Inverse-dynamics based controllers are introduced for control of the flying base, treating the whole tilt-rotor system as a single rigid body. Interaction controllers including axis-selective impedance and direct force control are developed for the system equipped with a fixed manipulator arm. A redundant control strategy is developed for the omnidirectional system with an attached translational parallel manipulator, where predicted reaction forces are fed to an independent base controller to compensate the manipulator dynamics. Several iterations of omnidirectional tilt-rotor aerial robots are designed and constructed, considering the requirements of aerial interaction tasks. Actuator selection, tilt-rotor mechanisms, and complete system assembly are presented, as well as design details for a parallel manipulator. Experimental trials evaluate the capabilities of the physical system and its control implementation to track omnidirectional trajectories. Aerial physical interaction tasks are demonstrated, involving point force application with the environment, push-and-slide tasks, and applications to non-destructive contact inspection of concrete. Fast end effector tracking and disturbance rejection experiments are performed to validate the macro-micro concept of an omnidirectional tilt-rotor parallel manipulator. Ranging from general modelling to control, design choices and complete system prototypes, the content of this work acts as a guide for envisioning and building innovative systems that will push the frontier of aerial manipulation.