Design and optimal control of a tiltrotor micro aerial vehicle for efficient omnidirectional flight

Abstract: Omnidirectional micro aerial vehicles are a growing field of research, with demonstrated advantages for aerial interaction and uninhibited observation. While systems with complete pose omnidirectionality and high hover efficiency have been developed independently, a robust system that combines the two has not been demonstrated to date. This paper presents the design and optimal control of a novel omnidirectional vehicle that can exert a wrench in any orientation while maintaining efficient flight configurations. The system design is motivated by the result of a morphology design optimization. A six degrees of freedom optimal controller is derived, with an actuator allocation approach that implements task prioritization, and is robust to singularities. Flight experiments demonstrate and verify the system's capabilities.

• The paper details the design of an optimized tiltrotor dodecacopter MAV capable of efficient, robust omnidirectional flight, addressing prior limitations.
• An optimal full state controller using a jerk-level LQRI is derived and implemented for 6-DOF control with task prioritization to manage singularities and dynamics.
• Flight experiments validate the MAV's trajectory tracking and controller performance, demonstrating effective handling of singular configurations and secondary tasks.

Design and Optimal Control of a Tiltrotor Micro Aerial Vehicle for Efficient Omnidirectional Flight

The paper presented by Allenspach et al. introduces a novel omnidirectional micro aerial vehicle (MAV) designed with an optimal control approach that enhances its dynamic capabilities while maintaining efficient flight configurations. The paper addresses the challenge of marrying full pose-omnidirectionality and high hover efficiency within a single MAV system, a junction that prior research had not fully realized.

Key Contributions

1. Design of Omnidirectional MAV: The paper details the design of a tiltrotor dodecacopter capable of exerting forces and torques in any direction. The authors utilized a morphology design optimization tool, made available open source, to strike a balance between omnidirectionality and efficient hover capabilities. The optimized design supports robust aerial interaction without compromising on payload capacity and efficiency.
2. Optimal Control: The paper derives a full state optimal controller, implemented as a jerk-level Linear Quadratic Regulator with Integral (LQRI). This controller is designed for six degrees of freedom (DOF) control. The task prioritization in actuator allocation allows for efficient control of dynamic trajectories while addressing complexities like singularities.
3. Experimental Validation: Through various flight experiments, the authors demonstrate the MAV's capabilities, evaluating controller performance in comparison to a benchmark PID controller. Emphasis is placed on tracking performance, handling of singularities, and fulfillment of secondary tasks within the overactuated system's null space.

System Design and Control

The design utilizes a tiltrotor approach, which allows individual rotor groups to tilt, optimizing for both hover efficiency and six DOF omnidirectionality. The morphological optimization focused on maximizing efficiency and versatility, confirming an optimized tiltrotor MAV design that surpasses fixed rotor designs in force and torque capabilities.

A comparative analysis highlighted the superior dynamic reachability and efficiency of the tiltrotor system over traditional fixed rotor systems. The tiltrotor system achieves high force and torque capabilities with efficient hover in specific orientations, balancing mechanical complexity with functional performance. While this introduces added inertia and singularity challenges, the enhanced control flexibility justifies these design choices.

Experimental Results

Flight experiments validated the theoretical design and control strategies. The MAV demonstrated proficient handling of defined trajectories, both in regular flight modes and under test conditions targeting singular configurations. The differential allocation strategy allowed for continuous tracking performance despite addressing singularities and additional control tasks like cable unwinding. Position and attitude errors observed during flights showed effective trajectory adherence with notable efficiency in controller design.

Implications and Future Work

This research contributes significantly to the domain of aerial robotics by introducing a sophisticated control strategy applicable to complex MAV systems. The authors suggest future work on refining system modeling and addressing unmodeled dynamics could further improve controller performance. Additionally, integrating constraints into the differential allocation and exploring nonlinear Model Predictive Control (MPC) approaches may enhance task scheduling and performance.

In conclusion, the paper by Allenspach et al. provides comprehensive insights into the advancement of MAV design and control for robust omnidirectional flight with efficiency and versatility. The proposed system architecture and control paradigm stand as robust frameworks for future explorations and practical implementations in aerial robotics.