FAST-Hex -- A Morphing Hexarotor: Design, Mechanical Implementation, Control and Experimental Validation (2004.06612v1)

Abstract: We present FAST-Hex, a micro aerial hexarotor platform that allows to seamlessly transit from an under-actuated to a fully-actuated configuration with only one additional control input, a motor that synchronously tilts all propellers. The FAST-Hex adapts its configuration between the more efficient but under-actuated, collinear multi-rotors and the less efficient, but full-pose-tracking, which is attained by non-collinear multi-rotors. On the basis of prior work on minimal input configurable micro aerial vehicle we mainly stress three aspects: mechanical design, motion control and experimental validation. Specifically, we present the lightweight mechanical structure of the FAST-Hex that allows it to only use one additional input to achieve configurability and full actuation in a vast state space. The motion controller receives as input any reference pose in $\mathbb{R}^3\times \mathrm{SO}(3)$ (3D position + 3D orientation). Full pose tracking is achieved if the reference pose is feasible with respect to actuator constraints. In case of unfeasibility a new feasible desired trajectory is generated online giving priority to the position tracking over the orientation tracking. Finally we present a large set of experimental results shading light on all aspects of the control and pose tracking of FAST-Hex.

• The paper introduces FAST-Hex, a hexarotor that uses a single servomotor to tilt all propellers for a seamless switch from under-actuated to fully-actuated configurations.
• It details a novel control strategy that re-computes feasible trajectories to ensure robust full pose tracking when direct tracking is unachievable.
• Experimental validation shows a mean position tracking error of approximately 8.7mm and minimal attitude errors, confirming the platform's precision.

Overview of "FAST-Hex -- A Morphing Hexarotor: Design, Mechanical Implementation, Control and Experimental Validation"

The paper "FAST-Hex -- A Morphing Hexarotor: Design, Mechanical Implementation, Control and Experimental Validation" introduces a novel micro aerial vehicle platform termed as the "FAST-Hex." This platform is characterized by its ability to transition seamlessly from being under-actuated to fully-actuated by employing a unique design that incorporates a single additional control input. This control input is a motor that can synchronously tilt all propellers, thereby morphing the aerial vehicle configuration.

Key Contributions

The significant contributions of this work are tri-fold:

1. Mechanical Design Innovation: The FAST-Hex utilizes a lightweight yet robust mechanical structure that allows seamless configurability with minimal additional input. The design entails a hexarotor configuration where the propellers can tilt synchronously using one servomotor, thus balancing between the efficiency of under-actuated and the full-pose-tracking capabilities of fully-actuated configurations.
2. Control System Development: The control strategy developed for the FAST-Hex enables full pose tracking, provided the reference pose is feasible within the actuator constraints. If a direct trajectory tracking is unfeasible, the system re-computes a feasible desired trajectory prioritizing position over orientation tracking. This ensures robust control of the vehicle in diverse operational scenarios.
3. Experimental Validation: Through extensive experimental validation, the paper demonstrates the efficacy of the FAST-Hex platform. The experiments include static hovering and dynamic trajectory tracking, establishing the platform's capabilities and the control scheme's effectiveness in transitioning between different configurations.

Numerical Results

The experimental results suggest that the FAST-Hex can maintain a mean position tracking error of approximately 8.7 mm during static hovering motions, showcasing the controller’s precision. Notably, when in the fully actuated configuration, this error reduces further, indicating enhanced control fidelity. The attitude tracking errors were also minimal with an average roll and pitch error of around 0.84 degrees and 0.92 degrees, respectively.

Implications and Future Work

The practical implications of this research are vast, considering the rapidly expanding applications of UAVs in areas like environmental monitoring, search and rescue, and aerial manipulation. The ability for a single platform to morph its physical configuration to adapt dynamically to operational demands represents a substantial advancement, offering increased versatility and energy efficiency.

Theoretically, this work contributes to the understanding of actuation and control in morphing multi-rotor systems. The approach of using a minimal input system for configurability highlights opportunities in optimizing design for UAVs in terms of simplicity and function.

Looking forward, advancements could escalate in fine-tuning adaptive control strategies that can facilitate even faster and more efficient morphing transitions. Extension to other UAV configurations and broader ranges of morphological adaptations could open new frontiers in UAV operations and applications. Further, integrating advanced onboard processing for real-time feedback and decision-making could enhance the autonomous capabilities of morphing UAVs. The prospects for integrating such platforms with AI systems to manage complex tasks autonomously could be a profound area of exploration in the development of intelligent aerial systems.