System identification of a robotic hummingbird in hovering flight
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2016 by the American Helicopter Society International, Inc. All rights reserved. This paper presents the first ever linear system identification of the flight dynamics of a hover-capable robotic hummingbird which utilizes only two wings for flying as well as for all its control and stabilization. The vehicle was developed in-house, using state-of-the-art materials, electronics, and innovative design/fabrication techniques, and a description of its development is provided. Systematic experimental studies were conducted to develop flexible, aeroelastically tailored wings, along with novel wing kinematic modulation mechanisms for controlling roll, pitch and yaw. Additionally, a custom, lightweight, autopilot implementing PID control was developed, and after a series of rigorous flight testing, the trim and feedback gains were determined which allowed stable, hovering flight. Once this was achieved, a motion capture camera system was used to track the position and attitude of the vehicle during flight tests which involved providing a series of inputs to excite the vehicle modes and measuring the response. A linearized six degree of freedom state-space model for hovering flight was then extracted from this data using time domain system identification. An analysis of the eigenstate of the model reveals four modes present: an unstable mode that excites all the vehicle states; a stable mode that excites yaw and translational motion; and two oscillatory modes, one stable and one marginally unstable, both responsible for translational and rotational excitation about all axes. This finally demonstrates experimentally for the first time the unstable nature of two-winged, hover-capable flapping flight. These four modes, which show a strong coupling between the longitudinal and lateral dynamics, suggest that assumptions which decouple longitudinal and lateral degrees of freedom may not be valid for this type of flight. Additionally, this unstable, coupled dynamics demonstrates definitively the superior agility inherent in this type of aerial locomotion.
