HiFly-Dragon: A Dragonfly Inspired Flapping Flying Robot with Modified, Resonant, Direct-Driven Flapping Mechanisms
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1. Introduction
Insect-inspired FW-MAVs have been actively investigated in recent decades. The flapping wings not only propel the vehicle to move but also generate aerodynamic forces and torques for attitude control [1] and even sense their surrounding environment by measuring and interpreting the variations of the wing loading, which ensures their exceptional stability and maneuverability at an extremely small scale [2,3]. Several notable bio-inspired flapping wing robots have achieved taking off and hovering in the air, including the Harvard RoboBee [4], the DelFly [5], the Nano Hummingbird [6], the KUBeetle [7], and the BionicOpter [8], inspired by the extraordinary flight capabilities of bees, fruit-flies, hummingbirds, beetles, and dragonflies.
Dragonflies have been recognized as the apex predators of the insect world [9], which have millions of years of adaptation [10], and almost 6000 species with wingspans ranging from 18 mm to 190 mm [11]. By virtue of the tandem wing configuration, they perform superior flight as they can hover in the air, glide with minimal energy consumption, and even maneuver in all directions [12,13,14,15], which makes them master all the flight conditions of helicopters, fixed-wing aircraft, and gliders, which aroused intense interest to study the aerodynamics of dragonflies in different flight modes [16] and develop dragonfly-inspired Micro Air Vehicles (MAVs) [17,18,19]. All four wings of a dragonfly are powered directly by the flight muscles attached to the wing bases; thus, they can independently adjust the flapping amplitude, the stroking phase, and the flapping amplitude offset of each wing to generate aerodynamic forces and torques [20,21,22]. To date, bionic engineers have devoted massive efforts to developing dragonfly-inspired flapping wing air vehicles, but very few untethered prototypes succeed in lifting off. Of these, the BionicOpter [10] from FESTO achieved active stable hovering flight, can maneuver in all directions, hover on the spot, and sail without flapping its wings, and was recognized as the first model capable of handling more flight conditions than helicopters, motorized, and non-motorized gliders combined. The BionicOpter adopts one brushless external rotor motor to drive wings flapping between 15 to 20 Hz and eight servo motors to independently actuate the flapping amplitude and the stroke plane inclination-regulating mechanisms of each wing for attitude control. The complex mechanical systems result in the prototype having a wingspan of up to 63 cm, a body length of 44 cm, and a total weight of up to 175 g, creating a huge gap for miniaturization. All of these prototypes adopt multi-DOF flapping driving mechanisms [23], so extra actuators and mechanisms are necessary, resulting in extremely complicated mechanical systems, a very large weight, and enormous size.
The resonant, direct-driven flapping mechanism is a flapping actuating system that achieves wing flapping motion by controlling the reciprocating rotation of the motors instead of the four-bar linkage or any other transformation mechanisms. In this process, springs are utilized to provide resonance, counteracting the inertial forces and storing energy during flapping. Eliminating the need for extra motor motion transformation and control mechanisms, resonant, direct-driven flapping mechanisms hold potential for light weight and miniaturization. In previous works, DC motors [24] have been tried for actuating the wings flapping directly; [25] studied the effectiveness of resonance for improving the driving efficiency and [26] modeled the dynamic of the resonant, direct-driven flapping. Although the resonant, direct drive mechanism based on a torsion spring is beneficial to simplify the mechanism, spring fatigue failure under resonant conditions tremendously reduces the durability of the direct drive system.
In this work, we introduced a systematic approach for developing a dragonfly-inspired flapping robot propelled by four independent, modified, resonant, direct-driven flapping mechanisms. Compared to the previous works that utilized a single spring to provide resonance, the proposed direct-driven flapping mechanisms in this paper were improved with two asymmetry cascaded torsion springs, which cancel out the spring distortion during stretching and compression. The issue of fatigue failure in torsion springs under alternating load conditions during flapping, which leads to lift damping and reduces the mechanical structure endurance within a few seconds, has been addressed by enhancing the springs’ linearity and system resonance with the proposed asymmetrical cascaded configuration of the torsion springs. The effectiveness of this modification was confirmed through several flapping tests, wherein the resonant, direct-driven flapping mechanism successfully generated a constant lift force of 10 g-force without any lift damping or structural failure. The Printed Circuit Boards (PCBs) of the avionics were designed to function as robot airframes for weight and size reduction. The two pairs of tandem wings were independently actuated by the resonant, direct-driven flapping system, which allows for mimicking the flight behaviors of natural dragonflies to enhance the flapping lift and generate multi-DOF aerodynamic control torques without an extra control mechanism. This research provides a platform for the development of bionic dragonfly aircraft, flapping flight controls, and bionics research.
Section 2 introduces the system design and fabrication methods, including the wings, the resonant flapping drive systems, and the avionics of attitude sensing and flight control. Section 3 introduces the attitude control torque generation strategies. Section 4 demonstrates the flapping propulsion test of the flapping robot. Section 5 concludes the article and reports on the future work of this project.
5. Conclusions
In this paper, we introduced a compact microrobot dragonfly with four tandem independently controllable wings that is directly driven by four modified, resonant, flapping mechanisms integrated on the Printed Circuit Boards (PCBs) of the avionics, called the HiFly-Dragon. The fatigue failure of the torsion springs under alternating load conditions was ameliorated by enhancing the spring linearity with an asymmetrically cascaded torsion spring resonant mechanism, which was verified to maintain a flapping amplitude of 180° at 28 Hz and generate a 10 gf lift for a single wing without attenuation. The indispensable hardware of subsystems for feedback flight control was developed and implemented on the onboard avionics, including attitude sensing, radio frequency wireless communication, four motor controllers, and onboard flight control. All of the parts of the prototype subsystems were integrated on the PCBs, and the robot demonstrated a substantial weight reduction, boasting a 180 mm wingspan and a total weight of 32.97 g (including three cells of LiPo batteries). The total lift of the robot was measured to be up to 34 gf with onboard power. And the robot lifted off powered by onboard batteries on the balance beam. This research provides a microrobot platform with compact structures for the development of bionic dragonfly aircraft, flapping flight control, and bionics research. Our current focus is on testing the attitude control torque responses, tuning the attitude controllers, and flight testing.
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