Fangyuan Liu

Investigations into animal locomotion across diverse environments have highlighted the universal applicability of adjustable reciprocal motion, offering insights into simplifying actuation systems for multi-modal robots. However, achieving unified and efficient reciprocal motion with environmental adaptability in miniature robotic systems is challenging due to constraints of size, weight, and the need for controlled degree of freedom. Here, we present the UniCore, a miniature unified actuation platform capable of flying, swimming, and running with modular appendages. This platform features bio-inspired motor-spring resonance actuation systems, with a central controller that generates four adjustable reciprocal control signals based on a central pattern generator model. Performance validation demonstrates UniCore’s proficiency in achieving three distinct modes of locomotion, underscoring the effectiveness of reciprocal motion for the locomotion of both animals and machines. All in all, this work demonstrates the potential of a unified reciprocal actuation platform to eliminate morphological and actuation redundancies commonly found in existing multi-modal robots, paving the way for more efficient and versatile miniature robotic systems.

Dragonflies show impressive flight performance due to their unique tandem flapping wing configuration. While previous studies focused on forewing-hindwing interference in dragonfly-like flapping wings, few have explored the role of asymmetric pitching angle in tandem flapping wings. This paper compares the aerodynamic performance of asymmetric dragonfly-like wings with symmetric hummingbird-like wings, both arranged in tandem. Using a three-dimensional numerical model, we analyzed wing configurations with single/tandem wings, advance ratios (J) from 0 to 0.45, and forewing-hindwing phase differences (ϕ) from 0° to 180° at a Reynolds number of 7000. Results show that asymmetric flapping wings exhibit higher vertical force and flight efficiency in both single and tandem wing configurations. Increasing the phase difference (ϕ) improves flight efficiency with minimal loss of vertical force in the asymmetric flapping mode, while the symmetrical flapping mode significantly reduces vertical force at a 180° phase difference. Additionally, symmetric tandem flapping wings unexpectedly gain extra vertical force during in-phase flapping. This study uncovers the flow characteristics of dragonfly-like tandem flapping wings, providing a theoretical basis for the design of tandem flapping wing robots.

Flapping wing rotorcrafts (FWRs) combine both the motion characteristics of flapping and rotary wings, exhibiting high aerodynamic efficiency at low Reynolds numbers. In this paper, the ceiling effect of FWRs has been studied through numerical and experimental methods to further investigate the aerodynamic performance of FWRs operating under a ceiling and to explore the feasibility of enhancing the flight efficiency of FWRs via ceiling-effect-based perching locomotion. Based on the momentum theory and blade element methods, a theoretical model is first established to predict the additional thrust generated by the FWR operating under the ceiling. Additionally, to uncover the detailed aerodynamic mechanisms of FWRs' ceiling effect, the computational fluid dynamics (CFD) simulations were conducted to analyze the changes in force production and flow field around the FWR at 75–115 mm distances from the ceiling. Furthermore, experimental methods were employed to validate the theoretical model and CFD simulation. The results demonstrate a continuous increase in the thrust from 19.18 to 22.15 gf as the ceiling distance decreases, while the total energy consumption remains relatively constant. Leveraging the ceiling effect, the tested FWR could achieve an additional lift force of up to 9.5% at 75 mm ceiling height with a 33 Hz flapping frequency. Finally, a ceiling perching demonstration was conducted to validate the feasibility of achieving FWRs' energy-efficient locomotion based on ceiling effects. Our study highlights the positive influence of ceiling effect on FWRs, showing a promising way to further improve the flight efficiency of FWRs.

Micro tandem bi-copters are capable of passing through narrow gaps owing to their particular slender shape. However, the introduction of the tilting servo motors leads to a non-minimum phase roll dynamics, which affects their flight stability when exploring environments with unpredictable disturbances. In this letter, we propose and design a re-configurable aerial platform consisting of two modular bi-copters with an undocking mechanism. In combined configuration, a crossover docking approach is employed to compensate for the poor stability in their servo-controlled attitude of each bi-copter. In bi-copter configuration, the minimum size (equal to ideal passable gap's width) of the system was reduced by 58% through mid-air separation. In detail, to compare the attitude response of the two configurations, a dynamic model considering servo response and non-minimum phase is established and simulated, and flying poking experiments were also conducted on them respectively. On the other hand, the performance of single bi-copter including trajectory tracking and passing through narrow gaps was demonstrated through flight tests. Finally, the feasibility of the undocking mechanism was verified by mid-air separation experiments. The proposed system is promising to be applied in scenarios containing both complex perturbations and confined spaces, while also having the potential to improve exploration efficiency through collaborative work.

Compared with traditional flapping motion, the flapping wing rotor (FWR) allows rotating freedom by installing the two wings asymmetrically, which introduces rotary motion characteristics and enables the FWR to have higher lift and aerodynamic efficiency at low Reynolds number. However, most of the proposed FWRs contain linkage mechanical transmission structures, the fixed degrees of freedom of which prohibit the wings from achieving variable flapping trajectories, limiting further optimization and controller design of FWRs. In order to fundamentally address the above challenges of FWRs, this paper presents a new type of FWR with two mechanically decoupled wings, which are directly driven by two independent motor–spring resonance actuation systems. The proposed FWR has 12.4 g of system weight and 165–205 mm wingspan. In addition, a theoretical electromechanical model based on the DC motor model and quasi-steady aerodynamic forces is established, and a series of experiments are conducted in order to determine the ideal working point of the proposed FWR. It is notable that both our theoretical model and experiments exhibit uneven rotation of the FWR during flight, i.e. rotation speed dropping in the downstroke and increasing in the upstroke, which further tests the proposed theoretical model and uncovers the relationship between flapping and passive rotation in the FWR. To further validate the performance of the design, free flight tests are conducted, and the proposed FWR demonstrates stable liftoff at the designed working point.

Real-time and drift-free state estimation is essential for the flight control of Micro Aerial Vehicles (MAVs). Due to the vibration caused by the particular flapping motion and the stringent constraints of scale, weight, and power, state estimation divergence actually becomes an open challenge for flapping wing platforms’ longterm stable flight. Unlike conventional MAVs, the direct adoption of mature state estimation strategies, such as inertial or vision-based methods, has difficulty obtaining satisfactory sensing performance on flapping wing platforms. Inertial sensors offer high sampling frequency but suffer from flapping-introduced oscillation and drift. External visual sensors, such as motion capture systems, can provide accurate feedback but come with a relatively low sampling rate and severe delay. This work proposes a novel state estimation framework to combine the merits from both to address such key sensing challenges of a special flapping wing platform—micro flapping wing rotors (FWRs). In particular, a cross-fusion scheme, which integrates two alternately updated Extended Kalman Filters based on a convex combination, is proposed to tightly fuse both onboard inertial and external visual information. Such a design leverages both the high sampling rate of the inertial feedback and the accuracy of the external vision-based feedback. To address the sensing delay of the visual feedback, a ring buffer is designed to cache historical states for online drift compensation. Experimental validations have been conducted on two sophisticated microFWRs with different actuation and control principles. Both of them show realtime and drift-free state estimation.