Classification of actuation mechanism designs with structural block diagrams for flapping-wing drones: A comprehensive review

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Abstract

Flying insects are interesting dipteras with an outstanding wing structure that makes their flight efficient. It is challenging to mimic flying insects and create effective artificial flapping drones that can imitate their flying techniques. The smaller insect-size drones have remarkable applications, but they need lightweight and minimal connecting structures for their transmission mechanism. Many operating methods, such as the traditional rotary actuation method and non-conventional oscillatory mechanisms with multiple transmission configurations, are popularly adopted. The classification and recent design innovations with flapping actuation mechanism challenges, particularly bio-inspired (biomimetics) and bio-morphic types of flapping-wing aerial vehicles from micro to pico-scale, are discussed in this review paper. For ease of understanding, we have attempted to depict the actuation mechanisms in the form of block diagrams. The ability of hybrid efficient mechanisms to improve the flapping frequency of wings and flapping actuation design process, including other parameters, such as flapping angle, lift generation, and hovering ability with current driving mechanisms, is also discussed. Depending on their endearing resemblance, we have segregated Flapping-Wing Micro Air Vehicle (FWMAV) design patterns like birds, small birds, nano hummingbirds, moths, bats, biomorphic types, flapping test bench models, and fully flyable models, which are characterized by their flight modes. Important flapping actuation systems that can be used to achieve hovering capability are highlighted. The actuation mechanisms' specifications and configurations are expanded by focusing on the need of flapping frequency and stroke angle controllability via the linkage mechanisms with insight into flapping patterns. Besides that, the requirements for the sustainability of flying patterns during manual and automatic launches were investigated. In addition, the different researchers' annual progress on their Flapping-wing models has been emphasized. The best performing prototypes with their flapping actuation mechanism contributions to achieving better lift and long-duration flight sustainability are articulated through ranking. An insight into some of the significant challenges and future work on flapping performance levels are also discussed.

Introduction

Insect movements depend on the synchronization of their flapping wings while in flight. Therefore, enabling the insect to move sideways is not an easy maneuver. The wings of fliers are generally linked to their dual muscles, one to enhance their strength and the other to control them [1]. During flight, these muscle parts deform the thorax by indirectly moving the wings via a complex hinge consisting of both compliant and rigid parts [2]. In general, some insects move their flight muscles explicitly, others implicitly. In insects with explicit flight, the wing muscles are attached to the wing base, so that a little downward progress of the wing base would lift the wing upwards. Potential energy is being accumulated due to the deformation of the thorax, wing, and wing hinge during flight. Flapping of the wing can be achieved by employing this potential energy, which can reduce the inertial effect [3]. Without elasticity in the wing, flapping wings will not be effective when compared to rotating propellers [4]. The impressive flight techniques associated with insects and birds, have inspired several researchers towards biological flying systems to evolve workable and efficient man-made flying mechanisms. The mechanism of flight has progressed from a fixed to a rotary, then to a flapping wing and an oscillating wing that mimics small flying creatures like birds and insects [5]. Based on parameters, such as size, mass, and volume, the small-sized Unmanned Aerial Vehicles (UAVs) are usually classified into four sub groups, namely, Micro Aerial Vehicles (MAV), Nano Aerial Vehicles (NAV), Pico Aerial Vehicles (PAV) [6] and Smart Dust (SD) [7]. All these vehicles are known as “extremely small and ultra-light air vehicle systems” [8]. For MAV, the overall wingspan is 1 m, and the weight is less than 2 kg. For NAV, the maximum wingspan is ≈ 15 cm, and the weight is below ≈50 g. For PAV, the overall wingspan is ≈ 2.5 cm, the weight is nearly under ≈3 g. Smart dust (SD) consists of several small micro-electromechanical systems (MEMS) and sensors inside it, the size for SD is ≈ 0.25 cm and the weight is ≈ 0.5 g. Explicitly, MAVs are operated by propellers that are flying wings, in which all avionics, stored resources, and propulsion in the wing environment are reduced. Rotary-wing implementations have yielded better performance compared to fixed wings, owing to their capability of flying slowly with even drift. Flapping-wings have the benefits of greater maneuverability, less noise, and improved concealment-compared to fixed and rotating wing drones. Thus, the main module for achieving the ideal bionic quality of an aerial vehicle is surely the flapping-wing drone [[9], [10], [11], [12]].

In short, the MAV's wing flapping actuation mechanisms are guided by two fundamental principles. One is to mimic the structure with mechanisms, just like actual living birds/insects, called the biomimetic approach of design, and the second is to use only limited characteristics observed in flying birds/insects with advanced design features of human-made aerial vehicles, referred to as the bio-morphic approach of design [13]. In 2005, Jones et al. attempted a biomorphic concept of design that involves fixed side wings to generate lift and two flapping wings at the back side (as foil) that could flap in the counter phase. Usually, whenever a bird flaps its wings, the body appears to fluctuate against the wing movements. But in this concept, the biplane framework is advantageous as it balances the aerodynamic limitations by concentrating on enhancing the thrust through mechanical parts [[14], [15], [16], [17]]. However, the general biomimetic approach to design has been followed by many researchers in projects like Microbat [18], Nano hummingbird [19] etc., where flapping of side wings is mainly focused. The flapping wing system is the most sustainable mechanism when compared to rotary and fixed types. This may be because its energy is transmitted via a larger chord, oscillating from a limit of zero thrusts that boost both ends of the flapping angle to a high (higher than in mid-stroke) [20]. The remarkable thing about these flapping structures is that they are designed to be a flexible and efficient oscillating system with reasonable elasticity. The flapping method involves control over the wing in such a way that the movement of the wing must stop and reverse the plane for every stroke [21]. The wing kinetic power will be held on the thorax walls, which will be utilized at the end of every stroke for better lift [22]. The versatility of wings is a major component of insect motion that efficiently generates aerodynamic power. In comparison to birds, insect wings are indeed controlled at the base, so in-flight it experiences great deflections and gradual structure adjustments accordingly. Insect wings serve as reconfigurable airfoils and have many associated supports, involving passive twist to achieve damping [23]. The structure and tensile mobility of the framework in a stretch and a chord-like orientation determine the functional resistance of a wing to a specified density [24]. As a result, researchers with the concept of imitating the insect flight technique on their MAVs usually opt for a better and simpler flapping mechanism on its wing. In 2012, the mechanism of flapping techniques being categorized by Ryan et al. [25] depending on the following conditions: workspace (generated wing trajectory), rigid body, compliant mechanism [26], type synthesis, mobility, and type of actuators used for wing actuation, etc. Phan and Park, in his review paper, elaborated on the motor unit, design mechanism, wing configuration, and control strategies utilized in micro-to-pico size fapping-wing drones. They described the associated electronics and power technologies that were used on the MAVs to drive actuators rather than traditional electromagnetic motors [27]. In 2019, Jiaxin et al. [28] reviewed various scientific approaches of driving actuation mechanism, such as piezoelectric, motor, and electromagnetic driven fapping-wing drones. In their recent work, the advancement of the FWMAV bio-inspired mechanism concerning the driving actuation methods is well explained. We surveyed the Scopus database from 2000 to 2021, using search keywords based on size (such as flapping wing micro, nano, and pico aerial vehicles) and driving mechanism (such as motor-driven flapping wing, hybrid flapping-wing aerial vehicle, and linear actuated flapping wing). This can be seen in Fig. 1, Fig. 2 respectively.

It can be observed from Fig. 1 that a large volume of research has been performed by researchers in the field of Micro Aerial Vehicles (i.e 94.83%) when compared to the Nano (4.37%) and Pico (0.79%) Aerial Vehicles. Fig. 2 depicts how FWAVs have captivated the interest of academic researchers in designing and testing various types of actuation mechanisms over the last two decades. As per the graph collected data, the average research on overall flapping-wing drones carried out by several academicians is of the order 49.50% on motor actuation mechanism, 30.69% on hybrid actuation mechanism, and 19.80% on linear actuation mechanism during the past 21 years.

The small portion of the work carried out is focused on dynamics, kinematics, measurements, control, and power supply. In any flapping-wing drone design, an efficient actuation mechanism for flapping, maneuvering, and modulating the wings is required from a strategic standpoint. As a result, the mechanisms used for flapping motion in MAV, NAV, and PAV till date should be classified systematically for the benefit of the study. Such a classification would also help future development of flapping methods, especially for very lightweight bioinsired aerial vehicles. Accordingly, in our literature review, we were able to detail the characteristics of several flapping-wing drones under each type of flapping actuation mechanism, which can be found in Appendix 1 of this paper. This paper has evaluated all classification based on size scales (the percentage of micro, nano, and pico aerial vehicles from Appendix 1 is represented in Fig. 3(a), and most types of flapping driven mechanism for free flight capabilities, such as fully flyable models and flapping test bench models in Appendix 1. Fig. 3(b) shows the percentage of fully flyable models and flapping test bench models available from our list.

A brief overview of the classification based solely on flapping actuation mechanisms, which are the core of insect-inspired flapping vehicles, has been carried out, to be primarily considered when designing any flapping-wing drone. Based on the similarity of the gear and linkage connections of the various aerial vehicle prototypes, we designed a standard combination of design mechanisms and designated it as Type I design, Type II design, etc., under diverse processes according to the classification in Fig. 4. The equivalent block diagram representation of those design mechanisms is implemented in this review, for ease of understanding of the mechanisms. It displays the depiction of design in the form of block diagrams with variants, add-ons, and enhancements provided by various authors to their prototypes, indicated with explanations. The block diagram approach was chosen here because it offers a comprehensive concept of design mechanisms with internal connections and parts in blocks. In addition, to block diagram representation, steps for the flapping-wing drones design process which include other parameters such as flapping angle, flapping frequency, and current driving mechanisms, chosen from various literature for the generation of lifts at different sizes or scales of flapping-wing drone are also discussed. We classified flapping-wing drones forms such as birds, small birds, nano hummingbirds, insects, bats, and biomorphic types into rotary, hybrid, and linear actuated flapping-wing drone based on their imitable similarity. Flight modes such as forward, backward, turn, take-off, hovering, and landing flight are described for the fully flyable model and the flapping test bench model. The best performing prototypes are addressed by assigning them a ranking based on their flight modes, flapping actuation structure designs, and long-duration flight affordability. The requirements and designs of most actuation mechanisms have been expanded by focusing on the new productivity of flapping frequency and stroke angle controllability via linkage mechanisms with awareness of flapping patterns. The sustainability of flying patterns after they have been manually or automatically launched is also being investigated. Furthermore, many researchers' annual progress on their FW models has been discussed.

The goal of this article is to perform a comprehensive and systematic review of the actuation mechanisms associated with flapping wing drones. The rest of the paper is organized as follows: in section 2, classification of flapping actuation mechanisms are discussed briefly followed by further sections. Section 3 describes, rotary driven flapping actuation mechanisms with various design strategies of motor driven system designs. Then in section 4, compliant structure of rotary actuated flapping mechanisms are well elaborated. While in section 5, hybrid driven Flapping mechanisms of the lates design methods with structural connections, are compared with other actuation designs and discussed. Section 6, clarifies on linear driven flapping mechanisms, with different material excitation and its impact on flapping actuation and lift. In section 7, the conceptual process flow required for the structural development of any flapping actuation mechanism are well elaborated with flow chart. In section 8, challenges and future works of flapping actuation design mechanisms with required future advancements are thoroughly discussed. Finally, the conclusions summarize the review's primary findings.

Section snippets

Classification of flapping actuation mechanisms

While developing any flapping-wing drone, there are numerous design options for the flapping actuation mechanism. In this paper, we have classified flapping actuation mechanisms by referring to the papers of the past two decades, as shown in Fig. 4. It contains detailed information on all of the actuation mechanisms used by many researchers on MAVs, NAVs, and PAVs. This classification also aids in the selection of a better and more efficient flapping mechanism for the design of lightweight

Rotary/motor-driven flapping mechanism

Obtaining the required resonance in FWs was the most essential aspect to boost the power capacity, generation of lifts, and efficiency in flight control. Rotary driven mechanism necessitates the use of a motor as a driver and translation of this rotary action to linear motion to get the flapping movement is challenging. For this conversion of rotary motion to linear motion, gears are a viable option to get better transmission, which is known as a rigid body mechanism. Under the geared motors,

Compliant structures for rotary actuated flapping mechanism

There are three types of compliant mechanisms available, including short compliant segments, long compliant segments, and flexural joints. In 2014, to mimic the Dipteran insect flight, Lau [58] came up with a compliant flapping mechanism, which could generate more than 25 Hz frequency, and its equivalent block diagram is represented in Fig. 12. They also tried with a rigid body mechanism by modifying a few things when compared to Fig. 12. To obtain better frequency and flapping angle, the

Hybrid driven flapping mechanism

Following possibilities of dual-lever mechanisms were considered to achieve high sweep amplitudes with some two combinations of mechanisms under rotary driven mechanisms. Like any linkage and string combination or any two combinations of rotary mechanism to obtain a hybrid technique. Few such combinations are considered here under the hybrid mechanism by referring to Fig. 4 classification as listed. 4-bar linkage and pulley string mechanism [35,37,132] single-crank and double-rocker mechanism [

Linear driven flapping mechanism

To date, many investigations have been carried out on flapping-wing drones by various institutes because of their tremendous mobility, less expensiveness, less complexity of circuits, and easy programming. It is necessary to obtain a mechanical harmonic or flapping actuation mechanism at the wing for producing efficient lift. This can be accomplished by applying any of the following four modes under a linear driven mechanism. i.e. material stretching, contraction, torsion, or bending. The first

Conceptual process flow for the structural development of any flapping actuation mechanism

To build any flapping-wing drone, it is necessary to opt for whether it should be of the bio-mimic type of bio-morphic type design. Because the biomimetic design perfectly mimics the physiology of living creatures such as birds, insects, and so on [251] The biomorphic designs are influenced by the concepts of living creatures, but they are not exact replicas of living organisms [252]. De Wagter's DelFly [32] and Platzer's biplane Flapping MAV [253] are two classic biomorphic models. In this

Challenges & future works of flapping actuation design mechanisms

While much has been accomplished in actuation mechanism designs, there are still a lot of challenges to create efficient flying, insect-inspired flapping-wing drones in real-world environments. Maintaining the least weight and compact design with Free flight capabilities using onboard or wireless components are still challenging for insect-scale flapping wing vehicles [27].

Conclusion

This paper summarizes and addresses various types of flapping-wing systems, with a focus on all flapping actuation mechanisms, structure-based designs for flapping enhancement, and hybrid flapping actuation techniques. A detailed classification of flapping-wing drones are defined by categorizing the various types of rotary/motor, hybrid, and linear driven flapping actuation mechanisms. The authors attempted to deconstruct a few basic and important actuation mechanisms into their equivalent

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors acknowledge the support received from Universiti Putra Malaysia and the Manipal Institute of Technology, MAHE. The authors would like to convey their gratitude to UPM for granting them the necessities required to advance in Bio-inspired researches through the University's Geran Putra Berimpak (GPB) research grant; UPM/800-3/3/1/GPB/2019/9677600.

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