Insect-inspired, tailless, hover-capable flapping-wing robots: Recent progress, challenges, and future directions

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Progress in Aerospace Sciences journal homepage: http://www.elsevier.com/locate/paerosci

Insect-inspired, tailless, hover-capable flapping-wing robots: Recent progress, challenges, and future directions Hoang Vu Phan *, Hoon Cheol Park * Artificial Muscle Research Center and Department of Smart Vehicle Engineering, Konkuk University, Seoul, 05029, South Korea

A R T I C L E I N F O

A B S T R A C T

Keywords: Insect-inspired flapping-wing air vehicle Hovering Biomimetics Multimodal locomotion Insect flight

Flying insects are able to hover and perform agile maneuvers by relying on their flapping wings to produce control forces, as well as flight forces, due to the absence of tail control surfaces. Insects have therefore become a source of inspiration for the development of tailless, hover-capable flapping-wing air vehicles (FWAVs). How­ ever, the technical difficulty involved in designing and building such a complicated and compact system within a limited takeoff weight for it to remain airborne is a major barrier. Consequently, among the many developed vehicles, only a few are capable of free flight. In this review paper, we survey recent developments of insectinspired tailless FWAVs in various sizes from micro-to pico-scale, with different types of driving actuator, mechanism design, wing configuration, and control strategy. We discuss the capability of free flight and flight endurance of the FWAVs, which are limited by current electronics and power technologies that severely constrain those vehicles using other driving actuators, rather than conventional electromagnetic motors, to freely take off. Achievements in the development of FWAVs demonstrate their potential for future applications, both in the military and civilian fields. In addition, further integration with other modes of locomotion, such as crawling, jumping, perching, self-wing-folding, and water-diving, can be a future direction of a FWAV to fully adapt the biologically locomotive strategies in nature, and to increase the range of applications.

1. Introduction In nature, both birds and insects flap their wings to produce flight forces. However, their underlying flight principles are quite different [1]. Most birds, except for small birds such as hummingbirds, fly by making the flapping stroke plane nearly vertical during cruise. Furthermore, useful aerodynamic forces are mostly produced during relatively low frequency and small amplitude downstroke motions [1,2]. On the other hand, most insects flap their wings faster with larger am­ plitudes, and produce forces during wing upstroke as well as downstroke motions, making a nearly horizontal stroke plane [3]. Therefore, insects can perform precise hovering flight, while most birds cannot. Birds and insects are also very different in the principles of attitude control and stability that they use. While a bird uses its tail, an insect, without a tail control surface, relies on its own flapping wings to produce control forces, by actively modifying wing kinematics during flapping motion [4,5]. Understanding the similarity and difference in bird and insect flights may inspire the creation of designs of various flapping-wing air vehicles

(FWAVs). Indeed, along with fixed-wing and rotary-wing unmanned aerial vehicles (UAVs), FWAVs have attracted growing interest from scientific researchers in the past few years (Fig. 1A) [6]. We may say that most existing vehicles that are capable of free flight adopt the flight principles of birds (Fig. 1B), because they use control surfaces located at the tail for attitude and flight controls, and flapping wings for flight force production (Fig. 2) [7]. Thus, generating an engineering design for a bird-inspired FWAV is relatively straightforward, since such FWAV may create only small sweep amplitude at low frequency. In addition, because of the inability to hover, bird-inspired vehicle operation re­ quires wide open space. Therefore, this type of FWAV is more suited to outdoor, rather than confined indoor space applications. On the other hand, insect-inspired FWAVs are able to stay in place in the air, and perform agile flight maneuvers with many degree-of-freedoms [8]. Thus, the vehicles have potential for applications in confined spaces, such as collapsed buildings or hazardous facilities, which are inacces­ sible by human, as well as outdoor use. However, the design of an insect-inspired tailless FWAV is technically more challenging than that of a bird-inspired tailed vehicle. To enable the tailless vehicle to remain

* Corresponding authors. E-mail addresses: [email protected] (H.V. Phan), [email protected] (H.C. Park). https://doi.org/10.1016/j.paerosci.2019.100573 Received 22 March 2019; Received in revised form 31 August 2019; Accepted 8 September 2019 Available online 13 September 2019 0376-0421/© 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Hoang Vu Phan, Hoon Cheol Park, Progress in Aerospace Sciences, https://doi.org/10.1016/j.paerosci.2019.100573

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airborne, the design requires a strategy to modulate flapping wings in the middle of the flapping motion, so that wings can produce a vertical force that is large enough to overcome the body weight, and control torques to maintain attitude as well. Additionally, with no tail stabi­ lizers, the flight of the vehicle is inherently unstable [8–10]. Thus, an active feedback control system should be implemented to stabilize the flight, which adds to the weight and complexity of the vehicle. Depending on the wingspan and body mass, all available FWAVs can be generally categorized as Micro Air Vehicle (MAV, size < 1 m, weight < 2 kg [11]), Nano Air Vehicle (NAV, size < 75 mm, weight < 10 g [8]) and Pico Air Vehicle (PAV, size < 50 mm, weight < 500 mg [12]). Due to the above mentioned difficulties and limitations of current power and actuation technologies, to date there is no vehicle at the scale of small insects, i.e. insect-inspired flapping-wing NAV (FW-NAV) or flapping-wing PAV (FW-PAV), that is capable of performing free flight, although several larger versions, i.e. insect-inspired flapping-wing MAVs (FW-MAVs), have flown success­ fully [8,9,13]. However, these hurdles do not reduce the desire to free fly a tiny insect-scale FWAV that can hover more efficiently than a similar scale rotary-wing vehicle [14,15]. At the small scale of insects, the use of conventional rotary actuators is restricted by low propulsive efficiency and technical challenges in manufacturing [16]. Other oscillatory ac­ tuators [10] are thus utilized to directly drive the flapping wings without the need for transmission systems, as found in the FWAVs that use rotary actuators [8,9,17]. However, these actuators require a source of power that is a challenge for onboard integration to produce sufficient lift for free flight [10,18]. Recent progress on the research and development of FWAVs has been well summarized and discussed in previous reviews [7,16,19,20]. Gerdes et al. [7] provided an overview of bird-inspired tailed FWAVs with the range of body weight from 10 to 100 g that performed suc­ cessful free flights, focusing on the design of flapping mechanisms and wings and flight control strategies with tail control surfaces. On the other hand, Helbling and Wood [16] discussed recent developments of small-scale FWAVs with weight less than 20 g, mimicking the flight of insects. The paper focused on the actuation and power technologies for tiny insect-scale vehicles, which can be represented by current progress on the tiny RoboBee PAV [12]. In this paper, we aim to survey the recent achievements of insectinspired tailless FWAVs at any scale in free flight ability. We first pro­ vide a brief summary on the principles of insect flight from aero­ dynamics to flight control and stability strategies, which are sources of inspiration for designing insect-inspired vehicles. We then focus on the recent developments available in the literature in insect-inspired tailless FWAVs at different scales, using different driving actuators and wing configurations for lift generation, attitude control and stability strate­ gies, and on key challenges to the free flight ability of an insect-scale FWNAV or FW-PAV, in addition to enhancement of the flight endurance of the FW-MAV. Finally, we discuss the potential and future directions of FWAVs to mimic the locomotive strategies of flying insects in nature.

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2. Overview of insect flight aerodynamics and control/stability strategies 2.1. Aerodynamic force mechanisms Unlike conventional fixed-wing or rotary-wing airplanes, insects flap their wings back and forth, utilizing unsteady aerodynamic force mechanisms to keep their bodies stay airborne (Fig. 3). These mecha­ nisms, which consist of attached leading edge vortex (LEV), added mass, wing-wake interaction or wake capture, rotational circulation, and clapand-fling effect, have been well summarized in previous reviews [29–31]. The attached LEV is an important feature to significantly enhance lift generation in insect flight, in which the wing is operated at high angles of attack (AoAs) [32]. For a conventional wing travelling linearly at high AoA, the LEV grows, and subsequently sheds into the wakes. The lift drops as a result of small difference in pressures above and below regions of the wing; and eventually the wing stalls. However, before the stall, the LEV remains attached for several chord lengths of travel resulting in high lift generation, which is called delayed stall. However, for the flapping wing operating at low Reynolds numbers, the LEV remains stably attached to the wing regardless of travel distance, avoiding the occurrence of stall (Fig. 3B). Ellington et al. [33] pointed out that the stable attachment of the LEV on the upper wing surface is a result of the appearance of axial flow or spanwise flow. The stable attachment of the LEV was also pointed out in the studies by Maxworthy [34], Van den Berg and Ellington [35], and Birch and Dickinson [36]. During acceleration and deceleration at the beginning and the end of stroke, respectively, the wing accelerates and decelerates the sur­ rounding air, and encounters a reaction force acting perpendicular to the wing surface in the reverse direction. This reaction force is called “added mass” [37] or “virtual mass” (Fig. 3 A and E) [38]. However, this added mass force is difficult to isolate by experimental and computational approaches, because it occurs simultaneously with the circulatory forces [30]. In previous works, the added mass was estimated using quasi-steady methods by the assumption of time-invariant added mass coefficient, due to the variable time history of the wing acceleration [39, 40]. At the end of the stroke, the flapping wing undergoes rapid pitch rotation about the spanwise axis, which can enhance lift generation (Fig. 3C) [41]. This is called the Kramer effect, which was demonstrated by Kramer [42] from an experimental work on the fluttering of airplane wing, or rotational forces [40]. Once the wing rotates about the span­ wise axis prior to the end of stroke, the wing performs an advanced rotation, resulting in the enhancement of lift. On the other hand, if the wing rotates after approaching the end of the stroke as a delayed rota­ tion, the lift is reduced [40,41]. By taking advantage of the Kramer ef­ fect, insects can control the lift generation for maneuvering [43]. Another mechanism, called wake capture, was first demonstrated by Dickinson et al. [41] from studies on 2D motion of an inclined plate and on the 3D model of a fruit fly. When the wing translates toward the end

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Fig. 1. (A) Papers published on flapping-wing, rotarywing, and fixed-wing vehicles from 2000 to 2018. Data were taken from the Scopus database using search keywords: (TITLE-ABS-KEY (ornithopter OR vehicle OR robot AND flapping-wing)) for flappingwing vehicles, (TITLE-ABS-KEY (unmanned OR UAV AND rotary-wing OR rotorcraft)) for rotary-wing ve­ hicles, and (TITLE-ABS-KEY (unmanned OR UAV AND fixed-wing)) for fixed-wing vehicles. (B) Papers pub­ lished on FWAVs capable of free flight. Data were manually selected from the Scopus database based on the search keyword: (TITLE-ABS-KEY (ornithopter OR flapping-wing AND flight)).

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of the stroke and reverses its direction, the LEV and trailing edge vortex (TEV) are shed. As the wing starts a new stroke in the reverse direction, the wing interacts with the shed vortices created during the previous stroke, resulting in an increase of flow velocity and higher generated aerodynamic forces (Fig. 3F) [41,44,45]. Previous studies indicated that the wake capture effect is significantly dependent on the duration of the stroke reversal [46,47]. However, this phenomenon consumes an amount of energy similar to that of enhanced lift, and thus it may not contribute to improving the efficiency of insect flight [48]. In contrast to the abovementioned mechanisms, clap-and-fling, which was first discovered by Weis-Fogh [49], occurs by the interac­ tion of the left and right wings at stroke reversals (Fig. 3D). The clap-and-fling was proven to improve lift generation in many insects, such as wasps [49,50], locusts [51], butterflies [52], whiteflies [53], thrips [54], and small flies [55]. However, it is not used frequently during insect flight, except during high-demand flight behavior, such as takeoff, climbing, or when carrying load [31]. Therefore, the presence of the mechanism was also hypothesized as a result of maximizing sweep amplitude, which acts to increase force generation [30]. In addition to these well-known mechanisms, some insects use other strategies of force generation for flight. Mosquitos were found to flap their wings with shallow sweep amplitudes utilizing unique mechanisms of force generation, which are rotational drag and trailing edge vortices formed at the stroke reversal, along with leading edge vortices during wing translation to support their body weight [56]. On the other hand, tiny insects produce weight-supporting force by flapping their wings with a very deep U-shape stroke, utilizing large drag force generation at the beginning of each stroke [57,58]. Not only the aerodynamic components, flapping flight is also impacted by wing inertia during acceleration and deceleration for stroke reversals. Although contribution of the wing inertia to the total cycleaverage vertical force is insignificant, it consumes energy for acceler­ ating the wing. However, how to precisely determine the amount of the inertial power contributing to the total power requirement is chal­ lenging. In insects, the possible presence of the storage of elastic energy in flight muscles eliminates the inertial cost. Many prior studies there­ fore have solely investigated the aerodynamics and have ignored the inertial effect of flapping insect wings [59–62]. On the other hand, study in Ref. [63] indicated that, even though with full storage of elastic

energy, the contribution of inertial power is still dominant. Moreover, most insects flap their wings at relatively high AoAs [32,64,65], which are aerodynamically inefficient [61,62,64,66]. Wing inertia was there­ fore proposed as the main cause [63,64]. In addition, wing inertia was considered as the cause of passive rotation at the stroke reversal not only in insects [67,68] but also in many robotic flapping wings [8,9,69]. Thus, studies on flapping wings, especially on robotic wings without implementation of the elastic storage, should consider the wing inertia for more accurate analysis. 2.2. Principles of flight control and stability Due to their inherent flight instability [70,71], insects rely on active feedback systems to remain airborne [20]. The feedback system involves sensors such as halters [72], ocelli [73], and antennae [74] to sense the attitude deviations, and control mechanisms to produce compensatory control forces. The sensory systems play an important role, as without them, insects cannot fly [20]. An experimental study on fruit flies indicated that when the halters are deactivated, they tumble quickly [75]. Moreover, in the absence of antennal flagellum, moths could not maintain stability [74]. The receiving signals in the sensors activate the control mechanisms to produce corresponding control forces that balance the body attitudes. Lacking tail control surfaces, insects therefore have the abilities to manipulate the wing kinematics by actively changing the sweep amplitude, angle of attack [5], flapping frequency [4], stroke-plane angle [76], or by modifying the location of the center of gravity (CG) by the change in posture [77]. For examples, Dickinson and Muijres [78] performed a study on the free flight of fruit flies Drosophila melanogaster, and found that fruit flies control pitch torque by shifting the flapping angle range forward or backward to relocate the position of the mean aerodynamic force center (AC), causing change in the relative distance between the CG and AC [78]. Fruit flies also manipulate the wing rotation angle or AOA and the deviation angle, which is the angle be­ tween the wing leading edge and mean stroke plane, to control pitch. However, their contributions are relatively small [78]. For roll control, fruit flies simultaneously modulate the sweep amplitudes, stroke angles, and wing rotations of the two wings. Meanwhile, yaw is controlled by asymmetrically regulating the wing rotation angles or AOAs of the two Fig. 2. Examples of existing bird-inspired tailed FWAVs. (A) Microbat from Aerovironment Inc [21]. (B) 32 g ornithopter from Konkuk University [22]. (C) DelFly Explorer from the Delft University of Tech­ nology [23]. (D) Smartbird from Festo AG & Co. KG [24]. (E). Robo Raven V from the University of Maryland [25]. (F) H2Bird ornithopter from the Uni­ versity of California, Berkeley [26]. (G) Robird from Clear Flight Solutions and the University of Twente [27]. (H) BionicBird from bionicbird.com. (I) 3.2 g flapping-wing platform from Harvard University [28].

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Fig. 3. Aerodynamic force mechanisms in insect flight (adapted from Ref. [29]).

wings [78,79]. Identification of these control principles in insect flight is thus useful for the development of insect-inspired tailless FWAVs.

3.1. Propulsive actuators 3.1.1. DC motors The DC motor is commonly used as the main propulsion system in conventional drones and bio-inspired FWAVs, because of its high effi­ ciency, robustness, low cost, and high-power density at low voltage operation that is suitable for onboard power sources, such as lithiumpolymer batteries. However, unlike in most miniature drones in which the motor directly drives the propeller, FWAVs typically require gear boxes to amplify the output torque of the motor, and transmission mechanisms to convert the rotary motion of the motor to the reciprocal motion of the wings, which cause weight increase, power loss, and complexity of the vehicles. Furthermore, at the small scale of insects, the performance of the DC motors is significantly degraded. Therefore, most available motor-driven FWAVs are at the MAV scale (Table 1, Figs. 4 and 5). Various types of transmission mechanism (Table 1 and Fig. 6) have been used to mimic the flapping motion of insects, such as the Scotchyoke [80], 4-bar linkage [111,156], slider-crank [106,118], crank-shaft [101,105], crank-rocker [17,113], and string-based [8,112]. In addition, to obtain high sweep amplitudes (>160 deg), dual lever mechanisms have been considered. They can be found in prototypes in Ref. [8], with dual series 4-bar linkage (Fig. 6A); in 62 g robotic hum­ mingbird, with modified 5-bar mechanism (Fig. 6B) [13,107]; in KUBeetle, with a combination of 4-bar linkage and pulley-string mech­ anism (Fig. 6C) [9,157,158]; and in Colibri, with slider-crank and 4-bar linkage (Fig. 6D) [100]. Although these mechanisms were successfully implemented in the flying vehicles, they still have some limitations such as mechanical wear and fracture of moving linkages that cause ineffi­ cient flapping motion [8]. To reduce the number of moving linkages and to improve the flapping efficiency and durability, Keennon et al. [8] proposed a lightweight string-based mechanism, as shown in Fig. 6E. It consists of a rotating crankshaft linked to a DC motor via a reduction gear system, strings connecting the crankshaft and the two pulleys to convert the rotating motion (crankshaft) to flapping motion (pulleys) (Fig. 6E). Two pulleys are also connected to each other through two additional strings to ensure synchronized flapping motions of the left and right wings. A flapping test demonstrated that the string-based mechanism produces more symmetric left-right motion and lower

3. Insect-inspired flying robots The capabilities of hovering and aggressive maneuvers of insects are sources of inspiration for the development of insect-inspired vehicles, which have potential for applications in confined spaces, surveillance, search and rescue, etc. Over the past few years, many insect-inspired systems have been developed (Table 1 and Fig. 4). They have appeared in various sizes from micro-, nano-, to pico-scale, using different power actuators from conventional rotary to unconventional oscillatory drivers, with a variety of wing configurations, such as twowing, X-wing, or tandem wing. However, only a few of them have demonstrated free controlled flight using an onboard control system and power source.

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Fig. 5. Motor-driven insect-inspired tailless FWMAVs capable of free controlled flight. (A) Nano Hummingbird developed by AeroVironment Inc [8]. (B) TechJect Dragonfly developed by TechJect Inc [85]. (C) BionicOpter developed by Festo AG & Co. KG [101]. (D) iMotionButterflies developed by Festo AG & Co. KG [97]. (E) Robotic Hummingbird devel­ oped by Texas A&M University [107]. (F) KUBeetle developed by Konkuk University [88]. (G) Colibri robot developed by the Universit� e Libre de Bruxelles [100]. (H) Robotic Hummingbird developed by Pur­ due University [155]. (I) Quad-thopter developed by Delft University of Technology [113]. (J) NUS-Robobird developed by the National University of Singapore [115]. (K) DelFly Nimble developed by the Delft University of Technology [17]. (L) Butterfly-type Ornithopter developed by Beihang University [117].

On the other hand, to avoid the use of complex mechanisms, the motor was designed to directly drive the flapping wing at resonant frequency based on elastic element, reducing power consumption (Fig. 6 G and H) [93,103,104,159]. Additionally, each flapping wing was driven separately by each motor, allowing the generation of control torques, without the need for additional control mechanisms, which add weight and complexity of the vehicle [93,104]. This design technique may allow the direct-driven FWAVs to perform controlled flight at the smaller scale of motor-driven NAV, which is unavailable as of yet. Indeed, among the many flight-capable FWAVs, the direct-driven Pur­ due hummingbird robot with a wingspan of 170 mm and weight of about 12 g is currently the smallest tailless FWAV that can perform controlled flight using DC motors [160] (Fig. 5H). 3.1.2. Piezoelectric actuators At the small scale of insects, inefficient performance and technical difficulty in manufacturing restrict the use of conventional magneticrotary actuators as an actuation option [16,162]. In 1998, a project named the Micromechanical Flying Insect (MFI) was launched to develop an insect-scale FWAV that was capable of autonomous flight, paving the way for the development of insect-inspired FWAVs using unconventional piezoelectric actuators, which are lightweight and offer high energy density [119–122,163]. The four degree-of-freedoms two-wing prototype of MFI (Fig. 8A) with the weight of about 100 mg and wingspan of approximately 25 mm could flap its wings at a fre­ quency of 275 Hz with an amplitude of about 120 deg, to produce a lift force more than twice the body weight [120]. At this scale, inefficient rotary joints were replaced by flexible hinges, which were fabricated using the smart composite microstructure process [164]. The oscillating motion of the actuator was amplified to the flapping motion of the wings, using a transmission mechanism with a combination of slider-crank and 4-bar linkage. Based on the MFI technology, Wood at Harvard University in 2007 demonstrated the first guided takeoff of a 60 mg FWAV, which was later named RoboBee (Fig. 8B), [130,165]. Afterwards, in 2013, the 80 mg RoboBee could successfully perform tethered-controlled flight

Fig. 6. Examples of various transmission mechanisms used in motor-driven FW-MAVs for high amplitude operation. (A to D) Dual level mechanisms: (A) dual series 4-bar linkage [8], (B) modified 5-bar mechanism [13,107], (C) 4-bar linkage and pulley-string mechanism [9,161], and (D) slider-crank and 4-bar linkage [98]. (E and F) String-based mechanisms in (E) Nano Hummingbird [8], and (F) insect-like FW-MAV developed by Seoul National University [112]. (G and H) Direct-driven mechanisms in (G) FW-MAV developed by Carnegie Mellon University [103,104], and (H) Purdue robotic hummingbird [94,160].

amplitude of wing tip acceleration, compared to the linkage-based mechanism (Fig. 7). Another string-based mechanism was also inven­ ted by a research group at Seoul National University, as shown in Fig. 6F [112]. Rotation of the crankshaft sequentially pulls and releases the two strings resulting in the flapping motion of the left and right pulleys, in which the wings are connected. To achieve phase matching in the mo­ tion of the left and right wings, two additional strings were also con­ nected between the two pulleys similar in Ref. [8] (Fig. 6F). 5

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Fig. 7. Flapping profiles of the dual series 4-bar linkage (left) and string-based (right) mechanisms [8].

Some circuit topologies have been proposed to create high-voltage power signals from low-voltage power sources [166–169]. Steltz et al. designed a hybrid converter, which consists of a boost converter and a cascaded charge pump circuit, for microrobots ranging from 2 to 4 g [166]. Lok et al. introduced a light-weight, high efficiency, low power consumption, two-stage power electronics unit (PEU) that was designed for the pico RoboBee (Fig. 9) [169,170]. It implements a step-up boost-flyback converter and a four-channel envelop tracking, charge sharing driver. On the other hand, Xu et al. created a 91 mg step-up converter, which can convert 3.7 V DC to 100 V AC at a frequency of 80 Hz [171]. However, these power electronics still require advance­ ments in power technologies to build an ultra-lightweight, high power density energy source fitting the scale of FWAVs, which energy source is currently commercially unavailable [172,173]. An alternative way was proposed to remotely power and lift-off the 190 mg piezo-driven RoboFly using laser beam, which is converted to 200 V high voltage bias by onboard power electronics (Fig. 8H) [138]. However, the power source is still offboard with a limited power range of about 1 m, requiring great effort and technique for flight autonomy.

Fig. 8. Examples of insect-inspired FWAVs driven by piezoelectric actuators. (A) Micromechanical Flying Insect (MFI) developed by the University of Cali­ fornia, Berkeley [121]. (B) Robobee developed by Harvard University [10]. (C) FW-NAV developed by the Air Force Institute of Technology [131]. (D) Flapping-wing platform developed by the US Army Research laboratory [132]. (E) Flapping-wing robot developed by Carnegie Mellon University [134]. (F) The 100 mg insect-scale flapping-wing robot developed by Shanghai Jiao Tong University [136]. (G) Flapping-wing robot with direct-driven piezoelectric actuation developed by Toyota Central R&D Labs [18]. (H) The 190 mg RoboFly developed by the University of Washington [138].

3.1.3. Electromagnetic actuators While piezoelectric actuators require high operating voltages, oscil­ latory electromagnetic actuators have emerged as an alternative driven strategy for the insect-scale FWAVs because of their low operating voltages, avoiding the use of sophisticated power electronics. Therefore, a number of electromagnetic-driven FWAVs have been developed (Table 1 and Fig. 10). Based on micro-electro-mechanical system (MEMS) technology, Bao et al. [174] presented a 41 mg insect-scale FW-PAV with a wingspan of 36 mm actuated by an electromagnetic actuator (Fig. 10A). The wings were made of SU-8 fiber as veins and polydimethylsiloxane as membranes. The flapping test showed that the vehicle could flap the wings at a resonant frequency of 51 Hz with an amplitude of 31 deg, without the use of a transmission mechanism

maneuvers using off-board power and attitude sensing [10]. Along with RoboBee, many other piezo-driven FWAVs have been developed, and most of them are within the scales of NAV and PAV, as listed in Table 1, and shown in Fig. 8. However, there are no vehicles at these scales that are capable of free controlled flight, although some could perform guided takeoff, such as the FWAVs developed by researchers at Shanghai Jiao Tong University (Fig. 8F) and Toyota Central R&D Labs (Fig. 8G) [18,136]. At small scales, besides the above mentioned hurdles, the power supply is another issue of the piezo-driven FWAVs. Even though the piezoelectric technology meets the requirements of lightweight, high power density, and low power consumption, it needs the application of hundreds of volts voltage, which is challenging for onboard integration. 6

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Fig. 9. Power electronics circuit topology with a step-up boost-flyback converter and four-channel envelop tracking, charge sharing driver (top) [169], and prototype of a full electrical system needed for flight of a FW-PAV (bottom) [170].

present a challenge for free controlled flights [16,148]. 3.1.4. Other actuators In addition to the abovementioned actuators, other forms of actua­ tion were also considered for the FWAVs. Tanaka et al. [150] developed a 0.4 g weight, 140 mm wingspan butterfly-type ornithopter driven by a rubber band. The vehicle could perform stable forward flight without active control. The 580 g Mentor developed by Zdunich et al. [175] used an internal-combustion engine as an actuator. By flapping the wings at about 30 Hz, the vehicle could perform hovering flight for more than 1 min. However, because it used tail control surfaces, it mimics bird flight, rather than insect flight. Liu et al. [152] utilized an electrostatic actuator with pivot-spar brackets to create flapping and rotational mo­ tions of a 50 mg flapping-wing platform. The platform with a wingspan of 56 mm powered by a DC voltage of 4 kV could generate a sweep amplitude of 35 deg at 35 Hz. Furthermore, Lau et al. [151], utilized a rolled dielectric elastomer actuator (DEA) to drive bio-inspired flappers through a thoracic mechanism and a support shell structure made of a lightweight cross-ply laminate of carbon fibre reinforced polymer. Also using DEA as an actuator, Cao et al. [153] created a flapping-wing platform with a wing length of 40 mm that could generate a sweep amplitude of 63 deg at 18 Hz. On the other hand, Zhao et al. [154] proposed a flapping-wing platform driven by an ionic polymer-metal composite actuator. Using a sinusoidal wave input voltage of 4.5 V AC, the platform flapped 42 mm wings with an amplitude of about 12.5 deg at 0.5 Hz.

Fig. 10. Examples of insect-inspired FWAVs driven by electromagnetic actua­ tors. (A) Flapping-wing prototype developed by IEMN [174]. (B) Flapping-wing robotic platform developed by Purdue University [145]. (C) The 80 mg insect-inspired flapping-wing robot developed by Shanghai Jiao Tong Univer­ sity [148]. (D) Insect-scale FWAV developed by Beihang University [149].

[174]. Also to minimize the cost of transmission, Roll et al. [145] pro­ totyped a 4 g direct-driven FW-NAV with a wingspan of 86 mm using an electromagnetic actuator (Fig. 10B), which consists of a wedge-shaped magnetic coil stator, a permanent magnet rotor, and a pair of magnets capable of restoring torque on the rotor. By flapping the wings at a frequency of more than 90 Hz with an amplitude of about 120 deg, the vehicle could produce a lift-to-weight ratio of about 1.3 to perform a tethered lift–off. On the other hand, Zou et al. [148] developed an electromagnetic-driven FW-PAV that has a weight of only 80 mg and wing span of 35 mm (Fig. 10C). Using a double planar 4-bar linkage as a transmission mechanism, the vehicle amplified the sweep amplitude of about 140 deg at a frequency of about 80 Hz, to become the first electromagnetic-driven FW-PAV that can perform guided takeoff at this scale [148]. Furthermore, Liu et al. [149] designed a 92.8 mg FWAV using a low voltage of 5.5 V, high-power density electromagnetic actu­ ator (Fig. 10D). The wings were excited by a cantilever plate though a slider-crank transmission mechanism to create a sweep amplitude of 20 deg at a frequency of 101.4 Hz. Thus, great progress has been achieved in developing FWAVs using electromagnetic actuators. However, the high power consumption of the actuators requires powerful onboard energy sources, which increase the total weight of the vehicle, and

3.2. Wing configuration Flying insects in nature show diversity in their wing configurations, varying from single pair to two pairs of wings, in different morphologies. Some insects, such as small flies (Diptera), have one pair of wings, while many other insects possess two pairs, consisting of forewings and hindwings. Insect-inspired FWAVs also display diversity in their wing configurations. They not only copy those in insects but also show more diversity (Fig. 11). We classify the FWAVs into three groups of wing configuration: two-wing, four-independent-wing, and X-wing, as shown in Fig. 11. 3.2.1. Two-wing In general, the primary function of the flapping wings is to create 7

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Four-wing Tandem

Perpendicular

Two-pair

Four-pair

Eight-pair

FWAV

Insect

Two-wing

X-wing

Fig. 11. Various wing configurations in insects and insect-inspired FWAVs.

useful aerodynamic forces for flight. However, in distinct insect species with two pair of wings that are coupled mechanically to flap synchro­ nously, the contribution of each pair to force generation is different [176]. A computational study of Chen et al. [177] indicated that on removing the hindwings, the forewings of the hawkmoth, bumblebee, and fruit fly still produced sufficient lift force for flight. Furthermore, Jantzen and Eisner [178] pointed out that the hindwings of Lepidoptera (moths and butterflies) play no role in flying force generation, but contribute to maneuver capability. On the other hand, the forewings of beetles (Coleoptera) function primarily as protection of hindwings and abdomen when at rest, rather than contributing significantly to flight force generation [60]. Therefore, to reduce complexity in the flapping mechanism, most developed FWAVs, especially FW-NAVs and FW-PAVs (Table 1, Figs. 8 and 10), have one pair of wings. In this category of vehicles, with the limitation of wing area (high wing loading, which is the ratio between body weight and wing areas), high flapping frequency and sweep amplitude are thus needed to produce sufficient lift force for flight. The 80 mg Havard RoboBee with a wingspan of 3 cm and wing loading of 15.1 N m 2 maintained its body airborne with a sweep amplitude of 110 deg and flapping frequency of 120 Hz [10]. With a wing loading of 49.5 N m 2, the 19 g Nano Hummingbird flaps its 7.4 cm long wings at a frequency of 30 Hz and amplitude of about 200 deg [8]. The 21 g KUBeetle can also flap its 7 cm long wings (wing loading of 50.7 N m 2) at 30 Hz and 190 deg [9]. Similarly, the robotic hum­ mingbird developed in Ref. [160] performed flight by operating its wings at a frequency of 34 Hz with 170 deg amplitude.

glide, and fly forwards and backwards [96]. In order to have room for onboard electronics and power systems, in this year, 2019, Fuller at University of Washington introduced a 143 mg, insect-sized, four-wing vehicle (Fig. 11) that can significantly improve vertical force generation [184]. The vehicle with a wingspan of 56 mm (tip to tip) has four perpendicular wings in which each wing is actuated by a single piezoelectric actuator, similar to a quadcopter. With the configuration, the vehicle is capable of steering around the body axis (heading control). 3.2.3. X-wing The X-wing is not considered a biomimetic configuration, because it does not mimic any wing configuration found in natural flyers. Using this configuration, FWAVs utilize the clap-and-fling effect at stroke reversal to improve lift generation [45,49,157,175]. Therefore, the X-wing can also be called “clapping-wing” [7]. Additionally, the wings of each pair flap in opposite directions, which cancel out inertial oscil­ lations, providing better pitch stability. Since the wings flap in the same stroke plane, the sweep amplitude in this category is typically small (less than 100 deg). In addition, with more wings, the vehicles with X-wing type have lower wing loading, compared to the two-wing vehicles. For examples, the tailless 31 g NUS-Roboticbird (Fig. 5J) flaps its 22 cm span wings with a wing loading of 12.5 N m 2 at an amplitude and frequency of 90 deg and 13.3 Hz, respectively, to stay airborne [115]. The tailless 28.2 g Delfly Nimble (Fig. 5K) with a wingspan of 33 cm and wing loading of 6.2 N m 2 performed remarkable flight, beating its wings at a frequency of 17 Hz with an amplitude of only 44 deg [17]. Researchers at TU Delft also created another version of the vehicle, which uses four pairs of X-wing configuration (Fig. 5I) arranged similarly to a rotary-wing quadcopter [113]. A similar concept with eight pairs of wings that was also developed is reported in Ref. [185].

3.2.2. Four-wing (tandem and perpendicular configurations) The tandem wing configuration has two pairs of forewings and hindwings that are actuated independently with variable phase angles. Insects with tandem wing configuration, such as dragonflies, demon­ strate remarkable flight capabilities, including gliding, fast forward, backward, sideways, powerful ascending, and hovering flights [179, 180]. Additionally, many previous studies have indicated that the tan­ dem configuration improves aerodynamic efficiency [181–183]. These benefits thereby motivate the development of dragonfly-inspired FWAVs with tandem wing configuration [84,96,101]. However, the vehicles in this category are mechanically more complex, compared to the two-wing types. A good example of the dragonfly-inspired FWAVs is BionicOpter (Fig. 5C), developed by Festo company [101]. It has a wingspan of 63 cm, and weighs 175 g. It is capable of adjusting the flapping frequency of the four wings cooperatively at 15–20 Hz, and the amplitude of each wing independently at 80–130 deg. Moreover, each wing is able to individually tilt its flapping stroke plane from horizontal to vertical. Therefore, BionicOpter can perform many flight modes that are similar to those of a real dragonfly in nature. Also capable of the independent control of frequency and amplitude in each wing, the palm-sized 25 g TechJet Dragonfly with a wingspan of 15 cm can hover,

3.3. Flight control and stability system Unlike the bird-inspired FWAV that uses tail stabilizers, the tailless FWAV requires an active control system that is able to modify the wing kinematics during the flapping motion resulting in control torque gen­ eration for attitude changes. We divide the control system into two blocks: control mechanism and feedback controller, as below. 3.3.1. Attitude control mechanisms Adapting the control strategies of insects, many control approaches have been proposed and successfully implemented in tailless FWAVs to achieve controlled flight (Fig. 12). To control pitch motion, the average lift vector can be shifted or tilted forward or aft of the center of mass, resulting in pitch torque generation. This effect can be achieved by modulating wing twist (Fig. 12A-A1), which is used in the Nano hum­ mingbird [8], KUBeetle [9], and Colibri [100]. On the other hand, 8

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Fig. 12. Examples of control methods used in tailless FWAVs. (A) Pitch control: (A1) Wing twist modulation to change AOA during flapping motion (top view). (A2) Bias of wing stroke to shift the mean location of thrust. (A3) Modulation of stroke-plane angles to tilt the thrust vectors of the two wings in the same direction (side view). (B) Roll control: (B1) Wing twist modulation to change the AOAs of the two wings asymmetrically (top view). (B2) Modulation of frequency in each wing (one wing flaps at a higher frequency than the other). (B3) Modulation of the sweep amplitudes in the two wings. (B4) Modulation of the stroke-plane angle to tilt the mean lift vector laterally (back view). (C) Yaw control: (C1) Asymmetric wing twist modulation (top view). (C2) Split-cycle change. (C3) Modulation of stroke-plane angles of the left and right wings in opposite directions (side view).

RoboBee [10], Delfly Nimble [17], Purdue robotic hummingbird [160], and the vehicle in Ref. [186] control pitch by varying the wing stroke centering as in fruit flies [78] (Fig. 12A-A2). Meanwhile, the 62 g robotic hummingbird [13], KUBeetle-S [88], and NUS-Roboticbird [115] change their stroke planes to tilt the lift vector. In KUBeetle-S, because the wing root spars are fixed, tilting the stroke plane causes change of wing twist, modulating the amount of average lift asymmetrically in front of, and behind, the vehicle’s center of mass. Roll control can be achieved by asymmetric lift force in the left and right wings. To achieve this, Nano hummingbird [8], KUBeetle [9], and Colibri [100] control the wing root spars to modulate the wing twist, causing differential variation of the angle of attack in the wings (Fig. 12B-B1). In a different way, Delfly Nimble [17], and NUS-Roboticbird [115] differentially change the flapping frequency of each wing (Fig. 12B-B2). Meanwhile, varying the sweep amplitudes of the left and right wings [78] (Fig. 12B-B3) was implemented in RoboBee [10], Purdue robotic hummingbird [93], and in the 62 g vehicle re­ ported in Ref. [13]. Another way to control roll motion is to change the stroke plane to produce lateral force (Fig. 12B-B4). This approach was utilized in KUBeetle-S, which rotates the flapping-wing mechanism to tilt the stroke planes of the two wings in the same direction [88]. The tilt thus leads to asymmetric wing twist in the two wings, due to the fixed wing-root spars, that support additional roll torque for roll control. Yaw motion does not affect the upright stability of the tailless FWAVs. This means that without yaw control, the FWAVs can still stay airborne. However, the vehicle may experience rotation around its body axis, due to the imperfect trim of the initial yaw torque produced by

asymmetric wing motions, causing difficulty in controlling heading di­ rection. Unlike pitch and roll controls, yaw motion can in general be controlled by the production of opposite horizontal forces in the left and right wings, resulting in yaw torque generation. In the Nano Hum­ mingbird [8], yaw torque is generated by asymmetrically adjusting the wing-root spars of the two wings to modify the AOA during flapping motion [78,79]. This asymmetric wing twist modulation (Fig. 12C-C1) was also used in KUBeetle [9], KUBeetle-S [88], and Delfly Nimble [17]. By asymmetrically modulating the flapping speeds of the downstroke and upstroke motions in the two wings (Fig. 12C-C2), yaw torque is generated in the RoboBee [10] and Purdue robotic hummingbird [93]. On the other hand, NUS-Roboticbird [115], the vehicle reported in Ref. [187], and the 62 g vehicle in Ref. [13] tilt the stroke planes of the two wings in the opposite directions, to produce yaw torque (Fig. 12C-C3). In FWAVs with four independent sets of flapping-wing mechanisms arranged similar to a quadcopter, in which each set is driven by a separate actuator, attitude control is achieved by regulating the flapping wing kinematics in each set. For examples, the Quad-thopter with four pairs of X-wings developed by researchers at TU Delft [113] controls pitch and roll by varying flapping frequency in two diagonally opposing pairs of wings. Meanwhile, yaw is controlled by tilting the stroke planes of the two wing pairs in opposite directions. Fuller at the University of Washington introduced a 143 mg insect-sized, four-wing vehicle that is capable of performing steering, and carrying more payload [184]. In the vehicle, pitch and roll motions are controlled by regulating the sweep amplitude in each pair of opposite wings, while yaw is controlled by 9

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Progress in Aerospace Sciences xxx (xxxx) xxx

varying the flapping speed of each stroke. Similarly, even though four wings are driven by only a brushless motor, BionicOpter [101], a dragonfly-inspired vehicle, can control each wing independently by changing sweep amplitude and stroke plane for attitude changes through installed servos.

where R is 3 � 3 rotational matrix representing the vehicle’s orientation. Thus, the time derivative of Lyapunov function can be described as V_ o ¼

In addition, lateral position and altitude of the vehicle were controlled using PD controller described as below:

3.3.2. Attitude feedback controllers To remain airborne, due to inherent flight instability as in insects [70,71], a tailless FWAV requires an active feedback system that senses body attitude as well as provides a fast corrective signal to activate the control mechanism for generating control forces and torques. However, implementation of the system requires a robust FWAV in both force generation and controllability because it costs more weight and complexity to the vehicle. On the demand to fly the tailless FWAVs without the need of corrective feedback, passive stabilizers were considered as an alternative way. It can be seen in Ref. [95], and early versions of RoboBee [188], Colibri [189] and KUBeetle [190]. The sta­ bilizers act as air dampers allowing the vehicle to maintain upright stability. However, the stabilizers are sensitive to wind disturbances and reduce maneuverable ability of the vehicle. To actively control a PAV, researchers at Harvard University have considered many approaches using either external motion-capture cameras [10,191,192] or onboard sensors such as conventional micro­ electromechanical systems (MEMS) gyroscope [193], magnetometer [194], and bioinspired ocelli [73]. On the basic of motion-capture sys­ tem (Fig. 13A) in which direct angular velocity feedback is unavailable, RoboBee was first stabilized using PD-like (proportional-derivative) control law based on modified Lyapunov function [10]: Vo ¼ kpa ð1

1 1 cos φÞ þ ωT J ω þ kva χ T χ ; 2 2

kpa Rzd

kva ET ðxÞχ ;

Sm Þ þ kd ðS_d

e ¼ kp ðSd

(4)

S_m Þ;

where e is the control output; kp and kd are the proportional and de­ rivative gains, respectively, Sd and Sm, are the desired and measured states, respectively. To deal with uncertain parameters such as torque bias caused by manufacturing imperfections, adaptive control approach was consid­ ered to improve the flight stability [192]. This controller employed Lyapunov function in addition to sliding mode control technique providing better estimation of uncertain parameters. A multi­ ple–input–multiple-output controller based on an experimental and model-free control strategy was also proposed in Ref. [191]. Neverthe­ less, the flight experiments using the above mentioned control ap­ proaches were performed inside a captured volume of the external cameras and activated by an off-board controller through electric wires (Fig. 13A) constraining the robot for its autonomous flight. On the other hand, onboard control systems have been also consid­ ered for the pico-vehicles. The system should integrate control sensors to sense the body attitudes, a microcontroller unit (MCU) to read and compute the sensor’s information for generating updated corrective feedback, and power electronics to drive actuators. However, mass re­ striction of the FW-PAVs requires a custom-built low-power microcon­ troller that fits to both size and payload capability of the vehicles. A study by Zhang et al. [195] proposed a full integrated 3 mg System-on-Chip (SoC) for close-loop control of the PAVs. The BrainSoC contains a 4:1 switched-capacitor regulator, a 32-bit ARM Cortex-M0, clock generators, hardware accelerators, an inter-integrated circuit (I2C) interface, a serial peripheral interface (SPI), a general-purpose input/output (GPIO) bus, and four analog-to-digital converter (ADC) channels. The results indicated that the BrainSoC associated with the PEU is able to perform open-loop control of flapping wing and better power efficiency [195]. Researchers have also investigated tiny onboard sensors to be integrated in the FW-PAVs. Fuller et al. [193] demon­ strated that, using an off-the-shelf MEMS gyroscope, upright orientation and attitude of the RoboBee can be stabilized based on either angular velocity feedback or attitude feedback (Fig. 13B). Angular velocities about the three body axes can be directly obtained by the gyroscope. However, attitude feedback requires a further estimation of attitude error defined by three Euler angles (θi, where i ¼ 1, 2, 3), which can be estimated based on the angular velocities ω using the following equation:

(1)

where kpa and kva are positive scalars and can be experimentally ob­ tained, φ is the angle between the measured (zm) and desired (zd) body orientations, J and ω are inertial matrix and angular velocity, respec­ s tively, χ ¼ sþγ x is the Laplace function in which x represents 3 � 1 Euler angle vector (x_ ¼ EðxÞω), and γ is the positive scalar. The control law for the command torque vector (τ) generated by the vehicle is as follows [10]:

τ¼

(3)

γkva χ T χ � 0:

(2)

θi;t ¼ θi;t

1

(5)

þ ωΔt;

where t and t-1 denote the current and previous time steps, respectively, and Δt is the time interval. Thus, the attitude control law for the torque vector is described as below [193]:

τ ¼ τo

Ka ðΓΔe þ ωÞ

ðΓΔe � J ωÞ

JΓΔ_ e ;

(6)

where τo is initial offset torque, Ka and Г are diagonal gain matrices, and Δe denotes the attitude error. Not only using gyroscope, the study in Ref. [194] showed that pitch and yaw (heading) angle control can be achieved using an onboard analog magnetometer in association with PD control law shown in Eq. (4). In addition, by mimicking the ocelli of insects, the study in Ref. [73] used a 25 mg light sensor consisting of four phototransistors arranged in a pyramid shape and showed that, the 106 mg RoboBee can remain upright during flight (Fig. 13C). In this study, the torque controller is proposed as a linear function of angular velocity following the form [73]:

Fig. 13. Feedback systems used in tailless PAVs for control and stabilization. (A) Motion tracking system activated by eight infrared cameras [10]. (B) On­ board MEMS gyroscope installed in RoboBee [193]. (C) Onboard vision sensor mimicking insect ocelli [73]. 10

H.V. Phan and H.C. Park

τ¼

kd ω;

Progress in Aerospace Sciences xxx (xxxx) xxx

(7)

� � gx ; θ1 ¼ atan gz

where kd is the control gain. In this case, the angular velocity can be estimated by ocelli signals through the relationship as follows [73]: � y_i;t ¼ κ’ dT si ðd � si ÞT ωt ; (8)

! gy θ2 ¼ atan qffiffiffiffiffiffiffiffiffiffiffiffiffiffi ; g2x þ g2z

(9)

� � sinθ1 sinθ2 mx þ cosθ2 my þ sinθ2 cosθ2 mz ; θ3 ¼ atan cosθ1 mx þ sinθ1 mz

where y_i;t denotes the derivative of the ith ocellus signal at an instant time t, κ represents the luminance sensitivity of an ocellus, d is the di­ rection of light source, and si is the direction of the ith ocellus. In contrast to the PAVs, larger size FW-MAVs produce sufficient lift to carry everything onboard from commercially available power sources to off-the-shelf electronic components, allowing them to achieve autonomous flight. Nano Hummingbird is the first tailless vehicle that can perform controlled flight using an onboard control system [8]. The custom-designed board (Fig. 14A) containing a single MCU, a 3-axis MEMS gyroscope, a receiver, power regulators, and driver circuits, weighs only 0.65 g. To fly the 62 g robotic hummingbird, Coleman et al. [13] also built a 1.3 g control board, as shown in Fig. 14B. It contains a 32-bit ARM Cortex M3 core, a 9-axis Invensense MPU-9150 Inertial Measurement Unit (IMU) with 3-axis gyroscope, accelerometer, and magnetometer, and a 2.4 GHz transceiver nRF24L01þ. The feedback control was implemented using a PD controller to sense the pitch and roll Euler angles and angular rates. On the other hand, the KUBeetle developed in Ref. [9] maintained upright during flight using separated off-the-shelf components, which consist of a BareDuino Nano micro­ controller with an ATmega328P microprocessor, a 3-axis L3GD20H MEMS gyroscope, and a 2.4 GHz Deltang DT-Rx35 receiver. To reduce the weight and improve stability of the vehicle, researchers at Konkuk University built an integrated board and installed it in the lighter version of the KUBeetle, 16.4 g KUBeetle-S [196]. The board consists of a 32-bit ARM Cortex-M4 microprocessor, a 9-axis MPU-9250 IMU, a 2.4 GHz transceiver nRF24L01þ, and power regulators, as shown in Fig. 14C. Similarly in Ref. [13], a PD controller was also implemented to stabilize the vehicle. Pitch and roll attitudes were estimated using both gyroscope readings ω ¼ [ ωx ωy ωz]T and accelerometer readings G ¼ [ gx gy gz]T, while magnetometer readings m ¼ [ mx my mz]T were used to control heading (yaw) of the vehicle. Moreover, to smooth the raw signals from the sensors reducing noises, low-pass and Kalman filters were used. Researchers at TU Delft [114] stabilized their tailless, X-wing vehicle by a custom-built control board featuring a 8-bit ATmega328P microcon­ troller, a 6-axis MPU9150 IMU (3-axis gyroscope and 3-axis acceler­ ometer), a 3-axis HMC5883L magnetometer, and a BMP180 barometer. Based on the accelerometer and magnetometer readings, three Euler attitude angles can be estimated as follows [114]:

where θ1, θ2, and θ3 represent the estimated pitch, roll and yaw angles, respectively. However, these estimated angles are highly disturbed by vibrations caused by fast flapping motion. Therefore, the study used a combination of estimated angles from gyroscope obtained using Eq. (5) and those in Eq. (9) via a complementary filter to obtain pitch, roll and yaw angles for using them as inputs of the PD feedback controller (Eq. (4)). The function of the complementary filter can be described as follows: _ θ 1;t _ θ 2;t _ θ 3;t

¼ μ θ 1;t

� þ ωy dt þ ð1

¼μ

þ ωx dtÞ þ ð1

¼μ

_ 1 _ ðθ 2;t 1 _ ðθ 3;t 1

þ ωz dtÞ þ ð1

μÞθ1;t ; μÞθ2;t ; μÞθ3;t ;

(10)

_

where θ denotes the output angle and μ is the filter coefficient. A custom-built electronic board was also found in the 12.5 g Purdue robotic hummingbird, as shown in Fig. 14D [92,155]. The 2.3 g board features two three-phase motor drivers, a 32-bit ARM Cortex-M4 microprocessor, a 9-axis MPU9150 IMU, a voltage regulator, a floating-point unit, a memory protection unit, and timers. Similar to the abovementioned FWAVs, the Purdue robotic hummingbird was also stabilized during flight using the PD controller at an updated rate of 100 Hz. In Ref. [17], Karasek et al. used a commercially available 2.8 g Lisa-S autopilot board running the open-source Paparazzi UAV software to fly the DelFly Nimble. The Lisa-S board (Fig. 14E) contains a 32-bit 72 MHz ARM Cortex-M3 microprocessor, a 6-axis MPU6000 IMU, a 3-axis Honeywell HMC5883L magnetometer, a MS5611 barometer, a U-Blox Max-7Q GPS module and others as detailed in Ref. [197]. The Paparazzi UAV software is based on the PD controller to stabilize the DelFly Nimble, as illustrated in Fig. 15. The modules for attitude esti­ mation and stabilization (pitch and roll) are the same as those used in Ref. [114]. For its rapid banked turns mimicking flight performance of fruit fly, open loop (OL) program was used instead of feedback loop. The heading was controlled using proportional (P) and feedforward (FF)

Fig. 14. Custom-built flight controllers installed in (A) Nano Hummingbird [8], (B) Texas A&M robotic hummingbird [13], (C) KUBeetle-S [196], (D) Purdue robotic hummingbird [155], and (E) DelFly Nimble [17] and NUS-Roboticbird [115].

Fig. 15. Block diagram of PD feedback controller implemented in the tailless DelFly Nimble [17]. 11

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Progress in Aerospace Sciences xxx (xxxx) xxx

terms to sense the yaw rate obtained by the gyroscope. However, the OL program was triggered for banked turn experiments.

et al. [175] used four wings implementing clap-and-fling effects at the stroke reversals. The experimental results indicated that the clap-and-fling effect improves as much as 50% lift and about 40% lift-to-power ratio. Benefits of the clap-and-fling effect also formed the X-wing configuration in many FW-MAVs from DelFly Nimble [17] to NUS-Roboticbird [115]. In particular, by using a large sweep amplitude of 190 deg, the two-winged KUBeetle could also implement the clap-and-fling effect to enhance lift generation [157].

3.4. Improvement of lift generation and payload capability To have room for all electronic components, the FWAVs are required to generate sufficient lift for staying airborne while consuming energy efficiently for long flight endurance. Recent works have shown remarkable improvements in the lift generation of insect-scale vehicles. A study by Ma et al. [198] showed that, by using a scaling heuristic, the 265 mg robot with a wing length of 25.5 mm can perform hover while carrying a payload of 115 mg, which is approximately the mass of the active control system and power electronics. Chen et al. [69], on the other hand, investigated the effects of wing morphology and inertia on flight performance and proposed a wing configuration that can enhance lift generation by 37%. Jafferis et al. [199] developed a nonlinear damping model of a passive-wing-rotation insect-scale vehicle. The re­ sults showed that, at an optimal rotation angle of about 70 deg, the vehicle increases mean lift by about 130% allowing higher payload ca­ pacity to carry onboard, while costing only about 55% more power. Furthermore, to have enough payload capacity for onboard electronic components, Fuller at the University of Washington [184] proposed a new flapping-wing concept with four perpendicular wings similar to a well-known quad-copter. The experimental result and flight test showed that the 143 mg vehicle can lift off while carrying a payload of 262 mg. By adding more wings, the RoboBee X-wing driven by two piezoelectric actuators could perform untethered flight with onboard power source and power electronics [200]. The new configuration allows the 259 mg vehicle to generate a peak lift of 372 mg and enhance its aerodynamic efficiency by about 29%. In order to efficiently fly the 19 g Nano Hummingbird, Keennon et al. [8] investigated many different wing configurations by varying mem­ brane material, wingspan flexibility, vein reinforcement, as well as aspect ratio and taper ratio. To reduce wing inertial power, Lau et al. [106] proposed a compliant thoracic flapping mechanism mimicking flight thorax of insects. The result showed that the compliant mechanism is capable of storing energy to save about 20–30% total power. Zhang et al. [94] developed an actuator that directly drives flapping wings at a resonant frequency using torsional springs as elastic elements to elimi­ nate the power expense for accelerating the wings (Fig. 6H). Compliant mechanisms have been also considered in many developed flapping-wing vehicles [19]. Researchers at Konkuk University investi­ gated wing deformation [161,201,202] and indicated that wing twist and camber with a proper arrangement of vein reinforcement is bene­ ficial for force generation and efficiency. Effect of wing aspect ratio was also investigated theoretically and experimentally to find the best wing configuration for economical flight [189,203–205]. Nan et al. [189] indicated that a wing with trapezoidal shape and aspect ratio of about 4.7 similar to a real hummingbird wing provides better flight perfor­ mance. To improve the lift generation for onboard components, Zdunich 1

Endurance (min)

10

9

2 3

Among the many developed tailless FWAVs, only a few at MAV scale can perform free flight, as shown in Fig. 16. At insect size (from pico-to nano-size), only RoboBee [10,192] could successfully demonstrate controlled flight, as shown in Fig. 17A. However, its power source and control system are still off-board. On the other hand, powered by on­ board solar cells, a recently released four-wing version of RoboBee could perform untethered (open-loop control) takeoff (Fig. 17B) [200]. RoboFly, which is mechanically based on RoboBee and powered by a laser beam, could also take off wirelessly in a very short time (Fig. 17C) [138]. Released in 2012, Nano Hummingbird [8] is the first tailless vehicle to successfully demonstrate agile flight performance, such as precise hover, fast forward flight up to 6.7 m/s, and autonomous 360� lateral flip, using onboard power source and electronics, as shown in Fig. 18A. It also demonstrates hovering for about 4 and 11 min (Saturn version) with and without payload of vision system and fairing, respectively. Up to now, the Nano Hummingbird is still the one that has made the longest flight. Of similar size to the Nano Hummingbird, the KUBeetle-S can hover and loiter in the air for about 3 min [88]. Meanwhile, the 62 g robotic hummingbird, which is more than three times heavier than Nano Hummingbird, developed at Texas A&M University lasts for less than 1 min, due to overheating of the motor [13]. Due to the same issue, Colibri can stay for several tens of seconds [100]. Mimicking the drag­ onfly, the TechJet Dragonfly with four tandem wings can sustain flight for about 8–10 min. Even though there is no information on the flight time, BionicOpter has demonstrated remarkable maneuvers in all di­ rections, hovering in one place, and even gliding without flapping wings. With an X-wing configuration, Delfly Nimble shows interesting maneuverability (Fig. 18B) [17]. It can hover for 5 min, fly forwards, backwards, and sideways, and perform banked turns as fruit flies, 360� flips and quick climbs. Also using an X-wing configuration, NUS-Roboticbird can loiter for 3.5 min with vision system onboard [115], and quickly fly in any direction as desired, demonstrating its readiness for applications. Quad-thopter with four pairs of X-wings can perform aggressive flight both in indoor and outdoor environments [113]. It also has a high flight endurance of about 9 min. Other than the Nano Hummingbird, which seems to be a unique vehicle that is ready for real application, in the viewpoint of the authors, some under-developing two-winged vehicles with body mass of less than 1. 2. 3. 4. 5. 6. 7.

Saturn Nano Hummingbird KUBeetle-S KUBeetle Colibri Purdue Hummingbird Butterfly-type Ornithopter 8. Texas A&M Robotic Hummingbird

13

12

14 4

1

10 11

3.5. Free flight capability and endurance

7

8

5

0.1

0.01 0

6

NAV

10

MAV

20

Multi-wing Two-wing

30 40 50 Body mass (g)

60

9. Techject Dragonfly 10. DelFly Nimble 11. eMotion Butterflies 12. NUS-Roboticbird 13. Quadthopter 14. DelFly

70

Fig. 16. Flight endurance of the free-flight capable, insect-inspired, tailless FW-MAVs. 12

H.V. Phan and H.C. Park

Progress in Aerospace Sciences xxx (xxxx) xxx

Fig. 17. Current achievements in flight of insect-scale FW-PAVs. (A) Controlled flight using off-board power source and electronics [192] and (B) untethered open-loop flight with onboard power source of RoboBee [200]. (C) Wirelessly powered RoboFly during uncontrolled takeoff [138].

excite the flapping wings, requiring lightweight power electronics to create high-voltage signals from low-voltage power sources. Addition­ ally, current commercially available power sources, including lithium batteries, solar cell, and fuel cell, are inappropriate (in terms of mass and power density) for the vehicles to fly autonomously, requiring demands for new fabrication technologies to develop lightweight, high-density power sources [208–212]. For example, Duduta et al. [173] intro­ duced a laser micro-machining technique to create 140 mg lithium-ion batteries, which can be 30 times lighter than the lightest commercial battery having similar power density. In addition to the power issue, tailless vehicles require onboard feedback sensors and processors, which cause increase of total body weight, for stabilization and control. Designing lightweight, low power consumption, robust electronic components is an additional challenge to overcome. For useful appli­ cations, the vehicles should also be able to carry vision systems onboard. The design of lightweight micro vision systems to fit the size and weight constraints of FWAVs is another requirement. Moreover, to have room for these onboard components, the aerodynamic performance of the vehicle should be improved. Further interesting discussions on the challenges and future directions of insect-scale vehicles can be found in Refs. [7,16]. Thus, with those abovementioned hurdles, insect-scale FWAVs may not be ready for real applications in the next few years. On the other hand, in the development of larger-scale FW-MAVs, current achievements of autonomous agile flights show their possibility of application in the near future. However, as we mentioned previously, the longest endurance of 11 min was made by Saturn, an early version of Nano Hummingbird; the flight time with onboard camera was reduced to about 4 min. Meanwhile, the similar size rotary-wing Black Hornet Nano with onboard cameras and the micro-quadcopter developed in

20 g may show their potentials for future applications both in confined indoor and outdoor spaces. Indeed, among many vehicles, 12 g Purdue Hummingbird [206,207] is the lightest vehicle (at scale of adult magnificent hummingbirds) that can perform controlled flight as aggressively as a real hummingbird [206], and even can autonomously detect the surrounding environments using its flapping wings as sensors [207]. In addition, mechanically simplified flapping and control mech­ anisms may allow the vehicle to carry more payloads and possibly endure longer than the Nano Hummingbird. However, to achieve those targets, the vehicle should be improved more in its flight efficiency for long flight endurance with an onboard power source. The 16 g KUBeetle-S is also a potential candidate for future applications with its simple and light weight but effective control mechanism [196]. Its current flight time of 3 min is limited by the overheating motor when operating at a high voltage of 7.4 V from two lithium polymer batteries connected in series. Thus, generating sufficient lift at operating range of the motor should be the next step for the vehicle to last longer in flight. 4. Challenges for future developments and directions of the insect-inspired FWAV Although great progress has been made, many challenges still lie ahead in developing insect-inspired FWAVs for real applications in complex environments. For insect-scale vehicles, free flight ability with all onboard components still presents challenges [16]. Developing on­ board microelectronics and power sources that fit the scale of a FWAV requires great effort for technological breakthrough. As we stated in the previous section, due to the limitation of low-voltage electromagnetic actuators at small scale, high-voltage smart actuators may be used to 13

H.V. Phan and H.C. Park

Progress in Aerospace Sciences xxx (xxxx) xxx

4.1. Low-noise flight ability Insect-inspired FWAVs leave a significant level of noise signature [8, 215], which may be a higher level of noise that nature’s flyers make. Reducing noise is another important aspect to enhance the ability to sneak into target areas. 4.2. Recovering stable flight after colliding flapping wings with obstacles In confined spaces or cluttered environments, FWAVs may possibly face wing collision with obstacles disrupting their flight or causing wing damage, even though they may be equipped with collision avoidance sensors to navigate the flight. The capability to recover stable flight after collision may thus help the vehicles continue their flight missions. One of the potential solutions is the design of collapsible wings, which was demonstrated to reduce the yaw rate by 40%, compared to a stiff wing [216]. 4.3. Performing various modes of locomotion Most insects can perform other various modes of locomotion than flight, such as jumping, running, walking, crawling, and swimming. Developing an FWAV with these locomotive abilities may help to save flight energy, and increase the range of applications [217,218]. More­ over, facing different situations in cluttered environments, vehicles may have more options to use appropriate locomotion modes. For example, in a confined space, where an FWAV has no room to operate its flapping wings, it may fold its wings along its body like most insects at rest, and use crawling, walking, or jumping legs to escape the space, before unfolding the wings for flying mode. Currently, several FWAVs have also been integrated with other locomotion modes, as shown in Fig. 19. RoboBee in Ref. [129] can perch on a leaf (Fig. 19A). In another research effort, it can fly, swim, and jump off the water surface (Fig. 19B) [128]. RoboFly in Ref. [219] is capable of flying and ground locomotion using its flapping wings, as shown in Fig. 19C. On the other hand, a centimeter-scale robot named Jump-flapper in Ref. [220] can use flapping wings to assist its jump (Fig. 19D). However, these multimodal vehicles should be further developed to fully adapt the locomotive strategies of natural flyers. In addition, adding other locomotion modes causes increase of total body mass, which reduces flight performance. Therefore, the design of such robots requires innovative ideas of simple and compact structures.

Fig. 18. Flight performance of tailless FW-MAVs. (A) Nano Hummingbird on its 360 deg flip and outdoor flight [8]. (B) Delfly Nimble on its rapid banked turn mimicking flight performance of fruit flies [17].

Ref. [213] can even sustain flight for about 25 and 30 min, respectively. Thus, flight endurance is still the most important target to improve to show the merits of hover-capable FWAVs over conventional rotorcrafts, which have successfully demonstrated their impacts on many fields of application, both confined indoor and outdoor. Other than that, re­ searches on FWAVs may pursue new scientific findings or specific ap­ plications for which rotorcrafts cannot meet the requirements. In such an application, FWAVs may enable the secrets of the agile flight ma­ neuvers of natural flyers to be uncovered. For example, the research in Ref. [17] used a tailless vehicle to discover how flies can perform rapid banked turns. Moreover, an experimental study in Ref. [185] indicated that flapping-wing propulsion shows better agility, compared to rotary-wings. Because of the large wing surfaces working as air resis­ tance, FWAVs can perform rapid transition from forward flight to hover [113]. In addition, FWAVs can be more human-friendly, due to their low flapping wing motion, compared to the high-speed rotors in rotorcrafts. For defense and military applications, insect-like FWAVs can be disguised as real insects in nature to conduct secret tasks, such as entering into enemy shelters without being detected. Although the current achievements of FWAVs are remarkable, they should show more capabilities to meet requirements for real applica­ tions. The requirements, as suggested in the discussion of [214], include capabilities of obstacle avoidance in confined spaces, long flight endurance to complete assigned tasks, coordination of multiple vehicles, autonomous flight, wind disturbance rejection, and interaction with the environment, such as perching on a tree branch. In addition, we recommend additional requirements and future research directions of insect-inspired FWAVs that can mimic a real flying insect:

4.4. Challenge of flight in low-density environment The development of small-scale vehicles that are capable of flying in low-density atmosphere for planetary exploration missions has received increasing attention [221]. However, in the condition of low density, the low range of Reynolds number causes inefficient flight of conventional fixed-wing and rotary-wing vehicles, due to the low lift-to-drag ratio [222]. On the other hand, flapping wing can be a promising solution to overcome the challenge of low-density environment, utilizing unsteady aerodynamic mechanisms to efficiently produce lift in low Reynolds number regimes [2]. Indeed, alpine bumblebees demonstrated hovering flight at a low air density environment equivalent to the condition of an altitude of 9000 m, which is higher than Mount Everest [223]. In addi­ tion, using numerical simulations, Bluman et al. [224,225] indicated that a bumblebee can hover on Mars, where air density is about 70 times lower than that on Earth [222], with the wing scale of a cicada. These researches may provide inspiration for the design of FWAVs that are capable of ultralow-density flight, such as the bumblebee-sized Marsbee, which is under ongoing development for flight on Mars [225,226]. 5. Conclusion This review paper presents recent progress on the development of 14

Name

Development group

Mass (g)

Transmission mechanism

Wings

Span (mm)

f (Hz)

Ψ (deg)

PS

FC

ACM

Refs.

Insect-like FW-MAV Insect-based FW-MAV Insect-inspired FW-MAV Flapping-Hovering MAV

2005 2005 2007 2008

Cranfield University University of Maryland University of Bristol Cornell University

50–100 – 46 24.2

Spherical double Scotch-yoke Scotch-yoke Parallel crank-rocker Crank-shaft

2 2 2 8 (4 pairs)

150 – 175 446

20 – 7.15 18

– 80–100 100 20–30

Off Off Off On

✕ ✕ ✕ ✕

[80] [81] [82] [83]

Nano Hummingbird

2007–2011

AeroVironment Inc.

19

String-based

2

165

30

180

On

✓

[8]

Dragonfly-Inspired robot

2009

Purdue University

–

Double Slider-crank

318

8

100

Off

✕

[84]

KUBeetle

2009Current 2011 Current 2011

Konkuk University

21.4

4-bar & pulley-string

4 (Tandem) 2

✕ ✕ ✕ Passively stable flight Free controlled flight (2011) ✕

160

30.5

190

On

✓

[9,85–90]

Purdue University

12

Direct-driven

2

170

30–40

170

On

✓

[19,91–94]

Cornell University

3.89

4-bar

4 (X-wing)

143

30

80

On

✕

[95]

2012

Georgia Institute of Technology

25

–

150

–

–

On

✓

[96]

University of Maryland Universite’ Libre de Bruxelles

12 22

Slider-crank Slider-crank & 4-bar

– 210

– 22

– 140–180

On On

✓ ✓

[97] [98–100]

BionicOpter

2012 2012 Current 2013

4 (Tandem) 2 2

Festo Company

175

Crank-shaft

630

15–20

80–130

On

✓

[101]

Flapping-wing robot

2013–2015

2.7

Direct-driven

220

10

80–150

Off

✓

[102–104]

Jellyfish-like flying machine Dipteran-Insect-Inspired thoracic mechanism Robotic Hummingbird

2014

Carnegie Mellon University & Nanyang Technological University New York University

4 (Tandem) 2

2.1

Crank-shaft

4

80*

19

�20 - 40

Off

✕

[105]

2014

Nanyang Technological University

3.51

Slider-crank

2

100

33

80

Off

Free controlled flight (2017) Wired controlled flight (2017) Passively stable flight Free controlled flight ✕ Free controlled flight (2017) Free controlled flight Tethered takeoff (2015) Passively stable flight ✕

✕

[106]

2015Current 2015

University of Maryland

62

Modified 5-bar

2

304.8

22

120

On

✓

[107]

Festo Company

32

–

500

1–2

–

On

✕

[108]

2015–2016 2017 2018 2018

KU Leuven Air Force Research Laboratory Seoul National University Delft University of Technology

3.39 35 10.8 37.9

Stroke-cam cable 4-bar String-based Crank-rocker

4 (Tandem) 2 2 2 8 (4 pairs)

50* 113* 150 280

40 16 22.6 15

175 170 140 44

On Off Off On

✕ ✕ ✓ ✓

[109,110] [111] [112] [113]

DelFly

2018

Delft University of Technology

19.7

Crank-rocker

4 (X-wing)

280

15

44

On

✓

[114]

NUS-Robobird

2018

National University of Singapore

31.0

Slider-crank þ Linkage

4 (X-wing)

220

13.3

90

On

✓

[115]

DelFly Nimble

2018

Delft University of Technology

28.2

Crank-rocker

4 (X-wing)

330

17

44

On

✓

[17]

FW-MAV Butterfly-type Ornithopter Beetle-type Ornithopter

2018 2018

Nanyang Technological University Beihang University

13.4 38.6

Crank-rocker Servo

4 (X-wing) 2

240 648

23 2

20–25 100

On On

✕ ✓

[116] [117]

2018

University of Tokyo

14

Slider-crank

4 (Tandem)

120*

14

160

Off

Free controlled flight (2015) Free controlled flight Guided takeoff ✕ ✕ Free controlled flight Free controlled flight Free controlled flight Free controlled flight Guided takeoff Free controlled flight Tethered flight

✕

[118]

2001–2007

UC Berkeley

0.1

Slider-crank & 4-bar

2

25

100–275

80–120

Off

✕

✕

[119–122]

DC motor-driven systems

Purdue Hummingbird Robot 3D-Printed Mechanical Insect TechJect Dragonfly Daedal Flapper Colibri

15

eMotion Butterflies KULibrie Tailless FW-MAV Insect-like FW-MAV Quad-thopter

Piezoelectric actuators MFI

(continued on next page)

Progress in Aerospace Sciences xxx (xxxx) xxx

Year

H.V. Phan and H.C. Park

Table 1 Timeline of the developments in insect-inspired flapping-wing systems with different driving actuators.

H.V. Phan and H.C. Park

Table 1 (continued ) Name

Year

Development group

Mass (g)

Transmission mechanism

Wings

Span (mm)

f (Hz)

Ψ (deg)

PS

FC

ACM

Refs.

FW-MAV Insect-inspired flapper RoboBee

2002 2004–2011 2007 Current 2011 2012 2012 2013 2017

Vanderbilt University Konkuk University Harvard University

7 9.6–10.3 0.06–0.1

5-bar 4-bar Slider-crank

2 2 2

150 114 25–30

20.5 12–17 110–120

30 90–130 110

Off Off Off

✕ ✕ ✓

Airforce Institute of Technology US Army Research Laboratory Carnegie Mellon University Pennsylvania State University Shanghai Jiao Tong University

0.35 0.03 0.16 0.112 0.084

4-bar Direct-driven Spherical 4-bar Slider-crank 4-bar

2 2 2 2 2

70 2.5* 37 45.78 35

30 156 37 37 100

110 120 �90 46 120

Off Off Off Off Off

✕ ✕ Wired controlled flight (2013) ✕ ✕ ✕ ✕ Guided takeoff

✕ ✕ ✕ ✕ ✕

[123] [124–127] [10,12, 128–130] [131] [132,133] [134] [135] [136]

2018 2018 2018

Toyota Central R&D Labs University of Toronto University of Washington

0.598 0.252 0.19

Direct-driven Slider-crank Slider-crank

2 2 2

114 74.9 13*

120 25 170

35 80 90

Off Off Off

Guided takeoff ✕ Untethered takeoff

✕ ✕ ✕

[18] [137] [138]

2008Current 2009 Current 2011 2013–2016

University of Valenciennes

0.022

Direct-driven

2

35

80

66

Off

✕

✕

[139–141]

Delft University of Technology

4.0

Ring-based

4 (X-wing)

120

27

–

Off

✕

✕

[142,143]

KAIST Purdue University

2.86 4.0

Ball-joint Direct-driven

2 2

75 86

68 90

– 120

Off Off

✕ ✕

[144] [145–147]

2016

Shanghai Jiao Tong University

0.08

4-bar

2

35

80

140

Off

✕ Tethered takeoff (2016) Guided takeoff

✕

[148]

2017

Beihang University

0.093

Slider-crank

2

�30

101.4

20

Off

✕

✕

[149]

Butterfly-type Ornithopter Bio-inspired flapper

2005

University of Tokyo

0.41

2

140

10

74

On

[150]

Nanyang Technological University

10.47

2

130

5–10

–

Off

Passively stable flight ✕

✕

2014

✕

[151]

Flapping-wing platform

2018

Beihang University

0.05

2

56

35

35

Off

✕

✕

[152]

FW-MAV

2019

University of Brisol

–

2

40*

18

63

Off

✕

✕

[153]

Beetle-inspired platform

2018

Xiamen University

–

Rubber-band þ4-bar Dielectric Elastomer Actuator þ Thoracic mechanism Electrostatic actuator þ Pivot-spar Dielectric Elastomer Actuator þ Slider crank Ionic Polymer-Metal Composite Actuator

2

42*

0.5–8

<12.5

Off

✕

✕

[154]

FW-MAV Insect-inspired FW-MAV Flapping-wing robot LionFly Insect-scale flapping-wing robot Bioinspired FW-MAV Dragonfly platform RoboFly Electromagnetic actuators OVMI Atalanta FW-MAV

16

FW-MAV FW-MAV Insect-inspired flappingwing robot FW-MAV Other actuators

Progress in Aerospace Sciences xxx (xxxx) xxx

f: frequency, Ψ: sweep amplitude, PS: power supply, Off: Offboard, On: Onboard, FC: Flight Capability, ACM: Attitude Control Mechanism. MFI: Micromechanical Flying Insect, OVMI (French): Flying Object Mimicking the Insect. * Wing length.

H.V. Phan and H.C. Park

Progress in Aerospace Sciences xxx (xxxx) xxx

Fig. 19. Insect-inspired FWAVs that are capable of other modes of locomotion. Robobee developed by Harvard University capable of (A) perching [129], and (B) water-diving and jumping [128]. (C) RoboFly developed by the University of Washington that is capable of ground locomotion [219]. (D) Jump-flapper developed by Konkuk University that is capable of jumping and flapping using only one driving actuator [220].

insect-inspired, tailless, hover-capable FWAVs, a new class of flyer, in the sense that both flight force and control torque are produced only by flapping wings, without control surfaces at tail, unlike the bird-like FWAV. Thus, the FWAV should be able to modulate the wing kine­ matics of each wing, so that asymmetric aerodynamic force can be produced. Therefore, its engineering design, underlying aerodynamics, and feedback control are all very challenging. We discuss the diversity of these vehicles with regard to size, propulsive actuator, wing configu­ ration, control and stability strategy, capability of autonomous flight, and flight endurance. At the scale of insects (pico- and nano-scale), recent technical challenges in the propulsion actuator, onboard micro­ electronics, and power supply prevent the free flight ability of the ve­ hicles, requiring the need for advanced technologies to achieve such free flight. Consequently for the next few years, freely flying an insect-scale FWAV will remain a challenging topic. Meanwhile, larger-scale motordriven FW-MAVs have demonstrated a great achievement in agile flight ability, showing their potential readiness for useful applications. How­ ever, they still need improvements in flight endurance to show their merits over similar-size conventional rotorcrafts, which are already predominant in the market. People conducting research on FWAVs may thus need to focus more on specific research, such as thrust enhance­ ment, efficiency, gust/side wing response, noise reduction, and agile maneuverability to avoid obstacles. The development of an insect-inspired FWAV may reveal its unique applications. Moreover, it is feasible to fully adapt locomotive strategies of natural flyers by adding other modes of locomotion, such as jumping, walking, swimming, and crawling. An insect-inspired FWAV can also be a promising candidate for exploring other planets, where the atmo­ sphere is more rarified than on Earth.

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