Corresponding author: Hoon Cheol Park E-mail: hcpark@konkuk.ac.kr

Journal of Bionic Engineering 7 (2010) 134–141

Improvement of Artificial Foldable Wing Models by Mimicking the Unfolding/Folding Mechanism of a Beetle Hind Wing

Azhar Muhammad1,2, Quoc Viet Nguyen1,2, Hoon Cheol Park1,2,4, Do Y. Hwang4,

Doyoung Byun2,3,4, Nam Seo Goo1,2,4 1. Department of Advanced Technology Fusion, Konkuk University, Seoul 143-701, Korea

2. National Research Laboratory for Biomimetics and Microsystems, Konkuk University, Seoul 143-701, Korea 3. Departmenet of Aerospace Engineering, Konkuk University, Seoul 143-701, Korea

4. Artificial Muscle Research Center, Konkuk University, Seoul 143-701, Korea

Abstract In an attempt to realize a flapping wing micro-air vehicle with morphing wings, we report on improvements to our previous

foldable artificial hind wing. Multiple hinges, which were implemented to mimic the bending zone of a beetle hind wing, were made of small composite hinge plates and tiny aluminum rivets. The buck-tails of rivets were flared after the hinge plates were assembled with the rivets so that the folding/unfolding motions could be completed in less time, and the straight shape of the artificial hind wing could be maintained after fabrication. Folding and unfolding actions were triggered by electrically-activated Shape Memory Alloy (SMA) wires. For wing folding, the actuation characteristics of the SMA wire actuator were modifiedthrough heat treatment. Through a series of flapping tests, we confirmed that the artificial wings did not fold back and arbitrarily fluctuate during the flapping motion.

Keywords: hind wing, unfolding, folding, shape memory alloy, folding ratio, artificial wing Copyright © 2010, Jilin University. Published by Elsevier Limited and Science Press. All rights reserved. doi: 10.1016/S1672-6529(09)60185-2

1 Introduction

Recent efforts to understand the mechanism of insect flight may help us to realize Nano- and Micro-Air Vehicles (NAV/MAV)[1]. For an insect-like flyer, aerodynamics in a low Reynolds number regime[2] and flexible wings[3] are important issues. To identify mechanisms of aerodynamic force generation in a low Reynolds number environment, some features of insect flight, such as the clap-and-fling mechanism[4], lead-ing-edge vortices[5], pitching-up rotation[6], and wake- capture[7] have been explored.

The wing is an essential member for all flying in-sects. Insect wings are membranous and fragile. Amaz-ingly, however, they are still strong enough to endure the aerodynamic forces produced by flapping wing motion. Moreover, some winged insects are able to fold and unfold their wings. Insects equipped with foldable wings are grouped into several orders including Coleoptera (beetle) and Dermaptera (earwig). They have two types

of wings: a pair of hardened forewings called elytra, and a pair of morphable hind wings. The hind wings are shielded underneath the fore wings during ground lo-comotion, and extended just before flying. Thus, they can fly in the air and crawl on the ground. This unique fea-ture has encouraged us to study and realize an artificial foldable hind wing. In this work, for easier observation with bare eyes, we selected a beetle called Allomyrina dichotoma (Fig. 1), a species in the order Coleoptera which is relatively larger than others.

Fig. 1 Allomyrina dichotoma (male).

Muhammad et al.: Improvement of Artificial Foldable Wing Models by Mimicking the Unfolding/Folding Mechanism of a Beetle Hind Wing 135

In our previous work[8], we demonstrated two types of artificial hind wings which can be unfolded by ap-plying an electric field to the installed Shape Memory Alloy (SMA) actuators[9]. One of the artificial wings was designed based on a so-called basic model[10] and could be unfolded in the out-of-plane mode. After observing the wing unfolding behavior of Allomyrina dichotoma, we found that it actually unfolds its hind wing nearly in a plane. Therefore, we made another artificial wing that can be unfolded in a plane by activating the installed SMA wires. Hass and Beutel explained the details of muscle actuation and principles for wing unfolding of Pachnoda marginata[11]. The wing unfolding is very similar to that of Allomyrina dichotoma, except Pach-noda marginata does not open the elytra during the hind wing unfolding. As Hass and Beutel suggested, we examined the change in angles between two major veins – the Radius Anterior (RA) and Medial Posterior (MP) – and tried to reproduce the so-called “scissor-like mo-tion” of the two major veins[11] in an artificial wing.

In this work, we first observed wing folding and unfolding of Allomyrina dichotoma, and then confirmed that wing folding is also occurring in a plane. The fold-ing motion of an artificial wing was demonstrated by reversing the actuation deformation of the SMA wire actuators, which were properly heat-treated before being installed in the artificial wing. We also modified the hinges such that the folding and unfolding motions be-come faster and wing veins remained straight after un-folding and during the flapping motion. All the charac-terization parameters were evaluated to confirm the similarity between the artificial wing and the real beetle wing. Afterward, a series of flapping tests was con-ducted to examine whether or not the artificial hind wing folds back during wing flapping, and to inspect its flexibility.

2 Unfolding and folding of beetle hind wing

We investigated the hind wing folding motions of a beetle, Allomyrina dichotoma, after tethering the beetle. A carbon rod was attached to a place at its dorsal and the other end was attached to a fixture. The unfolding process was captured by using a high speed camera recorder (Photron, Fastcam Ultima APX, model 120K) at 4000 frames per second, shown sequentially in Fig. 2. The folding images were captured by a digital camera (Canon G9) at 30 frames per second. Because a full set of sequential pictures for wing unfolding and time spent for the wing unfolding (~0.6 s) were reported in Ref. [8], we include only a few images in Fig. 2a. The figure shows that the beetle consecutively opens its forewings and folded hind wings, sweeps the hind wings forward, starts to flap the hind wings, and fully unfolds the hind wings nearly in a plane, just before flight. Thus, this species fully unfolds the outer part of its hind wings during the initial flapping motion within a relatively short time.

In contrast to unfolding, the folding process of the beetle’s hind wing is more random and requires more time. This may be because a beetle can still crawl even with partially-folded wing configuration, while the hind wing must be unfolded immediately for flight. Fig. 2b shows that the hind wings are completely folded after the elytra are closed. Thus, the folding also takes place in a plane.

The wing folding and unfolding motions are trig-gered by contracting various flight muscles in the thorax, as Hass and Beutel pointed out[11]. Similarly, a proper actuation of muscles may cause morphological change in the hind wings of Allomyrina dichotoma. Structural members of a hind wing, as shown in Fig. 3, create three main mechanisms with which to complete the wing

(a) Unfolding

(b) Folding

Fig. 2 Sequence of free unfolding and folding motions.

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shape change: (1) scissor-like motion between the RA and MP with the Medial Bridge (MB) as the pivot; (2) bending motion of the Bending Zone (BZ); and (3) ro-tation motion of the Marginal Joint (MJ). These are nearly the same mechanisms found in another beetle, Pachnoda marginata[11], even though it does not open its elytra during wing unfolding. Thus, the MP, RA, RA3, RP3+4, BZ, MJ, MB, and the movable joint work like an in-plane hinge structure, resulting in stable in-plane folding and unfolding motions of the hind wing.

Because details of morphological change during wing unfolding were explained in Ref. [8], we included only the final angle changes among vein structures in Fig. 4. As shown in Fig. 4, a hind wing of Allomyrina dichotoma is folded at two locations: the anal fold (fold of anal field to medial field in Fig. 3) and the apical fold (fold of apical field to medial field). A large elastic bending deformation in BZ is also noticeable in Fig. 4b.

(a) Regions and names of veins

(b) Joints

(c) Folding structures

Fig. 3 Real hind wing of Allomyrina dichotoma.

(a) Unfolding (b) Folding

Fig. 4 Unfolding and folding configurations of Allomyrina dichotoma hind wing[8].

3 Artificial foldable hind wing

3.1 Design principle As previously described, there are many muscles

and wing members involved in wing folding and un-folding motions. Hass and Beutel suggested other pas-sive mechanisms such as elastic energy storage during wing folding at specific locations in a hind wing, where resilin, a highly elastic material, is present[10]. Large elastic bending deformations of the BZ, in Fig. 3, is another important element in making in-plane wing folding and unfolding possible. However, exactly copying a beetle hind wing is almost impossible, be-cause no available material can reproduce real flight muscle and resilin. Therefore, we need to properly ap-proximate the wing folding and unfolding, and similarly replace the vein structures by using existing and avail-able engineering materials and elements.

In this study, we neglected the anal fold because of its relatively small contribution to the folding ratio. The vein structures were made from composite materials and the BZ was mimicked by using multiple hinges. Unlike our previous work[8], tiny aluminum rivets were used for the hinges to improve folding and unfolding perform-ance. Because two-way SMA wires were not available in the market, we separately demonstrated wing folding and unfolding by using two artificial wings equipped with SMA wires which have different one-way shape memory effects; i.e., one for folding and the other for unfolding. We did not attempt to exactly mimic the geometry and stiffness of a real hind wing, because the main purpose of the current work was to explore feasible methods of mimicking the folding and unfolding of a beetle hind wing. 3.2 Fabrication

For easier fabrication, we made artificial wings with a 10.5 cm span, which is about two times longer than a typical hind wing of Allomyrina dichotoma. The pattern of veins was taken from a real beetle hind wing as shown in Fig. 5. The vein structures were made of 0.27 mm thick glass/epoxy composites (GEP, SK Chemicals, Korea), which were attached to the upper and lower sides of an artificial membrane. The artificial membrane was made of 15 μm thick polypropylene film, even though a real beetle membrane has non-uniform thick-ness[12]. Because the size of each part was small, we

Muhammad et al.: Improvement of Artificial Foldable Wing Models by Mimicking the Unfolding/Folding Mechanism of a Beetle Hind Wing 137

constructed CAD models of all parts by using commer-cial CAD software (CATIA, Version 7, Dassault Systemes, France) prior to the cutting process. For high precision cutting, we used a CNC machine (Manix, MM300-S, Woosung E&I Ltd, South Korea).

In our previous work[8], multiple in-plane hinges were used to mimic the BZ of a real beetle hind wing, as shown in Fig. 3a. A piece of composite hinge plate and two short carbon rods, which are marked with white dotted circles in Fig. 5a, were used to form each in-plane hinge. As shown in Fig. 6a, each carbon rod was tightly fit (interference fit) to a hole in its own hinge plate be-cause the free ends of the rods could not be flared. Be-cause of this, friction in the in-plane hinges was rela-tively large, and fast wing folding and unfolding motions were impossible. In the current work, we replaced the carbon rod with a tiny rivet to reduce friction in the in-plane hinges, as shown in Fig. 5b and Fig. 6b. Short

carbon rods used to mimic the MB, MJ, and the move-able joint were also replaced by the same rivets as shown in Fig. 5b. In the current artificial wing, the diameter of each hole was slightly larger than 1.15 mm and the shaft diameter of an aluminum rivet was 1.15 mm. The rivet buck-tails were flared to obtain tight clearance in each in-plane hinge. Thus, a sliding fit was available. In this way, we could reduce the friction in each hinge and maintain a straight wing shape after assembly. Also, the artificial wing did not arbitrarily fluctuate during flap-ping motion.

3.3 Actuation system

For the actuation system to fold and unfold the proposed artificial hind wing, we used a set of SMA wires (Kantoc, Ltd.) which were also used for unfolding the previous artificial wing[8]. SMA wire, which is also known as “muscle wire,” is a metal alloy that can memorize its initial shape. SMA wire is typically weak and plastically deformable when its temperature is lower than its transformation range. A typical SMA wire available in the market is specially treated so that it has a one-way shape memory effect. Therefore, if we arbi-trarily bend a SMA wire and then increase its tempera-ture within or higher than the transformation range, the wire becomes stiffer and more elastic, and then returns to its original shape[9], as shown in Fig. 7a. In Fig. 7a, the original shape of the SMA wire is straight. The SMA was arbitrarily deformed and then heated up to recover its original shape. If we treat it differently, we can reverse the deformation, but the SMA then loses its original shape memory ability. Fig. 7b shows a heat-treated SMA wire whose original shape is arbitrary bent. The wire then goes back to its original shape when it is heated up after being straightened.

(a) Original SMA wire

(b) Heat-treated SMA wire

Fig. 7 SMA wire behaviors.

(a) Previous model

(b) Current model

Fig. 5 Artificial foldable hind wings.

(a) Previous model

(b) Current mode

Fig. 6 Configurations of in-plane hinges.

Journal of Bionic Engineering (2010) Vol.7 No.2 138

We installed the original SMA wires (used in the previous work[8]) in the current modified artificial wing to demonstrate the unfolding motion. Meanwhile, wing folding motion could be achieved by installing another set of SMA wires with a reversed shape memory effect. We properly treated the original SMA wire by following a heat treatment process described in Ref. [8], where a SMA wire was plastically deformed such that it had the desired shape and then baked at 250 ˚C for 30 min. This process was selected to make the wire’s shape trans-formation temperature the same as that of the original SMA wire. The process was carried out in an environ-mental chamber (Model EC1x, Sun Electronic Systems Inc.). Because a heat-treated wire loses its original shape memory effect, we had to fabricate another artificial wing to demonstrate wing folding using heat-treated SMA wire. Thus, the artificial wing folding and un-folding were separately demonstrated with two artificial wings equipped with differently heat-treated SMA wires.

Fig. 8 shows the two sets of SMA wires used in this work: the original SMA wires for wing unfolding, and the heat-treated wire used for wing folding. The shapes of the original SMA wires used for unfolding are shown at the bottom of Fig. 8. The bent angle of the SMA wire installed at the wing base is about 20˚, and the other SMA wire has a slight curvature so that it can stay along the leading edge vein (RA through RA3) when they are installed in wing vein structures. Then, the two pieces of SMA wire were properly bent to construct the folded configuration. The shapes of the two deformed SMA wires look like the shapes shown at the top of Fig. 8. In this case, the bent angle of the SMA wire installed at the folded wing base is about 10˚. When heated, this SMA wire created a scissor-like motion of the RA and MP, and the other SMA wire installed along the leading edge vein generated rotation as well as translation of the apical field (outer wing).

To demonstrate wing folding, we prepared the heat-treated SMA wires as previously described, and another artificial wing. The memorized shapes of the two heat-treated SMA wires looked like the shapes at the top of Fig. 8. They were installed in the unfolded wing after being properly deformed, just like the shapes at the bottom of Fig. 8. When heated, the two SMA wires re-covered their own shapes and thus wing folding was completed.

We raised the temperature of the SMA wires by applying 5 V and 1.5 A from an external power supply (Agilent, E3646A). The sequential unfolding and fold-ing motions of the artificial wing are shown in Fig. 9. The angle between RA and MP increased during un-folding and decreased during folding due to scissor-like motion triggered by the installed SMA wires at the wing base. During folding, the artificial BZ created a slight curvature so that the artificial RA could bend, and the artificial RA3 generated a large rotation about the mar-ginal joint. On average, wing unfolding was completed within about 3 s and the wing folded in about 4 s.

Fig. 8 Shapes of the SMA wires installed on the in-plane artificial hind wing when unfolding and folding.

(a) Unfolding

Fig. 9 Demonstration of artificial foldable hind wing.

Muhammad et al.: Improvement of Artificial Foldable Wing Models by Mimicking the Unfolding/Folding Mechanism of a Beetle Hind Wing 139

(b) Folding

Fig.9 Continued.

4 Comparison

We compared a real beetle hind wing, a previous artificial wing (only for unfolding), and two current modified artificial wings (for folding and unfolding) in terms of geometry, time spent for folding and unfolding,

and folding ratio. These data were acquired by examin-ing images taken by a high-speed camera and are sum-marized in Table 1. The current artificial wing is both larger and heavier than a real beetle hind wing. A typical unfolding time was about 3 s, which was less than half of the time spent by the previous wing[8], but is larger than that required by a real beetle wing (0.05 s). We believe that 3 s to 4 s for artificial wing folding and/or unfolding is still fast enough for a real application. We made other improvements in the current artificial wing model. After complete wing unfolding, the angle between RA and MP was 19˚ and another angle between RA and MJ through the tip of RA3 was 152˚ became closer to those of a real beetle wing, which are 20˚ and 154˚, respectively. In wing folding, the angle between RA and MP was 12˚, is close enough to that of a real beetle hind wing of 10˚, yet the angle between RA and MJ through the tip of RA3 was 50˚ which was still larger than the real one of 39˚. The discrepancy mainly came from a lack of actuation force produced by the SMA wires, which might not be enough for compact folding around marginal joint and moving joint areas. The folding ratio, which is the ratio between the areas of unfolded and folded configurations, was improved in the two current wings. Note that the folding ratio of a real beetle hind wing changes from 2.32 to 1.70 if the anal fold of the real wing is neglected for a fair comparison with the folding ratios of the arti-ficial wings, where the anal folds were ignored.

Table 1 Comparison between real and artificial hind wings

Artificial hind wing

Present Parameter Real hind wing Previous (unfolding only) Folding wing Unfolding wing

Length (cm) 5.2 10.5 10.5

Weight (g) 0.16 1.06 1.1

Thickness of membrane (μm) 3 ~ 6 15 15

Folding Random Not available 4 Not available Typical time spent (s) Unfolding 0.05 7 ~ 8 Not available 3

Folding 3.29 21.88 21.22 20.317 Projected wing area (cm2) Unfolding 7.62 32.79 35.76 34.93

Folding 10 12 12 10 Angle between RA and MP (˚) Unfolding 20 16 19 19

Folding 39 51 50 41 Angle between RA and MJ – tip of RA3 (˚) Unfolding 154 148 154 152

Folding ratio 2.32 (1.70*) 1.50* 1.70* 1.72*

*Without anal fold.

Journal of Bionic Engineering (2010) Vol.7 No.2 140

5 Flapping test

A beetle hind wing does not fold back during flight unless the beetle is exhausted. This is another curiosity because the leading edge of a hind wing is almost separated near the MJ. We conducted a series of flapping tests for the artificial wings after installing them in a motor-driven flapper to see if they would fold back during the flapping motion. The flapping system could flap at 30 Hz with 120˚ flapping angle without wings. When we installed the artificial wings, however, the flapping frequency happened to drop to 9 Hz for the same 120˚ flapping angle. Fig. 10 shows the flapping motions during upstrokes and downstrokes. From these figures, we note that the artificial hind wings do not experience folding back motion or arbitrary fluctuation during flapping. Because the wing vein structures re-main almost straight, we conclude that the artificial wings are stiff enough to sustain aerodynamic and iner-tial forces when the wings are under flapping motion at 9 Hz with a 120˚ flapping angle.

(a) Downstroke

(b) Upstroke

Fig. 10 Flapping test.

6 Conclusion

In this work, our previous artificial foldable hind wing was successfully improved by modifying the hinges. Through the modification, the current artificial wing demonstrated swifter folding/unfolding motions, closer angles among veins before and after folding, and a higher folding ratio. In addition, folding motion was demonstrated by reversing the actuation deformation of the SMA wires through a heat treatment process. Flap-ping test showed that the artificial hind wing did not fold back during the flapping motion, and possesses an adequate flexibility when flapped at a 9 Hz wing beat frequency with a 120˚ flapping angle. Information ac-quired from this research should be helpful in develop-ing portable flapping-wing MAVs with morphing wings.

Acknowledgement This research was financially supported by the

Korea Science and Engineering Foundation Grant (Na-tional Research Laboratory Program, R0A-2007-000- 200012-0) and the Korea Research Foundation (KRF- 006-005-J03301). This work was partially supported by the 2009 KU Brain Pool of Konkuk University. The authors gratefully acknowledge this support.

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