Bio-Mimetic Millimeter-Scale Flapping Wings for 

Micro Air Vehicles 

by Christopher Kroninger, Jeffrey Pulskamp, Jessica Bronson, 

Ronald G. Polcawich, and Eric Wetzel

ARL-TR-4729 March 2009 

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Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5066 

ARL-TR-4729 March 2009 

Bio-Mimetic Millimeter-Scale Flapping Wings for 

Micro Air Vehicles 

Christopher Kroninger 

Vehicle and Technology Directorate, ARL 

Jeffrey Pulskamp, Jessica Bronson, and Ronald G. Polcawich 

Sensors and Electron Devices Directorate, ARL 

Eric Wetzel 

Weapons and Materials Research Directorate, ARL 

Approved for public release; distribution is unlimited. 
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4. TITLE AND SUBTITLE 

Bio-Mimetic Millimeter-Scale Flapping Wings for Micro Air Vehicles 

5c. PROGRAM ELEMENT NUMBER 

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1L162618AH80 

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6. AUTHOR(S) 

Christopher Kroninger, Jeffrey Pulskamp, Jessica Bronson Ronald G. Polcawich, 

and Eric Wetzel 

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14. ABSTRACT 

In this report, we present designs of Micro-Electro-Mechanical System (MEMS) fabricated millimeter-scale flapping wings 

with some characterization of their structural properties and flapping performance. Wings of varied bio-mimetic design, 

including membrane and ribs-like structures, are flapped at resonance from lead zirconate titanate (PZT) bending actuators at 

their base. Laser Doppler vibrometer (LDV) measurements and finite element (FE) analysis predictions of the flapping 

frequencies and mode shapes and the flapping amplitude of the devices demonstrate the feasibility of fabricating wing systems 

analogous to insects of the same size scale. We also document the design and progress in implementing a photogrammetric 

setup, which will be employed for kinematic characterization in this ongoing work. 

15. SUBJECT TERMS

MEMS, micro air vehicles, robotics, flapping wings 

16. SECURITY CLASSIFICATION OF: 

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Christopher Kroninger 

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iii

Contents 

List of Figures iv

List of Tables iv

Acknowledgement v

1. Objective 1

2. Approach 1

2.1 Device Designs................................................................................................................1 

2.2 Characterization...............................................................................................................3 

3. Results 5

4. Conclusions 9

5. References 10

6. Transitions 11

Acronyms 12

Distribution List 13 
iv

List of Figures 

Figure 1. Optical image of a micro-fabricated wing structure (1.5 mm span) with three thinfilm PZT actuators at the base of the wing and gold vein-structures deposited on the wing 

surface........................................................................................................................................2 

Figure 2. Schematic diagram of fabrication of the wing and actuators (not to scale): (a) top 

view of a wing and (b–g) process flow......................................................................................2 

Figure 3. Four-camera photogrammetry system focused on a millimeter-scale wing....................4 

Figure 4. First two resonant mode shapes of the PiezoMEMS micro-wing (a) measured with 

an LDV and (b) FE model. ........................................................................................................5 

Figure 5. Fundamental resonant frequencies verses span for PiezoMEMS micro-wings. .............6 

Figure 6. Images of a wing (design 3, 2.5 mm length) (a) at 0 V and (b) at resonance (156 

Hz, 5 V p-p), showing that resonant behavior amplifies the stroke...........................................7 

Figure 7. Wing target triangulation showing (a) an example image of the wing and identified 

targets and (b) the triangulated surface “veins” and targets in space. .......................................8 

List of Tables 

Table 1. Materials and layer thicknesses of the millimeter-scale wings.........................................3 
v

Acknowledgement 

We would like to acknowledge the work of Richard Piekarz and Joel Martin of the U.S. Army 

Research Laboratory (ARL) and Brian Power of General Technical Services for their 

contributions in the fabrication of the lead zirconate titanate (PZT) actuators and structures. We 

would also like to acknowledge the work of Aaron Harrington of the University of Maryland for 

his contributions to the photogrammetry setup made during his summer internship at ARL. 
vi

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1

1. Objective 

The objective of the proposed research is to develop millimeter-scale flapping wings, created 

through a Micro-Electro-Mechanical System (MEMS) process, that produce lift and flight 

characteristics similar to those of insects in the same size class. Progress towards this objective 

will include a study of materials, MEMS design and processes, and wing aeromechanics for a 

prototype legged MEMS architecture already under development. 

2. Approach 

2.1 Device Designs 

Given the complexity of the aeromechanical design problem, the design of this wing system 

begins with the presumption that the aerodynamic forces required for flight can be produced by 

mimicking the kinematic patterns and using structures of similar mechanical properties to those 

of insects. This effort attempts to demonstrate stroke amplitudes and frequencies that are within 

the range of actual insect characteristics. 

The Drosophila fly (a common fruit fly) is an insect that has been well-characterized in the 

literature and was the primary model for these designs. A purely bio-mimetic design approach 

for these prototype wing structures dictated the choice of the wing shape. Additional design 

variables include the wing span and wing composition using materials currently available in the 

microfabrication facilities at the U.S. Army Research Laboratory (ARL), including thin-film 

silicon dioxide, silicon nitride, lead zirconate titanate (PZT), and several common evaporated 

metals (titanium [Ti], gold [Au], platinum [Pt], nickel [Ni], aluminum [Al]). The design seen in 

figure 1 features three PZT unimorph mechanically coupled actuators at the base of the wing that 

are used to directly drive the stroke angle. Altering the phase of the drive signal to the individual 

actuators permits a limited drive capability of the wing pitch. The actuator performance is a 

strong function of the underlying structural dielectric layer (figure 2). Gold or silicon veinstructures with patterns similar to those found on real insect wings were included in some 

designs and may be used to decouple the properties of the actuator from the stiffness of the wing. 

The fabrication process and materials are similar to the process used for PZT radio frequency 

(RF) MEMS switches (1) with an additional backside release etch. 
2

PZT 

Actuators 

Figure 1. Optical image of a micro-fabricated wing structure (1.5-mm 

span) with three thin-film PZT actuators at the base of the 

wing and gold vein-structures deposited on the wing surface. 

Figure 2. Schematic diagram of fabrication of the wing and actuators (not to scale): (a) top view of a 

wing and (b–g) process flow. 

(a)

(b)

(c)

(d)

(e)

(f)

(g)
3

Figure 2 shows a cross-section view of the micro-wing actuator taken longitudinally along the 

wing span. Starting with a bare silicon (Si) wafer (figure 2b), a thick composite elastic layer 

comprised of silicon dioxide and silicon nitride is deposited using plasma enhanced chemical 

vapor deposition (PECVD). The thickness of this layer was varied from 1 to 3 microns. The 

film is then annealed at 700 °C in a nitrogen atmosphere. An adhesion/diffusion barrier layer of 

Ti is sputtered on the PECVD oxide, immediately followed by a sputtered Pt thin-film to serve as 

the bottom electrode layer of the actuator. The sol-gel PZT is deposited in several layers to 

achieve the desired thickness of 0.5 micron. The top electrode of Pt is sputter deposited and 

patterned with argon ion-milling (figure 2c). The PZT for the actuators and the bottom electrode 

is defined by ion milling (figure 2d). With the PZT actuators patterned, the wing itself is 

lithographically defined and patterned using a reactive ion etch (RIE) of the composite elastic 

layer. A thin bi-layer of Ti/Au (200/7300 Å) is deposited and patterned using a liftoff process to 

provide metal to the bond pads and wing surface (figure 2e). Next, a 2-micron Au layer is 

deposited via electron beam evaporation and patterned by liftoff (figure 2f). The wing structure 

is released using a backside deep RIE (DRIE) process that creates a window of free space around 

the wing (figure 2g). Once the wings are released, the remaining photoresist is removed with an 

oxygen plasma. 

Table 1 summarizes the different combinations of materials and thicknesses used for the wings, 

including the thickness of the elastic layer as well as the composition of the wing materials, 

which may include the dielectric PZT layer. Variations of this process flow have been explored 

and include using silicon-on-insulator (SOI) substrates and using a xenon difluoride (XeF2) etch 

to ensure that all the backside Si is removed from the wing structure. 

Table 1. Materials and layer thicknesses of the millimeter-scale wings. 

Wing Materials Device 

Design Number 

Elastic Layer 

(μm) PZT 

(0.5 μm) 

Au Full 

(2 μm) Au V eins 

1 1 x — x 

2 1 — x — 

3 1 x x — 

4 3 — x — 

5 3 x — x 

6 3 x — — 

2.2 Characterization 

Characterization of the wing kinematics is performed using several different techniques. A laser 

Doppler vibrometer (LDV) is used to measure the resonant frequencies and mode shapes of the 

devices from surface point measurements of the velocity as the wings are excited by a pseudorandom signal. Dynamic angular displacement of the wings excited at resonance is measured by 
4

inspection from long exposure optical images. Finally, measurement of the wing surface 

displacements is made using a photogrammetric technique that is being developed for this 

project. 

Photogrammetry relies on triangulation from images taken simultaneously from different 

perspectives to determine the position of a target in space (2). The setup seen in figure 3 consists 

of four 2/3 in. Charge-Coupled Device (CCD) cameras positioned to view the wings from 

various perspectives. These cameras are controlled with an external trigger coordinated with a 

strobe light. The strobe light pulse duration is short enough to make the wing appear stationary 

in the image. As these devices are driven at resonance, the period of the flap stroke is known 

and any particular location in the flap stroke can be repeatedly imaged by strobing at whole 

fractions of the drive frequency. In this way, any number of phase-locked images can be 

acquired and the full wing stroke can be captured by stepping through the period in increments of 

the desired resolution. This process has been automated to collect all the images when provided 

a schedule of the three actuator amplitudes and phases as well as the desired number of repeated 

phase-locked images and resolution of the flap stroke. 

Figure 3. Four-camera photogrammetry system focused on a millimeter-scale wing.
5

3. Results 

The wings have been tested to determine the resonant frequencies and mode shapes using an 

LDV. These results have been compared to a finite element (FE) modal analysis. The modal 

analyses show that the fundamental resonant frequency exhibits an expected longitudinal flexural 

mode desirable as a large flapping stroke, while the second mode is torsional pitching of the 

wing about a longitudinal axis, as can be seen in figure 4. These are the two most fundamental 

motions characteristic of insect wing kinematics, a flapping motion to move the wing relative to 

the air and a pitching motion to set the appropriate angle of attack to generate lift. These 

motions occur at the same frequency with a precisely controlled phasing in insects, which will 

require further refinement of the device designs to achieve with the current actuation mechanism. 

1st mode 2nd mode

(a) 

(b) 

Figure 4. First two resonant mode shapes of the PiezoMEMS micro-wing 

(a) measured with an LDV and (b) FE model. 

The fundamental resonance frequencies for a set of devices are illustrated graphically, along with 

one point representing the average flying Drosophila, in figure 5. As would be expected by 

analogy with a cantilever beam, there appears to be an inverse relationship between the span of a 

wing and its resonant frequency. Comparing designs of the same span, designs 1–3 vs. 4–6 (see 

table 1), it seems that the thickness of the elastic layer is positively correlated to the natural 

frequency. The elastic layer covers the entire device, actuator and wing planform. It appears 
6

that the trend in frequency response is dominated by the stiffening effect of the thicker 

actuator/wing verses the softening of a higher wing mass. These basic conclusions provide some 

design trends but additional experiments are required to further understand how to achieve the 

desired kinematics. 

1 1.5 2 2.5 3 3.5 4

0

200

400

600

800

1000

1200

1400

Span [mm]

Frequency [Hz]

LDV: Devise 1

LDV: Design 2

LDV: Design 3

LDV: Design 4

LDV: Design 5

LDV: Design 6

FEM: Devise 1

FEM: Design 2

FEM: Design 3

FEM: Design 4

FEM: Design 5

FEM: Design 6

Drosophila

Figure 5. Fundamental resonant frequencies verses span for PiezoMEMS 

micro-wings. 

The FE model consists of a structural model of the wing and actuators. The model is created 

with solid, three-dimensional, 10-node tetrahedral elements. The default element size is 

specified as 50 microns in length along the surface boundaries. The boundary conditions for the 

FE model designate that the base of the actuators are fixed in all degrees-of-freedom. The 

theoretical approximations do not currently account for all of the geometry, such as vein 

structures and residual backside Si, which contributes to the error between the theoretical and 

experimental results. The model also neglects aerodynamic loads, including an apparent mass 

term, which will contribute to a higher predicted frequency. 

Inspection of long exposure optical images, as seen in figure 6, illustrates the range of wing 

displacement during wing flapping. The out-of-plane deformation apparent in figure 6a is a 

result of residual stress in the device due to fabrication. This stress will be mitigated in future 

designs by implementing the procedures described in reference 4. The flapping motion in figure 

6b is driven by a sinusoidal 5 V peak-to-peak (p-p) excitation at the resonance frequency of 

156 Hz with all three actuators in phase. The flapping motion is a result of both active bending 

of the piezoelectric actuators at the wing root, as well as passive deformations of the elastic wing 

structure due to aerodynamics and inertial forces. Depending on whether measured at the 

actuator or wing tip, the displacement of the wing is between 17° and 44°, respectively. As most 

of the aerodynamic force is produced on the outboard sections of the wing, where the speed is 
7

the greatest, the larger value is more relevant. Although not clearly evident in the images, video 

of these wings makes it fairly apparent that little pitching of the wing is occurring. As noted 

earlier, the incorporation of this pitching motion will be a necessary condition of producing lift. 

(a) 

(b) 

Figure 6. Images of a wing (design 3, 2.5 mm length) (a) at 0 V 

and (b) at resonance (156 Hz, 5 V p-p), showing that 

resonant behavior amplifies the stroke.

Comparison of this flapping amplitude can be made against the model Drosophila fly. The wing 

in figure 6 appears in figure 5 (design 3, the closest to the Drosophila data point). As can be seen 

in figure 5, this device of roughly the same wing span matches closely with the flapping 

frequency of this average Drosophila. The average flapping amplitude for the Drosophila is 

about 163°. As this 44° of flap amplitude is barely a quarter of that seen in Drosophila, this wing 

is not demonstrating the needed displacements to match this species. A larger flap amplitude 

may be produced with higher input voltage or a different wing composition but the most 

appropriate design will probably not be the same as this insect given the material difference that 

exist between insects and robots. There is a great diversity in the frequency, span, and amplitude 

across different species (5) and finding an appropriate combination to suit the constraints of 

MEMS fabrication seems very probable from these results. 
8

Figure 7 illustrates the spatial location of a wing derived from images of the wing captured by 

the photogrammetry system. Once a series of images have been captured by the system, an 

optimization is performed to minimize, in a least squares sense, the difference between the 

identified targets in the images and the known locations of the targets on the wing. The variables 

optimized to minimize this difference include three coordinates and three Euler angles 

representing the wing position and orientation in space as well as the amplitude of several shape 

functions, which model out-of-plane bending and twisting. Comparisons of the distance are 

made between respective targets in the pixel space of the acquired images and of the targets on 

the three-dimensional wing transformed into each camera’s image space. Target identification 

has proven to be difficult. Improvements to address this difficulty will address the optics and 

lighting of the system and the wing designs to try to reduce glare and improve image quality, as 

well as the implementation of a more robust algorithm for performing target identification. Once 

these issues have been resolved, the technique will allow the rapid collection and processing of 

data that can be used for parametric evaluation of the performance of various designs.

(a) 

(b) 

Figure 7. Wing target triangulation showing (a) an example 

image of the wing and identified targets and (b) the 

triangulated surface “veins” and targets in space.
9

4. Conclusions 

This work reveals that it is possible to fabricate MEMS flapping-wings that can achieve strokeamplitude and flapping frequencies similar to insects, and that this technology has great potential 

to produce millimeter-scale flying robots. Many additional challenges still need to be overcome 

before a fully operational vehicle can be produced. Solutions will be needed for full system 

integration of sensors, electronics, and power supply, which pose significant challenges to the 

long term success of these devices. 

In the short term, additional characterization of the current devices is needed, including testing of 

the wing material properties and stiffness. Additionally, the wing design should be optimized to 

have structural rigidity values that more closely mimic those of insects. Another area of interest 

is reducing the stiffness of the wings to more closely match properties of insect wings while 

maintaining structural integrity and robustness. This effort has established the feasibility of 

achieving insect-comparable stroke amplitudes; however, force production in insects is due, in 

large part, to the unsteady aerodynamic effects associated with the full 3-D wing kinematics that 

include the angle of attack. The next step is to control the angle of attack of the wing during the 

wingbeat. The kinematic data will be used to calculate the aerodynamic loads produced to make 

determinations of the flight forces generated by the millimeter-scale wings. 
10

5. References 

1. Polcawich, R.; Pulskamp, J.; Judy, D.; Ranade, P.; Trolier-McKinstry, S.; Dubey, M. 

Surface Micromachined Microelectromechancial Ohmic Series Switch Using Thin-Film 

Piezoelectric Actuators. IEEE Trans. Microwave Theory and Tech. 2007, 55, 2642–2654. 

2. Wolf, P; Dewitt, B. Elements of Photogrammetry with Applications in GIS, 3rd Edition, 

McGraw Hill Companies, Inc.: Boston, MA, 2000, 233–256. 

3. Lehmann, F.; Dickinson, M. The Changes in Power Requirements and Muscle Efficiency 

During Elevated Force Production in the Fruit Fly Drosophila Melanogaster. The Journal of 

Experimental Biology 1997, 200, 1133–1143. 

4. Pulskamp, J.; Wickenden, A.; Polcawich, R.; Peikarski, B.; Dubey, M.; Smith, G. Mitigation 

of Residual Film Stress Deformation in Multilayer Microelectromechanical Systems 

Cantilever Devices. J. Vac. Sci. Technol. B 2003, 21, 2482–8486. 

5. Ellington, C. The Aerodynamics of Hovering Insect Flight. VI. Lift and Power 

Requirements. Phil. Trans. R. Soc. London, B 1984, 305, 145–181. 
11

6. Transitions 

The following two conference presentations have been accepted: 

Bronson, J.; Pulskamp, J.; Polcawich, R.; Kroninger, C.; Wetzel, E. Thin Film PZT Flapping 

Wing Actuators for Insect-Inspired Robotics. 22nd IEEE International Conference on 

MEMS, Sorrento, Italy, Jan 2009. 

Kroninger, C.; Pulskamp, J.; Polcawich, R.; Bronson, J.; Wetzel, E. Design and Characterization 

of Millimeter-Scale Flapping Wings. AHS International 65th Annual Forum & Technology 

Display, Grapevine, TX, May 2008. 
12

Acronyms 

Al aluminum 

ARL U.S. Army Research Laboratory 

Au gold 

CCD Charge-Coupled Device 

DRIE deep RIE 

FE finite element 

LDV laser Doppler vibrometer 

MEMS Micro-Electro-Mechanical System 

Ni nickel 

PECVD plasma enhanced chemical vapor 

Pt platinum 

PZT lead zirconate titanate 

RF radio frequency 

RIE reactive ion etch 

Si silicon 

SOI silicon-on-insulator 

Ti titanium 

XeF2 xenon difluoride 
NO. OF NO. OF 

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14

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