The Development of a Miniature Mechanism for Producing Insect Wing Motion

The development of a miniature mechanism for producing insect wing motion

S. C. Burgess, K. Alemzadeh & L. Zhang Department of Mechanical Engineering, Bristol University, Bristol, UK

Abstract

Insects are capable of very agile flight on a small scale. If a man-made machine can be built that can fly like an insect, it would have important industrial, civil and military applications. Insects have complex wing motions including non- planar wing strokes and stroke reversal. This paper presents the design of a novel mechanism which can produce insect type wing motion. The mechanism is very light and compact and has potential for use in micro air vehicles. A prototype has been built and some preliminary tests have been carried out to characterize the mechanism performance. Keywords: insect flight, wing motion, novel mechanisms, wing reversal, lift.

1 Introduction

The ability for controlled flight has existed in at least four classes of creature: (1) birds; (2) mammals (bats); (3) reptiles (pterosaurs); and (4) insects. Pterosaurs are believed to have been the largest fliers with wingspans up to 12m. It is thought that pterosaurs were capable of flapping flight because fossil records show a similar type of shoulder joint to that found in birds. Birds currently have a large range of size from the large albatross with a wingspan of up to 3.5m, to the tiny bumble hummingbird which has a span of only 8cm. Most birds can perform flapping flight but not true hovering flight. However, hummingbirds have a flexible shoulder joint which enables them to hover in windless conditions. Bats have a relatively small range of size. Bats can perform flapping flight and some are capable of hovering flight. Insects are the smallest flying creatures with current sizes ranging from a few millimetres to several centimetres. Insects can perform flapping flight and hovering flight and are the most agile fliers. There is currently much research

Design and Nature II, M. W. Collins & C. A. Brebbia (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-721-3 238 Design and Nature II activity involved in producing a machine which can fly like an insect [1, 2, 3]. Cameras are now so small that insect sized air vehicles could carry sophisticated surveillance equipment. It is believed that small agile flying machines could have important applications such as search and rescue, remote observations (nuclear plant, volcanoes, etc) and surveillance (security and warfare).

2 Insect wing motion

2.1 Wing structure

An insect body has three parts: head, thorax and abdomen. The wings are attached to the thorax as shown in Figure 1. Insects are different to birds, bats and pterosaurs in that there is no skeleton and no discrete shoulder joint. In the case of insects there is just an elastic hinge where the wings join the body [4]. Another key feature of insect flight is that wings flap at a resonant frequency. At Bristol University, a vision system is being developed which will be used to attempt to digitally capturing the insect wing and hinge shape.

Elastic energy Elastic energy Notal hinge stored in muscle stored in hinge

Wing

Dorso ventral Dorso ventral muscle contracted muscle relaxed

Figure 1: Cross section of the thorax of a fly.

2.2 Flight lift mechanisms

Like hummingbirds, insects use the aerofoil effect in the down stroke and upstroke to generate lift. The leading edge of the wing is kept upwards because of wing reversal at the end of each stroke as shown in Figure 2. The aerofoil has an inclined angle as it cuts through the air. This profile and orientation of the wing are such that the air is travelling faster over the upper surface and therefore upwards lift is produced. Insects also gain lift from the rapid rotation of their wings at the end of each stroke whilst the wing still has linear motion. This latter mechanism is called the Magnus effect and is shown in Figure 2. Another lift producing mechanism is that of a leading edge vortex. A leading edge vortex also occurs when the insect sweeps its wings forwards at a very high angle of attack. In the case of an aircraft or bird, such a high angle of attack leads to stalling. However, in insects, the high angle of attack produces a leading edge

Design and Nature II, M. W. Collins & C. A. Brebbia (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-721-3 Design and Nature II 239 vortex which sits on top of the leading edge of the wing as shown in Figure 3. The leading edge vortex is a region of rapidly circulating air with a low pressure core and a helical structure. Because of the forward motion of the wings, the circulating air in the vortex produces lift due to the Magnus effect.

Down stroke

Rotation Rotation

Up stroke

Figure 2: Schematic of wing motion in hovering flight.

Figure 3: Schematic of leading edge vortex.

There are two basic wing motions in typical insect flight: (1) Flapping (rowing) motion; and (2) Wing twisting (for wing reversal). The flapping motion is not normally in one plane but traces out either a figure-of-eight or other path that prevents the wing form continually moving in its own wake. There are several adjustments that insects can make during flight: (1) Angle of flapping plane; (2) Angle of attack of wings; (3) Frequency of wing beat; and (4) Phase shift between wings. The angle of the flapping plane is adjusted in order to change forward speed or to hover. The angle of attack of the wings is adjusted to change the amount of lift. The frequency of wing beat can also be adjusted to

Design and Nature II, M. W. Collins & C. A. Brebbia (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-721-3 240 Design and Nature II change the lift. A phase shift between the two wings is produced to perform manoeuvres such as turns.

3 Comparison between man-made and natural flight mechanisms

Insect mechanisms are very different to the mechanisms normally found in man- made machines. It may be that normal man-made type mechanisms are inappropriate for producing insect flapping flight. If this is the case, it is very important to understand how and why insect mechanisms are different. Table 1 compares the basic characteristics of mechanisms found in aircraft, birds and insects. In the case of aircraft wings, the mechanisms that move the ailerons and other flaps are considered in Table 1. In the case of birds and insects, the mechanisms that move the wings are considered in Table 1. One key difference between man-made mechanisms and natural mechanisms is that man-made mechanisms are generally constrained to have a fixed motion. This fixed motion requires relatively simple actuators and control algorithms. In contrast, birds and insects generally have complex 3-D under-constrained mechanisms that require very complex actuators and control algorithms. One advantage of putting complexity into the control algorithms is that complex information and processing dos not require much space or weight. Another weight-saving aspect of insects is that they have simple flexible hinges.

Table 1: Comparison between man-made and natural flight mechanisms

AIRCRAFT BIRD INSECT AILERONS WINGS WINGS Mechanism motion Rigid Rigid Flexible 2-D 3-D 3-D Constrained motion Yes No No

Bearings Rolling Sliding Flexible

Actuator Linear/rotary Linear Linear

Control algorithms Simple Complex Very complex

4 Design of novel mechanism

Figure 4 shows a novel flapping mechanism developed at Bristol University which is capable of insect type flapping motion. The design consists of an oar which can produce flapping motion as well as twisting motion. The oar is driven by a crank mechanism via a link which has a ball joint at each end. A key feature

Design and Nature II, M. W. Collins & C. A. Brebbia (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-721-3 Design and Nature II 241 of the mechanism is that it does not have constrained motion. The motion depends on the relative resistance to motion around bearings X and Y. The resistance to motion is less in bearing Y than in bearing X and therefore rowing motion occurs in preference to twisting motion. The resistance to motion is dependent on the location of the centre of pressure on the wing. Figure 5 shows how the centre of pressure is below the wing axis and this produces more resistance to motion around bearing X than bearing Y. When the rowing motion reaches an end-stop then twisting motion takes over. Figure 5 shows a schematic of how the mechanism works for hovering flight. Images 1-6 show the wing being dragged in pure rowing motion in the forward direction. Since the leading edge is pointing upwards, lift is generated by the normal aerofoil effect in the upwards direction. At the end of stroke there is wing reversal from 5-8. During these stages the wing is still moving in a linear direction but it is also twisting. Since airflow is from right to left and the wing is twisting anti-clockwise, lift again occurs in the upwards direction. Images 9-13 show upward lift being produced again by the aerofoil effect in the backwards direction. Images 13-16 show upward lift being produced by the Magnus effect. It should be noted that Figure 4 shows the wing in position 13. The mechanism is relatively simple because it does not have constrained motion. A limitation of the mechanism is that there is only a certain amount of adjustment that can be made to the timing of the twisting motion. Another limitation of the device as shown is that it has planar flapping motion. However, non-planar motion such as banana shape could be produced by constraining the path of point c.

Lift

Bearing X

a Bearing Y

Centre of pressure c (Position 13)

Wormwheel

Ball joint Worm

Figure 4: Schematic of novel flapping mechanism.

b 1 2 3 4 5 6 7 8 c

1 2 3 4 5 6 7 8

16 15 14 13 12 11 10 9

Figure 5: Schematic of motion.

Figure 6: Prototype model of the flapping mechanism.

5 Prototype build and test

A prototype model of the novel flapping mechanism has been produced as shown in Figure 6. The demonstration mechanism is about twice the scale of a large insect. An electric motor is used to drive a worm gear set and a crank arm is used to drive the wing shaft. The crank arm has a ball joint at each end. Plain bearings are used for the wing shaft. It was found to be necessary to guide the root end of the wing shaft because bearing Y was found to be insufficient for supporting the wing. The guide shown in Figure 6 gives planar wing flapping. However, it would not be difficult to produce a guide that gives non-planar wing flapping motion. The prototype model was tested in order to determine the wing beat frequency as a function of power consumption. In order to perform this test the mechanism was driven with a power supply with adjustable voltage. The voltage was systematically increased and the current was measured at each voltage level. Figure 7 shows test results for an insect type wing. As expected, the power consumption increases rapidly with wing beat frequency. The reason for this is that the profile drag increases with the square of velocity.

3 Power W 2

0 00.511.522.53

Wing beat frequency, Hz

Figure 7: Power versus wing beat frequency.

6 Conclusions

One key difference between man-made mechanisms and natural mechanisms is that man-made mechanisms are generally constrained to have a fixed motion. This fixed motion requires relatively simple actuators and control algorithms. In contrast, birds and insects generally have complex 3-D under-constrained mechanisms that require very complex control algorithms. One advantage of putting complexity into the control algorithms is that complex information and

Design and Nature II, M. W. Collins & C. A. Brebbia (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-721-3 244 Design and Nature II processing does not require much space or weight. Another weight-saving aspect of insects is that they have simple flexible hinges. A novel mechanism has been developed at Bristol University which can produce insect type wing motion. The mechanism is very light and compact and has some potential for use in micro air vehicles. A prototype model has been built and some preliminary tests have been carried out to characterize the mechanism.

References

[1] Jones, K.D. and Platzer, M.F., On the design of micro air vehicles, First International Conference on Design and Nature, Udine, Italy 10-12 September 2002, pp 67-76. [2] Zbikowski R, Current Research on Flapping Wing Micro Air Vehicles at Shrivenham, Proceedings, NATO RTO AVT Panel Business Meeting Week, Ankara, Turkey, 9-13 October 2000. [3] Zbikowski R, Emergent Flapping Wing Technology for Micro Air Vehicles (MAVs), Proceedings, UV2000 Conference, London, UK, 19-21 July 2000. [4] Chapman, R.F., The insects, Cambridge University Press, 1998.