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Finding NEMO (novel electromaterial muscle oscillator): a polypyrrole powered robotic fish

with real-time wireless speed and directional control

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2009 Smart Mater. Struct. 18 095009

(http://iopscience.iop.org/0964-1726/18/9/095009)

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IOP PUBLISHING SMART MATERIALS AND STRUCTURES

Smart Mater. Struct. 18 (2009) 095009 (10pp) doi:10.1088/0964-1726/18/9/095009

Finding NEMO (novel electromaterialmuscle oscillator): a polypyrrole poweredrobotic fish with real-time wireless speedand directional controlScott McGovern1,2, Gursel Alici2,3,5, Van-Tan Truong4 andGeoffrey Spinks2,3

1 Intelligent Polymer Research Institute, AIIM Building Innovation Campus, University ofWollongong, Wollongong NSW, Australia2 ARC Centre of Excellence for Electromaterials Science, AIIM Building Innovation Campus,University of Wollongong, Wollongong NSW, Australia3 School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong,Wollongong NSW, Australia4 Maritime Platforms Division, Defence Science and Technology Organisation, Australia

Received 16 December 2008, in final form 17 April 2009Published 1 July 2009Online at stacks.iop.org/SMS/18/095009

AbstractThis paper presents the development of an autonomously powered and controlled robotic fishthat incorporates an active flexural joint tail fin, activated through conducting polymer actuatorsbased on polypyrrole (PPy). The novel electromaterial muscle oscillator (NEMO) tail finassembly on the fish could be controlled wirelessly in real time by varying the frequency andduty cycle of the voltage signal supplied to the PPy bending-type actuators. Directional controlwas achieved by altering the duty cycle of the voltage input to the NEMO tail fin, which shiftedthe axis of oscillation and enabled turning of the robotic fish. At low speeds, the robotic fish hada turning circle as small as 15 cm (or 1.1 body lengths) in radius.

The highest speed of the fish robot was estimated to be approximately 33 mm s1 (or 0.25body lengths s1) and was achieved with a flapping frequency of 0.60.8 Hz which alsocorresponded with the most hydrodynamically efficient mode for tail fin operation. This speedis approximately ten times faster than those for any previously reported artificial muscle baseddevice that also offers real-time speed and directional control. This study contributes topreviously published studies on bio-inspired functional devices, demonstrating thatelectroactive polymer actuators can be real alternatives to conventional means of actuation suchas electric motors.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Mobile platforms offer improved versatility for sensingsystems involved with environmental monitoring, pollutiondetection, video mapping, surveillance and other such tasks.For operation within aquatic environments, highly mobileswimming robots are an obvious form that can enable highmobility and versatility. While autonomous undersea vehicles

5 Author to whom any correspondence should be addressed.

(AUVs) in the form of miniature submarines already exist,a recent review of their performance [1] has highlightedtheir lack of low speed manoeuvring capabilities. Highlymanoeuvrable AUVs capable of hovering/station keeping andsmall radius turning at low speed are seen as ideal forunderwater inspections, particularly in confined spaces ordifficult to access areas.

Designing AUVs that mimic the swimming action of fishis one obvious way to improve their performance, as fishshow remarkable swimming abilities [2]. The wide range of

0964-1726/09/095009+10$30.00 2009 IOP Publishing Ltd Printed in the UK1

http://dx.doi.org/10.1088/0964-1726/18/9/095009http://stacks.iop.org/SMS/18/095009

Smart Mater. Struct. 18 (2009) 095009 S McGovern et al

Table 1. Summary of performance of actuator-driven robotic fish.

Description Top speed Dimensions/(mass) Drive voltage Reference

Konkuk University:ceramic piezo actuatorwith rack and pinionmechanism that flaps acaudal fin (various shapes)

2.5 cm s1 (0.09 bodylengths s1)

27 cm 5.0 cm 6.5 cm(550 g)

300 V (p-p) at 0.9 Hz(for max speed)

[16]

Michigan State University:IPMC operated caudal fin

0.63 cm s1 (0.027 bodylength s1)

23 cm 13 cm 6.5 cm(295 g)

3.3 V (p-p) at 2 Hz [11]

Kagawa University: atethered system using twoIPMC bending actuators(as parallel tails)


4.5 cm 1.0 cm indiameter (0.76 g)

5 V (p-p) at 1 Hz [17]

Hankuk AviationUniversity: untetheredsystem propelled with oneIPMC bender


9.6 cm 2.4 cm 2.5 cm(16.2 g)

5 V (p-p) at 4 Hz [9]

Auckland University:untethered systempropelled with onePPy.DBS bender


12.2 cm 3.5 cm 1.0 cm 1.6 V (p-p) at 1 Hz [12]

fish shapes and sizes highlight the many variables that affectfish swimming performance. Body size and shape; tail finsize and shape; size and placement of pectoral and dorsalfin(s) all affect the swimming speed and manoeuvrability.Different species of fish also produce propulsion throughdifferent combinations of movement of both the body and/orthe caudal (tail) fin [35, 2]. While still the subject of on-going research, it is clear that mimicking fish hydrodynamicsrequires a high degree of freedom of the flow control surfaces.In aquatic animals this control is afforded by their muscularsystem. For example, six major muscle groups controlthe pectoral fin on fish [6]. The same degree of freedomcannot be achieved in AUVs using conventional motorsbecause of size limitations. Bandyopadhyays analysis of fishhydrodynamics [1] concluded that mimicking the performanceof fish was most feasible by adopting multiple artificial muscleactuators with neuroscience based control.

Artificial muscles, or actuator materials, are attractivefor mobile robotics for a variety of reasons. As mentioned,actuator materials offer the ability to generate fish-likemovements by using multiple, small actuators to fine-tune bodyand fin movements. Because of their simple structure, artificialmuscles also provide the possibility of scaling down the sizeof the AUV. Small sized AUVs would be capable of enteringsmall spaces, expanding their surveillance capabilities to areassuch as pipe and tank inspections. Actuator materials canalso provide silent operation, which can be important to avoiddetection.

While a large number of actuating materials exist, wefavour the bending-type actuators that can be directly coupledto a tail or pectoral fin to produce a flapping action without anyother mechanical mechanism. Such a simple design will aid inthe future further miniaturization of the device. Bending-typepolymer actuators are available from piezoelectric materials,ionic-polymer metal composites (IPMCs) and conductingpolymers (CPs). The latter two materials operate at low

voltages, which can be an advantage for small mobile devicesas they can be powered directly from batteries.

A small number of prototype fish robot deviceshave already been demonstrated using artificial muscleactuators [712] and their performances are summarized intable 1. Most focus to date has been directed at producinguntethered systems that can achieve appreciable speeds. Thefastest speed reported to date is 2.5 cm s1 (or 0.245 bodylength s1), which is much lower than fish of similar size (forexample, the 140 g bluegill sunfish swims at 24 cm s1 [13],while the fastest swimming fish achieve up to 15 m s1 [14]).One prominent example of an artificial muscle powered roboticfish was a novelty aquarium product developed by EamexCorporation in Japan [15]. The small artificial fish wereneutrally buoyant and slowly moved about the aquariumpropelled by IPMC actuated tails. The system was wireless andpower was delivered to the fish via means of electromagneticinduction. Again, fish swimming speed was low and there wasno means provided for controlling the fish direction.

Most previous robotic fish using polymer actuators use asingle caudal (tail) fin to provide propulsion, which is knownas the ostraciiform swimming mode. However, fish propulsioninvolves a varied combination of movement of both the bodyand/or the caudal [35, 2]. Undulatory swimming utilizesmovements of the body to develop a wave displacement overthe entire or part of the fishs length to generate forwardpropulsion. In oscillatory swimming, the caudal fin movesabout its base without this body wave formation, and displaceswater generating forward thrust (figure 1). In terms of speed,manoeuvrability, acceleration and swimming efficiency, noone method of swimming excels in all of these areas. Forexample, acceleration and speed increases can be obtainedover the anguilliform method of swimming by better utilizingthe propulsive element of the caudal fin in the carangiformand thunniform modes of swimming, however, this benefit isachieved at the sacrifice of manoeuvrability. The ostraciiform

2


Figure 1. Various modes of swimming that are related to BCF propulsion [4].

mode has the lowest complexity (utilizing purely oscillatorymotions) [18] and as such is the easiest method of propulsionto mechanically implement and thus mimic in robotic fishdesigns.

Fish swimming efficiency is often assessed in terms ofthe dimensionless Strouhal number (St ) [19] that expresses therelationship between tail beat amplitude (a) and frequency (n)to the forward velocity (v):

St = nav. (1)

The product of tail beat amplitude and frequency is directlyrelated to the thrust force generated by the tail oscillation, sothe Strouhal number reflects how the tail thrust is converted toforward movement of the fish. Studies of various fish specieshas shown that the Strouhal number falls within the range0.2 < St < 0.4 [5]. Only one of the previously reportedIPMC-fish prototypes quoted a Strouhal number, which wasin the range 0.81.6. These high values suggest that furtherimprovements in actuator performance or fish body/tail designare needed to match the efficiency and speeds of real fish.

Significant improvements in CP actuator performancehave been reported recently, and these new formulationsnow offer the possibility for improving the speed andmanoeuvrability of robotic fish. Kaneto and co-workers haveshown that certain dopant ions and solvents enable very largelinear actuator strains (up to 40%) to be produced in thelength direction of films [2024]. These same formulationshave also been used in bending-type CP actuators and wereshown to increase the speed of response and amplitude ofbending [24]. The aim of the present study was to use thesehigh speed polypyrrole CP bending actuators in an untetheredfish swimming device and to assess the speed and efficiency.

2. Experimental details

The biorobotic fish design was based on previously reportedsystems and used a single caudal fin for propulsion andsteering. While probably not the optimal design to achievethe desired speed and manoeuvrability, this design allowsa direct comparison between a polypyrrole (PPy) propelledsystem with the previously reported CP and IPMC actuatedostraciiform swimming fish robots. The system used in thepresent study was designed to be simple to construct andoperate. In particular, the ability to readily change batteriesand actuator elements was incorporated into the fish design.

Bending-type PPy actuators were fabricated as previouslydescribed [24]. Polypyrrole (PPy) conducting polymer was

electrochemically deposited onto either side of a platinum-coated poly(vinylidene fluoride) membrane (PVDF) 100 mthick with a 0.45 m pore size (Immobilon Millipore)to manufacture a stand-alone actuator with a tri-layerconfiguration [7, 25, 24]. The advantage of this configurationis that both the working and counter electrodes (consisting ofeach of the individual PPy layers) are self-contained within thesystem and form the boundaries of the electrochemical cell(the PVDF separator). When fully wetted with electrolyte,these actuators may work stand-alone in air or other media andminimize the overall size of the device as the PVDF separatoris paper thin (100 m) and acts as a reservoir to hold theelectrolyte.

Extremely fast actuators have been realized with the useof bis(trifluoromethane sulfonimide) (TFSI) as dopant for thePPy [23, 24]. To form the PPy.TFSI, pyrrole monomer waselectropolymerized onto sputter-coated PVDF with an appliedcurrent density of 0.1 mA cm2 for 12 h at a temperatureof 33 C from a propylene carbonate (PC) solution thatcontained 0.1 M pyrrole monomer, 0.1 M lithium TFSI and1 w/w% water.

One design challenge encountered was to make a fish bodyto house the electronics that was waterproof but also enabledready access to the electronics and battery. Fully encapsulatedsystems provide good water-proofing, but do not allow easyaccess to the electronics for replacement of parts or batteries.A simple solution used in our prototype system, was to housethe control unit and the battery in a 25 ml syringe that wascut down and adapted to facilitate a tail fin for propulsion andpectoral and dorsal fins for stability. Further, the control unitand the battery are deliberately placed at the lower section ofthe syringe to ensure that the centre of gravity of the prototypewas below the centre of buoyancy to provide inherent stability,preventing any tilt unconditionally, during operation in liquid.The end of the syringe was plugged and the fixed pectoraland dorsal fins were attached with epoxy adhesive. Outputwires to power the actuators were attached to the electronicsboard and passed through a small hole in the fish body, whichwas also plugged with epoxy. The length of these wireswas made to enable complete removal of the electronics andbattery compartment from the fish body for any necessary lateradaptations. A photo of the apparatus may be seen in figure 2.The complete fish prototype was 20 mm in diameter125 mmin length and weighed 16.2 g.

The wireless capacity of the robot prototype was providedthrough a SCTX2B/RX2FS transmitter and receiver unitchips (IPS Japan). These units were contained in a boardthat produced a pulse width modulated constant 1 V

3


Figure 2. Photograph of the robotic fish prototype.

power supply to the tri-layer polymer actuators such that thefrequency of forward and backward bending of the actuator(causing a flapping of the tail) could be achieved withmanipulation of the forward and backward controls on theexternal transmitter unit (figure 3).

The final design challenge was the need to easily replacethe polymer actuators if they became damaged during thetesting phase. In our previous study [7], silver paste with aprotective epoxy adhesive was used to provide connections tothe polymer actuators through platinum wires. That previoussystem was non-serviceable making actuator replacementslow. For the current design, we introduce small (5 mmdiameter, 1.5 mm thick) 40 neodymium magnets with anickel coating that were soldered to the circuits output wiresto act as a removable clip to both hold the actuators inplace and provide the electrical connection. The neodymiummagnets are extremely powerful and can enable a strongbut detachable gripping force to the actuators and the nickelcoating helped provide corrosion resistance between theactuator and conducting surface. The magnets were housedin cavities in the adapted plunger of the syringe.

This connection system was used to attach the novelelectromaterial oscillator (NEMO) tail fin (used to propel therobotic fish). The tail was constructed from a sheet of highimpact polystyrene approximately 150 m thick. The finmaterial was chosen for its low weight and high stiffness(Youngs modulus 2 GPa) and a simple truncated shapewas chosen to simulate more closely a natural fin shape seenin aquatic environments (figure 4). Two actuators were cut

Figure 4. Schematic showing the fish fin dimensions and actuatorplacement on the NEMO.

25 mm long and 3 mm wide and were attached to the tailfin with Kapton tape (3M). The actuators were positionedon the fin with a spacing of 10 mm, allowing for a 10 mmoverlap for attachment to both the fin and the magnet clamps.As such, the NEMO was attached to the fish body via themagnet attachment to the actuators. This design allowed forfree bending of a 5 mm section within the centre of eachactuator upon application of electrical stimuli. Future studieswill consider in more detail the effects of tail shape and size,and actuator placement.

3. Results

The operation of this NEMO structure on the fish body wasinvestigated first in air and later in water to determine the effectthat the frequency of oscillation has on the sweep angle travel.These studies were used for the analysis of the resulting speedand manoeuvrability of the fish.

3.1. Operation of the NEMO in air

Operation of the NEMO tail fin in air was extremely responsiveand a large deflection of the fin tip was achieved upon

Figure 3. Electronics design of the fish robot prototype.

4


Figure 5. Peak to peak fin tip oscillation amplitude of the NEMOoperating in air and when immersed in propylene carbonate (PC).

excitation with the square wave voltage signal. For example,the application of a 1 V 1 Hz square wave signal wasseen to generate a peak to peak fin tip oscillation amplitudeof approximately 35 mm. The largest oscillation amplitudesoccurred at excitations using the lowest test frequencies (0.51 Hz), as shown in figure 5. The fin tip amplitude tendedto decrease when operated at higher operating frequencies,however a distinct peak in oscillation amplitude was observedat approximately 4.5 Hz (figure 5).

At the peak frequency the oscillation amplitude ofapproximately 35 mm is equivalent to that obtained when thedevice is operating at 1 Hz. This phenomenon relates to theresonance effect that occurs when the system is operated at itsnatural frequency. Previous studies of tri-layer PPy bendingactuators of similar length gave a resonance peak at 42 Hz [24].The natural frequency of the NEMO (polymer actuators + fin)is expressed as:

n =

3E I[(33

140

)mb + m

]L3

(2)

where n = natural frequency, E = elastic modulus of thecomposite actuator structure, I = area moment of inertia ofthe composite actuator structure, mb = mass of the NEMOactuators, m = mass of the NEMO tailfin and L = totalNEMO length (25 mm + 5 mm).

From this relation it can be seen that a tuning of thefrequency in the NEMO may be possible by changing themass and length of the attached tail fin. If the fin was madeeither shorter or lighter, an increase in the natural frequencyshould be observed. The effect of such geometry modificationson the net thrust force generated is the subject of on-goinginvestigations [26].

3.2. Operation of the NEMO in liquid

The NEMO tail fin was immersed in water to see the effect thatliquid dampening had on the oscillation amplitude of the tailfin at different operation frequencies. It was observed on theinitial attempt that the tail fin oscillation amplitude diminishedvery rapidly such that there was little or no movement afterapproximately 60 s operation. This degradation in oscillation

was due to a solvent exchange between the PC electrolytewithin the PVDF separator membrane and water within thetest bath, affecting such factors as the ionic conductivity andstiffness of the actuator.

Various encapsulants were investigated in an attempt tominimize or eliminate the electrolyte leaching with variedsuccess. For example, encapsulation of the actuators withinadhesive Scotch tape stopped this degradation, but significantlyincreased the stiffness of the tri-layer system and dramaticallyreduced the tip displacement amplitude. The best systemidentified to date was petroleum jelly applied to the outside ofthe actuators. This material formed a barrier coating betweenthe PC electrolyte and water without unduly increasing thestiffness of the actuator and reducing the oscillation amplitude.The coating enabled a stable performance with little or nodegradation within the first 10 min of operation, after whicha slow reduction in the oscillation amplitude occurred. Thesearch for longer lasting, flexible encapsulants is the subjectof on-going research. As such, further investigations into theliquid dampening of the NEMO were undertaken in a reservoircontaining a solution that was the same as the electrolyte withinthe actuatorsi.e. 0.1 M Li TFSI in PC, to ensure that amore accurate representation of the true oscillation amplituderesponse to frequency could be obtained.

The frequency response of the NEMO actuated fin in PC isshown in figure 5. It can be seen that operation in PC gave tipamplitudes significantly lower at all frequencies compared withoperation in air. The resisting force of the surrounding liquidsignificantly slows the bending of the tail fin so that smallerfin tip amplitudes were produced. The natural frequency of theNEMO was an order of magnitude lower when operated in PCcompared to air, such that the greatest amplitude of oscillationwas at approximately 0.4 Hz in PC. There was a very fast decayin the magnitude of the fin tip oscillation as the frequencywas increased such that when operating at 2 Hz the peak topeak fin tip oscillation was only 2 mm. The properties of thesurrounding liquid can be important factors to consider whenattempting to predict the effect of damping on the frequencyresponse of the NEMO. As the density of propylene carbonateis higher than water at 1.2 g ml1, this will have a dampingeffect on the frequency response of the fin. The higher is thedamping effect, the smaller is the resonant frequency of the finassemblyNEMO. This follows that the resonant frequencycan be shifted towards the left direction. Further, an increaseddamping effect will decrease the magnitude of the response atthe resonance frequency. On the other hand, the viscosity ofpropylene carbonate (2.5 cP) [27] is only slightly higher thanthat of water (1.0 cP) and is not expected to impart significantoperational changes in the fin response.

3.3. Operation of the robotic fish

Investigations were undertaken into the effectiveness of theNEMO for propulsion of the fish. A NEMO powered roboticfish prototype was placed in a tank 40 cm 60 cm thatwas filled with water to a depth of approximately 10 cm(figure 6). Petroleum jelly was used as an encapsulant forthe actuators and all measurements were taken within the first

5


Figure 6. A NEMO powered robotic fish being tested.

few minutes of operation to ensure that degradation in actuatorperformance due to leaching did not affect the results. The fishwas sufficiently buoyant that the dorsal fin extended above thewater surface. The entire NEMO fin was submerged.

The aim of the fish tests was to determine the operatingfrequency that generates maximum forward speed. It wasfound that operation at low frequencies (0.2 Hz or less)caused the actuator movement on the NEMO to becomedisabled. That is, the actuators bent in one direction butwere unable to bend back in the other direction. A similarphenomenon was encountered by Alici et al [7] where acurling of the actuators was seen to occur under heavy load.When the tri-layer actuator movement was hindered by a load,yet the voltage was still applied, the continued expansion ofthe polymer initiated a bending about the longitudinal axis(figure 7(b)). This curling had the effect of changing theflat plate design of the tri-layer actuator to be more tubular,significantly increasing the rigidity of the structure. Theincreased rigidity meant that the actuator could not bend backin the other direction when the voltage was reversed untilthe film had uncurled. The onset of these curling processessignificantly reduced the frequency of the tail fin oscillation.The actuator curling was seen to occur with oscillationfrequencies 0.2 Hz, and as such, an oscillation frequencyof 0.3 Hz was set as the minimum frequency for operation ofthe fish device. Under such conditions the problem of curlingwas eliminated, and traditional bending of the actuators wasreinstated (figure 7(a)).

Several different frequency regimes 0.3 Hz wereinvestigated. Low tail fin frequencies of approximately 0.30.4 Hz (enabling large flapping amplitudes) were utilized toobtain the maximum initial thrust. If once the fish began tomove, the frequency of oscillation was increased to 0.75 Hz(after 3 or 4 full cycles), the fish was able to obtain a top speedof approximately 3.3 cm s1 (or 0.25 times its body length persecond) after 30 cm of travel. With this regime, the cruisingspeed is higher than all other previously described polymer-actuator-propelled fish robots.

Interestingly, the highest cruising speeds were obtainedby operating the NEMO tail fin at frequencies higher thanthat which produced the peak flap amplitude, as previouslydetermined (figure 5). At lower frequencies, when the tailfin oscillation was highest it was found that the fish noseturned excessively, thereby limiting the forward speed. The

(a) (b)

Figure 7. Typical bending of the tri-layer actuators that occurs in (a)normal bending conditions under low load, and (b) under high loadsat large tip displacements.

propulsion produced by the tail fin must overcome the dragacting on the fish, which can be expressed by:

FD = 12 CDSV 2 (3)where: FD = drag force, CD = co-efficient of drag, =density of the fluid, S = frontal surface area of the body andV = velocity of the body through the fluid. Oscillation ofthe tail will cause the fish to accelerate until the point that itsvelocity causes the drag force to equal the thrust force. Atthis point, the fish will no longer accelerate, but will cruise ata constant velocity. While the magnitude of the thrust forcewill be determined by the amplitude of tail oscillation, so willthe direction of the thrust (figure 8). Large tail fin amplitudesproduced a turning moment on the fish body since the thrustforce contains a larger component in the transverse direction totravel (x-direction in figure 8). As the fish nose turns (figure 9),the effective frontal surface area (S) exposed to the liquid isincreased, which in turn causes the drag force to increase. Assuch, the maximum cruise speed obtainable is not as high foroperation at low fin frequencies as for high fin frequencieswhere the nose did not oscillate as much.

3.4. Directional control of the robotic fish

The transverse force described above could be used fordeliberate turning of the fish. In particular, the direction oftravel could be controlled by altering the duty cycle of thevoltage input to the NEMO. With an even oscillation aboutthe fish neutral axis (50% duty cycle), there is a net forwardtravel without turning. By changing the axis of oscillation,it was found that after each complete oscillation cycle, theresultant thrust force of the tail fin will contain a net xcomponent that causes a moment on the fish body enablingturning (figure 8(C)). Changing the duty cycle of the voltagesupply to the NEMO (giving a longer time at +1 V than at1 V) produces a greater bending of the tri-layer actuators toone side, and in turn shifts the axis of oscillation away from theneutral axis of the fish body.

The true responsiveness with turning of the fish wasdifficult to quantify within the small confines of the testing

6


Figure 8. Schematic diagram showing typical oscillation amplitudes of the NEMO at both (A) low and (B) high frequencies, and (C) with achange in the axis of oscillation that can enable turning.

Figure 9. Top view schematic illustration showing how the NEMO tail fin oscillations cause a turning of the nose of the fish.

tank. It was found that turning could be initiated withapplication of only small changes in the duty cycle to shiftthe axis of oscillation. For example, it was estimated thatthe turning circle of the fish at its top speed had a radius ofapproximately 30 cm (or 2.2 body lengths) by using a dutycycle of approximately 65% for 3 cycles. In this example, thefish was returned to forward propulsion without any turningby utilization of a duty cycle of 35% for a further 3 cycles.Tighter turns could be achieved at slower speeds by utilizinga larger duty cycle of approximately 75%, again for only2 or 3 cycles. The tightest turning circle had a radius ofapproximately 15 cm (1.1 body lengths) and was enabled ata forward speed of 10 mm s1. While these turning radii arelarger than found in some fish species (


Figure 10. Configuration of the body, actuators, and fin of theswimming device (not to scale). Fbdrag, F

fdrag, and Fthrust are the body

drag force, fin and actuator drag force, and the thrust force acting onthe fin, respectively. The actuator force F is reflected on thegeometric centre of the fin as F . L and L are 5 and 12.5 mm,respectively.

acting on the device consists of the body drag force and thecaudal fin drag force, where each drag force component is asummation of the form drag (due to the shape of the object) andskin friction drag (that is related to the viscous forces acting onthe surface of the object). If we approximate the body shapeto that of a cylinder, the form drag can be easily determinedfor forward motion without turning of the nose by usingequation (3), and taking the drag coefficient CD 0.81 as thatof a cylinder in axial flow [29]. The form drag force may beestimated as 0.138 mN. However, determination of the frontalsurface area upon turning of the nose of the fish (that occursduring normal swimming) is more difficult calculation that ismade easier using finite element packages such as ANSYS.Likewise, ANSYS may also be used to easily calculate the skinfriction drag acting on the surface at a given speed. The fishbody was generated in ANSYS with a mesh size of 3 mm andthe drag force acting on the body was calculated for movementat its maximum velocity of 33 mm s1 (figure 11(a)).

Using ANSYS, the form drag acting on the nose wascalculated as 0.146 mN and the total body drag force (includingskin friction drag) was estimated to be 0.303 mN. However,when the swimming device turns to one side by 15, this totaldrag force increases to 0.503 mN. Thus the orientation of theswimming body relative to the flow direction increases the dragforce significantly. The maximum drag force acting on therigid body is Fbdrag = 0.503 mN.

The form drag force acting on the caudal fin and thepolymer actuators may also be calculated using equation (3).The bending angle ( 2 ) of the fin was approximately 15

duringthe 33 mm s1 movement of the device in the water. Thismakes the frontal surface area of the fin and the two polymeractuators = 169.52 mm2. If we take the drag coefficient

CD for an oscillating plate as 0.75 [28], then the form dragacting on the fin is estimated as F fdrag = 0.0693 mN. The dragforce acting on the NEMO was again verified using ANSYS(figure 11(b)) and including skin friction drag, the maximumdrag was found to be 0.0782 mN. Hence, the total drag forceacting on the fish travelling at the top speed is calculated as thesum of the body drag and the fin drag to be 0.581 mN.

Two polymer actuators are used to generate the necessarythrust force to propel the device. The force generated bya bending-type polymer actuator with the dimensions of25 mm 3 mm 0.16 mm is approximately 1.3 mN under1 V [33, 34]. The total force generated by two polymeractuators is F = 2.6 mN. This force acts at the tip of thepolymer actuators and the reflection of this force onto thecentre of the caudal fin is F = ( L

L+L ) F = 2.229 mN. Thisforce acts perpendicularly onto the centre of mass of the fin, asdepicted in figure 10, and it follows that the net thrust forcefor propulsion is Fthrust = 2.229 sin( 2 ) = 0.577 mN.This estimated thrust force matches very closely with the totalestimated drag force calculated at a velocity of 33 mm s1.The analysis successfully predicts the cruising behaviour of thedevice and highlights that in order to increase the swimmingspeed, either the force generated by the actuators should beincreased or the total drag force acting on the device should bedecreased. Both are part of on-going investigations.

These force analysis results are in agreement with ourprevious research on experimental and theoretical performancecharacterization of polymer actuators, and show that polymeractuators can supply sufficient force to activate functionaldevices [30, 31, 25].

4.2. Mechanical efficiency of the NEMO device

The mechanical efficiency of the NEMO device to produceforward propulsion has been evaluated by the Strouhal number,given by equation (1). The Strouhal number represents theratio of unsteady to steady forces generated in the wake leftbehind bodies moving through fluids. St indicates how oftenthe reverse Karman vortices (that produce thrust) are beinggenerated within the wake of the flapping tail fin, and howclose they are to each other. Too low a Strouhal number(


Figure 11. Configuration of the body, actuators, and fin of the swimming device (not to scale). Fbdrag, Ffdrag, and Fthrust are the body drag force,

fin and actuator drag force, and the thrust force acting on the fin, respectively. The actuator force F is reflected on the geometric centre of thefin as F . L and L are 5 and 12.5 mm, respectively.

Table 2. Experimentally determined fin displacement amplitudes (a)obtained at different operating frequencies (n) and frequencyamplitude product (na), which is related to thrust force.

Frequency (n) Tail amplitude (a) na

0.3 28.5 8.60.4 29.5 11.80.5 24.5 12.30.6 17.0 10.20.7 13.5 9.50.8 11.0 8.80.9 8.5 7.71.0 7.5 7.5

at lower frequencies (table 2), the fish forward speed wasnoticeably lower than when operated at 0.70.8 Hz.

Thus, the Strouhal number at these frequencies would beoutside the optimal range, due to the increased drag forceassociated with the turning of the fish nose. To convertthe larger actuator movements produced at lower operatingfrequencies, it is probable that the fish design should bemodified away from the single caudal fin ostraciiform style ofswimming and towards the thunniform style characteristic offast swimming fish.

5. Conclusions

A prototype robotic fish device powered using polypyrroleartificial muscles and that embodies autonomous real-timecontrol over the devices speed and direction of swimminghas been demonstrated. The study has concentrated on thecharacterization of a novel electromaterial muscle oscillator(NEMO) tail fin propulsor, used to generate forward movementon the robotic fish device. The maximum forward speed of3.3 cm s1 is the fastest reported speed for robotic fish poweredby polymer actuators. The smallest turning radius of therobotic fish was 15 cm, or 1.1 body lengths. At its maximumspeed, the robotic fish operated at a Strouhal number of 0.28,which is within the optimum range identified from studies ofseveral fish species.

While the prototype used was based on previouslyreported ostraciiform swimming robots powered by a singlecaudal fin, some aspects of this design were observed to limitswimming speed. In particular, it was observed that large tipamplitudes caused excessive turning of the fish nose away fromthe desired direction of travel. Nose turning increased the dragforce and slowed fish speed. Hence, operating at larger tailamplitudes was counter-productive.

A major limitation of the NEMO powered tail fin wasthe deterioration in its performance when immersed in water.Better encapsulating materials for the actuators are needed thatdo not unduly stiffen the actuator and limit its bending. Furtheroptimizations in the fin design and shape would also improvethe thrust force generated from the NEMO and a streamliningof the body shape would reduce drag on the system, allowing amuch greater net propulsive force leading to faster movement.Miniaturization of the electronics can reduce weight in thedevice, improving acceleration rates and/or enabling the fishrobot to carry other electronics including sensors. Such adevice could see wide spread incorporation into autonomousenvironmental sensing devices.

Acknowledgments

The authors would like to thank Drs Stephen John andYanzhe Wu for their support in engineering design anddevelopment. Rahim Mutlu is also thanked for the ANSYSanalysis. The authors would also like to acknowledgethe financial support from the ARC Centre of Excellencefor Electromaterials Science (CE0561616) and the DefenceScience and Technology Organisation (DSTO).

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1. Introduction2. Experimental details3. Results3.1. Operation of the NEMO in air3.2. Operation of the NEMO in liquid3.3. Operation of the robotic fish3.4. Directional control of the robotic fish

4. Discussion4.1. Force analysis of swimming device4.2. Mechanical efficiency of the NEMO device

5. ConclusionsAcknowledgmentsReferences

Documents

finding nemo.pdf