Aurelia aurita bio-inspired tilt sensor

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Smart Mater. Struct. 21 (2012) 105015 (5pp) doi:10.1088/0964-1726/21/10/105015

Aurelia aurita bio-inspired tilt sensor

Colin Smith, Alex Villanueva and Shashank Priya

Center for Energy Harvesting Materials and Systems (CEHMS), Bio-Inspired Materials and DevicesLaboratory (BMDL), Virginia Tech, Blacksburg, VA 24061, USA

E-mail: spriya@vt.edu

Received 26 March 2012, in final form 17 July 2012Published 20 August 2012Online at stacks.iop.org/SMS/21/105015

AbstractThe quickly expanding field of mobile robots, unmanned underwater vehicles, and micro-airvehicles urgently needs a cheap and effective means for measuring vehicle inclination.Commonly, tilt or inclination has been mathematically derived from accelerometers; however,there is inherent error in any indirect measurement. This paper reports a bio-inspired tiltsensor that mimics the natural balance organ of jellyfish, called the ‘statocyst’. Biologicalstatocysts from the species Aurelia aurita were characterized by scanning electron microscopyto investigate the morphology and size of the natural sensor. An artificial tilt sensor was thendeveloped by using printed electronics that incorporates a novel voltage divider concept inconjunction with small surface mount devices. This sensor was found to have minimumsensitivity of 4.21◦ with a standard deviation of 1.77◦. These results open the possibility ofdeveloping elegant tilt sensor architecture for both air and water based platforms.

(Some figures may appear in colour only in the online journal)

1. Introduction

Many tilt sensors have been developed over the years usinga variety of macro- and micro-patterning methods combinedwith top-down assembly. Recently micro-electromechanicalsystems (MEMS) technology has been applied to thefabrication of tilt sensors that have opened the pathway formany different inclinometer technologies. Constandinou andGeorgiou (2008) demonstrated a sensor composed of onestationary and one moving part. The moving semicircularmass projects a shadow onto an array of optical sensors. Thesensor had a 5◦ resolution and up to 300 total degrees oftilt. Jung et al (2007) described an electrolytic version with aMEMS micromachined cavity and a moving electrolyte. Thedegree of tilt determined how much of the solution contactsthe electrodes, producing a varying electrical signal. Tang et al(2009) produced a structure that responds directly to gravitythrough the bending of extremely small piezoelectric beams.The sensor was able to achieve a sensitivity of 0.025 mV perdegree. A novel device has been proposed in the literature thathad a single MEMS heater with a silicon bridge to sense thetemperature by changes in resistance on either side (Billat et al2002). Due to free convective flow, when the tilt of the sensorchanges so does the temperature felt by each silicon bridgethermometer (Billat et al 2002). However, all these artificial

technologies ignore the fact that nature has already developedelegant tilt sensors which are used daily by aquatic animals.

Bio-inspired materials and systems research has receivedsignificant attention in the last decade. Some examples thatdemonstrate desired functions and structures are self-cleaning(Bhushan et al 2008), solar energy harvesting by replicatingphotosynthesis (Gust et al 2001), strong adhesives (Leeet al 2007) and silk, including technology for fiber spinning(Lazaris et al 2002), rapid locomotion in hard to reachplaces (Menciassi and Dario 2003), hard ceramics for armor(Yasrebi et al 1990), and self-healing characteristics of thebones and tissue (Trask et al 2007). These examples reflectupon the possibilities of achieving improved engineeringfunctions by mimicking fundamental mechanisms adoptedby nature. Jellyfish are attractive candidates for developingunmanned undersea vehicles (UUVs) due to attributes suchas their ability to consume little energy owing to a lowermetabolic rate than other marine species (Seibel and Drazen2007), survival in varying water conditions, and possessionof adequate morphology for carrying payload such as a largebell and trailing tentacles and oral arms. Jellyfish inhabitevery major oceanic area of the world (Cook 2010) and arecapable of withstanding a wide range of temperatures andsalinities (Arai 1997). Most species are found in shallowcoastal waters but some have been found at depths of 7000 m

10964-1726/12/105015+05$33.00 c© 2012 IOP Publishing Ltd Printed in the UK & the USA

Smart Mater. Struct. 21 (2012) 105015 C Smith et al

Figure 1. (a) Microscope image of a statocyst within the rhopalium and (b) SEM close-up view of a natural statolith ball.

(Kramp 1959). Furthermore, jellyfish encompass a widevariety of sizes ranging from a few millimeters to over 2 min diameter (Omori and Kitamura 2004) and also displaya multitude of shapes and colors. They have the ability tomove vertically, but depend mainly upon ocean currents forhorizontal movement (Cook 2010). Jellyfish (Cnidaria) haveno central nervous system (CNS) and use only a diffusednerve net to control movement. As such it is important forthem to have sensory organs that can provide information tothe nerve net and affect behaviors such as feeding, mating,and evasion of predators. For instance, most jellyfish havesimple light sensors called ocelli. When this sensor seeschanges in light intensity, the nerve net is programmed toassume that a predator has just passed over the jellyfish,initiating an escape mechanism. Similarly it possesses aninclinometer, the statocyst organ, which can detect pitch orroll. In the center of a ring of cells, a statolith (a mineralizedball) pushes against small sensing hairs called setae. Thispressure creates a feedback for change in orientation andhelps in maintaining balance (Arai 1997). The statocyst feedsinformation into the outer nerve ring, which is responsible forcollecting sensory data. The inner nerve ring then incorporatessensory input and produces asymmetric contractions of thesubumbrellar swimming muscles allowing the animal to turn.Pacemakers generate swimming gates, such as swimmingvertically, turning, and hovering. Horridge (1969) has pointedout that it is important for the gravity sensing organ to move inconjunction with the vibration sensing mechanism to localizethe information. In medusae and other lower invertebrates, thesensory role is served by motile or non-motile cilia which actas vibration receptors and their response is coupled with thesense of direction of gravity through statocysts. In this paper,we attempt to explain this mechanism and provide an artificialanalog of the statocyst which is to be integrated on Robojelly(Villanueva et al 2011), a biomimetic Aurelia aurita robot.

2. Aurelia aurita statocyst

Figure 1 shows SEM images of the natural jellyfish statolithtaken from an A. aurita sample that was acquired fromthe New England Aquarium. The adult animal 2.8 cmin diameter was dissected to retrieve a statocyst. Thiswas done by first locating one of the eight rhopalialocated sequentially along the bell margin. A rhopalium

was then opened and the statocyst was removed. Thesample was characterized using scanning electron microscopy(SEM). This was performed with a LEO (Zeiss) 1550high-performance Schottky field-emission SEM (FESEM)capable of resolution in the 2–5 nm size range. Figure 1(a)shows an image of the statocyst and figure 1(b) shows amagnified view of the statoliths. The number and geometryof statoliths is dependent upon the given species; statolithsprimarily synthesized from bismuth or calcium carbonate. TheA. aurita statoliths analyzed were made of calcium sulfatedihydrate and are primarily of spherical geometry as opposedto cubic, rectangular and other geometries found in differentspecies. The statolith has a rough surface caused by dispersedsodium chloride crystal formation. More than 100 statolithswere found per statocyst ranging from 10 to 20 µm indiameter. These findings are consistent with the data reportedin the literature. Sotje et al have investigated the structure ofstatoliths in Periphylla periphylla (Cnidaria, Scyphozoa) andChironex fleckeri (Cnidaria, Cubozoa) (Sotje et al 2011). Inthe case of P. periphylla the statocyst had a width of 110 µmwhich increased with increasing medusa diameter accordingto the relationship (60.691 × (diameter in mm)0.538 µm).The number of statoliths per statocyst also increasedwith increasing medusa coronal diameter following therelationship (5.494×(diameter in mm)1.379). Accordingly, fora diameter of 30 mm, the statocyst width will be ∼378 µmand the number of statoliths per statocyst will be ∼600.The relationship between the mean width of statoliths perstatocyst (in micrometers) and medusa coronal diameter wasfound to be 5.394 ln (diameter in mm) + 19.673. Thus, fordiameter of 30 mm, the mean width can be calculated to be∼38 µm. Detailed x-ray diffraction analysis was conductedon the statolith of C. fleckeri which was shown to have thecomposition of calcium sulfate hemihydrate (bassanite) withcrystal parameters given as: space group P 3121, a = 6.952 A,b = 6.952 A, c = 6.352 A, α = β = 90◦, γ = 120◦. Theinvestigated crystal had a plate shape with λ = 0.711 A (Sotjeet al 2011). It was noted in the study that the statolith structureis not continuous as it is composed of several oligocrystals.

Sensitivity experiments on the statocysts of medusae arelimited and not quantitative in nature. However, an estimationof the sensitivity can be made from the experiments conductedon other aquatic invertebrates. Williamson (Williamson 1988)conducted experiments on the vibration sensitivity in the

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Figure 2. Progression of the macro-scale prototype from biology to final electronic prototype. (a) Cross section of the natural statocystshowing the statolith (ball) and setae (hairs) (Huber 2007). (b) Conceptual schematic showing the artificial ball and circularly distributedsensors. (c) Circuit diagram of resistor distribution for the artificial statocyst. (d) Complete artificial statocyst.

statocyst of the northern octopus, Eledone cirrhosa. Theresults showed that the octopus statocyst has a vibrationsensitivity corresponding to a particle displacement of0.12 µm. This was found to compare well with the sensitivityof hair sensors in crayfish (Cherax destructor; ∼0.6 µm)and Procambarus clarkii (∼0.1 µm) (Williamson 1988, Tautzand Sandeman 1980, Wiese and 1976). Further, Williamsonshowed that E. cirrhosa exhibited peak sensitivity in thefrequency range of 70–100 Hz and the most sensitive unitresponded at a stimulus velocity of 60 µm s−1. These resultsdo not provide the angular sensitivity but they do provide uswith some comparative linear sensitivity that can be used as ametric in the design of artificial sensors.

3. Artificial Aurelia aurita statocyst

Figures 2(a) and (b) show the design of an artificial statocystbased upon the architecture identified in the jellyfish. Smallsurface mount resistors (.012′′ in width) were used to mimicthe ‘setae’ and a metal ball of 1.55 mm in diameter wasused to mimic the ‘statolith’. The statolith ball rolls aroundinside a circular cavity and contacts the resistors, therebycompleting the circuit with the base plate. Sixty 300� surfacemount resistors (Panasonic ERJ-1GEF3000C) were connectedin series as shown in figures 2(c)–(d). As the jellyfish tilts, themetal ball rolls and contacts various resistors—the resistanceof the sensor is then measured through the terminals as avoltage divider. The input signal is a small 5 V potential. Thecurrent drawn would thus be 278 µA and the power requiredis 1.389 mW. The output is an analog signal between 0 and5 V which linearly increases with tilt angle. Initially somereadings were unclear due to a thin oxide layer which wasbuilt up on the conductive surfaces. A gold coating was addedby sputtering to prevent corrosion as well as increase theconductivity, resulting in a more sensitive and longer lastingsensor. Optimization of the gold coating parameters was con-ducted by varying the sputtering parameters. Figure 3 showsa close-up view of the improved statocyst with gold plating ofthe metallic ball and base plate. It can also be noted from thisfigure that the coating provides a lower friction surface.

The relationship between resistance and inclinationfor the sensor was investigated by measuring resistanceindividually for each of the 60 connections with a multimeter(Fluke FLU87-5 Digital Multimeter). The analog voltage

Figure 3. Optical microscopy image of the gold plated artificialstatocyst.

output was recorded and plotted against resistor number infigure 4(a). This is a function of the changes in resistorvalue provided by the manufacturer. Resistor position ismeasured from the first resistor to the left of the input/outputconnections clock-wise around the sensor to the 60th positionimmediately to the right of the connectors. It was foundthat the sensor was quite linear with a slope magnitude of0.0831. If the sensor was perfectly linear the slope shouldbe 0.0833 since we are dividing the 5 V signal into 60individual voltages. Figures 4(b) and (c) show the sensitivityand accuracy of the sensor, respectively. An automaticallycontrolled tilt table was used to incline the sensor. The angleat which the sensor initially responded was recorded as wellas the final resting position of the artificial statolith. This testwas run 37 times to ensure an even statistical distribution.The frequency refers to the number of test runs where theresult matches that on the x-axis. The average angle neededfor reading was found to be 4.21◦ with a standard deviationof 1.77◦ and the average deviation from the true readingwas found to be approximately 4.85◦ with a high standarddeviation of 4.88◦. Since each resistor–sensor is equal to 6◦,

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Measured datalinear

Figure 4. (a) Voltage divider output versus position of the artificialstatocyst (in terms of resistor number). A linear fit demonstrates thelinearity of the system. (b) Angle at which the sensor must tiltbefore a reading can be obtained and (c) accuracy of the sensormeasured over a variety of readings. Frequency shows the numberof readings for each response.

this means that on average the sensor is accurate down to lessthan one fully resolved physical sensor position. We believethis is a good result given the simple design of the sensor. A

comparison with the sensitivity of the natural statocyst cannotbe made using these data as the representation here is interms of angular tilt and dimensions are at least two orders ofmagnitude larger than in the natural system. By using betterfabrication techniques one can incorporate a denser resistorarray and multiple balls instead of a single one which wouldsignificantly increase the sensitivity. For example, by usingthe three-dimensional (3D) MEMS fabrication one can designa better architecture for statolith and resistors that lowersthe contact time. The dimensions and 3D arrangement of theresistors and ball are a critical part of the sensitivity.

Artificial statocysts reduce the complexity involved inextracting the tilt angle as the voltage readings are obtaineddirectly. In comparison, a conventional off-the-shelf sensorchip needs several intermediate steps before the final outputcan be obtained (such as a Honeywell HMC634). In this case,first the tilt sensing occurs and data are stored momentarily onthe chip. Then the data are transferred through an I2C bus toa converter that allows the computer to use the USB protocol.This COM signal is then run into software such as LabViewwhere the raw data are collected. A LabView VI then storesthe data, which can be displayed on the screen in a variety ofways. In comparison, the data from the artificial statocyst canbe directly utilized in LabView.

4. Integration of the artificial statocyst on Robojelly

The artificial statocyst was integrated on the Robojelly(Villanueva et al 2011) as shown in figure 5(a). Thevehicle utilizes eight radially arranged bio-inspired shapememory alloy composite (BISMAC) actuators which mimicthe appearance, morphology, and kinematics of A. aurita.Actuation if the shape memory alloy was controlled as afunction of input current. The vehicle was able to propelitself in static water conditions and achieve a proficiency of0.19 s−1 while A. aurita achieves around 0.25 s−1. A circuitwas designed (figure 5(b)) to break up the large input signalfor all eight BISMACs into individual signals so that differentparts of the bell could be controlled independently.

Figure 5(c) shows the circuit diagram for the turningsignal board as well as the integration of the electronicswith the robotic jellyfish prototype. The board was designedto have a circular shape to match the natural shape of

Figure 5. (a) Prototype just after release from the mold. The power wire entrance has not yet been coated in additional silicone. (b)Integration of the board with the robotic jellyfish vehicle. (c) Circuit diagram of the power splitting board: U$1, 5 V power supply; U$2, I2CI/O expander; Q0–Q7, MOSFET chips.

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the jellyfish vehicle. The motion of the actuators was notrestricted because the upper portion where the electronicswas held has been shown to have negligible deformation. Ahole in the center of the circuit board allows wires to passthrough for software control and power. When a signal is sentto the board, an I2C I/O expander chip decodes the commandsand parses the signal into eight different individual on/offswitches. The switches are electrically controlled by eightMOSFETs with associated resistors and capacitors. There isalso an on-board 5 V supply, so that a variety of voltagescan be fed into the board. This allows the electronics touse the power feed wires intended for vehicle actuation.The addition of a tilt sensor with individualized actuationwill enable directional swimming which can be programmedwith bio-inspired motions such as prey capture and predatoravoidance. These motions and the ability to change directioncan also allow the vehicle to perform mission objectives. Ifsolar collection is needed in the future to add to on-boardpower, the robotic jellyfish can tilt its body to collect themaximum amount of radiant energy from the sun. Thus, thedevelopment of an artificial tilt sensor mimicking the formfactor and performance of natural statocyst brings us furthercloser to replicating A. aurita.

5. Summary

A tilt sensor based on the jellyfish statocyst was designedand characterized. The fabrication process consisted of theprocedure followed for printed circuit boards combined withreadily available surface mount components. The sensor wasfound to be highly linear in response. The average tiltangle needed for reading was found to be 4.21◦ with astandard deviation of 1.77◦ and the average deviation fromthe true reading was found to be approximately 4.85◦ witha standard deviation of 4.88◦. The sensor was integratedwith a robotic jellyfish vehicle to show the viability ofthis type of sensor with a UUV platform. Custom-builtelectronics were designed to split up a single input signalinto eight differing signals, allowing individual actuators tobe controlled independently. Future work will include testingthe integrated sensor in underwater conditions. Further, futuredesigns of the tilt sensor will use MEMS based architectureinstead of the surface mount resistors to enable much highersensitivity. It should be noted that the natural statocyst isthree-dimensional while the artificial one presented in thiswork is two-dimensional. Future work will include extendingthe above concepts to a three-dimensional artificial statocystsensor. This research is currently in progress in our laboratoryand will be published elsewhere.

Acknowledgments

This work is sponsored by the Office of Naval Researchthrough contract number N00014-08-1-0654 Jellyfish Au-tonomous Node and Colonies MURI.

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