7774 ieee sensors journal, vol. 16, no. 21,...

7774 IEEE SENSORS JOURNAL, VOL. 16, NO. 21, NOVEMBER 1, 2016

A 4-DOF MEMS Energy Harvester UsingUltrasonic Excitation

Anthony G. Fowler, Member, IEEE, and S. O. Reza Moheimani, Fellow, IEEE

Abstract— This paper presents a microelectromechanicalsystems (MEMS)-based energy harvester that is designed toharvest electrical energy from an external source of ultrasonicwaves. The harvester features a novel 4-degree-of-freedom (DOF)mechanical design that uses three translational resonance modesand one rotational mode, with electrostatic transducers beingimplemented to perform the conversion of mechanical to elec-trical energy. The 4-DOF design of the system allows energy tobe harvested regardless of the device’s orientation, which holdsparticular benefits for potential applications that include thepowering of implanted biomedical devices. Full characterizationof the fabricated MEMS harvester is performed, which showsthat the device is able to generate electrical power outputs of180, 189, and 49.1 nW due to ultrasonic excitation from thex-, y-, and z-directions, respectively.

Index Terms— Energy harvesting, MEMS, ultrasonic, 4-DOF,electrostatic transducer.

I. INTRODUCTION

THE field of energy harvesting remains a very active areaof research due to the significant benefits it potentially

holds for remote and standalone electronic devices. At present,the majority of portable, low-power systems including personalelectronic devices, wireless sensor nodes, and remote sensorsand actuators continue to rely on conventional batteries as theirmain source of electrical energy due to their high specificenergy density and ease of use [1], [2]. However, the needto replace batteries on a recurring basis represents a signifi-cant drawback, particularly when the application is deployedremotely or is in a location with limited accessibility [3]. Therehas therefore been much interest in developing techniques toreduce the current level of dependence on batteries, and theuse of energy harvesting represents one possible solution.

Energy harvesting involves generating electrical powerfrom external sources of energy that may include mechan-ical motion, solar energy, temperature gradients, and fluidicflow [4]–[6]. After appropriately conditioning the harvestedelectrical power, it can be used to supplement the energy

Manuscript received July 12, 2016; accepted August 12, 2016. Date ofpublication August 17, 2016; date of current version September 28, 2016. Thiswork was supported by the Australian Research Council and the Universityof Newcastle, Australia. The associate editor coordinating the review of thispaper and approving it for publication was Prof. Bernhard Jakoby.

A. G. Fowler was with the School of Electrical Engineering and ComputerScience, University of Newcastle, Callaghan, NSW 2308, Australia. He isnow with the Department of Mechanical Engineering, University of Texas atDallas, Richardson, TX 75080 USA (e-mail: [email protected]).

S. O. R. Moheimani is with the Department of Mechanical Engineer-ing, University of Texas at Dallas, Richardson, TX 75080 USA (e-mail:[email protected]).

Digital Object Identifier 10.1109/JSEN.2016.2601168

provided by a conventional battery, therefore increasing theeffective running time of the system and reducing the fre-quency of battery replacement. In certain cases, it may evenbe possible for the harvested energy to become the system’ssole source of electrical power, therefore removing entirely theneed for external charging.

Much of the current research involving energy harvestinghas focused on microscale, rather than macroscale imple-mentations [7], [8]. This has been driven largely by rapidreductions in the power consumption, size, and cost of sensingplatforms and other electronic systems [4]. As a result, therehas been a recent focus on the use of microelectromechanicalsystems (MEMS) to implement energy harvesting techniques,given their relatively straightforward fabrication processes andease of integration with external electronic systems [8], [9].

One example that stands to benefit from the use ofmicroscale energy harvesting is biomedical applications,which are expected to make increasing utilization of elec-tronic devices implanted within the human body [10].These devices include drug pumps, cardiac defibrillators, andpacemakers [11], and feature power consumptions that canrange from tens of microwatts to several milliwatts [11]–[13].Conventional batteries continue to be used as the primaryenergy source for such devices, meaning that surgery is usuallyneeded to replace the batteries on a periodic basis [14], [15].There is significant interest in developing techniques to wire-lessly transfer electrical energy to such implanted devices,and one such approach is the use of inductive transfer viaelectromagnetic waves [16], [17]. However, this method suf-fers from a number of potential limitations, including theeffect of electromagnetic coupling and greatly reduced transferefficiency at the distances typically associated with implanteddevices [18], [19]. As a result, the transmission of electricalpower through the body is of continuing research interest, andone possible solution is the direct integration of a MEMSenergy harvester with the implanted biomedical device.

This paper presents a MEMS-based electrical energyharvester that has been designed to use an externalsource of ultrasonic waves for mechanical excitation. Withultrasonic waves having previously been shown to be an effec-tive method to both transfer energy to a receiver embeddedwithin soft tissue [20], [21] and mechanically stimulate animplanted microdevice [22], [23], this represents a highlysuitable approach for implanted biomedical applications.

In a single-DOF system, any misalignment between theenergy harvester and the direction of the source of mechanicalexcitation can lead to a significant reduction in the energy

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FOWLER AND MOHEIMANI: 4-DOF MEMS ENERGY HARVESTER USING ULTRASONIC EXCITATION 7775

converted by the device. For example, the harvester presentedin [20] features 15 mm diameter disc-shaped PZT elements thatgenerate electrical energy from out-of-plane vibrations inducedby incident ultrasonic waves. While the use of a macroscalePZT structure allows relatively high levels of electrical powerto be obtained, experimental testing showed that the poweroutput of the device drops quickly if the transmitter andharvester are not optimally aligned. However, designing asystem with multiple mechanical degrees of freedom cansignificantly reduce this problem by increasing the harvester’sability to respond to excitation from different directions. Thisapproach is particularly relevant for an application such as animplanted harvester, where its orientation or position may notnecessarily be known with absolute precision.

The MEMS energy harvester presented in this paper imple-ments a novel 4-degree of freedom (DOF) mechanical designthat allows the harvester to respond to external ultrasonicwaves via multiple, independent mechanical mechanisms. Thedesigned device builds on previous work involving MEMSharvesters with multiple DOFs. In [24] and [25], a 2-DOF sys-tem was designed that allowed energy to be generated throughthe mechanical oscillations of a mass along two orthogonalin-plane directions. The system presented in [26] and [27]featured an additional out-of-plane harvesting mode, resultingin a 3-DOF design. The work presented in this paper extendsthe direction of this previous research through the design ofa novel MEMS-based system with four independent DOFsthat allow the harvester to generate electrical energy in anyorientation.

Given that the energy harvester’s proof mass comprisesthe majority of the device’s footprint, significant gains inpower density are achieved through the use of a multiple-DOF harvesting mechanism that uses the same mechanicalstructures to generate electrical energy via multiple reso-nance modes [28]. While multiple-DOF harvesting can beimplemented using several smaller 1-DOF harvesters arrangedin an orthogonal configuration, as noted in [19], the effi-ciency of ultrasonic power delivery falls with decreasingreceiver dimensions. Therefore, the implementation of a singleMEMS harvester with a multiple-DOF mechanical designresults in improved harvesting power density.

II. DESIGN AND FABRICATION OF THE

4-DOF MEMS ENERGY HARVESTER

The MEMS energy harvester presented in this paper is aresonant mechanical system that is designed to be used inconjunction with ultrasonic waves provided by an externaltransmitter. The harvester is mechanically designed such thatthe incident ultrasonic waves result in vibrations of the massstructures within the harvester, which are then used to generateelectrical energy that may be supplied to an attached electricalload.

Instead of possessing a basic, single-DOF mechanical struc-ture, the MEMS harvester has been developed to includetuned mechanical resonance modes in each of the x, y, andz directions, with an additional rotational yaw mode aroundthe device’s z axis. This structure allows resonant mechanicaloscillations to be generated within the harvester irrespective of

TABLE I

DESIGN PARAMETERS OF 4-DOF MEMS ENERGY HARVESTER

the direction of the ultrasonic excitation, and therefore enablesthe device to produce electrical power in any orientation.

Each of the four harvesting modes features independentelectrostatic transducers to convert the mechanical energy ofthe vibrating mass structures into electrical energy. Whilepiezoelectric transducers can provide higher electrical poweroutputs, the nature of the microfabrication process means thattheir use in MEMS energy harvesters is generally constrainedto out-of-plane operation. The 4-DOF MEMS energy harvestertherefore implements electrostatic transducers due to their abil-ity to be flexibly designed for both in-plane and out-of-planeoperation. These electrostatic transducers are comprised ofinterdigitated comb-finger electrodes, with the displacementsof the harvester’s mechanical structures being utilized to createvarying capacitances through variations in the overlap betweenthe electrodes. Table I provides a summary of the harvester’smain design parameters, while the operation of each of theharvesting modes is discussed below.

A. In-Plane Harvesting Modes

The MEMS harvester’s in-plane harvesting mechanism usesa parallel kinematic design, and features a square, centrally-positioned mass structure. The mass is suspended by a seriesof flexures, which are distributed around the edge of themass and allow it to mechanically displace along both thex and y directions when a source of ultrasonic excitationis directed towards the harvester. The dimensions of theflexures are tuned so that entire structure possesses symmet-ric mechanical resonance modes along both of the device’sin-plane directions.

The frequencies of these resonance modes are chosen to beequal to the frequency of the intended ultrasonic excitation,therefore maximizing the mechanical response of the harvesterand consequently maximizing its electrical power output. Forthe designed system, a resonance mode frequency of 25 kHzwas selected as it is a common frequency for off-the-shelfultrasonic transmitters.

The harvester’s resonance modes were simulated inCoventorWare, with Figs. 1a and 1b showing the finite elementsimulations of the x and y modes, respectively.

Electrostatic transducers are implemented around the edgesof the device to harvest electrical energy from the vibrationsof the mass, with separate electrodes being utilized for thex and y axes due to the decoupling nature of the flexures.The transducers are configured as in-plane, overlap-varying


Fig. 1. Finite element simulations of the harvester’s resonance modes.(a) X-axis mode. (b) Y-axis mode. (c) Z-axis mode. (d) Rotational mode.

comb fingers, which result in a linear change in capacitancewith respect to the displacement of the mass.

B. Out-of-Plane and Rotational Harvesting Modes

Rather than consisting of a single solid mass, the structurecomprising the in-plane harvesting mechanism is subdividedinto a number of separate mechanical elements that formthe MEMS device’s out-of-plane and rotational harvestingmechanisms. As a result, these mechanisms can be consideredto be serially nested within the in-plane harvesting structure.

The z-axis harvesting mode involves a circular disc at thecenter of the device, which is connected to a surrounding ringvia an additional set of flexures. These flexures are tuned suchthat a mechanical resonance mode is created that generates arelative out-of-plane vibration between the disc and the ring.The finite element simulation of this mechanical resonancemode is shown in Fig. 1c.

To provide the necessary surface area for the mechanismto be sufficiently excited by incident ultrasonic waves, thecircular disc has a relatively large diameter of 1.4 mm. How-ever, this results in the disc possessing a very low mechanicalstiffness in the z direction, making it difficult to design asuspension system that provides an out-of-plane mechanicalresonance at ultrasonic frequencies. This issue is resolved byusing the substrate layer of the MEMS die to provide extramechanical support for the disc, allowing it to remain rigidin the vertical direction. As shown in the scanning electronmicroscope (SEM) images of the device presented in Fig. 2,a cross-shaped structure fabricated from the substrate layer issuspended beneath the central disc. Additional flexures thenconnect the disc to the surrounding ring structure, with theflexures being designed to allow this out-of-plane mode toagain be located at approximately 25 kHz.

The electrostatic transducer for the out-of-plane harvestingmode is implemented by including comb finger electrodes

around both the edge of the disc and on the surrounding ringstructure. In this case, the vertical oscillations of the masscreate a changing capacitance due to out-of-plane variationsin the overlap between the electrodes.

The rotational harvesting mode is nested within the cen-tral mass structure in a similar manner. A set of flexuressurrounding the circular ring at the center of the deviceis designed so that the entire central harvesting structure,which includes the out-of-plane mechanism, has a rotationalresonance mode around the z axis of the device. Suspendedstructures fabricated from the substrate layer are again usedto mechanically reinforce the device layer and increase theout-of-plane stiffness of the mass structures. As with theother three resonance modes, this mode (shown in Fig. 1d)is designed to be located at approximately 25 kHz.

C. Electrical Routing

The MEMS harvester uses a silicon-on-insulator (SOI)fabrication process, which provides only a single conduc-tive layer that may be used to route electrical paths fromthe electrostatic transducer electrodes to bonding pads. Thismakes it difficult to implement an electrical connection withthe out-of-plane and rotational harvesting mechanisms, whichare serially nested within the in-plane harvesting structure.To overcome this issue, the suspended substrate structures usedto increase the rigidity of the device layer have also beenused to mechanically link adjacent structures in the devicelayer while maintaining their electrical isolation due to theburied oxide between the device layer and the substrate. Thismakes it possible to design more complex paths for electricalrouting within the device layer, as is necessary for the elec-trodes at the center of the device. An overview of the MEMSharvester’s electrical routing configuration is shown in Fig. 3.

D. MEMS Fabrication

The 4-DOF ultrasonic energy harvester was fabricatedusing the commercial SOIMUMPs silicon-on-insulator MEMSprocess provided by MEMSCAP [29]. This process providesa 25 μm device layer comprising n-doped silicon and allowsmechanical structures to be created with a minimum featuresize and spacing of 2 μm. A buried oxide layer with athickness of 2 μm is used to electrically isolate adjacent struc-tures within the device layer. SEM images of the fabricatedharvester are shown in Fig. 2.

The substrate of the MEMS die is a 400 μm thick layerof silicon, which is patterned using a deep reactive ion etchto release the mechanical structures within the device layer.While most of the substrate beneath the harvester is removedin this step, a number of suspended substrate structures areallowed to remain that are used to mechanically connectelectrically-isolated sections of the harvester, as describedin Section II-B.

III. EXPERIMENTAL CHARACTERIZATION

A. Mechanical Resonance Analysis

For testing and characterization, the MEMS die containingthe 4-DOF harvester was affixed to a small PCB incorporating


Fig. 2. Scanning electron microscope images of the fabricated 4-DOF MEMS harvester.

Fig. 3. Layout of electrical routing paths for the MEMS harvester. Each colordenotes the sections that are electrically connected, with etched channels inthe device layer being used to provide electrical isolation between sections.

a hole, which allows the harvester’s suspended substrate struc-tures to move freely during operation. Electrical connectionsto the harvester’s electrostatic transducers were made throughthe use of gold bonding wires.

The mechanical resonance modes of the fabricated har-vester were experimentally obtained with the use of a PolytecMSA-050-3D Micro System Analyzer (MSA). This systemuses three laser Doppler vibrometers to make high-resolutionmeasurements of the mechanical vibrations of a structure alongeach of the x, y, and z directions. For this test, the resonancemodes of the MEMS device were mechanically excited byadding a low-amplitude, wideband chirp signal to a 60 V DCbias and applying it to the electrostatic transducers. The MSAwas used to measure the resulting vibrations at a point close tothe center of the harvester’s mass, with the resulting frequencyresponse measurements being shown in Fig. 4. It is evidentthat this magnitude plot contains four significant peaks located

Fig. 4. Mechanical frequency response of the 4-DOF energy harvester alongthe x-, y-, and z-axis directions.

at 22.36 kHz, 24.56 kHz, 24.90 kHz, and 30.68 kHz, which arereasonably close to the desired harvesting frequency of 25 kHz.

B. Visualization of Resonance Modes

To confirm that the four identified resonances are thedesigned mechanical modes for energy harvesting, the MSAwas used to identify the shape of each of these modes. Thisis achieved by performing a scan that involves measuring thevibrations of the mechanically excited structure at a numberof defined points on the surface of the device. From thesemeasurements, a 3D visualization can be generated that showsthe nature of the harvester’s vibrations at each frequencywithin the excitation bandwidth. The obtained mode shapesat the four identified mechanical resonance frequencies areshown in Fig. 5.

It is clear that the shape of these experimentally obtainedresonance modes are very similar to the designed modes shownin Fig. 1. While the x and y modes are close to the desiredfrequency of 25 kHz, being located at 24.57 kHz and 24.90 kHzrespectively, there is a greater level of variation for the z androtational modes, which in the experimental system are locatedrespectively at 30.68 kHz and 22.36 kHz.

One potential factor for these variations is imperfect etchingof the suspended silicon substrate structures associated with


Fig. 5. 3D representations showing the shape of each of the 4-DOFharvester’s mechanical resonance modes. (a) X-axis mode. (b) Y-axis mode.(c) Z-axis mode. (d) Rotational mode.

the z and rotational harvesting mechanisms. The mechanicalstructures for these modes are serially nested at the center ofthe device, and their smaller geometric size compared withthe x and y harvesting mechanisms means that the suspendedsilicon substrate comprises a greater proportion of the mass ofthese structures. As a result, any variation in the dimensions ofthe suspended substrate components from their nominal valuesdue to inconsistencies in the fabrication process is likely toresult in more significant deviations in the frequencies of thez and rotational modes from their designed values.

Fig. 6. The experimental setup used to test the MEMS harvester with theultrasonic transducer.

C. Electrostatic Transducer Testing and Frequency Response

Having verified the frequency and shape of the fabricatedMEMS harvester’s mechanical resonance modes, the device’selectrostatic transducers were tested to confirm that they areable to generate an electrical output due to the mechan-ical oscillations of the mass structures. For this test, theMEMS harvester was mechanically excited using a Prowave250ST180 ultrasonic transmitter, which was selected as itscenter frequency of approximately 25 kHz is relatively wellmatched with the resonance frequencies of the device’s har-vesting modes. The transmitter was positioned close to theharvester and was electrically driven by a 20 Vrms periodicchirp signal. A photo of the experimental setup is shownin Fig. 6.

Each of the harvester’s four transducers were supplied witha 60 V DC bias provided by an external voltage source.As electrostatic transducers require an initial electrical chargeto generate electrical energy from mechanical vibrations,a number of methods for providing this bias are demon-strated in the literature. These include integrating electretmaterials with a quasi-permanent electrical charge [30], [31]or implementing a power conditioning circuit that uses aportion of the harvester’s converted electrical energy to primethe transducers [32]. For the purpose of testing the 4-DOFharvesting mechanism presented in this paper, an externallysupplied bias is sufficient, however a practical implementationof the system would likely utilize a standalone biasing method.

Each of the four transducers (x/y/z/rotational) was indi-vidually tested, being connected one at a time to the inputof a HP35670A signal analyzer. For each transducer, threefrequency response measurements of the transducer’s outputwere recorded while the ultrasonic transmitter was separatelyoriented along the x, y, and z axes of the harvester. Theresulting frequency responses for each transducer are shownin Fig. 7.

The obtained frequency responses show that the primary res-onance for each transducer is at essentially the same frequencyas its corresponding mechanical resonance mode, previouslyidentified via the MSA in Section III-B. These results thereforedemonstrate that the mechanical resonances of the MEMSharvester are successfully converted into electrical energy bythe electrostatic transducers.


Fig. 7. Frequency response measurements of the output of each electrostatictransducer. Excitation is provided by an ultrasonic transmitter directed alongeach of the three primary device axes. (a) X-direction. (b) y-direction.(c) z-direction. (d) Rotational transducers.

Furthermore, it can be seen that for the x, y, and z-axis trans-ducers, the dominant peak in the frequency response achievesits maximum amplitude only when the direction of the appliedultrasonic excitation matches the transducer being tested; forexample, the magnitude of the x-axis transducer’s responseis greatest when the ultrasonic waves are transmitted fromthe x direction, and similarly for the y and z-axis transducers.In contrast, the frequency response of the rotational transducerindicates that almost identical outputs are produced whenexcitation is provided from the x and y directions. Since the

direction of the rotational mode is around the z axis of thedevice, this result is to be expected as this mode is similarlysensitive to excitation from any in-plane direction.

These results show that the response of each of the MEMSharvester’s electrostatic transducers is strongly dependent onthe direction of the applied ultrasonic excitation, as expected,and therefore confirm that the mechanical modes of the deviceare effectively decoupled.

IV. ULTRASONIC TRANSMITTER BANDWIDTH TUNING

As described in Section II, the 4-DOF MEMS energy har-vester was designed such that the four mechanical resonancemodes used for energy harvesting would each be located atapproximately 25 kHz. This frequency allows the device to bewell matched with commercial ultrasonic transmitters such asthe Prowave 250ST180 used in this paper, which also featuresa 25 kHz center frequency. However, the previous charac-terization results indicate that in the fabricated device, thedesired mechanical resonance modes are located at 22.36 kHz,24.57 kHz, 24.90 kHz, and 30.68 kHz. With the 250ST180ultrasonic transmitter having a bandwidth of 1.5 kHz, thismeans that the mechanical response of the two modes locatedoutside of the transmitter’s bandwidth (the z-axis and rota-tional modes) will be significantly reduced, therefore loweringthe harvester’s potential energy conversion efficiency. Due tothe generally narrow bandwidths of ultrasonic transmitters ingeneral, it was not feasible to obtain a commercially availabletransmitter that is better matched with the fabricated device’sresonance modes than the model already used.

However, it was demonstrated in [33] that it is possible tomodify the frequency response of a piezoelectric ultrasonictransmitter through the addition of passive electrical compo-nents in series with the transmitter. By carefully selectingappropriate component values, the response of the ultrasonictransmitter used within these tests could therefore potentiallybe tuned to better match the dynamics of the fabricated MEMSenergy harvester, and thus improve the device’s electricalpower output. The procedure used to carry out this task isbriefly described here.

A useful electrical representation of a piezoelectric ultra-sonic transmitter is shown in Fig. 8a. To determine the corre-sponding component parameters Rs , Ls , Cs , and Cp for theProwave 250ST180 transmitter used for testing, the frequency-domain impedance of the transmitter was first obtained byperforming a frequency response measurement involving itsinput voltage and current. The impedance of the circuit shownin Fig. 8a is derived to be given by:

Z(s) =s2 1

C p+ s Rs

LsC p+ 1

Cs LsC p

s3 + s2 RsLs

+ s(

C p+CsCs LsC p

) (1)

A least squares method was then used to identify theparameters Rs , Ls , Cs , and Cp such that the impedance Z(s)best fits the experimentally obtained impedance measurement.The component values were obtained to be Rs = 550 �,Ls = 147 mH, Cs = 0.265 nF, and Cp = 2.31 nF, witha frequency response plot showing the match between theresulting Z(s) and the actual transmitter impedance being


Fig. 8. (a) Electrical model of a piezoelectric ultrasonic transmitter.(b) Transmitter model with added series resistor and inductor for bandwidthtuning [33]. (c) Final ultrasonic transmitter circuit used for measurement ofthe MEMS harvester’s power output. The circuit includes the transmitter,compensating elements, voltage amplifier, and a voltage source generatingfour sinusoidal outputs at the frequencies of the MEMS device’s resonancemodes.

Fig. 9. Measured impedance of Prowave 250ST180 ultrasonic transducerand fitted model.

shown in Fig. 9. It is clear that the fitted model provides agood match with the measured transmitter impedance.

Using the model shown in Fig. 8a, the acoustic powerproduced by the ultrasonic transmitter is proportional to thecurrent I flowing through resistor Rs [33]. The frequency

Fig. 10. Magnitude frequency response of transfer functions a) IVin

for

the uncompensated ultrasonic transmitter, and b) IcVin

for the compensatedultrasonic transmitter. The vertical lines indicate the frequencies of thefabricated harvester’s resonance modes.

response of the ultrasonic transmitter is therefore given by:

I

Vin=

1Ls

s

s2 + RsLs

s + 1LsCs

(2)

whose Bode plot is shown in Fig. 10a. As expected, thefigure shows a narrowband response with a peak at approxi-mately 25 kHz.

By adding a compensating resistor Rc and inductor Lc inseries with the ultrasonic transmitter (as in Fig. 8b), the circuitcan be tuned such that the frequency response of the ultrasonictransmitter features a second peak at a different frequency.In this case, the frequency response of the compensatedtransmitter is given by (3), shown at the bottom of this page.

By selecting Rc = 330 � and Lc = 17.7 mH, the frequencyresponse of the compensated circuit (shown in Fig. 10b) wastuned to widen the bandwidth of the transmitter and allow eachof the MEMS energy harvester’s four designed modes to beexcited with a similar amplitude. While the absolute amplitudeof the peaks in the compensated circuit are slightly lowerthan for the uncompensated circuit, the improved transmitterbandwidth is highly valued as it allows for a more useful

Ic

Vin=

1C p Lc Ls

s

s4 +(

Lc Rs + Ls Rc

Lc Ls

)s3 +

(Cp Lc + Cs Ls + Cs Lc + CpCs Rc Rs

CpCs Lc Ls

)s2 +

(CP Rc + Cs Rc + Cs Rs

CpCs Lc Ls

)s + 1

C pCs Lc Ls

(3)


Fig. 11. Frequency domain measurement of the signal used to drive theultrasonic transmitter while testing the harvester’s power output. The signalcontains four sinusoidal components at the frequencies of the harvester’sidentified resonance modes.

evaluation of the harvester’s ability to generate electrical powervia each of its mechanical modes. The compensated ultrasonictransmitter is therefore used for the power measurement testsperformed in the following section.

V. HARVESTED POWER MEASUREMENT

The 4-DOF MEMS harvester’s ability to provide electricalenergy to an external load was demonstrated by charginga storage capacitor. Each of the device’s four electrostatictransducers were again supplied with an external 60 V DCbias, and individual full-wave diode rectifiers were used toprovide a DC voltage from the harvested electrical energy.A 1 μF low-ESR electrolytic capacitor was used to store theenergy generated by the harvester, while a Stanford ResearchSystems SR560 low-noise voltage preamplifier connected toan oscilloscope was used to monitor the change in voltage onthe capacitor.

The ultrasonic excitation for the MEMS harvester was againprovided by the Prowave 250ST180 ultrasonic transmitter,using the compensating circuit as described in the previoussection. In order to maximize the mechanical response of theharvester, and thus maximize the generated electrical power,the driving signal for the transmitter was constructed bysumming four sine waves whose frequencies match those ofthe harvester’s mechanical resonances previously identifiedin Section III-A. This signal was generated by a ZurichInstruments HF2LI lock-in amplifier and amplified using anFLC Electronics A400DI voltage amplifier such that each ofthe four frequency components has an amplitude of 10 Vrms .A frequency domain measurement of the driving signal isgiven in Fig. 11, which shows the four frequency componentsat approximately 22.4 kHz, 24.5 kHz, 24.9 kHz, and 30.7 kHz.Fig. 8c shows a schematic diagram of the transmitter’s drivingcircuit.

The ultrasonic transmitter was positioned approxi-mately 5 cm from the MEMS harvester and aligned with thedevice’s x axis. Following the activation of the transmitter,the rising voltage on the storage capacitor was monitoreduntil reaching a steady state value. The test was repeated afterrealigning the transmitter with the y axis of the harvester, andsimilarly for the z axis.

Fig. 12. Waveforms showing the rising voltage on the storage capacitorfollowing the activation of the ultrasonic transmitter. The test was performedthree times, with the transmitter being separately aligned with the harvester’sx, y, and z axes.

Fig. 13. Instantaneous electrical power output of MEMS harvester, calculatedfrom the time varying voltage changes on the storage capacitor.

The results of this test are given in Fig. 12, which showsthe time-domain change in the capacitor’s voltage followingthe activation of the ultrasonic transmitter at t = 0. The threevoltage traces represent the results of the individual tests wherethe transmitter was separately aligned with each of the device’sprimary axes. Based on these results, the change in the energystored on the capacitor was calculated using the equationE = 1

2 CV 2, where C is the value of the capacitor and V is themeasured instantaneous voltage on the capacitor. The slope ofthe time varying change in energy was then used to obtainthe MEMS harvester’s instantaneous electrical power output,which is shown in Fig. 13. From this plot, the peak value of thepower generated by the harvester is determined to be 180 nW,189 nW, and 49.1 nW when the ultrasonic transmitter is alignedwith the x, y, and z axes, respectively.

These results confirm that the MEMS harvester has the abil-ity to generate electrical power regardless of the direction ofthe applied ultrasonic excitation. Fig. 14 shows a comparisonof the power densities of the current 4-DOF MEMS harvesterand previously reported 3-DOF and 2-DOF MEMS ultrasonicharvesters [25], [27]. From this comparison, it is evidentthat the current device provides a significant improvement ingenerated power when the ultrasonic waves are directed fromthe x and y directions. This can be attributed to the addition ofthe rotational harvesting mode that is unique to the design ofthis system, and which has been shown to respond effectivelyto excitation from the in-plane directions.


Fig. 14. Comparison of harvested electrical power densities for the 4-DOFMEMS energy harvester (this work), a 3-DOF harvester [27], and a 2-DOFharvester [25]. The performance of each device resulting from ultrasonicexcitation along the x, y, and z directions is shown.

Future work will involve adapting the energy harvester’spower management circuitry to be more specifically targetedtowards a practical biomedical implementation. This wouldlikely involve incorporating flyback components to allow theelectrostatic transducers’ bias to be provided from the har-vested energy, as well as a charging circuit that is optimizedfor trickle charging an energy reservoir such as a battery orsupercapacitor.

VI. CONCLUSION

Building on previous research into multi-DOF MEMSenergy harvesting systems, this paper has presented a 4-DOFMEMS harvester that uses externally generated ultrasonicwaves for mechanical excitation. The device has been mechan-ically designed to possess three mechanical resonance modesalong the x, y, and z axes of the device, as well as an additionalrotational mode around the z axis. This configuration allowsthe harvester to generate electrical power in any orientation,even when the external ultrasonic transmitter is not preciselyaligned with the MEMS device. Potential applications ofthis energy harvester include providing electrical power fordevices implanted within the human body, with an ultrasonictransmitter placed above the skin being used to provide therequired excitation.

Characterization of the fabricated MEMS device confirmedthat the shape of each of the designed resonances are asexpected, and the decoupled nature of each of these modeswas verified by exciting the device using ultrasonic wavesfrom multiple directions. To increase the mechanical excitationof the harvester, the frequency response of the ultrasonictransmitter was tuned according to the frequencies of theharvester’s modes through the implementation of a compen-sating electrical circuit. The charging of a storage capacitorwas demonstrated, with the MEMS harvester providing peakelectrical power outputs of 180 nW, 189 nW, and 49.1 nWresulting from ultrasonic excitation from the x, y, and zdirections, respectively.

While the present device is envisioned to be useful inapplications such as powering implanted biomedical devices,the general concept of a resonant mechanical structure withfour degrees of freedom is highly applicable to many forms

of MEMS vibration energy harvesting. Future research maytherefore include exploring how this structure can be imple-mented to harvest electrical energy from ambient vibrationswith both linear and rotational components.

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Anthony G. Fowler (S’10–M’15) received the bach-elor’s and Ph.D. degrees in electrical engineeringfrom University of Newcastle, Callaghan, NSW,Australia, in 2010 and 2014, respectively.

He was a Post-Doctoral Fellow with the Schoolof Electrical Engineering and Computer Science,University of Newcastle, from 2014 to 2015.He is currently a Research Scientist with the Depart-ment of Mechanical Engineering, University ofTexas at Dallas, Richardson, TX, USA. His currentresearch interests include the design, fabrication,

and analysis of novel microelectromechanical systems for energy harvesting,nanopositioning, and scanning probe microscopy applications.

S. O. Reza Moheimani (F’11) currently holdsthe James von Ehr Distinguished Chair in Scienceand Technology with the Department of MechanicalEngineering, University of Texas at Dallas. Hiscurrent research interests include ultrahigh-precisionmechatronic systems, with particular emphasis ondynamics and control at the nanometer scale, includ-ing applications of control and estimation in nanopo-sitioning systems for high-speed scanning probemicroscopy and nanomanufacturing, modeling andcontrol of microcantilever-based devices, control of

microactuators in microelectromechanical systems, and design, modeling,and control of micromachined nanopositioners for on-chip scanning probemicroscopy.

Dr. Moheimani is a Fellow of IFAC and the Institute of Physics, U.K.His research has been recognized with a number of awards, including theIFAC Nathaniel B. Nichols Medal (2014), the IFAC Mechatronic SystemsAward (2013), the IEEE Control Systems Technology Award (2009), the IEEETRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY Outstanding PaperAward (2007), and several best student paper awards in various conferences.He is the Editor-in-Chief of Mechatronics and has served on the editorialboards of a number of other journals, including the IEEE TRANSACTIONS

ON MECHATRONICS, the IEEE TRANSACTIONS ON CONTROL SYSTEMS

TECHNOLOGY, and Control Engineering Practice. He is currently Editor-in-Chief of IFAC Mechatronics Journal, the Chair of the IFAC TechnicalCommittee on Mechatronic Systems, and was the Chair of several internationalconferences and workshops.

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