A MEMS-based energy harvester for generating energy from non-resonant environmental vibrations

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Contents lists available at SciVerse ScienceDirect

Sensors and Actuators A: Physical

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MEMS-based energy harvester for generating energy from non-resonantnvironmental vibrations�

zge Zorlua,∗, Haluk Külaha,b

Middle East Technical University, METU-MEMS Research and Applications Center, Ankara, TurkeyMiddle East Technical University, Department of Electrical and Electronics Engineering, Ankara, Turkey

r t i c l e i n f o

rticle history:eceived 10 October 2012eceived in revised form 14 January 2013ccepted 15 January 2013vailable online xxx

eywords:nergy harvestingibrationEMS coilon-resonant harvesterechanical frequency up conversion

mFupC)lectromagnetic energy generation

a b s t r a c t

This paper presents a non-resonant vibration based electromagnetic MEMS energy harvester, whichgenerates energy from low frequency vibrations with low displacement amplitude. The harvester iscomposed of an energy harvester chip, housing two electroplated copper micro coils realized on pary-lene cantilevers and a miniature NdFeB magnet with two mechanical barrier arms. The structure usesthe mechanical frequency up conversion (mFupC) principle for energy generation. The non-resonantoperation is maintained by attaching the chip and the magnet to two different platforms, which movewith respect to each other. The prototype generates 2.1 mV RMS voltage and 18.5 nW RMS power fromboth coils on the average, under 10 Hz, 5 mm peak to peak (1 g) external vibrations. The RMS value ofthe generated voltage during the mFupC duration is calculated as 9.5 mV, leading to 363 nW power and1.1 �J energy delivery from each coil to equivalent resistive loads at each occurrence of the mFupC. Serialconnection of the coils is also studied and it is concluded that this configuration has a non-significanteffect on the generated power since the waveforms of the coil voltages have both phase and resonancefrequency differences, canceling out some portion of the signal when they are added together. During
the tests, it is observed that excessive stress around the cantilever fixed edges eventually break the coillines at this region. This is handled by applying an epoxy to this region, lowering the stress on the copperline. With this configuration, the generated power is slightly reduced due to the decreased resonancefrequency and increased damping ratio of the cantilevers. The epoxy-applied prototype has been testedunder various vibration conditions with no damage on the coil, and the non-resonant operation behaviorof the energy harvester has been verified.
© 2013 Elsevier B.V. All rights reserved.

. Introduction

Energy harvesting from the environmental vibrations has beenroposed as a major renewable energy source for mobile platformsogether with photovoltaic and thermoelectric sources [1]. The pro-osed energy harvesters for such applications utilize electrostatic,lectromagnetic, or piezoelectric energy conversion principles. Theenerated power from these systems is proportional with the

Please cite this article in press as: Ö. Zorlu, H. Külah, A MEMS-based energvibrations, Sens. Actuators A: Phys. (2013), http://dx.doi.org/10.1016/j.sna

alue of the vibration frequency [2]. However, ambient vibrationsre mostly at low frequencies (<20 Hz), limiting the generatedower from these systems for daily applications. Furthermore, the

� Manuscript submitted October 2012. An earlier version of this paper wasresented at the 2012 Eurosensors Conference (Paper ID 2200, A miniature andon-resonant vibration-based energy harvester structure); and was published in itsroceedings.∗ Corresponding author. Tel.: +90 321 210 4546; fax: +90 312 210 2304.

E-mail addresses: [email protected] (Ö. Zorlu), [email protected] (H. Külah).

924-4247/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.sna.2013.01.032

ambient vibrations mostly occur within a frequency band and theyare distributed in a random manner, hence have a non-resonantnature. Human motions, wind-induced movements of the branchesof the trees, and vehicle motions are examples for such vibrationsfor which the operation frequency may change in a wide range,depending on the motion [3–5].

On the other hand, most energy harvesters are designed to beconnected to a vibrating base through a spring-mass-damper sys-tem, and they rely on a mechanical resonance within a limitedbandwidth [6]. Furthermore, in order to meet the low resonancefrequency operation requirement, these devices are realized inlarge masses and volumes [7].

Several attempts have been made to increase the operationbandwidth of the energy harvesters. In some approaches, sev-eral cantilevers with different resonance frequency values have

y harvester for generating energy from non-resonant environmental.2013.01.032

been realized on the same energy harvester device [8,9]. In[8] a microfabricated electromagnetic energy harvester has beenreported with a number of serially connected coils on different can-tilevers. The device generates 0.4 �W power with 10 mV voltage

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n an external vibration frequency range of 4.2–5 kHz. The deviceeported in [9] has piezoelectric thick films on silicon cantileversf different lengths, and generates 3.98 �W with 3.93 V DC volt-ge within an 8 Hz bandwidth on 230 Hz. In a similar approach,echanical stoppers have been formed under the cantilevers of a

iezoelectric energy harvester so that the effective length of theantilever changes when it touches the stopper and resonates at aigher frequency [10]. It was reported that the operation band-idth of the device is 18 Hz (30–48 Hz), and the corresponding

ptimal power ranges from 34 to 100 nW at a base accelerationf 0.6 g.

It has also been shown that magnets moving in a tube and areevitated with other magnets show a wider bandwidth of motion,ence has a potential for wide bandwidth energy harvesting appli-ations [11–13]. The wide motion bandwidth of such structures isainly due to the non-linear magnetic forces on magnets, resulting

n a non-linear stiffness of the “magnetic spring” system. In [13], aagnet has been levitated in a vertically oriented tube by the help

f another magnet attached to the bottom edge. It has been shownhat the Type-C battery sized device delivers 54 �W to a 37 �A loadhrough a dual rail 1.46 V DC voltage with total system efficiencyf 81%, when subjected to external vibrations within a bandwidthf 8–12 Hz.

In another study on electrostatic energy harvesters, a conductiveod has been utilized as one of the capacitor plates [14]. The rodreely rolls over the inner surface of a tube, on which a comb shaped

etal is patterned. The energy is generated through the change inhe effective capacitive gap resulting from the displacement of theod inside the tube. The structure is non-resonant by its nature: anyind of motion of the tube creates a movement on the rod structure.he device generates 0.5 �W of output power, even under impulseype excitations.

All the reported prototypes above except [14] operate on a lin-ar or non-linear spring-mass-damper system. As an alternativeethod, different parts or edges of the moving parts of the energy

arvesting device may be attached to separate platforms in a sys-em, where these platforms move with respect to each other. Thisonfiguration enables energy generation through any kind of rel-tive displacement of the platforms, regardless of the frequencynd resonance of the motion. Such a device has been presented in15] and tested on insects. The device consists of a simple beam-r spiral-shaped piezoelectric material. One edge of the materials attached to the wing of the insect, whereas the other edge isttached to its neck. Since these two parts move independently dur-ng the flight of the insect, any kind of relative motion is transferredo electrical energy.

The frequency up conversion (FupC) method presented in16–18] is a promising method to be employed in wide bandwidthnergy harvesting applications. At least two moving structures,oving at low and high frequencies, are present in an FupC struc-

ure, and the energy is generated through their relative motion.he efficiency of the device is increased by FupC utilization, ashe motion of the low-frequency structure triggers the motion ofhe high frequency structure, increasing the coupling between the

echanical domain and electrical domain. Attaching these struc-ures to bases which move with respect to each other, results in annergy harvesting device operating independent of the frequencyf the motion: The high frequency structure is triggered wheneverhe other one is displaced more than a certain threshold distance.n the previous studies, energy harvesters utilizing magnetic or

echanical forces for realizing the frequency up conversion haveeen reported. The idea is presented with magnetic forces in [16]


ith a macro scale prototype, and a MEMS device has been pre-ented in [17]. In another macro scale prototype with around 3 cm3

olume, the frequency up conversion is realized mechanically, bysing barrier arms at the edges of the moving structures, and the

Fig. 1. (a) The structure of the energy harvester. The barrier arm touches, bend,and releases the cantilever. The mFupC occurs whenever the moving platform isdisplaced by a certain threshold distance (b) z1 or (c) −z2.

principle is named as mechanical frequency up conversion (mFupC)[18].

In this paper, we present a MEMS-based electromagnetic energyharvester using the mFupC principle. The targeted vibrations toharvest energy are not necessarily resonant, but with low fre-quency (<20 Hz) and low displacement amplitude (<10 mm peakto peak) characteristics. The moving structures in the energy har-vester have been attached to different bases, so the operationof the energy harvester does not rely on a resonant vibration.Another aim of the study is to realize energy generation in avery small volume (<250 mm3), enabling the utilization of theenergy harvester for portable and implantable self-powered sys-tems. Accordingly, microfabricated coils and cantilevers have beenutilized in the structure. The device structure and operation prin-ciple are presented in Section 2. Section 3 presents the theoreticalwork conducted for the design of the energy harvesters. Section 4gives the fabrication procedure of the device and the test setup, anddetailed test results of the devices are presented in Sections 5 and6. Section 7 concludes the paper.

2. Energy harvester structure


Fig. 1(a) shows the energy harvester structure. The harvesterconsists of two clamped-free support type cantilever beams,pick-up coils attached onto each cantilever, a magnet, and mechan-ical barrier arms extending from the sides of the magnet. The

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ig. 2. The schematics used for the modeling of the energy harvesters. (a) Top viewnitial position and just before the release moment.

antilever/coil structures are attached to a fixed base, whereas theagnet is placed on a moving platform. It should be noted that

he reverse configuration is also equivalent, where the magnet isxed and the cantilever/coil structure is moving. The former con-guration together with only one of the coil/cantilever structures isresented in Fig. 1 for the sake of simplicity. Fig. 1(b) and (c) illus-rates the operation principle of the energy harvester: With the

oment of the platform, the magnet is displaced and the barrierrms periodically touch, bend, and release the cantilevers. Whenhe cantilevers are released, they resonate at their resonant fre-uencies, realizing the mFupC. The electrical energy is generatedcross the coil terminals via electromagnetic induction, resultingrom the relative displacement of the resonating coil and the mag-et. In the proposed structure, the input vibration does not needo have a resonant or periodic characteristic for energy generation.he energy is generated at each time when the magnet is displacedore than a certain threshold distance to release the cantilever

rom the barrier arm.

. Theory and design

Both numerical methods and FEM simulations are utilized inhe modeling of the energy harvesters. Fig. 2 shows the schematicndicating the required dimensions for modeling the structure. Aingle cantilever with one layer of coils is used in the model; how-ver, there are 2 coil layers on two cantilevers. Hence the generatedoltage and coil resistance values should be doubled for each coil ofhe proposed structure. Furthermore, during the combined motion


f the magnet and the cantilever, it is assumed that the cantileveroes not bend, but maintain its planar form.

Once the cantilever is released from the barrier arm at a distance0 from its rest position, it starts resonating at its own resonance

ross section indicating the position of the cantilever with respect to the magnet at

frequency, and a time varying magnetic flux occurs around the coil.An induced voltage is generated across the coil as:

ε(t) = −dϕ

dt= −

(d�Bdt

· �A + d�Adt

· �B)

(1)

In (1), ϕ is the magnetic flux, B is the magnetic flux density, A isthe area of the coil, and t is time. The magnetic flux passing througha planar coil is expressed as:

ϕ =n∑

i=1

(�Bi · �Ai) (2)

where n is the number of coil turns and i is the index number of theevaluated turn. The magnetic field around the coil depends on thereciprocal positioning of the coil and the magnet during the motionof the coil. Hence, the distribution of the field around the magnetshould be known. Fig. 3 shows the distribution of the magnetic fluxdensity vector, B around a 2.5 mm × 0.5 mm magnet which is sim-ulated in Maxwell 2D. The magnetic flux density vectors along the3rd dimension (z-axis) are not available since the utilized FEM toolis a 2D solver (x–y plane). Hence, they are assumed to be equal to theones presented in Fig. 3 for the rest of the calculations. Accordingly,(2) is re-written as:

ϕ =n∑

i=1

((∫li

Bi · dl

)ki

)(3)


where the integral is taken on the line on which the coil turn lies.This value is then multiplied by the width of the coil turn, ki, givingthe flux passing though the coil.

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Fig. 3. (a) FEM simulation result of the distribution of the magnetic flux densityaround a magnet. The positioning of the coil with respect to the magnet both atinitial position and just before the release point are also indicated. (b) Zoomed viewa2t

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troplated copper layer and the heating of the substrate during theDRIE step.

Fig. 7(b) shows the picture of the utilized NdFeB magnetwith the polystyrene film which is shaped and glued onto

Table 1Summary of the physical properties and the calculated values of the generatedvoltage and power for two different energy harvesters.

Energy harvester 1 Energy harvester 2

Magnet propertiesMaterial NdFeBDimensions 2.5 mm × 2.5 mm × 0.5 mmSaturation magnetization 1.2 T

Coil propertiesMaterial CopperWidth × height 10 �m × 10 �m 15 �m × 10 �mSpacing 10 �m 5 �m# Coil turns 41 × 2 layersCoil resistance 54 � 36 �

Cantilever propertiesSize 2 mm × 2 mm × 15 �mMaterial ParyleneResonance frequency 1918 Hz 1764 Hz

Operational dimensionsOverlap (p) 200 �mZ0 870 �m

round the north pole of the magnet. The magnet size is 2.5 mm × 0.5 mm. Since aD simulator is used, the depth-dependent variations are cannot be obtained fromhe simulation.

The time derivative of the magnetic flux is calculated in dis-rete domain to find the generated voltage across the coil duringts movement as:

(tn) = −�ϕ

�t= − (ϕ(tn) − ϕ(tn−1))

tn − tn−1(4)

The position of the cantilever with respect to the coil for sev-ral time instants is determined, and the flux passing through theoil at these time instants are calculated by (3), and then insertednto (4) in order to calculate the generated voltage. The position ofhe cantilever at each time instant is related to its resonance fre-uency and the damping ratio. The position is evaluated by usinghe motion of the tip of the cantilever during resonance:

(t) = Z0e−�·2·�·fn·t sin(2�fnt + �) (5)

here, � is the damping ratio, fn is the resonance frequency of theantilever, and � is the phase angle. The resonance frequency of theantilever is simulated on Coventor MEM-Mech solver as shown inig. 4.

The calculation of the damping ratio � is given in [19]; how-ver, the actual value of the damping ratio is dependent on manyactors such as the velocity of the beam sections, the beam geom-


try, the ambient temperature etc. Thus, � is taken as 0.018 in thealculations regarding our previous works [18].

Table 1 presents a summary of the physical properties andhe calculated values of the generated voltage and power for two

Fig. 4. Conventor MEM-Mech modal simulation result of the cantilever of energyharvester 1. The properties of the cantilever are presented in Table 1.

different energy harvesters, which are discussed in the followingsections of the paper.

4. Fabrication

Fig. 5 shows the fabrication process flow of the cantilever/coilstructure of the micro energy harvester [20]. The structure is fabri-cated on a Si substrate, with a 0.5 �m SiO2 at the backside. The firstparylene layer with 10 �m thickness is deposited on the wafer, thefirst coil level is formed with a 10 �m-thick copper electroplatingprocess on a Ti/Au seed layer, and the seed layer is etched away.Then, 2nd parylene layer (5 �m-thick) is deposited and patternedand the 2nd coil layer is formed in a similar way. Afterwards, theparylene is patterned, forming the cantilevers and the via openings.Fig. 6 shows the SEM images of the cantilever/coil structure. Finally,the cantilever is released by patterning the backside SiO2 and etch-ing the Si substrate with a DRIE process. The dimension of eachcantilever is 2 mm × 2 mm, and the total chip size is 4 mm × 8.5 mm.Fig. 7(a) shows one of the energy harvester chips attached to a PCBhaving a hole which provides the space for the movement of themagnet. The deformation observed on the shapes of the cantileversin Fig. 7(a) is due to two main reasons: the internal stress of the elec-


Generated powerVpeak 65 mV 55 mVVRMS 12 mV 10 mVPRMS (for an equivalent load) 670 nW 700 nW

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(a)

(b)

(c)

(d)

(e)

(f)

(g)

SiO2

ParyleneSi

Copper

Ti/Au

Fig. 5. The fabrication process flow of the micro energy harvester cantilever/coilstructure. (a) Si wafer with SiO2 at the backside and parylene deposited to the frontside, (b) Ti/Au seed layer deposition, (c) copper electroplating for the 1st coil layerand seed layer strip (d) 2nd parylene layer deposition and via formation, (e) copperelectroplating for the 2nd coil layer and seed layer strip, (f) parylene patterning:cantilevers are formed, (g) device release via SiO2 RIE and Si DRIE.

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Table 2The summary of the test results of the energy harvester prototype. 1.1 �J of energyis transferred from the coils to equivalent resistive load for each occurrence of themFupC.

Cantilever 1 Cantilever 2

Dimensions 2 mm × 2 mmResonance frequency (Hz) 1950 2100Coil resistance (�) 61 64Vpeak (mV) 49.5 53.1Ppeak (�W) 10.0 11.0VRMS (all signal) (mV) 2.12 2.19PRMS (all signal) (nW) 18.4 18.7

generation within a very small duration before the signal dies out.The difference in the damping ratio is probably due to the very largeaspect ratio of the cantilever: the ratio of its width and length with

he magnet to realize the barrier arms required for mechan-cal frequency up conversion. The dimensions of the magnetre 2.5 mm × 2.5 mm × 0.5 mm, and the polystyrene barrier armsxtend by 350 �m from the two sides of the magnet. The gapetween the cantilevers is designed as 2.7 mm, resulting in 250 �mverlap between each barrier arm and cantilever. The alignmentf this overlap is done during the tests of the energy har-


ester.

Fig. 6. SEM images of the MEMS cantilever/coil: (a) 1st coil layer, (b) both coil lay

VRMS (during mFupC) (mV) 9.42 9.64PRMS (during mFupC) (nW) 364 363Generated energy for each occurrence (�J) 1.092 1.089

5. Experimental setup

Fig. 8(a) shows the schematic representation of the experimen-tal test setup. The setup is composed of a shaker table, an xyz-stage,an oscilloscope, and the energy harvester which includes the energyharvester chip and the magnet. The magnet is attached to the shakertable, while the energy harvester chip is fixed on an xyz-stage,enabling the positioning of the cantilevers and the magnet. Theshaker table is PC controlled, and the coil voltages are observedthrough an oscilloscope. Fig. 8(b) gives a close-up view showingthe alignment of the magnet and the cantilevers.

6. Results and discussion

6.1. Characterization of the generated voltage across the coils

The tests of the energy harvester are carried out under 10 Hz,5 mm peak to peak vibrations (1 g peak acceleration), and both coilvoltages are observed through the oscilloscope. Fig. 9 shows thegenerated voltages from the energy harvester coils. The resonancefrequencies of the cantilevers are 1950 and 2100 Hz for Cantilever1 and Cantilever 2, respectively. The peak and RMS values of thegenerated voltage across the coils are around 50 mV and 2.1 mV,corresponding to 10 �W peak and 18 nW RMS power, respec-tively for equivalent resistive loads (RL = Rcoil = ∼60 �). Table 2summarizes the test results of the energy harvester prototype. Theoperation frequency of the cantilever, the coil resistance, and thegenerated peak voltage are in good agreement with the theoret-ically predicted values presented in Table 1, however there is amismatch in the generated RMS voltage and power from the energyharvester. This is mostly due to the much higher damping ratio ofthe cantilever with respect to the predicted one, leading to energy


respect to its thickness. The other reason may be the non-perfect

ers separated by parylene, and (c) view of the coil/cantilever before release.

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Fig. 7. (a) Picture of the MEMS energy harvester chip attached to a PCB. The chip size is 4 mm × 8.5 mm. The hole on the PCB and the opening between the cantilevers providet .5 mm3

cc

ofaclodarai

bc

Fcx(t

he space for the magnet. (b) The picture of the NdFeB magnet (2.5 mm × 2.5 mm × 050 �m from the sides of the magnet.

lamping between the fixed edge of the cantilever and the siliconhip base, further increasing its damping ratio.

Fig. 9(b) and (c) show the zoomed waveforms after the releasingf the cantilevers from the barrier arms. The mFupC clearly occursor the case where the magnet is moving upwards, however, only

small peak is observed for the downward motion case, and theantilever motion is directly damped without making any oscil-ations at its resonance frequency. The stressed and bended formf the cantilevers has an effect on this issue, changing the motionynamics and the damping ratios according to whether the barrierrm touches the cantilever from its upper or bottom surface: Theelease of the cantilever from the barrier arm occurs very rapidlynd sharply while the magnet is moving upwards, however, theres a gentler and slower release for the downward motion case.


The duration of the power generation due to mFupC should alsoe discussed regarding Fig. 9. Due to the high damping ratio of theantilevers, the power is generated for about 2 ms after the mFupC

ig. 8. (a) The schematic of the experimental setup. The magnet is attached to aomputer controlled shaker table and the energy harvester chip is attached to anyz-stage. The generated voltage through the coils is measured by an oscilloscope.b), Close-up view showing the alignment of the magnet and the barrier arms withhe energy harvester cantilevers.

) and the polystyrene film shaped to form the barrier arms. The arms extend about

occurs, and this is a very small portion of the overall vibration dura-tion of 100 ms. Accordingly, the noise signal on the coil voltage hasa significant effect on the calculated RMS value. In order to mini-mize this effect, and to be able to make a better comparison withthe theoretically predicted values presented in Table 1, the gener-ated voltage is evaluated within a 3 ms duration (21–24 ms in Fig. 9)around which the mFupC occurs. It should also be noted that theenergy generation during the downward motion of the magnet isnot considered for the calculations since it has a negligible effect onthe generated power for the prototype under test. The RMS value ofthe generated voltage within the mFupC duration is around 9.5 mV(theoretically 12.5 mV), and the RMS power delivered to equivalentloads is 363 and 364 nW for Coil 1 and Coil 2, respectively. The gen-erated power is about the half of the theoretical one, however, itshould be noted that the differences in both coil resistance and gen-erated RMS voltage contribute to the deviation from the predictedpower value.

The generated power can also be interpreted in a way that foreach arbitrary occurrence of the mFupC, 1.1 mJ of energy is trans-ferred to equivalent loads through both coils within a duration of3 ms. Naturally, the generated energy increases with the increasednumber of occurrences, however, energy generation is not depend-ent on the frequency or the periodicity of the vibration. Hence theenergy harvester is said to be non-resonant.

6.2. Analysis of the serial connection of the coils

The generated voltage and power in the case of serial connectionof multiple coils is also a point of interest since this may lead to anincrease in the generated voltage and power levels (peak and RMSvalues). This may be beneficial for the rectifying electronics to beused, since a threshold voltage is needed for these circuits to startoperating [13]. Other alternatives are to use a very high efficiencytransformer to amplify the generated AC voltage, in the expense ofan increase in the volume [21], or to design the rectifying electron-ics in very advanced and costly CMOS technologies enabling theutilization low threshold transistors. Even if the generated voltagethrough a single coil is high enough for starting up the electron-ics, combining the signals and processing them in a single circuitincreases the rectification efficiency and decreases the required ICarea for the electronics. This may especially be important if there


are many coils in the energy harvester as in [9,17]. For such a case, ifall waveforms have the same shape and they are perfectly synchro-nized, the maximum peak and RMS values of the generated voltageare achieved as the summation of the individual peak and RMS

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-40

-20

0

20

40

60

Vou

t Coi

l 1 (

mV

)

-10 0 -80 -60 -40 -20 0 20 40 60 80 10 0-40

-20

0

20

40

60

time (ms)

Vou

t Coi

l 2 (

mV

)

-20

-15

-10

-5

0

5

10V

ou

t Co

il 1

(m

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67 68 69 70 71 72-20

-15

-10

-5

0

5

10

time (ms)

Vo

ut C

oil

2 (

mV

)

-40

-20

0

20

40

60

Vou

t Coi

l 1 (

mV

)

18 19 20 21 22 23 24 25 26-40

-20

0

20

40

60

time (ms)

Vou

t Coi

l 2 (

mV

)

10 Hz vibration

Time differenceof mFupCoccurances

1950 Hz

2100 Hz

Magnet

moving upMagnet

moving

down

VRMS=9.42 mV

VRMS=9.64 mV

VRMS=2.12 mV

VRMS=2.19 mV

(a)

(b) (c)

Fig. 9. (a) The generated waveform from the coils of the energy harvester under 10 Hz, 5 mm peak to peak vibration. The RMS values of the generated voltages are 2.12 and2 t at 1o

vp

P

cff

.19 mV, respectively. (b) The mFupC occurs for the upper movement of the magnenly a small peak is observed for the downward movement.

alues of each waveform. This leads to an increase in the generatedower as:

RMS,serial = (VRMS1 + VRMS2 + . . . + VRMSn )2

4(R1 + R2 + . . . + RN)(6)


However, mostly, the waveforms cannot be perfectly syn-hronized since there exist some differences in the resonancerequencies and the release moments of the cantilevers comingrom the fabrication non-uniformities or the limitations of the

950 and 2100 Hz with an RMS value of 9.42 and 9.64 mV within 3 ms, however (c)

assembly precision. Hence, while adding the waveforms, someportion of the signals cancel each other out and this lead to smallerpeak or RMS values for the overall waveform.

Such waveforms are also observed in Fig. 9 for the prototypeunder test. The time difference between the mFupC occurrencesand different resonance frequencies of the generated voltages over


Coil 1 and Coil 2 are shown in Fig. 9(b). In such a case, it is nec-essary to examine the serial connection of the coils consideringtwo alternatives: addition or subtraction of the waveforms, sinceone alternative may give a better result than the other one. Fig. 10

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Table 3The summary of the calculated results for serial connection of the coils.

Vcoil 1 + Vcoil 2 Vcoil 1 − Vcoil 2

Coil resistance (�) 125Vpeak (mV) 54.3 53.1Ppeak (�W) 5.90 5.64VRMS (all signal) (mV) 3.03 3.05PRMS (all signal) (nW) 18.3 18.6VRMS (during mFupC) (mV) 13.45 13.51

sst3iiiiwt

F(

Fs

PRMS (during mFupC) (nW) 362 365Generated energy for each period (�J) 1.086 1.095

hows the resulting waveforms after mathematical addition andubtraction of the coil voltages, and Table 3 gives the summary ofhe calculated values. The RMS values are calculated over the same

ms period described in Section 6.1. It is seen that the RMS valuesncrease by a factor of about 40%, while the peak values only slightlyncrease for both cases. Both connection alternatives give very sim-


lar results in terms of generated power and energy; however, thiss a case specific coincidence related to the timing of the signals

ith respect to each other. Furthermore, the generated power withhe serial connection stays in the same level since the equivalent

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t Coi

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Coi

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Vcoil1+Vcoil1- Vcoil2+

Vcoil2-

Vout=Vcoil1+Vcoil2

Vcoil1+Vcoil1- Vcoil2-

Vcoil2+

Vout=Vcoil1-Vcoil2

(a)

(b)

ig. 10. Mathematical analysis of the serial connection of the coils: (a) addition andb) subtraction of the coil voltages around the mFupC duration.

ig. 11. Pictures of the copper coil lines around the fixed edge of the cantilever: (a) an utress over the cantilever for 6 mN applied load corresponding to 140 �m tip displacemen

Fig. 12. The picture of a prototype in which the fixed edge of the cantilevers iscovered by an epoxy, in order to reduce the stress on the coil lines.

resistive load also increases (Req. = Rcoil 1 + Rcoil 2). For this specificcase, serial connection of the coils has no significant advantage ordisadvantage on the total generated power over treating both coilsignals separately.

6.3. Stress at the cantilever edges

During the tests of the prototypes it is observed that the stressaround the fixed end of the cantilevers result in a wearing onthe copper coil lines, and the line eventually breaks during thetesting of the devices. Fig. 11 presents one of these cases, whereFig. 11(a) shows an undamaged coil detail around the cantileveredge, whereas Fig. 11(b) shows a broken coil line. Fig. 11(c) showsthe simulation of the stress distribution over the cantilever with140 �m tip displacement (6 mN force applied to the cantilever tip),verifying the excessive stress on the edge of the 2nd metal line ofthe coil, which lies on the parylene cantilever. The stress is muchsmaller on the 1st metal line edge since this line is fully coveredwith parylene from all sides.

In order to reduce the stress on the coil line, an epoxy is appliedalong the fixed edge of the cantilever covering all the edge and somepart of the coil, as presented in Fig. 12. The prototype preparedwith this method was able to survive all the tests with different


vibration frequency and displacement profiles as will be presentedin Sections 6.4 and 6.5.

Alternatively, the stress on the copper line can be reduced bycovering the top surfaces of the devices by a 3rd parylene layer.

ndamaged coil, (b) a broken coil line due to excessive stress, (c) simulation of thet.

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1250 Hz

1250 Hz

VRMS=3.75 mV

VRMS=5.69 mV

VRMS=1.46 mV

VRMS=2.16 mV

VRMS=1.33 mV

VRMS=4.73 mV

Time differenceof mFupCoccurances

Fpp

Hts

6

w

TTa

ig. 13. The generated voltage waveform from the coils of the energy harvesterrototype with epoxy applied to the cantilever edges, under 10 Hz, 5 mm peak toeak vibration.

owever, this is not preferred here due to the need of an addi-ional photolithography step, which is not possible with the alreadyuspended structures.

.4. Characterization of the epoxy-covered prototype


Fig. 13 and Table 4 present the test results of a sample devicehose cantilever edges are covered with an epoxy. The effect of

able 4he summary of the test results of the energy harvester prototype with epoxypplied to the cantilever edges.

Cantilever 1 Cantilever 2

Dimensions 2 mm × 2 mmResonance frequency (Hz) 1250 (sim: 1764) 1250Coil resistance (�) 24 37Vpeak (mV) 22.3 27.1Ppeak (�W) 5.18 4.96VRMS (all signal) (mV) 1.46 2.16PRMS (all signal) (nW) 22.2 31.5VRMS (during mFupC) (mV) up/down 3.75/– 5.69/4.73PRMS (during mFupC) (nW) 146/– 219/151Generated energy for each period (�J) 0.584 0.876 + 1.035 = 1.911

PRESSuators A xxx (2013) xxx– xxx 9

the applied epoxy is seen in Fig. 13 as a decrease in the resonancefrequency of the cantilevers, leading to the conclusion that theincrease in the equivalent cantilever mass dominates the increasein the stiffness of the structure. In addition to this, the damping onCoil 1 is increased, while no significant evidence of this is seen onthe waveform of Coil 2.

Furthermore, it should be noted that the coil lines of the testedprototype are designed with 15 �m width and 5 �m spacing in thisdevice, leading to a reduced resistance of around 37 �, when com-pared to the coil resistance of 60 � for the former structure. The24 � resistance of Coil 1 indicate some short-circuited turns forthis coil structure.

The presented test results are taken under 10 Hz vibrations with5 mm peak to peak displacement. The peak and RMS values of thegenerated voltage for Coil 1 are smaller than the ones for Coil 2 dueto the smaller effective number of turns. The peak power for Coil 1is higher since the coil resistance is smaller; however RMS poweris still smaller than that of Coil 2.

Comparison of the test results with the ones presented in theprevious section in Fig. 9 and Table 2 shows that the generatedenergy is about the half for Coil 1 (0.584 �J with respect to 1.1 �J ofeither coil of the previous prototype), which is due to smaller num-ber of turns and smaller resonance frequency, both resulting in lessinduced voltage across the coil. On the other hand, there is only aslight decrease in the generated energy for the upwards motion ofthe magnet for Coil 2 (0.876 �J), while about 1 �J of energy is gener-ated as a result of the downward motion, which was not observedin the previous prototype. However, it should be noted that thisenergy generation during downward magnet motion does not showan mFupC characteristic: the waveform at this region is more like asmall induced voltage peak resulting from the displacement of themagnet with respect to the coil. The waveform dies slowly (within8 ms) when compared to 2 ms of the previous prototype or 4 ms ofthis prototype for the upper magnet motion, but without makingany oscillations, indicating a high damping ratio for this part of thecantilever motion.

As a conclusion, it can be said that the utilization of the epoxyon the cantilevers decrease the generated power levels since itdecreases the resonance frequency values. On the other hand, itprevents the failure of the copper lines due to excessive stress.Instead of using an epoxy, the fabrication process may be updatedwith a 3rd parylene layer coated and patterned on the cantilevers asmentioned in Section 6.3, to provide a more repeatable, wafer-levelprocess alternative to increase the life time of the energy harvestingcoils.

6.5. Operation under varying vibration conditions

The response of the energy harvester covered with epoxy hasbeen tested and characterized with different excitation conditions,and the non-resonant operation behavior is shown. During thetests, several data has been collected from the prototype for eachexcitation condition. Fig. 14 shows the generated RMS voltage ontwo coils of the harvester for a vibration at 10 Hz with 3, 4, and5 mm peak to peak amplitude. It is seen in Fig. 14 that the RMSvalue of the generated voltage does not change significantly withthe peak amplitude of the vibration. This is an expected resultsince the mFupC occurs after a certain displacement of the magnet,and the generated waveform is then related only to the oscillationof the cantilever. The magnet can safely be assumed as station-ary during this oscillation since the frequency of the magnet andcantilever motions are very different from each other. The peak


to peak displacement value may have some contribution to thevoltage waveform only if the vibration frequency is a few times(∼<20) smaller than the resonance frequency of the cantilever. Inthe presented case, there is more than two orders of magnitude

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1,2

1,4

1,6

1,8

2

2,2

2,4

2 3 4 5 6

Vout

RM

S (m

V)

peak to peak displacement (mm)

Coil 1 Coil 2

Fig. 14. The RMS value of the generated voltage on the coils of the energy harvesterunder 10 Hz vibration with 3, 4, and 5 mm peak to peak displacement, correspondingto 0.6, 0.8, and 1 g peak acceleration.

1,2

1,4

1,6

1,8

2

2,2

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International Electrical Machines and Drives Conference, Antalya, Turkey, May,

ig. 15. The RMS value of the generated voltage on the coils of the energy har-ester under 5, 10, and 20 Hz vibration with 5 mm peak to peak displacement,orresponding to 0.25, 1, and 4 g peak acceleration.

ifference between these frequency values (10 Hz and 1250 Hz),nd no contribution of the magnet motion is seen.

Fig. 15 shows the generated RMS voltage from the coils of thenergy harvester for 5 mm peak to peak vibrations at 5, 10, and0 Hz frequency for several measurements, with the associatedrend lines showing the increasing tendency of the generated volt-ge for each coil. Since the energy harvester does not require anyesonance frequency for operation, the energy is generated for anyibration frequency, however the value increases with increasingibration frequency, or number of mFupC occurrences, as explainedefore.

. Conclusion

In this paper, a miniature vibration based electromagneticEM) energy harvester, which generates energy from non-resonantibrations, has been presented. The structure combines two MEMS-ased coils realized on parylene cantilevers (energy harvester chip)nd a miniature NdFeB magnet with mechanical barrier armsttached on it. These arms have been employed to realize theFupC. The magnet and the chip have been attached to platformsoving independent of each other, eliminating the need for a res-

nant vibration for proper operation. With this configuration, therototype is able to work with any kind of motion as long as themplitude of the external vibration is high enough to release theantilever from the barrier arm.


The fabricated energy harvester chip has a size of mm × 8.5 mm, and the peak to peak motion of the magnetanges between 3 and 5 mm. The prototype generates 2.1 mV

[

PRESSuators A xxx (2013) xxx– xxx

RMS voltage and 18.5 nW RMS power with 10 Hz, 5 mm peak topeak (1 g) external vibrations, and 1.1 �J energy is transferred toequivalent resistive loads from each coil for each occurrence ofthe mFupC. The generated RMS voltage and power level for themFupC duration is also calculated to be around 9.5 mV and 363 nW,respectively. The non-resonant operation of the energy harvesteris also demonstrated by testing the device with different excitationfrequency values. The increased RMS value of the generatedpower with the increased operation frequency is also verified.The generated power can be further increased by decreasing thedamping ratio of the cantilevers. This can be achieved by usinga stiffer cantilever material such as Si, or operating the energyharvester under vacuum. However, in the former option, theenergy required to move the magnet up to the release point of thecantilever increases, and in a real application, this may affect theoperation of the part or junction on which the magnet is fixed.The latter case brings fabrication complexity and may limit theapplication areas of the energy harvester.

It should also be noted that the prototype utilizes electromag-netic transduction for energy generation; however, the mFupCtechnique can be adapted to piezoelectric and electrostatic typeof energy harvesters. The prototype is a good candidate for energyharvesting applications with non-resonant vibration characteris-tics including human motions, movement of the branches of thetrees, and vehicle motions.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.sna.2013.01.032.

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[7] K. Najafi, T. Galchev, E.E. Aktakka, R.L. Peterson, J. McCullagh, Microsystems forenergy harvesting, in: Proceedings of Transducers’11, Beijing, China, June 5–9,2011, pp. 1845–1850.

[8] I. Sarı, T. Balkan, H. Külah, An electromagnetic micro power generatorfor wideband environmental vibrations, Sensors and Actuators A 145–146(July–August) (2008) 405–413.

[9] J.Q. Liu, H.B. Fang, X.H. Mao, X.C. Shen, D. Chen, H. Liao, B.C. Cai, A MEMS-basedpiezoelectric power generator array for vibration energy harvesting, Micro-electronics Journal 5 (May) (2008) 802–806.

10] H. Liu, C. Lee, T. Kobayashi, C.J. Tay, C. Quan, Investigation of a MEMSpiezoelectric energy harvester system with a frequency-widened-bandwidthmechanism introduced by mechanical stoppers, Smart Materials and Struc-tures 21 (2012).

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netic energy harvesting system with highly efficient dual rail output, IEEESensors Journal 12 (June (6)) (2012) 2287–2298.

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Biographies

Özge Zorlu was born in Zonguldak, Turkey, in 1978. He received the B.Sc. and M.Sc.degrees in electrical and electronics engineering (with honors) from Middle EastTechnical University (METU), Ankara, Turkey, in 2000 and 2002, respectively, andthe Ph.D. degree in microtechnology from Ecole Polytechnique Federale de Lausanne(EPFL), Lausanne, Switzerland, in 2008. He was a Research Assistant with METUand EPFL between 2000 and 2008. In 2008, he joined the METU-MEMS Researchand Application Center, Ankara, Turkey, as a Research Fellow. His research inter-ests include MEMS-based energy harvesting, design and fabrication of microsensors,CMOS-integrated sensor design, MEMS fabrication technologies, fluxgate-type mag-netic micro sensors, magnetic thin films, and magnetic sensors and actuators.

Haluk Külah received the B.Sc. and M.Sc. degrees in electrical engineering with highhonors from Middle East Technical University, Ankara, Turkey, in 1996 and 1998,respectively, and the Ph.D. degree in electrical engineering from the University ofMichigan, Ann Arbor, in 2003. He was a Research Fellow with the Department of Elec-trical Engineering and Computer Science, University of Michigan, from 2003 to 2004.


In August 2004, he joined the Electrical and Electronics Engineering Department ofMETU as a faculty member. Since 2008, he is also working as the deputy directorof METU-MEMS Center. His research interests include MEMS sensors, mixed-signalinterface electronics design for MEMS sensors, BioMEMS, and MEMS-based energyscavenging.

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A MEMS-based energy harvester for generating energy from non-resonant environmental vibrations

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Transcript of A MEMS-based energy harvester for generating energy from non-resonant environmental vibrations