TECHNICAL PAPER

E. Koukharenko Æ S. P. Beeby Æ M. J. Tudor

N. M. White Æ T. O’Donnell Æ C. SahaS. Kulkarni Æ S. Roy

Microelectromechanical systems vibration powered electromagneticgenerator for wireless sensor applications

Received: 1 August 2005 / Accepted: 28 October 2005 / Published online: 25 April 2006� Springer-Verlag 2006

Abstract This paper presents a silicon microgenerator,fabricated using standard silicon micromachining tech-niques, which converts external ambient vibrations intoelectrical energy. Power is generated by an electromag-netic transduction mechanism with static magnets posi-tioned on either side of a moving coil, which is locatedon a silicon structure designed to resonate laterally in theplane of the chip. The volume of this device is approx-imately 100 mm3. ANSYS finite element analysis (FEA)has been used to determine the optimum geometry forthe microgenerator. Electromagnetic FEA simulationsusing Ansoft’s Maxwell 3D software have been per-formed to determine the voltage generated from a singlebeam generator design. The predicted voltage levels of0.7–4.15 V can be generated for a two-pole arrangementby tuning the damping factor to achieve maximum dis-placement for a given input excitation. Experimentalresults from the microgenerator demonstrate a maxi-mum power output of 104 nW for 0.4g (g=9.81 m s�1)input acceleration at 1.615 kHz. Other frequencies canbe achieved by employing different geometries or mate-rials.

1 Introduction

The rapid increase in demand for remotely locatedwireless devices has demonstrated the need for self-suf-ficient power sources. The growth in wireless sensornetworks has been greatly facilitated by developments inlow-power communications standards such as Zigbeeand IEEE 802.15.4. Similarly, low-power microelectro-

mechanical systems (MEMS) have allowed the devel-opment of highly accurate, portable sensors andinstruments for numerous applications in health care,condition monitoring (embedded sensors in building andbridges), consumer products, and defence [1]. Wirelessoperation enables such microsystems to be completelyembedded in a structure with no physical connection tothe outside. Typically, the energy necessary to powersuch systems is stored in batteries, which have draw-backs: they tend to be bulky, contain a finite amount ofenergy and have a limited shelf life. The replacement ofbatteries places an additional cost and environmentalburden on the use of wireless sensor systems and is notpossible in embedded applications. Alternative powersources can potentially overcome these drawbacks.

A potential alternative to batteries is miniature, self-contained, renewable power supplies that convert energyfrom an existing source in the environment into electri-cal energy. The most common ambient sources are solar,vibration and thermal. In this work, we have used theprinciple of electromagnetic induction for the generationof electrical energy from kinetic energy present in theenvironment in the form of vibrations [2–5]. An earlierdevice developed at the University of Southampton wasaround 3,000 mm3 in volume and was fabricated usingconventional machining techniques [6]. The motivationfor this work is the development and fabrication of aMEMS version of this generator that uses standard sil-icon-based fabrication techniques and a low-cost, batchprocess.

2 Microgenerator structure

The arrangement of the MEMS version of the device,based on four magnets, is shown in Fig. 1. The coil isrecessed in a silicon cantilevered paddle designed to vi-brate laterally in the plane of the wafer. Discrete mag-nets are positioned within etched recesses in cappingwafers that are bonded to each face of the middle wafer.The bonding process is aligned to ensure correct place-

E. Koukharenko (&) Æ S. P. Beeby Æ M. J. Tudor Æ N. M. WhiteSchool of Electronics and Computer Science,University of Southampton, SO17 1BJ Southampton, UKE-mail: ak@ecs.soton.ac.uk

T. O’Donnell Æ C. Saha Æ S. Kulkarni Æ S. RoyTyndall National Institute, Prospect Row, Cork, Ireland

Microsyst Technol (2006) 12: 1071–1077DOI 10.1007/s00542-006-0137-8

ment of the coil relative to the magnets. Figure 2 showsthe top view of the paddle layer, which is fabricated bydeep reactive ion etching (DRIE) through the totalthickness of the wafer.

3 Microgenerator mechanical modelling

The mechanical characteristics of the moving compo-nent of the generator (the cantilevered paddle) have beensimulated using ANSYS finite element analysis (FEA).The natural frequencies of different cantilever configu-rations machined in silicon have been determined usingmodal analysis. Mechanical stresses induced by thedeformation have also been simulated in order to ensurethat the structure can withstand repeated cyclicalstressing at maximum amplitude.

Three sets of supporting paddle beam dimensionshave been simulated, each 1 mm long and 500 lm thick.In model A the beam is 500 lm wide, model B 400 lmwide and C 300 lm wide. Table 1 lists the resonantfrequencies of the fundamental lateral modes of thegenerator, which is the operating mode of interest. Thismode is shown viewed from above in Fig. 3.

The amplitude of vibration is limited by the sur-rounding silicon frame thereby providing physical over-range protection. The corresponding maximum dis-placement of the centre point of the coil is 240 lm. Thereduced stiffness of the lower frequency designs meansthat the level of stress experienced at the maximumdeflection is lower than for the stiffer structures. In thecase of device C, the least stiff device, the maximumstress is 2.6 GPa which is well below the yield strength ofsilicon. The ultimate robustness of the generator will inpractice be limited by surface and/or material imper-fections and any impact stress arising from the paddlecontacting the frame. The influence of surface roughnessis discussed in more detail in the fabrication sectionbelow.

4 Magnetic modelling

Electromagnetic FEA simulations using Ansoft’s Max-well 2D software has been performed to determine themagnitude of the voltage which can be generated fromthe single beam generator designs. The coil used in theinitial simulation is a conventionally wound enamelledcopper coil with 600 turns of 25 lm wire. The outerdiameter of the coil is 2.4 mm and the inner diameter is0.6 mm. The separation between magnets and coil is0.1 mm. The resulting mass of the silicon paddle pluscoil is measured to be 2.8·10�5 kg. As described above,and shown in Fig. 1, the coil is positioned between themagnets. Four magnets are arranged in pairs to form atwo-pole arrangement as shown in Fig. 4. The magnetsused were sintered NdFeB with dimensions of1·1·3 mm. 3D FEA simulation has been used to verifythe flux density distribution between the magnets. Theflux density distribution for the two-pole configuration isshown in Fig. 5. As can be seen from the plot the fluxdensity varies from approximately +0.5 T under onepole to approximately –0.5 T under the other pole. Inorder to determine the voltage which would be inducedin the coil, given a certain vibration frequency, f, coildisplacement, Xmax, and velocity, v, a 2D transient finiteelement simulation is used. A sinusoidal velocity givenby v=Xmax2pf sin(2pft), is used as input to the transientsimulation to specify the coil movement.

Table 1 gives the predicted results for the voltages andpowers generated according to the FEA for the three

Fig. 1 Micromachined silicon generator

Frame

Coil

Paddle

Supporting cantilever

Fig. 2 Top view silicon paddle

Table 1 Simulation results for the natural frequency, fres, voltages,Vload, (peak) and power, Pload, delivered to the load resistance,Rload for three generator structures with 500 lm separation dis-tance and 240 lmmaximum displacement of the centre point of thecoil

Model fres(kHz)

Beam width(lm)

Rload

(kX)Vload

(V)Pload

(mW)

A 9.812 500 20.4 9.0 2.0B 7.149 400 14.8 6.5 1.45C 4.743 300 9.8 4.3 0.96

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beam geometries A, B and C and two-pole magnet con-figuration for a 240 lm coil displacement and a 100 lmseparation distance between the coil and the permanentmagnets. The simulation results indicate that sufficienthigh voltages, which allow rectification without signifi-cant loss, can be generated. The estimated power is basedon the power delivered to the load resistance indicatedin the table. This value of load resistance has been chosenso as to maximise the displacement for 1g vibrationacceleration as described below.

For the purposes of the 2D finite element simulationthe maximum displacement is fixed at 240 lm and it is

Fig. 5 3D FEA simulation offlux density for the two-poleconfiguration with 100 lmseparation distance betweencoil and permanent magnets

Fig. 4 Magnet configuration

Fig. 3 Generator mode ofoperation

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assumed that sufficient mechanical force, given by theproduct of mass and acceleration, exists to give the re-quired displacement. In order to illustrate how this isaffected by the choice of load resistance, we assume thatthe motion of the device can be described as a singledegree of freedom, damped mass-spring system, as de-scribed in [2, 4, 5]. If we assume that the device isoperated at resonance, i.e. the frequency of the drivingvibration is equal to the natural frequency of the device,then it can be shown that the amplitude of the move-ment of the mass is given by [4];

Xmax ¼ma

D2pfresð1Þ

where, D is the electromagnetic damping factor for thedevice, and m is the moving mass, and a is the accelera-tion of the input vibration. In this case only electro-magnetic damping is considered, and the electromagneticdamping factor, D can be estimated by [4]

D ¼ ðNlBÞ2

Rload þ Rcoil þ jxLcoilð2Þ

where N is the number of turns in the generator coil, l isthe side length of the coil (assumed square), and B is theflux density to which it is subjected and Rload, Rcoil, andLcoil are the load resistance, coil resistance and coilinductance, respectively. Equation 2 shows that thedamping factor can be adjusted by varying the loadresistance for a given device since all the other param-eters are fixed by the geometry. The expression in Eq. 1shows that the maximum displacement is inverselyproportional to the damping factor, and proportional tothe input acceleration with the other parameters fixed bythe device geometry. Provided, as it usual, the electro-magnetic damping is dominant. If the input accelerationis fixed at a value of 1g (9.81 m s�2), then the maximumdisplacement can be controlled by choosing the value ofload resistance, Rload. The value of load resistanceshown in Table 1 corresponds to the value required toachieve a 240 lm displacement of the paddle for 1g in-put acceleration.

After assembly, the devices were inspected and somedeviations from the simulated parameters were ob-served. In the fabricated devices the separation betweenthe magnet and coil is 0.5 mm as opposed to the 0.1 mmseparation assumed above. This was due to the use of aPerspex chip, which was glued to the generator chip asdescribed in Sect. 5. This increase in separation betweencoil and magnets will result in a decrease in the fluxdensity between the magnets. In order to compensate inpart for this, a mild steel yoke has been used across themagnets. The diagram in Fig. 6 shows the half model ofthe actual device which has been fabricated. Figure 7shows the plot of flux density between the magnets forthis configuration. The flux density varies from +0.2 Tunder one magnet to –0.2 T under the other. This issignificantly less than 0.5 T predicted for the closerseparation in the earlier simulations. In addition to these

changes, the actual coils used only have 600 turns due toa lower than expected coil filling factor. The measuredresistance of the coil is 112 X and the inductance is367 lH at 10 kHz. From experimental tests on the de-vices, the resonant frequencies were found to be 1.6 and9.5 kHz. Table 2 shows the predicted voltages from theactual devices, assuming that the full paddle displace-ment of 240 lm is achieved. The table also gives thepredicted power delivered to a 2 kX load resistance.

5 Micro generator fabrication

The prototype uses discrete components for the perma-nent magnets and coil, and employs seven masks in thefabrication process. The full device consists of threewafers bonded together. The central silicon wafer wasetched by DRIE to fabricate the silicon paddle, frameand beam. A KOH etched V groove is also includedalong the centre line of the beam to provide a channelfor the copper wire from the coil. In this way the wirescan be kept below the surface of the wafer and notinterfere with bonding. The full batch fabrication pro-cess is shown in Fig. 8. The capping wafers will even-tually be made from Pyrex wafers with etched recessesfor location of the permanent magnets and also toprovide clearance for the paddle to vibrate. These Pyrexwafers will be anodically bonded to each face of thesilicon wafer. However, the initial assemblies use Per-spex chips of similar dimensions fabricated using tradi-tional milling techniques. The Perspex chips wereindividually glued to the silicon chips during assembly.

A silicon paddle with a 500 lm wide beam, v-grooveand frame for the prototype microgenerator is shown inthe scanning electron microscope (SEM) image in Fig. 9.Figure 10 shows the components of the generator priorto assembly. The centre chip shows the coil integratedwithin the silicon paddle. The two permanent NdFeBmagnet pairs can be seen mounted in the outer Perspex

Fig. 6 Magnet configuration used in the simulation and the halfmodel used for the 3D simulation

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chips. Figure 11 shows an assembled microgeneratormounted into a standard package.

6 Testing results

In order to evaluate the output power from the micro-generators, devices were tested using a Bruel & Kjaertype 4810 mini shaker driven from a signal generator viaa 100 W RMS MOSFET amplifier. Excitation vibra-tions were measured using a PCB Piezotronics model

Fig. 9 SEM image of Silicon paddle chip with v-groove

Table 2 Voltages and power obtained from the 2D transientsimulation for two-pole configuration with 500 lm separationdistance and 240 lm maximum displacement of the centre point ofthe coil

Frequency(kHz)

Peakvoltage (V)

Power 2 kXload (mW)

1.6 0.7 0.119.5 4.15 3.35

Fig. 7 3D FEA simulation offlux density for the two-poleconfiguration with 500 lmseparation distance betweencoil and permanent magnets

1 m SiO2

160nm Si3 N4

Photolith trench,Dry etch SiO2/Si3N4

Wet KOH etch Si Strip SiO2/Si3N4

Evaporate 1 m AlPhotolith and wet etch Al

Photolith Si paddle SPR220 resist

DRIE Si through wafer Strip resist

Si

SiO2

Si3 N4

SPR 220-7

Fig. 8 Discrete coil generator fabrication process

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354C03 three-axis accelerometer. The tests were per-formed at acceleration levels of 0.4g (3.9 m s�2) and theresonant frequency of the generator observed on aHewlett Packard 3588 A spectrum analyser. The outputof the generator was applied across an impedance-mat-ched load and the voltage observed on a digital oscil-loscope. Typical generator outputs are shown in Fig. 12.Figure 12 shows the results from a model A cantilevergenerator with a beam width of 500 lm. This was foundto have a resonant frequency of 9.5 kHz and generated122 nW into a 100 X load with a Q-factor of approxi-mately 200. The difference between the measured andsimulated resonant frequencies is predominantly due tothe influence of the assembly process and the bonding ofthe enamelled copper wires along the beam or across themeander. It is also clear from the measured results thatmechanical damping mechanisms are dominating. Theseundesirable damping effects need to be substantiallyreduced in order to achieve the predicted power levelsgiven in Table 2.

7 Conclusions

This work presents the mechanical and electromagneticbehaviour of a silicon vibration-powered generator.

Different beam dimensions have been investigated.Magnetic finite element simulations predict that voltagelevels of 0.7–4.15 V can be generated for a full 240 lmdisplacement of the paddle. Simulations show thesevoltages can be increased if the separation between themagnets and moving coil is decreased to 0.1 mm and thenumber of turns in the coil is increased. With theseimprovements FEA predicts that voltages between 4 and9 V can be generated for a 1g acceleration input, with anoptimised choice of load resistance. Further work toimprove the performance of the generators by reducingthe gap between the magnets and the coil and reducingthe magnitude of undesirable damping effects is cur-rently underway. This will be achieved by optimisingfabrication and assembly process and operating the de-vices in a vacuum.

Acknowledgements The authors would like to acknowledge theEuropean Union for funding this Framework 6 STREP projectVIBES, project reference 507911.

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Fig. 12 Power extracted from generator model A

Fig. 10 Generator components(two Perspex chips withmagnets and silicon paddle andcoil)

Fig. 11 Assembled microgenerator

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