Electromagnetic Micro Power Generator - A Comprehensive Survey

Wong Chin Chye1, Zuraini Dahari1, Othman Sidek2, Muhammad Azman Miskam2

1School of Electrical and Electronic Engineering, 2CEDEC

Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Seberang Perai Selatan, Pulau Pinang, Malaysia.

Abstract-This paper presents a comprehensive survey on vibration powered electromagnetic micro generator, which harvest mechanical energy from environment and convert this energy into useful electrical power for micro system and sensor node. The on-going research works on electromagnetic micro generator are reviewed as a background of this paper. Basic theories of micro generator to produce power from ambient motion by damping the suspended proof mass system over the coils are presented. Several important parameters such as the shape, types of magnet, spring and coil property used in designing electromagnetic micro generator as well as power processing circuit are also included. Different designs of micro generator for different environment and application are discussed. Significant challenges in converting the power by each design are also highlighted in this paper. Keywords-Microelectromechanical System; micro power generator; electromagnetic; vibration-induced

I. INTRODUCTION Microelectromechanical (MEMS) technology is a result of a long history of technology development evolving with machine through the advent of microelectronics [1]. It were built upon the technological and commercial needs of latter part of the 20th century, as well as the driving force toward miniaturization of devices [2]. During the past decades, technological developments in the MEMS industry have lead to miniaturization of micro systems devices. With new technology, the power consumption of these micro devices has been reduced to the order of mW to µW level. These achievements have exposed a new and attractive research area; supplying power to micro systems as an alternative to batteries, which have a finite life and poisoning chemical [3]. Batteries seem to offer the optimum source of energy for micro systems as commercial battery technologies offering significant energy capacities in relatively small size. According to Cian [4], the future trend in battery technology is towards higher energy densities which is more suitable for portable micro system where increasing time between charges and size of micro system are important factors. Nevertheless, energy density is not essentially the significant factor for the choice of power solution for a sensor node in a “deploy and forget” application. In such a case, battery characteristics such as lifetime, self-discharge rate, lifetime under temperature variations and the number of allowable charge cycles are perhaps more important than energy density or capacity [4].

Recently, researchers are working and developing on renewable energy sources such as solar, thermal, vibration [4], and acoustic [5]. These sources are considered clean and have theoretically infinite life compared to batteries. These ambient energy sources are attractive alternative power solution for implantable and embedded micro-systems that operate and depend on their initial energy supply [4]. Common power generation from environment include solar (outdoor/indoor), vibration, acoustic noise, daily temperature variation, and temperature gradient while energy scavenging from chemical fuel are nuclear source, batteries (lithium), combustion (micro-engine), and fuel cells (methanol) [6]. In future, we will see rapid emergence of micro sensor system deployment throughout our environment. Energy solution are becoming significant factor for long period deployment sensors in our environment also known as “deploy and forgot” system, [4]. Micro systems that are able to convert energy present in their environment are one of the key components to self-powered system or “deploy and forgot” system for critical environment such as human body, deep sea and space where replacement of battery is nearly impossible or too costly [4, 7]. This paper presents an overview of vibration based micro generator which focuses on electromagnetic micro generator. General operation principles of electromagnetic generators and some of its important properties are discussed.

II. VIBRATION BASED GENERATORS Various techniques have been proposed as alternative to batteries in a sensor system or extract energy from the environment in order to increase the battery energy storage [4]. Vibration or motion energy harvesting is one of these techniques. Vibration sources are present in our surrounding such as automotive, buildings, structures (bridges, railways), industrial machines, and household appliances [4]. This vibration can be considered as an energy source for power up micro-system. Energy can be extracted from the vibration sources via mechanical-to-electrical power generator by using piezoelectric, electrostatic and electromagnetic principles [4]. Piezoelectric generators apply piezoelectric properties concept of some materials which produce a voltage when stressed. The piezoelectric material is stressed by the vibration and develops a voltage which can be used to

2010 IEEE Symposium on Industrial Electronics and Applications (ISIEA 2010), October 3-5, 2010, Penang, Malaysia

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power up micro-system [4]. Meanwhile, the electrostatic generators apply the concept of pull apart the plates of a charged capacitor against the force of electrostatic attraction by vibration energy and convert it into electric field energy [4]. Finally, electromagnetic generator makes use of Faraday’s law of induction [2-4, 8-25]. The vibration is used to move a magnetic mass move relative to a coil or vice versa, thus inducing a voltage and cause a current flow in the close circuit. Electromagnetic generator become useful at large volume and low frequency while piezoelectric and electrostatic produce significant power at low volume and high frequency [4].

III. PREVIOUS RESEARCH WORKS

As early as 1995, Williams et al. reported a design of magnetic energy harvester with mass-spring method [26, 27]. The device consists of millimeter-size SmCo permanent magnet attached to the underside of a flexible polyimide membrane of diameter 2mm, which placed above a cavity GaAs wafer. The gold planar coils were placed on the backside of wafer to cut flux linkage of the magnet and produced power. The device reported a maximum RMS power of 0.3 µW for 0.5 nm at resonant frequency of 4.4 kHz. Rajeevan et al. designed a cylindrical shape mass-spring system micro generator with moving coils which able to generate 180 mV of peak output voltage and ≈400 µW power at frequency 2 Hz with 2 cm of amplitude [8]. They also demonstrated that the generator is able to power up a low power DSP circuit. Meanwhile, Neil et al. reported a laser-micromachined multi-modal resonating power transducer by using a small NdFeB magnet cube attached to the laser-micromachined spiral copper spring [15]. This 1 cm3 micro generator is capable to produce maximum root mean square (RMS) power of 830 µW with up to 4.4 V peak-to-peak, which have loading resistance of 1000 Ω. Moreover, the spiral spring provide low resonant frequency so that the generator able to operate at 60-110 Hz with ≈200 µm of amplitude. They also demonstrated the capability of generator to power up low data rate infrared and RF wireless circuit. Furthermore, the generator is also combined with the power processing circuit such as quadrupler rectifying circuit and standard tripler circuit into a AA-battery sized form factor [11, 12]. Glynne-J et al. [16] developed a resonant cantilever beam base micro generator by using a pair/pairs of NdFeB permanent magnets connected on a U-shaped iron core to provide a magnetic field across an air gap and attached to the end of cantilever. Then, the air gap is replaced by the stationary coil winding. When the cantilever moves, the magnetic flux linkage cut the coil and induces EMF. Two prototypes were demonstrated by them, prototype A with a pair of magnets while prototype B consists of 2 pairs of magnets. Prototype A with 0.84 cm3 of volume produced 37 µW of power at 322 Hz under 0.36 mm of amplitude whereas prototype B with 3.15 cm3 of volume produced 157 µW of power

generation when mounted on the top of an automobile engine block. Furthermore, James et al. [2] reported that above micro generator are used to power a liquid crystal display to show system output and infrared link to transmit the data output by using step up transformer, rectifier and regulation circuitry. Another approach of this kind of generator in miniaturized form was reported by Torah et al. [19]. The miniaturized form generator with 150 mm3 of volume was capable to produce 17.8 µW of power at 56.6 Hz of resonant frequency with 52 mV of RMS voltage under 588.6ms-2 of acceleration and 150Ω of load resistance. Besides, Beeby et al. [21] also presented a small cantilever generator with 0.15 cm3 of volume. It is capable to produce 46 µW of power with 428 mV of RMS voltage in loading resistance of 4 kΩ from 0.59 ms-

2 acceleration levels at resonant frequency of 52 Hz. Recently, D. Zhu et al. [28] reported device miro generator with two pairs of magnets was capable modified to become frequency tunable vibration-based micro generator. A magnet is attached to an actuator positioned it axially in line with cantilever while another magnet is fixed to the free end of cantilever. Thus, the tuning force is caused by the attractive force between these two magnets with opposite poles facing each other. The axial load on the cantilever was changed by adjusting the distance between two turning magnets. The reported results of the change the distance between two tuning magnets from 5 to 1.5 mm caused the resonant frequency varied from 67.6 to 98 Hz. Meanwhile the power produced by generator decreased from 156.6 to 61.6 µW as resonant frequency increased at a constant acceleration level of 0.59ms-2. Meanwhile, K. Sasaki et al. [29] have explored, a micro generator with using rotation method. This generator utilized self-excited rotation of an eccentric rotor (semi-circle disk) to achieve continuously rotation of the rotor. Minimum vibration amplitude and maximum electro-mechanical damping in order to produce maximum power were studied by authors. They reported that at least 40 mm of amplitude in order to excite self rotation with electro-mechanical damping between 10-4 and 10-3 to obtain optimum power. Another approach of rotating unbalance of rotor with magnet was demonstrated by Spreemann et al. [30]. The generator consisted of a moving pendulum pair with 2 magnets on each side and stationary coils were placed between these pair of magnets. When the generator vibrates in linear direction, the pendulum pair rotate in one direction, the magnetic flux linkage change with coils and produce EMF. The prototype generator with 1.5 cm3 of volume is capable of producing 0.4-3 mW for vibration frequencies between 30 to 80 Hz based on 100 µm of vibration amplitude. Koukharenko et al. [10] reported a micromachined silicon paddle with discrete winding copper coils. Two magnets were fixed at the top and bottom of the paddle with coils forming a magnetic flux linkage through the coils. Power is generated by vibrating the paddle as the coils changing the magnetic flux linkage. This generator with 0.1cm3 of volume is able to produce 104 nW and

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122 nW of power at 1.615 and 9.5 kHz respectively. According to Santosh et al. [22], three prototypes are tested above silicon paddle micro generator. Prototype A had a wire-wound copper coil, is capable to generate 148 nW of maximum power output at 8.08 kHz of resonant frequency and 3.9 ms-2 of acceleration while prototype B had an electrodeposited copper coil on both beam and paddle, is able to produce a maximum load power of 23 nW for acceleration of 9.8 ms-2 at 9.84 kHz of resonant frequency and load resistance of 52 Ω. Prototype C consisted of two electroplated copper coil on silicon placed two side of the two oppositely polarized NdFeB on beryllium-copper beam. It produced a maximum power of 586 nW at frequency of 60 Hz and acceleration of 8.829 ms-2 across 110 Ω load resistance. Next, Thomas and Gerhard [24] developed a air-cored tubular linear generator with winding coils at the center of the tube. Multiple magnets was placed in the tube and fixed by flexible hinges spring. An optimized volume for stator and translator was 0.25 cm3 but a larger volume of 0.5 cm3 is constructed as a prototype and combined with the spring system in housing of 30.4 cm3. It produced mean output power of 35 µW when mounted below the knee for normal walking. Besides Wang et al. [18], reported a new vibration-based micro generator which is fabricated using MEMS technology. The micro generator composed of a NdFeB permanent magnet attached to a copper planar spring on silicon wafer and a lower two-layer copper coil on glass substrate. The reported prototype generator with 0.18 mm3 of volume is able to generate open-circuit voltage of 60 mV ac peak-to-peak with 121.25 Hz of frequency and 14.72 ms-2 acceleration levels. A group work by Saha et al. [9], has presented tubular micro generator which is suitable for human body motion. It consists of a tube with a magnet at each ends and two magnets stick to each other by a soft magnetic pole, this magnet is free to move inside the tube. Coils were winded around the center of the tube. The generator has an AA- sized battery form factor with ≈12.48 cm3 of volume. The generator with two ends magnets at tube was capable to generate 0.3 mW of power at frequency of 2 Hz and acceleration levels of 4.91 ms-2 for walk while generate 1.86 mW of power at frequency of 2.75 Hz and acceleration levels of 9.81 ms-2 for slow running. Meanwhile maximum load powers of generator without top fixed magnets were 0.95 and 2.46 mW for walking and slow running respectively. The generator was placed inside a rucksack during data collecting. In addition, Ibrahim et al. [3] has developed a wideband vibration micro generator was presented by the authors. The prototype generator composed of 35 parylene cantilevers with different length in order to have different resonant frequency of each cantilever. Coil turns were integrated on each cantilever and all cantilevers were surrounding a NdFeB permanent magnet. This generator was fabricated as 175 mm2 of area chip and produced a maximum output power of 0.4 µW with maximum voltage of 10 mV continuously within

frequency band of 800 Hz (4.2-5 kHz). According to Haluk and Khalil [13], a micro generator that change low-frequency vibrations to higher frequency is demonstrated. This generator consists of two resonating structures. The top resonator has a low resonant frequency that could be adjusted while the bottom resonator is a cantilever beam with higher resonant frequency and integrated coil for power generation as well as a magnetic tip that could be attracted to the large magnet. A macro scale prototype is demonstrated by using styrene cantilever and NdFeB permanent magnet which can produce maximum power and voltage of 120 nW and 6 mV respectively. It is cpable to generate maximum power of 3.97 µW at 25 Hz vibration for a single micromachined cantilever in micro scale implementation. The reviewed on-going researches are summarized in Table I. Based on above literature review, cantilever spring or spiral spring are commonly used in designing micro generator in order to achieve smaller size. Majority of the reported works utilized Neodymium Iron Boron (NdFeB) in their design. Copper coils are mostly reported by researchers as winding coils or electroplated coils. Different materials of cantilever spring have been reported by researchers depending on the desired resonant frequency. Low and high resonant frequency devices are demonstrated by authors. The future research trends are moving towards on developing a smaller size and wider bandwidth of resonant frequency of the electromagnetic micro power generator.

IV. PRINCIPLE OPERATION OF ELECTROMAGNETIC GENERATORS

Electromagnetic generators work on the principle of electromagnetic induction which is Faraday’s law of induction. Faraday’s law of induction states that the electromotive force around a closed path (loop) is equal to the negative of the time rate of change of the magnetic flux enclosed by that path.

demfdtΨ= − (1)

where emf in volts, and is the magnetic flux computed as though the coil is a one-turn coil (in Weber). The negative sign on the right side together with the right-hand screw rule ensures that Lenz’s law is always satisfied. Lenz’s law states that the sense of the induced electro-motive force (emf) is such that any current it produces tends to oppose the change in the magnetic flux producing it. The induced emf acts to oppose the change in the flux and not the flux itself. If the loop contains more than one turn, such as in an N-turn tightly wound coil, then this is equivalent to the situation in which N separate, identical, single-turn loops are stacked so that the emf induced in the N-turn coil is N times that induced in one turn. Thus, for an N-turn coil,

demf NdtΨ= − (2)

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TABLE I.

SUMMARY OF REVIEWED ELECTROMAGNETIC GENERATOR

Name Volume, cm3

Voltage/Power/ loading resistance

Resonant frequency

Amplitude Acceleration (g=9.81 ms-2)

Material

Williams [26], 1996 25 mm3 1 µW/0.1 mW 70 Hz/330 Hz 50 µm NA NA Raj [8], 1998 NA 180 mV/400 µW 94 Hz NA NA NA

Neil [15], 2002 1 cm3 4.4 Vpp/830 µW/ 1000 Ω 60-110 Hz 200 µm NA NA Glynne-J [16], 2004 0.84 cm3 37 µW 322 Hz 0.36 mm NA Neodymium Iron

Boron(NdFeB) 0.84 cm3 180 µW 322 Hz 0.85 mm NA NdFeB 3.15 cm3 157 µW NA NA NA NdFeB

Torah [19], 2006 <150mm3 52 mVrms/17.8 µW/ 150 Ω 56.6 Hz NA 60 mg NdFeB Spreemann [30], 2006 1.5 cm3 0.4-3 mW 30-80 Hz 100-75 µm NA NA E. Koukharenko [10],

2006 100 mm3 0.7 V/ 104 nW/2 kΩ 1.165 kHz 240 µm 0.4g NdFeB

Thomas [24], 2007 0.25 cm3 2-25 µW NA NA NA NA Beeby [21], 2007 0.15 cm3 428 mVrms/ 46 µW/4 kΩ 52 Hz NA 60mg NA Wang [18], 2007 0.18 mm3 60 mVpp 121.25 Hz NA 1.5g NdFeB Saha [9], 2008 12.48 cm3 14.55 µW/ 7.3 kΩ 8 Hz NA 0.38259 m/s2 NdFeB

12.48 cm3 0.3 mW 2 Hz NA 0.5g NdFeB 12.48 cm3 1.86 mW 2.75 Hz NA 1g NdFeB 12.48 cm3 0.95 mW NA NA NA NdFeB 12.48 cm3 2.46 mW NA NA NA NdFeB

Santosh [22], 2008 106 mm3 148 nW/ 52.7 Ω 8080 Hz NA 3.9 m/s2 NdFeB 106 mm3 23 nW 9837 Hz NA 9.81 m/s2 NdFeB 150 mm3 23.5mVpp/ 584 nW/ 110 Ω 60 Hz 1.5 mm 8.829m/s2 NdFeB

Ibrahim [3], 2008 10 mV/ 0.4 µW 4.2-5 kHz 1 µm NA NA Haluk [13], 2008 Macro size 6 mV/ 120 nW/ 300 Ω 64 Hz NA NA NA

Micro size 76 mV /3.97 µW 25 Hz NA NA NA Zhu[28], 2010 61.6-156.6 µW 67.6-98 Hz NA 0.59 ms-2 NA

Note: NA – Not Available.

Based on the literature review, researches conducted on electromagnetic micro generator are generally performed by varying some important properties of the electromagnetic micro generator in order to optimize its performance for instance the output power, acceleration and resonant frequency.

Generally, most researchers are focusing on the magnetic, spring-mass and coil property in designing the electromagnetic micro generator.

A. Magnetic Property According to the Faraday’s law of induction, the induced emf depends on the time rate of change of the magnetic flux through the circuit. Thus, magnetic flux density becomes a significant parameter in designing micro generator. Rare earth magnets produce a strong flux density and suitable for this application. It offers up to five times the magnetic energy density of conventional Alnico magnets. Neodymium Iron Boron (NdFeB) magnets is known as most powerful magnetic properties per cubic centimeter and able to operate at up to 140 °C [31]. Samarium Cobalt, a rare earth magnet with less powerful and less expensive compare to NdFeB, can be used if higher temperature operation is required as it has working temperature up to 300 °C [31]. Many pioneering

work related to electromagnetic micro generators were performed by using Neodymium Iron Boron (NdFeB) magnets to design their micro generator [2, 3, 10, 13-16, 18, 19, 22, 32]. This is due to the ability of NdFeB to provide the strongest magnetic flux density and to achieve smaller size of micro generator.

B. Spring-mass property Spring-mass system is referred as the system that provides oscillation for the magnet in order to achieve flux cutting mechanism and induce certain emf.

Figure 1. Simple harmonic motion of spring-mass system

Figure 1 illustrates a typical spring-mass system vibrating in vertical direction with amplitude, A and frequency, ω. The vertical direction represents the distance of the mass (x in the equation), and the horizontal axis represents time. Thus, the distance of the mass,

( ) ( )cosv t A tω ϕ= + (3)

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TABLE II. MATERIAL DATA FOR THE SPRING STRUCTURE [33]

Young’s modulus (GPa)

Yield stress (MPa) Ultimate stress (MPa)

Fatigue limits (MPa) Fatigue ratio

Aluminum 70 270 310 21 0.3 Brass 96-110 70-550 200-620 98-147 0.31 Copper 130 55-760 230-830 63 0.29 Nickel 200 100-620 310-760 109 0.35 Titanium 120 760-1000 900-1200 364 0.59 55-Ni-45-Ti 83 195-690 895 - - Silicon 160 - - - -

the velocity of the mass ,

( ) ( )sindxv t A tdt

ω ϕ= = − + (4)

the acceleration of the mass,

( ) ( )2

22 cosd xa t A t

dtω ω ϕ= = − + (5)

Where A is amplitude, ω is angular frequency and φ is the phase which determines the starting point on the wave. Spiral shape spring with lower spring constant and lower stress concentration produce larger displacement when oscillate [15], thus maximize flux cutting rate and induce higher emf. This spring is suitable for low frequency power generator. Published work have concluded that copper is much better than silicon in terms of reliability and power generation for spring structure [15, 34]. Table II shows the material properties of some metal used in micromachining processes. Although copper is better than silicon but it is not the best material in terms of yield stress and fatigue limit [15]. Titanium metal has higher yield stress, thus it is able to withstand higher stress and suitable for extremely large displacement of the spring. The 55-Ni-45-Ti has lower Young’s modulus value than the copper, thus it is more appropriate to work at low frequency range [15]. Most of previous researchers employed copper spring in their design as helical or spiral planar shape because the cost for copper is much economical compared to titanium and 55-Ni-45-Ti [11, 12, 15, 18]. Cantilever or paddle or beam springs are also preferred springs used in modeling micro generator by researchers [16]. Silicon and parylene cantilevers are fabricated in small packet size [3, 10, 13, 14, 19, 22]. Steel and copper beam cantilever as vibrator also reported respectively by [2] and [22] as well as using stainless steel beam as vibrator by [10]. Instead of using spring-mass system, some researchers employ the rotor and stator in order to perform magnetic flux cutting to induce power base on Faraday’s law [29, 30, 35]. The hybrid devices that convert linear vibration into rotational motion using an imbalanced rotor is presented by [30] to be non-resonant and wide frequency bands micro-generator.

C. Coil property Copper wire or coils are widely reported as induction coil used in micro generator [2, 9, 10, 12, 14, 18, 19, 22].

Either wire-wound copper coil or electrodeposited copper coil is used in reported micro generator device. Wire-wound copper coil is normally applied to the macro size prototype micro generator while electrodeposited copper coil is used for smaller size micro generator device. Besides, gold has also been used as coil material in fabrication process. It has been reported a square spiral coil fabricated by using gold metal via electroplating technique [23]. Copper metal as coil material is more commonly used than gold as the copper has better conductivity and more cost-effective.

D. Power processing circuit Power processing circuit has been introduced by some researchers [8, 15, 21]. The power generated from ambient vibration is going to be a time-varying voltage. Furthermore, this vibration source may not be reliable or periodic as the ambient vibration is nondeterministic. The generated voltage must be regulated to a certain level in order to maintain a specified performance level before it can be used to power a load circuit [8]. A very low power dc/dc switching converter is using for this propose. The power processing circuit is designed to produce a high voltage with only a down (Buck) converter [36]. These converters have been applied low power battery-base applications according to [37]. According to Seong [23], a step-up quadrupler circuit was proposed and the results of the circuit were recorded. It is measured that the generated voltage increased linearly without using the quadrupler circuit. The generated voltage increased nonlinearly after using the quadrupler circuit. The gain obtained by using the quadrupler circuit is less than the loss without using circuit until frequency of 4.5 Hz. Above frequencies of 4.5 Hz, the gain obtained by using quadrupler circuit become larger than device without using this circuit. Hence, the proposed quadrupler circuit becomes significant only after the vibration frequency of 4.5 Hz in the micro generator system. P.D. Mitcheson and et al. [38], have proposed dual polarity boost circuit for vibration-based electromagnetic micro generator [39]. The circuit is possible to achieve up-conversion to useful voltages with the efficiency in the region of 50% provided that the output of the low voltage micro generator in the range of 59 mW.

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V. CONCLUSIONS AND FUTURE PERSPECTIVE An overview and on-going research works on vibration based electromagnetic micro generator are presented. Basic designs of micro generator using suspended proof mass system over coils are included. Several important parameters such as the shape, types of magnet, spring and coil used in designing electromagnetic micro generator and also power processing circuit are discussed. Different designs of micro generator with different resonant frequency for different environment and application are included. Future challenges in converting the power by each design are also emphasized in this paper.

ACKNOWLEDGMENTS

The authors would like to express sincere appreciation of the assistance of Mr. Kusairay for his co-operation in supporting software. Financial support from the Universiti Sains Malaysia Fellowship, Short Term Grant, 304/PELECT/6039027 and Research University Grant, 1001/PELECT/814036 is gratefully acknowledged.

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