34th

International Electronic Manufacturing Technology Conference, 2010

Approaches and Developments in MEMS Power Harvesting Generators

Mohammed Dhia Shaker 1, Hanim Salleh 2

1 2Mechanical Eng. Dept, Universiti Tenaga Nasional

43009 Kajang, S’gor, Malaysia.

Email: 1Mdhia@uniten.edu.my, 2hanim@uniten.edu.my

Abstract

This paper presents designs and optimizations for two

types of micro electromechanical generators system, first one

is the narrow band generators which have a particular

resonant frequency the second one is wide band generators

(tunable generators) where the resonant frequency can be

controlled by adjusting the cantilever length, changing the

distance between magnets, and etc .The work and

developments done by researchers are presented and for

comparison, their results are listed below:

The Laser-micro generator was fabricated with a total

volume of 1cm3, output power of ~830µW, frequencies (60 -

110) Hz.

The output power of the (body-worn) generator was 2–25

µW, volume of 0.25 cm3.

The vibration-powered generator for intelligent sensor

systems has an overall volume of 0.84 cm3 with an average

power of 157 µW when tested on a car engine

.One type of Paddle generator produced an output power of

2 mW at a frequency of 9.81 kHz.

In the frequency sweeper generator, the device generates 0.4

µW with frequency range of 4.2–5 kHz.

For Tunable energy harvesting piezoelectric cantilever

generator, a natural frequency was successfully tuned over a

frequency range of 22–32 Hz to produce power output of

240–280 µW.

The resonant frequency of the vibration-based

electromagnetic micro-generator was tuned from 67.6 to 98

Hz to produce a power of 61.6–156.6 µW.

The tuning of the wide band generators seemed to be

effective.

1. Introduction

The past few years have seen an increasing focus on

energy harvesting issue, including power supply for portable

electric devices. Eliminating the need for batteries and

increasing portable device lifetimes indefinitely can be done

by utilize scavenging ambient energy from the environment.

Several different ambient sources, including solar, vibration

and temperature effect, have already exploited [1,2]. Each

energy source should be used in suitable environment,

therefore to produce maximum efficiency. Low-power Micro

electro mechanical Systems (MEMS) has allowed the

development of highly accurate, portable sensors and instruments for numerous applications in health care,

industrial, consumer products, and defense. In many of these

micro sensors, due to limited shelf life and replacement

accessibility power supplies from chemical energy sources

are undesirable [3,4]. To solve this power supply problem,

the conversion of electrical energy from a vibrating source

to a renewable storage device, such as rechargeable batteries

or super capacitors has been used[5,6,7] and showed to be of

a potential and promising alternative solution [8,9]. The

electrical energy stored in the storage device can be readily

used for low-power ICs or integrated distributed micro

sensors [10].

2- Narrow Band Generators

2.1 Reciprocating Vibration Generator

A reciprocating vibration generator has a structure

enabling force of a reciprocating vibrating body striking a

coil to be absorbed by magnetic fluid even if an external

impact force is applied. a reciprocating vibration generator

placed in a case body(1)and has an upper plate spring (4)

and a lower plate spring (5) for supporting a ring-shaped

weight (2) having a semi-ring-shaped permanent magnet

(3R,3L) at its inner circumference side.the ring and magnets

are able to reciprocally vibrate in a vertical direction.a

cylindrical excitation coil (6) passes through the inside of the

ring-shaped permanent magnet (3),hence a magnetic field is

generated.

The inner circumferential surface, top and bottom end faces

of the semi-ring-shaped permanent magnet (3R, 3L) are

covered by magnetic fluid F. The gap between the inner

circumferential surface and the outer circumferential surface

of the excitation coil (6) is also filled with the magnetic fluid

.

Reciprocating vibration generator is shown in Fig (1) and

used for a mobile phone, etc., [11]

Figure (1): Reciprocating vibration generator[11]

2.2 Laser-Micro Machined Multi-Modal Resonating

Power Transducer For Wireless Sensing Systems.

The design consists of a rare-earth magnet attached to a

spring Figs. (2,3). a coil is fixed on a rigid housing of the

device.

A generator of a total volume of 1cm3 was made and

was capable of producing up to 4.4V, has a maximum rms

power of ~830µW with loading resistance of 1000Ω. The

frequencies ranged from 60 to 110Hz. The amplitude was

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International Electronic Manufacturing Technology Conference, 2010

200µm. The generator was able to operate an IR transmitter

for sending 140ms pulse trains every minute.also for a

914.8MHz FM wireless temperature sensing system. [12]

Copper spiral spring structures as shown in Fig. (3) were

micromachined based on ANSYS modeling results. a Q-

switch Nd:YAG (1.06µm wavelength) laser was used.

.Different patterns of springs were modeled to optimize the

resonating spring design, i.e.,“zig-zag” and spiral structures

Fig. (4).ANSYS simulation results showed that spiral springs

having lower spring constant and lower stress concentration,

produce a larger displacement

(a) (b)

Figure (3): Laser-micro machined copper spring. (a) A

planar copper spring with total diameter of 4mm; (b) close-

up of the spring shown in (a).[12]

Figure (4): ANSYS modeling for different spring patterns:

(a) “zig-zag” spring; (b) spiral spring

Analyzing the motions of the mass-spring with 3 different

modes of resonant vibration were carried. a strobe light was

used to synchronize the vibration motion of the mass. the

mass was observed to have a 2nd and 3rd mode resonance

(Fig. 5b and c).

The mass appeared to cyclically rotate about an axis parallel

to the plane of the coil. Furthermore, it was observed that the

amplitude of the rotation was very small compared to the

vertical vibration at the 1st mode resonance (Fig. 5a)

Figure (5): FEA simulation and experimental results both

found 3different resonant vibration modes: (a) 1st mode

vibration (vertical); (b) 2nd mode vibration (horizontal); (c)

3rd mode vibration (horizontal); (d) The phase difference

between the output voltage of the generator and vertical

displacement of the magnet at the 3rd mode vibration[12]

2.3 Body -Worn Linear Electromagnetic Generator

The design consists of two parts, translator (magnet

assembly) and a stator part (armature coil) which is carried

by the translator.During operation,the relative motion

between stator and translator leads to a varying magnetic

flux through the armature coil windings. [13] A permanent

magnet air-cored tubular architecture was used. The

translator consists of several axially magnetized disc shaped

magnets separated by soft-magnetic spacers.

The optimized generator had an output power of (2–25)µW,

depending on its position on the human body where it is

worn. Stator and translator volume was 0.25 cm3. A proto

type working generator was built and validated the

simulations.

Fig (6) shows the generator and positions on the body where

it was worn.

Fig (2):Experimental setup for generator output ,

mass displacement measurement, “spiral” Cu spring

structure and a generator.

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International Electronic Manufacturing Technology Conference, 2010

Figure (6): Body worn linear electromagnetic

generator.[13]

2.4An Electromagnetic, Vibration-Powered

Generator For Intelligent Sensor Systems

A typical magnet-coil generator consists of a spring-mass

combination attached to a magnet or coil in such a manner

that when the system vibrates, a coil cuts through the flux

formed by a magnetic core. The overall volume of the

generator was 0.84 cm3. This generator has been tested on a

car engine and shown to produce a peak power of (3.9)mW

with an average power of 157uW.[14]

Two designs were invistigated, design (A) consists of a

round two magnets coupled together , attached to a

cantilever and a coil in between. Design (B) is based around

four magnets. Figure 7 shows photos of prototype A and B.

Prototype A prototype B

Figure(7):Electromagnetic generators prototype A&B

The configurations in Figs.8 A were modeled to predict

power output for various excitations.The prototype

generators were based around etched stainless steel

cantilever to which a hand wound coil was attached to form

an inertial mass. NdFeB magnets were held rigidly with

respect to the cantilever in an epoxy enclosure. Prototype A

has a cantilever length of 1.1 cm, a width of 0.9 cm and a

height of 0.85 cm giving an overall volume of 0.84 cm3.

Prototype B has a cantilever length of 2.1 cm, a width of 1.5

cm and a height of 1 cm giving and overall volume of 3.15

cm3.

prototype A. prototype B.

Figure (8): Electromagnetic generator geometry,

prototype A and B.[14]

Figure 9 shows data taken from a typical short drive in

Southampton. The generator was mounted on the top of the

engine block of a 5-year-old Volkswagen Polo

Figure (9): Output power from electromagnetic generator

prototype B on a car engine.[14]

2.5 Micro Electromagnetic Vibration-Powered

Generator For Low Power Mems

This generator produced a voltage of (9V)Load power of

(2) mW at a frequency of 9.812 KHz [15].

The device based on four magnets. The coil is located on a

silicon cantilevered paddle, designed to vibrate. Magnets are

positioned within etched recesses in Pyrex wafers, which are

then bonded to each face of the silicon layer. The bonding

process is aligned to ensure correct placement of the coil

relative to the magnets. The paddle is realized by deep

reactive ion etched (DRIE) through the thickness of the

wafer.

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International Electronic Manufacturing Technology Conference, 2010

Figure (10): the paddle generator [15]

The four magnets are placed so as to form a two pole

arrangement. Two magnets with opposite polarity are

placed above and below the coil to form the two poles of

opposite polarity as shown in figure 10. The magnets

used were sintered NdFeB..Fig (10) shows also a Silicon

paddle layer Frame,a Coil and Paddle Supporting

Cantilever .3D FEA simulation has been used to verify

the flux density distribution between the magnets. The

flux density distribution for the two pole configuration

varies from approximately +0.5T under one pole to

approximately –0.5T under the other pole.

Simulations for three models were obtained and Table(I)

gives the predicted results for the voltages and powers

for all three beam geometries for a 240 µ m coil

displacement.

Table I: Simulation results for the natural frequency

(Fres) , voltages( Vload ) and power ( P load) delivered to the

load resistance( R load) for three generator structures.[15]

Model F res (KHz)

Rload

(K Ω)

Vload (V)

Pload

(mW)

A 9.81 20.4 9 2

B 7.149 14.8 6.5 1.45

C 4.743 9.8 4.3 0.96

3 – Wide Band Generators

3.1 Electromagnetic Micro Power Generator for

Wideband Environmental Vibrations

The generator, named the “frequency sweeper”, consists of a

series of cantilevers with varying lengths and resonance

frequencies as shown in Fig 11.The device generates 0.4µW

of continuous power with 10mV voltage in an external

vibration frequency range of 4.2–5 kHz, covering a band of

800Hz.[16] Adjusting

the cantilever lengths will vary the natural frequencies as can

be seen in Fig 12.

Figure (11): the sweeper generator[16]

.

Figure (12): Natural frequency vs. cantilever length.[16]

Fig. 13 shows the measured voltage outputs from 20

consecutive cantilevers compared to a single cantilever. It

can be seen that the consecutive arms has a wider bandwidth

of about 500 Hz and a higher power output level compared

to the single arm, indicating the feasibility of band widening.

Figure(13): Measured voltage output from twenty

consecutive arms and a single arm[16]

Fig. 14 shows the measured voltage output from the

improved generator when 35 and 40 cantilevers were used.

The bandwidth of the generator was around 800 Hz and the

maximum voltage output was 10mV for the 35 cantilevers.

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International Electronic Manufacturing Technology Conference, 2010

Figure (14): Generators measured voltage and power

outputs.

3.2 Resonance Frequency Tunable Energy Harvesting

Generator Using A Magnetic Force Technique

This technique enables resonance tuning up to ±20% of

the unturned resonant frequency [17]. A piezoelectric

cantilever beam with a natural frequency of 26 Hz was

successfully tuned over a frequency range of 22–32 Hz to

enable a continuous power output of 240–280 µW over the

entire frequency range tested. Four magnets were placed on

top and bottom of the piezoelectric cantilever which

produced attractive and repulsive force on each side’s of the

cantilever. By changing the distance of the magnets, the

magnitude of the force induced was changed and in turn

increase or reduce the frequency of the vibrating system .

Figure (15): the vibrating cantilever,vibrating mass and

magnets of the magnetic force generator[17]

3.3 Frequency Up Conversion Arrangement Generator

The frequency arrangment device may be used to obtain

a tuned frequeny cantilever generator.

The device consists of a sharp probe, micro super elastic

NiTi ridges, a micro slider, and a piezoelectric bimorph

cantilever [18]. A probe tip attached on the edge of a

piezoelectric cantilever travels along the ridges attached onto

a micro slider mechanism. Experimental results showed that

power output per unit area is substantially larger than

conventional resonance approaches. The important two

features are that the amount of a voltage generated depends

on the depth of the ridge and ridge spacing. Fig (16)

illustrates the conversion arrangement and the variation in

frequency shape and magnitude

Figure (16): Arrangement of frequency converter and the

change in shape and magnitude in frequency obtained[18]

3.4 Tunable Vibration-Based Electromagnetic Micro-

Generator This generator has four-magnest.The four magnet

structure was fixed to a cantilever beam and vibrated with an

ambient vibration. [19]

The coil was attached to the housing of the generator Fig

(17).The resonant frequency was successfully tuned from

67.6 to 98 Hz when various axial tensile forces were applied

to the structure. The generator produced a power of 61.6–

156.6 µW over the tuning range when excited at vibrations

of 0.59m/s2.

Figure (17): Tunable electromagnetic micro generator

Figs (18) represents schematic diagram of the tuning

mechanism.

To control the frequency of the cantilever vibrations,

contact less magnetic force is provided by two tuning

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International Electronic Manufacturing Technology Conference, 2010

magnets and used to apply axial load. When the tensile load

applied to the cantilever becomes much greater than the

buckling force, the resonant frequency of the cantilever

approaches that of a straight tensioned cable with an off-

centre mass because the force associated with the tension in

the beam becomes much greater than the beam stiffness

Figure(18): Schematic diagram of tuning mechanism[19]

The variation of the resonant frequency of the generator

with the distance between the two tuning magnets is shown

in Fig. 19(a). The resonant frequency increases as the

distance between the two tuning magnets decreases, i.e.

when the tuning force increases as in Fig. 19(b).

(a)

(b)

Figure (19, a, b): variations in frequency against magnets

distance separation[19]

4. Conclusion

In the Laser-micro machined Multi-Modal generator, by

innovative spring designs, the mass can be made to vibrate

horizontally while the input vibration is applied vertically

and that this horizontal vibration gives significantly higher

output voltage for the generator.

The output power of the (body-worn) generator can be

increased by using a larger translator displacement limit, by

using a larger volume Vc, or by replacing the aluminum of

the flexible bearing with a stiffer spring material, i.e. a

material causing less internal friction and therefore less

parasitic damping.(Bronze ,Brass)

An electromagnetic generator based around a moving

coil between two magnets is capable of generating useful

level of power, however the output voltage is considered too

low for practical application although Simulations show

these voltages can be increased if the separation between the

magnets and moving coil is decreased to 0.1 mm and the

number of turns in the coil is increased.

The geometry requires coil winding around the magnets

which is cumbersome.

Using parylene as the structural material in the Micro

electric vibration powered generator allows much larger

deflections before mechanical failure compared to silicon,

and hence much larger generated power levels should be

expected.

In the Tunable energy harvesting generator, the amount

of a generated voltage depends on the distances of the

permanent magnets and the depth of the ridge.

The rectification of frequency in the up conversion

arrangment is a function of the ridge spacing.

For the Tunable vibration-based electromagnetic micro-

generator the tensile force due to adjusting magnets is much

greater than the buckling force. The resonant frequency

increases less than predicted from simulation and approaches

a finite value. This is because the force associated with the

tension in the beam becomes much greater than the beam

stiffness and the resonant frequency approaches that of a

straight tensioned cable.

Acknowledgments

This research was supported by a grant from Fundamental

Research Grant Scheme (FRGS)under the collaborative

effort of Universiti Tenaga Nasional.

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International Electronic Manufacturing Technology Conference, 2010

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