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: [email protected], [email protected]
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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