A MICRO ELECTROMAGNETIC VIBRATION ENERGY HARVESTER
WITH SANDWICHED STRUCTURE AND AIR CHANNEL
FOR HIGH ENERGY CONVERSION EFFICIENCY
Peihong Wang, Xuhan Dai, Xiaolin Zhao*, and Guifu Ding
Researc Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University,
Shanghai, China
Abstract: This paper presents the design, fabrication and performance of a novel electromagnetic vibration
energy harvester with sandwiched structure and air channel. It mainly consists of a top coil, a bottom coil, an
NdFeB permanent magnet and a nickel planar spring integrated with silicon frame. The prototype is fabricated
using silicon micromachining and microelectroplating techniques. Its sandwiched structure and air channel in the
silicon frame can make the induced voltage increased 69.6%. The resonant frequency of the prototype is 280Hz.
The load voltage generated by the prototype is 142.5mV when the prototype is at resonance and the input
vibration acceleration is 10m/s2. The maximal load power obtained is about 17.2μW at load resistance of 81Ω.
Keywords: Energy harvester, Vibration energy, Electromagnetic, Sandwiched structure, Air channel
energy conversion efficiency. The nickel planar spring
integrated with silicon frame can simplify fabrication
process and reduce production cost. Testing results
show that the fabricated prototype has better
performance comparing with our previous work [7, 8]
and other single coil structures.
INTRODUCTION
Wireless sensor networks （ WSN ） and
microelectromechanical systems (MEMS) are being
widely used in intelligent monitoring, health care,
automotive industry and even military field, with the
fast development of microelectronics technology.
These wireless sensor nodes and many MEMS
devices have very small volume, low power
consumption, are wireless and even embeddable.
However, power source is a great barrier to extending
the lifetime and reducing the cost for these devices,
because the developing speed of battery technology is
far slower than that of integrated circuit technology
described by Moore’s law [1]. Harvesting energy
from the ambient environment and then converting it
into electrical power is a very promising alternative to
batteries [2]. Mechanical vibrations seem to be the
most promising energy source since they are abundant
in many environments [3]. As a typical kind of energy
harvesting techniques, electromagnetic vibration
energy scavengers have been published on many
literatures [4, 5] since 1996. However, there is a large
gap between the performance of many prototypes and
practical requirement of wireless microsystems.
This paper presents the design, fabrication and
performance of a novel electromagnetic vibration
energy harvester based on microelectroplating and
silicon micromachining techniques. Its sandwiched
structure is clearly different with other published
electromagnetic vibration energy harvesters [5, 6].
The symmetrical arrangement of two coils can make
the magnetic field be utilized sufficiently and the air
channel on the silicon frame can decrease the air
damping efficiently, both of which can increase the
0-9743611-5-1/PMEMS2009/$20©2009TRF
DESIGN
The schematic of the sandwiched electromagnetic
vibration energy harvester is shown in Fig. 1. It
mainly consists of a top coil and a bottom coil on
glass substrates, an NdFeB permanent magnet, and a
nickel planar spring integrated with silicon frame. Top
coil and bottom coil can generate same electrical
energy because of the symmetrical arrangement of the
sandwiched structure. So the total electrical energy is
increased after top coil and bottom coil are connected
in series and the energy conversion efficiency is
higher comparing with single coil structure. If the
sandwiched structure is sealed, the air damping
generated by the inner air will decrease the amplitude
of the magnet greatly and then influence the energy
conversion efficiency. The air channel in silicon
frame can overcome this disadvantage, decrease the
air damping and then the output performance of the
energy harvester can be increased further. Moreover,
the gap between the magnet and the coil can be
controlled by using silicon frame with different
thickness or multiple silicon frames with same
thickness. The glass substrate with top coil is like a
cap so that it can protect the magnet-spring system
and does not increase the total volume additionally.
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PowerMEMS 2009, Washington DC, USA, December 1-4, 2009
coils on glass substrate, a Ni planar spring integrated
with silicon frame with air channel, an NdFeB
permanent magnet with dimension of 2×2×2mm3 and
several silicon frames for adjusting the distance
between magnet and coil, are assembled into the
sandwiched electromagnetic vibration energy
harvester prototype. Its dimension is 9×7×5mm3.
Fig. 1: Schematic structure of the sandwiched
electromagnetic vibration energy harvester
FABRICATION
The prototype is fabricated using MEMS
micromachining technique. The fabrication process of
the micro two-layer copper coil has been described
previously [7]. The nickel planar spring on the silicon
frame with air channel is fabricated using
microelectroplating and silicon micromachining
technique. The detailed fabricated process is shown in
Fig. 2 (a)-(h). (a): the photoresist is spin coated on the
backside of a double-side polished silicon wafer
which is thermally oxidized on both sides and then
patterned by photolithography; (b): the exposed SiO2
layer is wet etched using HF solution and then the
photoresist is removed; (c): the exposed Si in the
center of the silicon frame is wet etched to about 300
µm using KOH solution; (d): the photoresist is spin
coated on the backside again and patterned. And then
the SiO2 layer on the air channel is wet etched for next
Si etching; (e): the Cu/Cr seed layer is sputtered on
the upside of the silicon wafer and the photoresist is
coated and patterned for next electroplating; (f): the
Ni planar spring is electroplated and then the
photoresist is stripped; (g): the silicon wafer is wet
etched through to release the Ni spring; (h): the SiO2
layer and Cu/Cr seed layer under the Ni spring is wet
etched.
The fabricated Ni spring and the silicon frame
with and without air channel are shown in Fig. 3(a).
The thickness of the Ni planar is about 50µm, the
width of the spring beam 500µm and the gap between
the beams 200µm. The fabricated Cu planar square
coil and it SEM picture are shown in Fig. 3(b). The
inner/outer side length of the coil are 0.7/2.5mm,
respectively. The linewidth of the coil is 15µm, the
thickness 15µm, the turns of every layer 30, and the
resistance 40.4Ω.
After the fabrication of the Ni spring and Cu coil, two
Fig. 2: Fabrication process of nickel planar spring
Fig. 3: Photographs of (a) fabricated nickel spring
with silicon frame and (b) copper square coil
TESTING RESULTS
The assembled prototype is then mounted and
tested in a test stage. The experimental setup consists
of a vibrator (Sinocera JZK-5), a power amplifier
(Sinocera YE5872), a waveform generator (Agilent
33220A) a piezoelectric accelerometer (Sinocera CAYD-1107), a vibration monitor (Sinocera YE5932A)
and an oscilloscope (Agilent, MSO6034A).
In order to comparing the output performance of
sandwiched structure with that of single coil structure,
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structure. So the sandwiched energy harvester
prototype with air channel has higher output
performance and so higher energy conversion
efficiency.
testing is performed twice before and after the top coil
and bottom coil are connected in series. The testing
conditions are same, which are (1) the input vibration
acceleration is 10m/s2; (2) the prototype is at
resonance and (3) the load resistance is 1MΩ. Testing
result is shown in Fig. 4 when top coil and bottom coil
are separated. It can be seen from Fig. 4 that the
induced voltage through the coil is an AC signal but
the waveform is not sinusoidal. The magnetic flux rate
is different when the permanent magnet moves in
positive half period and negative half period, which
results in the no-sinusoidal waveform of the AC
signal. The relevant simulations have been made and
are shown in reference [7]. Fig. 4 also indicates that
the load voltage generated in top coil and bottom coil
are same and its peak-peak value is 84mV.
Fig. 5: Tested load voltage versus time in series coil
Fig. 6 and Fig. 7 are other testing results of the
sandwiched energy harvester prototype with air
channel. The relationship between the load voltage
and the input vibration frequency is shown in Fig. 6. It
indicates that the prototype has only a resonance
during the swept frequency process of 100~300Hz.
The resonant frequency is 280Hz and the
corresponding load voltage is 142.5mV. The load
voltage increases step by step before resonance point
and then decreases promptly after resonance point.
The different changing behaviors of load voltage at
the two side of resonant point result from the nonlinear effect of nickel spring with large deflection [9].
The no linear effect can reduce the load voltage
because the magnet is not able to move as far as it
would for a linear spring.
Fig. 4: Tested load voltage versus time in top coil
and bottom coil
Testing result is shown in Fig. 5 when the top coil
and bottom coil are connected in series. The testing
conditions have been given above. Fig. 5 shows that
the generated voltage signal through the coil in series
is an AC signal and the waveform is sinusoidal, which
is different with the result in Fig. 4. As can be seen
from Fig. 5 that the load voltage generated by the coil
in series is 142.5mV (peak-peak value). The induced
voltage through the coil in series is the synthesis of
the induced voltage through the separated coil but not
the simple addition of them. Since the induced voltage
through the top coil and bottom coil have a phase
difference which is shown in Fig. 4, the induced
voltage through the coil in series is smaller than
168mV (two times of 84mV).
According to the above analysis about Fig. 4 and
Fig. 5, the load voltage of 142.5mV generated by the
energy harvester prototype with sandwiched structure
and air channel is increased 69.6% comparing with
that of 84mV by the prototype with single coil
160
Load voltage (mV)
280Hz
142.5mV
120
80
40
0
50
100
150
200
250
Vibration frequency (Hz)
300
350
Fig. 6: Load voltage versus input vibration frequency
298
load power of 17.2μW when the input vibration
acceleration is 10m/s2 and the prototype is at
resonance.
The measured load voltage versus load resistance
for the prototype with sandwiched structure and air
channel are shown in Fig. 7. It indicates that the load
voltage increases with the load resistance very fast at
first and then increases slowly. The load voltage
hardly changes after the load resistance is bigger than
2000Ω and the maximum of load voltage is 142.5mV.
The calculating equation of the load power is
V pp2
(1)
Pload =
4 × Rload
ACKNOWLEDGEMENTS
This work was supported by the National High
Technology Research and Development Program of
China (2006AA04Z360) and National Natural Science
Foundation of China (50977056).
REFERENCES
[1] Paradiso J A, Starner T 2005 Energy scavenging
for mobile and wireless electronics IEEE
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[2] Glynne-fones P, White N M 2001 Self-powered
systems: a review of energy sources Sensor
review 21 91-97
[3] Roundy S, Wright P K, Rabaey J 2003 A study
of low level vibrations as a power source for
wireless sensor nodes Comput. Commun. 26
1131–1144
[4] Williams C B, Yates R B 1996 Analysis of a
micro-electric generator for microsystems Sens.
Actuators, A 52 8-11
[5] Arnold D P 2007 Review of microscale
magnetic power generation IEEE Trans Magn.
43 3940-3951
[6] Beeby S P, Tudor M J, White N M 2006 Energy
harvesting vibration sources for microsystems
applications Meas. Sci. Technol. 17 R175-R195
[7] Wang P, Tanaka K, Sugiyama S, Dai X, Zhao X,
Liu J 2009 A micro electromagnetic low level
vibration energy harvester based on MEMS
technology Microsys. Technol. 15 941-951
[8] Wang P, Dai X, Zhao X, Niu L 2009
Electromagnetic
self-powered
low-level
vibration
energy
scavenger
with
microelectroplated nickel resonator Electron.
Lett 45 832-833
[9] Williams C B, Shearwood C, Harradine M A,
Mellor P H, Birch T S, Yates R B 2001
Development
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148 337-342
Where V pp is the peak-peak value of load voltage,
Rload is the load resistance. The load power calculated
from the equation (1) is also given in Fig. 7. It can be
seen from the enlarged curve that the load power is
maximal when the load resistance is equal to the coil’s
resistance (81Ω) and the maximum is 17.2μW.
18
150
100
50
120
18
90
16
60
14
30
12
0
0
0
1000
0
2000
100
3000
200
4000
300
12
6
10
5000
Load power (μW)
Load Voltage (mV)
Load voltage
Load power
0
Load Resistance (Ω )
Fig. 7: Load voltage and power versus load resistance
CONCLUSION
A novel electromagnetic vibration energy
harvester with sandwiched structure and air channel is
presented. The prototype is fabricated using silicon
micromachining and microelectroplating technique.
Experimental results show that the load voltage
generated by the prototype with sandwiched structure
and air channel is increased 69.6% comparing with
the harvester with single coil structure. The resonant
frequency of the prototype is 280Hz. The prototype
can generate maximal load voltage of 142.5mV and
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