SIMULATION AND DESIGN OF MICRO INDUCTOR FOR
ELECTROMAGNETIC ENERGY SCAVENGING AT LOW AC FREQUENCY
IN WIRELESS SENSOR NETWORK
Y. Zhang*, J. Lu, H. Hiroshima, T. Itoh, and R. Maeda
Networked MEMS Technology Group, Advanced Manufacturing Research Institute, National Institute
of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan
Abstract: This paper presents a prototype of wireless sensor node for power management of information and
communication technology (ICT) devices at low AC frequency. Electromagnetic simulation and experiments
were carried out for discussing the miniaturization of the sensor node. The good fit between the simulation and
experimental results on the induction voltage of a commercial meso-scale inductor, suggested that energy
scavenging is possible at low AC frequency. However, the measured induction voltage deviated much from the
simulated ones with the reducing of inductor size. A careful selection of core material and design is required for
realizing micro inductor-based current sensing and power generation by MEMS technology.
Keywords: MEMS, Inductor, Energy scavenging
CO2. Wireless sensor network, one of the most
promising next generation of ICT applications, are
expected to be utilized for this purpose as well as
health and security applications. The wireless sensor
networks for facility energy controls can generate
positive economic impact of 25 billion US dollar or
INTRODUCTION
In the last two decades, there are rapidly increasing
interests and effort concentrated into developing
energy-efficient technology for less emission of
carbon dioxide and other green gases, especially after
the adoption of Kyoto Protocol. Information and
communication technology (ICT) plays important
roles in the development of the energy-efficient
technology for many fields including industry,
transportation, business and homes. A good example
is that radio frequency identification (RFID)
technology has assisted supply management in
reducing energy consumption by about 5% through
allocating vehicles to the most efficient routes [1]. It
is believed that the energy consumption of TV and
refrigerator for family usage could be reduced by 30%
in next three year and 50% in next ten year,
respectively, by using ICT devices [1-2].
As a matter of facts, the electricity power
consumption of ICT technology itself is increasing
enormously with the application of more ICT devices
such as servers, network equipments, displays and etc.
The electricity consumption of ICT devices is
estimated to grow to be 20% of total generated
electricity power in the year 2025 [1-2]. In Japan, the
electricity consumption of ICT devices would
increase by about 4.2 times by the year 2025. The
energy consumption of internet data center (IDC) is
also rapidly increasing with the increasing amount of
data traffic on the internet. It is estimated to grow by
two orders of its present value by the year 2025.
Therefore, ICT technology needs new energy
management technology for reducing the emission of
0-9743611-5-1/PMEMS2009/$20©2009TRF
Fig. 1 Conception of wireless sensor-based power
consumption monitoring system for ICT devices.
reduce 15 million ton CO2 in the year 2010. More
importantly, it could enable live visualization of
power consumption and CO2 emission so that energy
efficiency could be better. Fig. 1 shows conception of
wireless sensor-based power consumption monitoring
system for ICT devices. The system would have a
huge market potential with the rapid progress of
information society in both developed and developing
countries.
This research aims to develop a wireless sensor
node for the power management of IDC, office and
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PowerMEMS 2009, Washington DC, USA, December 1-4, 2009
current flow and environment temperature. Using the
sensor nodes and GUI, power monitoring of
residential and commercial ICT devices could be
easily enabled. The clamp-type current sensor has two
jaws so that it could detect the current flow through
appliance cords. The sensor node has to be set around
to only one cable of the appliance cords, as shown in
Fig. 2; otherwise, the output signal is very low. The
design has the advantage of high sensitivity resulted
from stronger alternating magnetic field surrounding
one single cable but its practical utilization becomes
inconvenient because most appliance cords are
difficult to be disassembled into single cables. Such
disassemble operation sometimes is even danger. In
addition, the prototype sensor is bulky because of the
magnetic core and the battery. Therefore, the sensor
node is expensive and needs regular maintenance.
These shortcomings halt the prototypes from
commercial applications. This paper would be focused
on the discussion of the miniaturization and selfpowering of the sensor node.
The key to the miniaturization of the sensor node
is to seeking to develop micro inductor with high
performance, which could be utilized as an energy
harvester for power generation without the cost of
current sensitivity. As the aforementioned, it is not
easy to dissemble appliance cords into single cables
for loading the sensor node. The appliance plug is
therefore interesting because it is easier for the sensor
node accessing to strong magnetic field at the
appliance plug if a suitable insulation method is used..
However, the inductor should be enough small
because of limited space near the plug.
home. The sensor node is not only for monitoring the
electric current flow in appliance cord but also for
power generation by the alternating magnetic field
surrounded so that it could be cheaper and
maintenance-free. Similar work has been reported by
Leland and Wright [3-5]. They developed a
cantilever-mount piezoelectric bimorph with magnets
attached to its free end. The magnets couple to the
alternating magnetic field surrounding the power cord,
driving the bimorph to generate a voltage that can be
used either for current measurement or power
generation. This paper presents a new solution based
on a clamp-type current sensor and our effort on its
miniaturization.
PROTOTYPE AND ELECTROMAGNETIC
SIMULATION
We have developed prototypes of a wireless
current sensor and graphic user interface (GUI),
which could monitor 50 sensor nodes simultaneously.
Fig. 2 is photos of the prototype of wireless sensor
node.
Fig. 3 Representative schematic of micro
inductor on appliance plug. The coil is Type B
with its meshing information.
Electromagnetic simulation is performed by using
Maxwell 3D simulator. Fig. 3 shows a representative
schematic of a small inductor on the appliance plug
and its meshing information. The magnetic core of the
small inductor was 1 mm thick. The core is 16 × 9
mm square with a square hole of 6 × 3 mm in the
Fig. 2 Prototype of wireless sensor node and
monitoring system.
The sensor node mainly consists of a clamp-type
current sensor, thermometer, antenna and other
components for the monitoring of the electricity
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CTL-6-S32-8F-CL).
600
Type A
400
Induced Voltage, mV
12
537 mV
8
Current in
appliance cord
200
Type B
152 mV
4
0
Type C
118 mV
0
-200
-4
-400
-8
-600
-12
10
20
30
40
50
Current, A
middle. The magnetic core was Mn-Zn ferrite, which
is commonly used in radio controlled clock or watch
[6]. It has the permeability of about 8000. Although
extensive simulation work had been carried out, the
effects of fabrication gap on the induction voltage
were only introduced here. The gap structure is
important for conventional fabrication process. It
assists in simplifying the fabrication process and
reducing cost. Three representative magnetic cores
were investigated, as shown in Fig. 4. Fig. 4 also
shows the simulated magnetic fields of the small
inductors. The gap structure resulted into obvious flux
loss. Figure 5 is simulated induced voltages. The
induced voltage was only 0.15 V for the Type C
inductor (5 mm long and 200 turns), which is not
enough for driving energy scavenging circuit.
However, if there were not the gaps, the induced
voltage could reach about 0.53 V. The calculated
value is based on the assumption that the winding
consists of 200 turns, which is corresponding to a
pitch of about 25 um. Such a small pitch is difficult to
be prepared by conventional machinery without the
presence of the gaps, suggesting that microfabrication
method should be developed.
60
Time, ms
Fig. 5 Simulated induced voltage of micro inductor
with different gap structure. Winding is 5 mm long
and 200 turns.
(a)
(b)
Fig. 6 Photo of the home-made inductor and
measurement set-up of induction voltage.
RESULTS AND DISCUSSION
Type A
Type B
Fig. 7 is measured induction voltage of the
commercial inductor. The simulated results were also
plotted. There was a good fit between the measured
and simulated values. The simulation also showed that
higher induction voltage could be achieved if there
were not gaps. It indicated that energy scavenging is
possible by using an inductor from the alternating
magnetic fields resulted from low AC frequency
current flow in the appliance cords.
Fig. 8 is measured voltage of the home-made
inductor. The measured value was much lower than
the simulated values shown in Fig. 5. The latter was
about ~ 285 μV/turn/A while the measured value was
only about ~ 18 μV/turn/A. Fig. 9 shows measured
waveform of the induction voltage by using the homemade one. It indicated that the saturation of magnetic
flux had occurred. This is supported by the simulation
results of Fig. 7. The difference between the induction
voltage of the core with and without gap tends to
small with decreasing AC current. We thought that the
phenomenon was mainly related to that a highpermeability core was used. High-permeability core is
Type C
Fig. 4 Magnetic field magnitude of coil with
different gap structure. Type A is gap-less. Type B is
of one gap. Type C is of two gap. The gap is shown
in dashed line.
EXPERIMENTAL
Home-made inductors were prepared. It had the
copper coil of 40 turns with the winding length of 6
mm. The core material was permalloy sheet from
Nakano Company, Japan.
It was machined into a 12 × 9 mm square with a
square hole of 6 × 3 mm in the middle. Fig. 6 (a)
shows the prepared inductor. The measurement of
induction voltage was measured by using a Tektronix
digital oscilloscope. Fig. 6 (b) shows the image of the
prepared inductor on the appliance plug. The
commercial inductor of the prototype node was also
characterized. The commercial inductor has a coil of
800 turns around a ferrite core (U. R. D. co., Ltd,
255
subject to flux loss. It was also supported by the fact
that the measured induction voltage of the home-made
inductor was not improved by using a bigger core.
Therefore, the low induction voltage might mainly
result from the poor fabrication quality of the coil.
Higher voltage output could be expected through
improving the fabrication of small inductor. In
particularly, with the size reducing of the inductor, a
careful design and precise fabrication are necessary.
3.0
Induction Voltage, V
2.5
Simulated; No gap
2.0
Simulated; 10 μ m gap
1.5
1.0
0.5
Measured
CONCLUSIONS
4
5
6
7
8
9
10
11
This research showed that the energy scavenging
was possible with a high-performance micro inductor.
The design of magnetic core was important as well as
the fabrication of coil. The simulation indicated that
microfabrication process is necessary for achieving
large number of turns in a limited volume in order to
develop a micro inductor for realizing either current
sensing or power generation.
12
Current, A
Fig. 7 Measured and simulated induction voltage
of the commercial inductor. The gap was
assumed to be 10 μm. The cross sectional area
of the core was 21 mm2.
Induction Voltage, mV
20
REFERENCES
[1] http://www.soumu.go.jp/s-news/2004/04086_4.html
[2] http://www.meti.go.jp/english/policy/GreenI
TInitiativeInJapan.pdf
[3] Leland E S, White R M, Wright P K, 2006
energy R. M. White, P. K. Wright, Energy
Scavenging Power Sources for Household
Electrical Monitoring Technical Digest
15
Cross section: 4 mm
2
10
Cross section: 2 mm
5
0
4
5
6
7
2
8
9
10
PowerMEMS 2006 (Berkely, USA, Nov. 29-Dec.
1 2006) 167-168
Current, A
[4] Leland E S, White R M, Wright P K, 2007
Design of A MEMS passive, Proximitybased AC Electric Current Sensor for
Residential and Commercial Loads Technical
Fig. 8 Measured induction voltage of the homemade inductors. Two kinds of inductors were
prepared.
Digest PowerMEMS 2007 (Freiburg, Germany,
28–29 November 2007) 77–80
8
Induction voltage, mV
6
[5] Leland E S, White R M, Wright P K, 2007 A
MEMS AC Current Sensors for Residual and
Commercial Electricity End-Use Monitoring
4
2
0
Technical Digest PowerMEMS 2008 (Sendai,
Japan, 9–12 November 2008) 497-500
[6] Abe K, Takada J 2007 Simulation and Design of
a Very Small Magnetic Core Loop Antenna for
an LF Receiver IEICE Trans. Commun. E90-B
(1) 122-130
-2
-4
-6
-8
-0.04
-0.02
0.00
0.02
0.04
Time
Fig. 9 Measured waveform of the induction voltage
by using the home-made inductor. The electrical
current was about 4.5 A at 50 Hz.
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