A SELF-POWERED MEMS SENSOR FOR AC ELECTRIC CURRENT
Eli S. Leland1, Christopher T. Sherman1, Peter Minor1,
Paul K. Wright1,2, and Richard M. White3,4
1
Dept. of Mechanical Engineering, University of California, Berkeley, USA
2
Center for Information Technology Research in the Interest of Society (CITRIS), Berkeley, USA
3
Dept. of Electrical Engineering and Computer Sciences, University of California, Berkeley, USA
4
Berkeley Sensor and Actuator Center (BSAC), Berkeley, USA
Abstract: A new type of MEMS sensor has been developed for measuring AC electric current. The sensor is
comprised of a piezoelectric aluminum nitride MEMS cantilever with a microscale composite permanent magnet
mounted to its free end. When placed in proximity to a wire carrying AC current, the alternating magnetic field
surrounding the wire induces a sinusoidal force on the sensor magnet, deflecting the piezoelectric cantilever and
thus producing a voltage signal proportional to the current in the wire. This sensor does not need to encircle the
conductor and its use of piezoelectric materials eliminates the need for a power source. This paper details an
improved micromagnet fabrication process using dispenser-printed epoxy and a neodymium alloy magnetic
powder. Also presented are the first test results of working MEMS current sensor devices, as well as the
development of a fully self-powered sensor package that uses a piezoelectric energy harvester to power circuitry
that amplifies the MEMS sensor’s signal.
Keywords: current sensor, MEMS, energy harvesting, piezoelectric cantilever, microscale permanent magnet
current in a common two-wire “zip cord” appliance
cord without separating the conductors as would be
required by other passive technologies.
Previous work detailed the theoretical background
for this new type of current sensor, as well as the
process employed to fabricate the MEMS device [3].
This paper presents an improved method for
micromagnet fabrication, test results from working
MEMS sensor devices that confirm earlier analytical
models, and the development of a self-powered
current sensor package employing a MEMS AC
current sensor, a piezoelectric energy harvester that is
driven by the wire whose current is being measured,
and circuitry for on-board power conditioning and
storage.
INTRODUCTION
Concerns about energy use as well as the need for
“smart grid” technologies that can improve the
efficiency and reliability of the electric power
distribution infrastructure present an opportunity for
new sensor technologies to measure electricity use in
homes, businesses, and electric power networks. This
paper presents a new type of MEMS (micro-electromechanical systems) sensor for AC electric current.
The sensor is comprised of a piezoelectric aluminum
nitride (AlN) MEMS cantilever with a microscale
composite permanent magnet (micromagnet) mounted
on its free end (Fig. 1). When placed in proximity to a
wire carrying AC current, the alternating magnetic
field surrounding the wire induces a sinusoidal force
on the sensor magnet, deflecting the piezoelectric
cantilever and thus producing a voltage signal that is
proportional to the current in the wire.
A variety of integratable sensors for electric
current are available [1, 2], however for certain
applications this new technology offers significant
advantages over the alternatives. The output signal is
produced through mechanical actuation of the
piezoelectric element, and thus the device does not
require an external power source for operation.
Additionally, while other passive sensors (current
transformers, Rogowski coils) must physically
encircle the conductor in order to function, this sensor
need only be placed in proximity to the current
carrier. This feature allows for measurement of AC
0-9743611-5-1/PMEMS2009/$20©2009TRF
DEVICE FABRICATION
Fabrication of this MEMS AC current sensor
Fig. 1: Schematic of MEMS current sensor design
(not to scale)
53
PowerMEMS 2009, Washington DC, USA, December 1-4, 2009
requires processes for creating both piezoelectric
MEMS cantilevers and microscale permanent magnets.
As described in [3], aluminum nitride MEMS
cantilevers were fabricated using a four-mask process
in the microfabrication facility at the University of
California, Berkeley. A micromagnet was deposited
onto the cantilever tip and then magnetized in situ
prior to the final release etch that freed the cantilever
from the substrate.
for each magnet, augmenting the magnet’s height with
each iteration. A final layer of epoxy was printed on
top of the magnet structure in order to provide
additional mechanical stability. The neodymium
powder is magnetically isotropic, and thus the
composite micromagnets have no net magnetic
moment as fabricated. The micromagnets were
magnetized in a 4 T field using a Quantum Design
Physical Property Measurement System.
Micromagnet Fabrication
A direct-write dispenser printer [4] was used to
fabricate microscale permanent magnets using
magnetic powder and a polymer binder. The
fabrication process described in earlier publications
was modified to improve the micromagnets’ magnetic
and mechanical properties, as well as ease of
fabrication. An epoxy resin was used because of its
inherent mechanical strength and good adhesion to the
cantilever surface. A neodymium alloy magnetic
powder (MQP S-11-9 from Magnequench, Inc.) was
chosen because of its strong magnetic properties and
corrosion-resistance.
Micromagnet Fabrication Results
The powder dispersion method produced
micromagnets approximately 150 µm in diameter and
100 µm tall. An SEM of a released AlN cantilever
with a printed composite micromagnet is shown in
Figure 2. Magnetic behavior of the micromagnets was
confirmed by their attraction to ferromagnetic metals
and was also characterized using a Quantum Design
MPMSXL-7 vibrating sample magnetometer.
Magnetic remanence (Br) of the sensor magnets was
found to be approximately 0.4 T, comparing well to
values for micromagnets found in the literature [5].
MEMS CURRENT SENSOR TESTING
Powder Dispersion Method
A “powder dispersion” fabrication method was
developed to achieve the desired magnet size. The
epoxy binder was prepared by mixing Hexion EPON
828 resin (72.3 wt. %) with EPICURE 3370 curing
agent (27.7 wt. %). A “dot” of uncured epoxy mixture
approximately 150 µm in diameter was first printed
on the cantilever tip. The magnetic powder was then
manually dispersed over the MEMS die with a small
spatula, adhering the particles to the substrate only
where epoxy had been printed. The epoxy was
allowed to cure for several hours at room temperature,
whereupon the excess magnetic powder was removed
and recycled for later use. The result was a small
cluster of magnetic particles firmly adhered to the
cantilever tip. This process was repeated three times
Test Assembly
The current sensor’s signal was amplified by a
non-inverting amplifier circuit built using a BurrBrown OPA129 op-amp from Texas Instruments. The
amplifier circuit was configured with a gain ratio of
100.6. The high output impedance of the MEMS
sensor required that a 1 GΩ resistor be placed
between the op-amp’s positive input and ground. Two
9 V batteries provided power to the amplifier circuit
during testing. Electrical connection to the sensor’s
electrodes was made using a Westbond 7400B
wirebonding tool. Twisted-pair wires were connected
to the amplifier circuit output terminals.
The MEMS sensor die, amplifier circuit, and
aluminum
enclosure
baseplate
"window"
MEMS amplifier
9V
sensor die circuit batteries
400 µm
400 µm
Fig. 2: Top and side view of MEMS AC current
sensor with magnet fabricated by powder dispersion
Fig. 3: Current sensor test apparatus
54
Amplified sensor signal, mVRMS
wire guide
spring
micrometer head
zip cord
bracket
enclosure
MEMS
sensor die
Fig. 4: Custom vise for sensor-wire positioning
1800
1600
1400
1200
1000
800
600
400
200
0
18 AWG
zip-cord
16 AWG
zip-cord
0
batteries were assembled inside an aluminum
enclosure in order to shield the device from
electromagnetic interference. The aluminum enclosure
baseplate on which the amplifier circuit and batteries
were mounted (Figure 3) was modified to include a
small 13 x 25 mm “window,” which was covered over
with aluminum foil tape. The MEMS die was placed
inside this window, device side up, in order to
position the sensor’s magnet as near as possible to the
current carrier for maximum signal. The sensor
magnet is separated from the outside of the enclosure
by the thickness of the foil tape, the thickness of the
sensor die, and the slight curvature of the released
MEMS cantilevers due to residual stress, a total of
roughly 600 µm. A custom vise was fabricated to hold
the aluminum enclosure and allow precise positioning
of the wire whose current is being measured relative
to the MEMS sensor. Figure 4 shows a schematic of
this configuration.
Data were taken using both a Tektronix DPO
4034 oscilloscope and a National Instruments USB6216 data acquisition board used with LabVIEW
SignalExpress software. The amplifier circuit
exhibited a DC offset of +200 mV, which was
removed before calculating the root-mean-square
(RMS) voltage of the sensor’s response. A current
transformer was used to generate variable currents for
testing and an Amprobe CT238A current probe was
5
10
15
20
Current in zip-cord, ARMS
Fig. 5: MEMS AC current sensor response
used to measure current in the wire for sensor
calibration.
MEMS Sensor Test Results
A MEMS current sensor measuring 1000 x 200
µm was selected for testing. Using an oscilloscope
and an impulse stimulus the sensor’s resonance
frequency was found to be 960 Hz. This sensor was
tested against both a 16 AWG and 18 AWG two-wire
“zip cord.” In each case the optimal position for the
sensor was found by holding the current in the zip
cord constant and using the micrometer head to adjust
the wire’s position relative to the sensor until a
maximum signal was observed. The current in the
cord was then adjusted incrementally up to the
maximum possible current as limited by resistance,
roughly 13 A for the 18 AWG cord and 20 A for the
16 AWG cord.
Response in both cases (Figure 5) was highly
linear (R2 > 0.99). Measured sensitivities were 0.87
mV/A for the 16 AWG cord and 1.08 mV/A for the
18 AWG cord (recall that the data presented in Figure
5 are amplified by a factor of 100.6). Greater
sensitivity was observed against the 18 AWG power
(a)
"scan" of
magnet
relative
to cord
(b)
appliance
cord
Amplified sensor signal, mVRMS
900
sensor
magnet
800
700
600
500
400
300
200
100
0
-12 -10 -8 -6 -4 -2
0
2
4
6
8 10 12
Wire position relative to MEMS sensor (mm)
Fig. 6:(a) Field plot surrounding zip cord. Darker regions indicate stronger signal from sensor,
(b) Response of MEMS sensor as sensor magnet is scanned past 16 AWG zip cord carrying 10 A
55
cord because its smaller conductor cross-section and
thinner insulation allow the closer positioning of the
sensor magnet than in the 16 AWG case.
The small size of the sensor magnet and precise
positioning afforded by the custom vise also allowed
for testing to verify previous analytical models
predicting the sensor’s response in various positions
surrounding a zip cord [3]. Figure 6(a) shows an
appliance cord cross-section combined with a contour
plot whose darker regions correspond to greater force
on the sensor magnet and thus increased sensitivity.
Holding current steady at 10 A, the custom vise was
used to move a 16 AWG cord past the MEMS sensor
in increments of 0.5 mm, recording the sensor’s
response at each step. Figure 6(b) shows good
correspondence between the analytical model
represented in the image on the left and the
experimental result. The slight asymmetry between
the two lesser peaks to either side of the central peak
in Figure 6(b) likely resulted from the lack of rigidity
in the aluminum foil tape on which the MEMS sensor
die is mounted, resulting in a slight rotation of the
sensor relative to the zip cord.
incorporating a bridge rectifier and a 10 mF
supercapacitor was constructed to store the harvester’s
output and use it to run the amplifier circuit
intermittently. Power transfer from the harvester to
the capacitor was 50-60 µW during operation,
presenting the possibility that a low-power radio [6]
could eventually be integrated to the self-powered
sensor package. A Maxim MAX 6778 battery monitor
was used to charge the storage capacitor to 5.6 V,
whereupon allowed the capacitor to power the
sensor’s amplifier until it had discharged to 5.1 V.
Figure 7 shows a photo of the self-powered sensor
package and its components. Testing of this device
continues as of this writing.
CONCLUSION
A MEMS AC current sensor has been
successfully fabricated and tested. This MEMS device
was used to experimentally confirm previously
derived analytical models. Work continues to more
fully characterize the MEMS sensor and test the
integrated self-powered sensor device.
ACKNOWLEDGEMENTS
SELF-POWERED SENSOR PACKAGE
This research was funded through the California
Institute for Energy and Environment grant number
500-01-43.
In the interest of building a sensor device that
truly needs no batteries, a sensor package was
developed that incorporates an energy harvester to
power the sensor’s amplification circuit. Like the
sensor, this energy harvester is constructed from a
piezoelectric cantilever with a magnet mounted on its
free end, but the harvester is much larger than the
sensor and is designed to resonate at 60 Hz in order to
couple most efficiently to the current in the zip cord.
The harvester was constructed from a Q220-A-103YB
piezoelectric generator from Piezo Systems, Inc., and
four D61-N50 neodymium disc magnets from K&J
Magnetics, Inc. When mounted next to a 16 AWG zip
cord carrying 13 ARMS, the open-circuited harvester
develops roughly 11 VRMS.
A power conditioning and storage circuit
sensor
die
amplifier
circuit
power conditioning
and storage
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[3] Leland ES, Wright PK, White RM 2009 A MEMS
AC Current Sensor for Residential and
Commercial End-Use Monitoring, J. Micromech.
and Microeng. 19 094018 (6 pp)
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Wake-Up Receiver With -72 dBm Sensitivity
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Journal of Solid State Circuits 44 (1) 269-280
energy
harvester
Fig. 7: Self-powered current sensor components
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