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A 2-DOF MEMS Ultrasonic Energy Harvester
Yong Zhu, Member, IEEE, S. O. Reza Moheimani, Senior Member, IEEE, and
Mehmet Rasit Yuce, Senior Member, IEEE
Abstract—This paper reports a novel ultrasonic-based wireless
power transmission technique that has the potential to drive implantable biosensors. Compared with commonly used radio-frequency (RF) radiation methods, the ultrasonic power transmission is relatively safe for the human body and does not cause electronic interference with other electronic circuits. To extract ambient kinetic energy with arbitrary in-plane motion directions, a
novel 2-D MEMS power harvester has been designed with resonance frequencies of 38 520 and 38 725 Hz. Frequency-response
characterization results verify that the device can extract energy
from the directions of X, Y, and diagonals. Working in the diagonal direction, the device has a bandwidth of 302 Hz, which is twice
wider than a comparable 1-D resonator device. A 1- F storage capacitor is charged up from 0.51 to 0.95 V in 15 s, when the harvester is driven by an ultrasonic transducer at a distance of 0.5 cm
in the X-direction, and is biased by 60 Vdc, indicating the energy
harvesting capability of 21.4 nW in the X-direction. When excited
along the Y-axis, the harvester has an energy-harvesting capacity
of 22.7 nW. The harvester was modeled and simulated using an
equivalent electrical circuit model in Saber, and the simulation results showed good agreement with the experimental results. The ultrasonic energy harvesting was also investigated using a 1-D piezoelectric micro-cantilever.
Index Terms—Energy harvester, implantable biosensor, microelectromechanical system (MEMS), ultrasonic transmission.
I. INTRODUCTION
I
MPLANTABLE devices and low-power biosensors, used
for identification, monitoring and treatment of patients, are
critical to future healthcare industries. Supplying energy continuously to a biosensor microsystem is a key requirement for its
continuous and reliable operation. Radio-frequency (RF) wireless powering and telemetry from outside the human body to
an implanted medical device has been shown to enable its uninterrupted operation without the need for a battery replacement
[1]. However, the inductive coupling method can cause electromagnetic coupling (EMC), which may interfere with other electronic devices in medical environment [2]. Also, exposing the
Manuscript received January 21, 2010; revised April 29, 2010; accepted
June 14, 2010. Date of publication September 20, 2010; date of current version
November 03, 2010. This work was supported by the Australian Research
Council (ARC) through Discovery Grant DP0774287. This is an expanded
paper from the IEEE SENSORS 2009 Conference. The associate editor
coordinating the review of this manuscript and approving it for publication was
Dr. Patrick Ruther.
The authors are with the School of Electrical Engineering and Computer Science, University of Newcastle, Newcastle, NSW 2308, Australia
(e-mail: [email protected], [email protected];
[email protected]).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JSEN.2010.2053922
patient to RF radiation may be unsafe for living tissue [3]. Alternatively, the ultrasound is free from the EMC and is believed
to be relatively safe for human’s health [4], making ultrasonic
wireless power transmission a promising candidate for future
implantable devices.
In a typical power harvester, kinetic energy is often converted
into electrical energy using electromagnetic, piezoelectric, or
electrostatic mechanisms [5]–[7]. However, the efficiency of vibration-to-electricity conversion is quite low in existing microgenerators, since: 1) most of the reported kinetic generator techniques suffer from scavenging in a narrow bandwidth from ambient sources due to their single resonance frequency and 2)
most devices are designed to harvest energy in a single direction, which wastes the vibration energy existing in other directions [8]. An exception is the multifrequency piezoelectric harvester reported in [9] that was shown to broaden the bandwidth
by using three piezoelectric bimorph cantilevers with identical
dimensions but different masses. Also, an 800-Hz wideband
electromagnetic micro generator has been realized by implementing 40 serially connected cantilevers in different lengths
[10]. Recently, Bartsch reported a 2-D electrostatic resonant
micro energy generator to extract energy from ambient vibrations with arbitrary motion directions. However, the level of harvested power was rather low (100 pW) [11].
In this paper, an ultrasonic energy transmission method is
introduced. A novel 2-degree-of-freedom (2-DOF) microelectromechanical system (MEMS) device has been designed, fabricated, and tested as a proof-of-concept. The fabricated device
vibrates due to incoming ultrasonic waves and can scavenge energy in arbitrary directions in a plane as well as broaden the
bandwidth with two resonance frequency peaks. A 1- F storage
capacitor is charged up from 0.51 to 0.95 V within 15 s in a
sample experimental test conducted, indicating the energy harvesting capability of 21.4 nW. Also, the energy-harvesting performance has been evaluated with an equivalent electrical circuit
model in Saber, showing good agreement between simulation
and experimental results.
II. DESIGN
To harvest vibration energy in arbitrary directions in a plane, a
2-DOF motion mechanism has been incorporated in our design,
as illustrated in Fig. 1(a). The seismic mass moves in the Xand Y-axes to absorb vibration energy in any in-plane direction.
The X and Y mechanisms and the seismic mass are all discrete
mechanical elements, attached to each other via elastic flexures. These flexures significantly decouple the X and Y movements of the mass. Electrostatic comb variable capacitors
and
collect and vibration contributions separately and
flow the currents into the load, as illustrated in Fig. 1(b). The
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Fig. 3. Mechanical structure design showing two movement modes (a) and (b)
along the - and -axes.
X
Y
TABLE I
DESIGN PARAMETERS FOR 2-DOF ENERGY HARVESTER
Fig. 1. 2-DOF motion mechanism to harvest any direction in-plane vibration
energy. (a) Schematic. (b) Equivalent circuit model.
Fig. 2. (a) System schematic in Architect. (b) 3-D view.
capacitors are polarized by a dc voltage source. In a practical
application, the polarization can be achieved using permanently
charged electrets, which remove the need for an external bias
voltage, as illustrated in [12], [13]. Also, an electrets-free power
harvester has been proposed by using an inductive fly-back circuit returning part of harvested charge back to the bias capacitor
[14]. A similar method may be incorporated into the design proposed here.
The electromechanical structure of the harvester was
designed and optimized using Architect Saber in a CoventorWare environment. The Architect parametric libraries allow
system-level designers to simulate and rapidly evaluate multiple
design configurations using a top-down, system-level approach
and to deliver precise behavioral models to the device designer.1
The detailed structure design was started from Schematic
Creation: building an accurate system-level model using Parametric Electro-Mechanical Library components (e.g., beams,
rigid plate mass, sense combs), as illustrated in Fig. 2(a). Then,
the schematic was processed, and a 3-D view of the device
was created as shown in Fig. 2(b). This structure topology is
inspired from the decoupled gyroscope structure described
in [15]. The central seismic mass is suspended by 16 beams
attached to four corner anchors. The specific topology of the
beams splits a certain direction of seismic mass movement into
the comb capacitance changing in the X and Y frames. The
movable comb fingers are restricted by these beams and only
1Online.
Available: http://www.coventor.com/
move in parallel with the fixed comb fingers, without the risk
of pulling in together. This way, symmetry of the structure is
preserved, while the X and Y sense capacitors are kept independent from each other but simultaneously contribute the charging
of the load. As the structures are totally symmetrical, and the dc
bias was applied to the central seismic mass, the associated electrostatic forces cancel each other in each axis. Thus, there is no
effect on the mechanical behavior caused by the dc bias.
Table I shows the key design parameters for the energy
harvester prototype. As a part of the design process, a modal
analysis via CoventorWare was carried out on the mechanical
structure which determined the primary resonance frequency
at 39238 Hz (X mode) and the secondary resonance frequency
at 39266 Hz (Y mode). The X and Y modes obtained from a
CoventorWare simulation are shown in Fig. 3. The simulation
results show that the mechanical coupling in the combs from
one mode to another is less than 2%. For example, in the X
mode, the combs in Frame Y move less than 2% of the movement of the combs in the X mode due to mechanical coupling
and vice versa. The -axis is less stiff than the - and -axes,
and the resonance frequency in the -direction is 22.48 kHz,
and
resonance frequencies.
which is far away from the
Thus, vibrations in the -direction can be ignored when the
harvester is excited by 38.5-kHz acoustic waves.
In Fig. 3, there are two primary resonance frequencies, one
for each mode. Each resonance frequency is determined by the
stiffness of the corresponding suspend beams and the mass in the
movement direction. For instance, in the X mode, the resonance
frequency depends on the stiffness of the beams in parallel with
the -axis, and the mass including the central mass and two
movable X comb fingers. In our design, all 16 beams were designed to have identical dimensions. However, the masses of X
ZHU et al.: 2-DOF MEMS ULTRASONIC ENERGY HARVESTER
157
sponse measurement was first conducted using a probe station
to obtain initial test results. To validate the design in terms of
energy harvesting, energy management was conducted using a
power rectification circuit. The energy-harvesting performance
was evaluated and then compared with a simulation results by an
equivalent electrical circuit model in Saber. The ultrasonic energy harvesting was also investigated using a 1-D piezoelectric
micro-cantilever, and a possible implantable ultrasonic power
transmission scheme is proposed.
A. Frequency Response Characteristics
Fig. 4. SOI fabrication process through MEMSCAP.
and Y movable comb fingers were separately adjusted to obtain
the requisite resonance frequencies. To broaden the bandwidth,
the masses were designed to have different dimensions along
m wider
the - and -axes (the width of the X frame is
than that of the Y frame, as shown in Table I).
III. FABRICATION
The device was fabricated in a commercial silicon-on-insulator (SOI) MEMS foundry (MEMSCAP) process2 with a
25- m-thick silicon device layer and minimum gap of 2 m.
Fig. 4 provides an illustrated summary of this process, which
is given here.
1) Surface metal pads are patterned on a highly doped n-type
25- m silicon device layer to allow for ohmic contact.
2) Deep reactive ion etch (DRIE) is performed from the front
side of the wafer to define both the anchored and movable
features of the structure.
3) A protective polyimide layer is applied to the front side.
4) A deep trench underneath the movable structures is created
by etching through the substrate using DRIE.
5) The exposed buried oxide is removed using a wet HF etch.
6) The polyimide coat on the front side is removed by oxygen
plasma, thereby allowing the movable structure to be fully
released. Then, a large contact metal pad (i.e., electrical
grounded contact) is patterned on the substrate to reduce
parasitics.
The image of the packaged device and a section of it taken
under a scanning electron microscope (SEM) are provided in
Fig. 5(a). A large number of holes have been incorporated into
the seismic mass to enhance the mechanical coupling between
ultrasonic and seismic mass. A photograph of the packaged device is provided in Fig. 5(b). To reduce the effect of parasitic
capacitances on measurement, the chip was glued mechanically
on a package using a conductive silver tape, and connected electrically to the pins of package by gold bonding wires.
IV. EXPERIMENTAL RESULTS AND DISCUSSION
Here, we describe the experimental setup and characterization of the fabricated MEMS energy harvester. Frequency re2Online.
Available: http://www.memscap.com/en_mumps.html
The measurement set up is illustrated in the inset of Fig. 6.
A SUSS PM5 Analytical Probe System was adopted for the initial test. The chip was fixed on the surface of an optical table,
and then probes were placed on the device pads for electrical
connection to provide access to the device for measurements.
An ultrasonic transducer (SQ-40 T) was utilized to generate ultrasonic waves. This transducer has a resonance frequency of
around 40 kHz, which is conveniently close to the resonance frequencies of the designed 2-D resonator. Thus, high energy coupling is expected to be achieved between the ultrasonic transducer and the power harvesting device. The ultrasonic transducer was driven by a 4-V peak-to-peak ac signal generated by
the HP35670A Dynamic Signal Analyzer to produce ultrasonic
waves. The seismic mass absorbs this ultrasonic energy and vibrates in the – plane. While the mass is moving, the combs
capacitance is also changing. The comb capacitors are polarized
by a 100 V DC voltage source, and the moving combs force the
currents into the input node of the signal analyzer (1-M input
impedance).
Fig. 6 shows the obtained frequency response of the harvester, with the ultrasonic transducer aiming at varied directions, as illustrated in the inset. With the transducer having an
angle of 0 with the -axis, one resonance frequency peak is
observed, located at around 38.52 kHz. The small peak exists
at 38.75 kHz, which is caused by the mechanical cross axis
coupling. The results of 90 case are similar to those of 0
case. When the direction comes to 45 , two resonance frequency
peaks are observed. The 10-dB bandwidth is 302 Hz, which
is twice wider than a comparable 1-D resonator. The results of
and
directions were the same, having two peaks
at 38.52 and 38.75 kHz. However, the magnitude between these
two peaks is lower than that in the 45 case, because of the antiphase currents contributed by the two connected combs.
Note that the measured resonance frequencies are different
to those predicted in the Table I. This is mainly due to the
fabrication process, which is not free of errors. For instance,
over-etching in DRIE tends to make the beams narrower than
what they are designed to be. Consequently, a lower stiffness in
beams results in lower resonance frequencies.
B. Energy Management
Ultrasonic energy is transformed into electrical energy in the
form of ac current in Fig. 6. However, in a real application,
the rectification of ac to dc voltage must be done for energy
storage prior to powering a biosensor. Fig. 7 shows the ac–dc
voltage-rectifier circuit diagram and the circuit board for the energy management. The rectifying bridge circuit consists of two
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Fig. 5. (a) SEM image of the fabricated energy harvester. (b) Photograph of the packaged device.
Fig. 6. Magnitude curves of the output voltage versus frequency.
Fig. 7. (a) Energy-harvesting circuit diagram. (b) Circuit board with ultrasonic
transducer and packaged MEMS chip.
Fig. 8. Voltage profiles of 1-F storage capacitor with different gaps (0.5, 1.0,
and 1.5 cm) between ultrasonic transducer and the MEMS energy harvester in
different directions.
STMicroelectronics 1N5711 small-signal Schottky diodes and a
1- F storage capacitor. These Schottky diodes have low forward
voltage drop of approximately 0.2 V, thereby allowing largest dc
.
voltage on the storage capacitor
Fig. 8 illustrates the voltage profiles of the 1- F storage
capacitor with different air gaps (0.5, 1.0, or 1.5 cm) between
the ultrasonic transducer and the MEMS energy harvester.
The bias voltage was 60 V and the ultrasonic transducer was
actuated by
, 38.53-kHz ac voltage, which matched the
X-mode resonance frequency of the harvester. The transducer
aimed at the -axis of the harvester to maximize the energy
coupling. A Stanford low-noise preamplifier with 100- input
impedance and unit gain is connected between
and an
oscilloscope.
Due to the ultrasonic attenuation through air, the converted
voltage on the storage capacitor decreases with the increasing
gap, as shown in Fig. 8. After the ultrasonic wave was applied
to the harvester, the dc voltage on storage capacitor was charged
up from 0.51 to 0.95 V in 15 s with an 0.5-cm air gap between
the transducer and harvester. Because of the stray capacitance
between the actuation ac signal and the MEMS harvester, the ac
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159
Fig. 9. Electrostatic harvester equivalent circuit model in SaberSketch.
signal was fed through to the rectification end, leaving an initial
0.51-V charge on the storage capacitor.
The power stored in the capacitor is calculated as
(1)
where is the charging time. Thus, the energy stored on the
storage capacitor changed from 0.13 to 0.451 J in 15 s, indicating that the power absorption capability of this energy harvester is 21.4 nW.
Fig. 8 also shows the harvesting results along the -axis. The
bias voltage was 60 V and the ultrasonic transducer was actuated
, 38.77-kHz ac voltage, which matched the Y-mode
by
resonance frequency of the harvester. The dc voltage on storage
capacitor was charged up from 0.51 to 0.97 V in 15 s with a
0.5-cm air gap between the transducer and harvester, indicating
that the harvester has an energy harvesting capacity of 22.7 nW.
In a practical device, a power supply of 0.95 V is enough
for powering low-power IC circuits. With the current state-ofthe-art IC technology, devices can work with dc voltages lower
than 1 V [16].
C. Equivalent Electrical Circuit Model
Modeling a MEMS variable capacitor is the key to simulating
electrostatic MEMS devices. Spice [17] and Verilog HDL [18]
models have been successfully developed to model the variable capacitors. However, both methods require the designer to
write complex codes. Saber in CoventorWare Architect provides
a variable capacitor component model, which is a building block
for multidomain system-level simulation in a user-friendly electrical simulation environment [19]. A schematic of the equivalent circuit model for our electrostatic harvester is shown in
Fig. 9. The variable capacitor component models an electrical
capacitor, whose capacitance value is the value on the input pin
cap, and the pin is connected to an ideal variable source
generating a sinusoidal signal. The output of
is a no-unit
variable, allowing us to define the variable capacitor’s function.
For instance, the displacement of moveable combs was measured to be 5 m at 38.53-kHz resonance frequency, with an
original overlap of 40 m. According to the combs dimensions,
the corresponding calculated capacitance amplitude is 0.053 pF
is set to be 0.053-pF
with an offset of 0.43 pF. Thus, the
amplitude, 0.43-pF offset, and 38.53-kHz frequency.
is the
parasitic capacitor and is set to be 4 pF. Fig. 10 illustrates the
simulation result for the electrostatic harvester operating under
a 1-cm air gap. This shows a good agreement between simulation and experimental results.
Fig. 10. Comparison between experimental results and simulation results for
the electrostatic harvester.
Fig. 11. Piezoelectric cantilever energy harvesting circuit diagram.
D. 1-DOF Piezoelectric Cantilever Harvester
To validate the ultrasonic energy harvesting in another way,
a harvester based on a commercially available Veeco DMASP
micro-actuated silicon probe3 was investigated. The device consists of a micromachined silicon cantilever designed for atomic
force microscopy. A piezoelectric zinc oxide (ZnO) layer sandwiched between two Ti/Au metal electrodes is deposited on the
surface of this micro-cantilever. Although it was originally designed for atomic force microscopy, this device has been used
in other applications including a wide range pressure sensor,
whose resonance frequency is a function of the pressure and
the temperature [20]. Piezoelectric transducers have been used
widely in vibration control applications due to their ability to
transform electrical energy to mechanical energy and vice-versa
[21]. Recently, there has been significant interest to use them as
power harvesting devices [22], [23].
The experimental setup and circuit connections are illustrated
in Fig. 11. The micro-cantilever has a resonance frequency
of 50.5 kHz. Thus, an ultrasonic transducer (PROWAVE
500 MB120) with a resonance frequency of 50 kHz was selected to maximize the energy coupling to the micro-cantilever.
The piezoelectric cantilever was actuated by this ultrasonic
transducer with a 1-cm air gap and aimed at the surface of the
cantilever. The beam absorbs the ultrasonic energy and vibrates
as shown in Fig. 11. The ZnO layer on the cantilever undergoes
a periodic stress due to the vibration, and thus AC current flows
into and charges the storage capacitor through the rectification
diodes. Note that, in this device, there is no dc bias connected
to the cantilever, which is an appealing feature compared with
3Online.
Available: https://www.veecoprobes.com/
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Fig. 12. Measured results comparison between electrostatic harvester and
piezoelectric harvester.
the electrostatic method. However, only one direction energy
can be harvested.
The measured charging voltage profiles of the storage capacitor for electrostatic and piezoelectric harvesters are depicted in
Fig. 12. For comparison, both devices operate under the same
1-cm air gap, and the storage capacitor in both experiments was
1 F. Herein we only plot the increasing voltage after ultrasonic
wave takes effect on the harvester, eliminating the feed through
offset voltage as shown at the beginning in Fig. 8. As illustrated
in Fig. 12, the piezoelectric harvester charges the storage capacitor faster than the electrostatic harvester.
V. CONCLUSION
The modeling, fabrication, and characterization of an ultrasonic-based transducer used for energy harvesting were
introduced and discussed. The 2-D resonator can extract ultrasonic energy from all directions in the device plane as well as
broaden the bandwidth, increasing the efficiency of the energy
scavenging. Using this device, a power level of 21.4 nW was
converted on a 1- F storage capacitor by ultrasonic energy
transmission in an air gap of 0.5 cm. The harvester was modeled
and simulated using an equivalent electrical circuit model in
Saber, and the simulation results showed good agreement with
the experimental results. A possible application for this device
is ultrasonic energy transmission through living tissue to power
implantable biosensors, which will be investigated in future
research.
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Yong Zhu (M’10) received the B.Sc. degree in
physics and M.Eng. degree in microelectronics from
the Dalian University of Technology, Dalian, China,
in 1999 and 2002, respectively, and the Ph.D. degree
in microelectronics from Peking University, Beijing,
China, in 2005.
He was a Research Associate with the Department of Engineering, University of Cambridge,
Cambridge, U.K., in 2006 and 2007, where he
was involved with the project of microfabricated
electrometer and mass sensor. He is currently a
Research Academic with the School of Electrical Engineering and Computer
Science, University of Newcastle, Newcastle, NSW, Australia. His research
interests include micromachined resonators, power harvesters, nanopositioners,
capacitive sensors, RF MEMS, as well as interface circuits design for MEMS.
ZHU et al.: 2-DOF MEMS ULTRASONIC ENERGY HARVESTER
S. O. Reza Moheimani (SM’00) received the Ph.D.
degree in electrical engineering from the University
of New South Wales, Australian Defence Force
Academy, Canberra, A.C.T., Australia, in 1996.
He is currently a Professor with the School of
Electrical Engineering and Computer Science, University of Newcastle, Newcastle, NSW, Australia,
where he holds an Australian Research Council
Future Felowship. He is Associate Director of
the Australian Research Council (ARC) Centre
for Complex Dynamic Systems and Control, an
Australian Government Centre of Excellence. He has held several visiting
appointments at IBM Zurich Research Laboratory, Switzerland. He has authored or coauthored two books, several edited volumes, and more than 200
refereed articles in archival journals and conference proceedings. His current
research interests include applications of control and estimation in nanoscale
positioning systems for scanning probe microscopy, control of electrostatic
microactuators in microelectromechanical systems (MEMS) and control issues
related to ultrahigh-density probe-based data storage systems.
Prof. Moheimani is a Fellow of the Institute of Physics, U.K. He is a recipient
of the 2007 IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY Outstanding Paper Award and the 2009 IEEE CSS Control Systems Technology
Award, jointly with a group of researchers from IBM Zurich Research Labs. He
has served on the Editorial Board of a number of journals including the IEEE
TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY and has chaired several
international conferences and workshops.
161
Mehmet Rasit Yuce (S’01–M’05–SM’09) received
the M.S. degree from the University of Florida,
Gainesville, in 2001, and the Ph.D. degree from
North Carolina State University (NCSU), Raleigh,
in 2004, both in electrical and computer engineering.
He is currently a Senior Lecturer with the School
of Electrical Engineering and Computer Science,
University of Newcastle, Newcastle, NSW, Australia. Between August 2001–October 2004, he
served as a Research Assistant with the Department
of Electrical and Computer Engineering, NCSU. He
was a Postdoctoral Researcher with the Electrical Engineering Department,
University of California, Santa Cruz, in 2005. He has author or coauthored
approximately 50 technical papers. His research interests include wireless
implantable telemetry, wireless body area network (WBAN), analog/digital
mixed-signal VLSI for wireless, biomedical, and RF applications.
Dr. Yuce was the recipient of received a NASA Group Achievement Award
in 2007 for developing an SOI transceiver.