vibrational energy harvesting mems review
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College class essay. Selection of a few publications, examine their faults and possibilities.TRANSCRIPT
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The Comparison Between Electromagnetic and Piezoelectric
Vibrational Energy Harvesters
Chi An Lu1 , Yu Tang Hu1 , Yu Chi Chang1 , and Weileun Fang1,2
1Power Mechanical Engineering dept., 2NEMS Inst., National Tsing Hua University, HsinChu, TAIWAN
ABSTRACT
In recent years, electronic components are starting to require low power
consumption, combined with electronic components getting tinier and tinier,
conventional batteries does not seem to be the ideal solution. The upcoming of
MEMS energy harvesters is a possible answer to the problem, and this paper
will attempt to determine if certain types of MEMS energy harvesters are up to
the demand. We find that batch produced vibrational piezoelectric energy
harvesters are applicable for low consumption devices, and manually
assembled large electromagnetic energy harvesters can be considered for larger
electronic devices.
KeywordsEnergy harvestElectromagnetic energy harvester
Piezoelectric energy harvester
IIntroduction
MEMS based energy harvesters are similar to
wireless battery chargers in the way that both do not
require a transfer cable to provide power to an electronic
component. However, unlike the limited life cycle of
batteries, a significant advantage the energy harvester has
is the possibility of allowing low power consumption
components to theoretically be able to operate infinitely,
which in turn allows continuous usage without worry of
power depletion.
The ability to operate components infinitely creates
possibilities of several new concepts, among them are
health monitoring devices [1], where energy harvesters
eliminates the need to replace the battery every now and
then. And another is reconnaissance cyborg insects [2],
with energy harvesters providing power to the sensors
and transmitters, it allows highly cost-effective
surveillance in harsh conditions over extended periods of
time.
In recent years, we have seen the rising of wearable
devices and the concept: Internet of Things (IoT), as well
as the rapid evolution of technology. We observed that
the overall design of electronic components leans
towards lowering the power consumption, a trend that
gives MEMS energy harvesters a chance to shine and
perform.
In general, the most common seen types of MEMS
energy harvesters are as follows: Photoelectric,
thermoelectric, electromagnetic, piezoelectric, and
electrostatic [3]. Amongst photoelectric harvesters, solar
harvesters are the most effective. Thermoelectric type
designs generate electricity by using temperature
differences, whereas the latter three mentioned takes in
kinetic energy and transforms it into electrical energy.
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Considering most wearable devices are meant to be
used indoors, and not all devices are in direct contact
with our skin, we shall select the most commonplace
kinetic energy as the harvesting energy and chose
electromagnetic and piezoelectric harvester devices as
the focus points of this review.
1.1 Electromagnetic Energy Harvesters
According to D.P. Arnold [5], we can deduce the
scale law of electromagnetic devices. The scaling
relationship between power P, length L and acceleration
a0 as follows:
{ P 50
2
P 702 0
(1)
Arnolds paper divides energy harvesters into two
categories, rotational and vibrational devices, since (1)
has an acceleration factor embedded within, it makes
rotational type harvesters difficult to analyze; as for
vibrational harvesters, power scales down with length at
a magnitude between 10-5 ~ 10-7.
Power density graphs for the two harvesters are
shown in Fig.1 and Fig.2. These figures contains datas of
electromagnetic harvesters before 2007 [5], where it can
observed that the scale law is not completely applicable.
Our guess is that the given specifications does not reflect
the best possible performance of the harvesters, which
might be a result of lack of advanced technology for
optimization, but it would seem certain that power
density will decrease at a faster rate as the scale goes
even lower.
Fig.1 Summary of rotational generator (a) power density
(W/cm3) and (b) normalized power density
(W/cm3*krpm ) versus size. The data shows no
significant trend with device size.[5]
Fig.2 Summary of oscillatory generator (a) power
density and (b) normalized power density (W/cm3*g2)
versus size. Two reference lines are shown on (b),
indicating the theoretical scaling trend of normalized
power density with L2 and L4 [5]
Other than the two presented above, other
electromagnetic harvesters designs include the Rolling
Rod Microgenerator [6] and the Linear Vibration
Harvester [7], but both designs face the problem of
stiction which occurs during operation, which is why we
only select no-contact vibration design types of
harvesters for our review.
1.2 Piezoelectric Energy Harvesters
Based on the piezoelectric harvester review done by
H. S. Kim, Joo-Hyong Kim and Jaehwan Kim [8], the
review lists the attributes of all kinds of piezoelectric
materials, the point of our interest amongst the database
is the conversion coefficient, which induces the energy
conversion efficiency to grow 4 times the original value
when the coefficient doubles in value. As we can see
from Fig.3, piezoelectric membrane thickness is not a
contributing factor to conversion efficiency, indicating
the usage of membranes as the energy absorber is an
acceptable design for piezoelectric materials. Although
the effect of volume decrease on power density is not
mentioned in the review, we believe that the ability to
use membrane structure is an important factor to take
into consideration, especially when integrating MEMS
fabricating processes.
Piezoelectric harvest methods are divided into
impulse and vibration modes, impulse harvesters will
encounter stiction problems due to repeated contact
between structures. To get a clean comparison with all
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our selected harvesters performing on an even footing,
we choose to take impulse harvesters out of the list,
which leaves vibration harvesters for piezoelectric
energy harvesters.
Fig.4 categorizes electrostatic, electromagnetic,
piezoelectric energy harvesters and their respective
power densities, where we find a relative low power
density for electromagnetic harvesting, which as
mentioned previously might be a problem with lack of
optimization. So we searched for some of the more
recent electromagnetic harvesters and piezoelectric
harvesters listed below, in hopes that technology has
advanced enough to get fully optimized energy
harvesters.
Fig.3 Energy conversion efficiency of a piezoelectric
PVDF nanogenerator[8]
Fig.4 Comparison of the energy density for the three
types of mechanical to electrical energy converters[8]
IIPaper Survey & Discussion
For this section, we will show both electromagnetic
and piezoelectric energy harvester designs in the past few
years, and present their differences in fabrication
process, design specifications and performance.
2.1 Vibrational Electromagnetic Energy Harvesters
2.1.1 S. P. Beeby et al. [9]
This team uses photolithography to define the shape
of the stainless steel cantilever beam, and manually
attaches the permanent magnet onto the beam as well as
the copper coil onto the stationary platform, Fig.5. The
sole MEMS fabricating process ends once the cantilever
beam is complete, and the paper focuses on effect of
number of coils has on electromagnetic harvesting,
shown in TABLE 2. Since the coil is stationary, its
effects on oscillating frequency is minimal to a few Hzs.
Something worthy of note is that in the three coil
numbers presented, generated energy all lands around 45
W, showing that generated power ceases to increase
after a certain number of coils. This type of vibrational
energy harvesting mostly use diodes to filter the
alternating electric current, since there will always be a
voltage drop after passing a diode [17], a higher generated
voltage means less energy loss, so a larger number of
coils for electromagnetic energy harvesters is optimal.
Fig.5 Micro cantilever generator[9]
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Harvester specifications are shown in TABLE 3.
From the fabrication perspective, this design cannot put
the batch production of MEMS to use, but the
performance at low frequency and low acceleration
achieves 300W/cm3 output energy, which is acceptable
to put into commercial use for wearable devices.
2.1.2 Qian Zhang et al. [10]
This design defines an array pattern before
electroplating copper to form a coil, then uses ICP for
backside dry etching to form a cavity for the magnet
array. Finally, the magnets are assembled onto the
component as the stationary end, distance between beam
and magnet is approx. 100m, as shown in Fig.6. As was
mentioned during lectures, not fully supported structures
suffer from residual stress; and to obtain a low natural
frequency, the beam must be long in length. From the
specifications of this design, the beam shows minimal
bending, meaning that the beam must be rather thick,
which can only be achieved with bulk micromachining.
The authors of this paper made a large scale model
in addition to the MEMS scale model. The models are
tested on a shaker providing vertical vibration, and
measurement is done with a Laser Doppler Displacement
Meter to find the natural frequency, the component specs
are shown in TABLE 3.
Fig.6 Brief fabrication process of the micro-electromagnetic energy harvester with magnet and coil arrays[10]
TABLE 3. The comparison between four electromagnetic energy harvester.
Volume
(cm3)
Power
(W)
Power
Density(W/cm3)
Natural
Frequency (Hz)
Voltage
(mV)
Acceleration (g)
S. P. Beeby et
al.[9]
0.15 46 307 52 428
(Vrms)
0.06
Qian Zhang et
al.[10]
0.09 2.6 28.9 290 30
(Vp-p)
4
Qian Zhang et
al.[10]
26 26,300 10,120 65 28,800 12
K. Tao et al.[11] 0.02 ----- ----- 365 0.0209 1
The result is that in comparison with the MEMS
fabrication processed model, the manually assembled
large model has a much higher power rating, which
supports Arnolds assumptions of scale down effects. But
the big advantage this design has over the design by S.P.
Beeby et al. is the lack of the final assembly process,
which is crucial for mass production.
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2.1.3 K. Tao et al [11]
This team uses LIGA process to make the copper
coil, macromolecule coating is coated afterwards as an
insulation layer, copper and nickel is then electroplated
as structure layer. And finally, integrates magnetic
macromolecule composites to the component, and a lift-
off process to remove the sacrificial layer, the process is
as shown in Fig.7. The most significant aspect of this
energy harvester is the fact that it is produced using only
MEMS fabrication processes, easily achieving batch
production.
Fig.7 Process flow for fabrication of fully-integrated
energy harvesters[11]
Measurement of this harvester is done on a shaker
as well, shown in TABLE 3. The paper does not provide
the power rating nor power density, but we could use
Power equation (2)[17] to estimate, giving us a power
density graded in degrees of nanoWatt/cm3.
Power = 2 (2)
The energy output of this design is significantly
smaller than the first two presented, but the possibility of
batch production taken into consideration gives this
design a slight edge if the manufacturing department.
2.2 Vibrational Piezoelectric Energy Harvesters
2.2.1 D. Shen et al. [12]
This team proposed in 2008 Journal of
Micromechanics and Microengineering, to use silicon
and piezoelectric materials for a cantilever beam and a
pure silicon vibrating mass together forming the
harvesting device. The beam length and silicon mass is
tweaked to operate at a lower frequency. Fabrication
process is shown in Fig.8, thermal oxide is first grown on
silicon wafer before depositing electrical layer and
piezoelectric PZT layer. A lift-off process difines the
electrode pattern, and double-side dry etching forms the
beam and mass. The main fabrication processes used are
thermal growth, sputtering, lift-off, and dry etching, all
of which were mentioned during lectures, indicating a
high probability to be batch produced.
Fig.8 Fabrication flow chart: (a) multilayer
deposition; (b) top electrode patterning by liftoff (mask
1); (c) bottom electrode opening via RIE (mask 2); (d)
cantilever patterning by RIE (mask3); (e) proof mass
patterning and cantilever release via backside RIE
(mask4).[12]
Measurements are done using the same methods as
-
electromagnetic energy harvesters using a shaker to
determine the natural frequency. The additional testing in
this paper is changing acceleration magnitudes to
simulate different energy input, calculate the damping
ratio of the beam, and record the output power and
voltage under different loadings (RLoad). We therefore
choose the maximum power output in this paper as a
comparison basis, listed in TABLE 4.
2.2.2 E. E. Aktakka et al. [13]
Aktakkas team bonds PZT bulk materials onto SOI
wafer and applies mechanical thinning to get a
piezoelectric membrane. Parylene is used as the
insulation layer, and the electrodes are patterned via
sputtering. Double-sided dry etching is used to create the
cantilever beam, and a Transient Liquid Phase (TPL)
bonding completes the fabrication process.
Fig.9 Process steps of thinned-PZT on SOI process
The fabrication processes involved is a lot more
complicated than the previous design by Shens team,
and is likely to have a lower yield. The main point of
selecting this paper is for discussion of the effects of
packaging. Since PZT is affected by residual stress,
which in turn changes piezoelectric performances, this
paper offers insight into System in Packaging (SIP).[13]
The authors of this paper made two types of energy
harvesting devices using different materials, silicon and
tungsten, as the vibrating mass. Measurement on a
shaker gets the data shown in TABLE 4. In comparison
with the previous harvester at Fn = 462 Hz, 1.5g; and this
design at Fn = 415 Hz, 1.5g; the power density for this
energy harvester is only half of the previous Shen et al.
design, which we predict to be a result of too many
fabrication procedures causing accumulation of residual
stress.
2.2.3 R. Elfrink et al. [14]
Elfrink et al. purposes using vacuum packaging to
decrease aerial dampening and achieve better power
density. The fabrication process is shown in Fig.10. The
team made 11 different components and took them all to
a shaker for measurement, the results are in Fig.11. We
can see that power density increases significantly by
100~200 times with vacuum packaging.
Fig.10 Wafer-scale vacuum package process flow. a. The
piezoelectric capacitor is formed by consecutive
deposition, lithography and etching steps of the Pt
bottom electrode, the AlN piezoelectric layer and the Al
top electrode. The Si mass and beam are shaped by
subsequent front- and backside etching. b. The cavities in
the glass wafer are etched with HF and the contact holes
are made with powder blasting. The SU-8 bonding layer
is applied with a wafer scale roller-coating process. c.
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The glass substrates are bonded to the Si device wafer in
two consecutive wafer scale vacuum bonding steps. d.
Single devices with the movable mass and beam in the
vacuum cavity are obtained after dicing.[14]
Fig.11 Resonance curves of packaged devices type 3 at
0.1 g and 1.0 g at atmospheric pressure and at vacuum,
showing a 200 and 140 fold increase in power.[14]
We selected the best power density component and
placed it into TABLE 4 to compare with the other two
designs, where we find the selected component to be
slightly weak in power density, but this might be due to
different piezoelectric materials [8]. After reconfiguration
[8] of each designs power rating, the Elfrink et al.
harvester is approximated at 340 microWatt/cm3, around
100 times the Shen et al. design, showing just how big an
effect vacuum packaging has for vibrational piezoelectric
energy harvesters.
TABLE 4 The comparison between four piezoelectric energy harvester
Volume
(cm3)
Power
(W)
Power
Density(W/cm3)
Natural
Frequency (Hz)
Voltage
(mV)
Acceleration (g)
D. Shen et al.[12] 0.000625 2.15 3,272 462.5 150
(Vp-p)
2
E. E. Aktakka et
al.[13]
0.1462 205 1,404 154 ----- 1.5
E. E. Aktakka et
al.[13]
0.1462 160.8 1,099 415 ----- 1.5
R. Elfrink et
al.[14]
2.5 85 340 325 ----- 1.75
IIIConclusion
We have now separately compared different
vibrational electromagnetic energy harvesters and
vibrational piezoelectric energy harvesters. For the
electromagnetic devices, it is observed that as the
manufacturing becomes MEMS based, the power density
goes down. Since the three designs we choose have
volume differences within an order, scale down effects
can be neglected, and the main factors to power density
drop are the two following reasons: One, multiple
MEMS processes means lower yield, so the coil number
is limited, and from the results of S. P. Beeby et al., coil
amount could lower the output voltage and power
density Two, the magnets are made using composites, the
energy product is 2 to 3 orders weaker than bulk
permanent magnets. Though MEMS production makes
electromagnetic energy harvesters capable of batch
producing, the problem remains when implementing
magnetic materials into MEMS [16], and the rapid
decrease of power rating due to scaling down is also an
extremely negative factor.
As for vibrational piezoelectric energy harvesters,
we selected three cantilever beam vibration designs, and
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observed a more stable power output from these
cantilever piezoelectric energy harvesters. The main 4
factors affecting output are: Natural frequency, decided
by structure distortion; Residual stresses, resulting from
multiple MEMS fabricating procedures and packaging
procedure; Application of vacuum packing, which
decreases the effect of aerial dampening and increases
power density; Piezoelectric material, due to power
rating being the piezoelectric converting coefficient
squared, making PZT a much more suitable material
rather than A1N
At the end of our review, we draft a graph of Power
density-Volume containing all the energy harvesters we
selected, Fig.12.
Fig.12 The comparison of power density between
piezoelectric & electromagnetic energy harvester
In conclusion, scale effects are neglect able for
vibrational piezoelectric energy harvesters, and
microscale piezoelectric harvesters have power densities
2~3 orders higher than vibrational electromagnetic
energy harvesters, plus the advantage of being
compatible with existing MEMS fabricating
technologies, if considering energy harvesting to power
low consumption wearable devices, piezoelectric
harvesters are much more suitable. Another aspect that
Qian Zhang et al.[10] provides is observed at larger scales
of >10cm3, where assembled electromagnetic energy
harvesters have a power density than piezoelectric
energy harvesters, so larger devices should consider
using electromagnetic energy harvesters for power.
IVAcknowledgement
A thank you to Professor Fang, for introducing us
to the world of MEMS, and providing us the knowledge
to understand and delve further into micro processes. A
deep gratitude to National Tsing Hua University for
providing access to databases of publications around the
world.
IVReferences
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