A Vibration-Based MEMS Piezoelectric Energy

A Vibration-Based MEMS Piezoelectric Energy
Sensors 2014, 14, 3323-3341; doi:10.3390/s140203323
OPEN ACCESS
sensors
ISSN 1424-8220
www.mdpi.com/journal/sensors
Article
A Vibration-Based MEMS Piezoelectric Energy Harvester and
Power Conditioning Circuit
Hua Yu 1,2,3,*, Jielin Zhou 1,2,3, Licheng Deng 1,2,3 and Zhiyu Wen 1,2,3
1
2
3
College of Optoelectronic Engineering, Chongqing University, Chongqing 400044, China;
E-Mails: [email protected] (J.Z.); [email protected] (L.D); [email protected] (Z.W.)
Key Laboratory for Optoelectronic Technology & Systems, Ministry of Education of China,
Chongqing 400044, China
National Key Laboratory of Fundamental Science of Micro/Nano-Device and System Technology,
Chongqing 400044, China
* Author to whom correspondence should be addressed; E-Mail: [email protected];
Tel./Fax: +86-23-6511-1027.
Received: 25 December 2013; in revised form: 21 January 2014 / Accepted: 21 January 2014 /
Published: 19 February 2014
Abstract: This paper presents a micro-electro-mechanical system (MEMS) piezoelectric
power generator array for vibration energy harvesting. A complete design flow of the
vibration-based energy harvester using the finite element method (FEM) is proposed. The
modal analysis is selected to calculate the resonant frequency of the harvester, and
harmonic analysis is performed to investigate the influence of the geometric parameters on
the output voltage. Based on simulation results, a MEMS Pb(Zr,Ti)O3 (PZT) cantilever
array with an integrated large Si proof mass is designed and fabricated to improve output
voltage and power. Test results show that the fabricated generator, with five cantilever
beams (with unit dimensions of about 3 × 2.4 × 0.05 mm3) and an individual integrated Si
mass dimension of about 8 × 12.4 × 0.5 mm3, produces a output power of 66.75 μW, or a
power density of 5.19 μW∙mm−3∙g−2 with an optimal resistive load of 220 kΩ from 5 m/s2
vibration acceleration at its resonant frequency of 234.5 Hz. In view of high internal
impedance characteristic of the PZT generator, an efficient autonomous power
conditioning circuit, with the function of impedance matching, energy storage and voltage
regulation, is then presented, finding that the efficiency of the energy storage is greatly
improved and up to 64.95%. The proposed self-supplied energy generator with power
conditioning circuit could provide a very promising complete power supply solution for
wireless sensor node loads.
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Keywords: energy harvester; PZT; MEMS; power conditioning circuit
1. Introduction
The use of wireless sensors and implanted medical electronic systems has developed rapidly in
recent years. They are usually powered by standard batteries which become depleted within a
relatively short timeframe, but the replacement or recharge of batteries is a major bottleneck for wide
deployment of wireless sensor nodes (WSN). Moreover, while the size of electronic circuitry has
shrunk thanks to the advent of integrated circuit technology, batteries are nowadays often the most
bulky devices in wireless sensor nodes. Vibration-based MEMS power generators present an amazing
solution to supply power for wireless sensor nodes, which can generate mW or μW level power.
Vibration energy harvesting has been exploited for converting ambient kinetic energy into electric
energy by several different transduction mechanisms, including piezoelectric, electromagnetic and
electrostatic methods. Among these three energy harvesters, the piezoelectric energy harvester (PEH)
has a high electromechanical coupling effect, and requires no external voltage sources, it is compatible
with MEMS technology, and accordingly has received much recent attention [1–4].
The piezoelectric material chosen for this research is PZT, because it is a well-known and
well-characterized material with a high piezoelectric coefficient. Vibration energy harvesters must be
able to respond to the low frequency and low acceleration vibrations that usually exist in the
environment. Moreover, the energy harvester should generate as much energy as possible in order to
supply enough power for the follow-up loads. Recently, MEMS technology has been applied toward
the development of energy harvesters, and many piezoelectric MEMS energy harvesters have been
developed. Renaud et al. proposed a fabricated MEMS-based PZT cantilever micro-generator with an
integrated proof mass that can generate an average power of 40 μW at 1.8 kHz vibration frequency [5].
Jeon et al. developed a d33 mode thin film PZT power generating device with interdigitated electrodes
that can generate an average power of 1.0 μW from 108 m/s2 vibration acceleration at its resonant
frequency of 13.9 kHz [6]. However, the above two energy harvesters’ resonant frequencies are very
high. Fang et al. fabricated a MEMS-based PZT cantilever power generator with a nonintegrated Ni
proof mass that can generate 2.16 µW from 10 m/s2 vibration acceleration at its resonant frequency of
609 Hz. The nickel metal mass on the tip of the cantilever is used to decrease the structure's resonant
frequency for the application under low-frequency vibration, but it cannot to be micro-machined by
MEMS technology [7]. Similar to the previous work, Liu et al. used the previous cantilever structure
to construct a power generator array to improve power output and frequency flexibility. Although they
demonstrated that power density was high, the proof mass was not also integrated with the cantilever
which will be an additional difficulty in production [8]. Muralt et al. designed and fabricated a micro
power generator of thin film PZT laminated cantilever with proof mass and interdigitated electrodes
which could generate a voltage of 1.6 V and power of 1.4 µW when excited under 20 m/s2 vibration
acceleration at 870 Hz resonant frequency [9]. Over the past two years, some new structures or new
piezoelectric materials have been applied in energy harvesters, which attain lower resonant frequency
and more output power, but the power conditioning circuit for the improvement of high internal output
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impedance about piezoelectric energy harvester has rarely been discussed in these references [10–12].
From these previous studies, at least three conclusions can be reached and summarized as follows:
Firstly, most MEMS vibration energy harvesters usually work under high frequency environment
conditions, which limits the scope of application for these devices, because the frequencies of available
ambient vibration sources are relatively low. Secondly, the delivered power output of those MEMS
micro-generators is relatively low, so the architecture and parameters of energy harvester must be
optimized as far as possible in order to increase output power. Thirdly, a power conditioning circuit is
absolutely necessary for piezoelectric energy harvesters in order to extract the maximum power from
the harvester, store energy and then regulate the voltage for meeting the demands of the loads, such as
wireless sensor nodes [13].
In this paper, a complete design flow of a self-powered MEMS PZT vibration energy harvesting
system for wireless sensor nodes is presented, which includes the design method of how to increase the
output power and voltage of the energy harvester. A design method of low resonant frequency for the
vibration energy harvester is also verified. This paper especially gives the theoretical calculation
formula of the resonant frequency, the simulation value using ANSYS FEM software, and the
experimental test results. It’s shown that their relative errors are very small, which provided a
reference design method of the resonant frequency for the vibrational energy harvester. Moreover, the
total output power and voltage are greatly improved by improving the PZT film material preparation
technology and MEMS cantilever array fabrication process, which lays a strong foundation of the
latter power condition circuit. Finally, in order to improve the high internal impedance characteristics
of the PZT generator, we design an efficient autonomous power conditioning circuit, with the
functions of impedance matching, energy storage and voltage regulation, finding that the efficiency of
the energy storage is greatly improved. The proposed whole energy harvesting system could provide a
complete self-powered supply solution to wireless sensor node loads, which is regarded as a reference
method for implementing a battery-less wireless sensor network.
2. Design of the MEMS Piezoelectric Energy Harvester
2.1. Architecture Design
The cross-section structure of the MEMS piezoelectric cantilever array is shown in Figure 1. The
cantilever array device is designed to resonate at a specific low vibration frequency by using a large
integrated Si proof mass. The bending results in a strain distributed along the beam, which is then
converted to electrical energy through the transverse mode (d31) piezoelectric effect. The Pt/Ti and Al
electrodes are patterned in order to generate strain in parallel to the electric field, which forms the d31
mode of the piezoelectric element. When an input vibration applies acceleration to the beam structure,
the effective mass converts the input acceleration into force. The relative displacement causes the PZT
layer to be tensed or compressed, which in turn induces a charge shift and accumulation due to the
piezoelectric effect. Electrodes collect the generated charge and electrical damping results. The
magnitude of the electric charge voltage is proportional to the stress induced by the relative
displacement. Material properties and structural parameters of the piezoelectric energy harvester are
listed in Table 1.
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Figure 1. Cross section structure of MEMS piezoelectric cantilever array.
SiO2
Al
PZT
Pt/Ti
SiO2
Si
SiO2
Top
electrode
Bottom
5
electrode
4
Si
3
2
2
1
1
Supporting
base
Supporting
beam
Proof
mass
Table 1. Material properties and structural parameters of the piezoelectric energy harvester.
Symbol
ρS
ρP
ES
EP
S
P
d31
ε0
S
ε11/ε0
S
ε33/ε0
lS × wS × hS
lP × wP × hP
wg
lm× wm × hm
Parameter
Density of Si beam
Density of PZT
Young’s modulus of Si beam
Young’s modulus of PZT
Poisson’s ratio of Si beam
Poisson’s ratio of PZT
Piezoelectric constant of PZT
Vacuum dielectric constant
Relative dielectric constant
Relative dielectric constant
Dimensions of Si beam
Dimensions of PZT
Gaps between the beams
Dimensions of the proof mass
Value
2,329 kg/m3
7,500 kg/m3
170 GPa
98.4 GPa
0.28
0.29
−274 × 10−12 C/N
8.85 × 10−12 F/M
1,703
1,433.6
3 × 2.4 × 0.05 mm3
3 × 2.4 × 0.002 mm3
100 μm
8 × 12.4 × 0.5 mm3
2.2. Output Power
Mechanical power is converted into electrical power when damping is present. For a sinusoidal
excitation vibration, the electrical power generated by the system is given in Equation (1) [14]:
(1)
where ζ is the damping ration (ζ = q/2mn); n and Y is the resonant frequency and the amplitude of
vibration respectively, and  is the excitation frequency. It’s obvious that the maximum output power
occurs at the resonant frequency of the generator if  is equal to n for the system without damping:
(2)
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It also can be concluded from Equation (2) that the value of the maximum power is indeed finite,
and reduction of the damping factor results in increased mass displacement, which is ultimately limited
by the size and geometry of the device. Equation (3) also expresses the power in terms of the
excitation acceleration magnitude of the input vibration:
(3)
where A is excitation acceleration magnitude, and its formula is A =  Y. Several conclusions may be
drawn from the above equations. Firstly, if the resonant frequency of the generator is set equal to the
proof mass frequency, the maximum power generated is limited by the movement of the proof mass.
Thus, the maximum power can be expressed in terms of maximum distance the proof mass can move.
Secondly, for applications where the frequencies of vibration are well defined and concentrated around
one point, a low damping factor will provide higher peak responses and power generation. Thirdly, it
can also be seen that power is proportional to the proof mass, so a large proof mass is always desirable
for energy harvesting. Finally, the harvester’s damping ζ is composed of the mechanical damping ζm
and electrical damping ζe. The maximum electrical output power is equal to half of the value in
Equation (2) when the electrical damping matches the mechanical damping [15].
2
n
2.3. Resonant Frequency
The resonant frequency of a vibration energy harvester is one of the most important design
parameters, because the maximum output power can be obtained when the vibration frequency
matches the resonant frequency. It is well-known that the power output will be dramatically reduced
when the driving vibration frequency deviates from the resonant frequency of a device [16]. When a
horizontal cantilever beam is subjected to a vertical harmonic excitation acting at the tip of the beam,
the equivalent stiffness is obtained from the following Equation (4):
(4)
where E is the equivalent elastic modulus, I is the equivalent rational inertia, and L is the effective
length of the cantilever beam. The value of EI can be calculated by Equation (5) [17]:
(5)
where zpN and zsN are the neutral axis of the piezoelectric layer and silicon substrate, zN is the neutral
axis of the total piezoelectric cantilever. They are calculated by Equations (6)–(8) respectively:
(6)
(7)
(8)
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where Ep1 and Es1 are the Young’s modulus of PZT material and Si beam under the plane strain
respectively, they are expressed by Equations (9) and (10) as follows:
(9)
(10)
where Ep and Es are the elastic modulus of PZT material and Si material respectively, vp and vs are the
Poisson’s ratio of the PZT layer and silicon cantilever beam layer. Based on the equivalent model of
the cantilever beam, the resonant frequency of the piezoelectric cantilever beam can be written as a
simple Equation (11) [16]:
(11)
where L is the equivalent length of the beam and the mass, m is the mass of the proof mass, and ms is
the mass of the piezoelectric cantilever.
3. Finite Element Method Model
The proposed piezoelectric energy harvester is analyzed using FEM to predict the relationships
between output characteristics and geometries of the harvester. The piezoelectric energy harvester is
modeled by the ANSYS14.0 software. Element types of the elastic base and piezoelectric patches are
selected as ‘SOLID45’ and ‘SOLID5’, respectively. A mapped meshing method is specified to mesh
the created model. In addition, the boundary conditions are applied to the clamped end of the
cantilever by applying zero displacement for all degrees of freedom at the nodes. The top and bottom
electrodes of the five piezoelectric patches are defined respectively [18]. Figure 2 shows the created
finite element model of the piezoelectric energy harvester.
Figure 2. Finite element model.
A modal analysis is conducted firstly for the established model to determine the vibration modes
and corresponding resonance frequencies. The resonance frequencies for the first four models are
222.3 Hz, 2,138.6 Hz, 4,057.4 Hz and 28,886.2 Hz, respectively, simulated by using ANSYS software
as shown in Figure 3. The horizontal color bar refers to the modal displacements of each point in the
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structure, and its unit is meters. However, the modal displacements here are not really physical
displacements but the relative displacements, and they illustrate the vibration characteristics of the
structure. The first resonant frequency (222.3 Hz) of the model makes it possible to use it in the low
frequency range, which is always chosen for dynamic analysis to achieve the maximum power.
Besides, there is a quite difference between the first frequency and the others, which ensures the
stability of the piezoelectric energy harvester.
Figure 3. The first four resonant frequencies of the piezoelectric energy harvester. (a) The
first resonant frequency. (b) The second frequency. (c) The third resonant frequency.
(d) The fourth resonant frequency.
(a)
(b)
(c)
(d)
Secondly, a harmonic analysis is implemented to calculate the response of the structure to cyclic
loads over a frequency range, which can provide the curves between the induced voltage and the
dynamic frequency. This can help the designer evaluate the dynamic traits of the results and any
possible fatigue and crack problems to maintain the stability of the device. In the harmonic response
analysis process, sinusoidal signal z(t) = u0sin(t) is forced on the whole structure in the poling
direction of the piezoelectric patch. Figure 4 shows the curves between the induced voltage and the
dynamic frequency in four cases. Case 1 and Case 2 show the electric potential of single beam and five
series connected beams at 5 m/s2 vibration acceleration, respectively. Case 3 and Case 4 present
separately the output voltages derived from single beam and five connected beams in series at 10 m/s2
vibration acceleration. All four simulation results indicate that maximum electric potential occurred at
the first resonance frequency of 222.3 Hz and then declined rapidly. That is to say, the piezoelectric
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cantilever is only sensitive to the harmonic frequency. If the acceleration is twice as much as before,
the value of output voltage is doubled, which is mainly due to the fact that harmonic analysis is a linear
analysis. Besides, the output voltage of five beams connected in series happened to be nearly five
times as much as that of single beam as a result of the symmetry of the piezoelectric vibration
structure. The output voltage response of piezoelectric energy harvester can be seen distinctly under
differing frequency exciting forces by the harmonic response, which provides the theory for further
systematic dynamic analysis and the structure design of the energy harvester. These also provide an
applied dynamic force analysis method, which can be used as reference for other analysis.
Figure 4. Harmonic curves.
35
Case 1
Case 2
Case 3
Case 4
Output voltage (V)
30
25
20
15
10
5
0
218
219
220
221
222
223
Frequeny (Hz)
224
225
226
227
Thirdly, the paper also studies how the resonant frequencies of the piezoelectric energy harvester
structure are affected by the length, the width and the thickness of the cantilever beam. Different
simulation results are plotted in Figure 5. Figure 5a shows that when the width and thickness of the
cantilever beam are fixed, the resonance frequency decreases with the increase of the length of the
cantilever beam. As shown in Figure 5b, with the increasing width of the cantilever beam, there is a
clear growth trend in the resonance frequency.
Figure 5. Resonance frequency versus dimensions of the beam. (a) Beam length. (b) Beam
width. (c) Beam thickness.
500
280
Resonance frequency (Hz)
Resonance frequency (Hz)
450
400
350
300
250
200
150
100
260
240
220
200
180
160
140
50
0
1
2
3
4
5
6
Beam length (mm)
(a)
7
8
9
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Beam width (mm)
(b)
4.0
4.5
5.0
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Figure 5. Cont.
Resonance frequency (Hz)
500
400
300
200
100
0
0
10
20
30
40
50
60
Beam thickness (m)
70
80
90
(c)
In Figure 5c, it can be seen that the resonant frequency increases nearly linearly with the thickness.
Accordingly, in order to make the piezoelectric energy harvester work in a low vibration frequency
environment, the cantilever beam should be designed to be longer, narrower and thinner on the
condition that it cannot be fractured.
Figure 6 presents the relationship between resonance frequency of the piezoelectric energy
harvester and the mass at the tip of the beam. The resonance frequency decreases sharply from
508.0 Hz to 165.53 Hz when the mass of the proof mass increases from 0.02 g to 0.2 g. As a result, by
adjusting the mass of the proof mass, the resonance frequency of the piezoelectric energy harvester can
be significantly changed to make it work better to meet the low frequency vibration needs.
Figure 6. The resonance frequency versus the proof mass.
550
Resonance frequency (Hz)
500
450
400
350
300
250
200
150
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22
Proof mass (g)
It is obvious that the targeted resonant frequency is attained by changing the lengths and widths of
the cantilever beam and the dimensions of the proof mass. Based on the simulation results, the
structural parameters of the piezoelectric energy harvester in Table 1 are chosen in this design.
Substituting the involved parameters into Equation (5), the value of EI is as follows:
EI = 4.95 × 10−6. Then, substituting the value of EI, L, m, mb into Equation (11), the theoretical value
of resonance frequency is as shown in Equation (12). Compared the theoretical value of resonance
frequency with the simulation resonance frequency of Figure 4, the relative error value between
theoretical and simulation resonant frequency is only 2%. This result shows the theoretical analysis has
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high prediction accuracy, which provides the basis of architecture parameters design for MEMS PZT
energy harvester:
(
)
(12)
Finally, to investigate the dependence of the output voltage on the parameters of the piezoelectric
patch, additional harmonic analyses are done. Three plots of the relationships between them are shown
in Figure 7. Figure 7a shows the dependence of the output voltage on the length of the piezoelectric
patch. While other parameters remain constant, the output voltage first increases and then decreases
with the increase of the length of piezoelectric patch. From Figure 7b, it is found that the output
voltage decreases when the width of the piezoelectric patch is changed from 0.25 mm to 2.4 mm. The
result in Figure 7c shows that the output voltage begins to increase first, and then decreases with the
growth of the thickness of the piezoelectric patch. The analysis results show that the piezoelectric
patch with the appropriate length and thickness should be less wide compared with the dimensions of
substrate, which is well in agreement with the previous literatures [19].
Figure 7. Output voltage versus piezoelectric patch dimensions. (a) Patch length. (b) Patch
width. (c) Patch thickness.
25
3.5
Output voltage (V)
3.3
3.2
15
10
5
3.1
0
0.0
3.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.5
1.0
1.5
(a)
(b)
20
18
16
14
12
10
8
6
4
2
0
0
10
2.0
Piezoelectric patch width (mm)
Piezoelectric patch length (mm)
Output voltage (V)
Output voltage (V)
20
3.4
20
30
40
50
60
70
Piezoelectric patch thickness (m)
(c)
80
90
2.5
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Based on these simulation results, a MEMS piezoelectric cantilevers array (five beams of
3 × 2.4 × 0.05 mm3) integrated with a large silicon proof mass of about 8 × 12.4 × 0.5 mm3 is designed
to improve the output voltage for the low frequency vibration condition.
4. Fabrication
The MEMS piezoelectric energy harvester consists of five piezoelectric PZT cantilevers and an
integrated silicon proof mass. The five PZT elements are electrically isolated from one another and
each PZT element is composed of a top electrode layer (Al), a PZT layer, and a bottom electrode layer
(Pt/Ti). Each of the top and bottom electrodes of the PZT element are connected to a bonding pad
individually. A large proof mass is designed and integrated at the end of the supporting beam to
achieve a low resonant frequency and increase output power. The fabrication process is as shown in
Figure 8 [20,21]:
(a) Grow thermal oxide on silicon on insulator (SOI) wafer.
(b) Sputter deposit Ti/Pt (bottom electrode), and Al (top electrode), spin coat sol-gel PZT
piezoelectric layer.
(c) Pattern and etch electrode, PZT and SiO2 layer, deep reactive ion etching (DRIE) Si
device layer.
(d) Plasma enhanced chemical vapor deposition (PECVD) deposit oxide, pattern and
etch via.
(e) Pattern and etch contact pads.
(f) DRIE from backside, dicing and releasing.
Figure 8. Fabrication process of MEMS piezoelectric energy harvester.
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5. Power Conditioning Circuit
The main goal of the power conditioning circuit is highly efficient energy transfer and energy
accumulation between the harvester and the electric load, because the typical output power of a MEMS
vibration energy harvester is in the μW range. Figure 9 shows the schematic of the proposed power
conditioning circuit, which includes AC-DC rectifying, impedance matching, energy storage,
instantaneous bleed-off and voltage regulator circuits. The basic design idea is to enable maximum
power extraction from the piezoelectric energy harvester by using the impedance matching circuit,
store the energy in a super-capacitor and supply power to the load when enough energy is accumulated
in super-capacitor. The different parts of the circuit are analyzed with detailed information as follows.
Figure 9. Power conditioning circuit for PZT energy harvester.
5.1. Rectifying Circuit
The output of the piezoelectric energy harvester is an alternating voltage signal which must be
rectified for supplying power to the sensor node load. The rectifying bridge circuit consists of four
small-signal diodes. These diodes are chosen specifically for rectification because they must have the
smallest forward voltage drop and leakage current.
5.2. Impedance Matching Circuit
The discontinuous control mode (DCM) buck-boost converter is chosen for the second stage
because of its ability to accommodate a wide range of input voltage and behave as a lossless resistor to
match the source impedance for the maximum power point tracking. The effective input resistance of a
DCM buck-boost converter is given in Equation (13) as follows [22]:
(13)
where Ts is switching period of the transistor Q1, D1 is duty cycle of the transistor Q1, and VRECT is the
rectified voltage. In order to achieve the resistive impedance matching, the effective input resistance
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RIN should be equal to the optimal resistive load RL,opt. Hence, the optimal duty cycle can be expressed
by using Equation (14):
(14)
An ultra-low power oscillator with an RC network is used to generate the pulse width modulation
(PWM) signal for turning the power switch Q1 on or off. The duty cycle and switching frequency can
be adjusted by choosing appropriate R1, R2 and C2 to achieve the maximum power delivery in Figure 9.
If the comparator output is high, the voltage at the non-inverting input of the comparator is two-thirds
of the supply voltage. Capacitor C2 is charged through resistor R1. As soon as the capacitor voltage
reaches two-thirds of the supply voltage, the comparator output acts and goes low. The voltage at the
non-inverting input of comparator is one-third of the supply voltage. Capacitor C2 is discharged
through resistor R2. The power supply of the chip connects to the cathode of diode D5.Once the
capacitor voltage decreases one-third of the supply voltage, the comparator output becomes high,
Capacitor C2 is charged again and the whole cycle repeats.
5.3. Energy Storage and Instantaneous Bleed-Off Circuit
In order to drive the load successfully, the weak harvested energy should be accumulated in the
energy storage element during the long period time because a wireless sensor node requires much
more peak power than a piezoelectric micro power generator can produce. A super-capacitor C1 of
33 mF is chosen as a storage element in this energy harvesting system, which accumulates energy to
enable efficient use for short power output bursts. The super-capacitor should be disconnected from
the load during the energy accumulation stage to prevent energy leakage to the load, and is supposed to
be connected to the load only if the accumulated energy is large enough to drive it. The instantaneous
bleed-off circuit is composed of an ultra-low power voltage comparator U2 with hysteresis, the
switching Q2, and Q3. The comparator monitors the super-capacitor’s voltage and controls the
super-capacitor’s charging and discharging process. The switch is in charge of powering on or off the
wireless sensor node. The hysteresis of a comparator creates two threshold voltage points: the upper
one for super-capacitor charge voltage and the other lower one for the super-capacitor discharge
voltage, which can control how much energy discharging from the super-capacitor.
5.4. Voltage Regulator
A voltage regulator is adopted as the output unit providing a stable voltage supply to the wireless
sensor node. In order to improve the efficiency of the voltage regulator, two key design points are
applied in the circuit. One is to reduce inductor power loss and the other is set to enable start-up signal
for DC-DC converter. If input voltage of the voltage regulator (i.e., the super-capacitor’s voltage) is
lower than a defined start-up voltage, the voltage regulator consumes unnecessary power because it is
not able to boost the output voltage. Thus, to overcome this drawback, we introduce a supervisory unit
that continuously checks the super-capacitor’s voltage and enables the voltage regulator output stage
only when it can be equal to the value of startup voltage. Otherwise, a complete shutdown with output
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disconnection suppresses any additional power consumption, reducing the charging time of the
super-capacitor and boosting the overall efficiency.
6. Testing and Analysis
6.1. Experimental Test Setup
In order to measure the output characteristics of the piezoelectric energy harvester, a vibration
testing system is employed. A schematic drawing of the experimental setup for PEH system is shown
in Figure 10. It consists of a vibration exciter, an accelerometer, a power amplifier, an oscilloscope, a
signal generator and a charge amplifier. An accelerometer is assembled on the vibration exciter
together with the cantilever array device for the acceleration measurement. The vibration signal is
generated from the signal generator, amplified via the power amplifier and finally utilized to control
the vibration amplitude and frequency of the shaker. Accordingly, the piezoelectric cantilever device
will undergo excitations and generate output voltage signal which is recorded to the oscilloscope.
Acceleration signals will be measured by the accelerometer and amplified by the charge amplifier, and
then shown on the monitor of the computer. The photo of the experimental system for testing the
MEMS piezoelectric energy harvester is shown in Figure 11.
Figure 10. Schematic drawing of the experimental setup for PEH system.
Sinusoidal signal to
vibration exciter
Power
amplifier
Signal
source
Signal
generator
Vibration
exciter with
PEH mounted
Voltage
output
from PEH
Oscilloscope
accelerometer
Charge
amplifier
Measured
acceleration
experienced
by PEH
Data acquisition
software&PC
Figure 11. Photo of the experimental setup for MEMS piezoelectric energy harvester.
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6.2. Test Results and Analysis
First, the respective characteristics of the MEMS piezoelectric energy harvester are measured,
including the resonant frequency, output voltage, output power and optimal load. Figure 12a shows the
output voltage of a single PZT beam and five beams connected in series as the vibration frequency
swept from 231 Hz to 237 Hz at 5 m/s2 vibration acceleration. As can be seen, the maximum open
circuit voltage is 1.62 V and 7.04 V at the resonant frequency of 234.5 Hz under 5 m/s2 acceleration,
respectively. The error between the simulated and measured resonant frequency is about 4.2%.
Figure 12b shows the load voltage and average power to the different loads at resonant frequency. As
expected, the load voltage increases with the increased load, up to 3.91 V at 230 KΩ load. However,
the power delivered to the load has a maximum value. The maximum output power of 66.75 μW is
obtained at 5 m/s2 acceleration with an optimal resistive load of 220 kΩ resulting in a maximum power
density of 5.19 μW∙mm−3∙g−2. The power density is calculated using the average power divided by the
effective volume of the device, which is the volume of the entire beam and the Si proof mass
calculated from the measured data in Table 1. Some main performance parameters of recent MEMS
piezoelectric energy harvesters are compared in Table 2.
Figure 12. The characteristics of MEMS piezoelectric energy harvester. (a) Open circuit
voltage vs. the frequency. (b) Load voltage and power vs. the load.
9
4.0
Single beam
Five series connected beams
Load voltage (V)
7
6
5
4
3
66.8
Load voltage
Power output
3.9
66.4
3.8
3.7
66.0
3.6
65.6
3.5
65.2
2
Power ouput (w)
Opencircuit voltage (V)
8
3.4
64.8
1
3.3
0
231
232
233
234
235
Frequency (Hz)
236
170
237
180
190
200
210
Load resisitance (K)
(a)
220
230
(b)
Table 2. Comparison of recent MEMS piezoelectric energy harvesters.
Reference
Material
[23]
[24]
[25]
[26]
This paper
PZT d31
PZT d31
PZT d31
PZT d31
PZT d31
Volume
mm3
1.063
12.775
27
0.608*
51.472
A
m/s2
5
9.3
15
10
5
f
Hz
243
615
154
78.7
234.5
Power
μW
2.33
51.3
205
0.803
66.75
Power Density
μW∙mm−3∙g−2
8.39
4.97
3.37
1.32
5.19
* Estimated from reference data.
The next experiment shows the performance of the power conditioning circuit. Figure 12 shows that
the proposed power MEMS system can successfully drive the wireless sensor load when the
temperature-humidity sensor node transmits signals. As shown in Figure 13, when the voltage across
Sensors 2013, 13
3338
the super-capacitor is charged to 2 V, the instantaneous bleed-off circuit begins to work and the
voltage regulator will start up, supplying 3.3 V to the wireless sensor node load. The super-capacitor’s
voltage will begin to drop while the energy is supplied to the load. When the voltage across the supercapacitor drops below 1.5 V, the output voltage level of instantaneous bleed-off circuit will become
low, so the voltage regulator shut off again. When the super-capacitor voltage is charged to 2.0 V next
time, the DC-DC converter will start up again. Through the test, the working cycle time of the wireless
sensor node is 0.185 h. In the proposed circuit, the energy of super-capacitor discharging is shown in
Equation (15) and the efficiency of the energy management is calculated by Equation (16), respectively:
(15)
(16)
Figure 13. The voltage across the super-capacitor and wireless sensor node.
4.0
Wireless sensor node voltage
3.5
Supercapacitor voltage
Voltage (V)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.0
0.5
1.0
T (h)
1.5
2.0
2.5
The maximum available power of 66.75 μW is measured as the power delivered to the 220 kΩ
optimal resistive load connected directly to the piezoelectric generator at the resonant frequency of
234.5 Hz. The extracted power of the proposed power conditioning circuit is the power delivered to the
super-capacitor during the charging process. The proposed circuit’s efficiency is about 64.95%, which
is better than the results reported in some previous literature [27–29].
7. Results
In this paper, a self-supplied MEMS piezoelectric energy generator with power conditioning circuit
is proposed. A complete design flow analyzing the architecture and parameters of the energy harvester
using the FEM is established. Based on the simulation results, a MEMS PZT cantilever array with an
integrated large Si proof mass is designed and fabricated. Test results show that the proposed energy
harvester can produce a maximum output power of 66.75 μW, or power density of 5.19 μW∙mm−3∙g−2
with an optimal resistive load of 220 kΩ from 5 m/s2 acceleration at its resonant frequency of
234.5 Hz, which represents a significant improvement of output power and power density. This paper
also offer a design method of a power conditioning circuit with the function of impedance matching,
energy storage and voltage regulation in order to improve high impedance characteristics of PZT
Sensors 2013, 13
3339
energy harvester and attain higher efficiency. The experimental results show that the proposed
self-supplied energy generator with power conditioning circuit could provide a more promising complete
power supply solution for wireless sensor node loads. Future planned work includes further improvement
of the energy harvester architecture and development of a monolithic power conditioning chip.
Acknowledgments
This work is sponsored by the National Natural Science Foundation of China (No. b61074177), the
Visiting Scholar Foundation of Key Laboratory for Optoelectronic Technology & Systems Ministry
of Education of China, and the Fundamental Research Funds for the Central Universities
(No. CDJZR10120006).
Author Contributions
The paper was completed in collaboration between all authors. Hua Yu proposed the research theme
and idea, performed the theoretical analysis of PEH, completed the circuit design, carried out the
laboratory experiments, analyzed the data, interpreted the results and wrote the paper. Jielin Zhou
performed the analysis of PEH, conducted the FEM simulations, measured the characteristics of the
MEMS PEH, and co-wrote the manuscript. Licheng Deng and Zhiyu Wen designed the fabrication
process of the MEMS PEH, co-designed the MEMS PEH testing experiments, and co-worked on
associated data collection and their interpretation. All authors have read and approved the final manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
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