27892069 a MEMS Based Piezoelectric Power Generator Array for Vibration Energy Harvesting

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Microelectronics Journal 39 (2008) 802806

A MEMS-based piezoelectric power generator array for vibration

energy harvesting

Jing-Quan Liua,, Hua-Bin Fanga, Zheng-Yi Xub, Xin-Hui Maob, Xiu-Cheng Shena,Di Chena, Hang Liaob, Bing-Chu Caia

aNational Key Lab of Micro/Nano Fabrication Technology, Key Lab for Thin Film and Micro-fabrication Technology of Ministry of Education,

Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200030, PR ChinabACS Sensors Lab, Honeywell, Shanghai 201203, PR China

Received 13 September 2007; accepted 15 December 2007

Available online 20 February 2008

Abstract

Piezoelectric power generator made by microelectromechanical system (MEMS) technology can scavenge power from low-level

ambient vibration sources. The developed MEMS power generators are featured with fixed/narrow operation frequency and power

output in microwatt level, whereas, the frequency of ambient vibration is floating in some range, and power output is insufficient. In this

paper, a power generator array based on thick-film piezoelectric cantilevers is investigated to improve frequency flexibility and power

output. Piezoelectric cantilevers array has been designed and fabricated. The cantilevers array can be tuned to the frequency and

expanded the excited frequency bandwidth in ambient low frequency vibration. Serial connection among cantilevers of the array is

investigated. The prototype generator has a measured performance of 3.98mW effective electrical power and 3.93 DC output voltage to

resistance load. This device is promising to support networks of ultra-low-power, peer-to-peer, wireless nodes.

r 2007 Elsevier Ltd. All rights reserved.

PACS: 84.60.Bk; 85.85.+j; 85.50.n

Keywords: Energy harvesting; MEMS; Cantilevers array; Vibration frequency; Serial connection; Low frequency

1. Introduction

With the recent advances in wireless and microelec-

tromechanical system (MEMS) technology, sensors have

the ability to be placed almost anywhere [1,2]. Due to the

nature of these wireless sensors, it is urgent that they

contain their own power supply. In most cases this power

supply is the conventional battery. However, the batteryhas a finite lifespan and once extinguished of its power, the

sensor must be retrieved and the battery replaced. With

these sensors being placed in remote location it can become

an expensive task to obtain and replace the battery.

Energy harvesting from ambient vibration by MEMS

technology is one of the promising alternatives. The

vibration can be converted to electric energy using three

types of electromechanical transducers: electromagnetic [1],

electrostatic [1], and piezoelectric [216]. Piezoelectric

vibration-to-electricity converters have high electromecha-

nical coupling, require no external voltage source and are

particularly attractive for use in MEMS especially for

volume-limited wireless sensor node [11]. Glynne-Jones

et al. [9,10] and Shu and Lein [11] provided an approach to

design, model and optimize the conversion efficiency of thevibration generator. Jeon developed a thin film piezo-

electric power generator based on MEMS using a d33

mode. Its resonant operation frequency is at 13.7 kHz [12].

Roundy reported a kind of prototype of tiny, piezoelectric

cantilever (925 mm in length) with a relatively heavy mass

on the free end, which can generate 375 mW from a

vibration source of 2.5 m/s2 at 120Hz. The scale of the

device, however, is larger than that of most MEMS devices

[13]. duToit et al. [14] proposed a prototype for low-

level ambient MEMS harvester. Another MEMS-based

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0026-2692/$ - see front matterr 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.mejo.2007.12.017

Corresponding author.

E-mail address: [email protected] (J.-Q. Liu).
http://www.elsevier.com/locate/mejohttp://dx.doi.org/10.1016/j.mejo.2007.12.017mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.mejo.2007.12.017http://www.elsevier.com/locate/mejo


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piezoelectric power generator designed to harvest low

frequency vibration energy by adding nickel proof mass

based on UV-LIGA technology [15,16].

So far, the developed MEMS power generators are

featured with fixed or narrow operation frequency and

power output in microwatt level, whereas the frequency of

ambient vibration is floating in some range, and microwattis insufficient to power current sensing node. In this paper,

a power generator array based on thick-film piezoelectric

cantilevers is investigated to improve power output and

frequency flexibility. Also, since many sources in the

ambient vibration are in the low frequency (o1000 Hz),

the cantilevers are designed working under low frequency

range.

2. Structure design and prototype fabrication

Cantilever beam configuration is chosen for its simpli-

city, compatibility with MEMS manufacturing processes,

and its low structural stiffness. The beam configuration is a

structure consisting of a silicon base frame, a single

piezoelectric element (layer sandwiched between a pair ofmetal (Pt/Ti) electrodes), and a proof metal mass in free

end, as illustrated in Fig. 1.

The cantilever device operates as follows. When base

frame of the device is vibrated by environmental ground-

work, simultaneous input force feeds into this second-order

mechanical system, then some parts of the cantilever will

move relatively to the base frame. That relative displace-

ment causes the piezoelectric material in the system to be

tensed or compressed, which in turn induces charge shift

and accumulation due to piezoelectric effect. Magnitude of

the electric charge voltage is proportional to the stress

induced by the relative displacement.

It is well known that resonant vibration can amplify the

relative displacement remarkably. Thus, the micro gen-

erators mechanically resonate at a frequency of the

ambient vibration can generate maximum electrical power.

Natural frequency of structure is approximately given as

$ ffiffiffiffiffiffiffiffiffik=m

pby its stiffness (k) and mass (m). This indicates

that varying structure dimension of the cantilever can

regulate the natural frequency of the power generator.

A single cantilever power generator device [10] reveals the

performance of its output voltage under different excited

frequency in Fig. 2. It shows that the output voltage drops

off dramatically when the excited frequency deviates from

resonant point. To locate in range of X1=ffiffiffi2p maximum

output voltage, the available bandwidth is just within

723 Hz. The narrow available bandwidth of the device

extremely restricts its practical application in ambient

vibration.

Under many circumstances, the driving frequency will be

known before the device is designed and fabricated. In

other situations, however, this frequency will not be known

a prior, or it may change over time. It is also relevant to

consider the mass fabrication of such devices. So it would

clearly be advantageous to create a device that can operate

effectively over a range of vibration frequencies.

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Fig. 1. Schematic configuration of single cantilever beam.

Fig. 2. Output voltage as a function of excited frequency.

Fig. 3. Fabrication process of micro power generator array: (1) functional films preparation: SiO2/Ti/Pt/PZT/Ti/Pt, (2) functional films pattern, (3) silicon

slot etching by RIE, (4) back silicon deep etching by KOH solution, (5) cantilever release by RIE, and (6) metal mass micro fabrication and assemblage.

J.-Q. Liu et al. / Microelectronics Journal 39 (2008) 802806 803


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Roundy et al. [13] developed two solutions to tune

resonance frequency of beam. One is to apply an axial

preload by set-screws or other devices that push on the

clamps at either end of the beam, which passively control

the stiffness (k) of the beam to tune its resonance point.

The second active tuning method is to design device with a

wider bandwidth that connects N springmassdampersystems in one long cantilever.

The two solutions are effective to address resonance

frequency issue, but they are not available in MEMS

fabrication due to its relative complicated structure.

The overlapping effect of resonance frequency is

introduced in our design. A MEMS array with multi-

cantilevers is designed with its single cantilever behaving

closer resonance frequency one after another. Each

cantilever is one springmassdamper system with one

degree of freedom. When cantilevers with closer resonance

frequency connected together as an array, the available

bandwidth will cover the range of minimum to maximum

resonance value of the cantilevers in the array. On the other

hand, MEMS fabrication technology ensures the advan-

tage of mass production of cantilevers with various

structure parameters in an array.The techniques of micro fabrication used here mainly

involve functional films preparation and pattern, bulk

silicon micromachining, structure release and mass assem-

blage [15,16]. Firstly, cantilevers with closer resonance

frequency are designed with proper structure parameter.

The center level of resonance frequency is determined by

the target vibration frequency level. And the structure

parameters can be selected through mature modal simula-

tion such as using ANSYS software. Based on the design

for structure parameters selection and cantilevers distribu-

tion in array, mask layout for MEMS fabrication can be

prepared. The detail of fabrication process can refer in

Ref. [16,17]. Although Ref. [15,16] only introduces one

single cantilever, the process of cantilevers-array is similar

with Ref. [15,16]. Fig. 3 illustrates the fabrication process.

A made-up power generator array is shown in Fig. 4. The

size of the cantilevers is of 12 mm silicon layer thickness,

3.2 mm PZT layer thickness, the length and width are in

range of 20003500 and 7501000 mm respectively. And the

natural frequency is in 200400 Hz range.

3. Testing and analysis

The vibration-electricity measurement is executed with

the fabricated power generator standing in a controllablevibration source (the vibration frequency is adjustable and

the vibration acceleration is 0.5 g). The metal pads on top

and bottom electrodes are connected to the load (resistor)

through the down-lead, using oscilloscope to monitor and

record the load voltage signal. Fig. 5 shows the schematic

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Fig. 4. Picture of power generator array prototype.

Fig. 5. Performance testing of piezoelectric power generator: (1) natural frequency and AC output voltage, (2) AC power output delivered to adjustable

resistor, (3) voltage after bridge rectification (4) capacitor charge.

J.-Q. Liu et al. / Microelectronics Journal 39 (2008) 802806804


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configuration of testing methods and measurements for

each single cantilever.

Three cantilevers (C1, C2, and C3) as an array are taken

as measurement sample in the testing and analysis. Therelated information is listed in Table 1. The bandwidth

covers from 226234 Hz, which indicates the cantilevers

array has wider bandwidth than that of single cantilever.

The respective performances are measured, including

natural frequency, output voltage, rectification property

and capacitor charge. Fig. 6 illustrates the performance of

cantilever C1.

As an array, these three cantilevers are electrically

connected. When they are connected directly, the AC

electrical power from different cantilevers can be counter-

acted as they have different phases. Fig. 7 shows the array

excited under frequency of 229 Hz, and the AC output

voltages of each cantilever are 2.01 V (C1), 1.64 V (C2) and

1.606 V (C3), respectively. The AC output voltage after

direct serial connection is about 3.06 V, which is far less

than the value of 2.01+1.64+1.606 5.256 V. As shownin Fig. 7, there exists phase difference of nearly 1201

between C2 and C1. The phase difference impairs the

electrical accumulation of three cantilevers. And the DC

voltage across capacitor after rectification is only 2.51 V,

and the maximum DC power output is about 3.15 mW.

One approach to this problem is provided by the

following. One DC output voltage can be attained by

rectifying an AC output voltage from each cantilever, and

DC output voltage terminals are connected in series to

achieve a higher voltage, just like serial connection of the

batteries. The output voltage from different cantilevers

cannot be counteracted. Fig. 8 shows the serial connection

method.

After ACDC rectification and serial connection of all

cantilevers together, the DC voltage goes up to 3.93 V and

the maximum DC power output is about 3.98mW.

Although the rectification circuit consumes some electrical

energy, it still takes advantage compared with the direct

connection under AC signal.

On the other hand, the cantilevers in the array connected

after ACDC rectification show wider bandwidth than that

of single one. It seems that the overlapping/accumulation

effect is effective to expand bandwidth of power generator.

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Fig. 6. Performance of one single cantilever power generator: (1) AC

output voltage on oscillograph, (2) output voltage after full wave

rectification, (3) voltage on a charging capacitor.

Fig. 7. AC output of three cantilevers in an array and their direct serial

connection.

Fig. 8. Electrical connection after ACDC rectification.

Table 1

Respective performance of the three cantilevers in the array

Cantilever

no.

Cantilever geometry (mmmm)

(different Ni mass)

Natural

frequency (Hz)

AC output

voltage (V)

DC voltage across

capacitor (V)

Maximum power

output (mW)

C1 Length3000mm; width1000mm 229 2.01 1.57 2.55

C2 234 1.86 1.43 2.1

C3 226 1.75 1.22 1.87

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4. Conclusion

In summary, a micro-power generator array has been

investigated by utilizing PZT film as the transducer to

harvest ambient low-level vibration. It is fabricated

successfully by the MEMS process. The prototype gen-

erator has a measured performance of 3.98 mW of effectiveelectrical power and 3.93 DC output voltage. The

experimental results show that the arrayed device is

promising in improving operation bandwidth and power

output of power generator. It is indicated that a potential

in the development of the power generator meets applica-

tions in wireless/embedded sensor networks.

Acknowledgements

This work is supported by Honeywell Company. The

authors also would like to thank Professor Jinrong Cheng

of Shanghai University for PZT film material preparation.

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27892069 a MEMS Based Piezoelectric Power Generator Array for Vibration Energy Harvesting

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