P0019

© 2012, IJARCSSE All Rights Reserved Page | 20

Volume 2, Issue 10, October 2012 ISSN: 2277 128X

International Journal of Advanced Research in Computer Science and Software Engineering Research Paper Available online at: www.ijarcsse.com

Special Issue: Recent Trend in Computing Conference Held in SRM University, NCR Campus, India

Modeling and Simulation of Vibration Energy Harvesting of

MEMS Device Based on Epitaxial Piezoelectric Thin Film Deepak Poria

1, Monika

2, Rajeev Sharma

3, Deepak Rohilla

4, Dr. Manoj kumar Pandey

5

1M.TECH, ECE, Vaish College of Engineering, Rohtak, Haryana

Contact no.9813429899, 2Assistant Professor, PDM College of Engineering for Women, Bahadurgarh, Haryana

Contact no.9813467857, 3Associate Professor, Vaish College of Engineering, Rohtak, Haryana

Contact no.9896013226, 4Assistant Professor, PDM College of Engineering for Women, Bahadurgarh, Haryana

Contact no.9467696406, 5Professor, SRM University, Modi nagar, Ghaziabad

Abstract: Low power wireless devices have been increasing in applications of industrial automation monitoring with the

limitation of manual battery replacement. This can be overcome by a miniaturized device that can convert natural

mechanical energies to power wireless devices. During the last decade piezoelectric cantilever energy harvesters have been

increasingly investigated for this application. In this paper unimorphs has been designed in COMSOL Multiphysics 3.5

in 3D view to decrease operating frequency and improve output power. Unimorphs are designed with two different non-

piezoelectric materials as Aluminium and Gold. Eigen frequency analysis has been performed to obtain resonant

frequency and generated voltage from the unimorphs with different piezoelectric epitaxial thin film. Unimorphs of

dimension 100mm×30mm×4mm has been modelled with 2mm thin film epitaxial layer of piezoelectric material. From the

simulation results Gold is preferred over Aluminium as about 100Hz less frequency response is observed. A Unimorph

with gold and PZT-5A material is considered the best model with resonance frequency of about 246Hz with generated

electric voltage of 2.51 volts when a load of 20 N/m2 is applied at the tip of unimorph.

Key words: MEMS, Unimorph, Piezoelectric materials, Cantilever, Energy harvesting.

1. Introduction

MEMS is basically a combination of electronics and mechanics. The micro sensors gather information by measuring

mechanical, thermal, biological, chemical, magnetic and optical signals from the environment. The microelectronic ICs act as

the decision-making piece of the system, by processing the information given by the sensors. Finally, the actuators help the

system respond by moving, pumping, filtering or somehow controlling the surrounding environment to achieve its purpose

[1]. Basic concept of MEMS and other technologies is shown in Fig.1.

Fig.1: Basic concept of MEMS.

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Deepak et al., International Journal of Advanced Research in Computer Science and Software Engineering 2 (10),

October- 2012, pp. 20-25


2. Device Configuration

The vast majority of piezoelectric energy harvesting devices uses a cantilever beam structure. A cantilever beam, by

definition, is a beam with a support only one end, and is often referred to as a “fixed-free” beam. When the generator is

subjected to vibrations in the vertical direction, the support structure will move up and down in sync with the external

acceleration. The vibration of the beam is induced by its own inertia, since the beam is not perfectly rigid, it tends to deflect

when the base support is moving up and down (see Fig.2). Typically, a proof mass is added to the free end of the beam to

increase that deflection amount. This lowers the resonant frequency of the beam and increases the deflection of the beam as it

vibrates. The larger deflection leads to more stress, strain, and consequently a higher output voltage and power [2].Electrodes

covering a portion of the cantilever beam are used to conduct the electric charges produced to an electrical circuit, where they

can be utilized to charge a capacitor or drive a load.

Fig.2: Strain is generated along the length of the beam, hence the use of the 3-1 mode [3].

Lowers the resonant frequency of the beam and increases the deflection of the beam as it vibrates. The larger deflection leads

to more stress, strain, and consequently a higher output voltage and power [4].

2.1. Cantilever

Cantilevered beams are the most ubiquitous structures in the field of microelectromechanical systems (MEMS). MEMS

cantilevers are also finding application as radio frequency filters and resonators. Two equations are key to understanding the

behavior of MEMS cantilevers. The first is Stoney's formula, which relates cantilever end deflection δ to applied stress σ:

𝜹 =𝟑𝝈 𝟏 − 𝒗

𝑬(𝑳

𝒕)𝟐 (1)

where ν is Poisson's ratio, E is Young's modulus, L is the beam length and t is the cantilever thickness. Very sensitive optical

and capacitive methods have been developed to measure changes in the static deflection of cantilever beams used in dc-

coupled sensors.

The second is the formula relating the cantilever spring constant k to the cantilever dimensions and material constants:

𝒌 =𝑭

𝜹=

𝑬𝝎𝒕

𝟒𝑳𝟑 (2)

where F is force and w is the cantilever width. The spring constant is related to the cantilever resonance frequency ω0 by the

usual harmonic oscillator formula

𝝎𝟎 = 𝑘

𝑚 (3)

A change in the force applied to a cantilever can shift the resonance frequency.

Different electrode lengths or shapes are frequently used to affect the output voltage, since strain is not uniform across the

beam [5].

2.2 Modes of Vibration and Resonance: A cantilever beam can have many different modes of vibration, each with a different resonant frequency. The first mode of

vibration has the lowest resonant frequency, and typically provides the most deflection and therefore electrical energy. A

lower resonant frequency is desirable, since it is closer infrequency to physical vibration sources and generally more power is

produced at lower frequencies [6]. Therefore, energy harvesters are generally designed to operate in the first resonant mode.

Each mode of vibration has a characteristic mode shape.

2.3 The Piezoelectric Effect The piezoelectric effect, in essence, is the separation of charge within a material as a result of an applied strain. This charge

separation effectively creates an electric field within the material and is known as the direct piezoelectric effect. The converse


October- 2012, pp. 20-25


piezoelectric effect is the same process in reverse: formation of stresses and strains in a material as a result of an applied

electric field.

2.4 Piezoelectric Materials

A majority of piezoelectric generators that have been fabricated and tested use some variation of lead zirconate titanate

(PZT). Typically, PZT is used for piezoelectric energy harvesters because of its large piezoelectric coefficient and dielectric

constant, allowing it to produce more power for a given input acceleration [7]. Another less common material is aluminum

nitride (AlN).Though it has a smaller piezoelectric coefficient and dielectric constant, aluminum nitride has advantages in

material deposition and in compatibility with the standard CMOS processes used for fabrication of integrated circuits [8].

2.5 Piezoelectric MEMS based on epitaxial PZT thin films:

Epitaxial PZT thin films grown on silicon substrates are receiving a lot of interest for many applications especially their

excellent properties are promising for the realization of high performance MEMS devices. The integration of high-quality

epitaxial PZT thin films on silicon substrates can be achieved through a buffer layer based on epitaxial oxide thin films.

Results and Discussion

Device Parameters used in all analysis:

A Bimorph has been designed with dimensions 100mm × 30mm × 4mm in COMSOL Multiphysics 3.5. Then providing it

boundary conditions as one side fixed and other side free constraints. A force of 20N/m2 along Y direction is made on the top

side. Table 1 shows the results of analysis 1 to analysis 7.

Table 2: Results of analysis 1 to analysis 7.

Base Material Piezoelectric

Material

Resonant

Frequency(Hz)

Potential

(volts)

Total Deflection

(m)

Aluminium Aluminium nitride 732.211 2.489 2.174E-10

Aluminium Tellurium Dioxide 246.423 3.896 6.127E-7

Aluminium Lithium Niobate 521.56 2.55 1.699E-9

Aluminium Gallium Arsenide 405.68 2.509 3.122E-10

Aluminium PZT-5 345.74 2.51 1.831E-9

Aluminium PZT-5A 339.272 2.51 3.415E-10

Aluminium PZT-5H 346 2.509 2.567E-10

From above seven analysis the Beam material is kept same as Aluminium with different piezoelectric layers on it. Fig.13

shows the variation of resonant frequencies obtained from eigen frequency analysis of unimorph for different Piezoelectric

materials.

Fig.13: Resonant Frequency of unimorph for different Piezoelectric materials.

Analysis Type 8:

This analysis has been done with three different piezoelectric materials PZT-5, PZT-5A and PZT-5H with Aluminium as base

material for obtaining modal analysis from these three models with same dimensions as 100mm × 30mm × 4mm. Modal

analysis is done for five modes in COMSOL. Table 2 shows modal frequencies for the three materials in all five modes.

0

200

400

600

800

mo

dal

fre

qu

en

cy (

Hz)

AiN TeO2 LiNi AlGaAs PZT-5 PZT-5A PZT-5H

material

frequency


October- 2012, pp. 20-25


Table2: Modal frequencies for PZT-5, PZT-5A and PZT-5H.

A graphical illustration is also shown in Fig.14 that shows lowest frequency response is obtained with PZT-5A i.e. 339 Hertz

and highest with PZT-5H i.e. 346 Hertz in the first mode.

Fig.14: Modal analysis for PZT-5, PZT-5A, PZT-5H for Al as base material.

Analysis Type 9:

Now designing a unimorph by replacing the base material Aluminium with Gold and using PZT-5, PZT-5A and PZT-5H as

piezoelectric materials a electric potential of 2.51V, 2.51V and 2.48V have been generated respectively keeping earlier

dimensions of unimorph same as 100mm × 30mm × 4mm. The solved unimorph showing modal frequency with

displacement obtained and electric voltage generated is shown in Fig.15. Fig.16 shows the modal analysis done with these

three models to obtain the finest result with a beam that can respond to very small fluctuation. From the first mode of PZT-5,

PZT-5A, and PZT-5H resonant frequencies of 249 Hz, 246 Hz and 250 Hz are obtained respectively.

Fig.15: Subdomain plot of unimorph showing displacement Fig.16: Modal analysis for PZT-5, PZT-5A, PZT-5 in

Y direction and Electric voltage. H for Au as base material.

Analysis Type 10: This analysis has been done by comparing different PZT materials on piezoelectric film with Aluminium and Gold as non-

piezoelectric layer. It is clear that the unimorph responds to lower resonant frequencies with gold as non piezoelectric

material as compare to piezoelectric material.

Modes\

Materials PZT-5 PZT-5A PZT-5H

1 345 Hz 339 Hz 346 Hz

2 1578 Hz 1535 Hz 1574 Hz

3 2063 Hz 2001 Hz 2052 Hz

4 2129 Hz 2090 Hz 2136 Hz

5 5850 Hz 5752 Hz 5882 Hz


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Fig.17: Comparison of PZT materials resonant frequencies with Al and Au as non piezo materials.

Analysis Type 11:

This analysis comprising of a bimorph designed with dimensions 100mm × 30mm × 6mm with both top and bottom layers of

piezoelectric materials PZT-5A with thickness of 2mm. The solution obtained from this bimorph with gold as non-

piezoelectric material is shown in: Fig.18.

Fig.18: Bimorph with Au as base material and PZT-5A as top and bottom layers.

A load of 20 N/m2 is applied on the tip of

bimorph then its displacement is observed in Y direction as 9.116E-9m. The

electric voltage generated from this bimorph is observed as 1.836 Volts.These analysis were done with the bimorph and

unimorphs with different MEMS materials.

3. Conclusion

This paper has presented the study of using piezoelectric cantilevers to effectively convert ambient vibrations and flows into

electricity. Such devices provide affordable, sustainable and maintenance-free power solutions for low power wireless and

portable devices. Upon review of published literature in this field, it was found the current design of the piezoelectric

cantilevers needs to be modified with an effective way of converting mechanical energy into electricity, from piezoelectric

cantilevers. From Analysis 1 to 7 it can be concluded that for designing a unimorph with low resonant frequency the

preference should be given to Tellurium Dioxide, Aluminium Gallium Arsenide, PZT materials respectively. Analysis 8 to 10

shows that the unimorph responds to lower resonant frequencies with gold as non piezoelectric material as compare to

piezoelectric material. As gold is much costly than Aluminium therefore when cost doesn’t matters much with the results

Aluminium can be preferred over gold.

4. Future Work

As shown by the results in this dissertation, the performance of the piezoelectric cantilevers can be largely improved by using

engineered extensions. Thus, it is worth further investigation of the effect of these extensions. The following future work may

be pursued :

(a) The beam extension was formed by using fixed lengths for the piezoelectric layer and nonpiezoelectric layer. Thus, the

extension beam had the same width and thickness as the mother beam. In fact, these parameters can also affect the

properties of the PUC, such as the resonant frequency and strain distribution. Thus, a further investigation of the

geometry of the extension beam is worth pursuing.

(b) The unimorph piezoelectric cantilevers were investigated. Bimorph piezoelectric cantilever is another commonly used

piezoelectric energy harvester. It is also of interest to investigate the effect of beam extensions on the performance of

0

100

200

300

400

PZT-5 PZT-5A PZT-5H

Al

Au


October- 2012, pp. 20-25


the bimorph cantilevers both experimentally and theoretically. The analytical model for the bimorph cantilever can be

obtained by including a second piezoelectric layer in the model of the unimorph cantilevers in this work.

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