Piezoelectric Vibration Energy Harvester

Visakh.V1, M.R.Baiju2

Dept. of Electronics and CommunicationCollege of Engineering ,Thiruvananthapuram.

visakh.l.vijayan87@gmail.com1,mrbaiju@gmail.com2

Abstract

Vibration energy harvesting can be done efficiently onlywhen the resonant frequency of the energy harvesting de-vice matches with the frequency of the ambient vibrations.We propose a piezoelectric energy harvester in the form ofa cantilever beam attached to an anchor at one end and afree hanging proof mass at the other end . Aluminium Ni-tride(AlN) is used as the piezoelectric material due to itshigh piezoelectric coefficients. The resonant frequency ofour divice is 120 Hz, which is the vibration frequency ofmost machines. Simulation results from Coventor MEMS+and Matlab Simulink show that the device vibrates with anamplitude of 1.5 µm, given an acceleration of 1g at the res-onant frequency.

1. IntroductionWireless Sensor Networks(WSN) are going to become

an indispensable component in our environment for ap-plications like automobile tire pressure sensors, tempera-ture sensors in buildings, monitoring crack formation inaircrafts,blood pressure monitoring in patients etc [7] [5][6].High energy storage density batteries like lithium ionbatteries has been the primary source for powering the WSNnodes [5].But the bulky size of batteries and the need forreplacement or recharging has hindered the extensive ap-plications of WSN in various domains. As a result, theidea of harvesting energy directly from the environment hasattracted immense interest in the recent years [7] [5].Theambient sources of energy in the environment includes so-lar energy, mechanical vibrations, acoustic nose, tempera-ture variations etc. Out of these sources, mechanical vi-brations is one of the most viable and ubiquitous sourceof energy [7].There are mainly three methods of convertingthe mechanical energy from vibrations to electrical energy -Electromagnetic Conversion, Electrostatic Conversion, andPiezoelectric Conversion [7] [5] [8].

Out of the various proposed and tested vibration en-ergy harvesting devices(EHD), piezoelectric converters has

received most attention due to various reasons like high en-ergy density, no separate voltage source is required, no me-chanical stops required etc. [7] [11]. Several piezoelectricEHDs were presented in literature. In [7], the author pro-posed an initial prototype for piezoelectric converter whichproduced a maximum output of about 200µ W.Anotherpiezoelectric EHD with cantilever beam using AlN piezo-electric material has been discussed in [2].This paper dis-cusses a standard manufacturing process flow for fabrica-tion of the device. An ultra wide bandwidth piezoelec-tric EHD is proposed in [1] which uses a doubly anchoredbeam for resonant frequency tuning. A passive resonant fre-quency adaptation capability for a piezoelectric converter isdiscussed in [3]. In [9], a doubly clamped structure has beenproposed which produces a power of 20µ W at 1.2 g accel-eration. This structure also has a frequency tuning propertyof about 3 Hz.

In this paper the design and analysis of a piezoelectricenergy harvester is presented.

2. Principle of Vibration Energy Harvesting2.1. Vibrations

Low level mechanical vibrations are present almosteverywhere like buildings, machines,manufacturing and as-sembly plant environments, vehicles, refrigerators, dryersetc. An extensive study of various commonly occurring lowlevel vibrations was carried out in [7]. Many interesting ob-servations were made in the study. Most of the vibrationenergy from the sources is concentrated at a few discretefrequencies. Also the fundamental vibration frequency foralmost all sources is between 70 and 125 Hz.All the vibra-tion sources can be characterized by the acceleration mag-nitude and frequency of the fundamental vibration mode.

2.2. Vibration to Electricity Conversion Model

A generic vibration to electricity conversion model hasbeen proposed by Williams and Yates in [10]. This modelis shown in fig.1

The model is described by the equation 1

Figure 1: Generic Vibration Converter

mz + (be + bm)z + kz = −my (1)

wherez is the spring deflection, y is the input displacement, m

is the mass, be is electrically induced damping coefficient,bm mechanical damping coefficient,and k is the spring con-stant.

The power converted to electrical system is given by

|P | =mζeωnω

2(ω

ωn)3Y 2

(2ζTω

ω n) + (1− (

ω

ωn)2)2

(2)

whereζe is the electrical damping ratio, ζm is the mechanical

damping ratio, ζT is combined damping ratio (ζe+ ζm), ωn

natural frequency of the mass spring system , ω frequencyof the driving vibrations,and Y input displacement.

The power output when the resonant frequency of themass spring system matches with the input vibration fre-quency is given by

|P | = mζeω3Y 2

4ζ2T

(3)

or

|P | = mζeA2

4ωζ2T

(4)

If the acceleration magnitude of the vibration remainsconstant, the output power is inversely proportional to thefrequency. So the converter should be designed to resonateat the lowest fundamental frequency. Also the power outputincreases with proof mass. The mechanical damping shouldbe as low as possible and the power is optimized for ζe equalto ζm.

2.3. Transduction Mechanisms

There are mainly three types of tranduction mech-anism for vibration energy harvesters- Electromagnetic ,Electrostatic and Piezoelectric transduction Mechanisms[7] [11].

In electromagnetic generators, the electrical damper ofthe microgenerator is implemented by using the principle ofFaraday’s law electromagnetic induction.A coil is allowedto move in a magnetic field.The change of magnetic fluxlinkage in the coil produces an voltage across the coil, driv-ing a current in the circuit The force produced by the mov-ing charges in the magnetic field opposes the relative motionbetween the coil and the magnet.The work done against thisopposing force is converted into electrical energy.

There are some practical difficulties in implementingelectromagnetic microgenerator in the MEMS scale. Thenumber of coils that can be manufactured is limited and thisresults in low output voltages which cannot be rectified us-ing diode rectifiers. Also the integration of permanent mag-nets and ferromagnetc materials in MEMS scale is difficult.

In electrostatic microgenerators, mechanical forcewill do work against the attraction of oppositely chargedplate of a capacitor.The voltage across a capacitor is givenby

V =Q

C

(5)

where Q is the charge, C is the capacitance which isgiven by

C =ε0lw

d

(6)

If the charge is held constant, the voltage can be in-creased by reducing the capacitance.If the voltage is heldconstant, the charge can be increased by increasing the ca-pacitance.The capacitance is varied by varying either ’l’ or’d’. In either case, the energy stored on the capacitor, whichis given by the following equation increases.

E =1

2QV =

1

2CV 2 =

Q2

2C

(7)

An excellent description of the various modes of opera-tion of electrostatic converters is given in [4].

The primary disadvantage of electrostatic convert-ers is that they require a separate voltage source to initi-ate the conversion process because the capacitor must becharged up to an initial voltage for the conversion process tostart.They also require mechanical stops which causes relia-bility problems and also increases the mechanical damping.

3. Piezoelectric Energy HarvestingIn piezoelectric converters, the property of piezo elec-

tricity is used as the conversion mechanism.ie when a me-chanical stress is produced in a piezoelectric material, anelectric field is induced across the material and vice versa.This effect is due to the spontaneous separation of chargewithin certain crystal structures under the right conditionsproducing an electric dipole.The constitutive equations fora piezoelectric material are given in equations 8 and 9 .

δ =σ

Y+ dE (8)

D = εE + dσ (9)

where δ is mechanical strain, σ is mechanical stress,Y is the modulus of elasticity (Young’s Modulus), d isthe piezoelectric strain coefficient, E is the electric field, Dis the electrical displacement (charge density),and ε is thedielectric constant of the piezoelectric material.

A circuit representation of a piezoelectric elementis given in [7]. The source voltage is simply defined as theopen circuit voltage resulting from equation 9. (The opencircuit condition means that the electrical displacement (D)

Figure 2: Circuit representation of piezoelectric element

is zero.) The expression for the open circuit voltage is givenby equation 10 .

VOC =−dtεσ (10)

If the piezoelectric material undergoes a periodic orsinusoidal stress due to external vibrations, an AC opencircuit voltage defined by equation 10 can be measuredacross the material.

3.0.1 Modes of Operation

There are two modes in which a piezoelectric materialis generally used. They are

• 33 mode

• 31 mode

Figure 3: (33 mode)

Typically, piezoelectric material is used in the 33 mode,meaning that both the voltage and stress act in the 3 direc-tion. However, the material can also be operated in the 31mode, meaning that the voltage acts in the 3 direction (i.e.

Figure 4: (31 mode)

the material is poled in the 3 direction), and the mechanicalstress / strain acts in the 1 direction. Operation in 31 modeleads to the use of thin bending elements in which a largestrain in the 1 direction is developed due to bending. Themost common type of 31 elements are bimorphs, in whichtwo separate sheets are bonded together, sometimes with acenter shim in between them. As the element bends, thetop layer of the element is in tension and bottom layer is incompression or vice versa.Therefore, if each layer is poledin the same direction and electrodes are wired properly, thecurrent produced by each layer will add. This is termedas Parallel polling.Conversely, if the layers are poled inopposite directions, the voltages add. This is termed seriespoling.

Although the electrical/mechanical coupling for 31mode is lower than for 33 mode, there is a key advantage tooperating in 31 mode. The system is much more compliant,therefore larger strains can be produced with smaller inputforces. Also, the resonant frequency is much lower. Animmense mass would be required in order to design apiezoelectric converter operating in 33 mode with a reso-nant frequency somewhere around 120 Hz. Therefore, theuse of bending elements operating in 31 mode is essential.

A bending element could be mounted in many ways toproduce a generator. A cantilever beam configuration witha mass placed on the free end has been chosen for two rea-sons. First, the cantilever mounting results in the loweststiffness for a given size, and even with the use of bendingelements it is difficult to design for operation at about 120Hz in less than 1 cm3. Second, for a given force input, thecantilever configuration results in the highest average strainfor a given force input.

Piezoelectric converters have certain advan-tages.Voltages in the range of two to several volts andcurrents on the order of tens to hundreds of microAmps areeasily obtainable.A second advantage is that no separate

voltage source is needed to initiate the conversion process.Additionally, there is generally no need for mechanicallimit stops. Therefore, these devices can be designed toexhibit very little mechanical damping.

4. Proposed Energy Harvesting StructureIn this paper, a piezoelectric microgenerator structure is

presented.

Figure 5: Proposed Structure

Most of the commonly occurring machine vibrations isaround 120 Hz [7].So we designed our structure to have aresonant frequency of 120 Hz , vibrating in the vertical di-rection in first mode. The structure has dimensions 1200µmlength, 500µm width and 400µm height.

The structure has an anchor part made of silicon onwhich the piezolayer and the electrical contacts are de-posited. The proof mass is at the end of the cantilever beamwhich is also made of silicon.The piezoelectric materialused is Aluminium Nitride(AlN), which has piezoelectriccoefficients of e33=1.55 cm−2 and e31=-0.58 cm−2. Theupper and lower electrode contacts are made of Aluminium.

5. Manufacturing Process FlowThe first step of the process flow is the deposition of

silicon with a thickness of 490 µm. This layer is then etchedwith a mask to define the anchor and the proof mass. Thena sacrificial layer is added above which the beam part is cre-ated. After this the lower electrode is deposited and etched.This is followed by Aluminium Nitride layer and then theAluminium upper electrode is deposited.

The floor plan of the device is as shown in fig 6.

Figure 6: Floor plan of the device

6. Simulation and Results

The structure is modeled in Coventor MEMS+ 2.0.The manufacturing process file is created in Process Edi-tor and structure is designed in Innovator. then the model isexported to Matlab Simulink for analysis.

A modal analysis is done in Simulink and the resonantfrequency of the device is found to be 120 Hz. Giving anacceleration of 1g, in the vertical direction at its resonantfrequency ,the displacement of the beam is found out to beabout 1.5 µm. The result is shown in the figure 7.

Figure 7: Displacement of the structure at the resonant fre-quency

7. ConclusionMost of the commonly occurring vibration sources

produce vibrations at very low frequencies, often lessthan 200 Hz. So microgenerators need to be designed tovibrate at such low frequencies.It is a challenge for thedesigners to design a micro device to vibrate at such lowfrequencies. In this paper , a piezoelectric microgeneratoris proposed for harvesting energy from ambient vibrations.Our device vibrates at a very low resonant freuency of 120Hz. Many commonly used electrical machines vibrate atthat frequency. So our structure finds application in suchenvironments.

The structure is designed in the MEMS designing soft-ware Coventor MEMS+ and the analysis of the structure isdone with Matlab Simulink.At its resonant frequency , thebeam displaces upto 1.5 µm at an input acceleration ampli-tude of 1g, which produces sufficient strain in the piezoelec-tric material.

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