1

Chapter 1

Introduction

During last few decades, booming of wireless sensor network (WSN) require a

reliable power source. Generally the power sources to these microsystems are the

conventional battery. However, the battery has a finite lifespan and once extinguished of its

power, these sensors must be retrieved and the battery replaced [1]. With these sensors being

placed in remote location it can become an expensive task to obtain and replace the battery.

Therefore, it becomes necessary to provide them source, which can provide them energy

reliably.

Technological developments in the MEMS industry have lead to miniaturization of

many of the transducer systems. With this effort, the power consumption of these devices has

been reduced to the order of W to mW level. These developments have opened a new source

for supplying energy to these micro systems as an alternative to batteries, which have a finite

life and are large in size. Researchers are working on alternative energy sources like solar,

thermal, acoustics, and vibration. These sources are clean and have theoretically infinite life

compared to batteries. Considering implantable and embedded microsystems that should

operate and survive on their initial energy supply, these ambient energy sources are attractive

alternatives. Among these alternative sources, environmental vibration is particularly

attractive because it is almost everywhere in our living environment and can be readily found

in the environment in abundance. Through these transducers ambient vibrational energy can

be efficiently converted into electrical energy.

In this report vibration based scavenging technique like piezoelectric and

electromagnetic conversions are explained and how the ambient vibrational energy can be

used to charge these microsystems through piezoelectric and electromagnetic transducer.

Inspite of development of MEMS industry, there are some of technical limitation of

vibration energy harvesting systems which are to be mitigated. Among these limitations, the

ways to remove narrow bandwidth frequency limitation are discussed.

2

1.1 Motivation for Energy Harvesting

Booming of wireless sensor network (WSN) and micro electro mechanical

system (MEMS) technology basing on development of low power device.

Power requirements must be scaled down, for size of <1cm3 the power

consumption goal is below 100 μW.

Wireless sensors require an efficient self-sustainable powering source because

batteries must be recharged/replaced and eventually disposed.

Long lasting operability – If on the basis of development of MEMS, ambient

energy based transducer are installed in microsystem, then there will not be any

need to replace battery because ambient source will provide them energy for

infinite life [2].

No chemical disposal – No use of battery will ensure no chemical disposal.

Cost saving - Battery has a finite lifespan and once extinguished of its power,

these sensors 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.

Maintenance free – Once the MEMS based transducers are installed then there is

no use of maintenance for lifetime.

No charging points – Unlike batteries there is no need of charging.

Sites operability – Since with these transducers,there is no of replacing battery

therefore they can be easily placed at remote sites..

Flexibility

90% of WSNs cannot be enabled without Energy Harvesting technologies (solar, thermal,

vibrations). Among these alternative sources, environmental vibration is particularly

attractive because of its abundance and can efficiently provide energy for micropowering.

3

Chapter 2

Principle to convert vibration into electricity

In a simplified approach, the structure consists of rigid mass M bonded on a spring K

corresponding to the stiffness of the mechanical structure, on a damper D corresponding to

the mechanical losses of the structure, and transducer to convert vibration energy into

electrical energy.[12]

Fig.2.1 Principle of energy harvesting[9]

2.1 Piezoelectric Conversion

2.1.1 Piezoelectric materials

Man-made ceramics

• Barium titanate (BaTiO3)—Barium titanate was the first piezoelectric ceramic discovered.

• Lead titanate (PbTiO3)

• Lead zirconate titanate (Pb[ZrxTi1−x]O3 0≤x≤1)—more commonly known as PZT, lead

zirconate titanate is the most common piezoelectric ceramic in use today.

• Lithium niobate (LiNbO3)

Naturally-occurring crystals

• Berlinite (AlPO4), a rare phosphate mineral that is structurally identical to quartz

• Cane sugar

• Quartz

• Rochelle salt

4

Polymers

• Polyvinylidene fluoride (PVDF): exhibits piezoelectricity several times greater than quartz.

Unlike ceramics, long-chain molecules attract and repel each other when an electric field is

applied

2.1.2 Piezoelectric generator principle

The piezoelectric effect exists in two domains, the first is the direct piezoelectric

effect that describes the material’s ability to transform mechanical strain into electrical

charge, the second form is the converse effect.

Piezoelectric materials belong to a larger class of materials called ferroelectrics. One

of the defining traits of a ferroelectric material is that the molecular structure is oriented such

that the material exhibits a local charge separation, known as an electric dipole. Throughout

the artificial piezoelectric material composition the electric dipoles are orientated randomly,

but when a very strong electric field is applied, the electric dipoles reorient themselves

relative to the electric field; this process is termed poling. Once the electric field is

extinguished, the dipoles maintain their orientation and the material is then said to be poled.

After the poling process is completed, the material will exhibit the piezoelectric effect.[3]

The mechanical and electrical behaviour of a piezoelectric material can be modelled

by two linearized constitutive equations. The direct effect and the converse effect may be

modelled by the following matrix equations:

Direct Piezoelectric Effect: D = d . T + εT . E (1)

Converse Piezoelectric Effect: S = sE . T + dt . E (2)

Where D is the electric displacement vector, T is the stress vector, εT is the dielectric

permittivity matrix at constant mechanical stress, sE is the matrix of compliance coefficients

at constant electric field strength, S is the strain vector, d is the piezoelectric constant matrix,

and E is the electric field vector. The subscript t stands for transposition of a matrix.

The piezoelectric material can be generalized for two cases.

The first is the stack configuration that operates in the 33 mode and the second is the

bender, which operates in the 13 mode. the material is strained in the "1" direction or

perpendicular to the poling direction. These two modes of operation are particularly

important when defining the electromechanical coupling coefficient such as d. Thus d13

5

refers to the sensing coefficient for a bending element poled in the "3" direction and strained

along "1"[3].

Fig.2.2 Piezoelectric bulk (33 mode)[10] Fig.2.3 Cantilever beam (31 mode)[10]

2.2 Electrimagnetic Conversion

2.2.1 Basic principle

The Faraday’s law states that

𝜀 = −𝑑∅𝑩

𝑑𝑡

for a coil moving through a perpendicular constant magnetic field, the maximum open circuit

voltage across the coil is

𝑉𝑂𝐶 = 𝑁𝐵𝑙𝑑𝑥

𝑑𝑡

N is the number of turns in the coil, B is the strength of the magnetic field, l is length of a

winding and x is the relative displacement distance between the coil and magnet.[10]

6

Fig.2.4 Electromagnetic generator[10]

2.3 Transduction Technique Comparison

Type

Advantage

Disadvantage

Electromagnetic • No need of smart material

• No concern fror

brittleness of material

• Bulky size magnaets and pick-up coil

• Difficult to integrat with MEMS

• Max voltage of 0.1V

Piezoelectric • High voltage of 2 to 10 V

• Compact configuration

• Compatible with MEMS

• High coupling in single

crystal

• Depolarization

• Brittleness in PZT

• Poor coupling in piezo-film(PVDF)

• Charge leakage

• High output impedence

7

Chapter 3

Prior Work and Future Scope

3.1 Energy Harvesting From Human Motion

The piezoelectric effect can be implemented to harvest mechanical energy from

walking. This energy can be converted into useful electrical energy that can be used to

power wearable electronic devices such as sensors and Global Positioning System (GPS)

receivers. Piezoelectric energy harvesting can also be used to power some consumer

electronic devices directly such as cellular phones, two-way communicators and

pagers[4].

Fig.3.1 Heel strike generator[4]

Description of heel strike system

The Heel Strike System consists of two major pieces – the Heel Strike Generator and

the power electronics circuit. The Heel Strike Generator has a mass of 0.455 kg and has

approximate dimensions of 8.89 cm (L) by 7.94 cm (W) by 4.29 cm (H). The principle

components of the Heel Strike Generator are four PZT-5A bimorph crystal stacks, lead

screw, bearing and rotary cam. The power electronics circuit is 5.2 cm2 with a height of 1.7

cm and has a mass of 10 g.[4] Its purpose is to convert unusable power from the Heel Strike

Generator to useable power. The power electronics circuit is connected to the Heel Strike

Generator to form the Heel Strike System. The power electronics circuit is designed to

accumulate infrequent pulses of power from the piezoelectric stacks (four phases), rectify

them, store them in a capacitor and convert that stored energy to a 12 VDC output when a

fixed voltage level has been reached on a storage capacitor. This circuit will store very small

8

energy pulses over a relatively long time, in a low-leakage storage capacitor and then

periodically discharge that capacitor into a load.

Principle of operation

The Heel Strike Generator uses Lead Zirconate Titanate (PZT-5A) piezoelectric

materials to transform mechanical energy into electrical energy. The input mechanical energy

is transformed into electrical energy through four PZT-5A bimorph stacks. Hence the Heel

Strike Generator has four phases of electrical energy generation. The Heel Strike System uses

a power electronics circuit to extract, store and regulate the electrical energy output from the

four phases and converts it into a 12 VDC pulse. When a user steps down and compresses the

Heel Strike Generator, a lead screw and gear train convert the linear motion into the rotation

of a cam, where the rotating cam causes the PZT-5A bimorph stacks to deflect sinusoidally.

The stacks are arranged in such a way that they oscillate 90 deg out of phase with one

another, recycling most of the elastic energy stored in the bimorph crystal stacks. Each

sinusoidally oscillating PZT-5A bimorph crystal stack produces an oscillating voltage that is

rectified and regulated by a power electronics circuit that is separate from and connected to

the Heel Strike Generator. The power electronics circuit takes in the AC voltage signals from

each phase of the Heel Strike Generator rectifies them and produces DC pulses that charge a

storage capacitor. Any stored charge in the capacitor is then discharged through a DC–DC

converter, which converts that stored energy into a regulated 12 VDC output pulse.[4]

Proposed Output and result

The goal of this research effort was to generate 0.5W of power at a 1 Hz step rate

since many electronic devices such as GPS receivers and communicators require power

within this range to operate. On average the Heel Strike System produced 0.0903 W of power

per compression. The average power produced by the Heel Strike System is much less than

0.5 W. It was found in the later stage of development that the mechanical forces resulting

from the oscillation of the bimorph crystal stacks were not completely cancelled, and as a

result an opposing toque from the unbalanced bimorph forces was applied to the cam. This

leads to a force opposing the input to the Heel Strike Generator so as the user steps down,

there is some resistance and not all of the downward force would be used to oscillate the

bimorph stacks. This results in lower mechanical to electric efficiency. There are two primary

causes for the bimorph forces not completely cancelling. One is the stiffness variations in the

9

bimorph stack assemblies and the other is due to the location of the bimorph crystal stack

assemblies relative to each other.

Future of research

Two methods of improvement would be to use bimorph materials with lower stiffness

and to maintain the uniformity of the stiffness across all four bimorph crystal stacks.

Reducing the bimorph stiffness can have a large impact in providing higher output power

since an additional gear system can be introduced that can increase the number of bimorph

blade deflections per strike and increase the Heel Strike System output power.[13] A lesser

bimorph stiffness would also result in less resistance as one presses down on the Heel Strike

Generator. A more uniform stiffness across all four bimorph stacks will result in more

cancellation of the bimorph forces leading to increased bimorph blade deflections per strike

and thus increased mechanical to electric efficiency and DC power output.

3.2 Energy harvesting from induced flow

The electromagnetic energy harvester for harnessing energy from flow induced

vibration is developed. It converts flow energy into electrical energy by fluid flow and

electromagnetic induction[5].

Fig.3.2 Component of energy harvester[5] Fig.3.3 Operation of energy harvester[5]

10

Description

It consists of a flow channel with two copper tubes, a PE diaphragm bonded to the

channel, and a permanent magnet glued to the PE diaphragm. The permanent magnet is

surrounded by a conducting coil which is guided around an inner housing. The inner housing

of the coil is fixed by an outer housing. The liquid pressure in the chamber drives the PE

diaphragm with the attached permanent magnet into vibration. It consists of an axisymmetric

slice of the diaphragm with a radius of 20mm and a thickness of 100 mm, and the magnet

with a radius of 5mm and a thickness of 10mm. The displacement in the r direction at r¼0

along the line z¼0 is constrained to represent the symmetry condition. The displacements in

the r and z directions at r¼20 mm clamped boundary condition. A pressure, p, from r¼0

to20 mm is applied in the +z direction.[5]

Principle of operation

This harvesting of flow energy via a flow-induced vibration is related to the response

of a flexible diaphragm to an internal flow. The flow is bounded by the flexible structure and

rigid walls. If the diaphragm has small inertia and is flexible enough to be able to respond

rapidly to the fluctuating pressure field set up by the flow, one may expect that the diaphragm

may oscillate with a frequency similar to that observed in the flow. When the fluctuating

pressure is applied on the surface of the diaphragm, the diaphragm oscillates up and down,

which causes the permanent magnet to vibrate at a frequency about the same as that of the

pressure in the pressure chamber. The relative movement of the magnet to the coil results in a

varying amount of magnetic flux cutting through the coil. According to the Faraday’s law of

induction, a voltage is induced in the loop of the coil. For convenience of analysis, finite

element models are developed to estimate the pressure in the pressure chamber, the deflection

of the PE diaphragm and the voltage generated in the coil.

Proposed output

The measurements conducted in various pressure differences in the pressure chamber

of the device show that the maximum output voltage is approximately 11mVpp, when the

excitation pressure oscillates with an amplitude of 254 Pa and a frequency of about 30Hz.[5]

11

3.3 Energy harvesting from wind

A low-speed wind energy harvesting system that transfers aerodynamically induced

flutter energy into electrical energy. A random airflow generates mechanical vibrations due to

the fluid structure interaction between a flexible belt and the airflow. An electromagnetic

resonator with copper coils and a permanent magnet is designed to efficiently harvest

electrical energy from the induced mechanical vibrations[6].

Description

The device consists of three parts:

(a) A wind-belt specifically designed to convert the wind flows into a periodic mechanical

vibration. (b) An electromagnetic resonant device which has two coils fixed to a support

housing and a permanent magnet inside a movable bolt (i.e., acting as a piston).

(c) A power management circuit which can store generated electrical energy into a super

capacitor and provide an appropriate output voltage level to support commercial electronic

devices.

Fig.3.4 Energy flow of the wind-driven flutter transducer[6]

Principle of operation

In the experiment, a thin polymer belt (width = 25 mm, thickness = 0.2 mm, and

length = 1m) is used to interact with the oncoming airflow. The electromagnetic resonator is

placed near the end of the belt due to the larger bending stiffness of the belt near the fixed

ends. This is to allow a larger magnet mass to be moved by the fluttering belt. Although there

is smaller vibration amplitude at the end-part than the central-part of the belt, the larger

bending stiffness enables a larger “actuation” force at the end-part, which allows the belt

to drive a larger mass of magnet, and hence provide more electrical power output and a

bigger magnet mass will give a larger flux density, and hence larger power output. For

convenience in adjusting the belt length and tension, the belt and the electromagnetic

resonator are fixed to the adjustable support beam. The bolt of the resonator is adhered to the

12

belt directly, so when the belt is driven to flutter, the magnet inside the bolt moves up and

down with the belt at the same frequency to couple with static coils. The belt fluttering

amplitude is assumed equal to the bolt displacement amplitude at the linked position.

Proposed output

The generated power is measured at different airflow conditions, the peak output

power is 7 mW at 2.8 m/s wind speed and 2.5 mW at 2.0 m/s with a 1 m long belt.

3.4 Future Application

• Medical implantations

• Medical remote sensing

• Body Area Network - monitor vital signs, control drug delivery according to need.

Implanted biomedical devices are the potential drug-dosing approach to the patients who

are suffer from severe or chronic diseases such as diabetes, colon cancer and heart disease.

To supply durable and stable power to implantable biomedical devices (IMDs) is one of the

most challenging issues[7]. For most cases, the IMDs have to be replaced owing to the dead

batteries inside. Unfortunately cutting into your body to change batteries brings with it a

significant percentage of mortalities, not just pain and infection Therefore, extracting energy

from ambient sources to extend the lifespan of power supply system for IMDs has attracted a

lot of attentions of researchers.

.

Fig.3.5 Electrodynamic energy harvesting to run pacemaker and defibrillator[10]

13

Chapter 4

Limitations and Remedies Of VEHs

4.1 Main technical limits of VEHs

Narrow bandwidth that implies constrained resonant frequency-tuned applications.

All of the reported generators so far focus on scavenging energy at a single

ambient vibration Frequency. As a result, they implement devices naturally with

small bandwidth (1–100 Hz).[11]

If the environmental vibration frequency deviates a little from the designed

frequency, which is most of the time the resonance frequency of the device, the

generated power decreases rapidly.

If the environmental frequency is constant, it is really hard to match the resonance

frequency of the device to that of the environment due to microfabrication

accuracy and variation in other physical parameters of the device[8].

Small inertial mass and maximum displacement at MEMS scale.

Low output voltage (~0,1V) for electromagnetic systems.

Versatility and adaptation to variable vibration sources.

Miniaturization issues (micromagnets, piezobeam).

14

4.2 Remedy for Narrow bandwidth frequency

Active and Passive tuning techniques

In active/passive tuning techniques, simply the parameters of the generator such as the

mass or the stiffness are altered so that the resonance frequency is tuned to match the

environmental frequency. In the active tuning technique, this adjustment is done

continuously, whereas in the case of passive tuning technique, the tuning actuators turn off

after the adjustment. But active tuning techniques are not feasible because the tuning

actuators will always require more power than the device can generate. On the other hand,

passive tuning techniques also require actuators and sensors, which increase the complexity

and the cost of the device.[8]

MEMS-based piezoelectric power generator array

This generator covers a wide band of external vibration frequency by implementing a

number of serially connected cantilevers in different lengths resulting in an array of

cantilevers with varying resonance frequencies to solve the bandwidth problem. By adjusting

the length increments sufficiently small, cantilevers will have an overlapping frequency

spectrum with the peak powers at close but different frequencies. This will result in widening

of the overall bandwidth of the device, as well as an increase in the overall generated

power[14].

One possible disadvantage of this approach is that the maximum power will be

smaller than the case of using identical cantilevers. On the other hand, this can be eliminated

by increasing the cantilever number at each incremental frequency without increasing the

overall chip area significantly. Another possible limitation can be that depending on the

cantilever material, fabrication may limit the minimum increment size, and hence

optimization of the cantilever lengths and uniform band coverage may become a problem.

This issue is resolved by choosing parylene as the cantilever structural material because

Parylene C is used as the structural material for the cantilevers due to its much lower modulus

of elasticity compared to silicon. This allows much larger deflections and increased power

generation. Also, using parylene permits adjustment of cantilever parameters (e.g. stiffness

and natural frequency) over a wide range.

The device generates 0.4 W of continuous power with 10mVvoltage in an external

vibration frequency range of 4.2–5 kHz, covering a band of 800Hz.[8]

15

Fig.4.1 Proposed electromagnetic generator[8]

16

Chapter 5

Conclusion

Vibrations represents one of the most promising renewable and reliable solutions for

mobile elctronics powering.

Scaling from millimeter down to micrometer size is important

90% of WSNs cannot be enabled without Energy Harvesting technologies.

Ambient vibration (human motion, hydro, wind ) can be alternative for

micropowering.

Most of vibrational energy harvesting system scavenging energy at single ambient

vibration frequency hence they implement device with small bandwidth hence if the

environmental vibration frequency deviates a little from the designed frequency,

which is most of the time the resonance frequency of the device, the generated

power decreases rapidly

Some measures can be taken to match frequency of vibration energy harvesting

system with that of ambient vibration for generation of more power like using array

of cantilever beam and by depositing parylene on beam in place of silicon.

17

References

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Vibrations‖ Procedia Engineering 25 (2011) 819 – 822.

[2] Roundy, S., P. K. Wright, et al. (2004). “Energy Scavenging For Wireless Sensor

Networks with special focus on Vibrations, Kluwer Academic Publisher”

[3] E. Minazara1, D. Vasic1,2 and F. Costa1,3 ―Piezoelectric Generator Harvesting Bike

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Paris 12

[4] Christopher A Howells “Piezoelectric energy harvesting‖ VA 22060-5816, United States

[5] D.-A. Wangn, K.-H.Chang ―Electromagnetic energy harvesting from flow induced

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[6] Fei Fei a, John D. Maib,1, Wen Jung Li a, ―A wind-flutter energy converter for powering

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[7] Chung-Yang Sue, Nan-Chyuan Tsai ―Human powered MEMS-based energy harvest

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[8] Ibrahim Sari a, Tuna Balkan a, Haluk Kulah b,∗ ―An electromagnetic micro power

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[9] Wikipedia

[10] Francesco Cottone Marie Curie Research Fellow ESIEE ― Introduction to Vibration

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[11] Jing-Quan Liua,_, Hua-Bin Fanga, Zheng-Yi Xub, Xin-Hui Maob, Xiu-Cheng Shena, Di

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[12] Songye Zhu ⇑, Wen-ai Shen, You-lin Xu ―Linear electromagnetic devices for vibration

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[13] C.R. Saha∗ , T. O’Donnell, N.Wang, P. McCloskey ―Electromagnetic generator for

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[14] Huicong Liua, Chenggen Quana, Cho Jui Taya, Takeshi Kobayashi b, and Chengkuo

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