seminar on energy harvesting from vibration
DESCRIPTION
Tte ways by which energy can be harvested from ambient vibration to power mini sensor and actuatorsTRANSCRIPT
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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.
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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.
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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
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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
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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]
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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
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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
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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
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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]
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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]
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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
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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]
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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).
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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]
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Fig.4.1 Proposed electromagnetic generator[8]
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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.
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References
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[9] Wikipedia
[10] Francesco Cottone Marie Curie Research Fellow ESIEE ― Introduction to Vibration
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[13] C.R. Saha∗ , T. O’Donnell, N.Wang, P. McCloskey ―Electromagnetic generator for
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