Sensor - Week 05 - Strain Gauge and Piezoelectric.pdf

8/18/2019 Sensor - Week 05 - Strain Gauge and Piezoelectric.pdf

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Strain Gauges

Strain gauge - The main tool in sensing force.

Strain gauges, measure strain

Strain can be related to stress, force, torque anda host of other stimuli including displacement,acceleration or position.

At the heart of all strain gauges is the change in

resistance of materials due to change in theirlength due to strain.


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Force sensors - Strain

Gauges Definition of strain: consider a length of metallic

wire L, of conductivity and cross-sectionalarea A.

The resistance of the wire is:

= L A

Taking the log on both sides:

log R= log 1 + log La

= log + log L A


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GaugesTaking the differential on both sides:

Change in resistance is due to two terms:Due to change in conductivity

Due to the deformation of the conductor.

For small deformations (linear deformation), both terms on theright hand side are linear functions of strain, . Bundling both

effects together (that is, the change in conductivity and

deformation) we can write:

dR

=

d

+

d ( L/ A)


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Gauges For small deformations (linear deformation), both terms

on the right hand side are linear functions of strain, .

Bundling both effects together (that is, the change in

conductivity and deformation) we can write:

dR = S s

Ss is the sensitivity of the strain gauge

Also known as the gauge factor


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Strain Gauge

For any given strain gauge the gauge factor is a

constant

Ranges between 2 to 6 for most metallic straingauges

From 40-200 for semiconductor strain gauges.

The strain gauge relation gives a simple linear

relation between the change in resistance of thesensor and the strain applied to it.


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Stress and Strain


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Strain and Stress

Given the conductor discussed above andapplying a force along its axis, the stress is :

= F = E dL = E

= stress [N/m2]

E = Young’s modulus of the material (modulus ofelasticity) [N/m2 ]

= dL/L = strain


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Strain and Stress

Strain is a normalized linear

deformation of the material

Stress is a measure of elasticity of thematerial.


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Strain gauges

Strain gauges come in many forms and types.

Any material, combination of materials orphysical configuration that changes its

resistance due to strain constitutes a straingauge.

Will restrict our discussion to two types thataccount for most of the strain gauges in use

today: wire (or metal) strain gauges - resistive

semiconductor strain gauges.


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Metallic strain gauge

In its simplest form:

A length of wire, held between two posts

When a force is applied to them, will deform thewire causing a change in the wire’s resistance.

This method was used in the past and is valid

It is not very practical (construction, attachment

to system, change in resistance is very small).

Sometimes, multiple lengths of wire were used.


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Wire strain gauge


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Metallic strain gauge common

form A more practical strain gauge - resistive

Built out of a thin layer of conducting material

Deposited on an insulating substrate (plastic,ceramic, etc.)

Etched to form a long, meandering wire (figure) Constantan (60% copper, 40% nickel) is most common

material

negligible temperature coefficient of resistance (TCR).

Other materials are commonly used (table)


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The resistive strain gauge


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Materials for resistive

strain gaugesTable 6.1. Materials for resistive strain gauges and their properties.

Material Gagefactor

Resistivity[mm2/m]

Thermalcoeff. of expansion

[10

/K]

Expansioncoeff.[10/K]

Maximumtemperatrure[C]

Constantan (Cu60Ni40) 2.0 0.48 5 12.5 400 Nichrome (Ni80Cr20) 2.0 1.3 100 18 1000Manganine(Cu84Mn12Ni4)

2.2 0.43 10 17

Nickel -12 0.11 6000 12Chromel (Ni65Fe25Cr10) 2.5 0.9 300 15 800Platinum 5.1 0.1 2450 8.9 1300

Elinvar (Fe55Ni36Cr8Mn0.5) 3.8 0.84 300 9

Platinum-Iridium(Pt80Ir20)

6.0 0.36 1700 8.9 1300

Platinum Rhodium(Pt90Rh10)

4.8 0.23 1500 8.9

Bismuth 22 1.19 300 13.4


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Metallic strain gauge common

form Strain gauges may also be used to measure

multiple axis strains by simply using morethan one gauge or by producing them in

standard configurations.


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Two-axis strain gauge


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120 degree rosette


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45 degree rosette


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45 degree stacked rosette


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membrane rosette


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Semiconductor strain gauges

Operate like resistive strain gauges

Construction and properties are different.

The gauge factor for semiconductors is much

higher than for metals. The change in conductivity due to strain is much

larger than in metals.

Are typically smaller than metal types Often more sensitive to temperature variations

(require temperature compensation).


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All semiconductor materials exhibit changes inresistance due to strain

The most common material is silicon because of

its inert properties and ease of production. The base material is doped, by diffusion of

doping materials (usually boron or arsenide for por n type) to obtain a base resistance as needed.

The substrate provides the means of strainingthe silicon chip and connections are provided bydeposition of metal at the ends of the device.


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Construction of a semiconductor straingauge:


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Other types of semiconductor strain gauges:


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One of the important differences betweenconductor and semiconductor strain gaugesis that semiconductor strain gauges are

essentially nonlinear devices with typically aquadratic transfer function:

dR

= S 1 + S 1 2

Also: PTC or NTC operation


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PTC and NTC operation


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Strain gauges - applications

Strain gauge must be made to react to a force. The strain gauge is attached to the member in which strain is

sensed, usually by bonding. Cannot be re-used!

Special bonding agents exist for different applications and types

of materials Usually supplied by the manufacturers of strain gauges or

specialized producers.

Strain gauges are often used for bending strain,

twisting (torsional and shear strain) andlongitudinal tensioning/deformation (axial strain)of structures (engine shafts, bridge loading,truck weighing and many many others)


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Strain gauges - properties

The properties of strain gauges vary by application

Most metal gauges have a nominal resistancebetween 100 and 1000, (lower and higherresistances are available)

Gauge factor between 2-5

Dimensions from less than 3x3 mm to lengths inexcess of 150 mm (almost any size may be fabricated ifnecessary).

Rosettes (multiple axis strain gauges) are availablewith 45, 90 and 120 axes as well as diaphragm andother specialized configurations.


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Strain gauges - properties

Typical sensitivities are 5m

Deformation is of the order of 2-3m/m.

Much higher strains can be measured with specialized

gauges. Semiconductor strain gauges

usually smaller than most resistive strain gauges

can be made with higher resistances.

their use is limited to low temperatures can be much less expensive than metal strain gauges.

often part of another device


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Strain gauges - errors

Strain gauges are subject to a variety of errors.

Due to temperatures - resistance, especially insemiconductors, is affected by temperature in

the same way as by strain. In metal gauges, this is usually small (materials

with low temperature coefficients of resistance).

In semiconductors, temperature compensation is

sometimes provided on board or a separatesensor may be used for this purpose.


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A third source of error is due to the strainitself, which, over time, tends, topermanently deform the gauge.

can be eliminated by periodic re-calibration

can be reduced by ensuring that the maximumdeformation allowed is small and below the

recommended for the device.


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Additional errors

Due the bonding process

Thinning of materials due to cycling.

Most strain gauges are rated for: given number of cycles (i.e. 106 or 107 cycles),

maximum strain (3% is typical for conducting straingauges, 1% for semiconductor strain gauges)

temperature characteristics specified for use with aparticular material (aluminum, stainless steel, carbonsteel) for optimal performance when bonded

Typical accuracies are of the order of 0.2-0.5%.


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Typical resistive strain

gauges


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Other Strain Gauges

Other strain gauges - for specializedapplications.

Optical fiber strain gauges.

The change in length of the fiber due to strain changesthe phase of the light through the fiber.

Measuring the light phase, either directly or in aninterferrometric method can produce readings ofminute strain that cannot be obtained in other straingauges.

The device and the electronics necessary is far morecomplicated than standard gauges.


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Other Strain Gauges

There are also liquid strain gauges which rely in theresistance of an electrolytic liquid in a flexible containerwhich can be deformed.

Another type of strain gauge that is used on a limitedbasis is the plastic strain gauge.

These are made as ribbons or threads based on graphiteor carbon in a resin as a substrate and used in a waysimilar to other strain gauges.

Very high gauge factors (up to about 300), they areotherwise difficult to use and inaccurate as well asunstable mechanically, severely limiting their practicaluse.


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Bridge Circuits


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Cantilever


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Force and tactile sensors

Forces can be measured in many ways The simplest - use a strain gauge

Calibrate the output in units of force.

Other methods include measuring acceleration of a mass (F=ma),

measuring the displacement of a spring under actionof force (x=kF, k is the spring constant),

measuring the pressure produced by force and somevariations of these basic methods.

None of these is a direct measure of force

most are more complicated than use of a strain gauge.


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Force and tactile sensors

One measures the tensile force by measuring the strainin the strain gauge. The sensor is usually provided with attachment holes

may also be used in compressive mode by pre-stressing the strain

gauge. This type of sensor is often used to measure forces in

locations such as machine tools, engine mounts and thelike.

Often it is called a load cell, especially when large forcesare measured.


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Force sensor


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PIEZOELECTRIC


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Acoustic waves

Sound waves are longitudinal elastic waves.

The pressure wave as it propagates, changes thepressure along the direction of its propagation.

Example: acoustic waves, impinging on oureardrums will push or pull on the eardrum toaffect hearing.

Any wave, including acoustic waves have three

fundamental properties:Frequency, wavelength and speed of propagation


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Acoustic waves

The frequency, f , of a wave is the number ofvariations of the wave per second.

Normally defined for harmonic waves and is

understood to be the number of cycles of theharmonic (sinusoidal for example) wave.

For example, if we were to count the number ofcrests in an ocean wave passing through a fixed

point in one second, the result would be thefrequency of the wave.


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Acoustic waves

Wavelength, l is the distance a wavepropagates in one cycle.

In the example of the ocean wave the

wavelength is the distance between two crests(or two valleys)

Velocity, c, of the wave is the speed with whichthe front of the wave propagates and, as

indicated above, is frequency dependent. These three quantities are related as: l = c/f


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Concept of wavelength


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Acoustic waves

Waves can be transverse waves, longitudinalwaves or a combination of the two.

Transverse waves are those waves which cause a

change in amplitude in directions transverse tothe direction of propagation of the wave.

Example: a tight string vibrates perpendicular tothe length of the string. The wave itself

propagates along the string. The wave propagates away from the source, in

all directions.


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Transverse waves on a tight

string


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Acoustic waves

Generation of longitudinalwaves:

Example: piston in a tube

Example: diaphragm in air

Effect: changes in volumecause changes in pressure.

These propagate - giverise to the wave.


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Acoustic waves - speed

The speed of an acoustic wave is directly related to thechange in volume and the resulting change in pressure

c = pV

V 0 m/ s

0 is the density of the undisturbed fluid,

V is the change in volume, p is the change in pressureV is the volume


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Acoustic waves - speed

In gasses, this simplifies to the following

0 is the density of the undisturbed fluid, is the ratio of specific heats for the gas,

p0 is the undisturbed gas pressureThus, the speed of acoustic waves is material, pressure andtemperature dependent

c = p0 0

m/ s


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Speed of sound

Table 6.1. Speed of sound in some materials at given temperatures.

Material Speed [m/s] Temperature [C]

Air 331 20

Fresh water 1,486 20

Sea water 1,520 20

Granite 6,000

Steel 5,200 20

Copper 3,600 20

Aluminum 6,320

Beryllium 12,900


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Acoustic waves - theory

Assuming a harmonic longitudinal wave of frequency f ,it may be written in general terms as:

p is pressure in the medium,P0 the pressure amplitude of the wavek is a constant.

The wave propagates in the x directionf is the angular frequency

= P 0 sin(kx

t


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The amplitude of the wave is:

ym is the maximum displacement of a particle duringcompression or expansion in the wave.The constant k is called the wave number or the phaseconstant and is given as:

0 = k 0c2 ym

k = 2 = c


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Waves carry energy. A shockwave (earthquake) can cause damage

A loud sound can hurt our ears.

A wave is said to be a propagating wave if itcarries energy from one point to another.

The wave can propagate in an unboundedmedium with or without attenuation (losses). Attenuation of a wave depends on the medium Attenuation reduces the amplitude of the wave.

Attenuation of waves is exponential


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When a propagating wave encounters adiscontinuity in the unbounded space (an objectsuch as a wall, a change in air pressure, etc) partof the wave is reflected and part of it is

transmitted into the discontinuity. Reflection and a transmission occur at any

discontinuity

These reflected and transmitted waves maypropagate in directions other than the originalwave.

Transmission causes refraction of the wave.


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Reflection, transmission and

refraction


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The reflected wave is reflected at an angle equal tothe angle of incidence ( r = i )

The transmitted wave propagates in the material atan angle t which is equal to:

sin t = c2c

sin i

c2 is the speed of propagation of the wave in the mediuminto which the wave transmitsc1 the speed in the medium from which the waveoriginates


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The reflected waves propagate in the samemedium as the propagating wave

Interfere with the propagating wave.

Their amplitude can add (constructiveinterference) or subtract (destructiveinterference).

The net effect is that the total wave can haveamplitudes smaller or larger than the original

wave. This phenomenon leads to the idea of a standing

wave.


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Standing waves


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Standing waves

Example of standing waves:vibrating tight strings reflections occur at the

locations the strings areattached.

This vibration at variouswavelengths, and itsinteraction with the air

around accounts for themusic we perceive when aviolin plays.


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Scattering is reflection of the waves in alldirections due to anything in the path of thewaves.

Dispersion is the propagation of variousfrequency components are different frequencycausing distortion in the received sound wave.

Wave impedance or acoustic impedance is theproduct of density and velocity:

Z = 0c


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The piezoelectric effect

Piezoelectric effect is the generation of electriccharge in crystalline materials upon applicationof mechanical stress.

The opposite effect is equally useful: applicationof charge across the crystal causes mechanicaldeformation in the material.

The piezoelectric effect occurs naturally in

materials such as quartz ( SiO2 - a silicon oxide) Has been used for many decades in so called

crystal oscillators.


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It is also a property of some ceramics and polymers

The piezoelectric effect has been known since 1880

First used in 1917 to detect and generate sound waves inwater for the purpose of detecting submarines (sonar).

The piezoelectric effect can be explained in a simplemodel by deformation of crystals:


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Deformation in one direction (B) displaces the

molecular structure so that a net charge occurs

as shown (in Quartz crystal - SiO2)

Deformation in a perpendicular axis (B) forms anopposite polarity charge


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The charges can be collected on electrodesdeposited on the crystal

Measurement of the charge is then a measure of

the displacement or deformation. The model uses the quartz crystal (SiO2) but

other materials behave in a similar manner.

Also, the behavior of the crystal depends on howthe crystal is cut and different cuts are used fordifferent applications.


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The piezoelectric effect -

theory

The polarization vector in a medium (polarization is

the electric dipole moment of atoms per unit

volume of the material) is related to stress throughthe following simple relation

= d C m2

d is the piezoelectric constant, the stress in the material.


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theory

Polarization is direction dependent in the crystal andmay be written as:

x, y, z are the standard axes in the crystal.The relation above now becomes.

= P xx + P + P zz

P xx = d 11 xx + d 12 yy + d 13 zz

P yy = d 21 xx + d 22 yy + d 23 zz

P zz = d 31 xx + d 32 yy + d 33 zz

d ij are the piezoelectric coefficients along the orthogonal

axes of the crystal.


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theory The coefficient depends on how the crystal is cut.

To simplify discussion we will assume that d issingle valued

The inverse effect is written as:

e= gPe is strain (dimensionless), g is called the constant

coefficient ( is permittivity)

g = d or : g ij = d ij ij


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theory The piezoelectric coefficients are related to the electrical

anisotropy of materials (permittivity).

A third coefficient is called the electromechanical couplingcoefficient and is a measure of the efficiency of theelectromechanical conversion:

k 2 = dgE or : k ij2 = d ij g ij E ij

E is the Young modulus.

The electromechanical coupling coefficient is simply theratio between the electric and mechanical energies per unit

volume in the material.

C t l i l t i


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Crystals - piezoelectric

properties

Table 7.2. Piezoelectric coefficients and other propertiesin monocrystals

Crystal Piezoelectriccoefficient dij, x10

[C/N]

Permittivity, ij Coupling coefficientk max

Quartz (SiO2) d11=2.31, d14=0.7 11=4.5, 33=4.63 0.1

ZnS d14=3.18 11=8.37 0.1CdS d15=-14, d33=10.3,

d31=-5.211=9.35, 33=10.3 0.2

ZnO d15=-12, d33=12,d31=-4.7

11=8.2 0.3

KDP (KH2PO4) d14=1.3, d36=21 11=42, 33=21 0.07

ADP (NH4H2PO4) d14=-1.5, d36=48 11=56, 33=15.4 0.1

BaTiO3 d15=400, d33=100,d31=-35

11=3000, 33=180 0.6

LiNbO3 d31=-1.3, d33=18,d22=20, d15=70

11=84, 33=29 0.68

LiTaO3 d31=-3, d33=7,d22=7.5, d15=26

11=53, 33=44 0.47

C i i l t i


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Ceramics - piezoelectric

properties

Table 7.3. Piezoelectric coefficients and other properties in ceramics

Ceramic Piezoelectric coefficient

dij, x10

[C/N]

Permittivity,

Coupling

coefficientk max

BaTiO3 (at 120C) d15=260, d31=-45,d33=-100

0.2

BaTiO3+5%CaTiO3 (at 105C) d31=43, d33=77 0.25

Pb(Zr0.53Ti0.47)O3+(0.5-3)%La2O2or Bi2O2 or Ta2O5 (at 290C)

d15=380, d31=119,d33=282

0.47

(Pb0.6Ba0.4)Nb2O6 (at 300C) d31=67, d33=167 0.28

(K0.5Na0.5)NbO3 (at 240C) d31=49, d33=160

0.45


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Piezoelectric devices

A piezoelectric device is built as asimple capacitor, (capacitance C)

Assuming force is applied on the x-axis

in this figure, the charge generated byforce is:

Q x = d 11 F x

Voltage developed across it is:

V= Q x =

d 11 F x = d 11 F x d = thickness

A = area


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Piezoelectric devices

The thicker the device the larger the voltage.

A smaller area has the same effect.

Output is directly proportional to force (or pressurewhich is force/area).

Most common piezoelectric materials for sensors

PZT (lead-zirconite-titanium-oxide)

Polymer films such as PVDF (PolyVinyliDeneFluoride).

Barium Titanate (BiTiO3) in crystal or ceramic form

Crystalline quartz are used for some applications.

Thin films of ZnO on semiconductors


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Piezoelectric resonator

Equivalent circuit of a

piezoelectric material.

This circuit has tworesonances – a parallel

resonance and a series

resonance (called

antiresonance)


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The resonant frequencies are given as:

s = 1

2 LC p =

12 LC C 0/ C + C 0

A single resonance is desirable

Materials or shapes for which the two resonant

frequencies are widely separated are used.

Therefore a capacitance ratio is defined as:

m = C C 0


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The relation between the two frequencies is:

The larger the ratio m, the larger the separation between frequencies.

The resistance R in the equivalent circuit acts as a

damping (loss) factor. This is associated with the

Quality factor of the piezoelectric material:

Q = 1 R

LC

p = f s 1 + m

i l i


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Piezoelectric actuators

One of the first actuator has been in use inanalog clocks for decades.

Essentially a cantilever beam made of a

piezoelectric crystal (quartz is common) thatengages a geared wheel.

When a pulse is connected across the beam itbends (downwards) and moves the wheel one

tooth at a time. This actuation only requires minute motion.

Its main importance - accuracy

i l i


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Piezoelectric actuators

Other actuators have been designed which can movemuch larger distances and apply significant forces aswell.

It is 70x90mm in size and when a 600V is applied across

the piezoelectric element (grey patch) one end movesrelative to the other (which must be fixed) about 8mm.

The rated force for this device is about 17kg force at ratedvoltage.

Some piezoelectric sensors and actuators can operate atlower voltages, large voltages are typical of piezoelectricactuators and is one serious limitation.

i i l i


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Linear piezoelectric

actuator

St k d i l t i


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Stacked piezoelectric

actuators Individual elements, each with its own electrodes can be

stacked to produce stacks of varying lengths.

In such devices, the displacement is anywhere between0.1 to 0.25% of the stack length, but this is still a small

displacement. One of the advantages of these stacks is that the forces

are even larger.


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S d i


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Saw devices

Surface waves or Rayleigh waves.

Surface waves propagate on the surface of anelastic medium with little effect on the bulk of

the medium Have properties which are significantly different

than longitudinal waves

The most striking difference is their much slower

speed of propagation. Propagation of surface waves is nondispersive

S d i


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Saw devices

The exact definition of Rayleigh wave is a wave thatpropagates at the interface between an elastic mediumand vacuum or rarefied gas (air for example) with littlepenetration into the bulk of the medium.

A good analogy for surface waves are ocean waves. Under most conditions this would seem to be a

disadvantage but, looking at the wavelength alone as theratio of velocity and frequency: l=c/f,

The lower the velocity of the wave, the shorter thewavelength in that medium.

The smaller the physical size of a device!

SAW d i


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SAW devices

Generation of surface waves:

In a thick sample, one can set up a surface waveby a process of wave conversion.

A longitudinal wave device is used and energycoupled through a wedge at an angle to thesurface.

At the surface of the medium there will be both a

shear wave and a surface wave This is an obvious solution but not necessarily

the optimal.

S f i lid


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Surface waves in a solid

S d i


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Saw devices

A more efficient method: apply metallic strips on thesurface of a piezoelectric material in an interdigital fasion(comblike structure)

This establishes a periodic structure of metallic strips.

When an oscillatory source is connected across the twosets of electrodes, a periodic electric field is establishedin the piezoelectric material,

Because of this electric field, an equivalent, periodicstress pattern is established in the piezoelectric medium.

This generates a stress wave (sound wave) that nowpropagates away from the electrodes in both directions.The generation is most efficient when the period of thesurface wave equals the inter-digital period.

SAW t


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SAW generator

SAW d i


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SAW devices

For example, a SAW device has a frequency of 400 Mhz.

The speed of propagation in a piezoelectric is of theorder of 3000 m/s.

This gives a wavelength of 7.5 m.

Making each strip in the structure l/4 gives 1.875mwidth for each strip and 1.875m distance betweenneighboring strips.

This calculation shows that the dimensions required are

very small (the same device, based on electromagneticwaves has a wavelength of 750mm).

SAW devices


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SAW devices

The comblike structure generates soundwaves in the piezoelectric medium

A sound wave in the piezoelectric medium

produces a signal in a comb-like structure. The structure can be used both for generation

and reception of surface waves which in turnmeans that the device can be used for

sensing or actuation

SAW Resonator


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SAW Resonator

By far the most common use of surface acoustic

waves (SAW) is in SAW resonators, filters and

delay lines.

The portion marked as In and Out are used as the

input and output ports of the resonator (i.e. theoutside connections of the resonator).

The parallel lines on each side are grooves

etched in the quartz piezoelectric.

SAW Resonator


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SAW Resonator

The input port establishes a surface wave

The wave is reflected by the grooves on eachside.

These reflection interfere with each otherestablishing a resonance which depends on thegrating of groves separation.

Only those signals that interfere constructively

will establish a signal in the output port, theothers cancel.

SAW Resonator


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SAW Resonator

This device is popular as the element thatdefines the oscillator frequency incommunication

A very small device can easily operate at lowfrequencies and can operate at frequenciesabove the limit of conventional oscillators.

The device may also be viewed as a very narrow

band filter and This is in fact another of its uses.

The basis of most sensors is a delay line

SAW resonators for


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SAW resonators for

communication

SAW delay line


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SAW delay line

SAW Resonator


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SAW Resonator

The device on the left generates a surface wave

This is detected after a delay in the device on theright.

The delay depends on the distance between thedevices and, because the wavelength is usuallysmall, the delay can be long.

Adding an amplifier in the feedback makes this

an oscillator with frequency dependent on thedelay.

SAW Resonator


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SAW Resonator

It is based on a delay line in which the delay is influencedby the stimulus.

An essentially identical sensor is shown in Figure 7.46which has two identical delay lines and the output is

differential. One line is used as the proper sensor, the second as a

reference to cancel common-mode effects such astemperature.

In most cases, the delay time is not measured but rather,a feedback amplifier is connected (positive feedback)which causes the device to resonate at a frequencyestablished by the time delay

SAW sensor


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SAW sensor

SAW sensor


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SAW sensor

SAW Resonator


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SAW Resonator

The stimuli that can be measured are many.

First, the speed of sound is temperature dependent.Temperature changes both the physical length of

the delay line and the sound speed as follows:

is the coefficient of linear expansion the temperature coefficient of sound velocity.

= L0 1 + T T 0 , c= c0 1 + T T 0

SAW Resonator


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SAW Resonator

These two terms are contradicting in that bothincrease and hence the delay and oscillatorfrequency are a function of the difference betweenthem.

The change in frequency with temperature is:

f =

This is linear and a SAW sensor has a sensitivity of about10 C.

SAW Resonator


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SAW Resonator

In sensing pressure, the delay in propagation isdue to stress in the piezoelectric as indicatedabove.

Measurement of displacement, force andacceleration are done by measuring the strain(pressure) produced in the sensor.

Many other stimuli can be measured including

radiation (through the temperature rise), voltage(through the stress it produces through theelectric field) and so on.

QCM Sensor


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QCM Sensor

A

m f f

qq

q

2

2

Sauerbrey

qq

l l ql f f

2/3

Kanazawa - Gordon

C 0

C 1

R 1

L 1

- C 0

'

Frequency Response


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Frequency Response

1

10

100

1000

10000

100000

1000000

9950000 9960000 9970000 9980000 9990000 10000000 10010000 10020000 10030000 10040000 10050000

I m p e d a n c e

Freqeuncy

Impedance

Im p ed ance Im p ed anc e

Frequency Change Response


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Frequency Change Response

0 50 100 150 200

-100

-50

0

f

i n H z

Time in s

TSM-Sensor


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TSM Sensor

02

1 l l

l j Z

q

L

Z

Z f f

Im0

q

L

Z

Z L f R

Re2

10

Quartz Loading

C 0

C q

Rq

Lq

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Liquid

Gl

qq

l l l f f

2/3

qq

l l l

f C K R

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28

f & R


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f & R

0 10 20 30

-5k

-4k

-3k

-2k

-1k

0

f /H

z

Zeit / s

0

100

200

300

400

500

R/

Ultrasonic sensors and


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actuators

In principle, identical to acoustic sensors andactuators

Somewhat different in construction

Very different in terms of materials used andrange of frequencies.

The ultrasonic range starts where the audiblerange ends,

Therefore ultrasonic sensor (i.e. microphone) oractuator for the near ultrasound range should bequite similar to an acoustic sensor or actuator.

24 kHz, UT transmitter and


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24 kHz, UT transmitter and

receiver



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actuators

Figure 7.31 shows an ultrasonic transmitter (left) and anultrasonic receiver (right) operating in air at 24 kHz.

Same size and essentially the same construction.

This is typical of piezoelectric devices in which the same

exact device can be used for both purposes Both use an identical piezoelectric disk

The only difference is in the slight difference in theconstruction of the cone.

Figure 7.31 shows a closer view of another device, thistime operating at 40 kHz, also designed to operate in airin which the piezoelectric device is square, seen at thecenter below the brass supporting member


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40 kHz ultrasonic


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40 kHz ultrasonic

transmitter/receiver for

ranging

Ultrasonic sensors


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Ultrasonic sensors

Scope of ultrasonic sensing is very wide.

Ultrasound is much better suited for use in solidsand liquids (higher velocities, lower attenuation)

Support waves other than longitudinal whichallow additional flexibility ultrasonics shear waves,

surface waves

Ultrasonic sensors exist at almost any frequencyand exceeding 1 GHz (especially SAW devices).

Most sensors operate below 50 MHz.

Ultrasonic sensors


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Ultrasonic sensors

Most ultrasonic sensors and actuators are based onpiezoelectric materials

Some are based on magnetostrictive materials

A particularly important property of piezoelectric

materials that makes them indispensable in ultrasound istheir ability to oscillate at a fixed, sharply definedfrequency called the resonant frequency.

The resonant frequency of a piezoelectric crystal (or

ceramic element) depends on the material itself, itseffective mass, strain and physical dimensions and is alsoinfluenced by temperature, pressure and the like.

Ultrasonic resonator


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Ultrasonic resonator

Resonance is important is two ways.

At resonance the amplitude of mechanicaldistortion is highest

In receive mode, the signal generated is largest Means the sensor is most efficient at resonance.

The second reason is that the sensors operate atclear and sharp frequencies

Parameters of propagation including reflectionsand transmissions are clearly defined as areother properties such as wavelength.


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Ultrasonic sensor -


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Ultrasonic sensor

construction

Ultrasonic sensors - sample


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Ultrasonic sensors sample

Specification sheet


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p


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Pulse-echo operation


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p

This reflection is an indication of the existence of the discontinuity

Amplitude of the reflection is a function of the size of thediscontinuity.

The exact location of the discontinuity can be found from the time ittakes the waves to propagate to and from the discontinuity.

Figure 7.32 shows an example of finding the location/size of a defectin a piece of metal.

The front and back surfaces are seen, usually as large reflectionswhile the defect is usually smaller.

Its location can be easily detected.

The same idea can be used to create an image of a baby in the womband for position sensing in industry.

Fault location by ultrasound


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y

Sensing fluid velocity


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g y

There are three effects that can be used.1. Sound velocity is relative to the fluid in which it

travels. (Our voice carries downwind faster (bythe wind velocity) than in still air). This speed

difference can be measured from the time ittakes the sound to get from one point toanother.

2. The second effect is based on the phase

difference caused by this change in speed3. Third is the doppler effect – the frequency of thewave propagating downwind is higher than thefrequency in still air.

Sensing fluid velocity


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g y

An example of a fluid speed sensing using method 1. In thiscase, the distance and angle of the sensors is known and thetransmit time, say downstream is:

T = Dc + v cos

c speed of sound

vf fluid speed

Magnetostrictive sensors


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g

In air or in fluids, piezoelectric sensors are best. In solids there is an alternative -

magnetostriction.

These sensors are collectively called

magnetostrictive ultrasonic sensors Used at lower frequencies (about 100 kHz) to

generate higher intensity waves.

All that is necessary is to attach a coil to the

material and drive it at the required frequency. The field generated in the material generates

stress which generates an ultrasonic wave

EMATs


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An even simpler method is to generate an acelectromagnetic field inside the material in which soundwaves are to be generated.

Because the induced electric currents, there is a force

acting on these currents due to an external magneticfield generated by permanent magnets.

The interaction generates stresses and a sound wave.

These sensors are called electromagnetic acousticsensors (EMAT – electromagnetic acoustic transducer).

These sensors are quite common because of theirsimplicity but they tend to operate at low frequencies(


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Sensor - Week 05 - Strain Gauge and Piezoelectric.pdf

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