1 sensors and measurements penderia & pengukuran ent 164 piezoelectric sensors hema c.r. school...
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Sensors and MeasurementsPenderia & Pengukuran
ENT 164Piezoelectric Sensors
Hema C.R.School of Mechatronics Engineering
Northern Malaysia University College of EngineeringPerlis , Malaysia
Contact no: 04 9798442Email: [email protected]
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General Structure of Measurement System
SENSING ELEMENT
SIGNAL CONDITIONING
ELEMENT
SIGNAL PROCESSING
ELEMENT
DATA PRESENTATION
ELEMENT
INPUT
TRUE VALUE
OUTPUT
MEASURED VALUE
Piezo-electric
Hall effect
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Sensing ElementsResistive
temperature & strain
Capacitive Pressure, level ,strain & humidity
Inductivestrain
Thermo Electrictemperature
Piezoelectricvibration , force & acceleration
Electro Chemicalgas composition & ionic concentration
Hall Effect SensorMagnetic field
silicon
pressure temperature
Flow
O2
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Piezoelectric Sensing
Elements
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The word piezo is derived from the Greek piezein, which means to squeeze or press.
The effect known as piezoelectricity was discovered by brothers Pierre and Jacques Curie in 1880.
Crystals which acquire a charge when compressed, twisted or distorted are said to be piezoelectric.
Piezoelectric materials also show the opposite effect, called converse piezoelectricity, where the application of an electrical field creates mechanical deformation in the crystal.
Further Reading : Crystal classes & Piezoelectric crystal classes
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CrystalsCrystals are naturally occurring material that can be induced to resonate or vibrate at an exact frequency.
Crystals are anisotropic materials
physical properties depend on the direction
Quartz, a piezoelectric crystal that provides excellent mechanical and electrical stability, acquires a charge when compressed, twisted, or distorted.
Quartz crystals are used as active elements in oscillators
A Quartz "Crystal" Isotropic materials have same physical properties in all directions
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Piezoelectric MaterialsQuartz (SiO2)
Barium Titanate (BaTiO3)
Gallium Orthophosphate (GaPO4),
Polymer materials like rubber, wool, wood and silk exhibit piezoelectricity to some extent
ApplicationsMicrophones, guitars, sonar, motors microbalances, clocks and vibration sensors.
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Piezoelectric Effect
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When force is applied to a crystal , the crystal atoms are displaced from their normal positions
Displacement x is proportional to applied force F
(1)
where k is the stiffness in the order of
The displacement can be summarised using a transfer function
Fk
x1
19 Nm102
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Transfer Function of an Element
sf
sfsG
i
0
sfsGsf i0
Element Transfer Function:
Transfer function of an output signal is the product of element transfer function and transfer function of the input
signal
When input signal of an element is changed suddenly the output signal will not change instantaneously. The way in which an element responds to sudden input changes are termed its dynamic characteristics, which can be conveniently summarised using a transfer function
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Transfer Function Of Second Order Elements Sensor converts force into
displacement , diagram shows the conceptual
model which has a mass m kg, a spring of stiffness k
N/m and a damper constant Ns/m.
The system is initially at rest at time t =0- so that the initial velocity and
the initial acceleration
. The initial input force F(0-) is balanced by
the spring force at the initial displacement x(0-)
Spring k
Damper
Mass
m
F
x
x=0
kx
.
xk
Model of an Elastic force sensor an analogous system to a piezo force sensor
00
x
00
x
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If input force is suddenly increased at t = 0, then element is
no longer in a steady state and its dynamic behavior is described by Newton ‘s second law
resultant force = mass x acceleration
and
Defining and to be deviations in F and x
)0()0( kxF
Fkxxxm
xmxkxF
F x
(i)
(ii)
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Fk
xdt
xd
kdt
xd
k
m
ei
Fxkxxm
FFxkkxxxm
xxxx
xxxFFF
1
.
)0()0(
,
)0(),0(
2
2
The differential equation now becomes
Which using equation (i) reduces to
(iii)
Second-order Linear Differential Equation
(iv)
Fkxxxm
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nn
n
kkm
km
m
k
/2/,/1/
22
If we define
Undamped natural frequency rad /s
and
Damping ratio
then
(v)
Eqn.(iv) can be expressed in standard form
Fk
xdt
xd
dt
xd
nn
1212
2
2
Second-order Linear Differential Equation
(vi)
(xi )
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)0()0()()(
)0()()(
22
2
fsfsfstfdt
d
fssftfdt
dLaplace Transform of Time functions f(t)
)(1
)()]0()([2
)]0()0()([1 2
2
sFk
sxxsxsxxssxsnn
To find transfer function of the element we use Laplace transform of equation (vi)
(vii)
0)0(
0)0()0(
x
xxSince
and
Equation (vii) reduces to
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)(1
)(
)(
)(1
)(121 2
2
sGk
sF
sx
sFk
sxssnn
Thus
Where 1/k =steady-state sensitivity K
(viii)
1
21
1)(
22 ss
sG
nn
Transfer Function for a second–order element
(ix)
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Using transfer function for a second order element
x and F can be represented by the second order transfer function
Where natural frequency is large
= 10 to 100 kHz and damping ratio
= 0.01
121
1
2
s)s(
k)s(F
x
nn
(2)
nn f 2
nf
Further Reading : Page 56 - Bentley
121
1
22
ss
)s(G
nn
Transfer Function of a Piezoelectric Element
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This deformation of crystal lattice results in crystal acquiring a charge q , proportional to x
q = Kx
(3)
From equation (1) and (3) we get
(4)
where is the charge sensitivity to force
dFFk
Kq
Direct Piezoelectric Effect
1 CNk
Kd
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A piezoelectric crystal gives a direct electrical output, proportional to applied force, so that a secondary displacement sensor is not required.
Piezoelectric crystals also produce an inverse effect where an voltage applied to the crystal causes a mechanical displacement.
(5)
inverse effect is used in ultrasonic transmitters is identical with
1CN 1mV
dVx Inverse Piezoelectric Effect
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Further Reading : Page 160 - Bentley
Metal electrodes are deposited on opposite faces of the crystal to form a capacitor to measure the charge q
Capacitance of the parallel plate capacitor formed
(6)
Metal PlatePiezoelectric crystal
t
Measuring ‘q’
t
ACN
0
0 Permittivity of free space (vacuum)
Relative permittivity or dielectric constant of the insulating material (here the piezo )
A Area of plate
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Further Reading : Page 82 - Bentley
The crystal can be represented as charge generator q in parallel with a capacitance
or a Norton equivalent circuit
consisting of current source in parallel with .
Magnitude of is
(7)
NC
NCNi
dt
dxK
dt
dqiN
Ni
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transfer function form of
(8)
where d/dt is replaced by the Laplace operator s
For steady force F ,
F and x are constant with time
Such that dx/dt and are zero.
Ks)s(x
iN
Ni
Further Reading: http://en.wikipedia.org/wiki/Laplace_transform#Formal_definition
Ni
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Piezoelectric Force
Measurement System
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Circuit of a force measurement system
Consider a piezoelectric crystal connected to a recorder where
is a pure resistive load
is pure capacitance of the cable
is the recorder voltage
Piezoelectric Crystal
Recorder
Capacitive Cable
Ni NC CC LRLV
LR
CC
LV
Figure 1. Piezoelectric Force measurement system
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Transfer function relating to and is
(9)
Overall system transfer function relating recorder voltage to input force is
(10)
LV Ni
sCCR
R
)s(i
)s(V
CNL
L
N
L
1
LV F
F
x
x
i
i
Vs
F
V N
N
LL
Further Reading : Page 84 - Bentley
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From equation (2),(8) and (9) we get
where
121
1
1 22
ss
kKssCCR
Rs
F
V
nn
CNL
LL
1
211
1
1
22
sssCCR
sCCR
CCk
K
nn
CNL
CNL
CN
121
1
1 22 ss
s
s
CC
ds
F
V
nn
CN
L
Transfer Function for basic Piezoelectric force measurement system
(11)
CNL CCR
121
1
2
s)s(
k)s(F
x
nn
Ks)s(x
iN
sCCR
R
)s(i
)s(V
CNL
L
N
L
1
(Tau )
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Disadvantages of the basic piezoelectric system
1.Steady state sensitivity is equal to . Thus the system sensitivity depends on the cable capacitance i.e. length and type of cable.
2.The dynamic part of the system transfer function is (ignoring recorder dynamics)
(12)
The second term is characteristic of all elastic elements and cannot be avoided , however it causes no problem if the highest signal frequency is well below
CN CC/d
CC
121
1
1)(
22 ss
s
ssG
nn
MAXn
(Tau )
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The first term indicates that system cannot be used for measuring d.c. and slow varying forces.
Illustration
Consider a frequency response
characteristics plot for and arg
of a typical measurement system
1s/s
jG jG
Piezoelectric Crystal
Recorder
Capacitive Cable
Ni NC CC LRLV
Figure 1. Piezoelectric Force measurement system
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2
22
2
222
41
1
1
nn
jG
Amplitude Ratio
Phase difference arg
2
2
110
1
290
n
ntantanjG
Figure 2: Approximate Frequency Response Characteristics Piezoelectric Measurement System with charge amplifier
(13)
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The term causes a low frequency
roll-off so that at and system cannot be used for frequencies much below
These disadvantages can be overcome by introducing a charge amplifier into the system as shown in Figure 2
1s
s
0jG 0
1
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This system gives an output proportional to
i.e. an output proportional to charge q .Since the system gives a non zero output for steady force input. From Figure 3 we Have
and charge on feedback capacitor is
For an ideal operational amplifier we have and In this case we have and
so that
dtiN
dtdqiN
iii F1 (14)
FC
OUTFF VVCq (15)
0 ii VV
0 VV dtdqi F
F
dt
dVCdt
dqii OUTF
FF 1 (16)
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Since the potential drop across and is zero
From equation and we have
From equation , and the overall transfer function for force measurement system is
NC CC
dt
dqii N 1 (17)
(16) (17)
(18)
FOUT
F
OUT
C
qVei
dt
dq
Cdt
dV ..
1
Transfer Characteristic Of Ideal Charge Amplifier
(18) (3) (2)
1
21
1)(
222
ssC
ds
F
V
n
F
OUT
Transfer Function for Piezoelectric system with Ideal Charge Amplifier
(19)
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The steady state sensitivity is now i.e. it depends only on the capacitance of the charge amplifier and is independent of transducer and cable capacitance
Common Piezoelectric materialsQuartz
Lead zirconium titanate (PZT)
Barium titanate (BaTi2O3)
PolyVinylidine DiFluoride (PVDT)
FCd
FC
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Piezoresistive Sensing
Elements
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Piezoresistivity is defined as the change in resistivity of a material with applied mechanical strain and is represented by the term in the equation (20)
Silicon doped with small amounts of n type or p type materials exhibits a large piezoresistive effect and is used to manufacture strain gauges.
e
e1
v is Poisson’s Ratio
evG 121
Gauge factor of Strain gauge
(20)
Further Reading : Page 158 - Bentley
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Poisson’s Ratio
When a sample of material is stretched in one direction, it tends to get thinner in the other two directions. Poisson's ratio (v) is a measure of this tendency. It is defined as the ratio of the strain in the direction of the applied load to the strain normal to the load. For a perfectly incompressible material, the Poisson's ratio would be exactly 0.5. Most practical engineering materials have v between 0.0 and 0.5..
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Reference
1. ‘Principles of Measurement Systems’ by John P Bentley. [Text book]
2.http://www.resonancepub.com/piezoele.htm3.http://hyperphysics.phy-astr.gsu.edu/hbase/solids/
piezo.html 4. ‘Piezoelectric Transducers and Applications’ by
Antonio Arnau