Emm 3242 Transduser (Week 4)
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EMM 3242
Transducers
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Takrifan
• A transduser is a device which, when
actuated by energy in one transmission
system, supplies energy in the same form
or in another form to a second
transmission system
• Transducers may be classified according
to their application, method of energy
conversion, nature of the output signal,
and so on.
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Prinsip-prinsip Transduser
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Prinsip Kemuatan (Capacitive)
The capacitance is defined as the ratio of the charge to the voltage for a given physical situation. If the capacitance is large, more charge is needed to establish a given voltage difference.
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Capacitance measurement is used to detect the motion of a sensor
element. A simple example would involve the motion of one electrode in
the plane parallel to the electrodes. Assume a pair of rectangular
electrodes, as shown in figure 3, with dimensions Length (L) and Width
(W). If one of the electrodes moves laterally a distance x, the
capacitance changes
from to
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Capacitive Sensors
e.g. An electrolytic
capacitor is made
of Aluminum
evaporated on
either side of a
very thin plastic
film (or electrolyte)
Electrolytic or
ceramic capacitors
are most common
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Capacitive SensorsOther Configurations
c. Differential Mode
b. Variable Dielectric Mode
a. Variable Area Mode
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Contoh
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Contoh
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Mechanical transducers : Inductive
CONTENTS]
• Differential core positions in
the coil produce changes in
inductance which can be
employed for displacement
measurements -----------------
• A linear variable differential
transformer (LVDT) has a
primary excitation coil and two
series-connected secondary
coils with a movable core --
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Contoh
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LVDT
Sensor/Transducer
Iron core moves (displaces) in magnetic field
Voltage generated
Signal Conditioner
Voltage amplified for readout
Readout
Change in voltage converted to displacement
(Linear Variable
Differential Transformer)
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Introduction
• Cheap, easy to use
way of measuring
deflection
• Measure deflections
from 12 in. to
millimeters
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Inductive Sensors
An inductor is basically a
coil of wire over a “core”
(usually ferrous)
It responds to electric or
magnetic fields
A transformer is made of at
least two coils wound over
the core: one is primary and
another is secondary
Primary Secondary Displacement Sensor
Inductors and tranformers work only for ac signals
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• When core slides through transformer, a certain
number of coils are affected
• This generates a unique voltage
Description
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Inductive Sensors - LVDT
LVDTLinear Variable
Differential Transformer
An LVDT is used as a sensitive displacement sensor: for example, in a cardiac assist device
or a basic research project to study displacement produced by a contracting muscle.
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Description
• An LVDT consists of a coil structure shaped into
a cylinder and a magnetic core
• Coil structure consists of one primary core and
two secondary cores
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Interpretation
• Voltage can be interpreted using formula
stated above
• V = Voltage and N = Number of coils
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Mounting
• Core attached to a stationary object
– E.g. Testing frame
• Rod attached to the object to be displacing
– E.g. Mid-span of girder
• Object moves and thus changes the
position of rod in core
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Benefits
• Easy way to measure deflection
• Very rugged piece of equipment
– Ideally a frictionless surface
– Expected life is 2,000,000 hours
– Can be used in adverse environmental
conditions
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Limitation
• The range of measurement is limited
– ~12 in.
– For longer stroke, a cable extension
transducer is preferable ~ 1700 in.
• Rod attached to magnetic core can be
bent
– Exact distance is no longer known
• Requires alignment to be correct
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Applications
• Displacements
– extensometers, temperature transducers,
butterfly valve control, servo valve
displacement sensing
• Deflections
• Measurements of thickness changes
• Fluid level (float attached to rod)
• Velocity and acceleration
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Contoh
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Piezoelectric Sensors
What is piezoelectricity ?
Strain causes a
redistribution of charges
and results in a net
electric dipole (a dipole
is kind of a battery!)
A piezoelectric material
produces voltage by
distributing charge
(under mechanical
strain/stress)
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A piezoelectric sensor is a device that uses the
piezoelectric effect to measure pressure,
acceleration, strain or force by converting them
to an electrical signal
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Piezoelectric Sensors
Above equations are valid when force is applied in the
L,W or t directions respectively.
31 denotes the
crystal axis
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Transduser Berintangan (Resistive)
• A resistive transducer is a device that
senses a change to cause a change in
resistance
Device Action Where used
Light Dependent Resistor Resistance falls with increasing light level
Light operated switches
Thermistor Resistance falls with increased temperature
Electronic thermometers
Strain gauge Resistance changes with force
Sensor in an electronic balance
Moisture detector Resistance falls when wet Damp meter
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Temperature sensors• Resistive thermometers
– typical devices use platinum wire (such a
device is called a platinum resistance
thermometers or PRT)
– linear but has poor sensitivity
A typical PRT element A sheathed PRT
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• Thermistors
– use materials with a high thermal coefficient
of resistance
– sensitive but highly non-linear
A typical disc thermistor A threaded
thermistor
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RTD (Resistive Temperature
Detector)
Resistance thermometers, also called resistance
temperature detectors (RTDs), are temperature sensors
that exploit the predictable change in electrical resistance
of some materials with changing temperature. As they are
almost invariably made of platinum, they are often called
platinum resistance thermometers (PRTs). They are
slowly replacing the use of thermocouples in many
industrial applications below 600 C.
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2-wire RTD 3-wire RTD
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Contoh (Gunakan Konsep Titi Wheatstone)
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Anjakan sudut
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Transduser Termoelektrik
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Jadual 4.2 (Type J with 0 deg
ref)Suhu ( C) Emf (mv)
0 0.00
50 2.61
100 5.28
150 8.01
200 10.77
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Contoh
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Transduser Photoelektrik
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Photocell– such devices do not
produce electricity, but simply change their resistance
– photodiode (as described earlier) can be used in this way to produce a linear device
– phototransistors act like photodiodes but with greater sensitivity
– light-dependent resistors (LDRs) are slow, but respond like the human eye
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PHOTOCELL
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Konsep sel foto
• Rintangan bahan ini berkurangan apabila
terdedah kepada keamatan cahaya. Rintangan
bahan ini sangat tinggi pada suhu bilik.
• Untuk mengurangkan nilai rintangannya, bentuk
sepertiRajah 4.19 digunakan dengan nilai
ketebalan l yang kecil dan A dimaksimumkan
dengan menggunakan jalur (strip) nipis dan
berlingkar(winding) supaya memenuhi
persamaan
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The photoconductive cell
structure
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Contoh
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Sel Suria
-Terdiri dari bahan semikonduktor
spt silikon,selenium dan
germanium
-Apabila gelombang cahaya
dipancarkan, elektron dalam sel
suria akan tercas keluar melalui
simpang diod didalam sel.
-Cas e akan menjanakan tenaga
elektrik secara berulang-ulang
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Sistem sel suria
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CONT..
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• Sejenis fotodiod yg boleh berfungsi dalam zero-bias (sistem sel suria) dan reverse bias (aliran arus balikan yg dikawal oleh keamatan cahaya)
• Diperbuat dari silikon
• Kelebihan : sambutan masa lebih cepat dari sel foto
PHOTO DIODE
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The photodiode uses the pn junction reverse current to measure radiation
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Soalan
Penyelesaian:
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Jadi:
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Foto Transistor
• Bekerja berasaskan
hubungan arus elektrik-
cahaya.
• Mempunyai 3 terminal spt
transistor biasa.
• Lebih peka dan arus
keluaran lebih tinggi
berbanding fotodiod.
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Fototransistor
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Litar untuk fototransistor
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Soalan
Penyelesaian
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Mechanical transducers : Level Transducers
CONTENTS]
• There are many level sensor
techniques ------------------
• Ultrasonic level measurement
needs no physical contact with
the material, just a transmitter
T and reciver R ----------------
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Mechanical transducers : Strain Gauge
CONTENTS]
• Tensile and compressional stress
can be defined in terms of forces
applied to a rod -----------------
• Stress =
• Strain =
• Typical semiconductor strain
gauge configuration ------------
A
F
l
l
00
0 21l
l
A
lR
strain
RR
GF
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Load Cell
Sensor/Transducer
Force elastically deforms the load cell structure
Strain gages attached to the structure deform,
causing change in resistance
Signal Conditioner
R causes V; V amplified
Readout
Change in resistance converted to V, then V
converted to load
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Accelerometer
Sensor/Transducer
Acceleration causes piezoelectric crystal to
deform
Signal Conditioner
Charge created by deformation converted to
voltage, amplified
Readout
Voltage converted to g’s
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Meter Pecutan (Accelerometer)
• Digunakan untuk mengukur pecutan.Juga
boleh digunakan untuk mengukur halaju
dan anjakan.
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Jenis Meter pecutan
• Potentiometric
• LVDT
• Keengganan boleh ubah (variable
reluctance)
• Piezoelectric
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Mechanical transducers : accelerometer
CONTENTS]
• An object in periodic motion about an
equilibrium at x=0, the peak motion is xo
txtx o sin)( f2
txtv o cos)(
txta o sin)( 2
opeak xa 2
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• A typical shock graph showing
acceleration versus time. Shock
duration is Td
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• The basic accelerometer is a spring –mass
system
maxk
xm
ka
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• Spring-mass system exhibits a natural
oscillation with damping as a transient
response to an impulse input
m
kf N
2
1
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• A spring-mass accelerometer driven by a periodic
acceleration of varying frequency shows a peak
mass displacement or resonance at the natural
frequency tfeXX N
t
oT 2sin
tk
mxx o sin2
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A piezoelectric accelerometer
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Contoh
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FINISH
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Contoh
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