Emm 3242 Transduser (Week 4)

EMM 3242

Transducers

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.

Classification of Transducers[CONTENTS]

Prinsip-prinsip Transduser

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.

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

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

Capacitive SensorsOther Configurations

c. Differential Mode

b. Variable Dielectric Mode

a. Variable Area Mode

Contoh

Contoh

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 --

Contoh

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)

Introduction

• Cheap, easy to use

way of measuring

deflection

• Measure deflections

from 12 in. to

millimeters

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

• When core slides through transformer, a certain

number of coils are affected

• This generates a unique voltage

Description

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.

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

Interpretation

• Voltage can be interpreted using formula

stated above

• V = Voltage and N = Number of coils

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

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

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

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

Contoh

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)

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

Piezoelectric Sensors

Above equations are valid when force is applied in the

L,W or t directions respectively.

31 denotes the

crystal axis

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

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

• Thermistors

– use materials with a high thermal coefficient

of resistance

– sensitive but highly non-linear

A typical disc thermistor A threaded

thermistor

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.

2-wire RTD 3-wire RTD

Contoh (Gunakan Konsep Titi Wheatstone)

Mechanical transducers : Potentiometric

CONTENTS]

Anjakan sudut

Transduser Termoelektrik

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

Contoh

Transduser Photoelektrik

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

PHOTOCELL

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

The photoconductive cell

structure

Contoh

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

Sistem sel suria

CONT..

• 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

The photodiode uses the pn junction reverse current to measure radiation

Soalan

Penyelesaian:

Jadi:

Foto Transistor

• Bekerja berasaskan

hubungan arus elektrik-

cahaya.

• Mempunyai 3 terminal spt

transistor biasa.

• Lebih peka dan arus

keluaran lebih tinggi

berbanding fotodiod.

Fototransistor

Litar untuk fototransistor

Soalan

Penyelesaian

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 ----------------

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

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

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

Meter Pecutan (Accelerometer)

• Digunakan untuk mengukur pecutan.Juga

boleh digunakan untuk mengukur halaju

dan anjakan.

Jenis Meter pecutan

• Potentiometric

• LVDT

• Keengganan boleh ubah (variable

reluctance)

• Piezoelectric

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

• A typical shock graph showing

acceleration versus time. Shock

duration is Td

• The basic accelerometer is a spring –mass

system

maxk

xm

ka

• Spring-mass system exhibits a natural

oscillation with damping as a transient

response to an impulse input

m

kf N

2

1

• 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

A piezoelectric accelerometer

Contoh

FINISH

Contoh