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					                                                               LDV 2008-1-TR-LEO05-03191




                       Chapter 6
                       INTRODUCTION TO SENSORS
                       AND TRANSDUCERS

Virtual Laboratory for Automation and Robotics
     A Transfer of Innovation Project funded by EU Commision
                                                                       LDV 2008-1-TR-LEO05-03191




Introduction

People percieve the changes in their surrounding via their sense organs and
move accordingly. If we give examples about this, we turn on the heater when
we get cold or we turn on the light when the environment gets dark. We
sometimes think that it would be nice if there were devices to perform these
works.
All these devices sensing physical environment changes (light, heat, pressure,
noise, etc.) on our behalf are called “sensors” and devices those converting
these information to electrical energy are called “transducers”. Data taken from
sensors after being converted to electrical signals are interpreted by electronic
circuits and control mechanical tools. By this way we are able to facilitate both
our daily lives and also industrial production processes. We, in this module will
come familiar with main sensors and transducers and recognize their area of
use.In fact, it is a little bit hard to allocate sensors and transducers ultimately.
Namely; the device percieving microphone sound is a sensor. On the other
hand, it is a transducer since it converts sound waves to electrical currencies
via a coil inside. For that reason we can accept these two words as equivalent.
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Sensor Terminology

Sensitivity
Sensitivity of sensors defined as outlet characteristic curve or more general an
accessible outlet variation being constituted with the smallest inlet of physical
parameters.

Sensitivity Fault
This fault occurs by the distinction of characteristic curves of faulty sensors
from ideal curve.

Value
Sensor’s values are the maximum and minimum values of applied measured
parameters.

Dynamic Values
These values are the total values between minimum and maximum.
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Sensor Terminology

Sensitivity
The term sensitivity is related with the productivity degree of measurement’s
reproducibility. If really sensor outlet signal remains the same when same values are
measured for a few times this is an ideal sensor.
Stability
This characteristic is the feature of detecting the smallest change (instrumental changes
are included) especially in inlet parameter in the outlet signal.
Accuracy
The accuracy for sensors is the difference between real values available in the system and
values founded as a result of measurement. This difference being less increases the
accuracy.
Offset
The offset faults of transducers is defined as the current outlet difference signal to be
zero.
Response Time
Sensors do not instantly change outlet status when inlet parameters change. Outlet, that
means outlet data changes a while after and this time is called the response time.
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Types
When we need to operate a mechanical machine or electronic circuit for the
reason of physical change in the environment we use sensors.

We can outline types of sensors as follows:

-Heat Transducer and Sensors
-Magnetic Transducer and Sensors
-Pressure (stress) Transducers
-Optical Transducer and Sensors
-Sound Transducer and sensors


Another issue that we must specify here is the classification of sensors as active
and passive sensors. Passive sensors are the elements requiring external energy
whil operating. Active sensors are the elements those not requiring any
external energy for operation.
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Various Sensor Applications
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HEAT SENSORS AND TRANSDUCERS
We call heat or temperature sensors for the devices which assist us for sensing
heat changes in the environment.

PTC

It is the circuit element increasing its electrical resistance as long as the
temperature of environment or surface it contacts increase.
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NTC

It is the circuit element decreasing its electrical resistance as long as the heat of
environment or surface it touches increase.
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Thermocouple

A movement occurs with the electrons inside all conductors when they are
heated. However this movement differs among various conductors. This is one
of the distinctive characteristics of material. We can be able to measure
temperature by beneting from the difference of conductors. When two edges of
two different conductor are welded or tightly connected to themselves and a
sensitive voltmeter is connected to its empty edges, if we heat the edge which
we have connected we obtain a DA voltage in mV’s level on voltmeter. The
value of voltage we obtained is proportional with the response given to heat by
metals we use.
Moreover, since thermocouples produce voltage they are active transducers.
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MAGNETIC SENSORS AND TRANSDUCERS
If we convert a wire to a coil and pass currency inside a magnetic field forms
inside and surrounding of this coil. This magnetic field is not visible. However
the movement of core inside the coil and metals approached around the coil
changes the inductance of metal coil. Magnetic sensors were devleoped by
moving ahead from this principle.
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PRESSURE (STRESS) TRANSDUCERS

The elements sensing every kind of physical power and pressure change and
converts this electrical change to signals are called pressure sensors.

Types
Pressure sensors can be investigated under four groups according to their
working principles. These are:

-Capacitive pressure measurement sensors
-Strain gage (deformation) sensors
-Load cell pressure sensors
-Pressure measurement sensors with piezoelectric
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OPTICAL TRANSDUCERS AND SENSORS
Electronic circuit elements operating with the effect of light are generally called
optical elements.

Photoresistance (LDR)

Some materials such as calcium sulphate and cadmium selenoid shows
resistance change with inversely proportional to light reflecting on them.
Circuit elements called photoresistance are made by benefiting from these
materials.
Circuit elements decreasing their resistance when light reflected on them and
showing high resistance in dark are called photoresistance.
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Photodiode

Photodiodes are diode being conductors to reverse direction with the effect of
light. They are used under reverse polarization.
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Photo transistor

These are the elements on which resistance values decrease between collector-
emitter terminals when light reflect on them. They are generally used as
switching elements for controlling of a load with light.
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Optocoupler

Elements obtained by combining luminous element and photosensor within the
same body are called optocoupler. “LED” as luminous element and
“photodiode”, “phototransistor”, “phototristor” “phototriac” etc elements
when using “Infrared LED” are used as light sensors for these elements.
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SOUND SENSORS AND TRANSDUCER

Microphone

The sound from mouth or sound broadcasted with any means creates pressure
change in air and this pressure change as similar to wave transmission created
by the stone dropped to water spreads as a wave transmission on air. Sound in
fact is a pressure change in air pressure.Microphones are tools sensing sound
waves spreading on air and converting them to electrical energy.

Types
-Dynamic microphones
-Capacitive microphones
-Microphones with strips
-Crystal microphones
-Microphones with carbon dust
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SOUND SENSORS AND TRANSDUCER

Loudspeaker
Elements converting electrical signals to sound singals high enough to be heard
by human ear are called “loudspeaker”.

Types
-Dynamic Loudspeaker
-Piezoelectric (With crystal) Loudspeaker
-Electrostatic Loudspeaker
-Electromagnetic Loudspeaker
-Loudspeaker with Ion

Despite loudspeakers have many types mostly used loudspeakers are dynamic
loudspeakers and loudspeakers with crystal.
LDV 2008-1-TR-LEO05-03191
                                          LDV 2008-1-TR-LEO05-03191




Sensor and Transducer Application Areas

Temperature Measurement and Control
Pressure Measurement and Control
Flow Measurement and Control
Weight Measurement and Control
Level Measurement and Control
Motor Control and Servo Systems

Numerical Control Systems
-Point-To-Point Control
-Contour Control

Programmable Controllers (PC)

				
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posted:1/2/2013
language:Latin
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