MEASURING DEVICES

1
CHAPTER 4

• MEASURING DEVICES
• (SENSOR TRANSDUCER)

2
OUTLINE

• Introduction
• What is sensor and transducer?
• Selecting Transducer
• Types of transducer
• Passive Transducer
• Self Generating Transducer

3
INTRODUCTION

• For many years, a transducer is a source of
  information.
• The operation of the transducer defines the
  reliability of the information.
• In spite of a wide variety of different systems
  containing transducer, they can be divided into
  two big groups i.e measuring system and control
  system.

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INTRODUCTION CONTD

• Component of instrumentation system

Electrical Signal
Sensor / Transducer
Physical Parameters

• Current
• Voltage

• Pressure
• Temperature
• Flow
• Light Intensity
• Sound
• Position
• Acceleration
• Force
• Strain

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WHAT IS SENSOR?

• Sensor is a device that detects, or senses, a
  signal or physical condition.
• Most sensors are electrical or electronic,
  although other types exist.
• A sensor is a type of transducer.
• Sensors are either direct indicating (e.g. a
  mercury thermometer or electrical meter) or are
  paired with an indicator (perhaps indirectly
  through an analog to digital converter, a
  computer and a display) so that the value sensed
  becomes human readable. Aside from other
  applications, sensors are heavily used in
  medicine, industry and robotics.

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WHAT IS TRANSDUCER?

• Transducer is a device that provides a usable
  output in response to a specific measured.
• In other word, transducer is a device that
  converts energy in one form to energy in another.
• Transducer that provide an electrical output are
  frequently used as sensors.
• The transducer is the most important portion of
  the sensor, in fact some sensor are merely
  transducer with packaging

7
SELECTING TRANSDUCER

• There are four factors to be considered in
  selecting a transducer in a system
• Operating range
• The transducer should maintain range requirements
  and good resolution
• Sensitivity
• The transducer must be sensitive enough to allow
  sufficient output

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SELECTING TRANSDUCER CONTD

• Ability to suite with the environment condition
  such as pressure
• Do the temperature range of the transducer, its
  corrosive fluids, the pressures, shocks, and
  interactions it is subject to, its size and
  mounting restrictions make it in application
• High accuracy to produce sufficient output
• The transducer may be subject to repeatability
  and calibration errors as well as errors expected
  owing to sensitivity to other stimuli

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TYPES OF TRANSDUCER

• Transducer can be classified into two types
• (i) Passive Transducer
• (ii) Self-Generating Transducer
• (Active)

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PASSIVE TRANSDUCER

• Require an external power and their output is a
  measure of some variation such as resistance or
  capacitance
• Examples
• LVDT
• POTENTIOMETER
• STRAIN GAUGE
• CAPACITIVE TRANSDUCER

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LVDT

• LVDT (Linear Variable Differential Transformer)
• The linear variable differential transducer
  (LVDT) is a type of electrical transformer used
  for measuring linear displacement
• The transformer has three solenoid coils placed
  end-to-end around a tube.
• The centre coil is the primary, and the two outer
  coils are the secondary.
• A cylindrical ferromagnetic core, attached to the
  object whose position is to be measured, slides
  along the axis of the tube.

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LVDT CONTD
A reliable and accurate sensing device that
converts linear position or motion to a
proportional electrical output.
13
LVDT CONTD

• Basic construction of LVDT as shown in figure
  below

• LVDT consists of
• a transformer with a single
• primary winding
• two secondary windings
• connected in the series-
• opposing manner
• (berlawanan arah)

Figure 1
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LVDT CONTD

• VOUT VA VB
• The core displacement determine the output
• If the core at the center, VAVB, VOUT0
• Core at the upper A
• VA max, VB min ? VOUT max ve
• Core at the lower B
• VA min, VB max ? VOUT max -ve

Relationship between displacement and output
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EXAMPLE 1

• LVDT has the following data
• Vin 6.3V, Vout 5.2V
• displacement range 0.5 in.
• Calculate the displacement when Vo is 2.6V.

5.2 V
2.6V
0.5
?
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EXAMPLE 2

• An ac LVDT has the following data input 6.3V,
  output 5.2V, range 0.50 in. Determine
• a) The plot of the output voltage versus core
  position
• for a core movement going from 0.45 in to
  -0.03 in.( 4.68V, -3.12V)
• b) The output voltage when the core is -0.25
  in. from
• center. (-2.6V)

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LVDT CONTD

• Applications of LVDT
• Used for measuring displacement and position
• Used as null detectors in feedback positioning
  systems in airplanes and submarines
• Used in machine tools as an input system

Example Measuring position
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POTENTIOMETER

• A potentiometer is a variable resistor that
  functions as a voltage divider
• Electromechanical device containing a resistance
  that is contacted by movable slider.
• Motion of the slider results in a resistance
  change depending on the manner in which the
  resistance wire is wound.

lT Shaft Stroke W Wiper
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POTENTIOMETER CONTD

• There are various type of potentiometer
• Low Power Types
• Liner potentiometers
• Logarithmic potentiometers
• High Power Types
• Rheostat
• Digital Control
• Digitally controlled potentiometers (DCP)

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POTENTIOMETER CONTD

• The output voltage under ideal condition

lT Shaft Stroke W Wiper
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POTENTIOMETER CONTD

• Theory of operation

The potentiometer can be used as a potential
divider (or voltage divider) to obtain a manually
adjustable output voltage at the slider (wiper)
from a fixed input voltage applied across the two
ends of the pot. This is the most common use of
pots
The voltage across RL is determined by the
formula
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EXAMPLE 3

• A resistive positive displacement transducer
  with a shaft stroke of 10cm is used in the
  circuit of figure below. The total resistance of
  potentiometer is 500? and the applied voltage Vi
  is 15V. If the wiper, W is 7.5cm from A, what is
  the value of
• (a) R2 (125?)
• (b) Vo (3.75V)

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POTENTIOMETER CONTD
Transducers Potentiometers are widely used as a
part of displacement transducers because of the
simplicity of construction and because they can
give a large output signal
Audio control One of the most common uses for
modern low-power potentiometers is as audio
control devices. Both sliding pots( known as
faders) and rotary potentiometer ( called knob)
are regularly used to adjust loudness, frequency
attenuation and other characteristics audio
signals
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STRAIN GAUGE

• A strain gauge is a metal or semiconductor
  element whose resistance changes when under
  strain.
• Strain gauge is a passive transducer that uses
  electrical resistance variation in wires to
  sense the strain produced by a force on the
  wires.
• It can measures
• Weight
• Pressure
• Mechanical Force
• Displacement

STRAIN GAUGE
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STRAIN GAUGE CONTD

• The function of strain gauge is to sense the
  strain produces by force on the wires.
• The strain gauge is generally uses as an arm of a
  bridge. This is only applicable when temperature
  variation in wire.
• Types of strain gauges

Wire gauge
Foil gauge
Semiconductor gauge
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STRAIN GAUGE CONTD

• Considering the factors that influence the
  resistance of the element a relationship between
  changes in resistance and strain can be derived.
• Resistance is related to length, l(m) and area of
  cross-section of the resistor ,A(m2) and
  resistivity, ?(Om) of the material as

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STRAIN GAUGE CONTD

• When external force are applied to a stationary
  object, stress and strain are the result.
• Stress is defined as the objects internal
  forces.
• For a uniform distribution of internal resisting
  forces, stress can be calculated by dividing the
  applied force (F) by the unit area (A)

N/m2
Where F? Force A? Area
Stress tekanan
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STRAIN GAUGE CONTD

• The effect of the applied stress is produce a
  strain.
• Strain is a fractional change (?L/L) in the
  dimensions of an object as a result of
  mechanical stress (force/area).
• Calculated by dividing the total deformation of
  the original length by the original length (L).

Unit-less
Where ?L? Change in length L?
Original unstressed length
Strain regangan
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STRAIN GAUGE CONTD

• The constant of proportionality between stress
  and strain for a linear stress-strain curve is
  known as Youngs Modulus, E.

? Youngs modulus in kilograms per-square meter
? The stress in kilograms per square meter
? The strain (no units)
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STRAIN GAUGE CONTD

• This changes its resistance (R) in proportion to
  the strain sensitivity of the wire's resistance.
  When a strain is introduced, the strain
  sensitivity, which is also called the Gauge
  Factor (GF), is given by

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EXAMPLE 4

• A resistant strain gauge with a gauge factor of
  2 is fastened to a steel member, which is
  subjected to strain of 1x10-6. If the original
  resistance value of the gauge is 130?, calculate
  the change in resistance. (260µ?)

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SOLUTION
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CAPACITIVE TRANSDUCER

• The capacitor consists of two parallel plates
  separated by an air space or by a dielectric
  (insulating material).
• The capacitance of the of the pair of the plates
  is measure of the amount of charge that can be
  transferred before a certain voltage is reached.

Plate 1
Dielectric material
Plate 2
The basic construction of capacitor
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CAPACITIVE TRANSDUCER CONTD
Schematic diagram of parallel-plate capacitor
k dielectric constant of the material in the
gap eo the permittivity of free space
8.854 x 10-12 farad/meter A Plate area (m2) d
the separation between plate (m)
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CAPACITIVE TRANSDUCER CONTD

• There are three criteria/conditions that can
  change the capacitor (variation of capacitance)
• (a) Changing the surface area
• (b) Changing the dielectric constant
• (c) Changing the spacing between plate

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CAPACITIVE TRANSDUCER CONTD

• (a) Changing the surface area

If one plate of the parallel plate capacitor is
displayed in a direction parallel to the plate,
the effective area of the plates will change
proportionally to the value of capacitance
C
Plate 1
Dielectric material
Plate 2
A
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CAPACITIVE TRANSDUCER CONTD

• (b) Changing the dielectric constant

The value of capacitance will increase when the
dielectric constant is increased
C
Plate 1
Dielectric material
Plate 2
k
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CAPACITIVE TRANSDUCER CONTD

• (c) Changing the spacing between plate

The value of capacitance will decrease when the
spacing between plate increased
C
Plate 1
Dielectric material
d
Plate 2
d
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EXAMPLE 5

• eo 8.854 x 10-12 Fm-1, kair 1, kmaterial
  5
• Two square metal plates, side 6 cm separated by
  a gap of 1 mm.
• Calculate the capacitance of the sensor when the
  input displacement of x is
• (a) 0.0 cm (159.38pF)
• (b) 3.0 cm (63.75pF)

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SOLUTION