MEASURING DEVICES

1
CHAPTER 4

• MEASURING DEVICES
• (SENSORS TRANDUCERS)

2
Structure For Chapter 4
General

• Introduction
• Definition
• Transducer Classification
• Choosing a Transducer

• Part 1

• Part 2

• Passive Transducer
• LVDT
• Potentiometer
• Strain gauge
• Capacitive transducer

• Active Transducers
• RTD
• Thermistor
• Thermocouple

3
INTRODUCTION

• Component of Instrumentation System
• Physical Parameter Pressure, Temperature, Flow,
  Light Intensity, Sound, Position, Acceleration,
  Force, Strain
• Electrical Signal Current, Voltage
• Example

4
DEFINITION

• Transducer
• A device that converts energy in one form to
  energy in another.
• OR
• A device that converts non electrical quantity to
  electrical quantity.
• Sensor
• A device that detects, or senses, a signal or
  physical condition.
• Transducers that provide an electrical output are
  frequently used as sensors.

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TRANSDUCER CLASSIFICATION

• Passive transducer
• Require an external power.
• Their output is a measure of some variation
  such as resistance, inductive or capacitance.
• Ex LVDT, potentiometer, strain gauge and
  capacitive transducer
• Self-generating / active transducer
• Do not required an external power.
• Generate an electrical output when it detects the
  measurand.
• Ex RTD, thermistor and thermocouple.

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CHOOSING A TRANSDUCER

• In selecting a transducer in a system, there are
  several factors to be considered
• Operating range the transducers should maintain
  range requirements and good resolution.
• Sensitivity the transducers must be sensitive
  enough to allow sufficient output.
• Environmental compatibility ability to suite
  with the environmental condition such as vibrant,
  pressure..
• Accuracy high accuracy to produce sufficient
  output.

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Part 1 Passive Transducer
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LVDT

• (Linear variable Differential Transformer)-
  class passive, output voltage
• The device is used for measuring displacement and
  position.

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

Basic Construction of LVDT
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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

• 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

• An ac LVDT has the following data input 6.3V,
  output 5.2V, range 0.50 in. Determine
• 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)
• The output voltage when the core is -0.25 in.
  from center. (-2.6V)

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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)

• The output voltage under ideal condition

lT Shaft Stroke W Wiper
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POTENTIOMETER (contd)
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

• 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
• R2 (125?)
• Vo (3.75V)

A
B
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STRAIN GAUGE

• Strain gauge is a passive transducer that uses
  electrical resistance variation in wires to
  sense the strain produced by a force on the
  wires.
• Measurement for
• Weight
• Pressure
• Mechanical force
• Displacement
• 
  
• 
  The Stain Gauge

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STRAIN GAUGE

• Resistance is related to length, l(m) and area of
  cross-section of the resistor ,A(m2) and
  resistivity, ?(Om) of the material as
• __________________________________________________
  __

Next slide
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STRAIN GAUGE

• When external forces are applied to a stationary
  object, stress and strain are the result.
• Stress is defined as the object's internal
  resisting forces.
• The effect of the applied stress is produce a
  strain.

Where F ?Force A?Area
N/m2
Where ?L ?Change in length
L?Original unstressed length
Unit-less
Stress tekanan Strain regangan
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STRAIN GAUGE

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

Where s ?Stress e?Strain
N/m2
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STRAIN GAUGE

• 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

• 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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CAPACITOR TRANSDUCER
Consist of two parallel plates separated by an
air space or by dielectric (insulating material)
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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CAPACITOR TRANSDUCER (contd)
Variation in Capacitance
The value of capacitance is determined by (a)
the area of the plates (b) the distance between
the plates (c) the type of dielectric between
the plates
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CAPACITOR TRANSDUCER (contd)
?k
?d
?A
Capacitive displacement transducers with
variation in a)dielectric constant b)gap
between plates c)area of capacitor's plates
Where 1 and 2 capacitor's plates
3 dielectric.
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CAPACITOR TRANSDUCER (contd)
Variation in Capacitance
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EXAMPLE

• 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
• 0.0 cm (159.38pF)
• 3.0 cm (63.75pF)

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EXAMPLE
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END OF PART 1