Lecture note on transducers

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.  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.
INTRODUCTION

4.  Component of instrumentation system
INTRODUCTION CONT’D
Sensor /
Transducer
Physical
Parameters
Electrical
Signal
 Pressure
Temperature
Flow
Light Intensity
Sound
Position
Acceleration
Force
Strain
 Current
Voltage

5.  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.
WHAT IS SENSOR?

6.  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
WHAT IS TRANSDUCER?

7.  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
SELECTING TRANSDUCER

8.  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
SELECTING TRANSDUCER
CONT’D

9. Transducer can be classified into
two types:
(i) Passive Transducer
(ii) Self-Generating Transducer
(Active)
TYPES OF TRANSDUCER

10.  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
PASSIVE TRANSDUCER

11.  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.
LVDT

12. LVDT CONT’D
A reliable and accurate sensing
device that converts linear
position or motion to a
proportional electrical output.

13.  Basic construction of LVDT as shown in figure below:
LVDT CONT’D
Primary Secondary
A
B
A
B
Displacement/
Figure 1
LVDT consists of :
• a transformer with a single
primary winding
• two secondary windings
connected in the series-
opposing manner
(berlawanan arah)

14. LVDT CONT’D
Primary Secondary
A
B
A
B
Displacemen
t/
Cor
e
Vo
ut
Core
positio
n
Relationship between
displacement and output
VOUT = VA – VB
 The core displacement determine
the output:
 If the core at the center,
VA=VB, VOUT=0
 Core at the ‘upper’A
VA max, VB min  VOUT
max & +ve
 Core at the ‘lower’ B
VA min, VB max  VOUT
max & -ve

15.  LVDT has the following data:
Vin= 6.3V, Vout= + 5.2V &
displacement range = + 0.5 in.
Calculate the displacement when Vo is +2.6V.
EXAMPLE 1
+5.2 V
+2.6V
0.5”
?
Vout
Core position

16. 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)
EXAMPLE 2

17.  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
LVDT CONT’D
Example: Measuring position

18. 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.
VO
W
R1
R2
Vi
ℓ1
ℓ2
ℓT
RT
ℓT = Shaft Stroke
W = Wiper

19.  There are various type of potentiometer:
 Low Power Types:
 Liner potentiometers
 Logarithmic potentiometers
 High Power Types:
 Rheostat
 Digital Control:
 Digitally controlled potentiometers (DCP)
POTENTIOMETER CONT’D

20.  The output voltage under ideal condition:
POTENTIOMETER CONT’D
Vi
Vo 








T
2
R
inal,
input term
at the
Resistance
R
,
minal
output ter
at the
Resistance
T
T
R
R 









1
1
T
T
R
R 









2
2
ℓT = Shaft Stroke
W = Wiper
VO
W
R1
R2
Vi
ℓ1
ℓ2
ℓT
RT

21. POTENTIOMETER CONT’D
 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:
s
L
L
L V
R
R
R
R
R
V .
||
||
2
1
2



22. 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 500Ω and the applied
voltage Vi is 15V. If the wiper, W is 7.5cm from A, what
is the value of
(a) R2 (125Ω)
(b) Vo (3.75V)

23. POTENTIOMETER CONT’D
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

24.  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
STRAIN GAUGE

25.  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:
STRAIN GAUGE CONT’D
Wire gauge Foil gauge Semiconductor gauge

26.  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, ρ(Ωm) of the material as
STRAIN GAUGE CONT’D

27. STRAIN GAUGE CONT’D
 When external force are
applied to a stationary object,
stress and strain are the result.
 Stress is defined as the object’s
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): A
F


Where; F Force
A Area
N/m2
*Stress – tekanan

28. STRAIN GAUGE CONT’D
 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).
L
L



Where; ∆L Change in length
L Original unstressed length
Unit-less
*Strain – regangan

29. STRAIN GAUGE CONT’D
 The constant of proportionality between stress
and strain for a linear stress-strain curve is
known as Young’s Modulus, E.



E
E


 Young’s modulus in kilograms per-square meter
 The stress in kilograms per square meter
 The strain (no units)

30. STRAIN GAUGE CONT’D
 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:
 

R
R
GF


L
L



L
L
R
R
GF


= gauge factor (unit less)
= the initial resistance in ohms (without strain)
= the change in initial resistance in ohms
= the initial length in meters (without strain)
= the change in initial length in meters
= gauge factor (unit less)
= the initial resistance in ohms (without strain)
= the change in initial resistance in ohms
= the initial length in meters (without strain)
= the change in initial length in meters
= gauge factor (unit less)
= the initial resistance in ohms (without strain)
= the change in initial resistance in ohms
= the initial length in meters (without strain)
= the change in initial length in meters

31. 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 130Ω,
calculate the change in resistance. (260µΩ)
EXAMPLE 4

32. SOLUTION

33.  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.
CAPACITIVE
TRANSDUCER
Plate 1
Plate 2
Dielectric
material
The basic construction of capacitor

34. CAPACITIVE TRANSDUCER CONT”D
d
kA
C o


k = dielectric constant of the material in the gap
εo = the permittivity of free space
= 8.854 x 10-12 farad/meter
A = Plate area (m2)
d = the separation between plate (m)
Plate 1
Plate 2
d
d
width
Length
Schematic diagram
of parallel-plate
capacitor

35. CAPACITIVE TRANSDUCER CONT”D
 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
x
Displacement
x=0
d
kA
C o



36. (a) Changing the surface area
CAPACITIVE TRANSDUCER CONT”D
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
Plate 1
Plate 2
Dielectric
material
C
A

37. CAPACITIVE TRANSDUCER CONT”D
(b) Changing the dielectric constant
The value of capacitance will increase when the dielectric
constant is increased
Plate 1
Plate 2
Dielectric
material
C
k

38. CAPACITIVE TRANSDUCER CONT”D
(c) Changing the spacing
between plate
The value of capacitance will decrease when the spacing
between plate increased
Plate 1
Plate 2
Dielectric
material
d
C
d

39. εo = 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)
EXAMPLE 5

40. SOLUTION