Transcript Theory Of Oscilloscope

Sensors and
Transducers
1
Objectives
At the end of this chapter, the students should
be able to:

describe the principle of operation of
various sensors and transducers;
namely..
Resistive Position Transducers.
Capacitive Transducers
Inductive Transducers
2
Introduction

Sensors and transducers are classified
according to;
 the physical property that they use
(piezoelectric, photovoltaic, etc.)
 the function that they perform
(measurement of length, temperature, etc.).

Since energy conversion is an essential
characteristic of the sensing process, the various
forms of energy should be considered.
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Introduction

There are 3 basic types of transducers namely
self-generating, modulating, and modifying
transducers.
The self-generating type (thermocouples,
piezoelectric, photovoltaic) does not require the
application of external energy.
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Introduction

Modulating transducers (photoconductive
cells, thermistors, resistive displacement devices) do
require a source of energy.
For example, a thermocouple is self-generating,
producing a change in resistance in response to a
temperature difference, whereas a
photoconductive cell is modulating because it
requires energy.

The modifying transducer (elastic beams,
diaphragms) is characterized by the same form of
energy at the input and output. The energy form on
both sides of a modifier is electrical.
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Definition

The words 'sensor' and 'transducer' are
both widely used in the description of
measurement systems.

The former is popular in the USA
whereas the latter has been used in Europe
for many years. The word 'sensor' is derived
from entire meaning 'to perceive' and
'transducer' is from transducer meaning 'to
lead across'.
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Definition

A dictionary definition of 'sensor' is
`a device that detects a change in a physical
stimulus and turns it into a signal which can
be measured or recorded;

The corresponding definition of
'transducer' is 'a device that transfers energy
from one system to another in the same or in
the different form'.
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Features of Sensors
The desirable features of sensors are:
1. accuracy - closeness to "true" value of variable;
accuracy = actual value - sensed value;
2. precision - little or no random variability in
measured variable
3. operating range - wide operating range; accurate
and precise over entire sensing range
4. calibration - easy to calibrate; no "drift" - tendency
for sensor to lose accuracy over time.
5. reliability - no failures
6. cost and ease of operation - purchase price,
cost of installation and operation
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Sensors Types
A list of physical properties, and sensors to
measure them is given below:
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Sensors Types
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Common Sensors
Listed below are some examples of common
transducers and sensors that we may encounter:
 Ammeter - meter to indicate electrical current.
 Potentiometer - instrument used to measure
voltage.
 Strain Gage - used to indicate torque, force,
pressure, and other variables. Output is change in
resistance due to strain, which can be converted into
voltage.
 Thermistor - Also called a resistance thermometer;
an instrument used to measure temperature. The
operation is based on change in resistance as a
function of temperature.
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Sensors Types
• There are several transducers that will be
examined further in terms of their
principles of operations.
• Those include :
1.
2.
3.
4.
5.
Resistive Position Transducers
Strain Gauges
Capacitive Transducers
Inductive Transducers
And a lot more…
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Resistive Position Transducers
• The principle of the resistive position transducer
is that the measured quantity causes a resistance
change in the sensing element.
• A common requirement in industrial
measurement and control work is to be able to sense
the position of an object, or the distance it has
moved.
• One type of displacement transducer uses a
resistance element with a sliding contact linked to the
object being monitored.
• Thus the resistance between the slider and one
end of the resistance element depends on the
position of the object.
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Resistive Position Transducers
•
•
•
The output voltage depends on the wiper position and
therefore is a function of the shaft position.
In figure below, the output voltage Eout is a fraction
of ET, depending on the position of the wiper.
The element is considered perfectly linear if
the resistance of the transducer is distributed
uniformly along the length of travel of wiper.
Eout
R2

ET
R1  R2
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Resistive Position Transducers
Example 1
An RPT with a shaft stroke of 5.5 inches is applied in
the circuit as below. The total resistance of the
potentiometer is 4.7kΩ. The applied voltage is
ET= 3V.
When the wiper is 0.9 in. from B, what is Eout?
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Strain Gauges
•
•
The Strain Gauge is an example of a passive
transducer that uses electrical resistance
variation in wires to sense the strain produced by
a force on the wire.
It is a very versatile detector and transducer for
measuring weight, pressure, mechanical force or
displacement.
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Strain Gauges
The construction of a bonded strain gauge shows a
fine wire looped back and forth on a mounting plate,
which is usually cemented to the element that
undergoing stress.
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Strain Gauges
•
•
•
For many common materials, there is a constant
ratio between stress and strain.
Stress is defined as the internal force per unit
area.
F
S
A
S – Stress (kg/m2)
F – Force (kg)
A - Area (m2)
• The constant of proportionality between stress
and strain for the curve is known as the modulus of
elasticity of the materials, E or Young’s Modulus.
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Capacitive Transducers
•
The capacitance of a parallel plate is given
by:
k= dielectric constant
A= area of the plate
o
o=8.854x10-12 F/m
d= plate spacing
kA
C
d
•
Since the capacitance in inversely
proportional to the spacing of the parallel
plates, any variations in d will cause a
variation in capacitance.
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Capacitive Transducers
•
Some examples of capacitive transducers
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Capacitive Transducers
Example 2:
An electrode-diaphragm pressure transducer has
plates whose area is 5x10-3 m2 and distance
between plates is 1x10-3.
Calculate its capacitance if it measures air
pressure with k=1.
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Inductive Transducers
• Inductive Transducers may be either the selfgenerating or the passive type transducers.
• In the Self-Generating IT, it utilises the basic
electrical generator principle that when there is
relative motion between conductor and magnetic
field, a voltage is induced in the conductor.
• An example of this is Tachometer that directly
converts speeds or velocity into an electrical
signal.
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Tachometers
•
Examples of a Common Tachometer
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Linear Variable
Differential Transformer (LVDT)
•
Passive inductive transducers require an external
source of power.
•
The Differential transformer is a passive inductive
transformer, well known as Linear Variable
Differential Transformer (LVDT).
•
It consists basically of a primary winding and two
secondly windings, wound over a hollow tube and
positioned so that the primary is between two of
its secondaries.
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Linear Variable
Differential Transformer (LVDT)
•
Some examples of LVDTs.
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Linear Variable
Differential Transformer (LVDT)
•
An example of LVDT electrical wiring.
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Linear Variable
Differential Transformer (LVDT)
• An iron core slides within the tube and therefore
affects the magnetic coupling between the primary
and two secondaries.
• When the core is in the centre , the voltage
induced in the two secondaries is equal.
• When the core is moved in one direction of centre,
the voltage induced in one winding is increased and
that in the other is decreased. Movement in the
opposite direction reverse this effects.
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Linear Variable
Differential Transformer (LVDT)
•In next figure, the winding
is connected ‘series opposing’
-that is the polarities of V1
and V2 oppose each other
as we trace through the circuit
from terminal A to B.
•Consequently, when the core
is in the center so that V1=V2,
there is no voltage output,
Vo = 0V.
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Linear Variable
Differential Transformer (LVDT)
• When the core is away from S1, V1 is greater than
V2 and the output voltage will have the polarity of V1.
• When the core is away from S2, V2 is greater than
V1 and the output voltage will have the polarity of V2.
• That is the output of ac voltage inverts as the core
passes the center position.
• The farther the core moves from the centre, the
greater the difference in value between V1 and V2,
and consequently the greater the value of Vo.
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Linear Variable
Differential Transformer (LVDT)
• Thus, the amplitude of Vo is a function of distance
the core has moved. If the core is attached to a
moving object, the LVDT output voltage can be a
measure of the position of the object.
• The farther the core moves from the centre, the
greater the difference in value between V1 and V2,
and consequently the greater the value of Vo.
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Linear Variable
Differential Transformer (LVDT)
Among the advantages of LVDT are as follows:
•
•
•
•
It produces a higher output voltages for small
changes in core position.
Low cost
Solid and robust -capable of working in a wide
variety of environments.
No permanent damage to the LVDT if
measurements exceed the designed range.
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Linear Variable
Differential Transformer (LVDT)
Example 3:
An ac LVDT has the following data; input 6.3V,
output 5.2V, range ±0.50 cm. Determine:
a) Plot of output voltage versus core position for a
core movement going from +0.45cm to -0.03cm?
b) The output voltage when the core is -0.35cm from
the center?
c) The core movement from center when the output
voltage is -3V?
d) The plot of core position versus output voltages
varying from +4V to -2.5V.
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Piezoelectric Transducers
When a mechanical pressure is applied to a
crystal of a Rochelle salt, quartz, or tourmaline type, a
displacement of the crystals that will produce a
potential difference will occur.
•
• This property is used in piezoelectric transducers; where a
crystal is placed between a solid
base and force-summing element,
as shown below:
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Piezoelectric Transducers
• When externally force is applied to the plates, a
stress will be produced in the upper part of the crystal.
• This deformation will produce a potential
difference at the surface of the crystal. This produces
an electromotive force across the crystal proportional
to the magnitude of the applied pressure. This effect is
called piezoelectric effects.
•
• The induced charge on the crystal is proportional
to the impressed force and given by:
Q = dF;
where d = piezoelectric constant.
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Temperature Transducers
•
The temperature transducers can be divided
into four main categories:
o
o
o
o
Resistance Temperature Detectors (RTD)
Thermocouples
Thermistors
Ultrasonic transducers
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Resistance Temperature
Detectors (RTDs)
• Detectors of resistance temperatures
commonly employ platinum, nickel, or
resistance wire elements, whose resistance
variation with temperature has a high intrinsic
accuracy.
• They available in many configurations and
sizes and as shielded and open units for both
immersion and surface applications.
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Resistance Temperature
Detectors (RTDs)
•
Some examples of RTDs are as follows:
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Resistance Temperature
Detectors (RTDs)
• The relationship between temperature and
resistance of conductors can be calculated from
this equation:
R  Ro (1  T )
where;
R= resistance of the conductor at temp t (oC)
Ro=resistance at the reference temp.
= temperature coefficient of resistance
= difference between operating and reference
temp.
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Resistance Temperature
Detectors (RTDs)
Example:
A platinum resistance thermometer has a
resistance of 220Ω at 20oC. Calculate the
resistance at 50oC?
Given that 20oC=0.00392.
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Thermocouples
• A thermocouple is a sensor for measuring
temperature. It consists of two dissimilar / different
metals, joined together at one end, which produce a
small unique voltage at a given temperature. This
voltage is measured and interpreted by the
thermocouple.
•The magnitude of this voltage depends on the
materials used for the wires and the amount of
temperatures difference between the joined end and
the other ends.
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Thermocouples
• Some examples of the thermocouples are as
follows:
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Thermocouples
•
Common commercially available
thermocouples are specified by ISA
(Instrument Society of America) types.
• Type E, J, K, and T are base-metal
thermocouples and can be used up to about
1000°C (1832°F).
• Type S, R, and B are noble-metal
thermocouples and can be used up to about
2000°C (3632°F).
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Thermocouples
• The following table provides a summary of basic
thermocouple properties.
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Thermocouples
•Calibration curves for several commercially
available thermocouples is as below:
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Thermocouples
• The magnitude of thermal emf depends on the
wire materials used and on the temperature difference
between the junctions.
• The effective emf of the thermocouple is given as:
E  c(T1  T2 )  k (T  T )
2
1
2
2
•Where;
c and k – constant of the thermocouple materials
T1
- temperature of the ‘hot’ junction.
T2
- temperature of the ‘cold’ or
‘reference’ junction.
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Thermocouples
Example
During experiment with a copper- costantan
thermocouple, it was found that
c= 3.75x10-2 mV/oC and k = 4.50x10-5 mV/oC.
If T1= 100oC and the cold junction T2 is kept
in the ice, compute the resultant electromotive
force, emf?
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