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Title: Sensors And Transducers
Description: All what u need to know about the relationship between sensors and transducers, and their applications.
Description: All what u need to know about the relationship between sensors and transducers, and their applications.
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CHAP 6:
SENSOR & TRANSDUCER
Objectives
n
n
n
Ability to understanding the definition,
functions & categories of transducers
...
Operations and applications for each
transducers
2
1
Sensor and transducer
n
n
‘Sensor' is `a device that detects a change in a physical
stimulus and turns it into a signal which can be measured or
recorded
...
g
...
E
...
Thermistor with it associate circuit convert heat to
electricity
...
All transducers
would thus contain a sensor and most (not all) sensors would
also be transducers
...
The majority either
convert electrical energy to mechanical
displacement or convert some
non-electrical
physical quantity, such as temperature, sound or
light to an electrical signal
...
To sense the presence, magnitude, change in, and frequency
of some measurand
...
To provide an electrical output that, when appropriately
processed and applied to readout device, gives accurate
quantitative data about the measurand
Measurand
Transducer
Electrical
output
Excitation
Measurand – refers to the quantity, property or condition which the
transducer translates to an electrical signal
...
A) Passive transducer:
- requires an external power
- output is a measure of some variation, such resistance and
capacitance
...
g
...
E
...
: Thermocouple
7
Selecting a transducers
1
...
3
...
5
...
7
...
Operating range
Sensitivity
Frequency response and resonant frequency
Environmental compatibility Minimum sensitivity measurand
...
Resistance Temperature Detectors (RTD)
2
...
Thermistor
4
...
R = R0 (1 + α ∆ T )
where;
R
R0
α
ΔT
= the resistance of the conductor at temperature t (0C)
= the resistance at the reference temperature, usually 200C
= the temperature coefficient of resistance
= the difference between the operating and the
reference temperature
11
Thermocouple
It consists of two wires of different metals are joined
together at one end, a temperature difference between
this end and the other end of wires produces a voltage
between the wires
...
12
6
Cont
...
Termistors have negative temperature coefficient
(NTC)
...
Types of thermistor
Disc
Washer
Rod
Resistance
1 to 1MΩ
1 to 50kΩ
high resistance
14
7
This figure shows
resistance versus
temperature for a
family thermistor
...
• Contact and lead resistance problems not
encountered due to large resistance
16
8
Limitations of thermistor
•
Non linearity in resistance vs temperature
characteristics
• Unsuitable for wide temperature range
• Very low excitation current to avoids self
heating
• Need of shielded power lines, filters, etc due to
high resistance
17
Comparison of Temperature
Transducers
18
9
Resistive Position Transducer
The principle of the resistance transducer is that the physical
variable under measurement causes a resistance change in the
sensing element
...
R
...
FIG 1 (a)
FIG 1 (b)
Figure shows the construction of a displacement transducer
uses a resistance element with a sliding contact or wiper
linked to the object being monitored
...
The
output voltage depends on the wiper position and therefore
is a function of the shaft position
20
10
Cont
...
854 x 10-12 F/m
= the plate placing in m
22
11
Types of Capacitance
Transducers
Rotary plate capacitor
Rectilinear Capacitance
Transducer
Thin diaphragm
23
Cont
...
This type of transducer will give sign
proportional to curvilinear displacement
or angular velocity
...
Rectilinear capacitance transducer:
It consists of a fixed cylinder and a moving
cylinder
...
25
Cont
...
The dielectric is either air or vacuum
...
26
13
Cont
...
Has excellent frequency response
2
...
Disadvantages:
1
...
the possibility of erratic or distortion signals owing to long
lead length
Applications:
1
...
In capacitance microphone
3
...
The self generating type utilises the basic electrical
generator principle, i
...
This relative motion between the field and the conductor
is supplied by changes in the measurand
...
Transducers of variable inductance type work upon
one of the following principles:
1
...
Variation of mutual inductance
28
14
Cont
...
The displacement
to be measured is arranged to cause variation in
any of three variables:
1
...
Geometric configuration
3
...
More specifically, strain is
defined as the fractional change in length
...
n
A strain gauge, a device whose electrical resistance
varies in proportion to the amount of strain in the
device
...
31
Cont
...
It is a very versatile detector
and transducer for measuring weight, pressure, mechanical
force, or displacement
...
A tensile stress tends to
elongate the wire and thereby increase its
length and decrease its cross-sectional area
...
The combined effect is an increase in resistance:
ρL
R=
A
Where,
ρ: the specific resistance of the conductor
material in ohm meters
L : length of conductor (meters)
A : area of conductor (m2)
As consequence of strain, 2 physical qualities are
particular interest:
• The change in gauge resistance
• The change in length
The relationship between these two variables called
gauge factor, K
...
Gauge factor:
K=
∆R / R
∆L / L
Where;
K = the gauge factor
R =the initial resistance in ohms (without strain)
∆R = the change in initial resistance in ohms
L = the initial length in meters (without strain)
∆L =the change in initial length in meters
Strain: ∆L/L = G, therefore
K=
∆R / R
G
34
17
Cont
...
S =
F
A
Where;
S= the stress in kilograms per square meter
F= the force in kilograms
A= area in square meters
Young’s modulus:
S
E =
G
Where,
E= Young modules in kg per square meter
S= the stress in kilograms per square meter
G= the strain (no units)
35
Cont
...
• Wire gauge
- to minimum leakage – for high T applications
• Semiconductor strain gauge
- high output transducers as load cells
36
18
Linear Variable Differential
Transformer (LVDT)
It consists a primary winding and two
secondary windings, wound over a
hollow tube and positioned as primary
winding is between two secondaries
...
The
voltage induced in the two secondaries is equal with core at
the center
...
37
Photoelectric Effect
A phenomenon in which electrons are emitted from
matter (metals and non-metallic solids, liquids or gases)
as a consequence of their absorption of energy from
electromagnetic radiation
...
Photoconductive the resistance of a material is
change when it’s illuminated
...
Examples of Photoelectric Transducer:
•
•
Photomultiplier Tube
Photoconductive Cells / Photocells
-
the electrical resistance of the materials varies with the
amount of light striking
...
40
20
Title: Sensors And Transducers
Description: All what u need to know about the relationship between sensors and transducers, and their applications.
Description: All what u need to know about the relationship between sensors and transducers, and their applications.