Transcript Module-2 - SNGCE DIGITAL LIBRARY

UNIT 2
Transducers
Transducer
• Devices used to transform one kind of energy to
another.
• Advantages of electrical transducers
Easy amplification, small power requirements,
easy data transmission & recording
• Disadvantages
Low reliability and high cost
• Requirement of a transducer
Linearity , Reproducibility , High reliability &
sensitivity, Good dynamic response, high SNR,
No hysteresis & ruggedness
TRANSDUCER
• ACTIVE
Self generating transducers or transducers
which develop their output in the form of
voltage or current without any auxiliary
source.eg: thermocouple, piezo electric
transducers
• PASSIVE
Transducers in which electrical parameters R,
L ,C changes with changes in input signal. It
requires power supply for its operation .eg:
resistive, capacitive, inductive trasducer
JOBY JOHN
3
TRANDUCER
• ANALOG
Transducer which converts input signal into an analog
output . Mainly divided into Electromechanical ,
opto electric Type
Eg: thermistor, LVDT, stain guages
• DIGITAL
Transducer which converts input signal into discrete
output /pulses .
2 types a) Frequency generating b) Digital encoders
Eg: digital encoders, optical encoders,
electromagnetic transducers
Electro Mechanical
Transducers
Input(Physical Variable) is
converted to Mechanical
Displacement Or Strain,
using Primary Sensors.
Then , it is converted to
Electrical Output with the
help of electromechanical
Transducers.
Electro Mechanical Transducer
Potentiometric Transducers
• Based on the linear dependence between total
impedance
and conductor length.
• Several technologies: wire bound, conductive
plastic, mixed,…
• Both for linear and angular displacement
measurements
• R(l) = (l/L) R0
• Angular and rotary Sensor, Linear Translation
Sensor
Electro Mechanical Transducer
• Potentiometers
Translational Type
(passive)
Rotational
Type
• Can be used to measure position, displacement &
level
• Advantages: Suitable for large amplitude of
displacement ,Electrical efficiency is high, inexpensive
• Disadvantages: Large force is required to move sliding
contacts and it also produce noise
Electro Mechanical Transducer
Inductive Transducers ( mainly for displacement)
• Two Types : Self Generating Type (Electrical Generator
Principle) Or Passive Type ( Change in Inductance)
• Active: Motion of conductor in a magnetic field
produces a voltage in the conductor
• Passive :Variation in Self Inductance and Variation in
Mutual Inductance
Implemented by 1.Number of lines
2. Geometric Configuration
3.Permeability of magnetic Material
• L= e/(di/dt) = N2/R= N2µA/l
a) Number of Turns
• Angular Inductive
input
output
• Linear Inductive
• As N Changes , L and
output Changes
b) Change in Permeability
• Iron core is surrounded by
the winding. When the iron
core is inside the winding
,its permeability is
increased ,hence the
inductance. When the iron
core is moved out of the
winding, permeability
decreases, resulting in
reduction in self inductance
of the coil
c)Variable Reluctance Type Transducer
• Coil is wound on a
ferromagnetic core.
Displacement to be measured
is applied to ferromagnetic
target ( No Physical contact
with core, but separated by an
air gap ).
• Reluctance of the magnetic
path is determined by the Size
of the air Gap.
• Self Inductance of the coil
L=N2/Ri+Rg, where
N - Number of turns
Ri - Reluctance of iron parts
Rg - Reluctance of air gap
c)Variable Reluctance Type Transducer
• L= N2/Rg , since Ri is negligible
• Rg = lg/µ0* Ag where lg – length of air gap, µ0 –
Permeability,
Ag – Area of the flux path
through air . Here µ0 and Ag are constants. Rg is
proportional to lg
• When the target is near the core, length is small
and hence self inductance is large , but when the
target is away from the core, reluctance is large
,resulting in smaller value of inductance
Linear Variable Differential Transducers
• Passive Inductive transducer Operation:
widely used for translating a • When core is in its null
position , equal voltages are
linear motion into an
induced in the two secondary
electrical signal
windings Es1 = Es2 . Hence E0 =
0.(ie,E = Es1- Es2 ).
• If the core is moved to the left
of the null position, E0= Es1Es2, in phase with Es1.
(ie,E0 =+ve, ø = 00 )
• If the core is moved to the
right of the null position,
E0=Es2-Es1, in phase with Es2
( ie,E0 = -ve, ø =1800 )
Linear Variable Differential Transducers
• Construction: Single
primary winding
P1(connected to AC Source)
and two secondary winding
S1 & S2 (equal No. of turns
and identically placed on
either sides of the primary
winding) wound on a
hollow cylindrical former. A
movable soft iron ( Nickel –
Iron alloy, movement is
attached to it) core slides
within the former and
affect the magnetic
coupling between primary
and two secondaries.
Linear Variable Differential Transducers
Various Core Positions and variation of output voltage & phase with displacement
Linear Variable Differential Transducers
Advantages
• Higher output volt for small
changes in core position(50300mv/mm)
• Output voltage is linear upto
5mm displacement
• Infinite resolution & High
sensitivity
• Low Power consumption (less
than 1 W) and low Hysteresis
• Less friction
Disadvantages
• Sensitive to stray magnetic
field.
• Temperature affect transducer
operation.
• Demodulator network must be
used if a dc output is required
Capacitive transducers
• Changes in position is
converted to changes in the
capacitance
• Capacitance C = KA/d
K- dielectric constant
A- Area of the capacitor Plates
D- distance between the plates
• Capacitance increases if
1.Effective area of the plate is
increased
2.If the material has high K value
3.If the distance between the
plates is reduced
A)Variable Distance
Type (pressure microphone)
• Here diaphragm is
deflected with respect
to the applied
pressure leads to
change in distance
between the plates,
hence the capacitance
Capacitive transducers
b) Variable Area Type
• Here one plate is kept
fixed called stator, and
the movable plate is
called rotor, to which
movement is
connected.
• Change in position of
the rotor with respect
to the stator causes
change effective area ,
hence capacitance
Capacitive transducers
C)Differential Capacitive Transducer
• A movable plate is kept in midway
between the two fixed plates.
Then C1= C2, and E1=E2 = E/2
Differential output = 0.
• Let the movable plate displaced x
in the upward direction, then C1=
KA/(d-x) & C2=KA/(d+x)
Then E1= C2E/(C1+C2) =(d-x)E/2d
and E2= C1E/(C1+C2) =(d+x)E/2d
Differential output = ± xE/d.
Capacitive transducers
D)Variable Dielectric Type
Initially C= Kwl1/d+ KKrwl2/d
• Let the Dielectric is moved
through a distance x, then
change in capacitance
∆C= Kw x(Kr-1)/d
Uses of Capacitive Transducers
• Can be used for the
measurement of presure,
force, Humidity etc
Advantages
• Good Frequency Response
• High Input Impedance
• Highly sensitive &
Operational with small
force
• Good resolution
Disadvantages
• Nonlinear behaviour due
to edge effect
• Temperature variation
,dust particle in moisture
affect the performance
Resistive Traducers
Resistance Temperature Detector(RTDs)
• Based on the temperature dependence of resistivity of all
metals and alloys.
• Although virtually all metals can be employed, platinum is
used almost exclusively: predictable response, long-term
stability and durability ,ultimate in accuracy . Other RTDs
are Nickel, Copper etc
• All RTD’s have positive temperature coefficients
Advantages
• Linearity over wide Temperature Range
• Better Stability at higher temperatures
Disadvantages
• Low sensitivity & high cost
• Can be affected by contact resistance, shock, acceleration
RESISTIVE TRANDUCERS
• Rt- Resistance of a conductor
at temperature t0C
• R0- Resistance of a conductor
at temperature 00C
• à- Temperature coefficient of
resistance
• Then
• Rt= R0(1+à∆t)
RESISTIVE TRANDUCERS
• Thermistor
• Thermo resistive sensors but fabricated with metal-oxide materials
that behave like semiconductors
• They usually present a Negative temperature coefficient (NTC),
although PTC (Positive temperature coefficient) ,thermistors are also
available.
• Low-accuracy and low stability sensors, but low cost, they present
an exponential-like dependence on temperature.
• Thermistors are available in the form of beads, disc,washer and rod
• Merits: Fast response, Good sensitivity in NTC region , Small size
,low Cost.
• Demerits: Non-linear behaviour, unsuitable for wide range, Self
heating
RESISTIVE TRANDUCERS
Thermistor
• R ref - Resistance of a thermistor
at temperature T ref.
• R- Resistance of a thermistor at
temperature T
• ß- Temperature coefficient of
resistance
Then
Resistance versus Temp .
Graph
RESISTIVE TRANDUCERS
• Strain guages ( variable resistance transducers)
• Passive transducer which is used to measure strain
produced by a Force by the changes in electrical resistance in
wires.
• Piezoresistive effect : The impedance( due to the change in
length or diameter) of a metal wire changes when the
material is mechanically deformed. This is the origin for a
widely used sensors known as strain gauges.
• When a stress is applied on wire , its length increases and
diameter decreases . i.e , R = ÞL/A
• In these sensors, the unitary change in resistance is
proportional to the elongation (strain) through a parameter
known as gauge factor (K).
• ∆R/R = K (∆L/L). Measurement of sensitivity of a material
to strain is called guage factor.
STRAIN GUAGES
Different Types Strain Guages
• Wire Strain Guage
• Foil Stain Guage
• Semiconductor Strain guage
Resistance Wire Gauges have 2
Basic forms: Unbonded & Bonded
• 1.UNBONDED
• It is connected in a bridge circuit
• With no load applied bridge is
balanced.
• When load is applied, resistance
change occurs resulting an
output voltage in the bridge
• In unbonded
Resistance strain
guages, Wire streched
Between two points in
an Insulating
Medium(D= 25µm) ,
STRAIN GUAGES
2)BONDED
• A wire is looped back and
forth on a mounting plate
(carrier) which is usually
cemented to a member
undergoing stress
• R = ÞL/A
• Different Types of Wire
Strain guages
•
•
•
•
Grid Type
Rosette Type
Torque Type
Helical type
STRAIN GUAGES
• Grid
Helical
• Torque
Rosette
STRAIN GUAGES
• Grid Type
Measuring axis of a strain gauge is along the longitudinal
axis, but in most applications , some degree of strain is
present along the transverse axis of the gauge .i.e,
transverse sensitivity can not be completely eliminated ,in
high accuracy measurements. So compensation is required…
• Rosette type
Compensation for transverse sensitivity is to place several
gauges on members surface with a known angles between
them(450)
• Characteristics of strain guages for better measurement
1.High value of K
2.R should be as high as possible
3.Low resistance temperature coefficient
4. No hysteresis
STRAIN GUAGES
Foil Strain Guage
Strain is measured with the help of a metal foil . Ex: Nickel,
Platinum Nichrome, Constantan(Ni+cu), Isoelastic (Ni+Cr+Mo)
Advantages
• Greater dissipation capability due to large surface area. Hence in
can be operated in higher Temperature range
• Better bonding & fabricated in any shape and etched on the
carrier.
• Longitudinal sensitivity is 5% more than wire type with equal K
value
• Transverse sensitivity & hysteresis is smaller to ½ to 1/3 of wire
Type
• Resistance in between 50-1000 ohms & thickness of the film is
about 0.2mm
STRAIN GUAGES
Semiconductor strain gauges
• Here change in resistance due to resistivity change rather
than dimensional change in metallic gauge eg: Ge, Si
Advantages
• High value of Gauge Factor (50 times more than wire type)
indicates higher change in resistance and hence good
accuracy
• Hysteresis characteristics of Semiconductor strain gauges are
excellent
• Better frequency response & small in size
Disadvantages
• Sensitive to temperature changes
• Linearity is poor
• Expensive
Piezo Electric Transducer
• Active tranducer used for the measurement of
force or strain
• Piezo electric effect: Crystalline materials such as
Quartz, barium titanate , produces an emf when
they are placed under stress.(reversible)
• Structure: Here crystal is placed between a solid
base and a force summing member. Force
entering the transducer through its pressure port
applies pressure vat the top of the crystal. This
produces an emf across the crystal proportional to
the magnitude of applied pressure
Piezo Electric Transducer
• Basic expression for output voltage E =Q/Cp, where Q –
generated Charge, Cp - shunt capacitance.
Let K- Coupling Coefficient
K= Mech.E converted to Ele. E/Applied Mech. E
K= Ele.E converted to Mech. E/Applied Ele. E
Properties of Piezo electric crystal
• Stability
• High Output
• Very good Frequency Response
• Sensitive to temperature and humidity
• Applications
• Measurement of force, High frequency accelerometer, as
mass to frequency converter, Measurement of temperature
•
Load
cell
(Pressure
cell)
Used to measure Heavy loads using strain guages
• As the stress/load is applied along Z direction, steel bar
experience a compression along this direction, and an
expansion along X & Y axis. As a result , gauge A experience
a decrease in resistance while the other B undergoes
increase in resistance. When these two gauges and gauges
on the other sides of the steel are connected to form a
bridge circuit, sensitivity is multiplied by four times. So it
load cell is sensitive to small values of stress as well as heavy
loads
Thermo Couple
Active transducer used for measurement of
temperature
Principle: When a pair of wires made of disimilar
metals is joined together at one end, a temperature
difference between the two ends of wires produces
a voltage between the two wires (Seebeck effect)
• The magnitude of this voltage depends on
the material used for the wire and
temperature difference between the
joined ends and other ends
• Temperature difference between the sensing junction
and other ends is a critical factor, the latter are kept at
constant reference temperature
Thermo Couple
• A series of thermocouple connected together is
called a thermopile
• Thermocouple is made of different alloys covering a
range of
-2700 C to 27000 C
• J- Iron Constantan, T-Copper Constantan
• E- Chromel-Constantan,
• K- Chromel-Alumel, S- Platinum-Rhodium etc
Thermo Couple
Advantages
• Rugged Construction
• Wide range (-2700 C to 27000
C)
• Good Accuracy
• No need of bridge circuit
• Good reproducibility
• Cheap
Limitations
• Cold junction & other
compensation are
required for accurate
measurement
• Nonlinear behavior
• Signal amplification is
required
Ultrasonic Transducers
• Ultrasonic sensors generate high frequency sound
waves(above 20khz) and evaluate the echo which is
received back by the sensor. Sensors calculate the
time interval between sending the signal and
receiving the echo based on the the ultrasound
frequency, and the sound velocity of the propagation
medium etc to determine the distance.
• Piezo electric transducers can be used as
transmitters and receivers
• When an alternating current is applied Piezo electric
transducers, causes them to oscillate at very high
frequencies, thus producing very high frequency
sound waves. Also piezoelectric crystals generate a
voltage when force is applied to them, So the same
crystal can be used as an ultrasonic detector
Ultrasonic Transducers
• It can be used to detect level of a fluid in a tank,
speed of an object through air or water etc
Ionization Transducer
• Most widely used for low-pressure measurement
• Operation: A regulated electron current (typically 10 mA) is
emitted from a heated filament. Electrons emitted from the
filament move several times in back and forth movements
around the grid before finally entering the grid (+150 volts).
During these movements, some electrons collide with a
gaseous molecule to form a pair of an ion and an electron
(Electron Ionization) and these ions pour into the collector (30 volts) to form an ion current. Since the gaseous molecule
density is proportional to the pressure, the pressure is
estimated by measuring the ion current .The number of
these ions is proportional to the gaseous molecule density
multiplied by the electron current emitted from the filament .
This current is amplified and displayed by a high-gaindifferential amplifier .
Ionization Transducer
• This ion current will differ for different gases at
the same pressure; that is, a hot filament
ionization gauge is composition-dependent
Proximity Transducers
• Detect the presence of nearby objects without
any physical contact
• Operation : A proximity sensor often emits
an electromagnetic field or electromagnetic
radiation and observes changes in the field or
return signal.
• An Inductive proximity sensor always requires a
metal target.
Proximity Transducers
Target
Signal
evaluator
Switching
Amplifier
• Advantages: Due their non-contact mode of operation
,direct contact with the object is avoided (No moving
parts or contacts, hence functions without wear).
• High degree of protection against vibration and shock,
stress as well as dirt, dust and humidity ( can also be
used in extreme conditions)
Proximity Transducers
• Operation
Proximity Transducers
• OPERATION : An inductive proximity sensor comprises an
LC oscillating circuit, a signal evaluator/trigger circuit and a
switching amplifier/output circuit .
• The coil of this oscillating circuit generates a high-frequency
alternating electromagnetic field. This field is emitted at
the sensing face of the sensor.
• If attenuating material/ Target nears the sensing face,
eddy currents are generated in the case of non ferrite
metals . In the case of ferro magnetic metals, hysteresis
and eddy current loss also occurs.
• These losses draw energy from the oscillating circuit and
reduce oscillation.
• The signal evaluator detects this reduction and converts it
into a switching signal.
Hall Effect sensors
• Hall Effect :When a current carrying conductor is subjected
to magnetic field, a voltage is developed across the
conductor transverse to the current flow and perpendicular
to the magnetic field.
• The Hall coefficient is defined as the ratio of the
induced electric field to the product of the current density
and the applied magnetic field
• No magnetic field - Straight line current flow
• With Perpendicular Mag. Field – Curved path and
accumulate on the surfaces ,produces an emf
For a simple metal strip( charge carriers:
Electrons only)
the Hall voltage VH= -IB/net
I= current, B= Mag.Flux
t= thickness, n= charge carrier density
Hall Effect sensors
• A Hall effect sensor : Analog/Digital transducer that varies
its output voltage in response to a magnetic field ( With a
known magnetic field, its distance from the Hall plate can be
determined).
• Hall sensor can be used to measure the current without
interrupting the circuit.
• Hall effect sensors are used for proximity switching,
displacement, positioning, speed detection, and current
sensing applications
• Digital Type: Two magnets are placed
on the disc .As the disc rotates ,LED tuns
ON/OFF depends on the position of
the coil & magnet
Hall Effect sensors
Photoconductive cell
• Passive transducer used for the measurement of radiation
intensity.
• Electrical resistance to the current flow varies as the light
intensity striking in it.
• When the photocell has appropriate light is incident upon it, its
resistance is low and the current flow through the relay is high to
operate the relay.
• When the light is interrupted shut off partially or completely,
resistance of the photocell increases ,thereby reducing the current
through the relay.
Photoconductive cell
• Photo resistors/LDR are basically photocells (CdS, PbS, InSb)
• A photo resistor is made of a high resistance semiconductor
. If light falling on the device is of high
enough frequency, photons absorbed by the
semiconductor give bound electrons enough energy to
jump into the conduction band. The resulting free electron
conduct electricity, thereby lowering resistance
Photovoltaic Cell
• Active transducer which directly converts EM radiations/light into
electrical energy.
• Generate a voltage proportional to light intensity (ex:
Si,Se,Ge,InAs,InSb)
• Advantages
Active Transducer
Temperature range -100 to 1250C
Extremely fast response
•
Operation :Photons striking the cell pass through the thin upper
layer and are absorbed by the electrons in the lower layer , causing
the generation of conduction electron and holes. The depletion
region separates conduction holes & electron causing a potential
difference across the junction
Photo Diode
• Symbol
• When the device operates with a reverse voltage, it
functions as a photoconductive device. When operating
without a reverse voltage, it functions as a photovoltaic
device
• Response time of photodiode is very fast so that it may be
used in applications where light fluctations occur at high
frequency
• PIN photodiode
Intrinsic layer in between
P and N region which
increases depletion layer
width thus reducing
Capacitance. So faster
response & low noise
Photo Transistor
• Symbol
• More Sensitive than a photodiode (as much as100 times), by
adding a junction resulting in NPN Device (but slower switching
time)
• Illumination of the central region causes the release of electron
hole pairs. This lower the potential barrier across both junctions,
causing an increase in the flow of electrons from left region to
central region and on to the right region.
Digital Encoder
A digital optical encoder is a device that converts
motion into a sequence of digital pulses
Encoders : linear and rotary (common type)
Rotary encoders are manufactured in two basic forms:
A) Absolute encoder
Here, a unique digital word corresponds to each
rotational position of the shaft
B) Incremental encoder
which produces digital pulses as the shaft rotates,
allowing measurement of relative position of
shaft
Digital Encoder (optical)
• Most rotary encoders are composed of a glass or
plastic code disk with a photographically deposited
radial pattern organized in tracks. As radial lines in
each track interrupt the beam between a photo
emitter-detector pair, digital pulses are produced.
The optical disk of the absolute encoder is
designed to produce a digital word that
distinguishes N distinct positions of the shaft.
Absolute Encoder
• For 8 tracks, the encoder is capable of producing
256 distinct positions or an angular resolution of
1.406 (360/256) degrees.
• The most common types of numerical encoding
used in the absolute encoder are gray and binary
codes.
• The gray code is designed so that only one track
(one bit) will change state for each count
transition, unlike the binary code where multiple
tracks (bits) change at certain count transitions
Absolute Encoder
• Gray code Absolute Encoder
• Binary code Absolute Encoder
Incremental Encoder
• The incremental encoder, sometimes called a relative
encoder
• It consists of two tracks and two sensors whose
outputs are called channels A and B. As the shaft
rotates, pulse trains occur on these channels at a
frequency proportional to the shaft speed, and the
phase relationship between the signals yields the
direction of rotation
• By counting the number of pulses and knowing the
resolution of the disk, the angular motion can be
measured.
• The A and B channels are used to determine the
direction of rotation by assessing which channels
"leads" the other
Incremental Encoder
• The signals from the two
channels are a 1/4 cycle
out of phase with each
other and are known as
quadrature signals.
• Third output channel,
called INDEX, yields one
pulse per revolution,
which is useful in
counting full revolutions.
• It is also useful as a
reference to define a
• Incremental encoder
Pattern
Incremental Encoder
• The direction of rotation(clockwise or counter-clockwise) is
determined by the level of one signal during an edge transition of
the second signal
• In the 1X mode,
A= with B =1 implies
a clockwise pulse, and
B=with A=1 implies a
counter-clockwise pulse