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BETL 305 ELECTRONIC MEASUREMENT &
INSTRUMENTATION
BY PRASHANT KUMAR
ASST. PROFESSOR
MITS GWALIOR
UNIT II TRANSDUCERSTransducer Block Diagram
A transducer will have basically two main components.
They are
1. Sensing ElementThe physical quantity or its rate of change is sensed
and responded to by this part of the transistor.
2. Transduction ElementThe output of the sensing element is passed on to the
transduction element. This element is responsible for
converting the non-electrical signal into its proportional
electrical signal.
• There may be cases when the transduction element
performs the action of both transduction and
sensing. The best example of such a transducer is a
thermocouple.
• There are transducers which don’t require external
Excitation
• measurand=quantity under measurement
TRANSDUCER
Defnition of transducer: A transducer is a device, usually electrical, electronic, electro-mechanical, electromagnetic, photonic, or photovoltaic that converts one type of energy or physical attribute (egdisplacement, change in velocity) to another (generally electrical or mechanical) for various measurement purposes including measurement or information transfer (for example, pressure sensors).
STATIC CHARECTERISTICS OF TRANSDUCERS1. Linearity
Its input vs output characteristics should be linear and it should produce these characteristics in balanced way.
(Linearity–can be defined as the variation in the constant of proportionality between the input physical quantity and the output electrical signal. A sensor or a transducer is therefore said to be linear when the constant of proportionality has the same value within the whole measurand range (i.e. when the graph relating input to output is a straight line))
2. Ruggedness
A transducer should be capable of withstanding overload and some safety arrangements
must be provided with it for overload protection.
3. Repeatability
The device should reproduce the same output signal when the same input signal is applied again and again under unchanged environmental conditions, e.g., temperature, pressure,
humidity, etc.
4. High Reliability and Stability
The transducer should give minimum error in measurement for temperature variations,
vibrations and other various changes in surroundings.
CONTINUED.......5. High Output Signal Quality (High SNR)
The quality of output signal should be good, i.e., the ratio of the signal to the noise should be high and the amplitude of the output signal should be enough.
6. No Hysteresis
It should not give any hysteresis during measurement while input signal is varied from its low value to high value and vice versa.
7. Residual Reformation
There should not be any deformation on removal of input signal after long period of use.
Resolution is the minimal change of the input necessary to produce a detectable change at the outputPrecision: The capacity of a measuring instrument to give the same reading when repetitively measuring the same quantity under the same prescribed conditionsPrecision implies agreement between successive readings, NOT closeness to the true valueTwo terms closely related to precision Repeatability : The precision of a set of measurements taken over a short time
interval Reproducibility : The precision of a set of measurements BUT i) taken over a long time interval or ii) Performed by different operators or iii) with different instruments or iv) in different laboratories
Hystheresis: The difference between two output values that correspond to the same input depending on the trajectory followed by the sensor (i.e., magnetization in ferromagnetic materials)
The basic equation is y = mX + B, where X is the scaling or multiplier, and B is the offset. Using a little basic algebra we can apply this same set up to calculate scale and offset of any linear sensor or device with a linear output
CLASSIFICATION OF TRANSDUCERS
ACTIVE TRANSDUCERSActive transducers are those which do not require any power source for their operation. They work on the energy conversion principle.
ACTIVE TRANSDUCER PARAMETER
1.PEIZOELCTRIC TRANSDUCER Pressure/strain/displacement to Electric potential
2.THERMOCOUPLE Temperature to electrical potential (emf)
3. Electromagnetic Change in area/velocity /displacement to Induced Emf
EXAMPLE OF PRECISION & ACCURACY (SHOOTING DART)
STRAIN GAUGE
MECHANICAL STRAIN =
ELECTRICAL STRAIN
ELECTRICAL OUTPUT
=
STRAIN GAUGE
l
l
R
R
Wheatstone bridge
R
R
GENERAL WORKING OF STRAIN
GAUGE
STRAIN: It is defined as extension or reduction per
unit lengthl
Sl
STRESS STRAIN CURVE
The change in the value of resistance by the application of force can be explained by the normal dimensional changes of elastic material.
If a positive strain occurs, its longitudinal dimension (x-direction) will increase while there will be a reduction in the lateral dimension (y-direction).
The reverse happens for a negative strain.
Since the resistance of a conductor is directly proportional to its length and inversely proportional to its cross sectional area, the resistance changes.
The resistivity of a conductor is also changed when strained. This property is known as piezoresistive effect.
GAUGE FACTOR
2
resistance of unstrained gauge=R = .....( )
to find R we need to differentiate R w.r.t stress S
...............( )
both side by R = we get
1 1 1
Li
A
dR L L A Lii
dS A S A S A x
Ldividing
A
dR L A
R dS L S A S
1
...............( )iiix
As we observe it from eq(iii)
The effective change in resistance w.r.t strain is due
to three factors Per unit change in length Per unit change in area
Per unit change in resistivity
(Peizoresitivity)
The piezoresistiveeffect is a change in
the electrical resistivity of a
semiconductor or metal when
mechanical strain is applied
L
L
A
A
GAUGE FACTOR…..
Continued..
2
2
2. .4 4
1 1 2.2. . ......( )
4
4
this value of eq (iv) in (iii) we get
1 1 2 1
poisson's ratio
strain =
/
D A DArea D
S S
A D DD iv
DA S S D S
substituting
dR L D
R dS L S D S S
now from
Lateral
Longitudnal axia
starin
.
D
DLl
L
D L
D L
1 1 2 1
when 0 it can be approximated as
1 1 2 1
2........( )
factor = G
eq (v) both side by
get
1 2
1
f
f
f
R L L
R S L S L S S
S
R L L
R S L S L S S
R L Lv
R L L
R
RGaugeL
L
Ldividing
L
we
GL
L
G
2 ( )L
strainS L
Continued…..1 1 2 1
factor = G
eq (v) both side by
get
1
1 2 ( )
2
f
f
f
dR L LR dS L S L S S
RRGaugeL
LL
dividingL
we
GL
L
LG strain
S L
Desirable characteristics in a material for strain
Gauge1.High Gauge factor2.High resistivity3. Low temp. sensitivity (Low temp coefficient of resistance)4. Low Hysteresis5. Linearity (Strain Vs Resistance should be linear)6. High yield point
STRAIN GAUGE
A bonded strain gauge will be either a wire type or a foil type as shown in the figure
below. It is connected to a paper or a thick plastic film support. The measuring leads are
soldered or welded to the gauge wire. The bonded strain gauge with the paper backing is connected to the elastic member whose
strain is to be measured.
Unbonded Strain Gauge is used at places where the Gauge is to be
detached and used again and again
BASIC STRAIN GAUGE WORKING USING WHEATSTONE BRIDGE
A quarter bridge output corresponding to the application of a force is shown below. Initially, the circuit will be balanced
without the application of any force. When a downward force is applied, the length of the strain gauge increases and thus a
change in resistance occurs. Thus an output is produced in the bridge corresponding to the strain.
SEMICONDUCTOR STRAIN GAUGEFor Semiconductor strain gauge material used is either n-type S.C or P-type S.C
It is based on Principle of Piezo-resistivity
Advantages :
1. High Gauge factor (>>130)
2. Very Low hystresis (<<0.05%)
Disadvandage
1. Poor linearity
2. prone to error due to high sensitivity to temp.
COMPARISON BETWEEN SEMICONDUCTOR VS METALLIC GAUGEPARAMETERS METALLIC STRAIN GAUGE SEMICONDUCTOR STRAIN
GAUGE
GAUGE FACTOR LOW HIGH
HYSTRESIS HIGH LOW
LINEARITY HIGH POOR
COST LOW EXPENSIVE
STRAIN GAUGE
PRINCIPLE OF WORKING OF S.GAUGE: Resistance of a
conductor changes when it is strained. This property can be
used to measure Pressure, Force and Displacement.
APPLICATION OF STRAIN GAUGE:1.TO measure pressure
2. To measure Displacement3. To measure force
THERMISTORS
MATHEMATICAL EXPRESSSIONS
APPLICATION OF THERMISTORS
1. measurement of temp.(thermistors’s large change of resistance with temp. provides good accuracy and resolution)
2. temperature compensator in in electronic circuits (because of NTC)
3. measurement of power at high frequency
4. measurement of level of liquid/ flow of liquid
5. measurement of time delay
MATHEMATICAL ANALYSIS OF THERMISTOR
0
0
0 0
1 1R=R exp ..
eq
....( )
(
i)
R is the resistance of thermistor at the temperature T in K
R is the resistance at given temperature T in K
is the material specific constant
differentiat
iT T
wher
i g
e
n
2
0
0
w.r.t Temp. T
we get
log
ln ln
1 1
dR R
dT T
taking weget
R R
T T
2
1 dR
R dT
T
2
1 dR
R dT T
CAPACITIVE TRANSDUCERS
• PRINCIPLE OF WORKING : A capacitive transducer works on principle of Capacitance of Parallel plate capacitor.
• Capacitance of a parallel plate capacitor is given by
• Capacitive transducer works on the principle that capacitance is varied by variation in any one of following parameters
I. Change in effective plate area A /(overlapping area)II. Change in sepration between parallel platesIII. Variation in dielectric constant
0 r AC
d
r
Change in capacitance can be converted into required electrical format using capacitor equation
by capacitor eqution
Q=CV......(i)
differentiating both side we get
( )C C
dQ dV dCC V
dt dt dt
dV dC dQI C V I
dt dt dt
VARIABLE AREA BASED CAPACITOR TRANSDUCERCapacitance of parallel plate capacitor is directly proportional to effective overlapping area between two plates.
0 r AC
d
C A
C A
C A
Used to measure Linear displacementUsed to measure angular displacement
LINEAR DISPLACEMENT MEASUREMENT USNGVARIABLE AREA BASED CAPACITOR
0 0
0
of overlapping part of plates
of overlapping part of plates
Sensitivity= F/m
r r
r
A wlC C
d d
l length
w width
wC
l d
ANGULAR DISPLACEMENT MEASUREMENT USNGVARIABLE AREA BASED CAPACITOR
0
2
2
0
2
0
2
2
F/radian2
r
r
r
AC
d
rA
rC
d
C rSensitivity C
d
VARIABLE DISTANCE BASED CAPACITOR TRANSDUCER
It is used for linear displacement/pressure measurement.
0
0
2
1
F/meter
r
r
AC
d
Cd
ACsensitivity S
d d
VARIABLE DIELECTRIC CAPACITIVE TRANSDUCER
Liquid level measuremen
t using dielectric variation
Displacement measurement
ADVANTAGE OF CAPACITIVE TRANSDUCER
1) High sensitivity
2) Very small power requirement
3) Good frequency response
4) High input impedence so minimum loading effect
DISADVANTAGE OF CAPACITIVE TRANSDUCER
1. Proper insulation of metallic parts required
2. Non linear behaviour due due to edge effect
3. Stray capacitance between metallic wires affect performance
Applications:
Measurement of i) force, ii) angular and linear displacement ,iii) moisture, iv) pressure v) water level indicator
THERMOCOUPLE
It is temperature to electrical (emf) energy conversion based transducer.WORKING PRINCIPLE:Themocouple is based on three effects i) Seeback effectii) Peltier effectiii) Thompson effectSeeback effect: when the temperature difference exists between junctions of two dissimilar metals the the thermo emf is generated across terminals.
Peltier effect when electric current passes junction of two dissimilar metals heat is absorbed at one junction and liberated at another junction
Thompson effects: when a homogeneous electrical conductor is subjected to a temperature gradient then a corresponding voltage gradient is generated along the conductor.
Materials used in thermocouple
POSITIVE WIRE
NEGATIVE WIRE
Temp (degree centigrade)
COPPER CONSTANTAN -253 to 400
Iron CONSTANTAN -200 to 850
platinum Platinum(87%)+Rhodium(13%)
0-1400
2
emf generated
( ) ( )
thermal
E a T b T
Adavantage:High temperature measurement rangeEasy calibrationFast responseLow costDisadvantage1. Reference junction is
necessary2. Non linearity
LVDT (LINEAR VARIABLE DIFFERENTIAL AMPLIFIER)WORKING PRINCIPLE:
LVDT is based on variation in inductance with differential output.
Inductance can be varied by variation in these three quantities
I. Magnetic permeability of material
II. Number of turns
III. Geometric configuration
Secondary winding are connected with opposite polarity to provide differential output
0 r AL
l
CONSTRUCTIONLVDT consists of a cylindrical transformer where it is surrounded by one primary winding (P1) on one side and two secondary windings (S1and S2 ) on the other side
The number of turns in both secondary windings are equal, but they are connected such that both have opposite polarity.
The primary winding is connected to an AC source.
A soft iron core moves within the hollow cylindrical core and changes the magnetic flux linked with the primary and two secondary windings.
WORKINGCase 1: when iron core is at centre( null position) E0=0, (Es1=Es2) because the flux linking with both secondary windings is equal, hence equal emf are induced in them. (Es1 = Es2 ) ( E0=Es1-Es2)
Case 2: When the core is moved to the left of null position (O - A) more flux links with winding S1 and less with winding S2.
Hence, output voltage is Es1 is greater than Es2. The output voltage is positive and in phase with input signal.
Case 3: When the core is moved to the right of null position (O- B) more flux links with winding S2 and less with winding S1
Hence, output voltage Es2 is greater than Es1.The output voltage is negative and 180o out of phase with input signal.
Equivalent
Circuit of LVDT
WORKING AND CHARECTERISTICS
..
Adavntage of LVDT
1. high linearity
2. high sensitivity
3. very low hystresis
4. low power consumption
Disadvantages
1.large offset(threshold)
2. easily affected by stray magnetic field
Applications:
1. Linear displacement measurement
2. Measurement of pressure, tension,
PEIZOELECTRIC TRANSDUCER
Working principle:
The main principle of a piezoelectric transducer is that a force, when applied on the quartz crystal, produces electric charges on the crystal surface.
The charge thus produced can be called as piezoelectricity. Piezo electricity can be defined as the electrical polarization produced by mechanical strain on certain class of crystals.
The rate of charge produced will be proportional to the rate of change of force applied as input. As the charge produced is very small, a charge amplifier is needed so as to produce an output voltage big enough to be measured