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TRANSDUCERS
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Transducers
The term transducer is applied to any
device which converts a mechanical orother measurable phenomenon into an
electrical one or vice versa.
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Resistive transducers Temperature sensors (metals)
Rt= Ro(1+t+t2+t3+.)where Rt is resistance of a length of wire at t
oC
Ro is the resistance at 0oC
, , are the temp coefficient of resistance with >>. For most metals resistance increases reasonably
linearly with temp and , etc. can be neglected.
Then Rt= Ro(1+t)
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Temperature sensors (Semiconductors)
Resistance changes with temp.
A group of transducers based on this are thermistors Made from mixture of metal oxides such as Cr, Co, Fe, Mn, Ni
Formed into various forms such as beads, discs and rods.
The resistance-temp graph is highly non-linear and described by
the exponential relationshipt
t eKR
=
where Kand are constant.
The resistance decreases with an
increase in tempchange in
resistance is larger than change in
temp
Decrease in resistance causes
increase in current which in turn
increases temp further.
This effect continues until heatdissipates and power supplied
equalizes.
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Sliding-Contact Devices
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Variable-Inductance Transducer Elements
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S i
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Strain gauges When a length of wire, or metal foil or
semiconductor, is stretched its resistancechanges. The fractional change in resistance,R/R is directly proportional to the strain
R/R = G (where G is a constant called gaugefactor)
For most material G is positivemeans resistanceincreases when strain is increased, i.e. tensionincreases resistance, compression decreases it.
The resistance of a strain gauge is changed notonly by a change in strain but also a change intemp.
Wires of Cu or Fe
Semiconductorstrips of Si doped with n-type or p-
type material.
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Capacitive Transducers
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Photoconductive cells It is light-dependent resistorits resistance decrease as
the intensity of light falling on it increases. Cadmium sulphidecommonly used, however, othersare also in use
Resistance variesmega ohms in the dark to few
hundred ohms in bright light Response timetypically of the order of 50 m.
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Photoemissive detectors:
A cathode-anode combination in an evacuated glass
In a proper circuit (several hundred volts required) light
impingement on the cathodecurrent produced and
amplified.
Photovoltaic cells
Sandwich of unlike
materials such as ironbase covered with thin
layer of iron selecide.
When exposed to light,a voltage is developed
across the sandwich.
Requires no external
power other than light.
Pi l t i
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Piezoelectric sensors
Piezoelectric effect
Certain materials can generate an electric charge when
subjected to mech. strain or, conversely can change
dimensions when subjected to voltage.
E.g. quartz, Rochelle salt (potassium sodium tartarate),
barium titanate, ammonium dihydrogen phosphate,
certain organic polymers, and even ordinary sugar.
None of the above materials possess all the desirableproperties, such as stability, high output, insensitivity to
temp extremes and humidity, and the ability to be formed
into desired shape Rochelle salt provides a high output, but requires protection from
moisture, and cannot be used above 45oC
Quartzmore stable but has low output15
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What is a crystal?
A class of materials arranged in a definite,geometric pattern in three dimensions(table salt and sugar are commonexamples)
Quartz Crystal is silicon and oxygenarranged in a crystalline structure (SiO2).
SiO2 is also found abundantly in nature in
a non-crystal structure (amorphous) assand.
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A method for predicting the
behavior of a crystal: The unit cell
+ Represents silicon atom
- Represents oxygen atom
Not actually correct, but this method allows
a good understanding of quartz crystals
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The unit cell of crystal silicon dioxide
-
+
-
+
-
+
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-
+
+
-
+
-
Unit Cell at Rest
-
+
+
-
+
-
Neutral Charge
Unit Cell Under MechanicalCompression (pushing force):
Electrical polarity as shown
-
+
+
-
+
-
Unit Cell Under mechanical
Tension (pulling force):
Electrical polarity reverses.
+ + +
- - -
- - -
+
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The Piezoelectric Effect
Crystal material at rest: No forces applied,
so net current flow is 0
Crystal
+ - + - + -
+ - + - + -
Current Meter= 0
Charges cancel
each other, sono current flow 21
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The Piezoelectric Effect
Crystal material with forces appliedin direction of arrows..
Crystal
- - - - -
+ + + + +
Force
Current Meter
deflects in +
direction
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The Piezoelectric Effect
Changing the direction of the
applied force..
Crystal
+ + + +
- - - - -
Force Current Meter
deflects in -
direction
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The electromechanical effect
Now, replace the current meter with a power source capable
of supplying the same current indicated by the meter.
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+ - + - + -
+ - + - + -
switch
Crystal
charges cancel power source
. With the switch open, the crystal material is now at rest again:the positive charges cancel the negative charges.
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The electromechanical effect
When the switch is closed, and you apply the exact amountof power to get the same current that resulted when you squeezed
the crystal, the crystal should deform by the same amount!!
Crystal
power source
(battery)
- side
+ + + +
- - - - -
+ side
. and, the crystal should get shorter and fatter.25
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The electromechanical effect
What will happen if you switched the battery around??
Crystal
power source
(battery)
+ side
- - - - -
+ + + + +
- side
. the crystal should get longer and skinnier.26
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Summary of the Piezoelectric &
Electromechanical Effect
A deformation of the crystal structure (eg: squeezing
it) will result in an electrical current.
Changing the direction of deformation (eg: pulling it)
will reverse the direction of the current.
If the crystal structure is placed into an electrical field,
it will deform by an amount proportional to the
strength of the field.
If the same structure is placed into an electrical fieldwith the direction of the field reversed, the
deformation will be opposite.
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Most of the piezoelectric materials are single
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Most of the piezoelectric materials are single
crystal; barium titanate is polycrystalline, hence
shaping and sizing is easy. Polarizing treatment is necessary to induce the effect:
Element heated above Curie point of 120oC
A high DC voltage (of the order of 10,000 V/cm)
Element cooled with the voltage applied.
Piezoelectric polymers, such as polyvinylidenefluoridelow cost piezoelectric transducer with
relatively high voltage outputs.
Formed into thin films (~30 micron thick)with silveredelectrodes on either side
Light, flexible and easily manipulated
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Hall Effect Sensors
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Hall Effect Sensors
The Hall effect is
the appearance ofa transversevoltage difference
on a conductorcarrying a currentperpendicular to a
magnetic field.Present in any
conductor carrying
a magnetic fieldbut morepronounced in
semiconductors.
Thermoelectric transducers
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Thermoelectric transducers
Thermocouples
There is a potential difference between
two different metals across the junction.
The potential difference depends on the
metals used and the temperature.
A thermocouple is just wires of two
different metals forming a complete circuit.
When same temp at both junctionssame potential difference across
each junction.
A temp difference between two junctionproduces a net e.m.f.the
value depends on the two metals and the temp. Usually one metal is held at 0oC
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Ass mption one of the j nction is at 0oC
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Assumptionone of the junction is at 0oC
If other temp.then law of intermediatetemperatures has to be used to determine the
e.m.f.
Statementemf of a thermocouple withjunctions at 1 and 3 is the algebraic sum of
the emfs of the two thermocouples of thesame materials with junctions at 1 and 2,
and 2 and 3.
Compensation circuits can be used when the
reference junction is not at a constant temp. but
allowed to vary with the ambient temp.
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Electrochemical transducers
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Ion selector electrodes It gives response whichdepends mainly on the
conc. of single type of ionin the solution.
A reference electrode is
also used. E.g. hydrogen ion
measurement for
measuring pH.
Elastic transducers
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Proving ring
A steel ring which deforms
from its circular shape under
the action of forces.
The amount of deformation
is a measure of forces and
can be measured by dial
indicator.
Are capable of high
accuracy
Are used for forces in therange of 2 kN to 2000 kN.
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Load CellsTh d f ti f li d b d th ti f f
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The deformation of a cylinder on a box under the action of forces canbe used as a measure of forces. Such a system is known as load
cell. The deformation is measured by means of strain gauges.
Four identical gauges
If the load cell is under compression then R1 and R3 are in
compressionthe strain = -(F/AE), where F is the force applied, A itscross-sectional area and E the tensile modulus of the cell material.
Strain gauges R2 and R4 are in tension, the strain being +(F/AE)where is Poissons ratio of the cell material.
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Diaphragms
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p g
The movement of the center
of a circular diaphragm whenthere is a pressure difference
between its two sides is the
basis of a pressure gauge.
Capsules and bellows Can be considered to be just
two diaphragms back-to-
back. Bellows being a stack
of capsules
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Bourdon tubes
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A bourdon tube is an almost
rectangular or ellipticalshaped tube made frommaterials such as stainlesssteel or phosphor bronze.In
one form the tube is C-shaped.
When pressure inside tube
increases the C opens outhence displacementmeasures pressure.
A helical form gives greater
deflections. Also exists in twisted form
the pressure changes the
tube to untwisted.37
Pneumatic transducers
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Flapper nozzle
Air at constant pressure Ps flows throughthe orifice and escapes through the
nozzle.
At flapper closed, i.e., x=0; no air
escapesWhen x increases pressure
drops so becomes measure of
displacement of the flapper
The transducer has high sensitivity but asmall range of measurement, typically
0.05 mm.
)/(161422
on
s
dxd
P
P +=
where dn is the dia of the nozzle
do is the dia of the orifice
Differential pressure transducers
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Flowmeters When a fluid flows from a wider dia. to a narrower dia. pipe its
velocity increases and the pressure drops.
If the fluid is incompressible, Bernoullis equation gives for ahorizontal pipe
g
P
g
v
g
P
g
v
2
2
21
2
1
22+=+
Where v1 is the fluid velocity, P1 the pressure at the pipe, v2 the
velocity and P2 the pressure at the constriction, and the fluid
density.
Hence, since the density does not change, the volume of the fluid Q
passing through the wide section per second must equal the volume
passing through the constriction. Hence Q=A1v1=A2v2 where A1 is the
area of the tube and A2 that at the constriction. In practice, the flow is not frictionless and the cross-sectional area of
the moving fluid may not be the same as the pipe. Hence, a
correction factor is usually applied.
There are number of forms of flowmeters based on the measurementsystem of the pressure difference between the flows.
Mechanical transducers
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