Transcript SENSORS AND TRANSDUCERS

Sensors – the term sensor is used for an
element which generates a signal which
is proportional to the quantity being
measured.
 Transducer – these are elements when
subjected to physical change
experience a related change.
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In measurement system a transducer in
addition to sensor is used to convert
signals in one form to another.
 A sensor/transducer is said to be
analogue if it gives output which is
analogue and is digital if output is in the
form of signals which is digital in nature.
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Smart sensors – is a device which has
inbuilt sensor and signal conditioning
embedded in a same package.
› Ability to compensate random errors
› Calculation of measurement accuracy
› Adjust for non-linearity and gives linear
output
› Self calibrate and self diagnosis
Range and span
o Error
o Accuracy
o Sensitivity
o Hysteresis error
o Non linearity error
o Repeatability error
o
Range and span
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› Range 0 KN to 50 KN
› Span is 50 KN
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Error
Range defines the limits between which
the inputs can vary.
Span = max value of input – min value
For a load cell
Error = true value – measured value
If a system gives a temperature reading
of true value 25 deg Celsius and
measured value is 24 deg Celsius then
error is +1 deg Celsius.
Accuracy
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Sensitivity
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Accuracy is the extent to which the value
indicated by measurement system might
be wrong.
It is the summation of all possible errors, as
well as accuracy to which transducer has
been calculated.
Accuracy is often expressed in terms of %
of full range output.
It is the relationship indicating how much
output is there for a given input. i.e
output/input.
For ex a resistance thermometer has
sensitivity 0.5 Ω / ºC.
Other than measurement sensitivity also
incorporates the external parameter.
Hysteresis error
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Transducers can give different outputs
from same value of quantity being
measured whether the value has
reached continuously increasing change
or continuously decreasing change.
Non linearity error
Repeatability error
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Repeatability is used to describe the
ability of transducer to give same output
for repeated applications of the same
input valve.
Repeatability = (max-min values) *100
full range
Transducers are elements which covert or
transform one form of energy to other.
 Transducer is a device used to measure
system parameters.
 Its is device that produces a measurable
response to a change in physical quantity
such as temperature, pressure, etc.
 It is used to quantify the parameters such as
temp, pressure, magnetic field, voltage,
flow, vibrations which are commonly called
a measurands.
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Transducers are the physical element which is a
part of sensor.
A sensor is merely a sophisticated transducer,
which contains signal conditioning circuits capable
of amplifying and refining weak and raw signals
that is available at output of transducers.
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Depending on type of conversion
› Primary transducer
› Secondary transducers
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Depending on output
› Active transducers
› Passive transducers
The Mechanical device which converts
physical quantity to be measured into a
mechanical signal.
 Ex – bourdon tube
 The Bourdon pressure gauge uses the
principle that a flattened tube tends to
change to be straightened or larger
circular cross-section when pressurized.
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Bourdon tube are instruments measuring
the pressure of liquids and gases of all
kinds, including steam, water and air.
 The tubes are C structured tubes.
 The angular of linear displacement is
proportional to applied pressure.
 One end is fixed and other is free and
closed.
 As pressure rises, the C structured tube
expands which in turn moves the needle.
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The output of the primary transducer is
again converted into a different form
using a secondary transducer.
 Ex – bourdon tube connected to a LVDT
to give a digital output
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LVDT is mainly used for displacement or
position measurement.
 It consists of three coils symmetrically
spaced along a insulated tube.
 The central coil is primary coil and other
two secondary primary coils are in series
whose end terminals act as outputs.
 A magnetic core is moved through the
central tube as a result of displacement
being monitored.
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When the core is moved or displaced right
of left the number of coils in the secondary
coil is exposed to primary coil changes
which is due to mutual inductance.
 Variation of mutual inductance of each of
coil, in effect, induce varied relative
voltages in secondary coils.
 Since two windings are connected in
opposition, as the core moves, the output
of one decreases and another increases.
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The emf induced in secondary coil by a
changing current i in primary coil is given
by =
 E = M (di/dt)
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Depending on output
› Active transducers
› Passive transducers
Passive equivalences are resistance,
inductance, and capacitance.
 Transducers of this category are made
up of specific materials, such as their
electrical properties like resistance,
inductance and capacitance vary in
response to change in input
measurands.
 Ex load cel, LVDT, capacitive
transducers.
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Strain gauges are resistive pick type
transducers.
 The resistance of the gauge changes in
accordance with the input mesurand.
 The strain gauge consists of metallic
filament (a resistor) of approximately 0.03
mm thickness, which is bonded or
pasted directly to the strained surface by
a layer of epoxy resin.
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When load is applied to the surface,
resulting change in surface length,
hence the strain.
 This is communicated to filament whose
resistance changes linearly with
measurand.
 The equation governing the strain and
resistance is
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› ▲R / R = g e
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Wheatstone Bridge,
a device for
measuring electrical
resistance.
It can be used to
convert resistance
change into voltage
change.
R1/R2 = R3/R4
The bridge is said to
be balanced.
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Load cell are transducers intended for
measurement of high values of pressure, load
and force.
Resistance load cell consists of at least four
strain gauges, which are adhered to cylinder.
Four strain gauges are used to obtain higher
sensitivity.
Two of the gauges are usually subjected to
tension and other two compression.
When the load applied, the strain changes the
electrical resistance of the gauges in
proportional to the load.
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The principle of capacitive transducers is based on
the change of distance, area and/or permittivity.
Capacitive transducers are non-contact type for
which accuracy and resolution are high.
These are employed for the measurement of
displacement, acceleration, force, level, pressure,
etc.