measurements and instrumentaion chapter 13
TRANSCRIPT
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Sensor technologies
Capacitive and resistive sensors
Capacitive sensors consist of two parallel metal plates in
which the dielectric between the plates is either air or some
other medium. C = 0rA/d,
Resistive sensors rely on the variation of the resistance of amaterial when the measured variable is applied to it
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Sensor technologies
Magnetic sensors
Inductive sensors
Variable reluctance sensors
Eddy current sensors
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Sensor technologies
Hall-effect sensors
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Sensor technologies
Piezoelectric transducers
Strain gauges
Piezoresistive transducers Optical sensor (air path)
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Optical sensors (fibre-optic)
The basis of operation of fibre-optic sensors is thetranslation of the physical quantity measured into achange in one or more parameters of a light beam.The light parameters that can be modulated are one
or more of the following: intensity
phase
polarization wavelength
transmission time.
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Optical sensors (fibre-optic)
Fibre-optic sensors usually incorporate either
glass/plastic cables or all plastic cables.
All glass types are rarely used because of their
fragility. Plastic cables have particular
advantages for sensor applications because they
are cheap and have a relatively large diameter of
0.51.0mm, making connection to thetransmitter and receiver easy.
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Optical sensors (fibre-optic)
Two major classes of fibre-optic sensor:
Intrinsic sensors, the fibre-optic cable itself isthe sensor
Extrinsic sensors, the fibre-optic cable is onlyused to guide light to/from a conventional
sensor
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Intrinsic sensors
Intrinsic sensors can modulate either the
intensity, phase, polarization, wavelength or
transit time of light.
Light intensity is the simplest parameter to
manipulate in intrinsic sensors because only a
simple source and detector are required.
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Intrinsic sensors
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Intrinsic sensors Modulation of the intensity of transmitted light takes place in
various simple forms of proximity, displacement, pressure, pH
and smoke sensors.
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Intrinsic sensors
A slightly more complicated method of effecting light intensitymodulation is the variable shutter sensor.
This consists of :
two fixed fibres
two collimating lenses a variable shutter between them
Working: Movement of the shutter changes the intensity oflight transmitted between the fibres.
Use: This is used to measure the displacement of variousdevices such as Bourdon tubes, diaphragms and bimetallic
thermometers.
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Extrinsic sensors
Extrinsic fibre-optic sensors use a fibre-optic
cable, normally a multimode one, to transmit
modulated light from a conventional sensor
such as a resistance thermometer and a
platinum resistance thermometer (PRT).
Extrinsic fibre-optic sensors provides excellent
protection against noise corruption.
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Ultrasonic transducers
Transmission speed
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Ultrasonic transducers
Direction of travel of ultrasound waves
Air currents can alter the direction of travel ofultrasound waves. An air current moving with a
velocity of 10 km/h has been shown
experimentally to deflect an ultrasound wave by
8mm over a distance of 1m.
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Ultrasonic transducers
Directionality of ultrasound waves
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Ultrasonic transducers
Relationship between wavelength, frequency
and directionality of ultrasound waves
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Ultrasonic transducers
Attenuation of ultrasound waves
X0 : the magnitude of the energy at the point of emission
F : the nominal frequency of the ultrasound
:the attenuation constant
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Ultrasonic transducers
Ultrasound as a range sensor
Use of ultrasound in tracking 3D objectmotion
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Ultrasonic transducers
Effect of noise in ultrasonic measurement
systems
Background ultrasound :
use of sound-absorbing material
Energy that is reflected off some object in the
environment:
transmission-time counter
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Ultrasonic transducers
Exploiting Doppler shift in ultrasound
transmission