Measurements and instrumentaion chapter 13

7/28/2019 Measurements and instrumentaion chapter 13

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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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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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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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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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Directionality of ultrasound waves


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Relationship between wavelength, frequency

and directionality of ultrasound waves


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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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Ultrasound as a range sensor

Use of ultrasound in tracking 3D objectmotion


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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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Exploiting Doppler shift in ultrasound

transmission

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Measurements and instrumentaion chapter 13