sensors iii.ppt

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EN 2850 APLLIED ELECTRONICSLECTURE 04 Sensors(3)

Level 2/Semester 1

05/10/2009

Kosala Jayasundara


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Position, Displacement and Level

detectors Position, displacement and level detectors belong to

the same domain of detectors.

These are important in areas such as security

systems, robotics, industrial automation applications,transportation applications, process feed backsystems etc.

Proximity sensors are also a special type of

displacement sensors. When the displacement isbeyond a given threshold they give an output.

The displacement sensors are sometimes part ofother complex sensors. e.g. Force, Pressure


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detectors (Contd) Some of the important factors that should be taken in

to account when selecting a position, displacementsensors, for a particular application.

Type of the measured object. (metal, non metal, liquid,ferromagnetic)

Accuracy and resolution needed?

Type of the displacement (linear or rotary)

The space required for mounting the sensor.

We will be briefly looking at several types ofposition displacement and level detectors.


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detectors - Potentiomertic Resistance of a conductor is directly proportional to

its length.

Therefore by making an object to control the length

of a conductor, displacement or the position can besensed.

This can either be linear displacement or rotarydisplacement.

Thus two types of potentiometer based displacementsensors can be identified as linear and rotary.

This is sometimes known as the pot


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detectorsPotentiomertic (Contd) This is essentially an active type of device.

The stimulus is the position or the displacement andit is obtained as a voltage upon an applied energy

source.Voutput voltage (proportional to the displacement)

EApplied voltage

DTotal length of the potentiometer (fixed)

d

length corresponding to the displacement

D can be identif

ied as the full scale input (FSI)

Assume no loading effect.


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detectorsPotentiomertic (Contd)

Potentiometric liquid level detectorIf there is loading effect with theinterface circuit, the output voltagewill no longer be proportional to the

displacement.


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detectorsPotentiometric (Contd) As because this is an ratiometric device, the

resistance of the conductor doesnt affect theaccuracy of the system.

Few disadvantages of these detectors, Speed of response is slow.

The sensor need to be physically coupled with the objectunder measurement.

Mechanical loading is there. (effect of friction)

Mechanical wear and tear.


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detectors Capacitive. Capacitive detectors are widely used as proximity,

displacement and level detectors because they canbe used with both conductive and non conductive

materials. Capacitance of a parallel plate capacitor (or rotary

type) can be controlled either by changing the width(d) between the plates or by the permittivity of themedium.

This capacitance variation can be converted to anelectrical signal. Therefore these are also activesensors.


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detectorsCapacitive (Contd)

The middle plate is attached with the object under measurement. Thetop and bottom plates are fed with equal but opposite phase signals.Depending on the movement of the center plate the output (voltage formthe middle plate) is obtained and its phase will give the direction of themotion. (whether it is up or down)


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detectorsCapacitive (Contd) Capacitive proximity sensors make use of the

property of permittivity of the medium in order todetect the objects.

In many practical applications when capacitivesensors are used as displacement detectors themeasured object it self forms the one plate of thecapacitor.


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Capacitive bridge typedisplacement detector

High noise immunity, increased reliability, increased linearity


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Capacitive liquid level detector

When the liquid level changes thepermittivity changes, causing the totalcapacitance to be changed.

Thus the level of the liquid can bedetected.


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detectors

Inductive Linear Variable Differential Transformer (LVDT)

This is a device used to measure the lineardisplacement by making use of the property of

mutual inductance. LVDT consists of three solenoidsplaced end to end inside a tube. One is the primaryand other two are secondary coils. A cylindricalferromagnetic core is attached to the object undermeasurement, and it causes the ferromagnetic core

to slide along the axis of the tube.

The primary is driven by an alternating

current.


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detectors

Inductive (Contd)

Alternating supply in primary causes voltages to

be induced in secondary windings

As the core slides, the mutual inductance changes, so

the induced voltages in secondary windings change.

Two secondary coils are connected in reverseseries so that the differential voltage is obtained

as the output.

When the core is in center position output iszero. Equal and opposite voltages are induced in

secondary coils.

Both positive and negative displacement can bemeasured, as the core moves left and right.


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Position, Displacement and LeveldetectorsInductive (Contd)

Rotary Variable Differential Transformer(RVDT)

This is the angular counterpart of the LVDT.

RVDT is used to measure the angulardisplacements.

Has a stator and a rotor.

The magnitude of the output voltage isproportional to the angle of rotation.

The direction of rotation (whether it isclockwise or anticlockwise) can be obtainedby the phase of the output voltage withrespect to the phase of the primary voltage.


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Advantages of LVDT and RVDT

The effect of friction is minimum because the core is not incontact with the windings.

Solid and robust construction have made it suitable forrugged use.

High resolution and precision can be achieved.

Very low output impedance.

High noise immunity. Hystereses effects are negligible.


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In addition to the above two types, proximity sensorsare widely used.

Mutual inductance proximity sensors and selfinductance proximity sensors are widely used asmetal and susceptive object detector.

Hall effect sensors are also used to detect the rotarydisplacement and the position.

Recall Hall effect ???


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The Hall voltage developed can be used to encode arotary motion.

Hall sensor is ONThe magnetic flux is shunted awayby the ferromagnetic object. Hall

sensor is OFF


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When the above assembly is enclosed in to a onehousing, the pulses produced with the presence andabsence of the ferromagnetic object, can be used to

measure the angular displacement, hence theangular speed.

Automobile distributor make use ofthis kind of detector !!


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Ultrasonic distance measurement

By using mechanical acoustic waves which are not

sensitive to humans (beyond 20kHz) a non contacttype distance measurement can be achieved.

Ultrasonic energy is reflected in a diffuse manner.That is regardless from the direction of incident

energy the reflection occurs in a considerable widerange. (wide solid angle).

Once the object under measurement is moving thereflected frequency will be different form that of the

incident. (Doppler effect)

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