position, displacement and level detectors

7/28/2019 Position, Displacement and Level detectors

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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, transporta

applications, process feed back systems etc.

Proximity sensors are also a special type of displacement sensors. When the displacement is beyond a given

threshold they give an output.

The displacement sensors are sometimes part of other complex sensors. e.g. Force, PressureSome of theimportant factors that should be taken in to account when selecting a position, displacement sensors, 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 of position displacement and level detectors.

Position, Displacement and Level 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 be sense This can either be linear displacement or rotary displacement.

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

This is sometimes known as the pot

This is essentially an active type of device.

The stimulus is the position or the displacement and it is obtained as a voltage upon an applied energy source

.

V output voltage (proportional to the displacement)

E Applied voltage

D Total length of the potentiometer (fixed)

d length corresponding to the displacement

D can be identified as the full scale input (FSI)

Assume no loading effect.

If there is loading effect with theinterface circuit, the output voltage

will no longer be proportional to thedisplacement. Potentio

c liquid level detector

As because this is an ratiometric device, the resistance of the conductor doesnt affect the accuracy of the sys

Few disadvantages of these detectors,

Speed of response is slow.


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The sensor need to be physically coupled with the object under measurement.

Mechanical loading is there. (effect of friction)

Mechanical wear and tear.

Position, Displacement and Level detectors Capacitive. Capacitive detectors are widely used as proximity, displacement and level detectors because they can be 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 the medium.

This capacitance variation can be converted to an electrical signal. Therefore these are also active sensors.

\

he middle plate is attached with the object under measurement. The top and bottom plates are fed with equal but

pposite phase signals. Depending on the movement of the center plate the output (voltage form the middle plate) isbtained and its phase will give the direction of the motion. (whether it is up or down)

Capacitive proximity sensors make use of the property of permittivity of the medium in order to detect the

objects.

In many practical applications when capacitive sensors are used as displacement detectors the measured objec

self forms the one plate of the capacitor.


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apacitive bridge type displacement detector, High noise immunity, increased reliability, increased linearity

Capacitive liquid level detector

When the liquid level changes the permittivity changes, causing the total capacitance to be changed.hus the level of the liquid can be detected.

osition, Displacement and Level detectors Inductive

Linear Variable Differential Transformer (LVDT)

This is a device used to measure the linear displacement by making use of the property of mutualnductance. LVDT consists of three solenoids placed end to end inside a tube. One is the primary and other two are

econdary coils. A cylindrical ferromagnetic core is attached to the object under measurement, and it causes the

erromagnetic core to slide along the axis of the tube.

he primary is driven by an alternating

urrent.

-ternating supply in primary causes voltages to be induced in secondary windings


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- the core slides, the mutual inductance changes, so the induced voltages in secondary windings change-o secondary coils are connected in reverse series so that the differential voltage is obtained as the output.

-en the core is in center position output is zero. Equal and opposite voltages are induced in secondary coils.

-th positive and negative displacement can be measured, as the core moves left and right

Rotary Variable Differential Transformer (RVDT)

This is the angular counterpart of the LVDT. RVDT is used to measure the angular displacements.

Has a stator and a rotor.

The magnitude of the output voltage is proportional to the angle of rotation.

The direction of rotation (whether it is clockwise or anticlockwise) can be obtained by the phase of th

output voltage with respect to the phase of the primary voltage.

Advantages of LVDT and RVDT

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

Solid and robust construction have made it suitable for rugged use.

High resolution and precision can be achieved.

Very low output impedance.

High noise immunity.

Hystereses effects are negligible.

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

Mutual inductance proximity sensors and self inductance proximity sensors are widely used as metal and

susceptive object detector.

Hall effect sensors are also used to detect the rotary displacement and the position. Recall Hall effect ???

The Hall voltage developed can be used to encode a rotary motion.


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Hall sensor is ONThe magnetic flux is shunted away by the ferromagnetic object. Hall sensor is OFF

When the above assembly is enclosed in to a one housing, the pulses produced with the presence and absence

the ferromagnetic object, can be used to measure the angular displacement, hence the angular speed.

Automobile distributor make use of

this kind of detector !!

By using mechanical acoustic waves which are not sensitive to humans (beyond 20kHz) a non contact typedistance measurement can be achieved.

Ultrasonic energy is reflected in a diffuse manner. That is regardless from the direction of incident energy threflection occurs in a considerable wide range. (wide solid angle).

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

(Doppler effect)

position, displacement and level detectors

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