measuremnt and instrumentation vii

7/30/2019 Measuremnt and Instrumentation VII

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MEASUREMENT AND INSTRUMENTATION

VII

Muhammad Sajjad


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ULTRASONIC TRANSDUCERS

Used particularly for measuring fluid flow rates, liquidlevels and translational displacements

Ultrasound is a band of frequencies in the range above 20kHz

Measurement devices that use ultrasound consist of onedevice that transmits an ultrasound wave and anotherdevice that receives the wave

Changes in the measured variable are determined eitherby measuring the change in time taken for the

ultrasound wave to travel between the transmitter andreceiver, or, alternatively, by measuring the change inphase or frequency of the transmitted wave

Piezo-electric crystal element is used as ultrasonictransducer


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In order to duplicate ultrasonic frequencies, humans have harnessed the

electrical properties of materials. When a specially cut piezoelectric quartz

crystal is compressed, the crystal becomes electrically charged and an

electric current is generated: the greater the pressure, the greater the

electric current. If the crystal is suddenly stretched rather than being

compressed, the direction of the current will reverse itself. Alternately

compressing and stretching the crystal has the effect of producing an

alternating current. It follows that by applying an alternating current that

matches the natural frequency of the crystal, the crystal can be made to

expand and contract with the alternating current. When such a current is

applied to the crystal, ultrasonic waves are produced.

How ultrasonic waves are produced? For your info!
http://science.jrank.org/pages/1902/Crystal.htmlhttp://science.jrank.org/pages/2310/Electric-Current.htmlhttp://science.jrank.org/pages/5479/Pressure.htmlhttp://science.jrank.org/pages/5479/Pressure.htmlhttp://science.jrank.org/pages/2310/Electric-Current.htmlhttp://science.jrank.org/pages/1902/Crystal.html


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ULTRASONIC TRANSDUCERTRANSMISSION

SPEED

The transmission speed of ultrasound varies

according to the medium through which it

travels

V = 331.6 + 0.6*T m/s where T is

temperature

Let discuss Air as a medium at 0 C and

standard condition


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ULTRASONIC TRANSDUCERDIRECTION OF

TRAVEL

Air currents can alter the direction of travel of

ultrasound 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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The peak emission always occurs along a line that is normal to thetransmitting face of the ultrasonic element

It is useful to treat the transmission as a conical volume ofenergyedges of

cone defined as transmission angles


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Ultrasound waves suffer attenuation in the amplitude of the transmitted energyaccording to the distance travelled


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The basic principles of an ultrasonic range sensor are to measure the timebetween

transmission of a burst of ultrasonic energy from an ultrasonic transmitter and

receipt

of that energy by an ultrasonic receiver. Then, the distance d can be calculated

from:

d = v*t

To counter the changing effect of ultrasound velocity, sometimes an extra

transmitter receiver pair has been inserted

Position of transmitter and receiver will be very important..note that it should

be properly oriented (coincident with the normal lines)


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Ultrasonic transmitter mounted

on the moving object TA, B andC are

receivers


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Apparent change in frequency of the wave when there is relative motion between

the transmitter and receiver


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Some part of this left intentionally!

measuremnt and instrumentation vii

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