PPT ON topic Piezoelectricity BY PROFESSOR Anurag soni

PPT ON topic Piezo-electricity BY PROFESSOR Anurag soni ELECTRONIC INSTRUMENTATION 6 TH SEM EX-604(A) Open elective Department of electrical and electronic engineering Lakshmi narain college of technology, Bhopal

Working of Piezo-electric transducer • The main principle of a piezoelectric transducer is that a force, when applied on the quartz crystal, produces electric charges on the crystal surface. The charge thus produced can be called as piezoelectricity. • Piezo electricity can be defined as the electrical polarization produced by mechanical strain on certain class of crystals. The rate of charge produced will be proportional to the rate of change of force applied as input. • As the charge produced is very small, a charge amplifier is needed so as to produce an output voltage big enough to be measured. • For example, if a force of 15 k. N is given to the transducer, it may only deflect to a maximum of 0. 002 mm. But the output response may be as high as 100 KHz. This proves that the device is best applicable for dynamic measurement. • The figure shows a conventional piezoelectric transducer with a piezoelectric crystal inserted between a solid base and the force summing member. • If a force is applied on the pressure port, the same force will fall on the force summing member. Thus a potential difference will be generated on the crystal due to its property. • The voltage produced will be proportional to the magnitude of the applied force. Theory of Piezo-Electric Transducer A piezoelectric crystal is shown in the figure below

The polarity of the charge depends on the direction of the applies forces. Where, d – charge sensitivity of the crystals F – applied force in Newton The force changes the thickness of the crystals. Where A – area of crystals in meter square t – the thickness of crystals in meter E – Young’s modulus N/m 2 The young modulus is,

where ω – width of crystals in meter l – the length of crystals in meter On substituting the value of force in the equation of charge, we get The output voltage is obtained because of the electrode charges. The g is the voltage sensitivity of the crystals.

Where E 0 – electric field strength, V/m The voltage sensitivity of the crystals is expressed by the ratio of the electric field intensity and pressure. When the mechanical deformation occurs in the crystals, it generates charges. And this charge develops the voltages across the electrodes. The Piezoelectric crystal is direction sensitive. The polarity of the voltage depends on the direction of the force which is either tensile or compressive. The magnitude and the polarity of the charges depend on the magnitude and the direction of the applied force. The advantages of piezoelectric transducers are • The transducer need no external power and is therefore is self generating. • It has a very good high frequency response. • Simple to use as they have small dimension and large measuring range. The disadvantages of piezoelectric transducers are: • It is not suitable for measurement in static condition • It is affected by temperatures • The output is low so some external circuit is attached to it • It is very difficult to give the desired shape to this material and also desired strength

Application of Piezoelectric Materials • In microphones, the sound pressure is converted into an electric signal and this signal is ultimately amplified to produce a louder sound. • Automobile seat belts lock in response to a rapid deceleration is also done using a piezoelectric material. • It is also used in medical diagnostics. • It is used in electric lighter used in kitchens. The pressure made on piezoelectric sensor creates an electric signal which ultimately causes the flash to fire up. • They are used for studying high-speed shock waves and blast waves. • Used infertility treatment. • Used in Inkjet printers

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