Aei405

DEPARTMENT OF TECHNICAL EDUCATION ANDHRA PRADESH Name Designation : Branch : Institution : Subject : Semester : Code No : Major Topic Duration : Sub Topic : Teaching Aids : diagrams

: A. Vinay Kumar HEIE AEIE Q.Q GPT, Hyderabad. Process Instrumentation – II IV AEI-405 : Pressure measurement 100 Mins. Piezo electric pressure transducer Photos, animations, AEI405.16TO17

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Recap In last class we have learnt about the • Force Balance pressure transducer

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Objectives Upon completion of this topic you will be able to know about Piezo electric pressure transducer • Description • Principle • Operation

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Introduction • When pressure, force or acceleration is applied to a quartz crystal, a charge is developed across the crystal • The fundamental difference between these crystal sensors and static-force devices such as strain gages is that the electric signal generated by the crystal decays rapidly

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Piezo electric effect • Some types of materials generates an electrostatic charge or voltage when mechanical stress is applied across them • An opposite effect is also observed when an electrostatic charge or voltage is applied to the crystal • It results mechanical deformation of the device • This is called piezo electric effect AEI405.16TO17

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Piezo electric effect • It is used for the design of pressure transducer • In this mechanical stress is generated by the diaphragm subjected to pressure • The stress distribution in the crystal will depend not only on the load applied but also manner in which it is applied, size and shape of the sensing element

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Classification

• Depending upon phenomenon is used, the crystal sensor can be classified as • Electrostatic • piezo resistive, or resonant

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Principle • When pressure is applied to a crystal It is elastically deformed • This deformation results in a flow of electric charge (which lasts for a period of a few seconds). • The resulting electric signal can be measured as an indication of the pressure which was applied to the crystal

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Principle… • The relation between output voltage and pressure is given by v =g t p Where, • V = output voltage • g = voltage sensitivity of crystal • t = thickness of crystal • p = applied pressure AEI405.16TO17

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Piezo electric pressure transducer

Fig,1 AEI405.16TO17

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Construction and working • It consists of a diaphragm by which pressure is transmitted to the piezoelectric crystal y1. • The crystal generates an electrical signal which is amplified by a charge amplifier. • A second piezoelectric y2 is included to compensate for any acceleration of the device during use.

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Construction and working… • Signals from the compensating crystal are amplified by a second charge amplifier. • A differential amplifier is used which subtracts pressure alone; all effects of acceleration are removed

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Bimorph beam mode

Basic element

Cantilever

Pinned end AEI405.16TO17

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Piezo electric crystal F

d Normal AEI405.16TO17

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Bimorph disc mode

Pinned end

Pinned end

Centrally supported AEI405.16TO17

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Piezoelectric Materials Materials used are •

Quartz

•

Rochelle salt

•

Synthetic ceramic material like a. Barium – Titanate b. Lead - Ziconatelianum

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Advantages and disadvantages Advantages • Need no external power (Self generating) • Very good high frequency response.

Disadvantages • Cannot measure static pressures • Affected by changes in temperature

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Applications

• Used to measure very high pressures that change very rapidly. • Used to measure the pressure inside the gasoline engine. • Used to measure the pressure in compressors, rocket motors etc.

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Applications… • • • • • • • •

Rapidly varying pressure Shock pressure Frequency response In aeronautics Turbines Pumps Hydraulics Acoustics

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Assembly of PEPT Diaphragm

P

PZT element

Backing piece

Lead Main case

Connector AEI405.16TO17

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Diagram Electrical connector Shrink tubing grooves I.C amplifier

Element lead

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Cross section

Element head

Seal surface Preload screw

Diaphragm AEI405.16TO17

Quartz crystal Electrode End piece 22

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Acceleration-Compensated Piezoelectric Sensor

Seal surface Compensation crystal Seismic mass Seal surface End piece Diaphragm AEI405.16TO17

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Piezo resistive pressure transducer

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Piezo resistive pressure transducer

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Piezo resistive pressure transducer Printed circuit

Sensing element

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Specifications

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PEPT

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Application

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Circuit diagram

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Piezo electric pressure transducer

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Piezo electric pressure transducer

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Comparison of piezo-electric transducers Principle Piezoelectric

Strain Sensitivity [V/µ*] 5.0

Piezoresistive 0.0001

Threshold [µ*]

Span to threshold ratio

0.00001

100,000,000

0.0001

2,500,000

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Limitations • Piezoelectric transducers are not capable of measuring static pressures • These sensors cannot detect static pressures • Temperature range -200 Deg.C To +300 Deg.C • 1 Hz -20 kHz

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Summary We have discussed about Piezo electric pressure transducer • Principle of operation • Construction and working • Advantages and disadvantages • Limitations

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Quiz 1.

Piezo electric pressure transducers are used to measure a) Static pressure b) Dynamic pressure c) Both d) None

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Frequently Asked Questions 1.

Explain the principle of operation of Peizo electric pressure transducer.

3.

Draw the diagram of Peizo electric pressure transducer.

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