Lecture10_Piezoelectric_Transducerx

8/3/2019 Lecture10_Piezoelectric_Transducerx

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Lecture10:Instrumentation 1


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Piezoelectric Effect Discovered in 1880 by Pierre Curie in quartz crystals.

The greek word piezein, which means to pressAppearance of an electric potential across certain

faces of a crystal when it is subjected to mechanical

pressure Examples --- Quartz, Barium titanate, tourmaline



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The effect is explained by the displacement of ionsin crystals

When the crystal is compressed, the ions in eachunit cell are displaced, causing the electricpolarization of the unit cell.

Because of the regularity of crystalline structure,these effects accumulate, causing the appearance

of an electric potential difference between certainfaces of the crystal.



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Quartz crystals (Silicon dioxide, SiO2) is one of the

most stable piezoelectric materials

The larger circles represent silicon atoms, while the

smaller ones represent oxygen



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Grey- test structure.

Red- piezoelectric crystals Blue- Sensor housing

The black electrode is where the charge from the crystals

accumulates before it is conditioned by the yellow, micro-circuit.

pressure sensors utilize a diaphragm to collect pressure,which is sim l force a lied over an area.



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The relationship between displacement (x)

and force (F) is

where k= stiffness of crystal is large

t icall = 2109 N/m

F

k

x1

=



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Piezoelectric sensors for measuring pressure,

force, and acceleration may be modeled by the

classical second-order differential equation

121

/1

2

2++

=

ss

k

F

x


n = 2fn is large typically fn = 10 to 100kHz

and is small, typically 0.01.

nn


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The deformation of crystal results in crystal

acquiring net charge q proportional tox:

d=K/kcoulombs/N, charge sensitivity to force F,2.310-12 coulombs/N

dFFk

KKxq ===

Electrical current is proportional to force F

Does not require power supply and the output

voltage is within 1 to 30 mV



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Quartz is preferred because:

Temperature resistance up to 930F

Very high rigidity, high linearity, and negligiblehysteresis

Almost constant sensitivity over a wide temperaturerange

Material stress limit of ~20,000 psi



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Typical Application-CombustionMonitoring

Pressures developed during thecombustion process is

con nuous y measure ysensors mounted on thecylinder heads



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Pros and Cons

Have a high Stiffness valueand produce a high outputwith very little strain.

Ideal for ru ed use.

Can be used only fordynamic pressure sensingas in case of static sensingthe signals will decay away.

Excellent linearity over awide amplitude.

Ideal for continuous online

condition monitoringsmart systems.

perat on over ong ca esmay affect frequencyresponse and introducenoise and distortion, thecables need to beprotected.



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Figure 1. The direct piezoelectric effect.


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Figure 2. The converse piezoelectric effect.


Conversely, when a piezoelectric crystal is placed in anelectric field, or when charges are applied by externalmeans to its faces, the crystal exhibits strain, i.e. the

dimensions of the crystal change.

When the direction of the applied electric field is reversed,the direction of the resulting strain is reversed.


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Sensor and Actuator

Transducers convert one form of energy to another. Piezo actuators convert electrical energy to

mechanical energy. This is why they are referred to

as motors o ten inear motors . Piezo sensors convert mechanical energy into

electrical energy. This is why they are referred to as

"generators". In most cases, the same element canbe used to perform either task.



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BASIC PRINCIPLE to generate an electric potential in response to applied

mechanical stress.

separation of electric charge across the crystal lattice


direct piezoelectric effect

production of electricity when stress is applied

converse piezoelectric effect

(the production of stress and/or strain when anelectric field is applied


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Key Properties

The ability to produce a voltage output in responseto an applied stress

The ability to produce a strain output (or.



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Piezoelectric materials are used inelectromechanical devices.

In the case of a microphone transducer, sound of a

particular frequency results in a strain in thematerial, which in turn induces an electric field.

Similarl in s eakers a volta e in ut into the

piezoelectric material can be converted into amechanical strain, such as in a speaker transducer.



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pp ca ons



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Application 1



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APPLICATION

CROWD FARM

Magic Carpet.

mechanical movement converted to electricity

though on a larger scale in which the mechanics would be supplied bya spongy floor in which embedded blocks move under the weight ofpassing pedestrians.

Grid ofpiezoelectric cables.


.

The conversion process - generator that uses a rotating coil andelectromagnets to produce an electric current from the mechanicalmovement.

lights and sound-depends on where you walked-different soundwould play.

examples : in some of the museums at MIT. At Georgia Tech -Smart Floor monitor and predict when people

were walking across it.


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COMMUTER A single human step can only power two 60W light bulbs for one flickering second.

multiply that single step by 28,527 steps, for example, and the result is enough energyto ower a movin train for one second.

APPLICATION


The East Japan Railway Company (JR-East) -train stations more eco-friendly.piezo elements that would generate electricity as commuters walk through.

Tested Shibuya, reception area. Pass through the gate, a lamp lights up, electricityproduced.

JR Easts- new- energy-generators under ticket wickets, a -milliwatt-tracking counter,and 700,000 daily commuters.

Vibrations of human footsteps at Tokyo Station to generate up to 100 milliwatts persecond per person that walks through.

To generate enough electricity to power the wickets themselves and their display

panels regularly.


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Blood Pressure Sensor


The Arterial Pressure Transducer (APT) is apiezoelectric transducer that provides fast, accurateblood pressure readings.


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Piezoelectric motor


Piezeoelectric motors are driven by ultrasonic

vibrations created from the piezoelectric transducer.


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Piezoelectric drill


A novel drive mechanism, which transfers ultrasonic vibrations of a

piezoelectric actuator into larger oscillations of a free-flying mass is

the central point of interest during approaches in understanding the

USDC: the free-mass impact on the drill bit creates a stress pulse at

the drill tip/rock interface causing fracture in the rock.


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