Microsoft PowerPoint - Vibration and Photoc

7/27/2019 Microsoft PowerPoint - Vibration and Photoc

1/22

Engin

etelegra

ph


2/22

Engin

etelegra

ph


3/22

vib

rometers

LVDT

is

used

to

m

easure

m

ass

displacement.In

these

instruments,the

L

VDTcoreitselfis

theseismicmass.

Displacementsofthe

core

are

converted

directly

into

a

linearlyproportionalA.C.

voltage

.The

yhaveana

turalfrequencyoflessthan

80H

zandarecommonlyu

sedforsteady-

state

andlow-freq

uencyvibration.


4/22

vib

rometers


5/22

vib

rometers

Vibro

metersare

notus

ed

near

the

resonance

at

theirnatu

ralfrequency

beca

useofhigh

nonlineartiesinoutput.

Ase

truleisf>2.5

fn

whe

refisapplied

frequ

encyand

fnresonantfrequency


6/22

vib

rometers

The

testmass

is

usually

a

permanent

magn

et.

The

measurementismadef

rom

thevoltage

induc

edinasurroundingcoil

asthemagn

etic

mass

movesunderthein

fluence

of

an

acceleration.

This

accelerome

teris

used

in

vibratio

n

,

becauseithasan

outputonly

whenthem

ass

isin

motion.Itsn

aturalfrequencyistypic

ally


7/22

V

ibrometers/piezoelectric

Thepiezoelectric

accelerome

terisbased

on

apro

pertyexhibit

edbycertaincrystalswh

ere

avo

ltage

isgenerated

acrossthecry

stal

when

stressed.

Here

,apiezoelectriccrystalisspringloa

ded

withatestmassincontactwit

hthecrystal.


8/22

V



9/22

V


Whe

nexposed

toanacc

eleration,

the

testmassstressesthecrystalbyafo

rce

(F

=

ma),r

esultingin

a

voltage

gene

ratedacrossthecrystal.

Am

easureofthisvolta

geisthena

measureofthe

acceleratio

n.


10/22

Photovo

ltaicdete

ctors

photodetectors

generatesa

voltagetha

tis

propo

rtionaltoincidentEMradiationintensity.

Thesedevicesa

recalledphotovoltaicc

ells

because

of

their

voltage-genera

ting

chara

cteristics.

They

actuallyconverttheEM

energy

into

electricalenergy.

Applicationsare

foundasbo

thEM

radia

tion

detec

torsandpowersources

convertings

olar


11/22

Pho

tovoltaic

detectors/principle

The

cellisactu

allya

gian

tdiode

tha

tis

constructed

usin

g

a

pnjunction

betw

een

dopedsemicondu

ctors.

Photonsstrikingthecellpass

throughthe

thin

p-dop

edupperlayerandareabsorbed

by

electrons

in

the

n

layer,

which

cau

ses

forma

tionofconductionelectronsandhole

s.


12/22

Pho

tovoltaic

detectors/principle


13/22

Pho

tovoltaic

detectors/principle

The

depletion-zone

potentialof

the

pn

junction

thenseparatesthese

conduc

tion

electrons

and

holes,

which

causes

a

differenceofpotentialtodev

elopacross

the

junction.

The

uppertermin

alispositive,andthelo

wer

negative.

Itisa

lsopossible

tobuildacellwithathin

,n-

dopedlayerontopsothatallpolarities

are


14/22

Pho

tovoltaic

detectors/principle


15/22

Photodiode

Thephotodiodeeffectrefers

tothefact

that

photo

nsimpingin

gonthep

n

junctionalso

alter

the

reve

rse

curren

t-versus-voltage

chara

cteristicofthediode.

The

reversecurr

entwillbein

creasedalm

ost

linearlywithlightintensity.

Thus.the

photo

diode

iso

perated

in

the

reverse-biasmode.


16/22

Photodiode


17/22

Phototransist

or

Itisa

nextension

ofthephotodiodeconceptis

thephototransisto

r.

Inthissensor,th

eintensityofEM

radia

tion

impin

gingonthe

collector-b

asejunction

of

thetr

ansistoracts

muchlikea

basecurrentin

produ

cing

an

amplified

collector-em

itter

current.

Figure

showstheschematicsymbolan

da

typicalgrounded

emitter

connection

of


18/22

Phototransist

or


19/22

Photoem

issivedetectors

Prin

ciples

To

understand

theba

sic

operationalm

echanism

of

ph

oto

emis

sivede-vices,

letus

consider

the

two-elementva

cuum

pho

totubesho

wn

inFigure.


20/22

Photoem

issivedetectors


21/22

Photoem

issivedetectors

Thecathodeism

aintainedatsomenega

tive

voltagewithrespecttothewireanodethatis

groun

ded

throug

h

resistor

R.

Thein

ner

surfaceofthecathodehasbeencoatedwith

aphoto

emissive

agent.Thismaterialisa

metalforwhichelectronsare

easilydetac

hed

from

themetalsurface.("E

asily"mean

sit

does

nottake

much

energ

yto

cause

an

electrontoleavethematerial.)


22/22

Photoem

issivedetectors

Inpa

rticular,then,aphoton

canstrike

the

surfaceandimp

artsufficien

tenergyto

an

electrontoejectitfromthiscoating.The

electronwillt

hen

bedrivenfr

omthecath

ode

toth

e

anodeb

y

the

potential,and

then

throughresistorR.Thus,we

haveacurrent

thatdependontheintensity

oflightstriking

thecathode.

Documents

Microsoft PowerPoint - Vibration and Photoc