microsoft powerpoint - vibration and photoc
TRANSCRIPT
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Engin
etelegra
ph
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Engin
etelegra
ph
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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.
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vib
rometers
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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
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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
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V
ibrometers/piezoelectric
Thepiezoelectric
accelerome
terisbased
on
apro
pertyexhibit
edbycertaincrystalswh
ere
avo
ltage
isgenerated
acrossthecry
stal
when
stressed.
Here
,apiezoelectriccrystalisspringloa
ded
withatestmassincontactwit
hthecrystal.
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V
ibrometers/piezoelectric
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V
ibrometers/piezoelectric
Whe
nexposed
toanacc
eleration,
the
testmassstressesthecrystalbyafo
rce
(F
=
ma),r
esultingin
a
voltage
gene
ratedacrossthecrystal.
Am
easureofthisvolta
geisthena
measureofthe
acceleratio
n.
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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
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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.
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Pho
tovoltaic
detectors/principle
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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
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Pho
tovoltaic
detectors/principle
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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.
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Photodiode
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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
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Phototransist
or
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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.
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Photoem
issivedetectors
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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.)
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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.