SR Technics
Basic Maintenance T r a i n in in g M a n u a l
13.8 Instrument Systems (ATA 31)
Se;:>041
Correspond in
WIth
EASA Part-66
Module 13 Aircraft Structures and Systems (A A 31) In tru ent Sys
Position Transmitting
Ratio System Moving co me er servo systems AC converters etc are all devices tha use signals simple way to show the level or amplit de of ra io gnal is with mov ing coi mete This type of in dica or has low to que avai able to dr ve other sys ems When othe systems need more orque a servo system is a better choice
DC Ratio Signals
AC an
Module 13 A rcraft Structures and Systems 13. s (A (ATA TA me nt Sy
Basic Maintenance Training Manual
SRTechnics
Introduction An AC or DC atio signal as va ri able amp itude or level. A certain pa ameter controls he amp ude or evel of such a s gnal. A device wit va iab e outpu lev makes such a signal. The plit de or evel changes under control o the pa ra er betwee a h gh and a low evel. These levels are diff different erent from device to device and depend n t he design f the t he device
Figure DC
Potentiometers sy chros RV Ts/LVDTs and rate generato s are examples of dev ces that make AC or DC rat o signa ers hi ch con rol he outpu of these dev ces are for example speed gu ar displ dis pl acement Figure
AC
AJD
Amplitude depending of Parameter
on
Ampl tude
n-""
Figure
°n--
u.
Analog Meter
arameter
Paramete AC
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TT
Indi ca ting
'/
DC
Corre
on ng with EA A Part
'/
te
Position Transmitting
Ratio System Moving co me er servo systems AC converters etc are all devices tha use signals simple way to show the level or amplit de of ra io gnal is with mov ing coi mete This type of in dica or has low to que avai able to dr ve other sys ems When othe systems need more orque a servo system is a better choice
DC Ratio Signals
AC an
Module 13 A rcraft Structures and Systems 13. s (A (ATA TA me nt Sy
Basic Maintenance Training Manual
SRTechnics
Introduction An AC or DC atio signal as va ri able amp itude or level. A certain pa ameter controls he amp ude or evel of such a s gnal. A device wit va iab e outpu lev makes such a signal. The plit de or evel changes under control o the pa ra er betwee a h gh and a low evel. These levels are diff different erent from device to device and depend n t he design f the t he device
Figure DC
Potentiometers sy chros RV Ts/LVDTs and rate generato s are examples of dev ces that make AC or DC rat o signa ers hi ch con rol he outpu of these dev ces are for example speed gu ar displ dis pl acement Figure
AC
AJD
Amplitude depending of Parameter
on
Ampl tude
n-""
Figure
°n--
u.
Analog Meter
arameter
Paramete AC
Sep
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Indi ca ting
'/
DC
Corre
on ng with EA A Part
'/
te
A variable es ce si gnal is ade y a evice of w ich a certa parameter con es sta ce e res ce va ies betwee a h gh a d a ow value hese ro s t ts depend on he ty e and r nge o the es sto Parame ers w ich contro essur ess ur l i n e a r an
no
8 Inst um nt Sy
s(
gure 6: Potent ometer
Variable Resistance Signals
Figure
Module 13 Aircraft Structures and Systems
Basic Maintenance Training
SRTechnics
Linear Potentiometer
es stance a e, for example temperature rotation o linear
Non-L near .,.
esistanc
Angu ar Pot ntiom te
Figure
Lin ea
'''' I--- Ra
Resistor Rheostat and Potent ometer
e--J
ara
er
Potent ometer Resistance Temperature Sensor in
igure
Circuit
Variable Res stor
~ - - - - -
- - - - - -
- - - - - - -
- - ;
Rheo ta Br dg Circui AID Conv rt
Etc
1---."
Output
Potentiometer
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TT
Co rr es ondIng
th EA A Part-66 Pa rt-66
Rheostat
31)
Module 13 Aircraft Structures and Systems 13 nstr ns tr nt Sys ems (AT 31)
Basic Maintenance Training Manual
SR Technics
Figure 9 Output of
Control Transformer gu
I Stator
Voltmeter
a)
26VAC Excitation
Rotor A trans ormer consis of a primary and a secondary co l. he pr mary co il produ es a cont y changing magnetic fl in the ron core n the se ondary co the changing flux nduces a alternat ng voltage e p imary co il exc ted by s ali gned wi h t e secondary coil. Outp ut has t e same p ase angle as he in ut vo lt age 90 clockwise rotated. The pr mary coi nu l. the secondary oil. Outpu
o magnet c fiux goes through
the rst position The primary co il s 180 the oppos te he phase ang e o the output voltage is opposite of the np t voltage il e p imary No magne ic f
70 ro ated The output s a so es hrough se ondary coi
ll
os tions n between between t he 4 shown card na pos ions wil l change the amplitude of he out ut not the phase ang e.
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on
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EASA Part 66
n
p
u
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Control Transformer
Module 13 Aircraft Structures and Systems
Basic Maintenance Training Manual
SR Technics Synchros
Figure 1 1 : .
- - - - -
Introduction
A typical synchro has a rotor three sta or coils The coils in he stator are a 120 degrees wi h respect o each her. his unit ac like 3 control tra sfo mers contained in e u t.
'----
Figure 10
5,
s , . - - - - - - - - - - - - - ~
53
s,
Symbol
1 ' - - - - \ - -- - - - , .
yn hr pl
ri nc
Sy chros se 26 V AC or 1 5 V AC exc tat on of he ro or The excita on akes a mag c field in the roto co l. his magnetic fie in uces a vol age in the stato coils he vol ages n the sta or coils a pha e r 80 egrees o t-ofse espec to each other The vo tage i the stator coi s depen s on the a between the rotor coil a d each s ator co l. Whe we turn the otor mag ic fie ld the tator also turn an d t e volt n the stato co ls change
Sep041THTT
Correspond ng with EASA Part-66
]Cifl
SRTechnics
Basic Maintenance Training Manual
Module 13 Aircraft Structures and Systems
13 8 Instrument Systems (AlA 31)
Figure 12:
Direct Torquer Systems The output signal of a synchro is an AC signal which has angular informatio n. The synchro's which make these signals are synchro transmitters These transm tters are of the old multi-coil type or of the latest sol d-state type The multi coil type makes from a mechanical input a synchro signa , the second from an e ectr ca input.
Synchro Receifer
In a synchro system we connect the hree output signals of a synchro transm tter to the three inputs of synchro (receiver) The field that is made by the rotor of the synchro transm tter s now repeated in the sta or of the synchro receiver. Before the rotor of the rece ver takes the pos tion the field the sta or we have to make a field the roto of the rece ver This field must be 180 out of phase with the field made by the synchro ransm tter. The rotor of the synchro rece ver now goes to the same pos ion as the rotor of the synchro transmitter Any time we change the posi ion of the rotor of the synchro transmitter the rotor of the receiver follows this turn.
" ~ , ~ = = = = = ~ ' u E "VJ'
TX TR
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TT
CorresponcMg
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EASA Pa rt-66
Torque Tran smitter Torque Aeceifer
"VJ' Synchro Transmitter Synchro Receifer
SRTechnics
Module 13 Aircraft Structures an
Basic Maintenance Training Manual
Servo Systems
Systems
13.8 In strument Sys ems (A
31)
Figure 13: Servo System
The rotor of a synchro receiver gives a li ted torque fo other systems When this torque is not high enough we have to use a servo system In a synchro-servo system the rotor of the synchro rece ver gives a signa to a ser vo amplifier. In this system the rotor of the rece ver s not connected to a supply source but t makes a signal from the stator field in the receiver synchro. The out put signa of the servo amplifier drives a motor The motor dr ves a a reduction gear the rotor of the synch o receiver and a load When the output signal of the rotor of the synchro receiver s not zero the servo amplifier drives the motor The motor adjusts the posit on of the rotor of the syn chro rece ver and the oad unt the output signal of th s rotor zero. This output signal is ze o when the angle between the rotor and the stator field is 90 degrees
_ _ _ _ _ .J
S e r v o m o or
Servo Amplifier
Figure 14: Monitoring
The outpu of the rotor of the synchro receive is also zero when the transmitter supply fails or a rotor wire is broken To make it possible to detect these fai ures there are synchro rece vers with two rotor ndings These two windings are at 90 degrees with respect to each other. VV'hen the rotor of the synchro rece ver is in the correct position. the output signal of one coil is zero and the output of the othe coil is maximum. With these two signals it is possible to see if the system works prop er y. A continuity detector monitors the two output signals of the rotor coils and when everyth ng is all righ it enables a valid signal 10 for example. a lag
~;~
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\ J t ~ l 1
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____
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Module 13 Aircraft Structures and Systems
Basic Maintenance Training Manual
SR Technics
3.8 Instrument Systems ATA 31)
Figure 16 OS used to synchronize
Differential Synchros With a different al synchro it is poss to add or subtract angles This synchro has three co il in the rotor and three coils in the stator at 120 degrees with respect to each other. When the rotor of th s synchro s turned toward left or right it adds or subtracts this angle from the angle the stator field has in the stator Figure
u.
Valve CT
Compass System
Slaving Amp
15 Symbol
x----------
flUX
'VALVE
____
:,
Input
Output
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--+--is:,;n@
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z----.J
Amplil
Ou
1
11
otOr
The posi on of the re ional gyro is transmitted via differentia synchro to the master shaft f the master shaft corresponds to the ux valve signal the annunci ator shows zero and the slaving s correct.
HOG Data Ou
VAC
} 4 . - - - 26 VA HOG
Trons ,
DG slaved compass system shows the usage of a differen
the mas ers aft is not the position which represents the madlnetic heading the slaving amplifier orques the direct onal gyro with a rate of per minute to the correct position
~ - - - H - O ~ G 4
Oir Gyro
Rndio Directio ndicotof RD 20 1)
) - > f - - ' - - 26 VAC
Ann nciator
" HOG Data
Ou'3
~ - - - H - O ~ G 4
)->f-..;'-
-
-
-
+ - ~ .
26 VAC
Remote Mon
HOG Data Ou
If the difference of mastershaft and earth magnet c field direction is to big synchro niz ng takes to ch time so the pilot changes the DG output s gnal w th the dif ferential synchro to synchronize the compass manually until the annunciator shows zero In th s case the DG will then maintain its own direction
THTT
@r
HOG Doto "-
The flux valve sends the direction of the earth magnetic field to the flu va ve con trol transformer
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Torque
nstrument
y - - - - - - - - - - - - - - - - - - ~ The next diagram of tial synchro
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----
Corresponding with EASA Part-66
r - - ~ " ' " HOG
H--'--
HOG 26 VAC
------
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___________ --
Remo
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- _ ~ -
~ H O G SEL
r: --
- - ~
"---------'
SR Technics
f;
Modu e 13 A rcraft Structures and Systems
Basic Maintenance Training Manual Figure 19
Resolvers
Resolver as Angular Transmitter
The resolver as two s ato coils a d a ro or coil. The two oto coils and he two re at 90 degrees w h res ect to each other st to coi esolve makes ro si ls in the s ator coils sine a nd cosine sig als Figure 17 Resolver
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gnal depending of existing Angle
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Figure 20 Resolver as Phase Angle Sh fter
·360
Phase Angle Selection •
36
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Phase Sh fting Network
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SR Technics
Module 13 Aircraft Structures and Systems 13.8 Instrument Sys ems (ATA 31
Basic Maintenance Training M a n u a l
Linear Variable Differential Transformer LVDTs
Figure 21 LVDT
A transformer nduces an output voltage he secondary coil. According the d re on of the wind ngs the phase angle is in zero degr. or oppos te phase 180 degr.) The phase angle can be determined w th dots
Po
:JIC Po
Po :JIC
Linear Variable Differential Transducers (LVDTs) change linear pOSit on informa tion nto ectrical s gnals An LVDT has one pr mary li near moveable iron core and two n seria connected secondary co
~ s e C O
The mechan ic input changes the position of the ron core The pos on of the core hanges the magnetic coupling between the primary and the secondary coils When the input moves the core in one direct one of the secondary coils re ce ves more magnetic flux This induces a higher voltage in the co il.
Electrical Output
Iron -Core (moveable)
The ther secondary coil rece ves less magnet c flux This induces a ower voltage The difference between vo tages induced in the secondary sta or co il s propor onal to the mechan cal pos on
A.
The position of the ron core is centred The magnetic fie d induced by
zero
ri mary is
The iron core has moved upward Now there is more coupling to the upper coil and less coup ng to the ower coil The output vo tage ncreases and is in phase with the excitat on
J r ~
C The i on core has turned downward Now there is more coupling to the ower co il and less coupl ng to the upper coil. The output voltage increases and is n op posite phase with the excitat on
Sep0
ary Coils
Co rres
nd ng Wit EA SA Pa rt·6S
Cat 82
13
10
Demodulation and Indication
TIle AC volt ge from the LVDT s proportional of the deflection of the core and the phase is depending of the core directi on e signa must be phase dependant de modulated Synchron demodula or) TI1e ampl fied signa goes via elec ro ic swi ch Th s closes periodica ll by the pos it ve phase of the re ence vo tage With o i t sign the ndicato sho s zero
Figure 22 Neutral Position Refe re
CT
Module 13 Aircraft Structures an
Basic Maintenance Training Manual
SR Technics
Volt
13
In trume nt Sys
Systems
s (A A 3
When the core moves to the r ght. the positive s ne wave passes d uri ng trans t time the electron c sw tch The pointer of strument sh ws to t e r gh side Figure 24 Deflection toward left
---
Di
VOT Aplifi
Electronic
> - - - - E ~ l ;
Ind ic a t o r
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&.
Vv'h n the core moves to the eft
he negat ve s ne wave passes during t ansit time the electronic sw tch The painte of e instrument shows to t e left s de Figure 25 Usage of l V D T
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Figure 3: Defle tion toward right
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SR
Basic Maintenance Training Manual
Technics
Rotary Variable Differential Transformer RVDT
Module 13 Aircraft Structures and Systems 13 nstrument Systems (AT 31) Figure 27
Figure 26
Zero Pos tion The position of the iron core s zero The magnet c f eld induced by primary coil equally div d Therefore the vo tage R-T is zero ed between L,and
RVDT ANGLE
~'
:~C~;~TlON 26 VA
- - E E i l l o ;J
Rotated ockwise The iro co e has turned clockwise Now there s more coupl ng between L3 and L and less coup in g between L3 and L, he ol tage between and reases and the voltage between Rand S de eases
Ro ary Variab e fferent Transducers RVDTs) cha nge angular pos on nfor mation nto electric signa An RVDT has a pr mary stator coil an iron rotor co re two secondary stator coils
The mechanical input changes the posit on of the iron core The position of the core changes the magnetic coupling between the primary and the secondary sta tor oils When the input rotates one of the secondary coils re eives mor mag ne ic flux This induces a h gher voltage in the coil. The othe secondary coil receives less magnet c flux Th is induces a lower voltage The difference between voltages nduced in the secondary stat coi ls s propor tional to the rotated angle This is an AC Ratio Signa l.
Rotated counter clockwise The core has turned counter-clockwise Now ther is more coupling between and L, and less coupli ng between L3 and The volt age between T and S decreases and the vo lt age be tween Rand in reases
The difference between ro ation directio is that he ou tpu voltage between Rand is of opposite phase The output measured between Rand an AC-RATIO signal which represents the rotated angle of the RVD
Sep04
THTT
Cor re spon
ng wIt
EASA Part 66
13 8 - 1 2
Basic Maintenance Training Manual
SRTechnics Servo Motors an
Module 13 Aircraft Structures and Systems
13 8 Instrument Systems ATA 31)
Figure 28 Servoloop with DC
Tacho Rate Generators
Motor
Servo Loops The DC servo motor oop is called a loop because of the closed nature of the sys tem operation Th DC source is connected to the variable control potentiometer and to the follow up pote omete r.
Servo Amplifier
Motor
A servo amplifier amp li fies the ra tio gna and drives a motor with t. The motor drives a feedback device and oad e sig from the feedback dev ce also es to the servo ampl er. he oad is n the corre pos ion when the d fference between the ra io and the eedbac signal is zero
Servo Loop with DC Motor
--;--8
Feedback Device
Any time there is fference between the two gnals the motor dr ves the oad and feedba k unti both signals are equa l. The po arity of he fference determ ne the d re tion of rotation gure 29: Servo Loop with AC
Servo Loop with AC Motor
en we must have even more orque, it is better to use an AC mot r. A chopper circu t makes AC from the DC signa l. To drive the AC motor with this signa we need an extra ampl fier. The rest of th s system works like the DC servo system
Motor
Source Servo Amplifier
.------'-----, Chopper
_____
In
_____ ____________
Feedback Device
SC
TT
CorrespondIng
th EA A P art 66
Module 13 Aircraft Structures and Systems 13 Instrument Systems ATA 31
Basic Maintenance Training Manual
SR Technics Two Phase Servo Motor
Figure 30: 2 Phase Motor running CCW CW and braked
The AC two phase inductance motor servo motor It may be very sm all or it may be quite powerful. The construction of the stator has tw fields Th ese two fields are repres en ed symbolically by two coils drawn at rig ht angles to each other.
Variable
~ 8 }
L ~ 0 '
Counter Clock Wise Rotation CC
r:T)
00 Hertz voltage with phase ang e o If is connected to the variable field and an th er 400 ertz voltage with a phase angle of 90 The resu ting field ro tates at 400 evolutions per second apaci or i series with the fixed fie ld sh fts the voltage 90 of the variable field
8"
Variable
(""1 Fixed
Fixed
L90
[9
Rotati on of the magnet field n the motor tends to rag t he rotor after in he same irectio How fast the moto oves depends upon its load and the strength of t he magnetic field, which effective dependent up n the strength e vari ab signa l. Clock Wise Rotation CW
he phase th var able signal has been reverse This reverse the direction of rotation of the resul ta nt fie ld he direct on of motor ro tation depends upon the phase of the variable sig nal, and he speed of rotation depends upon its amplitude Braked
Figure
Often times it s desirable to apply elect ical brake to a two-phase servo motor This can be done by isconnecting ei her th var able ie ld or ixed fie only one field is eft perative the motor does not otate because the fiel oes not o fi xed pos on tat Th tends to hold the rotor of the motor Servo motor l o o p USing
31
Servo-Loop with Control Transformer Ope,aled Item
iii
control synchro input
""
_ _ _ _ ..... ..
- - - - - - -
- - - - - = 5 --10-'
tlor
Control Transformer
- - - - -
- - - - - - - - - - ~
V-a-ri abl;
~ , . . . . . ~ r . ; . _
The operating s gnal will come from some remote sou ce whose mechanical posi tio we want to duplic ate the operated item For example the re mote source could be a direc ional gyro and the operated item could be compass ind ica to
C()(respondlng
--------------Servo Loop ..
:;
is yp al of many such loops used throughout aircraft systems Whenever the amplifier sees signa of a particular phase drives t motor n a parti ular di re tion until th e synchro ro or comes to a part ic ular null. signa of opposi phase rom the synchro rotor drives the se rvomotor th opposite direction The synchro nu ll. tor therefore always is dr ven to a particu
Sep041 THTT
Phase Motor in
[:A~Plifier
I
EASA Parl·56
/,
Fi ed L9
WIII'I
......
//
Tachometer
Rate Generator
Tachometer not running
The powered fixed fie d is perpendi per endicular to the va iable fie d the of he variab e f eld winding
to the variable field The magneti field is o transformer action and no s gnal out
Rotor turns in clockwise direction
The r tor as te ding to drag the magnet fi eld along with i t. A sl w rate of movement of the rotor does nol bend he magnetic field very much whereas a greater aie of mo tion moves the eld farther e gene ated voltages in reases n spee of the ro or
Headin itter
,
ra
r.'1 18
Variable
Er or
a d i n g ard os it on
or
...
-., __
Ho ·o er vo AMP
Rate Fe ed ba
Ta
er at or
Ai cr aft He !)l
In trument Amplifier
Output
Con rol Tran or
Co m m a n d
Const nt Frequency va able Amplitude Output
-----------G
ad
AO
of the greates uses of the rate ge erator s to provide nverse feedback s nals n servo motor systems or speed im ting a d smoothing functions Another use is o prov de rate signals
Sep04/ TH ff
Servo Loop with Tacho Generator
26
he field has been moved away from pe pe dic ar in the OPPOSite rec tion therefore he phase of the outp pposite. he ou t frequency is independent of rotation speed 400 Hz.
400
.,..
@-----------G r:'l Variable utput
COfres on (! mg
31)
Figure 33 Heading Dial Servo Loop with Rate Feedback
Rotor turns in counter clockwise direction
Fixed
Sy tems
s (A
The acho gene ator produces a signa proportio al ith the rolat g speed o the mo or wit the heading d The rate eedback is opposite applied t the amp lifie r. This educes he rotat ng speed o the indicato to preve nt ove oot g and osci lations when he scale eaches its final pos ition.
Speeding up the r tor displaces the magnetic fie d farther away fro perpendicuar. aus ng a large voltage o appear at the outpu t. The amplitude of the outpu s a d rect function of the rotor speed
Fi ed
rumen Sys
The ai craft heading. represented by the mas er aft angular pos on send as heading signal to the contro transformer. f the /lead ng d l of the cator does not co respon with the ai craft heading an e ror s gna resu s to the input o he amp ifier he command s gnal d ives he molor If the headi posit s ident ca with the a rcraft head ng the err or outpu of the ontro transformer goes to null so the motor s ops.
A low r tor speed the small t ansf rmer co pling results in a small ou put voltage At c ockwise direct on of rota on the output is a 400 z w th a phase angle 18 deg ees w th a small voltage amp litude
32
In
Example
Tachometer generato and ra e gene ato e the same de ice The output f om achometer generator s in phase or opposite phase ang e w th the exc tat on
Figure
Module 13 Aircraft Structures an
Basic Maintenance Training Manual
SR Technics
th EAS
l' rt·66
Compa ss Indi ca to
Hea
ng
Module 13 Aircraft Structures and Systems
Basic Maintenance Training Manual
SRTechnics
13
nstrumen Sys ems (
31)
Gage Pressure Instruments
Pressure Measuring Instruments Pressu is the amount of force acting on a given unit of area and all ressure must be measured fro some known eference Absolu e pressure is measured rom zero pressure a vacuum Gage p essure is measured rom the e ist ng at mosphe ic essure and different pressure s the difference between two pres res
Absolute Pressure Instruments Th s instrument uses a sealed evacuated conce trically corrugated metal cap su e as its pressure-sens ve mechanism co centr c corruga s provide a degree of spr nginess ha opposes the pressure of the a As the a r p ess re in creases the hick ess of t e capsule decreases d as the pressure decrease s, the capsule expands A rocking shaft sec or ge pini n m ti pl y t e change n d me sion of the capsu e and d ri ve a poi ter ac oss a ca ib ated ial.
gure 34 Aneroid
Gage p essure s measured rom the ex st ng barometric pressure a the pressu e t at has been added to a fiuid.
s a ua
Burdon Tube fla Bo rdon ube s typ ca y used to measure gage pressure This tube is tened th n-wall bronze tube orme to a curve One end f the tube is sea ed attached through a linkage to a sec or gear The o her e d is onnec to the i o be measured to en er st ment case through a f tting that al ows the fiu
When the pressure o the f in side the tube in creases t tries to hange the cross sect onal sha e of t e t be from fiat to round As the c oss sec changes the curved tube en s to stra ghten out. T is i urn moves the se or gear ich ota es he p nion gear on which t po nter s mounted ourdon tube instrumen s m asure relative y h gh pressures l ke those in engine lubr ca g systems and hydraulic sys ems Figure 35 Burdon Tube
An
Chamber
Pinion
..
.l!
- - . : : ~ L - . ~
Q.
.,
;{ Bourdon Tube
Sector
p0
THTT
Co respOndi g W
EASA
art-66
Cat B2
13
16
Basi Maintenance Training Manual
SRTechnics
Module 13 Aircraft Structures an
13
Systems
nstrument Systems ATA 31
Bellows
Differential Pressure Instruments
Lower p essures su ch as nstrument air p essure de cer a r pressure and suct on e often measu ed with a bellows mechan sm much li ke an anero d capsule The ess ur o be measu ed is taken into the bellows As the p essure in cr eases the bellows expands and i s expa on rotates the rock ng shaft and he se tor gea r. Movement of the sector gear rotates the p on gear and the sha ft n w hich the poi te is m unted
A differential pressu e is s mply the d fference etwe n tw pressu es A d ffere n tial be ll ows ike that in the figure below is a popu ar nstrument mechan sm that ca be sed to measure absolute diffe en l, or gage essure
Figure 36
can be used to meas re gage p essu
llow Mechani
When used to measure diffe ent
pressure as it s when used as a fuel p essure t. and the ot er gage, one bellows senses the ai pressure at the carb re or
and Instrument
le by eaving one of the be ll ows open to the
atmosphere and the other connected to the pressure to be measure Figure 37 Diff rential Bellows with Indicati n M ch ni
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Basic Maintenance Training Manual
Module 13 Aircraft Structures and Systems 31 13 Instrum nt S em
Strain Gages
Piezo-Resistive Sensors
is electr ic assive dev ces re sed to detect forces The resis ance o strain gages va es with the fo ce ap lied to it. The metallic wire co sist of a c rome ckel alloy he ength and he dia ete the conductor chang s as a func io xpand orce inc ses shorte orce decreases t e res st of he orce ance
ducting elemen s are diffused i to a pure silicon subst ate Th s so p- or called iezo-resist ve effect changes th e resistance wi h a much h gher sensit vi a m allic stra in gage d es Se ico ducto based se sors in m an fferent f ms he subst re sensor shown be ow as a imensio of 5 x 3 5 mm ns de the with ents
This sensors are sed fo ifferen applicat ons tru ure mo ni to ng ce so pres su re ans du cers an we ght meas in nside pressure sensors the pressure affec s c anged in o force
the pres is a bridge
Figure 40 Piezo Resistive Element
Figure 38 Strain Gage Force
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Figure 39 Pressure Indication using Strain Gage Bridge
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Module 13 Aircraft Structures and Systems 13.8 Inst ru ment Systems (A A 3
Basic Maintenance Training Manual
SRTechnics Variable Frequency Signals
variable freque cy signa as a frequency which is controlled y a certain pa rameter. device wi h a variab output frequency makes such a signa !. The fre quency varies under contro of the parameter, between hig and low frequency These li mit frequencies are d fferent from device to device and depend of
control voltage. a variab capacitor, or a variable esistor are for example pa rameters th t control the frequency_
Th is very sensitive and accurate p essure transducer is used ins de irda com pu ers The osci llator coil assemb ly oscillates the d ia phragm, ts resonan frequen cy increases wi h the app ied pressure against t vacuum efe ence nside the transducer The output frequency proportio nal to the pressure is easi change ins de the computer, into a digital signal. The tempe ature sens ng resisto compensates in fluences of the ambient tempe ature Figure 42: Vibrating Diaphragm Transducer
Frequency coun ers microprocessor system and specia mov ng co il mete s are all dev ices that wo k wit variable frequency signals Figure 41: Linear Param te Output after Conv rsion
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Basic Maintenance Manua Train
SR Technics Temperature Indication
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Module 13 Aircraft Structures and Systems
13 8 Instrument
ste s (A A 31)
Expansion
Temperat re is determined y measuring the pressu f the vapors above a high liquid The vapo press ure varies di ectly as the temperature of the quid ly vola
Temperature is one of the most important measurements in the aircraft opera ti on Operational emperatures range rom well be ow freezing for outside air fu el oil air-conditioning and pneumat c air to aro nd QQQ C for exhaust gas empera tures.
Bo urdo tube consists a hollow brass or bronze lli ptica shaped t be fo rm ed to a semi ircle One end of the tube is open and connec ed to the fluid to be meas red the opposite end of the ube is sea ed As pressure is applied el liptica tube changes shape a d tends to straighten the semi-circle curve The bourdon tube need to bee attached to a mec an ca nkage and pOinter to create a usefull inst ument.
Nonelectrical Temperature Measurements Most solids liquids and gases change dimensions proportional to their tempera ture c anges ese dimensional chan s may sed to move pointers ac oss a d to indica changes temperature
is connec ed to Bourdon ube by a capi ll ary tube, hat th wa ll hollow me al b has a very small nside diameter he bu b is filled with a volati e liquid su as me thyl chloride whic h has a high va or pressure and the enti bu capillary and ourdon tube are sea ed as a unit. The bulb is placed where he temperat re be measured and as its temperature cha ges the pressure of the vapors above he li quid c ges This p essu e change is sense by the Bourdon tube wh ch moves a pOin er across a dial that is calibrated in deg ees ahren or Ce us
Bimetallic
ost sm ll gene al aviation aircraft have an outside ai temperature gage protrud ing through he w ndshie d. This s mple the mome er is made of trips of two met als hav ng different coefficie ts of ex sio welded together side by side. and twisted into a elix or s al. When thi s bimetallic s rip s heated one str p expands more than he other a he s iral tries to strai ht ointer is ttached to t. the metal strip in ch a way that as the tempe ature cha ges the poin moves across a ial o indicate the temperatur
igure 45 Remote Temperature Indication with Bourdon Tube
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Basic Maintenance Training Manual
SR Technics Temperature Dependant Resistors
The NTC Negative Temperature Coefficient) resistor. It's esi tance decreases at ncreasing temperature s So it is calle High temperature conductor He issleiter
The PTe (positive Temperature Coefficient) res stor. It's resistance ncreases with nc reasing temperature So it s called Low temperature conductor: (Kaltleiter)
Figure 46: Resitance verus Temperature
Module 13 Aircraft Structures an Systems 13 Instrument Systems (ATA 31)
Tile temperature sensing bulb co ns ists of coi of fine ckekhrome ire encased and sealed in a thin stainless steel tube This bulb immersed in Ihe fluid whose
temperature being measured The resistance o the nickel-c rome ir ies directl ith its temperatu re At the low end te mperatu re, the bu res stance is ap proximate 20 ohms, at ts high end its resistance is about 200 ohms Figure 47: Resistance Temperature Bulb
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Module 13 A rcraft Structures an
Basic Maintenance Training Manual
SR Technics
13.8 I stru
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Systems
s (ATA 31)
Ratio Meter Circuits
Bridge Circuits
The instrument uses two coi s mounted on the ca or needle When he temper ature is low and the bulb resistance is ow more cur ent ows th gh he coil 1 and the bulb than flo s through coil 2 a d resis or R1 The resul ing magnet c field lls the needle toward he ow side of e dia l. When the emperatur is high a more urrent lows thro gh coil 2 and R 1 than coil 1 and the bulb, the needle de flects toward the high side of the l.
(Whea stone) Bri ge circ its are a specia type of comp ex circuit often used in ectri al measuring and controlli g devices e figure be ow shows a typic bridge ci it used to measure temperature Resisto is a te pe ature probe It esistance cha ges as its temperature changes
Figure
48
Ratiometer with Mov ng Coi Instrument
When the br dge s electrically powered. electrons nds two path throug which they an flow hey can flow through esistors R, and R2 or they can flow hrough reSis R3 and R f the fou resis ors have values such at the ratio of the resistance R, to 2 i the same as the at of then he vol age at poi nt wi ll be the same as the to vo age at pOi t D Because t ere is no voltage rop (no voltage difference) across the dica or no current will flo hrough t. In th condition dge s said to be balanced
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Thermocouples Higher temperatures, l ke those fo nd in the exhaust gases of both reciprocating and turbine engines are measur ed with thermocouples_
Module 13 Aircraft Structures an
: C linder Head Temperature (Sp rk plug ga ket)
u " " O ; : = " "
thermocouple is a loop made of two different kinds of wire welded together at one end to form a hot or measuring unction For example chromel and alumel wires The co of current measu ng i st ument is connected between the wires at he other end to form a cold or reference junction
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Module 13 Aircraft Structures and Systems
Basic Maintenance Training Manual
SR Technics
13.8 Instrument Systems (ATA 31)
STD (Standard) or QNE (Normal Elevation) setting
Altitude Measurement
QNH (Normal Height) setting
For vertical separa on between aircraft fl ing at igher alt tudes. pressu e a ti ude or ght evel s used. When the baromet ic essu sca s adjust ed standa d sea le el pres sure 29 92 inches of ercury o 1013.2 mba or hPa the altimeter measures the height above this standard pressure level. This is not an actual int. but is a con stantly changing re erence The reason that ll rcraft the upper eve have their al meters set to the same re erence
An alt meter can measure he ght above almost any convenient reference point. and for most fly ng it measures the lti tude above the exist ng sea evel pres sure level. This is called indicated altitude and is read directly from he indi cato when the altimete QNH sett in g is placed in the barometric window
Airport control towers ve the lot the al timeter selt ng which is their local baro metric pressure corrected to sea level
When the lot puts th baroselting the titude measurement starts at sea level pressure All elevations on aeronaut cal charts are measured from mean sea level SL) and therefore w th bit of simple arithmetic the pilot can easily and accurate fi nd the a rcraft's height above any charted posit on When the a rplane is o he ground w th the oca altimeter setting n the barometric window the altimeter should indicate the surveyed eleva ion of the rplane s parking space. Indi ated a tude gives us
measure of ter
n clearance at ow
If an
rplane fl ng at a constan 300 feet essu alti ud for example. may vary its height above the ex sting sea leve pressure all of the aircraft flying this same area w ll vary the same amou nt and the separation between the ir aft ll ema th e same. When an aircraft is fly ng w th he a lt imete set to ndicate pressure alii· tud it is operat ng at flight level. ight evel 32 is 32 '000 feet. essure al tude Figure
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