CA2050609A1 - Fiber-optic strain gauge manometer - Google Patents
Fiber-optic strain gauge manometerInfo
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- CA2050609A1 CA2050609A1 CA 2050609 CA2050609A CA2050609A1 CA 2050609 A1 CA2050609 A1 CA 2050609A1 CA 2050609 CA2050609 CA 2050609 CA 2050609 A CA2050609 A CA 2050609A CA 2050609 A1 CA2050609 A1 CA 2050609A1
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- optical fiber
- light beam
- hollow body
- strain
- polarization
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- Measuring Fluid Pressure (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE:
A fiber-optic strain gauge manometer and a method thereof are for measuring pressure of a fluid, to be respectively connected to a light source and a measurement apparatus. The manometer comprises: a cylindrical hollow body, acting as a pressure transducer, having an inlet by which the fluid can get inside, free ends such that longitudinal and circumferential strains are generated in the hollow body by the pressure of the fluid, thereby producing a dilatation of the hollow body; and a highly birefringent optical fiber having a sensing portion bonded to the outer surface of the hollow body along a longitudinal path such that its birefringence changes when the sensing portion is subjected to the dilatation of the hollow body.
A polarized light beam is generated by the light source and transmitted in a form of two polarization eigenstates each parallel to one of two parallel principal birefringence axes of the birefringent optical fiber. The state of polarization of the input light beam, being strain-modulated after a passage in the sensing portion of the birefringent optical fiber is collected by the measurement apparatus for detecting and measuring change in the birefringence as a function of the dilatation, thereby giving indication of the pressure inside the hollow body. This fiber-optic strain gauge manometer can be used for measuring pressure up to at least 100 MPa.
A fiber-optic strain gauge manometer and a method thereof are for measuring pressure of a fluid, to be respectively connected to a light source and a measurement apparatus. The manometer comprises: a cylindrical hollow body, acting as a pressure transducer, having an inlet by which the fluid can get inside, free ends such that longitudinal and circumferential strains are generated in the hollow body by the pressure of the fluid, thereby producing a dilatation of the hollow body; and a highly birefringent optical fiber having a sensing portion bonded to the outer surface of the hollow body along a longitudinal path such that its birefringence changes when the sensing portion is subjected to the dilatation of the hollow body.
A polarized light beam is generated by the light source and transmitted in a form of two polarization eigenstates each parallel to one of two parallel principal birefringence axes of the birefringent optical fiber. The state of polarization of the input light beam, being strain-modulated after a passage in the sensing portion of the birefringent optical fiber is collected by the measurement apparatus for detecting and measuring change in the birefringence as a function of the dilatation, thereby giving indication of the pressure inside the hollow body. This fiber-optic strain gauge manometer can be used for measuring pressure up to at least 100 MPa.
Description
.~5~
FI~D 0~ INV~IOM:
The present invention relates to a fiber-optic strain gauge ma~ometer and a method thereof.
5~ore precisely, the invention relates to a fiber-optic strain gauge manometer which exploits the e~fect o~ ~train-induced changes in the birefringence of a strained highly birefringent optical fiber bounded to a pres~ure tran~ducer based on a dilatin~ cylindrical element.
1~
Pre~ure tranæducers based on strain gauges fixed to an active m~chanical element that undergoes a deformation 15(dilatation, de~lection, etc.) under the influence of pres~ure are well known. ~his type of tran~ducer is particularly well suited ~or measurements of elevated or high pressures inside pipalines, storage vessels or proces~ing chambers. However, up to now, they have be~n all 20basad on bonded resistance stra~n gauges, characterized by the gauq~ ~actor G,:
G~ - Ro(~) T (l) ;25 :
w~ere: Ro is electrical re~istance of a strain gauge; and : ~ ia longitudinal strain ~ l/lo~
; The actual deformation of the element and the strain resulting fro~ it a~ the location o~ ~he strain gauge will obviously depend on the design of the astive element. For high-r pressures up to 100 MPa this element mo~t often taXes ' '' ' ' ' ' . ' :
..
, .
FI~D 0~ INV~IOM:
The present invention relates to a fiber-optic strain gauge ma~ometer and a method thereof.
5~ore precisely, the invention relates to a fiber-optic strain gauge manometer which exploits the e~fect o~ ~train-induced changes in the birefringence of a strained highly birefringent optical fiber bounded to a pres~ure tran~ducer based on a dilatin~ cylindrical element.
1~
Pre~ure tranæducers based on strain gauges fixed to an active m~chanical element that undergoes a deformation 15(dilatation, de~lection, etc.) under the influence of pres~ure are well known. ~his type of tran~ducer is particularly well suited ~or measurements of elevated or high pressures inside pipalines, storage vessels or proces~ing chambers. However, up to now, they have be~n all 20basad on bonded resistance stra~n gauges, characterized by the gauq~ ~actor G,:
G~ - Ro(~) T (l) ;25 :
w~ere: Ro is electrical re~istance of a strain gauge; and : ~ ia longitudinal strain ~ l/lo~
; The actual deformation of the element and the strain resulting fro~ it a~ the location o~ ~he strain gauge will obviously depend on the design of the astive element. For high-r pressures up to 100 MPa this element mo~t often taXes ' '' ' ' ' ' . ' :
..
, .
2~@)6~
the form o~ a dilating cylinderO The cylinder is usually closed at one end and has its other end diractly connected to a pressure apparatus in an arrangement that may generate some unwanted stres~ in the active area of the cylinder.
To suppress this unwanted stress, a pressure transducer based on a dilatillg cylinder with Pree ends ha~ been propos~d by one of the inventors, Mr. Roland WISNI~NSKI in a publication entitled "PQ~IARY, AUTOMA~IKA, KON~ROLA"
l1986) No. 3, page 60, to be used with an electrical strain ~auge ~or measuring pre~sure up to 100 MPa. The de~ormation o~ ~uch a dilating cylind~r is totally independent o~ the str~s~ induced by conn~çting it to the pressure apparatus and so ~epends exclusively on the value of ths internal pressura deliver~d ~rom the external pressura apparatus.
However, electrical strain gauges, although widely used, suffer ~ro~ ~lgnificant tempQratUre drift (thermally induced voltageæ cause~ by thermDcoupling and temperature efect~ on gauge rasistance and gauge ~actor)~ In addition, the low electri~al output level of such a strain gauge makes ~o it extremely su~ceptible to electromagnetie inter~erence (EHI~, especially in noisy industrial environ~ents.
D~nsitization o~ these gauge~ to EMI is very difficult and not alway~ possible, and the procedure is actually more costly than resistanae-strain ~nsing technology itself.
An ob~ect o~ the invention is to replace the standard electrical resistance ~traln gauge in the straln-gauge pres~ure manometer based on a dilating cylindrical ele~ent by an optical ~iber.
Another ob~ect o~ the inv~ntion is to provide a new ~iber-optic ~train gauge ~anometer and a mathod thereof ~or measurin~ pressur2 of a flui~ inside a dilating cylindrical element up to 100 ~Pa.
':
. ~ . .
: - ~:' ' - ' ~''',, ' .
Still another object of the invention is to provi~e a fiber-optic stra.in gauge manometer which i~ inherently immune to electromagnetic interferences, sa~e in electrically dangerous or explosive environments and h~ve a significantly greater ~ensitivity over prior art manometers.
Still anotller object of the invention is ko provide a method for measuring a preæ~ure which is directly compatible with optical data transmission system~ and optical multiplexing technology~
1~
S~,R~ ~O~:
According to the present invention, there is provided a fiber-optlc strain gaug~ manometer Por measurlng pressure o~ a ~luid, to be respectively connected ko a light source and a mea~urement apparatus, the manometer comprising:
a cylindrical hollow body of predetermined lengtb and acting as a pre~sure transducer, the hollow body having a central aXis, cylindrical inner and cuter surfaces, an end, and an opposi~e end provided with an inlet by which the fluid can get inside the hollow body, the end~ being free to move longitudinally with re~pect to the central axi~ ~uch that longltudinal and circum~erential strains are generated in the hollow body when the pres~ure i~
applied on the inner sur~ace, thereby producing a dilatation of the hollow body; and a highly bire~ringent optical fiber which can maintain only some polarlzation states, the bire~ringent op~ical fiber having a ~ensing portion bonded with bonding means to the outer surface oP the hollow body along a longitudinal path such that . - .
,- ~
- -ilD6~
birefringence o~ the sensing port1on changes wh~n the ~ensing portion i8 $ubjectsd to the dllatation of the hollow body, the sensing portion having a length smaller than the length of the hollow body, whlah is eelected in function of a desired sen~itivity;
whereby a polarized light beam i6 generated b~ the light sourc~ and tran~mitte~ in a form oP two polarization eigenætat~ each parallsl to on~ o~ two parallel principal bire~ringenae axes of ~he birefr~ngent op~lcal fiber, ~he state o~ polari~ation of t~e input light beam being strain-modulated a~ter a passage in the ~ensing portion, the strain-modulated output light beam being ~ollected by the : measurement apparatu~ for detecting and measuring change in the bire~ringence as a Punation of the dilatation, thereby giving indicati~n u~ th~ pressure lnside th~ hollow body.
According to the present invention, there i~ also provlded a method for ~aasuring pressure of a ~luid inside a ~ylindrical hollow body of pr~determin~d 10ngth and acting as a pr~ssure transducer~ the hollow body ~aving a central axis, cylindrical inner and outer surfaces, an end, and an opposit0 end provided wlth an inlet by which ~he ~luid can get inslde the hollow body, the ends being free to move : longitudinally with re~peat to the cenkral axis such that longitudinal and circumferential strains are generated in the hollow body when the pre~sure ~s applled on tha inner sur~a~e, thereby pro~uGing a ~ilatation of tha hollow body, a portion of highly bire~ringent optical ~iber which can : malntain only ome polarization states balng bonded to t~e outer sur~a~e of the hollow body alo~g a longitudinal path for sub~cting the portion to the dilatation, the method com~r1sinq steps o~:
:
,, :
,, transmi~ting a polarized light bea~ in a ~orm of two polarization eigens~ate~ each parallel to one of two parallel principal bire~ringen~e axes o~ said birefringent op~ical fiber/ the state of polarization o~ ~he input light beam being strain-modulated after a passage in the portion of the highly birefringent optical ~lber due to change in birefringence o~ the birefringent optical fiber a~
a resul~ o:e the dilatation~
collec~ing the 3train~modulated output light beam;
measuring change in the state of polari2ation between the input light bealn and the ~train-modulated output 1 ight beam; and determinillg a value indicative o~ the pressure as a ~unction of the ~n~a~ured change in the ~tat~ of polarization .
q~he present invention ~s well as its numerou~
advanta~ea will be better und~r~tot)d by the îollos~ g non-restrictive description of po~sible embodiment~ mada in re~erence to ~he appended drawing~.
B~ D ~0~ L~ a~
Figu~ a ~iagram illustrating the ~iber-optic ~train gauge manometer a¢co:rdlng to the pre~erlt invention;
Figure 2 is a ~iagram illustr~ting an instrumentatlon æystem for the ~iber-opti~ straln gauge manometer in a transmisslon con~iguration;
~i~ure 3 is a diagram illustrating an ~nstrumentation æyætem for the fiber-optic e~raln gauge manometer in a ~e~le~tlo~ con~iguration;
Figure 4 Is a diagra~ illustr~ting ~ range Or operation . :
.
.
:
~S~6~
of the fiber-optic strain gauge manometer in comparison with the full characteristic of an HB 600 fiber based strain-sensor;
Figure 5 i8 a diagram illustrating the relation between the value indicative of pressure generated by a HB~HB-LB
arrangement o~ opkical fibers for the manometer ~6ed in Figure 2 wlth respect to the pres6ure in ~Pa inside the pre6~ur~ transducer for two di~erent lengths o~ sensing por~ion;
Figure 6 is a diagra~ illustrating the relation between the value indicative o~ pressure ganerated by a LB-HB-LB
arrangement o~ optical ibers for the ~anometer ~s~d in Figure 2 with respect to the pre~sure in MPa inside the pr~ssure transducer, with and wi~hout ths Soleil~Babinet compen~ator;
Figure 7 i8 a diagram illustrating the hysteresis cycle the fi~er-optic strain gauge ~anometer used in Figure 6;
and Figure 8 i~ a diagram illustrating a temperature-20 compens~ted highly birefr~ngent optical fiber in the ~iber-optlc strain gauge manometer.
~T~ n~S~IP~0~ ~F T~ F~ E~R~
In the ~ollowing de~aription an~ in the drawings, the same numerals refer to sam~ elements.
Rs~crring ~ow to Figure l, ~he active ale~nt o~ the fibex-optic strain gauge mano~eter 2 is designed in the form . of a cylindrical hollow ~ody ~ having both it~ ends 6 and fre~, thus emulating the behaviour of an infinite cylinder.
; The cylindrical hollow body 4 acting a~ a pressura tran~ducer i6 built toge~her with a pres~ure inlet lO, .
:~501~
clo~ing element 12 (which can a~ well be replaced with an output pres~ure tu~e) and an external protective cover 14.
This concept of a free dilating cylinder pre~sure transducer wa~ proposed earlier by one o~ the inventors, Mr. Roland WISNIEWSKI ln a publ.ication entitled 'IPOMIARY, AUTOMATIKA, KONTROLA" (1~86) No. 3~ page 60, used with an elactrical ~train gauge for ~easuring pre~sure ~Ip to 100 ~Pa.
Th~ de~ormatiQn of the cylindrical hollow body 4 ~re~erred hereinafter as a free dilating cyllnder) depends exclu~ively on the value of the internal pressure delivered from outside the pre~sure tranæducer and is totally independ~nt of the ~tress lnduced by connecting it to a pressure system. The longltudinal ~1 and circum~erential ~
6train~ can bs found on the ba~is o~ Lame theory using the 15following expression~:
-- V 1;2 12 1 - 2 d E (2) ~0 =----~L~ (3) where; E i~ tha Young modulu~, ~ 25v i~ the Poisson ratio, : d i~ the thickness of the cylindrical hollow body;
: and D is $he in~ide diam~ter o~ the cyl~ndrical hollow body.
It ~hould be noted that longitudinal strain in the ca~e o~ the free dilating cylinder 4 u~ed as an active element is negative, contrary to th~ case o~ a clas~ioal cylinder fixed at one end.
ThQ sen~ing portion of a hi~hly bire~ringent (referred .~50~
hereinafter as HB) optical ~iher 16 serving as a strain-sensitive element is epoxied to the outer wall 18 of the free dilating cylinder 4 along a longitudinal path, and thus is totally isolated from the region of high pressure 20.
The axial strain to which the HB optical fiber 16 i5 exposqd will influence the relative phase retardation ~
between the two perpendicular eiyenmodes guided by the fiber 16 according to the equation:
L d~
d~ ~ ~ ( n d~ L d~ ) ~ d~ (4) where: ~ is the wavelength of the polarized light beam transmitted in the HB optical fi~er lS; and ~n is the di~ference between the effective indlces of the two polarization eigenstate6 o~ the HB fiber 16 (~ n = ny - nyl de~ined as ~iber birefringence B.
No high-pressure leadthrough is needed in this case which obviously simplifies th~ pressure transducer construction and avoids the uncertainties and false readings usually associated with such a leadthrough.
Before being permanently fixed to tha ~ree dilating cylinder 4, the HB optical fiber 16 is fusion-spliced with lead and aollecting optical quartz ~ibers 22 and 24, respectively. To optimi ZQ the propagation conditions for a strain-~odulated output light beam resulting ~rom a passage of a polarized input light beam in the HB optical ~iber 16, di~ferent kind of lead and collecting ~ibers 22 and 24 can ~e used. Among other kinds, here is some arrange~ents of particular inte~ests:
an HB-H~-SM arrange~ent: a lead HB ~iber 22 spliced at to the HB sensing fiber 16, and a collecting single-mode :~5~Q~9 fiber 24;
an HB-HB~LB arrangement: a lead HB ~iber 22 spliced at 45 to the HB sensing ~iber 16, and a collecting low-birefringence 24 (re~erred hereinafter as LB) fiber; and 5a LB-HB LB arrangement: lead and collecting LB fiberæ
22 and 24 ~pliced to the HB sensing fiber 16.
One of the preferred arrangement involve6 a lead ~B
fiber 22 of same type as in the sensing HB fiber 16 but having it~ birefringence axes precisely aligned at 45 lorelative to the axes of the sensing HB ~iber 16 in order to allow an incoming linearly polarized light beam to equally excite both perpendi~ular polarization eigenmodes inside the sensing portion. Alignment of both lead and sens~ng HB
~ibers 22 and 1~ with high accuracy can be achieved with a 15splicing ~acility having an an~ular reæolution of the rotati~n ~tage better than 0.1. To collect the stra~n-modulated output light beam, a collecting LB ~iber 24 which can maintain any polarization state with practically no perturbations may be used. The third mentioned arrangement 20(with LB ~ibers for both lead and collecting fibers 22 and 24) is very int~resting from ~he practical point of view since it doe~ not ~equire any angularly precise ~plices to the ~ensing HB fiber 16.
Re~erring now ~o Figure 2, a~ optical instrumentation 25~ystem 26 in a transmi~sion configuration for the m~nometer 2 is des~yned to generate a controlled polarlzatlon light :beam a~d to collect the pre~sure-modulated output light beam a~ter a pa6~age in the ~en~ing ~B fiber 16. A linearly polarized light beam is generated by a 3 mW HeNe la~er 28 : 30acting as a light source, emitting at 633 n~ and tran~mit~ed ;into a lead York (trademark) HB 600 optical ~ibar 22 with a cut-off wavelength o~ 550 nm, which assures a ~ingle-mode ~ 9 ,:
5~
operation of the optical ~lbers used in the manometer 2. To precisely allgn the polarization plane of the polariæed light beam parallel to one of its two parallel principal birefringence axes, a combination o~ polarization controllers i~ used~ including a ~uarter-wave plate 30 and a polarizer 32. Such align~nt i8 necessary to avoid any inf luence of Anvironmental parametars on light pr~pagation through the lead ~iber 2 The strain-modul~t~d output light beam transmi~ted by the collecting fiber 24 after its passag~ in th0 HB ~ensing fiber 16 is monitored by a ~ea~urement apparatus comprising a Soleil-Babinet compensator 34 and an appropriate analyser (as a Wollaston pri6m 36) combined with a two-datector syste~ formed of two photodetectors 38 and 40 configured using a synchronous detection scheme based on a lock-ln ampli~ier 42 in synahroni~m with the light source 28. If the strain-modulated output llght bea~ (beam irradiance) is monitored using only one of these detect~r~ 38 and 43, it can be correspondingly described by the equation6:
1 + sin~) 1 4 (5) I, = ~ (1 - sin~) ~5 4 ~ he U~Q of a di~erence-over-sum processing when both of the detectors 38 and 40 are us~d allows significant reducti~n o~ sy~ta~ fluctuations introduced by non-stable optical power Io e~itted by ~he light source ~8, according to the ~ormula:
L - s~nA~
I1 + I, (6) ~s~
The final stage of the analog signal recovery systam i5 a computer 4~ con~rolled digital volt~eter 46.
Focu~ of the light beam at the input of the lead optical Piber 22 and at the output o~ the collecting ~iber 24 is done w~th ob~ective l~ns 48 and 50.
Referring now to Figure 3, an optical instrumentation sys~em 52 in a re~lection con~igura~ion for the manometer 2 comprises almost the same element~ as the instrumentation sys~em 26 shown in Figure 2. The di~ferenca re~ides in ~he addi~ion o~ a polarization-preserving bi~ir~ctlonal coupler 54 separating th~ input light beam and the straln-modulate~
output light ~eam which tr~vel as two perpendicul~r polarization ~ighen ~odes of an interconnecting HB optical ~iber 56 placed between an end of the HB sensing fiber 16 and the bidirectional coupler 54. A re~lective mirror 58 i5 vacuum-deposited at ths other end of the sensing elem~nt 16.
It can be done by covering the ~ip o~ the 6en~ing element 16 b~ a gold compound cbtained ~rom Engelhardt Che~ical~
~trademark), ~ew Jersey, which a~t~r some heating and cooling proces~es makes a s~able layer ~cting as a mirror with a reasonable re~lective coe~ficient.
In this re~lection configuration, the e~fective interact~on di tance is twice the sensing elament }ength, resultlng in sensitivity o~ the manometer 2 being two times greater than that o~ the manometer 2 in the transmission configuration a~ shown in Figure 2.
R~erring now to Pigures 2 and 3, the length of the HB
optical fiber ~6 ~ervlng a~ a ætrain-sensltive element can obviou~ly not exceed th~ length of the ~ree dilating cylinder 4. Assuming tha~ f~r a cylinder 4 having a length o~ about 50 mm, ~/~ eguals 2 x lo-6 and ~/(2d) is about 1, ' , ' . ' .
.
, . ~ :
~1~5~
and according to equation (2)~ the maximum value of the longitudinal strain due to the deformation caused by an internal pressure of 100 MPa can be eætimated at ~ 200 ~t. If khe HB sensing ~lber 16 is a HB 600 York polarization-preserving bow-tie fiber having a diameter of 125 ~m and a length of 40 mm (alternatively, 48 m~), the longitudinal ~train required to induce a 2~ phase shi~t in the strain~modulated light boam observed at the output o~
the H~ ~an~ing fiber 16 (strongly dependent on ~ber's length) will amount to ~, ~ 1400 ~. A~ a measure of the periodicity of the phase shift wi~h strain, T, will be twiae dimini~hed when the strain-modified light beam is monitored with tha Wolla~ton prism 36 combined with the two-photodetector 3~ and 40 system in~tead of a ~ingle analyzer/datector configuration. Additionally, the total operation range of tha fiber-optic strain gauge manometer 2 (lOQ MPa, corresponding to 100 ~ in the strain scale) can be easily adjusted with the help of the Soleil Bab~net : compensator 34 to a quasi-linear steep region of a sin-like ~haracteristiG as shown in hatched lina~ in Figure 4.
Referrin~ now to ~i~ure 8, temperature compen~ation of the fiber-optic strain gauge manometer 2 in both trans~i~ion or reflection configuration ~hown in Figures 2 and 3) can be achieved wi~h a compensation portion 60 o~
HB optioal ~i~e~ besid~ the sensinq portion 16. The co~pen6ated ~anometer 2 con6i~ts of two identical ~sen6ing and co~pensating) parts of a ~B polarization maintaining optical fiber, spliced at 99 in relatlon to their polarization axe~. If both parts remain at the ~ame temperature ~ut only one i~ ~trained, their total temp~rature-induced phase retardation will cancel out. The degrea ~f cancellation depend~ heavily on preci~e angular ~5~
alignment and exact lengths of both partæ of tha ~ensorO
The phaæe retardation ~or each mode propagating in an optical fiber is giYen by:
~ = ~ n L (7) where: ~ $~ the wavelength u~ed;
n i~ the ef~ectiv~ refractive index of a given mode;
and ~ is the length.
For a re~lection con~iguration ~as shown in Figura 3) the e~ecti.ve length o~ th~ HB sen~ing element is twice its actual length.
~ he total relative phase retardation between the ~wo perpendicularly polari2ed eigenmodes propagating in a H~
optical ~iber will be given by:
~y = ~Ll) + ~(L2) (8, wh~r~ ( L, ) = -~ ~nlNLI
N is the number of ~ections o~ the ~B optical ~iber, ~; 25 e.g. N = 1 ~or tr~n6~ission con~iguration and N =
2 for re~lexio~ configuration; and i i~ th~ index of the ~ection.
~: ~emperature~induced ph~se ret~rd~ion ~an be e~pres6ed by: :
dT ~ ~dT (~nlL~ (Lld~lL~ ~ ~nl ~ ) (9) where nl 3 n~l - nyl, and nXl and nyl are the refra~tive :
.
, .' :
, ~CiiS06~k~
indices ~or two orthog~nal linear polarization modes of an i section. Since Ll = L~ = L and both parts are rotated one fiber relative to the other by 90 about their axes, we obtain n~ nl or ~An1 = O (1~) Henc~ d(~ L ~¢4n~ ~ ~L ~ n ) (11) dT ~ ~ ~ dT d~
Finally d~a~ d ~ _ dT ~ dT(L ,An1) - O (12) or~(Tl = constant ~rom ~14) it ~ollow~ that the strain sensor will be immune to ambient temperature or any other phy~ical parameter acting on both its parts. However, i~ only one part o~ the ~ensor is exposed to the influence of a physical parameter the sensor will mea~ure this parameter.
Referring a~ain to Figure 2, the fiber-optic strain ~auge ~anometer 2 ha~ b~en characterized at a conætant temperature for pressures up to 100 ~Pa in the three above-mentioned arrange~ents involving different combinations of lead and coll~cting optical fibers 22 and 24 fusion spliced at both ends of tha sensing ~iber 16.
Interestlng resuIt3 were obtained in the second arrangemen~ for which Figure 5 ~hows pressure characteristics o~ two ~ensing element~ 1~ havin~ 40 mm and 48 mm in length. However, the stable and repeatable re~ults 5~6~1 shown in Figur~ 6 were al60 obtained in the third arr~ngement when lead and collecting optical fibers 22 and 24 were made of LB fi~ers. Application o~ LB fibers allowed for more precise control of the state o~ polarization in the system than would be possible using a ~anometer 2 equipped with a standard single mode optical fiber thus diminishing signal fluctuations while at the same time signifiaantly decreasing the co~t o~ the device compared to that of a sensor e~uippad with a HB fiber input.
lo Shown in Figure 6 are two characteristics A and B of the same sensing element 16 (~hown in Figure 2) which wera displac~d ufiing a controlled phase shift introduced by the Soleil-Babinet compensator 34 ~hown in Figure 2). The sharp mini~um in the B characteristics corresponds to a ~train~modulated output light beam circular polarization whlch i~ ~eparated into both linear-polari2ation direction~
- I,) by pa~sln~ through the Wollaston prism 36 (shown in Figure 2).
Referring to Figure 2, by combining equation (4~ and the following one which r~sults from (2):
P.
dP 2 d E ~13) WQ can e~tabli6h the ~inal expre~sion for the phase pre~sure sensitivity of the manometer 2. The ~xpres~ion (13) clearl~ shows how to de~ign the manometer 2 with spsci~ied ~ensitivity in the predetermined pres6ure range:
dp d E ~ d~ (14) .
. . :
:
. ' . ' - ' ' , ' .. .
.~5~
The ~&nsitivity can be increased simply by increa~ing the length of the HB sensing fiber 16 (and the length of the free dilating cylinder 4), but this might be difficult without at the same time compromlsing the usual requirement for miniaturisation of the sa~sor head. Other means, how~ver, such as the choice of a shorter wa~elengt~, optimal geome~ry (D and d parameters), and/or appropriate ~ree dilating cyl~nder 4 t v and E para~eter~) and fiber (An para~etars) material~ would allow sufficient flexibility in HB sensing fibQr 2 de~ign to cover most of the potential applicatlons.
Figure 7 shows initial re~ult~ of pressure cycling on the ~etrological properties of the ~iber-optic strain gauge manometer 2 (shown in Figure~ 2 and 3). It appears that pres~ure~induced hysteresi~ o~ the ~iber-optic strain gauge mano~eter 2 ~shown in Fi~ure~ 2 and 3) ls residual, and generally i.~ due to the presence o~ adhesives. It i~ well known that the mechanical properties of the optical quartz ~ibers themselves are excellent and do not contribute to the eventual sensor hysteresis. It has been found that the hystere~1~ diminished as the number o~ pressure cycles increa~ed and it can be optimized through an appropriate choice of a~hesivQ materials.
Although the presant invention h~s been explained hereinabove by way o~ t~e pre~arred embodiments the~eo~, it ~hould be polnted out that any modifications to these prefer~ed ~b~d~ents, within the scope of the appended claim6 i8 not deemed to change or alter the nature and scope of the pre~ent invention.
: 16
the form o~ a dilating cylinderO The cylinder is usually closed at one end and has its other end diractly connected to a pressure apparatus in an arrangement that may generate some unwanted stres~ in the active area of the cylinder.
To suppress this unwanted stress, a pressure transducer based on a dilatillg cylinder with Pree ends ha~ been propos~d by one of the inventors, Mr. Roland WISNI~NSKI in a publication entitled "PQ~IARY, AUTOMA~IKA, KON~ROLA"
l1986) No. 3, page 60, to be used with an electrical strain ~auge ~or measuring pre~sure up to 100 MPa. The de~ormation o~ ~uch a dilating cylind~r is totally independent o~ the str~s~ induced by conn~çting it to the pressure apparatus and so ~epends exclusively on the value of ths internal pressura deliver~d ~rom the external pressura apparatus.
However, electrical strain gauges, although widely used, suffer ~ro~ ~lgnificant tempQratUre drift (thermally induced voltageæ cause~ by thermDcoupling and temperature efect~ on gauge rasistance and gauge ~actor)~ In addition, the low electri~al output level of such a strain gauge makes ~o it extremely su~ceptible to electromagnetie inter~erence (EHI~, especially in noisy industrial environ~ents.
D~nsitization o~ these gauge~ to EMI is very difficult and not alway~ possible, and the procedure is actually more costly than resistanae-strain ~nsing technology itself.
An ob~ect o~ the invention is to replace the standard electrical resistance ~traln gauge in the straln-gauge pres~ure manometer based on a dilating cylindrical ele~ent by an optical ~iber.
Another ob~ect o~ the inv~ntion is to provide a new ~iber-optic ~train gauge ~anometer and a mathod thereof ~or measurin~ pressur2 of a flui~ inside a dilating cylindrical element up to 100 ~Pa.
':
. ~ . .
: - ~:' ' - ' ~''',, ' .
Still another object of the invention is to provi~e a fiber-optic stra.in gauge manometer which i~ inherently immune to electromagnetic interferences, sa~e in electrically dangerous or explosive environments and h~ve a significantly greater ~ensitivity over prior art manometers.
Still anotller object of the invention is ko provide a method for measuring a preæ~ure which is directly compatible with optical data transmission system~ and optical multiplexing technology~
1~
S~,R~ ~O~:
According to the present invention, there is provided a fiber-optlc strain gaug~ manometer Por measurlng pressure o~ a ~luid, to be respectively connected ko a light source and a mea~urement apparatus, the manometer comprising:
a cylindrical hollow body of predetermined lengtb and acting as a pre~sure transducer, the hollow body having a central aXis, cylindrical inner and cuter surfaces, an end, and an opposi~e end provided with an inlet by which the fluid can get inside the hollow body, the end~ being free to move longitudinally with re~pect to the central axi~ ~uch that longltudinal and circum~erential strains are generated in the hollow body when the pres~ure i~
applied on the inner sur~ace, thereby producing a dilatation of the hollow body; and a highly bire~ringent optical fiber which can maintain only some polarlzation states, the bire~ringent op~ical fiber having a ~ensing portion bonded with bonding means to the outer surface oP the hollow body along a longitudinal path such that . - .
,- ~
- -ilD6~
birefringence o~ the sensing port1on changes wh~n the ~ensing portion i8 $ubjectsd to the dllatation of the hollow body, the sensing portion having a length smaller than the length of the hollow body, whlah is eelected in function of a desired sen~itivity;
whereby a polarized light beam i6 generated b~ the light sourc~ and tran~mitte~ in a form oP two polarization eigenætat~ each parallsl to on~ o~ two parallel principal bire~ringenae axes of ~he birefr~ngent op~lcal fiber, ~he state o~ polari~ation of t~e input light beam being strain-modulated a~ter a passage in the ~ensing portion, the strain-modulated output light beam being ~ollected by the : measurement apparatu~ for detecting and measuring change in the bire~ringence as a Punation of the dilatation, thereby giving indicati~n u~ th~ pressure lnside th~ hollow body.
According to the present invention, there i~ also provlded a method for ~aasuring pressure of a ~luid inside a ~ylindrical hollow body of pr~determin~d 10ngth and acting as a pr~ssure transducer~ the hollow body ~aving a central axis, cylindrical inner and outer surfaces, an end, and an opposit0 end provided wlth an inlet by which ~he ~luid can get inslde the hollow body, the ends being free to move : longitudinally with re~peat to the cenkral axis such that longitudinal and circumferential strains are generated in the hollow body when the pre~sure ~s applled on tha inner sur~a~e, thereby pro~uGing a ~ilatation of tha hollow body, a portion of highly bire~ringent optical ~iber which can : malntain only ome polarization states balng bonded to t~e outer sur~a~e of the hollow body alo~g a longitudinal path for sub~cting the portion to the dilatation, the method com~r1sinq steps o~:
:
,, :
,, transmi~ting a polarized light bea~ in a ~orm of two polarization eigens~ate~ each parallel to one of two parallel principal bire~ringen~e axes o~ said birefringent op~ical fiber/ the state of polarization o~ ~he input light beam being strain-modulated after a passage in the portion of the highly birefringent optical ~lber due to change in birefringence o~ the birefringent optical fiber a~
a resul~ o:e the dilatation~
collec~ing the 3train~modulated output light beam;
measuring change in the state of polari2ation between the input light bealn and the ~train-modulated output 1 ight beam; and determinillg a value indicative o~ the pressure as a ~unction of the ~n~a~ured change in the ~tat~ of polarization .
q~he present invention ~s well as its numerou~
advanta~ea will be better und~r~tot)d by the îollos~ g non-restrictive description of po~sible embodiment~ mada in re~erence to ~he appended drawing~.
B~ D ~0~ L~ a~
Figu~ a ~iagram illustrating the ~iber-optic ~train gauge manometer a¢co:rdlng to the pre~erlt invention;
Figure 2 is a ~iagram illustr~ting an instrumentatlon æystem for the ~iber-opti~ straln gauge manometer in a transmisslon con~iguration;
~i~ure 3 is a diagram illustrating an ~nstrumentation æyætem for the fiber-optic e~raln gauge manometer in a ~e~le~tlo~ con~iguration;
Figure 4 Is a diagra~ illustr~ting ~ range Or operation . :
.
.
:
~S~6~
of the fiber-optic strain gauge manometer in comparison with the full characteristic of an HB 600 fiber based strain-sensor;
Figure 5 i8 a diagram illustrating the relation between the value indicative of pressure generated by a HB~HB-LB
arrangement o~ opkical fibers for the manometer ~6ed in Figure 2 wlth respect to the pres6ure in ~Pa inside the pre6~ur~ transducer for two di~erent lengths o~ sensing por~ion;
Figure 6 is a diagra~ illustrating the relation between the value indicative o~ pressure ganerated by a LB-HB-LB
arrangement o~ optical ibers for the ~anometer ~s~d in Figure 2 with respect to the pre~sure in MPa inside the pr~ssure transducer, with and wi~hout ths Soleil~Babinet compen~ator;
Figure 7 i8 a diagram illustrating the hysteresis cycle the fi~er-optic strain gauge ~anometer used in Figure 6;
and Figure 8 i~ a diagram illustrating a temperature-20 compens~ted highly birefr~ngent optical fiber in the ~iber-optlc strain gauge manometer.
~T~ n~S~IP~0~ ~F T~ F~ E~R~
In the ~ollowing de~aription an~ in the drawings, the same numerals refer to sam~ elements.
Rs~crring ~ow to Figure l, ~he active ale~nt o~ the fibex-optic strain gauge mano~eter 2 is designed in the form . of a cylindrical hollow ~ody ~ having both it~ ends 6 and fre~, thus emulating the behaviour of an infinite cylinder.
; The cylindrical hollow body 4 acting a~ a pressura tran~ducer i6 built toge~her with a pres~ure inlet lO, .
:~501~
clo~ing element 12 (which can a~ well be replaced with an output pres~ure tu~e) and an external protective cover 14.
This concept of a free dilating cylinder pre~sure transducer wa~ proposed earlier by one o~ the inventors, Mr. Roland WISNIEWSKI ln a publ.ication entitled 'IPOMIARY, AUTOMATIKA, KONTROLA" (1~86) No. 3~ page 60, used with an elactrical ~train gauge for ~easuring pre~sure ~Ip to 100 ~Pa.
Th~ de~ormatiQn of the cylindrical hollow body 4 ~re~erred hereinafter as a free dilating cyllnder) depends exclu~ively on the value of the internal pressure delivered from outside the pre~sure tranæducer and is totally independ~nt of the ~tress lnduced by connecting it to a pressure system. The longltudinal ~1 and circum~erential ~
6train~ can bs found on the ba~is o~ Lame theory using the 15following expression~:
-- V 1;2 12 1 - 2 d E (2) ~0 =----~L~ (3) where; E i~ tha Young modulu~, ~ 25v i~ the Poisson ratio, : d i~ the thickness of the cylindrical hollow body;
: and D is $he in~ide diam~ter o~ the cyl~ndrical hollow body.
It ~hould be noted that longitudinal strain in the ca~e o~ the free dilating cylinder 4 u~ed as an active element is negative, contrary to th~ case o~ a clas~ioal cylinder fixed at one end.
ThQ sen~ing portion of a hi~hly bire~ringent (referred .~50~
hereinafter as HB) optical ~iher 16 serving as a strain-sensitive element is epoxied to the outer wall 18 of the free dilating cylinder 4 along a longitudinal path, and thus is totally isolated from the region of high pressure 20.
The axial strain to which the HB optical fiber 16 i5 exposqd will influence the relative phase retardation ~
between the two perpendicular eiyenmodes guided by the fiber 16 according to the equation:
L d~
d~ ~ ~ ( n d~ L d~ ) ~ d~ (4) where: ~ is the wavelength of the polarized light beam transmitted in the HB optical fi~er lS; and ~n is the di~ference between the effective indlces of the two polarization eigenstate6 o~ the HB fiber 16 (~ n = ny - nyl de~ined as ~iber birefringence B.
No high-pressure leadthrough is needed in this case which obviously simplifies th~ pressure transducer construction and avoids the uncertainties and false readings usually associated with such a leadthrough.
Before being permanently fixed to tha ~ree dilating cylinder 4, the HB optical fiber 16 is fusion-spliced with lead and aollecting optical quartz ~ibers 22 and 24, respectively. To optimi ZQ the propagation conditions for a strain-~odulated output light beam resulting ~rom a passage of a polarized input light beam in the HB optical ~iber 16, di~ferent kind of lead and collecting ~ibers 22 and 24 can ~e used. Among other kinds, here is some arrange~ents of particular inte~ests:
an HB-H~-SM arrange~ent: a lead HB ~iber 22 spliced at to the HB sensing fiber 16, and a collecting single-mode :~5~Q~9 fiber 24;
an HB-HB~LB arrangement: a lead HB ~iber 22 spliced at 45 to the HB sensing ~iber 16, and a collecting low-birefringence 24 (re~erred hereinafter as LB) fiber; and 5a LB-HB LB arrangement: lead and collecting LB fiberæ
22 and 24 ~pliced to the HB sensing fiber 16.
One of the preferred arrangement involve6 a lead ~B
fiber 22 of same type as in the sensing HB fiber 16 but having it~ birefringence axes precisely aligned at 45 lorelative to the axes of the sensing HB ~iber 16 in order to allow an incoming linearly polarized light beam to equally excite both perpendi~ular polarization eigenmodes inside the sensing portion. Alignment of both lead and sens~ng HB
~ibers 22 and 1~ with high accuracy can be achieved with a 15splicing ~acility having an an~ular reæolution of the rotati~n ~tage better than 0.1. To collect the stra~n-modulated output light beam, a collecting LB ~iber 24 which can maintain any polarization state with practically no perturbations may be used. The third mentioned arrangement 20(with LB ~ibers for both lead and collecting fibers 22 and 24) is very int~resting from ~he practical point of view since it doe~ not ~equire any angularly precise ~plices to the ~ensing HB fiber 16.
Re~erring now ~o Figure 2, a~ optical instrumentation 25~ystem 26 in a transmi~sion configuration for the m~nometer 2 is des~yned to generate a controlled polarlzatlon light :beam a~d to collect the pre~sure-modulated output light beam a~ter a pa6~age in the ~en~ing ~B fiber 16. A linearly polarized light beam is generated by a 3 mW HeNe la~er 28 : 30acting as a light source, emitting at 633 n~ and tran~mit~ed ;into a lead York (trademark) HB 600 optical ~ibar 22 with a cut-off wavelength o~ 550 nm, which assures a ~ingle-mode ~ 9 ,:
5~
operation of the optical ~lbers used in the manometer 2. To precisely allgn the polarization plane of the polariæed light beam parallel to one of its two parallel principal birefringence axes, a combination o~ polarization controllers i~ used~ including a ~uarter-wave plate 30 and a polarizer 32. Such align~nt i8 necessary to avoid any inf luence of Anvironmental parametars on light pr~pagation through the lead ~iber 2 The strain-modul~t~d output light beam transmi~ted by the collecting fiber 24 after its passag~ in th0 HB ~ensing fiber 16 is monitored by a ~ea~urement apparatus comprising a Soleil-Babinet compensator 34 and an appropriate analyser (as a Wollaston pri6m 36) combined with a two-datector syste~ formed of two photodetectors 38 and 40 configured using a synchronous detection scheme based on a lock-ln ampli~ier 42 in synahroni~m with the light source 28. If the strain-modulated output llght bea~ (beam irradiance) is monitored using only one of these detect~r~ 38 and 43, it can be correspondingly described by the equation6:
1 + sin~) 1 4 (5) I, = ~ (1 - sin~) ~5 4 ~ he U~Q of a di~erence-over-sum processing when both of the detectors 38 and 40 are us~d allows significant reducti~n o~ sy~ta~ fluctuations introduced by non-stable optical power Io e~itted by ~he light source ~8, according to the ~ormula:
L - s~nA~
I1 + I, (6) ~s~
The final stage of the analog signal recovery systam i5 a computer 4~ con~rolled digital volt~eter 46.
Focu~ of the light beam at the input of the lead optical Piber 22 and at the output o~ the collecting ~iber 24 is done w~th ob~ective l~ns 48 and 50.
Referring now to Figure 3, an optical instrumentation sys~em 52 in a re~lection con~igura~ion for the manometer 2 comprises almost the same element~ as the instrumentation sys~em 26 shown in Figure 2. The di~ferenca re~ides in ~he addi~ion o~ a polarization-preserving bi~ir~ctlonal coupler 54 separating th~ input light beam and the straln-modulate~
output light ~eam which tr~vel as two perpendicul~r polarization ~ighen ~odes of an interconnecting HB optical ~iber 56 placed between an end of the HB sensing fiber 16 and the bidirectional coupler 54. A re~lective mirror 58 i5 vacuum-deposited at ths other end of the sensing elem~nt 16.
It can be done by covering the ~ip o~ the 6en~ing element 16 b~ a gold compound cbtained ~rom Engelhardt Che~ical~
~trademark), ~ew Jersey, which a~t~r some heating and cooling proces~es makes a s~able layer ~cting as a mirror with a reasonable re~lective coe~ficient.
In this re~lection configuration, the e~fective interact~on di tance is twice the sensing elament }ength, resultlng in sensitivity o~ the manometer 2 being two times greater than that o~ the manometer 2 in the transmission configuration a~ shown in Figure 2.
R~erring now to Pigures 2 and 3, the length of the HB
optical fiber ~6 ~ervlng a~ a ætrain-sensltive element can obviou~ly not exceed th~ length of the ~ree dilating cylinder 4. Assuming tha~ f~r a cylinder 4 having a length o~ about 50 mm, ~/~ eguals 2 x lo-6 and ~/(2d) is about 1, ' , ' . ' .
.
, . ~ :
~1~5~
and according to equation (2)~ the maximum value of the longitudinal strain due to the deformation caused by an internal pressure of 100 MPa can be eætimated at ~ 200 ~t. If khe HB sensing ~lber 16 is a HB 600 York polarization-preserving bow-tie fiber having a diameter of 125 ~m and a length of 40 mm (alternatively, 48 m~), the longitudinal ~train required to induce a 2~ phase shi~t in the strain~modulated light boam observed at the output o~
the H~ ~an~ing fiber 16 (strongly dependent on ~ber's length) will amount to ~, ~ 1400 ~. A~ a measure of the periodicity of the phase shift wi~h strain, T, will be twiae dimini~hed when the strain-modified light beam is monitored with tha Wolla~ton prism 36 combined with the two-photodetector 3~ and 40 system in~tead of a ~ingle analyzer/datector configuration. Additionally, the total operation range of tha fiber-optic strain gauge manometer 2 (lOQ MPa, corresponding to 100 ~ in the strain scale) can be easily adjusted with the help of the Soleil Bab~net : compensator 34 to a quasi-linear steep region of a sin-like ~haracteristiG as shown in hatched lina~ in Figure 4.
Referrin~ now to ~i~ure 8, temperature compen~ation of the fiber-optic strain gauge manometer 2 in both trans~i~ion or reflection configuration ~hown in Figures 2 and 3) can be achieved wi~h a compensation portion 60 o~
HB optioal ~i~e~ besid~ the sensinq portion 16. The co~pen6ated ~anometer 2 con6i~ts of two identical ~sen6ing and co~pensating) parts of a ~B polarization maintaining optical fiber, spliced at 99 in relatlon to their polarization axe~. If both parts remain at the ~ame temperature ~ut only one i~ ~trained, their total temp~rature-induced phase retardation will cancel out. The degrea ~f cancellation depend~ heavily on preci~e angular ~5~
alignment and exact lengths of both partæ of tha ~ensorO
The phaæe retardation ~or each mode propagating in an optical fiber is giYen by:
~ = ~ n L (7) where: ~ $~ the wavelength u~ed;
n i~ the ef~ectiv~ refractive index of a given mode;
and ~ is the length.
For a re~lection con~iguration ~as shown in Figura 3) the e~ecti.ve length o~ th~ HB sen~ing element is twice its actual length.
~ he total relative phase retardation between the ~wo perpendicularly polari2ed eigenmodes propagating in a H~
optical ~iber will be given by:
~y = ~Ll) + ~(L2) (8, wh~r~ ( L, ) = -~ ~nlNLI
N is the number of ~ections o~ the ~B optical ~iber, ~; 25 e.g. N = 1 ~or tr~n6~ission con~iguration and N =
2 for re~lexio~ configuration; and i i~ th~ index of the ~ection.
~: ~emperature~induced ph~se ret~rd~ion ~an be e~pres6ed by: :
dT ~ ~dT (~nlL~ (Lld~lL~ ~ ~nl ~ ) (9) where nl 3 n~l - nyl, and nXl and nyl are the refra~tive :
.
, .' :
, ~CiiS06~k~
indices ~or two orthog~nal linear polarization modes of an i section. Since Ll = L~ = L and both parts are rotated one fiber relative to the other by 90 about their axes, we obtain n~ nl or ~An1 = O (1~) Henc~ d(~ L ~¢4n~ ~ ~L ~ n ) (11) dT ~ ~ ~ dT d~
Finally d~a~ d ~ _ dT ~ dT(L ,An1) - O (12) or~(Tl = constant ~rom ~14) it ~ollow~ that the strain sensor will be immune to ambient temperature or any other phy~ical parameter acting on both its parts. However, i~ only one part o~ the ~ensor is exposed to the influence of a physical parameter the sensor will mea~ure this parameter.
Referring a~ain to Figure 2, the fiber-optic strain ~auge ~anometer 2 ha~ b~en characterized at a conætant temperature for pressures up to 100 ~Pa in the three above-mentioned arrange~ents involving different combinations of lead and coll~cting optical fibers 22 and 24 fusion spliced at both ends of tha sensing ~iber 16.
Interestlng resuIt3 were obtained in the second arrangemen~ for which Figure 5 ~hows pressure characteristics o~ two ~ensing element~ 1~ havin~ 40 mm and 48 mm in length. However, the stable and repeatable re~ults 5~6~1 shown in Figur~ 6 were al60 obtained in the third arr~ngement when lead and collecting optical fibers 22 and 24 were made of LB fi~ers. Application o~ LB fibers allowed for more precise control of the state o~ polarization in the system than would be possible using a ~anometer 2 equipped with a standard single mode optical fiber thus diminishing signal fluctuations while at the same time signifiaantly decreasing the co~t o~ the device compared to that of a sensor e~uippad with a HB fiber input.
lo Shown in Figure 6 are two characteristics A and B of the same sensing element 16 (~hown in Figure 2) which wera displac~d ufiing a controlled phase shift introduced by the Soleil-Babinet compensator 34 ~hown in Figure 2). The sharp mini~um in the B characteristics corresponds to a ~train~modulated output light beam circular polarization whlch i~ ~eparated into both linear-polari2ation direction~
- I,) by pa~sln~ through the Wollaston prism 36 (shown in Figure 2).
Referring to Figure 2, by combining equation (4~ and the following one which r~sults from (2):
P.
dP 2 d E ~13) WQ can e~tabli6h the ~inal expre~sion for the phase pre~sure sensitivity of the manometer 2. The ~xpres~ion (13) clearl~ shows how to de~ign the manometer 2 with spsci~ied ~ensitivity in the predetermined pres6ure range:
dp d E ~ d~ (14) .
. . :
:
. ' . ' - ' ' , ' .. .
.~5~
The ~&nsitivity can be increased simply by increa~ing the length of the HB sensing fiber 16 (and the length of the free dilating cylinder 4), but this might be difficult without at the same time compromlsing the usual requirement for miniaturisation of the sa~sor head. Other means, how~ver, such as the choice of a shorter wa~elengt~, optimal geome~ry (D and d parameters), and/or appropriate ~ree dilating cyl~nder 4 t v and E para~eter~) and fiber (An para~etars) material~ would allow sufficient flexibility in HB sensing fibQr 2 de~ign to cover most of the potential applicatlons.
Figure 7 shows initial re~ult~ of pressure cycling on the ~etrological properties of the ~iber-optic strain gauge manometer 2 (shown in Figure~ 2 and 3). It appears that pres~ure~induced hysteresi~ o~ the ~iber-optic strain gauge mano~eter 2 ~shown in Fi~ure~ 2 and 3) ls residual, and generally i.~ due to the presence o~ adhesives. It i~ well known that the mechanical properties of the optical quartz ~ibers themselves are excellent and do not contribute to the eventual sensor hysteresis. It has been found that the hystere~1~ diminished as the number o~ pressure cycles increa~ed and it can be optimized through an appropriate choice of a~hesivQ materials.
Although the presant invention h~s been explained hereinabove by way o~ t~e pre~arred embodiments the~eo~, it ~hould be polnted out that any modifications to these prefer~ed ~b~d~ents, within the scope of the appended claim6 i8 not deemed to change or alter the nature and scope of the pre~ent invention.
: 16
Claims (18)
1. A fiber-optic strain gauge manometer for measuring pressure of a fluid, to be respectively connected to a light source and a measurement apparatus, said manometer comprising:
a cylindrical hollow body of predetermined length and acting as a pressure transducer, said hollow body having a central axis, cylindrical inner and outer surfaces, an end, and an opposite end provided with an inlet by which said fluid can get inside said hollow body, said ends being free to move longitudinally with respect to said central axis such that longitudinal and circumferential strains are generated in said hollow body when said pressure is applied on said inner surface, thereby producing a dilatation of said hollow body; and a highly birefringent optical fiber which can maintain only some polarization states, said birefringent optical fiber having a sensing portion bonded with bonding means to said outer surface of said hollow body along a longitudinal path such that birefringence of said sensing portion changes when said sensing portion is subjected to said dilatation of the hollow body, said sensing portion having a length smaller than said length of the hollow body, which is selected in function of a desired sensitivity;
whereby a polarized light beam is generated by said light source and transmitted in a form of two polarization eigenstates each parallel to one of two parallel principal birefringence axes of said birefringent optical fiber, the state of polarization of the input light beam being strain-modulated after a passage in said sensing portion, the strain-modulated output light beam being collected by said measurement apparatus for detecting and measuring change in said birefringence as a function of said dilatation, thereby giving indication of said pressure inside said hollow body.
a cylindrical hollow body of predetermined length and acting as a pressure transducer, said hollow body having a central axis, cylindrical inner and outer surfaces, an end, and an opposite end provided with an inlet by which said fluid can get inside said hollow body, said ends being free to move longitudinally with respect to said central axis such that longitudinal and circumferential strains are generated in said hollow body when said pressure is applied on said inner surface, thereby producing a dilatation of said hollow body; and a highly birefringent optical fiber which can maintain only some polarization states, said birefringent optical fiber having a sensing portion bonded with bonding means to said outer surface of said hollow body along a longitudinal path such that birefringence of said sensing portion changes when said sensing portion is subjected to said dilatation of the hollow body, said sensing portion having a length smaller than said length of the hollow body, which is selected in function of a desired sensitivity;
whereby a polarized light beam is generated by said light source and transmitted in a form of two polarization eigenstates each parallel to one of two parallel principal birefringence axes of said birefringent optical fiber, the state of polarization of the input light beam being strain-modulated after a passage in said sensing portion, the strain-modulated output light beam being collected by said measurement apparatus for detecting and measuring change in said birefringence as a function of said dilatation, thereby giving indication of said pressure inside said hollow body.
2. The manometer according to claim 1, wherein said hollow body can sustain pressure up to at least 100 MPa.
3. The manometer according to claim 1, wherein said means for bonding is epoxy.
4. The manometer according to claim 2, further comprising:
a lead optical fiber spliced to an end of said birefringent optical fiber, for guiding the polarized input light beam from said light source to said sensing portion; and a collecting optical fiber spliced to an opposite end of said birefringent optical fiber, for guiding the strain-modulated output light beam from said sensing portion to said measurement apparatus.
a lead optical fiber spliced to an end of said birefringent optical fiber, for guiding the polarized input light beam from said light source to said sensing portion; and a collecting optical fiber spliced to an opposite end of said birefringent optical fiber, for guiding the strain-modulated output light beam from said sensing portion to said measurement apparatus.
5. The manometer according to claim 2, further comprising:
a lead optical fiber for guiding the polarized input light beam from said light source to a bidirectional coupler;
a collecting optical fiber for guiding the strain-modulated output light beam from said bidirectional coupler to said measurement apparatus;
said bidirectional coupler for coupling the polarized input light beam from said lead optical fiber into an interconnecting birefringent optical fiber, and for coupling the strain modulated output light beam from said interconnecting birefringent optical fiber into said collecting optical fiber;
said interconnecting birefringent optical fiber having an end connected to said bidirectional coupler and an opposite end spliced at substantially 45° to an end of said birefringent optical fiber; and a reflective mirror at an opposite end of said birefringent optical fiber, said sensing portion being located at said opposite end.
a lead optical fiber for guiding the polarized input light beam from said light source to a bidirectional coupler;
a collecting optical fiber for guiding the strain-modulated output light beam from said bidirectional coupler to said measurement apparatus;
said bidirectional coupler for coupling the polarized input light beam from said lead optical fiber into an interconnecting birefringent optical fiber, and for coupling the strain modulated output light beam from said interconnecting birefringent optical fiber into said collecting optical fiber;
said interconnecting birefringent optical fiber having an end connected to said bidirectional coupler and an opposite end spliced at substantially 45° to an end of said birefringent optical fiber; and a reflective mirror at an opposite end of said birefringent optical fiber, said sensing portion being located at said opposite end.
6. The manometer according to claim 4, in combination with said light source and said measurement apparatus, wherein:
said light source comprises:
a laser for generating a polarized light beam:
polarization controller for aligning polarization planes of said polarized light beam parallel to one of two parallel principal birefringence axes of said lead optical fiber;
and said measurement apparatus comprises:
a compensator for adjusting given characteristics of the strain-modulated output light beam;
an analyzer combined with detector means for detection of the strain-modulated output light beam based on a lock-in amplifier in synchronism with said light source, and for generating an analog signal indicative of said pressure.
said light source comprises:
a laser for generating a polarized light beam:
polarization controller for aligning polarization planes of said polarized light beam parallel to one of two parallel principal birefringence axes of said lead optical fiber;
and said measurement apparatus comprises:
a compensator for adjusting given characteristics of the strain-modulated output light beam;
an analyzer combined with detector means for detection of the strain-modulated output light beam based on a lock-in amplifier in synchronism with said light source, and for generating an analog signal indicative of said pressure.
7. The manometer according to claim 6, further comprising:
an objective lens between said light source and said lead optical fiber for focussing said polarized light beam into said lead fiber; and an objective lens between said collecting optical fiber and said measurement apparatus for focussing said strain-modulated output light beam collected by said collecting optical fiber into said compensator;
wherein said polarization controller includes a quarter-wave plate and a polarizer, said compensator is a Soleil-Babinet compensator, said analyzer is a Wollaston prism, and said detector means are two photodetectors which are controlled by said lock-in amplifier.
an objective lens between said light source and said lead optical fiber for focussing said polarized light beam into said lead fiber; and an objective lens between said collecting optical fiber and said measurement apparatus for focussing said strain-modulated output light beam collected by said collecting optical fiber into said compensator;
wherein said polarization controller includes a quarter-wave plate and a polarizer, said compensator is a Soleil-Babinet compensator, said analyzer is a Wollaston prism, and said detector means are two photodetectors which are controlled by said lock-in amplifier.
8. The manometer according to claim 7, wherein said birefringent optical fiber has also a compensating portion beside said sensing portion, said compensating portion having birefrigence axes at 90° from the birefringence axes of said sensing portion, and a lenght substantially equal to said lenght of the sensing portion.
9. The manometer according to claim 8, wherein a passive optical fiber is added between said compensating portion and said sensing portion for ascertaining that only said birefringence of the sensing section which is subjected to said dilatation will change.
10. The manometer according to claim 7, wherein said lead optical fiber is a highly birefringent optical fiber spliced at substancially 45° to said end of the birefringent optical fiber, and wherein said collecting optical fiber is a single mode optical fiber or a lowly birefringent optical fiber which can maintain any polarization state, connected at said opposite end of the birefringent optical fiber.
11. The manometer according to claim 7, wherein said lead optical fiber and said collecting optical fiber are lowly birefringent optical fibers which can maintain any polarization state, respectively connected to said end and said opposite end of the birefringent optical fiber.
12. The manometer according to claim 5, in combination with said light source and said measurement apparatus, wherein:
said light source comprises:
a laser for generating a polarized light beam;
polarization controller for aligning polarization planes of said polarizer light beam parallel to one of two principal birefringence axes of said lead optical fiber;
and said measurement apparatus comprises:
a compensator for adjusting given characteristics of said strain-modulated output light beam;
an analyzer combined with detector means for detection of said strain-modulated output light beam based on a lock-in amplifier in synchronism with said light source, and for generating an analog signal indicative of said pressure.
said light source comprises:
a laser for generating a polarized light beam;
polarization controller for aligning polarization planes of said polarizer light beam parallel to one of two principal birefringence axes of said lead optical fiber;
and said measurement apparatus comprises:
a compensator for adjusting given characteristics of said strain-modulated output light beam;
an analyzer combined with detector means for detection of said strain-modulated output light beam based on a lock-in amplifier in synchronism with said light source, and for generating an analog signal indicative of said pressure.
13. The manometer according to claim 12, further comprising:
an objective lens between said light source and said lead optical fiber for focussing said polarized light beam into said lead fiber; and an objective lens between said collecting optical fiber and said measurement apparatus for focussing said strain-modulated output light beam collected by said collecting optical fiber into said compensator;
wherein said polarization controller includes a quarter-wave plate and a polarizer, said compensator is a Soleil-Babinet compensator, said analyzer is a Wollaston prism, and said detector means are two photodetectors which are controlled by said lock-in amplifier.
an objective lens between said light source and said lead optical fiber for focussing said polarized light beam into said lead fiber; and an objective lens between said collecting optical fiber and said measurement apparatus for focussing said strain-modulated output light beam collected by said collecting optical fiber into said compensator;
wherein said polarization controller includes a quarter-wave plate and a polarizer, said compensator is a Soleil-Babinet compensator, said analyzer is a Wollaston prism, and said detector means are two photodetectors which are controlled by said lock-in amplifier.
14. The manometer according to claim 13, wherein said birefringent optical fiber has also a compensating portion beside said sensing portion, said compensating portion having birefringence axes at so from the birefringence axes of said sensing portion, and a lenght substantially equal to said lenght of the sensing portion.
15. The manometer according to claim 14, wherein a passive optical fiber is added between said compensating portion and said sensing portion for ascertaining that only said birefringence of the sensing section which is subjected to said dilatation will change.
16. The manometer according to claim 13, wherein said lead optical fiber is a highly birefringent optical fiber, and wherein said collecting optical fiber is a single mode optical fiber or a lowly birefringent optical fiber which can maintain any polarization state.
17. The manometer according to claim 13, wherein said lead optical fiber and said collecting optical fiber are lowly birefringent optical fibers which can maintain any polarization state.
18. A method for measuring pressure of a fluid inside a cylindrical hollow body of predetermined length and acting as a pressure transducer, said hollow body having a central axis, cylindrical inner and outer surfaces, an end, and an opposite end provided with an inlet by which said fluid can get inside said hollow body, said ends being free to move longitudinally with respect to said central axis such that longitudinal and circumferential strains are generated in said hollow body when said pressure is applied on said inner surface, thereby producing a dilatation of said hollow body, a portion of highly birefringent optical fiber which can maintain only some polarization states being bonded to said outer surface of the hollow body along a longitudinal path for subjecting said portion to said dilatation, said method comprising steps of:
transmitting a polarized input light beam in a form of two polarization eigenstates each parallel to one of two parallel principal birefringence axes of said birefringent optical fiber, said input light beam being strain-modulated after a passage in said portion of the highly birefringent optical fiber due to change in birefringence of said birefringent optical fiber as a result of said dilatation;
collecting the strain-modulated output light beam;
measuring polarization change in the state of polarization between the input light beam and the strain-modulated output light beam; and determining a value indicative of said pressure as a function of the measured change in the state of polarization.
R O B I C .
transmitting a polarized input light beam in a form of two polarization eigenstates each parallel to one of two parallel principal birefringence axes of said birefringent optical fiber, said input light beam being strain-modulated after a passage in said portion of the highly birefringent optical fiber due to change in birefringence of said birefringent optical fiber as a result of said dilatation;
collecting the strain-modulated output light beam;
measuring polarization change in the state of polarization between the input light beam and the strain-modulated output light beam; and determining a value indicative of said pressure as a function of the measured change in the state of polarization.
R O B I C .
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2050609 CA2050609A1 (en) | 1991-09-04 | 1991-09-04 | Fiber-optic strain gauge manometer |
| PL29580192A PL168803B1 (en) | 1991-09-04 | 1992-09-01 | Strain gauge type pressure measuring transducer |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2050609 CA2050609A1 (en) | 1991-09-04 | 1991-09-04 | Fiber-optic strain gauge manometer |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2050609A1 true CA2050609A1 (en) | 1993-03-05 |
Family
ID=4148298
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2050609 Abandoned CA2050609A1 (en) | 1991-09-04 | 1991-09-04 | Fiber-optic strain gauge manometer |
Country Status (2)
| Country | Link |
|---|---|
| CA (1) | CA2050609A1 (en) |
| PL (1) | PL168803B1 (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN103604540A (en) * | 2013-11-13 | 2014-02-26 | 中铁四局集团第一工程有限公司 | Photoelectric stressometer |
| CN111288913A (en) * | 2020-03-26 | 2020-06-16 | 西北核技术研究院 | Non-contact measurement method and system for deformation of double-layer cylinder under internal explosion effect |
| CN113834527A (en) * | 2021-09-18 | 2021-12-24 | 重庆大学 | Crimping type power semiconductor structure and internal pressure online measurement method thereof |
-
1991
- 1991-09-04 CA CA 2050609 patent/CA2050609A1/en not_active Abandoned
-
1992
- 1992-09-01 PL PL29580192A patent/PL168803B1/en unknown
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN103604540A (en) * | 2013-11-13 | 2014-02-26 | 中铁四局集团第一工程有限公司 | Photoelectric stressometer |
| CN103604540B (en) * | 2013-11-13 | 2015-06-10 | 中铁四局集团第一工程有限公司 | Photoelectric stressometer |
| CN111288913A (en) * | 2020-03-26 | 2020-06-16 | 西北核技术研究院 | Non-contact measurement method and system for deformation of double-layer cylinder under internal explosion effect |
| CN111288913B (en) * | 2020-03-26 | 2022-01-04 | 西北核技术研究院 | Non-contact measurement method and system for deformation of double-layer cylinder under internal explosion effect |
| CN113834527A (en) * | 2021-09-18 | 2021-12-24 | 重庆大学 | Crimping type power semiconductor structure and internal pressure online measurement method thereof |
Also Published As
| Publication number | Publication date |
|---|---|
| PL295801A1 (en) | 1993-05-04 |
| PL168803B1 (en) | 1996-04-30 |
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