A novel ice-pressure sensor based on dual FBGs

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1 Confrn: Snsors and Smart Struturs Thnologis for Civil, Mhanial, and Arospa Systms, SPIE, San Digo, USA, 25 A novl i-prssur snsor basd on dual FBGs Zhi Zhou, Chunguang Lan, Taiming Lin, Jinping Ou Shool of Civil Enginring, Harbin Institut of Thnology, Harbin, 159, China ABSTRACT I prssur is on of th most important loads in high-latitud ara. It is hallngabl to build a durabl and stabl ral-tim strutural halth monitoring systm for i-prssur undr suh aggrssiv nvironmnt as windinss, oldnss, and vn vibrating, whih an not b mt by strain gaug basd snsors, whras FBG fits it wll du to its grat advantag of orrosion rsistan, absolut masurmnt, high auray, ltro-magnti rsistan, quasi-distribution snsing, absolut masurmnt and so on. In this papr, a novl FBG basd i-prssur snsor has bn dvlopd. Firstly, in onsidration of th monitoring of i-prssur of offshor platform, a novl i prssur snsor strutur has bn dsignd and it snsing prinipl is givn in dtails, whih thortially shows th proprtis of tmpratur slf-ompnsation and indpndn of th load position. And sondly, th proprtis of FBG-basd i-prssur snsor hav bn tstd by xprimnts. Finally, thortial snsitivity has bn ompard with that from xprimnts. Th rsarh rsults show that th FBG-basd i-prssur snsor has good linarity, rptition, immunity of tmpratur hangs and loading position. Suh kind of FBG-basd i-prssur snsor an b usd to monitor i load of offshor platform onvnintly. Ky words: FBG, tmpratur slf-ompnsation, i-prssur snsor, strutural halth monitoring, offshor platform 1. INTRODUCTION I prssur is an important load of th offshor platforms and vn bridgs aross rivrs in th high-latitud ara. Many offshor platforms wr damagd by i prssur, vn ollapsd. Th 2# xploitation platform in Bo Hai oil fild in China was push ovr by i in wintr, I-prssur data is th bas of dsign of offshor struturs. Morovr, du to that i-prssur is also th main fator of fatigu aumulativ damag, it is important to gt long trm information of i-prssur to valuat th safty and intgrity of offshor struturs. Traditional i-prssur monitoring dvi is basd on ltrial strain gaug, whih is asy to b damagd by moistur, orrosion, ltro-magnti disturban and so on. So, it is vry important to dvlop rliabl snsors for monitoring of i-prssur. Optial fibr snsors, spially th fibr Bragg grating (FBG) snsors, hav mor and mor bom th fous for strain snsor in strutural halth monitoring du to thir distinguishing advantags: ltro-magnti rsistan, small siz, rsistan to orrosion, multiplxing a larg numbr of snsors along a singl fibr, t. Fibr Bragg grating snsor is suitabl for th masur of i for. FBG strain snsor has now bn widly invstigatd and applid in infrastruturs (Ou and Zhou, 22, 2, 24). Som rsarhrs hav studid and dvlopd FBG strain snsing basd prssur snsors (Zhang, 21; Zhao, 22, 24). Howvr, FBGs ar snsitiv to both strain and tmpratur, and thus tmpratur ompnsation thniqu is unavoidabl for pratial nginring appliation of strain snsing basd snsors. Common mthods to distinguish th ross-snsitivity btwn strain and tmpratur dpnd on additional gratings and spial thniqu (Zhou, 24; Zhang, 21). As for i prssur snsor, morovr, th position of i load, whih is somtims hug, hangs along th sa lvl, so w must mak th i-prssur snsor adaptiv to or indpndnt of th hangs of loading position. In onsidration of th monitoring of i-prssur of offshor platform, a novl i prssur snsor strutur has bn dsignd and its snsing prinipl is givn in dtails. And th proprtis of FBG-basd i-prssur snsor hav bn tstd by xprimnts and analyzd thortially. Corrsponding author: oujinping@hit.du.n; phon ; fax

2 Confrn: Snsors and Smart Struturs Thnologis for Civil, Mhanial, and Arospa Systms, SPIE, San Digo, USA, SENSOR STRUCTURE AND BASIC SENSING PRINCIPLES 2.1 Snsor strutur To mt th dmand of i-prssur monitoring of platform, th snsor strutur should show: 1) Easy for onstrution; 2) Prssur snsing without additional tmpratur ompnsation, namly slf-ompnsation; ) Indpndnt of i loading position; 4) strong nough for durability; 5) Stability for long-trm srvi, t. Basd on abov points, w onfigurd our novl i-prssur snsor as figur 1. Optial fibr jumpr Elasti spring bam Rigid stainlss stl plat Equal strngth antilvr Dual FBGs Position rstritd pi Figur 1: Skth of novl i-prssur snsor strutur Th novl i-prssur snsor strutur is mainly omposd of: 1) Rigid stainlss stl plat usd to ndur i loading without too muh dformation; 2) Elasti spring bam usd to transfr th load to displamnt whih an b snsd by qual strngth antilvr; ) Equal strngth antilvr usd for snsing th displamnt; 4) Position rstritd pi usd to ontrol th maximum dformation of th load transfr bam, whih an assur th whol strutur avoidabl of too hug i loading.; 5) Rigid stainlss stl box usd to prott th whol snsor; 6) dual FBGs usd to sns strain of th bilatral antilvr indud by th displamnt of mid-point of th lasti spring bam. 2.2 Snsing prinipl of FBG-basd i-prssur snsor Th basi snsing prinipl of th FBG basd i-prssur an b dsribd lik that: whn i load ats on th rigid stainlss stl plat, th load an b transfrrd to th lasti spring bam, and th mid-point of th bam will bring displamnt to th antilvr, so th strain stat of th surfas of th bilatral antilvr will b hangd. Th strain an b snsd by th attahd FBGs. Th strain stat of th 2 surfas is opposit, but sam at valu. Baus th two FBGs ar vry los to ah othr, shifts of thir Bragg wavlngths du to tmpratur ar onsidrd to b idntial. Thus w an gt th strain stat by diffrntial mthod by th 2 FBGs. So w an gt th i loading aordingly. Th dtaild basi snsing prinipl is following. (I ) Mid-point displamnt of lasti spring bam I loading 1) Rigid stainlss stl plat L 2) Elasti spring bam

3 Confrn: Snsors and Smart Struturs Thnologis for Civil, Mhanial, and Arospa Systms, SPIE, San Digo, USA, 25 Figur 2: Skths of rigid stl plat and spring bam undr i prssur Figur 2 shows th skths of rigid stainlss stl plat and lasti spring bam undr i loading. Aording to matrial mhanis thory, th displamnts, V 1 and V 2, of mid-point of th lasti spring bam undr P 1 and P 2, rsptivly, an b xprssd as Pa 1 (L 4 a ) P2 a(l 4 a ) ν 1 = ν 2 = (1) whr L, E and I is th lngth, Young modul and inrtia momnt of lasti spring bam, rsptivly. So w an find that 2 2 P a(l 4 a ) v = v1+ v2 = (2) whih shows that displamnt of th mid-point of spring bam is indpndnt of i loading position. So on th a is givn, th displamnt is linar to i load. (ii) Exprssion of bilatral qual strngth antilvr undr displamnt Gnrally spaking, th span of th qual strngth antilvr bam (L C ) is biggr than hight (H C ), ovr 1 tims, and th antilvr is mainly undr miro bnding dformation, so w an nglt shar for and th bam satisfis th hypothss of plan stion. So th rlationship btwn strain (ε ) and nd point displamnt (v) of th antilvr an b givn as 2 LC V = H ε () C Hr, th nd point displamnt (v) is qual to that of mid-point of spring bam. Th xprssion shows that th strain of antilvr is linar to th displamnt of mid-point of lasti spring bam. (iii) Intration of antilvr and lasti spring bam Givn that th antilvr is atd by nd point displamnt, v, w an gt th strss (σ ) of surfa EHv σ = E ε = (4) 2 L Aording to th loading fatur of th antilvr, th load F ating on th nd point is EHBv F = (5) 6L whr B is th width of th fixd nd of th antilvr. So th displamnt ( v r ) of th spring bam atd by abov F an b givn as BHLE vr = v (6) 288LEI In gnral, onsidring of th intration of antilvr and lasti spring bam, ombining (2) and (6), w an gt 2 2 Pa(L 4 a ) BHL E v = v (7) 288LEI So w an gt BH L E P = (1 + ) v (8) LEI a(l 4 a )

4 Confrn: Snsors and Smart Struturs Thnologis for Civil, Mhanial, and Arospa Systms, SPIE, San Digo, USA, 25 Abov xprssion is th gnral rlationship btwn i load and displamnt of nd point of antilvr. (IV) Rlationship btwn i prssur and FBG wavlngth Aording to FBG thnology, th gnral xprssion of th 2 FBGs attahd on th bilatral antilvr, on up and th othr down, atd by strain and tmpratur an b dsribd as λ1 λ ε 1 λt 1 = + = (1 P ) ε + ( α + ς) T (9) 1 1 λ1 λ1 λ1 λ2 λ ε 2 λt 2 = + = (1 P ) ε2 + ( α + ς) T2 (1) λ λ λ whr λ,ζ,α, P and T is wavlngth, thrmal-optis offiint, thrmal xpansion offiint, optial lastiity offiint and tmpratur, rsptivly. And th sign, 1 and 2, is to show th diffrnt 2 FBGs. Hr T1 = T2, and onsidring th strain fatur of th antilvr, w an gt λ1 λ2 = 2(1 ) ε (11) λ λ 1 2 Combining (), (8) and (11), w an gt 2 BH L E L λ1 λ2 P = (1 + ) ( ) (12) LEI a(l 4 a ) 2(1 P) H λ λ P BH L E L Lt K = (1 + ), thn LEI a(l 4 a ) 2(1 P) H λ1 λ2 P = K( ) (1) λ λ 1 2 (V) Simplifiation of th FBG-basd i-prssur snsing xprssion Considring th pratial nginring appliation, if for indud by th antilvr is vry small, whih mans BH L E that 1; and th original wavlngth of th 2 FBGs is th sam or los nough, abov xprssion an b 288LEI hangd to P = ( k λ λ ) 1 2 (14) 2 L whr K = a 2 2 ( L 4 a ) 2(1 P ) λh.1 Snsor modl fabriation EXPERIMENTS AND ANALYSIS Considring th aggrssiv nvironmnt whr th i-prssur snsor will srv, w hos propr matrials and optimizd th siz of th FBG-basd i prssur snsor. Hr, th rigid stainlss stl plat is mad of high strngth stainlss alloy; th lasti spring bam and qual-strngth antilvr ar from spring stainlss stl whih show proprtis of high fatigu and orrosion rsistan; And th prottiv box must b strong nough to support th strutur of th i-prssur snsor. Dual FBGs ar bondd by high prforman glu namd J1. Th fabriatd FBG-basd i prssur snsor is dpitd as figur.

5 Confrn: Snsors and Smart Struturs Thnologis for Civil, Mhanial, and Arospa Systms, SPIE, San Digo, USA, 25.2 Calibration xprimnts Figur : Pitur of FBG-basd i prssur snsor In ordr to tst th rliability of th i-prssur snsor, w fixd it on an xprimntal tabl. Th load is givn through mass pis, and th wavlngth is monitord by SI425 providd by Miron Optis In. in USA. Th data of load and wavlngth of th 2 FBGs wr rordd simultanously. Th 4 yls data of rlativ wavlngth hangs and load ar givn as figur 4 a) ~) yl 1 linar fitting yl 2 Linar fitting load (N) P= λ P=.761 λ rlativ wavlngh hangs (pm) a) Cyl b) Cyl yl Liar fitting 1 12 yl 4 Linar fitting P=.7476 λ P= λ ) Cyl d) Cyl yl 1 yl 2 yl yl ) Comparison of th 4 yl data. Figur 4: Rlativ wavlngth hangs and load rlationship of th i prssur snsor

6 Confrn: Snsors and Smart Struturs Thnologis for Civil, Mhanial, and Arospa Systms, SPIE, San Digo, USA, 25 From figur4 a)~) w an find that th FBG-basd i prssur snsor shows good linarity and rptition, and th snsing offiint is stabl to b.74 N/Pm or so, whih shows that suh kind of snsor is good nough for pratial nginring appliation.. Loading position xprimnts In ordr to tst th stability of th i-prssur snsor undr diffrnt position loading, w arrid out abov xprimnts by hanging th loading positions. And th data of rlativ wavlngth hangs and load undr diffrnt position is givn as figur mid-point 1/4 on right 1/4 on lft asymmtry Figur 5: Comparison of i-prssur snsor undr diffrnt position loading From figur 5 w an find that th rlationship btwn rlativ wavlngth hangs and load undr diffrnt position agrs wll, whih shows that th i-prssur snsor is indpndnt of th styl and position of i-prssur loading..4 Tmpratur slf-ompnsation xprimnts In ordr to tst th stability of th i-prssur snsor undr diffrnt tmpraturs, w arrid out abov xprimnts by hanging th tmpraturs. And th data of rlativ wavlngth hangs and load undr diffrnt tmpratur is givn as figur t=14 t=21 t= Figur 6: Comparison of i-prssur snsor undr diffrnt tmpratur From figur 6 w an find that th rlationship btwn rlativ wavlngth hangs and load undr tmpratur agrs wll, whih shows that th i-prssur snsor is indpndnt of tmpratur. So th dual FBG-basd i-prssur snsor shows indpndnt of tmpratur, namly tmpratur slf-ompnsation.

7 Confrn: Snsors and Smart Struturs Thnologis for Civil, Mhanial, and Arospa Systms, SPIE, San Digo, USA, 25.5 Exprimnts of i-prssur snsor undr loading at diffrnt span (D) Aording to (12) and (14), th offiint of load and rlativ wavlngth hangs is afftd sriously by th span of th 2 ating points dpitd in figur 2, whih mans D = ( L 2 b)/2. In ordr to tst th rlationship btwn D and offiint K, th xprimnts of i-prssur snsor undr loading by hanging th loading span hav bn arrid out. Th omparison of rlationship btwn rlativ wavlngth hangs and loading undr diffrnt D is givn as figur D=5 linar fitting 1 12 D=1 linar fitting K= K= a) D=5 mm b) D=1 mm D=15 Linar fitting 8 D=2 Linar fitting 6 K= K= ) D=15mm d) D=2mm D=25 Linar fitting D= 5 D=1 D=15 D=2 D= K= ) D=25 mm f) Comparison Figur 7: Comparison of rlationship btwn rlativ wavlngth hangs and loading undr diffrnt D From figur 7, w an find that th rlationships ar vry diffrnt btwn rlativ wavlngth hangs and loading of diffrnt D, whih shows: th lss for th D, th biggr for K. And linarity is also muh bttr. So w an hang th rsolution or auray asily by hanging th loading span of D. Hr, if w hoos D=5, 1, 15, 2 and 25 mm, rsptivly, th K is.75 N/pm,.42 N/pm,.29 N/pm,.24 N/pm and.21 N/pm, rsptivly.

8 Confrn: Snsors and Smart Struturs Thnologis for Civil, Mhanial, and Arospa Systms, SPIE, San Digo, USA, 25 4 COMPARISON OF THEORECTIC AND EXPERIMENTAL RESULTS As for th fabriatd FBG-basd i prssur snsor, th basi paramtrs ar following: 5 2 L = 6mm, B = 5mm, H = 4mm, E = E = 2. 1 N / mm, a = 5mm, P =.22mm, λ = nm, H = 4mm, L = mm Using (1), w an gt th thortial K thory =.6788 N/pm, whr as th avrag xprimntal valu is K xp = N/pm, so th rror is Kt hor y Kxp = =.87 = 8.7% Kxp Th rsults from thory agr wll with that from xprimnts. And th rror is aptabl by pratial nginring appliation. 5 CONCLUSIONS It is vry important to dvlop rliabl snsors for monitoring of i-prssur for offshor platform. A novl i prssur snsor has bn dsignd and its snsing prinipl is givn in dtails. Morovr, th snsing proprtis of FBG-basd i-prssur snsor hav bn tstd by som xprimnts. Th rsarhs rsults show that th novl FBG-basd i prssur is indpndnt of loading position and tmpratur hangs, and has good linarity and rptition. Its rsolution and auray an b adjustd aording to th pratial nginring nd. Suh kind of FBG-basd i-prssur snsor an b usd to monitor i prssur of offshor platform onvnintly. ACKNOWLEDGEMENTS This papr is partially supportd by NSFC (Grantd no. 588) and high-th rsarh and dvlopmnt program of China. REFERENCES 1. Z. Zhou Optial Fibr Smart BRAGG Grating Snsors and Intllignt Monitoring Systm in Civil Infrastruturs, Dotor Dissrtation, Harbin Institut of Th., Harbin, China WG Zhang, XY Dong, QD Zhao. FBG-typ snsor for simultanous masurmnt of for (or displamnt) and tmpratur basd on bilatral antilvr bam. IEEE Photonis Th. Ltt. 21; 1(12):1-2.. Y Zhao, YB Liao, SR Lai. Simultanous masurmnt of down-hol high prssur and tmpratur with a bulk-modulus and FBG snsor. IEEE, Photonis Th. Ltt. 22;14(11): Zhi Zhou,OU Jinping. Thniqus of tmpratur ompnsation for FBG strain snsors. Proding of Asian Pasif Fundamntal Problms of Opto- and Miroltronis, Khabarovsk, Russia,Sptmbr 1-16, Zhi Zhou, Jinping OU. Dvlopmnt of FBG snsors for Strutural Halth Monitoring in ivil infrastruturs, Proding of North Amrian Euro-Paifi Workshop Snsing Issus in Civil Strutural Halth Monitoring Novmbr 11-1, 24, Waikiki Bah, Oahu,Hawaii,USA 6. Xiangdong Zhang, Yulin Li, Wiping Wang. Prssur and tmpratur snsing systm basd on Fibr-Optis Bragg Grating for oil and Gas downhol, SPIE,5279,24: Yong Zhao, Chngbo Yu, Yanbiao Liao. Diffrntial FBG snsor for tmpratur-ompnsatd high-prssur (or displamnt) masurmnt. Optis & Lasr Thnology, 6,24:9-42