·178· 智能系统学报 第3卷 sor pair,if one of the twin sensors is used as a nl。+∑nl=l6+∑l+X (2) strain sensor,the other sensor can be used as a temperature compensated sensor. The shift of the white light interference peak A Xe Scanning mirror corresponds to the variation of the k-th twimsen- sor,i.e. S S2 S LED 3 dB 92923 coupler GRIN △X= ,△(w.△m别 3) PD Lens For the case shown in Fig.2,the sensor array was Fiber attenuator 35889 embedded in the structure.The corresponding twimsensor array was put in a pipe near the sens Fig.I Working principle of the twirarray Michelson ing fiber sensors in order to compensate the varia- fiber optic interferometric strain sensing system tion of the refractive index of the fiber induced by Therefore,the proposed sensing scheme will temperature changes and the elongation of the fiber be useful for temperature compensation of distribu- caused by thermal expansion.The ambient tem- ted strain measurement.An important application perature could be considered the same because the of the sensing system could be deformation sensing twin sensor array has been arranged very close. in smart structures. (T) (T) (T) 2 Sensing principle Fig.2 shows a twimsensor-arrays embedded in (e.T）(e.T) ..(Em.T.) a concrete structure.A number of fiber segments pairs l and l/are connected in serial to form the twim sensor-arrays,which is further connected to a Fig.2 For the case of the twin sensor array arrangement lead in/out fiber of length Lo and Lo.In the sens- in the structure ing system,the length of the two lead in/out fiber In the Fig.2,the sensor array has been em- cables has been chosen as nearly equal,and the bedded in the concrete material,while the refer- same as each fiber segment pairs,i.e. ence array has been put on the pipe in free state L0≈L6, nearby the sensor array. h兰1k, (i,j=1,2,N0 (1) When the strain and its environmental temper- ≠， ature change,the length of the sensing fiber gauge 人4≠ will increase or decrease as a result of both The optical path length of the reference arm Lo can strain and elevated temperature.The optical path be varied through the use of a moving graded index elongation can be expressed as (GRIN)lens or a scanning right angle mirror.One △(nl)=[nAl,(g+△n(gla]+ adjusts the optical path of the reference arm to (nAk(T)+△n(T)h1, 4) match and trace the change of the twimfiber-sensor The compensation sensor array will surely increase gauge length in each sensing pairs.When the opti- as a result of elevated temperature cal-path difference (OPD)between the sensing △(nl0=Al()+△n(TD11.(5) branch and the reference branch falls within the co- Substitute equation (4)and (5)into equation (3) herence length of the light source,a white light and note the condition given in equation (1)1,=1/, fringe pattern is produced.The central fringe, we get which is located in the center of the fringe pattern △Xk= and has the highest amplitude,corresponds to the Pa9+△n91= exact path match of the two optical paths.Thus, nequivalent, 6) we have 1994-2008 China Academic Journal Electronic Publishing House.All rights reserved http://www.cnki.netsor pair , if one of t he twin sensors is used as a strain sensor , t he ot her sensor can be used as a temperat ure compensated sensor. Fig. 1 Working principle of the twin2array Michelson fiber optic interferometric strain sensing system Therefore , t he proposed sensing scheme will be usef ul for temperature compensation of distribu2 ted strain measurement. An important application of t he sensing system could be deformation sensing in smart struct ures. 2 Sensing principle Fig. 2 shows a twin2sensor2arrays embedded in a concrete struct ure. A number of fiber segments pairs li and l′i are connected in serial to form the twin2sensor2arrays , which is f urt her connected to a lead in/ out fiber of length L0 and L′0 . In t he sens2 ing system , t he lengt h of the two lead in/ out fiber cables has been chosen as nearly equal , and the same as each fiber segment pairs , i. e. : L0 ≈ L′0 , li µ l′i , li ≠lj , l′i ≠l′j . ( i , j = 1 ,2 , …, N) . (1) The optical pat h length of t he reference arm L′0 can be varied t hrough the use of a moving graded index ( GRIN) lens or a scanning right angle mirror. One adjusts the optical path of the reference arm to match and trace t he change of t he twin2fiber2sensor gauge lengt h in each sensing pairs. When t he opti2 cal2pat h difference ( OPD ) between t he sensing branch and t he reference branch falls wit hin t he co2 herence lengt h of the light source , a white light fringe pattern is produced. The central fringe , which is located in t he center of t he fringe pattern and has t he highest amplit ude , corresponds to the exact pat h match of the two optical pat hs. Thus , we have nL 0 + ∑ k i = 1 nl i = nL′0 + ∑ k i =1 nl′i + Xk . (2) The shift of t he white light interference peak ΔXk corresponds to t he variation of t he k2t h twin2sen2 sor , i. e. : ΔXk = ∑ k i = 1 Δ( nl i) - Δ( nl′i ) . (3) For t he case shown in Fig. 2 , t he sensor array was embedded in the struct ure. The corresponding twin2sensor array was p ut in a pipe near t he sens2 ing fiber sensors in order to compensate t he varia2 tion of t he refractive index of the fiber induced by temperat ure changes and t he elongation of t he fiber caused by thermal expansion. The ambient tem2 perat ure could be considered t he same because t he twin sensor array has been arranged very close. Fig. 2 For the case of the twin sensor array arrangement in the structure In the Fig. 2 , t he sensor array has been em2 bedded in t he concrete material , while t he refer2 ence array has been p ut on t he pipe in free state nearby the sensor array. When the strain and its environmental temper2 ature change , t he lengt h of t he sensing fiber gauge will increase ( or decrease ) as a result of bot h strain and elevated temperat ure. The optical pat h elongation can be expressed as Δ( nl i) = [ nΔli (ε) +Δn(ε) li ] + ( nΔli ( T) +Δn( T) li ]. (4) The compensation sensor array will surely increase as a result of elevated temperat ure Δ( nl′i ) = [ nΔl′i ( T) +Δn( T) l′i ]. (5) Substit ute equation (4) and (5) into equation (3) and note t he condition given in equation (1) li = l′i , we get ΔXk = ∑ k i = 1 [ nΔli (ε) +Δn(ε) li ] = ∑ k i = 1 nequivalent lεi i , (6) · 871 · 智 能 系 统 学 报 第 3 卷 © 1994-2008 China Academic Journal Electronic Publishing House. All rights reserved. http://www.cnki.net