JP2004061112A

JP2004061112A - Displacement measurement method based on optical fiber strain sensor

Info

Publication number: JP2004061112A
Application number: JP2002215356A
Authority: JP
Inventors: Yukio Miyakura; 宮倉　由紀夫; Kazuyuki Ichikubo; 一久保　和幸; Susumu Sasaki; 佐々木　進; Eiji Sakata; 坂田　栄治
Original assignee: NTT Infrastructure Network Corp; Airec Engineering Corp
Current assignee: NTT Infrastructure Network Corp; Airec Engineering Corp
Priority date: 2002-07-24
Filing date: 2002-07-24
Publication date: 2004-02-26

Abstract

<P>PROBLEM TO BE SOLVED: To provide a means for precisely detecting a displacement position, a displacement amount, and the displacement direction of a measurement object by means of an optical fiber strain sensor. <P>SOLUTION: The optical fiber strain sensor S1, in which first-fourth light guide paths F1-F4 formed of optical fibers and arranged along the axial line are sequentially arranged at equal intervals with a phase difference of 90° around a pipe 1, is used. When it is assumed that a strain value of the first light guide path F1, a strain value of the second light guide path F1, a strain value of the third light guide path F1, a strain value of the fourth light guide path F1 in the measured strain position are represented by ε<SB>1</SB>, ε<SB>2</SB>, ε<SB>3</SB>, and ε<SB>4</SB>, an angle θ made by the direction connecting the first light guide path F1 and the pipe center O together and the displacement direction P is calculated according to the following expression; θ = tan<SP>-1</SP>[¾ε<SB>2</SB>-ε<SB>4</SB>¾/¾ε<SB>1</SB>-ε<SB>3</SB>¾]. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、例えば土塊の地滑り等による移動のような測定対象物の変位を光ファイバ歪みセンサによって歪みとして捉え、この歪み位置と歪み値とから変位方向を特定するための測定方法に関する。
【０００２】
【従来の技術】
一般に、光ファイバ歪みセンサは、光ファイバに片端から入射したパルス光が内部で散乱して後方散乱光（ブリルアン散乱光）として入射端側に戻る際、光ファイバの歪み（伸縮）発生区間からの後方散乱光の周波数分布（パワースペクトラム）が歪みに比例してシフトすることを利用し、測定対象物の変位を光ファイバの歪みとして捉え、周波数分布がシフトした後方散乱光が戻るまでの時間から歪み位置を特定すると共に、その周波数分布の解析によって歪み値を測定するものであり、ＢＯＴＤＲ（Ｂｒｉｌｌｏｕｉｎ　Ｏｐｔｉｃａｌ　Ｔｉｍｅ　Ｄｏｍａｉｎ　Ｒｅｆｌｅｃｔｏｍｅｔｅｒ）と呼ばれている。このような光ファイバ歪みセンサによれば、光ファイバ自体がセンサとなるために線や面での広範囲な計測を行えると共に、センサへの電力供給用配線が不要であり、且つ雷や高圧線による電磁ノイズの影響を受けず、遠隔地からの高度な制御管理が可能になるという利点がある。
【０００３】
従来、光ファイバ歪みセンサを用いて土塊の地滑りによる移動を調べるには、長手方向に沿って１本の光ファイバを配設したパイプ等の変形可能な棒状体を地滑りが想定される地盤に垂直に貫設し、前記の後方散乱光の計測によって棒状体の変形に伴う光ファイバの歪みを測定するようにしている。すなわち、棒状体の軸線方向を横切る土塊の移動があれば、土塊の滑り面の位置で棒状体が変形し、この変形に伴って光ファイバの歪みを生じるから、その歪み位置から滑り面の位置（深さ）が判ると共に、その歪み値の大きさと経時的変化から地滑りの程度と進行度合を経験的に判断できる。しかしながら、このような測定では、滑り面の位置は特定できても、測定データから地滑りの変位方向は判定できないため、過去の傾向や地形等から予め変位方向が判っている場合を除き、有効な地滑り対策を講じる上で甚だ不充分であった。
【０００４】
そこで、本出願人らは先に、光ファイバ歪みセンサとして、測定対象物の変形に追従するフレキシブルな棒状体の周囲複数箇所に光ファイバからなる導光路を該棒状体の軸線に対して平行に形成したものを提案し、これを特願２０００−２９７５２５（特開２００２−１０７１２２）で開示している。しかして、このような光ファイバ歪みセンサは、前記棒状体に合成樹脂パイプを用いたものが光パイプ歪み計として、既に各地の地滑りや斜面崩壊の検知用として実際に使用されている。
【０００５】
この先行技術による光ファイバ歪みセンサでは、例えば図９（イ）で示すように、棒状体１の周囲４箇所に９０°の位相差で導光路Ｆ１〜Ｆ４を等配形成したものを用い、これを地盤Ａに垂直に貫設した状態において、固定地盤Ａ１に対して上部側土塊Ａ２が図示一点鎖線Ｌを滑り面として図示矢印Ｐのように棒状体１の導光路Ｆ１とＦ３を結ぶ径方向に移動した場合、棒状体１の変形に伴い、導光路Ｆ１の上側屈曲部ａ_１と導光路Ｆ３の下側屈曲部ｂ_２には伸張力が作用して引張歪みを生じると共に、導光路Ｆ１の下側屈曲部ａ_２と導光路Ｆ３の上側屈曲部ｂ_１には収縮力が作用して圧縮歪みを生じる一方、導光路Ｆ２及びＦ４は移動方向Ｐに対して直交する中立軸上に位置するために曲がっても伸縮作用を受けないことから、測定される導光路Ｆ１〜Ｆ４の歪み値は図９（ロ）に示すようなものとなる。なお、図９（ロ）において、縦軸は導光路の長さ方向であり、伸張歪みは（＋）の値、圧縮歪みは（−）の値として表れる。
【０００６】
この図９（ロ）より、導光路Ｆ１，Ｆ３に明瞭な歪み反転が認められることから、滑り面Ｌの位置（深さ）が歪み反転位置として特定されると共に、導光路Ｆ１，Ｆ３のみに有意的な歪みが表れ、導光路Ｆ１が（＋）歪みから（−）歪みへの反転、導光路Ｆ３が逆の歪み反転になっているので、地滑りの移動方向は導光路Ｆ１から導光路Ｆ３へ向かう方向であることが判明し、また歪み値は棒状体の変形度合に比例するために土塊の移動量も判定できる。しかして、移動方向が導光路Ｆ１，Ｆ３を結ぶ方向ならびに導光路Ｆ２，Ｆ４を結ぶ方向に一致しない場合は、導光路Ｆ１〜Ｆ４の全てに歪み反転が表れるが、変位方向との角度差が小さいほど歪み値は大きくなるため、導光路Ｆ１，Ｆ３の歪み値と導光路Ｆ２，Ｆ４の歪み値との比較と、歪み値の（＋）（−）反転の正逆から、大凡の移動方向を推定することが可能である。
【０００７】
【発明が解決しようとする課題】
しかしながら、現状では、前記先行技術に係る光ファイバ歪みセンサを用いても、地滑りによる土塊の移動方向が実際に導光路Ｆ１，Ｆ３を結ぶ方向又は導光路Ｆ２，Ｆ４を結ぶ方向と一致することは極めて稀であり、殆どの場合は導光路Ｆ１〜Ｆ４の歪み値の比較によって大凡の移動方向しか推定できないため、測定地域の地滑りメカニズムを詳細に解析したり、地盤状況に応じた的確な地滑り防止対策を具体的に策定する上で充分とは言えず、移動方向の判定精度を更に高めることが重要な課題になっている。
【０００８】
本発明者らは、上述の情況に鑑み、前記地滑りの移動方向を始めとする測定対象物の変位方向を精度よく検知する手段について、様々な観点から鋭意検討を重ねた結果、前記先行技術に係る光ファイバ歪みセンサを利用して、且つ格別な検出機構の付設やセンサの基本構成の改変を行うことなく、従来と同様にして後方散乱光の計測によって測定される光ファイバの歪み値から、変位方向を高精度で特定し得る測定方法を究明し、本発明をなすに至った。
【０００９】
【課題を解決するための手段】
すなわち、請求項１の発明に係る光ファイバ歪みセンサによる変位測定方法は、測定対象物の変位に追従して変形可能な棒状体の周囲に、軸線方向に沿う光ファイバからなる第１〜第４の４本の導光路が棒状体中心から等距離で且つ角度９０°の位相差で等配付設されてなる光ファイバ歪みセンサを用い、その光ファイバに計測光パルスを入射した際に検出される後方散乱光に基づいて各導光路毎の長手方向の歪み位置と歪み値を測定し、第１導光路の歪み値をε_１、第１導光路に対して棒状体の径方向反対側にある第３導光路の歪み値をε_３、第２導光路の歪み値をε_２、第２導光路に対して棒状体の径方向反対側にある第４導光路の歪み値をε_４としたとき、当該歪み位置の棒状体横断面における第１及び第３導光路を結ぶ方向と測定対象物の変位方向とのなす角度θを、次式（１）；
θ＝ｔａｎ^−１〔｜ε_２−ε_４｜／｜ε_１−ε_３｜〕　・・・（１）
にて算定することを特徴とするものである。
【００１０】
また、請求項２の発明に係る光ファイバ歪みセンサによる変位測定方法は、測定対象物の変位に追従して変形可能な棒状体の周囲に、軸線方向に沿う光ファイバからなる第１〜第６の６本の導光路が棒状体中心から等距離で且つ角度６０°の位相差で等配付設されてなる光ファイバ歪みセンサを用い、その光ファイバに計測光パルスを入射した際に検出される後方散乱光に基づいて各導光路毎の長手方向の歪み位置と歪み値を測定し、棒状体の径方向に対向配置した３対の導光路対の内、両導光路の歪み値の差の絶対値が最大となる導光路対の一方を第１導光路、他方を第４導光路とし、同絶対値が最小となる導光路対の一方を第３導光路、他方を第６導光路とし、残る導光路対の一方を第２導光路、他方を第５導光路とし、第１導光路の歪み値をε_１、第２導光路の歪み値をε_２、第３導光路の歪み値をε_３、第４導光路の歪み値をε_４、第５導光路の歪み値をε_５、第６導光路の歪み値をε_６としたとき、当該歪み位置の棒状体横断面における第１及び第２導光路を結ぶ方向と測定対象物の変位方向とのなす角度θを、次式（２），（３）；
ｃｏｓ　（６０　°＋θ）／ｃｏｓ　θ＝｜ε_２−ε_５｜／｜ε_１−ε_４｜・・・（２）
ｃｏｓ　（６０　°−θ）／ｃｏｓ　θ＝｜ε_３−ε_６｜／｜ε_１−ε_４｜・・・（３）
のいずれかによって算定することを特徴としている。
【００１１】
【発明の実施の形態】
本発明に係る光ファイバ歪みセンサによる変位測定方法の原理について、以下に説明する。まず、図１に示すように、外力によって変形可能な棒状体としてのパイプ１に中心Ｏを通る方向Ｐの曲げ力が作用した場合、この方向Ｐに一致する半径方向のパイプ端縁の点Ｆに生ずる歪みε_Ｆと応力σ_Ｆの関係は、パイプ１の弾性係数をＥとしたとき、フックの法則による線形弾性体の応力と歪みの関係式より、次式（４）で表される。
Ｅ＝σ_Ｆ／ε_Ｆ　　　　　　　　　　　　　　・・・（４）
また、パイプ１に発生する縁応力度σ_Ｆは、曲げモーメントをＭ、断面二次モーメントをＩ、曲げ方向Ｐと直交して中心Ｏを通る中立軸Ｎからの距離をＤとしたとき、次式（５）で表される。
σ_Ｆ＝Ｍ・Ｄ／Ｉ　　　　　　　　　　　　　・・・（５）
そして、これら（４）（５）式より、歪みε_Ｆ値は次式（６）；
ε_Ｆ＝Ｍ・Ｄ／ＥＩ　　　　　　　　　　　　・・・（６）
で表されるから、パイプ１の周面上に生じる歪み量は中立軸Ｎからの距離Ｄに比例することになる。
【００１２】
従って、光ファイバ歪みセンサとして、パイプ１の外周面にその軸方向に沿う光ファイバからなる４本以上で且つ偶数本の導光路を等配形成したものを使用する場合に、歪み位置の断面において、パイプ径方向に対をなす各導光路対で測定される歪み量（両導光路の歪み値の差の絶対値）の比と、各導光路対における一方の導光路の中立軸Ｎからの距離Ｄの比とに強い相関関係があれば、測定される歪み量の前記比から、基準とする導光路対の径方向と曲げ方向つまり変位方向Ｐとのなす角度を算出でき、もって変位方向Ｐを特定できることになる。
【００１３】
この仮定について、図２に示すように、パイプ１の外周面に角度９０°の位相差で第１〜第４の４本の導光路Ｆ１〜Ｆ４を等配形成した光ファイバ歪みセンサＳ１を例として説明する。なお、パイプ１の外周面の導光路形成位置には、光ファイバの取り付けと位置決めを容易にするために、予め断面半円形の軸方向に沿う溝１０…を形成している。この場合、導光路Ｆ１とＦ３、導光路Ｆ２とＦ４、がそれぞれパイプ径方向で対をなしており、導光路Ｆ１・Ｆ３の径方向と変位方向Ｐとのなす角度をθ、パイプ１の半径をｒ、パイプ中心Ｏを通って変位方向Ｐと直交する中立軸Ｎに対する導光路Ｆ１の距離をｄ_１、同じく導光路Ｆ２の距離をｄ_２とすれば；
ｄ_１＝ｒｓｉｎ（９０°−θ）＝ｒｃｏｓθ　　　　・・・（７）
ｄ_２＝ｒｓｉｎθ　　　　　　　　　　　　　　　　・・・（８）
であるから、導光路Ｆ１，Ｆ２の中立軸Ｎからの距離の比は；
ｄ_２／ｄ_１＝ｒｓｉｎθ／ｒｓｉｎθ＝ｔａｎθ　　・・・（９）
となる。
【００１４】
ここで、第１導光路Ｆ１の歪み値をε_１、第２導光路の歪み値をε_２、第３導光路の歪み値をε_３、第４導光路の歪み値をε_４として、導光路Ｆ１・Ｆ３及び導光路Ｆ２・Ｆ４の歪み量の比とｄ_２／ｄ_１が一致するとすれば；
｜ε_２−ε_４｜／｜ε_１−ε_３｜＝ｄ_２／ｄ_１＝ｔａｎθ・・・（１０）
となり、もって角度θは次式（１）によって求められる。
θ＝ｔａｎ^−１〔｜ε_２−ε_４｜／｜ε_１−ε_３｜〕　　・・・（１）
【００１５】
また、図３に示すように、パイプ１の外周面に角度６０°の位相差で第１〜第６の６本の導光路Ｆ１〜Ｆ６を等配形成した光ファイバ歪みセンサＳ２の場合、導光路Ｆ１とＦ４、導光路Ｆ２とＦ５、導光路Ｆ３とＦ６、がそれぞれパイプ径方向で対をなしており、変位方向Ｐに近く変位量が最大となる導光路Ｆ１・Ｆ４を通る径方向と当該変位方向Ｐとのなす角度をθ、パイプ１の半径をｒ、中立軸Ｎに対する導光路Ｆ１の距離をｄ_３、同導光路Ｆ２の距離をｄ_４、同導光路Ｆ３の距離をｄ_５とすれば；
ｄ_３＝ｒｓｉｎ（９０°−θ）＝ｒｃｏｓθ　　　　・・・（１１）
ｄ_４＝ｒｃｏｓ（６０°＋θ）　　　　　　　　　　・・・（１２）
ｄ_５＝ｒｃｏｓ（６０°−θ）　　　　　　　　　　・・・（１３）
であるから、導光路Ｆ１と導光路Ｆ２，Ｆ３との中立軸Ｎからの距離の比は；
ｄ_４／ｄ_３＝ｃｏｓ（６０°＋θ）／ｃｏｓθ　　　・・・（１４）
ｄ_５／ｄ_３＝ｃｏｓ（６０°−θ）／ｃｏｓθ　　　・・・（１５）
となる。
【００１６】
そして、第１導光路Ｆ１の歪み値をε_１、第２導光路Ｆ２の歪み値をε_２、第３導光路Ｆ３の歪み値をε_３、第４導光路Ｆ４の歪み値をε_４、第５導光路Ｆ５の歪み値をε_５、第６導光路Ｆ６の歪み値をε_６として、導光路Ｆ１・Ｆ４の歪み量に対する導光路Ｆ２・Ｆ５又は導光路Ｆ３・Ｆ６の歪み量の比と、ｄ_４／ｄ_３又はｄ_５／ｄ_３が一致するとすれば、次式（２）（３）；
ｃｏｓ　（６０　°＋θ）／ｃｏｓ　θ＝｜ε_２−ε_５｜／｜ε_１−ε_４｜・・・（２）
ｃｏｓ　（６０　°−θ）／ｃｏｓ　θ＝｜ε_３−ε_６｜／｜ε_１−ε_４｜・・・（３）
のいずれかによって角度θが求められる。
【００１７】
そこで、上記仮定を検証するために、前記の４本の導光路を設けた光ファイバ歪みセンサＳ１と、６本の導光路を設けた光ファイバ歪みセンサＳ２について、せん断試験によって各導光路の歪みを実際に測定したところ、両センサＳ１，Ｓ２共に前記歪み量の比と中立軸Ｎからの距離の比とが非常によく対応しており、前記歪み量の比から角度θを算定することによって変位方向Ｐを高精度で特定できることが判明した。以下、このせん断試験について、具体的に説明する。
【００１８】
〔光ファイバ歪みセンサＳ１，Ｓ２の構成〕
直径４０ｍｍ，長さ４ｍのＶＰ管からなるパイプ１の外周面に、ガラスＦＲＰ被覆タイプの光ファイバをエポキシ系接着剤にて接着することにより、センサＳ１では図４（イ）の如く９０°の位相差で４本の導光路Ｆ１〜Ｆ４を、センサＳ２では図５（イ）の如く６０°の位相差で６本の導光路Ｆ１〜Ｆ６をそれぞれ形成し、その上にアルミテープを貼着して保護した。。図４（ロ）及び図５（ロ）はパイプ１を外周面の展開図として表しており、両センサＳ１，Ｓ２共に各導光路は１本の光ファイバ２を折り返す形で配設して細長い矩形で示す区間を接着して構成され、その一端側がＢＯＴＤＲ歪み測定装置３（安藤電気社製ＡＱ８６０３：歪測定精度１００μｓｔ、光源パルス幅１０ｎｓにおける距離分解能１ｍ）に接続されている。しかして、このＢＯＴＤＲ歪み測定装置３は、パルス光源、光検出部、信号処理部、駆動制御部、表示部、モニタ等を含んでおり、光ファイバ２にパルス光を入射させ、戻って来る後方散乱光の周波数分布を解析して光ファイバの歪み位置と歪み値を表示できるようになっている。
【００１９】
〔せん断試験装置〕
図６に示すように、外周面に導光路を設けたパイプ１の長手方向両側部を半割型の保持枠４，５で抱持し、その一端側を固定部としてＨ形鋼からなる基台６に固定すると共に、他端側を可動部としてジャッキ７に取り付けて上方へ変位可能としており、その変位によって両保持枠４，５の内端位置がパイプ１の変形部１ａ，１ｂとなる。なお、パイプ１の固定部及び可動部の長さは１．５０ｍ、変形部１ａ，１ｂの間隔は１ｍに設定されている。
【００２０】
〔せん断試験〕
センサＳ１については、図７（イ）〜（ハ）に示すように、導光路Ｆ１−Ｆ３の径方向に対する変位方向Ｐの角度θを０°、２２．５°、４５°の各々に設定し、各設定角度において後記表１〜３に示す各変位量で光源パルス幅１０ｎｓによる歪み測定を行った。また、センサＳ２については、図８（イ）〜（ハ）に示すように、導光路Ｆ１・Ｆ４の径方向に対する変位方向Ｐの角度θを０°、１５°、３０°の各々に設定し、各設定角度において後記表４〜６に示す各変位量で光源パルス幅１０ｎｓによる歪み測定を行った。この測定結果から得られる歪み量と該歪み量の比を、中立軸Ｎからの距離比と共に後記表１〜６に示す。なお、各表における歪み量は、変形部１ａ，１ｂの平均で示した。
【００２１】
【表１】

【００２２】
【表２】

【００２３】
【表３】

【００２４】
【表４】

【００２５】
【表５】

【００２６】
【表６】

【００２７】
表１〜表６に示すように、両センサＳ１，Ｓ２共に、実測値に基づく前記歪み量の比は、理論値に相当する中立軸Ｎからの距離比と非常によく対応しており、、殆ど一致している。従って、導光路４本の光ファイバ歪みセンサＳ１では前記（１）式から、導光路６本の光ファイバ歪みセンサＳ２では前記（２）式又は（３）式から、測定した歪み値に基づいて前記角度θを算定することにより、変位方向Ｐを概ね±１０％という僅かな誤差内で特定できる。
【００２８】
なお、導光路６本の光ファイバ歪みセンサＳ２では、前記（２）式又は（３）式を適用する上で、パイプ１の径方向に対向配置した３対の導光路対の内、歪み量つまり両導光路の歪み値の差の絶対値が最大となる導光路対の一方を第１導光路Ｆ１、他方を第４導光路Ｆ４、同絶対値が最小となる導光路対の一方を第３導光路Ｆ３、他方を第６導光路Ｆ６、残る導光路対の一方を第２導光路Ｆ２、他方を第５導光路Ｆ５とすればよい。しかして、２つの導光路対の歪み量が同じであれば、該歪み量よりも残る導光路対の歪み量が大きい場合は図８（イ）の角度θ＝０°に相当し、逆に小さい場合は図８（ハ）の角度θ＝３０°に相当する。また、前記（２）式及び（３）式では角度θを単純には算出できないが、予めθに０°〜３０°までの角度を当てはめて前記（２），（３）式の左辺の数値を算出し、θの角度毎（例えば１°毎）の該数値を示す対照表を作成しておけば、実測した歪み量を該対照表に照らし合わすだけで簡単に角度θを特定できる。なお、導光路６本の場合のθは最大３０°である。
【００２９】
一方、このような歪み量の比と中立軸Ｎからの距離比との相関関係は、導光路を８本、１０本、１２本…と多くした場合にも当然に成立する。しかるに、変位方向Ｐは既述の導光路４本又は６本の構成で充分に精度よく特定でき、それ以上に導光路の数を増やしても別段の効果がない上、導光路の形成には非常に手間を要するため、導光路を８本やそれ以上にすることはコスト高になるだけで無意味である。
【００３０】
なお、例示した光ファイバ歪みセンサＳ１，Ｓ２では棒状体としてパイプ１を用いているが、測定対象物の変形に追従して変形可能なものであれば、中実の棒状体も使用可能である。また、棒状体としてパイプのような中空材を用いる場合は、導光路を棒状体の内周面に形成しても差し支えない。更に棒状体は断面円形のものに限らず、四角形や六角形等の他の断面形状を有するものであっても、４本又は６本の導光路を棒状体中心から等距離で等配配置できればよい。しかして、例示したパイプ１の外周面には導光路形成位置に溝１０を設けているが、棒状体の平坦周面に直接に又は介在物を介して光ファイバを貼着して導光路を形成してもよい。その他、導光路の保護や他の目的で、棒状体にフレキシブルなカバーを被せたり、その外周面に被覆層を形成してもよい。
【００３１】
本発明に係る変位測定方法は、地滑りや斜面崩壊の検知用として地盤に貫設した光ファイバ歪みセンサを利用して土塊の変位方向を特定するのに特に有用であるが、それ以外の光ファイバ歪みセンサを用いた様々な変位測定にも適用できる。例えば、橋脚や建造物等の基盤を支えるために地中に打ち込むコンクリート杭の中心孔に該光ファイバ歪みセンサを配設しておけば、その歪み測定からコンクリート杭に歪み（変位）を生じさせる地中圧力の水平方向の向きを特定できる。また、トンネル壁に沿って該光ファイバ歪みセンサを配設すれば、該トンネル壁に加わる異常地圧の位置と方向を知ることが可能となる。更に、様々な地中埋設管については、その埋設管自体を光ファイバ歪みセンサの前記棒状体として外周面に光ファイバによる導光路を形成し、歪み測定によって埋設管に加わる断面方向の圧力の向きを特定することが可能である。更に、水中等の流体中における測定対象物についても、該光ファイバ歪みセンサを配設により、流体圧による変位位置と変位方向を特定することができる。
【００３２】
【発明の効果】
本発明の変位測定方法によれば、例えば土塊の地滑り等による移動のような測定対象物の変位を光ファイバ歪みセンサによって測定する場合に、該光ファイバ歪みセンサに対して格別な検出機構の付設やセンサの基本構成の改変を行うことなく、本来の後方散乱光の計測によって測定される光ファイバの歪み値に基づいて、変位位置と共に変位方向を高精度で特定できるから、変位方向の予測がつかない部位での変位測定に極めて有効であり、また例えば測定地域の地滑りメカニズムを詳細に解析したり、地盤状況に応じた有効な地滑り防止対策を具体的に策定する上で非常に有用な情報を得ることができる。
【図面の簡単な説明】
【図１】曲げ歪みの説明に採用したパイプの断面図である。
【図２】本発明に係る変位測定方法の説明に採用した４本の導光路を備える光ファイバ歪みセンサの横断面図である。
【図３】本発明に係る変位測定方法の説明に採用した６本の導光路を備える光ファイバ歪みセンサの横断面図である。
【図４】せん断試験に用いた４本の導光路を備える光ファイバ歪みセンサを示し、（イ）は横断面図、（ロ）はパイプ外周面を展開して示す光ファイバの配線図である。
【図５】せん断試験に用いた６本の導光路を備える光ファイバ歪みセンサを示し、（イ）は横断面図、（ロ）はパイプ外周面を展開して示す光ファイバの配線図である。
【図６】光ファイバ歪みセンサのせん断試験装置を示す側面図である。
【図７】同せん断試験における４本の導光路を備える光ファイバ歪みセンサの配置を示し、（イ）は設定角度θ＝０°、（ロ）は設定角度θ＝２２．５°、（ハ）は設定角度θ＝４５°におけるそれぞれ横断面図である。
【図８】同せん断試験における６本の導光路を備える光ファイバ歪みセンサの配置を示し、（イ）は設定角度θ＝０°、（ロ）は設定角度θ＝１５°、（ハ）は設定角度θ＝３０°におけるそれぞれ横断面図である。
【図９】光ファイバ歪みセンサによる地滑りの測定を示し、（イ）は地滑りに伴う該センサの変形状態を示す概略縦断側面図、（ロ）は該センサの各導光路の長手方向に沿った歪みを示す特性図である。
【符号の説明】
１　　　　　パイプ（棒状体）
２　　　　　光ファイバ
３　　　　　ＢＯＴＤＲ歪み測定装置
Ｆ１　　　　第１の導光路
Ｆ２　　　　第２の導光路
Ｆ３　　　　第３の導光路
Ｆ４　　　　第４の導光路
Ｆ５　　　　第５の導光路
Ｆ６　　　　第６の導光路
Ｓ１，Ｓ２　光ファイバ歪みセンサ
Ｏ　　　　　パイプ中心
Ｐ　　　　　変位方向
Ｎ　　　　　中立軸
θ　　　　　角度
ｄ_１〜ｄ_５　中立軸からの距離[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a measurement method for identifying a displacement of a measurement object such as a movement of a soil mass due to a landslide or the like as a distortion by an optical fiber distortion sensor and specifying a displacement direction from the distortion position and the distortion value.
[0002]
[Prior art]
In general, when an optical fiber strain sensor scatters pulse light incident on an optical fiber from one end and returns to the incident end side as back scattered light (Brillouin scattered light), the optical fiber strain sensor generates a signal from the optical fiber strain (expansion / contraction) generation section. Utilizing the fact that the frequency distribution (power spectrum) of the backscattered light shifts in proportion to the strain, the displacement of the object to be measured is taken as the strain of the optical fiber, and the time until the backscattered light whose frequency distribution has shifted returns. A BOTDR (Brillouin Optical Time Domain Reflectometer) is used to specify a distortion position and measure a distortion value by analyzing its frequency distribution. According to such an optical fiber strain sensor, since the optical fiber itself serves as a sensor, a wide range of measurement can be performed on a line or a surface, wiring for power supply to the sensor is unnecessary, and lightning or high-voltage lines can be used. There is an advantage that advanced control and management can be performed from a remote place without being affected by electromagnetic noise.
[0003]
Conventionally, in order to investigate the movement of an earth mass due to a landslide using an optical fiber strain sensor, a deformable rod-shaped body such as a pipe having one optical fiber disposed along a longitudinal direction is perpendicular to a ground where a landslide is assumed. And the strain of the optical fiber accompanying the deformation of the rod is measured by measuring the backscattered light. That is, if there is a movement of the earth mass across the axial direction of the rod body, the rod body is deformed at the position of the slip surface of the earth mass, and the optical fiber is distorted due to this deformation. In addition to knowing the (depth), the degree and progress of the landslide can be empirically determined from the magnitude of the strain value and the change with time. However, in such a measurement, even though the position of the slip surface can be specified, the displacement direction of the landslide cannot be determined from the measurement data, so that it is effective unless the displacement direction is known in advance from past trends or terrain. It was extremely inadequate in taking landslide countermeasures.
[0004]
Therefore, the present applicants have previously described, as an optical fiber strain sensor, light guide paths made of optical fibers at a plurality of locations around a flexible rod that follows the deformation of an object to be measured in parallel with the axis of the rod. The formed one is proposed and disclosed in Japanese Patent Application No. 2000-297525 (JP-A-2002-107122). Such an optical fiber strain sensor using a synthetic resin pipe for the rod-like body has already been actually used as an optical pipe strain gauge for detecting landslides and slope failures in various places.
[0005]
In this optical fiber strain sensor according to the prior art, for example, as shown in FIG. 9A, a light guide path F1 to F4 is equally formed at four positions around the rod 1 with a phase difference of 90 °. Is vertically penetrated through the ground A, and the upper earth mass A2 is a radial surface connecting the light guide paths F1 and F3 of the rod-shaped body 1 as shown by the arrow P in the drawing with the dashed line L shown in the figure against the fixed ground A1. together when moving, with the deformation of the rod-shaped body 1, resulting in tensile strain stretch force acts on the lower bent portion b ₂ of the upper bent portions a ₁ and the light conducting path F3 conducting path F1 to, light guide path F1 while causing compressive strain acts shrinkage force to the upper bent portion b ₁ in the lower bent portion a ₂ and the light conducting path F3 of the light guiding path F2 and F4 are located on the neutral axis orthogonal to the moving direction P Is not affected by the stretching effect Distortion value of the light guide path F1~F4 is as shown in FIG. 9 (b). In FIG. 9B, the vertical axis is the length direction of the light guide path, and the extension distortion is represented by a value of (+) and the compression distortion is represented by a value of (-).
[0006]
From FIG. 9 (b), since clear distortion inversion is recognized in the light guide paths F1 and F3, the position (depth) of the slide surface L is specified as the distortion inversion position, and only the light guide paths F1 and F3 are provided. Since significant distortion appears, the light guide path F1 is reversed from (+) distortion to (−) distortion, and the light guide path F3 is reversed distortion reverse, so that the landslide moves from the light guide path F1 to the light guide path F3. It turns out that it is the direction toward, and since the distortion value is proportional to the degree of deformation of the rod-shaped body, the moving amount of the earth mass can also be determined. However, when the moving direction does not match the direction connecting the light guides F1 and F3 and the direction connecting the light guides F2 and F4, distortion reversal appears in all of the light guides F1 to F4, but the angle difference from the displacement direction becomes large. Since the smaller the value is, the larger the distortion value is, the approximate moving direction is determined from the comparison between the distortion value of the light guides F1 and F3 and the distortion value of the light guides F2 and F4 and the normal / reverse of (+) (−) inversion of the distortion value. Can be estimated.
[0007]
[Problems to be solved by the invention]
However, at present, even if the optical fiber strain sensor according to the prior art is used, the moving direction of the earth mass due to landslide does not match the direction actually connecting the light guide paths F1 and F3 or the direction connecting the light guide paths F2 and F4. It is extremely rare, and in most cases, only the approximate moving direction can be estimated by comparing the distortion values of the light guide paths F1 to F4. Therefore, the landslide mechanism in the measurement area can be analyzed in detail, and accurate landslide prevention according to the ground conditions can be performed. It is not enough to formulate a countermeasure specifically, and it is important to further improve the accuracy of the determination of the moving direction.
[0008]
In view of the above-described circumstances, the present inventors have conducted intensive studies from various viewpoints on means for accurately detecting the displacement direction of the measurement target including the landslide movement direction, and as a result, the prior art Using such an optical fiber strain sensor, and without attaching a special detection mechanism or modifying the basic configuration of the sensor, from the strain value of the optical fiber measured by measuring the backscattered light in the same manner as before, The present inventors have determined a measuring method capable of specifying the displacement direction with high accuracy, and have accomplished the present invention.
[0009]
[Means for Solving the Problems]
That is, in the displacement measuring method using the optical fiber strain sensor according to the first aspect of the present invention, the first to fourth optical fibers are formed along the axial direction around the rod-like body which can be deformed following the displacement of the measurement object. Are detected when an optical pulse is incident on the optical fiber using an optical fiber strain sensor in which the four light guide paths are arranged equidistant from the center of the rod and at a phase difference of 90 °. The distortion position and the distortion value in the longitudinal direction of each light guide path are measured based on the backscattered light, and the distortion value of the first light guide path is ε ₁ , which is on the opposite side of the rod-shaped body from the first light guide path in the radial direction. The strain value of the third light guide is ε ₃ , the strain value of the second light guide is ε ₂ , and the strain value of the fourth light guide on the opposite side of the rod from the second light guide in the radial direction is ε ₄ . When measuring the direction connecting the first and third light guides in the bar-shaped body cross section at the distortion position The angle θ between the displacement direction of the elephant was following formula (1);
θ = tan ⁻¹ [| ε ₂ −ε ₄ | / | ε ₁ −ε ₃ |] (1)
It is characterized by being calculated by.
[0010]
Further, the displacement measuring method using the optical fiber strain sensor according to the second aspect of the present invention is characterized in that the first to sixth optical fibers are formed along the axial direction around a rod-like body deformable following the displacement of the measurement object. Are detected when the measurement light pulse is incident on the optical fiber using an optical fiber strain sensor in which the six light guide paths are equidistantly arranged from the center of the rod-shaped body at a phase difference of an angle of 60 °. The strain position and strain value in the longitudinal direction of each light guide are measured based on the backscattered light, and the difference between the strain values of the two light guides among the three light guide pairs arranged in the radial direction of the rod is opposed. One of the light guide pairs having the largest absolute value is a first light guide, the other is a fourth light guide, one of the light guide pairs having the smallest absolute value is a third light guide, and the other is a sixth light guide. One of the remaining light guide pairs is a second light guide, the other is a fifth light guide, and the distortion of the first light guide is ₁ a value _epsilon, the distortion value of the second light guide path epsilon _2, the distortion value of the third light guide path epsilon _3, the distortion value of the fourth light guide path epsilon _4, the distortion value of the fifth light guide path epsilon _5, the when the distortion value of 6 light guide path and the epsilon _6, the angle θ between the displacement direction of the first and direction measurement object connecting a second light guide path in the rod-like body cross section of the strain position, the following equation (2 ), (3);
cos (60 ° + θ) / cos θ = | ε ₂ −ε ₅ | / | ε ₁ −ε ₄ | (2)
cos (60 ° −θ) / cos θ = | ε ₃ −ε ₆ | / | ε ₁ −ε ₄ | (3)
It is characterized by calculating by either of
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
The principle of the displacement measuring method using the optical fiber strain sensor according to the present invention will be described below. First, as shown in FIG. 1, when a bending force in a direction P passing through the center O is applied to a pipe 1 as a rod-like body deformable by an external force, a point F of a pipe edge in a radial direction corresponding to the direction P is applied. The relationship between the strain ε _F and the stress σ _F caused by the following equation (4) is given by the relationship between the stress and strain of the linear elastic body according to Hooke's law, where E is the elastic modulus of the pipe 1.
E = σ _F / ε _F (4)
The edge stress degree σ _F generated in the pipe 1 is represented by the following equation, where M is the bending moment, I is the secondary moment of area, and D is the distance from the neutral axis N passing through the center O perpendicular to the bending direction P. It is represented by equation (5).
σ _F = M · D / I (5)
From these equations (4) and (5), the strain ε _F value is calculated by the following equation (6);
ε _F = M · D / EI (6)
Thus, the amount of strain generated on the peripheral surface of the pipe 1 is proportional to the distance D from the neutral axis N.
[0012]
Therefore, when an optical fiber strain sensor having four or more and even-numbered light guide paths formed of optical fibers along the axial direction on the outer peripheral surface of the pipe 1 is formed, the cross section at the strain position is used. , The ratio of the distortion amount (absolute value of the difference between the distortion values of the two light guides) measured in each light guide pair forming a pair in the pipe radial direction, and the ratio of one of the light guides in each light guide pair to the neutral axis N. If there is a strong correlation with the ratio of the distance D, the angle between the radial direction and the bending direction, that is, the displacement direction P of the reference light guide path pair can be calculated from the ratio of the measured strain amount, and the displacement direction can be calculated. P can be specified.
[0013]
Assuming this assumption, as shown in FIG. 2, an optical fiber strain sensor S1 in which first to fourth four light guide paths F1 to F4 are equally arranged on the outer peripheral surface of the pipe 1 with a phase difference of 90 °. It will be described as. At the light guide path forming position on the outer peripheral surface of the pipe 1, grooves 10 having a semicircular cross section along the axial direction are formed in advance in order to facilitate attachment and positioning of the optical fiber. In this case, the light guides F1 and F3, and the light guides F2 and F4 form a pair in the pipe radial direction, respectively. The angle between the radial direction of the light guides F1 and F3 and the displacement direction P is θ, and the radius of the pipe 1 is θ. if the r, d ₁ distance light guide path F1 with respect to a neutral axis N perpendicular to the displacement direction P through the pipe center _O, also the distance light guide path F2 and d _2;
d ₁ = rsin (90 ° −θ) = rcos θ (7)
d ₂ = rsin θ (8)
Therefore, the ratio of the distance from the neutral axis N to the light guide paths F1 and F2 is;
_{_{d 2 / d 1 = rsinθ /}} rsinθ = tanθ ··· (9)
It becomes.
[0014]
Here, the distortion value of the first light guide F1 is ε ₁ , the distortion value of the second light guide is ε ₂ , the distortion value of the third light guide is ε ₃ , and the distortion value of the fourth light guide is ε ₄ . if the distortion quantity of the ratio of the optical path F1 · F3 and the light guiding path F2 · F4 and _d 2 / _{d 1} matches;
| Ε ₂ −ε ₄ | / | ε ₁ −ε ₃ | = d ₂ / d ₁ = tan θ (10)
Thus, the angle θ is obtained by the following equation (1).
θ = tan ⁻¹ [| ε ₂ −ε ₄ | / | ε ₁ −ε ₃ |] (1)
[0015]
As shown in FIG. 3, in the case of the optical fiber strain sensor S2 in which the first to sixth six light guide paths F1 to F6 are equally arranged on the outer peripheral surface of the pipe 1 with a phase difference of 60 °, The light paths F1 and F4, the light guide paths F2 and F5, and the light guide paths F3 and F6 are each paired in the pipe radial direction, and are close to the displacement direction P and have a maximum displacement amount. The angle formed by the displacement direction P is θ, the radius of the pipe 1 is r, the distance of the light guide path F1 to the neutral axis N is d ₃ , the distance of the light guide path F2 is d ₄ , and the distance of the light guide path F3 is d _5. given that;
d ₃ = rsin (90 ° −θ) = rcos θ (11)
d ₄ = rcos (60 ° + θ) (12)
d ₅ = rcos (60 ° −θ) (13)
Therefore, the ratio of the distance between the light guide path F1 and the light guide paths F2 and F3 from the neutral axis N is;
d ₄ / d ₃ = cos (60 ° + θ) / cos θ (14)
d ₅ / d ₃ = cos (60 ° −θ) / cos θ (15)
It becomes.
[0016]
The distortion value of the first light guide F1 is ε ₁ , the distortion value of the second light guide F ₂ is ε ₂ , the distortion value of the third light guide F ₃ is ε ₃ , the distortion value of the fourth light guide F ₄ is ε ₄ , Assuming that the strain value of the fifth light guide F5 is ε ₅ and the strain value of the sixth light guide F6 is ε ₆ , the ratio of the strain of the light guides F2 and F5 or F3 and F6 to the strain of the light guides F1 and F4. And d ₄ / d ₃ or d ₅ / d ₃ are equal, the following equations (2) and (3):
cos (60 ° + θ) / cos θ = | ε ₂ −ε ₅ | / | ε ₁ −ε ₄ | (2)
cos (60 ° −θ) / cos θ = | ε ₃ −ε ₆ | / | ε ₁ −ε ₄ | (3)
The angle θ is obtained by either of the above.
[0017]
Therefore, in order to verify the above assumptions, a shear test was performed on the optical fiber strain sensor S1 provided with the four light guides and the optical fiber strain sensor S2 provided with the six light guides, and the strain of each light guide was measured by a shear test. Was actually measured, the ratio of the amount of distortion and the ratio of the distance from the neutral axis N correspond very well for both sensors S1 and S2, and by calculating the angle θ from the ratio of the amount of distortion, It has been found that the displacement direction P can be specified with high accuracy. Hereinafter, the shear test will be specifically described.
[0018]
[Configuration of Optical Fiber Strain Sensors S1 and S2]
By bonding an optical fiber of a glass FRP coating type to an outer peripheral surface of a pipe 1 composed of a VP pipe having a diameter of 40 mm and a length of 4 m with an epoxy-based adhesive, the sensor S1 has a 90 ° angle as shown in FIG. Four light guides F1 to F4 are formed with a phase difference, and six light guides F1 to F6 are formed with a phase difference of 60 ° in the sensor S2 as shown in FIG. 5A, and an aluminum tape is adhered thereon. And protected. . FIGS. 4 (b) and 5 (b) show the pipe 1 as a developed view of the outer peripheral surface. In each of the sensors S1 and S2, each light guide path is disposed in a form in which one optical fiber 2 is folded back and is elongated. One end is connected to a BOTDR strain measuring device 3 (AQ8603 manufactured by Ando Electric Co., Ltd .: strain measurement accuracy 100 μst, distance resolution 1 m at a light source pulse width of 10 ns). The BOTDR distortion measurement device 3 includes a pulse light source, a light detection unit, a signal processing unit, a drive control unit, a display unit, a monitor, and the like. By analyzing the frequency distribution of the scattered light, the distortion position and the distortion value of the optical fiber can be displayed.
[0019]
[Shear test equipment]
As shown in FIG. 6, both ends in the longitudinal direction of a pipe 1 provided with a light guide path on the outer peripheral surface are held by half-shaped holding frames 4, 5, and one end of the base is made of an H-shaped steel as a fixed portion. In addition to being fixed to the base 6, the other end side is attached to the jack 7 as a movable part and can be displaced upward, so that the inner end positions of the holding frames 4 and 5 become the

deformed parts

1a and 1b of the pipe 1. . In addition, the length of the fixed part and the movable part of the pipe 1 is set to 1.50 m, and the interval between the

deformed parts

1a and 1b is set to 1 m.
[0020]
(Shear test)
As shown in FIGS. 7A to 7C, the angle θ of the displacement direction P with respect to the radial direction of the light guide paths F1 to F3 is set to 0 °, 22.5 °, and 45 ° for the sensor S1. At each set angle, distortion was measured with a light source pulse width of 10 ns at each displacement shown in Tables 1 to 3 below. As shown in FIGS. 8A to 8C, the angle θ of the displacement direction P with respect to the radial direction of the light guide paths F1 and F4 is set to 0 °, 15 °, and 30 ° for the sensor S2. At each set angle, distortion was measured with a light source pulse width of 10 ns at each displacement shown in Tables 4 to 6 below. Tables 1 to 6 below show the distortion amounts obtained from the measurement results and the ratios of the distortion amounts together with the distance ratio from the neutral axis N. In addition, the distortion amount in each table is shown by the average of the

deformed portions

1a and 1b.
[0021]
[Table 1]

[0022]
[Table 2]

[0023]
[Table 3]

[0024]
[Table 4]

[0025]
[Table 5]

[0026]
[Table 6]

[0027]
As shown in Tables 1 to 6, for both sensors S1 and S2, the ratio of the distortion amount based on the actually measured value corresponds very well to the distance ratio from the neutral axis N corresponding to the theoretical value, Almost agree. Therefore, based on the strain value measured from the equation (1) for the optical fiber strain sensor S1 having four light guide paths, and from the equation (2) or the equation (3) for the optical fiber strain sensor S2 having six light guide paths. By calculating the angle θ, the displacement direction P can be specified within a small error of approximately ± 10%.
[0028]
In the optical fiber strain sensor S2 having six light guide paths, when applying the above equation (2) or (3), the strain amount of the three light guide path pairs arranged in the radial direction of the pipe 1 is determined. That is, one of the light guide pairs having the largest absolute value of the difference between the distortion values of the two light guide paths is the first light guide F1, the other is the fourth light guide F4, and one of the light guide pairs having the minimum absolute value is the first light guide F1. The third light guide path F3, the other light guide path may be a sixth light guide path F6, one of the remaining light guide path pairs may be a second light guide path F2, and the other may be a fifth light guide path F5. However, if the distortion amount of the two light guide path pairs is the same, if the distortion amount of the remaining light guide path pair is larger than the distortion amount, it corresponds to the angle θ = 0 ° in FIG. If the angle is small, it corresponds to the angle θ = 30 ° in FIG. Further, although the angle θ cannot be simply calculated by the above equations (2) and (3), the angle on the left side of the above equations (2) and (3) is obtained by applying an angle of 0 ° to 30 ° to θ in advance. Is calculated, and a comparison table showing the numerical values for each angle of θ (for example, every 1 °) is created, so that the angle θ can be easily specified simply by comparing the measured distortion amount with the comparison table. In the case of six light guide paths, θ is 30 ° at the maximum.
[0029]
On the other hand, such a correlation between the ratio of the amount of distortion and the distance ratio from the neutral axis N naturally holds when the number of light guide paths is increased to 8, 10, 12,.... However, the displacement direction P can be specified with sufficient accuracy by the above-described configuration of four or six light guide paths, and even if the number of light guide paths is further increased, there is no particular effect. Since it takes a lot of trouble, it is meaningless to increase the number of light guide paths to eight or more, only to increase the cost.
[0030]
In the illustrated optical fiber strain sensors S1 and S2, the pipe 1 is used as a rod-shaped body, but a solid rod-shaped body can also be used as long as the pipe 1 can be deformed following the deformation of the measurement object. . When a hollow material such as a pipe is used as the rod, the light guide path may be formed on the inner peripheral surface of the rod. Furthermore, the rod-shaped body is not limited to a circular cross-section, and even if it has another cross-sectional shape such as a square or a hexagon, if four or six light guide paths can be arranged equidistantly from the center of the rod-shaped body. Good. Although the groove 10 is provided at the light guide path forming position on the outer peripheral surface of the illustrated pipe 1, the optical fiber is adhered to the flat peripheral surface of the rod-shaped body directly or through an intervening member to form the light guide path. It may be formed. In addition, a flexible cover may be put on the rod-shaped body, or a coating layer may be formed on the outer peripheral surface for protecting the light guide path or for other purposes.
[0031]
The displacement measuring method according to the present invention is particularly useful for specifying the displacement direction of the earth mass using an optical fiber strain sensor penetrating the ground for detecting landslides and slope failures, but other optical fibers It can be applied to various displacement measurements using a strain sensor. For example, if the optical fiber strain sensor is disposed in the center hole of a concrete pile driven into the ground to support a base such as a pier or a building, a strain (displacement) is generated in the concrete pile based on the measured strain. The horizontal direction of the underground pressure can be specified. Further, if the optical fiber strain sensor is disposed along the tunnel wall, it is possible to know the position and the direction of the abnormal ground pressure applied to the tunnel wall. Furthermore, for various buried pipes, the buried pipe itself is used as the rod-shaped body of the optical fiber strain sensor to form a light guide path with an optical fiber on the outer peripheral surface, and the direction of pressure in the cross-sectional direction applied to the buried pipe by strain measurement. Can be specified. Further, even with respect to an object to be measured in a fluid such as underwater, the displacement position and the displacement direction due to the fluid pressure can be specified by providing the optical fiber strain sensor.
[0032]
【The invention's effect】
According to the displacement measuring method of the present invention, when measuring the displacement of the measurement object such as the movement of the earth mass due to landslide by the optical fiber strain sensor, a special detection mechanism is added to the optical fiber strain sensor. And the displacement direction can be specified with high accuracy based on the strain value of the optical fiber measured by measuring the original backscattered light without modifying the basic configuration of the sensor or sensor. It is extremely useful for measuring displacement in unconnected areas, and is also very useful information for, for example, analyzing the landslide mechanism in the measurement area in detail and formulating effective landslide prevention measures according to the ground conditions. Can be obtained.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a pipe used for explaining bending strain.
FIG. 2 is a cross-sectional view of an optical fiber strain sensor having four light guide paths adopted for explaining a displacement measuring method according to the present invention.
FIG. 3 is a cross-sectional view of an optical fiber strain sensor having six light guide paths employed for explaining a displacement measuring method according to the present invention.
4A and 4B show an optical fiber strain sensor having four light guide paths used for a shear test, wherein FIG. 4A is a cross-sectional view, and FIG. 4B is a wiring diagram of an optical fiber in which a pipe outer peripheral surface is developed. .
FIGS. 5A and 5B show an optical fiber strain sensor provided with six light guide paths used in a shear test, wherein FIG. 5A is a cross-sectional view, and FIG. .
FIG. 6 is a side view showing a shear test device of the optical fiber strain sensor.
FIG. 7 shows an arrangement of an optical fiber strain sensor having four light guide paths in the shear test, wherein (a) is a set angle θ = 0 °, (b) is a set angle θ = 22.5 °, and (c) ) Are transverse cross-sectional views at a set angle θ = 45 °.
FIG. 8 shows the arrangement of an optical fiber strain sensor having six light guide paths in the same shear test, where (a) is the set angle θ = 0 °, (b) is the set angle θ = 15 °, and (c) is It is a cross-sectional view at a setting angle θ = 30 °.
FIG. 9 shows measurement of landslide by an optical fiber strain sensor, (a) is a schematic longitudinal side view showing a deformation state of the sensor due to landslide, and (b) is along a longitudinal direction of each light guide path of the sensor. It is a characteristic view showing distortion.
[Explanation of symbols]
1 pipe (rod)
2 optical fiber 3 BOTDR distortion measuring device F1 first light guide F2 second light guide F3 third light guide F4 fourth light guide F5 fifth light guide F6 sixth light guide S1, S2 optical fiber distortion Sensor O Pipe center P Displacement direction N Neutral axis θ Angles d _{1 to} d ₅ Distance from neutral axis

Claims

Around the rod that can be deformed following the displacement of the object to be measured, first to fourth four light guide paths formed of optical fibers along the axial direction are equidistant from the rod center and have an angle of 90 °. Using an optical fiber strain sensor equidistantly arranged with a phase difference, based on the backscattered light detected when a measurement light pulse is incident on the optical fiber, the longitudinal strain position and strain value for each light guide path. Measure
The strain value of the first light guide is ε ₁ , the strain value of the third light guide on the opposite side of the rod from the first light guide in the radial direction is ε ₃ , the strain value of the second light guide is ε ₂ , when the distortion value of the fourth light guide path in the radial direction opposite to the rod-like body was epsilon ₄ against 2 light guides,
The angle θ between the direction connecting the first and third light guides and the direction of displacement of the measurement object in the bar-shaped body cross section at the distortion position is represented by the following equation (1):
θ = tan ⁻¹ [| ε ₂ −ε ₄ | / | ε ₁ −ε ₃ |] (1)
A displacement measuring method using an optical fiber strain sensor.

Around a rod that can be deformed following the displacement of the measurement object, first to sixth six light guide paths formed of optical fibers along the axial direction are equidistant from the rod center and have an angle of 60 °. Using an optical fiber strain sensor equidistantly arranged with a phase difference, based on the backscattered light detected when a measurement light pulse is incident on the optical fiber, the longitudinal strain position and strain value for each light guide path. Measure
Of the three pairs of light guides disposed radially opposite to each other in the rod-like body, one of the light guide pairs that maximizes the absolute value of the difference between the distortion values of the two light guides is the first light guide, and the other is the fourth light guide. One of the light guide pairs with the minimum absolute value is the third light guide, the other is the sixth light guide, one of the remaining light guide pairs is the second light guide, the other is the fifth light guide, The distortion value of the light guide is ε ₁ , the distortion value of the second light guide is ε ₂ , the distortion value of the third light guide is ε ₃ , the distortion value of the fourth light guide is ε ₄ , and the distortion value of the fifth light guide is When ε ₅ and the distortion value of the sixth light guide path are ε ₆ ,
The angle θ between the direction connecting the first and fourth light guides and the direction of displacement of the object to be measured in the bar-shaped body cross section at the distortion position is expressed by the following formulas (2) and (3);
cos (60 ° + θ) / cos θ = | ε ₂ −ε ₅ | / | ε ₁ −ε ₄ | (2)
cos (60 ° −θ) / cos θ = | ε ₃ −ε ₆ | / | ε ₁ −ε ₄ | (3)
A displacement measuring method using an optical fiber strain sensor.