Patent Publication Number: US-2023152170-A1

Title: Measuring device and measuring method using tape core wire

Description:
TECHNICAL FIELD 
     The present disclosure relates to a measurement apparatus and a measurement method for measuring curvature and torsion. 
     BACKGROUND ART 
     Shape sensing using optical fibers has attracted attention. Curvature and torsion applied to an optical fiber are acquired for analysis of the shape (trajectory) of the optical fiber. 
     There has been proposed a technique using a multicore optical fiber with Fiber Bragg Gratings (FBG) in which the wavelength of reflected light is changed (Bragg wavelength is shifted) by fiber bending, measuring the longitudinal strain distribution of each core by Optical Frequency Domain Reflectmetry (OFDR) or the like, and then calculating curvature and torsion based on a cross-sectional strain distribution obtained from the strain of each core at an identical point (see NPLs 1 and 2, for example). 
     CITATION LIST 
     Non Patent Literature 
     
         
         NPL 1: “Shape sensing using multi-core fiber optic cable and parametric curve solutions”, Optics Express, Vol. 20, No. 3, pp. 2967-2973 
         NPL 2: “Bend measurement using Bragg gratings in multicore fibre”, Electronics letters, vol. 36, no. 2, pp. 120-121 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     Because special optical fibers such as a FBG multicore optical fiber are very expensive, there is a need for a method and an apparatus for acquiring curvature and torsion using a less expensive sensor medium. Thus, an object of the present disclosure is to provide a method and an apparatus for acquiring curvature and torsion using an inexpensive sensor medium. 
     Means for Solving the Problem 
     To achieve the above object, the present disclosure uses a fiber ribbon as a sensor medium for shape sensing using optical fibers, measures the longitudinal strain distribution of each coated fiber using a reflection measuring method such as Brillouin Optical Time Domain Reflectometry (B-OTDR) or OFDR, and then acquires the curvature and the torsion of the fiber ribbon based on the difference in a strain amount between optical fibers in the inner portion and in the outer portion of the fiber ribbon. 
     Specifically, a measurement apparatus according to the present disclosure includes: a fiber ribbon including a plurality of coated fibers arranged in parallel; a strain measurement unit that measures strain amounts of the plurality of coated fibers; and an arithmetic processing unit that calculates curvature and torsion of the fiber ribbon using a strain amount of a coated fiber arranged in a middle portion of the plurality of coated fibers and a strain amount of a coated fiber arranged in a marginal portion of the plurality of coated fibers. 
     Specifically, a measurement method according to the present disclosure is performed by a measurement apparatus, the measurement apparatus being connected to a fiber ribbon including a plurality of coated fibers arranged in parallel, the measurement method including measuring, by the measurement apparatus, strain amounts of the plurality of coated fibers, and calculating, by the measurement apparatus, curvature and torsion of the fiber ribbon using a strain amount of a coated fiber arranged in a middle portion of the plurality of coated fibers and a strain amount of a coated fiber arranged in a marginal portion of the plurality of coated fibers. 
     Effects of the Invention 
     The present disclosure uses a general-purpose fiber ribbon as a sensor medium and thus enables reduction of the cost of shape sensing. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    illustrates an example of a fiber ribbon, where  FIG.  1 ( a )  is a side view and  FIG.  1 ( b )  is a cross-sectional view. 
         FIG.  2    illustrates an example of a system configuration according to the present disclosure. 
         FIG.  3    illustrates an example of a relationship between curvature and a strain amount. 
         FIG.  4    illustrates an example of a relationship between torsion and a strain amount. 
         FIG.  5    illustrates an example of a flowchart of processing performed by a measurement apparatus according to the present disclosure. 
         FIG.  6    illustrates an example of strain amount distribution measured in step S 101 . 
         FIG.  7    illustrates an example of strain variation amount distribution. 
         FIG.  8    illustrates an example of measurement of curvature and torsion at each point. 
         FIG.  9    illustrates an example of a relationship between torsion and a strain amount in Embodiment 2. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments described below. These embodiments are just illustrative examples, and the present disclosure can be implemented in forms in which various modifications and improvements are added on the basis of knowledge of those skilled in the art. Note that constituent elements with the same reference signs in the specification and the drawings are assumed to be the same constituent elements. 
       FIG.  1    illustrates an example of a fiber ribbon. In the fiber ribbon, a plurality of coated fibers  91 - 1  to  91 - 4 , which are primary optical coated fibers, are arranged in parallel and coated with resin.  FIG.  1    illustrates an example in which four coated fibers  91 - 1  to  91 - 4  are arranged. The coated fibers  91 - 1  to  91 - 4  are bonded together by a coating. For this reason, while strains imposed on the coated fibers  91 - 1  to  91 - 4  are equal to each other in a state of displacement (bending) in the x-axis direction, strains imposed on the coated fibers  91 - 1  and  91 - 4 , disposed in an outer portion, are larger than strains imposed on the coated fibers  91 - 2  and  91 - 3 , disposed in an inner portion, in a state of rotation (twisting) on the xy plane. Further, considering the rigidity of the fiber ribbon, it can be assumed that the fiber ribbon is not bent in the y-axis direction but is just bent or twisted in the x-axis direction. 
     Thus, in the present disclosure, the curvature and the torsion of a fiber ribbon  91  are calculated based on the analysis of increase in a strain amount imposed on each of the coated fibers  91 - 1  to  91 - 4  and the analysis of difference in a strain amount between the coated fibers  91 - 2  and  91 - 3  disposed in the inner portion and the coated fibers  91 - 1  and  91 - 4  disposed in the outer portion. 
     Embodiment 1 
       FIG.  2    illustrates an example of a system configuration according to the present disclosure. A system according to the present disclosure includes a measurement apparatus  10  and a fiber ribbon  91 . The fiber ribbon  91  is used as a sensor medium and is laid on an object to be measured  92  subjected to shape sensing. The fiber ribbon  91  can be a general-purpose fiber ribbon including four coated fibers arranged as illustrated in  FIG.  1   . 
     The measurement apparatus  10  includes an optical meter  11  which functions as a strain measurement unit, an arithmetic processing unit  12 , and a storage unit  13 . The measurement apparatus  10  can also be achieved by a computer and a program, and the program can be recorded on a recording medium or provided via a network. 
     The optical meter  11  measures the strain of each of the coated fibers  91 - 1  to  91 - 4  included in the fiber ribbon  91 . The optical meter  11  is any optical meter capable of measuring the longitudinal strain distribution of optical fibers, for example, a Brillouin OTDR (B-OTDR) or an OFDR. 
     The arithmetic processing unit  12  measures the strain distribution of each of the coated fibers  91 - 1  to  91 - 4  using the strains measured by the optical meter  11 , and calculates the curvature and the torsion of the fiber ribbon  91  by using the strain distribution. 
     The storage unit  13  stores a relationship between curvature and a strain amount that are preliminarily measured using the fiber ribbon  91  and a relationship between torsion and a strain amount that are measured in the same way. 
       FIG.  3    illustrates an example of the relationship between the curvature and the strain amount of the fiber ribbon  91 . When the curvature of the fiber ribbon  91  increases, both the strain amount of the coated fibers  91 - 2  and  91 - 3  in the inner portion and the strain amount of the coated fibers  91 - 1  and  91 - 4  in the outer portion increase in the same manner. Thus, when the strain amount of the coated fibers  91 - 2  and  91 - 3  in the inner portion is substantially equal to the strain amount of the coated fibers  91 - 1  and  91 - 4  in the outer portion, the arithmetic processing unit  12  can use a strain amount of at least any one of the coated fibers  91 - 1  to  91 - 4  and derive curvature corresponding to the strain amount. 
       FIG.  4    illustrates an example of the relationship between torsion and a strain amount of the fiber ribbon  91 . When the torsion increases, the strain amount of the coated fibers  91 - 1  and  91 - 4  in the outer portion becomes more than the strain amount of the coated fibers  91 - 2  and  91 - 3  in the inner portion. Thus, when the strain amount of the coated fibers  91 - 1  and  91 - 4  in the outer portion is more than the strain amount of the coated fibers  91 - 2  and  91 - 3  in the inner portion, the arithmetic processing unit  12  can use a strain amount of at least any one of the coated fibers  91 - 1  to  91 - 4  and derive torsion corresponding to the strain amount. 
       FIG.  5    illustrates an example of a flowchart of processing performed by the measurement apparatus according to the present disclosure. 
     The measurement apparatus  10  performs steps S 101  to S 103 . 
     S 101 : The optical meter  11  measures the strain distribution of each of the coated fibers  91 - 1  to  91 - 4 .
 
S 102 : The arithmetic processing unit  12  compares the measured strain distribution with preliminarily measured reference strain data, and calculates the strain variation amount distribution of each of the coated fibers  91 - 1  to  91 - 4 .
 
S 103 : The curvature and the torsion of the fiber ribbon  91  are calculated using the strain variation amount distribution of each of the coated fibers  91 - 1  to  91 - 4 .
 
       FIG.  6    illustrates an example of the strain amount distribution measured in step S 101 . The optical meter  11  makes signal light incident on each of the coated fibers  91 - 1  to  91 - 4 , and measures a strain amount at each distance from the optical meter  11  using return light from each of the coated fibers  91 - 1  to  91 - 4 . This allows the arithmetic processing unit  12  to acquire the strain amount at each section S 1  to S 5 . 
     In step S 102 , the arithmetic processing unit  12  calculates the strain variation amount distribution by subtracting the preliminarily measured reference strain distribution (distribution data without strain) from the strain distribution measured in step S 101 . 
       FIG.  7    illustrates an example of strain variation amount distribution. In section S 2 , the strain amount of the coated fibers  91 - 2  and  91 - 3  in the inner portion is substantially equal to the strain amount of the coated fibers  91 - 1  and  91 - 4  in the outer portion. In section S 4 , the strain amount of the coated fibers  91 - 1  and  91 - 4  in the outer portion is more than the strain amount of the coated fibers  91 - 2  and  91 - 3  in the inner portion. There is no increase in the strain amount in sections S 1 , S 3 , and S 5 . 
     In step S 103 , the arithmetic processing unit  12  calculates the distributions of the curvature and the torsion of the fiber ribbon  91  at each point using “the strain variation amount” acquired in step S 102  and “the relationship between the curvature and the strain amount” and “the relationship between the torsion and the strain amount” stored in the storage unit  13 . For example, in section S 2 , the strain amount in the inner portion is substantially equal to the strain amount in the outer portion. Thus, the arithmetic processing unit  12  derives curvature corresponding to the strain amount using the relationship between the curvature and the strain amount as illustrated in  FIG.  3   . For example, in section S 4 , the strain amount in the inner portion is different from the strain amount in the outer portion. Thus, the arithmetic processing unit  12  derives torsion corresponding to the strain amount using the relationship between the torsion and the strain amount as illustrated in  FIG.  4   . This allows the measurement of curvature and torsion at each point from the optical meter  11  as illustrated in  FIG.  8   . 
     Embodiment 2 
     In the present embodiment, acquisition of twisting direction will be described. The present embodiment has a system configuration identical to Embodiment 1 and uses the optical meter  11  to measure longitudinal strain distribution of optical fibers. However, the fiber ribbon  91  is laid on the object to be measured  92  in a preliminarily twisted condition. Then, steps S 101  to S 103  are performed in the same manner as in Embodiment 1, but step S 103  differs as described below. 
     In the present embodiment, the fiber ribbon  91  is preliminarily twisted. For this reason, strain is preliminarily caused as illustrated in  FIG.  9   . Thus, when each strain amount increases from a reference thereof, the arithmetic processing unit  12  determines that twisting is added in a direction identical to a direction of the preliminary twisting. On the other hand, when each strain amount decreases from the reference, the arithmetic processing unit  12  determines that twisting is added in a direction opposite to the direction of the preliminary twisting. Note that the comparison between the reference and the strain amount may employ a difference in the preliminarily caused strain amount between the inner portion and the outer portion. 
     Points of the Invention 
     A general-purpose fiber ribbon is used as a sensor medium. 
     A longitudinal strain distribution of each coated fiber is measured by B-OTDR, OFDR, or the like. 
     The curvature and the torsion of the fiber ribbon are calculated based on a difference between strain amounts applied to optical fibers in the inner portion and in the outer portion of the fiber ribbon. 
     In consideration of the rigidity of the fiber ribbon, it is assumed that the fiber ribbon is not bent in a y-axis direction but is just bent or twisted in an x-axis direction. 
     Effects 
     Because a general-purpose fiber ribbon is used as a sensor medium, it is possible to provide a method and an apparatus each capable of acquiring curvature and torsion at a lower cost than a method that uses a special sensor medium such as a multicore fiber. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure can be applied in the information communication industry. 
     REFERENCE SIGNS LIST 
     
         
           10 : Measurement apparatus 
           11 : Optical meter 
           12 : Arithmetic processing unit 
           13 : Storage unit 
           91 : Fiber ribbon 
           91 - 1 ,  91 - 2 ,  91 - 3 ,  91 - 4 : Coated fiber 
           92 : Object to be measured