Patent Publication Number: US-2022236050-A1

Title: Member distinguish apparatus, member distinguish method, and computer-readable medium

Description:
TECHNICAL FIELD 
     The present disclosure relates to a member distinguishing apparatus, a member distinguishing method, and a computer readable medium. 
     BACKGROUND ART 
     When a reinforced concrete structure is built, it is necessary to carry out a bar arrangement inspection to inspect positions and thicknesses (diameters) of reinforcing bars placed in the structure. Patent Literature 1 discloses a technique for measuring reinforcing bars from a plurality of places in the structure using a three-dimensional laser scanner, integrating measurement data, and comparing the integrated measurement data with reference data to determine a reinforcing bar arrangement state. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2010-014693 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In the technique described in Patent Literature 1, it is necessary to measure reinforcing bars from a plurality of places in the structure, which causes a problem that much time and effort are needed to do this. Further, although the diameters of the reinforcing bars are standardized at intervals of, for example, 2 to 3 mm, in the technique of Patent Literature 1, it is necessary to perform measurement accurate enough to enable a distinction to be made between such diameters of the reinforcing bars (i.e., a degree of distinction precision on the order of millimeters). Thus, there is a problem that the cost of the measuring instrument is increased. 
     An object of the present disclosure is to provide a member distinguishing apparatus capable of automatically and accurately distinguishing a type of a member to be inspected by performing simple measurement on a member to be inspected at a low cost using a three-dimensional sensor. 
     Solution to Problem 
     A first example aspect of the present disclosure is a member distinguishing apparatus including: a three-dimensional sensor configured to irradiate one or more members to be inspected with a beam and acquire first point group data based on at least amplitude information of light; a point group data analysis unit configured to extract second point group data from the first point group data, the second point group data being point group data of the member to be inspected, and obtain, with respect to the second point group data, a distribution of the point group data in a vertical direction, the vertical direction being a longitudinal direction of the member to be inspected, a lateral direction orthogonal to an irradiation direction of the beam when an area of a surface of the member to be inspected irradiated by the three-dimensional sensor becomes the largest and also orthogonal to the vertical direction, and a depth direction orthogonal to the longitudinal direction and the lateral direction; and a member distinguishing unit configured to distinguish a type of the member to be inspected from the distribution. 
     A second example aspect of the present disclosure is a member distinguishing method including: a first step of, performed by a three-dimensional sensor capable of acquiring point group data based on at least amplitude information of light, irradiating one or more members to be inspected with a beam and acquiring first point group data; a second step of extracting second point group data from the first point group data, the second point group data being point group data of the member to be inspected, and obtaining, with respect to the second point group data, a distribution of the point group data in a vertical direction, the vertical direction being a longitudinal direction of the member to be inspected, a lateral direction orthogonal to an irradiation direction of the beam when an area of a surface of the member to be inspected irradiated by the three-dimensional sensor becomes the largest and also orthogonal to the vertical direction, and a depth direction orthogonal to the longitudinal direction and the lateral direction; and a third step of distinguishing a type of the member to be inspected from the distribution. 
     A third example aspect of the present disclosure is non-transitory computer readable medium storing a program for causing a computer to execute: a first step of, performed by a three-dimensional sensor capable of acquiring point group data based on at least amplitude information of light, irradiating one or more members to be inspected with a beam and acquiring first point group data; a second step of extracting second point group data from the first point group data, the second point group data being point group data of the member to be inspected, and obtaining, with respect to the second point group data, a distribution of the point group data in a vertical direction, the vertical direction being a longitudinal direction of the member to be inspected, a lateral direction orthogonal to an irradiation direction of the beam when an area of a surface of the member to be inspected irradiated by the three-dimensional sensor becomes the largest and also orthogonal to the vertical direction, and a depth direction orthogonal to the longitudinal direction and the lateral direction; and a third step of distinguishing a type of the member to be inspected from the distribution. 
     Advantageous Effects of Invention 
     According to the present disclosure, it is possible to provide a member distinguishing apparatus capable of automatically and accurately distinguishing a type of a member to be inspected by performing simple measurement on a member to be inspected at a low cost using a three-dimensional sensor. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of a member distinguishing apparatus according to a first example embodiment; 
         FIG. 2  is a block diagram showing a configuration of a member distinguishing apparatus according to a second example embodiment; 
         FIG. 3  is a schematic view showing an outer shape of a deformed steel bar which is a reinforcing bar serving as a member to be inspected by the member distinguishing apparatus according to the second example embodiment; 
         FIG. 4  is a schematic view for explaining definitions of directions of the member to be inspected; 
         FIG. 5  is a flowchart for explaining a flow of processing for distinguishing a type of a member to be inspected by the member distinguishing apparatus according to the second example embodiment; 
         FIG. 6  is a schematic view for explaining a method of extracting second point group data, which is point group data of each member to be inspected, from first point group data; 
         FIG. 7  is a schematic view for explaining a method of obtaining a distribution of the second point group data of each member to be inspected in a vertical direction, a lateral direction, and a depth direction in Step S 103  of  FIG. 5 ; 
         FIG. 8  is a flowchart showing a flow of processing for obtaining a distribution the second point group data of each member to be inspected in the vertical direction, the lateral direction, and the depth direction; 
         FIG. 9  is a schematic view for explaining a method of determining whether each of three principal components obtained in a principal component analysis method corresponds to the vertical direction, the lateral direction, or the depth direction; 
         FIG. 10  is a schematic view for explaining a method of determining whether each of three principal components obtained in a principal component analysis method corresponds to the vertical direction, the lateral direction, or the depth direction; 
         FIG. 11  is a flowchart showing a flow of processing for obtaining a variance of the second point group data in the vertical direction, the lateral direction, and the depth direction by the principal component analysis method; 
         FIG. 12  is a graph showing a relationship between a diameter and an eigenvalue of a second principal component of a reinforcing bar to be inspected; 
         FIG. 13  is a schematic view for explaining errors in the variance of reinforcing bars as members to be inspected in the lateral direction; 
         FIG. 14  is a schematic view for explaining errors in the variance of reinforcing bars as members to be inspected in the depth direction; 
         FIG. 15  is a schematic view for explaining a method of reducing the errors in the variance of reinforcing bars as members to be inspected in the depth direction; 
         FIG. 16  is a schematic view for explaining another method of reducing the errors in the variance of reinforcing bars as members to be inspected in the depth direction; 
         FIG. 17  is a schematic view for explaining an error occurring in the depth direction when the three-dimensional sensor is a ToF type sensor which is a mainstream 3D-LiDAR sensor for automated driving; 
         FIG. 18  is a block diagram showing a configuration of a member distinguishing unit; 
         FIG. 19  is a schematic view showing an example of reference information held in an information holding unit; 
         FIG. 20  is a block diagram showing the configuration of the member distinguishing unit according to the first modified example; 
         FIG. 21  is a schematic view showing a method of generating third point group data in the simulation unit; 
         FIG. 22  is a schematic view for explaining another method of generating third point group data in the simulation unit; 
         FIG. 23  is a graph in which variances in the lateral direction obtained by executing a simulating with a distance from the three-dimensional sensor to the reinforcing bar as 5 m are plotted; and 
         FIG. 24  is a graph in which variances obtained by executing a simulating with a distance from the three-dimensional sensor to the reinforcing bar as 5 m are plotted. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Example embodiments of the present disclosure will be described below with reference to the drawings. For clarity of description, the following descriptions and drawings have been omitted and simplified as appropriate. In each of the drawings, the same elements are denoted by the same reference numerals, and repeated descriptions are omitted as necessary. 
     [First Example Embodiment] 
     A first example embodiment will be described below. 
       FIG. 1  is a block diagram showing a configuration of a member distinguishing apparatus  10  according to a first example embodiment. As shown in  FIG. 1 , the member distinguishing apparatus  10  includes a three-dimensional sensor  11 , a point group data analysis unit  12 , and a member distinguishing unit  13 . 
     The three-dimensional sensor  11  can measure a distance based on at least amplitude information of light, and irradiates one or more members to be inspected with a beam to acquire first point group data. The point group data analysis unit  12  extracts second point group data, which is point group data of the member to be inspected, from the first point group data. The point group data analysis unit  12  obtains, for the second point group data, a distribution of the point group data in each of a vertical direction, which is the longitudinal direction of the member to be inspected, a lateral direction orthogonal to an irradiation direction of the beam when an area of a surface of the member to be inspected irradiated by the three-dimensional sensor becomes the largest and also orthogonal to the vertical direction, and a depth direction which is orthogonal to the vertical direction and the lateral direction. The member distinguishing unit  13  distinguishes a type of the member to be inspected from the distributions. 
     In this way, the type of the member to be inspected can be automatically and accurately distinguished by simple measurement of the member to be inspected at a low cost using the three-dimensional sensor  11 . 
     [Second Example Embodiment] 
     A second example embodiment will be described below. First, a configuration example of a member distinguishing apparatus according to the second example embodiment will be described.  FIG. 2  is a block diagram showing a configuration of a member distinguishing apparatus  110  according to the second example embodiment. As shown in  FIG. 2 , the member distinguishing apparatus  110  includes a three-dimensional sensor  111 , a point group data analysis unit  112 , and a member distinguishing unit  113 . 
     The three-dimensional sensor  111  can measure a distance based on at least amplitude information of light, and irradiates one or more members to be inspected with a beam to acquire first point group data. The three-dimensional sensor  111  is, for example, a 3D-LiDAR (Light Detection and Ranging) sensor. 
     Here, the members to be inspected are reinforcing bars called deformed steel bars (also called deformed reinforcing bars).  FIG. 3  is a schematic view showing an outer shape of one of the deformed steel bars. As shown in  FIG. 3 , the deformed steel bar is provided with uneven projections called “ribs” and “knots” on its surface. Standard names such as “D 10 ”, “D 13 ”, “D 16 ”, and “D 19 ” are given to the deformed steel bars depending on their respective diameters. The numbers in the standard names indicate approximate diameters of the deformed steel bars, for example, the diameter of D 10  is 9.53 mm and the diameter of D 13  is 12.7 mm. That is, the diameters of the deformed steel bars are standardized at intervals of 2 to 3 mm. 
     Referring again to  FIG. 2 , the point group data analysis unit  112  extracts the second point group data, which is the point group data of the member to be inspected, from the first point group data. The point group data analysis unit  112  obtains the distribution of the point group data, which is the second point group data, of the member to be inspected in each of the vertical direction, the lateral direction, and the depth direction. The distribution here is a variance. The definitions of the vertical, lateral, and depth directions of the member to be inspected will be described later. The member distinguishing unit  113  distinguishes the type of the member to be inspected from the distribution of the point group data in each of the vertical direction, the lateral direction, and the depth direction of the member to be inspected obtained by the point group data analysis unit  112 . 
       FIG. 4  is a schematic view for explaining the definitions of the directions of the member to be inspected. As shown in  FIG. 4 , the reference three-dimensional orthogonal coordinate system has an origin at the position of the three-dimensional sensor  111  and is composed of an x-axis as a first axis, a y-axis as a second axis, and a z-axis as a third axis. The reference three-dimensional orthogonal coordinate system is set, for example, in such a way that the z-axis as the third axis is vertical, and a plane including the x-axis as the first axis and the y-axis as the second axis is horizontal. 
     The reference three-dimensional orthogonal coordinate system can be transformed into a reference three-dimensional polar coordinate system having an origin at the position of the three-dimensional sensor  111 . It is assumed that a point P is a representative point of a part of the member to be inspected which is irradiated with the beam emitted from the three-dimensional sensor  111 . The point P is expressed as (x, y, z) in the three-dimensional orthogonal coordinate system and as (d, φ, θ) in the three-dimensional polar coordinate system. Here, d is a distance from the origin to the point P, i.e., a moving radius. φ is an angle formed by a projection of a radial vector (a vector from the origin to the point P) onto the x-axis and the xy plane.  0  is an angle formed by the xy plane and the radial vector. The relations of x=d·cosθ·coφ, x=d·cosθ·sinφ, and z=d·sinθ hold. Note that the representative point where the beam hits the member to be inspected is on an optical axis of the beam. 
     The vertical direction is the longitudinal direction of the member to be inspected. The lateral direction is a direction orthogonal to the irradiation direction of the beam when the area of the surface irradiated with the beam from the three-dimensional sensor  111  in the member to be inspected becomes the largest and also orthogonal to the vertical direction. The irradiation direction of the beam is a direction of an optical axis of the beam. The depth direction is a direction orthogonal to the vertical direction and the lateral direction. In the three-dimensional orthogonal coordinate system (x, y, z), the vertical direction is a direction parallel to the z-axis. In the three-dimensional orthogonal coordinate system (x, y, z), the lateral direction is a direction parallel to a vector e 2  which is orthogonal, on the xy plane, to a direction vector e 1  of projection of the beam emitted from the three-dimensional sensor  111  onto the xy plane. If the point P is a point of the member to be inspected where the beam emitted from the three-dimensional sensor  111  hits, the angle formed by the projection of the beam emitted from the three-dimensional sensor  111  onto the xy plane with the x-axis corresponds to an angle φ. 
     Next, a flow of processing for distinguishing the type of the member to be inspected in the member distinguishing unit  113  shown in  FIG. 2  will be described. In the following description,  FIG. 2  is also referred to as appropriate. 
       FIG. 5  is a flowchart for explaining a flow of processing for determining the type of the member to be inspected in the member distinguishing apparatus  110 . As shown in  FIG. 5 , first, the three-dimensional sensor  111  emits a beam to one or more members to be inspected to acquire the first point group data (Step S 101 ). That is, the three-dimensional sensor  111  emits a beam to a group of reinforcing bars placed at a construction site to acquire the first point group data. 
     Following Step S 102 , the point group data analysis unit  112  extracts second point group data, which is the point group data of each member to be inspected, from the first point group data (Step S 102 ). That is, the second point group data, which is the point group data of the individual reinforcing bar, is extracted from the first point group data. Next, the point group data analysis unit  112  analyzes the distributions of the second point group data of each member to be inspected in the vertical direction, the lateral direction, and the depth direction (Step S 103 ). Since the second point group data approximately represents the shape of the member to be inspected, the vertical, lateral, and depth directions of the second point group data correspond to the vertical, lateral, and depth directions of the member to be inspected described with reference to  FIG. 4 , respectively. Following Step S 103 , the member distinguishing unit  113  distinguishes the type of the member to be inspected from the analysis results of the distributions in the vertical direction, the lateral direction, and the depth direction (Step  104 ). 
     In Step S 102  of  FIG. 5 , the second point group data is extracted by segmenting the first point group data by clustering. General-purpose clustering algorithms, such as Euclidean Clustering and Region Growing, may be used as the clustering. Euclidean clustering is an algorithm in which if an Euclidean distance between points is equal to or less than a threshold, the points belongs to the same cluster, whereas if the Euclidean distance between the points is greater than the threshold, the points belong to different clusters. In the Euclidean clustering, it is necessary to determine the threshold of the Euclidean distance between the points in advance. The region growing method is an algorithm in which if an orientation of a surface formed by a point and a surrounding point is within a threshold, the points belong to the same cluster, whereas if the orientation of the surface formed by the point and the surrounding point is greater than the threshold, the points belongs to different clusters. In the region growing method, it is necessary to determine in advance the threshold of the orientation of the surface, and it is also necessary to determine in advance how many surrounding points are included to calculate the orientation of the surface. 
       FIG. 6  is a schematic view for explaining a method of extracting the second point group data, which is the point group data of each member to be inspected, from the first point group data. As shown in  FIG. 6 , in the first point group data, there is a region where the beam is obstructed and the point group data cannot be obtained therefrom. The first point group data is clustered by the distance between the points or the orientation of the surface. For example, the first point group data is segmented into clusters  1 ,  2 , and  3  according to the distance between the points, because the distance between the points is far in the region where the point group data cannot be obtained since the beam is obstructed on the region. The two clusters, cluster  2  and cluster  3 , are considered to be the same reinforcing bars, because the orientations of the surfaces are the same, the distances between the points are far, though. Therefore, the clusters  2  and  3  are integrated into one cluster based on the orientation of the surface, i.e., based on the coordinates of the center axis, the average of the coordinates in the depth direction and the horizontal direction, or the like. That is, the cluster obtained by integrating the cluster  2  and the cluster  3  is used as the second point group data. Further, since the orientation of the surface of the cluster  1  is different from that of the cluster  2  and that of the cluster  3 , the cluster  1  alone is used as the second point group data. In this way, the second point group data, which is the point group data of each member to be inspected, is extracted. 
       FIG. 7  is a schematic view for explaining a method of obtaining the distribution in each of the vertical direction, the lateral direction, and the depth direction of the second point group data of each member to be inspected in Step S 103  of  FIG. 5 . As shown in  FIG. 5 , the second point group data and the lateral direction and the depth direction are rotationally converted based on the angle φ (see  FIG. 3 ) formed by the x-axis (the first axis) and the projection of the radial vector on the xy plane (the plane including the first axis and the second axis) so that the lateral direction in the second point group data becomes parallel to the xz plane (the plane including the first axis and the third axis). After the rotation conversion, the variance of the second point group data in each of the vertical direction, the lateral direction, and the depth direction is obtained. 
       FIG. 8  is a flowchart showing a flow of processing for obtaining the variances as the distributions of the second point group data of each member to be inspected in the vertical direction, the lateral direction, and the depth direction. As shown in  FIG. 8 , it is determined whether the longitudinal direction of the second point group data is parallel to the xy plane (the plane including the first axis and the second axis) or parallel to the z-axis (the third axis) (Step S 201 ). If it is determined in Step S 201  that the longitudinal direction of the second point group data is parallel to the z-axis, the second point group data is rotated and converted based on the angle φ so that the lateral direction of the second point group data becomes parallel to the xz plane (the plane including the first axis and the third axis) (Step S 202 ). After the rotation conversion, the variance of the second point group data in the lateral direction is calculated (Step S 203 ). If it is determined in Step S 201  that the longitudinal direction of the second point group data is parallel to the xy plane, the variance of the second point group data as it is in the lateral direction is calculated (Step S 204 ). 
     According to the above-described method, when the longitudinal direction of the second point group data is parallel to the z-axis, the variances of the second point group data as the distributions in the vertical, lateral, and depth directions can be obtained. 
     In Step S 103  of  FIG. 5 , the Principle Component Analysis (PCA) method can also be employed as the method of obtaining the distributions of the second point group data of each member to be inspected in the vertical direction, the lateral direction, and the depth direction. In the principal component analysis method, the eigenvalues of the principal components (eigenvectors) are the variances. In the PCA method, the principal components are referred to as a first principal component, a second principal component, and so forth in order of the eigenvalues. The first to third principal components can be obtained from the second point group data, because the second point group data is composed of three parameters (x, y, z). 
     In the PCA method, it is necessary to separately distinguish whether each of the obtained three principal components corresponds to the vertical direction, the lateral direction, or the depth direction.  FIGS. 9 and 10  are schematic views for explaining a method of distinguishing whether each of the obtained three principal components corresponds to the vertical direction, the lateral direction, or the depth direction in the PCA method. As shown in  FIG. 9 , in the second point group data, the variance in the vertical direction becomes the largest. Further, since the point group data is present only on the surface facing the three-dimensional sensor  111 , the variance in the lateral direction is larger than that in the depth direction. Therefore, as shown in  FIG. 10 , among the obtained principal components, it can be determined that the first principal component having the largest eigenvalue is in the vertical direction and the second principal component having the second largest eigenvalue is in the lateral direction. 
       FIG. 11  is a flowchart showing a flow of processing for obtaining the variances of the second point group data in the vertical direction, the lateral direction, and the depth direction by the PCA method. As shown in  FIG. 11 , the PCA method is executed on the second point group data to calculate the eigenvalues of the three principal components (Step S 301 ). Next, the largest eigenvalue of the principal component among the eigenvalues obtained by the PCA method is determined as the variance in the vertical direction (Step S 302 ). Next, the second largest eigenvalue of the principal component among the eigenvalues obtained by the PCA method is determined as the variance in the lateral direction (Step S 303 ). Next, the smallest eigenvalue among eigenvalues of the three principal components is determined as the variance in the depth direction (Step S 304 ). 
     In the PCA method, it is not necessary to rotate and convert the second point group data regardless of the longitudinal direction of the member to be inspected as the member to be inspected. That is, by using the PCA method, it is possible to easily obtain the distribution of the second point group data in each direction, even if the longitudinal direction of the reinforcing bar as the member to be inspected is an oblique direction which is neither a horizontal direction nor a vertical direction. 
     The eigenvalue of the second principal component corresponding to the variance in the lateral direction of the reinforcing bar as the member to be inspected correlates with the diameter of the reinforcing bar as the member to be inspected.  FIG. 12  is a graph showing the relationship between the diameter and the eigenvalue of the second principal component in the reinforcing bar which is the member to be inspected. Here, the horizontal axis represents the diameter of the reinforcing bar [mm], and the vertical axis represents the eigenvalue of the second principal component [m 2 ]. As shown in  FIG. 12 , there is a clear difference between the ranges of eigenvalues of the second principal component according to the diameter of the reinforcing bar. Therefore, in Step  104  of  FIG. 5 , the member distinguishing unit  113  can distinguish the type of the reinforcing bar according to the range of the second main component obtained by analyzing the second point group data of the reinforcing bars (D 10 , D 13 , D 16 , and D 19 ) as the member to be inspected using the PCA method. In  FIG. 12 , a threshold P 1  is for distinguishing between D 10  and D 13 . The threshold P 2  is for distinguishing between D 13  and D 16 . The threshold P 3  is for distinguishing between D 16  and D 19 . That is, it can be determined that if the second main component is within the range below the threshold P 1 , the reinforcing bar is D 10 , if the second main component is within the range from the threshold P 1  or more and less than the threshold P 2 , the reinforcing bar is D 13 , if the second main component is within the range from the threshold P 2  or more and less than the threshold P 3 , the reinforcing bar is D 16 , and if the second main component is within the range of the threshold P 3  or more, the reinforcing bar is D 19 . 
     Next, an error of the variance in the lateral direction of the reinforcing bars as the member to be inspected will be described. 
       FIG. 13  is a schematic view for explaining errors in the variances of the reinforcing bars in the lateral direction as members to be inspected. As shown in  FIG. 13 , the diameter of the beam emitted from the three-dimensional sensor  111  increases as the distance from the three-dimensional sensor  111  increases. In the three-dimensional sensor  111 , even if a part of the beam hits the reinforcing bar, the point of the point group data may be obtained. Since the coordinate of the point acquired as the point group data is the coordinate of the center of the beam, the error of the variance of the second point group data in the lateral direction becomes larger for the reinforcing bars located farther from the three-dimensional sensor  111  than that for the reinforcing bars located near the three-dimensional sensor  111 . In this way, the variance of the reinforcing bar in the lateral direction is affected by the distance between the reinforcing bar and the three-dimensional sensor  111 . It is thus necessary to change the threshold of the range of the second main component for distinguishing the reinforcing bar described with reference to  FIG. 12  according to the distance between the reinforcing bar and the three-dimensional sensor  111 . 
     Next, the error of the variance of the reinforcing bar in the depth direction as the member to be inspected will be described. 
       FIG. 14  is a schematic view for explaining an error in the variance of the reinforcing bar in the depth direction as the member to be inspected. As shown in  FIG. 14 , when an angle (an incident angle X) at which the beam emitted from the three-dimensional sensor  111  enters the reinforcing bar is large, the beam is totally reflected by the surface of the reinforcing bar. For this reason, the beam totally reflected by the surface of the reinforcing bar may hit a surrounding object, and the reflected light may return to the three-dimensional sensor  111 . In such a case, a point group extending in the depth direction is obtained at both ends of the reinforcing bar in the lateral direction. Note that when the beam totally reflected by the surface of the reinforcing bar hits an object located at a considerable distance from the reinforcing bar and the reflected light returns to the three-dimensional sensor  111 , there is no problem, because the point is determined as being present in an unrelated region by the clustering described above. 
       FIG. 15  is a schematic view for explaining a method of reducing the error of the variance of the reinforcing bar as the member to be inspected in the depth direction.  FIG. 15  is a frequency distribution graph showing a frequency of appearances of points of the reinforcing bar in the vertical direction with respect to the distance from the three-dimensional sensor  111  to the reinforcing bar. In  FIG. 15 , the horizontal axis represents the distance from the three-dimensional sensor  111  to the reinforcing bar, and the vertical axis represents the frequency of appearances of points of the reinforcing bar in the vertical direction. As shown in  FIG. 15 , data of points in a section of the frequency distribution graph outside a predetermined range based on a section of the most frequent value is deleted from the second point group data. The predetermined range is set based on the measurement error of the three-dimensional sensor  111 . 
       FIG. 16  is a schematic view for explaining another method of reducing the error of the variance of the reinforcing bar as the member to be inspected in the depth direction. As shown in  FIG. 16 , first, the point group data of the reinforcing bar, which is the second point group data, is rotated and converted so that the lateral direction becomes parallel to the xz plane using the method described with reference to  FIG. 7 . Then, in the second point group data after the rotation conversion, a point group t 1  having a smallest coordinate of the x-axis and a point group t 2  having the largest coordinate are deleted. 
       FIG. 17  is a schematic view for explaining an error generated in the depth direction when the three-dimensional sensor  111  is a ToF (Time of Flight) type sensor which is a mainstream 3D-LiDAR sensor for automated driving. As shown in  FIG. 17 , when the three-dimensional sensor  111  is a ToF type sensor which is a mainstream 3D-LiDAR sensor for automated driving, measurement errors (on the order of cm), which are larger than the diameters of the reinforcing bars, are commonly generated in the measured values in the depth direction. Therefore, the coordinates in the depth direction are shifted back and forth by the error. This error may be corrected in the distribution of the second point group data in the depth direction. 
     Next, the processing of the member distinguishing unit  113  shown in  FIG. 2  will be described in detail. 
       FIG. 18  is a block diagram showing a configuration of the member distinguishing unit  113 . As shown in  FIG. 17 , the member distinguishing unit  113  includes, as functional blocks, an information holding unit  113   a  and a comparison unit  113   b.    
     The information holding unit  113   a  holds information about the type of the member to be inspected for each type of the member to be inspected. The information holding unit  113   a  holds reference information including information about the relationship between the distance from the three-dimensional sensor  111  to the member to be inspected and the range of values of the variances can take in a distinguishing direction. The distinguishing direction is a direction used for distinguishing the type of the member to be inspected, from among the vertical direction, the lateral direction, and the depth direction of the second point group data. The information holding unit  113   a  receives the variance of the second point group data in each direction. Note that when the variance of the second point group data is calculated by the PCA, the information holding unit  113   a  may hold information indicating which principal component is used to distinguish the type of the member to be inspected. 
     The comparison unit  113   b  distinguishes the type of the member to be inspected based on the reference information, the distance from the three-dimensional sensor  111  to the member to be inspected, and the variance of the second point group data in the distinguishing direction, and outputs information about the distinguished type of the member to be inspected. Note that the distance from the three-dimensional sensor  111  to the member to be inspected is obtained by the point group data analysis unit  112  based on the second point group data, and is input to the comparison unit  113   b.    
       FIG. 19  is a schematic view showing an example of the reference information held in the information holding unit  113   a  (see  FIG. 18 ). As shown in 
       FIG. 19 , the reference information is, for example, a table. In the table as the reference information, a range of a variance v in a direction for which the determination is made is shown for each type (with the standard names D 10 , D 13 , D 16 , and D 19 ) of the reinforcing bar to be inspected at predetermined intervals from 1 m to 10 m of the distance from the three-dimensional sensor  111  to the reinforcing bar as the member to be inspected. Here, the direction for which the determination is made is the lateral direction of the second point group data. Further, the predetermined interval is appropriately set to, for example, a 1 m interval or a 5 m interval according to the specification of the three-dimensional sensor  111 , the type of the member to be inspected, etc. For example, assume that the distance from the three-dimensional sensor  111  to the reinforcing bar is 1 m, and the variance v of the second point group data of the reinforcing bar in the distinguishing direction is 3.0 E-06 m 2 . In the reference information, the variance v of the second point group data of the reinforcing bar in the distinguishing direction falls within the range of 1.8 E-06 m 2 ≤v&lt;5.8 E-06 m 2 , where the distance from the three-dimensional sensor  111  to the reinforcing bar is 1 m. Therefore, the comparison unit  113   b  (see  FIG. 18 ) distinguishes that the reinforcing bar is “D 13 ”. 
     [Modified Example 1] 
       FIG. 20  is a block diagram showing a configuration of the member distinguishing unit  113  according to a modified example 1, i.e., a configuration different from that shown in  FIG. 18 . As shown in  FIG. 20 , the member distinguishing unit  113  includes, as functional blocks, a simulation unit  213   a  and a comparison unit  213   b.    
     The distance between the three-dimensional sensor  111  and a point group in the second point group data is input to the simulation unit  213   a . The simulation unit  213   a  generates, for each type of member to be inspected, third point group data that is assumed to be obtained by measurement based on the distance from the three-dimensional sensor  111  to the member to be inspected, information about the specification of the three-dimensional sensor  111 , and information about a shape of the member to be inspected. Then, the simulation unit  213   a  obtains the variance of the generated third point group data in the lateral direction. 
     The comparison unit  213   b  receives the variance of the second point group data in each direction. The comparison unit  213   b  compares the variance of the second point group data in the lateral direction with the variance of the third point group data in the lateral direction for each type of the member to be inspected corresponding to the distance from the three-dimensional sensor  111  to the member to be inspected obtained based on the second point group data. Then, the comparison unit  213   b  outputs information about the type of the member to be inspected corresponding to the third point group data whose variance has the smallest difference from the variance of the second point group data. 
       FIG. 21  is a schematic view showing a method of generating the third point group data in the simulation unit  213   a  (see  FIG. 20 ). As shown in  FIG. 21 , it is determined whether or not an intersection with the beam emitted from the three-dimensional sensor  111  is present within the range of a polygon for each polygon constituting the CAD drawing in which the member to be inspected is drawn. When it is determined that the intersection with the beam is within the range of the polygon, the intersection is defined as a point included in the third point group data. In this way, the simulation unit  213   a  virtually scans the member to be inspected by changing the irradiation direction of the beam in accordance with the specifications of the 3D-LiDAR to thereby generate the third point group data. 
       FIG. 22  is a schematic view showing a method of generating the third point group data in the simulation unit  213   a  (see  FIG. 20 ), which is different from the method described with reference to  FIG. 21 . As shown in  FIG. 22 , the member to be inspected is approximated to a simple shape which can be expressed by an equation, and the intersection of the shape and the beam emitted from the three-dimensional sensor  111  is stored as the point group data. 
     For example, when the member to be inspected is a reinforcing bar, the shape of the member to be inspected can be approximated as a cylinder. 
     Therefore, the equation of the member to be inspected is approximated by the following equation of the cylinder. 
       (x−a) 2 +(y−b) 2 =1, m&lt;z&lt;n   Equation of cylinder
 
     In this equation, (a, b) is a center coordinate of the cylinder of the member to be inspected in the three-dimensional orthogonal coordinate system described with reference to  FIG. 4 . m is a z-coordinate of a bottom surface of the cylinder of the member to be inspected, and n is a z-coordinate of a top surface of the cylinder of the member to be inspected. 
     The point P (see  FIG. 4 ) at which the beam emitted from the three-dimensional sensor  111  hits the member to be inspected is expressed as x=d·cosθ·cosφ, x=d·cosθ·sinφ, and z=d·sinθ, as described above. When these equations are substituted into the equation of the member to be inspected, a quadratic equation for d is obtained. If a solution exists to this quadratic equation, the coordinates (x, y, z) of the point P can be obtained. Note that when there are two intersections between the beam emitted from the three-dimensional sensor  111  and the member to be inspected, two solutions exist to the above-mentioned quadratic equation. In this case, the intersection on the side close to the three-dimensional sensor  111  is the point (the point P) where the beam hits the surface of the member to be inspected, and the intersection on the side far from the three-dimensional sensor  111  is a point where the beam is obstructed. Thus, the smaller one of the two solutions to the quadratic equation is referred to as d. 
     The simulation unit  213   a  shown in  FIG. 20  may execute a simulation for each type of member to be inspected a plurality of times and generate the third point group data which is estimated point group data a plurality of times. In this case, the simulation unit  213   a  obtains a range of variations of the variances of the generated third point group data in the lateral direction, and obtains the range of the variance of the third point group data in the lateral direction for each type of the member to be inspected. The comparison unit  213   b  refers to the range of the variations of the third point group data for each type of the member to be inspected corresponding to the distance from the three-dimensional sensor  111  to the member to be inspected obtained based on the second point group data. 
     Then, the comparison unit  213   b  outputs information about the type of the member to be inspected corresponding to the third point group data in which the variations of the variances of the second point group data in the lateral direction is included in the range of the variations. 
     Next, results of the simulations performed by the simulation unit  213   a  on a plurality of different types of members to be inspected when the distances from the three-dimensional sensor  111  to the reinforcing bars are 5 m and 10 m, respectively, will be described. 
     Various conditions of the simulations are as follows.
     1) The members to be inspected are four kinds of reinforcing bars (D 10 , D 13 , D 16 , and D 19 ).   2) The three-dimensional sensor  111  is a 3D-LiDAR sensor with the following specifications.   

     Horizontal angular resolution: 0.011° 
     Vertical angular resolution: 0.1° 
     Beam diameter at 0 m: 9.5 mm 
     Beam spread angle: 3.0 mrad 
     Ranging error: +/−3 cm (1σ, ranging error shall occur according to a normal distribution with a mean of 0 and a standard deviation of 3 cm.)
     3) If even a part of the emitted beam hits the reinforcing bar, it is considered that the beam has hit the reinforcing bar.   4) The light reflected from the reinforcing bar shall return stochastically to the 3D-LiDAR according to the area hit by the beam. More specifically, if the ratio of the area where the beam has hit to the beam diameter (area where the beam has hit/beam diameter) is equal to or greater than a random number generated between 0.0 to 1.0, the reflected light returns to the 3D-LiDAR.   5) The shape of the reinforcing bar is cylindrical.   6) A simulation is executed  1000  times for each of the four kinds of reinforcing bars which are member to be inspected.   

       FIG. 23  is a graph in which the variances in the lateral direction obtained by executing a simulation are plotted. In the simulation, the distance from the three-dimensional sensor  111  to the reinforcing bar is set to 5 m. In the graph, the horizontal axis represents the diameter of the reinforcing bar [mm], and the vertical axis represents the variance of the reinforcing bar in the lateral direction [m 2 ]. As shown in  FIG. 23 , in each of the four types of reinforcing bars, the variances obtained by executing the simulation a plurality of times vary. The ranges of variations of the variances of D 10 , D 13 , D 16 , and D 19  are different from each other. Therefore, the type of the measured reinforcing bar can be distinguished by determining where the variance of the second point group data in the lateral direction obtained by the measurement of the reinforcing bar by the three-dimensional sensor  111  is included in the ranges of the variations of the variances of the D 10 , D 13 , D 16 , and D 19  obtained by the simulations. 
       FIG. 24  is a graph in which the variances obtained by executing a simulation are plotted. In the simulation, the distance from the three-dimensional sensor  111  to the reinforcing bar is set to 10 m. In the graph, the horizontal axis represents the diameter of the reinforcing bar [mm], and the vertical axis represents the variance of the reinforcing bar in the lateral direction [m 2 ]. As shown in  FIG. 24 , the ranges of the variations of the variances of the second point group data in the lateral direction overlap in a region r 1  in D 10  and D 13 , in a region r 2  in D 13  and D 16 , and in a region r 3  in D 16  and D 19 . That is, the variances of the second point group data in the lateral direction obtained by the measurement by the three-dimensional sensor  111  may be included in any of the regions r 1 , r 2 , and r 3 . Therefore, when the distance from the three-dimensional sensor  111  to the reinforcing bar is 10 m, the reinforcing bar is measured by the three-dimensional sensor  111  a plurality of times. Then, the type of the measured reinforcing bar is distinguished by the variations of the variances of the second point group data in the lateral direction obtained by the measurement. 
     As described above, the member distinguishing apparatus  110  according to this example embodiment automatically distinguishes the type of the member to be inspected using the distribution, in each direction, of the second point group data that is the point group data of the member to be inspected acquired by the three-dimensional sensor  111 . By utilizing the distribution of the second point group data in each direction, the three-dimensional sensor  111  can distinguish the type of the member to be inspected even if the three-dimensional sensor  111  is an inexpensive sensor which cannot measure an accurate dimension of the member to be inspected. The degree of distinction precision of the inexpensive three-dimensional sensor  111  here is on the order of millimeters. Further, by automatically identifying the type of the member to be inspected, for example, when the member to be inspected is a reinforcing bar, the work load required for inspecting the reinforcing bar can be reduced. 
     Note that the members to be inspected to which the member distinguishing apparatus according to the above-described example embodiments can be applied are not limited to reinforcing bars, but include all members having similar shapes and different diameters such as reinforcing bars. 
     In the above example embodiments, the present disclosure has been described as a hardware configuration, but the present disclosure is not limited thereto. It is also possible to implement each processing by causing a CPU (Central Processing Unit) to execute a program. 
     The program for implementing the above processing can be stored and provided to a computer using any type of non-transitory computer readable media. Non-transitory computer readable media include any type of tangible storage media. Examples of non-transitory computer readable media include magnetic storage media (such as floppy disks, magnetic tapes, hard disk drives, etc.), optical magnetic storage media (e.g. magneto-optical disks), CD-ROM (Read Only Memory), CD-R, CD-R/W, and semiconductor memories (such as mask ROM, PROM (Programmable ROM), EPROM (Erasable PROM), flash ROM, RAM (Random Access Memory), etc.). The program may be provided to a computer using any type of transitory computer readable media. Examples of transitory computer readable media include electric signals, optical signals, and electromagnetic waves. Transitory computer readable media can provide the program to a computer via a wired communication line (e.g. electric wires, and optical fibers) or a wireless communication line. 
     Although the present disclosure has been described with reference to the example embodiments, the present disclosure is not limited to the above. The configurations and details of the present disclosure can be modified in various ways that can be understood by those skilled in the art within the scope of the disclosure. 
     REFERENCE SIGNS LIST 
     
         
           10 ,  110  MEMBER DISTINGUISHING APPARATUS 
           11 ,  111  THREE-DIMENSIONAL SENSOR 
           12 ,  112  POINT GROUP DATA ANALYSIS UNIT 
           13 ,  113  MEMBER DISTINGUISHING UNIT 
           113   a  INFORMATION HOLDING UNIT 
           113   b ,  213   b  COMPARISON UNIT 
           213   a  SIMULATION UNIT