JP2001289616A - Optical fiber displacement sensor and displacement measuring method using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reliably monitor a landslide, land subsidence, flexure of a structure such as a bridge girder by using an optical fiber and precisely finding the quantity of displacement of a position where the optical fiber is fixed. SOLUTION: The optical fiber is laid along 1st and 2nd lines 11 and 12 which extend almost in parallel and a 3rd line 13 which zigzags between them, base points No.1 and No.1 are fixed to a member which is easy to displace, and respective points succeeding the No.2 are so fixed to an object to be monitored that they move together with the object. The extension of the optical fiber at positions forming two sides of a triangle is measured to find the coordinates of the fixed points succeeding to the No.2, and the coordinate values before displacement are subtracted from the coordinate values after the displacement to measure the displacement quantities of the respective fixed points.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber displacement sensor for a displacement monitoring system for measuring the extension strain of an optical fiber and measuring the amount of displacement at a position where the optical fiber is fixed.
The present invention relates to a displacement measuring method using the same.

[0002]

2. Description of the Related Art The inventors of the present invention have already devised several methods for monitoring the state of displacement of the ground or the like by measuring the strain of an optical fiber. One such method is shown in FIG. This is because the optical fibers 1 are laid in a zigzag manner on the monitoring target portion, and the upper vertices are fixed to the anchor pile 2 or the like provided at predetermined intervals, and the lower vertex P of the optical fiber that moves together with the monitoring target portion. The displacement of
A measuring instrument 3 measures the elongation strain generated in the optical fiber between the right and left fixed points.
Measure and calculate. This method has the advantage that multipoint remote monitoring can be performed using a single optical fiber even if there is no power supply at the monitoring point.

[0003]

The method proposed so far predicts or predicts the movement of the monitoring target portion and calculates the displacement amount under certain assumptions. In such a case, there was a problem that the measurement accuracy was reduced.

[0004] The problem will be further described with reference to FIG. As shown in FIG. 10A, in the conventional method, the displacement amount δx is obtained by assuming only the displacement in the x direction at the point P. That is, it is considered that the displacement at the point P due to the extension of the optical fiber 1 occurs in the direction perpendicular to the longitudinal direction of the optical fiber, and the displacement δx is obtained by dividing the extension δl obtained from the measured strain value by the angle θ at the time of installation.

[0005] However, as shown in FIG.
Point is also displaced in the y-direction, if the y-direction displacement has occurred in the situations where increasing elongation of the optical fiber, since the rebate elongation amount δl in laying time of the angle theta _1, displacement δx which is obtained by calculation 'is The apparent displacement amount is smaller than the actually generated P point displacement amount δx, and the displacement is underestimated (in the reverse case, ie, the extension of the optical fiber is greater than the extension amount when the displacement in the y direction is zero). If so, the displacement is overestimated).

An object of the present invention is to improve the reliability of displacement monitoring by eliminating the above problems.

[0007]

In order to solve the above problems, the present invention provides the following optical fiber displacement sensor and displacement measuring method.

The optical fiber sensor includes first and second lines extending substantially in parallel in the same plane, and an optical fiber along a third line zigzag between the lines.
The optical fiber is laid in such a manner that a triangle having a base line and a triangle having a second line base are alternately drawn, and the intersections of the first, second and third lines of the optical fiber are defined as first and second intersections.
A fixed point provided at a predetermined interval on the line is shared and fixed to the monitoring target portion, and a point connected by an optical fiber to the same fixed point and serving as a base point for displacement measurement is defined as a first line and a second point.
One position is provided at each adjacent position on the line, and the two points are fixed to a member that holds the position.

When a plurality of sensors are combined and arranged in such a manner that the plurality of sensors are shifted in phase in the longitudinal direction of the first and second lines so that the third line of each sensor intersects, the monitoring area is subdivided. , Detailed monitoring becomes possible.

When one optical fiber is laid in a state where a predetermined pattern is drawn by making a single stroke between each fixed point, only one optical fiber is used, which is advantageous in terms of system cost. .

According to the displacement measuring method of the present invention, the displacement of each fixed point is measured using the above-described optical fiber displacement sensor of the present invention. Specifically, based on two reference points provided on the first and second lines of the sensor, an optical fiber between the reference points and a first fixed point connected to each reference point by an optical fiber is used. The amount of elongation is determined by measuring the elongation strain generated in the optical fiber, and an arc having a radius from one base point to the first fixed point after displacement, and a radius from the other base point to the first fixed point after displacement. The displacement of the first fixed point is obtained from the coordinate values before and after the displacement, and then the first fixed point and the first fixed point are determined with reference to the first fixed point and one of the base points. Third
From the amount of elongation of the optical fiber between the second fixed point connected through the line and between one base point and the second fixed point, the second
The displacement amount of the fixed point is obtained in the same manner as described above, and thereafter, similarly, of the fixed points alternately arranged on the first and second lines in succession, the two fixed points on the youngest side are used as a reference. To calculate the calculated displacement of the fixed point on the old side, and for each fixed point after the first fixed point, calculate the final displacement by integrating the displacement of the fixed point on the youngest side, which is the reference for measurement. It was.

There are several possible procedures for laying the optical fibers constituting the sensor as described later, but the procedure is not particularly limited. In short, it suffices that the final shape is such that triangles of opposite directions having the first line and the second line as bases are alternately drawn.

The term "fixed point" as used herein refers to a point at which the optical fiber is retained, and moves in accordance with the movement of the monitoring target. Only two base points are substantially fixed, and the position is maintained.

[0014]

According to the optical fiber sensor of the present invention, each of the fixed points on the first and second lines has a triangle at the next fixed point on the line on the opposite side and the next fixed point on the same line at the next line. Because it was made to be connected by the optical fiber that forms the two sides of
The displacement can be measured by the method of the present invention described above.

According to the displacement measuring method, the amount of elongation of two sides of the triangle, that is, the amount of elongation of the optical fiber on the first or second line from one reference point to the fixed point to be measured, and the other. Finding the coordinates of the fixed point after displacement from the amount of elongation of the optical fiber on the third line from the reference point to the fixed point,
Since the displacement amount is calculated from the coordinates before and after the displacement, correct calculation can be performed. However, if the fixed point serving as a reference is displaced, the influence thereof appears and the calculated displacement amount becomes inconsistent. Therefore, for each fixed point after the first fixed point, the displacement of the reference point is integrated to obtain the final displacement. In this case,
The influence of the displacement of the reference fixed point is removed, and the displacement amount of each fixed point can be calculated with high accuracy.

[0016]

1 to 4 show an embodiment of an optical fiber displacement sensor according to the present invention. The optical fiber displacement sensor 10 shown in FIGS. 1 and 2 shows a basic form, and includes a first line 11 and a second line 1 extending substantially in parallel.
2 and the optical fiber 1 is laid along the third line 13 zigzag between the first and second lines, and the intersections of the first, second lines 11, 12 and the third line 13 are shared. A fixed point is fixed to the monitoring target portion, and two base points 14 and 15 which are the origins of the displacement measurement are fixed to members 16 such as anchors.
And the positions of the base points 14 and 15 are held.

The sensors 10 shown in FIGS. 1 and 2 are both laid so that the optical fiber 1 goes around each fixed point in one stroke. The numbers with circles indicate the installation procedure at that time. Thus, the laying procedure is different, but the final shape is the same for both sensors. Note that the reference points 14 and 15 may be fixed to any point that can be regarded as immovable or immovable in the monitoring target portion, and the use of an anchor or the like may be performed as necessary.

The sensor 20 shown in FIG. 3 and FIG.
The two sensors 10 are combined, and the two sensors 10 are arranged such that the phases of the peaks and valleys of the third line are shifted by half a pitch and the third lines intersect in the middle.
The two sensors arranged crosswise are preferably formed of one continuous optical fiber. If you increase the number of measurement points (fixed points) in the same area, you can perform detailed monitoring,
When measuring strain with an optical fiber, the evaluation distance is limited due to the performance of the measuring instrument. This evaluation distance is currently 20km
When measuring the length exceeding the length as a series length, 2 m is the shortest evaluation length. Even if the distance between the fixed points is reduced to less than that, it is meaningless because the distortion between them cannot be measured accurately. The evaluation length can be as short as 10 cm, but at this time, the measurable distance is as short as 500 m. Therefore, the optical fiber is fixed with a distance of 2 m or more when the measurement distance is increased, but the monitoring becomes coarser as the distance between the fixed points increases. The sensors shown in FIGS. 3 and 4 deal with the problem, and are provided with a combination of n sensors having the same structure (n = 2 in the figure, but n is not limited to 2). 1 and 2, the number of measurement points can be increased n times. The symbols with circles in the figure indicate the optical fiber installation procedure for one sensor.

The principle of displacement measurement by the sensor according to the present invention will be described below with reference to FIG.

FIG. 5 (a) assumes a case in which both ends of the sensor 10 of FIG. 1 are fixed to a strong base, and the base point and the fixed point are numbered sequentially from the left end.

Now, one of the two base points serving as the origins of the displacement measurement is No. 1, the other is No. 2, and the fixed points are No. 3, No. 4 from the side closer to the base point. Then the fixed point N
As shown in FIG. 5B, o.2 is at a distance of L1 from the base point of No. 0, and is at a distance of L2 from the base point of No. 1.

Therefore, the coordinates of the fixed point No. 2 are two base points.
It is represented by the intersection of the arcs of the radii L1 and L2 with No. 0 and No. 1 as the centers. The coordinates of the fixed point No. 2 after the displacement are the radius when L1 is δL1 and L2 is δL2 extended ( L1 + δL
1), (L2 + δL2) can be defined as the intersection of the arcs (see FIG. 5C).

Similarly, the coordinates of the fixed point No. 3 can be calculated using one of the base points.
No.1 and radii L3 and L with reference to fixed point No.2 (center)
4, the coordinates of the fixed point No. 3 after the displacement can be defined as the intersections of the arcs of radii (L3 + δL3) and (L4 + δL4). Hereinafter, similarly, the coordinates of the fixed points after No. 4 are obtained, and the displacement amount is calculated from the coordinate values before and after the displacement.

The elongation of δL1, δL2, δL3, δL4, etc. can be calculated as a displacement amount based on the measurement result of the elongation strain of the optical fiber in the corresponding section and the initial section length between the fixed points. By measuring the elongation strain of the optical fiber in each section using the strain measuring device, it is possible to measure the displacement with high accuracy.

The displacement calculation at this time is performed by subtracting the coordinate value before the displacement from the coordinate value after the displacement of each fixed point. In this method, two base points among the reference points of the calculation are used. Excluding points may be displaced from their original positions. Therefore, the displacement of the fixed point on the younger side than the fixed point to be measured is integrated to determine the final displacement of each fixed point. In this case, the influence of the displacement of the fixed point as a reference at this time is eliminated, and the measurement accuracy is improved.

In FIG. 5C, the fixed point No. 2
If L1 becomes shorter than the original length due to the displacement of δ, δL1 becomes a negative value, but if initial tension is applied to the optical fiber between the fixed points, a negative value δL1 can be measured. Displacement measurement can be performed without any problem in the case.

The coordinates (x2, y2) in FIG.
Let δx2 and δy2 be the displacement amounts of the fixed point No. 2 indicated by
Using this as an example, a method of obtaining the displacement will be described in more detail.

A coordinate system and each point are taken as shown in FIG. Here, the coordinates of No. 2 after the movement are (x, y), the distance from the base point No. 0 of coordinates (x0, y0) to the fixed point No. 2 is L1, and the base point No. of coordinates (x1, y1) L1 is the distance from .1 to the fixed point No.2, and the base point No.0 based on the displacement
Assuming that the length change (displacement amount) between No. 1 and δL1 and δL2, the following equation is established.

X = x2 + δx2 (1) y = y2 + δy2 (2) (x2-x0) ² + (y2-y0) ² = L1 ² (3) (x2-x1) ² + ^{(y2-y1) 2 = L2} 2 ··· (4) L1 '= L1 + δL1 ··· (5) L2' = L2 + δL2 ··· (6) (x-x0) 2 + (y-y0) 2 = L1 ' ² ... (7) (x-x1) ² + (y-y1) ² = L2' ² ... (8) Expanding the above formulas (7) and (8), formula (7)-formula By rearranging in the form of (8), a linear expression of x and y is obtained.

[0030] 2 (x1-x0) x +2 (y1-y0) y + x0 2 -x1 2 + y0
²⁻ y1 ² = L1 ′ ² −L2 ′ ^{2 When} this is arranged for y, the following equation (9) is obtained.

Y = (1 / (2 (y1-y0) ｛L1 ′ ² -L2 ′ ² + x1 ² -x0 ² + y1 ² -y0 ² (x1-x0) x)) (9) Substituting into equation (7) and rearranging it as the equation for x yields equation (1
0) is obtained.

Ax ² −Bx + C = 0 (10) where A = 4 ｛(x1-x0) ² + (y1-y0) ² ＢB = 4 ｛(2x0) (y1-y0) ² − 2y0 (x1-x0) (y1-y0) + L (x1-x
0)｝ C = 4 ｛(y1-y0) ² (x0 ² + y0 ² -L1 ' ² ) -4y0 (y1-y0) L + L ² ｝ L = (L1' ² -L1 ² + x1 ² -x0 ^2- y1 ² -y0 ² ) As an analytical solution, x = (B ± SQRT (B ² −4AC) / (2A) (11) From the two solutions, the solution that is closer to the original position coordinates is selected to obtain the answer, and y is calculated again from equation (7) as (y-y0) ² = L1 ' ² − (x-x0) ² → y = y0 ± SQRT (L1 ′ ² − (x−x0) ² ) (12) This is also obtained by selecting a solution closer to the original position from the idea.

From this result, the position after the displacement is obtained by calculation using two points of each point closer to the base point than itself as the origin. Therefore, the displacement amounts δL1 and δL2 can be obtained by subtracting the initial coordinate values from the calculated values of x and y. Further, by using the calculated values of x and y as values for calculating the moving amount of the next fixed point (values of new reference points), new positions of the fixed points can be sequentially obtained.

By sequentially repeating this with respect to the position obtained by calculation for another section, the final displacement amounts δxn and δyn of each point away from the base point can be obtained.

When the result of equation (11) is obtained, if the solution is indeterminate, y is solved first, and x is calculated later.

Specifically, the coordinates of the base point and each fixed point are measured and determined in advance at the time when the optical fiber is laid.
1, L2 can be obtained by measuring the distance between the fixed points when the optical fiber is laid, or by directly measuring the length.

Since L1 'and L2' are obtained as the output of a device for measuring the distortion of the optical fiber laid between the fixed points, these values are used.

The advantage of this method is that the maximum displacement of the object to be monitored can be calculated without having to obtain the coordinates of each fixed point so precisely. If the section length between each fixed point can be measured, the initial coordinates can be obtained from the intersection point of the optical fiber, but even if it deviates slightly from the true value, the initial position is different because the initial position is different. Although the position becomes worse depending on its accuracy, the displacement itself can be measured with high accuracy.

The above-described displacement sensor and displacement measuring method of the present invention can be suitably used for monitoring a landslide or the like in which a skid occurs, monitoring for subsidence or a deflection of a bridge girder or the like in which a vertical displacement occurs. FIGS. 6 to 8 show examples of application to such an application.

In the landslide monitoring shown in FIG. 6, since the slope to be monitored is shifted obliquely in the horizontal direction, the slope is considered as a plane, and the first to third optical fiber lines, the base point, and each fixed point are located within the plane. Install a sensor to measure the slope displacement.

In the monitoring of the land subsidence shown in FIG. 7, the anchor pile 4 is driven into rock or the like where displacement is unlikely to occur, and the base point 1 of the sensor is set.
4 and 15 are fixed to the pile 4. In addition, a fixed point other than the base point is fixed to two points in the length direction of the pile 5 (which moves together with the ground) driven into the soft ground causing settlement, and the optical fiber, the base point, and each fixed point are perpendicular to each other. The sensor 10 is installed so as to be placed inside.

Further, in the deflection monitoring of the bridge girder shown in FIG. 8, the base points 14 and 15 may be set on the column 6 and the fixed points may be fixed to the side surfaces of the bridge girder 7.

[0043]

As described above, according to the optical fiber displacement sensor of the present invention and the displacement measuring method using the same,
An optical fiber is laid so that a triangle having the first line as the base and a triangle having the second line as the base are alternately drawn, and the coordinates after displacement of each fixed point are obtained from the extension of the two sides of each triangle. Since the displacement is calculated from the preceding and following coordinate values, and for the fixed point where the displacement of the reference point is considered, the final displacement is calculated by integrating the displacement of the fixed point on the younger side. The displacement can be measured with high accuracy, and the reliability of the displacement monitoring system can be increased to eliminate delays in avoiding danger due to underestimation of the displacement and wasteful measures due to overestimation.

[Brief description of the drawings]

FIG. 1 is a diagram showing a basic form of an optical fiber displacement sensor according to the present invention.

FIG. 2 is a diagram showing an example in which a procedure for laying an optical fiber is changed.

FIG. 3 is a diagram showing an example in which a plurality of sensors are arranged in a crossed manner.

FIG. 4 is a diagram showing an example in which a plurality of sensors are arranged in a crossed manner.

5A is a model diagram of a sensor for explaining the principle of measurement. FIG. 5B is a diagram illustrating a method of obtaining coordinates before displacement of fixed point No. 2 (c) After displacement of fixed point No. 2 Illustration of how to find coordinates

FIG. 6A is a cross-sectional view of a landslide monitoring unit using the sensor of the present invention. FIG.

FIG. 7 is a perspective view of a ground subsidence monitoring unit using the sensor of the present invention.

FIG. 8 is a side view of a deflection monitoring unit of a bridge girder using the sensor of the present invention.

FIG. 9 is a diagram showing an outline of a conventional displacement monitoring optical fiber sensor.

10A is a diagram showing how to calculate displacement in the X direction by a conventional sensor. FIG. 10B is an explanatory diagram of a problem when displacement occurs in two axial directions.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical fiber 3 Strain measuring device 4 Anchor pile 5 Pile 6 Prop 7 Bridge girder 10 Optical fiber displacement sensor 11 1st line 12 2nd line 13 3rd line 14, 15 Base point

Claims (4)

[Claims] A first and a second extending substantially in parallel in the same plane;
The third line zigzag between lines and between those lines
An optical fiber is laid along the line so that a triangle having the first line at the bottom and a triangle having the second line at the bottom are alternately drawn, and the first, second, and third lines of the optical fiber are arranged. The intersections are fixed to the monitoring target by sharing fixed points provided at predetermined intervals on the first and second lines, and are connected to the same fixed point by an optical fiber to be a base point for displacement measurement. An optical fiber displacement sensor in which one point is provided at each of adjacent positions on the first line and the second line, and the two points are fixed to a member that holds a position. 2. A light in which a plurality of sensors according to claim 1 are combined, and the plurality of sensors are arranged so as to be shifted in phase in the longitudinal direction of the first and second lines so that a third line of each sensor intersects. Fiber displacement sensor. 3. The optical fiber sensor according to claim 1, wherein one optical fiber is laid in a state of drawing a predetermined pattern by making a single stroke between each fixed point. 4. The optical fiber displacement sensor according to claim 1, wherein said optical fiber displacement sensor is provided on said first and second lines.
The amount of elongation of the optical fiber between those base points and the first fixed point connected to each base point by an optical fiber is determined by measuring the elongation strain generated in the optical fiber. And the coordinates of the intersection of an arc having a radius from the base point to the first fixed point after the displacement and an arc having a radius from the other base point to the first fixed point after the displacement are obtained, and the first fixed point is obtained from the coordinate values before and after the displacement. Of the first fixed point and the second fixed point connected to the first fixed point via the third line with respect to the first fixed point and one of the base points. From the amount of expansion of the optical fiber between the base point and the second fixed point, the amount of displacement of the second fixed point is obtained in the same manner as described above, and thereafter, in the same manner, the first and second lines are sequentially and alternately changed. Of the arranged fixed points, the calculated displacement of the fixed point on the old side is calculated based on the two fixed points on the young side. For each fixed point of the first and subsequent fixing point, the displacement measuring method to obtain the final displacement amount by integrating the amount of displacement of the fixing points of the young turn side becomes the reference measurement.