JP2771433B2 - Tube Magnetostrictive Stress Measurement Method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring the magnetostrictive stress of a large-diameter tube.

[0002]

2. Description of the Related Art In a magnetostrictive stress measurement method, when a load acts on a ferromagnetic material, anisotropy occurs in the magnetic permeability, and the magnetic permeability in the load direction increases, and conversely, the magnetic permeability in a direction perpendicular to the load direction decreases. Therefore, this method measures the direction and magnitude of the main stress by detecting the difference between the two magnetic permeability with a magnetostrictive sensor. In the prior art, the stress acting on the tube is measured while moving the magnetostrictive sensor in the circumferential direction of the tube. Such measurements indicate that the pipe is of small diameter, and therefore usually has a predominant bending deformation, the circumferential stress of the pipe is zero or negligible, so the output of the magnetostrictive sensor is the axial stress of the pipe. It corresponds to. However, when the pipe has a large diameter, for example, when the outer diameter is 400 mmφ or more,
The stress in the circumferential direction of the pipe cannot be neglected. This is also the case when the tube has a small diameter but the tube is deformed flat. In such a case, the conventional magnetostrictive stress measurement method can only obtain the stress of two axes in the tube axis direction and the tube circumferential direction, so that the stress of each axis cannot be obtained independently. In addition, when there is no location where the stress is known as in the case of a pipe, a conventionally known shear stress difference integration method cannot be applied as a technique for obtaining the biaxial stress. This is because this method is a method in which integration is sequentially repeated from a point where the stress is known, such as a free end, and a biaxial stress at a point where the stress is to be measured is obtained.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for measuring magnetostrictive stress in a pipe, which can accurately measure each stress in the circumferential direction and in the axial direction of the pipe.

[0004]

According to the present invention, an exciting coil is wound around a first core having a pair of magnetic poles at intervals in a direction intersecting at an angle other than 90 degrees with a tube axis direction of a tube to be measured. The excitation coil is excited by AC power, and a detection coil is provided on a second core having a pair of magnetic poles at intervals in a direction perpendicular to a straight line connecting the pair of magnetic poles of the first core. Using a magnetostrictive sensor that is wound and arranged, the magnetostrictive sensor is moved in the circumferential direction of the tube, and the induced electromotive force corresponding to the stress distribution from the detection coil in the circumferential direction is measured. Value is the angle θ about the pipe axis.
Is approximated using the least-squares method with the first cosine waveform corresponding to the following equation, and the coefficients A1, B1, and C1 of the following equation are obtained. F1 (θ) = A1 + B1cos (θ + C1) The deviation from the measured value is approximated by a second cosine waveform having a half cycle of the first cosine waveform to obtain coefficients A2, B2, and C2 of the following equation: f2 (θ) = A2 + B2cos (2θ + C2) coefficient B1 , B2 and the pipe material's Poisson's ratio ν to calculate the pipe axial stress σ1 and the pipe circumferential stress σ2

(Equation 1) This is a method for measuring the magnetostrictive stress of a pipe, wherein

[0005]

According to the present invention, the direction of the pipe to be measured is 9 mm.
An exciting coil that is excited by AC power is wound around a first core having a pair of magnetic poles spaced apart from each other at an angle other than 0 degrees, for example, 45 degrees. A second core having a pair of magnetic poles is provided at intervals in a direction perpendicular to a straight line connecting the pair of magnetic poles of the first core.
A detection coil is wound around the second core. Therefore, the induced electromotive force from the detection coil corresponds to the difference between the stress in the tube axis direction and the stress in the tube circumferential direction. Thus, the induced electromotive force corresponding to the stress distribution over the circumferential direction of the tube is measured by the detection coil. Among the stress measurement values, the first cosine waveform cos θ corresponding to the angle θ around the tube axis is approximated using the least squares method, and a deviation between the approximate value and the measured value is obtained. This deviation is calculated as a second cosine waveform cos2 corresponding to an angle 2θ around the pipe circumference having a half cycle of the first cosine waveform.
Approximate by θ. Since the stresses in the pipe axis direction and the pipe circumferential direction are determined by a predetermined calculation from the amplitude values of the first and second cosine waveforms and the Poisson's ratio ν of the pipe material, the pipes having a large diameter and a small diameter are used. However, it is possible to accurately determine the stress in the pipe axis direction and the pipe circumferential direction in a state where the pipe is deformed flat.

[0006]

FIG. 1 shows a tube 1 to which the measuring method of the present invention is applied.
FIG. 4 is a diagram schematically illustrating a cross section perpendicular to the axis when the shaft is bent. In a state where no stress acts on the pipe 1 and the pipe 1 is not bent,
The tube 1 has a right cylindrical shape, and the center line 2 of the ring is a perfect circle. When an external force acts on the tube 1 to cause the tube 1 to bend, the center line 2 becomes flat as indicated by reference numeral 3 in FIG.
The line of symmetry 6 of this center line 3 is perpendicular to another line of symmetry 7 and is offset by an angle C in the direction of angle θ with respect to a vertical line 5 passing through the horizontal axis 4 of the tube 1. Around center line 3,
The circumferential stress σ2 acting on the tube 1 becomes zero at positions 33 to 35 where the pair of straight lines 31 and 32 symmetrical with respect to the symmetry axis 6 intersect.

FIG. 2 is an overall perspective view of an apparatus for measuring the stress σ of the tube 1. An annular rail 10 is attached to the outer periphery of the tube 1 made of a ferromagnetic material such as steel, and the magnetostrictive sensor 8 is moved along the outer surface of the tube 1 along the rail 10 by driving means 11 including a motor and the like. The rail 10 and the driving unit 11 constitute a moving unit that moves the magnetostrictive sensor 8 along the outer peripheral surface of the tube 1.

FIG. 3 is a perspective view of the magnetostrictive sensor 8, and FIG.
FIG. 2 is a simplified plan view of the magnetostrictive sensor 8. The magnetostrictive sensor 8 has an inverted U-shaped first core 13, and an exciting coil 14 is wound around the first core 13. First core 13
Of the pair of magnetic poles 15 and 16 intersect with each other at an angle α (α = 45 degrees in this embodiment) other than 90 degrees in the direction of the tube axis 4.
Are provided at intervals. For example, a 50 Hz or 60 Hz, 100 V AC power supply 18 is connected to the excitation coil 14 to excite the excitation coil 14.
Further, a second core 19 is provided.
Are formed in an inverted U-shape. A detection coil 20 is wound around the second core 19. A pair of magnetic poles 2 of the second core 19
The reference numerals 1 and 22 are provided at intervals on a straight line 23 perpendicular to a straight line 17 connecting the pair of magnetic poles 15 and 16 of the first core 13. Each centroid of each magnetic pole 15, 16;
Lines 17 and 23 at the vertices of the virtual square
Matches the diagonal of the virtual square. The exciting coil 14 is excited by an AC power supply 18 and the detecting coil 20 is excited.
Is detected by voltage measuring means 24 such as a voltmeter. Assuming that the material of the pipe to be measured has a Poisson's ratio ν, within the range of elastic deformation, as an apparent stress, by bending the pipe in the longitudinal direction, (1) the stress in the pipe axis direction is B1 cos θ (2) The tube circumferential stress corresponding to the Poisson's ratio ν is νB
(3) The stress in the circumferential direction of the tube is B2cos2θ (4) The stress in the tube axial direction corresponding to the Poisson's ratio ν is νB
It is considered that these stresses are generated in a synthesized form.

In the magnetostrictive sensor 8, the magnetic pole 1 of the first core 13 is
5 and 16 are equidistant from the magnetic pole 21 of the second core 19,
Therefore, when magnetostrictive stress .sigma.1 is not generated in the pipe axis 4 direction of the pipe 1 and magnetostrictive stress .sigma.2 is not generated in the pipe circumferential direction, the magnetic permeability .mu. When the AC power supply 14 is excited, the magnetic flux entering the magnetic pole 21 from the magnetic pole 15 is equal to the magnetic flux exiting from the magnetic pole 21 to the magnetic pole 17, and the same holds true for the magnetic pole 22. The induced electromotive force V detected by the voltage measuring means 24 connected to the coil 20 is zero or a very small value. When the magnetostrictive stress σ1 in the pipe axis direction and / or the magnetostrictive stress σ2 in the pipe circumferential direction acts on the pipe 1, the respective magnetic permeability in the pipe axis direction and the pipe circumferential direction of the pipe 1 are different, so
The induced electromotive force V of 0 is a value corresponding to the magnetostrictive stresses σ1 and σ2. Here, the stress σ1 in the pipe axis direction and the stress σ2 in the pipe circumferential direction may be collectively referred to as stress σ.

FIG. 5 is a block diagram showing an electrical configuration of the magnetostrictive sensor 8 shown in FIGS. The output of the voltage measuring means 24 is provided to a processing circuit 25 realized by a microcomputer or the like. The processing circuit 25 controls the driving means 11 and stores the respective measurement results when the angle θ from the vertical line 5 is changed around the tube axis 4 by, for example, every 5 ° by the voltage measuring means 24.
Connected to. The stored contents of the memory 27 can be displayed by a visual display means 28 such as a cathode ray tube or a liquid crystal. As described above, for convenience of measurement, θ is usually measured at each angle of 5 °, so that there are 72 measurement points n.

FIG. 6 is a flowchart for explaining the operation of the processing circuit 25 which is the measuring method of the present invention. From step n1 to step n2, the induced electromotive force V of the detection coil 20 is measured for each angular displacement of the fixed angle while rotating the magnetostrictive sensor 8 in the circumferential direction of the tube 1 by the moving means 12, and in step n3 Store in the memory 27. Thus, the magnetostrictive sensor 8 moves around the entire circumference of the tube 1. The induced electromotive force V of the detection coil 20 stored in the memory 27, and thus the corresponding stress σ, can be displayed by the display means 28 to step n4.

According to the experimental results of the present inventor, the value of each induced electromotive force V when the angle θ displayed by the display means 28 is changed every 5 ° is the measured value F1 of the induced electromotive force V.
Using i (i = 1, 2,..., 72), a distribution waveform shown in FIG. 7 is obtained.

On the other hand, according to the result of measurement using other measurement methods, the pipe axial stress σ1 and the pipe circumferential stress σ2 are
These are represented by equations (1) and (2), respectively.

[0014]

(Equation 1)

Σ1 measured by a magnetostrictive stress measuring device
From the distribution waveform corresponding to the difference between σ1 and σ2, the pipe axial stress σ1 and the pipe circumferential stress σ2 are obtained using equations (1) and (2), respectively. For this reason, in step n5, the least squares approximation is performed on the measured value F1i using the first cosine function represented by Expression (3), and the coefficients A1, B1, and C1 of Expression (3) are obtained.
To determine.

F1 (θ) = A1 + B1cos (θ + C1) (3) When the expression (3) in which A1, B1, and C1 are determined is represented in a graph, the distribution waveform is as shown in FIG.

In step n6, the measured values F1i and f1
The deviation F2i (i
= 1, 2, ... 72).

In step n7, the least square method approximation is performed on the deviation F2i using the second cosine function equation (4), and the equation (3) is obtained.
Are determined.

F2 (θ) = A2 + B2cos (2θ + C2) (4) When the expression (4) in which F2i and A2, B2, C2 are determined is represented in a graph, the distribution waveform shown in FIG. 8 is obtained.

In step n8, the amplitude value B1 of cos θ obtained by the equation (3) and the amplitude c1 obtained by the equation (4)
From the amplitude value B2 of os2θ, the axial stress in the pipe and the stress in the circumferential direction of the pipe are obtained using the equations (1) and (2). Thereafter, the operation is ended in step n9.

The measured value F1i, the deviation F2i, and the equation f1
(Θ) and f2 (θ) are displayed in units of the induced electromotive force V,
Although σ1 and σ2 have a unit of stress, the induced electromotive force V and the stress σ are proportional to each other, so that the induced electromotive force can be converted into stress.

[0022]

As described above, according to the present invention, using the magnetostrictive sensor, the measured value from the detection coil caused by the distribution of the magnetostrictive stress in the tube axis direction and the tube circumferential direction is used to determine the angle around the tube axis. The first cosine waveform cos θ corresponding to θ is approximated using the least squares method, and the deviation between this approximated value and the measured value is calculated by the first
A second cosine waveform cos2θ having a half cycle of the cosine waveform
, And the stress is obtained from the respective amplitudes of the first and second cosine waveforms in accordance with a predetermined arithmetic expression. Therefore, it is possible to quickly, easily and accurately obtain each stress in the pipe axis direction and the pipe circumferential direction. Will be possible. In this way, the present invention is advantageously implemented when the pipe has a large diameter, and when the pipe is deformed flat even with a small diameter.

[Brief description of the drawings]

FIG. 1 is a view for explaining a state when a pipe 1 to which a measuring method of the present invention is applied is deformed.

FIG. 2 is a perspective view showing a state in which a stress is measured while moving in the circumferential direction of the tube 1 using a magnetostrictive sensor 8;

FIG. 3 is a perspective view showing a configuration of a magnetostrictive sensor 8;

FIG. 4 is a simplified plan view showing the configuration of the magnetostrictive sensor 8;

FIG. 5 is a block diagram showing an electrical configuration of the magnetostrictive sensor 8 shown in FIGS.

FIG. 6 is a flowchart for explaining the operation of the processing circuit 25 that is the measurement method of the present invention.

FIG. 7 is a diagram showing a measurement result F1i of the present inventor of the induced electromotive force V obtained from the detection coil 20 of the magnetostrictive sensor 8, and a distribution waveform obtained by approximating the measurement result F1i by f1 (θ) by the least square method.

8 is a deviation F2 between the measured value F1i and the approximate value in FIG.
It is a graph which shows the distribution waveform which carried out the least squares approximation by i and f2 ((theta)).

[Explanation of symbols]

 Reference Signs List 1 tube 4 tube shaft 8 magnetostrictive sensor 10 rail 11 driving means 12 moving means 13 first core 14 excitation coil 15, 16, 21, 22 magnetic pole 19 second core 20 detection coil 24 voltage measuring means 25 processing circuit 27 memory 28 display means

Continuation of the front page (72) Inventor Kuroda ▲ Takashi Tsukasa 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (72) Inventor Yoshiaki Sakai 1-1-2, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan (56) References JP-A-3-176627 (JP, A) JP-A-4-29848 (JP, U) (58) Fields investigated (Int. Cl. ⁶ , DB name) G01L 1/00 G01N 27/80

Claims (1)

(57) [Claims] An exciting coil is wound around a first core having a pair of magnetic poles at intervals in a direction intersecting at an angle other than 90 degrees with a tube axis direction of a tube to be measured. Is excited by AC power, and a detection coil is wound around a second core having a pair of magnetic poles at intervals in a direction perpendicular to a straight line connecting the pair of magnetic poles of the first core. The magnetostrictive sensor is moved in the circumferential direction of the tube, and the induced electromotive force corresponding to the stress distribution from the detection coil in the circumferential direction is measured. The first cosine waveform corresponding to θ is approximated using the least squares method to obtain coefficients A1, B1, and C1 of the following equation. f1 (θ) = A1 + B1cos (θ + C1) The first cosine waveform and the induced electromotive force Of the first cosine waveform Approximate by a second cosine waveform having a period of one minute, the coefficients A2, B2, C2 of the following equation are obtained. F2 (θ) = A2 + B2cos (2θ + C2) The pipe axial stress σ1 and the pipe circumferential stress σ2 are expressed as follows: A method for measuring the magnetostrictive stress of a pipe, wherein