JPH081464B2 - Non-destructive detection device for buried conductors - Google Patents

Description

Detailed Description of the Invention [Industrial applications]

The present invention relates to a non-destructive detection system for buried conductors, and in particular, the position, buried depth (hereinafter referred to as "cover thickness") of dielectrics such as reinforcing bars and various pipes buried in a dielectric medium such as concrete, and the like. The present invention relates to a device suitable for non-destructively detecting thickness. One example of the application of the device of the present invention is a bar arrangement inspection after construction of a reinforced concrete structure.

[Prior art]

Devices are known for detecting the position and size of defects on the surface of a metal or in a shallow interior by using eddy currents.
To explain the principle with reference to FIG. 12, when an alternating excitation current is applied to the inspection probe (hereinafter referred to as probe) 3, an alternating magnetic field is generated in the medium 1, which is a metal,
Eddy currents are induced on the surface of the medium or in a very shallow portion, and minute changes occur in the eddy currents in the nonuniform portion of the medium, that is, the defective portion. The magnetic field caused by this slight change in eddy current is generated by the probe 3
Since the apparent impedance of the probe 3 changes depending on the presence or absence of a defect, the presence or absence of a defect can be inspected nondestructively by detecting the impedance change ΔZ by the eddy current flaw detector 20. Utilizing this principle, there has been proposed a device for nondestructively detecting a reinforcing bar which is a conductor 2 which is arranged or embedded in a medium 1 which is concrete as shown in FIG.

[Problems to be Solved by the Invention]

However, in the conventional non-destructive detection device using eddy current, when actually trying to detect the bar diameter and the bar cover thickness at the same time, the bar thickness of the bar that can be accurately detected is limited to about 60-70 mm. . The measured value curve F-0 by the conventional probe in FIG. 11 shows this limit. If the concrete cover thickness over the reinforcing bar exceeds this limit, the accuracy of detecting the thickness of the reinforcing bar and the measuring accuracy of the reinforcing bar interval are significantly reduced, and it is often impossible to make a practically meaningful measurement. The conventional probe 3 uses, for example, a cylindrical magnetic core 4 as shown in FIG. 4 and a detection coil 6 wound around the magnetic core 4. A probe 3 having such a conventional structure is manufactured as a prototype, and the medium 1
The result of measuring the magnetic field generated inside is shown in FIG. Although the attenuation of the magnetic field in the depth direction D (see FIG. 1) of the medium depends on the magnitude of the current flowing in the detection coil 6, as an example, a typical conventional eddy current flaw detector 20 has this conventional structure. Probe 3
When is connected, an exciting current of 8.3 mA flows through the detection coil 6 at a frequency of 30 kHz, the depth D is 36 mm, and the magnetic flux density is attenuated to 25 mG or less. Also, the attenuation along the surface of medium 1 in the direction of distance H (see FIG. 1) is relatively gentle, and the magnetic flux density is 25 mG.
Ratio of concrete depth D and surface distance H (H / D)
Is 1.39 (= 50/36), which is a magnetic field spreading in the medium surface direction. In such a magnetic field, since the reaching depth of the magnetic field is shallow and the gradient of the magnetic field in the medium is gentle, it is not possible to accurately detect the reinforcing bar deeply embedded in the concrete. It is conceivable to increase the current applied to the probe 3 in order to nondestructively detect the reinforcing bars deeply embedded in the concrete, that is, having a large covering thickness. However, this alone cannot provide accurate detection. Probe 3
This is because the gradient of the magnetic field becomes more gradual as the distance from is increased, making it difficult to separate and identify adjacent reinforcing bars. Therefore, an object of the present invention is to provide a non-destructive detection device for a buried conductor which accurately detects the position and diameter of a rod-shaped conductor buried in a relatively deep position.

With reference to the embodiments shown in FIGS. 1 and 2, a nondestructive detection device for a buried conductor according to the present invention is designed so that a rod-shaped conductor 2 is embedded in a surface of a medium 1. A magnetic core 4 having a rectangular cross-section provided at one end with a medium contact surface 4A to be in contact with, and connected to the other end of the magnetic core 4 and terminated on the same plane as the medium contact surface 4A at a distance from the periphery of the medium contact surface 4A. A probe 3 having a magnetic side path 5 and a detection coil 6 wound around a magnetic core 4; a power supply circuit 13 for supplying an exciting current to the detection coil 6; and a measurement circuit for detecting an apparent impedance change ΔZ of the detection coil 6. 11
And a magnetic bypass 5 on the same plane as the medium contact surface 4A.
Is formed into an endless band having a width required for the required magnetic resistance of the magnetic side path 5, and the distance between the end surface and the long side of the medium contact surface 4A is the distance between the end surface and the short side of the medium contact surface 4A. As described above, the depth and the diameter of the embedded rod-shaped conductor are detected from the absolute value and the declination of the apparent impedance change ΔZ. A power supply circuit 13 for supplying an exciting current is connected to the detection coil 6, and an apparent impedance change ΔZ of the detection coil 6 is measured by the measurement circuit 11, whereby the depth of the rod-shaped conductor 2 and the magnetic core 4 are measured. The diameter (see diameter d in FIG. 7) of the rod-shaped conductor 2 in the short side direction of the rectangular cross section is detected. In the illustrated example, the power supply circuit 13 is connected to the detection coil 6 of the probe 3 via the detector 10, and the measurement circuit 11 is connected to the detector 10.
Be incorporated into. The present invention is not limited to the configuration including this detector 10. Since it is necessary to apply a relatively large high frequency current to the detection coil 6 in order to make the magnetic field reach the deep part of the medium 1, the power supply circuit 13 preferably includes a suitable booster amplifier. An example of the measuring circuit 11 is a bridge circuit. In the bridge circuit of the example of FIG. 2, the impedance of the detection coil 6 is represented by the inductance L1 for simplicity, and the reference coil (not shown) equivalent to the non-operating state of the detection coil 6 is shown.
The impedance of is represented by the inductance L2. A reference coil having an inductance L2 is installed in the magnetic core 4 or another suitable place, and in the illustrated example, the inductance L2 of the reference coil and the inductance L1 of the detection coil 6 are connected in series. Two resistors having the same resistance value are connected in series, and these resistors and the two coils are bridge-connected as shown in FIG. As shown in the figure, if an AC voltage is applied between the connection point of two resistors and the connection point of two coils, and the output is taken out between the connection point of R, L1 and the connection point of R, L2, Both inductance
It is obvious to a person skilled in the art that the output of the bridge circuit becomes zero when L1 and L2 are equal, that is, when the detection device is not operating. Further, it is clear to those skilled in the art that if the inductance of the reference coil is kept unchanged during the operation of the detection device, the output of the bridge circuit corresponds to the change of the inductance L1 of the detection coil 6. In short, the output of the bridge circuit in FIG. 2 corresponds to the change of the inductance L1 of the detection coil 6. In the bridge circuit, when it is necessary to consider the resistance component of the impedance of the detection coil 6 and the electrostatic capacitance (including the electrostatic capacitance of the cable and the electrostatic capacitance between the concrete and the detection coil). It suffices to replace the inductance L2 in the illustrated example with an impedance element that is exactly equivalent to the non-operating state of the probe 3 including resistance and capacitance. In the present invention, the impedance is used as described above instead of the impedance L2, and the impedance change ΔZ of the detection coil 6 is detected as the output of the bridge circuit. Preferably, the frequency and effective value of the exciting current of the detection coil are within the range of 2-100 kHz and 2-1000 mA.

[Action]

6A and 6B show the probe 3 of the device according to the invention.
2A and 2B respectively show a longitudinal section and a transverse section of an embodiment of the magnetic core 4 and the magnetic bypass 4 in FIG. In the figure, the unit of dimensions is mm. However, the detection device of the present invention is not limited to the shapes and dimensions of the magnetic core 4 and the magnetic side path 5 shown.
For simplicity, the detection coil 6 is not shown in FIG. 6B. FIG. 5 shows the magnetic field when a current of 131 mA at a frequency of 30 kHz is applied to the detection coil 6 on the magnetic core 4 of FIGS. 6A and 6B. As is clear from FIG. 5, the depth D at which the magnetic field reaches the medium 1 is significantly improved and the magnetic flux density is reduced to 25 mG.
It has reached 145 mm. This is because the applied current to the detection coil 6 is increased, and it is expected that the detection depth will be improved by about 300% (36-145 mm) compared with the conventional example shown in FIG. Further, in FIG. 5, the ratio (H / D) of the medium depth D at which the magnetic flux density decreases to 25 mG and the surface distance H at which the same decrease occurs is 0.79 (= 115/145). Considering that the value of the corresponding ratio in the conventional example of FIG. 3 is 1.39,
It will be understood that the present invention provides a sharp magnetic field distribution in the depth direction of the medium. This is because the magnetic bypass 5 having a specific shape according to the present invention is used together with the magnetic core 4. Fig. 11 shows the results of actual measurement of the relationship between the detectable rebar spacing and the cover thickness for a reinforcing bar diameter of 25 mm. Curve F-0 is when the probe with the conventional structure shown in Fig. 4 is used. When the cover thickness is 50 mm or more, it is not possible to distinguish the rebars with a rebar spacing of less than 50 mm. It cannot detect the diameter. Here, the bar arrangement interval is an interval between adjacent reinforcing bars, not a pitch between reinforcing bar centers. In the curve F-2 using the probe 3 of FIGS. 6A and 6B according to the present invention, when the cover thickness is 50 mm, adjacent rebars can be distinguished even if the bar arrangement interval is reduced to 40 mm. Even if the cover thickness reaches 160 mm, the diameter of the reinforcing bar can be detected. Measured value curve F- of another embodiment of the present invention for a reinforcing bar having a relatively small cover thickness
According to 1, when the cover thickness is 20 mm, the adjacent reinforcing bars can be distinguished even if the bar arrangement interval is narrowed to 10 mm. Therefore, since the magnetic field in the device of the present invention is (a) the reaching depth of the magnetic field is deep and (b) the gradient of the magnetic field in the medium is strong,
It makes it possible to detect rebar deeply embedded in concrete with high accuracy and high resolution, and significantly improve the separation and identification function of adjacent rebars. FIG. 7 shows a measuring circuit 11 when the probe 3 of the detector according to the present invention is moved to three positions A, B and C along the surface of the concrete which is the dielectric medium 1 in which the conductor 2 which is the reinforcing bar is embedded. The output generated by is shown as the impedance change ΔZ on the display 12 (see FIG. 1). The display includes the absolute value | ΔZ | and the phase angle θ. Since the change in impedance ΔZ at the position B, which is immediately above the conductor 2, is the maximum, the position of the conductor 2 is detected nondestructively, and the cover thickness is also determined by the absolute value | ΔZ |. It can be detected nondestructively. As described with reference to FIG. 5, according to the present invention, since the magnetic field deeply penetrates into the medium 1 and the magnetic field gradient in the medium is strong, the impedance between the positions A and B and the impedance between the positions B and C are increased. The position of the conductor 2 having a large change and a large fogging thickness can be accurately detected. Furthermore, even when a plurality of conductors 2 are present close to each other, the individual conductors can be identified with high resolution due to the strong magnetic field gradient. ,
The phase angle θ of the impedance change of the detection coil is the medium 1
It was experimentally found that it is a function of the cross-sectional area of the rod-shaped conductor in the direction of the magnetic field entering the inside, that is, the diameter of the reinforcing bar or metal tube. In the case of FIG. 7, since the buried conductor is one rebar having the diameter d and the same rebar is measured, the phase angle θ of the impedance change ΔZ even if the position of the probe 3 changes.
Is virtually unchanged. FIG. 8 shows the result of an experiment confirming that the phase angle θ increases as the diameter d becomes smaller in the case of a reinforcing bar. The relationship between the muscle diameter d and the phase angle θ is that the change ΔZ in the impedance is caused by (i) the eddy current induced in the reinforcing bar, and (ii) the magnitude of the eddy current from the probe 3. Due to the intersection of magnetic flux and rebar,
(Iii) It is considered that there is a phase difference between the current of the probe 3 and the reed bar eddy current. Thus, the object of the present invention is to provide a "nondestructive detection device for buried conductors that accurately and with high resolution detects rod-shaped conductors buried at relatively deep positions".

【Example】

In the embodiment shown in FIG. 2, the power supply circuit 13 includes a booster in order to apply a large current to the detection coil 6. Further, the output of the bridge circuit is applied to the synchronous detection circuit via the transformer T. This is to remove the DC component of noise at the output. Further, the synchronous detection circuit is constituted by the synchronous detection X for the in-phase component of the bridge circuit applied voltage and the synchronous detection Y whose phase is shifted by 90 ° from the applied voltage, but the reference phase is selected to be the same phase as the bridge circuit applied voltage. There is no need to select it, and any other phase may be selected as a reference. When it is not necessary to detect the diameter of the conductor 2, the circuit for measuring the phase angle θ can be omitted. In the configuration of the embodiment shown in FIG. 1, the output of the measuring circuit 11 is recorded by the recorder.
The data is recorded by 14, for example, the display graphic of FIG. However, the recorder 14 is not an essential requirement in the present invention. In the present invention, the frequency and effective value of the exciting current for the detection coil 6 are preferably 2-100 kHz and 20-1000 mA.
Select within the range of. If the frequency is 2 KHz or less, the phase information indicating the diameter of the conductor 2 cannot be obtained, and the excitation current 20
At mA or less, induction of eddy current is too weak to detect the above impedance change ΔZ, and at frequency of 100 KHz or more or exciting current of 1000 mA or more, the power supply circuit becomes too large due to increase in inductance of the detection coil, etc. Because it is lost. FIGS. 9 and 10 show the relationship between the output (mV) of the device of the present invention and the concrete cover depth (mm) for the reinforcing bar, and the relationship between the phase angle θ of the output and the reinforcing bar diameter (mm), respectively.

【The invention's effect】

As described in detail above, the non-destructive detection device for an embedded conductor according to the present invention has a rectangular cross-section magnetic core having a medium contact surface provided at one end for contacting a surface of a dielectric medium in which a rod-shaped conductor is embedded, A magnetic bypass connected to the other end of the magnetic core and terminating on the same plane as the medium contact surface at a position away from the magnetic core, a probe having a detection coil wound around the magnetic core, and an exciting current to the detection coil. Since the power supply circuit for supplying; and the measurement circuit for detecting the apparent impedance change ΔZ of the detection coil are used, the following effects are obtained. (B) The conductor embedded in the dielectric medium can be detected nondestructively. (B) The position of a concrete reinforcing bar having a large cover thickness and the cover thickness can be detected from the concrete surface with high accuracy and nondestructively. (C) The diameter of a concrete reinforcing bar having a large covering thickness can be detected with high accuracy as well. (D) For a wide range of cover thickness, the function of separating and identifying adjacent reinforcing bars can be improved. (E) Even when the reinforcing bars are arranged in two layers on walls, floor boards, eaves, etc., by measuring from both sides of the concrete, the covering thickness, position and reinforcing bar diameter of the reinforcing bars in each reinforcing bar layer are highly accurate and non-destructive. Can be detected.

[Brief description of drawings]

FIG. 1 is an explanatory view of an embodiment of the present invention, FIG. 2 is an explanatory view of an electric circuit thereof, FIGS. 3 and 4 are explanatory views of a magnetic field distribution and a magnetic core of a conventional example, and FIG. 6A and 6B are sectional views of the magnetic core and the magnetic bypass according to the present invention, FIG. 7 is an explanatory view of the action, and FIG. 8 is a diagram showing the reinforcing bar diameter and impedance phase angle. Explanatory diagram of the relationship, FIGS. 9 and 10 are operational characteristic diagrams of the device of the present invention, FIG. 11 is a diagram showing the relationship between the detectable cover thickness and the bar arrangement interval, and FIG.
The figure is an explanatory view of the prior art. 1 ... Medium, 2 ... Conductor, 3 ... Probe, 4 ... Magnetic core, 4A ...
Medium contact surface, 5 ... magnetic side path, 6 ... detection coil, 10 ... detector, 11 ... measurement circuit, 12 ... indicator, 13 ... power supply circuit, 14 ... recorder, 20 ... eddy current flaw detector.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshikazu Takei 2-19-1 Tobita-cho, Chofu-shi, Tokyo Kashima Construction Co., Ltd. Technical Research Laboratory (72) Inventor Masahiro Yoshinobu 1-2-2 Moto-Akasaka, Minato-ku, Tokyo No. 7 Kashima Construction Co., Ltd. (72) Inventor Yoshinobu Oda 16 Isogo, Miyata-cho, Kurate-gun, Fukuoka (72) Inventor Noriaki Hayashi 4-11-12 Shimizu, Kitakyushu-ku, Kitakyushu, Fukuoka (72) Invention Urakawa Hajime 10-20, Ishitsubo-cho, Yawatahigashi-ku, Kitakyushu, Fukuoka (56) References: Actual Development Sho 58-60279 (JP, U) Actual Public Sho 59-42709 (JP, Y2)

Claims (2)

[Claims] 1. A magnetic core having a rectangular cross section whose one end has a medium contact surface that comes into contact with the surface of a medium in which a rod-shaped conductor is embedded, and which is connected to the other end of the magnetic core and is on the same plane as the medium contact surface. A probe having a magnetic side path terminated at a distance from the periphery of the medium contact surface, and a detection coil wound around the magnetic core; a power supply circuit supplying an exciting current to the detection coil; and an apparent detection coil of the detection coil. A measuring circuit for detecting an impedance change, wherein the end surface of the magnetic side path on the same plane as the medium contact surface is formed into an endless band having a width required for a required magnetic resistance of the magnetic side path, and The distance between the long side of the medium contact surface and the distance between the end surface and the short side of the medium contact surface is equal to or more than the distance, and the depth and diameter of the embedded rod-shaped conductor are calculated from the absolute value and declination of the apparent impedance change. Detect each Comprising Te buried conductor nondestructive detector. 2. The apparatus according to claim 1, wherein the exciting current of the detection coil has a frequency and an effective value of 2-100 kHz and 20-1.
Nondestructive detection device for buried conductors within the range of 000mA.