JP3377169B2 - Method and apparatus for detecting paint film damage position of buried steel pipe - 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 and an apparatus for non-contact detection on the ground of a coating film damaged portion of a buried coated steel pipe.

[0002]

2. Description of the Related Art Generally, a steel pipe to be buried is coated around its periphery to prevent corrosion. However, if the coating is damaged for some reason, corrosion will progress from the damaged site, and eventually corrosion holes will be formed in the steel pipe. Therefore, in order to maintain the buried steel pipe, it is important to detect the presence or absence of coating film damage and its position at an early stage.

In response to this, various types of coating film damage detection have been conventionally proposed. Of these, the potentiometric method is well known as being excellent in workability and measurement accuracy. This method is shown in FIG. In FIG.
1 is a buried steel pipe, 2 is an AC power supply, 3 is a power amplifier, 4 is a ground electrode, 5 is a damaged portion of the buried steel pipe 1, and 6 is a probe.
Reference numerals a and 6b are wheel electrodes of the probe, 7 is a filter, and 8 is a potential difference detector.

A voltage is applied between the buried steel pipe 1 and the ground electrode 4 from the AC power source 2 through the power amplifier 3 to pass a current. In this state, the probe 6 is moved on the ground surface along the pipe axis direction of the buried steel pipe. The probe 6 is provided with a pair of wheel electrodes 6a and 6b in the front-rear direction thereof, and detects the potential of the ground with which they are in contact. The difference between these potentials is detected by the voltage detector 8 via the filter 7.

When the coating film of the buried steel pipe 1 has a coating film damaged portion 5, a current flows from the coating film damaged portion 5 in accordance with the applied voltage, and a potential gradient is created in the soil in the vicinity. The direction of this potential gradient is reversed at the position directly above the coating film damaged portion 5 as a boundary.
Therefore, when the explorer 6 is run along the pipe axis direction of the buried steel pipe and a graph is made with the running distance as the horizontal axis and the potential difference between the wheel electrodes 6a and 6b as the vertical axis, the true value of the coating film damaged portion 5 can be obtained. Draw an S-shaped curve above. Therefore, by detecting the center position (zero cross point) of this S-shaped curve,
The position of the coating film damaged portion 5 can be detected from the ground without contact.

FIG. 9 is a diagram showing a result of detection of a coating film damaged portion by this method. The target buried steel pipe has a diameter of 10
The PLP conduit has a length of 0 m and a length of 10 m, and is buried at a depth of about 1 m. The size of the coating damage is 5 mm x 4
mm. The space between the wheel electrodes of the spacecraft is 1 m, and the current flowing into the damaged coating film is 10 mA (440 H).
z). The detection of the potential difference was performed by spraying water on asphalt coating.

In FIG. 9, the horizontal axis represents the position of the probe and the vertical axis represents the potential difference between the wheel electrodes of the probe. In FIG. 9, the potential difference is positive in the left half and negative in the right half. This indicates that the direction of the potential gradient is reversed at the left half position and the right half position. Then, as described above, the center of the S-shaped curve in FIG. 9, that is, the position where the potential difference crosses zero can be determined to be directly above the coating film damage position.

However, the known potential difference method is
There were the following problems. That is, since the actual damage to the coating film is detected by running the probe on the asphalt, the ground resistance is large, and the detection signal detected as the potential difference between the wheel electrodes must be weak. On the other hand, a stray current is flowing in the ground, so that the soil has an intrinsic potential. This becomes noise when measuring the potential difference between the wheel electrodes. Further, the induced current due to the commercial frequency is superimposed on the potential difference detection, which also becomes noise.
Although these noises are removed to some extent by the filtering process, since the original signal S / N ratio is small,
It cannot be completely removed. Therefore, it is difficult to detect the damaged portion of the coating film accurately and reliably.

The inventors have used a random signal or a pseudo-random signal as a method for solving these problems,
An invention of a coating film flaw detection method and apparatus using correlation processing as signal processing was invented, and a patent application was filed as Japanese Patent Application No. 9-57102.

According to the present invention, an alternating current is applied to the buried pipe by applying an alternating voltage between the buried pipe and the ground, and the potential difference between the two points located along the pipe axis is determined by the two points. By sequentially detecting while moving, measuring the potential distribution generated on the ground by the AC current, in the method of detecting the coating film damage position of the buried pipe from the change in the potential distribution, the AC voltage applied to the buried steel pipe Using a random signal or a pseudo-random signal for cross-correlation processing between the waveform of the AC voltage applied to the buried steel pipe and the detection waveform of the potential difference between the two points, and the peak value between the two points. The potential distribution is obtained from the representative value of the potential difference, and the coating film damage position of the buried pipe is detected from the change in the obtained potential distribution.

The term "pseudo-random signal" as used herein refers to a signal that has a repeating period for a long period of time and loses randomness, but maintains randomness within the period.

According to this method, when the cross-correlation between the waveform of the applied voltage and the waveform of the detection signal is taken, the shift of the applied voltage and the shift of the detection signal are performed during the calculation of the cross-correlation. The detected signals have the same pattern, and a peak of cross-correlation appears at a synchronized position. The noise having a different pattern from the applied voltage is canceled by taking the cross-correlation and has almost no effect on the peak value. Since the applied voltage is a random signal or a pseudo random signal, noise that is not based on the applied voltage does not have the same pattern as the applied voltage pattern.

Therefore, according to this method, only the detection signal resulting from the applied voltage is detected as the representative value of the potential difference between the wheel electrodes, so that the S / N ratio is improved. The potential distribution is obtained using this representative value, and if the coating film damage position is detected from the change in the potential distribution by the same method as the above-mentioned conventional method, the coating film damage position is compared with the potential difference method. Can be detected accurately.

FIG. 10 is a diagram showing an example of an embodiment of this method. In FIG. 10, 1 is a buried steel pipe, 3 is a power amplifier, 4 is a ground electrode, 5 is a coating film damaged portion of the buried steel pipe 1, 6 is a probe, 6a and 6b are wheel electrodes of the probe, 10
Is an M-sequence signal generator, 11 is an M-sequence reference signal generator, 1
Reference numeral 2 is a correlation processing unit, and 13 is a personal computer arithmetic unit.

In this embodiment, an M-sequence signal is used as the pseudo random signal applied to the buried steel pipe 1.
FIG. 11 shows an example of an M-sequence signal generation circuit. The M-sequence signal can be easily created by a shift register having a feedback circuit as shown in FIG. FIG. 12 shows an example of the signal waveform of the M-sequence signal and its autocorrelation signal waveform. Figure 1
In FIG. 2, the horizontal axis represents time, the vertical axis represents the signal magnitude, and τa is the cycle of the clock given to the shift register that generates the M-sequence signal.

Since the M-sequence signal is a pseudo-random signal having a periodicity and has a period corresponding to the number of bits of the shift register, the peak value shown in FIG. To have. From this, it can be seen that if the cross-correlation with other signals is taken, only the signal whose pattern matches the M-sequence signal has the cross-correlation value having a high peak value. In the above embodiment, this property is utilized to reduce the noise signal.

In FIG. 10, the M-sequence signal from the M-sequence signal generator 10 is amplified by the power amplifier 3 and applied as a voltage between the buried steel pipe 1 and the ground electrode 4. In this state, the probe 6 is moved on the ground surface along the pipe axis direction of the buried steel pipe. The probe 6 is provided with a pair of wheel electrodes 6a and 6b in the front-rear direction thereof, and detects the potential of the ground with which they are in contact.

The difference between these potentials is taken, converted into a digital value by an A / D converter (not shown), and then taken into the personal computer arithmetic unit 13. The personal computer arithmetic unit 13 is provided with the M-sequence reference signal generator 11. This M-sequence reference signal generator 11 is
Although it is electrically independent of the sequence signal generator 10, it is configured to generate an M sequence signal having the same pulse pattern.

The correlation processing unit 12 in the personal computer arithmetic unit 13 receives the reference signal from the M-series reference signal generator 11 and the potential difference signal (detection signal) between the A / D converted wheel electrodes 6a and 6b. Cross-correlation calculation is performed. The cross-correlation calculation result has a peak value with the same period as the reference signal. FIG. 13 shows an example of the cross-correlation calculation result. By detecting this peak value and using it as the representative value of the detection signal, it is possible to detect the potential difference with high accuracy while suppressing the noise signal.

When the coating film of the buried steel pipe 1 has a coating film damaged portion 5, a current flows from the coating film damaged portion 5 in accordance with the applied voltage, and a potential gradient is created in the nearby soil. The direction of this potential gradient is reversed at the position directly above the coating film damaged portion 5 as a boundary.
Therefore, when a graph is made by traveling along the pipe axis direction of the buried steel pipe of the explorer 6 and plotting the traveling distance on the horizontal axis and the representative value of the potential difference between the wheel electrodes 6a, 6b on the vertical axis, Draw an S-shaped curve just above 5. Therefore, by detecting the center position of this S-shaped curve, the position of the coating film damaged portion 5 can be detected from the ground in a non-contact manner.

FIG. 14 shows the cross-correlation peak value (representative value of potential difference) and the moving distance (inspection position) of the spacecraft by the above method.
Shows the relationship with. The target buried steel pipe has a pipe diameter of 100.
A, a PLP conduit having a pipe length of 10 m, which is buried at a depth of about 1 m. Coating damage size is 5mm x 4m
It was m. The space between the wheel electrodes of the spacecraft was 1 m. The inflow current of the buried steel pipe is 1 mA (440 Hz), and although the condition is 1/10 of the condition of the potentiometric method, the representative value of the potential difference shows a clean S-shaped curve. The position of coating film damage can be known from the zero-cross position.

[0022]

As described above, according to the method of the present invention, which uses the random signal or the pseudo-random signal and uses the correlation processing as the signal processing, as compared with the conventional potential difference method. The position of coating film damage can be detected extremely accurately.

However, the method shown as an example in the above embodiment has the following problems. That is, since the pseudo-random signal feeding the buried steel pipe is a rectangular wave and has a DC component, it is not suitable for potential measurement in a system in which the DC component is attenuated. However, in the potential propagation characteristics in the ground, the DC component is significantly attenuated.
Therefore, it causes a large error in the method of the above embodiment based on the potential difference measurement.

FIG. 15 shows the actual applied voltage waveform and the detected voltage waveform between the wheel electrodes in the above embodiment. The target buried steel pipe is a PLP pipe having a pipe diameter of 100 A and a pipe length of 10 m, and is buried at a depth of about 1 m. The coating damage size was 5 mm x 4 mm. The space between the wheel electrodes of the spacecraft was 1 m. The inflow current of the buried steel pipe was 1 mA (440 Hz). The DC component of the applied voltage is exponentially attenuated at the detection voltage with a time constant CR = about 2 ms. Therefore, in the method of the above-described embodiment, the S / N ratio is significantly improved as compared with the conventional potential difference method, but this further prevents the S / N ratio from being further improved.

The present invention has been made in order to solve such a problem, and improves the S / N ratio of the signal as compared with the method shown in the above-mentioned embodiment, and the position of coating film damage can be accurately measured. An object of the present invention is to provide a reliable detection method and an apparatus suitable for carrying out this method.

[0026]

The skeleton of the present invention uses a random signal or a pseudo-random signal as an AC voltage applied to an embedded steel pipe, and performs cross-correlation processing to remove noise and reduce S / N cost. in the improvement to Japanese Patent Application No. 9-57102 invention, by generating a random signal or pseudo random signal by modulating the phase of the carrier
There is .

That is, the first for solving the above problems
Means applies an AC voltage between the buried pipe and the ground to cause an AC current to flow through the buried pipe, and the potential difference between the two points along the pipe axis direction is sequentially moved while moving the two points. Modulating the phase of the carrier wave in the method of measuring the potential distribution generated on the ground by the alternating current by detecting and detecting the coating film damage position of the buried pipe from the change of the potential distribution. Using the random signal or the pseudo-random signal generated by the method as an AC voltage signal applied to the buried steel pipe, a cross-correlation process is performed between the waveform of the AC voltage applied to the buried steel pipe and the detected waveform of the potential difference between the two points. The peak value is used as a representative value of the potential difference between the two points, the potential distribution is obtained from the representative value, and the coating film damage position of the buried pipe is detected from the change in the obtained potential distribution. Coating damage position detecting method 設鋼 tube is (claim 1).

The pseudo-random signal is a signal which has a repeating period for a long period of time and loses randomness, but maintains the randomness within the period.

When the waveform of the applied voltage and the waveform of the detection signal are cross-correlated, the applied voltage and the shifted detection signal are the same in the process of shifting and multiplying the detection signal during the calculation of the cross-correlation. A peak of cross-correlation appears at a synchronized position as a pattern. The noise having a different pattern from the applied voltage is canceled by taking the cross-correlation and has almost no effect on the peak value. Since the applied voltage is a random signal or a pseudo-random signal generated by modulating the phase of a carrier wave, noise that is not based on the applied voltage does not have the same pattern as the applied voltage pattern.

Therefore, according to the first means, only the detection signal resulting from the applied voltage is detected as the representative value of the potential difference between the wheel electrodes, so that the S / N ratio is greatly improved. If the potential distribution is obtained using this representative value and the coating film damage position is detected from the change in the potential distribution by the same method as the conventional method, the coating film damage position can be detected accurately and reliably.

In the first means, the random signal or the pseudo random signal is generated by phase-modulating the carrier wave. Therefore, if a signal containing no DC component or a small DC component is used as the carrier wave, the random signal or the pseudo-random signal applied to the buried steel pipe also becomes a signal containing no DC component or a small DC component. Therefore,
Since the signal attenuation in the ground is small, a detection signal similar to the applied voltage can be obtained.
The / N ratio is improved.

A second means for solving the above problems is
By applying an AC voltage between the buried steel pipe and the ground, an AC current is made to flow through the buried steel pipe, and the potential difference between the two points along the pipe axis direction is detected sequentially while moving the two points. , Measuring the potential distribution generated on the ground by the alternating current, in the method of detecting the coating film damage position of the buried steel pipe from the change in the potential distribution, a pseudo-random signal generated by modulating the phase of the carrier , Between the reference signal of the same pattern as the AC voltage generated independently of the AC voltage applied to the buried steel pipe, which is used as the AC voltage signal applied to the buried steel pipe, and the detection waveform of the potential difference between the two points. Correlation processing is performed, the peak value is used as a representative value of the potential difference between the two points, the potential distribution is obtained from the representative value, and the coating film damage position of the buried pipe is detected from the change in the obtained potential distribution. A coating damage position detecting method buried steel pipe characterized by Rukoto (claim 2).

The difference between the second means and the first means is that
In the second means, the AC voltage signal applied to the buried pipe is specified as a pseudo-random signal, and the reference voltage has the same pattern as the AC voltage generated independently of the AC voltage applied to the buried steel pipe. It is using the signal. In this case, the fact that the patterns are the same means that the repetition frequencies are also the same.

When a pseudo random signal is used for the applied voltage waveform, a signal having the same pattern as the AC voltage generated independently of the AC voltage applied to the buried steel pipe can be used as the reference signal.

That is, since the pseudo-random signal has a repeating period, if the detection signal is shifted, the reference signal and the shifted detection signal can be synchronized with each other by a certain shift amount. Therefore, if the cross-correlation is performed over a finite period corresponding to the cycle, it is guaranteed that a peak appears in the cross-correlation signal at the position of the shift amount that can be synchronized. Therefore, it is not necessary to synchronize the AC voltage with the reference signal.
The energization device and the detection device can be independent, and the device can be simplified.

A third means for solving the above problems is
The first means or the second means, wherein the carrier wave is a sine wave (claim 3).
The sine wave does not include a direct current component, and when a sine wave is used as a carrier, the oscillator circuit becomes simple and economical.

The fourth means for solving the above-mentioned problems is as follows.
It is any one of said 1st means to 3rd means, Comprising: A pseudo random signal is an M series signal (Claim 4). When the M-sequence signal is used as the pseudo-random signal, the signal for modulating the phase of the carrier wave at the time of creating the pseudo-random signal also becomes the M-sequence signal, and this M-sequence signal can be configured only by the shift register and a simple logic circuit. So it is economical.

The fifth means for solving the above-mentioned problems is as follows.
In any one of the first to fourth means, the random signal or the pseudo random signal generated by modulating the phase of the carrier wave multiplies the carrier wave with another random signal or the pseudo random signal. It is characterized by being generated by combining (Claim 5). When phase modulation is performed by multiplying a carrier wave with a random signal or a pseudo-random signal,
The phase modulation can be realized only by the multiplier, which simplifies the circuit configuration and is economical.

A sixth means for solving the above-mentioned problems is any one of the first to fifth means, in which a random signal or a pseudo-random signal generated by modulating the phase of a carrier wave is The carrier wave is generated so as to have two kinds of phase differences of 0 degree and 180 degrees with respect to the carrier wave (claim 6). This method of generating a random signal or a pseudo-random signal in two phases of 0 degree and 180 degrees in phase modulation not only simplifies the configuration of the oscillator, but also simplifies the frequency characteristic, which simplifies signal processing. It is so economical.

The seventh means for solving the above-mentioned problems is as follows.
Any one of the first to sixth means, characterized in that the frequency of the carrier wave is 1 kHz or less (claim 7). Carrier frequency is 1 kHz
If you do the following, it matches the potential propagation characteristics in the ground,
Since the signal attenuation is small, the detection sensitivity is good.

The eighth means for solving the above-mentioned problems is as follows.
A signal generator that generates a random signal or a pseudo-random signal generated by modulating the phase of a carrier wave ,
A power amplifier that generates a voltage proportional to the output of the signal generator and energizes between the buried steel pipe and the ground, and runs on the ground surface along the pipe axis direction of the buried steel pipe to isolate it in the pipe axis direction of the buried steel pipe. Probe having a plurality of wheel electrodes arranged in parallel, a voltage measuring device for measuring a potential difference between the plurality of wheel electrodes, a reference signal taken out from the signal generator and an output waveform of the voltage measuring device. Detecting the coating film damage position of the buried steel pipe, which comprises a correlation processing device that calculates the correlation and outputs the peak value, and means for detecting the coating film damage position of the buried steel pipe from the change in the output of the correlation processing device. It is a device (claim 8).

This device is suitable for realizing the method according to the first means. That is, a random signal or a pseudo-random signal generated by a signal generator is amplified by a power amplifier, and electricity is applied between the buried steel pipe and the ground.
When the coating film is damaged, an electric current flows into the damaged part, and as a result, a potential gradient is generated on the ground surface. This potential gradient is measured by running a probe along the buried steel pipe on the ground and detecting the voltage between the wheel electrodes. Then, the reference signal taken out from the signal generator by the correlation processing device (a signal having the same pattern as the random signal or the pseudo random signal)
And a cross-correlation with the detected voltage between the wheel electrodes are calculated, and the peak value is obtained. Then, the coating film damage position of the buried steel pipe is detected from the change of the peak value which is the output of the correlation processing device.

As the means for detecting the coating film damage position of the buried steel pipe from the change in the output of the correlation processing device, known means can be used without any change from those of the prior art. For example, a method using a circuit that detects a point where the potential difference is zero-crossed, a circuit that detects the midpoint between the point where the positive potential difference shows the maximum value and the point where the negative potential difference shows the maximum value, etc. can be appropriately selected and used. Good.

The ninth means for solving the above problems is as follows.
A first signal generator that generates a pseudo-random signal generated by modulating the phase of a carrier wave , and a power that generates a voltage proportional to the output of the first signal generator and that is applied between the buried steel pipe and the ground. An amplifier, a ground plane running along the pipe axis direction of the buried steel pipe, and a probe having a plurality of wheels arranged separately in the pipe axis direction of the buried steel pipe, and a potential difference between the plurality of wheel electrodes. The cross-correlation between the voltage measuring device to be measured, the second signal generator that outputs the waveform of the same pattern as the first signal generator, the output waveform of the second signal generator, and the output waveform of the voltage measuring device. An apparatus for detecting coating film damage position of an embedded steel pipe comprising a correlation processing device for calculating and outputting the peak value, and means for detecting the coating film damage position of the embedded steel pipe from a change in output of the correlation processing device ( Claim 9).

This device is suitable for realizing the method according to the second means. In this device, the reference signal used in the correlation processing device is not from the first signal generator but from the second signal generator which generates a pseudo-random signal of the same pattern. Only the points differ from the above-mentioned eighth means. As previously mentioned,
When the pseudo random signal is used, the reference signal does not need to be synchronized with the voltage waveform applied between the buried steel pipe and the ground, and if the pattern is the same, the same effect as the eighth means is obtained. Is obtained.

[0046] A tenth means for solving the above-mentioned problems is the eighth means or the ninth means, wherein the carrier wave of the signal generator is a sine wave. 10).

An eleventh means for solving the above-mentioned problems is the eighth means or the tenth means, wherein the pseudo-random signal is an M-sequence signal (claim 11). Is.

A twelfth means for solving the above-mentioned problems is the eighth means or the eleventh means, wherein a random signal or a pseudo-random signal generated by modulating the phase of a carrier wave is a carrier wave. , And is generated by multiplying by another random signal or a pseudo-random signal (claim 12).
Is.

A thirteenth means for solving the above-mentioned problems is the eighth means or the twelfth means, wherein a random signal or a pseudo-random signal generated by modulating the phase of a carrier wave is transmitted to the carrier wave. On the other hand, 0 degrees and 1
It is characterized in that it is generated so as to have two kinds of phase differences of 80 degrees (claim 13).

A fourteenth means for solving the above-mentioned problems is the eighth means or the thirteenth means, characterized in that the frequency of the carrier wave of the signal generator is 1 kHz or less ( Claim 14).

The operation of each of the tenth means to the fourteenth means is the same as the operation of each of the third means to the seventh means, and the description thereof will be omitted.

[0052]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing an example of an embodiment of the present invention. In FIG. 1, 1 is a buried steel pipe, 3 is a power amplifier, 4 is a ground electrode, 5 is a coating film damaged portion of the buried steel pipe 1,
6 is a probe, 6a and 6b are wheel electrodes of the probe, 10 is M
Sequence signal generator, 11 M sequence reference signal generator, 12 correlation processing unit, 13 personal computer arithmetic unit, 1
Reference numeral 4 is a sine wave generator, 15 is a multiplier, 16 is a sine wave generator, and 17 is a multiplier.

In this embodiment, as the signal applied to the buried steel pipe 1, the binary M-sequence signal generated by the sine-wave generator 14 and the M-sequence signal generator 10 synchronized with the sine wave is multiplied. A sine wave signal that is bi-phase modulated into an M-sequence pattern is used by multiplying by 3.

A comparison between the M-sequence signal (referred to as M-sequence original signal) described in the prior art and the sine wave phase modulation type M-sequence signal (referred to as M-sequence modulation signal) used here will be shown below.

The sine wave phase modulation is known as PSK (Phase shift keying) as a method of transmitting information by switching the phase of a carrier wave.
For example, as shown in FIG. 2, the carrier wave (here, a sine wave) is multiplied by an information waveform (here, a baseband waveform) to cause phase modulation of the carrier wave. The PSK waveform generated at that time is shown in FIG. (Yukiji Yamauchi, Denki Univ., Spread Spectrum Communications 1994)

FIG. 3 shows an M-series original signal and an M-series modulated signal. Figure 4 shows the frequency spectrum of both signals. Figure 3
(A) is an M-series original signal, which has 2 levels of H level and L level.
It is a value signal. The H level corresponds to 1 and the L level corresponds to -1. By multiplying this M-series original signal by a sine wave signal which is a carrier wave, an M-series modulated signal as shown in FIG. 3B is obtained. The M-sequence modulated signal is in phase with the carrier when the M-series original signal has a value of 1, and is out of phase with the carrier by π when the M-series original signal has a value of -1. Become a signal.

In the power spectrum of the M-sequence modulated signal, as shown in FIG. 4 (b), the DC and low frequency components, which are the main components of the M-series original signal, are shifted to the high frequency side by the clock frequency, Since the signal is composed of monocycle pulses for each clock, there is no DC component. Therefore, the influence of the attenuation of the direct current component in the ground is eliminated, and the waveform matches the propagation characteristics in the ground. As shown in FIG. 4B, the main component of the frequency of the M-sequence modulated signal is equal to the clock frequency.

FIG. 5 shows the autocorrelation function of the M-series original signal and the M-series modulated signal. The latter has cos in the former autocorrelation function.
It is a function multiplied by (2πfct). In FIG. 5, f
_c is the frequency of the M-sequence signal, and N is the code length for one cycle of the M-sequence signal. (For example, when the number of stages of the shift register that generates the M-sequence signal is n, N = 2 ⁿ −1)

In FIG. 1, the M-sequence signal (M-sequence original signal) generated by the sine-wave generator 14 and the M-sequence signal generator 10 synchronized with the sine wave is multiplied by the multiplier 3 to obtain the M-sequence. The sine wave signal (M-series modulation signal) that has become a pattern and is subjected to two-phase modulation is amplified by the power amplifier 3 and applied as a voltage between the buried steel pipe 1 and the ground electrode 4. In this state, the probe 6 is moved on the ground surface along the pipe axis direction of the buried steel pipe. The probe 6 is provided with a pair of wheel electrodes 6a and 6b in the front-rear direction thereof, and detects the potential of the ground with which they are in contact.

The difference between these potentials is taken, converted into a digital value by an A / D converter (not shown), and then taken into the personal computer arithmetic unit 13. The personal computer arithmetic unit 13 includes an M-sequence reference signal generator 11
Is provided. This M-sequence reference signal generator 11 is
Although electrically independent of the M-sequence signal generator 10, it is configured to generate an M-sequence signal (M-sequence original signal) having the same pulse pattern (same frequency).

The sine wave oscillator 16 is also provided in the personal computer arithmetic unit 13 and oscillates a sine wave having the same frequency as the sine wave oscillator 14 described above. By multiplying the sine wave generator 16 and the M sequence signal generated by the M sequence reference signal generator 11 synchronized with the sine wave by the multiplier 17, a sine wave signal having an M sequence pattern and two-phase modulated is obtained. There is a power amplifier 3 to a buried steel pipe 1
A signal having the same pattern as the voltage applied between the installation electrode 4 and the installation electrode 4 is generated as a reference signal.

The correlation processing unit 12 in the personal computer arithmetic unit 13 receives the A / D signal from the reference signal of the multiplier 17.
Cross-correlation calculation with the converted potential difference signal (detection signal) between the wheel electrodes 6a and 6b is performed. That is, the detection signal is f
(T) and the reference signal is g (t), the cross-correlation calculation result is as follows.

[0063]

[Equation 1]

Here, T is a repetition period. The detection signal f (t) includes an M-sequence signal component, and the reference signal g
Since (t) is an M-sequence signal having the same pattern as this,
The cross-correlation calculation result has a peak value with the same period as the reference signal.

FIG. 6 shows an example of the cross-correlation calculation result.
In FIG. 6, the M-series original signal is used for the voltage and the reference signal applied to the buried steel pipe 1, that is, the invention according to Japanese Patent Application No. 9-57102 shown in FIG. The one according to the invention, that is, the one using the M-sequence modulation signal for the voltage applied to the buried steel pipe 1 and the reference signal is shown in (b). In FIG. 6, the horizontal axis represents τ in equation (1) and the vertical axis represents Φ _fg (τ) in equation (1).
It can be seen from FIG. 6 that the S / N ratio is improved in the case of the present invention using the M-sequence modulated signal as compared with the case using the M-sequence original signal.

By detecting this peak value and using it as the representative value of the detection signal, it is possible to detect the potential difference with high accuracy while suppressing the noise signal. By performing processing using this representative value, the coating film damage position can be detected with high accuracy and reliability.

When the coating film of the buried steel pipe 1 has the coating film damaged portion 5, a current flows from the coating film damaged portion 5 in accordance with the applied voltage, and a potential gradient is created in the nearby soil. The direction of this potential gradient is reversed at the position directly above the coating film damaged portion 5 as a boundary.
Therefore, when a graph is made by traveling along the pipe axis direction of the buried steel pipe of the explorer 6 and plotting the traveling distance on the horizontal axis and the representative value of the potential difference between the wheel electrodes 6a, 6b on the vertical axis, Draw an S-shaped curve just above 5. Therefore, by detecting the center position of this S-shaped curve, the position of the coating film damaged portion 5 can be detected from the ground in a non-contact manner.

This processing method is the same as the prior art,
In the present invention, since the noise component is suppressed in the potential difference detection, highly accurate and highly sensitive detection is possible. A conventionally known circuit may be used to perform this processing method. For example, a method using a circuit that detects a point where the potential difference crosses zero, a method that sets a threshold value for the potential difference, and a method that detects a point that crosses zero after the absolute value of the potential difference exceeds this threshold, and the positive potential difference indicates the maximum value. There is a method using a circuit that detects the midpoint between the point and the point where the negative potential difference shows the maximum value.

FIG. 7 shows the relationship between the peak value of the cross-correlation (representative value of the potential difference) and the moving distance (inspection position) of the spacecraft by the above method, in which the voltage applied to the buried steel pipe 1 and the reference signal are M series original signals. Is used, that is, in comparison with the invention according to Japanese Patent Application No. 9-57102 shown in FIG. In FIG. 7, “original signal” is Japanese Patent Application No. 9-57.
No. 102 according to the present invention, and “modulation signal” indicates one according to the present invention.

The buried steel pipe to be measured had a pipe diameter of 100.
A, a PLP conduit having a pipe length of 10 m, which is buried at a depth of about 1 m. Coating damage size is 5mm x 4m
It was m. The space between the wheel electrodes of the spacecraft was 1 m. The inflow current into the buried steel pipe was 1 mA (440 Hz).

It can be seen from FIG. 7 that in the present invention, the S / N ratio is improved by a factor of 2 as compared with that using the M-sequence original signal. As can be seen from FIGS. 6 and 7, according to the present invention, the S / N ratio of the cross-correlation waveform alone is improved, and the graph showing the relationship between the cross-correlation peak value and the position of the spacecraft is also a clean S-shape. It has a curve, and the coating damage position can be accurately known from the zero-cross position of this S-shaped curve. Therefore, the method of the present invention is described in Japanese Patent Application No. 9-
It can be seen that, compared with the method according to the invention of No. 57102, it has a highly sensitive and highly accurate coating film damage position detection ability.

Especially, where the underground electrical physical constant changes, the method using the M-series original signal deteriorates the measurement accuracy remarkably, but this method considering the propagation characteristics in the ground has stable detection capability. Exert.

In the embodiment described above, the M-sequence signal is created using a personal computer, but it may be taken in from an external generator via an A / D converter or the like. The cross-correlation process may be performed using an electronic circuit instead of the personal computer.

In this embodiment, the M-sequence signal generator 10
The M-sequence reference signal generator 11 and the two sine wave generators 14 and 16 are independent. This allows the device for applying a voltage to the buried steel pipe 1 and the device for detecting and processing the signal to be independent, which is the most preferable, but of course, of the multiplier 3 supplied to the power amplifier 3. The signal can be used as a reference signal as it is or after being delayed. In this case, not only the pseudo random signal but also the random signal can be used as the applied voltage signal and the reference signal.

[0075]

INDUSTRIAL APPLICABILITY In the present invention, a noise component is removed by performing a cross-correlation process using a random signal or a pseudo-random signal generated by modulating the phase of a carrier wave as an AC voltage applied to a buried steel pipe. Remove and S
Since the / N ratio is improved, it is possible to detect the coating film damage position of the buried steel pipe with high sensitivity, accuracy and reliability. Also, because the detection sensitivity has improved, the applied current capacity has decreased,
It is possible to reduce the size of the probe by omitting the water spraying treatment on the inspection surface and reducing the distance between the wheel electrodes, which makes the system inexpensive.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of an embodiment of the present invention.

FIG. 2 is a diagram showing a configuration and output waveform of a PSK transmitter.

FIG. 3 is a diagram showing waveforms of an M-series original signal and an M-series modulated signal.

FIG. 4 is a diagram showing power spectra of an M-series original signal and an M-series modulated signal.

FIG. 5 is a diagram showing an autocorrelation function of an M-sequence original signal and an M-sequence modulated signal.

FIG. 6 is a diagram showing an example of a correlation processing result of an M-series original signal and an M-series modulated signal.

FIG. 7 is a diagram showing an example of a detection result of a coating film damaged portion according to the present invention.

FIG. 8 is a diagram showing an outline of a coating film damage detection method in the potential difference method.

FIG. 9 is a diagram showing an example of a detection result of a coating film damaged portion by a potential difference method.

FIG. 10 is a diagram showing an outline of a coating film damage detection method according to the prior application.

FIG. 11 is a diagram showing an example of an M-sequence signal generation circuit.

FIG. 12 is a diagram showing an example of an M-sequence signal and its autocorrelation signal waveform.

FIG. 13 is a diagram showing an example of a cross-correlation waveform of a reference signal and a detection signal according to the prior invention.

FIG. 14 is a diagram showing an example of a detection result of a damaged coating film according to the invention of the prior application.

FIG. 15 is a diagram showing an example of signal waveforms of an applied voltage and a detected voltage according to the M-sequence method.

[Explanation of symbols]

1: Buried steel pipe 2: AC power supply 3: Power amplifier 4: Ground electrode 5: Damaged part of coating film 6: probe 6a, 6b: Wheel electrodes 7: Filter 8: Potential difference detector 10: M-sequence signal generator 11: M-sequence reference signal generator 12: Correlation processing unit 13: Personal computer 14: Sine wave generator 15: Multiplier 16: Sine wave generator 17: Multiplier

Continuation of the front page (56) References Japanese Unexamined Patent Publication No. 10-239267 (JP, A) Japanese Unexamined Patent Publication No. 2-297089 (JP, A) Japanese Unexamined Patent Publication 63-191049 (JP, A) Akio Nagamune, Koichi Tezuka, M-sequence signal pulse compression method Underground exploration radar, Proceedings of the Society for Automatic Measurement Control, 1994, Vol. 30, No. 10, p. 1151-1157 (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 27/00-27/24 G01N 27/72-27/90 JISST file (JOIS)

Claims (14)

(57) [Claims] 1. An alternating current is applied to the buried steel pipe by applying an alternating voltage between the buried steel pipe and the ground, and the potential difference between the two points located along the pipe axis direction is moved while the two points are moved. By sequentially detecting the potential distribution generated on the ground by the alternating current and detecting the coating film damage position of the buried steel pipe from the change in the potential distribution, it was generated by modulating the phase of the carrier wave . A random signal or a pseudo-random signal is used as an AC voltage signal applied to the buried steel pipe, and the correlation processing is performed between the reference signal extracted from the AC voltage signal applied to the buried steel pipe and the detection waveform of the potential difference between the two points. The peak value is used as the representative value of the potential difference between the two points, the potential distribution is determined from the representative value, and the coating damage position of the buried pipe is detected from the change in the determined potential distribution. A method for detecting the position of coating film damage on a buried steel pipe, which is characterized by being put out. 2. An alternating current is applied to the buried steel pipe by applying an alternating voltage between the buried steel pipe and the ground, and the potential difference between the two points along the pipe axis direction is moved while the two points are moved. By sequentially detecting the potential distribution generated on the ground by the alternating current and detecting the coating film damage position of the buried steel pipe from the change in the potential distribution, it was generated by modulating the phase of the carrier wave . A pseudo random signal is used as an AC voltage signal applied to the buried steel pipe, and a reference signal having the same pattern as the AC voltage generated independently of the AC voltage applied to the buried steel pipe and the potential difference between the two points are detected. Correlation processing with the waveform,
The peak value is used as a representative value of the potential difference between the two points, the potential distribution is obtained from the representative value, and the coating film damage position of the buried pipe is detected from the change in the obtained potential distribution. Method for detecting the position of coating film damage. 3. The method for detecting a coating film damage position of a buried steel pipe according to claim 1, wherein the carrier wave is a sine wave. 4. The pseudo random signal is an M-sequence signal, according to any one of claims 1 to 3.
A method for detecting a coating film damage position of a buried steel pipe according to the item. 5. The random signal or the pseudo-random signal generated by modulating the phase of the carrier wave is generated by multiplying the carrier wave by another random signal or the pseudo-random signal. The coating film damage position detection method for a buried steel pipe according to any one of claims 1 to 4. 6. A random signal or a pseudo-random signal generated by modulating the phase of a carrier wave is generated so as to have two kinds of phase differences of 0 degree and 180 degrees with respect to the carrier wave. The method for detecting a coating film damage position of a buried steel pipe according to any one of claims 1 to 5. 7. The frequency of the carrier wave is 1 kHz or less, and any one of claims 1 to 6 is characterized.
A method for detecting a coating film damage position of a buried steel pipe according to the item. 8. A signal generator that generates a random signal or a pseudo-random signal generated by modulating the phase of a carrier wave , and a voltage that is proportional to the output of the signal generator and that is energized between the buried steel pipe and the ground. Between the power amplifier and the exploration machine having a plurality of wheel electrodes that travel along the pipe axis direction of the buried steel pipe on the ground surface and are separated in the pipe axis direction of the buried steel pipe, and the plurality of wheel electrodes. A voltage measuring device for measuring the potential difference, a correlation processing device for calculating the cross-correlation between the reference signal extracted from the signal generator and the output waveform of the voltage measuring device, and outputting the peak value thereof, and the output of the correlation processing device. And a means for detecting the coating film damage position of the buried steel pipe from the change of the above. 9. A first signal generator for generating a pseudo-random signal generated by modulating the phase of a carrier wave , and a voltage proportional to the output of the first signal generator,
A power amplifier that energizes between the buried steel pipe and the ground, a ground plane running along the pipe axis direction of the buried steel pipe, and a probe having a plurality of wheels arranged separately in the pipe axis direction of the buried steel pipe,
A voltage measuring device for measuring a potential difference between the plurality of wheel electrodes, a second signal generator for outputting a waveform having the same pattern as that of the first signal generator, an output waveform of the second signal generator, and It comprises a correlation processing device for calculating the cross-correlation of the output waveform of the voltage measuring device and outputting its peak value, and means for detecting the coating film damage position of the buried steel pipe from the change in the output of the correlation processing device. A coating film damage position detector for buried steel pipes. 10. The coating film damage position detection device for an embedded steel pipe according to claim 8, wherein the carrier wave of the signal generator is a sine wave. 11. The coating film damage position detecting device for a buried steel pipe according to claim 8, wherein the pseudo-random signal is an M-sequence signal. 12. The random signal or the pseudo random signal generated by modulating the phase of the carrier wave is generated by multiplying the carrier wave by another random signal or a pseudo random signal. The coating film damage position detection device for a buried steel pipe according to any one of claims 8 to 11. 13. A random signal or a pseudo-random signal generated by modulating the phase of a carrier wave is generated so as to have two kinds of phase differences of 0 degree and 180 degrees with respect to the carrier wave. The coating film damage position detection device for a buried steel pipe according to any one of claims 8 to 12. 14. The frequency of the carrier wave of the signal generator is 1k.
It is below Hz, Claims 8 to 13 characterized by the above-mentioned.
The coating film damage position detection device for a buried steel pipe according to any one of the above.