JP2001013111A - Paint film damaged position detecting method and device of buried coated steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a paint film damaged position detecting method and a device of a buried coated steel, capable of stable and high-sensitive signal detection by suppressing the influence of a frequency low band limit at the time of signal propagation. SOLUTION: A signal of an M-series signal generator 10 is amplified by a power amplifier 3 and applied on the interval between a buried steel 1 and an installed electrode. A survey instrument 6 is run on the ground surface along the longitudinal direction of the buried steel 1, and a potential difference between wheel electrodes 6a, 6b is detected. A reference signal having the same pattern as a signal to which a frequency band limit equal to that which the signal of the M-series signal generator 10 receives on its propagation route is given is outputted from a reference signal generator 11'. Mutual correlation between the potential difference between the wheel electrodes 6a, 6b and the reference signal is operated by a correlation processing part, and its peak value is adopted as a representative value of the potential difference, and operation processing is executed by using the representative value. Hereby, an S/N ratio is greatly improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting a damage position of a coating film of a coated steel pipe buried underground in a non-contact manner from the surface of the ground.

[0002]

2. Description of the Related Art Generally, a steel pipe to be buried is provided with a coating to prevent corrosion. However, if the coating is damaged for some reason, corrosion proceeds from the damaged portion, and eventually a corrosion hole is formed in the steel pipe. For this reason, it is important to detect the presence and location of paint film damage at an early stage in the maintenance of buried steel pipes. In order to respond to this, various types of coating film damage detection have been conventionally proposed. Among these, the potential difference method is known as being superior in terms of workability and measurement accuracy.

In general, in the potential method, a low frequency sine wave is used as a current flowing through a steel pipe, and a potential difference between two electrodes installed on the ground surface is detected. By moving the position of the electrode on the ground surface along the buried steel pipe, the change in the potential difference caused by the outflow current from the damage is measured, and the damage position is specified. In actual measurement, the current flowing into the ground from damage is very small, and the potential difference signal detected on the ground surface is also weak.
Furthermore, fluctuations in the detection signal due to fluctuations in the ground resistance between the ground surface and the electrodes, stray currents underground, and induced currents induced in the steel pipe by the commercial power supply become noise sources and deteriorate the SN ratio. Accurate damage of lining, especially minute damage,
It was difficult to reliably detect it.

[0004] As a method of solving such a problem,
Japanese Patent Application Laid-Open No. 10-239267 discloses that the present invention uses a pseudo-random signal as a signal to be supplied to and applied to a buried coated steel pipe, and a potential difference signal detected by an electrode on the ground surface. A method has been proposed in which the supplied and applied pseudo-random signal is correlated with the same signal, and the peak value resulting from the correlation processing is used as a detected potential difference signal to improve the SN ratio.

According to the present invention, an alternating current is applied to the buried pipe by applying an AC voltage between the buried pipe and the ground, and a potential difference between two points located along the pipe axis direction is determined. By sequentially detecting while moving, the potential distribution generated on the ground by the AC current is measured, and in the method of detecting the coating 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, a cross-correlation process is performed 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 is calculated between the two points. The potential distribution is determined from the representative value as a representative value of the potential difference, and the coating damage position of the buried pipe is detected from the change in the determined potential distribution.

Here, the pseudo-random signal refers to a signal which has a repetitive cycle and loses randomness over a long period of time, but maintains randomness within the cycle.

According to this method, when the cross-correlation between the waveform of the applied voltage and the waveform of the detection signal is obtained, during the calculation of the cross-correlation, for example, in the process of shifting and multiplying the detection signal, The cross-correlation peak appears at a position where the detected signals have the same pattern and are synchronized. Noise having a different pattern from the applied voltage is canceled by cross-correlation, and hardly affects 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 pattern of the applied voltage.

Therefore, according to this method, only the detection signal resulting from the applied voltage is detected as a 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 paint film damage position is detected from the change in the potential distribution by the same method as the conventional method, the paint film damage position can be compared with the potential difference method. It can be detected with high accuracy.

FIG. 6 is a diagram showing an example of an embodiment of this method. In FIG. 6, 1 is a buried steel pipe, 3 is a power amplifier, 4 is a ground electrode, 5 is a damaged portion of the coating of the buried steel pipe 1,
6 is a spacecraft, 6a and 6b are wheel electrodes of the spacecraft, 10 is M
A sequence signal generator, 11 is an M-sequence reference signal generator, 12 is a correlation processing unit, and 13 is a personal computer operation unit.

In this embodiment, an M-sequence signal is used as a pseudo-random signal applied to the buried steel pipe 1.
FIG. 7 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. 8 shows an example of the signal waveform of the M-sequence signal and its autocorrelation signal waveform. FIG.
, The horizontal axis is time, the vertical axis is signal magnitude, and τa is M
This is the cycle of the clock supplied to the shift register that generates the series signal.

The M-sequence signal is a pseudorandom signal having periodicity and has a period corresponding to the number of bits of the shift register. Therefore, when the autocorrelation is obtained, a peak value as shown in FIG. To have. From this, it can be seen that if a cross-correlation with another signal is obtained, only a signal whose pattern matches the M-sequence signal has a cross-correlation value having a high peak value. In the above embodiment, this property is used to reduce the noise signal.

In FIG. 6, an M-sequence signal from an M-sequence signal generator 10 is amplified by a power amplifier 3 and applied as a voltage between a buried steel pipe 1 and a 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 spacecraft 6 is provided with a pair of wheel electrodes 6a and 6b in the front-rear direction, and detects the potential of the ground with which each is in contact.

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

The correlation processing section 12 in the personal computer calculation section 13 includes a reference signal from the M-sequence reference signal generator 11, an A / D converted potential difference signal (detection signal) between the wheel electrodes 6a and 6b, and Is performed. The cross-correlation calculation result has a peak value in the same cycle as the reference signal. FIG. 9 shows an example of the cross-correlation calculation result. By detecting this peak value and using it as a representative value of the detection signal, it is possible to detect a potential difference with high accuracy while suppressing a noise signal.

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

FIG. 10 shows the cross-correlation peak value (representative value of the potential difference) and the moving distance (inspection position) of the probe according to the above method.
The relationship is shown below. The target buried steel pipe has a pipe diameter of 100
A, PLP conduit having a length of 10 m, which is buried at a depth of about 1 m. The coating damage size is 5mm x 4m
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 despite the extremely small amount, the representative value of the potential difference exhibits a clean S-shaped curve. You can know the position.

[0017]

A pseudo-random signal is used as a signal to be supplied to and applied to a steel pipe, and the same signal as the supplied and applied pseudo-random signal is supplied to a potential difference signal detected by an electrode on the ground surface. In a method of performing a correlation process with a signal and using a peak value of the result of the correlation process as a detected potential difference signal, a signal detected by an electrode installed on the ground surface is captured as detection signal data by a signal processing device and stored in advance. A method for performing correlation processing with reference signal data is described. Here, assuming that the detection signal is f (t) and the reference signal is g (t), the correlation processing result Φ (τ) is expressed by the following equation.

[0018]

(Equation 1) Here, T is the period of the pseudo random signal, and Φ (τ)
Is reset and repeated after this period. In the case where f (t) and g (t) are the same pseudo random signal, the correlation processing result is an autocorrelation function of the pseudo random signal, shows a periodic peak, and its period is equal to the period T of the pseudo random signal. .

However, in the actual coating film damage detection, the signal applied to the steel pipe propagates through the buried steel pipe and flows out of the steel pipe into the ground until it is detected by the electrode installed on the ground surface. In the process of propagating in the ground, the frequency band of the signal is limited. In particular, since the low-frequency band component is limited, the detection signal detected on the ground surface has a limited frequency band of the pseudo-random signal, causing waveform distortion. For this reason, the waveform of the pulse signal obtained as a result of the correlation processing between the detection signal and the reference signal is also distorted,
There is a problem that it is difficult to detect a change in the peak intensity and the signal phase of the signal waveform corresponding to the detected potential difference.

Further, when the condition of the place where the coating film damage detection is performed in the actual measurement changes, and the characteristic of the frequency band limitation at the time of signal propagation changes, the influence of the propagation waveform and the waveform distortion of the correlation processing result change. However, there is also a problem that the detection and discrimination of changes in the peak intensity and signal phase of the detected potential difference signal cannot be compared under the same conditions.

The present invention has been made to solve the above-mentioned problems, and suppresses the influence of low frequency band limitation at the time of signal propagation, thereby enabling stable and highly sensitive signal detection. It is an object of the present invention to provide a method and apparatus for detecting a damaged position of a coating film.

[0022]

A first means for solving the above-mentioned problem is that a pseudo-random signal or a pseudo-random signal is applied as an AC voltage between a coated steel pipe buried underground and the ground. Applying the signal modulated in the above, the current flows in the steel pipe, detects the potential difference between two points on the ground surface located along the pipe axis direction, performs correlation processing between the detected signal and the reference signal, This correlation process is sequentially performed while moving the positions of the two points to measure a change in the potential difference on the ground surface, and a method of detecting the position of the coating damage of the buried coated steel pipe from the change pattern of the potential difference. As a reference signal in the correlation processing, a signal having the same waveform as the signal applied to the buried steel pipe and a signal obtained by subjecting the signal applied to the buried steel pipe to the same frequency band limitation as that received on the propagation path is used. Specially Be a coating damage position detecting method buried coating-covering steel pipes, wherein (claim 1).

In this means, the reference signal for correlation with the detection signal is a signal having the same waveform as the signal applied to the buried steel pipe, and a frequency band equivalent to that received by the signal applied to the buried steel pipe in its propagation path. Since the restricted signal is used, the reference signal is a signal having the same signal waveform distortion as the detection signal. Therefore, the difference between the waveform of the detection signal and the waveform of the correlation signal is reduced, the distortion of the pulse waveform in the correlation calculation result is suppressed, and the signal peak value can be detected and the signal phase can be stably determined.

A second means for solving the above-mentioned problem is as follows.
The first means, wherein a signal between electrodes installed on the ground surface detected at a certain point in time is stored and used as a reference signal in the correlation processing (claim 2). It is.

In this means, the reference waveform is a signal propagated through a buried steel pipe, which is an actual signal propagation path, and through the ground, and a signal applied to the buried steel pipe is converted by the frequency characteristic of the actual propagation path. Since the signal is frequency band limited, the difference between the reference waveform and the detected waveform is smaller,
The distortion of the pulse waveform in the correlation operation result is suppressed. Therefore, it is possible to more stably detect the signal peak value and determine the signal phase.

A third means for solving the above-mentioned problem is:
In the first method, a signal obtained by performing a predetermined frequency band limitation on a signal between electrodes installed on the ground surface, which is detected at a certain time, is stored and used as a reference signal in the correlation processing. (Claim 3).

In this means, as in the second means, a signal having the same waveform as the signal applied to the buried steel pipe and a signal applied to the buried steel pipe are transmitted along the propagation path as a reference signal for correlation with the detection signal. A signal subjected to the same frequency band limitation as that received will be used.
Used as the actual reference signal. Therefore, even if the waveform changes due to a change in the condition of the signal propagation path, the predetermined frequency band is further limited, so that the waveform of the correlation calculation result is almost always the same. Therefore, it is possible to more stably detect the signal peak value and determine the signal phase.

A fourth means for solving the above-mentioned problem is:
A signal generator that generates a pseudo-random signal or a signal modulated by the pseudo-random signal, a power amplifier that generates a voltage proportional to the output of the signal generator, and that is energized between the buried steel pipe and the ground, A probe that travels along the pipe axis direction of the buried steel pipe and has a plurality of wheel electrodes arranged separately in the pipe axis direction of the buried steel pipe, and a voltage measuring device that measures a potential difference between the plurality of wheel electrodes. And the pseudo-random signal;
Or a reference signal having the same waveform as the signal modulated by the pseudo-random signal, and a cross-correlation between the output waveform of the voltmeter and a correlation processing device that outputs a peak value thereof, and an output of the correlation processing device. Means for detecting a paint film damaged position of the embedded steel pipe from the change, wherein the correlation processing device is the pseudo-random signal, or pseudo-random A shift register for storing an output waveform of the voltage measuring device corresponding to one cycle of a signal modulated by a signal, a shift register for storing a reference signal for one cycle, and corresponding components of these two shift registers An adder that calculates the product of the two and adds the products calculated for all the components of the shift register, and the correspondence between the information in these two shift registers,
A control device for sequentially shifting the components by one component, and means for calculating the maximum value of the output of the adder while the correspondence between the information in the two shift registers is shifted by one order. A coating film damage position detecting apparatus for a buried coated steel pipe (claim 4).

In this means, as means for calculating the correlation between the detection signal and the reference signal, one cycle of the detection signal and one
A shift register that stores reference signals for the period is used, a product-sum operation is performed on the corresponding components of these shift registers, and the contents of one of the shift registers are cyclically shifted by one component at a time. The maximum value of the output of the adder is obtained while the correspondence of the information in the two shift registers is shifted by one order. If the product-sum operation is performed by sequentially shifting the correspondence between the information in the two shift registers to one, the correlation between the detection signal and the reference signal can be calculated. In this means, since the pseudo-random signal, or a signal having the same waveform as the signal modulated by the pseudo-random signal is used as the reference signal,
The same operation and effect as those of the device described in JP-A-10-239267 can be obtained.

Note that the shift register and the product-sum operation device are
Although it can be realized by software such as a microcomputer, such a device is also included in the scope of the present invention. The same effect can be obtained by storing a signal for n cycles (n is an integer) instead of storing the detection signal and the reference signal for one cycle. It goes without saying that it is the equivalent of

A fifth means for solving the above problem is as follows.
In the fourth, in place of the pseudorandom signal, or the reference signal having the same waveform as the signal modulated with the pseudorandom signal, instead of the pseudorandom signal, or the signal having the waveform modulated with the pseudorandom signal, applied to the buried steel pipe A signal in which a signal is subjected to the same frequency band limitation as that received on the propagation path is used as a reference signal (claim 5).

In the present apparatus, a pseudo-random signal or a signal modulated by the pseudo-random signal is subjected to a signal obtained by subjecting a signal applied to a buried steel pipe to the same frequency band limitation as that received on the propagation path. Since the reference signal is used, the difference between the waveform of the detection signal and the waveform of the correlation signal is reduced, as in the first means, and the distortion of the pulse waveform in the correlation calculation result is suppressed. Therefore, it is possible to stably detect the signal peak value and determine the signal phase.

A sixth means for solving the above-mentioned problem is as follows.
In the fourth means, in place of the reference signal having the same waveform as the pseudo-random signal or the signal modulated by the pseudo-random signal, the voltage of the voltmeter detected at a certain point in time is used as a reference signal. This is a feature (claim 6).

In this means, the difference between the reference waveform and the detected waveform is smaller, as in the second means, since the voltage of the voltage measuring device detected at a certain point in time is used as the reference signal. The distortion of the pulse waveform in the correlation operation result is suppressed. Therefore, it is possible to more stably detect the signal peak value and determine the signal phase.

A seventh means for solving the above-mentioned problem is as follows.
A predetermined frequency band for the voltage of the voltmeter detected at a certain point in time, instead of the pseudorandom signal or the reference signal having the same waveform as the signal modulated by the pseudorandom signal in the fourth means. The restricted signal is used as a reference signal (claim 7).

In the present means, a signal detected at a certain time and subjected to a predetermined frequency band limitation on the voltage of the voltage measuring device is used as a reference signal. Therefore, similarly to the third means, even if the waveform changes due to a change in the condition of the signal propagation path, the predetermined frequency band is further limited, so that the waveform of the correlation operation result is almost always the same. Therefore, the signal peak value can be detected more stably,
The signal phase can be determined.

[0037]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing an example of a coating film damage position detecting device for a buried coated steel pipe according to 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 damaged portion of the coating 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. A generator, 11 'is a reference signal generator, 12 is a correlation processing unit, and 13 is a personal computer operation unit.

In this embodiment, an M-sequence signal is used as a pseudo-random signal applied to the buried steel pipe 1.
In FIG. 1, an M-sequence signal from an M-sequence signal generator 10 is amplified by a power amplifier 3 and applied as a voltage between a buried steel pipe 1 and a ground electrode 4. In this state, spacecraft 6
Is moved on the ground surface along the pipe axis direction of the buried steel pipe. The probe 6 has a pair of wheel electrodes 6 in the front-rear direction.
a and 6b are provided to detect 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 taken into the personal computer operation unit 13. The personal computer operation unit 13 is provided with a reference signal generator 11 '. The detailed configuration of the reference signal generator 11 will be described later in detail.

The correlation processing unit 12 in the personal computer operation unit 13 converts the reference signal from the reference signal generator 11 'and the potential difference signal (detection signal) between the A / D converted wheel electrodes 6a and 6b. Perform cross-correlation calculations. The cross-correlation calculation result has a peak value in the same cycle as the reference signal.

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

As described above, this embodiment is
6 is different from the prior art shown in FIG. 6 only in that the M-sequence reference signal generator 11 shown in FIG. 6 is replaced with a reference signal generator 11 '. Therefore, the difference will be described in detail below.

FIG. 2 shows an outline of the reference signal generator 11 ', the correlation processing unit 12, and the devices associated therewith. 2, reference numeral 21 denotes an AD converter, reference numerals 22 and 23 denote shift registers, reference numeral 24 denotes a multiplier, reference numeral 25 denotes an adder, reference numeral 26 denotes a signal discriminating device, and reference numeral 27 denotes a shift control device.

In this embodiment, a pseudo-random signal generated by the M-sequence signal generator 10 is applied between the terminal connected to the buried coated steel pipe 1 and the ground electrode 4 to bury the coated tubing. Electric current flows into the steel pipe. As a pseudo-random signal generator, a shift register having a feedback loop is used.
Generate a sequence signal. At this time, the period of the M-sequence signal is 1 /
(440 * 127) (seconds). The frequency and code length of the M-sequence signal to be used can be set to arbitrary values. By changing the frequency and code length, the SN improvement effect by the pseudo-random signal processing can be enhanced.

The potential difference signal detected by the probe 6 is
Before being input to the personal computer 13, it is input to the AD converter 21, converted into a digital signal, and input to the shift register 22 of the correlation processing unit 12 which is a part of the personal computer 13. Shift register 22
Operates in synchronization with the conversion cycle of the AD converter 21, inputs the latest digital data obtained by AD conversion to the first component f ₁ , sequentially shifts the components f _{1 to} f _n , by going discarding the oldest data from the component f _n of, during its components f ₁ ~f _n, always stores data of the number equal to one period of the detected signal. When data corresponding to one cycle of the M-sequence signal applied to the buried steel pipe 1 is accumulated, the operation of the AD converter 21 and the shift of the shift register 22 are temporarily stopped.

In the present embodiment, the conversion frequency of the AD conversion is set to 4400 Hz for the M-sequence signal having a frequency of 440 Hz, and the detection signal corresponding to one pulse constituting the M-sequence signal is converted to
A / D conversion of 10 points is performed and digital data is output. Therefore, the shift register 22 is configured in 1270 stages and driven in synchronization with the AD conversion by the clock CLK, thereby storing data of one cycle of the pseudo random signal.
accumulate.

The shift register 23 is a shift register 22
And has a loop shape. The shift register 23 is used to store a reference signal in the correlation processing.
A signal having the same waveform as the M-sequence signal applied to the signal or a signal having the same waveform as the M-sequence signal, and a signal obtained by subjecting the signal applied to the buried steel pipe to the same frequency band limitation as that received in the propagation path thereof, The digitized data for one cycle time is input as a reference signal.

That is, a signal having the same waveform as the 127 M-sequence signals, or a signal having the same waveform as the M-sequence signal, is subjected to the same frequency band limitation as the signal applied to the buried steel pipe in the propagation path. Are subjected to sampling and digital conversion at 10 times the frequency of the M-sequence signal, and the values are sequentially stored in the constituent elements g _{1 to} g _n ,
It circulates through the components g _{1 to} g _n according to the clock from the shift control device 27.

The frequency-band-limited M-sequence signal stored in the shift register 23 has a frequency band-limiting characteristic such that the M-sequence signal applied to the buried steel pipe 1 propagates through the steel pipe and the underground on the propagation path. Although it is desirable to have a characteristic close to the frequency band limitation received inside, in this embodiment, the low-frequency band limitation is performed on the M-sequence signal signal applied to the buried steel pipe 1 by a low-frequency limiting filter. You are using a signal. At this time, the cutoff frequency of the low-pass limiting filter is set to 100 Hz.

When the data corresponding to one cycle of the M-sequence signal is stored in the shift register 22, the shift register 23, which is the reference signal generator 11 ', the shift controller 27, and the multiplication, which is the correlation processor, Group 24 and adder 2
5 is started.

Shift register 22 and shift register 23
The contents of each component of is multiplied by the multiplier group 24, the number corresponding to the components of the shift register 22 and 23 f _m and g _{m (m} = 1~n) the multiplication result of each other are obtained. The multiplication result is input to the adder 25. The adder 25 adds the multiplication results and inputs the addition result to the signal discrimination device 26.

When the multiplication by the multiplier 24 and the addition processing by the adder 25 are completed, the contents 1 of the shift register 23 are shifted by the constituent elements by the shift control device 27, and the correspondence between the shift register 22 and the shift register 23 is shifted. . When the shift of the shift register 23 is completed, multiplication and addition processing are performed, and the result of the arithmetic processing is
6 are repeatedly performed.

The signal discriminating device 26 discriminates the maximum value of the absolute value of the addition result while the addition result from the adder 25 is input by the number of components of the shift register, and uses the corresponding addition result as a detection signal. Output. The personal computer 13 stores this value as a measured value. This completes the measurement at one point, and the next measurement is performed at the timing when the probe moves to another point.

In the present embodiment, these processes are performed by a personal computer, an AD converter, and software. However, these processes can be realized by configuring a dedicated electronic circuit.

In this embodiment, when a signal having the same waveform as the M-sequence signal applied to the buried steel pipe 1 is used as the reference signal, the same operation as the technique described in Japanese Patent Application Laid-Open No. 10-239267 is used. When an effect is obtained and a signal having the same waveform as the M-sequence signal and a signal applied to the buried steel pipe and subjected to the same frequency band limitation as that received in the propagation path is used as a reference signal, The functions and effects described in the first and fifth means are obtained.

FIG. 3 is a diagram showing another example of a paint film damage position detecting device for a buried coated steel pipe according to an embodiment of the present invention. The device shown in FIG. 3 is the same as the device shown in FIG. 1 except that the potential difference signal detected by the spacecraft 6 is input to the reference signal generator 11 ′. Except for this, the description is omitted.

When this apparatus is used, first, an artificial defect is provided in the coating layer of the buried steel pipe 1, and at that time, for example, a potential difference signal detected by the probe 6 immediately above the artificial defect is The data is stored in a shift register in the reference signal generator 11 'and used as a reference signal.

FIG. 4 shows an outline of the reference signal generator 11 ', the correlation processing unit 12, and the devices associated therewith. 4, the same components as those shown in FIG. 2 are denoted by the same reference numerals, and description thereof will be omitted. In FIG. 2, 2
8, 29 are changeover switches.

Since the operation of the circuit shown in FIG. 4 is almost the same as the operation of the circuit shown in FIG. 2, the description of the same parts will be omitted, and only the different parts will be described. A changeover switch 28 is provided on the input side of the shift register 23, and switches between the output of the AD converter 21 and the output from the final stage element g _{n of the} shift register 23 itself. A clock for driving the shift register 23 is provided by a changeover switch 29.
Thus, the clock pulse CLK for driving the shift register 22 and the clock from the shift control device 27 can be switched and input.

When the potential difference signal detected at the time of detecting an artificial defect is taken in as described above, the changeover switch 28 is switched to the output side of the AD converter 21 and the changeover switch 29 is switched to the clock pulse CLK. Side. Therefore, the potential difference signal detected by the probe 6 is input to the AD converter 21, converted into a digital signal, and input to the shift register 23. Shift register 23 operates in synchronism with the conversion period of the AD converter 21 inputs the latest digital data converted by the AD converter in the first component g _1, each component g ₁ to g _n
By shifting the inside sequentially, its component g ₁
During to g _n, for storing data of the number equal to one period of the detected signal.

One round of the M-sequence signal applied to the buried steel pipe 1
When the data corresponding to the period is accumulated, the switch
The input side of the switch 28 is necessary for the last stage of the shift register 23 itself.
Prime g _nAnd the input side of the changeover switch 29 is
It is connected to the shift control device 27. This allows for artificial defects
Work to capture the waveform when a defect is detected as a reference signal
Is completed.

The operation of the circuit thereafter is exactly the same as that described with reference to FIG. That is, at the time of defect search, data corresponding to one cycle of the M-sequence signal applied to the buried steel pipe 1 is taken into the shift register 22, and then the contents of the shift register 23 are shifted by the clock from the shift control device 27. , Each time, multiplier group 2
4 and the adder 25 calculate the sum of the products of the corresponding components of the shift register 22 and the shift register 23 and output the result to the signal discrimination device 26.

The signal discriminating device 26 discriminates the maximum value of the absolute value of the addition result while the addition result from the adder 25 is input for the number of components of the shift register, and uses the corresponding addition result as a detection signal. Output. The personal computer 13 stores this value as a measured value. This completes the measurement at one point, and the next measurement is performed at the timing when the probe moves to another point.

In this embodiment, since a detected waveform when an artificial defect is actually detected is used as a reference signal, a waveform close to a waveform actually obtained can be used as a reference waveform. Compared with the embodiment shown in FIGS. 1 and 2, it is possible to more stably detect a signal peak value and determine a signal phase.

Next, a description will be given of a third example of a paint film damage position detecting apparatus for a buried coated steel pipe according to an embodiment of the present invention.
In this example, the overall configuration of the device is the same as that shown in FIG. However, the outline of the reference signal generator 11 ', the correlation processing unit 12, and the devices associated therewith are as shown in FIG. In FIG. 5, reference numeral 30 denotes a frequency band limiting device. The circuit shown in FIG. 5 is different from the circuit shown in FIG. 4 only in that a frequency band limiting device 30 is provided. Therefore, only this different point will be described, and the description of the other parts will be omitted.

In this apparatus, measurement is performed in the same manner as in the apparatus shown in FIGS. 3 and 4. In this case, the detection signal and the reference signal are the potential difference signals themselves detected by the probe 6. Instead, it is a signal whose frequency band is limited by the frequency band limiting device 30. In this embodiment, the characteristics of the frequency band limiting device 30 are set so as to further limit the frequency band of the signal with respect to the frequency band limitation that the signal receives while propagating through the steel pipe and the ground that is the propagation path. When the characteristic of the propagation path is the low-frequency limiting characteristic and the cutoff frequency is Fc, the characteristic of the frequency band limiting device is also the low-frequency limiting characteristic, and the cutoff frequency fc is fc> Fc. Is set to

The transfer function of the signal in the low frequency region tends to be particularly unstable, and the signal detected when measuring the artificial defect and the signal detected when measuring the actual defect are:
It may vary depending on the underground electrical characteristics. Therefore,
Even the methods shown in FIGS. 3 and 4 are not always perfect. However, in the present embodiment, since the low-frequency component that tends to be unstable is cut, the waveforms of the detection signal and the reference signal can be made almost the same,
Further, it is possible to stably detect the signal peak value and determine the signal phase.

In each of these embodiments, an M-sequence signal is used as a signal to be applied to a buried steel pipe. However, other pseudo-random signals may be used, or a signal modulated by these may be used. The same effect can be obtained by using.

[0069]

As described above, in the present invention according to the first and fifth aspects of the present invention, the difference between the waveforms of the detection signal and the correlation signal becomes small, and the correlation calculation results Distortion of the pulse waveform is suppressed, and it is possible to stably detect a signal peak value and determine a signal phase.

According to the second and sixth aspects of the present invention, the difference between the reference waveform and the detected waveform becomes smaller, and the distortion of the pulse waveform in the correlation operation result is suppressed. Therefore, it is possible to more stably detect the signal peak value and determine the signal phase.

According to the third and seventh aspects of the present invention, even if the waveform changes due to a change in the condition of the signal propagation path, a predetermined frequency band limitation is further performed. Always have almost the same waveform. Therefore, it is possible to more stably detect the signal peak value and determine the signal phase.

In the invention according to claim 4, the method disclosed in
The same effect as the invention described in JP-A-239267 can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a coating film damage position detecting device for a buried coated steel pipe according to an embodiment of the present invention.

FIG. 2 is a reference signal generator 1 according to the first embodiment.
1 'is a diagram showing an outline of a correlation processing unit 12 and a device associated therewith.

FIG. 3 is a view showing another example of the coating film damage position detecting device for a buried coated steel pipe according to an embodiment of the present invention.

FIG. 4 is a reference signal generator 1 according to a second embodiment.
1 'is a diagram showing an outline of a correlation processing unit 12 and a device associated therewith.

FIG. 5 is a reference signal generator 1 according to a third embodiment.
1 'is a diagram showing an outline of a correlation processing unit 12 and a device associated therewith.

FIG. 6 is a diagram showing an example of a prior art apparatus for detecting a paint film damage position on a buried coated steel pipe.

FIG. 7 is a diagram illustrating an example of an M-sequence signal generation circuit.

FIG. 8 is a diagram illustrating an example of a signal waveform of an M-sequence signal and an autocorrelation signal waveform thereof.

FIG. 9 is a diagram illustrating an example of a cross-correlation calculation result.

FIG. 10 is a diagram showing a relationship between a cross-correlation peak value (representative value of a potential difference) and a moving distance (inspection position) of a probe according to a conventional technique.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Buried steel pipe, 3 ... Power amplifier, 4 ... Ground electrode, 5 ... Damaged coating film of buried steel pipe 1, 6 ... Probe, 6a, 6b ... Wheel electrode of probe, 10 ... M-sequence signal generator, 11 '... reference signal generator, 12 ... correlation processing unit, 13 ... personal computer operation unit, 21 ... AD converter, 22, 23 ... shift register, 24 ... multiplier, 25 ... adder, 26 ... signal discriminator, 27 ... Shift control device, 28, 29 ... Changeover switch, 30 ... Frequency band limiting device

Claims (7)

[Claims] 1. A pseudo-random signal or a signal modulated by a pseudo-random signal is applied as an AC voltage between a coated steel pipe buried underground and the ground to flow an electric current through the steel pipe. By detecting a potential difference between two points on the ground surface at positions along the axial direction, performing a correlation process between the detected signal and the reference signal, and sequentially performing the correlation process while moving the positions of the two points. A method of measuring a change in a potential difference on the ground surface and detecting a position of coating film damage of the buried coated steel pipe from a change pattern of the potential difference, wherein a signal applied to the buried steel pipe as a reference signal in the correlation processing. A signal having the same waveform as that of the signal applied to the buried steel pipe is subjected to the same frequency band limitation as that received in the propagation path, and a coating damage detection of the buried coated steel pipe is characterized by using Method. 2. The method according to claim 1, wherein a signal between the electrodes installed on the ground surface detected at a certain point in time is stored, and the correlation is stored. A method for detecting a paint film damaged position of a buried coated steel pipe, which is used as a reference signal in processing. 3. The method according to claim 1, wherein a predetermined frequency is applied to a signal between the electrodes installed on the ground surface, which is detected at a certain point in time. A method for detecting a paint film damaged position of a buried coated steel pipe, wherein the signal subjected to the band limitation is stored and used as a reference signal in the correlation processing. 4. A signal generator for generating a pseudo-random signal or a signal modulated by the pseudo-random signal, and a power amplifier for generating a voltage proportional to the output of the signal generator and supplying current between the buried steel pipe and the ground. And, travels along the pipe axis direction of the buried steel pipe on the ground surface, a probe having a plurality of wheel electrodes arranged in the pipe axis direction of the buried steel pipe, and a potential difference between the plurality of wheel electrodes. Calculate the cross-correlation between the voltmeter to be measured, the pseudorandom signal, or a reference signal having the same waveform as the signal modulated with the pseudorandom signal, and the output waveform of the voltmeter, and output the peak value thereof. A coating film damage position detecting apparatus for a buried coated steel pipe, comprising: a correlation processing device; and means for detecting a coating film damage position of the embedded steel pipe from a change in output of the correlation processing device. The processing device is a pseudo-laser. A shift register for storing an output waveform of the voltage measuring device corresponding to one cycle of a signal modulated by a random signal or a pseudo-random signal; a shift register for storing one cycle of a reference signal; An adder that calculates the product of the corresponding components of the register and adds the products calculated for all the components of the shift register; and the correspondence between the information in these two shift registers and the components one by one. A shift control device and means for calculating the maximum value of the output of the adder while the correspondence between the information in the two shift registers is shifted by one order. Detecting device for paint film damage on coated steel pipe. 5. A pseudorandom signal or a pseudorandom signal instead of a reference signal having the same waveform as a signal modulated by the pseudorandom signal in the apparatus for detecting a paint film damage position of a buried coated steel pipe according to claim 4. A buried signal characterized in that a signal or a signal having a waveform modulated by a pseudo-random signal is subjected to a frequency band limitation equivalent to that of a signal applied to a buried steel pipe in a propagation path thereof as a reference signal. Detecting device for paint film damage on coated steel pipe. 6. A pseudorandom signal or a reference signal having the same waveform as a signal modulated by the pseudorandom signal in the apparatus for detecting a paint film damage position of a buried coated steel pipe according to claim 4, wherein the signal is at a certain point in time. Wherein the voltage of the voltage measuring device detected in the step (c) is used as a reference signal to detect a paint film damage position on the buried coated steel pipe. 7. A pseudo-random signal or a reference signal having the same waveform as a signal modulated by the pseudo-random signal in the apparatus for detecting a paint film damage position of a buried coated steel pipe according to claim 4. Wherein a signal obtained by subjecting the voltage of the voltage measuring device to a predetermined frequency band limitation and detected as a reference signal is used as a reference signal.