JP2018124221A - Magnetic measuring method and magnetic measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic measuring method and a magnetic measuring device capable of accurately detecting amplitude of a secondary-side AC signal, without increasing the device or wiring cost and without being affected by noise or a ground drift.SOLUTION: In an analysis processing as one embodiment of the present invention, a waveform signal that has the same frequency as that of a secondary-side AC signal and has a phase 90 degrees different from that of the secondary-side AC signal is generated from the secondary-side AC signal, and amplitude of the secondary-side AC signal using the secondary-side AC signal and a waveform signal is calculated. Thus, the amplitude of the secondary-side AC signal can be accurately detected without increasing a device or wiring cost and without being affected by noise or a ground drift.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a magnetic measurement method and a magnetic measurement apparatus for detecting an induced current or a magnetic field generated in the vicinity of a subject as the subject is excited by an AC signal as a secondary AC signal.

  As a method for inspecting foreign matter such as a flaw existing in a steel plate or steel material, or inclusions inside, or a method for measuring magnetic properties of a material, an eddy current flaw detection method or a leakage magnetic flux flaw detection method is known. Note that the measurement of magnetic properties such as magnetic permeability is not flaw detection, but it will be referred to as “flaw detection” in the following because of the naming convention of those skilled in the art.

  In the eddy current flaw detection method, the eddy current induced in the subject is caused by foreign matter such as wrinkles present on the surface of the subject and inclusions present in the vicinity of the surface, or inhomogeneous electromagnetic characteristics such as conductivity and permeability. The phenomenon that is disturbed due to this is detected by installing a detection coil that is different from the induction coil in the vicinity of the subject, or is detected as a characteristic change of the induction coil itself. In other words, the eddy current flaw detection method uses the mutual induction phenomenon when the induction coil and the detection coil are different, and uses the self-induction phenomenon when the induction coil and the detection coil are the same.

  On the other hand, when the subject is a magnetic substance, the leakage magnetic flux flaw detection method is applied to the inside of the surface of the subject or near the surface when the subject is magnetized (excited) by applying a magnetic field to the subject. The flaw detection is performed by utilizing the change in the amount of magnetic flux that leaks to the surface of the subject due to the presence of foreign matter such as inclusions, or non-uniformity of the electromagnetic characteristics such as magnetic permeability. In this leakage magnetic flux flaw detection method, DC excitation has been frequently used in the past, but in recent years, AC excitation or a method using both DC excitation and AC excitation has come to be used in order to increase measurement sensitivity and response speed. Yes.

  A signal detected by such an eddy current flaw detection method or an AC leakage magnetic flux flaw detection method is an AC signal (secondary side AC signal) having a known induction or excitation (hereinafter collectively referred to as “excitation”) frequency. In addition, since the amplitude and phase include information such as the presence or absence of wrinkles and the magnetic characteristics of the material, it is necessary to analyze the secondary AC signal with high accuracy. For this reason, as a known method, quadrature detection using an excitation signal as a reference signal, a reference signal, a phase shifter that generates a signal obtained by advancing the phase of the reference signal by 90 degrees, and a secondary side alternating current between these two signals An apparatus has been proposed that includes two multipliers for multiplying signals and an arithmetic unit for calculating the amplitude and phase of the secondary AC signal from the square sum and quotient of the multiplication values.

  In addition, some of the above functions are commercially available as a lock-in amplifier, and many prior inventions have proposed using a lock-in amplifier or a phase detector equivalent to the lock-in amplifier (Patent Documents 1 to 3). 3). Furthermore, in Patent Document 4, a DC magnetic field and an AC magnetic field including a component orthogonal thereto are simultaneously applied to a steel plate, and a detection signal proportional to the magnitude of the internal energy of the steel plate is measured. An invention for calculating the crystal orientation of is described. Further, Patent Document 4 describes that there is effective value processing of a detection signal as a method for measuring the amplitude of the detection signal.

Japanese Patent No. 47368811 Japanese Patent No. 5293755 JP 2013-160739 A JP 2013-185902 A

  However, the methods described in Patent Documents 1 to 3 require a quadrature detection circuit or a lock-in amplifier in order to detect the amplitude of the secondary AC signal, and also the reference signal in addition to the secondary AC signal. Since wiring is necessary, the cost increases. In addition, when noise is mixed in the reference signal, it is not possible to ensure the accuracy of detecting the amplitude of the secondary AC signal. On the other hand, according to the method described in Patent Document 4, when noise or ground (zero point) drift occurs in the detection signal, the detection accuracy of the amplitude of the detection signal is affected.

  The present invention has been made in view of the above-described problems, and its purpose is to increase the cost of the apparatus and wiring, and to avoid the influence of noise and ground drift. An object of the present invention is to provide a magnetic measurement method and a magnetic measurement apparatus capable of accurately detecting an amplitude.

  The magnetic measurement method according to the present invention is a magnetic measurement method for detecting an induced current or a magnetic field generated in the vicinity of a subject as the subject is excited by an alternating current signal as a secondary side alternating current signal. A waveform signal having the same frequency as the frequency of the secondary AC signal from the secondary AC signal and having a phase difference of 90 degrees with respect to the phase of the secondary AC signal is generated. The method includes an amplitude calculating step of calculating an amplitude of the secondary AC signal using a signal and the waveform signal.

  In the magnetic measurement method according to the present invention as set forth in the invention described above, the amplitude calculating step includes a step of generating the waveform signal by performing a Hilbert transform on the secondary AC signal.

  In the magnetic measurement method according to the present invention as set forth in the invention described above, in the amplitude calculation step, the frequency component in which the amplitude of the secondary AC signal is maximized is extracted by executing frequency analysis, and the secondary AC signal is extracted. Generating a signal obtained by extracting only the frequency component from the signal as a noise removal signal, and calculating the amplitude of the secondary AC signal using the noise removal signal and the waveform signal.

  The magnetic measurement device according to the present invention is a magnetic measurement device that detects an induced current or a magnetic field generated near the subject as the subject is excited by an AC signal as a secondary AC signal. A waveform signal having the same frequency as the frequency of the secondary AC signal from the secondary AC signal and having a phase difference of 90 degrees with respect to the phase of the secondary AC signal is generated. An amplitude calculating means is provided for calculating the amplitude of the secondary AC signal using the signal and the waveform signal.

  According to the magnetic measurement method and the magnetic measurement device according to the present invention, the amplitude of the secondary AC signal can be accurately detected without increasing the cost of the device or wiring and without being affected by noise or ground drift. can do.

FIG. 1 is a block diagram showing a configuration of a magnetic measurement apparatus according to an embodiment of the present invention. FIG. 2 is a block diagram showing a configuration of a conventional magnetic measurement apparatus. FIG. 3 is a flowchart showing the flow of amplitude analysis processing according to an embodiment of the present invention. FIG. 4 is a diagram illustrating an example of an average value of amplitude errors calculated by the amplitude analysis processing according to the related art and the present invention. FIG. 5 is a diagram illustrating an example of variation in amplitude error calculated by the amplitude analysis processing according to the related art and the present invention. FIG. 6 is a diagram illustrating an example of a secondary AC signal, a secondary AC signal (restoration signal) that does not include noise, and noise.

  Hereinafter, the configuration and operation of a magnetic measurement apparatus according to an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a block diagram showing a configuration of a magnetic measurement apparatus according to an embodiment of the present invention. As shown in FIG. 1, a magnetic measuring device 1 according to an embodiment of the present invention uses a eddy current flaw detection method to inspect foreign matter such as wrinkles and internal inclusions in a steel plate S, or a steel plate S. The apparatus includes an excitation signal generator 2, an excitation coil 3, a detection sensor 4, an amplifier 5, and an analysis circuit 6 as main components. In this embodiment, the inspection or measurement is performed using the eddy current flaw detection method. However, the excitation coil 3 may be replaced with an AC magnetizer, and the inspection or measurement may be performed using the leakage magnetic flux flaw detection method. Good.

The excitation signal generator 2 is a device that outputs a sine wave signal having a predetermined excitation frequency f ₀ to the excitation coil 3 as an excitation signal.

  The excitation coil 3 is an apparatus that excites the steel plate S to be processed by the excitation signal output from the excitation signal generator 2.

  The detection sensor 4 is a device that detects an induced current or magnetic field generated in the vicinity of the steel sheet S as it is excited by the excitation coil 3 and outputs the detected induced current or magnetic field to the amplifier 5.

  The amplifier 5 is a device that amplifies the induced current or magnetic field detected by the detection sensor 4 into a signal having an appropriate amplitude and outputs the amplified signal to the analysis circuit 6 as a secondary AC signal.

  The analysis circuit 6 extracts information on the foreign matter and magnetic characteristics of the steel sheet S from the secondary AC signal output from the amplifier 5.

  The magnetic measuring device 1 having such a configuration can perform the following amplitude analysis processing without increasing the cost of the device and wiring, and without being affected by noise or zero point drift. By detecting the amplitude of the secondary AC signal with high accuracy, information regarding the foreign matter and magnetic characteristics of the steel sheet S is extracted with high accuracy. Hereinafter, the operation of the magnetic measurement apparatus 1 when executing the amplitude analysis process will be described.

[Amplitude analysis processing]
In general, the secondary AC signal x (t) output from the amplifier 5 is a noise signal n () with respect to a sine wave signal Asin (2πf ₀ t + ψ) having an excitation frequency f ₀ as shown in the following formula (1). t) and a ground drift signal d of the measurement circuit are superimposed on each other. The amplitude A including information related to the foreign matter and magnetic characteristics of the steel plate is accurately detected without being affected by the noise signal n (t) or the ground drift signal d from the secondary AC signal x (t) shown in the equation (1). It is an object of the present invention.

  Here, in the quadrature detection which is the prior art, as shown in FIG. 2, the excitation signal generated by the excitation signal generator 11 and the signal obtained by advancing the phase of the excitation signal by 90 degrees using the orthogonal detection circuit 15 are used. The secondary AC signal x (t) output from the amplifier 14 is multiplied. That is, signals p (t) and q (t) shown in the following mathematical formulas (2) and (3) are obtained. Then, by applying the product sum formula of the trigonometric function to the first term of the signals p (t) and q (t) shown in the equations (2) and (3), the following equations (4) and (5) Get.

Here, when the signals p (t) and q (t) shown in Equations (4) and (5) are averaged over a large time length compared to the reciprocal of the excitation frequency f ₀ , all the terms multiplied by sin and cos are 0. become. Therefore, if the average values of the signals p (t) and q (t) shown in the equations (4) and (5) are p _av and q _av , the amplitude A of the secondary AC signal x (t) is The amplitude A of the secondary AC signal x (t) can be accurately detected. However, in this case, it is necessary to install these circuits for each detection sensor that detects the secondary AC signal x (t), and the cost increases.

On the other hand, in another conventional technique, the amplitude a (t) of the secondary AC signal x (t) is calculated by an effective value calculation process using the following formula (7). However, the second term on the right side of equation (7), becomes zero when taking larger than the time length of the arithmetic processing to the inverse of the excitation frequency f _0, the third term of the right side of equation (7), the operation very not zero larger than the time length of the processing to the inverse of the excitation frequency f _0. For this reason, the bias resulting from the noise signal n (t) or the ground drift signal d remains in the calculation result of the amplitude a (t).

Therefore, in the present invention, the amplitude A of the secondary AC signal x (t) is calculated as follows. Now, when the noise signal n (t) and the ground drift signal d are superimposed on the secondary AC signal x (t), the secondary AC signal is expressed as the following formula (8). Then, the secondary AC signal x ₀ (t) shown in Expression (8) can be converted into a signal y ₀ (t) whose phase is advanced by 90 degrees by Hilbert transform. Here, the Hilbert transform is a well-known transform that delays the phase by 90 degrees, and details thereof are described in, for example, Oppenheim, Schafer. “Digital signal processing”, chapter 7, Prentice-Hall.

  Therefore, when the noise waveform signal n (t) and the ground drift d are not superimposed on the secondary AC signal x (t), the secondary AC is obtained by a simple calculation using the following formula (9). The amplitude A (t) of the signal x (t) can be calculated.

  Even if the noise signal n (t) or the ground drift signal d is superimposed on the secondary AC signal x (t), the secondary AC signal x (t) is excited at a single frequency. Therefore, the noise signal n (t) is similarly analyzed by using the noise removal signal obtained by extracting only the frequency component that maximizes the amplitude of the secondary AC signal x (t) by frequency analysis. And the influence of the ground drift signal d can be eliminated.

The flow of the amplitude analysis process according to the embodiment of the present invention described above is summarized as shown in FIG. That is, as shown in FIG. 3, in the amplitude analysis processing according to an embodiment of the present invention, first, the secondary AC signal x (t) is detected (step ST1), and the secondary AC signal x (t). (Step ST2), the frequency component (peak component) that maximizes the amplitude of the secondary AC signal x (t) is detected (step ST3). Then, a noise removal signal x ₀ (t) from which components other than the peak component detected from the secondary AC signal x (t) are removed is obtained (step ST4), and the Hilbert transform is performed on the noise removal signal x ₀ (t). The waveform signal y ₀ (t) whose phase is advanced by 90 degrees is calculated (step ST5). Then, the sum of squares of the noise removal signal x ₀ (t) and the waveform signal y ₀ (t) is calculated as the amplitude A (t) of the secondary AC signal x (t) using Equation (9) (step) ST6).

  Here, in order to compare the effects of the amplitude analysis processing according to the embodiment of the present invention, white noise is artificially changed to various amplitudes (noise amplitudes) in a sine wave signal simulating a secondary AC signal obtained in the field. (1) Quadrature detection, (2) RMS value calculation, and (3) Amplitude analysis processing results of the present invention were compared. In this amplitude analysis processing, the amplitude calculated when the amplitude of the sine wave signal is 1.5 V, the frequency of the sine wave signal is 100 kHz, the noise amplitude is 0 to 0.5 V, and the calculation is repeated 100 times with each noise amplitude. The average value and variation of the errors were compared. The comparison results are shown in Tables 1 and 2 and FIGS.

  Tables 1 and 4 are tables and diagrams showing the average values of amplitude errors measured by quadrature detection, effective value calculation, and amplitude analysis processing according to the present invention. Table 2 and FIG. 5 are diagrams showing variations in amplitude errors measured by quadrature detection, effective value calculation, and amplitude analysis processing according to the present invention. As shown in Tables 1 and 2 and FIGS. 4 and 5, when the noise amplitude increases, the amplitude analysis processing by the quadrature detection and the effective value calculation, which is the conventional method, particularly the amplitude analysis processing by the effective value calculation, Mean values and variability increased. On the other hand, in the amplitude analysis processing according to the present invention, both the average value and the variation of the amplitude error were stabilized near zero. From the above, according to the amplitude analysis processing according to the present invention, the amplitude of the secondary AC signal can be accurately detected without increasing the cost of the apparatus and wiring and without being affected by noise and ground drift. It was confirmed that it was possible.

  As is clear from the above description, according to the analysis processing according to an embodiment of the present invention, the secondary side AC signal has the same frequency as the frequency of the secondary side AC signal and the secondary side AC signal. A waveform signal having a phase difference of 90 degrees with respect to the phase of the signal is generated, and the amplitude of the secondary AC signal is calculated using the secondary AC signal and the waveform signal. Without being affected by noise and ground drift, the amplitude of the secondary AC signal can be accurately detected.

  Finally, examples of the present invention will be described. The configuration of the magnetic measurement device used in the examples is the same as that of the magnetic measurement device shown in FIG. 1, and the arrangement of the excitation means and sensors, the wiring length, and the like were appropriately selected according to the installation location. That is, the excitation signal is a sine wave with an excitation frequency of 100 kHz, the excitation coil is a top coil, and the detection sensor is a differential configuration with two hollow coils. Further, the distance (lift-off) between the sensor and the steel plate to be inspected was set to 1 mm, and the measurement was performed at a position where the steel plate was wound around the non-magnetic roll so that there was no flapping of the steel plate.

  FIG. 6 is a diagram illustrating a detection signal (secondary AC signal), a signal component that does not include noise detected by the present invention (restoration signal), and extracted noise. As shown in FIG. 6, since the detection signal is distorted by noise to the extent that it can be seen with the eyes, the amplitude cannot be obtained with high accuracy. On the other hand, according to the present invention, since a restored signal that does not contain noise can be extracted, the amplitude of the detection signal can be obtained with high accuracy from the restored signal. Further, this inspection was performed on a 1000 m steel plate, and the inspection could be continuously performed without the influence of noise.

  As mentioned above, although the embodiment to which the invention made by the present inventors was applied has been described, the present invention is not limited by the description and the drawings that constitute a part of the disclosure of the present invention according to this embodiment. For example, as described above, other embodiments, examples, operation techniques, and the like made by those skilled in the art based on the present embodiment are all included in the scope of the present invention.

DESCRIPTION OF SYMBOLS 1 Magnetic measuring device 2 Excitation signal generator 3 Excitation coil 4 Detection sensor 5 Amplifier 6 Analysis circuit S Steel plate

Claims (4)

A magnetic measurement method for detecting an induced current or a magnetic field generated in the vicinity of a subject as the subject is excited by an AC signal as a secondary AC signal,
Generating a waveform signal having the same frequency as the secondary AC signal from the secondary AC signal and having a phase difference of 90 degrees with respect to the phase of the secondary AC signal; A magnetic measurement method comprising an amplitude calculating step of calculating an amplitude of the secondary AC signal using a side AC signal and the waveform signal.   The magnetic measurement method according to claim 1, wherein the amplitude calculating step includes a step of generating the waveform signal by performing a Hilbert transform on the secondary AC signal.   The amplitude calculating step extracts a frequency component that maximizes the amplitude of the secondary AC signal by performing frequency analysis, and a signal obtained by extracting only the frequency component from the secondary AC signal is a noise removal signal. The magnetic measurement method according to claim 1, further comprising: calculating the amplitude of the secondary AC signal using a noise removal signal and the waveform signal. A magnetic measurement device that detects an induced current or a magnetic field generated in the vicinity of a subject as the subject is excited by an AC signal as a secondary AC signal,
Generating a waveform signal having the same frequency as the secondary AC signal from the secondary AC signal and having a phase difference of 90 degrees with respect to the phase of the secondary AC signal; A magnetic measuring device comprising amplitude calculating means for calculating the amplitude of the secondary AC signal using a side AC signal and the waveform signal.

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