JPS62225947A - Probe for measuring vortex - Google Patents

Abstract

PURPOSE:To obtain a high measuring accuracy, by arranging a material equivalent to an object to be inspected at a position separated from a reference coil by the liftoff of an active coil. CONSTITUTION:A probe body 3a contains an active coil 5 which causes a direct electromagnetic action directly upon a liner-covered tube 4 to obtain a change in the impedance corresponding to a linear layer thickness and a reference coil 6. The coil 5 is set to keep the distance from the surface of a liner layer 4b, namely, a lift off l constant. A plate 8 the same in the material as a base material layer of the tube 4 is arranged for the coil 6 through a spacer 7. With such an arrangement, the impedance ratio between the coils 5 and 6 is little varied with the frequency.

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to an eddy current measurement probe used for non-destructive testing of a test object such as a tube using the eddy current method. (Prior art and its problems) The principle of the eddy current method, which is used for non-destructive testing of pipe bodies, is that a coil carrying an alternating current is placed close to the surface of the test object, and an eddy current is applied to the surface of the test object. The disturbance of the eddy current due to defects near the surface of the specimen or changes in the thickness of the specimen is measured from the amount of change in impedance of the coil due to the induced magnetic field induced by the eddy current. Therefore, in principle, non-destructive testing is possible by measuring the absolute value of the impedance change, but since the impedance of the coil changes due to various factors such as those listed below, it is possible to detect disturbances among them. Eliminating these factors is essential to making accurate measurements. (1) Defects or thickness of the specimen (2) Magnetic permeability and electrical conductivity of the specimen (3) Electrical conductivity of the coil material (4) Magnetic permeability of the core around which the coil is wound (5) Lift-off (defects of the coil on the surface of the specimen) (6) If measurements are made based only on the external magnetic field, that is, the absolute value of the impedance change, there is a possibility that variations due to factors other than the detection objective in item (1) above may also be detected at the same time. In order to avoid this problem and improve the S/N ratio, the following methods have been put to practical use or proposed. For example, as shown in FIG. 2, an active coil 1 that is close to the subject and exerts an electromagnetic effect directly on it.
-1 and a reference coil L for picking up fluctuations due to disturbance factors other than the above-mentioned item (5), along with pure resistors R1 and R2, are used. It is common practice to try to improve N. In this case, the active coil L1 and the reference coil ``2'' are built into the probe 1 in a positional relationship as shown in FIG. 3, for example, and measurement is performed by bringing the probe 1 close to the surface of the subject 2. The bridge circuit is first adjusted to a balanced state, and then even if a disturbance acts on both coils L1 and L2 during measurement and their respective impedances change, the impedance ratio will hardly change, so the bridge balance will not collapse. , when the active coil L1 detects a defect or the like in the object 2, only the impedance of the active coil 1-1 changes, causing the bridge balance to collapse, and this unbalance is detected as a signal representing the defect or the like. It is known that the degree to which the The S/Na@ measure using the method (M) has also been attempted in the past.In this regard, the applicant has developed a method for developing nuclear fuel cladding tubes, such as nuclear fuel cladding tubes in which a pure zirconium liner is applied to the inner surface of a zirconium alloy tube. proposed a method of measuring the liner layer thickness of a liner cladding tube with a liner layer formed on the inner circumferential surface using multiple frequencies in an application separate from the present application. Publication No. 05 proposes a method of measuring the liner layer thickness of a liner cladding tube using an interpolation type probe while eliminating the influence of lift-off.Now, we have proposed a method for measuring the liner layer thickness of a liner cladding tube using an interpolation type probe while eliminating the influence of lift-off. For example, when measuring the liner layer thickness of a liner cladding tube (in this case, it would be measured using the above-mentioned interpolated probe), high accuracy of measurement is required for the following reasons. In other words, the impedances of the active coil L1 and the reference coil L2 not only change due to the disturbance factors listed above, but also change significantly with the frequency as a parameter, and especially due to the latter change, the impedance of both coils changes. This is because the impedance ratio of the probe coil changes greatly, which greatly disrupts the bridge balance.This is explained using Fig. 4. Fig. 4 shows the general impedance change of the probe coil, and the vertical axis of the figure is It shows an imaginary component and is affected by the metal plate.

[It is shown normalized by the imaginary component of the coil, which is sized so as not to cause any damage. Moreover, the horizontal axis shows the real number component, which is similarly normalized. In the figure, Ro is the specific resistance of the coil at an infinite distance from the subject, Lo is the coil impedance at an infinite distance from the subject, R4 is the pure resistance of the coil on the subject, and L is the coil resistance on the subject. The inductance, θ) indicates the angular frequency, bCTπ indicates the coil impedance, and the spear indicates the lift-off amount. As shown in the figure, the impedance of the probe coil is OA at frequency ω1. A difference in OB occurs at frequency ω2. AA', BB'
is the impedance change due to lift-off fluctuation. When measuring using a single frequency, 1. on the active coil. By connecting the impedance corresponding to the reference coil to the pure resistance side of the bridge circuit to maintain balance, highly accurate measurements can be performed. On the other hand, in multi-frequency measurements, since the impedance ratio of both coils changes greatly depending on the frequency, it is impossible to balance the bridge circuit over all frequencies as in the case of a single frequency described above. Therefore, as it is, it is not possible to measure high viscosity using the multi-frequency eddy current measurement method using the null method measurement method using the bridge circuit. (Objective of the invention) Does this invention solve the above problem? Even when performing eddy current measurements using multiple frequencies, the impedance ratio between the active coil and the reference coil does not change much depending on the frequency, making it suitable for measurements such as liner layer thickness of liner cladding. The purpose of the present invention is to provide an eddy current measurement probe that can be applied to and obtain high measurement accuracy. (Means for Achieving the Object) In order to achieve the above object, the eddy current measuring probe according to the present invention includes an active coil that exerts an electromagnetic effect on the object and obtains information about the object as an impedance change;
In an eddy current measurement probe that has a built-in reference coil that includes a bridge circuit for null method measurement together with this active coil and obtains information on disturbance factors as impedance changes, the probe is placed at a position away from the reference coil by the lift-off amount of the active coil. It is characterized by the arrangement of materials equivalent to the specimen. (Example) FIG. 1 is a schematic diagram showing an outline of an eddy current measuring probe which is an example of the present invention. This probe 3 has a base material layer 4 made of Zircaloy as an object to be tested.
This is a 1C pipe insertion type probe for measuring the liner layer thickness of a nuclear fuel liner cladding tube 4 having a liner layer 4b of pure zirconium formed on the inner circumferential surface by an eddy current method using a combination of two frequencies. The probe body 3a includes an active coil 5 for exerting a direct electromagnetic effect on the liner cladding tube 4 to obtain an impedance change corresponding to the liner layer thickness;
A reference coil 6 is built in together with the active coil 5 to form a bridge circuit for null method measurement and to obtain an impedance change corresponding to a disturbance factor. The active coil 5 is set so that the distance 11i11.71 from the surface of the liner layer 4b and the lift-off mtt are constant in the state from 1 to 1 during measurement when the active coil 5 is inserted into the liner cladding tube 4. On the other hand, a plate made of the same material as the base material layer of the liner cladding tube 4, that is, a Zircaloy plate 8, is arranged for the reference lens 6 via a spacer 7 having a thickness equal to the lift-off amount l, and the spacer 7
A material whose electrical characteristics are close to those of the air interposed between the active coil 5 and the liner cladding tube 4 is selected. That is, a material equivalent to the liner cladding tube 4 is placed apart from the reference coil 6 by the above-mentioned lift-off amount l. In order to ensure accurate measurements, it is desirable to use a plate having the same layer structure as the liner cladding tube 4, in which a pure zirconium liner layer is formed on a Zircaloy base material layer. When the liner layer thickness was measured by the eddy current method using dual frequencies under the following conditions using the probe 3 with the above configuration, the impedance of the active coil 5 and the reference coil 6 was calculated for each frequency. Table 1 shows a comparison with the case of the probe. (1) Liner cladding tube = 8 = Inner diameter of material layer (Zircaloy): 10.55 #φ Base material layer thickness: 0.87 mm Liner layer (pure zirconium) thickness: Approximately 90 μm. (2) Probe active coil: diameter 0. b Lie 1 ~ Thin copper wire of 0.07 mmφ is wound 85 times around the core. Reference coil: Active = Equivalent to il. Zircaloy plate: Area 3.5#lIn square, thickness 0.87 arm. Lift-off amount 1: Approximately 250 μm (3) Frequency 1): 4M1-1z High frequency <f2 (J) (blank below) Table 1 From this measurement result, it is clear that J: The absolute value of the impedance of the coil is almost the same regardless of the frequency, and the difference is as small as 1Ω at a frequency of 2MHz and 30 at a frequency of 4MHz. Therefore, the impedance ratio of both coils hardly changes depending on the frequency, and continues to maintain a value close to 1. In contrast, the conventional probe has a resistance of 15Ω at 2MHz and a resistance of 4MH2r18
It is recognized that there is an impedance difference of Ω, and that the impedance ratio also varies greatly depending on the frequency. This means that although it is impossible to maintain bridge balance over a wide frequency range in the conventional example, this is fully possible in this embodiment. Specifically, by using the probe of this example, it became possible to suppress the imbalance of the bridge circuit to within 3%, thereby achieving a significant improvement in S/H. With conventional probes, it is difficult to suppress the shift in bridge balance to within 5%. It is also known that the maximum sensitivity is achieved when <impedance of L)/impedance of N) = R/R2 = 1, and according to the present invention, as described above, the impedance ratio is approximately 1 regardless of the frequency. Therefore, the maximum sensitivity is 11
1 will be given. For these reasons, high measurement accuracy can be achieved according to the present invention. Incidentally, the measurement accuracy of the liner layer thickness in the above case is ±2.8 μm with the probe of the example.
It was confirmed that this was significantly improved compared to ±10.5 μι in the case of the conventional probe. (Effects of the Invention) As described above, according to the eddy current measurement probe of the present invention, even when performing eddy current measurement at multiple frequencies, the impedance ratio of the active coil and the reference coil does not change much depending on the frequency and is almost constant. It maintains a value close to 1 and has the effect of obtaining high measurement accuracy when applied to measurements of liner layer thickness of liner cladding tubes, etc.

[Brief explanation of drawings]

Fig. 1 is a conceptual diagram of an eddy current measuring probe which is an embodiment of the present invention, Fig. 2 is a wiring diagram of a bridge circuit used for null method measurement in the eddy current method, and Fig. 3 is a conceptual diagram showing a conventional probe. , FIG. 4 is a diagram showing a general impedance change of a probe coil. 3... Probe, 3a... Probe body, 5
... Active coil, 6... Reference coil, 8... Zircaloy plate (material equivalent to the test object), l...
・Lift-off amount

Claims (1)

[Claims] (1) In an eddy current measurement probe that has a built-in active coil that exerts an electromagnetic effect on the object to obtain information about the object, and a reference coil that forms a bridge circuit together with the active coil to obtain information on disturbance factors, the active coil An eddy current measurement probe characterized in that a material equivalent to the test object is placed at a position separated from a reference coil by an amount of lift-off.