JPS63311103A - Thickness measurement - Google Patents

Abstract

PURPOSE:To measure an accurate thickness regardless of temp. changes, by estimating the temp. of an object to be inspected from the difference between the measured values at two frequencies different in the depth of penetration of an eddy current and correcting the measured value at the first frequency based on said temp. CONSTITUTION:The measured values at two frequencies different in the depth of penetration of an eddy current show different changes with respect to the temp. changes of a material to be inspected. Hereupon, the first and second different frequencies F1, F2 are outputted from high frequency dividers 2, 3 and superposed by a mixer circuit 4 to be sent out to a measuring part 5. The measured values T(F1), T(F2) of the impedances of a sensor coil 6 at two frequencies are calculated in the measuring part 5 to be outputted to an operation part 7. The operation part 7 corrects the first measured value on the basis of the relation between the preliminarily calculated temps. and the measured values at two frequencies at the time of calibration. Therefore, an accurate thickness can be measured without using a reference pipe or thermostatic chamber.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for measuring the layer thickness or wall thickness of a material to be inspected, such as the liner thickness of a liner pipe or the wall thickness of a steel pipe.

[Prior art]

An eddy current method has been developed as a method for non-destructively measuring the liner thickness of a liner tube (for example, Japanese Patent Laid-Open No. 59-67405). This measures the liner thickness by detecting changes in the impedance of a coil to which alternating current is applied, and a disturbance factor that changes this impedance is a change in temperature of the material to be inspected.

For this reason, conventionally, errors due to temperature changes have been compensated for by calibrating using a reference tube at regular intervals or every measurement, or by providing a constant temperature room containing the material to be inspected and the entire apparatus.

[Problem that the invention seeks to solve]

However, in the above-mentioned method, calibration using a reference tube requires that the measuring device be temporarily stopped or inserted into the conveyance line of the material to be inspected in order to measure the reference tube, resulting in a decrease in inspection efficiency.

In addition, in the reference tube calibration method, in which a short reference tube is installed in the measuring device and calibration is performed each time a measurement is made, accurate calibration for purely temperature changes may not be possible because it is affected by the reproducibility of the reference tube measurement itself. There is.

The method of setting up a constant temperature room is that the time it takes for the temperature of the material to reach the temperature inside the constant temperature room varies depending on the material, and items that require a long time require storage space, making the handling work involved complicated and requiring a large amount of work. There is a problem in that it is costly.

The present invention has been made in view of the above circumstances, and utilizes the fact that the measured values of two frequencies with different eddy current penetration depths show different amounts of change in response to temperature changes in the inspected material. The present invention aims to provide a thickness measuring method that accurately measures the thickness of a material to be inspected by estimating the temperature of the material to be inspected and correcting the measured value.

[Means for solving problems]

A thickness measuring method according to the present invention is a method of measuring the thickness of an object to be inspected by an eddy current method, and includes: a first frequency whose eddy current penetration depth is associated based on the thickness to be measured; The thickness is measured using the first frequency and a second frequency different from each other, the temperature of the object to be inspected is estimated from the difference between the measured values, and the measured value of the first frequency is estimated based on the result. It is characterized by correcting and determining the thickness.

[Effect]

The temperature of the object to be inspected is estimated from the difference between a measured value of a first frequency, the eddy current penetration depth of which is associated based on the thickness to be measured, and a measured value of a second frequency different from this frequency;
The measured value of the first frequency is corrected based on the result,
Thickness is required.

〔Example〕

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on drawings showing embodiments thereof. FIG. 1 is a block diagram showing an implementation state of the present invention.

An oscillator 1 is an alternating current source that applies to a sensor coil 6 inserted into a liner tube 8 that is an object to be inspected, and its output is input to a high frequency frequency divider 2 and a low frequency frequency divider 3.

The high frequency divider 2 outputs a frequency F1 associated with the thickness of the liner layer 81 of the liner tube 8 to be measured, and the low frequency divider 3 outputs a frequency F2 different from the frequency F1. 1 output is frequency-divided in phase synchronization.

It has been obtained from actual measurement data that it is best to set the frequency F1 so that the eddy current penetration depth is 2 to 3 times the approximate thickness of the liner layer 8I, and this is determined by the penetration depth δ.
Using the fact that is proportional to the frequency f, that is, F,
can be determined. The frequency F2 is a frequency value different from this F, and in this embodiment, the low frequency divider 3
By using , the frequency is set on the lower frequency side than Fl.

Both the high frequency frequency divider 2 and the low frequency frequency divider 3 are connected to a mixer circuit 4, whereby the respective frequencies F1 and F2 are superimposed and the resulting signal is sent to the measurement section 5. The measuring section 5 is connected to the sensor coil 6, and provides the sensor coil 6 with superimposed signals of frequencies F1 and F2. The measurement unit 5 is composed of a combination of a bandpass filter, a bridge circuit, and a synchronous detection circuit (both not shown) that pass frequencies F1 and F2, respectively, and measures the impedance of the sensor coil 6 at each frequency F1 and F2. The value T(F,
) and T(F2) and output them.

The measured values “r(p,) and T(F
The output signal of 2) is input to the calculation section 7, where the following calculation processing is performed, and the measured value T (Fl) is corrected and output as an accurate thickness.

FIG. 2 is an example of a graph showing the relationship between the measured value detected by the measurement unit 5 and the temperature of the object to be inspected, where the horizontal axis represents the temperature ('C) and the vertical axis represents the measured value (μm). It shows.

High frequency measurement value T (Fl) and low frequency measurement value T (F2
) are both at temperature t. The liner thickness Mo of the reference tube is calibrated in advance by odor and measured by other means such as cross-sectional microscopic observation, and measured values corresponding to each temperature are obtained by changing the temperature.

In this graph, the low-frequency measured value T (F2) has a larger change in measured value with respect to temperature than T (Fl) because the eddy current penetration depth is deeper than T (F,) and the base material layer 8
2, the sensitivity of liner thickness measurement increases and is greatly influenced by resistance changes of the base material layer 82. On the other hand, the high frequency signal T (Fl) is the low frequency signal T (F
, ), so the effect on temperature is relatively small, but an error will occur if the high frequency is used alone for measurement. Therefore, by estimating the temperature of the object to be inspected at the time of measurement from the measured values of two different frequencies, the temperature and the temperature t at the time of calibration can be calculated. It is possible to correct the measured value from the relationship.

FIG. 3 is an explanatory diagram of this. First, the liner thickness M is determined in advance. Calibration is performed at the temperature t0 using the two frequencies F1 and F2 set as described above using the reference tube of
The relationship straight lines T (Fl) and T (F2) with the measured values of are created in advance.

Then, the measurement value outputted by the measurement unit 5 is M (F
2) and M(Fl), the temperature t of the object to be inspected is determined by the following equation.

1=1o+α(M (F2) −M (F,)1 where to is the temperature at the time of calibration mentioned above, α is the relational straight line T(Fl)
It is a constant determined from the difference between the slope of T(F2) and the slope of T(F2).

Once t is determined, calculate the temperature correction amount Δt using the following formula and divide Δ by β.
(tO−t) Here, the constant β is the slope of the relational straight line T(Fl).

Once Δt is determined, the measured value M(F,) is corrected using the following equation to determine the liner thickness T.

T=M(Fl)+Δt Note that in this example, the liner thickness is measured, but for example, it is also possible to measure the wall thickness of an unlined pipe to be inspected; in this case, the first By setting the frequency so that the penetration depth of the eddy current is approximately equal to the wall thickness, and setting the second frequency to a higher frequency side than the first frequency, the measured value at the first frequency can be similarly obtained. The thickness is determined by correction.

As described above, in the present invention, temperature correction of the material can be performed with high accuracy by selecting an appropriate frequency according to the object to be measured.

In addition, in this embodiment, the mixer circuit uses a superimposition type mixer circuit, but instead of this, a type in which the first and second frequencies are alternately switched and applied may be used.

〔effect〕

As mentioned above, in the method of the present invention, the first. The measured value of the first frequency is corrected based on the relationship between the measured value and the temperature of the two frequencies at the time of calibration, which is determined in advance from the difference in the measured value of the second two-week wave. Accurate thickness can be obtained without the need for frequent calibration with reference tubes.
There is no need to set up a constant temperature room. Therefore, there is no reduction in inspection efficiency, and the equipment cost does not increase. Furthermore, the method of the present invention has excellent effects, such as being an indispensable technique and very effective for, for example, measuring when there is a temperature change in the middle of a long object to be inspected immediately after manufacturing. .

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the implementation state of the method of the present invention, and FIG.
The figure is a graph showing the relationship between temperature and the measured value of thickness, and FIG. 3 is an explanatory diagram of the correction method. 1... Oscillator 2... High frequency divider 3... Low frequency divider 5... Measuring section 6... Sensor coil 8...
・Liner tube patent Applicant Sumitomo Metal Industries Co., Ltd. Agent Patent attorney Noboru Kono Lambda degree ('C) Figure 3

Claims (1)

[Claims] 1. A method for measuring the thickness of an object to be inspected by an eddy current method, comprising: a first frequency whose eddy current penetration depth is associated based on the thickness to be measured; The thickness is measured using the first frequency and a different second frequency, the temperature of the object to be inspected is estimated from the difference between the measured values, and the measured value of the first frequency is corrected based on the result. A thickness measuring method characterized by determining the thickness.