SU1298638A1 - Method of electromagnetic field scanning of articles in eddy-current checking - Google Patents

Abstract

) The invention can be used with an eddy current inspection of extended metal products using scanning electromagnetic fields. The aim of the invention is to improve the accuracy of control by obtaining a highly stable resulting excitation electromagnetic field of constant amplitude that moves smoothly at a given or controlled speed across the entire surface of the product. The field pathogen in the form of a system of coils extended in space relative to each other half the distance between the coil sides of the coils is fed by high-frequency currents and balanced by low-frequency currents shifted by 90 & which excludes the carrier frequency, and the coils supply only the currents of the side frequency, the modulating voltage of which is out of phase by 90. 2 Il. (L with N5 CO 00 O5 CO OO

Description

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The invention relates to a counter measuring and measuring technique and can be used for flaw detection by scanning a metal surface of an electromagnetic field,

The purpose of the invention is to increase the accuracy of control by creating an electromagnetic field, the greatest resultant intensity of which smoothly, with constant speed and amplitude, moves in space, whereby all surface areas are controlled, with constant sensitivity.

Fig. 1 shows a diagram of the arrangement of the coils of the electromagnetic field-exciting system and a block diagram of the device that implements the control method; in fig. 2 - graphs of the change in the vector potential of the exciting system for different values of time.

The excitation system consists of single-type extended coils, the extended edges of which are linear conductors with current arranged parallel to each other and shifted in the plane of their arrangement by half (0.5 D) distances (D) between their extended The sides of the excitation system of the coils is connected to a power device consisting of a high frequency generator 1, a low frequency generator 2, a phase converter

gate 3 and two balanced modules

Ditch 4 “The high-frequency generator 1 is connected to the first inputs of the balanced modulators 4, to the second inputs of which are connected in series the low-frequency generator 2 and the phase shifter 3.

The method is carried out as follows.

The system of long coils is locked by the high frequency current of the generator 1 and the low frequency current of the generator 2. Moreover, the low frequency currents supplied to the second inputs of the balanced modulators are phase shifted by the phase shifter 3 by 90 each relative to the other. The modulators 4 provide balanced modulation of the high frequency currents supplied by the coil system by low frequency currents.

Due to a phase shift of 90 modulating low frequency currents, the carrier frequency is eliminated, and the coils are fed by side frequency currents

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Voltages are shifted in phase by 90.

It is known that in balanced modulators, the output voltage is equal to V th V, where V and V are the voltages at its inputs. Consequently, given that both voltages are sinusoidal and differ in frequency, the current density in any of the long conductors of the coils can be expressed as

J 2MJ thfsin SJt) sin (g) t

m

where J is the maximum amplitude

tT

current density; M is the modulation depth; Sb, UJ - circular frequencies, respectively, of low-frequency and high-frequency oscillations; t is the time

Let x be the distance along the axis of distribution of the long sides of the coils relative to the zero point in the center of the exciting system. Since the coils of the excitation system, which are shifted to each other by D / 2, are fed from different balanced modulators, in each of the long conductors of these coils the phase of the low-frequency component of the current density differs by Ti / 2, T, es relative to the center, depending on from x, taking into account the fact that the distance to each of the conductors X nD / 2, where n is the conductor number, this phase is distributed in 31 23ix

conductors like --- n

2D

.Ras

The determination of the current density in the conductors can then be written K-ak

 th sin (at - I n) l sinuJt, a since the th function has an effect on the uniformity of the low-frequency component, the inputs of balanced modulators must be connected so that this function is taken from the high-frequency component. Then in the final form

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J 2MJ sin (t - -% - n) th (sinujt). f 2

It is known that the intensity of the electromagnetic field; d can be considered through the vector potential A. Assuming that the unprotected edges of the coils are extended from the object by a distance, at which they can be neglected for the long side of the coil, the voltage A is similar known expression for a linear conductor with current:

A - -2 41G

J. ,

where .f o is the magnetic constant; r is the distance to the conductor from the observation point in the cylindrical coordinate system.

Taking into account the principle of superposition, A for all the extensive conductors of the exciting system can be found as the sum of the vector potentials of each of them. Then, introducing the Cartesian coordinate system, produced by the norm o

roving to and entering relative

43T

2x

r parameters x ---;

Z --- where Z is the projection

on the axis.

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orthogonal to the distribution plane of the conductors of the exciting system, an expression is obtained for the vector potential of this system

 At A 5II.2MJ - sin (n) th (sinu3t

N-h

fn

 (Oj.) J + (x + (0.5 -

The values calculated for this formula for the vector potential at

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7. 1.5; - P / 2; :, i п oU - 1 Е various points along the distribution axis of the extensive conductors of the exciting system, for different times, are shown in the form of graphs (Fig. 2), where the circles indicate the locations of the conductors. It can be seen from the graphs that with time variation, the maximum value A evenly, with constant step, moves along the X axis. The step, and hence the scanning speed V 2 / 2jr, is determined by the frequency of low-frequency oscillations.

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F o r mula inventions

A method of scanning products with an electromagnetic field under eddy current control, which consists in exciting an electromagnetic field with a system of extended coils that are half-spaced relative to each other in half the distance between their extended sides by passing high-frequency currents modulated by phase low-frequency currents, characterized by the fact that, in order to increase the accuracy of control, they carry out a balanced modulation of high-frequency currents by low-frequency currents shifted in phase by 90, The carrier frequency is switched on, and the coils are fed by side-frequency currents whose modulation voltage is out of phase by 90.

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Claims (1)

Claim A method of scanning products by an electromagnetic field during eddy current control, which consists in exciting an electromagnetic field by a system of long coils, shifted in space relative to each other by half the distance between their pro. heavy sides, by transmitting high-frequency currents modulated by phase-shifted low-frequency currents, characterized in that, in order to improve the control accuracy, balance the modulation of high-frequency currents by low-frequency currents 90 ° phase-shifted, excluding the carrier frequency, and the coils supply currents of side frequencies, the modulating voltages of which are phase shifted by 90. FIG. 1 1298633