STUDY OF THE TWO-FREQUENCY METHOD UNDER CONDITIONS OF MOTION OF THE INSPECTED OBJECT

V. I. Urusov

(Presented by the scientific and technical seminar of the Department of Information and Measurement Technology)

A characteristic feature of the two-frequency method, in comparison with other electromagnetic methods, is the magnetization of a ferromagnetic object by two alternating fields: a weak high-frequency field and a sufficiently strong low-frequency field. As a result of such magnetization, an emf arises in the measuring coil of the transducer, containing both low-frequency and high-frequency harmonics. The possibility of using not only the fundamental harmonics but also higher harmonics significantly increases the amount of information about the inspected object and makes the two-frequency method an effective means for studying the properties and monitoring the parameters of ferromagnetic materials and products.

It should be noted, however, that despite fairly deep and extensive studies of the two-frequency method, the questions of the interaction of a two-frequency transducer with a moving object have remained little studied up to the present time. Meanwhile, the presence of a strong low-frequency field can have a substantial influence on inspection results when passing from stationary objects to rapidly moving ones. Taking into account the importance of this question in the development of inspection equipment for continuous production (for example, rolling production), we carried out corresponding experimental studies to investigate the influence of motion of the object on the output parameter of a two-frequency surface-mounted transducer, for which the mean value of the second harmonic was chosen, depending on the speed of motion of the object, the ratio of the frequencies of the high-frequency and low-frequency magnetizing fields, the radius of the low-frequency coil, and the gap between the transducer and the object.

The block diagram of the experimental setup and the transducer designs used in the investigations are shown, respectively, in Fig. 1, a, b. The setup contains two generators—a high-frequency generator \(G_{\omega}\) and a low-frequency generator \(G_{\Omega}\), a low-pass filter \(F\), a power amplifier \(PA\), a transducer having three coils—two magnetizing coils (\(W_{\omega}\), \(W_{\Omega}\)) and a measuring coil \(w_{2}\), a selective voltmeter \(SV\), an electronic oscilloscope \(EO\), and a device allowing the translational motion of the object to be replaced by rotational motion. The test object was a disk of carbon steel rotated by a DC motor. By changing the excitation current of the motor it was possible to re-

regulate the rotational speed of the disk from 0 to 30 rev/s, which corresponded to a linear velocity of disk motion at the center of the sensor from 0 to 15 m/s.

Fig. 1. Block diagram of the setup (a) and sensor design (b)

For a quantitative estimate of the magnitude of the “velocity effect,” a dimensionless coefficient was introduced:

\[ k_v=\frac{U_{2\omega\mathrm{cp}v}}{U_{2\omega\mathrm{cp}0}}, \]

where \(U_{2\omega\mathrm{cp}v}\) is the mean value of the second-harmonic voltage when the object moves with velocity \(V\), m/s;
\(U_{2\omega\mathrm{cp}0}\) is the same for \(V=0\).

The plots of the dependences of the velocity coefficient \(K_v\) on the velocity \(V\)

Fig. 2. Dependences of the velocity coefficient \(k_v\) on the velocity \(V\) under various test conditions

of motion of the object give a clear representation of the “velocity effect.” In the following figures, the curves \(K_v=f(V)\) are shown for variation of the magnitude of the low-frequency field \(H_{m\Omega}\) (Fig. 2, a) and its frequency \(\Omega\) (Fig. 2, b); the frequency of the high-frequency field (Fig. 2, c), the diameter of the low-frequency coil \(d_{\Omega\mathrm{n}}\) (Fig. 2, d), and the gap \(h\) (Fig. 2, e).

Analysis of the results obtained shows the following:

1. The magnitude of the change in the mean value of the second harmonic at the sensor output when the inspected object is moving depends on the speed of the latter, and this dependence is, in the general case, nonlinear. If, in the region of weak low-frequency fields, motion of the article causes an increase in the second harmonic in comparison with the static case, then in the region of medium and strong low-frequency fields, motion of the article leads, as a rule, to a decrease in the second harmonic.

2. The “velocity effect” depends both on the parameters of the magnetizing fields \((H_{m\Omega}, \Omega, H_{m\omega}, \omega)\) and on the diameter of the low-frequency coil and the gap between the sensor and the object. With an increase in the frequencies of the high-frequency and low-frequency fields, as well as in the diameter of the low-frequency coil and the gap, the “velocity effect” decreases. However, it does not appear possible to recommend increasing the frequency of the low-frequency field and the gap in order to reduce the “velocity effect,” since this sharply reduces the overall level of the output signal. Thus, to reduce the “velocity effect,” it is possible to recommend increasing two quantities: the frequency of the high-frequency field and the diameter of the low-frequency coil.