PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE
OF THE ORDER OF THE OCTOBER REVOLUTION AND THE ORDER OF THE RED BANNER OF LABOR
named after S. M. Kirov

Vol. 248 1975

CONTROL AND MEASUREMENTS

USING THE HIGHER HARMONICS METHOD

I. G. LESHCHENKO

(Presented by the Scientific and Technical Seminar of the Research Institute of Electrical Measurements and the Institute of Information Technology, TPI)

Of particular interest for scientific and industrial purposes is the development of methods and means for quality control of ferromagnetic materials and products that possess strongly expressed nonlinear characteristics during remagnetization. At present, for these purposes the eddy-current method is becoming widely used, with the higher harmonics of the electromotive force of control and measuring sensors used as measuring information [1]. This method of electromagnetic quality control of products and measurement of various nonelectrical quantities, which has been named the higher harmonics method (HHM), is based on the use of the nonlinear properties of ferromagnetic products and harmonic analysis of the electromagnetic parameters of the sensor [2]. Depending on the sensor connection circuit and its design, the harmonic composition of the e.m.f. of the secondary winding is most often used; however, no fewer possibilities can be obtained by harmonic analysis of the magnetizing current of a single-winding sensor supplied in a specified-voltage mode.

In contrast to known electromagnetic methods for quality control of products and materials (eddy-current, oscillographic, by characteristic points of the dynamic remagnetization loop, etc.), the HHM has significantly greater information capacity with respect to changes in particular parameters of the controlled region of a product.

It is known that the symmetric dynamic remagnetization loop of a ferromagnet is a carrier of multidimensional information about the properties of that layer of the product into which the electromagnetic field penetrates. The use of bias magnetization by a constant field and especially by an alternating field of another frequency leads to an additional increase in the information obtained about the properties of the remagnetized region of the product. As our studies have shown, the most complete information about one or simultaneously about several controlled parameters can be achieved by the HHM when the sensors are supplied with a current modulated in amplitude or frequency [1].

The advantages of the HHM consist not only in a quantitative increase in information, which already determines its promise for multiparameter nondestructive testing, but also in the fact that the sensitivity to the controlled parameters is thereby increased, and broader possibilities are opened for electromagnetic methods of quality control of products and materials for the purposes of structurescopy, flaw detection, and measurement of nonelectrical and magnetic quantities. The latter circumstance is due to the unequal dependence of the fundamental and higher harmonics of the sensor signal on the same para-

of the parameters of a ferromagnet [3, 4]. Questions of the study and application of the higher-harmonics method were reflected in candidate dissertations by M. M. Shelya, K. V. Markevich, A. M. Gruznov, G. M. Kurakin, D. I. Yakirevich, A. D. Pokrovsky, V. V. Chernyak, V. D. Sartakov, and Z. S. Kulysheva, as well as in works of the early period [5, 6, etc.].

A characteristic feature of the method is also that the higher harmonics arise in the ferromagnet itself and, consequently, the information obtained does not depend on the filling factor of the measuring coil in comparatively homogeneous magnetic fields, while the change in the e.m.f. harmonic components is associated with a change in the structure of the article or of its region, with the appearance of various types of defects, and with changes in geometrical dimensions and electrical conductivity. The presence of internal local defects leads to the appearance in this zone of higher harmonics, which are used to detect these defects by the higher-harmonics method; in this case the sensitivity to defects can be considerably increased, since in the region of a defect, as a rule, concentrations of internal stresses arise. When a ferromagnetic article is remagnetized simultaneously by two fields of different amplitudes and frequencies, commensurable and incommensurable in their values, new useful effects appear, which have begun to be widely used for purposes of flaw detection, structuroscopy, and measurement of nonelectrical quantities [1, 7, etc.]. The presence of electrodynamic forces of interaction between the magnetizing coil and the eddy-current contours leads, in strong fields, to micro-oscillations of the metal particles with double frequency and to distortion of the distribution of the eddy-current field, which also affects quality control of articles by the higher-harmonics method; wide application of this effect in flaw detection of various materials and articles is beginning [8].

The possibility of using higher harmonics in the e.m.f. signal of the secondary winding of transducers for analysis of the structural-mechanical properties of ferromagnetic materials and articles was first pointed out by N. S. Akulov in 1934 [5]. It was noted that the amplitudes and phases of the e.m.f. of the higher harmonics depend on magnetic permeability, coercive force, electrical conductivity, and the remagnetization regime, and, consequently, also on the mechanical properties of the articles being inspected. Later, in 1935–1939, only a few works were published reporting on methods and instruments of nondestructive inspection using higher harmonics [7]. However, the absence of harmonic-analysis equipment for about 25 years restrained the development of this very effective method of quality control of articles, although in implicit form work was developing on describing hysteresis loops and on studying the oscillographic method by Lissajous figures. And only thanks to the works of R. E. Ershov and the previously mentioned authors of dissertations did the higher-harmonics method gain recognition and begin to develop rapidly.

In the reference-bibliographic index of the literature on the theory and practice of control and measurements by the higher-harmonics method that we compiled, all reports, articles, dissertations, and other materials known to us are presented in chronological order, with abstracts or annotations to them [7].

There are very few foreign works on the study of the higher-harmonics method for quality control of articles, which indicates the leading role in this direction of the scientists of our country [7].

Analysis of published works, and of works on the higher-harmonics method that are at the research stage, makes it possible to formulate the following principal directions in the development of this method.

1. Development and study of the theoretical and physical foundations of the higher-harmonics method, consisting of an analytical representation of the dependence of induc-

depends on the magnetic-field intensity for the real shape of the remagnetization loop; investigation of the propagation of an electromagnetic field in ferromagnetic media; calculation of the amplitudes and phases of the harmonics of the secondary emf with allowance for eddy currents as functions of the material properties, geometrical dimensions, the presence of defects, etc., when the sensors are supplied both with sinusoidal and nonsinusoidal current, and also with the use of biasing by a constant and an alternating field for various sensor designs.

1. Development of methods and means for increasing the resolving power and sensitivity to particular monitored parameters through the choice of rational sensor designs; the form, amplitude, and frequency of the magnetizing current; harmonic analysis of the emf of the secondary winding, or of the emf difference in a differential circuit for connecting sensors; the use of rational methods for reducing the influence of interfering factors, etc.

2. Theoretical and experimental investigations of the HHM for the purposes of measuring various nonelectrical quantities (pressures, vibrations, mechanical stresses, motion parameters, etc.) and the properties of ferromagnetic materials and products. Development of appropriate measuring apparatus.

3. Development and investigation of methods and means for multiparameter monitoring and analysis of the measured information.

The theoretical direction of work on the HHM, especially for strong magnetic fields and with allowance for eddy currents, owing to the well-known difficulties of mathematically describing nonlinear processes in ferromagnets, lags far behind experimental investigations and the practical use of the HHM for the purposes of flaw detection, structuroscopy, and measurement of nonelectrical quantities. At present there are only a few works, and those dealing with the simplest approximations, which show the clear advantages of the HHM in comparison with other electromagnetic methods and qualitatively confirm the results obtained.

The practical use of the HHM for monitoring various parameters of ferromagnetic products was previously given in the article by M. M. Shelya [1]; therefore in the present work it is appropriate only to summarize the results achieved by the leading groups in the country and to indicate possible paths for research and application of the HHM.

The main direction of work carried out at the Institute of Physics of the Siberian Branch of the Academy of Sciences of the USSR under the direction of R. E. Ershov is connected with the development of the theory of propagation of the electromagnetic field of the fundamental and higher harmonics, and with the development of methods and means for increasing the sensitivity of the method when sensors are supplied with nonsinusoidal currents of various forms. The practical application of the HHM is the monitoring of structure, measurement of magnetic characteristics and defects of thin magnetic films, the hardness of steels, the thickness of the hardened layer, etc. [1, 9].

At IrkutskNIIKhimmash, under the direction of M. M. Shelya, the theory of the HHM and the propagation of an electromagnetic field in a ferromagnetic medium are being developed and investigated as applied to through-type and surface-type sensors [1, 10]. A number of important problems have been solved in the theory and practice of applying the magnetoelastic effect in the HHM for monitoring and measuring mechanical stresses in metals [4, 10].

A group of staff members of the Kiev Institute of Civil Aviation is carrying out and successfully solving a number of original and promising problems in the experimental investigation of the HHM as applied to monitoring the structure and quality of welded joints [11, 12], and to assessing stresses and fatigue characteristics of metals. Positive

the problem of multiparameter quality control of products using the higher harmonics of the emf is being solved. Investigations are being carried out on the HHM under pulsed excitation of magnetizing devices.

At Tomsk Polytechnic Institute, under our direction, positive results have been obtained in the flaw detection of ferromagnetic products by two-frequency through-type and surface-mounted transducers, in measuring hardness and other structural-mechanical characteristics of steels from the dynamic characteristics of the remagnetization loop, and with the use of a remote measuring coil [1, 12, 13, 14, etc.].

The two-frequency method makes it possible to detect small transverse and local defects both on the surface and in the depth of products. Studies are being conducted of harmonic analysis of the transducer current and of the form factor of the current and voltage curves.

The work of the Moscow Institute of Hydromelioration (supervisor T. Ya. Gorazdovskii) is connected with comprehensive theoretical and experimental studies of mechanical stresses in metals made of various ferromagnetic materials [1, 12, 14], the stability of metal structures, and fatigue cracks by the HHM.

At the Moscow Power Engineering Institute, theoretical and experimental work is being successfully carried out on approximations of hysteresis loops, calculation of harmonics of transducer emf, electronic modeling, and application of the HHM for purposes of product quality control [1, 8, 12].

In addition to the groups mentioned, a number of scientific institutions and higher educational establishments are beginning successfully to use and apply the HHM for purposes of flaw detection, structure inspection, and for measuring various nonelectrical quantities. An important role in the rapid development of the HHM is played by the works of M. A. Rosenblat, D. A. Savinovskii, I. D. Maergoyz, and others on the calculation of alternating magnetic fields in ferromagnetic media with allowance for the real shape of the hysteresis loop, the influence of eddy currents, and the use of biasing by direct and alternating current.

Combining the efforts of electromechanics specialists and specialists in electromagnetic methods of inspection for solving a number of theoretical and practical problems will undoubtedly have a positive effect on the more efficient use of the HHM not only for quality control of products made of structural materials widely used in industry, but also for the inspection of electrical steels, their magnetic characteristics, losses, etc., at various stages of production.

REFERENCES

1. Collection of materials of the All-Union Seminar “The Higher Harmonics Method in Eddy-Current Flaw Detection,” Publishing House of the Siberian Branch of the Academy of Sciences of the USSR, Krasnoyarsk, 1969.

2. R. E. Ershov, M. M. Shel, Defektoskopiya, No. 1, 1969.

3. R. E. Ershov, Izv. vuzov. Physics, No. 4, 1966.

4. Collection of reports “Studies in Metal Physics and Nondestructive Testing Methods,” Nauka i Tekhnika Publishing House, Minsk, 1968.

5. N. S. Akulov, Zavodskaya laboratoriya, No. 8, 1934.

6. N. I. Eremin, Vestnik mashinostroeniya, Nos. 1, 2, 1937.

7. I. G. Leshchenko, Control and Measurements by the Higher Harmonics Method. TsNTI Publishing House, Tomsk, 1970.

8. Yu. M. Shkarlet, Yu. A. Sazontov et al. In: Electromagnetic Methods of Control. MDNTP Publishing House, Moscow, 1969.

9. R. E. Ershov, Defektoskopiya, No. 1, 1966.

10. M. M. Shel, Nondestructive Testing by the Method of Higher Harmonics of Eddy Currents. Brochure, Irkutsk, 1970.

11. G. M. Kurakin, Avtomaticheskaya svarka, No. 12, 1968.

12. Abstracts of papers of the Fifth All-Union Conference on Nondestructive Testing. Moscow—Sverdlovsk, 1967.

13. A. M. Gruznov, I. G. Leshchenko, B. B. Vinokurov, Izv. TPI, Tomsk, vol. 194, 1971.

14. Reports of the 2nd All-Union Seminar “Nondestructive Testing of Ferromagnetic Materials and Products by Higher Harmonics Methods,” ed. I. G. Leshchenko, Tomsk, 1971.