STUDY OF A FLAW DETECTOR CIRCUIT WITH A PARAMETRIC SENSOR INCLUDED IN THE OSCILLATORY CIRCUIT OF A GENERATOR

V. K. Zhukov, D. V. Milyaev, A. M. Panasyuchenko

(Presented by the scientific seminar of the Research Institute of Electronic Introscopy)

Known designs of electromagnetic flaw detectors include, in the measuring part of the circuit, a voltage generator of a specified frequency, a two-winding eddy-current self-comparison sensor, and a selective amplifier that extracts the pulse of the envelope of the carrier frequency caused by a defect.

The sensitivity of the indicated types of flaw detectors is limited by industrial-frequency pickup at 50 Hz and by the voltage of the carrier frequency, including harmonic components. The effect of pickup is most pronounced when the lower boundary of the passband of the amplifying path is close to 50 Hz or even below it. In this case, pickup can be combated only by shielding, which does not always give the desired results.

The carrier-frequency voltage and its harmonics begin to limit the sensitivity if the upper frequency of the passband of the path approaches the carrier frequency. This circumstance occurs when detecting defects in rapidly moving products.

The flaw-detector circuit in which the sensor is included in the oscillatory circuit of the generator is, to a considerable degree, free of the shortcomings noted above (Fig. 1).

Fig. 1

The flaw-detector circuit described was developed for detecting defects by means of a through-type sensor in nonmagnetic, elongated products moving at high speed.

As the article passes through the inspection zone of sensor D, the frequency of generator G is modulated, caused by the appearance of defects, by vibrations of the article, and by chaotic changes in its geometric dimensions. The frequency-modulated oscillations are converted in frequency detector FD into corresponding changes in DC voltage. The frequency detector is designed according to the circuit of a ratio detector [1], which effectively suppresses various amplitude interferences acting on the detector and on the generator preceding it.

Unlike flaw detectors that use amplitude changes in eddy-current sensors, the frequency-based method of defect inspection is distinguished by high noise immunity and a small signal-to-noise ratio. The noise voltage level is usually limited by 50-Hz pickup and by harmonics of the carrier frequency.

At generator operating frequencies lying within several tens of kilohertz, frequency deviations due to modulation of the parameters of the active element of the generator by 50-Hz pickup may be practically neglected. Noise caused by harmonic components of the carrier frequency is much easier to combat, since their spectrum lies considerably above the frequency spectrum occupied by defects. On the other hand, in flaw-detector circuits using the frequency method, conversion of the useful signal of the sensor into a voltage takes place with additional amplification in the frequency detector. For this purpose it is necessary to use detectors with a high steepness of the conversion characteristic. In the flaw-detector circuit under consideration, the steepness was \(6 \text{ mV/Hz}\) with the width of the rectilinear portion of the characteristic about \(3 \text{ kHz}\). After detection the signal is amplified by amplifier U, which has a rise in the amplitude-frequency characteristic in the range \(300 \div 2000 \text{ Hz}\). This range corresponds to the frequency spectrum of defects at a speed of motion of the inspected article of \(10—20 \text{ m/sec}\). Extraction of the useful signal against the noise background is performed by threshold device TD, which is an amplitude selector.

Narrowing the linear portion of the detector characteristic increases the amplifying properties of the detector; however, when inspecting articles of different geometric dimensions, for each standard size it is necessary to set the generator frequency equal to the pass frequency of the detector.

When inspecting moving articles, false signals appear, caused by vibration of the article in the sensor. Such vibrations may also shift the operating point of the detector onto the linear part of the characteristic, as a result of which defects may be missed. To eliminate this shortcoming, the proposed circuit uses two automatic systems for tuning the generator frequency: an astatic system and a static system with low inertia. The astatic system, which includes low-frequency voltage amplifier LFA, a motor, and control element CE, carries out automatic frequency tuning under large disturbing influences arising from changes in the diameter of the inspected article, vibrations of the article, etc. Static regulation is carried out at the moment the frequency enters the working region of the detector. The static-regulation device includes filter F, which passes signals in the frequency range from 0 to 300 Hz, and control element CE\(_2\), for which a D808-type semiconductor Zener diode is used, reverse-biased by an initial voltage of 0.5 V. The static feedback considerably reduces the effect of interferences with frequencies below 300 Hz, i.e., disturbances that the astatic link does not compensate.

Experimental studies of the flaw-detector circuit were carried out in order to determine the maximum resolving power of the flaw detector and to reveal the quality of operation of the automatic-control system. From these studies it was established that the sensitivity of the flaw detector, expressed through the relative change in frequency, is an order of magnitude higher than that of a flaw detector using the amplitude method. Studies of the dynamic properties of the control system were carried out at a signal-to-noise ratio equal to 2. Figure 2 presents the dependence of the probability of the appearance of interference at the output of the threshold device of the flaw detector on the magnitude of the interference at different frequencies of its repetition. Figure 3 gives the dependences of the probability of the appearance of interference on the frequency of its appearance. Increasing the repetition frequency reduces the number of false signals. The appearance of a large number of false operations of the threshold device at low repetition frequencies (0.1 Hz) is explained by the imperfection of the astatical control system, which creates interference during tuning. When the amplitude of the interference is reduced, a static system is connected to the astatical system, and the probability of the appearance of interference decreases (Fig. 3, \(U_{\mathrm{p}} = 4; 1.5\) V).

Fig. 2

Fig. 3

As a result of the studies carried out, qualitative characteristics of the automatic frequency-control systems were established. For the purpose of further improving the flaw detector, it is proposed to abandon the astatical system, replacing it with a static one, while increasing the range of automatic tuning and the transmission coefficient of the frequency detector.