IMPROVING THE NOISE IMMUNITY OF A FEED-THROUGH EDDY-CURRENT FLAW DETECTOR

B. A. Dobner, V. K. Zhukov

(Presented by the Scientific and Technical Seminar of the Research Institute of Electronic Introscopy)

The problem of noise immunity of electromagnetic flaw-detection equipment is at present acquiring primary importance in connection with the introduction of automatic flaw detectors. In the absence of special measures to improve noise immunity, even a single interference pulse may cause the false rejection of a good product.

Mechanical interference in the form of periodic vibrations and individual impacts of the inspected article acts on the flaw-detector sensor. This causes the appearance of a false signal in the receiving part of the flaw detector and triggers its automation. Random electrical interference in the form of bursts of fluctuation noise, as well as 50-Hz pickup (mains interference), acts mainly on the input circuits of the amplifiers. Voltage surges in the flaw detector’s power supply, caused by the switching on and off of high-power process equipment in the shop, are also dangerous.

This paper describes a circuit for improving the noise immunity of an electromagnetic flaw detector, shown in Fig. 1. The flaw detector uses two feed-through self-balancing eddy-current sensors \((D_1, D_2)\), having a common magnetizing winding and two pairs of measuring windings located at a certain distance from one another and each connected to its own channel.

Fig. 1. Block diagram of the flaw detector

If the inspected item, passing through the transducer at a quite definite velocity, has a defect, then pulses are obtained at the outputs of both channels, shifted by the time

\[ t_3=\frac{l}{v}, \]

where

\(l\) is the distance between the measuring sections of the transducers,
\(v\) is the velocity of motion of the defect.

At the output of the flaw detector a time selector of pulses is used, consisting of three one-shot multivibrators, OM\(_1\)—OM\(_3\), and a coincidence circuit CC.

Fig. 2. Dependence of the number of false operations: a) on the magnitude of jumps in the collector supply; b) on the frequency of jumps in the collector supply; c) on the field strength of the mains interference

The appearance of a pulse in the first channel will cause OM\(_1\) to operate; it produces a pulse of duration \(\tau_1 < t_3\). After a time \(t_3\), in the second channel the one-shot multivibrator OM\(_2\) operates; it produces a pulse of duration \(\tau_2 = 0.5\,\tau_1\). This pulse is fed to one of the inputs of the coincidence circuit CC. The trailing edge of the OM\(_1\) pulse triggers the one-shot multivibrator OM\(_3\), which produces a pulse of duration

\[ \tau_3 \geq t_3-\tau_1+\tau_2, \]

fed to the second input of the coincidence circuit. Thus, two pulses will act simultaneously at both inputs of the coincidence circuit. In this case a rectangular pulse will appear at the output of the coincidence circuit; entering the automation unit, it will cause rejection of the given item.

In the case where interference acts simultaneously in the two channels, two pulses will appear simultaneously at the inputs of the pulse selector. As a result, the pulses fed to the coincidence circuit are separated in time, and the coincidence circuit does not produce a signal. In this case rejection of the item will not occur.

The study of noise immunity was carried out with the aid of a signal and interference simulator. To determine the influence of changes in the supply voltage of the flaw detector, jumps in the supply voltage were created artificially; moreover, both a change in the magnitude of the jumps and a change in their frequency were provided for. Figure 2a presents the dependence of the number of operations of the flaw detector on

the magnitude of the relative change in the supply voltage, with the total number of steps \(n = 100\).

It is evident from the graph that appreciable overrejection appears when the relative change in the supply voltage is 30%. Figure 2a gives the dependence of the number of flaw-detector actuations on the frequency of the supply-voltage steps (curve 1), and the same dependence under the combined action of the useful signal and the supply-voltage steps (curve 2). In this case the useful signal was simulated by means of a pulse generator and a polarized relay, while the number of actuations was recorded over a specified time interval by a counting instrument PP.

To investigate the influence of industrial-frequency pickup, the flaw-detector sensor was placed in a solenoid powered through an autotransformer from a 50-Hz network. Owing to the use of self-balancing sensors, the selective property of the carrier-frequency amplifiers UN, and the threshold device PU at the inputs of the pulse selector, under the action of pickup alone (without the useful signal) no flaw-detector actuations are observed over a comparatively wide range of pickup-field strengths (0–25 oersteds).

Figure 2b gives the dependence of the number of flaw-detector actuations on the field strength inside the solenoid. The dashed line corresponds to the normal number of actuations in the absence of interference. It is evident from the graph that the combined action of interference and the useful signal, depending on the magnitude of the interference, causes either underrejection or overrejection of the product.

A parallel investigation of one of the channels showed that the number of actuations of a single channel under the action of interference considerably exceeds the number of actuations of the two-channel circuit, which fully justifies the use of the second channel.

The investigations performed confirmed the expected substantial reduction in false actuations of the flaw detector caused by supply-voltage steps, vibrations of the item being inspected, and changes in its electromagnetic properties and dimensions.