Conductivity Testing of Nonmagnetic Metals by the Voltage Phase of a Surface-Mounted Sensor

V. K. Zhukov, A. M. Panasyuchenko, R. M. Zakirov

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

For monitoring the structural state and sorting various alloys by grade, industry uses mass-produced electromagnetic instruments—electrical-conductivity meters. At present there are modifications of the instruments (IE-1, IE-11, IE-20) that differ only in the frequency of the current exciting the electromagnetic field in the sensor. The operating principle of all the instruments is the same and consists in recording changes in the modulus of the complex resistance of the measuring winding of a surface-mounted sensor [1].

The modulus of the complex resistance of the sensor depends not only on the electrical conductivity of the product being tested, but also on the magnitude of the nonconducting gap between the surfaces of the sensor and the metal. The nonconducting gap may be formed by oxide films, scale, or a layer of paint. Fluctuations in the magnitude of the gap, inevitable in the course of operation, lead to changes in the resistance modulus and, consequently, to error.

To reduce the error caused by the gap, various methods are used to compensate for the influence of the gap. In IE-type meters, a special capacitor is used for this purpose; by selecting its value, a condition is achieved in which the resistance modulus of the circuit, consisting of the series-connected capacitor and the inductance of the measuring winding, becomes insensitive to small fluctuations of the gap. With this method of detuning, it is possible to eliminate the influence of the gap only in the case of small changes in the electrical conductivity of the metal. If the electrical conductivity being tested varies over wide limits, then the compensating capacitor must have variable capacitance, and its value must be set depending on the value of the electrical conductivity being tested.

The method of detuning from the influence of the gap used in mass-produced meters provides insensitivity to gaps (within the limits of the instrument error) not exceeding 200 μm; moreover, before each measurement it requires adjustment of the compensation circuit, which, taken together, reduces the quality characteristics of the instruments and makes them inconvenient in operation.

Considerably better results, both in terms of accuracy and in terms of ease of operation, can be obtained if

monitoring should be carried out by the voltage phase of a two-winding surface-mounted sensor. Theoretical studies show that the phase of the emf introduced into the measuring winding by the product does not depend on the distance between the surface of the product and the sensor; consequently, if monitoring is carried out by the phase of the introduced emf, it will be possible to eliminate completely the influence of the gap when it varies over wide limits.

The experimental studies we have carried out showed that the phase of the introduced emf, although only weakly, does depend on the magnitude of the gap (Fig. 1), and, when monitoring small changes in electrical conductivity, can lead to a significant error.

The influence of the gap on the emf phase can be substantially reduced by introducing, in series with the introduced emf, an additional compensating vector \(U_k\). The magnitude of this vector is determined with the aid of the constructions shown in Fig. 2. The hodograph of the vectors of the introduced emfs is continued until it intersects the abscissa axis; the segment \(OK\),

Fig. 1. Dependence of the introduced emf on the gap, in mm, between the sensor and the surface of copper (a), aluminum (b), and brass (c) products.

cut off by it on the axis, and will be the desired vector, but with the opposite sign. Changing the direction of the vector \(OK\) by \(180^\circ\), we obtain the vector \(U_k\).

As a result of adding the compensating vector \(U_k\) and the vectors of the referred emf \(U_1, U_2, U_3\), new vectors \(U'_1, U'_2, U'_3\) are obtained, lying on one straight line.

Thus, if a compensating voltage \(U_k\) is introduced in series with the measuring winding, then the resultant voltage obtained in the measuring circuit of the sensor will have a phase independent of the magnitude of the gap. An increase in the gap causes only a decrease in the amplitude of the resultant voltage and, when monitoring by the phase of this voltage, will not affect the inspection error.

Fig. 2. On determining the compensating vector \(U_k\)

The error in measuring electrical conductivity by the eddy-current method depends on the shape of the item. Calibration of instruments is carried out with the sensor placed on a flat surface whose area considerably exceeds the cross-sectional area of the sensor. If, in the course of operation, the sensor is placed on a pad whose dimensions are comparable with the diameter of the sensor, then the phase of the referred emf will differ from the phase of the emf obtained when the sensor is placed on a surface incommensurable with the dimensions of the sensor.

For the purpose of quantitatively estimating the effect of the finite dimensions of the pad, the dependence of the phase of the referred emf on the relative distance between the sensor and the edge of the pad was recorded. From the graph (Fig. 3), constructed from experimental results, it is seen that when the ratio of the distance between the center of the sensor and the edge of the item \(l\) to the diameter of the sensor \(d\) is greater than 1.5, the emf phase practically does not sense the edge of the item.

If the surface of the pad of the item on which the sensor is installed differs from a plane, an error also arises in the instrument readings. The magnitude of this error depends on the ratio of the sensor diameter to the radius of curvature of the item. Figure 4 shows the dependence of the phase of the referred emf on the diameter of the cylinder \(D\). To obtain the indicated dependence, five copper cylinders of different diameters were made; the surface-mounted sensor was successively placed on them and the values of the phase of the referred emf were recorded.

Fig. 3. Dependence of the phase of the referred emf on the relative distance between the sensor and the edge of a metal plate: \(l\)—distance between the sensor and the edge of the plate, \(d\)—diameter of the ferromagnetic core of the sensor

The phase of the emf when the sensor was placed on a flat surface \((d/D = 0)\) was conventionally taken as zero. From the graph it is seen that the emf phase is strongly sensitive to the curvature of the surface; therefore, to reduce the associated error when monitoring the electrical conductivity of items not having-

of large flat surfaces, it is necessary to strive to reduce the diameter of the sensor core.

The investigations carried out show that, when the phase of the induced emf is used as the working parameter of the sensor, the error in checking the electrical conductivity of metals can be significantly

Fig. 4. Influence of the diameter of the conducting cylinder \(D\) on the phase of the induced emf of a surface-mounted sensor having core diameter \(d\).

reduced, and the technical characteristics of the measuring instruments thereby improved.

REFERENCES

1. A. L. Dorofeev. Nondestructive Testing by Eddy-Current Methods. Oborongiz, 1961.