Subminiature eddy current transducers for studying metal- dielectric junctions

Based on an eddy current transducer (ECT), a probe has been designed to research metal-dielectric structures. The measurement procedure allowing one to detect defects in laminate composites with a high accuracy is described. The transducer was tested on the layered structure consisting of paper and aluminum layers with a thickness of 100 μm each in which the model defect was placed. The dependences of the ECT signal on the defect in this structure are given.


Introduction
A subminiature eddy-current transducer [1][2][3][4][5][6] has been designed to monitor physical parameters when studying the properties of metal-dielectric junctions. The material and its distribution over the surface and in the thickness of the studied object. The developed measuring system allows effective investigate the metal-insulator transitions in miniature metal-polymer composite objects. Similar composites may include several metal layers separated by thin dielectric layers of polymer. The typical defects of such materials are, for example, the disturbance of layers' continuity and the formation of link between the layers. The analysis of a recent investigations points to the miniaturization tendency for eddy-current transducers. Transducers with a size of 5 × 5 mm and a 0.15 mm diameter of the wire have been designed [7]. However, they do not provide the required penetration depth and localization of the magnetic field that are necessary for local measurements in different nonuniform media. Ferrite magnetic field concentrators are often used to increase the area of the magnetic field. A similar design provides an advantage that is related to the absence of the scatter of eddy currents [8]. In addition, a 2.5 mm penetration depth is attained. Experimental methods based on two eddy-current transducers that operate in the differential mode have been developed [9]. Similar connection circuits significantly decrease parasitic noise that arises during high-speed scanning in real time. The analysis of a recent investigations points to the miniaturization tendency for eddy-current transducers. L. Barbato et al. [10] scanned two aluminum plates with a model flaw in the centre and tested cracks between the plates. The diameter of the measuring winding was 7 mm. The scanning was performed at 1 and 5 kHz. In this case, the penetration depth of eddy currents into the studied plates at the abovementioned frequencies was 3.82 and 1.71 mm. In the past there were several attempts to use highly sophisticated magnetic field sensors like SQUID and Fluxgate sensors for sensitive low frequency eddy current testing to detect deep defects in metal parts [11][12]. Although very good results could be achieved such testing systems can hardly be used in real industrial applications because of their complexity, prices and insufficient robustness [13].
In this connection, the challenge is to design subminiature eddy-current transducers that provide a penetration depth of up to 5 mm and an area of 2500 μm 2 . Since the eddy-current inspection method is insensitive to non-conducting paint layers, it can also be used for diagnosing parts with paint coats.

The design of a subminiature eddy-current transducer and measuring system
The design of the measuring system includes two differentially connected subminiature transducers, which provide a large area of the magnetic field. The tested parameter is the electric conductivity of the material and its distribution over the studied object. Exciting subminiature transmitter winding consists of 10 turns, and its diameter is 0.13-0.12 mm. Measuring winding consists of 130 turns and its diameter is 0.05-0.08 mm. In order to minimize the influence of the exciting windings on the received signal, compensating winding is included into the circuit. The compensating winding is connected to the measuring winding so that the voltage of the exciting windings can be subtracted. The compensating winding consists of 20 turns. Copper wire 5 µm thick is used to the wind the turns. The windings are wound on the pyramidal core of the ferrite 2000 HM3 with relative magnetic permeability (µmax=500) or (if necessary, a higher localization of the magnetic field) of the annealed by a special technique 81NMA alloy. The core is a pyramid in the shape of a tetrahedron 1 mm in height, with a side of the bases 0.2 mm. The measuring winding is located on the tip of the pyramid, which improves the magnetic field localization. The scheme of the subminiature eddy-current transducer (ECT) is shown in Figure1. The eddy-current transducer is a transformer with measuring (1), exciting (2), and compensation (3) windings and a magnetic circuit 4, which is located inside the cylindrical platform 5 with tracks that are cut on the external side for windings. The platform is impregnated with a compound 6 at a temperature of 200°C to prevent the disintegration of the windings when the ferrite screen 7, which is intended for the localization of the electromagnetic field on the tested object, is put in place. From the outside the transducer is contained in a corundum washer 8, which protects the core 4 from contacting the tested object. The characteristics of the designed transducers allow one to efficiently localize the magnetic field within 2500 μm 2 and provide penetration of the magnetic field into the studied object at a depth of up to 1.4 mm. The eddy-current transducer is connected to a set of the designed amplifiers and band-pass filters and is controlled by the sound map of a personal computer with special software, which applies a voltage to the generator winding of the transducer. Digital signal from the virtual oscillator is fed to analog-to-digital converter (ADC) input of sound card, after which the analog signal through the power amplifier is fed to the exciting winding transducers. Passing through the excitation windings, sine wave signal creates electromagnetic field which induces voltage in the measuring windings of eddy-current transducer. The voltage depends on the parameters of the object under control. The transducers are included counter-currently, whereby the resultant signal represents the difference between two values of voltage. This signal is fed to a series of amplifiers and bandpass filters, and then supplied to the microphone input of the sound card and then through the preamplifier -to the input of an analog-to-digital converter (ADC) of sound card. The analog signal is converted to the digital one and transferred to software processing and control unit. The processing and control unit records the level of the digital signal in conventional units corresponding to the difference between the voltages on the measuring windings.

Experimental results
To demonstrate the proposed method operability, the structure of alternating aluminum foil of 100 µm thick and paper of 100 µm thick has been used. As a defect model between the layers a hollow parallelepiped with a wall of 300 µm thick has been placed. The defect was at a distance of 600 µm from the sensor in the depth of the layered structure.  At a frequency of measurement equal to 6000 Hz (Figure 3), the defect model is still clearly visible, however the amplitude of oscillation on a defectless portion of the sample, already exceed 7% of the signal level corresponding to the defectless portion of the sample. In laboratory and industrial measurements, such oscillation amplitude can be caused by external influences, unconditioned the presence of defects or small defects on the surface of the metal layer. In the study of the test object with unknown defects, similar changes amplitude may mistakenly be interpreted as a defect.  When operating frequency of the device is beyond the given limits the results of the measurements will be distorted by amplitude fluctuations caused by microcracks on the surface of the sample or by the decreasing of field localization inside the layered structure. Figure 4 shows the measurement results of the sample at a frequency of 7000 Hz. As seen from the graph, the amplitude changes, caused, in this case, by the microcracks on the sample surface, are much higher than the amplitude changes, caused by the defect directly.

Conclusion
Right up to the depth of the defect, equal to 1400 μm, there is a clear dependence of the response of transducer, on the position of the transducer over the defect. By fixing change in amplitude response in the converter caused by a defect, it is possible to change the frequency of the current in the exciting winding so that the eddy-currents are concentrated in the layers of composite placed above the defect. The solution of the inverse problem allows to determine the depth of the defect. After calibration, the sensor for typical defects, can use the eddy-current transducer for the diagnosis of composite laminates with a thickness of 1 to 1400 μm.