Usage of Magnetic Field Sensors for Low Frequency Eddy Current Testing

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 (EC) testing to detect deep buried defects in metal parts [1–3]. Although very good results could be achieved such testing systems can hardly be used in real industrial applications because of the complexity and costs of such systems and their insufficient robustness. This situation stimulated us to perform a comparative study of the application of traditional inductive coils with improved sensitivity and of commercially available magnetic field sensors like AMR and GMR sensors in order to produce low cost EC probes with high sensitivity, acceptable lateral resolution and with sufficiently high robustness for usage in real industrial environment. Some results of these studies were already reported in [4]. Very promising sensor performance could be obtained by using highly sensitive inductive coils. The problem is that such sensors have to be produced by skilful specially trained operators, which is resulting in low reproducibility and low productivity. In this paper we will present new results of using commercially available GMR and AMR sensors in low frequency EC probes allowing to increase reproducibility and productivity of sensor production and maintaining the performance of EC probes on the level of those probes using highly sensitive inductive coils. We will especially concentrate on the problem how to overcome limitations arising from low dynamic range, nonlinearity and hysteresis in sensor characteristics of commercially available AMR and GMR sensors. 2 Methods for Comparison of Results Dealing with high sensitive EC probes there always arises the question of objective comparison of results obtained by several groups of investigators. According to our experience there is no objective criteria. This is due to the high complexity of EC systems including excitation coils with different efficiency, different sensing elements (coils, magnetic field sensors), different probe geometry, different read out electronics, different mechanical systems for EC imaging, different post-processing of EC raw data etc. The organisation of Round Robbin tests can help to estimate the performance of EC probes of different working groups, but it seems to be quite difficult to organise such comparative tests. In order to compare the performance of our different sensors we perform tests on various test specimens specially produced for comparative studies. This helps us to estimate the behaviour of our probes in terms of their ability to detect various test defects and in terms of lateral resolution. To end up with this discussion we have to keep in mind that the main criteria for sensitivity of EC probes is their ability to successfully perform tests on real industrial applications.