Fast precise eddy current measurement of metals

Modern industries (incl. Industry 4.0 and beyond) expect fast, cost-efficient, trustable and precise data acquisition of sensors signals with appropriate data processing and inter-pretation. In metal processing industries eddy current sensorics can be used for fast (online) precise and efficient measurements of the various metal sheets and plates (e.g. of wide range of steel grades, but also other alloys), and also manufactured from these materials details and structures. The materials need to be identified and parts under test (PUT) to be characterized. Multifrequency measurement (up to full impedance spectroscopy) allows to characterize the properties of the PUT in the depth (or alternatively only at the surface layer), as the penetration depth of the electromagnetic fields depends on the measurement frequency. A solution, usable for measurement of electromagnetic properties of metals (on the example of steel specimen) is described. The solution using a small air-core planar coil sensor and a high accuracy data acquisition box USB-6281 (of National Instruments) with a Windows computer. In the software side the pre-calculated tables (according to Dodd-Deeds and further developed models) are exploited to convert the impedance values into electrical conductivity and sensor liftoff and magnetic permeability for the PUT. Advantage of the air-core coil is the absolute model-based accuracy and stability of the measurements. The solution is working in the frequency range 1 to 500 kHz, while the low frequency (100 Hz) measurement in the background is used to monitor the change of the ohmic (DC) coil resistance. By estimations the solution can measure the conductivity of the PUT in the range of 1 to 20 MS/m with spatial resolution of 5–8 millimeters. The relative magnetic permeability can be in the range of 1 to 15. As the real-life experiments show, the measurement speed of 100 depth-measurement profiles per second can be achieved in real-time. Also for measurement of the PUT there is n need to stop and fix the PUT under the sensor, but just online “on-the run” measurement can be applied, as the experiments confirm. As the variations of the measurement signals for the air-core sensor with relatively small number of turns is quite small, the impedance measurements of the sensor coil are needed to carried out with high resolution and accuracy, which is easily done by the mentioned USB box. Still, the low-pass filtering (e.g. by Savitzky-Golay filtering) of the longitudinal profile results can be reasonable.