Non-Invasive inspection - new electromagnetic induction tomography
Eddy current inspection using electromagnetic induction is important in a range of technological applications such as coating, surface treatment, crack detection and quality inspection. This often consists of a single sensor head which is manually robotically scanned over the surface of the test piece. However, the scanning operation of such an instrument involves frequent mechanical movements. Of course, several sensor heads can be used in parallel and current multi-channel eddy current testing systems normally arrange sensors in a simple grid and use them independently as in a single sensor system. Often these systems operate at a single frequency and the range of materials is limited to metallic components which have a relatively high electrical conductivity.
Tomographic inversion techniques can help extract more information to improve the inspection process and there are several possibilities, for example:
|Figure 1. A linear six coil array above an aluminium plate showing (left) induced eddy currents
when the second coil is active, and (right) inductance reconstruction of a defect.
1. More information could be obtained by acquiring the signals generated by the coupling between all the coil pair combinations. For example in Fig 1 a linear six coil array is placed above an aluminium plate. The diagram on the left shows the circulation of the induced eddy currents produced when the second coil is active. The image shows a defect reconstructed from the data from all combinations of self and mutual inductances from the coil array. Such an image would not be possible taking the conventional approach of using only the coil channels individually .
2. In addition to using the coils in a tomographic array, even more information can be obtained using spectroscopy. Fig 2 shows the phasor plots of a multi-frequency signal from an inductive sensor monitoring the microstructural change of steel rod on-line during hot transformation. At 136.8 sec (left), the steel is austenitic; whilst a short time later at 139.6 sec (right) the steel has transformed and is ferritic. There is a clear difference in the spectral phasor response which can be analysed to monitor the evolution of the steel micro-structure in real time on-line . Figure 3 shows an industrial deployment of the technology.
|Figure 2. Phasor plots of a multi-frequency signal from an inductive sensor monitoring microstructural change of steel rod on-line during hot transformation.
|Figure 3. Industrial employment of electro magnetic induction tomography|
 Yin, W. and Peyton, A. J. (2006) “A planar EMT system for the detection of faults on thin metallic plates.” Measurement Science and Technology. 17, 172130-2135
 Peyton, A. J., Yin, W., Dickinson, S. J., Davis, C. L., Strangwood, M., Hao, X., Douglas, A. J. and Morris, P. F. (2010) “Monitoring microstructure changes in rod online by using induction spectroscopy.” Ironmaking and Steelmaking. 37, pp. 135-139.