Analysis of the spectra of a multi-frequency electromagnetic sensor to determine the magnetic permeability of steel strip

Peyton, A J; University of Manchester; United Kingdom

Lu, M.; University of Manchester; United Kingdom
Zhu, W.; University of Manchester; United Kingdom
Yin, W.; University of Manchester; United Kingdom
van den Berg, F.D.; Tata Steel; Netherlands
Yang, H.; Tata Steel; Netherlands
Peyton, A.J.; University of Manchester; United Kingdom

ID: ECNDT-0212-2018
Download: PDF
Session: Data Processing - ET
Room: J2
Date: 2018-06-11
Time: 13:30 - 13:50

Multi-frequency electromagnetic sensors, such as EMspec, are now being used to non-destructively inspect the properties of strip steel on-line during industrial processing. These sensors measure the relative permeability of the strip during process operations such as controlled cooling and the permeability values are analysed in real time to determine important microstructural parameters such as the transformed fraction of the required steel phases. These parameters are critical to achieving the desired mechanical properties in the strip product.
The inductance spectra produced by the sensor are not only dependent on the magnetic permeability of the strip, but is also an unwanted function of the electrical conductivity and thickness of the strip and the distance between the strip steel and the sensor (lift-off). The confounding cross-sensitivities to these other parameters must be rejected by the processing algorithms applied to inductance spectra.
The paper considers cross-sensitivity of the complex EMspec spectra to the four variables, permeability, conductivity, thickness and lift-off. The paper then goes further to consider the solution of the inverse problem of determining unique values for the four variables from the spectra. Both the finite element method and the Dodd and Deeds formulation are used to solve the forward problem during the inversion process. Results from an industrial scale EMspec sensor are presented including the effects of noise. The results are backed up with measurements of selected cases.
The results are significant because they show for the first time that the inductance spectra can be inverted in practice to determine the key values for the thermal processing of steel strip.