Design of a multi-modal sensor for the in-process measurement of material properties based on inductive spectroscopy

Abstract

Ferromagnetic steel magnetic properties undergo isotropic and anisotropic changes during cold forming processes. Although their measurement provides an important basis for material property-controlled process control, it remains particularly challenging, as workpieces are subjected to several influencing effects, such as tilting and distance to the target. In this paper, we propose a robust contactless multi-modal sensor for the in-process measurement of material magnetic properties composed of a central excitation coil surrounded by eight receiving coils. The material magnetic permeability is measured based on a model-based evaluation of the central coil inductance spectrum compensating for the influence of the distance to the target. The sensor realizes a measurement of the magnetic anisotropy independently of the tilting effects based on an FFT-based analysis of the induced voltage in the peripheral coils. The sensor has been validated in a tensile test experiment on standard specimens. The measurement data of the tensile test reveal a systematic relation of permeability and magnetic anisotropy to the mechanical influences of stress and deformation. The measurements remained stable despite strong movements between the sensor and the target. A working distance increase of 80% resulted in variations of the magnetic permeability of only 3%. The observed changes in magnetic anisotropy are uncorrelated to tilting effects. Based on the investigation results a proposal for the design of an in-process measurement system is provided.