Microfabricated Eddy-Current Sensors for Non-Destructive Testing of the Micro Grinding Burn

The feasibility of using planar microfabricated eddy-current sensors for non-destructive testing of varying levels of grinding burn is presented. A 42CrMo4 workpiece is ground with varying degree of grinding burn as a test reference for the development of micro eddy-current sensors. Planar non-spiral and spiral micro-coils geometries are fabricated using electrochemically deposited nickel-cobalt alloys. The micro-coils are operated in an absolute sensor mode, i.e. as single coils, by recording their impedance as a function of the reference workpiece in its vicinity with varying degree of grinding burn. The micro-coil impedance shows typical inductor behavior as function of frequency (measured up to 10 MHz) and it decreases in vicinity of the workpiece when compared to its impedance in air. The presence of grinding burn, perceived as a change in the relative permeability in the grinding zone, is exploited by the eddy-current sensor for its detection. The inductance of 20 turns micro-coil with non-spiral and spiral geometry was found to be 0.22 μH and 1.5 μH, respectively. The impedance response of the micro-coils changed with the degree of grinding burn and the impedance difference increased with frequency. The impedance of spiral μcoil increased by ca. 3 % on moving it from low degree of grinding burn to high degree of grinding burn, which was found to be 10 times more than that of non-spiral μcoils. The proposed eddy-current sensors will be a part of sensor fusion system, working with an optical sensor for the real-time grinding process monitoring.