Design and Characterization of a Multilayer FPC Eddy-Current Sensor for Micrometric Distance Measurement Applications Exploiting a Low-Cost Commercial Converter

Condition monitoring in Industry 4.0 drives the shift from bulky eddy-current sensors to compact flexible printed circuit (FPC) coils paired with inductance-to-digital converters (LDCs). This letter presents the design, modeling, and validation of a miniaturized four-layer FPC sensor utilizing an LDC1101 converter. Coil impedance models were experimentally validated via frequency response analysis. Furthermore, three acquisition methodologies-equivalent parallel resistance (R-p), high-resolution inductance (LHR), and a time-difference (TD) approach-were compared. While R-p achieved the highest accuracy [root mean square error (RMSE) <0.20 mu m] within a constrained range, LHR exhibited instability due to resonance injection locking. Ultimately, the proposed TD method emerged as the optimal edge-computing paradigm, delivering robust accuracy (RMSE approximate to 0.37 mu m) while bypassing digital bounds and eliminating injection locking.