Rotational speed monitoring is essential in many industrial and electromechanical systems. This paper presents a rotational speed measurement method based on a wireless impedance sensing system leveraging the radio-frequency coupling between a passive resonant tag and a coplanar waveguide (CPW) probe. The sensing mechanism exploits periodic variations in the real part of the probe impedance caused by the relative alignment between the rotating tag and the stationary probe. While the impedance signal is inherently periodic, the usable speed range of sampling-based measurement systems is fundamentally constrained by their acquisition rate. To overcome this limitation without requiring higher-rate instrumentation, an equivalent-time sampling (ETS) reconstruction approach is proposed. Sparse and nonuniform impedance samples collected over multiple revolutions are mapped into an equivalent phase domain and combined to reconstruct the waveform associated with a single rotation period. The method is reader-agnostic in principle, as it only requires time-stamped monitoring of a periodic RF observable at a selected frequency; however, experimental validation in this work is performed using a vector network analyzer (VNA). Experimental results obtained on a rotating platform with speeds ranging from 150 RPM to 4000 RPM demonstrate that the proposed method reduces the mean relative estimation error to below 5% across the full range, compared to errors exceeding 70% for conventional peak-based estimation above 1000 RPM. These results highlight the effectiveness of the ETS approach in extending the operational range of RF impedance-based rotational sensing under severe undersampling conditions. The proposed framework is generalizable to other periodic RF sensing configurations where signal periodicity can be exploited across multiple acquisition cycles.
Gharibi et al. (Fri,) studied this question.