Lock-in Amplifier-based Contactless Detection of Impedance Variations

In this paper, we analyze the impedance characteristics of free-falling objects utilizing a cylindrical electrode sensor and propose a contactless material classification method. Steel, glass, and rubber beads were used as materials that freely fall into the cylindrical electrode sensor. Using a transimpedance amplifier (TIA), the current induced in the cylindrical electrode by impedance variations was converted into a voltage signal and measured. Using a lock-in amplifier-based (LIA-based) measurement system, we detected the impedance signal variations of the cylindrical electrode corresponding to the electromagnetic properties of materials. In measurement results, distinct signal patterns were observed: the steel bead exhibited a maximum amplitude change of 3.88 mV and a phase change of 1.00°, while glass and rubber beads showed changes of 2.75 mV/0.20° and 2.31 mV/0.14°, respectively. The measurements for glass beads of varying sizes demonstrated a clear correlation between the bead diameter and the signal magnitude. The amplitude and phase variations of 30 mm bead were 8.54 mV and 1.46°, while the 26 mm, 21 mm, and 16 mm beads exhibited sequentially decreasing amplitudes of 6.11 mV, 2.75 mV, and 1.07 mV, accompanied by phase changes of 1.17°, 0.21°, and 0.17°, respectively. The results are verified that the electromagnetic properties of the materials can be effectively distinguished by monitoring the simultaneous changes in the amplitude and phase of the impedance signal.