A Flexible Liquid-Level Positioning System Based on Dynamic Controlled Spoof Surface Plasmon Polaritons

An active liquid-level sensor system based on dynamically controlled spoof surface plasmon polaritons (SSPPs) is proposed for noninvasive liquid-level measurement. By integrating positive-intrinsic-negative (p-i-n) diodes into the split-ring resonator (SRR) unit of the SSPPs, the resonance frequency of the SRR on the transmission line can be tuned in real time by varying the bias voltage. This enables the dynamic control of electromagnetic wave propagation within the sensor. When the biased SRR units are positioned in air, at the air–liquid interface, and within the liquid medium, distinct response characteristics are generated by the system, facilitating precise liquid-level detection. To validate the performance of the proposed sensor, a prototype is fabricated, and an experimental platform is established. The experimental results demonstrate that the sensor achieves liquid-level localization and tilt angle measurement for nonmetallic containers with a tilt angle accuracy of 3° and a liquid-level measurement accuracy of 2 mm. The proposed liquid-level localization method is verified to be effective. Furthermore, the measurement range can be extended by increasing the number of SRR units.