Split-Ring Resonator Enhanced Detection of Liquids Using Backscatter Tags

Abstract

Wireless passive backscatter tags have become a popular choice for sensing due to their low cost and maintenance free operation. One application is binary liquid detection i.e., determining whether a liquid is present in proximity to the tag. Typically, the presence of the liquid alters the dielectric properties around the tag, which alters the reflected signal’s amplitude and phase. By monitoring these variations one can decide “presence” versus “absence” without any active sensing electronics on the tag itself. However, to induce a notable change in the received power or phase at the reader, the tag is typically detuned by a large amount of liquid, resulting in a reduced read range and poor liquid sensitivity. We propose an enhanced liquid detection method based on inter-tag channel estimation. A split-ring resonator (SRR) is introduced between two tags to suppress the inter-tag channel in the unloaded state. When loaded with liquid, the SRR’s resonant frequency shifts, producing measurable changes in the inter-tag channel that indicate the presence of a liquid. We present the complete system design and validate the approach experimentally with emulated tags and commercial UHF RFID tags. Results demonstrate inter-tag channel changes exceeding 12 dB in magnitude under loading of only 0.5 mL of deionized (DI) water.