16-Sensor passive wireless SAW humidity sensor system

This paper describes the development, operating principles, and experimental performance of sixteen uniquely identifiable, wirelessly interrogable, passive surface acoustic wave (SAW) humidity sensors, and a prototype wireless transceiver capable of reading the set of sensors and interpreting individual sensor responses. The set of sensors was produced using a combination of discrete frequency coding and time diversity, and can operate simultaneously in groups of 8 or more while retaining the ability to individually identify and read each sensor. Nanostructured polyvinyl pyrrolidone/lithium chloride doped titanium dioxide sensing layers developed in collaboration with researchers from Temple University were utilized on the SAW die to produce sensors that demonstrate large, rapid, reversible quantitative responses to humidity exposure in laboratory conditions. The transceiver implemented a differential time integrating correlator based system architecture. The system operates in a manual mode for sensor selection, and utilizes a PC-based LabView user interface for data acquisition and system control. This system was shown to be capable of reading the humidity sensors wired and wirelessly over short range (up to 10 feet) under laboratory conditions. Toggling of transmit (Tx) and receive (Rx) antenna operation was required to eliminate high Tx to Rx crosstalk levels caused by continuous transmission and reception, which otherwise would prevent wireless reading of passive sensors. The wireless sensor system was successfully demonstrated at NASA Kennedy Space Center in November, 2011.