A Battery-Less Wireless Embedded Sensor for Wide Area Monitoring

Due to its wireless and passive nature, the inductive-capacitive (LC) sensor has been used as an embedded sensor for applications such as monitoring conditions inside civil engineering structures. The sensor, consisting of an inductor and capacitor, measures the parameter of interest by tracking the shift in its resonant frequency wirelessly with a coil antenna. Although it can be detected from a distance, the sensing region of a typical LC sensor is still confined to its surrounding area. Therefore, a large array of LC sensors are often needed for wide area monitoring, which is costly and time consuming. Presented here is a new LC sensor design that can extend the sensing area of a singlespiral LC sensor by adding two parallel wires to its inductor. The parallel wires act as an additional capacitor in the resonant circuit of the sensor, effectively extending the sensing area throughout the whole length of the wires. For wide area monitoring, a sensor grid is formed by crossing the wires of two sets of perpendicularly aligned LC sensors. By determining the responses of these sensors, the sensor grid can measure the parameter of interest over a wide area and also pin-point the location where the parameter of interest has changed. The new sensor design allows a significant reduction of the number of sensors needed to cover an area. For example, if m × n traditional LC sensors are needed to cover an area, the new LC sensor design only requires m + n sensors. This paper presents the design and fabrication of the sensor, and demonstrates its application to detect water inside a block of concrete.