Coupled passive resonant circuits as battery-free wireless sensors

Detection and monitoring of the damage created by the corrosion of the steel reinforcement in concrete structures is a challenging and multidisciplinary problem. Economical monitoring strategy that is long-term and nondestructive requires low-cost, battery-free, wireless sensors. Our Electronic Structural Surveillance (ESS) platform uses battery-free passive resonant circuit (tag) as a sensor. The tag is magnetically coupled to an external reader coil. It is interrogated/read remotely in a non-contact (wireless) manner and the state of the sensor is determined from a swept frequency impedance measurement. When paired with the correct sensing element (transducer), the tag can be used for a variety of sensing applications for example, chemical & biochemical sensors. A circuit model of the reader and tag for such a universal battery-free wireless sensor platform is developed. The interaction between design and detection limit is examined. The dependence of the measured signal strength and read range on the various reader and tag circuit parameters is analyzed. Since the values of the circuit of the coils are dependent on their geometries, the effect of specific coil geometry is evaluated and design recommendations are made.