A wireless double planar coil sensor arrangement for monitoring capacitance changes due to water uptake embedded in a thin fiber-reinforced composite

A wireless, near-field coupled sensor based on two planar spiral coils embedded in a thin fiber-reinforced composite material is presented. The measurement effect exploits the fact that penetrating water changes the effective dielectric permittivity in the volume surrounding the planar coils. This leads to an increase of the sensor self-capacitance and to a decrease of the sensor self-resonant frequency. The sensor targets applications in which non tactile, in-situ monitoring of water uptake, within a confined volume of material, is of interest. In order to describe the general electric behavior a circuit model, considering the sensor and an inductively coupled detection coil, has been developed and verified. An analytic expression for the resonance frequency was deduced. Sensor prototypes were integrated in a glass fiber polypropylene composite. Applied measurements demonstrate the resonance frequency change due to water entering and leaving the material during immersi on and drying tests.