Estimation of Diaper Condition Based on Antenna Impedance Variation Through Rational Function Approximation

Real-time monitoring of a diaper is a key component in preventing skin rashes and skin eruption in babies. In this paper, we investigate the feasibility of sensing the urine and feces of a baby on a diaper using the input impedance variation of a single on-diaper antenna. As an RF transmitter requires an antenna by default, other sensors are not needed if the antenna also serves as a sensor. We designed a microstrip dipole antenna and located it on a diaper. When a change of medium properties around the antenna occurs due to the presence of urine or feces, the near field is perturbed, causing a variation in antenna impedance. We simulated and measured antenna impedance under diverse diaper conditions. Results showed significant impedance variation depending on diaper condition. The impedance is approximated as a rational function and the diaper condition is classified through investigating the parameter of the rational function.