PSC Optimization of 13.56-MHz Resistive Wireless Analog Passive Sensors

Patient and health monitoring in real-life settings can be realized with passive, wireless, unobtrusive, and low-cost physiological signal monitoring. We have previously reported a new fully-passive sensor system, namely resistive wireless analog passive sensors (rWAPSs), based on inductive coupling for physiological signal collection. Fabrication of thin body-worn sensors using rWAPS technology requires planar printed spiral coils (PSC) as the inductive link. This paper describes a formal method to design and optimize this PSC coil for such a system and presents the results of a comparison of a commercial PSC and the optimal design developed with the proposed method. Iterative process is required to reach the optimal solution of this complex multivariate optimization problem. After optimum design based on given coil dimensions and track specifications, a coil pair with a two-layer PCB was fabricated. The secondary coil size was restricted to a small footprint of 20 mm, while the carrier frequency was selected as 13.56 MHz within ISM frequency band. Lower power consumption and higher sensitivity were set as the key criteria for the comparison of these designs. Experimental results confirm that although the sensitivity varies with respect to sensor resistance, the optimized design has a smaller footprint and offers a higher sensitivity in most sensor ranges, while consuming slightly less power. This proposed optimization approach of PSC design is practical for rWAPS and other similar inductively coupled sensors and can be utilized in body-worn, wearable, and implanted sensors.