Simulation of coil separation and angle effects on the mutual inductance for 13.56 MHz WRAP sensors

Continuous patient monitoring in the normal daily life can be realized with body-worn wireless and fully passive sensors. We have previously developed and optimized a novel Wireless Resistive Analog Passive (WRAP) sensor technology with inductive link between a secondary Printed Spiral Coil (PSC) attached to the body and a wearable scanner with the primary PSC to continuously monitor physiological signal. The flexible and unobtrusive nature of wireless connection between primary and secondary coils causes the distance and the angle between the PSCs to change in real-life operation that can influence the mutual inductance and alter the sensitivity of the whole system. In this paper, we use COMSOL Multiphysics to simulate the effects of varying PSC separation and angle on the mutual inductance for our 13.56 MHz WRAP sensors. We simulate a pair of optimized coil in the cubic infinite boundaries (for magnetic and electric fields) using mef (magnetic and electric field) physics in a stationary study. The results show a decrease of 82% in coupling factor with increasing the tilting angle from 0° to 60°. The coupling factor decreases with increasing separation and angle (center-to-edge) in a quadratic function basis.