Monitoring Switch-Type Sensors via Inductive Coupling: Application to Occupancy and Belt Detection in Removable Vehicle Seats

Inductive links have been widely proposed for sensors placed in harsh or inaccessible environments, where wiring is unpractical. Usually, the sensor forms part of an LC resonant network. The resonant frequency is dependent on the quantity to be measured and is estimated from a coupled reader. This paper proposes the use of inductive links for switch-type sensors, i.e., those that can be roughly modeled as switches in response to the sensed parameter. First, we present a comprehensive analysis for an arbitrary number of sensors. Second, we show the feasibility of using inductive links for occupancy and belt detection in removable vehicle seats, where wiring is unpractical. The state (open or closed) of the related sensors is attained by first measuring the equivalent resistance of the readout inductor and then estimating its resonant frequency. Ferrite-core coils were used to increase the detection distance. Experimental tests were carried out using an impedance analyzer with the readout coil and commercial seat sensors at the resonant network. The resistance value at the resonant frequency decreased with an increasing distance between the coupled coils. Even so, detection of the sensors' state was feasible at all the tested distances, from 0.5 to 3 cm.