Occupancy and Belt Detection in Removable Vehicle Seats Via Inductive Power Transmission

This work proposes the use of inductive links to wirelessly power an autonomous sensor in a vehicle application. The selected application is intended for occupancy and belt detection in removable vehicle seats, where wiring the seat detectors from the vehicle chassis is impractical. The autonomous sensor includes seat detectors and a wireless transceiver to transfer the data on the state of detectors. To compensate the loose coupling between the coupled coils, resonant tanks were used. To drive the transmitting resonant network, a commercial class-D amplifier was used. Working frequency was restricted to 150 kHz. Commercial magnetic-core coils were selected as they provide high coil values and quality factors in a small-size factor, which is a requirement for the intended application. At the receiving network, a rectifier and a voltage regulator were used to provide dc voltage supply to the autonomous sensor. Three kinds of voltage regulators were compared from the point of view of the power efficiency. Both a theoretical analysis and experimental results are presented for different combinations of coils and working frequencies. Theoretical analysis shows that the operating points for the linear shunt regulator always lead to higher power efficiencies compared with other alternatives such as linear series and switching buck regulators. Experimental tests were carried out using a mechanical setup to fix the coil-to-coil distances. Experimental results agree with the theoretical analysis. Achieved power efficiencies ranged from around 50% to 10% for coil-to-coil distances from one to three times the inner diameter of the coils. Experimental tests also showed that the autonomous sensor was properly powered up to coil-to-coil distances of 2.5 cm, i.e., more than four times the inner diameter of the coils.