Monitoring switch-type sensors and powering autonomous sensors via inductive coupling: application to removable seats in vehicles

Abstract

This thesis explores the feasibility of using inductive links for a vehicle application where wiring an electronic control unit (ECU) to the sensors or detectors become unfeasible or unpractical. The selected application is occupancy and belt detection in removable vehicle seats. Two ways of using inductive links are considered: 1) passive detection of the state of the seat detectors from a readout unit and 2) remote power transmission to a detection unit and subsequent data transmission by a wireless transceiver. 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. We propose the use of inductive links for switch-type sensors. First, a comprehensive analysis for an arbitrary number of sensors is presented. Secondly, we show the feasibility of using inductive links for occupancy and belt detection in removable vehicle seats. The state (open or closed) of the related sensors was attained by first measuring the equivalent resistance of the readout inductor and then estimating its resonant frequency. Commercial ferrite-core coils were used to increase the detection distance. Experimental tests were carried out using an impedance analyzer connected to the readout coil and commercial seat detectors connected to the resonant network. The detection of the sensors¿ state was feasible at all tested distances, from 0.5 cm up to 3 cm. The second proposed alternative consists on remote powering, via an inductive link, the electronic device where the seat detectors are connected. Resonant coupled coils were used in order to increase the powering distance range and the power efficiency. Working frequency was restricted to 150 kHz. Commercial small-size magnetic-core coils were selected and their resistance and quality factor over frequency measured. At the receiving network, a rectifier and a voltage regulator were required to provide a DC voltage supply to the autonomous sensor. Four type of voltage regulators were compared from the point of view of the system power efficiency. Both a theoretical analysis and experimental results are presented. Results showed that shunt regulators provide the best power efficiency over the other alternatives, which are linear series and switching buck and boost regulators. On the other hand boost regulators led to an unstable behavior of the system in most of the cases. The use of rechargeable batteries was also considered in order to increase the power efficiency. Achieved power efficiencies were around 40%, 25% and 10% for coil distances of 1 cm, 1.5 cm and 2 cm respectively. Experimental tests also showed that the autonomous sensor was properly powered up to coil distances of 2.5 cm. Finally, different types of coils were assessed and the effect of metallic structures analyzed for the intended application. The final aim is, on the one hand, to increase the powering distance and, on the other hand, to minimize the influence of the metallic structures. Three different coil types, two with ferrite-core coils and one with an air-core coil were used. Numerical results showed that ferrite-core coils are less affected by the presence of metallic structures. Experimental results showed that the air-core coils provided a larger powering distance thanks to its much larger winding diameter. However, when approaching a metallic plate, the transferred power with the air-core coils to the load was insufficient for the intended application. On the other hand, ferrite-core coils barely noticed the presence of the metallic plate. As for the passive detection, the presence of a metallic plate below the primary air-core coil slightly affected the measured resistance values but detection for the four possible states of the seat detectors was still possible.

Algunes aplicacions a l'entorn de l'automòbil no són possibles si no és mitjançant la connexió sense fils dels seus dispositius a causa que el cablejat és difícil o inviable. Alguns exemples els trobem en el monitoratge de sensors situats en parts rotatòries, com les rodes, o en elements extraïbles, com els seients. Els sistemes de monitoratge de la pressió de l'aire en les rodes (TPMS) són d'obligat compliment als EUA i ho seran en breu també als països membres de la UE per als vehicles de categories M1 o N1 aprovats a partir de l'1 de novembre de 2012 o per als vehicles matriculats a partir de l'1 de novembre de 2014. Actualment, els sistemes TPMS existents al mercat estan alimentats per piles. Amb tot, la vida útil exigida per a les bateries és d'uns 10 anys, esdevenint el seu ús un autèntic repte. Un altre element en contra de l'ús de bateries és la directiva 2006/66/CE que limita el nombre màxim permès en els vehicles. D'altra banda, moltes furgonetes o mini furgonetes i vehicles familiars incorporen seients extraïbles amb l'objectiu d'aprofitar al màxim l'espai interior. Alguns models comercials incorporen en el seient extraïble el detector de cinturó de seguretat. Per evitar el cablejat, existeixen sistemes de detecció passiva mitjançant acoblament inductiu. A partir del present any 2012, una regulació de la agència nord-americana NHTSA requereix de l'ús de coixins de seguretat intel·ligents per minimitzar els riscos en nens. Aquests seients intel·ligents haurien de detectar si el passatger és un adult, una cadira infantil o si està lliure per evitar problemes ocorreguts en anterioritat amb els sistemes coixí de seguretat. Els sensors usats per a aquests coixins de seguretat intel·ligents requeririen d'energia per operar. Una opció per als seients extraïbles és la transmissió de potència via acoblament inductiu des del terra del xassís del vehicle fins al seient. També és possible usar l'acoblament inductiu per detectar l'estat de diversos sensors existents en els seients extraïbles mitjançant detecció passiva. Precisament, la detecció d'ocupació i de cinturó de seguretat en seients extraïbles ha estat seleccionada per aplicar la investigació present que consisteix, d'una banda, en el monitoratge de sensors de tipus commutat (dos possibles estats) via acoblament inductiu i, per una altra, en la transmissió mitjançant el mateix principi físic de la potència necessària per alimentar els sensors autònoms remots. En els dos casos, una primera bobina es fixaria en el seient extraïble, connectada als sensors, i una segona bobina se situaria sota la primera, en el terra del vehicle.