EMFi-based low-power occupancy sensor

Abstract The use of bed/couch occupancy sensors is imperative for certain ubiquitous computing systems such as ambient assisted living (AAL). DIA (Dispositivo Inteligente de Alarma, in Spanish) is an AAL system that allows to infer a potential dangerous action of a dependant person living alone at home. This inference is obtained by a specific sensitization with sensor nodes and a reasoning layer embedded in a personal computer. In this kind of systems, energy is a limited resource therefore sensor devices need to be properly managed to conserve energy. A first approach to solve the bed/couch occupancy detection problem is found in pressure mats, but several environmental dependencies make them weak to be an efficient and reliable solution for large volume deployments. Solutions based on force-to-resistor transducer imply a too high power consumption to be integrated on wireless sensor nodes. In our previous paper, an occupancy sensor based on force-capacitive transducer has been proposed, implemented and tested. This sensor is based on electro-mechanical film (EMFi) transducer which is able to detect force variations in a quasi-passive way and, besides, is a capacitor with variable capacitance depending on the static force exerted on its surface. This detection of force change is used to trigger an active mechanism to measure the weight by means of the transducer capacitance. In this paper, we present a new low-power circuit to measure weight, by means of the capacitance of the EMFi transducer, which enhances accuracy and power consumption, simplifies the signal sampling procedure and can be implemented as a standalone device. A low-power wireless sensor node prototype including this new design has been assembled and tested with a wide range of weights. The occupancy detection was successful and the power consumption of the occupancy sensor accounts for only 2%, which is acceptable for implementation.