B8.2 - System for 3D Localization and Synchronization of Embedded Wireless Sensor Nodes Based on AMR Sensors in Industrial Environments

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
B8 - Magnetic Sensors
S. Carrella, K. Iswandy, A. König - Universität Kaiserslautern (Germany)
349 - 354


In ambient intelligence, medical, and industrial applications, wireless sensor nodes are used for measuring variables of the environment or the process. In some applications, knowledge of the sensor localization is mandatory. While the RSSI from the radio module is widely used for the localization, there are some applications and environments, e.g. liquid filled containers, which hinder the establishment of a reliable radio link and the localization, accordingly. Even though there are alternative technologies, which are based on, for instance, ultrasonic or light, not all applications are covered by existing localization technologies. Therefore a technology based on magnetic quasi- DC fields has been developed and already presented in previous work. Coils, surrounding the relevant area, are switched sequentially and ternary, i.e. two directions and then resting. These fields are measured by tri-axial magnetic sensors. For each coil, the measurement results are subtracted from each other to remove offsets and the absolute emitted field strength values are calculated and used for computing the distances to the coils. These distances are used by a range-based algorithm to compute the positions of the sensor nodes. To enable short localization cycles, in particular for the localization of moving sensor nodes, the coils must be switched fast, e.g. less than 10ms per DC plateau. The resulting short periods of DC plateaus implicate precise clocks for measuring at the right time. Widely used 32786Hz quartz crystals have tolerances down to 10ppm, that can be reduced for environments with small changes of temperature by calibrating the real time counter to 2ppm, which still means 120μs per minute. Due to the high power consumption, size and costs of compensated oscillators, these oscillators may not be used. Therefore the localization concept is enhanced by a synchronization method, where the edges of the generated magnetic fields are used as time references. The first coil, that emits a field strength
beyond a certain threshold is used for the synchronization. The ADC of the sensor node keeps running for that coil and the edge from the latter DC plateau to the resting plateau is detected. Current implementation is able to detect the edges of the magnetic field. The precision of the synchronization that can be achieved is still matter of research.