Infrared local positioning system using phase differences

Abstract

In this paper an infrared (IR) indoor local positioning system (LPS) is presented. The most relevant low level design aspects are addressed. Using sinusoidal amplitude-modulation (AM) of an infrared carrier, differential distances between a mobile emitter, the position of which is to be obtained, and fixed receivers are measured. The system may yield accuracies at the level of a few cm and addresses applications for which the increasingly available wireless technologies and smart phone sensors are not sufficient. Such applications comprise e.g., positioning mobile-robots in a manufacturing plant or positioning tools on a construction site. The proposed system works with an IR LED emitter, with a wide emitting angle, resulting in a less complex system than a laser-based one, but requiring an elaborate sensor design in order to have a sufficiently high signal-to-noise ratio (SNR) for successful demodulation. A detailed description of the basic locating cells (BLC), composed of five receivers is given as well as a study including all the blocks that comprise the system: emitter and detector devices, sensor electronics, phase measuring electronic system and hyperbolic trilateration module. All these blocks are modelled numerically and their relevant parameters are discussed with respect to their effect on the position error. The numerical analysis provides a method to evaluate the system as a whole. The choice of parameter values is a trade-off between accuracy, coverage and admissible dynamics of the mobile robot, or - equivalently - between SNR, field of view and real time response. Multipath is one of the biggest challenges for current indoor positioning systems requiring line-of-sight observations. The proposed system achieves multipath mitigation through an additional spread spectrum modulation of the sinusoidal AM signal, in analogy to the modulation of the microwave carrier with GNSS. Finally, a numerical analysis and an experiment using a prototypical BLC are summarized. They indicate that the system achieves a precision of 5 cm (2σ) for the coordinates in a fixed local coordinate frame.