Maintaining Line-Of-Sight (LOS) between the receiver and the transmitter is an inherent challenge associated with light-emitting diode (LED)-based free space optical communication systems, especially when such systems are used by mobile robots. Due to constant movement of underlying robotic platforms and other unwanted disturbances, there is a need for an active alignment system that allows the receiver to constantly track the direction of the transmitting device. In this paper, we propose an active alignment control system, equipped with two degree-of-freedom (DOF) actuation and capable of tracking a transmitting source moving in the three-dimensional (3D) space. A 3D extension of a previously proposed Extended Kalman Filter-based algorithm is used to estimate the components of the angle between the receiver orientation and the receiver-transmitter line, which are used subsequently to adjust the receiver orientation. The algorithm uses only the measured light intensity from a single photo-diode, where successive measurements are obtained via a circular scanning technique. Simulation results are presented to illustrate the proposed approach and explore the tradeoffs in the design of the scanning pattern. In particular, a scheme with adaptively adjusted scanning amplitude is shown to deliver satisfactory alignment performance with actuation effort.

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