Extended Kalman Filter-Based 3D Active-Alignment Control for LED Communication

LED-based optical communication is emerging as a low-cost, high-data-rate alternative to the traditional acoustics mode of underwater communication. However, it is challenging to establish and maintain Line-Of-Sight (LOS) between the receiver and the transmitter, especially when such systems are used by mobile robots. Hence, there is a need for an active alignment system that enables the receiver to constantly align itself towards the direction of the transmitting device. In this paper, we propose and implement an active alignment control system capable of tracking a transmitting source moving in the three-dimensional (3D) space. An extended Kalman filter is used to estimate the components of the angle between the receiver orientation and the receiver-transmitter line. Using the estimate, a proportional-integral (PI) controller is implemented to adjust the receiver orientation. The algorithm uses one measurement of the light intensity from a single photo-diode, where successive measurements are obtained via a circular scanning technique. The amplitude of the scanning is adapted to the alignment performance, to achieve a sound trade-off between estimation accuracy, signal strength, and energy consumption. Simulation and experimental results are presented to illustrate the effectiveness of the proposed approach.