A Bidirectional Active-Alignment Control System for LED Communication

LED-based free-space optical communication is becoming a promising alternative to the traditional acoustics mode of underwater communication because of its low cost, high data rate, and low power consumption. However, a fundamental challenge associated with the free space optical communication systems is to establish and maintain line-of-sight (LOS) between the two communicating agents. It becomes even more challenging when the agents are in motion, requiring an alignment system that enables each agent to point itself towards the LOS direction continuously. This paper presents a bi-directional active alignment control approach for the LED-based communication system of two robots, where the robots navigate in a three-dimensional (3D) space. We propose an extended Kalman filter (EKF) based alignment approach, where the estimates of azimuthal and elevation components of heading bias with the LOS are used to correct the alignment. We introduce and implement a new circular scanning technique on a two-degree-of-freedom (2DOF) rotational system mounted on each robot that enables consecutive independent measurements from a single photo-diode to satisfy the observability constraints of the EKF. Furthermore, we explore a synchronized alternating scheme to address the bidirectional nature of the problem. The scanning amplitude is further adjusted based on the EKF estimation covariance, to balance the trade-off between estimation accuracy and actuation effort. The presented results support the efficacy of the proposed method in the presence of slow relative motion between two robots, and demonstrate its superiority over an extremum-seeking (ES) based approach across a wide range of distances between the robots.