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Active Alignment Control-based LED Communication for Underwater Robots

Achieving and maintaining line-of-sight (LOS) is challenging for underwater optical communication systems, especially when the underlying platforms are mobile. In this work, we propose and demonstrate an active alignment controlbased LED-communication system that uses the DC value of the communication signal as feedback for LOS maintenance. Utilizing the uni-modal nature of the dependence of the light signal strength on local angles, we propose a novel triangular exploration algorithm, that does not require the knowledge of the underlying light intensity model, to maximize the signal strength that leads to achieving and maintaining LOS. The method maintains an equilateral triangle shape in the angle space for any three consecutive exploration points, while ensuring the consistency of exploration direction with the local gradient of signal strength. The effectiveness of the approach is first evaluated in simulation by comparison with extremum-seeking control, where the proposed approach shows a significant advantage in the convergence speed. The efficacy is further demonstrated experimentally, where an underwater robot is controlled by a joystick via LED communication.