Laser acquisition, pointing, and tracking (APT) technology is prevalent in the realm of space optical communication, servicing platforms such as inter-satellites, satellite-ground, airborne, and ship-borne. By fusing coarse and fine tracking, it is possible to achieve picometer-scale, high-probability, swift, and accurate dynamic space-optical communications. However, the application of APT in underwater wireless optical communication remains underreported. This limited application stems from the APT system’s intricate, precise, sizable, and costly nature, which challenges the minimalist design needs of underwater wireless optical communication systems. Additionally, the underwater channel’s resistance, pressure, and environmental adaptability factors compromise the servo control system’s precision and pose engineering challenges. These challenges curtail the expansion and application of APT technology in underwater wireless optical communication. Thus, harnessing APT technology to enhance the stability and reliability of communication links by capturing and tracking the optical axis emerges as a promising avenue in underwater wireless optical communication’s future. Consequently, there’s a pressing need to devise a servo control system that’s both cost-effective and straightforward, catering specifically to the dynamic underwater wireless optical communication’s acquisition and tracking demands.

In this study, we first considered the basic concept of space optical communication acquisition, pointing, and tracking technology. Based on this, we proposed a set of acquisition and tracking systems grounded in servo control for underwater wireless optical dynamic communication. Subsequently, we studied key technologies, including the servo control system architecture, composition model, and motor control algorithm. For the servo control system we proposed, a tracking differentiator was introduced within the active disturbance rejection control algorithm to manage the motor’s acceleration and deceleration. Furthermore, we proposed a coarse and precise tracking strategy that utilized motor acceleration and deceleration control technology. Ultimately, we discussed the acquisition time, tracking accuracy, and acquisition probability of the servo control system we proposed, drawing insights from both simulations and actual indoor and underwater experiments.

In the simulation experiment, the upper computer receives miss distance information and transmits it to the lower computer via the virtual serial port, controlling the motor. The upper computer displays the motor’s working state in real-time on the upper computer (Fig.5) and simulates the spot capture and tracking of underwater wireless optical dynamic communication. When the simulated spot occupies a different position, the motor adaptively accelerates and decelerates, achieving both coarse and precision tracking. The motor operates stably before and after acceleration and deceleration, without missteps (Table 2, Fig. 7). The feasibility of the servo control system and tracking differentiator in executing motor acceleration and deceleration algorithm strategies is confirmed. In the indoor experiment, results indicate that the system captures and tracks the target spot within 4 s at its fastest rate. The azimuth motor’s tracking accuracy is 0.08 mrad (Fig.10) and that of the pitch motor’s is 0.27 mrad (Fig.11), aligning with tracking index requirements. The speed mutation curve for the azimuth and pitch motors during the experiment (Fig.12) reveals that the motor navigates via high-speed coarse tracking, variable-speed, and then low-speed precision tracking phases. The consistent operation surrounding the variable speed affirms the algorithm’s feasibility, suggesting this system’s potential for underwater wireless optical dynamic communications. The underwater experiment reveals that the system captures the target spot in 8 s before disturbance, which is more than the acquisition time of the indoor system. Post-disturbance, the spot experiences interference from water body scattering and refraction, showing a dynamic state. The system completes target spot capture and tracking within 10 s, a duration extended from its pre-disturbance counterpart. Data analysis highlights a 0.6 mrad tracking accuracy for the servo control system before introducing disturbance to the water tank and a 2 mrad accuracy post-disturbance (Fig.14). Additionally, experiments demonstrate a capture probability surpassing 99% for the system. If the spot’s moving speed falls below the specified range in both horizontal and vertical directions, then the servo control stabilizes tracking; otherwise, the tracking fails.

In this study, we examine an acquisition and tracking servo control system for an underwater wireless optical dynamic communication system. We design a servo control system architecture and explore its constitute and control algorithm. We propose a control algorithm based on a tracking differentiator to achieve motor acceleration and deceleration. Concurrently, we employ motor acceleration and deceleration technology to implement a coarse and precise tracking strategy for the underwater wireless optical dynamic communication optical axis. We conduct simulation verifications, indoor tests, and underwater laser spot acquisition and tracking experiments. The underwater laser spot acquisition and tracking experiment reveals that the system’s acquisition probability exceeds 99%, with an acquisition time of less than 10 s. The tracking accuracy, both before and after the water tank disturbance, registers at 0.6 mrad and 2 mrad, respectively. This experiment demonstrates that the designed servo control system aligns with the performance index requirements, setting the stage for further research into underwater wireless optical dynamic communication technology.