ESO-based super-twisting sliding-mode composite control for single-detector, pendulum-mirror PAT system in...

Achieving high-precision pointing and tracking is critical for inter-satellite laser communication under dynamic disturbances. In single-sensor pointing-acquisition-tracking (PAT) systems, the motions of the coarse-pointing assembly (CPA) and fine-pointing assembly (FPA) are coupled in the measured incident laser angle, which may degrade tracking performance. However, this issue has received limited attention in existing studies. To address this challenge, this study proposes a composite control strategy for single-sensor PAT systems, integrating an extended state observer (ESO) with a super-twisting sliding-mode controller. By incorporating ESO-estimated information from the fine-pointing loop into the coarse-pointing loop, the proposed framework addresses the inherent measurement coupling between the two stages. Furthermore, an ESO-based sliding surface is formulated for workload allocation. This surface establishes a low-pass disturbance-response characteristic for the CPA loop and enables adjustable workload distribution between the CPA and FPA through a single tuning parameter. Three sets of experiments were conducted to validate the proposed method. The first experiment, carried out under representative in-orbit disturbances, shows that in the fine-pointing mode the proposed method reduces the root-mean-square pointing error by at least 5.5% and 2.5% compared with proportional-integral-derivative-based composite control and twice-extended active disturbance rejection control, respectively. It also reduces the FPA driving voltage by more than 15.3% and 85.2% relative to the same two methods. The second experiment validated the effectiveness of the single-parameter tuning mechanism in achieving workload distribution, with measured CPA cutoff frequencies remaining within 19% of the theoretical predictions. The third experiment evaluated the computational cost of the proposed controller on an embedded platform, confirming its real-time feasibility for high-speed in-orbit applications.