Autonomous Nano Aerial Vehicles have been increasingly popular in surveillance and monitoring operations due to their efficiency and maneuverability. Once a target location has been reached, drones do not have to remain active during the mission. It is possible for the vehicle to perch and stop its motors in such situations to conserve energy, as well as maintain a static position in unfavorable flying conditions. In the perching target estimation phase, the steady and accuracy of a visual camera with markers is a significant challenge. It is rapidly detectable from afar when using a large marker, but when the drone approaches, it quickly disappears as out of camera view. In this paper, a vision-based target poses estimation method using multiple markers is proposed to deal with the above-mentioned problems. First, a perching target with a small marker inside a larger one is designed to improve detection capability at wide and close ranges. Second, the relative poses of the flying vehicle are calculated from detected markers using a monocular camera. Next, a Kalman filter is applied to provide a more stable and reliable pose estimation, especially when the measurement data is missing due to unexpected reasons. Finally, we introduced an algorithm for merging the poses data from multi markers. The poses are then sent to the position controller to align the drone and the marker's center and steer it to perch on the target. The experimental results demonstrated the effectiveness and feasibility of the adopted approach. The drone can perch successfully onto the center of the markers with the attached 25mm-diameter rounded magnet.

翻译：自主纳米飞行器因其高效性和机动性，在监视与监测任务中日益普及。一旦到达目标位置，无人机无需在任务期间始终保持工作状态。在此类情况下，飞行器可通过栖停并关闭电机来节省能量，同时在不佳飞行条件下维持静态位置。在栖停目标估计阶段，使用带标记的视觉相机的稳定性和精确性是一项重大挑战。使用大标记可从远处快速检测，但当无人机靠近时，标记会因超出摄像头视野而迅速消失。本文提出一种基于视觉的多标记目标位姿估计方法以解决上述问题。首先，设计一种内含较小标记的栖停目标（大标记内部嵌套小标记），以提升宽范围及近距离下的检测能力。其次，使用单目相机根据检测到的标记计算飞行器的相对位姿。然后，引入卡尔曼滤波器以提供更稳定可靠的位姿估计，尤其适用于因意外原因导致测量数据缺失的情况。最后，提出一种多标记位姿数据融合算法。这些位姿数据随后传输至位置控制器，以对准无人机与标记中心，并引导其栖停在目标上。实验结果表明所提方法的有效性与可行性，无人机可成功栖停在标记中心（附有直径25毫米的圆形磁铁）。