Abstract: Aiming at the problem that maps may be not consistent caused by accumulation of errors in visual Simultaneous Location and Mapping (SLAM), a Visual SLAM (V-SLAM) system based on improved closed-loop detection algorithm was proposed. To reduce the cumulative error caused by long operation of mobile robots, an improved closed-loop detection algorithm was introduced. By improving the similarity score function, the perceived ambiguity was reduced and finally the closed-loop recognition rate was improved. At the same time, to reduce the computational complexity, the environment image and depth information were directly obtained by Kinect, and feature extraction and matching was carried out by using small and robust ORB (Oriented FAST and Rotated BRIEF) features. RANdom SAmple Consensus (RANSAC) algorithm was used to delete mismatching pairs to obtain more accurate matching pairs, and then the camera poses were calculated by PnP. More stable and accurate initial estimation poses are critical to back-end processing, which were attained by g2o to carry on unstructured iterative optimization for camera poses. Finally, in the back-end Bundle Adjustment (BA) was used as the core of the map optimization method to optimize poses and road signs. The experimental results show that the system can meet the real-time requirements, and can obtain more accurate pose estimation.

Key words: Simultaneous Location And Mapping (SLAM), perceived ambiguity, Oriented FAST and Rotated BRIEF (ORB), closed-loop detection, pose estimation

摘要： 针对视觉同时定位与地图构建（SLAM）中容易由误差累积导致构建地图不一致的问题，提出了一种基于改进闭环检测算法的视觉SLAM（V-SLAM）系统。为了减小移动机器人长时间运行带来的累计误差，引入一种改进的闭环检测算法，改进相似性得分函数，减小感知歧义，提高闭环的识别率；同时为了减小计算量，通过Kinect直接获取环境图像以及深度信息，并采用计算量小、鲁棒性好的ORB特征进行特征提取和匹配；采用随机抽样一致性（RANSAC）算法进行误匹配删除，从而获得更准确的匹配点对，然后用PnP计算出相机位姿；更稳定、准确的初始估计位姿对后端处理至关重要，利用g2o对位姿进行无结构的迭代优化；最后在后端采用以集束调整（BA）为核心的图优化方法对位姿和路标进行优化。最终实验结果表明该系统能够满足实时性要求，并可以获得更加准确的位姿估计。

关键词: 同时定位与地图构建, 感知歧义, ORB, 闭环检测, 位姿估计