Simultaneous Localization and Mapping (SLAM) is used to solve the problem of autonomous localization and navigation of mobile robots in unknown environments. Loop closure detection is a key part of SLAM, which largely determines accuracy and stability of SLAM. In recent years, some experiments have proved that the loop closure detection system based on neural network is superior to the traditional loop closure detection in both accuracy and real-time performance. In this paper, we propose an adaptive real-time loop closure detection (AR-Loop) method based on monocular vision. A pre-trained convolutional neural network (CNN) is used to extract image features. Then features of different layers are concatenated as image descriptors. In addition, the adaptive candidate matching range algorithm and image-tosequence calibration algorithm are proposed to improve the performance of the algorithm. Extensive experiments have been conducted on several open datasets to validate the performance of AR-Loop. It has been demonstrated that the recall rate is increased by over 18% compared with other state-of-the-art algorithms when the precision is 100%.

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