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A B S T R A C TWe present the results of our monitoring the flux variability of 12 BL Lac objects, which have variabilities on time-scales ranging from hours to months. Individual sources are discussed in detail. Three of them, OY091, H0548 -322 and 0048 -097, show significant rapid variation (hours). Two of them, 3C 66A and Mrk 501, exhibit significant variability on time-scales of months. We find that 3C 66A has a variability period of 63^5 d, supporting the 65-d period obtained by Lainela et al. The periodicity seems to be weak according to the Jurkevich V 2 m test. We also find a correlation between B 2 V colour index and B magnitude for Mrk 421. We discuss possible physical mechanisms on the basis of these observational phenomena.

With the fast development of the power system, traditional manual inspection methods of a power transmission line (PTL) cannot supply the demand for high quality and dependability for power grid maintenance. Consequently, the automatic PTL inspection technology becomes one of the key research focuses. For the purpose of summarizing related studies on environment perception and control technologies of PTL inspection, technologies of three-dimensional (3D) reconstruction, object detection, and visual servo of PTL inspection are reviewed, respectively. Firstly, 3D reconstruction of PTL inspection is reviewed and analyzed, especially for the technology of LiDAR-based reconstruction of power lines. Secondly, the technology of typical object detection, including pylons, insulators, and power line accessories, is classified as traditional and deep learning-based methods. After that, their merits and demerits are considered. Thirdly, the progress and issues of visual servo control of inspection robots are also concisely addressed. For improving the automation degree of PTL robots, current problems of key techniques, such as multisensor fusion and the establishment of datasets, are discussed and the prospect of inspection robots is presented.