A comprehensive review of in-pipe robots suitable for small-diameter pipelines: passability, internal inspection, and localization
Abstract
Pipeline robots have gained significant research and industrial interest as essential tools for ensuring the safety of pipeline systems. This paper overviews the current state of in-pipe inspection robots for small-diameter pipelines, with a particular focus on three main aspects: passability, in-line inspection (ILI), and localization. Despite significant progress in related fields, challenges remain in achieving internal inspection of small-diameter pipelines. In terms of passability, most existing pipeline robots are unsuitable for small-diameter pipelines due to constraints imposed by their limited internal space and complex geometric configurations while maintaining locomotion efficiency. Regarding ILI, the detection technology must achieve precise alignment with the pipeline material, and single inspection methods are often limited in the types and locations of defects they can identify. Concerning localization, the complex surrounding environments of pipelines pose significant challenges to localization technologies, where single-method approaches frequently encounter issues such as signal loss or excessive localization errors. This review systematically analyzes the coupling mechanisms among propulsion, inspection, and localization. Based on this system-level analysis, future research trends are identified, including flexible/articulated modular configurations, on-chip integrated sensing, curvature-adapted probe design, multi-physics inspection fusion, AI-empowered signal enhancement, proactive slip suppression, physics-informed neural network-based co-design, low-power multi-sensor fusion, and advanced material applications. This review of recent research trends emphasizes the importance of passability, ILI, and localization strategies in the field of in-pipe robots suitable for small-diameter pipelines and highlights the potential of in-pipe robots for further development in enhancing the reliability and safety of pipeline infrastructures.