This study proposes a self-locking-type expansion mechanism for in-pipe robots. Previously, we proposed a highspeed locomotion mechanism using pneumatic hollow-shaft actuators; however, this mechanism lacked holding force and could not pass through a bent pipe. The proposed mechanism generates a large holding force and can easily pass through a bent pipe by invoking a self-locking phenomenon. We conceptualize and design the novel expansion mechanism and introduce its associated mathematical model to formulate the holding force and mechanism design. The characteristics and capabilities of the mechanism are elucidated by experiments. From the experimental results, we optimize the applied pressure and the design of the mechanism. The proposed mechanism generates a maximum holding force of 69.7 N, which is 5.2 times higher than that of the previous mechanism, and drastically improves the robot's bent-pipe-passing capability. Finally, the performance of this mechanism is confirmed in a simulated pipe test. In this trial, a robot equipped with the proposed mechanism smoothly and steadily moves through complex pipe configurations, including the vertical and bent pipes.

This study proposes a high-speed locomotion mechanism for a pipe-inspection robot. As a result of the narrow and complex structures of pipeline networks, it is difficult for robots to move quickly within the pipes. The new pneumatic mechanism proposed here realizes high-speed locomotion along with advantageous features for pipe inspection including a small diameter, flexibility, and low weight. First, we present the design concept of the novel locomotion mechanism using pneumatic flexible hollow-shaft actuators, which was previously developed by the authors. The prototype constructed to realize this concept and the associated mathematical model are then introduced. Second, the basic characteristics of the proposed mechanism are evaluated in terms of the holding force (generated by an expansion mechanism against the pipe wall) and the impellent force that induces forward motion in the robot. Finally, the in-pipe movement performance is confirmed. The experimental results show that the designed robot can be propelled inside a 53-mm-diameter pipe at a maximum speed of 250 mm/s, which is exceedingly faster than conventional designs.