Contents I. Introduction
Owing to the ability to relieve human beings from hazardous works, there has been increasing interest in developing various wall-climbing robots (WCRs) in construction, shipbuilding, chemical, petroleum and nuclear industries in the past decades [1]–[6]. With two basic functions: adhering and locomotion, WCRs can move on vertical walls and finish cleaning, inspecting, spraying paint and so on. The adhering method usually includes negative pressure, vacuum or permanent magnet. The locomotion mechanism can be roughly divided into legged, crawling, translation and wheeled and so on. Legged WCRs ranging from two, four, six to eight have good adaptability to the wall and can cross obstacles at the cost of the mechanical complexity and the moving speed due to its discontinuous movement [1], [2], [5]. Crawling WCRs can move quickly with permanent magnets, but cannot handle big cracks or obstacles. Besides, it is difficult for them to adjust the posture freely [1], [4]. A translation WCR, Cleanbot-I, is developed for cleaning glass walls [6]. Its basic mechanism is composed of a frame with a sliding section and the suction cups on the frame. The sliding feet enable the robot to move forwards and rotate. Although the working process is not complicated using a movement mode of sticking-moving-sticking, the movement of Cleanbot-I is also discontinuous. Remarkably, WCRs with sliding suction cups (SSCs), namely there is relative movement between the suction cups and the wall surface, have some predominance because they can attain continuous movement and high moving speed.
