## 1 Introduction  
A cable-stayed bridge is a common structural form. The cables which are as important bearing parts of the bridge, need to be inspected regularly due to the influences of extreme conditions, such as wind- and rain-induced vibration. At present, manual detection methods are widely used. This study designs a robot to replace workers for cable defect detection tasks. Robots for cable detection are non-standard automated items of equipment and many researchers have proposed various innovative mechanisms therein: for example, [PERSON] et al. used a six-wheel robot for bridge cable inspection [1; 2]. Such a robot can adjust its clamping forces according to the situation encountered on the surface of a cable, so as to adapt to cables of different diameters. Moreover, they designed a cable visual detection system and took pictures of the cable surface from different directions with three cameras. The pictures were sent to the ground-based monitoring station in real-time. [PERSON] et al. designed a cable crawling robot [3; 4]. The cables are automatically monitored by using non-destructive testing (NDT) technology. With the development of an eight-channel sensor for magnetic flux leakage inspection, the measured magnetic field signals were transformed into 3-d graphics to allow intuitive monitoring. [PERSON] et al. proposed a robot for cable detection [5]. By combining it with an efficient image-based damage detection system, the damage to the cable surface was automatically recognized through image processing and pattern recognition technology. Type-I and -II robots for cable maintenance are capable of carrying 250 kg payloads and designed by [PERSON] et al., which have been tested on Xupu Bridge in Shanghai, China for maintenance issues such as the painting of bridge cables [6]. There are a lot of researches on bar and pipeline climbing robots. For instance, [PERSON] et al. proposed a 3D CLIMBER bar climbing robot that can climb along a spatial tubular structure [7]. The 3D CLIMBER can measure position deviations against three different classifications by using a sensor and then the errors therein are corrected one-by-one. [PERSON] et al. proposed a method of computing operational regions for transition in global path searching for biped climbing robots [8]. Furthermore, [PERSON] et al. designed an oil and gas pipeline climbing robot to detect bent pipelines and T-shaped oil and gas pipelines [9]. [PERSON] et al. developed a wheeled bar climbing robot which is capable of climbing along a straight bar with heavy payloads [10]. [PERSON] et al. designed a climbing robot that can move along the bar and rotate around it [11]. In addition, [PERSON] et al. investigated a Treebot climbing robot with two claws [12]: when the robot climbs a tree, it detects the position of the trunk based on the information provided by an encoder, a contact sensor and an inclinometer to deduce the optimal climbing path. [PERSON] proposed a series of robots for pipeline detection and designed corresponding NDT methods [13]. [PERSON] proposed the use of a climbing robot with two claws [14]. By using three sensors (laser range scanner, color camera and ultrasonic ranging module) the deviation of different poses of the holder at the end is detected and then corrected step-by-step, thus realizing automatic gripping and grasping. A grasping module of cross-arranged claw is designed for the inspection of rough stone and concrete wall surfaces, which performs well in grasping vibrating walls with certain anti-rollover capability [15]. [PERSON] et al. designed a shape shifting rolling-crawling-wall-climbing robot with wall-climbing locomotion abilities [16]. [PERSON] et al. designed a robot for cable inspection with double wheels on both sides [17; 18]. Such a robot can overcome the influences of high-altitude wind loads and conduct NDT by entering the high-altitude inclined cable environment well beyond human reach.