# Design and Development of a Climbing Robot for Wind Turbine Maintenance
## 1 Introduction
The fourth type is electrostatic adhesion. It can provide adhesion on smooth surfaces, both conductor and insulator. The drawback of this method is that it is only suitable for smooth surfaces, including glass, acrylic, and plastic panels. The electrostatic adhesion can be enhanced by using flexible and soft elastic material as the dielectric to improve its resistance against the peeling force [10; 11]. The fifth type is propeller-type adhesion attraction force generators, based on aerodynamic principles [12; 13]. These propeller climbing robots depend only on the thrust force of the propellers as the only source for generating the  
Figure 1: Wind turbine maintenance and working condition  
adhesion force, which may cause a problem of stability unless a more sophisticated control technique is used [3]. Climbing robots can be classified into several types in terms of locomotion modes. Wheeled, tracked, legged, or arms with grippers and combined types have been applied in the field of climbing robots [2; 14; 15]. Typically, robots with wheels or tracks are used with few obstacles on continuous surfaces as they have a higher speed than legged climbing robots. The legged or arms climbing method is more applicable than other mechanisms if the surface has a complex structure or irregular obstacles. Robots have different climbing mechanisms, depending on the characteristics of different infrastructures. Various similar climbing methods have recently been developed to inspect cylindrical or circular truncated cone structures. For example, Figure 2a shows, the development of a \"ring\" climbing robot with a payload capability allowing it to climb around the cylindrical tower. The prototype has three modules that are completely identical and can be easily joined together to climb on any circumferential tube. The wheels depend on spring forces to grip around and climb on the tower. The innovation of the robot is that its adhesive forces are provided entirely by springs rather than conventional adhesion methods (such as vacuum suction, magnetic force, and dry adhesions) [16]. Figure 2b shows a robot with a cylindrical or conical shape. The robot utilizes a rolling self-locking concept: rolling allows continuous ascension whereas self-locking guarantees null energy consumption while staying still on the pole. However, the robot could only climb up a pole with a high coefficient of friction or else it will slip [17]. The development of a wheel-based pole climbing robot is shown in Figure 2c. It is designed for climbing stepped poles with circular or near-circular cross-sections [18] and consists of a triangular body and six limbs with ordinary wheels at their tips. Three lower wheels are actuated with DC motors. The upper limbs have passive wheels and act just to increase system stability [19]. Figure 2d shows the design of a tree-climbing robot which has anti-falling functionality and can climb tree trunks of various diameters vertically or in a spiral pattern [20; 21]. The RETOV robot Figure 2e can change its movement direction describing three different types of motion. Pure rotational and translational motion can be obtained independently when all three drive wheel systems are horizontally and vertically oriented, respectively (servomotor at 0\({}^{\circ}\) and 90\({}^{\circ}\) respectively). Pitch variable helical movements are also possible when the same acute orientation angle is defined for all the three-wheel systems [22]. A friction-based multi-robot approach is the climbing ring robot, see Figure 2f--also known as Scanning, Monitoring, Analyzing, Repair, and Transportation (SMART)--can carry high payloads and is equipped with an onboard robotic arm. The SMART robot claps the tower surface and climbs up and down with friction-based crawlers [23]. Table 1 shows the existing climbing robot considering the different methods in its locomotion, adhesion, structural design, application robot's weight and the diameter of the application of the climbing robot in pole, tower, tree and pipelines.