## 2 Materials and Methods
### Control System and Experimental Set-Up
#### 2.3.3 Experimental Set-Up  
An experimental setup of wind-turbine-tower-climbing robot has been established in Southern Taiwan University of Science and Technology shown in Figures 13a, 14a, 15a and 16a through the CAD design alongside the final prototype Figures 11b, 12b, 13b and 14b during the actual indoor experiments with the wind turbine tower mock-up, as the realization of the lower dimension from 2 MW wind turbine tower. Figure 13b depicts the actual physical view of the climbing robot attached to the tower, Figure 14b shows the top view of the four rubber wheels as the driving mechanism and the two-caster wheels, and the two body frames are made of aluminum alloy 6061 material, which is low in weight and economical to use. The body frame 1 holds the winding mechanism, power supply of the robot and other electronics modules, as shown in Figure 15b, and the body frame 2 holds  
Figure 12: Control flowchart. The two main mechanisms for the wind-turbine-tower-climbing robot were the locomotion and the adhesion mechanism. The locomotion comprises the four brushless DC planetary gear motorized rubber wheels, which have a 7-30-watt rating power and 80-100 round per minute rating speed, and two caster wheels used to support the body frame of the robot. Next, the adhesion mechanism uses a winding cable that is triggered from the distance sensor to attain the precise tension force to ensure enough adhesion to the tower circumference for climbing. From the distance sensor, an automatic setting in the microcontroller program was established to achieve the step motor speed adjustments in the winding and unwinding process of the cable and to have a synchronization with the four driving wheels ready for the wind turbine tower movement. Because there was only one step motor installed in the body frame of the robot, it was easy to control the cable adjustment to alter the distance between the tower center. When the step motor with twin pulley winds the Bowden cable inwardly, the robot body frame would tighten to the tower surface, the rubber wheels would contact the tower area and the corresponding normal force would increase. As a result, the normal forces provide the frictional force by multiplying the friction coefficient together with the angle of inclination of the tower, which overcomes the robot gravity force to make the robot climb straight up. When the step motor with twin pulley unwinds, the Bowden cable moves outwardly, causing the robot body frame to loosen to the tower surface, and the robot automatically goes straight down. The step motor gives good low speed performance in winding and unwinding the Bowden cable, and the winding rpm speed is around 19-25 rpm. Once the precise measurement in the adhesion mechanism was achieved from the distance sensor, the robot starts its movement on the tower. The robot needs 3 s to start the upward movement and another 3 s to start the downward movement of the tower height of 1200 mm. To verify the payload capacity of the prototype, as shown in Figure 17a,b, approximately 3.540 kg was implemented to test the load capacity of the robot. The results indicate that the climbing robot was able to climb straight up and down in the wind turbine tower mock-up regardless of its various speeds of climbing, whether low, medium, or fast, as well as the different tower diameters. The various payload capacities of the robot are 0.823 kg per body frame, with a total of 1.646 kg in the trial 1 test load, adding 0.947 kg per body frame, with a total of 3.540 kg in the trial 2 test load; regardless of the climbing speed of the robot, its climbing ability has no changes. The robot can withstand the maximum payload weight of 5.44 kg that requires high torque and velocity decrease. During the indoor experiment, the robot could handle climbing its maximum payload weight but, due to the light material that supported the body frame attached to the caster wheel, it tends to be the first to break after several instances of climbing.