# Research on Adhesive Coefficient of Rubber Wheel Crawler on Wet Tilted Photovoltaic Panel
Some of the cleaning methods for PV panels, such as rainfall cleaning [26; 27; 28], labor-based cleaning [29], and mechanized cleaning [30; 31], were introduced in the past. The rainfall cleaning method requires a large tilted angle of PV panels that are not costly to clean and mitigate loose dust on the PV surface. However, the disadvantage of this method is that it depends on the rainfall intensity at the installation site. If the rainfall is insufficient, it will cause sludge formation on the PV panel, or only large particles will be removed [32]. Thus, the rainfall cleaning method has low efficiency and is mainly adopted in areas where it is hard to reach for maintenance [33; 34]. The labor-based cleaning method requires costly time, human resources, water, and cleansing facilities [35]. This cleaning method can improve the conversion efficiency and cool down the PV panel [36; 37; 38; 39; 40], but it is an expensive cleaning cost compared to the rainfall cleaning method [41]. Furthermore, cleansing facilities might scratch the dry PV panel surfaces [35]. As well, human workers cannot maintain their focus on work due to declining physical conditions under the prolonged cleaning time. Mechanized cleaning methods employ motorized cleaning brushes or portable cleaning robots [42; 43; 44; 45] and water supply to clean the PV panel surface [47]. The study [48] pointed out that this cleaning method should be applied to clean the PV panels once per week or daily when the rate of dust is intense. The benefit of the mechanized cleaning method is to cool PV panels with water, use automated operation by cleaning robots, and save human resources [49]. Alternatively, the PV cleaning robot can be classified into two categories. The first is to use two motorized trolley carriers clamping on the top and bottom edge of the PV panel [15] to move throughout the PV panel arrays, which is well-suited for mainstream power with massive concentrated PV panel arrays. The other is based on the structure of rubber wheel crawler robots, which is preferred for small-scale PV systems [29]. The advantages of this type of robot are portability and the ability to work with misalignment and uneven surfaces of PV panels. The drawback is that it requires human teleoperation, and the cleaning robot might slip on a wet tilted PV panel. In Vietnam, the rooftop PV system has grown exponentially, with the capacity installations adding 9.3 gigawatts (GW) to its national grid at the end of 2020 compared to that of 378 megawatts (MW) in 2019. This leads to the increasing demand for cleaning accumulated dust on the rooftop PV panels. As aforementioned, a rubber wheel crawler robot with the anti-slip ability on the wet large tilted angle of PV panels seems to be the well-suited solution. This paper studies the anti-slip effect of a rubber wheel crawler equipped with cleaning robots on the wet large tilted angle of the rooftop PV panel. The objective of this study is to establish the relationship between the friction coefficient of the rubber wheel crawler and the surface of the PV panel with respect to various conditions such as the tilted angle of the PV panel, rubber hardness, and dry/wet surfaces.