# Design and Experiment of a Novel Facade Cleaning Robot with a Biped Mechanism
## 1 Introduction  
Robots have been advancing exponentially over the last three decades, moving beyond the traditional bounds of industrial applications into service missions and sharing social spaces with humans. [PERSON] and [PERSON] have estimated that 47% of total US employment will be replaced by robots and/or artificial intelligence (AI) in the near future [1]. Early evidence points to significant improvements in productivity and safety over a number of service tasks that are dull, dirty, and/or dangerous [2]. Facade cleaning of high-rise buildings and skyscrapers offers enormous opportunities for the use of robots. In recent decades, skyscrapers such as Burj Khalifa [3; 4; 5; 6] in Dubai and the Shanghai Tower [7; 8; 9] in Shanghai have been built as a result of improvements in construction technologies and processes. Even in such newfangled skyscrapers, the facades are generally cleaned by humans. In the case of some skyscrapers, including Burj Khalifa, equipment, such as gondolas, is not installed, and humans are employed to clean facades by hanging from a rope. The use of amanual workforce poses a high risk of fall, accidents, and even fatalities. Numerous incidences of accidents have been reported, even with the use of a gondola, in facade cleaning jobs. One such situation involved a gondola that lost control caused by a gust of wind at the Shanghai World Financial Center [10]. In another instance, a gondola became suspended in mid-air at a height of 240 m at the One World Trade Center in New York [11]. Robotic solutions offer enormous potential in significantly minimizing the risk to humans and improving productivity in facade cleaning jobs. To overcome such issues, research on glass facade cleaning robots has been reported. For example, a series of Skycleaners, which are entirely driven by pneumatic actuators, has been developed to move and clean on the glass wall by utilizing vacuum suction cups [12; 13]. The Skycleaner is installed with suction cups on both ends of the actuators in an X-Y stage, and these suction cups are connected to a vacuum pump [14]. [PERSON] et al. developed a wall climbing robot with an X-Y stage mechanism as well as a rotational joint and demonstrated the effectiveness of the developed robot and the proposed strategies through field trials at both the City University of Hong Kong and the British Broadcasting Corporation (BBC) [15]. The efficacy and validity of the developed system has been demonstrated through a range of experiments [16]. Serbot AG, an industrial company in Switzerland, developed a wall and solar panel cleaning robot called GEKKO [17]. GEKKO is equipped with a vacuum suction cup crawler which rotates horizontally. The crawler has a semi-circular shape and has a straight line on one side and an arc line on the other side, through the use of which both linear and rotational motions are realized. By changing the number of brushes and crawlers, the developed robot can be manually reconfigured to adapt to different wall areas. [PERSON] et al. developed ROPE RIDE [18]. ROPE RIDE enables one to climb up a vertical surface by utilizing a rope dropped down from the top of the building, two additional propeller thrusters are installed to securely adhere to the wall. A cleaning unit for a cleaning wall has been installed [19]. A new impedance control system has been proposed, and the effectiveness of the force control system to press brushes to the wall with a constant force has been shown through an experiment [20]. SIRIUS is a wall cleaning robot developed by Fraunhofer IFF, which is an industrial company in Germany [21; 22; 23; 24]. SIRIUS realizes up-and-down motion by utilizing a crane installed on buildings for facade maintenance. It adheres to the wall by a suction system which locomotes like a linear actuator [25; 26; 27]. [PERSON] et al. developed a wall climbing robot capable of spraying different cleaning and disinfecting liquids in hospital and clinic nursing environments assuming a full flat wall surface [28]. [PERSON] et al.