1 Introduction

Green technology leads to a better quality of life for humans. Humans try to improve their quality of life by observing their surrounding environment. Cleaning is a representative example of a process that improves the quality of life by preventing contamination and giving rise a feeling of well-being. Recently, the need for a clean life has led to the development of more technology, such as room cleaning robots [1, 2] and air purifying machines [3].

Contamination of a building’s façade is a significant problem worldwide, especially in Asian countries such as Korea and China. As industries are developed, more pollution is generated, dirtying building façades or glass. Human workers clean building façades manually by using suspended gondolas or ropes, which involves difficult and dangerous work. In Korea, an investigation indicates that 15 workers lost their lives during façade-cleaning work in 2016 [4]. It is also very tedious work, as it takes a long time to clean the façade of a large building.

Façade cleaning is a big market, estimated at 10 billion USD [5, 6]Footnote 1 in 2017, with potential for rapid increase. As the number of high-rise buildings has increased, the market is also increased, and high-rise buildings are being constructed worldwide, with an especially dramatic increase in construction in China [5]. This increasing trend will undoubtedly continue in the near future.

Many researchers have suggested different types of façade-cleaning robots. From scientific research, such as bio-inspired robots, to commercially available products, researchers have tried to penetrate the façade-cleaning robot market for the last few decades. In due course, the efforts of researchers will open up the façade-cleaning market to commercial robotic products, and it is expected that robots can fully replace humans, so they no longer have to do this hard and dangerous work.

The authors investigated façade-cleaning robots and analyzed their climbing mechanisms, cleaning methods, and application issues. Typically, the façade-cleaning robot consists of a wall-climbing device, and a cleaning device. The wall-climbing device can be classified by the locomotion and attachment mechanisms. Investigation methods and classification results on recently developed façade-cleaning robots are given in Sect. 2, while Sect. 3 presents the results gained from investigating the operating environments facing commercialized robots on the buildings in Gangnam, Korea. Sections 4–6 describe the investigation and analysis of robots based on their ability to overcome obstacles, and on their cleaning performance, mobility, and maneuverability, with analysis on recently developed robots presented in Sect. 7. Finally, Sect. 8 includes discussion and conclusion, and we identify the challenging issues that must be overcome for façade-cleaning robots to widen their applicability.

2 Investigation of Façade-Cleaning Robots and Classification

Façade-cleaning robots developed by researchers in the last few decades are summarized in Table 1 [7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62]. Both commercially available robots and research prototypes are included. Wall-climbing robots not currently used for cleaning purposes but with payload capacity on glass surfaces are included, in recognition of their potential availability as robots.

Table 1 Façade and glass cleaning robots including classification of their locomotion and attachment mechanisms
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The investigated robots are classified by two characteristics: locomotion and attachment mechanisms, as shown in Table 1. For locomotion, we classify the robots into four categories: track, wheel, walking, and hybrid types. Many commercially available façade-cleaning robots use either tracked or hybrid types of locomotion, and several examples are also found that use the walking style. For attachment mechanisms, four categories are defined: vacuum suction, vortex suction/magnetic adhesion, wire/rope, and bio-inspired types. For the bio-inspired attachment mechanisms, there is currently no commercial product example, but we include the mechanism as a potential future possibility. In all, sixteen categories are used to define the classifications of façade-cleaning robots; the types with functional indices are shown in Sect. 4, after first reviewing building environmental conditions in Sect. 3.

3 Environmental Investigation

To define the requirements for the robots’ functionality, investigating the environmental conditions in which they would operate is very important. As the numbers of high-rise buildings rapidly increase [6], so does their variation. To develop an efficient façade-cleaning robot, the robot must be able to clean all different types of buildings effectively.

We investigated high-rise buildings in Gangnam, Korea. Gangnam has many high-rise buildings, and they have many different types of façades, such as flat glass surface, rounded glass surface, obstacles, and inverted-sloped surface, as shown in Fig. 1. To collate detailed building information, we used “Daum Road View [63] ”, and measured the data based on these images: we also visited Gangnam sites to ground-truth the reliability of the data collected from the website. We investigated all buildings used as offices and retail premises, 276 in all, in Dec. 2017. We excluded apartments, as we found that they are not easy to define completely, in terms of their shapes and surfaces.

Fig. 1
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Examples of high-rise buildings in Gangnam. a Flat glass surface, b rounded glass surface, c 100-mm obstacle with signs, and d glass surface with inverse slope

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Table 2 provides the building data based on façade materials, existence of signs, shape of the bottom and tops, and the size of steps. As we expected, most large surfaces of high-rise building are composed of glass with zinc panels, indicating that cleaning glass surfaces is the most important aspect of the façade-cleaning mission. For the shape of bottoms and tops, many buildings have retail premises at ground level while most building tops are simply flat. In addition, many buildings have advertising signs on their facades, extending across the second to fifth floors.

Table 2 Statistical data for the surveyed buildings, Gangnam, Korea (n = 276)
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One of the most important environmental conditions for robot design is the profile variation, or “step size,” of buildings, which is shown in the bottom rows of Table 2. Half of the buildings include steps less than 100 mm on their vertical surfaces, including their signs, while the accumulated numbers are 88% for less than 400-mm steps and 96% for less than 500-mm steps. Therefore, the ability to overcome high obstacles is required for a robot cleaner to clean most high-rise buildings.

Based on this investigation of their operating environment, we concluded that the basic attributes a robot would need, to effectively clean façades, could be summarized as follows: (a) good cleaning performance (CP) for glass surfaces, (b) good “obstacle overcoming capability” (OOC), (c) high locomotion speed for fast cleaning, and (d) a high level of maneuverability, to assist cleaning speed and also to facilitate ease of installation and retrieval. We then applied these criteria to the robots inventoried in Sect. 2.

4 Obstacle Overcoming Capability/Cleaning Performance

The first two criteria to determine the performance of façade-cleaning robots (including some wall-climbing robots with potential for use as cleaning robots) are the obstacle overcoming capability (OOC) and cleaning performance (CP). As discussed earlier, the OOC is necessary, while CP is definitely the main function of a façade-cleaning robots. To analyze performance, we defined the levels of each criterion, as shown Table 3. The OOC is ranked according to five levels of obstacle height, and the CP is classed according to the cleaning principles applied.

Table 3 Criteria to analyze the OOC and CP of façade-cleaning robots
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Figure 2 shows the analysis results for the robots identified in Table 1, and as shown in Fig. 2, many of these just focused on flat surface climbing ability and did not have any cleaning devices. For the OOC, the multi-track robot, which comprises vacuum suction attachment and with tracked locomotion, has the highest score, by designing the tracks with multi-linked mechanism. The vacuum suction functioned robots, denoted in red, have relatively high OOC compared to the other robots, regardless of their locomotion method.

Fig. 2
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Obstacle overcoming capability (OOC) and the cleaning performance (CP) of façade-cleaning robots

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For the CP, there are some successful robot examples that use vacuum suction and wire/rope attachment mechanisms. Mostly, vortex suction robots use the dry clean or diatomite methods, and the wire/rope robots use water cleaning. Some examples in the right side of Fig. 2 use water cleaning with a squeezing mechanism to maximize the CP, while having limited OOC. The IPC Eagle [47] robot with wire/rope attachment was seen as a reliable solution, using the water cleaning method and with level one OOC, as shown in Fig. 2.

It is important to note that none of the robots examined showed both high OOC with high CP, and the authors believe this indicates that more design work is required to design a popular robotic solution that exhibits both high OOC and CP simultaneously.

5 Cleaning Speed Analysis

One important characteristic of façade-cleaning robots is their agility. We analyzed the agility using two indices, absolute speed and relative speed based on body length, and the results are shown in Fig. 3. As shown, the absolute speed and relative speed show a positive proportional relation. Generally, the vortex suction/magnet type is faster in both absolute and relative terms. The ROPERIDE [45] with wire/rope attachment and Kotay et al. [41]. with translational locomotion, show the highest absolute speed over all climbing robots. For relative speed, there are some good examples from the bio-inspired attachment types, with Waalbot [57] showing high agility by utilizing a wheel-leg mechanism. Combot [31] with magnetic track is also an important example of high speed movement on a vertical surface.

Fig. 3
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Climbing speed of façade-cleaning robots in terms of absolute speed (cm/s) and relative speed (absolute speed/body length)

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6 Maneuverability

The steering ability of a façade-cleaning robot is important for fast cleaning, as is ease of installation and retrieval. Typically, climbing robot design is focused on vertical climbing ability, with steering ability thought less important: nonetheless, our work indicates that steering ability, that is, maneuverability, is a very crucial aspect in the development of an efficient façade-cleaning robot.

Figure 4 portrays the maneuverability of façade-cleaning robots. It is important to note that the wire/rope suspension type robots (represented by cyan colors) do not have steering ability, although they have achieved high performances in the previous analyses of OOC, CP, and speed. Discounting wire/rope suspension type robots, no significant difference has been found in the steering ability exhibited by robots using different locomotion and attachment methods. The track and wheel robots all have relatively high maneuverability by skid steering compared with the wire/rope functioned robots.

Fig. 4
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Maneuverability of façade-cleaning robots

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7 Recent Directions for Façade-Cleaning Robots

To advance their dominance in the façade-cleaning robot market, many companies and laboratories have recently been trying to develop optimal solutions. Figure 5 shows examples of recently released commercial products that are mostly still at the prototype stage. Bisoh in Japan has suggested a solution using a rail system installed on the building to facilitate stable movement of the robot on the façade [64] (Fig. 5a), and also developed a specialized hub system, to insert and retrieve the robot from the guide rail (Fig. 5b). Korean researchers proposed a similar cleaning system concept a few years ago [65]. They also conducted research on control method of the robotic system and published paper recently [66]. Guide-rail types of robots have big advantages in stability, but there is a limitation in that the guide-rail needs to be constructed on buildings. The authors believe this method can be a good solution for the façade-cleaning automation, on the basis of the assumption that future high-rise buildings are designed with consideration of the need for the rail equipment to be installed.

Fig. 5
figure 5

Pictures of recently developed façade-cleaning robots. a Cleaning robot guided by external rail [64], b Curved shape robot insert/retrieve system [64], c cleaning robot using on a gondola [67], d hybrid wire and thruster climbing/attachment mechanism with squeezing cleaning device [69]

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Other approaches relate to the use of existing equipment on the high-rise building or using a hybrid type of locomotion, and different attachment mechanisms. Skypro in Cyprus has a conceptual design using a cleaning device on a gondola system [67] (Fig. 5c), while the authors are trying to develop a similar robotic device with support from the Korean government [68]. Pufeng Intelligent Tech. in China has announced a window cleaning robot that uses wire, tracked suction pads, and thrusters with tilting ability in the media [69] (Fig. 5d). By applying the three different adhesion mechanisms, this robot can maintain its position despite significant external disturbances.

As the complexity of design is increased, the possibility of failure and complexity of control are also increased; however, the authors note that the performance of recently developed cleaning devices has improved. For example, to save cleaning solvent, use of squeezing devices has recently received attention from researchers [70], and to achieve high cleaning performance overall, reliability is very important, and must be fully researched.

Portable cleaning robot with small scale is one direction of the commercialized robot such as Windowmate [32] and Winbot [30]. Recently, several robotic platform are suggested by researchers with similar suction mechanisms [71, 72]. Some modular concept to design a window cleaning robots are presented [73, 74]. There are many different types of tries to realize automated façade cleaning including green manufacturing of dry adhesives [75, 76], and they are continued until the big automated cleaning market is open.

8 Conclusions

The authors surveyed façade-cleaning robots for high-rise buildings in terms of their application in the replacement of human workers with robotic solutions. The investigated robots were classed in terms of their locomotion and attachment mechanisms, and information on their operating environment in Gangnam’s high-rise buildings was summarized to determine the feasibility of the robotic solutions in real world applications. The façade-cleaning robots were analyzed using several criteria, such as OOC, CP, climbing speed, and steering capability, with the latter two closely related to the locomotion and attachment methods used. Some recently developed robots were also analyzed.

The authors conclude that although many robotic solutions have been suggested by researchers, there is no single popular solution commercially available yet. Requirements for high OOC and CP are not easily fulfilled by existing solutions; speed is also very important, in terms of robotic solutions’ efficiency, and the wire/rope type has superior performance in this regard, although this type has limitations in terms of its steering capability.

Apart from the façade-cleaning robot itself, there are other issues to be overcome in order for a solution to be popular: ease of installation, retrieval, and operation in the real world are essential, and the business model shows that it is very important that the robot can be operated by a building’s owner, or by cleaning agents.

Overall, the authors hope these problems can be resolved by researchers, and that automated robotic solution can shortly be used in the dangerous and difficult working conditions currently faced by human workers. The automated façade-cleaning robotic system can help to improve the life quality of the human beings, and to make the environments to be cleaner. Also, some types of the façade cleaning robot can be easily applied to solar-cell cleaning to improve the energy efficiency [77].