State of the Art

As mentioned in the introduction, window cleaning robots are currently on the market in the form of domestic and professional robots. The most obvious difference between the two types of robots is the size. Window cleaning robots which are built for domestic use are much smaller than professionally used skyscraper robots. WINDORO [7, 9, 10] is the best-known example of a domestic window cleaning robot, with dimensions of 200 x 200 x 50 mm. With these dimensions it is small enough to fit on almost every window in an ordinary house. An example of a professional robot is IPC Eagle’s HighRise505 [2] which is 2908 x 930 x 2115 mm. A consequence of the size difference can also be seen in the performances of both types. The HighRise is able to clean surface area at a higher rate than the WINDORO. The main difference in behavior of domestic and professional robots is that domestic robots are only supposed to clean one window at a time and require to be manually transferred to a different window while most industrial robots are capable of cleaning an entire facade without human interaction.

There are, however, also guiding vehicles which are specially designed to use domestic window cleaning robots like the WINDORO. These tethered guiding robots, known as TGV’s [12], help to reduce the risk of unwanted events from happening and are specifically used for high rise buildings. The TGV’s help guide the window cleaning robots from window to window and determine the orientation of the robot itself. These tethered guiding systems are often sought for when the window cleaning robot has to move across window surfaces. Another example of a tethered robot is the SkyScraper-I [11]. This robot tackles, unlike the TGV, both issues of movement across windows and cleaning the windows itself. For the first issue, reels are installed on the top of a building to which tethers are attached, which are then in turn attached to the SkyScraper-I. This way the robot can access every window on a specific side of the building. For the cleaning of the windows, the SkyScaper-I uses long vertical oriented rods along with a squeegee can move up and down. At each end of these rods, a rotating arm is attached with a roller. These arms press the rollers to the window frame without making contact with the window. The squeegees move up and down to clean the window.

Getting back to domestic window cleaning robots, the WINDORO the best known domestic window cleaning robot, as mentioned before. The WINDORO adheres to the glass by means of two magnet units on each side on the glass. The inner unit is responsible for navigation and works via two silicon driving wheels which have a high coefficient of friction. The outer unit is responsible for cleaning and works via four motorized rotating disks with a pad for cleaning. The outer unit also consists of a water tank filled with liquid detergent. A nozzle sprays the volume of this tank on the glass surface by using a small water pump.

The WINDORO also has a successor, the Smart WINDORO [10]. This improved version has renewed magnetic adhesion, magnetic force control, vertical position control and corner cleaning mechanisms. The renewed magnetic adhesion reduces the energy needed in order to stick to the window. The magnetic force control helps with windows of different thickness. Furthermore, the vertical position control helps reducing time and energy by reducing double cleaned surfaces and the new corner cleaning mechanism, which has rectangular disk instead of a circular disk as seen in the old version, cleans window corners more properly.

There are more differences between window cleaning robots in general that can be considered, the water supply can be implemented through a water tank or hose, the movement is possible through tracks, external ropes or rails, energy through a battery or socket cable, cleaning through high-pressure water beams, brushing or movement of a wipe. Some robot are equipped with sensors to measure dirt levels on the surfaces those robots pass and other sensors to better understand the environment the robot is working in.
These are some of the possible characteristics of current window cleaning robots regarding mechanical features. Besides all these differences in geometry, the robots also differ in software. The way the robot moves over the window is called the motion planning of the robot. Motion planning is dependent on the mechanical possibilities of the robot in its environment, as well as the complexity of the algorithms it uses.

For this project, the focus is on domestic window cleaning robots, more specifically the motion planning of these domestic robots. The set specification regarding the hardware will be explained in the upcoming chapters. Because the focus is put on the improvement of the motion algorithms, the current technological state of the abilities on the terrain of motion planning are analyzed in the next section

Motion Planning Algorithms'