There are 4 types of hold principle for climbing window surface; magnetic force, counter weight, sticky material and vacuum system.
A system using magnetic force is useful on a steel wall, but on a window glass this is inconvenient since it needs that a window glass is wedged between two magnetic bodies, therefore at never be closed window the hold system does not work. In addition, thick window glass as a double glass window needs so much magnetism It means necessity of control of magnetic force, but it is complicating factor.
Counter weight can keep the robot on the vertical wall and also help the motion opposing gravity. But in the system, the robot must be towed using string-like thing as ropes or wires. It is not suitable for the concept of handy.
Sticky material also can keep the robot on the window surface, but it makes a glass dirty.
With the objective of window cleaning robot, vacuum suction force is most suitable for window cleaning robot. Hence it makes enough hold force with lightweight and small size equipments. Thus the vacuum suction principle has been selected for the hold principle. It allows handy design and enough performance.