In regards to window cleaning robots, there are 2 main types: Round-shaped & Square-shaped; both attach to the surface of a window with the help of a vacuum motor.

• Round-shaped have 2 cleaning wheels with microfiber (double side usable). There's a vacuum motor inside to rotate the wheels while the device is moving; similar to hand movement in cleaning of a window. They are very effective at cleaning heavy dust.
• Square-shaped clean windows with the help of a large microfiber cloth which presses on the window with auto-adjustable pneumatic pad. These devices move with assistance of caterpillar belts at the speed of 5 inches per second. They are more efficient for corner cleaning.

The most up-to-date models of round & square-shaped window cleaning devices feature automatic water-spray function. They spray water evenly, similar to the effect of human exhale on the glass, through a delicate ultrasonic chip nebulizing it to 15μm mist particles. You may refer to the video below for more information: 

HOBOT-298 Video

HOBOT-388 Video

Listed below is a comparison chart. The general guideline is that round-shaped units are better at cleaning heavy dust, and square-shaped ones are more efficient and more suitable for maintenance purposes. 

Model	HOBOT-298	HOBOT-388
Photo
Device size	240x240x100mm	295x148x95mm
Cleaning pattern	Liner clean, Zigzag then N-way	Rotating, Zigzag
Cleaning speed	2,4 min/m2	4,0 min/m2
Framed glass size	> 40 cm2	> 35 cm2
Glass thickness	Unlimited	Unlimited
Ultrasonic water spray	Yes	Yes
Replaceable water tank	40 ml	24 ml
Maximum cleaning area (per 1 ml)	1 m2	1 m2

In summary, you may choose a model according to your cleaning preferences, your specific environment, and your desired speed of cleaning. If you like to clean more frequently, we recommend you choose a square-shaped model. If your windows get dusty fast, we suggest you get a round-shaped one.