A window cleaning robot works by vacuum suction pulling its body against glass while drive wheels move it in a set path, with infrared sensors stopping it at the frame. A wiper pad or rotating brush scrubs, a small pump wets the glass, and a tether catches it if suction fails.
That is the whole machine in one sentence. What follows is the part the product pages leave out, which is why two robots with the same spec sheet behave completely differently on a skylight.
How does the vacuum suction actually hold the robot?
A centrifugal blower inside the body pulls air out from under the base plate. The pressure difference against atmospheric pressure holds the unit to the glass. Most consumer units run 2,500 to 3,000 Pa of vacuum; commercial glass-roof units push higher.
The number that matters more is flow rate. A strong vacuum with weak flow loses holding force fast if there is any leak, and on glass there is always a leak: a scratch, a hair, a frame notch. Sealing rings degrade with UV and heat, which is exactly what a roof gets.
This is why suction figures alone mislead. A machine rated 3,000 Pa with a tired seal holds worse on a dusty skylight than one rated 2,600 Pa with a fresh double lip.
What do the sensors do?
Four jobs. Infrared or ultrasonic rim sensors detect the frame and stop the robot. A pressure sensor watches suction and triggers an alarm if it drops. A bumper switch catches obstacles. Some units add a gyroscope or accelerometer to track orientation on pitched glass.
The rim sensors are the weakest link in real use. They sit close to the bumper where dirt collects. On glass roofs they also get confused by the shadow line where a frame meets a wet surface. A robot that stops 5 cm short of the edge on every pass leaves a dirt band you then clean by hand.
Our Mamibot iGLASSBOT W200 SkyBot review has a good example: rim behaviour changed noticeably after the sensor ports were cleaned, and the edge band shrank from about 6 cm to 2 cm.
What path pattern does it follow?
Three common patterns. The N pattern runs vertical strokes then horizontal, zigzag style. The Z pattern sweeps side to side with straight moves. Spiral starts at the centre and works outward. N covers a rectangle efficiently, Z handles wide short panes better.
None of them handle an L-shaped skylight, a roof light interrupted by a mullion, or a dome. That is why frame crossing matters more than path pattern for roof work. A robot that stops at every frame bar turns a 10-minute job into a 40-minute manual one.
This is the case for machines built around obstacle crossing. The Lingyun Y3 review describes a unit that steps over low frame bars instead of halting, which changes the economics of a multi-pane glass roof.
How does the cleaning part work?
Two approaches. A moving pad, where the whole body shifts and the microfibre pad wipes. Or a rotating brush or squeegee, usually on the front edge. Pad-only units are quieter and simpler. Brush units shift more dust but fling water and need more maintenance.
Water delivery is either a tank on board or a spray bottle you apply manually. Tank units dose 3 to 8 ml per square metre on typical settings. That is far less than most people expect, and doubling it does not clean better, it dries worse.
What happens when suction fails?
The robot drops, unless something stops it. On vertical glass a tether is the whole safety system. On a pitched skylight the robot slides before it drops, which is slower and more forgiving, and gives you a second or two to react. On a flat roof light it can do neither and just sits there while the alarm sounds.
The realistic failure triggers, in order of frequency: seal contamination, wet glass with a soap film, frame impact knocking the body loose, and battery sag on suction-heavy modes. Notice none of them are dramatic equipment faults. They are housekeeping.
What goes wrong on a frame edge?
The rim sensor fires late and the robot stops with a margin, or it fires early and stops short. Either way you get a band. On a skylight with a 4 cm aluminium frame, a 5 cm stop margin means the machine never touches the last strip of glass, and that strip then collects dirt faster because it is always the wettest and dirtiest edge.
What should you not assume?
Where do these machines still fail?
- Suction holds it, flow rate matters more than peak vacuum.
- Rim sensors stop it at frames, and they need cleaning every few runs.
- Path patterns assume rectangles, so frame crossing beats clever routing on roofs.
- Suction failure on glass is a tether problem, not a robot problem.
- Water dose is small by design, more water usually means more streaks.
If you are choosing between machines, our skylight robot reviews list suction, flow, path pattern and slope rating side by side.