How a robot vacuum maps your home
By Lachlan Ford
The first time a robot vacuum runs, it has no idea where anything is. By the end of one lap of the house it has drawn a floor plan, divided it into rooms and worked out a route. Most robots do this with a small laser that spins on top of them. The moving picture at the top of our homepage shows the idea, with a demo house drawn dot by dot as the laser finds each wall.
The short answer
A small laser spins on top of the robot and measures the distance to whatever is around it, many times a second. The robot joins those measurements into a floor plan and cleans room by room in rows. The laser looks out at about ankle height, so anything lying flat on the floor is invisible to it.
How the laser draws a map
Look at the top of most robot vacuums and you’ll see a round bump about the size of a jam-jar lid. Inside it is a laser scanner, which the box calls LiDAR or sometimes LDS. It sends out a thin beam of light that you can’t see, catches the reflection when the beam lands on something, and works out from that reflection how far away the something is.
The scanner spins, so the beam sweeps the whole room. Think of a lighthouse on the robot’s back, turning several times a second, except that this lighthouse keeps a note of how far its beam travelled in every direction before it hit a wall. One full turn gives the robot a ring of dots, which is the outline of everything around it at that height: walls, door frames, the legs of the dining table.
A single outline isn’t much use, because half the house is hidden around corners. So the robot drives, takes another outline, and lines it up with the last one where the two overlap. By the time it has been through every open door, the outlines have been stitched into one floor plan, and the robot knows where it is on that plan as well.
What the map is for
The map is what lets a robot clean in neat rows. It can see which parts of a room it has covered, turn at the wall, and come back along the next strip. A robot without a map tends to wander until the battery runs down. A robot with a map also knows where the dock is, so it can go home to charge halfway through a big house and then carry on from the spot where it stopped.
The rest of the benefit shows up in the app. Once the plan is drawn it gets divided into rooms, and you can send the robot to do the kitchen after dinner without it touring the bedrooms first. You can also draw a box around the dog’s water bowl or a floor-length curtain and tell the robot to stay out of it.
What “laser navigation” on the box doesn’t tell you
The map is a slice through your house at one height, roughly where your ankle is. The beam goes out sideways from the top of the robot, so anything shorter than the robot sits below the beam and never appears. A phone charger cable, a sock and a dog toy are all in that blind spot, and they happen to be the things a robot is most likely to choke on.
This is why many robots carry a second sensor low on the front, and why it’s listed separately from the navigation. It may be a small camera, or a line of light thrown onto the floor ahead with a sensor watching for the way that line bends over an object. The words “laser navigation” tell you the robot can find its way around the house. They say nothing on whether it will notice a cable in its path, so look for the front sensor as its own line on the spec sheet.
There are a couple of surfaces a laser can misread. A mirror that comes down to the floor bounces the beam away, and the robot may draw a room behind it that doesn’t exist. Clear glass can let the beam straight through, so a full-height window may show on the map as a gap.
How it plays out in a real house
A dark hallway is no trouble at all. The laser supplies its own light, so the robot maps as well at midnight with the lights off as it does at noon. Robots that find their way with a camera pointed at the room are different, because a camera needs light to see by.
Low furniture is where the bump on top costs you something. It’s part of the robot’s height, and a robot that’s a centimetre too tall for the gap under your couch will clean around the couch forever. Some robots get lower by tucking the scanner inside the body, and a few can drop the bump down when they need to. On most robots the height on the spec sheet includes the bump. On the few that can drop it, the figure is usually the height with the bump down. Either way, measure the gap under the couch and the bed and compare.
The map can’t help with door sills either. The laser sees the doorway as an opening and plans a route straight through it. Whether the wheels can climb the strip of timber or aluminium on the floor is a separate matter, with its own figure on the spec sheet. If your house has a raised sill or two, our guide to robots for door sills covers it.
Pets and children mostly change what’s lying on the floor. The more often there’s a toy or a stray shoelace about, the more that front sensor is worth having, although no sensor catches everything.
What to do with all this
For the first run, open every door you want cleaned behind, pick up what’s on the floor, and let the robot finish its lap without interruption. Later runs start from that first map, so it’s worth getting a good one. Then spend five minutes in the app naming the rooms and drawing a no-go box around anything that worries you.
When you’re choosing a robot, take the laser as a given. Look harder at the front sensor if your floor is rarely clear, and at the robot’s height if you want it to clean under the furniture. Every robot page on this site lists both, and the Matcher asks about them.
