How robot lawn mowers cut grass without whacking off your toes

Robot lawn mowers operate much like a robot vacuum cleaners, but with higher stakes. If a robovac runs over your feet, it might tickle. Equipped with sharp cutting blades, a robot mower run amuck can injure people and do real damage to common yard obstacles such as hoses, sprinklers, toys—children’s as well as pets’—and other things left out on the grass. And then there’s all the valuable shrubs and flowers you’ve painstakingly planted in your border gardens.

Robot lawn mowers are equipped with sensors, like the ones in robot vacuums, to help them avoid such calamities. But without the advantage of walls to establish a perimeter that helps them navigate and map the spaces they’re working in, less-expensive robot lawn mowers rely on the user to lay down a boundary wire that serves the same purpose. More recent—and more expensive—models have evolved and no longer rely on those wires.

Let’s explore how robot lawn mowers function.

Sensors on robot lawnmowers

Mikael Lindkvist

Like their vacuum cousins, the typical robot lawn mower is outfitted with a host of sensors to help them operate autonomously, safely, and effectively.

Grass sensors: These sensors detect the presence of grass below the mower. The sensors on more sophisticated mowers can also detect the height of the grass they’re cutting.
Rain sensors: Robot lawn mowers are designed to operate out of doors, but you don’t want to expose them to the elements unnecessarily. Besides, mowing grass when it’s wet can clog the mower and or leave clumps of grass than can smother your lawn and kill it. A rain sensor can prevent the mower from operating, or have it return to its charging base if it starts raining during a mowing job.
Ultrasonic, infrared, and bump sensors: Robot mowers use a combination of these sensors to detect obstructions. The bump sensor is the simplest: the mower taps an obstruction, and sensor data instructs the mower how best to work around it. Ultrasonics and infrared detect the obstruction before the mower hits it, allowing it to miss it completely.
Boundary wire sensors: Some robot mowers rely on you to lay down a boundary wire marking the edge of your lawn. Sensors on the bottom of the mower detect this current and keep the mower inside the perimeter.
Lift and tilt sensors: These sensors play two roles. During operation, lift sensors alert the mower that it’s caught on an object, while tilt sensors detect if the mower is operating at an unsafe angle. They also detect when the mower is picked up, which is particularly useful as an anti-theft mechanism.

How boundary-wire mowers navigate

Auto-Mow

With robot lawnmowers that rely on a boundary wire, you connect a wire to their charging station and then pull the wire in a loop around your lawn’s outer edges. The wire will either be secured to the ground with stakes or buried just below it. Sensors on the mower detect the current flowing through the wire and keep within that perimeter while it’s operating.

Laying boundary wire isn’t impossible, but I would strongly advise you to either have a professional do job, or rent tools that allow you to bury the wire in the ground yourself. Using ground stakes exposes the wire to the elements—and to animals—and they’re also a tripping hazard.

Some boundary wire mowers are also outfitted with GPS and/or cellular radios, not for the purposes of navigation but to help you locate the mower.

How boundary wire-free mowers work

While some boundary wire-free use grass sensors to detect the edges of your yard, most use GPS technology to establish their location in the world. They also use other sensors to detect their surroundings, identifying obstructions and fine-tuning how they mow around them. In most cases, you’ll need to do an initial walk around your property, using the mower’s mobile app to “teach” the robot mower the unique layout of your yard.

Ed Oswald/Foundry

Boundary wire-free mowers are typically equipped with these additional sensors:

Vision sensors: Without a boundary wire, a robot mower needs some way to find its way around. The most common technology I’ve seen is a camera mounted to the front of the mower.
Advanced grass sensors: While all robot mowers have some type of grass sensor, they have a more important role on boundary wire-free mowers: They detect the transitions between your sidewalk or driveway and the grass, keeping the mower where it should be.
Additional radios: Unless the mower is completely self-reliant for navigation (more on that in the next section), boundary wire-free mowers will have additional radios that connect to either a base station or an antenna.
Artificial intelligence: This is the special sauce that brings it all together. While most boundary wire-free mowers will need an initial walk-around of your yard, onboard AI takes over after that and your robot mower will navigate your yard more efficiently over time, even with slight changes to its layout.

Boundary wire-free robot mower navigation types

I’ve boiled down the various types of navigation methods that boundary wire-free mowers into the following three categories (there are no industry-standard definitions for these terms, and each type has its pros can cons):

Antenna dependent: You’ll set up an antenna at a location on your property that’s within the line of site of all areas that the mower will operate. While it might be able to operate for brief periods out of sight of the antenna, the antenna serves as its primary source for positioning. The mower’s charging station might provide a second reference point.
Mower dependent: This type of robot mower geolocates on its own, without depending on an antenna, and its charging station is just that. Mowers in this category often include cellular backup when Wi-Fi reception is poor—either as standard equipment or as an option.
Hybrid: This class of mowers uses a combination of both the methods above, either to make positioning more precise or for backup. These mowers are a great (and possibly a cheaper) option for more complex yards that have varied terrain and/or where a larger portion of the sky obstructed.

Robot mower navigation brand names

Since you won’t find those category names in spec sheets, here are the brand names that various manufacturers have come up with for them, so you’ll have real-world examples:

Husqvarna EPOS: The acronym stands for Exact Positioning Operating System), and this technology is best described as antenna dependent. Its antenna must be placed at a high enough point on the property where the mower’s work zones are generally within line of sight.

This can be a problem for more complex yards or where installing a rooftop antenna isn’t practical, but the system does works over large distances (up to 500 meters, according to Husqvarna claims), and it can support multiple mowers operating simultaneously, minimizing collision risks. The feature makes Husqvarna mowers popular for commercial venues, such as golf courses, as well as very large residential properties.

Husqvarna

Worx Vision: Worx went the mower-dependent route. As the name suggests, in addition to GPS, Worx Vision boundary wire-free mowers depend on an HD camera on the mower’s front side to make their way around. In addition, on-board AI analyzes what it sees, automatically detecting the edges of your lawn and avoiding any obstructions.

I haven’t an opportunity to test a Worx mower with this tech, but the manufacturer says you can simply “unbox and mow,” making it virtually setup-free.

Mammotion: Mammotion is a new manufacturer I’m just learning about, and I hope to review their mower soon. This is another example of a mower-dependent system. Like Worx’s Vision, Mammotion’s mowers use front-facing cameras and AI to make their way around your yard, with little if any setup.

Segway Navimow:Segway uses a hybrid approach with its H- and I-series Navimow robot mowers. They rely on a stationary antenna, but there’s a secondary GPS antenna inside the mower itself. Both antennas locate the same GPS satellite for enhanced accuracy.

This method also means the mower can navigate even if it temporarily loses site of the stationary GPS antenna. Vision sensors and onboard AI help it move around obstacles, allowing it to mow more efficiently over time.

Advantages of boundary wire-free robot lawnmowers

Ed Oswald/Foundry

Boundary wire-free mowers deliver the most benefits to larger suburban and rural properties, because boundary wire circuits are typically limited to around 2,600 feet, and many base stations can’t power more than 1,000 feet of wire. Very large properties might need a fleet of boundary wire robot mowers.

And if you ever decide to change your yard’s layout, it’s a relatively simple task to retrain a boundary wire-free—one with AI might even be able to teach itself. With a mower that depends on a boundary wire, you’d need to pull up the existing wire and redeploy it—or replace it if it’s not long enough to complete the perimeter loop.

The boundary wire-free mowers I’ve tested perform better overall, especially in terms of obstacle avoidance. The Husqvarna Automower 435X AWD I started with avoids obstacles by bumping into them and then moving around it; and it shows in the scuff marks from years of use.

Disadvantages of boundary wire-free robot lawnmowers

Boundary wire-free mowers generally cost considerably more than their boundary wire counterparts. To keep their price tags down, some manufacturers leave out things like all-wheel drive, which means they’re not good candidates for uneven lawns or steeper slopes.

This type of mower—and its antenna—relies on GPS satellites, so it needs a clear view of the southern sky (assuming you’re in the northern hemisphere), so not every yard will be compatible. Should the GPS signal degrade or fail, whether it be due to equipment failure, plan growth, a storm knocking your antenna down, or extreme solar storm activity, a boundary wire-free mower simply won’t be able to operate.

Is a boundary wire-free robot lawnmower right for you?

Although boundary wire-free robot lawnmowers have been around for several years now, the technology overall is still relatively new. If you’re willing to deal with a few hangups and have patience, we think it is.

The four platforms listed above are a good place to start. Simple yards with smooth slopes and few obstructions are best unless, you opt for one of the more expensive all-wheel drive models.

The more complex the yard, however, the more I’d still direct you toward traditional boundary wire-style models. While the more expensive boundary wire-free models here will likely do just fine, you’ll be able to get a similar boundary wire robot mower for a lot less.

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