How Do Robotic Lawn Mowers Work?

Before you buy a robotic lawn mower, it's worth knowing a bit about how they work so that you can make reasonable choices when confronted by a shop full of makes and models.

There are five general areas that are important to consider:

• wheels: these vary in compound, size and arrangement;
• control: how the mower drives around, and finds the edges;
• cutting: there are different blade types and arrangements;
• battery: all robot mowers are cordless;
• programming: how easy are they to customise?

A robot mower is supposed to be a time-saving device. A true modern convenience. However, they tend to get a bad press, usually due to poor choices leading from misunderstandings around how they work.

Wheels

The mower will only be as good as its wheels and blades.

Both depend on the type of lawn. If it is undulating, and located in a part of the world which is frequently wet, then the wheels will have a big impact on the result.

Most robot lawn mowers have two drive wheels, and then one or more balancing wheels. The most common arrangement seems to be three wheels: with the two drive wheels at the nominal 'front' of the mower.

This is great for all round mowing, but tougher terrains will need mowers with more sophisticated arrangements, bigger drive wheels, and wider treads.

For example, the Robomow RX12U is a cheap, highly capable little robotic mower. But one look at it will tell you that if you ask it to mow a golf course, it's going to struggle.

Control

With very few exceptions (mainly robotic mowers used to cut large areas, like golf courses, for example) robot grass cutters use a wire to delimit the area that they are allowed to cut.

This also helps them to cut around the perimeter (so-called edge cutting mode) in certain models.

Husqvarna mowers (including Flymo and Gardena) also have a return wire that has two functions:

• guide the robot home for charging;
• start the mower from a predefined point on the perimeter wire.

This is a handy innovation for people who have lawns that are divided into zones by way of a small entry, such as a gate, or who have fairly large or irregularly shaped lawns where the mower runs the risk of missing far flung areas.

In these cases, better control of the mowing area is gained by running a wire from the base station to a far point on the perimeter wire. The mower will then, on occasion (and the frequency can be adapted in the menu) follow the wire to the end point and start cutting from there, rather than a point nearer the base station.

Newer, and more expensive, models are starting to use apps to offer better control and management over the mower, which is a trend that is very likely to continue, as is using a combination of GPS and perimeter wire.

Localised radio beacons are another control option (think of the Roomba, but outside), but these have yet to make a dent on the market; due to expense and complaints of radio wave pollution.

Cutting

Robot mowers use various blade arrangements. It's worth remembering that a robot mower is a battery powered machine, and as such weight is at a premium.

Single blades as found on traditional mowers are heavy, take a lot of energy to spin up and stop, and can create huge vibrations that make control of an autonomous nature difficult at best, and dangerous at worst.

Manufacturers of home robot lawn mowers have opted for multi-blade (usually three) cutting, usually based on a spinning disc and freely rotating blades with two or more cutting surfaces. The blades are thin, break easily, and need replacing during the season, but deliver a great cutting result.

Larger mowers, designed for bigger spaces, use single blades for robustness (such as the John Deere range of robot mowers), and some makes (such as Robomow) have slightly more robust, powered spinning blades that cost a lot more to replace, but which may be less prone to breakage.

However, the most cost effective option at the time of writing is a three bade arrangement on a spinning disc.

Battery

It's important to pick a mower that has the right level of autonomy for the lawn it will be cutting.

Lithium ion batteries are expensive, but charge quickly and can stand repeated discharge/recharge cycles without losing capacity.

For medium to large lawns, an Li based mower is clearly the best option.

However, for smaller lawns, a different approach can be used. To go back to the current low cost end of the market, the Robomow RX12U takes about 18 hours to recharge, but on a small lawn that doesn't matter, as it will usually be able to cut the whole lawn in a single session.

Programming

However, these cheaper mowers lack finesse in the amount of programming that can be done.

The Gardena R40Li for example, has a flexible cutting schedule, but even that pales into insignificance when compared to the top of the range Worx Landroids which interface with a mobile phone and can be tweaked in a variety of ways.

However, the more options you have, the more time it will take to set up the mower, and the more expensive it will be.

My advice would be that, unless you really need the flexibility to control when, how, and using what pattern the mower should cut the grass, then an entry level mower that is pre-programmed to mow at random and gives you options that are aimed more at convenience than precision is the best bet.

To find out more, check out our free automatic robotic lawn mowers buyer's guide!

Installing the Perimeter Wire for your Automatic Robotic Lawnmower

One of the trickiest tasks when you first install your automatic robotic lawnmower is getting the perimeter wire right.In fact, there are usually two wires:

• perimeter wire round the outside (stops the robot leaving the lawn)
• guide wire (helps the robot get back to the charging base)

We'll deal with the guide wire in a later article, for now let's look at the perimeter wire. The easiest example is a rectangular lawn.

Placing the Perimeter Wire Around a Rectangular Lawn

Many lawns are basically rectangular - even our lawn, which has two zones (front and back) and an 'upper back' lawn, some swings, raised flower beds and a patio, is essentially rectangular.

The only concerns are:

• having enough wire
• what to do about corners

The first is easy enough - just measure the perimeter, and buy more wire if necessary! The second needs some explanation. The picture on the left shows my initial installation attempt, next to the patio.

Incorrect Perimeter Wire Placement

The blue line shows the wire, and you can assume that the area to the left and below the picture are out of bounds for the mower.

The red line shows the path that the mower consistently took. The result was that it would keep stopping with an "Outside mowing area" error!

In the manual, the instructions were quite clear to avoid sharp corners, but somehow I ignored this advice.

Clearly, you do need to follow it, so I adjusted the wire to look like the picture below.

Now, the robot doesn't exit the mowing area, and all is well. Of course, I had to buy extra clips because it's a more complex shape, but Husqvarna sells them in packs of 100, so although it was an extra expense, it wasn't insurmountable.

Correct Perimeter Wire Placement

Plus, it was cheaper than replacing the blades which were grinding themselves on the coping that runs around the edge of the lawn!

Another point to watch when laying the perimeter wire is the distance between the wire and the edge of the lawn.

The temptation is to get as close as possible to the edge, to avoid having to use a strimmer or edge cutter, but it's a false economy, as the robot needs space to turn, and not leaving enough space means that it will also exit the mowing area.

For those who have a raised edge, and are using that to limit the robot, that's fine, but it does cause wear and tear to the bumper.

Of course, if you are lucky enough to have a robot mower that uses ultrasound, that doesn't apply!

The Perimeter Wire and Obstacles

It is also possible to branch the perimeter wire out, and use it to go around an obstacle in the lawn (such as a valuable bush, or tree). The only thing to watch is that the wire is kept close to itself when it is doubled back (to return to the perimeter) so that the robot ignores it. Otherwise, it can get stuck in an area bounded by the outgoing wire, and the edge of the mowing area.

My preference was to build a small wall around things like shrubs and trees, because:

• it would take a lot of wire to go round every obstacle
• it increases the risk of problems with the wire breaking.

Again, following the instructions is paramount!

Setting Up The Perimeter Wire with Multiple Zones

Our lawn is essentially split into three parts, two are connected by a passageway, but the other is at the front of the house and not reachable by the robot mower automatically. We carry it between the back and front zones, manually!

The front zone is created by a perimeter wire that is taken off the main charging station, and runs completely around the lawn, bounding it.

At the back, the perimeter wire runs around the edge of the first zone, and then along the edge of the passageway, around the second zone, and back to the charging station.

The gap between the wires at the closest point of the passageway is less than a meter; this isn't ideal, as the robot sometimes doesn't quite make it through (in either direction) and therefore:

• gets stuck in the second zone, or
• never mows the second zone!

I'm still looking for the best solution for this, but I suspect I'll end up widening the passageway. I'll keep you posted...