robot lawn mower chassis

How Do Robot Lawn Mowers Work?

Robot lawn mowers are automatic machines that cut your grass for you. The popularity of these machines is increasing significantly and you will start to see more and more gardens having one. The most common question that I always get asked about my robot mower is “How does it work?”

A robot lawnmower is really quite a simple machine, and not the advanced robot that many predicted would be helping us around the home by now. It uses simple techniques to ensure that it does a reliable job of keeping your lawn cut. This article explains how robot lawn mowers work in a straight forward way, which gives should give you a good understanding of how they work. 

I think it is easiest to think of robot lawn mowers being made up of four main systems

The Four Main Systems That Make Robot Lawn Mowers Work

  1. A set of spinning blades that are powered by an on-board motor and battery.
  2. A chassis with wheels attached to a motor and battery, which enables the mower to travel around your garden.
  3. A basic and low power computer system which enables the mower to use logic and algorithms to determine what path to take across your lawn to ensure that it all gets cut.
  4. Boundary definition using a low voltage perimeter wire.

There is of course much more to them than this, but these are the essential features that combine to get the job done.

Generally, robots are capable of much more impressive feats than those displayed by the current crop of robot lawn mowers on the market. However, the designers and manufacturers are creating a consumer product and there are a number of competing requirements.

It is technically possible to build a robot that does a much more efficient job of cutting your grass, avoiding obstacles and cutting the lawn in the most efficient manner possible. Visual recognition systems could detect the height of the grass and the geography of your lawn. The computer could build a map of the garden over time, including mapping permanent and temporary obstacles and pro-actively avoiding them.

However, the complexity and cost of manufacturing such a machine would put it out of the reach of most consumers and result in the failure of robot lawn mower companies.

Therefore, there has been a conscious decision when designing robot lawn mowers to build machines that get the job done safely and effectively, while keeping costs competitive with conventional mowers.

As the costs of implementing more advanced technology decreases over time, it is progressively being implemented into robot mowers. This means that robot mowers will only get better over time and the switch from manual to robot mowers will likely accelerate over the next few years.

The Cutting System

robomow robot mower showing blades

Robot lawn mowers use significantly different cutting technology from manual lawn mowers. Manual lawn mowers tend to be energy inefficient and use high powered cutting blades to enable someone cutting their lawn to get it done quickly and efficiently.

As conventional lawn mowers cut a larger volume of grass per unit time, the blades need to be more powerful. This, along with the fact that conventional lawn mowers are usually petrol powered, makes them much louder than robot lawn mowers.

Robot lawn mowers work with much smaller cutting blades, and move slowly, cutting a much smaller volume of grass per minute than a manual mower. There are two main cutting systems that you will see on robot lawn mowers.

  1. A rotating disc, which has a number of freely rotating cutting blades attached to it. The blades tend to be very tiny, approximately the same size as razor blades. The cutting diameter of these mowers tends to be quite narrow, but are the least noisy option.
  2. Solid cutting blades, either alone or in multiples to increase the cutting width and hence rate at which your lawn is cut. The downside of these is that they are noisier and use more power, requiring either a larger battery for the same cutting time, or a trade off of battery capacity or operating time.

Robot lawn mowers are designed to cut your lawn much more frequently than a conventional mower. There are several reasons for this

  1. Robot lawn mowers mulch the clipping and return them to the soil. Delays of multiple days between cuts results in a greater volume of clippings, which are more likely to clump together and sit on the surface of your lawn, rather than fall into the lawn to act as fertiliser for the growing grass.
  2. The reduced power of the cutting blades means that it could take your mower an entire day or more to get round to cutting all areas of your lawn. If you only schedule your mower once per week, you may find there are areas of significant discrepancy in the look of your lawn
  3. As robot lawn mowers work with such low power requirements, they are designed for frequent use, ensuring that your lawn always looks freshly cut every day.

The cutting systems of robot lawn mowers needs minimal maintenance. Mowers will require blade changes every so often, but this is generally a simple and inexpensive task.

Robot lawn mowers are generally designed to have sharp blades which slice through the blades of grass. This is in contrast to conventional rotary mowers, which knock the tops off the blades. Sharp blades cause significantly less damage to the growing grass and ensures that the grass recovers quickly after being cut. This is another reason why a lawn cut by a robot lawn mower will generally be more green and vibrant than a lawn cut by a conventional rotary mower.

The Chassis And Wheels

robot lawn mower chassis

This is a fairly simple construction, but there are a few key points.

Robot lawn mowers have a low centre of gravity, which means they can generally handle considerable gradients without the risk of toppling over. Cheaper models may be limited by the power of the motor driving the wheels, but more premium models can manage slopes of up to 25 degrees.

If you have a particularly hilly garden, it can be a really unpleasant job hauling a conventional mower up and down steep hills. The way robot lawn mowers work on steep slopes may encourage people with more challenging lawns to take the plunge.

The wheels of robot lawn mowers are designed to have minimal damaging impact on the underlying grass. As robot lawn mowers are in use frequently, it is important that the wheels do not cause undue stress or leave tracks in the grass, which would spoil the look of the grass.

The chassis of robot lawn mowers usually has low ground clearance and the blades are tucked well under the mower, which reduces the risk of injury. It is very difficult to either intentionally or inadvertently access the spinning blades under the chassis without the automatic stop being activated by the on board tilt and lift sensors. This helps to make robot lawn mowers the safest option for cutting your lawn.

The Brains Of The Operation

robot lawn mower main board

As outlined at the start of this article, the computer system of robot lawn mowers is intentionally kept simple. Advanced pathing and robotic systems to aid navigation are omitted unless essential.

The mower is programmed with a fairly basic set of instructions which tell the mower what to do when it encounters an obstruction or the boundary wire. Robot mowers have a number of random and semi-random pathing techniques which ensure that the entirety of the lawn will be covered by the robot lawn mower within a reasonable time. However, compared to the most efficient route to cut all of the grass on your lawn, robot lawn mowers are very inefficient.

Some may see this as a negative, but as robot mowers are able to operate up to 7 days per week and are very unobtrusive while in use, there are very few negative consequences of this design decision. By keeping things basic, there are fewer systems to malfunction and it keeps the cost of robot mowers down to a reasonable level.

Robot mowers currently start at about £450 and go up to about £3100 for the top of the range model from the market leading brands. Even the cheapest ones are still quite expensive, so you can see why the manufacturers are keen to keep the cost semi competitive with traditional mowers.

The computer system of robot lawn mowers also controls the mow and charge scheduling. This ensures that the mower cuts when the owner wants and returns to the charging station when required, before resuming its duties once fully charged.

The safety features are also controlled by the on-board computer system. In my opinion, this is one of the most important features in giving consumers the confidence to purchase and use robot lawn mowers. I have written an entire article about the safety features of robot lawn mowers.

The safety features of robot lawn mowers are very comprehensive, making it almost impossible to sustain an injury from the blades while the robot mower is in use. One of the market leaders, Husqvarna, has sold more than a million robot lawn mowers since 1995, and yet there are reported to be no case of emergency department attendance due to injuries caused by robot lawn mowers.

Boundary Detection

installing robot lawn mower perimeter wire

All good robot lawn mowers currently use a boundary wire so that the mower can accurately detect the edge of the lawn. This has to be laid along the edge of your lawn before your first use your mower. It takes 1-2 hours to set up properly, but will not be visible on your grass once the grass grows over the top of it. Alternatively, you can bury the perimeter wire up to 20cm under the ground and it will therefore not be visible immediately.

I would have to admit that I was pretty disappointed when I discovered that this was the technique that robot lawn mowers use to detect the edge of your lawn. It’s hardly what you would expect from an advanced, high tech robot.

The reason for using this is because it is reliable and cost effective. There is one manufacturer that I am aware of which has released a robot mower which does not use a perimeter wire. However, there are considerable limitations as to the type of lawn that it can be used on, and for this reason, I couldn’t recommend it.

I am sure that in time, as the technology develops, perimeter wires will become a thing of the past, but for now we are stuck with them. To be honest, once you have it installed, there is no more input required, and any mower that I have ever tried works flawlessly with this system.

Three minor problems arise from using a perimeter wire, which you should be aware of.

1. If you change the shape of your lawn, you will need to move the perimeter wire to match the edge of your new lawn. This doesn’t take long, but is another job to do.

2. Perimeter wire breaks can happen. Burrowing animals, freeze thaw conditions or inadvertent damage from working in the garden can cause a break in your perimeter wire. The perimeter wire can be easily spliced using a wire coupler, but again, this is a frustrating problem.

The worst scenario is when you have a break in your perimeter wire, but you don’t know where and you are faced with the problem of digging up the wire till you find the break. Don’t panic if this happens, as I have a couple of tricks for finding a break in perimeter wire. This article is a really useful resource and will save you loads of time if you do have a break. There is a really easy solution, which is never mentioned in any of the robot mower manuals!

3. Dethatching/Scarifying can be more challenging. If you peg your wire down and let the grass grow over the top, you can have great difficulty dethatching your lawn. Unless you know precisely where the perimeter wire is, you are liable to catch the wire in your scarifier, ripping it up and causing a break.

It is for this reason, and a few others, that I would always recommend burying your perimeter wire. One option is to peg it down first, and then come back and bury it when you have more time, or once you are confident that the mower is working without any issues.

If you bury your perimeter wire at a depth of approximately 10cm, you can use a standard domestic scarifier without worrying about catching or damaging the perimeter wire. Lawn maintenance is not impeded and you keep your lawn looking great all year round.

If you have any questions about how robot lawn mowers work, please comment below, as I’d love to help.