robot lawn mower obstacles

How Does A Robot Lawn Mower Deal With Obstacles?

Robot lawn mowers need to have good ways of avoiding obstacles on your lawn. Most lawns will have something that a robot lawn mower will need to avoid. Some people have trees, shrubs, flower beds or ponds in the middle of their lawns.

What about temporary obstacles? If you want to put a trampoline or garden furniture on your lawn, it is important for a robot lawn mower to be able to detect and avoid this, while still getting on with its job of cutting your lawn. What about smaller items that are accidentally left on the lawn, such as toys, a hose, or gardening tools?

Robot lawn mowers use a combination of perimeter wire, which is laid during installation, and on board collision detection systems to avoid obstacles on your lawn. The user can decide whether to install perimeter wire around an obstacle or let the robot mower’s collision detection systems avoid an obstacle.

This article will discuss the ways that robot lawn mowers use to deal with obstacles. It will also discuss which obstacles it can’t avoid, and provide advice to ensure that your robot lawn mower operates without any problems.

Perimeter Wire

The most obvious way that robot lawn mowers used to avoid obstacles, is by the use of boundary or perimeter wire. This is wire that is installed around the perimeter of your lawn, prior to using your robot lawn mower for the first time. The perimeter wire is installed in a continuous loop,  and can also be used to demarcate obstacles within the lawn, such as flower beds, trees or ponds.

Each end of the perimeter wire is connected to the charging station, forming a continuous loop. A low voltage is applied through the perimeter wire, which produces an electromagnetic field, which the robot lawn mower is able to detect. The machine will be able to stop when it gets to the perimeter wire and then move in a different direction, to stay within the boundary of your lawn. Read more about the process of installing a robot lawn mower here. 

Laying the boundary wire around the perimeter of your lawn is very simple. The boundary wire can be pegged down to the grass, or buried a few centimetres under the grass. If you have permanent obstacles within your lawn, you will need to create boundary wire Islands.

To demarcate obstacles within your lawn, you should install the perimeter wire along the edge of your lawn, until you get to the point closest to the obstacle within the body of your lawn. At this point you should lay the boundary wire directly from the edge of the lawn towards the obstacle.  

You can then lay the boundary wire around the obstacle, completing a loop back to where you started. You then continue to peg the boundary wire to the grass between the obstacle and the edge of the lawn, and continue installing the boundary wire around the edge of the grass.

marking obstacles with perimeter wire my robot mower

It is very important to be precise when installing the boundary wire, taking particular care to ensure the wire never crosses. This is important, as a crossed boundary wire can create difficulties for the robot lawn mower detection system.

Once the obstacle within your lawn has been demarcated, the robot lawn mower will be able to detect the boundary wire around the obstacle and around the perimeter of the lawn. However, because the wires running to and from the obstacle are placed side by side, the signals cancel each other out and the robot lawnmower will ignore this section of perimeter wire, passing over it without issue. 

Collision Detection

Whilst the boundary wire is a very useful system for avoiding permanent obstacles, you do not have to use it to mark out all obstacles. Solid obstacles, such as trees, do not have to be marked out, as the robot lawn mower can use its collision detection system, to deal with these.

As robot lawn mowers move very slowly, they will not cause any damage to an obstacle that they encounter. The only factor that comes into play is whether the obstacle is solid enough to cause the collision detection systems to register and trigger a change in direction from the robot lawn mower. Any solid object, even if it is quite light, will do this.

The only obstacles which won’t be detected by the collision detection systems are very small or very light obstacles. Small obstacles, which are low to the ground will not impact with the outer shell of the robot lawn mower, and it will run over the top of them. Very light obstacles that may have been placed on the grass temporarily, such as a paper cup, for example, may result in the robot lawn mower pushing them along or out of the way, rather than stopping and changing direction.

Robot lawn mowers use a number of different systems to detect and avoid obstacles.

1. Floating Shell

Many robot mowers use a floating shell surrounding the mower chassis, that becomes displaced if an obstacle is encountered, and a Hall Effect sensor that detects shell movement.

2. Accelerometers

Most robot lawn mowers use a soft front bumper that has one or more accelerometers mounted to it. The accelerometers are able to detect obstacles due to the deceleration that accompanies an impact with an obstacle.

3. Changes in Wheel Speed

Some robot lawn mowers have sensors which monitor the speed at which the wheels are turning. A sudden and unexpected decrease in the wheel speed can indicate contact with an obstruction, which then triggers the robot lawn mower to back up and go in a different direction.

4. Sonar and Lidar

Some of the more advanced or expensive robot lawn mowers on the market are starting to incorporate sonar or lidar detection for ranging and obstruction detection. Sonar or Lidar enables the robot lawn mower to gain information about nearby obstacles and allows the robot lawn mower to slow down and even stop prior to coming into contact with the obstacle.

Overall, robot mowers have been developed with fairly advanced collision detection systems, which enable them to avoid or stop immediately on contact with an obstacle. This prevents any damage to the obstacle or to the robot mower.

Do Temporary Obstacles Cause Problems For A Robot Lawn Mower?

You may be concerned that a robot lawn mower would mean you would have to ensure that your garden is clear of temporary obstacles at all times. However this is generally not the case.  

In my personal situation, I have two small children that love playing in the garden. This results in a lot of toys being brought out into the garden. As they are getting more adventurous, we may consider getting a climbing frame, slide or trampoline. Thankfully, a robot lawn mower will be able to deal with these larger temporary obstacles with ease.

Larger temporary obstacles, such as a trampoline or garden furniture, are sufficiently solid that a robot lawn mower will simply use it’s collision detection systems to mow around them without causing any damage to the obstacle.

You do need to be a little bit more careful about small obstacles on the grass. Small toys will not be large enough to trigger the robot lawn mower collision detection systems. Plastic figures, building blocks or small garden tools are likely to be run over if they are left on the grass. Depending on the model of robot lawn mower that you choose, this may cause damage to the object to a greater or lesser extent.

Typically, robot lawn mowers that use small freely rotating blades, rather than solid fixed blades cut with lower force and will therefore cause less damage to any obstacles they encounter.  

I have had one experience where my robot lawn mower ran over a small plastic toy that belonged to my daughter. As my robot lawn mower is a model with three small freely rotating razor blades on a spinning disc, there was only superficial damage to the toy.

Models from other manufacturers such as Robomow, which uses solid fixed spinning blades would do a lot more damage. There are of course a lot of benefits to opting for a cutting system such as this, but running over small obstacles is not one of them.

Pine Cones, Leaves And Dog Poop

Obstacles don’t just have to be left by humans. At certain times of the year, there may be natural obstacles such as pine cones, leaves, branches, or fruit from trees. So how are these obstacles managed by a robot lawn mower?

Typically, the answer is that a robot lawn mower will cope with small obstacles such as this without issue. However, sticks, pine cones and fruit from trees may cause increased wear on the cutting mechanism of your robot lawn mower. For this reason, it is advisable that you periodically remove solid natural obstacles.

Regarding leaves in the autumn, I will typically let my robot lawn mower work through leaf litter as long as it is not too dense. My rationale is that the nutrients of the leaf litter, once mulched by the robot lawn mower, are likely to be beneficial for the nutrition of my lawn. If the leaf litter gets particularly dense, I will go out and either lift or rake any significant collections of leaves.

Another issue which may concern users with dogs, is how does a robot lawn mower cope with dog poop?

You would imagine that dog poop would be bad news for a robot lawn mower. However, they seem to deal with dog poop very well. True, it will cause some dog poop to adhere to the cutting mechanism  and underside of your robot lawn mower for a day or two. However, this seems to be quickly cleaned off by the frequent movement of the robot lawn mower and continued mulching of clippings which goes on under the robot lawn mower.

After the robot lawn mower has gone over a patch of dog poop a few times, it should disappear from view as it is flattened and the grass grows over the top. Personally, I would suggest that it is generally better to lift dog poop whenever you see it, as even once a robot lawn mower has been over the top of dog poop, there is still the possibility of stepping in a small patch, which would cause that awful experience of getting dog poop on your shoe.


Typically, I run my robot lawn mower 5 or 6 days per week to keep my lawn looking great and always freshly mowed. However, If I am out in the garden playing with my children, I don’t want the robot lawn mower getting in the way and running over small toys.

In these situations, I simply open the hatch of the robot lawn mower and activate the home command. This tells the robot lawn mower to return to it’s charging station. This doesn’t interrupt it’s normal schedule, but lets you have use of your lawn without having to worry about obstacles. Whenever I am done, I just set the robot lawn mower to auto again, and it continues to cut on it’s normal schedule.

Another thing I have considered, is to set the robot lawn mower to avoid cutting at times when I am likely to use the lawn, particularly with my children. For example, you could set your robot lawn mower to avoid cutting the grass at weekends, so that any temporary obstacles that would be on the lawn during the day on Saturday and Sunday would not cause problems for the robot lawn mower.


Overall, robot lawn mowers cope with obstacles, both temporary and permanent, extremely well. The only ones which will cause problems are the small obstacles which are too small to activate the collision detection systems. As long as you are mindful of this, you shouldn’t have any issues.