A Robotic Lawnmower

Make a robotic lawnmower

After delving into robotics briefly, it became apparent that it should not be too difficult to make a small, self propelled lawnmower that could fumble its way around the grass and cut it while in the process. A simple prototype would have very little intelligence, just a couple of bump sensors to tell it to stop and go elsewhere. This ramdomness should hopefully end up mowing the whole lawn but I suspect large tufts would get missed, but you never know till you try. If nothing else, it could mow under the trampoline which would certainly be useful.

Goal

Build a simple autonomous robotic lawnmower

If the lawn is cut short, a small robot should be able to just cut the tips of the grass regularly, keeping it short and removing the need to collect grass clippings; well not regularly anyway. This would mean the blades could be small and the torque required would be much less than its larger, human-propelled cousin. So its a maintenace or grooming lawnmower and not intended for major weed removal !

Requirements

The initial version will just use bump sensors. I may more intelligence and sensing after seeing how the initial version performs.

It would make sense to mount a solar panel over the mower as this can be used to charge the battery. Due to the small size of the machine, any solar panel that could be mounted would be very small so it may take several days to recharge the battery. However, mowing the lawn once or twice a week should still be sufficient to keep the grass under control.

Given the mower brains will be a microcontroller, I believe monitoring battery voltage will be a good investment of time. The mower can be shutdown when the battery voltage drops below a threshold to prevent damaging it by excessive discharge. This opens the door to solar charging or even navigating to a charge point. However, these are all nice to haves but distract from the initial goal so I'll investigate them later.

Time is always against me, I have very little free time so I decided to use an arduino uno as the brains of the system as it is easy to setup and program, and has a host of third-party libraries.

Initial Experiments

First and formemost, the robot needs to cut grass. With this in mind, I set about trying various motors and small blade arrangements. I decided a 12V motor run from a small lead-acid battery was the best option and set about doing some experiments.

Blades

I seem to spend a lot of time playing with blades, be they wind-turbine or mower blades!

The blades are a problem, they need to be sharp but not so sharp they cause major injury if the mower rolls over your foot. Initially, I tried a set of sickle-shaped plastic blades used by some strimming machines. I attached three of these to a central hub, attached it to a motor and sat on my hands an knees mowing the lawn much to my wife's amusement. Even with a fast rotating motor (10000 rpm), the blades did not really cut the grass, but rather batter it into submission, fraying the ends and not making much of an impression. I did consider changing them for stanley-knife blades but thought this was too lethal a machine to even contemplate.

I then made a blade using a 3mm piece of steel strip and ground down a cutting edge at either end, similar to a normal lawnmower blade. I was worried that I could not make this arrangement perfectly balanced and as the blade would be heavy, it may cause vibrations. However, it is still an option. Eventually, I settled on a very simple solution. It was a strip of very thin perforated steel (about twice as thick as a tin can and only about 5mm across). In fact, quite similar to the metal strips you once got in a mechano set. Mounting this on a plastic gear hub and letting it loose on the grass gave very encouraging results.

The Cutting Motor

I experimented with several handy model motors. The mower blade needs to turn very quickly and have reasonable although not excessive torque. Too much torque could prove dangerous in case it hits a foot, will use a lot of current and should be unnecessary for a 'maintenance cut'. However, I did find that small model moters were a little too puny so had to scale up slighty without going overboard. I settled on a 12V xxxxxx motor which passed the hands-and-knees-mowing-test to my satisfaction!

Blade Height

After seeing what model wheels I could find, I determined that a 4cm blade height above the grass was ideal and made a small wooden chassis to achieve this. A few trial runs showed this was rather optimistic on a lawn that is not like a bowling green. Any ruts or irregularities in the ground can cause the mower to ground and dig the blade into the soil so a much higher cut height of 10cm seemed a more realistic value.

This is an image of the prototype used for blade testing

August 2011