The remote control mow-bot
Most of the country has packed away lawn maintenance equipment for the year, but here in the central Texas there isn’t an off season for mowing. If the grass turns brown in the winter, it’s only because it’s tired from growing year round. So when the Arduino contest on Instructables turned up an entry for a remote controlled lawn mower I had to take a look.
For those who don’t know, Arduino is a small development board based on an Atmel ATmega microcontroller. There are a couple different versions of Arduino, but all offer the usual complement of analog and digital I/O, serial comms, timers, etc. You can think of it as the little brother to the mbed product I wrote about in a previous column.
It is very popular right now in the hobbyist community, and Instructables just ran a contest to find the best Arduino projects. There are some interesting entries in addition to the mower: Ghostbusters style proton pack with lasers, coffee bean roaster, an air guitar, a RepRap controller (RepRap itself is an interesting self-replicating machine worthy of a future column), a motion sensing Tesla coil, and more.
In this project an Arduino is teamed up with an RF remote control transmitter/receiver pair, two wheelchair motors, and a custom designed H bridge motor controller.
The frame is made from lengths of angle iron bolted together, with casters, wheels, sprockets, and chain to round the whole thing out.
The author created his own H bridge motor driver which allows him to run the motors in forward or reverse directions, and to vary the speed of the motors by supplying a Pulse Width Modulated signal to the driver. The author used 47 amp MOSFETS, with three in each leg, apparently giving him switching capacity of nearly 150 amps at 24 volts. This seems excessive for a gear motor that draws in the neighborhood of 12 to 15 amps. Perhaps it was done to be able to run without heat sinks or cooling.
The author goes to great pains to point out the dangers of a remote control lawnmower. It’s worth repeating here.
I have had 1 or 2 times during testing that the Arduino jammed up and I temporarily lost control of the mower for a few seconds!!!! Though I have added several filters to discard unwanted signals and I rarely have any issues, an un-manned lawnmower IS STILL A POTENTIAL DEATH TRAP and I assume no responsibility for anything that happens as a result of your use of this code or this tutorial.
The main addition for safety would be a relay that shorts the magneto, killing the motor, whenever the relay is de-energized. Having a de-energized relay kill the motor (or better, two in parallel) makes the motor cutoff fail safe. This relay would be controlled by a watchdog timer, energized while the counter is counting and de-energized when the counter reaches zero, shutting the motor off. The microcontroller would have to periodically refresh the timer before it counts down. If the firmware locks up or the microcontroller dies the timer and relay will shut the mower off within milliseconds.
Another needed improvement would be a dead man switch, a button that has to be held down continuously while the mower is in operation. Releasing the button would also de-energize the magneto relay.
For best results the magneto relay should also control a solenoid connected to the original engine brake so that the engine stops immediately instead of spinning down.
How could this design be improved? How about making it autonomous? One idea might be to take the guts of a Roomba and graft them onto this mower. It’s fascinating to watch Roomba maneuver around corners and chair legs, weaseling its way into tight spaces and then back out. A key element of Roomba is its circular shape and ability to spin in a circle without moving in any direction, so the frame would have to be modified to accomodate this.
Another idea would be one of those wireless fence systems sold for keeping dogs in their yards. Or a system of crisscrossing lasers. Or ultrasonic beacons and rangefinders. Or it could just memorize the pattern it drives while under remote control.
There could be a whole line of outdoor lawn care equipment: edgers, string trimmers, blowers, and pressure washers!
I would rate this project as medium difficulty. No aspect of it is particularly difficult, but it involves several different skills: Embedded programming, electronics and soldering, some metalwork and fabrication. Take it on, however, and you could find yourself relaxing in the hammock while the yard is mowed.