Lawnbot 2.0

Over the past few years, I’ve seen several renditions of remote control lawn mowers.  I LOVE THIS IDEA.  Yeah, I love the idea enough to use all caps.  Let me tell you why…

In high-school, I worked at a Subway.  This means that every day for months on end, I ate at Subway.  Eating at one place that frequently for that long will make a person quite sick of that food for a long time.  To this day, I still can’t eat there.  I don’t have any problem with Subway as a whole; I think they make a great product.  I’m just still sick of it after twenty years.  Similarly, I used to mow lawns in junior high and high school.  In an average week, I’d mow four yards.  Fast forward two decades and I still hate the idea of lawn mowers.  It just reminds me of wasting perfectly good summer days pushing a heavy object up a hill, smelling like gasoline, and not getting compensated fairly for the work I did.

Seeing articles of people who have circumvented the torment of mowing a lawn by leveraging technology, I couldn’t help but think, “This is the greatest idea ever.  Move aside fire and sliced bread.”  I began to plan my own implementation of a remote control lawn mower in a similar fashion to what I had seen before.  The style of implementation uses an existing lawnmower, but removes the wheels from the cutting deck and adds the cutting deck to a remote control platform.  For examples of this, see http://www.instructables.com/id/Arduino-RC-Lawnmower/ and http://www.robert-smith.net/my-projects/how-to-build-a-rc-lawn-mower/.

While there are benefits to this approach, I began to deviate.  I asked myself, “Why does it need to use a gas engine?”  At this point, I decided that maybe I’d make mine electric.  Then I asked, “Why does it need to use a lawnmower blade?”  If a string trimmer can cut grass, perhaps a heavy metal blade is overkill.  I lastly asked, “If it doesn’t have a gas engine or a blade, what does it have in common with a lawnmower deck?”  The answer is, “Nothing.”

During this time, I also thought about what I didn’t like about the existing style.  Mainly, this was that the wheels were too far out from the cutting deck.  Lawn mowers are difficult enough to turn when the wheels are bolted onto the deck itself.  When the deck gets bolted onto a longer frame, that puts the wheels much further out from  the deck, creating a longer wheel base.  Long wheel bases might be great for adding stability to cars, but it sounds terrible in terms of steering a lawnmower.  The extra depth also creates a longer depth of area between where the unit butts up against something and where it can cut.  If you drive it up to a fence, there is a certain distance between where it hits the fence and the blade.  Several inches along this fence will never get cut since the blade can’t reach it.

At this point, I felt liberated from the original platform.  I hate the phrase “paradigm shift”, but that is what happened.  I began to think of what needed to happen on a high level without considering form.  I put the cutting head in a place where it can get up to what I’m trying to cut.  This allowed me to decouple it from the rc robot base.  I eventually came up with a robot base with tank treads that has a mount onto which the cutting head is attached.

Diagram of the lawn bot

Diagram of the lawn bot

 The base unit is simply a plywood box.  It is 18″ wide x 24″ long x 12″ tall.  The wheels and tank treads are the more interesting part of the build.  I’ll cover that in a later post.

The cutting head is made up of three gears.  These gears are patterned off of Matthias Wandel’s gear generator.  The center gear is a 32-tooth gear and the two outer gears are both an 8-tooth gears.  As the center gear turns, the two outer gears turn at four times the speed.  Attached to the bottom of the small gears are hubs for string trimmer line.  The diameter of the string trimmer cutting path of each cutting head is 12″.  Having two side by side gives an overall cutting width of 24″, which is pretty good for a lawn mower.  See the full-size printable patter for these gears here.

Now the real fun begins.  How do we get all this stuff to move?  I need three points of rotation: cutting head, left wheel track, and right wheel track.  Let’s start with the cutting head. 

Since the cutting head is based on the principle of a string trimmer, the goal RPM is roughly that of a regular trimmer.  These run about 3,000 RPM.  Since the spool gears are 1/4th the size of the drive gear, the drive gear should run at 1/4th the final output rotation, which is roughly 750 RPM.  In this case, we need an electric motor that can spin at 750 RPM and handle enough torque to spin the cutting heads (considering they have been geared up for speed).  A basic cordless drill not only operates off of a batter that can be contained within the base unit, it also spins at roughly 800 RPM.  If the drive gear of the cutting head has an output shaft of 3/8″ or less, the drill can chuck directly up onto that.  Instead of having the battery in the drill, the battery is house in the base unit.  A solid state relay sits between the battery and the drill and is actuated by a microcontroller.  The trigger on the drill is held in with a zip tie.  When the microcontroller sends the 5 volt control signal to the solid state relay, the power from the battery is sent to the drill.  The drill turns the drive gear at 800 RPM.  The drive gear turns the outer gears at 3200 RPM, which spin the trimmer string to cut the grass.

And now for the wheels.  The base unit will be a tank-tread style vehicle.  This means there is a wheel that is covered by a tank tread.  The diameter of the wheel plus the tank tread above and below is a total of 8″.  At this diameter, one rotation of the drive wheel will move the unit 25.12″.  My goal speed is about 8″ per second, or 480″ per minute.  480 inches per minute / 25.12″ circumference=19 RPM.  This is pretty slow.  I like the idea of using a cheap rechargeable drill for each side, like I did with the cutting head, but going from 800 RPM to about 20 means a 40:1 gear reduction or a severe cut in voltage.  Assuming the drill motor is linear in its relationship between RPM and voltage and the drill is rated for 12 volts, this would mean 0.3 volts if we only went by voltage.  Since that would be difficult to create efficiently, a combination of voltage drop and gearing is necessary.  I’d much prefer using gears anyways since that will also increase the torque to the wheels.  I haven’t designed the gearbox yet, but watch for a later post.

Since this is a track-type vehicle, the left wheels turn together and the right wheels turn together.  In this case, only one of the two in each pair need to be powered.  For example, the shaft for the front left wheel is connected to a motor and the shaft for the rear right wheel is connected to another motor.  Both could be in the front, both could be in the back, it doesn’t really matter as long as one is driven on each side.  To turn the vehicle, each motor needs to be able to reverse.  As one side goes forward and the other in reverse, the vehicle turns.  An H-bridge will be used to control each motor.  An H-bridge is a type of electronic circuit that controls the speed and direction of a motor based on the output of a micro controller.  Much like the solid state relay in the cutting head, the H-bridge will sit between the battery and the drills that are used as the motors.  This will allow me to control the speed and direction of the rotation on each side of the unit.

Watch for updates.  I will also shed details on the controls in a later post.