Vehicle Kinematics

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Some different mower designs. From left to right, a one, two, and three mower blade design.

Picture yourself at your local grocery store. Our grocery store has two kinds of shopping carts: the big one that I grab for loading up 24 packs of Mountain Dew, and the small one I use when the Missus asks me to get a gallon of milk and some cans of soup. The small cart is half the length of the big one.

Which of these two carts is easier to navigate through the store? Or worded more technically, which one has a smaller turning radius? I think you can see where I’m going with this.

All else being equal, a smaller wheel base and foot print is going to be easier to control using our differential drive system. A longer mower is going to swing through a larger radius, and is going to be more difficult to control and position accurately. For a given amount of drive wheel rotation, you will get more angular travel at the front of the mower with a longer wheel base than a smaller one.

specs
Some high level measures for the three configurations. I didn’t design them in order of number of blades, that’s why the designations don’t match.

It turns out that there are other considerations that impact whether one cutting blade is the best choice for our mower design. A single blade mower allows for the most simple blade drive system. But as you can see from the table above, the single blade design is almost 9in longer than either of the other two designs.

The extra length tradeoff needs to be weighed against the simplicity gained by the cutting blade system. There is also a tradeoff between number of blades and cutting width. By going with a single blade design, you lose at least 5in of cutting width compared to a two or three blade design.

Long story short, of the three designs, the single blade is pretty inefficient by some measures. It is the longest of the three, has the smallest cutting width, and requires the most material to fabricate. If I’m going to be successful with the robot lawn mower build, these are serious considerations that need to be taken into account.

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