[mowbot] What is LawnBot?

KEN_REED nospam at hp-boise-om2.om.hp.com
Sat, 8 Mar 97 16:38:25 -0700

Greetings MowBot Heads, (: that's a compliment :);

Thou Shalst Mow, Hence I Shall Not!

Dave keeps asking me for photo's of my LawnBot, but that's not a good
medium for me and the hardware available to me. I do have the
mechanical drawings for my LawnBot available. Before I commit to any
hardware I always draw up the whole project so that I can get design
reviews done by my friends and robotics acquaintances. What do you
expect I'm a professional MSEE and BSCS.

First what is Ken's LawnBot like?

It's an autonomous lawn mowing robot that was started by me alone,
before the MowBot project started. It will be solar powered, that is,
charge for lots of hours then cut for an hour or two. It will be
completely unattended. That is, it lives on the lawn, like a really
shiny goat. I hope to maintain it only every month. It is intended
to replace the 2 hours of mowing that I do every week with my Lawn
Tractor, read: "lotsa lawn".

The chassis and body are made from aluminium sheet 0.062" thick. It is
22" x 22" x 12" high, roughly square, but actually octagonal in shape.
The body is not complete yet, but the whole thing will be able to stay
out in the rain/sprinklers if necessary.

It has two 12vdc gear motors that drive 10" drive wheels, directly in
the center of the bot. It mows at 1 foot/second with no speed control
wanted or needed. It has 6 other wheels, 4 of which are on suspension
to provide support and sensing fore and aft, two of which only sense.
(I have a long irrigation ditch along one side of my property and
other areas that have drop offs. All the extra wheels sense the drop
offs. LawnBot can't swim.) My whole mowing area is now contained by
drop offs or fences or buildings, or something physical.

There are two 12v grass cutting motors, turning two 5" aluminium
disks, each with 4 exacto knife blade cutters bolted to them. The
blades together mow a 12" wide path and are a full 5" from the edge of
the robot, which means two things. They are hard to get at by
accident, and I'll have to mow the edges of my lawn. I figured I'd
have to mow the edges anyway, so 2" or 5" or 10" is the same work to

The controller will be done in phases. The first phase is called "The
Dumbest Goat You Ever Saw". That is, it runs every day in a random
pattern, always eating when the battery is charged and the sun is out,
otherwise sleeping/charging. This is what I mean by it lives on the
lawn. I've written a simulation that shows how affective a random
mowing robot is, and how well it covers an area. My simulation showed
that the random coverage will work pretty well and cuts every square
inch of a 150 foot x 200 foot area every 24 hours of mowing. With a
charge to run ratio of about 6, that's a fully cut 30000 sq. ft. lawn
every 6 days. My calculated run to cut ratio is about 6:1. Now I have
more lawn than 30000 sq. ft. so it will take care of a portion of my
cutting this year.

The sensors this year will be limited to 40 bump sensors around the
whole perimeter at two heights, and full up or full down sensors on
each of the 6 sensor wheels. The bump sensors are SPST push button
switches, that are pressed on by the body if the body touches
anything. The control algorithm is simplicity itself. Mow straight
if no sensors are pressed. If front or side bump or wheel sensors are
"closed" then stop, pause for 1/4 second, backup 1/4 second, and then
make a random backup and random pivot, and proceed forward. If back
bump or wheel sensors are "closed" then do a mirror image of the
above. If any wheel sensor is "closed" turn off the cutting motors
and brake the blades to a stop, this takes about 1/2 second. My
simulation shows that this simple control algorithm not only covers
all the grass, it gets un-stuck from corners, traps, etc within about
2 iterations.

Next winter I'll build LawnBot2 with changes based on a whole summers
running, and possible failing to run. LawnBot2 will have a single
board computer to mow smarter than randomly.

I have the mechanical drawings for:

o A cover page showing LawnBot at work.
o The chassis, a concept page, i.e. all the pieces, kind of see
through drawing, giving a good space usage idea.
o The chassis, a cutting detail for the individual pieces, these are
what I use in my shop to measure out the material, mark, cut,
punch, drill, bend etc... There are two drawings here, one for the
floor of the chassis, and one for the skirt and bump sensor mounts.
o The body, an assembly detail, one drawing showing the pieces to cut
and the way to connect them.
o The hub that connects the grass cutting motors to the cutting disk.
o The hub that connects the gear motors to the drive wheels.
o The Charge/Run and Power Deliver schematic.
o The Phase 1 Brain, which is discrete components, not a computer.

I haven't drawn up everything, some ideas are re-used from previous
projects and don't need design review.

So what do you all think, is it worth figuring out how to show this

Dave: These drawings are done in "Designer" from "Micrographics" on a
PC. The only actual output I have is Color PostScript output ready for
a printer. I get this by printing to a file. The color is used in
many of the drawings to encode information, but monochrome prints are
pretty useful too. If there is sufficient interest, is it possible to
save the *.ps files on the website and let individuals download them
and print them?

Hope this helps someone out...

| \\| bye for now...... |
\//| |
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| electrical engineering --O-- machinist / metal working |
| software / computers / \ simplest possible solution |
| work hard & play hard /\ self sufficient living |
| autonomous robots /\ / \ /\ minimal government |
| observed trials / \/ \/ \ living in the country |
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| Disclaimer: the above opinions and thoughts are my own |
| or: my employer did not type this. |
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