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by
Dennis Clark
14 SERVO 09.2011
As the summer winds down (sigh), I always start to get
more interesting questions. One this month about robot
lawn mowers seems more in tune with the summer than
the fall, but (in some parts of the USA) I guess stuff grows
all year long (not in Colorado I might add, unless you are a
pine tree). It appears that some of you like to keep LOTS of
data on your robot from one question I got, which is kind
of cool — a sort of running diary for your robot, if you are
into that kind of thing! I’ll say more on these things later.
As hobbyists, we have been doing line followers, wall
bumpers, and mini Sumo and their ilk for quite a while
now. While they do entertain the myriad gawkers at our
shows and demos, they have ceased to entertain us any
more. How about folks doing robot “art,” like talking heads
that track motion, or voice-controlled helper bots carrying
your toolbox or ... In other words, are folks out there doing
useful things with their robots?
I’ve wanted to do a robot lawn mower for a while now
(one of my questions this month). I did recently get a
Roomba to vacuum my lab space for me (I hate to clean) —
thank you, Woot! I may not know the answer to every
question that I get, but I like to research those questions
and hopefully come up with answers, or even options to
investigate.
We should step up to the plate as robot hobbyists, and
do more than table-top conversation pieces if we want to
advance the hobby. So, I challenge folks out there to
knuckle their chins, scratch their heads, and generally get
creative to come up with more ubiquitous robots to let
loose into the wilds of our houses or yards, to actually do
something that we find useful or more widely entertaining
than say, a remote control mayhem monster or a table-top
line follower.
The principles that we learned with our line followers
and mini Sumo robots do have a use in this new world of
robots. It was not wasted time! The basic idea behind the
line follower can be used in our robot lawn mower to have
it, for instance, follow a buried cable that it can track. My
Roomba vacuum has a wall sensor (maze solver), cliff
detectors (mini Sumo), and a bumper sensor. With these
and the IR dome on the top, it does a pretty good job of
cleaning a room.
— Mark G.
Powell, OH
A. I have a suggestion for your platform. I’ve seen the “zero radius” turn mowers that the professionals use; they appear to have differential drive wheels in the
back and a pair of large castors on the front of the mowing
deck. That seems to be how you should build your mower.
There are some rules of thumb that you can use, or
you can use math. Let’s start with math. We need to know
how heavy your mower platform is to start with, so we
know what kind of power we need for the drive motors.
I’m going to give you the gist of Chapter 2 of Building
Robot Drive Trains that I wrote with Michael Owings a few
years ago. This is a thumbnail discussion of how you can
estimate what you’ll need for motors. If you are interested
in going into more depth, check this book out of your local
library, or, buy a copy of your very own (for which Michael
and I will thank you).
Even though this discussion will get into some math, I
promise, it won’t hurt a bit! First off, we want to estimate
what power we’ll need in our motors to move the mower
the way you want it to move. To move our robot, we’ll be
working against two basic forces: friction and gravity.
Together, we get this:
w f appFFF+=
