Re: [mowbot] Mowbot list

robin nospam at
Mon, 23 Sep 1996 17:10:41 +0100

On Mon, 23 Sep 1996 08:36:42 -0400 Monta Elkins <Monta nospam at> wrote:
> Is this for possible commercial application?
> or one-off gee whiz machines?

Not exactly either one. We certainly aren't planning a mass-produced
commercial product, but neither is it going to be full of uniquely
hand-crafted components.

> My platform was 'easily available' materials.
Although we haven't discussed it much, I think this is our basis too.
Certainly my goal is to design something that can be built from standard
parts bought from Maplin/RS/<your local electronics store> and put
together by a relatively low-tech hobbyist on a kitchen table in a weekend
or two.

> For "brains" I acquired a gameboy.
> 16k ram, z80 processor; serial ports, easy additions
> lcd display; linux and windows emulators; all for $25 (used)
This was certainly a bargain. But I had a look at Jeff Frohwein's web
pages and although there was lots of interesting stuff there, the GameBoy
isn't exactly tailor-made for the job. I don't relish the prospect of
having to reverse-engineer the workings of a propriatery single-purpose
device when (for example) Motorola will send me for free all I could
want to know about their chips which I could use in a design of my own.
Such a design is likely to be cheaper too since it'll have exactly
what is needed.

> I had planned to use a 12v string trimmer as the cutting
> device; assuming it was relatively safe a quiet.
We are going to have to do some research before we can decide on the
best cutting mechanism. A strimmer is a strong candidate. How close
was your design to working? We could do with knowing such things as
how fast it needs to rotate to cut, how big a motor is needed and how
much current it took. How much noise does it make? Also, to satisfy
the safety lobby, what damage does it do to non-grass? Any chance of
you doing some experiments?

> String trimmers may be slow, but the Mowbot doesn't care
> to work long hours.
Exactly. Whatever cutter we use, it's going to be slow. In fact, it
almost doesn't matter how slow it is, it just means the grass is longer
on average. Can we get some figures for this? Although I'm sure there
must be a minimum acceptable length for the grass left after cutting
(whether determined by nature or aesthetics), but this doesn't figure in
our calculation. What matters is the perhaps rather fuzzy upper bound
on how long the cuttings can be before you have to clean them up, c (m).
If we assume that grass grows g (m/s), then to keep its length within
acceptable bounds, it must be cut at least every c/g (s).

Given the width of the cutter, w (m), the average velocity of the
mowbot, v (m/s), and the area to be covered, a (m^2) together with the
implausible assumption that the coverage algorithm is perfect (i.e. the
cutter visits the whole area without overlap), we see that the mowbot
will take a/vw (s) to do the lot. If we put in an arbitrary factor,
e (0=useless<e<1=perfect), to account for the inefficiency in doubling
back and recharging periods, the time to complete goes up to ea/vw (s).
If this is less than or equal to c/g we have a workable system.

Let's make up some numbers and see what comes out. These really are
made up though, so if anyone has any figures at all they're better
than mine: c=0.01m?, g=1E-6m/s??, w=0.1m, v=0.1m/s, e=0.2 (generous)
then the largest area covered is:
=500 m^2
Amazingly, this is just a bit bigger than my back garden, and I didn't
even have to go back and fiddle the figures!


R.M.O'Leary <robin nospam at> +44 973 310035  P.O. Box 20, Swansea SA2 8YB, U.K.