Re: [mowbot] Mowbot list

Monta Elkins (Monta nospam at
Mon, 23 Sep 1996 14:19:35 -0400

robin nospam at wrote:

> > 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.

That sounds like a very useful goal

> > 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.

You might want to reconsider; price and ease of construction.
my 68hc11 from motorola was $68.11 512 bytes of ram
no power supply, no input devices, no LCD disply,
no stereo sound (whoops).

Development tools cost extra.

The gameboy is available new for $35
(they have a new slimline version coming out).

You can't compete on cost/performance with some
as mass produced as gameboy.

And availability... Ever try to actually order
Motorola parts in small quantities; it ain't easy.

>> > 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?

Mostly a thought experiment with various hardware.
I've rewired the 'big foot' platform;
that's about it so far (other projects keep me busy).

> 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?

I've got a 110 volt string trimmer.
I can't give you quantitive answers now, but I can give you some

Noise. Not a lot.
Damage to non-grass:
I've seen gas powered do a fair amount of damage to
a human leg. abrasions, cuts etc.
I suspect damage to non-living items would be minimal.
Hopefully the cutting could be placed under the vehicle
such that contact with anything non-grass would be very
Motor was relatively small (guess less than a pound).
I'll look for current requirements - guess about 1 amp at 120v.

> > > 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.

In general yes.
I suspect what ever our slow cut method turns out to be;
It will be very grass height sensitive.
That is, if the grass is very tall, it won't be able to mow
effectively. So the grass can't grow 'too much' between cuttings.

OTOH, Mowbot maybe able to cut at night too.
24 hours a day for a week, would seem to give us
quite a bit or leeway.

> 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:
> a=cvw/ge
> =(0.01*0.1*0.1)/(1E-6*0.2)
> =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!

offhand those figures seem reasonable.
After during a hard rain I'd guestimate g=1E-5m/s
But I might give w=.16m, c=.02,

I think I'll work up a quick spread sheet.

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

-Monta Elkins

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