Re: [mowbot] The grass keeps getting longer

david steinke (dsteink nospam at
Wed, 03 Feb 1999 14:02:14 -0800

> Robin (another list member) and I had warm exchanges about this several
> years ago.
> He maintains that a circular platform with centre axis wheels and a single
> caster at 90deg would be a stable platform for a mowbot. Robin argued that
> the battery could be placed over the caster to keep the centre of gravity
> in the middle of the tripod formed by the wheels and caster.
> While I agree that this works fine on level ground, my concern was that
> inclines can change the COG enough to place it over the unsupported region,
> or close enough so that if the robot stops, it's inertia would cause it to
> fall over.

I'll agree with Robin. By placing the batteries to the rear and as low
as possible would make it tip back even on a sloped surface. I have flat
surface anyway, and at a speed of 5 feet per minute (my planned movement
rate), my inertia will be so low that I don't think it could tip.

> >2: I am experienced in photovoltaics. I have a 12 volt nospam at 1 amp array
> >that will fit onto my 15" to 18" diameter top. Should I charge up large
> >batteries and then run or just try to make it low power, run when it
> >shines, or half way in between?
> You may do better trying to charge the battery and then run on the charge.
> Husqvarna's solar mower is limited in the types of lawn it can deal with,
> it can't climb an incline of 20deg or greater, the cutter had a windage
> shroud to reduce friction and could only just clip the top of the grass.
> Your array would only provide 12 watts at optimum and I think the solar
> mower used 50w and it had special motors. You can check the patent which is
> linked at the Domestic Droid website:

That's why I call a solar only a nipper. It would cut the grass every
day, so that it would not build up. I have experimented and I can keep
my loaded wheel drive current below 200 ma, and the unloaded cutter
motor current under 500 ma.

> If you decide to use solar cells, the exacto blade arrangement sounds like
> the best choice. To use a weed whacker head you need high revs, I think
> 6000 was the figure. I bought a head for experimentation, it feeds
> automatically if the revs are high enough which solved the usual feeding
> problem. If you want the part number let me know and I'll dig it out.
> I wasn't happy with the results, it mainly made a mess of the grass,
> tearing it instead of cutting. Also I have found out that the less clean
> the cut, the more likely the grass is to be infected by various lawn diseases.

I agree that a weed wacker rips the grass apart. Right now I am leaning
toward a high power standard blade (only 8" diameter though), with a
total power consumption of close to 150 watts.

> I don't see a problem with that, however you might find the grass looks
> patchy after the robot has wandered all over it. Is your grass area
> completely bounded by a wall or something else the robot can bump into?

2" or higher.

If you like nicads why not try NiMH or LIon. The NiMH can't form a
> so you'll get better runtime out of them. The LIon will give you at least
> twice the energy density of NiMH which is almost twice the NiCad.

Size or weight isn't the problem. It is cost and current draw. New
surplus nicads or gel-cels are about 3 watt-hours per dollar. There are
almost no surplus NiMH and LIon, so they are roughly 5 times as
expensive at least. The other problem is current draw. In the high power
mode (where I really need capacity), 150 watts represents a 12 amp
current draw from 12 volts. A 12 amp current draw of NiMH or LIon will
kill them quickly, and they run at much lower efficiency at higher
currents. Even if I just drew half the current, I would get a much
better life out the nicads.

Memory in Nicads and Gel-cels is not a problem in cells regularly cycled
and properly charged. In a robotic lawn mower they will constantly get

> Sounds great! Those brushless motors are much more efficient than normal ones.
> Dave Everett

I hope to keep the efficiency of the wheel drive system over 80% and the
mower blade itself over 85%.

I guess what I really have to experiment with is my cutting blade. I
would like to use an 8" diameter blade. 2" of horizontal cutting blade
on the outside, mounted on a 4" diameter disk. If I spin at 6000 rpm, I
can let the blades pivot clear if they hit an hard object, and swing
back out with centrifical force. Now the tough part. I can mill four
holes in the 4" diameter plate for air passage, and place air scoops to
force air down in the center, hopefully drawing up air and grass through
the cutting blades. Another choice is to simply mount a standard 8"
diameter aircraft prop above the cutting blades but offset at an angle.
I could mount the cutting blades at an angle to act as a prop, but I
what the cutting surface to be horizontal. I guess it's time to
experiment again.