Re: [mowbot] The grass keeps getting longer

Byron A Jeff (byron nospam at
Wed, 3 Feb 1999 19:26:11 -0500 (EST)

> > 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
> memory
> > 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
> cycled.
> > 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.
> David