Re: [mowbot] The grass keeps getting longer

Dave Everett (deverett nospam at
Wed, 03 Feb 1999 08:42:53 +1100

At 13:00 02-02-99 -0800, you wrote:
>My name is David Steinke and I am new to building domestic robots
>(except about 30 years ago with my erector sets). I have been working
>the past 22 years in scientific instrumentation. My goal is the build
>simple domestic robots for vacuuming and mowing.
Hi David, Welcome to the list.

>First I hope to build the mowing robot. Here is a basic description:
>1: Round platform mounted on 3 wheels, 2 mounted on center axis just
>inside edge, and one mounted 90 degrees from other two which could pivot
>in any direction. Now the problem is should I drive the two center axis
>wheels, or the pivot wheel? I want to be able to turn within itself, but
>the steering mechanism for the pivot wheel (+- 90 degrees) will be more
>work. If I drive the two center axix wheels in forward or reverse, I can
>spin like a top.
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.

The usually method with centre axis is to use 2 casters but this is
definitely out of the question on such uneven terrain as a lawn.

>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:
>3: How to cut the grass. I can make a nipping blade from exacto or
>utility blades, a minature standard mower blade, a mulching blade, or
>use weed wacker line. The mower blade, mulching blade, or weed wacker
>line will consume lots of power, so they would require charge for a day
>or two and then cut. The mower and mulching blade have suction to lift
>the grass as part of being power hunger (I have kids/dogs that trample
>the grass). I would need a fancy auto feed for the weed wacker line, but
>it is not impossible. A weed wacker line could cut all the way to the
>edge, where as the others probably could not get any closer than 1". I
>also could add suction to the nipper and weed wacker line. Any

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.
>4: Control. I was hoping to keep it simple and I use the PIC's at work.
>A simple bumper system to detect the edges, then rotate 120 degrees (?)
>and go again. In the solar only mode, I could make the robot stay in the
>sun (all my grass see's sun at some time). In the large power reserve, I
>could make it go the the corner of the yard where the sun shines most of
>the time with a electronic compass to charge. When fully charged, go
>forth and cut.

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?
>5: Batteries: I can use either nicad or lead acid gel cells. I prefer
>the nicads at work for environmental extremes, but the gel cells are
>easier to maintain charge. Any preference?
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.

>I hope to use brushless motors for all the motors because I hope this
>will run continuously for 5 years without my intervention. I already
>have the motors, controllers, gear drives, spindle for the mower blade
>to be driven by a timing belt, etc. Just have to decide on drive
>confirguration and mower blade (which determines everything else).
Sounds great! Those brushless motors are much more efficient than normal ones.

Dave Everett

Domestic Droids Website