Re: [mowbot] RFD: Define reference 'grassbox' for mowbot

Dave Everett (deverett nospam at
Wed, 13 May 1998 09:39:00 +1000

At 04:04 PM 12-05-98 -0400, you wrote:
>I've been on the list for three years. It has had some interesting
>However after all this time I still feel like I'm lacking to knowhow, skills,
>and equipment to develop a working mowbot.
>Early on in the list development, there were raging arguments about how
>to develop such a beast. Personally I think that was much more informative
>and productive than what followed, very few people discussing their own
>personal development.

I agree, it had a more community feel about it. Someone commented at the
time that the Mowbot, which at the time looked like being a standard unit
based on our discussions, might turn out to be a comprimise by being
committee driven.
>Secondly what about actual grass cutting? My last foray into this field
>(pun intended) was last spring. Someone on the list has experimented
>successfully with attaching Exacto Knife Blades to the end of a motor
>driven platform. I repeated the experiment, using a round plastic J-box
>cover with the attached blades. I found that if driven by a electric drill
>it was fairly effective. The only other addition it needed was some type
>of protection above the disk because the cut grass kept wrapping around
>the spindle. I envision something like an oil funnel. Cheesy ASCII graphics:
>Of course finding a powerful enough motor is another problem. I want to
>thank Paul for posting that motor driver board a while back. BTW is that
>a kit or is it fully assembled?
>Can anyone talk about motors, drivers, gears, pulleys, bearings, and other
>mechanically oriented stuff required to get a platform going? The drive
>electronics and sensors are rather ineffectual without a platform to
>carry it on.
The biggest problem with motors is that the most suitable one for your
project is always very expensive. Mostly I've restricted my projects to
what is cheaply available. This is not always a good idea, it's pointless
designing a great robot if it has trouble pushing itself along.

For drive motors I chose a wormdrive package with a speed close to what I
wanted. By choosing a speed close to my optimum I eliminate the need to PWM
the motor, which simplifies the software. I use differential steering and
relays to reverse the voltage for driving in reverse, since the robot is
usually going forward, even when turning, the relays will get used very
little and won't put much burden on the batteries. The wormdrive means I
don't have trouble with downhill runs as the motor has it's own top speed
and if power is removed the worms act as brakes.

One problem with direct drive motors to wheels is that going down an
incline, you can exceed your safe speed (for cutting and stopping). If you
were normally PWMing the motors to control speed, you now need to PWM the
dynamic braking input or imitate dynamic braking if your H-bridge doesn't
have one built in.

On your cutter motor problem. I will be using 2 cutters so I can monitor
average voltage to determine cut and uncut areas roughly. I have chosen a
Mabuchi motor that is readily available at hobby stores RS-380PH. The motor
is small, can run up to 14000rpm and has fair torque (I used the motor in a
lead screw portion of a walking robot some years ago). If you need more
torque, you could try the motors used for electric model aircraft.

>Sorry to ramble. If you're still here you're as interested as I am. What I
>thought about while mowing over the weekend was how to define a reference
>point to mowbot development. I finally hit upon the idea of the reference
>grassbox. The grassbox has these parameters:
>1) 20 foot square
I take that to be 20x20 not 4x5.
assuming a cut width of 12 inches and a pass efficiency of 50% (each pass
of the robot overlaps the previous cut by up to 50%), the robot needs to
cover about 600ft2, let's be safe and say 800ft2. To effectively cut all
this area, it would be neccessary to maintain some sort of map of cut and
uncut region, this could be implemented as an X-Y grid using odometry from
the wheels to count distance and compute vector. Odometry is inherently
error prone, especially on uneven surfaces like your average yard, but your
6 inch boundary walls could do some error correcting.

>2) Bounded by 6 in high boundaries
bump sensors would satisfy that requirement, and that is the first sensor
to be implemented on my mowbot.

>3) Sloped at a 40 degree angle.
If the robot weighs 20kg, then to the motors it may weigh 30kg going up
such a slope. I don't have the data, but it would seem to have to overcome
an extra 0.5g to climb the slope and since mass and weight are the same on
Earth, it would appear to weigh 1.5 times the horizontal weight. My robot
just climbs a 50deg slope, but it's real slow. Assuming that half the time
it is going up the slope you will increase current drain by about 3-4. My
robot draws about 1.5amps on level grass and 4 amps going up about 30deg,
on concrete the current is much lower.

>4) Contains grass 1 ft high
This is a hard one. I think you would need to hand cut the area first and
then use the Mowbot subsequently. 1ft high grass will be a serious
impediment to any robot, the grass will form a barrier that must first be
knocked down, then the grass under the wheels can cause slippage which will
affect any odometric data. As the grass is getting cut it will become a
thick mat putting great strain on your cutter motors.

>Given the grassbox here are some performance parameters for a mowbot:
>1) MB can cut the given area to a 2 inches.
Can't see too much trouble with that.

>2) MB must not tip over.
Keeping a low profile would eliminate that problem and will allow the robot
to go under low obstacles like branches on a bush.

>3) MB must cut all grass to within 4 inches of all boundaries
Mine will cut to about 2 inches for a fixed boundary and 0 for concrete

>4) MB must perform given task on a single charge.
That shouldn't be too hard, but we need to know vehicle weight, drive motor
and cutter motor current draw, wheel size, wheel speed.

>5) MB must be buildable using hobby available components.
That's always been my goal, although there will always be special
components, like axles or cutter hubs that are best constructed on a lathe.

>6) MB total cost must not exceed $X (not sure what X is. Subject to
>Probably would be nice if it were in the ballpark of a good power mower, say
>$300 to $400 US.)
HAHAHAHA! Well, we'll see ;-)

>Note that the performance parameters only define the powerplant (body, drive
>and cut motors, batteries) and not the electronics, which can be added later.
>If anyone can tell me how I can buy/salvage electric motors and connect
>them to lawm mower wheels, I'd be eternally grateful.

I had to have axles lathed up. I also had some castings made to take sealed
barings to take the weight off the motors. I can't see how you'll avoid
this unless the motors have substantial bearing built in, and I don't mean
bronze bearings. The casting could be eliminated, I found a flange bearing
that would do the trick.
>BTW the last post pointed out using a 500 oz-in ($250! Ouch!) servo. It seems
>to be a round hole (electric lawnmower designed for human operation) square
>peg (making it autonomous) problem. Can't steering simply be done by having
>centered, bi-directional drive motors? That's why I think building a platform
>from scratch is such an excellent idea.

Yes it can, in fact centred wheels aren't neccessary. I have my drive
wheels at the back and a single caster at the front. using the odometry and
the robot dimensions I can roughly compute the position of the nose and
sides, it avoids balance problems as I can keep the batteries very low on
the COG. Ground clearance is 6 inches at the front and 2 inches at the rear.

Dave Everett

Mowbot Project Website