[mowbot] What happened to "keep it simple"

KEN_REED nospam at hp-boise-om2.om.hp.com
Mon, 14 Oct 96 16:44:57 -0600

Item Subject: cc:Mail Text
Greetings fellow roboticists,

Having completed the experiment, design stage, and most of the building
of my LawnBot, I have followed the Mowbot mailings with great interest.

First of all let me say !!!!! BRAVO !!!!! I think the international
effort to work together toward a common engineering goal is great. I also
believe the concern for safety that the group has shown is excellent too.

Second, most of my comments are in the way of a reality check. I've been
doing hobby robotics since 1982, and following news.comp.robotics almost
since its inception. Clearly this Does Not Make me an Expert, just
someone that has lived through a lot of robot projects of my own and
with others in robot clubs. I'm concerned that some of the things that
the Mowbot group are taking for granted are really very difficult tasks.
It is my belief that a simple, reliable and efficient mower robot is
a huge task, not trivial to design or build, and would further the
knowledge base of hobby roboticists tremendously if done well.

Concern 1: Some requirements imply great navigation ability.
It is not trivial to navigate an indoor closed area and know where
you are, let alone outside. Indoors robots typically use odometry
and beacons to locate themselves. There are probably less than 25 hobby
robots in the world that know where they are, and can navigate
from there. Moving things outside makes odometry a greater
challenge, if not completely unviable. Beacons should still work.

I know some of the group are thinking about random movement
till you find grass then do a flood fill. That sounds pretty neat,
but flood fill implies navigation, knowing where things are. Flood
fill is great in CAD or PostScript, but I believe flood fill is
very ambitious given the sensor input proposed and the very
limited computing power available. How about searching randomly
until finding grass and then using a harvey-wall-banger approach
instead. Mow and always keep the mowed grass on one side. Sort of
spiraling down to a point. Certainly not a flood fill, but much
less complex, and not any less efficient with respect to total
distance traveled or number of turns required.

Finding a battery charger or cleaning station is non-trivial. My
own personal indoors robot does it fine almost all of the time, but it
is not an easy task, especially if the robot does not know where he is
or where the battery charger is. How about a robot with a battery
that charges with solar. Run about 2 hours, then charge for 8 to
10 hours.

Concern 2: Some requirements imply lots of power available.
Another thing I'm concerned about is power consumption. I believe
that the Mowbot proposed so far would discharge its batteries much
before a lawn is mowed. Some of you are thinking a 386 on board.
Even portables only run an hour or two on their batteries. Climbing
and mowing on inclines of 40 to 50 degrees will consume huge amounts
of power, assuming the robot could get traction. The mowing devices
proposed so far would consume allot of power. Is there a power
budget, or is something going to be built and then power requirements
addressed? I'd propose that a power budget be started, and the
necessary battery size, weight, and cost be figured out, before hand.

My own LawnBot uses 1.25 amps to cut grass, 1.6 amps for drive, (3.9
amps if both drive motors are at stall), and 0.240 amps for control.
This was all figured out in advance before anything was built, and
was within 11% of planned. Of course it helps to have experience
with power vs speed vs weight vs current vs type of drive.

Concern 3: Some requirements imply a different motion mechanism.
There is a reason that military vehicles are either tracked or have
4 wheel drive. Operating on slopes greater than 20 degrees require
lots of traction. As any of you that are 4 wheel drive enthusiasts
can attest to, grass is very slick. You can not always run
parallel to the slope, there will be objects to avoid. Tracked
vehicles take lots of power to move around. Four wheel drive is
not easy to work into a robot. If steep slopes are a must then
possibly more power needs to be budgeted and possibly tracks or 4wd
are necessary.

I'd recommend keeping the first robot as simple as possible. I know
everyone says this, but then goes merrily along, thinking of pooper
scoopers, or leaves pickup, crushing pine cones, or guard duty, or
gas powered, or RF this, and IR that, or calling home, or going across
the street and mowing the neighbors yard, lawn mower gates, detecting
the police, and so on......

In robotics I'm always rewarded when I rigorously, purposefully
and continually simplify the design and its requirements. Keep down
the part count, and re-use things that have worked well in the past.
When the first robot works grandly, celebrate, use it for a while,
learn from it, and then enhance it.

"Only the finest designs are simple."
or "Anyone can make things complex."

Okay, I've run down now, here's the disclaimer. I'm very interested
in the Mowbot group succeeding with this project. That is the only
reason I have raise my concerns. I'm not trying to rain on anyone, so
please take the above comments as some asking "Does this make sense?"

Here is a simple robot sensor that has
worked great for me, and is on my LawnBot.
Thought you might like to see it.

+ +v
\ pull up resistor
insulator |
_ \ +------+---- hook to sensing device
fill cap -> | | -|-
+-+ +---|||-------+
| -|- |
| small | | <- 2" copper tubing (standard )
| chain\| | w/cap soldered on (plumbing stuff)
| | | to top and bottom
| | ball | filled with motor
| + /bear- | oil.
| / \ ing |
| | | |
| \ / |
| - +--- -v
| |
mount to robot

No it is not a pipe bomb. It is a tilt sensor
for mobile robots. When the robot tilts too much
the ball bearing makes contact with the side of
the copper tube, thus closing the "switch". The
hollow portion of the tubing is filled with a relatively
non-conducting fluid like motor oil to dampen the swing
of the pendulum, and keep from registering false tilts
simply because the robot accelerated or decelerated
quickly, like touching a wall. The length of the
chain can be adjusted so the "switch" closes at the
correct tilt angle. If more or less damping is required
then the viscosity of the fluid can be adjusted, using
heavier or lighter fluid as needed.

| \\| bye for now, thanks for listening; |
\//| |
| digital control systems \ / simplest possible solution |
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| Disclaimer: the above opinions and thoughts are my own |
| or, my employer did not type this. |
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