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[DPRG] segway

Subject: [DPRG] segway
From: cbrenizer cbrenizer at socket.net
Date: Tue Dec 3 17:38:01 CST 2002

<I meant to send this to the entire list, too>

I had wondered if a third wheel device costing 3 or 4 hundred dollars
would have happier customers than the multi-thousand dollar segway (I
know I'd be happier paying less and it probably wouldn't need much
electronics, since it wouldn't have to be self balancing)

Did your demo prove out that the two wheel mobility was far superior to
what can be gotten from 3 or 4 wheels.  Is it worth the extra thousands
of dollars?

I would assume we'll be seeing late night infomercials selling knock
offs that are 3 wheeled, unless there truly is a benefit to the segway
design that I'm missing.

-----Original Message-----
>From: dprglist-admin at dprg.org [mailto:dprglist-admin at dprg.org] On Behalf
Of David P. Anderson
Sent: Sunday, December 01, 2002 12:59 PM
To: dprglist at dprg.org
Subject: [DPRG] segway


A fellow came by the university last week with a couple of Segway Human
Transport Vehicles and I had a chance to play with one and ride around a
bit, both indoors and outdoors.  

For anyone that's missed it, this is a two-wheeled dynamically balanced
platform very much like my two-wheeled robot nBot.  After playing with
it a bit I have the following observations:

1.  It's a LOT of fun to ride.   The balance is very stable and like
with nBot you make it move simply by leaning fore and aft a small
amount. Steering is controlled with a motorcycle style left-handed grip,
counter- clockwise for right and clockwise for left.  It can spin in
place and we easily maneuvered around the halls in the science building,
into folks offices and around their desks, etc.  

2. Top speed is about 12 mph, which seemed pretty fast indoors and
pretty slow outdoors, easily outpaced by folks on bicycles.  It handled
curbs and ramps with no problem, and had no tendency to roll downhill
unless you expressly leaned in that direction.  Very maneuverable.

3. Turning at any speed is a little counter-intuitive, as it does not
lean into the turn it almost feels like you are leaning out of the turn!
The fix is to lean your body into the turn, like on a four-wheeler or as
racers on motorcycles with sidecars do .  A bit awkward.

On the other hand, you can drive it fore and aft (no turning) without
touching the handlebars at all, just with your feet, with your hands in
your pockets.  It's that stable.

4. Of the two models we had to play with, one would balance by itself
with no rider (though we weren't allow to ride that one) and it had
saddle bags with books and batteries, ballast I suspect.  The other had
no "kick stand" mode and would not balance without the inverted-
pendulum weight of the human.  The one which would balance unattended
was not critically damped, and a small shove left it oscillating for a
long period as it slowly regained equilibrium.  The fellow who brought
them for demo got quite nervous when I started shoving it around.

5. The salesman (he insisted he was not, only "evaluating" them) was a
little defensive on the practical application of these vehicles, and had
no good answers to the questions posed him by the physics and geology
faculty (all of whom had a great time riding!) such as "why this rather
than walking?" 
and "why this rather than a bicycle or traditional electric scooter?"
Truth is for $6000-$9000 I not sure there are good answers to those

(If I was trying to sell them, I would concentrate on the sport market,
like skateboards and surfboards and dirtbikes and sailboats, rather than
trying to convince folks this was a practical replacement for cars or
motorcycles or bicycles or walking).

6. However, that being said, I am more convinced than ever that this is
the OPTIMAL PLATFORM FOR A ROBOT!  In concept at least if not in this
particular incarnation.  I believe our (robot builders') application for
this technology is extremely natural and logical, more so perhaps than
for human transport.

7. Most of us are already building three wheel robots, with two drive
and a tailwheel which is really only there for balance, and causes
certain manuvering problems.  The three wheel design is not really all
that stable, especially on inclines and irregular surfaces, and is
fairly miserable outdoors.

Making it more stable involves extended the tailwheel far behind the
drive wheels, and then it tends to run into things when the robot tries
to spin in place.  Four wheels are much more stable than three, and
allow sophisticated suspensions and such not possible with three wheels,
but gives up the ability to spin in place (0 turning radius) without
lots of complex technology.  NASA went to a very complex 6 wheels with
rocker boogies and independently steering wheels on their Mars rover to
address these problems.  Quite complex.

8. Military/contruction vehicles solve this problem with tank treads,
but these are 
very inefficent, hi-friction and hi-energy consumption.  And because
they require slippage in order to turn, it prevents the use of wheel
odometry for location and navigation, as well as causes problems indoors
with loose carpets and marks on the floor.  Treaded vehicles are also
very difficult to maneuver at high speed. Toy manufacturers in the 90s
tried to start a class of racing tanks, but found the problems of
steering at high-speed made it impractical.

For these and other reasons the two-wheel differential drive with
free-castering tailwheel has become the most common robot platform among
hobby builders.  The addition of an inertial measurement sensor and a
little software can get rid of the tail wheel without changing the rest
of the design, and _greatly_ increases the stability, off-road
capability, maneuvering, etc.  Hence my increasing confidence that the
two-wheel dynamic balancing technology is a natural platform for robots.

Plus it gets extremely high marks on the Scale of Absolute Coolness.

I shot some photos of the Segway and nBot side-by-side in the hallway of
the Heroy building, for comparison, and put them on


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