question about wheels

I don't have any experience with outdoor robots, but many of the ones
I've seen are differential drives like what you describe.  However,
instead of 2 drive wheels and casters, they are either tank like with
treads, or have multiple drive wheels, i.e. either 2 or 3 wheels on a
side.  So, the results are what is called "Skid steering", because when
it turns some of the wheels are forced to skid sideways.

The results are that the robot is not as precise when it turns.  Also,
I think you might not want those black wheels to be skidding sideways
on an interior floor because of the possibility of leaving marks.

Joe Dunfee

Thank you for the reply. Right now, this is still in the concept phase.
I did not plan on using the puny little wheels that came with the
wagon. I have a set of 10 inch inflatable wheels I salvaged from one of
those 2 wheel hand carts (used to move refrigerators and things like
that), that are rated at 900 lbs, and have treads like snowblower
tires, so they can go thru just about anything. I haven't decided what
I will use for the front wheels yet. I apologize for the delay in my
response, but earthlink for some reason has had problems with their
news server all weekend, and they keep telling me their engineers are
"working on it", needless to say, I am about ready to cancel them and
go with dsl. But that's another tale for another time.

Skid steering is Ok, this thing is not going to be used indoors anyway,
except for on the basement floor which is concrete. My backyard will
provide the initial field testing grounds, and then the woods. This is
why I wanted to use RC first. Gordon's book, I think it's chapter 8
shows that it is easier to convert to microcontroller from radio
control once the bugs have been worked out. Anyway, I will find out
once I try it. Thanks again

Joe

Yes, 2 wheels can work.  But most "wagons" tend to be long and narrow.  For
2 wheel drive, it will work better if it's short and wide with most the
weight on the drive wheels.  The further the front wheel is away from the
back drive wheels, and the more weight it has on it, the more traction and
torque the drive wheels need to make the bot turn.  If you intend to drive
in rough terrain (even something simple like thick grass) the rear wheels
might not be able to get enough traction to make it turn.  It's just a
question of weight and power.  You might consider making it run backwards
as well - i.e., put the drive wheels in the front and the caster in the
back.

Speaking of grass, most of the professional lawn mowers work using the rear
wheels to apply power and steer and just use small caster wheels on the
front. Here's a picture of one to give you an idea:

http://www.exmark.com/popups/mowerImage.asp?mower=viking

As Joe said however, it's more typical when making a bot for rough terrain
to use 4 or 6 wheels or to use treads and apply power to all wheels on the
same side.  ATVs work like that:

http://www.maxatvs.com/

As well as skid steer construction equipment like the bobcat loaders:

http://www.bobcat.com/

Their tires skid when they turn so they tend to use wide rounded wheels
that can easily skid sideways and they work better on dirt and gravel than
they do on pavement because of their need to skid.  It's common to turn
ATVs into bots because of the simplicity of their drive system.  The
CajunBot entry into the Dara Grand challenge 2004 for example was built on
an ATV:

http://www.darpa.mil/grandchallenge04/TeamTechPapers/TeamCajunbotFinalTP.pdf

A small 5 pound model robot skids a lot easier than a large 1500 pound bot
of course.

And then there are the bots that skip the caster and only use two wheels:

http://www.segway.com/products/rmp/

http://www.geology.smu.edu/~dpa-www/robo/nbot/

--
Curt Welch                                            http://CurtWelch.Com/
curt@kcwc.com                                        http://NewsReader.Com/

On Dec 9, 1:28 pm, c...@kcwc.com (Curt Welch) wrote:

For

Hide quoted text -- Show quoted text -

Hello Curt,

I had planned on using a deep cycle marine battery for the power
supply, and , driving a pickup truck all these years, I understand what
you are saying about keeping the weight over the back wheels. If you
read my previous post to Joe, you will get some idea of what I am
conceptualizing. The wagon platform is only about 10 lbs, but the
battery is about 30 lbs, the rest of the electronics I can build, along
with the actuators from the RC servos. I am estimating total weight at
about 50lbs. It is a very basic concept, but still in my head as I am
still getting ready for final exams that will go from the end of this
week up until 20 Dec. After that, I will have lots of time to devote to
this project. I know snow mobiles work with treads too. I have ruled
out nothiing yet. As I said, I am still conceptualizing. There is
nothing on paper because I have not really had time to do anything but
study and read Robot Bonanza. I am up to the chapter on
microcontrollers now, but that's out in the future. I just got a 40%
off discount coupon on any book from the book store that I have a
discount card with, and last time I was in there, I looked in a book
that was about outdoor bots, and wanted that one too, but budget
concerns prevented me from buying it at that time. The discount is only
for one day, and that day is tomorrow. I will be outside their door at
9AM waiting for the doors to open so I can buy it. I don't remember the
authors. It is in the faq for this group. This wagon is one of those
radio flyers, meant to hold one kid (probably 65-80 lbs, with the puny
wheels that they supply with it, but I won't be using those), My main
concern in my original post was, should the two wheel drive in the back
and 'caster type' in the front work for this application. I guess I am
going to find out. If not, I will investigate the options you cited
above. Oh, and thank you for the links. I will check them out. And, I
apologize for the delay in the response, but earthlink is having
trouble with their news server, so I signed up for google groups which
is how I am responding now. Thank you again for the info.

Joe