Question about wheels

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Question about wheels
Joe
12-09-2006

Joe posted on
December 9, 2006, 1:26 pm

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I am in the process of planning on building an (eventually ) autonomous
vehicle for exploring. I have the book Robot Bonanza and I was reading how,
on the small robot vehicles, usually what is done is the front wheel is a
caster, and the back wheels supply the drive power and steering by rotating
one faster then the other. My question is, will this work for an outdoor
robot?

I am planning on using a steel wagon for the platform, and I am going to rig
it for radio control first, since I have a few usable R/C transmitters and
servos. The problem with the wagon is that the wheels that come with it are
only 7" diameter by 1.5". In order to move over the terrain I want to, I
will probably need 10" wheels and more like 2 to 3 inches wide. I like the
idea of steering with the 2 back wheels, but is this practical on an outdoor
roving bot? Will the front wheel (or wheels) always recover to straight when
the rear motors are run at equal rpm? Or should I plan on somehow rigging a
steering motor to the front end too?

Then there's the question of motors, but I just want to deal with one thing
at a time since I am in no hurry. I will be using 12 V and then have a
circuit using LM 317's to get my 5, 6, and possibly 9 Volts from the battery
to drive the servos.

TIA,
Joe

noone posted on
December 10, 2006, 3:05 pm
Permalink

Re: Question about wheels (and shameless plug for engineers)

This is a less than desirable configuration.  Anyone who has ever been
mud stomping or rock crawling in a 4wd knows that off-road negotiation
involves having power to all wheels and a bit of intuition and experience
to negotiate obstacles.  Teaching computers to do this effectively isn't
quite there.

Skid steering is OK but it really needs to involve all wheels that have
contact with the ground.

Tracks are good for off-road but there are a couple of caveats:1) they
work better on large, heavy vehicles, 2) they are not very power
efficient.

Our vehicles are capable of omni-directional movement.  This allows a
vehicle to crab in any direction and simultaneously rotate around its
center.  See "ODIS at Superbowl" on a google search for relevant press.

Rob Prowel
Staff Program Engineer
Kuchera Defense Systems
814-467-9060x232

PS-I have openings for experienced software/firmware,
electro-mechanical, or RF engineers who are degreed, US citizens, and can
achieve a security clearance.  Work would involve living in rural
Pennsylvania, midway between Pittsburgh and Penn State .

Gordon McComb posted on
December 10, 2006, 3:59 pm
Permalink

Re: Question about wheels

You need to define vehicle size, wheel diameter, and typical terrain.
There are perfectly good power chairs used by the handicapped and
elderly outdoors that have a similar design as our typical caster-based
robvot -- though it's usually two free-wheeling casters on the front or
back, rather than just one. I wouldn't want to recommend using one over
rocky or uneven terrain, even without grandma sitting in the thing.

Generally the caster-based robots are best used indoors or outdoors over
flat, reasonably level surfaces. Less demanding applications can enjoy
the cheapest, easiest designs. Some variation in the surface can be
handled by spring-loading the casters, using oversize wheels, etc., but
generally another design is just easier all the way around if the
terrain is varied. Consider one of the rocker bogey arrangements in use.
Look at the Shrimp III for example, which is made for outdoors and can
handle some hefty terrain variations. There's also Mars Sojourner Rover
that you can study and steal.

-- Gordon