Page 334 - Robot Builder's Bonanza

BASIC DESIGN PRINCIPLES OF ROLLING ROBOTS 303

The size of the ball varies from pea- sized to over 3″ in diameter, and they
are available in steel, stainless steel, or plastic. Pololu sells a variety of small
ball casters for desktop robots; industrial supply outlets such as Grainger,
McMaster- Carr, and Reid Tool and Supply offer the bigger ones.
Omnidirectional Wheel
Omnidirectional wheels are basically rollers mounted on the tread of a tire.
The tire turns on an axis like any other, but the rollers allow for movement in
any direction. For what they do as casters, omnidirectional wheels mean
extra cost, size, and weight. I’ve not found that they work any better than a
ball transfer or even a well- made swivel caster.
Tail Wheel
One alternative to the swivel caster is the tail wheel, used on R/C model
airplanes (and, therefore, available at most hobby stores). The wheels come
in sizes ranging from about 3/4″ to over 2″ and are used with specific
mounting hardware.

In summary: For a small robot, under a couple of pounds and measuring 7″ in diameter or
less, a nonrotating skid is usually acceptable. For centerline motor mounting, use two skids:
one each in the front and rear of the bot. For larger or heavier robots, a skid may dig into soft
surfaces, or it may snag on bumps, cables, and other obstructions. For these, use a swivel
caster or a ball caster.

Successful Use of Casters
The casters on your robot must not impede the direction or speed of the machine’s travel.
Cheap swivel casters can catch and not swivel properly when the robot changes direction.
Keep these points in mind when selecting and using casters with your robots:

• Test them for smooth swivel action. Casters with ball bearings tend to give better results.
• In most cases, since the caster is provided only for support and not traction, the caster
wheel should be a hard material to reduce friction.
• When using two casters on either end of the base, there’s a possibility of the robot becom-
ing trapped if the casters touch ground but the drive wheels do not. You can fix this by

Base
Drive
wheel
Caster
Path
Figure 26- 4 Spring- loading can
overcome some of the common
Suspension
problems of using a caster over
uneven terrain. The height of the
caster adjusts to compensate for
Path bumps.

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