Page 333 - Robot Builder's Bonanza

302 BUILD ROBOTS WITH WHEELS AND TRACKS

3. Multiply the speed of the motor (in RPS) by the circumference of the wheel. The result
is the number of linear inches covered by the wheel in 1 second.

With a 100 RPM motor and 7″ wheel, the robot will travel at a top speed of 36.49″ per
second, or about 3 feet. That’s about 2 miles per hour! You can slow down a robot by decreas-
ing the diameter of the wheel. By reducing the wheel to 5″ instead of 7″, the same 100 RPM
motor will propel the robot at about 25 inches per second.
Bear in mind that the actual travel speed of your robot when it’s fully accessorized may be
less than this. The heavier the robot, the larger the load on the motors, so the slower they will
turn.
For your reference, here is handy table comparing travel speed, in inches per second,
against a variety of motor RPM and several common small wheel sizes.

See the Robot Speed Calculator on the RBB Online Support site (refer to Appendix A). Enter the
ON THE
motor RPM and the diameter of the wheels you’re using, and the calculator tells you the travel
W E B speed in inches per second.

SUPPORT CASTERS AND SKIDS FOR WHEELED ROBOTS
Differentially steered robots need something on the front and/or the back to prevent them
from tipping over. There are several common approaches, listed below.

Nonrotating Skid
The purpose of a skid is to glide over the ground without using any moving
parts. The skid is rounded (a cap or acorn nut works well) to facilitate a
smooth ride. Polished metal, hard plastic, or Teflon are common choices.
For obvious reasons, skids are not suitable for robots that may travel over
uneven surfaces or when there are many obstructions, like cables and old
socks.
Swivel Caster
Swivel casters are available with wheel diameters from 1″ to over 4″. Match
the size of the caster with the size of the robot. You’ll find the common
1- 1/4″- to 2″-diameter caster wheel is suitable for most medium robots. For
larger bases you can opt for the 3″ and even 4″ casters.
Swivel casters are commonly available with plate or stem mounting and in
the following wheel styles:
• Single wheel
• Dual wheel (“twin wheel”)
• Ball style
The ball style is used with furniture and tends to be heavy. If you use it at all,
reserve it for heavier robots. Single- wheel casters are the most common and
easiest to find. Look for a caster that swivels easily.
Ball Caster
Ball casters act as omnidirectional rollers. Unlike swivel casters, which must
rotate to point in the direction of travel, ball casters are ready to move in any
direction at any time. This makes them ideal for use as support casters in
robots.

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