| Omni-directional Wheel as a Caster |
Last Modified: 2006-11-17
This note describes how to use an omni-directional wheel as a caster. The omni-directional wheel is unique because it rolls freely in two directions. In one direction, it rolls like a normal wheel. It can also roll laterally because of the smaller wheels spread about its circumference.
Many robots use a 2-motor differential steering system with a caster. Hardware stores sell a wide variety of casters, but most are too big for small robots. Furthermore, the casters in hardware stores are designed to withstand large forces so they tend to have fairly coarse motion. Small robots usually exert a small amount of force on a caster. The caster must turn and roll easily to be effective.
One alternative to a hardware store caster is making one from scratch. However, this takes a significant amount of work and experience. Another choice is a simple skid. This works very well for lightweight robots on smooth surfaces, but the robot can get hung up on obstacles or dig into carpeting. A smooth wheel or ball rotating on a fixed axis works well in straight forward or backward motion, but acts like a skid when the robot is turning or rotating. Replacing the smooth wheel or ball with an omni-directional wheel eliminates most of the skidding problems.
Constructing an omni-wheel caster is fairly easy. You use just about any tube with a 5/16 inch outer diameter makes a good axle for the omni-directional wheel . A styrene tube from a hobby shop is inexpensive and works well. It is also possible to screw bolts directly into the ends of the styrene tube. An aluminum or brass strip bent into a 3-sided bracket can hold the tube and wheel. This basic construction plan is illustrated below.
Robot with Caster Wheel
Here are some still photos of the robot which give a better view of the caster. Such a caster provides effective support for the robot platform and good maneuverability.
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