The term omni-directional is used to describe the ability of a system to move instantaneously in any direction from any configuration. A variety of designs of omni-directional or near omni-directional vehicles have been developed. These can be broken down into two approaches: special wheel designs and conventional wheel designs. An omni-directional vehicle is usually formed using three or more of such wheels.
Most special wheel designs are based on
a concept that achieves traction in one direction and allow passive motion in
another. The Mecanum wheel is an example of the special wheel design that has a
number of small passive rollers mounted on the periphery of a normal wheel. The
axes of the rollers are perpendicular to that of the wheel. The wheel is driven
in a normal fashion, while the rollers allow for free motion in the
perpendicular direction. By controlling the four wheels attached to a platform,
omni-directional mobility can be achieved.
Another special wheel design of note is
the ball wheel mechanism. In the ball wheel design, power from a motor is
transmitted through gears to an active roller ring and then to the ball via
friction between the rollers and the ball (much like a the action in a computer
mouse).
Although special wheel designs demonstrate
good omni-directional mobility, they generally have complex mechanical
structures and can have limited load capacity. Designs with passive rollers
also can generate unwanted vibrations as the rollers make successive contact
with the ground.
Conventional wheels are inherently
simple, have high load capacity, and high tolerance to floor irregularities
such as bumps, cracks, dirt and debris. The most common designs are those using
steered wheels. Vehicles based on this design have at least two active wheels,
each of which has both driving and steering actuators. They can move in any
direction from any configuration. However, this type of system is not truly
omni-directional because it needs to stop and re-orient its wheels to the
desired direction whenever it needs to travel in a trajectory with
non-continuous curvatures.
One major drawback of conventional
wheel designs is the high friction and scrubbing during the steering as the
wheel is actively twisted around a vertical axis. This reduces positioning accuracy
and increases power consumption and tire wear. The fundamental cause of the
scrubbing problem is that the wheel generates larger frictional forces when
steered actively around a vertical axis than when it is rolling. The scrubbing
problem can be reduced using the dual wheel design similar to that found in
aircraft front landing gear. Wheels in the dual wheel design are always rolling
even during steering.
The design presented in this report expands upon a previous active split castor design, using a similar control system, to include a two-link pivot ball suspension system and added wheel compliance.