FIGURE 5. This Nexus robot
utilizes mecanum wheels and
supports sonar sensors
on all four sides. Notice also
that the front wheels have a
of the rear wheels, giving
it better traction in
to drift either left or right,
depending on which wheel is
moving the fastest.
Using a similar philosophy,
the robot can be moved in any
of the six directions shown by
simply choosing which two
wheels to energize. These six
directions are special because
the chosen direction will
remain the same, even if the
speed changes. For example,
if you double the speed of
both motors the robot will just
move faster in the designated
direction. This linear relationship is unique to the six
With experimentation, you can find a combination of
motors and motor speeds to move the robot in almost any
direction. If you double (or half, for example) the speed,
the direction will also change slightly, often moving the
robot in an arc rather than a straight line. This non-linearity
generally means that a three-wheel omni robot is easier to
control for some applications if you restrict its
movement to the six linear directions.
You may feel like confining the robot to six
directions is too restrictive, but remember, as long
as you are willing to accept a single speed for any
given direction, you can create the desired
motion by experimenting with various motor
speeds until you achieve your goal. This can be an
interesting activity for many hobbyists, especially
if they have never used multi-directional wheels.
For such experimental purposes, it would be nice
to have an easy and inexpensive way of building
a usable robot base. In addition to the
mechanical assembly, each motor must have the
electronics to control both speed and direction. It
would also be nice — especially for quick
experimentation — to be able to write the control
programs in a high-level language.
Since LEGO NXT motors have built-in speed
control circuitry and the LEGO system can control
three motors, it seemed like an excellent
candidate for experimenting with omni wheels.
Unfortunately, we found it difficult to find multi-directional wheels that would mate with the NXT motors.
We discovered an emerging company in China called Nexus
Automation (see www.NexusRobot.com) that offers omni
wheels that attach directly to LEGO NXT motors as shown
in Figure 4.
FIGURE 6. Foam board can be cut as needed
with only a knife or razor blade.
SERVO 12.2011 43