either side of the robot as shown in Figure 1. It’s called
differential steering because the robot is steered by
changing the speed and direction (“difference”) between
these two wheels. One of the key benefits of differential
steering is that the robot can spin in place by reversing one
wheel relative to the other.
A feature of most differentially-steered robots is that
they use one or two casters (or skids) placed center-line
over the robot in the front and/or back to provide
balancing support for the base.
In dual caster/skid designs, the wheels are
placed in the center of the vehicle and a separate
caster on each end (see Figure 2). This
arrangement provides maximum stability, but
requires a relatively flat traveling surface. On
rough terrain, the casters may cause the drive
wheels to lose contact with the ground.
In single caster/skid designs, the wheels are
placed on one side of the vehicle, and a caster on
the other. This has the benefit of creating a good
three-point balance without causing as much of a
problem on rough ground. The disadvantage is
that the robot’s turning radius isn’t as tight as it is
when the wheels are mounted in the center.
Variations of the two-wheeled base include four or six
wheels (4WD, 6WD). On bases that use more than two
wheels, support casters or skids aren’t generally needed.
In 4WD and 6WD systems, each wheel can be powered
by a separate motor. The motors on each side are operated
as if they were one unit; all the motors on the left turn on
or off at the same time, for example. Figure 3 shows a
4WD robot base that uses four motors, each connected to
By Gordon McComb
SERVO 05.2014 43
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Figure 2. Two casters (or skids) are needed on differentially-steered robots with motors in the center. This provides maximum
balance and support, but reduces the robot's
ability to navigate rough terrain.
Figure 3. Four- and six-wheel drive robots use multiple
wheels to provide traction and stability. The wheels on
each side of the robot may be individually driven by
separate motors, or driven through linkages or gears
by a central motor.
Figure 1. Differential steering involves using two motors on
either side of the robot. The robot steers by changing the
speed and direction of each motor.