require a lower gear speed with
greater torque for the same
traveling speed.
RoboMagellan robots are just one
of many varieties of mobile machines
that operate outdoors and need some
special design considerations. A roving
video platform, an autonomous lawn
mower, a cave explorer, or even a
dog-walking robot can make for an
interesting project.
Needless to say, you will probably
encounter dirt, grass, and moisture
along the way, so you need to plan a
way of sealing and protecting
electronics, encoders, motors and
gears, batteries, and sensors.
Ackermann Steering
for the Robot's Base
Let’s discuss Ackermann steering
next (refer to Figure 8). Notice that
the pivot arms on each of the front
wheels are not parallel with the sides
of the wheels, but actually form a ‘V’
with lines extended to a point
centered between the rear wheels as
shown in the left drawing. This is
called the Ackermann angle. The
drawing on the right shows how the
two front wheels are not parallel
when turned to an extreme to allow a
single turning point for all four
wheels.
As you can see, only one servo
would be required to turn both
wheels, but the pivot arms must be
set at the proper angle when
connected to the cross bar. Many
ready-built RC cars chassis’ will already
be configured this way, unless you
change the wheel base for your robot.
This style of steering requires an
entirely different type of control than
many robot builders are used to. Your
robot will require a steering
motor/servo and one or two driving
motors for the rear wheels. Most
robot builders place shaft encoders on
the steered wheels for feedback as
they just ‘skate’ across the ground
without any torque skidding.
If the base is a true 4WD setup,
the encoders can be placed on the
rear. You should not use a single rear
shaft connecting the rear wheels to
a single motor, but rather use a
differential between the two wheels
like those used in automobiles. Since
these types of gear arrangements are
difficult to build or expensive to buy
for a hobbyist, most robot builders
use two separate motors — one for
each rear wheel.
Figure 9 shows a set of front and
rear wheel axle/steering assemblies
from an online supplier that uses a
single motor and a differential. In
most cases, one motor will operate
either faster or slower as required
when equal drive voltages are applied
to both motors, without skidding one
or both wheels.
Not all off-road RC car chassis
configurations utilize Ackermann
steering. Many use four or six wheel
designs, with each side motor sets of
two or three wheels being driven
together to form a differential driving
configuration. Figure 10 shows an
arrangement of six sets of servo-driven
wheels — each with another servo
mounted on top to a plastic base
plate to individually steer each one.
The Mars Curiosity rover is an
example of a combination of six
individually steered wheels that allow
the robot to steer 360º about its
center. Figure 11 shows Curiosity in a
lab setup with its left-front wheel
turned at an angle. Notice that the
rotating shaft is directly above the
center of turning rotation. This one
ton robot is powered by a plutonium
SERVO 12.2013 77
Figure 8. Ackermann steering.
Figure 9. Ready-built Ackermann steering
for RC cars.
Figure 10. Six-wheel drive with
individual six-wheel steering.
Figure 11. Curiosity Rover in JPL lab setup.
