Differential steering — also known
as tank-style or skid steering — utilizes
different wheel speeds on each side
of the vehicle to turn the vehicle; take
a look at Figure 2. This type of
steering is popular with many robot
bases. If the wheel (or tank-style
tread) on the right moves faster than
the one on the left, the robot will turn
to the left, and vice versa.
Drive Train and
Power Considerations
Wheel and drive motor mounting
issues can be obstacles in mobile
robot design. There are so many
different ways in which to mount
gearmotors to robot bases that I will
point out some key issues and refer
you to the Internet for further
information.
Reliable mounting of wheels to
servos or larger gearmotors must take
into consideration the intended
weight of the robot and possible
slopes or uneven ground to cover.
Small robots weighing just a few
ounces can use wheels directly
mounted on small servos.
Smaller gearmotors and servos
frequently have only a single bearing
or bushing on the output spur gear.
However, robots of several pounds or
more require two bearings on the
output shaft of the gearmotor in
order to handle the downward
bending movement on the shaft
caused by the robot’s weight and fore
and aft bending as it travels.
Figure 3 shows a set of high
quality HDPE (high density
polyethylene) pneumatic wheels with
a gearmotor, quadrature encoder, and
mount from Parallax that is ready to
attach to a larger robot.
Intended use, the available power
source, the construction budget, the
builder’s mechanical skill level, and
even aesthetics are all important
issues to consider. If your desire is for
an SRS (Seattle Robotics Society)
RoboMagellan robot, you might want
to look at an off-road RC car chassis
with Ackermann steering to cover a
rough outdoor environment.
If you are designing a tall indoor
robot to serve you or guests, you
would look at differential steering for
quick turns and a wide base for
stability (to prevent tipping over).
Most small kit robots also utilize
differential steering. These are
just two examples of many
designs to consider; I’ll delve
into the two steering types a bit
more later.
Robot Base
Structural
Materials
Robots can be made of
almost any material, though many
hobbyist-built robots end up being
constructed out of aluminum. My first
robot (years ago) was made from a
large gallon size ‘tin’ can. A later one
I designed for Boys Life Magazine was
made from a plastic trash can.
The largest robot that I built as a
high school student was made from
3/4” plywood with jukebox motors
and parts for the mechanisms. It was a
huge beast, and I would never
recommend anyone making a robot
from such heavy materials.
Wood, however, is a great
material — especially thinner types of
craft plywood and even cabinetry
wood. Wood is easy to cut, trim, drill,
and sand, and it is relatively cheap and
readily available. Figure 4 shows a
prototype of the old MadeUSA base
from Parallax that is made from a high
grade furniture plywood. I highly
recommend using screws rather than
SERVO 12.2013 75
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Advances in robots and robotics over the years.
Figure 2. Differential steering from Robotix.in site.
Figure 4. Parallax robot with plywood base.
Figure 3. Parallax HDPE wheels for the Arlo robot.
