Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

Chapter 6 Steering History 195
ding of the wheels or tracks. This is where the name “skid steer” comes
from.
The fact that the wheels or tracks skid means this system is wasting
energy wearing off the tires or track pads, and this makes skid steering an
inefficient design. Placing the wheels close together or making the tracks
shorter reduces this skidding at the cost of fore/aft stability. Six-wheeled
skid-steering vehicles can place the center set of wheels slightly below
the front and back set, reducing skidding at the cost of adding wobbling.
Several all-terrain vehicle manufacturers have made six-wheeled vehicles
with this very slight offset, and the concept can be applied to indoor
hard-surface robots also. Eight-wheeled robots can benefit from lowering
the center two sets of wheels, reducing wobbling somewhat.
The single wheel drive/steer module discussed earlier and shown on a
tricycle in Figure 6-6 can be applied to many layouts, and is, in general,
an effective mechanism. One drawback is some inherent complexity
with powering the wheel through the turning mechanism. This is usually
accomplished by putting the drive motor, with a gearbox, inside the
wheel. Using this layout, the power to the drive motor is only a couple
wires and signal lines from whatever sensors are in the drive wheel.
These wires must go through the steering mechanism, which is easier
than passing power mechanically through this joint. In some motor-inwheel
layouts, particularly the syncro-drive discussed next, the steering
Figure 6-6
on tricycle
Drive/steer module