Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

Chapter 4 Wheeled Vehicle Suspensions and Drivetrains 139
Figure 4-8 Reversed tricycle, all
drive, all steer
The most complicated and highest mobility three-wheeled layout is
one where all wheels are powered and steered. This layout is extremely
versatile, providing motion in any direction without the need to be moving;
it can turn in place. This ability is called holonomic motion and is
very useful for mobile robots because it can significantly improve mobility
in cluttered terrain. Of the vehicles discussed so far, all, except the
front steer reversed tricycle, can be made holonomic if the third wheel
lies on the circumference of the circle whose center is midway between
the two opposing wheels, and the steering or passive wheel can swing
through 180 degrees. To be truly holonomic, even in situations where the
vehicle is enclosed on three sides, like in a dead-end hallway, the vehicle
itself must be round. This enables it to turn at any time to find a path out
of its trap. See Figure 4-8.
Before we investigate four-wheeled vehicles, there is a mechanism
that must be, at least basically, understood—the differential. The differential
(Figure 4-9) gets its name from the fact that it differentiates the
rotational velocity of two wheels driven from one drive shaft. The most
basic differential uses a set of gears mounted inside a larger gear, but on
an axis that lies along a radius of the larger gear. These gears rotate with
the large gear, and are coupled to the axles through crown gears on the
ends of the axles. When both wheels are rolling on relatively high fric-