Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

Chapter 4 Wheeled Vehicle Suspensions and Drivetrains 135
Three wheels are the minimum required for static stability, and threewheeled
robots are very common. They come in many varieties, from
very simple two-actuator differential steer with fixed third wheel types,
to relatively complex roller-walkers with wheels at the end of two or
even three DOF legs. Mobility and complexity are increased by adding
even more wheels. Let’s take a look at wheeled vehicles in rough order
of complexity.
The most basic vehicle would have the least number of wheels.
Believe it or not, it is possible to make a one-wheeled vehicle! This vehicle
has limited mobility, but can get around relatively benign environments.
Its wheel is actually a ball with an internal movable counterweight
that, when not over the point of contact of the ball and the
ground, causes the ball to roll. With some appropriate control on the
counterweight and how it is attached and moved within the ball, the vehicle
can be steered around clumsily. Its step-climbing ability is limited
and depends on what the actual tire is made of, and the weight ratio
between the tire and the counterweight.
There are two obvious two wheeled layouts, wheels side by side, and
wheels fore and aft. The common bicycle is perhaps one of the most recognized
two-wheeled vehicles in the world. For robots, though, it is quite
difficult to use because it is not inherently stable. The side by side layout
is also not inherently stable, but is easier to control, at low speeds, than a
bike. Dean Kamen developed the Segway two-wheeled balancing vehicle,
proving it is possible, and is actually fairly mobile. It suffers from
Figure 4-4
Bicycle