Page 48 - Introduction to Autonomous Mobile Robots

Locomotion

2.3.1.2 Wheel geometry 33
The choice of wheel types for a mobile robot is strongly linked to the choice of wheel
arrangement, or wheel geometry. The mobile robot designer must consider these two issues
simultaneously when designing the locomoting mechanism of a wheeled robot. Why do
wheel type and wheel geometry matter? Three fundamental characteristics of a robot are
governed by these choices: maneuverability, controllability, and stability.
Unlike automobiles, which are largely designed for a highly standardized environment
(the road network), mobile robots are designed for applications in a wide variety of situa-
tions. Automobiles all share similar wheel configurations because there is one region in the
design space that maximizes maneuverability, controllability, and stability for their stan-
dard environment: the paved roadway. However, there is no single wheel configuration that
maximizes these qualities for the variety of environments faced by different mobile robots.
So you will see great variety in the wheel configurations of mobile robots. In fact, few
robots use the Ackerman wheel configuration of the automobile because of its poor maneu-
verability, with the exception of mobile robots designed for the road system (figure 2.20).
Table 2.1 gives an overview of wheel configurations ordered by the number of wheels.
This table shows both the selection of particular wheel types and their geometric configu-
ration on the robot chassis. Note that some of the configurations shown are of little use in
mobile robot applications. For instance, the two-wheeled bicycle arrangement has moder-
ate maneuverability and poor controllability. Like a single-legged hopping machine, it can
never stand still. Nevertheless, this table provides an indication of the large variety of wheel
configurations that are possible in mobile robot design.
The number of variations in table 2.1 is quite large. However, there are important trends
and groupings that can aid in comprehending the advantages and disadvantages of each
configuration. Below, we identify some of the key trade-offs in terms of the three issues we
identified earlier: stability, maneuverability, and controllability.

2.3.1.3 Stability
Surprisingly, the minimum number of wheels required for static stability is two. As shown
above, a two-wheel differential-drive robot can achieve static stability if the center of mass
is below the wheel axle. Cye is a commercial mobile robot that uses this wheel configura-
tion (figure 2.21).
However, under ordinary circumstances such a solution requires wheel diameters that
are impractically large. Dynamics can also cause a two-wheeled robot to strike the floor
with a third point of contact, for instance, with sufficiently high motor torques from stand-
still. Conventionally, static stability requires a minimum of three wheels, with the addi-
tional caveat that the center of gravity must be contained within the triangle formed by the
ground contact points of the wheels. Stability can be further improved by adding more
wheels, although once the number of contact points exceeds three, the hyperstatic nature of
the geometry will require some form of flexible suspension on uneven terrain.