Page 46 - Introduction to Autonomous Mobile Robots

Locomotion

a) b) c) d) 31

Swedish 90° Swedish 45°

Swedish 45°
Figure 2.19
The four basic wheel types. (a) Standard wheel: two degrees of freedom; rotation around the (motor-
ized) wheel axle and the contact point.(b) castor wheel: two degrees of freedom; rotation around an
offset steering joint. (c) Swedish wheel: three degrees of freedom; rotation around the (motorized)
wheel axle, around the rollers, and around the contact point. (d) Ball or spherical wheel: realization
technically difficult.

sufficient traction and stability for the robot to cover all of the desired terrain, and does the
robot’s wheeled configuration enable sufficient control over the velocity of the robot?

2.3.1 Wheeled locomotion: the design space
As we shall see, there is a very large space of possible wheel configurations when one con-
siders possible techniques for mobile robot locomotion. We begin by discussing the wheel
in detail, as there are a number of different wheel types with specific strengths and weak-
nesses. Then, we examine complete wheel configurations that deliver particular forms of
locomotion for a mobile robot.

2.3.1.1 Wheel design
There are four major wheel classes, as shown in figure 2.19. They differ widely in their
kinematics, and therefore the choice of wheel type has a large effect on the overall kinemat-
ics of the mobile robot. The standard wheel and the castor wheel have a primary axis of
rotation and are thus highly directional. To move in a different direction, the wheel must be
steered first along a vertical axis. The key difference between these two wheels is that the
standard wheel can accomplish this steering motion with no side effects, as the center of
rotation passes through the contact patch with the ground, whereas the castor wheel rotates
around an offset axis, causing a force to be imparted to the robot chassis during steering.