Page 47 - Introduction to Autonomous Mobile Robots

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Figure 2.20 Chapter 2
Navlab I, the first autonomous highway vehicle that steers and controls the throttle using vision and
radar sensors [61]. Developed at CMU.

The Swedish wheel and the spherical wheel are both designs that are less constrained by
directionality than the conventional standard wheel. The Swedish wheel functions as a
normal wheel, but provides low resistance in another direction as well, sometimes perpen-
dicular to the conventional direction, as in the Swedish 90, and sometimes at an intermedi-
ate angle, as in the Swedish 45. The small rollers attached around the circumference of the
wheel are passive and the wheel’s primary axis serves as the only actively powered joint.
The key advantage of this design is that, although the wheel rotation is powered only along
the one principal axis (through the axle), the wheel can kinematically move with very little
friction along many possible trajectories, not just forward and backward.
The spherical wheel is a truly omnidirectional wheel, often designed so that it may be
actively powered to spin along any direction. One mechanism for implementing this spher-
ical design imitates the computer mouse, providing actively powered rollers that rest
against the top surface of the sphere and impart rotational force.
Regardless of what wheel is used, in robots designed for all-terrain environments and in
robots with more than three wheels, a suspension system is normally required to maintain
wheel contact with the ground. One of the simplest approaches to suspension is to design
flexibility into the wheel itself. For instance, in the case of some four-wheeled indoor robots
that use castor wheels, manufacturers have applied a deformable tire of soft rubber to the
wheel to create a primitive suspension. Of course, this limited solution cannot compete with
a sophisticated suspension system in applications where the robot needs a more dynamic
suspension for significantly non flat terrain.