Page 55 - Introduction to Autonomous Mobile Robots

Chapter 2
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spheric bearing motor

Figure 2.23
The Tribolo designed at EPFL (Swiss Federal Institute of Technology, Lausanne, Switzerland. Left:
arrangement of spheric bearings and motors (bottom view). Right: Picture of the robot without the
spherical wheels (bottom view).

2.3.2.2 Omnidirectional drive
As we will see later in section 3.4.2, omnidirectional movement is of great interest for com-
plete maneuverability. Omnidirectional robots that are able to move in any direction
(xy θ,, ) at any time are also holonomic (see section 3.4.2). They can be realized by either
using spherical, castor, or Swedish wheels. Three examples of such holonomic robots are
presented below.

Omnidirectional locomotion with three spherical wheels. The omnidirectional robot
depicted in figure 2.23 is based on three spherical wheels, each actuated by one motor. In
this design, the spherical wheels are suspended by three contact points, two given by spher-
ical bearings and one by a wheel connected to the motor axle. This concept provides excel-
lent maneuverability and is simple in design. However, it is limited to flat surfaces and
small loads, and it is quite difficult to find round wheels with high friction coefficients.

Omnidirectional locomotion with four Swedish wheels. The omnidirectional arrange-
ment depicted in figure 2.24 has been used successfully on several research robots, includ-
ing the Carnegie Mellon Uranus. This configuration consists of four Swedish 45-degree
wheels, each driven by a separate motor. By varying the direction of rotation and relative
speeds of the four wheels, the robot can be moved along any trajectory in the plane and,
even more impressively, can simultaneously spin around its vertical axis.