Page 52 - Introduction to Autonomous Mobile Robots

Locomotion

Figure 2.21 37
Cye, a commercially available domestic robot that can vacuum and make deliveries in the home, is
built by Aethon Inc. (http://www.aethon.com). © Aethon Inc.

In general, the ground clearance of robots with Swedish and spherical wheels is some-
what limited due to the mechanical constraints of constructing omnidirectional wheels. An
interesting recent solution to the problem of omnidirectional navigation while solving this
ground-clearance problem is the four-castor wheel configuration in which each castor
wheel is actively steered and actively translated. In this configuration, the robot is truly
omnidirectional because, even if the castor wheels are facing a direction perpendicular to
the desired direction of travel, the robot can still move in the desired direction by steering
these wheels. Because the vertical axis is offset from the ground-contact path, the result of
this steering motion is robot motion.
In the research community, other classes of mobile robots are popular which achieve
high maneuverability, only slightly inferior to that of the omnidirectional configurations.
In such robots, motion in a particular direction may initially require a rotational motion.
With a circular chassis and an axis of rotation at the center of the robot, such a robot can
spin without changing its ground footprint. The most popular such robot is the two-wheel
differential-drive robot where the two wheels rotate around the center point of the robot.
One or two additional ground contact points may be used for stability, based on the appli-
cation specifics.
In contrast to the above configurations, consider the Ackerman steering configuration
common in automobiles. Such a vehicle typically has a turning diameter that is larger than
the car. Furthermore, for such a vehicle to move sideways requires a parking maneuver con-
sisting of repeated changes in direction forward and backward. Nevertheless, Ackerman
steering geometries have been especially popular in the hobby robotics market, where a
robot can be built by starting with a remote control racecar kit and adding sensing and
autonomy to the existing mechanism. In addition, the limited maneuverability of Ackerman