Page 57 - Introduction to Autonomous Mobile Robots

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Figure 2.25 Chapter 2
The Nomad XR4000 from Nomadic Technologies had an arrangement of four castor wheels for holo-
nomic motion. All the castor wheels are driven and steered, thus requiring a precise synchronization
θ
and coordination to obtain a precise movement in x y, and .

2.3.2.3 Tracked slip/skid locomotion
In the wheel configurations discussed above, we have made the assumption that wheels are
not allowed to skid against the surface. An alternative form of steering, termed slip/skid,
may be used to reorient the robot by spinning wheels that are facing the same direction at
different speeds or in opposite directions. The army tank operates this way, and the
Nanokhod (figure 2.26) is an example of a mobile robot based on the same concept.
Robots that make use of tread have much larger ground contact patches, and this can sig-
nificantly improve their maneuverability in loose terrain compared to conventional
wheeled designs. However, due to this large ground contact patch, changing the orientation
of the robot usually requires a skidding turn, wherein a large portion of the track must slide
against the terrain.
The disadvantage of such configurations is coupled to the slip/skid steering. Because of
the large amount of skidding during a turn, the exact center of rotation of the robot is hard
to predict and the exact change in position and orientation is also subject to variations
depending on the ground friction. Therefore, dead reckoning on such robots is highly inac-
curate. This is the trade-off that is made in return for extremely good maneuverability and
traction over rough and loose terrain. Furthermore, a slip/skid approach on a high-friction
surface can quickly overcome the torque capabilities of the motors being used. In terms of
power efficiency, this approach is reasonably efficient on loose terrain but extremely inef-
ficient otherwise.