Page 53 - Introduction to Autonomous Mobile Robots

Chapter 2
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steering has an important advantage: its directionality and steering geometry provide it with
very good lateral stability in high-speed turns.

2.3.1.5 Controllability
There is generally an inverse correlation between controllability and maneuverability. For
example, the omnidirectional designs such as the four-castor wheel configuration require
significant processing to convert desired rotational and translational velocities to individual
wheel commands. Furthermore, such omnidirectional designs often have greater degrees of
freedom at the wheel. For instance, the Swedish wheel has a set of free rollers along the
wheel perimeter. These degrees of freedom cause an accumulation of slippage, tend to
reduce dead-reckoning accuracy and increase the design complexity.
Controlling an omnidirectional robot for a specific direction of travel is also more diffi-
cult and often less accurate when compared to less maneuverable designs. For example, an
Ackerman steering vehicle can go straight simply by locking the steerable wheels and driv-
ing the drive wheels. In a differential-drive vehicle, the two motors attached to the two
wheels must be driven along exactly the same velocity profile, which can be challenging
considering variations between wheels, motors, and environmental differences. With four-
wheel omnidrive, such as the Uranus robot, which has four Swedish wheels, the problem is
even harder because all four wheels must be driven at exactly the same speed for the robot
to travel in a perfectly straight line.
In summary, there is no “ideal” drive configuration that simultaneously maximizes sta-
bility, maneuverability, and controllability. Each mobile robot application places unique
constraints on the robot design problem, and the designer’s task is to choose the most
appropriate drive configuration possible from among this space of compromises.

2.3.2 Wheeled locomotion: case studies
Below we describe four specific wheel configurations, in order to demonstrate concrete
applications of the concepts discussed above to mobile robots built for real-world activities.

2.3.2.1 Synchro drive
The synchro drive configuration (figure 2.22) is a popular arrangement of wheels in indoor
mobile robot applications. It is an interesting configuration because, although there are
three driven and steered wheels, only two motors are used in total. The one translation
motor sets the speed of all three wheels together, and the one steering motor spins all the
wheels together about each of their individual vertical steering axes. But note that the
wheels are being steered with respect to the robot chassis, and therefore there is no direct
way of reorienting the robot chassis. In fact, the chassis orientation does drift over time due
to uneven tire slippage, causing rotational dead-reckoning error.