Page 137 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Chapter 8

At first glance, this capability would seem to offer a total navigation solution, but
this is not the case. GPS signals can be blocked by overhead structures, severe weather,
and even heavy vegetation. GPS is of no use at all in most indoor environments. If a
robot is to depend totally on GPS, it will not be capable of operating in many envi-
ronments and situations. GPS is an immensely powerful tool for outdoor navigation,
but even this technology must be integrated into a navigational architecture with
other techniques.

Understanding why good dead reckoning is crucial

Live reckoning is the heart of any good navigation system, providing continuous
position updates between fixes. This position estimate is then used as a basis from
which to evaluate incoming navigation data. If a platform provides good odometry,
then it will have even better live reckoning.

If our robot is navigating from GPS, and suddenly passes under a highway overpass,
good odometry can keep it rolling until the GPS signal is restored or the vehicle
receives some other kind of navigational data. All else being equal, the better the
odometry, the more robust the navigation will be. In indoor navigation, robots tend
to depend on odometry even more significantly as they move from one partial fix to
another.

Platform inherent odometry
The better a vehicle’s odometry, the better it will be able to navigate. The quality of
a mobile platform’s inherent odometry is dependent on the drive system configura-
tion. The most critical axis is steering and the sense of direction because errors in
this parameter will accumulate geometrically as the vehicle drives.
The less coupling a platform has between steering and driving, the more predictable will be
its response to a steering command.

Vehicles using Synchro-drive have independent motors for driving and steering. The
drive motor causes all of the wheels to turn at the same rate, while the steering motor
causes all the wheels to perpetually steer in the same direction. Since driving and
steering are decoupled, the vehicle responds precisely to steering commands, even
on very slippery surfaces. A well-aligned Synchro-drive platform will seldom require
additional heading sensors.

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