Page 183 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Chapter 11

served him well his whole life. The third of these lines was, “It was that way when I found
it, boss.” In the security business, an incident report is an account of a serious event that
contains adequate detail to totally exonerate the writer. In short, it always says, “It was
that way when I found it, boss.”
This incident report stated that the robot had been sent to its charger and had mysteri-
ously driven under the shelving unit with the aforementioned results. In those days we
had a command line interface that the guard at the console used to control the robot. A
quick download of the log file showed what had happened.
Instead of entering the command, “Send Charger,” the operator had entered the com-
mand “Reference Charger.” This command told the robot its position was in front of the
charger, not in a dangerous aisle with overhead shelves. The reference program specified
two very close reflectors for navigation. The robot was expected to fine tune its position
from these reflectors, turn off its collision avoidance, and move forward to impale itself
on the charging prong. Since there was no provision in the program for what to do if it
did not see the reflectors, the robot simply continued with the docking maneuver.

The oversight was quickly corrected, and later interlocks were added to prevent the rep-
etition of such an event, but the damage had been done. Although this accident resulted
in no damage to the robot or warehouse, it had a long-lasting impact on the confidence
of the customer’s management in the program.

If the robot’s uncertainty estimate is smaller than the true error in position, then the
navigation agents will reject any position corrections. If this happens, the robot will
continue to become more severely out of position as it moves. At some uncertainty
threshold, the robot must be set to an unreferenced state automatically.

If the robot successfully finds the referencing features, then—and only then—can it
be considered referenced and ready for operation. Certain safety related problems
such as servo stalls should cause a robot to become unreferenced. This is a way of
halting automatic operations and asking the operator to assure it is safe to continue
operation. We found that operators ranged from extremely competent and conscien-
tious to untrainable.

Reducing uncertainty

As our robot cavorts about its environment, its odometry is continually increasing
its uncertainty. Obviously, it should become less uncertain about an axis when it has
received a correction for that axis. But how much less uncertain should it become?

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