Page 155 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Chapter 10

Additionally, there may be areas in which it is necessary to specify more than one
method of navigation concurrently. For example, there may be a warehouse aisle defined
by crates along the aisle boundaries. If the crates are removed, walls or reflectors may be
exposed behind them. For this reason, our navigator may be looking for multiple
features at the same time.

Add to this the fact that almost any feature we are looking for may be mimicked by a
false reading, and you begin to see the challenge. For example, a series of boxes along a
wall may look just like the wall to our sensor, indicating that we are out of position
by the width of the boxes. The navigation process therefore becomes one of selective
belief in things that are most likely to be right. As we will see later, most of the time our
machine will not believe anything entirely!
There are several metaphors that come to mind when trying to describe this process.
In some ways, a good navigator is like a police detective, sorting through clues and
contradictions trying to find the truth. The metaphor that I like best, however, is
that of an electronic filter that locks in on and passes good data while blocking false
information. For optimal performance, this filter will become more and less selective
as the platform becomes more and less uncertain of its position estimates.

The importance of uncertainty

Last spring we experienced some terrible flooding. As I dug out from all of the debris
I decided to order about 100 tons of rock and gravel to fill in the holes. When the
truck driver called with the first load, I gave him instructions on how to find our prop-
erty. I told him that after the road makes a sharp left turn, he should go about 1.25
miles and find a gazebo on the right-hand side of the road. Just after the gazebo was
my driveway. I told him to come across the low-water bridge and drop his load to the
right of the driveway just past the bridge.

When he finally arrived, he informed me that there was another gazebo and bridge
about half a mile after the turn. Apparently, this gazebo had recently been erected.
He had turned into this drive, and was preparing to drop the first 10 tons of rock
when a frantic man emerged from the house and wanted to know why he was going
to drop the rock on his vegetable garden!

In retrospect, the driver said that it did not seem like a mile and a quarter from the
turn to the gazebo, but that the coincidence had been too much for him. The driver
had opened his window of acceptance to allow the first gazebo to be taken as the
landmark. On the other hand, if he had refused to turn into my driveway because his

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