Page 187 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Chapter 12

The advantages to physical paths are their technical simplicity and their relative ro-
bustness, especially in feature-challenged environments. The sensor system itself is
usually quite inexpensive, but installing and maintaining the path often is not. Other
disadvantages to physical paths include the lack of flexibility and in some cases cos-
metic issues associated with the path.

A less obvious disadvantage for physical paths is one that would appear to be a strength:
their precise repeatability. This characteristic is particularly obvious in upscale envi-
ronments like offices, where the vehicle’s wheels quickly wear a pattern in the floor.
Truly autonomous vehicles will tend to leave a wider and less obvious wear footprint,
and can even be programmed to intentionally vary their lateral tracking.

Sonar

Most larger indoor robots have some kind of sonar system as part of their collision
avoidance capability. If this is the case, then any navigational information such a
system can provide is a freebee. Most office environments can be navigated exclu-
sively by sonar, but the short range of sonar sensors and their tendency to be specularly
reflected means that they can only provide reliable navigation data over limited angles
and distances.

There are two prevalent sonar technologies: Piezo-electric and electrostatic. Piezo-
electric transducers are based on crystal compounds that flex when a current passes
through them and which in turn produce a current when flexed. Electrostatic trans-
ducers are capacitive in nature. Both types require a significant AC drive signal (from 50
to 350 volts). Electrostatic transducers also require a high-voltage bias during de-
tection (several hundred volts DC). This bias is often generated by rectifying the
drive signal.

Both of these technologies come in different beam patterns, but the widest beam widths
are available only with Piezo-electric transducers. Starting in the 1980s the Polaroid
Corporation made available the narrow beam Piezo-electric transducer and ranging
module that they had developed for the company’s cameras. The cost of this combi-
nation was very modest, and it quickly gained popularity among robot designers.

The original Polaroid transducer had a beam pattern that was roughly 15 degrees wide
and conical in cross-section. As a result of the low cost and easy availability, a great
number of researchers adopted this transducer for their navigation projects. To many
of these researchers, the configuration in which such sensors should be deployed
seemed obvious. Since each transducer covered 15 degrees, a total of 24 transducers

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