Page 96 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Communications and Control

the communications cable becomes shorted to ground. The high-speed version can
operate up to 1 Mbps, but is not as robust. Both standards are often used in different
areas of a single automobile.

CAN has the advantage of being developed especially for the automotive industry,
but it does not have the wide support outside this application arena that Ethernet
enjoys.

Ethernet

The Ethernet protocol has become widespread and is probably the most popular proto-
col for new robotics projects. There are several electrical standards for coax, twisted
pair, and even fiber-optic connectivity. Ethernet is gradually replacing dedicated
protocols in applications such as security monitoring and building controls. Simple
interfaces are available to rates of 100 Mbps. The availability of low cost wireless
Ethernet (802.11) make Ethernet almost ideal for mobile robotics.
The biggest disadvantage to Ethernet is that it was not designed as a control or data
acquisition protocol, and it offers limited control over message latency. Even so,
with proper time stamping of data, Ethernet can be used for most of the data acquisi-
tion and control functions a mobile robot will need to perform.
In the final event, most robots will contain a combination of these networks. For
example, a lidar may have a serial interface that is controlled by a sub-processor that
in turn communicates via Ethernet to a higher-level computer. This translation may
be dictated by the fact that the lidar is only available with a serial interface. Even if
the lidar has an Ethernet interface, it will undoubtedly have its own application
protocol.

Unfortunately, the necessity of changing protocols causes road blocks to communi-
cations across these boundaries. Only requests and commands supported at both
sides of the interface will be available through it.

Basic requirements of the application protocol

It is absolutely essential that the application protocol for a mobile robot support the
monitoring and manipulation of almost every function of the robot, and that it do so
efficiently. This is true even if the robot is not expected to communicate with a base
station during normal operation. It is even true if the final design will have no
means of communications at all! I have watched in horror as engineers doomed their
projects with ill-advised protocols.

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