Page 106 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Communications and Control

With a peer-to-peer architecture, the managing process must be prepared to receive
and act on a wide variety of messages at any time. This can greatly complicate the
software design. For this reason, some systems are actually hybrids that allow only
limited message initialization by the slaves.

There may be more than one communications link onboard the robot. For example,
at Cybermotion we chose to use two links: a supervisory link and a control link. The
supervisory link was mastered by the base station, and all of the computers on the
robot responded as slaves. This link was used to program and monitor the robot. The
control link was mastered by the mobile base and all of the other computers were
slaves. The control link allowed the base to read sensors and direct subsystems. This
architecture worked very effectively for us.

Wrappers, layers, and shells

The application protocol is almost never sent directly to the robot from the base sta-
tion. Even if the message is sent by wire, it will be coded in a format such as RS-232.
In reality it will travel over radio modems, telephone modems, the internet, or other
means, and each of these will have its own protocol.

Some layers have to do with the way data is modulated onto carriers, while other la-
yers are concerned with traffic handling, blocking, routing, and other matters. When
the various protocols are part of a single master architecture (such as the factory
MAP architecture), then these various protocols are usually called layers.

Most of these protocols are invisible to the user, but it is important to realize that the
data stream may be broken into blocks for transmission. In fact, if you use a dial-up
modem, it may use one of a variety of encoding protocols depending on line condi-
tions. The timing of the application protocol decoders must be tolerant of this fact if
the application protocol is to take advantage of all of the wondrous means of com-
munications available today.
The last link from the base station to the robot will almost always be wireless. This
can be infrared, FSK radio, or spread spectrum. The advent of inexpensive 802.11
Ethernet radios has opened up many possibilities for communications with mobile
robots. The Ethernet protocol allows multiple data protocols to be exchanged with
the robot at the same time but for different purposes.

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