Page 74 - Anatomy of a Robot

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CONTROL SYSTEMS 59
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FIGURE 2-28 It’s hard to control two variables at the same time (such as
speed and direction).

What do we do if one of the control systems determines that it is hopelessly out
of control? If it loses track of the black line, should it slow down?
If the robot is moving very rapidly, does it need to look farther ahead for bends in
the black line?

All the scenarios argue for sending information back and forth between the two con-
trol systems. Further, the ways in which they interact can become very complex. At
some point, if more and more control systems are added to the robot, the following can
occur:

Multiple control systems get expensive.
Communication between the control systems can get expensive and slow things
down. In the worst case, communication errors can occur.
Interactions between the control systems can get unpredictable. In the worst cases,
instabilities can arise. These instabilities can take the form of unexpected delays
or thrashing. Thrashing arises when two control systems disagree and fight over
the control of parts of the system. Each control system sees the actions of the other
as creating an error.
Designs can become very complex to accommodate all cases.
Designs can become difficult to maintain. As one control system is changed, other
control systems may cease to function. Retesting the robot becomes a large task.

Many years ago, in the primordial soup of engineering history, engineers began to
consider control systems that had more than one variable. We need only look at old
drawings of steam engines to appreciate this. They had to regulate speed, pressure, tem-
perature, and several other variables all at the same time. The general approach back