Page 73 - Anatomy of a Robot

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58 CHAPTER TWO
We really haven’t worried about acceleration at all so far. But various restrictions
on acceleration will take place:
Traction Wheels, if we use them, can only accelerate the robot a certain
amount. Beyond the traction that the wheels provide, the robot will burn
rubber!
Balance The robot might pop a wheelie.
Mechanical stress Acceleration imposes force on all the parts of the robot.
The robot might rip off a vital part if it accelerates too fast. More on this later.
Mechanical wracking The robot will change shape as it accelerates. This
happens in loose joints and connections. More on this later.

So with all these variables to control at one time, what do we do?

Multivariable Control Systems

Up to this point, we’ve been trying to build a control system for the robot that could
serve to maintain a single variable, such as position. We should recognize that the math-
ematics of the control system are very general and apply just as well to robots that want
to control other single variables like speed or acceleration. Although cruise control sys-
tems are very complex, they are simply control systems that regulate speed to suit the
driver’s needs.
But what happens if we want to control two or more variables simultaneously?
Suppose we want the robot to follow a black line and move at a safe speed. Control of
both position (relative to the black line) and velocity (so the robot does not veer too far
off course during high-speed turns) puts us in the position of controlling two variables
at the same time. How do we do this? (See Figure 2-28.)
One solution is to put two separate control systems into the robot. One system will
control the position relative to the black line. The other control system will make sure
the robot moves at the appropriate speed. Such a control system is inherently a distrib-
uted control system such as the ones we discussed earlier. Cars do, in fact, have multi-
ple computers handling these tasks. Each control system has its own set of issues that
we have discussed, such as steady state error, overshoot, ringing, and settling time.
However, as we discussed in the section on distributed control systems, things can
become complex very rapidly. Here’s some points to consider:

Wouldn’t it make sense to slow the robot down if it is very far off the black line?
Would it be a good idea to speed up if the robot has been on course for quite a
while?