Page 70 - Anatomy of a Robot

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1.8 CONTROL SYSTEMS 55
1.6
1.4
1.2
Position X 0.8
1
0.6
0.4
0.2
0
0 2 4 6 8 10
-0.2
Time t
FIGURE 2-25 A control servo system exhibiting unwanted sine-wave
hunting

Think for a minute how upsetting it would be if the elevator door opened and the
height of the elevator oscillated up and down while you were trying to get off! In many
systems, hunting is not acceptable. Hunting behavior can be avoided by refraining from
using any nonlinear elements:
Digital actuators that are on-off (like a solenoid) introduce nonlinear motion into
a system.
Don’t use digital sensors that report only on and off. The sensors that turn on night
lights are like this. They do not bring the lights on slowly as it gets dark.
Avoid mechanical wracking. The mechanical parts of the robot may make sudden
moves if all the bolts are not tight. The control system cannot compensate for this
very well.
Decrease v. Often, if we decrease the frequency response of the system, we can
avoid oscillations. Of course, this comes at the expense of slower performance.
Add a hysteresis element to the control system; such an element is defined as “a
retardation of an effect when the forces acting upon a body are changed.” The
common way to look at a hysteresis element is that it behaves differently depend-
ing on the direction. We are including here a few nonlinear control system ele-
ments that we can make a case for grouping with the hysteresis topic. Here are
some examples of hysteresis elements:
A friction block that drags more easily one direction than the other.
A spring system that puts two springs into service when moving one way, but
releases one spring when moving the other way.