Page 71 - Anatomy of a Robot

02_200256_CH02/Bergren 4/17/03 11:24 AM Page 56
56 CHAPTER TWO
An object with a ratchet mechanism on it so it moves one tick mark easily in
one direction but will not move one tick mark the other way unless it’s being
forced to move two tick marks that way. Such a system is great for keeping the
object still when it comes close to equilibrium (see Figure 2-26).
Gain changes based on position are another example. Elevators typically have
powerful motors pulling them up and down when they are between floors. When
they get very near the desired floor, they switch to less powerful motors to make
the final adjustment before stopping. When the door opens, they may even turn
off the motors completely. These sorts of gain changes make it much easier to
avoid hunting in the final position of the control system (see Figure 2-27).

Y

Hysteresis

X

FIGURE 2-26 Mechanical (or electrical) hysteresis prevents symmentrical
movement.

2
Zone of Lower Gain
1.5
Position x Position x Position x Position x 1

0.5
0
0 2 4 6 8 10
Time tTime t
Time t
Time t
FIGURE 2-27 Control system gain can be decreased near equilibrium.