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02_200256_CH02/Bergren 4/17/03 11:24 AM Page 54
54 CHAPTER TWO
Battery
Wire wrapped
around a nail
Paper clips
are attracted
PULL
FIGURE 2-23 Electromagnets exert pull inside relays, soleoids, and electric
motors.
Temperature (F)
80
70
60
Time
FIGURE 2-24 Thermostats are control systems that exhibit hunting.
In linear control systems with a great deal of power and some weaknesses in the high-
frequency response, the output response will actually have a hunting sine wave on it.
This disturbance can be quite annoying, much like the buzz in a stereo system. It’s not
unlikely that the oscillations would be at v unless governed by a nonlinear element in
the system (see Figure 2-25).
