Page 66 - Modern Control Systems

40 Chapter 1 Introduction to Control Systems

FIGURE P1.5 A photocell is mounted in each tube. The light reaching
each cell is the same in both only when the light source is exactly in the
middle as shown.

P1.7 The story is told about the sergeant who stopped at
Process
the jewelry store every morning at nine o'clock and
Initial Acmaj compared and reset his watch with the chronometer in
wages + s-* wages Pri :cs
Industry the window. Finally, one day the sergeant went into
the store and complimented the owner on the accuracy
• •
of the chronometer.
"Is it set according to time signals from Arlington?"
asked the sergeant.
Automatic "No," said the owner, "I set it by the five o'clock can-
cost of living * i 4— non fired from the fort each afternoon. Tell me, Sergeant,
W a n e Cost of
increase why do you stop every day and check your watch?"
The sergeant replied, "I'm the gunner at the fort!"
Is the feedback prevalent in this case positive or
FIGURE P1.6 Positive ; feedback.
negative? The jeweler's chronometer loses two minutes
each 24-hour period and the sergeant's watch loses
three minutes during each eight hours. What is the net
Figure PI .6, adds the feedback signal to the input sig-
nal, and the resulting signal is used as the input to the time error of the cannon at the fort after 12 days?
process. A simple model of the price-wage inflation- PI.8 The student-teacher learning process is inherently a
ary spiral is shown in Figure PI.6. Add additional feedback process intended to reduce the system error
feedback loops, such as legislative control or control to a minimum. With the aid of Figure 1.3, construct a
of the tax rate, to stabilize the system. It is assumed feedback model of the learning process and identify
that an increase in workers' salaries, after some time each block of the system.
delay, results in an increase in prices. Under what P1.9 Models of physiological control systems are valu-
conditions could prices be stabilized by falsifying or able aids to the medical profession. A model of the
delaying the availability of cost-of-living data? How heart-rate control system is shown in Figure PI.9
would a national wage and price economic guideline [23,24,48]. This model includes the processing of the
program affect the feedback system? nerve signals by the brain. The heart-rate control

Nerve
Stretuh frequency frequency
Medulla, Heart
Heart
ill
V brain ir X rate
lungs
z
Nerve
frequency Pressure >' Vascular • * —
receptors Pre ssure system

FIGURE P1.9 Heart-rate control.