8 CHAPTER 1. A PRELUDE TO CONTROL THEORY
The operator is instructed to maintain the level offluid in the tank at a
particular level. A porthole on the side of the tank provides the operator a
window to observe thefluid level. A reference marker is placed in the window
for the operator to see exactly where thefluid level should be. If thefluid level
drops below the reference marker, the human sees thefluid level and compares
it with the reference. Sensing whether the height offluid is above or below
the reference, the operator can turn the valve either in the clockwise (close) or
counterclockwise (open) direction and control theflow rate of thefluid output
from the tank.
Good feedback control action can be achieved if the operator can contin-
uously adjust the valve position. This will ensure that the error between the
reference marker position, and the actual height offluid in the tank, is kept to
a minimum. The controller in this example is a human operator together with
the valve system. As a component of the feedback system, the human operator
is performing two tasks, namely, sensing the actual height offluid and compar-
ing the reference with the actualfluid height. Feedback comes from the visual
sensing of the actual position of thefluid in the tank.
Example 1.8 (Temperature control)The temperature inside a home is in-
fluenced by the outside temperature. In order to maintain the inside temper-
ature at a comfortable level, the desired room temperature is set on the ther-
mostat. If the room temperature is lower than the desired temperature, a relay
Figure 1.10. Thermostat controlling room temperaturecloses and turns on the furnace to produce heat in the room. When the room
temperature reaches the desired temperature, the relay opens, and in turn shuts
offthe furnace.
As shown in Figure 1.10, a comparator is used to determine whether or not
the actual room temperature is equal to the desired room temperature. The
relay/switch and furnace are the dynamic elements of this closed-loop control
system shown in thefigure.