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CONTROL SYSTEMS 25
power to the TV circuits. Actually, two open-loop control systems are at work. They are
concatenated and operate as a single open-loop control system (see Figure 2-3).
In open-loop control systems, the information tends to flow only one way. For exam-
ple, the control system inside the remote never finds out if the TV goes on or not.
Furthermore, the power button on the remote never indicates if the infrared beam was
sent out or not. If your finger is over the optical opening, nothing happens at all and the
remote never knows the TV has not gone on.
Let’s run an experiment illustrating an open-loop control system within your body.
Glance over to your right and locate an object in the room. Remember where it is and
then look back here to the book. Now close your eyes, point to the object, trying to put
your finger right on the object in your field of vision. Open your eyes, and see how close
you came (see Figure 2-4).
You’ll notice that you never really get it right with your eyes closed. When you open
your eyes, you can see your finger is a little off. The error will never go away and is
called the steady state error. It’s an error that will persist long after the control system
has settled on the final output and will make no further corrections. We’ll see steady
state error as a term in the equations that we develop later. All control systems have this
error. It’s an important parameter because when you are designing a control system, you
must keep the steady state error below acceptable bounds.
You can perform another experiment if you have a dimmer in your home. Wait until
dark and turn off the dimmer, making the room dark. Close your eyes and then turn on
the dimmer to where you think the minimum acceptable reading light level is.

Infrared Signal
Power Butt Power to TV
TV Remote TV

FIGURE 2-3 Concatenated open-loop control systems

FIGURE 2-4 The open-loop control error is large with eyes closed.