TABLE OF CONTENTS In our previous discussions in this chapter, we have used system components that are interconnected in series. These are referred to as open-loop control systems. An example of an open-loop system is a microwave oven which is controlled by a timer. However, most control systems are closed-loop control systems where the control action ia affected by the output. An example of a closed-loop system is the autopilot subsystem in an airplane which continuously measures the actual airplane direction and automatically adjusts other subsystems of the airplane to change the airplane heading to the desired direction. Feedback is the characteristic of a closed-loop control system which distinguishes it from open-loop systems. It is beyond the scope of this text to describe feedback control systems in detail. We will only describe some basics to aid the reader in understanding some of the advanced designs of the systems provided by the Simulink demos.
Figure 20.68 shows a simple feedback control system with two elements represented as blocks, the first of which is generally known as the control element or controller, and the second is known as the plant. The feedback (the line connecting the output to the summing point) is the same as the output and for this reason the entire system is referred to as unity feedback system. The path E represents the error, that is, the difference E = X - Y.
In Figure 20.52, the...
