1.3. CLOSED-LOOP CONTROL SYSTEMS
Closed-loop control systems derive their valuable accurate reproduction of the input from feedback comparison. An error detector derives a signal proportional to the differences between the input and output. The closed-loop control system drives the output until it equals the input and the error is zero. Any differences between the actual and desired output will be automatically corrected in a closed-loop control system. Through proper design, the system can be made relatively independent of secondary inputs and changes in component characteristics. This section illustrates the closed-loop control system versions of the open-loop control systems considered in Section 1.2.
Figure 1.7 illustrates an automatic tank-level control version of the system shown in Figure 1.1. It can maintain the desired tank level h within quite accurate tolerances even though the output flow rate through valve V1 is varied. If the tank level is not correct, an error voltage is developed. This is amplified and applied to a motor drive which adjusts valve V2 in order to restore the desired tank level by adjusting the inlet flow rate. A block diagram analogous to this system is shown in Figure 1.8. Because feedback comparison is present, the term closed loop is used to describe the system’s operation.