TABLE OF CONTENTS The purpose of an automatic control on a system is to produce a desired output when inputs to the system are changed. Inputs are in the form of commands, which the output is expected to follow, and disturbances, which the automatic control is expected to minimize.
The various aspects of automatic control can best be described by use of an example. Consider a process such as that shown in Fig. 12.1. The flowing liquid is to be heated to a desired temperature by steam flowing through heating coils. The temperature of the exit flow is affected by factors (process variables) such as the temperature of the incoming liquid, the flow rate of the liquid, the temperature of steam, the flow rate of steam, heat capacities of the fluids, heat loss from the vessel, and mixer speed.
The system shown in Fig. 12.1 is normally classified as "open-loop." Open-loop control systems are those in which information about the controlled variable (in this case, temperature) is not used to adjust any of the system inputs to compensate for variations in the process variables. The term open loop is often encountered in discussions of control systems to indicate that the uncontrolled process dynamics are being studied.
A closed-loop control system implies that the controlled variable is measured and that the result of this measurement is used to manipulate one of the process...
