Figure 2 Closed-Loop Control System

Process Controls PRINCIPLES OF CONTROL SYSTEMS A closed-loop control system is one in which control action is dependent on the output. Figure 2 shows an example of a closed-loop control system. The control system maintains water level in a storage tank. The system performs this task by continuously sensing the level in the tank and adjusting a supply valve to add more or less water to the tank. The desired level is preset by an operator, who is not part of the system. Figure 2 Closed-Loop Control System Feedback is information in a closed-loop Figure 3 Feedback in a Closed-Loop Control System control system about the condition of a process variable. This variable is compared with a desired condition to produce the proper control action on the process. Information is continually "fed back" to the control circuit in response to control action. In the previous example, the actual storage tank water level, sensed by the level transmitter, is feedback to the level controller. This feedback is compared with a desired level to produce the required control action that will position the level control as needed to maintain the desired level. Figure 3 shows this relationship. Rev. 0 Page 3 IC-07