An automatic system usually consists of a control system and an information system. A control system is a system which measures the condition of some entity and, with the information, governs the state of a variable; i.e., speed, temperature, pressure, position, etc. The open-loop system is the st form a control system may assume. It is distinguished from the closed-loop system by the lack of an input which measures the state of the controlled variable; such an input is called a feedback. In modern ships the open-loop system has practically disappeared and the closed-loop system which operates with a feedback is extensively used.
An information system serves the parallel function of monitoring system or plant performance. The monitoring function may serve a number of purpose; specifically it may indicate values of controlled variables to guide remote control operations, warn of off-limit conditions and provide record of performance. In many cases a visual and audible alarm may be provided to warn of off-limit conditions on board ship and, associated with this, the important variables will be recorded on demand. The automatic control system can be more sophisticated by incorporating digital-processing equipment, that is, a computer. This computer is usually fed with signals from all sensors. It scans the values of the sensors and generates alarm signals for off-limit conditions, acting as a super-supervisor. Sometimes, it can even make necessary and take the necessary action to optimize performance without human intervention. (1)The closed-loop system differs from the open-loop system by_____. A. the lack of a feedback in a circuit B. the presence of a feed back C. the lack of a portable computer D. the presence of a portable computer (2)A feedback is an input_____. A. which governs the state of a variable B. which provides a visual and audible alarm C. which indicates values of controlled variables D. which measures the state of the controlled variable (3)The monitoring function may serve all of the following purposes except_____. A. instigating (激起;煽动) corrective action in a control system B. indicating values of controlled variables C. warning of off-limit conditions D. providing record of performance (4)The word "off-limit" in the passage may mean_____. A. off-set B. off-state C. within limits D. beyond the limit