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 simplest 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 ware 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 ation to optimize performance without human intervention.