In the mid-1970s,complete decentralization was the operation of a controller to complete a loop.At that time,because people were not familiar with digital technology and were accustomed to analog instruments,loop controllers were popular in the late 1970s and 1980s.The PID operation is digitized internally.An instrument(a computer)completes the control task of a loop.It is more expensive,but the danger is diffused.Then use the communication network to connect each controller and the man-machine interface based on CRT to form a system.At this time,the network structure is usually a star structure.The manufacturing cost of the controller of the loop is too high,and the price/performance ratio is not good.Later,in order to reduce costs,there are two-loop and four-loop controllers,which have a slightly better price/performance ratio.
70年代中期,彻底分散就是一个控制器完成一个回路的运算。当时由于人们对数字技术不太熟息,习惯于模拟仪表,70年代末、80年代曾经风行回路控制器,把数字控制器做成和原来模拟仪表在外观上几乎完全一样,不改变操作习惯,内部把PID运算数字化。一块仪表(一台计算机)完成一个回路的控制任务。其价格较为昂贵,但危险是分散了。然后用通讯网络把各个控制器和以CRT为基础的人机界面连成一个系统。这时网络结构通常都是星形结构。回路的控制器的制作成本太高,价格/性能比不好。后来为了减低成本,就有两回路的、四回路的控制器,它的价格/性能比稍好一些
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