制各类机械或生产过程。用户编制的控制程序表达了生产过程的工艺要求,并事先存入plc的用户程序存储器中。运行时按存储程序的内容逐条执行,以完成工艺流程要求的操作。plc的cpu内有指示程序步存储地址的程序计数器,在程序运行过程中,执行一步该计数器自动加1,程序从起始步(步序号为零)起依次执行到**终步(通常为end指令),然后再返回起始步循环运算。plc每完成一次循环操作所需的时间称为一个扫描周期。不同型号的plc,循环扫描周期在1微秒到几十微秒之间。程序计数器这样的循环操作,这是dcs所没有的。这也是使plc的冗余不如dcs的原因。dcs是在运算放大器的基础上得以发展的。把所有的函数和过程变量之间的关系都做成功能块(有的dcs系统称为膨化块)。dcs和plc的表现的主要差别是在开关量的逻辑解算和模拟量的运算上,即使后来两者相互有些渗透,但是还是有区别。80年代以后,plc除逻辑运算外,控制回路用的算法功能已经大大加强,但 plc用梯形图编程,模拟量的运算在编程时不太直观,编程比较麻烦。但在解算逻辑方面,表现出**的优点,在微秒量级,解算1k逻辑程序不到1毫秒。它把所有的输入都当成开关量来处理,16位(也有32位的)为一个模拟量。而dcs把所有输入都当成模拟量,1位就是开关量。解算一个逻辑是在几百微秒至几毫秒量级。对于plc解算一个pid运算在几十毫秒,这与dcs的运算时间不相上下。 Spectrum B-5002 ENI B-5002-01 AMAT 0920-00062
DCG-200Z ENI DC24M-Z141300110A AMAT 0190-07964
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ENI OEM-1250 RF Generator 1250W 13.56MHz
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OEM-10B ENI OEM-10B-8 RF Generator 1250W
MKS Instruments EPCA-24592 2-Channel
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模拟量输入模块 AB-1794-IE12模拟量输入模块 AB-1794-IE12模拟量输入模块 AB-1794-IE12
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