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The introduction and application of plc WWW_P※LCJS_COM－PLC－)技.术_网
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1.1 Motivation WWW_PLCJS_COM－PLC－技.术_网
Programmable Logic Controllers (PLC), a computing device invented by Richard E. Morley in 1968, have been widely used in industry including manufacturing systems, transportation systems, chemical process facilities, and many others. At that time, the PLC replaced the hardwired logic with soft-wired logic or so-called relay ladder logic (RLL), a programming language visually resembling the hardwired logic, and reduced thereby the configuration time from 6 months down to 6 days [Moody and Morley, 1999]. WWW_P※LCJS_COM－PLC－)技.术_网
Although PC based control has started to come into place, PLC based control will remain the technique to which the majority of industrial applications will adhere due to its higher performance, lower price, and superior reliability in harsh environments. Moreover, according to a study on the PLC market of Frost and Sullivan [1995], an increase of the annual sales volume to 15 million PLCs per year with the hardware value of more than 8 billion US dollars has been predicted, though the prices of computing hardware is steadily dropping. The inventor of the PLC, Richard E Morley, fairly considers the PLC market as a 5-billion industry at the present time. WWW_PL※CJS_COM－PLC－技.术_网
Though PLCs are widely used in industrial practice, the programming of PLC based control systems is still very much relying on trial-and-error. Alike software engineering, PLC software design is facing the software dilemma or crisis in a similar way. Morley himself emphasized this aspect most forcefully by indicating [Moody and Morley, 1999, p. 110]: WWW_PLC※JS_COM－PLC－技.术_网(可编程控※制器技术门户)
`If houses were built like software projects, a single woodpecker could destroy civilization.” WWW※PLCJS_COM－PL＃C－技.术_网(可编※程控※制器技术门户)
Particularly, practical problems in PLC programming are to eliminate software bugs and to reduce the maintenance costs of old ladder logic programs. Though the hardware costs of PLCs are dropping continuously, reducing the scan time of the ladder logic is still an issue in industry so that low-cost PLCs can be used. WWW_PLCJS@_COM%－PLC－技.术_网