1、 1 PLC development 1、 PLC overview Programmable controller is the first in the late 1960s in the United States, then called Plc programmable logic controller (Programmable Logic Controller) is used to replace relays. For the implementation of the logical judgment, timing, sequence number, and other
2、control functions. The concept is presented Plc General Motors Corporation. Plc and the basic design is the computer functional improvements, flexible, generic and other advantages and relay control system simple and easy to operate, such as the advantages of cheap prices combined controller hardwar
3、e is standard and overall. According to the practical application of target software in order to control the content of the user procedures memory controller, the controller and connecting the accused convenient target. In the mid-1970s, the Plc has been widely used as a central processing unit micr
4、oprocessor, import export module and the external circuits are used, large-scale integrated circuits even when the Plc is no longer the only logical (IC) judgment functions also have data processing, PID conditioning and data communications functions. International Electro technical Commission (IEC)
5、 standards promulgated programmable controller for programmable controller draft made the following definition: programmable controller is a digital electronic computers operating system, specifically for applications in the industrial design environment. It used programmable memory, used to impleme
6、nt logic in their internal storage operations, sequence control, timing, counting and arithmetic operations, such as operating instructions, and through digital and analog input and output, the control of various types of machinery or production processes. Programmable controller and related periphe
7、rals, and industrial control systems easily linked to form a whole, to expand its functional design. Programmable controller for the user, is non-contact 2 equipment, the procedures can be changed to change production processes. The programmable controller has become a powerful tool for factory auto
8、mation, widely popular replication. Programmable controller is user-oriented industries dedicated control computer, with many distinctive features. First, high reliability, anti-interference capability； Second， programming visual, simple； Third, adaptability good； Fourth, functional improvements, st
9、rong functional interface. 2、 History of PLC 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
10、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. Although PC based control has started to come into
11、 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
12、 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 prese
13、nt time. 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 3 crisis in a similar way. Morley himself emphasized this aspect
14、most forcefully by indicating. If houses were built like software projects, a single woodpecker could destroy civilization.” 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
15、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. In general, the productivity in generating PLC is far behind compared to other domains, for instance, VLSI design, where efficient computer aided design tool
16、s are in practice. Existent software engineering methodologies are not necessarily applicable to the PLC based software design because PLC-programming requires a simultaneous consideration of hardware and software. The software design becomes, thereby, more and more the major cost driver. In many in
17、dustrial design projects, more than of the manpower allocated for the control system design and installation is scheduled for testing and debugging PLC programs. In addition, current PLC based control systems are not properly designed to support the growing demand for flexibility and recon figurabil
18、ity of manufacturing systems. A further problem, impelling the need for a systematic design methodology, is the increasing software complexity in large-scale projects. The objective of this thesis is to develop a systematic software design methodology for PLC operated automation systems. The design methodology involves high-level description based on state transition models that treat automation control systems as discrete event systems, a stepwise design process, and set of design rules providing guidance and measurements to
