ONE、PLC overview
Programmable controller is the first in the late1960s 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 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 hardware 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 microprocessor,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) 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 implement logic in their internal storage oper
ations,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 peripherals,and industrial control systems easily linked to form a whole,to expand its functional design.Programmable controller for the user,is a
non-contact equipment,the procedures can be changed to change production processes.The programmable controller has become a powerful tool for factory automation,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,strong functional interface.
TWO、History of PLC
Programmable Logic Controllers(PLC),a computing device invented by Richard E.Morley in1968,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 from6months down to6days[Moody and Morley,1999].
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 to15million PLCs per year with the hardware value of more than8billion 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 a5-billion industry at the present 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 s
controller翻译中文
oftware dilemma or crisis in a similar way.Morley himself emphasized this aspect 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 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 tools 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 industrial 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 reconfigurability 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
achieve a successful design.The tangible outcome of this research is to find a way to reduce the uncertainty in managing the control software development process,that is,reducing programming and debugging time and their variation, increasing flexibility of the automation systems,and enabling software reusability through modularity.The goal is to overcome shortcomings of current programming strategies that are based on the experience of the individual software developer.
Three、now of PLC
From the structure is divided into fixed PLC and Module PLC,the two kinds of PLC including CPU boar
d,I/O board,display panel,memory block,power, these elements into a do not remove overall.Module type PLC including CPU module,I/O modules,memory,the power modules,bottom or a frame,these modules can be according to certain rules combination configuration.
In the user view,a detailed analysis of the CPU's internal unnecessary,but working mechanism of every part of the circuit.The CPU control works,by it reads CPU instruction,interprets the instruction and executes instructions.But the pace of work by shock signal control.
Unit work under the controller command used in a digital or logic operations.In computing and storage register of computation result,it is also among the controller command and work.CPU speed and memory capacity is the important parameters fot PLC.its determines the PLC speed of work,IO PLC number and software capacity,so limits to control size.
Central Processing Unit(CPU)is the brain of a PLC controller.CPU itself is usually one of the microcontrollers.Aforetime these were8-bit microcontrollers such as8051,and now these are16-and32-bit microcontrollers.Unspoken rule is that you’ll find mostly Hitachi and Fujicu microcontrollers in PLC controllers by Japanese makers,Siemens in European controllers,and Motorola microcontrollers in American ones.CPU also takes care of communication,interconnectedness among other parts of PLC controllers, program execution,memory operation,overseeing input and setting up of an output.
System memory(today mostly implemented in FLASH technology)is used by a PLC for a process control system.Aside form.this operating system it also contains a user program translated forma ladder diagram to a binary form. FLASH memory contents can be changed only in case where user program is being changed.PLC controllers were used earlier instead of PLASH memory and have had EPROM memory instead of FLASH memory which had to be erased with UV lamp and programmed on programmers.With the use of FLASH technology this process was greatly shortened.Reprogramming a program memory is done through a serial cable in a program for application development.
User memory is divided into blocks having special functions.Some parts of a memory are used for storing input and output status.The real status of an
input is stored either as“1”or as“0”in a specific memory bit/each input or output has one corresponding bit in memory.Other parts of memory are used to store variable contents for variables used in used program.For example, time value,or counter value would be stored in this part of the memory.
PLC controller can be reprogrammed through a computer(usual way),but also through manual programmers(consoles).This practically means that each PLC controller can programmed through a co
mputer if you have the software needed for programming.Today’s transmission computers are ideal for reprogramming a PLC controller in factory itself.This is of great importance to industry.Once the system is corrected,it is also important to read the right program into a PLC again.It is also good to check from time to time whether program in a PLC has not changed.This helps to avoid hazardous situations in factory rooms(some automakers have established communication networks which regularly check programs in PLC controllers to ensure execution only of good programs).
Almost every program for programming a PLC controller possesses various useful options such as:forced switching on and off of the system input/outputs (I/O lines),program follow up in real time as well as documenting a diagram. This documenting is necessary to understand and define failures and malfunctions.Programmer can add remarks,names of input or output devices, and comments that can be useful when finding errors,or with system maintenance.Adding comments and remarks enables any technician(and not just a person who developed the system)to understand a ladder diagram right away.Comments and remarks can even quote precisely part numbers if replacements would be needed.This would speed up a repair of any problems that come up due to bad parts.The old way was such that a person who developed a system had protection on the program,so nobody aside from this person could understand how it was done.Correctly documented ladder diagram allows any technician to understand thoroughly how system functions.
Electrical supply is used in bringing electrical energy to central processing unit.Most PLC controllers work either at24VDC or220VAC.On some PLC controllers you’ll find electrical supply as a separate module.Those are usually bigger PLC controllers,while small and medium series already contain the supply module.User has to determine how much current to take from I/O module to ensure that electrical supply provides appropriate amount of current. Different types of modules use different amounts of electrical current.
This electrical supply is usually not used to start external input or output. User has to provide separate supplies in starting PLC controller inputs because then you can ensure so called“pure”supply for the PLC controller. With pure supply we mean supply where industrial environment can not affect it damagingly.Some of the smaller PLC controllers supply their inputs with voltage from a small supply source already incorporated into a PLC.
Four、PLC design criteria
A systematic approach to designing PLC software can overcome deficiencies in the traditional way of programming manufacturing control systems,and can have wide ramifications in several industrial applications. Automation control systems are modeled by formal languages or,equivalently, by state m
achines.Formal representations provide a high-level description of the behavior of the system to be controlled.State machines can be analytically evaluated as to whether or not they meet the desired goals.Secondly,a state machine description provides a structured representation to convey the logical requirements and constraints such as detailed safety rules.Thirdly,
well-defined control systems design outcomes are conducive to automatic code generation-An ability to produce control software executable on commercial distinct logic controllers can reduce programming lead-time and labor cost.In particular,the thesis is relevant with respect to the following aspects.
In modern manufacturing,systems are characterized by product and process innovation,become customer-driven and thus have to respond quickly to changing system requirements.A major challenge is therefore to provide enabling technologies that can economically reconfigure automation control systems in response to changing needs and new opportunities.Design and operational knowledge can be reused in real-time,therefore,giving a significant competitive edge in industrial practice.
Studies have shown that programming methodologies in automation systems have not been able to match rapid increase in use of computing resources.For instance,the programming of PLCs still relies
on a conventional programming style with ladder logic diagrams.As a result,the delays and resources in programming are a major stumbling stone for the progress of manufacturing industry.Testing and debugging may consume over50%of the manpower allocated for the PLC program design.Standards[IEC60848,1999; IEC-61131-3,1993;IEC61499,1998;ISO15745-1,1999]have been formed to fix and disseminate state-of-the-art design methods,but they normally cannot participate in advancing the knowledge of efficient program and system design.
A systematic approach will increase the level of design automation through reusing existing software components,and will provide methods to make large-scale system design manageable.Likewise,it will improve software quality and reliability and will be relevant to systems high security standards,especially those having hazardous impact on the environment such as airport control,and public railroads.
The software industry is regarded as a performance destructor and complexity generator.Steadily shrinking hardware prices spoils the need for software performance in terms of code optimization and efficiency.The result
is that massive and less efficient software code on one hand outpaces the

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