1、西安科技大学毕业设计（论文）外文翻译7.5 PLC System Functions system has advanced to become an open PLC system that can meet the following requirements:1. Portability: The PLC program can be operated and is reusable regardless of PLC system and maker.2. Connectivity: Communication (data transmission) between PLC syste
2、ms whose makers are different should be guaranteed.3. Standardization: The user interface and programming language are unified regardless of system and maker. For example, the PLC systems and programming languages mentioned in previous sections are not compatible with other systems and languages tha
3、t other makers provide. Therefore, users should learn the makers own programming languages. It is also very difficult for third parties to understand and modify PLC programs. In addition,when a new function is added, it is almost impossible to guarantee successful execution within a specified time.
4、To overcome these problems, the activity for standardizing programming environments for industrial automation equipment was started and the IEC, (International Electromechanical Commission), established IEC1131-3 in 1993. The standard IEC1131, is the international standard for PLC, consisting of fiv
5、e parts and IEC1131-3 is one of the parts of which IEC1131 is composed.1. IEC1131-1: PLC General information.2. IEC1131-2: Equipment and test requirements3. IEC1131-3: PLC programming language4. IEC1131-4: User guidelines5. IEC1131-5: Communications IEC1131-3 is the international standard for progra
6、mmable controller programming languages. It specifies the syntax, semantics and display for the following suite of PLC programming languages: 1) Ladder diagram (LD), 2) Sequential Function Charts (SFC), 3) Function Block Diagram (FBD), 4) Structured Text (ST), and 5) Instruction List (IL). If we use
7、 IEC1131-3 to edit a PLC program, it is possible to obtain the following advantages:1. Because syntax and semantics are unified, it is possible to generate a program that can be operated on all makers systems and the program can be executed regardless of maker.2. It is easy to maintain the program.3
8、. Because the standard supports the structured programming method, any complex program can be edited in easily understandable and structured format and can easily be maintained.4. Due to the rigorous syntax and semantics it is possible to reduce program error.5. The standard makes modulation of a pr
9、ogram easy and it is possible to increase the efficiency of programming using program modules. However, the following disadvantages have been identified:1. Compared with the sequence programming method, the programming procedure is complex due to computer programming.2. Much effort is needed to unde
10、rstand and know the grammar of the programming language.3. Due to the rigorous grammar, the flexibility of programming is restricted.4. Because IEC1131-3 is heavy, it is not appropriate for application in small-sized PLC systems. IEC1131-3 consists of the configuration model of a PLC system, five pr
11、ogramminglanguages, and the common generality of programming languages.The software model and communication model address the name and definition of the parts from which a PLC system is composed and the data transmission mechanism between running programs. In the Programming model, not only basic el
12、ements such as identifiers, keywords, data types, and variables but also program elements such as functions, function blocks, programs, resources, and tasks are described as the common factors of the programming language. To understand IEC1131-3, it is necessary to undertake a study of the configura
13、tion model, which represents the design concept of a PLC system and includes a software model, communication model, and programming model.7.5.1 Software Model and Communication Model In the introductory part of IEC1131-3 the software model is described and represents the PLC system as a controller w
14、ith multitasking-enabled architecture. In the software model,1. Configuration is the top-most concept that represents the PLC system and includes all the software that is contained in one PLC system.2. Resource is the element that makes up the configuration and means the functions that a processor b
15、oard provides. It consists of the software that is needed to execute a PLC program.3. Program means the logical management unit of a user program and is edited by one of the languages specified by IEC1131-3.4. Function block is a key concept of IEC1131-3 and makes a program structured and modularize
16、d. It is the logical management unit for data transmission and consists of the data for defining input/output parameters and the algorithms for performing pacific functions.5. Task represents how a program or function block works. It begins iteratively or by a specific trigger.6. Function is one of
17、the elements of which a program is composed. It is different from the function block, and it denotes the software that generates a single output from a specific input.7. Access path does not exist in a single resource system. In multiple resource systems it manages the data of elements and the commu
18、nication between elements.There are various ways to transmit data in a PLC system. In a program, internal variables are used. To input and output the data to the program, function, and function block, input variables and output variables are used. To share resource or configuration information betwe
19、en programs, external variables that specify them are used. In addition, data transmission between configurations is done by the communication object defined by an access path. Using the access path, it is possible to exchange data between the functions and the programs that are located in different
20、 resources or configurations. Comparing PLC systems with the software model in IEC-1131-3, we can regard the controlled system as configuration. Configuration exchanges data or information with other configurations via Access paths (only specific variables can be transmitted via access paths and ext
21、ended communication functions, as specified in IEC1131-5). This configuration consists of one or multiple resources and each resource consists of one or multiple tasks. Because of the high functionality of PLC systems,multiple processing is required and the CPU board can be regarded as a resource.Ea
22、ch task executes a program or function block based on regular interrupts or irregular triggers. Consequently, this systematic structure makes it possible to execute a lot of individual programs synchronously. In addition, resource has a function for supporting the interface between I/O channel and a
23、 program. Furthermore, small-sized PLC systems consist of one processor and one piece of software that controls single operations. A single configuration is formed, even in the case of large-sized PLC systems with multiple processors, a variety of resources (multiple processors) are controlled in re
24、al-time. In the case of complicated distributed systems, a system is composed of more than one configuration connected by a network and each configuration can include any resource or program on the network. As is known from the software model, the key concept of IEC1131-3 is to support the structure
25、d programming concept. Actually, by using the task, function, and function block mentioned in the software model, it is possible to change the programming style of the user. Therefore, by using IEC1131-3 it is possible to design a PLC system by distinguishing an iterative task and an interrupt-drive
26、n task (or event-driven task). Furthermore, it is possible to divide the iterative tasks into tasks with the same iteration time. In addition, by implementing common tasks or programs as functions or function blocks, it is possible to decrease the program size.Consequently, this structured programmi
27、ng method enables the modulation of programs and this modulation increases the productivity and maintainability of large-sized PLC systems.7.5.2 Programming Model The programming model describes the relationship between the common elements of the programming language specified in IEC1131-3. The prog
28、ramming model is based on the concept of derivation and reuse. In other words, the programmer can define new data types from basic data types, new functions or function blocks from basic functions (or standard functions) and can make libraries using them. These libraries can be used not only in an i
29、dentical system but also in other non-identical systems.The reusability of programs enables advanced programming techniques such as libraries, functions, and function blocks and provides ease and reliability of programming. As mentioned above, as IEC1131 is the international standard for PLC, it con
30、sists of a configuration model and a programming model. To build the standard and open system, it is necessary for the PLC system developer to design the system and the functions based on IEC1131. It is also necessary to design the interpreter for the standard programming language. As well as progra
31、mmers or system designers, it is also necessary for system users (operators) to understand the key concepts and Functions.7.5.3 User Programming Languages In IEC1131-3, five programming languages (actually, four languages and one common element) are specified; LD (Ladder Diagram), IL (Instruction Li
32、st), ST (Structured Text), FBD (Function Block Diagram), and SFC (Sequential Function Chart).The user can select the appropriate language depending on the characteristics of the program. Actually, SFC is not a programming language for implementing the control program,but the representation tool to d
33、epict all sequences of a control program. SFC classifies continuous tasks into Steps as well as Transitions, which are the conditions for shifting between steps, and Actions, being the job to be performed at a step. In other words, SFC is composed of multiple steps, the module of a program, an actio
34、n block associated with a particular step, and a transition to represent the condition for shifting between steps. This graphical representation method is based on Petri-nets or IEC848. SFC can be used not only by itself but also with other program languages specified in IEC1131-3. Therefore, SFC is
35、 used as a common element in IEC1131-3. SFC supports not only conditional sequencing but also parallel sequencing where one sequence monitors or executes a background task simultaneously with another performing the main control. For effective programming, it is necessary to reuse functions or partia
36、l programs. For this, functions, function blocks, and programs are reused in application programs. A function is composed of basic functions such as ADD, ABS, SQRT, SIN, COS and user-defined functions. A function block contains data and algorithms, as semiconductor hardware, which enables the specif
37、ic function. It can be reusable in other application programs. In addition, Functions, function blocks, and programs are common elements that can be used in all programming languages specified in IEC1131-3. Besides SFC, the possibility of powerful data addressing is another common element.It is used
38、 to prevent a programmer from substituting (allocating) a different data type to a variable. Boolean type, integer type, byte type, word type, date type,and date-time type are defined as the data types of IEC1131-3. Local variables and global variables can be used as variables in IEC1131-3. A local
39、variable is a variable that can be used only within software elements where it is defined. Global variables mean variables that can be used over whole software elements. It is also possible to define direct pointer variables that refer directly to memory locations. The key characteristics of the fou
40、r languages specified in IEC1131-3 are summarized in Fig 7.10. With the above-mentioned common elements. For convenience of understanding, the program that sets or resets the output port depending on the type of input is edited by four languages and the basic format of each language will be describe
41、d. Among the four languages, IL (Instruction List) and ST (Structured Text) are textual languages, while the Ladder Diagram and Function Block Diagram are graphical languages. In the late 1960s, after GE powertrain introduced the concept of the PLC system that replaced the relay board, PLC systems t
42、hat control a variety of processes by using simple sequence programs has been widely used for 40 years. However, as the controlled systems have become more complex, faster, and larger, the demand for openness and standardization of PLC systems has increased. To meet this requirement, the PLC system
43、has been changed from a hardware-based system to a software-based system. Consequently, Soft PLC systems, which operate from personal computers and enable logical sequence control functions in real time, were introduced. With the advancement of the PCs performance, Soft PLC systems have come to prov
44、ide not only conventional sequence control functions but also easy user interfaces, network communication functions, and advanced functions for factory automation. In Soft PLC systems, the basic and advanced functions of PLC and communication functions are executed by one processor module, except fo
45、r input and output modules. It is possible to make a standardized PLC system based on the software model and programming languages specified in IEC1131-3. Figure 7.11 shows an example of a Soft PLC that has been applied to the transfer line in Ford Motors. In this system, the interface board that ca
46、n be connected to various I/O devices is built into a PC that contains the Soft PLC system. This system is a good example of a Soft PLC system that satisfies the openness requirement by using PC hardware. To develop a soft PLC system, real-time operation and reliability of response, which are key re
47、quirements for industrial PLC systems, should be guaranteed. Basically, though, PC operating systems cannot satisfy these. However, the non realtime property of DOS or Windows OS can be overcome by various methods and the method of designing a soft PLC system will be described together with design o
48、f Soft-NC in the later in this textbook. In Soft-NC, a PC is used as the hardware platform and all CNC functions including PLC functions are implemented in software. In this point, Soft PLC is very similar to Soft-NC. Furthermore, Soft NC includes more functions than Soft PLC, used for NCK control a
49、nd MMI. Therefore, if NCK functions and MMI functions are omitted or simplified from Soft NC, Soft NC and Soft PLC can be regarded as the same system. Soft PLC which is made by a user interface and the PLC kernel based on the IEC1131-3 can be regarded as partial systems of Soft-NC. Figure 7.12 shows the open CNC system of MDSI. The figure shows that the CNC system consists of Interpreter , Servo controller , Int
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