In the late 1990s, people gradually realized that traditional PLC (herein referred to as “hard PLC”) has its own shortcomings: it is difficult to build an open hardware architecture; the staff must undergo a long period of professional training to master a certain one. The programming method of the product; The production of traditional PLC is monopolized by several manufacturers, resulting in slow growth of the price-performance ratio of PLC. These problems have become factors restricting the development of traditional PLC. In recent years, with the rapid development of computer technology and the formulation of international standards for PLC, an emerging technology that breaks the limitations of traditional PLC has developed. This is soft PLC technology. Its characteristics are: on the premise of retaining PLC functions, it adopts a fieldbus network-oriented architecture, adopts open communication interfaces, such as Ethernet, high-speed serial ports, etc.; adopts various related international industrial standards and a series of de facto Standard; all use software to realize the functions of traditional PLC.
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Soft PLC Development System
The soft PLC development system is actually a PLC programmer with debugging and compilation functions. This part has the following functions: ①The programming language is standardized, follows the IEC61131-3 standard, and supports multi-language programming (a total of 5 programming methods: IL, ST, LD, FBD and SFC), programming languages can be interchanged; ②Rich control modules, support a variety of PID algorithms (such as conventional PID control algorithm, adaptive PID control algorithm, fuzzy PID control algorithm, intelligent PID control algorithm, etc. ), also includes some popular control algorithms, such as neural network control; ③open control algorithm interface, support users to embed their own control algorithm modules; ④simulation operation, real-time online monitoring, online program modification and compilation; ⑤powerful network Features. Support TCPIP network, through the network to achieve PLC remote monitoring, remote program modification.
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Operating system and application characteristics of soft PLC
1. Soft PLC operating system
This part is the core of the soft PLC, which completes work such as input processing, program execution, and output processing. It usually consists of IO interface, communication interface, system manager, error manager, debugging kernel and compiler: ①IO interface, which can communicate with any IO system, including local IO system and remote IO system. Remote IO is mainly through the field bus InterBus, Realization of ProfiBus, CAN, etc.; ②Communication interface. Through this interface, the operating system can communicate with the development system or HMI (or MMI) software according to various protocols, such as downloading PLC programs or data exchange; ③The system manager handles different tasks and coordinates the execution of the program, and from IO Image read and write variables; ④Error manager, detect and handle various errors that occur during program execution; ⑤Debug the kernel, provide multiple debugging functions, such as rewriting, forcing variables, setting breakpoints, setting variables and address status; ⑥ Compiler, usually the development system compiles the written PLC source program into intermediate code, and then the compiler of the running system translates the intermediate code into machine executable code (ie object code) related to the hardware platform.
2. Application characteristics of soft plc
The application features are: ① It embodies the integration of advanced technologies of IPC, PLC and DOC. It can make full use of the hardware and software resources on the PC platform to make the control system more distinctive; ②The system is more open and the application is more convenient. The software PLC can connect to Office software or software developed with VB and VC through the OPC function and Active control provided by its own development tools; ③The control system based on PC+fieldbus+distributed IO simplifies the complex control system The structure improves communication efficiency and speed, and reduces investment costs.
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Technical advantages and development constraints of soft PLC
1. Advantages of soft PLC technology
The hardware architecture of the soft PLC is no longer closed, and the user can choose the appropriate hardware to form a soft PLC that meets the requirements.
The instruction set of the traditional PLC is fixed, and the algorithm may need to be defined in actual industrial applications. The soft PLC instruction set can be more abundant, and users can use standard operating instructions.
The fierce competition among PC manufacturers has improved the cost performance of PC-based soft PLCs.
Traditional PLC is limited to a few manufacturers and is private. Therefore, it is difficult to adapt to existing standard computer networks. PLCs and computers are often in different types of networks. Soft PLC can not only join the existing private PLC network, but also join the standard computer network. This makes a lot of research results of existing computer networks easily applied to PLC control technology.
The technology of soft PLC is based on the IEC61131-3 standard, so it is easier to develop after mastering the standard language.
2. Factors restricting the development of soft PLC
Although soft PLC technology has great development potential, the realization of this technology needs to solve some important problems. Among them is mainly the real-time problem of PC-based control engine. The preferred operating system for soft PLC is Windows NT, but it is not a hard real-time operating system. Traditional PLC has hard real-time, because of this it can provide fast, deterministic and repeatable response. To make WindowsNT hard real-time, it must be extended so that the control task of the PC has the highest priority, and it will not be preempted because of NT system functions and user program calls. Now, we can add real-time performance to the NT system through some methods. For example, modify the hardware abstraction layer of NT, or combine NT with a hard real-time operating system that has been practically verified. In addition, operating systems such as WindowsCE have features that NT does not have in terms of hard real-time performance. In actual development, other operating systems can also be used as platforms.