With the advancement of technology and the intensification of market competition, the control system of dedicated architecture has exposed its inherent defects more and more. Because the software PLC based on the PC platform does not need a special programmer, it can make full use of the software and hardware resources of the PC, directly use ladder diagram or language programming, and has good human-machine interface advantages. Therefore, it gradually replaced the hardware PLC and became a research. hot spot.
1 Introduction to electrical test system principles
The life test system can use a timer to control the operating frequency of electronic component products and a timer to control the number of actions. The basic principle is shown in Figure 1.
Figure 1 Basic principles of electrical testing
PLC uses OMRON CQM1H-CPU11 PLC plus expansion module, its I / O points can reach 26, can meet multi-channel test, use control
Relay timer and load feedback Relay timer time difference is used to judge the bad state of the product or load, and the system is shut down to protect. After the experiment is completed, the product or load is evaluated for its mechanical and electrical characteristics.
Among them, the interruption protection, action relay and load feedback relay are all controlled by PLC internal high-precision timers, the principle of which is to control high voltage with low voltage, to achieve high and low voltage separation control engineering network copyright, to ensure the test system For safety, Fuji Electric Co. Ltd is used for the interruption protection. JapanSC-13AA type contactor, withstand voltage up to 600V, single contact rated current is 32A, action Relay and load feedback Relay (you can also use photocoupler or current transformer) are OMRON's MY series relays.
The actual measurement with DL750 scoper found that these two kinds of accuracy are not very high, and the error is often 8-10ms.
In practice, product evaluation requires a lot of data, and many products need to be tested. This requires multiple independent circuits, wiring and wiring difficulties, and parasitic circuits. Therefore, considering the compatibility of PLC products and test costs, the current advanced embedded Soft PLC technology can greatly shorten the experimental cycle and reduce costs.
2 Soft PLC technology
After more than ten years of development, the traditional PLC has been very mature, and it has been widely used in the field of industrial control with high speed, high stability, and high performance. At the same time, with the development of modern computer technology and electronic technology, Engineering has gradually shown its shortcomings: traditional PLC manufacturers monopolize the market, their products are incompatible, they lack clear and consistent standards, and it is difficult to build an open hardware architecture; the programming methods of each manufacturer's products vary greatly, and the technical proprietary Strong, long staff training time, resulting in a slow growth in the price-performance ratio of PLC.
In the mid-1990s, the rapid development of computer and microelectronic technology and the formulation of the international standard IEC61131 of PLC produced soft PLC technology. All "soft PLC technology" is to use PC as a hardware support platform and use software to implement standard hardware PLC The basic function is to encapsulate the control function of the PLC in the software and run in the environment of the PC. The PC-based control system provides the same functions of the PLC but has various advantages of the PC.
Soft PLC technology is generally composed of two parts: development system and operation system. The development system runs on the PC platform, and the running system runs on the embedded hardware platform. Generally speaking, the embedded hardware platform must run some embedded operating system, such as μC / OS-II, μClinux, etc.
The characteristics of modern development systems are generally integrated development environment, friendly interface and easy to use. The soft PLC embedded system is based on the IEC61131 international standard, so it must support several languages. The system runs on an embedded hardware platform and is responsible for interpreting and executing the target files generated by the development system after compilation and linking. The operating system is generally composed of The operation core, communication interface and system management are composed of three parts. Its design and implementation accuracy are very high. Its execution efficiency will directly affect the system's response speed in field control.
3 Virtual operating system of embedded platform
The structure of the operating system is shown in Figure 2. The main tasks of the system manager are to handle different tasks, coordinate program execution, and read and write variables from the I / O image. The I / O interface can communicate with local I / O systems and remote I / O systems. The communication interface enables the operating system to communicate with the development system or other devices. The kernel interpreter is the core part of the running system and is responsible for interpreting and executing the target code of the PLC application program compiled by the development system.
Figure 2 The structure of the operating system
The operating system can run on a variety of supporting platforms. It separates the data area from the code area to achieve dynamic update of programs and efficient and dynamic download of codes.
(1) Implementation steps of soft PLC virtual machine on μClinux platform
Translate the ladder diagram instructions into macro assembly and build a macro library;
Compile the ladder program written by the user into a PLM file in the form of intermediate code;
Turn PLM files into assembly programs;
Use the assembler to assemble the successful assembler, convert the interpreted and executed internal instructions (PLM files) into programs, assemble them into machine code for execution, realize the PLC instruction machine code execution scheme, greatly improve the execution speed. Greatly saves memory space;
The machine code corresponding to the PLC instruction is transplanted into the embedded operating system for execution.
(2) Two major modules of the operating system
The operating system is mainly composed of two major modules: 1) the conversion from ladder diagram to machine code program; 2) the execution of machine code files. The functions of each module are as follows:
1) Conversion of ladder diagram to machine code program. First, compile the ladder program written by the user into a PLM file in the form of intermediate code, then convert the PLM file into an assembler, and finally use the assembler to assemble the successfully converted assembler. CONTROL ENGINEERING China Copyright, generate machine code to generate executable PLE file.
2) Execute the machine code file. Embed and execute the PLE file in the configured execution environment, and monitor the correctness of the program through the panel indicator of the industrial control computer.
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