The international electrotechnical commission to the definition and the characteristic of PLC of PLC ( PLC why would receive such a widely used)

Since the introduction of PLC PLC concept, although time is not long, but has been developing rapidly. In order to make the standardization of the production and development, the international electrotechnical commission ( IEC) Successively introduced the PLC standard draft of the first draft, a second draft, and in February 1987 by the definition of it: 'a programmable controller is a kind of digital computing operations electronic systems, specially designed for applications in industrial environment. It USES a programmable memory for its internal storage procedures, to perform the logic operation, sequence control, timing, counting and arithmetic operations such as user-oriented instructions, and through the digital or analog input/output control various types of machinery or production process. Programmable controller and related peripheral equipment, industrial control systems installed by easily with a whole and to expand its function on the principle of design. 'In order to avoid and PC (personal computer 个人电脑) Confused, so instead of PLC ( 可编程逻辑控制器) Can programming logic controller, but tell from the function, the PLC is not original already in the sense of 'PLC'. In a word, programmable controller is a computer, it is designed for industrial environment application and design and manufacture of computer. It has a wealth of input and output interface, and has strong driving ability. But programmable controller products are not on a specific industry application, in practical application, its hardware should be carried out according to the actual need to choose the configuration, the software design should be carried out according to the control requirements. 2. Characteristics of PLC, 1) High reliability, strong anti-interference ability generally for high requirement of control equipment, industrial production should have very strong anti-interference ability and high reliability, can work reliably in harsh environment, the average time between failures, fault repair time is short. This is an important characteristic of PLC control is superior to the microcomputer control. For example, Japan's mitsubishi F1, F2 series MTBF for 300000 h, and A series of reliability is higher than F1, F2 series. PLC control system failure usually has two kinds: one kind is accidental failure, namely because of bad environment ( Electromagnetic interference, ultra high temperature, overvoltage, undervoltage) Cause, this kind of failure as long as it doesn't cause the damage of the system components, once back to normal environmental conditions, the system is supposed to be back to normal, then but after PLC is influenced by the outside world, internal storage of information are destroyed, must start again from the initial state. Another kind is the permanent fault, it is caused by components unrecoverable damage. In the design of PLC, can take measures from two aspects of hardware and software, prevent the occurrence of the above failure, in order to improve its reliability. Main measures are: (1) hardware measures are: shield: the power transformer, CPU programmer main components, such as conductivity, permeability good materials for shielding, in case of interference. Filtering: the power supply system and the input/output circuit adopts various kinds of filter, such as LC type PI type filter network, to eliminate the high frequency interference and weaken the interaction between the various modules. : the adjustment of the power supply and protection of the microprocessor core parts needed to + 5 v power supply, using multilevel filter, and the integrated voltage regulator to adjust, to adapt to the ac power grid volatility and overvoltage, undervoltage influence. Isolation: between the microprocessor and the input/output circuit, using photoelectric isolation, effectively isolate the power relation between the input/output, reduces the likelihood of failure and misoperation. Interlocking: all controlled by the open signal output module, and this signal is only effective when the regulation of various conditions are met, thus effectively preventing the possibility of produce abnormal output. Adopts modular structure: this structure will help repair fault cases short. Set up the environment detection and diagnosis circuit: this kind of circuit and software, can realize the flexible protection and fault indicator, etc. (2) software measures are: the failure detection: software testing environment on a regular basis, such as power, strong interference signal, and so on and so forth can timely processing. Data protection and recovery: the accidental failure as long as the failure condition occurs, does not destroy the PLC internal information, once the fault conditions disappear, can return to normal, continue to the original work. Set the alarm clock WDT: if the program execution time per cycle than the WDT regulation time, announced a program into an infinite loop, immediately report to the police. To strengthen the inspection and test of program: once the program is wrong, immediately report to the police and stay of execution. To battery back-up procedures and dynamic data: after the blackout, using the backup battery power, information about status and would not be so lost. In this way, the reliability of the PLC, anti-interference ability is greatly increased. Such as American ge company made of PC control module, on average, no failure rate can reach more than 10 million hours, an average failure rate of system can be up to 4 to 50 million hours. （ 2) PLC programming is simple, easy to use this is better than another characteristic of microcomputer. Most current PLC controlled by relay in the form of 'ladder diagram programming method', namely the traditional control circuit of clear and intuitive, and suitable for electrical technical personnel's habit of reading and the computer application level, easy to accept, but compared with the commonly used assembly language, the more popular. This further simplify programming, today's PLC is designed for specific problems, such as the step ladder instructions, functions, etc. PLC is designed for workshop operators, usually as long as the training is to learn how to use a very short time. And the microcomputer control system with a certain knowledge of personnel operations. Of course, the function of PLC development, needs the help of software experts.