PLC system, anti
1 Introduction Programmable controller is designed specifically for industrial control, in the design and manufacturing process manufacturers to take a multi-level anti-jamming measures, the system can be harsh industrial environments with strong electrical devices work together. High stability and reliability of operation, PLC machine MTBF of time up to tens of hours. With the development of computer technology, PLC features are also growing, more and more convenient to use, so in the industrial control system becomes more extensive. However, the machine's reliability is the prerequisite to ensure systems work reliably, but also to the design and installation of systems used in the course PLC appropriate measures, to ensure that systems work reliably. This paper discusses the design and installation of PLC systems in the course of the interference measures.
2 PLC system of basic structures
PLC programmable logic controller hardware system consists of the host, functional I / O modules and external device component, shown in Figure 1. In which the host PLC by the CPU, memory, basic I / O modules, I / O expansion interface, peripheral interface and power supply components, each part of the connection between the internal system bus.
3 PLC system design of anti-interference measures
3.1 hardware measures (1) Shield: The power adapter, CPU, programmers and other major components, using electrical conductivity, permeability good shielding material handling, to prevent outside interference signal.
(2) Filtering: The total input power supply system circuits using various forms of filtering, to eliminate and suppress high frequency interference signals, but also weakens the interaction between modules.
(3) the power to adjust and protection: voltage distortion caused by power fluctuations or spikes, would PLC and I / O modules have a negative impact. The core components of the microprocessor needed to +5 V power supply with multi-stage filtering, with integrated voltage regulator to adjust to fluctuations in exchange and over-voltage power grid, due to voltage. Level when the power cord as walking line, when the power cord on the ground was low-impedance, in order to reduce power supply noise. The shield ground in different ways, not the same inhibitory effect on the interference, the general secondary coil can not be grounded. Input and output lines and application of twisted pair shield grounding is required to suppress interference of touch.
(4) Isolation: In the microprocessor and I / O circuit between the use of optical isolation, effectively leaving them all to prevent external interference signal and the noise produced by ground loop signals into the PLC through the local public ground, thus affecting its normal work.
(5) with modular structure: This structure helps in the short period when fault repair, if found to fail a module can be quickly replaced, allowing the system returned to normal work, but also help to speed up Find System failure.
3.2 software measures Order to improve the input signal to noise ratio, often using software digital filtering to improve the useful signal authenticity. For large random interference system, using procedures limiting law, that continuous sampling 5 times, if a sample of a sample support is much larger than the amplitude of the other several times, then take care of. For the flow, pressure, liquid level, displacement and other parameters are frequent fluctuations in a certain range, then the arithmetic average. The samples with n times the average instead of the current value. General view: Flow n = 12, the pressure the most appropriate n = 4.
(1) fault diagnosis: the system software on a regular basis to detect the external environment, such as power-down, under voltage, lithium battery voltage is too low and strong interference signals for timely reflected and addressed.
(2) protection and restoration of the signal: when the occasional failure occurs, the PLC does not undermine the internal information, once the fault disappears, you can return to normal, to continue their jobs.
(3) set the alert clock WDT: If the program execution time exceeds the scan cycle WDT time required, indicating the program enters an infinite loop, call the police.
(4) to strengthen the inspection and verification process: Once the program was wrong, call the police, and stop the implementation of the program.
(5) of the procedures and dynamic data, battery back-up: When in power, using backup battery power to keep the information and state data are not lost.
4 PLC system installed in the anti-interference measures
PLC parts of the composition and method of connection and assembly instructions must be installed in strict accordance with the requirements, it is important to ensure the basic conditions for reliable operation.
4.1 Power wiring and ground wiring
Power cord to a reasonable arrangement, strong power and weak to be strictly separated, and the weak to try to increase the power cord. Grounded in the elimination of interference
played a significant role. Exchange of land is necessary to supply PLC control system, which constitutes a power supply through the transformer center of one of the two circuits. This will be the way of current, such as the various harmonic current is a serious interference element. Therefore AC ground, DC ground, analog ground and digital ground and so must be separated. Digital ground and analog ground common point to the best way home suspension. Ground potential difference between points as small as possible, as bold ground, ring ground conditions can be used.
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