With the development and application of microprocessor technology, computer network technology, information communication technology, power electronics technology and artificial intelligence control technology, it has brought new vitality to traditional electrical appliances. Intelligent electrical appliances are new electrical products developed on the basis of these related new technologies. On the one hand, it enables the electrical appliance to have an intelligent function, that is, it can automatically select and control the best mode for control and protection according to the operating state through sensing, reasoning, learning, and decision-making means; on the other hand, the electrical appliance can realize two-way communication with the central control computer. The informationization and automation degree of the distribution control system are improved.
As the degree of automation of power systems increases, intelligent appliances that require microprocessors as the core technology have a reliable reliability. Since intelligent electrical appliances are mostly operated in a harsh electromagnetic environment, the anti-interference ability of intelligent electrical appliances largely determines the reliability of the entire system. Generally, the intrusion system interference can not directly cause hardware damage, but often causes the microprocessor system program to run out of control, resulting in control failure, resulting in equipment and production accidents. Therefore, how to improve the anti-interference ability and reliability of the system is an important part of the development of microcomputer application system, and it is also one of the keys to the application and promotion of microcomputer. This paper combines scientific research experience to summarize the reliability design and anti-interference measures of some intelligent electrical control systems. From the overall solution, device selection, printed circuit board layout to software programming, anti-jamming technology is applied throughout the application process design process.
2 The main interference source interference in the microcomputer system is the disturbance caused by external noise and unwanted signals in the reception. Intrusion into the microcomputer control system by way of conduction and field coupling. There are many sources of interference, mainly including: electromagnetic radiation interference Electromagnetic radiation disturbance mainly refers to the radiation, absorption and modulation of electromagnetic fields on lines, wires and casings. Interference comes from inside and outside the application system. When the current changes greatly or the current is high, the main interference source that generates inductive coupling noise is the main interference source that produces capacitive coupling noise when the voltage changes greatly or the high voltage operates. The switchgear itself is also a very strong source of interference. When the switching electrical appliance is opened and closed, its main circuit current will generate strong electromagnetic radiation and the electromagnetic system operating overvoltage will form an interference signal. Generally, the voltage di/dt of the current di/dt is the source of interference.
A process channel is a path for information transfer between a forward interface, a backward interface, and a host or host and host. Interference from long-haul transmissions in the process channel is a major factor. Distortion, attenuation, and delay occur when the signal is subjected to series mode interference and common mode interference during transmission.
The power supply interference mainly comes from serious pollution of the AC power grid.
The noise of the power grid interferes with the microcomputer control system through the power supply circuit, which is one of the main causes of interference of the microcomputer control system. Experience has shown that good power system anti-jamming measures are the key to the success of microcomputer system applications. Common disturbances in the power supply system are: (1) overvoltage, undervoltage, instantaneous power loss (on the order of seconds); (2) surge, drop (on the order of milliseconds); (3) spike voltage pulse (on the order of microseconds) (4) Radio frequency interference (on the order of nanoseconds).
3 hardware system reliability technology 3.1 hardware system reliability design - ElectromagneticCompatibility) design: electromagnetic compatibility simply means that various electronic devices and systems can work in the same electromagnetic environment, not affected by the surrounding electromagnetic environment The impact does not affect the surrounding environment, nor does it cause the system performance to deteriorate or malfunction due to the electromagnetic environment, and the ability to work reliably according to the original design requirements. Consideration of electromagnetic compatibility issues in control systems must be throughout the entire process of product development.
According to the basic principle of electromagnetics, the formation of electromagnetic interference (EMI) must have three elements at the same time: electromagnetic interference source; electromagnetic interference coupling path; system sensitive to electromagnetic interference. The electromagnetic interference received by the microcomputer control system has internal and external interference.
Electromagnetic interference (EMI) is mainly propagated in two ways: Radiated mode energy is coupled to a victim device through a magnetic field or electric field; Conducted mode—Energy is transmitted along a power line or data line.
MPU' system reliability design: mainly considers the interference noise generated inside the system or the device itself, such as thermal noise, induced noise, spike noise and so on. In the MPU system, the clock signal, the reset signal, the interrupt signal, and the control signal are most sensitive to noise interference signals. The MPU system selects CPU chips with program memory, A/D conversion, and WDT circuits as much as possible. Its anti-interference ability is strong. Under the system performance requirements, low-speed and low-frequency devices are selected to reduce high-order harmonics. The peripheral circuit device selects digital circuit and CMOS device as much as possible, and its noise capacity is large; the peripheral device expansion tries to select the serial expansion mode as much as possible, and the bus is easy to be closed and the data transmission reliability is high.
Printed Circuit Board (PCB) Reliability Design: Printed circuit boards are an important means of electromagnetic interference propagation. Proper design of PCB, reasonable wiring is the most important measure to improve the electromagnetic compatibility of microcomputer systems. Printed circuit board design principle: separate EMC sensitive components and non-sensitive components, single point grounding to reduce common impedance coupling of grounding; grounding should be as thick as possible, network grounding system can be used; wiring should be as short as possible, no branch And sudden turn, which may cause reflections and harmonics; minimize the effective enveloping area of ​​the power line trace to reduce electromagnetic coupling; add decoupling bypass capacitors to the integrated circuit and op amp power pins; crystal and resonant capacitor wiring To be as close as possible to the CPU chip, the crystal case should be grounded.
Reliability design of regulated power supply: Mainly consider suppressing external noise such as impulse noise, arc discharge, and lightning wave.
The discrete components are used for conventional AC voltage regulation, isolation, filtering, DC voltage regulation, power supply decoupling, and spike suppression. Although they can suppress interference, they are not the best EMC solution. Strict systems can choose either a switching power supply or a standard power supply filter. The power supply filter has strong bidirectional suppression on the serial-mode interference and the common-mode interference signal. It not only effectively attenuates various external interferences, but also attenuates the external interference of the device itself, thereby improving the reliability of the application system and itself. Quality and compliance with electromagnetic compatibility requirements t 3.2 Hardware system anti-interference technology Hardware anti-interference is the most basic and most important anti-interference method of the application system. Generally, from the two aspects of prevention and resistance to suppress interference. The general principle is: suppress or eliminate the interference source; cut off the coupling channel of the interference to the system; reduce the sensitivity of the system to the interference signal. Specific measures include isolation, grounding, shielding, filtering, amplitude discrimination, and improvement of signal-to-noise ratio.
Isolation technology is primarily used for isolation of process channels. The optocoupler can effectively suppress spikes and various noise interferences and improve the signal to noise ratio. The optocoupler is used to isolate the microprocessor control system from the peripheral interface on the input and output channels.
The power isolation transformer should use a noise isolation transformer. The primary and secondary coils of the transformer are shielded by a shield to reduce the distributed capacitance between the primary and secondary, and the primary shield is connected to the ground to provide common mode noise, and the secondary shield is connected to the ground, so it is very high. The common mode rejection ratio can effectively prevent noise interference in the power grid from entering the control system.
Shielding technology The so-called shielding is the metal isolation between the two spatial regions, which is the most effective way to suppress electromagnetic field coupling. The shielding is divided into electrostatic shielding and electromagnetic shielding. Capacitive coupling can electrically shield the metal shell from the ground, and inductive coupling uses a low-resistance metal shell for electromagnetic shielding. The twisted pair used in long-distance communication is also a form of electromagnetic shielding, and its shielding effect increases as the number of twists per unit length increases. If the twisted pair wire is added with a metal woven mesh, the electrostatic induction can be overcome, so that the shielding effect is better. At low frequencies (<1MHZ), the shielded twisted pair is grounded at one end to avoid the current flowing through the shield caused by the ground potential difference caused by grounding at both ends. This current will seriously affect the shielding effect. Practice shows that the shielding effect is closely related to the grounding system, so every link of the grounding system should be emphasized.
Filtering Technology Power system interference sources are mainly higher harmonics. The power filter is an ideal low-pass filter that only passes the 50HZ fundamental wave in the power grid, and has a sharp attenuation of higher harmonics. It has strong two-way suppression of serial-mode interference and common-mode interference signals. It has been shown that the control system has a suitable power supply filter installed at the input of the power supply to effectively solve the fast transient burst test (EMC immunity test) coupled to the power supply line. There are two points to note when installing the power filter: (1) The input and output lines of the power supply filter should not be bundled together, and the distance from the output line to the transformer should be as short as possible; (2) The metal shield of the power supply filter must be connected to the chassis of the device. Ground reliably and with low impedance.
Suppressor) is a clamp type overvoltage protection component. When the voltage exceeds the breakdown voltage of the component, the TVS diode can change the high impedance between the two poles into a low impedance at a nanosecond speed, absorbing up to several kilowatts of surge power, so that the voltage between the two poles is clamped to a predetermined value, effectively Protect the electronic circuit. TVS is particularly suitable for secondary protection of low voltage circuits that do not require large energy bypass, such as power lines, signal lines, communication lines, and the like. Bipolar TVS and unipolar TVS transient voltage suppressors can be added to the input and output of the DC stabilized power supply to eliminate transient waves such as lightning, electromagnetic pulse (EMP), electrostatic discharge (ESD), and electricity on the power line. Transient (EFT), etc.
For the correct selection of TVS, please refer to the relevant product manual.
When the transmission line frame is used outdoors, the interface chip and even the whole system are often attacked by lightning. Even if an antistatic or lightning-resistant chip (MAX1487E, SN75LBC184) is used, such loss is inevitable. Therefore, it is also necessary to install a limit current limiting, clamping, high energy bleed function in one lightning protection device.
Grounding technology Microcomputer application system grounding methods are: floating ground, single point grounding and multi-point grounding; grounding structure: system ground, chassis ground, digital ground and analog ground. Good and correct grounding often eliminates or at least reduces various forms of interference. The grounding principle is single-point grounding; the digital ground and the analog ground are respectively connected to the power ground wire; the ground wire should be as thick as possible.
Noise desensitization technology With the development of chip technology, in hardware system design, we must pay attention to low power consumption, low noise, wide voltage, anti-£11 circuit, £50 suppression, 0 (:-0 (: isolation, fault protection) Chips such as Shutdown and Sleep are used to improve the reliability of the system itself.
Software system reliability design to improve the reliability of the operation of the microcomputer system mainly adopts two methods: one is to improve the reliability design of the hardware system to resist the influence of external interference; the second is to improve the reliability design of the software system to enhance the self-defense ability of the microcomputer system. Because the microcomputer system is inevitably subject to various unpredictable interferences during the operation, the system's faults and errors are objective. Some faults cannot be solved by hardware measures. Only software methods can be used to suppress and eliminate the effects. The main contents of software system reliability design are: system initialization, other reliability; system self-diagnosis and processing scheme; - reliability of system boundary parameters; safety of system program out of control without disturbance self-recovery; consequences of interference on microcomputer system There are: (1) the program runs and enters the endless loop; (2) tampering with the data information content; (3) the forward channel data acquisition error increases; (4) the backward channel control state fails. Therefore, software anti-jamming measures should be added during software programming to detect, intercept and correct the effects in time.
Self-diagnosis technology self-diagnosis is generally divided into: self-diagnosis, self-diagnosis and self-diagnosis.
Boot self-diagnosis content: RAM area data, timer function, read and write of mutual channel, etc. Check whether the RAM is correctly read or written or whether the RAM area of ​​the running process is damaged. If so, the RAM data is automatically restored. Check whether the mutual channel (such as 8155, 8250, and communication interface chip) is correctly read or written. Give an alarm prompt.
Periodic self-diagnosis content: zero drift self-test, automatic zero calibration, automatic compensation, etc. The performance parameters of the control system during operation will be affected by other factors, causing zero offset and drift, which will affect the accuracy of the measured data. Therefore, the subroutine such as zero drift self-test and automatic compensation should be automatically transferred during system program operation to realize the automatic calibration function.
Key-controlled self-diagnosis content: Set special self-diagnosis function through man-machine dialogue interface.
Program fault tolerance techniques include trap trapping, program rollback, instruction redundancy, WDT, timer warm reset, system reset processing, and more. It can be used to solve the program running or entering an infinite loop and can be restored to normal operation. This article is related to many of the articles and will not be repeated here. However, it is necessary to emphasize two points: First, pay attention to identify different reset states during system reset processing. For system hot start, first turn off the interrupt, clear the interrupt response flag, and set the I/O port to a safe state to avoid misoperation. The second is that the system is undisturbed and restored to the program module entrance when the loss occurs. When re-entering, the correctness of the important data (redundant data structure) and the security of the output port state (output port lock) must be guaranteed.
Information Redundancy Technology redundancy technologies include hardware redundancy, time redundancy, and information redundancy.
It is inevitable that errors occur in the information transmission process in the microcomputer system. Reliable and effective coding of information data is an important means to reduce the bit error rate. Information redundancy refers to adding redundant information codes according to a certain regularity in the transmission data sequence, making the original irrelevant data become relevant, and transmitting these redundant symbols together with the supervised code and related information codes. At the receiving end, according to the encoding rule of the transmitting end, the additional information symbol can automatically detect the error generated in the transmission and take corrective measures. The more general redundant information codes, the stronger the error detection and error correction capabilities.
Parity codes, accumulation sum codes, Hamming codes and cyclic check codes (CRC check) commonly used in microcomputer systems have strong error detection and correction capabilities. Among them, the cyclic check code is the most effective redundancy check method. It removes the information code by a specific generator polynomial at the transmitting end, and attaches the remainder to the information code as a redundant bit to form a new frame. The information code, if the transmission is error-free, this frame can be divisible by this particular generator polynomial, otherwise it is wrong. The error detection capability of the CRC check depends on the power of the generator polynomial. The international standard CRC-CCITT recommends the use of a 16th degree polynomial, which has an error rate of 100% for the information code within 16 bits, and other sudden error detection rates. It is 99.99%. Therefore, CRC verification has become the standard way of data transmission error control. The CRC check can be implemented by both hardware circuit and software programming. The hardware circuit uses a shift register as a dividing circuit.
The digital filtering technology digital filtering is to reduce the proportion of noise interference superimposed on the useful signal by a certain method of calculating or judging the program, thereby improving the quality of the collected signal. Digital filtering is implemented by the program. It does not need to add hardware. Different filtering methods or filtering parameters can be used depending on the signal, so it has the characteristics of good stability and strong function. Commonly used digital filtering methods are: arithmetic mean method to calculate the average value of data acquisition N times. Suitable for filtering with random interference signals.
The anti-pulse interference average method removes the maximum and minimum values ​​for data acquisition N times, and then calculates the arithmetic mean of the remaining N-2 data. Suitable for filtering with pulsed interference and random interference signals.
The first-order recursive digital filtering uses an algorithm to complete the RC low-pass filter algorithm. Since the frequency of the signal is low and the frequency of the noise or interference signal is high, the low-pass filter is suitable for filtering of random interference signals with periodic interference and high frequency.
Based on the reliability design of intelligent electrical control system, this paper comprehensively summarizes the hardware anti-interference and software anti-interference methods and measures. The above reliability design and anti-jamming technology have been applied in many products such as the medium voltage switch rejection joint temperature online monitoring device, intelligent contactor and distributed computer automatic meter reading system. The products have passed the Ministry of Power Technology. Standard tests have now been extensively promoted.
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