The universality of electronic product development must be considered. As a product, it should be applicable to a variety of occasions, that is to say, it is not only used in laboratories and offices, but also in the field, or other harsh working conditions. Occasionally, the surrounding conditions may be superior, or the environment may be frequently interfered with frequently. To survive, an enterprise must improve the quality and reliability of its products. Quality is life. How to ensure that the developed electronic products have extremely high anti-interference ability and reliability has always been a problem that developers have been dreaming about.
The power supply ripple in the vehicle power supply system will interfere with the normal operation of electronic equipment. The transient wave voltage or spike-like interference will cause such as MCU main chip crashes, high and low potential malfunctions in digital logic circuits, data processing errors and Failures such as control system failures cause inestimable losses to in-vehicle electronic products, so measures must be taken to suppress them.
The generation of automobile electromagnetic interference
There are many wires, coils and components in automotive electrical equipment, which have different capacitances and inductances. And any closed loop with inductance and capacitance will form an oscillating circuit. When the electrical equipment on the car generates sparks, it will generate high-frequency oscillations and be emitted into the air in the form of electromagnetic waves. The large impact is particularly serious. The center electrode of the spark plug has a certain capacitance to its shell, and the center electrode itself has a certain inductance. In this way, the capacitance and inductance of the spark plug form an oscillating circuit. When the spark plug is discharged, high-frequency oscillations occur. The wire connected to the spark plug acts as an antenna and emits the oscillation into space in the form of electric waves.
In fact, there are many similar circuits in automobiles. Like generator brushes, horns, regulator contacts, etc. These appliances will produce electromagnetic interference when they are working, and the oscillation frequency due to the different capacitance and inductance in the loop is also different. The frequency range is very wide, and the interference to other appliances is very large. Data show that the frequency range of electromagnetic interference on cars is 0.15 ～ 1000MHz. For example, when a car passes by, it has a significant impact on the reception of radios and televisions. The dot matrix road signs and information controllers produced by our company are precision products integrating electronic microcomputers. It is normal for automobile electromagnetic waves to interfere with their work.
Inductive loads produce interference that propagates along the power line
Various types of motors used in automobiles are inductive loads. For example, wiper drive motor; car start motor; air conditioner motor, etc. When the power supply of the inductive load is suddenly cut off, a reverse transient voltage will be generated. The larger the initial energy storage of the coil, the faster the shutdown speed and the higher the transient overvoltage. The measured result is generally -100V ～ 300V; the time is 0.2s ～ 0.5s.
Although this type of interference does not have continuity, the amplitude of its transient voltage is quite large, and the occasional occurrence will have a serious impact on the electronic modules of vehicle electrical appliances, or even damage.
The harsh operating environment of in-vehicle electronic products is mainly manifested in: there are many large energy-consuming devices on the in-vehicle system, and the start and stop of high power, especially large induced loads, will often cause spike interference of tens of volts or even hundreds of volts for the entire vehicle circuit. The undervoltage or overvoltage of the on-board circuit system often reaches 15% of the rated voltage, and some even more than 50%. The duration of this harsh power supply can sometimes be as long as tens of seconds or even minutes. For example, when the generator is running at high speed, the voltage of the whole machine will be in an overvoltage state, and when the car starts to ignite. The vehicle voltage will always be undervoltage. In the vehicle circuit, various signal lines and power lines are bundled together or take the same multi-core cable. Due to the capacitive coupling and inductive coupling between the wires, the electric field and magnetic field are coupled together. This is also the main cause of interference in the vehicle circuit system. One of the reasons, especially when the signal line and the car power supply line are in the same long pipeline, the interference is even worse. The performance of the multi-way switch or the car relay is not good, and it can also cause the interference of the adjacent channel signals of the car electronic products. In addition, the control changes of various electromagnetic components in the vehicle-mounted system will also interfere so that the electronic products cannot work normally. Load dump refers to the situation where the battery is suddenly turned off when the on-board generator is charging the battery or when the power is supplied to the large vehicle load. This is the typical current pulse caused by a sudden load drop. This sudden disconnection may produce transient voltage spikes of up to tens of volts or even hundreds of volts because the DC generator attempts to fully charge the battery. The transient voltage suppressor on the DC generator usually clamps the bus voltage between 12V or 24V of the on-board battery, thus causing a transient spike of hundreds of V on the power supply main end of the downstream electronic product of the DC generator. In addition, the changes in the temperature and humidity on the site have also triggered the reliability of in-vehicle electronic products.
There are various interferences encountered by on-site electronic products during on-site operation. The specific situation is analyzed and different measures are taken for different interferences. This is the principle of anti-interference. This flexible strategy and universality are undoubtedly contradictory. The solution should be a modular approach. In addition to basic components, for different operating occasions, electronic products can be equipped with different options to effectively resist interference and improve reliability.
Peak interference suppression technology
The most serious harm to the normal operation of vehicle-mounted electronic products is the peak pulse interference of the vehicle. The electrical equipment that produces spike interference are: starting motor, automobile air conditioning generator, battery charging generator, relay contactor, and vehicle-mounted inverter circuit Ballast or inflatable lighting. Peak interference often causes the microcomputer system in intelligent vehicle electronic products to have program or crash for such problems. It can be suppressed by a combination of hardware and software.
1 The impact of peak interference with hardware lines
There are three common methods as follows: or in the power input terminal of vehicle electronic products, a jammer designed according to the principle of balanced spectrum is connected in series to distribute the peak voltage concentrated energy to different frequency bands, thereby reducing its destructiveness, or in the vehicle A super isolation transformer is added to the power input end of the electronic product to suppress the spike pulse using the principle of ferromagnetic resonance; or a varistor is connected in parallel to the input end of the DC power supply of the electronic product to reduce the resistance value when the spike pulse arrives, reducing the separation of the electronic product from the power source The resulting voltage, thereby weakening the effects of interference.
2 Using hardware and software combined watchdog technology to suppress the impact of spike
The software arrangement is as follows: as long as the timer expires, the CPU accesses the timer once and allows the timer to restart timing. When the normal program runs, the timer will never generate an overflow pulse and the spike pulse will not work. Once the spike interference occurs, the CPU will not access the timer before the timer expires. Therefore, the signal will appear when the timing is up, causing an interruption during the system reset and ensuring that the intelligent instrument returns to the normal procedure.
3 Noise interference suppression technology
Broadly speaking, all useless signals are called noise. Noise exists objectively. In a sense, the process of developing electronic products is a process of fighting noise from beginning to end.
4 Shielding technology
According to the occurrence of mutual interference between the electrical coupling, magnetic coupling and electromagnetic coupling between the wires, in the wiring of in-vehicle electronic products, it should be done: the feeder of strong electricity must go alone, and must not be mixed with the signal line; try to make the strong signal line It is orthogonal to the weak signal line; it can't be avoided by the flat track. For the interference caused by the electric field coupling, the non-mesh aluminum foil shielded cable is used and the exposed portions of the leads at both ends are short. For interference caused by coupling, twisted pairs are used to equalize the areas of small loops with opposite normals, so that the effects of magnetic field interference cancel each other; for interference caused by electromagnetic coupling, twisted pairs that combine the advantages of both twisted pairs and coaxial cables can be used Shielding pair, in addition to adding a shield around the interference source and grounding the shield at one point, the interference source formed by the electric field can be shielded, so that it does not interfere with adjacent wires or circuits and can inhibit the magnetic field from causing weak signal circuits. Interference.
5 Filtering technology
It is recommended to use LC resonance filter circuit for vehicle power supply system. At least two-stage filtering measures are adopted, and the first section adopts the mutual inductance filtering method. The second stage adopts the resonant filtering method composed of LC to strictly control the interference of the spike pulse on the electronic products of the subsequent stage of the circuit.
6 Electrical isolation technology
In order to prevent various interference signals generated by high voltage, high current, high power or transmission lines from entering the inside of the intelligent instrument and preventing normal operation, the information transmission path can be electrically isolated, that is, between the two parts of the line before and after isolation There is no electrical connection, which is electrically independent of the two systems. The independent power supply and reference potential are not related to each other and rely on non-electrical methods (such as magnetic and optical) to transmit information.
(1) Method of magnetic isolation
Common types of magnetic isolation include: isolation transformers, current regulators, various voltage and current transformers, and modulation touch isolation amplifiers. It can be used to transmit judgment signals of industrial level, and can also be used to transmit analog signals of AC, gradual change or even DC, to achieve ideal transmission in the low frequency range, but there are also slow speeds and large power consumption, which will cause loss of higher frequency signals Shortcomings of the frontal earness of the switching signal drop.
(2) Optical isolation method
The method of optical isolation is to use optical transmission information for electrical isolation. Because the electrical insulation performance of optical isolation is excellent, the signal transmitted by the optical cable will not be affected by strong magnetic fields, etc., so it is an ideal isolation method. Common optical isolation devices include: photoelectric switch, photoelectric isolator, optical cable, optical trigger thyristor and analog signal photoelectric isolation device. Because of its small power consumption, simple structure, and reliable operation, this method should be preferred for the development of smart instruments.
7 Other anti-interference technologies
(1) Voltage stabilization technology
At present, there are two kinds of regulated power supplies commonly used in the development of intelligent electronic products: one is the series-regulated power supply provided by the integrated voltage-stabilizing chip, and the other is the DC-DC isolated regulated power supply. Normal work is effective. However, it is recommended that the company adopt the latter, because the DC-DC type regulated power supply can make the peak pulse strictly controlled at the primary of the switching transformer of the inverter power supply. Although in-vehicle electronic products are common ground, they have been strictly controlled on their VCC positive power supply. The ground potential in the automotive circuit is constant, so signals such as spikes and other low-frequency pulse interference will not be effective for low potentials.
(2) Suppression of common mode interference technology
Using a differential amplifier, increasing the input impedance of the differential amplifier or reducing the internal resistance of the signal source can greatly reduce the impact of common mode interference.
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