Electromagnetic interference phenomenon in automobiles and measures to reduce automobile's radio interference

The source of electromagnetic interference in automobiles is not just the ignition system, but also various electronic and electrical equipment used in vehicles. Interference not only affects the radio equipment outside the vehicle, but also has adverse effects on various electronic components inside the vehicle.

1. Electromagnetic interference phenomenon in automobiles

The electromagnetic interference generated by the car will cause mutual influence inside the car, for example:

Example 1: A mid-to-high-end sedan has a high-performance ABS system. During a live test, the prototype car encountered rainy weather and the wipers were started. When the vehicle was running at a certain speed, the ABS suddenly lost its function.

Example 2: The generator regulator of a certain model of mini-car produced in China is often easily damaged by breakdown. After investigation, it was found that this damage phenomenon is easy to occur when the wiper is working. The main reason for this phenomenon is that the wiper drive motor is an inductive load. When the power is cut off, a reverse current will be generated and transmitted to the power supply system through the power line, thereby generating interference pulses in the power supply system, causing some electronic components to malfunction or even be damaged.

Example 3: A domestically developed and produced airbag suddenly exploded on a car assembly line. It was found that the electronic detonation controller of the airbag could not withstand the strong electromagnetic field of environmental radiation, and would malfunction when electrostatic discharge occurred.

1) Characteristics of electromagnetic interference in cars

The characteristics of electromagnetic interference inside the vehicle are different from the interference of the vehicle to the outside. Electromagnetic interference inside the vehicle can be transmitted through various connecting cables, and can also be transmitted in the form of coupling and space auxiliary emission. Typical forms include: interference conducted along the power line; interference of human electrostatic discharge on electronic components; interference energy radiated through space, etc. The following analyzes the characteristics of some typical interference sources.

2) The engine ignition system generates interference that is conducted along the power line

The circuit diagram of the engine ignition system is shown in Figure 1. The sensor obtains the ignition signal Va, and the drive circuit generates an on-off pulse current Ib in the primary of the ignition coil. The secondary of the coil generates a high-voltage pulse to discharge the spark plug and ignite the engine fuel mixture to work. When the primary circuit of the coil is on and off, the primary winding will generate a transient voltage, and the secondary winding will generate a high voltage to discharge the spark plug. The residual energy forms a high-frequency electromagnetic wave and radiates into space. The transient voltage in the primary circuit is transmitted to the power supply system along the power line, interfering with the power supply system and generating a fluctuating voltage △V. As shown in Figure 2. Generally speaking, the situation is different. The experimental measurement shows that △V is 2~4V.

Various electronic control units used in automobiles require a stable power supply voltage to work properly. When voltage fluctuations (such as △V) occur in the power supply system, it will affect the normal operation of the electronic module.

3) Inductive loads generate interference along the power line

All types of motors used in cars are inductive loads. For example: wiper drive motor; car starter motor; heater motor, etc. The characteristic circuit of this type of load is shown in Figure 3. When the power supply of the inductive load is suddenly cut off, a reverse transient voltage Vc will be generated, as shown in Figure 4. The greater the initial energy storage of the coil, the faster the shutdown speed, and the higher the transient overvoltage. According to the actual measurement results, Va is generally -100V~300V; is is 0.2s~0.5s.

Although this type of interference is not continuous, its transient voltage amplitude is quite large, and its occasional occurrence will cause serious impact on the electronic module, or even damage it. The generator regulator breakdown damage mentioned in the previous article is a serious consequence caused by this reverse transient voltage.

5. Interference of electrostatic discharge on electronic components in the car

The human body generates static electricity, especially in the northern part of my country, where the climate is dry in winter and the human body is prone to static electricity. Static electricity on the human body will be released when it encounters some conductors. There is direct discharge, which people cannot feel. When static electricity is stored to a certain extent, it will be discharged through the air, and even sparks will be generated, and people will have a strong sense of discharge. This static discharge phenomenon is inevitable when people use cars.

2. EMI hazards and characteristics of automotive electronic equipment

Industrial development not only brings some visible pollution to people's living environment, such as water, air and noise pollution. However, with the development of electronic technology, especially the popularization of digital circuits, mobile communications and switching power supplies, there is another invisible pollution that cannot be felt by the senses, which is electromagnetic interference (EMI), or electromagnetic noise.

The electromagnetic waves radiated and leaked by electronic equipment not only cause serious interference to the electronic equipment themselves, but also threaten human health and safety.

The various electrical appliances in modern cars work in different ways and can interfere with each other in different ways. Usually all automotive electrical appliances are compatible, that is, they can work together in the car without interfering with the normal operation of other appliances, and they also have the ability to resist interference from other appliances.

For the circuits of automotive electronic equipment, any oscillation in the circuits stimulated by any factors will be emitted in the form of electromagnetic waves through wires, etc., which will not only interfere with radios and communication equipment, but also cause electromagnetic interference to electronic systems on the vehicle with high-frequency response characteristics. At the same time, radio waves emitted by radio equipment such as transceivers outside the vehicle, radars, radio stations, etc. will interfere with the instruments on the vehicle and cause the electronic control device to lose control. Therefore, computers (controllers) used in automobiles should have good electromagnetic shielding measures. Once the shielding is damaged, it will also cause abnormal operation.

Characteristics of electromagnetic interference propagation in vehicles:

(1) Inductive loads generate interference along the power line. Various inductive loads used in cars, such as wiper drive motors, car starter motors, heater motors, etc. When the power supply is suddenly cut off, a reverse transient voltage Uc will be generated. The greater the initial energy storage of the coil, the faster the shutdown speed, and the higher the transient overvoltage. Generally, Uc is 100~300V; ts is 0.2~0.5s. Although this type of interference is not continuous, its transient voltage amplitude is quite large, which will have a serious impact on the electronic module and even damage it. The breakdown damage of the generator regulator is a serious consequence caused by this reverse transient voltage.

(2) Interference of electrostatic discharge on electronic components in the car. It will be released when it encounters a conductor. When static electricity is stored to a certain level, it will be discharged through the air, and even sparks will be generated. People will have a strong sense of discharge. When using a car, this electrostatic discharge phenomenon will inevitably produce interference characteristics of electrostatic discharge: high voltage, short time, and small current. The interference impact is huge, which can cause some electronic control units to malfunction, and in severe cases, damage the electronic units.

(3) Mutual coupling interference between components or cables. In cars, various cables are often bundled together and arranged along the inside of the car. Transient interference in the power line will couple into the signal line or control line to form a differential signal, which will affect the electronic modules such as the ECU in the car.

(4) Radiated interference. The electromagnetic wave radiation form of interference energy has a frequency range of 150kHz~1000MHz. The sources of EMI interference in automotive electronic equipment are: ① Ignition system, whose interference is manifested as rhythmic popping or ticking sounds in the receiver audio, and the tone is directly related to the engine speed. When the engine load increases, the interference amplitude also increases. The usual way to solve ignition noise is to install resistor spark plugs and wires. At present, most cars are equipped with resistor spark plugs and wires as standard. Usually, replacing new spark plugs and wires will help reduce noise, because much noise comes from ignition system component failure. ② Charging system, including AC generator, is controlled by solid-state regulator. Because AC power is only rectified and not filtered in the AC generator, there is ripple in the output. Charging system noise is transmitted to the device through the vehicle wiring, affecting the audio part of the receiver and transmitter. This noise can be identified from the humming sound in the receiver audio or the transmitted signal. A more accurate method is to temporarily disconnect the charging system. The tone and intensity of the charging system noise are related to the engine speed and charging system load. When the light is on, the load on the charging system increases, and the humming sound becomes louder. At this time, check whether the connection between the alternator and the battery is corroded or in poor contact, and whether the solid-state voltage regulator is in good condition. If both are normal, connect 0.47μF and 0.01μF capacitors in parallel between the output and the ground line for filtering.