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Automotive electromagnetic shielding instructions, conductive shielding materials introduction

Author:admin   AddTime:2021-03-16

With the popularization of smart devices in cars in the current society, the magnitude of electromagnetic interference has further increased, and electromagnetic interference in cars has become a driving force for cars.

One of the hidden dangers. For this reason, we will analyze the harm of electromagnetic interference and the possibility of using conductive cloth, conductive foam and other shielding materials to prevent it.

1. Electromagnetic interference phenomenon in the car
The electromagnetic interference generated by the car will cause mutual influence inside the car. Examples are as follows:
 Example 1. A certain mid-to-high-end car has a high-performance ABS system. In a live test, the prototype car encountered a rainy day, and the wiper was activated. At a certain speed, the ABS suddenly lost its function.
Example 2: A certain type of domestically produced mini-car, its generator regulator is often easily broken down and damaged. After investigation, this kind of damage is likely to happen 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, which will generate interference pulses in the power system and make some electronic components unable to work normally. , Even damaged.
Example 3, an airbag developed and produced in China, suddenly detonated on an automobile assembly line. After investigation, it was found that the electronic detonation controller of the airbag could not withstand the strong environmental radiation electromagnetic field, and it would malfunction when electrostatic discharge occurred.

2. EMI hazards and characteristics of automotive electronic equipment
Industrial development not only brings some tangible pollution that can be recognized by the senses, such as water, air and noise pollution, to the living environment of people. However, with the development of electronic technology, especially the popularization and application of digital circuits, mobile communications and switching power supplies, there has been an invisible pollution that cannot be felt by the senses. This 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 itself, but also threaten human health and safety.
Various electrical appliances on modern cars work differently, and they invade each other in different ways. Generally, all automotive electrical appliances are compatible, that is, they can work together in the car without interfering with the normal operation of other electrical appliances, and they also have the ability to resist interference from other electrical appliances.
For the circuit of automotive electronic equipment, the oscillations in the circuit triggered by any factor will be emitted in the form of electromagnetic waves through wires, etc., which not only interferes with radios and communication equipment, but also affects the electronic systems with high-frequency response characteristics on the car. Will produce electromagnetic interference. At the same time, radio waves emitted by radio equipment such as transceivers, radars, and radio stations outside the car will interfere with the instruments on the car and make the electronic control device out of control. Therefore, the application computer (controller) in the car should have good electromagnetic shielding measures. Once the shielding is damaged, it will also cause abnormal operation.

Principles of electromagnetic shielding and shielding materials
 
Characteristics of electromagnetic interference propagation in the car:
(1) The inductive load produces interference conducted along the power line. Various inductive loads used in automobiles, such as: wiper drive motor, automobile starter motor, heater motor, 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 turn-off speed, and the higher the transient overvoltage. Generally, Uc is 100~300V; ts is 0.2~0.5s. Although this kind of interference does not have continuity, its transient voltage amplitude is quite large, which will seriously affect or even damage the electronic module. The breakdown of the generator regulator is a serious consequence of this reverse transient voltage.
(2) The interference of electrostatic discharge on the 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 feeling of discharge. When using a car, this kind of electrostatic discharge will inevitably produce electrostatic discharge interference. The characteristics are: high Voltage, short time, tiny current. The degree of interference is huge, which will cause some electronic control units to malfunction, and severely damage the electronic units.
(3) Mutual coupling interference between components or cables. In cars, various cables are often bundled into a bundle and arranged along the inside of the car. Transient interference in the power line will merge into the signal line or control line to form a differential mode signal, which will affect the electronic modules such as ECU in the car.
(4) Radiation interference. The electromagnetic wave radiation form of interference energy, the frequency range is 150kHz~1000MHz. The EMI interference sources of automotive electronic equipment are:
① Ignition system, its interference is shown as a rhythmic pop or ticking sound in the receiver audio, and the pitch 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 resistance spark plugs and wires.
At present, most cars are equipped with resistance spark plugs and wires as standard. Usually replacing the spark plugs and wires with new ones will help reduce noise, because a lot of noise stems from the failure of the ignition system components.

②The charging system, including the alternator, is controlled by a solid-state voltage stabilizer. Since the alternating current is only rectified in the alternator and not filtered, the output has ripples. The noise of the charging system is transmitted to the equipment through the car wiring and affects the audio part of the receiver and transmitter. The noise can be distinguished from the whine 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 noise of the charging system are related to the engine speed and the load of the charging system. When the light is turned on, the load of the charging system increases, and you can find that the whine is louder. At this time, check whether the connection between the alternator and the battery is corroded or has poor contact, and whether the solid-state voltage stabilizer is in good condition. If they are normal, use 0.47μF and 0.01μF capacitors in parallel, and filter between the output and ground.

Since cars use multiple different motors, these motors may generate EMI, and it is difficult to determine which is the problem from the noise. It is generally expressed as a crackling sound, and there is also a whining sound similar to the charging system. The diagnosis of motor interference must rely on special instruments. Interference can not only be conducted, but also radiated. Therefore, filter processing should be done near the interference source. The simple processing method can use conductive cloth, conductive foam and other shielding materials for processing. Generally, special equipment such as filters are used.

The microprocessor (microcontroller) used in the car needs to be driven by a clock. The clock generation circuit is an oscillating circuit. Since the oscillation waveform is a square wave, its harmonic frequencies are rich and can be extended to very high frequencies, so the receiver is likely to be affected by interference signals with equal frequency intervals, or it can be heard in the entire band Broadband digital noise. You can use the receiver to adjust the interference frequency to detect which control board has the problem, and then take the method of adding a shielding cover or properly grounding the shielding cover to reduce the interference. In addition, putting a magnetic ring on the wire can also help reduce the interference. . The electronic equipment of the car will affect the radio equipment, and the transmitting equipment will also affect the electronic equipment of the car. It is necessary to ensure that the power line, antenna feed line and the wiring and electronic system of the car are as far away as possible. Install the antenna correctly, preferably on the roof or at the back of the car. Try to make the standing wave ratio (SWR) of the large-line system the lowest. Check whether the antenna feeder shielding is good and whether the shielding net is dense enough.

3. Classification and causes of radio interference
Radio interference refers to radio interference in the radio frequency (9kHz~3000GHz) frequency band. According to the nature of the interference source, radio interference is divided into natural interference (from natural phenomena, which is uncontrollable) and man-made interference (from machines or other human devices, which can be controlling). Man-made interference can be divided into two types: radio equipment interference and non-radio equipment interference.

Non-radio equipment interference includes industrial, scientific research, medical and other electrical equipment interference. Power line interference and other radio equipment interference mainly include:

(1) Co-channel interference. Any interference sent by other signal sources that has the same frequency as a signal and enters the intermediate frequency passband of the receiver in the same way is called co-channel interference. When there is a carrier frequency difference between the two signals, it will cause beating interference; when the modulation of the two signals is not large or there is a phase difference, it will also cause distortion interference. The larger the interference signal, the smaller the output signal-to-noise ratio of the receiver. When the interference signal is large enough, it can cause jamming of the receiver. This kind of interference is mostly caused by too small co-frequency multiplexing distance.

(2) Let frequency interference. Any signal in or near the radio frequency passband of the receiver, the interference caused by falling into the intermediate frequency passband after frequency conversion is called adjacent frequency interference. This interference will reduce the signal-to-noise ratio and sensitivity of the receiver; strong interference signals can cause blocking interference in the receiver. This kind of interference is mostly caused by the technical indicators of radio equipment that do not meet national standards.

(3) Out-of-band interference. The interference caused by the spurious radiation of the transmitter and the spurious response of the receiver is called out-of-band interference.

① The stray radiation interference of the transmitter. In the low frequency bands of VHF and UHF, mobile communication equipment, especially base station transmitters, mostly use crystal oscillators to obtain higher frequency stability. This kind of interference is usually caused by the excessive number of stray radiation of the transmitter due to factors such as the large number of frequency multiplications, the poor selectivity of the output circuit of the frequency multiplier, and the poor shielding isolation between the frequency multipliers.

② The spurious response of the receiver. The receiver not only receives useful signals, but also unwanted signals. The ability to "response" to unwanted signals is usually called spurious response, which is usually caused by the spurious radiation of the transmitter and has a direct relationship with the local oscillator frequency purity of the receiver itself and the selectivity of the high-amplification circuit. .

4. Measures to reduce the car's radio interference
The most serious interference from the car to the radio receiver is the ignition system, and the interference radius can reach several hundred meters. The suppression of electromagnetic interference should adopt different suppression methods according to the characteristics of different interference sources. Secondly, consider the propagation path of interference. The way of interference is: through the cables, antennas or various wires of the power supply system, through coupling, direct radiation of electromagnetic waves in space, etc. Interference suppression should consider the cost. The general approach is to limit the interference noise generated by the interference source to the specified reasonable range; at the same time, the interfered body should have a certain ability to resist interference, so as to achieve mutual coexistence and non-influence.

For the interference from the power supply system in the car, a simple and effective method is to use the battery as a very low impedance, large-capacity transient voltage suppressor to absorb the interference energy generated by various transient voltages. The battery cable should be well connected. If the negative pole is grounded, ensure that the grounding resistance is the smallest. Should try to ensure that the line resistance R0 reaches the minimum value, or even zero. For interference caused by coupling between cables, the best way is to separate the ECU control line or signal line from the power line to reduce the intrusion of interference signals caused by coupling. In addition, the use of shielded cables, conductive cloth, and conductive foam is also a good way to prevent external electromagnetic interference from invading the control lines and signal lines. For the interference caused by inductive load, the suppression method can be to use a capacitor of appropriate value in parallel to eliminate the reverse overvoltage.

The reason for the interference is that the self-inductance and capacitance of the wires, coils and other parts of the electrical equipment system form an oscillating circuit. When the discharge is in the form of sparks, high-frequency oscillations are generated, and electromagnetic waves are emitted into the air through high-voltage wires (or wires) to cut and receive The antenna of the machine causes interference. The following methods are used on modern cars to prevent this interference:

(1) Install interference reducing resistors. In circuits that form high-frequency oscillations, for example, in the high-voltage circuit from the distributor to the ignition coil and the distributor to the spark plug, the interference reduction resistor of 6000~15000Ω is connected in series, because the resistance of the oscillation circuit is large enough to prevent it from Oscillation discharge occurs, electromagnetic waves no longer occur and interfere with the structure of the radio damping resistor.

(2) Install interference reducing capacitors. Connect a capacitor with a capacity of 0.5~0.1μF in parallel between all contact points that may cause spark discharge to absorb sparks, avoid reflection of high-frequency oscillating electromagnetic waves, and avoid oscillating discharge.
(3) Install metal shielding. Wrap all electrical appliances and conductors that are easy to emit electromagnetic waves with metal mesh or shielding cover. In this way, when electromagnetic waves or high-frequency electromagnetic oscillations encounter the metal screen wall, electromagnetic induction generates eddy currents in the metal screen wall, so that the electromagnetic waves are consumed in the heat effect of the eddy currents and cannot be emitted outward, thereby avoiding interference with radio waves. However, to avoid interference well, it is necessary to completely cover up, prevent leakage, and make all joints contact with the frame well. In addition, the copper wire braid for the engine body is reliably connected to the metal part of the cab, and it can also be used as a metal shield. In order to prevent interference, the above methods can also be used in combination. See the anti-jamming device when equipped with high-sensitivity radio equipment, and see the anti-jamming system of the car equipped with radio.
Anti-interference device when equipped with high-sensitivity radio equipment Car anti-interference system equipped with radio

5. Examples of automobile failures caused by electromagnetic interference

In the automobile electronic control system, the weak electric signal below 1V generated by the sensor is easily affected by electromagnetic interference and becomes an error signal, so shielded wires are installed to prevent electromagnetic interference. Once the shielding wire is damaged, the ECU will receive the disturbed signal and lose its normal control, and the alarm light of the self-diagnostic system will flicker.



Example 1 A Chevrolet car installed a tweeter near the oxygen sensor. Soon after the power cord was taken from the ignition switch, it was found that the engine warning light had alarms from time to time. The fault code was 13 (oxygen sensor) and the oxygen was measured. The output voltage of the sensor keeps changing between 0.1 and 0.3V, indicating that the oxygen sensor is normal, but when the horn is pressed, the output signal of the oxygen sensor is chaotic, and the operation of the engine is also abnormal instantaneously. After the speaker is removed, the fault is rectified. It turns out that this is a typical example of man-made interference sources. There are certain design requirements for the installation location and wiring layout of automotive electrical components. Random installation of alarm and anti-theft devices will cause abnormal operation of the electronic control system.

Example 2 A Toyota Crown 3.01 sedan has driven 120,000 km. After overhauling the engine, the fault warning light alarms after only a few minutes of operation. The read fault code is 55 (ie, the explosion-proof sensor is faulty), and the code is removed before starting. Engine, the fault remains. Measure the working voltage of the explosion-proof sensor and it is a normal pulsating voltage below 0.5V, indicating that the explosion-proof sensor is working normally. When testing along the line, it is found that the shielding wire is open. Connect the shielding wire and restart the engine after removing the code. exclude.

The control system failure caused by electromagnetic interference mainly occurs during the operation of the engine. Once the engine stops running, the failure phenomenon disappears by itself. After the fault self-diagnosis system alarms, if the fault code is detected, it can be implemented smoothly due to the memory function of the ECU. If the fault of the control system is indeed caused by electromagnetic interference, statically measuring the voltage signal of the component will find that the sensor, circuit, ECU, etc. are all normal.

Example 3 A Toyota Camry car driving tens of thousands of kilometers, the ABS fault self-diagnosis system alarms, and the fault codes are 31 (front right wheel speed sensor), 32 (front left wheel speed sensor), 33 (rear stone wheel speed sensor) , 34 (rear left wheel speed sensor), considering that the 4 wheel speed sensors and related lines are less likely to be damaged at the same time, because other control systems of the whole vehicle are working normally, the possibility of ECU failure is also small, and finally it will fail The reason is focused on electromagnetic interference. After searching, it was found that the shielding wire of the wheel speed sensor was seriously damaged. After repairing, clear the fault code, and everything is normal in the road test. It turns out that because the ABS sensor of this car is electromagnetic, the signal voltage generated by the low-speed area is extremely weak, and the ABS needs to rely on highly sensitive signal voltage.

The pressure can be adjusted by the ECU to adjust the wheel braking force. In order to ensure the accuracy of the signal, there is a shielding net on the wheel speed sensor. Once the shielding net is damaged, the high-frequency electromagnetic waves on the car will interfere with the normal operation of the wheel speed sensor, resulting in ABS failure or malfunction or malfunction. Self-diagnosis.

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