Electromagnetic compatibility (EMC, Electro-MagneTIc CompaTIbility) means that Electrical and electronic products can work normally in an electromagnetic environment and do not generate excessive electromagnetic interference (EMI, Electro-Magne TIc Interference) in other products in the environment. This involves two requirements: one is to require the product to withstand the external electromagnetic interference; the other is to require the product to be in the normal operation process, the electromagnetic interference generated by the product to the surrounding environment can not exceed a certain limit. The electromagnetic compatibility of automobile electrical appliances means that in the vehicle and its surrounding space, under the condition of available spectrum resources during the running time, the vehicle itself and the surrounding electrical equipment can coexist without causing degradation.
The normal and reliable operation of ABS anti-lock braking system, engine fuel ignition electronic control system, GPS global positioning system and other electronic equipment must pay attention to the design and research of electromagnetic compatibility technology, which can be from traditional automotive electrical appliances (such as starter, scraping Water motor, flasher, air conditioner starter, fuel pump, etc.), the connection of the alternator cable and intermittent cutoff are also the sources of interference for generating high-power electromagnetic radiation, but other equipments have more impact on their operational reliability than those. Small-power high-band electronic devices are small. Nowadays, the alternator's regulator, like the electronic ignition system, has been designed to integrate modular structures and is also subject to interference immunity.
2 Automotive Electromagnetic Compatibility Introduction With the increase in the number of automotive electronics and the increasing distribution of complex electronic modules throughout the vehicle, engineers are facing increasingly severe electromagnetic compatibility design challenges. The problems are mainly in three aspects:
How to minimize electromagnetic susceptibility (EMS)? To protect electronic products from harmful electromagnetic radiation from other electronic systems such as mobile phones, GPS or infotainment systems.
How to protect electronic products from the harsh automotive environment? This includes disturbances caused by large changes in the supply voltage, heavy loads, or inductive loads such as lights and starters.
How to minimize the EME that may affect other automotive electronic circuits?
These issues will be even more challenging as system voltages, the number of in-vehicle electronic devices, and the frequency increase. In addition, many electronic modules will interface with inexpensive, less linear, and highly offset low-power sensors that operate in small-signal states, and the effects of electromagnetic interference on their operating conditions can be catastrophic.
With the increase in electronic equipment in modern automobiles, more and more good designs are required to ensure compliance with electromagnetic compatibility standards. At the same time, as integration increases, automotive design engineers need system-on-chip ASIC and ASSP solutions to replace multiple discrete components.
3 Sources, transmission routes and detection of electromagnetic interference
3.1 Source of electromagnetic interference
The electromagnetic interference inside the car mainly comes from various actuators in the engine ignition system, power supply system, instrument system and control system. The spectrum of the interference signal is usually between 0.1 and 1000 MHz, and the electronic equipment inside and outside the car will be Have a certain impact. The most serious disturbance is the engine ignition system. The ignition system usually works in the pulse ignition state. The Fourier series can be used to decompose the ignition pulse into many frequency components. The ignition coil will generate high frequency electromagnetic radiation under the action of the higher frequency components of these harmonics; the center electrode of the spark plug The high-frequency ignition pulse can be equivalent to an inductor with a certain inductance, which can be equivalent to a capacitor between its housing, thus forming an LC parallel circuit, so it will be a certain harmonic in the ignition pulse. A high-frequency oscillation is formed to radiate electromagnetic waves externally. In addition, the high-voltage connection from the ignition pulse output end of the engine ECU to the ignition coil and the ignition coil to the spark plug may radiate electromagnetic waves. The longer the wire, the longer the spark duration. The more engine cylinders and the higher the speed, the higher the number of sparks. More, the electromagnetic interference generated is also stronger.
Secondly, various relays, solenoid valves, contacts and voltage regulators in the car will also produce different levels of electromagnetic interference. The spark generated by the relay and solenoid valve is the main source of interference. For example, EFI main relay, fuel pump control relay, solenoid valve (ISC valve) in idling control, EGR valve and other solenoid valves are mostly directly or indirectly controlled by the CPU in the ECU, so the interference they generate will directly affect the ECU. normal work.
In addition, it is the interference of various motors. The electric motor in modern automobiles is mainly used as an actuator for various electric functions, including generators, starters, wiper motors, electric door and window motors, ventilation motors, heater motors, and the like. Some of these motors use silicon rectifying AC motors, and some use DC motors with commutators. These motors produce strong electric sparks between the carbon brushes and the commutator, and the spectrum range is wide, so Electromagnetic interference occurs over a wide frequency range. Especially when such a motor has poor contact, insulation damage and shaft misalignment, the electromagnetic interference generated by it will be greatly enhanced.
3.2 Classification and propagation of electromagnetic interference Electromagnetic interference (EMI) can be roughly divided into: 0.02 ~ 2kHz, harmonic interference; 2 ~ 300kHz, conducted interference or carrier frequency interference; 0.3 ~ 300MHz, RF interference; 0.3 ~ 300GHz, microwave interference. From the way of interference, 0 to 300 kHz is accompanied by near-field induced interference caused by conducted interference and alternating electromagnetic field; both radio frequency and microwave interference are radiated interference in the far field. When the length of the device and the wire are shorter than the wavelength, the main problem is conducted interference; when their size is longer than the wavelength, the main problem is radiation interference.
3.3 Detection of electromagnetic interference The automotive circuit system consists of a battery and a rectified alternator as the core power source. The vehicle body acts as a shared grounding, and the various electrical devices are connected in parallel. The connected harness causes conduction interference between the electrical appliances; there is induced interference between adjacent wires; because of the antenna effect, there is radiation interference between adjacent conductors. Therefore, electromagnetic interference combines three ways to cover a wide interference frequency.
The spectrum analyzer is used for electromagnetic compatibility analysis, and the electromagnetic interference is intuitive and quantifiable. It is a 30-1000MHz narrow-band sweep receiver that can receive energy in a certain frequency range at a certain time and record the amplitude of interference in different frequency bands. The disadvantage is that for a very short time range, the spectrum range is very wide. For example, the transient pulse interference such as electrostatic discharge cannot reflect the actual interference situation in a wide frequency band. Often only a few units can be enabled for observational analysis at the same time.
4 suppression of electromagnetic interference In modern automobiles, various electronic control systems may be independent of each other and dispersedly installed at different positions, so the above-mentioned circuit interference will inevitably exist. In order to reduce this interference, the following measures can be taken:
1. Power supply decoupling. Decoupling circuit is set up at the output end of the common power supply to make the power supply itself have certain ability to suppress interference; different power supply decoupling filter circuits are adopted for different systems (such as different ECUs) to reduce the mutual power supply between the systems. Circuit interference, at the same time in the circuit design, should be connected to the decoupling filter capacitor near the power supply terminal of the CPU and other integrated circuits to reduce the circuit interference introduced by the power lead resistance; the circuit between different current loops in the same circuit Sexual interference should be grounded at one point, which should be fully considered when designing the PCB.
2. Limit the coupling impedance to make it as small as possible. To do this, the wire resistance and its equivalent inductance must be reduced. When designing the PCB, the power supply lead and the common ground should be properly distributed and should be straight, short and thick; the input and output leads of the signal should be as short as possible. Straightening; grounding the analog signal processing circuit such as a temperature sensor and the ground of a digital processing circuit such as a CPU, and then grounding a little.
3. Give the ECU a separate power supply. In modern cars, the electrical environment is harsh. Many devices are pulsed power supply, and some devices have large voltage changes (such as when the engine starts, the voltage can be reduced from 12V to 8V). If the ECU is connected to the same power supply, it will make The ECU is malfunctioning or even damaged. Therefore, the ECUs should be powered by noise-free isolating joints, separating their power supply lines from the grounding joints and the main load of the battery. The ECU preferably uses an independent power supply, voltage regulation and filtering system.
4. Potential isolation. For systems with large level differences, such as signal transmission equipment, solenoid valve drive circuit, high-voltage ignition circuit and other high-power, high-voltage circuits, Transformers, optocouplers and other devices are used for potential isolation to suppress corresponding interference. In order to suppress capacitive interference, transmission lines such as wires and cables should be arranged reasonably. For signal input/output lines with high sensitivity, shielded lines are used for signal transmission, which can effectively reduce capacitive interference.
Another type of interference that is more influential in modern cars is electromagnetic radiation interference. It mainly manifests as the radiation interference formed by the weak current and small signal circuit part of the high current or high voltage circuit part, among which the pulse radiation interference is more prominent. The ignition pulse, relay, solenoid valve, etc. in the car may be electromagnetic pulse interference. This electromagnetic energy will oscillate through the LC parallel circuit formed by the distributed capacitance at both ends, and the electromagnetic waves interfere with other systems through the corresponding wires and body radiation, especially the communication system. Therefore, it is necessary to select an in-vehicle communication system with better anti-interference performance. When the sensor is disturbed, it can cause the corresponding ECU system to malfunction or even have serious consequences. For this reason, when writing ECU software, you can use software anti-interference technology to write effective anti-interference programs, such as commonly used digital filtering; use a series of anti-jamming technologies for the CPU, such as program operation monitoring system, software trap, and Technologies such as software and hardware fault-tolerant design can be used.
The suppression of electromagnetic radiation interference usually adopts measures such as shielding, damping, grounding or filtering in hardware. For example, in the high-voltage circuit of the ignition device, a suitable amount of resistance is connected with a resistive cable to reduce high-frequency oscillation caused by spark interference; in a distributorless engine, a shielded spark plug is often used to suppress ignition interference; The electrical appliances (or arcs) are shielded by a metal cover, and the lead wires are made of wires with a shield mesh and the shield mesh is grounded. Each ECU system should be kept away from strong magnets such as horns and installed in metal shielded boxes to prevent direct interference from the ECU. Between 0. 2~0. 8μF capacitors, a capacitor of 0. 01~0. 8μF is connected in parallel between the positive and negative poles of the power supply and the terminals of the various instruments. , can better suppress interference.
5 Summary China's research on electromagnetic compatibility systems for automotive electrical and electronic equipment started late and has received increasing attention, which is crucial for improving the export competitiveness of automotive products. The EMC or FCC certification for electromagnetic compatibility export is very expensive. Raising awareness and timely adjusting various parameters can shorten the certification cycle and reduce costs, so that China's export products have excellent EMC quality and the quality is steadily improved.
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