Technical column



The role of earthing in automobile EMC

  1 Overview

  Since electric energy was used for ignition in 1860, automobiles have a history of emitting electromagnetic disturbances to the surrounding electromagnetic environment. Today's automobiles, especially the engine ignition system, are facing great challenges of electromagnetic compatibility. In the design process, in order to ensure that all parts of the car work in harmony and are compatible with each other, various international, national and local electromagnetic compatibility standards must be followed. In addition, EMC engineers must also consider passenger comfort, including radio entertainment equipment, car phones , GPS, anti-explosion devices (radar and lidar), and other advanced equipment that will be applied to future functional vehicles.

  The "ground" in the automotive electronic system does not only perform one function. This term is usually used to refer to some conductors on the car. They have the following functions: 1. Provide a current loop 2. Provide reference voltage for analog sensors and digital logic circuits 3. Frequency modulation and amplitude modulation Antenna ground grid 4 is used to shield radio frequency bypass current 5 electrostatic protection


  2 Vehicle grounding system

  2.1 Overview

  As shown in the figure below, the automotive power supply system consists of a battery, a generator and an elegant regulator. It is the power supply system of the automotive electrical system, providing sufficient electric power and stable operating voltage for the automotive electrical system. The vehicle power supply system is powered by a battery and a generator connected in parallel. When the generator is running, it outputs current to drive a DC load, but the internal capacitance of the generator is too large, so it cannot output fast switching current. Then this task has to be done by the battery.

  In addition, the primary consideration of the automotive grounding system is to ensure the correct operation of the starter, so we have to consider the requirements of the starter. In order to ensure that the starter motor can operate in cold weather, its current loop must present a DC low-resistance characteristic between the negative pole of the battery and the ground of the starter (generally the engine body). The installation position of the starter and the battery can create additional unfavorable current loops between this earth conductor.

  2.2 Grounding element

  In a car, many components are dedicated to grounding, including the body metal, engine block, wiring, and battery.

  2.2.1 Body metal body

  Although there is an increasing trend in body materials to use composite materials or plastic panels, today's automobile bodies still use metal in large areas. Because of its size, shape and convenient layout (eg, close to most electrical equipment), the body metal body is unique in providing current return, reference potential, shielding and attenuation of noise. The goal in designing a good automotive grounding system is to maximize the effect of the body metal.

  Due to its unique shape and large surface area, the body itself is a huge capacitor. It has been verified that the metal body of the body can absorb high-frequency and small-amplitude currents. However, doing so presents a common resistive coupling, especially if there is current passing through the joints on the body. In addition, if the metal body of the body is used as a general current loop instead of some specific current loops, it will not only fail to exert its strengths, but will increase an important radiation source. The Ohio State University of the United States and the Daimler-Chrysler Company of the United States have conducted experiments and found that if the metal body of the body only provides a loop for the general ignition current, the surface electric field is not stable and there are obvious interference glitches. Noise can be generated in radio entertainment equipment when baffled. However, if an additional component is added to the car so that the current loop avoids the metal body of the body near the baffle, the surface electric field is very stable at this time.

  The metal body of the body can also shield the low-frequency capacitors. When the frequency exceeds kilohertz, the metal body of the body itself has a certain thickness, so it will also play a certain electromagnetic shielding effect on the engine box. In addition, it is also the main conductor for the shielding of automotive and lighting systems, external ESD interference, and other radiation sources outside the vehicle. However, such shielding cannot provide reliable electrical integrity if the body is of general construction. The connection between the metal plates may be by solder joints, non-conductive anti-corrosion inserts, or non-conductive paint. Ideally, the fixed metal panels are electrically bonded and the movable metal panels (roof and doors) are connected by auxiliary conductors (ground straps). The metal body of the body should be connected to the negative terminal of the battery through a solid low impedance wire harness.

  2.2.2 Engine block and cylinder head

  There are a variety of electrical signals in the engine block and cylinder head of an automobile. In addition to the possibility of conductive coupling between them, some signals can generate voltage between the engine device and the metal body of the body through the loop lead inductance. This voltage can radiate or capacitively couple to electronics within the engine.

  The engine and the cylinder head are physically tightly connected by many large-diameter bolts, but the electrical characteristics of the connection are generally poor. Cylinder head gaskets are often insulated and usually have poor quality conductors on their exterior. The connection between the engine block and other engine components (including the generator frame, valve cover and throttle body) is not controllable and may change at any time during the operation of the engine.

  2.2.3 Wiring

  Due to the large size of engine and automotive components, ground impedance must be created at high frequencies. Moreover, the distance between these components is large (on the order of meters), so the inductance problem of the current loop is also difficult to eliminate. Normally, the inductance of wiring reaches tens of nH/inch, and the inductive capacitance of many automotive components can reach the order of μH. Then a few amperes or more current with a frequency higher than 100KHz in the automotive system may cause a large voltage drop in the current loop.

  2.2.4 Battery

  In addition to storing electrical energy, batteries can also suppress some noise. At low frequencies, lead-acid batteries present a capacitive reactance of 1-2F per 100Ah. When placed in parallel with the generator, this reactance will help reduce low-frequency noise generated by the car's charging system. In the high-frequency state, the reaction in the battery is more complicated. In the range of 25kHz to 250kHz, the battery exhibits capacitance (tens of nF), and basically does not change with frequency. It has been observed that the change order of the magnitude of the effective capacitance is related to the load state and size of the battery. In high frequency state, it is best to use series LC circuit to equivalent the reactance of battery. At this time, if the frequency is higher than 1MHz, series resonance is likely to occur. Because of this, batteries cannot be used to suppress AM broadcast band or higher radio frequency noise.

  3 Issues to be considered in automotive EMC

  According to the above, the car itself has many sources of radiation and sensitive components. In this section, we only discuss the more specific elements of the automotive environment.

  3.1 Ignition device

  A car's primary and secondary ignition circuits are the most common sources of emissions. Since the two stages of circuits have completely different waveforms after they are coupled by the ignition coil, they are introduced separately here.

  The primary ignition circuit consists of a current source (battery or alternator), an ignition coil, and a trigger (usually an engine controller). The schematic diagram of the ignition system used in most of the current automobiles is shown in Figure 2. For this type of circuit, the primary concern with EMC is the falling edge of the coil current waveform.


  Figure 2 Schematic diagram of the ignition system


  Figure 3 shows the primary current and voltage waveforms of the ignition coil. This part of the waveform contains effective high-frequency energy (100KHz and above), which is affected by the inductance of the ground return harness.

  The secondary ignition circuit includes the ignition coil, spark plug, cylinder head and other conductors. The equivalent circuit is shown in Figure 4. This circuit includes the engine block and the engine control module. It also includes a 1pH grounding inductance, which can touch the engine block. There is a potential difference of several volts between the battery and the control module. Here, the primary voltage is represented by an equivalent series voltage source. Although capacitive and radiation losses also occur at sufficiently high frequencies, the current loop to the battery can be represented by an equivalent inductance; Cc represents the parasitic capacitance through the coil. In the diagram, it is not visible that the secondary is capacitively coupled to the primary through the ignition coil. This actual coupling method can not only increase the radiation voltage, but also introduce more conductive coupling into the electronic system.

  This problem can be alleviated to some extent if we minimize the area of ​​the closed-loop loop and arrange the components reasonably to shorten the lead distance between them. In addition, in the current loop flowing through the negative pole of the battery, no other conductor can be used as a reference potential, nor can the metal body of the vehicle body.

  The signal from the secondary ignition circuit is unique among automotive-generated signals because of its spectral range up to around 1 GHz. It is therefore especially likely to be a source of radiation, but its conductive coupling to other parts of the engine is also a source of radiation.

  Figure 2 and Figure 4 show the generation mechanism of the high-frequency transmission signal of the secondary ignition circuit. The battery supplies direct current to the primary winding of the ignition coil. The primary and secondary windings of the engine adopt an autotransformer structure. It takes a certain amount of time for the electronic switch in the engine controller to drive the primary winding on. It is known that the primary voltage is amplified by the turns ratio (generally several hundred), and a voltage of tens of kV is generated on the secondary side. Therefore, an RF suppression capacitor Crf is added between the high side of the coil and the cylinder head.


  Figure 3 The primary voltage and current waveforms of the ignition coil


  Figure 4 Secondary equivalent circuit of ignition system


  The RF characteristics of the spark plug can be described by the following model: (1) an equivalent arc impedance L, which can be used to describe the nonlinear time-varying current flowing through the spark gap, (2) the external threaded metal sheath of the spark plug and the internal iron Core composed of coaxial capacitors. In fact, the spark gap is open when the arc is not discharging, which is represented here by a switch. After the switch is closed, the capacitor Cp is rapidly discharged through the gap.

  3.2 Car Wireless Entertainment System

  In-vehicle wireless entertainment consists of a shielded high-gain amplifier connected to a monopole antenna. Wireless devices are sensitive to noise in the kHz to MHz range, which is the range in which automotive electrical and mechanical systems generate noise.

  The coaxial cable connects the wireless device to the receiver, and its outer conductor forms a continuous shield between the baffle (antenna ground) and the receiver frame. Although the car's designer is generally unaware of the connections inside the receiver rack, it can be assumed that the receiver rack is also connected to the body metal via the ground strap. There are several types of grounding in in-vehicle wireless entertainment systems. First, the outer shielding layer of the coaxial cable is connected to the metal body of the vehicle body to provide a ground grid for the monopole antenna. It is also necessary to connect the coaxial cable to the wireless equipment rack, which can divert the surface noise current of the former to the latter, providing a loop for the antenna signal. If there is no connection between the wireless equipment rack and the body, its large metal parts will capacitively couple with the body metal, and noise on the body metal may affect the receiver's reference voltage. If the outer conductor of the coaxial cable does not make a good connection to the receiver and wireless equipment chassis, noise currents will couple into the receiver. Finally, if the receiver and radio rack do not have a common ground, noise currents on the radio rack will capacitively couple at the receiver.

  3.3 Power supply system

  3.3.1 Power supply system structure

  In the traditional power supply system shown in Figure 1. The starter between the engine body and the negative pole of the battery has a large diameter, but has low resistance characteristics, and provides a circuit for the current to return to the engine body and the negative pole of the battery. There are two possible challenges to designing a good grounding system. First, due to the size and mounting location of the battery and starter, very long leads are required in the circuit, which can create a large ground inductance; second, the resistance of the engine block is variable because it is composed of many structurally independent It is composed of parts (such as the fuselage, cover, manifold and its brackets), and the electrical characteristics of the connections between them are not controllable during the assembly process, nor are they constant in one layer.

  3.3.2 Power supply system components

  Noise generated by generators and voltage regulators should also be considered in grounding design. There are several sources of this noise: 1. The brush noise caused by the intermittent contact of the motor slip ring; 2. The harmonic noise generated during rectification; 3. The pulse noise generated by the reverse connection of the rectifier diode; 4. The excitation winding through PWM signal, high frequency noise is generated on the voltage regulator. The generator's drive belt is also a source of noise as friction and the mechanical movement of the belt cause static charges to move. This small noise source can exacerbate other large capacitive and radiated signals produced by the engine.

  As mentioned earlier, the internal capacitance of the battery can be used to eliminate generator noise. This structure can absorb low-frequency noise, but it is ineffective for high-frequency noise, because the battery has a complex frequency response and a large wire inductance. To improve high-frequency rejection, the noise should be shunted to ground as close as possible to the generator casing. The impedance of the path between the shell and the negative pole of the battery should be low. If you use the method of shortening the connection and reducing the coupling with other circuits, you must be cautious!

  3.4 Non-electrical components

  While the foregoing discussion has focused on the electrical components of a car, many non-electrical components on a car are also significant sources of noise radiation due to their size, shape, and mounting characteristics. These components include engine blocks and cylinder heads, radiators, radiator cores and exhaust pipes. Noise signals can couple through inductive devices, capacitive devices, conductive elements, and radiative processes.

  In many cars the radiator is attached to the body with an insulating rubber bushing so that the radiator is electrically isolated from the reference potential. The radiator core is also the same principle. Also, flow vanes can exacerbate capacitively coupled noise. Since the in-vehicle entertainment system is also nearby, this interference affects the radiator core particularly hard. To suppress these noise sources, the radiator and radiator core are grounded at least one point to the body. Now more and more aluminum heat sinks are used, which has caused new problems. Particular care must be taken in the selection of materials at the joints, and galvanic corrosion must be minimized when the metal materials are in contact.

  3.5 Exhaust pipe

  The exhaust pipe is connected to the engine cover through the exhaust manifold, which is equivalent to a monopole antenna and is the ground grid of the engine. This kind of ground grid can also generate radiation, and jumper conductors can be used to reduce these radiation to the metal body of the vehicle body.

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