Detailed description of the grounding design of automotive wiring harness

Wire harness grounding distribution is also called grounding distribution. Whether it is necessary to have a separate grounding or whether it can be shared, etc., these are all the contents of grounding distribution design. This article mainly talks about the design of wire harness grounding distribution. We know that the power distribution and grounding design of the whole vehicle are the core parts of the automotive wiring harness design. A good wire harness grounding design is an important guarantee for power transmission and signal transmission. If the grounding point is not selected properly, it is easy to cause signal interference, thereby affecting the functional realization of electrical devices. This article will elaborate on the design of wire harness grounding. The following is the main text.

1 Single-wire system and negative grounding

There are two important concepts in the vehicle grounding system: single-wire system and negative grounding. The single-wire system means that in the automotive electronic system, only one wire is used to connect the power supply to the electrical equipment, and the metal body such as the car body, chassis, engine, etc. is used as another common wire. Since the single-wire system saves wires, the lines are simplified and clear, installation and maintenance are convenient, and the electrical components do not need to be insulated from the car body, modern automotive electrical systems generally adopt the single-wire system. When the automotive electrical system adopts the single-wire system, one electrode of the battery is connected to the car body, commonly known as "grounding". If the negative pole of the battery is connected to the car body, it is called negative grounding, otherwise it is called positive grounding. According to national standards, domestic automotive electrical systems all use negative grounding.

2 Classification and introduction of grounding points

1) Power grounding The zero potential on the negative pole of the battery. 2) Vehicle grounding The conductive body sheet metal, chassis or engine parts on the vehicle that are interconnected. 3) Power signal grounding The power feeder of various electrical components on the vehicle. According to the size/waveform of the current in the loop, it can be divided into "dirty grounding" or "clean grounding". Clean grounding: grounding with a peak current less than 1 A, such as sensor signal feedback or control signals between different components (such as network communication). Dirty grounding: pulse width modulation loads with a peak current greater than 1 A and switch loads greater than 1 A, such as motor and switch loads. 4) RF grounding is often used as a grounding to control RF interference. This type of grounding is generally installed directly on the body sheet metal through assembly and cannot be used as a bypass for any grounding current. 5) Antenna grounding, such as radio antenna grounding.

3 Grounding design process

Ground distribution design process (Figure 1)

4 Grounding Distribution

Consider the installation position of the electrical components in the vehicle, and combine the load type of the electrical components and the specific grounding category to design the grounding distribution for each electrical component. 4.1 General principles 1) Grounding is done nearby. The length of the grounding loop should be shortened as much as possible to reduce the voltage drop, cost and quality of the loop. 2) Minimize unnecessary mutual interference between subsystems. 4.2 Principles of interference generation Ideally, each electrical component needs to be grounded nearby, but this will result in too many groundings in the vehicle, greatly increasing the complexity of assembly, and increasing the quality and cost of the vehicle. Therefore, this method is not the best choice. When setting the grounding, it is necessary to consider sharing and merging the grounding.

Figure 2 is the grounding combined interference diagram. When only part 1 is working, I1=U/(L1+R1); when only part 2 is working, I2=U/(L1+L2+R2); when both parts are working, I1+I2=U/(L1+R1)+U/(L1+L2+R2). At this time, if I2 is significantly greater than I1, the potential difference between part 1 and the grounding will increase greatly, which will affect its normal operation. R1—Resistance of part 1, L1—Resistance of the wire from the rivet point to the grounding point, R2—Resistance of part 2, L2—Resistance of the wire from part 2 to the rivet point

Figure 2 Grounding merging interference diagram. Usually, design specifications prohibit the dirty grounding and clean grounding from being riveted together. If the difference between the high and low grounding currents exceeds 1/5, the two grounding loops cannot be riveted together. But please note that this is only a very conservative design specification. If the two loops are thoroughly analyzed, they can be riveted together. From the above analysis, it can be seen that in order to minimize the interference caused by riveting, the value of L1 should be reduced as much as possible, that is, the resistance between the riveting point and the grounding should be reduced. Therefore, the riveting point should be as close to the grounding sheet as possible, and if necessary, the wire diameter of this section should be increased. 4.3 Connection methods of several grounding loops 4.3.1 Grounding merging using riveting points (Figure 3)

Figure 3: The first type of riveting between clean grounding loops is allowed. The second type of riveting between dirty grounding loops needs to be analyzed according to the method in 4.2. If it passes, it is completely acceptable. The third type of riveting dirty grounding and clean grounding loops is usually not allowed unless it is approved by experts. 4.3.2 The grounding combination form using grounding sheets (Figure 4)

Figure 4: Combined grounding method through lugs. In the first method, overlapping of grounding terminals between clean grounding circuits is allowed; in the second method, overlapping of grounding terminals between dirty grounding circuits is also completely acceptable; in the third method, overlapping of dirty grounding and clean grounding terminals is usually acceptable, but it needs to be reviewed and approved by relevant professionals. 4.3.3 The form of crimping to the grounding terminal at the grounding end (Figure 5) In the first method, parallel connection of clean grounding circuits is allowed; in the second method, parallel connection of dirty grounding circuits is also completely acceptable; in the third method, parallel connection of dirty grounding and clean grounding circuits is usually acceptable, but it needs to be reviewed and approved by relevant professionals.

Figure 5: Grounding diagram by crimping terminals in parallel

5 Requirements for grounding points

1) The location of the grounding point should be convenient for installation and maintenance, and the lap joint should be able to meet the torque requirements. 2) Each grounding point should not be connected to more than 2 wiring lugs. 3) Grounding points should not be arranged in strong splash areas and water accumulation areas. 4) Grounding points are not allowed to be arranged on metal parts connected by bolts, such as car doors. When setting grounding points on the dashboard frame, it is only allowed when it is welded to the body. 5) For areas where the body sheet metal thickness is less than 3mm, it is recommended to use a welding nut solution. For thick plates greater than 3mm, it is recommended to use self-tapping screws for grounding installation.

6 General grounding principles

1) The engine ECU, ABS and other electrical components that have a great impact on the performance and safety of the vehicle and are easily interfered by other electrical equipment (such as audio, oil level sensor, etc.) must be grounded separately. 2) For the airbag system, the grounding point must not only be set separately, but also use a composite grounding point. The purpose is that when one of the grounding points fails, the system can be grounded through another grounding point to ensure the safe operation of the system. 3) Radio frequency signals need to be grounded separately to avoid interfering with other systems. 4) It is best to ground the weak signal sensor independently, and the grounding point is best to be close to the sensor to ensure the true transmission of the signal. 5) Other electrical components can be combined with each other to share the grounding point according to the specific layout. The principle is to ground nearby to avoid too long a grounding line, which causes unnecessary voltage drop. 6) The negative electrode line of the battery, the engine grounding line, etc. have a large cross-sectional area, so the line length and direction must be controlled to reduce the voltage drop; to increase safety, the engine and the body are generally connected to the negative electrode of the battery separately. 7) It is necessary to distinguish the electronic ground and the power ground, and to separate the analog signal ground and the digital signal ground to avoid mutual interference between signals.

7 Conclusion

The above are some contents of the design of the grounding of the low-voltage wiring harness of the automobile. In the actual application process, it should be used flexibly. After the design is completed, some necessary tests are required to ensure the reliability of the design.