Discussion on Design Method of Automobile Harness System

As the nerve of the automobile, the wiring harness system plays an important role in connecting the central control components and various electrical appliances of the automobile, and the design quality of the wire harness system is directly related to the safety of the vehicle. With the increasingly hot automobile market, the rapid development of electronic and electrical industry, the rapid increase of automobile electronic appliances, and the improvement of people's requirements for automobile safety, comfort, economy and emission, the wiring loom is more and more widely used in automobiles, and the design of automotive wiring harness system becomes more important and strict.  

I. Design process

The design of wire harness system is a rigorous project, which pays attention to step by step.

Second, the layout principle

2.1 Pre-planning

In the conceptual design and structural design stage of the whole vehicle, it is necessary to integrate the overall layout consciousness of the wiring harness and make early planning. The battery, fuse box and central control unit should be arranged in a centralized way, which can not only save the cost of trunk wiring harness, reduce the overall quality of the whole vehicle, improve the aesthetic feeling of wiring harness arrangement, but also greatly reduce the risk rate of car burning.

2.2 Modularization

The wiring harness system should be modularized according to the function and characteristics of the whole machine, which is convenient for disassembly and maintenance and saves the time of vehicle maintenance and repair. The wiring harness system of a passenger car is divided into 11 modules: engine wiring harness, starting cable wiring harness, front wiring harness, instrument wiring harness, ceiling lamp wiring harness, left front door wiring harness, left middle door wiring harness, right front door wiring harness, right middle door wiring harness, tailgate wiring harness and chassis wiring harness.

2.3 Layout environment

When arranging the wire harness, try to avoid sharp edges, high temperature, oil circuits, water pipes and moving parts to ensure the reliability of the arrangement. If you can't avoid it, you need to take corresponding protective measures. For example, in the engine compartment, the engine vibration and heat radiation around the exhaust branch pipe should be fully considered in the engine harness, and a fixed point must be set every 200mm, and the distance from the heat source should be not less than 100 mm. A heat shield should be attached in front of the harness and the heat source, and the wire harness should be wrapped with a high-temperature corrugated pipe. The door wiring harness should focus on the factors such as through hole, sealing, assembly, etc., and plastic protective sleeve should be used when passing through the hole to prevent the functional failure of the wiring harness caused by wear; Try to arrange it on the dry side of the door to prevent the influence of water leakage on the wiring harness; Fix as many wire harnesses as possible with as few buckles as possible, and reduce the opening on the car body and the assembly complexity.  

2.4 electromagnetic interference prevention

In areas with strong electromagnetic interference or signal sensitivity, attention should be paid to increasing anti-magnetic protection for the wiring harness, and the conductor should be twisted pair or shielded wire, such as CAN bus. Because the internal electronic control components of the automobile interfere with the CAN bus, it is in an external electromagnetic environment that changes from low frequency to high frequency, so higher requirements are put forward for the anti-interference ability of the CAN bus. As a reliable data transmission CAN bus, it is particularly important to ensure its good anti-electromagnetic interference characteristics.

2.5 grounding design principles

Automotive electrical failures are mostly caused by grounding failure, usually due to poor grounding contact or open circuit of grounding wire. Therefore, the design of grounding points of electrical equipment in automobile circuits should be arranged reasonably according to the nature and function of electrical equipment to ensure the good working condition of electrical equipment in automobiles.  

(1) The grounding points of electrical appliances that have a great influence on vehicle performance and safety and are easily interfered by other electrical appliances must be set up separately, such as engines, ECU, ABS, etc. Weak signal sensors or easily disturbed electrical appliances should set up separate grounding points to make their electrical circuits as short as possible to ensure the true transmission of signals, such as audio and other radio systems; The grounding point of high-current electrical appliances should also be set up separately to ensure that the circuit is grounded nearby, shorten the length of the circuit and reduce voltage drop and electromagnetic interference, which is safe and economical.  

(2) In order to improve the reliability of the key electrical equipment on the car, which affects personal safety, multiple grounding, such as airbag system, is adopted to ensure its accurate and timely work.  

(3) General electrical appliances can selectively share ground according to their three-dimensional layout position and environment, thus reducing assembly complexity and material cost.

Three, two-dimensional harness diagram design

3.1 Overview of 2D drawing design

The data parameters of interface control files such as automobile sensors, actuators and central control components and the schematic diagram of automobile circuit are used to select the harness components, and then the two-dimensional harness diagram is completed by combining the three-dimensional wiring diagram. Two-dimensional wiring diagram is mainly oriented to debugging and maintenance, which should include in detail: fixing mode, fixing position and binding mode of wiring harness; Local special protection measures; Connector model and manufacturer; Harness part number and harness version; Type, wire diameter, color, starting and ending positions of each loop; Technical requirements of wiring harness, etc. Ensure that the wiring harness has sufficient margin to avoid subjecting the wiring harness to tension or tension. In view of the fact that the wire harness is a deformed part in automobile parts, its actual length and branch position have to be verified by real vehicle measurement.  

3.2 Line color and line marking

With the increase of automobile electrical appliances, the number of wires is increasing. In order to facilitate maintenance, low-voltage wires are often distinguished by different colors. There are two kinds of wire colors: monochrome and bicolor. Monochrome is the best, followed by bicolor. The marking of wire colors is indicated by color codes. The first color in bicolor marking is the main color, and the second color is the auxiliary color. Black wires should be used as grounding wires for all kinds of automobile electrical appliances. Black wires are not used for other purposes except grounding wires. Due to the large amount of power cords of various electrical appliances, generally, double-color wires with red as the main color are selected. When actually making drawings, considering the number of wire harnesses and the accuracy of making, Excel and SolidEdge can be directly used to generate the electrical circuit table.

3.3 conductor cross-sectional area selection

Generally, the allowable voltage drop of 12V power supply line is not greater than 1V, and the calculation of conductor cross-sectional area can be made according to the following formula:

S=IρL/U 

Where: s-conductor cross-sectional area, mm; I-current flowing through the conductor, a; ρ-conductor resistivity, generally 0.0185 mm/m; L-conductor length, m; U-allowable voltage drop of conductor. It should be pointed out that the larger the wire diameter, the greater the current that can pass, the less likely the wire harness is to burn out, and the higher the safety factor of the wire harness, but the larger the wire diameter, the higher the cost of the wire harness, which is not conducive to the lightweight of the car. Factors such as voltage drop, heating after wire binding, mechanical strength, cost, etc. should also be considered for the cross-sectional area of the rounded wire. Generally, the wire area should not be less than 0.5mm². Different types of wires have different standards and experience values.

3.4 Selection of connectors

(1) The vehicle wiring harness system should use as few connectors as possible to improve its reliability. There will be poor contact between connectors due to vibration, friction, coating oxidation, water ingress and other factors, and there will also be electrical function blocking caused by human factors.  

(2) The quality of the connector directly affects the overall performance of the wiring harness. For the selection of connectors, it is necessary to ensure good contact and reliability, and the appropriate connectors should be selected according to the number of wires, line current and layout space. The material of connector terminal is generally brass, and the coating is selected according to the specific coating of electrical equipment terminal at the butt end. Without special requirements, tin plating is generally selected. With the intelligentization of automobiles, the number and types of connectors are increasing day by day, which causes a lot of inconvenience to maintenance and assembly. The unification of connectors will become the future development trend.  

Fourth, design verification

According to its mechanical performance, electrical performance and environmental performance, the wire harness connector needs to be tested for terminal bonding force, terminal pull-out force, oscillating current, insulation strength, thermal aging resistance, high temperature resistance, salt spray resistance, temperature/humidity cycle, etc. The wires need to be inspected and tested for insulation peeling performance, insulation wear resistance and hot pressing. Before mass production, the wiring harness assembly should also complete relevant tests, including vibration resistance, salt spray resistance, voltage drop resistance, industrial solvent resistance, temperature and humidity cycle resistance, and electrical performance test. Due to customer needs, different countries and regions' related test items and standards, the experimental requirements will be different.  

Concluding remarks

The intellectualization and lightweight of automobile development have brought unprecedented challenges to automotive wiring harness. From the electrical schematic diagram to the two-dimensional wiring diagram, from the design concept to the completion of the wiring harness design, it is necessary to be well-founded. The selection and matching of related parts of the wiring harness should be strictly calculated and checked, and the wiring harness system should be well designed, so that the failure of the wiring harness will no longer be the chief culprit of automobile failures.