Automotive Circuit Harness Design Method

As an important proportion of the wiring harness cost, the automobile wiring harness circuit has a significant impact on the optimization of the wiring harness cost. It is also directly related to the safety and reliability of the vehicle’s electrical appliances. Based on work practice, this article summarizes many considerations for loop design from both the aspects of cost and performance, and proposes specific control strategies and implementation approaches, providing technical support for efficient and accurate wire harness loop design. It has strong guiding significance for wiring harness designers.

Wiring harness circuits for various levels of vehicle models

1 Introduction
Wire harness products serve as the connection carrier for automotive electrical functions and realize the circuit connection between various electrical components. The starting point and end point of each circuit connection constitute the loop within the wiring harness product. It can be said that the wiring harness circuit is the core of the wiring harness product. The quality of the circuit design of the wiring harness product directly determines the safety and reliability of the vehicle wiring harness. As the degree of electrification of vehicles increases, the data of electrical components increases, the signal interaction between electrical appliances becomes more and more intimate, and the number of automotive wiring harness loops also increases sharply. The wiring harness circuit data of general vehicle models has reached nearly 1,000 (Figure 1).
How to optimize and coordinate such a large number of circuits is a difficult problem faced by automotive wiring harness design.
The existing technical information on automotive wire harness design mainly provides design guidance for wire harness design in the selection of wire harness materials and manufacturing and processing links, but lacks systematic analysis of the planning and design concepts of wire harness circuits. This article explains the relevant key points of wire harness loop design from two aspects: cost and performance, and provides specific control paths. It has a certain guiding role in the design of wiring harness circuits.

Different designs of BCM loops

2 Cost-based loop design method
The wiring harness loop accounts for about 90% of the wiring harness material cost, including wires and connectors. To control the cost of wire harness design, we must start from the optimization of wire harness loop design.
Regarding the use of wires, how to achieve the loop connection function with the minimum wire length is the first issue to be considered in loop design. This involves two aspects of design elements: the placement location of electrical components and the selection of wiring harness layout paths. These two factors are independent but interrelated and have a significant impact on the use of wire length.
First, it is necessary to determine the connection method of the circuit based on the principle of components, and then determine the preliminary position of the layout of each component in the vehicle environment. The selection of the wiring harness layout path is based on the layout position of the components, using the shortest wire harness length to cover as much of the component layout area as possible. This is also the prototype of the vehicle electrical topology.

Fuse box and relay wiring harness circuit

After completing the vehicle topology construction, it needs to be designed and verified. By calculating the specific wire usage, we can determine whether the layout position of components and the wiring harness layout path are reasonable (there are currently a large number of software on the market that can realize this function). The specific method is to compare by adjusting the parts one by one. As shown in Figure 2 and Figure 3, the designs of different BCM layout positions are compared to check the length and amount of wires used in the vehicle, and then determine which position of BCM layout is better.
In this process, mutual influence often occurs: the adjustment of the layout of component A will affect the selection of the location of component B. Therefore, after determining the impact of each component and wiring harness path on the length of the wire one by one, the one that has a greater impact on the length of the wire will be selected as the first-round preferred solution. On this basis, the topology is rebuilt and other secondary solutions are compared and analyzed again. This achieves a topology design platform with the smallest wire length.
A perfect topology can ensure the minimum amount of wire usage. At the same time, regarding the use of wires, the traditional design concept has clear requirements for the selection of wires. In order to avoid confusion in terminal plugging, more wire colors are often used to distinguish them. However, as the manufacturing level and inspection methods continue to improve, the wire colors of the wires can actually be appropriately designed and adjusted to realize the loop function with the minimum number of wire types, which is also a method to reduce the loop design cost from a design perspective.
For connectors, how to minimize the use of connectors and reduce transfer loops is what needs to be focused on in loop design. Here, the wiring harness design engineer needs to transform into a system design engineer, and the design work of reducing the use of connectors and transfer loops needs to be moved to the design and planning of electrical components. There are two main aspects to consider:
On the one hand, functional circuits of electrical components can be distinguished according to vehicle model configurations. For example, for airbag controllers, basic functional circuits can be designed in the same connector, while advanced or extended functions can be arranged in another connector. In this way, only one connector can be used on low-end models, and the electrical circuit function can also be realized.
On the other hand, it can also be planned according to the connection area of the circuit, such as the airbag controller. Some designers will consider designing the functions of the chassis in the same connector and the functions of the instrument panel in another connector. This kind of planning can reduce the mutual transfer of circuits in various regions. This area-based functional loop design is particularly effective for electrical components with a large number of connected pins (such as BCM controllers).

Interference forms of automobile wire coupling

3 – Circuit design method based on wire harness performance
The wiring harness loop is the core of realizing circuit connection. The safety and reliability of its circuit connections are requirements that must be met. The wires and connectors in the circuit design must comply with the requirements of the load and environment. These contents have been described in detail in other design materials. This article only explains how to ensure the design of loop performance from the perspective of loop path selection.
First, the circuit design must avoid undetectable failure modes. As shown in Figure 4, the rear part of the fuse is connected in parallel with the relay coil end and contact end. This kind of design is very common in vehicle circuit design. This design is obviously reasonable when the relay coil end and contact end terminals are different. However, when the relay coil end and contact end terminals are the same, current electrical inspection equipment cannot identify such a failure mode when the terminals are inserted into the relay holes in the wrong position.
Therefore, this loop design method cannot be used in some cases. Of course, different design engineers face different design environments and manufacturing environments, and the specific failure modes will also be different, but the avoidance of failure modes in circuit design is the first thing to consider.
On the other hand, the current level of automotive electronics has improved significantly. As an electronic carrier, the electromagnetic environment faced by automobiles is also more complex, and how to reduce electromagnetic interference in wire harness circuit design is an inevitable topic. Wire coupling interference (Figure 5), power supply interference, ground interference, radiation interference, etc. will all have adverse effects on the normal operation of electrical devices. The circuits in the wire harness are bundled together, and the wire coupling interference generated between the wire harness loops and between the wire harness and the metal conductor is particularly prominent in the wire harness.
To reduce wire coupling interference in loop design, we must first distinguish between interference loops and sensitive loops. To put it simply, inductive load circuits such as ignition coils, speakers, motors, etc. are interference circuits, while circuits such as imaging, radar probes, low-power LED lights, and various sensors are sensitive circuits. Interference loops and sensitive loops need to be arranged separately during the design process. Tests have shown that increasing the distance between wires can reduce high-frequency interference (Figure 6). If the distinction cannot be made, functional testing needs to be carried out by injecting interference into the wires to determine the correctness of the circuit design.
At the same time, in order to reduce the influence of wire harness radiation and coupling, the circuit loop area and wire harness length should be reduced as much as possible. In the design of the entire vehicle, it is necessary to minimize the loop area of the wiring harness, especially the power lines and ground lines. It is required that the wire harnesses in the loop should be routed in parallel as much as possible and fixed as close to the metal body as possible to reduce the loop area, and the distance between the wires should not exceed 50cm.
In addition to the consideration of the layout of interference loops and sensitive loops, the anti-interference components twisted pairs and shielded wires used on the wire harness also need to be paid attention to in the loop design to achieve shielding expectations. The two wires of the twisted pair should have the same diameter and length, and the twist distance should be 10~20mm. The specific twist length is subject to experimental testing. The shield ground terminal should connect the shield 360° to the shield shell at both ends. The shielding layer and shielding shell form a complete shield on the signal line. If the shell of the component connected to the shielded cable is not of metal structure, metal conductive clips can be used to press the shielding layer onto the metal plate that is reliably connected to the car body. The shielding effectiveness should reach 60dB.

The relationship between high frequency interference and conductor spacing

4 Conclusion
This article analyzes the automotive wiring harness circuit design method from two aspects: cost and performance, and explains the application of specific methods based on work practice. The key points of wire harness loop design are extracted, which has guiding significance for wire harness design engineers in the loop design process.