Design Idea and Parts Selection of Automobile Harness
December 20, 2022
It is an important part of automotive wiring harness's automobile circuit, and the reasonable design of automotive wire harness is one of the conditions to ensure the safe driving of automobiles. This paper introduces the design process of automotive wiring loom and the selection of core raw materials. In terms of design principle, 3D layout is an essential link in wiring harness design. The correct selection of key raw materials such as connectors, wires and protective materials is the basis for automotive wiring harness to play its due role.
I introduction
The basic idea of automotive wiring harness design is to determine the circuit circuit and calculate the voltage and current of each circuit, especially the high-power equipment, according to the demand of the electrical equipment, select the matching connector according to the electrical equipment, determine the cross-sectional area of the wiring harness according to the current, and then design the wiring harness direction.
II. Overview of Harness Design
2.1 The basic process of harness development and design is as follows:
(1) According to the function table required by the OEM, list all the functional parts connected to the wiring harness, and confirm the component functions with the corresponding electrical engineer to obtain ICD information and determine the electrical principle layout. Check the principle of each ECU system, and integrate each functional part into a completed wiring harness schematic diagram. Generally, the power supply system and the grounding part can be combined by riveting points. At the same time, check the current of each functional system (including over-current or locked-rotor current, etc.) to ensure that it can meet the working requirements of electrical systems such as motors, switches, fuses, blowers, etc.
(2) Design the preliminary 3D distribution diagram, and complete the route distribution of the harness by CATIA or UG software according to the processing and assembly sequence of the general process of the harness. In this process, the installation position of fuse box should be basically defined, and some allowance should be reserved in the initial 3D distribution drawings.
(3) Convert 2D drawings, which refer to industry standards, such as QC29106.
(4) According to the initial 2D drawings, make a manual harness sample, and try to install it in a car.
(5) According to the actual trial installation results and trial operation results, the 3D wiring engineer of the wire harness will optimize and adjust the direction or length of the wire, taking into account the dynamic situation.
(6) The wiring harness is used for the installation and connection of the electrical functional parts of the whole vehicle. Due to the installation requirements of each electrical system or the whole vehicle, the wiring harness generally needs to be continuously adjusted and optimized with each system, that is, the whole development process is also a process of continuous optimization and update of the design.
Third, the design method
3.1 Principle design of wiring harness
(1) Demand the complete vehicle electrical functions, current requirements and other special requirements from the complete vehicle factory. And confirm the installation location of electrical devices, the assembly method of electrical devices and wiring harnesses, working environment and other information.
(2) Refer to the functions and methods that customers need to realize, and make electrical schematic diagram and circuit layout.
(3) Energy distribution for each electrical component system, including distribution of power line and grounding line.
(4) Determination of wire diameter. Firstly, the current required by the wire is obtained from the actual power of the electrical device; Long-term working parts use wires with 60% of actual current carrying capacity (such as heating system, entertainment system and safety system); Short-time working parts use wires with actual current carrying capacity between 60% and 100%. (e.g. seat adjustment motor, window lifting motor, angle adjuster, etc.).
3.2 3D wiring design
Generally, the three-dimensional wiring harness layout is based on the overall layout, combining with the layout of the body sheet metal and the design of opening and slotting, on the premise of satisfying the installation of the whole vehicle's electrical appliances. Especially, the sectional design and trunk positioning of the harness should consider the overall layout.
3.2.1 Requirements of automotive wiring harness 3D Wiring
Automotive wiring harness 3D cabling from scratch is a complex project, mainly referring to the following points:
(1) Ensure that all electrical functional areas are easy to assemble, and the final assembly section is as simple as possible for easy installation. If there are parts that are not suitable for direct installation, you can consider the built-in counterpart parts, which can be installed separately and then assembled together (this will increase the cost). Tail lines such as door panels and interiors can be assembled separately. Ensure the traditional technological process and tooling as much as possible to realize the assembly process.
(2) In addition to assembly, attention should be paid to maintenance and repair, and it also needs to be easy to disassemble, which is commonly referred to as DFD(designfordisassembly). When repairing, only one section of wire is dismantled, which is much easier to operate than the whole wire harness, and at the same time, it saves costs and reduces potential risks.
3.2.2 Other details of wiring harness layout
Besides the above-mentioned convenience for assembly and disassembly, the following details should be considered for wiring harness layout:
(1) Leave enough allowance (even in the case of lower tolerance limit), especially for the wire harness between two or more parts with relative movement. The length required for the extreme position must be considered.
(2) The wire harness can't be in a tight state all the time, otherwise it will continuously form internal stress on the copper sheet of the wire, and accelerate its aging process.
(3) The harness generally needs to be fixed by buckles or slots at intervals, and cannot be in a free state for too long.
IV. Part Selection
4.1 wire selection
The wire selection of the harness focuses on the function and environment that the harness needs to achieve. For example, the engine harness, the cabin temperature is very high and there are many corrosive substances, so it is necessary to choose materials that are resistant to high temperature and oil corrosion.
Or the trunk door moves frequently, choose the wiring harness with high elasticity, such as TPE or rubber. For some weak signals, compound shielded wires, such as knock sensor harness, are generally used. The requirements in the cab are relatively low, so PVC harness can be considered, which can save the cost and be beneficial to light weight.
4.2 connector
The connector is the most important component of the wiring harness, which directly determines whether the wiring harness can realize the core connection function and plays a decisive role in the stability of the electrical system.
4.2.1 Selection requirements of connectors
First of all, it is the cooperation with electrical components, and the mechanical insertion retention meets the use requirements. The design with secondary lock is preferred. The contact resistance is as low as possible. The insulation resistance and overcurrent meet the working current requirements of the conductor. For wiring harnesses in wet areas, it is necessary to select waterproof sheath, with appropriate sealing ring or blind plug, to meet the requirements of different waterproof grades in different areas.
4.2.2 Properties of raw materials for connectors
① Sheath material (plastic parts):
At present, the commonly used materials of connectors are PA66, PBT, ABS, etc. Generally, some additives need to be added to increase the properties of the materials, such as glass fiber, strength, plasticizer and softness.
② Material of terminal (copper):
Generally, the commonly used materials of terminals are brass, bronze and copper alloy. Brass has strong wear resistance, good castability of bronze, wear resistance and stable chemical properties, and copper alloy has excellent electrical conductivity, thermal conductivity, ductility and corrosion resistance. In addition, considering salt spray and aging, terminals generally need to add different coatings, such as tin plating and gold plating.
4.2.3 Classification of connectors
According to the mating form of connectors, it can be divided into: line-to-line and line-to-board.
① Line to line
Line-to-line connection includes the form of line-to-cable or cable-to-cable connection, which is defined as the permanent connection between two single-wire individuals or corresponding wires in two cables.
② Line to board
Line-to-board connection is mainly a kind of connection in which one connector is connected to a wire or cable, the other connector has a fixed terminal, and is connected to the substrate by welding.
V. Harness coating design
Automotive wiring harness is affected by various external factors in the car for a long time, such as oil pollution, dust, friction, water-like rust, etc. If there is no protective material on the wiring harness, the wires exposed to the outside are easily damaged, such as broken wires and short circuits, which may cause functional failure. Several commonly used bandaging materials are as follows
5.1 Bellows
Bellows are the most important wire harness protection materials used in automotive wiring harness, which are generally divided into single-sided openings and sealed sleeves.
Made of PE, PA6/66, PP, etc., the temperature resistance is generally-40℃ ~ 85℃,-40℃ ~ 125℃ and-40℃ ~ 150℃. Good wear resistance, high temperature resistance, flame retardancy and heat resistance in high temperature zone. PA is good in flame retardant and wear resistance, while PP is superior in bending fatigue resistance.
5.2 adhesive tape
Adhesive tape is the most widely used coating method in wire harness, and it is mainly divided into three types: PVC tape, flannel tape and duct tape. PVC tape has good insulation performance, and its temperature resistance is generally 80℃, but it is only 105℃ after improvement. The noise reduction is not good, and the wiring harness is hard after coating. At present, the VOC effect is generally poor. The base materials of flannelette tape and duct tape are generally PET. The flannelette tape has a good noise reduction performance with a temperature resistance of about 105℃, and the wire harness is soft after the coating is completed. Duct tape has the best wear resistance, which can generally reach D or E grade, and its temperature resistance can reach 150℃, but its price is high, so it is generally used in vias or places with relative motion.
5.3 Weaving network management
The woven mesh tube is generally made of PA66 monofilament or PET monofilament, and generally has open type and closed type, which is characterized by very high wear resistance, but high noise reduction and cost. It is generally used in parts with long-term relative movement.
VI. Conclusion.
Automotive wiring harness is known as the vascular system of automobiles. With the rapid development of automobiles and the continuous improvement of use experience, the requirements for automotive wiring harness are also constantly improving. It is even more necessary for automotive wiring harness people to continuously research and develop new schemes in order to meet the ever-changing needs.
