Automotive wire harness twisted pair technical parameter setting

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Abstract: Twisted Pair (Twisted Pair) is a telecommunication transmission line consisting of two metal wires twisted periodically at a distance. A pair of mutually insulated metal wires are twisted together to resist part of the external electromagnetic interference, and more importantly, to reduce the external interference of their own signals.



Automotive application of twisted-pair cable system is many, such as the EFI system, audio-visual entertainment system, airbag system, CAN network and so on. Twisted pair is divided into shielded twisted pair, unshielded twisted pair. Shielded twisted pair has a metal shield between the twisted pair and the outer insulating envelope. The shield reduces radiation, prevents information leakage, and also prevents external electromagnetic interference, and the use of shielded twisted-pair cable has a higher transmission rate than similar unshielded twisted-pair cable.



Shielded twisted pair, the harness is generally used directly with the finished shielded wire. Unshielded twisted-pair cable, manufacturers with processing capabilities, are generally stranded using a stranding machine for stranding. In the stranded wire processing or use of the process, especially need to pay attention to two important parameters are stranding distance and untwisted distance.

| Stranding distance

The stranding distance of a twisted pair of wires is the distance between two adjacent peaks or valleys on the same wire (which can also be regarded as the spacing of two homogeneous stranded sections), see Fig. 1, stranding distance = S1 = S2 = S3.



Figure 1 Stranding distance S

The strand pitch has a direct impact on the signal transmission capability, and different strand pitches have different anti-interference capabilities for signals of different wavelengths. However, except for CAN bus, international and domestic standards do not specify the stranding distance of twisted-pair cables. In GB/T 36048 Passenger Vehicle CAN Bus Physical Layer Technical Requirements, it is stipulated that the CAN wire stranding size ranges from 25±5mm (33-50 strands/meter), which is the same as that stipulated for CAN stranding in SAE J2284 High-speed CAN for 250kbps Vehicles.

Generally, each automobile company has its own stranding distance setting standards, or comply with the requirements of each subsystem on the stranding distance of the stranded wires. For example, Foton uses a stranding distance of 15-20mm; some European OEMs recommend selecting a stranding distance according to the following standards:

1. CAN bus 20±2mm

2. signal lines, audio lines 25±3mm

3. drive line 40±4mm

Generally speaking, the smaller the stranding distance for the magnetic field of the better anti-interference ability, but need to take into account the diameter of the wire and the skin material can be bent range, and according to the transmission distance and the signal wavelength to determine the most appropriate stranding distance. When more than one twisted pair of wires laid together, different signal lines are best used twisted pair of different distances, in order to attenuate the interference generated by mutual inductance. The damage to the insulation of the wires caused by too tight a twisting distance can be seen in the following figure:



Figure 2 Deformation or cracking of the conductor due to too tight a stranding distance.

In addition, the twisted pair should be twisted at a uniform distance. Errors in the stranding distance of a twisted pair have a direct effect on the level of interference immunity, and the randomness of the stranding distance error causes uncertainty in the prediction of crosstalk in twisted pair cables. Twisted pair production equipment parameters rotary axis angular velocity is the key factor affecting the size of the twisted pair inductive coupling, in the twisted pair production process must be considered to ensure that the twisted pair of anti-jamming ability.

| Uncoupling distance

Untwisted distance refers to the twisted pair of end wires due to the installation of the sheath needs to be open and untwisted parts of the size. See Figure 3.



Figure 3 Untwisting distance L

The untwisting distance is not stipulated in the international standard, the domestic industry standard QC/T29106-2014 "Automotive Wire Harness Technical Condition" stipulates that: the untwisting distance should be no more than 80mm, see Figure 4, while the American standard SAE 1939 for CAN wire twisted pair of the provisions is: the untwisted size should not exceed 50mm. so the domestic standard for the CAN line of the provisions of the size of a larger wire is not It is not applicable to CAN cables. At present, each automobile enterprise or wiring harness factory for high-speed CAN line untwisted distance is limited to 50mm or 40mm or less, in order to ensure the stability of the CAN signal. For example, Delphi's CAN bus requires an untwisting distance of less than 40mm.



Figure 4 The untwisting distance specified in QC/T 29106

In addition, in the process of wiring harness, in order to prevent the stranded wires from becoming loose and leading to a larger untwisting distance, the stranded wires should be wrapped with rubber at the untwisting point. The American standard SAE 1939 stipulates that: in order to maintain the stranded state of the wire, need to be installed in the untwisted heat shrink tubing. Domestic standard QC/T 29106 provides for the use of tape wrapping treatment.

| Summary

Twisted-pair cable as a signal transmission carrier, need to ensure the accuracy and stability of signal transmission, it should be better anti-interference ability. Stranded wire stranding distance size, stranding distance uniformity and untwisted distance for its anti-interference ability has an important impact, so need to focus on the design and processing.
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