Talk about how to set the car wire harness twisted pair technical parameters

Scatman said that there are many twisted pair systems used in cars, and twisted pair cables use a pair of mutually insulated metal wires twisted each other to resist part of the external electromagnetic wave interference, and more importantly, reduce the external interference of their own signals. This paper mainly introduces the setting of automotive wire harness twisted pair technical parameters, the following is the text.

There are many twisted pair systems used in automobiles, such as electric injection system, audio-visual entertainment system, airbag system, CAN network and so on. Twisted pair is divided into shielded twisted pair and unshielded twisted pair. The shielded twisted pair wire has a metal shielding layer between the twisted pair wire and the outer insulation envelope. The shielding layer can reduce radiation, prevent information leakage, and also prevent external electromagnetic interference, and the use of shielded twisted pair has a higher transmission rate than similar unshielded twisted pair.

Shielded twisted pair wire, wire harness is generally used directly with the finished shielded wire. Unshielded twisted pair, manufacturers with processing capacity, generally use a strander for stranding. In the process of wire processing or use, the two important parameters that need to be paid attention to are the stranding and the stranding distance.

| lay

The twist of a twisted pair refers to the distance between two adjacent peaks or troughs on the same wire (also seen as the distance between two twisted joints in the same direction), as shown in Figure 1, twist =S1=S2=S3.

Figure 1 Lay length S

The skein directly affects the ability of signal transmission, and different skein has different anti-interference ability to different wavelength signals. However, in addition to the CAN bus, the international and domestic relevant standards have not made a clear provision for the twisted pair's skein. In the GB/T 36048 passenger car CAN bus physical layer technical requirements, the CAN wire lay size range is 25±5mm (33-50 lay/m), which is the same as the CAN lay in SAE J2284 250kbps high-speed CAN for vehicles.

Generally, each car company has its own lay standard, or comply with the requirements of each subsystem for the strand lay. For example, Foton automobile adopts a lay of 15-20mm; Some Oems in Europe recommend the following criteria for hinge selection:

1. CAN bus 20±2mm

2. Signal cable and audio cable 25±3mm

3. Drive line 40±4mm

In general, the smaller the skein, the better the anti-interference ability for the magnetic field, but the diameter of the wire and the bendable range of the skin material need to be considered, and the most suitable skein distance is determined according to the transmission distance and signal wavelength. When multiple twisted pairs are laid together, it is best to use twisted pairs with different strands for different signal lines to weaken the interference caused by mutual inducement. The damage to wire insulation caused by too tight stranding can be seen in the following figure:

FIG. 2 Deformation or cracking of wires caused by too tight lay

In addition, the skein of the twisted pair should be maintained evenly. The skein error of twisted pair will directly affect its anti-interference level, and the randomness of skein error will cause the uncertainty of crosstalk prediction of twisted pair. The angular speed of twisted pair is the key factor that affects the inductive coupling of twisted pair. It must be considered in the process of twisted pair making to ensure the anti-interference ability of twisted pair.

| unhinge distance

The stranding distance refers to the size of the unstranded part of the twisted pair end conductor due to the split inside the sheath. See Figure 3.

FIG. 3 Detaching distance L

The stranding distance is not specified in the international standard, and the domestic industry standard QC/T29106-2014 "Automotive wiring harness technical conditions" stipulates that the stranding distance should not be greater than 80mm. See Figure 4. The American standard SAE 1939 for the CAN wire twisted pair is: the untwisted size should not exceed 50mm. Therefore, the domestic standard is not applicable to the CAN line because of its large size. At present, the detach distance of high-speed CAN lines is limited to 50mm or 40mm to ensure the stability of CAN signals. For example, Delphi's CAN bus requires a stranding distance of less than 40mm.

FIG. 4 Unstranding distance specified in QC/T 29106

In addition, in the process of wire harness processing, in order to prevent the stranding loose lead to greater untwisting distance, the stranding should be covered at the stranding. SAE 1939 states that in order to maintain the stranded state of the wire, a heat shrink tube must be installed at the unstranded point. Domestic standard QC/T 29106 stipulates the use of adhesive tape for adhesive processing.

| general junction

As a signal transmission carrier, twisted pair wire needs to ensure the accuracy and stability of signal transmission, and it should have better anti-interference ability. The lay size, lay uniformity and lay distance of the stranded wire have important effects on its anti-interference ability, so it should be paid attention to in the design and processing process.