With the continuous progress of automobile electrification, intelligence, networking and sharing, the number of vehicle communication loops and data transmission frequency continue to increase.

The complex loop network will release plenty of electromagnetic signals to the surrounding environment, and the increase of loop signal transmission frequency increases the strength of the released electromagnetic signal, and the problem of electromagnetic interference of the whole vehicle is becoming more and more serious.

As the carrier of signal transmission, the problem of shielding failure is particularly prominent.

A large number of vehicle shielding failure data show that the shielding failure of wire harness has become the most important part of vehicle electromagnetic interference.

However, the electromagnetic shielding failure of vehicle wiring harness has many interference sources and different interference modes, which leads to many difficulties in the investigation of vehicle interference failure.

Based on the above problems, this paper summarizes the ways of wire harness shielding failure, and provides a working method to reduce shielding failure, which improves the reliability of wire harness design and has a certain guiding significance for solving the problem of wire harness shielding failure.

1. Wire harness shielding failure mode

1.1 Electromagnetic shielding of vehicle wiring harness

Good shielding performance has two meanings: one is that the system does not release electromagnetic waves that affect other systems, and the other is that the system itself is not affected by external electromagnetic fields.

There are three elements in the occurrence of shielding failure, namely, interference source, coupling path and sensitive equipment, as shown in figure 1.

The interference sources on the whole vehicle mainly include ignition system, inverter, relay and other equipment which can send out electromagnetic signals.

The coupling path consists of two parts: the air and the whole vehicle circuit, and the electromagnetic wave is transmitted in the coupling path.

Sensitive devices are easy to interfere with.

The wire harness system is special, it can be not only the carrier of conduction coupling, but also the equipment easy to be interfered with, so the research on electromagnetic shielding of wire harness is of great significance.

As an important transmission way of conducting coupling interference, the electromagnetic interference signals generated by various electronic devices directly enter the sensitive equipment through the wire harness, or enter the control loop and signal loop in the wire harness through capacitive coupling or inductive coupling.

There is also a coupling between the wire harnesses, which makes the electromagnetic compatibility of wires more complex.

The wire harness is a very efficient electromagnetic wave receiving antenna. The electromagnetic wave in the space around the wire harness can be directly coupled to the wire harness, and then enter the sensitive equipment through conduction coupling.

At the same time, the change of the signal inside the wire can produce electromagnetic wave and transmit it outward, which leads to the problem of shielding failure of the whole vehicle.

1.2 Wire harness shielding failure modes

There are three main ways of shielding failure of vehicle wiring harness, namely, the abnormal code caused by controller local area bus (CAN) failure, the abnormal video signal and the abnormal control circuit.

1.2.1 Abnormal code caused by CAN failure

The traditional point-to-point communication connection requires a separate loop between each pair of communication nodes.

With the increase of the vehicle ECU, the vehicle wiring harness is becoming more and more complex, and continuing to use the traditional point-to-point communication will make the vehicle cost and quality higher and higher.

In order to solve this problem, the vehicle network arises at the historic moment.

Among many vehicle networks, CAN has been widely used because of its unique design, excellent performance and high reliability.

Due to the limitation of vehicle space, the CAN bus is often wired with other wire harnesses, which makes the CAN bus vulnerable to electromagnetic interference.

After the CAN bus is subjected to electromagnetic interference, the module will report the code because the signal does not conform to the definition of the protocol.

The main forms of shielding failure of CAN bus are signal delay, amplitude change, signal jump, timing disorder and so on.

Signal delay means that the time for the CAN receiver to receive the signal is greater than the time defined by the CAN bus protocol.

When the clock frequency increases, the bus load is too high, similar to the “traffic jam” site, and the signal transmission rate decreases.

At the same time, too long wire harness can also cause signal delay.

Amplitude change refers to the difference between the maximum and minimum value of voltage or current and the set value after the current or voltage signal is affected by the external electromagnetic field.

When the coupling effect of the external signal and the internal signal of the wire harness is not consistent, the result of the differential signal will be inconsistent with the preset value, and the amplitude will rise or decrease.

The signal jump indicates the abnormal slope of the rising edge or falling edge of the signal, and the rising edge and falling edge are related to the clock frequency of the signal.

When the clock frequency of the signal is high, the current changes rapidly, the intensity of the electromagnetic wave released from the outside is large, and the coupled electromagnetic field also increases, which has a stronger weakening effect on the original signal, resulting in signal jump.

Time sequence is an important parameter of the signal, and the signals of different time sequences carry different information.

After the wire harness receives the electromagnetic interference, the local information appears abnormal such as lag or amplitude change, which leads to the timing disorder of the signal and the transmission of wrong information.

1.2.2 Abnormal video signal

In the process of video signal transmission, the image information is processed by the sensor into an electrical signal for transmission, and then the current or voltage signal is converted into a digital signal through an analog-to-digital converter, and finally imaging on the display screen.

The essence of video signal transmission is electrical signal transmission, and the carrier of transmission is still wire harness, which determines that video signal transmission can not escape electromagnetic interference.

In the process of video signal transmission, if the voltage or current signal is affected by electromagnetic interference, the amplitude and time sequence of the voltage or current signal is abnormal, which will lead to the distortion of the transmitted signal and eventually lead to the abnormal video signal.

1.2.3 Abnormal control loop

The control loop of electronic equipment often performs different working states through the change of voltage or current.

And the signal of the control loop is often sensitive.

In the process of coupling, the current or voltage of the control loop in the wire harness is vulnerable to electromagnetic interference, resulting in abnormal operation.

2. Source of wire harness shielding failure

Wire harness is easily affected by vehicle interference sources.

Typical vehicle interference sources include ignition system, motor, converter, antenna and other parts.

2.1 Ignition system

The engine ignition system is mainly composed of storage battery, ignition coil, spark plug, capacitor and so on.

When the car is ignited, the current in the battery enters the primary coil, and the secondary coil produces induced current under the action of the coil core.

When the induced current in the secondary coil increases so much that it breaks through the gap of the spark plug, the energy stored in the spark plug capacitor is quickly released.

The discharge process will cause the current on the side of the secondary circuit to increase suddenly and radiate electromagnetic waves to the surrounding environment.

2.2 Motor

The motor is widely used in the whole vehicle, including wiper, water pump, seat adjustment, rearview mirror adjustment and so on.

The commutator of the motor commutes in the magnetic field and the switch contacts open and close repeatedly when working, which make the electric machine release a large number of electromagnetic waves to the surrounding environment.

Even with the shielding of the metal object, the electromagnetic wave transmitted by the motor through conduction can not be underestimated.

2.3 DC/DC converter

DC/DC converter consists of switch tube, rectifier tube, inductor and capacitor.

The sudden change of current or voltage produced by the switch tube and rectifier tube in the converter is the fundamental reason why the converter becomes the source of interference.

When the switch tube works, the turn-on time is short and the current spike is formed.

When the switch is turned off, the turn-off time is short, which will cause voltage abrupt change and form voltage spike.

Both current spikes and voltage spikes release a large number of electromagnetic waves.

2.4 Antenna

The vehicle antenna is the carrier for the whole vehicle communication electronic equipment or entertainment equipment to send and receive electromagnetic signals.

The antenna receives the electromagnetic signal in the space and converts it into an electrical signal, which is adjusted by the amplifier and transmitted to the host of the vehicle entertainment system through the wire harness.

This means that the electromagnetic waves in the space environment may be coupled by antennas.

At the same time, the nature of the antenna is still a wire, and it will also release electromagnetic waves to the surrounding environment and interfere with the electronic equipment of the surrounding environment.

3. Solution

Based on the analysis of the mode and root cause of wire harness shielding failure, it is not difficult to summarize the solution, that is, to clarify the contribution of wire harness parameters to electromagnetic interference in the current vehicle environment in the design stage, and design the wire harness layout scheme based on this.

Then modeling and analyzing whether the layout scheme can meet the shielding requirements, and bench verification.

Finally, the endurance test of the whole vehicle verifies the electromagnetic shielding performance of the wire harness.

In the verification stage, we can strictly implement the test outline, and the most difficult solution is to establish the shielding model.

The establishment of the shielding model needs to strictly consider the number of vehicle interference sources, the general intensity of interference, the characteristics of interference sources and the corresponding failure mode.

After establishing the model, the simulation results are compared with the actual performance of the bench to ensure that the simulation results are consistent with the actual performance.

When the results are inconsistent, the parameters of the model are adjusted to make the model consistent with the actual performance.

The adjustment is based on the current research results of wire harness shielding, that is, with the increase of wire length, or the increase of interference source intensity, or the decrease of the distance between wires, or the increase of wire height from the ground, the wire induced current increases. Electromagnetic interference is obvious.

The effect of wire diameter on induced current is frequency dependent. In some frequencies, the benefit of electromagnetic interference increases with the increase of wire diameter, while others are on the contrary.

At the same time, in order to reduce the shielding failure of the wire harness, the layout and design of the wire harness need to follow the following principles.

The easily disturbed wire harness is as close to the grounding as possible, and fast and good grounding can effectively reduce electromagnetic shielding.

At the same time, the wire harness is arranged as close to the sheet metal as possible, the direction of the wire harness is adjusted to avoid the strong coupling area, away from the interference source, the distance between the wire harness is increased, the shielding layer is used to isolate, and the wire diameter is adjusted according to the transmission frequency.

These design principles can also guide the troubleshooting of wire harness shielding failure.

4. Conclusion

The wire harness plays an important role in the signal transmission and voltage and current transmission of the whole vehicle, and the on-board electrical equipment is all over the vehicle, and the wire harness will inevitably be exposed in the interference source area when connecting each equipment, which leads to the failure of the automobile wire harness from time to time.

How to troubleshoot the wire harness shielding failure and how to strengthen the wire harness shielding design has gradually become an important issue in the whole vehicle research.

This paper first discusses the failure mode of wire harness shielding, then focuses on the analysis of vehicle interference sources, and puts forward the methods to solve the shielding failure according to the characteristics of various failure modes and interference sources.

That is, the model is constructed according to the characteristics of the interference source, and the sub-parts design and vehicle wire harness layout design are carried out according to the results of the model analysis.

These methods can not only improve the reliability of harness design, but also provide a reference for solving the problem of wire harness shielding failure.