Solve the electromagnetic compatibility problem by ground offset test and vehicle system grounding design

With the development and application of more and more high-tech automotive electronic products, how to solve the electromagnetic compatibility problem of automotive electronic systems and improve the reliability and safety of automobiles has become a very important and urgent issue. However, as one of the methods to solve the electromagnetic compatibility problem, ground offset test is particularly important. Therefore, this article explains ground design and ground offset test.

1. Vehicle system grounding design

1. The significance of ground wire

The ground wire is not just a connection point in the car, it is a comprehensive system of the car electrical system, its main functions are:

● Provides current loops for DC loads, AC loads and transient loads, and connects to the negative terminal of the battery or generator;

●Provide voltage to sensors, communication systems, single-ended digital inputs, etc.;

●Electrostatic shielding to isolate external RF radiation;

●Provide electrostatic discharge leakage, ESD protection;

●Ground plane for car antenna;

●Reduce the electrical level and reduce corrosion.

2. Ground wire reliability

The reliability of the ground loop is mainly determined by the following key factors:

●The connection surface of the grounding metal, including the connection between the grounding plates, and between the grounding wire and the grounding plate;

●The influence of coating and lubricating oil on the conductive ground connection plate and its fasteners;

● Potential for corrosion;

●Potential mechanical degradation.

3. The grounding symbol and grounding circuit on the car are shown below

Vehicle electrical ground: mainly the generator and battery in the DC circuit, and the RF ground of all products in the AC circuit;

Vehicle structure ground: marked as the ground mark of the vehicle structure (such as engine, body in white, etc.);

Product circuit grounding: This symbol can be used for product circuit grounding, including analog ground and digital ground;

4. Grounding structure diagram used in actual vehicle

Figure 1: Grounding structure of a real vehicle

This is a diagram of the grounding structure used in a real vehicle, where all grounding ultimately returns to the negative terminals of the battery and generator. As the frequency increases, the impedance of the loop will also increase, which will eventually cause current to flow through an undesirable loop, resulting in common-mode interference, and thus EMC effects that damage the product.

Then everyone will ask why ground offset will produce common-mode interference? When talking about common-mode interference, we have to talk about differential-mode interference. Let's learn about it together.

2. Common mode interference and differential mode interference

The communication line of electrical equipment, the communication line for exchanging with other equipment or peripheral equipment, has at least two wires. These two wires are used as round-trip lines to transmit power or signals. In addition to these two wires, there is usually a third conductor, which is the "ground wire". There are two forms of voltage and current changes when they are transmitted through wires. One is that the two wires are used as round-trip lines for transmission, which we call "differential mode"; the other is that the two wires are used as the outbound path and the ground wire is used for return transmission, which we call "common mode".

figure 2

As shown in the figure above, the blue signal is transmitted back and forth between the two wires, which we call "differential mode"; and the yellow signal is transmitted between the signal and the ground wire, which we call "common mode".

The interference between any two power lines or communication lines can be expressed as common mode interference and differential mode interference: common mode interference is transmitted between the wire and the ground (casing), which is asymmetric interference, and it is defined as the unwanted potential difference between any current-carrying conductor and the reference ground; differential mode interference is transmitted between two wires, which is symmetrical interference, and it is defined as the unwanted potential difference between any two current-carrying conductors. In general, common mode interference has a large amplitude and high frequency, and can also radiate through the wire, causing greater interference. Differential mode interference has a small amplitude and low frequency, and causes less interference. Figure 3 shows common mode interference.

image 3

The current magnitude of common mode interference is not necessarily equal, but the direction (phase) is the same. The external interference of electrical equipment is mostly common mode interference, and the external interference is also mostly common mode interference. Common mode interference itself generally does not cause harm to the equipment, but if the common mode interference is transformed into differential mode interference, the interference will be serious because the useful signals are all differential mode signals. Figure 4 shows differential mode interference.

Figure 4

The currents of differential mode interference are equal in magnitude and opposite in direction (phase). Due to the distributed capacitance and inductance of the traces, discontinuous impedance of the signal traces, and the signal return path flowing through unexpected paths, the differential mode current will be converted into common mode current.

In order to ensure that the vehicle network can communicate normally without error frames when ground offset occurs, a ground offset test of the entire vehicle is required.

3. CANDT ground offset test

1. Test Purpose

This test case is used to check the CAN bus communication status of the DUT during the ground offset fault state, and to check whether the DUT can restore the CAN bus communication after the fault is repaired.

2. Judgment basis

When the ground offset voltage changes from 0V to 2V (the user can set this range), the DUT is not allowed to have CAN bus communication failures (such as sending error frames, etc.).

3. Test principles and steps

Figure 5

●Configure the control board to ground offset test mode;

●Configure and turn on the DUT power supply;

●CANScope normal mode connection;

● Adjust the voltage source to increase the ground voltage of the DUT until the DUT stops CAN bus communication;

●Restore the ground voltage of DUT to normal state (around 0V), and use CANScope to record the CAN bus data sent by DUT;

●Analyze whether the message can be received correctly and determine the result.

This test is used to check the reliable operation of components. For example, if a component is powered by both the battery and the engine, the power supply voltages may be inconsistent, resulting in ground offset. In order to detect whether the CAN signal can communicate normally, the CANDT consistency test system of ZLG Zhiyuan Electronics can be used.

CANDT Conformance Test System

The CANDT consistency test system can automatically complete the consistency test of the physical layer, link layer and application layer of the CAN node. It is the only instrument in the current CAN bus test field that can perform complete physical layer automatic testing and export reports. The detailed architecture is shown in the figure below.

Figure 6

According to the test standard, CANDT is used to perform consistency test on the ground offset of each node on the bus, and the test results can be generated into a report, as shown in Figure 7.

Figure 7

●After the test is completed, users can export the automated test report. For the test items that failed, the CAN bus analyzer can be used to eliminate the interference of the fault and perform reliability tests to enhance the robustness of the tested equipment. In addition, ZLG Zhiyuan Electronics' CAN bus experts will assist users to analyze the failed items and give rectification suggestions;

●According to the test report, you can also track the standard source of the test items, test steps, and judgment basis;

●The OEM can evaluate the quality of CAN nodes based on this report, which can be used as the basis for OEM access.