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EMC test and design based on vehicle T-BOX
Release time:
2022-03-22 00:00
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Abstract: This article introduces the working principle of vehicle communication terminal (T-BOX), an EMC test method and equipment that can automatically and parallelly verify all functions of T-BOX. And the common problems such as RE, BCI and ESD encountered in the electromagnetic compatibility test of automotive electronics; detailed analysis is carried out from the aspects of principle design, device selection, PCB layout and structural design, etc., and the EMC of vehicle communication terminals is listed. The design rules ensure the good performance of the product.
Key words: automotive electronics; vehicle communication terminal; electromagnetic compatibility
foreword
As the main component of the Internet of Vehicles, the vehicle communication terminal (English name is Telematics BOX, TBOX for short), undertakes important functions such as data communication between the vehicle intranet and the vehicle external network, vehicle body data collection, remote control, etc., so the application is more and more The more widespread, especially the emergence of the national standard "Technical Specifications for Electric Vehicle Remote Service and Management System" (GB/T32960), the pre-installation rate of T-BOX will be greatly improved. At the same time, we must also deal with the challenges brought by EMC design. This article mainly introduces T-BOX and analyzes EMC design.
1 Introduction of Vehicle Communication Terminal
As an automotive electronic product, T-BOX has strict requirements on reliability, operating temperature, and anti-interference. The vehicle battery is used for power supply, and the built-in backup battery can be used as an alarm in an emergency environment. T-BOX is mainly composed of host, antenna (satellite + mobile antenna), power harness and external accessories, among which there is a mobile communication module in the host (built-in integrated satellite communication function). When the main engine is powered on, the MCU processor reads the CAN bus message data of the vehicle body, uploads the analyzed position information together with the mobile communication module to the background server through the mobile network, and sends it to the background for analysis and processing; at the same time, the vehicle After T-BOX obtains the downlink control command, it sends a control message through the CAN bus and realizes the control of the vehicle, which can help users realize various online application functions such as remote control of the vehicle, data monitoring, and remote alarm. The T-BOX system mainly includes: 4G module, MCU processor, CAN bus, audio processing, power management, interface circuit, etc. See Figure 1 for the system block diagram.
The purpose of these interfaces is summarized in Table-1 below:
Interface Type Purpose
CAN transceiver High-speed CAN is mainly used in the control of power systems with high real-time requirements; low-speed CAN is mainly used in the control of comfort systems and body systems with low real-time requirements.
LIN transceivers are used in occasions that do not require high speed and safety performance, such as body electronic accessories (window lifts, etc.).
Ethernet transceivers are currently mainly used in non-CAN parts to interconnect with gateways. It mainly solves the problems of high-speed data transmission, protocol compatibility and cost constraints.
RS232/RS485/USB2.0 transceivers are used for system debugging and testing.
Figure 1 T-BOX system block diagram
2 EMC test based on vehicle T-BOX
At present, many vehicles are equipped with Internet of Vehicles terminals, which can conveniently and efficiently connect to satellites to realize vehicle positioning, and can automatically connect to communication base stations, and send the analyzed information to third parties. For the Internet of Vehicles terminal, it is necessary to use electromagnetic waves as the carrier to realize its functions. Therefore, the EMC test of wireless terminal equipment has become more and more important. Only by passing the EMC test can the stability of T-BOX (Telematics-BOX) be guaranteed. However, the Internet of Vehicles terminal has various functions and complex technologies, and it is a technical difficulty to simulate its normal working conditions in EMC testing. This article will discuss in detail how to simulate the normal working conditions of T-BOX in EMC testing.
Vehicle T-BOX includes: 4G module, MCU processor, CAN bus, audio processing, power management, and external interface.
3 EMC test method, equipment and process of vehicle T-BOX
A kind of EMC testing method of vehicle-mounted T-BOX, is used for testing the anti-disturbance ability of the vehicle-mounted T-BOX arranged in EMC darkroom, this test method comprises sending interference signal by the interference source being arranged in EMC darkroom, this test method also The method comprises the steps of: sending the original test data of the vehicle-mounted T-BOX function to the T-BOX through the upper computer arranged outside the EMC darkroom; receiving the test data sent by the T-BOX through the upper computer; judging the received Whether the test data is consistent with the original test data sent by the host computer; if the received test data is consistent with the original test data sent by the host computer, it is judged that the function of the T-BOX is normal. The method provides a test device and method for automatically and parallelly verifying the T-BOX function based on real wireless signals.
Figure 2 Test setup diagram
Equipment construction and testing process:
The car networking system includes a smart host, a car T-BOX, a mobile APP and a background system. Among them, T-BOX is the abbreviation of telematics-box. On the one hand, it communicates with the controllers in the car through the CAN bus to collect vehicle status information; on the other hand, it communicates wirelessly with the background system and mobile phone APP. , to realize the function of displaying vehicle information on the mobile phone APP and remotely controlling the vehicle. The main functions of T-BOX include: receiving GPS signals, receiving/sending 4G data, receiving/sending CAN bus data.
At present, T-BOX uses manual serial testing in the EMC testing process. For GPS signal testing, run a simulated navigation program on the device under test, and only verify the sound and image playback functions during navigation; for 4G signal testing, use The comprehensive tester establishes a simulated interaction with the device under test and only measures level parameters.
The EMC test equipment using the on-board T-BOX includes the interference source set in the EMC dark room. The test equipment also includes: the host computer set outside the EMC dark room; the CAN optical bridge set inside and outside the EMC dark room; connecting the EMC The optical fiber of the CAN optical bridge inside and outside the anechoic chamber; the connecting device installed outside the EMC anechoic chamber to establish communication with the host computer; the waveguide connected to the connecting device connected to the inside and outside of the EMC anechoic chamber; respectively connected to the waveguide The GPS transmitting antenna and the second 4g antenna arranged in the EMC anechoic chamber; the vehicle-mounted T-BOX containing the GPS module arranged in the EMC anechoic chamber; the vehicle-mounted T-BOX passes through the CAN optical bridge, the The optical fiber, the CAN optical bridge outside the EMC anechoic chamber are connected with the host computer.
Further, both the first 4G antenna and the second 4G antenna are bidirectional antennas.
Further, both the GPS receiving antenna and the GPS transmitting antenna are unidirectional antennas.
The following beneficial effects can be obtained through the above technical solution:
(1) The real signal is introduced into the EMC laboratory to meet the requirements of verifying the actual function.
(2) Use the computer outside the EMC dark room as the upper computer, which can not only simulate the 4G data interaction between the mobile APP and T-BOX, but also perform CAN data interaction with T-BOX through the CAN optical bridge. The upper computer can automatically, parallel, and repeatedly test all functions, obtain feedback data in real time, and judge the test results without manual intervention, saving time and efficiency.
4 EMC design of vehicle communication terminal
The EMC test for T-BOX mainly includes radiation, immunity and ESD tests. Among them, RE test, BCI test and ESD test are the key points. If not considered properly at the beginning of product design, the product will appear NG during the test. The test results lead to individual frequency points exceeding the set standard; when the product is disturbed during the BCI test, the monitoring data will be garbled; ESD will cause interface damage or drive IC failure, etc. Countermeasures can reduce the EMC risk of products.
4.1 Vehicle communication terminal RE problems and solutions
The RE test is to measure the ability of the tested product to harass other equipment through space radiation. The RE test frequency of automotive electronics generally starts from 0.15MHz, and can reach up to 6GHz, covering almost all frequencies in the communication industry. There are many reasons why RE exceeds the standard, such as defective design circuits, PCB layout problems, low structural shielding efficiency, The problem of device selection and so on, so Lei has to analyze specific problems. In one of the author's projects, the frequency point of 1.2MHz in the low frequency band was seriously exceeded. After the inductor is closed, it is verified again through experiments that the radiation is significantly reduced. As shown in Figure 3 and Figure 4.
Figure 3 Test picture before rectification
For RE radiation problems, common solutions are as follows:
4.1.1 Reduce the radiated energy of interference sources
The working clock CLK frequency of each device in the internal circuit of T-BOX is also different, ranging from hundreds of kHz to tens of MHz. Usually, in order to reduce the external energy radiation of the clock, resistors ranging from 10-229Ω are connected in series on the transceiver clock line of the device, so that The rising edge of the clock pulse becomes a slow rise; at the same time, the clock line must be grounded; the space and other wiring must meet the 3W principle; other analog signal circuits should be kept away from clock interference sources.
4.1.2 PCB layout should be optimized
The loop layout area of the power supply and ground should be small, and the power inlet line should be the shortest; the differential signal lines should be on the same layer, parallel in equal length, and have the same impedance, and there should be no other traces between the differential lines; there should be fewer holes for high-frequency signals to go through; individual DC -DC power supply plus shielding shell grounding treatment; important signal lines do not go in broken lines.
4.1.3 Device layout should be reasonable
Strong radiation devices such as crystal oscillators and crystals should be kept away from sensitive devices, and away from the edge of the PCB. An isolation ground should be added around the crystal oscillator, and the lead wire between the crystal oscillator and the MCU should be short; the filter protection device of the interface signal should follow the principle of protection before filtering.
Through the above multiple treatment measures, the RE interference energy can be reduced, and the problem of RE radiation exceeding the standard can be effectively solved.
Figure 4 Test picture after rectification
4.2 BCI problems and solutions of vehicle communication terminals
BCI "Bulk Current Injection" (large current injection) is used to measure the ability of each vehicle equipment to resist the radio frequency interference signal coupled to the wiring harness, and it is an anti-interference test method commonly used in automotive electronic products. The working principle of BCI is to use the current injection probe transformer to induce the interference signal to the wiring harness. After the radio frequency interference signal is coupled by the current injection probe, it is converted into a radio frequency current and flows through the wiring harness in a common mode. Therefore, the interference mode is mainly common Mode interference seems to exist, and the frequency range of the general test is from 1MHz to 400MHz. According to the test standard IS011452-4:2005 "Road Vehicles. Component Test Methods for Electrical Interference Generated by Narrowband Radiated Electromagnetic Energy Part 4: Large Current Injection (BCI) Method", the T-BOX is tested and tested. Including 4G communication, CAN communication and other main functions for real-time monitoring.
During the BCI test, it is often found that the CAN message data and the USB interface data appear garbled; the button interface occasionally malfunctions. To analyze the above experimental principles, the following two methods are generally adopted:
(1) For USB, CAN, etc., which belong to differential mode signal transmission, and the applied loop interference is of common mode type, common mode inductors on the string can better suppress common mode interference, and common mode inductors have no attenuation to useful differential signals. .
(2) For single-ended signals, such as buttons, LEDs and other differential-mode loop types, the filter method is used, and the filter capacitor is placed at the cable entrance. According to the frequency range of the applied interference, the effective filter frequency band and capacitor value are selected. Reduce and filter out interference signals.
Therefore, for BCI test, according to different circuit design patterns, adopting effective countermeasures can achieve obvious results.
4.3 ESD design of vehicle communication terminal
ESD is the most important link in the EMC design of T-BOX. The ESD test is to evaluate the anti-disturbance ability of the vehicle-mounted equipment when it is subjected to direct contact discharge from the operator and indirect discharge from nearby objects. SD failure will cause some interface functions of the T-BOX host Failure or device damage directly affects the use of the product. For ESD protection design, it is mainly considered from the perspective of hardware, including comprehensive processing methods such as schematic design, PCB layout and structural design. The relevant design experience is listed below for your reference.
(1) In the early stage of design, it is necessary to determine the ESD withstand voltage level of T-BOX, and the selected anti-static device should be higher than the test voltage to ensure that static electricity will not damage the internal components of the T-BOX interface during the ESD experiment.
(2) In the schematic design stage, it is necessary to reserve ESD protection measures such as TVS tubes at all interfaces of T-BOX interacting with the outside world to improve the anti-ESD interference ability of the interface.
(3) When laying out PCB components, it is necessary to ensure that the path of static discharge through the TVS tube to the ground is as short as possible and the impedance of the discharge circuit is low enough; ESD protection measures such as TVS tubes should be placed near the interface as much as possible. When transmitting from the outside to the inside along the signal line, it must first pass through the TVS tube and protective measures, and a small resistor can be connected on the signal line to effectively intercept the static current from flowing into the board. When laying out all the components of the host, the distance between the device and the edge of the surrounding board should be ≥5mm, and the signal line should not be laid in this area, but should be covered with ground layers. For multi-layer PCBs, more ground holes should be placed on the edge of the board.
(4) When designing the structure, the shell gap should be small enough to prevent static electricity from disturbing the internal PCB board of the shell during air discharge and damaging the device.
5 Conclusion
With the development of the automobile industry, the production and ownership of automobiles are constantly increasing. EMC design plays an increasingly important role in automotive electronics. The EMC standards of automotive electronics are stricter than other types of products. Through EMC tests, T -BOX electromagnetic compatibility design capability. Aiming at the problem of EMC testing, we can start from the design source and take effective countermeasures to reduce the EMC risk of the product and ensure the product quality. The EMC solution mentioned in this paper can also be applied to other types of electronic products.
references
(1) Wang Xi, Electronic Component Selection Technology in EMC Design of Automotive Electronics [J]. Electronic Components and Materials, 2014.9:99-100.
(2) Li Yanqiang, Zhang Xiaofang. Electromagnetic compatibility test method for the whole life of automotive electronic products [J]. Shandong Science, 2017.4:133-138.