Analysis of several EMI test methods for automotive components

For many years, the effect of electromagnetic interference EMI test has been a matter of great concern in modern electronic control systems. Especially in today's automobile industry, cars use many critical and non-critical on-board electronic modules, such as module management module, anti-lock system, electronic power steering program modules, in-car entertainment system and thermal controller.

At the same time, the electromagnetic environment of the vehicle is more complex. The electronic components on the vehicle must coexist with the frequency transmitter. Some of these transmitters are installed and set properly (for example, in emergency service vehicles), but some are not (for example, some transmitters are installed after delivery) CB transmitters and mobile phones). In addition, the vehicle will enter some strong magnetic field areas generated by external transmitters, with the intensity of dozens or even hundreds of Ford per meter. The automobile industry has been aware of various problems for many years, and all well-known manufacturers have taken certain measures to reduce the impact of electromagnetic interference according to the established test standards and legal provisions. Therefore, today's vehicles have strong resistance to such interference. EMI has a great impact on the characteristics of on-board modules, so we must be alert again.

The testing of vehicles and parts is a highly specialized industry, which has been carried out by the manufacturer. In some countries, many automobile manufacturers will support these professional test laboratories together. With the increasing number of commonly used subcomponents in automobiles, the trend of auto manufacturers outsourcing parts is becoming more and more obvious. Therefore, EMC testing has gradually become the obligation of parts manufacturers. Various test methods and test levels are described in the sub-chapters of national automotive parts immunity test standards such as ISO11452 (International Organization for Standardization) SAEJ1113 (Association of Automotive Engineers), with overlapping frequencies. In the absence of any higher legal requirements, automobile manufacturers can formulate their test provisions on the premise of this general standard. That is to say, when an auto manufacturer wants to develop the parts dealer's requirements for parts level test, he can select the appropriate account, including a variety of test methods, test frequency range and test level details, to form his own test specifications. Finally, a manufacturer providing sub-parts for many automobile manufacturers may have to choose different methods according to different specifications and test the same parts in the same frequency range.

Figure 1. Typical radiated interference test device

In order to meet the customer's test requirements, the component manufacturer can take a series of measures to help complete the work, including the RF test standards contained in ISO11452 and SAEJ1113 and the designed vehicle component test system. This kind of test system is usually self-contained, which should meet the highest level test standards specified in all specifications. After selecting this system, many test instruments used by parts manufacturers in testing their multiple standards are the same, so a lot of money can be saved. We will discuss the following categories of RF test methods and some test parameters specified in the test requirements of automobile manufacturers, and discuss how the parts manufacturers build corresponding test systems according to the test requirements of different customers, so as to achieve the effect of only testing the necessary items.

Several RF test methods

图2.电流注入法测试装置

If you want to test the RF anti-interference of an automobile part, you need to increase the RF interference with the interference in the vehicle in an unusual way. This leads to the first variable. The vehicle may be exposed outside the field, or may carry transmitters and antennas, which may produce interference signals, but in any case, the interference field can be directly applied to the location of components. For example, when a component is installed on or near the open area of the instrument panel, the interference caused by it is much greater than that caused by the shielding area installed around the vehicle chassis or in the module box. On the other hand, in order to meet the needs of power and signal connection, all electronic modules are connected to the vehicle's wiring system.

The wiring equipment is equivalent to an efficient antenna, which can be coupled with RF interference no matter where the components are assembled. RF current is transmitted to the components through its connectors. Therefore, two test methods are generally adopted: radiation interference test and transmission interference test.

Test radiated interference test

Each radiation test method also does not add strength to the equipment to be tested externally to obtain the corrected RF, so that the RF current and voltage can be introduced into the internal structure of the RF current and voltage sensitive nodes of the device, and the current and voltage will appear again, resulting in interference in the electronic circuit. Different methods are applied to RF fields in different forms, and each has its own advantages, disadvantages and limitations.

Measurement method of microwave anechoic chamber radiation antenna

The simplest and clearest way to form the RF field is to inject kinetic energy into the antenna and deflect it to the equipment to be tested (EUT). The antenna can convert RF energy into radiation field and fill the test area. Because the high level RF signal must be tested in a very wide frequency range in a shielded room to prevent mutual interference with other legitimate radio customers around. But this will introduce the reflection of the wall, and then change the spread of the indoor field. In order to solve this problem, it is necessary to conduct radio noise reduction on the surface of the shielding chamber and create an "absorber linear chamber" environment, which greatly increases the cost of the test equipment. The antenna used in the test shall have a wide frequency response. The test frequency in the vehicle test may be 10kHz to 18GHz, so there are a variety of antennas required (see Figure 1). In addition, EUT should be as symmetrical as possible on the field and well controlled. The site during the test will seriously affect the specification of the anechoic chamber, so the antenna cannot be too close to the EUT, and the specificity cannot be too strong, otherwise the field will only be concentrated in one area of the EUT. At the same time, the distance between the antenna and EUT is too close, which will also cause the mutual feeling between them to expand, thus increasing the difficulty of signal manipulation on the antenna. The more physical specifications of the tested object, the more difficult it is to achieve this spacing regulation. In addition, according to the formula P=(E · r) 2/30watts (when the antenna has module gain), the farther the antenna is away from the EUT, the greater the power required to make a strong exit.

Please note that the formula gives the square-rate correlation between field strength and distance, that is, when the distance between field strengths is given, when the field strength is expanded from 10V/m to 20V/m, the required power is 4 times of the original, or when the field strength is expanded from 10V/m to 20V/m, the distance is only one quarter of the original. EUT measures the field intensity through anisotropic broadband field sensors. The purpose of anisotropy is to ensure that the sensor is not sensitive to the direction, while the purpose of broadband is to ensure that it can obtain the correct measurement value at each frequency.

TEM element method

Figure 3. Interference direct injection method test device

According to the provisions of ISO11452-3 and SAEJ113/24, if the module is only a simple closed transmission line, then a transmission line will feed RF at one end and a load impedance at the other end. The propagation of electromagnetic waves in transmission lines creates a magnetic field between conductors. TEM (horizontal electromagnetic wave) describes the magnetic field generated in the effective area of these units. When the transmission line length is fixed, the field strength is symmetrical on a certain cross section, and it is easy to measure or calculate. EUT is placed in the functional area of TEM unit. TEM modules generally appear in the form of shells, with a protective surface inside. The wall of the shell is used as one end of the transmission line, and the protective surface (or diaphragm, diaphragm) is used as the other end. The geometry of TEM cells has a decisive impact on the characteristic impedance of transmission lines. The enclosure is closed. Except for small leakage, there is no magnetic field outside the module, so this type of module can be used in all environments without external shielding. Its main disadvantage is that there is frequency limit, which is inversely proportional to its physical specification (see Table 1). When the frequency is higher than this limit, the higher-order mode and field uniformity begin to appear in its internal magnetic field structure, especially the uniformity of TEM resonance frequency is determined by the accurate specification of the unit, and also gradually decreases. TEM can more effectively measure the EUT specification of the module, which is limited by the volume of the symmetrical area of the internal available field strength, so the larger EUT directly affects the maximum frequency of the module. The minimum measurement frequency of TEM unit can be DC, which is also the difference between TEM unit and radiation antenna measurement.

Strip line method and three-plane method

These methods are essentially different from the TEM module method. The TEM modular method belongs to the closed measurement method, and the test device used by the stripline method and the three-plane method is an open transmission line. In other words, when selecting these two methods, although the larger site is located in the middle of the plane, there is still kinetic energy radiation to the outside of the test device, so the test must be conducted in a shielded room. ISO11452-5 and SAEJ stripline tests are described in 1113/23, while the three-plane test is only mentioned in SAEJ1113/25.

In the stripline test, the component module to be tested is only connected with the cable device connected to the relevant machine, and is not exposed to the maximum field strength between the planes. As the source conductor of the transmission line, the stripline plane is placed at the lower part of the 1.5m cable device, and the test reference plane is used as the conductor at the other end. The field generated by the stripline will sense the leading current in the cable device, and then the lotus root will enter the EUT. Therefore, the stripline test is almost a mixture of two methods: radiation field test and transmission test.

In the three-plane test device, an active inner conductor is sandwiched between two outer planes, and the resulting impedance can be measured. The module to be tested is placed between the core conductor and an outer plane, and the other side of the core conductor is empty. Because the structure of all test devices is symmetrical, the EUT can be placed on the empty side to place a field intensity probe in the mirror area. TEM is the same as module test, and the upper frequency limit of stripline test and three-plane test device is limited by its specifications. When it is equal to or higher than the resonant frequency determined by the principle specification, uncontrolled high order mode of magnetic field will be generated. Compared with the radiation antenna method, the advantages of these three methods are that only a small amount of power can produce a stronger field than the radiation antenna method when selecting these three methods. Because the field strength is equal to the voltage between the conductor planes, except for the distance between them.

Conducted interference test

The second test method is called conducted interference test. It does not need to add magnetic field at the position of the module to be tested, but directly adds it to the module to be tested. RF is in the cable device or the device connecting the module of the component to be tested, and the influence is increased. During the intermediate transmission of the current in the RF circuit structure (such as the printed circuit board) PCB, the connection between the component module and the external equipment will generate the current, which will affect the electronic circuit. Although this method is similar to the radiation field test method, there is no equality between them, so these methods are usually used for detailed testing, and sometimes the frequency categories of the two tests will overlap. The two most commonly used lotus methods in conducted interference test are the current injection method (BCI) and the immediate injection method. The former should inject the influence current into the EUT and control the injection current, and the latter should inject the power and control the injection power.

Table 1. Upper frequency limit of TEM cell method

Current injection method (BCI)

BCI method is described in ISO11452-4 and SAEJ113/4. When this method is selected, the current is injected into the probe, placed above the cable device connecting the component to be tested, and then the probe RF influence is injected. At this time, the probe acts as the first current converter and the cable device acts as the second current converter. Therefore, the RF current first flows through the cable device in a common mode (that is, the current flows in all conductors of the device in the same way), and then enters the EUT connection port.

The real current flow is determined by the common-mode impedance of the current injection device, but at low frequencies, this is almost entirely determined by the common-mode impedance of the current injection device. The EUT determines the impedance of the equipment related to the other end of the cable device to the ground. Once the cable length reaches a quarter wavelength, the impedance change becomes very important, which may reduce the accuracy of the test (see Figure 2). In addition, due to the loss caused by current injection into the probe, it is necessary to have greater driving capacity EUT to build a reasonable impact level. Even so, BCI law may have a great advantage, that is, its non-intrusive, because the probe can be simply clamped on all cables of acceptable diameter and diameter, and it does not need to connect all direct cable conductors, so it will not affect the working circuit of cable connection.

Immediate injection method

The BCI law stipulates that the driving capacity is too high, and the protection of relevant machines is not good in the testing process. The purpose of immediate injection method is to get rid of its BCI defects. The specific method is to directly connect the test equipment EUT to the cable, inject RF power into the EUT cable according to the Broadband Artifical Network (BAN), and do not affect the EUT and the socket of sensor and load (see Figure 3). The RF impedance energy control presented by the BAN EUT within the test frequency range. BAN auxiliary equipment can provide at least 500 W impedance impedance in the inflow direction. Electromagnetic interference is directly coupled to the measured line through direct capacitance. ISO11452-7 and SAEJ113/3 describe this method.

EMI test parameters of automobile components

In the process of EMI testing of vehicle components, according to different regulations put forward by different automobile manufacturers, there are many different parameters besides the basic methods of introducing electromagnetic interference. How the RF impact is caused is related to these parameters.

Frequency category

The test method itself and the commonly used transducer (transducer) limit that each of the above methods is only applicable to a given frequency category. Table 2 lists the available frequency categories of various methods published in the corresponding standards. In the test process, it is usually necessary to scan the test signal in all frequency ranges, and then detect that the EUT test conclusion is different from the expected function and performance. If the minimum dwell time for each test is generally 2 seconds, if the EUT time constant is large, the dwell time may be longer. If the test signal generator controlled by software is used, the test signal usually does not scan all frequency categories, but selects the step method, so the step size of frequency step should be defined. The retention time and frequency steps depend on the time required to perform a single scan, which also determines the time required for all tests.

Amplitude control

No matter which test method is adopted, it shall be applied to the range of the above test signals that the EUT must carefully operate. According to different principles, the strength control method can be divided into two types, one is called closed-loop control method, and the other is called open-loop control method. In the stripline test and TEM in the module test, the field can be calculated from the known net input power and transmission line parameters. However, in addition to these two methods, the closed-loop method should also be used to complete the strength control. In the radiation interference test, the electromagnetic interference unit selects volts/meter. In the current injection test, the enterprise selects milliamps. In the instant power injection test, the enterprise selects watts.

Closed loop method

Table 2. Applicable frequency range of different test methods in different standards

When selecting the closed-loop control method, a field strength meter or current monitoring probe has been detecting and applying EUT to adjust the power to the target. There is a problem with this method, that is, the EUT intervention has disrupted the magnetic field that we encourage, so we can't find a field that can properly reflect the field strength we have obtained. This problem is particularly obvious when all types of EUT radiation interference tests are conducted in the microwave anechoic chamber. Promote the test frequency EUT. Compared with the wavelength, the specification and wavelength may be significantly reduced in some parts. If the field strength meter is placed in such a position, then when we maintain the required electromagnetic field strength according to the reading of the field strength meter, it will certainly cause serious over-testing around the EUT. When there are similar problems in the test of BCI, when the common-mode input impedance of EUT resonates with the test signal, the current to be maintained will lead to over-testing. In fact, in such an environment, many times the amplifier cannot provide the required level. Once the amplifier is loaded, it will cause more test problems.

Open-loop method

The open loop method can avoid the above problems. The open loop method is sometimes called the replacement method. When selecting the open-loop method, first send the signal with clear strength to the test equipment for correction and setting. At each frequency, the output power of the amplifier is monitored by the auxiliary power meter and recorded when the output level of the amplifier reaches the target. Finally, in the real test, the pre-corrected power record is strictly replayed. Generally speaking, because the EUT field or current (voltage per meter or milliamps) measurement applied during the application is not within the scope of test requirements, the open-loop method does not measure them, but only monitors them to ensure the normal operation of the system. However, due to the reasons mentioned in the previous section, we can't see the truly correct measurement value. In the radiation interference test, the EUT is required to place the field intensity meter at the exact position that the microwave anechoic chamber should occupy during the calibration setting process. However, in the conducted interference test, the correction equipment is the load given by the impedance value, and everyone measures the power or current on both sides. The common power parameters of the open-loop method include the net power, or the difference between the forward power of the input transducer and the reverse power reflected by the transducer. Assuming that there is no other important consumption, this error is equal to the power actually sent into the EUT. Therefore, when using the immediate coupler, it is necessary to measure two power at each frequency. At this time, you can use a power meter to measure the forward and reverse output of the coupler, or you can use two power meters to measure at the same time. The net power is used to represent the voltage standing wave ratio (VSWR) of the transducer, because the VSWR will change when the EUT is introduced. However, when the EUT is strictly matched with the test equipment, the forward power required to maintain the net power may vary significantly from the power required for correction. In order to prevent excessive measurement, the forward power increased to maintain the required net power shall not exceed 2dB. Even if 2dB cannot meet the requirements, it cannot be further expanded and can only be recorded in the test report.

Deployment frequency and depth

Each RF immunity test shall apply CW (unmodulated continuous wave) and modulated continuous wave) AM signals to EUT at each frequency, and EUT response is usually more susceptible to adjustment. Generally, the deployment signal specified in the test standard is 80%, and the frequency is 1kHz sine wave. However, some vehicle manufacturers may have different regulations. The purpose of defining deployment parameters is to define deployment parameters. AM and CW tests require a constant peak level. This is related to business. (IEC61000-4 series) RF different immunity tests. In business. RF In the anti-interference test, the peak power of the deployed signal is 5.3dB higher than that of the unadjusted signal. However, in the constant peak level test, the power of the modulated signal with the deployment depth of 80% is only 0.407 times that of the unmodulated signal. The application process of such signals is clearly defined in ISO11452:

● At each frequency point, expand the signal strength linearly or mostly until the signal strength meets the requirements (the net power of the open-loop method meets the requirements, and the signal level of the closed-loop method strictly meets the requirements). The 2dB rule detects the forward power.

● Apply the adjusted signal as required and keep the test signal equal to the EUT minimum response time.

● Slowly reduce the test signal strength, and then conduct the next frequency test.

Test EUT

When the test signal is applied, the EUT response must be tested and compared with the performance rules to confirm whether the components to be tested are tested. For different reasons. EUT function and performance rules must be different, so it is impossible to summarize these regulatory methods. If the test software can automatically perform part or all of the test work, all tests will be simpler and more reliable. The test process may only need to simply measure and record the output voltage at each frequency point, or it may involve some special EUT that can identify the software when errors are found in the test.

Report test results

In the EMI test, EUT has observed that the work of the test engineer is only half completed. Then he or she must establish a test report in the format specified by the vehicle manufacturer. One component manufacturer may provide products for multiple automobile manufacturers, so for the same group of tests, component manufacturers may need to submit test reports in various formats. Some packages contain optional report forming modules, which can provide standard report templates for different automobile manufacturers. Although most test engineers like the test process, few people like to write test reports, so all test laboratory supervisors understand that providing test reports to users is the most difficult task. By forming software modules through automatic reports, not only do test engineers not have to bear the pain of writing test reports, but also can quickly meet customer requirements. Generally speaking, although the EMC test of components in the automotive industry contains many variable parameters, we can still carry out efficient coverage according to different automobile manufacturers.