Abstract: With the increase of electronic components in vehicles and the increasingly complex interior environment, automotive manufacturers are increasingly demanding component testing. This article aims to introduce the various methods of EMI immunity testing of automotive electronic components, and summarizes the advantages and disadvantages of various methods to help test engineers choose the best test method.
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introduction
Electromagnetic interference (EMI) effects have long been a concern in modern electronic control systems. Especially in the automotive industry of the day, the vehicle uses a series of in-vehicle electronic modules, such as engine management module, ABS system, electronic power steering function module, in-vehicle entertainment system and thermal control module.
At the same time, the electromagnetic environment in which the vehicle is located is also more complicated. The in-vehicle electronic components must coexist with the in-vehicle RF transmitter. These transmitters are partially installed and set up appropriately. For example, in ambulances, some are not. For example, some CB transmitters and car mobile phones installed at the factory. In addition, the vehicle may enter some areas of strong electromagnetic fields generated by external transmitters, and the intensity can reach tens or even hundreds of volts per meter. The auto industry has been aware of these problems many years ago, and all well-known manufacturers have taken certain measures to reduce the impact of electromagnetic interference by developing automotive test standards and legislative requirements. Therefore, today's vehicles have strong resistance to such interference. However, since EMI has a great impact on the performance of the vehicle module, it is necessary to continue to be vigilant.
Testing of vehicles and their components is a highly specialized field that has always been done by the manufacturers themselves. In some countries, many vehicle manufacturers will jointly fund professional test labs. With the increasing number of electronic components used in vehicles, the trend of outsourcing of components by automobile manufacturers is becoming more and more obvious. Therefore, EMC testing has gradually become the responsibility of component manufacturers.
A number of different test methods and test levels for overlapping frequencies are described in subsections of the International Standard for Immunity Testing of Automotive Components, such as ISO 11452 (International Standards Organization) and SAE J1113 (Institute of Automotive Engineers). Automotive component testing In the absence of any higher legislative requirements, vehicle manufacturers can individually develop their testing requirements based on these common standards.
That is, when an automobile manufacturer wants to set component-level test requirements for its component suppliers, he can select the appropriate amount from the list of test methods, test frequency ranges, and test levels to form his own test standard. Thus, a manufacturer that provides sub-assemblies for multiple automotive manufacturers may have to use different methods to test the same components in the same frequency range according to different standards.
Component manufacturers can design automotive component test systems with a range of RF test specifications included in ISO 11452 and SAE J1113 to help with their work to meet customer testing needs. These test systems are primarily self-contained systems that follow the highest level of test specifications specified in all standards. With such a system, many of the test instruments used by component manufacturers when testing multiple standards are the same, thus saving a lot of money. Below we will discuss several RF test methods and some of the test parameters specified in the automotive manufacturer's test requirements, and explore how component manufacturers can build a test system according to the test needs of different customers, to achieve the purpose of testing only customers.
1. RF test method
To test the RF immunity of an automotive component, RF interference must be applied to it in a manner comparable to how the in-vehicle interference occurs. This imports the first variable. The car may be exposed to an external field or may carry a transmitter and antenna with interfering signals. In any case, the interference field can act directly on where the component is located. For example, when the component is installed in an open area, such as on or near the dashboard, it creates more interference than when it is installed near the chassis of the vehicle or in a shielded area inside the engine compartment. Much more.
On the other hand, all electronic modules are connected to the wiring system of the vehicle for the purpose of power supply and signal connection. The wiring device is equivalent to an effective antenna that can be coupled to RF interference, so RF current can be conducted through the connector to the component regardless of where the component is mounted. There are two test methods we usually use: radiation interference test and conducted interference test.
1.1 Radiation interference test
All radiation tests apply a calibrated RF field to the device under test so that the RF current and voltage can be directed into the internal structure of the device, which in turn can appear on the sensing node of the active component. , thus causing interference in the electronic circuit. Different methods differ in the way the RF field is applied.
1.1.1 Radiation antenna measurement in darkroom
The simplest way to generate an RF field is to inject energy into an antenna and point it to the device under test (EUT). The antenna converts the RF energy into a radiation field and fills it with the test area. Due to the need to generate high level RF signals over a wide spectrum, the test should be performed in a shielded room to avoid interference with other legitimate radio users in the vicinity. But this will cause reflections from the walls, which will change the field distribution in the room. To solve this problem, the surface of the shielded room needs to be treated to create an 'absorber lined chamber' environment, which in turn greatly increases the cost of the test equipment. The antenna used in the test should have a wide frequency response over the measured frequency range. Test frequencies in vehicle testing may range from 10 kHz to 18 GHz, so many different types of antennas are required. Figure 1 is a typical radiation device.
Figure 1 Radiation interference test device
The field distribution applied to the EUT is also as uniform as possible and has good control. The field at the time of testing may affect the size of the shielded room. Therefore, the antenna should not be too close to the EUT, and the directivity should not be too strong. The generated field should be concentrated only in one area of ​​the EUT. If the distance between the antenna and the EUT is too close, the mutual inductance will increase, which will affect the control difficulty of the signal added on the antenna. The larger the size of the object being measured, the more difficult this requirement is. In addition, according to the formula P = (E * r) 2 / 30W (when the antenna has unit gain), the farther the antenna is from the EUT, the greater the power required to reach a certain field strength.
Note that this formula gives the squared rate relationship between field strength and distance, that is, when the field strength at a certain distance increases from 10V/m to 20V/m, the required power is 4 times, or When the field strength increases from 10V/m to 20V/m, the distance is only a quarter of the original power. The field strength at the EUT position is measured by an isotropic wide-band field sensor. Isotropic is to ensure that the sensor does not sense the direction, while broadband is to ensure that it can get the correct measurement at different frequencies. .
1.1.2 TEM unit method
According to ISO 11452-3 and SAE J1113/24, the transverse electromagnetic wave (TEM) unit is only a section of transmission line, feeding a certain RF power at one end and a load impedance at the other end. As electromagnetic waves propagate through the transmission line, an electromagnetic field is established between the conductors. TEM describes the dominant electromagnetic field generated in the active area of ​​such a unit. When the length of the transmission line is given, the field strength is uniform within a certain area, and it is easy to measure or calculate. The EUT is placed in the active area.
The TEM unit is generally in the form of a box with a partition surface inside, so the wall of the box serves as one end of the transmission line, and the isolation surface (or septum) is used as the other end. The geometry of the TEM cell has a decisive influence on the characteristic impedance of the transmission line. Because the enclosure is closed (except for small leaks), there is no electromagnetic field outside the unit, so this unit can be used in any environment without external shielding.
The main disadvantage of the TEM unit is its upper frequency limit, which is inversely proportional to its physical size (see Table 1). When the frequency is above this upper limit, high-order modes begin to appear in the structure of the internal electromagnetic field, and the uniformity of the field, especially the field uniformity at the resonant frequency determined by the exact size, also begins to deteriorate. The maximum EUT size that the TEM unit can measure is limited by the amount of field strength uniformity available within it, so there is a direct relationship between the maximum EUT size and the highest frequency that can be measured by the unit. The lowest measurement frequency of the TEM unit can go to DC, which is why it differs from the radiation antenna measurement method.
Table 1 Frequency upper limit of TEM unit method
1.1.3 Stripline method and three plane method
These two methods are fundamentally different from the TEM unit method. The TEM cell method is a closed-type measurement method, and the test device used in the strip line method and the three-plane method is an open transmission line. That is to say, when using these two methods, the maximum field is located between the planes, but there is still energy radiated to the outside, so it is necessary to test in a shielded room. Stripline testing is described in both ISO 11452-5 and SAE J1113/23, while the three-plane test is only mentioned in SAE J1113/25.
In the stripline test, the module under test exposes only the cable assembly that connects it to the associated equipment and does not expose the maximum field strength between the planes. The strip line plane serves as the source conductor of the transmission line, and a 1.5 m long cable device is placed under it, and the reference ground plane of the test is used as the other end conductor. The field created by the stripline induces a longitudinal current in the cable assembly and then enters the EUT coupling. Therefore, the stripline test is almost a mixture of the two methods of radiation field test and conduction test.
In a three-plane test set, an active inner conductor is sandwiched between two outer planes to produce an impedance that can be calculated by operation. The module under test is placed between an outer plane and the center conductor, and the other side of the center conductor is left blank. Since the structure of the entire test is symmetrical, a field strength probe can be placed on this side in a mirrored position with the EUT.
As with the TEM unit test, both the stripline test and the three-plane test set have an upper frequency limit limited by their size. When it is equal to or higher than the resonance frequency determined by the size, an uncontrolled electromagnetic field higher order mode is generated. The advantage of these three methods over the radiated antenna method is that with these three methods, only a proper power is required to produce a much larger field strength than the radiating antenna method because the field strength is equal to the voltage between the conductor planes. Divide by the distance between them.
1.2 Conducted interference test
The second type of test method, called conducted interference test, directly applies RF interference to the cable assembly, instead of applying an electromagnetic field where the module under test is placed. As the RF current is transmitted in a circuit structure (such as a printed circuit board PCB), a current is generated at the junction of the component module and the external device, causing interference in the electronic circuit. Although this method is similar to the radiation field test, there is no equivalent between the two, so these two methods are often used for complete testing, and sometimes the frequency ranges of the two tests overlap.
The two coupling methods most commonly used in conducted interference testing are the injection of a bulk current injection (BCI) that controls the magnitude of the interference current, and the injection of a direct injection method that controls the power of its size. .
1.2.1 Current Injection Method (BCI)
When the BCI method is used, a current injection probe is placed on the cable device connected to the device under test, and then RF interference is added to the probe. At this point, the probe acts as the first current transformer and the cable device acts as the second current transformer, so that the RF current flows first in the cable arrangement in a common mode, ie the current flows on all conductors of the device in the same manner Circulate and then enter the connection port of the EUT.
The actual current flowing is determined by the common mode impedance of the device at the current injection. In the case of low frequencies, this is almost entirely determined by the impedance of the EUT and the associated equipment connected to the other end of the cable assembly to ground. Once the cable length reaches a quarter of a wavelength, the change in impedance is important and reduces the repeatability of the test. 
Figure 2 Current injection test device
Current injection probes introduce losses and require a large drive capability to establish a reasonable source of interference on the EUT. Despite this, the BCI method has the great advantage that it is non-invasive because the probe can be simply clamped to any cable that does not exceed its maximum acceptable diameter without any direct cable conductors. The connection does not affect the working circuit to which the cable is connected.
1.2.2 Direct injection method
The BCI method has too high requirements for driving capability, and the isolation from related equipment during the testing process is not good. The purpose of the direct injection method is to overcome these two shortcomings of the BCI method. This is done by connecting the test equipment directly to the EUT cable and injecting RF power into the EUT cable through a Broadband Artificial Network (BAN) without interfering with the EUT's interface to its sensors and loads (see Figure 3).
Figure 3 Interference direct injection test device
The BAN can control the RF impedance over the test frequency range. The BAN can provide at least a blocking impedance of 500 W in the direction of flow to the auxiliary device. The interference signal is directly coupled to the line under test through a DC blocking capacitor. This method is described in ISO 11452-7 and SAE J1113/3.
2. Test parameters for EMI testing
In the EMI test of vehicle components, depending on the different requirements of different vehicle manufacturers, in addition to the basic methods of introducing interference signals, there are many different parameters. But regardless of how RF interference occurs, these parameters are relevant.
2.1 frequency range
Depending on the method itself and the transducers used, any of the above methods are only applicable to a given frequency range. Table 2 lists the applicable frequency ranges published by the various methods discussed in this paper in the corresponding standards.
Table 2 Applicable frequency ranges for different test methods in different standards
During the test, it is usually necessary to scan the test signal for changes or step changes over the entire frequency range. The speed of the test is important because the EUT must reflect each test frequency accordingly. The minimum residence time for the test is typically 2 seconds, and if the time constant of the EUT is large, the residence time may be longer. If a software-controlled test signal generator is used, it is usually stepped rather than scanned over the entire frequency range, so the step size of the frequency step is also defined. The combination of the residence time and the frequency step determines the time it takes to perform a single scan and thus the time required for the entire test.
2.2 amplitude control
Regardless of the test method used, the amplitude of the test signal applied to the EUT must be carefully controlled. The method of amplitude control can be divided into two categories according to different principles, one is called closed-loop control method and the other is called open-loop control method. In stripline testing and TEM unit testing, the applied field can be calculated by known net input power and transmission line parameters. In addition to these two methods, it is necessary to use a closed loop method to achieve amplitude control. In the radiated interference test, the unit of the interference signal is volts/meter. In the current injection test, the unit uses milliamps. In the direct power injection test, the unit uses watts.
2.3 closed loop method
With the closed-loop control method, a field strength meter or current monitoring probe constantly monitors the excitation applied to the EUT, thereby adjusting the power to the target value. This method has a drawback, and this problem is particularly noticeable when performing radiation interference tests in a microwave darkroom. That is, the intervention of the EUT disrupts the electromagnetic field that interferes with the excitation, so that a field strength that correctly reflects the resulting field strength and is generally applicable to all types of EUTs can be found. When the test frequency is such that the EUT size can be compared to the wavelength, the distribution of the field may be greatly reduced at some locations. If the field strength meter is placed in this position, then maintaining the required electromagnetic field strength based on the reading of the field strength meter will inevitably cause severe over-testing at the location near the EUT.
A similar problem exists in the BCI test. When the common mode input impedance of the EUT resonates with the test signal, maintaining the required current will cause over-testing. In fact, in such an environment, many times the amplifier does not provide the power needed to maintain the specified level, and once the amplifier is overloaded, it will cause more test problems.
2.4 open loop method
The open-loop method (also called the replacement method) can avoid the above problems. When using the open loop method, a signal of a given intensity is first sent to the test equipment for calibration settings. At each frequency, the output power of the amplifier is monitored by an auxiliary power meter, which is recorded when the amplifier output level reaches the target value. Finally, during the actual test, the pre-calibrated power record is replayed. In general, since the measurement of the field or current (volts per meter or milliamps) applied to the EUT is not within the requirements of the test, the open loop method does not measure them, but only monitors them to confirm the system operation. normal. But for the reasons mentioned in the previous section, we are not likely to see the true correct measurement.
In the radiation interference test, the calibration setup process requires that the EUT be placed in an accurate position in the microwave darkroom. In the conducted interference test, the calibration device is a load with a specific impedance value, and we measure the output power or current at both ends.
The power parameters used in the open loop method include the net power, or the difference between the forward power of the input converter and the reverse power reflected from the converter. Assume that in the absence of other significant losses, this difference is equal to the power actually delivered to the EUT. Therefore, when using a directional coupler, two powers must be measured at each frequency. At this time, a power meter can be used to sequentially measure the forward output and the reverse output of the coupler, or two power meters can be simultaneously measured.
The net power is used to describe the voltage standing wave ratio (VSWR) of the converter, which changes when the EUT is introduced. However, when the EUT is matched to the test set, the forward power required to maintain the net power may vary significantly relative to the power required for calibration. In order to avoid over-testing, the forward power that is increased by maintaining the required net power cannot exceed 2dB. Even if 2dB is not enough, it should not continue to increase, and this can only be recorded in the test report.
2.5 modulation frequency and modulation depth
All RF immunity tests require CW (unregulated continuous wave) and modulated AM signals on the EUT at each frequency, and the EUT's response is generally more susceptible to modulated interference. In general, the modulation signals specified in the test standard are sine waves with a modulation depth of 80% and a frequency of 1 kHz. However, there are also individual vehicle manufacturers that may have different requirements. The purpose of defining modulation parameters is to specify a constant peak level for AM and CW testing. This is different from the commercial (IEC 61000-4 series) RF immunity test. In the commercial RF immunity test, the peak power of the modulated signal is 5.3 dB higher than the unmodulated signal. In the test with constant peak level, the modulated signal power with a modulation depth of 80% is only 0.407 times of the unmodulated signal power.
The application of this signal is clearly defined in ISO 11452:
1. At each frequency, the signal strength is increased linearly or logarithically until the signal strength meets the test requirements (for the open loop method, the net power meets the requirements, and for the closed loop law, the level of the test signal meets the requirements), according to the +2dB standard. Monitoring forward power;
2. Apply the modulated signal as required and keep the test signal hold time equal to the EUT minimum response time;
3. The test signal strength should be slowly reduced before testing for the next frequency.
2.6 Monitoring EUT
When interfering signals are applied, the EUT's response must be monitored and compared to the performance criteria it should meet to determine if the device under test passes the test. Since each EUT has different functions and meets different performance standards, it is not possible to generalize these monitoring methods. But if the test software can automate some or all of the monitoring work, the whole test will be simpler and more reliable. In this process, it may be necessary to simply measure and record the output voltage at each frequency point, or it may involve special EUT software that can give a mark when the test finds an error.
2.7 Report test results
After the test was completed and the EUT's response was observed, the test engineer's work was only half completed. Engineers must also produce test reports in the format specified by the vehicle manufacturer. A component manufacturer may provide products for multiple vehicle manufacturers, so component manufacturers may need to submit test reports in multiple formats for the same set of test results.
Some packages include an optional report generation module that can be customized for each vehicle manufacturer's standard report templates. The managers of all test labs know that providing test reports to customers is one of the most difficult tasks. Although everyone enjoys the testing process, few people like to write test reports. With the automatic report generation software module, not only test engineers are relieved of the pain of writing test reports, but also can meet customer requirements faster.
Although the EMC test of components in the automotive industry contains many variable parameters, it is still possible to efficiently perform tests covering a wide frequency range for different vehicle manufacturers.
Corn Sheller,Hand Corn Sheller,Corn Sheller Machine,Hand Crank Corn Sheller
Hunan Furui Mechanical and Electrical Equipment Manufacturing Co., Ltd. , https://www.thresher.nl