- Buyers Guide
An Update on Automotive EMC Testing
Electronic components are increasingly critical in the safety and functional features of automotive vehicles. Moreover, the world is also increasingly connected through electromagnetic communications. Electromagnetic compatibility (EMC) of various electronic parts internal to automobiles as well as compatibility to the environment that they operate in are becoming more challenging to designers in automotive industries. EMC testing at both full vehicle and component levels serves as one of the critical links to the overall integrity and functional verification of automotive vehicle designs. This article provides an overview of the recent trends in the industry and EMC testing requirements.
Starting from the mid-1990s, electronic components had already outweighed their mechanical counterparts in the overall content of a vehicle. This trend continues and there is no reason that it will stop. “Today’s cars are 4-wheel vehicles with dozens of computer systems,” summarized Professor Todd Hubing of Clemson University at the 2011 Asia Pacific EMC (APEMC) Symposium. “Tomorrow’s cars will be computer systems with 4 wheels,” he added.1 Owing to the extensive design and testing efforts by the auto industries, today’s cars are safer than ever, but there is also room for further improvements.
A sharp increase of fossil fuel burning cars in the recent years has stressed the world’s limited supply of fossil fuels. Sparked by the rising cost of fossil fuels, green vehicles and related technological developments are increasingly desired. Hybrid and electric vehicles are strongly incentivized by governments and welcomed by consumers around the world. One of the most common approaches to increasing the energy efficiency is to recover kinetic energy and to convert it into electric energy to store in batteries. The stored energy can then be converted to kinetic energy on demand. Throughout the process, a series AC/DC, DC/DC, and DC/AC conversions will be required. To enhance the efficiency in the conversions, faster switching time is desired. However, abrupt switching during the conversions will generate harmonic components that might emit unintentional electromagnetic energy through interface wires and/or through radiations. These may generate internal and external EMC problems in a vehicle design. Additional design effort and verification testing are required to ensure optimum balance is achieved.
On the other hand, vehicles with advanced safety features and intelligent cars are in demand by consumers. These additional safety and operational features are most commonly achieved through the use of electronic systems installed in the vehicle. EMC challenges for the critical safety and operational functions are also new topics to designers in the auto industries.
Facing the increased complexity of the automotive EMC challenges, both system and component designers of automotive vehicles are relying more and more on both signal integrity planning on the PCB and module levels to reduce the risk level of EMC issues, and on testing for final verifications. Although EMC testing is not the final solution of the automotive EMC problems, it serves as an early warning system to detect system design EMC problems, and provides a means to simulate post-accident problems to aid the development of solutions. Therefore, EMC testing also plays an increasingly important role in automotive system developments.
Auto EMC Testing Requirements Updates
Automotive EMC testing requirements can be separated into two distinctive categories, full vehicle or component levels. The full vehicle testing requirements are most commonly practiced by major automakers, while the component level testing requirements are applied primarily by suppliers of electronic parts contracted by the automakers. Most automotive testing requirements are defined and issued by regional organizations, i.e. European Union (EC) and American (SAE), international (CISPR, IEC and ISO), and manufacturers (GM, Ford, VW, etc.). More detailed illustrations of these EMC test standards and their recent development can be found in articles and symposium digests by Wiles2 and Shin.3
Among the standards shown in Table 1, recent development activities have focused on automatic EMC testing in the ISO, CISPR, and IEC standard bodies more than the other regional bodies. These standard bodies are focused on EMC issues on electric vehicles and/or fossil fuel electric hybrid vehicles. The ISO-series of test standards are focused on the immunity issues while the CISPR-series test standards are focused on the emission limits on these vehicles at charging modes. Also noted is that the European communities are faster in developing test standards addressing the advent of the EMC problems presented by the electric vehicles and by their operating environment.
Automotive EMC Test Facilities
Figure 1 One of four General Motors' Milford full vehicle EMC test chambers. (Image courtesy of ETS-Lindgren.)
Figure 2 Chassis dynamometer turntable (a) and its support filtered AC/DC power system (b). (Image courtesy of ETS-Lindgren.)
Full vehicle EMC test facilities are normally built by major automakers and government agencies around the world. Figure 1 shows one of the General Motors’ full vehicle EMC test chambers with most of the vehicle electromagnetic immunity test setups released. The EC directive and some manufacturers’ internal standards also require that the vehicle’s Electromagnetic Immunity (Susceptibility, EMS) be tested at simulated motion speed(s) and at braking mode. This will require that the full vehicle EMC test chamber be equipped with a chassis dynamometer turntable. As shown in Figure 2, the chassis dynamometer and its driving system will add to a substantial amount of complexity of the overall facility construction. In addition, to accommodate the required EMC testing on electrical and/or hybrid vehicles, the charging station and its interfaces to vehicle at charging mode will be needed to complete the full vehicle test capability. The full vehicle test chambers and its test instrumentation are most frequently used for EMS testing, especially the safety features of the vehicles at most known test conditions. Therefore, efficiency in test setups for EMS test is often the most critical utilization factors of the test facility.
The component test chambers are also often called Absorber Lined Shielded Enclosures (ALSE) and a detailed definition of ALSE dimensions and absorber treatment requirements can also be found in the references.4,5,6 To incorporate the EMC test capability on the electric motor/generator module of the electric/hybrid vehicle, a shielded drive shaft will be needed to provide the drive/load simulation for testing the EMC of the control unit in the motor generator module. The shielded shaft must be designed to transfer and withstand the maximum torque output by the electric motor through ALSE
Figure 3 Simplified component level ALSE test chamber with drive/load simulator interface.
As shown in Figure 3, the ALSE room is generally setup for near-field test distance at 1 m separation between the antenna and the EUT interface cables.
Figure 4 Total electric field vs. tangential and radial components.
The ALSE specifications for the absorber treatment only start at 80 MHz and above. The specification assumes that the near-field coupling dominates the overall energy transfer between the test antenna and the test object in the frequency range below 80 MHz. However, a simple electric field dipole coupling model analysis presented by Liu7 shows a different result. Figure 4a presents the ratio of the total amount of transferred energy to the energy from the radiating component of two electrically small dipoles at 1 m separation distance. As presented, the near-field coupling overwhelmingly dominates the energy transfer for test frequencies below 20 MHz (greater than 10 dB). Above 200 MHz, the radiating far-field component dominates the energy ratio (by more than 10 dB since the ratio shows <-10 dB). To examine the transition frequency range between 20 and 200 MHz, a ratio of reactive energy to the radiating components demonstrates an alternating variation of radiating energy in this frequency range with a peak at 50 MHz. Again, at above 100 MHz, the near-field coupling diminishes to an insignificant amount.
Figure 4 (a and b) illustrates the importance of good ALSE boundary conditions in the overall measurement uncertainty component EMC testing in an ALSE in the frequency range of 20 to 200 MHz. Since most of the electric inverters and converters for electric and/or hybrid vehicles operate in lower MHz frequency range, a good ALSE room that provides good termination conditions of the shielded wall can be very important for improving the measurement uncertainty and the repeatability of the test results. Thus, an ALSE treated with ferrite tile-based hybrid absorbers can be a preferred choice for automotive component test chambers, especially for testing such vehicles’ components between 20 to 80 MHz.
Figure 5 Component (a) and full vehicle reverb test chambers (b). (Image courtesy of ETS-Lindgren.)
In addition to ALSE and anechoic chambers, reverberation chambers have also been introduced for automotive EMC testing. The reverberation chambers, test method has been adopted into SAE J1113-27 and the IEC 61000-4-21:2011. The most common use of reverberation chambers is for radiated immunity testing because the reverberation chambers are capable of generating high field intensity level requiring much lower amplifier power than that of free space test setup. A well-built reverberation chamber is capable of generating > 25 V/m using just 1 W of drive power at the antenna port at prior to EUT loading. Large test volume of up to 8 percent total space of the chamber can also be achieved through the introduction of one or more stirrers. Figure 5 shows typical configuration of automotive EMC testing utilizing reverberation methods for both component and full vehicle radiated immunity testing. Typical constructions of reverberation chambers are for testing frequencies above 80 MHz to be efficient in both cost and building space utilization.
Widespread use of computer and energy conversion systems present new EMC challenges in modern car design. An efficient EMC test facility can provide fast-track design validation for faster completion of both component and full vehicle integrity. Just as traffic laws cannot avoid auto accidents, EMC design planning and testing cannot totally eliminate the automotive EMC problems as the electronic systems in the car get more and more complex. However, we can minimize automotive system malfunctions due to EMC problems through our design and testing efforts. Having said that, it does not mean that the cars we drive are not safe because of EMC issues. On the contrary, cars are much safer and much more fuel efficient than before due to the introduction of electronic control systems. Our efforts in automotive EMC are to aid the speedier introduction of more sophisticated computer systems to the car for further improvement in the safety and efficiency of the future vehicles.
- Todd Hubing, “Ensuring the Electromagnetic Compatibility of Safety Critical Automotive Systems,” Invited Plenary Speaker at the 2011 APEMC, Jeju, S. Korea, May 2011.
- Martin Wiles, “An Overview of Automotive EMC Testing Facilities,” Automotive EMC Conference 2003, Milton Keynes, UK, November 6, 2003.
- Jaekon Shin, “Automotive EMC Standards and Testing,” Tutorial Workshop Digests on “Introduction to Automotive EMC Testing” at the 2011 APEMC, Jeju, S. Korea, May 2011.
- ISO 11452 Components - Vehicle Test Methods for Electrical Disturbances by Narrowband Radiated Electromagnetic Energy.
- CISPR 25 Ed 3.0(2008-03) Vehicles, Boats and Internal Combustion Engines-Radio Disturbance Characteristics - Limits and Methods of Measurement of Radio Disturbance Characteristics for the Protection of Receivers.
- SAE J1113 Electromagnetic Compatibility Measurement Procedures and Limits for Vehicle Components.
- Kefeng Liu, “Site Definition and Validation for Automobile Component Test Chambers,” Invited Presentation at Society of Automotive Engineers (SAE) Work Group Meeting, Detroit, MI, April 2002.
- IEC 61000-4-21:2011 Ed. 2.0 Electromagnetic compatibility (EMC) - Part 4-21: Testing and Measurement Techniques - Reverberation Chamber Test Methods.