Observations on International Automotive EMC Testing
September 10th, 2019
By Derrick Gabalewicz, Sr. EMC Engineer.
At Elite, we perform a significant number of Automotive EMC tests. This includes a wide range of test methods developed by standards organizations like SAE, ISO, and CISPR. We also test to dozens of specifications developed by domestic OEMs as well as requirements from International OEMs around the world. There are easily several hundred unique industry and corporate tests we run for International Automotive EMC and electrical validation.
The majority of all OEM specifications include familiar core methods such as those described in ISO 11452, ISO 7637, or CISPR25. However, nearly all corporate OEMs tailor these common methods, and many also create their own uniquely developed tests to meet their specific quality standards and markets. International OEMs in particular have some very specialized tests that require custom-built equipment and test procedures. This month, I offer my observations on some unique international OEM requirements and how Elite configures our services to meet the challenge based on my 15 years of experience.
1) Lengthy Surge Testing: One major international OEM specifies a conducted transient immunity test of 50,000 positive and negative polarity pulses associated with the alternator field coil and other inductive loads. At 2 seconds per pulse, a single sample in one mode of operation requires approximate 28 hours of testing. It’s a good test in that it confirms transient voltage suppressors (TVS) are properly designed to shunt voltage spikes. But given the high number of applied pulses, it’s also effective as a TVS durability test where component weaknesses, heating effects, and dielectric breakdown over time are evaluated.
Test plans often call for multiple samples and test modes, so Elite developed two sets of custom test equipment to handle throughput. Also, because this particular OEM specification allows for a wide tolerance for the spark surge duration (0.1 to 1.5ms) it’s important to confirm this parameter with the OEM prior to testing. Our test stand is flexibly configured to vary the duration, but a variation across the range of allowable surge duration can mean imparting pulses with energy that can vary by over 100 joules. It’s a good test, but depending on the defined surge duration, a product may or may not meet this requirement.
2) 10-meter Harness Noise Coupling Test: In this international OEM test, a 10 meter long (30 feet) harness is run on a 12ft x 6ft bench with the harness connected to typical vehicle electronic subassemblies (ESA). The specification calls for a car horn, headlamp, windshield wiper motor and other components that create the electrical transient environment of a vehicle harness. Once the test environment harness is set up, the test item is connected to its own length of harness tightly coupled alongside the 10m test harness and subjected to the transients created by the on/off cycling of each ESA.
While many OEMs have versions of this coupled noise test, this one is unique in that it requires a 10m long harness and uses actual vehicle components as the noise generators. It’s a very comprehensive evaluation of all possible conditions on the vehicle and the transients are more representative of the actual harness environment. At Elite, we optimize this test by dedicating a test bench and whole area having the ESA generators permanently fixed along with the 10m harness constrained in a static configuration. This permanent arrangement provides more repeatability each time we run the test and reduces setup time.
3) Magnetic Field (Noise Box) Test: This is generally referred to as the “Noise Box” test in which a loop of wire is formed around a low dielectric frame, such as Styrofoam. The OEM standard specifies a 0.5m x 0.3m dimension for the test but vehicle components are often too large. In order to test large components, Elite increased its box dimensions to 1m x 1m. A correction factor is used to account for the larger box size dimensions.
4) Emissions Measured in dBm: The majority of all conducted and radiated emissions automotive EMC testing are performed using an antenna cabled to the input of a measurement receiver having a 50 ohm input impedance. A few international OEMs specify emissions measurements in terms of power (dBm). While emissions measured as a power metric is not conventional for the automotive markets, it is often used in regulatory testing for wireless transmitters. Most harmonized European Union standards such as those for WiFi, Bluetooth, and cellular transceivers require testing for spurious emissions measured in power (dBm).
We also observe that international OEMs often times do not publish the limits for emissions with their common specifications; rather they provide a test method document then in a separate product document they define the emissions limits in some cases for very specific modules on the vehicle. Elite has created automation and software that allows for setting unique emissions limits for a particular product.
5) Stripline, TEM Cell, and BCI Immunity: Nearly all OEMs include Bulk Current Injection (BCI) immunity testing within the span of 10kHz up to 400MHz because inductively coupling RF energy onto a vehicle harness is very efficiently accomplished over this range.
In addition to BCI, some international OEMs also require TEM cell and Stripline RF immunity tests. TEM and Stripline use capacitive rather than inductive coupling to impart RF energy to harnesses and circuit elements. Running a BCI test along with TEM and Stripline would appear to be redundant but it illustrates the importance that international OEMs place on having robust electronic performance, in particular for this frequency range. Typical vehicle harnesses can be resonant in the frequency range where BCI, TEM, and Stripline are applied. Plus, harnesses are almost always unshielded and can act as tuned antennas capable of receiving RF energy and creating susceptibility problems. The rigor of multiple RF immunity tests over this range reduces the risk of immunity problems at the vehicle level.
At Elite, our TEM cell operation is optimized with proven software and by dedicating the amplifier and cabling for the setup. Some TEM cell requirements run up to 400MHz where the VSWR of the chamber becomes an issue. However, by continuously monitoring VSWR during the test we can gage the operation of the test and evaluate the relevance of anomalies if they occur.
Our Stripline is configured on a wheeled base that allows for quick placement in an absorber lined chamber. Test setup time for TEM or Stripline is very quick.
Finally, we have one OEM specification that requires BCI testing up to 2GHz. Testing to this high frequency is not a conventional test but we have the injection probes tuned for this range along with all other hardware to complete testing.
In summary, even though the EMC physics of Faraday, Maxwell, and Hertz are the same regardless of product, industry, or electromagnetic environment, the actual test methods we run for our wide range of clients are quite different depending on the application and environment.
The variety of testing makes for a challenging professional work day, especially when you have to quickly transition between tests and from one OEM’s specification to another. Fortunately, we have software, test fixtures, and quality processes that help keep all 35 of Elite’s EMC test engineers accurate and efficient in our tasks.
One last observation to make is that I’m fortunate to work with my brother, Mark, because he’s a great person and (in case you haven’t noticed by the photo) we have a lot in common. Mark works in the Environmental Stress Testing lab and we often get into very “lively Gabalewicz discussions” as to which technical career is more difficult - EMC or Mechanical testing. Since this is my article, EMC is far more challenging. End of story.