The rising scourge of EMI continues to vex automakers
On the long list of design issues faced by automakers, the need to keep stray electromagnetic energy in check has been near the top since the first vehicles rolled off the assembly line.
Over the years, sources of electrical and electromagnetic interference have multiplied, and the result is an environment rich with signals spanning very low frequencies to the microwave region that can, in the worst cases, render a vehicle immobile or cause a critical subsystem to malfunction.
This problem will only worsen with the “digitalization” of vehicles that in the not-too-distant future will make them rolling IoT platforms. To understand these interference issues, it is important to revisit the root causes of interference and how they are being solved today and will affect future vehicles.
First, it is important to clarify the confusing definitions of interference. The International Telecommunication Union (ITU) defines interference as "the effect of unwanted energy due to one or a combination of emissions, radiations or inductions upon reception in a radiocommunication system, manifested by any performance degradation, misinterpretation or loss of information which could be extracted in the absence of such unwanted energy.”
Note that this definition refers to radio communication systems, which are just one of many subsystems in a vehicle. A broader definition includes the hundreds of other subsystems affected by interference. Complicating things further is that interference from electromagnetic energy splits into two subsets: electromagnetic interference (EMI) and radio frequency interference (RFI).
For practical purposes, EMI translates into very-low-frequency emissions while RFI relates to emissions in the RF spectrum that is, unfortunately, often defined as from 1 Hz to 300 GHz, the lower frequency actually residing within the domain of EMI.
RF emissions are mostly generated by systems where the frequency is higher and concentrated within the “sweet spot” for telecommunications, a spectral region between about 600 MHz and 6 GHz (and higher in the future) that offers the best signal propagation characteristics.
There are two means by which these signals can enter a system: radiated or conducted. Radiated signals travel through the air while the conducted signals transfer through wires or some other physical medium.
The takeaway from this is that interference can arise anywhere inside or outside a vehicle, making it crucial for component manufacturers through automakers to thoroughly test them at various stages from design to production to ensure electromagnetic compatibility (EMC).
Keeping interference out, or sometimes in
To be compliant, a device must not cause interference at a level high enough to cause interference to nearby devices and thus be compatible with them. A certified facility with an anechoic chamber can conduct EMC testing to isolate the vehicle from electromagnetic energy at any frequency. EMC testing for conducted emissions evaluates a device or subsystem in which emissions can enter through a power supply cord.
This is a complex process that requires extremely precise, calibrated test equipment performed in a standards-compliant manner to ensure that measurement results are comprehensive and accurate. They specify the levels of both its susceptibility to ingress of emissions from outside the vehicle and egress from inside the vehicle.
Vehicles sold worldwide must meet requirements of standards from the U.S. Federal Communications Commission, the European Telecommunications Standards Institute and others. Automakers impose even stricter standards on their suppliers. Measures to improve electromagnetic compatibility include filtering, shielding, and optimizing wiring performance.
Designers traditionally use physical prototypes for EMC testing. However, using simulation testing achieves significant benefits in both cost and time. Simulation testing at any stage of design models currents and eclectic fields. Because problems can be squelched before a physical prototype is built, simulation reduces the risk of problems emerging later in the design process — or in the worst-case scenario — after product launch. This is a growing trend that offers great promise.
Satisfying all requirements before vehicle production is already incredibly challenging. Requirements include multiple signal buses, up to 150 electronic control units (ECUs) and various wireless communications systems (such as navigation, Wi-Fi and cellular), as well as radar, cameras, ultrasonic sensors, keyless entry, and infotainment systems.
Each one of these is a potential contributor to or recipient of EMI or RFI, and as more of these are added, the difficulty of ensuring that they work without interfering with each other will increase exponentially. Keep in mind that this list covers just the internal systems and not external interference caused by power lines and other electrical systems, cellular and other communications systems, and natural causes such as lightning.
The possibilities for interference are endless. What’s more, all radio transmitters and other RF components can emit not just the desired frequency but others, typically at lower signal strengths but still strong enough to cause problems. Ideally, designs will filter out these spurious and harmonic signals before they emerge from the antenna, but this is not an ideal world.
More interference sources to come
As everyone knows, the auto industry is facing two huge paradigm shifts: vehicles powered and enabled exclusively by electricity and vehicles that drive themselves. As an electric vehicle (EV) has fewer components than its gas-powered counterparts, it would appear to be easier to design from an EMC standpoint.
The heart of an EV is the powertrain called an electric drive unit (EDU) that consists of an electric motor, a power electronics module and a transmission. Rather than using cables, EVs use a busbar-based EDU because it is strong, can handle high current levels and is easy to manufacture, dramatically reducing the number of places where EMI can play havoc. The busbar approach also shortens the length of the cable between high-power switches and the motor terminals, and the mechanical stability of busbars also eliminates EMC testing issues caused by the random arrangement of power cables.
Autonomous vehicles are likely to be electric when they arrive, so they would also presumably be less of an EMC headache, but as always, the devil is in the details. For example, autonomous vehicles will use more ECUs, sensors and wireless systems than today, and they will be processing and transferring data (most likely via Ethernet) at very high data rates. At a speed of 10 Gb/s, this translates into an operating frequency around 5 GHz, so designers must attenuate signals at this high frequency in the devices as well as along their path to the processors. This will place new demands on automotive EMI shielding such as gaskets and housings that reduce radiated emissions because shielding materials will have to attenuate much higher frequencies without adding much weight or cost to the vehicle.
In addition, autonomous vehicles will be continuously connected to roadside infrastructure such as wireless-enabled street signs and cameras to enable vehicle-to-vehicle (V2V), vehicle-to-cloud (V2C) and vehicle-to-person (V2P) communication, most likely via cellular networks. This “vehicle-to-everything” paradigm adds one more significant possibility for EMI.
Electrical interference has been a problem for automakers since 1930 when Ford put the first Motorola radio in the Model A Deluxe coupe at the staggering cost of $130 (the car itself cost only $540). Back then, the problem was keeping it out of vacuum-tube radios and reducing high-voltage arc between chassis parts so fuel would not demolish the vehicle and its occupants.
Today, the problem is arguably more problematic and less dangerous but nevertheless an enormous challenge as cars and trucks become more electronic than mechanical. When autonomous vehicles take the road, safety will once again come to the forefront as interference with mission-critical components will have catastrophic consequences. In short, interference is the gift that keeps on giving...headaches.