EMC design in tomorrow’s semi- and fully autonomous vehicles
With the introduction first of Advanced Driver Assistance Systems (ADAS) and other semi-autonomous driving technologies, and now with live testing of fully autonomous vehicles in full swing (see Figure 1), a different concept of the vehicle is emerging: in future, it seems, the car will be a media playback centre, telephone, office, and extension of the home’s living room which also happens to be able to convey passengers from A to B at speed.
This evolution is having a profound effect on the characteristics and on the sheer number of electronics systems in new vehicles. And this in turn will dramatically extend the demands on the EMI shielding devices, such as EMI gaskets and EMI housings, used to attenuate the radiated emissions that could affect circuits in the car. In future, EMI shielding materials will be required to perform over a wider range of frequencies, in more applications, while adding as little as possible to the weight of the vehicle.
It might not be viable simply to continue to use shielding materials and products from the past in these new conditions. The time to consider the options for achieving EMC in new car designs is as close as possible to the start of a new design project, before the electrical and mechanical features of the vehicle’s systems have been irreversibly decided: this gives the OEM’s chosen supplier of shielding products the opportunity to bring considerations of EMI and shielding devices into the design process, and enable optimisation of the size, cost and performance of EMI shielding in the final system.
These considerations are changing markedly with the introduction of new and sophisticated sensing, control and communications modules in new vehicle designs. This article outlines the EMC challenges that automotive system designers face today, and the implications for their choice of EMI shielding devices.
A two-fold problem
The challenge for EMC design in automotive systems is two-fold.
First, the range of frequencies which need to be attenuated will be far greater in new cars than it was in the past. Until recently, the main frequencies of interest were the AM and FM bands used by radio broadcasts, and frequencies below 3GHz used by Bluetooth radios and mobile phone networks.
With the planned introduction of 5G mobile phone network coverage possibly starting as early as 2019 in some countries, the frequency coverage of EMI shielding materials will need to be extended. A higher frequency range is not the only issue, however. Cars are also going to support a much greater number of wireless communications systems to support demand for vehicle-to-vehicle (V2V), vehicle-to-cloud (V2C) as well as vehicle-to-person (V2P) communication. High-speed communications buses will also connect the growing number of electronics modules such as LIDAR (optical) detection and ranging systems, park-assist systems and safety and monitoring systems to central electronic control units (ECUs). And sophisticated infotainment devices, such as high-definition video displays in the front seat headrests, also require their own high-speed, high-frequency networks.
Broad frequency coverage, then, is one aspect of the EMC designer’s challenge: the second dimension is that the effectiveness of EMI shielding is likely to be more tightly specified in future as automotive manufacturers move towards a strict view of the functional safety of the electronics systems in cars, codified in the ISO 26262 functional safety standard. ISO 26262 requires car makers to identify the ‘failure modes’ of electronics systems, to quantify the risk of failure attributable to each mode, and to take steps to limit the probability of failure, known as the ‘failure in time’ or FIT rate, to a specified maximum value.
Since EMI is a known failure mode for almost any electronic circuit, measures to attenuate RF emissions to safe levels are likely to be more strictly implemented under the ISO 26262 regime than ever before. This is all the more likely as electronic systems take over more and more aspects of the car’s road-going operations. In an autonomous car, the communications link between a LIDAR object-detection camera and the ECU responsible for control of the speed and direction of movement is as safety-critical as the hydraulic link between the brake pedal and the brakes in a non- or semi-autonomous car.
So the developers of tomorrow’s automotive systems will need to provide for higher attenuation of a wider range of frequencies than before. And always in the automotive industry, the pressure to minimise space, weight and cost is intense. What does this mean for the specification of EMI shielding materials?
Development of new elastomer fillers
Various types of electrically conductive elastomers are commonly used in EMI gaskets for shielding. Special elastomers offer useful properties, including resistance to high temperatures and contaminants, and the ability to provide environmental sealing to protect circuits from the ingress of liquids.
For instance, CHO-SEAL conductive elastomer from Parker Chomerics consists of a silicone, fluorosilicone, EPDM or fluorocarbon-fluorosilicone binder with a filler of pure silver, silver-plated copper, silver-plated aluminium, silver-plated nickel, silver-plated glass, nickel-plated graphite, nickel-plated aluminium or non-plated graphite particles. These elastomer gaskets resist compression set, accommodate low closure force, and help control air flow. They are available as standard extruded products or in custom shapes (see Figure 2).
Today, these products are widely used in automotive systems. As demand emerges for wider frequency coverage, however, automotive designers will do well to consult with their EMI shielding product supplier to determine whether existing products will perform adequately.
Parker Chomerics maintains an intensive research and development programme aimed at producing new filler materials for electrically conductive elastomer products such as EMI gaskets. An important goal for this research programme is to produce elastomer EMI gaskets that can cover the broader frequency range of interest in autonomous vehicles, while maintaining the desirable mechanical characteristics of existing CHO-SEAL products such as high abrasion resistance, high chemical resistance, and high temperature rating.
New opportunities for weight saving
The development of autonomous and semi-autonomous vehicles is leading to a huge increase in the number of electronics modules per vehicle. This increases the scope for car makers to reduce weight by replacing conventional metal (aluminium or steel) housings with lighter conductive plastic housings. While the weight saving on each module might appear small, when multiplied across the 100 or more electronics modules which may be found in new car designs, the total weight saving becomes valuable.
Here, Parker Chomerics has made a breakthrough with the introduction of its PREMIER PBT-225 product, a single-resin conductive plastic for use in automotive housings. In contrast to the two-pellet (or ‘salt and pepper blend’) conductive resins available elsewhere, PREMIER PBT-225 provides for easy processing and uniform filler dispersion (see Figure 3). As a result, EMI housings made from PBT-225 offer tightly controlled electrical and mechanical performance throughout complex geometries.
Attenuation of EMI is good across the spectrum of RF and microwave frequencies, and is as high as 55dB at frequencies between 6GHz and 12GHz. Weight savings of 30% are possible when replacing an equivalent aluminium housing, and the PBT-225 material offers excellent resistance to hydrolysis when exposed to extreme temperatures and humidity.
New era for EMC design in vehicles
This article has described the scale of the challenge facing the designers of next-generation vehicles arising from the increasing use of high-frequency, high-bandwidth communications links and the growing number of electronics modules in the car.
By collaborating early in development projects with a trusted supplier of EMI shielding products, automotive systems designers can ensure that their electronic and mechanical design is optimised for shielding functions, and can take advantage of the latest advances in materials science pioneered by innovators in the field such as Parker Chomerics.
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