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In-vehicle networks gain by harnessing plastic optical fibre technology

Feature

By César Esteban, Applications and Support Manager, and Juergen Schachtschneider, Automotive Manager Central Europe and Greater China, KDPOF

Electric and autonomous driving architectures are substantially pushing the challenges for wiring systems. Issues include electromagnetic interference, electromagnetic susceptibility and weight reduction. On top of this, automotive applications, utilisation and safety requirements are boosting the necessary network speed tremendously. The new 48V electric architecture in cars additionally pushes the envelope in terms of cross-domain isolation requirements. Copper links for communication rates above 100Mb/s need heavy and expensive solutions to comply with the stringent OEM EMC specs, resulting in high cost and very difficult engineering. Moreover, the weight of the ever-growing diameter of the required cables plays against the race for increased range in electric powertrains.

Optical network technology overcomes these trends thanks to its inherent galvanic isolation, robustness, low cost and low weight. Carmakers benefit from optical links for communications between the 48V and the 12V domains. For weight, an optical network will save more than 30% of the weight of the equivalent copper-based harness. Optical Ethernet provides 100Mb/s and 1Gb/s network solutions today, and multi-gigabit Ethernet is the significant upcoming breakthrough for in-vehicle networks. The standardisation effort for optical multi-gigabit is already in progress within the IEEE as an amendment to the Ethernet standard 802.3.

Low weight and robustness

Plastic optical fibre (POF) cables are the most reliable solution: plastic optical fibre can withstand harsh environments, vibrations, misalignments, dirtiness, humidity, wide temperature range, and so on. In addition, POF allows fast dynamic bending, tight static bending and immersion in liquid. Additionally, optical Ethernet generates very low noise and can operate in noisy environments, such as in RF electronic boards. With optical and copper in parallel, the optical network provides ASIL-D safety architecture ASIL-D = ASIL-B + ASIL-B. Concerning weight, fibre optics also has a significant advantage. Since it weighs 10g/m with 2 x 2.3mm diameter, POF reduces the weight by over 30%, compared to shielded twisted pair of copper wires (STP), where JTP copper is at 13g/m and STP copper sums up to 25g/m with a diameter of 5mm.

Electromagnetic compatibility

Optical data networks, with their intrinsic electromagnetic compatibility (EMC), are the best technology choice for new powertrains – fully electric or hybrid. POF communications have excellent performance in electromagnetic interference (EMI) and electromagnetic susceptibility (EMS). POF is inherently immune to and robust against electromagnetic fields due to its native galvanic isolation. In numerous use cases, optical networking can be used to solve issues caused either by EMI/EMS or lack of galvanic isolation on copper-based networks in the powertrain of hybrid (HEVs) and electric vehicles (EVs).

The powertrains of hybrid and electric vehicles require multiple electronic control units (ECU) placed all around the car, which (among other tasks) also regulate and control the electric flow of energy between the batteries, converters and motors/generators. However, energy flow and conversion generate electric noise, which affects other areas of the car like the infotainment or navigation systems, or worse in autonomous cars. By optically connecting the ECUs, it is possible to confine noise to the ECU it originates in, avoiding its propagation to all the other ECUs. Achieving a similar isolation with a copper-based network is difficult and expensive, and it translates into long engineering development cycles and more complex – and possibly less reliable – ECUs.

Isolation in battery management systems

Propulsion batteries in H/EVs are grouped into clusters controlled by the battery management system (BMS). Although the volume of data moving back and forth between the clusters and the control module is not very high (typically below 100Mb/s), the communication between all of them is crucial and must be very reliable. BMS links are critical to avoid battery damage, and must be suitable in emergency situations like crashes or fires. Optical links between the BMS control module and battery clusters prove highly reliabile, since copper-based communications create parasitic loops, which, in the case of emergency events, may translate into dangerous conditions for the driver and occupants.

For sensing the battery cells and monitoring, wireless communication is ideal in coexistence with optical Ethernet links to connect the HV and the LV ECUs.

48V jump-start parasitic high energy pulse

48V-based energy networks or mixed 12-48V topologies are and will keep being the mainstream of HEVs and plug-in hybrid electric vehicle (PHEVs) powertrains. The electric ground, which is connected to the vehicle chassis and is common to high- and low-voltage ECUs, creates problems on start-up events that continuously take place in such powertrains. For example, the infotainment system shares the electric ground with the energy-generation and control systems. The high return currents flowing through the chassis on start-up couple into the infotainment low-voltage system through the cable shielding, which is connected to the same electric ground as other systems in the vehicle. Copper-cable shielding provides a parallel return path (alternative to the chassis) for the currents of the diverse ECUs. Due to this, currents higher than 8A can be measured in the cable shielding during a typical jump-start, as reported by some carmakers. If the communications link between the ECUs in the low-voltage systems like infotainment or ADAS is optical, then the native galvanic isolation will isolate them from the high-voltage/high-energy systems and their associated events, thus preserving their reliability.

Fault protection

In mixed 48/12V energy systems, 48V are reserved for power-hungry electric modules like starters, alternators and battery modules, whereas 12V are dedicated for the less powerful electronic modules like infotainment or ADAS processing units; both domains share the same ground system, the car chassis. The ECU in the 48V domain is designed with electronic components sized for such voltages, components typically rated to over 70V, whereas the 12V ECUs are designed to withstand to about 60V.

In case of an event like loss of ground in a 48V ECU, and if there are non-galvanic isolated links between the 48V and the 12V domains, there will be an electric path between the two, exposing the 12V ECUs and its components to high voltages, causing failures or a reduction of service life.

EMC compliance

EMC qualification is one of the critical steps of a platform validation by Tier1 and OEMs. Copper links for communication rates above 100Mb/s need sophisticated and expensive solutions to comply with the stringent OEM EMC specs, such as high-quality shielding, controlled pair twisting, complex in-line connectors, and more. Optical ports can pass both EMI and EMS much more easily. This directly impacts the cost of the harness and the connectors, not to mention the engineering resources assigned to the development and debugging stages.

Outlook

The key advantages of the optical solution for specific applications using multi-gigabit speeds with in-vehicle connectivity are, among others, EMC, thanks to the inherent galvanic isolation, low weight and low cost. Relevant use cases from different carmakers in Europe and the US incorporate the comprehensive features and benefits of optical network technology. Key leading optoelectronic, connector and wire-harness vendors worldwide already provide components and solutions needed for multi-gigabit networking in the car: Physical Layer (PHY), Fibre Optic Transceiver (FOT), fibres, connectors and light sources. The technology will be scaleable to enable even higher data rates in the future.

By combining optimisation in all areas of the new standard, the right balance of complexity and cost among all parts (CMOS IC, VCSEL, PD, ferrules, sleeves, cable, in-line connection technology, optics, and lenses, etc.) can be achieved to deliver the lowest-cost, most-reliable and highly-scaleable solution to the automotive market.

Standardisation and compliance

IEEE 802.3 standard

As an amendment to the IEEE 802.3 standard, IEEE Std 802.3bv for gigabit Ethernet over POF defines physical layer specifications and management parameters for automotive, industrial and home-networking applications using POF. KDPOF technology entirely fulfills the preconditions of the new IEEE amendment, providing reliable and proven solutions for automotive applications. The KD1053 transceiver perfectly meets the requirements of carmakers by providing high connectivity with a flexible digital host interface, low latency, low jitter and low linking time. The transceiver is optimised for low power and small footprint and transmits data at 1000/100Mb/s on standard SI-POF, MC-POF, or PCS, according to 1000BASE-RH (IEEE 802.3bv).

ISO 21111

With the supplementary parts ISO 21111-3:2020 and ISO 21111-5:2020, the International Organization for Standardization (ISO) specifies further features for the reliable in-vehicle data transmission of one gigabit per second over POF technology. With the new ISO 21111 sections complementing the existing IEEE Std 802.3bv, optical gigabit connectivity is now entirely standardised.

Optical multi-gigabit

Automotive applications, with technological leaps such as electric vehicles, automated driving and V2X interconnection, call for tremendous network speeds, moving in-vehicle networks from one to multiple gigabits per second. Optical communication supports fast multi-Gigabit Ethernet speeds in the car, and is close to standardisation and wide-spread implementation. With the approval of the IEEE 802.3 working group, a team of individuals affiliated with more than 15 key carmakers and components suppliers, including KDPOF, has started the standardisation of an IEEE 802.3 Automotive Optical Multi-Gigabit Standard with strong support from the industry. The working group, headed by KDPOF’s CEO, Carlos Pardo, kicked off in 2019, with the first prototypes expected at the end of this year. The study group is evaluating the creation of an IEEE Ethernet standard for the automotive industry, with speeds starting at 2.5 Gb/s and going up to 25 or 50 Gb/s. It will leverage the existing 10GBASE-SR, which is the current standard by IEEE for industrial use, to get a new technology that is suitable for the stringent automotive requirements.

The key development objectives for the new standard target outranging performance. The high speed will reach up to 50Gb/s per line, scaleable to 100Gb/s with multiple lines. The temperatures range from -40°C to 125°C, at distances of 15m with four inline connectors for cars and 40m with four inline connectors for buses and trucks for 25Gb/s. Meeting OEM reliability requirements, the failure rate is below 10 FIT (failures in time: 1FIT equals one failure per 1,000 devices operating one million hours).

With cost-down and consistency in focus, optics, fibres, connectors and electronics already developed for nGBASE-SR will be reused. Further specifications include 850nm VCSEL, multimode OM3 fibre, PAM2 receivers and connectors. Standardisation work focuses on the automotive requirements: VCSEL reliability for the operation temperature range, connector development with quality grades, and an adaptive DSP to cope with the large parametric deviation of the VCSEL. Increasing the yield percentage results in cost reduction.

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