Analysis of optical connection technologies for multi-gigabit Ethernet in autonomous driving

introduction

Electrical and autonomous driving architectures have significantly driven the challenges of cabling systems. Issues include electromagnetic interference, electromagnetic susceptibility, and weight reduction. In addition, automotive applications, usage, and safety requirements have significantly increased the necessary network speeds. The new 48-volt electrical architecture in the car has also driven the scope of cross-domain isolation requirements. Copper wire links with communication rates exceeding 100 Mb/s require heavier and more expensive solutions to meet stringent OEM EMC specifications, resulting in high costs and very difficult engineering. In addition, there is a conflict between the weight of the increasing diameter of the required cables and the range of the electric powertrain. Optical network technology overcomes these trends thanks to its inherent electrostatic isolation, robustness, low cost, and low weight. Automakers can use optical fiber links to communicate between 48V and 12V domains. In terms of weight, optical networks can save more than 30% of the weight of copper wire harnesses. Optical Ethernet provides 100 Mb/s and 1 Gb/s network solutions, and multi-gigabit Ethernet is a major breakthrough in in-vehicle networks. Standardization work on optical multi-gigabit Ethernet has begun at the IEEE as an amendment to the Ethernet standard 802.3.

Low weight and robustness

POF fiber is the most reliable solution: plastic optical fiber can withstand harsh environments, vibrations, misalignment, contamination, humidity, wide temperature ranges, etc. In addition, POF allows fast dynamic bending, tight static bending, and immersion in liquids. In addition, fiber optic Ethernet produces very low noise and can operate in noisy environments, such as in RF electronic boards. With fiber and copper wire in parallel, optical networks provide ASIL-D safety architecture, ASIL-D=ASIL-B+ASIL-B.

Optical fiber also has significant advantages in terms of weight. Compared to shielded twisted pair (STP), plastic optical fiber can reduce weight by more than 30%: POF weighs 10g/m and has a diameter of 2 x 2.3mm. In comparison, JTP copper wire is 13g/m and STP copper wire is a total of 25 g/m with a diameter of 5mm.

Electromagnetic compatibility

Optical data networks are the best technology choice for fully electric or hybrid powertrains, with their inherent electromagnetic compatibility (EMC). Fiber optic communications have excellent performance in terms of electromagnetic interference (EMI) and electromagnetic susceptibility (EMS). POF is naturally immune and stable to electromagnetic fields due to its inherent electrical isolation. In many use cases, fiber optic networks can be used to solve problems caused by EMI/EMS in hybrid electric vehicle (HEV) and electric vehicle (EV) powertrains or the lack of potential isolation of copper-based networks. In the paper "Experimental Investigation of Degradation of 1000BASE-T1 Communication Quality Due to Pulse Interference" from the Graduate School of Engineering, Kyoto University, the relationship between the communication quality of 1000BASE-T1 and the pulse width of interference was experimentally investigated. Due to common-mode reflection and mode conversion, the FER (frame error rate) increased by more than two orders of magnitude at a specific pulse width. They concluded that pulse interference with sharp changes can significantly degrade communication quality "Susceptibility of 100Base-T1 communication lines to coupled fast switching high voltage pulses" investigates the coupling effects between high voltage (HV) systems and 100Base-T1 twisted pair Ethernet communications and the impact of pulse interference on communication performance. With battery voltages up to 850V, coupled with faster switching times of the inverter, the interference potential of the HV system may be sufficient to have a performance-degrading effect on communication quality. Starting with 200V amplitude pulses, data rates are reduced due to the reduction in amplitude and band-limited power (limited to 66.6 MHz) of the capacitive coupling clamp (CCC) pulses. Pulses greater than 400V reduce the data rate to 20%.

Noise propagation in hybrid/electric powertrains

The powertrain of hybrid and pure electric vehicles requires multiple electronic control units (ECUs) installed throughout the vehicle. These ECUs regulate and control the flow of electrical energy between the battery, inverter, and motor/generator. The flow and conversion of energy generates electrical noise, which will affect other parts of the vehicle, such as infotainment or navigation systems today and autonomous driving systems in the future. By optically connecting the ECUs, each noise can be confined to the ECU that generates it, preventing it from propagating to other ECUs scattered throughout the vehicle. Achieving similar isolation with copper-based networks is very difficult and expensive, which means longer engineering development cycles, more expensive and complex ECUs, which may in turn mean lower reliability.

In the paper “Effect of Emissions from HV Battery Cables on Signal Integrity of Two-Wire Ethernet Communications in Automotive Applications”, EMC Testing GmbH Dortmund, Germany, investigated the coupling between HV traction systems and 100Base-T1 communications. The voltage of the inverter is close to the limit of 400V. The PHY port voltage in differential mode measured in the time domain has a peak of 2V. This shows that UTP cables are very susceptible to electromagnetic interference caused by EMC coupling between these high-voltage (HV) traction systems and communication channels. Another study examined “EMC Impact of High-Efficiency Power Electronics on Electric Vehicles with Autonomous Driving Functions”. To improve the energy efficiency of electric vehicles, silicon carbide (SiC) and gallium nitride (GaN) semiconductors are used. These semiconductors can switch the battery voltage faster (4 ns) compared to the currently deployed silicon semiconductors (120 ns). Another way to improve efficiency is to increase the battery voltage, which can reduce current losses in the traction system. Due to both methods, the electromagnetic radiation of the high-voltage (HV) components of the car increases significantly.

Isolation in Battery Management Systems

The propulsion batteries in an electric or hybrid vehicle are grouped and controlled by a battery management system (BMS). Although the amount of data transmitted between the battery cluster and the control module is not very high (typically less than 100 Mb/s), the communication between the BMS control module and the individual battery clusters is crucial and needs to be very reliable. These BMS links are essential to avoid battery damage and must also be suitable in emergency situations such as crashes or fires. Fiber optic links between the BMS control module and the battery clusters are the best way to ensure the required high reliability. Copper-based communications create parasitic loops that can lead to dangerous situations for drivers and passengers in the event of an emergency.

Traditional BMS Architecture For sensing and monitoring of battery cells, wireless communication is ideal and can coexist with optical Ethernet links to connect high-voltage and low-voltage ECUs. The benefits of wireless monitoring include link reliability because the links are short and internal to the high-voltage equipment. Periodic monitoring information is not critical because it is continuously sampled. In addition, wiring complexity is reduced. It also allows sensors to be installed in locations that were previously not suitable for wiring tubes. In addition, size, weight and cost are minimized. Compared to the isolated CAN bus, the advantages of optical Ethernet include higher electromagnetic compatibility in very noisy environments such as power electronics with high voltages and very low switching times. Safety can be guaranteed due to its inherent ground isolation. Engineering is simple and easy because the battery pack can be distributed over a large vehicle area. Ethernet is a standard solution. Integrating all ECUs into the same network, together with other ECUs of the BMS system, can increase the integration of the system in the overall vehicle architecture, thereby reducing costs.

New BMS architecture combines the advantages of both technologies

48V jump start parasitic high energy pulse

Energy networks based on 48V or hybrid 12-48V topologies will continue to be the mainstream for HEV and plug-in hybrid electric vehicle (PHEV) powertrains. High and low voltage ECUs are connected together to the electrical ground of the vehicle chassis, creating problems at startup events that occur constantly in the powertrain. For example, the infotainment system shares the electrical ground with the energy generation and control system. At startup, currents above 8 amps can be measured through the cable shields connected to the vehicle chassis with the same electrical ground, and the copper cable shield provides a parallel return path for the currents of the different ECUs. According to a report by a major European automaker, if the communication link between low voltage systems (such as infotainment systems or ADAS) is optical, then local electrostatic isolation will isolate them from high voltage/high energy systems and their related events, thereby protecting their reliability.

Fault protection for 48/12V systems

In a 48/12V hybrid energy system, 48V is used for "hungry" electronic modules such as the starter, generator or battery module, while 12V is dedicated to more "delicate" electronic modules such as infotainment or ADAS processing units. Both domains share the same ground system, which is the vehicle chassis. The ECU in the 48V domain uses electronic components designed for this voltage, which are typically rated at more than 70 volts. The electronic components used in the 12V ECU can support voltages up to 60V. If a ground loss event occurs in the 48V ECU and there is a non-electrostatically isolated connection between the 48V and 12V domains, there will be a circuit between the two domains. This will subject the 12V ECU and its components to voltages beyond their rated voltages, causing failures or reducing their service life.