“As the automotive industry moves towards the concept of semi-autonomous and fully autonomous vehicles, the variety of complex and sensitive Electronic systems has increased dramatically. From a societal benefit perspective, automation promises to make roads safer, have fewer accidents, and proactively alleviate traffic congestion. Autonomous driving requires multiple high-performance interconnected sensors and subsystems to operate reliably and safely.
As the automotive industry moves towards the concept of semi-autonomous and fully autonomous vehicles, the variety of complex and sensitive electronic systems has increased dramatically. From a societal benefit perspective, automation promises to make roads safer, have fewer accidents, and proactively alleviate traffic congestion. Autonomous driving requires multiple high-performance interconnected sensors and subsystems to operate reliably and safely. Electric or hybrid vehicle applications are electrically harsh and noisy, further complicating possible technical challenges. The system may affect the operation of other critical systems due to internal sources of interference (such as EMI), transients, and external influences (such as roadside C2X infrastructure).
Advanced driver assistance systems (ADAS) are the basis for autonomous vehicle driving, and when fully autonomous, it can work in tandem with other more advanced and complex applications. This ADAS system is also able to directly notify the driver of impending situations when the car is operating in any semi-autonomous mode (Level 1 to Level 3). The different autonomous driving levels of the vehicle are shown in Figure 1.
Figure 1: Vehicle autonomy levels as defined by the Society of Automotive Engineers (SAE). (Source: SAE)
This article will focus on the architecture of advanced autonomous vehicle sensing systems, as well as the protocols and interfaces used to transmit data within the vehicle subsystems. We will also explore sources of interference and propose techniques to mitigate the impact these unwanted signals can have on safe vehicle operation.
Autonomous Vehicle System Architecture
In a self-driving car, the eyes and ears of the “electronic driver” contain a myriad of different sensors, see Figure 2, all networked to a central computer system responsible for safely navigating the vehicle in any driving situation.
Figure 2: Advanced self-driving vehicle sensing system. (Source: Littelfuse)
Sensors include long-range RF radar capable of detecting other vehicles, pedestrians, various moving objects at long distances in front of the vehicle, and a video subsystem with machine learning convolutional networks to detect pedestrians, road signs and lane departures. Other sensors include a 360-degree video camera system to keep the vehicle aware of moving or stationary objects around it. To determine vehicle position accurately at all times, high-performance GNSS navigation provides centimeter-level positioning accuracy and Dead Reckoning capability, enabling precise navigation even when the vehicle enters typical urban canyons or through tunnels in cities.
Reliable and robust network protocols CAN and Ethernet interconnect subsystems with acceptable (
HD BaseT is emerging as a viable network protocol to meet vehicle infotainment and video transmission requirements. HDBaseT combines the best of HDMI and Ethernet, bringing together audio, video, Ethernet, 100W Power over Ethernet (PoE), system controls and USB on a single cable. This protocol is optimized for different layers, and a single unshielded cable pair is recommended for device connections up to 15m. Significant savings in cable weight, installation effort and material costs can also be achieved by consolidating different transmissions into one cable.
For autonomous vehicles to operate safely and reliably, all interconnected and interdependent systems need to operate continuously and without failure. If a sensor begins to fail, or if the monitoring circuit subsystem detects sub-par performance, an alarm must be raised immediately, notifying the central computer to initiate a failsafe safe shutdown.
Technical challenges and solutions
Electronic sensors and related subsystems operate by utilizing highly complex analog and digital components, and these devices are susceptible to disturbances such as electrical transients, electromagnetic interference (EMI), and electrostatic discharge (ESD). Transients originate from the supply rails, originate from fast switching in dV/dt, and produce voltage spikes many times larger than the nominal supply voltage. High-power motors and other inductive loads can cause fast surge transients during operation, and the associated drive chain in electric vehicles is also a source of transients. Also, the small motors used for electric steering, comfort and body control, as well as the electric parking brake, also produce significant transients. These transients can enter the subsystem by conduction along a common power line, and possibly through mutual induction in adjacent cables connecting sensors, into a subsystem or a network of subsystems connected to a central computer. Without adequate protection, transients can cause microprocessors to reset, lock up, or in extreme cases physically damage critical components.
Likewise, unpredictable and erratic system behavior can result from induced EMI, which can originate from different sources, including wireless access points and smartphones, to name a few. Likewise, protection is required to prevent EMI from significantly interfering with critical system operations.
Electrostatic discharge is a particular concern for sensitive electronic components. These sensitive electronic components require special handling throughout the supply chain and during production, but also require protection in the final application circuit. Like transients, electrostatic discharges can cause large voltage spikes and can be caused by energy build-up from the friction of a vehicle’s rubber tires against the road, and a person’s contact with fabric. Vehicles driving in low humidity areas are prone to static electricity.
Securing ADAS communication and control subsystems
Figure 3 highlights the main functional blocks of the ADAS communication and control subsystem, including the protection devices that should be used.
Figure 3: ADAS communication and control subsystems and requirements for protection devices. (Source: Littelfuse)
Each communication link (2, 3, 4, and 5) requires transient and ESD protection and should fit the electrical specifications and data rates of the respective protocol. The fastest protocol is Ethernet, which typically has a bit rate in the range of 100Mbps to 10Gbps. For high-speed differential Ethernet interfaces, recommended ESD and transient surge protection methods include the use of polymer ESD suppressors such as the AEC-Q200 qualified Littelfuse AXGD Xtreme-Guard series.
Figure 4 shows where to configure the AXGD family of devices in an Ethernet connection from differential twisted pair to Ethernet PHY. The AXGD series has a fast response time and is capable of absorbing transients up to 30kV and 50A, protecting differential pairs with a single package. Also, due to its extremely low capacitance value, ESD protection does not affect Ethernet data rates up to 1Gbps.
Figure 4: Functional block diagram of a polymer ESD protection device for Ethernet transceivers. (Source: Littelfuse)
For CAN transceiver interfaces, a diode array is recommended to protect the system from fast transients and ESD.
Another example of using Zener diodes for Transient Voltage Suppression (TVS) is the Littelfuse SZ1SMB Series 600W TVS Diode. Ideal for power supplies such as Item 1 in Figure 3, the SZ1SMB series has excellent clamping capability, fast response time, and can absorb high surge transients.
Automotive Electronics Standards
When selecting transient and ESD protection devices for automotive electronic systems, it is recommended to be aware of applicable international standards. There are three most basic ISO standards, namely ISO7637-2, ISO16750-2 and ISO10605:2008. In addition to these standards, the Automotive Electronics Council (AEC) has developed a series of quality standards that define mechanical, electrical, and environmental stress requirements for devices used in automotive electronic systems. Relevant standards include AEC-Q101 for discrete components such as semiconductors and diodes, and AEC-Q200 for passive components such as capacitors, resistors and inductors.
Protecting the electronic systems of autonomous vehicles from transients, EMI, and ESD is critical to ensuring continuous, reliable, and trouble-free operation of the vehicle. Autonomous vehicles are expected to significantly reduce traffic congestion and provide safer roads for drivers, passengers and pedestrians. By using surge and electrostatic protection components, designers will have greater confidence that their systems will be robust enough to withstand any unwanted electrical disturbances.
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