Grundlagen FlexRay - Institut fÃ¼r Automatisierungs- und ...

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Grundlagen FlexRay BasicsV 1.1 10 1 FlexRay 1.1 Motivation Because of the huge potential for innovation in automotive electronics, and the resulting possibilities to make driving safer, more comfortable and more economic, the automotive sector is undergoing a trend towards electrification since the 90s. This trend continues up until today [Joch07]. To provide the desired level of functionality in a modern day car, a variety of electronic control units (ECU) have to interact with each other. For historic reasons, and because of the low data transfer rates needed in the past, the CAN bus system has been established as the primary bus system in modern cars. For special use cases, there are additional bus systems. As an example, the connection between the actuators and sensors inside of a door is realized by the low-cost LIN bus [LIN11], and the MOST bus is used for multimedia applications [MOST11]. Since the requirements for electronic and mechatronic control units are getting higher for modern-day and future cars (e.g. for steer-by-wire), there is a need for a real-time communication with high data rates. Since such a communication is not possible with CAN, a consortium has been founded in the year 2000, to develop a new bus which complies with these requirements. Founding members were the companies BMW, Daimler, Motorola and Philips. During the years, the structure of the consortium has been divided in Core, Premium Associate, Associate and Development members. In the year 2009, the consortium has been closed, as the FlexRay specification 3.0 had been released. At this point, the Core members were BMW, Bosch, Daimler, Freescale, GM, NXP and VW. There were 28 Premium members, more than 60 Associate members, and several hundred Development members. Currently, the FlexRay specification 2.1 is being used. It can be downloaded from the official homepage [Flex11], until the specification 3.0 has been successfully merged into the following ISO standards: ISO 10681-1:2010 Road vehicles – Communication on FlexRay – Part 1: General information and use case definition ISO 10681-2:2010 Road vehicles – Communication on FlexRay – Part 2: Communication layer services 1.2 Requirements and properties While defining the requirements for the new bus systems, the following points have played an important role: • Transmission rate – a transmission rate significantly higher than CAN (500 Kbit/s) should be achieved • Redundant communication channels – by implementing two separate physical communication channels (A/B), the system offers to transmit data redundant, or to double the data rate.. • Global synchronous time base – the communication protocol should provide a global time base, which can be used to synchronize the various ECUs. Bäurle 12.10.2012
Grundlagen FlexRay BasicsV 1.1 11 • Reliability of transmission – Transmission of information should be reliable and have deterministic timing, since the CSMA method used by the CAN bus can lead to massive fluctuations in timing. • Physical layer – the physical layer should be robust and simple. • Extendibility – The network should be easily extendable. Single ECUs should be able to be removed or added to the network.. • Support for time- and event-based communication – since – depending on the application - both of these methods of communication may be suited better, both methods should be supported. CSMA: TDMA: Figure 1.1 Examples for time- (CSMA) and event-based (TDMA) communication [Göhn10] As a result of these requirements, the properties of the FlexRay system have been specified. The key parameter of any communication system is the data rate. In respect to current and future demand for bandwidth, a sufficiently high data rate of 10 Mbit/s per channel has been selected. A synchronous time base is provided, which ensures the deterministic character of the system. Because of that, the event-based communication process using priorities (CSMA/CA), as used in the CAN bus, had to be abandoned. Instead, a time-based method (TDMA) is used for data transmission, see Figure 1.1. By using scheduling, an accurate time plan is possible, while still allowing for a jitter of 0.5 to 10 µS (1 to 3µS typical).. Because the communication process is organized in cycles, providing each ECU with defined slots, a deterministic delay for messages can be guaranteed. The focus during the development was to make the system flexible and extendable. As a result, it is possible to choose which messages should be transmitted redundant, and the system can be optimized for reliability or performance, using static or dynamic bandwidth allocation. As many as 60 parameters – e.g. message length or cycle length - in the network design allow an optimization of the network for the specific application or task. Bäurle 12.10.2012
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