New developments

Due to the rapid developments in automotive technology, faster, near real-time performance capability for data transmission networks will be essential (for example drive or brake-by-wire systems). New bus systems are being developed and proposed for these applications. The leading technology is FlexRay which has already

been implemented in production (to an extent on the suspension control of the BMW X5). This technology has been developed by a consortium including Volkswagen, BMW, Daimler-Chrysler, General Motors and Bosch. FlexRay

FlexRay has been designed to support the high-bandwidth needs of current and future in-car control applications. At the core of the system is the communications protocol. The protocol provides flexibility and performance and has the following features:

● time- and event-triggered communication schemes allowing deterministic, real-time performance of the data bus

● high error detection and error diagnosis capability ● sophisticated power down and wake-up mechanisms

● flexible extendibility and full scalability to enable upgrades

● collision-free bus access ● guaranteed message latency

● message oriented addressing via identifiers

● scalable system fault-tolerance via the support of

either single or dual channels.

A hardware layer incorporating an independent bus monitoring feature provides further support for error management. The FlexRay system is targeted to support data rates of up to 10 Mbps with a gross of up to 20 Mbps possible. The system consists of a bus network and processors (ECU, electronic control units) similar to the CAN bus system. Each ECU has an independent clock and these are resynchronised frequently in order to guarantee high performance. The FlexRay network provides scalable fault tolerance by allowing single or dual-channel communication. For security-critical applications, the devices connected to the bus may use both channels for transferring data. However, it is also possible to connect only one channel when redundancy is not needed, or to increase the bandwidth by using both channels for transferring non-redundant data. Within the hardware layer, the FlexRay protocol provides fast error detection and signalling as well as

140 Power distribution Fundamentals of Motor Vehicle Technology: Book 3

error management via an independent bus guardian which monitors traffic on the data bus for errors. Why FlexRay and not CAN?

The main benefits of FlexRay are:

● it provides up to 10 Mbps data rate on two channels,

or a gross data rate of up to 20 Mbps

● it significantly increases frame length (compared to CAN – 8 bytes per frame)

● synchronous and asynchronous data transfer is

possible

● guaranteed data throughput performance during synchronous transfer (deterministic, real-time performance)

● it provides prioritisation of messages during

asynchronous transfer

● it provides fault-tolerant clock synchronisation via a global time base

● it has error detection and signalling capability ● it enables error containment on the physical layer

through the use of an independent bus guardian mechanism

● it provides scalable fault-tolerance through single or

dual channel communication.

FlexRay has been specifically developed to support future requirements in the industry and it will become

Requirements

Data rate LIN

FlexRay CAN

Figure 6.35 Relative performance comparison of bus systems

6.5

FUTURE DEVELOPMENTS IN VEHICLE POWER DISTRIBUTION AND

NETWORK SYSTEMS

commonplace in the high-performance control systems mentioned above. In addition, FlexRay has the performance to support active and passive safety systems, collision avoidance and driver assistance systems.

Vehicle networking allows sharing of data and the consequent reduction in cabling and complexity. This technology is essential in order to reduce the amount of cables in a modern vehicle to a manageable level with respect to cost and weight. It has now been adopted by most manufacturers in most models currently on the market

The most commonly used in-vehicle network technology is CAN. This was invented by Bosch In order to cope with the amount of data, modern vehicles normally have more than one bus network. Each one runs at a different data transfer speed according to the requirements of the components connected to that bus. This reduces the load and hence increases the response time for critical components such as powertrain or dynamic safety systems

The buses are connected together at one point called a gateway. This allows a single access point for diagnostics

Other network technology seen in current vehicles is LIN. This is a lower-performance bus system suitable for body components such as door control systems (windows, mirrors etc.) It is cheaper to implement than CAN but is fully compatible with it

New technology that will be implemented in the future is FlexRay. This is a high-performance bus for safety-critical systems that need a high degree of performance and reliability. It has been specifically developed for X-by-wire systems (where X = brake, steering, drive, etc.)

K ey P oints K ey P oints

The adoption of hybrid drive systems will have the biggest impact on future developments. We have mentioned these before with respect to the effect they will have on the design of traditional engine starting, power generation and battery systems. The adoption of this technology in mainstream production vehicles will be inevitable (if not essential) to achieve targets set by future emissions legislation. It will not only change the shape of all the above-mentioned sub-components, it will also alter the way that they interact with each other.

For the wiring and communication system, a major deviation from the current norm will be the adoption of higher-voltage power networks. This will be absolutely necessary in order to be able to store and provide the large amounts of power needed for vehicle traction in hybrid-drive powertrains. In addition, the power requirements of current and future vehicles will increase as an ever-growing number of comfort and control-related consumers are fitted. This is a trend which has been clearly demonstrated in the industry over the years. Higher

power at 12 volts means more current and this leads to heavier, more expensive cables as well as lower efficiency in transferring this power.

Over the last few years, 42 volt systems have been discussed, and it is likely that this standard will be adopted in the future but initially in a complementary form. It is not likely that the 12 volt system will be replaced immediately, but it is likely that vehicle systems will use two power supply networks (see Figure 6.36), both of which will be controlled by a single energy management system. This system will coordinate the operation and function of the system components (ISG, batteries, converters and inverters), to optimise operation and control of the power supply system in such a way as to improve efficiency and cope effectively with peak demands. It is thought that the 42 volt system will become the standard once manufacturers of components in the industry adapt to it.

(Data leads not shown) 42 V subsystem: 1 Alternator 2 Starter motor 3 Electrical consumer 4 Battery 5 Convertor (42 V/14 V) 14 V subsystem: 6 Electric motor 7/8 Consumer groups 9 EEM 10 Battery

Figure 6.36 System architecture with 14 and 42 volt systems

The latest generation of vehicle communication networks have been discussed above, and these have been and will be an important part of the development of future systems. However, these systems have high demands with respect to safety-critical operation, and in this respect the automotive industry is cautious and will only adopt this technology when it is well proven. The change and developments in these systems becomes less frequent over time, but the adoption of the technology in current vehicles and the advantages that this can bring is now well accepted by all manufacturers. The interaction of vehicle control systems via data buses is essential to achieve the overall performance required to fulfil customer/ driver expectations. The harmonised operation of sub-systems and components becomes more critical with a sophisticated hybrid-drive powertrain (see Figure 6.37) and this presents the main challenge for vehicle communication networks in the future.

(Data leads not shown) 42 V subsystem: 1 Starter alternator 2 Consumer groups (42 V) 3 Battery

4 EEM with integral convertor (42/14 V) 14 V subsystem: 5 Electric motor 6 Consumer groups (4 V) 7 Battery

Figure 6.37 Hybrid drive with ISG and 14/42 volt power network