PowerNets for next generation cars

Demands for new features, increasing electrical loads, and improved fuel economy are driving development of 42V PowerNets in automobiles. Joachim Langenwalter of Avant! Corporation explains.

It always takes several prototype iterations to design a new system, especially with a new voltage. Without an integrated design (2D/3D CAD) and analysis (CAE) flow, this would take too long because hardware components have to be ordered, assembled, and tested.

A more complex and even more robust product is the key to success for a new voltage system. It took more than 30 years to debug our existing 14V systems, and create databooks and design rules for them. In general, the same flow should apply to 14V system, but it was not necessary to follow it because design reuse was possible in most cases. Because it is such a huge step, a higher voltage requires an integrated flow. We do not have the time for the hardware iterations again.

 

Designing industrial networks iQBus is an integrated package of design, simulation and analysis tools which can be used by engineers to develop systems for use in the transportation industry. Although first introduced into the automotive industry, it can also be used for designing boats, trains, aircraft and other types of vehicles. It is being used now to design electrical and electromechanical systems and bus systems, including CAN, TTP, LIN MOST and D2B networks for a wide range of companies. In many of these cases electrical design is extremely complex and cannot easily be tested. The ability to simulate systems within the computer means that designs can be checked out before any actual wire is cut. In the automotive industry, this is a great time-saver and for ships and aeroplane design this may be the only way to test configurations that are just too complex or large to mock-up and test by hand. Comprehensive testing of the electronics and networking can be carried out through simulation and the results added to designs as a quality check. This is of greatest importance when changes have been made as a design evolves. The iQBus design environment has been developed specifically for the electrical and network systems in the latest 42V automotive applications. Very few cars have yet appeared built to the 42V standard, but the ability to design and simulate the wiring systems reliably will smooth the path for their introduction, which will undoubtedly include systems relying on residual 14V components. In-car networking has been a reality now for several years, at first for maintenance and diagnostics but now for much of the on-board control. It will only become an every-day reality when the systems are shown to be safe and reliable, and the first step towards this is the use of a sophisticated design platform. Aircraft designers require similar degrees of certainty. Whilst this is a view into the top end of an industry under extreme competitive pressure, this type of design technique will one day be commonplace for all networks. The demand for industrial networking is growing as more and more applications are being found for it, so automation is bound to follow.

Simulation is always the best compromise between accuracy and simulation time. The longer the simulation time, the more accurate the model. Use the simplest model, which can answer questions applied to the system.

The key for an advanced development process is an integrated flow from the first idea down to the manufacturing of the product. The flow starts with the first idea for a new function in the car such as Steer-by-Wire, which can be selected from a functional database. From that, a knowledge-based tool places the subsystems into a functional topology schematic. The topology analysis leads to requirements for the component design. Components are then analysed and connected by wires. Then, many of the wires are bundled in a bundle drawing from which the formboard drawing and manufacturing are driven. This process is tightly integrated to the in-house PDM system. All steps must be followed by a virtual verification with simulation to check the functionality against the specifications and apply an iteration if the system does not pass the tests.

The system design needs to be analysed under working conditions, and depending on all the requirements, many systems must be evaluated within a short time. It is not possible to test them all with real hardware lab cars - metal frames, shaped like a car in which all electrical and mechanical parts can be mounted, connected, and tested. Because the hardware is also not available yet, virtual prototypes must be built and simulated.

 

System simulation - software-in-the-loop The electronics content of today's vehicles is increasing. For example, in current 7-series BMWs, between 20 and 40 electronic control units (ECUs) are used. And at the heart of each is at least one microcontroller. By intelligent combination of various technologies one can increase functionality and achieve higher integration: on the other hand the complexity rises substantially. Therefore, in order to be able to control the resulting systems and also to optimise design cycles, the automobile industry has come to rely more and more on simulation. Two essential types of simulation tools can be identified: hardware development tools, which cover mechatronic issues, and software simulation tools for the model based development of processor/controller algorithms. Using typical software design tools an algorithm is first modelled which can then be tested within the simulation environment. Then a code generator converts the model into efficient target code. On the hardware side, a 'top down' approach results in a physical model description of the hardware. However, in the simulation, the software and hardware design flows are very much isolated from each other, so interactions between software and hardware, as they occur in a typical ECU, can not be analysed properly. It would seem obvious to combine both hardware and software functions in order to consider the complete system. Here, two solutions are possible: l Co-simulation - two simulators are configured in a master/slave arrangement, with synchronisation being the major headache. However, co-simulation solutions are usually pedantic in operation and the user must be able to handle both simulators. l Model Export / Import - the model devel-oped in the source simulator is converted into a format compatible with the modelling language of the target simulator. With only one simulator working on the complete system, the synchronisation problem is eliminated and performance increases, whilst the user only needs to understand one simulator. The disadvantage is that additional conversion is necessary.

In developing the simulation, it is important to apply accurate user profiles. They are unique for every car manufacturer and are defined by monitoring real drive cycles that occur in Asia, Europe, and the United States. The user profile defines which load is switched on and off at certain times. It is important to test the different power management algorithms because they do make sure that essential loads get enough power in case of a low state of charge (SOC) in the batteries. This is implemented by grouping the loads into independent priority groups. For example:

X-by-Wire = Highest Priority
Comfort Loads = Lowest Priority

If one battery is low on SOC, the lower priority loads are reduced or switched off.

System analysis Performing system analyses allows us to consider various topology options - varying the numbers of DC/DC converters and batteries and applying the 42V bus. Such analyses quickly illustrate the benefit of the pure 42V system as the final goal for creating a future car. It does not contain additional expensive parts like the DC/DC converter. It does not require costly short-circuit protection for a short circuit between supply voltages. It consists of one CSG (crank shaft mounted starter generator) and one battery. There will be smaller DC/DC converters for the logic voltage level of 5V, which are not included in the topology studies.

Further optimisation The system allows the topology to be optimised further: for the given loads, it might be the best solution to decrease the battery size or the generator size. The best solution is the cheapest system which can deliver the power installed. It is possible to apply several virtual iterations to find an optimum solution.

After the topology is developed, the components are designed and placed, all in software. The functional schematic links all components in their logical order with ideal connections. The cable drawing then links all components in the layout. The nodes are now real wires with physical dimensions like area, colour and variants. The 'connector manager' software controls the connectors and their pinning. It keeps track of how each subsystem interacts, what is comprised in each drawing (wires and connectors), variant information, layout details and everything else needed to implement that function in a vehicle. It also controls all revisions and versions of the subsystems, with a tight integration of electrical, mechanical, cost and manufacturing data aimed at electrical systems and harness design. It must be possible to test and validate the system in every design iteration.

With all the data, it is possible to generate a virtual system prototype, which can be analysed under all ranges of operation. After that, real prototypes are built. The data from the build is then ready for production. INOC

• Avant! Corporation
  d147@industrialnetworking.co.uk