Developing an automotive electrical distribution system: Part 1 - System design

The electronic content of a modern car has become one of the key brand differentiators in the market place. Alongside the obvious problems in ensuring that the latest technologies are available to be included is making sure that the associated systems supporting deployment into the vehicle can be developed and integrated into the final production vehicle.

Any problems are compounded by the pressure of ever decreasing design cycle times. As can be seen from the graph below, cycles have now been reduced to between 24 and 18 months, with a clear goal for most OEMs to reduce this to 12 months by 2010. Within this cycle the electrical distribution system supporting the electronic content must be designed, validated, and deployed. Doing this is increasingly dependent on developing new processes and tools to support those processes.

In order to preserve a reasonable margin, automotive OEMs must also see that an appropriate return is gained from the technology, by specifying vehicle models with an appropriate set of options in order to support accurate pricing. Recently the market has witnessed a dramatic increase in the amount of optional content in vehicles. This trend pushes the need to make the latest technologies available, whilst ensuring basic car models are still available in an extremely price competitive market.

Re-use caveats
A key part of these new processes is increasing the amount of re-use of previous designs. Traditionally this has involved a painstaking process of taking existing wiring designs and carefully updating them for new vehicle platforms. This manual process inevitably introduces errors, and demands significant validation and verification.

Re-use of previous designs tied to a particular platform is difficult. Almost all details of the wiring will change as it is integrated with a different physical platform. For example, all of the wire lengths will need to change, as will locations of in-lines and splices. However, logical systems can be designed which are re-usable and provide a sufficient level of abstraction to significantly increase the amount of direct re-use whilst still not requiring large amounts of manual re-work.

Process overview
We will consider each stage of the high-level process shown below in more detail later in this article, but this diagram provides a useful overview of the broad context in which an electrical distribution system (EDS) is designed.

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First, systems are selected from any available libraries that will implement the required functionality. Of course new vehicles will typically deliver at least some new content, and those systems must be developed (sometimes by a Tier 1 supplier).

All captured systems are normalized (option and variant expressions from the particular platform applied) and shared connectivity identified. Then the designs must be placed in the context of the harness topology (the physical routes the wires will take through the vehicle), and the nets (signals) converted into actual wires inside harness bundles.

Components such as fuses and wires can now be sized appropriately based on the amount of current they are expected to carry.

Once this process has been completed, individual wiring diagrams can be generated which are then passed on to service groups, who will re-layout the diagrams ready for service manuals or other distribution channels. The harness designs themselves will be passed onto the harness engineering groups who will further refine the detail of the physical implementation, adding mechanical components such as clips and grommets, as well as specifying terminals, harness dressings, and coverings.

We will now consider each of the stages shown in the process overview diagram in more detail.