Electrical Simulation, MCAD and PLM Team Up for Automotive Design

A modern vehicle is in truth a large, complex electromechanical system. To design these systems, many companies are turning to virtual prototyping technologies in which software “models” are created, studied and optimized at far lower cost than hardware prototypes. The virtual realm encompasses not only mechanical assemblies but also electrical simulation, in which electrical behavior and performance are predicted. The two cannot be isolated from each other; designers must account for cross-domain integration and synchronization, data management, and design change management in the electrical and mechanical domains.

ELECTRICAL SIMULATION STARTS EARLY
Electrical simulation during the initial design phase can reveal problems that require redesign. The electrical system and the physical design are interdependent, so changes in the electrical system may require changes in the mechanical packaging too. Architectural changes like these—whether electrical or mechanical—are easier and cheaper to make in the early design phase.

Simulation reduces the need for physical prototyping, and it is less costly in both time and money. Importantly, computer-based methods also take the process of validating design integrity far beyond what can be achieved using physical prototypes. An automobile’s electrical subsystems, embedded software and data networks support a huge range of interactions, which can all too easily include combinations that will cause a major problem in service. Physical prototyping is not likely to find these problems, while simulation can reveal even the most obscure problems before the product goes into service.

MORE THAN JUST DC VALIDATION
Simulation tools are available to validate and support many stages in the electrical design process. DC simulation, the most common tool, validates the sizing of wires, fuses and power supplies. The foundation for this type of electrical simulation is an “electrical model” for each of the devices and wires in the electrical system. The models are relatively simple resistive models for DC analysis.

Continuity/Qualitative simulation is a variant of DC simulation that uses even simpler electrical models than DC models, yet provides powerful design support to engineers creating the wiring design. This type of simulation identifies continuity and misconnection problems as each device and wire is placed.

Transient simulation is widely used to model inductive circuits containing components such as motors and fluorescent-discharge lighting, where there are significant differences between start-up and steady state power consumption. The simulation tools guide the design to accommodate these worst-case electrical currents.

Simulation tools can validate the correct operation of vehicle networks by modeling analog, digital and mixed-signal aspects of transceivers and the behavior of the transmission lines and more. The physical geometry of these digital networks and the distances between components can affect their performance, so tight integration between the electrical and physical design tools is essential.

WIRE SIZING, SNEAK CIRCUITS, AND MORE
In the early days of electrical simulation, the tools primarily ensured that components such as fuses and wires were correctly sized. Present-day systems confirm that the electrical system will behave properly under all circumstances, and also may verify benign behavior when components fail.

The simulation tool must exercise the electrical design in multiple scenarios to represent each of the electrical-switch states and/or component-failure modes that may arise in service. Here again there are many, many permutations. Simulation is a computation-intensive task but it can reveal unforeseen failures that, if left unresolved, might affect the comfort or safety of the end user.

Poorly-designed electrical systems can contain “sneak circuits.” This is the result when a particular combination of switch positions or component failures allows current to flow from an active circuit into one that should be inactive. Components may activate or de-activate, or lamps may dim or switch off, apparently at random. Today, more companies are adding sneak circuit simulation to their evaluation processes.

Electrical failure mode effects analysis (FMEA) is critical for ensuring that component failures do not impact the safety or integrity of the product. In a complex automobile a single component failure can create cascading side-effects in other parts of the electrical system. These can be predicted only by computational simulations that evaluate millions of possibilities and identify serious problems.

MCAD AND ECAD WORKING TOGETHER
Electrical simulation tools have been available since the earliest days of computing but, until recently, these tools have not been widely used in the automotive design process. This is in part because electrical design is interdependent with physical packaging. For example, the resistance of a wire is dependent on both its resistivity and its length. The electrical analysis engineer must ensure that all wire lengths are correctly defined in the analysis model. Historically this has been an intensive manual process, and it has become infeasible with advances in design complexity.

The modern solution is a “round-trip” interface between the ECAD and MCAD tools. The ECAD tool sends to the MCAD tool the “from-to” information for each wire. The MCAD tool routes the wire through the 3D cabling network, and then sends the wire lengths back to the ECAD tool.

There is also the issue of synchronicity between electrical and mechanical designs. Realistically, both electrical and mechanical designs change almost daily during the most intense parts of the design phase, and each design is composed of many subsystem designs. The electrical simulation engineer must ensure that the simulation and analysis results are correct for the current design phase. This data management challenge impacts the electrical simulation engineer, component engineers, manufacturing engineers and others.

Simple file management tools initially met this challenge. But with hundreds of files and multiple revisions for each file, PLM systems now provide the needed high-level control. Locally within the electrical design tool, further data management controls have emerged to ensure synchronization between electrical wiring and harness designs, component definitions, and electrical simulation models. As a result engineers can manipulate complete, synchronous sets of designs, with the ECAD system automatically validating and checking the set and reporting any out-of-date designs.

The physical and electrical designs are differing views of the same physical product. The MCAD designer models a wire as a tubular entity curving in 3D space. The ECAD engineer models the same wire as a connection between two terminals with a cross-sectional-area and a length. There is commonality between the MCAD and ECAD tools in the definition of wires and other entities, but each tool also maintains its own set of attributes that are not needed by the other tool.

When exchanging common data between MCAD and ECAD, which should be the “master” for wires and other elements? In practice each system needs to be the master of particular attributes. The challenge is to merge the definitions correctly as changes occur in one tool or the other. Until recently importing an update in the MCAD system would overwrite the definition in the ECAD system, destroying any changes made to ECAD-specific attributes in the ECAD environment.

Fortunately the latest tools include change management and merging facilities that precisely control the flow of information between the two systems. Filters define which system is the “master” for each and every attribute, allowing changes to flow seamlessly between systems without overwriting data.

READING THE COMPONENT LIBRARY
Some of today’s tightly integrated simulation tools can automatically define wire sizing. Figure 1 shows the physical topology of a car with its cable routing. The cable routing network has been defined within the MCAD system and imported into the ECAD tool. The inset image in Figure 1 is an electrical schematic relating to the car’s audio system. The ECAD tool uses engineering rules to place the components from the electrical schematic and to route the connections to generate the full wiring design. Correct wire lengths are assured because the topology of all cable routing is defined by the MCAD system.


Figure 1: Integrating the electrical and mechanical views of the design supports “What-if?” architectural analysis.

Now the designer can initiate the electrical simulation to exercise all switch position permutations and identify the maximum worst-case current in each of the wires. The simulator uses these values to select the appropriate wire types from the component library and updates the wire attributes on the schematic to conform to the library component data. The data can also be back-propagated to the MCAD system to ensure that the correct wire sizes are defined in the MCAD design. Both MCAD and ECAD systems can remain synchronized during successive design changes via the change-manager merge interface. The most up-to-date feedback from simulation is always available in both environments.

THE PLM ENVIRONMENT
The role of PLM systems is well established in large companies, providing the means to effectively manage the large number of data files associated with the product data—particularly the MCAD data. Typically, the MCAD data is stored in a tree structure, analogous to the structured assembly and sub-assembly composition of the physical product.

ECAD design data is structured rather differently. A particular wiring design includes components from many different physical sub-assemblies, which means that ECAD design data cannot be linked directly and simply to equivalent items in the existing tree structure. Instead, the ECAD designs are defined in their own tree structure. The ECAD designs are stored in both their native ECAD file format and as drawing files, allowing PLM users to view the electrical designs without activating the ECAD tool. The PLM system’s Check-out/Check-in facilities manage access to the ECAD designs. More advanced facilities can provide sophisticated reporting and linking of the related MCAD and ECAD files, providing tight integration between the two domains.

CONCLUSION
The complexity of today’s vehicles has passed the point at which physical prototyping can be used to validate electrical systems. Simulation and virtual prototyping have become an essential part of the design process. Today’s ECAD tools have bonded with MCAD engineering installations in automotive companies and other design/manufacturing enterprises to help improve design quality, reliability and cost.