Model-driven design of vehicle wiring systems

  • 16-Nov-2008 07:47 EST
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The physical manufacture of a wire harness is a complex and labor-intensive activity. Electrical design software can automate each step in the design process from architectural schematics to physical harnesses.

Model-driven development has been standard practice in the design of electronics, integrated circuits (ICs), and printed circuit boards (PCBs) for many years. According to Mentor Graphics, model-driven processes have allowed companies to move from a manual design process for ICs and PCBs to an automated process that allows designers to create more complex designs much faster. It has been the model-driven approach, as much as improvements in manufacturing science, that has delivered technology improvements to consumers.

The model-driven approach is not just for electronic design. It works for electrical design, too. The last few years have seen the emergence of this methodology into the wiring-systems development domain because wiring integration challenges are a perfect fit for this approach.

Issues that make electrical-systems integration complex and challenging include:

• The systems and subsystems data to be integrated is typically described in diverse, heterogeneous artifacts: text documents, spreadsheets, sketches, design handbook guidelines, etc. Not only is the ability to re-use this data compromised, but the data itself does not naturally serve virtual prototyping activities.

• Engineering intellectual property (IP) for wiring integration often resides in the heads of skilled, experienced engineers, not in a form that allows crucial best practice to be captured and leveraged.

• Design change occurs constantly, and reacting rapidly but accurately to design changes is difficult. Any delay within the integration group often causes a knock-on effect with manufacturing planning because systems integration is typically one of the latest tasks to be completed.

Significant process improvements can be realized when the entire wiring system’s development process is considered as a whole, rather than as independent steps. True process innovation is enabled only by improving the entire process.

For example, when systems and subsystems design data is captured in a form that makes re-use easy and able to drive downstream decision making, then wiring integration can be simplified. When IP is captured as re-usable design constraints, then all this information can be used by wiring integration to repeatedly and consistently generate the physical wiring implementation with each vehicle program.

Such factors are the underlying principles of the model-driven process: to provide engineers with the ability to describe the electrical design at a high level, and use software to generate the detail.

The model-driven approach is often referred to as the generative design process, and the software tools used to support the approach are called generative design tools. Generative design tools provide the wiring designer with a framework to capture all aspects of the design process at a high level and then provide functions that can automate each step in the design process from architectural schematics to physical harnesses.

At the start of the generative process, electrical systems and subsystems are captured in a form that pictorially represents signal connectivity. Added to this connectivity are parameters that describe the operating characteristics of the equipment and signals.

For example, power and signal characteristics, EMC classifications, and other attributes are added to the high-level connectivity description. At this point, there is a fully defined logical signal model. Integrated simulation functions allow the design to be analyzed for FMEA, sneak circuits, and a multitude of other effects.

Since there is no physical or wiring information as yet, any changes that need to be made can be identified and implemented quickly and cheaply. The logical signal model does not contain any vehicle specific information; it can be used and re-used repeatedly on successive vehicle programs, reducing development time and ensuring that best practice is captured and applied.

Proceeding along the design cycle, a physical harness model is created complete with corresponding geometric and environmental constraints, usually extracted from the 3-D MCAD domain. There may be areas or zones that have special requirements for the wiring system: high heat, wet areas, etc. This information is described with codified constraints that are applied to the locations within the physical representation. These constraints are reusable and can be put in a library for future use on other programs.

Constraints are core to a model-driven development process. Constraints drive part selection and physical distribution of some components (such as fuses), determine correct signal routing based on EMC compatibility, define splicing, and implement grounding strategies and many other systems integration design tasks. From a quality perspective, constraints are the way to automate decision making so that it is guaranteed to adhere to regulatory and other design mandates.

At this point, the designer can generate the physical wiring designs by invoking the wiring synthesis function. Wiring synthesis is a process of integrating the systems and subsystems information with geometric, environmental, and electrical design constraints. Signals defined in the high-level-connectivity diagrams are routed in the 3-D physical definition, obeying the associated routing and electrical constraints, to create the wiring paths. Integrated electrical simulation algorithms within the wire synthesis process ensure that the wires are correctly rated, and other algorithms ensure that the customer-option/variant requirements are correctly engineered.

Wiring diagrams are important outputs from wiring system generation. These diagrams depict the physical “as-built” engineering configuration of the wiring information. In a model-driven development process, this wiring information has already been determined via constraints and other design decisions as previously described. The wiring diagram is essentially a visual representation of the wiring generation results.

Since all the wiring data is fully defined, wiring diagram creation becomes an automated documentation step. Instead of the diagram being a manually created document that is susceptible to data re-entry errors, it becomes a generated artifact of the wiring model. If the model data changes, a subsequent regeneration of the document is easily done. Any generation of the wiring diagram will reflect the current state of the wiring data.

At any time along the development process, electrical virtual prototyping and analysis can be done. The electrical data is in a form that makes analysis simple. Logical and physical steady-state or time domain simulation can be undertaken while the actual design is evolving, ensuring that design changes can be implemented when it is most cost-effective and convenient.

John Wilson, Product Marketing Manager, Integrated Electrical Systems Division, Mentor Graphics Corp., wrote this article for SAE Off-Highway Engineering.

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