With each new generation of aircraft, electrical systems have been called upon to provide more capability, with the result that the number and complexity of electrical systems has increased dramatically.
The task of integrating the multitude of systems and subsystems into a platform is a time intensive and difficult process, says Mentor Graphics. Often the impact of a design decision is not discovered until much later in the process when a wiring harness that implements the integration has been manufactured, tested, and installed. Despite all the efforts to reduce development risk and a desire to follow disciplined and rigorous development programs, program objectives slip and electrical system integration frequently ends up on the critical path.
While electrical systems complexity is increasing, emerging regulatory standards are also increasing the focus on the wiring system. The new Electrical Wiring Interconnect System (EWIS) requirements in Federal Aviation Regulation Part 25 Subpart H address the decisions made during the system-integration process, and the subsequent maintenance and service of the interconnect wiring interconnect.
Issues that make electrical systems integration complex and vulnerable to risk include:
• 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 last tasks to be completed.
• 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.
Significant process improvements can be realized if the entire wiring system’s development process is considered, not just parts of the process. Wiring integration is a key part of the wiring systems development, but true process innovation is enabled by improving the entire process.
For example, if systems and subsystems design data is captured in a form that makes re-use easy and is able to drive downstream decision making, then wiring integration would benefit. Ideally, the underlying data should be representative of the actual electrical model being described, allowing early design verification at any point in the design process, which, in turn, addresses the design change issue. If IP is captured as design constraints, then all this information can be used by wiring integration to synthesize, or derive, the physical wiring implementation.
Leading-edge commercial off-the-shelf (COTS) software suppliers currently provide tools and technology to support this model-driven development process. Using this approach allows the designer to capture the systems and subsystems design data in a form that describes the connectivity, its operating characteristics, and associated design constraints, for example, electromagnetic compatibility (EMC) requirements. This logical design data is an electrical model that can be used to study behavior at a very early stage. The model can then be analyzed together with other models (to identify unintended electrical interactions, for example) to understand combined effects before wiring integration starts.
As the design process advances into wiring integration, the logical systems and subsystems data, together with a customer’s wiring integration guidelines and IP constraints, is used in conjunction with geometric constraints to synthesize the wiring model. This wiring model is a complete platform-level wiring architecture that reflects optimized signal routing, correctly sized wires, and proper grounding implementations. At any point in time, the data is in a form that enables quick and efficient electrical verification, occasionally called electrical virtual prototyping.
To use an actual example of COTS tools that are being used today, at the start of the 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 that can be analyzed for FMEA, sneak circuits, and a multitude of other effects. Since there is no physical information (i.e., actual wiring) as yet, any changes that need to be made can be implemented quickly and cheaply.
Proceeding along the design cycle, a topological 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, SWAMP areas, etc. This information is described with easy-to-read, codified constraints that are applied to the locations within the topological 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 will drive part selection, determine correct signal routing based on EMC compatibility, define splicing, implement grounding strategies, and many other systems integration design tasks. From an EWIS 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 in the process, the integrated wiring model is synthesized. Wiring synthesis is a process of evaluating the systems and subsystems information, considering geometric, environmental, and electrical design constraints and then automatically implementing the wiring model.
An important output from wiring system integration is wiring diagrams. 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. The wiring diagram is essentially a visual representation of the wiring integration results.
Since all the wiring data is fully defined, wiring diagram creation now 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.
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.
This article was written for Aerospace Engineering by John Low, Aerospace Product Marketing Manager, Mentor Graphics Corp.