By Elisa Pouyanne, Business Development Manager, Integrated Electrical Systems Division, Mentor Graphics Corporation
The electrical distribution system (EDS) design process for automobiles and other transportation platforms is basically a sequence of steps performed in more or less serial fashion. The stages in the process begin with requirements definition, systems design and engineering and extend all the way through harness engineering including manufacturing documentation. The process as a whole must be organized to create manufacturable harness products that support the requirements (design intent) and meet the enterprise’s quality and cost constraints.
Historically each stage has been an “island” with its own design tools and a complex local dialect that describes the components, inputs and outputs of the particular stage. Communication between the stages has often been cumbersome, requiring conversions and/or manual data re-entry at the input to each step.
Unfortunately the historical methods cannot meet the time, cost, and competitive pressures of the modern design realm. It is simply no longer feasible for consumer vehicle makers to live with the redundancy and delays that result from passing “lumped” data from island to island in the process. As a result, innovative software-based solutions have emerged to address these challenges.
The Concept of Digital Continuity
The solution for today’s time, cost, and competitive pressures is a concept known as digital continuity. In such an environment, data flows from one development stage to another without duplication, disconnection, or data re-entry. True digital continuity also enables critical information to cross to, and from, related processes occurring in parallel within the enterprise. These include: concurrent design using MCAD tools; harness configuration management; component inventory processes and databases; and product lifecycle management via Product Data Management (PDM) systems.
Two other issues have an impact on the nature of digital continuity. First, design changes are inevitable—sometimes a daily occurrence. A toolset that supports intelligent rules-driven design change management is essential. Only then can reliable up-to-date data propagate across domains and teams and over organizational and geographical boundaries.
And secondly, reaching the final stage in the design flow doesn’t guarantee that the product will comply with the original design intent, reliability guidelines, and manufacturability objectives. But with the data accumulated over the course of the entire flow, it is relatively easy to confirm that goals and cost expectations have been met.
Where does digital continuity begin? At the very earliest planning and requirements definition stage. Enterprise-wide databases are a medium for continuity, as are consistent, compatible interfaces among the various development stages. A key enabling factor is a design toolset that raises the level of abstraction and automates more tasks. If an engineer can work with higher-level entities and symbols, then these entities can bring embedded knowledge of their own characteristics and behavior to the design. This in turn supports real-time rules checking, “what-if” prototyping, and more. Data continuity embodies data unification, integration, verification, and accessibility.
The Harness Design Flow, From Specification to Manufacturing
Harness development is a transition point in the vehicle design and manufacturing process. It is the environment in which critical design data matures into a buildable product. Modern ECAD software supports this process by enabling and imposing digital continuity. It spans process stages ranging from the earliest design effort (where electrical functional requirements and physical requirements are captured) to implementation. The deliverable is a completed harness product and ideally, thorough documentation to accompany it.
Figure 1 illustrates the concept of the design flow and symbolizes the digital continuity as data passes from stage to stage. Each step in the flow receives and matures relevant information from all preceding stages. New design data is of course generated in each successive step for use within that step. But each step also evolves the data received from the steps that precede it. As Figure 1 implies, the volume of data increases with each step forward. The data from the Logical Architecture stage, for example, passes to the Systems Design stage where it becomes an integral part of that stage’s data, which in turn gets passed on. This flow underscores the importance of both consistent interfaces from step to step and data versioning and release management. Typically the data content is communicated via one or more databases.
Figure 1: Digital continuity implies a series of smooth transitions among all the steps of the design flow. Each process step (stage) contributes data to the subsequent steps. This information is preserved and reused, maturing as it moves ahead in the process.
Interpreting and Applying the Design Intent
“Design intent” arises from the Requirements and Architecture stages at the beginning of the process depicted in Figure 1. Intent includes not only a description of the functional needs that must be satisfied, but also details such as weight and above all, cost. It also includes methods to help designers satisfy these functional requirements. Harness engineers are responsible for achieving in general terms the design intent and requirements, including some constraints on installation into the vehicle, all in the context of a manufacturable product.
A harness definition is at the crossroads of data from the electrical wiring design and the digital harness layout mock up. Ideally it accounts for the diverse configurations that will be needed to support variations in the harness’ electrical content. The design department’s deliverable is a completed drawing that may form the basis of contracts between OEMs and Tier 1 suppliers, and is ready to be further processed by the harness engineering group.
The distinction between the disciplines of harness design and harness engineering may not be clear to those unfamiliar with harness development. The harness emerges from the design department complete in concept but not necessarily ready for production:
· It may not contain all the data required to physically build the harness.
· It may need to be enriched with certain parts necessary to support assembly of the harness into the vehicle.
· It may call out specification criteria rather than actual part numbers to describe wires, connectors and components.
· It may be authored as a superset/composite with the expectation that these will be broken down into variants/derivatives.
· It may embody some requirements that are unachievable and others that don’t conform to best practices.
Therefore the next step is harness engineering, which transforms the design intent into usable manufacturing data. Figure 2 shows how multiple inputs are combined in the harness design and engineering stages to create manufacturing and business data that is ready to use throughout the enterprise.
Figure 2: The harness design and engineering process accepts data in diverse formats and delivers the drawings, costing data, and BOMs needed for manufacturing.
Moving from Design to Reality
The harness engineer is assigned to produce a 100% accurate, error free data set—per variant/derivative—that can be fully costed and passed on to production for assembly. He or she must generate the full BOM and validate it against the design intent. Again, accuracy is crucial because the product is destined for high-volume manufacturing, where the cost of an error is multiplied by thousands or even millions of units.
In effect, the engineering phase of the process brings theory and planning face-to-face with reality, where “reality” includes laying out formboards, programming test and production equipment, defining tooling requirements and wire cutting charts, and making cost calculations.
Software tools selected for the process must provide a 2D design environment supporting the large variety of harness components including bundle protections, clips and grommets, cavity components, and more. In turn this environment must be automated to help harness engineers manage discretionary steps including:
· Component selection
Wires, terminals, seals, plugs, tapes, tubes must accommodate OEM specifications. For example, an OEM might specify a basic clip position but the harness maker must define the taping that surrounds it.
· Splice position optimization and balancing
Here the harness builder must meet manufacturing and quality rules, including customized rules such as the choice of waterproofing materials (i.e., heat-shrink sleeves). Similarly the OEM might specify a shielded cable while leaving it to the harness maker to define the solder-sleeve position and the drain wire connection.
· Wire-color optimization
The harness provider must allocate wire-color definitions for manufacturing and service while taking the wire inventory into account:
· Component sourcing verification
· Assembly time calculation/prediction
· BOM calculation
“Engineering for manufacturability” means different things to different people but most agree that it encompasses activities such as splice optimization and component selection; that part of the task is to confirm the availability of the selected components from preferred suppliers; and that there is an overarching responsibility to ensure that all contractual obligations with the OEM are met.
Figure 3 summarizes all these steps and processes.
Figure 3: One harness design forms the basis for many variants. Modern design tools automatically manage configurations for dozens or even hundreds of minutely varying harness types.
All this must be done within the constraints of the very low margins that characterize the contract harness market. Production is often geographically widespread and logistically complex, making it difficult, but critical, to accurately predict product cost. But a small amount saved on each harness can save many thousands of dollars when volume production begins.
Fortunately, a full-featured harness design environment can manage a composite harness that encompasses all the anticipated variations in electrical content. Essentially this is the master design. All the variations are managed as a single entity but the differences—in electrical content as well as tubing, taping, and so forth—are calculated for every single harness product.
Importantly, this approach provides economies of scale in tooling and manufacturing planning. If a dozen harness variants use a specific length of black plastic corrugated tubing, then a single inventory of that part, prepared as a batch with one tooling setup, one cutting operation and one purchasing transaction, can serve the needs of all twelve variants. The data about the tubing can be used most efficiently when it is maintained in a components library that keeps track of components’ individual attributes as well as all relevant compatibility and dependency rules. Figure 4 illustrates a hierarchy of derivatives. Note that this simple image intentionally depicts far fewer derivatives than a real-world platform might require.
Composite designs also allow design changes to be proliferated across a whole family of harness derivatives. If the tubing length in the master harness design gets changed, then that change automatically “trickles down” to the derivatives.
Figure 4: Small variations can create a large number of derivatives. These three derivatives represent just a fraction of the hundreds that may arise.
Comparing the Engineered Product with the Design Intent
The technique of embedding design rules within the software is the surest way to achieve practical compliance with the design intent. Design rules are a form of business logic that, together with a rigorous validation regime, ensures the production of fully-detailed harness designs data in minimal time. A rule is exactly what the term suggests: a requirement that, if violated, will cause an alert at the very least. A rule might state for example that gold-plated terminals will be selected for all safety-related function. Of course this implies that “safety-related functions” must be defined as well, and so on through a hierarchy of definitions.
Design rules checking (DRC) relies on basic definitions that are consistent throughout the harness. Examples include “all wires are allocated into a connector or splice at each end;” “the path of each wire is unambiguous so that the length can be calculated,” and many more. In addition DRCs also monitor manufacturability, ensuring that terminal and crimping combinations are correct for the wire cross-section
It is important to note that DRCs are not uniform across the whole industry. The rules and their severity are always customized to a harness maker’s specific best practices, and the design toolset must allow for this customization.
From Digital Format to Physical Formboard
All the foregoing steps pay off when it is time to prepare the drawings that will be used in manufacturing. At this point all of the components in the harness are selected and their placement is established. The bill of materials is accurately calculated and costed. A completed virtual harness exists in digital form. What is needed is a template for the actual layout and construction of the “hardware” harness.
While there is much talk about levels of abstraction in the design automation realm, one of the key deliverables of the design process is something that is not abstract at all: a full-scale map of the harness layout. Today’s advanced designed tools can produce this “Formboard” document automatically, and it is more than just a map. It includes all necessary visual aids to support speedy and error-free manufacturing via steps such as wire insertion, bundle covering instructions etc. It also lays out certain fixturing components that guarantee the harness will integrate into its mechanical environment. Considerations include the correct orientation of clips for easy assembly into the vehicle’s panels and passages; orientation of connectors for safe insertion into ECUs, and so on. Figure 5 depicts a formboard in use on the harness production line.
Figure 5: The formboard is one of the ultimate deliverables of the design-for- manufacturing flow.
Here again, digital continuity proves its worth. All necessary data accumulated throughout the design flow is ready and waiting to inform the final formboard drawing. No data re-entry is necessary. If there are late-breaking design changes in some part of the harness, these can be applied easily and will propagate throughout the affected branches and wires as well as all derivatives.
The design data accumulated over the span of the process has yet another use in manufacturing. Information can be delivered in the correct format to directly drive production equipment such as test systems, laser markers, and wire cutting machines. This eliminates the need for transcription or re-entry, bypassing a time-consuming step and ensuring error-free execution of the respective operations.
Conclusion
After decades of harness engineering performed manually, the old ways are fast becoming obsolete in transportation product manufacturing enterprises of every scope and size. Today’s technical requirements, complexity, and time pressures are bringing electronic design methods to the head of the class. Solutions such as Mentor Graphics VeSys and CHS apply proven data-centric technologies to harness design and engineering for manufacturability, simplifying the whole process:
· Data-centric software enables digital continuity from requirements through implementation
· Change management is thorough and automated at every step thanks to a purpose-built data-model specialized for electrical harness engineering applications
· Configurable harness engineering and Design Rule Checks validate compliance with design intent
· Automated tools provide both formboard layouts and visual assembly aids for manufacturing
· Design data accumulates over the length of the harness engineering process and feeds production equipment with current and correct information
ELISA POUYANNE is Business Development Manager for the Integrated Electrical Systems Division of Mentor Graphics Corporation.
