Large mechatronic systems rely on complex wiring to integrate their various functions. Pre-designing and pre-building machine wiring and cabling can offer many benefits. Whilst good cabling and wire routing can improve reliability and performance of the system, as well as providing an opportunity to improve cost and material planning.
Mechanical factors such as clearance, collisions, and access should also be considered, in addition to electrical factors like EMI, crosstalk and voltage drop. The constant growth of electrical content in machines makes wire and cable harness design increasingly complex and error-prone. However, despite increasing challenges, engineering schedules remain largely unchanged and using inappropriate tools and work-flows can result in costly and time-consuming rework and sub-optimal designs as well as inaccurate cost and material estimates.
Traditional design method
The conventional harness design process requires a physical prototype. Wire lengths are measured manually on the prototype, often by the electrical/harness engineers, and the information transferred to production documents. A prototype harness is built and checked for fit on the mechanical prototype. This approach can be acceptable for systems with low-complexity electrical systems and simple mechanical geometries. However, problem identification occurs relatively late in the schedule. Rework at this stage can be costly, and quick fixes can mean compromises resulting in a less than optimum design.
Another drawback to this approach is the inherent delay. A wire harness cannot be developed until virtual prototyping has finished and a physical prototyping is built, and documentation cannot be finalised until harness design is completed. 
Using a 2D methodology
With a dedicated 2D design tool electrical schematic information can be imported from ECAD tools to develop the wire harness. Length information can be obtained from Mechanical CAD (MCAD) tools using add-on routing functions to define wire paths. However, it may be difficult for electrical engineers to perform this task. MCAD tools are specialized and require significant training before the user is proficient. In addition, representing wires in an MCAD environment adds complexity to the mechanical model and can affect the tool performance (speed).
Benefits of a 3D methodology
Adopting a 3D approach, by using a solution like EPLAN’s Harness, schematic information is taken from ECAD tools and a mechanical model from the MCAD tool in a single environment for harness design.
EPLAN Harness proD is not a simple MCAD tool, rather a stand-alone 3D wire harness design tool, developed specifically for purpose. Native 3D mechanical models from all popular MCAD tools can be imported and schematic information can be read from any ECAD tool including EPLAN Electric P8. Harness proD is easy and intuitive for electrical designers to use, allowing them to assume more responsibility for the mechanical considerations of harness design.
Schedule Improvements
Wire harness development can begin as soon as a 3D mechanical model is available and can occur in parallel with virtual prototype testing of the mechanical design. Waiting times involved in the traditional method are eliminated and change requests to accommodate wiring requirements can be made before the mechanical design is ‘frozen’.
Wire harness design requires collaboration between electrical and mechanical departments, so engineering change management is vitally important. This is enforced by the Harness proD. The 3D mechanical model cannot be changed inside Harness proD, so harness designers cannot compromise the original mechanical design.
Harness engineers can export their designs as a STEP file assembly which can be viewed by mechanical engineers, but not modified, thereby protecting the integrity of the harness design. Teams can collaborate and produce a design that is optimal from both a mechanical and electrical perspective. Late modifications to the documentation should be reduced since changes will have been incorporated earlier in the process.
Ensures harness design quality
Using a 3D design method like Harness proD will identify problems caused by poor fit in the mechanical structure. An extensive set of checks are included that highlight problems with bend radius, pin/wire diameter mismatches, collisions and fill grades (e.g. of flex tubing). Identifying problems early in the process will reduce the cost of rework and improve quality.
Starting development sooner
Early in the development schedule not all the components in the Bill of Materials (BOMs) may be defined. It may be known what component type is needed, but it may not have been fully specified. Alternatively, perhaps a component is specified, but a model is not available for the design. Being able to design the harness, route it around the mechanical geometry, then finalise the components when they are known and available would be an ideal situation. With the ‘rapid prototyping’ feature of Harness proD this can be achieved. Generic library parts are used which can later be substituted for the real ones. Wires can be connected to these parts and they can be used in the same way as other library components.
Further productivity improvements
Creating BOMs, wire lists and nailboard diagrams can be a time consuming, and error prone process, so it makes sense to generate these automatically. Once a harness design is completed in Harness proD, a 1:1 nailboard diagram can be generated at the push of a button. This diagram can be modified and scaled down if necessary. BOM reports and wire lists can also be generated with a simple push-button operation.
Conclusion
The conventional wire harness design process contains manual activities, is prone to error, and introduces inefficiencies into project schedules. It is not conducive to collaboration between mechanical and electrical teams.
2D wire harness tools improve matters, but depend on MCAD tools for bundle diameter and wire length information. Iterations between 2D Harness and MCAD tools may be required to converge on a good design. What’s more, MCAD tools are not intuitive for electrical engineers resulting in the responsibility for wire routing on occasion passing to the mechanical design team.
Using a 3D design process with EPLAN Harness proD enables electrical wire harness engineers to consider mechanical factors and influence changes in structural design that will benefit the electrical wire harness. Waiting times are eliminated by concurrent mechanical and electrical design and wire harness error checking and productivity features reduce project schedules and ensure quality.
One EPLAN user transitioned from an extensively manual process using physical prototypes for harness design to a fully automated one, even exporting data from Harness proD to wire processing machines. Schedule improvements were around 90%, and product quality was improved because of collaboration between mechanical and electrical teams. Of course, improvements in schedule time will depend on the nature of the current process, but the savings can be significant.
Author: Kevin Edwards, Product Manager EPLAN Harness proD
