Cable Harness Design — Making the Connection

As every cable harness designer knows, the design of electrical cabling systems — often exceeding 100 miles in length in large-scale aerospace and defense systems — poses enormous challenges. The cabling design process involves both electrical and mechanical design. Two dimensional electrical schematics and 3D solid CAD models require very different methods and tools, and bridging the gap is often the most difficult part. Pro/ENGINEER Routed Systems Designer 7.0 integrates the two disciplines, making the cable design process faster and easier.

The electrical side of cabling design. On the electrical side, engineers must consider the electrical viability of the design. That means checking the voltage drops across the entire power distribution system to be sure there’s enough power at the far end of the cable for safe operation of critical equipment. It also means calculating the resistance, power dissipation, and temperature rise of each conductor to ensure it will never become hot enough to melt the insulation.

Engineers also have to consider the potential for electromagnetic interference (EMI) to generate spurious currents that would interfere with safe operation. For example, you must evaluate inrush currents that can occur when circuits are turned on and have the potential to generate EMI. You also have to protect against external EMI sources such as lightning strikes and other electronic equipment.

Transition to the mechanical side. As an engineer working on the electrical design, you have software tools for addressing these issues. But when its time to move the electrical design into the physical world, typically another engineer must start from scratch and create a virtual solid model of each of the tens of thousands of wires in the electrical design, then snake them through the mechanical design, correct interferences, address critical safety issues such as temperatures and chafing, and ultimately calculate the length of each wire.

This is the point where things usually get difficult. Here, we have two designs — one electrical and one mechanical — each with tens of thousands of conductors providing both power and data to hundreds of modules. Since data entry errors are possible, and changes might be applied to either design at any time, we have to worry about how closely these two models match up.

If the mechanical design does not accurately interpret the electrical information, the associated modules may not function correctly and manual checking of data between the two disciplines is time consuming.”

In addition, calculations made on the electrical design, such as voltage drops and inrush currents, depend both on the resistive and reactive characteristics of the wire, as well as the reactive load of the models being supplied. The length of the cable run can only be estimated before the cable is routed, and mechanical designers may make changes that affect the electrical side, such as replacing a pump because of supplier problems.

The instant the routing is completed; the electrical design must be updated with the correct physical specifications, checked for accuracy, reevaluated, and in many cases redefined. And of course, this needs to be repeated whenever the mechanical design changes.

Using current methods and tools, the electrical and mechanical aspects of cabling design must be addressed separately in different environments. The problems arise from the interdependence of these two environments. The electrical design must be completely transferred, at least once, from the electrical world to the mechanical world, and data from the mechanical design — such as wire length — must be transferred back to the electrical world. The iterative nature of engineering means that many additional data transfers must be made in both directions.

Bridging the gap between electrical and mechanical. There is an easy and comprehensive solution to this problem with the newest version of Pro/ENGINEER Routed Systems Designer 7.0.

Pro/ENGINEER Routed Systems Designer 7.0 provides a single, fully integrated environment for creating a top-level system design and electrical schematic that integrates the flow of information throughout the entire cabling design process. You create your electrical schematics in Routed Systems Designer, and the information required by electromechanical designers is sent via XML to automate the routing within the digital mockup.

Pro/ENGINEER Routed Systems Designer 7.0 finds the best possible path for cabling. And using two optional add-ons — RSD Simulate and Virtual Interconnect’s SMARTSymbol libraries — you can even simulate the schematic from an electrical standpoint. Once the schematic data has been transferred to the mechanical design for routing within the digital mockup routing information, such as wire length, can be extracted out of cabling design and reused for further, more accurate simulation.

Let’s take a closer look at how this new software can improve the cable design process. Pro/ENGINEER Routed Systems Designer 7.0 provides an environment to develop the block interconnection diagram (BID) that’s often used to define the top-level system design, as well as the wiring interconnection diagram (WID). The BID describes the different modules at a high level and how they are to be interconnected. It provides a high-level representation of the critical requirements of the electrical design which serves as a guide to the electrical engineers responsible for the detailed design process. In the normal design flow, a single engineer or a small team creates the BID, which is then divided into sections and parceled out to a larger group of engineers who create the schematic diagram that forms the basis of the WID.

BID linked to WID. Normally, a considerable amount of time must be spent at various stages of the design process checking the BID against the WID to ensure that all the required interconnections are made correctly between modules. But Pro/ENGINEER Routed Systems Designer provides a link between the BID and WID to automate this checking process, saving a considerable amount of time and eliminating the risk of errors.

Pro/ENGINEER Routed systems Designer 7.0 also supports concurrent engineering, each engineer has full access to the BID, making it easy to define design rules such as diameter, thickness, grouping, packing, strip lengths and terminators, and enforcing these rules during the cable routing process.

Engineers can use 2D symbols from the SMARTSymbol library to define the electrical and mechanical aspects of the cable, such as resistance, inductance, and capacitance per unit length, voltage and current rating. Virtual Interconnect’s SMARTParts library also provides 3D geometry. These libraries seamlessly integrate with RSD Simulate, which determines voltage drops as a function of loading or current everywhere on the circuit.

Electrical schematic drives 3D routing. At a suitable point, well before the schematic is completed, you can leverage the knowledge contained within the schematic to automate the creation of harnesses within the 3D CAD assembly. This streamlines the design process by removing the tedious manual process of interpreting 2D schematic diagrams. By automating this process, you virtually eliminate inconsistencies between the electrical and mechanical designs by ensuring adherence to the schematics.

Upon importing the XML file containing the 2D electrical information into the 3D CAD model of the product, you have the option of routing the cable either manually or automatically. The design rules that were defined in the electrical schematic, transferred along with the connectivity information, are used to drive both manual and automated routing. The SMARTParts library saves additional time by providing 3D CAD models of the various types of connectors that are automatically invoked based on the schematic.

When the cable routing process is completed, you can export the length of each cable run and other parameters back to the 2D routed systems design environment in order to update the schematic diagram. This, in turn, makes it possible to re-run the electrical simulation with accurate cable lengths. These operations can be performed without manually entering data. Whenever a design change occurs, the new design information can easily be transferred electronically.

By bridging the gap between the electrical and mechanical disciplines, Pro/ENGINEER Routed Systems Designer 7.0 and the Virtual Interconnect libraries can dramatically reduce the time and risk of error involved in cabling design.