Linius Technologies Inc.
Until recently, most computer- aided design software for producing virtual prototypes of wire harnesses had been part of a mechanical engineer’s toolbox. Mechanical 2D and 3D CAD packages just adequately handle the spatial features of a harness design such as wire bundle length and position, but fail miserably in quantifying combined electrical and mechanical features such as wire resistance and electromagnetic field interference. Moreover, repeatedly hooking into electrical CAD software to fill the gaps slows the design process and cannot satisfy all harness designers’ needs for a totally integrated, start-to-finish design. To make matters worse, the electrical features of many mechanical CAD packages are nothing but addon modules that don’t blend to make a coherent harness-design process.
COMPUTERIZING THE ESSENTIALS
Because harnesses are becoming increasingly more complex and usually the last components to be fabricated and installed, they often become the pacing item in new product development. What’s needed then to accelerate the design process for large systems is a dedicated electromechanical software package that automatically handles merging and continuously changing electrical and mechanical feature information. It should update schematics and nailboard drawings, store logic and algorithms for 3D displays of virtual prototypes, and maintain the computer database. The software should also generate enough accurate information to manufacture harnesses without depending on numerous prototypes to verify designs. A new program called EMbassy meets these requirements. It quickly helps wire harness engineers assemble accurate information for fabricating harnesses, including:
• finished dimensions
• wire lengths
• cable groupings
• lists of wires and connection points that define logical connections
• component specifications for ordering individual harness parts.
Though input for the program comes from many different sources, such as engineers designing power-distribution systems, computer displays, and mechanical cabinets, as well as workers in information system and marketing departments, all the data logically merges to form a single harness model. For instance, electrical engineers define the point-to-point wiring requirements, define various types of electrical circuits such as high or low ac or dc power, and specify a variety of connectors and wires that the program might select to make connections. The mechanical engineering group in turn designs the structural assemblies that determine wire routing, and specifies minimum bend radii and mechanical interferences. This input lets the program calculate the amount of wire needed to fit the space available in the assembly and decide where specific wires should be protected or specially routed. Both design groups also contribute a variety of material specifications for all components.
The final piece the program provides is component data. This includes basic information for ordering wires and connectors and definitions of many other parts of the harness, such as shrink tubing, tape, lugs, pin inserts, clips and many similar objects.
In many cases, 3D models of housings, frames, and connectors can be used from outputs of other mechanical solid-modeling packages. In cases where models are not available, the software has the option to create basic 3D enclosures and connectors. Wire connections can be derived from 2D schematic software programs, spreadsheet files, ASCII lists, or manually inserted on the fly.
The program combines this information in the software model with the electrical wiring connections and 3D physical models of components to create an accurate virtual harness prototype. Using this, the software intelligently manages the position of the wire and connectors on the software model. For instance, as engineers reposition a harness, the components appear to behave as an intelligent group of related objects, that is, the wires remain connected to their respective connector pins in all views of rotation.
After completing the virtual prototype, the software quickly provides manufacturing reports and drawings such as bills of materials, wire-run lists, and 2D nailboard drawings. Because it is based on a virtual prototype, this automated process produces the data much more accurately than traditional manual methods.
LEVERAGING THE ENVIRONMENT
After the program organizes the aggregate information in its database and produces a virtual prototype harness, design and manufacturing teams can select special features to explore or troubleshoot. For example, designers often trace electrical signals throughout a circuit to verify and troubleshoot defective circuits either before or after system production. The teams also check for illegal grounding schemes, dangerous misconnections, or inaccessibility to certain wires that were not part of the original design guideline input.
Similarly when harness designers look up connector properties in the computer database, all related part information, manufacturing properties, 3D locations, and pins and wires are readily available. Because the program intelligently maintains electromechanical relationships between harness objects such as wires, pins, connectors, and wire bundles, designers can select a segment of a harness and quickly find which wires are part of that segment.
In addition, large assemblies often contain multiple harnesses, many of which are variations on a basic design — further justifying the need for a central database. For instance, products designed for various countries require different wiring configurations such as for right-hand steering columns in European vehicles and left-hand columns for the US. Also, many automobiles offer add-on features such as antilock brakes, digital dashboards, navigation systems, cruise control, and sunroofs that require harness options. In both examples, basic point-topoint wiring is the same, only the positions change or parts are omitted. Users need only click on the option desired in a display tree to set up the software for managing these multiple harnesses and configurations.
VERIFICATION BOOSTS ACCURACY
With all data contained in a single database, comprehensive design verification algorithms take advantage of the logical, physical, and component information available. These data, combined with engineering design guidelines, improve wire-harness accuracy. The algorithms verify bend radii, part selection, and other design rules automatically.
The software also calculates conductor impedance as well as harness weight. The program tags critical wires with maximum impedance and checks them against the calculated value based on wire length and resistance per unit length. Wires that fail the check are shortened or rerouted until they pass. (This step is becoming increasingly necessary for verifying that the high-speed signals handled by many onboard computer control systems won’t degenerate.) The program also calculates the overall weight of the harness using wire lengths and weight per unit length. Weight is often an issue in vehicles such as airplanes and automobiles, but it also determines how to fasten a harness to a chassis in a 19-in. rack, for example. Large harnesses that are not fastened securely can stress connection points and break under low-level shock and vibration.
In the midst of managing different wire harness designs and dealing with engineering changes, the software ensures that the parts mate properly. For example, gage checks verify that lugs or pin inserts and wire sizes are correct for the connector body. The program also checks relationships between connectors, wires, and pins for maximum voltage, current, and power ratings, and it verifies designs before sending data to manufacturing to ensure that all required information is included.
The program checks another quality, wire formation, before handing a design off to manufacturing. These checks verify that the virtual prototype is a valid physical representation of the harness. First, the minimum bend radius is checked on each wire based on internal design rules to ensure it is not out of limits, eliminating overstressed conductors. This is especially critical for fiber-optic wiring and semirigid cables.
The software makes a second check to determine if wire bundles can fit into a product. For example, as wires are added to a bundle it becomes more difficult to bend, and forcing the conductors to bend beyond predetermined limits stresses the wire-to-pin connections. Algorithms in the software use wire material type, shielding, and wrap information for all wires in a bundle to calculate limits and compare them to the actual bundle geometry.
Mechanical and electrical interference is another big issue. Mechanical interference can become a serious problem in moving systems. Conductors that rub against sharp edges can break and bring down an entire system. At a minimum, interference between harnesses and other objects can generate audible noise which is perceived as poor product quality. The software checks all aspects of the harness, including size, weight, fastener points, and flexibility to determine when the harness interacts with other harnesses or other objects. Thus, users can set tolerances and limits to identify acceptable levels of interference.
Electrical interference comes from high-power signals on one conductor influencing nearby low-level signals in another. The corrupted low-level circuits can then produce intermittent electrical problems or entire system failures. In most cases, it’s easy to identify signals which are likely to affect other circuits or are susceptible to noise. Armed with this information, the software provides guidelines for the spacing to be maintained between these circuits, and automated checks verify that the guidelines are followed. Without such checks, many harnesses are unnecessarily shielded, which drives up costs.
PUSHING THE ENVELOPE
Computer algorithms derived from user input data automatically optimize the wire harness for its environment by balancing cost, impedance, weight, minimum and maximum length data, and other user-defined constraints. Two design techniques include wire autorouting and component configuration. The benefits of autorouting are that a smart algorithm explores all possible paths and chooses the optimum by examining user-defined constraints. Each wire can have individual constraints such as maximum length or minimum bend radius. Priorities can also be set for the entire design, such as maximum bundle diameter, shortest path routing, and least weight. By using the information already in the harness design database, a wire harness router can check design rules and optimize system priorities simultaneously. For instance, users can specify that high-power signals and high-frequency signals cannot be routed into the same channels, or if there is no other solution, the system can notify users that special shielding must be added. Using computers and a harness-oriented data model, these design trade-offs can be explored more quickly and many different options can be examined before choosing the best layout.
The choice of wire, connectors, and associated parts can have a significant impact on harness cost, quality and weight. The software can explore trade-offs using a routine that analyzes all component data and makes suggestions to improve the harness based on the factor that demands highest priority.
For example, suppose a specific connector family is too expensive. The design can be reanalyzed after substituting another connector family to see if quality goals are still met. The program decides if the substituted family of connectors can handle the wire and pin gauges that have already been selected or if more changes are needed. It also determines if the new connectors cause mechanical interference problems when they are larger. Issues like these and many others can be systematically solved using the combined electromechanical data and built-in analysis modules targeted to handle an individual user’s design guidelines.
HARNESSING THE FUTURE
Maintaining critical design knowledge in a central location throughout the harness design cycle is also a factor in reducing costly post-production failures. This may be especially true for problems such as electromagnetic interference. “When working on EMI problems, electrical designers usually know in advance which areas are sensitive and likely to cause problems,” says Michael Prussel, principle engineer at Sanders, a Lockheed Martin Co. in Nashua, N.H. Prussel specializes in solving EMI problems on various military electronics systems at Sanders. “Many potential interference problems can be minimized or eliminated by providing down design rules and constraints to EMbassy’s database. The electrical engineer’s time can then be freed to focus on more difficult EMI problems or signal integrity issues that often aren’t detected until well into the prototype stage or even until field testing.
Quality Assured Cables (QAC), one of the Northeast’s largest cable and harness manufacturers, also uses EMbassy for working more efficiently and cost effectively with its large system design customers. Dick Dyer, president of QAC, states, “When designers and suppliers can easily exchange more accurate design data within a central database, the time needed to deliver final production harnesses significantly decreases. Assessing total project time is also more predictable.