"Harnessing the Job," Feature Article, November 2007

harnessing the job

Software tools convey the 2-D world of electrical schematics into the third dimension.

This article was prepared by staff writers in collaboration with outside contributors.

there was a time, farther in the past than any of us can remember, when mechanical systems were largely that. It was a time before anyone put electric generators or radios into vehicles, before diesel-electric locomotives, and long before avionics, inertial guidance systems, or remotely controlled aircraft. There was a time before SCADA and the wealth of electronics that control and monitor production lines in factories and generating equipment in power plants, but it is a time that has passed.

You can still buy a can opener that you crank by hand, but it seems that, in general, very little is simply mechanical any more. That's because very little of our world is simple.

But for all their electrical components, the function of machines is still fundamentally mechanical. Often machines are designed on two tracks-one the province of the mechanical engineer, and the other of the electrical. So you have a mechanical design and an electrical design. Somewhere the twain must meet.

Electrical cabling systems can be a particularly challenging point of meeting, according to Jim Barrett-Smith, Routed Systems product manager for PTC, the software development company in Needham, Mass. Cabling can exceed 100 miles in length in large-scale aerospace and defense systems. Packaging and managing so much cable requires a lot of planning.

What's more, Barrett-Smith said, the issue is complicated by the differences between electrical and mechanical design. Electrical schematics are two-dimensional and CAD models are increasingly becoming 3-D, especially in the aerospace industry.

Schematics and 3-D models require very different methods and tools, and bridging the gap between these two worlds is often the most difficult part of the design process, he said.

Electrical engineers must consider issues like voltage drops across the entire power distribution system to be sure there's enough power at the far end of the cable to ensure safe operation of critical equipment.

They must also calculate the resistance, power dissipation, and temperature rise of each conductor to be sure it will never become hot enough to melt the insulation. They also have to consider the potential for electromagnetic interference, the spurious currents that can interfere with safe operation. For example, the inrush of currents when circuits are turned on has the potential to generate interference. Circuits must be shielded from surges, lightning, and electrical fields generated by other electrical equipment.

Electrical meets mechanical: Complexities arise when machines need to be wired.

Engineers working on the electrical design have software tools for addressing these issues. But when it's time to put the electrical design into the physical product, another engineer often must start from scratch and create a virtual solid model of each length of wire in the electrical design, then snake them through the mechanical design, correct interferences, address critical safety issues such as temperatures or chafing, and ultimately calculate the length of each wire.

This is the point where things usually get difficult, Barrett-Smith said. There are two designs-one electrical and one mechanical-each with 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, everyone has to worry that these two models match. That's because, if the mechanical design does not accurately interpret the electrical information, the associated modules probably won't function correctly. So, engineers need to perform an exhaustive manual process of comparing and checking the data between the two disciplines, a process that usually must be repeated many times as the design iterates.

Another concern is that all the calculations made on the electrical side of the design, such as voltage drops and inrush currents, depend on both the resistive and reactive characteristics of the wire, as well as on the reactive load of the models being supplied.

The final length of the cable to be run can only be estimated before the cable is routed. Mechanical designers may make changes that affect the electrical side. For instance, they may find it necessary to replace one pump with a different one because of supplier problems. The change of equipment may require different connections.

When routing is completed, the electrical design must be updated. This requires another major job of updating the electrical schematic with the new physical specifications, verifying its accuracy, and reevaluating-and, in many cases, refining-the electrical design. And, of course, this must be repeated, at least partially, whenever the mechanical design changes.

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. In addition, the iterative nature of engineering means that many additional data transfers must be made in both directions. Each of these manual updates consumes substantial work-hours, and is often fraught with errors.

To address the complexities of mixing the electrical and mechanical, PTC markets a product it calls Pro/Engineer Routed Systems Designer. Now in version 6.0, it also serves for other complex design challenges, such as hydraulic systems and piping that runs through process plants, where 2-D diagrams must eventually be interpreted for construction in the solid world of three dimensions.

Cassandra DeLeon-Kemp, who owns Cedel Solutions in Dedham, Mass., not only uses Routed Systems Designer to design applications for products ranging from air conditioners to fuel cells, but she is also sufficiently expert in the software to advise others how best to use it. The consulting end of her business has been far-ranging, from aerospace to automotive and medical. She says her business is about half design and half consulting.

According to DeLeon-Kemp, Routed Systems Designer speaks to both the electrical and mechanical engineer so they are both informed.

Diagrams of electrical systems cover details such as wire gauge and data on types of connectors, including cost, rating, and size-in general, information that will be used in the bill of materials, as well as details that the harness router will need to fit parts or direct cable and wire through machinery.

Working in a 3-D model permits the harness designer to consider alternative solutions. The efficiency of such a system becomes apparent in time saved. DeLeon-Kemp said the user of the system can expect to reduce cycle time by 25 to 65 percent.

PTC says that Routed Systems Designer provides an environment for creating the top-level system design and electrical schematic that integrates the flow of information throughout the entire cabling design process. Connectivity information required by electromechanical designers is sent electronically via XML to automate the routing within the digital mockup.

Using optional add-ons-RSDSimulate, SMARTParts, and SMARTSymbol from PTC's partner Virtual Interconnect Ltd. in Glasgow-an engineer can simulate the schematic from an electrical standpoint.

Engineers can use 2-D symbols from Virtual Interconnect's SMARTSymbol library to define the electrical and mechanical aspects of the cable such as resistance, inductance, and capacitance per unit length, and voltage and current rating. Virtual Interconnect's SMARTParts library provides 3-D geometry. These libraries integrate with RSDSimulate, which analyzes the electrical performance of the wire harness design.

Even before the schematic is completed, designers can leverage the knowledge contained in the schematic to automate the creation of harnesses within the 3-D CAD assembly. This streamlines the design process by removing the tedious manual process of interpreting 2-D schematic diagrams. Plus, by automating this process, they can virtually eliminate inconsistencies between the electrical and mechanical designs by ensuring adherence to the schematics.

Upon importing the XML file containing the 2-D electrical information into the 3-D CAD model of the product, the engineer has 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 extra time by providing 3-D CAD models of the various types of connectors that are automatically invoked based on the schematic.

Whenever a change occurs, the new design information can be transferred electronically. When the cable routing process is completed, specific information, such as the length of each cable run and other parameters, can be sent to the 2-D design environment to update the schematic diagram. This, in turn, makes it possible to rerun the electrical simulation with accurate cable lengths.

As products become more complex, so will the job of designing them, of integrating the electrical, mechanical, and other disciplines that combine in the sophisticated products of our world.

Is it a challenge for design engineers? Yes, it is. But it is also an opportunity for engineering toolmakers to come up with new products of their own that make the job at hand go more smoothly.