Interconnect design

In order to gain an understanding of the interconnect design procedures currently in use we visited a number of leading European companies in the defence, rail, aerospace, naval and motor sport industries. Our discussions with engineers in these companies revealed a number of common problems with existing electrical interconnect design processes, including:

  • Poorly defined design procedures.
  • Little integration between the various engineering groups or their design tools.
  • Limited means of ensuring the use of best practice design procedures.
  • Incomplete procedures for validating designs.

These problems may result from the way in which interconnect design processes have evolved over the past century. Electrical systems originally comprised a few simple circuits that were only documented in the form of a basic electrical circuit schematic. The interconnect design was derived by cabling a prototype of the product and then measuring cable lengths. Historically wiring harnesses were considered as little more than a bunch of wires taped together.

Electrical and electronics equipment is now considerably more complex, the number of circuits has grown dramatically and wiring harnesses are expensive assemblies which are difficult to design. To the casual observer it might also appear that the electrical interconnect design process has been significantly enhanced by the use of computer aided design (CAD). However, we found these CAD systems simply automate some parts of the basic interconnect design process and do not address the more fundamental design requirements.

In one of the companies we visited, separate CAD systems were used to draw circuit schematics, wiring diagrams, harness drawings and lay-up board designs. Although much of the same data was required for these various drawings, in each case information was manually re-entered. The engineers “integrate” the systems and have to ensure each of the individual software tools is properly updated after every design change which provides many opportunities for errors to occur.

The simple drawing, spread sheet and database systems that were used for interconnect design in all but one of the companies we visited, require large amounts of time-consuming human interaction. General purpose CAD drawing systems provide some help to the basic drawing process but none to the more important design or analysis tasks. Especially on large design projects, this “bottom-up” approach can result in engineers missing important issues as they become lost in unnecessary and time consuming details.

Managing the interconnect design process is increasingly challenging. Not only are electrical designs becoming more complex, design changes are becoming more frequent and must be managed throughout the process. Interconnect and harness designs must be continually revised and documented while engineers are also expected to reduce cost, weight and size and enhance performance, reliability, safety and robustness.

The interconnect design process typically starts with a generalised, high-level functional specification and gradually progresses through to the specification and design of all devices and harnesses in consultation with suppliers. But it is often not until the prototype stage, when the physical components are connected for the first time, that many interconnect design problems become apparent.

Currently engineers define interconnect systems “wire by wire”. This is increasingly time consuming and costly when, for example:

  • The number of wires in one railway carriage exceeded 10,000.
  • In one helicopter there were 250 harnesses each containing up to 2,000 wires.
  • In one army land vehicle there were 200 harnesses containing 2,000 connectors and 16,000 contacts.

There are immense difficulties in processing the volume of data and the complex interrelationships. This means that designers are forced to accept the first feasible design solution that emerges and have little opportunity to analyse alternatives or optimise the interconnect design.

Sometimes the timely creation of accurate electrical drawings is a mission-critical function. For example, in motor sport accurate documentation has to be punctually provided to the regulatory bodies in order to avoid penalties. It is, therefore, surprising that many motor sport companies use highly inappropriate software such as PowerPoint to produce interconnect drawings. This is inefficient, unreliable and in marked contrast to the sophisticated 3D CAD systems which these companies use for mechanical design.

Interconnect drawings and documentation could, if held in an easy-to-use form, be an invaluable resource which should be accessible to everyone with a need to use the information. This need often extends beyond other staff within a company to include suppliers, customers and regulatory bodies.

Surveys of reliability in many industries have highlighted electrical system problems as the most common cause of failure. For example, in the automotive industry over 20% per cent of warranty problems are  attributed to an electrical system. More reliability problems result from poor design processes than from weaknesses in electrical components. It seems likely that better CAD tools could help engineers prevent many electrical system problems.

Value engineering exercises often identify large savings that can be made through the use of improved design tools and processes. Typically wiring harnesses are designed without optimisation and frequently with design faults. These shortcomings often require costly rectification work.

Electrical and electronic systems are the most complex aspect of many products. The interconnect design must integrate a large number of components and systems. The interactions between components, wire, environment, ageing effects, and so on, are often impossible to predict and are difficult to formalise in any reliable design methodology. Typically the interconnect design process is started late, takes many months to complete and the resulting design is rarely optimum.

The interconnect design process in defence, rail, aerospace, naval and motor sport companies involves a number of separate engineering departments who approach the work from different and often conflicting viewpoints. For example, the interconnect system can be seen as:

  • a network of electric circuits which need to be designed to ensure electrical performance or;
  • a maze through which wires have to be routed in order to minimise weight and size to ensure cables can be installed or;
  • an assembly of components whose cost has to be optimised by the ingenious matching of parts, materials and adhesives.

As each department has a different viewpoint, they can be tempted to work in isolation. However, especially with frequent design changes, working in isolation results in duplication of effort and expensive delays. But as working in parallel is often impossible, interconnect design can only really start when the electrical and mechanical design are almost complete.

If the interconnect design task is started when specifications are still changing the engineers will, because of mechanical and electrical design interdependencies, find much of their previous work becomes obsolete after every change. For example, moving a device may appear a simple modification to the electrical designer because the connectivity of the wires will remain the same. But changing wire lengths, affects their resistance, voltage drops and temperature rises, which may require a change in wire size, which could require the use of different connectors, cable jackets, boots, adaptors, etc. Thus a simple positional change can have a wide ranging impact on the interconnect design.

The complex interactions between the electrical design, the physical design and the cost implications of the many different design alternatives, demand a design tool that can integrate these factors. But we were surprised to find that only one of the companies we visited in the defence, rail, aerospace, naval and motor sport industries had, or even knew of, any CAD system which could support any significant part of the interconnect design process. The one exception was a company that had developed software to suit their own particular interconnect design requirements over a 20 year period.

Clearly a better approach is required and it should be one that matches the way engineers think about interconnect design and which can be automated with CAD technology.

We would like to think that HarnConnect will provide the better approach that is so obviously required.

HarnConnect

The starting point for many interconnect and harness designs!!!

HarnConnect

HarnConnect

Two partial single line diagrams.

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Just a few examples of the vast amount of drawing and other documentation required to define an interconnect design in sufficient detail to allow manufacture and test.