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WIRING DESIGN

Wiring harness designcan a computer help?

by Roger Billsdon and Ken Wallington

Raychem and ADE have developed a computer-aided wiring harness design system to help engineers select the most suitable parts and undertake many of the necessary analysis operations. This article

describes some key features of the wiring harness design process and a little of the background on how the system was developed.

Features of the harness designprocess

Designing wiring harnesses, especially thosefor demanding applications such as defenceand aerospace, is a surprisingly complexbusiness. There are many different parts to

select while taking into account various environmentaland mating part conditions. There can, therefore, be alarge number of potential design solutions to analyse.

The constraints imposed on harness design are alsobecoming more demanding. For example:

5 Weight and space saving considerations are especiallyimportant in missile and aerospace applications. Withthe additional electrical equipment now being added toproducts such as cars and public transport, the samefactors are becoming an issue in these industries.

5 Electromagnetic interference (EMI). Harnesses caneither radiate interference to, or they can pick upinterference from, equipment that is close to any pointalong the route of a harness. With the increasing use ofsensitive electronic equipment in cars, aircraft andmilitary equipment this is becoming a major designconsideration.

5 Resistance to environmental hazards including hightemperature or fire, chemical and nuclear agents. Alsoin products such as military vehicles, differentcharacteristics may be specified for each area. Forexample, zero halogen materials in the crew area andmaterials able to withstand high temperature andresistant to fluids in the engine area.

5 Corrosion has always been a problem in electricalsystems. Many factors may need to be considered,including humidity, corrosive fluids and temperaturecycling which can lead to condensation. The additional

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costs of totally sealed wiring harness systems arebecoming easier to justify as products and the lives ofthose who use them become more dependent on thefault-free operation of electrical systems.

5 Repair and maintainability. Some electrical andelectronic systems in military vehicles, naval vesselsand other products now need to be upgraded ormodified several times during the life of the mainmechanical platform. There are parts and harnessdesign techniques available to make this work easier toaccomplish on harnesses installed in a very integralmanner in the equipment.

The harness designerHarness design is not generally considered to be a

glamorous role or one which confers a particularly highstatus. Nevertheless a particular blend of electrical andmechanical design skills is required and the design tasksdemand careful, even grinding attention to detail.Unfortunately the detail is often skimped with onlyvague attempts at optimising designs to the particularrequirements of an application. Furthermore harnessdesign is often left until a very late stage and the resultingtime pressures can increase the likelihood of expensivemistakes being made. For example:

5 Electrical interference disrupting the gun stabilisationsystem in a main battle tank is a problem withassociated costs that far outweigh those of the wiringharness.

5 In civil aviation there have been some recent accidentsattributed to such wiring faults as wrong connectionsor inadequate protection for the operating environ-ment.

5 Wiring harnesses are assembled into many products

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such as cars and military vehicles, at an early stage,and testing is not possible until the final stages ofassembly. It is then extremely difficult to rectify anyproblems in the wiring harness and again the costs ofmistakes can far outweigh those of the wiring harness.

Because of these risks and the somewhat thanklessnature of the work, harness designers often tend to besomewhat cautious. They are certainly not susceptible toglib sales patter from computer salesmen and they, quiterightly, demand extensive proof of the capabilities andbenefits of using computer design aids before trying suchtechniques. Moreover, due to the time pressures underwhich they work, harness designers do not have thepatience or forgiveness that many CAD systems seem toexpect of their users.

Computer solutionsGiven the type of analysis work and volume of detail

involved, wiring harness design would seem a naturalcandidate for computerisation. But there are no standardsoftware packages available, or at least none thatprovides more than a limited amount of functionality to help the wiring harness designer. This is because,compared with the market for general-purpose 2Ddraughting, wiring harness design is a highly specialisedand small segment of the computer-aided-design (CAD)market. Therefore, we decided to develop our own wiringharness design software for use with a general-purposeCAD system for the drawing work.

The first attempt we made was over 12 years ago andused a CAD system which ran on DEC minicomputersand engineering workstations; Unfortunately the equip-ment was so expensive as to preclude its use by all but afew dedicated operators. Also in the 1980s only limitedmeans were available for interfacing application softwarewith CAD systems.

We began our second attempt three years ago using

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low-cost PCs running Microsoft Windows. To provide thenecessary drawing functions we selected Visio as it wasthe only CAD system able to fulfil all our criteria, namely:

5 Affordable: The standard Visio system costs less than£100.

5 Easy to use: Unlike many CAD systems, it wasdesigned from the outset to run under Windows andthe user interface is familiar to anyone who has usedany of the Microsoft Office applications. Ease of use was a major consideration as we wanted to makethe system suitable even for engineers who onlyoccasionally need to do some design or to work withdesigns from their colleagues.

5 Schematic drawing capabilities including connectivityand attribute handling facilities: The drag and dropVisio drawing system is ideally suited to the schematicstyle of harness drawings. Connectivity and attributemanagement facilities are also very well implemented.

5 Programmability: Object linking and embedding(OLE) automation facilities allow other programs toaccess the Visio drawing objects, their properties andmethods. Visio was one of the first CAD systems toadopt this technology. Previously CAD systems had to be programmed using special purpose macrolanguages, files containing command sequences or byusing calls to subroutine libraries. OLE automationprovides a much more flexible and powerful method ofintegrating custom software which can be written inany of the general-purpose programming languagesthat support OLE automation.

5 Drawing, text and spreadsheet functionality: AlthoughVisio is primarily a drawing system, it includes manyof the basic functions of a word processor and spread-sheet. This is important because our harness designdocuments include an assembly drawing, a wiringschematic, parts list, wire list, labour estimate spreadsheet and design notes. Also most CAD systems

generate files containing singledrawings which we believe is toorestrictive for harness designdocuments. The multi-page docu-ments that can be held in a singleVisio file mean that all the detailsassociated with a harness design areeasier to manage and to E-mail.

Harness design systemHarnWare is the name chosen for

the Raychem harness design system(Fig. 1). The software is written inMicrosoft Visual Basic andcurrently comprises about 50 000lines of code. The design databaseincludes 60 000 Raychem wiringharness products plus manymaterial, finish and other permu-

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aticctions

ttesate

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Fig. 1 HarnWare harness design system

HarnWare

Visio

HarnWarehelp

HarnWardatabas

drawingwire list

wiring schemcable cross-se

parts lisdrawing no

labour estimmarker pa

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Fig. 2 Some pages from a sample HarnWare document set

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tations. Also included isonline help for guidanceon using the system andon wiring harnessdesign procedures andproducts.

The starting point forthe system is a simpleharness assembly draw-ing. This is created usingVisio to drag and dropshapes into the drawing.The shapes automat-ically snap and gluetogether, so that it doesnot take very long toproduce a high-qualitydrawing. Fig. 2 showssome pages from a sample HarnWare document set.

Factors such as minimum and maximum operatingtemperature and any required Military or DefenceStandards are entered. This, along with such informationas the need for fire resistance or the need to withstandnuclear agents, is used to narrow down the choice ofharness material systems.

A design wizard (Fig. 3) is used to analyse the drawingand to generate a suggested design sequence. When themouse is moved over the parts listed by the wizard,HarnWare outputs such details as dimensions, materialsand connections between parts. By clicking a part in thelist the appropriate shape in the drawing is highlightedand the form used for selecting that particular class of

Fig. 3 Harness

design wizard

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part is loaded. Forexample, the form usedto select wire containsoptions for various typesof wire suited to differ-ent temperatures andenvironments. Guidanceis also available forchoosing the wire gaugemost suited to givencurrent loading, ambienttemperature and otherconditions (Fig. 4).

Wire lengths arecalculated automaticallyby tracing routesthrough the harness andapplying allowances for

certain parts through which the wires pass. For example,if a wire is routed through a right-angled adaptor a smallamount is added to the length to allow for the wirebending around the corner.

The key steps in selecting the optimum part to use in aparticular harness design situation include (Fig. 5):

5 On clicking the part in the drawing or design wizard,HarnWare obtains design data from the shape andthose with which it is mated in the assembly drawing.For example, in the case of a Raychem boot, the styleof part is determined by the drawing shape and therequired diameters are obtained from the matingharness leg and adaptor.

Fig. 4 Guidance on choosing wire type and wire gauge

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Fig. 5 Route to selection of optimum part

5 The database is then searchedfor parts which are suited to allthese dimensional, materialand other constraints. In manycases several parts that couldsatisfy the constraints arefound and are listed in order ofpreference.

5 Information on each part isoutput and the designer is freeto choose any of the partslisted. Therefore, when forexample space is limited thedesigner may choose a smallerpart than the one that wouldnormally be preferred.

Five more examples of thevarious analysis options are:

(i) The design checker analysesthe contents and structure ofa design against a set ofrules. Where potentialproblems or deviations from‘best practice’ are found, thedrawing is flagged with warning messages. Thesemessages are each linked to the online help systemwhich contains additional explanations and detailsof the problem.

(ii) The labour estimator analyses the parts contained in the design and certain aspects of the structure ofthe harness. The results are automatically insertedinto a spreadsheet containing standard assemblytime synthetics. While estimating harness assemblytimes can never be an exact science, the estimatesproduced are sufficiently accurate for such purposesas comparing the cost effectiveness of alternativedesign solutions.

(iv) Wiring schematics can be generated automaticallyfrom the information included in the wire list. Theseschematic diagrams show the pin-to-pin wiring forall the connectors and wires in a harness design.

(v) Alternative lays of cables containing mixed diameterwires can be automatically analysed to identify thesmallest diameter and most even construction.

Benefits achievedHarnWare was originally intended for in-house use by

Raychem engineers. But even during the first year whenthe system was at an early stage of development, anumber of customers expressed an interest in using thesystem. It is now used by many companies in countriesaround the world including Europe, the USA, Japan,Israel, South Africa and Australia. We have recentlyadded a mechanism for translating harness designphrases included in HarnWare documents into other

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languages, including those such as Japanese, Korean andTaiwanese that use extended character sets.

One of the key features of the system is that it is used interactively. The choices and calculations canalways be modified by the engineer in order to suitspecific requirements. It is also easy to modify the design constraints and to analyse alternative design solutions.

Users of the system have reported benefits in four keyareas:

(i) Selection of preferred parts to ensure best deliveryand price.

(ii) More detailed and accurate designs and betterquality documentation.

(iii) Faster designs and quotations, in some cases up to 20times faster.

(iv) Increased confidence that each part is compatiblewith the intended service conditions and with themating parts.

We believe we have proved, even to the most scepticalharness designer, that computers can help to producebetter designs more quickly.

© IEE: 1998

Roger Billsdon is Director, ADE Analysis & Design EngineeringLtd., 47 Rookery Place, Fenstanton, Cambridge PE18 9LZ, UK,Tel: 01480 466209, E-mail: roger_billsdon@compuserve.com.Ken Wallington is Harness Design Systems Manager, Raychem,Cheney Manor Industrial Estate, Swindon, SN2 2QS, UK, Tel:01793 573912, E-mail: kwalling@raychem.com

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