Designing cable harness assemblies in virtual environments

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  • Designing cable harness assemblies in virtual environments

    F.M. Ng, J.M. Ritchie*, J.E.L. Simmons, R.G. DewarDepartment of Mechanical and Chemical Engineering, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK

    Abstract

    Cable harness assemblies are amongst the most costly items in any electro-mechanical product. The domain is not widely recognised as

    an area for academic research. Internationally, some efforts have been made to automate or semi-automate the choice of cable harness path

    through the use of artificial intelligence (AI) via CAD systems, but with little success. Common themes voiced are that the problem is too

    open-ended and it is very difficult to capture the design intent of the activity. Human input is still very much required to guide the computer

    systems to reach an optimum solution. Case study investigations were carried out at five advanced manufacturing organisations to

    determine the current industrial practice. The investigations revealed that the cable harness design and planning (CHDP) process is

    essentially sequential in nature and consists of lengthy activities carried out late in the overall product development cycle. It was also found

    that there has been little attempt to integrate any of the core activities involved. This paper describes work undertaken at Heriot-Watt

    University to research the effectiveness of immersive virtual reality for designing and routing cable harnesses by enhancing the expertise of

    the cable harness designer rather than by replacing the individual via an automated system. The new virtual cable design system developed

    in the course of this work has now undergone some pilot trials to test its usability. The system will subsequently be used to carry out full

    industrial trials in conjunction with a number of high technology equipment manufacturers. These pilot trials, combined with the case

    studies of current practice carried out at the companies, have highlighted a number of issues regarding cable design, particularly that

    immersive VR has a potentially unique role to play in the integration of cable harness electrical and mechanical design activities.

    # 2000 Elsevier Science B.V. All rights reserved.

    Keywords: Immersive virtual reality; Cable harness design

    1. Introduction

    Cable harnesses are a vital part of all electro-mechanical

    systems from aircraft and automobiles to personal compu-

    ters and domestic appliances. In many instances the cable

    harness is one of the most costly items in the overall

    engineered system. In spite of this the detail design and

    planning of cable harnesses are often only addressed almost

    as afterthoughts at the end of the product design process.

    Cable harness design and planning (CHDP) in fact cover a

    set of manually intensive, time-consuming and costly activi-

    ties. There is the obvious problem of determining satisfac-

    tory routes for bundles of cables in crowded spaces. The

    wires themselves will vary in size depending on their duties.

    The stiffness and mass distribution of the bundle is deter-

    mined by the size and type of cables involved. Acceptable

    bend radii must be defined as well as the position and

    distribution of the fasteners used to constrain the harness.

    One important concern for harness designers is that of

    voltage drop. Voltage drop is directly proportional to cable

    length and inversely proportional to cable cross-sectional

    area. Ideally, the designer must find a routing configuration

    that maintains a suitable voltage drop for all cables in the

    bundled harness. Fig. 1 shows an example of a completed

    cable harness ready for assembly into a final product.

    Current industrial practice, confirmed in case study inves-

    tigations at five leading UK companies, often requires the

    building of a physical prototype of a new design before

    engineers are able to manually determine the correct cable

    lengths and routes, as well as the numbers and positions of

    fasteners. Once a set of suitable cable paths have been

    chosen and the associated components selected, the results

    are entered into a database that allows the production of two-

    dimensional drawings and parts lists together with assembly

    instructions. It is vital that this information is accurate and

    well-proven since the actual manufacture of the harness

    assembly is often carried out by an external specialist

    supplier.

    The routing problem is further complicated by the vulne-

    rability of the cable harness to decisions made upstream. The

    cable harness may have to be reconfigured after only minor

    changes that affect, say, the chassis and the individual

    modules within a prototype product. The routing process

    Journal of Materials Processing Technology 107 (2000) 3743

    * Corresponding author.

    E-mail address: j.m.ritchie@hw.ac.uk (J.M. Ritchie).

    0924-0136/00/$ see front matter # 2000 Elsevier Science B.V. All rights reserved.PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 7 2 5 - 1

  • can even result in the late and expensive re-design of the

    machine chassis to allow the cables to reach their terminal

    points.

    2. Background

    In spite of its industrial importance, cable harness design

    is not widely recognised as an area for academic research.

    Most investigators who have explored the subject have

    attempted to semi-automate or automate the choice of

    harness path through the use of artificial intelligence (AI)

    in conjunction with CAD systems. Such systems are used as

    a review tool for use after the equipment has been designed.

    Park et al. [1] recognised that cable harness design

    requires in depth three-dimensional spatial reasoning. They

    proposed the use of agents to produce different cable

    configurations that satisfy the pin-to-pin connections of a

    typical harness circuit layout and automate routine opera-

    tions such as moving a section of bundles from one position

    to another. Conru and Cutkosky [2,3] report that they have

    incorporated into Park et al.s system a set of algorithms that

    attempt to automate cable routing in a 3D environment. Two

    genetic algorithms were developed to route cable harnesses

    in a 3D environment. Finally, Petrie et al. [4] report the

    development of a harness design system called Next-Link

    that allows different designers to create different harness

    layout concurrently. Next-Link is essentially a manage-

    ment tool that uses a software agent to co-ordinate, update

    and keep track of the work of individual designers, evaluat-

    ing all the routings developed by each designer based on

    satisfying global constraints.

    Much more recently, Cerezuela et al. [5] carried out a case

    study on cable harness design at a helicopter manufacturing

    company. From the case study they found that harness

    design is an iterative process involving schematic, routing

    and component design. It is postulated that harness design is

    a dynamic process and it is not feasible to automate the

    entire activity by computers. Thus, Cerezuela et al. propose

    a conceptual knowledge based decision support system to

    assist in the design of cable harnesses.

    In summary, the review of published academic literature

    in the design and planning of cable harnesses shows that

    much of the limited amount of research in the area has been

    concerned with developing automated or semi-automated

    systems for determining cable routings. The algorithms

    developed tend to be demonstrated in simple geometric

    layouts of components and little evidence is provided that

    the work has been applied in industry.

    3. Industrial case studies

    As part of the present research, case study investigations

    were carried out carried out at five UK advanced electro-

    mechanical technology businesses. These were carried

    through extensive visits, discussions and meetings with

    practitioners and managers. The results were documented

    and returned to the companies involved for their verification.

    Taken together, the five case studies show that the CHDP

    process is essentially sequential in nature and consists of

    lengthy activities carried out late in the overall product

    development cycle. The investigations revealed that there

    has been little attempt to integrate any of the core activities

    involved. It was also found that companies are increasingly

    using CAD based systems to support the design of harnesses.

    There was also no evidence to suggest the use of automated

    or semi-automated harness design tools in use by the

    companies, confirming prior impressions obtained from

    the literature survey.

    The case studies results were used to create a generic

    model shown by Fig. 2 for the CHDP process; this provides

    Fig. 1. Complete cable harness prior to assembly.

    Fig. 2. General stages in the harness design and planning process.

    38 F.M. Ng et al. / Journal of Materials Processing Technology 107 (2000) 3743

  • an outline picture of how manufacturing companies in the

    electro-mechanical sector address the cable harness design

    problem. The model is of course subject to detail change in

    particular cases dependent on the types of product manu-

    factured and the required electrical specifications.

    The contention of the work described in this paper is that

    companies prefer to have cable harness design as an inter-

    active technique under the control of the designer. The

    remaining sections of the paper describe a prototype immer-

    sive virtual reality demonstrator system, developed to assist

    designers in producing feasible virtual prototype harness

    assemblies, and the corresponding pilot trial results.

    4. Cable layout using immersive virtual reality

    The virtual design and planning cable routing system at

    Heriot-Watt University is implemented on a Hewlett-Pack-

    ard workstation with additional VR hardware and software

    from Division Ltd. CAD models of a prototype assembly can

    be imported directly into the system which negates the need

    for any extra component modelling. As illustrated in Fig. 3,

    the user interacts with the system by means of a head

    mounted display (HMD). This provides a stereo image of

    the virtual environment. A three-dimensional mouse (3D) is

    used as an input device.

    The ability to touch and feel objects in the real world is

    one that is taken for granted. However, the development of

    viable systems to provide this haptic feedback in virtual

    environments is still the subject of much research [68]. For

    this reason, the system described here makes use of alter-

    native visual and audio cues to highlight collisions. A full

    polygonal collision detection algorithm is available in the

    software. Thus, when a collision occurs, the system utilises

    messages sent from the algorithm to make images of objects

    in the virtual world turn to wire-frame representations. This,

    along with a simple audio cue, informs the user that some-

    thing is amiss (Fig. 4).

    The virtual cable router has five key design tools in its

    operation namely, point-to-point, continuous path,

    way-point routing, rubber banding and size manage-

    ment. Collision detection is inherent within the first three

    features. Point-to-point and continuous path are creation

    functions, whereas way-point routing allows the creation of

    cable bundle assemblies along existing routes. Rubber

    banding is normally used during editing and size manage-

    ment enables the user to amend the size of the model relative

    to the system user. All the features are activated through a

    virtual toolbox as shown in Fig. 5.

    4.1. Point-to-point

    The point-to-point technique of routing cables provides

    the capability to generate outline cable routes rapidly by

    picking positions or nodes in the virtual environment. The

    user simply probes ports located on cable connectors, or a

    point in space, and a section of cable appears between this

    and the last node created as shown in Fig. 6. Once an existing

    node has been picked in an operation it can be moved around

    in three dimensions, stretching or contracting the associated

    cables as required. This editing facility within point-to-point

    is called rubber banding and is described later. The picking

    Fig. 3. A user interacting in the virtual environment.

    Fig. 4. Wire-frame collision warning of a clash with a cable.

    Fig. 5. A virtual toolbox.

    F.M. Ng et al. / Journal of Materials Processing Technology 107 (2000) 3743 39

  • of another node makes that node active and any subsequent

    point chosen in space will create a section of cable

    between it and the active node. By choosing existing nodes,

    multiple spliced or breakaway cable branches can emanate

    from a single node. Some examples of these are shown in

    Fig. 7.

    4.2. Continuous path

    Continuous path generates a cable route by extruding a

    new section from a user-selected node. Thus, by picking an

    existing node, or a port on a connector, a new node is created

    and attached to the virtual hand until the node is released.

    This method has rubber banding implicit within it also; the

    new section changes in length and position as the virtual

    hand moves. In a fashion similar to point-to-point, multiple

    spliced or breakaway cable branches can be produced.

    The user can observe collisions and immediately take

    action to move the section and so as to avoid a clash. Again,

    this method allows for creating nodes with multiple

    branches.

    4.3. Way-point routing

    Having laid one cable, it is possible quickly to lay bundles

    of cables along the same route by using way-points. This is

    achieved by simply choosing the relevant beginning and end

    nodes along the common length of an existing cable between

    which a new cable is to run.

    4.4. Rubber banding

    Once the entire cable layout is produced some modifica-

    tions may be required. The rubber banding facility allows

    the user to re-position either entire sections of cable or

    bundles that are knitted together simply by holding on to and

    subsequently moving a node. Although this editing facility

    stands alone, it has already been mentioned that it is avail-

    able in the point-to-point and, to some extent, the continuous

    path tools.

    4.5. Size management

    The final VR cable layout tool developed and defined as

    part of this research is size management. This provides the

    user with the ability to enlarge or shrink the virtual prototype

    to enable human-scale ergonomic access to either fine

    geometry details or large-scale geometric features within

    the virtual environment as well as deal with any scale of

    product.

    5. System architecture

    The set of nodes and cable sections created by the user are

    stored in a multi-linked graph structure containing a linked

    list of nodes and a further linked list of joins for each node

    [9] (Fig. 8).

    At the end of the routing session, the system generates a

    text file by traversing the graph structure and extracting

    useful information which details the bills-of-materials and

    process planning information associated with the physical

    cable harness. These outputs include the types of end

    connectors and cable configurations selected as well as

    the positions and liaisons that exist between the virtual

    nodes as shown in Fig. 8. The connector type and liai-

    sons/cable configurations indicated in the text file are spe-

    cified by the user during the immersive routing session. The

    numbers highlighted on the connector type list describe the

    physical configuration of the connector, i.e. actual size,

    number of crimps found on the connector. The liaisons/

    cable configurations, on the other hand, describe the types of

    bundles of wires that are specified for use within certain

    sections of the cable harness layout. A post-processor has

    been developed to convert the data within the text file into a

    two-dimensional layout of the cable harness in AutoCAD

    DXF format. This drawing can be used in the manufacture of

    the physical cable harness.

    Fig. 6. Cables leaving a connector via ports.

    Fig. 7. A cable harness laid out on an assembly in the virtual world.

    40 F.M. Ng et al. / Journal of Materials Processing Technology 107 (2000) 3743

  • 6. Pilot study

    A pilot study was carried out to evaluate:

    1. the usability and robustness of the VR routing tools

    developed;

    2. the effects of learning by comparing repetitions for each

    methods and the key differences between the two cable

    creation methods.

    Six participants took part in the pilot study, aged between

    23 and 30, all were male post-graduate students from the

    Department of Mechanical and Chemical Engineering at

    Heriot-Watt University. None of them had used an immer-

    sive virtual reality system before. A description of the

    experiment and the results now follows.

    6.1. Experimental procedure

    The participants task was to produce a cable route from

    one side of the wall of a virtual component to the other side

    while immersed in the virtual environment as shown in

    Figs. 9 and 10. They were asked to develop a route by

    following the contours of the component. The six partici-

    pants were divided into two groups of three denoted as PTP

    and CP group. The first group used point-to-point and the

    other group used continuous path to develop the cable paths.

    Each participant performed the task for 10 consecutive trials

    and the time to complete the task was measured for each one.

    6.2. Results

    In order to detect improvement in performance for all

    trials, one-way analysis of variance (ANOVA) was carried

    out for both PTP and CP participants. The one-way ANOVA

    tests were applied to the task completion time (TCT) scores

    of both groups. The TCT defines the total time required for

    completing a trial in an experiment. The analysis revealed

    significant differences between the trials within the PTP

    group based on the scores, F9; 20 7:19; p 0:0001,suggesting that there is overall improvement in performance

    between the trials. Statistical differences were not detected

    between the trials for the CP group, F9; 20 0:89; p 0:55, confirming that the CP method of routing cable path ismuch more difficult to learn.

    Subsequently, student t-tests were applied to identify

    improvement in performance from the later trials when

    compared with trial 1 for scores from TCTs both PTP

    and CP groups. The results from the t-tests on TCT data

    are tabulated in Tables 1 and 2. The t-tests revealed sig-

    nificant differences within the PTP group for trials 35, 710

    Fig. 8. An example of the output from the system.

    Fig. 9. Layout of the assembly. Fig. 10. An example of a laid cable in place for the experiment.

    F.M. Ng et al. / Journal of Materials Processing Technology 107 (2000) 3743 41

  • when compared with trial 1, suggesting that participants

    were learning quickly. On the other hand, no significant

    differences were detected between trial 2 and 6 when

    compared with trial 1. The trial 2 finding indicates

    that participants are still learning to use the method.

    From the video recordings, it was found that participants

    tend to explore alternative routes in trial 6, thus statistical

    significance was not detected as participants were spending

    more time planning the routes. Direct observations also

    indicated that the participants were looking for alternative

    designs.

    The t-tests revealed no significant differences between

    trial 1 when compared with successive trials for the CP

    group, suggesting that there were no obvious improvements

    over all subsequent trials. This finding suggests that the CP

    method is much more difficult to master and takes longer to

    learn.

    6.3. Participants feedback

    Participants were asked about the functionality of the

    CHIVE system after performing the trials. In general the

    participants from the CP group found that the CP method for

    cabling is tiring to operate as the constant holding of the

    cable node is necessary when laying the cable sections.

    Participants from the PTP group using the PTP technique

    felt that the method was easy to use. Participants from

    both group felt that the two-dimensional virtual toolbox

    was blocking their routing operations thus hampering their

    performances.

    To summarise, the pilot study found that:

    It is easier to learn PTP than CP. As the number of trials performed increases the TCT

    decreases.

    There is a significant difference between the 10 trials inPTP indicating that the participants have learned the

    method.

    There is no significance difference detected for CP groupsuggesting that participants are still learning the opera-

    tions of the CP method and that more trials are required

    before operators become fully proficient.

    It has been observed that participants in both groups tendto require more guidance in the operation of the VR

    cabling tools and also navigating in the VE during the

    earlier trials.

    Direct observations and feedback from participants alsofound that the CP group was having considerably more

    difficulties in routing than the PTP group.

    All subjects experienced fatigue whilst conducting thevirtual experiments. Fatigue was experienced more

    quickly in the CP group than the PTP group.

    The pilot tests fulfilled their purpose of testing systemsoperation and usability.

    It was possible to develop feasible cable routes using bothPTP and CP methods.

    The pilot tests and the industrial case studies together also

    pointed out a number of issues related to both the industrial

    trials and future system development, namely:

    Future industrial trials will have to be restricted to pre-defined routing paths in order to compare the efficiency of

    different designers and alternative cable harness design

    solutions, e.g. CAD.

    A generic assembly will be used which incorporatesfeatures that can be designed on both the virtual cable

    designer and the companies CAD systems.

    Longer training times are required to allow users tobecome proficient with the virtual cable design tools.

    A new paradigm can now be researched whereby adesigner could design feasible cable routes connections,

    using PTP or CP, for electrical connectivity on the virtual

    table top (in, say, 212D). In parallel with this, these con-

    nections would be automatically mapped using straight

    line connections onto the 3D model in the same virtual

    space. Rubber banding would be used to tailor the routes

    around obstacles. This means that, potentially, cable

    harness electrical and mechanical design can be carried

    out concurrently instead of sequentially as is the case at

    present. Thus the potential for reduction in design lead-

    time is substantial.

    Table 1

    Comparing subsequent PTP task completion times (TCTs) with trial 1: t-

    tests t- and p-values

    Trial No. t p

    2 2.24 0.09

    3 3.54* 0.02

    4 3.8* 0.02

    5 3.63* 0.02

    6 2.46 0.07

    7 3.33* 0.03

    8 3.20* 0.03

    9 3.85* 0.02

    10 4.07* 0.02

    * Significantly different if p < 0:05.

    Table 2

    Comparing subsequent CP task completion times (TCTs) with trial 1: t-

    tests t- and p-values

    Trial No. t p

    2 0.79 0.47

    3 0.53 0.62

    4 0.87 0.44

    5 1.03 0.36

    6 1.18 0.30

    7 0.87 0.43

    8 1.25 0.28

    9 1.01 0.37

    10 1.39 0.24

    42 F.M. Ng et al. / Journal of Materials Processing Technology 107 (2000) 3743

  • 7. Conclusion

    This paper has described a novel software tool to assist

    users to perform cable routing in a virtual environment. The

    system here has been successfully tested in pilot trials. The

    recommendations made by the participants during the pilot

    study were noted and changes had been incorporated into

    virtual cable routing system. Firstly to enable easier user

    selection of the cabling tools, the dimension of the virtual

    toolbox for choosing cable routing methods has been

    enlarged to the size of a billboard within the virtual

    environment and the virtual buttons in the toolbox were

    also spaced widely to facilitate easier user selection. To

    overcome obstruction caused by the virtual toolbox that was

    blocking the routing operations, the user is now transported

    to another position within the virtual environment with only

    the toolbox in view so that they can select the required

    cabling tools for subsequent cable layout routing. Once the

    required tool or tools are selected, the user can return to their

    original position by hitting the return option on the toolbox

    in the original location of the user inside the assembly before

    the toolbox was invoked and continue the cabling operation

    as per normal.

    The pilot study also indicated that the CP method is more

    difficult to learn since statistical analysis were unable to

    detect obvious improvements in performance for all trials.

    More repetitions would have to be carried out in order to

    bring participants to a level where they are confident in using

    the tool for laying cables. Direct observation and feedback

    from participants also indicated that it was tiring to use the

    CP method when performing the routing experiment. Unlike

    the PTP method, the CP cable creation method does not have

    the editing facility rubber-banding available to it. To alter

    the cable layout, users were required to select the rubber-

    banding function by invoking the virtual toolbox. Once the

    modification was completed, users were required to invoke

    the toolbox again so as to select the CP method and carry on

    laying the cable as per normal. Thus from a user-friendly

    interface point of view CP has two extra redundant steps

    when modifications are required to be performed to the cable

    path. The feedback from participants and the results from the

    pilot trials have suggested that training on the general usage

    of the 3D mouse for navigating and interacting with the VE

    might be useful prior to the actual industrial trials. This

    exercise may make it easier for the participants to concen-

    trate on learning the VR cabling tools since the basic

    methods of navigation will have been learnt through the

    training exercise.

    In the future full scale industrial trials will be carried out

    to investigate the viability of this approach to complete the

    cable harness routing task as compared to current commer-

    cial CAD systems. However, this work did show conclu-

    sively that CHDP will be possible in an immersive VR

    environment.

    Combined with the industrial case study investigations, a

    new and novel concurrent electrical and mechanical design

    paradigm has been recognised. The technology and applica-

    tion of immersive VR in this environment provides a new

    solution to a traditionally difficult, costly and tail-end part of

    the overall product design process.

    Acknowledgements

    The authors are grateful to the five companies that

    collaborated in this research, for their support of this

    work and for access to their expertise and knowledge.

    The support of the EPSRC, through access to the equipment

    provided under grant GR/K41823, is also very gratefully

    acknowledged.

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