designing cable harness assemblies in virtual environments

7
Designing cable harness assemblies in virtual environments F.M. Ng, J.M. Ritchie * , J.E.L. Simmons, R.G. Dewar Department 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) 37–43 * Corresponding author. E-mail address: [email protected] (J.M. Ritchie). 0924-0136/00/$ – see front matter # 2000 Elsevier Science B.V. All rights reserved. PII:S0924-0136(00)00725-1

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Page 1: Designing cable harness assemblies in virtual environments

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 arti®cial intelligence (AI) via CAD systems, but with little success. Common themes voiced are that the problem is too

open-ended and it is very dif®cult 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 ®ve 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 de®ned 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 ®nd a routing con®guration

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 ®nal product.

Current industrial practice, con®rmed in case study inves-

tigations at ®ve 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 recon®gured 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) 37±43

* Corresponding author.

E-mail address: [email protected] (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

Page 2: Designing cable harness assemblies in virtual environments

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 arti®cial 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

con®gurations 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 ®ve 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 veri®cation.

Taken together, the ®ve 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, con®rming 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) 37±43

Page 3: Designing cable harness assemblies in virtual environments

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 speci®cations.

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 [6±8]. 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 ®ve key design tools in its

operation Ð namely, `point-to-point', `continuous path',

`way-point routing', `rubber banding' and `size manage-

ment'. Collision detection is inherent within the ®rst 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) 37±43 39

Page 4: Designing cable harness assemblies in virtual environments

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 modi®ca-

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 ®nal VR cable layout tool developed and de®ned 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 ®ne

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 ®le 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 con®gurations 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 con®gurations indicated in the text ®le are spe-

ci®ed by the user during the immersive routing session. The

numbers highlighted on the connector type list describe the

physical con®guration of the connector, i.e. actual size,

number of crimps found on the connector. The liaisons/

cable con®gurations, on the other hand, describe the types of

bundles of wires that are speci®ed for use within certain

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

been developed to convert the data within the text ®le 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) 37±43

Page 5: Designing cable harness assemblies in virtual environments

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 ®rst 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 de®nes the total time required for

completing a trial in an experiment. The analysis revealed

signi®cant differences between the trials within the PTP

group based on the scores, F�9; 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, F�9; 20� � 0:89; p �0:55, con®rming that the CP method of routing cable path is

much more dif®cult 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-

ni®cant differences within the PTP group for trials 3±5, 7±10

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) 37±43 41

Page 6: Designing cable harness assemblies in virtual environments

when compared with trial 1, suggesting that participants

were learning quickly. On the other hand, no signi®cant

differences were detected between trial 2 and 6 when

compared with trial 1. The trial 2 ®nding 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

signi®cance 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 signi®cant 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 ®nding suggests that the CP

method is much more dif®cult 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 in

PTP indicating that the participants have learned the

method.

� There is no significance difference detected for CP group

suggesting 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 tend

to 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 also

found that the CP group was having considerably more

difficulties in routing than the PTP group.

� All subjects experienced fatigue whilst conducting the

virtual experiments. Fatigue was experienced more

quickly in the CP group than the PTP group.

� The pilot tests fulfilled their purpose of testing system's

operation and usability.

� It was possible to develop feasible cable routes using both

PTP 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 incorporates

features that can be designed on both the virtual cable

designer and the companies' CAD systems.

� Longer training times are required to allow users to

become proficient with the virtual cable design tools.

� A new paradigm can now be researched whereby a

designer 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

* Signi®cantly 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) 37±43

Page 7: Designing cable harness assemblies in virtual environments

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

dif®cult 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 con®dent 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

modi®cation 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 modi®cations 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 dif®cult, costly and tail-end part of

the overall product design process.

Acknowledgements

The authors are grateful to the ®ve 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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