Development of an Expert System for Cable Harness Design · PDF fileDevelopment of an Expert System for Cable Harness Design ... of the applications life cycle. ... The feasibility

Development of an Expert System for CableHarness DesignP. MehtaDepartment of Engineering,Glasgow Caledonian University,Glasgow, G4 0BA, [email protected]

ABSTRACT

Cable harnesses in systems such as computers, aeroplanes and automobiles are usually the longest

parts of the systems and major contributors to EMC related and other problems. However

currently, the design of cable harness is an after thought in the design of the whole system. Design

of cable harnesses is normally a manual process and a performance study of individual cables is

not undertaken. Any problems which arise are addressed after the whole system design has been

completed by which time the remedy for theses problems becomes more difficult. The ability to

predict performance of cables at the early design stage would lead to a better overall design of the

system. In association with a Teaching Company Scheme, a joint research programme was

launched to investigate the possibility of computerising the process of designing cable harnesses.

As a result of this programme, an expert system for Cable Harness Designer (CHD) has been

developed. This system couples a qualitative evaluation with a quantitative analysis of

performance aspects of cable harnesses. The qualitative analysis is carried out by the Design Rule

Checker within the CHD whereas the performance evaluation is carried out by the Performance

Predictor. As part of this research programme, various computer models have been developed to

predict the effects due to the inductive and capacitive coupling in parallel cables under different

circumstances. The validity of the models has been demonstrated by comparing the computed

values with experimental results. The successful implementation of the cable harness design

system invites wider application in industry.

Transactions on Information and Communications Technologies vol 16, © 1996 WIT Press, www.witpress.com, ISSN 1743-3517

INTRODUCTION

Electronic subsystems are often connected by several wires. For economy of

space, neatness, and ease of identification, the wires are grouped and tied together.

Each group of wires is called a cable harness. A cable harness consists of an

assembly of wires of various types and colours, bundled together in a distinct

shape terminated by one or more connectors and ready for insertion into a product,

such as a computer frame. Industries, such as computer, electronic, aerospace,

automobile, and automatic machines, spend considerable resources on cable

harnesses [ 1 ].

Due to the variation and required sophistication in harness configurations and

patterns in different applications, the majority of cable harness designs and

assemblies are carried out manually, which makes the harnesses expensive, and

prone to defects [ 2 ].

For more than a decade, the automation of cable harness design and manufacture

has been under investigation by a variety of companies and research agencies in

the aerospace and automobile industry [ 3, 4, 5 ]. In 1982, Henderson suggested

several approaches to factory automation of electrical cable harness manufacture [

4 ], one of these being the need to establish interactive graphics design. However

because the manual production of cable harnesses is a time consuming and high

cost process, cable harness manufacturers have been looking at the use of

industrial robots to automate the process and most of this effort has been

concentrated on the manufacture of cable harnesses. The design problem appears

to be more intractable.

From the earliest times when EMC was recognised as a subject in its own right,

engineers have been aware of the problems created by coupling between wires.

Wires which carry excessive electrical noise whether real or spurious will

interfere with wires which are sensitive to such noise, causing malfunctions in

some of the subsystems.


There are a large number of parameters involved in the coupling mechanism. Due

to the complexity of the coupling mechanism, many coupling models have been

developed using a limited number of parameters [ 6, 7, 8, ]. A number of studies

have reported on the problems of coupling between coaxial cables and coupling

within multiconductor cables [ 9, 10, 11 ].

CRYSTAL - Expert System Shell

CRYSTAL was chosen to be the vehicle for the development of Cable Harness

Designer Expert System. CRYSTAL is a commercially available expert system

shell that can be used to build expert system applications. This is a rule-based

shell and has built-in features which facilitates development of knowledge bases.

CRYSTAL has been designed for the IBM PC and compatibles and combines

cheapness with the ability to build highly sophisticated systems.

Rule-based programming circumvents many of the difficulties experienced with

conventional programming environments, such as, if an application is difficult to

specify or if the specification changes frequently. This is because the rules are

inherently self-explanatory and it is the natural way people express themselves

when describing a solution to a problem. CRYSTAL combines rule editing and

processing in a natural language with all the conventional features such as screen

painting, graphics, mathematical functions and database handling.

Maintaining an application after it has been developed can be the most costly part

of the applications life cycle. Since applications in CRYSTAL are implemented

as structured rules in a top-down hierarchical manner they are inherently self-

explanatory. CRYSTAL also provides facilities such as rule tracing, rule listing

and graphical representation of the rule structure, which allows easy maintenance

of the system.

A great strength of CRYSTAL is its ability to deliver a finished product.

CRYSTAL is designed so that a prototype can be extended into a finished


application and a run-time version allows the final rules to be locked into a low-

cost interpreter that requires less memory to execute.

Having considered the above points, together with resources available, and the

possible cost (hardware and software) to the potential customers (medium sized

electronic companies), the expert system builder CRYSTAL was chosen to

develop the system for cable harness design. The forward chaining inference

method has been used in order to analyse the design and possible coupling path

and subsequently identify the interference levels and bending levels of a particular

harness from the given parameters.

Any design system must address both the electrical and mechanical requirements

of the cable harness. One of the prime electrical requirements is to limit

unwanted signals due to interference. Crosstalk within and between cables and

harnesses is a major problem which degrades the performance of cable

harnesses.Attention should also be paid to the mechanical aspects such as,

optimisation of cable harness layout, solution to physical bend of cables, and

computer-aided process planning to enable the use of CAM in cable harness

manufacture.

Cable Harness Design System - CHD

To address the problems discussed, a cable harness design system, Cable Harness

Designer, CHD, has been developed in CRYSTAL. This system has two main

components: the Design Rule Checker and the Performance Predictor. There is a

separate knowledge base for each of these, so that they can be maintained

independently. The Design Rule Checker provides qualitative rule checking for

cables, connectors, harness layout and accessories. whereas the Performance

Predictor provides quantitative analysis of the performance of harnesses. CHD

contains approximately one thousand and nine hundred rules. Of these, one

hundred and forty-eight rules are concerned with providing guidelines and rules

for cable harness design, the rest are related to assessing electrical and mechanical


performance of a harness, such as coupling calculations and physical bend level

checking.

In this rule-based system, the rules contained in the first level and referred to as

master rules are:

1. Welcome to the system

2. Introduction to CHD

3. Initialise variables

4. Main menu

5. Design Rule Checker

6. Performance Predictor

7. Restart system

The features of the master rules are:

1. The system starts with a logo to welcome users working with the system.

2. The structure of CHD and the main functions of each component are

explained.

3. The system initialises all the variables used before the start of a new

design.

4. A main control menu is provided for the user, which hands over the

system to the user. The user will then have the full control of the system

for designing a cable harness.

5. If the user chooses to check the design rules and requirements for the

harness, the system will invoke the Design Rule Checker.

6. After checking the design rules or if the user chooses to start the

Performance Predictor in the first instance (the user is free to do so), then

the prediction of crosstalk and physical bend levels of a harness will be

accessed.

7. Once evaluation of the design has been completed, the system allows the

user to choose to either quit the system or start a new design.


In each of the master rules, there are further rules at lower levels. The rules are

structured like a tree.

Figure 1 illustrates the system's organisational flowchart of the design process.

The knowledge base is the heart of an expert system, and CHD consists of two

knowledge bases; the Design Rule Checker and the Performance Predictor.

Start

Design Rule Checker Performance Predictor

evaluate design

report results

new design?

End

no

yes

Figure 1 System Flowchart of CHD

The Design Rule Checker

The expertise of cable harness design resides with specialist researchers and

practitioners in this field and it is natural to consider how this expertise can be


captured and made available for wider use. Some of the features of the current

practice in cable harness design are:

• Design is largely a trial and error process based on designers' initiatives;

• The knowledge includes heuristics and rules of thumbs;

• Design is a complex process involving a number of possibilities at most of

the decision points;

• The designer has the insight of the actual design process;

• The knowledge available to the designer is in the form of guidance only.

Design Rule Checker is a qualitative knowledge base, which provides the basic

guidelines for the design of cable harnesses. It includes a set of design rules to

effectively address various problems occurring in the harness design process.

The basic design requirements and guidelines included in design rules are partly

based on those currently in use in industry and partly extracted from handbooks

on electronic system design [ 12 ]. These rules and guidelines have all followed

IEE Wiring Regulations [16th Edition].

Design Rule Checker has incorporated the following requirements and rules for

the harness design:

1. Design rules and guidelines for cables.

There are basic requirements for a.c./d.c. power distribution lines, data

cables, as well as EMI related issues. Furthermore, special design rules

have also been included for different types of cables, viz.: single wires,

twisted pairs, coaxial cables, shielded cables, multiconductor cables and

ribbon cables.

2. Design rules and guidelines for connectors.


Design guidelines for connectors consist of electrical, mechanical,

environmental requirements and material considerations. Special rules for

a range of connectors are incorporated. Some suggestions on selecting pin

locations have also been incorporated.

3. Design guidelines for harness layout and others.

There are some basic requirements for harness layout. As an important

issue, the minimum and the preferred bend radii of a harness are analysed

and calculated.

Conclusions

The feasibility of using Artificial Intelligence techniques for design of cable

harnesses has been clearly demonstrated. This work presents a new development

for the previously neglected area of cable harness design.

Further work

The system is being continually refined and extended to allow more effective

design of cable harnesses.

References1. Ryan, J.(1980) Electronic Assembly, Reston Publishing Co., Reston2. Schraft, R.D.; Schlaich, G.(1988) A Survey of the assembly of wire harness inindustry, Assem-Auto(UK) vol 8, no 1, pp29-323. Goldstein, B.; Payne, W.(1980) Semi-automation techniques improve wireharness board assembly, Insul-Cicuits,USA,vol 26, no 9, pp 33-354. Henderson, J.A.(1985) Westinghouse Technology modernisation for electronicassembly, Flexible Manufacturing Systems '85 Conference, USA5. Sugimura, N. et al (1989) Wire harness CAD system for automobile, Fujitsu,Japan, vol 39, no. 5, pp 362-8.6. Mohr, R.J.(1967) Coupling between open and shielded wire lines over aground plane, IEEE Trans. on EMC, vol. 9, no. 3


7. Paul C. R.(1992) The concept of predominant effect in EMC, IEEE Trans. onEMC, vol. 34, no. 38. Badr, A.H. et al(1981) Interference between braided coaxial cables, IEEE proc.vol 128, pt A, no 5, pp 347-539. Vance, E.F.(1978) Coupling to shielded cables, John Wiley & Sons Inc.10. White, D.R.J., et al(1988) Electromagnetic interference test methodology andprocedures, vol 6, Interference Control Technologies Inc., Gainesville, Virginia11. Moser, J. R. et al(1968) Predicting the magnetic fields from a twisted-paircableIEEE Trans. on EMC, vol. 10, no. 312. Harper, C. A., Ed(1972) Hanbook of wiring, cabling and interconnecting forelectronics, McGraw Hill


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Development of an Expert System for Cable Harness Design · PDF fileDevelopment of an Expert System for Cable Harness Design ... of the applications life cycle. ... The feasibility