An expert system for building technological pathways of initial processing of furniture elements

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Akademia Rolnicza w Poznaniu

Summary

The objective of this paper was to elaborate a concept of an expert system al-lowing the development of technological routes at the department of initial process-ing of solid wood element in a furniture factory. The concept was developed by building example rules of inference written down in a pseudo-language which repre-sented individual areas of operation of the expert system. In the designed solution the entire set of input data derives from the MRP system. The results of the operation of the expert system are to be written down in the form of a tabular report which could be collected back by the integrated system.

Keywords: expert system, technological pathway, furniture, MRP 1. Introduction

In recent years we have observed a strong inclination in furniture factories to realise produc-tion in a discrete form. It has become an increasing common practice that, in the course of negoti-ating contracts, manufacturers offer their customers the choice of such traits of the finished prod-ucts as the surface colour, type of the applied fittings and fillings of front frame sub-assemblies, some additional functional elements and, last but not least, possibilities of modification of furni-ture dimensions. This leads not only to increased diversity of the manufacfurni-tured elements and finished products but also to higher proportion of non-standard furniture elements and semi-finished articles.

Another recent phenomenon is the dispersion of plants belonging to the same enterprise which carry out similar, often complementary, production programs or linked by mutual cooperation ties. This carries with it a dispersal of specialist technological expertise and difficulties in efficient exchange of information.

The above highlighted problems indicate that there is a need for the elaboration of an expert informatics system which, on the one hand, would streamline the process of preparation of techno-logical pathways in furniture factories and, on the other, make it easier to better organise the dis-persed technological know-how. So far there is no literature dealing with this type of solutions dedicated to furniture, although there are a number of studies for this industrial sector focused on the management of bill of materials in the MRP/ERP class systems [1,3,4].

2. Aim of the study

The objective of these investigations was to elaborate a concept of an expert system allowing the development of technological routes at the department of initial processing of solid wood ele-ment in a furniture factory.

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3. Research methodology

The outline of the considered technological processes is presented in Figure 1. It was assumed that they would comprise preparation of planed all round blanks and double-planed curvilinear strips planed to a definite thickness.

Unedged timber

Cross cutting

Longitudinal cutting

All round planing Basic planing Marking off Cross cutting Fretting Basic planing Thicknessing

All round planed blanks

Blanks planed on two sides

Fig. 1. Schematic diagram of the considered technological processes

It was further assumed that the elaborated expert system should operate fully automatically and the user would only need to interfere with it periodically to update inference rules and testing its operation following the introduction of updates. Therefore, it was decided that the entire set of input data should derive from the integrated MRP class system of an enterprise. This is possible only in the case when the semi-finished article index is associated with essential technological-construction information. Bearing in mind the need for the modification elasticity of the proposed system as well as the transparency of the database and the desire for the minimalisation of the number of material items associated with it [2], the author decided to employ indices referring only to the general shape of the semi-finished articles and to describe specific construction and technological parameters by numerical and dictionary-based characteristics associated with the index. Hence, for the needs of this study, the index structure which corresponds to the following key was adopted:

AB-CC, where:

• A – index type: A = 1 – index of the final product, A = 2 – index of the assembly, A = 3 – in-dex of the sub-assembly, A = 4 – inin-dex of the element, A = 5 – inin-dex of the semi-finished product; A = 6 index of the raw material;

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• B – type of semi-finished product: B – 1 - semi-finished product of solid wood, B = 2 – board semi-finished product;

• CC – in the case of the semi-finished product of solid wood, it determines the form of the semi-finished product, C = 01 - planed all round blanks, C = 02 double-planed curvilinear blanks.

The following set of numerical traits will be associated with the index: • THICKNESS – thickness of the semi-finished product,

• WIDTH – width of the semi-finished product, • LENGTH - length of the semi-finished product, as well as dictionary-based characters:

• PROFILE – profile of the rectilinear blank planed all round with the assistance of profile heads,

• SHAPE – shape of the double-planed curvilinear blanks,

• MATERIAL – type of material - solid wood, it was assumed that this trait can adopt the fol-lowing values: beech, oak, birch and alder.

Table 1. Operation catalogue

Index Name 111 Marking off 121 Transverse cutting 122 Longitudinal cutting 123 Fretting 131 Basic planning

132 All round planning

133 Thicknessing

Source: own elaboration

Table 2. Workstation catalogue

Index Name

1111 Marking off station

1211 Cross-cut saw

1212 Numerically controlled cross-cut saw

1221 Gang saw

1231 Band saw

1311 Surfacer

1321 All round planer

1331 Thicknesser

The results of the operation of the expert system will be written down in the form of a tabular report which could be collected by the MRP class system. It was assumed that the rows of the generated report would represent consecutive technological operations, while individual columns – would contain:

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• Column 1 – number of the operations – in compliance with the principles adopted by many MRP class systems, operations will be designated using consecutive numbers which constitute multiple of 10,

• Column 2 – operation index,

• Column 3 – index of the workstation,

• Column 4 – technological parameters of the operation, • Column 5 – time standard.

In order to elaborate the proposed concept, the author adopted catalogues of operations and work stations shown in Tables 1 and 2.

The concept of an expert system elaborated on the basis of the above described assumptions will be developed by building example rules of inference written down in a pseudo-language which will represent individual areas of operation of the expert system.

4. Description of the proposed expert system

The proposed expert system will be activated separately for each semi-finished product, whenever it will be necessary to introduce or update its technological route. The system activation will be preceded by the generation of a set of initiating facts. The basic initiating fact will include information about the index of the semi-finished article in the form of:

(INDEX <index>).

The consecutive initiating facts will reflect index traits according to the following pattern: (<name of trait> <value of trait>).

Table 3 gives inference rules determining properties of the semi-finished product and mate-rial. The index serves to establish the form of the semi-finished article: a rectilinear blank/strip or a curvilinear blank/bolt. The analysis of the type of timber material consists in its allocation to soft or hard species which, later on, affects processing technological parameters.

Table 3. Inference rules for the determination of properties of the semi-finished product and material

No. IF: THEN:

1 (INDEX 51-01) assert(blank-type rectilinear)

2 (INDEX 51-02) assert(blank-type curvilinear)

3 (MATERIAL BIRCH) assert(wood-hardness soft)

4 (MATERIAL ALDER ) assert(wood-hardness soft)

5 (MATERIAL OAK) assert(wood-hardness hard)

6 (MATERIAL BEECH) assert(wood-hardness hard)

The consecutively presented rules (Table 4) make it possible to select technological allow-ances in each course of planning operations. The dimensions of selected allowallow-ances depend on the final dimensions of the blank as well as on whether soft or hard timber species is used to manufac-ture it. The rules presented in Table 4 refer to the allowances used for widths dimensions and were elaborated on the basis of values proposed in the BN-67/7103-02 standard on the assumption that the length of semi-finished products does not exceed 1500 mm and their breadth – 500 mm. Rules determining the remaining dimension allowances can be developed in the same way.

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Table 4. Inference rules for the specification of technological material allowances

No. IF: THEN:

7 (wood-hardness soft) AND (LENGTH 500) AND (WIDTH 100)

assert(width-allowance 2.0)

8 (wood-hardness soft) AND (LENGTH 500) AND (WIDTH > 100)

9 (wood-hardness soft) AND (LENGTH > 500) AND (LENGTH 1000)

10 (wood-hardness soft) AND (LENGTH > 1000) AND (WIDTH 100)

11 (wood-hardness soft) AND (LENGTH > 1000) AND (WIDTH > 100)

12 (wood-hardness hard) AND (LENGTH 500) AND (WIDTH 100)

13 (wood-hardness hard) AND (LENGTH 500) AND (WIDTH > 100)

14 (wood-hardness hard) AND (LENGTH > 500) AND (LENGTH 1000)

assert(width allowance 3.0)

15 (wood-hardness hard) AND (LENGTH > 1000) AND (WIDTH 100)

16 (wood-hardness hard) AND (LENGTH > 1000) AND (WIDTH > 100)

Table 5. Rules for the specification of processing parameters

No. IF: THEN:

17 (blank-type rectilinear) AND (LENGTH > 900)

assert(additional-base-planning) 18 (blank-type rectilinear) AND

(wood-hardness soft)

assert(all-round-planning-feed 18) 19 (blank-type rectilinear) AND

(wood-hardness hard)

assert(all-round-planning-feed 14) 20 (blank-type rectilinear) AND

(WIDTH s) AND (width-allowance n)

assert(rip-saw-distance s+2n)

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Consecutive operational stages of the expert system aim at determining the course parameters of the technological process as well as the parameters of individual operations. Rule No. 17 (Table 5), in the case of rectilinear blanks aim at introducing into the technological process an additional operation of basic planning which precedes the all round planning if the length of the semi-finished product exceeds 900 mm. Rules No. 18 and 19 determine the feed rate for the all round planning on the basis of the classification of the processed material carried out earlier. Rule No. 20 determines the appropriate distance of saws for the longitudinal sawing on the basis of the width of the semi-finished product and the selected technological allowances.

Table 6. Inference rules which for estimating time rates for all round planning

No. IF: THEN:

21 (all-round-planning-feed w) AND (LENGTH l) assert(thru-feed-time l/w) 22 (planning-thru-feed-time t1) AND (planning-operator-time t2) assert(planning-time max(t1,t2))

One of the most difficult tasks facing the proposed expert system is the determination of time limits for individual operations because there is a certain amount of operations, at each of the workstations anticipated in the technological process, which must be done manually. Therefore, the implementation of the proposed expert system should be preceded by a certain number of ob-servations and inspections which would make it possible to quantify these types of operation and then to generalize the obtained results for various operations and blank dimensional classes. On the other hand, times of operations carried out mechanically can be determined on the basis of the set feed rates. Table 6 presents some examples of inference rules which allow estimating time rates for all round planning. It was assumed that the constant initiating facts comprise the following information:

(planning-operator-time t),

where t is the mean time necessary to transfer the required element by the operator to the machine. This time is measured from the moment the given passed element is touched by the operator to the moment of touching the next element and is established on the basis of a day’s inspection. On the other hand, rule No. 21 (Table 6) determined the time the mechanical feed moves a given element by the distance equal to its length which will allow the passage of the next element. The unit op-eration time will correspond to the longer of the determined times which finds its reflection in the rule No. 22.

It will certainly be much more difficult to determine time standards for the operations in which the feeding of material is conducted by hand. One of the examples of such operations is sawing on the belt saw. The additional complexity of this operation results from the fact that there are several tasks in one procedure as well as from the fact that the path of the movement of the processed material is usually curvilinear. Therefore, the elaboration of inference rules allowing the determination of reliable timing standards for these types of operations requires conducting statis-tical analyses of the processing unit times of various, example elements. Such analyses should aim at the establishment of correlations between the measured unit time of processing and such con-struction parameters of the semi-finished product as overall dimensions or the circumference

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length of the blank which would allow employing these parameters as indicators for the determina-tion of appropriate time standards.

The final stage of the operation of the expert system will be to generate the technological pathway in the form of a tabular report adopted earlier. Some examples of inference rules respon-sible for this action, in the case of rectilinear blanks are presented in Table 7.

Table 7. Rules for technological pathway building

No. IF: THEN:

23 (blank-type rectilinear) AND (LENGTH l)

report(10 121 1212 (length: l) cross-sawing-time)

24 (blank-type rectilinear) AND (rip-saw-distance r)

report(20 122 1221 (saw distance: r) sawing-time)

25 (blank-type rectilinear) AND NOT (additional-base-planning) AND (all-round-planning-feed w)

report(30 132 1321 (feed: w) planning-time)

26 (blank-type rectilinear) AND (additional-base-planning)

report(30 131 1311 ()) 27 (blank-type rectilinear) AND

(additional-base-planning) AND (all-round-planning-feed p)

report(40 132 1321 (feed: p) planning-time)

Source: own elaboration 5. Conclusions

Recapitulating the above described considerations, the following conclusions can be drawn: 1. Building of technological pathways on the basis of traits of semi-finished products

written down in the MRP class system should release engineers from the time-consuming and onerous activities associated with the elaboration and feeding in of these data.

2. The determination of times of technological operations with the assistance of the ex-pert system can cut dramatically the labor-consuming activities necessary to deter-mine time standards and to increase the accuracy of scheduling the realisation of non-standard orders.

3. The proposed method and linking of the expert system with the MRP class system can support effectively the operation of the product configurator.

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Bibliography

1. Smardzewski J. (2002): Representation of production types in integrated systems. [in] Engineer Workshops. Department of Furniture Design, Agricultural University in Pozna, Pozna, 22.03.2002, pp. 49-62 (in Polish).

2. Smardzewski J. (2002): Production management in the ERP class system Exact Globe for Windows. [in] Engineer Workshops. Department of Furniture Design, Ag-ricultural University in Pozna, Pozna 22.03.2002, pp. 49-62 (in Polish).

3. Smardzewski J., Gawroski T. (2003): Non-standard product configurator for the MRP environment. Annals of Agricultural University in Pozna CCCLI. Wood Technology 38, pp. 19-26 (in Polish).

4. Smardzewski J., Gawroski T. (2006): Product configurator for furniture industry. Virtual Design and Automation. New Trends in Collaborative Product Design. Pozna University of Technology Pub., pp. 415-422.

Tomasz Gawroski

Akademia Rolnicza im. Augusta Cieszkowskiego Wydział Technologii Drewna

Katedra Meblarstwa

Wojska Polskiego 38/42, 60-627 Pozna e-mail: tgawronski@au.poznan.pl