Computers ind. Engng Vol. 19, Nos 1-4, pp. 299-303, 1990 0360-8352/90 $3.00 +0.00 Printed in Great Britain. All rights reserved Copyright © 1990 Pergamon Press plc

An AU~oCAD-Based ExPert Svstem For Plant Lavout

Chin-Sheng Chen& Khokiat Kengskool

Department of Industrial Engineering Florida International University

Miami, Florida

ABSTRACT

This paper describes the development of a plant layout system implemented in AutoLISP on an IBM PS/2 computer. This system combines computational algorithms, human expertise, and computer graphics in one program for plant layout design and drafting. The objective is to develop a computer system for automation of the layout design and documentation process.

This system emulates a plant layout expert in the last step of plant layout process. The input to the system includes department dimensions, between-department material flows, and aisle width. The system generates and evaluates layout alternatives according to given objectives. AutoCAD commands are called to plot the layout when an alternative layout is being created. This system ensures that each department has an access to the aisle and the travel distance between two departments are measured along the aisle. As both design expertise and computer graphics are programmed in the same environment, the interfacing problem between them is easily eliminated.

INTRODUCTION

A plant layout may be viewed as a collection of department blocks interconnected by an aisle reserved for material handling activities. The aisle pattern and the location of each department along the aisle are mainly determined by pre- estimated material handling activities between these departments. Typical layout objectives are minimum total material handling cost and minimum total space requirement, various production and equipment information must be known to the facilities planner. Among them, equipment specifications, product rout ings, and production quantity are the most important to the planner. They form the basis for determining space for required machine tools, material

flows, production methods, workstation layouts, and department formation.

The quality of a plant layout has been heavily reliant on the experience of the planner because a large number of factors need to be considered in the design process. Many of them are either uncertain or difficult to specify in mathematical terms. In the past a few decades, computer programs have been developed to facilitate the design process. There are basically two types of computer applications. One is to capture and implement design expertise in a computer program for layout creation or improvement. Many such programs have been made available. Among them, CRAFT, CORELAP, PLANET, ALDEP, and COFAD [i] are best known to us. The other application is the use of existing computer graphics programs as a drafting tool for layout drawing and documentation. A wide variety of 2- D and 3-D computer graphics packages are available on PCs, computer workstations, and main frame computers. Some packages are equipped with machine templates and management features to improve layout drawing and management efficiency.

More recently various efforts have been made to further automate the plant layout process. For instance, Fisher and Nof [2] developed a program called FADES to demonstrate the feasibility of capturing qualitative design expertise in a computer program. The program could select workstations, perform economic analysis, develop relationships among workstations, and assign workstations to the layout. Instead of pre-programming all design expertise in a computer system, Ketcham and Malstrom [3] developed a set of programs called FLAG to create a design environment. FLAG had graphic templates and a built-in design algorithm. It could create a more realistic layout because of its flexibility of incorporating layout details of the particular design. The user,

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however, was required to specify the details.

SCOPE and ASSUMPTIONS

This paper presents an effort aimed at the integration of layout design and plotting activities in a computer program. The scope and assumptions are detailed in this section, followed by a description of the design procedure employed in the system and its implementation.

The current system is developed to design and plot the final plant layout. The input to the system includes department dimensions, aisle width, and material flows between departments. The design objective is to minimize a combination of the total material handling cost and the space requirement. Major assumptions are listed as follows:

i. Each department has a fixed and rectangular shape. One entry/exit point is allowed for each department. This point is fixed at the middle point on one border line of the department facing the aisle.

2. One aisle is reserved for material handling activity. The aisle starts at the upper- left corner of the layout (the only exit/entry point in the facility designated for material handling). The aisle connects all the departments. It makes rectangular turns within the facility.

3. Material handling cost is proportional to its volume and travel distance, which is measured from door to door along the aisle.

4. The available space for layout development is rectangular with its maximum dimensions known.

PLANT LAYOUT PROCEDURE

The plant layout procedure implemented in this system seeks the best layout among the alternatives based on its given objective. As shown in Figure I, the procedure begins with generating a departmental entry sequence. Each department is entered to the layout in that order and placed along either side of the aisle. Each entering department is oriented to have its door open to the aisle. The aisle extends itself to link all

generate an entry sequence

i Assign depts to the layout along the aisle

Calculate the location of each dept.

l Estimate distances between two depts along the aisle

l Compute the total MH cost

and space util ization

Evaluate the alternative and update incumbent ~ Yes

Figure 1: Plant Layout Procedure

the departments and makes turns to stay within the facility. The locations of both the departments and the aisle is not fixed at the initial assignment. It is finalized only after all departments are entered. The relative location of each department on the aisle and its coordinates are calculated according to their accumulated lengths on each section of the aisle.

With all the department locations and coordinates calculated, distances between departments are estimated along the aisle. Finally the total material handling cost and space utilization are computed for this layout alternative. Each layout alternative is evaluated according to its material handling cost and space requirement. These two measures are converted into cost and weighed equally at default but the user is given the option to change it. The process continues until no more layout alternative is desired. During the design process, the layout alternative may be plotted on the monitor. The user is given an opportunity to save the layout and key in an entry sequence,

Chen and Kengskooh An AutoCAD-based System 301

after viewing the current layout. The best alternative is automatically saved.

Two approaches are implemented to generate a departmental entry sequence in this system. For the first entry sequence, the receiving and shipping department is the first one to enter and placed at the exit/entry point of the aisle. It is considered appropriate because all materials and products should go through this department before they enter or leave the facility. Other departments are selected to enter based on a rule similar to the one implemented in the CORELAP program [4]. The only difference is that instead of using letters, actual flow volumes are used to indicate their closeness. Subsequent entry sequences are generated randomly or keyed in by the user.

Each department entering the layout is assigned to the aisle starting from the upper-left corner of the layout to provide passage for each department. It extends horizontally and makes right-angle turns as required. A department may be assigned to either side of the aisle according to the accumulated assignments on each side. Records are kept for each department in terms of its aisle section number, aisle side, and the entry order. These records are used to estimate the available space on both side of the aisle section for further assignment, decide where to put the next department and when to make a turn, and measure the distance between two adjacent sections of the aisle. An example layout is given in Figure 2.

The location of each department in the layout is defined by the location of its exit/entry point on the aisle. The x- and y-coordinates of each department are estimated separately. Each x-coordinate is calculated with its department dimensions, entry sequence, the aisle side and section where it was assigned to. The y-coordinates are estimate in the same fashion. They are determined by the width of the aisle and the depth of the departments along related aisle sections.

The department locations and the aisle pattern are the basis for measuring the distance between two departments. The locations of the two departments in question along the aisle are first identified before their relative distance can be estimated. The distance along x-

C " , , ' / / / / , ~ ~ /-,4.

o Io! ,o o

(o,o) I ,

I

@ @ ®

--(28' ,62')

~ 5 '

///// Q r (so'.25'>

e o e 6 10'

,o'~ 15'

Figure 2: A Layout Alternative

coordinate is a summation of the length of the aisle sections in between plus the partial aisle lengths on which the two departments are respectively located. The travel distance along y-axis is simply the difference between the two y-coordinates.

The distances between these departments are the basis for calculating the total material handling cost for this layout alternative. The finalized location of each department and aisle sections are used to estimate the total required rectangular space. Total material handling cost is the summation of the products of unit handling cost, volume, and travel distance between two departments. The total required space is defined by the given length (P) and the width required by the actual layout. These two measures are combined into one criterion to compare this layout alternative with the incumbent layout. The details of these procedures for department assignment, location calculation, distance estimation, and cost computation are explained in Chen and Kengskool [5].

IMPLEMENTATION

This design procedure is implemented in AutoCad/AutoLISP, version i0 on an IBM PS/2 computer. As shown in Figure 3, this system is centered at the AutoLISP interpreter [6].

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Figure 3: S=hemati¢ of the System

AutoLISP is a subset of Common Lisp consisting of more than one hundred internal functions for programming and access to AutoCAD commands. The plant layout procedure described above is programmed into six AutoLISP functions. These six major functions are entry sequence generation function, department placement function, coordinates generation function, distance estimation function, layout evaluation function, and layout plotting function. Input data and facts are stored in separate files. The user is allowed to assign relative weights to the two evaluation criteria, give an entry sequence, save a desired layout alternative, or run the system with default settings. AutoCAD commands are called through the layout plotting function, which receives data including department dimensions, coordinates, and aisle pattern for each layout alternative, calculates all extreme points, and then calls AutoCad commands to plot the layout on the monitor. It saves the layout alternative upon the request from the user.

Shown in Figure 2 is the layout alternative generated by using the CORELAP-Iike entry rule. There are 15 departments. The first department is shipping/receiving and

therefore selected as the first one to enter. Others are entered following the CORELAP-I ike entry rule. As a result, the entry sequence is department i, 15, 14, 2, Ii, 9, 13, 12, 5, 4, 7, 6, 10, 8, and finally department 3. The aisle extends horizontally from the upper- left corner. The departments are sequentially placed along both side of the aisle. It makes right-angle turns as needed to accommodate more departments. In this layout, the aisle has two sections connected by a vertical aisle segment where no department is assigned to. The distance between the two horizontal aisle sections are determined by the maximum vertical dimension of the departments on the aisle. In this layout alternative, it is determined by departments ii and 6. The distance between two departments is measured from door to door which is located at the middle point of the department front facing the aisle. The doors of departments ii and 5, for example, are located at (28' ,62') and ((50' ,25') , respectively. The distance between these two departments is therefore 64' The total rectangular space occupied by this layout is 55'x 65'.

CLOSING REMARKS

A plant layout system has been presented. This system was developed to integrate the plant layout design and plotting activities. This system was implemented in AutoLISP. AutoLISP was chosen because it is a subset of Common LISP and specially developed for use with the AutoCad software. The AutoLISP interpreter was employed in this system to execute the layout design procedures and interact with AutoCAD commands to plot the layout, as required. The problem of integrating layout design expertise and a graphics package was easily solved for this system because the programming language and the CAD software were available in the same environment. For further study, more flexibility should be built in this system to consider various types of material handling systems and aisle patterns. It is also desirable to allow the user to pause the system, make changes to the layout alternative in process, and add constraints. The interface with a data base system such as DBASE III should also prove beneficial to the design of workstations and departments.

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REFERENCES

1. Tompkins, J. and White, J., Facilities Plannina, New York: John Wiley and Sons, 1984, pp. 286-291.

2. Fisher, E. and Nof, S., "FADES: Knowledge-Based Facility Design, " The Proceedinqs of the Annual International Industrial Enqineerinq Conference, 1984.

3. Ketcham, R. and Malstrom, E., "A Computer Assisted Facilities Layout Algorithm Using Graphics, " The 1984 Fall Industrial Enaineerina Conference Proceedinqs.

4. Lee, R. and Moore, J., CORELAP- Computerized Relationship Layout Planning," Journal of Industrial Enqineerinq, 18(3), pp. 147-159.

5. Chen, C. and Kengskool, K., "An Intelligent Computer Aided Design System for Industrial Facilities Planning," accepted for presentation at the ASEE 4th International Conference on Computer Graphics and Descriptive Geometry, Miami, Florida, June, 1990.

6. Autodesk Inc., AutoLISP Proqrammer's Reference, CA: Autodesk Inc., 1987.