Total Manufacturing Assurance; Chapter 5, System Definition
          As in any major undertaking, no great structure is built without first laying a firm foundation.  For our task of implementing the concepts of TMA, this translates to the solid base of a well-defined operating environment.  Men, machines, and materials, as well as information and instructions, must be organized in an efficient manner, according to the best standard industrial techniques.  Without a sound basic structure, with all parts interacting as efficiently as possible, there is little point in striving toward TMA.
          This chapter examines the basics of product system definition.  This includes basic management and organizational functions as in any modern industrial engineering curriculum, plus the new innovations that industry is currently adopting.

PLANT LAYOUT
          In any manufacturing system, a large amount of equipment of various types is employed.  This can include nuts-and-bolts type of equipment such as that for machining, assembly and material handling, as well as storage, loading docks, repair facilities, computers, and personnel offices.  Where are these to be located?  Plant layout, or in broader terms, facilities design, can be defined as the analytical and design techniques aimed at maximizing efficient organization and interaction between these various assets.
          This results in more than a mere floor plan.  There must also be adequate power supply, lines of communication for both voice and data, and, most importantly, room to grow and adapt to the inevitable changes of the future.
          Many of the techniques used in facility planning involve minimizing necessary evils.  Minimizing the amount of equipment needed, for example, facilitates economical production.  Decreasing distance to be traveled by material in process saves both time and energy, as well as lowering the inventory cost of having material tied up in production.  Obviously, a layout which allows fewer workers to attend a greater number of tasks will result in manpower savings.  An incomplete list of items to minimize is presented here:
          1. Investment in equipment
          2. Production time
          3. Material handling
          4. Work-in-process
          5. Floor space
          6. Variation in equipment
          7. Inflexibility
          8. Manpower
          9. Employee inconvenience, discomfort, and danger


Plant Layout Analysis
          We have discussed some of the goals of plant layout design above, but have yet to address the fundamental question: how do we arrive at the layout which achieves those goals?  Many techniques exist, both new and old, but they all follow one of two basic schemes:

1. Select a number of candidate layouts.  Analyze each layout according to some grading method.  Select the layout with the highest score.

2. Define a list of goals, constraints, and requirements for the target layout.  Synthesize the best possible layout using some optimization technique.

          We can refer to these two basic schemes as the analytic and synthetic methods of plant layout.  This section will discuss some analytic techniques.

The From-To Chart
          The from-to chart, also called a travel chart, is a matrix which tracks flow between the various locations in a plant.[1]  Each activity location (machine, storage center, entry or exit point, etc.) is listed across the top row of the chart, as well as down the first column of the chart.  Each element in the interior of the chart lists some fact concerning the relationship between the corresponding row and column locations [Figure 5-1].

          One use of a from-to chart is to list distances.  An element in row 4 and column 2 would show the distance from location 4 to location 2.  Naturally, the diagonal would have all zeros, and the chart would be symmetrical about this diagonal.
          Another from-to chart could be used to tabulate traffic volume.  The numbers in the interior of the chart would indicate the flow of parts from each location to each other location.  In this application, of course, there would not necessarily be diagonal symmetry.
          Combining these two charts, the distance from-to and the traffic from-to, gives the total material handling burden of the proposed layout.  Slight changes in the layout could be easily documented by altering one row and column.  In this way, many candidate layouts can be compared.

The REL Chart
          Another analytic tool is the REL chart, or activity relation chart.[2]  This is a more subjective technique than the from-to chart, and is useful for considering the more qualitative influences, as well as distance and traffic volume.  The REL chart uses a closeness rating instead of a numerical score for each location pair [Figure 5-2].

          Unlike the from-to chart, which listed facts about a specific layout, the REL chart is a method for expressing desired distance relationships.  The closeness rating for each pair of locations is filled in based on a qualitative judgment of how important it is that the two locations be situated near, or even adjacent, each other.  The standard ratings are:
          A = Absolutely necessary
          E = Especially important
          I = Important
          O = Ordinary closeness OK
          U = Unimportant
          X = Undesirable
          
          These ratings are selected intuitively, based on the subjective judgment of the plant layout engineer.  Often, a reason code is listed as well, indicating the rationale for the closeness rating.  Typical reason codes might be sequence of work flow, sharing of records, personnel, or equipment, similarity of function, or ease of supervision.  Undesirable ratings may arise between noisy operations and those that need quiet surroundings, or particularly dirty or sooty operations and those that require a clean environment.

The Space-Relationship Diagram and Block Plan
          We are now ready to start constructing some candidate layouts and evaluating them.  The REL chart is our starting point for generating the candidates, and the from-to chart will enable us to judge them.
          The space-relationship diagram is the first step.  This combines two sources of information: our REL, or activity relation chart, and our space requirements.  For each activity listed in the REL chart, we must calculate the necessary amount of floor space.  This can be derived from anticipated production volume, time standards, and machine capacities.
          Next, a square in drawn on a piece of paper for each activity.  The size of each square is proportional to the area needed for that activity, and this area is listed in the square, along with the activity number [Figure 5-3].  Each square is connected to each of the other squares with a line.

          The type of line used is based on the closeness rating assigned in the REL chart:  An A pair is connected by a quadruple line, an E pair by a triple line, an I pair by a double line, an O pair by a single line, and a U pair by no line at all.  Activity pairs with an X rating are connected by a zig-zag type line.
          This space-relationship diagram is the tool for generating the candidate layouts.  Think of the lines connecting each activity as attractive forces.  The more lines between two blocks, the stronger the attraction, like so many tightly stretched rubber bands.  The zig-zag lines, of course must be thought of as repulsive forces.
          The next step is to try to assemble the activities into a block plan.  Using your imagination, let the attractive forces bring the activities as close together as the relationship lines indicate.  Think of the process as piecing together a jigsaw puzzle with rubber bands connecting the pieces.  The final shape should approximate the shape of the plant or building that will house the activities.  A typical block plan is shown in Figure 5-4.
          Naturally, many block plans are possible from any one given space-relationship diagram.  Try to generate as many reasonable plans as possible.  The more candidates that are generated, the greater the probability that the best possible is among them.

          When you have sufficient candidate plans, the from-to chart can be used to evaluate them.  The traffic-volume from-to chart will be the same for each candidate, but the distance chart will not.  Measure the distances for each pair of activities in each candidate, and construct the appropriate distance chart.  Combining this with the (constant) traffic volume chart, each candidate can be given a score for material handling burden.
          A simple final step would be to merely select the candidate with the lowest material handling score, but this can sometimes lead to undesirable results.  It is better to select the best three or four candidates and scrutinize each closely.  Are the closeness relationships implemented as desired?  Are the A pairs and X pairs situated as desired?  It is possible, especially in complex systems, for one or two important relationships to be sacrificed in favor of many small improvements in the less important relationships.  This results in a better numerical score, but lets an "Achilles's heel" slip into the design.  Use sound engineering judgment to select the final winner from the top three or four contenders, based on overall adherence to closeness ratings, and any other subjective considerations not programmed into the ratings scheme.

Plant Layout Synthesis
          The preceding section focused on analysis of candidate layouts, with only minimal effort directed at the generation of those candidates.  This is a tried and true method of plant layout, and works well for small plants with few departments.  Where large, complex facilities are concerned, however, the method of creating these candidates becomes more important.
          Synthesis of candidate layouts requires many complex decisions to be made as each function or activity is placed into a layout.  Consequently, computer methods are often used.  Many programs for plant layout exist, three of the most popular being CORELAP [3], ALDEP [4], and CRAFT.[5]  Some programs employ a generative approach, building up each candidate layout "from scratch", while others use a derivative approach, which continually modifies an existing layout, making incremental improvements at each step.  Some programs are capable of using either approach, or a combination of the two.
          While different programs use different methods of generating layouts, they all have certain functions in common.  Most of them require as input the REL chart, as described in the preceding section.  They also require a grading scale, which translates the codes in the REL chart to a score for each candidate layout.  Some programs assume a constant grading scale, others allow the user to supply one.
          The programs must also be told how much area is required for each department, and what the size and shape is of the available facility.  Some programs can handle multiple floors at one time, and some allow the user to fix certain activities or departments in specific locations before synthesis begins.
          Most generative programs begin by selecting one activity from the REL chart.  The method of selection of the first activity varies from one program to another, but is usually based on some function of the REL chart which allows greatest flexibility in subsequent selections.  This activity is placed in the layout, and the REL chart is searched again for an appropriate second activity.  Again, each program uses its own criteria for selection of the order of placement, based on some function of the REL chart and the grading scale.  Each successive activity is placed in the layout at a location which will minimize or maximize some program-specific function.  The program will generate a large number of possible layouts, giving each a score based on the grading scale.
          The best of these "first cut" layouts will be stored as a benchmark, and the program will then attempt to do better.  Some programs will start over with a different first activity.  Others will go into a derivative mode, and attempt to make improvements on the benchmark, using various internal functions.
          Finally, after several iterations, the program will report its latest and greatest layout.  The layout engineer must then decide if this layout meets all the non-numeric requirements that were not programmed into the analysis.  The engineer also has the option of supplying a different grading scale, or a modified REL chart, and seeing if a preferable layout results.  As always, the engineer should use his own judgment in making the final decision, and not take the program's recommendation as gospel.