Computation Section
Subunit Facility Layout

For more extensive searches for the optimum of a sequential layout we use the capabilities of the Optimize add-in. To use the methods described on this page the Optimize add-in must be installed. When the sequential layout worksheet is created, two additional buttons are placed above the graphical display of the layout. The Optimize Form button, shown in column O of the figure below, constructs a form used for the combinatorial search process and creates the necessary links between the form and the layout data. The Optimize button, in column T, calls the dialog that sets the parameters for a combinatorial search and initiates the search.

We use the example described earlier for illustration. The initial layout below has the departments sequenced in numerical order.


Clicking the Optimize Form button brings a dialog that controls whether the model includes assignment costs and restrictions or not. For this example we choose to not include the assignment costs and illustrate the other option later.

Clicking the OK button creates the form shown below at the right of the layout. The form shows the initial permutation describing the layout. Rows 3 through 5 hold information used by the search procedure. Cell AL has a link to Cell B8, where the computed value of the layout is calculated. The range of cells AJ8 through AS8 are manipulated by the search algorithms. They are linked by Excel formulas to the range G11 through G20, the cells that define the sequence for the Layout add-in. The cells in row 10 are constructed by the Optimize add-in but are not used when assignment costs are not considered. The Feasibility conditions defined by cells AN3 and AN4 are not used for the example.


Clicking the Optimize button presents the Search dialog with various options for searching for the optimum permutation and the associated layout. For the illustration we have chosen to randomly generate 10 permutations or layout sequences. Notice that the form does not allow a Greedy solution. We have disabled this button because the algorithm of the Optimize add-in used for the greedy solution of permutations does not work for the layout application.


The Optimize add-in generates 10 random permutations and the Layout add-in evaluates them. The best of the 10 are placed on the combinatorial form. The 10 solutions appear to the right of this display (shown below the form on this page).


The layout associated with the best of the solutions is shown below. It happens that this solution is not as good as the initial solution.


We continue in our search for the optimum by starting from the best random result and choosing the improvement option that tries all 2 and 3-change variations of the layout. The process first tries all 2-change variations and whenever a change results an improvement, the two permutation positions are switched in value. The process continues until no 2-change switch results in improvement, then all 3-change switches are evaluated. The program terminates when a complete run through the changes results in no improvement.


The results are shown below. The layout measure has been improved, but there is no guarantee that the solution is optimal. Along with the results, the improving solutions encountered during the search are listed starting in column AU. The best solution is repeated as run 381. The time required for the 381 evaluations was 28 seconds on the author's computer. Each evaluation requires a call to the subroutine eval_layoutcomb in the Layout add-in. Each evaluation requires significant computation effort.


The final layout with the interdepartmental flows superimposed is shown below.


We see on the worksheet starting in column A, various quantities used in the evaluation. When the combinatorial search procedures are in control, the sequence defining the layout in column G is controlled by the the combinatorial algorithms. For example, cell G11 holds the formula


The value in cell AJ8 is the first element of the permutation. The other cells in column G are similarly linked to the combinatorial variables.


The combinatorial procedures of the Optimize add-in are much more powerful than the random search and 2-way switches available in the Layout add-in. There are limitations to the Optimize search however. Only the Sequential layout is defined by a permutation, so the option is not available for the Traditional layout.

The combinatorial form (cells AN3 and AN4) allows feasibility conditions on permutations. These are not used for the example, but the feasibility conditions might be useful for other layout applications.


Assignment Costs and Restrictions


Clicking the Optimize Form button brings a dialog that controls whether the model includes assignment costs and restrictions. We consider the same example as above, but decide to include assignment costs. The Random data button indicates whether the program is to provide random data for the costs. The Cost Density indicates the proportion of cells that are to contain numeric cost data. The alternative is for a cell to contain the string ***. This indicates that an assignment is not allowed.

The figure below shows the portion of the worksheet containing the combinatorial form after 10 random solutions were generated and the best of these improved with 2-change assignment swaps. The assignment cost matrix is labeled C(i, j). A component in column j and row i indicates the cost of assigning department j to sequence position i. To illustrate the possibly fixed assignments, disallowed assignments are indicated by *** in the associated cells. The example illustrates the case where department 1 is fixed as the first department in the sequence and department 10 is fixed as the last. Department 5 is required to have an even numbered position. Disallowed cells reduce the search effort because solutions that use disallowed cells are not enumerated.

The assignment costs are computed in row 10 and the sum is added to the layout cost in cell AL5. The combinatorial optimization minimizes this sum.

  The form below shows several solutions found during the improvement process. Run 1 in row 17 holds the best solution obtained in 10 randomly generated solutions. During the improvement process only solutions that result in an improvement in the incumbent solution are included in the solutions presented. The best solution is solved as the final run, so two identical solutions appear at the top of the sorted list.
  The final layout is shown below.

It must be emphasized that the search processes provided by the Optimize add-in do not guarantee optimality. They do provide a method to find good solutions to hard problems. Using the random generation plus improvement options and few hours of computation time, one can probably find good answers to problems of moderate size.

The effort to evaluate an individual layout grows approximately as the square of the number of departments and linearly with the number of cells in the layout. The effort of generating random solutions is approximately linear with the number of solutions generated. The effort of one pass through the 2-change improvement process is approximately quadratic with the number of departments. The number of passes through the process is hard to estimate, but one would also expect that to grow with the number of departments. The number of solutions evaluated for exhaustive enumeration grows exponentially with the number of departments.

With these rough estimates, one could try exhaustive enumeration with up to 10 departments. From 10 -30 departments the various heuristics probably would yield results in reasonable time. With more than 30 departments, quadratic growth begins to become painful. With more than 30 departments, the cost of a commercial solver or programming a stand alone application in an efficient programming language is probably justified. The Excel worksheet can probably hold a problem with 100 departments, but computation would be painfully slow.

Return to Top

tree roots

Operations Management / Industrial Engineering
by Paul A. Jensen
Copyright 2004 - All rights reserved