User's guide chapter 9



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Chapter 9 – Optimization

See Optimizing Models Using SimRunner video tutorial.

9.1 – What is SimRunner?

SimRunner is a decision support tool used to help optimize simulated processes. SimRunner takes your existing ProcessModel models, evaluates them for you, then performs tests to find better ways to achieve the results you desire.

Typically, most people use simulation tools to predict and improve system performance or to establish the relationships between various system elements. By modeling the actual facility or process, you can conduct what-if analyses to determine the best way to improve system performance—this is optimization. Although SimRunner cannot guarantee it will identify the optimal solution for all processes every time, it will find better solutions than you would likely get on your own.

With each optimization project, SimRunner runs sophisticated optimization algorithms on your model to help you optimize multiple factors simultaneously. Each project requires a validated model, a standard by which to measure system performance (an objective function), and a group of factors that SimRunner may change to improve system performance.

9.1.1 – Benefits

Simulation allows you to find solutions to complex problems without incurring the tremendous costs associated with a trial and error approach. Optimization helps ensure that the solutions you implement are at or near their optimal values.

Unlike manual simulation experimentation which answers only specific what-if questions, SimRunner automatically seeks a course of action that will help optimize your entire system’s performance—essentially answering how to meet your objectives.

To help you present your findings, SimRunner allows you to output various types of reports:

Data reports (for spreadsheets)

Analysis reports (for text and word processing reports)

Graphical reports (for charts and graphs)

You can print each of these reports or export them to other applications through common clipboard functions.

9.2 – Where Do I Begin?

Start with a validated model

Once you complete and validate your simulation model, you are ready to begin an optimization project. If you are not working with a valid model, you don’t need to perform an optimization until the output from the simulation is valid.

Identify your simulation type

It is important to properly identify the simulation type—terminating or nonterminating. What does it mean to refer to a simulation as terminating or nonterminating? A terminating system stops when some key event occurs like the end of the day. When you come back the next day, you start fresh again. A non-terminating system is not necessarily a system that never stops production, rather it is a system that resumes from the point it left off. For both terminating and nonterminating simulations, you need to determine the appropriate run length and number of replications. For non-terminating simulations it is also necessary to determine the warm-up period.

Determine if model is a true candidate for optimization

Not every simulation model is built with the express purpose of optimizing some particular element. Many simulation models are built to demonstrate the relationships that exist between various elements of your system. If optimization is appropriate, define the objective function—the output statistics used to measure the performance of proposed solutions.

Use simulation to identify and examine potential solutions

Simulation has always been trial and error when it comes to optimization. We have some sort of optimization method that we apply to the model and we examine the model’s output statistics to see if we achieved the desired outcome. This is not a bad approach if you have one decision variable you are trying to optimize—but what if you are trying to optimize multiple decision variables at once? Interaction becomes very complex and requires more advanced optimization methods like SimRunner.

Define scenario parameters

When you build your model, you must define a scenario parameter for any variable you want to optimize. This provides SimRunner with a series of values it can change as it seeks to optimize your model. In SimRunner, these values are called factors. For information on how to add and modify Scenario Parameters see Chapter 3, Section 3.13.1, Scenario Parameters.

Screen factors

Part of the simulation process is to evaluate the relationships that exist between model elements, or factors. Often, you will take the time to adjust some part of your model to find that the adjustment has no impact on system performance. Factor screening is the process of identifying which model elements (factors) do not affect the output of your model, and narrowing your search to include only those factors that affect the model’s output. Be discriminant in your selections.

Relax and wait

While some models contain relatively few factors that you can quickly optimize, others contain many. In a previous inventory reduction project, a high-end computer took approximately 24 hours to compute what it estimated to be the optimal value for the model. Although it took a long time to produce this result, the net savings were tremendous.

Consider the results

While there is no promise that SimRunner will identify the optimal solution to your process, it is possible. SimRunner will, however, find better solutions than you would likely get with your own trial and error experimentation. The surest way to know the optimal solution to any model is to run an infinite number of replications of all possible inputs. Since this is not an option, take into consideration the number of experiments you are able to perform and act accordingly.

9.2.1 – Concepts and Theories

SimRunner uses both genetic and evolution strategies algorithms with its primary algorithm being evolution strategies. The specific design of these algorithms for SimRunner is based on the work of Dr. Royce Bowden and other experts. Evolutionary algorithms have been extensively evaluated by academics and have proved reliable in optimizing simulated systems.

Evolutionary algorithms

An evolutionary algorithm is a numerical optimization technique based on simulated evolution. Solutions generated from the evolutionary algorithm must adapt to their environment in order to survive. Since each potential solution returns a specific result, you must establish an objective function to measure the performance of each solution. If the returned value falls within the acceptable range defined by the objective function, SimRunner will continue to examine the value as it searches for the optimal solution.

Example

To help you better understand how SimRunner’s optimization process works, consider the following analogy. If you and a group of explorers found yourselves on the slopes of a mountain, in the dark, with nothing but radios and altimeters, how would you find the summit?

simrunner example 1

The first step would be to establish the current altitude of everyone in your group by recording each person’s altimeter reading.

Topographical view Reading 1 simrunner

Next, you would direct your group members to wander out in any direction for various distances, then stop and review their new altimeter readings. At the second reading, you find that some of your group has moved higher and some have moved lower.

Topographical view Reading 2 simrunner

By comparing the results of each altimeter reading, you can determine the general direction in which to proceed. Since you want to climb to the summit, you proceed in the general direction that had the highest reading (in this case, 2). Again, after everyone moves various distances, you stop to take another altimeter reading.

Topographical view Reading 3 simrunner

By taking the average of the group’s new altimeter readings, you can see that the overall reading increased. This confirms that you are moving in the right direction. Just to be sure, you repeat the process and take another reading.

Topographical view Reading 4 simrunner

As you will notice from the example, your group is beginning to converge upon a single point. The more you repeat the data collection process, the closer the group will get to one another, and the more certain you will be that you are at or near the summit. Just to be on the safe side, however, you even send a few members of your group out to remote parts of the terrain to be sure that there is nothing higher than the point you already identified.

Topographical view Reading 5 simrunner

Once your average altimeter reading is equal to the best in your group, you have converged upon the summit and can be confident that you have achieved your goal.

Topographical view Reading 6 simrunner

Conceptually, this is how SimRunner works—only the terminology is different. Instead of altimeters, explorers, and tests, you will use objective functions, input factors, and replications. Each time the explorers move, SimRunner calls this a generation. The concepts are the same.

Not an exhaustive search

In spite of what some may think, you don’t want an algorithm that will do an exhaustive search. In a previous inventory reduction project, modelers estimated the number of possible solutions to be around 9.38 x 10^37 —it was a big project. To perform an exhaustive search for the optimum value would take a lifetime. More than likely, you will never get the answer.

What you want is an algorithm that can efficiently explore the response surface (the output from the model) and focus on those areas returning good answers—without evaluating everything.

9.2.2 – General Procedure

The following is an overview of the process you will use to perform an optimization of your system.

Step 1: Create, verify, and validate

The most important preparation you can make for an optimization project is a validated model. It is not enough to simply create a model—it will profit you nothing if the model does not reflect the real operation. Once you validate the model, you are ready to begin.

Step 2: Build a project

With your model prepared for evaluation, create a new SimRunner project and identify the response statistic you wish to target. Using these response statistics, define an objective function by which to gauge system performance. SimRunner will use this objective function to measure system improvement. Next, select the input factors you will allow SimRunner to use as it determines how best to achieve system improvement. When you optimize the model, SimRunner tests each input factor to seek the combination of factors that will result in the greatest improvement of model performance.

Step 3: Run experiments

Once you select the input factors and define the objective function, you can use SimRunner to automatically conduct a series of experiments on your model. SimRunner runs your model for you and tests a variety of possible combinations of values. After it completes the tests, SimRunner lists the test results in order of the most to the least successful combination of factor values.

Step 4: Evaluate suggestions

The fourth step is to consider and evaluate SimRunner’s suggestions. This is crucial because SimRunner will often identify several candidate solutions to your problem and you may, for reasons not addressed in the model, prefer one solution over another. You may also wish to make additional model runs (replications) and look at confidence intervals to further evaluate SimRunner’s list of possible solutions.

Step 5: Apply solution

Once you identify the solution that best fits your needs, implement the solution.

9.2.3 – Pitfalls

If you follow the general procedure stated previously, your chances of success are very good. Typically, projects fail because the:

• Model is not valid

• Analysis considers insignificant factors

• Analysis ignores significant factors

• Objective Function is inappropriately formulated

• Test results are not scrutinized

9.2.4 – Suggested Readings

If you would like more information on optimization, see the following references:

Bowden, R. O. and J. D. Hall. 1998. Simulation Optimization Research and Development. Proceedings of the 1998 Winter Simulation Conference 1693-1698.

Hall, J. D. and R. O. Bowden. 1997. Simulation Optimization by Direct Search: A Comparative Study. Sixth International Industrial Engineering Research Conference 298-303.

Hall, J. D.; R. O. Bowden; and J. M. Usher. 1996. Using Evolution Strategies and Simulation to Optimize a Pull Production System. Journal of Materials Processing Technology 61:47-52.

Goldberg, D. 1989. Genetic Algorithms in Search, Optimization, and Machine Learning. Addison Wesley, Massachusetts.

Schwefel, H. P. 1981. Numerical Optimization of Computer Models. Chichester: John Wiley and Son.

9.3 – The SimRunner Interface

The SimRunner interface provides you with easy access to every step necessary to create an optimization project. The project building process is divided into three phases, each containing a series of steps necessary to complete the phase. As you move from phase to phase (displayed at the top of the dialog), a list of steps for the phase appears in the left pane.

simrunner interface for process optimization

9.3.1 – Set Up a Project

The first thing you must do to create an optimization project is to start a ProcessModel simulation of your model. You don’t need to complete it. The simulation just needs to be started in order to load the data required by SimRunner. After ending the simulation and returning to the modeling window, click the Tools menu and select SimRunner.

Tools menu simrunner option

The model data will be loaded automatically.

9.3.2 – Define Objectives

The objective of your project is the final outcome you want to achieve. SimRunner measures your progress toward this goal using an objective function. An objective function is composed of response statistics, a min/max or target range, and a specific weight you wish to apply to each response statistic.

Define objectives in SimRunner

Response Category

The response category is the type of statistic you wish to use to evaluate your model. Response categories include model elements such as locations, entities, resources, and variables.

Performance measure in simrunner optimization tool

Response Statistics

By clicking on one of the Response Categories, you will see a list of Response Statistics. Simply put, response statistics are those values you wish to improve. Once you define your targeted improvements, you are ready to define how you want them to perform—the objective for the response statistic.

Response statistics of simrunner

Objective for Response Statistic

The objective for the response statistic refers to the way in which you want to effect change for that item. If you are trying to increase the overall output of a system, you would maximize the response statistic. Likewise, you could minimize the statistic or target a specific range within which you want the result. Finally, enter the reward or weight for each response statistic—larger numbers signify greater rewards in situations that require more than one objective.

objective for response statistics in simrunner

Max Check this option if you want to maximize the final value of this statistic.

Min Check this option if you want to minimize the final value of this statistic.

Please Note

Maximized objective functions return positive values. Minimized objective functions return negative values.

Target Range Check this option to enter a specific target range within which you want the final result.

Statistic’s weight Weights serve as a means of load balancing for statistics that might bias the objective function. Since most simulation models produce a variety of large and small values, it is often necessary to weight these values to ensure that the objective function does not unintentionally favor any particular statistic. For example, suppose that a run of your model returns a throughput of .72 and an average WIP of 4.56. If you maximize throughput and minimize WIP by applying the same weight to
both ( W 1 = W 2 ), you will bias the objective function in favor of WIP:

Maximize [(W 1 )*(Throughput)] = .72

Minimize[(W 2 )*(WIP)] = 4.56

weighed statistics in simrunner

In this case, since you want to ensure that both statistics carry equal weight in the objective function, you will apply a weight of 6.33 ( W 1 =6.33) to throughput and 1.0 ( W 2 =1.0) to WIP to make them of equal weight in the objective function.

Maximize[(W 1 )*(Throughput)] = 4.56

Minimize[(W 2 )*(WIP)] = 4.56

weighed statistics in simrunner

In situations where it is necessary to favor one statistic over another, balancing the statistics first will make it easier to control the amount of bias you apply. For example, if you apply a weight of 12.67 ( W 1 =12.67) to throughput and 1.0 ( W 2 =1.0) to WIP, the objective function will consider throughput to be twice as important as WIP (adapted from Harrell, Ghosh, and Bowden 2000).

Important information to be aware of Typically, you will need to experiment with your model to identify the weight ratio necessary to balance statistics.

Response statistics selected for objective function

After you define the objective for the response statistic, you may click the Add button to include the statistic as part of the objective function. SimRunner combines the statistics into a linear combination and displays the updated objective function for the project.

Response statistics selected for objective function

Objective Function

The objective function is an expression used to quantitatively evaluate a simulation model’s performance. By measuring various performance characteristics and taking into consideration how you weigh them, SimRunner can measure how well your system operates. However, SimRunner knows only what you tell it via the objective function. For instance, if your objective function measures only one variable, Total_Throughput, SimRunner will attempt to optimize that variable. If you do not include an objective function term to tell it that you also want to minimize the total number of operators used, SimRunner will assume that you don’t care how many operators you use. Since the objective function can include multiple terms, be sure to include all of the response statistics about which you are concerned.<