Simulation Master Class

Being a simulation modeler can often be a lonely proposition. Often alone, you are called on to build a model of a complex process, analyze that model, and present the results to management. As simulation modeling is as much an art as a science, we thought applying the musician/artist Master Class concept to simulation would be helpful to our customers. 

A team of simulation experts from a range of backgrounds have been invited to join the Imagine That Inc. technical staff to discuss their simulation techniques, general research, and specific ExtendSim features that they find intriguing. Some of the Master Classes will be specific to ExtendSim while others will be more general and useful to all simulation modelers.

We have a number of speakers lined up. However, if you are interested in presenting at one of our Master Class sessions or simply have some suggestions, let us know. We are looking for speakers who can address the issues of the simulation community as a whole as well as ExtendSim modelers.

Imagine That’s simulation Master Class sessions are open to everyone at no charge. Classes begin May 2011. Check our web site for more details.

Reviewing the Review

If you are evaluating simulation software, a third-party review or paper comparing the features of different simulation software programs can be very useful. However, not all software reviews are created equal. Some are truly excellent, well researched, informative, and accurate. Others contain factual errors, use outdated software, and are even sometimes biased by economic considerations of the author.

The gold standard for a comparison is Tom Schriber and Dan Brunner’s perennial “Inside Simulation Software: How it Works and Why it Matters”. This paper does not attempt to recommend one software product over another, but it does give insight into the inner workings of a variety of simulation packages. When Tom and Dan were including ExtendSim (Extend) into their paper, we exchanged nearly 100 emails detailing the precise behavior of our software. The authors have taken care to update the paper as new versions of ExtendSim have come out. There are other good reviews as well. I have not always agreed with their conclusions, but I do respect their methodology.

I won’t reference the paper on the other end of the spectrum. However, I did find 14 obvious technical errors in a little more than a page of text describing ExtendSim. These included the wrong web site, incorrectly stating that there were limitations on the number of levels of hierarchy, and even the product name was incorrect. The author never contacted us or asked us to comment on his review.

So, if you are looking at simulation software reviews, take the following steps to make sure that you are getting a thoughtful, accurate commentary:

• Contact the author and ask them if the paper was reviewed by the simulation software vendors before publication.
• Look for obvious technical errors. Generally, this is an indication that the author did not do their homework.
• Did they use the latest version of the software?
• Does the author have any connection with a simulation software vendor, currently or in the past?
• If you can, contact modelers who use the software in the paper. Ask if they agree with the conclusions.

As software developers, you can expect us to be biased towards our own creations. Unfortunately, you cannot always find that even in neutral third party evaluations. Caveat emptor.

Bias block’s use in optimization problems

I feel the bias block does not get the use it deserves and would like to make a case for it.

If you do not use the Rate library, there may be no point trying to understand the Bias block…However, lack of familiarity rather than true functionality might be the reason you’re not using Rate. The ExtendSim manual has a good discussion of ExtendSim AT’s discrete rate capabilities; you may want to read up on it a bit. For now let focus on this little block called the Bias block.

The block is small and simple but carries a powerful concept. The bias block allows the rate of flow through particular model sections to be maximized. In other words, after defining where the flow should “preferably” go, the model optimizes the direction of the flow.

To illustrate my point lets give a practical example:

-Goal: The model purpose is to properly allocate power supply so that each customer gets what they need at the lowest possible cost.

-Setting: 3 different sources of power, 4 pools of users and a distribution network.

-Logic: Each source of power has an associated cost and is linked to the distribution network. Each pool of users needs a certain amount of power and is connected to the distribution network.

Supply information	Maximum Megawatt-hour	Cost per Megawatt	Bias priority
Supply 1	50 MW-h	35 $/MW-h	1
Supply 2	50 MW-h	50 $/MW-h	2
Supply 3	50 MW-h	60 $/MW-h	3

Demand information	Megawatt-hour Demand
User a	20 MW-h
User b	30 MW-h
User c	20 MW-h
User d	10 MW-h

-Implementation: The modeler will construct the distribution system, the suppliers and the consumers… When all of that is done, the modeler can position a bias block next to each power supply; the lowest cost producer will be given the highest priority and so on…. And that’s it; the model is done and will distribute energy at the lowest cost.

The concept of bias block allows a modeler to look at his model as a global system. What is flowing through the system will follow the preferences provided by the bias block.

If the Bias block gets used more, I have in mind some fun upgrades to make the block even more powerful. Right now, the Bias parameter only authorized priorities between different parts of the model. It would be a fun new development to allow a full “objective function” to be defined which would be either maximized or minimized.