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Control System Modeling: Purpose

I’m going to use model and simulation as synonyms in this post.

The purpose of modeling in any discipline, including control systems, is to answer a question; often a very specific question is answered.  There are several reasons for why any given model only answers a small set of questions.  Budget and Schedule.

Modeling Complexity

Budget and schedule force engineers to model only those aspects deemed necessary to answer the question posed.

Modeling the universe in detail - even the very localized universe around a small object - takes a lot of work and time.  Budget and schedule concerns always force engineers to start with first principles and then model progressive deeper levels of details and fidelity.  The deeper layers are only modeled if the desired level of result accuracy requires this extra fidelity.

There are several reasons for keeping a model as simple as possible:

1. Initial time to development goes up with complexity
2. Time required for maintanence goes up with complexity
3. Odds of a mistake go up with complexity
4. Time between simulation start and delivery of results goes up with complexity

My observation is that items #1 through #3 increase roughly exponentially with complexity.  Turn around time (#4) increases but the amount of increase is highly dependent on the slowest part of the model as it exists prior to the increase in fidelity.

Control System Modeling: Pitfalls

Expanded Purpose

Engineers and other professionals who do not create or run simulations on a regular basis often forget about the narrow focus of a good model.  As a result these people often ask for results the model is not designed to produce.  Obviously the engineer being asked for the results needs to consider the request very carefully.  There may be an assumption built into the model which invalidates its use for this expanded purpose.

Juggling Programs

Each day that I work on a model I go through a process of “loading my RAM” or short term memory.  In order to work on the model and produce meaningful results a certain number of details and parameters must be loaded up into short term memory.  I find this process takes no more than 30 minutes and rarely takes more than 45 minutes.

The pitfall is in assuming you can juggle certain types of work.  Last summer I was asked to juggle modeling work and hardware maintanence work.  The hardware work needed me for 30 minutes at a time about 4 or 5 times a day.  As a result the hardware work repeatedly interrupted my efforts on the modeling work.  The interruptions came about every hour and a half.  So I used half of my time in between “loading my RAM”.

After about 2 or 3 weeks of trying to juggle the hardware and the modeling work I realized I was never gonig to get anything done on the model if I didn’t set some limits.  I asked the two programs how they wanted me to handle the problem.  The basic response was just deal with it.  So I decided to tell the hardware guys that 2 days a week they couldn’t bother me, except for emergencies.  No one was happy but it was the best I could do.