Wednesday, March 30, 2016

Simulating Active Feedback Control in Engineered Systems

Posted by Rick Kossik

Most real-world models contain many interacting (positive and negative) feedback loops. Some feedback loops are “natural” and cannot be readily managed or controlled (e.g., water evaporating from a pond is an example of a process that forms a negative feedback loop). Others, however, are “intentional” negative feedback loops that are used for active feedback control of a system. That is, these are feedback loops that are part of the design of the system, and are used to make the system behave in a specified manner. A heating system controlled by a thermostat is the classic example of such feedback control. Pumping water to maintain a target water level is a simple water resource example. We discussed using GoldSim to represent such systems in our January webinar.

Of course, representing such feedback systems is a critical part of many modeling approaches. System dynamics, an approach developed by Professor Jay W. Forrester at MIT in the late 1950s and early 1960s puts particular emphasis on understanding the feedback structure of systems. Although GoldSim cannot strictly be described as a system dynamics package (system dynamics utilizes a highly standardized "stock and flow" paradigm, in which models are built using three principal element types: stocks, flows, and converters), it is similar to system dynamics programs in many ways (and can simulate any system that can be modeled by these tools).  The stock and flow syntax of system dynamics is too restrictive for many complex engineering models, and as such, GoldSim moves beyond the standard "stock and flow" paradigm. GoldSim also focuses much more on creating probabilistic models that represent uncertain and stochastic systems. (You can read more about the differences between GoldSim and system dynamics approaches here).

Nevertheless, the system dynamics community is an excellent resource for simulation modelers. There is a very rich system dynamics literature (much of which is completely applicable to GoldSim modeling) and a number of excellent textbooks have been published. Although much of the system dynamics literature tends to be a bit academic, there is a tremendous amount of interesting and valuable work that has been done in the arena over the past 60 years that is worth exploring.  An excellent starting place to learn more is the System Dynamics Society website.

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