System Dynamics Calculator

The Model Simulator

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The Model Simulator loads a model and runs the model simulation. The model simulator accepts user inputs, calculates variable and component values, detects threshold crossings and component errors, animates symbols, draws simple graphs and logs all pertinent information for the user. The logs are sufficient for the user to play back the simulation at a later time if network connectivity is lost or computing resources are insufficient to perform the simulation calculations at a later time or in the required timeframe. The model simulator can produce a movie from a simulation. The model simulator also produces finely formatted detailed graphs and reports suitable for publication on the web or in print media.

Loading a Model to Simulate

The Model Simulator begins the user session with a prompt for the user to select and load a model to run a simulation on. The user has several options:

•select a model from a library of models stored locally in the system dynamics calculator
•select a model from local media such as a memory card, USB storage device or a SAN or NAS device attached or bonded to the system dynamics calculator
•select a model from remote libraries in model warehouses available over the Internet
•optionally build a new model from scratch with the system dynamics calculator model builder

These options may require the following actions:

•accessing the Internet, browsing for a model and downloading it into the SD Calculator memory
•verifying a credential of the source server to confirm the integrity of the source server
•verifying a checksum or a hash to confirm the integrity of the model file
•checking the model file out of a version control system and returning it revised to the control system
•encrypting or decrypting the model file

Once a model is loaded, the model simulator identifies and loads the data that is necessary to define variable values and in the model. This initial data includes the following:

•user-input values of constants in model formulas and for initial stock levels
•local datasets such as TAL files exported from personal finance software
•remote datasets from websites such as US Census Bureau or the New York Stock Exchange

To accomplish the import of data from files and websites, the simulator may be required to generate queries and parse responses with a set of rules that the user may have to define.

Defining the Model Simulation

The Model Simulator operates each model component according to a set of characteristics that the user defined when the model was built. The following Model Simulation Base attributes are defined in the Model Simulator process for the model:

•SIM NAME STRING - the simulation scenario’s user assigned plain language label
•SIM NAME REG - the label assigned to the simulation scenario by the Model Builder
•SIM OWNER - the Model Simulator user who defined the simulation method
•SIM VERSION NUMBER - the simulation base reference version number
•SIM VERSION OWNER - the name of a person or organization name, a URL
•SIM VERSION DATE - the published date
•SIM REVISION NUMBER - an appended simulation revision number
•SIM REVISION OWNER - the user who last revised the simulation revision
•SIM REVISION DATE - the date of simulation revision changes
•SIM COMPLIANCE - the version of the global system dynamics modeling standard followed by the Model Simulator process
•SIM SCRIPT LIST - (optional) a list of simulation initialization and execution scripts
•SIM NUMBER SYSTEM - the number system set for the model – typically base 10 (default)
•SIM CURRENCY SYSTEM - the currency system set for the model – typically dollars (default)
•SIM UNIT SYSTEM - the unit system set for the model – typically Metric (CGS default)
•SIM MODE - (optional) a value of n (the number of simulations to run useful with random variables), “ACCURATE” (default) or “QUICK”
•SIM PAUSE - the amount of time to pause between intervals (optional);a value in seconds of 0 (default) or a number
•TIME START - the starting model time for the simulation with a value of “now” or a specified date and time or “null” (not a time-dependent model)
•TIME INTERVAL - the basic time increment for each trial of the model such as a number of years, months, days, hours, minutes, seconds, or “null” (not a time-dependent model)
•TIME PERIOD - the number of time intervals in a report period, or alternatively
•TRIAL PERIOD - the number trials in a report period
•TIME FIRST PERIOD - the number of time intervals in the first period, or alternatively
•TRIAL FIRST PERIOD - the number of trials in the first period
•TIME END - the ending model time for the simulation such as a specified date and time, or alternatively
•TRIAL LAST - the number of trials (time intervals) simulated
•LOCK SIM - a value of “NO” (default) or “YES” to prevent Model Builder changes by a user who is not the model owner, model version owner, model revision owner, simulation owner, simulation version owner, simulation revision owner or authorized by a password
•SIM DESCRIPTION - an abstract of the simulation’s scope and use – a “Read Me”

The default currency system is associated with the user location.

The model simulator checks the MODEL MODE attribute to determine if simulation attributes for the model are calendar / clock-based or unitless interval trials for model calculations.

Basic Simulator Control Strip Functions

The simulator control strip performs the following functions: