FMI for Model Exchange

The Functional Mock-Up Interface (FMI) for Model Exchange specifications (ref. 1) defines a standard format for system-simulation models and the corresponding standard interface between these models and a simulation environment. The simulation model is wrapped in a software component known as a Functional Mock-up Unit (FMU) for Model Exchange. The standard thus defines a two-step process for exchanging models between an exporting environment and an importing environment, as shown in the figures below:

1. The Simcenter Amesim model is first exported as an FMU for Model Exchange using the FMU Export Assistant.

   Figure 64: Creation of the FMU from Simcenter Amesim (step 1)

2. The FMU is thus delivered to the third-party simulation software environment. The FMU is imported into the environment, and is connected by the user to the surrounding model (that may contain some other imported FMUs) through its input and output variables. The result of this step is a unique simulation model that can be simulated using the numerical solver of the third party tool. Results are also post-processed in this environment.

   Figure 65: Use of the FMU in the third party software (step 2)

The Model Exchange specifications assume that the model to be exported is explicit (i.e. described only by a system of ordinary differential equations) and that the simulation environment is equipped with a numerical solver that is able to numerically integrate this system of ordinary differential equations with event handling. Many FMUs for Model Exchange connected together in the environment constitute a block-diagram model. Indeed, each FMU is described by enumerating its input and output variables, its continuous state variables, and possibly some result variables and independent parameters that are exposed to the environment. The standard thus specifies for the simulation environment:

• how to set the value of the input variables, and how to get the value of the output variables and the exposed result variables;

• how to evaluate the model dynamics by setting the value of the continuous state variables and retrieving the value of the state variable derivatives;

• how to initialize the model by setting the value of the exposed independent parameter;

• how to deal with the different types of event that arise in the model

Although the interactions between the model and the environment are formalized by means of a deterministic finite automaton, the Model Exchange specification does not address the model-solver adaptation issue, which is required for convergence. Simulation models are created, tuned and tested with the numerical solver of their originating tools. Once exported as an FMU, it is expected that the numerical solver of the importing environment has the same numerical properties regarding step-size selection, numerical stiffness and discontinuity handling. Nothing in the FMI for Model Exchange specification can guarantee that the numerical solver of the environment actually verifies these properties. When exchanging models between two simulation software tools it is up to the user to ensure that the targeted solver fits the model, with his a-priori knowledge of the numerical properties of the model and of the solvers of both the exporting tool and the importing tool.