FMUs

FMU stands for Functional Mock-up Unit. It is a self-contained software component used to represent a dynamic model that can be exchanged and integrated across simulation tools. FMUs are defined by the Functional Mock-up Interface (FMI) standard, which aims to enable interoperability among modeling and simulation environments.

An FMU is typically packaged as a zip archive containing a modelDescription.xml file, platform-specific binaries for the included solver or code, and optional resources such as external data or scripts. The modelDescription.xml describes the model name, GUID, variables (inputs, outputs, states), units, initial values, and the capabilities of the FMU. The binaries implement the FMI application programming interface (API) that allows a host tool to initialize, set or get variable values, perform time stepping, and manage events during simulation. FMUs can be platform-specific, so separate binaries exist for different operating systems.

There are two main types of FMUs: FMU for Model Exchange and FMU for Co-Simulation. In Model Exchange, the importing tool provides the solver and time management, while the FMU supplies the model equations and state information. In Co-Simulation, the FMU contains its own solver and can advance its internal state independently through DoStep calls. This distinction affects how simulations are composed and scheduled within a co-simulation environment.

FMUs are widely used to facilitate model reuse and collaboration in domains such as automotive, energy systems, robotics, and control. They enable model exchange between tools like OpenModelica, Dymola, Simulink, and PyFMI, supporting a modular and vendor-neutral simulation workflow. The FMI standard continues to evolve to address new capabilities and interoperability requirements.