# 6. Graphical Modelling

OMSimulator provides a graphical modelling environment through OMEdit, the OpenModelica Connection Editor. This feature requires a full OpenModelica installation that includes OMSimulator.

Composite models are imported and exported in the System Structure Description (SSD) format, which is part of the System Structure and Parameterization (SSP) standard.

See also FMI documentation and SSP documentation.

## 6.1. Architecture

OMEdit communicates with OMSimulator through a Python-based ZMQ server:

- OMSimulatorGuiServer.py — handles all GUI-driven requests (model management, element queries, solver settings, connections, etc.)

- OMSimulatorSimulationServer.py — handles simulation execution and result streaming

The GUI server is started automatically when the first SSP model is opened or created. A notification in the Messages Browser confirms the server is running, including the script path and ZMQ endpoint.

## 6.2. New SSP Model

A new and empty SSP model can be created from `File -> New -> SSP` menu item.

OMEdit: New SSP Model

A dialog opens to enter the model name and the name of the root system.

OMEdit: New SSP Model Dialog

OMEdit: Newly created empty root system of SSP model

## 6.3. Open SSP Model

An existing SSP file (`.ssp`) can be opened from `File -> Open Model/Library`. If a model with the same name is already loaded, an error is reported and the file is not loaded again.

## 6.4. Add System and Sub-Systems

A root system is always created together with the model. Additional subsystems can be added inside the root system via `SSP -> Add System`.

OMEdit: Add System

A dialog opens to enter the name of the new system.

OMEdit: Add System Dialog

## 6.5. Add SubModel

A sub-model is typically an FMU, but it can also be a result file (CSV). To add a sub-model, select the target system in the Libraries Browser and choose `SSP -> Add SubModel`.

OMEdit: Add SubModel

A file browser opens to select an FMU (`.fmu`) or result file (`.csv`). A dialog then opens to set the name for the new sub-model.

OMEdit: Add SubModel Dialog

OMEdit: Root system with added FMU.

## 6.6. Replace SubModel

An existing sub-model can be replaced with a different FMU by right-clicking on the component in the diagram view and selecting `Replace SubModel` from the context menu.

OMEdit: Replace SubModel Dialog.

A file browser opens to select the new FMU. The replacement is performed in two steps:

1. Dry run (`dryRun=true`) — OMEdit first performs a dry run without making any changes to the model. It checks for interface differences between the old and new FMU, such as added, removed, or modified connectors and parameters. If any changes are detected, they are reported in the Messages Browser and a warning dialog is shown. The model remains unchanged at this stage, allowing the user to review the impact before deciding to proceed.

2. Replacement (`dryRun=false`) — If the user confirms, the replacement is carried out and all detected interface changes are applied to the model. Existing connections to removed or renamed connectors are dropped, and new connectors are available for connecting.

OMEdit: Replace SubModel context menu.

## 6.7. Simulation Setup

Select `Simulation -> Simulation Setup` to configure simulation parameters before running. The dialog has two tabs: General and Solver Settings.

The General tab covers start time, stop time, and result file settings.

OMEdit: Simulation Setup — Solver Settings tab with a solver configuration.

The Solver Settings tab is divided into two parts:

Solver Configurations

This table lists the named solver configurations available in the model. Each configuration has a name and a method. Click Add to create a new solver configuration — a new row appears in the table with a default method of `oms-ma`. The method can be changed using the combo box in the Method column.

OMEdit: Simulation Setup — Solver Settings tab with a solver configuration.

To fine-tune a solver’s numerical parameters, select the row and click Edit. This opens the Solver Settings dialog:

OMEdit: Solver Settings dialog.

The available fields depend on the solver method:

- Fixed-step solvers (`oms-ma`, `euler`) — Fixed Step Size is editable; Initial Step Size, Minimum Step Size, and Maximum Step Size are disabled.

- Variable-step solvers (`oms-mav`, `oms-mav-2`, `cvode`) — Initial Step Size, Minimum Step Size, Maximum Step Size, and Relative Tolerance are editable; Fixed Step Size is disabled.

Click Remove to delete the selected solver configuration.

Component Assignments

This table lists all FMU components in the model. Each component can be assigned one of the named solver configurations using the combo box in the Solver column. The available solvers are filtered by the FMI kind of each component:

- Co-Simulation FMUs — only co-simulation masters are selectable (`oms-ma`, `oms-mav`, `oms-mav-2`).

- Model-Exchange FMUs — only ODE integrators are selectable (`cvode`, `euler`).

- me+cs FMUs — all solvers are available.

OMEdit: Solver Assignments in Components

Incompatible solvers are shown as disabled in the combo box with a tooltip explaining why. If no solver is assigned, `(none)` is used and the system default applies.

## 6.8. Simulate

Select the simulate button (green arrow) or `Simulation -> Simulate` to run the SSP model. Results are streamed back in real time via the simulation server.

## 6.9. Dual Mass Oscillator Example

The dual mass oscillator example from the test suite demonstrates a typical SSP workflow. The model is split into two sub-models, each exported as an FMU, and then connected in an SSP.

Dual mass oscillator Modelica model (diagram view) and FMUs

OMEdit: Simulate Dual Mass Oscillator SSP model