Attach and Run an FMU

An FMU (Functional Mock-up Unit, FMI 2.0 Co-Simulation) is a simulation model packaged as a .fmu file. Examples include a hydraulic axis, a thermal model or a controller exported from Modelica, Simulink or a similar tool. In realvirtual you attach the FMU to a node of your model. Its inputs and outputs then appear as signals below that node. Any component that can use a PLC signal can use them. A drive can follow an FMU output, a lamp can show one, and a button can write an FMU input.

The FMU is stored inside the GLB. Saving, sharing and reloading the model preserves it without a separate file.

Without CONNECT, the FMU stays standalone and uses start values.

Requirements

• realvirtual CONNECT on a Windows x64 machine. The browser never executes an FMU — CONNECT runs it and streams the values to the viewer. Without CONNECT the model still loads; the FMU just does not run (see Without CONNECT).
• The viewer must be connected to that CONNECT as its live data source.
• The FMU must be FMI 2.0 Co-Simulation and contain a win64 binary (binaries/win64/<model>.dll). Anything else is refused when you attach it, with the reason.

Attaching an FMU

FMUs are attached in the Editor:

1. Open the asset in the Editor
2. Right-click the node the FMU belongs to — in the 3D view or in the hierarchy — and choose FMU anhängen…
3. Pick the .fmu file. The editor reads the model description in the browser (nothing is uploaded yet) and shows the model name, GUID and variables in the Inspector
4. Save the asset — the FMU is now part of the GLB

Attaching is one undoable step, like every editor change. FMU entfernen in the same context menu removes it again. A node that already carries an FMU gets the new one in place, keeping its step size and tolerance.

Signals below the FMU node

For every supported variable, the viewer creates a signal below the FMU node:

Ball
└── Signals
    ├── h        Ball.h        (output → read by the viewer)
    ├── v        Ball.v        (output)
    ├── e        Ball.e        (tunable parameter → written by the viewer)
    └── Restart  Ball.Restart  (restart control, see below)

• The row name in the hierarchy is the FMI variable name; the signal name is <node>.<variable>, so two FMUs with the same variable never collide. In a layout, the placement name is added in front, like for every signal.
• Outputs (and calculated parameters) come from the FMU and are read-only in the viewer. Inputs and tunable parameters are written by the viewer — by you in the signal list, or by a component bound to them. They start at the FMU’s start values.
• Real becomes a float signal, Integer and Enumeration an integer signal, Boolean a bool signal.

Bind these signals like any other. In the demo model, the ball’s Drive Follow Position points at Ball/Signals/h, so the drive position follows the FMU output. The FMU calculates metres; a drive moves in millimetres. The viewer converts the units for you, as described in the next section.

Units and automatic scaling

Every FMU signal carries the unit its model declares (m, m/s, rad, …). The picker shows it as a chip behind the signal name. The signal tooltip shows it on a Unit line. Drive behaviors work in millimetres and degrees. Binding a signal with a known unit to a slot with a scale field sets that field’s conversion factor. For example, binding Ball.h (m) to Drive Follow Position › Position sets Scale = 1000 for m → mm. The factor is a normal field value, visible in the Inspector. You can change it or undo it together with the binding.

A small icon next to the scale field tells you where the value comes from:

Icon	Meaning
link	The value is the automatic factor (×1000 from unit m → mm)
link off	You changed the value by hand. The tooltip names the automatic factor; a click sets it back
warning	The units do not fit (for example rad to a linear drive), the unit is unknown to the viewer, two slots share the field with different factors, or the behavior does not apply the field (ScaleFeedbackPosition off). The binding exists, the field is left as it is, and the Problems panel explains what to do

The viewer never overwrites a value you typed yourself. Rebinding a slot recalculates the factor only while the field still holds the previous automatic value. Rotary drives get degrees instead of millimetres; the slot’s unit follows the drive’s Direction.

Units come from the FMU’s modelDescription.xml. Model-specific unit names that are not in the viewer’s built-in table (metres, millimetres, radians, degrees and their speeds and accelerations) are resolved through the FMU’s own UnitDefinitions, which the viewer stores on the FMU node when the package is attached. Signals from PLCs and other interfaces carry no unit yet, so binding them changes nothing — the scale field stays as it was. Placed layout assets are not scaled automatically either; the Problems panel shows the factor to enter by hand.

The Inspector of an FMU node shows its state, model, GUID, generator, package size and SHA-256, step size and tolerance. The FMU badge in the hierarchy takes the colour of the state:

State	Meaning
standalone	No CONNECT connection — the signals show start values
needs trust	CONNECT holds the FMU but waits for your consent
starting / running	CONNECT runs the FMU; values are live
error	CONNECT could not start or run it — the message is in the Inspector and the Problems panel
unsupported	No FMU package on the node, or its bytes are missing from the file

Stop ends the FMU run for this browser tab; Re-provision sets it up again in CONNECT. While the FMU runs, the line Run shows start #<n> · t = <s> s: how often CONNECT has started this instance and the simulation time since the last start.

Change parameters

The Inspector of an FMU node has a section Inputs & Parameter with one row per input and parameter you may change: the variable’s description (or name), its unit, and an editor that fits its type — a number field, a checkbox, or a dropdown with the names of an enumeration.

• Change a value and it goes to the running FMU at once. A number is kept inside the range the FMU declares (for example 0.5…1); the range is shown next to the field.
• Als Startwert übernehmen stores the current value as the variable’s start value in the model. From now on the FMU starts with it — after a reload, in another browser, on another CONNECT. The row shows an Override badge.
• Zurück auf Start removes the stored start value and sets the variable back to the start value of the FMU itself.
• A dot marks a value that differs from the start value; the badge marks a stored start value. Both are independent: you can try a value live without storing it.
• Storing a start value is an ordinary edit: Undo takes it back, and it is saved with the model.

Some parameters are fixed: the FMU reads them only while it starts (the gravity g of the BouncingBall, for example). Their row says fest · wirkt nach Neustart. Edit the value, then use Als Startwert übernehmen to store it. The FMU restarts automatically with the new value. Nothing is sent while you type.

Not every variable has a row: outputs, constants and calculated parameters cannot be changed, and string variables are not supported.

If the FMU node has Allow Viewer To Fmu switched off, the live editors are locked, as a hint indicates. You can still store start values, and CONNECT applies them when the FMU starts. Without a CONNECT connection the rows stay editable, but only stored start values have an effect later.

Restart the FMU

An FMU restart puts the model back to its start values — the ball is lifted to its start height and falls again. There are three ways:

• Reset (the Reset button or Shift+R) restarts every running FMU whose Restart On Reset is on (the default). Switch it off in the Inspector to keep an FMU running through a reset.
• Storing a fixed parameter restarts it (see above).
• The Restart signal below the FMU node: when it changes from off to on, the FMU restarts, and the signal goes back to off by itself. Anything that can write a bool signal can use it — a button, a PLC, or a LogicStep.

Restart from a LogicStep

To restart an FMU regularly, add a sequence of two LogicSteps:

Sequence        LogicStep_SerialContainer (top level: runs in a loop)
├── RestartBall LogicStep_SetSignalBool   Signal = Ball/Signals/Restart, Set To True
└── Wait        LogicStep_Delay           Duration = 12 s (the ball starts with e = 0.95 as a saved start value)

The sequence switches Restart on, waits four seconds, and starts over — so the FMU restarts every four seconds. Change Duration in the Inspector of Wait and the new interval applies at once.

A restart takes a moment. Further restart requests are ignored during the restart and for the first second after the FMU runs again. This prevents a very short Duration from flooding CONNECT with restarts.

Reset restarts the sequence from its first step, so the FMU keeps restarting every four seconds after a reset too. The sequence’s first restart request usually arrives while the reset’s own restart is still under way and is ignored; the next one follows after Duration.

The consent dialog — and why it exists

An FMU contains native code. CONNECT runs it inside its own process, with the rights of the CONNECT service. A faulty or malicious FMU can therefore crash the gateway or do anything that process may do. That is why nothing runs without your explicit consent:

• The first time a model with an FMU opens against a CONNECT, the viewer uploads the package and asks. Vertrauen & starten starts it; Abbrechen leaves it stopped (you can start it later from the Inspector).
• Consent is given per FMU package (identified by its SHA-256) and per CONNECT installation. A changed FMU is a new package and asks again. There is no “trust everything” switch.
• Once given, later loads of the model start the FMU automatically.
• Only attach FMUs from sources you trust.
• Consent is only accepted from a page CONNECT knows: localhost, the machine’s IP, https://web.realvirtual.io, or an origin listed in AllowedOrigins of CONNECT’s appsettings.json. If the viewer runs on your own web address, add it there and restart CONNECT — otherwise Vertrauen & starten is refused.

Without CONNECT

In a browser without a CONNECT connection — a shared link, the public demo, a static deployment — the model loads normally and the FMU nodes show their signals with start values. Nothing is calculated. The Problems panel lists the FMU as not runnable (info). The same happens when the viewer is connected to Unity instead of CONNECT: Unity does not run FMUs attached in the web editor.

Several instances

Every FMU node is a separate instance with its own state. This also applies when you use the same FMU twice or place an asset with an FMU several times. Each browser tab keeps its instances alive in CONNECT. When the last tab using an instance closes, CONNECT stops it within about a minute.

Try it: the FMU demo

The demo project contains the document FMU Demo (BouncingBall):

• Ball carries the BouncingBall FMU. Its output h drives the ball’s height through a drive — connect to CONNECT, confirm the dialog, and the ball drops and bounces.
• Sequence restarts the ball every four seconds through Ball.Restart, as described in Restart from a LogicStep.
• Try the parameters: set Coefficient of restitution e to 0.95 and the ball bounces almost back to its start height; store g = -3 as a start value and it falls slowly, like on another planet.

The FMU is the unmodified Modelica Reference FMU BouncingBall v0.0.40 (BSD 2-clause license).

Known limits

• String variables get no signal. The Inspector lists them as not supported.
• One instance per process: some FMUs allow only one instance in a process. A second instance of such an FMU reports an error (restarting CONNECT clears it).
• No FMU-to-FMU coupling yet: an FMU exchanges values with realvirtual signals only. To feed one FMU’s output into another, route it through a component or a PLC.
• Windows x64 only: CONNECT on Linux or ctrlX does not run FMUs.
• FMI 3.0 and Model Exchange FMUs are not supported.

See also

• Scenes and Models
• Editor overview
• realvirtual CONNECT