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Create a Digital Twin

A digital twin is built from the assets available in the workspace. To bring assets or files from a local computer into the DTaaS workspace, follow the instructions in library assets.

The library assets depend on one another as shown below.

Relation between reusable assets Relation between reusable assets

A digital twin is created by linking assets in a meaningful way. The relationship can be written as:

\[ D_t: \{ D{^*},M^{*},(FT)^{+} \}C_{dt} \]

where D denotes data, M denotes models, F denotes functions, T denotes tools, \(C_{dt}\) denotes DT configuration, and \(D_t\) is a symbolic notation for a digital twin itself. The \(\{ D{^*},M^{*},(FT)^{+} \}C_{dt}\) expression denotes composition of a DT from D, M, T, and F assets. The \(*\) indicates zero or more instances of an asset, and \(+\) indicates one or more instances of an asset.

The DT configuration specifies which assets to use and any parameters to set for them. When a DT requires RabbitMQ, InfluxDB, or similar services supported by the platform, the DT configuration must include access credentials for these services.

This generic DT definition is based on DT examples observed in practice, and a DT may deviate from it. The only requirement is that the DT can be executed from either the command line or a graphical desktop environment.

Tip

If you are new to digital twins and do not have separate digital twin assets, only a single directory containing all components, upload this monolithic digital twin into the digital_twins/your_digital_twin_name directory.

Example

The Examples repository contains a co-simulation setup for a mass-spring-damper system. The example shows how co-simulation can be used in a digital twin.

The file system contents for this example are:

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workspace/
  data/
    mass-spring-damper
        input/
        output/

  digital_twins/
    mass-spring-damper/
      cosim.json
      time.json
      lifecycle/
        analyze
        clean
        evolve
        execute
        save
        terminate
      README.md

  functions/
  models/
    MassSpringDamper1.fmu
    MassSpringDamper2.fmu

  tools/
  common/
    data/
    functions/
    models/
    tools/
        maestro-2.3.0-jar-with-dependencies.jar

The workspace/data/mass-spring-damper/ directory contains input and output data for the mass-spring-damper digital twin.

The two FMU models required for this digital twin are in the models/ directory.

The co-simulation digital twin requires the Maestro co-simulation orchestrator. Maestro is reusable across all co-simulation-based DTs, so it is placed in the common/tools/ directory.

The digital twin configuration is in the digital_twins/mass-spring-damper directory. The co-simulation configuration is defined in two JSON files: cosim.json and time.json. Documentation for the digital twin goes in digital_twins/mass-spring-damper/README.md.

The launch program for this digital twin is digital_twins/mass-spring-damper/lifecycle/execute. It runs the co-simulation until completion and then exits. The programs in digital_twins/mass-spring-damper/lifecycle manage the lifecycle of this digital twin. The lifecycle page explains these programs.

Execution of a Digital Twin

A frequent question concerns the run time of a digital twin. The natural intuition is that a digital twin must operate as long as its physical twin is in operation. If a digital twin runs for a finite time and then ends, can it be called a digital twin? Yes. The Industry 4.0 use cases seen among SMEs have digital twins that run for a finite time. Users often run these digital twins when they choose to.

To execute this digital twin:

  1. Open the Workbench page of the DTaaS website and click the Desktop tool. The desktop opens in a new browser tab.
  2. The tab shows the VNC Desktop with a connect button. Click Connect to connect to the Linux desktop of the workspace.
  3. Open a terminal (the black rectangular icon in the top left region of the tab) and enter the commands below.

  4. Download the example files by following the instructions in the examples overview.

  5. Go to the digital twin directory and run:

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cd /workspace/examples/digital_twins/mass-spring-damper
lifecycle/execute

The last command executes the mass-spring-damper digital twin and stores the co-simulation output in data/mass-spring-damper/output.