Three-Tank System Digital Twin

Overview

The three-tank system is a simple case study of three components coupled in a cascade: the first tank is connected to the input of the second tank, and the output of the second tank is connected to the input of the third tank.

This example contains only simulated components, so there is no configuration for a connection to the physical system.

The TwinManager manages the three-tank system. It is packaged as a jar library in the tools and run from a Java main file. The TwinManager uses Maestro as a slave for co-simulation, so it generates the output of the co-simulation.

The main file can be changed to suit the application, that is, /workspace/examples/tools/three-tank/TankMainDTaaS.java can be modified to get a different result.

The /workspace/examples/models/three-tank/ folder contains Linear.fmu, a non-realistic model of a tank with an input and an output, and TankSystem.aasx, the schema representation as an Asset Administration Shell. The three tank instances use the same .fmu file and the same schema because they belong to the same object class. The TwinManager reads the values from the co-simulation output.

Example Structure

Digital Twin Configuration

This example uses two models, three tools and one data file. The specific assets used are:

Asset Type	Names of Assets	Visibility	Reuse in Other Examples
Model	Linear.fmu	Private	No
	TankSystem.aasx	Private	No
Tool	TwinManagerFramework-0.0.2.jar (wraps Maestro)	Common	Yes
	maestro-4.1.0-jar-with-dependencies.jar (used by TwinManager)	Common	Yes
	TankMainDTaaS.java (main script)	Private	No
Data	outputs.csv	Private	No

The DT has several configuration files. The Maestro tool uses coe.json and multimodel.json. The files tank1.conf, tank2.conf and tank3.conf configure three instances of one model (Linear.fmu).

Lifecycle Phases

The example covers these lifecycle phases:

Lifecycle Phase	Completed Tasks
Create	Installs Java Development Kit 21 and Maven, and builds the TwinManager
Execute	The TwinManager executes the three-tank digital twin and produces output in data/three-tank/output directory
Terminate	Stops the background processes
Clean	Removes the output

Run the example

To run the example, change to the example directory.

cd /workspace/examples/digital_twins/three-tank

If required, give the lifecycle scripts execute permission, for example:

chmod +x lifecycle/create

Maestro 4.1.0 needs Java 21 or newer. The create script installs Java 21. If other Java versions are already installed, the default java may point to another version. Check and change the default version with these commands.

java -version
update-alternatives --config java

Then run the following scripts.

Create

Installs Open Java Development Kit 21 and Maven, then builds the TwinManager tool (TwinManagerFramework-0.0.2.jar) from source code.

lifecycle/create

Execute

Executes the three-tank digital twin using the TwinManager, which in turn runs the co-simulation using Maestro. The co-simulation output file outputs.csv is written to /workspace/examples/data/three-tank/output.

lifecycle/execute

Terminate

Stops Maestro running in the background, along with any other Java process started during the execute phase.

lifecycle/terminate

Clean

Removes the output generated during the execute phase.

lifecycle/clean

Examining the results

Executing this digital twin generates a co-simulation output. The results can also be monitored by adding a set of getAttributeValue calls to /workspace/examples/tools/three-tank/TankMainDTaaS.java, as shown in the code.

The main file runs the digital twin and its internal components online.

The co-simulation output is written to the /workspace/examples/data/three-tank/output folder.

In the default example, the co-simulation runs for 10 seconds in steps of 0.5 seconds. Both the duration and the step size can be changed. The output stored in outputs.csv contains the level, in/out flow and leak values.

The example neither generates nor uses data from the physical twin.

References

More information about the TwinManager is available at:

D. Lehner, S. Gil, P. H. Mikkelsen, P. G. Larsen and M. Wimmer,
"An Architectural Extension for Digital Twin Platforms to Leverage
Behavioral Models," 2023 IEEE 19th International Conference on
Automation Science and Engineering (CASE), Auckland, New Zealand,
2023, pp. 1-8, doi: 10.1109/CASE56687.2023.10260417.