The Modelica CombiTimeTable block provides the ability to load time-series data into a model. In this post, we will show you how to flexibly manage the data both in Modelica and in compiled models.
In a previous post from our Dymola tips and tricks series, we covered a way to parameterize models using external data with the DataAccess package included with the Modelon Base Library. Providing model data externally is a powerful and popular approach that allows the separation of model from data.
Many of our customers leverage DataAccess to not only parameterize the model with external data files but also change the model parameterization in the data files with compiled models either as executables or Functional Mockup Units (FMUs).
In this post, we will share an approach to load time series data into Dymola from a file and also maintain the flexibility to change the file name reference along with the actual data in the file after compilation. The CombiTimeTable block in the Modelica Standard Library provides the basic ability to load time series data into Dymola.
The component allows data to be entered directly into the model or read from a file by setting the tableOnFile parameter to true. With tableOnFile set to true, the user can enter the fileName and tableName parameters which give the file reference and the specific table reference to be used for a given table instance. The info layer for the model provides details on all the configuration options and external file formats supported.
While there are many different source blocks that can provide inputs to a model, an external file input can provide maximum flexibility in that you can simply create whatever input signal you want in the file, perhaps loading experimental data or even constructing input signals from requirements.
For this demonstration, we will take the CoolingLoop example from Liquid Cooling Library and modify it so that the inputs are provided in an external file via the CombiTimeTable block. The model is shown below.
To make it convenient to load the same data file into each CombiTimeTable block, we can propagate a string parameter for the file name as shown in the parameter dialog for the heat_input block and in the source code snippet below. The pump_speed and airflow blocks reference the same string parameter.
In the source code, the fileName parameter has a default file name “cooling_loop_ramps.txt”. As a first step, let’s create this input file with values corresponding to the same ramps as the original model in Liquid Cooling. The file is shown below.
So now we have a model that references an external file for inputs. While we can always change the file reference via the filename parameter in the Modelica code for simulating in Dymola, this approach requires the model to be re-compiled. Furthermore, when we are using the model in compiled form (i.e. as an executable or FMU), we need the flexibility to change the data file after the model is compiled.
In Dymola 2016 FD01, we do have the ability to access and modify string parameters, but you will need to enable an advanced option for string parameter handling. Let’s go through the workflow. First, we will create a second input file called cooling_loop_newinputs.txt with different values for the inputs. Note that both the size of the data traces and the actual data have changed.
Now let’s go through the steps to compile the model and change the data file reference. These steps are described below and also shown in the Dymola command window for clarity.
To confirm that the model ran with the different input files, the plot below shows the engine volume temperature (i.e. input to the radiator) and the prescribed pump speed specified in the input file for the two simulations.
Now let’s test the model as compiled into an FMU. First, we can create an FMU from our model and then import into FMI Add-in for Excel to run the simulations in Excel with different file inputs by changing the string parameter in Excel. With the string support in our Excel tool, the screenshot below shows the experiment sheet with the string parameter for the fileName set to our two different input files for Case1 and Case 2 and then a comparison of the results. So far so good- we are getting the same behavior in an FMU as expected.
Finally, let’s load the FMU into FMI Toolbox for MATLAB/Simulink. As shown in the screenshot below, the fileName parameter appears in the parameter dialog for the FMU. And when the string value is changed to cooling_loop_newinputs.txt, the model results change as expected. So with the FMU exported from Dymola with string parameters enabled, we can certainly conclude that our FMI tools handle the string parameters properly and that this approach works both inside Dymola and when the models are compiled into FMUs.
The ability to change input files by changing the file reference in the CombiTimeTable after compilation (rather than just changing the data in the default file as was the limitation without proper string parameter support) is a common request, and now it can certainly be done in the latest version of Dymola. Will you utilize this approach? Feel free to leave a comment as I’d love to hear from you.
Contact us if you would like more information on the use of external data to streamline modeling and simulation workflows, including the use of DataAccess for model parameterization using external data. DataAccess is part of the Modelon Base Library which is provided along with nearly all Modelon commercial libraries.
John Batteh is Group Manager and Technical Specialist at Modelon Inc. He holds a PhD in Mechanical Engineering from University of Michigan and has 15 years experience in physical modeling and simulation, with expertise in powertrain and thermofluid systems. In his current position, he is focused on helping customers leverage the power of Modelica and FMI for model-based systems engineering.