Time-varying model parameters remain constant when loading FMU into DaeBuilder

[b-mare]

Hi,

I am using the DaeBuilder functionality to read an FMU into CasADi. I have multiple parameters in my model that are function of time. When I create an integrator, these parameters do not change over time as I would expect. When I simulate with the FMU itself, the parameters do change over time. Is this expected behavior?

In attachment a small example of a moving mass where I made the acceleration function of time. Unfortunately I cannot share the FMU itself because it is not in my license plan. The plot shows how the acceleration changes as a function of time when simulating with the FMU, but remains constant (but does calculate the initial value correctly) when using the integrator `

Currently I have a workaround where I change the model in a way that time becomes an input variable for the time-varying parameters. I then also have to provide the time as an input to the integrator and output function.

Best regard accelerated_mass.zip

[donkikos]

Hi @b-mare did you manage to have a model with some time var as input variable that can also work in OpenModelica? I add a var 'modelTime', then add an equation "modelTime = time" OpenModelica gives an error that the "Equation has no unknown variables to solve for".

[donkikos]

We were able to work around the issue by introducing an internal variable for time, e.g. modelTime, and adding a differential equation that will equate it to the Modelive 'time' - der(modelTime) = 1. In the case of the model used in this issue the model would become:

model accelerated_mass
  Modelica.Mechanics.Translational.Sources.Accelerate accelerate;
  Modelica.Mechanics.Translational.Components.Mass mass(L=1, m=1);
  Real acceleration_input = Modelica.Math.cos(10*modelTime)+1; // Use modelTime instead of `time`

  Real modelTime(start = 0); // New internal var for the time

  Modelica.Blocks.Interfaces.RealOutput mass_x;
  Modelica.Blocks.Interfaces.RealOutput mass_v;
  Modelica.Blocks.Interfaces.RealOutput mass_a;

equation
  der(modelTime) = 1; // Equation to make modelTime = time

  mass_x = mass.s;
  mass_v = mass.v;
  mass_a = mass.a;
  acceleration_input = accelerate.a_ref;
  connect(accelerate.flange, mass.flange_a);
end accelerated_mass;