maximum time step of IDA solver in Open Modelica

Question

In Open Modelica, in the settings of IDA solver, we can define the initial and maximum time step as well. In a simulation, I have set these parameters, but after simulation and checking the solutions points, I found the IDA has not bounded to the defined maximum time step. is there anyone to advise me why the parameter did not work?

Answer 1

You can check which maximum allowed step size your simulation executable is using by adding -lv=LOG_SOLVER to the simulation flags and search in the log for maximum step size. If its not there IDA will be use with the default setting.

To change the maximum step size for IDA you can't use -maxstepsize, it's not implemented for IDA. I had to check the source code to find out how to do it correctly. It's possible but way to complicated.

You need to:

• Disable equidistant time step with -noEquidistantTimeGrid
• Set a value to -noEquidistantOutputTime=value

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So the following example script will use IDA with a maximum stepsize of π to simulate a pendulum from the MSL:

loadModel(Modelica); getErrorString();
setCommandLineOptions("-d=newInst"); getErrorString();
simulate(Modelica.Mechanics.MultiBody.Examples.Elementary.Pendulum, method="ida", simflags="-noEquidistantOutputTime=3.14159265358979 -noEquidistantTimeGrid -lv=LOG_SOLVER"); getErrorString();

Save this script to runTest.mos and run it with omc $omc runTest.mos &> some.log. Check some.log and you will find

LOG_SOLVER        | info    | recognized solver: ida
LOG_SOLVER        | info    | Initializing IDA DAE Solver
LOG_SOLVER        | info    | ## IDA ## Initializing solver of size 2 .
|                 | |       | | The relative tolerance is 1e-06. Following absolute tolerances are used for the states: 
|                 | |       | | IDA uses internal root finding method YES
|                 | |       | | Maximum integration order 5
|                 | |       | | use equidistant time grid NO
|                 | |       | | maximum step size 3.14159
|                 | |       | | as the output frequency control is used: 0
|                 | |       | | as the output frequency time step control is used: 3.141593
|                 | |       | | IDA linear solver method selected ida use sparse direct solver KLU. (default)
|                 | |       | | Jacobian is calculated by \"Colored numerical Jacobian, which is default for dassl and ida. With option -idaLS=klu a sparse matrix is used.\"
|                 | |       | | initial step size is set automatically.

You can do the same in OMedit with the simflags -noEquidistantOutputTime=3.14159265358979 -noEquidistantTimeGrid added to your simulation.

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Note: Of course this is ridiculous and the documentation of -maxstepsize is wrong:

Value specifies maximum absolute step size, used by the methods: dassl, ida.

Please open a ticket about this on the OpenModelica GitHub so it get's fixed.