Improved stability of algebraic loop solutions #1525
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refs OpenModelica#1330 It seems that simply calling CVode with mode CV_NORMAL does not work correctly to return at the given stopping time, if the previous call stopped due to a root.
- Some errors made in commit 4c3fe79. - All discrete inputs should be updated before going back to continous time mode in any FMU. refs OpenModelica#1326
Even though the inputs values shouldn't change in continuous time mode, calling setters for such inputs is against the FMI standard and may result in error responses from FMUs. refs OpenModelica#1326
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These changes are cherry picked from the branch "work", which includes changes originally made to the master branch in October 2024. I hadn't had the time earlier to rebase my code on the major refactoring done to SystemSC.cpp on 19.11.2024 (6d7a397).
Note:
Related Issues
Purpose
This request introduces a mechanism that prevents solutions to algebraic loops from intermediate model evaluations by CVODE to lead into the results. It starts the solution of each algebraic loop from values that the input variables had at the start of the solver time step and/or event iteration.
This prevents noise from difference quotient evaluations leaking as a strange form of noise through the inexact algebraic loop solutions. This happens, when the algebraic loop solution for a step change inside CVODE during a difference quotient-based Jacobian matrix estimation ends up being accepted as the solution for an algebraic loop at the end of the step. This results in non-zero, seemingly random changes in system states, even if the inputs variable values for the entire system would indicate a stationary state with zero derivatives.
Approach
At the beginning of each solver step, the values of algebraic loop inputs are stored into a member named
initial_guess. They are restored from there before each call toupdateInputs().