A Composable and Extensible Environment for Equation-based Modeling and Simulation of Variable Structured Systems in Modelica

Sammanfattning: Modeling and Simulation are usually used to solve real-world problems safely and efficiently by constructing digital models of Cyber-Physical Systems. The models can be simulated and analyzed with respect to requirements, and decisions about their design can be based on this analysis. In the latest years, the field of Modeling and Simulation has grown massively and is tackling systems with increased complexity. Thus, the process of modeling and simulating Cyber-Physical systems is becoming more and more complex. This increase requires modeling languages that can express systems with increasing complexity.Modelica is an open-standard declarative equation-based object-oriented language used to model various systems expressed using equations. Modelica tools can read the models, process them, and simulate them. However, the Modelica language and tools cannot express some concepts such as structural changes to the components or behavior of Cyber-Physical Systems during Simulation.In this thesis, we propose extensions of the Modelica language to support modeling so-called variable structure systems, that is, systems where the structure of the system varies during Simulation. The full Modelica language and the new extensions are supported by a novel composable programming environment framework called OpenModelica.jl written in the Julia language. The proposed Modelica language extensions can handle explicit and implicit modeling of variable structure systems by introducing new operators and, consequently, new semantics to the Modelica language.The explicit modeling is based on extensions that switch at runtime between continuous modes of operations with operators similar to the ones used in the specification of Modelica state-machines. The implicit modeling supports reconfiguration during runtime via recompilation. A Just-in-time compiler was implemented to handle the new semantics using the symbolic-numeric programming language Julia.We investigate the performance of our new framework and compare it with existing state-of-the-art Modelica tools on models with thousands of equations and variables. The results show that our extensions and proposed runtime framework is viable for simulating both usual Modelica models and models with variable structure systems.The conclusion is that the Modelica language can be extended further to support systems with variable structures with the addition of a few operators and JIT enhanced runtime system support. Based on the result of this thesis, we propose several directions for future work.