OMJulia: An OpenModelica API for Julia-Modelica Interaction

Abstract

Modelica is an object oriented, acausal equation-based language for describing complex, hybrid dynamic models. About ten Modelica implementations exist, of which most are commercial and two are open source; the implementations have varying levels of tool functionality. Many Modelica implementations have limited support for model analysis. It is therefore of interest to integrate Modelica tools with a powerful scripting and programming language, such as Julia. Julia is a modern and free language for scientific computing. Such integration would facilitate the needed analysis possibilities and can speed up the development of effient simulation models. A number of design choices for interaction between Julia and Modelica tools are discussed. Next, Julia package OMJulia is introduced with an API for interaction between Open-Modelica and Julia. Some discussion of the reasoning behind the OMJulia design is given. The API is based on a new class ModelicaSystem within package OMJulia, with systematic methods which operate on instantiated models. OMJulia supports handling of FMU and Modelica models, setting and getting model values, as well as some model operations. Results are available in Julia for further analysis. OMJulia is a further development of a previous OMPython package; a key advantage of Julia over Python is that Julia has better support for control engineering packages. OMJulia represents a first effort to interface a relatively complete Modelica tool to Julia, giving access to an open source set-up for modeling and analysis, including control synthesis, easily installable from a unified package manager. Some possibilities of OMJulia are illustrated by application to a few simple, yet industrially relevant problems within control design. Keywords: Modelica, FMI, FMU, OpenModelica, Julia, Julia API, OMJulia.