Handling Multimode Models and Mode Changes in Modelica

Since its version 3.3, the Modelica language offers the possibility to model multimode systems having different DAE-based dynamics in each mode, thanks to the introduction of state machines. When the differentiation index and structure varies with mode changes, compilers generate erroneous simulation code, often resulting in runtime exceptions. We propose in this paper a multimode structural analysis for both multiple modes and mode change events and we show how correct code for restarts can be generated. Our approach is illustrated on two simple but representative mechanical systems.