Development of a Simplified Transmission Hydraulics Library based on Modelica.Fluid

Modeling of hydraulic systems often leads to systems of equations that are stiff and difficult to solve. In many cases, stiffness of these systems can be traced to orifices and relatively small volumes within the model. Frequently, such volumes and orifices are only present to facilitate explicit statespace formulations of the underlying conservation principles. In an effort to create more efficient models and to eliminate the need for insignificant or nonphysical contributions from such components, the new Modelica.Fluid library [1] introduces a structured set of base classes (leveraging new features in the Modelica language) from which fluid component models can be built. These base classes allow for a wider range of component configurations by eliminating the need for extraneous volumes and orifices in hydraulic schematics. Using the Modelica.Fluid library as a foundation, another library has been developed that includes hydraulic components for hydraulic transmission modeling. The models are aimed at addressing lingering performance and robustness issues with hydraulic circuits in transmission models and include several useful simplifications. Because these component models use a first-principles formulation (i.e. conservation of mass and energy), it is possible to mix simplified or idealized components with models that include complex dynamics. As such, model developers can focus on the dynamics of interest (e.g. dynamics associated with the design of a specific spool valve in a transmission) while still capturing the basic functional behavior for the other components in the system. The result of this approach is a practical continuum between functional and predictive modeling.