Software For Fluid Power Technology

From Editor

AMESim, a simulation program for engineering applications

The software is based on a full multiport approach, which allows connections between components to exchange power flow information. The software has an easy and intuitive interface for parameter set-up, and good interactive tools for model building and simulation. Similarly to many other software tools on the market, AMESim makes model construction easy and intuitive by allowing dragging and dropping icons, representing components, from selectable sets of libraries. As AMESim icons use ISO symbols, the resulting system model resembles the appearance of a fluid power systems circuit. Fluid, mechanical and signal connections are made automatically and use different line styles to improve readability of a model.  A useful feature for initial functional evaluation of systems is that a system model  can be quickly constructed using supplied “primer” library. At a later stage, individual com-ponents can be associated with more complex models also selected from a menu of available items or build by the user. The complexity of a system model is limited only by the available computational power and to improve the readability of a complex system, models of components can be grouped in submodels.

On the numerical side, this software shows its best performance. The analyst can easily control the numerical accuracy and the detail level of each component. Non-linearities, end-stops and stiffness ratio of the system are no longer a problem; all the components’ models are designed with emphasis on numerical consistency of the underlying mathematical assumptions. This considerably reduces the risk of problems in assembling systems where some components require special attention due to the presence of non-linear (or bi-lin-ear) states, which can cause some integrators to behave unpredictably. A special feature of the software is the availability of specialized solution algo-rithms for fluid transmission lines. The provision of easy-to-use and well developed fluid transmission models, which can be selected from pull-down menus is one of the best features of the software.

AMESim is supplied with standard libraries which include mechanical, hydraulic and pneumatic components, and which make the software a complete tool for the simu-lation of hydromechanical systems. In addition it offers, as options, spe-cialized libraries (pneumatic, thermal, thermal hydraulic, hydraulic resistance, cooling and transmission etc.) which were developed over the years in co-operation with major industrial clients. However, although the number of components in each library and their flexibility are remarkably high, there is often a requirement for new or improved models. One approach used in AMESim is to build complex components using basic elements from Hydraulic Component Design Library  (previously called Basic Elements Library). This is a relatively simple task not requiring any programming skills.

A second approach, functional programming, is based on a dedicated tool (Submodel Editing Tool) which gener-ates C or Fortran code skeleton equipped with appropriate calls and description of inputs and outputs and proper interface with the other AmeSim components, which only need to be edited to include equations describing state variables.

This feature is one of the most powerful tools a simulation software can have as it makes it possible of having virtually any compo-nent or subsystem, modelled and properly interfaced connected with the integration routines of the solver. This is definitely a non trivial aspect of modelling, especially when non-linearities require a special integration technique for stiff, variable-step algorithms. Implementation of such models, however, require a sufficiently good knowledge of dynamics, programming skills in either C or Fortran and adequate knowledge of ODE systems integration. A developer of such a model must be able to access the same subroutines of  basic functions (area functions, end-stop handling, Reynolds number transition in orifices) as the standard AMESim components. However, building of mathematical models should be left to “experienced” users; inexperienced users or users not having sufficient experience in program-ming languages should be encouraged to use the hydraulic component design library, with all the lim-itations imposed by this choice.

Most of us feel comfortable when we can do everything within a single environment, even if this means that we must use only a reduced subset of the options available in the performance of some tasks. In modelling of complex systems there are often requirements for more detailed (often referred to as “local”) approach to component behaviour and/or a different perspective when looking at the system itself. AMESim is structured in order to be “easily” interfaced with “complementary” software like CFD, linear control and multi-body dynamics packages, in order to create a consistent environ-ment for virtual prototyping.

External program interfaces include Matlab, Simulink, Matrixx and ADAMS, and in many cases the interface includes the possibility of a co-simulation, where a complex model includes parts described in different environments. For instance, the active suspension system of a pas-senger car may be modelled using the capability of ADAMS to visualize the rigid body dynamics of a system whose internal behaviour is described by an AMESim model of the active suspensions hydraulics. Although the protocols for data exchange amongst different programs have evolved tremendously in the last years, they still remain a critical point in the simulation process. As soft-ware development is carried out independently by different teams this may cause unex-pected problems in the continuous process of software improvement and development. Thus I was pleased, attending a pre-release demonstration of new software, to see substantial improvements in this area as planned enhancement of  the interfaces with other software will provide improved multi-domain simulation capability taking advantage of a pow-erful numerical core which represents the best quality of AMESim.

Recently a number of linear anal-ysis tools like Bode, Nichols and Nyquist diagrams, Root locus analysis were added. The graphical output from simulations is now considerably enhanced in comparison with previous versions and represents a good com-promise between effectiveness and appearance. Also worth noting is the presence of a number of components’ models which include thermal effects as well as a remarkably structured library of pneumatic components.

The manuals provided with AMESim are strongly driven by a problem solving approach, easy to read and easy to be applied. The examples reported are in general very simple, and easy to follow, but a trade-off is that many potential problems do not show up at first sight, and are not explained in the manuals. Some aspects could have been discussed more detailed, as for instance the theory behind the models used. A documentation of the approximations implied by many of the models used in the standard libraries would be very useful.

In summary, the software is a very good simulation tool for experienced users and at the same time, by reducing work load in modelling and simulation tasks, it is an easy tool to use at the simulation entry level. As a general purpose tool the program has a relatively good degree of flexibility, and as a specialized tool it has some specific features to tackle prob-lems typical in fluid power systems.