Software For Fluid Power Technology

From Editor

This is the general statement found at the very beginning of the Web site of the product, and proves that it can be broadly classified in the same group of software as AmeSim (introduced in the first issue of the Journal). The package is not a rookie; actually, the name appeared in 1976 as a results of the effort of the initial development team (most members of which are still active) and was used internally by Boeing for some years before being offered as a commercial product in 1980.

The best way to introduce the software is to make a fundamental distinction between its General Purpose part and its specialized parts, i.e. application Libraries. The general purpose (gp) part is intended to support the modelling and simulation process of a generic dynamic system. The model is developed by means of one or more interacting blocks (in EASY5 they are usually named "components"), which are available in four ways: (1) from a built-in general purpose (gp) library (inclusive of subgroups as continuous transfer functions, discrete functions, integrators, controllers, switches, functions generators, tabular functions, non linear effects, logic functions, math functions, data analysis, and some more) and visible in the working area of the screen through predefined icons; (2) as a Fortran or C block written by the user for a specific or temporary use within the model, and visible through a generic icon; (3) as a Macro block written by the user for general use, stored in a private or public library, and having no predefined icon (to be prepared by means of a built-in icon editor); (4) as Standard block, which is similar to a Macro block but is only visible as an external subroutine. No matter how generated, any block has an external interface made of three sets: "inputs" (constant or coming from other blocks), "variables" (outputs resulting from algebraic operations) and "states" (outputs coming from time integrations, i.e. differential variables). The fact that both inputs and variables can be grouped in one or more "ports" proves that the model generation process is closely related with the power port method. A single model can include up to 1500 blocks, but fifteen to twenty icons tend to saturate the working area; in this case, two or more icons can be grouped under a collective icon, then generating a hierarchy of sub-models.

If a model is assembled with the gp library, as for instance in Fig. 1, the famous statement applies no line of code has to be written (though I am a bit skeptic, because the relevant equations are to be written at least on a piece of paper, which is not much different). Conversely, if Fortran or Macro blocks are used, the relevant codes are to be written (I refer to Fortan because I am familiar with it, but the same applies to the C language). The syntax rules are practically the same as the original language, but are complemented by a few specific features. Two of them deserve an explicit mention: (a) the control of the "sorting" process, i.e. how the lines of code are ordered in the final subroutine; (b) the use of the so called "switch states", which are not so easy to understand but at the end one discovers that they are an elegant and efficient way of dealing with almost any type of discontinuity in the model (and this is particularly useful in hydraulics). When a dynamic model is programmed, the user should keep in mind that the expected result is normally a set of ordinary differential equations; EASY5, however, offers the possibility of handling the so called "implicit loops", i.e. the description made with implicit equations (the detailed description of this option is outside the scope of this presentation).

Fig. 1: Example of a model assembled with (gp) library (with a sub-model block)

Once the model is ready, it has to be analyzed. Here, I concentrate on the primary tool offered by EASY5, i.e. nonlinear simulation, but it is not the only one; actually, others are available, for instance, in the linear analysis domain (linear model generation, root locus - which I found very useful in tuning critical parameters -, transfer functions, and some more). A number of integration methods are included, from the classic Adams Runge-Kutta Euler to the advanced Gear (standard or modified) and Radau (special for implicit loops), but the user is allowed to use an external subroutine with a proper calling sequence. I normally use the Gear family, though it is probably better to take the habit of certifying the results by running the same problem with different methods. In my experience, I found two types of simulation failures. The first occurs at the beginning of the simulation and an explicit error condition is declared; here, the rather poor diagnostics (the debug is suggested, but it is so boring and often useless) is partially compensated by the fact that experience teaches the appropriate tricks. The second occurs when the simulation starts, proceeds for some time and then gets stuck without issuing any error message; this is strange, because the integration routines are generally coded in such a way that several explicit error conditions are identified and reported to the user. But let's resume the optimistic attitude, and suppose that the simulation has been successful and we are anxious to look at the results. Now, one of the best features of EASY5 appears, i.e. the plotter. And it is not matter of graphical beauty, because it allows an efficient investigation and presentation of data: just to mention a few things, it offers cartesian views, polar views, multiple axes, multiple diagrams, comparison of curves, full editing, and more (the User Guide takes 46 pages to describe the whole thing). Figure 2 shows an example of the plotter window, though the try-and-see would be the best way to appreciate the tool. My ranking is 99 out of 100, the missing point being an encouragement to develop a 3-D display.

Fig. 2: Appearance of the plotter windows

Two additional capabilities are associated with the general purpose part of the software. The first is more or less shared with other packages, i.e. the interface with a number of CAE tools found in the market. Once again, I take the listing from the Web page: EASY5 models can be co-simulated with controller models in MATLAB/Simulink® and the MATRIXx® product family. Multibody mechanical models in ADAMS®, DADS®, and Pro/Mechanica® can be tightly integrated with EASY5's system models. Data can be exchanged between EASY5 and structural analysis tools like NASTRAN® and ANSYS®. Integrated circuit simulator like VANTAGE® have been tied to EASY5. And, finite state machine models from STATEMATE®, or auto-generated controller code from BEACON® can be brought into the EASY5 environment.  Any sound comments about the success of the interaction with third party products should be supported by an extensive experience. Anyway, a couple of remarks are applicable: (a) the relevant "extensions" (i.e. the special EASY5 blocks which are able to exchange inputs and outputs with the external code) do not have a common source, because they are sometimes provided by Boeing and sometimes by the external code developer; (b) the operation of the assembled simulation is not always straightforward as one might think (this conclusion is drawn from a short experience I'm having with EASY5 and Adams). The second additional capability is a powerful tool named MAT (Matrix Algebra Tool), which is a language for numerical calculus and data manipulation (with special emphasys on matrices). It is able to work by itself but it is particularly useful in conjunction with EASY5 to preprocess inputs and post-process outputs, and possibly drive the simulation code at higher level (for instance, controlling a sequence of simulation runs to find the maximum or minimum of a certain figure of merit).

As anticipated, the general purpose part of the software is complemented by a number of specialized or application Libraries (other shared libraries can be downloaded from the Web site): Aerospace Vehicle, Electric Drive, I.C. Engine, Power Train, Multiphase Fluid, Thermal Hydraulics (TH), Thermal Pneumatics, Valve Actuators (VA, hydraulics) and Multiphase Fluid. Actually, this is not an up-to-date list, because the latest release of the package (expected to come in few weeks time) features a rearrangement of the material: in particular, the two hydraulics related Libraries will be organized into a Basic and Advanced Library, while the pneumatic components will be included in a more extended Gas Dynamics Library. Presently, the TH library includes 65 components with several sub-groups (pumps, actuators, pipes, valves, splits and merges, orifices, etc...) and offers the choice of using allusive or ISO related icons, as in the simple example of Fig. 3; personally, I prefer the abstract icons, but I understand that industrial customers may have a different attitude. The VA library includes about 40 components, intended to be the building blocks useful to assemble valves or linear actuators which are not included in the TH library. A significant aspect of the modelling approach is the embedded integration of the energy equation, and consequently the analysis of the fluid temperature in the circuit (the presence of enthalpy gives a lot of dignity to fluid power); as far as I know, the EASY5 staff did a pioneer job in this area. Moreover, both libraries have a menu of 16 fluids, from water to mineral oils and synthetic fluids, modelled with a full set of properties.

Fig. 3: Example of a model assembled with TH library (with a sub-model block)