Software for Fluid Power Technology

From Editor

The purpose of the Software Review section of the Journal is to present information to the reader about engineering software, including simulation programs, to highlight their specific features and their "fitness to purpose" in the unique field of fluid power and motion control. It is, of course, impossible to establish evaluation criteria matching the needs of all readers, therefore readers should not look for absolute ratings but more or less "fuzzy" opinions of the reviewer. A software program is like a wrench, just a tool to solve problems. It is good to solve some problems and not so good for others and this depends on both the nature of the problem and the users' attitude - and generally when we review software we do not know either. A software tool can be highly specialised and great for a some applications but not so well suited for others, on the other hand another software tool can be more flexible and generally applicable but without outstanding features. It is impossible, and even misleading, to say which one is better. What we hope to accomplish is to give the reader information necessary to take his/her own decision.

Simulation Environment for Fluid Power Systems
Pc_bathfp

Abstract

The purpose of this review is to describe the history and current implementation of the Pc_bathfp simulation software and the rationale behind its provision as a free download from the web-site of the Centre for Power Transmission and Motion Control (PTMC) at the University of Bath, UK.

Software History and Rationale

The Pc_bathfp software is a direct development from the UNIX BATHfp software package. This was an influential development in fluid power simulation software, combining a convenient graphical user interface (GUI) with a powerful numerical computation engine (integrator). The system for analysis was specified graphically using the familiar component symbols corresponding to their physical equivalents. Mathematical models, attached to these symbols, represented the properties of the components so that their dynamic behaviour could be simulated.
The software design was an attempt to emulate the process that the typical fluid power engineer would follow in constructing the physical system. However, without the need to construct the physical system, the engineer was free to concentrate on the analysis of the system under investigation.
From a mathematical viewpoint the dynamics in BATHfp component models are represented by first-order differential equations. The system under investigation is therefore represented by a set of differential equations and the simulation process involves the solution of those equations by the integrator. The chosen integrator was LSODA designed by Linda R. Petzold (Petzold, 1983) and Alan C. Hindmarsh (Hindmarsh, 1983) at the Lawrence Livermore laboratories. This is a variable method, variable order variable time-step package which is highly suitable for the solution of the mathematically stiff systems characterised by this type of application. However, fluid power systems exhibit characteristics which upset the smooth running of integrators, namely the discontinuities which occur when, for example, valves suddenly close, oscillatory behaviour occurs, etc. Consequently both the integrator code and the construction of the mathematical component models were modified to better handle these discontinuities.
The graphical interface and the powerful integrator constituted the basis of the primary component of the package which was the simulation module. This had a large range of predefined components and their associated mathematical models, referred to as ‘the standard model library’. These models were validated against experimental data to ensure they provided a good correspondence to their physical equivalents.
To allow the standard model library to be augmented a number of component model development modules completed the package. Newly developed models could be combined with, or replace, those in the standard library. The total package provided a powerful simulation environment which was used extensively for research within the department and also commercially on a consultancy basis and as a software product in its own right.
Some of the research and consultancy topics on which BATHfp was used on include:

the design and validation of hydraulic systems prior to construction,
trouble-shooting of existing hydraulic systems,
the modelling of mechanical system on motor vehicles, for example suspension, transmission and power-steering systems,
the modelling of electrical components,
the detailed modelling of individual components such as axial piston pumps,
the thermal modelling of vehicle cooling systems
the modelling of the human respiratory system for diving, submarine escape and medical applications.

Although BATHfp was commercially successful for the centre of PTMC, the proliferation of similar simulation packages resulted in the decision to withdraw the software from sale. However, one of the goals of the centre is to promote the use of simulation in fluid power system design and analysis and it was decided that the simulation module from BATHfp could provide the basis of a software product for that purpose. The first such release was bathfreesim which was essentially a port of the UNIX simulation module to Linux. Anyone familiar with BATHfp would be familiar with bathfreesim, however, the aim was to provide software to the widest audience and it was decided to develop a product which could be installed on PCs running some version of Microsoft Windows®. That product is Pc_bathfp.
The rationale behind this initial release of Pc_bathfp is to provide a simple tool for the fluid power community which will allow users to become acquainted with simulation but which is underpinned by the same powerful integration and model codes of the parent software so that ‘real’ simulation can be performed. In addition the software should be provided free as a download from the centre’s website. The software is intended for a range of users:

to users new to simulation,
to occasional users of simulation who cannot, or who do not wish to, invest in costly commercial software,
to application engineers wishing to demonstrate fluid power designs to clients or others,
to course tutors teaching about fluid power systems.

The software consumes few computer system resources and has been run on a range of PCs from low-powered machines running Windows® 95 to modern machines running Windows® XP. The license, which the user accepts as a precondition for downloading the software, has few restrictive conditions other than that the user may not distribute or sell the software as a product in its own right.

Features of Pc_bathfp

The Pc_bathfp software is a native Microsoft Windows® program and as such it possesses the basic features typical of those applications, for example menus, toolbars, tooltips, etc. It is also a multiple-document program which enables whole or part circuits to be copied from one document and paste into another document within the program. The main application window is shown in Fig. 1.

Fig. 1: Components are selected from an extensive library of fluid power components; examples here are pumps, directional valves, control system elements and miscellaneous items

The toolbar, down the left-hand side of the application, provides access to a variety of components from the standard model library. These categories of component provided are:

pumps,
motors,
pressure control valves,
flow control valves,
actuators,
tanks and reservoirs,
directional and proportional valves,
prime movers,
loads,
filters and reservoirs,
miscellaneous components such as pressure and flow sources.

In addition the package includes a small range of pneumatic components. Figure 2 shows a selection of the available components.

Fig. 2: Pc_bathfp allows the creation of the circuit to be evaluated using familiar component symbols

Components selected from the library are positioned in the drawing area and either snapped to other components or connected by signals or lines. A signal is the equivalent of an electrical wire and simply transfers data from one connected component to the next. Lines are the equivalent of hydraulic or pneumatic pipes and within Pc_bathfp they have dynamic properties. A number of pipe models are available and the user needs to select the desired type depending on their requirements. Two approaches are possible:

the simplest approach is to apply a ‘multi-line’ model to a group of pipes. This models a dynamic, frictionless, constant volume, compressible pipe and can accept flow from up to 20 component ports. A model of cavitation and air release is employed but pipe friction and fluid inertia effects are not taken into account,
the more complex approach is to model each line separately and a variety of models are available for this. The simplest models are constant volume models, similar to the multi-line model, but including pipe friction. In addition there are more complex types which include wave dynamics.

All components and lines have reasonable default parameters but it is usual to change some of these some of these to alter and analyse the effects on the system under investigation. Figure 3 shows typical property pages for a component and line model.

Fig. 3: Parameter property pages for a component and line models

Having modified the required component and/or line model parameters the user is almost ready to run the simulation. However, Pc_bathfp also has one other parameter which can be modified and which can have a significant effect on the behaviour of the system. That is the choice of hydraulic fluid. Figure 4 shows the dialog for selecting a fluid other than the default, and there is also the option to manually adjust the characteristics of the fluid should that be necessary.

Fig. 4: Dialog to allow an alternate fluid to be selected, the fluid properties to be manually modified or be reset to the default values

Although the drawn circuit, or more accurately the component and line mathematical models mapping onto that circuit, uniquely identifies the system under investigation, changing any of the available parameters will modify the operation of that system. Consequently, at simulation run-time, Pc_bathfp gives the user the option to save a separate results file for each run, or to overwrite the current results file. A dialog is provided to select the result of a particular run so that the stored data can be visualised. A title and short description are automatically added to a results file to identify it a later time or date but there is the option to modify these defaults at simulation run-time.
Even quite simple drawn circuits can be mathematically complex and require considerable computing power to solve, with a consequently long run time. To ensure that the user can easily control the simulation it is run in a separate ‘thread’ from the main user interface. The operating system then alternates a share of computing power between the two program threads. As the computation progresses a progress window is updated to allow the user to monitor the simulation. However, the user can also select output variables from components in the circuit and display a small graph for each called a ‘thumb-plot’. This allows the simulation to be monitored from a dynamic, data-driven viewpoint, especially as the thumb-plots are automatically updated as the simulation progresses. Figure 5 shows an example of this type of plot.

Fig. 5: A simple thumb-plot graph used to monitor a data value generated by the simulation
In addition to the thumb-plots a larger graph is available which allows one variable to be plotted against another or for up to six variables to be plotted against a time axis, with the option of separate axes for each data set. Optionally these graphs can also be automatically updated on simulations with long run-times. Figure 6 shows an example of this type of graph.

Fig. 6: A multi-axis graph used to display data values generated by the simulation
These graphs also have a popup menu which allows:

graph elements to be selected,
the graph to be zoomed and panned,
labels to be added,
cross-hairs to be selected so that data values can be read from anywhere on the graph,
printing of the graph.

Also this menu accesses property pages for the graph which allow all aspects of the graph to be customised:

chart properties - background colour etc.,
axes properties - colour, font size, font weight, etc.,
curve properties - colour, shading, symbols, etc.

Another feature worth mentioning is the online help which provides articles on using simulation, describes the user interface and provides example tutorials. However, it also includes the ‘Model Reference Guide’ which describes the mathematical basis for the various component models supplied with the software and provides a useful reference source on modelling using this approach.

Summary

In summary, Pc_bathfp is a simple entry level simulation tool based on a powerful underlying computational engine. The goal of this release is to introduce simulation to the fluid power community and this is emphasised by the free distribution of the software. However, the Pc_bathfp project is very much a ‘work in progress’ and the intention is to gradually add features so that it will ultimately rival or surpass the power of its parent software.

References

Hindmarsh, A. C. 1983. ODEPACK, A Systematized Collection of ODE Solvers. Stepleman, R. W. et al [ed.] Scientific Computing, North-Holland, Amsterdam, pp. 55–64.
Petzold, L. R. 1983. Automatic Selection of Methods for Solving Stiff and Nonstiff Systems of Ordinary Differential Equations. Siam J. Sci. Stat. Comput. 4, pp. 136–148.
Richards, C. W., Tilley, D. G., Tomlinson, S. P. and Burrows, C. R. 1990. Bathfp - A Second Generation Package for Fluid Power Systems. BHRA 9th International Symposium on Fluid Power, Cambridge, UK.

Vendor	Centre for PTMC (Power Transmission & Motion Control), University of Bath
Contact Person:	Colin Brain
Address	Department of Mechanical Engineering Bath BA2 7AY, UK
Phone	+44 1225 385960
Fax	+44 1225 389268
Email	C.Brain@bath.ac.uk
Internet	http://www.bath.ac.uk/mech-eng/ptmc
Platforms	Win9X, WinNT, Win2000, WinXP

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