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.
CASPAR: A Multi-Domain Model for Predicting the
Performance and Losses of Swash Plate Type Axial Piston Machines
1. Introduction
In the past, the design of piston machines has relied on very
simplified models and the experience of the de-signer. Due to
the limitations associated with these traditional methods, achieving an
optimal pump/motor design was only possible at a single operating
condi-tion. Current technology, however, calls for machines
that have minimal losses over a broad range of operat-ing
conditions. To meet these demands in a cost effec-tive
manner, engineers in the field of pump/motor de-sign can no longer rely
on traditional design methods and require sophisticated models that
predict machine performance and losses across a broad range of
operat-ing conditions. The simulation model, CASPAR, is a
model for predicting the performance and losses of swash plate type
axial piston pumps and motors con-sidering specific machine geometry
and operating con-ditions. The model has been developed to
meet the need of the next generation of pump and motor re-searchers and
designers and represents a powerful de-sign tool for this kind of
displacement machine. The following review discusses some of
the theory and assumptions behind the model as well as its many
capabilities, options, and outputs.
Contact Information
2. Model Description
CASPAR describes the flow of a
compressible and viscous fluid from the ports through the valve plate
to the displacement chamber. It further considers the gap flow through
each of the three lubricating gaps that seal the displacement chamber.
The change of pressure in the displacement chamber resulting from the
basic working process of the displacement machine causes fluctuating
forces and moments leading to oscillating micro motion of moveable
parts of the rotating group which are considered by the model.
CASPAR is
a stand alone tool developed using the C++ programming language. Models
implemented and solved in CASPAR consider the time dependent change of
gap heights due to oscillating forces, the interaction between machine
parts, the dependency on design and operating parameters and the energy
dissipation within the gaps.
CASPAR
also considers the influence that surface deformation of parts forming
the gaps has on machine performance and behavior. The mathematical
descrip-tion of the fluid flow from the ports to the displacement
chamber and through the sealing and bearing gaps leads to a system of
partial and ordinary differential equations. A new numerical method
based on iterative coupling of separate solvers for fluid/solid domains
has been developed to solve this transient nonlinear system consisting
of the Reynolds equation and the energy equation for the fluid domain,
the equation of elasticity for the solid domain and the determination
of gap heights by solving the motion equation of the multi-body system
of the rotating group. The initial boundary conditions such as
instantaneous cylinder pressure are obtained by solving the fluid flow
from the displacement chamber to the ports.
CASPAR
models the gap flow as the situation where a balance of external and
fluid forces is present, i.e. full film lubrication. No mixed
friction model is therefore present. Additionally, the model
does not assume any surface roughness of the solid parts, assum-ing
rather ideally smooth sliding surfaces.
3. Input Parameters
CASPAR
requires several input files, most of which can be easily created using
a graphical user interface. The input files describe the
machine geometry and operating conditions, as well as any specific
options, such as the deformation model, that the user wishes to
consider. An example of the GUI for building the operating
condition input file (also known by its file extension, the bpd file)
is shown below.
Fig. 1: Illustration
of the GUI for building the operating condition input file
Using the
interface shown above in Fig. 1, the user can specify
either pumping or motoring mode as well as the operating condition of
the machine. Other parameters that can be specified in the
input
files are listed below.
• Hydraulic Oil properties
• Temperatures at the input and
output ports and in the fluid film
• General machine geometry
• Operating parameters
• Grid settings for all solid
parts
• Simulation settings (e.g.
number of revolutions, initial gap height values, etc)
Much
work has been done to develop a versatile model. CASPAR is
able to
consider the effect of precompression grooves, as well as
precompression filter volumes. Additionally, CASPAR
incorporates a
module for considering spherical valve plates.
Finally, CASPAR
includes the capability of considering macro-geometry alterations to
the solid parts. This allows for investigations into the
effect of
piston, slipper, and cylinder block shaping.
4. Using CASPAR
The program CASPAR is separated into two modules; the
pressure module and the gap flow module. After creating the
input files, which can be done using either the GUI or directly
creating appropriately formatted text files, the pressure simulation is
run. The instantaneous cylinder pressure, an output from the
pressure simulation, is then used as an input (i.e. boundary condition)
to the gap flow simulation. The leakage output from the gap
flow simulation can be input back into the pressure module for a second
iteration of the pressure simulation. This second iteration
of the pressure module allows for an accurate calculation of volumetric
losses.
Double-click the images to
enlarge them
and click once to make them thumbnail size again.
Fig. 2: User interface of CASPAR
The initial user interface, shown above in Fig. 2, allows the user to
choose between the pressure module simulation and the gap flow module
simulation. Also in this window, the user can either resume
an existing simulation or begin a new simulation.
Fig. 3: Pressure Module user interface
In the pressure module
user interface, shown above in Fig. 3, the user loads the necessary
input files and specifies the leakage option. Either constant
leakage is assumed, or leakage data from the gap flow simulation is
used. If the user possesses measurement data, this can be
input into the pressure simulation as well in place of constant leakage
or predicted leakage from the gap flow simulation.
Fig. 4: Gap Flow Module user interface
In the gap flow module user interface, shown above in Fig. 4,
the user loads the necessary input files. Here, the user may
also specify the gaps to be considered, any macro-geometry of the solid
parts, as well as pressure data. This data is either read
from the output of the pressure simulation, or from measurement
data. If no input is specified, an ideal pressure profile is
assumed. Also included in this interface is the option to
consider non-isothermal gap flow. This option requires a
special input file as well as the appropriate thermal module for
calculating the boundary conditions (i.e. surface temperature
distribution) of the energy equation.
5. Outputs of
CASPAR
Many outputs are generated by CASPAR for user
analysis. A summary list of the outputs from both the
pressure simulation and gap flow simulation are listed below, as well
as a brief discussion concerning visuali-zation of the output data.
5.1
Pressure Module Outputs
•
Instantaneous cylinder pressure
•
Effective instantaneous flow and real flow ripple at pump outlet or
motor inlet
•
External and internal volumetric losses, includ-ing their origin
5.2
Gap Flow Module Outputs
•
Gap heights between the valve plate and the cyl-inder block, between
piston and cylinder and be-tween slipper and swash plate
• Viscous
friction between piston and cylinder, valve plate and cylinder block
and slipper and swash plate as well as the resulting torque loss.
(torque
loss only considers viscous friction)
•
Forces and moments applied on the swash plate
•
Pressure, temperature and velocity fields for the above mentioned
lubricating gaps of a swash plate machine
5.3
Visualizing CASPAR Outputs
The model incorporates a viewer, CASPARView, which allows the user to
visualize many of the outputs of the model while the simulation is
running. Two examples of the viewer are shown below.
Fig. 5: User interface of CASPARView
Fig. 6: Sample output of CASPARView,
illustrating the Swash plate moment about the x-axis over one shaft
revolution of the cylinder block
To meet the specific
needs of the user, the raw output data may be post processed in a
secondary software of the user’s choice. Some of
the information returned by CASPAR, such as three dimensional gap
heights and pressure fields, is not displayed in CASPARView and must be
post processed in a secondary software, such as MatLab.
Examples of these outputs are shown below.
Fig. 7: Illustration of the gap height
between the piston cylinder interface unwrapped along its
circumfer-ence for one angular position of the cylinder block
Fig. 8: Illustration of the pressure
distribution in the cylinder block valve plate interface at one angular
position
6. Summary and Further
Information
The
simulation program CASPAR, which has been developed at the Institute
for Aircraft Systems Engi-neering in Hamburg, Germany, is based on a
non-isothermal gap flow model considering the change of gap heights due
to micro motion of parts and surface deformations for the coupled
lubricating gaps of swash plate axial piston machines. The program
calculates real flow ripples at both the inlet and outlet ports, the
instantaneous displacement chamber pressure, the in-ternal and external
volumetric losses, viscous friction forces, gap heights, and
oscillating forces and moments exerted on the swash plate.
The program is a powerful tool for predicting machine performance and
losses based on a given pump/motor design. Some selected
publications regarding the development and application of CASPAR are
listed below.
Huang, C.
and Ivantysynova, M.
2003. A new ap-proach to predict the load carrying ability of the gap
between valve plate and cylinder block. Bath Work-shop on Power
transmission and Motion Control PTMC 2003, Bath, UK, pp. 225 - 239.
Ivantysynova, M.,
Huang, C.
and Behr, R. 2005.
Measurements of elastohydro-dynamic pressure field in the gap between
piston and cylinder. Bath Workshop on Power Transmission and Motion
Con-trol PTMC 2005, Bath, UK, pp. 451 - 465.
Lasaar, R.
and Ivantysynova, M.
2004. An Investiga-tion into Micro- and Macrogeometric Design of
Pis-ton/Cylinder Assembly of Swash Plate Machines. International
Journal of Fluid Power, Volume 5, No. 1, pp 23-36.
Wieczorek, U.
and Ivantysynova, M. 2002.
Computer Aided Optimization of Bearing and Sealing Gaps in Hydrostatic
Machines – The Simulation Tool CAS-PAR. International Journal
of Fluid power, Volume 3, No. 1, pp. 7-20.
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