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
Power-Split Drives (PSD) are known for more than four decades. As the
name suggests, there are two parallel paths available for power flow
from the primary energy source (usually combustion engine) to the
wheels, mechanical and a continuously variable path. There are many
diverse technologies to realize the continuously variable path; for
example, mechanical, electrical, hydrodynamic and hydrostatic. PSDD
models a combination of planetary gear and hydrostatic transmission
(Fig. 1 shows output coupled PSD) that achieves the dual objective of
highly efficient mode of power transfer (mechanical path) and
continuously variable speed control (hydrostatic path).
Fig. 1: An output coupled power-split transmission
Apart from these advantages from system design point of view, PSD also
offers the possibility of optimal engine management since engine is
decoupled from the wheels.
Selection of the most suitable PSD configuration for a particular
application and the sizing of the com-ponents is an iterative process.
It requires a simulation-based tool that achieves the right balance
between complexity and accuracy. PSDD is such a Matlab-Simulink based
software that can assist the design engineer select the optimal system
structure and components sizing. It allows incorporating the precise
loss models of the hydraulic units in their entire operating using a
program called POLYMOD. POLYMOD uses pure mathematical approximation of
the measured steady state loss characteristics of the displacement
machines. Apart from the steady state predictions, the dynamic nature
of the system modelling makes the PSDD suitable for the controller
development for the PSD. 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
PSDD describes the torque and speed of each
component of the model in response to the vehicle trying to follow a
given drive cycle. The drive cycle specifies the reference vehicle
speed under the external load condition as dictated by the rolling
friction, air drag, vehicle inertia and road grade etc. Depending on
the control strategy at the supervisory (vehicle) level and at the
local (component) level, PSDD parameters, e.g. fuel consumption,
power-split ratio, amount of brake energy captured, may be different
for the same drive cycle. Hence, PSDD is an effective tool to compare
the performance of various control strategies, either against each
other or against a benchmark solution, based on certain parameters of
interest. At the system level, various layouts of the power-split
transmission can also be compared before the designer selects the most
appropriate one for the given application. Once the suitable
configuration has been selected, PSDD-aided components sizing is the
next step. The effects of components sizes and loss characteristics of
the hydraulic units can be seen in the simulation results for the
per-formance and fuel economy. Other parameters of interest such as
units’ speeds and displacements and system pressure can also be
observed which may help the designer further in the design process.
PSDD is modular in structure. Engine, gears, hydraulic units and wheels
are modelled as blocks in Simulink. It makes it possible to realize
various configurations of PSD through different arrangements of
components. To facilitate the process of building these models, there
are component libraries in the Simulink tree browser. Appropriate
blocks can be dragged and dropped in the model file. New model blocks
can also be added in these libraries. Besides libraries, an evaluation
module serves for the calculation of indirect drive parameters (power,
efficiency) from operating parameters obtained during simulation and
the graphical representation of results. For further analysis, this
evaluation module contains blocks for mechanical power, hydraulic power
and available and recovered braking energy.
3. Input Parameters
PSDD requires the precise steady-state
model of all the components. Most importantly the complex loss
behaviour of displacement machines must be described with sufficient
precision within the simulation model. The displacement machines in
power-split transmissions usually run in an extremely wide range of
operating parameters, i.e. speed, pressure and displacement volume vary
between zero and its maximum value. Therefore the loss behaviour and
its dependency on changing operating parameters must be considered in
an appropriate way. POLYMOD loss models achieve very high accuracy and
are independent of the type of displacement machine.
Hydraulic units’ sizes and hydraulic oil properties also inputs.
Engine torque and speed characteristics and fuel map are other
important inputs needed if fuel con-sumption is a parameter of interest
or a fuel saving control strategy is one of the design objectives. PSDD
also requires the various vehicle dynamics inputs for rolling friction
coefficient, air drag coefficients, road grade, tire radius and vehicle
mass for an accurate description of the wheel model.
All the mechanical gears require the gear ratios, expressed in terms of
number of teeth on each gear as well as the efficiency of power
transfer. Inertia of all the mechanical components, including the
engine shaft are also inputs to the Simulink model.
All the inputs parameters can easily be entered into the user-interface
windows by double clicking the com-ponent model mask. Component masks
contain the appropriate description of all the inputs needed to
exe-cute that block successfully. Example of one such mask for a gear
is shown in Fig. 2.
Fig. 2: User interface mask for entering input parameters
4. PSDD libraries
Given below is the list of libraries available in Simulink.
Following the list, there is brief description of each library.
• Evolution module
• Gear sets
• Hydraulic components
• Hydrostatic transmission
• Planetary gear sets
• Power sources
• Power consumers
• Brakes and clutches
The evaluation module is used for the calculation of indirect drive
parameters like power and efficiency. This calculation is based on the
operating parameters obtained during the simulation. The graphical
representation of the results is automatically calculated. Once the
simulation is finished, a command bar offers the possibility to show
the graphical representation of the results for the different
components of the system as well as for the entire transmission.
The gear sets library offers the models for 2 unit negative/positive
gear and also a 3 unit gear with a neutral middle gear and 3 unit gear
with 3 active shafts.
The hydraulic components library contains the five basic building blocks:
− A generic variable displacement machine (PU)
− A user defined loss model (Loss_XXX)
− A generic charge pump model (CHP)
− The simplified models of the relief valves for the low pressure and high pressure lines (LP & HP)
− The lumped parameter model of hydraulic pipes and hoses (Vol)
Fig. 3 has a graphical representation of these models.
Double-click the images to
enlarge them
and click once to make them thumbnail size again.
Fig. 3: Hydraulic components library
Hydrostatic transmission (HT) library contains three different blocks
of hydrostatic transmissions. One consists of two displacement machines
in parallel ar-rangement and two consist of three displacement
ma-chines in parallel arrangement. Simple HT with one primary (pump)
and one secondary (motor) unit is shown in Fig. 4.
Fig. 4: Hydrostatic transmission with single motor
Planetary gear sets library consists
of six blocks of different planetary gears (Fig. 5). The user can
define the gear ratios by changing the number of teeth in the mask.
User can also select among the different sets of inputs and outputs
available for each block. These inputs and outputs are the torque and
speed of each gear in the planetary gear set. For example, in Fig. 5
below, “k” stands for speed and “M” for the
torque (kC ≡ speed of gear C and MC ≡ torque on gear C)
Fig. 5: Planetary gears library
Power sources library consists of two blocks of combustion engines.
The first block represents a speed controlled combustion engine and the
second block represents a speed and torque controlled combustion
engine. Engine model requires the speed-torque characteristics of the
engine.
Power consumers library contains a wheel block to model the
vehicle’s longitudinal dynamics in response the torque applied by
the power-split transmission and the external load dictated by road
conditions and vehicle inertia. Usually the objective is to follow a
user defined drive cycle and the speed output of the wheel block
represents the actual vehicle velocity. This output is an important
feedback signal for the vehicle controller.
Brakes and clutches library contains a clutch model that either returns
the torque at the input shaft and the speed of the output shaft or the
speed of the input shaft and the torque at the output shaft.
5. Outputs of PSDD
Outputs from PSDD can be broadly divided into two categories:
• Direct simulation parameters
• Drive parameters computed indirectly
Speed and torque at the input and output shafts of all the components
as well as hydraulic units’ displacements and system pressure are
examples of the parameters that are direct results of running the
simulation. Flow rates in the high pressure and low pressure lines and
the leakage flows also belong to this category. These outputs provide
valuable insight at the component level in terms of how efficient each
component is and whether any of the physical constraints are being
violated.
Many system level parameters such as efficiency of the transmission,
percentage of braking energy captured (in the hybrid model), losses
incurred in the hydrostatic transmission and fuel consumed, for
example, are calculated based on the direct parameters. Results of
these indirect calculations are represented graphically by the
Evaluation model. These parameters provide valuable information about
the overall system and help the designer find an optimal system
structure and combination of components to achieve the highest possible
efficiency in the whole operating parameter range.
6. Summary and Further Information
The simulation program PSDD
was developed by the research team headed by Dr. Monika Ivantysynova.
It is a powerful tool for simulating the power-split transmission in
the MATLAB-Simulink environment. Simulation results generated by PSDD
are valuable for the designer in selecting the most suitable layout,
complete with component-level details, for a given application. Some
selected publications regarding the development and application of PSDD
are listed below.
Ivantysynova, M. 2000. Power-Split Drive Technology – Trends and Requirements. 2nd International Scientific Forum. Crakow, Poland.
Ivantysyn, J. and Ivantysysnova, M. 2001. Hydrostatic Pumps and Motors. Akademia Books International. New Dehli.
Ivantysynova, M. 2001. Energy Losses of Modern Displacement Machines – A New Approach of Modeling. Proceedings of Seventh Scandinavian In-ternational Confernece on fluid Power, Linköping, Sweden, pp. 377 – 395.
Kress, H. J 1968. Hydrostatic
Power-Splitting Transmissions for Wheeled Vehicles-Classification and
Theory of Operation, Society of Automotive Engineers, USA.
Mikeska, D. and Ivantysynova, M. 2002. Virtual Prototyping of Power Split Drives. Bath Workshop on Power Transmission and Motion Control PTMC 2002, Bath, UK, pp. 95 - 111.
Mikeska, D. 2002. A
Precise Steady State Model of the Displacement Machines for the
Application in Virtual Prototyping of Power Split Drives. Proceedings
of 2nd FPNI PhD Symposium 2002, Modena, Italy.
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