Paweł BACHMAN
APPLICATION OF VIRTUAL REALITY IN DESIGN
OF ELECTROHYDRAULIC SERVO DRIVES
AND THEIR CONTROLLERS
Keywords: design process, virtual reality, electrohydraulic servo drives.
Abstract
The article aims to get its readers familiar with virtual reality. It presents how useful it
may be while designing electrohydraulic servo drives control systems. In the beginning of
the article a definition of virtual reality and its brief history have been presented. There has
been also shown its usefulness in various fields, among others medicine, aviation,
mechanics, computer games and designing process. The following part of the article is a
description of chosen applications used in virtual reality creation. Basic vices and virtues of
VRML, Oxygen, CAD programs and fields in which they are most frequently applied have
been described. In the next part of the article it has been shown how to combine simulation
existing in Simulink with virtual reality created with Virtual Reality Toolbox. The rules of
constructing virtual world in a specially adapted editor attached to VR Toolbox-V-Realm
Builder 2.0 have also been described. The whole process of creating virtual reality is based
on a simulation of a model of electro-hydraulic servo-mechanism system altogether with a
proportional valve and with PID control system. The end of the article is a summary of
information concerning designing of electro-hydraulic servo drives with the use of virtual
reality.
1. INTRODUCTION
Virtual reality (from latin word virtualis – effective, virtus – perfection) is a
world created with the aid of computer technology, where a sample of natural
environment is reconstructed with greater or smaller simplification.
Virtual reality is used in the following fields:
– computer games: from simple 3-D type, to complicated systems where the
players are equipped with helmets and stereoscope spectacles, stereophonic
headphones and location sensors thanks to which a computer „knows” in
which direction is a player looking at in a given moment , and also special
devices: gloves, joysticks for communication with a computer and sensors
Paweł BACHMAN
fixed to a player’s body for telling the computer about the player’s moves
[1],
– movies production: particularly movies where the action takes place in not
existing worlds („Matrix” is a good example here),
– aviation: pilots training – flights simulators; navigation database allows to
do flights within optional airports and routes and a system of cylindrical
visualization, fixed to the ground, gives the cabinet staff the impression of
movement [2],
– medicine: training of surgeons (operations performed on virtual patients),
treatment of schizophrenia and different kinds of phobia,
– reconstructing of extinct, disappeared worlds and antiquities – in the Vasa
Museum in Stockholm one can see a virtual model of boat „Vasa” that
sunk over 350 years ago,
– designing products and production processes, e.g. visualization of the
process of machining (programs of MTS: Topturn and Topmil for
simulation of the process of turning and milling) [3].
As it is shown in the above examples virtual reality has quite a wide use and is
present in almost every field of life. In this article more pressure will be put on
applying virtual reality in a process of project designing with reference to
electrohydraulic servo drives.
2. APPLYING VIRTUAL REALITY IN DESIGNING PROCESS
Several years ago designing basically meant creating a prototype and
undergoing tests on it. At present, more and more often designing phase consists of
undergoing numerous tests for different parameters of the system on mathematical
models and selecting the best solution. This is the best solution which is launched
on the market. While constructing a model, we have a possibility to examine how a
mechanism works without building it which considerably reduces the costs.
During project design virtual reality, also called artificial reality, is used for
simulating the sense of sight and provoking the effect of 3D vision and showing the
move of the object. Additionally, all these might be combined with audio effects
that boost the sense of reality.
There are many applications of virtual reality. Among them one can mention
VRML, Oxygen, Cad described below and others, e.g. 3D Dreams by Shells,
Cult3D (Cycore Computers), Depthcharge (Vrex), Meta Stream (Inlet), Open
Space (Virtus) or Quick Time VR (Apple). Work with all the applications consists
of similar stages:
1. Creating of three-dimensional model in virtual reality.
2. Attributing movement to suitable elements.
Scientists have been interested in development of virtual product designing for
several years. The result is creating the project IVIP [4] (Integrated Virtual Product
Application of virtual reality in design of electrohydraulic
servo drives and their controllers
Creation in 1998 where virtual reality is used in simulation of product behaviour.
The partners of that project are BMW, BOSCH, Dimler Chrysler, Porsche,
Siemens, and many others (51 firm’s altogether). Problems of virtual design were
also discussed in scientific project EU-Mech [5], which aimed at working out
integrated engineering environment supported by a computer for the development
and design of mechatronic products and MechaSTEP [6] aimed at working out
digital models for simulation of mechatronic systems.
3. REVIEW OF CHOSEN APPLICATIONS FOR CREATION
OF VIRTUAL REALITY
One of the programs used most often for describing virtual reality is VRML
(Virtual Reality Modelling Language). It allows to define three-dimensional
objects, their location in three-dimensional area, their interactions, and also their
reaction to particular events that are the result of human being performance in such
defined world. Thanks to it, we are able to create interactive virtual worlds where
we have a possibility to walk among defined objects in a real world, to meet other
participants of the world and to define any amount of interactive sensors that
change the look of the world according to its participant’s wish. With the aid of the
system of references and using the mechanism similar to that which is present in
HTML we can define connections between elements of virtual reality described in
VRML document, altogether with other documents VRML, HTML or other types
[7]. Many examples of virtual worlds created with the aid of the programming
language can be found in Internet. To watch them, one must have installed plug-in
co-operating with Internet browser, e.g. Cosmo Player, World View, FreeWrl or
Casus Presenter.
Oxygen is the next program worth mentioning. It distinguishes itself with a
perfect visualization of objects presented on a monitor screen. Oxygen may be
particularly useful for such tasks as presenting products which still have no
prototypes, or for training installation and maintenance of not existing yet devices.
With the use of Oxygen medical simulators for training surgery (Clarus Virtual
Shoulder Arthroscopy) have also been built. Simulator of a new model of Volvo
and military simulators (tank Leopard 2 for Swedish Army) have also been created
in this program. Virtual reality created with the program Oxygen and with the use
of sensing gloves and virtual helmets has been used in BMW mainly for examining
assembly matters. New conception of packing have been simulated for Tetra Pak.
Digital eyehole made it possible to go inside the packing machine and observe its
work in place that can not be seen in reality. Oxygen operates on Silicon Graphics
computers under IRIX operating system which is a variety of UNIX system [8].
The last program that is going to be described widely in this article is CAD
system. The system, particularly 3D-CAD systems (forms design) is used in virtual
Paweł BACHMAN
creation of a product in its early stage of preliminary design. Except geometric
representation of the product and its structure 3D-CAD systems also contain other
product data, such as: material and the way of processing. CAD systems often
communicate with each other with the DMU (Data Management Unit) systems.
They use product structure and approximate geometric representation data
transforming them into kinematic data. It allows to make analysis of kinematic
product behaviour as well as analyze instalment and uninstalment processes.
Virtual prototypes include physical data of products taking into account their use in
specific fields such as mechanics, electronics as well as logistics.
The programs described above are only exemplary applications used for virtual
reality creation. They are so comprehensive that it is possible to use them in
different domains. There are also programs concentrated on creating virtual reality
in strictly specified field, e.g.:
– ADAMS – allows to perform complicated kinematic and dynamic analysis
of the objects moving in real time. It consists of many packages, among
others Rail allowing fast and easy parametric building of rail vehicles.
– Medical Multimedia Systems company have worked out the programs
showing how human body works: Brainiac – Interactive Neuroanatomy
Atlas – human brain work, Visible Human Atlas – virtual atlas presenting
the inside of the human body; SimBioSys Simulators – simulating human
body performance,
– Prosolvia Research & Technology uses software LS-DYNA3D in
combination with CAD Real-Time Link for simulating car crashes.
– CAE Electronics Ltd. have built software for simulating atoming power
stations and carbon power stations. It is also the author of professional
system of civil flights simulators, Flight Training Devices, Visual Systems
Simulators, Computer Based Trainings and Computer Assisted Training
Systems.
4. DESIGNING OF ELECTROHYDRAULIC SERVO DRIVES WITH
THE USE OF VIRTUAL REALITY THANKS TO SIMULINK
VIRTUAL REALITY TOOLBOX
Mathworks company following designers needs have also worked out a tool for
virtual reality creation –Virtual Reality Toolbox. Designing with the use of virtual
reality in Matlab – Simulink is based on mathematical models created before. In
the article electrohydraulic servo drives model (third degree) with proportional
valve simulated with the help of inertialess element have been taken advantage of
[11, 12]. The system is characteristic of transmittance described with a formula 1.
Application of virtual reality in design of electrohydraulic
servo drives and their controllers
G( s) 
k s s2
y( s)

x( s) s( s 2  2 s s s   s2 )
(1)
where: ks– reinforcement ratio, ωs– undamped natural frequency, ζs– damping
ratio.
The above quantities can be specified on the grounds of servo-motor producers’
catalogue data and they depend on servo-motor dimensions, pressure and type of
working liquid, friction, system stiffness and mass shifted. Taking these parameters
into consideration and using a formula it is possible to design a servo-motor and
create in Simulink a corresponding model.
The model already existing is connected to virtual world thanks to special
elements which are in Virtual Reality Toolbox: VR Signal Expander and VR Sink.
VR Signal Expander converts simulation signal into a signal compatible with a
VRML language. In case of simulation described in the article it is a shifting of a
servo-motor piston. VR Sink, on the other hand, includes graphic interface of the
user combining simulation with virtual reality. In it, it is defined what type of move
must be performed, file access path is given, and editor and browser VR are
evoked.
Fig. 1. View of editor V-Realm Builder 2.0
Paweł BACHMAN
Virtual world is designed in a specially adapted editor V-Realm Builder 2.0
(fig. 1). The editor uses VRLM environment. A model of a simulated mechanism is
built of three-dimensional solids. There is also a possibility of importing a ready
model made in other editor or Cad program. The program includes a 3D library of
objects, however there are no hydraulic elements. Particular parts can be given
dimensions corresponding to real mechanisms dimensions, of course suitably
diminished. Three-dimensional model used during simulation has been build
according to a real servo-motor of Walmet firm.
While building a model some simplification has been made focusing on the
most essential elements of a servo-motor. The elements are presented in
fig. 2.
Cylinder
Joint
bearing
Lug
Piston rod
Piston
Piston seal
Fig. 2. Components and a view of the internal structure of a servo-motor
Possibility of viewing created object in four projections is considerably helpful
while building a three-dimensional model. The model can be optionally illuminated
and particular elements may be given different surface quality and colours. Then,
the move of particular elements is added. It may be a linear slip, rotation or object
distortion. Whole virtual world created with the aid of V-Realm Builder is stored in
one file with *.wrl. expansion Virtual world made in V-Realm Builder can be
accessible for other computers in the net. In that case Matlab-Simulink package
Application of virtual reality in design of electrohydraulic
servo drives and their controllers
must be installed on Windows NT server and domain server must be inscribed into
Internet browser of a computer-client and port number, where virtual reality z has
been made accessible. One of the plug-ins, mentioned in point 3, or joined to
Matlab plug-in Blaxxun must be installed on computer-client for viewing virtual
reality. It is possible to observe the move of the objects in virtual world only during
started simulation thanks to a browser Virtual Reality Toolbox Viewer (fig. 3)
included into a package.
y
3.1806
To Workspace
Kp
0.0379
Step
Ki
1
s
Integrator1
0.0083
du/dt
Kd
Derivative
-KSum
KQ=12
1
s
1
s
Integrator5
1
s
Scope
Integrator2 Integrator3
40
ruchome.translation
D=0.2
VR
10000
VR Signal Expander
VR Sink
KQ=1
w=100
40
Fig. 3. View of a Virtual Reality Toolbox Viewer browser altogether with electrohydraulic
servo drives model with PID controller build in Simulink.
In the example presented the move of servo-motor piston is done according to
servo-motor data taken from a model created in Simulink program. The signal of
piston location is converted into a format suitable for a VRML language.
Additionally, PID controller is included into a system. Within this simulation
setting of regulator may be changed applying the method of Ziegler – Nichols
selection, and then applying different piston locations one can observe how is a
move performed and a piston rod positioned.
Paweł BACHMAN
5. CONCLUSION
The examples described in the article show that virtual reality may be
successfully applied in the process of designing and testing all sorts of products. A
number of different programs and easy access to all sorts of information in this
field in Internet prove popularity of virtual reality applications. There are no
obstacles to apply them in designing steering systems of electrohydraulic drives. Of
the example presented in the article one can see that applying virtual reality in the
process of designing steering system of electrohydraulic servo drives is a simple
and clear way of visualization of the whole process.
In the future they will come into effect activities aiming at creating database of
three-dimensional models of hydraulic elements (valve, servo drives), which might
be useful for designers in the process of designing or for teachers as didactic help.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
http://www.open-mind.pl/Ideas/WirtualnaR.htm
http://www.prz.rzeszow.pl/pl/aktualnosci/index.php?page=symul
http://www.mts-cnc.com/
http://www.ivip.de/
http://wwwhni.upb.de/rip/projekte/EUMECH/
http://www.dik.maschinenbau.tu-darmstadt.de/Kompetenzen/Forschung/
Projekte/projekt_html?project=19970930-100016
[7] http://www.enter.pl/vrml/
[8] http://www.cadcamforum.pl/11'98/oxygen.htm
[9] http://www.rtcn.org.pl/1340.xml
[10]Ihttp://www.engin.umd.umich.edu/~yilu/ECE577_PROJECTS/portell/applicati
ons/applications.htm
[11] Milecki A., Wybrane metody poprawy właściwości liniowych serwonapędów
elektrohydraulicznych, Politechnika Poznańska 1999.
[12] Pizoń A., Elektrohydrauliczne analogowe i cyfrowe układy automatyki, WNT 1995.
Author:
mgr Paweł BACHMAN
Uniwersytet Zielonogórski, Instytut Edukacji Techniczno –Informatycznej
65-077 Zielona Góra, Al. Wojska Polskiego 69
e-mail: P.Bachman@eti.uz.zgora.pl