Object-oriented virtual environment for visualization of
Contents
1. User Interface Objects Support Objects Geometric Entities Finite Elements Container Nodal Coordinates Boundary Rep Solid Truss Beam Shell Observer Virtual Material Box Cone Sphere Controllers Actuators 3 D Selection Physical Property Surface 2 D Selection Physical Material i p Pointer Line oly line Hinge Joint Light ala Ss Ato Cirle Prismatic Joint Graphical Sens point Prescribed Motion Objects FE Data Constraints B tton Variable Check Box Fou Label Text Box 2 D Graph 3 D Graph Picture Dial Table Color Key Fig 3 The four types of objects Typically three computers are used a display computer a support computer for sound input output and a support computer for tracking and navigation Fig 1 The display computer is either an SGI Onyx station with an Infinite Reality 3 multi pipe rendering engine or a Pentium III PC with an open GL accelerated graphics card The sound support computer is a Pentium II PC with a stereo digital sound card The tracking and navigation support computer is also a Pentium III PC with interface boards for the track ing system and optional haptic feedback gloves Communi cation between the three computers is achieved by using a standard Ethernet connection 3 Features of the obje2. 6 7 0 8 Hz ee ea 4 6 Hz ke E Pa T 4H g lt moot i D J X S24 E A l fi fs zit VLN J UN say SA V Vi WO edee ee seh pies 0 H d 2 4 6 8 10 Frequency Hz Freq 0 8 Hz Fig 9 The total power spectrum plot for the NGST and two typical mode shapes shaded using the total vibration amplitude 6 2 NGST model element model is shown in Fig 6b Fig 6c shows the finite element model with the finite elements exploded in order to Fig 6 shows a rendered model of the NGST The model is delineate the individual elements Table 5 lists the main rendered by using components of the NGST Each component is a container These components include sub components which are also e light sources containers The components of the NGST are connected via e ambient specular and diffuse material colors revolute joints prismatic joints and latches which are also e image textures mapped on the components surfaces placed inside containers The observer object enables the use of the wand to exam A VRML geometric model is shown in Fig 6a The finite ine the NGST model from any angle and scale in the VE 310 T M Wasfy A K Noor Advances in Engineering Software 32 2001 295 315 Finite Element Model Reaction 7 wheels 5 Revolute joints a Attitude control system pyram
3. L 30 A 4 420 on ET A A HAS aie 4 ay Ps tte es ie ue a ae HS eu aa an e NO n D A E 1 Dmr a ic A L a oN Wey Ps a oR o n np e Step I The sunshield angle is adjusted from time 0 to 314 T M Wasfy A K Noor Advances in Engineering Software 32 2001 295 315 time 40s Two revolute joints along with two linear actuators are used to perform this motion e Step 2 The isolation truss is deployed from time 40 to time 70 s A revolute joint along with a rotary actuator at the mid point of each of the isolation truss members are used to deploy the isolation truss e Step 3 The primary mirror is deployed from time 70 to time 100 s Each primary mirror petal is attached to the main structure using two revolute joints A rotary actua tor located at the center of the petal axis is used to deploy the petal e Step 4 The secondary mirror is deployed from time 100 to time 120 s The secondary mirror is mounted on eight prismatic joints A linear actuator is used to deploy the secondary mirror PD tracking controllers are used to control the linear and rotary deployment actuators A constant acceleration decel eration trajectory for the motion of all deployable compo nents is selected with a zero dwell time Fig 13 shows snapshots of the deployment motion of NGST rendered using light sources Fig 14 shows the user s view of the VE during deployment of the NGST The figure shows a snapshot of the
4. Advances in Engineering Software 32 2001 295 315 ADVANCES IN ENGINEERIN SOFTWARE www elsevier com locate advengsoft Object oriented virtual environment for visualization of flexible multibody systems T M Wasfy A K Noor Center for Advanced Computational Technology Mail Stop 201 NASA Langley Research Center University of Virginia Hampton VA 23681 USA Accepted 4 August 2000 Abstract An object oriented event driven virtual environment VE for viewing the simulation results of flexible multibody systems FMS is developed The VE interfaces with the following output devices immersive stereoscopic screen s and stereo speakers and a variety of input devices including head tracker wand joystick mouse microphone and keyboard The VE incorporates the following types of primitive software objects user interface objects support objects geometric entities and finite elements Each object encapsulates a set of properties methods and events that define its behavior appearance and functions A container object allows grouping many objects into one object which inherits the properties of its children objects The VE allows real time viewing and fly through of photo realistic models vibrational mode shapes and animation of the dynamic motion of FMS An application of this VE is presented for visualization of the dynamic analysis results of a large deployable space structure NASA s Next Ge
5. 98 Halifax Nova Scotia Canada May 1998 14 Kuschfeldt S Holzner M Sommer O Ertl T Efficient visualization of crash worthiness simulations IEEE Computer Graphics and Applica tions 1998 18 4 60 5 15 Bryson S Virtual reality in scientific visualization Communications of the ACM 1996 39 5 62 71 16 Bryson S Johan S Schlecht L An extensible interactive visualization framework for the virtual windtunnel Proceeding of IEEE VRAIS 97 1997 17 Wesche G Three dimensional visualization of fluid dynamics on the 18 19 20 21 22 23 24 25 26 27 28 29 T M Wasfy A K Noor Advances in Engineering Software 32 2001 295 315 315 responsive workbench Future Generation Computer Systems 1999 15 4 469 75 Halme A Suomela J Savela M Applying telepresence and augmen ted reality to teleoperate field robots Robotics and Autonomous Systems 1999 26 2 3 117 25 Gao S Wan H Peng Q An approach to solid modeling in a semi immersive virtual Computers and Graphics 2000 24 191 202 Ferretti G Filippi S Maffezzoni C Magnani G Rocco P Modular dynamic virtual reality modeling of robotic systems IEEE Robotics and Automation Magazine 1999 Dec 13 23 Deering M The HoloSketch VR sketching system Communications of the ACM 1996 39 5 54 61 Brown J Enabling educational collaboration Computers and Graphics 2000 24 289 92 Foley J van Dam A Feiner S Hughes J Computer graphic
6. NGST along with time history plots of the trajectory for some of the deployment joints and the user s menu Fig 15 shows a snapshot of the NGST during deploy ment shaded using strain energy density 7 Concluding remarks An object oriented event driven VE for viewing the simulation results of FMS is developed The VE incorpo rates the following types of primitive objects UI objects support objects geometric entities and finite elements Each object encapsulates a set of properties methods and events which completely define its behavior appearance and function A container object allows grouping of many objects into one object and this object inherits the properties of its children objects The VE interfaces with the VR facilities human input and output devices The output facil ities include stereoscopic screen s and stereo speakers The input devices include head tracker wand joystick mouse microphone and keyboard The VE allows real time view ing of photo realistic models pre computed mode shapes and pre computed dynamic motion of FMS The application of the VE to the visualization of the dynamic response of a large deployable space structure the NGST is described A detailed finite element model for the NGST is constructed Each component of the model is a container object which includes finite elements trusses beams shells and solids geometric entities as well as other containers The vibrational respo
7. VEs Among the toolkits that enable the devel opment of custom VE applications are SGI s Inventor 25 SGI s IRIS Performer 2 0 26 WorldToolKit WKT from Sense8 27 MR Toolkit 28 wuse SDK 29 Karma VI 11 for GIS visualization and Lego Toolkit 30 These toolkits consist of a collection of C C functions and classes for interfacing with the various hardware compo nents and navigation in the VE The Virtual Reality Model ing Language 2 0 VRML 2 0 31 is a file format specification for scene graph description of VEs on the internet VRML includes a set of primitive geometry group ing sensor interpolator texture map and lights objects that allow construction of dynamic interactive multimedia VEs 1 4 Application of VEs to flexible multibody systems A flexible multibody system FMS is a group of interconnected rigid and deformable solid bodies or T M Wasfy A K Noor Advances in Engineering Software 32 2001 295 315 297 Output Devices Speakers l Stereoscopic Display Glasses with Stereo Receiver LCD Shuttered Stereo Emitter Support Computer Display Computer Support Computer Ethernet Port Ethernet Port Ethernet Port Microphone Keyboard Input Devices Mouse Head Touch Pad Tracking Receiver Wand Gloves Wand Tr
8. for the multibody system appli cation An IVRESS script file consists of subroutines and objects Each subroutine has a name a list of input output parameters and a main body of script commands The script is interpreted and executed one command at a time All objects have the same basic structure Each object defined in the script file has a name and may be followed by a list of properties and property values Property values that are not explicitly defined are set to a default value Also each object is automatically assigned a unique reference number The object can be referenced either by its name or reference number Each object has properties that 300 Table 1 UI objects T M Wasfy A K Noor Advances in Engineering Software 32 2001 295 315 UI object Description Container Observer 3D selection tool Direction vector 2D selection tool Light Selection The container is a special type of UI object that is used to group children objects Children objects are displayed using the homogeneous 4 X 4 geometric transformation matrix of the parent container Other UI objects including other containers geometric entities and finite elements can be children objects Each multibody system component is represented by a container that holds geometric entities for representing the geometry of the component and finite elements for representing the numerical model of the component A sha
9. Exit Graphs Top 2 EEE oO O Shading Key 2 O Pause Model Front Explode Elms O Back Elms Edges 0 Primary Mirror ji j ii o Isolation Truss fom ome En et b O Shading Key 3 O1 O4 o8 Wand Control O Model View O Model Select O Slider for Speed Control O Menu Select W Fig 7 a User s Menu and b Menu tree the deployment actuators are used to adjust the position of the space support module isolation truss primary mirror and secondary mirror The present VE is used for the visualization of the geometric and FE models of the NGST as well as the afore mentioned dynamic simulation results A menu system is designed using the UI objects in order to allow changing the viewing parameters inside the VE 6 1 Menu The menu allows changing the viewing parameters settings T M Wasfy A K Noor Advances in Engineering Software 32 2001 295 315 307 Table 5 Main components of the NGST structure Module Sub module Description Model Optical OTA reaction The reaction structure is a shell structure that telescope structure represents the main frame of the OTA assembly OTA Secondary mirror conical mast e Center rings Primary mirror The primary mirror is composed of eight petals Each petal is composed of two layers 4 cm apart Each layer is 2 mm thick Beryllium shell Each petal is connected to the O
10. TA supporting structure through revolute joints Rotary actuators along with PD controllers located at each revolute joint are used to deploy the primary mirror petals In the deployed configuration the petals are latched together and to the supporting structure Secondary mirror The secondary mirror is connected to the conical mast through longerons which are mounted on prismatic joints A linear actuator along with a PD controller allows deployment of the secondary mirror Once the secondary mirror is deployed latches are engaged to hold the longerons in the final deployed position Integrated This module comprises the science instruments of science the NGST along with its supporting structure instrument ASIM Space support Support module This module contains all the power controls module SSM including attitude control and communication systems of the NGST It contains the reaction wheels and the thrusters Also the sunshield is mounted on this module Attitude Control The ACS is inside the SSM Attitude control is System achieved using four reaction wheels mounted in a pyramid configuration The inertia of each wheel is 0 0948 kg m The maximum angular velocity of a wheel is 100 rev s Inflatable A simplified beam type model of the inflatable sunshield sunshield is used 308 T M Wasfy A K Noor Advances in Engineering Software 32 2001 295 315 Table 5 continued Module Sub module Description Isol
11. VE e Recognizing the actions of the user such as touching or clicking an object Many VEs have included standard graphical user interface objects such as buttons dials menus and checkboxes 3 e Generating secondary motion effects where a user s motion and actions in the VE generate motion of other objects in the environment This also includes computa tional steering where the user can change the parameters of the model in the VE and watch the simulation respond to that change 5 2 6 In Ref 7 lumped masses springs a simplified fluid dynamics model and collision detec tion were used to model leaves clothing a flexible floor mat and stepping in a puddle In that reference two way one way and hybrid coupling were used between the primary motion and the secondary motion objects As expected two way coupling requires much more compu tational power than the one way or hybrid coupling approaches For practical real life systems computational steering was only achieved when the model was grossly simplified For example in Ref 8 a virtual molecular simulation system with computational steering was developed Real time simulation was obtained for a model of up to 450 atoms They conclude that a massively parallel computer is required for the simula tion of 10 000 atoms Today s computers do not have enough speed to simulate in real time the dynamics of 104 10 DOF systems which is the size of practical models e Ste
12. aced in MBS 302 Table 3 Geometric entities T M Wasfy A K Noor Advances in Engineering Software 32 2001 295 315 Table 4 Finite elements Geometric entity Description Finite element Description Solid brick Shell brick Eight noded solid brick element Eight noded shell element The top shell surface is Surfaces Elevation Curves Polyline Solids Box Defined by its center point and three dimensions width length and height Cone Defined by its center point top radius bottom radius and height A cylinder is a special case of a cone with equal top and bottom radii Sphere Defined by its center point and radius Indexed face Defined by a list of position coordinates and set connectivities In addition a list of normals and texture coordinates can also be specified These lists are stored as variables arrays and support objects of an appropriate type and referenced by the indexed face set using their names If the indexed face set forms a set of unintersecting closed surfaces it represents a boundary rep solid Extrusion Defined by using a closed curve for defining the cross section and a path curve for defining the extrusion path of the cross section Defined by the length and width of the surface surface the number of nodes along the length and width and a list of elevation values Surface Defined by a 2D ordered matrix of nodes 3Dface Defined by either three t
13. acking Receiver Tracking Emitters Fig 1 Schematic diagram of the VR system hardware components each of which may undergo large 3D motions Typical connections between the components include revolute spherical prismatic and planar joints gears and cams The bodies can be connected in closed loop configurations e g linkages and or open loop or tree configurations e g manipulators A large number of practical devices and systems can be modeled as an FMS These include ground air and space transporta tion vehicles automobiles trains airplanes and space craft manufacturing equipment machines manipulators and robots mechanisms articulated earth bound struc tures such as cranes and draw bridges articulated space structures such as deployable satellites space tele scopes and space stations microelectro mechanical systems MEMS and bio dynamical systems human body and animals Physics based techniques for model ing the dynamics of FMS generate the time histories of quantities of interest in design and control of these systems such as motion strain stress and internal forces In addition the models can also generate the frequency response of the FMS including mode shapes and natural frequencies The majority of high fidelity physics based models of large FMS are based on the finite element method Solid shell beam and truss elements are used to model the various flexible c
14. at property for the container sets it for all children objects inside that container Set the SHADE _FILE property of the MBS container to the name of an appropriate FE data support object Also in the case of displaying mode shapes set the MODES _ FILE property of the MBS container to the name of an appropriate FE data support object Also set the FILE property for the graphs to the name of an appropriate FE data support object Step 6 Create a subroutine to handle the draw event of MBS called MBS_DRAW In the case of displaying a motion animation a function to set the support object T M Wasfy A K Noor Advances in Engineering Software 32 2001 295 315 303 Define World container Load default script file Run entry point subroutine Open raster display nn Set display parameters Load true type fonts v Load included script files v Place objects in appropriate containers Run other initialization subroutines Display loop 4 Draw World object Check event for World object Script file Objects Definitions Names Initial properties Subroutines Definitions Name Parameter list Body Entry point Subroutine Fig 4 IVRESS execution flow chart pointer to the next nodal positions is called Linear inter polation is used to obtain the positions at any arbitrary time between stored time steps 5 Vis
15. ation truss The purpose of the isolation truss is to thermally isolate the cold ISIM from the warm SSM and sunshield This is a deployable truss which is deployed using revolute joints rotary actuators and PD controllers The truss is mounted on a connection disk that is connected to the ISIM 77 Linear actuators inside the VE The menu system consists of containers with the shape property set to form A main container is used to hold the entire menu system Children containers are used to hold the sub menus Each menu can consist of UI objects such as buttons check boxes slider bars and pictures A script routine is associated with the object s events For example the click T 1 Hz SR 15 Normalized Amplitude 12 Hz ae nee event for a check box can execute a function to change a display property The user utilizes the menu by clicking touching or dragging the various UI objects with the 3D or 2D selection tools A picture of the menu system used with the NGST application is shown in Fig 7a and the menu tree struc ture is shown in Fig 7b 10 f 20 30 Frequency Hz Fig 8 The total power spectrum plot for the NGST primary mirror and two typical mode shapes of the NGST primary mirror shaded using the total vibration amplitude T M Wasfy A K Noor Advances in Engineering Software 32 2001 295 315 309
16. ct oriented VE Fig 3 shows the objects that can be used in the VE These can be divided into four categories UI objects support objects geometric entities and finite elements see Tables 1 4 for descriptions of the various available objects and are described subsequently e UI objects Table 1 provide various functions in the VE Typical UI objects include container button check box dial and graph The container is a special type of object that is used to group children objects Typical uses of these objects include displaying information to the user in the VE as well as allowing the user to input data or commands in the VE e Support objects Table 2 contain data that can be refer enced by other objects For example a finite element refers to a nodal positions list a physical material and a material color support object Operations such as arith metic addition multiplication and division and logical and or not can be performed on support objects e Geometric entities Table 3 represent the geometry of the physical components of the multibody system Typi cal geometric entities include boundary representation solid box cone and sphere e Finite elements Table 4 represent the numerical model of the physical components of the multibody system Typical finite elements include beam shell and solid elements In addition the scripting language IVRESS script can be used to customize the VE
17. e supported The dial points to a specified dial position in a continuous range The dial object can be used as a slider bar by dragging the dial pointer to change the position of the dial Linear and rotary dials can be displayed The table is a spreadsheet object used for displaying or editing text in a 2D tabular form The color key is a bar which displays the mapping between the value of a shading variable and the shading color The color key data is read from a color key data support object determine its state and behavior methods which are functions that it can perform and events that are triggered when certain conditions are met Common properties of UI objects include translation orientation scale foreground material color name background material color name and visibility Common properties of geometric entities include names of the material color physical material and image texture support objects Common properties of finite T M Wasfy A K Noor Advances in Engineering Software 32 2001 295 315 301 Table 2 Support objects Support object Description Geometric property This support object has the following properties Shell thickness Beam cross sections and moments of inertia Physical material This support object has the following properties Material type linear isotropic orthotropic general Values for the material parameters Young s modulus Poisson ratio density t
18. ear modeling 4 e Solid modeling 19 21 e Virtual product development including design and virtual prototyping e g automotive assembly and design 4 robotics modeling and design 20 Virtual manufacturing and factory simulation 4 Tele collaboration Tele presence 18 Training and education 22 Most of the aforementioned VE studies use an object oriented approach to represent the various virtual objects which simulate the behavior of the real objects In an object oriented paradigm each object encapsulates a set of prop erties data which determines its appearance and behavior The objects are polymorphic which means that objects of different types can contain or respond to the same function without regard to the object type The objects are persistent which means that they behave in a natural predictable way Also their properties can be modified they can be deleted and new objects can be added In addition different objects can be grouped together into one group object The last characteristic allows a hierarchical directed tree type representation of the VE The group object can be trans formed translated rotated and scaled as one entity This hierarchical object oriented representation including the transformation hierarchy is called the scene graph 23 Several general purpose toolkits based on the scene graph approach 24 have been developed for construction and display of
19. el m is the mass of the ith node and x is the modal displacement of node i in direction j Fig 8 shows two typical mode shapes of the primary mirror along with the total power spectrum plot for the vibration of the primary mirror of the NGST as they are displayed in the VE The mirror is shaded using the nodal values of A A mode shape is animated using the following formula X t Xo S cos at x f xi where is the global time f is the selected frequency xo is the initial position of the node S is the scale factor for plotting the mode shape and a is a time scale factor The values for x f and xo are stored in a response data support object A slider bar allows selection of the frequency and corresponding mode shape The user selects the frequency by dragging the slider pointer using the 3D selection tool Figs 8 and 9 show respectively two typical mode shapes of the primary mirror of the NGST and of the entire NGST structure along with the total power spectrum 6 4 Attitude control simulation The NGST attitude control system ACS is composed of four reaction wheels that provide the torque necessary for orienting the NGST around three axes in space This allows an extra degree of freedom which can be used to minimize the mean wheel speed The reaction wheels are mounted in a pyramid configuration Fig 10a The attitude is sensed using gyros and a star tracker camera 40 A PD controller is used for attit
20. ength spring Used to impose an equal position constraint on two nodes It is used to define hinge joints Defined by two nodes which define the path of the joint and a third node which is restricted to move on that path Defined by the value of the point mass at a node Defined by the direction of the rotation axis and three nodes corresponding to the actuator action points Defined using two nodes corresponding to the actuator action points multibody system Each container groups a number of finite elements and corresponding geometric enti ties which form the multibody system component see Fig 5 All of the root containers should be placed inside the MBS container The motion of the geometric entities can be interpolated from the motion of the corresponding finite elements Step 4 Load time history and or modal FE data support objects These support objects hold the position velocity or acceleration vs time or frequency or selected para meters e g stresses strains internal forces for the nodes of the multibody system The properties of these objects include the total number of variables N the number of time steps S and the value of each variable at each time step Step 5 Set the file POSITION_FILE property of the MBS container to the name of an appropriate FE data support object Although the container does not have a property POSITION_FILE the children finite elements have that property Setting th
21. hermal conductivity Contains the color information which includes ambient color diffuse color specular color and shininess Material color Image Stores a single picture or a series of pictures a movie An image object has the following properties Red green blue and transparency intensities for each pixel Horizontal and vertical pixel size of the image Number of pictures 1 for a still image and gt 1 for a movie Font Connectivity and control point positions for each character Nodal positions Stores a list of nodal positions This support object has the following properties The total number of nodes N A list of 3 x N floating point values Each three of these numbers correspond to the position coordinates of a node FE data For storing finite element vectors such as nodal positions velocities and accelerations nodal values stress strain displacement element values stress strain other response quantities of interest relative angles internal force and torque controller actions It can store time independent time dependent or frequency dependent data Variables This support object can store either single values or arrays of values The data type for these variables include variant string integer single precision float and real Color key data Contains a list of colors and corresponding normalized values between 0 and 1 which are used for shading a model using a
22. id assembly b Finite element model of the attitude control system Fig 10 Attitude control system T 88 6 sec a m b Command slew Goiz c Slew error at the ACS 24 0 002 23 Angle Angl deg T deg 33 i deg 2000 21 0 002 205 20 40 60 80 100 Time Sec DDE o d Slew error at the OTA conical 500 Reaction wheel angular velocity 400 300 Angular Velocity200 rad sec 100 20 40 60 100 i00 Time Sec Fig 11 a A snapshot of the VE display at the end of the attitude control maneuver rendered using light sources b Time history of the command slew c Slew error at the reaction wheel pyramid d Slew error at the OTA conical mast e Reaction wheel angular velocity T M Wasfy A K Noor Advances in Engineering Software 32 2001 295 315 311 Fig 12 A snapshot of the VR display with some components moved to reveal the components underneath 6 3 Vibrational response The vibrational response was generated by applying a short duration disturbance force on the structure An FFT algorithm is then applied to the time history of the subse quent motion of all the FE nodes The resulting frequency response of the nodes gives the mode shapes at discrete frequencies The total amplitude of vibration A at a pre selected frequency fis given by N A gt mix x2 xh i 1 where N is the total number of nodes in the mod
23. ided in IVRESS e The observer object interfaces with the wand in order to allow the user to naturally move and rotate in 3D so that he can examine the model from any angle The 3D selection tool can be used to move the various components of the multibody system which allows viewing hidden parts of the model The 3D selection tool is also used to interact with the menu and with various UI objects Multiple observers can be defined For each observer a 3D selection tool is defined The user can switch between observers by clicking a function key on the wand This can be used to define an observer for the menu and an observer for the model such that when the user is exam ining the model the menu is not hindering his view and vice versa 6 Case study application to the NGST The foregoing object oriented VE was used to view the dynamic simulation results of a large deployable space Properties Ref to Physical Material Support Object Physical Property Support Object Material Color Support Object Nodal Positions Support Object Container Object Component of a Flexible Multibody System Finite Finite Element 1 Element n Physical Material Support Object Geometric Entity 1 Material Color Support Object Geometric Entity n Material Color Support Object Physical Property Support Object Material Color Support Object Nodal Positions Support Object Tex
24. iles In addition IVRESS has facil ities for voice commands and communication with other computers A flow chart of a typical execution sequence is shown in Fig 4 When IVRESS is executed an initial container called World is created Then a default script file is loaded Inside the script file an entry point command is used to define the starting subroutine The starting subroutine executes the following functions open the raster display set display parameters load true type fonts load other included script files place objects in appropriate containers run the initialization subroutines start the display loop All root containers should be placed inside the World container using the Add container method The Draw and Check events methods for the World container are called during the display loop at each frame update These in turn call the Draw and Check events methods for all objects inside World 4 Application of the VE to multibody systems After starting the VE the user can load a multibody system simulation A typical simulation script consists of six main steps Step 1 Create a root container for example MBS for the multibody system and place it in the World container Step 2 Create UI objects such as graphs dials checkboxes buttons and containers for organizing and grouping other UI objects for displaying custom data for the multibody system Those objects are also pl
25. in a plane The motion of the 2D selection tool can be controlled using the wand s joystick a mouse or a touch pad Light sources can be defined in the VE The properties of a light source include position spot direction spot angle color and intensity Geometric entities and finite elements of the multibody system are rendered using the active light sources Table 1 continued UI object Description Button Check box Label Text box 2D and 3D graphs 0 E ea o o Z s o 188 tiw SEC Picture Dial o 5s Geo ee gS 12 179 9 29 m wee Sasot ai Table Color key The button is used to perform predefined functions which are triggered by user generated events including clicking and touching the button using a 2D or 3D selection tool The button changes its appearance when it is touched The button appearance properties include color shape and texture picture The user toggles the check box on and off by clicking on it using a selection tool The label displays single or multi line text The text box displays single or multi line editable text The graph object displays static or animated time dependent plots The graph reads its data from a FE data support object Displays a single picture or multiple pictures movies The picture reads the image data from an image data support object Standard image and movie formats ar
26. ities and finite elements Each object has a set of properties and methods that determine its appearance behavior and actions Also associated with each objectis a set of events that are triggered when certain condi tions initiated by the user or the passage of time are met UI objects provide the functionality in the VE Typical UI objects include buttons check boxes slider bars text boxes labels graphs tables light sources and selection tools The scene graph capability is enabled by using the container object which is a special type of UI object that can contain children objects Children objects are displayed using the homogeneous geometric transformation of the parent container Children objects can be other UI objects includ ing other containers geometric entities and finite elements Support objects contain information that can be referenced by other objects Typical support objects include material properties time history data and mode shape data Geometric entities represent the geometry of the physical components of the FMS Finite elements represent the numerical model of the physical components of the FMS 2 Hardware of the virtual reality facilities A review of the input navigation and output display devices of virtual reality VR systems is presented in Ref 2 A VR system includes output and input facilities for interfacing with users computers for generating the VE and facili
27. m screen is used FOV 45 and in the VisionDome one hemispherical 5 10 m diameter screen is used FOV 90 180 Two or four speakers The speakers can be used to output spoken messages sound effects and data sonification The following input facilities were used A position and orientation tracking device for tracking the position and orientation of the user This can be achieved by using six or more fixed electromagnetic or ultrasonic emitters and a receiver placed on the part of body to be tracked The data from the receiver indicates the distance between the receiver and each emitter which is triangulated to obtain three position coordinates and three orientation angles of the receiver Tracking recei vers are usually placed on the stereo glasses for head tracking in order to calculate the correct perspective view as well as a hand held wand for navigating and pointing in the VE e Tracked 3D navigation and selection device such as the wand The wand has a pressure sensitive 2D joystick that can be used to control the speed and direction of motion Also the wand has two or more buttons that can be programmed to perform special functions e 2D navigation device such as a mouse touch pad or joystick e Microphone for voice commands e Keyboard T M Wasfy A K Noor Advances in Engineering Software 32 2001 295 315 299
28. neration Space Telescope Published by Elsevier Science Ltd Keywords Virtual environment Flexible multibody systems Finite elements 1 Introduction 1 1 Definition of a virtual environment Virtual or synthetic environments VEs are three dimen sional computer generated environments that occur in real time as manipulated by the user 1 A VE is a projection of either some real environment a fairly realistic environment that does not exist or an unreal environment e g for enter tainment and games VEs provide a natural interface between humans and computers by artificially mimicking the way humans interact with their physical environment A VE includes facilities for interfacing with humans through output of sensory information and input of commands Output facilities include an immersive stereoscopic display and stereo sound Input facilities include a hand held 3D navigation device such as a wand joystick or 3D mouse a 2D navigation device such as a mouse or a touch pad a haptic feedback device such as gloves devices for position and orientation tracking of parts of the user s body such as the head and hands a microphone for voice commands and a keyboard Corresponding author Tel 1 757 864 1978 fax 1 757 864 8089 E mail address a k noor larc nasa gov A K Noor 0965 9978 01 see front matter Published by Elsevier Science Ltd PII S0965 9978 00 00091 0 1 2 Major components of a virtual e
29. nse mode shapes attitude control and deployment for the NGST were computed using a flexible multibody dynamics finite element code DIS and then were displayed in the VE Acknowledgements The present research is supported by NASA Cooperative Agreement NCC 1 263 The IVRESS and DIS codes were provided by Advanced Science and Automation Corp The authors would like to thank Gary Mosier of NASA Goddard Space Flight Center for providing information about the NGST structure Jeanne Peters of the University of Virginia for her assistance in performing the computer simulations Steve Irick of NASA Langley Research Center for provid ing the geometric VRML model of the NGST and Richard McGinnis Larry Matthias and Jim Frenzer of the Distrib uted Active Archive Center DAAC at NASA Langley Research Center for the use of the DAAC CAVE facility References 1 Mills S Noyes J Virtual reality an overview of user related design issues revised paper for special issue on virtual reality user issues in interacting with computers Interacting with Computers 1999 11 375 86 2 Nash E Edwards G Thompson J Barfield W A review of presence and performance in virtual environments International Journal of Human Computer Interaction 2000 12 1 1 41 3 Stanney KM Mourant RR Kennedy RS Human factors issues in virtual environments a review of the literature Presence Teleopera tors and Virtual Environments 1998 7 4 327 51 4 Dai F edi
30. nvironment In order for a VE to mimic the real environment it must be able to couple the sensory output of the environment to the real time actions navigation of the user s Recent review articles 2 3 and a book 4 provide an overview of the current research on coupling the visual output for recogni tion tracking of moving objects distance judging search and size estimation auditory output for recognition and sound localization and kinesthetic haptic output with the user s navigation fly through and manipulation of objects in the VE The studies reviewed conclude that in order to achieve a realistic VE a VE in which the user is fully immersed and feels as if he she is actually present the following capabilities are needed e high resolution minimum 1280 x 1028 24 bit color flicker and ghosting free stereoscopic display frame rate of at least 15 frames s head tracking large field of view FOV gt 40 light source based rendering photo realistic textures consistency the object s position and appearance are predictable as like in a real environment e no disturbance from the real world environment 296 T M Wasfy A K Noor Advances in Engineering Software 32 2001 295 315 e navigation tool that allows accurate direction pointing and fly through or walk through in the environment In addition the following capabilities are not essential but can enhance the realism of the
31. om ponents FMSs naturally lend themselves to the object oriented as well as the scene graph representation 32 20 VEs can be used to visualize geometric and finite element models of FMS as well as dynamic simulations of the motion of the FMS calculated using physics based simulation codes Users can move around the model of the FMS while it is moving and look at it from any position or angle The close to real life visualization allowed by the VE helps to quickly attain a better understanding of the FMS and its dynamic response Typical ways of displaying an FMS in a VE include any combination of the following e photo realistic rendering of the FMS in its final environment e finite element mesh of the FMS with exploded elements and or element boundaries e animation of the motion of the FMS e shading of the FMS using a scalar response quantity such as a stress strain component strain energy density and displacement component etc e animated mode shapes e 2D or 3D graphs of time histories of response quanti ties of interest A user interface inside the VE allows the user to control the way the model is displayed Typical user actions include controlling the animation speed setting shading parameters and selecting the shading variable loading a model moving objects e g light sources and model parts selecting natural frequencies and mode shapes selecting graph variables etc 1 5 Objectives and scope of
32. or and isolation truss are deployed using revolute and prismatic joints along with rotary and linear actuators The attitude control system ACS is composed of four reaction wheels mounted on revolute joints in a pyramid configuration These provide the torque necessary for orienting the NGST The attitude is sensed using gyros and a star tracker camera The ACS can maintain pointing accuracy of 0 4 arcs 40 Detailed dynamic numerical simulations were performed using the DIS code 38 for the vibrational response attitude control and deployment of the NGST 41 The vibrational response was evaluated by converting the time domain vibrational response of the NGST to the frequency domain using an FFT algorithm In the attitude control simulation the four reaction wheels along with a proportional derivative PD tracking attitude controller were used to rotate the structure 5 around the Y axis In the deployment simulation PD controllers placed on 306 T M Wasfy A K Noor Advances in Engineering Software 32 2001 295 315 Virtual Desk Main Shading Menu Color Key Shading a Main Menu Range Min 0 _ Max OD m D es D Advanced IGomp tational Technology O MODEL VIEW X MODEL SELECT D MENU VIEW D MENU SELECT File Display Open Background q Side 1 Close Lights Side 2 ice Events Anim Sinn Min L Max C Shading Key 1 Save o Color Key Top 1
33. oration 1997 ABAQUS Standard User s Manual Version 5 7 Hibbit Karlsson amp Sorensen Inc 1997 Dynamic interactions simulator DIS User s Manual Version 1 1 Advanced Science and Automation Corporation 2000 Mosier G Femiano M Kong H Bely P Burg R Redding D Kissil A Rakoczy J Craig L An integrated modeling environment for systems level performance analysis of the next generation space telescope Space Telescopes and Instruments V SPIE vol 3356 1998 Mosier G Femiano M Kong H Bely P Burg R Redding D Kissil A Rakoczy J Craig L Fine pointing control for a next generation space telescope Space Telescopes and Instruments V SPIE vol 3356 1998 Wasfy T Noor AK Multibody dynamic simulation of the next generation space telescope using finite elements and fuzzy sets Computer Methods in Applied Mechanics and Engineering 2000 in press
34. pe property for the container determines its shape appearance It can be set to 2D form box room or no shape The observer is used to define the viewer s head position and orientation The observer object is controlled by using the wand The user points the wand in the direction he wants to move then uses the pressure sensitive joystick to control the speed of motion in that direction Also by holding a function key on the wand and using the pressure sensitive joystick the user can rotate around the axis of the wand The 3D selection tool is used to select move and touch objects in the VE The 3D selection tool is controlled using the wand The object consists of a selection bounding box and two perpendicular vectors indicating the current spatial orientation of the wand The user points the wand in the direction he wants the selection box to move the direction vector and then uses the pressure sensitive joystick to control the speed of motion in that direction Once the selection bounding box touches an object a touch event for that object is triggered which in turn executes an associated sub routine Also a click event is triggered when the selection box is touching the object and the user clicks the first wand function key Similar to the 3D selection tool the 2D selection tool is used to select move touch and click on objects The 2D selection tool consists of a selection bounding box which can only move
35. reo realistic sound effects which can help the user localize the sound producing objects e Two way natural language communication including voice commands The VE can feature an intelligent voice enabled virtual assistant which can respond to the user s speech 9 e Support for haptic feedback devices such as gloves and a pressure sensitive joystick with force feedback e Tracking multiple points on the user s body This allows the user to use multiple parts of his body e g hands head and legs to interact with the VE It also allows displaying realistic avatar for the user which closely mimics the user s actual movement 1 3 Brief review of previous VE applications and studies Several applications of VEs have been reported in the literature in engineering medicine and entertainment In engineering VEs have been used for e Visualization of 3D models of engineering systems e g geographic information systems for agriculture geology urban planning and telecommunications 10 11 archi tectural visualization and walkthrough 12 e Visualization of numerical and experimental simulation results e g automotive crash visualization 13 14 fluid flow visualization for automotive and aerospace applica tions 15 17 e Computational steering and simulation based design e g molecular simulation for material characterization 8 visualization and computational steering for a tractor lift arm 5 landing g
36. response variable Texture map Contains texture parameters for mapping an image texture on a geometric entity These include name of the image support object algorithm for wrapping the picture on the geometric entity rotation and repetition of the picture elements include names of the physical material material color and nodal positions support objects as well as the element nodal connectivity Common methods of all objects include Draw and Check events The container methods invoke the methods of all the children nodes For example the container Draw method invokes all Draw methods of all the objects contained within including other containers Typical events include Touch Press and Click For exam ple the Touch event is invoked when a selection tool touches the object The Click event is invoked when a selec tion tool is touching the object and the user clicks on the first wand button An event is triggered by calling a subroutine associated with that event The subroutine name consists of the object name concatenated with an underscore and the event name e g object name_event name IVRESS can read and write file formats for geometry data e g VRML 2 0 31 Open Inventor 25 DXF 33 and LightWave 34 finite element information e g MSC NASTRAN 35 MSC DYTRAN 36 ABAQUS 37 and DIS 38 pictures e g Bitmaps PNG JPEG and GIF movies e g MPEG and AVI and user interfaces e g VRML 2 0 f
37. riangle or four polygon 3D points An ordered list of 3D points that are connected by straight line segments to form the polyline Line Two 3D points connected by a straight line segment Spline An ordered list of 3D control points define a Bezier spline Circle Defined by its center radius and a normal vector to the plane of the circle Arc Defined by its center starting point arc angle and a normal vector to the plane of the arc Ellipse Defined by its center two radii a normal vector to the plane of the ellipse and a vector in that plane defining the direction of the first radius Point A 3D point Step 3 Load the multibody system data file This file includes the following objects o FE nodal positions support objects see Table 2 o Physical material support objects see Table 2 o Material color support objects see Table 2 o Finite elements see Table 4 Each finite element has the following properties names of the nodal positions physical material material color support objects and element nodal connectivity o Geometric entities see Table 3 o Containers Each container is a component of the Prismatic joint Point mass Rotary actuator Linear actuator defined by the first four nodes and the bottom surface by the last four nodes The normal to the shell surface is the vector connecting the two surfaces Truss Two noded truss element Beam Three noded beam element Zero l
38. s princi ples and practice 2nd ed Reading MA Addison Wesley 1990 Green M Malliday S A geometric modeling and animation system for virtual reality Communications of the ACM 1996 39 5 46 53 Strauss P Carey R An object oriented 3D graphics toolkit Proceed ings of Siggraph 92 Chicago IL 26 31 July 1992 p 341 9 Eckel G IRIS performer programmer s guide Silicon Graphics Inc 1997 WorldToolKit Reference Manual Sense8 Corporation 1998 Shaw C Green M Liang J Sun Y Decoupled simulations in virtual reality with the MR toolkit ACM Transactions on Information Systems 1993 11 3 287 317 http www cs ualberta ca graphics MRToolkit html puSE Software Development Environment 2000 MUSE Technolo gies Inc 2000 environment 30 31 32 33 34 35 36 37 38 39 40 41 Ayers MR Zeleznik RC The lego interface toolkit Proceedings of the ACM Symposium on User Interface and Software Technology UIST 1996 ISO IEC 14772 1 Virtual reality modeling language VRML97 The VRML Consortium Incorporated 1997 Kunz DL An object oriented approach to multibody systems analy sis Computers and Structures 1998 69 209 17 AutoCAD 2000 DXF Reference AutoDESK Corp 2000 LightWave 3D Object File Format NewTek Corp 1996 1999 MSC NASTRAN User s Manual Version 67 The MacNeal Schwendler Corporation 1991 MSC DYTRAN User s Manual Version 4 0 The MacNeal Schwend ler Corp
39. the present study The objective of the present study is to describe an object oriented scene graph toolkit IVRESS Integrated Virtual Reality Environment for Synthesis and Simulation which is specifically designed for visualization of FMS finite element simulation results The Cave Automatic Virtual Environment CAVE facility at NASA Langley Research Center is used for demonstrating the effectiveness of this toolkit IVRESS can be used to construct a realistic VE which supports the aforementioned ways of viewing FMS and their simulation results Unlike other VE development toolkits that consist of a set of C C 298 T M Wasfy A K Noor Advances in Engineering Software 32 2001 295 315 Projectors __ A N Projection d Mirrors Screens Fig 2 CAVE facility functions and classes IVRESS is a stand alone program An object oriented scripting language IVRESS script allows describing the various objects and writing custom event handling routines Custom objects can be added to IVRESS by writing C C code for the object and linking that code to IVRESS either dynamically using a dynamic link library or statically by linking with IVRESS object files Four types of modular objects are used to define the FMS components the data used for displaying the defor mation motion and mode shapes of the FMS as well as the user interface These are user interface UI objects support objects geometric ent
40. ties for communication with other computers Fig 1 shows a schematic diagram of the hardware configuration of a typical VR facility that was used in conjunction with IVRESS A typical VR facility the CAVE is shown in Fig 2 The follow ing output facilities were used e Immersive stereoscopic displays Stereoscopic viewing is achieved by displaying the correct perspective view of the model for both eyes of the user This can be achieved by using LCD shuttered glasses which are synchronized with the screen refresh rate When the correct perspective view for the right eye is displayed the left eye is black ened and vice versa An infrared emitter which is linked to the display output signal sends the screen refresh trig ger signal to infrared receivers in the glasses to shutter them A refresh rate above 72 Hz usually 96 Hz will result in a flicker free stereoscopic display This techni que is used in the CAVE and ImmersaDesk Stereoscopic display can also be achieved by using head mounted displays These consist of two small LCD screens which display the correct perspective view for each eye In order to achieve a high level of immersion one or more large flat or curved screens which allow an FOV larger than 45 are used For example in the CAVE four flat 3 X 3 m screens arranged as a cubical room one front screen two side screens and a floor or a ceiling screen are used FOV 90 180 In the ImmersaDesk one flat 2
41. tor Virtual reality for industrial applications Berlin Springer 1998 5 Ryken MJ Vance JM Applying virtual reality techniques to the interactive stress analysis of a tractor lift arm Finite Elements in Analysis and Design 2000 35 141 55 6 Ginsberg M Influences challenges and strategies for automotive HPC benchmarking and performance improvement Parallel Comput ing 1999 25 12 1459 76 7 O Brien JF Zordan VB Hodgins JK Combining active and passive simulations for secondary motion IEEE Computer Graphics and Applications 2000 July August 86 96 8 Suzuki A Kamiko M Yamamoto R Tateizumi Y Hashimoto M Molecular simulations in the virtual material laboratory Computa tional Materials Science 1999 14 227 31 9 Tarau P Bosschere K Dahl V Rochefort S LogiMOO an extensible multi user virtual world with natural language control The Journal of Logic Programming 1999 38 331 53 10 Losa A Cervelle B 3D topological modeling and visualization for 3D GIS Computers and Graphics 1999 23 469 78 11 Germs R Maren GV Verbree E Jansen FW A multi view VR inter face for 3D GIS Computers and Graphics 1999 23 497 506 12 Nomura J Sawada K Virtual reality technology and its industrial applications Control Engineering Practice 1999 7 1381 94 13 Schulz M Ertl T Reuding T From high end VR to PC based VRML viewing supporting the car development process by adapted virtual environments Proceedings of ATED CGIM
42. ture Map Support Object Texture Map Support Object Ref to Image Support Object Image Support Object Fig 5 Object representation of a component of a flexible multibody system Container Container Object 1 Object n Bae o0 ea a ave a E POE SIE S6Z T007 ZE aivuyfog SupsaauSuq uy saouvapy 400N J V MOM WL T M Wasfy A K Noor Advances in Engineering Software 32 2001 295 315 305 Spacecraft Support Module Isolation Truss a Integrated Science Instrument Sunshield Conical Mast Secondary Mirror b Fig 6 Rendered model of the NGST a VRML model b FE model and c exploded elements view for the FE model structure NASA s NGST Fig 6 The NGST is designed as a deployable structure in order to fit in the shroud of the launch vehicle The NGST s components can be folded and unfolded either by using mechanical joints e g revolute prismatic and spherical or by inflation Low hysteresis joints and actuators along with actuators for active shape control are used to maintain the precise shape of the structure The NGST has an aperture diameter of 8m which is 10 times the collecting area of the Hubble Space Telescope HST and mass of about 3100 kg which is 28 of the mass of the HST 39 40 It will be passively cooled to 70 K by shading it from the sun using a large inflatable sunshield The NGST primary mirror secondary mirr
43. ualization capabilities The object oriented approach allows simultaneous view ing of multiple models in the same VE Each model can be loaded into its own container A virtual model of the multi body system can be viewed in the following ways e A detailed geometric model including surface textures transparency and light sources e The finite element mesh including the element edges and exploded elements in order to delineate the element boundaries e Animation of the motion of the finite element model and the geometric model The motion of the geometric model is interpolated from the motion of the underlying nodes of the FE model The FE nodal positions at each time instant are obtained from a FE data support object Linear interpola tion between two successive node positions is used to obtain the node position at any arbitrary time in between e Shading of the finite element model using a scalar response quantity such as a stress strain component combined stress strain strain energy density displacement component and combined displacement etc The FE data support object is used to store the time history of the scalar response quantity e 2D and 3D graphs of time histories of response quantities e 2D and 3D static graphs of response quantities e Viewing animated mode shapes of the FE model shaded using the displacement or strain stress magnitudes The following tools for enhancing the visualization experi ence are prov
44. ude control of the NGST The pyramid struc ture of the ACS is modeled using beam and truss elements Fig 10b The reaction wheels are also modeled using beam and truss elements along with lumped masses to account for wheel inertia Each reaction wheel is connected to the pyramid structure using a revolute joint A rotary actuator along each revolute joint provides the wheel torque Each wheel can rotate at a speed of up to 100 rev s An attitude control maneuver of 5 around the inertial Y axis was simulated using DIS The total time for the maneuver is 312 T M Wasfy A K Noor Advances in Engineering Software 32 2001 295 315 T 1 52 sec T 41 e T 69 9 seg T 111 sec Fig 13 Snapshots of the NGST during the deployment maneuver rendered using light sources 70 s A constant acceleration deceleration slew angle profile is selected The rise time is 35 s and the fall time is 35 s with zero dwell time The attitude is measured at the reaction wheel pyramid assembly such that the attitude control actuators and the sensors are collocated Initially all reaction wheels are at rest The sampling frequency for attitude measurement is 10 Hz Fig 11 shows a snapshot at the end of this maneuver The figure also shows time history plots of the command slew the slew error at the reaction wheel pyramid assembly the slew error at the OTA conical mast and the angular velocit
45. y of the two active reaction wheels Note that only two reaction wheels are active during this maneuver The torque applied to the two active wheels and the resulting angular velocity profiles of the two wheels are almost the same The other T M Wasfy A K Noor Advances in Engineering Software 32 2001 295 315 0 30 60 90 120 150 Time sec Ang Rad 0 50 100 150 Time sec Virtual Desk Main T 86 6 sec NGST DEPLOYMENT MODEL SELECT MENU VIEW MENU SELECT 2 0 315 1 0 20 5 lt 0 50 100 Time sec 150 4 0 3 0 a 2 0 G 1 0 O 50 100 150 Time sec Fig 14 Simultaneous viewing of motion and response time history plots two wheels remain at almost rest In Fig 12 the 3D selec tion tool was used to grab some of the NGST components and move them in order to better reveal the components underneath 6 5 Deployment simulation The simulation starts with all the deployable compo a 48 o oT a ae Fig 15 Snapshot of the NGST during the deployment maneuver shaded using strain energy density ae ie a 99 yt e T aa er 313 nents of the NGST in the retracted position Fig 13a Deployment is performed in 120 s and involves adjust ing the sunshield angle angle between SSM and OTA then deploying the isolation truss primary mirror and secondary mirror These steps are executed in the following order
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