exam question Flashcards
GEOMETRY MODELING
Mention three different aspects that has to be included in an assembly model (in e.g. a CAD
system)
An assembly model needs to include \:Hierarchical relations --- assembly -> sub-assembly -> part \:Mating conditions --- geometrical restrictions, etc \:Mechanical degrees of freedom
GEOMETRY MODELING
Mention two advantages of using solid models instead of e.g. surface models
:Solid models support higher levels of functionality and automation than surface models
— Example: Calculation of mass and moments of inertia
:Solid models allow the designer to work with higher level objects rather than points, curves
and surfaces
GEOMETRY MODELING
Describe how trimmed parametric surfaces are defined/created
:Parametric surfaces are defined in a similar way as parametric curves but with two
parameters u and v.
:Parametric curves are defined in the same 2D parametric space.
— They are used to create holes (inner trim curves)
— Or the outer boundary of the surface (outer trim curve)
:The trimmed parametric surface is transformed to 3D space
GEOMETRY MODELING
Bézier and B-spline are two types of curves used in geometry modeling. What are the
advantages of B-spline curves?
:Better local control of the curve
:Order of the polynomial does not increase with the number of control points
:Easier to define joined curve segments
GEOMETRY ASSURANCE
How does stability analysis work and for what is it used?
Simulation:
:Each locating point is disturbed with a unit disturbance
:The amplification to the output, color-coding, part position or critical product dimension, is
calculated
:The amplification for each individual locating point is summarized with RSS to give a value
for the locating scheme
It is used to identify sensitive areas and sensitivity factors and guides optimization of locator
position
COMPUTER GRAPHICS AND VIRTUAL REALITY
Describe what LOD (Level of Detail) is and how it can be used to increase the performance
of a visual simulation.
Many representations with different precision is stored. Depending on the distance to the object, one
suitable representation is shown, i.e. when an object is far away a simple representation is shown.
When an object is very far away it might not even be drawn at all.
Three rings with point in the middle(VO) rings from point, LOD 0, LOD 1, LOD 2, -LOD 3(not visible)
COMPUTER GRAPHICS AND VIRTUAL REALITY
Describe how a shadow buffer can be used to create shadows.
:Create a buffer, corresponding to the depth buffer, containing the distance from the light
source to the closest surface
:When drawing the point P on a surface:
- – Find the corresponding position in the shadow buffer for the point that is being drawn
- – Compare the value in the shadow buffer with the distance from the light source to P
- – If the value is less than the distance: draw only with ambient light
MISCELLANEOUS
What are the six most central functions of a PLM system?
The most central functions of PLM systems are:
- – Item (part, assembly, component) management
- – Product structure management
- – Document management
- – Change management
- – Retrieval of information
- – Workflow and distribution management
MISCELLANEOUS
On a higher level, a PLM system have two major tasks. One is to manage information, what
is the other? Give some examples of tools used for the second task.
Create Information, e.g. with CAD systems or simulation software.
MISCELLANEOUS
What are homogenous coordinates and why are they used in computer graphics?
Homogenous coordinates are created by adding an extra coordinate, w, to the Cartesian
coordinates: P_(cartes) [x y z] -> P_(homogen) [x y z w] normally (in computer graphics) w = 1.
They are used in order to simplify the calculation of transformations. With homogenous coordinates
all transformations can be calculated as the same matrix multiplication which can be optimized for
speed in the graphics processor.
VIRTUAL GEOMETRY ASSURANCE
What is virtual geometry assurance?
Using computer tools to perform geometry assurance tasks on virtual product models
VIRTUAL GEOMETRY ASSURANCE
What are the benefits of using it (VIRTUAL GEOMETRY ASSURANCE)
- – Minimizing the need for costly physical prototypes
- – Finding problems as early as possible in the development process (easier and cheaper to fix)
- – Faster development process with efficient tools (time to market)
- – Increased quality
VIRTUAL GEOMETRY ASSURANCE
Describe how it can be used in some tasks in the geometry development process.
(Virtual Geometry Assurance)
- – Analyze different assembly concepts
- – Analyze and define robust locating systems
- – Simulate and verify the final demands
- – Virtual matching for trimming the production process
GEOMETRY MODELING
Describe the steps necessary to create a solid, using surface modeling, in a modern CAD
system
- – Create wireframe elements (points, lines, planes, curves) in 3D or sketches
- – Create surfaces from the wireframe geometries (sweep, revolve, …)
- – Trim the surfaces together
- – Join the surfaces together to a uniform element
- – Transform into a solid (Thick, Closed Surface, …)
- – (Add fillets ), can also be done before transforming into a solid.
GEOMETRY MODELING
What characterizes a feature (in the geometry modeling context)
A feature:
- – is a physical part of a detail
- – can be linked to a generic form
- – has a specific engineering role (function, manufacturing method, simulation method, …)
- – has predictable properties
GEOMETRY ASSURANCE
In order to perform a 3D variation analysis a 3D assembly model is needed. Describe the
necessary components and inputs for a 3D assembly model.
A 3D assembly model consist of:
- Parts
- Subassemblies
- Positioning systems
- Input tolerances with range and type of distribution
- Critical measures
GEOMETRY ASSURANCE
How does contribution analysis work and for what is it used?
Model consist of:
- – 3D assembly model with defined locating schemes
- – Input tolerances with range and type of distribution
- – Critical measures
Simulation:
- – All input parameters are varied (one at the time) within their tolerances on 3 levels
- – Max output is registered for all measures
- – Contribution is calculated in percent as ( % contribution_i = 100 (delta,output^2_i)/(sum, deltaoutput^2_i, n and i=1)
It is used to calculate a ranked list of how all input tolerances contributes to the variation in the
critical measures
COMPUTER GRAPHICS AND VIRTUAL REALITY
Describe the RGB color model
The color is accomplished with a mixture of three primary colors
- – Red [0 – 1] (or [0 – 255])
- – Green [0 – 1]
- – Blue [0 – 1]
COMPUTER GRAPHICS AND VIRTUAL REALITY
Bump-mapping is a way of using textures to give a smooth surface an irregular appearance.
Describe how it works.
- – It works by modifying the surface normal that is used in the illumination model.
- – The value of the modification is obtained from a texture.
- – The color (RGB value) of each pixel indicates the surface normal modification.
MISCELLANEOUS
PLM and PDM are two acronyms used in relation to the product realization process. What
do they stand for?
- – Product Lifecycle Management
- – Product Data Management
MISCELLANEOUS
Describe what PLM and PDM systems are and how they relate to each other.
A PDM system manages all information needed to manage the product definition during its
lifecycle. It is not only a database but also a toolbox for rationalizing engineering work.
A PLM system aims to support the creation and management of all information related to a product
throughout its lifecycle. That means that it includes a PDM system for data management buT also
tools to create data, e.g. a CAD, simulation software, etc.
MISCELLANEOUS
The Method of Influence Coefficients can be used to perform Monte Carlo variation
simulations on non-rigid parts. How does it work?
- – When doing a Monte Carlo variation simulation more than 1000 iterations have to be done
- – To do this with traditional FE methods would take too long time
- – Therefore, a FE solver is used to create a linearized model of the assembly
- – The linearized model is then used in the simulations
- – This gives a very large reduction of the simulation time (~ a factor 1000)
- – If the locators or support points are moved, a new linearization has to be done
MISCELLANEOUS
Mention four different standards for geometry exchange and what type of geometry data
they can handle.
(four of) IGES o Mathematical description STL o Triangulated format VRML o Triangulated format JT o Both mathematical and triangulated STEP o Mathematical description
MISCELLANEOUS
Mention two different tasks in the product realization process where off-line programming
can be used
(two of)
Programming of industrial robots
Programming of NC-machines
Programming of CMMs
MISCELLANEOUS
Three different types of coordinate systems are used in virtual reality models (and also in
CAD models). Specify these and what they are used for
- World coordinate system (w)
• Only one in each model
• The position of other objects are related to this - Object coordinate system (o)
• One coordinate system per object in the model
• Positions the object relative to the world coordinate system or relative to a
superseding object - The coordinate system of the observer (Virtual Observer)
• Makes it possible to travel around in the model
GEOMETRY MODELING Describe the following three types of solid models, mention some advantages or disadvantages for each (6p) Decomposition models Constructive models Boundary representation
Decomposition models:
Can be made of:
— Voxels: the solid is composed of a number of cubes
— Cell based: the solid is built up by polygons
It is an approximate model and requires a lot of memory for high precision.
It is suitable for different types of calculations
Constructive models:
Solid models are created by manipulating primitives with Boolean operators.
It is hard to handle general surfaces
It is very compact (do not require a lot of memory)
Boundary representation:
The solid is defined with points, curves and surfaces plus a definition of what is inside the
model
Uses graphical methods e.g. sweep and rotate
Can use parametric surfaces
Can use Boolean methods
GEOMETRY MODELING
Curves used in geometry modeling can be of different order. What are the advantages and
disadvantages of higher order curves?
Advantage: increased precision
Disadvantages: risk for corrupt curves, increased calculation time
GEOMETRY MODELING
What is a digital mockup (DMU) and for what is it used?
A special type of component based assembly model, developed to be able to handle large
assemblies (> 1000 parts) from different CAD-systems
Can be used for e.g. packaging studies and assembly simulation but not for e.g. calculation of
mass etc.
GEOMETRY ASSURANCE
Describe how a 3-2-1 locating scheme works
Six DOF are locked by six points
Primary points A1, A2 and A3 defines a plane and locks the geometry in space in two
rotations and one translation: TZ, RX, RY
Secondary points, B1 and B2, defines a line and locks the geometry in space in one rotation
and one translation: TY, RZ
Tetriary point C1 locks the geometry in space in one translation: TX
COMPUTER GRAPHICS AND VIRTUAL REALITY
Ray-Tracing and Radiosity are two different methods for advanced 3D computer graphics rendering.
Describe how these methods work
Ray-tracing
Follow light rays from the observer, one for each pixel
Calculate how the ray is mirrored and refracted (if the object is transparent) when it hits
surfaces in the model, and save the way in a tree graph
Calculate the intensity at the initial points with illumination models
Use the tree to calculate the intensities at each pixel
Handles automatically shadows, hidden surfaces and clipping
Radiosity
Based on the effect that light sources and surfaces are emitting light
Is done by iterating the emission of light
In the first step only surfaces with direct light are visible
In the next step these surfaces emits light to other surfaces
This is repeated until the result is satisfactory
COMPUTER GRAPHICS AND VIRTUAL REALITY
Ray-Tracing and Radiosity are two different methods for advanced 3D computer graphics rendering
Mention two advantages and two disadvantages of each of the two methods.
Ray-tracing Advantages -- Good at transparent objects -- Handles specular reflection well -- Good at point light sources
Disadvantages
- Unnaturally sharp shadows
- Bad at diffuse reflection
- Not good at global light sources
- Slow
Radiosity Advantages -- Good at diffuse reflection -- Gives realistic shadows -- Good at global light sources
Disadvantages
- Not good at specular reflection
- Not good for transparent objects
- Not good at point light sources
- Slow
COMPUTER GRAPHICS AND VIRTUAL REALITY
Three different types of coordinate systems are used in virtual reality models (and also in CAD
models). Specify these and what they are used for.
- World coordinate system (w)
• Only one in each model
• The position of other objects are related to this - Object coordinate system (o)
• One coordinate system per object in the model
• Positions the object relative to the world coordinate system or relative to a superseding
object - The coordinate system of the observer (Virtual Observer)
• Makes it possible to travel around in the model
MISCELLANEOUS
Mention the five basic needs of a PDM system.
Capture information at the source
Organize information
Distribute the information – when, where, what, to whom
Search, re-use and present information
Secure storage of information over a long time
MISCELLANEOUS
Geometry models, created in a CAD-system, are used by a number of different functions
(departments etc.) within the product development process. Describe some problems related to
this.
Different functions uses different software requiring different file formats: problem with file
conversion
Problem with access to the right models
MISCELLANEOUS
Mention three reasons for the increased industrial need for IT support for product development
Shorter lead-times and product lifecycles
Increased complexity: variants, functions, components etc.
Collaborative product development
MISCELLANEOUS
What characterizes a geometrically robust assembly concept?
A geometrically robust design is a design that allows manufacturing and assembly variation without
jeopardizing function or aesthetics.
MISCELLANEOUS
What are the advantages of a geometrically robust assembly concept?
Easier process adjustment and tuning
Shorter start and ramp-up times
Global production with high and equal quality level
MISCELLANEOUS
Mention two advantages of using off-line programming.
Avoid costly mistakes with real machines
Faster and more efficient programming
Possibility to make new programs without stopping the production
GEOMETRY MODELING
What order is usually used in CAD-systems?
3: rd. grads kurva
GEOMETRY MODELING
Specify, describe and exemplify three different types of solid models.
- Decomposition models
Voxels: a solid is composed by a number of cubes
Cell based: built up by polygons - Constructive models
Solids are created by manipulating primitives with Boolean operators
Example: a circular hole in a cube is created by subtracting the cylinder from the cube - Boundary representation
The solid is defined with points, curves and surfaces plus a definition of what is inside the model
Example: a solid can be created by rotating a curve
GEOMETRY MODELING
Bézier curves are defined by the following equations: P(u) = ∑ p_iB_i,n(u) i=0 to n
Describe the different components of the formula and what they are used for
p_i = control points, defines the curve
n = order of the curve
B_i,n = weight function, defines how the different control points affect the curve
n + 1 = number of control points
GEOMTRY MODELING
NURBS is the most commonly used type of curves in modern CAD systems. What geometrical forms can be
represented with NURBS but not with Bézier or B-splines?
Bézier and B-splines cannot represent conical and circular forms exactly
GEOMTRY MODELING
Describe how solid models are created with CSG (Constructive Solid Geometry)
Created by manipulating “primitives” with Boolean operators (union, sections, subtraction)
GEOMETRY MODELING
Geometry models, created in a CAD-system, are used by a number of different functions (departments etc.) within
the product development process. Describe some problems related to this
- – Different functions uses different software requiring different file-formats: problem with file conversion
- – Problem with access to the right model
GEOMETRY MODELING
Describe the concepts of Component based and Feature based assembly modelling
Component based:
— The position of a part in the assembly is determined by specifying its position and orientation in global or relative
coordinates
Feature based:
- – Associates form features on different parts
- – Handles restrictions on form, position, orientation etc. between mating form features
GEOMETRY MODELING
Mention three benefits of using virtual product models in the product realization process
- – Minimizing the need for costly physical prototypes
- – Finding problems as early as possible in the development process (easier and cheaper to fix)
- – Faster development process with efficient tools (time to market)
GEOMETRY ASSURANCE
Variation analysis (with Monte Carlo simulation), Contribution analysis and Stability analysis are three different
types of analyses used in CAT (Computer Aided Tolerancing) software. Describe how these methods work and what
they are used for.
Variation Analysis:
- – Calculates a statistical prediction of the variation in critical measures
- – Statistical method – random data
- – Tolerances on parts (inputs) are randomly generated within defined distributions, tolerancing and Cp
- – Distributions for critical measures (outputs) are generated from thousands of iterations
- – All kinematical relations and sensitivities are captured in a 3D assembly model
Contribution Analysis:
— Calculates a ranked list of how all input tolerances contributes to the variation into the critical measures
— All input parameters are varied (one at a time) within their tolerances on 3 levels
— Max output is registered
Contribution is calculated in percent as
% contribution_i = 100
(∆output^2_i )/ (∑(i=0 ->n) ∆output^2)
Stability Analysis:
a. Can be used to analyse the influence of each part locating scheme on
i. Variation amplification, color-coding
ii. Position stability of parts
iii. Critical product dimension (measures)
b. It is done by disturbing each locating point with a unit disturbance
c. And summarising their contribution with RSS
GEOMETRY ASSURANCE
What are the factors that contribute to variation in critical dimensions on a product?
\:Component variation --- Process variation --- Machine precision --- Manufacturing process \:Assembly variation --- Process variation --- Assembly presicion --- Assembly process \:Design concepts --- Robustness
GEOMETRY ASSURANCE
What characterizes a geometrically robust design?
A geometrically robust design is a design that allows manufacturing and assembly variation without jeopardising function or aesthetics
GEOMETRY ASSURANCE
In order to perform a 3D variation analysis a 3D assembly model is needed. Describe the necessary components and
inputs for a 3D assembly model.
A 3D assembly model consists of:
- -Parts
- -Subassemblies
- -Positioning systems
- -Input tolerances with range and type of distribution
- -Critical measures
COMPUTER GRAPHICS AND VIRTUAL REALITY
Describe how so called shutter glasses can be used to obtain stereo projection on an ordinary computer screen
–Gives 3D by “blocking” one eye at a time
–This is synchronised with the computer which alternatives between views for the left and the right eye
respectively
– The view is calculated with respective projection
–The view for the left eye is calculated by moving the picture half of the distance of the eyes to the left
–For the right eye the picture is moved the same distance to the right
COMPUTER GRAPHICS AND VIRTUAL REALITY
Describe three different visualization systems for VR. Mention advantages and disadvantages with the different
systems.
Desktop (“Fish tank”) VR (Pc (+tracker)(+glove)(+stereo glove)): \+ High display resolution \+ Cheap \+ Simple to use - Narrow field of view (FOV) - Low degree of immersion - Not the natural scale
Helmet (HDM) (some sort of helmet or glasses with one display for each eye): \+ wider FOV \+ stereo viewing \+ high degree of immersion \+ cheap \+ simple to install - helmet weight - isolation from the world - only one user - low screen resolution
Powerwall ( two or more projectors working together, special software to coordinate the picture): \+ wide FOV \+ natural size \+ stereo display \+ high resolution - not so high degree of immersion -advanced computers -advanced technology -expensive
COMPUTER GRAPHICS AND VIRTUAL REALITY
Gouraud and Phong are two methods for rendering over multiple polygon surfaces. Describe how they works.
Gouraud:
- -Calculate normal vector for all polygon surfaces
- -Calculate “mean normal vectors” for the vertices of the polygon
- -Mean normal + illumination model => calculation of intensities in the vertices
- Intensity values are interpolated line by line over the polygon
Phong:
- Calculate normal vector for all polygon surfaces
- Calculate “mean normal vectors” for the vertices of the polygon
- Surface normal are interpolated line by line over the polygon
- Surface normal + illumination => calculation of intensities in each pixel
COMPUTER GRAPHICS AND VIRTUAL REALITY
Light from a point light source can be reflected in two ways from a surface in a computer model, diffuse and
specular. Describe the difference between diffuse and specular reflection.
Diffuse:
Light hitting the surface is spread equally in all directions => placement of the light source influences but not the
placement of the observer.
Specular:
The appearance of smooth, polished, surfaces is dependent of both the placement of the light source and the position
of the observer i.e. the light is reflected mostly in one direction.
COMPUTER GRAPHICS AND VIRTUAL REALITY
Gouraud and Phong are two different methods to obtain smooth shading of triangulated surfaces. Why is Gouraud
called vertex shading and Phong pixel shading?
Gouraud vertex shading:
–The colour of a triangle is calculated at each vertex (corner) with the normal at that corner. The colour is then
interpolated over the surface of the triangle
Phong pixel shading:
–The normal at the vertices (corners) are interpolated over the surface of the triangle and the colour is calculated for
each pixel.
USE OF GEOMETRY DATA
Mention four different types of geometrical models and what they are used for.
: CAD-model
–Defines the geometry of the product
: Mechanical model
–View of the product as a mechanical system, used by design to evaluate mechanical behaviour
: Visual model
–View of the product appearance, used by design, management, marketing
: Ergonomic Model
–View of the products ergonomic properties, used by design, manufacture, service
USE OF GEOMETRY DATA
Mention two different usages of geometry models within production
- -Ergonomic simulation
- - Off-line programming of industrial robots, NC-machines, CMMs
USE OF GEOMETRY DATA
Give two examples of usage of ergonomic simulation with computer manikins.
- -Evaluation of user interfaces
- -Evaluation of concept for manual assembly
USE OF GEOMETRY DATA
What type of geometry representation is usually used in a digital mockup (DMU)?
Triangulated surface model
PLM/PDM
What is the aim of a PLM system?
PLM systems aim to support the creation and management of all information related to a product throughout its
lifecycle
PLM/PDM
How can PLM systems support an Engineering Change Management process?
Automation, e.g.:
- Workflow functions automate information transfer
- Standard templates for change documents
- Parameterized CAD/CAM/CAE models
Monitoring, e.g.:
- Follow-up of engineering changes status
- Overview of all engineering changes
Information access & retrieval, e.g.:
- All have access to engineering changes in PDM database
- All info and doc’s related to an engineering change is connected
Quality assurance, e.g.:
- Only authorized individuals may promote change requests
- Control of versions, status and effectivity
Review, e.g.:
- Check change history, rollback possible
PLM/PDM
Describe two different types of Bill of Material that are used in PLM systems.
- The engineering Bill of material (E-BOM)
Organizing the various components of a product including software - Manufacturing Bill of Material (M-BOM)
Structured according to the way a product is sourced and manufactured.
Primarily supports supply chain including manufacturing
PLM/PDM
Mention two important tasks for a PLM systems functions for version control.
- -Keep track of all versions of a specific piece of information
- -Pick up the currently valid version as well as at given earlier dates – the information’s history
MISCELLANEOUS
What are the industrial benefits of using parameterization to reuse engineering knowledge?
Shorter time – means more iterations. More iterations means better products because:
– Larger amount of designs considered and evaluated in concept phase
– Knowledge related to lifecycle aspects (design for: assembly, manufacturing, serviceability, environment…) can
be integrated in detail design phase
– Synthesis – analysis loops can be shortened – verification feedback instant
MISCELLANEOUS
What does perceived geometrical quality depend on?
- -Visual sensitivity
- -Geometrical form
- -Split-line placement
- Geometrical sensitivity (locator placement)
- -Tolerances/distributions
MISCELLANEOUS
What are the three types of modeling components in a multi-body system dynamic simulation?
- -Rigid bodies
- Constraints (joints, motions)
- -Forces (gravity, spring/dampers, friction etc.)
