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
