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Exam Flashcards

1
Q

Curves used in geometry modelling can be of different order. What are the advantages and disadvantages of higher order curves?

A

Advantages: increased precision
Disadvantages: risk of corrupt curves, increased calculation time

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2
Q

What order is usually used in CAD-systems?

A

3: rd

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3
Q

Bézier and B-spline are two types of curves used in geometry modelling. What is the advantage of B-spline curves?

A
  • Better control of the curve
  • Order of the polynomial does not increase with the number of control points
  • Easier to define joined curve segment
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4
Q

Specify, describe and exemplify three different types of solid models.

A
  1. Decomposition models
    Voxels: a solid is composed by a number of cubes
    Cell based: built up by polygons
  2. 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
  3. 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
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5
Q

Bézier curves are defined by the following equations:

p(u)=Sum of (pi Bi,n(u))

Describe the different components of the formula and what they are used for.

See question 5 in PDF

A 
pi = control points, defines the curve
n = order of the curve
Bi,n = weight function, defines how the different control points affect the curve 
n+1 = number of control points
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6
Q

Describe how trimmed parametric surfaces are defined/created.

A
  • Parametric surfaces are defined in similar way as parametric curves but with two parameters u and v.
  • Parametric curves are defined in the same 2D space
  • -> They are used to create holes (inner trim curve)
  • -> Or the outer boundary of the surface (outer trim curve)
  • The trimmed parametric surface is transformed to 3D space
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7
Q

What do C0, C1 and C2 continuity between two curve segments mean?

A
  • C0-continuity = Two curve segments are joined without constraints (point continuity)
  • C1-continuity = The curve segments have the same direction at the common point (tangent continuity)
  • C2-continuity = The curve segments have the same curvature at the common point (curvature continuity)
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8
Q

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?

A

Bézier and B-splines cannot represent conical and circular forms exactly.

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9
Q

What characterizes a feature (in the geometry modelling context)?

A

A feature

  • Is a physical part of a detail
  • Can be linked to a generic form
  • Has a specific engineering role
  • Has predictable properties
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10
Q

Describe the steps necessary to create a solid, using surface modelling, in a modern CAD system.

A
  • Create wireframe elements
  • Create surfaces from the wireframe elements
  • Trim the surfaces together
  • Join the surfaces together to a uniform element
  • Transform into a solid
  • (Add fillets)
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11
Q

Mention three different aspects that have to be included in an assembly model (in e.g. a CAD system).

A
  • Hierarchical relations (Assembly -> sub-assembly -> part)
  • Mating conditions (geometrical restrictions)
  • Position and orientation of sub-assemblies and parts, globally and locally
  • Mechanical degrees of freedom
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12
Q

Mention two advantages of using solid models instead of e.g. surface models.

A
  • Solid models support higher levels of functionality and automation than surface models
  • Solid models allow the designer to work with a higher level objects rather than points, curves and surfaces.
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13
Q

Describe how solid models are created with CSG (Constructive Solid Geometry).

A

Created by manipulating “primitives” with Boolean operators (union, sections, subtraction)

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14
Q

In CSG the concept of half spaces is used. Describe/exemplify how they work and how they are used to define geometry.

A

Real analytical functions f(x,y,z) defined in 3D which splits the space in two half spaces:

One half space where f(x,y,z)<0
One half space where f(x,y,z)>0

Example: Cylindrical half space x^2 + y^2 − r^2 < 0
Construction of the cylinder C:
H1: x^2 + y^2 − r^2 < 0
H2: z > 0
H3: z − h < 0 
C: H1 ∩ H2 ∩ H3
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15
Q

Describe the following three types of solid models, mention some advantages and disadvantages for each.

  1. Decomposition models
  2. Constructive models
  3. Boundary representation
A

Decomposition models
Can be made of:
Voxels: the solid is composed by 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 calculation

Constructive models
Solid models are created by manipulating Boolean operators
- It is hard to handle general surfaces
- It is very compact

Boundary representation
The solid is defines by points, curves and surfaces plus a definition of what is inside the model
- Uses graphical methods e.g. sweep, rotate
- Can use parametric surfaces
- Can use Boolean methods

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16
Q

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.

A
  • Different functions uses different software requiring different file-formats: problem with file conversion
  • Problem with access to the right model
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17
Q

Describe the concepts of Component based and Feature based assembly modelling

A

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
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18
Q

Mention three benefits of using virtual product models in the product realization process

A
  • 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
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19
Q

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.

A

Variation Analysis
- Calculates a statistical prediction of the variation in critical measures
Simulation:
- Randomly assign one value for each input within its defined tolerance
- Assemble the model according to the defined locating schemes
- Calculate the critical measures and store the result for each iteration
- Repeat this at least 1000 times
- Calculate the distribution of the critical measures
- 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 in critical measures.
  • All input parameters are varied (one at a time) within their tolerances on 3 levels
  • Max output is registered for all measures
  • Contribution is calculated in percent

Stability Analysis
It is used to identify sensitive areas and sensitivity factors and guides optimisation of locator position
- 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 summarised with RSS to give a value for the locating scheme

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20
Q

Describe how a 3-2-1 locating scheme works

A
  • 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 one rotation and one translation: TY, RZ
  • Tertiary point C1, locks the geometry in space in one translation: TX
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21
Q

What are the factors that contribute to variation in critical dimensions on a product?

A

Look in PDF file, Q21

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22
Q

What characterizes a geometrically robust design?

A

A geometrically robust design is a design that allows manufacturing and assembly variation without jeopardising function or aesthetics

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23
Q

How does Monte Carlo variation simulation work?

A

Model consist of:
- 3D assembly model with defined locating schemes

Simulation:

  • Randomly assign one value for each input within its defined tolerance
  • Assemble the model according to the defined locating schemes
  • Calculate the critical measures and store the result for each iteration
  • Repeat this at least 1000 times
  • Calculate the distribution of the critical measures
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24
Q

How does stability analysis work and for what is it used?

A

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 summarised with RSS to give a value for the locating scheme

It is used to identify sensitive areas and sensitivity factors and guides optimisation of locator position.

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25
Q

Define a 3-2-1 locating scheme for the box in the figure

A

Look in PDF file, Q25

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26
Q

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

A 3D assembly model consists of:

  • Parts
  • Subassemblies
  • Positioning systems
  • Input tolerances with range and type of distribution
  • Critical measures
27
Q

How does contribution analysis work and for what is it used?

A

Model consists of:
- 3D assembly model with defined location schemes

Model consists of:

  • All input parameters are varied (one at a time) within their tolerances on 3 levels
  • Max output is registered for all measures
  • Contribution is calculated in percent

It is used to calculate a ranked list of how all input tolerances contributes to the variation in the critical measures

28
Q

Ray-Tracing and Radiosity are two different methods for advanced 3D computer graphics rendering.

a) Describe how these methods work.
b) Mention two advantages and two disadvantages of each of the two methods.
c) Describe how so called shutter glasses can be used to obtain stereo projection on an ordinary computer screen.

A

a)
Ray-Tracing
- Follow the light rays from the observer, one for each pixel
- Calculate how the ray is mirrored and refracted 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 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
b)
Ray-tracing
- Good at transparent objects
- Handles specular reflection well
- Good at point light sources

Radiosity

  • Good at diffuse reflection
  • Gives realistic shadows
  • Good at global light sources

The disadvantages is the others advantages. Both are slow

c)
- 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

29
Q

Describe three different visualization systems for VR. Mention advantages and disadvantages with the different systems.

A 
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
30
Q

Describe the RGB colour model

A

The colour is accomplished with a mixture of three primary colours
Red [0 – 1]
Green [0 – 1]
Blue [0 – 1]

31
Q

What are homogenous coordinates and why are they used in computer graphics?

A

Homogenous coordinates are created by adding an extra coordinate, w, to the Cartesian coordinates:
Pcartes [x y z] –>
Phomogen[x y z w]

They are used in order to simplify the calculation of transformations. With homogenous coordinates all transformations can be calculated as a matrix multiplication which can be optimised for speed in the graphics processor

32
Q

SKIP THIS QUESTION

Describe (with text, figures and equations) the four steps for collision detection between two objects defined by triangle surfaces

See question 32 in PDF

A
  • Do a coarse “mini-max-test” with bounding boxes/spheres to determine if a collision might be possible at all
  • If yes, check if any of the points on body A is inside body
  • Test if any of the edges on body A intersects an infinite plane defined by some of the polygons on body B
  • Test if the endpoints of the edge are on different sides of the plane. This is done by using the
    equation of the plane
  • If yes, calculate the intersection point between the edge and the plane:
  • Use the equation for the line between the points and the equation of the plane 

Check if the intersection point is on the polygon:

  • Calculate the area of the three sub triangles defined by the intersection point and the original vertices of the triangle
  • If all of these areas have the same sign, the intersection point is inside the triangle and it is a collision
  • To speed up the calculation this can be cone I 2D by projecting the triangle on a plane
33
Q

Bump-mapping is a way of using textures to give a smooth surface an irregular appearance. Describe how it works.

A

It works by modifying the surface normal that is used in the illumination model. The value of the modification is obtained from a texture

34
Q

Gouraud and Phong are two methods for rendering over multiple polygon surfaces. Describe how they works.

A

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
35
Q

Describe how a shadow buffer can be used to create shadows.

A
  • 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
36
Q

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.

A

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.

37
Q

Gouraud and Phong are two different methods to obtain smooth shading of triangulated surfaces. Why is Gouraud called vertex shading and Phong pixel shading?

A

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.

38
Q

What is a digital mock up (DMU) and for what is it used?

A
  • 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.
39
Q

Mention four different standards for geometry exchange and what type of geometry data they can handle.

A 
STEP – mathematical
IGES – mathematical
STL – triangulated
VRML – triangulated
JT – both
40
Q

Mention two different tasks in the product realization process where off-line programming can be used.

A
  • Programming of industrial robots
  • Programming of NC-machines
  • Programming of CMMs
41
Q

Mention two advantages of using off-line programming.

A
  • Avoid costly mistakes with real machines

- Faster and more efficient programming

42
Q

Mention four different types of geometrical models and what they are used for.

A

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

43
Q

Mention two different usages of geometry models within production.

A
  • Ergonomic simulation

- Off-line programming of industrial robots, NC-machines, CMMs

44
Q

Give two examples of usage of ergonomic simulation with computer manikins.

A
  • Evaluation of user interfaces

- Evaluation of concept for manual assembly

45
Q

What type of geometry representation is usually used in a digital mockup (DMU)?

A

Triangulated surface model

46
Q

Mention three reasons for the increased industrial need for IT support for product development.

A
  • Shorter lead-times and product lifecycles
  • Increased complexity: variants, functions, components etc.
  • Collaborative product development
47
Q

What is the aim of a PLM system?

A

PLM systems aim to support the creation and management of all information related to a product throughout its
lifecycle

48
Q

Mention three things that a Product Data Management (PDM) database should provide.

A
  • A database “vault” for secure storage of information
  • Product structure management
  • Process management
49
Q

PLM and PDM are two acronyms used in relation to the product realization process. What do they stand for?

A

Product Lifecycle Management and Product Data Management

50
Q

Describe what PLM and PDM systems are and how they relate to each other.

A

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 bur also tools to create data, e.g. a CAD, simulation software, etc.

51
Q

How can PLM systems support an Engineering Change Management process?

A

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

52
Q

Describe two different types of Bill of Material that are used in PLM systems.

A
  1. The engineering Bill of material (E-BOM)
    Organizing the various components of a product including software
  2. Manufacturing Bill of Material (M-BOM)
    Structured according to the way a product is sourced and manufactured. Primarily supports supply chain including manufacturing
53
Q

Mention two important tasks for a PLM systems functions for version control.

A
  • 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
54
Q

SKIPPA

Describe how the Z-buffer algorithm for depth sorting of object drawn on a computer screen works.

A
  • Initiate video memory and a Z-buffer for each pixel on the screen
  • depth [x, y]= inf, refresh [x, y] = Ibackground
  • Polygons are rendered in an arbitrary sequence
  • For each pixel: check if the z-coordinate of the current polygon is less than the stored one (use the plane
    equation for the polygon)
    Ax+By+Cz+D=0, Zx,y= -(Ax+By+D)/C
  • In that case the video memory is updated for the pixel
    Zx,y < depth[x,y] –> depth[z,y]=Zx,y
    refresh [x, y] =Isurf
  • When all polygons are checked the screen is updated with the colour closest to the observer
55
Q

What are the industrial benefits of using parameterization to reuse engineering knowledge?

A

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
56
Q

The Method of Influence Coefficients can be used to perform Monte Carlo variation simulations on non-rigid parts. How does it work?

A
  • 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 (approx. a factor 1000)
  • If the locators or support points are moved, a new linearization has to be done
57
Q

What does perceived geometrical quality depend on?

A
  • Visual sensitivity
  • geometrical form
  • split-line placement
  • Geometrical sensitivity (locator placement)
  • Tolerances/distributions
58
Q

What characterizes a geometrically robust assembly concept?

A

Geometrically robust concepts are insensitive to manufacturing variation

59
Q

What are the advantages of a geometrically robust assembly concept?

A
  • Easier process adjustment and tuning
  • Shorter start and ramp-up times
  • Global production with high and equal quality level
60
Q

What are the three types of modeling components in a multi-body system dynamic simulation?

A
  • Rigid bodies
  • Constraints (joints, motions)
  • Forces (gravity, spring/dampers, friction etc.)
61
Q

Mention four of the essential tasks for a PDM-system

A
  • Capture information at the source
  • Organize information
  • Run the information flow – when, where, what, to whom (distribute information)
  • Search and re-use information
  • Secure storage of information over a long time
62
Q

a) What is virtual geometry assurance?

b) Describe how it can be used in some task in the geometry development process.

A

a) Using computer tools to perform geometry assurance tasks on virtual product models

b)

  • Analyse different assembly concepts
  • Analyse and define robust locating systems
  • Simulate and verify the final demands
  • Virtual matching for trimming the production process
63
Q

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.

A
  1. World coordinate system
    - Only one in each model
    - The position of other objects are related to this
  2. Object coordinate system
    - One coordinate system per object in the model
    - Positions the object relative to the world coordinate system or relative to a superseding object
  3. The coordinate system of the observer (Virtual Observer)
    - Makes it possible to travel around in the model

Advanced CAD (1 decks)

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