Exam 2021 & 2022 Flashcards
Geometry Modelling:
Describe the following types of solid models, mention some advantages or
disadvantages for each (6p)
Decomposition models
Decomposition models:
Can be made of:
o Voxels: the solid is composed of a number of cubes
o 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
Geometry modeling
a) Describe the following types of solid models, mention some advantages or
disadvantages for each (6p)
Constructive models
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)
Geometry modeling
a) Describe the following types of solid models, mention some advantages or
disadvantages for each (6p)
Boundary representation
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 modelling:
What are half spaces and how can they be used to create geometric primitives?
In geometry, a half space is a region of space that includes all the points on one side of a flat, infinite plane. The term “half” is used because it divides space into two parts: one on the side of the plane, and the other on the opposite side. This flat, infinite plane is usually defined by an equation
Solid primitives are created by combining half spaces with Boolean operators
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
Note:
1. Select a plane, choose 3 points in it that covers largest area
2. Select 2 other points that follows that line both in the “depth” of the solid
3. Select the last point to lock the figure in translating direction, the edge surface
Computer graphics and virtual reality: Describe how the Z-buffer algorithm for depth sorting of objects drawn on a computer
screen works.
The Z-buffer algorithm sorts objects on a computer screen based on their depth. It works by assigning a depth value (Z) to each pixel. When rendering objects, it checks if the Z value of the current object is closer to the viewer than the Z value already stored for that pixel. If so, it updates the pixel’s color. This process ensures that closer objects appear in front of farther ones, creating a 3D effect in computer graphics.
When all polygons are checked the screen is updated with the color closest to the observe.
Gouraud and Phong are two methods for rendering over multiple polygon surfaces. Describe
how they work.
Gourad:
Calculate normal vector for all polygon surfaces
Calculate ”mean normal vectors” for the vertices of the polygon
Mean normal + Illumination model => calculation of intensities (colors) 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 normals are interpolated line by line over the polygon
Surface normal + Illumination model => calculation of intensities (colors) in each pixel
Describe how Parallel and Perspective Projections are calculated. What is the visible
difference (on screen) between the two projections?
In perspective projection, objects appear smaller when they are farther away. It’s like when you see a road and the lines converge in the distance. We use similar triangles to figure this out.
In parallel projection, objects keep the same size no matter how far they are. We do this by setting the z-coordinate to zero, like when you’re looking at a map, and all the places have the same size.
So, in short, perspective makes far things look smaller, while parallel keeps things the same size no matter how far they are.
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 use different software requiring different file formats: problem with file
conversion
Problem with access to the right models
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)
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
What is a digital mockup (DMU) and for what is it used?
A special type of component based assembly model developed to be able 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. Tringulated surface
Curves used in geometry modeling can be of different order. What are the advantages and
disadvantages of higher order curves? (2p
Advantage: increased precision
Disadvantages: risk for corrupt curves, increased calculation time
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.
Describe four different aspects that has to be considered when designing for injection
molding.
For example:
Assign proper draft angles, rule of thumb: 1 deg. per inch cavity depth
Wall thickness should be uniform, e.g. coring out could be used to avoid thick walls
Compensate for shrinkage
Locate parting line, ejector pins and gate appropriately
Avoid sharp corners and small corner radiuses, gives stress concentration and hinders a
streamlined flow for the molten plastics
Describe how solid models are created with CSG (Constructive Solid Geometry). (2p)
Solid models are created by manipulating “primitives” with Boolean operators (union, sections,
subtraction)
Bézier curves are defined by the following equation: p(u)=Sum (i=o, n antal) p_i, B_i,n(u)
Describe the different components of the formula and what they are used for.
Pi: control points, defines the curve
Bin: weight functions, defines how the different control points affect the curve
n: Order of the curve
n+1: number of control points
u independent variable 1
In CSG the concept of half spaces is used. Describe/exemplify how they work and how they
are used to define geometry. (3p
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 0222 ryx
Solid primitives are created by combining half spaces with Boolean operators
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
Computer aided tolerance: Describe Variation analysis:
Variation analysis (with MC)
Calculates a statistical prediction of the variation in critical measures
Statistical method – random data
Tolerances on parts (inputs) are randomly generated within defined distributions, tolerances
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
Computer aided tolerance: Describe Contribution analysis:
Calculates a ranked list of how all input tolerances contributes to the variation in the critical
measures
All input parameters are varied (one at the time) within their tolerances on 3 levels
Max output is registered
Contribution is calculated in percent as output_2/output_1
Computer aided tolerance: Describe Stability analysis:
Can be used to analyze the influence of each part locating scheme on
o Variation amplification, color-coding
o Position stability of parts
o Critical product dimensions (Measures)
It is done by disturbing each locating point with a unit disturbance
And summarizing their contribution with RSS
Is often used to evaluate different positioning systems
Computer graphics and VR: describe Desktop VR w pros and cons
Desktop (”Fishtank”) VR
PC (+tracker) (+glove) (+stereo glasses)
Advantages
o High display resolution (1600x1200)
o Cheap
o Simple to use
Cons:
- lack of realism
- Interactive ;(
Computer graphics and VR: describe Helmet w pros and cons
Function
o Some sort of helmet or glasses with one display for each eye
Advantages
o Wider FOV
o Stereo viewing
o High degree of immersion
o Relatively cheap
o Simple to install
Disadvantages
o Helmet weight 1 - 4 kg
o Isolation from the world
o Only one user
o Relatively low screen resolution
Computer graphics and VR: describe Powerwall w pros and cons
Function
o Two or more projectors working together
o Special software to coordinate the picture
Advantages
o Wide FOV
o Natural size (cars)
o Stereo display (with shutter or polarized glasses)
o High resolution (3200x1024)
Disadvantages
o Not so high degree of immersion
o Advanced computers
o Advanced technology to obtain invisible edges between projectors
o Expensive
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.
(2p
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
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:
The color of a triangle is calculated at each vertex (corner) with the normal at that corner. The color
is then interpolated over the surface of the triangle.
Phong: The normal at the vertices (corners) are interpolated over the surface of the triangle and the
color is calculated for each pixel.
Give two examples of usage of ergonomic simulation with computer manikins. (2p)
Evaluation of user interfaces
Evaluation of concepts for manual assembly
What are the tree types of modeling components in a multi-body system dynamic
simulation? (3p)
Rigid bodies
Constraints (joints, motions)
Forces (gravity, spring/dampers, friction etc.)
Mention two different usages of geometry models within production (2p)
Ergonomic simulation
Off-line programming of industrial robots
Off-line programming of NC-machines
Off-line programming of CMMs
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)
Describe the concepts of Component based and Feature based assembly modeling (2p)
Component based assembly modeling:
The position of the part in the assembly is determined by specifying its position and
orientation in global or relative coordinates
Feature based assembly modeling:
Associates form features on different parts
handles restrictions on form, position, orientation etc between mating form features
b)
A feature
o is a physical part of a detail
o can be linked to a generic form
o has a specific engineering role (function, manufacturing method, simulation method, …)
o has predictable properties
c)
Shorter lead-times and product lifecycles
Increased complexity: variants, functions, components etc.
Collaborative product development
d)
Applying guidelines for manufacturing already at the design stage leads to:
Reduced manufacturing cost
Shortened lad time
What characterizes a feature (in the geometry modeling context)? (3p)
A feature
o is a physical part of a detail
o can be linked to a generic form
o has a specific engineering role (function, manufacturing method, simulation method, …)
o has predictable properties
Mention three reasons for the increased industrial need for IT support for product
development (3p)
Shorter lead-times and product lifecycles
Increased complexity: variants, functions, components etc.
Collaborative product development
Mention two reasons why it is important for the designer to have knowledge about the
manufacturing process.
Applying guidelines for manufacturing already at the design stage leads to:
Reduced manufacturing cost
Shortened lead time
