Exam 2012

Question 1

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

Answer 1

1. Hierarchical relations
   - -> assembly–> subassembly –> part
2. Mating conditions
   - -> geometrical restrictions
3. Mechanical degrees of freedom

Question 2

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

Answer 2

1. Solid models support higher levels of functionality and automation than surface models
   - -> example calculation of mass and moments of inertia
2. Solid models allow the designer to work with higher level objects rather than points, curves and surfaces

Question 3

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

Answer 3

Solid models are created by manipulating “primitives” with Boolean operators (union, sections, subtraction)

Question 4

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

Answer 4

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
Solid primitives are created by combining half-spaces with Boolean operators

Construction of a cylinder:
H1: x^2+y^2+r^2<0
H2: z>0
H3: z-h<0

C=H1∩ H2∩ H3
(mängden av alla element som tillhör bade H1, H2 och H3)

Question 5

How does monte carlo simulation work?

Answer 5

Model consist of

3D assembly model with defined locating schemes
Input tolerances with range and type of distribution
critical measures
Simulation:

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

Question 6

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

use the points in the figure
try to make it as robust as possible
Motivate your selection of points

Answer 6

Select P311, P321 and P318 as A1, A2 and A3 defining the plane.
- -> This will maximize the area of the triangle which makes it robust.
Select P311 and P321 as B1 and B2
- -> This gives a long line which makes it robust.
The selection of C1 does not have a large influence on the robustness we can e.g. select P321 which make it easier to make a fixture.
(Many other solutions might be correct, some even more robust. In order to get 6 opints the shoises should be motivated in a correct way)

Question 7

Describe the difference between diffuse and specular reflection.

Answer 7

Diffuse reflection:
Light hitting the surface is spread equally in all directions
–> placement of the light source and the position of the observer i.e. the light is reflected mostly in one direction

Specular reflection:
The appearance of smooth, polished surfaces is dependent of both placement of the light source and the position of the observer, i.e. the light is reflected mostly in one direction.

Question 8

In what ways can light from a point light source be reflected from a surface in a computer model?

Answer 8

1. Specular reflection
2. Diffuse reflection

Question 9

Name two different methods to obtain smooth shading of triangulated surfaces!

Answer 9

1. Gouraud
2. Phong

Question 10

Why is gouraud called vertex shading?

Answer 10

The color of the triangle is calculated at each vertex with the normal at that corner. The color is then interpolated over the surface of the triangle

Question 11

Why is Phong called pixel shading?

Answer 11

The normal at the vertices are interpolated over the surface of the triangle and the color is calculated for each pixel

Question 12

Describe desktop VR and mention some advantages and disadvantages

Answer 12

Desktop VR
- PC (+tracker) (+glove) (+stereo glasses)
ADVANTAGES:

high display resolution
Cheap
Simple to use
DISADVANTAGES:

Narrow field of view (FOV)
Low degree of immersion
Not the natural scale (for e.g. cars)

Question 13

Describe the Helmet as visualization system for VR and mention some advantages and disadvantages

Answer 13

Helmet (HMD)
- Some sort of helmet or glasses with one display for each eye

ADVANTAGES:

Wider FOV
Stereo viewing
High degree of immersion
Relative cheap
Simple to install
DISADVANTAGES:

Helmet weight (1-4 kg)
Isolation from the world
Only one user
Relatively low screen resolution

Question 14

Describe the Powerwall as visualization system for VR and mention some advantages and disadvantages

Answer 14

Powerwall (Large Volume Display)

Two or more projectors working together
Special software to coordinate the picture
ADVANTAGES:

Wide FOV
Natural size
Stereo display (with shutter or polarized glasses)
High resolution (3200x1024)
DISADVANTAGES:

Not so high degree of immersion
Advanced computers
Advanced technology to obtain invisible edges between projectors
Expensive