Exam 121025 Flashcards
Mention three different aspects that have to be included in an assembly model (in e.g. a CAD system)
- Hierarchical relations
- -> assembly–> subassembly –> part - Mating conditions
- -> geometrical restrictions - Mechanical degrees of freedom
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
Describe how solid models are created with CSG (Constructive Solid Geometry)
Solid models are created by manipulating “primitives” with Boolean operators (union, sections, subtraction)
In CSG the concept of half spaces is used. Describe/exemplify how they work and how they are used to define geometry
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)
How does monte carlo simulation work?
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
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
- 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)
Describe the method for defining a 3-2-1 locating scheme.
- use the points in the figure
- try to make it as robust as possible
- Motivate your selection of points
- Select three points that will maximize the area of the triangle which will make it robust
- Select two points creating a long line which makes it robust
- Select the last point, which doesn’t have such large influence on the robustness but which can make it easier to make a fixture
Describe the difference between diffuse and specular reflection.
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.
In what ways can light from a point light source be reflected from a surface in a computer model?
- Specular reflection
2. Diffuse reflection
Name two different methods to obtain smooth shading of triangulated surfaces!
- Gouraud
2. Phong
Why is gouraud called vertex shading?
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
Why is Phong called vertex shading?
The normal at the vertices are interpolated over the surface of the triangle and the color is calculated for each pixel
Describe desktop VR and mention some advantages and disadvantages
- 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)
Describe the Helmet as visualization system for VR and mention some advantages and disadvantages
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
Describe the Powerwall as visualization system for VR and mention some advantages and disadvantages
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
