Thesis (PhD)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: Broadly speaking Reverse Engineering is the process of digitising a physical object
and creating a computer model of the object. If sharp edges formed by two surfaces
can be extracted from a point cloud (which is the set of measured points) it can speed
up the segmentation of the point cloud and the edges may also be used to construct
swept surfaces (or various other types of surface that best captures the design intent).
A strategy is presented to "scan" edges. The strategy simulates a CMM (Coordinate
Measurement Machine) as it would scan a sequence of short lines straddling the edge.
Rather than measuring on a physical object, the algorithm developed in this
dissertation "scans" on the points in the point cloud. Each line is divided in two parts,
or line sections, belonging to the surfaces fanning the edge. The points of the line
sections are then approximated with polynomials. Each edge point is the intersection
of two such polynomials. In many engineering components sharp edges are replaced
with fillet radii or the edges become worn or damaged. This algorithm is capable of
reconstructing the original sharp edge without prior segmentation.
A simple analytical model was developed to determine the theoretically achievable
accuracy. This Analytical accuracy was compared with the accuracy of edges
extracted from point clouds. A series of experiments were done on point clouds. The
input parameters of the experiments were chosen using the technique of Design of
Experiments. Using the experimental results the parameters that most significantly
influences the accuracy of the algorithm was determined. From the Analytical and
experimental analysis guidelines were developed which will help a designer to specify
sensible input parameters for the algorithm. With these guidelines it is possible to find
an edge with an accuracy comparably with an edge found with the traditional method
of finding the edges with NURBS surface intersections.
Finally the algorithm was combined with a swept surface fitting algorithm. The
scanned edges are used as rails and profile curves for the swept surfaces. The
algorithms were demonstrated by reverse engineering part of another core box for an
inlet manifold. If the edge detection parameters are specified according to the guidelines developed
here, this algorithm can successfully detect edges. The maximum gap size in the point
cloud is an important limiting factor, but its effect has also been quantified. / AFRIKAANSE OPSOMMING: In Truwaartse Ingenieurswese word 'n fisiese voorwerp opgemeet en 'n rekenaar
model word daarvan geskep. Die segmentering van die puntewolk (dit is die
versameling gemete punte) sal aansienlik vergemaklik word indien dit moontlik is om
skerp rante in die puntewolk te identifiseer. Die rante sal dan gebruik kan word om
veegvlakke (swept surfaces), of enige ander tipe oppervalk wat die ontwerp die beste
beskryf, te konstrueer.
Hierdie proefskrif beskryf 'n strategie wat die rante kan opmeet. Dit simuleer die
manier waarvolgens 'n Koërdinaatmeetmasjien 'n reeks lyne, wat oor die rant lê, sou
meet. In plaas van op 'n fisiese voorwerp op te meet, "meet" die algoritme op 'n
puntwolk. Elke lyn word dan in twee dele verdeel (elke deel word 'n meetlynseksie
genoem). Elke meetlynseksie behoort aan een van die twee oppervlaktes wat die rant
vorm. Die rant punte word bereken as die interseksie van twee polinome wat deur die
punte van die meetlynseksie gepas is. Dit is dikwels die geval met meganiese
onderdele dat skerp rante vervang word met 'n vulstraal of dit kan ook gebeur dat die
rant verweer het of beskadig is. Die algoritme, wat hier beskryf word, kan selfs die
oorspronklike skerp rant in sulke gevalle herkonstrueer.
'n Eenvoudige analitiese model is ontwikkelom die teoretiese akkuraatheid van die
algoritme te bepaal. Die teoretiese akkuraatheid is vergelyk met die akkuraatheid van
rante wat uit puntewolke bepaal is. 'n Reeks eksperimente is op puntwolke gedoen.
Die parameters vir die eksperimente is gekies deur van Eksperimentele Ontwerp
gebruik te maak. Met behulp van hierdie tegniek kon bepaal word watter meetparameters
die grootste invloed op die akkuraatheid van die gemete punte het. Die
teoretiese en eksperimentele resultate is gebruik om riglyne daar te stel waarmee die
intreeparameters van die algoritme gekies kan word. Met hierdie riglyne is dit
moontlik om 'n rant te vind met 'n akkuraatheid vergelykbaar met die tradisionele
metode om die rante te vind met behulp van NURBS oppervlakte interseksies.
Laastens is die algoritme gekombineer met 'n algoritme wat veegvlakke deur punte
kan pas. Die gemete rante word gebruik as spore en profiele vir die veegvlakke. Die tegnieke is gebruik om 'n CAD model van 'n sandkernvorm (vir die giet van 'n
inlaatspruitstuk) te maak.
Deur die riglyne te gebruik om die intreeparameters vir die algoritme te spesifiseer,
kan rante suksesvol uit puntewolke bepaal word. Die maksimum afstand tussen
naburige punte in die puntewolk beperk die gebruik van die algoritme, maar die effek
hiervan is ook vasgevat in die riglyne wat ontwikkel is vir die algoritme.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52250 |
| Date | 12 1900 |
| Creators | Schreve, Kristiaan |
| Contributors | Basson, A. H., Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 1 v. (various foliations) : ill. |
| Rights | Stellenbosch University |
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