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represented by a set of Boolean operations applied to primitive solids such as cylinder, cube, sphere, torus, wedge, etc. B-Rep and CSG representations lack explicit information in terms of functional features and non-geometric part attributes (face roughness, tolerances, dimensions, etc.). Therefore, a suitable part representation scheme named FORM of part data based on the rich B-Rep provided by the STEP file of the part is developed for part recognition algorithm in this study. This part representation scheme describes the part in terms of both geometric and topological data which are necessary for part recognition and next procedures. FORM is sized in accordance with the number of faces on the part. For example, if the number of faces on the part is 15, FORM uses a 15 15 square matrix.
That is to say, the size of the FORM can expand or shorten. In the FORM, face adjacency relations, attributes belonging to each face are explicitly represented. FORM represents the part in terms of both geometric and topological data in the square matrix format. For construction of the FORM, attributes and adjacent faces belonging to each face on the component should be extracted from the stitching faces. First, edge loops of all faces on part with stitched faces of any part are extracted. Adjacent faces of each face are found through edge curves in their edge loop. Each face is connected to the adjacent faces that share a common edge. Other face which shares common edge is adjacent face to that face. As shown in Fig. 5, edge loop of the face 1 is edges a–d and edges a–d share faces 2–5, respectively.
A. C¸ ic¸ek, M. Gu¨lesin / Computers in Industry 58 (2007) 733–746 739
plain rivet in Fig. 7 and the plain rivet in Fig. 8 are the same. But, the radii of the cylindrical surfaces on both plain rivets are different. Part recognition algorithm differs from another through the radius attribute of the face 5 (cylindrical surface) and recognizes them as different plain rivets according to part names written in the result sections of (THEN) rules belonging to the plain rivets.
As shown in Fig. 9, there are various types of 62 faces of the piston. The program decreases the number of faces into 42 faces by performing of stitching 20 equivalent faces and finally constructs face oriented relation matrix belonging to piston as 42 42 square matrix. The representation scheme constructed in this stage is input to part recognition system along with rules represented in the knowledge base.
5. Expert system based part recognition
Fig. 10. Retaining ring for bores.
Therefore, faces 2–5 are adjacent faces of the face 1. In this way, adjacent faces of the all faces on the component are found and face attributes are extracted.
After adjacent faces and face attributes are determined, FORM is constructed according to number of faces, adjacent faces, face attributes on the component. Face numbers and their corresponding abbreviations are respectively located from top to down and left to right in the matrix. The abbreviations for the faces are as follows:
cylindrical face: cyl;
conical face: con;
planar face: pla;
spherical face: sph;
toroidal face: tor;
b_spline face: bsp;
bounded face: bon;
quasi uniform face: qua.
Then, the program handles the first face and investigates adjacency relations with other faces on the component. If the first face is adjacent to any face, the program places value ‘‘1’’ into the cell corresponding to adjacent two faces in FORM. If the first face is not adjacent any face, the program places value ‘‘0’’ into the cell corresponding to non-adjacent two faces in FORM. Then, attributes belonging to that face are associated with first face in FORM. In this way, face attributes and adjacency relations are extracted, and FORM is constructed for all faces on the component. Intake valve and face oriented relation matrix of the intake valve is shown in Fig. 6. FORM of the intake valve is 12 12 square matrix because there are 12 faces in stitched faces. In fact, there are 19 faces in original STEP file. Since sevens are equivalent, the program has stitched these faces. Also, as shown in Fig. 6, attributes of the face 11 of the intake valve are associated with related to face number in the FORM.