Photoplotter accurate film locating mechanism

Claim 1

In a photoplotting machine having an optical head including a light source shining through a lens; having a rotatable aperture disc containing a circle of spaced-apart apertures in which are mounted transparent images of symbols and characters, said disc positioned between the light source and the lens; having a movable table positioned beneath the lens, said table having cassette mounting means on its upper surface; and having a removable cassette in which photographic film, having precisely prepunched registration holes, is positioned to be exposed, said cassette mounted on the movable table and fixed in place by the cassette mounting means; the light source, the rotatable aperture disc, and the movable table all being actuated in response to an interfaced computer program to photograph the desired plot on the film; improved means for determining the exact film location on the movable table comprising: a. at least two registration pins mounted on and extending upwardly from the film supporting surface of the cassette, said cassette pins being sized and shaped to precisely engage the prepunched film holes; b. a first retaining wall mounted on the cassette having a vertical face, said first retaining wall vertical face being horizontally aligned with the optical head; c. a second retaining wall mounted on the cassette having a vertical face disposed at an angle to said first retaining wall vertical face, said second retaining wall vertical face being horizontally aligned with the optical head; d. sensor means for precisely registering the exact instant the optical head physically contacts a retaining wall vertical face; and e. computer interfaced drive means for storing the known geometrical data cOncerning the precise location coordinate plot of said registration pins and said retaining wall vertical faces, and for moving the cassette-carrying table horizontally to cause said first and said second retaining wall vertical faces to sequentially contact the optical head whereby the computer calculates and stores in its memory the location coordinates of the table at the instants of contact between the optical head and said retaining wall vertical faces, and further calculates therefrom and stores the location coordinates of the registered film.