Information playback systems frequently utilize a stylus for reading signals from the surface of an information record, typically a plastic disc, that contains stored video and audio information. In some systems the information record has a fine spiral groove to guide the tip of a stylus that contains a thin electrode. In these systems the stylus is typically made from natural or synthetic diamond by several critical shaping and lapping processes, normally referred to as micromachining. The stylus tip is tapered to form the prow of the tip, and is also lapped to form a keel having a V-shaped shoe for its bottom portion. This keel-shaped tip has a shoe length of about 3 to 5 micrometers and a thickness of about 2 micrometers. These micromachining steps are normally performed while the stylus is mounted on an arm attached to a cartridge, wherein the arm is capable of flexible movement with respect to the cartridge.
In manufacturing stylus tips for use in such video disc playback systems, the tip of the diamond stylus is inspected at various stages in the fabrication of the stylus. Optical inspection or measurement after prismatic coning, faceting or keel lapping first requires that the stylus tip be accurately positioned at the center of the field of view of a microscope or other optical system. Since the field of view of a high-power optical microscope having 1000.times. to 1500.times. magnification is so small, it is laborious and time consuming to bring the stylus tip into focus by manual adjustments. One possible method for facilitating alignment of the stylus tip is to insert the tip into a conical or grooved structure, made of glass, for guiding the tip into the field of view. An index-matching immersion oil is utilized to permit high-resolution examination of the tip through the glass structure. Such a method not only raises the possibility of mechanical damage during the alignment, but necessitates removal of the immersion oil after the inspection has been made. Also, in certain applications, manufacturing tolerances are such that the required degree of positioning accuracy cannot be achieved by locating the stylus arm or cartridge against mechanical stops.
An optical method and apparatus for positioning a tapered body is described in U.S. Pat. No. 4,341,472 issued to I. Gorog, M. A. Leedom and J. P. Wittke on July 27, 1982, and assigned to RCA Corporation. For maximum image intensity, the preferred body illumination is in transmission, with the illuminating and imaging optics on opposite sides of the body. The above-mentioned patent discloses a technique for positioning a body having a tapered profile when viewed along a first axis. The technique includes exposing the body to a first beam of light oriented along the first axis in a manner such that the tapered profile is projected as a first image onto a first linear array of photodetectors disposed along a direction orthogonal to both the first axis and the direction of the taper. The body is exposed to a second beam of light oriented along a second axis orthogonal to the first axis in a manner such that a second profile of the body is projected as a second image onto a second linear array of photodectors disposed along a direction parallel to the first axis. The body is then moved along the first and the second axes until the first and the second images strike predetermined locations along the first and second photodetector arrays. The location of the body is also changed along a third axis, orthogonal to the first and the second axes, until the width of the first image equals a predetermined width measured by the linear magnitude of the first image along the first photodetector array. Free access to both sides of the body is thus required in the two orthogonal directions. In many applications, however, access is restricted to one linear direction by the surrounding stylus cartridge or mounting or the processing equipment. An example is the inspection or measurement of the stylus in its cartridge assembly, as required at the keel lapping stage.
The present invention uses only a single photodetector array and requires free access only in one direction. The novel system enables a body, such as the tip of a stylus, to be accurately and automatically positioned within a volume of space having dimensions in the order of one micrometer.