Patent GB-1156969-A (GB)
Title: Non-Contacting Gauge
Inventor: ZOOT ROBERT MARTIN
IPC: G01S17/89
Filed: 1967-12-14
Abstract: 1,156,969. Photo-electric measurement of distance. HUGHES AIRCRAFT CO. Dec. 14, 1967 [Jan. 3, 1967], No. 56907/67. Heading G1A. [Also in Division G2] The distance of an object, along a predetermined path, from a photo-electric system is measured by detecting the amount by which the surface of the object is displaced from the focal plane of lenses forming part of the system. In the arrangement of Fig. 1, the displacement of the surface of the object 11 from the focal plane of lenses 14 is detected photo-electrically and used to move either the object or the optical system (plus detector) so as to null the harmonic content of the detector output, at the same time the necessary displacement of object of optical system to accomplish this being monitored by a micrometer or a linear displacement transducer. The optical system comprises a source of ultra-violet, visible or infra-red radiation and is preferably a continuous laser 10. A diverging lens 12 directs light through a lens and reticle system 13. As shown in Fig. 2, this comprises a convex lens 14 masked by a crossshaped light-absorbing member 15 so that only four quadrants 14a-d are available for transmitting light and thus the beam is split up into four parts. The centre of the beam from lens 12 is masked by the back of mirror 18. The four beams from the structure 13 come to a focus in a common focal plane and if the surface of object 11 is at this plane, the image is a finely focussed point of light. If the surface is displaced from this plane then the image is wider and forms four defined quadrant shaped portions. The image is focussed on a receiving reticle 21 associated with a photo-electric detector 17 by passage through lens 16, and via mirrors 18, 19. A rotatory plate 20 causes the image to execute a circular scan over the reticle 21 which comprises a disc having a cross-shaped aperture, Fig. 3 (not shown), and is thus the converse of reticle 15. A filter 22 passes only the wavelengths of the source. The detector output is of pulsed form since during one revolution of plate 22, the image is passed four times by the gaps of the reticle, and as the image at the object surface deviates further from a finely defined point, the output waveform increases in its harmonic content. The detector output is sensed and used to move either the detector or object to minimize the harmonic content. The apparatus can be used in contour mapping, for example of the surface of a model, e.g. buildings, airframes or automobiles, Fig. 7 (not shown). The system of Fig. 1 is moved continuously or in stepped fashion relatively to the model (object 11) in an X-Y plane, the Z distances (along the light beam of the object) being measured and recorded e.g. on punched cards. A system for such contour mapping, Fig. 8 (not shown), includes a fluid pressure system for maintaining the photoelectric measuring system a constant distance from the object surface, error signals being converted to displacement signals for recording. A dither signal is superimposed on the error signal even when the error signal is zero. Means are also provided to prevent the measurement system to object distance becoming too great.