Patent GB-1410613-A (GB)
Title: Device for photo-electric measurement of object speed or distance
IPC: G01P3/36
Filed: 1972-12-13
Abstract: 1410613 Photo-electric measurement of speed and/or distance ERNST LEITZ GmbH 13 Dec 1972 [20 Dec 1971] 57452/72 Heading H4D Relates to an electro-optical system for measuring (i) the velocity component v of an object transverse to the optical axis of the system and/or (ii) any displacement d of the body along said optical axis from a predetermined position thereon, in which an image of the object is projected on to a photo-cell 5, Fig. 1, through a gating 3 which is (i) moved continuously perpendicularly to the optical axis and/or (ii) reciprocated along said axis thereby modulating the output .signal from the photocell, the frequency of phase of said modulation signal being dependant on said velocity or axial displacement. According to the present invention the modulation depth is increased by using a grating having alternate reflecting and transmitting portions, the components of the radiation transmitted through and reflected by the grating being received respectively by two photo-cells 5 and 7 to produce output modulation signals in anti-phase which are then combined in a known manner. As shown in Fig. 1 the incident radiation is directed at the grating 3 via a polarizing splitter 2 and a (2n-1)#/4 retarder plate 8, the splitter 2 also producing an image component for visual observation. Fig. 3 shows an embodiment for measuring transverse velocity v utilizing a circular grating 14 having n radial grating lines which is rotated at a speed of w r.p.s. so that the modulation frequency fs produced by a stationary object is given by fs= n.w, the corresponding modulation frequency f produced by a moving object being less or greater than fs according to whether the grating lines move across the image field in the same or opposite direction to the movement of the corresponding image, and a signal indicative of the magnitude and sense of the velocity v being obtained by comparison of the frequency f with a reference frequency fs produced by projecting light on to an auxiliary photo-cell 18<1> via a fixed grating 17<1> and the rotating grating 14; the transverse distance moved by the object can also be determined by counting the cycles of the difference frequency f#fs. With such an arrangement an axial displacement d of the object can also be measured if the plane of the grating 14 is not perpendicular to its rotational axis; in this case the effective portion of the grating in the image field reciprocates along a path parallel to said axis (so called swash plate action) and each of the outputs from the photo-cells 5 and 7 will include a component of frequency w, the phase of said component being dependent on the object displacement d. Fig. 4 (not shown) illustrates an embodiment for measuring two orthogonal components of the transverse velocity v in which two images of the object are formed on separate gratings (3<1>, 3<11>) each associated with a respective pair of photocells, the gratings being moved in orthogonal directions, and Figs. 2 and 5-9 (none shown) show six more embodiments each using a single rotating grating. In Fig. 2 the rotation axis of the grating is inclined to the optical axis and Figs. 7 and 7a show two alternative methods of producing the visually observed image. Figs. 5, 6 and 9 all show arrangements for measuring two orthogonal components of the transverse velocity v utilizing two pairs of photo-cells as in Fig. 4. In Fig. 5 a single image is split into two separate components which are projected on to quadrantally spaced areas (14<1>, 14<11>) of the grating and Fig. 9 shows a similar arrangement in which the two images are produced separately (so called binocular variant); in Fig. 6 two image components are projected on to the same area of the grating, one of the images being rotated through 90 degrees and the two images being differently polarized. Fig. 8 shows a binocular variant of Fig. 6 in which the relative orientation of the two images can be set at any angle.