GB762021A - Improvements in or relating to velocity meter - Google Patents

Abstract

762,021. Photo-electric measurements of velocity. NORTH AMERICAN AVIATION, Inc. May 3, 1954, No. 12776/54. Class 40 (3). Means for measuring the velocity component of a moving object comprises a source of modulated light, a reflective element on the object and a timer actuated by signals from filter means responsive only to reflected light from the source. Light from source 22 (Fig. 3), is modulated by a chopper disc 23 driven at constant speed, on which it is focused by lens 24 and is formed into a slightly convergent beam by lens 30. The beam is split by prism 31 into two divergent beams which are formed by concave cylindrical lens 20, 21 and reflecting prism 18, 19 respectively into two parallel fans of light which are only slightly divergent in the plane containing the prisms but broadly diverging in the horizontal planes, normal to this. The fans of light respectively illuminate the horizontal substantially laminar fields of view 5, 7 of two optical systems. Each optical system (Fig. 2) comprises a stop 16, 17 objective lens 14, 15 a narrow slit 10, 11 parallel to the plane of the associated fan of light, second lens 12, 13 and a photo-electric cell 8, 9 situated in the plane of the exit pupil of the optical system so that light from any point in the object field is uniformly distributed over it. The thickness of the field of view of each optical system is defined by the overall height of the projected image of the slit 10, 11 and the diffraction patterns formed on either side of it. If the width of aperture 16 is adjusted to be equal to the total height of the image and patterns at the point at which a sharp image is formed the total thickness of the field of view is substantially constant between this point and the aperture. The aircraft carries a retroflective element such as a corner prism so that when the latter falls within the laminar field of view of one of the optical systems illuminated by the projected modulated light the corresponding photo-cell is energized. In the system shown for measuring the vertical velocity of aircraft at touch down the descending aircraft passes successively through the laminar fields of view of the upper and lower optical systems. When the retroflective element enters the field of view of the upper optical system photo-cell 8 gives an output which is amplified and passed through a filter tuned to pass a band of frequencies centred around the modulation frequency, and fires thyratron 43 (Fig. 9) which is in series with a diode 46 and R.C. network 40, 39. The capacitor therefore begins to charge at a rate determined by the voltage across the network, which is stabilized by tube 44. When the reflector enters the field of view of the lower optical system the output of photo-cell 9 which is similarly amplified and filtered fires thyratron 47 to short-circuit tube 44 and thereby stop the charging. The charge built up on capacitor 39, which is measured by a valve voltmeter with a current meter in its output is proportional to the time of charging and therefore inversely proportional to vertical velocity component. The extent of the fields of view of the optical systems in the horizontal directions enables the vertical velocity component of aircraft with high horizontal velocity components to be measured. The optical systems may be orientated differently to allow measurement of velocity components of any vehicle in any direction.