Patent GB-847901-A (GB)
Title: Improvements in or relating to apparatus for determining information about a moving body
IPC: G01P3/68
Filed: 1956-11-06
Abstract: 847,901. Photo-electric sensing apparatus. CHICAGO AERIAL INDUSTRIES Inc. Nov. 6, 1956 [Jan. 26, 1956], No. 33875/56. Class 40(3). Apparatus for determining information about a moving body comprises radiant-energy sensitive means arranged to receive a pair of mutually inclined radiant-energy beams defining a parallax angle there-between and each interrupted by movement of said body there-across to produce an electric signal at each interruption and each restoration of said beam by said body, means for processing a selected pair of signals to produce an electric pulse having a characteristic depending on the interval between said pair of signals and indicating means responsive to the characteristic of said pulse. In Fig. 1, a sensing device comprises an opaque disc 2 having a pair of apertures 3, 4 located in the focal plane of a lens 1 so as to define ray bundles 7, 8 which are arranged to be cut by an opaque object 13 moving in the direction of the arrow 14. The ray bundles impinge on a photo-cell 5 of the sensing device which, in response to the cutting of the ray bundles 8, 9 by the object 13, produces an output signal as shown in Fig. 2. The time t between the steps 15-16 and 16-17 in the photo-cell output represents the parallax interval, that is the time taken for the leading edge of the object to travel from the ray bundle 8 to the ray bundle 9. The output of the photocell is applied to an electronic circuit which produces an output pulse having an amplitude proportional to the time t, the output pulse being fed to an indicating device. In an application for determining the relative blade track positions of  the blades a, b, c of a helicopter rotor, the sensing device is mounted on the fuselage beneath the rotor, and the output of the photo-cell is applied to the circuit shown schematically in Fig. 9. The block 75 contains amplifying and differentiating circuits which produce sharp negatively going pulses at B in response to each sharp rise in the input voltage at A. The input at A due to the blades a, b and c is shown at A in Fig. 10, and the corresponding differentiated signals at B is shown at B in Fig. 10. The pulses at B are applied to a flip-flop circuit 77 which produces pulses shown at Cin Fig. 10, whose width is a measure of the parallax interval t for the respective blades a, b and c. The signals at C are fed to an amplifier 78 which produces pulses shown at D in Fig. 10 whose amplitudes ea, eb, ec respectively are proportional to the duration of the pulses at C and thus provide a measure of the respective parallax intervals t. The signals ea, eb, ec are stored in storage gates 81, 83, 85 respectively which are opened in sequence by pulses from a three digit ring counter circuit represented by blocks 80, 82, 84 operated by pulses from the point C and by a pulse from a reset generator 87 derived from a signal generator 86 on one of the blades of the rotor which insures correct phasing. The difference between the discharge voltages of two of the storage gates, such as gates 81, 83 corresponding with blades a and b, can be measured on a zero centre meter of a comparison circuit 88, and by throwing a switch 90 the difference between the voltages of the gates 81, 85 can be measured thus providing comparative measurement of the track positions of the blades a, b and c. In a modification, the gating of the storage circuit is controlled by a commutator driven by the rotor shaft. A modification in the indicator circuit comprises providing a high input impedance to the comparator 88 to facilitate reduction of meter pulsations. The height of pulses at C may be controlled until a reference voltage reading is obtained at D whereby meter readings at 88 become a percentage change in the reference value, related to a percentage change in the distance s, Fig. 1. Two meter circuits may be used for differences between blades such as a, b and b, c or separate meter circuits or a single switched meter circuit may be used for each blade of the rotor. The light source may be solar, electrically or chemically excited, flourescent, flames, or the radiation of the object. The spectral energy of the source may be entirely in the visible range, or in the ultra violet or infrared ranges. An alternative sensing device, Fig. 11  comprises two units 93, 94 fixed so that their optical axes 96, 97 define a parallax angle # therebetween. By adjusting the angle #, or the unit separation d, the distance So can be adjusted. Since an object at the distance So produces a zero parallax interval, the distance of the object can be determined. The system is capable of separating data resulting from cutting of the beams by the leading and trailing edges of an object, which data can be used to determine the interval between the transit of the leading and trailing edges to obtain a measure of angular inclination of the object, e.g. the pitch of a helicopter rotor blade in motion or to provide automatic control or correction of rotor blade pitch. More than two apertures may be used in the opaque disc. For example, a disc 2, Fig. 21, having three apertures 3, 118, and 4, in conjunction with suitable electronic circuits, can be used for determination of the inclination of the plane of travel and the direction of travel of the object 13. Other arrangements of apertures are described with reference to Figs. 22-28 (not shown).