| 摘要 |
750,168. Photo-electric guard arrangements. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. July 23, 1954 [July 17, 1953], No. 19937/53. Class 40 (3). A photo-electric guard arrangement comprises means directing a light beam on to a mirror of parabolic section or a converging lens through its focus and angularly rotating it about the focus to give a projected beam parallel to the optical axis sweeping transversely to the axis to define a protected area, and a similar mirror or lens with its axis parallel to said optical axis to receive the beam and focus it on the photosensitive surface of a photo-electric device forming part of a circuit responsive to changes in its output. In Fig. 1A a series of rotating beams which successively traverse mirror 3 to give a parallel scanning beam 6 is formed by the rotating slotted cylinder 2, the spacing of the slots 4 being such that just before one beam leaves one end of the mirror the following one illuminates the other end. The mirror is a section of a paraboloid bounded by two parallel planes which are parallel to but off its axis so that the source 1 and cylinder 2 do not form a shadow in the swept area. The scanning beam is reflected by a corresponding part of a second paraboloidal mirror 7 on to a photo-cell 8 in the grid circuit of one V1 of a pair of cathode coupled valves V1, V2. A trip circuit which when broken operates an alarm or brings into operation some safety measure includes the normally closed contact RL1<SP>1</SP> of relay RL<SP>1</SP> and RL1 of relay RL. When the illumination of photo-cell 8 is cut off by an object in the protected area, the current through V2 falls and a positive pulse is delivered to the grid of V3 which forms with V4 a flip-flop circuit. V4 is normally conductive and V3 cut off, but on receiving the pulse V3 conducts and V4 is cutoff and remains cut off for a time determined by the charging time of capacitor C. This time exceeds the interval between two successive scannings of any part of the protected area so that further positive pulses arrive at the grid of V3 before V4 recommences conducting. So long, therefore, as the object remains V4 is cut off and relay RL is de-energized to open the trip circuit. Relay RL<SP>1</SP> is responsive to the mean anode current of V2 and remains energized with its contacts closed unless the average illumination of the cell falls below a certain level due to light source failure, or when a large object blocks the beam for an appreciable part of the scanning cycle, in which case it releases to open the trip circuit. In a modification, a single beam is angularly oscillated to scan the surface of the first mirror alternately in opposite directions. Parabolic mirrors or converging lenses may be used instead of the paraboloidal mirrors. Specification 715,444 is referred to. |
