Optics: measuring and testing – Range or remote distance finding – With photodetection
Patent
1992-07-07
1994-03-22
Buczinski, Stephen C.
Optics: measuring and testing
Range or remote distance finding
With photodetection
356141, G01C 308, G01C 100, G01B 1126
Patent
active
052969096
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
This invention concerns a suspended cable detector specially suited for avionic applications. It is well known that some risks are encountered mainly by light aircrafts and helicopters when they are flying in areas traversed by suspended cables. The scope of this invention is of supplying the aircraft pilot with a warning system which, when suspended cables are present, allows the immediate selection of the evasive move to be effected. This alarm is the result of a very effective and reliable scan of the search area and of an extractor which can generate in time the alarm signal and supply the data related to the position parameters of the detected suspended cable.
In current practice the types of scan generally used in these applications were producing continuous curves in the specific search area by means of adjacent or partially superposed "spots" (pulsed emission beams).
These scans were obtained by moving the laser beam by means of easily made servomechanisms with two degrees of freedom. The most common curves used were the spiral, the rose curve and daisy curve. A cable inside the search area was detected, with a good probability figure, when the Lidar spot was passing over it owing to the continuity of the scan characteristic curve. The cable could be detected by observing a set of aligned intersection points. It must be admitted that these scan types shoved a very good performance, in terms of detection probability, when the cable was in the central part of the search area. In this case a large number of intercepts was obtained because the scan curve crossed many times the cable. The performance degraded as the cable moved off the center of the search area since the crosspoints were reduced rapidly. To improve the above performance the number of pulses emitted by the Lidar should be greatly increased in the scan time. This however could mean an increase of the emitted average power and consequently the increase of the overall dimensions and weight of the device.
A scan type which may be defined as "quasi-random" is proposed in the invention which is the subject of this patent application.
Its performance in the search area is more uniform than the previous solutions. The results of simulated tests show that there is a high probability of detecting a cable lying both in the central and peripheral portions of the search area. On the contrary a spiral scan in the same conditions and with the same number of spots gives a high detection probability in the central part but a low probability in the peripheral part of the search area. This scan is obtained by modifying, by means of suitable random signals, a uniform scan of conventional type. For instance a television Raster scan may be used.
The scan parameters are so selected that the search area is divided into many signal small areas equal in number to the Lidar PRF (Pulse Repetition Frequency) if the scan time is one second. At the center of each areola a Lidar spot is located. Only a small portion of this areola is covered by the spot due to its dimensions. The spots are anything but adjacent or superposed as in the scan types above montioned. The areolas may have equal or different dimensions so that consequently the spots may be concentrated in zones where the maximum performance is requested. The scan so far obtained is not very much effective to detect cables since the Lidar shots are concentrated along a limited number of diagonals inside the search area. An almost rectilinear cable, which is outside the areas defined by such diagonals and the width of the Lidar beam, has a low probability of being detected. On the other hand a cable which is inside the above areas has a number of intercepts exceeding those needed for its detection. By summing a small amplitude random movement to the Raster type movement of the beam the actual position of the spot inside the elementary area is made quasi-random. This feature breaks the intrinsic periodicity of the Raster scan and distributes the intercept probability on the whole scan frame.
The ban
REFERENCES:
patent: 4183640 (1980-01-01), Abe
patent: 4572662 (1986-02-01), Silverman
patent: 4902126 (1990-02-01), Koechner
R. Kleehammer et al., Optical Engineering, vol. 19, #6, Nov.-Dec. 1980, p. 92.
Fazi Marco
Modestini Filippo
Alenia Aeritalia & Selenia S.p.A.
Buczinski Stephen C.
C.N.R.
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