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Description  |
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FIELD OF THE INVENTION
Our present invention relates to a system for monitoring the movement of a
virtual point source of luminous radiation relative to a detection site.
BACKGROUND OF THE INVENTION
Systems of this general type have been used in the past for automatically
tracking the sun from a vehicle; see U.S. Pat. No. 3,084,261. More recent
uses have been in the field of keeping track of the head motion of a
person operating a vehicle, specifically the pilot of an aircraft holding
a target in sight. Thus, as described for example in U.S. Pat. No.
3,917,412, a pilot's helmet may be equipped with two radiation emitters
irradiating a pair of detectors whose outputs, fed to a computer, enable
the determination of the pilot's line of sight paralleling the line of
intersection of two planes, each of these planes being defined by the two
emitters and by the location of the respective detector.
In the case of a single point source, its direction as seen from a sensing
device can also be established with the aid of two slits perpendicular to
each other in a mask otherwise opaque to the emitted radiation, these
slits being traversed by flat beams or sheets of incident rays lying in
mutually orthogonal planes whose orientation can be ascertained with the
aid of two elongate radiation detectors respectively intersected by these
beams. See, in this connection, U.S. Pat. No. 3,951,550. A modification of
that system, described in French Pat. No. 2,322,356, utilizes a V-shaped
slit training two flat beams from the same source upon a single elongate
radiation detector whose points of intersection with these beams indicate
the direction of the source.
OBJECTS OF THE INVENTION
The principal object of our present invention is to provide an improved
radiation-monitoring system of the general type described, utilizing only
one elongate radiation detector for direction determination while avoiding
the use of slitted masks so as to make available a broad field of radiant
energy for the localized illumination of that detector.
Another object is to provide means in such a system for locating the center
of a narrow beam impinging upon such a radiation detector.
SUMMARY OF THE INVENTION
In accordance with our present invention, a radiation sensor coacting with
one or more virtual point sources comprises a first and a second
cylindrical lens member extending parallel to each other across an
elongate radiation detector to focus incident rays from such a source onto
certain portions of the detector, e.g. one or more photocells closely
juxtaposed in a linear array. The path of radiation incident on the first
lens member includes beam-rotating means for turning a first sheet of
rays, lying in one of two mutually orthogonal planes, through 90.degree.
into a third plane which includes the axis of the first lens member. A
second sheet of rays transverse to the first sheet passes through the axis
of the second lens member whereby the two sheets of rays intersect the
radiation detector at respective points defining with the lens axes the
positions of the two mutually orthogonal planes. The locations of these
points of intersection are ascertained by output circuitry coupled to the
radiation detector and connected to a processor which determines from
these locations the direction of the source with respect to the radiation
detector.
Advantageously, the two cylindrical lens members are of planoconvex
configuration with coplanar flat faces confronting the radiation detector.
The latter, pursuant to a more particular feature of our invention,
preferably comprises a pair of overlappingly juxtaposed, relatively
inverted prisms with planes of symmetry inclined at 45.degree. to each
other.
In order to compensate for parallactic errors due to differences in the
path lengths of the rays traversing the two lens members, a transparent
body of suitable refractive index may be disposed in the path of radiation
incident on the second lens member.
It will be understood that the two cylindrical lens members intercept not
only the aforementioned planar sheets of incident rays, whose relative
orientation has been discussed above, but an entire field of radiation
respectively focused by each lens member upon a line intersecting the
radiation detector, preferably at right angles to the detector axis. Our
improved system, therefore, is considerably more sensitive than one using
a slitted mask for the establishment of two narrow beams.
The output circuitry associated with the radiation detector may comprise a
scanner of the charge-soupled type sequentially sampling the cells of the
linear array in a succession of cycles. Such charge-coupled devices (CD),
whose mode of operation resembles that of a shift register, are well known
in the art; see, for example, an article by James Meek in "Journal of
Electronic Engineering," February 1975, pages 38-44, and an article by
John Mavor in "Wireless World," January 1975, pages 13-16. The scanner,
feeding its output signals to a threshold circuit, is advanced by clock
pulses from a timer which also stops a pulse counter or, preferably, a
pair of such counters alternately loading two buffer registers for the
storage of respective counts indicating the ranks of illuminated
photocells--i.e. their positions within the array--respectively irradiated
by the beams traversing the two lens members.
Even with a pair of carefully designed cylindrical lenses, the beams
respectively focused by them upon the radiation detector will generally
illuminate several adjoining cells simultaneously since, with the sources
located at finite and variable distances from the sensor, their linear
images will not precisely coincide with the focal planes of the lenses.
According to an advantageous further feature of our invention, therefore,
we control the stepping of the counter or counters by the timer in such a
way that the counting rate is halved in response to an output signal from
the threshold circuit indicative of detected radiation whereby the rank
indicated by the count refers to a cell at or near the middle of a series
of simultaneously illuminated cells.
The radiation emitted by the source or sources may lie outside the visible
spectrum, e.g. in the infrared range, and may be sorted out by wave
filters interposed between the lens members and the detector.
BRIEF DESCRIPTION OF THE DRAWING
The above and other features of our present invention will now be described
in detail with reference to the accompanying drawing in which:
FIG. 1 is an isometric view of a radiation sensor embodying our invention;
FIGS. 2A, 2B and 2C are, respectively, a top view and two side views of a
beam rotator included in the sensor of FIG. 1;
FIG. 3 schematically illustrates the overall monitoring system according to
our invention, with the beam rotor omitted for the sake of simplicity;
FIG. 4 shows part of the sensor of FIG. 1 modified by the addition of a
path-length compensator;
FIG. 5 is a diagram illustrating the pinpointing of a radiation source with
the aid of a pair of sensors according to our invention;
FIG. 6 is a circuit diagram of the electrical components of our monitoring
system; and
FIG. 7 is a set of graphs relating to the operation of the circuitry shown
in FIG. 6.
SPECIFIC DESCRIPTION
Reference will first be made to FIG. 3 showing a pilot's helmet 9 having a
group of light-emitting diodes or equivalent radiation sources S.sub.A,
S.sub.B, S.sub.C mounted on one side thereof; a similar group, not shown,
may be carried on the opposite side of the helmet. Sources S.sub.A and
S.sub.B lie on a line LOS representing or closely paralleling the line of
sight of the pilot. At D.sub.A, D.sub.B and D.sub.C we have indicated the
directions of these sources as seen from a detection site at a fixed
location in the cockpit of an aircraft piloted by the wearer of helmet 9;
those directions may be related to a reference point O at the center of an
elongate radiation detector 1 consisting of a linear array of closely
juxtaposed photocells generally designated E.
Sources S.sub.A, S.sub.B and S.sub.C are sequentially energized, as more
fully described hereinafter, to facilitate their tracking by a radiation
sensor RS including the detector 1, centered on an axis x.sub.1, and a
pair of planoconvex cylindrical lenses 2, 3 with mutually parallel axes
y.sub.2, y.sub.3 and focal lines F.sub.2, F.sub.3 lying in planes of
symmetry perpendicular to axis x.sub.1. The flat lower faces of lenses 2
and 3 are coplanar and are separated from a confronting upper face of
detector 1 substantially by their focal length f as indicated in FIG. 1,
focal lines F.sub.2 and F.sub.3 thus lying close to the level of that
face.
Upon excitation of source S.sub.A, the radiation emitted by that source
contains two sets of light rays lying in mutually orthogonal planes
P.sub.1 and P.sub.2. In the simplified presentation of FIG. 3, plane
P.sub.1 includes the axis y.sub.2 of lens 2 whereas plane P.sub.2 includes
an axis x.sub.3 ' of a virtual image 3' of lens 3 which is formed by a
beam rotator not shown in FIG. 3 but illustrated in detail in FIGS. 1 and
2A-2C. The same beam rotator produces a virtual image 1' of radiation
detector 1 perpendicular to plane P.sub.2, the image of axis x.sub.1 being
shown at y.sub.1 '. Actually, however, the rays in plane P.sub.2 are
transposed by the beam rotator to a plane P.sub.3 which includes the lens
axis y.sub.3 and, like plane P.sub.1, is thus generally transverse to
detector axis x.sub.1. Lenses 2 and 3 have midpoints M.sub.2, M.sub.3, the
latter being imaged at M.sub.3 '; points M.sub.2, M.sub.3 and axis x.sub.1
define a plane bisecting these lenses.
The two planes P.sub.1 and P.sub.3 intersect the detector 1 at two points
represented by respective cells E.sub.j and E.sub.k, the last-mentioned
cell having an image E.sub.k ' in line with plane P.sub.2. In practice,
however, because of unavoidable optical distortions, the rays incident
upon lenses 2 and 3 will not exactly converge upon their respective focal
lengths F.sub.2 and F.sub.3 so that more than one cell at a time may be
illuminated. The image of focal line F.sub.3 has been shown at F.sub.3 '.
FIGS. 1, 2A, 2B and 2C show the aforementioned beam rotator as comprising a
pair of prisms 4 and 5 with profiles in the form of right isosceles
triangles, the broad prism faces of the hypotenuses of the triangles being
coplanar and in contact with each other at a level above that of lenses 2
and 3, separated by a distance L.sub.1 from the plane of the lower lens
faces. The planes of symmetry of these prisms include a 45.degree. angle
with each other, as best seen in FIG. 2A. The midplane of prism 4 is
parallel to the yz plane of the cartesian coordinate system xyz (with
origin O) and thus to the axes of lenses 2 and 3.
With about half the broad upper face of prism 5 overlain by the prism 4,
the other half of that face intercepts incident rays from any sources
S.sub.A 14 S.sub.C and directs them, by internal reflection, into the
adjoining prism 4 of like refractive index where they are further
reflected to leave by the unobstructed half of the broad lower face of the
latter. This has been particularly illustrated for two rays r.sub.I and
r.sub.II, parallel to the z axis, impinging perpendicularly upon prism 5
at points registering in FIG. 2A with the image axis x.sub.2 '. After
their reflection, the rays emerge from prism 4 in registry with lens axis
y.sub.3 as viewed in FIG. 2A. Thus, the plane of these two rays has been
rotated through 90.degree. on their passage through the prism pair.
Naturally, the entire ray bundle striking the exposed part of the upper
face of prism 5 experiences the same noninverting rotation.
As illustrated in FIG. 4, a prismatic transparent block 6 of height L.sub.2
and width L.sub.3 is advantageously interposed in the path of the light
rays which bypass the prism assembly 4, 5 to impinge upon lens 2. Oblique
light rays reaching lens 3 via the beam rotator constituted by these
prisms have their paths seemingly foreshortened by their refraction at the
entrance and exit faces even though these paths are actually lengthened by
the multiple internal reflections; thus, the emitting sources appear to be
closer to detector 1 than they are in fact so that their angles of
incidence are greater than they would be with direct illumination. While
this phenomenon affords easier distinction between different sources or
different positions of the same source, it also gives rise to a
parallactic error. A similar apparent foreshortening of the light path for
the rays falling upon lens 2 is achieved by a suitable choice of the
dimensions and the refractive index of block 6. Thus, we have illustrated
in FIG. 4 two planar sheets of rays P.sub.1A, P.sub.1B with different
angles of incidence .THETA..sub.1A, .THETA..sub.1B traversing the lens
axis y.sub.2 by way of block 6, on opposite sides of the longitudinal
midplane of lens 2, and two similar sheets of rays P.sub.2A, P.sub.2B with
angles of incidence .THETA..sub.2A, .THETA..sub.2B traversing the lens
axis y.sub.3 by way of prisms 5, 4 (only the latter being seen in FIG. 4).
The parallactic error introduced by transparent bodies 4-6 will, of
course, have to be taken into account in determining the true location or
direction of the active source. The interposition of block 6 places the
apparent positions of the flat faces of lens 2 and lens image 3'
substantially at the same level.
FIG. 5 diagrammatically illustrates two sensors RS', RS", each of the type
described with reference to FIGS. 1-4, which are separated by a distance
L.sub.4 and receive light from a source S.sub.i along respective
directions of incidence D.sub.i ', D.sub.i ". With such an arrangement the
source S.sub.i can be pinpointed by simple triangulation.
With the use of a single sensor and three sources S.sub.A, S.sub.B, S.sub.C
(FIG. 3) excited at different times but in close succession, the
establishment of their respective directions of incidence D.sub.A,
D.sub.B, D.sub.C also enables the location of each source to be determined
inasmuch as the distance between any two sources is invariable and known.
The combined use of three or more sources and at least two sensors on each
side of helmet 9 provides immediate verification of the obtained readings
with the aid of the two procedures described; moreover, with the sources
necessarily close to one another, a triangulation with the aid of two
relatively widely separated sensors is more exact than a calculation of
distance based upon the directions of incidence D.sub.A, D.sub.B, D.sub.C.
If the lens 2 and the lens image 3' are presumed to be respectively
bisected by the xz and yz planes, and if the receiving face of detector 1
lies in the xy plane so that the origin O coincides with the center of
that face as assumed above, the coordinates of intersection points
E.sub.j, E.sub.k ' and midpoints M.sub.2, M.sub.3 ' will be E.sub.j
(x.sub.j,O,O), E.sub.k '(O,y.sub.k, O), M.sub.2 (x.sub.m2,O,z.sub.m2) and
M.sub.3 '(O,y.sub.m3,z.sub.m3). The two planes P.sub.1 and P.sub.2
respectively including the axes of lens 2 and lens image 3' can then be
defined by
z.sub.m2 x+(x.sub.j -x.sub.m2)z=z.sub.m2 x.sub.j
for the plane P.sub.1 and
z.sub.m3 y+(y.sub.k -y.sub.m3)z=z.sub.m3 y.sub.k
for the plane P.sub.2. From these two equations the direction of incidence
D.sub.A can be derived in terms of analytic geometry. The other directions
D.sub.B and D.sub.C are obtainable in an analogous manner upon excitation
of the respective sources S.sub.B and S.sub.C in lieu of source S.sub.A.
Reference will now be made to FIG. 6 for a description of an electrical
circuit arrangement designed to evaluate the information available from a
given radiation sensor RS. A rectangle 9, symbolizing the pilot's helmet
of FIG. 3, carries a power supply 50 adapted to feed the several sources
S.sub.A, S.sub.B, S.sub.C through a switching circuit 51 under the control
of a switchover signal S.sub.15 periodically emitted by a processor 40;
the excitation of any source is reported back to the processor via a
respective line 52A, 52B, 52C. Radiation detector 1 is here shown to be
illuminated from block 6 and lens 2 via a slit diaphragm 81 and a filter
71; a similar slit diaphragm 82 and filter 72 are interposed between the
detector and lens 3. The two slit diaphragms 81, 82, provided for sharper
focusing, should of course give passage to rays from any source position
within the range to be monitored. Filters 71 and 72 serve to exclude
ambient illumination by passing only the spectral band (preferably
infrared light) emitted by any sources S.sub.A -S.sub.C.
A scanner 21, advantageously an integrated shift-register circuit of the
CCD type discussed above, has a multiplicity of stages respectively
connected to the several photocells of detector 1 and is steppable by
clock pulses H from a timer 20 to transfer the charges received from the
detector through an amplifier 22 to one input of a comparator 23 whose
other input receives a threshold voltage from an adjustable supply 53. The
output signal SV of scanner 21, after amplification in component 22, gives
rise to a signal S.sub.10 including a pair of flat-topped peaks SV.sub.j,
SV.sub.k in the output of threshold comparator 23. These peaks are
differentiated in an R/C network 54, 55 feeding a pair of antiparallel
diodes 26, 27; diode 26 passes only their rising flanks S.sub.11, S.sub.16
(FIG. 7) whereas diode 27 passes only their falling flanks S.sub.12,
S.sub.17. The positive spikes from diode 26, corresponding to flanks
S.sub.11 and S.sub.16, are respectively directed by a switching circuit 29
to control inputs of two second-state switching circuits 24, 31 also
having other control inputs connected to receive the negative spikes from
diode 27, corresponding to flanks S.sub.12 and S.sub.17, which are
respectively directed thereto by the first-stage switching circuit 29.
Timer 20 also generates another set of clock pulses H/2, of half the
cadence of clock pulses H, the two sets of clock pulses being transmitted
in parallel to respective inputs of switching circuits 24 and 31 for
alternate delivery to a pair of pulse counters 25, 32 as stepping pulses
S.sub.13, S.sub.18. These pulse counters, working into respective buffer
registers 28 and 33, are periodically zeroized by a resetting signal
S.sub.14 emitted by the scanner 21 at the end of each sweep cycle in which
the charges received from any photocell of detector 1 are transferred in
the form of output signal SV to amplifier 22 and comparator 23. Pulse
counters 25 and 32 are arrested by spikes S.sub.12 and S.sub.17
respectively applied to stopping inputs of these counters as well as to
loading inputs of the associated buffer registers 28 and 33. With the
stage outputs of registers 28 and 33 connected to processor 40, the latter
thus receives the readings of both pulse counters after each sweep cycle.
The switchover signal S.sub.15 may be generated after every sweep cycle or
after a certain number of consecutive cycles.
In FIG. 7, whose graph (a) shows the output signal S.sub.10 of threshold
comparator 23 with its rectangular peaks SV.sub.j and SV.sub.k, we have
indicated in graphs (b) and (c) the trains of counting pulses S.sub.13 and
S.sub.18 applied to the stepping inputs of pulse counters 25 and 32. As
long as signal S.sub.10 is low, the two counters are stepped rapidly at
the high rate 1/H of clock pulses H. When the leading edge S.sub.11 of the
first peak SV.sub.j is encountered, spike S.sub.11 reverses the switching
circuit 24 whose control input it reaches by way of circuit 29 which
assumes its illustrated position under the control of resetting signal
S.sub.14 before the start of any sweep cycle. Counter 25, therefore, is
now stepped at the slower rate 2/H by the clock pulses H/2 while
registering a count representing the rank of the first illuminated cell
E.sub.j of detector 1. Actually, as indicated in FIG. 7, the signal peak
SV.sub.j spans six consecutive cycles of clock pulses H corresponding to
an illumination of six neighboring photocells by way of lens 2; the
reduction of the stepping speed of pulse counter 25, however, insures that
buffer register 28 is loaded upon the occurrence of the spike S.sub.12
with a count which identifies the third rather than the last one of the
illuminated neighboring cells. Spike S.sub.12 also restores the original
position of switching circuit 24, yet counter 25 arrested by that same
spike is no longer stepped by the pulses S.sub.13 now again coinciding
with the fast clock pulses H.
It will be apparent that the above-described halving of the counting rate
in the presence of detected radiation may also be used in a system in
which the pulse counters 25, 33 determine the positions of illuminated
cells on two separate radiation detectors respectively irradiated through
mutually orthogonal slits, for example.
Through a delay line 34, spike S.sub.12 is also transmitted to a control
input of switching circuit 29 which therefore directs the next two spikes
S.sub.16 and S.sub.17, derived from peak SV.sub.k, to switching circuit
31, pulse counter 32 and buffer register 33. Thus, stepping pulses
S.sub.18 are slowed to half their previous cadence in response to the
leading edge of peak SV.sub.k as the counter 32 loads the buffer register
33 with a count identifying the rank of the third of the six neighboring
cells illuminated via lens 3. Spike S.sub.17, which stops the pulse
counter 32 and discharges the contents of register 33 into processor 40,
also returns the switching circuit 31 to its illustrated normal position.
The three sources S.sub.A -S.sub.C on each side of helmet 9 are
advantageously represented by LED's of the fast-switching type with a rise
time of less than 1 ms, emitting their radiation over a wide area
encompassing close to 2.pi. steradians. The scanner 21 may have a sweep
cycle of less than 2 ms; in practice, the number of photocells of detector
1 sampled in such a cycle may be as high as 1,700. Lenses 2 and 3 may have
axial lengths on the order of 2 cm, focal lengths of about 15 to 20 mm and
an aperture of 3 to 5 mm. Naturally, each of these lenses could also be
replaced by a more elaborate optical system.
Processor 40 evaluates the pulse counts received in each cycle from
registers 28 and 33, as well as from corresponding registers of other
sensors, if any, and performs the calculations required for establishing
the instantaneous locations of sources S.sub.A -S.sub.C together with the
line of sight LOS (FIG. 3). This information may be transmitted to a
visual indicator or some other load aboard the aircraft or elsewhere.
* * * * *
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