Multiple aperture ir sensor

A multi-aperture, non-imaging sensing system is disclosed, for guiding a moving projectile toward a source of infrared energy. The detectors are arranged peripherally about a forward portion of the projectile, with their individual fields of view overlapping one another to selectively separate a composite field of view into a plurality of separate sectors, with a central sector including all individual fields of view. Infrared sensing elements and electrical circuitry are operatively associated with the detectors, for generating a binary one or zero, depending upon whether an infrared energy source is sensed by the associated detector. The combined results are generated as a digital word, utilized to indicate the location of the source within the composite field of view, and to generate a control signal. The control signal adjusts the projectile orientation to locate and maintain the source within the central sector of the composite field of view.

BACKGROUND OF THE INVENTION 
This invention relates to apparatus for sensing sources emitting infrared 
radiation, and more particularly to multi-aperture sensors placed on 
projectiles for precisely guiding the projectiles toward such sources. 
A number of known systems have been developed to locate and track objects, 
for example in the precision guiding of munitions fired toward a target. 
One approach involves single aperture sensors utilizing a single lens to 
receive and focus incident energy on a plurality of detector elements, for 
example a quad-detector with four separate elements. The target or 
infrared energy source is deemed centered when the response of all four 
detectors is the same. While this system is relatively low cost and 
affords a narrow acquisition field of view for high center tracking 
accuracy, it lacks wide acquisition field of view for capture of a target. 
Further, such a system fails to recognize the presence of two or more 
targets within its field of view, and therefore steers the munition 
towards the centroid of multiple targets. 
Alternatively, sensor systems utilizing a focal plane array have high 
center tracking accuracy, in that they can create a real or pseudo image 
of the target or targets. A disadvantage, however, is that such sensors 
collectively have a narrow field of view in spite their large number. 
Moreover, the electronics necessary to process the numerous signals is 
complex and expensive. For example, a project at the University of Florida 
involves multiple apertures with fiber optic bundles behind each lens. The 
fiber optic bundles are brought together and a focal plane array is used 
to sense their output. However, the bundles require a "training" process 
involving placement of different potential target shapes and at various 
temperatures in front of the sensor and storing the pseudo image in 
computer memory. The needs for high speed computation and memory capacity 
substantially increase the processor cost. 
Therefore, it is an object of the present invention to provide a low-cost 
detector having a large acquisition field of view, yet with high 
resolution at the center of the field of view. 
Another object is to provide a multi-aperture sensor utilizing multiple 
lenses, each lens with a single detector, arranged in overlapping fields 
of view. 
Yet another object is to provide a multiple lens sensor in which the fields 
of view of the lenses are selectively positioned for an optimum 
overlapping in the center of a composite field of view consisting of the 
fields of view of all lenses. 
SUMMARY OF THE INVENTION 
To achieve these and other objects, there is provided an apparatus for 
determining the location of a source of energy relative to an object. The 
apparatus includes an object having a longitudinal axis extended forwardly 
and rearwardly thereof. A detecting means is provided including a 
plurality of energy detectors, each detector having an individual field of 
view and adapted to receive energy from an energy-emitting source located 
within its individual field of view. The detectors are mounted with 
respect to the object to position the individual fields of view of the 
detectors in an array about the longitudinal axis to form a composite 
field of view of said detecting means. A plurality of signal generating 
means are provided, one associated with each of the detectors. Each signal 
generating means produces a signal indicating the detection of the energy 
emitting source within the individual field of view of its associated 
detector. A composite of the signals indicates which of the individual 
fields includes the source, and thereby indicates the approximate location 
of the source within the composite field of view. 
Preferably the individual fields of view are conical, substantially the 
same size, and all circumscribing the longitudinal axis, thereby forming a 
mutual intersection field of view at the composite field center, and 
including all of the individual fields and the longitudinal axis. 
In one aspect of the invention, the object is an elongate projectile and 
the apparatus further includes a control means, responsive to signals 
generated by the plurality of signal generating means, for selectively 
altering the orientation of the projectile to move the mutual intersection 
field toward coincidence with the energy emitting source. More 
particularly, threshold means cause each of the detectors to generate a 
binary one when sensing the source, and a binary zero when not sensing the 
source. The binary ones and zeros are combined to form a digital word, 
which is processed in a read-only memory (ROM). The ROM output is a 
digital control signal which causes the control means, preferably a pitch 
correction motor and a yaw correction motor, to correct the trajectory. 
The detecting means can include a plurality of lenses fixed to the 
projectile, and a plurality of infrared radiation detecting elements, one 
for each lens. The lenses surround the longitudinal axis and are 
positioned so that their fields of view form a Venn optical pattern, with 
selective overlapping of neighboring fields of view and with all fields 
intersecting at the center, about the longitudinal axis. This forms a 
number of sectors or Venn areas in the composite field substantially 
greater than the number of detectors, with each sector uniquely identified 
by a binary word. 
One advantageous embodiment utilizes six such detectors in a symmetrical 
array about the longitudinal axis. The corresponding Venn optical pattern 
includes 31 discrete sectors, which tend to be smaller in size as they 
approach the mutual intersection field. This feature provides the 
advantage of combining a relatively large acquisition field of view or 
capture range, with high resolution at the center of the field of view. In 
effect, the sensors cooperate to function much as the human eye, with high 
central resolution and wide "peripheral vision" for acquisition of the 
target. A direct, real time readout of each detector is provided, 
eliminating the need to integrate or multiplex signals from many sensors. 
Hence, the processing electronics are comparatively simple, resulting in a 
more reliable, less expensive tracking system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning now to the drawings, there is shown in FIG. 1 a munition or 
projectile 16 incorporating an infrared energy detection system 
constructed in accordance with the present invention. Projectile 16 
includes an elongate body 18, a rounded or blunt nose cone or tip 20 at 
its forward end, and four substantially identical fins 22 disposed about a 
rearward portion of the body and angularly spaced apart from one another 
90.degree.. 
The optical system carried by projectile 16 provides it with a generally 
conical acquisition field of view 24. Shown within acquisition field of 
view 24 is an armored combat vehicle 26. When its engine is operating, 
vehicle 26 is a source of infrared energy, and is sensed by the optical 
system of projectile 16 whenever it is within the acquisition field of 
view. 
In FIG. 2, projectile 16 is shown in end elevation with tip 20 removed to 
better illustrate the infrared energy detecting means, which is a 
non-imaging multi-aperture system. More particularly, six double convex 
lenses 28a-28f are positioned in a symmetrical array about the periphery 
of body 18, each spaced apart radially from the projectile center. A 
projectile diameter of three inches readily accommodates six lenses with a 
diameter of, e.g., three-fourth of an inch. 
As seen from FIG. 3, each of lenses 28 is part of an individual infrared 
energy detector 30, with each detector further including an infrared 
energy detecting element 32 fixed behind its associated lens. While only 
detectors 28a and 28d are illustrated, it is understood that the remaining 
detectors are substantially identical. Such detectors are known, and 
available from Honeywell Inc. With reference to detector 30a, lens 28a 
includes a focal axis or lens axis 34a passing through the lens center and 
containing the focal points on opposite sides of the lens (not shown). 
Detecting element 32a is located along lens axis 34a and preferably on or 
near the focal point behind the lens. 
Each of detectors 30a-f is selectively oriented with respect to a 
longitudinal axis 36 of projectile 16. For example, lens 28a is oriented 
to define a selected offset angle A between lens axis 34a and longitudinal 
axis 36. Further, lens 28a has a field of view angle B, i.e. the angle 
between lens axis 34a and a radially inward edge 38a of an individual 
field of view 40a for the lens. The field of view angle B is slightly 
greater than the offset angle A, resulting in an overlap angle C between 
longitudinal axis 36 and radially inward edge 38a, so that the individual 
field of view 40a for lens 28a circumscribes the longitudinal axis. 
While the individual field of view and field of view angle B, along with 
angles A and C, have been described only in connection with lens 28a, it 
is to be appreciated that the remaining lenses have substantially 
identical fields of view and are inclined from longitudinal axis 36 in 
substantially the same manner. As a result of this arrangement, the 
individual fields of view for detectors 30-30f cooperate to form composite 
or acquisition field of view 24 as illustrated in FIGS. 6 and 8. 
Preferably, field of view angles B and offset angle A are substantially 
equal and therefore quite large compared to the overlap angle. For 
example, field of view angle B can be 10.degree., while overlap angle C is 
substantially smaller, for example 0.5.degree.. Accordingly, each 
individual field of vision spans 20.degree., with the composite or 
acquisition field of view approaching 40.degree.. The angle at which the 
individual fields of view intersect one another is twice the overlap angle 
or 1.degree.. 
The large acquisition field of view, in combination with a substantially 
smaller mutual overlap or intersection of the individual fields of view, 
provides a tracking system having a wide capture range but with very high 
resolution at the center of the field of view. To achieve this advantage, 
the composite field of view is divided into a plurality of discrete Venn 
diagram areas or sectors. FIG. 4 illustrates a pattern of four individual 
fields of view I-IV, as would be formed by a simplified optical system 
utilizing four detectors. The individual fields of view appear 
substantially circular, as if projected upon a plane surface perpendicular 
to a longitudinal axis surrounded by the four detectors. 
A composite field of view 42 is divided into thirteen Venn diagram areas or 
sectors, labeled a-m. Sectors a-d include only one of individual fields of 
view I-IV. Sectors e-h represent intersections of two adjacent individual 
fields, while sectors i-l include intersections of three fields. Sector m 
is the intersection of all four individual fields of view, and can be 
considered an intersection field of view at the center of the composite 
field. 
The table in FIG. 5 illustrates the manner in which each of sectors a-m is 
uniquely identified for target tracking. Each of four columns corresponds 
to one of individual fields of view I-IV. A binary one indicates detection 
of a source of infrared energy within the individual field of view, while 
a binary zero indicates the lack of any such detection. For example, 
detection of an infrared source within fields of view I and II, but not 
within either field III or field IV, yields the digital word 1100 and thus 
locates the infrared energy source within sector f. Similarly, the digital 
1111 locates the infrared source within the intersection field of view, 
sector m. 
Composite field of view 24 of the optical system carried by projectile 16 
is divided into sectors in the same manner, as seen from FIGS. 6 and 8. 
More particularly, composite field of view 24 includes thirty-one Venn 
diagram areas or sectors. An optical system including N detectors, with 
overlapping individual fields of view as described, forms an acquisition 
field of view having S sectors, according to the formula S=N.sup.2 -N+1. 
From this equation, it is readily apparent that a substantial improvement 
in resolution is gained from a relatively small increase in the number of 
detectors. Also, when the detectors are positioned to define a small 
overlap angle C, the smallest sectors are positioned adjacent the 
intersection field of view, thereby providing a particularly high 
resolution near the center of the acquisition field of view. 
Projectile 16 carries circuitry operatively associated with detectors 
30a-30f for generating a six-bit digital word indicating the location of 
vehicle 26 within field of view 24, and further for correcting the 
trajectory of projectile 16 in accordance with the digital word. As seen 
in FIG. 7, the circuitry includes an analog processor 42 and a digital 
processor 44 associated with a detection system 45 including the 
detectors. The analog processor includes six pre-amplifiers 46a-46f and 
six comparators 48a-48f, with a pre-amplifier/comparator pair associated 
with each one of detecting elements 32a-f. The analog processor further 
includes a reference voltage source 50 for providing a reference voltage 
to each of the operational amplifiers. 
Each of detecting elements 32a-f generates a voltage in accordance with the 
infrared energy received through its associated one of lenses 28a-f, which 
of course depends upon the nature of the infrared energy sources, if any, 
within the associated individual field of view. In any event, the voltage 
output of each detecting element is amplified by its associated 
pre-amplifier 46a-46f, with the output of each pre-amplifier provided to 
the positive input terminal of the associated comparator. The reference 
voltage from reference source 50 is provided to the inversion terminal of 
each comparator. Consequently, each of comparators 48a-f functions as an 
IR energy threshold detector. More particularly, each comparator generates 
a binary one if the voltage received from its associated pre-amplifier 
exceeds the reference voltage, i.e. whenever the associated detecting 
element receives IR energy exceeding a predetermined threshold. 
Digital processor 44 includes a sample and hold circuit 52 for receiving 
the output of comparators 48a-f, and providing the resultant six-bit 
digital word to a programmable read-only memory (PROM) 54. A read-only 
memory (ROM) or any device providing the same function (e.g. Programmable 
Logic Array or Programmable Array Logic) would suffice. In either event, 
memory 54 is preprogrammed (in the sense of storing a map) to provide an 
error input to a control signal generator 56, which input depends upon the 
digital word provided to the memory. 
The output of control signal generator 56 is provided to a motor controller 
58, operatively connected to a pitch correction motor 60 and a yaw 
correction motor 62. In a manner known to those skilled in the art and 
therefore not further described herein, motors 60 and 62 operate 
individually or in concert to manipulate the physical guidance means of 
projectile 16, namely fins 22, to selectively alter the projectile 
orientation. 
As indicated by a line 64 from motors 60 and 62 to detecting system 45, the 
detectors, processors, controller and motors work as a closed loop system, 
with new digital words continuously generated based on what is sensed by 
detecting elements 32a-f following each pitch and/or yaw correction. The 
operation of this system is perhaps best understood in connection with 
FIG. 6, where a line 66 including a plurality of vectors illustrates the 
movement of vehicle 26 with respect to acquisition field of view 24. Of 
course, it is to be understood that most of this relative movement occurs 
as the projectile orientation is adjusted, with perhaps some movement of 
the vehicle, relative to the ground as well. Vehicle 26 is not sensed 
until it enters acquisition field of view 24. As shown, entry is into 
individual field of view 40e of detector 30e. The digital word provided to 
PROM 54 is 000010, which identifies a sector 68. The resultant pitch and 
yaw thrust commands are provided to control signal generator 56 which 
provides an analog signal to motor controller 58 for pitch and/or yaw 
correction predetermined for and unique to sector 68. 
Preferably, the correction associated with sector 68 shifts field of view 
24 to in effect "move" vehicle 26 radially inward, directly to the center 
of the acquisition field of view from a point at the center of sector 68. 
Vehicle 26 as shown is sensed in sector 68 at an off-center location. 
Hence, a vector 70, indicating the direction of projectile orientation 
adjustment corresponding to sector 68, is offset from the radially inward 
direction. However, the correction rapidly repositions vehicle 26 within a 
sector 72, which is the intersection of individual fields of view 40e and 
40f of detectors 30e and 30f, whereby the digital word generated changes 
to 000011, giving rise to an altered direction of correction represented 
by vector 74 in sector 72. This tracking process continues, with each 
correction in projectile orientation triggered by a digital word, the word 
in each case being uniquely associated with the sector in which vehicle 26 
is detected. 
Eventually, the orientation of projectile 16 is adjusted to position 
vehicle 26 in a central sector 76 comprising an intersection field of 
view, resulting in a digital word of 111111, in which event no correction 
is required. As indicated by that portion of line 66 in the intersection 
field of view, projectile 16 and field of view 24 can move slightly with 
respect to vehicle 26 without any correction, so long as the vehicle 
remains within the intersection field of view. This slight drift, however, 
need not diminish tracking performance, so long as overlap angle C of 
detectors 30a-f is chosen to create an acceptably small intersection field 
of view. 
One feature of the present invention as embodied in projectile 16 is that 
detection system 45, analog processor 42, digital processor 44, control 
signal generator 56, motors 60 and 62, and motor controller 68 are 
conveniently housed within the projectile. Further, the circuitry 
intermediate the detectors and motors, including the analog and digital 
processors and the control signal generator, can be embodied as one or 
more integrated circuit chips. Consequently, all of these components can 
be built into a relatively small projectile, for example having a length 
of about one foot and a diameter of about three inches. 
At the same time, the invention is readily applied elsewhere. For example, 
a stationary infrared sensitive tracking device can utilize a portion of 
the circuitry shown in FIG. 7 to assist in tracking an approaching IR 
energy radiating object. For this purpose, an LED display, indicated in 
broken lines at 78, is operatively connected to receive the output of 
sample and hold circuit 52, whereupon display 78 provides the digital word 
to indicate the object's position. Alternatively, an intermediate digital 
processing means (not illustrated) can be provided to convert the digital 
word into a sector number or pictorial representation of a given location 
within the acquisition field of view. In yet another application of the 
invention, a joy stick or other manual ground based control means can be 
employed to remotely control pitch and yaw correction motors 60 and 62, 
based on the information on LED display 78. 
In FIG. 8, two vehicles 80 and 82 are shown in composite field of view 24, 
in sectors 84 and 86, respectively. The look-up table in FIG. 9 
illustrates the digital words for these sectors, and also a resultant 
based on boolean logic, i.e. for each of the Columns a-f, if there is a 
binary one present in connection with either sector 84 or sector 86, then 
the resultant also shows a binary one. A feature of the present invention 
is that, given a sufficient number of detectors, the tracking circuitry 
can be programmed to indicate the presence of more than one infrared 
energy source within the acquisition field of view. With reference to FIG. 
9, the resultant 101101, if assumed to apply to a single source, would 
indicate its presence within the individual fields of view 40a, 40c, 40d 
and 40f of detectors 30a, 30c, 30d and 30f, but outside of the individual 
fields 40b and 40e of detectors 30b and 30e. From an inspection of FIG. 8, 
it can be recognized that such positioning would be impossible. Hence the 
resultant not only indicates the presence of more than one IR energy 
source, but can be decomposed to indicate where the sources are located. 
The value of this feature is readily apparent from a comparison with a 
prior art quad-detector. Upon sensing vehicles 80 and 82, a quad-detector 
would tend to direct a projectile toward the centroid of the two vehicles, 
in other words to the ground half-way between them. In contrast, the 
resultant obtained in accordance with the present invention indicates the 
presence of more than one source, so that a manual override or other 
appropriate action can be taken. 
It should be noted that the presence of more than one infrared energy 
source in field of view 24 does not always result in an indication of the 
presence of more than one source. For example, individual sources in 
sectors 88 and 90 would yield the same reading as a single source in 
sector 84. At the same time, the probability of correctly recognizing the 
presence of two or more sources is substantially increased by adding 
detectors. This is because, while the number of sectors increases in 
accordance with the formula N.sup.2 -N+1, the number of possible digital 
words is 2.sup.N, with N in both cases being the number of detectors. When 
six detectors are employed, the number of sectors is 31 while the number 
of possible combinations is sixty-four, resulting in thirty-three possible 
digital words which do not identify a sector. With ten detectors, the 
number of sectors is ninety-one, while the number of combinations 
increases to one thousand twenty-four. Along with an increased ability to 
recognize a plurality of sources, the additional detectors provide 
increased tracking accuracy. Alternatively, if at some time during the 
tracking process the supposed single target is resolved as two targets, 
appropriate action can be taken. 
Thus, significant advantages arise from use of multiple aperture, 
non-imaging detectors positioned in accordance with the present invention 
to track sources of infrared or other sensible energy. More particularly, 
multiple detectors, with their individual fields of view selectively 
overlapping as described, provide a wide acquisition field of view or 
initial capture range, and also a high resolution center for precise 
tracking. This result is achieved with relatively few detectors and simple 
electronics, particularly as compared to the focal plane array approach. A 
thirteen element focal plane array would be necessary to yield the same 
number of sectors as a four detector multi-aperture system. Also, the 
division of the acquisition field of view into Venn diagram areas or 
sectors permits use of a relatively simple binary control system capable 
of indicating the presence of more than one energy source within the field 
of view.