Dual field image scanner

A self-stabilizing optical scanner system comprising a multi-faceted mirror includes a plurality of large and small facets formed on or attached to a gyro rotor, which rotor is adapted for being gimbal mounted, such that as each facet is rotated through the optical path of an energy receiving lens system, a detector is optically scanned across a field of view along a first dimension. In one embodiment, each mirror facet is inclined at a different angle to the rotor spin axis such that as the rotor revolves a two-dimensional pattern is scanned; and in a second embodiment a driven mirror is disposed in the optical path of the received energy so as to provide scanning along the second dimension.

BACKGROUND OF THE INVENTION 
This invention relates generally to optical imaging systems and more 
particularly to such systems embodying self-stabilizing image scanners. 
Heretofore optical imaging systems requiring space stabilization, e.g. 
vehicle mounted systems, have generally employed a separate gyroscopic 
system for stabilization. Although such systems have been satisfactory for 
many applications, cost and size penalties imposed by the separate 
stabilization system have proven to be quite significant in some 
applications and prohibitive in still others. For example, in airborne 
vehicles, space is generally at a premium; and in guided missiles, size 
and cost in such systems are both of prime importance. 
A significant aspect of the subject invention relates to a feature thereof 
which allows a major portion of the optical scanning mechanism and the 
rotor of a free gyro to be combined into a single element so as to provide 
cost and size savings. 
SUMMARY OF THE INVENTION 
Therefore, it is a primary object of the subject invention to provide an 
improved optical imaging system. 
A more particular object is to provide a self-stabilizing optical image 
scanner. 
Another object is to provide a compact self-stabilizing optical image 
scanner capable of providing television type imagery data in response to 
received IR (infrared) energy. 
A further object is to provide a self-stabilizing optical image scanner of 
improved quality and high reliability, and which is sufficiently compact 
and inexpensive for use in applications such as missile guidance systems. 
Image scanners in accordance with the subject invention provide both 
inertial stabilization and image scanning while enabling generation of 
high resolution pictorial data with relative few detector elements. In 
accordance with one configuration, a multi-faceted mirror comprising a 
plurality of large and small groups of facets is formed on or attached to 
a gyro rotor, which is adapted for gimbal mounting, so as to enable 
rotation of each facet through the optical path of an energy receiving 
lens system. A detector, a detector array or a plurality of detector 
arrays are optically scanned across the field of view along a first 
dimension. Each mirror facet is inclined at a different angle to the rotor 
spin axis such that as the rotor revolves a two-dimensional pattern is 
scanned. In accordance with another configuration, a driven mirror is 
disposed in the optical path of the received energy so as to provide 
scanning along the second dimension. 
The revolving rotor includes an annular ring having a first plurality of 
abutting reflectors integral with and at a first portion of the periphery 
of the ring, and also includes a second plurality of abutting reflectors, 
integral with and at a second portion of the periphery of the ring. Each 
of the second plurality of reflectors are smaller in area than each of the 
first plurality of reflectors. Said second plurality of reflectors as a 
group are interposed between the first plurality of reflectors.

DETAILED DESCRIPTION 
Referring first primarily to FIGS. 1 through 3, the self-stabilizing image 
scanner there shown includes a support bulkhead 20 which provides a 
mounting for a gimbal pedestal 22 and torquer assemblies 24 and 26. 
Support bulkhead 20 also serves as a bulkhead interface for a dome cover 
29 and an IR dome 30, see FIG. 3. Pedestal structure 22 has four legs 
which extend forward from bulkhead 20 to accommodate outer gimbal 32 and 
bail 34. A pair of flanged duplex ball bearings (not shown) mount the 
outer gimbal between two lateral legs of pedestal 22. An inner gimbal 36 
is mounted at 90.degree. to the outer gimbal axis inside circular outer 
gimbal 32 by a pair of flanged duplex ball bearings 38 (FIG. 3). 
The entire sensor package including a gyro stator 40 (FIG. 4) gyro rotor 
42, detector/dewar/preamp assembly 44, synchronization (sync.) reticle 46, 
sync. generator 48, and delay line assembly 50, is assembled aligned and 
balanced (static and dynamic) prior to mounting to the gimbaled system. 
The inside diameter of inner gimbal 36 (see FIG. 4) is large enough to 
allow the large diameter front objective lens 52 (4 inches in diameter, 
for example) to pass through. 
Bail ring 34 is mounted between vertical legs of pedestal 22 with threaded 
pivot ball bearings 54. The outer races of the bearings are threaded into 
the bail and preloaded to reduce the radial play of the bail. The bail is 
independently balanced about the inner gimbal axis to reduce drift torques 
caused by g-sensitive unbalance. A tripod structure 56 protrudes from the 
rear of the inner gimbal and connects to the bail ring 34 through a pair 
of preloaded ball bearings. The outer diameter of the bearings registers 
inside the U-shaped bail, thus coupling the inner gimbal axis (azimuth, 
for example) to the bail ring while decoupling the outer gimbal axis 
(elevation, for example). 
An inner gimbal axis push-rod 58 is attached to bail 34 by a torque arm 60 
as shown best in FIG. 1. An outer gimbal push-rod 62 is connected directly 
to outer gimbal 32 as shown best in FIG. 2. To prevent damage to the 
scanner caused by inadvertent gyro tumbling, a mechanical stop is provided 
between a protrusion 64 on the rear of the inner gimbal bail drive 
structure and a thin hemispherical structure 66 attached to pedestal 22. A 
properly shaped hole is provided in hemispherical structure 66 to allow 
the desired circular look-angle capability in elevation and azimuth. 
Gimbal position sensors 68 and 70, which may be film-type DC 
potentiometers, are mounted on the inner and outer gimbal axes, 
respectively. Potentiometer 68 is shown in FIG. 3 and potentiometer 70 is 
indicated in the electrical block and schematic diagram of FIG. 11. 
With reference to FIGS. 6 and 8, the optical train of the subject invention 
includes an IR telescope which comprises objective lenses 52 and 72, 
folding mirror 74 and eyepiece lenses 76. The IR telescope directs 
received IR energy to a multi-faceted scan mirror 78 located on an 
internal surface of gyro rotor 42. The energy reflected from scan mirror 
78 having facets 78' and 78" is applied through viewing lenses 80 and a 
dewar window 82 in detector-dewar/preamp assembly 44 to detector arrays 
88, disposed therein. 
It is obvious that facets similar to 78' and 78" may be fixed on the outer 
and side surfaces of annular ring or scan mirror 78. 
It is noted that although the herein disclosed embodiments relate to IR 
image scanners, that the subject invention is applicable to optical 
systems in general; and as used herein the term "optical system" includes 
systems which respond to infrared or ultraviolet energy as well as those 
which respond to energy in the visible portion of the spectrum. 
Thermal compensation of the IR telescope is provided by mounting the rear 
objective lens 72 in a temperature compensator assembly 84. Assembly 84 
responds to temperature changes to move lens 72 so as to reduce focus 
errors due to thermally induced changes in the index of refraction and 
form of the lenses and in the dimensions of the supporting structures. 
Compensator assembly 84 is located and held by opposing spring forces of 
partially compressed bimetallic Belleville washers. These washers are made 
with opposite bimetal surfaces such that both surfaces move in the same 
direction with temperature changes while retaining the same holding spring 
force at all times. 
Similarly, the optical focus in the detector image plane is maintained 
against thermally induced changes by temperature compensator assembly 86. 
A significant aspect of the subject invention relates to the performance of 
space stabilization and optical scanning (at least along one scan 
dimension) functions by a single element. The large polar moment of 
inertia resulting from the rotation of gyro rotor 42 provides space 
stabilization for the system; and the mirror facets of scan mirror 78, the 
optical scan along at least one dimension. Folding mirror 73 allows the 
spin axis, which is the axis stabilized in direction, to be substantially 
coincident with the optical axis. Gyro stator 40 is adapted to be rigidly 
mounted to the bottom side of inner gimbal 36, see FIG. 4, and the 
combination of gyro stator 40 and gyro rotor 42 forms a squirrel-cage 
3-phase induction type motor. 
Referring momentarily to FIGS. 4 and 5, the main housing 41 of gyro rotor 
42 may be constructed of titanium with the scan mirror portion 78 being 
nickel. One preferred method of fabricating scan mirror 78 consists of 
machining a metal master (or mandrel) 77, having facets 77' and 77", see 
FIG. 5, and electroforming a nickel mirror from the mandrel. This 
fabrication technique has proven to be costeffective inasmuch as a 
plurality of scanning mirrors may be formed from a single master. For a 
scan mirror of the type shown in FIG. 5, which could be approximately 6.75 
inches in outside diameter, there could be, for example, 20 
rectangularly-shaped mirror facets 78' and four smaller facets 78". 
The received optical path or train is illustrated in FIG. 6. As there 
shown, received IR energy is directed by optical elements which include IR 
dome 30 and the telescope portion (elements 52, 72, 74 and 76) of the 
optical system, onto the scanning mirror 78 comprising mirror facets 78' 
and 78". The energy reflected from scanning mirror 78 is directed by 
lenses 80 and window 82 onto the detector arrays disposed in 
detector/dewar/preamp assembly 44. Scanning mirror 78 may be located at or 
about the exit pupil of the telescope. 
According to one embodiment of the subject invention, scanning along one 
dimension such as azimuth, for example, is provided by the rotation of the 
mirror facet assembly 78; and scanning along a second dimension such as 
elevation, for example, is provided by forming the mirror facets such that 
they make different angles with the spin axis of rotor 42. Two-dimensional 
scanning by means of a mirror drum scanner with tilted mirror facets is 
described in U.S. Pat. No. 3,626,091. In the embodiment of FIG. 3, 
interlace fields are provided by shifting the position of folding mirror 
74 between fields (i.e., between revolutions of rotor 42). The 
displacement of folding mirror 74 by interlace actuator 73 (FIG. 3) is 
diagrammed in FIG. 7. 
This invention extends the art to include scan of two fields of view of 
different sizes per revolution of the scan wheel. A sequence of mirror 
facets 78' on the interior of a spinning wheel sequentially scans a raster 
pattern over the larger field of view of the sensor. A second sequence of 
mirror facets 78" scanning a smaller field of view is integral with wheel 
78 so that a smaller field of view is covered during the vertical retrace 
time of the main (larger) field of view. This gives a doubling of the data 
rate for small targets near the center of the main field of view compared 
to the field of view seen when facets 78" are not present. 
In the herein-disclosed embodiment, detector assembly 88 (FIGS. 6 and 8) 
comprises four detector arrays 90 through 93 oriented traversely to the 
direction of mechanical scan, such as the azimuth direction, for example. 
Each array comprising a plurality of detector elements, for example array 
90 comprises elements 90a, 90b, 90c and 90d. Each detector array is 
similarly structured and produces one line of output video per active 
mirror facet, per field. Therefore, four lines of video displaced along a 
direction transverse to the direction of mechanical scan are produced 
during the period a single mirror facet passes through the path 79 (FIG. 
6) of the received optical energy. 
The processing of the signals from a single one of the detector arrays (90) 
is illustrated in FIG. 9. As thereshown the output signals from each 
detector element 90a, 90b, 90c and 90d are individually amplified in 
preamplifier units 95 and are processed through multiple tap delay line 
96. In delay line 96, differential time delay is applied to the output 
signals from the various detector elements; and the combined signals are 
applied through a buffer amplifier 98 to a output video channel labeled 
channel A in FIG. 9. Buffer amplifier 98 couples signals out of the delay 
line while preventing undesirable signal reflections, i.e., provides 
isolation. A termination 97 prevents undesirable reflections from the 
other end of the delay line. Similarly, the output signals from arrays 91 
through 93 are processed to provide video output signals for channels 
designated B, C, D, see FIG. 11. It is important to note that the relative 
time or propagation delays imposed upon the output signals from the 
detector elements of a single array are selected to match the azimuth 
mechanical scan speed with respect to the displacement of the elements 
along the scan direction. In this manner at the output of delay line 96 
the signals originating from the same segment of the scanned field of view 
are combined. This type of signal processing improves the signal-to-noise 
ratio and a defective or faulty element does not produce a dark or missing 
line in the resulting display -- i.e., the channels degrade gracefully. 
The above-summarized processing techniques and further advantages obtained 
therefrom are explained more fully in U.S. Pat. No. 3,723,642. 
Synchronization of the display and tracking systems which may be associated 
with the image scanner of the subject invention is accomplished by 
horizontal (H.sub.s) and vertical (V.sub.s) sync. signals generated by 
electro-optical sync. generator 48 (FIG. 3) in conjunction with reticle 
46. Reticle 46 is ring-mounted on the gyro rotor assembly, and it is 
encoded with two tracks of alternately clear and opaque spokes. As shown 
in FIG. 10, an outer track 100 which may have 525 spokes, for example, 
generates the horizontal sync. pulses; and an inner track disposed between 
a mounting shoulder 104 and the outer track 100 includes a single spoke 
for generating the vertical sync. pulses. 
OPERATION 
For further explaining the operation of image scanners in accordance with 
the subject invention reference is now primarily directed to FIG. 11 which 
shows such an image scanner in functional block diagram form. As 
thereshown, received IR energy passes through dome window 30, is processed 
by optics, designated in FIG. 11 by reference numeral 53 and described 
hereinabove relative to FIG. 6. The received energy is then reflected from 
one of the mirror facets of scanning mirror 78 onto detector/dewar/preamp 
assembly 44. The output signals from each detector element of assembly 44, 
after being preamplified therein, are processed in the delay line unit (96 
of FIG. 9) associated with the respective array unit so as to provide four 
parallel channels of video data on an output cable designated 106. 
As described hereinabove relative to the discussion of FIG. 10, sync. 
reticle 46 and electro-optical sync. generator 48 provide the vertical 
sync. pulses (V.sub.s) and horizontal sync. pulses (H.sub.s). 
As explained above, scanning along at least one dimension is accomplished 
in accordance with the subject invention by a set of plane mirror facets 
located on an internal surface of the spinning gyro rotor 42, see in 
particular FIGS. 3 and 6. As each mirror facet passes through optical path 
79 (FIG. 6) of the lens system, the detector assembly 88 is optically 
scanned along a first dimension, azimuth for example, across a field of 
view. Accordingly, each facet is inclined at a different angle to the spin 
axis of gyro rotor 42 so as to produce stepped displacement in a 
dimension, elevation for example, transverse to said first dimension, from 
facet to facet. The sequence of facet angles is chosen to cover the 
elevation field of view in equal steps, separately for the larger and 
smaller fields of view, once per revolution. 
The width of facet 78' is 16.4571.degree., which at 3.125 inches from spin 
(optical) axis 79 providing a width W' of 0.904 inches. The four retrace 
facets 78" subtend 7.7143.degree. each and are 0.423 inches wide at W". 
In this invention, use of the vertical retrace time, which was formerly 
unused for signal processing and electronic tracking, allows scanning the 
smaller field of view without changing the signals received during scan of 
the larger field of view. 
An example of one possible configuration of multi-faceted scan mirror 78 is 
an embodiment having 20 active contiguous mirror facets 78' each 
subtending 2.pi./21.875 radians. The remaining 2.pi.(1.875)/21.875 radians 
coincide with the vertical retrace time and contain four smaller facets 
78" which scan, for example, a rectangle 0.25 of the width and 0.2 of the 
height, of the full field of view. The total number of facets equivalent 
to 78' may be chosen to provide 525 TV lines per frame after scan 
conversion. Note that 525 television lines/24 = 21.875 mirror facets. The 
factor of 24 rises from the use of two fields per frame with four lines 
per facet and three TV lines per IR line. The above-described scanner 
supplies 80 active IR lines per field and with a 2 to 1 interlace supplies 
160 active IR lines per frame. 
In accordance with a second preferred embodiment of the subject invention, 
all of the mirror facets on gyro rotor 42 make the same angle with the 
spin axis of the rotor and scanning along a second dimension such as 
elevation, for example, is obtained by positioning folding mirror 74 to a 
plurality of discrete positions; for example, 20 such positions per field 
and 40 positions per frame (two interlaced fields). A functional diagram 
of such an elevation scanning configuration is illustrated in FIG. 12. As 
there shown, an elevation scan generator 75, which is synchronized in 
response to horizontal and vertical sync. pulses, produces a drive current 
to interlace actuator 73' which may be a galvanomotor, for example. Mirror 
74 is mechanically coupled to galvanomotor 73' and is thereby positioned 
to a new discrete angle during the scan time of each of the mirror facets 
of scan mirror 78. Galvanomotor 73' is well-known in the art inasmuch as 
the electrical-mechanical components thereof are substantially identical 
to a conventional galvanometer -- the difference in nomenclature being 
derived from the function of the unit i.e., to provide a mechanical drive. 
Again referring primarily to FIG. 11, three-phased power is applied to gyro 
stator 40 from a speed control unit 108. The speed control unit responds 
to the horizontal sync. pulses (H.sub.s) to control the speed of rotor 42 
to a preselected value, such as 3600 rpm. 
During periods when the seeker is non-operational, torquers 24 and 26 are 
held in a rigid position by braking units 112 and 114, respectively. 
During periods of operation of the scanner, the brakes are deactivated 
(the torquers are free to be driven) by the application of an enabling 
signal on lead 110 from a control source (not shown). 
Gimbal angle sensing potentiometers 68 and 70 are energized by the 
application of a supply source voltage (not shown) across leads 116 and 
118. 
The temperature of the detector assembly 88 (FIG. 6) is indicated by output 
signals on leads 120 from the assembly 44. In response to the measured 
temperature a cooling system (not shown) provides a coolant liquid such as 
liquid nitrogen, for example, through a coolant line 122 as required to 
maintain the desired temperature of the detector assembly. 
An interlace control unit 124 applies control signals to interlace actuator 
73 such that in the embodiment of FIG. 3 the folding mirror 74 is 
positioned to provide field-to-field scan interlace. The interlace control 
unit 124 is synchronized by the vertical sync. pulses (V.sub.s). In the 
embodiment of FIG. 12, field-to-field interlace scan, as well as the total 
or part of the elevation seen, is provided by elevation scan generator 75. 
A missile seeker system suitable for using an image scanner in accordance 
with the subject invention is shown in block diagram form in FIG. 13. It 
is noted that the system of FIG. 13 is not part of the invention claimed 
herein, but is presented to illustrate one possible application for the 
invention. As shown in FIG. 13, the four channels of video data from delay 
lines 50 (FIG. 11) are applied on a cable 106 to a signal processor 128. 
Processor 128 provides conventional signal processing and then applies the 
four channels of data in parallel to a scan convertor unit 130. Scan 
convertor 130 converts the data applied thereto (four parallel channels in 
the time frame of the image scanner) to output data in conventional 
television video format. The output signals from convertor 130 are applied 
to a television display unit 132 and to a video tracker unit 134. 
The tracker 134 may be of any suitable type such as that disclosed in U.S. 
Pat. No. 3,586,770. Tracker 134 processes the applied data so as to 
produce output signals (steering signals) indicative of the displacement 
of a designated target from the center of the scanned field of view. 
During a track mode of operation, the steering command signals are applied 
through track/slave switch 136 to torquer amplifiers 138. Torquer 
amplifiers 138 apply the required signals to torquers 24 and 26, see FIGS. 
1 through 3, such that the image scanner is positioned (gyro is precessed) 
to maintain the designated target at the approximate center of the field 
of view. 
During an acquisition mode of operation, switch 136 is in the opposite 
position from that shown in FIG. 13 and the torque amplifiers are 
controlled by the output signals from a summation unit 140. One input 
signal to summation unit 140 is a command signal applied from slave 
control unit 142 and a second input signal is the output from demodulators 
144. The output signals from demodulators 144 are indicative of the gimbal 
angles of the seeker and any difference between the command position 
indicated from unit 142 and the actual gimbal angles of the seekers is 
used to produce drive signals, through switch 136 and torquer amplifiers 
138, which position the gimbals to the commanded position. 
It is noted that in the interest of clarity a single-dual control channel 
is illustrated in FIG. 13; but it is understood that two separate 
channels, one for the inner and one for the outer gimbals, are 
implemented. 
The steering command signals from the video tracker 134 are also applied to 
a vehicle flight controller unit 146, which in response thereto positions 
the control surfaces of a missile, for example, such that its flight path 
is adjusted to null the steering command signals i.e., so that its flight 
path will intercept the target. 
It is noted that suitable implementations for and more detailed 
descriptions of tracker 134 and vehicle flight controller 146 are 
presented in the above-cited U.S. Pat. No. 3,586,770. 
Scan convertor 130 (FIG. 13) may be any suitable unit for transforming the 
four parallel channels of IR video into a single channel of composite TV 
video for display on a standard TV monitor, and for use by video tracker 
134. One suitable implementation is generally shown in FIG. 14 as 
comprising read-in control circuits 150, field No. 1 video storage unit 
152, field No. 2 video storage unit 154 and read-out control circuits 156. 
Units 150 and 152 and 156 are controlled and synchronized in response to 
output signals from timing and control circuits 158 which themselves are 
synchronized by the horizontal and vertical sync. pulses provided by the 
image scanner of the invention. In response to the output signals from 
circuits 158, the read-in control circuits 150 apply the four video 
channels of data produced by the scanner to one of the storage units 152 
or 154; and read-out control circuit 156 reads the data which was 
previously stored in the other storage unit 154 or 152 in a conventional 
525 line television format. Timing and control circuits 158 also 
synchronize TV sync. and blanking generator 160 and the output signal 
therefrom is combined with the video data from unit 156 in a combiner 162. 
The composite television video and sync. signals from combiner 160 are 
applied to TV display 132 and video tracker 134 of FIG. 13. 
Thus there has been herein described a new and useful image scanner wherein 
spatial stabilization is obtained by having the scanning and detector 
elements as an integral part of a two degree of freedom gyro assembly. 
In order to provide full and complete disclosure, the preferred embodiments 
have been described herein with particularity; however, it is understood 
that many variations and modifications thereto may be readily implemented 
by those skilled in the art without departing from the scope of the 
subject invention. For example, although in the preferred embodiment the 
detector assembly was disposed on the inner gimbal, in certain 
implementations the detector assembly may be ungimbaled and located at the 
gimbal center. Such an arrangement although resulting in a certain amount 
of defocusing at the ends of the array could be acceptable in many 
applications. Also, it is noted that although the preferred optics and 
detector arrangements of the disclosed embodiments were specified in 
detail herein, that any suitable optics and detector configurations may be 
utilized in accordance with the invention. For example the optical and 
rotor spin axes need not coincide. Further, certain of the lens 
arrangements could be replaced by concave or convex mirrors; and the 
folding mirror could be a deviating prism -- i.e., a refractive rather 
than reflective configuration.