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BACKGROUND OF THE INVENTION
Multi-faceted scanners usually comprising multi-faceted rotating mirrors
are employed in well known techniques for erecting optical scanning
between a light source and a pohtocell. Typically, a light illuminates a
silvered mirror, for example, at an angle of 45.degree. to direct light
toward a facet that is reflected from the facet toward the object being
scanned. Normally the object reflects this light back along the same path
upon a photocell. The duration of the scan corresponds to the time for a
facet to pass the light beam along the object being scanned. It is usually
preferred that the object path scanned is independent of which facet is
then in the light beam path.
In connection with television equipment, it is known to use mirror prisms
for image scanning along one dimension, usually for line scanning. Since
the advent of television, cameras operating in accordance with the image
storage system, the need for such mirror prisms has become greatly
increased. Recently television cameras have been designed for operation
within the infrared radiation range, for example, within the range of 2 to
5.5 microns. Television cameras operating within this wave-length require
mirrors or similar light deflecting optical means for scanning an image.
Usually one means, for instance, a light deflecting mirror, is used for
vertical scanning image division. Rotary mirror prisms which are generally
prisms composed of several plane mirrors such as glass mirrors are
conventionally employed by suitably mounting them on a shaft or other
rotary support. These mechanically composed rotary prisms are found to
have many disadvantages, both as to their optical characteristics and
their mechanical reliability. In particular, they have been found
mechanically difficult to mount the several planed mirrors so that they
accurately form a polygonal shape of predetermined dimensions. For short
optical path lengths, slight misalignment of the facets is found to be of
little practical significance. However, when the distance between the
scanning mirror and the object being scanned is many feet, slight
misalignment of the facets results in the path of scan changing from one
facet to the other. Such a result is especially disadvantageous when
scanning labels with an encoded stripe arrangement. If there is
misalignment of the facets one facet might make a perfect scan of the
coded stripes while the next facet would register no scan at all or only
scan a few of the stripes.
Moreover, it is difficult to mount the mirrors so that they accurately
retain their spatial positions when subjected to the stressed of high
speed rotation. The last mentioned mounting problem entails a danger of
injury to persons close to the spinning mirror prism which is often
unavoidable. Obviously when the mirror prism should disintegrate shrapnel
is produced which may cause serious injury to a bystander.
Thus, many methods have been investigated to produce multi-faceted scanners
so that the materials from which they are composed would have high modulus
to density ratio, low thermal expansion, low Poisson's ratio, good
workability and possess the ability to be readily polishable or coatable
with a substance which in turn can be polished to produce high quality
optical surfaces. Unfortunately, the imposition of these material
restrictions result in the requirement of a material which is not readily
available. Presently, in view of these material restrictions and
limitations, scanners are now being manufactured from glass, stainless
steel, beryllium and chromium carbide. The latter two materials are the
most widely used since they more nearly meet the requirements of the
predicated material limitations. Of these two, beryllium is found to best
satisfy the material requirements of the predicated material limitations
and consequently is found to perform in a superior fashion when employed.
However, the use of beryllium to provide multi-faceted scanners in and of
itself results in still other problems among which are exorbitant cost of
the material and the extreme difficulty of working the material into the
desired configurations. Chromium carbide scanners, although not as
expensive as beryllium scanners, possess very high density and therefore
require in the overall general construction of the scanner a driver motor
and bearings which are much heavier and much more costly to provide.
There is therefore a demonstrated need to provide multi-faceted scanner
systems which may be precisely machined, inexpensively, and with great
facility than known scanner systems enabling these multi-faceted scanners
to be considered for employment in a vast number of applications other
than military or development laboratories where the exorbitant costs of
currently available scanner systems can only be justified.
It is therefore an object of this invention to provide a novel
multi-faceted scanning system devoid of the above noted deficiencies.
It is another object of this invention to provide a novel multi-faceted
scanner capable of operation at high rotational speeds.
It is another object of this invention to provide a novel multi-faceted
scanner system characterized by precise alignment of the facets.
Another object of this invention is to provide a novel scanning system
which achieves precise alignment of the different facets with techniques
that are relatively easy to perform.
These and other objects are accomplished, generally speaking, in accordance
with the principles of the system of the instant invention, by
centrifugally casting a thin film plastic against an aluminum preform
which is placed in a precisely machined polygonal master in a conventional
replica process. The plastic, which may be for example an epoxy material,
is applied to the preform-master assembly with centrifugal force causing
it to impact in the cavity between the preform and the master to produce a
bubble-free layer.
In prior art replica techniques the production of large optical surfaces
has been contemplated so that the application of a thin film plastic to
the preform is found to be relatively easy. However, the ease of
accessibility unfortunately of these surfaces does not always assure a
bubble-free plastic layer. Since the thickness of these layers may be
only, for example, approximately 0.005 inches, it can readily be seen that
to fill the narrow cavity between the master and the polygon preform
without the introduction of air bubbles is either extremely difficult if
not impossible.
The master of the system of the instant invention should be shaped such
that when it is rotated the centrifugal force exerts pressure on the
liquid epoxy and compacts it into a bubble-free layer. The master is
rotated only during the filling operation on a vertical axis which
provides the most convenient arrangement.
The general premise of the system of the instant invention having been
described, the specifics of the system of the instant invention will be
more readily understood with reference to the drawings which follow of
which:
FIGS. 1 and 1A illustrate schematically a system for applying epoxy
centrifugally between a preform and a precisely machined multi-facet
scanner master.
FIG. 2 illustrates schematically a finished multi-facet scanner in a high
speed scanning application.
In FIG. 1 there is seen a master 20 which is concentrically located on a
rotational hub 21 carrying a preform 22 which rotates with the master 20.
Epoxy 23 is applied to the gap 24 between the preform 22 and the master 20
while the hub 21 rotates the preform 22 thus applying a bubble-free layer
25 between the preform 22 and the master 20. The preform 22 is fabricated
of an aluminum alloy as hereinafter designated.
Epoxy 23 is applied while the hub 21 rotates the entire assembly shown
until the gap 24 is completely filled with a bubble-free layer 25. The
multi-facet scanner thus fabricated is completed when the epoxy 23 is
cured and then is removed by breaking the master 20 away at the
disengagement points 26, and two others not shown each located at
120.degree. intervals. With a draft-angle of 30.degree. or more 28, the
master 20 can be made in one piece since the master 20 can be removed by
screws (not shown) using the threaded holes 29 shown in FIG. 1-A. There
should be six holes to remove the master 20 and six bolts (not shown) to
hold the master 20 to the base plate 30 during operation. If there is no
draft-angle 28 (facets are parallel with scanner axis) the master 20 must
be made in three parts. During operation they are held in place with
precisely located dowel pins and bolts (not shown). When operation is
completed, dowels and bolts are removed and the master is being removed
radially using screw 26 and two others spaced at 120.degree. intervals not
shown.
In FIG. 2 there is seen a conventional application for the high speed
multi-facet scanner thus provided. A laser 5 emits light through a
modulator 6 which is reflected by a mirror 7 to a beam expander 8. The
expanded beam impringes on the scanner facets 4 which are rotated at high
speed by motor 9. The scanned beam then passes through a focusing lens 10
and is directed to the photoreceptor surface 11.
Typical applications of this system include deflection of a light beam such
as laser in such a manner that it produces a "flying spot". When this
bright spot is moved across an object-document having high and low density
areas by rotation of the scanner, a light detector (placed in the
vicinity) provides an electronic signal which is low or non-existent when
the spot is in a dark area, and high when the spot is in a light area of
the document. This type of scanning system is used in facsimile devices
and in optical character readers. Since this system can be used (in
conjunction with other hard and software) to decode alpha numerics, it is
also known as a "reader". Another system which also uses multi-faceted
scanners is the so-called "write" system. The overall arrangement in
general is the same except that in the stationary path of the beam (before
the scanner) a light switch known as a modulator is used to "write" the
inage on a xerographic photoreceptor. The signal going into the modulator
can come either from the light detector of the "read" station, or from a
character generator which is the case with computer printers.
Any suitable epoxy may be employed in the system of the instant invention.
Typical epoxies include EPON 828, among others. Preferred of these are
EPON 828 with the addition of 5% by WT DEAPA (Diethylaminopropylamine).
Any suitable master may be employed in the system of the instant invention.
Typical masters are produced employing the following technique: facet
segments are fabricated from stainless steel or glass and then held in a
common frame. The segments are then polished to a high degree of provide
the necessary surface quality.
Any suitable aluminum alloy preform of the type recited may be provided for
use in the system of the instant invention. Typical techniques for
providing such aluminum alloy preforms include: turning the aluminum disk
to the desired diameter and thickness on a precision lathe and boring the
mounting hole. Employing this method no facet milling is required. The
outside diameter of the preform is round. The preferred aluminum alloy is
7075-T651 having the following specifications:
7075-T651 Aluminum Alloy
This alloy is recommended when extra strength and hardness are required. It
is used primarily for aircraft and ordinance applications.
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Nominal Chemical Composition
Zinc 5.6%
Magnesium
2.5%
Copper 1.6%
Chromium 0.3%
Aluminum Balance (incl. normal impurities)
Typical Tensile Strength, psi
83,000
Mechanical
Yield Strength, psi 73,000
Properties
Elongation, % in 2" 11
Shear Strength, psi 48,000
Brinell Hardness 10/500
150
Typical Density, Lbs./Cu. In.
0.101
Physical
Melting Range, approx. .degree. F
890-1180
Properties
Electrical Conductivity,
% IACS at 20.degree. C (68.degree. F)
33
Thermal Conductivity, btu
at 25.degree. C (77.degree. F)
900
Average Coefficient of Thermal
Expansion at 68.degree. to 212.degree. F
0.0000131
These typical properties are average values.
Fabricating Performance
Cold Forming:
Poor
Machining: Good
Brazing: Not suitable
Welding:
Arc, Poor
Gas, Poor
Resistance,
Good
Government and Industry Specifications
Cold Finished-Rolled
Extruded
A.M.S. 4122C, 4123A 4154F, 4168A, 4169B
A.S.T.M.
B211 B221
Federal
QQ-A-225-9b(QQ-A-282)
QQ-A-200/11b(QQ-A-277)
Military
None None
S.A.E. AA7075 AA7075
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Any suitable rotational mechanism may be employed in the system of the
instant invention. Typical such rotational mechanisms include a vertical
shaft mounted in a structure supported by bearings, and driven by an
electric motor.
The epoxy may be applied employing any suitable conventionally available
dispensing system. Typical dispensing systems include a container with
adjustable flow mounted in a structure in such a manner that the nozzle is
in the feeding groove or the nozzle may be located above the feeding
groove in which case the epoxy must be applied before rotation.
The master may be machined and assembled employing any suitable technique.
The master is specifically provided so that it may be disassembled from
around the epoxy-coated preform. Separation between master and preform may
be achieved by another method, namely by quick cooling of the preform
after curing. Since during the curing the entire assembly is at the same
temperature, a quick cooling of the preform would shrink its diameter
faster than the steel of the master. This is due to the fact that the
aluminum thermal expansion coefficient is approximately twice that of the
steel. This type of separation technique would result with a
well-distributed stress in the master.
To further define the specifics of the present invention, the following
examples are intended to illustrate and not limit the particulars of the
present system. Parts and percentages are by weight unless otherwise
indicated.
EXAMPLE I
An aluminum alloy preform of the type previously recited is placed on a
vertical rotational mechanism which is capable of rotating the preform at
500 rpm. A master previously vacuum coated with 500 to 10000A gold is
placed over the hub and around the preform. The master is fabricated of
stainless steel having been precisely machined by conventional polishing
techniques and is assembled around the preform. A liquid epoxy dispensing
system begins to dispense the liquid epoxy in the gap between the rotating
preform and master. Dispensing continues with rotation until the gap is
filled with a layer which is observed to be bubble-free. The thickness of
the layer applied in the gap is found to be 0.005 inches of liquid epoxy.
The epoxy is then allowed to cure for 16 hours at 120.degree. F and after
that the master is broken away from the preform by either pushing or
pulling with screws. If the scanner has draft-angle, the former applies,
if there is no draft-angle, the latter is done (see FIG. 1 and 1A). The
separation-film of gold deposited earlier on the master stays with the
epoxy film due to its higher adhesion to that material. Subsequent
finishing coating of 1000A aluminum for enhanced reflectivity and a 200A
of silicon monoxide coating complete the fabrication. The multi-facet high
speed scanner thus provided is employed in a conventional scanning system
as in FIG. 2.
Although the present examples were specific in terms of conditions and
materials used, any of the above listed typical materials may be
substituted when suitable in the above examples with similar results. In
addition to the steps used to carry out the process of the present
invention, other steps or modifications may be used if desirable. In
addition, other materials may be incorporated in the system of the present
invention which will enhance, synergize, or otherwise desirably affect the
properties of the systems for their present use.
Anyone skilled in the art will have other modifications occur to him based
on the teachings of the present invention. These modifications are
intended to be encompassed within the scope of this invention.
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