Note: Descriptions are shown in the official language in which they were submitted.
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OPTI~AL P~INTING SYSTEM WITH I~ISCONTINUOUS PRINT MEDIUM
BACKGROUND QF THE INVENTION
This invention relates to the focusing of an optical print
head upon a discontinuous print medium secured to a print drum by a
ciamp and, more particularly, to the use of coaxially disposed focusing
and write laser beams to enable close positioning of an objective lens to
the print medium during writing, and including lens-positioning apparatus
for locating the lens at a reference position for passage over the clamp
during rotation of the drum.
Op~ical printers employing a photosensitive recording
medium and a laser beam for writing alphanumeric and pictorial data on
the recording medium are in wide use today, these printers including
laser printers for outputting hard copy from computers and facsimile
machines. The printers employ various optical systems for focusing and
directing a laser beam upon the recording medium. The printers employ
various transports for moving the recording medium past a print station
for illumination by the laser beam, and various electro-optical devices for
controlling the intensity of the laser beam and for modulating the laser
beam. It is of particular interest to increase the speed of the writing,
and also to increase the resolution of an image produced on the print
medium so as to provide a higher output printing rate and improved
image quality.
While various forms of photosensitive materials may be
employed four the print medium, it is advantageous to employ a
photosensitive material having a binary characteristic in terms of its
response to incident radiation such that, below a threshold intensity of
incident radiation, there is no imprinting of marks on the material. For ;;
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radiation incident upon the material with an intensity above thè
threshold, marks are imprinted upon the material. The binary
characteristic may include also a frequency response in that, by way of
example, a minimum threshold valIJe of frequency is required to activate
5 the photosensitive material and that, for radiation of a lower frequency,
there is no imprinting of marks. Such a binary characteristic enhances
the production of half-tone images formed by an array of loosely spaced
dots, as well as gray-scale images wherein the density of dots is varied
within each pixel to ~roduce a gray scale.
1Q A problem arises in that there is a need for higher resolution
images than is available with present printing systems. Also, it is
desirable to include numerous shades of gray by the distribution of dots
of varying density within the pixels of the image. However, currently
available printers are limited in their ca~cacity for performing
15 high-resolution gray-scale images, and are also limited in the speed at
which such images can be produced.
SUMMARY OF TH~ INVENTION
The aforementioned problem is overcome and other
advantages are provided by an optical printing system employing a beam
20 of radiation incident upon a photosensitive recording, or print medium
wherein, in accordance with the invention, the incident radiation is
focused as a write beam by an objective lens which is placed close to
the print medium for illuminating the medium with a field of light present
directly in front of a radiation source comprising an array of laser diodes
25 for optimizing uniformity of illumination. Such a positioning of the
medium relative to the lens provides for a distribution of radiation
intensity which differs from that in the far fields or Fraunhofer region.
Of particular interest in the practice of the invention is the fact that the
distribution of the radiation intensity in the near field is more uniform
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then in the far field. This is of particular importance in the case of
finer-grain pixels which are, themselves, created by a distribution of
minute dots, possibly of various shapes, so as to provide greater c~ntrol
and fidelity in the creation oF these dots. Thereby, the optical system of
the invention enables the production of an accurate multiple-level gray
scale to images produced by the system of the invention.
The invention provides for a high speed of scanning by
employing a print drum carrying a sheet of photosensitive material, the
print medium, secured to an outer surface of the drum by a clamp
~ xtending parallel to a rotational axis of the drum. A print head contains
a laser which serves as the source of the radiation of the write beam,
and an optical system including the foregoing objective lens for
collimating rays of the radiation and for focusing the radiation upon the
print medium. The print head advances along the drum in a direction
parallel to the rotational axis as the drum rotates. The concurrent
translation of the print head and rotation of the drum produces a spiral
path of travel of the objective lens along the surface of the print
medium. The spiral path allows the printing to be accomplished with
increased rapidity.
The close spacing of the objective lens to the print medium
enables printing with a resolution of better than 250 pixels per inch. To
produce a gray scale within each pixel, smaller dots, or rectangular
spots, of 3 by 30 micrometers or 3 by 5 micrometers may be employed,
by way of example. The invention employs a telecentric optical system
for guiding laser radiation to the print medium, with a lens positioning
system capable of a positional accuracy within 5-10 microns to maintain
desired spot size and uniforrnity of illumination throughout each pixel.
In accordance with the invention, the objective lens is
positioned within a housing of the print head by means of an electric
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coil and magnet assembly, analogous to a speaker coil, wherein
electrical excitation of the coil produces a small displacement of the
objective lens relative to the housing. By varying the arnplitude and
sense of the excitation current, the lens can be moved forward towards
5 the print medium or retracted from the print medium by small
increments in position as are required to track slight changes in distance
which may occur between the print medium and the objective lens
- during rotation OT the drum.
A further optical system including focusing apparatus is
responsive to the distance between the lens and the medium for
producing a signal which drives the lens-positioning coil. However, the
signal of the focusing apparatus is disabled during passage of the clamp
at the print head. A memory stores an updated value of reference
position for the objective lens to ready the lens for resumption of
printing subsequent to passage of the clamp, the updating being in
accordance with signals of a shaft-angle encoder outputting the angular
posiiion of the drum. Another optical system sights the position of the
objective lens relative to the housing, and activates the coil to place the
objective lens at the reference position during passage of the clamp.
A feature of the invention is the use of the objective lens
for projecting both the write beam concurrently with a projection of a
further laser beam which serves as a focus beam for use with the
focusing apparatus. The focus beam has radiation characteristics of
intensity which do not excite the print medium to produce a mark, this
being in contradistinction to the write beam which has radiation
characteristics of frequency and intensity which do excite the print
medium to produce a mark. While most of the incident radiation is
absorbed by the print medium, there is some reflection, approximately
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4% of the incident power. Accordingly, the reflected radiation includes
radiations of both the write and the focus beams.
In order to separate reflections of the write beam frorn
reflections of the focus beam to allow operation of the focusin0
5 apparatus without interference from the write beam, the optical system
includes dichroic prisms rr~ounted with optical coatings therebetween to
provide for reflections and transmissions of radiation based on
polarization of the electric vector. A quarter-~vave plate Is employed to
reverse the direction of polarization of a reflected wave from that of an
10 incident wave. This enables the waves of radiation outputted by the
write laser and a focus laser ~o be directed by the prisms on a patn
which leads to the medium and away from a detector assembly of the
focusing apparatus. During propagation along the return path from the
print medium, the prisms function with the altered polarization to direct
15 the reflected light toward the detector assembly. A color filter blocks
passage of the write beam, while permitting passage of the focus beam
to the detector. The radiations of the two beams are separated in
frequency such that their wavelength differ by approximately 40
angstroms, this being sufficient to allow for separation by the color
20 filter.
The resulting configuration of the print head allows for
rapid positioning of both the head and the objective lens to
accommodate a rapid scanning of the print medium in conjunction with
rotation of the print drum. Also, accurate focus of the objective lens is
25 maintained with the print medium located within the near field of the
objective lens. This provides the printing system of the invention with
the requisite speed and resolution desired for high-q~ality gray-scale
imaging.
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BRIEF DESCRIPTION OF THE DRAWING
The aforementioned aspects and other features of the
invention are explained in the following description, taken in connection
with the accompanying drawing wherein:
Fig. 1 shows a printing system incorporating the inven~ion,
portions of the system being shown in sectional view, and portions
being shown diagrammatically;
Fig. 2 is a flow chart showing operation of the system,
includin~ a computer thereof, during ~he printing of an image on a sheet
of print medium carried by a ro~ating drum of Fig. 1;
Fig. 3 is a block diagram of a data source, operatively
coupled to a write laser array of Fig. 1, for producing gray-scale pixels in
a printed image;
Fig. 4 is a graph showing a sequence of events in the
positioning of an objective lens of the system of Fi~. 1 as a function of
drum position;
Fig. 5 is a block diagram of a clamp bypass circuit of Fig.1;
Fig. 6 shows variation in detected light intensity as function
of position of the objective lens; and
Fig. 7 is a stylized perspective view of a lens-supporting
carriage including a showing of a position sensing optical system located
beneath the carriage.
DETAILED DESCRIPTION
In Fig. 1, a printing system 10 includes a circular cylindrical
drum 12 rotatable about a shaft 14 disposed along an axis of the
cylindrical drum, and a print head 16 including a source 18 of radiation
focused by an objective lens 20 upon a sheet 22 of photosensitive
material carried by the drum 12. The sheet 22 is wrapped about an
outer cylindrical surface 24 of the drum 12 with opposed ends 26 and
20~2~ ~3
28 of the sheet 22 being pressed against the drum surface 24 by a
clamp 30.
While the clamp 30 may have various configurations, one
such configuration which is readily implemented includes a pair of
opposed wings 32 and 34 which extend perpendicularly from a stem 36
to envelop the sheet ends 26 and 28, respectively. The stem 36 is
disposed parallel to the axis of the drum 12, and extends inwardly from
the drum surface 24 towards the shaft 14 so as to be secured to the
drum 1~ during rotation of the drum 12. The clamp wings 32 and 34
are resilient so as to exert a spring force upon the sheet ends 26 and
28, and thereby secure the sheet 22 to the drum 12.
In accordance with a feature of the invention, the print
head 16 includes a housing 38 which supports the radiation source 18.
A portion of the housing 38 is cantilevered in the form of a carriage 40
which supports the objective lens 20 via a pair of spring legs 42 and 44,
the legs 42 and 44 connecting between the carriage 40 and a frame 46
which holds the lens 20. In order to view components which are hidden
in the view of Fig. 1, some of the components in the carriage 40 are
shown displaced, similar to an exploded view so that all functions of the
carriage can be understood from the figure. The components of the
carriage 40 are shown in their true positions, but in stylized form, in Fig.
7. The housing 38, the carriage 40, and the legs 42 and 44 may be
constructed of a metal such as aluminum. The frame 46 has a circular
cylindrical form, and is constructed of a relatively light-weight material
such as fibrous glass embedded in a bonding agent. The frame 4~
carries the lens 20 at a forward end thereof, near the drum 12, with the
opposed sides of the frame 4~ being used to support coils 4~ of electric
wire, the coils 48 having a construction similar to that of a voice coil
employed in the well-known construction of loud speakers. The total
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mass of the frame ~6 with the lens 20 and the coils 48 is sufficiently
small so as to permit rapid displacement of the lens 20 along a
cylindrical axis of the frame 46, in the manner of a dither movement for
precise focusing of radiation of the source 18 upon the moving sheet
5 22. In accordance with the invention, any small perturbations on the
outer surface of the sheet 22, such as a perturbation due to a variation
of thickness of the sheet 2~, is compensated for by a rapid retraction of
the lens 20 from the sheet 22 or a rapid advancement of the lens 20
towards the sheet 22 so as to maintain radiation of the source 18
10 focused upon the sheet 22.
The coils 48 are activated electrically with current provided
by a focus drive amplifier 50, the amplifier 50 being part of a feedbacl~
loop including a filter 52, such as a lead-lag filter, for loop stability.
Further components of the loop will be described hereinafter. Electric
signals for the amplifier 50 are coupled from a switch 54 by terminal J
and the filter 52 during a focussing of the lens 20. The housing 38 also
supports permanent magnets 56 adjacent the coils 48, the magnets 56
Iying alongside the frame 46 for interacting magnetically with the coils
48 upon energiza~ion of the coils 48 with electric current. Activation of
20 the coils 48 with current produces a magnetic force relative t~ the
magnet 56, the force displacing the coil 48 with the frame 46 and the
lens 20 along a common axis of the frame 46, the coil 48 and the lens
20 for retracting or advancing the lens 20 relative to the drum 12. The
magnitude and direction of the force depends in well-known fashion
upon the magnitude and direction of the current in the coil 48.
As will be described hereinafter, it is useful in the practice
of the invention to provide a reference position on the lens-carrying
frame 46 relative to the housing 38. For this purpose, lens-location
detection apparatus 62 is carried by carriage 40 of the housing 38
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alongside the frame 46 for detecting the location of the lens 20 relative
to the housing 38. The apparatus 62 includes a lens 64 and a pair of
photodetectors 66 supported by an extension 68 of the carriage 40.
The detection apparatus 62 further comprises a target 70 mounted on a
5 bottom 72 (indicated diagrammatically in Fig. 1, but shown in Fig. 7) of
the frame 46, and a pair of LED's 74 held by the carriage ex~ension 68
for illuminating the target 70. By way of example in the construction of
the detection apparatus 62, interior portions of the extension 68 and the
bottom 72 may be provided by with a nonreflecting black color, while
10 the target 70 may be provided with a reflective white color.
The operation of the detection apparatus 62 in outputting a
signal dependent on the location of the target image is well known. The
photodetectors 66 are located one behind the other for viewing
successive positions of the target 70. The amount of light received by
15 each of the photodetectors 66 varies with the position of the target 70.
Each of the photodetectors 66 outputs an electrical signal to a signal
combiner 76 in response to light impinging upon the photodetectors 66,
the light being provided by the lamps 74 and reflected by the target 70
through the lens 6~ to the photodetectors 66. The signal combiner 76
20 outputs a signal at terminal P equal to the difference between two
signals outputted by respective ones of the photodetectors 66. The
photodetectors 66 may be identified further by the legends A and B in
which case the ~lifference signal is given by A - B. The normalized
difference signal is outputted at terminal P, this being given by the
25 difference signal A - B divided by the sum of the detector signals A + B.
The two signals of the photodetectors 66 are of equal amplitude when
the image of the target 7Q is positioned equally distant between the two
photodetectors 66. Displacement of the image of the target 70 closer
to one or the other of the two photodetectors 66 results in a nonzero
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signal outputted at terminal P to the switch 54 via a clamp bypass
circuit (to be described hereinafter). The sense of the signal at the
terminal P depends on which of the photodetectors 66 is closest to the
target image, the amplitude of the signal at the terminal P depending on
5 the difference in spacing between the target 70 and the photodetectors
66.
In accordance with an important feature of the invention,
the printing system 10 further comprises a focus laser 78 and a focus
sensor 80 coupled via an optical system 82 to the objective lens 20.
The laser 78, the sensor 80, and the optical system 82 are mounted
within the housing 38. The optical system 82 also serves to couple
radiation from the source 18 to the objective lens 20. Thus, a portion
of the optical system 82 provides a common path for radiation of both
the source 18 and the focus laser 78. The focus laser 78 may employ
15 radiation in a part of the electromagnetic spectrum which is readily
focused, the radiation employed in a preferred embodiment of the
invention being in the near infrared region of the spec~rum, and being
provided by a laser diode (LD) 84. The focus laser 78 further comprises
a lens 86 for collimating rays of radiation emitted by the diode 84 to
20 provide a focus beam. Radiation emitted by the diode 84 is linearly
polarized .
Similarly, radiation provided by the source 18, in the
preferred embodiment of the invention, is also in the near infrared
region, but differing in frequency from the frequency of the focus
25 radiation of the LED 84 so as to permit separation of the two radiations
by a color sensitive filter, or interference filter, 88, as will be described
hereinafter. The radiation source 18, in a preferred embodiment of the
invention, comprises a plurality of !ight-emitting iaser diodes (LD's)
located at 90, the diodes 90 emitting linearly polarized radiation, and
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being disposed in an array for produGing a plurality of gray-scalè levels
of darkness to each pixel imprinted upon the photosensitive sheet 22.
Also included within the radiation source 18 is an optical system shown
schematically by lens 92 for collimating rays of radiation emitted by the
diodes 90 to produce a beam of radiation, this being the write beam,
which propagates through the optical system 82 and the objective lens
20 to impinge upon the sheet 22.
The focus sensor 80 comprises an array of two
photodetectors indicated generally at ~4, and a lens 96 for focusing
radiation of the focus beam upon the photodetectors 9~. It is noted
that light received at the photodetectors 94 emanates at the focus laser
78, is directed by the optical system 82 to the photosensitiv,e sheet 22
from which the radiation is reflected, and is redirected by the optical
system 82 to the focus sensor 80.
The optical system 82 comprises four dichroic prisms 98,
100,102, and 104 which are arranged serially along a path of
propagation of the focus beam. An optical coating 106 lies along an
interface between the prisms 98 and 100. An optical coating 108 lies
along an interface between the prisms 102 and 104.
The optical system 82 further comprises a quarter-wave
plate 110 disposed between the prism 102 and the frame 46, a
corner-reflecting prism 112, and a knife edge optical element 1 14
comprising a transparent plate 116 supporting an opaque layer 118
extending halfway across collimated rays 120 of the reflected focus
beam. The plate 116 is contiguous to a face of the prism 112.
In operation, elements of the optical system 82 are held in
their respective posi~ions by a housing 38. Polarization of the focus
radiation is parallel to polarization of the write radiation within the prism
100. Radiation of the write beam is reflected perpendicularly by the
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- prism I00, and undergoes a second perpendicular reflection by the
prism 1û2 to pass through the quarter-wave plate 110 and the objective
lens 20 to impinge upon the photosensitive sheet 22. Most oF the
radiation of the write beam is absorbed within the material of the sheet
22, but a small fraction, approximately 4% by way of example, reflects
back through the lens 20, the plate 1 10, the prism 102, and the prism
104, and is finally stopped by the filter 88, the latter Iying contiguous a
face of the prism 104. Radiation of the focus beam passes along a
straight path through the prisms 98, 100, and 102, and is reflected
perpendicularly by the prism 102 to pass through the plate 110 and the
lens 20 to reflect off of the photosensitive sheet 22. Reflected radiation
of the focus beam passes along a straight path through the lens 20,
through the plate 1 10, through the prisms 102 and 104, through the
filter 88, through the knife-edge element 1 14, and into the prism 1 12.
The prism 112 reflects the focus beam perpendicularly a!ong a path
which carries the focus radiation into the focus sensor 80.
It is noted that the quarter-wave plate 110 converts the
plane polarized light from both the focus laser 78 and the radiation
source 18 to circularly polarized radiation. Upon reflection from the
photosensitive sheet 22, the circularly polarized reflected radiation
interacts further with the quarter^wave plate 110 to produce linearly
polarized radiation which is perpendicular to the radiation incident upon
the plate 1 10 from the prism 102. As a result of the reorienta~ion of
the plane of polarization of the reflected radiation relative to the incident
radiation by the plate 110, reflected radiation propagates along a linear
path through the interface of the prisms 102 and 104. This is in
contradistinction to the perpendicular reflection undergone by the
incident beam of light at the interface between the prisms 102 and 104.
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Assuming that all of the reflected radiation of the write
beam has been stopped by the filter 88, only the radiation of the focus
beam interacts with the knife-edge element 114. The knife-edge
element 114 interacts with the radiation of the reflected Focus beam in a
5 manner dependent ~Jpon the degree of focus of incident radiation of the
focus beam by the objective lens 20 upon the photosensitive sheet 22.
As a result of the interaction of the knife-edge element 114 with the
radiation of the reflected focus beam, which relationship is well-known,
the distribution of radiation incident upon the array of photodetectors 94
10 of the sensor 80 varies in a well-known fashion in accordance with the
degree of focus of the incident focus beam upon the photosensitive
sheet 22. With correct focus, both of the photodetectors 94 output
equal signals in response to the radiation incident upon the
photodetectors 94.
The radiations produced by the focus laser 78 and the
source 18 are sufficiently close in frequency such that the presence of a
focus by the focus beam indicates adequate focus by the write beam. It
is noted that, in the case of a print medium formed of a laminated
structure of an outer transparent film which allows transmission of
radiation of the writs laser to impinge upon an inner layer, wherein the
outer film is reflective of radiation of the focus laser, then radiations of
the two lasers are reflected from siightly different locations, namely, the
outer film and the inner layer. This can be compensated by optically
offsetting the focus beam relative to the write laser beam.
Upon a correct focusing of the print head 16, all of the
photodetectors 94 of the sensor 80 output equal signals. However,
upon a shift of the focus such that the objecth/e lens 23 is either too
close or too far from the photosensitive sheet 22, the signals outputted
by the photodetectors 94 differ from each other. The output signals of
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the photodetectors 94 are connected to a two-channel signal combiner
126 which forms surns and differences of the photodetector signals to
output at terminal ~ a signal representing focus error of the print head
16. The signal at terminal E is applied, via the filter 52, to the switch
54, and to the clamp bypass circuit as will be described in further detail
hereinafter. The photodetectors 94 may be identified further by the
legends A and B in which case the difference signal is given by A - B.
The normalized difference signal is outputted at terminal E, this being
given by the difference signal A - B divided by the sum of the datector
signals A + B. The sum signal A + B is outputted at terminal S.
The printing system 10 includes the clamp bypass circuit
122 connected betwee!~ terminals E and P, a switch 124 connected
between terminal E and the focus drive filter 52, and an interface 128
for coupling of signals with a computer 130 and a memory 132. The
computer 130 is coupled to both the interface 128 and the memory
132. Also included in the system 10 are two stepping motors 134 and
138, two drivers 140 and 1~4 having circuitry for energizing,
respectively, the motors 134 and 138, and a shaft-angle encoder 146.
The motor 134 rotates the drum shaft 14, the latter being rotatably
mounted in a frame 148. The encoder 146 connects with the shaft 14
for outputting the angular orientation of the drum 12 to the computer
130 via the interface 128. Upon activation of the printing system 10 by
an operator inputting a start command to the computer 130 in a manner
to be described, the computer 130 signals the driver 140, via the
interface 128, to activate the motor 134 to rotate the drum 12. By
virtue of the feedback of drum orientation via ~he encoder 146 to the
computer 130, operation of the computer 130, and of the entire system
10, can be synchronized with rotation of the drum 12.
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The print head 16 is movable in a parallel path re!ative to
the axis of the drum 12 by a servo system 150. The servo system 150
,; connects the housing 38 of the print head 16 with the frame 148 whichholds the print drum 12. The servo system 150 includes a rigid member
152 which is connected by a lead screw 156 to the frame 148. The
Iead screw 156 is powered by the motor 138. During operation of the
printing system 10, the motor 138 is activated by the driver 144 in
response to signals of the computer 130 coupled via the interface 128
to the driver 144.
Operation o~ the lead screw 156 translates the member
152 and the print head 16 in a direction parallel to the axis of the drum
12 during rotation of the drum 12 to effect a spiral scan of the
photosensitive sheet 22, the spiral scanning path being indicated by a
dashed line at 160.
Relatively small displacements of the lens 20 as may be
necessary to maintain focus on small undulations in the surface of the
- sheet 22, is accomplished by electrical activation of the coils 48.
Activation of the coil 48 is accomplished in synchronism with rotation of
the drum 12 by virtue of drum orientation angle data provided by the
encoder 146 to the computer 130. Use of the computer 130 to direct
activation of the rapidly responding coil 48, as well as activation of the
switch 54, is accomplished with the aid of the computer 130 by the
interface 128 as will be described hereinafter with reference to Fig. 2.
The feedback loop employing the coil 48 for focussing the
objective lens 20 includes the amplifier 50 and the filter 52, disclosed
above, the focus sensor 80, and the signal combiner 126, the latter
being coupled to the filter 52 via the switch 54 and terminals J, K and
E. The feedback loop is completed by operatin~ the switch 54 to
connect terminals J and K. The feedback loop is disabled by operating
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the switch 54 to disconnect terminals J and K. During operation of the
feedback loop, an energization of the coils 48 to advance the objective
lens 20 towards the drum 12 by an excessive amount results in the
yeneration of an error signal at terminal E which activates the coils 48
5 to retract the objective lens 20. Similarly, upon an excessive retraction
of the objective lens 20 from the drum 12, the loop produces an error
signal at terminal E which acts to energize the coils 48 to advance the
lens 20 towards the drum 12. The error signal at terminal E has zero or
near zero amplitude when the lens 20 is properly focused upon the
sheet 22. The feedback loop of the coils 48 with the focus sensor 80 is
operative during a scanning of the sheet 22 by the print head 16 during
those portions of a scan in which the clamp 30 is distant from the
objective lens 20. However, upon an approach of the clamp 30 to the
lens 20, the feedback loop is disabled by operation of the switch 54,
15 and remains disabled until the clamp 30 has cleared the lens 20.
Operation of the switch 54 is accomplished by a signal of the computer
130, the switch control signal being coupled from the cornputer 130 via
the interface 128 to the switch 54. The foregoing feedback loop
including the coils 48 and the focus sensor 80 may be referred to as the
20 focus loop.
A further feedback loop, which may be referred to as the
position loop serves to position the objective lens 20 at a parking
location during disablement of the focus loop, such as occurs during
passage of the clamp 30 by the lens 20. The position loop is enabled
25 by operation of the switch ~4 to connect terminals J and R, the latter
being connected via the bypass circuit 122 to terminal P. The position
loop includes the photodetectors 66 and the signai combiner 76 which
are serially connected via the bypass circuit 122, the switch 54 and the
amplifier 50 to the coils 48. Displacement of the objective lens 20 from
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the reference position produces an error signal at the output of the
difference amplifier 234 which activates the coils 48 to drive the lens
20 back towards the reference position. The reference position is at a
specific distance of the lens 20 from a front wall 162 of the housing 58.
5 The reference position is a suitable location from which enablement of
the focus loop can be initiated upon resumption of scanning after
passage of the clamp 30 past the lens 20.
During the production of an image upon the photosensitive
sheet 22, data is provided ~o the LDs 90 of the radiation source 18 from
a data source 164, the data source 164 outputting electric signals
which activate a drive circuit 166 to drive individual laser diodes of the
array of diodes 90. The selection of specific ones of the diodes 90 to
be activated will be described in further detail with reference to Fig. 3 to
produce gray-scale printing of pixels of the ;mage on the sheet 22.
1~ Synchronization of the rate of data flow from the source 164 to the
radiation source 18 with the speed of rotation of the drum 12 is
accomplished by connection of the computer 130 to the data source
164. Connection of the computer 1 3û to the source 164 is
accomplished via terminal D.
Fig. 2 is a flow chart showing operation of ~he computer
130 in directing a scanning of the drum 12 by the print head 16 to
accomplish a printing of an image upon the photosensitive sheet 22 of
Fig. 1. Included within the flow chart are references to the operations
of the focus loop and the position loop, as well as enablement of data
flow from the source 164 to the radiation source 18 during the printing
of an image on the sheet 22.
With reference to the flow chart, the procedure begins with
a start at block 168 and proceeds to block 170 for rotation of the drum
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12 to home position. Home position is immediately after the trailing
edge of the clamp 30 where the sheet 22 is again visible to the
objective lens 20. Thereupon, at block 174, the computer 130 disables
the focus loop by operating the switch 54 connecting terminal J to
5 terminal R. ~peration proceeds to block 176 to activate a ramp
generator in the bypass circuit 122, as will be described hereinafter, for
energizing the coils 48 with a ramp signal to slew the lens 20 relative to
the carriage ~0. During the slewing process, the focus error signal at
~erminal E of the signal combiner 126 is observed to determine the lens
10 position at which focus of the sheet 22 is obtained.
The lens position of zero focus error is stored at
block 180. The drum then begins its rotation at block 181.
A clamp disable signal is used to tell the focusing system when to
disable the focusing servo. This signal is derived from the drum shaft
15 angle encoder. The clamp disable signal is true for an angular segment
beginning just prior to the leading edge of the clamp and ending just
after the trailing edge of the clamp. At 182, the clamp disable signal
thus will become true just prior to the leading edge of the clamp
appearing in front of the focusing device. At 183, the focus loop is
20 disabled by switching to terminal R. After the trailing edge of the clamp
passes the focus head, the clamp disable signal becomes false at 184.
At block 186, the focus loop is enabled by switching to terminal K.
Immediately after enabling the focus loop, the lens position for zero
focus error is sampled and stored at block 188.
2~ We then wait for block 182 to occur and then repeat blocks 183 to 188
until the drum stops spinning at the end of the print.
During the preceding, the writing lasers are activated when
the sheet is present in front of the focusing device after the drum has
reached its correct rotational speed.
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Fig, 3 shows an implementation of the data source 16~ for
providing individual pixels of the ;mage imprinted upon the
photosensitive sheet 22. They may be black, white, or a blend of black
and white areas which give the visual impression of a gray scale. This
is accomplished in the following manner. The data source 164
comprises a data memory 206, a gray-scale memory 208 and a coun-ter
210 for addressing the gray-scale memory 208. The gray-scale memory
208 may be constructed as a read-only memory (ROM). Output signals
of the gray-scale memory 208 serve as output signals of the data source
164, and are applied via the drive circuit 166 to the radiation source 18
(shown in both Figs. 1 and 3) for activating the LDs 90. Also shown in
Fig. 3 is a portion of the computer 130, namely a clock 212 and an
address generator 21~. Connections of the computer 130 to the angle
encoder 146 and to the data source 164 are accomplished via the
interface 1~8, as shown in Fig. 1. The interface 128 is deleted in Fig. 3
to simplify the drawing. The embodiment of the gray-scale feature, as
depicted in Fig. 3, is presented by way of example, it being understood
that the principles of the invention can be practiced with other
implementations of the gray-scale feature.
In operation, the data memory 206 stores data of each
pixel, and the location of each pixel, of the image to be imprinted on the
photosensitive sheet 22. The location in the image is designated by the
address of storage of each pixel within the da~a memory 206. In the
case of a printing system employing only white or black pixels, the pixel
data consists of a single bit indicating whether a pixel is to be white or
black. However, in the case of gray-scale presentation of the pixels, the
data stored in the data memory for each pixel comprises a multiple-digit
word designating the level of gray scale to be applied in the imprinting
of each pixel. With respect to the array of LDs 90 in the radiation
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source 18, in a preferred embodiment of the invention, the LDs 90 are
arranged as a group of linear subarrays of LDs. This is depicted
diagrammatically in Fig. 3. In the diagrammatic representation of Fig. 3,
the source 18 is represented as a pixel Iying within ~he spiral scan path
160 upon the drum 12, the path 160 having been presented previously
in Fig. 1. The array of diodes 90 includes a linear subarray 90A of
individual diodes, the subarray measuring, by way of example, 3
microns in width by 90 microns in length. The subarray 90A extends
one third of a pixel width, this being one third of the width o~ the scan
path 160, and is centered within the scan path 160. Two additional
diode subarrays 90B and 90C are provided, the subarrays 90B and 90C
being identical to the subarray 90A, but being spaced to the right and to
the left sides of the subarray 90A as shown in the diagrammatic view of
Fig. 3. Illumination of all of the light-emitting diodes, or laser diodes, of
the three subarrays 90A, 90B and 90C would produce a continuous
swath upon the photosensitive sheet 22 during a generation of the spiral
scan path 160. Illumination of only the center subarray 90A would
produce a stripe along the center of the path 160, while illumination of
either of the side subarrays 90B or 90C would produce a black stripe on
the right or on the left side, respectively, of the scan path 160 in the
sheet 22.
Also included in the array of L~Ds 90 is a relatively short
subarray 90D measuring 5 microns in length by 3 microns in width, and
a single diode 90E measuring 3 microns by 3 microns. The subarray
90D and the single diode 90E are centered along a center line of the
scan path 160. A flashing on and off of any of the subarrays 90A-D or
the single diode 90E produces a marking within a region of the scan
path, the extent of the marking depending upon the duration of a flash
of radiation produced by the activa~ed ones of the LEDs 90. ~or
.
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example, by flashing the subarrays 90A and 90B on and off several
times during the scanning of a length of the path 160 e~ual to the
extent of a single pixel, there is imprinted a pixel having a checkerboard
appearance (viewed under a microscope). However, when viewed by
the human eye, the appearance is a uniform gray. By selecting the
intervals of time during which selected ones of the diodes 90 are to be
illuminated, various patterns of black and white can be imprinted within
each pixel of the image. This gives numerous values of gray scale.
In the operation of the data source 164, for each pixel, the
data memory 206 outputs an address identifying the level of gray scale
desired, the level ranging from pure white to pure black. The gray-scale
address is applied to the gray-scale memory 208, which, in response to
the address, outputs a digital word to the driver circuit 166 directing
activation of specific ones of the subarrays 90A-E. In addition, the
presence of the address signal resets the counter 210 to count clock
pulses of the clock 212. The count provided by the counter 210 serves
as an additional sequence of partial addresses to output from the
gray-scale memory 208 signals which direct a pulsing or flashing of light
from selected ones of the subarrays ~OA-E for a succession of intervals
within the time allocated for the scanning of a single pixel. Thereby, the
radiation source 18 is able to output a checkerboard format, or other
arrangement of light and dark areas wherein the average amount of
darkness apparent across the surface of a pixel is selectable to produce
the numerous levels of gray scale. Also shown in Fig. 3 is an outpu~ting
of angle data from the encoder 146 to synchronize the clock 212 with
rotation of the drum 12 to ensure that the rate of presentation of data is
locked to the rotation of the drum 12.
In order to accurately produce the gray scale upon the print
medium, and to produce well defined pixels upon the print medium, it is
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important to maintain uniformity of illumination in the write beam. The
use of the telecentric optics in the optical system 82 (Fig. 1), provides
for the imaging of a near field region spaced apart from the diode array
and located in front of the diode array. This region is characterizetl by
5 contributions of light from the various diodes and has substan~ial
uniformity of illumination. The telecentric optics preserves this
uniformity of illumination and projects the uniform illumination upon the
print medium to attain the desired accuracy in presentation of the gray
scale, as well, well defined pixels, in printing of an image of a desired
1 0 subject.
Fig. 4 is a graph showing thickness of the sheet 22 as a
function of angular position around the drum 12, shown on the
horizontal axis of the graph. The sheet presents a uniform depth except
at the location of the clamp 30 where the clamp 30 presents a
15 protuberance in the otherwise smooth surface. Also, the graph shows
the location of the lens 20 which is essentially constant relative to the
drum surface, except for minor variations too small to show in the
figure. The nominal value of the lens position is maintained by the focus
loop wherever the print medium is in view. At the location of the clamp
20 30, the lens position is rnaintained by disabling the focus loop, and by
retaining the lens in its parking position, until the print medium again
becomes visible to the focus loop sensor 80. The shaft angle encoder
146 outputs drum position, as has been described hereinabove, to
enable the various steps in the activation of the focus loop and in the
25 updating of stored parking position and reading out the value of the
parking position from storage. Thus, the horizontal axis shows various
points at which steps are taken, namely, reading from storage of a
previously stored value of lens parking position before the appearance of
the clamp 30, disabling the focus loop and retaining the lens at the
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-23- ~
read-out parking position immediat~ly before and during passage of the
clamp 30, reactivating the focus loop and then initiating or updating the
parking position.
- Fig. 5 shows details in the construction of the clamp
. 5 bypass circuit 122. the bypass circuit 122 comprises a scaler 220, a
summer 222, an analog-to-digital converter 22~, a storage unit 226, a
ramp counter 228, a switch 230, a digital-to-analog converter 232, a
-- differential amplifier 234, a switch 236 and filter 238. In operation,
the bypass circuit 122 is operative in the lens position feedback loop fcr
positioning the lens 20 (Fig. 1) at a suitable parking position during
passage of the clamp 30 by the lens 20. The parking position is
established initially in an initialization procedure in which the lens 20 is
slewed through the carriage 40 to find a position at which the focus
error signal at terminal E is zero. This is accomplished with the drum 12
held stationary at a home position located immediately after passage of
the clamp 30 by the lens 20, the home position corresponding to the
closest region at the top edge of the sheet 22 at which printing can
commence. Thereafter, with each rotation of the drum 12 through the
home position, the parking position is updated to correspond to any
undulations which may appear in the sheet 22 as the prin~ head
translates along the drum 12. By proper selection of the parking
position, the lens 20 is located optimally for resumption of the printing
process after passage of the clamp 30 by the lens 20.
Initialization and updating of the lens parking position is
accomplished as follows with the aid of the focus error signal provided
at terminal E of the signal combiner 126 (Fig. 1), the position error
signal provided at terminal P Q~ the signal combiner 76 (Fig. 1), the sum
channel signal provicled at terminal S of the signal combiner 12~, and
via connections with the computer 130 indicated by terminal H. During
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initialization, the ~:omputer 130 directs the drum 12 to the home
position which is ~ndicated by the angle encoder 146. Thereupon, via
terminal H, the computer 130 operates the switch 230 to connect the
counter 228 to the converter 232, resets the counter 228, and activa~es
the counter 228 to count clock pulses provided by the computer 130 at
terminal C. The resulting output count of the counter 228 is a digital
ramp (staircase waveform) which is converted by the converter 232 to
an analog ramp voltage fed to a positive ;nput terminal of the differential
amplifier 234. The position signal is applied from terminal P to the
negative input terminal of the differential amplifier 234. The position
slgnal is in the form of a ramp voltage which increases with translation
of the lens 20 in the carriage 4(:). Therefore, the position signal also
serves to identify the position of the lens 20 within the carriage 40.
The output signal of the amplifier 234 is the difference between the
commanded ramp position and the actual position of the lens 20 relative
to the carriage 40.
- The clamp bypass circuit 122 further comprises a detector
240 responsive to signals at the terminals E and S for noting the
presence of a zero focus error which occurs during the slewing
20 movement of the lens 20. The sheet 22 is correctly focussed by the
Iens 20 upon an outputting of a zero error signal by the signal combiner
126. the position error signal (terminal P) and the focus error signal
(terminal E) are summed together by the summer 222 and converted to
a digital sum signal by the converter 224. It is noted that the focus
error signal is also a ramp voltage, similar to the position error signal,
However, the slope of the ramp of the focus error signal, in the
preferred embodiment of the invention, has been found to be greater
than the slope of the ramp of the position error signal by a factor of 5.
Before summing together the focus and the position error signals, it is
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-25-
desirable to equalize the two slopes. This is accomplished at the scaler
220 which scales the arnplitude of the focus error signal by a factor of
one fifth. Thereby, the digital sum signal outputted by the converter
224 is influenced equally by the focus ancl the position error signals.
Upon the occurrence of the zero focus error, the detector
240 outputs a logic-1 signal and the focusing servo is enabled. After a
delay for settling, the storage unit 226 to store the sum signal. The
logic-1 signal outputted by the de~ector 240 is also applied via terminal
H and the interface 128 to the computer 130 to signal the end of the
initialization process. The computer then activates the drum 12 to
rotate.
- The zero detector 240 comprises two comparators 244 and
246, a source 248 of positive reference voltage, and an AND gate 250.
- The comparator 244 is operative to compare the focus error signal
amplitude with zero volts provided by a connection of ground to the
negative input terminal of the comparator 244. The focus error signal is
applied to the positive input terminal of the comparator 244. During the
initial stage of the slewing of the lens 2~, the focus error signal is
-- negative, and the signal outputted by the comparator 244 is relatively
low, a logic-0. The signal outputted by the comparator 244 goes high
to a logic-1 as the ~ocus error signal reaches zero.
The voltages outputted by the photodetectnrs 94 (Fig. 1 )
vary in amplitude with lens position as is shown in ~he graph of Fig 6.
The maximum signal strength is obtained with the sheet ~!2 in correct
focus by the lens 20. In order to prevent an occurrence of a false zero
detection, which might during a lens position of low light upon the
photodetectors 94, the sum of the two output signals of the
photodetectors 94 (at terminal S) is applied to the comparator 246 to
check for the presence of adequate signal strength. The sum-channel
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signal at terminal S is applied to the positive input terminal of the
comparator 246, and the positive reference voltage of the source 248 is
applied to the negative input terminal of the comparator 246. the
reference voltage of the source 2~8 serves as the threshol~ shown in
5 Fig. 6. The output signal of the comparator 246 goes high (logic-1 ) for
values of sum-channel signal above the threshold, and serves to activate
the AND gate to pass the output signal of the comparator 244 to the
switch 242.
Also included in ~he bypass circuit 122 is a decoder unit
10 252 having a plurality of decoders for decoding the signal outputted by
the shaft angle encoder 146. The first decoder output is a signal
designating the attainment of a drum position immediately after passage
of the clamp 30 at which time the value of stored lens reference
position is to be read out of the storage unit 226 for updating the lens
15 parking position. The read command, in terms of drum angle, is shown
in the graph of Fig. 4. The second of the decoders of the decoder unit
252 provides the initialize or update signal, shown in Fig. ~, which
indicates that the drum position is appropriate for initializing the lens
parking reference position, as has been described above, or for updating
20 the iens parking reference position as is described below.
After initialization of the lens position, the drum 12
proceeds to rotate and the print head 16 translates along the drurn 12,
as has been described above. With each rotation of the drum 12, upon
passage of the clamp 30 by the lens 20, the lens 20 is placed in the
25 parking position. This occurs upon disabling of the focus control loop,
immediately prior to the passage of the clamp 30, as indicated in Fig.4.
It is desirably to update the lens parking reference position with each
rotation of ~he drum 12, as noted above, to compensate f~r any
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-27-
undulations which may appear in the sheet 22 as the print head
translates along the drum 12.
The updating of the lens parking position is accomplished
by operating the switch 242 to connect the second output of the
decoder unit 252 to the storage unit 226. Also, the switch 230 is
operated to connect the output terrninal of the storage unit 226 to the
converter Z32. Upon occurrence of the update command signal from
the decoder unit 252, the storage unit 226 stores the present value of
the sum signal outputted by the converter 224. No slewing of the lens
- 10 20 is necessary during update because the lens 20 is already a~, or
close to, the optimum parking position. With each rota~ion of the drum
12, the latest value of the lens parking position is read from the storage
unit 226 prior to passage of the clamp 30, the latest value of the
parking position then being used during disablement of the focus loop to
position the lens 20 until after passage of the clamp 30, whereupon the
focus loop is reactivated.
For best accuracy in positioning ~he lens 20 it is desirable
to employ different filters for controlling dynamics of the position control
loop and the focus control loop. The focus drive filter 52 ~Fig. 1) is
employed in the focus control loop, and the position drive filter 238 (Fig.
5) is employed in the position control loop. During a period of inactivity
of the focus drive filter 52, the filter is completely disconnected from
other circuitry by the switch 124 at the input to the filter 52 and by the
switch 54 at the output of the filter 52. During a period of inactivity of
the position drive filter 238, the filter is completely disconnected from
other circuitry by the switch 236 at the input to the filter 238 and by
the switch 54 at the output of the filter 52~ This allows various
voltages which may be present in the filters, such as the voltage of a
charged capacitor, to drain off. This avoids the possibility of a large
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-28- ~
transient behavior takin~ place upon a switching of the filters. It is
understood that each of the filters may comprise suitable Frequency
control circuitry for loop stability, such as a lead-lag circuit or an
. integrator, and may also comprise amplifiers to provide adequate gain as
is well known in feedback design. Each of the foregoing switches is
operated under command of the computer 130.
Fig. 7 presents the underside of the carriage 40 (Fig. 1) in a
; stylized perspective view to show the locations of the various
components of the carriage 40 used in positioning the lens 20, such as
the frame 46, the coils 48 and the photode1:ectors 66 which were
presented by way of exploded diagrammatic view in Fig. 1. In Fig. 7
the carriage 40 includes a back plate extending in a vertical plane from
the housing 38, the plate 254 extending in a plane parallel to the axis of
the drum 12. The extension 68 protrudes from a lower portion of the
plate 254 for supporting the photodetector 66 and the lamps 74 in their
respective positions about the lens 64. The legs 42 and 44 extend
downwardly from an arm 256 which protrudes forwardly from a top
cushion of ~he plate 254. The legs 42 and 44 are resilient and pivot
forwardly and backwardly about their respective junctions with the arm
256 so as to allow translation of the frame 46 in a direction
perpendicular to the axis of the drum 12. The frame 46 is secured to
lower portions of the legs 42 and 44 which carry the frame ~6. The
strip 258 of damping material is secured to the front leg 42 for damping
movement thereof, and a similar strip (not shown) is secured to the rear
leg 44.
Struts 260 and 262 protrude forward from the back plate
254 to support the magnets 56 on one side of the carriage 40, there
being a similar set of struts (not shown) for supporting the magnets 56
on the opposite side of the carriage 40. An opening 264 in the back
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~9
plate 254 provides space for translation of the frame 46 and passage for
the beam of laser radiation which propagates through the lens 20. The
location of the target 70 upon the bottom 72 of the frame 46 permits
illum;nation of the target 70 by the lamps 74 and a viewing of the targe~
70 by the photodetectors 66 during translation of the target 70 past the
lens 64.
In view of the foregoing description, the printing system 10
of the invention is able to accurately print an image of a subject upon a
discontinuous recording medium such as a sheet of photosensitive
material. Data of the subject is presumed to have been previously
stored in a memory, such as memory of the data source 164. If
;~ desired, the data can be provided by an optical scanner of a subject
being imaged, which data would be digitized and stored temporarily in
the data source 164 which would serve then as a buffer storage. The
feature of retracting the print head from the print drum allows the
convenient use of a clamp carried by the drum for seGuring ends of the
discontinuous medium to the drum during the implementation of a spiral
scanning of the print medium. Furthermore, the use of the optical
focusing apparatus provides for an accurate focusing of a write beam
upon the photosensitive sheet, the focusing compensating rapidly for
any undulations which may be present in the surface of the print
medium.
It is to be understood that the above described embodiment
of the invention is illustrative only, and that modifications thereof may
occur to those skilled in the art. Accordingly, this invention is not to be
regarded as limited to the embodiment disclosed herein, but is to be
... .
limited only as defined by the appended claims.
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