Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates generall~ to non-impact
printers and more specifically to an electrophotographic
prin-ter adapted to print copy from a source of lrnage in-
formation.
This application is related to applicant's Canadian
Application Serial No. ~76,421, filed April 25, 1981 and
entitled "Light Emitting Diode Assembly" and to Canadian
Application Serial No. 409,447, filed August 13, 1982 and
entitled "Control of a Light ~mitting Diode Array".
In electrophotographic printing, a photoreceptor
in the form of a photoconductive surface on a moving belt
or drum is uniformly charged and exposed to a light image
from a source of image information. ~xposure of the photo-
conductive surface to a light source discharges localized
portions of the surface and thereby records thereon an
electrostatic laten~_ image corresponding to the original
image. The electrostatic latent image is then developed and
rendered visible by depositing toner particles which adhere
electrostatically thereto in an image configuration.
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Subsequently, the toner powder is transferred to a sheet of
support material which may be plain paper or transparent
plastic sheet, among other materials. The toner powder i8
then affixed to the support material and fused thereto so as
to produce a so-called hard copy of the original image.
As a source of illumination for exposing the photo-
conductive surface in medium to high speed applications
requiring high image quality, it is desirable to employ an
array of light emitting diodes (LEDs). This permits the
construction of a printer which is compact, light in weight,
economical and having low maintenance while maintaining high
quality printing. Such a non-impact printer employing light
emitting diodes as a source of illumination can be used as
peripheral equipment for the output of a computer where the
array of light emitting diodes is driven by a digital signal
from a character generator containing image or character
information received from the computer. The source of image
information for the light emitting diode array can also be
from a hard copy reader such as an optical character reader,
or from a high speed facsimile scanner, word processor or
other image information generating device.
In such printers, the photoreceptor may be either a
belt or a drum having a photoconductive surface thereon in
optical registration with the light emitting diode array.
Selective energization of individual groups of the light
emitting diode array illuminates and discharges selected
portions of the charged photoconductive surface in the image
pattern of a line of print characters or other image associ-
ated with the image code signals output from the character
generator or other source of image information. The selec-
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tive energization of individual groups of the light emittingdiode array produces an array of dot sized discharged por~
tions on the photoconductive surface o the photoreceptor
which has been charged prior to exposure to the diode array.
~he image created on the photoreceptor is in the form of
depleted charge portions on the previously charged photo-
receptor. As noted above, the latent image on the photo-
receptor is developed electrophotographi~ally by means well
known in the art and not forming part of this invention.
It has been determined that a major cause of poor
image quality in the printing of hard copy by such photo-
printers is irregular motion by the photoreceptor. The
photoreceptor is driven by an electrical or an electromechani-
cal drive and as with any such mechanisms develops occasional
iitter or other non-uniform irregular motion which, though
slight, is capable of di3torting or diminishing image quality
of the output from the printer. It has been discovered that
such poor image quality can be overcome by coordinating or
synchronizing the energization of individual groups of
the diode array with the motion of the photoreceptor even
though the photoreceptor motion be non-uniform. In this way
the dots forming the image on the photoreceptor will always
be in the correct position to insure a high quality image
despite irregular or non-uniform motion of the photoreceptor.
Accordingly, it is an object of this invention to
provide a method and apparatus for synchronizing the ener-
gization of individual groups of the light emitting diode
array with the motion of the photoreceptor to coordinate
the exposure of selected localized areas of the photocon-
ductive surface to produce a high quality latent image
71) ~
even though movement o~ -the photoreceptor is irregularly
non-uniform.
SU~MARY OF THE INVEN~ION
Briefly stated, and in accordance witn one embodi-
ment of the present invention, there is provided a printing
apparatus and method for exposing a charged photoconductive
surface to light emitted from individual groups of diodes
of a light emitting diode array energized selectively by means
o~ a signal containing image information.
Specifically, the invention is used in a non-impact
printer having a moving charged photoreceptor, an image
information data signal source and a light emitting diode
array operatively connected to the data signal source ~or
selec_ive ene-gizaticn of individual groups of diodes within
the diode array in a cycle in response to the data signal
recei~Jed from the source, such cycle including a predetermined
inter~a~ of diode energization followed by an inter~al of
diode non-energiza~ion, the diode array being located in
optical registration with the photoreceptor, the method of
compensating for non-uniform photoreceptor motion co~prising
the steps of: monitoring the motion of the photoreceptor
to generate a iiming signal representative of the photo-
receptor motion, and delaying input of the data signal to
the diode array in response to variations in the timing
signal by varying the duration of the interval of diode
non-eneryization while maintaining the predetermined interval
of diode energization, whereby energization of individual
groups of the diode array is synchronized wi-th mo-tion of the
photoreceptor.
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In i-ts apparatus aspect, the invention relates
to a non-impact printer comprising a moving photoreceptor
having a charged pho-toconductive surface thereon, means for
providing an image informa-tion containing data signal, means
for selectively discharging ].ocalized areas of the photo-
receptor to form a laten-t image thereon according to a pre-
determined -ti.minq cycle in response to the data signal, means
for generating a timing signal representative oE the motion
of the pho-toreceptor and means for varyiny the -time of input
of the data signal to the discharging means in response to
variations in the timing signal whereby actuation of the
discharge cycle is synchronized with motion of the photo-
receptor without changing the internal timing of the pre-
determined cycle.
Pursuant to one embodiment of the present invention
a character generator or other source of lmage data selectively
energizes individual groups of diodes within an array of
solid state light emitting diodes (LEDs) to expose and there-
by discharge selected localized areas of a previously charged
photoconductive surface on a photoreceptor drum or belt to
produce thereon a latent image corresponding to the image
transmitted from the source of image data. A motion encoder
is employed to develop a timing signal indicative of the
movement of the pho-toreceptor and a logic circuit is
employed to synchronize the selective energization of the
individual diode groups with movemen-k of the photoreceptor
to produce a high resolution printed image.
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7(~10
BRIEF DESCRIPTION OF THE DRAWI~lGS
O-ther objects and advantages of the present
invention will become apparent upon readinc3 the following
detailed description and upon reference to the drawings
wherein:
FIG. 1 is a block diagram of the essential elements
of the prin-ter according to this invention;
.
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FIG. 2 is a timing and logic circuit employed in this
invention together with a specific form of motion encoder;
FIG. 3 is a timing diagram showing the relationship
of the electrical signals at various points in the circuit;
FIG. 4 is a graphic showing of the effect oE typical
uncompensated irregular motion of the photoreceptor;
FIG. 5 is a graphic showing of the effect of compen-
sation for non-uniform movement of the photoreceptor.
FIG. 6 is a block diagram of the circuit elements
energizing the diode array and receiving the output from the
circuit of FIG. 2.
While the present invention will be described in
connection with a preferred embodiment thereof, it will
be understood that it is not intended to limit the invention
to that embodiment only. On the contrary, it is intended to
cover all alternatives, modifications and equivalents as may
reasonably be included within the spirit and scope of the
invention as defined by the appended claims.
DETAILED DESCRIPTION OF T~E INVENTION
Referring now to the drawings, particularly FIGS. 1
and 2, an electrophotographic printing device 10 incorpo-
rating non-uniform motion compensation according to this
invention is shown in schematic form. The electrophotographic
printer adapted to employ the present invention therein
comprises a photoreceptor 11 having a belt 12 with a ~hoto-
conductive surface 1~ on one side thereof. The belt 12
is mounted on a rotat`able drive roll 16 and idler roll 18
which rotate in direction A as shown by the arcuate direction-
al arrow. Photoconductive surface 14 is moved continuously
_ 5
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in direction B at approximately 10 in¢hes per second. In
an alternate embodiment as is well known in the art and is
not shown in the drawing, the photoreceptor 11 can be in the
form of a rotating drum having a photoconductive surface
14 thereon. Motive power for the rotation of the drum or
belt can be by any suitable electrical or electromechanical
drive mechanism, not shown.
A motion encoder 20 is employed to monitor the motion
of the photoreceptor and to produce a digital timing signal
indicative of the photoreceptor motion. It will be under-
stood that the motion encoder 20 can be any suitable device
for sensing the motion of the photoreceptor 11 and generating
a digital signal representative of the motion including, for
example, magnetic or capacitive devices for sensing the
displacement of the photoreceptor. As shown schematically
in FIG. 1, the motion encoder 20 may engage the drive roll
16 to track the movement of the photoreceptor and generate a
digital signal corresponding thereto which is fed through
logic circuit 32 to the LED drive circuit 34. As will be
explained more fully, the output of the motion encoder 20 is
combined with the timing signals employed in the LED drive
circuit to selectively energize individual groups of LED's
contained in the dicde array and selectively discharge
localized areas of the photoconductive surface of the photo-
receptor as is shown schematically in FIG. 1.
A motion encoder 30 is shown schematically in FIG.
2 where the drive roll 16 of the photoreceptor 11 is mounted
on a shaft 22, and mounted on the same axis or shaft, is
a light interruptor in the form of an opaque rotating wheel
24 having light permeable radial slots 26 therethrough.
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~otation of the wheel and the drive roll are identical since
both are mounted on the same axis or shaft. A light source
28 is mounted on one side of the light interruptor 24 and a
photodetector 31 is mounted on the opposite side. r~Ov~ ~t
of the drive roll 16 is accompanied by identical movement of
the light interruptor wheel 24 which interrupts the light
rays impinging on the photodetector 31 to form a digital
timing signal representative of the motion of the photo-
receptor 11.
As shown in FIG. 1, the timing signal representing
the motion of the photoreceptor 11 is directed to a logic
circuit 32 and, as is discussed more fully in connection
with FIGS. 2, 3 and 6, to an LED driver circuit 34 which
also receives an image data signal from a source 36. The
drive circuit 34 selectively energizes individual groups of
diodes in LED array 38. Diode array 38 may be of the type
and construction described in the above-noted copending
Canadian Application Serial No. 876,42l. In general, the diode
array may be in an assembly approximately eight and one-half
inches in length to span the width, in a direction perpen-
dicular to the direction of travel B, of the image bearing
portion of photoconductive surface 14. By way of example,
if 2048 LEDs are employed, four individual groups of 32
contiguous LEDs are energized simultaneously and the light
emitted therefrom is directed through a focusing assembly 90
to photoconductive surface 14 of the photoreceptor 11.
Focusing assembly 40 may be a lens or fiber assembly to
focus light emitted from the diode groups onto surface 14.
The photoconductive surface 14 is given an electrostatic
charge by a conventional assembly, not shown. Selective
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energization of individual LED groups discharges localized
areas, preferably in the form of dots, of the photoconductive
surface 14 in a pattern forming an electrostatic latent
image corresponding to the image data signal received from
source 36. The latent image is subsequantly developed by
means, not shown, in a conventional manner by the application
of toner to the surface 14. Thereafter, the image is
transferred to sheet paper or transparent plastic and fused
as is well known in the art.
Referring now to FIGS. 2, 3 and 6, the signal repre-
sentative of the motion of the photoreceptor 11 is directed
to amplifier 50, the output of which is connected to a
digital differentiator circuit 51. Within the digital
differentiator circuit 51, the output of the amplifier
50 is connected to one input of a first J-R flipflop 5Z
and also through an inverter 53 to the other input of the
first J-K flipflop 52. One output of the first J-K flipflop
52 is connected to one input of a second J-R flipflop 54,
while the other output is connected to the other input of
the second J-K flipflop 54 and also to one input of a first
NAND gate 56. The output of the J-K flipflop 54 is con-
nected to the other input of the first NAND gate 56, the
output of which is connected to one input of second NAND
gate 58.
A crystal oscil1ator 60 provides a clock signal such
as a 6 MHz clock signal to each of the first and second
J-K flipflops 52 and 54 and is also connected to one input
of AND gate 62, the other input of which is received from
the output of second NAND gate 50. The output of AND
gate 62 is connected to a binary counter 64, the multiple
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outputs N of which are connected through a decoder 66, one
set of multiple outputs M of which are the timing signal~
connected to shift registers 68 (FIG. 6). The remaining
output from decoder 66 is labelled TERMINAL COUNT STOP and
is connected to the other input to second NAND gate 58.
The source of image information 36 shown in FIG. 1
may, as shown in FIG. 6, be in the form of a host computer
70, the output of which is connected through lead 7~ to a
character generator 72. The host computer 70 provides the
necessary intelligence or c ~nd~ relative to tbe text or
characters to be reproduced. An example of such a computer
is shown for example in U.S. Patent No. 3,737,852, but does
not form a part of this invention and for this reason,
details as to the functioning of the computer are not given.
It will be understood that pictorial and graphic material,
as well as alphanumeric characters, may form the image to be
reproduced. The character generator 72, in response to
information received from computer 70, determines the
location and arrangement of the characters to be reproduced.
Such characters are generated by a plurality of data signals,
each of which will, in the embodiment shown here, produce a
dot sized discharged area on photoreceptor 11. Dot producing
data signals from character generator 72 are fed to a
programmable read only memory 76 over data input leads 74.
Dot location information is supplied by the character
generator over leads 73 to a second programmable read only
memory 78 which has been pre-programmed with information
relating to the individual intensity characteristics of each
of the light emitting diodes to be energized. The PROM 76
combines the dot producing data received from PROM 78
_ 9 _
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through leads 75 with the individual LED intensity charac-
teristics information signals to produce signals to shift
registers 68 through leads 77 for energizing array 38 so
that uniform classes of dot sizes are created by combining
data signals received from PROM 78 and character generator
72. Image data is supplied to and stored in shift registers
68 together with timing signals M received from the decoder
66. The timing signals M time the operation of the latc~
circuits within the shift registers 68, under the control of
clock pulse signals from oscillator 60, to effect ener-
gization of individual groups of diodes within the light
emitting diode array 38 by sourcing drivers 80. It will be
understood that connections 71, 73, 74, 75 and 77 are
multiple lead lines. A more complete description of the
details of FIG. 6 may be found in the above noted Canadian
serial number 409,447.
A description of the operation of the invention
follows. Referring to FIGS. 2; 3, and 6, the photoreceptor
11 is in continuous motion in direction B, and continually
presents the freshly charged photoconductive surface 14 to
LED array 38, the motion of the photoreceptor 11 being
monitored by a motion encoder in the form of light interrupter
30 as shown in FIG. 2. The motion encoder provides a
digital signal representative of the motion of photoreceptor
11, including non-uniform or irregular motion, to amplifier
50. When the signal output from amplifier 50 (as shown in
FIG. 3 at G) goes high, the output from first J-K flipflop
52 at F goes 1GW at the next occurring rising edge of clock
pulse A and the output from second J-K flipflop 54 at E goes
high one clock cycle later.
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Binary counter 64 is a multi-state, multi-output
device, a timing diagram for the outputs N of which is
shown in FIG. 3 at B. In the embodiment shown in FIG. 2,
counter ~4 has N binary bit outputs and 2 states.
Counter 64 will sequence or count from a ~ero state to
state 2N 1, where it will stop counting or sequencing and
remain in i~s highest state, that is state 2N 1. The
sequencing of binary counter 64 is timed by the clock signal
A from oscillator 60 through AND gate 62 so that the period
of one binary counter state equals one complete clock cycle
i.e. a high and a low as shown in FIG. 3.
When the signal at G from the light detector amplifier
50 goes low, the state change of the signal received from
amplifier 50 is sensed by the digital differentiator circuit
51. This change in state of amplifier 50 is input to first
J-K flipflop 52 which outputs the signal received from the
amplifier 50 in inverted form as shown at F on the next
occurring clock pulse rising edge. Second J-K flipflop
54 receives the output of the first J-R flipflop 52 and
outputs that siynal in inverted form as shown at E in Figure
3 with a one clock cycle delay. The J-K flipflops 52 and 54
operate as a two bit shift register and delay the signal by
one clock cycle as shown in Fig. 3. It will be understood
that the state change of amplifier 50 sensed by digital
differentiator circuit 51 can occur at any time within the
clock cycle preceeding the rising edge clock pulse at which
J-R 52 operates. The state change of amplifier 50 indicates
that photoreceptor 11 has moved a preselected distance.
The inverted signal from J-K flipflop 52 and the
non.inverted signal from J-K flipflop 54 are input to NAND
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gate 56 to produce a low going signal of one clock cycle
duration as shown by the RESTAXT CO~NTER signal D in Fig. 3.
When the next leading edge of a rising clock pulse signal A
occurs while the RESTART CO~NTER signal D is low, the
RESTART signal D goes high on the next clock pulse and
results in the binary counter 64 reverting to state zero to
begin the counting sequence at zero through 2N 1 as
shown by signal B. When the highest state of the counter
64, state 2N 1 is reached, one output of the decoder 66,
the TERMINAL CO~NT STOP signal shown at C goes from low to
high (as shown at the left hand portion of FIG. 3) at the
same clock pulse and disables the clock signal A to the
binary counter 64 to disable the counting sequence, thus
maintaining the highest state, 2N 1, of the counter 64
until a subsequent low going signal from amplifier 50 is
sensed by the digital differentiator circuit 51. When the
binary counter 64 is in its counting sequence, timing
signals M exit from decoder 66 to tbe control circuit of
Figure 6 to control loading of data information to shift
registers 68 with data signals from image information
source 70, 72. The timing signals M initiate the energiza-
tion cycle in which the sourcing drivers 80 are enabled to
selectively energize or excite individual diode groups
within array 38. Further, when binary counter 64 is in its
highest state 2N 1 and sequencing has stopped, the timing
signals enabling energization of LED array 38 are also
inhibited.
It will be understood that one energization cycle of
a line of the LED array 38 takes place during a relatively
shoPt interval of time followed by a delay interval (binary
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counter 64 at state 2 ) until the photoreceptor 11 hasmoved a predetermined distance. The function of the motion
encoder 30 and digital differentiator 51 is to monitor the
movement of the photoreceptor including the non-uniform
motion and effectively to vary the delay interval between
the LED energization cycles referred to above.
It should also be noted that clock pulse signal A
determines the pulse rate of timing signals M and controls
the initiation of the energization cycle of LED array 38.
The clock frequency is selected to achieve optimum dot size
and density consistent with a preselected photoreceptor
speed. By energization cycle is meant both the timing for
initiation of the energization of individual LED's as well
as the duration of such energizations. The LED array 38
begins its energization cycle at state O of the binary
counter 64 and terminates its cycle at state 2 2 The
interval between those states being fixed by the frequency
of clock signal A. This frequency, as noted, is selected
based on the desired dot size and density. Varying the
delay interval between energization cycles of array 38,
according to this invention, is accomplished without affect-
ing the timing within or duration of the energization cycle
of array 38. In this way, the delay interval between
energization cycles is varied in accordance with photo-
receptor 11 motion while the total duration and timing of
the energization cycle for array 38 is maintained in order
to achieve the desired predetermined dot size and density.
~ eferring now to Pigures 4 and 5, the curve 90
represents the speed of the photoreceptor 11. In Figure 4
there is graphically represented the dot printing intervals
~:Z07~
on the distance axis in the absence of non-uniform motion
compensation according to this invention. As the diode
array 38 is energized at regular intervals of time 92 on the
time axis, the image dots 98 are placed on the photoreceptor
11 at irregular intervals as a result of non-uniform photo-
receptor motion thus degrading image quality. In Figure 5,
the dot printing intervals 96 are represented graphically on
the distance axis. When the timing of the energi~ation of
diode groups within array 38 is varied in response to a
change in speed or non-uniform motion by the photoreceptor
11 as shown at selected points 9~ on the time axis, the
image dots 96 are located at regular evenly spaced intervals
as shown on the distance axis thus insuring a high quality
image even though the photoreceptor 11 motion is irregular
or uneven.
It will be understood that the method and apparatus
disclosed herein for compensating for erratic motion by an
image receiving surface, although disclosed in an embodiment
employing an LED array to discharge a charged photoconductive
image receiving surface, is applicable to other forms of
non-impact printers employing an image transferring device
such as an ink jet head or a thermal print head for conveying
an image from a data signal to a moving image receiving
surface such as plain paper, or a heat sensitive paper,
where the image receiving surface is subject to erratic
non-uniform motion and the image transfer process is a
function of the length of time in which the image conveying
device is actuated, there being a delay interval between
actuation cycles which is predetermined based on the desired
image receiving surface speed.
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In recapitulation, the method and apparltus of the
present invention defines a non-impact printer of the
electrophotographic type in which irregularly non-uniform
motion of the photoreceptor is sensed and encoded for
synchronization with the drive circuits for energizing
selected diode groups of a light emitting diode array
illumination assembly in order to compensate for such
non-uniform motion to produce a high quality image.
It is, therefore, evident that there has been pro-
vided in accordance with the present invention a non-impact
printing method and apparatus that fully satisifies the
objects, aims and advantages set forth abo~e. While this
invention has been described in conjunction with specific
embodiments thereof, it is evident that many alternatives,
modifications and variations will be apparent to those
skilled in the art. Accordingly, it is intended to embrace
all such alternatives, modifications and variations that
fall within the spirit and scope of the appended claims.
