Reproduction and enlarging imaging system and method using a pulse-width modulated air stream

A system for copying images onto a long sheet of paper or vinyl while. A flow of constant-pressure air is pulse-width modulated in accordance with the control signals and passed over an ink meniscus maintained on the end of a small nozzle. The pulse-modulated air flowing across the meniscus causes the ink to be sprayed onto the recording medium. The dot size on the image remains constant and the pulse width within each pixel is varied to produce the desired density of color. Pressure surges in the ink supply system, produced for example by vibration or acceleration of the ink, are suppressed by a second meniscus in the ink-supply system, formed by a gas bubble or interface near the nozzle meniscus. The second meniscus should have a surface area at least as large as that of the jet meniscus. At the end of each pixel, the ink flow is interrupted for a period of about 100 microseconds to restore system equilibrium. A second and continuous air flow around the air nozzle supplies air to the turbulent flow across the ink jet and reduces recirculation of atomized ink and minimizes contamination of the nozzle and adjacent surfaces. The supply roll is mounted at each end on independently movable arms.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to systems for reproducing color images by scanning 
an original and using electrical signals from the scanner to control the 
paint-spraying of a larger duplicate image. More particularly the 
invention relates to such a system in which a number of spray heads 
reciprocate across the imaging medium while the duration of pulses of ink 
or paint ejected by each spray head are controlled by pulse-width 
modulation of a constant pressure air stream that passes over a meniscus 
of the ink or paint. 
2. Description of Related Art 
Systems have been in use for making enlarged prints, such as for 
billboards, in which an original image is scanned to produce control 
signals that operate the reproduction equipment. In most systems, the 
recording medium is supported by a rotating cylinder while one or more 
spray heads slowly traverse the width of the cylinder and spray paint or 
ink in accordance with the scanning signals to reproduce the image on the 
medium carried by the cylinder. One such system is described in U.S. Pat. 
No. 1,709,926 in which the original and the recording medium are each 
carried by a rotating cylinder. A color duplicate of the original is 
produced by mechanically controlling three ink jets in accordance with 
signals produced by scanning the original while the ink jets are moved 
slowly across the width of the recording medium. 
U.S. Pat. No. 1,817,098 describes a facsimile system using a first drum for 
scanning and a second drum for recording. The initial signal is divided 
into color components from which electrical control signals are generated. 
The control signals cause electric potentials to be applied to a pair of 
deflection electrodes positioned on opposite sides of a stream of atomized 
ink particles so that the ink stream is deflected toward the recording 
medium when an electrical potential is applied to the electrodes. When no 
voltage is present on the electrodes, the ink spray is prevented from 
reaching the recording medium. 
U.S. Pat. No. 3,553,371 describes another dual-cylinder system in which 
multi-color images are reproduced by using one or more ink-jet heads 
operated in synchronism with the scanning signals. The pressure of the air 
is amplitude modulated in accordance with the scanner signals and the 
modulated air controls a mechanical valve that regulates the rate of ink 
flow. The extent to which the ink valve is opened is a function of the air 
pressure. The same patent also describes spray heads in which the ink jet 
is controlled by a mechanical valve operated directly by the scanner 
signals without pressure modulation of the air stream. 
The above patent suggests the elimination of the recording cylinder by 
transferring the medium from one roll to another, forming an arcuate 
surface in the medium between the two rollers and paint spraying the 
medium by the use of a rotating head. 
In most spray applications, it is preferred to use an internal system in 
which the ink and air are expelled together. Such an arrangement provides 
better atomizing of the spray, but it is not satisfactory where a specific 
pattern of color is to be reproduced requiring instantaneous response time 
in the control of the ink delivery. 
Jets that include a mechanical ink valve operated by the recording signals 
are slow and suffer from problems associated with contamination, clogging 
and wear of the valve mechanism. Various attempts to amplitude modulate 
the air stream and avoid the problems associated with variable ink valves 
have not been commercially successful, in part because the character of 
the atomized ink is a function of the pressure of the air that produces 
the ink spray. 
SUMMARY OF THE INVENTION 
As used here and in the accompanying claims, the word "ink" is to be 
interpreted to mean ink, pigmented paint, or other colored liquid capable 
of producing an image. The present system for preparing large images, such 
as are used for outdoor advertising, artistic representations and other 
purposes, has a number of important advantages by producing the image on a 
long sheet of medium, for example of paper or vinyl, while it is being 
transported from a supply-roll to a take-up roll. The width and length of 
the image can be varied readily without major modifications of the system; 
and the tedious task of securing the recording medium to a cylinder is 
avoided. Moreover, very long images can be produced readily whereas a 
cylinder to accommodate such length would be very large with all the 
attendant difficulties of mounting the recording medium and precisely 
controlling the speed of rotation of the cylinder. 
The use of separate ink and air supply jets provides for easier cleaning or 
replacement of the ink jet, faster response time and improved repetitive 
performance. 
In the present invention, a flow of air, supplied at a constant pressure, 
is turned on and off in accordance with the control signals and passed 
over an ink meniscus. In other words, when the air pressure is turned on 
to cause ink to be sprayed on the medium, it is always at the same 
pressure. To achieve the desired imaging, the air is pulse-width 
modulated. The dot size on the image remains constant and the time the air 
is allowed to remain on within each pixel is varied to produce the desired 
density of color. 
A nozzle is supplied with ink that forms a small meniscus on the tip of the 
nozzle. The pulse-modulated air flowing across the meniscus causes the ink 
to be sprayed onto the recording medium. It is important that the nozzle 
tip be formed of material that is wetted by the ink so that the meniscus 
formed on the end of the nozzle remains attached to the end of the nozzle. 
The meniscus is thus maintained at all times at the tip of the nozzle, 
which is not true of systems in which the ink is forced by pressure from 
the nozzle or withdrawn into the nozzle at the end of each ink pulse. 
Systems in which the ink must be drawn through a supply tube at the 
beginning of each pulse, for example because the meniscus is allowed to 
recede into the supply tube at the end of each pulse, the response time is 
excessive. Such a system is also erratic because the response time for 
each pulse is a function of how far the meniscus has been allowed to 
recede into the supply tube. 
The maintenance of the meniscus at the tip of the nozzle limits the maximum 
diameter of the nozzle with a given ink with a higher surface tension 
coefficient increases the maximum permitted diameter of the nozzle 
However, the diameter of the ink jet nozzle is related to the viscosity of 
the ink. If the nozzle diameter is too small, the response time and rate 
of flow of the ink are adversely affected and clogging of the noze may 
become a problem. Moreover, because the force that can be resisted by the 
meniscus is an inverse function of its area, the use of a larger diameter 
nozzle reduces the amount of negative head that can be tolerated in the 
ink supply. Higher viscosity ink increases the maximum permitted diameter 
of the nozzle, but excessive viscosity has a deleterious affect on the 
response time and rate of flow. It is preferred that the nozzle diameter 
be between 0.005 and 0.020 inches, with a preferred diameter of about 
0.010 inches. The overall arrangement minimizes the problems of 
contaminated, clogged and worn valves and materially increases the speed 
and fidelity of the imaging process. 
The rapid transverse movement of the spray heads necessary for a system of 
this type causes imperfections in the reproduced image. For example, 
pressure surges in the ink supply system, produced by vibration or 
acceleration of the ink supply system, cause changes in the rate at which 
the ink is delivered with resultant banding or other defects in the 
reproduced image. These imaging problems are overcome by providing a 
second meniscus in the ink-supply system, formed by a gas bubble or 
interface preferably as near as possible to the nozzle meniscus. The 
second meniscus should have a surface area at least as large as that of 
the jet meniscus. Preferably the second meniscus has an area several times 
larger than the meniscus at the nozzle tip so that internal pressure 
surges in the ink system are effectively damped by the second meniscus 
rendering the quantity of ink being delivered to the recording medium 
independent of pressure surges in the ink supply such as are typically 
caused by vibration and acceleration. The tube supporting the second 
meniscus should be sealed so that an air bubble is trapped at the meniscus 
thus permitting the necessary movement of the meniscus while preventing it 
from being withdrawn into the ink supply. For most applications, the level 
of ink supply for each spray head is maintained between that of the 
associated ink nozzle and about one inch lower. 
The ink could be allowed to flow continuously to produce a color band 
across the image surface. However, in the present system, the ink flow is 
interrupted at the end of each pixel and the system allowed to come to 
equilibrium before the ink is again turned on. For example, in a system 
having say, 12 pixels per inch and in which the head is traveling at a 
rate of 40 inches per second so that the head traverses one pixel in 
approximately 2 milliseconds, the ink spray is never allowed to continue 
uninterrupted for as long as 2 milliseconds. At the end of each pixel, the 
ink flow is interrupted for a period of about 100 microseconds to restore 
system equilibrium before the next ink pulse. This arrangement provides 
continuing stable operation during the imaging process and produces an 
improved image. 
In any arrangement, there is a finite period of time between the start of 
an air pulse and the initiation of the ink spray. This time interval must 
be constant and it must be short relative to the maximum pulse period. If 
pulses of ink are amplitude modulated as a function of the image being 
reproduced, the response time interval will be variable as a function of 
the air pressure. In the present system the air pressure is constant 
resulting in a constant response time interval. The use of constant 
velocity air and an ink meniscus that remains at the end of the ink nozzle 
provides a minimum response time interval that is constant from pulse to 
pulse. In the system described here, the minimum pulse width to produce a 
flow of ink is about 100 microseconds. 
The high velocity air stream that passes over the ink meniscus to produce 
the spray is turbulent in nature and draws relatively large amounts of air 
into the stream from the surrounding air. This air flow creates a 
feed-back air stream that recirculates the ink spray into the stream 
adjacent the ink nozzle. The result of this air flow is to contaminate the 
head in the area of the ink nozzle and to cause a build-up of ink on the 
nozzle and adjacent surfaces To minimize this ink build-up, a separate 
flow of air is provided around the ink nozzle. This flow is continuous, 
that is, it is not modulated on and off as is the ink control spray and 
provides a continuous supply of clean air to join the ink spray. This 
secondary air flow is not of sufficient velocity to cause ink to be 
withdrawn from the ink supply nozzle. 
With systems in which wide strips of flexible medium are used, it is 
important to prevent wrinkling or excessive stress in the medium. It is 
important for that reason that the drive and idler rollers over which the 
medium passes be precisely aligned with the supply roll. This is difficult 
to achieve with rigid mountings unless the medium is precisely rolled at 
constant tension onto the supply roller, an unusual condition with 
commercial supplies. To eliminate the problems of wrinkling and stress in 
the medium, the supply roll is mounted on independently movable arms at 
each end of the supply roll. The supply roll is thus free to move, against 
gravitational forces, angularly with respect to the linear direction of 
medium movement as required to maintain uniform stress and wrinkle-free 
condition of the medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIGS. 1-3, a roll 2 of imaging medium 4, which may be paper, 
vinyl or other sheet material, is supported by a frame 6 of the image 
reproducing system. The medium passes over an idler roller 8 (shown only 
in FIG. 1), around a drive roller 12 and onto a take-up roller 14. 
The drive roller 12 is driven by a stepper motor 16 (FIG. 2). A series of 
rubber rollers 18 press against the outside of the medium 4 to prevent 
slippage between the medium and the outer surface of the drive roller 12. 
The motor 16 is coupled also, by a chain or other suitable drive means 
(not shown), to the take-up roller 14 through a slip clutch (not shown) 
that applies sufficient torque to maintain the medium 4 under tension. 
Four ink spray heads, generally indicated at 22, are positioned adjacent 
the surface of the medium and are supported by a carriage 24. The carriage 
24 is slidably mounted on a rail 26 (FIG. 3) and is driven back and forth 
across the medium by means of a motor drive 28 (FIG. 2) and a reversing 
drive cable 32. Ink for the four spray heads 22 is provided from a 
compartmented ink reservoir, generally indicated at 34, carrying four 
different colors of ink, typically cyan, magenta, yellow and black. The 
operation of each of the ink spray assemblies is the same so only one unit 
is described. 
As shown in FIGS. 4 and 5, the ink reservoir 34a is connected by a flexible 
tube 36a to an ink spray nozzle 38. The ink in the reservoir 34a travels 
through the conduit 36a to form a meniscus at the end of the nozzle 38. An 
air nozzle 42 is positioned so that a stream of air flows across the 
meniscus at the end of the ink nozzle 38 causing the ink to be extracted 
from the nozzle 38 and atomized into a fine spray, as indicated at 44, and 
deposited on the medium 4. A conventional source of compressed air (not 
shown) is applied at constant pressure through a conduit 46 to a control 
valve 48. The valve 48 is opened and closed by the action of a 
piezo-electric actuator, to be described later. When voltage is applied to 
the valve through the leads 52, the valve opens to permit the air to flow 
through the nozzle 42. When the voltage is removed, the valve closes and 
no air flows through to the nozzle 42. 
The maximum level of the ink in the reservoir 34a is positioned at 
approximately the same level as the nozzle 42 so that the level of the ink 
cannot rise above the level of the nozzle which would cause excessive flow 
of ink to the nozzle. The bottom of the ink reservoir 34a is approximately 
one inch below the level of the nozzle 42. 
The conduit 36a that carries the ink to the nozzle 42 communicates with a 
second meniscus 54 that damps pressure surges in the ink supply system. 
The second meniscus arrangement is identical for each of the spray heads, 
so the description of the spray head 22a of FIG. 4 applies to all. A short 
branch of tubing 56 extends upwardly from the conduit 36a at a point near 
the ink spray nozzle 38. The upper end of the branch tubing 56 is closed 
and traps a small amount of air above the surface of the ink. The trapped 
air forms when the conduit 36a, which is initially filled with air, is 
connected to the ink reservoir 34a and filled with ink. The inner 
cross-sectional area of the branch tubing 56 is at least as large as the 
inner cross-sectional area of the nozzle 38 at the point where the first 
ink meniscus is formed. Improved damping is obtained if the area of the 
second meniscus is at least several times as large as the area of the 
meniscus formed on the end of the nozzle 38. It is preferred that the 
diameter of the second meniscus be 5 to 10 times the diameter of the 
nozzle 38. 
In operation, the image to be reproduced is placed on a conventional 
commercial scanner 58 (FIG. 5) and the image is scanned in conventional 
manner. The signals from the scanner are modified by a computer 62 to 
achieve the desired color effects with the particular inks being used. 
These signals control the operation of the piezo-electric valve 48 (FIG. 
4). 
In this example, the image is divided into square pixels, each about 1/12 
of an inch on each side. Each composite signal from the scanner 58 
corresponds to one pixel from the image. The pixel signal from the image 
is divided by the computer 62 into appropriate signals representing the 
color components using conventional techniques. These signals then control 
the pulse widths of the air flows and thereby the duration of the spray of 
ink from each of the heads 22a, 22b, 22c and 22d. Each of the compartments 
of the ink reservoir 34 associated with one of the heads 22 carries a 
different color ink. The rate of ink flow is not changed as a function of 
the color component, only the time during each pixel that the ink is 
allowed to spray onto the medium 4. 
At the beginning of each pixel, a signal from the original image is 
transmitted to the appropriate spray head which is turned on for a length 
of time required to give the desired color perception. For example, at the 
beginning of a pixel, the control signals might indicate that the cyan 
head 22a is to be turned on for a period of 600 microseconds, the yellow 
head 22c is to be turned on for a period of 1200 microseconds, the magenta 
head 22b is to be turned on for a period of 300 microseconds, and the head 
22d carrying the black ink is not turned on during this particular pixel. 
At the end of each pixel, each of the spray heads is turned off for a 
period of about 100 microseconds to bring the system into stable 
equilibrium before the next pixel begins. 
Obviously, one or more spray heads may not be used for a considerable 
period of time as a function of the colors being reproduced. To prevent 
the drying of the ink meniscus on the nozzles 38 during such a period, the 
control circuits cause the carriage 24 periodically to pass beyond the 
edge of the image being reproduced. Each spray head is then automatically 
discharged into a purge station for a short period to supply fresh ink to 
its meniscus. The drive roller 12 is driven incrementally by the motor 16 
to advance the recording medium by one line at the end of each scan of the 
carriage 24. 
Alternate head constructions are illustrated by FIGS. 6, 7 and 8,.in which 
certain parts corresponding to parts previously described are indicated by 
the same or similar numbers. In FIG. 6, a second air stream is provided to 
supply air to the jet stream from the air nozzle. A housing 64 forms a 
cavity 66 that is supplied with compressed air at constant pressure 
through an inlet opening 68. An air jet nozzle 42 is coupled to the cavity 
66 through a length of conduit 74. The inner end of the conduit opens into 
the cavity 66, but can be sealed by a pad 76 that is actuated by a 
conventional piezo-electrically driven arm 78. When voltage is applied to 
the arm through the leads 82, the arm 78 flexes toward the right as viewed 
in FIG. 6 moving the pad 76 from the end of the conduit 74 allowing the 
air to escape from the nozzle 42 and draw ink from the ink jet nozzle 38. 
As in the previous example, a meniscus of ink is maintained on the end of 
the nozzle 38 and provides ink for the atomized spray whenever the air is 
allowed to exit from the nozzle 42. 
An air channel 83 is provided around the conduit 74 where it passes through 
the wall of the housing 64. Air which is under continuous pressure within 
the cavity 66 passes through this channel to form a flow of air that 
surrounds the air nozzle 42. This continuous air flow provides a source of 
air for the turbulent jet stream formed by the nozzle 42 and minimizes 
recirculation of the atomized ink spray and materially decreases the 
amount of deposition around the ink jet nozzle 38 and adjacent surfaces. 
In the arrangement of FIG. 7, the air is not turned on and off by means of 
valves as in the previous construction, but rather the air stream is 
diverted from the meniscus The result is that the air is turned on and off 
so far as the ink meniscus is concerned, but the air stream continues to 
flow at a constant rate at all times. This arrangement eliminates the 
problems invariably associated with mechanical valves that open and close, 
and provides a constant air pressure that is unaffected by changes in the 
rate of flow. A bracket 84 supports an inlet conduit that provides a 
constant flow of pressurized air to form a first jet indicated at 88. A 
second length of conduit 92 is positioned to receive the air impressed 
upon it by the air jet 88. This air passes through the conduit 92 to the 
air spray nozzle 42. As previously described this air jet draws the ink 
from the ink jet nozzle 38 and atomizes it for application to the imaging 
medium. 
A baffle 94 is secured to the end of a conventional piezo-electric arm 78 
that is supported by the bracket 84. When voltage is applied to the arm 78 
through the leads 82, the arm flexes into the position shown in FIG. 7 and 
permits the free flow of air through the air nozzle 42. When the voltage 
is removed from the arm, the arm returns to its unflexed position and 
moves the baffle 94 into position to intercept the flow of air to the 
nozzle 42. Thus, the air flow is constant and the control of the ink 
pattern is accomplished by pulse width modulation of the arm 78. 
FIG. 8 illustrates a similar arrangement in which the inlet air conduit 86 
includes a flexible section 86a. The opposite end of this flexible section 
is connected to another rigid conduit section 86b that is supported by an 
extension 96 secured to the end of the piezo-electric arm 78. When the 
voltage applied to the arm 78 causes it to flex in a downward direction, 
as viewed in FIG. 8, the end of the conduit 86b is directed downwardly at 
an angle such that its output of air does not impinge on the open end of 
the conduit 92 and thus no air flows from the nozzle 42. When the voltage 
is removed, the arm 78 returns to the position shown and the air flow and 
ink atomization are resumed. As in the previous examples an ink supply jet 
is positioned appropriately in the air spray pattern from the, nozzle 42. 
It is important to maintain the paper or other medium 4 for movement 
precisely perpendicular to the axes of the idler and drive rollers 8 and 
12 to prevent wrinkling or uneven tension in the medium. For that reason, 
the supply roll 2 (FIG. 2) is supported by two spindles 98 and 98a which 
extend into the ends of the roll 6. The spindle 98 is supported by an arm 
102 the opposite end of which is pivotally mounted by a support 104 on the 
frame 6. The other spindle 98a is similarly mounted on the frame 6 by an 
arm 102a and a support 104a . With this arrangement, each end of the 
supply roll 2 is independently supported so that the forces of gravity 
maintain the width of the medium 4 under equal and constant tension. 
In operation, the scanner 58 (FIG. 5) inspects the image to be reproduced 
laterally pixel-by-pixel and vertically line-by-line. For each pixel, a 
composite signal is generated carrying the color information. These 
signals are fed into a color look-up table 106 that forms part of the 
computer 62 by which the scanner signals are converted into data that 
controls the intensity of each of the four output colors. The scanned 
image is displayed on a computer display 108 so that the operator may make 
any desired adjustments in color balance. The signals from the computer 
are fed into a pulse width modulator 112. For each pixel of the scanned 
image, four signals are generated having a width that is a function of the 
intensity of that particular color for that particular pixel. The wider 
the pulse, the longer the corresponding spray will stay on and the more 
intense the color. The signals from the pulse width modulator 112 are 
amplified by four amplifiers 114a, 114b, 114c, and 114d. The signals from 
each of the amplifiers is fed into a corresponding spray head 22a, 22b, 
22c or 22d, where a pulse of air is produced whose duration is a function 
of the width of the pulse from the modulator 112. 
A servo controller 116 provides signals that control the drive motor 28 
that moves the carriage 24 and the spray heads 22 on the rail 26. An 
encoder 118 feeds back to the servo controller 116 a signal representing 
the actual position of the carriage 24 on the rail 26 to maintain precise 
control of the printing action. The servo controller also provides the 
signals that operate the paper drive 16 to move the recording medium one 
line at the end of each sweep of the carriage.