Variable rate ink jet printing

A method of simulating variable rate printing in an ink jet printing system employing a fixed frequency vibrating reed. By varying several printing parameters, a range of quantized values is derived for character size and the rate of character generation.

BACKGROUND OF THE INVENTION 
This invention relates to printing systems, and in particular to printing 
systems involving projection of ink from a nozzle point to a recording 
medium. 
Prior art devices for recording with liquid ink may be categorized into 
those involving continuous or sporadic contact between a stylus and a 
recording medium, and those involving projection of ink onto a recording 
surface. 
The latter devices, known as "ink spitters" or "ink jets", may be further 
classified as to the manner in which the flow of ink is regulated, and as 
to the method by which the ink is targeted onto the recording medium. Ink 
flow is generally regulated through electrical means. By contrast, a 
variety of techniques have been utilized in directing the ink stream. One 
approach has been to apply a charge to the ink drops, and to employ 
electrical fields to deflect the charged drops by an amplitude 
corresponding to an applied potential difference. Another method utilizes 
mechanical means of placing the ink drops, by inducing oscillations of the 
ink jet nozzle transverse to the axis of the nozzle. 
The oscillating nozzle technique, as disclosed, for example, by Carl Hertz 
in U.S. Pat. No. 3,737,914, uses a recording sheet which moves 
continuously in a direction substantially transverse both to the axis of 
the nozzle and the axis of oscillation. The result is that the ink jet 
traces a sinusoidal scanning pattern, and the image may be controlled by 
such factors as the spread of the ink drops as they impinge on the 
recording surface, whether the ink jet is on or off, and the frequency and 
amplitude of oscillation. 
Because the aerodynamic properties of the projected ink stream create 
distortions from the pattern described by the nozzle tip, these devices 
typically project ink either on the up or down stroke. Furthermore, the 
segments of the sinusoidal pattern near the peaks are customarily omitted. 
These techniques rely on the reasonable approximation that the targets of 
all ink drops thus projected will be aerodynamically shifted by an equal 
displacement. Customarily, the ink stream is broken up into a line of 
discrete drops, which trace a pattern of dots on the recording medium. 
The parameters of the frequency and amplitude of oscillation are subject to 
physical limits defined by the device employed to induce nozzle 
oscillation. One such device is comprised of a galvanometer attached near 
the tip of a glass capillary tube, with the tip bent at a right angle. The 
periodic torsion induced by an AC current through the galvanometer 
windings causes the nozzle tip to oscillate. A range of frequencies may be 
obtained through this device, typically with a 1 to 2 KHZ upper limit, and 
no lower limit. This broad band device involves serious control problems, 
however, in that there is a phase lag at high frequencies between the 
actual location of the galvanometer and that perceived by a control 
mechanism, and this phase lag varies from frequency to frequency, making 
this device quite difficult to calibrate. 
An alternative approach which avoids the drawback achieves the desired 
oscillation by means of a vibrating metal reed. The reed carries a 
capillary tube from which the ink is projected, with the oscillations of 
the device confined to the resonant frequency of the reed. This avoids the 
phase lag calibration problem. There is, however, a commensurate inability 
to continuously vary the frequency of the scan, resulting in difficulties 
in providing variable rate printing with this device. A factor which must 
be considered in this regard is the range of intended applications for the 
ink jet printing system. For applications such as production lines coding, 
a lower degree of image control will suffice. In this use, for example, 
some variance in the size of alphanumeric characters is permitted, and a 
system which gives approximate frequency control would meet the 
requirements. 
Accordingly, it is an object of this invention to employ an ink jet 
printing apparatus of the oscillating nozzle type which may be accurately 
controlled. Another object of the invention is to avoid phase lags of 
unknown magnitude in the oscillating mechanism. A related object of the 
invention is to utilize for this purpose a vibrating reed which may be 
easily calibrated to adjust for phase shifts. 
A further object of the invention is to employ a vibrating reed ink jet 
system which is capable of variable frequency printing. Yet another object 
is to use a method for this purpose which will meet the tolerances 
required for production line coding. 
SUMMARY OF THE INVENTION 
In accomplishing the above and related objects, the ink jet printing method 
of the present invention entails a choice of vibrating reed of a given 
frequency, and provision of a matrix of character dimensions, a number of 
cycles skipped between printed strokes, and a number of space cycles 
between characters. 
In accordance with one aspect of the invention, the reed frequency becomes 
a constant of the system, and the latter three parameters may be modified 
to yield different values for the number of characters generated per 
second. By factoring in the speed of horizontal motion of the recording 
medium, which may be varied continuously, corresponding values may be 
derived for the number of characters per inch. 
In accordance with another aspect of the invention, these output values are 
quantized due to the integral values of the input parameters. In 
accordance with a physical embodiment of the invention, the output values 
are confined to a range beyond which image degeneration occurs, such range 
being determined empirically for a given ink jet system.

DETAILED DESCRIPTION 
Reference should be had to FIGS. 1 and 2 and to the accompanying table for 
a detailed description of the ink jet printing method of the invention. In 
its simplest mode of operation, the fixed frequency vibrating reed system, 
to which the present invention pertains, prints a pattern of discrete dots 
which repeats once every scanning cycle and which is confined to somewhat 
less than 180.degree. of each cycle. This printing pattern results in 
approximately straight segments of dots which are uniformly slanted with 
respect to the vertical, this slant varying with the amplitude and 
frequency of oscillation and with the horizontal speed of the recording 
sheet (See FIG. 2). 
The manner in which an ink jet system employing a reed head traces a 
scanning pattern is illustrated in FIG. 1. Ink flows through capillary 
tube 10 (partially shown), emits from nozzle 20, and passes between 
control electrodes 30 where the ink stream breaks up into individual 
drops. The capillary tube oscillates in conjunction with metal reed 40 
(partially shown), this oscillation induced by means not shown at the 
resonant frequency of the reed 40. The oscillation occurs in a direction 
indicated by arrows B. As the ink stream passes through the control 
electrodes 30, and breaks up into discrete drops, these drops are allowed 
to pass through selectively. The passage of ink drops is controlled by 
character generator 50, which is connected to the electrodes 30 by wires 
60. The impulses produced by character generator 50 are coordinated with 
the oscillation of the reed 40 by means not shown, so as to adjust for 
possible phase lags between the impulses delivered by the character 
generator 50 to the control electrodes 30 and the actual oscillatiion 
pattern of the ink jet 70. The impulses produced by character generator 50 
are used to control the printing parameters which are the subject of the 
present invention. The character generator 50 contains logic circuitry 
which may be programmed to perform these control functions, as will be 
apparent to those skilled in the art. 
After the ink stream has been screened by the control electrodes, the 
discrete drops which now comprise the ink stream 70 continue their passage 
until they impinge upon the recording medium 80. The recording medium 80 
moves in a direction A which is transverse to the axis of oscillation B. 
The result is a trace 90, which ideally is sinusoidal but actually is 
subject to the aerodynamic distortions of the ink stream 70. The control 
electrodes 30 are programmed by the character generator 50 to allow the 
ink stream 70 to pass for less than half of each cycle. The result is that 
segments 100 of dots are printed, and no ink drops impinge on the 
recording medium 80 over the balance of the trace 90. (Compare FIG. 2). 
Several characteristic reed frequencies are available. The variable 
printing rate technique of the present invention is a result of 
experimentation with reeds of approximately 500, 1000 and 1800 cycles per 
second. Other frequencies within this range are equally valid, bearing in 
mind that the frequency is determined by the resonant frequency of the 
vibrating reed. Generally, at higher frequencies print quality degrades, 
and the maximum practicable distance of the ink jet nozzle from the 
recording medium decreases. There is, therefore, a tradeoff between print 
speed and print quality to be considered by the user in choosing an 
appropriate reed. 
Having chosen a reed of a given frequency, one may also assume a given 
amplitude of oscillation to avoid operational difficulties. Under the 
method of the present invention, it is advantageous to decide on 
dimensions of the alphanumeric characters. The relevant quantities here 
are the number of strokes used in forming a character, and the number of 
discrete dots per stroke. Symmetry considerations make it advisable to 
choose odd numbers for both dimensions. Experience dictates that the 
minimum for characters of reasonable quality is 5.times.7, where the 
former figure is the number of strokes, the latter figure the dots per 
stroke. Somewhat finer resolution may be obtained with a 7.times.9 matrix. 
These two character sets are in fact considered standard, but higher 
dimensions are feasible. FIG. 2 illustrates a 5.times.7 matrix printout of 
the letter `H` with a pronounced character tilt (In practice, character 
tilt is less noticeable). The blurring of vertically contiguous drops is 
indicative of a high mark/space ratio, which gives the effect of a solid 
stroke. Discrete drops may be discerned by comparing adjacent vertical 
strokes, however. 
Another parameter which may be varied to advantage is the number of strokes 
or cycles which are skipped between printed strokes. In the simplest case 
this is zero. One may combine this factor with a choice of matrix to 
arrive at a provisional figure for character cycles, or cycles per 
character. For example, with a 5.times.7 matrix and no stroke skipping, 
the provisional character cycle figure is 5. If three strokes are skipped, 
the figure is 20. 
Finally, the ink jet printing method allows the operator to modify the 
number of space cycles between characters. The standard figures here are 
one space for a 5.times.7 matrix and two spaces for a 7.times.9 matrix, 
assuming no stroke skipping. This figure is added to the above provisional 
figure to derive the total number of cycles per character. 
Reference may now be had to the accompanying table for a synthesis of the 
above factors. This table pertains to a reed with a frequency of 1020 
cycles per second. One key figure is found in column 4, that of 
CPS=characters per second, which may be derived by the formula: 
EQU CPS=1020.div.Cycles Per Character 
Thus, for example, for the first entry with a total cycle per character 
figure of six (the sum of columns two and three), the CPS is 
1020.div.6=170.0. The CPS figure, of course, indicates the rate at which 
the characters are printed. This figure is proportional to reed frequency, 
all other things being equal. For a given reed, the CPS is largest for the 
5.times.7 matrix, no stroke skipping and one space cycle. 
Another figure of interest to the operator is that in column 5 of 
CPI=characters per inch, which is derived by the formula: 
EQU CPI=CPS.times.(ft./min.).sup.- .times.1 ft./12 in..times.60 sec./1 min. 
The feet per minute figure represents the rate of horizontal motion of the 
recording sheet. An increase in this rate will naturally cause a decrease 
in the number of characters per inch. The CPI figure is of primary 
importance, as it quantifies character spacing. This proportion may be 
computed by taking the ratio of the figures in columns 2 and 3. A given 
CPI may include varying proportions of character size to space between 
characters. The figures are tabulated for binary stroke skipping values 
(printing every one, two, or four strokes). Other strokes skipping values, 
however, are equally valid. 
The efficacy of this method depends on the ability of the ink jet equipment 
to generate characters with the desired properties without a significant 
degradation of character appearance. This problem occurs primarily in the 
context of the choice of space cycle. The table values were compiled for a 
range of space configurations which represent the product of 
experimentation with image quality as a function of the space cycle. The 
resulting rule of thumb is that, for both matrices, the space cycle value 
may be raised or lowered by one cycle from its standard value (one cycle 
for 5.times.7, and two cycles for 7.times.9). This variance may go up 
proportionately with stroke skipping (for example, a permissible variance 
of three cycles if two strokes are skipped). 
The table for any given reed frequency may be utilized by an operator in a 
number of ways. Given a certain speed of the recording medium, a desired 
CPI may be derived by choosing an appropriate combination of matrix, 
stroke skipping, and space cycles. Furthermore, given such a combination, 
the user may derive an interpolated range of CPI values, as the speed of 
the recording medium may be varied continuously. Such interpolation is not 
possible for a given recording medium speed, however, as this would 
require non-integral character cycles. Hence, the method is essentially a 
quantized one, which should nevertheless be sufficient for applications 
such as production line coding. 
The table gives equivalent ways of arriving at almost identical images. If 
space size is increased proportionately with the provisional character 
cycle, and the speed of the recording medium is decreased by the same 
proportion, the characters in both configurations should appear identical 
but for character tilt. Thus, for example, almost identical characters 
should be yielded by a 5.times.7 matrix with one space at 170.0 ft. per 
second, and by a 5.times.7 matrix with one stroke skipped and two space 
cycles at 85.0 ft. per second. 
While various aspects of the invention have been set forth by the drawings 
and the specification, it is to be understood that the foregoing detailed 
description is for illustration only and that various changes in parts, as 
well as the substitution of equivalent constituents for those shown and 
described, may be made without departing from the spirit and scope of the 
invention as set forth in the appended claims. 
TABLE 
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(Reed Frequency = 1020 Hz.) 
5 
Prov. 
3 CPI 
1 Char. 
Space 
4 Recording Medium Speed (ft./min.) 
Matrix 
Cycles 
Cycles 
CPS 170 
150 
120 
100 
85 50 40 30 
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5 .times. 7 
5 1 170.0 
5.00 
5.67 
7.08 
8.50 
10.0 
17.0 
21.25 
28.33 
5 .times. 7 
5 2 145.7 
-- 4.86 
6.07 
7.28 
8.57 
14.57 
18.21 
24.28 
7 .times. 9 
7 1 127.5 
-- -- 5.31 
6.38 
7.5 
12.75 
15.94 
21.25 
7 .times. 9 
7 2 113.3 
-- -- 4.72 
5.66 
6.66 
11.33 
14.16 
18.88 
7 .times. 9 
7 3 102.0 
-- -- -- 5.1 
6.0 
10.2 
12.75 
17.0 
5 .times. 7 
10 1 92.7 
-- -- -- 4.64 
5.45 
9.27 
11.59 
15.45 
5 .times. 7 
10 2 85.0 
-- -- -- -- 5.0 
8.50 
10.62 
14.17 
5 .times. 7 
10 3 78.5 
-- -- -- -- -- 7.85 
9.81 
13.08 
5 .times. 7 
10 4 72.9 
-- -- -- -- -- 7.29 
9.11 
12.15 
7 .times. 9 
14 1 68.0 
-- -- -- -- -- 6.8 
8.5 
11.33 
7 .times. 9 
14 2 63.8 
-- -- -- -- -- 6.38 
7.98 
10.63 
7 .times. 9 
14 3 60.0 
-- -- -- -- -- 6.0 
7.5 
10.0 
7 .times. 9 
14 4 56.7 
-- -- -- -- -- 5.67 
7.09 
9.45 
7 .times. 9 
14 5 53.7 
-- -- -- -- -- 5.37 
6.71 
8.95 
7 .times. 9 
14 6 51.0 
-- -- -- -- -- 5.1 
6.38 
8.5 
5 .times. 7 
20 1 48.6 
-- -- -- -- -- 4.86 
6.08 
8.1 
5 .times. 7 
20 2 46.4 
-- -- -- -- -- 5.8 
7.73 
5 .times. 7 
20 3 44.3 
-- -- -- -- -- 5.54 
7.38 
5 .times. 7 
20 4 42.5 
-- -- -- -- -- 5.31 
7.08 
5 .times. 7 
20 5 40.8 
-- -- -- -- -- 5.1 
6.8 
5 .times. 7 
20 6 39.2 
-- -- -- -- -- 4.9 
6.53 
5 .times. 7 
20 7 37.8 
-- -- -- -- -- 6.3 
5 .times. 7 
20 8 36.4 
-- -- -- -- -- 6.07 
7 .times. 9 
28 4 31.9 
-- -- -- -- -- 5.32 
7 .times. 9 
28 6 30.0 
-- -- -- -- -- 5.0 
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