Electrostatic printer head structure and styli geometry

Each of the plurality of styli contained in an electrostatic print head and used for placing an electrostatic charge on a dielectric coated medium has a rectangular cross section. The thickness of the cross section is selected such that the difference between the lowest voltage on the stylus at which the amount of electrical charge deposited on the dielectric medium for any increase in voltage on the stylus increases substantially in a linear fashion and the lowest voltage on the stylus at which an unwanted background image is formed on the medium is selected to have a value equal to or less than a selected number. In one embodiment this thickness is selected to minimize this difference. In another embodiment this thickness is selected to ensure that the electrostatic print head performs for some minimum lifetime. In another embodiment this thickness is selected to ensure that the distance between adjacent styli in the print head is selected such that current flowing from a stylus having a print voltage applied thereto to a directly adjacent stylus with a lower voltage applied thereto is below a selected number.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to electrostatic printers and in particular, to an 
electrostatic print head which utilizes a stylus of unique cross section 
which reduces background noise produced on the recording medium over that 
produced by a stylus of the prior art. 
2. Prior Art 
Electrostatic printers are well known. Such printers are described for 
example in U.S. Pat. Nos. 4,672,399, 4,672,400 and 4,731,542. 
Electrostatic printers use print heads containing electrically-conductive 
styli. Typically, these styli are round in cross section, placed close 
together and separated by an insulative material. When a medium passes by 
the styli and a voltage is applied to selected styli, electrostatic charge 
is deposited on the medium adjacent the selected styli such that the 
medium will subsequently attract and hold a toner and thereby make the 
charge images visible. This mechanism is well known in the art. While 
styli with rectangular cross section have been disclosed, to ensure that 
the print on the medium is dense and further to ensure that the medium can 
be printed with an optimum speed for the resulting desired image quality, 
rectangular styli have usually been selected with an aspect ratio that is 
made nearly one to one (i.e., the ratio of width to length of the cross 
section is approximately one to one). Rectangular styli whose thickness is 
much less than their width have also been used. See, for example, U.S. 
Pat. No. 4,521,790 entitled "Electrostatic Printer of Video Pictures with 
Grey Tones" which discloses use of a writing head having electrodes with a 
"Substantially elongated cross-sectional geometry such that a width 
dimension is significantly less than a length dimension." (Abstract, lines 
4-6.) Unfortunately, styli of the '790 patent when used in a multiplexed 
arrangement require a considerable sacrifice in the speed at which the 
medium moves past styli of the electrostatic print head. 
Electrostatic printing typically uses two voltage sources to effect 
printing. One source, which can be described as a bias source, produces a 
voltage difference between the styli and the print medium of a level below 
that necessary to effect transfer of charge between the styli and the 
print medium. This bias voltage is generally applied to the media through 
a conductive back plate arrangement to a conductive layer of the print 
medium which is well known in the art. Bias voltage can also be applied to 
the media via capacitative coupling from a set of front plates. Such 
arrangements are described, for example, in U.S. Pat. No. 3,611,419. It is 
also possible to place a DC bias voltage on styli directly. In either 
event, the additional voltage required to effect printing is supplied to 
the selected styli via an additional electronic pulsing circuit that is 
controlled in such a way as to produce the desired image on the print 
medium. 
One of the problems of the prior art styli is that with a D.C. bias voltage 
applied to head styli that is below the voltage at which printing occurs, 
a spurious discharge occurs from some styli to the print medium which upon 
subsequent toning causes discoloration of the medium in random ways 
bearing no relationship to the image desired to be formed on the medium. 
This "background noise" is undesirable and must be reduced to a very low 
level if not totally eliminated. To do this, the DC bias voltage must be 
reduced to a lower level and consequently the pulsing circuitry, that is 
used in conjunction with the D.C. bias circuitry to provide the required 
printing voltage to the various styli in the printing head must be made 
capable of a larger voltage swing than before necessary. This increases 
cost and complexity of the electronics and shortens the life of the print 
head due to degradation of the insulative material between styli due to 
the new higher voltages between adjacent styli. This effect will be 
explained in detail later. 
SUMMARY OF THE INVENTION 
In accordance with this invention, certain disadvantages of the prior art 
electrostatic print heads are overcome by providing an electrostatic print 
head using styli of a new configuration. This new styli configuration 
allows reduced amplitude of the voltage swing required to provide high 
quality images on the print medium and consequently decreases cost, 
complexity and size of the electronic components used to drive the styli. 
Additionally, the lower pulsing voltage required by this invention reduces 
degradation of the insulative material between styli in the electrostatic 
printing head and thus increases the operational life of electrostatic 
print heads using styli made in accordance with this invention. Because of 
this lower pulsing voltage this invention allows construction of print 
heads with styli that are placed closer together than has heretofore been 
possible. 
In accordance with this invention, each stylus is fabricated having a 
rectangular shape wherein thickness of the stylus is selected to allow use 
of a stylus drive voltage swing of a smaller amplitude than in the prior 
art while still eliminating or substantially reducing background noise. 
I have discovered that background noise appearing on the print medium is to 
a great extent a function of the roughness of the surfaces and edges of 
the styli. Rough projections protruding from the surface and edge of each 
stylus serve as points of increased electrical fields thereby resulting in 
breakdown of the atmosphere between random points on the stylus and the 
print medium on which the image is to be formed at voltage levels lower 
than would be experienced by smooth-surfaced styli. Such random breakdown 
occurs when the difference between styli voltage and print medium voltage 
is at the bias voltage level prior to the time when voltage on the styli 
is increased to a level required to cause sustained breakdown across the 
entire surface of the styli in order to effect dense printing When such a 
random breakdown occurs, electrons are transferred to those portions of 
the surface of the medium directly adjacent projections on the styli to 
thereby form areas of spurious charge images on those portions of the 
medium. These charge images are subsequently developed by suitable toning 
apparatus so as to produce visible but spurious images. Thus, as the 
medium moves past a stylus held at a DC bias voltage, unwanted 
discolorations and streaks may appear on the surface of the medium 
adjacent this stylus. 
I performed an experiment whereby styli of considerable width but of 
various selected thicknesses were connected to a voltage source. I 
increased the voltage applied to each stylus linearly with time between 
two selected values. Typically, I started with -100 volts applied to a 
stylus and changed voltage on the stylus linearly to -600 volts over a 
time period of five minutes. Simultaneously, I moved a print medium past 
the stylus. Thus, as the print medium moved past the stylus, voltage on 
the stylus was changed (i.e., increased or decreased) as a function of 
time. As time proceeded and the voltage increased on the stylus, small 
discolorations and streaks appeared on the surface of the print medium 
adjacent the stylus prior to the voltage on the stylus reaching a voltage 
level required to form a sharp, clear and dense image. One example of such 
discolorations is shown in FIG. 1a where a stylus of a thickness 
consistent with styli that are used in heads of the prior art was made to 
print on an electrostatic medium. That portion of FIG. 1a labeled 
"Background" represents the discoloration of the print medium occurring as 
a result of premature partial discharge of the stylus. The portion of FIG. 
1a labeled "Linearity" shows charge being deposited by the stylus on the 
print medium in such a way that charge on the print medium (i.e., the 
voltage) increases linearly with a subsequent increase in stylus voltage. 
In this portion of the figure the amount of charge applied to that portion 
of the print medium adjacent the stylus is directly (i.e., linearly) 
proportional to the incremental increase in voltage on the stylus. FIG. 1b 
(placed directly adjacent FIG. 1a) illustrates coloration on the print 
medium from the application of a voltage to a stylus of the present 
invention in the same manner as applied to a prior art stylus. The white 
portion indicates that no discharge of electrons occurs from the stylus to 
the medium when voltage on the stylus is below the threshold for 
linearity. The dark portion on the right of FIG. 1b is the printed image 
that corresponds to voltages on the stylus sufficient to cause the number 
of electrons discharged from the stylus to the print medium to vary 
linearly as a function of change in stylus voltage. Note that the print 
medium as shown in FIGS. 1a and 1b is moving to the left (i.e., time 
increases to the right in both FIGS. 1a and 1b) and the horizontal bands 
shown formed in FIGS. 1a and 1b are each formed by one extended stylus 
having a width of about one-half inch. 
In accordance with one embodiment of this invention utilizing copper styli 
where the styli are fabricated to be less than five ten thousandths of an 
inch (i.e., 0.0005") thick, a head can be fabricated that significantly 
reduces background noise on the medium on which the image is to be formed 
while using a pulsing voltage to produce a sharp, clear, dense image that 
is lower than pulsing voltages used in the prior art. 
This invention will be more fully understood in conjunction with the 
following detailed description taken together with the attached drawings:

DETAILED DESCRIPTION 
This invention will now be described in conjunction with several 
embodiments of the stylus. This description is not intended to be limiting 
but is intended to be merely illustrative of the concepts of the 
invention. 
FIG. 2 illustrates stylus 10 (one of a plurality of such styli in an 
electrostatic printing head such as disclosed for example in U.S. Pat. No. 
4,419,679 issued Dec. 6, 1983 entitled "Quadrascan Styli for Use in 
Staggered Recording Head") of round cross section with projections 11a, 
11b and 11c formed on the surface thereof. Print medium 12 is shown moving 
past surface 11 of stylus 10 to the left. When voltage is applied to 
stylus 10, it is thought that projections (such as 11a, 11b and 11c) on 
the stylus (these projections are not drawn to scale but are exaggerated 
for illustrative purposes) serve as sites for spurious discharge of 
electrons. These spurious discharges result in background noise on print 
medium 12 of the type shown in FIG. 1a in the portion labeled 
"Background." Because the electric field is higher at the edges of a 
stylus (whether round or square) spurious breakdown occurs first at the 
edges of the stylus rather than in the center of the stylus. This is well 
known in the art. This same spurious breakdown is believed to be initiated 
at any small localized stylus surface irregularity. It is my further 
belief that as the size of the irregularity decreases the probability of a 
spurious breakdown occurring will increase. However, at some point, a 
further decrease in irregularity size causes no further change in 
probability of a spurious discharge to initiate. 
I proceeded with the assumption that by making each stylus of a rectangular 
cross section and then by reducing the thickness of the stylus, ultimately 
the thickness of the stylus would approach the size of the surface 
irregularities that cause spurious discharge. As the stylus itself 
approaches a dimension equal to the dimensions of the surface 
irregularities, the average voltage for stylus discharge approaches the 
average voltage at which the irregularities discharge thereby reducing the 
backgrounding region. To verify this, I investigated discharge 
characteristics of a stylus of rectangular cross section as a function of 
its thickness. FIGS. 3, 4 and 5 illustrate certain of the results of this 
investigation. FIG. 3 illustrates part of this investigation wherein 
voltage A, increasing with time, is applied to a stylus and voltage B is 
thus induced on a moving print medium above the stylus. At time T0, 
voltage A, increasing linearly with time, is applied to the stylus. 
Voltage B on the moving print medium (typically a dielectric coated paper) 
opposite the stylus remains approximately zero until time T1. During the 
portion of time between T1 and T2 denoted by the portion of curve B in 
FIG. 3 labeled "C" , voltage B on the print medium demonstrates "noise" 
reflecting the formation of random background images on the print medium. 
This is due to spurious intermittent breakdown of the atmosphere between 
the stylus and the print medium as previously discussed. I believe this 
spurious breakdown is caused by portions of the surface of the stylus 
having high electric fields due to imperfections such as 11a, 11b and 11c 
in surface 11 of stylus 10 shown in FIG. 2. At time T2 voltage A on the 
stylus reaches the value A2. After time T2 voltage B on the print medium 
begins to increase approximately linearly with time at substantially the 
same rate as voltage A on the stylus. This is the linear portion of 
operation of the system. When voltage A on the stylus reaches A3, voltage 
B on the print medium reaches B3 and with subsequent toning the image will 
be cleanly, densely and sharply formed on the medium. 
FIG. 3 was generated as part of an experimental test to determine the point 
at which background noise on the print medium is generated as voltage is 
increased relatively slowly on a stylus. In actual operation, the stylus 
operates in a digital fashion and is either on (at which point the voltage 
on the stylus is that shown at point A3) or off (at which point the 
voltage on the stylus is typically that shown at point A4 selected to be 
beneath A1 so as to eliminate background noise entirely). As a general 
rule it is desirable to minimize the voltage swing between turning on and 
turning off a stylus. In order to accomplish this, voltage difference 
A3-A4 on the stylus should be made as small as possible. Voltage on the 
print medium corresponding to the stylus being on is that voltage shown as 
B3 and voltage on the print medium when the stylus is off is that voltage 
shown as B4 which is essentially zero. In a conventional head driven by 
conventional electronics, when a stylus is being turned "on", stylus 
voltage increases rapidly with time (a typical rise from A4 to A3 takes 1 
to 10 micro seconds) while the time during which the voltage on the stylus 
is "on" (i.e., is at point A3) is typically 80 micro seconds. The fall 
time of voltage on the stylus from point A3 to point A4 is also rapid 
(typically 1 to 10 micro seconds). Looking at FIG. 3, if the voltage on a 
stylus when the stylus is off is at a level slightly higher than the level 
depicted by A1, background noise would be generated on the print medium 
and as a result voltage B on the paper would fluctuate slightly with time 
due to the presence of voltage A on the stylus, together with the process 
of spurious discharges caused by stylus irregularities. Naturally, the off 
voltage on the stylus could be lowered to A4 such that voltage B on the 
print medium moves to voltage B4, to the left of region C, on the curve 
shown in FIG. 3 and therefore no background noise would be formed on the 
print medium. However, doing this will cause the voltage difference 
between printing and not printing (A3-A4) to increase and the drive 
circuitry will be more complex and expensive due to the larger voltage 
swing required. Indeed, even if voltage difference A3-A1 were used by the 
stylus to print, the voltage swing required to be provided by the 
electronics would include voltage difference A2-A1 which would be 
necessary to reduce background noise on the print medium and yet would not 
contribute to creating a clean, dense, sharp image on the print medium. 
Accordingly, this is undesirable. Indeed, the goal to minimize cost and 
complexity is to make the voltage swing from "printing" to "no printing" 
as small as possible An additional goal is to produce, when printing, a 
"printing" voltage B3 on the medium which provides a clean, dense and 
sharp image and to provide, when not printing a "no printing" voltage of 
zero volts on the print medium which leaves the medium free from 
background discolorations. 
This problem is complicated in the usual electrostatic print head where 
printing is controlled by a multiplexed series of drive voltages. In such 
a multiplexed system, a plurality of styli uniformly spaced from each 
other are connected together in a number of groups such that when one 
stylus of a particular group of styli is selected to print and is 
consequently pulsed, all corresponding styli in the other groups are 
likewise pulsed. Actual printing is effected by a second pulsing voltage 
being coupled to the medium via a series of backplates. This second 
pulsing voltage is applied to the medium locally and serves to establish 
the "bias" voltage in a spatially defined region around a particular group 
of styli. When a stylus in one group is energized, the corresponding styli 
in each of the other groups will similarly be energized even though 
backplates associated with the other groups of styli are not activated and 
therefore styli in the nonselected groups will not print clear, dense, 
sharp images on the print medium. If the level of pulsing voltage on the 
styli are in the range between A1 and A2 in FIG. 3, faint images called 
"ghosts" are often formed on the print medium beneath these other 
nonselected groups of styli. Ghosting was addressed in U.S. Pat. No. 
3,792,495 issued Feb. 12, 1974 on an application of Art Bliss, et al. 
Ghosts can also be formed if the electrical characteristics of the print 
medium are not correct. If resistance of the conductive layer of the print 
medium is not optimized then the locally applied "bias" voltage from the 
selected backplate will unduly spread and affect styli outside of the 
styli group selected, so as to cause a voltage difference between the 
print medium and corresponding selected styli in nonselected groups near 
the selected group to be in the range between A1 and A2. To reduce or 
eliminate this ghosting, the pulsing voltage supplied to the backplates 
can be increased and the pulsing voltage applied to the styli being used 
to deposit charge on the medium reduced to below voltage A1 in FIG. 3. 
Unfortunately, the increase of pulsing voltage on the backplate adjacent 
the group of selected styli will cause a voltage difference between styli 
and the print medium such that those styli in the selected group which are 
selected not to print on the print medium will now experience a voltage 
difference between the print medium and themselves in the region between 
A1 and A2 thereby causing backgrounding under those nonselected styli. 
This trade off is well known in the art. 
As will be seen from a further description of this invention, using 
beryllium copper or another material of equivalent properties for the 
styli will allow an electrostatic print head to be constructed which will 
allow a wider range of stylus voltages and backplate voltages to be used 
in the prior art multiplexed print system, while at the same time 
eliminating and minimizing the above-described ghosting and spurious 
writing. Such a prior art multiplexed structure is shown in FIG. 8. 
In FIG. 8, four driver circuits D1 through D4 are utilized to drive each of 
four sets 71, 72, 73 and 74 of styli which together comprise the styli in 
one electrostatic print head. Five additional driver circuits P1 through 
P5 are shown schematically to drive respectively field plates (or 
backplates) 75-1, 75-2, 75-3, 75-4 and 75-5 which contact the conductive 
back side of print medium 76. The print medium is of a type that is well 
known in the art of electrostatic printing. Field plates 75-1 through 75-5 
are shown on the back side of print medium 76, but this is not essential. 
As is well known in the art, plates 75-1 through 75-5 could be placed on 
the same side of print medium 76 as the styli and the potentials on plates 
75-1 through 75-5 would then be capacitively coupled into print medium 76 
through the dielectric layer to the conductive layer on the back side of 
print medium 76. In operation, when a selected stylus, such as stylus 
71-1, is to print, the group 71 styli (containing stylus 71-1, 71-2, 71-3, 
71-4 and 71-5) in which stylus 71-1 is located are driven to a selected 
drive voltage E3 (typically -275 V) and backplate 75-1 is then driven to a 
voltage E4 (typically +275V). Thus the voltage difference between the 
styli in group 71 and the conductive layer of print medium 76 in the 
region near backplate 75-1 is such as to deposit charge on the print 
medium adjacent to stylus 71-1. Should other styli in group 71 be desired 
to print, then after backplate 75-1 has been returned to zero backplates 
opposite these other styli must be activated in sequence from backplate 
75-2 through backplate 75-5 to cause selected styli in group 71 adjacent 
these four backplates to print. In practice, all styli adjacent a given 
backplate, such as backplate 75-1, which are selected to print on print 
medium 76 are driven by their appropriate driver circuits simultaneously 
such that when backplate 75-1 is activated and raised to voltage E4, all 
of styli 71-1, 72-1, 73-1 and 74-1 adjacent backplate 75-1 which are 
selected to print in fact do so. Subsequently, selected styli in groups 71 
through 74 adjacent backplate 75-2 are simultaneously activated such that 
when backplate 75-2 is raised to voltage E4, those styli which are 
selected to print actually do so. The remainder of the styli adjacent the 
remaining backplates are similarly activated and the selected styli are 
driven to print in coordination with the sequential activation of 
backplates 75-3, 75-4 and 75-5. 
Experiments were run to determine the effect of thickness of a stylus on 
voltages A1 (shown in FIG. 3 as the voltage on a stylus at which 
background noise on the print medium begins), voltage A2 (the voltage on a 
stylus at which charge begins to be linearly deposited on the print medium 
in response to an increase in voltage on the stylus) and voltage A3 (the 
voltage on a stylus at which the stylus is normally operated to deposit 
charge on a print medium such that the subsequent application of toner 
produces a crisp, dense, clear image on the print medium). Note that 
voltage A2 achieves essentially zero voltage (B2) on the print medium and 
thus theoretically a zero image on the print medium but any subsequent 
increase in voltage on the stylus above voltage A2 produces a 
corresponding increase in charge on the print medium and thus an increase 
in image density on the print medium. 
Turning now to FIG. 4, the advantage of reducing the thickness of styli 
used in electrostatic print heads in accordance with this invention is 
conceptually shown. FIG. 4 shows that by reducing stylus thickness, the 
voltage difference between A3 (the voltage on the stylus at which a sharp, 
dense clean image is formed) and A1 (the voltage on the stylus when no 
background image is to be formed) is reduced. The lower curve A1 
corresponds to the voltage on a stylus as a function of stylus thickness 
at which background noise begins to appear on the print medium. As stylus 
thickness decreases below point F1 on FIG. 4, threshold voltage A1 for 
backgrounding increases The curve labeled A3 corresponds to the voltage on 
the stylus at which the desired image density is formed on the print 
medium. This voltage A3 is relatively constant FIG. 4 shows that by 
reducing thickness of the stylus, the difference between these voltages 
(A1 and A3) is significantly reduced. The curve A2 in FIG. 4 represents 
voltage on the stylus at which charge begins to be deposited linearly upon 
the print medium. At point D, the two voltages A1 and A2 become 
approximately the same. At stylus thickness D1 corresponding to point D, 
it is assumed that the thickness of the stylus is of a dimension on the 
order of the size of the projections that cause spurious charge transfer. 
Thus for a stylus of thickness D1 or less, substantially no spurious 
streaks or undesired marks are caused on paper for stylus voltage A (FIG. 
3) slightly below the threshold (A2) for linear printing. At point D, 
voltage A2 is substantially equal to voltage A1. Consequently, voltage 
difference A3-A1 will be minimized. In one embodiment, a copper stylus was 
utilized. Stylus thickness F1 at which the two curves A1 and A2 on the 
graph in FIG. 4 start to come together (point F on curve A1) was 0.0005 
inches. Therefore, a thickness of styli less than about 0.0005 inches 
results in a reduction of voltage difference A3-A1 required to produce 
clean, dense, sharp printing on the one hand, and produce a print medium 
that is free from unwanted spurious discolorations on the other hand. 
EXPERIMENT 1 
FIG. 7 illustrates schematically the experimental structure employed to 
determine the effect of thickness of styli on the image formed on the 
print medium. 
A 0.0005 inch thick copper film 72 (see FIG. 7) was formed on Printed 
Circuit (PC) board made of electrically insulating substrate 71. The 
laminated PC board-copper structure was then mounted between thick 
glass-epoxy laminate such that regions 73a and 73b of the glass-epoxy 
material were adjacent to the copper-PC board laminated structure. Top 
surface 74 of the composite glass epoxy-copper-substrate laminate 
containing stylus 72a was then rounded to give a radius of approximately 
two to three inches (the radius was not crucial to the experiment). A 
print medium 75 (dielectric paper, type 2089, supplied by James River 
Graphics, South Hadley, Ma., was then passed over region 74 which 
contained stylus 72a (the end surface of conductive material 72) as shown 
in FIG. 7. Voltages A1, A2 and A3 defined above and shown in FIGS. 3 and 4 
were then determined. This was done by linearly increasing stylus voltage 
in a negative direction using a Wavetek function generator (Model 801, 
supplied by Wavetek, San Diego, Calif.) connected to a TREK high voltage 
Op Amp (Model 601 supplied by Trek Inc. Medina, N.Y.) from a nominal 
voltage (sometimes zero but sometimes a voltage other than zero such as 
-200 volts) at a rate of about -100 volts per minute while passing 
dielectric paper past and adjacent to the stylus at a rate of one half 
inch per second. The test structure used was installed in a machine such 
as described in U.S. Pat. No. 4,731,542 issued Mar. 15, 1988 on an 
application of David Doggett. The back of the print medium was contacted 
by a conductive fabric available from Schlegel Corp., Rochester, N.Y. that 
also served to press the medium against the stylus test structure so as to 
supply a normal force on the medium and cause the medium to maintain an 
average distance of 10 microns from the stylus. This is well known in the 
art. Voltage B on the dielectric paper was measured by an electrostatic 
voltmeter (TREK model 565), in a manner well known in the art. Voltage B 
was then plotted as a function of time, together with voltage A on the 
stylus. Results are shown in FIG. 3. For the 0.0005 inch thick copper 
stylus (approximately 0.5 inches wide) voltage A1 was about 235 volts and 
voltage A2 was about 360 volts. Thus the difference between voltage A1, at 
which backgrounding becomes apparent on the dielectric paper, and voltage 
A2, at which charge becomes linearly deposited on the paper, is about 125 
volts. Consequently, to avoid backgrounding, voltage A on the stylus must 
be dropped beneath 235 volts (voltage A1) when the stylus is "off" and 
must be raised above 360 volts (voltage A2) by an additional voltage 
necessary to produce dense printing. The additional voltage necessary to 
produce dense printing is typically 150 volts. Voltage A3 necessary to 
produce dense printing is shown in FIG. 3 and corresponds to voltage B3 on 
the print medium. Consequently, a voltage swing of about 275 volts is 
required on the stylus to go from a state of no printing on the print 
medium to a state of dense, acceptable printing on the print medium. Such 
a large excursion requires expensive electronics. 
In addition, an electrostatic print head contains a plurality of styli, 
each stylus being closely spaced and separated by insulative material from 
the directly adjacent stylus or styli. The requirement that one stylus 
have a voltage such as A3 impressed upon it, for it to print as shown in 
FIG. 3, and that the directly adjacent stylus be at a voltage such as A1 
or beneath, in order to eliminate backgrounding, creates a large voltage 
difference between the two adjacent styli. Voltage difference A3-A1 can 
cause a current flow between the two directly adjacent styli. This is 
shown in FIG. 9. In FIG. 9 styli 90 has impressed upon it voltage 
substantially equal to A1 (see FIG. 3) such that stylus 90 does not print 
to medium 95. Stylus 91 has impressed upon it voltage that is 
substantially equal to A3 such that it will transfer sufficient charge to 
medium 95 to produce a dense, clean, sharp image. Voltage difference A3-A1 
causes current to flow between styli 91 and 90 in the presence of the 
excited atmosphere caused by the discharge between stylus 91 and medium 
95. This atmosphere is trapped between electrostatic printer head 96 and 
medium 95. Because of the magnitude of voltage difference A3-A1, the 
resulting current that flows from stylus 91 to stylus 90 is of a level 
that can degrade insulating material 98 and produce an area 94 of pitting 
between styli 91 and 90 such that performance of the print head 96 is 
adversely affected. In response to the decrease in performance of 
electrostatic print heads caused by pitting, styli of the prior art have 
of necessity been placed farther apart so that resulting increased 
distance will decrease unwanted current flow and thereby increase lifetime 
of the print head. The requirement of increased distance serves to limit 
resolution that can be achieved in a print head and also serves to limit 
spot size produced by a stylus of the prior art so as to create areas of 
white between the toned images of two adjacent styli that are selected to 
print thereby limiting the density of the image produced. This effect is 
described in U.S. Pat. Nos. 3,798,609 and 3,157,456. In FIG. 10, several 
rows of styli (101-104) are drawn to illustrate the effect of increased 
distance between styli on the resolution attainable with styli of the 
prior art. In a low resolution printer, styli 101 are large and distance d 
between styli is a small percentage of the total area that would be 
printed if, for instance, two adjacent styli printed simultaneously. In a 
higher resolution printer, styli need to maintain the same distance d 
between them in order to limit unwanted current between styli, but because 
the print head has a higher number of styli for a given head length (i.e., 
higher resolution) the area between styli becomes a larger percentage of 
the total printed area and the consequent print density will decrease. 
This effect is clearly seen in styli 103 where the resolution desired is 
closely related to distance d required between styli to reduce unwanted 
current. Indeed, this problem has led prior art print heads to be 
constructed with two, three and even four rows of styli in order to 
produce a print head that simultaneously offers high resolution and dense 
printing of defined images together with long print head life. 
A styli configuration such as 104 can achieve high resolution while 
maintaining a large separation between adjacent styli, but such a head 
would require accurate matching of rows of styli during manufacture and 
would require storing (buffering) of information between the two rows. 
This is well known in the art. Considering that each styli is of the order 
of 0.0025 inch and the length of a print head could be 36 inches or even 
greater, the difficulty of manufacturing a head with matched rows of styli 
is considerable. Such difficulties are mentioned in copending application 
entitled "Improved Electrostatic Printhead" filed on the same day as this 
application on an invention of Doggett, Mitchard and Dahlquist. 
Styli of the present invention allow a head to be constructed where the 
distance between styli can be a smaller number than styli of the prior art 
thereby allowing a head of higher resolution to be produced with a single 
row or with multiple rows of styli while still producing a print of high 
optical density. This is accomplished by reducing or eliminating 
background region C (see FIG. 3). Reduction of backgrounding region C with 
the consequent reduction of voltage difference A2 minus A1 (see FIGS. 3 
and 4) would reduce the required voltage difference between adjacent styli 
and would therefore lower unwanted current flow between adjacent styli 
which would increase lifetime of insulating material between styli, 
thereby increasing lifetime of the print head. Therefore, it is desirable 
that a change in voltage on a stylus from a state of no printing to a 
state of printing be made as small as possible. In accordance with this 
invention I have discovered that it is possible to significantly reduce 
the voltage difference between styli by making a stylus much thinner than 
heretofore thought appropriate in the art. 
EXPERIMENT 2 
My second experiment was conducted using a 1500 angstrom thick stylus 
consisting of essentially copper attached to a Kapton film substrate by a 
thin chrome adherent layer. 
In this experiment conductive layer 72 (FIG. 7) was comprised of a thin 
layer of chromium (thickness of this layer is not known exactly but is 
believed to be around 50 to 100 angstroms) upon which was formed a thicker 
layer of copper (thickness of copper was such that total thickness of the 
composite layer is believed to be about 1500 angstroms). The chrome 
underlayer between copper and Kapton was required because copper does not 
adhere well to Kapton but chrome does. Copper, on the other hand, adheres 
well to chrome. Formed on both sides of the resulting laminate of Kapton, 
chrome and copper were glass epoxy composite materials 73a and 73b. This 
glass epoxy material 73a, 73b is commonly obtained from printed circuit 
board manufacturers and suppliers. Thickness of this material on each side 
of the styli was nominally about three-eighths (i.e., 3/8") inch. Ends 71a 
and 72a of the Kapton and conductive layer respectively, which form the 
test stylus and backing laminate, protrude from glass epoxy materials 73a 
and 73b upon initial lamination. These ends and the adjacent glass epoxy 
materials subsequently were smoothed and polished so that a radius of 
about two to three inches was formed at top surface 74 of the structure. 
This radius allowed the medium (typically dielectrically coated paper of 
the same manufacture used in Experiment 1) to easily pass over surface 74 
and the experimental stylus 72a. Connection was made to conductive layer 
72 by means of an alligator clip as in Experiment 1 and voltage A was 
supplied to layer 72 and was varied as in Experiment 1 while dielectric 
paper 75 was moved past the stylus. Dielectric paper 75 was held an 
average distance of about ten (10) microns from stylus 72 by either spacer 
particles in the surface of the paper or by the average roughness of the 
paper stock as is well known in the art of making paper for use in 
electrostatic printers. Voltage A1, A2 and A3 (FIGS. 3 and 4) were 
determined. Voltages A1 and A2 coincided and were about 385 volts as shown 
in FIG. 5 by point 2 on the graph. Voltage A3 was about 535 volts. The 
voltage difference A3-A1 was about 150 volts. 
Thus, the threshold for linear deposition of charge on the media (such that 
an increase in voltage on stylus 72 (FIG. 7) would increase linearly the 
charge deposited on print media 75) was equal to the threshold for 
background noise formation on the print media. Consequently, there was in 
fact no substantial backgrounding voltage region (corresponding to the 
region C on curve B in FIG. 3) and thus no background noise on the media 
below stylus voltage A2 where a linear increase in charge transfer begins. 
Thus region C (FIG. 3), where backgrounding occurs on the media, (i.e., 
where there is not a linear relationship between an increase in voltage on 
stylus 72 and an increase in charge deposited on media 75) has been 
eliminated. Note that while charge is deposited linearly on the print 
medium in response to an increase in stylus voltage from A2 to A3 in FIG. 
3, toned areas created by stylus voltages between A2 and A3 result in 
images formed on the media which are of poorer quality than images formed 
with a stylus voltage substantially equal to A3. In practice, when a 
stylus image is desired to be printed on the media, voltage on the stylus 
is rapidly raised to voltage A3 to ensure that charge deposited on the 
print medium is sufficient to give a good quality image. Several other 
thicknesses of copper were investigated and resulting voltages A1 and A2 
are graphed in FIG. 5. This study indicates that styli thinner than 12,000 
angstroms would exhibit little additional reduction in the voltage 
difference A2-A1. 
EXPERIMENT 3 
A 0.00017 inch thick titanium stylus was fabricated on Kapton using first 
an intermediary adherent layer believed to be chrome. The resulting 
structure is identical to that shown in FIG. 7 with titanium electrode 
material 72 being substituted for copper. A dielectric paper 75 was passed 
over top surface 72a of stylus 72. The experiment yielded voltage A2 of 
about 365 volts and voltage A1 of 300 volts. These results were similar to 
the results obtained for a 0.00017 inch thick copper stylus. (See FIG. 5). 
As a practical matter, thin styli (on the order of 12,000 angstroms) are 
difficult to work with and manufacture. Therefore, I have determined that 
one can achieve a reduction in backgrounding region C (FIG. 3) and at the 
same time use thicker styli than 12,000 angstroms shown in FIG. 5 in order 
to reduce voltage difference A2-A1. My discovery allows an engineer to 
pick the thickness of the styli to be compatible with the amount of 
backgrounding which is acceptable in the system being developed and the 
complexity and cost of the electronics allowed. As shown by FIG. 5, the 
magnitude of voltage difference A2-A1 that can be tolerated can be 
identified and then the thickness of the styli required to obtain that 
voltage difference can be determined. Accordingly, great flexibility in 
designing a system is achieved using my invention by allowing the designer 
to select both the amount of backgrounding which is acceptable and the 
stylus-to-stylus voltage difference allowable in the system and then 
determine the appropriate stylus thickness to achieve that result. Among 
factors which should be considered in determining stylus thickness will 
be; cost of fabricating the electrostatic print head, cost of the 
electronic circuitry used to drive the styli in the print head, desired 
life of the print head and quality of the images formed on the medium. 
Cost of the system includes a measure of useful life of the system which 
in turn is dependent upon maximum voltage between adjacent styli. This 
stylus to stylus voltage determines the lifetime of the electrostatic 
print head because degradation of dielectric material between adjacent 
styli is a direct function of voltage difference between adjacent styli. 
Although voltage A1 shown in FIGS. 3, 4 and 5 is a fairly substantial 
voltage (this voltage is in the range of -200 to -400 volts) this voltage 
can be applied to all styli and styli pulsing electronics so that the 
voltage difference between styli represents only the difference between A1 
and A3 and not the absolute magnitude of the voltage applied to the styli. 
Indeed, either the entire circuitry can be biased up to voltage A1 or 
alternatively the print medium can be biased up to voltage level A1 by use 
of a biased backplate. 
While styli made of titanium and styli made of copper seem to give results 
which are totally compatible, an additional test on a stylus believed to 
be at least a substantial part zinc and on another stylus of aluminum gave 
different results. In fact, zinc, and aluminum styli gave results which 
were not compatible with results obtained by copper and titanium. However, 
enough tests have not been run yet on these materials (i.e., zinc, and 
aluminum) to determine whether the general shape of curves obtained using 
these materials as styli would in fact be similar to the shape of curves 
obtained using copper or titanium styli. 
Further tests were conducted using beryllium and beryllium-copper styli. 
While complete experimental results for a wide range of thicknesses have 
not been obtained using beryllium or beryllium copper styli, FIG. 5 shows 
the difference between voltages A2 and A1 for a 0.0003 inch beryllium 
stylus. As shown in FIG. 5, this difference is approximately 55 volts. 
Thus, electronic circuitry for use 
with an electrostatic print head containing styli made of 0.0003 inch 
beryllium would be such as to required a voltage swing between an off and 
an on condition of about 205 volts if 150 volts are desired on the surface 
of the dielectric media prior to toning. Such circuitry is less expensive 
than circuitry used with prior art electrostatic print heads requiring 275 
volts. Therefore, accepting voltage difference A2-A1 of about 55 volts 
allows the use of a 0.0003 inch beryllium styli in an electrostatic print 
head and achieves the results contemplated by this invention but with a 
thicker stylus than would be required if copper was used as a stylus 
material. Unfortunately beryllium is quite expensive. 
A 0.0005 inch beryllium-copper stylus and 0.002 inch beryllium-copper 
stylus likewise achieved lower values for voltage difference A2-A1 than 
would be achieved by the use of pure copper. Accordingly, beryllium-copper 
styli can be used in accordance with the principles of this invention to 
form part of an electrostatic print head. By discovering that voltage 
difference A2-A1 associated with beryllium-copper is lower than for copper 
of similar thickness and yields relatively low cost electronic drive 
circuitry, I have discovered that beryllium-copper is an appropriate 
material for use as styli in electrostatic print heads in accordance with 
my invention. Indeed, the results from a stylus of 0.002 inch 
Beryllium-copper stylus indicate that a superior electrostatic print head 
could be constructed even if the styli were thick enough to be virtually 
square. The results from 0.002 inch Beryllium-copper also suggest that an 
electrostatic print head constructed by winding round wire on a mandrel 
would also produce superior print quality. 
While experimental work with respect to use of a copper, titanium, 
aluminum, beryllium or beryllium-copper stylus is as described above, 
those skilled in the art will recognize in view of this disclosure that 
different materials and different thicknesses styli can also be 
implemented in accordance with my discovery.