U.S. patent number 3,778,690 [Application Number 05/235,346] was granted by the patent office on 1973-12-11 for electrostatic copying machine.
This patent grant is currently assigned to Copy Research Corporation. Invention is credited to David P. Bujese, Francis N. Rothacker.
United States Patent |
3,778,690 |
Rothacker , et al. |
December 11, 1973 |
ELECTROSTATIC COPYING MACHINE
Abstract
In a copying machine employing two conductive rollers to which
zinc oxide coated copy paper is fed, the power source provides a DC
voltage between the rollers that is derived from a frequency (of 25
kilocycles, for example) which is relatively high compared to line
frequency. The relatively high frequency ripple that is on the DC
output eliminates much of the problem with discernible lines on the
end product. One of the two rollers has a rubber sleeve which is
selected to provide a low capacitance such as 75 micro-microfarads.
The resistance of the sleeve is such that a current flow of 50
microamperes is obtained with a 3,000 volt potential between the
two rollers. The low capacitance means that when a sheet of copy
paper is fed between the rollers, the change in the potential
across the rubber sleeve and the change in current flow will occur
very rapidly so that an even charge is applied on the copy paper.
Because of the relatively high frequency employed, the ripple
factor is kept low and the voltage magnitude applied is kept down
to a point where the amount of ozone generated is minimized.
Inventors: |
Rothacker; Francis N.
(Valhalla, NY), Bujese; David P. (Toms River, NJ) |
Assignee: |
Copy Research Corporation
(Elizabeth, NJ)
|
Family
ID: |
22885114 |
Appl.
No.: |
05/235,346 |
Filed: |
March 16, 1972 |
Current U.S.
Class: |
361/225; 361/235;
250/325 |
Current CPC
Class: |
G03G
15/0233 (20130101) |
Current International
Class: |
G03G
15/02 (20060101); G03g 015/02 () |
Field of
Search: |
;317/262A
;256/49.5ZC |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Miller; J. D.
Assistant Examiner: Moose, Jr.; Harry E.
Claims
What is claimed is:
1. In a two-roller mechanism for applying a charge to coated copy
paper wherein the copy paper passes between a first conductive
roller and a second conductive roller having a di-electric sleeve
thereon, the improvement comprising,
pressure means to apply pressure between said first roller and said
sleeve,
the material of said sleeve being such that substantial area
contact is established between said sleeve and whatever paper is
fed between said sleeve and said first roller under said pressure
established by said pressure means,
said area of contact being sufficiently great to provide a current
path that substantially eliminates the generation of ozone,
the capacitance and resistance of said sleeve when positioned
between the rollers and when subject to said pressure providing an
electrical time constant sufficiently short in duration that the
introduction of copy paper between the rollers results in a
sufficiently rapid adjustment of the voltage division between the
sleeve and the copy paper that the charge density in the leading
portion of the copy paper is substantially equal in magnitude to
the charge density in the trailing portion of the copy paper,
said sleeve being mounted on said second roller such that said
second roller applies a uniform potential at any one moment to the
surface of said sleeve in contact with said second roller.
2. The improvement of claim 1 wherein said time constant is no
greater than 5 milli-seconds.
3. The improvement of claim 1 wherein said resistance of said
di-electric sleeve is great enough to limit the DC current level
between said rollers to no more than 60 microamperes.
4. The improvement of claim 2 wherein said resistance of said
di-electric sleeve is great enough to limit the DC current level
between said rollers to no more than 60 microamperes.
5. The improvement of claim 1 wherein the material of said sleeve
is a butadiene-acrylonitrile.
6. The improvement of claim 2 wherein the material of said sleeve
is a butadiene-acrylonitrile.
7. The improvement of claim 3 wherein the material of said sleeve
is a butadiene-acrylonitrile.
8. The improvement of claim 4 wherein the material of said sleeve
is a butadiene-acrylonitrile.
9. The improvement of claim 1 further comprising:
a DC power supply having its output across the rollers and having a
relatively high frequency ripple current on its DC output,
the relationship between
a. the output capacity of said power supply,
b. the capacity of the di-electric sleeve when subject to said
pressure and
c. the amplitude and frequency of the ripple providing a
substantially even charge density along the surface of copy paper
introduced between the rollers.
10. The improvement of claim 9 wherein the frequency of said ripple
current is at least 10 kilohertz.
11. The improvement of claim 10 wherein the magnitude of the ripple
is less than 5 percent of the DC voltage level.
12. The improvement of claim 9 wherein said time constant is no
greater than 5 milli-seconds.
13. The improvement of claim 12 wherein said resistance of said
di-electric sleeve is great enough to limit the DC current level
between said rollers to no more than 60 microamperes.
14. The improvement of claim 13 wherein the frequency of said
ripple current is at least 10 kilohertz.
15. The improvement of claim 14 wherein the magnitude of the ripple
is less than 5 percent of the DC voltage level.
16. The improvement of claim 13 wherein the material of said sleeve
is a butadiene-acrylonitrile.
17. The improvement of claim 15 wherein the material of said sleeve
is a butadiene-acrylonitrile.
Description
BACKGROUND AND OBJECT OF THE INVENTION
This invention relates in general to electrostatic copying machines
and more particularly to an improvement in the manner in which an
electric charge is placed on the copy paper.
Most commonly employed techniques of applying an electrostatic
charge on zinc oxide coated copy paper in electrostatic office
copying machines is through the use of what is called a corona.
This corona is essentially two sets of spaced apart parallel wires
across which 15,000 volts are applied. The paper is passed between
the two sets of wires and a discharge across the gap between the
two sets of wires carries an electric charge that is picked up in
part by the paper. The discharge is of a sort that is known as a
corona discharge. Thus, this part of the equipment has been termed
the corona.
Among the disadvantages of the corona are the generation of
appreciable amounts of ozone; the high voltages required; the high
power required; the variability of charge applied as a function of
ambient conditions and in particular as a function of humidity; and
the substantial amount of servicing required for the corona. For
all of these reasons and further in order to provide a smaller,
lighter and less expensive office copying machine, it has been
proposed to substitute for the corona two conductive rollers, one
of which has an insulating sleeve on it. These rollers are in
contact with one another and copy paper is fed through the rollers.
A voltage between the two rollers results in a small current flow
across the high resistance sleeve, thereby applying an
electrostatic charge to the paper passed between the rollers. U.S.
Pat. No. 2,980,834 issued in April of 1961 to Tregay and U.S. Pat.
No. 3,521,126 issued in July of 1970 to Granzow et al both
illustrate this two-roller type of design. Although the basic
two-roller design has been known for over a decade, it has not
achieved any widespread application because this design, although
it solves many of the problems of the corona design, has certain
problems of its own.
For example, the two roller design has required a sufficiently high
voltage that some ozone is generated. Although the rate of ozone
generation may be considerably less than when a corona is employed,
it is sufficient to deteriorate the high resistance, rubber-type
sleeve employed on one of the two rollers. Furthermore, the even
and repeatable application of a charge across the entire sheet of
copy paper is a problem when a two-roller design is employed
particularly when relatively high copy paper feeding speeds are
employed.
Accordingly, it is a purpose of this invention to provide an
improved two-roller design from which superior performance will be
obtained, and by which improvements will be achieved in the areas
of cost, weight, evenness with which charge is applied,
repeatability of charge application and limiting of ozone
generation. More particularly, it is a purpose of this invention to
provide a design in which lower voltages and lower currents are
required and in which a much more efficient use of the power supply
is effected.
It is an important purpose of this invention to obtain these
particular results while at the same time providing a technique for
applying a charge to copy paper whereby the results will be
repeatable from sheet to sheet and the result will be uniform so
far as density and contrast are concerned.
BRIEF DESCRIPTION OF THE INVENTION
In brief, this invention is an improvement in a two-roller type of
copier. The di-electric sleeve on one of the two rollers is
selected to have a capacitance and resistance that will provide a
sufficiently low time constant that when paper is introduced
between the rollers, the change in the voltage distribution across
the di-electric sleeve will occur quickly and will thus avoid a
gradually varying magnitude of charge laid down on the copy paper.
Yet, the resistance of the sleeve is high enough that a very low
level of DC current is provided. The DC power supply has a high
frequency ripple that permits easy filtering so that the amplitude
of the ripple can be kept at a low level. The frequency of the
ripple is sufficiently high that a series of discernable lines in
the end product, that are a result of a varying level of charge
laid down on the copy paper, are avoided.
Pressure adjustment between the rollers together with proper
selection of the material for the di-electric sleeve on one of the
rollers in the context of a high frequency, low level ripple on the
output of the DC power supply makes possible a long life copying
machine with minimum sleeve degradation that provides a uniform and
repeatable product at a reasonably high rate of speed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective, fragmentary view, illustrating copy paper
feeding through two rollers, one of which is provided with a rubber
sleeve.
FIG. 2 is an elevation view of one embodiment of this
invention.
FIG. 3 is a left side view of FIG. 2.
FIG. 4 is a right side view of FIG. 2.
FIG. 5 is a longitudinal cross sectional view of FIG. 2.
FIG. 6 is a schematic representation of the invention.
FIG. 7 is an electrical schematic of the power supply.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The drawings all relate to the same embodiment. Basically, this
invention involves, as shown in FIG. 1, passing copy paper 10
between two rollers 12, 14. The machine can be designed to employ
either sheet or roll fed copy paper. The rollers 12 and 14 are
electrically conductive, preferably made of metal. The roller 14 is
surrounded by a high resistivity di-electric sleeve 16 which in one
embodiment is a one-sixteenth inch thick, 10 inch long, rubber
sheet. Two springs 18 bias the two rollers 12, 14 toward one
another so that when in operation, the sheet 10 is in physical
contact with the periphery of the roller 12 and the sleeve 16.
A DC power supply 20 provides a positive voltage to the roller 12
and a negative voltage to the roller 14. For convenience, safety
and to maintain minimum charge on the paper 10, the roller 12 is
preferably held at zero potential and the roller 14 has a negative
potential. What counts is that the rollers 12, 14 have a DC
potential between them. As a consequence when copy paper is passed
between the rollers, a small leakage current flows between the two
rollers through the high resistance rubber sleeve 16.
In one embodiment with the DC power supply providing a voltage of
approximately 3,000 volts, the amount of current flow is in the
range of 50 microamperes. In that embodiment, the paper 10 is a
standard zinc oxide coated copy paper in which the coated surface
10c is in contact with the rubber sleeve 16 of the negative roller
14. In that embodiment, the rubber sleeve is Buna N with a
thickness of approximately one-sixteenth of an inch and provides a
very high resistance and a low capacitance between the roller 12
and the roller 14. The resulting 50 microampere leakage current
provides an adequate electron supply to negatively charge the
coated surface 10c of the copy paper 10. Thus, as contrasted with
the use of a corona, this invention not only substantially reduces
the voltage level required but also greatly reduces the amount of
current required.
The embodiment illustrated in the Figures, with reference to an
actual reduction to practice of the present invention, shows a 3/4
inch diameter metal roller 12 and a 10/16th inch diameter metal
roller 14, both mounted for rotational movement about their
respective axes in bearings 21a, 21b, respectively. The bearings
21a are directly mounted in electrical insulating end plates 22R,
22L which in turn are connected to supporting framework (not
shown). The bearings 21b, which support the lower roller 14, float
in the end plates 22R, 22L and are supported by the springs 18. The
ends of each spring 18 are connected to respective projections 22p
of the end plates 22L, 22R. Thus, the springs 18 hold the roller 14
and tend to cause the rubber sleeve 16 to bear against the roller
12.
A gear 26 on a shaft extension 28 of the upper roller 12 provides
coupling to a mechanical drive mechanism (not shown) so that the
roller 12 can be rotated. Because of the frictional contact, due to
the rubber sleeve 16 and copy paper 10, rotation of the roller 12
will result in rotation of the sleeve 16 and thus rotation of the
roller 14. Thus it may be seen that the rubber sleeve 16 provides
not only the electrical function of limiting the current flow
between rollers 12 and 14, but also traction that aids in pulling
the copy paper 10 between the two rollers 12, 14. This rubber
sleeve 16 also provides a cushioning contact against the coated
surface 10c of the paper 10 to avoid damage to the coating.
The DC power supply 20 has its positive terminal connected through
a lead 30 and terminal 32 to the right bushing 21a which is in
contact with the upper roller. Similarly, the negative terminal of
the DC power supply 20 is connected through lead 34 and contact 36
to the right bushing 21a, which bushing is in contact with the
lower metal roller 14.
Certain features of the embodiment illustrated are preferred
relationships in order to provide an optimum result. For example,
it is important that the coated surface 10c of the copy paper 10
face toward the negative roller 14 so that the coated surface will
receive an electron or negative charge. This is the polarity of the
charge that is required in order to operate properly with the
standard copying machines for which this invention is an
improvement.
Furthermore, it is desirable that the roller with which the copy
paper 10 is in direct contact (that is, the roller 12) electrically
floats relative to the chassis so that the copy paper 10 will not
become so charged relative to the chassis as to be difficult to
handle. With the known copiers, this background voltage level for
the copy paper 10 will mean a minimum background shading of the
copy paper 10 when it is inked.
FIG. 6 is a schematic representation of how the invention operates
and provides an explanation of some of the more significant
operating aspects of the invention that result in the objectives
stated above.
The power supply 20 provides 3,000 volts which is applied to the
inner surface of the di-electric sleeve 16. The di-electric sleeve
16 presents a resistive parameter R and a capacitive parameter C to
the circuit. The outer part of the di-electric sleeve is in direct
contact with the coated upper surface 10c of the paper 10. The
lower surface 10b of the paper 10 is in direct electrical contact,
through the roller 12, with the other side of the power supply 20.
Preferably, the power supply 20 and roller 12, 14 combination is
electrically substantially isolated from the rest of the
chassis.
The capacitive and resistive parameters of the sleeve 16 are
represented as a resistor R and capacitor C in parallel between the
negative terminal of the power supply 20 and the upper surface 10c
of the paper 10. Assuming an ideal DC power supply 20 (and thus no
ripple) there will be a current flow to and from the two surfaces
of the paper 10 in much the same fashion as there is when the
plates of a battery are being charged from a power supply. The DC
current will flow through the resistor R from the top surface 10c
to charge the top surface 10c negative and will flow to the lower
surface 10b from the power supply 20 to charge the lower surface
10b positive. As the copy paper 10 is moved, as to the right, the
portion of the surfaces 10c and 10b in contact with the di-electric
sleeve and lower roller, respectively, continuously changes and new
charge is laid down on the paper so that there is a continuous
current flow.
In one embodiment, the DC current flow is 50 microamperes and the
voltage at the positive surface of the paper 10 is approximately
500 volts. Thus there is a 2,500 volt drop across the sleeve 16 and
the resistance of the sleeve 16 is 50 megohms.
When the paper 10 is not between the rollers, the capacitor C
becomes effectively charged to a voltage level of 3,000 volts. When
a sheet of copy paper 10 is introduced between the rollers, that
sheet of paper is equivalent to applying another capacitor in
series with the capacitance C. As a result, the charge and thus the
voltage drop is divided between the capacitance C and the effective
capacitance of the paper 10. The transient conditions that will
exist until a stable charging condition is obtained will produce a
non-uniform charge on the paper 14. Thus it is important that the
value of the capacitance C be as low as possible so that it can
discharge from 3,000 volts across the sleeve to 2,500 volts across
the sleeve as rapidly as possible. Because the sleeve in one
embodiment has a 75 micro-microfarad capacitance parameter, this
discharge occurs very rapidly and permits rapid movement of the
paper through the rollers while applying a uniform charge on the
paper. A linear speed of 275 inches per minute is employed in this
embodiment.
In one embodiment the rubber sleeve 16 is molded onto the roller 14
from a butadiene acrylonitrile rubber, commonly known as Buna N, a
material which may be purchased from the Dyco Rubber Company of
Belleville, N.J. However, a number of materials can be employed as
long as they provide the required resistive and capacitive
properties. It is preferred that the sleeve 16 be resilient so that
a surface rather than a line contact between rollers is established
and so that the tension of the spring 18 can be adjusted to obtain
the amount of area contact which will provide the required current
flow. In addition, a resilient sleeve 16 has the advantage of
providing traction to assure proper feed through of the copy paper
10.
The DC power supply 20 is preferably variable to some extent so
that once a machine has been set up and adjusted, the voltage can
be varried to adjust the current flow and thus adjust the charge on
the copy paper 10 to affect the intensity of the image.
The sleeve 16 resistance is preferably in order of magnitude
greater than the copy paper 10 used so that most of the voltage
drop is across the sleeve 16 and it is the resistance of the sleeve
16 that primarily controls charging current magnitude.
Power Supply (FIG. 7)
The nature of the power supply 20 is an important factor in the
proper operation of this invention. FIG. 7 illustrates one
embodiment of a power supply that constitutes part of this
invention.
Broadly, the FIG. 7 power supply converts a 50 or 60 Herz, 110 volt
AC line input to a 3,000 volt DC output that has a relatively low
level of ripple (2 percent) at a relatively high frequency
(25KHz.). Furthermore, the FIG. 7 design for a power supply
provides an impedance output (specifically, sufficiently low
capacitance) from power supply to the roller system so that
meaningful efficiency is obtained.
The 100 ohm variable resistor R1 is a dropping resistor that
provides the ability to manually adjust the output voltage level.
The diode D1, resistor R2 and capacitor C1 constitute a rectifying
circuit. The capacitor C2 and small inductor L1 tune the primary P1
of the transformer T1 to 25 KHz. Similarly, the capacitor C3 is
selected to tune the secondary of the transformer T1 to 25 KHz. The
capacitor C2, inductor L1 and transformer primary P1 constitute the
tank circuit for the transistor Q1, thereby providing an oscillator
at 25KHz. A few turns of primary winding P 2 are connected to the
base of transistor Q1 to provide regenerative feedback. The number
of turns of this feedback coil P2 are selected as a trade-off
between degree of regulation and magnitude of DC output.
The diode bridge 40 provides full wave rectification of the 25,000
Hz. output of the step-up transformer T1. Because of the high
impedance of the load consisting of the sleeve 16 and paper 10
between the two rollers, efficient operation of this power supply
20 requires that the output impedance of this bridge must also be
high. Accordingly, diodes were selected that have a 4,000 peak
inverse voltage rating; and thus a peak inverse voltage rating
above that of the DC output voltage. Furthermore, in order to
provide an efficient impedance match, it is important that the
capacitance of the diodes be small as possible. However, a small
capacitor C5 of about 5,000 micro-microfarads is employed in order
to reduce the magnitude of the ripple.
One of the important aspects of this FIG. 7 power supply design is
that its design arises out of a recognition that the capacitance as
well as the resistance of the di-electric sleeve must be considered
in designing an efficient, fast operating copying device.
Among the major problems that are faced in the design of an
efficient, fast operating copying device is that the occurrence of
AC ripple in the DC power supply output causes such a variation in
the instantaneous voltage applied that an uneven charge is applied
so that the resultant product has a series of alternating darker
and lighter lines laid out over the actual information that is
copied. This undesirable effect can be eliminated by so filtering
the power supply output that there is no ripple. However, such an
approach is uneconomic in that it requires larger current levels in
the power supply. In part in order to overcome the effect of the
ripple, and in particular to overcome the cancellation of the DC by
the AC ripple at its negative peak, there is a tendency to increase
the voltage output of the power supply which again is uneconomic
and inefficient and further tends to produce excessive ozone
because of the high voltage levels required.
In order to provide a design in which minimum current and voltage
levels are required, the device of this invention provides a very
substantial increase in the frequency of the signal that is
rectified to produce the DC output. Accordingly, as shown in FIG.
7, a 25 KHz input is applied to the rectifier 40 so that the ripple
output is at a high frequency. This high frequency alone means that
the lines mentioned above that are created in the copy paper tend
to be much more closely spaced and thus that the copy paper can be
run through more quickly than otherwise might be the case. Perhaps
more importantly, this high (25 KHz) frequency makes it much easier
to filter the output so that the magnitude of the ripple is very
substantially reduced. In one embodiment, this ripple is as low as
2 per-cent of the DC level.
FIG. 7 does not show a ground connection in the power supply. This
is on purpose, since the device of this invention makes possible
isolating the power supply output from the chassis of the copying
machine. As a consequence of so isolating the power supply, the
potential charge between paper and chassis is minimized and the
electrostatic charge that causes the paper to hang up in the
machine and makes the paper more difficult to handle is minimum.
This advantage is made possible only because of the very low
current level required by the device of this invention. Since the
power supply's output current capacity is in the range of 50 to 100
microamperes, physical contact by a user with the roller having the
sleeve 16 is in effect a contact through a very high impedance. The
resultant voltage drop and current level in the body of the person
making such contact is completely harmless.
Although one specific embodiment has been described, there is a
preferred range of values over which many of the parameters
mentioned may vary. The preferred ripple frequency range is from 10
KHz to 100 KHz. The preferred power supply output voltage range is
from 1,500 volts to 4,000 volts. The preferred range of sleeve 16
capacitance is from 25 micro-microfarads to 150 micro-microfarads.
A preferred RC time constant for the sleeve 16 in use is less than
5 milli-seconds. A preferred current level is one that is less than
60 microamperes.
Protection by Letters Patent of this invention in all its aspects
as the same are set forth in the appended claims is sought to the
broadest extent that the prior art allows.
* * * * *