Photoconductor charging technique

In an electrophotographic copying device having charging, imaging, developing, transferring, precleaning and cleaning facilities, the arrangement being in the conventional sense, but incorporates a combined charging and precleaning unit that is operable to perform either a charging function or a precleaning function at the proper time during a copying/cleaning cycle. A combined precharging/transferring unit is also incorporated to facilitate precharging or transferring at a predetermined time during the copying process.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to an electrophotographic copying device and more 
specifically, to an improvement over the charging and cleaning of the 
support surface on which the latent image of an original is developed. 
2. Prior Art 
The following U.S. Patents are representative of the prior art: U.S. Pat. 
Nos. 3,647,293; 3,637,306; and 3,736,055. 
Numerous prior art teachings in the field of electrophotographic or 
xerographic copying teaches various methods and devices for preparing the 
surface of a photoconductor so as to obtain a latent image from an 
original copy. Prints are then transferred from the latent image on the 
surface of the photoconductor, to a transferring media. 
To enable the development of the latent image on the photoconductor and the 
transferring of said latent image to a transferring media, several 
stations are arranged in proximity to and to cooperate with the 
photoconductor to perform certain functions. At the charging station, the 
photoconductor is charged negatively. The photoconductor then moves to the 
exposing or imaging station where a latent image is copied from an 
original. Next, the electrostatic latent image is developed at a developer 
station to form a toner image on the photoconductor. The toner image is 
then transferred from the photoconductor to another media at the 
transferring station. To complete the cycle, the photoconductor is erased, 
precleaned, and cleaned and is then ready for another cycle. 
Although the prior art electrophotographic devices function adequately for 
the intended purpose, several problems plague the systems. 
Probably one of the pressing problems is the fact that the charging, 
transferring and precleaning functions are all performed by separate 
coronas at separate stations. With this type of prior art design, the cost 
of the electrophotographic device is relatively high, due to the 
individual cost of each corona. Since the general trend is to minimize the 
cost of electrophotographic devices without sacrificing efficiency, any 
reduction in the number of component counts in the prior art devices will 
be welcomed. 
Another problem relating to the separate processing station is the fact 
that each of the separate coronas requires a separate power supply. The 
aggregate cost of these power supplies further augments the overall cost 
of the unit. As such, any reduction in the number of power supplies will 
result in cost reduction of the unit. 
It is common knowledge that conventional electrophotographic devices may be 
either a single cycle process or a two cycle process. In the typical two 
cycle process, the photoconductor is charged, imaged and developed during 
the first cycle; while the image is transferred and the photoconductor is 
cleaned in the second cycle. For satisfactory operations, some of the 
stations which render necessary functions during the copying process are 
active during the first cycle, while others are inactive and vice versa. 
On account of the rapid speed at which the photoconductor accesses each of 
the stations. It is, therefore, necessary for high speed switching to 
occur at these stations. The conventional 60 cycle power supply which is 
used for supplying power to these stations cannot withstand high speed 
switching. With these drawbacks, it is clear that a more efficient device 
is needed. 
Several attempts have been made to improve the prior art 
electrophotographic devices by solving some of the above identified 
problems. For example, attempts have been made to combine the charge and 
the transfer corona stations. At first blush, this combination seems to be 
workable and logical; since the function of both stations is to supply 
negative charges. However, the combination instead of solving the above 
described problems creates additional problems. 
One of the additional problems stems from the fact that the combined charge 
transfer station is designed with a grid structure to enhance the charge 
operation. However, transferring media which is fed into the machine at 
the charge/transfer station for transferring the latent image from the 
photoconductor jams into the grid wires. This jam results in machine 
breakdown. 
For proper operation, a negative charge has to be deposited onto the 
transferring media so that the positively charged toner particles will be 
attracted. With the presence of the grid assembly in the combined 
charge/transfer station, the negative charge cannot be uniformly 
distributed onto the transfer media. With an uneven distribution of 
charges, the quality of the final copy is less than satisfactory. 
OBJECTS OF THE INVENTION 
It is, therefore, the object of the invention to design a more efficient, 
low cost electrophotographic device than has heretofor been possible. 
It is another object of the present invention to build an 
electrophotographic device with fewer coronas than has heretofor been 
possible. 
It is a further object of this invention to combine the preclean and charge 
coronas into a single unit. 
It is still a further object of the present invention to use the transfer 
corona station to render the precharging and the transferring functions. 
SUMMARY OF THE INVENTION 
The present invention overcomes the aforementioned drawbacks in the prior 
art by means of a unique structural combination of processing stations 
within the copying process. More specifically, the invention discloses a 
unique two cycle process for an electrophotographic copying device. In one 
feature of the invention during the first cycle, the photoconductor is 
overcharged to a first polarity by the combined precharge transfer corona 
(Corotron), the overcharge is then reduced by an opposite polarity 
combined charge/precleaned corona (Scorotron). Imaging and developing also 
occurs during this first cycle. 
During the second cycle, the toned image is transferred to the transferring 
media using the same precharge transfer corona (Corotron). Following 
transfer, the drum is charged by the charge/preclean corona to a second 
potential for cleaning. In order to place the second charge level or 
potential on the photoconductor drum, the grid of the charge/preclean 
corona is switched to a different voltage (either the same or opposite 
polarity or ground as required to obtain heat cleaning). The drum is then 
(optionally) erased by the erase lamp, and cleaned by the developer. 
In another feature of the invention, the photoconductor is overcharged by a 
first or auxilliary corona which may (optionally) have a grid for smoother 
precharging, at a precharge station. This first or auxilliary corona is 
separate and distinct from the final charge/preclean corona and the 
overcharge/transfer corona. The charge is then reduced to a uniform value 
by a second corona of opposite polarity at a final charge station. The 
photoconductor is then ready for imaging and developing. In this 
embodiment the system is characterized as a three (3) corona overcharge 
system. 
In another feature of the invention, the photoconductor is charged to a 
very uniform negative value from the precharge level by means of a 
positive final charge corona which yields more uniform emission than a 
negative corona. 
Another feature of the invention is the use of a gridded corona (Scorotron) 
to perform the preclean function. The increased control of the preclean 
photoconductor voltage, because of the grid structure, may eliminate the 
need for the preclean erase lamp function. The improved cleaning action 
has reduced the hole and electron carrier intensities in the 
photoconductor which also reduces the fatigue effects of the 
photoconductor. 
The foregoing and other objects, features and advantages of the invention 
will be apparent from the following more particular description of the 
preferred embodiment of the invention, as illustrated in the accompanying 
drawings.

DETAILED DESCRIPTION 
The term corotron as used in this application means a type of corona having 
either limited or no grid structure. In effect the corotron may be 
considered analogous to a current source. 
The term scrotron as used in this application means a type of corona having 
a grid structure. The scorotron may be considered as a voltage source. 
For explanation purposes, the photoconductor in the preferred embodiment of 
the present invention will be described as a rotating drum. However, this 
should not be construed as a limitation on the scope of the invention; 
since it is well known in the art to design a photoconductor having a 
different shape, size and mechanical configuration. For example, the 
photoconductor may be a continuous belt or a plate rather than a rotating 
drum structure. 
Although the preferred embodiment of the invention is described in 
association with a two cycle copying process, this should be interpreted 
as only illustrative rather than restrictive, since it would be obvious 
for one skilled in the art to modify the inventive feature as disclosed 
hereinafter to make said concept operable in a one cycle copying process. 
In the drawings, similar elements are identified by identical numerals. 
Referring now to FIG. 1 and FIG. 3, a pictorial view of an 
electrophotographic copying system 10 which embodies the present invention 
is shown. A cylindrical drum 12, hereinafter called a photoconductor, is 
mounted for rotation on shaft 14 and having on its outer periphery a 
photoconductive insulating layer which contains an organic or inorganic 
photoconductor material. The drum 12 is rotated to bring the 
photoconductive layer in space relationship with various stations 
associated with the electrophotographic process; each of said stations 
being positioned in proximity to the rotating drum. 
A negative corotron 18 is positioned within the orbit of cylindrical drum 
12 to define the so called precharge/transfer station 32. Negative 
corotron 18 of the precharge/transfer station serves two functions, 
namely: to deposit an excess of negative ions on the surface of the 
photoconductor (for example, -1300 volts) and to deposit negative ions on 
a transferring media, for example, paper so as to transfer a latent toner 
image from the surface of the photoconductor. As will be explained 
subsequently, the negative charge which is deposited to the photoconductor 
by negative corotron 18 is rough; i.e., the charge is unevenly distributed 
on the surface of the photoconductor. 
After precharge/transfer station 32, the next station in order is the 
combined final charge/preclean station 20. Final charge/preclean station 
20 is the facility which supplies the final charge to the surface of the 
photoconductor and renders the preclean function. This final charge is 
referred to as smooth due to the fact that the charge is evenly 
distributed over the surface of the photoconductor because of the cutoff 
characteristic produced by the control grid 24. As will be explained 
subsequently, the polarity of the emission wires in the final 
charge/preclean corona is opposite to the voltage applied by the 
precharge/transfer corona. In the preferred embodiment, a positive 
emission voltage is used so that positive ions are generated. 
Final charge/preclean station 20 comprises a positive scrotron 22. Scrotron 
22 supplies positive ions at station 20. The positive ions reduce the 
rough charge on the photoconductor surface to a smooth charge. Grid 
structure 24 is positioned between scrotron 23 and the photoconductor 12. 
The function of grid structure 24 is to control the flow of positive ions 
which are deposited on photoconductor 12 and hence, the resulting smooth 
photoconductor voltage. 
As will be explained subsequently, and as shown in FIG. 2, a switching 
circuit is connected to grid 24 to control the voltage on the grid. For 
example, in one instance the voltage on the grid is very negative 
(approximately-700 volts), while in another instance the grid is slightly 
positive (approximately +50 volts). Still in another instance, the voltage 
may be slightly negative (approximately -50 volts) or ground. 
The other station in order is the so called interimage station 26. The 
interimage station comprises high intensity lamp 20 and the function is to 
erase images on the sides of the photoconductor depending on the size of 
the document to be copied. During the second cycle, this lamp can be 
optionally turned on to erase the photoconductor and, therefore, aids in 
the cleaning process of said photoconductor. 
The next station in order is the image station 30. Image station 30 
comprises a conventional optical system which functions to transfer a 
latent image of a document onto the photoconductor. With the latent image 
on the photoconductor, the most station in line is the developer cleaner 
station 60. Developer cleaner station 60 is conventional. For example, the 
developer cleaner station is analogous to the developer cleaner station as 
disclosed in the above identified U.S. Pat. No. 3,637,906, entitled 
"Copying System Featuring Alternate Developing and Cleaning of Successive 
Image Areas for Photoconductor" and assigned to the same assignee of the 
present invention. 
Referring now to FIG. 2, the control means which controls the negative 
corotron 18 of precharged station 32 is disclosed. Also the control means 
for switching the polarity of grid structure 24 from a first potential to 
a second potential is disclosed. 
As was mentioned previously, negative corotron 18 of precharge/transfer 
station 32 supplies negative ions to the photoconductor in one cycle and 
in another cycle supplies negative ions to a transfer medium 62 (FIG. 4). 
In order to supply negative ions, a negative high voltage power supply 34, 
also called control means 34, is connected to corotron 18. 
In one embodiment of precharge/transfer station 32, the same amount of 
negative ions (negative charge) is applied to the photoconductor and the 
transfer media. With this design there is no need for a switching 
mechanism to switch the control means 34 so as to supply different current 
levels to corotron 18. In an alternative embodiment, the magnitude of the 
negative charge which is applied to the photoconductor and the transfer 
media is different. This design requires a switching means analogous to 
the one which will be subsequently described. 
Still referring to FIG. 2, grid structure 24, also called control means 24, 
functions as a limiting means for controlling the positive ion (positive 
charge) which is deposited on the surface of photoconductor 12 from 
scorotron 22. The resulting photoconductor voltage is a function of the 
grid voltage. In order to effectuate this limiting or controlling 
function, a switching means is operably connected to the grid for 
switching its voltage between two (or more) levels. 
Switching means 36 comprises a diode 38 hereinafter called unidirectional 
device 38. One terminal of the unidirectional device is connected to grid 
24 while the other terminal is connected to positive terminal 40 
hereinafter called third reference voltage source 40. Third reference 
voltage source 40 may be any positive value, negative value or ground. For 
example, in the preferred embodiment of this invention the value was 
ground. 
Resistor 42, hereinafter called third resistor means 42 connects third 
reference voltage source 40 to a lower or equal potential. In the 
preferred embodiment of this invention, the low potential is ground. 
Likewise, another resistor 44, hereinafter called second resistor means 
44, connects third reference voltage means 40 to a higher potential. In 
the preferred embodiment of the invention, the higher potential was chosen 
to be 120 volts. 
In an alternate embodiment of the invention, third reference voltage source 
40 is connected to a switchable preclean level supply. The preclean level 
supply can be adjusted to one of a plurality of voltage potentials. For 
example, typical voltage levels would be +100 volts to -100 volts or 
ground. 
Reference voltage source 46, hereinafter called first reference voltage 
source 46, is positioned in parallel with third reference voltage source 
40. The potential of first reference voltage source 46 is negative. In the 
preferred embodiment of this invention, a 1000 volts negative potential 
was chosen. First reference voltage source 46 was established by a 
conventional bank of neon tubes 48. Of course, it would be obvious to one 
skilled in the art to substitute conventional devices to establish first 
reference voltage source 46 without departing from the scope of this 
invention. 
Resistor 50, hereinafter called first resistor means 50, is connected in 
series with first reference voltage source 46 so as to establish second 
voltage source 52. In the preferred embodiment of this invention, source 
52 is chosen to be 1500 volts negative. In an alternate embodiment, second 
voltage source 52 was connected to a negative grid supply means. The 
negative grid supply means has a typical value of approximately -1500 
volts. Switching means 54 interconnects unidirectional device 38 and first 
reference voltage source 46. The connection is such that by activating 
switching means 54 either the voltage at third reference voltage source 40 
or the voltage at first reference voltage means 46 is rendered operative 
(that is, appears on grid 24). Of course, several conventional switching 
devices may be used for switching means 54. However, in the preferred 
embodiment of this invention, switching means 54 was a high voltage read 
relay switch. Positive high voltage supply 58 supplies power to scorotron 
22 via terminal 56. 
In the preferred embodiment, high voltage corona supplies 34 and 58 are 
current regulated so that they deliver a constant total current to the 
corona emission wires. 
Referring now to FIG. 4 an alternative preferred embodiment of the 
invention is shown. In this preferred embodiment both the final charge and 
the preclean function are performed by dual bay corona charging station 
100 with a common grid 102. As mentioned before the electrophotographic 
copying system 10 includes photoconductor 12 which is rotated on shaft 14 
to position the photoconductive insulating layer in space relationship 
with various stations associated with the electrophotographic process and 
positioned in proximity to the photoconductor. 
As photoconductor 12 rotates clockwise in the direction shown by arrow 16, 
it encounters precharge/transfer station 32. Precharge/transfer station 32 
includes negative corotron 18. Negative corotron 10 performs two 
functions. During the first cycle of the two cycle process, negative 
corotron 18 charges the photoconductor to a negative voltage polarity. 
This negative voltage polarity may be less than the voltage which has to 
be placed on the photoconductive drum for satisfactory operation (or may 
be zero if the corona is turned off during this cycle). For example, in 
the preferred embodiment a negative charge of minus 800 volts is placed on 
the photoconductive surface. Of course, it is within the skill of the art 
and the teaching of this invention to vary the voltage both in magnitude 
and polarity without departing from the scope of this invention. 
During the second cycle of the two cycle process, negative corotron 18 
charges transfer media which is supplied along paper path 62 to an 
acceptable voltage level so that latent image which is developed as the 
photoconductive surface of photoconductor 12 is transferred to said 
transfer means. In this embodiment of the invention negative corotron 18 
is set at one voltage level as opposed to switching the voltage level of 
corotron 18 relative to whether the surface of the photoconductor or the 
transfer media is being charged. Stated another way, in the embodiment 
stated above, if corotron 18 is charging the photoconductor surface, the 
magnitude of the voltage is set at one level. Alternately, if corotron 18 
is charging transfer media which is supplied along paper path 62, then the 
emission voltage from corotron 18 is set at a second level, generally less 
than the first level. For satisfactory operation, high frequency power 
supply is required, since the conventional 60 cycle power supply is not 
easily switched. However, with the alternative embodiment depicted in FIG. 
4, corotron 18 may be set at one current level. Generally, the setting is 
dictated by the voltage level which has to be placed on the transfer media 
which is supplied along paper path 62. 
Positioned downstream from the precharge transfer station is the dual bay 
corona charging station 100. Dual bay corona charging station 100 performs 
two functions, namely to smooth and increase the negative charge which is 
deposited on photoconductor 12 by negative corotron 18 (or to do the total 
precharge function if the transfer corona 18 is turned off during this 
cycle) and to deposit a charge on the photoconductor which precleans the 
photoconductive surface. The smoothing function occurs during the first 
cycle of the two cycle process while the preclean function occurs during 
the second cycle of the the two cycle process. The dual bay corona of 
charging/preclean station 100 includes a leading bay 104 and a trailing 
bay 106. The small leading bay is negative and supplies negative ion to 
the photoconductive surface while the trailing bay supplies positive ions 
to the photoconductive surface. Of course, this arrangement can be changed 
without departing from the scope of the present invention. The emission 
from the leading bay and the trailing bay is controlled by common grid 
structure 102. Common grid structure 102 is tied to grid control means 
108. Several conventional grid control means can be used to switch the 
voltage on the grid. For example, the switching means previously described 
above is one of the many configurations which may be used. 
The physical design of dual bay corona charging station 100 can take 
several forms. For example, as is shown in FIG. 4 a common case 109 can be 
used to form a uniform structure having the two bays. Alternately, the 
dual bay corona charging station 100 may be separate coronas having 
separate grounding case and separate grids and grid supplies. However, 
irrespective of what configuration is used, grid 102 is common to both the 
positive and negative coronasof charging/preclean station 100. Still 
referring to FIG. 4 the emission wires of leading bay 104 and the emission 
wire of negative corotron 18 is tied in parallel to a negative high 
voltage supply 110. Assume that a negative charge having a magnitude of 
minus 800 volts is placed on the photoconductor by precharge transfer 
station 32. As the conductive surface approaches leading bay 104, negative 
ions are supplied from the leading bay of the dual bay corona charging 
station 100. This negative ion augments the negative charge which was 
previously deposited on the photoconductive surface and increased said 
charge to an approximate value of minus 1100 volts. As the photoconductor 
approaches trailing bay 106 of dual bay corona charging station 100, 
positive ions are emitted which further smooth the charge to a voltage 
level which is acceptable to perform the electrophotographic process. For 
example, the charge is neutralized to minus 870 volts. During this 
process, common control grid 102 is tied at the approximate voltage of 
minus 1000 volts by grid control means 108. The magnitude of the voltages 
are only descriptive and does not limit the scope of this invention. 
Positioned downstream from dual bay corona charging station 100 is the 
interimage erase station 26. Interimage erase station 26 includes erase 
lamp 28. Erase lamp 28 discharges the border area or no copy area of the 
photoconductor to approximately minus 150 volts. 
Positioned downstream and in order is the illumination station 30 and the 
developing cleaning station 60. These stations have already been described 
above and will not be described in any detail here. Suffice it to say that 
at station 30 the photoconductor is selectively discharged in accordance 
with the document to be copied. While at station 60 the latent image which 
is placed on the photoconductor surface is toned and subsequently the 
photoconductor surface is cleaned. This completes the detailed description 
of the preferred embodiment of the invention. 
OPERATION 
In describing the operation of the two cycle process, the position of the 
processing station in relation with rotating cylindrical drum 12 will be 
equated with positions on the face of a clock (see FIGS. 1 and 3). In 
operation, cylindrical drum 12 rotates in the direction shown by arrow 16. 
During the first cycle of the two cycle process (FIG. 1), step 1A occurs 
at 6:00. At 6:00, the precharge/transfer constant current negative corona 
18 of precharge/transfer station 32 will precharge the photoconductor of 
cylindrical drum 12 to a rough negative voltage. For example, the 
overcharge voltage is -1300 volts. 
The second step 1B occurs at 11:00 where the final charge/preclean 
scorotron 22 of final charge/preclean station 20 reduces the 
photoconductor charge to approximately -800 volts as controlled by grid 
24. At 12:00 step 2 occurs; lamp 28 of interimage station 26 performs the 
interimage erase. At 1:00, step 3 occurs; the photoconductor is imaged at 
image station 30, such that the photoconductor charge in a black image is 
approximately -720 volts, the photoconductor charge in a gray image is 
approximately -400 volts, and the erase background and white charge is 
from -170 to -200 volts. 
At approximately 4:00 step 4 occurs; the latent image is developed by 
magnetic brush 58 of developer/cleaner station 60. The bias of magnetic 
brush 58 is approximately -300 volts. Thus, magnetic brush 58 is positive 
relative to the latent image and negative relative to the erased 
background. This completes the first drum cycle. 
At 6:00 during the second drum cycle (FIG. 3), step 5 occurs; transfer 
media 62 is gated so that it moves between the corona and the drum. 
Negative corotron 18 of precharge/transfer station 32 provides the 
electrostatic force causing the toned image on cylindrical drum 12 to be 
transferred to transfer media 61. The transfer media, for example, paper, 
is held against drum 12 by electrostatic force only. In one embodiment of 
the invention, the same corotron current setting was used for both 
precharge and transfer functions so that switching the current level was 
not necessary except at the end of a multicopy run when the unit must be 
turned off for the final clean cycle. Of course, one alternative 
embodiment would be to switch the current setting depending on whether the 
precharge function or the transfer function was being performed. 
At approximately 11:00 during the second drum cycle step 6 occurs; 
switching means 36 (FIG. 2), switches grid 24 so that the voltage from 
third reference source 40 appears on the photoconductor surface of 
rotating drum 12 so that the charge on said drum is reduced to a voltage 
near ground. This change in voltage accomplishes the preclean function. 
At approximately 12:00 step 7 occurs; lamp 28 of interimage station 26 is 
energized to illuminate the entire photoconductor surface of rotating drum 
12 which changes the voltage to approximately 0 volts. This is an optional 
step and may be eliminated because of the improved control of the preclean 
photoconductor voltage achieved with gridded preclean corona. At 1:00 
during the second cycle, imaging station 30 may be on or off. The 
photoconductor then rotates to developer/cleaner station 60 where magnetic 
brush 58 removes residual toner from the photoconductor surface. This 
completes the two cycle process. 
This unique configuration as described above has distinct advantages over 
prior art configurations, in that the requirement of high voltage ferro 
switching in short time intervals is eliminated. In addition, the 
combination of the two corona units requires one less power supply and one 
less corona unit for a sizable cost reduction. 
Another advantage of this configuration is the fact that the transfer 
corona can be made smaller than would have been possible if the combined 
charge and transfer coronas had been used. This is important in that 
significant reduction in the overall machine dimension is achieved. 
While the invention has been particularly shown and described with 
reference to a preferred embodiment thereof, it will be understood by 
those skilled in the art that various changes in form and details may be 
made therein without departing from the spirit and scope of the invention.