Cleaning commutator brushes for an electroded donor roll

A self-cleaning commutator brush structure for use with electroded donor members. The donor member is provided with embedded electrodes to which an electrical bias is supplied for creating toner clouding in the development zone of a toner imaging system. Electrical power is supplied to the electrodes via a pair of commutator brushes which contact the electrodes. A pair of bumps positioned at different locations on one of the surfaces of the donor member impede movement of the brushes. Each time the donor member is moved through a complete cycle of movement each brush is caused to be flicked by the bump to thereby liberate toner therefrom. The flicking action and consequent toner liberation serve to keep the interface between the brushes and the electrodes clean which prevents electrical resistance buildup therebetween.

BACKGROUND OF THE INVENTION 
This invention relates generally to the rendering of latent electrostatic 
images visible. More particularly, the invention relates to 
non-interactive or scavengeless development systems and a self-cleaning 
commutating structure for transmitting electrical power to electrodes of a 
toner donor roll. 
The invention can be utilized in the art of xerography or in the printing 
arts. In the practice of conventional xerography, it is the general 
procedure to form electrostatic latent images on a xerographic surface by 
first uniformly charging a photoreceptor. The photoreceptor comprises a 
charge retentive surface. The charge is selectively dissipated in 
accordance with a pattern of activating radiation corresponding to 
original images. The selective dissipation of the charge leaves latent 
charge patterns on the imaging surface corresponding to the areas not 
exposed by radiation. 
This charge pattern is made visible by developing it with toner. The toner 
is generally a colored powder which adheres to the charge pattern by 
electrostatic attraction. 
The developed image is then fixed to the imaging surface or is transferred 
to a receiving substrate such as plain paper to which it is fixed by 
suitable fusing techniques. 
Low scavenging, high quality, low noise, powder xerographic development is 
important for a number of reasons. The high quality low noise operation 
is, of course, desirable for any development system but the low scavenging 
behavior is if particular importance for single pass process color and 
highlight color applications. Low scavenging development can be achieved 
by applying an AC/DC bias to one or more wires which are self spaced over 
a toned donor roll as disclosed in U.S Pat. No. 4,868,600 granted to Hays 
et al on Sep. 19, 1989. Excitation of the toner layer on the donor roll 
causes a charged toner cloud to form from which toner can be developed to 
an electrostatic latent image. The is a very effective way to achieve low 
scavenging development but there are problems in doing so reliably using 
this approach, Wire vibration has been seen to cause strobing in the 
images. Such vibration has been shown to be affected by too much tension 
in the wires and the presence of toner material. Streaks in images are 
seen to occur when debris gets caught between the wire and the donor roll. 
Another approach which achieves low scavenging development is the use of a 
donor roll with an embedded electrode structure of the type disclosed in 
U.S. Pat. No. 5,172,170 granted to Hayset al on Dec. 15, 1992. These 
electrodes lie parallel to the longitudinal axis of the donor roll and are 
embedded in the surface of the donor roll. These electrodes or conducting 
strips are excited using a combination AC/DC electrical bias which causes 
the charged toner on the surface of the donor roll to form a cloud of 
toner in the manner similar to the method using the self-spaced wire or 
wires. 
The AC bias must be transmitted to the embedded electrodes from one or both 
ends of the roll. One way to transmit this bias to the electrodes is via a 
conductive brush contact through exposed electrodes at one end of the 
roll. In the case of interdigitated electrode structures, brush contact is 
made at both ends of the roll. Toner from the development process can get 
caught in the brush thereby causing causing a high resistance at the 
interface between the electrode and contact brush. Eventually such a 
condition causes failure of the toner jumping process due to the inability 
to transmit high enough potentials to the electrodes on the donor roll 
surface. 
The invention is particularly useful in highlight color imaging such as 
tri-level imaging. The concept of tri-level, highlight color xerography is 
described in U.S Pat. No. 4,078,929 issued in the name of Gundlach. The 
patent to Gundlach teaches the use of tri-level xerography as a means to 
achieve single-pass highlight color imaging. As disclosed therein the 
charge pattern is developed with toner particles of first and second 
colors. The toner particles of one of the colors are positively charged 
and the toner particles of the other color are negatively charged. In one 
embodiment, the toner particles are supplied by a developer which 
comprises a mixture of triboelectrically relatively positive and 
relatively negative carrier beads. The carrier beads support, 
respectively, the relatively negative and relatively positive toner 
particles. Such a developer is generally supplied to the charge pattern by 
cascading it across the imaging surface supporting the charge pattern. In 
another embodiment, the toner particles are presented to the charge 
pattern by a pair of magnetic brushes. Each brush supplies a toner of one 
color and one charge. In yet another embodiment, the development systems 
are biased to about the background voltage. Such biasing results in a 
developed image of improved color sharpness. 
In highlight color xerography as taught in the '929 patent, the xerographic 
contrast on the charge retentive surface or photoreceptor is divided into 
three levels, rather than two levels as is the case in conventional 
xerography. The photoreceptor is charged, typically to -900 volts. It is 
exposed imagewise, such that one image corresponding to charged image 
areas (which are subsequently developed by charged-area development, i.e. 
CAD) stays at the full photoreceptor potential (V.sub.cad or V.sub.ddp). 
The other image is exposed to discharge the photoreceptor to its residual 
potential, i.e. V.sub.dad or V.sub.c (typically -100 volts) which 
corresponds to discharged area images that are subsequently developed by 
discharged-area development (DAD) and the background areas exposed such as 
to reduce the photoreceptor potential to halfway between the V.sub.cad and 
V.sub.dad potentials, (typically -500 volts) and is referred to as 
V.sub.white or V.sub.w. The CAD developer is typically biased about 100 
volts closer to V.sub.cad than V.sub.white (about -600 volts), and the DAD 
developer system is biased about 100 volts closer to V.sub.dad than 
V.sub.white (about -400 volts). 
The viability of printing system concepts such as tri-level, highlight 
color xerography requires development systems that do not scavenge or 
interact with a previously toned image. Since commercial development 
systems such as conventional magnetic brush development and jumping single 
component development interact with the image receiver, a previously toned 
image will be scavenged by subsequent development. Since the present 
commercial development systems are highly interactive with the image 
bearing member, there is a need for scavengeless or non-interactive 
development systems. 
The present invention is especially suited for use in hybrid scavengeless 
single component development (SCD) systems wherein a confined toner cloud 
is formed in a 250 micron development zone gap by applying an AC bias of 
several hundred volts to one or more small diameter wire electrodes 
carried by a toner donor roll positioned adjacent a photoreceptor. The AC 
bias, which has a frequency is in the kilohertz range, acts upon the 
charged toner to induce a mechanical agitation which is sufficient to 
overcome adhesive forces that hold toner to the donor roll. Once freed, 
the toner is readily available to develop the electrostatic latent image 
on the photoreceptor. Toner delivery in a hybrid system differs from 
conventional SCD systems, in that, toner is delivered to a donor roll via 
a magnetic brush loader. 
In earlier renderings of this type of development system, the electrodes 
consisted of taut wires supported intermediate a photoreceptor and a toner 
donor roll. See for example U.S. Pat. No. 5,010,367 granted to Dan A. Hays 
on Apr. 23, 1991. Unfortunately, it has proven difficult to devise a 
mechanical design for the fragile taut wire array that is both robust, and 
free of development artifacts. For example, the wires tend to entrap toner 
agglomerates and spurious paper fibers which can cause streaks in the 
developed image. 
The problems attendant taut wires may be obviated by using an array of 
small diameter wires or electrodes embedded in the surface of the donor 
roll. In this approach, the AC bias is applied to the wires in the 
development zone through commutating brushes at the ends of the donor 
roll. Such a construction is described in U.S. Pat. No. 3,996,892 granted 
to Parker et al on Dec. 14, 1976. The '892 granted to Parker et al on Dec. 
14, 1976 discloses a spatially programmable electroded donor roll wherein 
an DC voltage is applied to the wire electrodes in the development nip or 
zone, pre-nip and post-nip zones through commutating brushes at the ends 
of the donor roll. Such an arrangement allows the bias profile around the 
circumference of a two component magnetic brush development roll to be 
tailored in a way that promotes good development. Thus, a pre-nip voltage 
of 100 volts, a nip voltage of 250 to 300 volts and a post-nip voltage of 
1000 volts are provided. The electrodes on the donor roll were constructed 
by first plating a thin layer of copper on the outer surface a phenolic 
roll, and then by etching 0.01" wide electrode strips, on 0.02 centers, 
axially along the length of the roll. Next, the roll was overcoated with a 
semi-conductive rubber sheath, except for a short length at the ends where 
the bias was applied to the electrodes through a commutating bushes. The 
voltage profile around the circumference of the roll was determined by the 
IR voltage drop due to current flow through the semi-conductive sheath 
from one commutator to another. Such a construction is know to have had 
problems with wear and pitting of the thin electrodes where they made 
contact with the commutating brushes. Nickel plating the electrodes helped 
alleviate the wear problem somewhat, but the electrode damage problem was 
never completely solved. 
The '892 patent, in a second embodiment, discloses the use of a ring-like 
resistive member mounted for rotation with a donor roll. A plurality of 
stationarily mounted electrical contacts ride on the ring-like member 
which, in turn, is seated on the coating free portions of conductors and 
mounted for rotation with a sleeve upon which the conductors are carried. 
U.S. Pat. No. 4,568,955 granted to Hosoya also discloses a development or 
donor roll having electrode structures incorporated therein. Copper 
electrode structures are deposited on the insulated surface of a donor 
roll. In one rotational position of the Hosoya et al donor roll, a DC 
voltage is supplied to alternate ones of the copper electrodes while an AC 
voltage is supplied to the electrodes intermediate the electrodes having 
the DC voltage applied thereto. In another rotational position of this 
donor roll the AC and DC voltages are applied to the opposite electrodes. 
In other words, each electrode when positioned in the development nip 
first has one kind of voltage applied and then the other. The AC voltage 
establishes an alternating electric field for liberating toner particles 
on the surface of the donor roll. According to the Hosoya et al 
description, when the AC voltage is greater than the DC voltage the toner 
particles move from one electrode to an adjacent electrode and when the AC 
voltage is less than the DC voltage the toner particles move in the 
opposite direction between two adjacent electrodes. 
U.S. Pat. No. 5,031,570 granted to Hays et al on Jul. 16, 1991 and assigned 
to the same assignee as the instant application discloses a scavengeless 
development system for use in highlight color imaging. AC biased 
electrodes positioned in close proximity to a magnetic brush structure 
carrying a two-component developer cause a controlled cloud of toner to be 
generated which non-interactively develops an electrostatic image. The 
two-component developer includes mixture of carrier beads and toner 
particles. By making the two-component developer magnetically tractable, 
the developer is transported to the development zone as in conventional 
magnetic brush development where the development roll or shell of the 
magnetic brush structure rotates about stationary magnets positioned 
inside the shell. 
U.S. Pat. No. 4,868,600 granted to Hays et al on Sep. 19, 1989 discloses a 
scavengeless development system in which toner detachment from a donor and 
the concomitant generation of a controlled powder cloud is obtained by AC 
electric fields supplied by self-spaced electrode structures positioned 
within a development nip. The electrode structure is placed in close 
proximity to the toned donor within the gap or nip between the toned donor 
and image receiver, self-spacing being effected via the toner on the 
donor. Such spacing enables the creation of relatively large electrostatic 
fields without risk of air breakdown. 
U.S. Pat. No. 5,010,367 granted to Dan A. Hays on Apr. 23, 1991 discloses a 
scavengeless/non-interactive development system for use in highlight color 
imaging. To control the developability of lines and the degree of 
interaction between the toner and receiver, the combination of an AC 
voltage on a developer donor roll with an AC voltage between toner cloud 
forming wires and donor roll enables efficient detachment of toner from 
the donor to form a toner cloud and position one end of the cloud in close 
proximity to the image receiver for optimum development of lines and solid 
areas without scavenging a previously toned image. 
U.S. patent application Ser. No. 07/724,242 filed on Jul. 1, 1991 in the 
name of Dan A. Hays and assigned to the same assignee as the instant 
application discloses a scavengeless or non-interactive development system 
for use in image formation such as highlight color imaging. A toned donor 
roll structure having two sets of interdigitated electrodes physically 
supported by an insulative support structure is provided. Both sets of 
electrodes have a DE bias applied thereto while the other set has an AC 
bias applied thereto. The AC and DC biases are such as to preclude 
background development without creating fringe DC fields between adjacent 
electrodes. 
U.S. Pat. No. 5,172,170 granted to Hays et al on Dec. 12, 1992 relates to 
an apparatus in which a donor roll advances toner to an electrostatic 
latent image recorded on a photoconductive member. A plurality of 
electrical conductors are located in grooves in the donor roll. The 
electrical conductors are spaced from one another and adapted to be 
electrically biased in the development zone to detach toner from the donor 
roll so as to form a toner cloud in the development zone. In the 
development zone, toner is attracted from the toner cloud to the latent 
image. In this way, the latent image is developed with toner. 
It is known in the prior art, as illustrated in U.S. Pat. No. 3,766,593 
granted to Becker et al on Oct. 23, 1973, to clean residual particulate 
material remaining after transfer of a developed image from an insulator. 
As disclosed in the '593 patent, the cleaning apparatus comprises rotating 
and stationary brushes for cleaning both surfaces of the insulator, both 
brushes enmeshing after the cleaning cycle to provide a brush 
self-cleaning action for the stationary brush. The apparatus is designed 
such that air flow is at a maximum at the point where the particulate 
material would normally be centrifuged out of the brush housing, thereby 
minimizing leakage of the particulate material despite a gap between the 
insulator and the brush housing. 
BRIEF SUMMARY OF THE INVENTION 
In accordance with the present invention, a commutator brush structure and 
the interface between the brush structure and a donor roll structure are 
kept free of toner particles in order to avoid the build up of a high 
electrical resistance between the brush and the electrodes carried by the 
donor roll. To this end, a pair of bumps are provided either on the 
surface of the donor roll or on its end. The bumps or protrusions serve as 
flicker members for a pair of commutator brushes. For each revolution of 
the donor roll, each brush is lifted out of contact with the electrodes on 
the donor roll. The bumps are angularly staggered such that only one brush 
is out of contact with the donor electrodes at any given time. The 
bristles bend due to their engagement with the bumps but return quickly 
via their natural restoring force. Each brush cleans the area of contact 
with the electrodes and is detoned by the flicking action effected by the 
bumps.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
As shown in FIG. 5, a highlight color printing machine of the prior art in 
which the invention may be utilized comprises a charge retentive member in 
the form of a photoconductive belt 10 consisting of a photoconductive 
surface and an electrically conductive substrate and mounted for movement 
past a charging station A, an exposure station B, developer station C, 
transfer station D and cleaning station F. Belt 10 moves in the direction 
of arrow 16 to advance successive portions thereof sequentially through 
the various processing stations disposed about the path of movement 
thereof. Belt 10 is entrained about a plurality of rollers 18, 20 and 22, 
the former of which can be used as a drive roller and the latter of which 
can be used to provide suitable tensioning of the photoreceptor belt 10. 
Motor 23 rotates roller 18 to advance belt 10 in the direction of arrow 
16. Roller 18 is coupled to motor 23 by suitable means such as a belt 
drive. 
As can be seen by further reference to FIG. 5, initially successive 
portions of belt 10 pass through charging station A. At charging station 
A, a corona discharge device such as a scorotron, corotron or dicorotron 
indicated generally by the reference numeral 24, charges the belt 10 to a 
selectively high uniform positive or negative potential, V.sub.0. Any 
suitable control, well known in the art, may be employed for controlling 
the corona discharge device 24. 
Next, the charged portions of the photoreceptor surface are advanced 
through exposure station B. At exposure station B, the uniformly charged 
photoreceptor or charge retentive surface 10 is exposed to a laser based 
input and/or output scanning device 25 which causes the charge retentive 
surface to be discharged in accordance with the output from the scanning 
device. Preferably the scanning device is a three level laser Raster 
Output Scanner (ROS). Alternatively, the ROS could be replaced by a 
conventional xerographic exposure device. An electronic subsystem (ESS) 27 
provides for control of the ROS as well as other subassemblies of the 
machine. 
The photoreceptor, which is initially charged to a voltage V.sub.0, 
undergoes dark decay to a level V.sub.ddp equal to about -900 volts. When 
exposed at the exposure station B it is discharged to V.sub.c equal to 
about -100 volts which is near zero or ground potential in the highlight 
(i.e. color other than black) color parts of the image. The photoreceptor 
is also discharged to V.sub.w equal to approximately -500 volts imagewise 
in the background (white) image areas. 
At development station C, a development system, indicated generally by the 
reference numeral 30 advances developer materials into contact with the 
electrostatic latent images. The development system 30 comprises first and 
second developer apparatuses 32 and 34. The developer apparatus 32 
comprises a housing containing a pair of magnetic brush rollers 36 and 38. 
The rollers advance developer material 40 into contact with the latent 
images on the charge retentive surface which are at the voltage level 
V.sub.c. The developer material 40 by way of example contains color toner 
and magnetic carrier beads. Appropriate electrical biasing of the 
developer housing is accomplished via power supply 41 electrically 
connected to developer apparatus 32. A DC bias of approximately -400 volts 
is applied to the rollers 36 and 38 via the power supply 41. With the 
foregoing bias voltage applied and the color toner suitably charged, 
discharged area development (DAD) with colored toner is effected. 
The second developer apparatus 34 comprises a donor roll structure in the 
form of a roller 42. The donor structure 42 conveys developer 44, which in 
this case is a single component developer comprising black toner deposited 
thereon via a combination metering and charging device 46, to an area 
adjacent an electrode structure. The toner metering and charging can also 
be provided by a two component developer system such as a magnetic brush 
development structure. The donor structure can be rotated in either the 
`with` or `against` direction vis-a-vis the direction of motion of the 
charge retentive surface. The donor roller 42 is preferably coated with 
TEFLON-S (trademark of E.I. DuPont De Nemours) or anodized aluminum or a 
polycabonate coating with a suitable additive to enable charge to relax 
through the coating. The donor roll structure 42 is provided with a 
plurality of embedded electrodes 44. (FIGS. 2 and 3). A DC biased AC power 
source 46 serves to supply a suitable electrical bias to the electrodes 44 
via a pair of commutator brushes 48 and 50. The brushes are supported 
relative to the donor roll 42 such that they contact some of the embedded 
electrodes as the electrodes pass through a development zone 52 (FIG. 5). 
The purpose of applying an electrical bias to the electrodes in the 
development zone is to liberate toner from the surface of the donor roll 
for development of latent images on the photoreceptor. For a more detailed 
description of an electroded donor roll structure reference may be had to 
'170 patent noted above. A pair of protrusions or bumps 55, 56 on the 
circumference of the donor roll interfere with the engagement of the 
commutator brushes 48 and 50 once for each revolution of the donor roll. 
The bumps are positioned or staggered such that when one section of brush 
is out of contact with the electrodes, i.e. being cleaned, the other brush 
section will still be supplying a voltage to the development zone. 
A sheet of support material 58 (Figure FIG. 5) is moved into contact with 
the toner image at transfer station D. The sheet of support material is 
advanced to transfer station D by conventional sheet feeding apparatus, 
not shown. Preferably, the sheet feeding apparatus includes a feed roll 
contacting the uppermost sheet of a stack copy sheets. Feed rolls rotate 
so as to advance the uppermost sheet from stack into a chute which directs 
the advancing sheet of support material into contact with photoconductive 
surface of belt 10 in a timed sequence so that the toner powder image 
developed thereon contacts the advancing sheet of support material at 
transfer station D. 
Because the composite image developed on the photoreceptor consists of both 
positive and negative toner, a positive pre-transfer corona discharge 
member 56 is provided to condition the toner for effective transfer to a 
substrate using negative corona discharge. 
Transfer station D includes a corona generating device 60 which sprays ions 
of a suitable polarity onto the backside of sheet 58. This attracts the 
charged toner powder images from the belt 10 to sheet 58. After transfer, 
the sheet continues to move, in the direction of arrow 62, onto a conveyor 
(not shown) which advances the sheet to fusing station E. 
Fusing station E includes a fuser assembly, indicated generally by the 
reference numeral 64, which permanently affixes the transferred powder 
image to sheet 58. Preferably, fuser assembly 64 comprises a heated fuser 
roller 66 and a backup roller 68. Sheet 58 passes between fuser roller 66 
and backup roller 68 with the toner powder image contacting fuser roller 
66. In this manner, the toner powder image is permanently affixed to sheet 
58. After fusing, a chute, not shown, guides the advancing sheet 68 to a 
catch tray, also not shown, for subsequent removal from the printing 
machine by the operator. 
After the sheet of support material is separated from photoconductive 
surface of belt 10, the residual toner particles carried by the non-image 
areas on the photoconductive surface are removed therefrom. These 
particles are removed at cleaning station F. A magnetic brush cleaner 
housing 70 is disposed at the cleaner station F. The cleaner apparatus 
comprises a conventional magnetic brush roll structure for causing carrier 
particles in the cleaner housing to form a brush-like orientation relative 
to the roll structure and the charge retentive surface. It also includes a 
pair of detoning rolls for removing the residual toner from the brush. 
Subsequent to cleaning, a discharge lamp (not shown) floods the 
photoconductive surface with light to dissipate any residual electrostatic 
charge remaining prior to the charging thereof for the successive imaging 
cycle. 
A donor roll structure 90 directed to the features of this invention is 
illustrated in FIG. 1. The donor roll structure is preferably used in 
place of the donor structure 34 illustrated in FIG. 5. As illustrated in 
FIGS. 1 and 2, the donor roll structure 90 comprises a roll structure 92 
having a plurality of electrodes 94 embedded in a surface 96 thereof. A 
pair of commutator brushes 98 and 100 (FIG. 2) are provided for applying a 
suitable electrical potential to the electrodes 94. To this end, an AC 
power source 102 biased by a DC power source 104 is provided for applying 
an alternating current to the electrodes. The applied AC serves to cause 
toner particles to be liberated form the surface of the donor roll in a 
development zone 112 intermediate the photoconductive belt 10 and the 
donor roll structure 90. 
In order to obviate the problem of electrical resistance build up between 
the brushes and the electrodes and the brushes, which build up is due to 
toner accumulation in the brushes, means for causing a periodic flicking 
action of the brush bristles is provided. Accordingly, a pair of bumps or 
protrusions 114 and 116 are carried by the surface 96. As can be seen from 
FIG. 2, the bumps or protrusions are positioned in a staggered 
relationship vis-a-vis the surface 96. As the donor roll structure 90 is 
rotated the brushes 98 is first contacts the bump 114 thereby causing a 
flicking action of the brush bristles which liberates toner particles 
therefrom. Similarly, a flicking action of the brush bristles 100 is 
effected through contact thereof with the bump or protrusion 116. Since a 
brush's bristles are out of contact with the electrodes when engaging its 
respective bump, the staggered relationship of the bumps or protrusions 
ensures continuous commutation of electrical power to the electrodes. 
A modified embodiment of the donor structure 90 is illustrated in FIGS. 3 
and 4 comprises a donor roll structure 120. The donor structure 120 is 
provided with a plurality of embedded electrodes 122. A pair of commutator 
brushes 124 and 126 are similar to brushes 98 and 100 but they contact 
their associated electrodes 122 at an end 128 of the donor structure 120 
in lieu of contacting electrodes on a surface such as in the case of the 
embodiment disclosed in FIGS. 1 and 2. A pair of bumps or protrusions 130 
and 132 similar to bumps 1114 and 116 are integral with the end 128 of the 
donor roll structure. An electrical potential is applied to the brushes 
124 and 126 via AC power source 134 biased via DC power source. Toner 
liberated through the action of the flicking the brush commutators is 
carried away from the area of the brushes and their contact with the bumps 
or protrusions by means of a vacuum source and plenum, not shown.