Title: Image forming apparatus
Abstract: This invention provides an image forming apparatus where the useful life of the image carrier does not become significantly short. This system has a charging unit, a charge voltage loading unit, an exposure unit, a development unit, an image transfer unit and a control unit, wherein, when the transport interval of the plural recording materials is shorter than a predetermined time the AC charge voltage applied to the image carrier during the transport interval being a first AC charge voltage, and when the transport interval is longer than the predetermined time the ac charge voltage applied to the image carrier during the transport interval being a second AC charge voltage, the control unit makes the current running in the charging unit to which the second charge voltage is applied lower than the current running in the charging unit to which the first AC charge voltage is applied.
Patent Number: 7,016,619 Issued on 03/21/2006 to Ito, et al.
| Inventors:
|
Ito; Mitsuhiro (Shizuoka, JP);
Sakai; Hiroaki (Shizuoka, JP)
|
| Assignee:
|
Canon Kabushiki Kaisha (Tokyo, JP)
|
| Appl. No.:
|
982808 |
| Filed:
|
November 8, 2004 |
Foreign Application Priority Data
| Jul 05, 2002[JP] | 2002-197743 |
| Jul 12, 2002[JP] | 2002-204877 |
| Current U.S. Class: |
399/50; 399/45; 399/82 |
| Current Intern'l Class: |
G03G 15/02 (20060101) |
| Field of Search: |
399/50,43,45,82,16
|
References Cited [Referenced By]
U.S. Patent Documents
| 5450180 | Sep., 1995 | Ohzeki et al.
| |
| 5717979 | Feb., 1998 | Senba et al.
| |
| 5835818 | Nov., 1998 | Hoshika et al.
| |
| 5845172 | Dec., 1998 | Saito et al.
| |
| 5970302 | Oct., 1999 | Yamane.
| |
| 6081679 | Jun., 2000 | Yamane et al.
| |
| 6266151 | Jul., 2001 | Tachibana et al.
| |
| 6496660 | Dec., 2002 | Takahashi et al.
| |
| 6539184 | Mar., 2003 | Shimura et al.
| |
| 6806895 | Oct., 2004 | Jeong.
| |
| 2001/0026694 | Oct., 2001 | Sakaizawa et al.
| |
| 2002/0006289 | Jan., 2002 | Takami et al.
| |
| 2002/0159782 | Oct., 2002 | Tsuruya et al.
| |
| Foreign Patent Documents |
| 8-320642 | Dec., 1996 | JP.
| |
| 10-39691 | Feb., 1998 | JP.
| |
| 2001-88370 | Apr., 2001 | JP.
| |
| 2001-88406 | Apr., 2001 | JP.
| |
| 2001/-192132 | Jul., 2001 | JP.
| |
| 2002-46876 | Feb., 2002 | JP.
| |
| 2002-91102 | Mar., 2002 | JP.
| |
Primary Examiner: Chen; Sophia S.
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper & Scinto
Parent Case Text
This application claims the priority of Japanese Patent Application Nos. 2002-197743
filed Jul. 5, 2002 and 2002-204877 filed Jul. 12, 2002, which are incorporated
hereinto by reference.
This application is a divisional of U.S. patent application Ser. No. 10/609,469,
filed Jul. 1, 2003 now U.S. Pat. No. 6,898,385, and allowed on Sep. 15, 2004.
Claims
What is claimed is:
1. An image forming apparatus that forms an image in a plurality of print modes
differing in a transport interval of recording materials, the image forming apparatus comprising:
charging means for charging an image carrier;
charge voltage loading means for applying a charge voltage to the charging means;
image transfer means for transferring an image formed on the image carrier onto
a plurality of recording materials; and
control means for controlling the charge voltage applied by the charge voltage
loading means to the charging means,
wherein according to the print modes, during a period in which an image is not
formed on the plurality of recording materials, the control means sets a charge
voltage which differs from a charge voltage which is set during a period in which
an image is formed on the plurality of recording materials.
2. The image forming apparatus according to claim 1, wherein the print modes
are modes in which an image is formed on both sides of the plurality of recording materials.
3. The image forming apparatus according to claim 1, wherein the print modes
are modes in which the transport intervals are changed according to types of the
recording materials.
4. The image forming apparatus according to claim 1, wherein the charge voltage
includes an AC charge voltage.
5. The image forming apparatus according to claim 1, wherein the charge voltage
is a voltage including a DC charge added to an AC charge voltage.
6. The image forming apparatus according to claim 1, wherein the charge voltage
loading means applies a predetermined charge voltage to a non-image formation area
of the image carrier.
7. The image forming apparatus according to claim 1, wherein the period in which
an image is not formed on the plurality of recording materials is a period in accordance
with the transport interval of the plurality of recording materials.
8. An image forming apparatus that forms an image in a plurality of print modes
differing in a transport interval of recording materials, the image forming apparatus comprising:
a charging portion configured to charge an image carrier;
a charge voltage loading portion configured to apply a charge voltage to the
charging portion;
an image transfer portion configured to transfer an image formed on the image
carrier onto a plurality of recording materials; and
a control portion configured to control the charge voltage applied by the charge
voltage loading portion to the charging portion,
wherein in accordance with the print modes, during a period in which an image
is not formed on the plurality of recording materials, the control portion sets
a charge voltage which differs from a charge voltage which is set during a period
in which an image is formed on the plurality of recording materials.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electro-photographic image forming apparatus.
More particularly, the invention relates to an image forming apparatus for forming
images by the electro-photographic process using copiers and printers.
2. Description of the Related Art
Many electrographic copiers and printers form images on one side of a recording
material such as recording paper. Now, however, what is called the double-sided
image forming apparatus, which is capable of forming images on both sides of a
sheet for environmental protection and savings of natural resources, has been commercialized.
The double-sided image forming apparatus prints images on a first side and then
on the other side, utilizing a paper turn-over mechanism that turns over the sheet
of which one side has been printed and a re-feeder mechanism that feeds the the
sheet of which one side has been printed and a re-feeder mechanism that feeds the
sheet again.
FIG. 1 is a diagram illustrating an example of the structure of the prior art
electro-photographic laser beam printer. This laser beam printer has a sheet turn-over
unit and a re-feeder unit near the center of the printer
100, and has a
detachable transfer unit D for double-sided printing in the body. A paper cassette
101 that houses sheets of paper P is located at the bottom of the body.
Sheets P are transported by a transport roller
108 to a process cartridge
112 via a pickup roller
104, a feeder roller
105 and a retard
roller
106 that feed paper, separating sheets P one by one. Upstream of
the process cartridge
112 are a pre-resist sensor
110 that detects
the sheets P and resist rollers
109 that transport the sheets P synchronously.
The process cartridge
112 is detachably attached to the body and forms
an electrostatic latent image with laser light from a scanner
111 on a photosensitive
drum
1 working as the image carrier. A visible image or toner image is produced
by developing this latent image. The scanner
111 is generally comprised
of a laser unit
129 that emits laser light, a polygon mirror
130
that scans the laser light from the laser unit
129 on the photosensitive
drum
1, a polygon motor
131, an image formation lens assembly
132
and a return mirror
133. The process cartridge
112 is equipped with
the photosensitive drum
1, a charger
2, a developer
134 and
a cleaner
6 that are all needed in common electro-photography. Conventionally,
the charger
2 is usually a non-contact type corona charger that charges
the photosensitive drum
1 surface by providing corona produced by high-voltage
applied to a thin corona discharge wire. In recent years, however, contact-type
chargers have been most preferably used because of their advantages of lower pressure
process, less ozone emission and lower cost. This is a method of, for example,
contacting a roller charger material (hereinafter, a roller charger) to the surface
of the photosensitive drum
1 and charging the photosensitive drum
1
by applying voltage to this roller charger
2. Although voltage applied to
the roller charger
2 may be DC voltage alone, charging becomes uniform if
AC voltage is additionally applied to repeat a plus/minus discharge alternatively.
By exposing the uniformly charged photosensitive drum
1 to laser light using
the scanner
111, the desired latent image is formed thereon and this latent
image is transformed into a toner image by the developer
134.
A development bias is applied to the development roller constituting the developer
134. As the bias voltage for development, only DC voltage is applied when
the development roller
134 contacts the photosensitive drum
1, while
AC voltage is added to DC voltage during non-contact operation. The toner image
on the photosensitive drum
1 is transferred to a sheet P by a transfer roller
113.
Downstream of the process cartridge
112 a fixer F affixes the toner
image transferred to a sheet P by applying heat and pressure thereto. The fixer
F is generally comprised of a fixer roller
117, a heater
116 that
heats the fixer roller
117, a pressure roller
118 and a temperature
sensor
140, such as a thermistor. The pressure roller
118 is pressed
against the fixer roller
117 by a spring unit (not shown). Downstream of
the fixer F are fixer exit rollers
139 and a fixer unit sensor
119
that detects the passage of a sheet P.
Downstream of the fixer exit rollers
139, the transport path is
branched and a flapper
120 decides the way of paper transport. In usual
single-sided printing, a sheet P is conveyed to the outside of the body by the
output rollers
122, while for double-sided printing it is sent to the transport
unit D.
The transport unit D for double-sided printing has a sheet turn-over unit equipped
with reverse rollers
123 and a reverse sensor
124, and a re-feeder
unit equipped with a D-cut roller
125, a sensor
126 and transport
rollers
127.
The transport path is branched upstream of the reverse rollers
123, and
the reverse sensor
124 is installed near the branching point. A sheet P
is stopped in the position where the end of the sheet P has traveled a prescribed
distance passing the reverse sensor
124, and then sent to the re-feeder
unit by reverse rotation of the reverse rollers
123.
When the turn-over unit sensor
126 has detected the passage of the sheet
P, the transport rollers
127 convey sheet P to the transport roller
108
again for re-feeding. Later, the sheet P passes the resist rollers
109 again,
and the transfer roller
113 conducts image formation on the other side of
the sheet P. Then the sheet P is guided by the flapper
120 to output rollers
122 for output after toner is fixed by the fixer F.
In this type of image forming apparatus, the number of sheets waiting in the
transport
path in the sheet turn-over mechanism and re-feeder mechanism is determined according
to sheet sizes, and their printing sequence is optimized for efficient double-sided
printing (for example, as discussed in Japanese Patent Application Laid-open No.
2002-091102). If a large number of sheets are to be printed double-sided, their
printing sequence is changed so that the number of sheets waiting in the transport
path in the sheet turn-over mechanism and re-feeder mechanism is maximized according
to sheet sizes. Such changes of printing sequence are conducted by altering the
page sequence based on printing information that is sent from a PC, for example,
and stored in the memory of the printer.
However, when the memory capacity in the printer is small, it cannot hold
the printing information of many pages and thus the printing sequence cannot be
changed. When the memory capacity is small, the sheet is turned over after its
first side is printed and then re-fed for printing on the other side (rear face).
Each of two or more sheets is printed in this manner. Then, instead of plural sheets,
only one sheet is held in the transport path of the sheet turn-over mechanism and
the re-feeder mechanism.
Regardless of memory capacity, when only one sheet is printed double-sided,
the sheet is turned over after one side is printed and re-fed for printing on the
other side (rear face). In addition, when a double-sided copy is made by scanning
a document with a scanner, printing is done while the document is being scanned.
Since the page sequence cannot be changed in this case, it is repeated in many
cases to turn over the sheet after one side is printed and then re-feed it for
printing on the other side, when two or more document pages are scanned for double-sided copying.
When the sheet is turned over after one side is printed and then re-fed for
printing on the other side and therefore the transport path in the sheet turn-over
mechanism and the re-feeder mechanism holds only one sheet at a time, it takes
time to turn over and re-feed the paper. Then the power to the charger for the
electro-photographic process is suspended, or the heater for fixing is deactivated
to prevent the image carrier from wearing and unnecessary heater operation (for
example, as discussed in Japanese Patent Application Laid-open No. 8-320642).
However, in such a double-sided image forming apparatus, there will be a
significant difference in the rotation time of the photosensitive drum per sheet
between continuous double-sided printing and double-sided printing on only one sheet.
FIG. 2 is a timing chart for continuous double-sided printing in the prior art
image forming apparatus, and it illustrates the timing for continuous 4-sheet double-sided
printing. FIG. 3 is a timing chart for one-sheet double-sided printing in the prior
art image forming apparatus.
In general, after AC voltage and DC voltage for charging are raised to prescribed
values, DC high-voltage is applied as the bias voltage for development in the pre-rotation
process, and then AC high-voltage is applied in the printing process as the bias
voltage for development. Transfer high-voltage is applied when a sheet P passes
the transfer unit. During the interval of sheet printing, the AC high-voltage for
development is lowered and the transfer high-voltage is also lowered to a level
for the interval. When the last page is printed, the post-rotation process starts,
and the transfer high-voltage, DC high-voltage for development, DC high-voltage
for charging and AC high-voltage for charging are lowered in this order.
In FIG. 2, when a first side of the first sheet is printed and the sheet has
reached
the turn-over point, a first side of the second sheet is printed. When the first
sheet has reached the transport unit in the turn-over unit and the second sheet
has reached the turn-over point, a first side of the third sheet is printed, and
then the second side of the first sheet, a first side of the fourth sheet and the
second side of the second sheet are printed sequentially. When the second side
of the third sheet and the second side of the fourth sheet are printed in a row,
the double-sided printing on four sheets is over.
Referring now to FIG. 2, because printing is completed in a short time
in continuous double-sided printing, the interval period of time per sheet does
not much affect the life of the photosensitive drum
1. The life is as long
as that of the drum used in continuous single-sided printing.
On the other hand, when double-sided printing is repeated for each single sheet,
the steps of printing on a first side, paper interval, and printing on the second
side are repeated, as shown in FIG. 3. Such operation is seen when the memory does
not have a capacity large enough to store the image data of plural pages or when
an image forming apparatus equipped with a read scanner conducts double-sided copying.
During the time interval between printing on a first side and printing on the other
side, namely, the period of time from the turn-over of a sheet P to its re-feeding,
the photosensitive drum
1 keeps rotation. Because usually it takes as much
time as printing two or three pages to turn over sheet P and re-feed it, the life
of the photosensitive drum
1 becomes equally shorter.
Image forming apparatuses are expected to run faster and faster. Thus if the
next feed process is started after the feeding of each previous sheet is completed,
the feeding speed itself must be raised. Otherwise, even if the feeding speed is
raised, there will be a limit to throughput.
To solve such problems, printing data is stored in a printing data reservation
memory, and as soon as the printing requirements are met paper is fed for printing
based on the data stored in the memory, in order to feed not only the next sheet
but also further latter sheets at a time (hereinafter, preliminary feeding; for
example, as discussed in Japanese Patent Application Laid-open Nos. 2002-046876,
2001-192132, 2001-088406 and 2001-088370). By virtue of this improvement, throughput
can be easily maximized without raising the paper feeding speed too much or raising
print cost, even when the transport path for recording sheets is rather long.
In many printers, a single driving source (motor) is used to rotate the image
carrier and transport rollers for lower cost. The motor is directly connected to
the driver of the image carrier, while its connection to transport rollers is switched
by a clutch. In the image forming apparatus of such structure, the sheet is turned
over after its first side is printed and then re-fed for printing on the other
side. Then a single sheet is held for double-sided printing in the transport path
in the sheet turn-over mechanism and the re-feeder mechanism. If the abovementioned
preliminary feeding is adopted in this system to maximize throughput, the following
problems arise.
If a single sheet is to be printed double-sided, it is possible to stop the rotation
of the image carrier by suspending high-voltage for electro-photography while the
one-side printed sheet is turned over and fed again. However, in the case of continuous
double-sided printing of plural sheets, the transport rollers must be kept rotating
for preliminary feeding of the subsequent sheets, while the one-side printed sheet
is turned over and fed again. Since the image carrier shares the driving source
with the transport rollers, its rotation cannot be stopped during preliminary feeding.
As a result, throughput can be maximized with no increased cost, but such a problem
results that the image carrier wears fast and comes to the end of its life early
because it keeps rotating and receives a high-voltage while the one-side printed
sheet is turned over and re-fed.
In cases other than double-sided printing, a similar problem will arise when
the
paper interval is long in usual single-sided printing.
SUMMARY OF THE INVENTION
The present invention has been made to solve such problems, and provides an image
forming apparatus where the life of the image carrier does not become significantly
short even when the distance between individual sheets is rather long.
Another object of the invention is to provide an image forming apparatus
that can extend the life of the image carrier while maintaining maximized throughput.
To attain these objects, forming an electrostatic latent image on an image carrier,
in one aspect of the present invention an image forming apparatus includes: a charging
unit for charging the image carrier; a charge voltage loading unit for applying
charge voltage to the charging unit; an exposure unit for exposing the image carrier
charged by the charging unit to form an electrostatic latent image corresponding
to image signals; a development unit for forming a toner image by developing the
electrostatic latent image formed on the image carrier by the image carrier; an
image transfer unit for continuously transferring the toner image formed by the
development unit onto a plurality of recording materials; and a control unit for
controlling AC charge voltage applied by the charge voltage loading unit to the
charging unit, wherein, when the transport interval of the plural recording materials
is shorter than a predetermined time the AC charge voltage applied to the image
carrier during the transport interval is a first AC charge voltage, and when the
transport interval is longer than the predetermined time the AC charge voltage
applied to the image carrier during the transport interval is a second AC charge
voltage, the control unit makes the current running in the charging unit to which
the second AC charge voltage is applied lower than the current running in the charging
unit to which the first AC charge voltage is applied.
In another aspect, the image forming apparatus that forms an electrostatic latent
image on an image carrier includes: a charging unit for charging the image carrier;
a charge voltage loading unit for applying charge voltage to the charging unit;
an exposure unit for exposing the image carrier charged by the charging unit and
forming an electrostatic latent image corresponding to image signals; a development
unit for forming a toner image by developing the electrostatic latent image formed
on the image carrier by the image carrier; an image transfer unit for continuously
transferring the toner image formed by the development unit onto a plurality of
recording materials; a fixer unit for fixing the toner image transferred by the
image transfer unit to the recording material; a transport unit for transporting
the recording material to the image transfer unit to transfer a toner image onto
the other side of the recording material where a toner image has been fixed by
the fixer unit; and a control unit for controlling AC charge voltage applied by
the charge voltage loading unit to the charging unit. While the transport unit
is not transporting the recording material the AC charge voltage is a first AC
charge voltage, and while the transport unit is transporting the recording material
the AC charge voltage is a second AC charge voltage, and the control unit makes
the current running in the charging unit to which the second AC charge voltage
is applied lower than the current running in the charging unit to which the first
AC charge voltage is applied.
In another aspect, the image forming apparatus that forms an electrostatic latent
image on an image carrier includes: a charging unit for charging the image carrier;
a charge voltage loading unit for applying charge voltage to the charging unit;
an exposure unit for exposing the image carrier charged by the charging unit and
forming an electrostatic latent image corresponding to image signals; a development
unit for forming a toner image by developing the electrostatic latent image formed
on the image carrier by the image carrier; an image transfer unit for continuously
transferring the toner image formed by the development unit onto a plurality of
recording materials; a fixer unit for fixing the toner image transferred by the
image transfer unit to the recording material; a feeder unit for feeding the recording
material from a recording material container where a plurality of recording materials
are loaded; a transport unit for transporting the recording material to the image
transfer unit to transfer a toner image onto the other side of the recording material
where a toner image has been fixed by the fixer unit; a control unit for controlling
AC charge voltage applied by the charge voltage loading unit to the charging unit;
and a memory unit for storing the image formation conditions about the plural recording
materials based on the command sent from an external device. While the transport
unit is not transporting the recording material, the AC charge voltage is a first
AC charge voltage, while the transport unit is transporting the recording material
and the feeder unit is feeding the recording material subsequent to said recording
material based on the image formation conditions stored in the memory unit, the
AC charge voltage is a second AC charge voltage, and the control unit makes the
current running in the charging unit to which the second AC charge voltage is applied
lower than the current running in the charging unit to which the first AC charge
voltage is applied.
According to the above configurations, it becomes possible to prevent the
image carrier from wearing by an optimized control based on individual print conditions
such that only a single side is printed, alternative double-sided print holding
plural sheets in a standby status in the turn-over unit, and double-sided printing
is conducted while only one sheet is held in the turn-over unit.
According to the above configurations, it becomes possible to prevent the
image carrier from wearing while minimizing the decrease in throughput by conducting
preliminary paper feeding upon the resumption of image carrier rotation even when
a print reservation is made during the period while the paper is under transport
for double-sided printing and the rotation of the image carrier is suspended.
According to the present invention related with an image forming apparatus
that charges the image carrier by contacting a voltage-loaded charging material
thereto, it becomes possible to reduce the wear of the image carrier and thereby
significantly extend its useful life by lowering AC voltage or AC current applied
to the charging unit when it is known in advance that the paper interval during
continuous printing becomes longer than usual.
Furthermore, if any subsequent print job is reserved, the preliminary
feeding of paper is conducted for the reserved job during the time while the first
sheet is turned over and transported to the position of re-feeding for double-sided
printing in the interval between printing on its first side and printing on the
other side to maximize throughput with no rise in cost. No preliminary paper feeding
becomes necessary when no subsequent print job is reserved when the first sheet
is turned over and transported to the position of re-feeding for double-sided printing
in the interval between printing on its first side and printing on the other side.
Thus, during this period, both DC and AC voltages are terminated and the rotation
of the photosensitive drum is suspended to further reduce the wear of the photosensitive
drum. As a result, the throughput is maintained high with no rise in cost, and
the wear of the photosensitive drum is prevented in the optimized manner by controlling
the drum rotation and voltage output for charging corresponding to individual conditions
for double-sided printing. In addition, energy saving effects are provided by eliminating
unnecessary drum operation and charging power.
The above and other objects, effects, features and advantages of the present
invention will become more apparent from the following description of embodiments
thereof taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic structure of the prior art image forming apparatus;
FIG. 2 is a timing chart for continuous double-sided printing in the prior art
image forming apparatus;
FIG. 3 is a timing chart for single-sheet double-sided printing in the prior
art image forming apparatus;
FIG. 4 is a schematic structure of the image forming apparatus of a first embodiment
of the invention;
FIG. 5 is a diagram of an embodiment of the high-voltage output circuit for charging;
FIG. 6 is a characteristic chart of AC voltage for charging and charge current;
FIG. 7 is a characteristic chart of charge current and potential of the photosensitive drum;
FIG. 8 is a timing chart for the image forming apparatus of the first embodiment;
FIG. 9 is a schematic structure of the image forming apparatus of a second embodiment
of the invention;
FIG. 10 is a characteristic diagram illustrating the step-down and step-up of
charge current;
FIG. 11 is a timing chart for continuous single-sided printing in the second
embodiment of the image forming apparatus equipped with a plurality of paper feeder ports;
FIG. 12 is a timing chart for continuous double-sided printing in the second
embodiment of the image forming apparatus equipped with a plurality of paper feeder ports;
FIG. 13 is a timing chart for the image forming apparatus of a third embodiment;
FIG. 14 is a schematic structure of the image forming apparatus of a fourth
embodiment and a fifth embodiment of the invention;
FIG. 15 is a block diagram (No. 1) illustrating the functions of the
fourth and fifth embodiments;
FIG. 16 is a block diagram (No. 2) illustrating the functions of the
fourth and fifth embodiments;
FIGS. 17A-17K are diagrams illustrating the print reservation tables for the
image forming apparatus of the fourth embodiment;
FIG. 18 is a timing chart for printing in the image forming apparatus of the
fourth embodiment;
FIG. 19 is a flowchart showing the relationship of FIGS. 19A and 19B;
FIG. 19A is a flowchart (No. 1) illustrating the printing operation of
the engine controller of the image forming apparatus of the fourth embodiment;
FIG. 19B is a flowchart (No. 2) illustrating the printing operation of
the engine controller of the image forming apparatus of the fourth embodiment;
FIGS. 20A-20K are diagrams illustrating the print reservation tables (double-sided
printing on two pages) for the image forming apparatus of the fifth embodiment;
FIG. 21 is a timing chart (double-sided printing on two sheets) in the image
forming apparatus of the fifth embodiment;
FIGS. 22A-22M are diagrams illustrating the print reservation tables (double-sided
printing on two pages plus single-sided printing) for the image forming apparatus
of the fifth embodiment;
FIG. 23 is a timing chart (double-sided printing on two pages and single-sided
printing) in the image forming apparatus of the fifth embodiment; and
FIG. 24 is a flowchart showing the relationship of FIGS. 24A and 24B;
FIG. 24A is a flowchart (No. 1) illustrating the printing operation of
the engine controller of the image forming apparatus of the fifth embodiment; and
FIG. 24B is a flowchart (No. 2) illustrating the printing operation of
the engine controller of the image forming apparatus of the fifth embodiment.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Now the preferred embodiments of the present invention will be described with
reference to the accompanying drawings.
[Embodiment 1]
FIG. 4 is a schematic structure of a laser beam printer that is an embodiment
of the image forming apparatus of the invention.
The laser beam printer
100 of this embodiment has a paper cassette
101
holding recording material, namely, recording paper P, a paper cassette paper detection
sensor
102 that detects the presence/absence of recording paper P in the
paper cassette
101, a paper size sensor
103 that detects the size
of recording paper P in the paper cassette
101, a pickup roller
104
that picks up recording paper P from the paper cassette
101, a transport
roller
105 that conveys recording paper P picked up by the pickup roller
104, and a retard roller
106 that is paired with the transport roller
105 and prevents recording paper P from being conveyed in a stack.
Downstream of the feeder roller
105 are a paper feeder sensor
107
that monitors the state of paper sheets transported from a turn-over unit D (to
be described later), a paper transport roller
108 that conveys recording
paper P further downstream, a pair of resist rollers
109 that convey recording
paper P in synchronization, and a pre-resist sensor
110 that monitors the
state of recording paper P transported to the resist roller pair
109.
Downstream of the resist roller pair
109 are a process cartridge
112 that forms a toner image on the photosensitive drum
1 by the
use of laser light from a laser scanner
111 (to be described later), a transfer
roller
113 that transfers the toner image formed on the photosensitive drum
1 onto the recording paper P, and a discharge unit
114 (hereinafter,
discharge wire) that facilitates the charge removal from the recording paper P
and thereby helps it leave the photosensitive drum
1.
Further downstream of the discharge wire
114 are a transport guide
115, a fixer unit F having a pressure roller
118 and a fixer roller
117 equipped therein with a halogen heater
116 for thermally affixing
the toner image transferred to the recording paper P, fixer exit rollers
139,
a fixer unit sensor
119 that monitors the state of paper sheets transported
from fixer unit F, and a flapper
120 that switches the path of recording
paper P sent from fixer unit F to either an output unit or the turn-over unit D
for double-sided printing. Downstream on the output side, a paper output sensor
121 that monitors the state of paper sheets sent to the output unit and
a pair of output rollers
122 for ejecting recording paper are installed.
The turn-over unit D for double-sided printing turns over the recording paper
P, of which either side has been printed, for printing on the other side, and sends
it to the image forming unit again. This turn-over unit D has a pair of reverse
rollers
123 that switch back the recording paper P by rotating in forward/reverse
directions, a reverse sensor
124 that monitors the state of the recording
paper P transported to the reverse roller pair
123, a D-cut roller
125
that transports recording paper P from a transverse resist unit (not shown) that
aligns recording paper P in the transverse direction, a turn-over unit sensor
126
that monitors the state of recording paper P in turn-over unit D for double-sided
printing, and a pair of transport rollers
127 in turn-over unit that transport
recording paper P from turn-over unit D to the feeder unit.
The scanner
111 has a laser unit
129 that emits laser light modulated
by image signals sent from an external device
128 (to be described later),
a polygon mirror
130 and a scanner motor
131 for scanning laser light
of the laser unit
129 on the photosensitive drum
1, an image formation
lens assembly
132, and a return mirror
133.
The process cartridge
112 has a photosensitive drum
1 needed for
common electro-photography, a charging roller
2 working as a charger, a
development roller
134 and a toner cassette
135 that work as a developer,
and a cleaning blade
6 that is a cleaning unit. The process cartridge is
attached to the laser printer
100 detachably.
The laser beam printer
100 has a high-voltage power supply
3 and
a printer controller
4. The high-voltage power supply
3 has a high-voltage
output circuit for charging
30 (shown in FIG. 5) (to be described later),
the developer roller
134, the transfer roller
113, and a high-voltage
output circuit that supplies a desired voltage to the discharge wire
114.
The printer controller
4 that controls the laser beam printer
100
has a CPU
5 equipped with a RAM
5a, a ROM
5b,
a timer
5c, a digital I/O port (hereinafter, I/O port)
5d,
an analog-digital converter input port (hereinafter, A/D port)
5e and
a digital-analog output port (hereinafter, D/A port)
5f, as well
as input-output control circuits (not shown). The printer controller
4 is
connected to the external device
128, such as a personal computer, via an
interface
138.
FIG. 5 is a diagram illustrating the structure of an embodiment of the charging
high-voltage output circuit in the high-voltage power supply. The control of high-voltage
output for charging by CPU
5 of the invention is explained with reference
to this charging high-voltage output circuit
30.
The charging high-voltage output circuit
30 produces high-voltage for
charging by overlapping charging AC high-voltage Vcac onto charging DC high-voltage
Vcdc, and provides the output from the output terminal
31 of FIG. 5. The
output terminal
31 is connected to the charging roller
2 that contacts
the photosensitive drum
1.
When the I/O port
5d of CPU
5 provides clock pulses (PRICLK),
a transistor Q
1 switches via a pull-up resistor R
1 and a base resistor
R
2, and the pulses are amplified to have amplitudes corresponding to the
output of an operation amp OP
1 connected to a pull-up resistor R
3
via a diode D
1. The operation amp OP
1 is part of a current detection
unit
35 and will be explained in detail later. When the amplitudes of clock
pulses are large, the amplitudes of sinusoidal driving voltage waves (voltage from
peak to peak) provided to a high-voltage transformer TR (to be described later)
become also large. Thereby, voltage from peak to peak, indicating the level of
charging AC high-voltage Vcac, is raised.
The clock pulses (PRICLK) are provided to the primary coil of high-voltage transformer
TR via a filter circuit
32 and a high-voltage transformer driver circuit
33 of a push-pull type. Namely, the clock pulses(PRICLK) amplified by operation
amp OP
1 are sent to the filter circuit
32 via a capacitor C
1,
with the filter circuit
32 consisting of resistors R
4-R
14,
capacitors C
2-C
6 and operation amps OP
2, OP
3 providing
sinusoidal waves across +12V.
The output from the filter circuit
32 is entered to the primary coil of
the high-voltage transformer TR via the push-pull type high-voltage transformer
driver circuit
33, which includes a transistor Q
2, a Zener diode
D
2, resistors R
15-R
19 and transistors Q
3, Q
4,
and via a capacitor C
7, to produce sinusoidal waves of charging AC high-voltage
Vcac on the secondary coil side. One of the terminals of the secondary side of
the high-voltage transformer TR is connected to a charging DC high-voltage generator
circuit
34 via a resistor R
20. Thus, the charging high-voltage V
where charging AC high-voltage Vcac is overlapped on charging DC high-voltage Vcdc
is provided from the output terminal
31 via an output protection resistor
R
21, and then supplied to the charging roller
2.
Next explained is the current detection unit
35 of the charging AC high-voltage
circuit
30.
As described above, the charging AC current Iac produced by the charging AC high-voltage
generator circuit
30 is provided to the current detection circuit, namely,
the current detection unit
35. In this current detection unit
35,
the charging AC current Iac from the charging AC high-voltage generator circuit
30 passes a capacitor C
8, and the half-waves of direction A run through
a diode D
3, while the half-waves of direction B run through a diode D
4.
The half-waves of direction A that have passed the diode D
3 are provided
to an integral circuit composed of an operation amp OP
4, a resistor R
22
and a capacitor C
9, and then converted into DC voltage. Additionally, a
resistor R
28 is provided.
Voltage at output (V
1) in the operation amp OP
4 is expressed by:
V1=-(
Rs×Imean)+
Vt (Eq.1)
where Imean is the mean of the charging AC current Iac half-waves, Rs the resistance
of resistor R
22, and Vt the voltage supplied to the positive input of operation
amp OP
4. This voltage Vt is a voltage provided by splitting an output (PRION)
from the I/O port
5d of CPU
5 by resistors R
25, R
26,
and thereafter, inputting it into a transistor Q
5 so that the output of
the transistor Q
5 is split by resistors R
23, R
24.
The output from operation amp OP
4 is connected to the positive input of
operation amp OP
1 for comparison with the level of a current control signal
(PRICNT) at the minus input. The current control signal (PRICNT) is a signal used
to set the current level of the charging AC current Iac.
If the output voltage (V
1) from operation amp OP
4 is larger than
setting voltage (Vc) used to set by the current control signal (PRICNT), the output
from operation amp OP
1 grows. As explained previously, when the output from
operation amp OP
1 grows, the amplitudes of clock pulses provided to the
filter circuit
32 also grow and thereby voltage from peak to peak of the
charging AC high-voltage Vcac becomes large. Here, a capacitor C
10 and a
resistor R
29 are provided for the operation amp OP
1. In addition,
a resistor R
27 is provided to adjust an input resistance of the operation
amp OP
1.
Under such configuration, the peak to peak voltage of the charging AC high-voltage
Vcac is controlled so that the charging AC current Iac has a value corresponding
to the setting voltage Vc used to set by the current control signal (PRICNT). In
other words, a constant current control is conducted according to the current control
signal (PRICNT)
FIGS. 6-8 are diagrams illustrating the charging control in this embodiment.
FIG. 6 is a characteristic chart of the charging AC high-voltage Vcac and the charging
current Iac. FIG. 7 is a characteristic chart of the charging current Iac and the
surface potential Vd of the photosensitive drum
1. FIG. 8 is a timing chart
for the image forming apparatus.
In FIG. 6, graph AA shows the characteristics of early stages of the photosensitive
drum
1, while graph BB shows the characteristic of the state of the photosensitive
drum
1 after a lapse of significant time.
The charging AC current (Iac) running in the charging roller
2 steps up
straightforwardly when the applied charging AC voltage Vcac of the charging roller
2 has low peaks, and the charging AC current (Iac) increases after passing
a threshold for starting of discharge. Namely, the difference between the solid
line and the broken line extrapolated from the straight line of the early-stage
of the photosensitive drum
1 becomes a discharge current Is for charging.
The constant current is controlled so that this discharge current Is for charging
falls in a prescribed range. In general, when the discharge current Is for charging
is low the image quality is impaired because of shortage of charging, while if
the discharge current Is for charging is large then damage to the photosensitive
drum
1 grows and it quickly wears.
In this embodiment, by setting the current control signal from the D/A port
5f
to Vc
1 at early stages of the photosensitive drum
1, the AC current
Iac
1 (applied AC voltage: Vpp
1) as shown in FIG. 6 is held constant
by the CPU
5 to provide a discharge current Is
1. Meanwhile, when
significant time has passed for the photosensitive drum
1, it shows the
characteristics of graph BB. If the applied AC voltage Vpp
1′ is set
so that the charge current Iac becomes Iac
1, the discharge current of the
early stage of the photosensitive drum
1 increases to Is
1′
from Is
1, and damage to the photosensitive drum
1 also increases.
As a result, after a predetermined time of use, the CPU
5 controls such
that the discharge current is set to Is
2 (>>Is
1) by changing
the current control signal from the D/A port
5f to Vc
2 from
Vc
1 and the constant current (changing AC current) Iac to Iac
2 (applied
AC high-voltage Vcac>>Vpp
2).
Now the relationship between the charge AC current Iac and the photosensitive
drum potential Vd is explained with reference to FIG. 7. When the current control
signal (PRICNT) increases to the setting voltage Vc by CPU
5, the discharge
current Is for charging also increases from an initial current IacO according to
the characteristics shown in FIG. 6 and the potential Vd of the photosensitive
drum
1 increases, approaching the charging DC high-voltage Vcdc applied
to the charging roller
2. With the charge current Iac
1 (Iac
2)
for setting the discharge AC current Is for changing at a prescribed value Is
1
(Is
2), the potential Vd of the photosensitive drum
1 is sufficiently
stabilized and poor charging does not occur (region indicated by arrow as shown
FIG. 7).
Charging control by the CPU
5 conducted during double-sided printing
of recording paper P is explained with reference to FIG. 8. Much like FIG. 3, FIG.
8 shows a timing chart for double-sided continuous printing to print either side
and then print the other side on each of three recording papers P.
When it has been decided to print either side of the recording paper P and then
print the other side of the recording paper P like this example, the charging AC
high-voltage Vcac for charging is kept, while the period of time the sheet (hereinafter
transporting for double-side printing) is printed one-sided, turned over and re-fed,
at a value (hereinafter, LOW value) lower than that running during the printing process.
This LOW setting is a setting of voltage Vc in the current control signal (PRICNT)
provided from the D/A port
5f of CPU
5 at a voltage VcZ which
is lower than the voltage Vc
1 adopted during printing by the photosensitive
drum
1 onto the recording paper P. As described later, a predetermined time
is needed from the time the voltage Vc in the charge current signal (PRICNT) is
switched to the time the charge current Iac running in the charging roller
2
has stabilized at a constant value. Thus, during the step-down of charge voltage,
the charge current Iac changes from iac
1 to IacZ after a predetermined time
Tdn has passed since the CPU
5 switched voltage Vc in the current control
signal (PRICNT) from Vc
1 for printing (Vc
2 after the photosensitive
drum
1 has been used for a sufficiently long time) to VcZ for the LOW setting.
Meanwhile, during the step-up of charge voltage, the charge current Iac changes
from IacZ to Iac
1 (Iac
2 after the photosensitive drum
1 has
been used for a sufficiently long time) after a predetermined time Tup has passed
since CPU
5 switched voltage Vc in the current control signal (PRICNT) from
VcZ for the LOW setting to voltage Vc
1 for printing (Vc
2 after the
photosensitive drum
1 has been used for a sufficiently long time). Thus,
from FIG. 6, at an early stage of the photosensitive drum
1, when charge
current value Iac changes from Iac
1 to IacZ (the charge AC voltage Vcac
changes from Vpp
1 to VppZ), a discharge current Is drops from Is
1
to IsZ. After a significant lapse of time for the photosensitive drum
1,
the charge current value Iac changes from Iac
2 to IacZ (the charge AC voltage
Vcac changes from Vpp
2 to VppZ′), and there occurs a drop from is
2
to IsZ′.
This charging AC current Iacz at LOW value as shown in FIG. 7 (hatched area)
is a current level that causes poor charging if adopted during printing and sufficiently
lower than the charging AC currents Iac
1 and Iac
2 during printing.
Then the discharge current Is for early stages where the charging AC current
Iac is IacZ and the discharge current Is running after a sufficient time of using
the photosensitive drum
1 becomes IsZ and IsZ′. The discharge current
Is becomes IsZ or ISZ′, during printing. Since the difference in discharge
current between ISZ and ISZ′ is lower than that between Is
1 and Is
2
during printing, the discharge current Ic increases is reduced after a sufficient
time of using the photosensitive drum
1, to reduce wear of the photosensitive
drum
1.
Even when two or more values for constant current control can be set in the
charging roller
2, the system structure and control sequence are simplified
in the first embodiment by setting only one value for the AC voltage for charging
during the interval during double-sided printing.
Meanwhile, by setting photosensitive drum potential Vd at a value larger
than DC voltage Vdc for development, it becomes possible to prevent toner pick-up
to the white areas of the photosensitive drum
1 and to avoid both contamination
of the transfer roller
113 by toner and waste of toner. In other words,
by setting (LOW value) the charging AC current Iac for paper interval (during double-sided
printing) at a value in the hatched area of FIG. 7, such troubles can be avoided
and wear of the photosensitive drum
1 can be reduced.
Furthermore in this embodiment, switching of the charging AC current
Iac to the LOW value is carried out between the time the first side is printed
and the time the paper is re-fed for printing on the second side, with reference
to the vertical synchronization signal of image (VSYNC). This switching may be
done based on the signals from the fixer unit sensor
119, the reverse sensor
124 in the turn-over unit and the turn-over unit sensor
126.
In this embodiment, the period of time of LOW setting of the charging AC current
during double-sided printing on one recording paper P accounts for 50% of the total
charge time. Wear of the photosensitive drum
1 during the LOW setting is
less by 30% than that during the regular setting. As a result, the life of the
photosensitive drum
1 is extended by 15% in total at double-sided printing
on one recording paper P.
When using an image forming apparatus equipped with such a life detection means
for estimating the useful life of the photosensitive drum
1 as shown in,
for example, Japanese Patent Application Laid-open No. 10-039691, the wear coefficient
corresponding to wear of the photosensitive drum
1 per use-time during the
LOW setting may be set at 0.7, considering the above 30% improvement in life, in
comparison with 1.0 that is the wear coefficient for regular setting (unless LOW setting).
[Embodiment 2]
Now a second embodiment of the present invention is described below. In the above
first embodiment for double-sided printing, what will be printed after a first
side of a sheet has been printed is the other side of the same sheet. In other
words, when double-sided printing is conducted sheet by sheet, the charging AC
high-voltage Vcac is lowered while the period of time the sheet is printed one-sided,
turned over and re-fed, and wear of the photosensitive drum
1 can be reduced.
The second embodiment will describe to wear of the photosensitive drum
1
can be reduced that can be used one-sided printing with regular printing operation
unless double-sided printing on recording paper P.
FIG. 9 is a schematic sectional view of the laser beam printer of the second
embodiment of the invention. Its structure is very similar to that of the laser
beam printer of the first embodiment shown in FIG. 4. It has three paper feeder
cassettes
101-
1,
101-
2 and
101-
3 for
paper feeding. Corresponding to each of the paper feeder cassettes
101-
1,
101-
2, and
101-
3 are paper cassette detection sensors
102-
1,
102-
2,
102-
3, respectively, paper
size sensors
103-
1,
103-
2, and
103-
3,
respectively, pick-up rollers,
104-
1,
104-
2, and
104-
3,
respectively, transport rollers
105-
1,
105-
2, and
105-
3,
respectively, and retard rollers
106-
1,
106-
2, and
106-
3, respectively. The components of the same structures and functions
of the laser beam printer of the second embodiment have the same reference numbers
throughout the figures, and their descriptions are not repeated.
In the second embodiment, the paper feeder cassettes
101-
1 and
101-
2
have the same specifications, while the cassette
101-
3 is a deck
type cassette of a larger capacity.
FIG. 10 shows the characteristics of the step-down and step-up of an AC charge
current observed when an AC high voltage for charging Vcac is switched. When the
CPU
5 switches the AC charge current Iac
1 for printing to IacZ for
the LOW setting for the transport interval (paper interval) between a preceding
recording paper P and a subsequent recording paper P by controlling the AC high-voltage
for charging Vcac, which is loaded to the charging roller
2, the AC current
Iac
1 for printing reaches the AC charge current IacZ after step-down time
Tdn has passed. Meanwhile, when IacZ for the LOW setting is switched to the AC
charge current Iac
1 for printing, the AC charge current IacZ reaches the
AC charge current Iac
1 after the step-up time Tup has passed.
A transport interval Tr represents the time between the moment the back end of
the preceding recording paper P passes an image transfer nip where the transfer
roller
113 contacts the photosensitive drum
1 and the moment the
front edge of the subsequent recording paper P reaches the image transfer nip.
This transport interval Tr must be long enough to cover both step-down time and
step-up time of the AC charge current Iac to conduct printing on each recording
paper P with no problem.
In general, during continuous printing for preceding page data printing and subsequent
page data printing, a print reservation (discussed further in connection with the
description of fourth embodiment) is made and paper feeding is completed earlier
for higher throughput (output sheet number of recording paper P per use-time) when
the next sheet to be printed is decided. The paper feeding operation of the subsequent
recording paper P is completed before the preceding recording paper P is ejected
out of printer. The recording papers P are held by the resist rollers
109,
and the paper is re-fed with a predetermined timing to secure transport interval
Ts for continuous printing.
A transport interval Tt for feeding paper is the time between which a tip of a
recording paper P is picked up from the paper feeder cassette
101 by the
pick-up roller
104 and the time at which it reaches the resist rollers
109.
A waiting time Tw is the time the recording paper P waits in the resist rollers
109. These intervals are decided by the specifications of the employed image
forming apparatus. The transport interval of the feeder paper becomes longer depending
on the distance from the outlet of each of the paper cassettes
101-
1,
101-
2 and
101-
3 to the resist rollers
109, where
Tt
1 is a transport time of feeder paper from the outlet of the paper cassette
101-
1 to the resist rollers
109, and Tt
2 and Tt
