Image formation apparatus

An image formation apparatus includes an image carrier on which a developer image is to be formed, an image transfer device configured to transfer the developer image formed on the image carrier to a medium at an image transfer position, a controller configured to control drive of the image carrier and the image transfer device, a first medium feeder configured to feed the medium to the image transfer position along a medium conveyance path extending from the first medium feeder to the image transfer position, and a medium detector provided between the first medium feeder and the image transfer position in the medium conveyance path. The controller is configured to control the drive of the image carrier on the basis of a medium-detection result by the medium detector.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority based on 35 USC 119 from prior Japanese Patent Application No. 2011-016101 filed on Jan. 28, 2011, entitled “IMAGE FORMATION APPARATUS”, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to an image formation apparatus of electrophotography or the like.

2. Description of Related Art

A conventional image formation apparatus transfers a toner image as a developer image to a sheet as a medium by means of a photosensitive drum as an image carrier and an image transfer roller as an image transfer device as disclosed, for example, in Japanese Patent Application Laid-open No. 2006-124058.

SUMMARY OF THE INVENTION

The conventional image formation apparatus, however, starts driving the photosensitive drum or the image transfer roller at the same time as the feeding of a sheet from a sheet feeder cassette. Accordingly, the photosensitive drum or the image transfer roller of the conventional image formation apparatus, in some cases, lacks a sufficient service life.

An aspect of the invention is an image formation apparatus including: an image carrier on which a developer image is to be formed; an image transfer device configured to transfer the developer image formed on the image carrier to a medium at an image transfer position; a controller configured to control drive of the image carrier and the image transfer device; a first medium feeder configured to feed the medium to the image transfer position along a medium conveyance path extending from the first medium feeder to the image transfer position; and a medium detector provided between the first medium feeder and the image transfer position in the medium conveyance path. The controller is configured to control the drive of the image carrier on the basis of a medium-detection result by the medium detector.

Another aspect of the invention is an image formation apparatus including: an image carrier on which a developer image is formed; an image transfer device configured to transfer the developer image formed on the image carrier to a medium; a controller configured to control drive of the image carrier and the image transfer device; a first medium feeder configured to feed the medium in the medium-conveyance direction to convey the medium to the image transfer device; a medium-size detector configured to detect a conveyance-direction dimension of the medium; and a leading-end detector provided at a first distance from the image transfer device in a downstream direction and configured to detect the leading end of the medium discharged from the image transfer device. The controller is configured to stop driving the image carrier when the medium is conveyed over a distance determined by subtracting the first distance from the conveyance-direction dimension after the leading-end detector detects the leading end of the medium.

DETAILED DESCRIPTION OF EMBODIMENTS

Descriptions are provided hereinbelow for embodiments based on the drawings. In the respective drawings referenced herein, the same constituents are designated by the same reference numerals and duplicate explanation concerning the same constituents is omitted. All of the drawings illustrate the respective examples only.

First Embodiment

Configuration of First Embodiment

FIG. 1is a schematic diagram illustrating the configuration of image formation apparatus10according to a first embodiment of the invention.

Image formation apparatus10is a printer of a tandem type. Image formation apparatus10includes a main body including medium feeder F, as well as first optional tray11-3and second optional tray11-2that are additionally provided to the image formation apparatus main body. Thus, medium feeders of image formation apparatus10include: medium feeder F as a third medium feeder that image formation apparatus10is originally equipped with and provided in a bottom portion of the main body of image formation apparatus10; second optional tray11-2as a second medium feeder that is additionally provided below medium feeder F of the image formation apparatus main body; and first optional tray11-3as a first medium feeder that is additionally provided below second optional tray11-2.

The main body of image formation apparatus10includes medium feeder F, image formation section20, fixation unit40, discharger unit50, and stacker55or face-up stacker56. Medium feeder F is configured to feed print medium100, which may be a recording sheet. Image formation section20is configured to form a toner image as a developer image. Fixation unit40is configured to fix the toner image to a surface of print medium100. Discharger unit50is configured to discharge print medium100. Stacker55or face-up stacker56is configured to contain discharged print medium100. In addition, image formation apparatus10includes various motors that are configured to rotate the rollers and the like (described later) and various clutches configured to turn ON and OFF the transmission of power to the rollers provided in a medium conveyance path. Furthermore, image formation apparatus10includes high-voltage power supply63and a low-voltage power supply. High-voltage power supply63shown inFIG. 3and described later supplies high voltages ranging from 200V to 5000V to charger roller24, image transfer roller21, and the like in image formation unit22. The low-voltage power supply supplies DC electric power with voltages of 5 V, 24 V, and the like to the circuits and motors.

Medium feeder F of the image formation apparatus main body is detachably set in a lower portion of the main body of image formation apparatus10. Medium feeder F includes sheet cassette110-1, pick-up roller12-1, sheet-feeder roller13-1, sheet-feed sensor14-1, first IN-sensor15-1, second IN-sensor17, WR sensor19, first resist-roller pair16-1, and second resist-roller pair18. Sheet cassette110-1as a medium storage detachably set in a lower potion of the main body of image formation apparatus10is capable of storing print media100therein. Pick-up roller12-1works together with a blade-shaped separator or the like to pick up print media100one by one from sheet cassette110-1. Sheet-feeder roller13-1is configured to feed print medium100thus taken out of the sheet cassette110-1. Sheet-feed sensor14-1is configured to judge whether print medium100is fed. First IN-sensor15-1, second IN-sensor17, and WR sensor19are configured to judge the position of print medium100. First resist-roller pair16-1and second resist-roller pair18are configured to convey print media100to image formation section20.

Second optional tray11-2as a second medium feeder, includes sheet cassette110-2, pick-up roller12-2, sheet-feeder roller13-2, sheet-feed sensor14-2, conveyance sensor15-2, and conveyance rollers16-2. Sheet cassette110-2as a medium storage is capable of storing print media100therein. Pick-up roller12-2works together with a blade-shaped separator to pick up print media100one by one from sheet cassette110-2. Sheet-feeder roller13-2is configured to feed print medium100thus taken out of the sheet cassette110-2. Sheet-feed sensor14-2is configured to judge whether print medium100is fed. Conveyance sensor15-2is configured to judge the position of print medium100. Conveyance rollers16-2are configured to convey print medium100to image formation apparatus10.

First optional tray11-3(serving as a first medium feeder) has a configuration that is similar to the configuration of second optional tray11-2.

Each of sheet cassettes110(=110-1to110-3) can store a plurality of print media100therein. Print media100is used to print either monochrome or color images, and have various predetermined sizes. For example, print media100is a sheet of high-quality paper, recycled paper, gloss paper, or matte paper. In addition, an OHP (over head projector) film may also be used as print media100.

Pick-up rollers12(=12-1to12-3) are provided respectively in medium feeder F of the main body of image formation apparatus10, in second optional tray11-2, and in first optional tray11-3. Each pick-up roller12is capable of rotating while being pressed onto the top surface of the stacked print media100. In the medium conveyance path, sheet-feeder rollers13(=13-1to13-3) are provided downstream of their corresponding pick-up rollers12(=12-1to12-3). In addition, sheet-feed sensors14(=14-1to14-3) are provided downstream of their corresponding sheet-feeder rollers13(=13-1to13-3).

Inside the medium feeder F, first IN-sensor15-1is provided downstream of sheet-feed sensor14-1along the medium conveyance path in a manner that first IN-sensor15-1can detect print medium100. Downstream of first IN-sensor15-1, first resist-roller pair16-1, second IN-sensor17, second resist-roller pair18, and WR sensor19are provided in this order.

Inside second optional tray11-2, which is provided at the upstream side of medium feeder F, conveyance sensor15-2is provided downstream of sheet-feed sensor14-2along the medium conveyance path in a manner that conveyance sensor15-2can detect print medium100. In the medium conveyance route, conveyance sensor15-2is provided at a substantially linear section of the medium conveyance path where print medium100is conveyed stably. Conveyance rollers16-2are provided downstream of conveyance sensor15-2. Conveyance rollers16-2of second optional tray11-2convey print medium100towards image formation section20through medium feeder F of the main body of image formation apparatus10. To put it differently, the downstream side of conveyance rollers16-2of second optional tray11-2is connected to an upstream-side position of first IN-sensor15-1and first resist-roller pair16-1provided in medium feeder F of the main body of image formation apparatus10.

Conveyance sensor15-3is provided along the medium conveyance path in the first optional tray11-3and downstream of sheet-feed sensor14-3in a manner that conveyance sensor15-3can detect print medium100. Conveyance rollers16-3are provided downstream of the conveyance sensor15-3. Conveyance rollers16-3convey print medium100via conveyance sensor15-2and conveyance rollers16-2provided in second optional tray11-2to the position of first IN-sensor15-1and first resist-roller pair16-1provided in medium feeder F of the main body of image formation apparatus10.

Image formation section20of the main body of image formation apparatus10includes four image formation units22(=22-1to22-4) arranged in the order of of black (K), yellow (Y), magenta (M), and cyan (C) from the left hand side ofFIG. 1. Image formation section20also includes image transfer rollers21(=21-1to21-4) respectively including photosensitive drums23-1to23-4provided therebelow. Medium conveyance mechanism30is provided below image formation section20. Medium conveyance mechanism30includes driven roller32, belt drive roller33, and conveyor belt31for medium conveyance wound across driven roller32and belt drive roller33. Belt drive roller33is configured to drive conveyor belt31. Driven roller32, on the other hand, rotates along with the rotation of conveyor belt31and rotates conveyor belt31.

Each of four image formation units22respectively corresponding to black (K), yellow (Y), magenta (M), and cyan (C) includes photosensitive drum23, charger roller24, light-emitting diode (hereinafter referred to as LED) head25, development roller26, developer supply roller27, developer storage cartridge29, an unillustrated toner regulation member, and cleaning blade28. Photosensitive drum23is configured to carry an electrostatic latent image based on image information. Charger roller24is configured to charge photosensitive drum23. LED head25is configured to irradiate the surface of photosensitive drum23with light based on the image information. Development roller26is configured to develop the electrostatic latent image on the photosensitive drum2by using toner as the developer. Developer supply roller27is configured to supply the toner to development roller26. Developer storage cartridge29can be detachably set in image formation unit22. Cleaning blade28shown inFIG. 2(described in detail later) is configured to scrape off the toner remaining on the surface of the photosensitive drum23. Between each of photosensitive drums23(=23-1to23-4) and the corresponding one of image transfer rollers (=21-1to21-4), the upper line of conveyor belt31of medium conveyance mechanism30and is in contact with the image carriers and with the image transfer devices. Conveyor belt31rotates to convey print medium100to the nip portions between photosensitive drums23(=23-1to23-4) and image transfer rollers21(=21-1to21-4) one after another. Image formation section20serves as a development device configured to develop toner image on print medium100.

Fixation unit40includes fixation roller41, back-up roller42, and heater43. Heater43, which is provided in fixation roller41, is a halogen lamp or the like. Fixation unit40serving as a fixation device is configured to apply heat and pressure to print medium100, thereby fixing the toner image.

Discharger unit50includes EXIT sensor51as a leading-end detector and pairs of discharger rollers52to54. EXIT sensor51is positioned downstream of image formation unit22-4provided on the most downstream side among the four image formation units22in the image formation section20. EXIT sensor51and image formation unit22-4are separated away from each other by a first distance. EXIT sensor51thus positioned detects discharge of print medium100from image formation section20. Pairs of discharger rollers52to54are provided downstream of fixation unit40along the medium conveyance path so as to nip print medium100. Unillustrated motors drive these pairs of discharger rollers52to54.

Incidentally, point P between conveyance rollers16-2and first IN-sensor15-1inFIG. 1is the most downstream position of print medium100in a zone that allows the toner-image transfer at image transfer position B to be performed in time. To put it differently, point P is located immediately before a zone that does not allow any more the toner-image transfer at image transfer position B to be performed in time. Hence, if the image formation process to form a toner image on the surface of photosensitive drums23-1is started while print medium100is being conveyed on the downstream side of this point P in the medium conveyance path, the toner image fails to reach image transfer position B in time for the arrival of print medium100at image transfer position B. This means that the toner image fails to get ready for the toner-image transfer operation that occurs at image transfer position B. In contrast, if the image formation process to form a toner image is started while print medium100is still being conveyed at the upstream side of this point P in the medium conveyance path, the arrival of the toner image at image transfer position B can be synchronized with the arrival of print medium100at image transfer position B. This means that the toner image is ready for the toner-image transfer operation to be performed at image transfer position B.

When distance L2shown inFIG. 1represents the distance between point P and the most upstream image transfer position B (i.e., nip B between the most upstream photosensitive drum23-1and the most upstream image transfer roller21-1), and distance L1represents the distance between the most upstream image transfer position B (the most upstream nip B) and contact point A where pick-up roller12-1of medium feeder F is in contact with print medium100, preferably, L1=L2.

FIG. 2is a schematic diagram illustrating the configuration of image formation unit22of image formation apparatus10shown inFIG. 1.

Each of image formation units22includes photosensitive drum23, charger roller24, LED head25, development roller26, supply roller27, developer storage cartridge29, and cleaning blade28. Charger roller24is pressed onto photosensitive drum23. LED head25is provided above photosensitive drum23. Development roller26contacts photosensitive drum23. Supply roller27is in contact with development roller26. Developer storage cartridge29is provided above supply roller27. Cleaning blade28contacts the surface of photosensitive drum23.

Photosensitive drum23includes a conductive base layer made of aluminum or the like. Photosensitive drum23also includes a photosensitive layer formed on the conductive base layer and including a photoconductive layer and a charge transportation layer. Photosensitive drum23has a cylindrical shape, and is supported rotatably. Photosensitive drum23is in contact with charger roller24, with image transfer roller21, and with development roller26. In addition, the leading end portion of cleaning blade28contacts the photosensitive drum23. Electrical charges are accumulated on the surface of the photosensitive drum23, and thereby photosensitive drum23serves as an image carrier that is configured to carry a toner image. Photosensitive drum23rotates anticlockwise inFIG. 2. Components of each image formation unit22are described below in an order of the rotation of photosensitive drum23.

Charger roller24is made of a conductive metal shaft coated with a semiconductor rubber such as silicone rubber, has a cylindrical shape, is supported rotatably, and is pressed onto photosensitive drum23. Charger roller24is electrically charged by high-voltage power supply63shown inFIG. 3and described later. Charger roller24rotates while being pressed onto photosensitive drum23, and thereby applies a predetermined voltage to photosensitive drum23. Consequently, the surface of photosensitive drum23accumulates electrical charges uniformly.

LED head25includes LEDs, a lens array, and an LED driver element and is provided above photosensitive drum23. LED head25radiates light based on the image information onto the surface of photosensitive drum23, and thereby serves as a light-exposure device that is configured to form an electrostatic latent image on the surface of the photosensitive drum23.

Supply roller27is made of a conductive metal shaft coated with rubber. Supply roller27has a cylindrical shape and is in contact with development roller26. Supply roller is electrically charged by high-voltage power supply63shown inFIG. 3and described later. Supply roller27is pressed onto development roller26, and thereby supplies toner to development roller26.

Development roller26is made of a conductive metal shaft coated with a semiconductor urethane rubber material or the like and has a cylindrical shape. Development roller26is in contact with photosensitive drum23and the leading end portion of the unillustrated toner regulation member at the circumferential surface. Development roller26is electrically charged by high-voltage power supply63shown inFIG. 3and described later. Development roller26is pressed onto supply roller27, and thereby toner is supplied to the developer roller26.

The unillustrated toner regulation member is made of stainless steel or the like and has a plate shape. The toner regulation member has a leading end portion that contacts the surface of development roller26. The toner regulation member scrapes off the excess portion of a predetermined amount of the toner on the surface of development roller26. Thus, the toner regulation member regulates the thickness of the toner on the surface of the development roller26in a manner that a toner layer with a uniform thickness can always be formed on the surface of the development roller26.

Cleaning blade28is made of a rubber material or the like and has a plate shape. Cleaning blade28is provided so that the leading end portion of cleaning blade28contacts the surface of photosensitive drum23. After the toner image formed on the surface of photosensitive drum23is transferred to the surface of print medium100, cleaning blade28cleans the surface of photosensitive drum23by scraping off the toner remaining on the surface.

FIG. 3is a diagram illustrating the circuit configuration of image formation apparatus10shown inFIG. 1.

I/O port68is connected to sheet-feed sensor14-1, first IN-sensor15-1, second IN-sensor17, WR sensor19, and EXIT sensor51. VIDEO processor circuit (counter)69is connected to LED heads25(=25-1to25-4) and DRAM70.

I/O port71is connected to sheet-feeder motor91via driver circuit72, and is also connected to resist motor93via driver circuit73. In addition, I/O port71is connected to drum motor94via driver circuit74, and is also connected to belt motor95via driver circuit75. Moreover, I/O port71is connected to fixation motor96via driver circuit76, and is also connected to heater43via heater driver circuit77.

Controller60is configured to entirely control image formation apparatus10. By monitoring the detection results of sensors14,15,17,19, and51, controller60controls the driving of, and the application of voltages to, rollers12,13,16,18,52,53, and54, as well as fixation unit40, photosensitive drum23, image transfer roller21, and conveyor belt31. In addition, controller60controls the image formation process. Image processor circuit61is configured to take in image data sent via control signal line87as a connection device from a host or an external image transfer apparatus that is connected to image formation apparatus10and convert the image data into a printable data format. Display unit62is configured to monitor the state of image formation apparatus10, and to prompt the user to take appropriate actions. ROM65is configured to store control programs that are necessary for the operation of this first embodiment. RAM66serves as a working memory for the control programs of this first embodiment. Non-volatile memory67is configured to store information which is needed for the control of this first embodiment and which must be kept even after image formation apparatus10is powered OFF.

I/O port68monitors the states of sheet-feed sensor14-1, first IN-sensor15-1, second IN-sensor17, WR sensor19, EXIT sensor51, and other unillustrated sensors. VIDEO processor circuit69is configured to output the image data that have been converted by image processor circuit61to LED heads25as the light-exposure devices. In addition, VIDEO processor circuit69is configured to count, by using DRAM70, the number of dots that are outputted (emitted) in the printing. DRAM70is configured to store temporarily the image data outputted by image processor circuit61.

I/O port71is configured to output, to driver circuits72to76, control signals that make these driver circuits72to76drive sheet-feeder motor91, resist motor93, belt motor95, drum motor94, and fixation motor96. I/O port71also outputs, to heater driver circuit77, a control for driving heater43of fixation unit40. Timer78is configured to perform timer processes that are necessary for the controls.

High-voltage power supply63is configured to output high-voltage signals that are necessary for image formation. High-voltage power supply63is configured to output voltages CH-1to CH-4applied respectively to charger rollers24-1to24-4, voltages DB-1to DB-4applied respectively to development rollers26-1to26-4, output voltages SB-1to SB-4applied respectively to supply rollers27-1to27-4and output voltages TR-1to TR-4applied respectively to image transfer rollers21-1to21-4. Optional tray IF circuit79is configured to communicate with each of optional trays11(first optional tray11-3and second optional tray11-2in this embodiment) shown inFIG. 4and described later.

FIG. 4is a diagram illustrating the circuit configuration of optional tray11of image formation apparatus10shown inFIG. 1.

Each of optional trays11(first optional tray11-3and second optional tray11-2in first embodiment) includes optional tray controller80, main board interface circuit (herein after, simply referred to as “main board IF circuit”)81, optional tray ROM82, RAM83, I/O ports84and85, driver circuit86, sheet-feed sensor14(14-2or14-3), conveyance sensor15(15-2or15-3), tray motor92, and control signal line87.

Optional tray controller80is configured to perform overall control of optional tray11. Main board IF circuit81is communicably connected to optional tray IF circuit79of image formation apparatus10.

Optional tray ROM82is configured to store programs that are used to control optional tray11. I/O port84is configured to monitor sheet-feed sensor14and conveyance sensor15in optional tray11. I/O port85is configured to output, to driver circuit86, control signals for driving tray motor92.

A circuit board of optional tray11is mounted in each of second and first optional trays11-2and11-3. The circuit boards of second and first optional trays11-2and11-3are capable of communicating individually with controller60of the main body of image formation apparatus10.

Operations of Comparative Example

Case of Using Medium Feeder F of Image Formation Apparatus Main Body

By referring toFIG. 1, description is given of the operations of a comparative example of a case where print medium100is fed from medium feeder F of the main body of image formation apparatus10.

Print medium100is conveyed from the upstream side to the downstream side along the medium conveyance path. Sheet cassette110-3is located at the upstream end of the medium conveyance path whereas stacker55is located at the downstream end of the medium conveyance path.

Image formation apparatus10is connected to a host computer or an external apparatus (not illustrated) via a cable or wirelessly. When image formation apparatus10receives, from the host computer or the external apparatus, print data and an instruction to make the printing using print media100stored in medium feeder F of the main body of image formation apparatus10, image formation apparatus10makes sheet-feeder motor91rotate pick-up roller12-1and sheet-feeder roller13-1. With the rotation of pick-up roller12-1, each of print media100is separated from the others. Then, print media100thus separated are sent one by one to the downstream side of the medium conveyance path.

Print medium100sent from pick-up roller12-1is further conveyed by sheet-feeder roller13-1. Note that the driving of resist motor93, belt motor95, drum motor94, and fixation motor96is started substantially at the same time as the driving of sheet-feeder motor91.

Print medium100that has been conveyed by the drive force of sheet-feeder motor91is then conveyed substantially at the same speed by first resist-roller pair16-1and then by second resist-roller pair18. To this end, resist motor93is rotated substantially at the same speed beforehand.

In addition, the image formation process is started by the charging operation to electrically charge the surface of photosensitive drum23as the image carrier at a certain potential or even higher. Hence, photosensitive drums23and conveyor belt31are made to rotate at the same speed by drum motor94and belt motor95, respectively.

Note that, if the temperature of fixation roller41is at a target temperature, the driving of fixation motor96is started substantially at the same time as the feeding of print medium100is started as described earlier. If, conversely, the temperature of fixation roller41is lower than the target temperature, the driving of fixation motor96is started before the feeding of print medium100is started to warm up fixation roller41.

Once sheet-feed sensor14-1detects that print medium100is fed properly, butting control to correct skew of print medium100is performed by using first IN-sensor15-1and first resist-roller pair16-1. While skew is corrected, the drive force of resist motor93is cut off by an unillustrated drive-force transmission device. When the butting action of a predetermined amount is completed, the drive-force transmission device is switched to a transmission state, and thus the conveyance of print medium100is resumed.

Print medium100passes through second IN-sensor17, and then is conveyed to second resist-roller pair18. Print medium100is then conveyed by second resist-roller pair18to pass through WR sensor19, and then gets on top of conveyor belt31to be conveyed by conveyor belt31.

To be more specific, print medium100turns ON WR sensor19, and then is conveyed to conveyor belt31located downstream in the medium conveyance path. A certain time after WR sensor19is turned ON, LED heads25of image formation units22of black (K), yellow (Y), magenta (M), and cyan (C) start radiating light to form electrostatic latent images of their respective colors on their respective photosensitive drums23.

Belt drive roller33rotates to drive the conveyor belt31wound across belt drive roller33and the driven roller32to rotate along the medium conveyance path. Print medium100is conveyed by the driving of conveyor belt31sequentially to the four image formation units22(=22-1to22-4) arranged in the order of black (K), yellow (Y), magenta (M), and cyan (C).

Photosensitive drum23of each of the four image formation units22of black (K), yellow (Y), magenta (M), and cyan (C) rotates anticlockwise, and the surface of each photosensitive drum23is uniformly charged by the corresponding charger roller24. Each uniformly charged photosensitive drum23is then irradiated with light based on the image data received from the host computer or the external apparatus radiated by LED head25. Thus, an electrostatic latent image is formed on photosensitive drum23. Photosensitive drum23with the electrostatic latent image formed on the surface is then subjected to a development of the toner image performed by supply roller27and development roller26. Image transfer roller21and photosensitive drum23sandwich (nip) therebetween print medium100and conveyor belt31and convey print medium100. A voltage of approximately +3000 V that is applied to each image transfer roller21attracts, to print medium100, the toner on the surface of photosensitive drum23. Thereby each toner image is transferred to the surface of print medium100. Print medium100with the transferred toner images is then sent to fixation unit40. The toner remaining on photosensitive drum is scraped off by cleaning blade28, and thus photosensitive drum23is made ready for the formation of another toner image.

The image formation process includes: a step of electrically charging the surface of photosensitive drum23by charger roller24; irradiating the surface of photosensitive drum23with light radiated by LED head25to form an electrostatic latent image on the surface of photosensitive drum23; developing the electrostatic latent image formed on the surface of photosensitive drum23as the image carrier by development roller26to form a toner image on the surface of photosensitive drum23; and transferring the toner image formed on the surface of photosensitive drum23to the surface of print medium100by image transfer roller21.

After toner images of the four colors of black (K), yellow (Y), magenta (M), and cyan (C) are transferred to the surface of print medium100, print medium100is conveyed to fixation unit40. In fixation unit40, print medium100is nipped by and conveyed through nip portion formed by fixation roller41and back-up roller42. In the nip portion, the heat from fixation roller41and the pressure caused by the biasing force of back-up roller42are applied to print medium100. Thus, the toner is melted and the toner images are fixed to the surface of print medium100.

After the toner images are fixed to the surface of print medium100, the leading end of print medium100is detected by EXIT sensor51. Then, print medium100is conveyed by the rotations of pairs of discharger rollers52to54. A predetermined time after print medium100passes through EXIT sensor51, belt motor95and drum motor94are stopped. A predetermined time after that, fixation motor96is stopped. Print medium100thus conveyed is then discharged to stacker55or face-up stacker56through a discharge route selected by the user.

Operations of Comparative Example

Case of Using First Optional Tray11-3

By referring toFIG. 1, description is given of the operations of the comparative example of a different case where print medium100is fed from first optional tray11-3of image formation apparatus10.

Firstly, image formation apparatus10receives, from an external apparatus or a host computer (not illustrated), print data and an instruction to perform the printing using print media100stored in first optional tray11-3. Then, image formation apparatus10makes first-optional-tray motor92-3rotate pick-up roller12-3and sheet-feeder roller13-3. With the rotations of pick-up roller12-3, each of print media100is separated from the others. Then, the print media100thus separated are sent, one by one, to the downstream side of the medium conveyance path.

After print medium100passes through pick-up roller12-3, sheet-feeder roller13-3further conveys print medium100. Note that the driving of resist motor93, belt motor95, drum motor94, fixation motor96, and second-optional-tray motor92-2is started substantially at the same time as the driving of first-optional-tray motor92-3is started.

The image formation process is started by the start of the charging operation to electrically charge the surface of photosensitive drums23at a certain potential or higher.

Note that, if the temperature of fixation roller41is at a target temperature, the driving of fixation motor96is started substantially at the same time as the feeding of print medium100is started as described earlier. If, conversely, the temperature of fixation roller41is lower than the target temperature, the driving of fixation motor96is started before the feeding of print medium100is started to warm up fixation roller41.

When sheet-feed sensor14-3detects that print medium100is fed properly, butting control to correct skew of print medium100is performed by using conveyance sensor15-3and conveyance rollers16-3. Print medium100passes through conveyance sensor15-2as a medium detector, conveyance rollers16-2, first IN-sensor15-1, first resist-roller pair16-1, and second IN-sensor17to be conveyed to second resist-roller pair18. After print medium100is conveyed to second resist-roller pair18, print medium100passes through WR sensor19, and then gets on top of conveyor belt31to be further conveyed by conveyor belt31.

The operations performed thereafter are the same as those performed in the case where print medium100is fed from medium feeder F of the main body of image formation apparatus10. Print medium100turns ON WR sensor19, and then is conveyed to conveyor belt31located downstream of WR sensor19in the medium conveyance path. Then, print medium100is conveyed sequentially to the four image formation units22of black (K), yellow (Y), magenta (M), and cyan (C) arranged in this order.

Toner images of the four colors are conveyed to print medium100by the four image formation units22, and then the toner images are fixed by fixation unit40.

After the toner images are fixed to the surface of print medium100, the leading end of print medium100is detected by EXIT sensor51, and print medium100is conveyed by the rotations of pairs of discharger rollers52to54. A predetermined time after print medium100passes through EXIT sensor51, belt motor95and drum motor94are stopped. A predetermined time after that, fixation motor96is stopped. Print medium100thus conveyed is then discharged to stacker55or face-up stacker56through a discharge route selected by the user.

Operations of First Embodiment

A feature of the first embodiment lies in the control method of a case where print medium100is conveyed over a long distance. So, the following description is given of the operations of a case where print medium100is fed from first optional tray11-3. Note that the operations of the first embodiment is also applicable to a case where print medium100is fed from second optional tray11-2. If print medium100fed from medium feeder F has to be conveyed over a long distance, the operations of the first embodiment is also applicable to a case where print medium100is fed from medium feeder F.

Image formation apparatus10receives, from an unillustrated external apparatus, print data and an instruction to perform the printing using print media100stored in first optional tray11-3. Then, image formation apparatus10makes first-optional-tray motor92-3rotate pick-up roller12-3and sheet-feeder roller13-3. With the rotation of pick-up roller12-3, each of print media100is separated from the others. Then, the print media100thus separated are sent, one by one, to the downstream side of the medium conveyance path.

In this first embodiment, on the basis of the print data sent from the host computer or the external apparatus, image processor circuit61as a size detector detects the conveyance-direction dimension of print medium100.

After print medium100passes through pick-up roller12-3, sheet-feeder roller13-3further conveys print medium100. Note that in this first embodiment, the driving of resist motor93, fixation motor96, and second-optional-tray motor92-2is started substantially at the same time as the driving of first-optional-tray motor92-3. Unlike the comparative example, the driving of neither belt motor95nor drum motor94is started at that timing.

The rotations of neither belt motor95nor drum motor94of this first embodiment are started substantially at the same time as the rotations of first-optional-tray motor92-3are started because first optional tray11-3is provided at the upstream side, in the medium-conveyance direction, of point P as the most downstream position for print medium100within the zone that allows the toner-image transfer at image transfer position B to be performed in time. Thus, the distance from this first optional tray11-3to image transfer position B is long. Accordingly, at the time when the feeding of print medium100from first optional tray11-3is started, the position of print medium100while being conveyed in the medium-conveyance path is still within a zone that allows the toner-image transfer at image transfer position B to be performed in time. The above-mentioned zone that allows the toner-image transfer at image transfer position B to be performed in time refers to a position that satisfies the following condition. The distance measured along the medium-conveyance path from each of the positions to image transfer position B of the most upstream image formation unit22-1is not shorter than the distance by which photosensitive drum23rotates for a period starting from the beginning of the image formation process to form a toner image on the surface of photosensitive drums23-1and ending with the transferring of the toner image thus formed onto the surface of print medium100at image transfer position B.

Note that, if the temperature of fixation roller41is at the target temperature, the driving of fixation motor96is started substantially at the same time as the feeding of print medium100is started as described earlier. If, conversely, the temperature of fixation roller41is lower than the target temperature, the driving of fixation motor96is started before the feeding of print medium100is started to warm up fixation roller41.

When sheet-feed sensor14-3detects that print medium100is fed properly, butting control to correct skew of print medium100is performed by using conveyance sensor15-3and conveyance rollers16-3. Print medium100passes through conveyance sensor15-2, conveyance rollers16-2, first IN-sensor15-1, first resist-roller pair16-1, and second IN-sensor17. Then, print medium100is conveyed to second resist-roller pair18.

In this first embodiment, when conveyance sensor15-2detects the leading end of print medium100, controller60makes optional tray controller80of second optional tray11-2count the number of drive pulses outputted from the detection position through I/O port85to driver circuits86-2of second-optional-tray motor92-2. With this counting, the driving of drum motor94and belt motor95is started at the time when the distance from the leading end of print medium100to the position of nip portion B (i.e., image transfer position) between photosensitive drum23-1and image transfer roller21-1becomes substantially equal to the distance L1from medium feeder F of the main body of image formation apparatus10to the position of nip portion B (at the time when the leading end of print medium100arrives at the most downstream position P within the above-described zone that allows the toner-image transfer to be performed in time).

In this first embodiment, the delayed start of the rotations of belt motor95and drum motor94reduces the number of rotations of photosensitive drum23and conveyor belt31rotating wastefully in the above-described comparative example. Hence, the wear of photosensitive drum23and conveyor belt31caused by their rotations can be reduced. In addition, even if print medium100is fed from optional tray11-3, the rotations of conveyor belt31and photosensitive drum23is started at the same timing as in the case where print medium100is fed from medium feeder F of the main body of image formation apparatus10. Hence, the surface of photosensitive drum23can be electrically charged reliably. As a consequence, the degradation of image quality due to charging failure can be avoided.

After that, print medium100is conveyed by second resist-roller pair18, and then passes through WR sensor19. Then print medium100gets on top of conveyor belt31to be conveyed further by conveyor belt31.

Print medium100turns ON WR sensor19, and then is conveyed to conveyor belt31that is located downstream in the medium conveyance path. Then, print medium100is conveyed sequentially to the four image formation units22of black (K), yellow (Y), magenta (M), and cyan (C) arranged in this order

Toner images of the four colors are transferred to print medium100by the image formation units22, and then the toner images are fixed to the surface of print medium100by fixation unit40.

After the toner images are fixed to the surface of print medium100, the leading end of print medium100is detected by EXIT sensor51, and print medium100is conveyed by the rotations of pairs of discharger rollers52to54. Note that in this first embodiment, image processor circuit61as a medium-size detector detects the conveyance-direction dimension of print medium100on the basis of the print data sent from a host computer or an external apparatus. Hence, the rotations of conveyor belt31and photosensitive drum23can be stopped before the trailing end of print medium100completely passes through fixation roller41and then through EXIT sensor51. Specifically, the number of drive pulses of belt motor95after the detection of the leading end of print medium100by EXIT sensor51is measured, and then whether or not the trailing end of print medium100has passed through the image transfer position of the most downstream image formation unit22-4is judged on the basis of the measured number of drive pulses. Then, belt motor95and drum motor94are stopped. The judgment relies on a threshold value of the number of drive pulses. The threshold value of the number of drive pulses is the number of drive pulses of a case where the print medium100is conveyed over a distance obtained by subtracting the first distance between the most downstream image formation unit22-4and EXIT sensor51from conveyance-direction dimension of print medium100.

A predetermined time after that, fixation motor96is stopped. Print medium100thus conveyed is then discharged to stacker55or face-up stacker56through a discharge route selected by the user.

FIG. 5is a flowchart illustrating the operations of image formation apparatus10according to the first embodiment of the invention.FIG. 6is a time chart illustrating the operations of image formation apparatus10according to the first embodiment of the invention.

In the time chart ofFIG. 6, the vertical axis represents an ON state at the upper position and an OFF state at the lower position. The horizontal axis represents the passage of time. The thick solid lines are of the control performed in the first embodiment while the thick dashed lines represent the control performed in the comparative example.

At the beginning of the process, at step S1, controller60of image formation apparatus10turns ON first-optional-tray motor92-3, second-optional-tray motor92-2, resist motor93, fixation motor96, and high-voltage power supply63not illustrated in the time chart but needed for the electrophotographic process.

At step S2, controller60of image formation apparatus10waits for the leading end of print medium100fed from first optional tray11-3to turn ON conveyance sensor15-2of second optional tray11-2.

At step S3, controller60of image formation apparatus10instructs optional tray controller80to continue the conveyance of print medium100until it is judged that the conveyance of print medium100over distance D1is completed. Distance D1is obtained by adding a safety margin to the distance over which print medium100is conveyed since the leading end of print medium100turns ON conveyance sensor15-2of second optional tray11-2until the trailing end of print medium100passes through conveyance rollers16-3of first optional tray11-3.

Upon detecting the turning ON of second IN-sensor17at step S5, controller60of image formation apparatus10waits for print medium100to be conveyed over distance D2after turning ON second IN-sensor17at step S6. Distance D2is obtained by adding a safety margin to the distance over which print medium100is conveyed after the leading end of print medium100turns ON second IN-sensor17until the trailing end of print medium100passes by conveyance rollers16-2of second optional tray11-2.

If controller60judges that print medium100is conveyed over distance D2after turning ON of second IN-sensor17(Yes at step S6), then at step S7controller60of image formation apparatus10instructs optional tray controller80to turn OFF second-optional-tray motor92-2thereby stopping the rotation of conveyance rollers16-2. Accordingly, from then onwards, print medium100is conveyed without being driven by second-optional-tray motor92-2(conveyance rollers16-2).

Upon detecting the turning OFF of WR sensor19by the trailing end of print medium100at step S8, controller60of image formation apparatus10waits for print medium100to be conveyed over distance D3at step S9. Distance D3is the safety margin after the trailing end of print medium100passes by WR sensor19.

If controller60of image formation apparatus10judges that print medium100is conveyed over distance D3after turning OFF WR sensor19(Yes at step S9), at step S10, controller60turns OFF resist motor93to stop the rotations of first resist-roller pair16-1and second resist-roller pair18. Hence, print medium100is conveyed toward the downstream by the conveyance force of photosensitive drum23and image transfer roller21and by the conveyance force of fixation unit40.

Upon detecting the turning ON of EXIT sensor51by the leading end of print medium100, at step S11, controller60of image formation apparatus10waits for print medium100to be conveyed over distance D4at step S12. Distance D4is obtained by adding a safety margin to the distance over which print medium100is conveyed after the leading end of print medium100passes through EXIT sensor51until the trailing end of print medium100passes through the most downstream image formation unit22-4.

If controller60of image formation apparatus10judges that print medium100is conveyed over distance D4after turning ON of EXIT sensor51(Yes at step S12), then at step S13, controller60stops drum motor94and belt motor95and turns OFF high-voltage power supply63.

If, at step S14, controller60of image formation apparatus10judges that the discharge of print medium100by fixation motor96has been completed and that the temperature of fixation unit40is low enough to stop the rotation, then at step S15, controller60stops fixation motor96, thereby terminates the print operations shown inFIG. 5.

Effects of First Embodiment

Image formation apparatus10of this first embodiment has the following effects (A) to (C).

(A) In the comparative example, even if image formation process is started while print medium100is within the zone that allows the transferring of the toner image to be performed in time (i.e., even if print medium100is at the upstream side of the most downstream position P of the above-mentioned zone for in-time toner-image transfer), photosensitive drum23and conveyor belt31rotate wastefully.

In contrast, in this first embodiment, the driving of drum motor94and belt motor95is started at the time when print medium100arrives at the most downstream position P of the zone that allows the toner-image transfer to be performed in time. Accordingly, photosensitive drum23and conveyor belt are prevented from rotating wastefully, and the service lives of these consumable members can be prolonged.

(B) In this first embodiment, belt motor95and drum motor94are stopped if it is judged that the trailing end of print medium100passes through the image transfer position of the most downstream image formation unit22-4. Accordingly, photosensitive drum23and conveyor belt31are prevented from rotating wastefully, and the service lives of these consumable members can be prolonged.

(C) Controller60controls the driving of photosensitive drum23and image transfer roller21on the basis of the detection results of conveyance sensor15-2provided at a substantially linear section of the medium conveyance path which allows print medium100to be conveyed stably and located at the upstream side, in the medium-conveyance direction, of photosensitive drum23-1and downstream, in the medium-conveyance direction, of first optional tray11-3. Accordingly, even if print medium100is fed from first optional tray11-3not smoothly, the driving of photosensitive drum23and image transfer roller21can be controlled stably.

Second Embodiment

Configuration of Second Embodiment

In a second embodiment of the invention, image formation apparatus10has an identical configuration to the one in the first embodiment shown inFIG. 1, but has different operations.

In the second embodiment, if it is judged that photosensitive drums23has a surface potential that is equal to or higher than a predetermined threshold, the charging step performed at the beginning of the above-described image formation process in the first embodiment is omitted. As a consequence, the start of the image formation process is delayed further.

FIG. 7is a chart illustrating the relationship between the surface potential of photosensitive drum23shown inFIG. 2and the elapsed time. The vertical axis represents the surface potential of photosensitive drum23whereas the horizontal axis represents the elapsed time.

The graph shown inFIG. 7illustrates how the surface potential of photosensitive drum23attenuates as time elapses since the application of voltage to photosensitive drum23is stopped. Threshold voltage Vx is the limit value of the surface potential that does not adversely affect the image quality. Threshold time Tx is the elapsed time until the surface potential is lowered down to threshold voltage Vx.

The data on the attenuation characteristics of the surface potential of photosensitive drum23shown inFIG. 7are stored in non-volatile memory67. Note that photosensitive drum23of this second embodiment preferably has favorable surface-potential attenuation characteristics so that the surface potential of photosensitive drum23attenuate slowly.

Operations of Second Embodiment

Operations of this second embodiment are described by referring toFIG. 1.

As in the first embodiment, a feature of the second embodiment lies in the control method of a case where print medium100is conveyed over a long distance. So, the following description is given of the operations of a case where print medium100is fed from first optional tray11-3. Note that the operations of the second embodiment is also applicable to a case where print medium100is fed from second optional tray11-2. If print medium100fed from medium feeder F of the main body of image formation apparatus10has to be conveyed over a long distance, the operations of the second embodiment is also applicable to a case where print medium100is fed from medium feeder F.

Image formation apparatus10receives, from an unillustrated external apparatus, print data and an instruction to make the printing be performed using print media100stored in first optional tray11-3. Then, image formation apparatus10makes first-optional-tray motor92-3rotate pick-up roller12-3and sheet-feeder roller13-3. With the rotations of pick-up roller12-3, each of print media100is separated from the others. Then, the print media100thus separated are sent, one by one, to the downstream side of the medium conveyance path.

As in the first embodiment, in this second embodiment, on the basis of the print data sent from the host computer or the external apparatus, image processor circuit61as a size detector detects the conveyance-direction dimension of print medium100.

After print medium100passes through pick-up roller12-3, sheet-feeder roller13-3further conveys print medium100. Note that, as in the first embodiment, in this second embodiment, the driving of resist motor93, fixation motor96, and second-optional-tray motor92-2is started substantially at the same time as the driving of first-optional-tray motor92-3is started. Unlike the comparative example, the driving of neither belt motor95nor drum motor94is started at that timing. In this second embodiment, the rotations of neither belt motor95nor drum motor94are started substantially at the same time as the rotations of first-optional-tray motor92-3are started. This is because of a similar reason to the one in the first embodiment.

Note that, if the temperature of fixation roller41is at the target temperature, the driving of fixation motor96is started substantially at the same time as the feeding of print medium100is started as described earlier. If, conversely, the temperature of fixation roller41is lower than the target temperature, the driving of fixation motor96is started before the feeding of print medium100is started to warm up fixation roller41.

When sheet-feed sensor14-3detects that print medium100is fed properly, butting control to correct skew of print medium100is performed by using conveyance sensor15-3and conveyance rollers16-3. Print medium100passes through conveyance sensor15-2, conveyance rollers16-2, first IN-sensor15-1, first resist-roller pair16-1, and second IN-sensor17. Then, print medium100is conveyed to second resist-roller pair18.

As in the first embodiment, in this second embodiment, when conveyance sensor15-2detects the leading end of print medium100, controller60makes optional tray controller80of second optional tray11-2count the number of drive pulses outputted from the detection position through I/O port85to driver circuits86-2of second-optional-tray motor92-2. On the basis of this counting, controller60of this second embodiment judges whether or not the distance from the leading end of print medium100to the position of nip portion B (image transfer position) between photosensitive drum23-1and image transfer roller21-1is equal to distance L1from medium feeder F to the position of nip portion B. If controller60judges that the above-mentioned two distances are substantially equal to each other, controller60then judges whether or not the charging-process suspension period measured by timer78as a charge-voltage judgment portion exceeds threshold time Tx shown inFIG. 7. Note that the charging-process suspension period is the time elapsed since the application of voltage to photosensitive drum23is stopped.

If controller60judges that the charging-process suspension period exceeds the threshold time Tx (i.e., that the image quality deteriorates when no extra charging process is performed), the driving of drum motor94and belt motor95is started, and high-voltage power supply63is turned ON, as in the case of the first embodiment.

If, in contrast, controller60judges that the charging-process suspension period does not exceed the threshold time Tx, that is, that the surface potential of photosensitive drum is high enough to allow image formation process to be performed without an extra charging process, controller60turns OFF only first-optional-tray motor92-3. Then, if controller60judges that print medium100has been conveyed over distance D5after the detection of the turning ON of WR sensor19, controller60starts driving drum motor94and belt motor95and turns ON high-voltage power supply63. Distance D5is the distance from the position where the leading end of print medium100reaches WR sensor19to a predetermined position that guarantees the in-time arrival of print medium100for image formation process. Note that, unlike the image formation process of the first embodiment, the image formation process of this second embodiment is not commenced by the start of the charging process but by the start of latent-image formation process to form an electrostatic latent image on the surface of photosensitive drum23as an image carrier.

In this second embodiment, controller60measures the length of time the charging process to electrically charge the surface of each photosensitive drum23is suspended. If the result of measurement does not reach threshold time Tx, controller60judges that the image quality is not adversely affected even when no extra charging process is performed. Hence, controller60begins the image formation process with the start of rotation of photosensitive drum23to start light exposure and with the start of the rotations of the conveyor belt31without performing any preceding charging process. The start of the driving of belt motor95and drum motor94at this time delays the start of rotations of photosensitive drums32and conveyor belt31in comparison to the cases of the control in the comparative example or of the control in the first embodiment. Accordingly, the numbers of rotations of photosensitive drum23and conveyor belt31are reduced from their respective counterparts in the case of the control of the comparative example or in the case of the control of the first embodiment. Consequently, the wear of these members can be reduced more effectively.

After that, print medium100is conveyed to second resist-roller pair18, and then passes through WR sensor19. Then print medium100gets on top of conveyor belt31to be conveyed further by conveyor belt31.

Print medium100turns ON WR sensor19, and then is conveyed to conveyor belt31that is located downstream in the medium conveyance path. Then, print medium100is conveyed sequentially to the four image formation units22of black (K), yellow (Y), magenta (M), and cyan, arranged in this order. Toner images of the four colors are transferred to print medium100by the image formation units22, and then the toner images are fixed to the surface of print medium100by fixation unit40.

After the toner images are fixed to the surface of print medium100, the leading end of print medium100is detected by EXIT sensor51, and then print medium100is conveyed by the rotations of pairs of discharger rollers52to54. Note that, as in the first embodiment, in this second embodiment, image processor circuit61as a medium-size detector detects the conveyance-direction dimension of print medium100on the basis of the print data sent from a host computer or an external apparatus. Hence, the rotations of conveyor belt31and photosensitive drum23can be stopped before the trailing end of print medium100completely passes through fixation roller41and then through EXIT sensor51. Specifically, the number of drive pulses of belt motor95after the detection of the leading end of print medium100by EXIT sensor51is measured, and then if it judged that the trailing end of print medium100has passed through the image transfer position of the most downstream image formation unit22-4on the basis of the measured number of drive pulses, belt motor95and drum motor94are stopped. A predetermined time after that, fixation motor96is stopped.

Print medium100thus conveyed is then discharged to stacker55or face-up stacker56through a discharge route selected by the user.

FIGS. 8 and 9are flowcharts (Part1and Part2) illustrating the operations of the image formation apparatus according to the second embodiment of the invention. Elements therein that are the same as the ones that appear inFIG. 5showing the first embodiment are denoted by their respective reference numerals.

FIG. 10is a time chart illustrating the operations of the image formation apparatus of the second embodiment of the invention of a case where the charging-process suspension period does not exceed threshold time Tx. Elements inFIG. 10same as the ones those inFIG. 6showing the first embodiment are denoted by their respective reference numerals. In the time chart ofFIG. 10, the vertical axis represents an ON state at the upper position and an OFF state at the lower position. The horizontal axis represents the passage of time. The thick solid lines are of the control performed in the second embodiment while the thick dashed lines represent the control performed in the comparative example.

At the beginning of the processes, the processes performed at steps S1to S3are the same as those the first embodiment. At step S20, controller60of image formation apparatus10judges whether or not the charging-process suspension period measured by timer78exceeds threshold time Tx shown inFIG. 7.

If charging-process suspension period does not exceed threshold time Tx, first-optional-tray motor92-3is turned OFF, and the rotations of pick-up roller12-3and sheet-feeder roller13-3are stopped at step S4A. From then onwards, print medium100is conveyed without being driven by first-optional-tray motor92-3(pick-up roller12-3and sheet-feeder roller13-3). The processes at steps S5A to S7A are similar to the processes at steps S5to S7in the first embodiment. If, at step S21A, the turning ON of WR sensor19is detected, controller60of image formation apparatus10waits for print medium100to be conveyed over distance D5at step S22. Distance D5is the distance from the position where the leading end of print medium100reaches WR sensor19to a predetermined position that guarantees the in-time arrival of print medium100for image formation process. If, at step S22, controller60of image formation apparatus10judges that print medium100has been conveyed over distance D5after turning ON of WR sensor19, then at step S23, controller60starts driving drum motor94and belt motor95and turns ON high-voltage power supply63.

If, in contrast, at step S20, the charging-process suspension period exceeds threshold time Tx, the processes at step S4to S7are performed, and then at step S21controller60waits for the turning ON of WR sensor19to be detected. Then at step S8, the process that is similar to that at step S8in the first embodiment is performed. After that, the processes of node A shown inFIG. 9are performed.

Once the processes of node A shown inFIG. 9are started, the processes of step S9to S15are performed as in the first embodiment. Then, the print process shown inFIG. 9is terminated.

Effects of Second Embodiment

Image formation apparatus10of this second embodiment can delay further the start of the driving of photosensitive drum23and conveyor belt31. Hence, the period in which the driving of photosensitive drum23and conveyor belt31can be suspended can be prolonged. Accordingly, the idle driving of photosensitive drum23and conveyor belt31can be reduced even from the case of the first embodiment. As a consequence, the service lives of these members can be prolonged even further.

Third Embodiment

Configuration of Third Embodiment

The configuration of image formation apparatus10of a third embodiment of the invention is identical to the configuration of image formation apparatus10of the first embodiment shown inFIG. 1.

Operations of Third Embodiment

A feature of the third embodiment lies in the control method of a case where print medium100is conveyed over a long distance. So, the following description is given of the operations of a case where print medium100is fed from first optional tray11-3. If the conveyed distance is long, the operations of the third embodiment is also applicable to a case where print medium100is fed from second optional tray11-2and to a case where print medium100is fed from medium feeder F.

Image formation apparatus10receives, from an unillustrated external apparatus, print data and an instruction to make the printing be performed using print media100stored in first optional tray11-3. Then, image formation apparatus10makes first-optional-tray motor92-3rotate pick-up roller12-3and sheet-feeder roller13-3. With the rotations of pick-up roller12-3, each of print media100is separated from the others. Then, the print media100thus separated are sent, one by one, to the downstream side of the medium conveyance path.

As in the first and the second embodiments, in this third embodiment, on the basis of the print data sent from the host computer or the external apparatus, image processor circuit61as the size detector detects the conveyance-direction dimension of print medium100.

After print medium100passes through pick-up roller12-3, sheet-feeder roller13-3further conveys print medium100. In this third embodiment, the driving of resist motor93, fixation motor96, second-optional-tray motor92-2, belt motor95, and drum motor94is started, and high-voltage power supply63is turned ON substantially at the same time as the driving of the first-optional-tray motor92-3is started. In this case, the drive speed (first speed) of belt motor95and drum motor94are slower at the beginning of the drive than the drive speed (second speed) of these motors95and94at the time of the toner-image transfer. The drive speed of these motors95and94is switched from the first speed to the second speed at a time when the leading end of print medium100reaches point P in the medium conveyance path. Accordingly, in comparison to a case where photosensitive drum23and conveyor belt31always rotate at the normal rotational speeds (moving speeds), the wear of photosensitive drum23and conveyor belt31can be reduced, and the service lives of photosensitive drum23and conveyor belt31can be prolonged.

Note that if the temperature of fixation roller41is at the target temperature, the driving of fixation motor96is started substantially at the same time as the feeding of print medium100is started as described earlier. If, in contrast, the temperature of fixation roller41is lower than the target temperature, the driving of fixation motor96precedes the start of the feeding of print medium10to warm up fixation roller41.

When sheet-feed sensor14-3detects that print medium100is fed properly, butting control to correct skew of print medium100is performed using conveyance sensor15-3and conveyance rollers16-3. Print medium100passes by conveyance sensor15-2, conveyance rollers16-2, first IN-sensor15-1, first resist-roller pair16-1, and second IN-sensor17, and then is conveyed to second resist-roller pair18.

As in the first and the second embodiments, if, in this third embodiment, the conveyance sensor15-2detects the leading end of print medium100, the counting of the number of drive pulses of driver circuit86-2of second-optional-tray motor92-2is started by using optional tray controller80of second optional tray11-2. Specifically, the counting starts at the time when the print medium100is at this detection position. On the basis of this counting, the drive speed of the drum motor94and that of belt motor95is switched to the drive speed (the second speed) at the time of toner-image transfer. Specifically, the switching of the speeds is done at a time when it is judged that the conveyance distance of the print medium100measured from the position of the leading end of print medium100to the position of nip portion B (image transfer position) becomes substantially equal to the conveyance distance (L1) measured from medium feeder F of main body of image formation apparatus10to the position of nip portion B. To put it differently, the speed is switched at the time when it is judged that the leading end of print medium100reaches point P.

In this third embodiment, the drive speed of belt motor95and drum motor94is switched from the slow first speed to the normal second speed at the above-described timing. Accordingly, the number of rotations of photosensitive drum23and that of conveyor belt31before switching of the speed can be reduced. Consequently, the wear of photosensitive drum23and conveyor belt31can be reduced. In addition, in this third embodiment, the rotational speed of belt motor95and drum motor94is slowed down before this timing, but the surface of photosensitive drum23is electrically charged reliably by high-voltage power supply63. Consequently, the deterioration of image quality due to charging failure can be avoided.

Print medium100is then conveyed by second resist-roller pair18to passes through WR sensor19, and then gets on top of conveyor belt31to be conveyed by conveyor belt31.

To be more specific, print medium100turns ON WR sensor19, and then is conveyed by conveyor belt31located downstream in the medium conveyance path sequentially to the four image formation units22of black (K), yellow (Y), magenta (M), and cyan (C) arranged in the order of black.

Toner images of the four colors are transferred to the surface of print medium100by image formation units22, and then the toner images are fixed to the surface of print medium100by fixation unit40.

After the toner images are fixed to the surface of print medium100, the leading end of print medium100is detected by EXIT sensor51, and then print medium100is conveyed by the rotations of pairs of discharger rollers52to54. Note that, as in the first and the second embodiments, in this third embodiment, image processor circuit61as the medium-size detector detects the conveyance-direction dimension of print medium100on the basis of the print data sent from a host computer or an external apparatus. Hence, the rotations of conveyor belt31and photosensitive drum23can be stopped before the trailing end of print medium100completely passes through fixation roller41and then through EXIT sensor51. Specifically, the number of drive pulses of belt motor95after the detection of the leading end of print medium100by EXIT sensor51is measured, and then if it is judged that the trailing end of print medium100has passed through the image transfer position of the most downstream image formation unit22-4on the basis of the measured number of drive pulses, controller60stops belt motor95and drum motor94. A predetermined time after that, controller60stops fixation motor96.

Print medium100thus conveyed is then discharged to stacker55or face-up stacker56through a discharge route selected by the user.

FIG. 11is a flowchart illustrating the operations of the image formation apparatus according to the third embodiment of the invention. Elements therein that are the same as the ones that appear inFIG. 5showing the first embodiment are denoted by their respective reference numerals.

FIG. 12is a time chart illustrating the operations of the image formation apparatus of the third embodiment of the invention of a case where the charging-process suspension period does not exceed threshold time Tx. Elements inFIG. 12same as the ones those inFIG. 6showing the first embodiment are denoted by their respective reference numerals. In the time chart ofFIG. 12, the vertical axis represents an ON state at the upper position and an OFF state at the lower position. The horizontal axis represents the passage of time. The thick solid lines are of the control performed in the third embodiment while the thick dashed lines represent the control performed in the comparative example.

At the beginning of the process, controller60of image formation apparatus10turns ON first-optional-tray motor92-3, second-optional-tray motor92-2, resist motor93, fixation motor96, and high-voltage power supply63not illustrated in the time chart at step SIC. In addition, controller60drives drum motor94and belt motor95at the slow speed mode. The processes at steps S2and S3are the same as those in the first embodiment.

At step S4C, controller60of image formation apparatus instructs optional tray controller80to stop first-optional-tray motor92-3and thereby to stop the rotations of pick-up roller12-3and sheet-feeder roller13-3. From then onwards, print medium100is conveyed without being driven by first-optional-tray motor92-3(pick-up roller12-3and sheet-feeder roller13-3). The processes at steps S5to S7are the same as those in the first embodiment.

At step S30, controller60waits for the turning ON of WR sensor19by the leading end of print medium100to be detected. Then at step S31, controller60waits for print medium100to be conveyed over distance D5. Then at step S32, controller60speeds up the drive speed of drum motor94and belt motor95to the printing speed. The processes at steps S8to S15are the same as those in the first embodiment.

Effects of Third Embodiments

Image formation apparatus10of this third embodiment slows down the drive speed of photosensitive drum23and conveyor belt31and thereby can prolong the service lives of these members, even if image formation apparatus10of this third embodiment is not equipped with special photosensitive drum23like the one in the second embodiment whose surface potential is less likely to be attenuated. In addition, image formation apparatus of this third embodiment can electrically charge the surface of each photosensitive drum23so sufficiently that the transfer can provide fine image quality.

Fourth Embodiment

Configuration of Fourth Embodiment

The configuration of image formation apparatus10of a fourth embodiment of the invention is identical to the configuration of image formation apparatus10of the first embodiment shown inFIG. 1.

Operations of Fourth Embodiment

A feature of the fourth embodiment lies in the control method of a case where print medium100is conveyed over a long distance. So, the following description is given of the operations of a case where print medium100is fed from first optional tray11-3. If the conveyed distance is long, the operations of the fourth embodiment is also applicable to a case where print medium100is fed from second optional tray11-2and to a case where print medium100is fed from medium feeder F of the main body of image formation apparatus10.

Image formation apparatus10receives, from an unillustrated external apparatus, print data and an instruction to make the printing be performed using print media100stored in first optional tray11-3. Then, image formation apparatus10makes first-optional-tray motor92-3rotate pick-up roller12-3and sheet-feeder roller13-3. With the rotations of pick-up roller12-3, each of print media100is separated from the others. Then, the print media100thus separated are sent, one by one, to the downstream side of the medium conveyance path.

As in the first to the third embodiments, in this fourth embodiment, on the basis of the print data sent from the host computer or the external apparatus, image processor circuit61as a size detector detects the conveyance-direction dimension of print medium100.

Print medium100conveyed from pick-up roller12-3is further conveyed by the rotation of sheet-feeder roller13-3. Note that in this fourth embodiment, the driving of resist motor93, fixation motor96, and second-optional-tray motor92-2is started substantially at the same time as the driving of first-optional-tray motor92-3is started. Unlike the comparative example, the driving of neither belt motor95nor drum motor94is started at that timing.

Note that, if the temperature of fixation roller41is at the target temperature, the driving of fixation motor96is started substantially at the same time as the feeding of print medium100is started as described earlier. If, conversely, the temperature of fixation roller41is lower than the target temperature, the driving of fixation motor96is started before the feeding of print medium100is started to warm up fixation roller41.

When sheet-feed sensor14-3detects that print medium100is fed properly, butting control to correct skew of print medium100is performed by using conveyance sensor15-3and conveyance rollers16-3. Print medium100passes through conveyance sensor15-2, conveyance rollers16-2, first IN-sensor15-1, first resist-roller pair16-1, and second IN-sensor17. Then, print medium100is conveyed to second resist-roller pair18.

In this fourth embodiment, if time T1elapses after controller60starts the feeding of print medium100from first optional tray11-3, an interrupt processing by timer78is performed. The interrupt processing is performed at a time when the conveyance distance of the print medium100measured from the position of the leading end of print medium100to the position of nip portion B becomes substantially equal to the conveyance distance measured from medium feeder F of main body of image formation apparatus10to the position of nip portion B. The position of the leading end of print medium100varies depending upon the structure of the apparatus and other factors. The timing of the interrupt processing may be delayed as long as the start of the rotations of photosensitive drum23and conveyor belt31allows the in-time arrival of print medium100for the transfer of toner images. The start of the rotations of belt motor95and drum motor94at this timing can reduce the rotations of photosensitive drum and conveyor belt31which rotate wastefully under the control in the comparative example. Consequently, the wear of these members due to the idle rotations can be avoided. In addition, even if print medium100is fed from optional tray11-3, the rotations of conveyor belt31and photosensitive drum23is started at the same timing as in the case where print medium100is fed from medium feeder F of the main body of image formation apparatus10. Hence, the surface of photosensitive drum23can be electrically charged reliably. As a consequence, the degradation of image quality due to charging failure can be avoided.

After that, print medium100is conveyed to second resist-roller pair18, and then passes through WR sensor19. Then print medium100gets on top of conveyor belt31to be conveyed further by conveyor belt31.

Print medium100turns ON WR sensor19, and then is conveyed to conveyor belt31that is located downstream in the medium conveyance path. Then, print medium100is conveyed sequentially to the four image formation units22of black (K), yellow (Y), magenta (M), and cyan (C), arranged in this order

Toner images of the four colors are transferred on print medium100by the image formation units22, and then the toner images are fixed to the surface of print medium100by fixation unit40.

After the toner images are fixed to the surface of print medium100, the leading end of print medium100is detected by EXIT sensor51, and print medium100is conveyed by the rotations of pairs of discharger rollers52to54. Note that, as in the first to the third embodiments, in this fourth embodiment, image processor circuit61as a medium-size detector detects the conveyance-direction dimension of print medium100on the basis of the print data sent from a host computer or an external apparatus. Hence, the rotations of conveyor belt31and photosensitive drum23can be stopped before the trailing end of print medium100completely passes through fixation roller41and then through EXIT sensor51. Specifically, the number of drive pulses of belt motor95after the detection of the leading end of print medium100by EXIT sensor51is measured, and then if it is judged that the trailing end of print medium100has passed through the image transfer position of the most downstream image formation unit22-4on the basis of the measured number of drive pulses, controller60stops belt motor95and drum motor94. A predetermined time after that, controller stops fixation motor96.

Print medium100thus conveyed is then discharged to stacker55or face-up stacker56through a discharge route selected by the user.

FIG. 13is a flowchart illustrating the operations of image formation apparatus10according to the fourth embodiment of the invention. Elements therein that are the same as the ones that appear inFIG. 5showing the first embodiment are denoted by their respective reference numerals.

FIG. 14is a time chart illustrating the operations of image formation apparatus10of the fourth embodiment of the invention of a case where the charging-process suspension period does not exceed threshold time Tx. Elements inFIG. 14same as the ones those inFIG. 6showing the first embodiment are denoted by their respective reference numerals. In the time chart ofFIG. 14, the vertical axis represents an ON state at the upper position and an OFF state at the lower position. The horizontal axis represents the passage of time. The thick solid lines are of the control performed in the fourth embodiment while the thick dashed lines represent the control performed in the comparative example.

The process at step S1is similar to that in the first embodiment. At step S3C, controller60waits until time T1(first time) measured by timer78elapses. Time T1is the length of time starting from the time when the conveyance of print medium100begins and ending at a predetermined time by which photosensitive drum23and conveyor belt31have to start rotating unless toner images cannot be transferred to the surface of print medium100.

In this fourth embodiment, time T1refers to the length of time starting from the feeding of print medium100, for example, from second optional tray11-2and ending at the time when the print medium100is conveyed to point P shown inFIG. 1. Point P shown inFIG. 1is a position, the distance from which to the position B where the toner image is transferred by image transfer device21is equal to the distance measured from pick-up roller12-1of medium feeder F of the main body of image formation apparatus10to the position B. The processes at steps S4to S15are the same as those in the first embodiment.

Effects of Fourth Embodiment

In image formation apparatus10of the fourth embodiment, the timing at which the rotations of photosensitive drum23and conveyor belt31are started is controlled by the interrupt processing by timer78only. Accordingly, the control can be made simpler and more accurate.

Modifications

The invention is not limited to the embodiments described above, but tolerates various other forms of use and modifications. Such various other forms of use and modifications include (a) to (e).

(a) The description of the first to the fourth embodiments is given with a color electrophotographic printer as an example. The invention is not limited to such a case, but is also applicable to other cases where the image formation apparatus is a monochrome photocopier, a color photocopier, a monochrome multifunction printer, a color multifunction printer, or the like with a similar structure.

(b) The description of the first to the fourth embodiments is of the case of direct-transfer image formation apparatus10where the toner image is transferred directly from each photosensitive drum23to print medium100conveyed by conveyor belt31. The invention, however, is not limited to such a case, but is also applicable to a case where the image formation apparatus is an intermediate-transfer image formation apparatus where the developer image is transferred firstly from each photosensitive drum23to an intermediate transfer belt serving as an image carrier and then the developer image on the intermediate transfer belt is transferred to the surface of print medium100that is being conveyed. In addition, the invention is also applicable to a case where image formation apparatus10is an image formation apparatus that uses no conveyor belt31.

(c) When there are plural medium feeders in a lower portion of image formation apparatus10, conveyance of the media and image formation may be performed as follows. First, source of print medium100is determined from the following: medium feeder F of the main body of image formation apparatus10; second optional tray11-2additionally provided to the main body of image formation apparatus10at the upstream side, in the medium-conveyance direction, of medium feeder F; and first optional tray11-3additionally provided to the main body of image formation apparatus10at the upstream side, in the medium-conveyance direction, of second optional tray11-2. Then the rotations of photosensitive drum23and conveyor belt31may be suspended in the preliminary operations for a length of time that is predetermined depending upon from which feeder print medium100is fed.

For example, if print medium100is fed from medium feeder F of the main body of image formation apparatus10, the driving of neither photosensitive drum23nor conveyor belt31is suspended while print medium100is being conveyed from medium feeder F to image transfer device21. Alternatively, in the above-described case, the driving of photosensitive drum23and conveyor belt31is suspended temporarily at the same time as the feeding of print medium100is started, and then the driving of photosensitive drum23and conveyor belt31is resumed after a first time elapses since the start of the suspension.

If second optional tray11-2is additionally provided and print medium100is fed from this second optional tray11-2, the driving of photosensitive drum23and conveyor belt31is suspended temporarily along with the start of the feeding of print medium100while print medium100is being conveyed from second optional tray11-2to image transfer position B. Then, the driving of photosensitive drum23and conveyor belt31is resumed after a second time that is longer than the first time elapses.

If first optional tray11-3is additionally provided and print medium100is fed from this first optional tray11-3, the driving of photosensitive drum23and conveyor belt31is suspended temporarily along with the start of the feeding of print medium100while print medium100is being conveyed from first optional tray11-3to image transfer position B. Then, the driving of photosensitive drum23and conveyor belt31is resumed after a third time that is longer than the second time elapses.

(d) In the first to the fourth embodiments, both in the case where print medium100is fed from second optional tray11-2as the second medium feeder and in the case where print medium100is fed from first optional tray11-3as the first medium feeder, the driving of photosensitive drum23and conveyor belt31is suspended temporarily while print medium100is being conveyed. Even if, however, print medium100is fed from medium feeder F of the main body of image formation apparatus10as the third medium feeder, the driving of photosensitive drum23and conveyor belt31may be suspended temporarily on condition that the distance from medium feeder F to image transfer position B is sufficiently long or that the time from the start of conveyance of print medium100to the arrival of print medium100to image transfer position B is sufficiently long.

(e) In the first to the fourth embodiments, the conveyance-direction dimension of print medium100is identified on the basis of the print data sent from a host computer or an external apparatus, but this is not the only method of identifying the dimension. The conveyance-direction dimension of print medium100may be detected by detecting the conveyance speed of print medium100and the length of time between the turning ON and turning OFF of any one of sheet-feed sensor14, conveyance sensor15, first IN-sensor15-1, second IN-sensor17, and WR sensor19. Alternatively, the conveyance-direction dimension of print medium100may be detected by special sheet-size sensors provided in medium feeder F and optional trays11(11-2and11-3).