JP5366508B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus, and more particularly to an apparatus that conveys sheets in a superimposed manner.

  In recent years, in image forming apparatuses such as copiers, printers, and facsimiles, there is an increasing demand for forming images on various sheets (media). Here, when an image is formed on a sheet in the image forming apparatus, the sheet S is conveyed to a transfer portion constituted by the photosensitive drum 21 and the transfer charger 22 as shown in FIG. I have to.

  However, for example, when an image is formed on a thin sheet S having a basis weight of 60 g or less, since the thin sheet S has low rigidity, when passing through the transfer portion, as shown in FIG. There is a case where the photoconductor drum 21 is wound as it is without being separated.

  Therefore, in order to prevent such wrapping around the photosensitive drum 21, as shown in FIG. 11B, the leading end portions of the succeeding sheets S2 and S3 are overlapped below the trailing end portions of the preceding sheets S1 and S2. In order to prevent the sheet from being wound, there has been proposed (see Patent Document 1).

JP-A-5-238590

  By the way, in such a conventional image forming apparatus, after the sheet is conveyed from the cassette, the succeeding sheet is superimposed on the preceding sheet before reaching the transfer unit. However, when the succeeding sheet is overlapped with the preceding sheet, the leading edge of the sheet serving as a reference for image and sheet alignment can be detected by a sensor (not shown) in the case of the first sheet S1, The second and subsequent sheets S2 and S3 cannot be detected by the sensor. For this reason, after detecting the leading edge of the first sheet S1, the leading edge positions of the succeeding sheets S2 and S3 are calculated on the basis of the sheet length, the overlapping amount, and the like.

  However, when the leading edge position of the succeeding sheet is determined by the calculation in this way, if a deviation occurs in the sheet conveyance timing after the calculation, the alignment between the image and the sheet that requires high accuracy cannot be achieved. Even if a jam occurs, it is difficult to detect the occurrence of the jam.

  In the case of a thin sheet, a transmissive sensor is used as a sensor for detecting the leading edge of the sheet, so that the leading edge can be detected even when the succeeding sheet is stacked on the preceding sheet. The tip cannot be detected with this sensor.

  Accordingly, the present invention has been made in view of such a situation, and provides an image forming apparatus that can prevent the sheet from being wound around the photosensitive drum and can reliably detect the leading edge of the succeeding sheet. It is intended.

The present invention is a sheet conveying apparatus for conveying an image forming section that have a transfer unit for transferring the toner image formed on the photosensitive drum and the photosensitive drum a toner image is formed on the sheet, the sheet to the transfer section In the image forming apparatus, the transfer unit is provided upstream of the transfer unit in the sheet conveyance direction , and detects a conveyed sheet, and conveys a sheet having a predetermined thickness and a thickness thinner than the predetermined thickness. If the sheet is thicker than a predetermined thickness, the sheet conveying device is controlled so that the trailing edge of the preceding sheet and the leading edge of the succeeding sheet are overlapped and conveyed to the transfer unit . and a control unit for controlling the sheet conveying apparatus to the preceding sheet and a succeeding sheet spaced conveyed to the transfer unit, wherein the control unit, destination based on the detection signal from the sheet detecting portion It is characterized in that for detecting the leading edge of the subsequent sheet transported from the state or the preceding sheet stacked on the sheet in a state in which a gap.

  As in the present invention, by detecting the leading edge of the succeeding sheet based on the magnitude of the detection signal from the transmission type sheet detecting unit, the winding of the sheet around the photosensitive drum is prevented and the leading edge of the succeeding sheet is reliably secured. Can be detected.

  The best mode for carrying out the present invention will be described below in detail with reference to the drawings.

  FIG. 1 is a cross-sectional view illustrating a configuration of a printer which is an example of an image forming apparatus according to an embodiment of the present invention.

  In FIG. 1, reference numeral 1 denotes a printer. This printer 1 is loaded with a printer main body 2, a scanner 11 disposed on the upper surface of the printer main body 2, and a large number of sheets S arranged on one side of the printer main body 2. A feeding deck 12 is provided.

  Here, the printer main body 2 is fed by the image forming unit 3 including the photosensitive drum 21 as an image carrier, the sheet feeding devices 16 and 17 for feeding the sheet S, and the sheet feeding devices 16 and 17. A sheet conveying device 4 for conveying the fed sheet S to the image forming unit 3 is provided.

  The sheet feeding devices 16 and 17 include cassettes 13 and 14 in which a predetermined amount of sheets S are stacked and stored, feeding rollers 16a and 17a, retard separation type separation units 16b and 17b, and the like. Further, the sheet conveying device 4 includes conveying rollers 42 to 46, a leading end detection sensor 19, and a skew correction unit 18.

  For example, the sheet S stored in the cassette 14 and fed from the sheet feeding device 17 passes through the leading edge detection sensor 19 and is guided to the skew feeding correction unit 18 by the conveyance rollers 42 to 46. Yes. The sheets S stored in the feeding deck 12 pass through the leading edge detection sensor 19 and are guided one by one to the skew correction unit 18 by the sheet feeding device 15 and the conveying roller 12a. .

  Here, when the leading edge of the sheet reaches the leading edge detection sensor 19, the control unit 50 shown in FIG. 4 to be described later irradiates the photosensitive drum 21 with laser light based on a detection signal from the leading edge detection sensor 19. Determine when to start the latent image on the drum. In addition, after the skew correction of the sheet S is corrected by the skew correction unit 18, the sheet S is sent to a transfer unit including the photosensitive drum 21 and the transfer charger 22 of the image forming unit 3. A toner image previously formed on the photosensitive drum 21 is transferred.

  The skew feeding correction unit 18 is of a side registration type including a side guide 181 and skew feeding rollers 182 and 183 arranged in parallel with the sheet conveying direction as shown in FIG. When skew correction is performed, the sheet S is obliquely fed by the skew feeding rollers 182 and 183 and the end surface of the sheet S is abutted against the side guide 181 so that skew correction is performed and the side end surface of the sheet S is corrected. Positioning is performed.

  Further, as described above, the timing of forming an image on the photosensitive drum 21 is determined based on the result of detecting the leading edge of the sheet S by the leading edge detection sensor 19, so that the position of the sheet and the image at the transfer portion is determined. Fits with high accuracy.

  Thereafter, the sheet S on which the toner image has been transferred is sent to the fixing device 24 by the conveying belt 23, and the fixing process on the sheet surface of the toner image transferred by the fixing device 24 is performed. Here, the printer 1 has a double-sided copying mode in which double-sided copying is performed on the sheet S and a duplicated copying mode in which multiple copying is performed. The paper is discharged onto a discharge tray 27 outside the machine by the roller pair 26.

  In the duplex copying mode and the multiple copying mode, the inner discharge roller pair 25 or the switchback roller pair 29 is temporarily stacked and stored on the intermediate tray 31 via the refeed path 28 and the duplex conveyance path 30. The Then, the sheet S stored on the intermediate tray 31 is then conveyed again to the skew feeding correction unit 18 for image formation by the refeed device 32, and thereafter goes out of the machine through the same process as single-sided copying. Discharged.

  By the way, in this Embodiment, the thin sheet | seat with a basic weight of 60 g or less is accommodated in the cassette 14, for example. When an image is formed on such a thin sheet, the sheet is wound around the photosensitive drum 21 when passing through the photosensitive drum 21 as shown in FIG. There is a fear.

  Therefore, in addition to the normal conveyance mode, which is the second conveyance mode in which the printer 1 conveys the preceding sheet and the succeeding sheet with an interval, the sheets are stacked when the sheet is equal to or less than a predetermined thickness. It has a superposition conveyance mode which is a first conveyance mode for conveying together. When an image is formed on a thin sheet, the sheet is conveyed in this overlapping conveyance mode.

  Here, in order to convey the sheets in such a manner, the conveying roller pair 44 and 45 can be accelerated as shown in FIG. 3, and the light emitting unit and the light receiving unit are provided between the conveying roller pair 42 and 43. A reflection type timing sensor 41 is arranged.

  The timing sensor 41 is connected to a control unit 50 as shown in FIG. 4, and the control unit 50 drives a pair of conveying rollers 44 and 45 based on a signal from the timing sensor 41. The rotation speed is controlled. In FIG. 4, reference numeral 51 denotes an operation unit for setting the type of sheet stored in the cassette 14. Based on information on the thickness of the sheet from the operation unit 51, the control unit 50 determines the conveyance mode. To decide.

  In the case of a thin sheet, when the preceding sheet S1 is detected by the timing sensor 41 as shown in FIG. 3A, and the trailing end of the preceding sheet S1 comes out as shown in FIG. 3B. A detection signal is input from the timing sensor 41 to the control unit. Here, the control unit controls the rotation speed of the drive motor M so as to catch up the succeeding sheet S2 with the preceding sheet S1 based on the detection signal, that is, based on the sheet leading edge detection, and conveys the sheet by the conveying roller pairs 44 and 45. Increase speed.

  As shown in FIG. 3B, immediately after the trailing edge of the preceding sheet S1 passes through the timing sensor 41, the trailing edge of the preceding sheet S1 is moved to the retreat space by a force that combines the rigidity and gravity of the preceding sheet S1. Run away. For this reason, if the subsequent sheet S2 is accelerated, the leading end of the subsequent sheet S2 can be superimposed below the rear end of the preceding sheet S1. In the present embodiment, the predetermined time for conveying the succeeding sheet S2 at an increased speed is determined in advance, and the overlapping amount is set to be about 5 mm.

  Then, due to the speed increase of the pair of conveying rollers 44 and 45, the succeeding sheet S2 catches up with the preceding sheet S1 as shown in FIG. 5A, and eventually, after the preceding sheet S1 as shown in FIG. 5B. The end portion and the leading end portion of the succeeding sheet S2 overlap each other.

  When the trailing edge of the preceding sheet S1 and the leading edge of the succeeding sheet S2 overlap in this way, that is, when a predetermined time has elapsed, the control unit controls the rotational speed of the drive motor M, and the conveyance roller pair 44, 45 The sheet conveyance speed is set to the same steady speed as the conveyance roller pairs 42 and 43. As a result, the preceding sheet S1 and the succeeding sheet S2 are conveyed to the leading edge detection sensor 19 which is the sheet detecting unit shown in FIG.

  The leading edge detection sensor 19 is a transmissive optical sensor in which a light emitting portion 19a and a light receiving portion 19b are separated as shown in FIG. 6, and the sheet conveyance of the transfer portion constituted by the photosensitive drum 21 and the transfer charger 22 is performed. It is provided upstream in the direction.

  Then, as shown in FIG. 8 to be described later, for example, the leading edge detection sensor 19 outputs 5 [V] when there is no shielding object, and outputs one thin sheet having a basis weight of 60 g which is an upper limit for performing the overlapping conveyance mode. When transmitted, 1 [V] is output.

  Here, in the present embodiment, the threshold value V3 for determining the sheet leading edge of the overlapping portion in the overlapping conveyance mode is obtained by the following equation.

V3 = V1- (V2-V1) × 2 (1)
However, when V3 ≦ 0, V3 = 0

  V1 is an output voltage when one sheet having a predetermined basis weight of 60 g shields the leading edge detection sensor 19, and V2 is when there is nothing to shield, that is, the leading edge detection sensor 19 detects the sheet. This is the output voltage that is output when not.

  Then, the control unit can determine that the leading edge of the succeeding sheet has passed when the output of the leading edge detection sensor 19 reaches the threshold value V3 obtained by the above equation (1).

  Next, the sheet conveying operation of the sheet conveying apparatus having such a configuration will be described with reference to the flowchart shown in FIG.

  In the present embodiment, as described above, when a sheet having a basis weight of 60 g or less is conveyed, the sheet is wound around the photosensitive drum 21, so that the sheet having a basis weight of 60 g or less is conveyed in the overlap conveyance mode. A sheet having a basis weight greater than 60 g is conveyed in the normal conveyance mode.

  Here, assuming that a thin sheet S having a basis weight of 60 g is stored in the cassette 14, the user first sets the sheet basis weight in the cassette by the operation unit 51, that is, a thin sheet is stored in the cassette 14. This is preset (STEP 1). When printing on a thin sheet, the user selects the cassette 14 as a cassette to be used by the operation unit (STEP 2), and then starts printing by operating a start button (not shown) ( (Step 3).

  Accordingly, the control unit first starts feeding (STEP 4), and feeds the first sheet stored in the cassette 14 by the feeding roller 17a, and then sequentially feeds the second and subsequent sheets.

  Next, a control part judges whether the basic weight of the sheet | seat (use sheet | seat) to print is 60 g or less (STEP5). When the basis weight is 60 g or less (Y in STEP 5), when the timing sensor 41 detects that the rear end of the sheet has passed as shown in FIG. 3 (STEP 6), a signal from the timing sensor 41 is displayed. Based on this, the speed of the conveying roller pair 44, 45 is increased (STEP 7). As a result, as shown in FIG. 5A, the leading edge of the succeeding sheet S2 is made to catch up with the trailing edge of the preceding sheet S1.

  The predetermined time t for conveying the succeeding sheet S2 at an increased speed is determined in advance, and after the predetermined time t seconds, the preceding sheet S1 and the succeeding sheet S2 overlap each other by a predetermined amount (about 5 mm). (STEP8).

  After the predetermined time t seconds, the sheet conveying speed of the conveying roller pair 44, 45 is returned to the steady speed (STEP 9). Accordingly, the succeeding sheet S2 is also transported at a steady speed, and as shown in FIG. 4B, the rear end portion of the preceding sheet S1 and the leading end portion of the succeeding sheet S2 are overlapped, and the transport roller pair 42, 43 is conveyed to the tip detection sensor 19.

  Next, when the leading edge detection sensor 19 detects the leading edge of the sheet (STEP 10), it is determined how many sheets this sheet is (STEP 11). In this case, since the preceding sheet S1 is the first sheet (Y in STEP 11), after that, the first sheet is transported to the threshold V3 [V] for determining the leading edge of the second and subsequent sheets. It is determined from the output result of the sensor at that time (STEP 12).

Specifically, when conveying a sheet having a basis weight of 60 g, from the above-described formula (1),
Threshold value V3 = 1− (5-1) / 2 = −1
Since the threshold value V3 = −1 <0, V3 = 0 [V] is determined.

  FIG. 8 shows the output of the leading edge detection sensor 19 when, for example, three sheets having a basis weight of 60 g are stacked and conveyed. The output of the leading edge detection sensor 19 is 5V until the first sheet is detected, as shown in FIG. 8, and 1V when the first sheet is detected. The first sheet and the second sheet overlap. When this part passes, the output becomes 0V.

  In addition, when the first sheet and the second sheet overlap each other and only the second sheet is detected, the output becomes 1V, and then the second and third sheets overlap. When the part passes, the output is 0V. Further, when only the third sheet is detected, the output becomes 1V, and when the third sheet passes, the output becomes 5V.

  After determining the threshold value V3 in this way, the sheet S1 is conveyed to the fixing unit without image formation (STEP 13). In the case of the second and subsequent sheets (N in STEP 11), the laser writing timing Tn for the photosensitive drum is determined according to the determined threshold value V3 (STEP 14). That is, when the output voltage of the leading edge detection sensor 19 shown in FIG. 8 changes from 1 V to 0 V, it is determined that the leading edge of the second and subsequent sheets has passed the leading edge detection sensor 19, and this timing is determined with respect to the photosensitive drum. The laser writing timing is Tn.

  By the way, in the present embodiment, the upper limit range of the thickness of the sheet that can be transmitted by the tip detection sensor 19 is larger than the thickness of a sheet having a predetermined thickness, for example, one sheet having a basis weight of 60 g. This is a range that is thinner than the thickness of two thin sheets. For this reason, even when a thin sheet having a basis weight of less than 60 g, for example, 30 g, is transported, the tip detection sensor 19 can be transmitted.

  Here, the threshold value V3 [V] for determining the leading edge of the second and subsequent sheets can be similarly determined from the output result from the leading edge detection sensor 19 when the first sheet is conveyed. Further, the basis weight of the sheet and the output voltage of the leading edge detection sensor 19 are substantially proportional, and when one sheet having a basis weight of 30 g is transmitted, the output (V1) of the leading edge detection sensor 19 is 3V. For this reason, the threshold value V3 [V] when a thin sheet having a basis weight of 30 g is conveyed is determined as follows.

Threshold value V3 = 3- (5-3) / 2 = 2
Therefore, the threshold value V3 = 2 [V] is determined.

  FIG. 9A shows the output of the leading edge detection sensor 19 when, for example, three sheets having a grammage of 30 g are stacked and conveyed. The output of the leading edge detection sensor 19 is 5V until the first sheet is detected, as shown in FIG. 9A, and 3V when the first sheet is detected. The first sheet and the second sheet are detected. When the sheet overlaps, the output is 1V.

  In addition, when the first sheet and the second sheet overlap each other and only the second sheet is detected, the output becomes 3V, and then the second and third sheets overlap. When the part passes, the output is 1V. Further, when only the third sheet is detected, the output becomes 3V, and when the third sheet passes, the output becomes 5V.

  As described above, in the present embodiment, in the case of the second and subsequent sheets, the leading edge of the sheet is detected by detecting T2, T3... Whose output is the threshold value V3, and image formation is performed based on the leading edge detection. The timing is determined. Thereby, the image forming position on the sheet can be adjusted with high accuracy. After this, since the leading edge is held at the trailing edge of the preceding sheet, the second and subsequent sheets are not wound around the photosensitive drum 21 as shown in FIG. 24 (STEP 17).

  On the other hand, when the basis weight of the used sheet is 60 g or more (N in STEP 5), when image formation is performed on this sheet, the sheet is conveyed while forming an interval between the sheets in the normal conveyance mode. In the case of a thick sheet, as shown in FIG. 9B, the output voltage at the leading edge detection sensor 19 is 0 V, and this is used as a trigger to determine Tn and start image image formation. Since a thick sheet having a basis weight of 60 g or more has sufficient rigidity, it is thereafter sent to the fixing device 24 without being wound around the photosensitive drum 21 and discharged outside the printer.

  As shown in FIG. 10, a jam sensor 53 including a reflection type optical sensor as a jam detection unit is disposed in the sheet conveyance path of the printer 1, and the jam sensor 53 is described above. As shown in FIG. 4, it is connected to the control unit 50.

  In the overlap conveyance mode, the preceding sheet S1 and the subsequent sheet S2 are conveyed in an overlapped state as shown in FIG. 10A, so that the jam sensor 53 always detects the sheets S1 and S2. On the other hand, when the succeeding sheet is jammed by the upstream conveying roller 54, an interval is generated between the preceding sheet S1 and the succeeding sheet S2, and accordingly, the jam sensor 53 does not detect the sheet.

  When the jam sensor 53 no longer detects a sheet in this way, the control unit determines that a jam has occurred. In this configuration, a slight delay occurs in jam detection. However, since this configuration is only when a thin sheet is being conveyed, damage to the apparatus is small. In the normal conveyance mode, if the sheet is jammed, the leading edge detection timing of the sheet is delayed. Therefore, it is determined that the sheet is jammed based on the leading edge detection timing of the sheet.

  As described above, in the present embodiment, the leading edge of the sheet can be detected by detecting that the magnitude of the detection signal from the transmissive leading edge detection sensor 19 has reached the threshold value V3. Accordingly, not only in the normal conveyance mode, but also in the case where the succeeding sheet is overlapped and conveyed in the overlapping conveyance mode, the leading edge of the subsequent sheet can be reliably detected, and the sheet is stably conveyed. be able to.

  That is, in the present embodiment, the leading edge of the succeeding sheet is detected based on the magnitude of the detection signal from the transmission-type leading edge detection sensor 19, thereby preventing the sheet from being wound around the photosensitive drum 21. The leading edge of the sheet can be reliably detected.

  In the present embodiment, as the sheet detection unit to which the present invention is applied, a front end detection sensor for detecting the front end of the sheet and determining the timing of image formation has been described as an example. Not limited to this. For example, the present invention may be applied to a sheet detection unit such as a sensor that detects skew (posture) of a sheet or a jam sensor. And when this invention is applied to such a sheet | seat detection part, the conveyance timing of a sheet | seat can be controlled with high precision by controlling a roller peripheral speed.

1 is a cross-sectional view illustrating a configuration of a printer that is an example of an image forming apparatus according to an embodiment of the present invention. FIG. 3 is a diagram illustrating a configuration of a skew feeding correction unit provided in the sheet conveying device of the printer. The figure explaining the structure of a part of the said sheet conveying apparatus. FIG. 3 is a control block diagram of the printer. The figure explaining the sheet conveyance operation | movement of the said sheet conveying apparatus. The figure explaining the front-end | tip detection sensor provided in the said sheet conveying apparatus. 6 is a flowchart illustrating a sheet conveying operation of the sheet conveying apparatus. The figure explaining the output in the case of detecting the sheet | seat whose basic weight of the said front-end | tip detection sensor is 60g. The figure explaining the output in the case of detecting the sheet | seat with a grammage of 30g and a thick sheet | seat of the said front-end | tip detection sensor. The figure explaining the jam detection sensor provided in the said sheet conveying apparatus. FIG. 10 is a diagram illustrating a problem during sheet conveyance of a conventional image forming apparatus and countermeasures thereof.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Printer 2 Printer main body 3 Image formation part 4 Sheet conveying apparatus 19 Front-end | tip detection sensor 21 Photosensitive drum 50 Control part 53 Jam sensor S1 Prior sheet S2 Subsequent sheet S3 Subsequent sheet

Claims (7)

An image forming section that have a transfer unit for transferring the toner image formed on the photosensitive drum and the photosensitive drum a toner image is formed on the sheet, and a sheet conveying apparatus for conveying a sheet to the transfer section In the image forming apparatus,
A transmission type sheet detection unit that is provided upstream of the transfer unit in the sheet conveyance direction and detects a conveyed sheet;
When conveying a sheet having a predetermined thickness and a thickness thinner than the predetermined thickness, the sheet conveying device is controlled so that the trailing edge of the preceding sheet and the leading edge of the succeeding sheet are overlapped and conveyed to the transfer unit, and the sheet In the case where the sheet is thicker than a predetermined thickness, a control unit that controls the sheet conveying device to convey the preceding sheet and the succeeding sheet to the transfer unit with a gap therebetween, and
Wherein the control unit, image formation and detecting the leading end of the succeeding sheet conveyed from the detection signal state or the preceding sheet stacked on the preceding sheet on the basis of the between the sheet detecting unit in a state in which a distance apparatus. The sheet conveying apparatus includes a plurality of rollers arranged in a sheet conveying direction,
  When the control unit conveys a sheet having a predetermined thickness and a sheet thinner than the predetermined thickness, the plurality of rollers are configured so that the trailing edge of the preceding sheet and the leading edge of the succeeding sheet are overlapped and conveyed to the transfer unit. When the sheet is thicker than a predetermined thickness, the sheet conveyance speed of the plurality of rollers is controlled so as to convey the preceding sheet and the succeeding sheet to the transfer unit with a gap therebetween. The image forming apparatus according to claim 1. 3. The image forming apparatus according to claim 1, wherein a threshold value of the sheet detection unit for detecting a leading edge of a succeeding sheet in a sheet overlapping unit is different depending on a thickness of the sheet. Wherein the control unit, any one of claims 1 to 3, wherein the controller controls the image forming unit to form the toner image on the basis of the sheet front end detection based on signals from the sheet detecting portion the image forming apparatus according to. Wherein the control unit, the image forming apparatus according to any one of claims 1 to 4, wherein the controlling the posture of the seat based on the sheet front end detection based on signals from the sheet detecting portion. It has a jam detector that detects the jam of the sheet.
When the sheet is a sheet having a predetermined thickness or less, a first conveyance mode for conveying the trailing edge of the preceding sheet and the leading edge of the succeeding sheet in an overlapping manner;
In the case where the sheet is a sheet having a predetermined thickness or more, a second transport mode for transporting the preceding sheet and the succeeding sheet with a gap therebetween, and
In the first conveyance mode, the jam detection unit determines that the sheet is jammed when the sheet interval is detected. In the second conveyance mode, the jam detection unit determines whether the sheet is based on the leading edge detection timing of the sheet. the image forming apparatus according to any one of claims 1, characterized in that it is determined that the jam 5. The upper limit range of the thickness of the sheet that can be transmitted by the sheet detection unit is a range that is thicker than the thickness of one sheet having a predetermined thickness and smaller than the thickness of two sheets having a predetermined thickness. the image forming apparatus according to any one of claims 1 to 6, characterized in that.

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