Image forming apparatus

Abstract

To provide an image forming apparatus that can achieve a target printing speed more easily than before.SOLUTION: An image forming apparatus forms an image on a sheet conveyed through a first conveyance path. A second conveyance path is branched from the first conveyance path at a branch point provided downstream of image forming means in a conveyance direction of the sheet. A third conveyance path connects the second conveyance path and the first conveyance path to each other, and conveys the sheet having the image formed on a first surface from the second conveyance path to the first conveyance path. Inversion means conveys the sheet in a first direction to draw in the sheet from the first conveyance path to the second conveyance path in order to change a surface of the sheet on which an image is formed by the image forming means from the first surface to a second surface, and inverts the conveyance direction of the sheet from the first direction to a second direction to feed the sheet from the second conveyance path to the third conveyance path. Control means controls the inversion timing for the inversion means according to an actual length of the sheet.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] An image forming apparatus can form images on both the front and back sides of a sheet. This is called double-sided printing. The sheet with an image formed on the front side passes through a loop-shaped conveying path (double-sided conveying path) provided in the image forming apparatus, and an image is formed on the back side. Note that a method of reversing the conveying direction of the sheet in order to reverse the front and back sides of the sheet is known (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2006-290582 A Summary of the Invention [Problem to be solved by the invention]

[0004] In double-sided printing, the cycle at which a sheet passes through the image forming unit is kept constant, thereby achieving the target printing speed (throughput) of the image forming device. If the timing for reversing the conveying direction of a sheet with an image formed on its front side deviates from the specified timing, misprints occur, making it difficult for the image forming device to achieve the target printing speed. Research by the inventors has revealed that when the actual length of a sheet differs from the expected length (nominal length), the reversal timing becomes slightly inappropriate, making it impossible to achieve the target printing speed. Therefore, the present invention provides an image forming device that is easier to achieve the target printing speed than ever before. [Means for solving the problem]

[0005] The present invention relates to, for example, A storage means for storing the sheet; A discharge means for discharging the sheet; a first conveying path connecting the storage means to the discharge means; an image forming unit that forms an image on the sheet transported through the first transport path; a second conveying path branching off from the first conveying path at a branching point provided downstream of the image forming means in a conveying direction of the sheet; a third conveying path that connects the second conveying path and the first conveying path and conveys the sheet having an image formed on a first surface thereof from the second conveying path to the first conveying path; a reversing means for transporting the sheet in a first direction to draw the sheet from the first transport path to the second transport path, and for reversing a transport direction of the sheet from the first direction to a second direction to send the sheet from the second transport path to the third transport path, in order to change a surface on which an image is formed by the image forming means from the first surface to a second surface of the sheet; a control means for controlling a timing of reversing the sheet in accordance with an actual length of the sheet; The present invention provides an image forming apparatus having the above structure. Effect of the Invention

[0006] According to the present invention, an image forming apparatus that can achieve a target printing speed more easily than ever before is provided. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram illustrating an image forming apparatus. [Diagram 2] Block diagram illustrating an engine control unit [Diagram 3] Flowchart showing transport control [Figure 4] Flowchart showing acceleration / deceleration control [Diagram 5] Flowchart showing a method for adjusting inversion timing [Figure 6] Timing chart explaining the case where the actual length of the sheet is shorter than the nominal length [Figure 7] Timing chart explaining the case where the actual length of the sheet is longer than the nominal length [Figure 8] Enlarged view of the inverted part [Figure 9] Block diagram illustrating an engine control unit [Figure 10] Flowchart showing the actual length measurement method [Figure 11] Flowchart showing a method for adjusting inversion timing DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, the embodiments will be described in detail with reference to the attached drawings. Note that the following embodiments do not limit the invention according to the claims. Although the embodiments describe a number of features, not all of these features are essential to the invention, and the features may be combined in any manner. Furthermore, in the attached drawings, the same reference numbers are used for the same or similar configurations, and duplicated descriptions are omitted.

[0009] <Example 1> (1) Image forming device Image forming apparatus 100 is, for example, an electrophotographic laser printer. The letters YMCK suffixed to the reference numbers indicate the toner colors yellow, magenta, cyan, and black, respectively. When matters common to the four colors are explained, the letters YMCK are omitted from the reference numbers.

[0010] The image forming unit 20 is a printer engine that forms a full-color image on the sheet P using YMCK toner. The image forming unit 20 has a removable process cartridge 6. The process cartridge 6 is an integrated unit of a photosensitive drum 1, a charger 2, and a developing unit 4. The photosensitive drum 1 is an image carrier that is rotated by a motor. The charger 2 uniformly charges the surface of the photosensitive drum 1. The exposure unit 3 irradiates the surface of the photosensitive drum 1 with laser light in response to an image signal to form an electrostatic latent image. The developing unit 4 develops the electrostatic latent image using toner to form a toner image. The primary transfer member 5 transfers the toner image from the photosensitive drum 1 to an intermediate transfer belt 7. The intermediate transfer belt 7 is rotated to transport the toner image to a secondary transfer roller 8.

[0011] The sheet cassette 10 is a container that stores a large number of sheets P. The feeding rollers 11 feed the sheet P from the sheet cassette 10 to the first conveying path R1. The registration rollers 12 convey the sheet P to a secondary transfer portion.

[0012] The secondary transfer section 30 is a nip portion formed by the intermediate transfer belt 7 and the secondary transfer roller 8. When the sheet P passes through the secondary transfer section 30, the toner image is transferred from the intermediate transfer belt 7 to the sheet P. A fixing device 9 is provided downstream of the secondary transfer section 30 in the conveying direction of the sheet P. The fixing device 9 has a fixing roller 14 and a pressure roller 15. The fixing roller 14 supplies heat to the sheet P and the toner image. The pressure roller 15 applies pressure to the sheet P and the toner image. This fixes the toner image on the sheet P. In single-sided printing, the flapper 17 guides the sheet P from the first conveying path R1 to the fourth conveying path R4. The fourth conveying path R4 may be called an ejection conveying path. The fourth conveying path R4 may be understood to be a part of the first conveying path R1, or the first conveying path R1 and the fourth conveying path R4 may form a main conveying path. The flapper 17 is a guide member that switches the conveying direction of the sheet P. The discharge rollers 18 discharge the sheet P guided to the fourth conveying path R4 onto a discharge tray 27.

[0013] In double-sided printing, the flapper 17 guides the sheet P from the first conveying path R1 to the second conveying path R2. The second conveying path R2 may be called a reversing conveying path. The reversing roller 19 is a roller that can rotate forward and backward in the second conveying path R2. The reversing roller 19 rotates in a first direction to pull the sheet P from the first conveying path R1 to the second conveying path R2. When the rear end of the sheet P reaches a position (reversing position) where it can be sent to the third conveying path R3, the reversing roller 19 rotates in a second direction. As a result, the rear end of the sheet P becomes the leading edge, and the sheet is sent to the third conveying path R3. The third conveying path R3 may be called a conveying path for double-sided printing or a sub-conveying path. The surface of the sheet P on which an image is first formed is called a first surface. The surface of the sheet P opposite to the first surface is called a second surface. In the conveying direction of the sheet P, the sheet P has a first end and a second end. When the sheet P enters the second conveying path R2 from the first conveying path R1, the first end of the sheet P becomes the leading edge. When the sheet P enters the third transport path R3 from the second transport path R2, the second end of the sheet P becomes the leading edge. In this way, the transport direction of the sheet P is reversed, so that the leading edge of the sheet P is switched from the first edge to the second edge. Furthermore, the surface of the sheet P onto which the image is transferred is switched from the first surface to the second surface. This type of reversal method is sometimes called a switchback reversal method.

[0014] At the same time that the reversing roller 19 is reversed, the flapper 17 is switched, and the sheet P is guided from the second conveying path R2 to the third conveying path R3. Conveying rollers 21 and 22 provided in the third conveying path R3 convey the sheet P toward the first conveying path R1. The third conveying path R3 is connected to the first conveying path R1 downstream of the registration roller 12. The registration roller 12 conveys the sheet P, which has been conveyed from the third conveying path R3 to the first conveying path R1, again to the secondary transfer unit 30. As a result, a toner image is formed on the second surface of the sheet P.

[0015] The sheet P is conveyed to the fixing device 9, where the toner image is fixed on the second side. The flapper 17 guides the sheet P from the first conveying path R1 to the fourth conveying path R4. The discharge rollers 18 discharge the sheet P onto the discharge tray 27.

[0016] The sheet sensor 13 is provided between the registration roller 12 and the secondary transfer roller 8, and monitors the timing at which the sheet P is transported to the secondary transfer unit 30. If the transport timing of the sheet P is later than a specified timing, the registration roller 12 temporarily increases its speed. If the transport timing of the sheet P is earlier than a specified timing, the registration roller 12 temporarily decelerates. This synchronizes the timing at which the toner image arrives at the secondary transfer unit 30 with the timing at which the sheet P arrives at the secondary transfer unit 30.

[0017] The sheet sensor 16 is disposed between the fixing unit 9 and the flapper 17, and is a sensor that detects the leading and trailing ends of the sheet P. For example, the timing at which the sheet sensor 16 detects the trailing end of the sheet P is used as the starting point for timing at which the rotation direction of the reversing roller 19 is reversed.

[0018] (2) Engine control unit 2 shows an engine control unit 200. A CPU 201 realizes various functions by executing control programs stored in a memory 220. Note that some or all of these functions may be implemented by hardware such as an application specific integrated circuit (ASIC) or a field programmable gate array (FPGA).

[0019] The print control unit 202 controls the motors M1, M2, and M3 through the transport control unit 205, the clutch CL1 through the reversal control unit 206, and the exposure unit 3 through the exposure control unit 207. The print control unit 202 starts the print process based on a print instruction input from the operation unit 250 or the communication circuit 260. The interval control unit 203 controls the interval of image formation to achieve a target print speed. The interval of image formation refers to, for example, the interval of exposure start timing for continuously creating yellow toner images on multiple pages. In double-sided printing, this interval is the time from the exposure start timing of the front side (first side) to the exposure start timing of the back side (second side). The timer 204 is used to monitor and manage the timing of image formation control, transport control, and the like.

[0020] The conveyance control unit 205 controls the motor M1 to drive the conveyance roller 21 and the feed roller 11. The conveyance control unit 205 also controls the motor M2 to drive the fixing roller 14, the pressure roller 15, the discharge roller 18, and the reversing roller 19. The conveyance control unit 205 controls the motor M3 to drive the registration roller 12. The motor M3 and the registration roller 12 are used for acceleration / deceleration control of the sheet P. For example, the conveyance control unit 205 may temporarily increase or decrease the conveyance speed of the sheet P based on the timing when the sheet sensor 13 detects the leading edge of the sheet P. This acceleration / deceleration control is performed to match the timing when the toner image arrives at the secondary transfer unit 30 with the timing when the sheet P arrives at the secondary transfer unit 30.

[0021] The inversion control unit 206 controls the clutch CL1 to switch the rotation direction of the inversion roller 19 and the position of the flapper 17. The inversion control unit 206 may include, for example, an arrival determination unit 211, a time determination unit 212, and an inversion determination unit 213. The arrival determination unit 211 determines whether the sheet sensor 16 detects the leading edge or trailing edge of the sheet P. The time determination unit 212 determines the timing to switch the conveying direction of the sheet P. For example, the time determination unit 212 determines the conveying time from the timing when the trailing edge of the sheet P is detected by the sheet sensor 16 to the timing to switch the conveying direction of the sheet P. The inversion determination unit 213 monitors the timer 204 and waits for the timing (inversion timing) determined by the time determination unit 212 to arrive. For example, when the time measured by the timer 204 matches the conveying time determined by the time determination unit 212, the inversion determination unit 213 switches the clutch CL1 from on to off. This switches the inversion roller 19 from forward rotation to reverse rotation. The conveying direction of the sheet P is reversed, and the sheet P is sent from the second conveying path R2 to the third conveying path R3. Furthermore, the position of the flapper 17 is switched from the position where the sheet P is guided to the fourth conveying path R4 to the position where the sheet P is guided to the third conveying path R3. On the other hand, when the clutch CL1 is switched from off to on, the reversing roller 19 rotates forward, and the flapper 17 moves to the position where the sheet P is guided to the fourth conveying path R4. As a result, the sheet P is guided from the first conveying path R1 to the fourth conveying path R4 and discharged to the discharge tray 27.

[0022] The exposure control unit 207 controls the exposure unit 3 based on an instruction from the print control unit 202. As shown in FIG. 1, the exposure unit 3Y for yellow toner starts exposure first. Next, the exposure unit 3M for magenta toner starts exposure. Next, the exposure unit 3C for cyan toner starts exposure. Finally, the exposure unit 3K for black toner starts exposure. Therefore, in the following, the start timing of image formation is based on the exposure unit 3Y. Based on the exposure start timing of the exposure unit 3Y, the exposure start timings of the remaining exposure units 3M, 3C, and 3K are determined. As a result, the yellow toner image, magenta toner image, cyan toner image, and black toner image are accurately aligned and transferred on the intermediate transfer belt 7.

[0023] (3) Flowchart (3-1) Basic image formation processing FIG. 3 shows the transport control that the CPU 201 executes in accordance with the control program.

[0024] In S301, the CPU 201 (print control unit 202) determines whether a print instruction has been received via the operation unit 250 or the communication circuit 260. When a print instruction is received from a host computer or the like via the communication circuit 260, the CPU 201 proceeds from S301 to S302.

[0025] In S302, the CPU 201 (print control unit 202, transport control unit 205) drives the motors M1 and M2.

[0026] In S303, the CPU 201 (print control unit 202) starts preparation for image formation. The preparation for image formation includes, for example, adjusting the amount of laser light from the exposure device 3 to a target amount of light, controlling the rotation speed of the rotating polygon mirror of the exposure device 3 to a target speed, and controlling the rotation speed of rotating bodies such as the photosensitive drum 1 and the intermediate transfer belt 7 to a target speed.

[0027] In S304, the CPU 201 (print control unit 202, exposure control unit 207) starts image formation on the first side (front side) of the sheet P. For example, the print control unit 202 supplies an image signal to the exposure control unit 207. The exposure control unit 207 starts outputting laser light according to the image signal. As a result, an electrostatic latent image for the first side is formed on the photosensitive drum 1. Thereafter, the electrostatic latent image is developed into a toner image, and is transported to the secondary transfer roller 8 by the intermediate transfer belt 7. Below, a series of processes from S305 to S313 and a series of processes from S331 to S332 are executed in parallel.

[0028] (3-1-1) From S305 to S313 In S305, the CPU 201 (print control unit 202) determines whether a predetermined time X has elapsed from the start of formation of the electrostatic latent image for the first surface. The predetermined time X is the time from the start of formation of the electrostatic latent image to the start of feeding of the sheet P. The predetermined time X is determined in advance and stored in the memory 220. The predetermined time X may be calculated from the following formula.

[0029] X = Xi - Xp (Eq1) Here, Xi is the ideal time from time t0 when formation of the electrostatic latent image starts to time t2 when the toner image reaches the secondary transfer roller 8. Xp is the ideal time from time t1 when the sheet P is fed from the sheet cassette 10 to time t2 when the leading edge of the sheet P reaches the secondary transfer roller 8. In the first embodiment, the time Xi is longer than the time Xp. Therefore, after the formation of the image starts, the feeding of the sheet P to which the image is to be transferred starts. When the predetermined time X has elapsed, the CPU 201 proceeds from S305 to S306.

[0030] In S306, the CPU 201 (print control unit 202, conveyance control unit 205) controls a solenoid (not shown) or the like to lower the feed roller 11, and the feed roller 11 starts to feed the sheet P. At this time, the feed roller 11 is already rotating at a rotation speed that can convey the sheet P at a predetermined conveying speed (specified speed V). Therefore, when the feed roller 11 comes into contact with the sheet P, the sheet P is conveyed at the specified speed V.

[0031] In S307, the CPU 201 (print control unit 202) determines whether or not the leading edge of the sheet P has been detected by the sheet sensor 13. When the leading edge of the sheet P has been detected, the CPU 201 proceeds from S307 to S308.

[0032] In S308, the CPU 201 (conveyance control unit 205) executes acceleration / deceleration control in response to the timing at which the leading edge of the sheet P is detected. Details of the acceleration / deceleration control will be described later with reference to FIG. 4. While the sheet P passes through the secondary transfer unit 30, the sheet P is conveyed at a specified speed V. The toner image is also conveyed at the specified speed V by the intermediate transfer belt 7.

[0033] In S309, the CPU 201 (print control unit 202) determines whether a predetermined time Z has elapsed since the time when the acceleration / deceleration control ended. The predetermined time Z is stored in advance in the memory 220. The predetermined time Z is the time from the time when the acceleration / deceleration control ended to the time when the clutch CL1 is switched from OFF to ON. When the predetermined time Z has elapsed, the CPU 201 proceeds from S309 to S310.

[0034] In S310, the CPU 201 (reversal control unit 206) turns on the clutch CL1. This causes the flapper 17 to move to a position where the sheet P is guided from the first conveying path R1 to the second conveying path R2. Furthermore, the rotation direction of the reversing roller 19 is switched from reverse to forward. This causes the sheet P to be drawn from the first conveying path R1 to the second conveying path R2.

[0035] In S311, the CPU 201 (arrival determination unit 211) determines whether or not the rear end (second end) of the sheet P has been detected by the sheet sensor 16. If the rear end of the sheet P is detected by the sheet sensor 16, the CPU 201 proceeds from S311 to S312.

[0036] In S312, the CPU 201 (reversal determination unit 213) determines whether a predetermined time B0 has elapsed since the time when the rear end of the sheet P was detected by the sheet sensor 16. The predetermined time B0 is calculated by the following formula.

[0037] B0 = Lb ÷ V (Eq2) Here, V is the conveying speed (prescribed speed) of the sheet P. Lb is the distance from the sheet sensor 16 to the reversal position, and is calculated by the following formula.

[0038] Lb = Lfr - Lr (Eq3) Here, Lr is the distance from the reversing position to the reversing roller 19. Lr is, for example, 15 mm. Lfr is the distance from the sheet sensor 16 to the reversing roller 19. When Lr is 15 mm, the reversing position is a position 15 mm upstream from the reversing roller 19. When the rear end (second end) of the sheet P reaches the reversing position, the conveying direction of the sheet P is reversed. When a predetermined time B0 has elapsed, the CPU 201 proceeds from S312 to S313.

[0039] In S313, the CPU 201 (reversal determination unit 213) turns off the clutch CL1. As a result, the flapper 17 moves to a position where the sheet P is guided from the second conveying path R2 to the third conveying path R3. Also, the rotation direction of the reversing roller 19 switches from forward to reverse, and the conveying direction of the sheet P is reversed. Therefore, the sheet P is conveyed through the third conveying path R3 with the second end as the leading edge, toward the first conveying path R1. After that, the CPU 201 proceeds from S313 to S314.

[0040] (3-1-2) From S331 to S332 In S331, the CPU 201 (print control unit 202) determines whether a predetermined time Y has elapsed from time t2 when the leading edge of the sheet P reaches the secondary transfer roller 8. The predetermined time Y is the time from time t2 when the leading edge of the sheet P, onto which a toner image is to be transferred on the first side, reaches the secondary transfer roller 8 to time t9 when formation of an electrostatic latent image for the second side starts. The predetermined time Y is determined in advance and stored in the memory 220. The predetermined time Y may be calculated from the following formula.

[0041] Y = Yp1 + Yp2 + Yp3 (Eq4) Here, Yp1 is the ideal transport time required for the leading edge (first edge) of the sheet P with an image formed on its first surface to move from the secondary transfer roller 8 to the reversal position. Yp2 is the ideal transport time obtained by dividing the nominal length of the sheet P by the transport speed (prescribed speed V). Yp3 is the ideal transport time required for the leading edge (second edge) of the sheet P with an image formed on its first surface to travel from the reversal position through the third transport path R3 to reach the secondary transfer roller 8. Note that since the transport direction of the sheet P is reversed, the leading edge of the sheet P in the transport direction is switched from the first edge to the second edge. When the predetermined time Y has elapsed, the CPU 201 proceeds from S331 to S332.

[0042] In S332, the CPU 201 (print control unit 202, exposure control unit 207) starts image formation on the second side (rear side) of the sheet P. For example, the print control unit 202 supplies an image signal to the exposure control unit 207, and the exposure control unit 207 starts outputting laser light according to the image signal. As a result, an electrostatic latent image for the second side is formed on the photosensitive drum 1. Thereafter, the electrostatic latent image is developed into a toner image, and is transported to the secondary transfer roller 8 by the intermediate transfer belt 7. The CPU 201 proceeds from S332 to S314.

[0043] (3-1-3) From S314 to S318 In S314, the CPU 201 (conveyance control unit 205) determines whether or not the leading edge (second edge) of the sheet P has been detected by the sheet sensor 13. When the leading edge (second edge) of the sheet P is detected, the CPU 201 proceeds from S314 to S315.

[0044] In S315, the CPU 201 (conveyance control unit 205) executes acceleration / deceleration control. The acceleration / deceleration control in S315 is the same process as the acceleration / deceleration control in S308. This synchronizes the time when the toner image to be transferred to the second side reaches the secondary transfer roller 8 with the time when the sheet P conveyed through the third conveyance path R3 reaches the secondary transfer roller 8.

[0045] In S316, the CPU 201 (print control unit 202) determines whether or not the rear end (first end) of the sheet P has been detected by the sheet sensor 16. If the rear end (first end) of the sheet P has been detected, the CPU 201 proceeds from S316 to S317.

[0046] In S317, the CPU 201 (print control unit 202) determines whether a predetermined time W has elapsed from the time when the rear end (first end) of the sheet P is detected by the sheet sensor 16. The predetermined time W is the transport time from the time when the rear end (first end) of the sheet P is detected to the time when the sheet P is discharged to the discharge tray 27. When the predetermined time W has elapsed, the CPU 201 proceeds from S317 to S318.

[0047] In S318, the CPU 201 (conveyor control unit 205) stops the motors M1 and M2.

[0048] (3-2) Acceleration / deceleration control 4 shows the acceleration / deceleration control executed according to the control program by the CPU 201. This acceleration / deceleration control corresponds to S308 and S315 shown in FIG.

[0049] In S401, the CPU 201 (the transport control unit 205) obtains the amount of misalignment of the leading edge of the sheet P relative to the position of the leading edge of the toner image. For example, the transport control unit 205 obtains the difference between the time when image formation starts and the time when the leading edge of the sheet P reaches the sheet sensor 13. Furthermore, the transport control unit 205 obtains the amount of misalignment by multiplying this difference by a specified transport speed (specified speed V).

[0050] In S402, the CPU 201 (conveyance control unit 205) determines whether the amount of misalignment is within a predetermined range. The predetermined range is determined according to the acceleration / deceleration performance of the motor M1. In other words, the predetermined range is a range of the amount of misalignment that can be eliminated by acceleration / deceleration control while the leading edge of the sheet P is conveyed from the sheet sensor 13 to the secondary transfer roller 8. If the amount of misalignment is not within the predetermined range, the CPU 201 proceeds from S402 to S410. In S410, the CPU 201 executes a misprint process. For example, the CPU 201 stops the motors M1 and M2, and displays a message on the operation unit 250 prompting the user to remove the misprinted sheet P from the image forming apparatus 100. If the amount of misalignment is within the predetermined range, the CPU 201 proceeds from S402 to S403.

[0051] In S403, the CPU 201 (conveyor control unit 205) obtains the target speed of the motor M1 and the maintenance time T. The maintenance time T is the time during which the rotation speed of the motor M1 is maintained at the target speed.

[0052] In S404, the CPU 201 (conveyor control unit 205) changes the rotation speed of the motor M1 to the target speed. Furthermore, the CPU 201 uses the timer 204 to measure the elapsed time from the time when the rotation speed was changed to the target speed.

[0053] In S405, the CPU 201 (transport control unit 205) determines whether or not the maintenance time T has elapsed based on the timer 204. When the maintenance time T has elapsed, the CPU 201 proceeds from S405 to S406.

[0054] In S406, the CPU 201 (the conveyance control unit 205) returns the rotation speed of the motor M1 to the specified speed V. As a result, the sheet P passes through the secondary transfer unit 30 at the specified speed V.

[0055] (4) Difference between the actual length and nominal length of sheet P 3 is based on the assumption that the actual length of the sheet P fed from the sheet cassette 10 matches the length (nominal length) of the sheet P specified by the print instruction. However, a user may mistakenly operate the operation unit 250, or may store a sheet P with a length different from the nominal length in the sheet cassette 10. In this case, the above assumption is violated. Furthermore, due to manufacturing variations in the sheet P, the actual length of the sheet P may differ from the nominal length.

[0056] For example, there are cases where the actual length of the sheet P stored in the sheet cassette 10 is longer than the nominal length of the sheet P specified by the user in the print instruction. In this case, the timing at which the rear end (second end) of the sheet P is detected by the sheet sensor 16 in S311 is delayed from the specified timing. As a result, the timing at which the front end (second end) of the sheet P with an image formed on its first surface reaches the sheet sensor 13 again is also delayed.

[0057] Meanwhile, the CPU 201 determines the predetermined time Y from the size information of the sheet P included in the print instruction in order to provide the user with a printed matter in the shortest time. Then, the CPU 201 starts forming an image for the second side when the predetermined time Y has elapsed. If the actual length of the sheet P is longer than the nominal length, the above-mentioned deviation amount becomes large, and the acceleration / deceleration control may not be able to eliminate the deviation amount. Note that if the actual length of the sheet P is shorter than the nominal length, the above-mentioned deviation amount becomes large, and the acceleration / deceleration control may not be able to eliminate the deviation amount. Thus, the greater the difference between the actual length and the nominal length, the greater the deviation amount and the higher the possibility of a misprint occurring.

[0058] (5) Improved Flowchart Fig. 5 is a flowchart for solving the problem based on the difference between the actual length and the nominal length of the sheet P. Note that Fig. 5 shows only a part of the flowchart shown in Fig. 3. Steps not shown in Fig. 5 are the same as the steps shown in Fig. 3. According to Fig. 5, the processes from S311 to S313 are improved as follows. When the trailing end of the sheet P is detected by the sheet sensor 16 in S311, the CPU 201 proceeds from S311 to S501.

[0059] In S501, the CPU 201 (arrival determination unit 211) determines whether the rear end (second end) of the sheet P has been detected earlier than expected by the sheet sensor 16. For example, the CPU 201 (time determination unit 212) determines the expected timing Tp at which the rear end of the sheet P will arrive at the sheet sensor 16, based on the start time (time t0) of image formation.

[0060] Tp = Xi + Xf + Yp2 (Eq5) Here, Xi is the ideal time from time t0 when the formation of the electrostatic latent image starts to time t2 when the toner image reaches the secondary transfer roller 8. Xf is the ideal time required for the leading edge of the sheet P to be transported from the secondary transfer roller 8 to the sheet sensor 16. Yp2 is the ideal transport time obtained by dividing the nominal length of the sheet P by the transport speed (prescribed speed V). The arrival determination unit 211 monitors the scheduled timing Tp using the timer 204. The arrival determination unit 211 determines whether the timing (actual timing Tr) when the sheet P actually reaches the sheet sensor 16 is earlier than the scheduled timing Tp. If the actual timing Tr is earlier than the scheduled timing Tp, the CPU 201 proceeds from S501 to S502.

[0061] In S502, the CPU 201 (time determination unit 212) calculates the predetermined time C1. The predetermined time C1 is calculated by the following formula.

[0062] C1 = B0 + (Tp-Tr)÷2 ···(Eq6) In S503, the CPU 201 (reversal determination unit 213) determines, based on the timer 204, whether a predetermined time C1 has elapsed since the rear end of the sheet P reached the sheet sensor 16. When the predetermined time C1 has elapsed, the CPU 201 proceeds from S503 to S313 and turns off the clutch CL1.

[0063] On the other hand, if the trailing edge of the sheet P is not detected earlier than expected in S501, the CPU 201 proceeds from S501 to S511.

[0064] In S511, the CPU 201 (arrival determination unit 211) determines whether the rear end (second end) of the sheet P is detected later than expected by the sheet sensor 16. For example, the arrival determination unit 211 determines whether the actual timing Tr is later than the scheduled timing Tp. If the rear end (second end) of the sheet P is not detected later than expected, the CPU 201 proceeds from S511 to S312. On the other hand, if the rear end (second end) of the sheet P is detected later than expected, the CPU 201 proceeds from S511 to S512.

[0065] In S512, the CPU 201 (time determination unit 212) calculates the predetermined time D1. The predetermined time D1 is calculated by the following formula.

[0066] D1 = B0 - (Tr-Tp)÷2 ···(Eq7) In S513, the CPU 201 (reversal determination unit 213) determines, based on the timer 204, whether a predetermined time D1 has elapsed since the rear end of the sheet P reached the sheet sensor 16. When the predetermined time D1 has elapsed, the CPU 201 proceeds from S513 to S313 and turns off the clutch CL1.

[0067] (6) Timing chart (6-1) When the actual length of sheet P is shorter than the nominal length 6(A), 6(B), and 6(C) show the position of sheet P during the reversal process. The horizontal axis indicates time. The vertical axis indicates the position of sheet P. As shown in FIG. 6(A), the dashed line indicates the case where the actual length of sheet P matches the nominal length. The trailing end of sheet P reaches the detection position Ys of sheet sensor 16 at scheduled timing Tp. When a predetermined time B0 has elapsed from scheduled timing Tp, the trailing end of sheet P reaches the ideal reversal position Yb. At this point, the conveying direction of sheet P is reversed. As a result, the second end is replaced from the trailing end to the leading end, and the first end is replaced from the leading end to the trailing end. After that, sheet P is conveyed to third conveying path R3.

[0068] 6B shows a case (solid line) where the actual length of the sheet P is shorter than the nominal length and the reversal timing is not corrected. When the actual length of the sheet P is shorter than the nominal length, the timing Tr at which the rear end of the sheet P reaches the sheet sensor 16 occurs earlier than the scheduled timing Tp.

[0069] The conveying direction of the sheet P is reversed at a timing when a predetermined time B0 has elapsed from the actual timing Tr. As a result, the reversal timing in FIG. 6B occurs earlier than the reversal timing in FIG.

[0070] 6C shows a case (solid line) where the actual length of the sheet P is shorter than the nominal length and the reversal timing is appropriately corrected. When a predetermined time C1 has elapsed from the actual timing Tr, the conveying direction of the sheet P is reversed. As a result, the position of the leading edge (second end) and the conveying timing (dashed line) in the case where the actual length is equal to the nominal length become consistent with the position of the leading edge (second end) and the conveying timing (solid line) in the case where the actual length is shorter than the nominal length.

[0071] (6-2) When the actual length of sheet P is longer than the nominal length 7(A), 7(B), and 7(C) show the position of sheet P in the reversal process. The horizontal axis indicates time. The vertical axis indicates the position of sheet P. Since FIG. 7(A) is the same as FIG. 6(A), its description will be omitted.

[0072] 7B shows a case (solid line) in which the actual length of the sheet P is longer than the nominal length and the reversal timing is not corrected. When the actual length of the sheet P is longer than the nominal length, the timing Tr at which the rear end of the sheet P reaches the sheet sensor 16 is later than the scheduled timing Tp.

[0073] The conveying direction of the sheet P is reversed at a timing when a predetermined time B0 has elapsed from the actual timing Tr. As a result, the reversal timing in FIG. 7B is later than the reversal timing in FIG.

[0074] 7C shows a case (solid line) where the actual length of sheet P is longer than the nominal length and the reversal timing is appropriately corrected. When a predetermined time D1 has elapsed from the actual timing Tr, the conveying direction of sheet P is reversed. As a result, the position of the leading edge (second end) and the conveying timing (dashed line) in the case where the actual length is equal to the nominal length become consistent with the position of the leading edge (second end) and the conveying timing (solid line) in the case where the actual length is longer than the nominal length.

[0075] (7) Upper and lower limits for the predetermined time C1 and the predetermined time D1 8 is an enlarged view showing the reversing unit 800 of the image forming apparatus 100. The most upstream reversing position Pt1 indicates the most upstream position of the rear end (second end) of the sheet P at which the conveying direction of the sheet P conveyed to the second conveying path R2 can be reversed. If the rear end is located at a position upstream of this, the rear end is still located inside the first conveying path R1. At this time, if the rotation direction of the reversing roller 19 is reversed, the sheet P will be conveyed to the first conveying path R1 without being conveyed to the third conveying path R3. The sheet P will attempt to enter the fixing unit 9 and a jam will occur.

[0076] The most downstream reversal position Pt2 indicates the most downstream position of the rear end (second end) of the sheet P at which the conveying direction of the sheet P conveyed to the second conveying path R2 can be reversed. If the rear end is conveyed downstream of the most downstream reversal position Pt2, the sheet P will pass through the reversing rollers 19 and be discharged to the outside of the image forming apparatus 100.

[0077] Therefore, the lower limit of the predetermined time C1 and the predetermined time D1 is the transport time required for the rear end of the sheet P to be transported from the sheet sensor 16 to the most upstream reversal position Pt1. The upper limit of the predetermined time C1 and the predetermined time D1 is the transport time required for the rear end of the sheet P to be transported from the sheet sensor 16 to the most downstream reversal position Pt2.

[0078] In this way, when the actual length and the nominal length of the sheet P differ, the CPU 201 adjusts the reversal timing, thereby reducing the variation in the transport timing of the sheet P in the third transport path R3. As a result, the image forming apparatus 100 will be able to achieve the target printing speed (throughput).

[0079] <Example 2> (1) Overview of Example 2 In the first embodiment, the timing of reversing the sheet P is determined according to the actual length of the sheet P. More specifically, in the first embodiment, the predetermined time B0 is corrected to the predetermined time C1 or D1 according to a comparison result between the actual timing Tr at which the rear end of the sheet P reaches the sheet sensor 16 and the scheduled timing Tp. In other words, the predetermined time C1 or D1 is calculated based on the actual timing Tr which changes according to the actual length of the sheet P.

[0080] In the second embodiment, the timing of reversing the sheet P is basically determined according to the actual length of the sheet P. More specifically, in the second embodiment, the sheet sensor 13 is used to directly measure the actual length of the sheet P, and the predetermined time B0 is adjusted to the predetermined time C2 or D2 according to the actual length. The explanation of the first embodiment is cited for the matters common to the first embodiment in the second embodiment.

[0081] (2) CPU Functions Fig. 9 shows functions realized by the CPU 201 executing a control program. Explanation of matters common to Fig. 2 will be omitted in Fig. 9. A measuring unit 901 measures the actual length of the sheet P based on the timing at which the rear end of the sheet P is detected by the sheet sensor 13. An actual length determining unit 911 determines whether the actual length measured by the measuring unit 901 is shorter than the nominal length. A time determining unit 212 determines conveying times C2 and D2 based on the determination result of the actual length determining unit 911. An inversion determining unit 213 controls the inversion of the sheet P based on the conveying times C2 and D2.

[0082] (3) Flowchart (3-1) How to measure actual length Fig. 10 is a flowchart showing an actual length measuring method in the embodiment 2. S1001 and S1002 are added between S308 and S309 in the flowchart shown in Fig. 3. When the acceleration / deceleration control in S308 ends and the conveying speed of the sheet P returns to the specified speed V, the CPU 201 proceeds from S308 to S1001.

[0083] In S1001, the CPU 201 (measurement unit 901) determines whether or not the rear end of the sheet P has been detected by the sheet sensor 13. When the rear end of the sheet P is detected, the CPU 201 proceeds from S1001 to S1002.

[0084] In S1002, the CPU 201 (measurement unit 901) measures the actual length of the sheet P. The actual length Pb may be calculated, for example, from the following formula so as not to be affected by acceleration / deceleration control.

[0085] Pb = (Z3+(Z2-Z1))×V (Eq8) Here, Z1 is the time when the leading edge of the sheet P reaches the secondary transfer roller 8. Z1 is obtained by dividing the distance (known) of the section from the sheet sensor 13 to the secondary transfer roller 8 by the average value of the conveying speed of the sheet P in that section to obtain the conveying time, and adding the conveying time to the time when the leading edge is detected by the sheet sensor 13. Alternatively, Z1 may be the time when the toner image reaches the secondary transfer roller 8. This is because acceleration and deceleration control is performed so that the arrival timing of the leading edge of the sheet P and the arrival timing of the toner image coincide with each other. Y1 may be obtained by adding a fixed value stored in the memory 220 to the time when image formation is started. Z2 is the time when the trailing edge passes the sheet sensor 13. Z3 is the ideal conveying time obtained by dividing the distance from the sheet sensor 13 to the secondary transfer roller 8 by the specified speed V.

[0086] Thereafter, the CPU 201 proceeds from S1002 to S309, and determines whether or not a predetermined time Z has elapsed since the end of the acceleration / deceleration control.

[0087] (3-2) How to determine the timing of reversal 11 shows a method for determining the timing of reversal. In the processes from S311 to S313, S1101 to S1113 are added. When the trailing edge of the sheet P is detected by the sheet sensor 16 in S311, the CPU 201 advances from S311 to S1101.

[0088] In S1101, the CPU 201 (actual length determination unit 911) determines whether the actual length Pb is shorter than the nominal length. If the actual length Pb is shorter than the nominal length, the CPU 201 proceeds from S1101 to S1102.

[0089] In S1102, the CPU 201 (time determination unit 212) determines the predetermined time C2. The predetermined time C2 is a transfer time used in place of the predetermined time B0. The time determination unit 212 determines the predetermined time C2 using, for example, the following formula.

[0090] C2 = B0 + (Yp2-Zp)÷2 ···(Eq9) Here, Yp2 is the ideal transport time obtained by dividing the nominal length of the sheet P by the transport speed (prescribed speed V). Zp is the ideal transport time obtained by dividing the actual length Pb by the prescribed speed V.

[0091] In S1103, the CPU 201 (reversal determination unit 213) determines, based on the timer 204, whether a predetermined time C2 has elapsed since the rear end of the sheet P reached the sheet sensor 16. When the predetermined time C2 has elapsed, the CPU 201 proceeds from S1103 to S313 and turns off the clutch CL1.

[0092] On the other hand, if the actual length Pb of the sheet P is not shorter than the nominal length in S1101, the CPU 201 proceeds from S1101 to S1111.

[0093] In S1111, the CPU 201 (actual length determination unit 911) determines whether the actual length Pb is longer than the nominal length. If the actual length Pb is longer than the nominal length, the CPU 201 proceeds from S1111 to S1112. If the actual length Pb is equal to the nominal length, the CPU 201 proceeds from S1111 to S312.

[0094] In S1112, the CPU 201 (time determination unit 212) determines the predetermined time D2. The predetermined time D2 is a transfer time used in place of the predetermined time B0. The time determination unit 212 determines the predetermined time D2 using, for example, the following formula.

[0095] D2 = B0 - (Zp-Yp2)÷2 ···(Eq10) In S1113, the CPU 201 (reversal determination unit 213) determines, based on the timer 204, whether or not a predetermined time D2 has elapsed since the rear end of the sheet P reached the sheet sensor 16. When the predetermined time D2 has elapsed, the CPU 201 proceeds from S1113 to S313 and turns off the clutch CL1.

[0096] In this way, according to the second embodiment, when the actual length Pb of the sheet P differs from the nominal length, the reversal timing is adjusted to reduce the variation in the transport timing of the sheet P in the second transport path R2. As a result, the image forming apparatus 100 will be able to achieve the target printing speed (throughput).

[0097] <Modification> In the first embodiment, it is assumed that the conveying speed of the sheet P before the conveying direction is reversed is the same as the conveying speed of the sheet P after the conveying direction is reversed. However, this is only an example. When the conveying speeds are different, the predetermined time C1 and the predetermined time D1 are simply calculated taking into account the difference in the conveying speeds. In other words, the predetermined time C1 and the predetermined time D1 are calculated so that the timing at which the sheet P arrives at the secondary transfer unit 30 again is the specified timing. Alternatively, it is sufficient that the deviation of the conveying timing of the sheet P relative to the conveying timing of the toner image when the second end of the sheet P arrives at the sheet sensor 13 is within a range that can be eliminated by acceleration / deceleration control. Therefore, the predetermined time C1 and the predetermined time D1 may be calculated so that the deviation amount is within a range that can be eliminated when the leading edge (second end) of the sheet P arrives at the sheet sensor 13. The same applies to the predetermined time C2 and the predetermined time D2 in the second embodiment.

[0098] The conveying speed in the third conveying path R3 may differ depending on the type of sheet P (e.g., plain paper, thin paper, thick paper, coated paper). The conveying speed in the third conveying path R3 may also differ depending on environmental conditions (e.g., temperature, humidity). The conveying speed in the third conveying path R3 may also differ depending on the durability deterioration of the conveying rollers 21, 22, etc. In these cases, the CPU 201 may obtain the conveying speed in the third conveying path R3 depending on the type of sheet P, environmental conditions, or deterioration state (e.g., number of sheets passed), and correct the reversal timing depending on the obtained conveying speed. The CPU 201 may use a formula or table for obtaining the conveying speed in the third conveying path R3 from the type of sheet P, environmental conditions, or deterioration state (e.g., number of sheets passed). Such a formula or table may be obtained in advance by experiment or simulation and stored in the memory 220.

[0099] In the second embodiment, the actual length Pb is measured based on the timing when the trailing end of the sheet P passes the sheet sensor 13, but this is merely an example. For example, if the motor M1 that drives the feed roller 11 is a pulse motor, the measurement unit 901 may measure the actual length Pb by counting pulses supplied to the pulse motor. For example, the measurement unit 901 may measure the actual length Pb from the difference between the count value when the leading end of the sheet P passes the sheet sensor 13 and the count value when the trailing end of the sheet P passes the sheet sensor 13.

[0100] In the second embodiment, the predetermined time C2 and the predetermined time D2 are determined based on the timing when the rear end of the sheet P reaches the sheet sensor 16, but this is also just one example. Another sheet sensor provided in the first conveying path R1 or the second conveying path R2 may be used instead of the sheet sensor 16. Also, instead of the timing when the rear end of the sheet P is detected, the timing when the leading edge of the sheet P is detected may be used as the reference. For example, another sheet sensor may be provided in the second conveying path R2, and the timing when the leading edge or trailing edge of the sheet P reaches that sheet sensor may be used as the reference.

[0101] <Technical ideas derived from examples> (Item 1) A storage means for storing sheets (e.g., a sheet cassette 10); Discharge means (e.g., discharge roller 18) for discharging the sheet; A first conveying path (e.g., a first conveying path R1, a fourth conveying path R4) connecting the storage means to the discharge means; an image forming unit (e.g., an image forming section 20) that forms an image on the sheet conveyed through the first conveying path; a second conveying path (e.g., second conveying path R2) branching off from the first conveying path at a branching point provided downstream of the image forming means in the conveying direction of the sheet; a third conveying path (e.g., third conveying path R3) that connects the second conveying path and the first conveying path and conveys the sheet having an image formed on a first side from the second conveying path to the first conveying path; an inversion means (e.g., inversion roller 19) that transports the sheet in a first direction to draw the sheet from the first transport path to the second transport path, and inverts the transport direction of the sheet from the first direction to a second direction to send the sheet from the second transport path to the third transport path, in order to change the surface on which an image is formed by the image forming means from the first surface to a second surface of the sheet; A control means (e.g., CPU 201) for controlling the timing of inversion of the inversion means in accordance with the actual length of the sheet; An image forming apparatus comprising:

[0102] As described above, according to this embodiment, the reversal timing is controlled in accordance with the actual length of the sheet P, so that the image forming apparatus 100 that can more easily achieve the target printing speed than the conventional image forming apparatus is provided. (Item 2) The sheet conveying device further includes a detection unit (e.g., a sheet sensor 16) for detecting the sheet conveyed through the first conveying path, 2. The image forming apparatus according to claim 1, wherein the control unit controls a timing of reversing the reversing unit in response to a timing at which the trailing end or leading end of the sheet is detected by the detection unit, the timing varying in response to the actual length of the sheet.

[0103] 6(A), 6(B), and 7(B), the timing at which the rear end of the sheet P arrives at the sheet sensor 16 differs depending on the actual length of the sheet P. Therefore, the reversal timing is controlled depending on the timing at which the rear end of the sheet P arrives at the sheet sensor 16. As a result, the reversal timing is controlled depending on the actual length of the sheet P, making it easier to achieve the target printing speed than before. (Item 3) The device further includes a designation unit (e.g., an operation unit 250, a communication circuit 260) for designating the nominal length of the sheet, The control means If the detection timing (e.g., Tr) by the detection means according to the actual length of the sheet is earlier than the detection timing (e.g., Tp) by the detection means predicted based on the nominal length, the reversing timing of the reversing means is delayed (e.g., FIG. 6(C)). If the detection timing by the detection means according to the actual length of the sheet is later than the detection timing by the detection means predicted based on the nominal length, the reversing timing of the reversing means is advanced (e.g., FIG. 7(C) ). 3. The image forming apparatus according to item 2.

[0104] In this way, by delaying or advancing the timing of reversal depending on the timing at which the trailing edge of the sheet P is detected, it becomes easier to achieve the target printing speed than before. (Item 4) The device further includes a designation unit (e.g., an operation unit 250, a communication circuit 260) for designating the nominal length of the sheet, The image forming apparatus according to item 2 or 3, wherein the control means controls the reversal timing of the reversing means in accordance with a difference (e.g., Tr-Tp or Tp-Tr) between the detection timing by the detection means predicted based on the nominal length and the detection timing by the detection means in accordance with the actual length of the sheet.

[0105] As exemplified by equations (6) and (7), the reversal timing may be accurately determined according to the timing at which the trailing edge of the sheet P is detected. (Item 5) The control means When the actual length of the sheet is equal to the nominal length, a time from when the trailing end of the sheet is detected by the detection means to when the reversing means is reversed is controlled to a predetermined time (e.g., B0); 5. The image forming apparatus according to item 4, wherein, if the actual length of the sheet is not the nominal length, the predetermined time is corrected according to the difference.

[0106] As exemplified by equations (6) and (7), the reversal timing may be accurately determined according to the timing at which the trailing edge of the sheet P is detected. (Item 6) 6. The image forming apparatus according to item 5, wherein the control means corrects the predetermined time by subtracting half of the difference from the predetermined time when the actual length of the sheet is not the nominal length.

[0107] As exemplified by equations (6) and (7), the reversal timing may be accurately determined according to the timing at which the trailing edge of the sheet P is detected. (Item 7) 7. The image forming apparatus according to any one of items 1 to 6, wherein the control means maintains a constant interval between the timing at which the image forming means forms an image on the first side of the sheet and the timing at which the image forming means forms an image on the second side of the sheet.

[0108] In this way, by controlling the intervals at which images are formed between multiple pages to be constant, the print speed of the image forming apparatus 100 is controlled to a target print speed. For example, by appropriately adjusting the timing at which the sheet P is turned over, the timing at which an image is formed on the first side of the sheet P and the timing at which an image is formed on the second side of the sheet P are kept constant. As a result, misprints, such as the transfer position of a toner image on the sheet P being shifted from the ideal position, are less likely to occur. (Item 8) The image forming means includes: A photoconductor that is rotated (e.g., photoconductor drum 1), An exposure means (e.g., an exposure device 3) for exposing the photoconductor to light to form an electrostatic latent image; a developing means (e.g., a developing device 4) for developing the electrostatic latent image to form a toner image; a primary transfer means (e.g., a primary transfer member 5) for transferring the toner image from the photoreceptor to an intermediate transfer member; a secondary transfer means (e.g., a secondary transfer roller 8) that transfers the toner image from the intermediate transfer body to the sheet transported through the first transport path; A fixing means (e.g., a fixing device 9) for fixing the toner image onto the sheet, the image forming apparatus further includes a guide member (e.g., a flapper 17) that guides the sheet having the image formed on the first surface thereof from the first conveying path to the second conveying path, and guides the sheet guided to the second conveying path to the third conveying path; 8. The image forming apparatus according to any one of items 2 to 7, wherein the detection unit is disposed between the fixing unit and the guide member in the first transport path.

[0109] When the detection means is disposed at such a position, the timing of reversing the sheet P is accurately corrected, because the guide member is located near the reversing position of the sheet P and contributes to the reversing of the sheet P. (Item 9) A motor (e.g., motor M2) that drives the reversing means; A clutch (e.g., clutch CL1) that transmits the driving force of the motor to the reversing means and controls the rotation direction of the reversing means, 9. The image forming apparatus according to claim 8, wherein the clutch further controls the guide member.

[0110] The clutch for switching the rotation direction of the reversing means and the clutch for switching the guide member may be provided separately, but by controlling both with a single clutch CL1, the number of parts can be reduced. (Item 10) When the clutch is in a first state (e.g., on or off), the reversing means rotates to pull the sheet from the first conveying path to the second conveying path, and the guide member guides the sheet from the first conveying path to the second conveying path; When the clutch is in a second state (e.g., off or on), the reversing means rotates to feed the sheet from the second conveying path to the third conveying path, and the guide member guides the sheet from the second conveying path to the third conveying path. Item 10. The image forming apparatus according to item 9.

[0111] In the first and second embodiments, the clutch CL1 ON and OFF are described as being associated with specific transport directions, but this is merely an example. The relationship between the operating state (ON, OFF) of the clutch CL1 and the transport direction may be reversed. By adopting such a clutch CL1, the number of motors can be reduced. Note that if the number of motors is reduced, the number of rotating bodies driven by one motor increases. In this case, the section in the transport path where the transport speed of the sheet P can be adjusted decreases, making it difficult to absorb the variation in the arrival timing of the sheet P by acceleration / deceleration control. Therefore, it would be technically meaningful to adjust the reversal timing according to the actual length of the sheet P. (Item 11) The sheet conveying device further includes a first detection means (e.g., a sheet sensor 13) for detecting the sheet conveyed through the first conveying path, 2. The image forming apparatus according to claim 1, wherein the control means controls the inversion timing of the inversion means in accordance with the actual length of the sheet calculated from the timing at which the leading edge of the sheet is detected by the first detection means and the timing at which the trailing edge of the sheet is detected by the first detection means.

[0112] For example, when the acceleration / deceleration control of the sheet P is not applied, the actual length of the sheet P is obtained by multiplying the difference between the timing when the leading end is detected by the sheet sensor 13 and the timing when the trailing end is detected by the sheet sensor 13 by the specified speed V. Alternatively, the actual length can also be obtained by integrating the conveying speed to which the acceleration / deceleration control is applied between these two timings. (Item 12) a second detection means (e.g., a sheet sensor 16) is disposed downstream of the first detection means in the sheet conveying direction in the first conveying path and detects the trailing edge of the sheet; Item 12. The image forming apparatus according to item 11, wherein the control unit adjusts the time from the timing at which the rear end of the sheet is detected by the second detection unit to the timing at which the inversion unit is inverted, in accordance with the actual length of the sheet.

[0113] In this manner, the sheet sensor 16 is disposed downstream of the sheet sensor 13. In other words, it is sufficient that the sheet sensor 13 is disposed upstream of the sheet sensor 16. (Item 13) The method further includes a designation means for designating a nominal length of the sheet, The control means If the actual length of the sheet is shorter than the nominal length, delaying the reversing timing of the reversing means; If the actual length of the sheet is longer than the nominal length, the reversing timing of the reversing means is advanced. Item 13. The image forming apparatus according to item 12.

[0114] As illustrated in FIG. 11 and the like, the reversal timing may be adjusted in accordance with the measurement result of the length of the sheet P in the transport direction. (Item 14) The image forming means includes: A photoconductor that is rotated; an exposure unit for exposing the photoreceptor to light to form an electrostatic latent image; a developing means for developing the electrostatic latent image to form a toner image; a primary transfer means for transferring the toner image from the photoreceptor to an intermediate transfer medium; a secondary transfer unit that transfers the toner image from the intermediate transfer body to the sheet transported through the first transport path; a fixing means for fixing the toner image to the sheet, the image forming apparatus further includes a guide member that guides the sheet having the image formed on the first surface thereof from the first conveying path to the second conveying path, and guides the sheet, which has been guided to the second conveying path, to the third conveying path; the first detection unit is disposed upstream of the image forming unit in a conveying direction of the sheet, the second detection means is disposed in the first transport path between the fixing means and the guide member; Item 13. The image forming apparatus according to item 12.

[0115] 1, the sheet sensor 13 may be disposed upstream of the secondary transfer roller 8. The sheet sensor 16 may be disposed between the fixing unit 9 and the flapper 17. (Item 15) Item 15. The image forming apparatus according to item 14, wherein the control means maintains a constant interval between the timing when the exposure means forms an electrostatic latent image for the first surface of the sheet and the timing when the exposure means forms an electrostatic latent image for the second surface of the sheet.

[0116] In the second embodiment, the interval between the exposure start timing for the first side and the exposure start timing for the second side is also controlled to be constant. This allows the print speed of the image forming apparatus 100 to be maintained at the target print speed. In other words, by adjusting the reversal timing according to the actual length of the sheet P, misprints are reduced even if the interval between the exposure start timing for the first side and the exposure start timing for the second side is maintained constant. (Item 16) The image forming apparatus further includes a conveying means (e.g., registration rollers 12) that is disposed upstream of the first detection means in the conveying direction of the sheet, temporarily increasing or decreasing the conveying speed of the sheet so that the timing at which the sheet conveyed along the first conveying path arrives at the image forming means coincides with a specified timing, and returns the conveying speed of the sheet to a predetermined speed, Item 16. The image forming apparatus according to item 14 or 15, wherein the control means calculates a differential time between a time when the leading edge of the sheet arrives at the secondary transfer means and a time when the trailing edge of the sheet arrives at the first detection means, calculates a transport time by dividing a distance from the first detection means to the secondary transfer means on the first transport path by the predetermined speed, and calculates the actual length of the sheet by multiplying the sum of the transport time and the differential time by the predetermined speed.

[0117] For example, the actual length Pb may be calculated according to equation (8). (Item 17) 17. The image forming apparatus according to claim 16, wherein the control unit uses the time when the toner image arrives at the secondary transfer unit as the time when the leading edge of the sheet arrives at the secondary transfer unit.

[0118] As explained in relation to formula (8), there are various ways to find the time when the leading edge of the sheet P arrives at the secondary transfer roller 8. For example, by acceleration / deceleration control, the timing when the sheet P arrives at the secondary transfer roller 8 coincides with the timing when the toner image arrives at the secondary transfer roller 8. The toner image is transported at a specified speed V by the intermediate transfer belt 7. Therefore, the timing when the toner image arrives at the secondary transfer roller 8 can be calculated from the exposure start time, the transport distance of the toner image, and the specified speed V. Therefore, the timing when the toner image arrives at the secondary transfer roller 8 may be used as the timing when the sheet P arrives at the secondary transfer roller 8.

[0119] The invention is not limited to the above-described embodiments, and various modifications and variations are possible without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0120] 10...sheet cassette, 18...discharge roller, R1...first conveying path, R4...fourth conveying path, 3...exposure device, 8...secondary transfer roller, R2...second conveying path, R3...third conveying path, 17...flapper, 19...reversing roller

Claims

1. A storage means for storing the sheet; A discharge means for discharging the sheet; a first conveying path connecting the storage means to the discharge means; an image forming unit that forms an image on the sheet transported through the first transport path; a second conveying path branching off from the first conveying path at a branching point provided downstream of the image forming means in a conveying direction of the sheet; a third conveying path that connects the second conveying path and the first conveying path and conveys the sheet having an image formed on a first surface thereof from the second conveying path to the first conveying path; a reversing means for transporting the sheet in a first direction to draw the sheet from the first transport path to the second transport path, and for reversing a transport direction of the sheet from the first direction to a second direction to send the sheet from the second transport path to the third transport path, in order to change a surface on which an image is formed by the image forming means from the first surface to a second surface of the sheet; a control means for controlling a timing of reversing the sheet in accordance with an actual length of the sheet; An image forming apparatus comprising:

2. a detection unit for detecting the sheet conveyed through the first conveying path; 2. The image forming apparatus according to claim 1, wherein the control means controls a reversing timing of the reversing means in response to a timing at which the trailing end or leading end of the sheet is detected by the detection means, the timing varying in response to the actual length of the sheet.

3. The method further includes a designation means for designating a nominal length of the sheet, The control means delaying a reversing timing of the reversing means if a detection timing by the detection means corresponding to the actual length of the sheet is earlier than a detection timing by the detection means predicted based on the nominal length; if the detection timing by the detection means according to the actual length of the sheet is later than the detection timing by the detection means predicted based on the nominal length, the reversing timing of the reversing means is advanced. The image forming apparatus according to claim 2 .

4. The method further includes a designation means for designating a nominal length of the sheet, 3. The image forming apparatus according to claim 2, wherein the control means controls the reversal timing of the reversing means in accordance with a difference between the detection timing by the detection means predicted based on the nominal length and the detection timing by the detection means corresponding to the actual length of the sheet.

5. The control means When the actual length of the sheet is equal to the nominal length, a time from the timing at which the trailing end of the sheet is detected by the detection means to the timing at which the reversing means is reversed is controlled to a predetermined time; 5. The image forming apparatus according to claim 4, wherein when the actual length of the sheet is not the nominal length, the predetermined time is corrected in accordance with the difference.

6. 6. The image forming apparatus according to claim 5, wherein said control means, when the actual length of said sheet is not the nominal length, corrects said predetermined time by subtracting half of said difference from said predetermined time.

7. 2. The image forming apparatus according to claim 1, wherein the control means maintains a constant interval between the timing at which the image forming means forms an image on the first side of the sheet and the timing at which the image forming means forms an image on the second side of the sheet.

8. The image forming means includes: A photoconductor that is rotated; an exposure unit for exposing the photoreceptor to light to form an electrostatic latent image; a developing means for developing the electrostatic latent image to form a toner image; a primary transfer means for transferring the toner image from the photoreceptor to an intermediate transfer medium; a secondary transfer unit that transfers the toner image from the intermediate transfer body to the sheet transported through the first transport path; a fixing means for fixing the toner image to the sheet, the image forming apparatus further includes a guide member that guides the sheet having the image formed on the first surface thereof from the first conveying path to the second conveying path, and guides the sheet guided to the second conveying path to the third conveying path; The image forming apparatus according to claim 2 , wherein the detection unit is disposed in the first transport path between the fixing unit and the guide member.

9. A motor for driving the reversing means; a clutch that transmits the driving force of the motor to the reversing means and controls the rotation direction of the reversing means, The image forming apparatus according to claim 8 , wherein the clutch further controls the guide member.

10. When the clutch is in the first state, the reversing means rotates to pull the sheet from the first conveying path to the second conveying path, and the guide member guides the sheet from the first conveying path to the second conveying path; When the clutch is in the second state, the reversing means rotates to feed the sheet from the second conveying path to the third conveying path, and the guide member guides the sheet from the second conveying path to the third conveying path. The image forming apparatus according to claim 9 .

11. a first detection unit that detects the sheet being conveyed through the first conveying path; 2. The image forming apparatus according to claim 1, wherein the control means controls the inversion timing of the inversion means in accordance with the actual length of the sheet obtained from the timing at which the leading edge of the sheet is detected by the first detection means and the timing at which the trailing edge of the sheet is detected by the first detection means.

12. a second detection means arranged downstream of the first detection means in a sheet conveying direction in the first conveying path and configured to detect the trailing edge of the sheet; 12. The image forming apparatus according to claim 11, wherein the control unit adjusts a time from the timing at which the rear end of the sheet is detected by the second detection unit to the timing at which the reversing unit reverses the sheet, in accordance with the actual length of the sheet.

13. The method further includes a designation means for designating a nominal length of the sheet, The control means If the actual length of the sheet is shorter than the nominal length, delaying the reversing timing of the reversing means; If the actual length of the sheet is longer than the nominal length, the reversing timing of the reversing means is advanced. The image forming apparatus according to claim 12.

14. The image forming means includes: A photoconductor that is rotated; an exposure unit for exposing the photoreceptor to light to form an electrostatic latent image; a developing means for developing the electrostatic latent image to form a toner image; a primary transfer means for transferring the toner image from the photoreceptor to an intermediate transfer medium; a secondary transfer unit that transfers the toner image from the intermediate transfer body to the sheet transported through the first transport path; a fixing means for fixing the toner image to the sheet, the image forming apparatus further includes a guide member that guides the sheet having the image formed on the first surface thereof from the first conveying path to the second conveying path, and guides the sheet guided to the second conveying path to the third conveying path; the first detection unit is disposed upstream of the image forming unit in a conveying direction of the sheet, the second detection means is disposed in the first transport path between the fixing means and the guide member; The image forming apparatus according to claim 12.

15. 15. The image forming apparatus according to claim 14, wherein the control means maintains a constant interval between the timing at which the exposure means forms an electrostatic latent image for the first surface of the sheet and the timing at which the exposure means forms an electrostatic latent image for the second surface of the sheet.

16. a conveying means arranged upstream of the first detection means in a conveying direction of the sheet, the conveying means temporarily increasing or decreasing a conveying speed of the sheet so that the timing at which the sheet conveyed on the first conveying path arrives at the image forming means coincides with a specified timing, and the conveying speed of the sheet is returned to a predetermined speed; The image forming apparatus according to claim 14, wherein the control means calculates a differential time between the time when the leading edge of the sheet arrives at the secondary transfer means and the time when the trailing edge of the sheet arrives at the first detection means, calculates a transport time by dividing the distance from the first detection means to the secondary transfer means in the first transport path by the specified speed, and calculates the actual length of the sheet by multiplying the sum of the transport time and the differential time by the specified speed.

17. 17. The image forming apparatus according to claim 16, wherein the control unit uses the time when the toner image arrives at the secondary transfer unit as the time when the leading edge of the sheet arrives at the secondary transfer unit.

Images