Image forming apparatus

Abstract

To provide an image forming apparatus that can detect the amount of a plurality of types of bending and has improved accuracy of detecting the amount of bending.SOLUTION: An image forming apparatus comprises: a transfer section that transfers a toner image to a sheet; a fixing section that fixes the toner image transferred by the transfer section to the sheet; a detection unit that detects the amount of bending of the sheet sandwiched by the transfer section and the fixing section; and a control section. The detection unit has a rotating section that is pressed by the sheet sandwiched by the transfer section and the fixing section to rotate in a rotation direction from a standby position, a rotary encoder that outputs a pulse signal according to the amount of rotation of the rotation section, and a rotation detection section that detects that the rotation section reaches a detection position downstream of the standby position in the rotation direction. The control section controls the conveyance speed of the sheet conveyed by the fixing section on the basis of the pulse signal output by the rotary encoder after the rotation detection section detects that the rotation section reaches the detection position.SELECTED DRAWING: Figure 7

Description

Technical Field

[0001] The present invention relates to an image forming apparatus that forms an image on a sheet.

Background Art

[0002] Conventionally, a loop detection sensor has been proposed for reversing whether or not a loop formed on a sheet by a transfer unit and a fixing roller has reached a certain amount (see Patent Document 1). The loop detection sensor includes a mechanical flag that rotates by contacting the sheet, and a photointerrupter that can transition between a light-shielding state and a light-transmitting state as the mechanical flag rotates.

[0003] By the way, sheets used in image forming apparatuses include various types such as thin paper, plain paper, and thick paper. And it is known that an appropriate loop amount varies depending on the type of sheet. However, the loop detection sensor in Patent Document 1 can only detect one type of loop amount, and it has been difficult to control the loop amount (amount of deflection) according to various types of sheets.

[0004] Also, an image forming apparatus has been proposed in which two light transmission type loop detection sensors are provided between a secondary transfer unit and a fixing device (see Patent Document 2). The image forming apparatus includes an actuator that rotates by contacting the sheet, and the actuator has two protruding pieces that can each shield the optical axes of the two loop detection sensors. These two loop detection sensors output an off signal when the optical axis is shielded by the protruding piece, and output an on signal when the optical axis is in an open state. This image forming apparatus can detect four types of loop amounts based on the combination of the signals of these two loop detection sensors.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0006] In recent years, there has been a demand for an image forming apparatus that can detect a plurality of types of deflection amounts (loop amounts) and improve the detection accuracy of the deflection amounts.

[0007] Therefore, an object of the present invention is to provide an image forming apparatus that can detect a plurality of types of deflection amounts and improve the detection accuracy of the deflection amounts.

MEANS FOR SOLVING THE PROBLEMS

[0008] The present invention provides an image forming apparatus including a transfer unit that conveys a sheet while sandwiching it and transfers a toner image onto the sheet, a fixing unit that conveys the sheet while sandwiching it and fixes the toner image transferred by the transfer unit onto the sheet, a detection unit that is disposed between the transfer unit and the fixing unit in the sheet conveyance direction and detects the deflection amount of the sheet sandwiched by the transfer unit and the fixing unit, and a control unit. The detection unit includes a rotating unit that rotates in a rotation direction from a standby position by being pressed against the sheet sandwiched by the transfer unit and the fixing unit, a rotary encoder that outputs a pulse signal corresponding to the rotation amount of the rotating unit, and a rotation detection unit that detects that the rotating unit has reached a detection position downstream of the standby position in the rotation direction. The control unit controls the conveyance speed of the sheet by the fixing unit based on the pulse signal output by the rotary encoder after the rotation detection unit detects that the rotating unit has detected the detection position.

EFFECTS OF THE INVENTION

[0009] According to the present invention, it is possible to detect a plurality of types of deflection amounts and improve the detection accuracy of the deflection amounts.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0011] [Overall Configuration] FIG. 1 is an overall schematic diagram showing the cross-sectional configuration of an image forming apparatus 201 according to the present invention. The image forming apparatus 201 is a full-color laser beam printer using an electrophotographic method. As shown in FIG. 1, the image forming apparatus 201 includes a printer main body 201A which is the apparatus main body, and a reading device 202 provided above the printer main body 201A for reading image data of a document.

[0012] Note that the image forming apparatus includes a printer, a copier, a facsimile machine, and a multifunction machine, and refers to an apparatus that forms an image on a sheet used as a recording medium based on image information input from an external PC or image information read from a document. In addition, in addition to the main body having an image forming function, the image forming apparatus may be connected with accessory devices such as an optional feeder, an image reading device, and a sheet processing device. However, the entire system to which such accessory devices are connected is also a kind of image forming apparatus.

[0013] The printer main body 201A includes an image forming unit 201B that forms an image on the sheet S, a fixing unit 220 that fixes the image on the sheet S, and the like. A discharge space U for discharging the sheet S is formed between the reading device 202 and the printer main body 201A, and a discharge tray 230 on which the discharged sheet S is stacked is provided in this discharge space U. Further, the printer main body 201A is provided with a sheet feeding unit 201E that feeds the sheet S to the image forming unit 201B. The sheet feeding unit 201E includes cassette feeding devices 100A, 100B, 100C, 100D arranged at the lower part of the printer main body 201A, and a manual feeding device 100M arranged at the right side part of the printer main body 201A.

[0014] The four-stage cassette feeding devices 100A, 100B, 100C, 100D and the manual feeding device 100M have the same configuration. For this reason, only the cassette feeding device 100A will be described, and the description of the other cassette feeding devices 100B, 100C, 100D and the manual feeding device 100M will be omitted.

[0015] The cassette feeding device 100A includes a cassette 3 on which the sheet S is stacked and which can be pulled out and attached to the printer main body 201A, a pickup roller 4 that feeds the sheet stacked on the cassette 3, a feed roller 5, and a retard roller 6. The feed roller 5 conveys the sheet fed by the pickup roller 4. The retard roller 6 forms a separation nip N1 together with the feed roller 5 and separates the sheets one by one.

[0016] The image forming unit 201B is an image forming unit of a so-called four-drum full-color system, which includes a laser scanner 210, four process cartridges 211, and an intermediate transfer unit 201C. These process cartridges form toner images of respective colors of yellow (Y), magenta (M), cyan (C), and black (K). Each process cartridge 211 includes a photosensitive drum 212, a charger 213, a developer 214, and a cleaner (not shown). Above the image forming unit 201B, a toner cartridge 215 containing toner of each color is detachably attached to the printer main body 201A.

[0017] The intermediate transfer unit 201C has an intermediate transfer belt 216 wound around a driving roller 216a and tension rollers 216b, 216c, etc. The intermediate transfer belt 216 is disposed above the four process cartridges 211. The intermediate transfer belt 216 is disposed so as to contact the photosensitive drum 212 of each process cartridge 211, and is rotationally driven in the counterclockwise direction (the direction of arrow Q) by a driving roller 216a driven by a driving unit (not shown). The intermediate transfer unit 201C includes a primary transfer roller 219 that abuts on the inner peripheral surface of the intermediate transfer belt 216 at a position facing each photosensitive drum 212, and a primary transfer portion T1 is formed as a nip portion between the intermediate transfer belt 216 and the photosensitive drum 212. Further, the image forming unit 201B includes a secondary transfer roller 217 that abuts on the outer peripheral surface of the intermediate transfer belt 216 at a position facing the driving roller 216a. As a nip portion between the secondary transfer roller 217 and the intermediate transfer belt 216, a secondary transfer portion T2 is formed where the toner image carried on the intermediate transfer belt 216 is transferred to the sheet S.

[0018] The fixing unit 220 includes a pressure roller 220a and a heating roller 220b with a built-in heater, and the pressure roller 220a and the heating roller 220b form a fixing nip N2 as the fixing unit. The pressure roller 220a is biased to be in pressure contact with the heating roller 220b. Instead of the heating roller 220b, an endless film or belt heated by a heater such as a ceramic heater may be applied. Also, instead of the heating roller 220b, a belt having a heat generating layer that generates heat by electromagnetic induction heating may be applied.

[0019] Above the fixing unit 220, a first discharge roller pair 225a, a second discharge roller pair 225b, and a reverse conveyance unit 201D are provided. The first discharge roller pair 225a and the second discharge roller pair 225b discharge the sheet S into the discharge space U and stack it on the discharge tray 230. The reverse conveyance unit 201D has a reverse roller pair 222 that can rotate forward and backward, and the reverse roller pair 222 conveys the sheet S to the re-conveyance path R by performing a switchback.

[0020] In each process cartridge 211 configured as described above, after an electrostatic latent image is drawn on the surface of the photosensitive drum 212 by the laser scanner 210, toner images of each color charged negatively are formed by supplying toner from the developing device 214. These toner images are sequentially multi-transferred (primary transferred) to the intermediate transfer belt 216 at each primary transfer unit T1 by applying a positive transfer bias voltage to the primary transfer roller 219, and a full-color toner image is formed on the intermediate transfer belt 216.

[0021] In parallel with such a toner image forming process, the sheet S fed from the sheet feeding unit 201E is conveyed toward the registration roller pair 207 and is skew-corrected by the registration roller pair 207. The registration roller pair 207 conveys the sheet S to the secondary transfer unit T2 at a timing synchronized with the transfer timing of the full-color toner image formed on the intermediate transfer belt 216. The toner image carried on the intermediate transfer belt 216 is secondarily transferred onto the sheet S in the secondary transfer unit T2 by applying a positive transfer bias voltage to the secondary transfer roller 217.

[0022] The sheet S onto which the toner image has been transferred is heated and pressurized at the fixing nip N2 of the fixing unit 220, and the color image is fixed on the sheet S. The sheet S on which the image has been fixed is discharged to the discharge tray 230 by the first discharge roller pair 225a or the second discharge roller pair 225b and stacked. When forming images on both sides of the sheet S, after passing through the fixing unit 220, the sheet S is switched back by the reversing roller pair 222. Then, the sheet S is conveyed again to the image forming unit 201B via the re-conveying passage R, and an image is formed on the back surface.

[0023] Note that the above-described image forming unit 201B is configured to transfer a full-color toner image onto the sheet S via the intermediate transfer belt 216, but is not limited thereto. For example, the image forming unit 201B may use a direct transfer type electrophotographic unit that transfers the toner image formed on the photoreceptor onto the sheet without passing through the intermediate transfer body.

[0024] Also, an operation unit 730 for receiving operations from the user is provided above the image forming apparatus 201. The operation unit 730 is composed of, for example, a touch panel, physical keys, and the like.

[0025] [Deflection sensor] Next, with reference to FIG. 2, the detection unit 180 disposed between the secondary transfer unit T2 and the fixing nip N2 and its peripheral configuration will be described. FIG. 2 is a cross-sectional view showing the detection unit 180.

[0026] As shown in FIG. 2, the sheet S is conveyed in the sheet conveyance direction D1 by the secondary transfer unit T2. Between the secondary transfer unit T2 and the fixing nip N2 in the sheet conveyance direction D1, conveyance guides 300 and 301 and a detection unit 180 are arranged. The conveyance guides 300 and 301 are arranged only on the non-image surface side of the sheet S so as not to disturb the unfixed image on the sheet S. That is, on the image surface side where the sheet S contacts the intermediate transfer belt 145, no conveyance guide is provided, and the image surface of the sheet S does not rub against any member of the printer main body 201A between the secondary transfer unit T2 and the fixing nip N2.

[0027] The conveyance guide 301 smoothly guides the sheet S conveyed by the secondary transfer unit T2 as a transfer unit to the conveyance guide 300. The conveyance guide 300 guides the sheet S toward the fixing nip N2. These conveyance guides 300 and 301 may be integrally formed with each other or separately formed.

[0028] The conveyance guides 300 and 301 are not configured to guide the sheet S linearly between the secondary transfer unit T2 and the fixing nip N2, but are configured such that the sheet S can bend toward the conveyance guides 300 and 301. Thus, the conveyance guides 300 and 301 form a bending formation space SP in which the sheet S can bend. Here, in the present embodiment, when viewed in the rotation axis direction of the secondary transfer roller 217, the state in which the sheet S is not linear but curved is referred to as "bending" or "forming a loop". That is, the "loop" in the present embodiment means the bending of the sheet S.

[0029] The detection unit 180 is disposed at the center of the conveyance path between the secondary transfer unit T2 and the fixing nip N2 in the width direction orthogonal to the sheet conveyance direction D1. The width direction is a direction parallel to the rotation axis direction of the secondary transfer roller 217 and the pressure roller 220a. The detection unit 180 can detect the amount of deflection of the sheet S in the deflection formation space SP. The amount of deflection indicates the amount (distance) by which the sheet S is deflected toward the deflection formation space SP from the straight state when viewed in the width direction. For example, the amount of deflection can be expressed as the distance from the straight line connecting the secondary transfer unit T2 and the fixing nip N2 to the apex of the deflection (loop) formed in the sheet S.

[0030] The detection unit 180 is disposed at a substantially intermediate position between the secondary transfer unit T2 and the fixing nip N2 in the sheet conveyance direction D1, and can accurately detect the amount of deflection of the sheet S. Further, since the detection unit 180 is disposed at the center in the width direction of the deflection formation space SP, it can detect the amount of deflection of sheets from the minimum size to the maximum size that can be used in the image forming apparatus 201.

[0031] The detection unit 180 includes a first flag 80 as a first rotating member, a deflection amount detection sensor 81, and a detection position sensor 82 as a rotation detection unit. The deflection amount detection sensor 81 includes a second flag 81a as a second rotating member and a detection unit 81b. The first flag 80 is rotatably supported about a first rotation axis 801, and has a contact portion 801a and a light shielding portion 801b. Further, the first flag 80 is configured such that the tip portion protrudes from the conveyance guide 301 into the deflection formation space SP in the standby position shown in FIG. 2. For example, an opening through which the first flag 80 passes is formed in the conveyance guide 301. Further, the first flag 80 is biased by a spring (not shown) so as to be positioned at the standby position in the natural state. The contact portion 801a is configured to be able to contact or rub against the second flag 81a.

[0032] The second flag 81a is rotatably supported about a second rotation axis 811 different from the first rotation axis 801 and has a detected portion 812. When the detection unit 180 is in the standby state, the contact portion 801a and the second flag 81a are in contact at the contact point C. At this time, the first flag 80 and the second flag 81a are each located at the standby position. Note that at this time, the sheet S is not in contact with the first flag 80.

[0033] The first flag 80 is pressed by the sheet S sandwiched between the secondary transfer unit T2 and the fixing nip N2, and rotates in the direction of arrow J1 about the first rotation axis 801. The second flag 81a rotates in the direction of arrow K1 from the standby position in conjunction with the rotation of the first flag 80 being pressed by the sheet S. These first flag 80 and second flag 81a constitute a rotating portion 240 that rotates by being pressed by the sheet S. And the detection unit 81b can detect the amount of rotation of the second flag 81a in the direction of arrow K1 from the standby position. For example, the detection unit 81b is composed of a photo interrupter having a light emitting element and a light receiving element, and the detection unit 81b and the second flag 81a constitute an optical rotary encoder 260. The light emitting element of the photo interrupter is, for example, a light emitting diode, and the light receiving element is, for example, a photo transistor.

[0034] When the rotary encoder 260 is a transmissive encoder, the detected portion 812 has a plurality of slits capable of transmitting the light emitted from the light emitting element. When the rotary encoder 260 is a reflective encoder, the detected portion 812 has a plurality of unevenness or slits capable of reflecting or not reflecting the light emitted from the light emitting element. The light receiving element of the detection unit 81b outputs a current corresponding to the amount of received light, and the waveform shaping circuit in the rotary encoder 260 converts the waveform of the current into a pulse signal and outputs it as a voltage signal. In other words, the rotary encoder 260 outputs a pulse signal corresponding to the amount of rotation of the rotating portion 240.

[0035] Also, when the amount of deflection of the sheet S sandwiched by the secondary transfer unit T2 and the fixing nip N2 decreases, the first flag 80 and the second flag 81a perform operations opposite to the above-described operations. That is, when the amount of deflection of the sheet S decreases, the first flag 80 rotates in the direction of arrow J2 opposite to the direction of arrow J1, and the second flag 81a rotates in the direction of arrow K2 opposite to the direction of arrow K1. The rotary encoder 260 can detect the amounts of rotation of the second flag 81a in the directions of arrows K1 and K2, respectively.

[0036] The detection position sensor 82 is composed of, for example, a photo interrupter having a light emitting element and a light receiving element, and has an optical path 82a between the light emitting element and the light receiving element. When the optical path 82a is blocked by the light shielding portion 801b of the first flag 80, the output value such as the signal of the detection position sensor 82 changes. Here, the positions of the first flag 80 and the second flag 81a when the optical path 82a is blocked by the light shielding portion 801b are set as the detection positions, respectively. That is, the first flag 80 and the second flag 81a can be moved to the standby position and the detection position, respectively. The detection position is a position downstream of the standby position in the direction of arrow J1 as the rotation direction. That is, the detection position sensor 82 can detect that the first flag 80 and the second flag 81a have reached the detection position. In other words, the detection position sensor 82 can detect that the rotating unit 240 constituted by the first flag 80 and the second flag 81a has reached the detection position.

[0037] FIG. 3 is a control block diagram of the control unit 401 of the image forming apparatus 201. The control unit 401 includes a CPU (Central Processing Unit) 402a, a ROM (Read Only Memory) 402b, and a RAM (Random Access Memory) 402c. The CPU 402a reads out and executes various programs stored in the ROM 402b. The RAM 402c is used as a working area for the CPU 402a.

[0038] On the input side of the control unit 401, a deflection amount detection sensor 81 and a detection position sensor 82 are connected. On the output side of the control unit 401, a fixing motor M1 that drives the pressure roller 220a is connected. Also, an operation unit 730 and a host device 900 are connected to the control unit 401. Information regarding the stiffness of the sheet S used in the image forming apparatus 201, that is, sheet information 403 such as size and basis weight, is input to the control unit 401 through the operation of the operation unit 730 by the user. Further, the control unit 401 outputs an image formation signal to the laser scanner 210 based on the image data output from the host device 900. The host device 900 is a personal computer, an image scanner, a facsimile machine, or the like.

[0039] Note that the amount of deflection of the sheet S increases when the conveyance speed of the sheet S by the fixing nip N2 (hereinafter referred to as the fixing conveyance speed) is smaller than the conveyance speed of the sheet S by the secondary transfer unit T2 (hereinafter referred to as the transfer conveyance speed). Also, the amount of deflection of the sheet S decreases when the fixing conveyance speed is greater than the transfer conveyance speed. Then, the control unit 401 can adjust the amount of deflection of the sheet S by controlling the fixing motor M1 based on the detection result of the detection unit 180. The fixing conveyance speed is the same as the peripheral speeds of the pressure roller 220a and the heating roller 220b.

[0040] [Variation in the standby position of the first flag] FIG. 4 is a diagram for explaining the variation in the standby position of the first flag 80. As described above, the first flag 80 is biased to the standby position by a spring (not shown). However, increasing the biasing force of the spring may affect the formation of the deflection (loop) of the sheet S, so it is desirable to reduce the biasing force of the spring. When the biasing force of the spring is reduced, it is conceivable that the standby position of the first flag 80 varies as shown by positions P1, P2, and P3 in FIG. 4. For example, even if the target standby position is position P1, it is also conceivable that the first flag 80 waits at a downstream position P3 in the direction of arrow J1 from position P1 or at a downstream position P2 in the direction of arrow J2 from position P1.

[0041] In such a case, if the amount of deflection of the sheet S is detected only using the pulse signal from the detection unit 81b of the deflection amount detection sensor 81, when the standby position of the first flag 80 varies, the detection result of the amount of deflection will vary. Then, a deviation will occur between the actual amount of deflection of the sheet S and the detection result of the amount of deflection, and there may be a case where appropriate deflection control cannot be performed.

[0042] For example, if appropriate deflection control cannot be performed and the intermediate transfer belt 216 receives an external force from the sheet S in the secondary transfer unit T2, the transfer position of the toner image in the primary transfer will shift, resulting in a color shift problem. Also, if the posture of the sheet S becomes unstable upstream of the fixing nip N2 in the sheet conveyance direction D1, wrinkles may occur on the sheet S when passing through the fixing nip N2. Further, if the sheet S is in a tension state between the secondary transfer unit T2 and the fixing nip N2, when the rear end of the sheet S passes through the upstream roller pair in the sheet conveyance direction D1 of the secondary transfer unit T2, the sheet S may sway and image distortion may occur in the secondary transfer unit T2. Thus, if appropriate deflection control cannot be performed, the likelihood of color shift, paper wrinkles, image distortion, etc. occurring will increase.

[0043] FIG. 5 is a timing chart showing the detection timings of the detection position sensor 82 and the deflection amount detection sensor 81. As shown in FIG. 5, when deflection amount detection starts at time t1, the deflection of the sheet S sandwiched by the secondary transfer unit T2 and the fixing nip N2 increases, and at time t2, the surface of the sheet S reaches the first flag 80. However, as described above, when the standby position of the first flag 80 varies, time t2 will also vary. And the variation in the standby position appears as the variation (H) of the detection unit 81b of the deflection amount detection sensor 81.

[0044] [Deflection amount control] Regarding the deflection amount control (loop amount control) of the present embodiment for addressing such problems, it will be described with reference to FIGS. 5 to 7. FIG. 6 is a timing chart showing the speed control of the fixing motor M1 in the deflection amount control. FIG. 7 is a flowchart showing the deflection amount control.

[0045] Next, the control of the bending amount of the sheet S by the control unit 401 will be described. The control unit 401 can execute bending amount control for controlling the conveyance speed of the sheet by the fixing nip N2 based on the detection result of the detection unit 180.

[0046] Note that the user inputs sheet information 403 (see FIG. 3) such as the size and basis weight of the sheet S to be used from the operation unit 730 before starting the print job. Then, after the sheet information is input, the print job is started. Then, as shown in FIG. 7, the control unit 401 starts driving the fixing motor M1 so that the fixing conveyance speed becomes the speed V0 (step S11).

[0047] In the present embodiment, the fixing conveyance speed (V0) at the start of the print job is set to be slightly slower than the transfer conveyance speed. For this reason, the sheet S conveyed by the secondary transfer unit T2 is conveyed so as to gradually bend after reaching the fixing nip N2. Thereby, the sheet S is not pulled by the fixing nip N2, and scattering of the unfixed toner image transferred onto the sheet S can be suppressed.

[0048] Next, the control unit 401 starts detecting the bending amount of the sheet S by the detection unit 180 (step S12). In other words, the control unit 401 starts counting the pulse signal output from the bending amount detection sensor 81 of the detection unit 180. Then, the control unit 401 determines whether or not the detection position sensor 82 is turned on (step S13). In the present embodiment, when the optical path 82a of the detection position sensor 82 is blocked by the light shielding portion 801b of the first flag 80, the detection position sensor 82 is turned on, but the present invention is not limited thereto. For example, when the optical path 82a of the detection position sensor 82 is blocked by the light shielding portion 801b, the detection position sensor 82 is turned off, and in step S13, the control unit 401 may determine whether or not the detection position sensor 82 is turned off.

[0049] As described above, since the fixing conveyance speed (V0) is slower than the transfer conveyance speed, the amount of deflection of the sheet S increases, and the first flag 80 rotates in the direction of arrow J1 in FIG. 4. Then, when the amount of deflection of the sheet S becomes equal to or greater than a predetermined amount, the first flag 80 reaches the detection position, and the detection position sensor 82 is turned on.

[0050] When it is determined that the detection position sensor 82 has been turned on (step S13: No), the control unit 401 resets the detection result by the deflection amount detection sensor 81, that is, the count of the pulse signal output by the deflection amount detection sensor 81 (step S14). Then, the control unit 401 newly starts detecting the deflection amount. In other words, the control unit 401 newly starts counting the pulse signal output by the deflection amount detection sensor 81 of the detection unit 180.

[0051] As shown in FIG. 5, by resetting the detection result of the deflection amount, that is, the count of the pulse signal of the deflection amount detection sensor 81 when the detection position sensor 82 is first turned on after starting the deflection amount control, the influence of the variation in the standby position of the first flag 80 can be reset. In other words, since the pulse signal output by the deflection amount detection sensor 81 before the first flag 80 rotates from the standby position to the detection position has variations, this pulse signal is reset. Then, the control unit 401 detects the deflection amount of the sheet S and controls the fixing conveyance speed based on the pulse signal output by the deflection amount detection sensor 81 (rotary encoder 260) after the first flag 80 reaches the detection position.

[0052] As shown in FIG. 7, next, the control unit 401 determines whether the detected deflection amount of the sheet S is greater than the first threshold value TH1 (step S16). When it is determined that the deflection amount of the sheet S is greater than the first threshold value TH1 (step S16: Yes), the control unit 401 switches the fixing conveyance speed to a speed V1 that is faster than the speed V0 (step S17), and proceeds to step S20.

[0053] Also, when it is determined that the amount of deflection of the detected sheet S is smaller than the first threshold value TH1 (step S16: No), the control unit 401 determines whether the amount of deflection of the sheet S is smaller than the second threshold value TH2 (step S18). The second threshold value TH2 is smaller than the first threshold value TH1. When it is determined that the amount of deflection of the sheet S is smaller than the second threshold value TH2 (step S18: Yes), the control unit 401 switches the fixing conveyance speed to a speed V2 slower than the speeds V0 and V1 (step S19), and proceeds to step S20. When it is determined that the amount of deflection of the sheet S is larger than the second threshold value TH2 (step S18: No), it proceeds to step S20 without changing the fixing conveyance speed.

[0054] In step S20, the control unit 401 determines whether it is the end timing of the deflection amount control (step S20). For example, the end timing of the deflection amount control is the timing when the trailing edge of the last sheet of the job has passed through the secondary transfer unit T2. When it is determined that it is not the end timing of the deflection amount control (step S20: No), it returns to step S15.

[0055] The relationship among the first threshold value TH1, the second threshold value TH2, and the speed of the fixing motor M1 is shown in the timing chart of FIG. 6. Note that the first threshold value TH1 and the second threshold value TH2 are determined based on the sheet information 403 (see FIG. 3). For example, a table showing the relationship between the sheet information 403, the first threshold value TH1, and the second threshold value TH2 may be stored in advance in the ROM 402b. In this embodiment, as shown in FIG. 6, when conveying a sheet of the first stiffness, the fixing motor M1 is controlled using the first threshold value TH1 and the second threshold value TH2.

[0056] That is, when the detected deflection amount of the sheet S exceeds the first threshold value TH1, that is, when the deflection amount becomes excessive, the control unit 401 switches the fixing motor M1 to the speed R1 in order to reduce the deflection amount. Further, when the detected deflection amount of the sheet S is below the second threshold value TH2, that is, when the deflection amount becomes too small, the control unit 401 switches the fixing motor M1 to the speed R2 in order to increase the deflection amount. When the fixing motor M1 is switched to the speed R1, the fixing conveyance speed becomes the speed V1, and when the fixing motor M1 is switched to the speed R2, the fixing conveyance speed becomes the speed V2.

[0057] When it is determined that the end timing of the deflection amount control has arrived (step S20: Yes), the control unit 401 stops the detection of the deflection amount by the detection unit 180 (step S21) and ends the deflection amount control.

[0058] As described above, in the present embodiment, by resetting the count of the pulse signal of the deflection amount detection sensor 81 when the detection position sensor 82 is turned on, the influence of the variation in the standby position of the first flag 80 can be reset. Thereby, while detecting a plurality of types of deflection amounts, the detection accuracy of the deflection amount can be improved. Then, the control unit 401 controls the fixing conveyance speed based on the pulse signal output by the deflection amount detection sensor 81 (rotary encoder 260) after the first flag 80 reaches the detection position, so that the deflection amount of the sheet S can be maintained within an appropriate range. As a result, the occurrence of color misregistration, paper wrinkles, image distortion, etc. can be suppressed.

[0059] In the above-described FIGS. 6 and 7, an example of controlling the fixing motor M1 using the first threshold value TH1 and the second threshold value TH2 when conveying the sheet S with the first stiffness was described. More specifically, when the detection unit 180 detects the amount of deflection of the sheet S exceeding the first threshold value TH1, the control unit 401 sets the fixing conveyance speed to the speed V1 as the first conveyance speed. Further, when the detection unit 180 detects the amount of deflection of the sheet S less than the second threshold value TH2, the control unit 401 sets the fixing conveyance speed to the speed V2 as the second conveyance speed. The transfer conveyance speed is slower than the speed V1 and faster than the speed V2. That is, the control unit 401 controls the fixing conveyance speed so that the amount of deflection of the sheet S becomes the first amount of deflection.

[0060] On the other hand, for example, when conveying the sheet S with the second stiffness smaller than the first stiffness, as shown in FIG. 6, the control unit 401 uses the third threshold value TH3 larger than the first threshold value TH1 and the fourth threshold value TH4 larger than the second threshold value TH2 to control the fixing motor M1. More specifically, when the detection unit 180 detects the amount of deflection of the sheet S exceeding the third threshold value TH3, the control unit 401 sets the fixing conveyance speed to the speed V1. Further, when the detection unit 180 detects the amount of deflection of the sheet S less than the fourth threshold value TH4, the control unit 401 sets the fixing conveyance speed to the speed V2. That is, the control unit 401 controls the fixing conveyance speed so that the amount of deflection of the sheet S becomes the second amount of deflection larger than the first amount of deflection.

[0061] For example, the sheet with the first stiffness is cardboard, and the sheet with the second stiffness is thin paper or plain paper. The thin paper is, for example, a sheet with a basis weight of 52 to 59 [g / m 2 , and the plain paper is, for example, a sheet with a basis weight of 64 to 105 [g / m 2 , and the cardboard is, for example, a sheet with a basis weight of 106 to 300 [g / m 2 .

[0062] Incidentally, the resolution of the rotary encoder 260 may be arbitrarily set, and the rotary encoder 260 can detect the deflection amount (loop amount) of at least three or more sheets S. Therefore, the deflection amount control can be performed so that an appropriate deflection amount is obtained according to the type of the sheet, and even when various types of sheets are conveyed, image defects and wrinkles of the sheet can be suppressed.

[0063] <Second Embodiment> Next, a second embodiment of the present invention will be described. The second embodiment is obtained by changing the configuration of the detection unit 180 of the first embodiment. For this reason, regarding the same configuration as that of the first embodiment, illustration is omitted or the same reference numerals are given in the drawings and described. FIG. 8 is a cross-sectional view showing a detection unit 280 according to the second embodiment.

[0064] As shown in FIG. 8, the detection unit 280 according to the second embodiment includes a first flag 80 as a rotating part, a detection part 81b, and a detection position sensor 82. That is, the detection unit 280 according to the second embodiment omits the second flag 81a of the detection unit 180 according to the first embodiment, and the rotation amount of the first flag 80 is detected by the detection part 81b.

[0065] The detection position sensor 82 detects that the first flag 80 has reached the detection position, in the same manner as in the first embodiment. The first flag 80 has a detected part 812, and the rotary encoder 260 is configured by the detected part 812 and the detection part 81b. Then, the rotary encoder 260 outputs a pulse signal according to the rotation amount of the first flag 80. Since the deflection control using the detection unit 280 is the same as that of the first embodiment, the description thereof is omitted.

[0066] As described above, even if the rotating part that rotates by being pressed by the sheet S is constituted only by the first flag 80, the same effect as that of the first embodiment can be obtained.

[0067] <Other Embodiments> In any of the above-described embodiments, the control unit 401 controls the amount of deflection of the sheet S by switching the fixing conveyance speed to the speed V1 or the speed V2. However, the present invention is not limited to this. For example, when the amount of deflection corresponding to the stiffness of the sheet S is detected, the control unit 401 may set the fixing conveyance speed to be the same as the transfer conveyance speed.

[0068] In the first embodiment, the detection position sensor 82 detects that the first flag 80 has reached the detection position. However, the present invention is not limited to this. For example, the detection position sensor 82 may detect that the second flag 81a has reached the detection position.

[0069] The present invention can also be realized by supplying a program that realizes one or more functions of the above-described embodiments to a system or apparatus via a network or a storage medium, and having one or more processors in the computer of the system or apparatus read and execute the program. Further, it can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

Explanation of Reference Numerals

[0070] 80: Rotating unit, first rotating member (first flag) / 81a: Second rotating member (second flag) / 82: Rotation detection unit (detection position sensor) / 180, 280: Detection unit / 201: Image forming apparatus / 240: Rotating unit / 260: Rotary encoder / 401: Control unit / N2: Fixing unit (fixing nip) / T2: Transfer unit (secondary transfer unit) / TH1: First threshold / TH2: Second threshold / TH3: Third threshold / TH4: Fourth threshold / V1: First conveyance speed (speed) / V2: Second conveyance speed (speed)

Claims

1. A transfer unit that conveys while sandwiching a sheet and transfers a toner image onto the sheet, A fixing unit that conveys while sandwiching a sheet and fixes the toner image transferred by the transfer unit onto the sheet, A detection unit that is disposed between the transfer unit and the fixing unit in the sheet conveyance direction and detects the amount of deflection of the sheet sandwiched by the transfer unit and the fixing unit, A control unit, and is provided with, The detection unit includes a rotating unit that rotates in a rotation direction from a standby position by being pressed against the sheet sandwiched by the transfer unit and the fixing unit, a rotary encoder that outputs a pulse signal corresponding to the amount of rotation of the rotating unit, and a rotation detection unit that detects that the rotating unit has reached a detection position downstream of the standby position in the rotation direction, The control unit controls the conveyance speed of the sheet by the fixing unit based on the pulse signal output by the rotary encoder after the rotation detection unit detects that the rotating unit has detected the detection position, An image forming apparatus characterized by the above.

2. The rotating unit includes a first rotating member that rotates by being pressed against the sheet sandwiched by the transfer unit and the fixing unit, and a second rotating member that rotates by being pressed against the first rotating member, The rotary encoder outputs a pulse signal corresponding to the amount of rotation of the second rotating member, The image forming apparatus according to claim 1, characterized by the above.

3. The first rotating member and the second rotating member are each movable to the standby position and the detection position, The rotation detection unit detects that the first rotating member has reached the detection position, The image forming apparatus according to claim 2, characterized by the above.

4. When the control unit conveys a sheet having a first stiffness, it controls the conveyance speed of the sheet by the fixing unit so that the amount of deflection of the sheet becomes a first amount of deflection, and when conveying a sheet having a second stiffness smaller than the first stiffness, it controls the conveyance speed of the sheet by the fixing unit so that the amount of deflection of the sheet becomes a second amount of deflection larger than the first amount of deflection, The image forming apparatus according to any one of claims 1 to 3, characterized by the above.

5. The control unit, When transporting the sheet with the first stiffness, if the detection unit detects a deflection amount of the sheet exceeding the first threshold value, the conveyance speed of the sheet by the fixing unit is set to the first conveyance speed. If the detection unit detects a deflection amount of the sheet less than the second threshold value which is smaller than the first threshold value, the conveyance speed of the sheet by the fixing unit is set to a second conveyance speed slower than the first conveyance speed. When transporting the sheet with the second stiffness, if the detection unit detects a deflection amount of the sheet exceeding a third threshold value which is larger than the first threshold value, the conveyance speed of the sheet by the fixing unit is set to the first conveyance speed. If the detection unit detects a deflection amount of the sheet less than a fourth threshold value which is larger than the third threshold value, the conveyance speed of the sheet by the fixing unit is set to the second conveyance speed. The conveyance speed of the sheet by the transfer unit is slower than the first conveyance speed and faster than the second conveyance speed. The image forming apparatus according to claim 4, characterized in that.

Images