Image forming apparatus

Abstract

To appropriately maintain the speed of conveyance of a sheet performed by a fixing device.SOLUTION: A speed adjustment device 86 changes the rotation speed of a pressure member 50 of a fixing device 46. A sheet detection unit 32 detects a sheet at positions on a conveyance path from a mechanism that feeds the sheet to at least a fixing position up to the fixing position or positions from the fixing position up to a mechanism that takes over the conveyance of the sheet from the fixing device 46. A timer unit 8e checks the time required for a result of detection performed by the sheet detection unit 32 to indicate a predetermined change. A speed control unit 8c controls the speed adjustment device 86 according to the time checked by the timer unit 8e.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an image forming apparatus capable of changing the conveyance speed of a sheet by a fixing device.

Background Art

[0002] An electrophotographic image forming apparatus includes an image forming device, a transfer device, and a fixing device. The image forming device forms a developer image on an image carrier. The transfer device transfers the developer image from the image carrier to a sheet.

[0003] The fixing device fixes the developer image on the sheet by heating and pressurizing the developer image on the sheet.

[0004] The fixing device includes a heater, a fixing member, a pressurizing member, and a biasing mechanism. The fixing member is heated by the heater. The biasing mechanism elastically biases one of the fixing member and the pressurizing member toward the other.

[0005] The pressurizing member has an elastic outer layer that presses against the fixing member. A portion where the fixing member and the pressurizing member contact each other forms a nip portion through which the sheet passes.

[0006] In the fixing device, there may be a case where the biasing mechanism biases the fixing member toward the pressurizing member, the pressurizing member is rotationally driven, and the pressurizing member rotates passively. In this case, depending on the degree of thermal expansion of the pressurizing member, the peripheral speeds of the fixing member and the pressurizing member in the nip portion change. As a result, the conveyance speed of the sheet by the fixing device changes.

[0007] When the conveyance speed of the sheet by the fixing device is significantly different from the conveyance speed of the sheet by the conveyance mechanisms before and after the fixing device, inconvenient situations such as poor transfer of the developer to the sheet or breakage of the sheet may occur.

[0008] Also, in the image forming apparatus, it is known that a deflection detection unit detects the deflection of the sheet between the transfer position and the fixing position in the conveyance path (see, for example, Patent Document 1). In this case, the conveyance speed of the sheet by the fixing device is switched according to the detection result of the deflection detection unit.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0010] By the way, when the conveyance speed of the sheet by the fixing device is too fast, the sheet does not deflect at a position before the fixing position. Also, even when the conveyance speed of the sheet by the fixing device is too slow, depending on the hardness of the sheet, the sheet may not deflect significantly at a position before the fixing position.

[0011] That is, in adjusting the rotational speed of the pressing member in the fixing device according to the detection result of the deflection of the sheet, it is difficult to appropriately maintain the conveyance speed of the sheet by the fixing device.

[0012] An object of the present invention is to provide an image forming apparatus capable of appropriately maintaining the conveyance speed of a sheet by a fixing device.

Means for Solving the Problems

[0013] An image forming apparatus according to one aspect of the present invention includes a transfer device, a fixing device, a speed adjusting device, a sheet detection unit, and a control device. The transfer device transfers a developer image onto a sheet while transporting the sheet at a transfer position on a transport path. The fixing device fixes the developer image onto the sheet by heating and pressing the developer image on the sheet while transporting the sheet at a fixing position on the transport path. The speed adjusting device can change the transport speed of the sheet by the fixing device. The sheet detection unit detects the sheet at a predetermined position on the transport path. The control device controls the fixing device. The fixing device includes a heater, a fixing member, a pressing member, a motor, and a biasing mechanism. The fixing member is heated by the heater and is supported rotatably and displaceably in a direction intersecting the sheet transport direction. The pressing member is disposed to face the fixing member in the intersecting direction and has an elastic outer layer that presses against the fixing member, and is rotationally driven to rotationally drive the fixing member. The motor rotationally drives the pressing member. The biasing mechanism elastically biases the fixing member toward the pressing member along the intersecting direction. The sheet detection unit detects the sheet at least at a position from a mechanism that feeds the sheet to the fixing position on the transport path to the fixing position, or at a position from the fixing position to a mechanism that takes over the transport of the sheet from the fixing device. The control device includes a timing unit and a speed control unit. The timing unit measures the time required for the detection result of the sheet detection unit to show a predetermined change. The speed control unit controls the rotational speed of the motor according to the measured time of the timing unit.

Advantages of the Invention

[0014] According to the present invention, it is possible to provide an image forming apparatus capable of appropriately maintaining the transport speed of a sheet by a fixing device.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Mode for Carrying Out the Invention

[0016] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the following embodiments are examples of embodying the present invention and do not limit the technical scope of the present invention.

[0017] [Configuration of Image Forming Apparatus 10] The image forming apparatus 10 according to the embodiment includes a printing device 4 that performs printing processing for forming an image on a sheet 9.

[0018] The printing device 4 performs the printing process by an electrophotographic method. The sheet 9 is an image forming medium such as paper or a sheet-like resin member.

[0019] As shown in FIG. 1, the image forming apparatus 10 includes a sheet conveyance device 3 and a printing device 4 provided in a main body 1, a control device 8, an operation device 801, and a display device 802.

[0020] The printing device 4 includes one or more image forming devices 4x, an optical scanning device 40, a transfer device 44, and a fixing device 46. The image forming device 4x includes a drum-shaped photoreceptor 41, a charging device 42, a developing device 43, a drum cleaning device 45, and the like.

[0021] The operation device 801 is a device that receives human operations and includes, for example, operation buttons and a touch panel. The display device 802 is a device that displays information and includes, for example, a panel display device such as a liquid crystal display unit.

[0022] The sheet conveying device 3 includes a sheet feeding device 30 and a plurality of pairs of conveying rollers 31. The sheet feeding device 30 feeds the sheet 9 accommodated in the sheet storage unit 2 into the conveying path 300 within the main body unit 1. The conveying path 300 forms a passage through which the sheet 9 is conveyed.

[0023] The plurality of pairs of conveying rollers 31 are rotationally driven by a motor (not shown), and convey the sheet 9 along the conveying path 300 by rotating while sandwiching the sheet 9, and further discharge the sheet 9 to the discharge tray 101.

[0024] The sheet 9 is discharged to the discharge tray 101 after passing through the registration position P1, the transfer position P2, and the fixing position P3 in the conveying path 300.

[0025] In the following description, the direction in which the sheet 9 is conveyed along the conveying path 300 is referred to as the sheet conveying direction D1. The registration position P1 is a position upstream of the transfer position P2 in the sheet conveying direction D1. The fixing position P3 is a position downstream of the transfer position P2 in the sheet conveying direction D1.

[0026] Also, the direction intersecting the sheet conveying direction D1 and along the conveying path 300 is referred to as the width direction D2. Also, the direction intersecting the sheet conveying direction D1 and the width direction D2 is referred to as the intersecting direction D3. In the present embodiment, the width direction D2 is a direction orthogonal to the sheet conveying direction D1.

[0027] The plurality of pairs of conveying rollers 31 include a registration roller pair 31a and a post-fixing roller pair 31b.

[0028] The registration roller pair 31a is disposed at the registration position P1 in the conveying path 300. The registration roller pair 31a temporarily stops the sheet 9 at the registration position P1 and then feeds the sheet 9 to the transfer position P2.

[0029] The post-fixing roller pair 31b is disposed at a position downstream in the sheet conveyance direction D1 with respect to the fixing position P3 in the conveyance path 300. The post-fixing roller pair 31b is an example of a mechanism that takes over the conveyance of the sheet 9 from the fixing device 46.

[0030] The sheet conveyance device 3 further includes a sheet detection unit 32 that detects the sheet 9 at a predetermined position in the conveyance path 300. In the present embodiment, the sheet detection unit 32 includes a first sheet detection unit 32a and a second sheet detection unit 32b.

[0031] The first sheet detection unit 32a detects the sheet 9 at a position upstream in the sheet conveyance direction D1 with respect to the registration position P1 in the conveyance path 300.

[0032] The registration roller pair 31a temporarily stops when a predetermined time has elapsed after the sheet 9 is detected by the first sheet detection unit 32a, and then rotates again. Thereby, the timing at which the sheet 9 is sent out to the transfer position P2 is adjusted.

[0033] The second sheet detection unit 32b detects the sheet 9 at a position from the fixing position P3 in the conveyance path 300 to the post-fixing roller pair 31b.

[0034] The plurality of sets of conveyance roller pairs 31 stop when a predetermined time has elapsed after the last sheet 9 to be conveyed is detected by the second sheet detection unit 32b. The second sheet detection unit 32b is used to confirm that the sheet 9 has been discharged from the conveyance path 300.

[0035] For example, the first sheet detection unit 32a and the second sheet detection unit 32b each include a displacement member and a detection sensor (not shown).

[0036] The displacement member is displaced from a reference position to a retracted position by contacting the sheet 9 conveyed along the conveyance path 300. The detection sensor detects the displacement member displaced to the retracted position.

[0037] The printing device 4 forms a toner image on the sheet 9 conveyed along the conveyance path 300 by the sheet conveyance device 3. The toner image is a developer image using toner as a developer. The toner is an example of the granular developer.

[0038] The image forming apparatus 10 shown in FIG. 1 is a tandem type color image forming apparatus. Therefore, the printing device 4 includes four image forming devices 4x corresponding to the four colors of toner, namely yellow, cyan, magenta, and black.

[0039] In the image forming device 4x, the photoreceptor 41 rotates, and the charging device 42 charges the surface of the photoreceptor 41. Further, the optical scanning device 40 writes an electrostatic latent image on the surface of the photoreceptor 41 by scanning with laser light.

[0040] Furthermore, the developing device 43 supplies the toner to the surface of the photoreceptor 41 to develop the electrostatic latent image as the toner image. The photoreceptor 41 is an example of an image carrier that rotates while carrying the toner image.

[0041] The transfer device 44 transfers the toner image to the sheet 9 at the transfer position P2 on the conveyance path 300. The transfer device 44 includes an intermediate transfer belt 441, four primary transfer devices 442 corresponding to the four image forming devices 4x, a secondary transfer device 443, and a belt cleaning device 444.

[0042] In the transfer device 44, the primary transfer device 442 transfers the toner image on the surface of the photoreceptor 41 to the surface of the intermediate transfer belt 441. Thereby, the color toner image is formed on the surface of the intermediate transfer belt 441.

[0043] The secondary transfer device 443 transfers the toner image formed on the intermediate transfer belt 441 to the sheet 9 in the conveyance path 300.

[0044] In addition, when the image forming apparatus 10 is a monochrome image forming apparatus, the secondary transfer device 443 transfers the toner image on the photoreceptor 41 to the sheet 9 in the conveyance path 300.

[0045] The drum cleaning device 45 removes the waste toner remaining on the surface of the photoreceptor 41. The belt cleaning device 444 removes the waste toner remaining on the intermediate transfer belt 441.

[0046] [Fixing device 46] The fixing device 46 heats and presses the toner image on the sheet 9 while conveying the sheet 9 at the fixing position P3 in the conveyance path 300. Thereby, the fixing device 46 fixes the toner image to the sheet 9.

[0047] As shown in FIG. 2, the fixing device 46 includes a pressure roller 50, a drive mechanism 500, a fixing member 51, a support member 52, a heater 53, a biasing mechanism 54, and a temperature sensor 55.

[0048] The fixing member 51 is a flexible cylindrical member. In other words, the fixing member 51 is an endless belt-shaped flexible cylinder. For example, the fixing member 51 is a cylindrical film member.

[0049] The pressure roller 50 is disposed to face the fixing member 51 in the crossing direction D3. The pressure roller 50 is an example of a pressure member. The pressure roller 50 has a shaft core part 50a and an elastic outer layer 50b.

[0050] The shaft core part 50a is rotatably supported. The elastic outer layer 50b is formed around the shaft core part 50a. The elastic outer layer 50b is a layer of an elastic member such as rubber. The elastic outer layer 50b forms a nip Np1 with the fixing member 51 by being pressed against the fixing member 51.

[0051] The support member 52 rotatably supports the fixing member 51 and is displaceable along the crossing direction D3. The support member 52 has a facing part 52a that contacts the inner surface of the fixing member 51 and faces the pressure roller 50 via the fixing member 51.

[0052] The heater 53 is incorporated in the opposing portion 52a. The pressure roller 50, the fixing member 51, and the support member 52 are formed to extend in the width direction D2. For example, the heater 53 is composed of a plurality of heating elements arranged along the width direction D2. Each of the heating elements generates heat when power is supplied thereto.

[0053] The heater 53 is in contact with the inner surface of the fixing member 51. Thereby, the heater 53 heats the portion of the fixing member 51 that forms the nip Np1.

[0054] The temperature sensor 55 measures the temperature of the heater 53. For example, the temperature sensor 55 is a thermistor.

[0055] The detected temperature of the temperature sensor 55 is used for fixing temperature control. The fixing temperature control is a feedback control that controls the power supplied to the heater 53 by comparing the temperature of the heater 53 with a preset target temperature.

[0056] The temperature sensor 55 measures a temperature that serves as an alternative indicator of the temperature of the portion of the fixing member 51 that forms the nip Np1. Therefore, the temperature sensor 55 may be arranged at a position where it measures the temperature of the fixing member 51.

[0057] The fixing member 51 is heated by the heater 53 and is rotatably supported by the support member 52. Further, the fixing member 51 is supported by the support member 52 so as to be displaceable in the crossing direction D3.

[0058] The drive mechanism 500 includes a motor 501 and a gear mechanism 502 that transmits the rotational force of the motor 501 to the pressure roller 50. The motor 501 is a drive source that rotationally drives the pressure roller 50.

[0059] The pressing roller 50 is rotationally driven by a motor 501 to rotationally drive the fixing member 51 passively. When the fixing member 51 rotates passively, the inner surface of the fixing member 51 slides with respect to the heater 53 and the opposing portion 52a.

[0060] The support member 52 has a plurality of ribs 52b for reducing the frictional resistance with the fixing member 51. The plurality of ribs 52b are arranged at intervals in the width direction D2. Each of the ribs 52b is formed to extend along the rotational direction of the fixing member 51. A lubricant is applied to the inner surface of the fixing member 51.

[0061] Furthermore, the support member 52 has a pair of edge guide portions 52c that guide the circumferential tracks at both ends in the width direction D2 of the fixing member 51 along a predetermined curved path.

[0062] For example, the support member 52 is a member in which one member including the opposing portion 52a and two members each including one of the pair of edge guide portions 52c are combined.

[0063] The biasing mechanism 54 elastically biases the fixing member 51 toward the pressing roller 50 along the intersecting direction D3. In the present embodiment, the biasing mechanism 54 biases the fixing member 51 toward the pressing roller 50 by elastically biasing the support member 52 toward the pressing roller 50 along the intersecting direction D3. The biasing mechanism 54 includes a pressing member 541, a spring 542, a spring holding member 543, and a biasing force adjusting mechanism 544.

[0064] The spring holding member 543 holds one end of the spring 542. The spring 542 elastically biases the support member 52 toward the pressing roller 50 via the pressing member 541. The pressing member 541 biases the support member 52 toward the pressing roller 50 by receiving the elastic force of the spring 542. Further, the pressing member 541 also serves to reinforce the support member 52.

[0065] The biasing force adjusting mechanism 544 is a mechanism capable of changing the biasing force with which the biasing mechanism 54 biases the fixing member 51 toward the pressure roller 50. For example, the biasing force adjusting mechanism 544 is a cam mechanism that displaces the spring holding member 543 along the crossing direction D3.

[0066] When the cam mechanism displaces the spring holding member 543 in the direction approaching the pressure roller 50, the biasing force increases. On the other hand, when the cam mechanism displaces the spring holding member 543 in the direction moving away from the pressure roller 50, the biasing force decreases.

[0067] [Control device 8] The control device 8 executes various data processes and controls devices such as the sheet conveyance device 3, the printing device 4, and the display device 802. The control target of the control device 8 includes the fixing device 46.

[0068] As shown in FIG. 3, the control device 8 includes a CPU (Central Processing Unit) 81 and peripheral devices such as a RAM (Random Access Memory) 82, a secondary storage device 83, and a signal interface 84.

[0069] Furthermore, the control device 8 includes a communication device 85, a motor drive circuit 86, and a heater power supply circuit 87.

[0070] The CPU 81 is a processor that executes various data processes and controls by executing a computer program. The RAM 82 is a computer-readable volatile storage device. The RAM 82 primarily stores the computer program executed by the CPU 81 and the data output and referred to by the CPU 81 during the execution of various processes.

[0071] The CPU 81 includes a plurality of processing modules realized by executing the computer program. The plurality of processing modules include a main control unit 8a, a heating control unit 8b, a drive control unit 8c, a print control unit 8d, and the like.

[0072] The main control unit 8a executes start control for starting various processes in response to operations on the operation device 801, control of the display device 802, and the like.

[0073] The heating control unit 8b controls the power supply amount to the heater 53 of the heating device 5 by the fixing temperature control. The heating control unit 8b adjusts the power supply amount to the heating device 5 by controlling the heater power supply circuit 87.

[0074] The heater power supply circuit 87 supplies power according to the power supply command from the heating control unit 8b to the heater 53. For example, when the power supply command represents the degree of increase or decrease in the power supply amount to the heater 53, the heater power supply circuit 87 adjusts the power supply amount to the heater 53 according to the power supply command.

[0075] The drive control unit 8c controls the sheet conveyance device 3, the drive mechanism 500, the biasing force adjustment mechanism 544, and the like.

[0076] The motor drive circuit 86 supplies power according to the speed command from the drive control unit 8c to the motor 501 of the drive mechanism 500. Thereby, the motor 501 rotates at a speed corresponding to the speed command. For example, the motor drive circuit 86 is an inverter drive circuit.

[0077] The print control unit 8d causes the printing device 4 to execute the printing process in synchronization with the conveyance of the sheet 9 by the sheet conveyance device 3.

[0078] The secondary storage device 83 is a computer-readable non-volatile storage device. The secondary storage device 83 can store and update the computer program and various data. For example, one or both of a flash memory and a hard disk drive are adopted as the secondary storage device 83.

[0079] The signal interface 84 converts the signals output by various sensors such as the temperature sensor 55 into digital data, and transmits the converted digital data to the CPU 81. Further, the signal interface 84 converts the control commands output by the CPU 81 into control signals, and transmits the control signals to the devices to be controlled.

[0080] The communication device 85 executes communication with other devices such as a host device that transmits a print job to the image forming apparatus 10. The CPU 81 communicates with the other devices through the communication device 85.

[0081] Incidentally, the volume of the elastic outer layer 50b of the pressure roller 50 changes due to the heat received from the heater 53.

[0082] In the fixing device 46, the peripheral speeds of the fixing member 51 and the pressure roller 50 at the nip Np1 change according to the degree of thermal expansion of the elastic outer layer 50b of the pressure roller 50. As a result, the conveyance speed of the sheet 9 by the fixing device 46 changes.

[0083] When the conveyance speed of the sheet 9 by the fixing device 46 is too fast, the sheet 9 does not bend at a position before the fixing position P3. Also, when the conveyance speed of the sheet 9 by the fixing device 46 is too slow, depending on the hardness of the sheet 9, the sheet 9 may not bend significantly at a position before the fixing position P3.

[0084] That is, it is difficult to appropriately maintain the conveyance speed of the sheet 9 by the fixing device 46 by adjusting the rotation speed of the pressure roller 50 in the fixing device 46 according to the detection result of the bending of the sheet 9.

[0085] On the other hand, the drive control unit 8c of the image forming apparatus 10 executes fixing drive control described later. Thereby, it is possible to appropriately maintain the conveyance speed of the sheet 9 by the fixing device 46. Hereinafter, the fixing drive control will be described.

[0086] The plurality of processing modules in the CPU 81 further includes a timing unit 8e (see FIG. 3). The driving control unit 8c and the timing unit 8e execute the fixing driving control. In the following description, the conveyance speed of the sheet 9 by the fixing device 46 is referred to as the fixing conveyance speed.

[0087] [Fixing Driving Control] Each time the printing process on one sheet 9 is executed, the timing unit 8e executes a timing process as part of the fixing driving control. The timing process is a process of measuring the time required for the detection result of the sheet detection unit 32 to show a predetermined change.

[0088] Specifically, in the timing process, the timing unit 8e detects a first time point and a second time point, and measures a trailing edge passing time TR1, which is the time from the first time point to the second time point. The trailing edge passing time TR1 is an example of the timing time of the timing unit 8e.

[0089] The first time point is the time when the first sheet detection unit 32a changes from a state of detecting the sheet 9 to a state of not detecting it. The second time point is the time when the second sheet detection unit 32b changes from a state of detecting the sheet 9 to a state of not detecting it.

[0090] The first time point is the time when the trailing edge of the sheet 9 passes the position of the first sheet detection unit 32a. The second time point is the time when the trailing edge of the sheet 9 passes the position of the second sheet detection unit 32b.

[0091] The slower the conveyance speed of the sheet 9 by the fixing device 46, the longer the trailing edge passing time TR1 becomes. That is, the trailing edge passing time TR1 has a negative correlation with the conveyance speed of the sheet 9 by the fixing device 46.

[0092] Also, as part of the fixing driving control, the driving control unit 8c executes a fixing conveyance control for controlling the motor driving circuit 86 according to the trailing edge passing time TR1. The driving control unit 8c and the motor driving circuit 86 are an example of a speed control unit that controls the rotation speed of the motor 500 according to the timing time of the timing unit 8e.

[0093] Here, let the rotation speed of the current motor 501 be V1, and the preset reference time be TS1. The reference time TS1 is the trailing-edge passing time TR1 when the fixing device 46 conveys the sheet 9 at an ideal speed corresponding to the conveyance speed of the sheet 9 by the sheet conveyance device 3.

[0094] For example, the drive control unit 8c sets the adjusted rotation speed V2 of the motor 501 by applying the trailing-edge passing time TR1 to the following formula (1). Note that k1 in formula (1) is a predetermined positive coefficient. For example, k1 = 1.

[0095]

Equation

[0096] Each time the trailing-edge passing time TR1 is measured, the drive control unit 8c controls the motor drive circuit 86 to rotate the motor 501 at the rotation speed V2. Thereby, when the printing process on the next sheet 9 is executed, the rotation speed of the motor 501 of the fixing device 46 is adjusted to the rotation speed V2.

[0097] For example, it is conceivable that the reference time TS1 is a constant. Also, the timing unit 8e may measure the leading-edge passing time TF1 in the timing process, and the drive control unit 8c may derive the reference time TS1 from the leading-edge passing time TF1.

[0098] The leading-edge passing time TF1 is the time from the third time point to the fourth time point. The third time point is the time point when the first sheet detection unit 32a changes from a state of not detecting the sheet 9 to a state of detecting it. The fourth time point is the time point when the second sheet detection unit 32b changes from a state of not detecting the sheet 9 to a state of detecting it.

[0099] The third time point is the time point when the leading edge of the sheet 9 passes the position of the first sheet detection unit 32a. The fourth time point is the time point when the leading edge of the sheet 9 passes the position of the second sheet detection unit 32b.

[0100] The slower the conveyance speed of the sheet 9 by the sheet conveyance device 3, the longer the leading edge passing time TF1 becomes. That is, the leading edge passing time TF1 has a negative correlation with the conveyance speed of the sheet 9 by the fixing device 46.

[0101] For example, the drive control unit 8c derives the reference time TS1 by applying the leading edge passing time TF1 to the following formula (2). k2 in formula (2) is a predetermined positive coefficient. For example, k2 = 1.

[0102]

Equation

[0103] The drive control unit 8c may set the rotation speed V2 of the motor 501 after adjustment by applying the current trailing edge passing time TR1 and the previous trailing edge passing time TR2 to the following formula (3). Note that k3 in formula (3) is a predetermined positive coefficient. For example, k3 = 1.

[0104]

Equation

[0105] Formula (3) represents setting the rotation speed V2 of the motor 501 after adjustment according to the reciprocal of the change rate of the trailing edge passing time TR1.

[0106] The actual conveyance speed of the sheet 9 by the fixing device 46 is reflected in the leading edge passing time TF1. By executing the fixing drive control, the conveyance speed of the sheet 9 by the fixing device 46 is appropriately maintained.

[0107] Further, when the elastic outer layer 50b of the pressure roller 50 thermally expands due to the heat received from the heater 53, the length of the nip Np1 in the sheet conveyance direction D1 tends to become longer. That is, when the elastic outer layer 50b thermally expands, the area of the nip Np1 tends to become larger.

[0108] When the area of the nip Np1 increases, even if the temperature of the fixing member 51 does not change, the amount of heat transferred from the fixing member 51 to the toner image on the sheet 9 increases. When the amount of heat transferred from the fixing member 51 to the toner image increases, the sheet 9 is likely to stick to the fixing member 51.

[0109] Excess or deficiency in the amount of heat transferred from the fixing member 51 to the toner image is likely to cause disturbance of the toner image on the sheet 9.

[0110] Therefore, the drive control unit 8c may control the biasing force adjustment mechanism 544 according to the trailing edge passing time TR1. The drive control unit 8c that controls the biasing force adjustment mechanism 544 is an example of a biasing control unit.

[0111] Specifically, the drive control unit 8c controls the biasing force adjustment mechanism 544 such that the biasing force with which the biasing mechanism 54 biases the fixing member 51 toward the pressure roller 50 weakens as the trailing edge passing time TR1 becomes shorter than the reference time TS1.

[0112] On the other hand, the drive control unit 8c controls the biasing force adjustment mechanism 544 such that the biasing force strengthens as the trailing edge passing time TR1 becomes longer than the reference time TS1.

[0113] By controlling the biasing force adjustment mechanism 544, the occurrence of inconvenient situations such as the sheet 9 sticking to the fixing member 51 or the toner image being disturbed is avoided.

[0114] Alternatively, instead of the drive control unit 8c controlling the biasing force adjustment mechanism 544, the heating control unit 8b may control the power supplied to the heater 53 according to the trailing edge passing time TR1.

[0115] For example, the heating control unit 8b corrects the target temperature in the fixing temperature control to be lower as the trailing edge passing time TR1 becomes shorter than the reference time TS1. Thereby, the power supplied to the heater 53 is reduced.

[0116] On the other hand, the drive control unit 8c corrects the target temperature in the fixing temperature control to be higher as the trailing edge passing time TR1 becomes longer than the reference time TS1. As a result, the power supplied to the heater 53 increases.

[0117] By correcting the target temperature, it is possible to avoid an inconvenient situation such as the sheet 9 sticking to the fixing member 51 or the toner image being disturbed.

[0118] [First Application Example] Hereinafter, a first application example of the image forming apparatus 10 will be described.

[0119] In this application example, the timing unit 8e measures the one-point passing time instead of the trailing edge passing time TR1.

[0120] The one-point passing time is the time during which the second sheet detection unit 32b continuously detects the sheet 9. In other words, the one-point passing time is the time from the point when the second sheet detection unit 32b changes from a state where it does not detect the sheet 9 to a state where it detects the sheet 9 to the point when the second sheet detection unit 32b changes back to a state where it does not detect the sheet 9.

[0121] Similar to the trailing edge passing time TR1, the one-point passing time also has a negative correlation with the conveyance speed of the sheet 9 by the fixing device 46.

[0122] For example, the drive control unit 8c in this application example sets the adjusted rotational speed V2 of the motor 501 by applying the one-point passing time to the formula in which the trailing edge passing time TR1 in the above-mentioned formula (1) is replaced with the one-point passing time.

[0123] In this application example, it is conceivable that the reference time TS1 is a constant. Further, the timing unit 8e may measure the resist position passing time in the timing process, and the drive control unit 8c may derive the reference time TS1 from the resist passing time.

[0124] The resist position passing time is the time obtained by adding a predetermined time to the time from when the resist roller pair 31a resumes rotation after a temporary stop until the state changes from when the first sheet detection unit 32a detects the sheet 9 to when it no longer detects it.

[0125] For example, the drive control unit 8c derives the reference time TS1 by applying the resist position passing time to the equation in which the tip passing time TF1 in the aforementioned equation (2) is replaced with the resist position passing time.

[0126] Even when this application example is adopted, the same effects as when the image forming apparatus 10 is adopted can be obtained.

[0127] [Second Application Example] Hereinafter, a second application example of the image forming apparatus 10 will be described.

[0128] In this application example, the second sheet detection unit 32b detects the sheet 9 at a position in the conveyance path 300 from the secondary transfer device 443 to the fixing position P3. The secondary transfer device 443 is an example of a mechanism that feeds the sheet 9 to the fixing position P3.

[0129] In this application example, the timing unit 8e and the drive control unit 8c execute the same processing as in the embodiment. Even when this application example is adopted, the same effects as when the image forming apparatus 10 is adopted can be obtained.

Explanation of Reference Numerals

[0130] 3: Sheet conveyance device 4: Printing device 10: Image forming apparatus 31: Conveyance roller pair 31a: Resist roller pair 31b: Post-fixing roller pair 32: Sheet detection unit 32a: First sheet detection unit 32b: Second sheet detection unit 46: Fixing device 50: Pressing roller (pressing member) 50a: Core metal part 50b: Elastic outer layer 51: Fixing member 86: Motor drive circuit (speed regulator) 300: Conveyor path 500: Driving mechanism

Claims

1. A transfer device that transfers a developer image onto the sheet while transporting the sheet at the transfer position of the transport path; A fixing device that fixes the developer image onto the sheet by heating and pressing the developer image on the sheet while transporting the sheet at the fixing position of the transport path; A sheet detection unit that detects the sheet at a predetermined position in the transport path; A control device that controls the fixing device, and includes: The fixing device includes: A heater; A fixing member that is heated by the heater, rotatably supported, and displaceable in a crossing direction intersecting the sheet transport direction; A pressure member that is disposed to face the fixing member in the crossing direction, has an elastic outer layer that presses against the fixing member, and is driven to rotate to passively rotate the fixing member; A motor that rotationally drives the pressure member; A biasing mechanism that elastically biases the fixing member toward the pressure member along the crossing direction; A resist roller pair that temporarily stops the sheet at the resist position in the transport path and then sends the sheet to the transfer position; The sheet detection unit includes: A first sheet detection unit that detects the sheet at a position upstream of the resist position in the transport path in the sheet transport direction; A second sheet detection unit that detects the sheet at a position from the fixing position in the transport path to a mechanism that takes over the transport of the sheet from the fixing device; The control device includes: A timing unit that measures a trailing edge passing time, which is the time from when the first sheet detection unit changes from a state of detecting the sheet to a state of not detecting the sheet to when the second sheet detection unit changes from a state of detecting the sheet to a state of not detecting the sheet; A speed control unit that controls the rotational speed of the motor according to the trailing edge passing time. An image forming apparatus. **Claim 2**: The image forming apparatus according to claim 1, wherein the speed control unit corrects the current rotational speed of the motor according to the reciprocal of the change rate between the trailing edge passing time last measured by the timing unit and the trailing edge passing time currently measured by the timing unit, thereby setting the adjusted rotational speed of the motor. **Claim 3**: The timing unit further measures a leading edge passing time, which is the time from the point when the first sheet detection unit changes from a state of not detecting the sheet to a state of detecting the sheet to the point when the second sheet detection unit changes from a state of not detecting the sheet to a state of detecting the sheet. The control device derives a reference time according to the leading edge passing time. Further, the control device corrects the current rotational speed of the motor according to the ratio between the trailing edge passing time and the leading edge passing time, thereby setting the adjusted rotational speed of the motor. The image forming apparatus according to claim 1. **Claim 4**: A transfer device that transfers a developer image onto a sheet while transporting the sheet at a transfer position on a transport path; A fixing device that fixes the developer image onto the sheet by heating and pressing the developer image on the sheet while transporting the sheet at a fixing position on the transport path; A sheet detection unit that detects the sheet at a predetermined position on the transport path; And a control device that controls the fixing device. The fixing device includes: A heater; A fixing member that is heated by the heater, is rotatably supported, and is displaceable in a crossing direction that intersects the sheet transport direction; A pressing member that is disposed to face the fixing member in the crossing direction, has an elastic outer layer that presses against the fixing member, and is driven to rotate to drive the fixing member to rotate passively; A motor that rotationally drives the pressing member; And a biasing mechanism that elastically biases the fixing member toward the pressing member along the crossing direction. A resist roller pair that temporarily stops the sheet at the resist position in the conveyance path and then feeds the sheet to the transfer position. The sheet detection unit A first sheet detection unit that detects the sheet at a position upstream of the resist position in the conveyance path in the sheet conveyance direction. A second sheet detection unit that detects the sheet at a position from the fixing position in the conveyance path to a position up to the mechanism that takes over the conveyance of the sheet from the fixing device. The control device A timing unit that measures a one-pass time, which is the time from when the second sheet detection unit changes from a state of not detecting the sheet to a state of detecting the sheet to when the second sheet detection unit changes from a state of detecting the sheet to a state of not detecting the sheet. An image forming apparatus including a speed control unit that controls the rotational speed of the motor according to the one-pass time.

5. The timing unit further measures a resist position passing time, which is the time obtained by adding a predetermined time to the time from when the resist roller pair starts rotating after the temporary stop to when the first sheet detection unit changes from a state of detecting the sheet to a state of not detecting the sheet. The speed control unit derives a reference time according to the resist position passing time and corrects the current rotational speed of the motor according to the ratio of the one-pass time and the resist position passing time, thereby setting the adjusted rotational speed of the motor. The image forming apparatus according to claim 4.

6. The biasing mechanism includes a biasing force adjustment mechanism capable of changing the biasing force that biases the fixing member toward the pressing member. The control device further includes a biasing control unit that controls the biasing force adjustment mechanism according to the measured time of the timing unit. The image forming apparatus according to any one of claims 1 to 5.

7. The image forming apparatus according to any one of claims 1 to 5, wherein the control device further includes a heating control unit that controls the power supplied to the heater according to the time measured by the time measuring unit.

8. The fixing member is a flexible cylindrical member. The fixing device has a facing portion that contacts the inner surface of the fixing member and faces the pressing member via the fixing member, and includes a support member that rotatably supports the fixing member and is displaceable in the intersecting direction. The image forming apparatus according to any one of claims 1 to 7, wherein the biasing mechanism biases the fixing member toward the pressing member by elastically biasing the support member toward the pressing member along the intersecting direction.

Images