Heating device and image forming apparatus

Abstract

To provide a heating device which can suppress a malfunction due to load torque at the driving start of the heating device with a configuration advantageous for miniaturization and cost reduction and an image formation apparatus including the same.SOLUTION: A heating device 120 comprises: a control unit 104 which controls a drive unit 150 so as to maintain a rotational speed of a first rotor 130 at a first rotational speed VI in a first period from the start of rotation of the first rotor 130 to the start of heating processing in a nip part Nt and maintain the rotational speed of the first rotor 130 at a second rotational speed Vh larger than the first rotational speed VI in a second period subsequent to the first period; and an acquisition unit 105 which acquires torque information in correlation with the drive torque when rotating the first rotor 130 with the driving force generated by the drive unit 150. The control unit 104 can change timing of starting the change in the rotational speed of the first rotor 130 to the second rotational speed Vh from the first rotational speed VI on the basis of the toque information acquired by the acquisition unit 105 in the first period.SELECTED DRAWING: Figure 7

Description

[Technical field]

[0001] The present invention relates to a heating device, typically a fixing device, used in an image forming apparatus such as a printer, a copier, or a facsimile machine that utilizes an electrophotographic method, and to an image forming apparatus equipped with the same. [Background technology]

[0002] Conventionally, there are image forming apparatuses that form a toner image corresponding to image information on a recording material such as paper or glossy film by an image forming process such as an electrophotographic method, an electrostatic recording method, or a magnetic recording method. In such image forming apparatuses, a heating device (image heating device, sheet heating device) typified by a fixing device for fixing the toner image onto the recording material is used. The fixing device performs a fixing process (heating process) that melts the toner image and brings it into close contact with the recording material. The fixing device will be described below as an example.

[0003] As a fixing device, a film heating type fixing device is known (Patent Document 1). In terms of a typical configuration, a fixing device of the film heating type has an endless belt-like fixing film, a heater that contacts the inner surface of the fixing film, and a pressure roller that pressurizes and contacts the heater through the fixing film to form a nip portion for fixing. The pressure roller is driven to rotate by a driving force from a driving motor as a driving source constituting a driving device. The fixing film rotates in accordance with the rotation of the pressure roller. The fixing film also rotates while sliding against the heater that contacts its inner surface. Between the inner surface of the fixing film and the heater, a heat-resistant lubricating grease (hereinafter also referred to as "heat-resistant grease") is interposed in order to reduce sliding friction and wear. Such a fixing device of the film heating type has a small heat capacity, and therefore has excellent quick start properties and power saving properties.

[0004] Also, the following method is known for increasing the rotation speed of the fixing film when starting to drive the fixing device (Patent Document 2). That is, the fixing film is first rotated at a lower rotation speed than that used for the fixing process, and then the rotation speed of the fixing film is switched to a higher rotation speed used for the fixing process. Switching the rotation speed of the fixing film when starting to drive the fixing device can be done for various reasons, such as reducing the number of rotations of the fixing film and pressure roller to extend the life of the fixing device. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 4-44075 [Patent Document 2] JP 2015-43020 A Summary of the Invention [Problem to be solved by the invention]

[0006] In a fixing device using the film heating method, the fixing film rotates while sliding against the heater, and the load torque is generated due to the sliding friction. Therefore, the load torque of the fixing device using the film heating method is larger than that of a heat roller fixing device that does not have a sliding part.

[0007] For example, when the fixing device starts to operate from a low temperature state or when the cumulative usage of the fixing device increases, the load torque of the fixing device tends to increase. When the load torque increases, the load torque may exceed the allowable torque of the drive motor, causing the drive motor to stop, or the rotation of the fixing film may become unstable, causing uneven gloss of the image due to uneven temperature. Therefore, an increase in the load torque of the fixing device may be a factor that determines the life of the fixing device. To address this issue, it may be possible to use a larger, more powerful drive motor. However, this would lead to an increase in the size of the fixing device and an increase in costs.

[0008] Thus, for example, the reason why the load torque of the fixing device increases at low temperatures or after an increase in cumulative usage is thought to be the influence of the heat-resistant grease between the fixing film and the heater. The viscosity of the heat-resistant grease also changes depending on the temperature. At high temperatures during the fixing process, the viscosity of the heat-resistant grease is low, and the lubricating properties of the heat-resistant grease contribute to reducing the sliding resistance between the fixing film and the heater. On the other hand, at low temperatures, the viscosity of the heat-resistant grease is high, which causes an increase in the load torque of the fixing device. In addition, depending on the usage status (cumulative usage) of the fixing device, the lubricity and viscosity of the heat-resistant grease change due to the depletion of the oil components that contribute to the lubricity of the heat-resistant grease and the increase in the ratio of solid components, or due to the inclusion of foreign matter in the heat-resistant grease and contamination.

[0009] Here, when the method of increasing the rotation speed of the fixing film as described in the above-mentioned Patent Document 2 is used, for example, the fixing film is rotated at a low speed at low temperatures where the viscosity of the heat-resistant grease is high, so that the load torque of the fixing device can be reduced. However, if the timing of switching the rotation speed to high is too early, the rotation speed is increased in a state where the viscosity of the heat-resistant grease is high. As a result, a larger load torque is applied due to the viscous stress (shear stress due to viscosity) that increases according to the rotation speed, and the load torque may exceed the allowable torque of the drive motor. On the other hand, if the timing of switching the rotation speed to high is too late, the timing of starting the fixing process is delayed, and the waiting time becomes long. Note that the above description has been given by taking as an example the change in the load torque due to the heat-resistant grease, which is particularly likely to be noticeable. However, as will be described later, the load torque due to sliding friction also changes due to variations in the configuration, so the timing of switching the rotation speed may be too early or too late. When switching the rotation speed at a predetermined timing, the same problem as described above may also occur when the rising gradient of the rotation speed is too large or too small.

[0010] Therefore, an object of the present invention is to provide a heating device that has a configuration advantageous for miniaturization and low cost, and is capable of suppressing malfunctions caused by the load torque when the heating device starts to operate, and an image forming apparatus equipped with the same. [Means for solving the problem]

[0011] The above object is achieved by the heating device and image forming apparatus according to the present invention. In summary, the present invention provides a heating device that includes a rotatable first rotating body, a rubbing member that contacts the inner surface of the first rotating body and rubs the inner surface of the first rotating body as the first rotating body rotates, a rotatable second rotating body that contacts the outer surface of the first rotating body to form a nip portion, and a drive unit that generates a driving force to rotate the first rotating body, and that performs a heat treatment to heat a recording material while nipping and conveying the recording material in the nip portion, the heating device being configured to: a control unit that controls the drive unit to maintain the rotational speed of the first rotating body at a first rotational speed during a first period and at a second rotational speed higher than the first rotational speed during a second period after the first period; and an acquisition unit that acquires torque information that correlates with a drive torque when the first rotating body is rotated by a drive force generated by the drive unit, and the control unit is capable of changing a timing to start changing the rotational speed of the first rotating body from the first rotational speed to the second rotational speed based on the torque information acquired by the acquisition unit during the first period. and when a torque value indicated by the torque information acquired by the acquisition unit during the first period becomes equal to or less than a threshold value, the control unit starts changing the rotation speed of the first rotating body from the first rotation speed to the second rotation speed, and the control unit corrects the threshold value based on the torque information acquired by the acquisition unit when the rotation speed of the first rotating body was changed from the first rotation speed to the second rotation speed during a previous or previous rotation of the first rotating body. The heating device is characterized by the above.

[0012] According to another aspect of the present invention, in a heating device that performs a heat treatment for heating a recording material while nipping and conveying the recording material in the nip portion, the heating device has a rotatable first rotating body, a rubbing member that contacts an inner surface of the first rotating body and rubs the inner surface of the first rotating body as the first rotating body rotates, a rotatable second rotating body that contacts an outer surface of the first rotating body to form a nip portion, and a drive unit that generates a drive force to rotate the first rotating body, the heating device further includes a drive unit that generates a drive force to rotate the first rotating body, and the drive unit generates a drive force to rotate the first rotating body, the drive unit generates a drive force to rotate the first rotating body, and the heat treatment is performed by nipping and conveying the recording material in the nip portion ... and the heat treatment is performed by nipping and conveying the recording material in the nip portion, and the heat treatment is performed by nipping and conveying the recording material in the nip portion, and the heat treatment is performed by nipping and conveying the recording material in the nip portion, and the heat treatment is a control unit that controls the drive unit to maintain the rotational speed of the first rotating body at a first rotational speed during a first period and at a second rotational speed higher than the first rotational speed during a second period after the first period; and an acquisition unit that acquires torque information that correlates with a drive torque when the first rotating body is rotated by a drive force generated by the drive unit, and the control unit is capable of changing an increase rate of the rotational speed when changing the rotational speed of the first rotating body from the first rotational speed to the second rotational speed, based on the torque information acquired by the acquisition unit during the first period. and the control unit changes the increase rate such that the increase rate when the torque value indicated by the torque information acquired by the acquisition unit at a predetermined timing in the first period is a first value is smaller than the increase rate when the torque value is a second value that is larger than the first value. A heating device is provided.

[0013] According to another aspect of the present invention, there is provided an image forming apparatus comprising an image forming section for forming an image on a recording material with toner, and the heating device of the present invention. Effect of the Invention

[0014] According to the present invention, it is possible to suppress problems caused by the load torque at the start of driving the heating device with a configuration advantageous for miniaturization and cost reduction. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Diagram 2] FIG. 2 is a schematic cross-sectional view of the fixing device. [Diagram 3] FIG. 2 is a schematic diagram of a drive motor. [Figure 4]FIG. 2 is a schematic block diagram showing a motor control unit. [Diagram 5] 5 is a graph showing the relationship between the coil current and the drive torque of the drive motor. FIG. [Figure 6] FIG. 4 is a graph showing the relationship between load torque and heater temperature. [Figure 7] 5 is a graph showing changes over time in heater temperature, pressure roller rotation speed, and pressure roller drive torque in relation to the control of the first embodiment. FIG. [Figure 8] FIG. 4 is a flow chart of control in the first embodiment. [Figure 9] 5 is a graph showing changes over time in the heater temperature, the rotation speed of the pressure roller, and the driving torque of the pressure roller in a low-temperature environment in relation to the control of the first embodiment. FIG. [Figure 10] 11 is a graph showing changes over time in heater temperature, pressure roller rotation speed, and pressure roller driving torque in the control of the first embodiment after an accumulated usage amount of the fixing device increases. FIG. [Figure 11] FIG. 11 is a flow chart of control in the second embodiment. [Figure 12] FIG. 11 is a diagram showing a change over time in the driving torque of the pressure roller in relation to the control of the second embodiment. [Figure 13] 13 is a graph showing changes over time in heater temperature, pressure roller rotation speed, and pressure roller drive torque in relation to the control of the third embodiment. FIG. [Figure 14] FIG. 11 is a flow chart of control according to the third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The heating device and image forming apparatus according to the present invention will be described in more detail below with reference to the drawings.

[0017] [Example 1] 1. Configuration and operation of image forming apparatus 1 is a schematic cross-sectional view of an image forming apparatus 100 according to this embodiment. The image forming apparatus 100 according to this embodiment is a tandem type laser beam printer that employs an intermediate transfer method and is capable of forming a full-color image using an electrophotographic method.

[0018] The image forming apparatus 100 has four stations SY, SM, SC, and SK that form images of each color, yellow (Y), magenta (M), cyan (C), and black (K), as a plurality of stations. Elements having the same or corresponding functions or configurations in each image forming section SY, SM, SC, and SK may be generally described by omitting the Y, M, C, or K at the end of the reference numeral indicating that the element is for any color. In this embodiment, the image forming section S is configured with photosensitive drums 1 (1Y, 1M, 1C, 1K), charging rollers 2 (2Y, 2M, 2C, 2K), laser scanner units 3 (3Y, 3M, 3C, 3K), developing devices 4 (4Y, 4M, 4C, 4K), primary transfer rollers 5 (5Y, 5M, 5C, 5K), and drum cleaning devices 6 (6Y, 6M, 6C, 6K), which will be described later.

[0019] The photosensitive drum 1, which is a rotatable drum-type (cylindrical) photosensitive body (electrophotographic photosensitive body) serving as an image carrier, is rotated in the direction of arrow R1 (clockwise direction) in the figure at a predetermined peripheral speed (process speed). The surface (outer periphery) of the rotating photosensitive drum 1 is charged to a predetermined potential of a predetermined polarity (negative polarity in this embodiment) by a charging roller 2, which is a roller-type charging member serving as a charging means. The surface of the charged photosensitive drum 1 is scanned and exposed according to image information by a laser scanner unit (exposure device) 3 serving as an exposure means, and an electrostatic latent image (electrostatic image) is formed on the photosensitive drum 1. The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) by a developing device 4 serving as a developing means, which supplies toner as a developer, and a toner image (toner image, developer image) is formed on the photosensitive drum 1. In this embodiment, toner charged with the same polarity as the charging polarity of the photosensitive drum 1 (negative polarity in this embodiment) adheres to the exposed area (image area) on the photosensitive drum 1, which has been uniformly charged and then exposed to light, thereby reducing the absolute value of the potential.

[0020] An intermediate transfer belt 7, which is an endless belt serving as an intermediate transfer body, is disposed so as to face the four photosensitive drums 1Y, 1M, 1C, and 1K. The intermediate transfer belt 7 is wound around a plurality of tension rollers (support rollers) and tensioned with a predetermined tension. The intermediate transfer belt 7 rotates (moves around) in the direction of arrow R2 (counterclockwise direction) in the figure as a result of a driving roller among the plurality of tension rollers being rotated. On the inner peripheral surface side of the intermediate transfer belt 7, a primary transfer roller 5, which is a roller-type primary transfer member serving as a primary transfer means, is disposed in correspondence with each photosensitive drum 1. The primary transfer roller 5 presses the inner peripheral surface of the intermediate transfer belt 7 toward the photosensitive drum 1, forming a primary transfer portion N1 where the photosensitive drum 1 and the intermediate transfer belt 7 come into contact with each other. In this embodiment, the primary transfer roller 5 comes into contact with the photosensitive drum 1 via the intermediate transfer belt 7. The toner image formed on the photosensitive drum 1 is transferred (primary transfer) at the primary transfer section N1 onto the intermediate transfer belt 7 rotating at a circumferential speed (process speed) corresponding to the circumferential speed of the photosensitive drum 1. During the primary transfer, a primary transfer voltage of a polarity opposite to the normal charging polarity of the toner (charging polarity during development) (positive polarity in this embodiment) is applied to the primary transfer roller 5. For example, when a full-color image is formed, the toner images of each color, yellow (Y), magenta (M), cyan (C), and black (K), formed on each photosensitive drum 1 are transferred sequentially onto the intermediate transfer belt 7 so as to be superimposed on each other.

[0021] On the outer peripheral surface side of the intermediate transfer belt 7, a secondary transfer roller 8, which is a roller-type secondary transfer member serving as a secondary transfer means, is disposed at a position facing the secondary transfer opposing roller (which may also serve as a drive roller) among the above-mentioned multiple tension rollers. The secondary transfer roller 8 is pressed toward the secondary transfer opposing roller and abuts against the secondary transfer opposing roller via the intermediate transfer belt 7, forming a secondary transfer section N2 where the intermediate transfer belt 7 and the secondary transfer roller 8 come into contact with each other. The toner image formed on the intermediate transfer belt 7 is transferred (secondarily transferred) onto the recording material P, which is sandwiched and conveyed between the intermediate transfer belt 7 and the secondary transfer roller 8, at the secondary transfer section N2. In this embodiment, during the secondary transfer, a secondary transfer voltage of a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment) is applied to the secondary transfer roller 8. The recording material (transfer material, sheet) P, such as recording paper, is stored in a tray 11, which serves as a recording material storage section. The recording material P in the tray 11 is sent out one by one by a feeding roller 12 as a feeding member, and is transported to a registration roller pair 14 as a transport member by a transport roller pair 13 as a transport member. The recording material P transported to the registration roller pair 14 is supplied to the secondary transfer portion N2 by the registration roller pair 14 so as to be synchronized with the toner image on the intermediate transfer belt 7.

[0022] The recording material P onto which the toner image has been transferred is transported to a fixing device 120 as a fixing means disposed downstream of the secondary transfer portion N2 in the transport direction of the recording material P. The fixing device 120 applies heat and pressure to the recording material P bearing the unfixed toner image, thereby fixing (melting and bonding) the toner image onto the recording material P (fixing process, heating process). The recording material P onto which the toner image has been fixed is discharged (output) onto a discharge tray 9 as a discharge portion provided at the top of the device main body 101 of the image forming apparatus 100.

[0023] Adherents such as toner (primary transfer residual toner) remaining on the photosensitive drum 1 after the primary transfer are removed from the photosensitive drum 1 and collected by a drum cleaning device 6 as a photosensitive body cleaning means. The drum cleaning device 6 has a cleaning blade as a cleaning member arranged to abut against the surface of the photosensitive drum 1, and a cleaning container. The drum cleaning device 6 scrapes off adherents such as primary transfer residual toner from the surface of the rotating photosensitive drum 1 with the cleaning blade, and stores them in the cleaning container. Adherents such as toner (secondary transfer residual toner) remaining on the intermediate transfer belt 7 after the secondary transfer are removed from the intermediate transfer belt 7 and collected by a belt cleaning device 15 as an intermediate transfer body cleaning means. The belt cleaning device 15 has a cleaning blade 15a as a cleaning member arranged to abut against the surface of the intermediate transfer belt 7, and a cleaning container 15b. The belt cleaning device 15 scrapes off adherents such as secondary transfer residual toner from the surface of the rotating intermediate transfer belt 7 with the cleaning blade 15a, and stores them in the cleaning container 15b.

[0024] In this embodiment, at each station S, the photosensitive drum 1, and the charging roller 2, developing device 4, and drum cleaning device 6 acting as process means thereon, are integrally configured as a process cartridge 10 that is detachably attached to the main body 101 of the apparatus.

[0025] In this embodiment, each station S, the intermediate transfer belt 7, the secondary transfer roller 8, etc. constitute an image forming section 110 that forms an image on the recording material P with toner.

[0026] The image forming apparatus 100 also has a printer control unit 102 that controls the operation of each unit of the image forming apparatus 100. The printer control unit 102 is configured to have a CPU as an arithmetic control means (arithmetic control unit) that is a central element that performs arithmetic processing, and memories (storage media) such as ROM and RAM as storage means (storage unit). The RAM, which is a rewritable memory, stores information input to the printer control unit, detected information, arithmetic results, etc., and the ROM stores a control program, a data table obtained in advance, etc. The CPU and memories such as the ROM and RAM can transfer and read data to and from each other. The printer control unit 102 also has an input / output circuit that controls communication between the printer control unit 102 and external devices.

[0027] 2. Fixing device Next, the fixing device 120 of this embodiment will be further described. The image forming apparatus 100 of this embodiment has a film heating type fixing device 120 as a heating device (image heating device, sheet heating device). The fixing device 120 fixes an unfixed toner image formed on the recording material P by the image forming section 110 to the recording material P. Fig. 2 is a schematic cross-sectional view of the fixing device 120 of this embodiment (a cross-section substantially perpendicular to the rotation axis direction of a pressure roller 201 described later).

[0028] The fixing device 120 has a heating unit 200. The heating unit 200 has a fixing film 130 as an endless belt-like heating rotator (first rotator) made of a flexible cylindrical film (endless film). The heating unit 200 also has a rubbing member that contacts the inner surface of the fixing film 130 and rubs the inner surface of the fixing film 130 as the fixing film 130 rotates. In this embodiment, the heating unit 200 has a ceramic heater (hereinafter, also simply referred to as "heater") 131 as a heat generating member (heating body) that heats the fixing film 130 as a rubbing member. The heating unit 200 heats the fixing film 130 by bringing the heater 131 into contact with the inner surface of the fixing film 130 to transfer heat, thereby heating the image on the recording material P that contacts the outer surface of the fixing film 130.

[0029] In this embodiment, the fixing film 130 is formed by providing a 200 μm thick silicone rubber layer as an elastic layer on a cylindrical base layer made of polyimide resin with a thickness of 65 μm, an inner diameter of 24 mm, and a length of 235 mm, and further covering the outside of the elastic layer with a 20 μm thick PFA resin tube as a release layer. Other heat-resistant resin materials or metal materials such as nickel and SUS can also be used for the base layer of the fixing film 130. The release layer can also be formed by coating with fluorine resin or the like. In this embodiment, the silicone rubber of the elastic layer has a thermal conductivity of 1.0 W / m·K or more, which is high for silicone rubber. This reduces the thermal resistance in the thickness direction of the fixing film 130.

[0030] In this embodiment, the heater 131 is a substrate made of ceramics such as alumina or aluminum nitride molded to a length of 270 mm, width of 8 mm, and thickness of 0.6 mm, on which a resistance heating element pattern is formed. In this embodiment, a resistance heating pattern is formed by printing on the surface of the substrate opposite to the fixing film 130, and a 70 μm thick heat-resistant insulating glass protective layer is provided thereon. In addition, a 10 μm thick smooth glass layer is formed on the surface of the substrate facing the fixing film 130 in order to maintain good sliding properties with the fixing film 130 and reduce wear of both. In order to control the temperatures of the heater 131 and the fixing film 130 to target values, a thermistor 324 as a temperature detection means (temperature detection member) for detecting the temperature of the heater 131 is provided so as to be in contact with the surface of the heater 131 that does not come into contact with the fixing film 130. The thermistor 324 can be provided in contact with the inner surface of the fixing film 130 or in non-contact with the surface of the fixing film 130. In response to the temperature detection result by thermistor 324, the amount of heat generated by heater 131 (power input) is controlled by a heater control unit (not shown).

[0031] The heating unit 200 also has a guide member 132 made of a heat-resistant resin (such as a liquid crystal polymer). The guide member 132 holds the heater 131 and also serves to guide the running of the fixing film 130. The heating unit 200 also has a metal skeleton 151 that serves as a skeleton supporting the guide member 132 in the longitudinal direction. A total pressure of 196 N in this embodiment, which is applied to the metal skeleton 151 from a pressure mechanism (not shown), is transmitted to the guide member 132. As a result, the guide member 132 and the heater 131 held by the guide member 132 both press the fixing film 130 against the pressure roller 201, which will be described later. Thus, in this embodiment, the guide member 132 and the heater 131 function as support members that support the fixing film 130 from the back and press it against the pressure roller 201, which will be described later.

[0032] In the heating unit 200, the fixing film 130, the heater 131, the guide member 132, and the metal skeleton 151 extend along a direction (the width direction of the fixing film 130) substantially perpendicular to the conveying direction of the recording material P. The fixing film 130 is flexible, and is loosely wrapped around the guide member 132. The heater 131, the guide member 132, and the metal 151 are contained inside the fixing film 130 (hollow portion).

[0033] The fixing device 120 also has a pressure roller 201 as a pressure rotating body (second rotating body) that contacts the outer surface (surface) of the fixing film 130 constituting the heating unit 200. In this embodiment, the pressure roller 201 is provided with an elastic layer 141 made of silicone rubber and having a thickness of 4 mm on a stainless steel core metal 140 having an outer diameter of 17 mm, and a surface layer 142 made of PFA resin and having a thickness of 50 μm as a release layer on the elastic layer 141. Therefore, in this embodiment, the outer diameter of the pressure roller 201 is about 25 mm. In this embodiment, the product hardness of the pressure roller 201 is 55 degrees (ASKER-C, load 1 kgf ≒ 9.8 N). The pressure roller 201 is disposed so that its rotation axis direction is approximately parallel to the width direction of the fixing film 130 (direction approximately perpendicular to the conveying direction of the recording material P).

[0034] The elastic layer 141 of the pressure roller 201 is deformed by the pressure from the heater 131, forming a fixing nip portion (fixing nip area) Nt, which is a contact area between the fixing film 130 and the pressure roller 201. In this embodiment, the width of the fixing nip portion Nt in the conveying direction of the recording material P (the moving direction of the surface of the fixing film 130) is about 9 mm. The pressure roller 201 is driven by a drive motor 150 connected via a gear train (not shown). In this embodiment, the drive motor 150, the gear train, etc. constitute a drive device (drive mechanism) 160 (FIG. 1) as a drive means for the fixing device 120.

[0035] The fixing film 130 rotates in the direction of the arrow R3 in the figure due to the frictional force acting at the fixing nip portion Nt, in association with the rotation of the pressure roller 201 in the direction of the arrow R3 in the figure. That is, the fixing film 130 rotates in the direction of the arrow R4 in the figure at a rotational speed substantially equal to that of the pressure roller 201 while sliding against the heater 131 and the guide member 132 in a pressure-contact manner. In this embodiment, in order to reduce the frictional force generated between the fixing film 130 and the heater 131 or the guide member 132, heat-resistant grease is interposed between them as a lubricant. For example, a mixture of fluorine oil and fluorine resin can be used as the heat-resistant grease. In this embodiment, HP-300 grease manufactured by Dow Corning Toray Co., Ltd. was used as the heat-resistant grease. In this embodiment, the amount of heat-resistant grease used was 400 mg.

[0036] In this embodiment, for example, the basis weight is 60 to 100 g / m 2 During normal image formation on plain paper or the like, the pressure roller 201 is driven at a peripheral speed of 300 mm / sec. At the same time, power is supplied to the heater 131 so that the surface temperature of the fixing film 130 rises to 180° C. or higher, at which the toner image can be fixed.

[0037] After the transfer process, the recording material P with the unfixed toner image T thereon is guided to the fixing nip Nt. The toner is melted and fixed onto the recording material P by the pressure applied at the fixing nip Nt and the heat transmitted from the fixing film 130 and the heater 131.

[0038] In addition, after the fixing process is completed, the recording material P discharged from the fixing nip portion Nt is guided by a discharge guide (not shown) to a pair of discharge rollers (not shown) serving as discharge members, and is discharged outside the apparatus main body 101 as described above.

[0039] 3. Method for detecting driving torque of driving motor and pressure roller of fixing device <Drive motor of fixing device> A description will be given of a driving motor 150 serving as a driving source constituting a driving device 160 of the fixing device 120 in this embodiment. Fig. 3 is a schematic diagram of the driving motor 150 in this embodiment. Fig. 4 is a schematic block diagram showing a motor control unit 220 that controls the driving motor 150 in this embodiment.

[0040] In this embodiment, a brushless motor is used as the drive motor 150 that drives the pressure roller 201. In this embodiment, the motor control unit 220 that controls the drive motor 150 is provided with a mechanism for detecting the current (coil current, drive current) that is passed through the coil of the drive motor 150 when the drive motor 150 is driven.

[0041] The structure of the drive motor 150 in this embodiment will be described with reference to Fig. 3. The drive motor 150 has a six-slot stator 240 and a four-pole rotor 241. The stator 240 has U-phase, V-phase, and W-phase coils 235, 236, and 237, respectively. The rotor 241 is made of a permanent magnet and has two pairs of N poles and S poles. The coils 235, 236, and 237 of the U-phase, V-phase, and W-phase are connected to an inverter output 233, which will be described later.

[0042] With reference to FIG. 4, the motor control unit 220 constituting the motor drive circuit that drives the drive motor 150 will be described. The motor control unit 220 has, for example, a microcomputer 221 as a calculation processing unit. The microcomputer 221 has a communication port 222, an AD converter 229, a counter 223, a non-volatile memory 224, a reference clock generation unit 225, a PWM port 227, and a current calculation unit 228 built in. The reference clock generation unit 225 generates a reference clock based on the output of a crystal oscillator provided in the motor control unit 220. The counter 223 performs a counting operation based on the reference clock generated by the reference clock generation unit 225, and the microcomputer 221 measures the period of the input pulse and generates a PWM signal based on the count value of the counting operation. The PWM port 227 has six terminals and outputs PWM signals of three high (H) side signals (UH, VH, WH) and three low (L) side signals (UL, VL, WL). The motor control unit 220 has a three-phase inverter 231 composed of three high-side and three low-side switching elements. For example, a transistor or a FET can be used as the switching elements. Each switching element is connected to a PWM port 227 via a gate driver 232, and can be controlled to be turned on / off by a PWM signal output from the PWM port 227. Each switching element is turned on when the PWM signal is H, and turned off when the PWM signal is L.

[0043] The U-, V-, and W-phase outputs 233 of the inverter 231 are connected to the coils 235, 236, and 237 of the drive motor 150, respectively, and can control the coil currents flowing through the coils 235, 236, and 237. The coil currents flowing through the coils 235, 236, and 237 are detected by a current detection unit 238. In this embodiment, the current detection unit 238 is configured to include a current sensor 230, an amplifier unit 234, an AD converter 229, and a current value calculation unit 228. First, the currents flowing through the coils 235, 236, and 237 are converted into voltages by the current sensor 230. The voltages are amplified and an offset voltage is applied to them by the amplifier unit 234, and then input to the AD converter 229 of the microcomputer 221. For example, if the current sensor 230 outputs a voltage of 0.01V per 1A, the amplification factor of the amplifier unit 234 is 10 times, and the applied offset voltage is 1.6V, the output voltage of the amplifier unit 234 when a current of -10A to +10A flows is 0.6 to 2.6V. The AD converter 229 outputs a voltage of, for example, 0 to 3V as an AD value of 0 to 4095. Therefore, when a current of -10A to +10A flows, the AD value is approximately 819 to 3549. The positive and negative of the current is defined as + when the current flows from the three-phase inverter 231 to the drive motor 150. The current value calculation unit 228 performs a predetermined calculation on the AD-converted data (hereinafter also referred to as "AD value") to calculate the current value. That is, the offset value is subtracted from the AD value, and the current value is further multiplied by a predetermined coefficient to obtain the current value. The offset value is the AD value with an offset voltage of 1.6V, so it is approximately 2184, and the coefficient is approximately 0.00733. The offset value is read from the AD value when no coil current is flowing, and is stored in the non-volatile memory 224. The coefficient is saved in advance in the non-volatile memory 224 as a standard coefficient.

[0044] The microcomputer 221 controls the three-phase inverter 231 via the gate driver 232, thereby causing current to flow through coils 235, 236, and 237 of the drive motor 150. The microcomputer 221 detects the coil current using the current sensor 230, amplifier unit 234, and AD converter 229 that configure a current detection unit 238, and calculates the rotor position and speed of the drive motor 150 from the detected coil current. In this way, the microcomputer 221 can control the rotation of the drive motor 150.

[0045] In this embodiment, the printer control unit 102 controls the drive motor 150 of the fixing device 120 via the motor control unit 220 (FIG. 4) and controls the rotation speed of the pressure roller 201 (fixing film 130). In this embodiment, the printer control unit 102 can be seen as constituting the fixing device 120. As shown in FIG. 4, in this embodiment, the printer control unit 102 has a speed control unit 104 that controls the drive motor 150 of the fixing device 120 via the motor control unit 220 and controls the rotation speed of the pressure roller 201 (fixing film 130). In addition, in this embodiment, the printer control unit 102 has a torque information acquisition unit 105 that acquires torque information correlated with the drive torque of the pressure roller 201 based on the coil current (drive current) detected by the current detection unit 238 of the motor control unit 220 as described later. In this embodiment, the speed control unit 104 and the torque information acquisition unit 105 as functional blocks are realized by the CPU of the printer control unit 102 executing processing according to a program stored in the ROM. As shown in FIG. 4, in this embodiment, the printer control unit 102 is provided with a non-volatile memory 103 as a storage unit (storage unit) for storing information on threshold values, which will be described later.

[0046] <Method for detecting the driving torque of the pressure roller> A method for detecting the driving torque of the pressure roller 201 in this embodiment will be described below. Fig. 5 is a graph showing the relationship (TI characteristic) between the driving torque of the driving motor 150 and the coil current that drives the driving motor 150 in this embodiment.

[0047] 5, in the configuration of the drive motor 150 and the motor control unit (motor drive circuit) 220 in this embodiment, there is a correlation between the drive torque of the drive motor 150 and the coil current that drives the drive motor 150. Therefore, the drive torque of the drive motor 150 can be obtained based on the TI characteristic of the drive motor 150.

[0048] Specifically, a conversion formula obtained based on the measurement result of the TI characteristic of the drive motor 150 is stored in advance in the non-volatile memory 103 of the printer control unit 102. This conversion formula is configured so that the drive torque of the drive motor 150 can be calculated from the coil current based on the TI characteristic. Then, the printer control unit 102 is configured to calculate the drive torque of the drive motor 150 from the coil current detected by the current detection unit 238 based on this calculation formula. More specifically, in this embodiment, the axial torque of the pressure roller 201 is calculated based on the drive torque of the drive motor 150 and the gear ratio of the gear train from the drive motor 150 to the pressure roller 201, and the value is set as the drive torque of the pressure roller 201. That is, more specifically, in this embodiment, the conversion formula is configured so that the drive torque of the pressure roller 201 can be calculated from the coil current based on the TI characteristic.

[0049] When the drive motor 150 rotates at a substantially constant speed, the drive torque of the pressure roller 201 is in balance with the load torque of the fixing device 120. In other words, the drive torque of the pressure roller 201 detected as described above can be substituted for the load torque caused by the sliding friction between the fixing film 130 and the heater 131, which occupies a major proportion of the load torque of the fixing device 120. Here, the drive torque of the pressure roller 201 detected as described above is substituted for the load torque of the fixing device 120, particularly the load torque caused by the sliding friction between the fixing film 130 and the heater 131, and is also simply referred to as the "load torque".

[0050] 4. Change in load torque with temperature Fig. 6 is a graph showing the relationship between the temperature of the heater 131 and the load torque (temperature characteristics of the load torque). A solid line 501 in Fig. 6 shows the relationship between the load torque and the temperature of the heater 131 when the fixing film 130 is rotating at a substantially constant speed. As described above, due to the influence of the temperature characteristics of the viscosity of the heat-resistant grease, the load torque is large when the temperature of the heater 131 is low, and the load torque decreases as the temperature of the heater 131 increases.

[0051] The load torque is also affected by the rotation speed of the fixing film 130. When the fixing film 130 rotates, the sliding friction acting between the heater 131 and the fixing film 130 includes shear stress caused by the viscosity of the heat-resistant grease. The value of the shear stress increases as the rotation speed of the fixing film 130 increases. Therefore, as shown by the dashed line 502 in FIG. 6, even at the same temperature of the heater 131, the faster the rotation speed of the fixing film 130, the greater the sliding friction and the higher the load torque. Conversely, as shown by the dashed line 503 in FIG. 6, even at the same temperature of the heater 131, the slower the rotation speed of the fixing film 130, the less the sliding friction and the lower the load torque.

[0052] 5. Timing to increase rotation speed Next, a description will be given of the timing at which the rotation speed of the fixing film 130 is switched from the first rotation speed to the second rotation speed, which is faster than the first rotation speed, when the fixing device 120 starts to be driven (started up) in this embodiment.

[0053] In this embodiment, the fixing device 120 has a first period during which the rotation speed of the fixing film 130 is maintained substantially constant at a first rotation speed when the fixing device 120 starts to drive, and a second period during which the rotation speed is maintained substantially constant at a second rotation speed higher than the first rotation speed after the first period. In this embodiment, the printer control unit 102 acquires information on the load torque of the fixing device 120 when the fixing film 130 is rotating at the first rotation speed when the fixing device 120 starts to drive. In this embodiment, the printer control unit 102 changes the timing at which the rotation speed of the fixing film 130 is switched from the first rotation speed to a second rotation speed higher than the first rotation speed according to the information. Specifically, in this embodiment, the printer control unit 102 controls the rotation speed of the fixing film 130 to be increased from the first rotation speed to the second rotation speed when the load torque of the fixing device 120 falls below a predetermined threshold ("torque threshold"). In this embodiment, the torque threshold value is preset and stored in the non-volatile memory 103 of the printer control unit 102. In this embodiment, the first rotation speed of the fixing film 130 is a rotation speed lower than that when performing the fixing process. Also, in this embodiment, the second rotation speed of the fixing film 130 is a rotation speed when performing the fixing process that is faster than the first rotation speed. Here, in this embodiment, as described above, the printer control unit 102 obtains the driving torque of the pressure roller 201 from the coil current of the driving motor 150 detected by the current detection unit 238 of the motor control unit 220 based on the TI characteristic obtained in advance. This driving torque of the pressure roller 201 can be substituted for the load torque of the fixing device 120, in particular, the load torque caused by the sliding friction between the fixing film 130 and the heater 131. Also, in this embodiment, as described above, the fixing film 130 rotates following the rotation of the pressure roller 201, and the rotation speed of the pressure roller 201 corresponds to the rotation speed of the fixing film 130.

[0054] FIG. 7 is a graph for explaining a method for increasing the rotation speed of the fixing film 130 at the start of driving the fixing device 120 of this embodiment. In FIG. 7, (a) shows the temperature of the heater 131 (solid line 401), (b) shows the rotation speed of the pressure roller 201 (solid line 402), and (c) shows the driving torque of the pressure roller 201 (solid line 404) over time. Note that FIG. 7 is a graph when the fixing device 120 is started to be driven from a state in which the fixing device 120 has become accustomed to a normal environment (23° C.). Also, FIG. 7 is a graph when the fixing device 120 is driven in a state before the cumulative usage of the fixing device 120 increases (for example, at the beginning of the life period (before durability)).

[0055] 7, the pressure roller 201 starts to be driven to a first rotation speed Vl (38 rpm (low speed)) corresponding to a circumferential speed of 50 mm / sec. At almost the same time, the heater 131 starts to be energized, and the temperature of the heater 131 starts to rise.

[0056] Here, as shown by the dashed line 403 in FIG. 7B, when the driving of the pressure roller 201 is started so that the second rotation speed Vh (230 rpm (high speed)) corresponding to the peripheral speed of 300 mm / sec when performing the fixing process from the beginning, the following occurs. In this case, the fixing film 130 rotates at high speed at a low temperature, and the load torque becomes high according to the temperature characteristic of the load torque in FIG. 6. Furthermore, the acceleration torque when increasing the rotation speed is also added, and an even higher load torque is generated. As a result, as shown by the dashed line 405 in FIG. 7C, the driving torque of the pressure roller 201 increases sharply. Then, before the rotation speed of the pressure roller 201 reaches the second rotation speed Vh, the driving torque of the pressure roller 201 exceeds the upper limit Tlm (in this embodiment, 20 kgf cm ≈ 1.96 N m). When the driving torque of the pressure roller 201 exceeds the upper limit, the driving motor 150 stops driving the pressure roller 201 to protect the device. The upper limit Tlm of the driving torque of the pressure roller 201 is set in advance in accordance with the allowable torque of the driving motor 150 .

[0057] In contrast, in this embodiment, the pressure roller 201 starts to be driven at a sufficiently slow first rotation speed Vl at first. As a result, the drive torque of the pressure roller 201 once reaches a maximum value (hereinafter also referred to as "peak torque") T11, but does not exceed an upper limit Tlm (in this embodiment, 20 kgf cm ≈ 1.96 N m).

[0058] Thereafter, in this embodiment, the driving torque of the pressure roller 201 is detected while the rotation speed of the pressure roller 201 is maintained substantially constant at the first rotation speed (38 rpm (low speed)). While the detection of the driving torque of the pressure roller 201 is continued, the driving torque of the pressure roller 201 gradually decreases with the rise in temperature of the heater 131, according to the temperature characteristic of the load torque in FIG. 6. Then, in this embodiment, the rotation speed of the pressure roller 201 is switched to the second rotation speed (high speed) Vh at the timing B when the driving torque of the pressure roller 201 becomes equal to or lower than a predetermined torque threshold value Tth. In this embodiment, the torque threshold value Tth is 3 kfg·cm (≈0.29 N·m). In addition, in this embodiment, the second rotation speed Vh is 230 rpm, which corresponds to a peripheral speed of 300 mm / sec when performing the fixing process. In this embodiment, in order to remove minute noise components, a moving average of the immediately preceding five sampled values ​​sampled at a period of 10 msec was used as the torque detection result (measured value).

[0059] Immediately after timing B when the rotation speed of the pressure roller 201 is switched, an acceleration torque is applied in the same manner as when the pressure roller 201 starts to be driven, so that the drive torque of the pressure roller 201 reaches a second peak torque T12. In this embodiment, this peak torque T12 is 15 kgf cm (≈1.47 N m), and is suppressed to be equal to or less than the upper limit Tlm of the drive torque of the pressure roller 201. Thereafter, the rotation speed of the pressure roller 201 is maintained approximately constant at the second rotation speed (230 rpm) until the fixing process is completed.

[0060] In this embodiment, the torque threshold Tth is set to the value of the driving torque of the pressure roller 201 in a state in which the rotation speed of the pressure roller 201 is maintained substantially constant at a first rotation speed, as follows. That is, the torque threshold Tth is a value of the driving torque of the pressure roller 201 such that the peak torque T12 is equal to or less than the upper limit Tlm of the driving torque of the pressure roller 201 even if the rotation speed of the pressure roller 201 is switched immediately after the driving torque of the pressure roller 201 reaches the torque threshold Tth. This keeps the peak torque T12 always equal to or less than the upper limit Tlm of the driving torque of the pressure roller 201. As a result, it is possible to prevent the driving motor 150 from stopping or the rotation speed of the fixing film 130 from becoming unstable, which causes uneven glossiness in the image.

[0061] Fig. 8 is a flow chart for explaining a method for increasing the rotation speed of the fixing film 130 at the start of driving the fixing device 120 in this embodiment. Fig. 8 shows an outline of a procedure focusing mainly on the process related to the control of driving the fixing device 120 among the processes executed by the printer control unit 102 when the image forming apparatus 100 performs an image forming operation (printing operation).

[0062] When the printer control unit 102 receives an image formation start instruction, it issues a command to the drive motor 150 and starts driving the pressure roller 201 so that the rotation speed of the pressure roller 201 becomes the first rotation speed (38 rpm (low speed)) (S101). In addition, almost simultaneously with starting the drive of the pressure roller 201, the printer control unit 102 issues a command to the heater control unit (not shown) to start energizing the heater 131 and start heating the heater 131. Thereafter, while maintaining the rotation speed of the pressure roller 201 substantially constant at the first rotation speed, the printer control unit 102 detects the drive torque of the pressure roller 201 based on the detection result of the current detection unit 238 of the drive motor 150 as described above (S102). In addition, every time the printer control unit 102 obtains the detection result (the above-mentioned moving average value) of the drive torque of the pressure roller 201, it determines whether the drive torque is equal to or less than a predetermined torque threshold value Tth stored in advance in the non-volatile memory 103 (S103). The processes of S102 and S103 are continued until the driving torque of the pressure roller 201 becomes equal to or less than the torque threshold value Tth. When the printer control unit 102 determines in S103 that the torque is equal to or less than the torque threshold value Tth, it issues a command to the driving motor 150 to switch the rotation speed of the pressure roller 201 to a second rotation speed (230 rpm (high speed)) (S104). Then, the printer control unit 102 waits until the image forming operation (fixing process) for the designated number of sheets is completed (S105). During this time, the rotation speed of the pressure roller 201 is maintained approximately constant at the second rotation speed. Then, when the image forming operation (fixing process) for the designated number of sheets is completed, the printer control unit 102 performs a predetermined end process such as stopping the driving of the pressure roller 201, stopping the energization of the heater 131, and stopping the driving of the photosensitive drum 1 and the intermediate transfer belt 7.

[0063] 6. Effects of this embodiment Next, the effects of this embodiment will be further described. Here, the rotation state of the pressure roller 201 in this embodiment will be compared with the following Comparative Example 1 and Comparative Example 2 for each of the following two cases (Case 1 and Case 2) in which the driving torque of the pressure roller 201 is likely to reach the upper limit.

[0064] Case 1 is a case where the fixing device 120 starts to operate after it has become accustomed to a low-temperature environment (10° C.). Case 2 is a case where the fixing device 120 starts to operate after the cumulative usage of the fixing device 120 has increased (for example, at the end of its life (after durability test)) (in case 2, the fixing device 120 starts to operate after it has become accustomed to a normal environment (23° C.)).

[0065] In Comparative Example 1, the rotation of the pressure roller 201 is switched to high speed rotation a predetermined time after the start of low speed rotation (here, the same timing as timing B in FIG. 7). That is, in Comparative Example 1, unlike this embodiment, the control of changing the timing of switching the rotation speed of the pressure roller 201 based on the detection result of the drive torque of the pressure roller 201 is not performed.

[0066] In Comparative Example 2, the rotation of the pressure roller 201 is switched to high speed rotation when the temperature of the heater 131 rises to or exceeds a predetermined threshold value ("temperature threshold value") Hth during low speed rotation. Here, the temperature threshold value Hth is set to a temperature equivalent to the temperature of the heater 131 at timing B in FIG. 7(a).

[0067] The configurations and operations of the image forming apparatus 100 and the fixing device 120 in the comparative examples 1 and 2 are substantially the same as those of the image forming apparatus 100 and the fixing device 120 in this embodiment, except for the points mentioned above.

[0068] The comparison results are shown in Table 1. In Table 1, the cases where the rotation of the pressure roller 201 was normally increased from a low speed to a high speed and the fixing process was performed without uneven gloss of the fixed image due to unstable rotation of the fixing film 130 were rated as "no problem." On the other hand, the cases where the rotation of the pressure roller 201 was not normally increased from a low speed to a high speed and the driving was stopped were rated as "driving stopped."

[0069] [Table 1]

[0070] In a normal temperature environment and before the cumulative usage has increased (for example, at the beginning of the lifespan (before endurance)), the rotation state of the pressure roller 201 changes over time as shown in FIG. 7 in this embodiment, comparative example 1, and comparative example 2, and no difference occurs. However, differences occur between Case 1 and Case 2. Case 1 and Case 2 will be described in detail below.

[0071] (Case 1: Low temperature environment) 9(a), (b), and (c) are graphs similar to those of FIG. 7(a), (b), and (c), respectively showing the time changes of the temperature of the heater 131, the rotation speed of the pressure roller 201, and the drive torque of the pressure roller 201 in case 1. In FIG. 9(b) and (c), the time changes are shown superimposed on each other, with a solid line for this embodiment, a dashed line for comparative example 1, and a dashed line for comparative example 2. In case 1, the time changes for comparative example 2 are almost overlapped with the time changes for this embodiment.

[0072] When the fixing device 120 starts to operate after it has become accustomed to a low-temperature environment (10° C.), the temperature rise of the heater 131 is slower, as shown by the solid line 406 in Fig. 9(a), than in a normal environment (23° C.), as shown by the solid line 401 in Fig. 7(a). This results in an unfavorable state in which the temperature of the heat-resistant grease is low and the load torque is high.

[0073] Therefore, as in Comparative Example 1, when the rotation of the pressure roller 201 is switched to high speed rotation at the same timing B as in the normal environment shown in Fig. 7, the driving torque Tb1 of the pressure roller 201 immediately before the switching becomes high at 6 kfg cm, as shown in Fig. 9(c). Accordingly, the peak torque of the driving torque of the pressure roller 201 when the rotation of the pressure roller 201 is switched to high speed rotation also becomes high. As a result, as shown by the dashed line 411 in Fig. 9(c), immediately after the rotation of the pressure roller 201 is switched to high speed rotation, the driving torque of the pressure roller 201 exceeds the upper limit Tlm, and the driving motor 150 stops.

[0074] On the other hand, in this embodiment, the rotation of the pressure roller 201 is switched to high speed rotation after the driving torque of the pressure roller 201 during low speed rotation falls below a predetermined torque threshold Tth. Therefore, as shown by the solid line 410 in FIG. 9C, the timing at which the rotation of the pressure roller 201 is switched to high speed rotation can be set to timing C, which is appropriately delayed from timing B in the normal environment shown in FIG. 7. As a result, the peak torque T22 of the driving torque of the pressure roller 201 immediately after the rotation of the pressure roller 201 is switched to high speed rotation does not exceed the upper limit value Tlm of the driving torque of the pressure roller 201. As a result, problems such as the drive motor 150 stopping do not occur.

[0075] In the case of Comparative Example 2, as shown in Fig. 9(a), as the temperature rise of the heater 131 becomes slower, the rotation of the pressure roller 201 is switched to high speed rotation at the timing when the temperature rises to or above the predetermined temperature threshold Hth. As a result, as shown in Fig. 9(c), similarly to the case of this embodiment, the rotation of the pressure roller 201 is switched to high speed rotation at timing C, and problems such as the drive motor 150 stopping do not occur.

[0076] (Case 2: After the cumulative usage of the fixing device increases) 10(a), (b), and (c) are graphs similar to those in FIG. 7(a), (b), and (c), respectively showing the time changes of the temperature of the heater 131, the rotation speed of the pressure roller 201, and the driving torque of the pressure roller 201 in case 2. In FIG. 10(b) and (c), the above-mentioned time changes are shown superimposed on each other, with a solid line for this embodiment, a dashed line for comparative example 1, and a dashed line for comparative example 2. In case 2, the above-mentioned time changes for comparative example 2 are almost superimposed on the above-mentioned time changes for comparative example 1. Also, the curve of temperature rise indicated by the solid line 413 in FIG. 10(a) is the same as the curve of temperature rise indicated by the solid line 401 in FIG. 7(a), which is the same under the normal environmental conditions (23° C.).

[0077] In case 2, the lubricating property of the heat-resistant grease is reduced due to an increase in the cumulative usage amount of the fixing device 120, so that the driving torque of the pressure roller 201 is generally high.

[0078] Therefore, as in Comparative Example 1, when the rotation of the pressure roller 201 is switched to high speed rotation at the same timing B as in Fig. 7, the driving torque Tb2 of the pressure roller 201 immediately before the switching becomes high. Accordingly, the peak torque of the driving torque of the pressure roller 201 when the rotation of the pressure roller 201 is switched to high speed rotation also becomes high. As a result, as shown by the dashed line 418 in Fig. 10(c), immediately after the rotation of the pressure roller 201 is switched to high speed rotation, the driving torque of the pressure roller 201 exceeds the upper limit value Tlm, and the driving motor 150 stops.

[0079] In the case of Comparative Example 2, similarly to Case 1, the rotation of the pressure roller 201 is switched to high speed rotation at the timing when the temperature of the heater 131 rises to or above the predetermined temperature threshold Hth. However, in Case 2, the temperature rise of the heater 131 shown by the solid line 413 in FIG. 10(a) is not delayed as much as the temperature rise of the heater 131 shown by the solid line 406 in FIG. 9(a). Therefore, the timing of switching the rotation of the pressure roller 201 to high speed rotation cannot be appropriately delayed. As a result, the temperature of the heater 131 reaches the temperature threshold Hth at approximately the same timing B as in Comparative Example 1, and the rotation of the pressure roller 201 is switched to high speed rotation, so that the drive motor 150 stops as in Comparative Example 1.

[0080] On the other hand, in this embodiment, the rotation of the pressure roller 201 is switched to high speed rotation at timing D, which is an appropriate timing when the drive torque of the pressure roller 201 during low speed rotation falls below a predetermined torque threshold Tth, as in case 1. As a result, the peak torque T32 of the drive torque of the pressure roller 201 when the rotation of the pressure roller 201 is switched to high speed rotation is also suppressed to be equal to or lower than the upper limit Tlm of the drive torque of the pressure roller 201. As a result, problems such as the drive motor 150 stopping do not occur.

[0081] Here, the viscosity and sliding friction of the heat-resistant grease may change due to various factors other than the temperature and the usage status of the device, and affect the load torque (driving resistance, driving load). Such factors include variations in the viscosity and amount of application of the heat-resistant grease itself, variations in the surface roughness (variation in configuration) of the sliding surface of the heater 131 with the fixing film 130 and the sliding surface of the fixing film 130 with the heater 131, and the like. Therefore, the operation of Comparative Example 1 and Comparative Example 2 cannot cope with this, and it is necessary to switch the rotation speed of the fixing film 130 based on the actual measurement value of the load torque (driving torque of the pressure roller 201) as in this embodiment.

[0082] Thus, in this embodiment, the heating device 120 has a rotatable first rotor 130, a rubbing member 131 that contacts the inner surface of the first rotor 130 and rubs the inner surface of the first rotor 130 as the first rotor 130 rotates, a rotatable second rotor 201 that contacts the outer surface of the first rotor 130 to form a nip portion Nt, and a drive unit 150 that generates a driving force to rotate the first rotor 130, and performs a heat treatment in which the recording material P is heated while being clamped and transported in the nip portion Nt. In this embodiment, the heating device 120 has a control unit 104 that controls the drive unit 150 to maintain the rotation speed of the first rotating body 130 at a first rotation speed Vl during a first period from when the first rotating body 130 starts rotating to when the heating process starts at the nip portion Nt, and to maintain the rotation speed of the first rotating body 130 at a second rotation speed Vh higher than the first rotation speed Vl during a second period after the first period, and an acquisition unit 105 that acquires torque information that correlates with the drive torque when the first rotating body 130 is rotated by the drive force generated by the drive unit 150. In this embodiment, the control unit 104 can change the timing at which the rotation speed of the first rotating body 130 starts changing from the first rotation speed Vl to the second rotation speed Vh based on the torque information acquired by the acquisition unit 105 during the first period. In particular, in this embodiment, when the torque value indicated by the torque information acquired by the acquisition unit 105 during the first period becomes equal to or less than a threshold value, the control unit 104 starts changing the first rotation speed Vl of the first rotating body 130 to the second rotation speed Vh. Here, in this embodiment, the drive unit 150 drives the second rotating body 201 to rotate, and the first rotating body 130 rotates in accordance with the rotation of the second rotating body 201. In this embodiment, the first rotating body 130 is a cylindrical film having flexibility. In this embodiment, the rubbing member 131 includes a heat generating member that applies heat to the recording material P through the first rotating body 130 at the nip portion Nt. In this embodiment, heat-resistant grease is interposed between the rubbing member 131 and the first rotating body 130. In this embodiment, the first rotation speed Vl is a rotation speed that is initially maintained after the first rotating body 130 starts rotating. In this embodiment, the second rotation speed is the rotation speed when the heat treatment is performed.In this embodiment, the driving unit 150 has a motor, and the acquiring unit 105 acquires torque information based on a driving current of the motor. In this embodiment, the heating device 120 is a fixing device that heats the recording material P carrying an image formed with toner at the nip portion Nt to fix the image onto the recording material. In this embodiment, an image forming apparatus 100 is provided that has an image forming unit 110 that forms an image on the recording material P with toner, and the heating device 120.

[0083] As described above, in this embodiment, when the fixing device 120 starts to be driven, the load torque of the fixing device 120 is first detected while the fixing film 130 is rotating at a low speed. Then, when the load torque of the fixing device 120 falls below a predetermined threshold, the rotation speed of the fixing film 130 is increased. This makes it possible to suppress the maximum value of the load torque of the fixing device 120 to be equal to or less than the allowable torque of the driving motor 150. As a result, it is possible to suppress the driving motor 150 from stopping, or the rotation of the fixing film 130 from becoming unstable, causing uneven gloss on the image. In other words, according to this embodiment, even if the fixing device 120 is in a low temperature state or the cumulative usage amount has increased, the rotation of the fixing film 130 can be switched from a low speed rotation to a high speed rotation at an optimal timing when the fixing device 120 starts to be driven. This makes it possible to suppress problems caused by the load torque at the time when the fixing device 150 starts to be driven, while employing a relatively small driving motor 150. As a result, it is possible to contribute to a longer life of the fixing device 120.

[0084] In this embodiment, the control is performed based on the torque value, but the present invention is not limited to this embodiment, and the control may be performed based on an index value (other parameters or measured values) correlated with the torque. For example, the control may be performed using the drive current value of the drive motor 150, a value calculated by calculation from the drive current value, or the like. Also, a dedicated torque sensor for detecting the torque may be provided.

[0085] In this embodiment, the rotation speed is switched between two stages, but the present invention is not limited to this embodiment. The rotation speed may be switched between two or more stages (e.g., three stages, four stages, etc.), and the same control as in this embodiment may be performed between some or all of the rotation speeds.

[0086] In this embodiment, the rotation of the pressure roller 201 is switched to high speed rotation when the driving torque of the pressure roller 201 becomes equal to or lower than a predetermined threshold value while the pressure roller 201 is rotating at a low speed. In contrast, the load torque state of the fixing device 120 may be predicted based on the detection result of the driving torque of the pressure roller 201 at a predetermined timing set in advance while the pressure roller 201 is rotating at a low speed, and the timing of switching the rotation speed may be determined. For example, if the detected driving torque of the pressure roller 201 is high, the timing of switching the rotation speed may be controlled to be delayed. Conversely, if the detected driving torque of the pressure roller 201 is low, the timing of switching the rotation speed may be controlled to be advanced.

[0087] [Example 2] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are given the same reference numerals as those of embodiment 1, and detailed explanations are omitted.

[0088] In this embodiment, the torque threshold value Tth for determining the timing of switching the rotation speed of the fixing film 130 is corrected based on information about the load torque of the fixing device 120 acquired at the start of driving the fixing device 120 before the previous time (hereinafter, simply referred to as "previous time"). Thereby, the timing of switching the rotation of the fixing film 130 from low speed rotation to high speed rotation is optimized. Here, as in the first embodiment, the driving torque of the pressure roller 201 is substituted for the load torque of the fixing device 120. Also, as in the first embodiment, the rotation speed of the pressure roller 201 corresponds to the rotation speed of the fixing film 130. Specifically, the peak torque (for example, T12 in FIG. 7C) of the driving torque of the pressure roller 201 when the rotation of the pressure roller 201 is switched from low speed rotation to high speed rotation is controlled so as to approach and stabilize the optimal peak torque target value Tc (FIG. 12B).

[0089] Fig. 11 is a flow chart for explaining a method for increasing the rotation speed of the fixing film 130 at the start of driving the fixing device 120 in this embodiment. Fig. 11 shows an outline of a procedure focusing mainly on the process related to the control of driving the fixing device 120 among the processes executed by the printer control unit 102 when the image forming apparatus 100 performs an image forming operation (printing operation).

[0090] When the printer control unit 102 receives an image formation start instruction, it detects the initial temperature Hini of the heater 131 using the thermistor 324 and stores it in the non-volatile memory 103 (S201). After that, the printer control unit 102 issues a command to the drive motor 150 to start driving the pressure roller 201 so that the rotation speed of the pressure roller 201 becomes a first rotation speed (38 rpm (low speed)) (S202). Furthermore, almost simultaneously with starting the drive of the pressure roller 201, the printer control unit 102 issues a command to a heater control unit (not shown) to start energizing the heater 131 and start heating the heater 131.

[0091] After that, the printer control unit 102 judges whether the previous torque threshold value Tthold and the previous peak torque T42old are stored in the non-volatile memory 103 (S203). Here, the previous torque threshold value Tthold is the torque threshold value Tth used to judge the timing of switching the rotation speed of the pressure roller 201 from the first rotation speed to the second rotation speed when the fixing device 120 was last driven. Also, the previous peak torque T42old is the peak torque of the driving torque of the pressure roller 201 when the rotation speed of the pressure roller 201 was switched to the second rotation speed (230 rpm (high speed)) when the fixing device 120 was last driven. For example, when the fixing device 120 (image forming device 100) is used for the first time, the previous torque threshold value Tthold and the previous peak torque T42old are not stored in the non-volatile memory 103. In this case, the printer control unit 102 sets the initial value of the torque threshold Tth previously stored in the non-volatile memory 103, for example, 3 kgf cm (≈0.29 N m) as the same as in the first embodiment, as the current torque threshold Tthnew (S204). Here, the current torque threshold Tthnew is the torque threshold Tth used to determine the timing to switch the rotation speed of the pressure roller 201 from the first rotation speed to the second rotation speed when the current drive of the fixing device 120 starts. On the other hand, when the previous torque threshold Tthold and the previous peak torque T42old are stored in the non-volatile memory 103, the printer control unit 102 obtains the current torque threshold Tthnew as follows (S205).

[0092] In this embodiment, the current torque threshold value Tthnew is calculated based on the previous torque threshold value Tthold, the previous peak torque T42old, and the peak torque target value Tc. Here, the peak torque target value Tc is a target value of the peak torque of the driving torque of the pressure roller 201 when the rotation speed of the pressure roller 201 is switched from the first rotation speed to the second rotation speed, and is set in advance.

[0093] When the previous peak torque T42old is higher than the peak torque target value Tc, in order to bring the peak torque closer to the target value, it is desirable to perform a correction to make the current torque threshold Tthnew smaller than the previous torque threshold Tthold. This makes it possible to control in the direction of suppressing the peak torque. Conversely, when the previous peak torque T42old is lower than the peak torque target value Tc, it is desirable to perform a correction to make the current torque threshold Tthnew larger than the previous torque threshold Tthold. This makes it possible to control in the direction of suppressing the startup time of the rotational speed. In this embodiment, the printer control unit 102 performs the above correction according to the following formula 1. Tthnew = Tthold + α × (Tc - T42old) (Formula 1)

[0094] α in the above formula 1 is a coefficient that determines the strength of the correction. In this embodiment, when T42old > Tc, α = 0.2, and when T42old < Tc, α = 0.1. That is, when the previous peak torque T42old is high, the above correction is performed relatively strongly so that the torque quickly decreases and approaches the peak torque target value Tc. On the other hand, when the previous peak torque T42old is low, priority is given to stability, and the above correction is performed relatively weakly so that the torque gradually increases and approaches the peak torque target value Tc. The information of the above formula 1 is stored in advance in the non-volatile memory 103 of the printer control unit 102.

[0095] After that, the printer control unit 102 detects the driving torque of the pressure roller 201 based on the detection result of the current detection unit 238 of the drive motor 150 in the same manner as in the first embodiment while maintaining the rotational speed of the pressure roller 201 substantially constant at the first rotational speed (S206). Further, each time the printer control unit 102 obtains the detection result (the moving average value described in the first embodiment) of the driving torque of the pressure roller 201, it determines whether the driving torque is less than or equal to the current torque threshold Tthnew set in S204 or S205 (S207). The processes of S206 and S207 are continued until the driving torque of the pressure roller 201 becomes less than or equal to the torque threshold Tthnew.

[0096] Then, when the printer control unit 102 determines in S207 that the torque threshold value Tthnew is equal to or lower than the torque threshold value Tthnew, it issues a command to the drive motor 150 to switch the rotation speed of the pressure roller 201 to the second rotation speed (230 rpm (high speed)) (S208). The printer control unit 102 also detects the current peak torque T42new, which is the peak torque of the drive torque of the pressure roller 201 when the rotation speed of the pressure roller 201 is switched from the first rotation speed to the second rotation speed (S209). After that, the printer control unit 102 waits for the completion of the image forming operation (fixing operation) for the specified number of sheets (S210). During this time, the rotation speed of the pressure roller 201 is maintained substantially constant at the second rotation speed.

[0097] Thereafter, when the image forming operation for the specified number of sheets is completed, the printer control unit 102 determines whether the temperature of the heater 131 detected in S201 is equal to or lower than a predetermined threshold value ("initial temperature threshold value") Hini pre-stored in the non-volatile memory 103 (S211).

[0098] If the printer control unit 102 determines in S211 that the temperature is equal to or lower than the initial temperature threshold value Hini, it proceeds as follows: That is, it replaces the current torque threshold value Tthnew and the current peak torque T42new with the previous torque threshold value Tthold and the previous peak torque T42old, and stores them in the non-volatile memory 103 (S212). This is because it can be determined that a reliable peak torque has been detected in this case.

[0099] On the other hand, when the printer control unit 102 determines in S211 that the temperature is higher than the initial temperature threshold Hini, it performs the following. That is, it discards the current torque threshold Tthnew and the current peak torque T42new, and leaves the previous torque threshold Tthold and the previous peak torque T42old stored in the non-volatile memory 103 as they are (S213). In this case, it is assumed that the viscosity of the heat-resistant grease is originally low, and the drive of the fixing device 120 is started from an advantageous state in terms of the load torque. For example, it is possible that the drive torque of the pressure roller 201 is below the torque threshold Tthnew when the drive torque of the pressure roller 201 starts to be detected while the pressure roller 201 is rotating at a low speed. In this case, the peak torque T42new detected when the rotation of the pressure roller 201 is switched to a high speed is lower than the original value (when the drive is started in a low temperature environment). Therefore, it is not suitable as data to be used for correcting the torque threshold Tth when the drive of the fixing device 120 is started next time. Therefore, in such a case, the previous torque threshold value Tthold and the previous peak torque T42old stored in the non-volatile memory 103 are used for the next correction.

[0100] Fig. 12 is a graph for explaining a method for increasing the rotation speed of the fixing film 130 at the start of driving the fixing device 120 of this embodiment. Fig. 12(a) shows an example of the change over time in the driving torque of the pressure roller 201 at the start of driving the previous fixing device 120 (solid line 420), and Fig. 12(b) shows an example of the change over time in the driving torque of the pressure roller 201 at the start of driving the current fixing device 120 (solid line 421).

[0101] 12A, when the fixing device 120 started driving the previous time, the pressure roller 201 was switched to high speed rotation at the timing E when the driving torque of the pressure roller 201 became equal to or less than the torque threshold value Tthold = 3 kgf·cm (≈ 0.29 N·m). The peak torque T42old at that time was 17 kgf·cm (≈ 1.67 N·m), which does not reach the upper limit value Tlm (20 kgf·cm ≈ 1.96 N·m) of the driving torque of the pressure roller 201, and there is no problem. However, the peak torque T42old is shifted to the higher side than the peak torque target value Tc (14 kgf·cm ≈ 1.37 N·m), and the margin 601 to the upper limit value Tlm of the driving torque of the pressure roller 201 is reduced.

[0102] 12B, when the fixing device 120 starts to be driven this time, the torque threshold Tth is corrected and lowered to the torque threshold Tthnew, and the timing at which the pressure roller 201 is switched to high speed rotation is delayed. As a result, the peak torque is lowered to the peak torque T42new, which is closer to the peak torque target value Tc. By repeating this control, the peak torque becomes almost equal to the peak torque target value Tc.

[0103] As described above, in this embodiment, the control unit 104 can correct the torque threshold value based on the torque information acquired by the acquisition unit 105 when the rotation speed of the first rotating body 130 was changed from the first rotation speed Vl to the second rotation speed Vh during the previous or previous rotation of the first rotating body 130. In this embodiment, the control unit 104 corrects the torque threshold value to be smaller when the torque value indicated by the torque information acquired when the rotation speed of the first rotating body 130 was changed during the previous or previous rotation of the first rotating body 130 is larger than a preset target value. In other words, the control unit 104 can correct the timing of starting to change the rotational speed of the first rotating body 130 from the first rotational speed Vl to the second rotational speed Vh with respect to the torque information acquired by the acquisition unit 105 during a first period during which the rotational speed of the first rotating body 130 is maintained at the first rotational speed Vl, based on the torque information acquired by the acquisition unit 105 when the rotational speed of the first rotating body 130 was changed from the first rotational speed Vl to the second rotational speed Vh during the rotation of the first rotating body 130 prior to the previous time. Also, in this embodiment, the torque information acquired when the rotational speed of the first rotating body 130 was changed during the rotation of the first rotating body 130 prior to the previous time indicates a peak torque value immediately after the rotational speed was changed.

[0104] In the first embodiment, a torque threshold value is set based on the torque detection result during low-speed rotation, predicting the peak torque when switching to high-speed rotation. However, the actual measured value of the peak torque may deviate from the ideal value. In this embodiment, learning control is performed to correct the torque threshold value based on the actual measured value of the peak torque in the past. Therefore, the peak torque when switching to high-speed rotation is more stable than in the first embodiment, and the margin to the upper torque limit value is stabilized at a predetermined value. As a result, according to this embodiment, it is possible to improve the reliability of the control compared to the first embodiment.

[0105] In addition, by applying this embodiment, the first peak torque (for example, T11 in FIG. 7C) when starting to drive the fixing device 120 can also be stabilized. In particular, when using a heat-resistant grease with a relatively high viscosity at low temperatures, the first peak torque (for example, T11 in FIG. 7C) may become as large as the second peak torque (for example, T12 in FIG. 7C). In such a case, similar to the case of the second peak torque in this embodiment, the first peak torque detected at the start of the previous drive is compared with a target value, and the rotation speed at the low speed rotation at the start of the current drive can be corrected according to the difference. For example, if the first peak torque is higher than the target value, the rotation speed at the low speed rotation can be controlled to be lowered to lower the first peak torque. Conversely, if the first peak torque is lower than the target value, the rotation speed at the low speed rotation can be controlled to be increased to increase the first peak torque. Alternatively, the time from the start of energization of the heater to the start of the drive motor at the start of the current drive can be corrected according to the difference between the first peak torque detected at the start of the previous drive and the target value. For example, if the first peak torque is higher than the target value, the time from when the heater is energized to when the drive motor is started can be lengthened so that the drive motor is started after the viscosity of the heat-resistant grease has decreased to a certain degree. Conversely, if the first peak torque is lower than the target value, the time from when the heater is energized to when the drive motor is started can be shortened.

[0106] In this embodiment, the peak torque is detected and correction is performed based on the detected value, but the torque in a steady state, which is more stable and easier to detect, may be detected and correction may be performed based on the detected value. For example, the torque may be detected near timing G in FIG. 12, when the rotation speed and the heater temperature are stable after switching to high speed rotation. For example, if the torque is high, it can be determined that the cumulative usage of the fixing device is increasing and the peak torque is likely to increase. In that case, the torque threshold value can be corrected to a lower value and the peak torque can be controlled to be suppressed. Conversely, if the torque is low, it can be determined that the cumulative usage of the fixing device is not increasing and the peak torque is not likely to increase. In that case, the torque threshold value can be corrected to a higher value and the rotation speed ramp-up time can be controlled to be suppressed.

[0107] [Example 3] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are given the same reference numerals as those of embodiment 1, and detailed explanations are omitted.

[0108] In the first embodiment, the timing of switching the rotation of the fixing film 130 from low speed rotation to high speed rotation is changed based on the detection result of the load torque of the fixing device 120 while the fixing film 130 is rotating at low speed. In contrast, in the present embodiment, the rising gradient of the rotation speed when the rotation of the fixing film 130 is increased from low speed rotation to high speed rotation is changed based on the detection result of the load torque of the fixing device 120 while the fixing film 130 is rotating at low speed. In particular, in the present embodiment, the load torque of the fixing device 120 is detected at a predetermined timing while the fixing film 130 is rotating at low speed, and the rising gradient of the rotation speed when the rotation of the fixing film 130 is increased from low speed rotation to high speed rotation is changed according to the detected load torque. Here, as in the first embodiment, the driving torque of the pressure roller 201 is substituted as the load torque of the fixing device 120. Also, as in the first embodiment, the rotation speed of the pressure roller 201 corresponds to the rotation speed of the fixing film 130.

[0109] FIG. 13 is a graph for explaining a method for increasing the rotation speed of the fixing film 130 at the start of driving the fixing device 120 of this embodiment. In FIG. 13, (a) shows the temperature of the heater 131, (b) shows the rotation speed of the pressure roller 201, and (c) shows the change over time of the driving torque of the pressure roller 201. The solid lines 423 and 426 in FIG. 13(b) and (c) show the case where the load torque of the fixing device 120 is a standard load torque, and the dashed lines 424 and 427 show the case where the cumulative usage amount of the fixing device 120 has increased and the load torque of the fixing device 120 has increased. Also, the dashed lines 425 and 428 in FIG. 13(b) and (c) show the case where the control of this embodiment is not executed in the same state as the dashed lines 424 and 427. Note that FIG. 13 is a graph when the fixing device 120 starts to drive from a state where the fixing device 120 is accustomed to the normal environment (23° C.).

[0110] The following description will be given taking as an example the dashed line graph in which the cumulative usage amount increases.

[0111] First, in this embodiment, at timing A in FIG. 13, driving of the pressure roller 201 is started so as to reach the first rotation speed Vl (38 rpm (low speed)). At almost the same time, power is started to be supplied to the heater 131, and the temperature of the heater 131 starts to rise. As the rotation speed of the pressure roller 201 increases and reaches the first rotation speed (38 rpm (low speed)), the driving torque of the pressure roller 201 reaches a peak torque T51 once and then decreases. Thereafter, the driving torque Th of the pressure roller 201 at a preset predetermined timing H (in this embodiment, 700 msec after the start of driving) is detected.

[0112] In this embodiment, the increase gradient of the rotation speed when the rotation of the pressure roller 201 is switched from low speed rotation to high speed rotation is determined according to the value of the detected driving torque Th. In this embodiment, the increase gradient of the rotation speed is determined by the following formula 2. β×γ×Tref / Th (Formula 2)

[0113] In the above formula 2, Tref is the standard driving torque of the pressure roller 201 at timing H when the load torque of the fixing device 120 is the standard load torque (3 kgf cm (≈0.29 N m) in this embodiment). In addition, in the above formula 2, β is the optimal rotation speed increase gradient at which the driving torque of the pressure roller 201 does not exceed the upper limit value Tlm even if the rotation speed of the pressure roller 201 is switched at timing H when the load torque of the fixing device 120 is the standard load torque. In this embodiment, this rotation speed increase gradient is 0.6 rpm / msec. In addition, γ in the above formula 2 is a coefficient that determines the strength of correction, and is set to 1 in this embodiment. The information of the above formula 2 is pre-stored in the non-volatile memory 103 of the printer control unit 102.

[0114] As shown by the dashed line 427 in Fig. 13(c), when the driving torque Th of the pressure roller 201 detected at timing H is higher than the standard driving torque Tref, it is expected that the peak torque will increase when the rotational speed is switched thereafter. In such a case, it is desirable to correct so as to reduce the rising gradient of the rotational speed (reduce the amount of increase in the rotational speed per unit time). Therefore, in this embodiment, Equation 2 is configured such that the rising gradient of the rotational speed is inversely proportional to the torque Th detected at timing H.

[0115] On the other hand, when the driving torque Th of the pressure roller 201 detected at timing H is lower than the standard driving torque Tref, it is not necessarily necessary to correct so as to further increase the rising gradient of the rotational speed (increase the amount of increase in the rotational speed per unit time). In this embodiment, when Th < Tref, β = 0.6 rpm / msec, which is the standard rising gradient of the rotational speed, is used. In this case, it may be corrected so as to increase the rising gradient of the rotational speed.

[0116] After the rising gradient of the rotational speed is determined as described above, the start of the startup of the rotation of the pressure roller 201 from the low-speed rotation to the high-speed rotation is started according to the rising gradient of the rotational speed.

[0117] In this embodiment, when the load torque of the fixing device 120 is high as shown by the dashed lines 424 and 427 in FIG. 13B and FIG. 13C, the driving torque Th of the pressure roller 201 detected at the timing H is high, for example, 5 kgf cm (≈0.49 N m). As a result, the rising gradient of the rotation speed when the rotation of the pressure roller 201 is increased from a low rotation speed to a high rotation speed is calculated to be 0.36 rpm / msec by the above formula 2. In this case, the rotation speed increases at a gentler gradient than the standard rotation speed rising gradient of 0.6 rpm / msec. As a result, the load torque of the fixing device 120 caused by the acceleration torque is reduced. In addition, the time until the rotation speed reaches a high speed is also delayed, and the temperature of the heater 131 increases more during that time, so the load torque caused by the viscosity of the heat-resistant grease is also reduced. In this embodiment, for example, in this case, the peak torque T52h of the driving torque of the pressure roller 201 when the rotation speed is switched is 14 kgf·cm (≈1.37 N·m), which can be suppressed to be equal to or less than the upper limit value Tlm.

[0118] 13B and 13C, even when the load torque of the fixing device 120 is high, if the rotation of the pressure roller 201 is increased from a low speed to a high speed at a standard increasing gradient of the rotation speed, the following occurs: In this case, the drive torque of the pressure roller 201 when the rotation speed is switched exceeds the upper limit Tlm before reaching the peak torque, and the drive motor 150 stops.

[0119] Fig. 14 is a flow chart for explaining a method for increasing the rotation speed of the fixing film 130 at the start of driving the fixing device 120 in this embodiment. Fig. 14 shows an outline of a procedure focusing mainly on the process related to the control of driving the fixing device 120 among the processes executed by the printer control unit 102 when the image forming apparatus 100 performs an image forming operation (printing operation).

[0120] When the printer control unit 102 receives the image formation start instruction, it issues a command to the drive motor 150 and starts driving the pressure roller 201 so that the rotation speed of the pressure roller 201 becomes the first rotation speed (38 rpm (low speed)) (S301). In addition, almost simultaneously with starting the drive of the pressure roller 201, the printer control unit 102 issues a command to the heater control unit (not shown) to start energizing the heater 131 and start heating the heater 131. Thereafter, the printer control unit 102 detects the drive torque of the pressure roller 201 based on the detection result of the current detection unit 238 of the drive motor 150 at a predetermined timing while the rotation speed of the pressure roller 201 is maintained substantially constant at the first rotation speed (S302). In addition, the printer control unit 102 determines the increase gradient of the rotation speed of the pressure roller 201 based on the detected drive torque of the pressure roller 201 by the above-mentioned formula 2 that is preset and stored in the non-volatile memory 103 (S304). As described above, in this embodiment, when the detected torque is smaller than the standard torque pre-stored in the non-volatile memory 103, the rising gradient of the standard rotation speed pre-stored in the non-volatile memory 103 is used. Then, the printer control unit 102 issues a command to the drive motor 150 so that the rising gradient of the rotation speed becomes the gradient determined in S303, and increases the rotation speed of the pressure roller 201 from the first rotation speed to the second rotation speed (230 rpm (high speed)) (S304). After that, the printer control unit 102 waits until the image forming operation (fixing process) for the designated number of sheets is completed (S305). During this time, the rotation speed of the pressure roller 201 is maintained approximately constant at the second rotation speed. After that, when the image forming operation (fixing process) for the designated number of sheets is completed, the printer control unit 102 performs a predetermined end process such as stopping the driving of the pressure roller 201, stopping the energization of the heater 131, and stopping the driving of the photosensitive drum 1 and the intermediate transfer belt 7.

[0121] In this manner, in this embodiment, the control unit 104 can change the rate of increase in the rotation speed when changing the rotation speed of the first rotating body 130 from the first rotation speed Vl to the second rotation speed Vh, based on the torque information acquired by the acquisition unit 105 during a first period in which the rotation speed of the first rotating body 130 is maintained at the first rotation speed Vl. In this embodiment, the control unit 104 changes the rate of increase such that the rate of increase when the torque value indicated by the torque information acquired by the acquisition unit 105 at a predetermined timing set in advance during the first period is a first value is smaller than the rate of increase when the torque value is a second value larger than a first value.

[0122] As described above, according to this embodiment, similarly to the first embodiment, the maximum value of the load torque of the fixing device 120 when the rotation of the fixing film 130 is increased from a low speed rotation to a high speed rotation can be suppressed to be equal to or less than the allowable torque of the driving motor 150. This makes it possible to suppress the driving motor 150 from stopping or the rotation of the fixing film 130 from becoming unstable, resulting in uneven gloss of the image.

[0123] In addition, when changing the rising gradient of the rotation speed, it is possible to perform learning control such as comparing the detection result of the previous peak torque with the peak torque target value and correcting the rising gradient of the current rotation speed, as in the second embodiment. For example, when the peak torque is higher than the peak torque target value, the rising gradient of the rotation speed can be corrected in a direction to decrease. Conversely, when the peak torque is lower than the peak torque target value, the rising gradient of the rotation speed can be corrected in a direction to increase. This stabilizes the peak torque and makes it easier to secure a margin to the allowable torque, thereby improving the reliability of the control. In addition, as explained in the second embodiment, the torque in a steady state, which is more stable and easier to detect, may be detected and correction may be performed based on the value. That is, the control unit 104 can correct the increasing speed for the torque information acquired by the acquisition unit 105 during the first period during which the rotation speed of the first rotating body 130 is maintained at the first rotation speed Vl, based on the torque information acquired by the acquisition unit 105 when the rotation speed of the first rotating body 130 is changed from the first rotation speed Vl to the second rotation speed Vh during the rotation of the first rotating body 130 before the previous time. When the torque value indicated by the torque information acquired when the rotation speed of the first rotating body 130 is changed during the rotation of the first rotating body 130 before the previous time is larger than a preset target value, the control unit 104 can correct the increasing speed to be smaller. At this time, the torque information acquired when the rotation speed of the first rotating body 130 is changed during the rotation of the first rotating body 130 before the previous time may indicate a peak torque value immediately after the rotation speed is changed.

[0124] In addition, in this embodiment, the rotation speed is increased linearly with a predetermined gradient, but the present invention is not limited to this embodiment. As a result, it is sufficient that the time until the rotation speed increases to the target value is changed. In other words, it is sufficient that the rate of increase of the rotation speed until the rotation speed increases to the target value is changed. Therefore, the gradient of the increase of the rotation speed from when the rotation speed starts to increase to when it reaches the target value may be a gradient that changes in a broken line (in stages) or a curved gradient that changes continuously with time.

[0125] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-mentioned embodiments.

[0126] For example, the heating device is typically a fixing device that fixes an unfixed toner image onto a recording material, but it may also be a device that reheats the recording material on which the toner image has been fixed to control gloss, for example.

[0127] In addition, in the above-described embodiment, the first rotating body (fixing film) rotates in conjunction with the rotation of the second rotating body (pressure roller), but the first rotating body may be rotationally driven by a driving unit separate from or common to the second rotating body.

[0128] In the above embodiment, the first rotating body is made of a cylindrical film having flexibility, but may be, for example, an endless belt (film) stretched over a plurality of tension rollers. In the above embodiment, the second rotating body is made of an elastic roller, but may be, for example, an endless belt stretched over a plurality of tension rollers. In this case, the second rotating body may be pressed against the first rotating body by a pressing member provided on the inner peripheral surface side of the second rotating body. [Explanation of symbols]

[0129] 100 Image forming device 102 Printer control unit 110 Image forming section 120 Fixing device 130 Fixing film 131 Ceramic heater 150 Drive motor 201 Pressure roller 220 Motor control unit P Recording material

Claims

1. a rotatable first rotor; a rubbing member that comes into contact with an inner surface of the first rotating body and rubs against the inner surface of the first rotating body as the first rotating body rotates; a rotatable second rotor that contacts an outer surface of the first rotor to form a nip portion; A drive unit that generates a drive force to rotate the first rotating body; having a heating device for performing a heat treatment for heating a recording material while nipping and conveying the recording material in the nip portion, a control unit that controls the drive unit to maintain a rotation speed of the first rotating body at a first rotation speed during a first period from when the first rotating body starts to rotate until when the heating process starts in the nip portion, and to maintain the rotation speed of the first rotating body at a second rotation speed higher than the first rotation speed during a second period after the first period; an acquisition unit that acquires torque information that correlates with a driving torque when the first rotating body is rotated by a driving force generated by the drive unit; having the control unit is capable of changing a timing to start changing a rotation speed of the first rotating body from the first rotation speed to the second rotation speed, based on the torque information acquired by the acquisition unit during the first period; the control unit starts changing the rotation speed of the first rotating body from the first rotation speed to the second rotation speed when a torque value indicated by the torque information acquired by the acquisition unit during the first period becomes equal to or less than a threshold value; and The control unit corrects the threshold value based on the torque information acquired by the acquisition unit when the rotation speed of the first rotating body was changed from the first rotation speed to the second rotation speed during the previous or previous rotation of the first rotating body.

2. The heating device according to claim 1, characterized in that the control unit corrects the threshold value to be smaller when the torque value indicated by the torque information acquired when the rotational speed of the first rotating body was changed during the rotation of the first rotating body prior to the previous time is larger than a predetermined target value.

3. The heating device described in claim 1, characterized in that the control unit is capable of correcting the timing for the torque information acquired by the acquisition unit during the first period based on the torque information acquired by the acquisition unit when the rotational speed of the first rotating body was changed from the first rotational speed to the second rotational speed during the previous or previous rotation of the first rotating body.

4. The heating device according to any one of claims 1 to 3, characterized in that the torque information acquired when the rotational speed of the first rotating body is changed during the previous or previous rotation of the first rotating body indicates a peak torque value immediately after the rotational speed is changed.

5. A rotatable first rotor; a rubbing member that comes into contact with an inner surface of the first rotating body and rubs against the inner surface of the first rotating body as the first rotating body rotates; a rotatable second rotor that contacts an outer surface of the first rotor to form a nip portion; A drive unit that generates a drive force to rotate the first rotating body; having a heating device for performing a heat treatment for heating a recording material while nipping and conveying the recording material in the nip portion, a control unit that controls the drive unit to maintain a rotation speed of the first rotating body at a first rotation speed during a first period from when the first rotating body starts to rotate until when the heating process starts in the nip portion, and to maintain the rotation speed of the first rotating body at a second rotation speed higher than the first rotation speed during a second period after the first period; an acquisition unit that acquires torque information that correlates with a driving torque when the first rotating body is rotated by a driving force generated by the drive unit; having the control unit is capable of changing an increase rate of the rotation speed when changing the rotation speed of the first rotating body from the first rotation speed to the second rotation speed, based on the torque information acquired by the acquisition unit during the first period; The control unit changes the rate of increase when the torque value indicated by the torque information acquired by the acquisition unit at a predetermined timing during the first period is a first value, so that the rate of increase is smaller when the torque value is a second value greater than the first value than when the torque value is a first value.

6. The heating device described in claim 5, characterized in that the control unit is capable of correcting the increase speed for the torque information acquired by the acquisition unit during the first period based on the torque information acquired by the acquisition unit when the rotational speed of the first rotating body was changed from the first rotational speed to the second rotational speed during the previous or previous rotation of the first rotating body.

7. The heating device according to claim 6, characterized in that the control unit corrects the rising speed to be smaller when the torque value indicated by the torque information acquired when the rotation speed of the first rotating body was changed during the rotation of the first rotating body prior to the previous time is larger than a preset target value.

8. The heating device according to claim 6 or 7, characterized in that the torque information acquired when the rotational speed of the first rotating body is changed during the previous or previous rotation of the first rotating body indicates a peak torque value immediately after the rotational speed is changed.

9. 9. The heating device according to claim 1, wherein the drive unit rotates the second rotating body, and the first rotating body rotates in accordance with the rotation of the second rotating body.

10. 10. The heating device according to claim 1, wherein the first rotating body is a cylindrical film having flexibility.

11. 11. The heating device according to claim 1, wherein the rubbing member includes a heat generating member that applies heat to the recording material via the first rotating body in the nip portion.

12. 12. The heating device according to claim 1, wherein heat-resistant grease is interposed between the friction member and the first rotating body.

13. 13. The heating device according to claim 1, wherein the first rotation speed is a rotation speed that is initially maintained after the first rotating body starts to rotate.

14. 14. The heating device according to claim 1, wherein the second rotation speed is a rotation speed when the heat treatment is performed.

15. 15. The heating device according to claim 1, wherein the drive unit has a motor, and the acquisition unit acquires the torque information based on a drive current of the motor.

16. 16. The heating device according to claim 1, which is a fixing device for fixing an image formed of toner on a recording material by heating the recording material at the nip portion.

17. an image forming unit that forms an image on a recording material with toner; A heating device according to any one of claims 1 to 16, An image forming apparatus comprising:

Images