JP2017097192A - Sheet conveying device and image forming apparatus including the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sheet conveying device that can dynamically determine the functional relationship between tangential velocities and peripheral velocities at a nip resulting from deformation of a soft roller at the nip, so as to improve separability of a sheet from the soft roller while maintaining the high quality of an image on the sheet.SOLUTION: A sheet conveying device (200) causes a pressure roller (32) to be driven by a heating roller (31) during a preparation period (PRT) to measure the peripheral velocities (VC1 and VC2) of the rollers. An estimation part (242) estimates, from the measurements, the functional relationship (α) between the peripheral velocities and the tangential velocities (VN1 and VN2) at a nip (NP). A determination part (243) selects a target value of a ratio of the tangential velocities at the nip between both rollers according to an environmental condition or an operation condition of a fixing part (30), and determines target values of the rotation speeds (N1 and N2) of the rollers during an operation period (DRT) by using the target value and the estimated functional relationship.SELECTED DRAWING: Figure 6

Description

  The present invention relates to a sheet conveyance technique, and more particularly to a nip conveyance.

  The “nip conveyance” is one of methods for conveying a sheet such as paper, a film, a cloth, and a thin plate using a rotating body such as a roller and a belt. In nip conveyance, a pair of rotating bodies whose rotation axes are parallel to each other rotate in opposite directions with the outer peripheral surfaces in contact with each other. Since the tangential speed is the same in the contact part (nip) of both rotating bodies, the sheet inserted from one side into this nip is pulled into this nip by the frictional force from the outer peripheral surface of both rotating bodies and sent to the opposite side. Is done. Nip transport is used in many systems that process sheets, including image forming devices such as printers and copiers, scanners, vending machines, automatic teller machines (ATMs), automatic ticket vending machines, automatic ticket gates, etc. Is done.

  In these systems, a sheet conveying apparatus generally uses a soft roller as a rotating body itself or a roller (referred to as a “pulley”) that drives or tensions a belt that is a rotating body. The “soft roller” refers to a roller made of a metal shaft such as aluminum or a hard resin shaft or cylinder shaft (referred to as “core metal”) covered with an elastic layer such as silicone rubber. . On the other hand, a roller including only a core metal and not including an elastic body layer is referred to as a “hard roller”. The soft roller is advantageous in comparison with the hard roller, for example, in that the noise is low, the sheet is hardly damaged, and a sheet having a large paper thickness or material such as an IC card can be conveyed. The soft roller also enlarges the nip width due to the high elasticity of the outer peripheral surface and easily adheres the outer peripheral surface to the sheet. Accordingly, the soft roller is often used in a fixing unit (for example, see Patent Documents 1 to 3) or a transfer unit (for example, see Patent Documents 4 and 5) of an electrophotographic image forming apparatus such as a laser printer. . The higher the adhesion between the rotating body and the sheet, the higher the effect of both fixing and transfer. Therefore, the soft roller is advantageous for improving the quality of the toner image. However, this high adhesion, on the contrary, tends to lead to the sheet being difficult to separate from the outer peripheral surface of the roller at the exit of the nip, leaving wrinkles due to curling when leaving the nip on the sheet. It can also cause paper jams.

  As a device for improving the ease of separation (separability) of a sheet from a roller, for example, the following technique is known. (1) A soft roller is combined with a hard roller to increase an angle formed with the outer peripheral surface of the hard roller when the sheet leaves the nip (hereinafter referred to as “release angle”). (2) The outer peripheral surface of the roller is covered with a release layer such as a fluororesin (see, for example, Patent Document 1). Furthermore, it is also effective to control the difference in peripheral speed between the soft roller and the other roller that forms a pair with the soft roller. Roughly, the greater the difference, the higher the sheet separation. However, if this difference is excessive, the sheet separability deteriorates. In addition, there is a risk that the image quality of the toner image is impaired. Therefore, maintaining the difference in peripheral speed within an appropriate range is important in order to achieve both high sheet separation and high image quality of the toner image.

  For example, in the technique disclosed in Patent Document 2, a shaft between a heating roller (also referred to as a “fixing roller”) and a pressure roller is connected by a gear so that one of the two rollers is used as the other driven roller. Designed. This gear ratio mechanically maintains a 1% to 7% difference between the rotational speeds of both rollers. In the technique disclosed in Patent Document 3, both the heating roller and the pressure roller are driven independently. In this case, the control target value of the rotation speed of each roller is determined so as to cancel out the warpage of the sheet caused by the difference in the toner amount between the front and back of the sheet, the difference between which is estimated from the image data. . In the technique disclosed in Patent Document 4, both the intermediate transfer belt and the transfer roller are driven independently. In this case, first, the intermediate transfer belt rotates between job processes, the transfer roller rotates as a driven roller, and the number of rotations of the transfer roller at that time is measured. Next, this measured value is set as a control target value for the number of rotations of the transfer roller in job processing. In the technique disclosed in Patent Document 5, both the intermediate transfer belt and the secondary transfer belt are driven independently. At this time, the circumferential speed of each belt is measured by an optical sensor, and the rotation speed of the driving roller of each belt is independently controlled based on these measured values.

Japanese Patent Laid-Open No. 07-092840 JP 2003-149969 A JP 2004-029563 A JP 2010-134061 A JP 2011-095368 A

Noriaki Okamoto, Kazuhiro Otani, Keiichiro Misawa, Kenji Yoshida, “Elucidation of Speed Characteristics and Control Mechanism of Paper Transport by Rubber Roller”, Transactions of the Japan Society of Mechanical Engineers (C), Vol. 67, No. 654 (2001-2)

  In recent nip transport technology, as the sheet transport speed increases and the transport interval (between sheets) decreases, it is important to control the difference in peripheral speed between pairs of rotating bodies with higher accuracy. Yes. However, none of the known techniques is sufficient for increasing the accuracy of this control. Actually, for example, in the techniques disclosed in Patent Documents 2 to 4, the peripheral speed of each rotating body is indirectly monitored through the rotational speed of the drive motor, and in the technique disclosed in Patent Document 5, the peripheral speed is measured by an optical sensor. Directly monitored. Any of these peripheral speeds of the rotating body to be monitored includes an error with respect to the tangential speed at the nip when the rotating body is a soft roller. This is because the tangential speed at the nip deviates from the peripheral speed accurately as the outer peripheral surface of the soft roller is deformed so as to be crushed by the nip strictly due to its high elasticity (for example, see Non-Patent Document 1). ). The effects on sheet separation and transfer / fixing image quality are all determined by the tangential speed at the nip. To improve these, the error between this tangential speed and the peripheral speed must be grasped with higher accuracy. is necessary. However, the functional relationship representing this error depends on, for example, changes in the surface shape of the soft roller due to deterioration such as wear, or changes in the radius or elastic modulus of the soft roller due to changes in environmental temperature or humidity. Change. Therefore, this functional relationship cannot be determined uniformly by experiments or simulations performed at the time of manufacturing the sheet conveying apparatus. Therefore, it is difficult to control the difference in the tangential speed at the nip between the soft roller and another rotating body with higher accuracy by using a known technique.

  The object of the present invention is to solve the above-mentioned problem, in particular, by dynamically determining the functional relationship between the tangential speed and the peripheral speed at the nip caused by the deformation of the soft roller at the nip. An object of the present invention is to provide a sheet conveying apparatus capable of improving the separation property of a sheet from a soft roller while maintaining high quality of an image formed on the sheet.

  A sheet conveying apparatus according to one aspect of the present invention is a sheet conveying apparatus that performs nip conveyance with respect to a sheet, rotation axes are parallel to each other, and outer peripheral surfaces contact each other to form a nip, at least one of which is The first roller and the second roller including the elastic layer, and a drive unit that rotates these rollers around each rotation axis at a target rotation number, and one of the first roller and the second roller is a drive roller. It is possible to switch between an adjustment mode in which the other roller is rotated as a driven roller and an actual operation mode in which both the first roller and the second roller are rotated independently of each other. A driving unit that rotates the one roller and the second roller at a target number of rotations, a measuring unit that measures each peripheral speed of the first roller and the second roller, and a driving unit Based on the peripheral speed measured by the measurement unit while rotating the roller and the second roller in the adjustment mode, the functional relationship between the peripheral speed of the first roller and the second roller and the tangential speed at the nip is estimated. While the drive unit rotates the first roller and the second roller in the working mode using the estimation unit and the functional relationship estimated by the estimation unit, the target value of each rotation number of the first roller and the second roller And a determination unit that determines and instructs the drive unit.

An estimation part may set the preparation period which should rotate a 1st roller and a 2nd roller in an adjustment mode to a drive part between the operation | movement periods when a 1st roller and a 2nd roller convey a sheet | seat. The determination unit may cause the driving unit to rotate the first roller and the second roller in the operation mode during the operation period.
The first roller may include an elastic layer, and the second roller may have a negligible outer peripheral surface with respect to the first roller. In this case, the estimation unit obtains a ratio between the circumferential speed of the first roller and the circumferential speed of the second roller from the circumferential speed measured by the measurement unit during the preparation period, and calculates the ratio at the circumferential speed of the first roller and the nip. The proportional coefficient may be estimated as a proportional coefficient between the tangential speed and the proportional coefficient may be used to determine the target value of the rotation speed.

  Both the first roller and the second roller may include an elastic layer. In this case, the estimation unit causes the driving unit to alternately rotate the first roller and the second roller as a driving roller and a driven roller during the preparation period, and from the peripheral speed measured by the measurement unit during the preparation period, the peripheral speed of the driving roller When the first roller and the second roller are each a driving roller, the ratio of the peripheral speed of the driven roller and the peripheral speed of the driven roller is obtained, and the ratio when the first roller is the driving roller and the second roller is the driving roller The geometrical mean of the ratio at the first ratio of the first proportional coefficient between the peripheral speed of the first roller and its tangential speed, and the second ratio between the peripheral speed of the second roller and its tangential speed at the nip. The ratio may be estimated as a ratio with the two proportional coefficients, and the determination unit may use the ratio between the first proportional coefficient and the second proportional coefficient to determine the target value of the rotation speed.

  This sheet conveying apparatus has a target value for a ratio between a tangential speed at the nip of the first roller and a tangential speed at the nip of the second roller, and a detection unit that detects an environmental condition or an operating condition of a system in which the sheet is conveyed. And a storage unit that stores a table in which the system is associated with the environmental conditions or operating conditions of the system. In this case, the determination unit searches the storage unit for a target value corresponding to the environmental condition or operation condition of the system detected by the detection unit, and uses this target value and the functional relationship estimated by the estimation unit to determine the operation period. The target value of the number of rotations of the first roller and the second roller may be determined from the tangential speed at the nip of at least one of the first roller and the second roller measured by the measuring unit.

The measurement unit periodically repeats the measurement of the respective peripheral speeds of the first roller and the second roller during the operation period, and the determination unit calculates the peripheral speed each time the measurement unit measures the peripheral speed during the operation period. The target value of the rotational speed may be updated using the functional relationship estimated by the estimation unit.
An image forming apparatus according to one aspect of the present invention includes an image forming unit that forms an image on a sheet, and the above-described sheet conveying device that forms an image or conveys the formed sheet. Yes.

  The image forming unit forms an image on the sheet with toner, and the first roller and the second roller apply heat and pressure to the sheet passed from the image forming unit to the nip, and the image forming unit The toner image formed on the sheet may be heat-fixed.

  The sheet conveying device according to the present invention first measures the circumferential speed of each roller by rotating the first roller as a driving roller and rotating the second roller as a driven roller during the preparation period, Estimate the functional relationship between the tangential velocity of Next, the sheet conveying apparatus determines a target value of each rotational speed of the first roller and the second roller during the operation period using this functional relationship. Therefore, even if the shape at the nip of either the first roller or the second roller changes according to the change in the environmental condition or operating condition of the system, the tangential speed at the nip and the peripheral speed at the corresponding nip The rotational speed of each roller changes according to the change in the functional relationship, and the difference in the tangential speed at the nip is maintained within an appropriate range. In this way, this sheet conveying apparatus can improve the separability of the sheet from these rollers while maintaining the high quality of the image formed on the sheet.

1 is a perspective view illustrating an appearance of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a front view schematically showing an internal structure of the image forming apparatus shown in FIG. 1. FIG. 3 is a schematic diagram illustrating a sheet conveyance path built in the image forming apparatus illustrated in FIG. 2. (A) is a schematic perspective view of a roller pair included in the fixing unit shown in FIG. 2 and their drive mechanism, and (b) is a straight line (b)-() of the roller pair shown in (a). It is sectional drawing along b). FIG. 2 is a block diagram of an electronic control system of the image forming apparatus shown in FIG. 1. FIG. 6 is a block diagram of an electronic control system of the sheet conveying apparatus shown in FIG. 5. It is a graph which shows the time-dependent change of the temperature of the heating roller which FIG. 4 shows. (A) is a schematic diagram showing the tangential speed at the nip of the roller pair shown in FIG. 4, (b) is the frictional force received by the front and back of the sheet sandwiched between the nip shown in (a), and their It is a schematic diagram which shows the shearing force which acts on a sheet | seat resulting from the difference between them. (A) is a parameter value indicating the environmental condition and operating condition of the fixing unit shown in FIG. 2 as an index value (point) of the difficulty of separating the sheet from the roller pair shown in FIG. 8 (a). (B) is a correspondence table between the sum of points indicated by (a) and the target value of the nip speed ratio. (A) is a schematic diagram showing the peripheral speed and the tangential speed at the nip in the case where the roller pair shown in FIG. 4 is a combination of a soft roller and a hard roller, and (b) shows (a). It is an enlarged view of a nip. (A) is a schematic diagram showing a peripheral speed and a tangential speed at the nip shown in FIG. 7 when the roller pair shown in FIG. 4 is a combination of a soft roller and a hard roller. (B) shows the peripheral speed and tangential speed at the nip when the roller pair shown in FIG. 4 is a combination of soft rollers and the heating roller is the driving roller and the pressure roller is the driven roller during the preparation period. (C) is a schematic diagram showing the peripheral speed and the tangential speed at the nip when the heating roller is a driven roller and the pressure roller is a driving roller in the same preparation period. . (A) is a schematic diagram showing the peripheral speed and the tangential speed at the nip in the operation period shown in FIG. 7 for the combination of the soft roller and the hard roller shown in (a) of FIG. 11, (b) FIG. 12 is a schematic diagram showing the peripheral speed during the operation period and the tangential speed at the nip for the combination of soft rollers shown in FIG. It is a flowchart of the process which estimates the internal speed ratio of a soft roller among the combinations of the soft roller and hard roller which (a) of FIG. 11 shows. It is a flowchart of the process which estimates the ratio of the internal speed ratio between the combinations of the soft rollers which (b) of FIG. 11 shows. It is a flowchart of rotation control of a roller pair in an operation period.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[Appearance of image forming apparatus]
FIG. 1 is a perspective view showing an external appearance of an image forming apparatus according to an embodiment of the present invention. The image forming apparatus 100 is a multi-function peripheral (MFP) that has a function of a scanner, a color copier, and a color laser printer. Referring to FIG. 1, an automatic document feeder (ADF) 110 is mounted on the upper surface of the casing of MFP 100 so as to be openable and closable. A scanner 120 is built in the upper part of the casing located directly below the ADF 110, a printer 130 is built in the lower part, and a plurality of paper feed cassettes 133 are attached to the bottom of the scanner 130 so that they can be pulled out. A gap DSP is opened between the scanner 120 and the printer 130, and a paper discharge tray 46 is disposed therein. A paper discharge port 42 is installed in the back of the gap DSP, and a sheet is discharged from there to a paper discharge tray 46. An operation panel 51 is attached to the front portion of the housing located beside the gap DSP. A touch panel is embedded in the front surface of the operation panel 51, and various mechanical push buttons are arranged around the touch panel. The touch panel displays a GUI screen such as an operation screen and an input screen for various information, and accepts a user's input operation through gadgets such as icons, virtual buttons, menus, and toolbars.

[Printer structure]
FIG. 2 is a front view schematically showing the structure of the printer 130. In FIG. 2, the elements of the printer 130 are depicted as if they were seen through the front of the housing. Referring to FIG. 2, the printer 130 is an electrophotographic color printer, that is, a color leather printer, and includes a feeding unit 10, an image forming unit 20, a fixing unit 30, and a paper discharge unit 40. The feeding unit 10 to the paper discharge unit 40 cooperate to function as an image forming unit of the MFP 100, and form an image on a sheet based on the image data.

  The feeding unit 10 feeds sheets SHT one by one from the sheet feeding cassettes 11a and 11b or the manual feed tray 16 to the image forming unit 20 using the transport roller groups 12P, 12F, 12R, 13, and 15. The material of the sheet SHT is, for example, paper or resin, the paper type is, for example, plain paper, high-quality paper, color paper, or coated paper, and the size is, for example, A3, A4, A5, or B4.

  The image forming unit 20 forms a toner image on the sheet SH2 sent from the feeding unit 10. Specifically, each of the four image forming units 21Y, 21M, 21C, and 21K first charges the surfaces of the photosensitive drums 25Y, 25M, 25C, and 25K, and uses the laser light from the optical scanning unit 26 to The surface is exposed with a pattern based on the image data. Thereby, an electrostatic latent image is created on the surface. Each image forming unit 21Y,... Then develops the electrostatic latent image with yellow (Y), magenta (M), cyan (C), and black (K) toners. The obtained four-color toner images are formed on the surface of the intermediate transfer belt 23 in order from the surface of the photosensitive drums 25Y,... By the electric field between the primary transfer rollers 22Y, 22M, 22C, 22K and the photosensitive drums 25Y,. Transferred to the same position above. Thus, one color toner image is formed at that position. This color toner image is further transferred to the surface of the sheet SH2 that has passed through the nip between the intermediate transfer belt 23 and the secondary transfer roller 24 through the nip between the two toner images 23 and 24. Thereafter, when a separation voltage is applied to the sheet SH2, the sheet SH2 is peeled off from the secondary transfer roller 24 and sent to the fixing unit 30.

  The fixing unit 30 heat-fixes the toner image on the sheet SH2 sent out from the image forming unit 20. Specifically, when the sheet SH2 is passed through the nip between the heating roller 31 and the pressure roller 32, the heating roller 31 applies heat of a built-in heater to the surface of the sheet SH2, and the pressure roller No. 32 applies pressure to the heated portion of the sheet SH2 and presses it against the heating roller 31. The toner image is fixed on the surface of the sheet SH <b> 2 by the heat from the heating roller 31 and the pressure from the pressure roller 32. Thereafter, the fixing unit 30 sends the sheet SH2 from the top along the guide plate 41 to the paper discharge port 42.

The paper discharge unit 40 discharges the sheet SH2 sent from the fixing unit 30 from the paper discharge port 42 by the paper discharge roller 43 and stores it in the paper discharge tray 46.
[Sheet Conveying Section]
As shown in FIG. 2, in the MFP 100, in addition to the feeding unit 10 and the paper discharge unit 40, an image forming unit such as a driving roller 23 </ b> R of the intermediate transfer belt 23, a secondary transfer roller 24, a heating roller 31, and a pressure roller 32. Part of the unit 20 and the fixing unit 30 functions as a sheet conveying unit.

  FIG. 3 is a schematic diagram illustrating a sheet conveyance path formed by the conveyance unit. Referring to FIG. 3, the transport path is configured as follows. First, the three paper feed paths from the paper feed cassettes 11a and 11b and the manual feed tray 16 are gathered into one path at the junction MP, and this path passes through the image forming unit 20 and the fixing unit 30 and is discharged. Connected to mouth 42. On this transport path, a plurality of optical sensors 1FS, 2FS, CS, TS, ES are installed in addition to the roller group 12P shown in FIG. Each optical sensor 1FS,... Detects a sheet passing through the installation location. Specifically, each optical sensor includes a light emitting unit and a light receiving unit. The light emitting unit emits light of a predetermined wavelength such as infrared rays, and the light receiving unit detects light of that wavelength. While one sheet passes through the installation location of each optical sensor, the sheet blocks the light emitted from the light emitting unit before the light receiving unit or reflects it toward the light receiving unit. Since the output of the light receiving unit changes according to the blocking or reflection of the emitted light, the sheet passing through the installation location of each optical sensor is detected. The conveyance units 10,... Notify these detections to a main control unit 60 (see FIG. 5) described later. In response to this notification, the main control unit 60 determines whether or not there is an abnormality in the conveyance timing due to a jam or the like.

  In the vicinity of the boundary between the feeding unit 10 and the image forming unit 20, a timing roller 27 and a timing sensor TS are installed downstream of the junction MP. The timing roller 27 is generally stopped, and the sheet moving from any of the sheet feeding cassettes 11a and 11b and the manual feed tray 16 is temporarily stopped on the spot. The timing roller 27 further starts to rotate at the timing indicated by the drive signal from the main control unit 60, and sends the stopped sheet to the image forming unit 20 at that timing. Thus, the sheet passes through the nip between the intermediate transfer belt 23 and the secondary transfer roller 24 simultaneously with the toner image on the surface of the intermediate transfer belt 23. Depending on whether or not there is a delay in the sheet passing timing indicated by the output of the timing sensor TS, whether or not those sheets have arrived at the timing roller 27 at a normal timing, and sent from the timing roller 27 at a normal timing. It is determined whether or not it has been done.

  Drive motor groups M1, M2, M3, M4, TM, MM, FM1, FM2, and DM for the transport roller group 12P,... Are installed around the transport path. Each of the drive motors M1,... Is, for example, a direct current brushless (BLDC) motor, and applies a rotational force to a roller to be driven through a transmission system such as a gear and a belt. In the vicinity of the paper feed cassettes 11a and 11b, the feed motors M1 and M2 rotate the feed roller groups 12P, 12F, and 12R. In the vicinity of the path from the second-stage sheet feeding cassette 11b, the longitudinal transport motor M3 rotates the longitudinal transport roller 13. In the vicinity of the path from the manual feed tray 16, the feed motor M <b> 4 rotates the feed roller 15. The timing motor TM rotates the timing roller 27 near the boundary between the feeding unit 10 and the image forming unit 20. In the image forming unit 20, the main motor MM rotates the driving roller 23 </ b> R of the intermediate transfer belt 23. In the fixing unit 30, the first motor FM1 rotates the heating roller 31, and the second motor FM2 rotates the pressure roller 32. In the paper discharge unit 40, the paper discharge motor DM rotates the paper discharge roller 43.

[Fixing part structure]
FIG. 4A is a schematic perspective view of a roller pair included in the fixing unit 30 shown in FIG. 2 and their drive mechanism, and FIG. 4B is a straight line of the roller pair shown in FIG. It is sectional drawing along b)-(b). Referring to FIG. 4, the fixing unit 30 includes a heating roller 31, a pressure roller 32, a first circumferential speed sensor 33, a second circumferential speed sensor 34, a heater 35, a first transmission mechanism 36, a second transmission mechanism 37, A first motor FM1 and a second motor FM2 are included.

  The heating roller 31 and the pressure roller 32 have rotational axes arranged in parallel with each other, and their outer peripheral surfaces are in contact with each other. As shown in FIG. 4B, the contact portion, that is, the nip NP is in a direction perpendicular to a virtual plane including the rotation axes of the rollers 31 and 32 (up and down direction in FIG. 4B). It spreads with a width of several mm. The sheet SH2 sent out from the image forming unit 20 is sandwiched in the nip NP.

  The heating roller 31 includes a cored bar 311, an elastic body layer 312, and a release layer 313. The cored bar 311 is a cylindrical member having a diameter of several tens mm, for example, and is mainly made of a metal such as aluminum or iron. The elastic body layer 312 is a layer mainly made of a highly elastic heat-resistant resin such as silicone rubber and covering the outside of the core metal 311 and has a thickness of, for example, less than 1 mm. The release layer 313 is a thin film such as a fluororesin that covers the outer side of the elastic body layer 312, and forms the outer peripheral surface of the heating roller 31. A heater 35 is installed in a hollow portion inside the cored bar 311. The heater 35 is a thin rod-shaped halogen lamp, and heats the metal core 311 from the inside by heat radiation accompanying light emission. Since this heat is transmitted to the outer peripheral surface through the elastic body layer 312 and the release layer 313, the temperature is maintained in the range of hundreds of degrees Celsius to several hundred degrees Celsius, for example. Due to this high temperature, the toner adhering to the surface of the sheet SH2 sandwiched between the nips NP melts. The release layer 313 plays a role of preventing a phenomenon (offset phenomenon) in which the melted toner is transferred from the surface of the sheet SH2 to the outer peripheral surface of the heating roller 31.

The pressure roller 32 includes a cored bar 321, an elastic body layer 322, and a release layer 323. The cored bar 321 is a cylindrical member having a diameter of several tens mm, for example, and is mainly made of a metal such as aluminum or iron. The elastic body layer 322 is a layer mainly made of a highly elastic heat-resistant resin such as silicone rubber that covers the outside of the cored bar 321. The thickness of this layer is several mm, for example, and is larger than the thickness of the elastic body layer 312 of the heating roller 31. The release layer 323 is a thin film such as a fluororesin that covers the outside of the elastic body layer 322, and forms the outer peripheral surface of the pressure roller 32. The pressure roller 32 receives a force toward the heating roller 31 from an urging member such as a spring or an electromagnet not shown in FIG. As a result, the portion of the pressure roller 32 positioned at the nip NP is pressed against the heating roller 31 with a pressure on the order of, for example, about 10 ⁶ Pa, so that the portion is deformed so as to be depressed as shown in FIG. Due to this pressure and deformation of the pressure roller 32, a sufficient amount of heat is transmitted from the heating roller 31 to the portion of the sheet SH2 sandwiched between the nips NP, so that the toner attached to the portion SH2 does not leave unevenness. It settles on the surface.

  The peripheral speed sensors 33 and 34 are, for example, laser Doppler velocimeters, which irradiate a measurement target with laser light to detect reflected light from the target, and detect the target from the difference in frequency generated between the irradiated light and the reflected light. Find the speed of. The first circumferential speed sensor 33 measures the tangential speed of the portion of the outer peripheral surface of the heating roller 31 that is out of the nip NP, and the second peripheral speed sensor 34 is out of the nip NP of the outer peripheral surface of the pressure roller 32. Measure the tangential velocity of the part. In the case of a rotating body, the “tangential speed” refers to the speed of one point in the tangential direction of the locus drawn by one point on the surface with the rotation. A tangential velocity at a portion of the locus forming a circular arc with a constant radius is particularly called a “circumferential velocity”.

  Motors FM1 and FM2 are, for example, BLDC motors. The shaft of the first motor FM <b> 1 is connected to one end of the first transmission mechanism 36, and the other end of the first transmission mechanism 36 is connected to one end of the core metal 311 of the heating roller 31. The shaft of the second motor FM <b> 2 is connected to one end of the second transmission mechanism 37, and the other end of the second transmission mechanism 37 is connected to one end of the metal core 321 of the pressure roller 32. Each transmission mechanism 36, 37 includes, for example, a clutch and a plurality of gears. The clutch connects the gears to the cores 311 and 321 of the rollers 31 and 32 in a separable manner. The gear maintains the rotation speeds of the rollers 31 and 32 to be connected at a predetermined ratio with respect to the rotation speeds of the shafts of the motors FM1 and FM2. The first motor FM1 transmits torque to the heating roller 31 through the first transmission mechanism 36 and rotates it. The second motor FM2 transmits torque to the pressure roller 32 through the second transmission mechanism 37 and rotates it.

  There are two modes of rotation of the heating roller 31 and the pressure roller 32: an adjustment mode and an operation mode. In the “adjustment mode”, one of the transmission mechanisms 36 and 37 separates the gear from the cores 311 and 321 of the rollers 31 and 32 by the clutch. Accordingly, one of the rollers 31 and 32 rotates as a driving roller by receiving torque from the motors FM1 and FM2, and the other rotates as a driven roller by receiving a frictional force at the nip NP. In the “actual operation mode”, both transmission mechanisms 36 and 37 continue to connect the gears to the cores 311 and 321 of the rollers 31 and 32 by the clutch. Thereby, both rollers 31 and 32 rotate mutually independently.

[Electronic control system of image forming apparatus]
FIG. 5 is a block diagram showing the configuration of the electronic control system of MFP 100. Referring to FIG. 5, in this system, in addition to the ADF 110, the scanner 120, and the printer 130, the operation unit 50 and the main control unit 60 are connected through a bus 90 so as to communicate with each other.
-Printer drive unit-
Each element 10, 20, 30, 40 of the printer 130 includes drive motor groups 10A, 20A, 30A, 40A and their drive units 10D, 20D, 30D, 40D. The drive motor groups 10A,... Are transport roller groups 12P, 12F, 12R, 13, 15, 23R, 24, 27, 31, 32, 43, the photosensitive drums 25Y,. The transfer roller 22Y is rotated. Each drive unit 10D is an electronic circuit mounted on a single printed circuit board built in MFP 100, and includes a control circuit and a drive circuit for the drive motor. The control circuit is a logic circuit such as a microprocessor (MPU / CPU), an application specific integrated circuit (ASIC), or a programmable integrated circuit (FPGA), and sets a target value for the rotational speed of the motor to drive the circuit. To instruct. Specifically, for example, the target value of the voltage applied to the motor is instructed to the drive circuit based on the actual rotational speed fed back from the motor. The drive circuit is an inverter and supplies power to the motor using a switching element such as a power transistor (FET).

  Further, the driving unit 10D,... Monitors the operation state of the element 10-40 of the printer 130 and the sheet conveyance state using various sensors, and controls the element 10-40 according to these states. When a failure is detected from any one of the elements 10,..., The drive unit 10D notifies the failure to the main control unit 60. These sensors include, in addition to the optical sensor 1FS shown in FIG. 3, a position sensor for detecting the position or posture of a movable member such as the photosensitive drum 25Y,. Includes a temperature sensor for detecting overheating of the actuator to be driven or its drive circuit, a sensor for detecting a paper out in the paper feed cassettes 11a, 11b, a sensor for detecting a toner shortage in the image forming unit 21Y,. It is. In particular, the drive unit 30D of the fixing unit 30 monitors the temperature of the heating roller 31 through a temperature sensor installed in the fixing unit 30, and adjusts the amount of heat generated by the heater 35 in accordance with the change in the temperature. Thereby, the temperature of the heating roller 31 is maintained at the target value.

-Operation part-
The operation unit 50 receives a job request and image data to be printed through a user operation or communication with an external electronic device, and transmits them to the main control unit 60. Referring to FIG. 5, the operation unit 50 includes an operation panel 51 and an external interface (I / F) 52. As shown in FIG. 1, the operation panel 51 includes a push button, a touch panel, and a display. The operation panel 51 displays a GUI screen such as an operation screen and an input screen for various parameters on a display. The operation panel 51 also identifies a push button pressed by the user or detects a position on the touch panel touched by the user, and transmits information related to the identification or detection to the main control unit 60 as operation information. In particular, when the input screen for a print job is displayed on the display, the operation panel 51 displays the size of the sheet to be printed, the type of paper, the orientation (separately between portrait and landscape), the number of copies, color / monochrome, Operation conditions related to printing such as image quality are received from the user, and items indicating these conditions are incorporated in the operation information. The external I / F 52 includes a USB port or a memory card slot, through which image data to be printed is directly read from an external storage device such as a USB memory or HDD, or an image captured by the scanner 120 into those storage devices. Export data. The external I / F 52 is also wired or wirelessly connected to an external network (not shown in FIG. 5), receives image data to be printed from other electronic devices on the network, or scans 120 to the electronic device. Send the image data captured in.

−Main control unit−
Main controller 60 is an integrated circuit mounted on a single printed circuit board built in MFP 100, and includes a CPU 61, a RAM 62, and a ROM 63. The CPU 61 is composed of one MPU, and implements various functions as a control subject for the other elements 50 and 110-130 by executing various firmware. For example, the CPU 61 displays a GUI screen such as an operation screen on the operation unit 50 to accept a user input operation. In response to this input operation, the CPU 61 selects one of operation modes such as an operation mode, a standby mode, and a sleep mode, and instructs each element 110, 120, and 130 to perform processing corresponding to the operation mode. The RAM 62 is a volatile semiconductor memory device such as a DRAM or an SRAM, and provides a work area when the CPU 61 executes firmware to the CPU 61 and stores image data to be printed received by the operation unit 50. In particular, the RAM 62 displays the operation conditions such as printing indicated by the operation information from the operation unit 50, the current operation mode selected by the CPU 61, and the environmental conditions such as temperature and humidity detected by the CPU 61 through the sensors in the MFP 100. Keep referenceable from the element. The ROM 63 is configured by a combination of a non-writable nonvolatile storage device and a rewritable nonvolatile storage device. The former stores firmware. The latter includes a semiconductor memory device such as EEPROM, flash memory, SSD, or HDD, and provides the CPU 61 with a storage area for environment variables and the like.

  When the operation unit 50 receives a print job from the user, the main control unit 60 first causes the operation unit 50 to transfer image data to be printed to the RAM 62. Next, the main control unit 60 designates the type of sheet to be fed and the timing of feeding in accordance with the printing conditions indicated by the job, and the toner to be formed in the image forming unit 20. Image data representing the image is provided. In particular, the main control unit 60 selects the target value of the sheet conveyance speed in accordance with the image quality, power consumption, or the sheet type or sheet thickness indicated by the printing conditions, and instructs each element 10 of the printer 130. For example, when the paper type to be printed is thick paper, the sheet conveyance speed is set lower than the value for plain paper. As a result, the power consumption of the drive motor group M1,... Is stabilized regardless of the weighing of the sheet to be conveyed. The main control unit 60 further notifies the drive unit 30D of the fixing unit 30 of the operation conditions for job processing. The operating conditions include, for example, the target temperature of the heating roller 31 to be maintained, the weighing of the sheet to be printed, the image density to be formed (black and white (BW) ratio), and the size of the margin to be left at the leading edge of the sheet.

[Electronic control system of sheet conveying device]
The driving unit 30D of the fixing unit 30 illustrated in FIG. 5 includes a heating roller 31, a pressure roller 32, peripheral speed sensors 33 and 34, a transmission mechanism, and motors FM1 and FM2 illustrated in FIG. The transport apparatus 200 is configured.
FIG. 6 is a block diagram of the electronic control system of the sheet conveying apparatus 200. Referring to FIG. 6, drive unit 30 </ b> D includes a control circuit 210, a first drive circuit 221, a second drive circuit 222, and a rotational speed actual measurement unit 230. The control circuit 210 is a logic circuit such as an MPU / CPU, ASIC, or FPGA, and sets a target value for each rotation speed of the motors FM1 and FM2 and instructs the drive circuits 221 and 222. The control circuit 210 also functions as a detection unit that detects environmental conditions and operation conditions of the fixing unit 30. The environmental conditions include, for example, the temperature of the heating roller 31 detected by the temperature sensor and the humidity sensor installed in the fixing unit 30 and the humidity of the atmosphere in the fixing unit 30. The operating conditions include the operating conditions notified from the main controller 60, that is, the target temperature of the heating roller 31, the weighing of the sheet to be printed, and the like. Each drive circuit 221 and 222 is an inverter, and supplies electric power to the motors FM1 and FM2 using a switching element such as an FET. The rotational speed actual measurement unit 230 monitors the output signals FGP of the encoders built in the motors FM1 and FM2, calculates the actual rotational speed Nms of each motor from these signals FGP, and feeds it back to the control circuit 210.

  With further reference to FIG. 6, the electronic control system of the sheet conveying apparatus 200 includes a measurement unit 241, an estimation unit 242, a determination unit 243, and a storage unit 244. The previous three 241 to 243 are logic circuits such as MPU / CPU, ASIC, or FPGA. The storage unit 244 is a non-writable nonvolatile storage device or a rewritable nonvolatile storage device such as an EEPROM, a flash memory, an SSD, or an HDD. These elements 241 to 244 are integrated on a single chip and mounted on the same printed circuit board as the driving unit 30D.

  The measurement unit 241 monitors the outputs of the peripheral speed sensors 33 and 34 and notifies the estimation unit 242 of the measured values of the peripheral speeds of the heating roller 31 and the pressure roller 32 represented by these outputs. The estimation unit 242 calculates the difference between the peripheral speed of the rollers 31 and 32 measured by the measurement unit 241 while the drive unit 30D rotates the rollers 31 and 32 in the adjustment mode, and the tangential speed at the nip NP. Estimate functional relationships. Using this functional relationship, the determination unit 243 determines a target value for each rotational speed and instructs the drive unit 30D while the drive unit 30D rotates both rollers 31 and 32 in the working mode. In particular, the determination unit 243 selects a target value for the ratio between the tangential speed of the heating roller 31 at the nip NP and the tangential speed of the pressure roller 32 at the nip NP (hereinafter referred to as “nip speed ratio”). The target value is used to determine the target value of the rotation speed of each of the rollers 31 and 32. The storage unit 244 stores a speed ratio table. The “speed ratio table” is a table that defines a correspondence relationship between the target value of the nip speed ratio between the rollers 31 and 32 and the environmental condition or operating condition of the fixing unit 30. From this speed ratio table, the determination unit 243 searches for a target value of the nip speed ratio corresponding to the environmental condition or operation condition of the fixing unit 30 detected by the control circuit 210.

[Preparation period and operation period]
The “operation period” refers to a period during which the heating roller 31 and the pressure roller 32 convey a sheet as the printing process is performed by the printer 130, for example. The “preparation period” refers to a period during which the estimation unit 242 should rotate the rollers 31 and 32 to the driving unit 30D in the adjustment mode between the operation periods.

  The estimation unit 242 actually starts the processing of the print job, that is, the printing process from the time when the temperature of the heating roller 31 reaches the target value in the operation mode according to the power-on of the MFP 100 or the shift to the operation mode. Set the period up to the point in time. Specifically, the estimation unit 242 monitors the temperature of the heating roller 31 and sets the start point of the preparation period at the time when the temperature reaches the target value in the operation mode.

The determination unit 243 sets the time when the print processing start instruction is received from the main control unit 60 as the start point of the operation period. During the operation period, the determination unit 243 repeatedly instructs the drive unit 30D to specify the target value of the rotational speed of both the rollers 31, 32, and rotates both the rollers 31, 32 in the working mode.
FIG. 7 is a graph showing the change over time of the temperature of the heating roller 31. Referring to FIG. 7, at power-on of MFP 100, drive unit 30D of fixing unit 30 activates heater 35 and starts increasing the temperature of heating roller 31 from initial value T0. This temperature then reaches the target value Ttg in the operation mode at the first time t1. A period during which the heating roller 31 continues to rise in temperature, such as a period WUT from when the MFP 100 is turned on until the first time t1, is referred to as a “warm-up period”. After the first time t1, the drive unit 30D increases or decreases the amount of heat generated by the heater 35 and maintains the temperature of the heating roller 31 at the target value Ttg in the operation mode unless the main control unit 60 instructs to shift to the standby mode. Keep doing. At the second time t2, the main control unit 60 instructs each element 10,... To start the printing process. In response to this instruction, the driving unit 30D starts sheet conveyance processing and toner image fixing processing by both rollers 31 and 32. In this case, the estimation unit 242 sets the period PRT from the first time t1 to the second time t2 as the preparation period, and the determination unit 243 sets the period DRT after the second time t2 as the operation period.

  Further referring to FIG. 7, at the third time t3, the main control unit 60 instructs each element 10,... To shift from the operation mode to the standby mode with the end of the print job. In response to this instruction, the driving unit 30D of the fixing unit 30 suppresses the amount of heat generated by the heater 35, drops the temperature of the heating roller 31 to the target value Twt in the standby mode, and maintains that value. Thus, in the standby mode period WTT, the temperature of the heating roller 31 is maintained at the target value Twt in the standby mode. Thereafter, at the fourth time t4, the main control unit 60 instructs each element 10,... To shift from the standby mode to the operation mode in response to the reception of the print job by the operation unit 50. In response to this instruction, the drive unit 30D increases the amount of heat generated by the heater 35 and raises the temperature of the heating roller 31 to the target value Ttg in the operation mode. In the warm-up period WUT after the fourth time t4, the temperature of the heating roller 31 reaches the target value Ttg in the operation mode again at the fifth time t5. After the fifth time t5, the drive unit 30D adjusts the amount of heat generated by the heater 35 to maintain the temperature of the heating roller 31 at the target value Ttg in the operation mode, unless the main control unit 60 is instructed to shift to the standby mode. to continue. At the sixth time t6, the main control unit 60 instructs each element 10,... To start the printing process. In response to this instruction, the driving unit 30D starts sheet conveyance processing and toner image fixing processing by both rollers 31 and 32. In this case, the estimation unit 242 sets the period PRT from the fifth time t5 to the sixth time t6 as the preparation period, and the determination unit 243 sets the period DRT after the sixth time t6 as the operation period.

[Rotation control of heating / pressure roller]
-Outline of control-
The sheet conveying apparatus 200 controls the number of rotations of the heating roller 31 and the pressure roller 32 as follows. First, the control circuit 210 detects an environmental condition or an operating condition of the fixing unit 30, and the determination unit 243 searches the target value of the nip speed ratio corresponding to this condition from the speed ratio table stored in the storage unit 244. . Next, the estimation unit 242 causes the driving unit 30D to rotate both rollers 31 and 32 in the adjustment mode in the preparation period PRT, and the measurement unit 241 measures the peripheral speed of each of the rollers 31 and 32. From the measured value at this time, the estimation unit 242 estimates a functional relationship between the peripheral speeds of the rollers 31 and 32 and the tangential speed at the nip NP. Subsequently, in the operation period DRT, using this functional relationship and the target value of the nip speed ratio, the determination unit 243 determines the rotational speed of each of the rollers 31 and 32 from the measured value of the circumferential speed of at least one of the rollers 31 and 32. A target value is determined and instructed to the drive unit 30D. In response to this instruction, the drive unit 30D rotates both rollers 31 and 32 in the actual operation mode, and controls the motors FM1 and FM2 so that the rotation speeds of the rollers 31 and 32 coincide with the target values.

During the operation period DRT, the determination unit 243 periodically repeats determination of the target value of the rotation speed of each of the rollers 31 and 32. Thereby, each rotation speed is sequentially corrected, and the nip speed ratio is maintained at the target value. As a result, high separation of the sheets from both rollers 31 and 32 is compatible with high image quality of the toner image.
-Reason why nip speed ratio is related to sheet separability and toner image quality-
8A is a schematic diagram showing tangential velocities VN1 and VN2 at the nip NP between the heating roller 31 and the pressure roller 32, and FIG. 8B is a front and back view of the sheet SH2 sandwiched between the nip NP. FIG. 6 is a schematic diagram illustrating frictional forces FF1 and FF2 received by the sheet and a shearing force SHF acting on the sheet SH2 due to a difference between them. Referring to FIG. 8, since there is a difference in the tangential velocities VN1 and VN2 at the nip NP between the rollers 31 and 32, the frictional force FF1 between the rollers 31 and 32 and the sheet SH2 is caused by this difference. , FF2 also has a difference. Due to this difference, a shear force SHF is generated in the sheet SH2 in a direction perpendicular to the surface. Depending on the direction of the shearing force SHF with respect to the outer peripheral surfaces of the rollers 31 and 32, one of the separation angles θ1 and θ2 between the sheet SH2 and each outer peripheral surface expands and the other narrows. Therefore, if the difference between the tangential velocities VN1 and VN2 at the nip NP is maintained within an appropriate range, any of the separation angles θ1 and θ2 is maintained at a certain level or more. Further, as the shearing force SHF is increased, a larger amount of wax exudes from the toner attached to the portion of the sheet SH2 sandwiched between the nips NP. As a result, roughly, the greater the difference between the tangential velocities VN1 and VN2 at the nip NP, the higher the separation of the sheet SH2.

  However, if the difference between the tangential velocities VN1 and VN2 at the nip NP is excessive, the image quality of the toner image may be impaired. This is because the difference in the frictional forces FF1 and FF2 between the front and back surfaces of the sheet SH2 due to this difference causes the toner to be displaced along the surface of the sheet SH2. If the amount of toner is excessive, the distortion of the toner image may increase to a level where it can be visually recognized. Further, since the outer peripheral surfaces of the rollers 31 and 32 directly rub against each other when the sheet is not sandwiched in the nip NP, an excessive difference between the tangential speeds VN1 and VN2 excessively increases the frictional force between the two outer peripheral surfaces. Thus, unevenness due to wear is increased on each outer peripheral surface. These irregularities can also cause distortion of the toner image.

  As described above, if the difference between the tangential velocities VN1 and VN2 at the nip NP between both the rollers 31 and 32 is large to some extent, the both rollers 31 and 32 maintain high sheet separation. Can be damaged. Therefore, in order to achieve both high sheet separation and high image quality of the toner image, it is important to maintain the difference between the tangential velocities VN1 and VN2 in the nip NP within an appropriate range. The value of the nip speed ratio when this difference is maintained in the proper range is set in the speed ratio table as a target value.

-Selection of target value of nip speed ratio-
The sheet separability between the heating roller 31 and the pressure roller 32 depends on the environmental conditions of the fixing unit 30 when processing a print job and the operating conditions in the processing. Therefore, the appropriate range of the difference to be secured between the tangential velocities VN1 and VN2 at the nip NP between the rollers 31 and 32 varies depending on these environmental conditions and operating conditions. The speed ratio table defines this appropriate range as follows, for example.

  FIG. 9A associates parameter values representing the environmental conditions and operating conditions of the fixing unit 30 with index values (points) of difficulty in separating sheets from the heating roller 31 and the pressure roller 32. It is a table | surface and (b) is a correspondence table | surface between the total of the point which (a) shows, and the target value of nip speed ratio. Hereinafter, the former is called “scoring table” and the latter is called “total table”. The speed ratio table is composed of a combination of these tables.

  Referring to FIG. 9A, the point is represented by an integer value from “1” to “6”, and the larger the value, the more difficult the sheet is separated from the roller. Points are associated with each parameter as follows, for example. (A) The higher the humidity of the atmosphere in the fixing unit 30 is, the softer the sheet contains more water, so it is more difficult to separate from the roller. Therefore, the higher the humidity, the larger the point is defined. (B) The smaller the weighed sheet, the softer the sheet, the harder it is to separate from the roller. Therefore, the smaller the sheet is weighed, the larger the point is defined. (C) The higher the image density (BW ratio) is, the more toner adheres to the sheet, so the viscosity of the surface of the sheet with respect to the surface of the roller is high, and the sheet is difficult to separate from the roller. Therefore, the higher the image density, the larger the point. (D) The narrower the margin at the front end of the sheet, that is, the region where the toner is not attached, the higher the viscosity of the front end of the sheet with respect to the surface of the roller. Therefore, the smaller the margin at the front end of the sheet, the larger the point is defined. (E) The shorter the preparation period PRT shown in FIG. 7, the shorter the elapsed time from the time points t1 and t5 when the temperature of the heating roller 31 reaches the target value Ttg in the operation mode by heating in the warm-up period WUT. The shorter this time is, the higher the possibility that the temperature distribution on the outer peripheral surface of the heating roller 31 is left uneven, and the outer peripheral surface has a lower effect of thermally fixing the toner to the sheet. The lower this effect is, the more unfixed toner remains on the surface of the sheet, so the viscosity of the surface of the sheet with respect to the surface of the roller is higher, and the sheet is less likely to be separated from the roller. Therefore, the shorter the preparation period, the larger the point.

  Referring to FIG. 9B, the target value of the nip speed ratio increases as the total value of points increases. This is due to the following reason. As understood from the rules of the scoring table shown in FIG. 9A, the higher the total value of the points, the more the conditions that make it difficult to separate the sheet from the roller. Therefore, by setting the target value of the nip speed ratio high, priority is given to the improvement of sheet separation over the maintenance of the high image quality of the toner image for both rollers 31 and 32.

  When the total value of the points is sufficiently small, such as “9” or less, the environmental condition and the operation condition of the fixing unit 30 indicate that the separation of the sheet from the roller is quite easy. In this case, the target value of the nip speed ratio is set to “0”. This means that the drive unit 30D only needs to keep one of the heating roller 31 and the pressure roller 31 driven. As a result, both the rollers 31 and 32 autonomously stabilize the nip speed ratio to a value substantially equal to “1”. As a result, the adhesion between the rollers 31 and 32 is improved, and the high image quality of the toner image is reliably maintained.

When the determination unit 243 selects the target value of the nip speed ratio, the determination unit 243 obtains the environmental conditions and operation conditions of the fixing unit 30 from the control circuit 210, and stores points corresponding to the values of parameters representing these conditions in the storage unit 244. Search from the scoring table stored by. For example, the environmental condition defines “humidity = 78%”, the operation conditions are “sheet weighing = 52 g / m ² ”, “image density (BW ratio) = 86%”, “sheet margin length = 2. 7 mm ”and“ elapsed time from the end of warm-up = 45 seconds ”, point“ 4 ”corresponds to“ humidity 70-80% ”as indicated by a circle in FIG. The point “5” is searched according to “weighing 50-60 g / m ² ”, the point “6” is searched according to “image density ≧ 80%”, and according to “margin 2-3 mm”. Point “5” is searched, and point “3” is searched according to “elapsed time 40 to 100 seconds”. The determination unit 243 adds up the points searched in this way, and searches the total value stored in the storage unit 244 for the target value of the nip speed ratio corresponding to the total value. For example, as indicated by a circle in FIG. 9B, when the total value of the points is 4 + 5 + 6 + 5 + 3 = 23, the target value “1.03” is searched.

-Estimating the functional relationship between peripheral speed and tangential speed at the nip-
The estimation unit 242 rotates the heating roller 31 and the pressure roller 32 during the preparation period PRT, and estimates the functional relationship between the peripheral speed and the tangential speeds VN1 and VN2 at the nip NP from the measured value of the peripheral speed at that time. To do. The details of this estimation process differ between the case where the heating roller 31 and the pressure roller 32 can be handled as a combination of a soft roller and a hard roller, and the case where it should be handled as a combination of soft rollers.

<Combination of soft roller and hard roller>
In the example shown in FIG. 4B, since both the heating roller 31 and the pressure roller 32 include elastic body layers 321, 322, they are strictly soft rollers. Therefore, since any outer peripheral surface is deformed so as to be crushed at the nip NP, the tangential speed at the nip NP deviates from the peripheral speed accurately. However, in this example, the elastic body layer 322 of the pressure roller 32 is thicker than the elastic body layer 312 of the heating roller 31. Therefore, the deformation at the nip NP is larger at the pressure roller 32 than at the heating roller 31, and the error between the tangential speed and the peripheral speed at the nip NP due to the deformation is also greater at the pressure roller 32 than at the heating roller 31. large.

  In addition to this example, in general, in the fixing unit, one of the heating roller and the pressure roller is a soft roller. The other may be a hard roller or a soft roller. When a soft roller and a hard roller are combined, deformation at the nip is negligible with the hard roller as compared with the soft roller, so that the tangential speed at the nip can be regarded as equal to the peripheral speed with the hard roller. Even when soft rollers are combined, the elasticity of the outer peripheral surface can be ignored by the other roller with respect to one roller, for example, because the difference in thickness of the elastic layer is sufficiently large. That is, the error between the tangential speed and the peripheral speed due to the deformation at the nip may be negligible with a low roller compared to a highly elastic roller.

Thus, the case where a heating roller and a pressure roller are the combination of a soft roller and a hard roller, or the case where it can be considered that is equivalent to the combination is assumed. Specifically, in the example of FIG. 4B, the pressure roller 32 is handled as a soft roller as it is, while the heating roller 31 is regarded as a hard roller.
FIG. 10 (a) shows that when the heating roller 31 is a hard roller and the pressure roller 32 is a soft roller, the rotation speeds N1 and N2, the peripheral speeds VC1 and VC2, and the nip NP of both rollers 31 and 32 Is a schematic diagram showing the tangential velocities VN1 and VN2, and (b) is an enlarged view of the nip NP shown in (a). Referring to FIG. 10, the outer peripheral surface of the heating roller 31 is not deformed at the nip NP. Therefore, the tangential speed VN1 at the nip NP is equal to the peripheral speed VC1: VN1 = VC1. On the other hand, the outer peripheral surface of the pressure roller 32 is crushed and dented by the outer peripheral surface of the heating roller 31. With this depression, the contour of the outer peripheral surface of the pressure roller 32 is deformed from the original arc shape LC indicated by the broken line in FIG. 10B to the actual curved shape LN indicated by the alternate long and short dash line. In any of the shapes LC and LN, the central angle φ at which these shapes are expected is constant. Therefore, if the rotation speed N2 of the pressure roller 32 is constant, the time required for one point on the outer peripheral surface of the pressure roller 32 to move along any of the shapes LC and LN is equal. On the other hand, the curve shape LN after deformation is generally longer than the arc shape LC before deformation: LC <LN. This means that the speed in the tangential direction of the curve shape LN, that is, the tangential speed VN2 at the nip NP is larger than the speed in the tangential direction of the arc shape LC, that is, the circumferential speed VC2: VN2> VC2.

  Thus, since the pressure roller 32 is a soft roller, an error occurs between the tangential speed VN2 and the peripheral speed VC2 at the nip NP. As a functional relationship representing this error, the estimation unit 242 considers that, for example, the tangential velocity VN at the nip is proportional to the peripheral velocity VC: VN = α × VC. This proportionality coefficient α, that is, the ratio α = VN / VC of the tangential speed VN at the nip with respect to the peripheral speed VC is hereinafter referred to as “internal speed ratio”.

  As described for FIG. 10, the hard roller does not deform at the nip, so the tangential speed VN at the nip is equal to the peripheral speed VC: VN = VC. Therefore, the internal speed ratio α of the hard roller is substantially equal to “1”: α≈1. On the other hand, since the soft roller is recessed at the nip, the tangential speed VN at the nip is larger than the peripheral speed VC: VN> VC. Therefore, the internal speed ratio α of the soft roller is higher than “1”: α> 1. In the example of FIG. 10, the internal speed ratio α1 of the heating roller 31 is substantially equal to “1”, and the internal speed ratio α2 of the pressure roller 32 is higher than “1”: α1≈1, α2> 1.

  Since the peripheral speed VC of the roller depends on the radius R of the roller, and the tangential speed at the nip depends on the degree of depression of the outer peripheral surface at the nip, the internal speed ratio α is a function of the radius R of the roller and the elastic modulus. . The radius R and the elastic modulus are, for example, the amount of water contained in the elastic body layer due to aging of the surface shape of the roller due to deterioration such as wear, expansion / contraction of the core metal due to change in environmental temperature, or change in environmental humidity. It fluctuates according to increase and decrease. Therefore, the estimation unit 242 reestimates the internal speed ratio α from the new measured value of the peripheral speed VC for each preparation period PRT as described below.

  First, the estimation unit 242 causes the driving unit 30D to rotate the heating roller 31 and the pressure roller 32 in the adjustment mode in the preparation period PRT. Specifically, for example, when the heating roller 31 is a driving roller and the pressure roller 32 is a driven roller, the driving unit 30D first uses a clutch for the first transmission mechanism 36 and a gear to the core metal 311 of the heating roller 31. The gear is separated from the cored bar 321 of the pressure roller 32 by connecting the second transmission mechanism 37 with a clutch. In this state, the drive unit 30D causes the first drive circuit 221 to supply power to the first motor FM1. Thereby, the heating roller 31 rotates by receiving torque from the first motor FM1. On the other hand, the pressure roller 32 rotates with the frictional force received from the heating roller 31 through the nip NP.

The drive unit 30D rotates both rollers 31 and 32, for example, several times. During this time, the measuring unit 241 measures the peripheral speeds of the rollers 31 and 32 with the peripheral speed sensors 33 and 34. The estimation unit 242 acquires this measurement value from the measurement unit 241 and sets the ratio thereof as the estimated value of the internal speed ratio α2 of the pressure roller 32. This estimated value is stored in the storage unit 244, for example.
FIG. 11A is a schematic diagram showing the peripheral speeds VC1 and VC2 between the heating roller 31 and the pressure roller 32 and the tangential speeds VN1 and VN2 at the nip NP during the preparation period PRT. In FIG. 11A, the heating roller 31 that is a driving roller is indicated by oblique lines, and the pressure roller 32 that is a driven roller is indicated by white. When the heating roller 31 rotates at the rotation speed ND due to the torque received from the first motor FM1, the pressure roller 32 rotates at the rotation speed NF due to the frictional force received from the heating roller 31 at the nip NP.

Here, the ratio γ of the tangential speed at the nip of the driven roller to the tangential speed at the nip of the driving roller is called “slip rate”. Since the driven roller is rotated by the frictional force received from the driving roller at the nip, the tangential speed at the nip of the driven roller does not exceed that of the driving roller. That is, the slip ratio γ is generally “1” or less: γ ≦ 1.
In the example of FIG. 11A, the pressure roller 32 is sufficiently in close contact with the heating roller 31 due to deformation at the nip NP. Therefore, it may be considered that the slip ratio γ is equal to “1” between the rollers 31 and 32: γ≈1. That is, the tangential velocities VN1 and VN2 at the nip NP are substantially equal between the rollers 31 and 32: VN1≈VN2. Since the heating roller 31 is a hard roller, the internal speed ratio α1 is equal to “1”. That is, the tangential speed VN1 at the nip NP is equal to the peripheral speed VC1: VN1 = α1 × VC1 = VC1. On the other hand, since the pressure roller 32 is a soft roller, the tangential speed VN2 at the nip NP is equal to the product of the internal speed ratio α2 and the peripheral speed VC2: VN2 = α2 × VC2. Therefore, the internal speed ratio α2 of the pressure roller 32 is represented by the ratio of the peripheral speeds VC1 and VC2 of both the rollers 31 and 32 as shown in the following equation (1):
α2 = VN2 / VC2 = VN1 / VC2 = VC1 / VC2. (1)
The estimation unit 242 substitutes the estimated value of the internal speed ratio α2 of the pressure roller 32 by substituting the measured value of the peripheral speeds of the rollers 31 and 32 in the preparation period PRT acquired from the measurement unit 241 into the right side of the equation (1). Calculate.

For example, when the measured value of the circumferential speed VC1 of the heating roller 31 is 150 mm / second and the measured value of the circumferential speed VC2 of the pressure roller 32 is 147 mm / second, the estimated value of the internal speed ratio α2 of the pressure roller 32 Is 150/147 = 1.02.
<Combination of soft rollers>
Since both the heating roller 31 and the pressure roller 32 shown in FIG. 4B are actually soft rollers, both outer peripheral surfaces are deformed so as to be crushed at the nip NP. When the elasticity of the outer peripheral surface between the two rollers 31 and 32 cannot be ignored, the error between the tangential speed and the peripheral speed accompanying the deformation at the nip NP cannot be ignored. Therefore, both rollers 31 and 32 are handled as a combination of soft rollers as they are.

  First, the estimation unit 242 causes the driving unit 30D to rotate the rollers 31 and 32 alternately as a driving roller and a driven roller in the preparation period PRT. Specifically, for example, the drive unit 30 </ b> D first connects the gear to the first transmission mechanism 36 by a clutch to the cored bar 311 of the heating roller 31, and the second transmission mechanism 37 connects the gear by the clutch to the pressure roller 32. The first driving circuit 221 is separated from the cored bar 321 and supplies power to the first motor FM1. Thereby, the heating roller 31 rotates by receiving torque from the first motor FM <b> 1, and the pressure roller 32 rotates by a frictional force with the heating roller 31. After rotating the rollers 31 and 32 several times, the drive unit 30D causes the first transmission mechanism 36 to separate the gear from the core metal 311 of the heating roller 31 with the clutch, and the second transmission mechanism 37 to shift the gear with the clutch. The second driving circuit 222 is connected to the metal core 321 of the pressure roller 32 to supply power to the second motor FM2. Thereby, the pressure roller 32 rotates in response to the torque from the second motor FM <b> 2, and the heating roller 31 rotates with a frictional force with the pressure roller 32.

The drive unit 30D rotates the rollers 31 and 32, for example, several times each using the rollers 31 and 32 as drive rollers. During this time, the measuring unit 241 measures the peripheral speeds of the rollers 31 and 32 with the peripheral speed sensors 33 and 34. The estimation unit 242 acquires these measurement values from the measurement unit 241.
Next, the estimation unit 242 calculates the ratio between the peripheral speed of the driving roller and the peripheral speed of the driven roller from the measured value of the peripheral speed of each of the rollers 31 and 32 in the preparation period PRT, when each of the rollers 31 and 32 is a driving roller. Ask for. The estimation unit 242 further sets the geometric mean of these ratios as an estimated value of the ratio of the internal speed ratio between the rollers 31 and 32. This estimated value is stored in the storage unit 244, for example.

FIG. 11B shows the circumferential speeds VC1 and VC2 and the tangential speeds VN1 and VN2 at the nip NP when the heating roller 31 is a driving roller and the pressure roller 32 is a driven roller in the preparation period PRT. It is a schematic diagram. In FIG. 11B, the heating roller 31 that is a driving roller is indicated by oblique lines, and the pressure roller 32 that is a driven roller is indicated by white lines. When the heating roller 31 rotates at the rotation speed ND due to the torque received from the first motor FM1, the pressure roller 32 rotates at the rotation speed NF due to the frictional force with the heating roller 31. At this time, the product of the driving roller = the driven roller with respect to the heating roller 31 = the slip rate γ of the pressure roller 32 and the tangential velocity VN1 at the nip NP of the heating roller 31 is the tangential velocity VN2 at the nip NP of the pressure roller 32. Equal: γ × VN1 = VN2. The tangential speeds VN1 and VN2 at the nips NP of the rollers 31 and 32 are equal to the product of the internal speed ratios α1 and α2 and the peripheral speeds VC1 and VC2: VN1 = α1 × VC1, VN2 = α2 × VC2. Therefore, the slip ratio γ is expressed by the following equation (2) using the internal speed ratios α1 and α2 of the rollers 31 and 32 and the peripheral speeds VC1 and VC2.
γ = VN2 / VN1 = (α2 × VC2) / (α1 × VC1)
= (Α2 / α1) × (VC2 / VC1). (2)
FIG. 11C shows the peripheral speeds VC1 ^* , VC2 ^* and the tangential speeds VN1 ^* , VN2 at the nip NP when the heating roller 31 is a driven roller and the pressure roller 32 is a driving roller in the preparation period PRT. It is a schematic diagram which shows ^* . In FIG. 11C, the pressure roller 32 that is a driving roller is indicated by oblique lines, and the heating roller 31 that is a driven roller is indicated by white. When the pressure roller 32 rotates at the rotational speed ND ^* by the torque received from the second motor FM2, the heating roller 31 rotates at the rotational speed NF ^* by the frictional force with the pressure roller 32. At this time, the tangential speed VN1 ^* of the heating roller 31 at the nip NP is the driving roller = the driven roller with respect to the pressure roller 32 = the slip rate γ ^* of the heating roller 31 and the tangential speed VN2 ^* of the pressure roller 32 at the nip NP. VN1 ^* = γ ^* × VN2 ^* . The tangential speeds VN1 ^* and VN2 ^* at the nips NP of the rollers 31 and 32 are equal to the product of the internal speed ratios α1 ^* and α2 ^* and the peripheral speeds VC1 ^* and VC2 ^* : VN1 ^* = α1 ^* × VC1 ^* , VN2 ^* = α2 ^* × VC2 ^* . Accordingly, the slip ratio γ ^* is expressed by the following equation (3) using the internal speed ratios α1 ^* and α2 ^{* of the} rollers 31 and 32 and the peripheral speeds VC1 ^* and VC2 ^* :
γ ^* = VN1 ^* / VN2 ^* = (α1 ^* × VC1 ^* ) / (α2 ^* × VC2 ^* )
= (Α1 ^* / α2 ^* ) × (VC1 ^* / VC2 ^* ). (3)
Here, it is assumed that the slip ratio between the rollers 31 and 32 and the internal speed ratio of the rollers 31 and 32 are constant regardless of which of the rollers 31 and 32 is the driving roller: γ = Γ ^* , α1 = α1 ^* , α2 = α2 ^* . Under this assumption, equations (2) and (3) yield equation (4) as follows:
(Α2 / α1) × (VC2 / VC1) = (α1 ^* / α2 ^* ) × (VC1 ^* / VC2 ^* )
= (Α1 / α2) × (VC1 ^* / VC2 ^* )
∴α1 / α2 = {(VC2 / VC1) × (VC2 ^* / VC1 ^* )} ^1/2 . (4)
As expressed by equation (4), the ratio α1 / α2 of the internal speed ratio between the rollers 31 and 32 is the ratio of the peripheral speed VC2 / VC1 and the pressure roller 32 when the heating roller 31 is a driving roller. Is equal to the geometric mean of the peripheral speed ratio VC2 ^* / VC1 ^* . The estimation unit 242 calculates the estimated value of the ratio α1 / α2 of the internal speed ratio by substituting the measured value of the peripheral speeds of the rollers 31 and 32 in the preparation period PRT acquired from the measurement unit 241 into the right side of the equation (4). To do.

For example, the circumferential speed VC1 of the heating roller 31 measured when the heating roller 31 is a driving roller is 150 mm / second, the circumferential speed VC2 of the pressure roller 32 is 144 mm / second, and the pressure roller 32 is driven. When the peripheral speed VC1 ^* of the heating roller 31 measured when it is a roller is 150 mm / second and the peripheral speed VC2 ^* of the pressure roller 32 is 150 mm / second, the ratio α1 / α2 of the internal speed ratio is estimated. The value is {(144/150) × (150/150)} ^1/2 = 0.98 = 1 / 1.02.

-Determination of target value of roller rotation speed-
The determination unit 243 uses the functional relationship estimated by the estimation unit 242, specifically, the internal speed ratio α of the soft roller or the ratio α1 / α2 of the internal speed ratio between the rollers 31 and 32, to determine the operation period DRT. From the measured values of the peripheral speeds of both the rollers 31, 32, the target value of the rotational speed of each of the rollers 31, 32 is determined. The details of this determination process differ depending on whether the heating roller 31 and the pressure roller 32 can be handled as a combination of a soft roller and a hard roller, and a case where they should be handled as a combination of soft rollers.

<Combination of soft roller and hard roller>
For example, when the pressure roller 32 is a soft roller, the estimation unit 242 estimates the internal speed ratio α2. The determining unit 243 uses the estimated value as follows to determine the target value of the rotational speed of each of the rollers 31 and 32 from the measured values of the peripheral speeds of both the rollers 31 and 32 during the operation period DRT.

FIG. 12A is a schematic diagram showing the peripheral speeds VC1 and VC2 between the heating roller 31 and the pressure roller 32 and the tangential speeds VN1 and VN2 at the nip NP during the operation period DRT. In FIG. 12A, since both rollers 31 and 32 are drive rollers, both are shown in white. The heating roller 31 rotates at the rotation speed N1 by the torque received from the first motor FM1, and the pressure roller 32 rotates at the rotation speed N2 by the torque received from the second motor FM2. Since the rollers 31 and 32 are driven independently of each other in this manner, the tangential velocities VN1 and VN2 at the nip NP are generally different between the rollers 31 and 32: VN1 ≠ VN2. That is, the nip speed ratio RVN is generally different from “1”: RVN = VN1 / VN2 ≠ 1. Since the heating roller 31 is a hard roller, the tangential speed VN1 at the nip NP is equal to the peripheral speed VC1: VN1 = VC1. On the other hand, since the pressure roller 32 is a soft roller, the tangential speed VN2 at the nip NP is equal to the product of the internal speed ratio α2 and the peripheral speed VC2: VN2 = α2 × VC2. Therefore, the nip speed ratio RVN is expressed by the following equation (5) using the internal speed ratio α2 of the pressure roller 32 and the peripheral speeds VC1 and VC2 of both rollers 31 and 32:
RVN = VN1 / VN2 = VC1 / VN2 = VC1 / (α2 × VC2). (5)
For example, when the rotation speed N1 of the heating roller 31 is fixed to a value corresponding to the sheet conveyance speed, the determination unit 243 obtains the target value RTG of the nip speed ratio RVN and the internal value obtained from the estimation unit 242 in Expression (5). By substituting the estimated value of the speed ratio α2 and the measured value of the peripheral speed VC1 of the heating roller 31 in the operation period DRT acquired from the measuring unit 241, the target value of the peripheral speed VC2 of the pressure roller 32 is calculated. That is, the target value of the peripheral speed VC2 is expressed by the following equation (6):
VC2 = VC1 / (α2 × RTG). (6)
For example, when the target value RTG of the nip speed ratio is “1.03”, the estimated value of the internal speed ratio α2 is “1.02,” and the measured value of the peripheral speed VC1 of the heating roller 31 is 135 mm / sec. The target value of the circumferential speed VC2 of the pressure roller 32 is calculated as 135 / (1.02 × 1.03) = 128 mm / second, whereas the measured value of the circumferential speed VC2 of the pressure roller 32 is 130 mm. / Sec, the determination unit 243 calculates the product of the ratio 128/130 of the target value to the measured value of the peripheral speed VC2 and the current rotation speed N2 of the pressure roller 32, and the target value of the rotation speed of the roller 32. To decide.

<Combination of soft rollers>
The estimation unit 242 estimates the ratio α1 / α2 of the internal speed ratio between the rollers 31 and 32. The determining unit 243 uses the estimated value as follows to determine the target value of the rotational speed of each of the rollers 31 and 32 from the measured values of the peripheral speeds of both the rollers 31 and 32 during the operation period DRT.
FIG. 12B is a schematic diagram showing the peripheral speeds VC1 and VC2 between the heating roller 31 and the pressure roller 32 and the tangential speeds VN1 and VN2 at the nip NP during the operation period DRT. In FIG. 12B, since both rollers 31 and 32 are drive rollers, both are shown in white. The heating roller 31 rotates at the rotation speed N1 by the torque received from the first motor FM1, and the pressure roller 32 rotates at the rotation speed N2 by the torque received from the second motor FM2. The tangential speeds VN1 and VN2 at the nips NP of the rollers 31 and 32 are equal to the product of the internal speed ratios α1 and α2 and the peripheral speeds VC1 and VC2: VN1 = α1 × VC1, VN2 = α2 × VC2. Accordingly, the nip speed ratio RVN = VN1 / VN2 is equal to the product of the internal speed ratio ratio α1 / α2 between the rollers 31 and 32 and the peripheral speed ratio VC1 / VC2 as represented by the following equation (7):
RVN = VN1 / VN2 = (α1 × VC1) / (α2 × VC2)
= (Α1 / α2) × (VC1 / VC2). (7)
For example, when the rotation speed N1 of the heating roller 31 is fixed to a value corresponding to the sheet conveyance speed, the determination unit 243 obtains the target value RTG of the nip speed ratio RVN and the internal value obtained from the estimation unit 242 in Expression (7). The target value of the peripheral speed VC2 of the pressure roller 32 is calculated by substituting the estimated value of the ratio α1 / α2 of the speed ratio and the measured value of the peripheral speed VC1 of the heating roller 31 during the operation period DRT obtained from the measuring unit 241. To do. That is, the target value of the peripheral speed VC2 is expressed by the following equation (8):
VC2 = (α1 / α2) × (VC1 / RTG). (8)
For example, the target value RTG of the nip speed ratio is “1.03”, the estimated value of the ratio α1 / α2 of the internal speed ratio is 1 / 1.02, and the measured value of the peripheral speed VC1 of the heating roller 31 is 135 mm. / Second, the target value of the peripheral speed VC2 of the pressure roller 32 is calculated as (1 / 1.02) × (135 / 1.03) = 128 mm / second. On the other hand, if the measured value of the circumferential speed VC2 of the pressure roller 32 is 130 mm / second, the determining unit 243 determines the ratio 128/130 of the target value to the measured value of the circumferential speed VC2 and the current pressure roller 32. The product of the rotational speed N2 is determined as a target value of the rotational speed of the roller 32.

  At the start of the operation period DRT, the drive unit 30D rotates the rollers 31 and 32 in the working mode, and controls each rotation number to a default value. That is, the drive unit 30D rotates the heating roller 31 at the default rotation speed Nd1 with the torque from the first motor FM1, and rotates the pressure roller 32 at the default rotation speed Nd2 with the torque from the second motor FM2. These default values Nd1 and Nd2 are set based on, for example, the sheet conveyance speed. When the rotation speeds N1 and N2 of the rollers 31 and 32 are stabilized to the default values Nd1 and Nd2, or when the time has passed to such a degree that the rollers 31 and 32 are regarded as stabilized, the measuring unit 241 has the circumferential speed VC1 of the rollers 31 and 32, Start measurement of VC2. The measured values of the peripheral speeds VC1 and VC2, the internal speed ratio α of the soft roller estimated by the estimation unit 242 or the ratio α1 / α2 of the internal speed ratio between the rollers 31 and 32, and the target value of the nip speed ratio Using the RTG as described above, the determination unit 243 determines target values for the rotation speeds N1 and N2 of the rollers 31 and 32 and instructs the drive unit 30D. The drive unit 30D controls the motors FM1 and FM2 so that the rotation speeds N1 and N2 of the rollers 31 and 32 coincide with these target values.

For example, in a state where the rotation speed N1 of the heating roller 31 is fixed to the default value Nd1 = 3200 rpm and the rotation speed N2 of the pressure roller 32 is the default value Nd2 = 3000 rpm, the target value of the peripheral speed VC2 of the pressure roller 32 = When 128 mm / second is calculated and the measured value of the peripheral speed VC2 = 130 mm / second is obtained, the target value of the rotation speed N2 of the pressure roller 32 is expressed by the following equation (9):
N2 = (target value of VC2) / (measured value of VC2) × Nd2
= 128/130 x 3000 rpm = 2954 rpm.
The drive unit 30D controls the second motor FM2 so that the rotation speed N2 of the pressure roller 32 matches the target value.

[Rotation control flow]
FIG. 13 is a flowchart of processing in which the estimation unit 242 estimates the internal speed ratio of the soft roller when the heating roller 31 and the pressure roller 32 are a combination of a soft roller and a hard roller. This process is started in the warm-up period WUT shown in FIG.

In step S101, the estimation unit 242 monitors the temperature of the heating roller 31 and confirms whether the temperature has reached the target value Ttg in the operation mode. If so, the process proceeds to step S102. If not, the process repeats step S101.
In step S102, since the temperature of the heating roller 31 has reached the target value Ttg, the estimation unit 242 causes the driving unit 30D to rotate one of the heating roller 31 and the pressure roller 32 as a driving roller, and the other is driven. Rotate as a roller. Thereafter, the process proceeds to step S103.

In step S <b> 103, the measurement unit 241 measures the peripheral speeds VC <b> 1 and VC <b> 2 of the heating roller 31 and the pressure roller 32 with the peripheral speed sensors 33 and 34. Thereafter, the process proceeds to step S104.
In step S104, the estimation unit 242 acquires the measurement values VC1 and VC2 of the circumferential speeds of the rollers 31 and 32 from the measurement unit 241, and sets the ratios to the estimated value of the internal speed ratio α of the soft roller according to the equation (1). Calculate. Thereafter, the process ends.

FIG. 14 is a flowchart of processing in which the estimation unit 242 estimates the ratio of the internal speed ratio between the two rollers when the heating roller 31 and the pressure roller 32 are a combination of soft rollers. This process differs from the process shown in FIG. 13 in that it includes steps S102 ^* , S103 ^* , and S104 ^* instead of step S104.
In step S101, the estimation unit 242 monitors the temperature of the heating roller 31 and confirms whether the temperature has reached the target value Ttg in the operation mode. If so, the process proceeds to step S102. If not, the process repeats step S101.

In step S102, since the temperature of the heating roller 31 has reached the target value Ttg, the estimation unit 242 causes the driving unit 30D to rotate one of the heating roller 31 and the pressure roller 32 as a driving roller, and the other is driven. Rotate as a roller. Thereafter, the process proceeds to step S103.
In step S <b> 103, the measurement unit 241 measures the peripheral speeds VC <b> 1 and VC <b> 2 of the heating roller 31 and the pressure roller 32 with the peripheral speed sensors 33 and 34. Thereafter, the process proceeds to step S102 ^* .

In step S102 ^* , the estimation unit 242 causes the driving unit 30D to rotate the one that was the driven roller in step S102 as a driving roller, and in step S102, rotate the one that was the driving roller as a driven roller. Thereafter, the process proceeds to step S103 ^* .
In step S <b ^> 103 ^* , the measurement unit 241 measures the peripheral speeds VC <b ^> 1 ^* and VC <b ^> 2 ^* of the heating roller 31 and the pressure roller 32 with the peripheral speed sensors 33 and 34. Thereafter, the process proceeds to step S104 ^* .

In step S104 ^* , the estimation unit 242 obtains the measured values VC1, VC2, VC1 ^* , VC2 ^* of the circumferential speeds of the rollers 31, 32 from the measurement unit 241, and between the rollers 31, 32 according to the equation (5). The geometric average of the ratios of the peripheral speeds is calculated as an estimated value of the ratio α1 / α2 of the internal speed ratio. Thereafter, the process ends.
FIG. 15 is a flowchart of rotation control for the heating roller 31 and the pressure roller 32 during the operation period. This process is started when the control circuit 210 detects an environmental condition or an operating condition of the fixing unit 30.

In step S <b> 111, the determination unit 243 acquires the environmental condition and the operation condition of the fixing unit 30 from the control circuit 210. Thereafter, the process proceeds to step S112.
In step S <b> 112, the determination unit 243 obtains a point representing difficulty in separating the sheet from the heating roller 31 and the pressure roller 32 based on the condition acquired in step S <b> 111. Specifically, the determination unit 243 searches the score corresponding to each value of the parameter specified by the condition from the scoring table stored in the storage unit 244 (see FIG. 9A). Thereafter, the process proceeds to step S113.

In step S113, the determining unit 243 sums up the points searched in step S112, and the target table RTG of the nip speed ratio RVN = VN1 / VN2 corresponding to the total value is stored in the storage table 244 (FIG. 9). (Refer to (b)). Thereafter, the process proceeds to step S114.
In step S114, the driving unit 30D causes the rollers 31 and 32 to start rotating. Thereafter, the process proceeds to step S115.

In step S115, the measurement unit 241 measures the peripheral speeds of the rollers 31 and 32. Thereafter, the process proceeds to step S116.
In step S116, the determination unit 243 acquires the estimated value of the internal speed ratio α of the soft roller or the ratio α1 / α2 of the internal speed ratio from the estimation unit 242, and acquires the measurement value in step S115 from the measurement unit 241. . By substituting these values and the target value RTG of the nip speed ratio searched in step S113 into the formula (5) or the formula (7), the determination unit 243 calculates the target values of the peripheral speeds of the rollers 31 and 32. To do. Thereafter, the process proceeds to step S117.

In step S117, the determining unit 243 determines a target value for the number of rotations of each of the rollers 31 and 32 based on the measured value of the peripheral speed in step S115 and the target value of the peripheral speed calculated in step S116, and the driving unit 30D. To instruct. The drive unit 30D matches the rotation speed of each of the rollers 31 and 32 with the instructed target value. Thereafter, the process proceeds to step S118.
In step S118, the control circuit 210 checks whether or not the main control unit 60 has instructed the end of the print job processing. If instructed, the process ends. If not instructed, the process is repeated from step S115.

[Advantages of the embodiment]
In the MFP 100 according to the embodiment of the present invention, as described above, the sheet conveying apparatus 200 rotates one of the heating roller 31 and the pressure roller 32 as a driving roller and rotates the other as a driven roller during the preparation period PRT. , And 32, the functional relationship between the peripheral speed and the tangential speed at the nip NP is estimated. Using this functional relationship and the target value of the nip speed ratio, the sheet conveying apparatus 200 uses the measured values of the peripheral speeds of the rollers 31 and 32 during the operation period DRT to set the target values of the rotation speeds of the rollers 31 and 32. Is determined so that the nip speed ratio matches the target value.

  In particular, the sheet conveying apparatus 200 determines the internal speed ratio α of the soft roller that defines the above functional relationship or the ratio α1 / α2 of the internal speed ratio between the rollers 31 and 32 for each of the rollers 31 and 32 for each preparation period PRT. Re-estimate from the new measured value of the peripheral speed. Therefore, even if the shape of the rollers 31 and 32 at the nip NP changes in accordance with changes in the environmental conditions or operating conditions of the fixing unit 30, each of the rollers 31 and 32 follows the change in the internal speed ratio ratio α1 / α2. The number of rotations changes to maintain the nip speed ratio at the target value. In this way, the sheet conveying apparatus 200 can improve the separability of the sheet from the rollers 31 and 32 while maintaining the high quality of the toner image.

[Modification]
(A) The image forming apparatus 100 according to the above embodiment of the present invention is an MFP. In addition, the image forming apparatus according to the embodiment of the present invention may be any single function machine such as a printer of another type such as a laser printer or an inkjet, a copier, a scanner, or a FAX. The image forming apparatus also has a mechanism for double-sided printing, such as a reversing roller capable of rotating both forward and reverse to switch back a sheet that has undergone fixing processing, and a reversing path for turning the sheet back from the fixing unit 30 and returning it to the image forming unit 20. May be included.

  (B) The heating roller 31 and the pressure roller 32 are both soft rollers including elastic layers 321 and 322, and the elastic layer 322 of the pressure roller 32 is more than the elastic layer 312 of the heating roller 31. thick. In addition, either the heating roller or the pressure roller may be a hard roller that does not include an elastic body layer, and the elastic body layer of the heating roller may be thicker than the elastic body layer of the pressure roller.

  (C) The circumferential speed sensors 33 and 34 are both laser Doppler velocimeters. In addition, the peripheral speed sensor may be another non-contact type speed sensor such as a sensor that continuously captures images to be detected by an image sensor and calculates the speed of the detection target from a change between successive images. The peripheral speed sensor may also be a contact type speed sensor such as a rotary encoder that contacts the outer peripheral surface of each roller and rotates by frictional force with the outer peripheral surface.

  (D) The heater 35 built in the heating roller 31 is a halogen lamp. In addition, the heater may be an induction heating device. The fixing unit 30 may also include a combination of a fixing belt that contacts the sheet and a device that heats the fixing belt, instead of the combination of the heating roller 31 and the heater 35. In this case, the rotation control according to the above-described embodiment may be applied to nip conveyance performed by a combination of a pulley and a pressure roller that drives or tensions the fixing belt.

(E) Both transmission mechanisms 36 and 37 use gears to connect the shafts of the motors FM1 and FM2 to the cores 311 and 321 of the rollers 31 and 32. In addition, the transmission mechanism may use a combination of a belt and a pulley.
(F) In the adjustment mode, the driving unit 30D of the fixing unit 30 causes one of the heating roller 31 and the pressure roller 32 to be separated from the cored bars 321 and 322 by the clutch in any of the transmission mechanisms 36 and 37. Is driven by the other. In addition, for example, if the torque when rotating the motors FM1 and FM2 separated from the power source is sufficiently small, the drive unit 30D does not cause any of the transmission mechanisms 36 and 37 to disengage the clutch. One of the pressure roller 32 and the other may be driven.

  (G) In the combination of the soft roller and the hard roller shown in FIG. 11A, it is considered that the slip ratio γ between both rollers is equal to “1”. In addition, the slip ratio γ between the two rollers may be set to be similar to a value obtained by an experiment or simulation during manufacturing. In this case, the product of the slip ratio γ of the pressure roller 32 with respect to the heating roller 31 and the tangential velocity VN1 at the nip NP of the heating roller 31 is equal to the tangential velocity VN2 at the nip NP of the pressure roller 32: γ × VN1 = VN2. Therefore, the product of the set value of the slip ratio γ and the right side of the equation (1) may be estimated as the internal speed ratio α2 of the pressure roller 32.

In the combination of the soft rollers shown in FIGS. 11B and 11C, the slip ratio γ when the heating roller 31 is a drive roller, and the slip ratio γ ^* when the pressure roller 32 is a drive roller. Equation (4) is derived under the approximation that are equal to each other. In addition, the slip ratios γ and γ ^* may be set to values obtained by experiments or simulations during manufacturing. In this case, each set value and the measured values VC1, VC2, VC1 ^* , VC2 ^* of the peripheral speeds of the rollers 31, 32 are substituted into the formulas (2), (3), and the interior between the rollers 31, 32 is substituted. The ratios α1 / α2 and α1 ^* / α2 ^* of the speed ratio may be obtained, and the geometric average or arithmetic average thereof may be calculated as the estimated value of the ratio of the internal speed ratio.

  (H) The rotation control according to the above embodiment targets the heating roller 31 and the pressure roller 32 of the fixing unit 30. In addition, the rotation control may be performed on all roller pairs that perform nip conveyance, such as the conveyance roller pairs 12F and 12R of the feeding unit 10 and the paper discharge roller 43.

  The present invention relates to the control of a rotating body used for nip conveyance, and as described above, between the peripheral speed and the tangential speed at the nip when one rotating body is driven by the other. And the target value of the rotational speed of each rotating body is determined from the measured value of the peripheral speed of each rotating body when both rotating bodies are rotated independently of each other using this functional relation. Thus, the present invention is clearly industrially applicable.

DESCRIPTION OF SYMBOLS 100 Image forming apparatus 10 Feeding unit 20 Image forming unit 30 Fixing unit 200 Sheet conveying device 30D Fixing unit driving unit 210 Control circuit 221 First driving circuit 222 Second driving circuit 230 Rotational speed measurement unit 241 Measuring unit 242 Estimation unit 243 Determination unit 244 Storage unit 31 Heating roller 32 Pressure roller 33 First circumferential speed sensor 34 Second circumferential speed sensor 35 First transmission mechanism 36 Second transmission mechanism FM1 First motor FM2 Second motor FGP An encoder built in each motor Output signal Nms Actual speed of each motor VC1 Peripheral speed of heating roller VC2 Peripheral speed of pressure roller

Claims (8)

A sheet conveying device that performs nip conveyance for a sheet,
A first roller and a second roller, the rotation axes of which are parallel to each other and whose outer peripheral surfaces are in contact with each other to form a nip, at least one of which includes an elastic layer;
An adjustment mode in which one of the first roller and the second roller is rotated as a driving roller and the other is rotated as a driven roller, and an operation mode in which both are rotated independently of each other can be switched, and the adjustment mode and A drive unit that rotates the first roller and the second roller at a target number of rotations in any one of the actual modes;
A measuring unit for measuring each peripheral speed of the first roller and the second roller;
Based on the peripheral speed measured by the measurement unit while the driving unit rotates the first roller and the second roller in the adjustment mode, the peripheral speed between the first roller and the second roller and the nip An estimator for estimating the functional relationship between the tangential velocity of
Using the functional relationship estimated by the estimation unit, while the driving unit rotates the first roller and the second roller in the working mode, the number of rotations of the first roller and the second roller A determination unit that determines a target value and instructs the drive unit;
A sheet conveying apparatus. The estimation unit includes a preparation period in which the driving unit rotates the first roller and the second roller in an adjustment mode between operation periods in which the first roller and the second roller convey a sheet. Set,
The sheet conveying apparatus according to claim 1, wherein the determination unit causes the driving unit to rotate the first roller and the second roller in an operation mode during the operation period. The first roller includes an elastic body layer, and the second roller has negligible outer peripheral surface elasticity with respect to the first roller,
The estimation unit obtains a ratio between the circumferential speed of the first roller and the circumferential speed of the second roller from the circumferential speed measured by the measurement unit during the preparation period, and calculates the ratio as the circumferential speed of the first roller. Estimated as a proportional coefficient between the tangential velocity at the nip,
The sheet conveying apparatus according to claim 1, wherein the determining unit uses the proportional coefficient for determining a target value of the rotation speed. Each of the first roller and the second roller includes an elastic layer,
The estimation unit causes the driving unit to rotate the first roller and the second roller alternately as a driving roller and a driven roller during the preparation period, and from the peripheral speed measured by the measurement unit during the preparation period, the driving roller The ratio between the peripheral speed of the roller and the peripheral speed of the driven roller is obtained for the case where the first roller and the second roller are drive rollers, respectively, and the ratio when the first roller is a drive roller and the second The geometric mean of the ratio when the roller is a drive roller is the first proportionality factor between the peripheral speed of the first roller and the tangential speed at the nip, the peripheral speed of the second roller and the nip. Estimated as the ratio of the second proportionality factor to the tangential velocity at
3. The sheet conveying apparatus according to claim 1, wherein the determination unit uses a ratio between the first proportional coefficient and the second proportional coefficient for determining a target value of the rotation speed. 4. A detection unit for detecting an environmental condition or an operating condition of a system in which the sheet conveying apparatus is mounted;
A storage unit storing a table in which a target value of a ratio between a tangential speed at the nip of the first roller and a tangential speed at the nip of the second roller is associated with an environmental condition or an operating condition of the system;
Further comprising
The determination unit searches the storage unit for a target value corresponding to the environmental condition or operating condition of the system detected by the detection unit, and uses the target value and the functional relationship estimated by the estimation unit, A target value of the number of rotations of the first roller and the second roller is determined from a tangential speed at the nip of at least one of the first roller and the second roller measured by the measurement unit during the operation period. The sheet conveying device according to any one of claims 1 to 4, wherein the sheet conveying device is characterized in that: The measuring unit periodically repeats the measurement of the peripheral speeds of the first roller and the second roller during the operation period,
The determination unit updates the target value of the rotational speed by using the peripheral speed and the functional relationship estimated by the estimation unit each time the measurement unit measures the peripheral speed during the operation period. The sheet conveying apparatus according to any one of claims 1 to 5. An image forming unit for forming an image on a sheet;
The sheet conveying apparatus according to any one of claims 1 to 6, wherein the image forming unit forms an image or conveys the formed sheet;
An image forming apparatus. The image forming unit forms an image on a sheet with toner,
The first roller and the second roller apply heat and pressure to the sheet passed from the image forming unit to the nip, and heat the toner image formed on the sheet by the image forming unit. The image forming apparatus according to claim 7, wherein the image forming apparatus is fixed.

Images