JP2006313207A - Image forming apparatus and its control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To appropriately restrain deterioration of an image, even if feeding a recording medium, such as an envelope whose thickness or shape is uneven on a plane or which tends to be creased because it has creases. <P>SOLUTION: When the reliability of a result of loop detection is relatively high, control is performed, to switch the conveying speed or the revolving speed of a fixing device according to the result of the loop detection. Meanwhile, when the reliability of the result of the loop detection is relatively low, the control for switching the conveying speed or the revolving speed of the fixing device is stopped, and control to set the conveying speed or the revolving speed to a predetermined value is performed. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus and a control method thereof.

  Generally, an image forming apparatus includes an electrophotographic image forming apparatus having a transfer unit that transfers a toner image on a photosensitive drum to a recording medium, and a fixing unit that fixes a toner image on the recording medium sent from the transfer unit. It has been known. Such an image forming apparatus controls a plurality of conveying units such as rollers so as to form a certain degree of deflection (loop) on the recording medium. By providing a loop in the recording medium, for example, skewing of the recording medium during conveyance can be suppressed.

  By the way, for example, a change in the roller diameter of the fixing roller or the like constituting the fixing unit may cause a difference between the conveyance speed of the transfer unit and the conveyance speed of the fixing unit. As a result, the transfer unit and the fixing unit may pull the recording medium, and an unnecessarily large loop may be formed on the recording medium. This is not preferable because it causes image deterioration such as color misregistration and rubbing of an unfixed image.

In order to solve this problem, a method has been proposed in which a loop amount formed on a recording medium is detected and a plurality of conveying units such as rollers are controlled so as to keep the loop amount constant (see Patent Document 1).
Japanese Patent Laid-Open No. 5-107966

  However, according to the image forming apparatus described above, the loop amount detection unit may not operate normally depending on the type of the recording medium. For example, a recording medium such as an envelope having a non-uniform thickness or a wrinkle due to a crease or the like is passed.

  Thus, if the loop amount detection means does not operate normally, the loop amount cannot be controlled appropriately, and image degradation such as color misregistration and image rubbing occurs.

  Therefore, an object of the present invention is to solve at least one of such problems and other problems. Other issues can be understood throughout the specification.

  According to the present invention, when the reliability of the loop detection result is relatively high, control is performed so as to switch the conveyance speed or the rotation speed of the fixing unit according to the detection result of the loop detection unit. On the other hand, when the reliability of the loop detection result is relatively low, the control for switching the conveyance speed or the rotation speed of the fixing unit is stopped and the conveyance speed or the rotation speed is controlled to be a predetermined value. Further, based on the detection result of the loop detection means, it is determined whether or not the recording medium is in an abnormal state. Also good.

  According to the present invention, it is possible to suppress the occurrence of image degradation by changing the control method of the number of revolutions between when the reliability of the loop detection result is relatively low and when the reliability is relatively high. For example, even when a recording medium having a non-uniform thickness or shape in a plane or a wrinkle caused by a fold is fed like an envelope, image deterioration can be suitably suppressed.

  FIG. 1 is an exemplary cross-sectional view of an image forming apparatus according to an embodiment. The image forming apparatus 100 of the present embodiment is a color image forming apparatus including four color process cartridges 10m, 10c, 10y, and 10k (hereinafter referred to as process cartridge 10). Further, an optical unit 20 for irradiating the photosensitive drum 12 with the laser light 22, an electrostatic adsorption conveyance unit 30 that electrostatically adsorbs and conveys a recording medium (such as paper) S, a fixing unit 50, and the like are provided. .

  Each process cartridge 10 may basically have the same structure except that the color of the toner is different. Also provided are a photosensitive drum 12, which is an electrophotographic photosensitive member, a charging roller 13, a developing device 14, a cleaning device, and the like.

  The electrostatic adsorption conveyance unit 30 is arranged on an endless electrostatic adsorption conveyance belt (hereinafter referred to as conveyance belt 31), two rollers 32 and 33 that rotatably support the conveyance belt 31, and the roller 33 side. It is comprised by the spring etc. which stretch 31. The roller 32 is rotated by a driving force from a driving unit (not shown), and the conveying belt 31 and the roller 33 are rotated accordingly. The recording medium S is transported by being electrostatically attracted to the transport belt 31. The toner image formed on each photosensitive drum 12 is transferred to the recording medium S.

  The recording medium feeding unit includes a pickup roller 42, a registration roller 44, a recording medium detection sensor 45, and the like. The pickup roller 42 feeds the recording medium S from each paper feed cassette 41 to the conveyance belt 31 via the registration roller 44. The recording medium detection sensor 45 is a sensor that detects the presence or absence of a recording medium conveyed by the registration roller 44.

  The fixing unit 50 is a unit that fixes the toner image on the recording medium S by a fixing roller and a pressure roller. Finally, the recording medium S is discharged out of the apparatus by the paper discharge roller 54.

  A general image forming process will be described. When the recording medium S fed from the recording medium transport unit is detected by the sensor 45, writing of an image is started according to the detected timing. First, in the magenta process cartridge 10 m, the photosensitive drum 12 is uniformly charged by the charging roller 13, and a latent image is formed by the laser light 22 irradiated from the optical unit 20. The latent image is developed by the developing device 14 to form a toner image. The toner image formed on the photosensitive drum 12 is transferred to the recording medium S conveyed by the conveyance belt 31. After the transfer, the photosensitive drum 12 is cleaned by a cleaning device and used for the next image formation. A similar image forming process is performed in each of the process cartridges 10c, 10y, and 10k. The recording medium S onto which the toner image has been multiplex-transferred is fixed at the nip portion of the fixing unit 50 and is discharged by the paper discharge roller 54.

  FIG. 2 is a diagram illustrating a relationship between the control unit and the fixing unit according to the embodiment. The fixing unit 50 includes a fixing roller 51 and a pressure roller 52. The loop detection sensor 56 detects the loop amount of the recording medium S by the fixing loop sensor lever 55. The lever 55 is disposed at a predetermined position between the fixing unit 50 and the conveyance belt unit 30. The paper discharge sensor 58 detects the recording medium S discharged from the nip portion by the paper discharge sensor lever 57.

  The controller 62 is a control unit that comprehensively controls each unit of the image forming apparatus 100. The cartridge drive motor 60 is a drive unit that drives the cartridge 10. The fixing unit driving motor 61 is a driving unit that drives the pressure roller 52 of the fixing unit 50. The drivers 63 and 64 drive the drive motors 60 and 61 in response to a control command from the controller 62, respectively. The image forming counter 65 is a counting unit that counts the number of images formed. The timer 66 is a measuring means for measuring various time intervals. The thermistor 67 is an element that is disposed inside the fixing unit 50 and detects the temperature of the pressure roller 52. Needless to say, the detection results of the loop detection sensor 56, the paper discharge sensor lever 57 and the thermistor 67 are transmitted to the controller 62.

  As described above, the loop detection sensor 56 can be realized by the sensor lever 55 that contacts the recording medium S that is being transported and another sensor (such as a photo interrupter) that detects the operation of the sensor lever 55. However, the present invention is not limited to this, and a non-contact sensor that detects the position of the recording medium S with a laser beam or the like may be employed as the loop detection sensor 56. In the present embodiment, the contact type thermistor 67 is used, but the surface temperature of the pressure roller 52 may be directly detected by a non-contact type thermistor.

  FIG. 3 is an exemplary flowchart illustrating a method for controlling the image forming apparatus according to the embodiment. This control method can be said to be a basic concept of the invention according to the present embodiment.

  In step S <b> 301, the controller 62 detects the loop amount of the recording medium S by the loop detection sensor 56. For example, it is detected whether a predetermined amount of loop is formed.

  In step S <b> 302, the controller 62 performs control so that the rotation speed of the fixing unit drive motor 61 is switched according to the detection result of the loop detection sensor 56. Thereby, the conveyance speed of the recording medium S is changed.

  In step S303, the controller 62 determines whether or not the reliability of the loop detection result is relatively high. If it is relatively high, the flow returns to step S301 to apply the rotation speed switching control according to the loop detection. On the other hand, if the reliability is relatively low, the process proceeds to step S304. There are various methods for determining reliability. For example, as will be described later, when the loop detection result is substantially the same for a predetermined time or longer, the controller 62 determines that the reliability is relatively low. However, the present invention is not limited to these. In short, any technique can be adopted as long as it can determine whether the reliability of the loop detection result (which can be said to be normality of fixing loop control) is relatively high or low.

  In step S304, the controller 62 performs control so that the rotation speed of the fixing unit drive motor 61 becomes a predetermined value. The controller 62 may determine dynamically according to the number of continuous image formations counted by the counter 65. Specifically, the predetermined value may be determined so that the speed decreases stepwise as the number of continuous image formations increases. This is because the pressure roller 52 expands with an increase in the number of image formation sheets (thus, the conveyance speed increases).

  The controller 62 may determine a predetermined value according to the temperature of the fixing unit 50 (particularly, the pressure roller 52) detected when the image formation command is received by the thermistor 67. This is because the degree of expansion of the pressure roller 52 varies depending on the temperature at the start of image formation.

  As described above, according to the present embodiment, the method of controlling the rotation speed (conveying speed) of the fixing unit drive motor 61 depending on whether the reliability of the loop detection result is relatively low or relatively high. By changing, the occurrence of image degradation can be suppressed. For example, even if the loop amount of the recording medium differs in the longitudinal direction of the fixing unit 50, in this embodiment, image deterioration can be suitably suppressed. That is, the image quality can be stabilized.

  For example, when the reliability of the loop amount detection result is relatively high, the rotation speed of the fixing unit drive motor 61 is controlled according to the loop detection result. On the other hand, when the reliability is low, the rotational speed is controlled to be a predetermined value. As a result, image degradation can be suitably suppressed.

  The predetermined value is dynamically determined according to the number of continuous image formations counted from the start of image formation, so that the expansion of the pressure roller 52 can be suitably reflected. Thereby, image degradation can be suppressed more suitably.

  Since the pressure roller 52 expands as the number of continuous image formations increases, for example, the predetermined value can be determined so that the speed gradually decreases as the number of continuous image formations increases. Would be desirable.

  Further, the temperature of the pressure roller 52 detected when the image forming command is received can be an initial value or an offset. Therefore, it is desirable to determine the predetermined value according to the temperature of the pressure roller 52 detected at the time when the image forming command is received.

  FIG. 4 is an exemplary flowchart illustrating a method for controlling the image forming apparatus according to the embodiment. This control method can be said to be a subordinate concept of the invention according to the present embodiment.

  In step S401, the controller 62 monitors whether image data transmitted from a video controller (not shown) or the like has been received. When received, the process proceeds to the next step. In step S <b> 402, the controller 62 detects the temperature of the pressure roller 52 at the start of image formation by the thermistor 67.

  In step S403, the controller 62 starts image formation and time measurement by the timer 66, and starts feeding the recording medium S. In step S <b> 404, the controller 62 determines the thermal expansion state of the pressure roller 52 according to at least one of the number of formed images and the temperature of the pressure roller 52. The controller 62 determines the conveyance speed Vc of the fixing unit 50 corresponding to the thermal expansion state. The transport speed Vc is basically substantially equal to the process speed Vp. The controller 62 sends a command to the motor driver 64 so that the conveyance speed becomes Vc. The motor driver 64 drives the motor 61 in response to this command.

  In step S405, the controller 62 monitors the detection result of the loop detection sensor 56, and determines whether or not the loop sensor is ON. If it is ON, the process proceeds to step S406, and if it is OFF, the process proceeds to step S407. In step S406, the controller 62 controls the conveyance speed of the fixing unit 50 to be Vpu higher than the process speed Vp so that the loop amount decreases. For example, the controller 62 transmits an acceleration command to the motor driver 64 and increases the rotation speed of the fixing unit drive motor 61. On the other hand, in step S407, the controller 62 controls the conveyance speed of the fixing unit 50 to be Vpd higher than the process speed Vp so that the loop amount increases. That is, the controller 62 transmits a deceleration command to the motor driver 64 and decreases the rotation speed of the fixing unit drive motor 61.

  In this embodiment, the fixing conveyance speeds Vpu and Vpd at the time of loop amount control are set to 104% and 96%, respectively, with respect to the process speed Vp. This takes into account variations in the process speed Vp and the outer diameter of the pressure roller 52 and thermal expansion. Thereby, the relationship of Vpu> Vp, Vpd <Vp is maintained regardless of the variation of each component and the temperature of the pressure roller 52.

  In step S408, the controller 62 monitors switching of the fixing conveyance speed and determines whether or not the switching has been performed. For example, by storing the previous fixing conveyance speed in a memory or the like and comparing it with the current fixing conveyance speed, the presence or absence of switching can be determined. When the conveyance speed is switched, the process proceeds to step S409, and the controller 62 resets the timer 66. The timer 66 starts counting from zero. On the other hand, if not switched, it progresses to step S410. Needless to say, the count of the timer 66 is continued.

  In step S410, the controller 62 determines whether or not the leading edge of the recording medium S has reached the paper discharge sensor lever 57 and the output of the paper discharge sensor 58 has been turned on. If it is ON, the process proceeds to step S411. On the other hand, if it is OFF, the process proceeds to step S413.

  While the paper discharge sensor 58 is ON, the controller 62 monitors the time counted by the timer 66 in step S411. Then, it is determined whether or not the time measured by the timer 66 exceeds a predetermined threshold value Tf. Tf is a time for determining a failure in loop detection or fixing loop control. It is preferable to obtain a suitable Tf for each model in advance through experiments or the like.

  When the time measured by the timer 66 exceeds Tf, there is a possibility that the loop detection by the loop detection sensor lever 55 and the loop detection sensor 56 is not performed normally (the reliability is relatively low). Is expensive. Accordingly, the process proceeds to step S412, and the controller 62 stops the feedback control of the loop amount by switching between the fixing conveyance speeds Vpu and Vpd, and sets the subsequent fixing conveyance speed to Vc (determined in S404).

  In the present embodiment, monitoring of the time measured by the timer 66 is started after the paper discharge sensor is turned on. However, the time from the start of image formation may be measured using another timer, and monitoring of the time measured by the timer 66 may be started from the timing when the leading edge of the recording medium reaches the fixing nip.

  In step S413, the controller 62 determines whether or not the image formation is completed. For example, the controller 62 monitors the discharge end sensor 58 that the rear end of the recording medium has been discharged from the fixing unit 50. If the image formation has been completed, the process proceeds to step S415. On the other hand, if the image formation has not ended, the process proceeds to step S414.

  In step S414, the controller 62 determines whether or not the fixing conveyance speed is set to Vc. This is the same as determining whether or not the reliability of the loop detection result is relatively low. If Vc is set, the process returns to step S413 to wait for the end of image formation. On the other hand, if the fixing conveyance speed is not set to Vc, the process returns to step S405. Thereafter, the loop amount detection and the fixing conveyance speed switching control are repeated until the image formation is completed or the fixing conveyance speed is set to Vc in step S412.

  In step S415, the controller 62 determines whether image formation has been completed for all pages. If completed, the process advances to step S416, and the controller 62 resets the image forming counter 65. Further, the controller 62 stops the time counting by the timer 66. Thereafter, the process returns to a waiting state for image data (S401, etc.). On the other hand, if there is a page to continue image formation, the process proceeds to step S417. In step S417, the controller 62 increments the number of formed images and returns to step S402.

  As described above, according to the control method shown in FIG. 4, when the loop detection result is substantially the same over the predetermined time Tf, it is determined that the reliability of the loop detection result is relatively low. it can. On the other hand, if the loop detection result substantially changes before the predetermined time Tf, it can be determined that the reliability of the loop detection result is relatively high. In this way, by using the timer 66 and the predetermined threshold value Tf, the reliability of the loop detection result can be determined with a relatively simple configuration.

<Outline of loop detection>
FIG. 5 is a schematic configuration diagram for explaining operations of the loop detection sensor lever and the loop detection sensor according to the embodiment. In the figure, the loop detection sensor 56 is constituted by a photo interrupter. The photo interrupter is turned ON / OFF according to the displacement of the loop detection sensor lever 55.

  If the output signal of the loop detection sensor 56 is in the OFF state, the fixing conveyance speed is set to Vpd. Vpd is a value lower than the process speed Vp. Therefore, when the conveyance of the recording medium S proceeds, a loop as shown by the dotted line in FIG.

  When the loop detection sensor lever 55 is rotated by this loop, the loop detection sensor 56 is shielded from light. As a result, the output signal of the loop detection sensor 56 is turned ON, and the fixing conveyance speed is switched to Vpu faster than the process speed Vp. If the fixing conveyance speed becomes relatively fast, the loop of the recording medium S is eliminated. That is, the recording medium S is in the state shown by the solid line in FIG.

  As described above, when the reliability of the loop detection result is relatively high, the loop amount of the recording medium S is apparently kept constant by repeating the ON / OFF control of the loop detection sensor. Note that the target value of the loop amount can be adjusted by changing the contact position between the loop detection sensor lever 56 and the recording medium S when the loop detection sensor 56 is shielded from light. If the loop amount is too small, the recording medium S is pulled between the fixing unit and the transfer unit. On the other hand, if the loop amount becomes too large, a force for pushing back the recording medium S to the transfer portion is generated. Therefore, it is important to set the target value of the loop amount so that the loop is not too small and not too large.

<Principle of reliability judgment>
FIG. 6 is a diagram illustrating an example of an output signal of the loop detection sensor according to the embodiment. As shown in FIG. 6A, if ON and OFF are repeated at a cycle of 100 ms or less, it can be said that the loop detection result is normal. That is, the loop amount is kept near the target value.

  However, when a recording medium such as an envelope or the like whose shape and thickness are not uniform in the surface or wrinkled by a fold is passed, loop detection may not be performed normally. This is because the loop amount of the recording medium S differs in the longitudinal direction of the pressure roller 52. When switching control (S405, S406 and S407) between the conveyance speeds Vpu and Vpd described above is performed in such a state, the loop amount of the recording medium S is smaller than a desired loop amount in a part of the pressure roller 52 in the longitudinal direction. turn into. As a result, the recording medium S is pulled between the fixing unit and the transfer unit. Alternatively, a phenomenon occurs in which the recording medium S is pushed back to the transfer portion because the loop amount becomes too large. As a result, color misregistration or an unfixed image is rubbed on the guide portion or the like, and image degradation occurs.

  At this time, for example, when the tension occurs, the recording medium S slides at the transfer portion. This is because the transfer unit has a smaller holding force of the recording medium S than the fixing unit. Therefore, even if the fixing conveyance speed is set to Vpu, the loop cannot be solved immediately. Therefore, the output signal of the loop detection sensor 56 has a waveform in which the ON state continues for a while as shown in FIG.

  If the control of switching between the conveyance speeds Vpu and Vpd is continued with reliability of the loop detection result, the recording medium S continues to slip at the transfer portion, and the image deterioration is further deteriorated.

  Therefore, in the control method shown in FIG. 4, when the output signal of the loop detection sensor 56 is the same for a longer time than the predetermined time Tf, the control of the loop amount using the output signal of the loop detection sensor 56 is normally performed. It is determined that the abnormal state has not been performed. The control fixing conveyance speed is set to Vc, and the recording medium S is conveyed at a constant conveyance speed until the image formation is completed.

  The predetermined time Tf must be longer than the duration of the ON signal and the OFF signal in the normal state. The normal time is when the switching control between the fixing conveyance speeds Vpu and Vpd is normally performed. Also, Tf needs to be set shorter than the time until pulling or pushing occurs as the ON signal or OFF signal continues. For example, in this embodiment, it is set to 500 ms. Of course, it goes without saying that the value may be other than 500 ms depending on the model.

<Setting method of fixing conveyance speed Vc>
Next, a method for setting the fixing conveyance speed Vc used when the reliability of the loop detection result is relatively low (when fixing loop control is determined to be defective) will be described.

  FIG. 7 is a diagram illustrating a relationship between the number of image formations and the rotation speed of the fixing drive motor according to the embodiment. In this example, continuous image formation is started when the temperature of the pressure roller 52 is equal to room temperature (so-called cold state).

  As described above, as the number of continuous image formations increases, the fixing conveyance speed increases due to the expansion of the pressure roller 52. Therefore, in consideration of this increase, it is desirable to perform control so that the fluctuation of the loop amount is minimized. Therefore, the rotation speed of the fixing drive motor 61 is set to be gradually reduced (slow) in accordance with the continuous increase in the number of images formed. Actually, in the example shown in FIG. 7, it can be understood that the rotational speed decreases in order from the first stage to the fifth stage.

  FIG. 8 is a diagram illustrating an example in which the initial value of the image forming counter is changed according to the temperature of the pressure roller according to the embodiment. When the pressure roller 52 is not in a cold state, such as when image formation is performed intermittently, it is necessary to correct the relationship between the number of image formations and the number of rotations (conveyance speed) shown in FIG.

  Therefore, the controller 62 detects the temperature of the pressure roller 52 at the start of image formation by the thermistor 67, and from which stage in FIG. 7 the count of the number of image formation is started based on the detected temperature. To decide. That is, the controller 62 reads the initial value of the counter corresponding to the detected temperature from the table shown in FIG.

  For example, if the detected temperature at the start of image formation is 80 ° C., Vc is set to the third stage speed (100.7%) for the first to seventh sheets. Further, Vc is set to the fourth stage speed (100.3%) from the 8th sheet to the 19th sheet. Further, for the 20th and subsequent sheets, Vc is set to the fifth stage speed (99.9%).

  As described above, by determining the fixing conveyance speed in accordance with the temperature of the fixing device (especially a pressure roller), it is possible to perform image formation more suitably.

1 is an exemplary cross-sectional view of an image forming apparatus according to an embodiment. It is a figure which shows the relationship between the control part which concerns on embodiment, and a fixing unit. 5 is an exemplary flowchart illustrating a method for controlling the image forming apparatus according to the embodiment. 5 is an exemplary flowchart illustrating a method for controlling the image forming apparatus according to the embodiment. It is a schematic block diagram for demonstrating operation | movement of the loop detection sensor lever and loop detection sensor which concern on embodiment. It is a figure which shows an example of the output signal of the loop detection sensor which concerns on embodiment. FIG. 6 is a diagram illustrating a relationship between the number of image formations according to the embodiment and the rotation speed of a fixing drive motor. It is a figure which shows the example which changes the initial value of an image formation counter according to the temperature of the pressure roller which concerns on embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Process cartridge 20 ... Optical box 30 ... Electrostatic adsorption conveyance belt unit 40 ... Transfer material conveyance unit 50 ... Fixing unit 51 ... Fixing roller 52 ... Pressure roller 55 ... Fixing loop sensor lever 56 ... Loop detection sensor 57 ... Exhaust Paper sensor lever 58 ... Paper discharge sensor 60 ... Cartridge drive motor 61 ... Fixing unit drive motor 62 ... Controller 63, 64 ... Motor driver 65 ... Image formation counter 66 ... Timer 67 ... Thermistor

Claims (9)

Transfer means for transferring a toner image onto a recording medium;
Fixing means for fixing the toner image to the recording medium;
A loop detection unit that is disposed between the transfer unit and the fixing unit and detects whether a loop amount formed on the recording medium is a predetermined amount;
When the reliability of the detection result and the detection result of the loop detection unit are relatively high, the conveyance speed or the rotation speed of the fixing unit is switched according to the detection result of the loop detection unit. When the reliability of the detection result is relatively low, switching control of the conveyance speed or the rotation speed of the fixing unit is stopped so that the conveyance speed or the rotation speed becomes a predetermined value. And an image forming apparatus.   The image forming apparatus according to claim 1, wherein the determination unit determines that the reliability is relatively low when the detection results are substantially the same for a predetermined time or more.   3. The control unit according to claim 1, wherein the control unit further includes a determination unit that dynamically determines the predetermined value in accordance with the number of continuous image formations counted from the start of image formation. Image forming apparatus.   The image forming apparatus according to claim 3, wherein the determining unit determines the predetermined value so that the speed decreases stepwise as the continuous image forming number increases. A temperature detecting means for detecting the temperature of the fixing means;
5. The image forming apparatus according to claim 3, wherein the determining unit determines the predetermined value according to the temperature detected when the image forming command is received. 6. A control method for an image forming apparatus, comprising: transfer means for transferring a toner image onto a recording medium; and fixing means for fixing the toner image to the recording medium,
Detecting a loop of a predetermined amount or more formed on the recording medium conveyed between the transfer unit and the fixing unit;
Determining the reliability of the detection result of the loop, and when the reliability of the detection result is relatively high, the conveyance speed or the rotation speed of the fixing unit is switched according to the detection result of the loop detection unit. If the reliability of the detection result is relatively low, switching control of the conveyance speed or rotation speed of the fixing unit is stopped so that the conveyance speed or rotation speed becomes a predetermined value. A control method for the image forming apparatus. Transfer means for transferring a toner image onto a recording medium;
Fixing means for fixing the toner image to the recording medium;
A loop detection unit that is disposed between the transfer unit and the fixing unit and detects a loop amount formed on the recording medium;
Control means for controlling the conveying speed or the rotational speed of the fixing means so as to form a predetermined amount of loop on the recording medium in accordance with the detection result of the loop detecting means;
Determination means for determining whether the recording medium is in an abnormal state based on a detection result of the loop detection means;
The image forming apparatus according to claim 1, wherein the control unit controls the conveyance speed or the rotation speed of the fixing unit to be switched to a predetermined value when the determination unit determines that the state is abnormal.   The image forming apparatus according to claim 7, wherein the determination unit determines that the state is abnormal when the detection result of the loop detection unit indicates the same state for a predetermined time or more. A control method for an image forming apparatus, comprising: transfer means for transferring a toner image onto a recording medium; and fixing means for fixing the toner image to the recording medium,
Detecting a loop of a predetermined amount or more formed on the recording medium conveyed between the transfer unit and the fixing unit;
A step of determining whether or not the recording medium is in an abnormal state based on a detection result of the loop; and a conveyance speed or a rotation speed of the fixing unit when the determination unit determines that the recording medium is in an abnormal state Switching to a predetermined value;
A control method for an image forming apparatus.

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