JP2011237656A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of conveying sheets while forming a proper loop and preventing deterioration of image qualities.SOLUTION: While sheets to which toner images are transferred are conveyed between a transferring part 16 and a fixing device 11 with a loop of a size within a predetermined range being formed, a sheet conveying speed of the fixing device 11 is controlled so that the size of the loops which are formed in the sheets between the transferring part 16 and the fixing device 11 falls within the predetermined range, based on the detected sheet conveying speed of the transferring part 16 and the detected sheet conveying speed of the fixing device 11.

Description

  The present invention relates to an image forming apparatus, and more particularly to an apparatus that conveys a sheet on which a toner image is transferred while forming a loop between a transfer unit and a fixing unit.

  Conventionally, in an image forming apparatus using an electrophotographic system, a toner image formed on an image carrier is transferred to a sheet as a transfer material by a transfer unit, and then the sheet is guided to a fixing unit having a fixing roller. Thus, the sheet is heated to fix the toner image on the sheet.

  In such a conventional image forming apparatus, depending on the length of the sheet, when the leading end of the sheet enters the fixing unit, the trailing end may not yet pass through the transfer unit. Normally, the sheet conveyance speed of the fixing unit and the sheet conveyance speed of the transfer unit are set to be approximately equal. However, the fixing unit and the transfer unit may be affected by thermal expansion, solid difference, or aging of the fixing roller provided in the fixing unit. There may be a difference between the sheet conveying speeds.

  Here, when the sheet conveyance speed of the fixing unit exceeds the sheet conveyance speed of the transfer unit, a sheet carrying an unfixed toner image is pulled to the fixing unit side between the fixing unit and the transfer unit. May occur, leading to image degradation. Therefore, in order to prevent such a phenomenon that the sheet is pulled, a loop is formed in the sheet conveyed between the transfer unit and the fixing unit by providing a sheet conveyance speed difference between the fixing unit and the transfer unit. The sheet is bent.

  However, in recent years, image forming apparatuses have been required to increase the speed, and as a result, many sheets are continuously conveyed to the fixing unit in a short time. Here, when many sheets are continuously conveyed in such a short time, it is necessary to raise the fixing temperature in order to fix the toner image. However, when the fixing temperature is increased in this way, even if the sheet conveyance speed difference between the transfer unit and the fixing unit is within a predetermined range immediately after the start of paper feeding, the variation in expansion of the outer diameter of the fixing roller occurs during continuous printing. It becomes larger than before. Since the transfer roller is not subjected to a thermal load even if the speed is increased, the outer diameter does not change during continuous printing, and the sheet conveyance speed of the transfer portion remains constant. For this reason, if the fluctuation in the sheet conveyance speed of the fixing unit increases due to the increase in speed, the difference in sheet conveyance speed between the transfer unit and the fixing unit greatly varies, so the amount of sheet loop between the transfer unit and the fixing unit increases. It will fluctuate.

  Therefore, for example, a loop detection sensor that detects the loop of the sheet is provided in the conveyance guide disposed between the fixing unit and the transfer unit, and the conveyance speed of the fixing roller is controlled so that the loop amount formed on the sheet is constant. There is an image forming apparatus configured to do so (see Patent Documents 1 and 2). In such an image forming apparatus, a predetermined amount of loop is formed by controlling the control clock cycle of a stepping motor, which is a driving motor for the fixing roller, according to the detection result of the loop detection sensor. .

  FIG. 8 is a diagram illustrating such a loop amount control method. 8A shows a case where the loop amount is small, and FIG. 8B shows a case where the loop amount is large. In FIG. 8, reference numeral 20 denotes a fixing entrance guide. The fixing entrance guide 20 rotates on the conveyance surface according to the loop amount, and a detection flag 21 a for detecting the loop amount can be rotated to the swing center O. Is arranged.

  Further, from the swing center O of the detection flag 21a, a flag 21c that is interlocked with the detection flag 21a and that turns on / off the photo interrupter 22 extends to the photo interrupter 22 side. A control unit (not shown) detects whether or not the loop amount has exceeded a certain value based on the change in the interruption state of the photo interrupter 22. Note that the flag 21c is biased by a spring (not shown) and is normally in a state where the photo interrupter 22 is turned off (a state where the flag is not detected by the photo interrupter) as shown in FIG. Then, the loop of the sheet P becomes a predetermined size, and is set so as to turn on the photo interrupter 22 only when a force is applied to the flag 21c (a state where the flag is detected by the photo interrupter). Has been.

  In this case, when the loop amount increases, the photo interrupter 22 is blocked (a flag is detected) as shown in FIG. 8B, so that the loop amount is reduced (eliminated). In this way, control is performed in the direction in which the conveyance speed of the fixing unit is increased. On the other hand, when the loop amount is small, the interruption of the photo interrupter 22 is released (the flag is not detected) as shown in FIG. 8A, so that the conveyance speed of the fixing unit is decreased so as to increase the loop amount. Control in the direction you want.

JP-A-10-097154 JP 2000-344385 A

  However, in such a conventional image forming apparatus, the fixing roller having a thick rubber layer, which is a factor of thermal expansion, has a large temperature difference between the surface temperature during continuous paper passing and the vicinity of the core metal. Therefore, it is difficult to predict the outer diameter of the fixing roller, that is, the conveyance speed. Further, the temperature variation due to the sheet passing is different depending on the material and basis weight of the sheet, and also the variation in the sheet passing interval due to the delay of the image processing or the like caused by the different image pattern is added, so that the form is complicated. As a result, the expansion variation greatly fluctuates, and if a tolerance of parts such as a fixing roller is added to this, the conveyance speed variable width used in the loop control may be deviated, resulting in excessive loops.

  The excessive loop is a phenomenon in which, when performing loop control, the sheet is separated from the loop amount detection unit after the start of loop control. When the loop amount is excessive as described above, the sheet P is separated from the detection flag 21a after the start of the loop control as shown in FIG. 8C, and as a result, the photo interrupter 22 is turned OFF (the flag is not detected). State). In this case, although the loop amount is excessive, it is determined that the loop amount is small because the photo interrupter 22 is OFF, and the control proceeds while the sheet conveyance speed of the fixing unit is reduced. As a result, the loop excess further advances, and the unfixed surface rubs against various places in the image forming apparatus, for example, the lower part of the cleaning unit 10 as shown in FIG. 8C, and noticeable image deterioration occurs.

  Accordingly, the present invention has been made in view of such a current situation, and provides an image forming apparatus capable of conveying a sheet while forming an appropriate loop and preventing deterioration in image quality. It is intended.

  The present invention provides an image forming apparatus comprising: a transfer unit that transfers a toner image formed on an image carrier to a sheet; and a fixing unit that fixes the toner image transferred by the transfer unit to the sheet. A first detection unit that detects a sheet conveyance speed of the sheet conveyed by the first detection unit, a second detection unit that detects a sheet conveyance speed of the sheet conveyed by the fixing unit, and the transfer detected by the first detection unit. A loop formed on the sheet between the transfer unit and the fixing unit is within a certain range on the basis of the sheet conveyance speed in the unit and the sheet carry-out speed in the fixing unit detected by the second detection unit. And a control unit that controls a sheet carry-out speed in the fixing unit so as to be a size.

  The present invention conveys a sheet while forming an appropriate loop by controlling the sheet conveyance speed of the fixing unit based on the sheet conveyance speed of the sheet passing through the transfer unit without using a conventional loop detection sensor. Image quality degradation can be prevented.

1 is a diagram showing a schematic configuration of a laser beam printer which is an example of an image forming apparatus according to an embodiment of the present invention. The figure which shows the correlation (before control introduction) of the fixing temperature and Tfout at the time of the continuous paper passing of the said laser beam printer. The schematic diagram of the loop shape of the transfer part-fixing part of the said laser beam printer. FIG. 4 is a schematic diagram of a sheet conveyance path from a registration sensor to a paper discharge sensor of the laser beam printer. FIG. 3 is a control block diagram of the laser beam printer. 6 is a flowchart illustrating sheet conveyance speed control of the fixing device. The figure which shows the correlation (after control introduction) of the fixing temperature and Tfout at the time of continuous paper passing. The figure explaining the control method of the loop amount by the conventional loop sensor.

  Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram showing a schematic configuration of a laser beam printer which is an example of an image forming apparatus according to an embodiment of the present invention. In FIG. 1, 50 is a laser beam printer, 51 is a printer unit 52 which is an electrophotographic image forming unit, a sheet feeding unit 53, a fixing device 11 constituting a fixing unit, and the like (hereinafter referred to as a laser beam printer main body). The printer itself).

  Here, the printer unit 52 includes a photosensitive drum 1 as an image carrier, and a charging unit 2 such as a contact charging roller disposed around the photosensitive drum 1 to primarily charge the photosensitive drum 1. Further, a laser beam that scans and exposes (irradiates) the charged surface of the photosensitive drum 1 by outputting a laser beam L that is on / off modulated in accordance with an image signal input from an external device such as an image scanner or a computer (not shown). A scanner 3 is provided. Further, a developing device 4 that develops an electrostatic latent image by supplying developer (toner) to the surface of the photosensitive drum 1 from a developing sleeve 15 that is a developer carrying member, and a transfer that transfers the developed toner image to a sheet. A transfer roller 9 and a cleaning unit 10 forming the portion 16 are provided.

  The photosensitive drum 1 is rotated at a predetermined peripheral speed (process speed) in the clockwise direction indicated by an arrow, and the circumferential surface is uniformly charged to a predetermined polarity and potential by the charging means 2 (primary Charged). Further, the scanning exposure by the laser beam scanner 3 removes the charge from the exposed portion of the photosensitive drum 1, and an electrostatic latent image is formed on the surface of the photosensitive drum 1.

  The sheet feeding unit 53 includes a sheet feeding cassette 5 for stacking and storing sheets, and a feeding roller 6 for feeding the uppermost sheet in the sheet feeding cassette.

  Next, an image forming operation of the laser beam printer 50 having such a configuration will be described. First, the feeding roller 6 is driven based on a signal for starting image formation, and the uppermost sheet P in the sheet feeding cassette 5 is separated and fed one by one, passes through the sheet conveying rollers 7a and 7b, and the sheet conveying path. It is conveyed along 8a. Next, at the stage where the flag of the registration sensor 12, which is the first sensor at the front end of the sheet, is turned ON (the flag is turned down), the laser beam scanner 3 performs scanning exposure on the surface of the photosensitive drum 1, and the photosensitive member. An electrostatic latent image is formed on the surface of the drum 1. Thereafter, the electrostatic latent image is developed by the developing device 4 to form a toner image on the photosensitive drum 1. As described above, the image forming operation in the image forming unit is started based on the detection of the sheet by the registration sensor 12.

  Next, at the timing when the leading edge of the sheet reaches the transfer section 16, the leading edge of the image forming area on the photosensitive drum (on the image carrier) also reaches the transfer section 16 at the same time. Then, a transfer bias is applied to the transfer roller 9 with a polarity opposite to the toner charging polarity on the photosensitive drum, whereby the toner image is electrostatically transferred onto the surface of the sheet P at the transfer nip portion T of the transfer portion 16. The Thereafter, the sheet P to which the toner image has been transferred is separated from the photosensitive drum 1 and conveyed to the fixing device 11 through the sheet conveying path 8b, and the unfixed toner image on the sheet is transferred to the fixing roller 20a and the pressure roller 20b. In the fixing nip F formed by the above, heating and pressure fixing processing is performed.

  The sheet P that finally passes through the fixing device 11 is a second sensor, and is a sheet discharge tray while turning on a sheet discharge sensor 13 for detecting sheet discharge (while turning a sheet discharge sensor flag not shown). 14 is discharged. After the separation of the sheet, that is, after the transfer of the toner image to the sheet P, the photosensitive drum 1 is cleaned by the cleaning unit 10 after removing the transfer residual toner, paper dust, and the like, and repeatedly used for image formation.

  By the way, in the present embodiment, the sheet conveyance speed in the transfer unit 16 and the fixing device 11 is detected, and the sheet conveyance behavior between the transfer unit and the fixing device is stabilized based on the detection result. The sheet conveyance speed is controlled.

  Next, an experiment conducted for examining the conveyance speed control between the transfer unit 16 and the fixing device 11 will be described. When examining the conveyance speed control, an experiment was conducted by paying attention to the conveyance speed Vfu of the fixing device 11 having a large variation, not the sheet conveyance speed Vtr at the transfer nip T where the variation in the conveyance speed is very small. In this experiment, as a substitute characteristic of the conveyance speed Vfu, the sheet is brought into contact with the ON time of the sheet discharge sensor 13 on the downstream side of the fixing device 11 in the conveyance direction, that is, the sheet discharge sensor flag, and the sheet discharge sensor flag falls. The measured time Tfout was measured.

Furthermore, in this measurement, the data acquisition was performed for the components of the laser beam printer 50 after varying the following component tolerances and sensor detection (accuracy and responsiveness) variations.
(1) Transfer roller outer diameter upper and lower limit products (2) Transfer roller hardness upper and lower limit products (3) Fixing roller and pressure roller outer diameter upper and lower limit products (4) Fixing roller and pressure roller hardness upper and lower limit products (5) Registration sensor flag size upper and lower limit products (6) Paper discharge sensor flag size upper and lower limit products (7) Registration sensor detection (accuracy, responsiveness) variation upper and lower limits (8) Paper discharge sensor detection ( Accuracy and responsiveness variation upper and lower limits (9) Fixing temperature thermistor detection (accuracy and responsiveness) variation upper and lower limits (10) Paper type (basis weight, surface property), image pattern.

  As a result, the relationship between the measured Tfout and the fixing temperature (detection temperature of the thermistor) is as shown in FIG. 2, and the following results were obtained.

  (Result 1) Even when the above variation was incorporated at the start of printing, the fixing temperature converged in the range of 183.2 to 187.1 ° C., so that neither excessive loop nor excessive loop occurred.

  (Result 2) Even when the fixing target temperature was 185.0 ° C. during continuous paper feeding, the fixing temperature fluctuated in the range of 179.2 to 189.1 ° C. when the above variation was incorporated. As a result, there were cases where excessive loops and excessive loops occurred.

(Result 3) If Tfout is within the following range, excessive loops and insufficient loops did not occur even when component tolerances and sensor detection (accuracy, responsiveness) variations were incorporated.
1.422 s ≦ Tfout ≦ 1.575 s (Formula 1)
In addition, outside the area | region of Formula 1, the loop excess and the loop excess generate | occur | produced reliably as follows.
When Tfout <1.422 s: loop shortage (fixing device sheet conveyance speed high)
When Tfout> 1.575 s: excessive loop (low sheet conveyance speed of fixing device)

  Next, modeling was performed focusing on Tfout so that the control derived from the above experimental results can be applied to a general apparatus. Hereinafter, the results will be described.

(Result 1) Relationship between the amount of loop formed (and the amount of loop fluctuation) and loop excess and excess The occurrence of loop excess and loop excess is correlated with the amount of loop formed, and if the loop amount is within a predetermined range Loop over and loop under will not occur. In other words, the amount of loop formed, the loop excess and the loop excess have the following relationship.
Under loop amount <Loop amount formed <Excessive loop amount (Formula 2)

  Therefore, the loop amount is defined as shown in FIG. In FIG. 3, Lmin is the minimum loop length that does not cause the loop amount to be excessive, and Lmax is the maximum loop length that does not cause the loop amount to be excessive.

  Further, the length of the moment when the leading edge of the sheet arrives at the fixing nip portion along the fixing entrance guide from the transfer portion (loop formation start) is Lin, and when this Lin is investigated, it is found that the length is constant.

Here, when the loop amount formed in Equation 2 is transformed using the loop fluctuation amount Lprint, Equation 2 becomes as follows. Lprint represents the amount of loop fluctuation during the loop formation (the time from when the leading edge of the sheet reaches the fixing nip F until the trailing edge of the sheet passes through the transfer nip T).
(Lin−Lmax) ≦ Lprint ≦ (Lin−Lmin) (Formula 3)

(Result 2) Correlation between the sheet conveyance speed and Lprint in the transfer unit and the fixing device 11 FIG. 4 is a diagram for explaining the correlation. In FIG. 4, Tfin is a sheet passing time at the registration sensor 12, Vtr is a sheet conveying speed of the transfer nip T, and Vfu is a conveying speed of the fixing nip F (fixing device). A time required for the leading edge of the sheet to reach the fixing nip F from the transfer nip T was defined as Ttr_fu as a related parameter.

Further, the length in the sheet conveyance direction was defined as follows.
Length in sheet conveyance direction before fixing: Lpaper
Length in sheet conveyance direction after fixing: Lpaper × α (α: contraction rate of sheet length accompanying fixing)

  Note that the sheet conveyance time of the registration sensor 12 and the transfer unit is adjusted to be the same. Therefore, Vtr = Lpaper / Tfin.

Here, Lprint is the following characteristic value derived from the above parameters:
Loop variation per unit time: Vfu-Vtr
Loop formation time: Tfin-Ttr_fu
It is shown by the following formula.
Lprint (loop fluctuation amount) = (Tfin−Ttr_fu) × (Vfu−Vtr)

Here, Lprint is at most Lprint = Tfin × (Vfu−Vtr) (Formula 4)
Therefore, it was decided to define with the expression 4.

(Result 3) Appropriate region in transport control using Tfout Substituting Equation 4 into Equation 3 results in the following.
Lin−Lmax ≦ Tfin × (Vfu−Vtr) ≦ Lin−Lmin (Formula 5)

  If this inequality relationship is satisfied, excessive and insufficient loops will not occur.

here,
Transfer section conveyance speed Vtr = Lpaper / Tfin
Fixing part conveyance speed = Lpaper × α / Tfout
Media length Lpaper = Tfin × Vtr
Is transformed using the following equation, Tfout is defined by the following inequality region.
Tfin ² × Vtr × α / [Tfin × Vtr + (Lin−Lmin)]
≦ Tfout ≦
Tfin ² × Vtr × α / [Tfin × Vtr + (Lin−Lmax)] (Formula 6)

And the average value of each parameter shown below actually examined this time Tfin = 1.485 [sec]
Vtr = 200 [mm / sec]
Lin−Lmax = −18 [mm]
Lin−Lmin = 10 [mm]
α = 0.993
When substituting into the above equation 6, the range of Tfout is as follows.
1.427 s ≦ Tfout ≦ 1.570 s

  This is a Tfout region (Equation 1) of experimental values incorporating variation, and even in a general apparatus, Tfout only needs to be within the numerical value derived by Equation 6.

  Based on the above experiment and verification results, in the present embodiment, the sheet discharge of the fixing device 11 that forms a loop having a size within a predetermined fixed range on the sheet based on the sheet conveyance speed conveyed to the transfer unit 16. A speed range of the speed is obtained, and the sheet carry-out speed of the fixing device 11 is controlled.

  FIG. 5 is a control block diagram of the laser beam printer 50. In FIG. 5, reference numeral 60 denotes a control unit. The control unit 60 forms a loop having a size within a predetermined fixed range between the fixing device 11 and the transfer unit 16 on the sheet on which the toner image is transferred. The sheet carry-out speed of the fixing device 11 is controlled so that the sheet is conveyed while being conveyed.

  The control unit 60 is connected to a registration sensor 12 for calculating the sheet conveyance speed of the sheet conveyed to the transfer unit 16. Further, the control unit 60 is connected with a paper discharge sensor 13 for detecting the sheet carry-out speed of the sheet carried out from the fixing device 11. Further, a heater 61 that heats the fixing roller 20a and a drive motor M that drives the fixing roller 20a and the pressure roller 20b of the fixing device 11 are connected to the control unit 60. The control unit 60 is connected to an input unit 62 for inputting the length of the sheet in the sheet conveyance direction and a fixing temperature thermistor 63 that is a temperature detecting unit for detecting the fixing temperature of the fixing device 11. The input unit 62 includes an operation unit (not shown), a sensor that detects the length of the sheet in the sheet conveyance direction, and the like.

  Here, the control unit 60 calculates the sheet conveyance and sheet discharge speeds of the transfer unit 16 and the fixing device 11 based on signals from the registration sensor 12 and the sheet discharge sensor 13. Specifically, the calculation unit 60 a provided in the control unit 60 passes through the transfer unit 16 based on the signal output period from the registration sensor 12 and the sheet conveyance direction length information from the input unit 62. The sheet conveyance speed of the sheet is calculated. Although the sheet conveyance speed of the sheet passing through the transfer unit 16 is not directly detected, the sheet conveyance speed based on the signal output period in the registration sensor 12 and the sheet conveyance speed of the sheet in the transfer unit 16 are substantially equal. Are the same.

  Further, based on the signal output period from the sheet discharge sensor 13 and the sheet conveyance direction length information of the sheet from the input unit 62, the calculation unit 60 a provided in the control unit 60 determines the sheet to be carried out from the fixing device 11. The sheet carry-out speed is calculated. Although the sheet conveyance speed of the sheet discharged from the fixing device 11 is not directly detected, the sheet conveyance speed based on the signal output period of the sheet discharge sensor 13 and the sheet conveyance speed of the sheet discharged from the fixing device 11 Are substantially the same.

  In the present embodiment, the first detection unit includes the registration sensor 12 and a calculation unit 60 a provided in the control unit 60, and the second detection unit includes the discharge sensor 13 and the control unit 60. It is comprised with the calculating part 60a provided. Then, the drive motor M is controlled based on the sheet conveying speed and the sheet unloading speed of the fixing device 11 and the transfer unit 16 obtained in this way, and the sheet unloading speed of the fixing device 11 is controlled. Further, the heater 61 is controlled so that the temperature of the fixing roller 20a is a predetermined temperature.

  In the present embodiment, as will be described later, the control unit 60 determines the rotation speed of the drive motor M so that the Tfout of the nth sheet becomes a median value that does not cause excessive and insufficient loops, for example, 1.500 sec. After calculating Rfu_ (n), adjust. Here, the rotational speed (hereinafter referred to as a fixing target rotational speed) Rfu_ (n) of the drive motor M is calculated by the following method.

First, a deviation Δ_ (n−1) of Tfout_ (n−1) from the control target value in the (n−1) th sheet is defined by the following equation.
Δ_ (n−1) = Tfout_ (n−1) −1.500 sec

Then, the fixing target rotation speed Rfu_ (n) is calculated by the following method.
Rfu_ (n) = Rfu_ (n−1) × [1 + K × (Δ_ (n−1) / Tfout_ (n−1)] (Expression 7)
Note that K in Equation 7 is a value optimized from convergence and responsiveness, and in this embodiment, K = 0.6.

  Next, the sheet carry-out speed control of the fixing device 11 by the control unit 60 will be described with reference to the flowchart shown in FIG.

  With respect to the control of the first sheet to start feeding, fixing is performed in the temperature range of the fixing roller where Tfout exists in the range of Expression 5 even if each variation affecting the carry-out speed is taken into consideration. In this embodiment, the control unit 60 controls the heater 61 so as to satisfy Equation 5 described above, and the fixing temperature of the fixing device 11 is set to a temperature adjustment target temperature of 182 to 187 ° C., and this fixing temperature is set. The fixing temperature for the first sheet is used. Here, the temperature adjustment target temperature is a predetermined temperature at which the sheet carry-out speed in the fixing device 11 can be controlled.

  Thereafter, the control unit 60 determines that the fixing temperature is within the temperature adjustment target temperature range (within a predetermined temperature range) before printing the first sheet based on the fixing temperature information from the fixing temperature thermistor 63, and this temperature adjustment target temperature (182). It is determined whether or not fixing at ˜187 ° C. is possible (S01). If it is determined that fixing at 182 to 187 ° C. is possible (Y in S01), the first print sequence is started (S02). In this way, by determining whether or not the fixing temperature of the fixing device 11 is in a temperature range in which the sheet carry-out speed can be controlled at the start of printing in this way, it is possible to prevent an excessive loop amount or an insufficient loop amount from the first sheet. be able to.

  Next, the time Tfout (1) during which the paper discharge sensor flag is tilted at the time of printing the first sheet is measured (S03). Thereafter, it is determined whether or not printing is completed for the first sheet (S04). When printing is completed for the first sheet (Y in S04), printing is terminated as it is. If the printing is not completed (N in S04), the control is performed for the nth sheet (n ≧ 2 or more integer) at the start of sheet passing. That is, in order to shift to the next print, the print count is increased by 1 (n = n + 1) (S05), and the next (n-th) print is started (S06).

  Next, the fixing target rotation speed Rfu_ (n) is set using the above-described equation 7 so that Tfout (n) becomes the median value (1.500 sec in the present embodiment) at which the loop excess and the excess do not occur simultaneously with the start of printing. Then, adjustment (correction) is performed (S07). And by such rotation speed control, Tfout comes close to the target value of 1.500 sec. In other words, the sheet carry-out speed approaches the target sheet carry-out speed set within the speed range of the sheet carry-out speed of the sheet that forms a loop having a size within a predetermined range on the sheet.

  Next, Tfout_ (n) at the time of printing is measured (S08). Then, a print end determination is made (S09). If the print has not ended (N in S09), then S05 to S08 are repeated until the print ends (Y in S09).

When the above control was incorporated after varying component tolerances and sensor detection (accuracy, responsiveness) variations, there was no occurrence of excessive or insufficient loops. FIG. 7 shows the correlation characteristics between Tfout and fixing temperature at this time. As can be seen from FIG. 7, the Tfout region during the current control is independent of the fixing temperature.
1.463 ≦ Tfout ≦ 1.512
It was converged to.

  As described above, in the present exemplary embodiment, the sheet carry-out speed of the fixing device 11 is controlled based on the sheet conveyance speed of the sheet conveyed to the transfer unit 16. Thus, conventionally, without using a loop detection sensor provided between the transfer unit and the fixing unit, it is possible to convey the sheet while forming an appropriate loop regardless of the outer diameter change accompanying the fixing temperature fluctuation, Degradation of image quality can be prevented.

  Further, as in the present embodiment, the registration sensor 12 and the paper discharge sensor 13 detect the sheet conveyance and unloading speeds in the fixing device 11 and the transfer unit 16, so that the loop amount is excessive regardless of the loop shape. It is possible to prevent the loop amount from being too small. Further, the loop sensor is not necessary, and the sheet conveyance speed of the sheet passing through the fixing device 11 and the transfer unit 16 can be detected without providing a separate sensor, so that the cost can be reduced.

  In this embodiment, the control is performed so that Tfout satisfies Expression 6. However, the control satisfying Expression 5 may be performed after directly measuring the conveyance speed Vfu of the fixing device. In this embodiment, Equation 7 is used as an example of the control method of Rfu_ (n). However, if the control is effective, the control method can be changed to another control method (for example, PID control). However, the control method in the present embodiment does not limit the scope of the present invention.

  In this embodiment, the sheet passing time is detected by the registration sensor 12 and the paper discharge sensor 13, and the sheet transport speed is calculated based on the detection result and the length in the sheet transport direction. The sheet conveyance speed may be detected by another configuration. For example, using a roller made of resin or the like that rotates following contact with the sheet being conveyed, the sheet conveyance speed is calculated based on the diameter of the roller and the number of rotations of the roller (for example, detected by an encoder) when driven. You may make it do.

  Furthermore, in the present embodiment, a registration sensor and a paper discharge sensor are used as a method for detecting Tfin and Tfout, but the present invention is not limited to this. For example, a non-contact optical sensor may be used as long as it can detect the sheet conveyance speed or the passage time of the fixing unit and the transfer unit. Furthermore, Tfin and Tfout may be detected by monitoring fluctuations in driving torque while the sheet is held between the transfer unit and the fixing unit.

DESCRIPTION OF SYMBOLS 11 ... Fixing device, 12 ... Registration sensor, 13 ... Discharge sensor, 16 ... Transfer part, 50 ... Laser beam printer, 51 ... Laser beam printer main body, 52 ... Printer part, 60 ... Control part, 61 ... Heater, 62 ... Input unit, 63 ... Fixing temperature thermistor, M ... Drive motor, P ... Sheet

Claims (5)

An image forming apparatus comprising: a transfer unit that transfers a toner image formed on an image carrier to a sheet; and a fixing unit that fixes the toner image transferred by the transfer unit to the sheet.
A first detection unit that detects a sheet conveyance speed of a sheet conveyed by the transfer unit;
A second detection unit for detecting a sheet carry-out speed of the sheet carried out by the fixing unit;
Based on the sheet conveyance speed at the transfer unit detected by the first detection unit and the sheet carry-out speed at the fixing unit detected by the second detection unit, between the transfer unit and the fixing unit. A control unit that controls the sheet carry-out speed in the fixing unit so that a loop formed in the sheet has a size within a certain range;
An image forming apparatus comprising: A motor for driving the fixing unit;
The control unit obtains a target sheet carry-out speed of the sheet carried out by the fixing unit based on the sheet carrying speed of the sheet in the transfer unit detected by the first detection unit, and the second detection unit The image forming apparatus according to claim 1, wherein the motor is controlled such that the detected sheet discharge speed of the sheet at the fixing unit becomes the target sheet discharge speed.   Based on the sheet conveyance speed detected by the first detection unit and the sheet conveyance speed detected by the second detection unit, the sheet conveyance speed that causes the sheet to form a loop having a size within the certain range. The image forming apparatus according to claim 2, wherein a range is obtained, and the target sheet carry-out speed is obtained within the speed range. The first detection unit includes a registration sensor for starting an image forming operation in an image forming unit having the transfer unit,
The sheet conveyance speed in the transfer unit is calculated based on a signal output period from the registration sensor and a length of the sheet in the sheet conveyance direction. The image forming apparatus described.   The second detection unit includes a paper discharge sensor for detecting the discharge of the sheet that passes through the fixing unit and is discharged to the paper discharge tray, and includes a signal output period from the paper discharge sensor and a sheet conveyance direction. The image forming apparatus according to claim 1, wherein a sheet carry-out speed at the fixing unit is calculated based on the length.

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