JP4920987B2 - Image forming apparatus - Google Patents

Description

The present invention relates to an image forming equipment, and more particularly relates to an image forming equipment comprising a fixing device.

  2. Description of the Related Art Conventionally, in an electrophotographic image forming apparatus provided with a fixing device, the conveyance speed at a transfer unit that transfers a toner image onto a sheet such as recording paper may fluctuate and affect the image. This is because the sheet conveyance speed at the transfer portion and the sheet conveyance speed at the fixing device that fixes the toner image on the sheet are slightly different, and the sheet is pulled between the transfer portion and the fixing device. It is. In particular, the fixing device during printing is heated for fixing the toner image on the sheet, and the pressure roller in the fixing device expands due to the heat, so that the sheet conveyance speed in the fixing device gradually increases. Tend. As a result, the difference in the conveyance speed between the transfer unit and the fixing device gradually increases, and the sheet is pulled to adversely affect the image such as transfer deviation.

  In view of this, there has been proposed a sensor (referred to as a loop sensor) that detects a deflection state (loop state) of a sheet in front of the fixing device on the conveyance path (see, for example, Patent Document 1). In such an apparatus, the conveyance speed of the fixing device is controlled based on the detected loop state of the sheet (referred to as loop control) so as to minimize the speed difference between the transfer unit and the fixing device. That is, the conveyance speed of the fixing device is set to two speeds, a high speed and a low speed, with respect to the conveyance speed of the transfer unit, and the two speeds are switched to perform loop control. In order to control the conveyance speed of the fixing device, the transfer unit and the fixing device are provided with different drive sources so that they can be driven independently.

JP-A-7-234604

  However, in the above conventional example, when controlling the conveyance speed of the fixing device, if the drive source is a servo motor such as a DC motor or a DC brushless motor, overshoot or undershoot is caused when the speed is switched due to the influence of the motor inertia or the like. Shooting occurs. When this overshoot or undershoot occurs, a large speed difference occurs with respect to the conveyance speed at the transfer unit, and the loop state of the sheet changes to adversely affect the image.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an image forming method and apparatus capable of suppressing an adverse effect on an image when the conveyance speed of the fixing device is switched to perform loop control.

  In order to achieve such an object, an image forming apparatus according to the present invention is an image forming apparatus that creates an image on a recording material, and is disposed on a transport path through which the recording material is transported, and transports the recording material. Transporting means, a second transporting means disposed on the downstream side of the first transporting means on the transporting path, for transporting the recording material transported by the first transporting means, the first transporting means, and the second transporting means A detecting unit that is arranged between the conveying unit and detects a deflection of the recording material conveyed on the conveying path, and the second conveying unit is set to the first speed and the first according to the detection result of the detecting unit. And a control means for controlling the conveyance of the recording material by switching to a second speed different from the speed, and the control means is driven by a servo motor by feedback control, and the first speed and the second speed To switch the second conveying means to the first speed By switching to a third speed, which is a speed between the first speed and the second speed, driving for a certain period of time at the third speed, and switching from the third speed to the second speed. The change width of the speed when switching to the speed is reduced.

  According to the present invention, when switching between the first speed and the second speed, since the speed is temporarily switched to an intermediate speed between the first speed and the second speed, the conveyance speed of the fixing device is switched to perform loop control. Can adversely affect the image.

The image forming apparatus and method according to the present invention will be described below with reference to the drawings.
(First embodiment)
FIG. 1 shows the configuration of an image forming apparatus according to this embodiment. Only one sheet, which is a recording material stacked on the paper feed cassette 1, is sent out from the paper feed cassette 1 by the pickup roller 2, and conveyed toward the registration roller 4 by the paper feed roller 3. Further, the sheet is conveyed to the process cartridge 5 by the registration roller 4 at a predetermined timing. The process cartridge 5 is integrally configured by a charging unit 6, a developing roller 7, a cleaner 8, and a photosensitive drum 9, and an unfixed toner image is formed on a sheet by a series of processes of a known electrophotographic process. To do. The surface of the photosensitive drum 9 is uniformly charged by the charging unit 6, and then image exposure based on an image signal is performed by the scanner unit 11. The laser light emitted from the laser diode 12 in the scanner unit 11 is scanned in the main scanning direction by the rotating polygon mirror 13 and the reflecting mirror 14 and in the sub-scanning direction by the rotation of the photosensitive drum 9. A two-dimensional latent image is formed on the surface.

  The latent image on the photosensitive drum 9 is visualized as a toner image by the developing roller 7, and the toner image is transferred onto the sheet conveyed from the registration roller 4 by the transfer roller 10. Each of the above rollers is driven via a gear by a main motor (not shown) formed of a DC brushless motor. Subsequently, the sheet on which the toner image is transferred is conveyed to the fixing device 15. The fixing device 15 is provided with a fixing roller 16 having a built-in heater and a pressure roller 17 that rotates while being in pressure contact with the fixing roller 16. The sheet is heated and pressed by passing between the fixing roller 16 and the pressure roller 17, and the unfixed toner image on the sheet is fixed to the sheet. The sheet is further discharged out of the image forming apparatus main body by the intermediate discharge roller 18 and the discharge roller 19 and the series of printing operations is completed.

  Each of the fixing roller 16, the intermediate paper discharge roller 18, and the paper discharge roller 19 is driven via a gear by a fixing motor (not shown) including a DC brushless motor provided separately from the main motor. . Further, a loop sensor 20 serving as a detection unit is provided on the conveyance path in front of the fixing device 15 to detect the loop state of the conveyed sheet. The series of processing is controlled by a CPU (not shown).

  Next, the loop sensor 20 in the present invention will be described with reference to FIG. The loop sensor 20 is provided on the sheet conveyance path between the transfer unit including the photosensitive drum 9 and the transfer roller 10 and the fixing device 15. A lever 22 that is rotatable about a shaft 21 is rotated by being pushed by the sheet, and the photo interrupter 24 is shielded by a flag 23 at the rotation origin portion to detect the loop state of the sheet. The operation of the loop sensor during sheet conveyance will be described below.

  First, when the sheet does not reach the loop sensor 20, the lever 22 of the loop sensor 20 stands as shown in FIG. 2A by a spring (not shown), and the flag 23 shields the photo interrupter 24 from light. It is in a transparent state. Next, when the sheet is conveyed from the transfer unit and reaches the loop sensor as shown in FIG. 2B, the sheet pushes down against the lever 22 of the loop sensor 20. At the same time, the flag 23 is rotated so that light is not transmitted, and the photo interrupter 24 is in a light shielding state.

  Further, when the sheet is conveyed, the sheet is drawn into the gap between the fixing roller 16 and the pressure roller 17 of the fixing device 15 and is conveyed by both the transfer unit and the fixing device 15. When the time further elapses, the conveyance speed of the fixing device 15 gradually increases with respect to the conveyance speed of the transfer portion due to the thermal expansion of the pressure roller 17, so that the sheet eventually becomes fixed as shown in FIG. Pulled to move upward and the loop decreases. At the same time, the lever 22 of the loop sensor 20 gradually rises, and the flag 23 does not block the photo interrupter 24 and becomes transparent again. In the present embodiment, the light blocking / transmitting state of the photo interrupter 24 is detected to detect the sheet loop state, and the speed control of the fixing motor is performed based on the detection result.

  Next, control of the fixing motor that drives the fixing roller 16 of the fixing device 15 and determines the conveyance speed of the fixing device 15 will be described with reference to FIG. The fixing motor of this embodiment is a three-phase DC brushless motor, and has three coils 25 to 27 of U, V, and W phases and a rotor. Three Hall elements 28 to 30 are provided for detecting the rotational position of the rotor, and their outputs are input to the motor control IC 31. The three coils 25 to 27 are connected to the midpoints of the high-side FETs 32 to 34 and the low-side FETs 35 to 37, respectively. Accordingly, the motor control IC 31 sequentially turns on / off the FETs 32 to 37 based on the rotational position information of the rotor detected by the Hall elements 28 to 30, and causes the coils 25 to 27 to flow to rotate the fixing motor. The current flowing through the fixing motor is converted into a voltage by the current detection resistor 44, and the motor drive IC 31 performs PWM control when the high-side FETs 32-34 are turned on so that the detected current value is equal to or less than a predetermined current value. Yes.

  Further, fine multipolar magnetization is applied to the end face of the rotor, and a print pattern (FG pattern) 45 on a rectangular wave is provided at a position facing it. When the rotor rotates, an AC voltage is induced at both ends of the FG pattern 45 and is detected by the motor control IC 31. Since the detected frequency of the alternating voltage is proportional to the speed of the rotor, it is shaped into a digital waveform in the motor control IC 31 and transmitted to the CPU 46 as an FG signal representing the speed of the motor. The CPU 46 compares the frequency of the FG signal received from the motor with the target frequency of the FG signal calculated from the set target speed. Then, an acceleration (ACC) signal is transmitted to the motor control IC 31 if the actual speed obtained from the received FG signal with respect to the target speed is slow, and a deceleration (DEC) signal is transmitted to the target speed. The motor control IC 31 performs acceleration or deceleration according to the received signal, and performs control so as to rotate at a constant speed at a target speed.

  FIG. 4 shows a time chart of the change in the speed of the fixing motor and the acceleration signal and the deceleration signal when the target speed of the present embodiment is switched. For example, when the target speed is switched from low speed to high speed, when the target speed is switched in the CPU, the CPU transmits an acceleration signal to the motor control IC because the actual speed obtained by the FG signal is slower than the target speed. The motor control IC that has received the acceleration signal starts to accelerate the motor. The CPU constantly monitors the FG signal and transmits a deceleration signal when the actual speed reaches the target speed. The motor control IC immediately starts decelerating, but actually starts decelerating after overshooting due to the influence of the inertia of the motor and the like. Next, when decelerating and falls below the target speed, the CPU transmits an acceleration signal again, and the motor starts to accelerate. In this way, control is performed to rotate at a constant speed at the target speed while switching between the acceleration signal and the deceleration signal in accordance with the FG signal.

  Next, the loop control method of this embodiment will be described. The feature of the loop control of the present embodiment is that, when switching the speed of the fixing motor, speed switching is performed so that overshoot and undershoot of the speed do not occur as much as possible. The operation of the loop sensor and the speed control of the fixing motor of this embodiment will be described with reference to the time chart shown in FIG.

  Here, the fixing motor speed when the conveyance speed of the fixing device is slower than the conveyance speed of the transfer unit is called a first speed, and the speed of the fixing motor when it is faster than the conveyance speed of the transfer unit is called a second speed. The speed is set in advance. If the conveyance speed of the fixing device is slower than the conveyance speed at the transfer unit, the sheet can bend and bend (loop), so the lever of the loop sensor is pushed down, and the photo interrupter changes from the transmission state to the light shielding state. Accordingly, when the light shielding state is detected, the sheet conveyance is eliminated by driving the conveyance speed of the fixing device faster than the conveyance speed of the transfer unit. Here, when the speed is switched, if the speed of the fixing motor is switched from the first speed to the second speed at once, a large overshoot occurs as shown by a dotted line in FIG. Since a large speed difference occurs, the image is adversely affected. Note that the state detection of the loop sensor eliminates chattering that occurs due to minute movements of the lever, so if the sensor remains in the same state for a certain period of time, it is determined in that state.

  In the present embodiment, in order to reduce such overshoot, the first speed → the third speed, which is an average speed of the first speed and the second speed, as shown by the solid line in FIG. The speed is switched in two stages in the order of the third speed → the second speed. As a result, the speed change width per speed change is halved, and the amount of overshoot is also reduced. In the present embodiment, the time t controlled at the third speed is longer than the time until the actual speed of the fixing motor reaches the third speed, so the actual speed of the fixing motor converges to the third speed. Set the time to be stable.

  Next, when the speed of the fixing motor increases and the conveying speed of the fixing device becomes faster than the conveying speed of the transfer unit, the deflection of the sheet disappears, so that the lever 22 of the loop sensor rises and the photo interrupter 24 changes to the transmissive state. Since the transmission state indicates a state in which there is no deflection, this time, the conveyance speed of the fixing device is lowered to be slower than the conveyance speed of the transfer unit so that the sheet bends. When switching the speed, similarly to the above, the speed of the fixing motor is not changed from the second speed to the first speed once as shown by the dotted line in FIG. 5, but as shown by the solid line in FIG. Switching is performed in two stages in the order of second speed → third speed → first speed. By controlling in this way, the change width of the speed is reduced, and the amount of undershoot at the time of speed switching can be suppressed. Thus, every time the speed is switched, the speed is switched via the third speed in accordance with the movement of the loop sensor. In the present embodiment, only one third speed is set between the first speed and the second speed, but a plurality of intermediate speeds may be set. When a plurality of intermediate speeds are set, they can be set at arbitrary intervals. For example, the effects of the present invention can also be achieved by setting them at equal intervals.

  As described above, in this embodiment, when switching the speed of the fixing motor in the loop control, an average speed between the current speed and the target speed is set, and the average speed is always inserted between the speeds when switching to the target speed. Switch. As a result, overshoot and undershoot at the time of speed switching can be suppressed to a lower level, and image deterioration due to this can be prevented.

(Second Embodiment)
In the first embodiment described above, when switching from the first speed to the second speed, the intermediate speed is the average speed of the first speed and the second speed, but in order to achieve the effects of the present invention. Need not be average. Therefore, the present embodiment is characterized in that the intermediate speed is flexibly set so as to further reduce overshoot and undershoot. Since the configuration of the image forming apparatus in the present embodiment is the same as that in the first embodiment, description thereof is omitted.

  This embodiment will be described with reference to the time chart of FIG. First, when the loop sensor 24 changes from the transmissive state to the light-shielded state, deflection occurs, so the fixing motor is switched from the first speed to the second speed to eliminate the deflection. At this time, the intermediate speed is set to a fourth speed that is faster (closer to the second speed) than the average speed (the third speed in the first embodiment) of the first speed and the second speed, Switching is performed in two stages in the order of the first speed → the fourth speed → the second speed. Note that the time t controlled at the fourth speed is set to a time at which the actual speed of the fixing motor converges and stabilizes at the fourth speed as in the first embodiment.

  The change from the first speed to the fourth speed has a larger speed change width than the first embodiment, so the overshoot amount at this time becomes large. However, the change from the fourth speed to the second speed has a small speed change width, so the overshoot amount can be reduced. Since it is the speed difference between the conveyance speed of the fixing device and the conveyance speed of the transfer unit that affects the image, the present embodiment in which the overshoot amount is reduced as a whole can further suppress adverse effects on the image. In other words, in the speed difference, the overshoot from the first speed to the fourth speed is larger than the overshoot from the fourth speed to the second speed, so that adverse effects on the image can be further suppressed. It is.

  Next, the case where the loop sensor changes from the light shielding state to the transmission state will be described. In this case, the second speed is switched to the first speed. At this time, the intermediate speed is set to a fifth speed that is lower than the average speed of the second speed and the first speed, and switching is performed in the order of the second speed → the fifth speed → the first speed. Do. At this time, similarly to the above, the undershoot can be reduced because the acceleration time at the time of switching from the fifth speed to the first speed at which the speed difference with the transfer speed of the transfer portion is the largest is short. It is possible to further suppress the adverse effect on the image.

  As described above, in the present embodiment, the intermediate speed when switching the speed is minimized so that overshoot and undershoot are minimized when the speed difference between the transport speed of the fixing device and the transport speed at the transfer unit is the largest. Set the speed asymmetrically. As a result, it is possible to suppress image deterioration due to switching of the conveyance speed of the fixing device.

(Third embodiment)
In addition to image degradation due to overshoot and undershoot as described above, when switching the conveyance speed of the fixing device in loop control, a shock due to speed fluctuation at the time of speed convergence is transmitted to the transfer section via the sheet, and there is an adverse effect on the image. May occur. The present embodiment is characterized in that control is performed not only to reduce the amount of overshoot and undershoot when switching the speed of the fixing motor, but also to reduce the shock of speed switching. The configuration of the image forming apparatus in the present embodiment is the same as that in the first embodiment, and a description thereof will be omitted.

  With reference to the time chart of FIG. 7, the present embodiment will be described in comparison with the first embodiment. In the first embodiment, as indicated by the dotted line in FIG. 7, for example, the loop sensor changes from the transmission state to the light shielding state, and when the fixing motor is switched from the first speed to the second speed, The time for driving at this speed was until the actual speed converged to the third speed and stabilized. On the other hand, in the present embodiment, the time t during driving at the third speed as shown by the solid line in FIG. 7 is set to be shorter than the time at which the actual speed converges to the third speed. Here, the time until the actual speed exceeds the third speed and reaches the vicinity of the top of the overshoot is measured in advance and set to that time. As a result, the target speed is switched to the second speed before the actual speed reaches the third speed and converges, and the shock due to speed fluctuation when the speed converges at the third speed can be reduced. . Furthermore, since the deceleration signal is transmitted to the motor when the third speed is reached by setting the third speed once even for a short time, the acceleration is weakened. Therefore, it is more than switching from the first speed to the second speed. The amount of chute can be reduced. Here, the intermediate speed is the third speed, which is the average of the first speed and the second speed, but is not limited to this. For example, the fourth speed or the fifth speed of the second embodiment Speed may be used.

  As described above, in this embodiment, when the speed of the fixing motor is switched, the time for setting the intermediate speed is set shorter than the time for the actual speed to converge and stabilize at the intermediate speed. As a result, the number of times the shock due to speed switching is transmitted to the seat can be reduced, and the amount of overshoot and undershoot at the target speed can also be reduced.

1 is a diagram illustrating a configuration of an image forming apparatus according to an exemplary embodiment. It is a figure explaining the loop sensor in this embodiment. It is a figure explaining control of the fixing motor of this embodiment. It is a time chart of the change of the speed of the fixing motor when the conventional target speed is switched, and the acceleration signal and the deceleration signal. 6 is a time chart of a change in the speed of a fixing motor when an object speed is switched according to the first embodiment, and an acceleration signal and a deceleration signal. 12 is a time chart of a change in the speed of the fixing motor when the target speed of the second embodiment is switched, and an acceleration signal and a deceleration signal. It is a time chart of the change of the speed of the fixing motor when the target speed of the third embodiment is switched, and the acceleration signal and the deceleration signal.

Explanation of symbols

9 Photosensitive drum 10 Transfer roller 15 Fixing device 16 Fixing roller 17 Pressure roller 20 Loop sensor 31 Motor control IC
46 CPU

Claims (8)

An image forming apparatus of making images on a recording material,
Disposed on the transport path of the recording material is conveyed, a first conveying means for conveying the record member,
A second conveying unit disposed on the downstream side of the first conveying unit on the conveying path and configured to convey the recording material conveyed by the first conveying unit;
A detecting unit disposed between the first conveying unit and the second conveying unit, and detecting a deflection of the recording material conveyed on the conveying path;
In accordance with a detection result of said detecting means, said second conveying means and the first speed, switch to the second speed different from the first speed drive, control means for controlling the transport of the record material The control means is driven by a servo motor by feedback control , and when switching between the first speed and the second speed, the second transport means is moved from the first speed to the first speed . speed and switched to the third speed of the Ru velocity der between the second speed, a certain time driving in the third speed, by switching from the third speed to the second speed, An image forming apparatus, wherein a change width of a speed when switching to the second speed is reduced .   The image forming apparatus according to claim 1, wherein the third speed is an average speed of the first speed and the second speed. Wherein, you switch from the first speed to the third speed, the transport speed of the recording material after the convergence to reach the speed of the third, the second from the third speed The image forming apparatus according to claim 1, wherein the image forming apparatus is switched to a speed. The control unit switches from the first speed to the third speed, and before the recording material conveyance speed reaches the third speed and converges, the control unit shifts the second speed from the third speed to the second speed. The image forming apparatus according to claim 1, wherein the image forming apparatus is switched to a speed. When the second conveying unit is driven at the first speed and the detecting unit detects a deflection of the recording material, the control unit moves the second conveying unit to the second conveying unit. 2. The recording material is controlled by switching to a speed and switching the second transport means to the first speed when the detection means no longer detects deflection after the switching. 5. The image forming apparatus according to any one of 4 above. When the control means switches from the first speed to the second speed, the control means drives the second transport means at a fourth speed different from the third speed, and starts from the second speed. when switched to the first speed, any one of claims 1 to 5, characterized in that for driving the second conveying means at different fifth speed and the third speed and the fourth speed The image forming apparatus described in 1. The fourth speed is faster than the third speed, which is an average speed of the first speed and the second speed, and the fifth speed is slower than the third speed. The image forming apparatus according to claim 6 . The first conveying means is a transfer means for transferring an image to the recording material;
It said second conveying means, an image forming apparatus according to any one of claims 1 to 7, characterized in that the image transferred by said transfer means is a fixing means for fixing to the recording material.

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