JP2019020658A - Image formation apparatus - Google Patents

Abstract

To detect the current in the state where the voltage is stable not in the fall of the voltage even when the electric resistance value of a transfer roller changes in the inter-sheet ATVC, and correct the reference voltage based on the detection.SOLUTION: An image formation apparatus starts changeover of the voltage applied to a secondary transfer outer roller from the secondary transfer voltage to the reference voltage before correction (S2) in a case where the potential difference between the secondary transfer voltage and the reference voltage before correction is smaller than a threshold in the inter-sheet ATVC (NO in S1), starts changeover of the voltage applied to the secondary transfer outer roller from the secondary transfer voltage to the changeover voltage (S3) in a case where the potential difference between the secondary transfer voltage and the reference voltage before correction is equal to or greater than the threshold (YES in S1), detects the current at a prescribed interval (S5) when the current detection start timing arrives (YES in S4), and can properly correct the reference voltage based on the detection because the current is detected in the stable state of reaching the reference voltage before correction or changeover voltage not in the fall of the voltage.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an image forming apparatus using electrophotographic technology, such as a printer, a copying machine, a facsimile machine, or a multifunction machine.

  As an image forming apparatus, an intermediate transfer type image forming apparatus is known in which a toner image formed on a photosensitive drum is primarily transferred to an intermediate transfer belt, and the toner image primarily transferred to the intermediate transfer belt is secondarily transferred to a recording material. ing. There is also known a direct transfer type image forming apparatus that directly transfers a toner image formed on a photosensitive drum onto a recording material. The toner image formed on the intermediate transfer belt or the photosensitive drum is transferred onto a recording material at a transfer nip portion formed by bringing a conductive transfer roller into contact with the intermediate transfer belt or the photosensitive drum. In order to transfer the toner image to the recording material, a transfer voltage is applied to the transfer roller by a high voltage power source.

  The transfer roller has an electrical resistance value that changes due to a change in environment (for example, temperature and humidity) or deterioration due to long-term use. When an image forming job for continuously forming images on a large number of recording materials does not change the transfer voltage even though the electric resistance value of the transfer roller changes, a target current suitable for transfer flows to the transfer nip portion. Transfer defects may occur. In view of this, an apparatus has been proposed that performs an inter-sheet ATVC (Active Transfer Voltage Control) between a recording material and a recording material that passes through a transfer nip portion each time an image is formed on a predetermined number of recording materials during an image forming job. (Patent Document 1). In the inter-sheet ATVC, a current detected in response to the application of a reference voltage that allows a target current to flow through the transfer nip when there is no recording material, and a transfer roller obtained in advance during an image forming job pre-rotation, etc. The reference voltage is corrected on the basis of the voltage-current characteristics. Then, the sum of the corrected reference voltage and a predetermined voltage (referred to as a paper sharing voltage or the like) determined in advance by the type of recording material, for example, is set as a new transfer voltage.

Japanese Patent No. 3847875

  By the way, depending on the reference voltage and the paper sharing voltage, it may take a long time to switch the voltage applied to the transfer roller during the inter-paper ATVC. Therefore, in the inter-sheet ATVC, when the voltage applied to the transfer roller is switched from the transfer voltage to the reference voltage, a current is detected during the fall of the voltage before the actual voltage applied to the transfer roller reaches the reference voltage. It becomes easy. Conventionally, even if the reference voltage is corrected based on the current detected during the voltage fall, the correction is not properly performed, and it is easy to cause an image defect when a transfer voltage based on the corrected reference voltage is applied. . In particular, in order to further increase the productivity of the image forming apparatus, this is remarkable when the distance between the recording material that is continuously conveyed and the recording material (between sheets) is further shortened.

  The present invention has been made in view of the above-described problem, and an image capable of suppressing adverse effects caused by setting the transfer voltage based on the current detected during the fall of the voltage applied to the transfer roller during the ATVC between sheets. An object is to provide a forming apparatus.

  The image forming apparatus according to the present invention includes an image carrier that carries and rotates a toner image, a transfer nip formed in contact with the image carrier, and a transfer voltage applied to the image on the recording material. A transfer member for transferring a toner image on the carrier, a power source for applying a voltage to the transfer member, current detection means for detecting a current flowing through the transfer member, and a recording material passing through the transfer nip portion. A reference voltage for setting the transfer voltage is obtained based on a detection result of the current detection means when a voltage is applied to the transfer member when there is no voltage, and is set according to the reference voltage and the recording material And a control unit that controls the transfer voltage based on the shared voltage, and the control unit performs subsequent recording after the preceding recording material passes through the transfer nip during an image forming job. Material in the transfer nip The voltage applied to the transfer member during the period until the transfer member is changed based on the current detected by the current detection means when the transfer voltage is switched from the transfer voltage to a predetermined voltage lower in absolute value than the transfer voltage. When the setting mode for setting the voltage is executable and the setting mode is executed when the reference voltage is the same, the shared voltage of the recording material transferred immediately before the setting mode is executed is the first voltage. In the case of one voltage, the predetermined voltage is set to the reference voltage, and the shared voltage of the recording material transferred immediately before executing the setting mode is a second voltage whose absolute value is lower than the first voltage. Is a potential difference between the transfer voltage applied immediately before the setting mode is executed and the predetermined voltage, the transfer voltage applied immediately before the setting mode is executed, and the reference Set to a value smaller than the potential difference between the pressure, characterized in that.

  The image forming apparatus according to the present invention includes an image carrier that carries and rotates a toner image, a transfer nip formed in contact with the image carrier, and a transfer voltage applied to the image on the recording material. A transfer member for transferring a toner image on the carrier, a power source for applying a voltage to the transfer member, current detection means for detecting a current flowing through the transfer member, and a recording material passing through the transfer nip portion. A reference voltage for setting the transfer voltage is obtained based on a detection result of the current detection means when a voltage is applied to the transfer member when there is no voltage, and is set according to the reference voltage and the recording material And a control unit that controls the transfer voltage based on the shared voltage, and the control unit performs subsequent recording after the preceding recording material passes through the transfer nip during an image forming job. Material in the transfer nip The voltage applied to the transfer member during the period until the transfer member is changed based on the current detected by the current detection means when the transfer voltage is switched from the transfer voltage to a predetermined voltage lower in absolute value than the transfer voltage. A setting mode for setting a voltage can be executed, and when the setting mode is executed for recording materials having the same shared voltage, when the reference voltage is a first reference voltage, the predetermined voltage is When the reference voltage is set to the reference voltage and the reference voltage is a second reference voltage whose absolute value is lower than the first reference voltage, the transfer voltage and the predetermined voltage applied immediately before the setting mode is executed The potential difference is set to a value smaller than a potential difference between the transfer voltage applied immediately before the setting mode is executed and the reference voltage.

  According to the present invention, even if the electrical resistance value of the transfer member changes, the current is detected in a state where the voltage applied to the transfer member is stable. Therefore, the transfer voltage is appropriately set using the detected current. be able to.

1 is a schematic diagram illustrating a configuration of an image forming apparatus according to an embodiment. Schematic which shows a control part. The flowchart which shows the inter-paper ATVC of this embodiment. The graph explaining a switching voltage table. The timing chart explaining the inter-paper ATVC of this embodiment. 6 is a timing chart for explaining a case where the current-current characteristic of the outer secondary transfer roller is used. The figure explaining correction | amendment of the reference voltage using the voltage-current characteristic of the present secondary transfer outer roller. The figure explaining front rotation ATVC. The timing chart explaining the conventional inter-paper ATVC. The figure explaining correction | amendment of the reference voltage using the voltage current characteristic of the secondary transfer outer roller obtained by pre-rotation ATVC. 9 is a timing chart for explaining a conventional inter-paper ATVC when the inter-paper interval is short. FIG. 2 is a schematic diagram illustrating a direct transfer type image forming apparatus.

<Image forming apparatus>
The image forming apparatus of this embodiment will be described. First, the configuration of the image forming apparatus of this embodiment will be described with reference to FIG. An image forming apparatus 1 shown in FIG. 1 is a tandem intermediate transfer type full color printer in which image forming units 3Y, 3M, 3C, and 3K of yellow, magenta, cyan, and black are arranged along an intermediate transfer belt 2. Of course, the image forming apparatus 1 is not limited to this, and may be a mono-color printer including only one black image forming unit 3K.

  In the image forming unit 3Y, a yellow toner image is formed on the photosensitive drum 4Y and is primarily transferred to the intermediate transfer belt 2. In the image forming unit 3M, a magenta toner image is formed on the photosensitive drum 4M, and transferred onto the yellow toner image on the intermediate transfer belt. In the image forming units 3C and 3K, a cyan toner image and a black toner image are formed on the photosensitive drums 4C and 4K, respectively, and are sequentially transferred onto the intermediate transfer belt 2.

  The image forming units 3Y, 3M, 3C, and 3K are configured in substantially the same manner except that the color of toner used in the developing devices 7Y, 7M, 7C, and 7K is different from yellow, magenta, cyan, and black. Therefore, hereinafter, the image forming unit 3Y will be described in detail, and the image forming units 3M, 3C, and 3K will be described by replacing Y at the end of the symbol with M, C, and K.

  In the image forming unit 3Y, a charging roller 5Y, an exposure device 6Y, a developing device 7Y, a primary transfer roller 8Y, and a drum cleaning device 9Y are disposed so as to surround the photosensitive drum 4Y. The photosensitive drum 4Y is a drum-shaped electrophotographic photosensitive member in which a photosensitive layer is formed on the outer peripheral surface of an aluminum cylinder, for example, and is rotated in the direction of arrow R1 in the drawing at a predetermined process speed by a motor (not shown).

  The charging roller 5 </ b> Y charges the surface of the photosensitive drum 4 </ b> Y to a uniform negative dark potential by applying a charging voltage obtained by superimposing an AC voltage on a negative DC voltage. The exposure device 6Y writes an electrostatic latent image of the image on the surface of the charged photosensitive drum 4Y by scanning with a rotating mirror a laser beam obtained by ON-OFF modulating the scanning line image data obtained by developing the separation color image of each color.

  The developing device 7Y supplies toner to the photosensitive drum 4Y to develop the electrostatic latent image into a toner image. In the developing device 7Y, the developing sleeve 7S disposed on the surface of the photosensitive drum 4Y with a slight gap is rotated in the counter direction with respect to the photosensitive drum 4Y at a predetermined process speed. The developing device 7Y contains, for example, a two-component developer containing a non-magnetic toner having a negative charging characteristic and a magnetic carrier having a positive charging characteristic. The two-component developer is carried on the developing sleeve 7S, and the photosensitive drum 4Y. To the opposite part. By applying a developing voltage, in which an AC voltage is superimposed on a DC voltage, to the developing sleeve 7S, the negatively charged toner is transferred to the exposed portion of the photosensitive drum 4Y having a relatively positive polarity and electrostatically charged. The latent image is reversely developed. In FIG. 1, the developing sleeve 7S is described only in the developing device 7Y, but the developing devices 7M, 7C, and 7K also have the developing sleeve 7S.

  The primary transfer roller 8Y presses the intermediate transfer belt 2 to form a primary transfer portion T1 between the photosensitive drum 4Y and the intermediate transfer belt 2. A primary transfer power supply (not shown) is connected to the primary transfer roller 8Y, and the primary transfer power supply applies a positive primary transfer voltage to the primary transfer roller 8Y, whereby a negative electrode on the photosensitive drum 4Y (on the photosensitive member). The toner image charged to be neutral is transferred to the intermediate transfer belt 2. In the drum cleaning device 9Y, for example, a cleaning blade made of a polyurethane material is in contact with the surface of the photosensitive drum 4Y, and the transfer residual toner that passes through the primary transfer portion T1 and remains on the photosensitive drum 4Y is collected by the cleaning blade. .

  The intermediate transfer belt 2 as an image carrier is an intermediate transfer body that can rotate in contact with the photosensitive drums 4Y to 4K. The intermediate transfer belt 2 is supported around a tension roller 31, a driving roller 32, and a secondary transfer inner roller 33, and is driven by the driving roller 32 to rotate. The intermediate transfer belt 2 moves in the same direction (arrow R2 direction in the figure) as the rotation direction of the photosensitive drums 4Y to 4K (arrow R1 direction in the figure) at a position where it contacts the photosensitive drums 4Y to 4K.

  The four color toner images transferred onto the intermediate transfer belt (on the intermediate transfer member) by the image forming units 3Y to 3K are conveyed to the secondary transfer unit T2 which is a transfer nip unit, and are recorded on the recording material P (paper, OHP sheet). Secondary transfer to a sheet material). The recording material P is taken out from the recording material cassette 101 by the pickup roller 102, separated one by one, and sent out to the conveyance path. The recording material P on the conveyance path is sent to the secondary transfer portion T2 in time with the toner image on the intermediate transfer belt 2. Then, the recording material P on which the four color toner images are secondarily transferred is sent to the fixing device 40, and the toner image on the recording material is heated and fixed. The recording material P on which the toner image is fixed is discharged out of the machine body.

  The secondary transfer portion T2 as a transfer nip portion is formed by pressing the secondary transfer outer roller 34 toward the secondary transfer inner roller 33 with the intermediate transfer belt 2 interposed therebetween. The secondary transfer outer roller 34 as a transfer member is made of, for example, an ion conductive foamed rubber (for example, NBR, EPDM, urethane or the like in which a surfactant is injected on a metal shaft, or an ion conductive polymer. A roller formed with an elastic layer (such as a rubber layer). A secondary transfer power supply 50 with a variable supply voltage is connected to the secondary transfer outer roller 34. In the present embodiment, the secondary transfer inner roller 33 is connected to the ground potential (0 V), while the secondary transfer power supply 50 applies a positive secondary transfer voltage having a polarity opposite to that of the toner to the secondary transfer outer roller 34. By applying this, a transfer electric field is generated in the secondary transfer portion T2. The secondary transfer outer roller 34 responds to the transfer electric field by transferring the four color toner images transferred to the intermediate transfer belt 2, that is, the toner images charged with negative polarity of yellow, magenta, cyan, and black, to the secondary transfer unit. Secondary transfer can be performed collectively to the recording material P conveyed to T2.

<Control unit>
The image forming apparatus 1 according to the present embodiment includes a control unit 200 as a control unit. The control unit 200 will be described with reference to FIG. The controller 200 may be connected to various devices such as a motor and a power source for operating the image forming apparatus 1 other than those shown in the drawing, but the illustration and description thereof are omitted here because they are not the gist of the invention.

  The control unit 200 as a control unit performs various controls of the image forming apparatus 1 such as an image forming operation, and includes a CPU 201 (Central Processing Unit) and a memory 202 such as a ROM, a RAM, or a hard disk device. The memory 202 stores various data such as various programs such as an image forming job, a reference voltage, a paper sharing voltage, a secondary transfer voltage, which will be described later, and a plurality of current values to be passed during a pre-rotation ATVC which will be described later. The control unit 200 can execute various programs stored in the memory 202, and can execute the various programs to operate the image forming apparatus 1. Note that the memory 202 can also temporarily store arithmetic processing results and the like accompanying the execution of various programs.

  The image forming job is a series of periods from the start of image formation to the completion of the image forming operation based on a print signal for forming an image on the recording material P. Specifically, it refers to the period from pre-rotation (preparation operation before image formation) after receiving a print signal (job input) to post-rotation (operation after image formation). It is a period that includes the interval. In this specification, the interval between sheets (during non-passage) means that the recording material P preceding in the image forming job passes through the secondary transfer portion T2 (see FIG. 1) and the subsequent recording material P is secondary. This is a period until the transfer portion T2 is reached. In other words, the sheet interval is a predetermined period in which a corresponding region between the recording material P and the recording material P passes through the secondary transfer portion T2 during the image forming job.

  In addition to the CPU 201 and the memory 202, the control unit 200 includes a constant voltage control circuit 301, a constant current control circuit 302, a voltage detection circuit 303, a current detection circuit 304, and a voltage generation circuit 305. These constant voltage control circuit 301, constant current control circuit 302, voltage detection circuit 303, current detection circuit 304, and voltage generation circuit 305 are arranged on a substrate (not shown) and control the secondary transfer power supply 50 (see FIG. 1). . The CPU 201 can transmit a constant voltage setting signal to the constant voltage control circuit 301, a constant current setting signal to the constant current control circuit 302, and a transfer clock signal to the voltage generation circuit 305. On the other hand, the CPU 201 can acquire a voltage detection signal from the voltage detection circuit 303 and a current detection signal from the current detection circuit 304. The CPU 201 can perform sampling by performing A / D conversion on the voltage detection signal acquired from the voltage detection circuit 303 and the current detection signal acquired from the current detection circuit 304.

  Based on the constant voltage setting signal and constant current setting signal transmitted from the CPU 201, the constant voltage control circuit 301 and the constant current control circuit 302 operate. In response to this, the voltage generation circuit 305 generates a voltage (output signal) to be applied to the secondary transfer outer roller 34 (see FIG. 1). In this way, a voltage is applied to the secondary transfer outer roller 34 by the secondary transfer power supply 50. A voltage detection circuit 303 as voltage detection means detects a voltage from the voltage generation circuit 305, and the detected voltage is output to the constant voltage control circuit 301 and the CPU 201 as an analog voltage detection signal (Vsns). A current detection circuit 304 as current detection means detects a current from the voltage generation circuit 305, and the detected current is output to the constant current control circuit 302 and the CPU 201 as an analog current detection signal.

  The operation of the control unit 200 will be described in more detail. The constant voltage control circuit 301 includes an operational amplifier (IC301) and a diode (D301). The constant current control circuit 302 includes an operational amplifier (IC302) and a diode (D302). The voltage detection circuit 303 includes resistors (R303 and R304). The current detection circuit 304 includes an operational amplifier (IC303) and a resistor (R305). The voltage generation circuit 305 includes a resistor (R301, R302), a transistor (Q301), a transformer (T301), and an FET (Q302).

  The voltage generation circuit 305 adjusts the voltage on the primary side of the transformer (T301) by the transistor (Q301) based on the constant voltage setting signal or the constant current setting signal, and drives the primary side of the transformer (T301) according to the transfer clock signal. Thus, a voltage (output signal) is generated. The voltage detection circuit 303 detects the voltage applied to the secondary transfer outer roller 34 by dividing the voltage generated by the voltage generation circuit 305 with resistors (R303, R304), and sends the voltage to the constant voltage control circuit 301 and the CPU 201. Output (voltage detection signal). The constant voltage control circuit 301 can perform feedback control so that the voltages of the constant voltage setting signal and the voltage detection signal match. That is, if the voltage detection signal is larger than the constant voltage setting signal, the output is controlled by the operational amplifier (IC301) so that the voltage output from the voltage generation circuit 305 is reduced. On the other hand, if the voltage detection signal is smaller than the constant voltage setting signal, the output is controlled by the operational amplifier (IC301) so that the voltage output from the voltage generation circuit 305 is increased.

The current detection circuit 304 detects the current flowing through the secondary transfer outer roller 34 based on the voltage generated by the voltage generation circuit 305 (current detection signal). The current (Ib) flowing through the secondary transfer outer roller 34 can be expressed by the following equation 1 using the current detection signal (Isns) detected by the current detection circuit 304. Note that “Vref” in Equation 1 is a predetermined voltage applied to the non-inverting input terminal (+) side of the operational amplifier (IC 303) of the current detection circuit 304.
Isns = Ib * resistance value of resistance (R305) + Vref Equation 1

  The constant current control circuit 302 can perform feedback control so that the currents of the constant current setting signal and the current detection signal match. That is, if the current detection signal is larger than the constant current setting signal, the output is controlled by the operational amplifier (IC 302) so that the voltage output from the voltage generation circuit 305 is reduced. On the other hand, if the current detection signal is smaller than the constant current setting signal, the output is controlled by the operational amplifier (IC 302) so that the voltage output from the voltage generation circuit 305 is increased. Thus, the voltage applied to the secondary transfer outer roller 34 is adjusted.

  The diode (D301) of the constant voltage control circuit 301 and the diode (D302) of the constant current control circuit 302 compare the output of the constant voltage control circuit 301 and the output of the constant current control circuit 302. A signal for increasing the voltage output from the voltage generation circuit 305 is input to the base of the transistor (Q301) of the voltage generation circuit 305. During constant voltage control, the output of the operational amplifier (IC301) operates so as to be larger than the output of the operational amplifier (IC302), and the output of the operational amplifier (IC302) is ground (GND) by comparing the constant current setting signal and the current detection signal. Works like sticking to a level. On the other hand, during constant current control, the output of the operational amplifier (IC302) operates so as to be larger than the output of the operational amplifier (IC301). The output of the operational amplifier (IC301) is ground level by comparing the constant voltage setting signal and the voltage detection signal. It works to stick to.

  Next, a method for setting the secondary transfer voltage will be described. Since the toner image on the intermediate transfer belt 2 (on the image carrier) is transferred to the recording material P during the image forming job, the control unit 200 applies a constant secondary transfer voltage to the secondary regardless of the amount of toner related to image formation. Constant voltage control applied to the outer transfer roller 34 is performed. However, it is necessary to apply a secondary transfer voltage so that a target current for transferring the toner image flows to the secondary transfer portion T2. If the current flowing through the secondary transfer portion T2 is smaller than the target current, a transfer failure may occur in which the toner image is not sufficiently transferred from the intermediate transfer belt 2 to the recording material P. Conversely, the current flowing through the secondary transfer portion T2 Is larger than the target current, an abnormal discharge may occur in the secondary transfer portion T2. In order to avoid this, it is necessary to flow a current that does not cause a transfer failure or abnormal discharge in the secondary transfer portion T2 to the secondary transfer portion T2 as a target current.

  Therefore, the control unit 200 performs the pre-rotation ATVC when the image forming job is pre-rotated. The pre-rotation ATVC is control for setting, as a reference voltage, a voltage that allows a target current to flow through the secondary transfer portion T2 when the recording material P does not pass through the secondary transfer portion T2. Done in Since the reference voltage that allows the target current to flow changes according to changes in the environment (for example, temperature and humidity) and changes in the electrical resistance value of the secondary transfer outer roller 34 due to long-term use, the control unit 200 is Rotate ATVC is performed. Also in the present embodiment, the pre-rotation ATVC is executed at the time of the first image forming job that is started after the cumulative number of image-formed recording materials P exceeds a predetermined number (for example, 1000 sheets), for example. Is done.

<Pre-rotation ATVC>
The pre-rotation ATVC will be briefly described with reference to FIG. 8 with reference to FIG. 1 and FIG. The control unit 200 sequentially applies a plurality of currents (I1 and I2 in FIG. 8) stored in the memory 202 to the secondary transfer outer roller 34 in order, and corresponding test voltages (V1 in FIG. 8). , V2) are applied in order. However, one current (I1) has a current value smaller than the target current, and the other current (I2) has a current value larger than the target current. Then, the control unit 200 performs linear approximation using the P1 point (I1, V1) and P2 point (I2, V2) in FIG. 8 corresponding to these (Y = (I2-I1) / (V2-V1)). This is regarded as the voltage-current characteristic (VI characteristic) of the secondary transfer outer roller 34 and stored in the memory 202. Then, according to the voltage-current characteristic (Y), the control unit 200 calculates the difference (ΔI) between the target current and the current (I1) smaller than the target current, the voltage (V1) applied when the current (I1) is passed, A reference voltage (Vb = V1 + ΔI / Y) is obtained and stored in the memory 202.

  As described above, the reference voltage (Vb) obtained by the pre-rotation ATVC is a voltage that allows a target current to flow through the secondary transfer portion T2 when the recording material P does not pass through the secondary transfer portion T2. It is. On the other hand, the secondary transfer voltage applied to the secondary transfer outer roller 34 during the image forming job is a target current applied to the secondary transfer portion T2 when the recording material P is passing through the secondary transfer portion T2. If the voltage is not able to flow, there is a risk of causing a transfer failure or the like. Therefore, when setting the secondary transfer voltage, in addition to the electrical resistance value of the secondary transfer outer roller 34, it is necessary to consider the electrical resistance value of the recording material P that passes through the secondary transfer portion T2. Therefore, the control unit 200 determines the secondary transfer voltage applied to the secondary transfer outer roller 34 at the time of the image forming job as the paper sharing voltage (Vp) in consideration of the reference voltage (Vb) and the electrical resistance value of the recording material P. ) And the sum. That is, the reference voltage becomes a reference when setting the secondary transfer voltage. The paper sharing voltage (Vp) is pre-assigned a voltage value that varies depending on the temperature and humidity obtained by an environmental sensor (not shown) mounted in the apparatus main body, the type of recording material P, and the front or back side of the paper. And stored in the memory 202.

  However, as described above, when images are continuously formed on a large number of recording materials P, the electrical resistance value of the secondary transfer outer roller 34 may change. Nevertheless, if the secondary transfer voltage based on the reference voltage (Vb) set by the above-described pre-rotation ATVC is continuously applied, it is difficult for the target current to flow to the secondary transfer portion T2, and image defects may occur. . Therefore, the control unit 200 executes the inter-sheet ATVC and corrects the reference voltage every time image formation is continuously performed on a predetermined number (for example, 100) of recording materials P. In a general paper interval ATVC, a secondary voltage is detected between papers based on the current value detected by performing constant voltage control and the voltage / current characteristics of the secondary transfer outer roller 34 obtained by the above-described pre-rotation ATVC. The reference voltage is corrected to a voltage value that allows a target current to flow through the transfer portion T2.

<Conventional paper-to-paper ATVC>
Here, the conventional inter-sheet ATVC will be described with reference to FIGS. Here, the reference voltage (Vb1) set by the pre-rotation ATVC is 800V, the reference voltage (Vb2) corrected by the inter-paper ATVC is 700V, and the paper sharing voltage (Vp) of the recording material P is 1200V. Further, it is assumed that the interval between sheets is 100 ms, the time required for the voltage fall associated with the voltage switching applied to the secondary transfer outer roller 34 is 40 ms, and the time required for the voltage rise associated with the voltage switching is 20 ms. In this case, the time devoted to current detection in the stable state where the reference voltage is reached is 40 (100-40-20) ms. In FIG. 9, black circles indicate the current detection timing by the CPU 201 (see FIG. 2).

  In view of the fact that the voltage-current characteristic of the secondary transfer outer roller 34 is actually non-linear, in order to obtain the reference voltage (Vb1) with high accuracy, a voltage at which a current equivalent to that at the time of transfer can be passed between sheets, that is, the reference voltage. It is desirable to detect the current while applying a voltage as close as possible to (Vb1). The control unit 200 (see FIG. 2) executes the inter-sheet ATVC after completing the transfer to the recording material P1 with the secondary transfer voltage. In the example shown in FIG. 8, the secondary transfer voltage of the recording material P1 immediately before executing the inter-paper ATVC is the first of the sum of the reference voltage (Vb1: 800V) and the paper sharing voltage (Vp: 1200V). The target voltage (2000V) is set.

  The control unit 200 switches the voltage applied to the secondary transfer outer roller 34 from the first target voltage (2000V) to the reference voltage (Vb1: second target voltage (800V)) set in the pre-rotation ATVC. The control unit 200 detects a current after a current detection start timing after a predetermined time from the start of voltage switching from the secondary transfer voltage (first target voltage) to the reference voltage (before correction: second target voltage). To start. The current detection start timing is stored in advance in the memory 202 for “+ α (for example, 0 to 10 ms)” longer than the time required for the voltage fall associated with the voltage switching for the new outer transfer roller 34 before being deteriorated. Has been. Accordingly, here, the detection result is divided into five times at intervals of 8 ms, for example, within the time devoted to current detection after the second target voltage has been reached, that is, at least the time taken for the voltage to fall. A current value is detected (see solid line A in the figure). When there are a plurality of detected currents, the detected current values are averaged. As the averaging process, for example, the remaining three current values excluding the maximum current value and the minimum current value among the detected five current values are averaged.

  As shown in FIG. 10, the control unit 200 obtains a difference (ΔIb1) between the averaged current value (Ib1) and the target current (Itrg). A reference voltage correction amount (ΔVb = ΔIb1 / Y) is obtained according to the obtained current difference (ΔIb1) and the voltage-current characteristic (Y) obtained by the pre-rotation ATVC. This reference voltage correction amount (ΔVb: -100V, for example) is added to the second target voltage (Vb1) (Vb2 = Vb1 + ΔVb) to obtain a corrected reference voltage (Vb2). The secondary transfer voltage for the next recording material P2 passing through the secondary transfer portion T2 is a third target voltage (1900V) that is the sum of the corrected reference voltage (700V) and the paper sharing voltage (1200V). It is set (see FIG. 9). According to this, the current flowing through the secondary transfer outer roller 34 becomes a current indicated by “Ib2” in FIG. 10, and an error from the target current is “ΔIb2”. That is, by correcting the reference voltage, the error between the current flowing through the outer secondary transfer roller 34 and the target current can be reduced from “ΔIb1” before correction to “ΔIb2” after correction.

  Recently, in order to further increase the productivity of the image forming apparatus, the process speed at the time of image formation is increased and the gap between sheets is made as short as possible. When the paper interval is shortened (for example, 80 ms), the time taken for the voltage fall (40 ms) and the time taken for the voltage rise (20 ms) do not change, so the time devoted to current detection in a stable state where the reference voltage has been reached. Becomes shorter (20 ms). FIG. 11 shows the current detection timing of the conventional inter-sheet ATVC when the inter-sheet interval is shortened (black circle in FIG. 11). Here, the number of current values that can be detected between one sheet of paper is one.

  As shown in FIG. 11, when the interval between sheets is shortened, the current can be detected only once or twice at most in a state where the second target voltage (Vb1) is applied in one interval between sheets. This makes it difficult to average the current values and obtain a current value (Ib1: see FIG. 10) suitable for correcting the reference voltage. Therefore, as indicated by a solid line A in FIG. 11, the correction of the reference voltage is performed by averaging a plurality of current values obtained by performing the inter-sheet ATVC between a plurality of (for example, five times) sheets. The current value used for is obtained. In the example shown in FIG. 11, the secondary transfer voltage is set to the third target voltage (1900 V) from the sixth sheet that is the first recording material after the correction of the reference voltage.

  By the way, depending on the reference voltage and the paper sharing voltage, it may take a long time to switch the voltage applied to the secondary transfer outer roller 34 during the ATVC between papers. In addition, the time required for the voltage to fall with the voltage switching from the secondary transfer voltage to the second target voltage may change due to a change in the electrical resistance value of the secondary transfer outer roller 34 or the like. In the case where the time required for switching the voltage becomes longer in this way, conventionally, even if a plurality of currents can be detected between one sheet of paper as shown by a dotted line B in FIG. There was a time before reaching. In particular, when the interval between the sheets is shortened, as indicated by a dotted line B in FIG. 11, it is remarkable that the current is detected before the second target voltage (Vb1) is reached. Conventionally, since the current detection start timing is determined in advance, particularly when the electrical resistance value of the secondary transfer outer roller 34 changes, the current detection is started after waiting for the actual voltage to reach the second target voltage. It could be difficult. In this case, the current may be detected before reaching the second target voltage. Conventionally, when the reference voltage is corrected based on the current detected before reaching the second target voltage, it is difficult to correct the reference voltage appropriately. This is because, in the prior art, the reference voltage is corrected using a current measured on the premise that the stable state has reached the reference voltage. Therefore, even when a secondary transfer voltage based on the corrected reference voltage is applied, transfer failure, abnormal discharge, etc. are caused, and image defects such as “strong missing” and “weak missing” may occur.

<Inter-paper ATVC of this embodiment>
In the present embodiment, in view of the above points, in the paper-to-paper ATVC, even when the current can be detected only during the fall of the voltage due to voltage switching, the voltage reaches the predetermined voltage, not during the fall of the voltage. The current can be detected in a stable state. In order to do so, in the present embodiment, in the inter-sheet ATVC, the voltage is switched from the secondary transfer voltage to a switching voltage having a smaller potential difference from the secondary transfer voltage than the reference voltage before correction. FIG. 3 shows a flowchart of the inter-sheet ATVC of this embodiment. In the inter-sheet ATVC of this embodiment, the control unit 200 (see FIG. 2) sets a predetermined value (for example, 100) as the number of recording materials P on which images are continuously formed as information relating to image formation time during an image formation job. This is the control (setting mode) that is executed between the sheets whenever it exceeds.

As shown in FIG. 3, when starting the inter-sheet ATVC (in the setting mode), the control unit 200 determines the potential difference between the secondary transfer voltage and the reference voltage (second target voltage) before correction before switching the voltage. Is a threshold value (S1). When the potential difference between the secondary transfer voltage and the reference voltage before correction is the first potential difference smaller than the threshold value (NO in S1), the voltage applied to the secondary transfer outer roller 34 is set at the time of passing the previous recording material P. Switching from the applied secondary transfer voltage to the reference voltage before correction is started (S2). In this case, the reference voltage stored in the memory 202 (see FIG. 2) is switched. On the other hand, when the potential difference between the secondary transfer voltage and the reference voltage before correction is equal to or greater than the threshold value, that is, when the second potential difference is larger than the first potential difference (YES in S1), the voltage is applied to the secondary transfer outer roller 34. The voltage to be switched starts from the secondary transfer voltage to the switching voltage (S3). In this case, the threshold value and the switching voltage are defined in the switching voltage table shown in Table 1. The switching voltage table is stored in the memory 202.

  The switching voltage table of Table 1 will be described with reference to FIG. In the present embodiment, as described above, the time from the start of switching from the secondary transfer voltage to the reference voltage before correction in the inter-sheet ATVC until the current detection start timing for starting the current detection depends on the fall of the voltage. Over time. FIG. 4 shows the relationship between the paper sharing voltage (Vp) and the time until the actual voltage applied to the secondary transfer outer roller 34 reaches the reference voltage (Vb) before correction (response time in the figure). As can be understood from FIG. 4, when the reference voltage (Vb) before correction is smaller than 2000V (500V, 1000V, 1500V in FIG. 4), the response time exceeds the current detection start timing depending on the paper sharing voltage (Vp). Sometimes.

  Therefore, in the present embodiment, when the reference voltage (Vb) before correction is less than 2000 V, for example, and is a paper sharing voltage whose response time does not exceed the current detection start timing, the secondary transfer voltage is changed to a predetermined voltage (switching voltage). ) To start switching. At this time, when the reference voltage before correction is the same, if the paper sharing voltage of the recording material P transferred immediately before executing the setting mode is the first voltage, the predetermined voltage is set as the reference voltage. . On the other hand, if the paper sharing voltage of the recording material P transferred immediately before executing the setting mode is the second voltage that is lower in absolute value than the first voltage, the secondary transfer voltage before switching than the reference voltage before correction. Is set to a predetermined voltage (switching voltage) with a small difference. When the paper sharing voltage is the same, if the reference voltage before correction is the first reference voltage, the predetermined voltage is set as the reference voltage. On the other hand, if the reference voltage before correction is a second reference voltage that is lower in absolute value than the first reference voltage, a predetermined voltage (switching voltage) having a smaller difference from the secondary transfer voltage before switching than the reference voltage before correction. ). If the switching voltage can be reached after the switching from the secondary transfer voltage until the current detection start timing, the switching voltage is preferably a voltage closer to the reference voltage before correction.

  As shown in Table 1, when the reference voltage before correction is “1500 or more and 1999 or less”, and the paper sharing voltage is “1800 or more”, the switching voltage is determined to be “2000” V. When the reference voltage before correction is “1000 or more and 1499 or less”, if the paper sharing voltage is “1000 or more and 2299 or less”, the switching voltage is “1500” V, and if the paper sharing voltage is “2300 or more”, “ 2000 "V. When the reference voltage before correction is “500 to 999”, if the paper sharing voltage is “800 to 1499”, the switching voltage is “1000” V, and the paper sharing voltage is “1500 to 2799”. If the paper sharing voltage is “2800 or more”, “2000” V is determined.

  Returning to FIG. 3, the control unit 200 waits until the above-described current detection start timing after voltage switching (NO in S <b> 4). When the current detection start timing has arrived (YES in S4), the control unit 200 proceeds with the processes after S5, and first detects current at a predetermined interval (S5). In the case of the present embodiment, the current is detected in a stable state that reaches the reference voltage or the switching voltage before correction. In this case, the voltage may be detected in accordance with the current detection. The control unit 200 corrects the reference voltage based on the current detected in the stable state that has reached the reference voltage or the switching voltage before correction (S6). The corrected reference voltage is stored (updated) in the memory 202. The controller 200 adds the paper sharing voltage to the corrected reference voltage to obtain the secondary transfer voltage (S7). Then, the controller 200 starts switching the voltage applied to the secondary transfer outer roller 34 to the secondary transfer voltage (third target voltage) obtained from the reference voltage before correction or the switching voltage (second target voltage). (S8).

  Concretely, it demonstrates using FIG.5 and FIG.10. Here, a case will be described in which the reference voltage (before correction, Vb1) set in the pre-rotation ATVC is 800V, and the paper sharing voltage (Vp) of the recording materials P1 and P2 is 1200V. Further, it is assumed that the interval between sheets is 100 ms, the time required for the voltage fall associated with the voltage switching applied to the secondary transfer outer roller 34 is 40 ms, and the time required for the voltage rise associated with the voltage switching is 20 ms. In FIG. 5, black circles indicate the current detection timing by the CPU 201 (see FIG. 2). In FIG. 5, the time required for the voltage fall due to the voltage switching from the secondary transfer voltage to the second target voltage is changed due to the change in the electrical resistance value of the secondary transfer outer roller 34. (Same as the dotted line B of the conventional example shown in FIG. 9).

  In this case, when the secondary transfer with respect to the recording material P1 is completed, the voltage applied to the secondary transfer outer roller 34 is switched from the secondary transfer voltage (2000V) to the switching voltage (second target voltage). . That is, in FIG. 5, since the reference voltage before correction is 800 V and the paper sharing voltage is 1200 V, the switching voltage is switched to 1000 V as the switching destination from the secondary transfer voltage based on the switching voltage table of Table 1 described above. Be started. If switching from the secondary transfer voltage to the switching voltage is started, even if the time taken for the voltage fall due to voltage switching due to the change in the electrical resistance value of the secondary transfer outer roller 34 is indicated by the dotted line B, the actual voltage Becomes a stable state in which the switching voltage is reached before the current detection start timing. Therefore, in this case, as indicated by a solid line C in FIG. 5, the current is detected in a stable state where the actual voltage reaches the switching voltage. At this time, if there are a plurality of detected currents, they are averaged, and the reference voltage is corrected using the averaged current value. If the detected current is not plural, the averaging process is not performed and the reference voltage may be corrected using the detected current value.

  In this way, based on the current value (Ib1) detected when the actual voltage is in a stable state, the reference voltage correction amount (ΔVb) is obtained as described above (see FIG. 9). A corrected reference voltage is obtained. However, in this case, in order to obtain the corrected reference voltage (Vb2 = Vb1 + ΔVb), the switching voltage as the second target voltage (Vb1) is added to the reference voltage correction amount (ΔVb). The secondary transfer voltage of the next recording material P2 is set to a third target voltage (for example, 1900 V) that is the sum of the corrected reference voltage and the paper sharing voltage.

  As described above, in this embodiment, when the voltage is switched from the secondary transfer voltage to detect the current in the inter-sheet ATVC, the voltage is switched to either the reference voltage before correction or the switching voltage depending on the case. The switching voltage is a voltage whose potential difference from the secondary transfer voltage immediately before execution of the inter-sheet ATVC is smaller than the reference voltage before correction. When switching from the secondary transfer voltage to the switching voltage is started, the actual voltage applied to the secondary transfer outer roller 34 reaches the switching voltage earlier than when the secondary transfer voltage is switched to the reference voltage before correction. Therefore, the current can be detected in a stable state where the switching voltage is reached. According to this, even if the electrical resistance value of the secondary transfer outer roller 34 changes, the voltage reaches the reference voltage or the switching voltage before correction without using the current detected during the voltage falling caused by the voltage switching. The reference voltage can be corrected using the current detected in the stable state. Therefore, a secondary transfer voltage that does not cause transfer failure or abnormal discharge can be applied, and image defects such as “strong missing” and “weak missing” are less likely to occur.

<Other embodiments>
In the above-described embodiment, the reference voltage correction amount is obtained using the voltage-current characteristics of the secondary transfer outer roller 34 obtained by the pre-rotation ATVC (see FIG. 10), but the present invention is not limited to this. An example will be described with reference to FIGS. In FIG. 6, the reference voltage (Vb1) set by the pre-rotation ATVC is 800V, the reference voltage (Vb2) corrected by the inter-paper ATVC is 700V, and the paper sharing voltage (Vp) of the recording materials P1, P2, and P3 is 1200V. . In this case, the secondary transfer voltage (first target voltage) of the recording material P1 immediately before execution of the inter-sheet ATVC is 2000V.

  As shown in FIG. 6, since the reference voltage set in the pre-rotation ATVC is 800V and the paper-to-paper sharing voltage is 1200V, according to the switching table of Table 1 described above, the voltage is the switching voltage (second voltage). The target voltage is 1000 V). Then, the current is detected in a stable state where the actual voltage reaches the switching voltage (first predetermined voltage: VbA) (first current: IbA). However, in this case, after the current is detected, the voltage is changed from the switching voltage (1000 V) to the first target before the next recording material P2 is conveyed to the secondary transfer portion T2 without correcting the reference voltage. It is switched to return to the voltage (2000V). Then, switching between the first target voltage (2000 V) and the previous sheet (first non-transfer period) is performed between the sheets (second non-transfer period) between the recording material P2 and the recording material P3. Switching to the third target voltage (VbB) higher than the switching voltage (VbA: 1000 V) is started. The third target voltage (VbB) may be any voltage that is higher in absolute value than the switching voltage switched between the front sheets and lower in absolute value than the secondary transfer voltage. In the case of this embodiment, it was set to 1500V which is an intermediate value between 2000V and 1000V. Then, a current is detected in a stable state where the actual voltage reaches the third target voltage (second predetermined voltage: VbB) (referred to as second current: IbB).

  Then, a corrected reference voltage (Vb2) is obtained using the current detected according to the switching of the voltage between the sheets and the switched voltage. As shown in FIG. 7, linear approximation is performed using two points of the switched voltages (VbA, VbB) and the current values (IbA, IbB) corresponding to them (Y = ΔIbAB / ΔVbAB). This linear approximation is regarded as the voltage-current characteristic of the current secondary transfer outer roller 34. Then, a difference (ΔIb) between the current value (IbA) and the target current (Itrg) is obtained, and according to the relationship between the obtained current difference (ΔIb) and the current voltage-current characteristic (Y) linearly approximated, A correction amount (ΔVb = ΔIb / Y) is obtained. By adding the correction amount (ΔVb) of the reference voltage to the lower voltage (VbB) that is relatively close to the reference voltage before correction among the switched voltages (Vb2 = VbB + ΔVb), the corrected reference voltage (Vb2) )

  As described above, the voltage / current characteristics of the secondary transfer outer roller 34 may be obtained by using the voltage switched by the inter-sheet ATVC and the current detected at that time, without depending on the pre-rotation ATVC. In such a case, it is possible to correct the reference voltage more accurately according to the current electrical resistance value of the secondary transfer outer roller 34. On the other hand, when the voltage-current characteristic obtained by the pre-rotation ATVC is used, there is an advantage that the time required for performing the inter-paper ATVC can be shortened, and the inter-paper interval can be further shortened.

  In the above-described embodiment, the averaging process is not performed when there are not a plurality of currents detected in one paper interval ATVC, and the reference voltage is corrected using only the current value. Not exclusively. If there is not a plurality of currents detected in one paper-to-paper ATVC, as described above, the paper-to-paper ATVC is performed a plurality of times (for example, five times), and an averaging process is performed on the plurality of currents obtained thereby. You may do that.

  In the above-described embodiment, the intermediate transfer type image forming apparatus has been described as an example, but the present invention is not limited thereto. In the embodiment described above, for example, as shown in FIG. 12, a direct transfer method in which toner images are directly transferred from a plurality of photosensitive drums 4 </ b> Y to 4 </ b> K as image carriers onto a recording material P conveyed on a conveyance belt 250. The present invention can also be applied to other image forming apparatuses.

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 2 ... Image carrier (intermediate transfer body, intermediate transfer belt), 4Y-4K ... Image carrier (photosensitive body, photosensitive drum), 34 ... Transfer member (2) Secondary transfer outer roller), 50 ... power supply (secondary transfer power supply), 200 ... control means (control unit), 303 ... voltage detection means (voltage detection circuit), 304 ... current detection means ( Current detection circuit), P ... recording material, T2 ... transfer nip (secondary transfer)

Claims (9)

An image carrier that carries and rotates a toner image;
A transfer member that abuts against the image carrier to form a transfer nip, and a transfer voltage is applied to transfer a toner image on the image carrier to a recording material;
A power source for applying a voltage to the transfer member;
Current detection means for detecting a current flowing through the transfer member;
A reference voltage for setting the transfer voltage is acquired based on a detection result of the current detection unit when a voltage is applied to the transfer member when no recording material passes through the transfer nip, Control means for controlling the transfer voltage based on a reference voltage and a shared voltage set according to the recording material,
The control means applies a voltage to be applied to the transfer member during a period from when the preceding recording material passes through the transfer nip portion until the subsequent recording material reaches the transfer nip portion during an image forming job. A setting mode for setting the transfer voltage based on a current detected by the current detection means when the transfer voltage is switched from the transfer voltage to a predetermined voltage having an absolute value lower than the transfer voltage;
When the setting mode is executed when the reference voltage is the same, and the shared voltage of the recording material transferred immediately before the setting mode is executed is the first voltage, the predetermined voltage is set to the reference voltage. When the shared voltage of the recording material transferred immediately before executing the setting mode is a second voltage that is lower in absolute value than the first voltage, it is applied immediately before executing the setting mode. Setting the potential difference between the transfer voltage and the predetermined voltage to a value smaller than the potential difference between the transfer voltage and the reference voltage applied immediately before executing the setting mode;
An image forming apparatus. An image carrier that carries and rotates a toner image;
A transfer member that abuts against the image carrier to form a transfer nip, and a transfer voltage is applied to transfer a toner image on the image carrier to a recording material;
A power source for applying a voltage to the transfer member;
Current detection means for detecting a current flowing through the transfer member;
A reference voltage for setting the transfer voltage is acquired based on a detection result of the current detection unit when a voltage is applied to the transfer member when no recording material passes through the transfer nip, Control means for controlling the transfer voltage based on a reference voltage and a shared voltage set according to the recording material,
The control means applies a voltage to be applied to the transfer member during a period from when the preceding recording material passes through the transfer nip portion until the subsequent recording material reaches the transfer nip portion during an image forming job. A setting mode for setting the transfer voltage based on a current detected by the current detection means when the transfer voltage is switched from the transfer voltage to a predetermined voltage having an absolute value lower than the transfer voltage;
When executing the setting mode for recording materials having the same shared voltage, if the reference voltage is a first reference voltage, the predetermined voltage is set to the reference voltage, and the reference voltage is the first reference voltage. In the case of the second reference voltage having an absolute value lower than one reference voltage, the potential difference between the transfer voltage and the predetermined voltage applied immediately before executing the setting mode is applied immediately before executing the setting mode. Set to a value smaller than the potential difference between the transfer voltage and the reference voltage.
An image forming apparatus. In the setting mode, the control means includes a current detected by the current detection means after a predetermined time from the start of switching from the transfer voltage to the predetermined voltage, the predetermined voltage, and a voltage-current characteristic of the transfer member. Correcting the reference voltage based on
The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. Voltage detecting means for detecting a voltage applied to the transfer member from the power source;
The control means includes a plurality of voltages detected by the voltage detection means in response to sequentially passing a plurality of currents having different sizes through the transfer member during a pre-rotation of the image forming job, and the plurality of currents. The voltage-current characteristic is obtained based on
The image forming apparatus according to claim 3. The control unit switches the voltage applied to the transfer member from the transfer voltage to a first predetermined voltage that is lower in absolute value than the transfer voltage in a first non-transfer period during the image forming job. In the second non-transfer period different from the first non-transfer period and the first current detected by the current detection means, the voltage applied to the transfer member is changed from the transfer voltage to the transfer voltage. The voltage-current characteristic is obtained based on the second current detected by the current detection means when switching to a second predetermined voltage that is low in absolute value and different from the first predetermined voltage.
The image forming apparatus according to claim 3. The control unit starts the setting mode every time information relating to an image forming time exceeds a predetermined value during an image forming job.
The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. The information relating to the image formation time is the number of recording materials on which image formation has been performed.
The image forming apparatus according to claim 6. The image carrier is a photoconductor in which an electrostatic latent image is developed into a toner image by a developer.
The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. The image carrier is an intermediate transfer member to which a toner image on a photosensitive member developed with a developer is transferred.
The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.

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