JP2006208497A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of enhancing transferability than in the conventional case even at a sheet tip part. <P>SOLUTION: In a printer constituted so that secondary transfer of a toner image carried on an intermediate transfer belt is performed to paper by passing the paper to be fed through a contact part between the intermediate transfer belt and a secondary transfer roller, voltage Vt' which is smaller than voltage Vt by a prescribed value ΔV is applied to the secondary transfer roller from when the tip of the paper runs into the contact part to when predetermined time t1 elapses (between D and E) and afterward (after the point of time E), the regulated voltage Vt is applied as secondary transfer voltage. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus configured to pass a conveyed sheet through a contact portion between an image carrier such as a photosensitive drum and a transfer member such as a transfer roller, and to transfer an image formed on the image carrier onto the sheet. In particular, the present invention relates to a technique for improving the transferability of the leading edge of a sheet.

In such an image forming apparatus, for example, a specified transfer voltage, for example, 2000 V is applied to the transfer roller by constant voltage control in accordance with the timing at which the sheet reaches the transfer position, and an electric field generated between the transfer roller and the photosensitive drum is applied. The toner image on the photosensitive drum is transferred onto the sheet by the action.
JP-A-5-119646

  However, in the image forming apparatus, the impedance between the photosensitive drum and the transfer roller changes instantaneously due to the sheet interposed at the moment when the sheet enters the contact portion between the photosensitive drum and the transfer roller. For this reason, even if constant voltage control is performed, a so-called overshoot phenomenon occurs, that is, for example, a momentary increase of the transfer voltage for a few tens of milliseconds. There is a problem in that transfer failure is likely to occur within a range of several millimeters at the front end in the transport direction, and the image quality is deteriorated.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide an image forming apparatus capable of improving transferability at a sheet front end portion as compared with the related art.

  In order to achieve the above object, an image forming apparatus according to the present invention includes an image carrier and a transfer member disposed in contact with the image carrier at a transfer position, and the sheet passing through the contact portion is provided with the above-described sheet. An image forming apparatus configured to transfer an image formed on an image carrier, the transfer voltage applying unit applying a transfer voltage to the transfer member, and controlling the transfer voltage applying unit to convey the sheet Control means for outputting a rush voltage that is lower than a specified voltage by a predetermined value during a period from when the tip of the direction enters the contact portion to when a predetermined time elapses, and for outputting the specified voltage thereafter. It is characterized by providing.

Further, the control means acquires information indicating a type of a sheet to be used, and changes the predetermined value according to the type of the sheet.
Here, the type of the sheet indicates the thickness of the sheet, and when the sheet is a sheet having a first thickness, the control means sets the predetermined value to Vm1, and In the case of a thick second sheet, Vm2 is larger than Vm1.

The type of sheet indicates the material of the sheet, and the control means sets the predetermined value to Vp1 when the sheet is a sheet, and Vp1 when the sheet is a resin film. It is characterized by setting it to Vp2 larger than this.
The information processing apparatus further comprises receiving means for receiving an input of the sheet type, and the control means acquires information received by the receiving means as information indicating the sheet type.

In addition, a detection unit that detects the humidity around the apparatus is provided, and the control unit changes the predetermined value in accordance with the detected humidity level.
Further, the control means sets the predetermined value to Vs1 when the humidity is the first humidity, and smaller than the Vs1 when the humidity is the second humidity higher than the first humidity. It is characterized by Vs2.

  Further, the transfer member is a transfer roller.

  Thus, only when the leading edge of the sheet enters the contact portion between the image carrier and the transfer member, the transfer voltage is changed to an inrush voltage lower than a specified voltage. The voltage fluctuation is suppressed, so that good transfer can be performed also at the front end portion of the sheet, and the image quality can be improved.

Hereinafter, an embodiment of an image forming apparatus according to the present invention will be described using a tandem color digital printer (hereinafter simply referred to as “printer”) as an example.
FIG. 1 is a diagram illustrating the overall configuration of the printer 1.
As shown in the figure, the printer 1 includes image forming units 10Y, 10M, 10C, and 10K corresponding to each of yellow (Y), magenta (M), cyan (C), and black (K), and arrows. An intermediate transfer unit 20 including an intermediate transfer belt 21 that rotates in the direction a, a paper feeding / conveying unit 30, a fixing unit 40, and a control unit 50 are provided, connected via a network, and printed from an external terminal (not shown). When an instruction is received, printing is executed based on the instruction. Hereinafter, the reproduction colors of yellow, magenta, cyan, and black are represented as Y, M, C, and K, and Y, M, C, and K are added as subscripts to the numbers of the components related to the reproduction colors.

  The image forming units 10Y to 10K are arranged in series at predetermined intervals along the intermediate transfer belt 21, and the photosensitive drums 11Y to 11K as image carriers and the print heads 12Y to 12K arranged around the photosensitive drums 11Y to 11K. And charging devices 13Y to 13K, developing devices 14Y to 14K, primary transfer rollers 15Y to 15K that face the photosensitive drums 11Y to 11K across the intermediate transfer belt 21, and a cleaner (not shown).

The intermediate transfer unit 20 includes an intermediate transfer belt 21, a driving roller 22 and a driven roller 23 around which the belt is stretched.
The paper feeding / conveying unit 30 includes a paper feeding cassette 37 that stores the paper S as a sheet for recording, a feeding roller 31 that feeds the paper S in the paper feeding cassette 37 one by one, and a paper feeding path 36 for the paper S. A timing roller pair 32 that takes the timing of feeding to the secondary transfer position 24 along with the intermediate transfer belt 21 at the secondary transfer position 24 and a metal secondary transfer roller 34 that is pressed against the drive roller 22 with a predetermined pressure. And a high voltage output unit 35 as transfer voltage applying means for applying a specified voltage to the secondary transfer roller 34, here, 2000 VDC as a secondary transfer voltage.

In such a configuration, when a print instruction is received from an external terminal, the control unit 50 receives the transmitted image signal, converts it to a digital image signal of Y, M, C, and K colors, and prints it. A drive signal for driving the heads 12Y to 12K is generated.
The print heads 12Y to 12K emit a laser beam in response to a drive signal from the control unit 50, and expose and scan the photosensitive drums 11Y to 11K in the main scanning direction.

The photosensitive drums 11Y to 11K are subjected to the removal of residual toner on the surface with a cleaner before being subjected to the exposure described above, and after being discharged by irradiating an unillustrated eraser lamp, they are uniformly charged by the chargers 13Y to 13K. Thus, when exposed to the laser beam in a uniformly charged state, electrostatic latent images are formed on the surfaces of the photosensitive drums 11Y to 11K.
The electrostatic latent images are developed by developing units 14Y to 14K for the respective colors, whereby toner images of Y, M, C, and K colors are formed on the surfaces of the photosensitive drums 11Y to 11K, and are primary at the primary transfer position. The images are sequentially transferred onto the rotating intermediate transfer belt 21 by the electrostatic action of the transfer rollers 15Y to 15K. At this time, the image forming operation for each color is executed while shifting the timing in the order of Y, M, C, and K colors so that the toner images are superimposed on the same position on the intermediate transfer belt 21.

  On the other hand, the sheet S is fed from the sheet feeding / conveying unit 30 via the timing roller pair 32 in accordance with the image forming timing of each color toner image on the intermediate transfer belt 21, and the secondary transfer position 24. The toner image on the intermediate transfer belt 21 is secondarily transferred onto the paper S by the electrostatic action generated between the secondary transfer roller 34 and the drive roller 22 in FIG. The sheet S that has passed through the secondary transfer position 24 is conveyed to the fixing unit 40 where the toner image is pressurized and heated to be fixed on the sheet S and then discharged onto the discharge tray 41.

Between the timing roller pair 32 and the secondary transfer position 24 in the sheet conveyance path 36, a sheet detection sensor 33 including a photoelectric sensor for detecting the sheet S is disposed. The paper detection sensor 33 is used to control the output timing of the secondary transfer voltage of the high voltage output unit 35.
FIG. 2 is a diagram illustrating a circuit configuration of the high-voltage output unit 35.
As shown in the figure, the high-voltage output unit 35 is a circuit that outputs a secondary transfer voltage as a constant voltage by so-called feedback control, and includes an output unit 351, a transistor Q1, resistors R1 to R4, and the like.

The output unit 351 includes an inverter circuit 352 and a rectifying / smoothing unit 353.
The inverter circuit 352 includes a transistor Q 2, a transformer 354, and a PWM (pulse width modulation) circuit 355.
One end of the primary coil of the transformer 354 is connected to the power supply VCC, and the other end is connected to the collector terminal of the transistor Q2. A secondary coil of the transformer 354 is connected to the rectifying / smoothing unit 353.

The transistor Q2 is used as a switch element that switches the primary coil of the transformer 354, the base terminal is connected to the output terminal 3552 of the PWM circuit 355, and the emitter terminal is grounded.
The PWM circuit 355 is a circuit that outputs a drive pulse for turning on and off the transistor Q2 from the output terminal 3552, and the pulse width of the drive pulse according to the voltage (input voltage) of the signal input to the input terminal 3551. Is configured to modulate.

  Specifically, a circuit that generates a sawtooth signal synchronized with the reference clock, a circuit that compares the generated sawtooth signal and the input voltage of the input terminal 3551, and a time when the sawtooth wave is larger than the input voltage are set. It consists of a circuit that generates a rectangular wave as a drive pulse with high level (on) and low time (low) as the drive pulse. The ratio of the drive pulse on time to off time (duty ratio) depends on the input voltage value. ). This pulse width modulation method itself is a known method, and is described in detail, for example, in Japanese Patent Application Laid-Open No. 06-317978.

The transistor Q2 becomes conductive (on) when the drive pulse is on and non-conductive (off) when the drive pulse is off. Therefore, when the input voltage to the input terminal 3551 is lowered, the on-time of the drive pulse is lengthened, the on-time of the transistor Q2 is lengthened, so that the energy accumulated in the primary side coil of the transformer 354 is increased. If the output voltage from the secondary coil can be increased, and conversely, if the input voltage is increased, the ON time of the drive pulse is shortened, and the output voltage from the secondary coil can be decreased.

The rectifying / smoothing unit 353 is a circuit composed of a diode, a capacitor, and the like that rectifies and smoothes the AC voltage output from the secondary coil of the transformer 354, and the output terminal 359 is a rotating shaft of the secondary transfer roller 34. This output becomes the secondary transfer voltage.
The output terminal 359 of the rectifying / smoothing unit 353 is grounded via a series circuit composed of resistors R3 and R4, and the voltage divided by the resistors R3 and R4 is the A / D input port 512 of the CPU 51 in the control unit 50. Is input.

The emitter terminal of the transistor Q1 is connected to the input terminal 3551 of the PWM circuit 355 and is connected to the power supply VCC via the resistor R2, and the base terminal is connected to the D / A output port 511 of the CPU 51 via the resistor R1. It is connected. The collector terminal is grounded.
In such a configuration, when an analog control signal is output from the D / A output port 511 of the CPU 51, a current corresponding to the voltage value flows to the transistor Q1, and a voltage corresponding to the magnitude of the current is the emitter voltage. Is input to the input terminal 3551 of the PWM circuit 355. From the PWM circuit 355, a drive pulse having a duty ratio corresponding to the voltage of the input terminal 3551 is output from the output terminal 3552.

Thereby, the transistor Q2 is switched and driven, an AC voltage is output from the transformer 354, and the output AC voltage is rectified by the rectifying / smoothing unit 353 and output to the secondary transfer roller 34 as a DC secondary transfer voltage Vt. Is done.
At this time, a voltage obtained by dividing the secondary transfer voltage Vt by the resistors R3 and R4 is input to the A / D input port 512 of the CPU 51 as a monitor signal.

  The CPU 51 samples the monitor signal, and when the voltage value and a specified voltage (2000 volts) are output, the value Va obtained in advance as a voltage to be input to the A / D input port 512. Taking the difference from the (target value), the voltage of the control signal output from the D / A output port 511 is changed so that the difference becomes as small as possible, and the output voltage is controlled to be maintained at the specified voltage. (Feedback control).

Specifically, when the sampling voltage is larger than the target value Va, the output voltage exceeds the specified voltage, so the voltage of the control signal is increased, and conversely, the sampling voltage is smaller than Va. In this case, since the output voltage is lower than the specified voltage, the voltage of the control signal is lowered.
The controller 50 performs a calibration operation for obtaining a voltage value (initial value Vo) of a control signal to be output first at the time of secondary transfer and a value such as a change width when the voltage of the control signal is changed by feedback control. The secondary transfer can be performed more stably. Details of the calibration operation will be described later.

FIG. 3 is a flowchart showing the secondary transfer control by the control unit 50, and FIG. 4 is a diagram showing an example of the output waveform of the secondary transfer voltage output from the high voltage output unit 35.
As shown in FIG. 3, the control unit 50 detects the leading edge of the sheet S in the transport direction (hereinafter referred to as “sheet leading edge”) based on the detection signal from the sheet detection sensor 33 (paper sheet). It is determined whether or not the position of the detection sensor 33 has passed (step S10).

When it is determined that the leading edge of the sheet has been detected (“YES” in step S10) (time A in FIG. 4), the internal timer starts to count and it is determined whether time T1 has elapsed (step S11).
If it is determined that the time T1 has elapsed ("YES" in step S11), the output of the secondary transfer voltage is started from the high voltage output unit 35 (step S12) (time B in FIG. 4). Specifically, a control signal whose voltage is Vo (initial value) is output from the D / A output port 511 of the CPU 51. At that time, the output is controlled so that the voltage gradually increases from 0 volts and reaches Vo after a predetermined time, for example, 100 milliseconds (between time B and time C in FIG. 4). As a result, it is possible to stabilize the rising output voltage at the start-up rather than outputting Vo at a time.

It is assumed that the leading edge of the sheet has not yet arrived at the secondary transfer position 24 when the secondary transfer voltage reaches the specified voltage Vt (time C). Further, the feedback control described above is executed after time C when the start-up is completed.
Next, it is determined whether or not the time T2 has elapsed (step S13). Here, the time T <b> 2 corresponds to the time from when the leading edge of the paper is detected by the paper detection sensor 33 until it enters the contact portion between the intermediate transfer belt 21 and the secondary transfer roller 34.

  When it is determined that the time T2 has elapsed ("YES" in step S13), a voltage (1800 volts, which is smaller than the specified voltage Vt (2000 volts) by a predetermined value ΔV (here, 200 volts)) (Step S14) (at time D in FIG. 4) and when a predetermined time t1 (here, 30 milliseconds) has elapsed (“YES” in step S15), the original specified voltage is applied thereafter. Vt is output (step S16) (time E in FIG. 4).

Specifically, assuming that the current voltage value of the control signal from the D / A output port 511 of the CPU 51 is V1, at the time of inrush voltage output, the voltage value is reduced to a value V2 obtained by subtracting δV from V1. Switching is performed in the order of V1, V2, and V1. Here, δV is a value corresponding to the amount of decrease ΔV in the output voltage as the secondary transfer.
In the present embodiment, the circuit is configured so that the voltage of the control signal and the output voltage from the high voltage output unit 35 are in a substantially proportional relationship. For example, the above V2 is expressed by an equation of V1 × (Vt ′ / Vt). Can be obtained using

The reason why the inrush voltage Vt ′ is provided in this way is to suppress the fluctuation of the transfer voltage when the paper enters. This will be specifically described below.
FIG. 5A is a diagram illustrating an example of a waveform of a secondary transfer voltage output from a conventional high-voltage output unit, and FIG. 5B is a diagram illustrating an example of a voltage waveform of a secondary transfer roller. is there.
As shown in FIG. 5A, conventionally, no inrush voltage is provided, and only the specified voltage Vt is output even when the leading edge of the paper enters. In this case, when the actual voltage of the secondary transfer roller 34 is measured, as shown in FIG. 5 (b), the transfer voltage greatly changes when the leading edge of the sheet enters due to overshoot, for example, increases or decreases by about 200 volts. This is because, as described above, the impedance between the driving roller 22 and the secondary transfer roller 34 instantaneously varies when the sheet enters, and even if the constant voltage control is performed, the variation cannot be completely followed.

For example, when the paper conveyance speed is 100 (mm / second), the above t1 (30 milliseconds) is the range of 3 mm from the leading edge to the trailing edge of the conveyed paper passing through the secondary transfer position 24. The secondary transfer voltage fluctuates within the range of 3 mm, and transfer failure occurs.
Therefore, in this embodiment, only when the sheet enters the sheet, the value of the secondary transfer voltage is set lower than the specified voltage Vt by a voltage corresponding to the increase due to the overshoot (providing the entry voltage), thereby overshooting. Therefore, when the voltage is increased, the voltage of the secondary transfer roller 34 becomes just the same value as the specified voltage Vt so as to suppress fluctuations in the transfer voltage.

FIG. 6 is a diagram illustrating an example of a voltage waveform of the secondary transfer roller 34 when the inrush voltage Vt ′ is provided in the present embodiment.
As shown in the figure, the secondary transfer voltage after paper entry is substantially constant at the specified voltage Vt, and voltage fluctuation can be suppressed as is apparent from the conventional FIG. 5B. Become.
The magnitude of voltage fluctuation due to overshoot varies depending on the material of the secondary transfer roller and the intermediate transfer belt, the electrical characteristics such as impedance, the value of the secondary transfer voltage, etc. As for the voltage value ΔV (voltage Vt ′ at the time of inrush) and the value of the application time t1 (here, t1 ≦ t2 (FIG. 5)), appropriate values are set in advance for each device through experiments or the like. Stored in a ROM or the like. The same applies to the modified examples described later.

  Returning to FIG. 3, in step S17, it is determined whether or not the time T3 has elapsed. Here, the time T3 corresponds to the time from when the leading edge of the paper is detected by the paper detection sensor 33 until the trailing edge of the paper passes through the secondary transfer position 24 (secondary transfer is completed). This time T3 varies depending on the paper size (length in the paper conveyance direction), and the data is obtained in advance from experiments or the like and stored in the ROM or the like.

If it is determined that the time T3 has not yet elapsed ("NO" in step S17), it is determined that the sheet is passing the secondary transfer position 24 (secondary transfer is in progress), and the process returns to step S16 and the secondary transfer voltage is reached. A specified voltage Vt is output.
When the elapse of time T3 is determined (“YES” in step S17), the output of the secondary transfer voltage is stopped as the end of the secondary transfer to one sheet (step S18) (time F in FIG. 4), and the processing is performed. Exit. Specifically, the output of the control signal from the D / A output port 511 of the CPU 51 is stopped.

Next, the calibration operation executed by the control unit 50 will be described. This calibration operation is executed when power is supplied to the apparatus, and the operation is completed before image formation becomes possible.
First, the control unit 50 outputs a control signal from the D / A output port 511 of the CPU 51 when the apparatus is powered on. At this time, as shown in FIG. 7, the voltage is gradually increased from 0 volts for a predetermined time, for example, every several milliseconds, for example, by a predetermined value, for example, 0.1 volts, and a voltage corresponding to a specified percentage of the rated voltage value. For example, when it reaches 3 volts, proportional control is performed thereafter. In this proportional control, the voltage value of the control signal is varied so that the difference between the specified voltage (2000 volts) as the secondary transfer voltage and the actual output voltage is as small as possible.

Specifically, the voltage of the monitor signal input to the A / D input port 512 of the CPU 51 is sampled every predetermined time, for example, every 500 microseconds, and for each sampling, the control signal is expressed using the following (Equation 1). Update the voltage value.
Control signal voltage = current control signal voltage + (target value Va−current monitor signal voltage) × Gain (Expression 1)
Here, the target value Va is an input voltage to the A / D input port 512 when a specified voltage (2000 volts) is output as the secondary transfer voltage as described above, and is about 3.5 volts. The resistance values of the resistors R4 and R5 are determined in advance so that the voltage of Gain is a value corresponding to a coefficient for determining the change width when the voltage of the control signal is changed for updating, and is a fixed value, for example, 0.1 here.

When the sampling proceeds a predetermined number of times, the average voltage of the monitor signal so far is averaged, and it is determined whether or not the average value is within a predetermined tolerance range with respect to the target value Va. Here, if it is determined that the input is present, the output is stabilized and the voltage value of the control signal at that time is stored as an initial value Vo to be output first, and the data is stored in an internal nonvolatile memory or the like. Output is stopped (end of calibration operation). If it is determined that it is not included, the determination process is performed until a predetermined time elapses. When the predetermined time elapses, the voltage value of the control signal at that time is stored as Vo, and the calibration is terminated. The initial value data obtained by the calibration is read at the start of secondary transfer voltage output at the time of image formation in step S12 of FIG.

The reason why the initial value Vo is obtained by calibration in this way is to shorten and keep the startup time until the output voltage becomes stable.
That is, in the case of a configuration in which the calibration is not performed and the initial value Vo is not obtained, the same processing as the above sampling must be executed first every time the secondary transfer is performed, and the time required for the output must be stabilized. Only the secondary transfer timing is delayed, and the time varies and becomes unstable. Further, the time until the output is stabilized may vary from circuit to circuit due to variations in the load and circuit components, particularly the current amplification factor hfe of the transistor.

On the other hand, in the case of a configuration in which calibration is performed, if Vo is output as the voltage of the control signal at the start of secondary transfer, the output voltage is immediately stabilized at a specified voltage without performing the above processing such as sampling. The startup time can be shortened and made constant.
The controller 50 samples the voltage of the monitor signal input to the A / D input port 512 of the CPU 51 every predetermined time after activation (after the time point C in FIG. 4), according to the above (Equation 1). Although feedback control for updating the voltage value of the control signal is performed, when the inrush voltage Vt ′ is output, the output time is very short. Therefore, the feedback control is not performed only during this time, and the voltage value V2 for inrush voltage output is set. When it is obtained, the control signal is output with the value fixed. Of course, feedback control may be performed during this time in order to stabilize the inrush voltage Vt ′.

In addition, instead of Gain in the above (Equation 1), the value of Gain ′ obtained by the following (Equation 2) is used for feedback control after activation.
Gain ′ = [Vo / V (min)] × Gain (min) (Formula 2)
Here, V (min) is a configuration in which a transistor having the smallest current amplification factor hfe within the range of variation is incorporated in the circuit and a secondary transfer roller having the largest load resistance is disposed (the worst output condition). In this configuration, the initial value obtained when calibration is performed is shown, and Gain (min) is a value determined as the optimum Gain for stabilizing the output in the same configuration. These are fixed values, measured in advance by experiment, and the data is stored in an internal memory. In addition, about Gain and Gain ', it is not restricted to the above, You may set an appropriate value according to an apparatus.

  As described above, in this embodiment, it is assumed that the output voltage from the high-voltage output unit 35 to the secondary transfer roller 34 is generated at the time of entry only when the paper leading edge enters the secondary transfer position 24. Since the voltage is changed to the rush voltage that is lower than the specified voltage by the amount corresponding to the voltage rise, the voltage fluctuation of the secondary transfer roller 34 at the time of the paper rush is suppressed, and therefore the range of several millimeters of the paper leading edge. The image quality can be improved since the transfer can be performed well.

The present invention is not limited to the image forming apparatus, and may be the output control method for the secondary transfer voltage. Furthermore, the method may be a program executed by a computer. The program according to the present invention includes, for example, a magnetic disk such as a magnetic tape and a flexible disk, an optical recording medium such as a DVD-ROM, DVD-RAM, CD-ROM, CD-R, MO, and PD, and a flash memory recording medium. It can be recorded on various computer-readable recording media, and may be produced, transferred, etc. in the form of the recording medium, wired and wireless various networks including the Internet in the form of programs, In some cases, the data is transmitted and supplied via broadcasting, telecommunication lines, satellite communications, or the like. In some cases, all or part of predetermined processing can be executed using dedicated hardware.

(Modification)
As described above, the present invention has been described based on the embodiment. However, the present invention is not limited to the above-described embodiment, and the following modifications may be considered.
(1) In the above embodiment, the paper to be used is not particularly defined. However, in the case of a configuration in which different types of paper such as plain paper and cardboard can be selectively used, the entry is made according to the type. It is also possible to change the magnitude of the hourly voltage.

Specifically, when the type of paper is, for example, the thickness of the paper, the thick paper usually has a higher electric resistance in the thickness direction, and therefore, the width of change in impedance when entering the paper should be larger.
Therefore, the paper used, plain paper (for example 64 g / m ^2), to distinguish it from cardboard (e.g. 100 to 120 g / m ²⁾ or the like, in the case of thick paper, as shown in FIG. 8 lowers during paper rush It is conceivable that the power value is set to ΔVm larger than ΔV in the case of plain paper.

  Here, as a method for determining the type of paper to be used, for example, input of the type of paper (discrimination of paper such as plain paper, a numerical value indicating thickness, etc.) is input from the user via an operation panel (not shown). A method of accepting or setting the type of paper that the user wants to use on the external terminal, and determining that the information on the paper is accepted from the external terminal when the control unit 50 accepts a print request in the printer 1. Can be taken.

(2) Further, as the type of paper, for example, paper and other special paper, specifically, a transparent resin film for OHP (overhead projector) may be distinguished. This is because the resistance value differs between the paper and the film even though the thickness is the same.
Therefore, when the paper to be used is an OHP film, for example, as shown in FIG. 9, it is conceivable that the voltage value to be reduced when entering the paper is set to ΔVp larger than ΔV in the case of plain paper. The figure shows an example in which the magnitude relationship between the voltage values is ΔV <ΔVp <ΔVm.

(3) Further, even when the same type of paper is used, the magnitude of the inrush voltage may be changed according to, for example, the humidity around the apparatus.
Normally, when considering an environment of normal humidity (for example, 55% relative humidity) and high humidity (for example, 85% relative humidity), the moisture content of the paper should be higher and the resistance value should be lower. In a normal humidity (first humidity) environment, the voltage value to be decreased is ΔVs1, and in a high humidity (second humidity higher than the first humidity) environment, the voltage value is smaller than ΔVs1. It can be considered to be ΔVs2. In this case, for example, a humidity sensor may be arranged in the paper feed cassette 37 or around the paper transport path 36 and detected as the apparatus peripheral humidity.

  (4) Further, as the type of paper, for example, the magnitude of the inrush voltage may be changed according to the width of the paper used (the length in the direction orthogonal to the paper transport direction). That is, when the paper size is the first size, the A3 paper having a long paper width has a larger impedance change and a larger overshoot than the paper of A5 paper having a short paper width. In this case, it is conceivable that the voltage value to be decreased is Δv1, and in the case of the second size larger than the first size (paper width is wide), Δv2 larger than Δv1.

As a paper size detection method, for example, there is a method of accepting an input of a paper size from a user via an operation panel, or a known paper size detection sensor or the like in the paper feed cassette 37. Note that when different types of paper (sheets) are used, a configuration may be adopted in which the specified voltage is changed and the voltage value to be reduced when the paper enters is changed according to the type.

  (5) In the above embodiment, an analog voltage is output as a control signal from the D / A output port 511 of the CPU 51, and this is sent to the PWM circuit 355 via the transistor Q1, but as a secondary transfer voltage, As long as it is a circuit that can output a specified voltage at a constant voltage and can output a voltage that is smaller than the specified voltage by a predetermined value for a predetermined time as an inrush voltage, it is not limited to the above configuration, and any configuration is possible. It may be taken. For example, a control circuit that outputs a pulse signal for directly switching driving the transistor Q2 from the CPU 51 can be used. Further, in the sense that the configuration is not limited to the above configuration, for example, a configuration in which the calibration is not performed may be employed.

  (6) In the above embodiment, an example in which the present invention is applied to secondary transfer of a tandem color digital printer has been described, but the present invention is not limited to this. An image forming apparatus having a configuration in which an image carrier and a transfer member for transfer are in contact at a transfer position, and an image formed on the image carrier is transferred to a sheet passing through the contact portion. Can be applied as long as overshoot occurs when entering the contact portion between the image carrier and the transfer member.

  For example, as a tandem type printer, a sheet (sheet) is conveyed while adsorbed on the surface of an intermediate transfer belt, and on each sheet of conveyed sheet, on each photosensitive drum (image carrier) for each color. The formed toner image is sequentially transferred onto a sheet using the electrostatic force of each color transfer roller (transfer member) arranged in pressure contact with each photoconductor drum and an intermediate transfer belt. It can be applied to the transfer section.

  In addition, as an image forming apparatus that forms a monochrome image, not limited to color, an image carrier and a transfer member are arranged to face each other in a contact state at a transfer position, and a conveyed sheet is passed through the contact portion to form an image. The present invention is generally applicable to an image forming apparatus that transfers an image on a carrier to a sheet. Further, the transfer member is not limited to a roller-shaped member. A member that can be transferred in contact with the image carrier, such as a blade, may be used as the transfer member.

  Further, the image forming apparatus can be applied to a copying machine, FAX, MFP (Multiple Function Peripheral), and the like. Furthermore, the above embodiment and the above modification examples may be combined.

  INDUSTRIAL APPLICABILITY The present invention can be widely applied to an image forming apparatus configured to pass a conveyed sheet through a contact portion between an image carrier and a transfer member and transfer an image formed on the image carrier to the sheet. .

1 is a diagram illustrating an overall configuration of a printer 1. FIG. FIG. 3 is a diagram illustrating a circuit configuration of a high voltage output unit 35 disposed in the printer 1. 4 is a flowchart showing control of secondary transfer by a control unit 50. It is a figure which shows the example of the output waveform of the secondary transfer voltage output from the high voltage | pressure output part. (A) is a figure which shows the example of the waveform of the secondary transfer voltage output from the conventional high voltage | pressure output part, (b) is a figure which shows the example of the voltage waveform of a secondary transfer roller. It is a figure which shows the example of the voltage waveform of the secondary transfer roller 34 at the time of providing the rush voltage Vt 'concerning this invention. It is a figure for explanation of calibration operation. It is a figure which shows the modification of the output waveform of a secondary transfer voltage when using thick paper. It is a figure which shows the modification of the output waveform of a secondary transfer voltage when using the film for OHP.

Explanation of symbols

1 Printer 21 Intermediate transfer belt (image carrier)
24 Secondary transfer position 33 Paper detection sensor 34 Secondary transfer roller (transfer member)
35 High-voltage output (transfer voltage application means)
50 control unit 51 CPU
351 output unit

Claims (8)

An image forming apparatus comprising: an image carrier; and a transfer member disposed in contact with the image carrier at a transfer position, wherein an image formed on the image carrier is transferred to a sheet passing through the contact portion. A device,
Transfer voltage applying means for applying a transfer voltage to the transfer member;
Controlling the transfer voltage applying means to output a rush voltage that is lower than a predetermined voltage by a predetermined value from the time when the leading edge of the sheet in the conveying direction rushes into the contact portion until the lapse of a predetermined time; Thereafter, an image forming apparatus comprising: control means for outputting the prescribed voltage. The control means includes
The image forming apparatus according to claim 1, wherein information indicating a type of a sheet to be used is acquired, and the predetermined value is changed according to the type of the sheet. The type of the sheet indicates the thickness of the sheet,
The control means includes
When the sheet is a sheet having a first thickness, the predetermined value is Vm1, and when the sheet is a sheet having a second thickness that is thicker than the first thickness, Vm2 is set to be larger than Vm1. The image forming apparatus according to claim 2. The type of the sheet indicates the material of the sheet,
The control means includes
3. The image forming apparatus according to claim 2, wherein when the sheet is paper, the predetermined value is Vp1, and when the sheet is a resin film, Vp2 is larger than the Vp1. Receiving means for receiving the input of the sheet type;
The control means includes
The image forming apparatus according to claim 2, wherein the information received by the receiving unit is acquired as information indicating the type of the sheet. Equipped with detection means for detecting the ambient humidity of the device,
The control means includes
The image forming apparatus according to claim 1, wherein the predetermined value is changed according to the detected humidity level. The control means includes
When the humidity is the first humidity, the predetermined value is Vs1, and when the humidity is the second humidity higher than the first humidity, the predetermined value is Vs2 smaller than the Vs1. The image forming apparatus according to claim 6.   The image forming apparatus according to claim 1, wherein the transfer member is a transfer roller.

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