JP2005221622A - Color image forming apparatus and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To correct an image writing start position from the leading end of transfer paper and to optimally correct color shift. <P>SOLUTION: In the color image forming apparatus, voltage changes or current changes which occur when the leading end of transfer paper enters a plurality of transfer rollers are sequentially detected by at least two primary transfer current detection means. The speed at which a recording medium is conveyed is detected by a difference in time needed for each detection. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a color misregistration correction unit and a writing position correction unit in a color image forming apparatus.

  A tandem type image forming apparatus, which is an example of an image forming apparatus targeted by the present invention, will be described with reference to FIGS. FIG. 8 is an overall explanatory diagram of the image forming apparatus, and FIG. 9 is an explanatory diagram of the color misregistration detection sensor mounting position.

The image forming operation of the laser beam printer will be described with reference to these drawings.
The tandem type image forming apparatus includes an image forming unit for each color of a black image (Bk), a yellow image (Y), a magenta image (M), and a cyan (C) image.

  Each image forming unit includes a photosensitive drum 18, a primary charger 16 for uniformly charging the photosensitive drum, a scanner unit 11 for forming a latent image on the photosensitive drum, and developing the latent image into a visible image. A developing device 14 for transferring the visible image onto the transfer paper, a cleaning device 15 for removing residual toner on the photosensitive member, and the like.

  When there is an image formation instruction from an external device (not shown) such as a personal computer, the image information is converted into an image signal (VDO signal) for turning on and off the laser beam as the exposure means in a controller (not shown) in the image forming apparatus. Is done.

  The image signal (VDO signal) is input to the laser unit in the scanner unit 11. The laser unit irradiates a laser beam that is on / off modulated by an image signal (VDO signal). In addition, a scanner motor is provided in the laser unit, and the rotary polygon mirror is regularly rotated. The laser beam irradiated from the laser unit is polarized by the polygon mirror and irradiated onto the photosensitive drum 18 which is the surface to be scanned through the imaging lens.

  Therefore, the laser beam modulated by the image signal is horizontally scanned (scanned in the main scanning direction) on the photosensitive drum 18 to form a latent image on the photosensitive drum 18.

  Reference numeral 29 denotes transfer paper as a transfer material, which is fed from the cassette 22. When the leading edge of the transfer paper fed by the registration sensor 24 is detected, the paper feeding operation is temporarily stopped and the heater (not shown) temperature of the fixing device 23 is controlled to be a predetermined temperature.

  Then, the transfer of the transfer sheet is resumed at the same timing as the image forming process by the image forming unit, and the transfer sheet is fed to the registration roller 21. When the top sensor 25 detects the leading edge of the transfer paper, it outputs a TOP signal, and an image of the first color (cyan in the example in the figure) is output on the photosensitive drum 18 as an image carrier in accordance with a predetermined time from the TOP signal output. Form. Reference numeral 29 denotes a suction roller, which applies a suction bias to the shaft of this roller and electrostatically sucks the transfer paper onto the transport belt 20.

  The transfer paper adsorbed on the conveyance belt 20 disposed so as to penetrate each color image forming unit is conveyed by the conveyance belt 20 and the first color image is transferred onto the transfer paper by the transfer device 19.

  Similarly, the image of the second color (magenta in the example in the figure) takes the timing of the detection result of the top sensor 25 and the second color image formation process, and the first color on the transfer paper conveyed on the transfer belt 20. Is superimposed and transferred onto the image. Similarly, the image of the third color (yellow in the example in the figure) and the image of the fourth color (black in the example in the figure) are sequentially superimposed and transferred onto the transfer paper in time with each image forming process.

  The transfer paper onto which the four color toner images have been transferred is heated and pressurized by a fixing device 23 having a built-in halogen heater, whereby the toner images are fused and discharged outside the apparatus.

  On the other hand, the toner remaining on the surfaces of the photosensitive drums 18a to 18d after the toner image transfer is removed by the cleaning devices 15a to 15d, and the toner attached on the transport belt 20 is removed by the transport belt cleaning device 41.

  In the color image forming apparatus having the above configuration, as a method of forming a color image in a state where toner images of a plurality of colors are matched, a color misregistration detection mark is formed on a transfer belt and read by a sensor. A method of correcting color misregistration based on a mark detection result is known (see, for example, Patent Document 1).

  Next, the color misregistration correction method targeted by the present invention will be described with reference to FIGS.

  10 is an explanatory diagram of various color misregistrations, FIG. 11 is an explanatory diagram of color misregistration detection patterns, FIG. 12 is an explanatory diagram of color misregistration detection means, FIG. 13 is an explanatory diagram of a circuit configuration of a color misregistration detection pattern reading processing unit, and FIG. FIG. 15 is an explanatory diagram of an output waveform of the color misregistration detecting means.

  Problems with an apparatus having a plurality of image forming units, such as the conventional example, include uneven movement of a plurality of photosensitive drums and conveyor belts due to mechanical accuracy and the like, and photosensitive at the transfer position of each image forming unit. For example, the relationship between the outer peripheral surface of the drum and the amount of movement of the conveyor belt may occur for each color, and may not match when the images are superimposed, resulting in color misregistration. In addition, a change in the photosensitive drum and the conveyor belt with time, fatigue, humidity in the apparatus, temperature, and the like may cause color misregistration.

  In particular, in an apparatus having a plurality of image forming units having a laser scanner and a photosensitive drum, there is an error in the distance between the laser scanner and the photosensitive drum in each image forming unit. A difference occurs in the scanning width of the laser on the drum, and color misregistration occurs.

  An example of color misregistration is shown in FIG. Reference numeral 207 denotes an original image position, and 208 denotes an image position when color misregistration occurs. Further, (a), (b), and (c) are cases where there is a color shift in the main scanning direction, but for the sake of explanation, two lines are drawn apart in the transport direction. (A) shows an inclination shift of the main scanning line, and occurs when there is an inclination between the optical unit and the photosensitive drum. For example, the position is corrected in the direction of the arrow by adjusting the position of the optical unit or the photosensitive drum or the position of the lens. (B) shows color misregistration due to variations in the main scanning line width, which occurs due to a difference in distance between the optical unit (laser) and the photosensitive drum. It tends to occur when the optical unit is a laser scanner. For example, the image frequency is finely adjusted (if the scanning width is too long, the frequency is increased), and the length of the scanning line is changed to correct in the arrow direction. (C) shows a writing position error in the main scanning direction. For example, when the optical unit is a laser scanner as in this embodiment, the correction is made in the direction of the arrow by adjusting the writing start timing from the beam detection position. (D) shows a writing position error in the paper transport direction. For example, it is corrected in the direction of the arrow by adjusting the writing start timing of each color from the detection of the leading edge of the paper.

  As one of the means for correcting these color misregistrations, in the conventional image forming apparatus, the total number of printed sheets is counted to obtain a color misregistration correction judgment value that represents the degree of change with time of each part that is a cause of color misregistration. . When the color misregistration correction determination value is equal to or greater than a predetermined value, the following color misregistration detection is performed, and various adjustments as described above are performed according to the detected misregistration amount.

  The color misregistration is detected by forming a color misregistration detection pattern for each color on the transport belt 20 and detecting with a pair of optical sensors 206 provided on both sides of the downstream portion of the transport belt.

FIG. 11 shows an example of a color misregistration detection pattern. Reference numerals 209 and 210 denote patterns for detecting a color misregistration amount in the paper conveyance direction (sub-scanning direction), and 211 and 212 denote patterns for detecting a color misregistration amount in the main scanning direction orthogonal to the paper conveyance direction. With an inclination of 45 degrees, a to d represent black (hereinafter Bk), yellow (hereinafter Y), magenta (hereinafter M), and cyan (hereinafter C). Reference numerals tsf1 to 4, tmf1 to 4, tsr1 to 4, and tmr1 to 4 indicate detection timings of the respective patterns, and arrows indicate the moving direction of the conveyor belt 20. The moving speed of the conveying belt 20 is vmm / s, Bk is the reference color, the theoretical distance between each color of the paper conveying direction pattern and the Bk pattern is dsYmm, dsMmm, dsCmm, and the paper conveying direction pattern for each color and the main scanning direction. The measured distances between the patterns are dmfBkmm, dmfYmm, dmfMmm, dmfCmm, dmrBkmm, dmrYmm, dmrMmm, and dmrCmm, respectively. With Bk as a reference color, the positional deviation amount δes of each color in the transport direction is
δesY = v * {(tsf2−tsf1) + (tsr2−tsr1)} / 2−dsY (formula 1)
δesM = v * {(tsf3−tsf1) + (tsr3−tsr1)} / 2−dsM (Expression 2)
δesC = v * {(tsf4-tsf1) + (tsr4-tsr1)} / 2-dsC (Formula 3)
It becomes. With respect to the main scanning direction, the positional deviation amounts δemf and δemr for each of the left and right colors are
dmfBk = v * (tmf1-tsf1) (Formula 4)
dmfY = v * (tmf2-tsf2) (Formula 5)
dmfM = v * (tmf3-tsf3) (Formula 6)
dmfC = v * (tmf4-tsf4) (Formula 7)
And dmrBk = v * (tmr1-tsr1) (Formula 8)
dmrY = v * (tmr2-tsr2) (Formula 9)
dmrM = v * (tmr3-tsr3) (Formula 10)
dmrC = v * (tmr4-tsr4) (Formula 11)
From
emfY = dmfY−dmfBk (Formula 11)
δemfM = dmfM−dmfBk (Formula 12)
δemfC = dmfC−dmfBk (Formula 13)
And δemrY = dmrY−dmrBk (Formula 14)
δemrM = dmrM−dmrBk (Formula 15)
δemrC = dmrC−dmrBk (Formula 16)
Thus, the direction of deviation can be determined from the sign of the calculation result, the writing position is corrected from δemf, and the main scanning width is corrected from δemr−δemf. If there is an error in the main scanning width, the writing position is calculated not only by δemf but also by taking into account the amount of change in the image frequency that has changed with the main scanning width correction.

  FIG. 12 is a diagram for explaining the color misregistration detection means 206. Reference numeral 251 denotes a light emitting element, for example, an LED. Reference numeral 252 denotes a light receiving element, for example, a photo sensor. Reference numeral 20 denotes a conveyance belt, and 209, 210, 211, and 212 are patterns for detecting color misregistration. Reference numeral 253 denotes emitted light from the light emitting element 251, and reference numeral 254 denotes received light received by the light receiving element 252 among reflected light from the transport belt 20 or the color misregistration detection patterns 209, 210, 211, and 212. The light emitting unit and the light receiving unit are configured by a regular reflection optical system with the conveyance belt 20 as a reflection surface, and the color of the light reflected by the difference in regular reflection light reflectance between the conveyance belt 20 and the color misregistration detection pattern, that is, the difference in reflectance. The position of the deviation detection pattern is detected.

  FIG. 13 is a diagram for explaining the pattern reading processing unit. A pattern detection unit including an LED light-emitting unit and a photosensor light-receiving unit, a calculation unit that calculates detection data and calculates a color misregistration amount and a correction value, an image output unit that forms an image according to the calculation result, and each unit It consists of a timer and CPU that perform timing adjustment and various settings.

  FIG. 14 shows a circuit configuration of the light receiving unit of the detecting means of FIG. The detected current from the light receiving element 252 is converted into a voltage by the I / V conversion circuit, the maximum value and the minimum value are detected by the upper peak hold circuit and the lower peak hold circuit, respectively, and divided by the resistors 1 and 2. The A comparison unit determines the magnitude relationship between the output waveform of the I / V conversion circuit and the voltage value divided by the resistors 1 and 2, and generates a pulse signal. Further, it has a function of resetting information held in the upper peak hold circuit and the lower peak hold circuit that operate based on a reset signal from the CPU.

  Here, in order for the upper peak hold circuit and the lower peak hold circuit to obtain the maximum value and the minimum value, it is necessary to read one line in advance. A pulse conversion reference voltage is created from the maximum and minimum values acquired here. That is, it is necessary to create the color misregistration detection patterns 213 and 214 for obtaining the maximum and minimum values immediately before the color misregistration detection pattern 209 shown in FIG.

  FIG. 15 shows a detection waveform when a color misregistration detection pattern is detected by the detection means of FIG. The horizontal axis is time t. (A) shows the color misregistration patterns 209, 210, 211, 212 detected by the light receiving element 252. FIG. (C) is a threshold value serving as a reference of the comparison circuit generated from the output of the maximum value and minimum value detection circuit, and (d) is the output (position information pulse) of the detection circuit.

The conveyance belt 20 has a high reflectivity, and most of the emitted light 253 becomes regular reflection light 254 and is received by the light receiving element 252. The color misregistration detection pattern, which is a toner image, has low reflectance, most of the emitted light 253 is scattered, and the specularly reflected light 254 received by the light receiving element 252 is slight. The output waveform (b) is high when the conveyance belt 20 is detected, and is low when the color misregistration detection pattern is detected. The value of the threshold (c) is determined by the voltage dividing ratio of the resistors 1 and 2. This relationship is expressed by the following equation.
A: B = resistance 1: resistance 2

  The color misregistration detection patterns black a, yellow b, magenta c, and cyan d shown in FIG. 11 are formed on the conveyor belt 20 and detected by the color misregistration detection sensor 206 shown in FIG. The light receiving element 252 outputs four waveforms corresponding to FIG. The first corresponds to a black color shift detection pattern, the second corresponds to yellow, the third corresponds to magenta, and the fourth corresponds to a cyan color shift detection pattern. At this time, the waveform for detecting the pattern of four colors shows the same shape. The first to fourth valleys are lower than the threshold value (c), and a pulse corresponding to (d) in FIG. 15 is output from the comparison unit. 14 calculates the center position of each of the first to fourth pulses in FIG. 15D, and further determines the time difference between the center positions. The amount of color misregistration is calculated from the difference between the obtained time difference and a preset time difference value.

In a conventional image forming apparatus, the total number of printed sheets is counted to obtain a color misregistration correction determination value that represents a change with time and a degree of fatigue of each part that is a cause of color misregistration. When this color misregistration correction judgment value is equal to or greater than a predetermined value, color misregistration detection is performed, various adjustments are performed according to the detected misregistration amount, and color misregistration correction judgment (which serves as a reference for performing color misregistration correction processing) Clear the value. The color misregistration correction determination value is stored in a storage unit such as an EEPROM provided in the image forming apparatus, and reading / writing of the color misregistration correction determination value is performed by the image forming control unit.
JP-A-63-271275

  However, there were the following problems.

  The top sensor 25 is arranged between a suction roller and a registration roller, and includes a sensor flag supported so as to be swingable and a photo interrupter for monitoring the movement thereof. In the case of the top sensor having such a configuration, an apparatus difference is likely to occur in the paper leading edge detection timing for each image forming apparatus due to variations in sensor flag mounting position, sensor flag movement, and the like. Further, the deflection state of the leading edge of the paper may change depending on the thickness of the transfer paper to be conveyed, the stiffness of the paper, and the like. That is, since the image formation timing is based on the paper leading edge detection signal of the top sensor, the variation in the output time of the signal becomes the variation in the image writing position from the leading edge of the transfer paper.

  Therefore, in order to improve the mechanical time error due to the movement of the sensor flag and the time error due to the type of paper, an optical sensor having a configuration in which a light emitting element and a light receiving element are paired (not using a sensor flag) is used. It is possible to use it. However, it is difficult for such an optical sensor to detect a transparent transfer paper such as an OHP sheet.

  In addition, in the color misregistration correction, one of the preconditions for correction is that the speed of the conveyor belt 20 is constant. The belt conveyance speed depends on the rotation speed of the driving roller 30 that drives the conveyance belt and the support roller 31 that is driven by the driving roller. That is, the belt conveyance speed is kept constant by keeping the rotation speed of the drive roller constant. However, when the diameter of the drive roller changes depending on the use environment or life of the image forming apparatus, the conveyance speed is slow when the diameter is small, and the conveyance speed is high when the diameter is large. Therefore, when the diameter of the driving roller 30 changes after the color misregistration correction, the writing position error in the paper transport direction in FIG. 10D increases, and thus appears as a color misregistration image during image formation.

  Conventionally, since the color misregistration correction processing is performed only when the color misregistration correction determination value is equal to or greater than a predetermined threshold value, even if color misregistration due to the change in the diameter of the driving roller 30 occurs, Color misregistration correction could not be performed unless the color misregistration correction judgment value has reached a predetermined threshold value. Further, in order to detect the speed of the transfer paper itself, a sensor flag supported so as to be able to swing and a photo interrupter for monitoring its movement are provided between the four image forming units, like the top sensor. Even if the sensor is provided, there is a problem that the cost is increased by adding the sensor.

  As described above, there is room for improvement in the variation in the image writing position from the leading edge of the transfer paper and the optimum color misregistration correction timing.

  The present invention has been made paying attention to the above points, and an object of the present invention is to provide a color image forming apparatus and an image forming apparatus that perform an image writing position correction from the leading edge of a transfer sheet and an optimum color misregistration correction. To do.

  In the image forming apparatus, a color image forming apparatus capable of detecting the transfer paper conveyance speed at low cost by using an existing circuit and correcting the image writing position from the leading edge of the transfer paper and performing the optimum color misregistration correction. provide.

  That is, the technical contents of the present invention can solve the above-described problems by including the following configuration.

  (1) Image forming means for forming toner images of different colors on a plurality of image carriers, and recording that is transferred to the plurality of image carriers and copies the toner images formed on the plurality of carriers. A medium, a conveying unit that conveys the recording medium to the plurality of image carriers, a plurality of transfer units that respectively transfer the toner images of different colors to the recording medium conveyed by the conveying unit, and a transfer unit In an image forming apparatus having a first detection means for detecting a high voltage applied or a current flowing through the transfer means, fluctuations in voltage or current generated when the leading edge of the transfer paper enters the plurality of transfer means, respectively. A color image forming apparatus, wherein the color image forming apparatus detects the conveyance speed of the recording medium by detecting at least two of the first detection means sequentially and detecting a difference in detection time. .

  (2) The color image forming apparatus described in (1) detects an adsorption unit that electrostatically adsorbs a recording medium to a conveyance unit, and a high voltage applied to the adsorption unit or a current flowing through the adsorption unit. A second detecting means, and the second detecting means and the first detecting means sequentially detect fluctuations in voltage or current generated when the leading edge of the transfer paper enters the attracting means and the transferring means, and the detection The color image forming apparatus according to (1), wherein the conveyance speed of the recording medium is detected by a time difference.

  (3) The color image forming apparatus described in the above (1) or (2) includes a misregistration measurement toner pattern (hereinafter referred to as a color misregistration detection pattern) formed for each color on a conveying unit, and a color misregistration detection pattern. A position detection unit for detecting the position of the color, and when a predetermined start condition is satisfied, the color misregistration detection pattern is formed, and based on the position information of the color misregistration detection pattern detected by the position detection unit, the image forming position of each color A color misregistration correction unit that corrects the recording medium, and when the first detection unit or the second detection unit detects that the conveyance speed of the recording medium is out of a predetermined speed range, The color image forming apparatus according to (1) or (2), wherein a start condition is satisfied.

  (4) Image forming means for forming a toner image on an image carrier, a recording medium transported to the image carrier and copying the toner image formed on the carrier, and the recording medium as the image carrier A conveying means for conveying to the body; a third detecting means for outputting a leading edge detection signal for determining an image forming start timing of the image forming means by detecting a leading edge of the transfer paper conveyed by the conveying means; An image forming apparatus comprising: a transfer unit that transfers the toner image to a recording medium conveyed by the conveyance unit; and a fourth detection unit that detects a high voltage applied to the transfer unit or a current flowing through the transfer unit. In the above, the fourth detection means detects a change in voltage or current generated when the leading edge of the transfer paper enters the transfer means from the output timing of the leading edge detection signal. By the time difference at, detecting the transport speed of the recording medium An image forming apparatus comprising.

  (5) The image forming apparatus described in (4) includes a suction unit that electrostatically attracts the recording medium to the transport unit, and a high voltage that is applied to the suction unit or a current that flows through the suction unit. A time difference from the output timing of the leading edge detection signal to the time when the fluctuation of the voltage or current generated when the leading edge of the transfer paper enters the attracting means is detected by the fifth detecting means. The image forming apparatus according to (4), wherein a conveyance speed of the recording medium is detected.

  (6) When it is detected that the detection speed of the recording medium described in (4) or (5) is out of a predetermined speed range, the image formation start timing is corrected according to the detection speed. The image forming apparatus as described in (4) or (5) above.

  Constant current bias control for the timing at which the leading edge of the transfer paper rushes into at least two of the suction roller or multiple transfer rollers, and for constant voltage bias control, the current fluctuation when the leading edge of the transfer paper rushes. In this case, the transfer time of the transfer paper is detected by detecting the time difference of voltage fluctuation with an existing circuit, and color misregistration correction processing is executed according to the detection result, thereby correcting the writing position error of each color. It is possible to provide an image with high color misregistration accuracy at low cost.

  In addition, the detection of the top sensor and the time difference between the current fluctuation or voltage fluctuation when the leading edge of the transfer paper enters the suction roller or the plurality of transfer rollers is detected by the existing circuit, resulting in the movement of the sensor flag. By improving the mechanical time error and the time error due to the type of paper, the variation in the image writing position from the leading edge of the transfer paper can be corrected at low cost.

  Furthermore, it is also possible to detect the conveyance speed of transparent transfer paper such as an OHP sheet, which is difficult to detect with an optical sensor.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to examples.

  A first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a schematic wiring diagram of an adsorption bias and transfer bias circuit used in a tandem type laser printer which is an example of an image forming apparatus to which the present invention is applied. FIG. 2 is a schematic wiring diagram focusing on one image forming unit in FIG. FIG. 3 is an explanatory diagram of the transfer current detection circuit.

  A feature of the present embodiment is that a transfer current variation time difference in a plurality of image forming units is detected to detect a paper conveyance speed, and color misregistration correction is performed based on the conveyance speed information.

  Since the image forming operation of the laser printer is as described in the conventional example, the description thereof is omitted.

  The transfer roller 19d electrostatically transfers the toner image formed on the photosensitive drum 18d to the transfer paper 40 on the conveyance belt 20, and for example, a metal cored bar has a volume resistivity of 10 ^ 5 to 10 ^. A material covered with an elastic body such as EPDM (ethylene-propylene-diene terpolymer), urethane rubber, NBR (nitrile butadiene rubber) adjusted to about 8 Ω · cm can be used. The transfer roller 19d is installed inside the conveyance belt 20 and presses the conveyance belt 20 against the photosensitive drum 18d, whereby a transfer point (transfer nip) N is formed between the photosensitive drum 18d and the conveyance belt 20. Has been.

  The conveyor belt 20 is stretched around two rollers 30 and 31, and is rotated by the drive roller 30 to convey the transfer paper 40 carried on the surface thereof. The transport belt 20 is made of a resin material or a thickness such as PVdF (polyvinylidene fluoride), polyamide, polyimide, PET (polyethylene terephthalate), polycarbonate having a thickness of 50 to 300 μm and a volume resistivity of about 10 9 to 10 16 Ω · cm Rubber materials such as chloroprene rubber, EPDM, NBR, and urethane rubber having a thickness of 0.5 to 2 mm and a volume resistivity of about 10 ^ 9 to 10 ^ 16 Ω · cm are used. Further, if necessary, conductive fillers such as carbon, ZnO, SnO2, and TiO2 may be dispersed in these materials to adjust the volume resistivity to about 10 ^ 7 to 10 ^ 11 Ω · cm.

  Reference numeral 68 denotes a suction bias application power source, and the suction bias application power source 68 is connected to the suction roller 29 provided in contact with the conveying belt 20, and the suction roller 29 is a constant voltage high voltage from the suction bias application power source 68. Is mainly provided for electrostatically adsorbing the transfer paper 40 onto the transport belt 20. Further, similarly to a transfer bias applying power source 63d described later, a high voltage according to the ON width of the PWM clock signal S65 from the control circuit 61 can be variably output.

  In this embodiment, the transfer roller 19d is connected to a transfer bias application power source 63d, which is a transfer constant voltage power source (constant voltage power source), and a transfer current detection circuit 64d is connected to the transfer bias application power source 63d. ing. The transfer bias application power source 63d can variably output a high voltage from the control circuit 61 according to the ON width of the PWM clock signal S61. Depending on the environmental state of the printer, the paper type, the printing timing, etc. The control circuit 61 controls the constant voltage bias to a predetermined value. When a constant voltage bias is applied from the transfer bias application power source 63d, most of the transfer current I61a is transferred from the transfer roller 19a, which is the transfer load 62a, to the transport belt 20, transfer paper, and the photosensitive drum 18a, and then to the ground. Flowing. The transfer current I61a flowing to the ground follows a current loop that returns to the transfer bias application power source 63a via the current detection circuit 69.

  The operation of the transfer current detection circuit 64 using the OP-AMP 94 will be described below with reference to FIG.

  The transfer current I61a flows from the ground of the OP-AMP 94 of the current detection circuit as shown by the output terminal, the protective resistor 96, the resistor 92 and the arrow in the figure. The + terminal of the OP-AMP 94 is connected to the ground by an impedance matching resistor 95. Since the + terminal and the − terminal of the OP-AMP 94 are generally imaginary shorts, the − terminal can also be considered as a ground potential. When the transfer current I61a flows, a voltage of Vi is generated with reference to the virtual ground at the negative terminal of the OP-AMP. This Vi is expressed by the following equation when the transfer current is Itr and the resistance 92 is Ra.

Vi≈Itr × Ra
Therefore, the value of the output signal S63d from the current detection circuit 64a becomes Vi, and as a result, the current detection circuit converts the magnitude of the transfer current I61a flowing through the first image forming unit into an analog voltage. The output signal S63d output from the transfer current detector 64a is transmitted to the A / D port in the control circuit 61. The control circuit 61 performs digital conversion based on the output signal S63d and calculates a transfer current value. Since the switching noise of the transfer bias applying power source 63d is often superimposed on the output signal S63d, the noise component is removed by inserting a noise filter or averaging the output signal S63d as necessary. preferable.

  The detection result of the transfer current is generally [1] impedance detection of a current loop through which the transfer current I61a flows, or [2] a reference for changing the PWM setting value so as to keep the transfer current I61a at the time of transfer constant. Used. The former [1] outputs a predetermined constant voltage bias from the transfer bias application power source 63d before the printing operation, and the control circuit 61 calculates I61a at that time. Based on the calculation result, the control circuit 61 selects an optimum transfer bias setting value (PWM signal) from a predetermined transfer bias table with respect to the impedance that changes depending on the use environment and life of the apparatus and the type of transfer paper. Thereafter, a printing operation is started, and transfer is performed using the selected constant voltage bias.

  In the latter [2], in order to keep the transfer current I61a constant during transfer (constant current bias control), the control circuit 61 based on the output signal S63d changes the set value (PWM signal) of the transfer bias application power source 63d as needed. change. That is, when the transfer current obtained from the output signal S63d is small, the predetermined constant voltage bias is increased, and when the transfer current is large, the predetermined constant voltage bias is decreased to stabilize the transfer current I61a.

  In the present embodiment, the former [1] will be described. An example of [2] will be described in the second embodiment.

  The image forming stations for other colors, that is, the magenta, yellow, and black image forming units have the same configuration as the cyan (first color) image forming unit and operate in the same manner. In the drawing, members having the same configuration as the first color in each image forming unit are assigned the same number as the first color image forming unit, and the magenta ( C is assigned to the second color), b is assigned to the yellow color (third color), and a is assigned to the black color (fourth color).

  In the above configuration, a method for detecting the paper conveyance speed will be described with reference to FIG.

  FIG. 4 shows transfer voltages and current values for the first and fourth colors from the start of image formation. Before the transfer paper reaches the transfer point N of the first color, Vt11 lower than a predetermined transfer bias Vt12 is applied in advance. At this time, a transfer current It11 is flowing. When the transfer paper 40 is conveyed and reaches the transfer point of the first color, the transfer current is reduced to It13 because the transfer bias is controlled at a constant voltage. Therefore, when the transfer current detection signal S63d is monitored, the timing t1 when the transfer paper 40 reaches the transfer point N can be known (a threshold value for determination is set between It11 and It13).

  As described above, since the transfer bias voltage is selected from a transfer bias table determined in advance according to the use environment and life of the apparatus and the type of transfer paper, the timing t1 depends on the applied transfer bias and other conditions. It is also possible to determine a threshold value of the transfer current for detecting the above.

  After the leading edge of the transfer paper reaches the transfer point N and before the image formation area from the leading edge of the paper reaches the transfer point N (no image defect due to transfer occurs), the switching is made to the predetermined transfer bias Vt12. Here, in order to detect the leading edge position of the transfer paper, Vt11 is set to a low voltage because the transfer bias applied to the photosensitive drum 18d in the absence of the transfer paper causes a memory in the photosensitive drum 18d. This is to prevent a so-called drum memory that appears as a shading image defect in a subsequent print job.

  Similarly, the timing at which the transfer sheet 40 reaches the transfer point N can be recognized for other colors. In the fourth color, as shown in FIG. 4, the timing when the transfer paper 40 reaches the transfer point N is detected as t4. In the figure, ta is a predetermined time required to reach the transfer point N of the first color to the fourth color obtained from the conveyance speed of the conveyance belt 20, and a predetermined time ta has elapsed since t1 was detected. However, if t4 is not detected, or if t4 is detected before reaching the predetermined time ta after t1 is detected, the control circuit 61 determines that the transfer paper conveyance speed has changed.

  As described above, when t4 is detected after a predetermined time ta or more, it is possible that the driving roller diameter of the conveyor belt has changed, so that the writing position error of each color is large. It may have become. For this reason, although it is difficult to correct the transfer paper when the conveyance speed is detected, the color misregistration correction process is executed after the current print job is completed or before the second print of the current print job is performed. By doing so, it is possible to provide an image corrected for color misregistration at the time of the second print job or the next print job.

  However, if the color misregistration correction is repeated frequently, such as for each print job, it will only unnecessarily increase the waiting time of the user, so the color misregistration correction processing is appropriately performed based on the time history of the previous color misregistration correction. It should be executed.

  In this embodiment, the transfer speed of the transfer paper is detected from the time difference between the fluctuations in the transfer currents of the first color and the fourth color. However, similarly, in any combination of the first to fourth colors, similarly, the transfer paper conveyance speed can be detected by detecting the time difference of the fluctuation of the transfer current. However, the larger the time difference is, the smaller the influence of the error at the time of detecting the transfer current fluctuation is. Therefore, it is preferable to detect the time difference of the fluctuation of the transfer current of the first color and the fourth color having the longest time difference.

  Furthermore, as shown in FIG. 5, when the current detection circuit 69 is also connected to the suction bias application power source 68, the change in the suction current when the leading edge of the paper enters the suction roller is detected by the detection signal S68. The paper transport speed may be detected based on the time from when the change in the adsorption current is detected to the change in the transfer current of any of the first to fourth colors.

  As described above, by detecting the paper speed at low cost because of the existing transfer current detection circuit, and correcting the color misregistration according to the detection speed (at the necessary timing), the color misregistration is improved. The image can be provided to the user.

  A second embodiment of the present invention will be described with reference to FIG.

  Since the image forming operation and the color misregistration correction processing of the tandem type laser printer, which is an example of the color image forming apparatus targeted by this embodiment, are as described in the conventional example, the description is omitted.

  A characteristic of this embodiment is a constant transfer current bias control in which the PWM setting value is changed so as to keep the transfer current I61a at the time of transfer constant.

  That is, in order to keep the transfer current I61a constant during transfer, the control circuit 61 changes the set value (PWM signal) of the transfer bias application power source 63d as needed based on the output signal S63d. That is, the control circuit 61 increases the predetermined constant voltage bias when the transfer current obtained from the output signal S63d is small, and reduces the transfer current by decreasing the predetermined constant voltage bias when the transfer current is large. It is kept at a predetermined value (hereinafter, constant current bias control). In addition, about the structure in Example 2, it is the same as that of Example 1, About the same structure, the same code | symbol is provided and duplication description is abbreviate | omitted.

  In the above configuration, a method for detecting the paper conveyance speed will be described with reference to FIG.

  FIG. 6 shows transfer voltages and current values for the first and fourth colors from the start of image formation. Before the transfer sheet reaches the transfer point N of the first color, a transfer constant current bias is applied so that a current It15 lower than a predetermined transfer current It16 flows constantly. When the transfer sheet 40 is conveyed and reaches the transfer point of the first color, the transfer current It15 decreases because the transfer current hardly flows. In the constant current bias control, the control circuit 61 changes the set value of the PWM signal to increase the transfer voltage in order to compensate for the decreasing current and keep the It15 stable. Therefore, when the PWM signal S61d is monitored, the timing t1 when the transfer paper 40 reaches the transfer point N can be known.

  As described above, when the constant current bias control is performed, when the transfer sheet 40 reaches the transfer point N, the voltage increases as shown in FIG. The conveyance speed of the paper 40 can be detected.

  In the present embodiment, the transfer bias application power supply circuit that outputs the constant voltage of the first embodiment is used to perform the transfer bias constant current control by software control.

  On the other hand, as another constant current bias control method, a transfer bias application power source (constant current power source) 63 instead of the transfer bias application power source (constant voltage power source) 63 of FIG. 1 and a voltage detection instead of the current detection circuit 64 are detected. It goes without saying that the same effect as in this embodiment can be obtained even when a circuit is provided and a configuration (not shown) capable of controlling the constant current of the transfer bias in hardware is provided.

  A third embodiment of the present invention will be described with reference to FIG.

  Since the image forming operation of the tandem type laser printer, which is an example of the color image forming apparatus targeted by this embodiment, is as described in the conventional example, the description thereof is omitted.

  The feature of the present embodiment is that the image writing position from the leading edge of the transfer paper is corrected according to the time from the detection of the leading edge of the transfer paper by the top sensor that is a reference for starting image formation until the transfer point N is reached. That is.

  The top sensor 25 is disposed between the suction roller and the registration roller, and includes a sensor flag supported so as to be swingable and a photo interrupter for monitoring its movement. Since the other end of the sensor flag blocks the light of the photo interrupter in conjunction with the contact of the transfer paper with one end of the sensor flag, the output voltage of the top sensor is when the paper is in the sensor section. When the paper is not in the sensor unit, the low level is indicated and the high level is indicated.

  As shown in FIG. 7, the time t0 when the leading edge of the transfer paper comes into contact with the sensor flag of the top sensor and the output signal from the sensor becomes the low level is detected.

  On the other hand, the lower part of FIG. 7 shows the transfer voltage and current value of the first color from the start of image formation. Details regarding the fluctuations in voltage and current are the same as those described in the first embodiment, and are therefore omitted.

  Tc in the figure is a predetermined time required for reaching the transfer point N of the first color from the time when the output signal becomes low level, and is a predetermined time tc after detecting t0. If t1 is not detected even after elapse of time t, or if t1 is detected before reaching the predetermined time tc after detecting t0, the control circuit 61 determines that the transfer paper conveyance speed is different from the set value. Judge that

  As described above, when t1 is detected at or after a predetermined time from the predetermined time tc, the leading edge position of the paper may not be accurately detected. The error may be large.

  For this reason, although it is difficult to correct the transfer paper when the transport speed is detected, the time from t0 to t1 after the end of the current print job or before the second print of the current print job is performed. By correcting the image formation timing according to the above, it is possible to provide an image in which the writing position is corrected at the time of the second or next print job.

  In this embodiment, the leading edge position of the transfer paper is detected based on the time difference from the time when the output signal from the top sensor becomes the low level to the fluctuation of the transfer current of the first color. However, it is also possible to detect the leading edge position of the transfer paper by detecting the time difference between the adsorption current and the current variation of any of the second to fourth colors instead of the variation of the first color transfer current. is there.

  As described above, the position of the leading edge of the transfer paper can be detected at low cost by using existing sensors and current detection circuits, and the mechanical time error and paper type caused by the movement of the sensor flag according to the detection result By correcting the time error due to, the writing start position can be corrected.

  Needless to say, this embodiment is effective not only in a color image forming apparatus but also in a monochrome image forming apparatus having a single image forming unit.

  Further, in the present embodiment, in combination with the first embodiment or the second embodiment, the threshold value of the color misregistration correction determination value and Needless to say, it is effective to change the correction value.

  In the first to third embodiments, an example of a laser beam printer using a laser beam as an exposure apparatus is shown. However, it goes without saying that the effects of the present invention can be obtained even in a printer using an LED as an exposure apparatus.

Schematic diagram of transfer / adsorption bias wiring of image forming equipment Wiring schematic diagram focusing on one image forming unit in FIG. Illustration of current detection circuit Timing chart for explaining the first embodiment Adsorption bias circuit Current detection circuit wiring explanatory diagram Timing chart for explaining the second embodiment Timing chart for explaining the third embodiment Overall view of the image forming apparatus Color misregistration detection sensor installation position explanatory diagram Explanation of various color shifts Color misalignment detection pattern illustration Explanation of color misregistration detection means Circuit configuration explanatory diagram of the color misregistration detection pattern reading processing unit Configuration explanatory diagram of the light receiving portion of the color misregistration detection means Output waveform explanatory diagram of color misregistration detection means

Explanation of symbols

18 Photosensitive drum 11 Laser scanner 20 Conveying belt 206 Color shift detection sensor 207 Original image position 208 Image positions 209 to 212 when there is color shift Pattern 251 for color shift detection Light emitting element 252 Light receiving element

Claims (6)

Image forming means for forming toner images of different colors on a plurality of image carriers, and
A recording medium that is conveyed to the plurality of image carriers and copies the toner images formed on the plurality of carriers;
Conveying means for conveying the recording medium to the plurality of image carriers;
A plurality of transfer means for transferring the toner images of different colors to the recording medium transported by the transport means;
An image forming apparatus having a first detection means for detecting a high voltage applied to the transfer means or a current flowing through the transfer means;
Changes in voltage or current generated when the leading edge of the transfer paper respectively enters a plurality of transfer means are sequentially detected by at least two first detection means, and the conveyance speed of the recording medium is detected by the difference in detection time. A color image forming apparatus. The color image forming apparatus according to claim 1, wherein an adsorbing unit that electrostatically adsorbs the recording medium to the conveying unit, and a second detection that detects a high voltage applied to the adsorbing unit or a current flowing through the adsorbing unit. Means,
Changes in voltage or current generated when the leading edge of the transfer paper enters the suction means and transfer means are sequentially detected by the second detection means and the first detection means, and the conveyance speed of the recording medium is detected by the difference in detection time. The color image forming apparatus according to claim 1, wherein: A color image forming apparatus according to claim 1 or 2, wherein a misregistration measurement toner pattern (hereinafter referred to as a color misregistration detection pattern) formed for each color on the conveying means
Position detecting means for detecting the position of the color misregistration detection pattern;
When a predetermined start condition is satisfied, the color misregistration detection pattern is formed, and based on position information of the color misregistration detection pattern detected by the position detection unit, color misregistration correction means for correcting the image forming position of each color is provided. And
2. The predetermined start condition is satisfied when the first detection unit or the second detection unit detects that the conveyance speed of the recording medium is out of a predetermined speed range. Or the color image forming apparatus of 2. An image forming means for forming a toner image on the image carrier;
A recording medium that is transported to the image carrier and copies a toner image formed on the carrier;
Conveying means for conveying the recording medium to the image carrier;
Third detection means for outputting a leading edge detection signal for determining the image formation start timing of the image forming means by detecting the leading edge of the transfer paper conveyed by the conveying means;
Transfer means for transferring the toner image to the recording medium conveyed by the conveyance means;
In an image forming apparatus having a high voltage applied to the transfer means or a fourth detection means for detecting a current flowing in the transfer means,
The conveyance speed of the recording medium is detected by the time difference from the output timing of the leading edge detection signal until the fourth detecting means detects a change in voltage or current that occurs when the leading edge of the transfer paper enters the transferring means. An image forming apparatus. 5. The image forming apparatus according to claim 4, wherein an adsorption unit that electrostatically adsorbs the recording medium to the conveyance unit and a fifth detection unit that detects a high voltage applied to the adsorption unit or a current flowing through the adsorption unit. And
The conveyance speed of the recording medium is detected by the time difference from the output timing of the leading edge detection signal to the detection of the voltage or current fluctuation generated when the leading edge of the transfer paper enters the suctioning means by the fifth detecting means. The image forming apparatus according to claim 4.   6. The image forming start timing is corrected according to the detected speed when it is detected that the detected speed of the recording medium is outside a predetermined speed range. Or the image forming apparatus according to 5;

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