JP2003131443A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device that surely detects the position of a toner image held by a sensor regardless of scratch or soil on an image carrier to be subjected to the detection and an error in the mounting of a regular reflection type photosensor. SOLUTION: A reference pattern image P for density detection and reference pattern images pP1 and pP2 for position detection, which are formed on a transfer carrying belt 60, are detected by the same reflection type photosensor 69. A threshold Vth used for the position detection is set on the basis of a base part output voltage Vsg obtained by the density detection and the minimum output voltage Vmin.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a facsimile, a printer, etc.
The present invention relates to an image forming apparatus that forms a position detection visible image on an image carrier and detects the position of the position detection visible image by a position detection unit.

[0002]

2. Description of the Related Art Conventionally, as an image forming apparatus of this type, based on a change in an electric signal output from an optical sensor such as a reflection type photo sensor that irradiates an image bearing member such as a photoconductor or an intermediate transfer member with light, It is known to calculate the position of a reference toner image as a position-detecting visible image on an image carrier and detect the position of the reference toner image from the calculation result. Based on this, the image forming conditions such as the toner image forming position on the image carrier can be corrected.

The correction of the toner image forming position is effective in the following cases. For example, as in a so-called tandem type image forming apparatus in which a plurality of photoconductors are arranged side by side and the toner images formed on the photoconductors are sequentially superimposed and transferred onto a transfer paper or the like to form a composite image, This is a case where the toner image formation position relative to is relatively displaced and the image displacement is likely to occur. In the case of such a tandem type image forming apparatus, after transferring the reference toner image formed on each photoconductor to an image carrier such as an intermediate transfer body or a paper transport belt, based on the output signal value from the optical sensor. The position of each reference toner image on the image carrier is calculated, the relative positional deviation of each toner image is determined based on the calculation result, and the toner image forming position is corrected in the direction of correcting the positional deviation. . With such a configuration, it is possible to suppress the image shift due to the positional shift of each toner image by adjusting the position where the toner image is formed on each photoconductor based on the determination result of the positional shift of each toner image. Further, it is effective to improve the reproducibility of a color image by correcting the toner image forming position in a color image forming apparatus that superposes a plurality of color images.

Conventionally, as an invention using the above-mentioned optical sensor, the optical sensor detects the passing time at a predetermined position of a multi-color toner line which is exposed, developed, and transferred at a predetermined timing. There has been proposed a color image forming apparatus or a color image forming method which detects a positional deviation of a toner image by detecting a passage time difference (see JP-A-10-142895, 10-198110, etc.).

By the way, the optical sensor includes a reflection type sensor and a transmission type sensor. Among these, the reflection type sensor irradiates the detection target with light and detects the reflected light, and the light emitting part and the light receiving part can be housed together in one housing. On the other hand, the transmissive sensor measures the amount of transmitted light of an object to be detected, in which the light emitting portion and the light receiving portion are housed in different housings and are arranged to face each other through the light transmitting object. Among these, the transmissive sensor requires a plurality of housings, and since it is necessary to secure an optical path between both housings, the degree of freedom in layout within the device is lower than when a reflective sensor is used. It may end up. Further, in the transmissive sensor, it is necessary to use a transmissive member for the image carrier that is the detection target, and the material that can be used as the detection target is limited. For example, since the transfer belt contains a resistance control agent such as carbon, it is often not a transmissive member, and it is difficult to use a transmissive sensor. For these reasons, the reflective sensor is generally used more often.

Further, the reflection type sensor includes a regular reflection type sensor for receiving regular reflection light and a diffused light type sensor for receiving diffused light. Among these, the diffused light type sensor has a lower S / N ratio than the specular reflection type sensor, and there is a phenomenon that the magnitude relationship between the toner density and the output value is reversed between the BK toner and the color toner. Further, the specular reflection type has a lower limit to the output value when the toner density becomes high to some extent, but the diffused light type has an endless increase in the output value as the toner density increases. For various reasons as described above, the regular reflection type is relatively easy to use for detecting the position of the toner image.
In this specular reflection type sensor, the light emitting axis and the light receiving axis are arranged symmetrically with respect to the image plane.

Here, an example of a method for detecting the position of the toner image on the transfer belt by using the reflection type sensor will be described. First, 100 to 1000 with a predetermined time difference
Images of multiple color toner lines of about μm are formed on the transfer belt. When the toner image passes the sensor position, the output voltage gradually decreases with a predetermined gradient after the sensor starts detecting the front end of the toner image, and finally reaches the minimum value. When the output voltage reaches the minimum value, it stabilizes for a while, but thereafter, when the sensor starts detecting the trailing edge of the toner image, the output voltage gradually increases with a predetermined gradient and returns to the original value. This is because the output voltage changes due to the difference in the intensity of light detected by the light receiving unit depending on the reflectance of the light emitted by the reflective sensor on the transfer belt. Output voltage when this toner image passes the sensor position and reference voltage as a preset threshold value (hereinafter referred to as reference value)
And the timing when the output voltage drops below the reference value (hereinafter referred to as the falling edge), and when the output voltage reaches the minimum value and rises above the reference value (hereinafter referred to as the rising edge), etc. The relative position between each color is detected from. As a method of comparison, calculating between the falling edges, or predicting the center position of the toner image from the falling edge and rising edge portion of a predetermined color, the passing time of the predicted center position between each color There are methods such as calculation.

[0008]

However, the output voltage at the time of detecting a toner image also has a low S / N ratio due to scratches or stains on the transfer belt or mounting error on the optical sensor. Come on. FIG. 22 is a graph showing the relationship between the number of printouts and the minimum value Vmin of the output voltage when a toner image is detected by an optical sensor in which the output of the light emitting unit is adjusted so that the sensor output value for the background portion is almost constant. Is. As shown in this figure, as the number of printouts increases, the output voltage also increases.
This is because scratches and dirt adhere to the surface of the transfer belt as the printouts are piled up, unexpected irregularities occur on the originally smooth background area, and the light reflectance decreases, resulting in an output voltage corresponding to the background area. It is thought that this is due to the decrease. Many optical sensors adopt a configuration in which the output voltage to the background portion of the transfer belt is raised to the original output value and then the detection is performed. Also in FIG. 22, the sensor having such a configuration is used for detection. As a result, the output voltage when the toner image is detected is increased. In addition, FIG.
7 is a graph showing the relationship between the distance between transfer belts and the minimum value Vmin of output voltage. As shown in this figure, when the distance between the sensor and the transfer belt changes, the output voltage of the sensor changes rather than being constant. Therefore, it can be seen that the detection result is different due to the mounting error of the optical sensor.

From the above, if the reference value is a fixed value, it becomes difficult to reliably detect the toner image. For example, the output voltage of the sensor does not sufficiently decrease in a line image having a low toner concentration, but when detecting such an image, a falling edge or a rising edge may not be detected in some cases. In addition, since the reflectance of light decreases and the output voltage decreases in the area where scratches or dirt are attached on the surface of the transfer belt, that area may be erroneously detected as a part having a toner image, or the toner image may be detected in that part. In some cases, it may not be possible to detect when there are formed.

It has already been proposed in JP-A-2000-089541 to change the reference value when the toner image cannot be detected due to such a decrease in the S / N ratio. However, this Japanese Patent Laid-Open No. 2000-
Japanese Patent No. 089541 does not describe a specific method of how to optimize the reference value, and it cannot be said that the above-described erroneous detection can be reliably avoided.

Further, as a method of setting the reference value to an appropriate value,
It is easy to think of a method of predicting a proper reference voltage from the waveform of the output voltage of the toner image during the position detection operation of the optical sensor. However,
Since the pattern toner image for position detection requires high accuracy in the detection position, a toner line relatively thinner than the density detection pattern is often used. For this reason, it is considered that it is difficult to obtain an appropriate reference value because it is easily affected by the edge effect and density unevenness.

The present invention has been made in view of the above background. The first object of the present invention is to provide a specular reflection type optical sensor even if the image carrier to be detected is scratched or soiled. An object of the present invention is to provide an image forming apparatus capable of surely detecting the position of a toner image. A second object of the invention is to provide an image forming apparatus capable of surely detecting the position of a toner image by means of a specular reflection type optical sensor even if an optical sensor has an attachment error. .

[0013]

In order to achieve the above first and second objects, an image forming apparatus according to a first aspect of the present invention includes an image carrier for carrying a visible image, and an image carrier for the visible image. A visible image forming unit for forming a visual image, a light emitting unit for irradiating light onto the image carrier, and a light receiving unit for receiving reflected light from the surface of the image carrier are provided, and the light emitting axis and the light receiving axis are An optical sensor in which the light emitting portion and the light receiving portion are arranged so as to be symmetrical with respect to a normal line of the surface of the image carrier, and which outputs an electric signal having a value according to the optical characteristics of the image carrier, and the image. An image forming apparatus having position detecting means for detecting the position of the visible image by utilizing that the output signal value from the optical sensor for the visible image formed on the carrier becomes lower than a predetermined threshold value. And a density detection visible image forming means for forming a density detection visible image on the image carrier, Of the background output signal value storage means for storing the background output signal value, which is the output signal value for the background of the image carrier by the optical sensor, and the output signal value for the visible image for density detection by the optical sensor. A minimum output signal value storage means for storing the minimum output signal value that is the minimum value is provided, and a threshold value setting means for setting the threshold value by using at least one of the minimum output signal value and the bare skin output signal value. It is characterized by being provided.

An image forming apparatus according to a second aspect of the present invention is the image forming apparatus according to the first aspect, in which the light emitting section of the light emitting unit is in accordance with the light reflectance of the visible image for density detection formed on the image carrier. The light emitting unit output adjusting means for adjusting the light output value is provided.

In order to achieve the first object, the image forming apparatus according to a third aspect of the present invention is an image carrier for carrying a visible image, and a visible image forming for forming a visible image on the image carrier. Means, a light emitting portion for irradiating the image bearing member with light, and a light receiving portion for receiving reflected light from the surface of the image bearing member, and the light emitting axis and the light receiving axis are the normals to the surface of the image bearing member. An optical sensor in which the light emitting portion and the light receiving portion are arranged so as to be symmetrical with respect to each other, and which outputs an electric signal having a value according to the optical characteristics of the image carrier, and a visible light formed on the image carrier. Position detection means for detecting the position of the visible image by utilizing the fact that the output signal value from the optical sensor for an image becomes lower than a predetermined threshold value, and the output of the optical sensor at the background portion of the image carrier. A light emitting unit output adjusting means for adjusting the light output value of the light emitting unit so that the signal value becomes constant. In the image forming apparatus, in which the threshold value, characterized in that a threshold value setting means to reset high so with the increase in the cumulative number of the image forming operation.

An image forming apparatus according to a fourth aspect is the image forming apparatus according to the first aspect.
The second or third image forming apparatus is characterized in that it is provided with a positional deviation correcting means for correcting the visible image forming position on the image carrier based on the detection result of the position detecting means. .

An image forming apparatus according to a fifth aspect is the image forming apparatus according to the fourth aspect, in which an endless moving body is provided as the image bearing body and a latent image bearing body that carries a latent image as the visible image forming means. And a plurality of developing means for developing the latent image on each latent image carrier, and a transfer means for sequentially transferring the visible image developed on each latent image carrier onto the endless moving body. It is characterized by that.

In the image forming apparatus according to any one of claims 1 to 5, when the position of the visible image is detected by the specular reflection type optical sensor, at least one of the minimum output signal value and the background portion output signal value is previously set. It is detected and the threshold is set to an appropriate value between them. As a result, the threshold value is set lower than the output signal value of the background detected by this optical sensor in consideration of the case where the output signal value is reduced due to scratches or background stains on the background, and the background image is erroneously visualized. Prevent it from being detected. In addition, the threshold value is set higher than the minimum output signal value in consideration of the decrease in sensor sensitivity in the high toner adhesion area, so that the output signal value for the visible image does not fall below the detection start threshold value so that it is not detected. To Further, by setting the threshold value to an appropriate level between the minimum output signal value and the background portion output signal value, the visible image for position detection can be reliably detected. Further, how to determine the threshold value from at least one of the minimum output signal value and the background output signal value is determined in consideration of various conditions.

Further, in the image forming apparatus according to the third aspect, the output signal value of the optical sensor in the background portion of the image carrier is kept constant by the light emitting portion output adjusting means.
Even if the image carrier has scratches or stains, only the output signal value of the image portion changes, so that the threshold value is increased as the number of image forming operations increases. The way to raise the threshold value may be to grasp beforehand the rising tendency of the output signal value of the image section with respect to the number of image forming operations. By increasing the threshold value as the number of image forming operations increases, even if the output signal value in the high toner adhesion area increases as the number of image forming operations increases, the output signal value for the visible image remains It will not be detected because the threshold does not fall below the threshold at which detection is started.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A tandem color laser printer (hereinafter referred to as "laser printer") will be described below as an embodiment of a tandem image forming apparatus to which the present invention is applied.

First, the basic configuration of the laser printer will be described. [Overall Configuration] FIG. 1 is a schematic configuration diagram of a laser printer according to the present embodiment. This laser printer has four sets of toner image forming units 1Y for forming images of yellow (Y), magenta (M), cyan (C), and black (K).
1M, 1C, 1K (hereinafter, subscripts Y, M, C, K
Indicate members for yellow, magenta, cyan, and black, respectively), which are sequentially arranged from the upstream side in the moving direction of the transfer paper (not shown). The toner image forming units 1Y, 1M, 1C and 1K include photoconductor drums 11Y, 11M, 11C and 11K as latent image carriers.

In addition to the toner image forming units 1Y, 1M, 1C and 1K, the present laser printer has an optical writing unit 2 as latent image forming means, paper feed cassettes 3 and 4, registration roller pair 5, transfer. A unit 6, a fixing unit 7 of a belt fixing system, a paper discharge tray 8, a manual feed tray (not shown),
It also has a toner supply container, a waste toner bottle, a double-sided / reversing unit, and a power supply unit.

[Optical Writing Unit] The above optical writing unit 2
Includes a light source, a polygon mirror, an f-θ lens, a reflection mirror, and the like, and each photosensitive drum 11 is based on image data.
The surface of Y, 11M, 11C, 11K is irradiated with scanning laser light.

[Photoreceptor Drum and the Like] FIG. 2 is an enlarged view showing a schematic structure of the yellow toner image forming section 1Y among the toner image forming sections 1Y, 1M, 1C and 1K. In addition,
The other toner image forming units 1M, 1C, and 1K have the same configuration, and therefore description thereof will be omitted. In FIG. 2, the toner image forming unit 1Y includes the photoconductor unit 10Y and the developing device 20Y as described above. The photoconductor unit 10Y includes the photoconductor drum 11
In addition to Y, a brush roller 12Y that applies a lubricant to the drum surface, an oscillating counter blade 13Y that performs cleaning, a charge eliminating lamp 14Y that performs charge eliminating processing, and a non-contact type charging roller 15Y that performs uniform charging treatment. And so on. As the photoconductor drum 11Y, one having an organic photoconductor (OPC) layer on its surface is used.

In the photoconductor unit 10Y, the surface of the photoconductor drum 11Y uniformly charged by the charging roller 15Y to which an AC voltage is applied is scanned by the laser beam modulated and deflected by the optical writing unit 2. When irradiated while being irradiated, an electrostatic latent image is formed on the drum surface.

[Developing Device] In the developing device 20Y, the developing roller 22Y, the first transport screw 23Y, and the second developing screw 22Y are provided so as to be partially exposed from the opening of the developing case 21Y.
The transport screw 24Y, the developing doctor 25Y, the toner concentration sensor (T sensor) 26Y, the powder pump 27Y and the like are provided.

The developing case 21Y contains a developer containing a magnetic carrier and a Y toner having a negative charging property. This developer is the first transport screw 23Y,
After being stirred and conveyed by the second conveying screw 24Y, the toner is electrostatically charged and then carried on the surface of the developing roller 22Y as a developer carrying member. Then, after the layer thickness is regulated by the developing doctor 25Y, the layer is conveyed to a developing area facing the photoconductor drum 11Y, where Y toner is attached to the electrostatic latent image on the photoconductor drum 11Y. Due to this adhesion, a Y toner image is formed on the photoconductor drum 11Y. The developer, which has consumed the Y toner by the development, is rotated by the developing roller 22Y and the developing case 21 is rotated.
Returned to Y.

A partition wall 28Y is provided between the first transport screw 23Y and the second transport screw 24Y, whereby a first supply for accommodating the developing roller 22Y, the first transport screw 23Y and the like is provided. Part 29Y,
A second supply unit 30Y for accommodating the second transport screw 24Y
And are separated in the developing case 21Y.

Y developed on the photosensitive drum 11Y
The toner image is transferred onto a transfer paper that is transferred by a transfer transfer belt 60 described below.

The first transport screw 23Y is rotationally driven by a driving means (not shown), and transports the developer in the first supply section 29Y from the front side to the back side in the figure along the surface of the developing roller 22Y. While supplying to the developing roller 22Y.

FIG. 3 is a vertical sectional view showing the developing device 20Y. As shown in the figure, the partition wall 28Y has two openings that allow the first supply unit 29Y and the second supply unit 30Y to communicate with each other near both ends of each transport screw.

The developer transported to the vicinity of the end of the first supply section 29Y by the first transport screw 23Y enters the second supply section 30Y through one of the openings provided in the partition wall 28Y. To do.

In the second supplying section 30Y, the second conveying screw 24Y is rotationally driven by a driving means (not shown) so that the developer entering from the first supplying section 29Y is fed in the opposite direction to the first conveying screw 23Y. Transport to. The second feeding unit 30 by the second transport screw 24Y
The developer transported to the vicinity of the end of Y is separated by the partition wall 28Y.
And returns to the inside of the first supply unit 29Y through the other opening provided in the.

The T sensor 26Y which is a magnetic permeability sensor.
Is provided on the bottom wall near the center of the second supply unit 30Y and outputs a voltage having a value according to the magnetic permeability of the developer passing thereabove. Since the magnetic permeability of the developer has a certain degree of correlation with the toner concentration of the developer, the T sensor 26Y outputs a voltage having a value according to the Y toner concentration. The value of this output voltage is sent to a control unit (not shown).

The control unit includes a RAM, in which Vtref for Y, which is a target value of the output voltage from the T sensor 26Y, and the T sensor 26 mounted in another developing device.
Data of Vtref for M, Vtref for C, and Vtref for K, which are target values of the output voltages from M, 26C, and 26K, are stored. Regarding the developing device 20Y, the value of the output voltage from the T sensor 26Y is compared with the Vtref for Y,
The powder pump 27Y connected to a Y toner cartridge (not shown) is driven for a time corresponding to the comparison result, and Y
The Y toner in the toner cartridge is replenished in the second supply unit 30Y. By controlling the drive of the powder pump 27Y (toner replenishment control) in this way, the second toner is consumed by the development to reduce the Y toner concentration.
An appropriate amount of Y toner is replenished in the supply unit 30Y, and the Y toner concentration of the developer supplied to the first supply unit 29Y is maintained within a predetermined range. Other developing devices 20M, 20C, 20
The same toner replenishment control is performed for K as well.

[Transfer Unit] The photoconductor drum 11 described above.
Y, 11M, 11C, and 11K contact a transfer conveyance belt (described later) of the transfer unit 6 disposed below the Y, 11M, 11C, and 11K to form a transfer nip as a transfer position.

FIG. 4 is an enlarged view showing a schematic structure of the transfer unit 6. The transfer conveyer belt 60 used in the transfer unit 6 has a volume resistivity of 10 ⁹ to 10 ¹¹ Ω.
It is a high resistance endless single layer belt of cm and PVDF (polyvinylidene fluoride) is used as its material. The transfer / conveying belt 60 as an endless moving body is provided on the photosensitive drum 11 of each of the toner image forming sections 1Y, 1M, 1C and 1K.
It is wound around four support rollers 61 that are grounded so as to pass through the transfer positions that contact and face Y, 11M, 11C, and 11K.

Of these supporting rollers 61, the one on the rightmost side in the drawing is arranged so as to face the electrostatic attraction roller 62 to which a predetermined voltage is applied from a power source 62a.
Between the support roller 61 and the electrostatic attraction roller 62,
The transfer paper 100 is sent by the registration roller pair 5 and electrostatically adsorbed on the transfer conveyance belt 60.

The leftmost support roller 61 in the drawing is a drive roller that is rotated by a drive means (not shown) to frictionally drive the transfer / conveyance belt 60.

A power source 63a is provided on the outer peripheral surface of the transfer / conveying belt 60 located between the two lower supporting rollers 61 in the figure.
Is arranged so that the bias roller 63 to which a predetermined cleaning bias is applied comes into contact with.

Below the transfer nips, transfer bias applying members 65Y, 6Y which contact the back surface of the transfer / conveying belt 60.
5M, 65C, 65K are provided. These transfer bias applying members 65Y, 65M, 65C and 65K are composed of fixed brushes made of Myra, and transfer bias is applied from the transfer bias power sources 9Y, 9M, 9C and 9K. The transfer bias applied by the transfer bias applying member applies a transfer charge to the transfer / transport belt 60, and the transfer / transport belt 60 is transferred at each transfer position.
A transfer electric field having a predetermined strength is formed between the surface of the photosensitive drum and the surface of the photosensitive drum.

FIG. 5 is a schematic view showing the transfer pressure adjusting means of the transfer unit 6. In the figure, each transfer bias applying member 65Y, 65M, 65C, 65K is rotatably supported by one support base 66, and this support base 66 is further supported by two solenoids 67, 68. By driving these two solenoids 67, 68, the transfer bias applying members 65Y, 65M, 65
C and 65K move up and down to adjust the contact pressure (nip pressure) between the photosensitive drum 11 and the transfer / conveying belt 60 at each transfer position. When the toner images of the respective colors are superimposed and transferred, the transfer / conveying belt 60 moves the photosensitive drums 11Y and 11Y so that the contact pressure becomes a predetermined value.
Pressed by M, 11C, 11K.

The one-dot chain line in FIG. 1 described above indicates the transport path of the transfer paper. The transfer sheet (not shown) fed from the sheet feeding cassettes 3 and 4 is conveyed by a conveying roller while being guided by a conveying guide (not shown), and is sent to a temporary stop position where a pair of registration rollers 5 is provided. The transfer paper sent out at a predetermined timing by the registration roller pair 5 is carried on the transfer / transport belt 60 and passes through each transfer nip that can contact the toner image forming portions 1Y, 1M, 1C, and 1K.

Toner image forming sections 1Y, 1M, 1C, 1K
The toner images developed on the photoconductor drums 11Y, 11M, 11C, and 11K are superposed on the transfer paper at the respective transfer nips, and transferred onto the transfer paper under the action of the transfer electric field and the nip pressure. By this superposition transfer, a full-color toner image is formed on the transfer paper.

[Electrification] In FIG. 2, the surface of the photosensitive drum 11Y after the toner image is transferred is the brush roller 1
After a predetermined amount of lubricant is applied in 2Y, it is cleaned by the counter blade 13Y. And the static elimination lamp 1
The light emitted from 4Y removes the charge, and prepares for the formation of the next electrostatic latent image.

[Fixing Unit] On the other hand, the transfer paper 100 on which the full-color toner image is formed is fixed in the fixing unit 7 (see FIG. 1) having a heating roller, and then the paper discharge tray 8 is provided. Discharged on top. The fixing unit 7 includes a temperature sensor (not shown) that detects the temperature of the heating roller.

[Control Unit] FIG. 6 is a block diagram showing a part of an electric circuit of the laser printer. In the figure, a control unit 150 includes toner image forming units 1Y, 1M, 1C, 1K electrically connected to each other, an optical writing unit 2, paper feed cassettes 3, 4, registration roller pairs 5, a transfer unit 6, and an optical sensor. The reflective photosensor 69 and the like are controlled. In addition, the control unit 150 executes C for performing arithmetic processing.
It has a PU 150a and a RAM 150b for storing data.

In the RAM 150b, data of the Y developing bias value, the M developing bias value, the C developing bias value, and the K developing bias value corresponding to the toner image forming portions 1Y, 1M, 1C, and 1K, and Y. Data of the drum charging potential for M, the drum charging potential for M, the drum charging potential for C, and the drum charging potential for K are stored.

In the printout process, the control unit 150 controls the charging rollers 15Y, 15M, 15
The control is performed such that the charging biases of the Y drum charging potential, the M drum charging potential, the C drum charging potential, and the K drum charging potential are supplied to C and 15K. By this control, the photosensitive drums 11Y, 11M, 11C, 11K
However, it is uniformly charged to the Y drum charging potential, the M drum charging potential, the C drum charging potential, and the K drum charging potential. Further, the control unit 150 controls the developing roller 22Y,
22M, 22C, and 22K are controlled to supply the Y development bias value, the M development bias value, the C development bias value, and the K development bias value.

Immediately after the main power source (not shown) is turned on, 60
When the temperature of the heating roller below [° C.] is detected or when the printout of a predetermined number or more is performed, the control unit 1
Reference numeral 50 tests the image forming performance of each toner image forming unit 1. Specifically, first, the photosensitive drums 11Y, 11M, 11C,
Charge 11K while rotating 11K. Regarding the electric potential in this charging, unlike the uniform drum charging electric potential in the printout process, the value is gradually increased to the negative polarity side. Then, the electrostatic latent image for the reference pattern image is formed on the photoconductor drums 11Y, 11M, 11C and 11K by the scanning of the laser beam, and is developed by the developing devices 20Y, 20M, 20C and 20K. By this development, the photosensitive drums 11Y, 1
Reference pattern image Py, reference pattern image Pm, reference pattern image Pc, reference pattern image P on 1M, 11C, and 11K.
k is formed. At the time of development, the control unit 150 controls the value of the development bias applied to the development rollers 22Y, 22M, 22C, and 22K to gradually increase to the negative polarity side. Immediately after the main power source is turned on, when a heating roller temperature exceeding 60 ° C. is detected, the image forming performance is not tested. Therefore, when the time from the turning off of the main power supply to the turning on of the main power supply is relatively short, from a few minutes to a few tens of minutes, the test is omitted, and the user is unnecessarily waited by the test, or power and toner are wastefully consumed. It is possible to eliminate the situation such as doing.

By the way, in the printer, it is required that the toner images of the respective colors formed by the toner image forming portions 1Y, 1M, 1C and 1K are formed at correct positions. If there is a misregistration between the toner images of the respective colors, a reproducibility of the color image such as a misregistration of the superimposed toner images formed on the transfer paper, a misregistration of the toner images as a whole, a color misregistration, or the like occurs. Therefore, conventionally, there has also been proposed an apparatus that forms a pattern toner image for position detection, detects the position using a sensor, and corrects the position according to the detection result. However, the pattern toner image may not be accurately detected due to an error in mounting the sensor on the detection surface or a scratch or stain on the transfer conveyance belt, which is the detection surface. Therefore, the present embodiment is configured so that the position of the pattern toner image can be accurately detected even if there is a sensor mounting error, scratches, stains, or the like on the transfer / conveyance belt 60. The characteristic part of the laser printer will be described below.

In the present embodiment, the same reflective photosensor 69 is used, and a reference pattern image P as a density detection visible image formed on the transfer / conveying belt 60 and a position detection visible image are formed. The reference pattern images pP1 and pP2 are configured to be detected, and the threshold value used in position detection is set based on the output signal value of the reflection type photosensor 69 in density detection.

FIG. 7 shows a reference pattern image P (Py, Pm,
It is a schematic diagram which shows Pc, Pk). In the figure, the reference pattern image P is composed of five reference images 101 arranged at intervals L4 from each other. In this laser printer, each reference image 101 has a length of 10 [mm] and a width (L3) 1.
A square having a size of 0 [mm] is formed with L4 with a gap of 20 [mm]. Reference pattern images Py, Pm,
Unlike the toner images of the respective colors formed during the printing process, Pc and Pk are transferred on the transfer / conveying belt 60 so as to be lined up without overlapping. By such transfer, the reference pattern image P of each color is formed on the transfer / transport belt 60.
One pattern block PB composed of y, Pm, Pc, and Pk is formed.

FIG. 8 is a schematic diagram showing the installation pitch of the photosensitive drum 11. As shown, the photosensitive drum 11
Y, 11M, 11C, and 11K are arranged at equal intervals with a pitch of L1. The installation pitch L1 of the photosensitive drum 11 is set to the above-mentioned reference pattern images Py, Pm, Pc,
It is set longer than the length of Pk. Therefore, the reference pattern images Py, Pm, Pc, and Pk can be independently transferred so that their end portions do not overlap each other.

FIG. 9 is a schematic diagram showing the pattern blocks formed on the transfer / transport belt 60. On the transfer / conveyance belt 60, four reference patterns Pk, Pc, P
Two pattern blocks PB composed of m and Py are formed. Specifically, the reference pattern images Pk1, Pc1, Pm
A pattern block PB1 composed of 1 and Py1,
A pattern block PB2 including the reference pattern images Pk2, Pc2, Pm2, and Py2 is formed.

The pattern blocks PB1 and PB2 are formed as follows. That is, the control unit 150 controls the reference pattern image Pk in the first pattern block PB1.
From the time point when 1, Pc1, Pm1, Py1 have been transferred to the transfer / conveying belt 60, the reference pattern P on the most upstream side is obtained.
Until y1 finishes passing through the transfer nip of the most downstream photosensitive drum 11K, the solenoids 67 and 68 (see FIG. 5) of the transfer unit 6 are driven to keep the transfer pressure at a predetermined level (separation Decompress to (including). By this pressure reduction, the reference pattern images Pc1, Pm1, Py1
While the reverse transfer to the photosensitive drum 11 at the transfer nip on the downstream side is suppressed, the transfer transport belt 60 is
Move with. Therefore, the pattern block PB1
The reference pattern images Pc1, Pm1, Py1 in the
Each of the density patterns has a state in which the reverse transfer to the photosensitive drum 11 is suppressed.

Further, the control unit 150 forms the respective reference pattern images Pk2, Pc2, Pm2, Py2 of the second pattern block PB2 on the photoconductor drums 11Y, 11M, 11C, 11K at a predetermined timing. . Specifically, the predetermined timing means the rear end (reference pattern image Py) of the first pattern block PB1.
From the time point when 1) has passed through the transfer nip of the most downstream photosensitive drum 11K and moved by a predetermined amount, the reference pattern images Pk2, Pc2, Pm of the pattern block PB2 have been moved.
This is the timing when the transfer of Py2 onto the transfer / conveying belt 60 can start.

Further, the control section 150 causes the second pattern after the rear end (reference pattern image Py1) of the first pattern block PB1 passes through the transfer nip of the most downstream photosensitive drum 11K. By the time the respective reference pattern images P of the block PB2 start to be transferred to the transfer / conveying belt 60, the solenoids 67 and 68 are driven to press the transfer pressure to the original value. This pressurization enables good transfer of each reference pattern image P for the pattern block PB2.

Further, the control section 150, like the first pattern block PB1, also controls the solenoid 67 and the second pattern block PB2 so as to suppress the reverse transfer to the photosensitive drum 11. The drive of 68 is controlled.

Each of the pattern blocks PB1 and PB2 includes four reference pattern images Py, Pm, Pc, and Pk, and each of the reference pattern images includes five reference images 101. Sensor 69
5 for each color (Y, M, C, K)
The image densities of the x2 = 10 reference images 101 are detected.

The toner image forming sections 1Y, 1M, 1C,
In 1K, the ten reference images 101 in the pattern blocks of each color have a developing bias of 100, 120, 140, 160, 180, 200, 2 from the front end of the pattern block PB1 to the rear end of the pattern block PB2.
40, 280, 320, 560V are formed while switching to a gradually higher value. Further, in this embodiment, the maximum exposure potential is all set to 50V. by this,
The ten reference images 101 are developed with a higher development potential (development bias-maximum exposure potential) as they are formed later, so that the image density becomes higher and the whole becomes one gradation pattern.

[Reflective Photo Sensor] In FIG. 9, a reflective photo sensor 69 as an image detecting means is disposed on the left side of the transfer unit including the transfer / transport belt 60 in the figure. Each reference pattern image P on the transfer / transport belt 60 moves along with the endless movement of the belt and is detected by the reflection type photosensor 69, and then the contact position with the bias roller 63 (see FIG. 2) of the transfer unit 6. And is electrostatically transferred to the bias roller 63 and removed.

The reflection type photosensor 69 measures the amount of light reflected from each reference image 101 forming the reference pattern images Pk1, Pc1, Pm1 and Py1 from the front end to the rear end of the first pattern block PB1 as follows. It detects in order. That is, the five reference images 10 of the reference pattern image Pk1
1. The five reference images 101 of the reference pattern image Pc1, the five reference images 101 of the reference pattern image Pm1, and the five reference images 101 of the reference pattern image Py1 are detected in this order.
At this time, an output voltage as an output signal value corresponding to the light reflection amount of each reference image 101 is sequentially output to the control unit 150.
The control unit 150 controls each reference image 101 based on the output voltage sequentially sent from the reflective photosensor 69.
The image densities of the above are sequentially calculated and stored in the RAM 150b. The reflection type photo sensor 69 includes a diffused light detection type and a regular reflection light detection type, but the regular reflection light detection type is used in the present embodiment. Therefore, in the reflection type photo sensor 69, the light emitting portion and the light receiving portion are arranged so that the sensor light emitting axis and the light receiving axis are symmetrical with respect to the normal line of the surface of the transfer / conveying belt 60.

In addition, the reflection type photo sensor 69 has 2
From the leading end to the trailing end of the second pattern block PB2, the amount of reflected light from each reference image 101 forming the reference pattern images Pk2, Pc2, Pm2, and Py2 is detected in the same order as the pattern block PB1. Control unit 150
Is similar to the case of the first pattern block PB1,
The image density of each reference image 101 is sequentially calculated based on the output voltage sequentially sent from the reflective photosensor 69, and stored in the RAM 150b.

FIG. 10 is a perspective view showing the transfer / conveyance belt 60 together with the reflection type photosensor 69. As shown in the figure, the laser printer includes two reflective photosensors 69a and 69b. The two pattern blocks PB1 and PB2 are respectively provided on the transfer / conveyance belt 60.
Is formed in the vicinity of the end on the near side in the figure, and is detected by the reflective photosensor 69a. The vicinity of this end portion is a portion corresponding to the region R2 of the developing device 20Y shown in FIG. 3 described above. In FIG. 3, the width W2 is a portion corresponding to the width of the transfer paper (not shown), and this region R2 is the first portion than the width W2.
It is on the upstream side of the developer transport direction of the supply unit 29Y. During the normal printout process, the developer existing in the area R2 on the developing roller 22Y does not contribute to the development, and the area R2 on the developing roller 22Y or the area R of the first supply unit 29Y is not used.
The developer existing in No. 2 has a toner concentration maintained within a predetermined range by the above toner replenishment control. Therefore, even when the Y toner image having a high image area ratio such as a solid pattern image or a photographic image is continuously developed during the printout process, the reference pattern image Py is developed by the developer having the regular toner density. In addition, another reference pattern image P
For the same reason, m, Pc, and Pk are also developed with a developer having a regular toner concentration.

Now, a method of performing toner density correction control based on the detection result of the reflection type photo sensor 69 will be described. In the present embodiment, the light emitting unit output adjusting means of the reflection type photo sensor 69 is configured so that the LED current value can be adjusted by PWM control. Specifically, it is turned on with 128 values (/ 255) starting from PWM toward the background portion of the transfer / conveying belt 60, and it is determined whether the output signal value at the light receiving portion of the sensor is higher or lower than 4.0V. And 128-128 / 2 if higher than 4.0V
Value. If it is low, it is set to 128 + 128/2. Furthermore, using the same two-division method, PWM closest to 4.0V
The value is calculated, and the LED current value is adjusted according to the result. Further, the developing roller 22 is turned off so that the transfer and transport belt 60 can be prevented from being contaminated by toner.

When the reference image 101 of (1) to (10) as described above reaches the detection position facing the reflection type photosensor 69, the LED current is turned on and the output signal obtained from the reflected light is output. Read the value Vsp [n]. The reading of the output signal value is performed at the timing calculated in advance from the layout. For example, the distance from the exposure position to the detection position is 400 mm, and the belt linear velocity is 100 mm /
If it is sec, reading is started 4.0 seconds after the start of exposure. Further, since the length of the reference image 101 can be made sufficiently long as the reading interval of the output voltage by the reflective photosensor 69, a sufficient number of data can be obtained even at an interval of 2 to 4 msec. 11A and 11B are graphs showing an example of the reading result of the output signal value at this time. In this figure, FIG. 11A shows output signal values Vsp [n] of five reference images 101 of a predetermined color in PB1.
FIG. 11B shows the results of reading (n = 1 to 5).
Is a read result of output signal values Vsp [n] (n = 6 to 10) of five reference images 101 of the same color in PB2. Next, the adhesion amount conversion for obtaining the adhesion amount of toner is performed from the result of reading the output signal value. The conversion of the adhesion amount is first performed based on the following equation 1 by using the output signal value Vsp of each reference image 101.
Normalize [n].

[Equation 1] (Vsp [n] -Vmin) / (Vsg-Vmin) = MA [n] ... [Equation 1]

Next, the normalized data MA [n] is converted into the toner adhesion amount from the reference table (FIG. 12). 12
Is a reference table for obtaining the toner adhesion amount from the normalized data of the output voltage value, which is obtained in advance by an experiment. The reference table is normalized data MA [n]
It is composed of a table of 100 steps in which the reference value of 1 is decomposed into 0.01 steps in the range of 0 to 1, and the toner adhesion amount corresponding to each reference image 101 can be obtained from this table.

FIG. 13 shows that for each color Y, M, C, K,
6 is a graph in which toner adhesion amount per unit area (hereinafter, referred to as toner adhesion amount) y [n] at each development potential x [n] in the reference image 101 of (1) to (10) is plotted. There is a positive correlation between the development potential x [n] and the toner adhesion amount (hereinafter referred to as toner adhesion amount) y [n], and since it can be approximated to a straight line graph, the following equations 2 to 4 are obtained by the least square method. Using the above calculation, the slope a and the intercept b of the linear function y = ax + b showing a linear graph were obtained as follows and linear approximation was performed.

## EQU00002 ## Development .gamma. (Gradient) =. SIGMA. (X [n] -X) (y [n] -Y) /. SIGMA. (X [n] -X) ² ... [Formula 2] intercept = Y-development γ × X ... [Formula 3] Development start point Vk = −intercept / development γ ... [Formula 4] x [n]: development potential data X: development potential average value y [n]: toner adhesion amount data Y: Average amount of adhesion

However, due to the characteristics of the reflection type photosensor 69, the upper limit is set in anticipation of a decrease in sensor sensitivity in the high adhesion region. Further, a lower limit value is similarly set in order to avoid the influence of the detection error due to the background stain. The upper and lower limits of these ranges from 0.05 to
0.40 [mg / cm ² ]. The respective colors Y, M, C, which are linearly approximated from the development γ and the intercept thus obtained,
The results for each K are shown in FIG.

Next, a method of calculating the developing potential capable of obtaining the target toner adhesion amount will be described. 14
4 is a graph showing only a graph of the toner adhesion amount with respect to the development potential of the black reference image 101, out of the toner density detection results of each color reference image 101. here,
For example, when the target value of the toner adhesion amount is 0.5, the development potential when the toner adhesion amount becomes 0.5 [mg / cm ² ] may be obtained from the graph. Even if it is not obtained on the graph, the toner adhesion amount y on the black reference image 101 is expressed by a linear function of the development potential x below,
By substituting y = 0.5 into and calculating with Equation 6,
It can be seen that the required development potential x is 515V.

[Equation 3] y = 0.9579x + 0.0071 ... [Formula 5] x = (0.5-0.0071) /0.9579=515V [Equation 6]

In this way, the target value of the image density is substituted to calculate an appropriate developing potential, and the developing bias value is calculated from this value to obtain the above-mentioned corrected developing bias value for Y, M, C or K. It is stored in the RAM 150b. Then, the image forming conditions of the toner image forming means during the printout process are corrected so that a toner image having a desired image density can be formed.

By using the reflection type photosensor 69 used for detecting the toner density as described above, a visible image for position detection can be obtained even if there is a sensor mounting error, scratches or stains on the transfer / conveyance belt 60. The characteristic part of the present embodiment, which is capable of accurately detecting the position of a certain reference pattern image, will be described. FIG. 15 is a flowchart for performing the position detection according to the present invention. In the figure, first,
The drive system and the high-voltage system of the apparatus are turned on (S1), the photosensitive member, the transfer belt, the developing roller 22 and the polygon motor are rotated, and charging, the application of the same bias as the printout to the developing roller 22 and the transfer brush is performed. . Further, the LEDs in the light emitting portion and the light receiving portion of the reflective photo sensor 69 are turned on.
To do. Then, the LED current value of the light emitting unit is adjusted by PWM control (S2). Then, the output signal value in the light receiving portion of the sensor is made to be close to 4.0 V, and the output voltage at this time is the background portion output voltage Vs which is the background portion output signal value.
It is stored in the RAM 150b as g (S3). Therefore,
The RAM 150b serves as a background portion output signal value storage means. The LED current value (IF) used at that time is also RA
Save to M150b. Next, exposure is started so that the reference pattern images Py, Pm, Pc, and Pk, which are visible images for density detection, have the gradation patterns as described above (S).
4). Exposed reference pattern images Py, Pm, Pc, P
The k is sequentially developed and transferred to the transfer / transport belt 60, and is transported to a position facing the reflective photosensor 69. Then, the LED is turned on at the previously calculated timing when the reference pattern image developed as described above comes to the sensor position.
N, the reading of the image is started (S5). As a result, 1 is set for each color as shown in FIGS.
The output voltage in the zero reference pattern images Py, Pm, Pc, and Pk can be detected. Each of the reference pattern images Py, P
The output voltages corresponding to m, Pc, and Pk are RAM150b.
To Vsp [n] respectively (S6). Further, the minimum output voltage as the minimum output signal value which is the output voltage at the time of detecting the maximum density developed with the developing bias of 560 V (see FIG.
Vmin in 1 (b) is RAM as Vmin
It is stored in 150b (S7). Therefore, RAM150b
Serves as a minimum output signal value storage means. Then, the toner adhesion amount conversion is performed using these output voltages (S8). Further, the development γ is calculated (S9). The methods of converting the toner adhesion amount in S8 and calculating the development γ in S9 are as described above. Then, using these results, the developing bias value and the like of the image area are set so that the toner density of the image area becomes a desired density (S10). Next, alignment control is performed. First, the background output voltage Vsg and the minimum output voltage Vmi stored in the RAM 150b during the density control.
n is called and referenced (S11). These Vsg and Vm
The threshold Vth is determined by substituting in and Equation 6 (S)
12). This is the threshold setting means.

[Equation 4] Vth = (Vsg + Vmin) × 0.40 ... [Equation 6]

Here, for example, Vsg = 4.05, Vmi
If n = 0.80, the threshold value Vth1 = 1.94.
In Expression 6, the multiplier 0.4 with respect to the sum of Vsg and Vmin is an example of a multiplier that can be used to optimize the threshold value, and the present invention is not limited to this. However, if the multiplier is made too large, steps S16 to S18 described below will be performed.
In this case, the number of samplings of the line passing time in the line is increased and a large amount of memory is required, which is not preferable. On the other hand, if the multiplier is made too small, the sampling as the line passage time is reduced, and the line passage time error is likely to occur. Next, exposure of the reference pattern images pP1 and pP2 for detecting the positional deviation is started on the transfer / conveying belt 60 (S1).
3). Then, the reference pattern images pP1 and pP2 for position detection as shown in FIG. 16 are formed.

[Reference pattern image for position shift detection pP1,
pP2] Here, the reference pattern images pP1 and pP2 for detecting the positional deviation in the present embodiment will be described. The reference pattern images pP1 and pP2 are, as shown in FIG. 17, four reference images d extending straight in the belt width direction.
101K, d101C, d101M, d101Y, and reference images s101K, s inclined by 45 [°] from the belt width direction.
101C, s101M, and s101Y. In the reference pattern images pP1 and pP2, the reference image d10
1K, d101C, d101M, d101Y, s101
K, s101C, s101M, and s101Y are formed at a pitch of the distance d, and the length of the entire reference pattern is L3. Of these reference images, reference images d101K and d1
01C, d101M, and d101Y are formed to have a length A and a width W. In addition, the reference images s101K and s101
C, s101M, and s101Y are formed to have a length A√2 and a width W. Further, reference images d101K, d101C, d101M, d101Y of the reference pattern image pP1,
s101K, s101C, s101M, s101Y,
Reference images d101K, d101C of the reference pattern pP2,
d101M, d101Y, s101K, s101C, s
101M and s101Y are formed to face each other in the belt width direction. Photoconductor drums 11Y, 11
The M, 11C, and 11K have an inclination due to an assembly error, and the Y, M, C, and
It is assumed that there is no situation in which the reflection mirror for K is tilted in the longitudinal direction, or the driving timing of the polygon mirrors for Y, M, C, K or the light source is deviated from the regular timing. . Then, as shown in FIG. 16, the reference images 101 are formed so as to maintain a parallel state at equal intervals.

Then, both reflective photosensors 69
Reading of the reference pattern image is started at substantially the same time as a and 69b (S14). FIG. 18 shows reference images d101K, d101C, d101 composed of horizontal lines as shown in FIG.
M, d101Y and s101K, s10 composed of diagonal lines
It is the figure which showed an example of the output waveform of the output voltage obtained by the reflection type photosensors 69a and 69b, when 1C, s101M, and s101Y are detected alternately. This shows the case of Vsg = 4.05 and Vmin = 0.80. In some cases, the transfer / conveying belt may be further scratched or soiled to reach Vmin = 2.0 (one-dot chain line Vp2 in the figure).
The threshold value Vth is set again at 2 and position detection is performed with Vth2 = 2.412. This reference pattern image pP1, pP
The position detection method of No. 2 will be described. FIG. 19 is a partially enlarged view of the output waveforms of the reflective photosensors 69a and 69b when the reference pattern images pP1 and pP2 for detecting the positional deviation are detected. Immediately before the tips of the reference pattern images pP1 and pP2 reach the sensor position, the LED is turned on at the same current value that is adjusted and set to detect the reference pattern image P for toner density detection. As a result, a voltage value of the background output voltage Vsg of the transfer / conveyance belt 60 is approximately equal to the output voltage 4.05V at the time of density detection. Next, in a predetermined cycle, the output voltage Vp from the sensor is
To read. The reading interval depends on the required detection accuracy, but since the limit value of the line deviation is generally about 100 μm, if the linear velocity is 100 mm / sec, 1
Considering that the deviation of 100 μm is caused by the error of msec, it is desirable that the time is about 10 to 100 μsec. The read output voltage Vp is compared with the threshold value Vth set in S12. If Vp <Vth is not satisfied (N in S15), the read output voltage Vp is read again to update Vp. If Vp <Vth is not satisfied within the predetermined time, it is notified that there is an abnormality. When Vp <Vth (Y in S15),
At that timing, the measurement of the line passing time, which is the reference image, is started (S16). This is the first V in FIG.
It is the timing H1 when p exceeds the position where it intersects Vth. After that, Vp is constantly compared with Vth, and Vp>
When it does not reach Vth (N in S17), Vp is updated again. When Vp> Vth (Y in S17), the measurement of the line passage time is completed (S18). This is the timing H2 when Vp in FIG. 19 becomes higher than Vth again. Then, the time from H1 to H2 required to pass one reference image d101K (hereinafter,
As the measurement time T, the passing time TB at the sensor position of the reference image d101K is calculated from the measurement time T / 2 (S1).
9). Hereinafter, similarly, reference images d1 of other colors C, M, and Y
01C, d101M, d101Y passing time TC, T
Find M and TY. Then, if the last reference image for which the passage time has been obtained is not the fourth color Y or the final color Y input in advance in the memory (N in S20), the process returns to S14 to detect the position of the next reference image. . If the final reference image is the final color Y (Y in S20), the obtained reference images d for each color d101K, d
101C, d101M, d101Y passing times TK, T
The mutual intervals of the color pattern images are calculated from C, TM, and TY (S21). Then, the exposure start timing is adjusted based on this (S22). Also, the diagonal reference image s
For 101K, s101C, s101M, and s101Y, position detection is performed in the same procedure. Although not described above, the transfer conveyor belt surface position that enters the sensor detection position before the reference pattern images pP1 and pP2 for each color is:
In addition to the reference pattern image, the reflection type photo sensor 69
A trigger line (not shown) serving as a signal to start detection by a and 69b is provided. Then, Vp <Vt for the first time after the detection by the reflective photosensors 69a and 69b is started.
The time when it becomes h is set to 0, and the measurement of time is started from here. As a result, each color reference image d101 from the start of measurement
K, d101C, d101M, d101Y Passing time T
By comparing the time required for K, TC, TM, and TY (referred to as transit time) with the ideal transit time originally determined, it is possible to detect how much each color image is displaced. FIG. 20 shows reference images d101K, d101C, d101M, d101 obtained from detection data.
Based on the passage time (μsec) of Y, each color reference image d101
9 is a table showing an example when a shift amount (mm) indicating how much the positions of K, d101C, d101M, and d101Y are shifted is detected.

In the above embodiment, as shown in FIG. 10, the density detection reference pattern images Py, Pm, Pc,
Although Pk is shown only at one end in the width direction of the transfer / conveyance belt, the same density detection reference pattern image P is once again provided on the side.
The reflective photosensor 6 is formed by forming y, Pm, Pc, and Pk.
The density detection by 9b is performed, the minimum output voltage Vmin and the background output voltage Vsg are stored, and the threshold value Vth peculiar to the reflective photosensor 69b can be set.

As described above, the reference pattern image pP1,
When the position detection of pP2 is performed, the threshold Vth is set to Vmin even when Vmin is gradually increased due to scratches or stains on the transfer / conveying belt 60 during overlapping prints as shown in FIG. Since it changes in conjunction with
The positions of the reference pattern images pP1 and pP2 can be reliably detected. Further, as shown in FIG. 23, the threshold value Vth is changed in association with Vmin and Vsg even if there is a change over time in the orbit due to a mounting error of the reflective photosensor 69, a slack of the transfer / conveyance belt 60, or the like. Therefore, it is possible to reliably detect the positions of the reference pattern images pP1 and pP2. In the present embodiment, when it becomes necessary to detect the position, Vmin and Vsg detection and position detection are continuously performed after the last printout operation is completed. This is the latest Vmi
It is effective for setting the threshold value Vth in order to obtain a value higher than n or Vsg. However, it is not limited to such drive control. For example, Vmin and Vsg may be obtained and stored from the information of the density detection performed last before the position detection. As a result, it is not necessary to obtain the values of Vmin and Vsg immediately before position detection, and the time required for position detection and drive control of the device can be simplified.

[Displacement Correcting Means] Next, an example of the positional deviation correcting means for correcting the positional deviation of the image based on the result of the positional deviation detection of the reference pattern image by the reflective photosensors 69a and 69b as described above will be described. To do. For example, in FIG.
, Two reference images d101C facing each other are shown in FIG.
A positional deviation due to skew occurs. When the displacement due to the skew occurs in this way, the reflective photosensor 69
A time lag Δt occurs between the timing at which a detects the reference image d101C and the timing at which the reflective photosensor 69b detects the reference image d101C. The skew angle θ can be obtained based on this time lag Δt and the moving speed of the transfer / conveying belt 60. Further, instead of the reference image d101C, other reference images d101K, d10
Even if a skew occurs in 1M and d101Y, the skew angle θ can be similarly obtained.

Therefore, the control unit 150 sets the reference image d1 for each of the two reference toner images pP1 and pP2.
The detection timings of 01K, d101C, d101M, and d101Y are sequentially stored in the RAM 150a, and the detection intervals t1a, t2a, t3a, t1b, t2b, t3b are obtained. Then, the skew angle θ of the reference image having the time lag Δt is calculated, and based on the calculation result, the corresponding reflecting mirror is tilted by the mirror tilting means to suppress the skew.

The correction of the positional deviation due to the skew has been described above as an example of the positional deviation correcting means. However, it is possible to perform the positional deviation correction other than the skew by using the positional deviation detection result according to the present embodiment. . Further, instead of the method of correcting the latent image forming position on the photoconductor drum 11 by correcting the conditions in the optical writing unit 2 such as the inclination angle of the reflection mirror, a latent image carrier such as the photoconductor drum is carried. The latent image forming position may be corrected by correcting the position of the body or the endless moving body such as the transfer / transport belt 60. And by correcting various misalignment,
Disturbance of the full-color toner image formed during the printing process can be suppressed.

[Modification 1] Next, Modification 1 of the above embodiment will be described. In the above embodiment, the threshold value V
Reference patterns Pk and P for density detection are used as reference pattern images used to obtain data of the background output voltage Vsg and the minimum output voltage Vmin required when setting th.
Although c, Pm, and Py are used, these data may be obtained from other images. In this modification 1,
When it becomes necessary to perform position detection, the background portion output voltage is provided at a predetermined distance from the reference image d101K at the tip of the position where the reference pattern images pP1 and pP2 for position deviation detection are formed, to the downstream side in the transfer conveyance belt conveyance direction. A reference pattern image for data acquisition of Vsg and the minimum output voltage Vmin is formed. Then, the background output voltage Vsg and the minimum output voltage Vmin obtained from the reference pattern image are substituted into the above equation 6 to obtain the threshold value Vth. The rest of the configuration is the same as that of the embodiment, so the description is omitted.

According to this modification 1, the background output voltage V
Although it is necessary to form a new reference pattern image for obtaining sg and the minimum output voltage Vmin, when it is necessary to detect the position shift, the background output voltage V
It suffices to form a reference pattern image having a shape that is the minimum required to obtain sg and the minimum output voltage Vmin. Therefore, it is effective when there is no need to detect the image density and only the position detection needs to be performed.

In the above embodiment and the first modification, the threshold value Vth is set, so that the background image output voltage Vsg and the minimum output voltage Vm from the toner image k previously formed on the transfer / transport belt are set.
Although both data of in are referred to, either one may be used depending on the case. For example, in the configuration in which the LED current value is not adjusted as in the present embodiment, the threshold value Vth may be set with reference to only the background output voltage Vsg that decreases as the number of printouts integrated increases. It is possible.

[Modification 2] Next, Modification 2 will be described. In the embodiment and the modification 1, the data of the background output voltage Vsg and the minimum output voltage Vmin obtained in advance for setting the threshold Vth is used, but in the present modification 2, the background output voltage Vsg and the minimum output are used. The threshold value Vth is corrected as needed without using the voltage Vmin. In the case of a configuration in which the output voltage of the reflection type photosensors 69a and 69b maintains the output voltage of the background portion at a substantially constant value, FIG.
Since it is known that the number of laser printers increases as the number of laser printers used increases as shown in 2, the threshold value Vth is set to a high value as predicted in advance with the increase in the number of printouts accumulated by the printer. Do it again.

First, the cumulative number of printouts in the printer is sequentially updated and stored in the RAM 150b. And
An appropriate threshold value Vth is set in advance according to the cumulative number of printouts at predetermined intervals and stored in the RAM 150a. Then, every time printout is performed, the data is compared with the stored data, and when the number of times of integration coincides with the data, the threshold value Vth is changed to a value one rank higher that is set corresponding to the number of times of integration. By repeating this, the threshold value Vth is sequentially increased.

According to the second modification, even if the output signal value in the high toner adhesion area rises as the number of times of printout integration increases, the threshold value Vth rises accordingly. Accurate position detection can be performed without being not detected.

According to the above-described embodiment and the first and second modifications, since it is possible to use the reflection type photosensors 69a and 69b in which the light receiving portion and the light emitting portion are housed in the same housing, both portions can be used. It is possible to solve problems such as deterioration of the maintainability and layout freedom caused by using the transmissive photosensors housed in separate housings. Although the reference pattern image for detecting the positional deviation is formed only near both ends of the belt in the above-described embodiment, instead of this, the reference pattern image may be formed by straightening from one end of the belt to the other end. .

In the above-described embodiment and modifications 1 and 2, the output adjusting means keeps the output voltage Vsg of the background portion of the reflection type photosensor 69 constant. Therefore, the background output voltage Vsg, which is reduced in advance due to scratches, dirt, and the like attached to the transfer / transport belt, is increased. Although this also increases the output voltage of the image forming unit, the output voltage of the image forming unit and the output voltage of the image forming unit increase as the output voltage of the image forming unit increases at the same rate.

As described above, in the embodiment and the first modification, the reference pattern images d101K, d101C, and d101K are output from the output pattern images of the position detecting reference pattern images pP1, pP2.
The threshold value Vth used for specifying the start and end of the measurement of the passage time of d101M and d101Y is set to the reference pattern image pP using the same reflection type photosensors 69a and 69b.
1 and a value lower than the background output voltage Vsg when detecting pP2 and higher than the minimum output voltage Vmin. As a result, it can be prevented that the output voltage for the reference pattern images pP1 and pP2 does not become lower than the threshold value Vth and thus is not detected. Further, in the present embodiment, the threshold value Vth is set to the background output voltage Vsg.
The value between the minimum output voltage Vmin and the minimum output voltage Vmin is set to a value that can be detected more reliably by using the above equation 6, and therefore the reference images d101K, d101C, d for position detection are set.
Even if 101M and d101Y are thin linear lines, they can be reliably detected. In addition, since the threshold value Vth can be set to an appropriate value in consideration of both data of the background output voltage Vsg and the minimum output voltage Vmin, it is possible to set the threshold value Vth from only one of the data. In comparison, it is possible to perform reliable position detection corresponding to various situations. Further, the LED current value is configured to be adjustable by PWM control as the light emitting unit output adjusting means. As a result, the reference image d101K due to the output voltage,
The positions of d101C, d101M, and d101Y can be detected more accurately. This is because the difference between the output voltages of the light emitting units is adjusted so that the portions of the reference pattern images pP1 and pP2 in which the reference images are formed and the background portion can be clearly distinguished so that the S / N ratio does not drop too much. Because you can. For example, when the number of printouts increases and the light reflectance of the transfer / conveying belt decreases, the output voltage to the background portion decreases and the background portion and the reference pattern image pP decrease.
1, the difference in output voltage between the pP2 forming portion is less likely to occur. In such a case, the LED current value can be increased and the output voltage of the background portion can be increased so that the difference in output voltage between the non-image portion and the image portion can be clearly obtained. Further, since the positional deviation correction of the toner image is performed based on the positional deviation detection result by the reflective photosensors 69a and 69b, the image can be returned to the correct position. Further, since the above various inventions are applied to the color image forming apparatus,
It is possible to reliably correct the deviation of the toner image position between the respective colors, and to form a good full-color toner image without causing image distortion such as color misregistration in a color image formed by superposing toner images of a plurality of colors. be able to.

[0091]

According to the image forming apparatus of the present invention, since the threshold value is set to an appropriate level between the background output signal value and the minimum output signal value, the image carrier to be detected. It is possible to provide an image forming apparatus capable of reliably detecting the position of a toner image by a specular reflection type optical sensor even if the body is scratched or soiled or an optical sensor has a mounting error. It has an excellent effect.

Further, according to the image forming apparatus of the third to fifth aspects, the number of times of image formation is increased, the scratches or stains are formed on the image carrier, and the output signal value in the high toner adhesion region is increased. Even if the output signal value for the visible image is less than the threshold value, the position of the toner image can be reliably detected by the specular reflection type optical sensor even if the image carrier is scratched or soiled. There is an excellent effect that it is possible to provide an image forming apparatus capable of performing the above.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a laser printer according to an embodiment.

FIG. 2 is an enlarged view showing a schematic configuration of a toner image forming unit 1Y of the laser printer.

FIG. 3 is a vertical cross-sectional view showing a developing device 20Y of the toner image forming unit 1Y.

FIG. 4 is an enlarged view showing a schematic configuration of a transfer unit of the laser printer.

FIG. 5 is a schematic view showing a transfer pressure adjusting means of the transfer unit.

FIG. 6 is a block diagram showing a part of an electric circuit of the laser printer.

FIG. 7 is a schematic diagram showing a reference pattern image for density detection formed by the laser printer.

FIG. 8 is a schematic diagram showing an installation pitch of each photosensitive drum in the laser printer.

FIG. 9 is a schematic view showing a pattern block formed on a transfer / conveying belt of the laser printer.

FIG. 10 is a perspective view showing the transfer conveyance belt together with a reflective photosensor.

11A and 11B are graphs showing an example of a result of reading an output signal value by a reflective photosensor.

FIG. 12 is a reference table for converting the normalized data MA [n] into a toner adhesion amount.

FIG. 13 is a graph in which the toner adhesion amount y [n] at each development potential x [n] in the reference image is plotted for each color.

FIG. 14 is a graph of toner adhesion amount with respect to the development potential of a black reference image.

FIG. 15 is a flowchart showing a main flow for performing position detection according to the present invention.

FIG. 16 is a reference pattern image pP1, p for position shift detection.
Explanatory drawing of P2.

FIG. 17 is an enlarged explanatory diagram of a reference pattern image for detecting a positional deviation.

FIG. 18 is a diagram showing an example of an output waveform obtained by a reflective photosensor.

FIG. 19 is a partially enlarged view of an output waveform of the reflective photosensor when a reference pattern image for detecting a positional deviation is detected.

FIG. 20 is a table showing an example in which the deviation amount (mm) of each color image is detected from the passage time (μsec) of the reference pattern image.

FIG. 21 is an explanatory diagram showing a state in which the reference pattern image d101C is displaced due to skew.

FIG. 22 is a graph showing the relationship between the number of printouts and the minimum value Vmin of the output voltage from the optical sensor.

FIG. 23 is a graph showing the relationship between the distance between the sensor and the transfer belt and the minimum value Vmin of the output voltage.

[Explanation of symbols]

1 Toner image forming unit 2 Optical writing unit (latent image forming means) 3, 4 paper feed cassette 5 Registration roller pair 6 Transfer unit 7 fixing unit 8 Output tray 11 Photoconductor drum (latent image carrier) 20 Developing device 22 Development roller (developer carrier) 27 powder pump (toner replenishing means) 29 First supply unit 30 Second supply unit 60 Transfer conveyor belt (endless belt) 69 Reflective photo sensor 101 Reference image (reference toner image) 150 control unit

Claims (5)

[Claims] 1. An image bearing member carrying a visible image, a visible image forming means for forming a visible image on the image bearing member, a light emitting unit for irradiating the image bearing member with light, and the image. The light-emitting portion and the light-receiving portion are arranged so that the light-emitting axis and the light-receiving axis have a light-receiving portion that receives reflected light from the surface of the carrier, and the light-receiving axis and the light-receiving axis are symmetrical with respect to the normal line of the surface of the image carrier. An optical sensor that outputs an electric signal having a value according to the optical characteristics of the image carrier, and an output signal value from the optical sensor for a visible image formed on the image carrier becomes lower than a predetermined threshold value. In the image forming apparatus having a position detecting means for detecting the position of the visible image by utilizing the above, a visible image for density detection for forming a visible image for density detection on the image carrier. And a background part output signal value which is an output signal value for the background part of the image carrier by the optical sensor. And a minimum output signal value storage means for storing the minimum output signal value which is the minimum value among the output signal values for the density detection visible image by the optical sensor. An image forming apparatus comprising: a threshold value setting unit that sets a threshold value using at least one of the minimum output signal value and the background output signal value. 2. The image forming apparatus according to claim 1, wherein the light emitting section adjusts an output value of light in the light emitting section according to a light reflectance of the visible image for density detection formed on the image carrier. An image forming apparatus comprising an output adjusting unit. 3. An image bearing member carrying a visible image, a visible image forming means for forming a visible image on the image bearing member, a light emitting section for irradiating the image bearing member with light, and the image. The light-emitting portion and the light-receiving portion are arranged so that the light-emitting axis and the light-receiving axis have a light-receiving portion that receives reflected light from the surface of the carrier, and the light-receiving axis and the light-receiving axis are symmetrical with respect to the normal line of the surface of the image carrier. An optical sensor that outputs an electric signal having a value according to the optical characteristics of the image carrier, and an output signal value from the optical sensor for a visible image formed on the image carrier becomes lower than a predetermined threshold value. The position detection means for detecting the position of the visible image and the output value of the light from the light emitting unit are adjusted so that the output signal value of the optical sensor at the background of the image carrier becomes constant. In the image forming apparatus having a light emitting unit output adjusting means for An image forming apparatus characterized in that a threshold value setting means to reset high so with the increase. 4. The image forming apparatus according to claim 1, 2 or 3, further comprising a positional deviation correcting means for correcting a visible image forming position on the image carrier based on a detection result of the position detecting means. An image forming apparatus characterized by the above. 5. The image forming apparatus according to claim 4, wherein an endless moving body is provided as the image carrier, and a latent image carrier that carries a latent image and each latent image carrier are provided as the visible image forming means. Image formation characterized by comprising a plurality of developing means for developing each of the above latent images and a transfer means for sequentially transferring the visible image developed on each latent image carrier onto the endless moving body. apparatus.