JPH1063046A - Method for detecting image density, and device therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve a precise concentration detection in a wide concentration range necessary for improvement of the temporal stability of image quality by detecting the concentration of toner on an image carrier and controlling conditions under which an image is formed, in an image forming device. SOLUTION: On the peripheral edge of a rotary photoreceptive drum 10, a photosensor 31, consisting of a light emitting element and a light receiving element, and a potential sensor 32 are disposed. In the measurement of the toner concentration of a reference patch formed on the drum, one in a low concentration range is measured by the photosensor 31, and one in a high concentration range is measured by switching to the potential sensor 32. To switch to the potential sensor 32, the detection characteristic of the potential sensor 32 is corrected based on the detection value of the photosensor 31.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an image density detecting method and an image density detecting apparatus for detecting the density of a toner image formed on an image carrier, used in an image forming apparatus such as a copying machine or a laser beam printer. About.

[0002]

2. Description of the Related Art Conventionally, in order to detect the density of a toner image on an image carrier provided inside an image forming apparatus, an optical sensor comprising a light emitting element and a light receiving element has been used. Image density detection methods for optically detecting the reflection density of an image are often used. On a uniformly charged image carrier, for example, a test patch latent image is formed by exposing with a laser beam of a constant luminance, and this is developed into a test patch image, and the reflection density is detected by an optical sensor. The data is used to control the toner concentration of the developer and other image forming conditions (for example, the maximum image exposure amount and γ correction). By performing such image density detection and control based on the detection before the start of copying or at the time when a predetermined number of copies have been made, a high-quality copy can be stably obtained over time.

There is also used a potential sensor provided with a surface voltmeter facing the surface of the image bearing member and detecting the density by measuring the potential formed by the charge of the toner adhered to the image bearing member.

[0004]

The detection of image density and the control of toner density and image forming conditions based on the detection result are indispensable for obtaining a high-quality copy over time in an image forming apparatus. There is a need for an image density detection method and apparatus that can accurately detect attached toner. Both the optical sensor and the potential sensor are capable of detecting toner concentration, but both have drawbacks.

The density detection by the optical sensor is performed by utilizing the difference between the reflectance of the surface of the image carrier and the reflectance of the toner portion of the adhered toner. V _PG and the value V _PS detected by the light receiving element on the surface of the image carrier on which the toner to be measured is attached.
From this, the optical sensor output ratio _VP is calculated.

[0006] and _{V P = (V PS -V PG} ) / V PG Figure. 6 (a) toner adhesion amount M / A, shows the relationship between the optical sensor output ratio V _P, the sensitivity setting (generally There is a density region of the toner image suitable for detection due to the amplifier gain), and the detection sensitivity of a so-called high density portion where the exposed portion of the surface of the image carrier that is not blocked by the toner is in a small state is not good.

[0007] The density detection by the potential sensor is performed by detecting the amount of the adhered toner in the form of the image carrier by detecting the potential formed by the charge held by the adhered toner. Adhered toner obtained by developing the latent image on the image carrier has a charge due to friction with the carrier in the developing device, and the charge held by the toner varies. Even if the potential is immediately measured for the adhered toner after development, highly accurate detection cannot be performed. To enhance the detection accuracy, the toner adhering to the image carrier before the detection is recharged by corona discharge, followed by uniform exposure (light elimination) to remove the charge held by the image carrier. detects the toner adhesion amount by the measurement of the potential V _T by potential sensor then it was. FIG. 6B shows the adhesion amount M / A and the potential V _T.
The relationship is shown. The image carrier has a residual potential under the influence of recharging and light elimination, and the potential does not become 0 V. Therefore, the detection density in the low density portion is unstable.

The present invention has been made after various studies have been made on the detection accuracy of the optical sensor and the potential sensor. The present invention has been applied to the case where the toner adhering to the image carrier is covered from the low density region to the high density region. It is an object of the present invention to provide an image density detection method and apparatus in which high-precision density detection is performed over a long period.

[0009]

An image density detecting method according to the present invention is directed to an optical sensor for irradiating light onto an image carrier, receiving reflected light thereof and measuring the reflection density, and an image sensor. Using a potential sensor for measuring the surface potential of the carrier, the toner sensor detects the toner density by using an optical sensor for a low density region of the toner attached to the image carrier and by using a potential sensor for a high density region. An image density detecting method is provided. In a preferred embodiment, the switching of the density measurement from the optical sensor to the potential sensor is performed based on a density detected value by a predetermined optical sensor, and the detection characteristic of the potential sensor is corrected based on the detected value of the optical sensor. is there.

Further, an image density detecting apparatus according to the present invention comprises an optical sensor comprising a light-emitting element and a light-receiving element and a potential sensor disposed on the periphery of a rotating image carrier. The present invention provides an image density detecting device characterized by having a control unit for measuring the toner density of the toner formed in the low density range by using an optical sensor and measuring the high density range by switching to a potential sensor. It is. In a preferred embodiment of the present invention, the density of the developed reference patch formed on the image carrier is sequentially detected from the low-density reference patch, and the potential at a predetermined density position measured by the optical sensor is measured. When switching to the sensor, the image density detecting device corrects the detection characteristics of the potential sensor held in advance as a memory based on the detection value of the optical sensor, and performs measurement by the potential sensor based on the correction.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior to the description of the present invention, an image forming apparatus to which the present invention is applied will be described. FIG. 1 shows a color image forming apparatus in which a toner image is superposed and formed on an image carrier, and transfer and fixing are performed.
The image forming apparatus to which the present invention is applied is not limited to this, and is very effectively applied to, for example, a digital monochrome printer.

In FIG. 1, reference numeral 10 denotes a photosensitive drum serving as an image carrier, which is formed by applying an OPC photosensitive member on the drum, grounded, and driven and rotated clockwise. 12 is a scorotron charger, provided by a corona discharge by a grid and the corona discharge wire which uniform charging has been potential held in the V _G of the V _H the photosensitive drum 10 peripheral surface. Prior to the charging by the scorotron charger 12, in order to eliminate the history of the photoconductor up to the previous print, exposure is performed by the PCL 11 using a light emitting diode or the like to eliminate the charge on the peripheral surface of the photoconductor.

Image exposure means 13 after uniformly charging the photosensitive member
Performs image exposure based on the image signal. The image exposure means 13 performs scanning (main scanning) by bending the optical path by a reflecting mirror 132 via a polygon mirror 131, an fθ lens, and the like, which rotate by using a laser diode (not shown) as a light emitting light source. A latent image is formed by sub-scanning. In this embodiment, the character portion is exposed to form a reversal latent image such that the character portion has a lower potential _VL .

Around the periphery of the photoreceptor drum 10, there are provided developing units 14 each containing a developer containing a toner such as yellow (Y), magenta (M), cyan (C), black (K) and a carrier. First, development of the first color is performed by the developing sleeve 141 which rotates while holding the developer with a built-in magnet. The developer consists of a carrier with ferrite as the core and an insulating resin coated around it, and a toner with polyester as the main material and a pigment according to the color, a charge control agent, silica, titanium oxide, etc. The agent is regulated to a layer thickness (developer) of 100 to 600 μm on the developing sleeve 141 by the layer forming means, and is conveyed to the developing area.

[0015] As 0.2~1.0mm gap is greater than the layer thickness (developer) to the developing sleeve 141 in the developing zone between the photosensitive drum 10, the DC bias of the AC Baaisu and V _DC of V _AC during which It is superimposed and applied. V _DC and V _H, for charging the toner are the same polarity, the toner given the opportunity to leave from the carrier by V _AC and V
_It does not adhere to the _VH portion having a higher potential than _DC, but adheres to the _VL portion having a lower potential than _VDC , and visualization (reversal development) is performed.

After the visualization of the first color is completed, the image forming process for the second color is started, and the uniform charging is again performed by the scorotron charger 12, and a latent image based on the image data of the second color is formed by the image exposure means 13. Is done. At this time, the charge removal by the PCL 11 performed in the image forming process of the first color is not performed because the toner attached to the image portion of the first color scatters due to a sharp drop in the surrounding potential.

The latent image similar to that of the first color is formed on the portion of the photoconductor having the potential of _VH which is again at the potential of _VH over the entire peripheral surface of the photoconductor drum 10 and the image is not developed. is carried out, 1 again part for developing image to the portion where there is the color, the latent image of V _M 'by the charge possessed by the shading of the toner itself by the toner adhering the first color is formed, V
Developing in response to the potential difference between the _DC and V _M 'is performed. In the overlapping portion of the first color image and the second color image, development of the first color is performed by V
_When the latent image of _L is formed, the balance between the first color and the second color is lost, so that the exposure amount of the first color is reduced and V _H > V _M >
In some cases, the intermediate potential may be V _L.

An image forming process similar to that for the second color is performed for the third color and the fourth color, and the fourth color is formed on the peripheral surface of the photosensitive drum 10.
A visible color image is formed.

On the other hand, a half moon roller 16
The recording paper P carried out via the printer is temporarily stopped, and is supplied to the transfer area by the rotation of the paper supply roller 17 when the transfer timing is adjusted.

In the transfer area, the transfer roller 18 is pressed against the peripheral surface of the photosensitive drum 10 in synchronization with the transfer timing, and the fed recording paper P is sandwiched to transfer the multicolor image at once. You.

Next, the recording paper P is discharged by a pointed electrode 19 provided with a slight gap, and the photosensitive drum 10
The toner is conveyed to the fixing device 20 after being separated by the peripheral surface of the heat roller 201, and the toner is welded by heating and pressing of the heat roller 201 and the pressure roller 202, and then discharged to the outside of the device via the paper discharge roller 21. The transfer roller 18 is retracted from the peripheral surface of the photosensitive drum 10 after the recording paper P has passed, and is ready for the formation of the next toner image.

On the other hand, the photosensitive drum 10 from which the recording paper P is separated
Removes and cleans the residual toner by pressing the blade 221 of the cleaning device 22 and receives the charge removal by the PCL 11 and the charge by the charger 12 again to start the next image forming process. The blade 221 moves immediately after the cleaning of the surface of the photoconductor and is retracted from the peripheral surface of the photoconductor drum 10.

In the image density detecting device of the present invention, an optical sensor 31 and a potential sensor 32 are provided on the peripheral portion of the photosensitive drum 10 downstream of the developing device 14 in the rotation direction. FIG.
In the embodiment shown in FIG. 5, the optical sensor 3 is located between the sharp electrode 19 and the cleaning device 22 at the peripheral portion of the photosensitive drum 10.
1 and a potential sensor 32 between the scorotron charger 12 and the developing device 14. In this embodiment, an erasing lamp 33 is provided between the scorotron charger 12 and the potential sensor 32 to irradiate the photoreceptor surface to erase the potential.

The image density detecting apparatus of this embodiment is automatically switched to the image density detecting mode immediately after the power is turned on or after a predetermined number of images have been formed in the image forming apparatus. Conditions are controlled. FIG.
Shows a circuit between members related to image density detection.

In the image forming apparatus of this embodiment, the mode is switched from the image forming mode to the image density detecting mode immediately after the power switch 41 is turned on, and the memory is stored in the ROM (A) 35 under the control of the control unit 30. The toner density detection is performed by the following process.

When the power switch 41 is turned on, the photosensitive drum 10 starts rotating at a constant speed.

Next, the optical sensor 31 composed of a light emitting element and a light receiving element is turned on to detect the output of the light receiving element (LED) by receiving the reflected light on the surface of the photoreceptor drum 10. Times, drum 1
Find the average value of the circumference. When the obtained average value deviates from the reference value (7 ± 0.2 V in this embodiment), LE
The voltage applied to D is adjusted to adjust the output (average value) of the LED to the reference value (optical sensor adjustment step). The voltage applied to the LED is stored in the RAM 37,
Until switching to the next image density detection mode, toner density detection by the optical sensor 31 is performed under this applied voltage.

With respect to the rotating photosensitive drum 10,
Uniform charging is performed by the scorotron charger 12, and the potential sensor 32 detects the surface potential of the drum. The value detected by the potential sensor 32 is a reference value (−8 in this embodiment).
If it deviates from 50 ± 5V), the charging condition (grid voltage) of the scorotron charger 12 is adjusted so that the output from the potential sensor 32 becomes a reference value (charging adjustment step). The newly adjusted charging condition of the scorotron charger 12 is stored in the RAM 37, and the charging condition stored in the RAM 37 is called out during the charging by the scorotron charger 12 until the mode is switched to the next image density detection mode. , Charging is performed.

The rotating photoreceptor drum 10 is uniformly charged by a scorotron charger 12, then the reference patch (1) is exposed by an exposure amount set in advance by an image exposure unit 13, and is inverted by a developing unit 14. After the development, the density of the reference patch (1) is detected by the optical sensor 31. If the detection value of the optical sensor 31 deviates from the set value set in advance for the reference patch (1), the developing conditions (developing bias voltage, developing sleeve 1
41, etc.) (maximum density adjustment step).
In the color image forming apparatus of the present embodiment, yellow (Y),
This maximum density adjustment step is performed for four colors of magenta (M), cyan (C), and black (K). The adjusted development conditions for these four colors are stored in the RAM 37, and the adjusted development conditions corresponding to the respective colors are set to R until the next image density detection mode is switched.
Called from the AM 37, development is performed under these development conditions.

[0030] The optical sensor 31 is ON, the detection output V _PG drum sober the photosensitive drum 10. Then, the optical sensor 31 is turned off, and the photosensitive drum 10 is charged by the scorotron charger 12 and the latent image of the reference patch (2) is formed by the image exposure means 13. The reference patch (2) is, for example, an 8-bit digital signal of 0 to 255.
In the case of 256 levels, a PWM signal skipped by 8 levels is sent to the semiconductor laser of the image exposure means 13, and the latent images of the 32 test patches as shown in FIG. Are formed in a line from a high-exposure patch. This latent image is subjected to reversal development by the developing device 14 under the development conditions set as described above, and becomes a plurality of tone correction test patch images p _{0 to} p _{32 having} different densities and passes through the retracted transfer roller 18 position. The output after development is detected by the optical sensor 31 which has been turned ON. In the detection after the development, the detection start timing is regulated so that the detection of the first drum bare surface and the detection after the development start at the same position on the photosensitive drum 10. The detection value (V _PS ) after development by the optical sensor 31 and the detection value (V
_PG ), the control unit 30 determines the optical sensor output ratio ( _VP ).
Is calculated (output step of density detection by an optical sensor).

V _P = (V _PS -V _PG ) / V _PG Next, the optical sensor 31 is turned off, and the developed reference patch (2) of the rotating photosensitive drum 10 is the cleaning device 22 with the blade 221 retracted. , And is recharged by the scorotron charger 12. Thereafter, the erasing lamp 33 is turned on to perform light elimination on the photosensitive member. Next, the potential sensor 32 is turned on to detect the potential of the developed reference patch (2). Also at this time, the detection start timing is regulated so that the detection of the drum surface by the first optical sensor 31 and the detection at the same position on the drum are started. The control unit 30 calculates the density output (V _T ) of the potential sensor 32 by subtracting the output of the drum substrate at the same position from the output of the test patch unit by the potential sensor 32 (output step of density detection by the potential sensor). FIG. 4 shows the toner adhesion amount (M / A) and the toner layer potential (density output) V
FIG. 4A is a graph showing the relationship with _T when developing with magenta (M) toner, and FIG. 4B is a graph showing the characteristics when developing with black (K) toner. ing. As is apparent from FIG.
It is necessary to form reference patches (2) for the colors and determine the density output for each. In the graph,
The characteristics when the recharge potential is set to 850 V (shown by a solid line) and 950 V (shown by a dotted line) are shown.

Optical sensor 31 and potential sensor 32
Are compared, and the detection value of the potential sensor 32 is corrected based on the detection value of the optical sensor 31. That is, a correction coefficient is calculated so that the output ratio of the optical sensor 31 and the detection value of the potential sensor 32 have a predetermined relationship between the output ratios of the optical sensor 31 of 0.3 to 0.7. To correct the detection value of the potential sensor 32 (calculation of a correction coefficient).

[0033] To calculate the concentration output V _T of the output ratio V _P and the potential sensor 32 of the optical sensor 31 in 20-point average for each of the control unit 30 in the 32 test patches p ₀ ~p _32. Next, the sensor output data is converted into a toner adhesion amount using a preset coefficient stored in the ROM (B) 36. At this time, the output ratio of the optical sensor 31 is less than 0.7, for example. Optical sensor 31 for patches
Using the output ratio toner adhesion amount obtained from the V _P, using the toner adhesion amount obtained from the density output V _T of the potential sensor 32 with respect to the patch output ratio is 0.7 or more optical sensors 31, 32 Floor The tonality data of the tone is obtained, and the obtained tonality data is stored in the RAM 37 (step of creating the tonality data).

FIG. 5 shows that the gradation data obtained by the present invention is detected with high accuracy over the entire density range, and is an example in which development is performed using magenta (M) toner. . In the figure, white circles indicate optical densities on printed paper, and black squares indicate calculation results from the above detected values.

In the image forming apparatus of this embodiment, γ correction is performed for each of the four colors based on the gradation data obtained by the above steps.

[0036]

According to the present invention, an optical sensor is used to detect a high-sensitivity toner density in a low-density portion, and a high-density portion having a problem in sensitivity is detected by a potential sensor and detected by an optical sensor. By calibrating the detection data of the potential sensor based on the data, it is possible to provide an image density detection method and an image density detection device in which density detection is accurately performed in all density ranges from low density to high density. Was.

[Brief description of the drawings]

FIG. 1 is a cross-sectional configuration diagram of an image forming apparatus to which the present invention is applied.

FIG. 2 is a circuit diagram between members related to image density detection of the present invention.

FIG. 3 shows an example of the shape of a reference patch (2).

FIG. 4 is a graph showing a relationship between a toner adhesion amount and a toner layer potential.

FIG. 5 is a graph showing a relationship between PWM and optical density.

6A is an explanatory diagram illustrating a relationship between a toner adhesion amount and an output ratio of an optical sensor, and FIG. 6B is a diagram illustrating a relationship between a toner adhesion amount and an output potential of a potential sensor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Photoconductor drum 12 Scorotron charger 13 Image exposure means 14 Developing device 30 Control part 31 Optical sensor 32 Potential sensor 33 Erase lamp 35 ROM (A) 36 ROM (B) 37 RAM

Claims (4)

[Claims] 1. An image carrier comprising: an optical sensor that irradiates light onto an image carrier, receives reflected light thereof, and measures a reflection density; and a potential sensor that measures a surface potential of the image carrier. An image density detection method, wherein the toner density is detected by an optical sensor for a low density area of toner adhered thereon and by a potential sensor for a high density area. 2. The method according to claim 1, wherein the switching of the density measurement from the optical sensor to the potential sensor is performed based on a density detected value by a predetermined optical sensor, and the detection characteristic of the potential sensor is corrected based on the detected value of the optical sensor. The image density detecting method according to claim 1, wherein 3. An optical sensor comprising a light-emitting element and a light-receiving element and a potential sensor are provided at the periphery of the rotating image carrier, and a low density is used for measuring the toner density of the toner formed on the image carrier. An image density detection device, comprising: a control unit that measures an area by an optical sensor and measures a high density area by switching to a potential sensor. 4. A density patch is sequentially detected from a reference patch on a low density side with respect to a developed reference patch formed on an image carrier, and switching to a potential sensor is performed at a predetermined density position measured by an optical sensor. 4. The image density detection method according to claim 3, wherein the detecting is performed by correcting the detection characteristics of the potential sensor stored in a memory in advance based on the detection value of the optical sensor, and performing measurement by the potential sensor based on the correction. apparatus.