JP3386274B2 - Image forming device - Google Patents

Description

Description: 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, and a printer. 2. Description of the Related Art Conventionally, as an apparatus for forming an image by exposing a photoreceptor to an image with a light beam, a pixel density is switchable so that an image can be recorded at a plurality of pixel densities. Several devices have been proposed (Japanese Unexamined Patent Application Publication No.
"Image forming apparatus" described in JP-A-197263, "Optical printing apparatus" described in JP-A-2-79060,
"Laser beam recording apparatus" described in US Pat. Switching of the pixel density in these image forming apparatuses is basically performed as described in Japanese Patent Application Laid-Open No. 5-197263, in which the frequency of the pixel clock, the rotation speed of the polygon mirror, that is, the exposure scanning speed of the image, and the like. This is achieved by changing the combination with the rotation speed of the photoconductor. Conventionally, a two-component magnetic brush developing type developing device using a developer in which a toner and a carrier are mixed has been widely used as a developing device of an electrophotographic image forming apparatus. It is also well known to control the mixture ratio of toner and carrier, that is, the toner concentration of the developer, for stabilization. In order to control the toner density, a toner density sensor (hereinafter, referred to as a T sensor) for magnetically detecting the toner density in the developing device has been proposed (Japanese Patent Publication No. 3-71068 and Japanese Utility Model Publication No. Hei 3-71068). No. 55895, “Magnetic detector” and the like). [0004] Incidentally, the T sensor in the developing device of the conventional image forming apparatus has a different detection value due to the flow of a magnetic material (developer) on the sensor surface. In the state where the developing device is being driven stably, an initial setting for setting the driving voltage of the T sensor so that the developer having a known toner concentration is detected at a certain specific value is performed. However, in an image forming apparatus provided with a drive system that drives both the photoconductor and the developing unit by the main motor, when the rotation speed of the photoconductor is changed in order to switch the pixel density of an image formed on the photoconductor. Since the driving speed of the developing device also changes with the change in the rotation speed of the photoconductor, the flow (velocity, flow rate, etc.) of the developer in the developing device changes. If the detection is not performed correctly, the toner density in the developing device becomes inappropriate, and a problem occurs in which a stable image cannot be output. SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has as its object to provide an image forming apparatus capable of accurately detecting the toner density in a developing device and outputting a stable image. Is to do. In order to achieve the above object, according to the present invention, there is provided a two-component image sensor comprising a toner concentration sensor for magnetically detecting a mixing ratio of toner and carrier. In an electrophotographic image forming apparatus having a magnetic brush developing type developing unit and at least a variable speed main motor for driving a photosensitive member, the developing unit has two developing units.
And a dedicated motor for driving all the driven members in the developing unit at a constant speed regardless of the rotation speed of the main motor. An embodiment of the present invention will be described below in detail with reference to the drawings. FIG. 2 shows a configuration example around a photoconductor of an image forming apparatus in which the present invention is implemented. In FIG. 2 , an image forming means for forming an image on the photoconductor 1 by a well-known electrophotographic method is arranged around the photoconductor 1. The image forming means includes a first charger 2, a first exposure device 3, a first developing device 4, a second charger 5, a second exposure device 6, a second developing device 7,
Transfer / separation device 8, P sensor 9, cleaning device 1
0, a static eliminator 11, and the like. Here, the first charger 2 operates in the dark (-).
The corona discharge is controlled by a grid to charge the photoconductor 1 to a constant potential. The first exposure device 3 forms an electrostatic latent image of a black image on the photoconductor 1 by emitting a laser diode corresponding to the black image and erasing the (−) charge on the photoconductor 1. The first developing device 4 has a (-)
A (-) charged black toner is adhered to the portion (black image portion) from which the charge has been erased to form a visible image. Second charger 5
Indicates that the potential of the photoconductor 1 attenuated by the formation of a black image is (-)
Is restored by corona discharge. The second exposure device 6 forms an electrostatic latent image of a red or blue image on the photoconductor 1 by causing the LED array to emit light for the red or blue image and eliminating the (−) charge on the photoconductor 1. I do. Second developing device 7
The non-magnetic one-component non-contact development allows the negatively charged red or blue toner to adhere to the electrostatic latent image portion of the red or blue image without disturbing the previously formed black image. Form a visual image. The transfer / separation device 8 applies a (+) bias to the transfer belt to transfer a black and red (or blue) visible image (toner image) charged to (−) to the transfer paper 12 and convey it. . In addition, it has a separation claw for reliably separating the transfer paper 12 from the photoconductor 1. P sensor 9
Reads a black image density of a fixed pattern developed on the photoreceptor 1 with a photo sensor, and feeds back a detection signal to a toner replenishment control circuit of a black toner replenishment device (for the first developing device 4) to supply a black image. Image control. The cleaning device 10 scrapes off the toner remaining on the photoreceptor 1 after transfer with a cleaning blade, and collects waste toner (residual toner) with a cleaning brush. Static eliminator 11
, The entire surface of the photoreceptor 1 is exposed (static elimination) by the LED, and the (-) charge remaining on the photoreceptor 1 is erased to prepare for the next copy. Although the above-described image forming apparatus has a structure capable of two-color copying, in the present invention, two-color copying is not directly related. The configuration may be such that the second exposure device 6 and the second developing device 7 are omitted. FIG. 3 shows a configuration example of a general developing unit (first developing unit 4 in FIG. 2 ) of an image forming apparatus in which the present invention is implemented. The developing unit 40 is a dry type two-component magnetic brush developing type developing device using a developer in which a toner and a carrier are mixed. In FIG. 3 , the developer is a paddle roller 41.
The photosensitive drum 1 is conveyed by being jumped up to the upper developing sleeve 42 and regulated to a certain thickness by the doctor blade 43, and the upper developing sleeve 42 and the lower developing sleeve 44.
Is transported to the corresponding developing unit. Then, in this state, the first developing P.P. By applying a developing bias to the upper developing sleeve 42 and the lower developing sleeve 44 by P, the electrostatic latent image formed on the photoconductor 1 is developed (visible image). The remaining developer regulated by the doctor blade 43 flows toward the separator, and is agitated from the near side to the back by a diagonal guide (not shown) provided on the separator 46 and circulated to the paddle roller 41. You.
Here, a part of the developer on the back side of the separator 46 flows directly to the transport screw 47 without passing through the separator 46, and is agitated from the back to the front by the transport screw 47, and the paddle roller 41 and the stirring roller Recirculated to 48. The upper part of the developing unit 40 includes
An air filter 49 for releasing pressure in the unit is provided. A toner replenishing unit 50 for replenishing new toner by an amount corresponding to the amount of toner consumed by the development is disposed above the stirring roller 48 of the developing unit 40 (the broken line in FIG. 3 ). Display). The toner supply unit 50 includes a toner hopper 51 for storing new toner, a toner stirring screw (agitator) 52 for stirring the toner, and a toner supply roller 5 for supplying a fixed amount of toner from the toner hopper 51.
3 is comprised. The toner concentration of the developer in the developing unit 40 is determined by a T
A sensor (toner density sensor) 60 is magnetically detected to detect the mixture ratio of toner and carrier, and the toner replenishing roller 53 is rotated according to the detected value of the T sensor 60 to timely By supplying new toner to the stirring roller 48, control is performed so as to always maintain an appropriate density. Incidentally, the formation of an electrostatic latent image of a black image in the image forming apparatus of the present embodiment is performed by a raster scan in which a laser beam emitted from a laser diode is polarized by a polygon mirror. In the image forming apparatus of the present embodiment, recording is performed at a pixel density of, for example, 400 dpi at the time of normal copying as a copying machine, but when this is used as a printer or a facsimile, for example, 240 dpi, 300 dpi, 360 dpi, 60
Recording can be performed even at a pixel density of 0 dpi or the like. As a method of switching each pixel density according to the application, for example, when recording at a pixel density of 400 dpi or less, the clock frequency is changed in the main scanning direction and the sub scanning direction without changing the speed of the photosensitive member 1. The pixel density is switched by making it proportional to the product of the pixel density in the scanning direction and making the rotation speed of the polygon mirror proportional to the pixel density in the sub-scanning direction. That is, when the pixel density is 300 dpi, for example, the clock frequency is 9/16 times that at the pixel density of 400 dpi, and the rotation speed of the polygon mirror is 3/4.
Double it. However, in this method, when the pixel density is changed to, for example, 600 dpi and the pixel density is switched in the same manner as described above, the clock frequency and the rotation speed of the polygon mirror become very high. I will.
Therefore, when switching to such a high pixel density, a method of reducing the rotation speed of the photoconductor 1 is adopted. That is, this method does not change the speed of the polygon mirror,
The rotation speed of the photoconductor 1 is made inversely proportional to the pixel density in the sub-scanning direction, and the clock frequency is made proportional to the pixel density in the main scanning direction. Specifically, the rotation speed of the photoconductor 1 is increased to 2/3 times,
Increase the clock frequency by 3/2. However, in an image forming apparatus provided with a drive system for driving both the photosensitive member 1 and the developing unit 40 by the same main motor, the pixel density of an image formed on the photosensitive member 1 is switched. When the rotation speed of the photoconductor 1 is changed, the driving speed of the developing unit 40 also changes in accordance with the change in the rotation speed of the photoconductor 1, and the flow (velocity, flow rate, etc.) of the developer in the development unit 40 changes. Resulting in. Therefore, in this type of image forming apparatus, although the toner concentration of the developer in the developing unit 40 is actually maintained at an appropriate value, the T sensor 60 does not operate at the time of 400 dpi and 600 dpi. Since the toner concentration detection value is detected as a different value, the toner concentration of the developer in the developing unit 40 cannot be correctly detected, and the toner concentration in the developing unit 40 becomes improper, so that a stable image can be output. The problem that disappears occurs. In the image forming apparatus of the present invention, FIG.
As shown in (2), the developing unit 40 is driven at a constant speed by using the developing motor 70 independently of the rotation speed of the main motor. That is, in FIG. 1, the development-dedicated motor 70 is connected to the drive system of the photoconductor 1 through the first gear train 71, the timing belt 72, the second gear train 73, the development drive clutch 74, and the toner supply clutch 75. Independently, the developing unit 40 is driven appropriately. Therefore, in the image forming apparatus of the present invention, even if the rotation speed of the photoconductor 1 changes due to the switching of the pixel density, the developing unit 40 is always driven at a constant speed by the developing motor 70. The flow (flow velocity, flow rate, etc.) of the developer in the developing unit 40 does not change, the toner concentration of the developer in the developing unit 40 is always accurately detected, and a stable image is output. As described above, according to the present invention, according to the first aspect, even if the rotation speed of the photosensitive member changes due to the switching of the pixel density, the developing unit can develop the image. Since it is always driven at a constant speed by the dedicated motor, the flow of the developer in the developing unit (flow velocity, flow rate, etc.) is constant, and the developing unit 40
It is possible to accurately detect the toner concentration of the developer inside, and to output a stable image. [0025]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a drive system of a developing unit for explaining a first embodiment of an image forming apparatus of the present invention. FIG. 2 shows the periphery of a photoconductor of an image forming apparatus in which the present invention is implemented .
It is the schematic which shows the example of a structure. FIG. 3 is a development unit of the image forming apparatus in which the present invention is implemented.
FIG. 3 is a cross-sectional view illustrating a configuration example of a computer. [Description of Signs] 1 Photoconductor 40 Developing unit 60 Toner density sensor (T sensor) 70 Developing motor 80 Sensor driving voltage control means

Claims (1)

(57) Claims: 1. A two-component magnetic brush developing type developing device provided with a toner density sensor for magnetically detecting a mixing ratio of toner and carrier, and a speed changer for driving at least a photosensitive member. In the electrophotographic image forming apparatus having a possible main motor, the developing device includes two developing sleeves, and drives all driven members in the developing device at a constant speed regardless of the rotation speed of the main motor. An image forming apparatus comprising a development-specific motor to be driven.