JP2872271B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus for forming a pulse width modulation signal based on an input image signal and modulating a laser based on the pulse width modulation signal to form an image. It concerns the device.

[Conventional technology]

Conventionally, this type of apparatus is configured to form a latent image by irradiating light to a photosensitive drum, and develop the formed latent image with a developer to visualize the latent image.

In general, the sensitivity of a photosensitive drum is not constant due to a change in a use atmosphere or a deterioration of the drum.

Therefore, a surface condition sensor is provided to measure the surface condition of the photosensitive drum in a timely manner, and by comparing the measured value with the grid voltage of the corona charger and the exposure energy during image exposure, the surface condition of the photosensitive drum is changed. Had control.

Further, in order to keep the density of the output image constant, for example, by adding a temperature and humidity detecting means to the image forming apparatus, the image forming conditions are changed according to the detected temperature and humidity to make the output density constant. there were.

FIG. 4 shows a conventional example in which the image forming conditions are set by controlling the grid voltage of the corona charger, and when the range of the forming conditions exceeds the control range of the grid voltage, the exposure amount is switched. Shows an example of what is set to the optimal condition.

This is a method for controlling the potential contrast on the surface of the photosensitive drum as an image forming condition. When the exposure amount is fixed, the grid bias is changed to change the surface potential (V _D ) of the photosensitive member, and the required V contrast is changed. (V _D -V _L )
Is used to calculate the grid bias for obtaining.

However if the change range of potential contrast in need (V _D -V _L) is wide, as shown in FIG. 5 because it is not possible to sufficiently wide variation range in a single exposure, low contrast side and high contrast A method of switching the exposure amount on the side is performed. That is, when the potential contrast is higher than the point B, an operation of switching the exposure amount from (L _O ) to (H _P ) is performed. As shown in FIG. 4, the method of controlling the potential contrast when the exposure amount changes is to change the grid bias with the change in the exposure amount to obtain the same potential contrasts B and B '. _D ) will be controlled. According to the method described above, for example, when the potential contrast changes from the low A side to the high C side, A → B → B ′ → C, and conversely, C → B ′ → B → A
This makes it possible to perform a wide range of potential contrast control.

[Problems to be solved by the invention]

However, in the above conventional example, the following problems occur when the laser exposure amount is switched.

In other words, the pulse width modulation (PWM) of the laser drive signal based on the input image signal changes the amount of laser light emitted, and when recording an image on a photoconductor, the pulse width given as the laser drive signal and the laser light emission The relationship with the light amount is as shown in FIG.

In FIG. 8, the image data levels 00 _H (white) to FF _H (black) [hex] indicate the minimum level of the image data in order to use the linear portion of the characteristic curve as widely as possible.
00 _H is the amount of light when the curve starts to become linear,
The maximum level FF _H of the image data is set to be the light amount immediately before the curve deviates from linear.

However, when the laser power is switched, the laser starts to emit light when it exceeds a predetermined threshold current. Therefore, even if the same pulse is given as shown in FIG. Come. Therefore, the relationship between the pulse width and the amount of light applied to the laser driver when the laser power is switched changes in the above-described linear region as shown by the curves shown in FIG. Here, the curve is when the laser power is high, and the curve is when the laser power is low.

Therefore, in order to obtain the same image density for the same image data even if the laser power is changed, it is necessary to change the pulse width along with the switching of the laser power. The problem of having to have had arisen.

However, for example, even if a modulation circuit for two-stage switching between high power and low power is provided, the adjustment work becomes complicated. There is a problem that the change in image quality accompanying the change in the EV characteristic of the body becomes remarkable. In view of this, it is conceivable that the exposure amount is changed in multiple stages to reduce the change in the exposure amount during one switching to reduce the occurrence of image quality change, and that the potential contrast control of the photoconductor is stably and widely performed. It was difficult to realize due to problems such as adjustment.

The present invention has been made in view of the above points, and provides an image forming apparatus capable of obtaining a high-quality image without taking a complicated configuration.

[Means for solving the problem]

For this purpose, according to the image forming apparatus of the present invention, a correction unit for correcting characteristics of an input image signal;
Image exposure means for forming an image on a photosensitive member by forming a pulse width modulated laser drive signal based on the image signal corrected by the correction means, and the image at the time of image formation based on surface condition control data of the photosensitive member Switching means for switching the exposure energy of the exposure means; and control means for controlling the correction characteristic of the correction means in accordance with the switching of the exposure energy by the switching means, wherein the control means controls the exposure energy by the switching means. Is reduced, the correction characteristic is controlled so that at least the low density level value of the image signal is higher than the level value when the exposure energy is large.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a configuration diagram of an image forming apparatus using a contact type color CCD sensor of the present embodiment.

The copying apparatus 80 includes a reader unit 100 and a printer unit 200. An original scanning unit 83 moves and scans in the direction of arrow A in order to read the image of the original 84 on the original table, and at the same time, an exposure lamp 85 in the original scanning unit 83.
Lights up. The reflected light from the original is guided to a converging rod lens array (not shown), and the contact type color CCD sensor 87
Is collected. 62.5 μm for contact type color CCD sensor 87
(1 / 16mm) as one pixel, 1024 pixel chips are arranged in a staggered pattern with 5 chips, each pixel is 15.5μm × 62.5μ
m, and C, G, Y color filters are attached to each of them.

The optical image collected on the contact type color CCD sensor 87 is converted into an electric signal for each color. These electric signals are subjected to predetermined processing described later by a processing block 88. The color separation image electric signal formed by the image processing block 88 is transmitted to the printer unit 200 and printed.

The color image data from the reader unit 100 is subjected to PWM processing and the like, and finally drives the laser. The laser light modulated according to the image data is scanned at high speed by a polygon mirror 89 rotating at high speed, and is reflected by the mirror 90 to perform dot exposure corresponding to the image on the surface of the photosensitive drum 91. One horizontal scan of the laser beam corresponds to one horizontal scan of the image, and has a width of 1/16 mm in this embodiment. On the other hand, since the photosensitive drum 91 is rotating at a constant speed in the direction of the arrow, a plane image is successively exposed by the above-described laser beam scanning in the main scanning direction and the constant speed rotation of the photosensitive drum 91 in the sub-scanning direction. The photosensitive drum 91 is uniformly charged by the charger 97 prior to exposure,
A latent image is formed by exposing the charged photoconductor to light. A latent image based on a predetermined color signal is visualized by developing units 92 to 95 corresponding to a predetermined color.

For example, considering the first document exposure scanning in a color reader, first, a dot image of a yellow component of a document is exposed on a photosensitive drum 91 and developed by a yellow developing unit 92. Next, this yellow image is placed on the photosensitive drum on the paper wound on the transfer drum 96.
At the contact point between the transfer drum 96 and the transfer drum 96, a transfer toner 98 transfers and forms a yellow toner image. The same process is repeated for M (magenta), C (cyan) and Bk (block), and by superimposing each image on paper,
A color image is formed by the four-color toner.

Next, a method of switching the laser exposure energy will be described with reference to FIG. FIG. 2 is a detailed view of the laser drive system including the processing block 88. 26 is a potential sensor for detecting the potential charged on the photoreceptor 91, 27 is a converter for converting the output from the potential sensor 26 into a digital signal,
This is a potential measurement unit input to the CPU 25-1.

Also, the data of the humidity sensor 98 and the temperature sensor 99 for measuring the environment inside the device and feeding back the potential control are A / D converted by the A / D converter 25-3 and input to the CPU 25-1. Then, the necessary potential contrast is calculated from the result of the current environment measurement.

As a result, as shown in FIG. 5, the laser exposure amount is determined from the required potential contrast, and the potential control of the photosensitive drum is started using the exposure amount. This embodiment shows an example of two-step exposure amount switching.

[Description of Gradation Control Circuit (FIG. 3)] FIG. 3 is a block diagram of the gradation control circuit 21.

The 8-bit image data 11 output from the processing block 88 of the reader unit 100 is input to the buffer memory 30 in synchronization with the synchronization signal RHSYNC and the image clock RCLK from a synchronization signal processing unit (not shown). The image data stored in the buffer memory 30 is transmitted from the synchronization control unit 31 to HSYNC and CLK.
The data is read from the buffer memory in synchronization with the signal 32. As a result, synchronization and speed conversion between the reader unit 100 and the printer unit 200 are performed, and image data is output to the selector 33.

The selection signal 34 from the CPU 25-1 of the control unit 25 is
When the A input of 3 is selected, the image data is input to the address of the look-up table RAM (LUTRAM) 38. At this time, when the CPU 25-1 reads the LUTRAM 38 by the control signal 36, the LUTRAM 38 outputs data corresponding to the address input. The data output from the LUTRAM 38 is the selector 39
To the next selector 40 according to the selection signal 34 described above.
Is input to When the selection signal 42 of the selector 40 selects the A input, the image data from the selector 40 is output to the D / A converter 4
1 and converted to an analog signal 41-1.

The image signal 41-1 converted into the analog signal is binarized by the binarization circuit 44. A specific example of the binarization circuit 44 is the sixth example.
Shown in the figure. CLK of a predetermined cycle output from the synchronization control unit 31
A triangular wave is generated by the triangular wave generating circuit 44-1 based on the signal 51, and the gain and offset level of the triangular wave are respectively adjusted.
Adjust by the volume indicated by 44-3 and 44-5. The adjusted triangular wave is compared with the analog image signal 41-1 by the comparator 44-6 to perform pulse width modulation (PW).
M) signal, and this PWM signal is input to the gate circuit 45.

Note that the relationship between the output pulse width and the amount of emitted laser light has the characteristics shown in FIG. Image data level 00 _H
By to ff _F (16 hex), for use as wide as possible part of the linear of the characteristic curve, the minimum level 00 _H of the image data to the power when the curve starts to become linear,
Further, the above-mentioned gain and offset volume are manually adjusted by using a device capable of measuring the light energy in the optical path so that the maximum level FF _H of the image data becomes the power immediately before the curve deviates from linear.

However, when the laser power is switched as described later, the laser starts to emit light when the laser current exceeds a predetermined threshold current. Therefore, even if the same pulse is given as shown in FIG. Will be different. Therefore, the relationship between the pulse width and the amount of light applied to the laser driver 22 when the laser power is switched changes in the above-described linear region as shown by the curves in FIG. Here, the curve is when the laser power is high, and the curve is when the laser power is low.

Therefore, in order to obtain the same image density for the same image data even when the laser light amount is changed, it is necessary to change the pulse width along with switching of the laser light amount.

Here, further switching of the 8 See FIG When the laser light quantity, the effects of changes in linear region with respect to the pulse width appears strongly is mainly data 00 near _H begins to start laser light emission.

The linearity of the change in the light amount with respect to the light amount and the pulse width near 00 _H has a great influence on the image quality of the output image. In particular, when the reversal development method is used, the image corresponds to a highlight portion and cannot be ignored as a change in image quality.

Therefore, in the present embodiment, the above problem is solved by correcting the pulse width of data instead of having a plurality of binarization circuits shown in FIG. 6 for each light amount when switching the laser light amount.

This embodiment will be described with reference to FIG. The horizontal axis in FIGS. 9 and 8 indicates the video data 00 _{H based on} the pulse width of the laser drive.
Which was changed to to ff _H (16 hex), at the time of actual image forming using only the good region Riniaritei shown in Figure 9.

In FIG. 9, when the laser power of the curve is large, the PWM adjustment, that is, the pulse width setting of the 00 _H light amount and the FF _H light amount is performed by using the binarization circuit 44 of FIG.

Then when the laser power is small as shown in the curve, simply reducing the laser drive current, reducing the laser power, around data 00 _H curve is still without being outputted sufficiently light intensity, and have reached the linear region It turns out there is no. Become linear region it can be seen that in the Figure 9 curve is near data 10 _H.

Therefore, it may be used a linear region of 10 _H to ff _H as a video data in the case where the laser power and small in a state where the PWM adjustment (00 _H, the pulse width setting of the FF _H light amount) in the laser power Univ Will be. That will be the case of a laser power small it may be corrected video data 00 _H to → 10 _H.

In this embodiment, the LUTRAM in the gradation control circuit 21 shown in FIG.
The input data is corrected within 38. The contents
Figure 10 shows.

FIG. 10 shows the contents of a lookup table (LUT) referred to when correcting output image data with respect to input image data.

In FIG. 10, the LUT outputs the output image data to the input image data without correction, and is used when the laser power is large.

LUT output image data 10 for input image data 00 _H
H is a table for correcting so that the output image data 10 _H to ff _H relative to output the input image data 00 _H to ff _H is outputted, which is used in the case of the aforementioned laser power small.

Therefore, even if the laser power is changed, by outputting correction data for the same image data, the same gradation property,
Image density can be obtained.

FIG. 11 shows the laser driver 22 used in this embodiment.
FIG. Changing the constant current value supplied to the laser element 23 can be achieved by changing the voltage of the + input of the operational amplifier 22-5, and the laser power is switched.

[Other embodiments]

In the above-described embodiment, an example in which the laser light amount is switched in two stages has been described. However, the present invention can be applied to a case in which the laser light amount is switched in multiple stages.

FIGS. 12 and 13 show the output image data characteristics for input image data and the laser light amount characteristics for image data when the laser light amount is switched in four steps.
This is the explanation for T ~. As described above, even if the number of switching stages of the light amount increases, even if the number of the binarization circuits is not the same, the LUTRAM has the correction LUTs of the number of the switchings in the LUTRAM.
Images can be reproduced with high quality.

It is also possible to change the laser light amount continuously instead of stepwise.

〔The invention's effect〕

As described above, according to the present invention, even when the amount of laser light is changed, it is possible to apply a drive signal having an optimum pulse width, and a high-quality and stable image can be formed.

[Brief description of the drawings]

1 is a sectional view of a color copying machine according to the present embodiment, FIG. 2 is a detailed view of a laser driving system including a processing block of the color copying machine of the present embodiment, FIG. 3 is a block diagram of a gradation control circuit, 4 and 5 are diagrams showing a conventional potential control method, FIG. 6 is a detailed diagram of a binarization circuit of this embodiment, FIG. 7 is a diagram showing a relationship between laser power and a light emission waveform, FIG. The figure shows the relationship between the pulse width and the amount of laser light emission. FIGS. 9 and 12 show the relationship between the video data and the amount of laser light emission. FIGS. 10 and 13 show the contents of the LUT of this embodiment. FIG. 11 is a detailed view of the laser driver. In the figure, 11 ... image data, 20 ... BD detector, 21 ... gradation control circuit, 22 ... laser driver, 23 ... laser, 25 ... control unit, 26 ... potential sensor, 27 ... Potential measurement unit, 29 Photoconductor, 30 Buffer memory, 31 Synchronous control unit, 33, 39, 40 Selector, 38 Lookup table RAM (LUTRAM), 41 D / A converter .., 44-6... A comparator, 44-1... A triangular wave generator, 100... A reader, 200.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) B41J 2/44 G03G 15/00 303 G03G 15/36 G03G 21/00 370-540 G03G 21/02 G03G 21 / 04 G03G 21/14 G03G 21/20

Claims (1)

(57) [Claims] A correcting means for correcting characteristics of an input image signal; and an image exposure for forming a laser drive signal pulse-width modulated based on the image signal corrected by the correcting means to form an image on a photosensitive member. Means for switching the exposure energy of the image exposure means during image formation based on the surface state control data of the photoreceptor; and controlling the correction characteristics of the correction means in accordance with the switching of the exposure energy by the switching means. Control means, wherein when the exposure energy is reduced by the switching means, at least the low density level value of the image signal is made higher than the level value when the exposure energy is large. An image forming apparatus for controlling the correction characteristic.