JPH0631976A - Image forming device - Google Patents

Abstract

PURPOSE:To accurately set a timing for starting scanning of an image signal for a photosensitive body even when quantity of light of laser beam to be injected into a photo-detector changes. CONSTITUTION:Although laser beam reflected by a polygon mirror scans a photosensitive drum, the laser beam is injected into a photo-detector 41 before that. A peak value detecting circuit 133 detects a peak of an output of the photo-detector 41 and a threshold value Vth of a comparator 134 is changed dependent on the magnitude of the peak. From the comparator 134, an SOS signal 61 indicating a start of formation of image in a photosensitive body will be outputted at an accurate timing.

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 for forming an electrostatic latent image according to image information by scanning a laser beam on a photosensitive member, such as a copying machine or a laser printer. In particular, the present invention relates to an image forming apparatus in which a laser beam is detected outside an image scanning area of a photoconductor to set an image scanning timing.

[0002]

2. Description of the Related Art An image forming apparatus for recording and displaying an image by forming an electrostatic latent image on a photoconductor can obtain a high quality image at a high speed, and a copying machine, a laser printer, or an information display on a display. It is widely used as a display device for displaying on.

FIG. 12 shows an outline of a laser printer as an example of such a device. The laser printer 11 is connected to a printer control device 12 represented by a workstation or a computer by a cable 13 and is supplied with image data to print out an image.

Most of such laser printers 11 are provided with a photosensitive drum 15 which rotates at a constant speed. Around the photoconductor drum 15, a charge corotron 16 for uniformly charging the drum surface, a developing device 17 for developing an electrostatic latent image, and a toner image obtained by the development on a recording paper 18. Transfer corotron 19 to transfer
Further, a discharge corotron 21 for removing charge from the drum surface after transfer and a cleaning device 22 for removing toner remaining on the drum surface are provided. The semiconductor laser control device 24 is configured to perform on / off control (modulation) of the semiconductor laser 25A according to image data. Laser beam 2 output from semiconductor laser 25A
6 enters a polygon mirror 28 through a shaping optical system 27 including a lens, is reflected by the polygon mirror 28, and then forms an image on the photosensitive drum 15 through an imaging optical system 29.

Since the polygon mirror 28 is rotated at a high speed by the polygon mirror drive motor 31, the laser beam reflected from the polygon mirror 28 is deflected and the drum surface between the charge corotron 16 and the developing device 17 is scanned line by line. It will be. As a result, the photosensitive drum 1
An electrostatic latent image corresponding to the image data is formed on the surface of No. 5, and this is developed by the developing device 17.

The semiconductor laser controller 24 is controlled by a device controller 33 for controlling the entire laser printer 11. The paper feeding system of the laser printer 11 is also subjected to the same control. That is, the recording paper 18 accommodated in the cassette tray 35 is fed by the feed roll 36.
They are sent out one by one and proceed along the route shown by the dotted line. Then, first, the photosensitive drum 15 and the transfer corotron 1
The toner image is transferred through the space 9 and the toner image is fixed by heat or pressure through the space between the fixing devices 37 including a pair of rolls. The recorded recording sheet thus obtained is discharged from the discharge roll 38,
The paper is discharged into the discharge tray 39.

An image forming apparatus typified by such a laser printer may be used in a one-to-one connection with an image information source such as the printer controller 12 shown in FIG. In some cases, it may be selectively connected to various image information sources via a local area network, etc. Especially in the latter case where the image information source is selectively connected to a plurality of image information sources, it is necessary to switch the pixel density according to the image information source to form an image.

Therefore, in the apparatus disclosed in Japanese Unexamined Patent Publication No. 52-119938, the rotation speed of the polygon mirror for deflecting the laser beam is changed and the frequency of the image signal is changed, thereby changing the pixel density. I am trying to make changes. Further, in JP-A-59-198076 and JP-A-60-29720, the number of surfaces used by the polygon mirror is controlled according to the image density.

Among them, in Japanese Patent Laid-Open No. 59-198076, the SOS (Start of Scan) signal for detecting the scanning start position of the deflected laser beam is appropriately ignored according to the scanning density to modulate the image signal. According to Japanese Patent Laid-Open No. 60-29720, the use surface itself of the polygon mirror is controlled.

When the pixel density is changed or switched as described above, it is necessary to change the effective diameter of the laser beam applied to the photosensitive member corresponding to each pixel. The energy density of the laser beam per unit area of the irradiated portion of the photoconductor should depend on the scanning density.
Therefore, in order to change the effective diameter of the laser beam, it is necessary to change the emitted light amount of the laser beam accordingly.

FIG. 13 shows the relationship between the scanning density and the required energy density for a certain photoconductor. The horizontal axis shows the scanning density (spi; scan / inch),
The vertical axis represents the energy density of the laser beam as a relative value from "0" to "1". As can be seen from this figure, the higher the scanning density, the smaller the required energy density. In this figure, the required energy density when the scanning density is 240 spi is set to "1". In this case, the required energy density is reduced to about "0.35" when the scanning density becomes 600 spi.

FIG. 14 shows in principle how the laser beam scans both the SOS sensor and the photosensitive member. The laser beam 26 output from the semiconductor laser 25A is reflected by the polygon mirror 28 and deflected at a predetermined angle. At this time, first, the laser beam is detected by the photodetector (SOS sensor) 4 arranged near the photosensitive drum 15.
1 and then the photosensitive drum 15 is scanned in the direction of arrow 42. The polygon mirror 28 slightly deviates in the timing of deflecting the laser beam 26 due to a subtle error in processing each surface and a variation in the rotational speed of each surface. Therefore, the synchronization signal is generated at the detection timing of the photodetector 41 and the start timing of each scanning line is set.

Depending on the image forming apparatus, in order to detect the end of the scanning line, another optical sensor (EOE (End Of
Scan) sensor) may be used together.

[0014]

The laser beam deflected by the deflecting means such as the polygon mirror 28 as described above is used both by the photodetector such as the photodetector 41 and the photoconductor such as the photoconductor drum 15 or the photoconductor belt. Will be incident on. Then, the light quantity of the laser beam 26 emitted from the semiconductor laser 25A is changed in accordance with the change or switching of the pixel density.

FIG. 15 shows how the amount of light incident on the photodetector changes. In this figure, the horizontal axis is the same as that shown in FIG. The upper curve 51 shown in the drawing shows the amount of light incident on the photodetector 41 when the polygon mirror 28 is scanned on a partition surface in order to make the image density relatively rough. On the other hand, the curve 52 shown on the lower side shows the amount of light incident on the photodetector 41 when the entire surface of the polygon mirror 28 is scanned in order to make the image density relatively dense. In this example, since the scanning of the other surface and the scanning of the entire surface are switched according to the image density, the solid line portion represents the amount of light incident on the photodetector 41.

When the image densities are switched in this manner, the photodetector 41 side needs to detect the amount of incident light that greatly changes in level, which causes a problem that the precision at the time of generating the synchronization signal is lowered. Occur.

FIG. 16 shows a conventionally used synchronizing signal generating circuit. The detection output of the photodetector 41 upon incidence of the laser beam 26A is
P) 55, and the detected signal 56 is input to the comparison terminal side of the comparator (COMP) 57. The other terminal of the comparator 57 has 2
The voltage V _TH divided by the two voltage dividing resistors 58 and 59 is input as a threshold value. The comparator 57 outputs the comparison result as the SOS signal 61.

FIG. 17 is a diagram for explaining a conventional problem when such a synchronizing signal generating circuit is used. 16A shows the amount of incident light on the photodetector 41 (FIG. 14), and FIG. 17B shows the signal level incident on the comparator 57 shown in FIG. Further, FIG. 17C shows the SOS signal 61. The waveform (a) on the left side of these figures is the laser beam 2
6 (FIG. 14) is the case where the scanning density is 600 spi, and the incident light amount is the smallest. Waveform (c) on the right is 2
In the case of 40 spi, the amount of incident light is the largest. The waveform (b) in the center is for 400 spi, and the incident light amount is an intermediate value between these.

Further, of the three types of waveforms (a) to (c) shown in FIG. 9A, the first small pulse-shaped waveform 71 is not the original laser beam reception,
It represents stray light that is detoured and received for some reason. The relatively large pulse-shaped waveform 72 that follows shows the original reception of the laser beam. The tip portion 72A of the waveform 72 represents a phenomenon (a light-receiving flaw) in which a part of the beam that should reach the detection element does not reach due to a flaw or the like on the surface of the photodetector 41.

FIG. 17 shows such changes in the incident light quantity.
The detection signal 56 as shown in (c) is input to one terminal of the comparator 57, respectively. That is, each detection signal 56 has the power supply voltage V _CC as the upper limit value, but the signal level toward the lower limit value changes according to the amount of input light. The voltage V _TH is 400 spi in the same figure (b).
If the setting is made to be suitable for the case, as a result, the rising of the SOS signal 61 varies in the portion of the light receiving scratch in the case of FIG. 9A, and stray light is generated in the case of FIG. This causes a problem that the SOS signal 61 rises with the waveform 71 of the part.

As a means for solving such a problem due to the difference in sensitivity between the photoconductor and the photodetector, Japanese Patent Publication No.
There are technologies described in Japanese Patent No. 77506 and Japanese Patent Publication No. 9285/1992. Of these, Japanese Examinations 3-77506
In Japanese Patent Laid-Open Publication No. 2004-242242, a light intensity attenuating means having a predetermined attenuation rate is arranged between a polygon mirror and a photosensitive member to relatively increase the amount of light incident on a photodetector, and to provide a laser with an optimum intensity for both. The beam is made to enter.

In the technique disclosed in Japanese Patent Publication No. 9285/1992, the intensity of the laser beam is changed when the laser beam is deflected by the polygon mirror so that the laser beam scans the photoconductor and enters the photodetector. The intensity of the laser beam to be used is set to the optimum one.

However, the techniques described in these two publications are premised on that the intensity of the laser beam scanned on the photoconductor is one, and this occurs when the intensity of the laser beam with respect to the photoconductor is changed. It is not possible to solve the problem of deterioration of the detection accuracy of the photodetector.

As described above, when the range of the amount of light incident on the photodetector 41 (FIG. 14) is set to a large extent, there is a problem that the detection accuracy of the SOS signal 61 as a synchronizing signal is lowered. It explained using. In such an image forming apparatus, as described with reference to FIG. 15, when the partition surface scanning is performed at a low pixel density and the whole surface scanning is selected at a high pixel density, the change range of the incident light amount to the photodetector 41 is changed. As a result, the detection accuracy of the sync signal is further reduced.

On the other hand, a technique relating to the method of suppressing jitter of the photodetector 41 is disclosed in Japanese Patent Laid-Open No. 63-267908. Here, the jitter is suppressed by controlling the threshold value (threshold level) or the amplification factor. In addition, JP-A-63-256918 and JP-A-63
No. 44618 discloses a method of setting a threshold value from the peak light amount for each surface of a polygon mirror. Further, Japanese Patent Laid-Open No. 61-232765 discloses a technique of controlling the amplification factor of the photodetector based on the intensity of the beam detected by the photodetector. However, these technologies have different problems from the present invention, and the problems of the present invention cannot be solved.

Therefore, an object of the present invention is to accurately determine the timing of starting the scanning of the image signal even when the light amount of the laser beam incident on the photodetector is changed as in the case of switching the image density. To provide an image forming apparatus that can be set to.

[0027]

According to a first aspect of the present invention, there is provided (a) a laser oscillator, and (b) a deflecting unit such as a polygon mirror for injecting and deflecting a laser beam output from the laser oscillator, (C) A photoreceptor such as a photoreceptor drum that forms an electrostatic latent image by moving the surface at a relatively constant speed and scanning the laser beam after deflection by the deflection means, and (d) after deflection. A photodetector that detects the laser beam, (e) a peak detection circuit that detects the peak value of the output of the photodetector, and (f) a threshold value that corresponds to the peak value detected by the peak detection circuit is set to perform photodetection. The image forming apparatus is provided with a comparator for setting the timing of starting scanning of the image signal with respect to the photoconductor when the output of the container exceeds this threshold value.

That is, according to the first aspect of the present invention, the peak value of the output of the photodetector is detected by the peak detection circuit, and the threshold value of the comparator is adjusted according to the peak value to start the scanning of the image signal. The timing can be set accurately.

According to a second aspect of the present invention, (a) a laser oscillator, (b) a deflection means such as a polygon mirror for injecting and deflecting a laser beam output from the laser oscillator, and (c) a surface thereof. Move at a relatively constant speed,
A photoconductor such as a photoconductor drum which scans the laser beam after being deflected by the deflecting means to form an electrostatic latent image, (d) a photodetector for detecting the deflected laser beam, and (e) photodetection. Peak detection circuit that detects the peak value of the output of the instrument,
(F) An amplifier that amplifies the output of the photodetector so that the amplification factor decreases as the peak value detected by the peak detection circuit increases, and (g) has a predetermined threshold, and the output of the photodetector has this threshold. The image forming apparatus is provided with a comparator for setting the timing of starting the scanning of the image signal with respect to the photoconductor when the value exceeds the limit.

That is, in the second aspect of the present invention, the peak value of the output of the photodetector is detected by the peak detection circuit, and the output of the photodetector is amplified so that the amplification factor decreases as the peak value increases. The output of the photodetector after the amplification is compared with a fixed threshold value of the comparator so that the timing of starting the scanning of the image signal can be accurately set.

According to the third aspect of the invention, (a) a laser oscillator, (b) a deflecting means for injecting and deflecting a laser beam output from the laser oscillator, and (c) a surface thereof is relatively constant. A photoconductor that moves at a speed and scans the laser beam after deflection by the deflecting means to form an electrostatic latent image, (d) a photodetector that detects the laser beam after deflection, and (e) this light. A comparator for comparing the output of the detector with a predetermined threshold value and setting the timing of starting scanning of the image signal with respect to the photoconductor when it exceeds this threshold value; and (f) the threshold value of the comparator in the first stage before image formation. And (g) threshold value of the comparator in the second stage before the image formation. A threshold setting means for setting a desired value of the inner is provided in the image forming apparatus.

That is, in the third aspect of the invention, in the first step before image formation, the threshold value of the comparator is sequentially increased or decreased for every plural scanning lines, and when the threshold value is within the range, the scanning is started. The timing of is set correctly. Then, the threshold value is set to, for example, the central value of the threshold allowable range, for example, 1
Even when the amount of light incident on the photodetector changes for each page image, this is dealt with appropriately.

According to a fourth aspect of the present invention, (a) a laser oscillator, (b) a deflection means for injecting and deflecting a laser beam output from the laser oscillator, and (c) a surface thereof is relatively constant. A photoconductor that moves at a speed and scans the laser beam after deflection by the deflecting means to form an electrostatic latent image, (d) a photodetector that detects the laser beam after deflection, and (e) this light. An amplifier that amplifies the output of the detector,
(F) A comparator for comparing the output of this amplifier with a predetermined threshold value and setting the timing of starting scanning of the image signal with respect to the photoconductor when the output value exceeds this threshold value; and (g) in the first step before image formation. Amplification factor allowable range determining means for checking the allowable range of the amplification factor for changing the amplification factor of the amplifier so that the scanning start timing is correctly set for each scanning line,
(H) The image forming apparatus is provided with a threshold value setting unit that sets the amplification factor of the amplifier to a desired value within the allowable range in the second stage before the image formation.

That is, in the fourth aspect of the invention, the amplification factor of the amplifier arranged on the output side of the photodetector is increased or decreased in order at every first scanning line before forming an image, for example. A determination is made as to which range of the amplification factor the timing for starting scanning output from the comparator having a fixed threshold is correct. Then, by setting the amplification factor of the amplifier to, for example, the central value of the allowable range of the amplification factor, it is suitable for this even when the amount of light incident on the photodetector changes for each image of one page. I am trying to respond.

[0035]

EXAMPLES The present invention will be described in detail below with reference to examples.

FIG. 1 shows an outline of a circuit configuration of a laser printer as an image forming apparatus according to an embodiment of the present invention. The same parts as those in FIG. 12 or FIG. 14 are designated by the same reference numerals, and the description thereof will be appropriately omitted. The laser printer of this embodiment includes a pixel density control circuit 81 for controlling the pixel density. In this embodiment, the pixel density control circuit 81 sets the pixel density to 240 spi, 300 spi, 400
It is designed to perform the control necessary for switching in five stages of spi, 480spi and 600spi, but especially 30
Switching between 0 spi and 600 spi will be described.

The pixel density control circuit 81 uses five types of signals 8
2 to 86 are created and supplied to the corresponding circuits. The laser beam is reflected by the polygon mirror 28, then passes through the fθ lens 89, and then passes through the photosensitive drum 15.
To reach. A part of the laser beam other than this is reflected by the reflection mirror 90 and is incident on the photodetector 41. The output of the photodetector 41 is
The SOS signal 61 is input to the waveform shaping circuit 91 as shown in FIG. 16, and is output from here.

The SOS signal 61 is input to both the SOS gate generation circuit 92 and the counter 93. The SOS gate generation circuit 92 outputs an SOS gate signal 94 for SOS detection from the rising edges of the SOS interval signal 82 and the SOS signal 61, and supplies this to one terminal of a 2-input OR gate 95.
A video signal 98 corresponding to image information is supplied from the image buffer 97 to the other input terminal of the 2-input OR gate 95. That is, the 2-input OR gate 95 is SO
The S gate signal 94 and the video signal 98 are combined, and this is supplied to the laser oscillator 25 as video data 99.

On the other hand, the motor control signal 83 as the first signal output from the pixel density control circuit 81.
Are supplied to the polygon mirror drive motor 31. The polygon mirror drive motor 31 switches and controls the rotation speed of the polygon mirror 28.

The intensity signal 84 as the second signal output from the pixel density control circuit 81 is supplied to the laser oscillator 25 to control its light quantity. The relationship between the pixel density and the light amount on the photosensitive drum 15 is as shown in, for example, FIG. 13. Therefore, such a light amount adjustment is performed by the variable attenuator 88 and the intensity signal 84 of the laser oscillator 25. The output light quantity will be.

The set counter signal 85 as the third signal output from the pixel density control circuit 81 is supplied to the counter 93 to preset it. The counter 93 counts down the preset value every time the SOS signal 61 is supplied, and when the value becomes zero, the operation of returning to the preset value again by the arrival of the next SOS signal 61 is repeated.
The video clock 102 is output from the clock generator 101 when the count value is zero. This video clock 102
Is a cycle corresponding to the pixel density based on the clock control signal 86 as the fourth signal output from the pixel density control circuit 81.
It is designed to be supplied to 7.

The image buffer 97 receives print data from a circuit (not shown) and sequentially stores the print data. When the supply of the video clock 102 is started, the video signal 98 is read for one line in synchronization with this. The read video signal 98 passes through the 2-input AND gate 95 and is supplied to the laser oscillator 25.

As a result, for example, a pixel density of 600
In the case of spi, each time the SOS signal 61 is generated, the video data 99 for one line from the 2-input OR gate 95 is generated.
Is output, and the polygon mirror 28 sequentially scans the image line by line. When the pixel density is 300 spi, which is half the pixel density, the SOS signal 61 becomes 1
It will be ignored every other time.

The video signal 98 read from the image buffer 97 on the validated scan line is output as video data 99 from the 2-input OR gate 95 and is sent to the laser oscillator 25 for modulation. In this way, when the pixel density is 300 spi, the polygon mirror 28
Will be scanned on the other side.

As is clear from FIG. 1, when the polygon mirror 28 rotates, the laser beam reflected thereby rotates the entire length of the photosensitive drum 15 in the axial direction. The light receiver 113 does not obstruct the laser beam incident on the photoconductor drum 15 and is arranged in the scanning range of the laser beam by the rotation of the polygon mirror 28. The laser beam 26 is incident on the light receiver 113 for each scanning line.
The intensity of the beam actually delivered to 5 will now be detected. That is, a laser beam having a significantly different intensity is made incident on the light receiver 113 based on the switching of the scanning density.

FIG. 2 shows the logic for controlling the number of faces of the polygon mirror employed in the laser printer of this embodiment. The image density of the polygon mirror 28 is 2
From 00spi to 400spi, the scanning on the other side is performed. Such control is performed by the counter 93 supplied with the set counter signal 85 shown in FIG.

FIG. 3 specifically shows the pixel density control circuit of the laser printer circuit and its surroundings. The laser beam output from the laser oscillator (semiconductor laser) 25 is detected by the photodetector 41 at a predetermined timing, and after being shaped by the waveform shaping circuit 91, the SOS signal 6
It becomes 1. The SOS signal 61 is supplied to the CPU (Central Processing Unit) 111 and transmitted from there to the counter 93 shown in FIG.

The CPU 113 has video data 112 for recording an image and resolution data 1 for instructing the resolution.
We are supposed to receive 13 supplies. Of these, the video data 112 is expanded in the image buffer 97 also shown in FIG. 1, and the video signal 98 read therefrom is supplied to a 2-input OR gate 95 as a switching circuit to control ON / OFF of the laser oscillator 25. Intensity signal 84 for performing

The level of the intensity signal 84 is designated by the CPU 111 as the strength instruction signal 115. The D / A converter 116 corrects this instruction to an analog level and outputs the intensity control signal 117.
As a result, the constant current source 118 is supplied to the constant current source 118, and the constant current 119 adjusted to a desired level is supplied to the 2-input OR gate 95.

On the other hand, the waveform shaping circuit 91 inputs the output of the photodetector 41 and uses the threshold value data 121 supplied from the CPU 111 to create the SOS signal 61. Further, the peak of the output of the photodetector 41 is detected as the data that becomes the basis of the threshold data 121, and this is used as the peak data 1
22 is supplied to the CPU 111.
For example, the CPU 111 reads data for setting an optimum threshold value using the peak data 122 as address information from a look-up table (not shown), and sends this as threshold value data 121 to the waveform shaping circuit 91. Note that a part of the laser beam output from the laser oscillator 25 is detected by the monitor diode 125. This detection output 126 is the CPU 11
By being fed back to 1, the intensity of the laser beam output from the laser oscillator 25 is maintained at a desired value.

FIG. 4 specifically shows a waveform shaping circuit to which a photodetector is connected. The waveform shaping circuit 91
An amplifier 131 that amplifies the output of the photodetector 41, a peak value detection circuit 133 that inputs the output signal 132 of the amplifier 131 and detects the peak thereof, and a comparator 134 that inputs the output signal 132 to one input terminal. The comparator 134 is composed of a D / A converter 135 whose output side is connected to the other input terminal and an A / D converter 136 which is arranged at the output side of the peak value detection circuit 133.

The peak value detection circuit 133 outputs a peak signal 138 representing the peak value of the output signal 132. The A / D converter 136 converts this into the peak data 122 as a digital signal and supplies it to the CPU 111. Further, the threshold data 121 sent from the CPU 111 is converted into an analog level by the D / A converter 135 and supplied to the comparator 134 as a threshold signal 139 for comparison. This threshold signal 13
9 is set to a voltage level which is about half the signal level of the peak signal 138, for example.

FIG. 5 and FIG. 6 are for explaining the situation in which the SOS signal is created according to the change in the peak value. These, FIG. 5, the signal level V _peak of the high resolution image recording is selected peak signal 138 indicates a relatively close state to the power supply voltage V _CC. In this case, the signal level V _th of the threshold signal 139 is substantially in the middle of the power supply voltage V _CC and the signal level V _peak , so that even if the output signal 132 has the stray light or the light-receiving damage described in FIG. SOS in these parts
The signal 61 is never created in error.

FIG. 6 shows a case where low resolution image recording is selected. At this time, the signal level V _peak of the peak signal 138 is the power supply voltage V _CC and the level to widely separated. Also in this case, the CPU 111 sets the signal level (threshold value) V _th of the threshold signal 139 to a value approximately in the middle between the power supply voltage V _CC and the signal level V _peak . Therefore, the signal level V _th is lower than that in FIG.
Even if there are stray light and light-receiving scratches described in FIG. 17 in 32, the SOS signal 61 is not mistakenly created in these parts.

First Modification

FIG. 7 corresponds to FIG. 4 and shows a modification of the waveform shaping circuit to which a photodetector is connected.
In the previous embodiment, the CPU 111 outputs the threshold data 121 to set the threshold V _th . In the modification shown in FIG. 7, the output side of the peak hold circuit 133A that holds the peak value is connected to the power supply line (not shown) of the voltage V _CC via the series circuit of the two resistors 141 and 142. . Then, the voltage divided by the resistors 141 and 142 is input to the comparator 134 as the threshold value V _th .

That is, in this modified example, two resistors 14 are provided.
The voltage divided by 1, 142 becomes the threshold value V _th .
Therefore, without intervention of the CPU 111, the signal level V
_The threshold value V _th corresponding to the fluctuation of _peak is set.

Second modification

FIG. 8 shows another example of a waveform shaping circuit to which a photodetector is connected as a second modification of the present invention. In the second modified example, the peak value detection circuit 133 detects the peak signal 138 and sends it to the CPU 111 (see FIG. 3) via an A / D converter (not shown) as in the previous embodiment. Has become. The CPU 111 creates a gain signal 151 so that the peak value becomes a predetermined signal level, converts the gain signal 151 by a D / A converter (not shown), and then supplies the gain signal 151 to the amplifier 131 so that the amount of light incident on the photodetector 41. The amplification factor is adjusted based on the initial peak value of.

Therefore, in the waveform shaping circuit of this modified example, the threshold value V _{th of the} comparator 134 is _{equal to} that of the two resistors 15.
The partial pressure value is 2,153, which is fixed. The variation of the threshold value V _th for each waveform shaping circuit can be corrected on the CPU 111 side of the image forming apparatus. of course,
One or both of these resistors 152 and 153 can be variable resistors.

Third Modification

In the above-described embodiments and modified examples, it is known what the peak of the output of the photodetector should be, and the value of the amplification factor of the amplifier, and the actually measured peak. Based on the above, the threshold value and the amplification factor of the amplifier are set to appropriate values. In the third modification of the present invention, the comparator is actually operated under various conditions, and based on this, the threshold value of the comparator is set to an appropriate value.

FIG. 9 shows a work flow for printing in the third modification. When the CPU 111 shown in FIG. 3 receives print data (step S
101; Y), and decode the control data (step S1).
02) Set the decoded resolution and start preliminary scanning (pre-scan) of the laser beam before scanning the image. At this stage, the CPU 111 sets the lowest predictable value as the threshold value V _th as the initial value (step S10).
4).

In this state, the CPU 111 sends the SOS signal 6
The interval from the output of 1 (see FIG. 1) to the output of the next SOS signal 61 is counted by the clock. This counting is performed over a time corresponding to a plurality of lines, and each count value obtained thereby is stored in a RAM (random access memory) (not shown) corresponding to the threshold value V _th at that time (step S105).

FIG. 10 is for explaining the relationship between the SOS signal and the count value. In FIG. 3A, the circled numbers represent the surface numbers of the polygon mirror 28 (see FIG. 1) to which the laser beam is applied. Here, a polygon mirror 28 having a six-sided structure is used. The first half of FIG. 9B shows a state in which the threshold value V _{th of the} comparator 134 is appropriate and the SOS signal 61 is normally output. In this case, for example, when the SOS signal 61 is output for the first surface (here, SO
The S signal 61 is displayed in negative logic. ) To the same figure (c)
As shown in, the counter control signal is H (high).
It becomes a level.

As a result, the counter 93 (see FIG. 3) starts counting the video clock 102 (FIG. 10 (d)) output from the clock generator 101.
The counting operation of the counter 93 continues until the next SOS signal 61 is generated. In the example shown in FIG. 10, since the SOS signal 61 is normally generated in the first half, the SOS signal 61 continues until the generation of the SOS signal 61 at the head of the second surface of the polygon mirror 28. The CPU 111 will write this count value in the RAM.

SOS signal 61 generated at the beginning of the second surface
Has been used to stop the counting of the video clock 102, the next counting starts from the head of the third surface of the polygon mirror 28. In this way, in the example shown in FIG. 10, the respective count values for the odd-numbered surfaces of the polygon mirror 28 are stored in the RAM. In the latter half of FIG. 10, the polygon mirror 2
Video clock 10 continuously on the first and second sides of 8
2 occurs, but this shows an example in which the SOS signal 61 does not occur at the beginning of the second surface because the threshold value V _th is inappropriate.

Returning to FIG. 9, the description will be continued. Predetermined number of lines
Count values over the threshold value V_thAgainst
When the data is stored in the RAM in response to the request, the CPU 111 sets the threshold value V_th
Δ _thOnly (step S106). And
Threshold V after this increase_thIs the maximum threshold V
_{th (max)}Unless it exceeds (step S107;
N), returning to step 105, the increased threshold value V_thso
Similarly, the count value is obtained and stored in the RAM.

[0069] After storing the count value corresponding thereto and each of the threshold V _th of up threshold V _th this way _(max) to the RAM (step S107; Y), CPU111 counts each stored in the RAM compared to count value corresponding the value to the original one line, and removing the threshold value V _th caused the error, to determine the threshold V _th the correct count is obtained range (allowable range) (step S1
08). Then, the average value of the maximum value and the minimum value of the threshold value V _th in these allowable ranges is calculated, and this is set as the threshold value V _th at the time of the current image formation (step S109). That is, the threshold value data 121 indicating the threshold value V _th is output and the threshold value V _th of the comparator 134 is set. Then, the printing operation is executed based on this (step S110).

Fourth Modification

FIG. 11 shows a work flow for printing in the modified example in which the amplification factor is adjusted instead of adjusting the threshold value V _th in the third modified example. When the print data is received (step S201; Y), the CPU 111 shown in FIG. 3 decodes the control data (step S202), sets the decoded resolution and sets a preliminary laser beam before scanning the image. Start the scan. At this stage, the CPU 111 has the amplifier 1 shown in FIG.
The lowest value that can be predicted as the amplification factor B of 31 is set as an initial value (step S204).

In this state, the CPU 111 sends the SOS signal 6
The interval from the output of 1 (see FIG. 1) to the output of the next SOS signal 61 is counted by the clock. This counting is performed over a time period corresponding to a plurality of lines, and each count value obtained thereby is stored in the RAM described above in association with the amplification factor B at that time (step S205).
In the fourth modified example, the threshold value V _{th of the} comparator 134 is fixed as shown in FIG.

After that, the CPU 111 increases the amplification factor B by Δ _B (step S206). Then, unless the amplification factor B after this increase exceeds the expected maximum amplification factor B _max (step S207; N), step 205
The count value is similarly obtained with the increased amplification factor B and stored in the RAM.

In this way, when each amplification factor B up to the maximum amplification factor B and the corresponding count value are stored in the RAM (step S207; Y), the CPU 111 RAM
Comparing each count value stored in the original count value corresponding to one line, the amplification factor B in which the error occurred
Is removed, and the range (allowable range) of the amplification factor B for which the correct count value is obtained is determined (step S20).
8). Then, an average value of the maximum value and the minimum value of the amplification factor B in these allowable ranges is obtained, and this is set as the amplification factor B of the amplifier 131 at the time of the present image formation (step S2).
09). Then, the printing operation is executed based on this (step S210).

Although the image forming apparatus centering on the laser printer has been described in the above-described embodiments and modified examples, it goes without saying that the present invention can be applied to other image forming apparatuses.

[0076]

As described above, according to the first aspect of the invention, the photodetector detects the laser beam with which the photoconductor is irradiated, and the threshold value of the comparator for generating the SOS signal is set according to the peak value. Therefore, not only when the intensity of the laser beam is changed due to switching of the resolution, but also the timing of starting the scanning of the image signal can be accurately set not only when the laser oscillator changes over time. There is an effect.

According to the second aspect of the present invention, the photodetector detects the laser beam irradiating the photoconductor, and the amplification factor of the output signal of the photodetector input to the comparator according to the peak value is set. I adjusted it. Therefore, the threshold value of the comparator can be fixed, and the circuit configuration can be simplified.

Further, according to the invention of claim 3, the threshold value of the comparator is actually changed to obtain the allowable range,
Since the appropriate threshold value is set, there is an effect that the timing of starting scanning of the image signal can be accurately set even if the characteristics of each circuit element change.

According to the invention described in claim 4, the amplification factor of the amplifier for inputting the output of the photodetector is actually changed to obtain the allowable range for the threshold value of the comparator, and the appropriate amplification factor is determined. Therefore, there is an effect that the timing of starting scanning of the image signal can be accurately set even if the characteristics of each circuit element change.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an outline of a circuit configuration of a laser printer according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing the logic of the switching of the number of faces of a polygon mirror used in the laser printer of this embodiment.

FIG. 3 is a circuit diagram specifically showing a pixel density control circuit of the laser printer circuit of the present embodiment and its periphery.

FIG. 4 is a block diagram specifically showing a waveform shaping circuit to which the photodetector of this embodiment is connected.

FIG. 5 is a waveform diagram showing the setting state of the threshold value when the peak value is high in the present embodiment.

FIG. 6 is a waveform diagram showing the setting state of the threshold value when the peak value is low in the present embodiment.

FIG. 7 is a circuit diagram of a waveform shaping circuit to which a photodetector as a first modified example of the present invention is connected.

FIG. 8 is a circuit diagram of a waveform shaping circuit to which a photodetector as a second modified example of the present invention is connected.

FIG. 9 is a flowchart showing a work flow for printing in a third modified example of the present invention.

FIG. 10 is various waveform charts for explaining the relationship between the SOS signal and the count value in the third modified example of the present invention.

FIG. 11 is a flowchart showing a work flow for printing in a fourth modified example of the present invention.

FIG. 12 is a schematic configuration diagram showing an outline of a laser printer as an example of an image forming apparatus.

FIG. 13 is a characteristic diagram showing a relationship between a scanning density and a required energy density for a certain photoconductor.

FIG. 14 is a perspective view showing in principle how the laser beam scans both the SOS sensor and the photoconductor.

FIG. 15 is a characteristic diagram showing how the amount of light incident on the photodetector changes in both the scanning of the space and the scanning of the entire surface.

FIG. 16 is a circuit diagram showing a synchronizing signal generating circuit which has been conventionally used.

FIG. 17 is a waveform diagram for explaining various operations of the synchronization signal generating circuit shown in FIG.

[Explanation of symbols]

15 ... Photosensitive drum, 25 ... Laser oscillator, 25A ... Semiconductor laser, 26, 26A, 26B ... Laser beam, 2
8 ... Polygon mirror, 41 ... Photodetector, 81 ... Pixel density control circuit, 90 ... Reflecting mirror, 93 ... Counter, 99 ...
Video data, 111 ... CPU, 131 ... Amplifier, 13
3 ... Peak value detection circuit, 133A ... Peak hold circuit, 134 ... Comparator, 135 ... D / A converter, 1
36 ... A / D converter

Claims (4)

[Claims] 1. A laser oscillator, a deflection means for injecting and deflecting a laser beam output from the laser oscillator, and a surface thereof is moved at a relatively constant speed, and the laser beam deflected by the deflection means is scanned. To form an electrostatic latent image, a photodetector that detects the laser beam after deflection, a peak detection circuit that detects the peak value of the output of the photodetector, and a peak that the peak detection circuit detects. An image forming apparatus comprising: a threshold value corresponding to a value, and a comparator for setting a timing of starting scanning of an image signal with respect to the photoconductor when the output of the photodetector exceeds the threshold value. 2. A laser oscillator, a deflection means for injecting and deflecting a laser beam output from the laser oscillator, and a surface thereof is moved at a relatively constant speed, and the laser beam deflected by the deflection means is scanned. To form an electrostatic latent image, a photodetector that detects the laser beam after deflection, a peak detection circuit that detects the peak value of the output of the photodetector, and a peak that the peak detection circuit detects. An amplifier that amplifies the output of the photodetector so as to decrease the amplification factor as the value increases, and has a predetermined threshold, and when the output of the photodetector exceeds this threshold, An image forming apparatus comprising: a comparator that sets a timing of starting scanning. 3. A laser oscillator, a deflection means for injecting and deflecting a laser beam output from the laser oscillator, and a surface thereof is moved at a relatively constant speed, and the laser beam deflected by the deflection means is scanned. To form an electrostatic latent image, a photodetector that detects the deflected laser beam, and compare the output of this photodetector with a predetermined threshold value. And a comparator for setting the scanning start timing of the image signal with respect to, and a threshold permissible range in which the scanning start timing is correctly set for each scanning line by changing the threshold value of the comparator in the first stage before the formation of the image. And a threshold value setting means for setting the threshold value of the comparator to a desired value within the allowable range in the second step before image formation. And an image forming apparatus. 4. A laser oscillator, a deflection means for injecting and deflecting a laser beam output from the laser oscillator, and a surface thereof is moved at a relatively constant speed, and the laser beam deflected by the deflection means is scanned. To form an electrostatic latent image, a photodetector that detects the laser beam after deflection, an amplifier that amplifies the output of this photodetector, and the output of this amplifier is compared with a predetermined threshold value. A comparator that sets the scanning start timing of the image signal to the photoconductor when the lever threshold is exceeded, and the scanning start timing is one scan by changing the amplification factor of the amplifier in the first stage before image formation. Amplification factor allowable range determination means for checking the allowable range of the amplification factor set correctly for each line, and the amplification factor of the amplifier to a desired value within this allowable range in the second step before image formation. An image forming apparatus characterized by comprising a threshold setting means for constant.