JP2002137443A - Imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably operate a sensor for detecting a scanning timing of light beams. SOLUTION: When receiving a light quantity control signal outputted from an MCU 36, an LD drive circuit 44 turns on an LD 40 before an SOS signal is outputted so as to control a quantity of light and to obtain the SOS signal. The quantity of light is controlled so that an output quantity of light of the LD 40 becomes a predetermined quantity of light. When the SOS signal is outputted, the LD drive circuit 44 turns off the LD 40. The light quantity control signal outputted from the MCU 36 is also supplied as a gain-switching signal to an SOS sensor 54. During a light quantity control period of time, the SOS sensor 54 is set to a high gain to have a high sensitivity. When the SOS signal is outputted, the SOS sensor 54 is returned to a low gain to have a low sensitivity.

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, and more particularly to an image forming apparatus that scans a light beam output from a light source to irradiate a photosensitive member to form an image.

[0002]

2. Description of the Related Art An image forming apparatus, such as a laser printer or an electrophotographic copying machine, for recording an image by scanning a light beam on a photoreceptor by an optical scanning device has become widespread. 2. Description of the Related Art In an optical scanning device, generally, a light beam modulated based on image data output from a light source such as a semiconductor laser is incident on a reflection surface of a polygon mirror rotating at a predetermined speed via a collimator lens or the like. By the rotation of the polygon mirror, the incident angle of the light beam is deflected while continuously changing, and the light beam scans on the photoconductor to form an electrostatic latent image on the photoconductor.

In such an optical scanning device, an optical sensor such as an SOS (Start of Scan) sensor for detecting a scanning timing of a light beam is generally provided outside an image area. In the image forming apparatus, the writing of an image is started after counting the clock for several pixels after detecting the scanning timing by the optical sensor, that is, by using the output signal of the optical sensor as a synchronization signal, The image writing position is controlled.

At this time, if noise is mixed on the transmission line of the output signal from the optical sensor, the generation timing of the synchronizing signal changes, and the image writing timing in the main scanning direction changes due to the change, thereby distorting the output image. Will occur. For this reason, conventionally, there has been proposed a technique for estimating a cycle at which an output signal is obtained from an optical sensor and, when a signal is input at a time other than the predicted cycle, cutting off the signal, thereby eliminating the above-mentioned disadvantages. I have.

For example, in Japanese Patent No. 2794682, a permissible range of light beam detection by an optical sensor is set in advance, and only when a detection signal from the optical sensor is output in this range, this is set to a proper optical sensor. Output and
There is disclosed a technique for preventing a malfunction due to noise by turning off the light beam a predetermined time after the light sensor detects the light beam and outputs a light beam detection signal.

[0006] In such a conventional technique, although it is strong against electric noise, it is weak against noise of light incident on the optical sensor, and the sensitivity remains limited. In particular, with the recent increase in the sensitivity of the photoconductor, it has become possible to form an image with a small amount of light beam, and the output light amount from the optical scanning device has been reduced. A response cannot be made with a low light quantity. This is because if the optical sensor is set to a sensitivity suitable for low light quantity, it operates without problems on the low light quantity side, but on the high light quantity side, stray light generated in the optical scanning device (after being reflected by the edge of the optical sensor or the like). This is because the optical sensor malfunctions due to a light beam that enters the optical sensor via a non-regular optical path such as incident on the optical sensor.

For this reason, Japanese Patent Laid-Open No. 6-286217 discloses a light beam detecting means for detecting the light beam intensity, and a peak value holding means for holding the peak value of the detection result of the light beam intensity detecting means for one scanning period. Means, a level changing means for changing a detection result of the light beam intensity detecting means to a predetermined level according to an output value of the peak value holding means, and comparing the output value of the level changing means with a predetermined threshold value. A conversion means for converting the output value to a logical level, and using an output signal of the conversion means as a synchronization signal for image formation in the main scanning direction. That is, since the beam detection value is corrected by the level varying means, an optimum synchronization signal can be obtained even if the light amount of the light beam changes.

Incidentally, in recent optical scanning devices, the light amount of a semiconductor laser is controlled so that a predetermined output light amount is obtained for each scanning of a light beam. In addition, the scanning speed of the light beam has been increasing in order to increase the speed of image formation.
The light quantity control requires that the semiconductor laser be turned on for a predetermined period. As the scanning speed of the light beam increases, the distance that the light beam scans (moves) when the semiconductor laser is turned on for the light quantity control is increased. Becomes longer, so that stray light is easily generated.

[0009]

However, in the technique described in Japanese Patent Application Laid-Open No. Hei 6-286217, the gain (amplification factor) when amplifying the photoelectric conversion output of the optical sensor, that is, the sensitivity of the optical sensor is related to the amount of incident light. Since the noise due to the light reflected from the frame of the optical sensor is also automatically amplified by the gain, it is easily affected by stray light, and a stable synchronization signal can be obtained when stray light occurs frequently. Did not.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to provide an image forming apparatus capable of stably operating a sensor for detecting a scanning timing of a light beam.

[0011]

According to one aspect of the present invention, there is provided an image forming apparatus for forming an image by scanning a light beam output from a light source and irradiating the same with a photosensitive member. It is arranged at a position where the light beam is incident, and detects a scanning timing of the light beam,
Scanning timing detecting means having a function of changing the detection sensitivity, and a detection sensitivity change for changing the detection sensitivity when the scanning timing is detected by controlling the function for changing the detection sensitivity and at other times. And control means.

According to the first aspect of the invention, the detection sensitivity of the scanning timing detecting means for detecting the scanning timing of the light beam of the SOS sensor or the like is determined when the scanning sensitivity is detected by the detection sensitivity change control means and when the scanning timing is detected. It is changed by other than. Thereby, only when the scanning timing is detected, the appropriate detection sensitivity at which the scanning timing detecting means operates by the incidence of the light beam is used.
The detection sensitivity can be set so that the scanning timing detecting means does not operate even when the light beam is incident, and the malfunction of the scanning timing detecting means due to the stray light entering the scanning timing detecting means at a timing other than detecting the scanning timing can be set. Can be prevented.

More specifically, as described in claim 2, the detection sensitivity change control means preferably sets the detection sensitivity higher when detecting the scanning timing than in other times.

According to a third aspect of the present invention, there is provided an image forming apparatus for forming an image by scanning a light beam output from a light source and irradiating the photosensitive member with the light beam, wherein the image forming apparatus is arranged at a position where the light beam is incident. Scanning timing detecting means having a function of detecting a scanning timing of the light beam and changing detection sensitivity, and controlling an image writing timing based on the scanning timing detected by the scanning timing detecting means. Image writing timing control means, based on the scanning timing detected by the scanning timing detection means, to end the lighting of the light source for detecting the scanning timing, after a lapse of a predetermined time from the detected scanning timing, A scanning timing detection point for starting lighting of the light source for detecting a scanning timing in a next scanning line Control means for controlling a function of changing the detection sensitivity based on lighting control by the scanning timing detection lighting control means, and a detection sensitivity change control means for changing the detection sensitivity. I have.

According to the third aspect of the present invention, in the image forming apparatus, the scanning timing detecting means detects the scanning timing of the light beam, and the image writing timing control means synchronizes with the detected scanning timing. The image writing timing (image writing position) is controlled.
This scanning timing detection means has a function of changing the detection sensitivity stepwise or continuously, and this detection sensitivity is
The detection sensitivity is changed by the detection sensitivity change control unit based on the lighting control of the light source for detecting the scanning timing by the scanning timing detection lighting control unit.

According to this, only at the time of the lighting control of the light source for detecting the scanning timing, the appropriate detection sensitivity is set so that the scanning timing detecting means operates by the incidence of the light beam,
Except during the lighting control, the detection sensitivity can be set such that the scanning timing detecting means does not operate even when the light beam enters, so that the scanning timing detecting means can be operated stably even when stray light occurs. Further, there is no fear that the writing timing of the image is shifted due to the malfunction of the scanning timing detecting means due to the stray light, and it is possible to form a high quality image.

[0017] As described in claim 4,
The detection sensitivity change control means may set the detection sensitivity higher during a period in which the light source is turned on by the scanning timing detection lighting control means than during a period other than the period.

Alternatively, as set forth in claim 5, the detection sensitivity change control means is delayed from the start of lighting of the light source by the scanning timing detection lighting control means, and is provided with the scanning timing detection control means. The detection sensitivity may be higher in a period until the lighting of the light source by the lighting control unit is completed than in other periods.

Further, as described in claim 6,
The detection sensitivity change control means may change the detection sensitivity only when the amount of light incident on the scanning timing detection means is equal to or more than a predetermined value.

Alternatively, as described in claim 7, when the amount of incident light on the scanning timing detecting means is less than a predetermined value, the detecting sensitivity change controlling means controls the light source by the scanning timing detecting lighting control means. Is turned on, the detection sensitivity is made higher than the period other than the period, and when the amount of light incident on the scanning timing detecting means is equal to or more than a predetermined value, the starting of lighting of the light source by the lighting control means for scanning timing detection is started. The detection sensitivity may be set to be longer than a period until the lighting of the light source is completed by the scanning timing detection lighting control unit later than the period other than the period.

Further, as described in claim 8,
The function of changing the detection sensitivity may change an amplification factor of a photoelectric conversion output of a light beam incident on the scanning timing detecting means.

[0022]

Next, an example of an embodiment according to the present invention will be described in detail with reference to the drawings.

First Embodiment (Overall Configuration) FIG. 1 shows a schematic configuration of an image forming apparatus 10. As shown in FIG. 1, the image forming apparatus 1
0 is covered by a casing 12. An image forming unit 14 is provided in the casing 12. The image forming unit 14 mainly rotates a cylindrical photoconductor 16 that rotates at a constant speed in the direction of arrow A shown in FIG. 1 and directs a light beam toward the photoconductor 16 based on desired image data (see arrow B). And an optical scanning device 18 (detailed later) for irradiating while scanning.

A charger 20 is provided near the peripheral surface of the photoreceptor 16. The charger 20 charges the photoconductor 16 uniformly. Photoconductor 16 uniformly charged by charger 20
Is rotated in the direction of arrow A, so that the optical scanning device 1
The light beam from 8 is applied. Thereby, the photosensitive member 1
6, a latent image is formed on the peripheral surface.

Further, on the downstream side in the rotation direction of the photoconductor 16 from the irradiation position of the light beam by the optical scanning device 18,
A developing unit 22, a transfer charger 24, and a cleaner 26 are provided. The developing device 22 supplies the toner and
The latent image formed on the peripheral surface of the photosensitive member 16 is developed to form a toner image on the peripheral surface of the photoconductor 16. This toner image is transferred from the recording paper tray 28 to the recording paper 30 guided between the photosensitive member 16 and the transfer charging member 24 by the transfer charging member 24, and remains on the peripheral surface of the photosensitive member 16 after the transfer. The remaining toner is removed by the cleaner 26. The recording paper 30 to which the toner image has been transferred is conveyed to a fixing device 32 and subjected to a fixing process, and then discharged to a discharge tray 34.

The image forming apparatus 10 includes a main control unit (hereinafter, referred to as “MCU”) 36. The MCU 36 allows the image forming section 14
The image forming operation (so-called printing operation) is controlled.

(Detailed Configuration of Optical Scanning Apparatus) Next, the schematic configuration of the optical scanning apparatus 18 will be described with reference to FIG.

The optical scanning device 18 includes a semiconductor laser (hereinafter, referred to as “LD”) 40 as a light source, a polygon mirror 42 that reflects a light beam output from the LD 40 and irradiates the photosensitive member 16 with a light beam. It has.

The LD 40 is an LD drive circuit (LDD: LD D
river) 44, and is lit by the LD drive circuit 44 with a predetermined output light amount to output a light beam.
The light beam output from the LD 40 is incident on the polygon mirror 42 via the cylindrical lens 46. The polygon mirror 42 is formed in a regular polygonal shape (a regular hexagon in the present embodiment) having a plurality of reflection surfaces 42A provided on the side surface, and an incident light beam is converged on the reflection surface 42A. ing.

The polygon mirror 42 is pivotally mounted on a scanner motor 48, and is rotated at a predetermined rotational speed (predetermined speed) in the direction of arrow C by the driving of the scanner motor 48. By this rotation, the angle of incidence of the light beam on each reflecting surface 42A changes continuously and is deflected. As a result, the photosensitive member 16 is scanned in the axial direction (see arrow D, hereinafter referred to as “main scanning direction”), and the photosensitive member 16 is irradiated with a light beam.

In the traveling direction of the light beam reflected by the polygon mirror 42, the first lens 50A and the second lens 5
FB lens 50 is disposed. The light beam transmitted through the fθ lens 50 is output from the optical scanning device 18 and irradiated on the photoconductor 16. This fθ
The scanning speed when irradiating the light beam to the photoconductor 16 is made uniform by the lens 50, and an image point is formed on the peripheral surface of the photoconductor 16.

In the optical scanning device 18, a mirror 52 is disposed downstream of the fθ lens 50 in the traveling direction of the light beam and outside the image forming area on the upstream side in the scanning direction of the light beam. In the direction of reflection of the light beam by the mirror 52, an SOS sensor 54 (described in detail later) including a photodetector or the like is arranged as a scanning timing detecting unit.

Each time the photosensitive member 16 is scanned in the axial direction of the SOS sensor 54, a light beam outside the image forming area on the upstream side in the scanning direction is guided by the mirror 52 by the mirror 52.
That is, the SOS sensor 54 detects the timing at which the optical scanning device 18 starts scanning the photoconductor 16 for each scan, and outputs the result as an SOS signal.

(Detailed Configuration of SOS Sensor) Next, SOS
The detailed configuration of the sensor 54 will be described.

As shown in FIG. 3, the SOS sensor 54
A scanning light beam reflected by the mirror 52 is incident thereon, and a light detection unit 60 that converts the incident light beam into an electric signal and a circuit that generates an SOS signal based on the electric signal from the light detection unit 60 are provided. Provided substrate part 62
And a wire 64 for electrically connecting the light detection unit 60 and the substrate unit 62. The light detection unit 60 is covered with a box-shaped casing 66 as shown in FIG.

As shown in FIG. 3, the light detecting section 60 includes two PDs (photodiodes) 6 as light receiving elements.
8A and 68B. In addition, this PD68
A, 68B, with the light receiving surface facing the incident direction of the light beam,
In addition, they are arranged side by side in the main scanning direction. PD68A,
68B respectively output a photocurrent according to the amount of incident light.

As shown in FIG. 5, the outputs of the PDs 68A and 68B are output from a current / voltage converter (I / V)
70A, 70B, and PD68A, 6
8B is output from the current / voltage converter 70.
A and 70B respectively perform voltage conversion. Outputs of the current / voltage converters 70A and 70B are supplied to amplifiers 72A and 72A.
B. Current / voltage converters 70A, 7
0B, the signal Va converted from the voltage of the photocurrent and output.
1, Vb1 are input to amplifiers 72A and 72B, respectively.
It is amplified with a predetermined gain (amplification rate). That is, the photoelectric conversion output of the incident light beam is amplified by the amplifiers 72A and 72B.

The gains of the amplifiers 72A and 72B can be switched in a plurality of stages, and are switched by an external gain switching signal. In the present embodiment, as an example, a case will be described in which switching is possible between two stages of high gain and low gain. Hereinafter, the gain of the amplifiers 72A and 72B is referred to as the gain of the SOS sensor 54.

The signal Va2 amplified and output by the amplifier 72A is input to the clamp circuit 74 so that the signal level when the light beam is not input to the SOS sensor becomes a predetermined level (clamp level). Clamped. The signal Va3 clamped and output by the clamp circuit 74 is input to the comparator 76. On the other hand, the signal Vb2 amplified and output by the amplifier 72B is directly input to the comparator 76.

The comparator 76 compares the signal Va3 from the clamp circuit 74 with the signal Vb2 from the amplifier 72B, and outputs a logical level signal based on the comparison result as an SOS signal.

More specifically, the signal V from the clamp circuit 74
The level of the logical level signal is switched at the cross point between a3 and the signal Vb2 from the amplifier 72B. When the signal Va3 becomes lower than the signal Vb2, the output voltage of the comparator 76 becomes L level. Become
When the potential becomes high, the output voltage of the comparator 76 becomes H
Become a level. Thereby, the position of the scanning light beam is PD
At the time of the transition from 68A to PD 68B, an SOS signal which is a falling edge is generated.

The clamp level of the clamp circuit 74 is higher than the expected noise level (noise caused by stray light or the like), and a cross point between the signal Va3 and the signal Vb2 can be obtained even when the signal is applied to the PDs 68A and 68B. It is set as follows.

(Peripheral Configuration of MCU) Next, the peripheral configuration of the MCU 36 will be described with reference to FIG.

As shown in FIG. 6, the MCU 36 is connected to the LD drive circuit 44 and the SOS sensor 54.
The MCU 36 outputs to the LD drive circuit 44 a light amount control signal for controlling the output light amount of the LD 40 for each scanning line and an image signal for forming an image.

When the execution of the light amount control is instructed by the light amount control signal from the MCU 36, the LD drive circuit 44 turns on the LD 40, and turns on the LD 40 for the lighting.
The drive light value supplied to the LD 40 is adjusted, and the light amount is controlled so that the output light amount of the LD 40 becomes a predetermined light amount. Also,
When the lighting is instructed by the image signal from the MCU 36, the LD 40 is turned on with the output light amount after the light amount control, and
A light beam based on the image data is output from D40.

The MCU 36 includes an SOS sensor 54.
The SOS signal output from is input. The MCU 36 counts the elapsed time from the output of the SOS signal from the SOS sensor 54 by a counter in the MCU 36, and outputs an image signal to the laser drive circuit 44 when a predetermined time has elapsed. In other words, the MCU 36 sends the image signal to the laser drive circuit 44 in synchronization with the SOS signal, so that the LD drive circuit 4
4 starts lighting based on the image data of the LD 40 to control the writing position of the image, and corresponds to an image writing timing control means.

The MCU 36 outputs a signal from the SOS sensor 54 to the SO
The elapsed time from the output of the S signal is counted by a counter in the MCU 36, and the light amount control signal is turned ON only for a predetermined period during which the output of the next SOS signal is expected. Specifically, in the present embodiment, the light amount control signal of the L level is output during the period when the output of the SOS signal is predicted, and the H level is output during the other periods. In the circuit 44, the LD 40 is turned on for controlling the light amount and acquiring the SOS signal.
That is, the MCU 36 sets the SO
Lighting for acquiring the S signal, that is, lighting for detecting the scanning timing of the light beam, is controlled, and corresponds to lighting control means for scanning timing detection.

The light amount adjustment signal output from the MUC 36 is inverted and input to the SOS sensor 54 as a gain switching signal. That is, the MCU 36
By outputting the light amount adjustment signal, the gain of the SOS sensor 54 is switched between a period in which the output of the SOS signal is predicted from the SOS sensor 54 and a period other than the period, and the sensitivity of the SOS sensor 54 is changed and controlled. Therefore MCU36
Corresponds to the detection sensitivity change control means (more specifically, the detection sensitivity change control means of claim 4).

More specifically, the SOS sensor 54 switches the gain to a high gain when the gain switching signal is at the H level and to a low gain when the gain switching signal is at the L level. As described above, by switching the gain of the amplifiers 72A and 72B to high / low gain by the gain switching signal, the detection sensitivity (hereinafter, simply referred to as “sensitivity”) of the SOS sensor 54 can be switched to high / low sensitivity. .

(Operation) Next, the operation of the first embodiment will be described with reference to FIG. FIG. 7 shows a timing chart of various signals in the MCU 36.

As shown in FIG. 7, in the MCU 36, the light intensity control signal is set to the L level before the SOS signal is output, and the light intensity control signal is set to the H level when the SOS signal is output, for the light intensity control and the acquisition of the SOS signal. Return to level.

In response to the light quantity control signal, the LD drive circuit 44 turns on the LD 40 before the SOS signal is output, and supplies a drive current supplied to the LD 40 so that the output light quantity of the LD 40 becomes a predetermined light quantity. Adjust the value (light intensity control). In addition, the lighting at this time causes the SOS sensor 5
When the light beam is incident on 4, a light beam is output from the SOS sensor 54. When the SOS signal is output, LD4
0 is turned off.

The light quantity control signal is inverted and input to the SOS sensor 54 as a gain switching signal. This allows
In the SOS sensor 54, when the light quantity control signal is L level and LD
Only when the light amount control is performed, the H level gain switching signal is supplied, and the gain of the SOS sensor 54 is switched to the high gain. When the SOS signal is output, the gain switching signal becomes L level, and the gain of the SOS sensor 54 is switched to a low gain.

That is, the SOS sensor 54
The sensitivity is set to be high only during a period from before the light beam enters the S sensor 54 until the light beam enters and the SOS signal is output. When the SOS signal is output, the sensitivity is returned to low.

The MCU 36 outputs an image signal to the LD drive circuit 44 after a lapse of a predetermined time α from the output of the SOS signal. Then, before the output of the next SOS signal after a lapse of a predetermined time β from the output of the SOS signal, the light intensity control signal is set to L level again, and the light intensity control and the SOS
A signal is acquired (the LD 40 is turned on), and the gain of the SOS sensor 54 is switched to a high gain.
Set 4 to high sensitivity.

Next, a comparison with the prior art will be made with reference to FIG.
In FIG. 8, for example, a light beam reflected inside the SOS sensor 54 is applied to a light detection unit 60 (PD 68) of the SOS sensor 54.
A, 68B) (see FIG. 4).
Comparator 76 when stray light occurs after S signal output
5 shows input / output signals for the input device.

If the gain of the SOS sensor, that is, the sensitivity of the SOS sensor is kept constant as in the prior art, FIG.
As shown in (A), the signal Va3 and the signal V
A cross point occurs at b2. This allows
After the output of the normal SOS signal, a non-normal SOS signal is also output, and the SOS sensor 54 malfunctions.

On the other hand, as in the first embodiment, the SOS sensor 54 is set to a high gain only during the light amount control from before the SOS signal is output until the SOS signal is output, and the SOS signal output is performed. Thereafter, by controlling the SOS sensor 54 to return to a low gain, the photoelectric conversion output due to stray light generated after the output of the SOS signal is amplified at a low gain. As a result, as shown in FIG. 8B, the amount of stray light component in the signal Va3 becomes very small, and it is possible to prevent a cross point from occurring between the signal Va3 and the signal Vb2.

That is, only during the light amount control from before the SOS signal is output until the SOS signal is output, the SO
By switching the S sensor 54 to a high gain to make the SOS sensor 54 high sensitivity, and by switching the SOS sensor 54 to a low gain after outputting the SOS signal to make the SOS sensor 54 low sensitivity, stray light is generated after the SOS signal is output. Even, SOS
The sensor 54 can be operated stably.

In the first embodiment, the case where the malfunction of the SOS sensor due to the stray light after the output of the SOS signal is prevented has been described, but the present invention is not limited to this.

When the light amount control of the LD 40 is performed for each scanning line, a predetermined time is required for controlling the light amount for each scanning of the light beam. When the scanning speed of the light beam increases, the physical distance when the LD 40 is turned on from before the timing at which the light beam enters the SOS sensor 54 becomes longer than the timing at which the light beam enters the SOS sensor 54, so that the light beam enters the SOS sensor 54, That is, stray light is likely to occur before outputting the normal SOS signal. The present invention may be applied to prevent the malfunction of the SOS sensor due to the stray light before the output of the SOS signal.

[Second Embodiment] Next, as a second embodiment, a case will be described in which a malfunction of an SOS sensor due to stray light before outputting an SOS signal is prevented. The first
The same parts as in the first embodiment are denoted by the same reference numerals as in the first embodiment, and the detailed description is omitted here.

FIG. 9 shows an MC according to the second embodiment.
The peripheral configuration of U36 is shown. As shown in FIG.
In the second embodiment, the switch 80 and the delay circuit 8
2. A NOR circuit 84 is provided to constitute a circuit for delaying the L level time of the light quantity control signal.

Specifically, the light amount control signal output from the MCU 36 is directly input to one input side of the NOR circuit 84, and the light amount control signal is directly input to the other input side by switching the switch 80, or Circuit 82
Is entered via

The switching of the switch 80 is performed based on a switch switching signal from the MCU 36. Specifically,
In the present embodiment, when the switch switching signal from the MCU 36 is at the H level, the light amount control signal is directly input,
When the signal is at the L level, the selection of the switch 80 is switched so that the signal is input via the delay circuit 82.

The NOR circuit 84 performs a NOR operation on two signals input from both input sides, and supplies the result to the SOS sensor 54 as a gain switching signal. That is, by switching the switch in response to the switch switching signal from the MCU 36, the gain switching signal for switching the SOS sensor 54 to a high gain is switched in synchronization with the light amount control signal or is delayed. I have.

When the switch switching signal is at the H level, that is, when the light amount control signal is
When the input to the R circuit 84 is selected, the gain switching signal is output in synchronization with the light amount control signal, so that the operation is the same as that of the first embodiment, and the description is omitted here. I do.

When the switch switching signal is at the L level and the switch 80 selects the input of the light quantity control signal to the NOR circuit 84 via the delay circuit 82, the gain switching signal is delayed from the light quantity control signal. Output. FIG. 10 shows a timing chart of the light amount control signal (see S1 in FIG. 9), the output signal of the Delay circuit 82 (see S2 in FIG. 9), and the gain switching signal (see S3 in FIG. 9) in this case. I have.

The delay circuit 82 generates and outputs a signal obtained by delaying the light quantity control signal by a predetermined time γ. N
In the OR circuit 84, the light amount control signal and the N of the delayed signal
An OR operation is performed to generate a gain switching signal that sets the H level from the fall of the delayed signal to the rise of the light amount control signal.

Using such a gain switching signal, the SOS
By controlling the gain of the sensor 54, the timing of increasing the gain of the SOS sensor 54
The high gain period can be shortened compared to the embodiment. That is, the MCU 36, the switch 80, the delay circuit 82, and the NOR circuit 84 constitute a detection sensitivity change control unit (more specifically, a detection sensitivity change control unit according to claim 7).

The delay amount (shortening amount) is determined by the delay circuit 8
2 can be easily changed by controlling the delay time. Further, a general delay circuit may be used for delaying.

As described above, by delaying the gain switching signal from the light amount control signal, as shown in FIG.
With a delay of a predetermined time γ after the light intensity control signal output from the L level becomes L level, the gain switching signal switches to H level, and the SOS sensor 54 is set to a high gain.
When the SOS signal is output, the light amount control signal returns to the H level, and accordingly, the gain switching signal also returns to the L level, and the SOS sensor 54 returns to the low gain.

That is, during the light quantity control period, the SO
The SOS sensor 54 has high sensitivity only during the period from immediately before the output of the S signal to the output of the SOS signal, so that the SOS sensor 54 is always high during the light amount control period. Also, the high sensitivity period is shortened.

Next, a comparison with the first embodiment will be made with reference to FIG. FIG. 12 shows input / output signals to the comparator 76 when stray light is generated before the SOS signal is output after turning on the LD 40 for light amount control.

If the SOS sensor 54 is always set to a high gain during the light amount control period as in the first embodiment, as shown in FIG. 12A, stray light is detected by the SOS sensor 54 during the light amount control. When the light enters the unit 60, the stray light causes a cross point between the signal Va3 and the signal Vb2. For this reason, before the output of the normal SOS signal, the non-normal SOS signal is also output.
4 malfunctions.

On the other hand, as in the second embodiment, the SOS sensor 54 is set to a high gain only immediately before the SOS signal is output until the SOS signal is output, and
After the output of the S signal, the SOS sensor 54 is controlled to return to the low gain, so that the photoelectric conversion output due to the stray light generated before the output of the SOS signal during the light amount control period is amplified with a low gain.

As a result, as shown in FIG. 12B, the amount of stray light component in the signal Va3 becomes very small, and it is possible to prevent the occurrence of a cross point between the signal Va3 and the signal Vb2. That is, the SOS sensor 54 can operate stably even if stray light occurs before outputting the SOS signal.

After the SOS signal is output, S
Since the OS sensor 54 is returned to the low gain, the SOS sensor 54 can operate stably even if stray light occurs after the output of the SOS signal, as in the first embodiment.

Since the light quantity control signal and the gain switching signal are switched to the H / L level depending on the elapsed time from the output of the SOS signal, the scanning speed of the light beam is determined by the rotation error of the scanner motor 48 rotating the polygon mirror 42. During the period in which the SOS sensor 54 is set to a high gain by the gain switching signal, the SOS sensor 54
When the main light beam scans the value on the low gain side of the SOS sensor 54, that is, the sensitivity on the low sensitivity side of the SOS sensor 54, it operates normally,
It is preferable to set a value that does not operate when stray light is incident. Accordingly, when the LD 40 is turned on at the timing of the light amount control signal, even if the speed of scanning the light beam varies due to a rotation error of the scanner motor 48,
The main light beam does not jump over the SOS sensor 54.

Next, the switching signal control by the MCU 36, that is, the switching timing of the switch 80 will be described. The switching timing of the switch 80 is determined by the LD 40
Is determined by the light amount value to be controlled. FIG.
4 shows an example of a gain switching range of the OS sensor 54. In addition,
In FIG. 13, the vertical axis represents the output light amount of the LD 40 (the output light amount from the optical scanning device 18), and the horizontal axis represents the set value used by the MCU 36 to set the output light amount of the LD 40.

Here, when a normal SOS sensor is used, there is an influence of stray light or the like, so that 3 to 10 nJ / m
The incident light amount range can be only up to about m ² .

The MCU 36 has a normal 3 to 10 nJ / mm.
^In the case of the light amount range up to ² , the switch switching signal is set to the H level, and the light amount control signal
The input to the circuit 84 is selected. As a result, the gain switching signal is output in synchronization with the light amount control signal, and S is synchronized with the light amount control signal in the same manner as in the first embodiment.
The gain of the OS sensor 54 is switched, and the sensitivity of the SOS sensor is switched.

A light amount range of 10 to 12 nJ / mm which is higher than usual
^In the case of ² , since stray light is likely to occur, the MCU 36
Sets the switch switching signal to the L level, and causes the switch 80 to select the input of the light amount control signal to the NOR circuit 84 via the Delay circuit 82. As a result, the gain switching signal is output after being delayed from the light amount control signal, and by controlling the high gain period of the SOS sensor 54, that is, the high sensitivity period, to be short, the influence of stray light can be prevented.

Incidentally, regardless of the light amount of the light beam,
The gain switching signal may be output after being delayed from the light amount control signal, and the high gain period of the SOS sensor 54, that is, the high sensitivity period may be controlled to be short (corresponding to claim 5 of the present invention). In this case, the switch 80 can be omitted.

As described above, in the first and second embodiments, the gain of the SOS sensor 54 is switched at a timing synchronized with the SOS signal, so that the SOS signal is obtained and the other times. The sensitivity of the SOS sensor 54 can be changed. Further, by making the SOS sensor 54 highly sensitive only when acquiring the SOS signal, malfunctions due to stray light entering the SOS sensor 54 can be prevented, and a stable SOS signal can be obtained. Stable operation).

By changing the gain of the amplifiers 72A and 72B, that is, the amplification factor of the photoelectric conversion output,
Since the sensitivity of the SOS sensor 54 is changed to prevent malfunction due to stray light, for example, it is not necessary to provide a wall or the like near the SOS sensor 54 to shield stray light. Further, since a complicated gate circuit is not required, it can be realized at low cost.

In the first and second embodiments,
The case where the gain of the SOS sensor 54 is switched for each scanning of the light beam to switch the sensitivity of the SOS sensor 54 has been described as an example, but the present invention is not limited to this.
For example, the light amount range is normal (3 to 10 n in the example of FIG. 13).
J / mm ² ), the sensitivity of the SOS sensor 54 is not switched, and the light amount range (usually 10 to 10 in the example of FIG.
Only in the case of 12 nJ / mm ² ), the sensitivity switching of the SOS sensor 54 may be performed (corresponding to claim 6 of the present invention).

In the first and second embodiments,
The gain of the SOS sensor 54, that is, the SOS sensor 54
Has been described as an example in which the sensitivity of the image is switched between two stages.
The present invention is not limited to this.

For example, the sensitivity of the SOS sensor 54 is determined by changing the light beam from the light detection unit 6 after the light amount control signal becomes L level.
It is also possible to switch between a period until scanning over zero, a period when the light beam scans on the light detection unit 60 of the SOS sensor 54, and several other periods, thereby preventing malfunction due to stray light. The effect can be obtained. Also,
The sensitivity may be changed continuously.

[0090]

As described above, the present invention has an excellent effect that the sensor for detecting the scanning timing of the light beam can be operated stably.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an image forming apparatus.

FIG. 2 is a detailed configuration diagram of an optical scanning device.

FIG. 3 is a diagram showing a top view of an SOS sensor and an arrangement of light beams.

FIG. 4 is a cross-sectional view of the SOS sensor.

FIG. 5 is a circuit diagram of the SOS sensor.

FIG. 6 is a peripheral configuration diagram of an MCU according to the first embodiment.

FIG. 7 is a timing chart for explaining an MCU control process according to the first embodiment.

8A is a conventional SOS sensor, and FIG. 8B is a first SOS sensor.
FIG. 9 is a diagram showing input / output signals of a comparator of the SOS sensor according to the embodiment.

FIG. 9 is a peripheral configuration diagram of an MCU according to the second embodiment.

FIG. 10 is a timing chart for explaining generation of a gain switching signal according to the second embodiment.

FIG. 11 is a timing chart for explaining an MCU control process according to the second embodiment.

FIG. 12A is an SOS according to the first embodiment.
FIG. 7B is a diagram illustrating input / output signals of a comparator of the SOS sensor according to the second embodiment.

FIG. 13 is a diagram for explaining switch switching by a switch switching signal according to the second embodiment.

[Explanation of symbols]

 10 Image Forming Apparatus 16 Photoconductor 18 Optical Scanning Device 36 Main Control Unit 40 Semiconductor Laser 44 LD Drive Circuit 54 SOS Sensor 60 Photodetector 68A, 68B Photodiode 72A, 72B Amplifier 74 Clamp Circuit 76 Comparator 80 Switch 82 Delay Circuit 84 NOR circuit

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2C362 AA54 AA56 BB29 BB30 BB31 BB32 BB33 BB37 2H045 AA01 CA88 CA98 CB63 2H076 AB05 AB12 AB16 AB18 AB32 AB67 DA17 5C072 AA03 BA11 HA02 HA13 HB01 HB08 HB11 UA05 X

Claims (8)

[Claims] 1. An image forming apparatus for forming an image by scanning a light beam output from a light source and irradiating the photosensitive member with the light beam, the image forming apparatus being arranged at a position where the light beam is incident, and a scanning timing of the light beam And a scanning timing detecting means having a function of changing the detection sensitivity, and controlling the function of changing the detection sensitivity to detect the scanning timing and the detection sensitivity at other times. An image forming apparatus comprising: a detection sensitivity change control unit that changes the detection sensitivity. 2. The image forming apparatus according to claim 1, wherein the detection sensitivity change control unit sets the detection sensitivity higher when detecting the scanning timing than at other times. 3. An image forming apparatus which scans a light beam output from a light source and irradiates a photosensitive member to form an image, wherein the image forming apparatus is arranged at a position where the light beam is incident, and scans the light beam. Scanning timing detection means having a function of changing detection sensitivity, and an image writing timing control means for controlling writing timing of an image based on the scanning timing detected by the scanning timing detection means. The lighting of the light source for detecting the scanning timing is terminated based on the scanning timing detected by the scanning timing detecting means, and after a lapse of a predetermined time from the detected scanning timing, the scanning timing in the next scanning line Scanning timing detection lighting control means for starting lighting of the light source for detecting the light source; Based on the lighting control by the timing detection lighting control unit controls the function of changing the detection sensitivity,
An image forming apparatus comprising: a detection sensitivity change control unit that changes the detection sensitivity. 4. The detection sensitivity change control unit sets the detection sensitivity to be higher during a period in which the light source is turned on by the scanning timing detection lighting control unit than in a period other than the period. Item 4. The image forming apparatus according to Item 3. 5. The detection sensitivity change control unit is configured to delay the lighting of the light source by the scanning timing detection lighting control unit and to end the lighting of the light source by the scanning timing detection lighting control unit. 4. The image forming apparatus according to claim 3, wherein the detection sensitivity is set higher during a period until the detection is performed than during the period other than the period. 6. The detection sensitivity change control unit changes the detection sensitivity only when the amount of light incident on the scanning timing detection unit is equal to or more than a predetermined value. The image forming apparatus according to claim 1. 7. The detection sensitivity change control unit, when the amount of light incident on the scanning timing detection unit is less than a predetermined value, sets a period during which the light source is turned on by the scanning timing detection lighting control unit to the period. If the amount of incident light on the scanning timing detecting means is equal to or greater than a predetermined value, the detection timing is delayed from the start of lighting of the light source by the scanning timing detecting lighting control means, and the scanning timing is increased. 4. The image forming apparatus according to claim 3, wherein the detection sensitivity is set higher during a period until the lighting of the light source by the detection lighting control unit is finished than during the period other than the period. 5. 8. The apparatus according to claim 1, wherein the function of changing the detection sensitivity changes an amplification factor of a photoelectric conversion output of a light beam incident on the scanning timing detecting means. 2. The image forming apparatus according to claim 1.