JPH04149482A - Image formation device - Google Patents

Abstract

PURPOSE:To accurately defect the intensity of laser beam and to stably drive a laser by detecting the intensity of the laser beam and holding it in a 1st and a 2nd holding means, and detecting the difference between both their outputs and controlling the driving current of the laser. CONSTITUTION:Th image formation device is equipped with a beam intensity detecting means 2 which detects the intensity of the laser beam, the 1st holding means 4 and 2nd holding means 5 and 6 which hold the output of the beam intensity detecting means 2, a differential amplifying means 7 which detects the difference between the outputs of the holding means 4 and 6, and a control means 9 which controls the driving current of the laser with the output of the differential amplifying means 7. The differential amplifying means 7 detects the difference between the outputs of the holding means 4 and 6 holding the detected value of the intensity of the laser beam and the driving current of the laser is controlled according to the output of the differential amplifying means 7. Consequently, the intensity of the laser beam is accurately detected to stably drive the laser.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to an image forming apparatus that scans a photoreceptor with laser light and forms an image using an electrophotographic process.

[Conventional technology]

Conventionally, image forming apparatuses widely known as LBPs (laser beam printers) scan laser light onto a photoreceptor to form a latent image, which is then visualized using a developer to obtain an image. . In such a device, when the amount of laser light changes due to the influence of ambient temperature, the formed pixels become thicker or thinner, resulting in a change in final image density. In order to reduce this impact,
That is, in order to stabilize the laser beam output against temperature changes, a heater or a Peltier element has been used to maintain the ambient temperature of the laser generating element constant. In addition, the laser beam output is detected by an optical output detection circuit near the laser, and this output signal is fed back to the laser drive circuit to control the laser beam intensity so that the laser output is always equal to the set value. , some have automatic power control (APC). This APC includes a device that detects the laser beam output between printed recording sheets using an optical output detection circuit, and controls the intensity of the laser beam using software.
Some devices detect the laser beam output once per horizontal scan using an optical output detection circuit, and control the intensity of the laser beam using hardware.

[Problem to be solved by the invention]

However, the above conventional example has the following problems. First, in the former case, by controlling the temperature of the laser generating element, the output of the laser generating element itself can be stabilized. However, because of the temperature characteristics of the laser drive circuit and peripheral circuits, even if the ambient temperature of the laser generation element is constant, the laser drive current will fluctuate depending on the temperature characteristics of the peripheral circuit that drives the laser generation element. , the laser output cannot be stabilized. In the latter case, by installing an optical output detection circuit near the laser generating element, the laser beam output is detected and this output signal is fed back to the laser drive circuit.
Due to changes in the dark current due to temperature changes in the photodiode connected to the optical output detection circuit, the standard of the laser beam output changes, making it impossible to perform normal detection.
There was a problem that the laser output could not be stabilized. The present invention has been made in view of the above conventional example, and an object of the present invention is to provide an image forming apparatus that can accurately detect the intensity of laser light and drive the laser stably.

[Means to solve the problem]

In order to achieve the above object, the image forming apparatus of the present invention has the following configuration. That is, it is an image forming apparatus that scans a photoreceptor with a laser beam and forms an image using an electrophotographic process, and includes a beam intensity detection means for detecting the intensity of the laser beam, and an output of the beam intensity detection means. a first holding means and a second holding means for holding, a differential amplifying means for detecting the difference between the outputs of the first holding means and the second holding means, and an output of the differential amplifying means. and control means for controlling the driving current of the laser.

[For use]

In the above configuration, the detected value of the intensity of the laser beam is
and held by the second holding means. The difference between the outputs of the first holding means and the second holding means is detected by the differential amplifying means, and the output of the differential amplifying means operates to control the driving current of the laser.

【Example】

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Description of Laser Beam Printer (Fig. 1)> Fig. 1 is a block diagram showing a schematic configuration of a laser beam printer according to an embodiment. In the figure, 101 is a data input unit that performs interface control with the host computer, which inputs image data from an external device such as the host computer, and inputs image data to the control unit 101.
It is output to 2. When the control unit 102 receives this image data, for example, in the case of code data, it develops it into image data and stores it in an image memory provided in a RAM. Reference numeral 105 is a recording signal output according to the image data of the image memory, and a semiconductor laser of the laser control circuit 103 is driven according to this recording signal 105. 10
3 is a laser control circuit whose details are shown in FIG. 2, which detects the intensity of the laser light output from the semiconductor laser and controls the driving current of the semiconductor laser. 104 is the recording signal 10
This is a recording section that prints on recording paper using a laser beam modulated according to No. 5. Next, the configuration of the recording unit 104 will be explained. 110 is a polygon mirror, 111 is a photosensitive drum, and the polygon mirror 11 is a photosensitive drum.
An electrostatic latent image is formed on the surface of the laser beam reflected by the laser beam. This electrostatic latent image is developed by a developing device (not shown), and is transferred to recording paper by a fixing device and printed. 112 is a motor for rotationally driving the polygon mirror 110, and 113 is a motor for rotationally driving the photosensitive drum 111. Reference numeral 114 denotes a photosensor for detecting laser light, and a signal detected by this photosensor 114 is input to the control unit 102 as a beam detect (BD) signal and processed as a horizontal synchronization signal. <Laser Control Circuit (FIGS. 2 and 3)> FIG. 2 is a block diagram of the laser control circuit of this embodiment. In FIG. 2, 1 is a semiconductor laser, and 2 is a photodiode for detecting the intensity of laser light output from the semiconductor laser 1. In FIG. This photodiode 2 has a pulse width of 30 (
(Fig. 3), and the scanning light outside the recording area by the laser beam of the laser diode 1 is detected in accordance with the recording signal 105 of pulse width 31 (Fig. 3) at which the operating current of the light emitting region of the laser 1 reaches a predetermined value. are doing. Then, this detected signal is output as a beam intensity monitor current (I,). The monitor current (1,) is converted to a voltage (1,) by the current-voltage converter 3.
■2) Converted into a signal. At this time, a voltage value corresponding to the pulse width 31 is sampled and held by the sample and hold circuit 4 in synchronization with the gate signal 21 for l horizontal scanning period. This held signal is input to the non-inverting input terminal of the differential amplifier 7. Further, the monitor voltage M (corresponding to the photodiode dark current) corresponding to the pulse width 30 is sampled and held by the sample and hold circuit 5 in synchronization with the gate signal 22 for one horizontal scanning period. This held voltage is input to the sample and hold circuit 6, and then inputted to the inverting input terminal of the differential amplifier 7 with timing with the sample and hold circuit 4 corresponding to the pulse width 31 determined by the gate signal 21. . Thereby, the output signal of the differential amplifier 7 becomes the subtraction value of the monitor voltage (predetermined amount of light) corresponding to the pulse width 31 and the monitor voltage (photodiode dark current) corresponding to the pulse width 30. This voltage value is applied to the constant current source 9, and the LED light emitting area current (bias current) of the laser diode 1 is
control is performed. FIG. 4 is a diagram showing the current-optical output temperature characteristics of the semiconductor laser 1. In FIG. 4, the horizontal axis is the drive current of the semiconductor laser 1,
The vertical axis indicates the optical output P. - When fishing on a boat, a semiconductor laser exhibits a temperature characteristic of optical output as shown by T1 when the temperature of the semiconductor laser is low and T2 when the temperature is high. That is, when the temperature of the semiconductor laser 1 is low, even a small drive current reaches the laser emitting region, but as the temperature of the semiconductor laser 1 becomes high (as the temperature of the semiconductor laser 1 becomes high), this threshold value increases and the The amount of light output with respect to the drive current decreases (the slope becomes smaller). In FIG. 2, the operation of the constant current source 9 will be explained using FIG. 4. When the temperature of the semiconductor laser 1 changes from T1 to Ti ("rz>T,)
When this happens, the current threshold for light emission from the semiconductor laser 1 is I
Changes from II to Engineering to □. In other words, the larger the output of the differential amplifier 7, the smaller the driving current of the semiconductor laser 1 is operated. Here, since the input of the differential amplifier 7 similarly includes the dark current of the photodiode, the output of the differential amplifier 7 has this dark current canceled out. In this way, the dark current of the photodiode is removed per horizontal scan, and a voltage corresponding to the beam intensity of the semiconductor laser 1 is detected. Thereby, the intensity of the laser beam of the laser 1 is controlled. The operation will be explained with reference to FIG. 2 again. A recording signal 105 (image signal) is input to the switching circuit 12, and the operating current of the light emitting region of the semiconductor laser 1 is turned on and off. This setting of the operating current is performed by inputting the output signal of the operating current setting circuit 10 to the constant current source 11. FIGS. 5A and 5B are diagrams for explaining the drive current flowing through the laser diode 1 through the series of control systems described above. FIG. 5(A) shows the state when the temperature is Tli, and FIG. 5(B) shows the state when the temperature is T2. When the temperature is as low as T1, when the recording signal is on with respect to the threshold current Is+, Ip+ is superimposed, and the recording signal moves to the laser drive region and emits a laser beam. On the other hand, when the temperature is as high as T2, the threshold current 111
When the recording signal is on for 2, the process is superimposed,
Laser light is emitted. However, in the case of this embodiment, the operating current Ip+=I, □. FIG. 3 is a time chart showing waveforms of each part of the laser control circuit 103 of this embodiment. In the figure, gate signals 21 and 22 are pulse signals output from the control section 102 in synchronization with the main scanning period. By this signal, the laser bias current controller is controlled in the direction of performing temperature correction for each main scan. At timing T1, the gate signal 22 causes the sample-and-hold circuit 5 to hold the voltage value ■2 corresponding to the pulse width 30, and at timing T2, the sample-and-hold circuit 4 holds the voltage value ■2 corresponding to the pulse width 31. At this time, the output voltage of the sample and hold circuit 5 is also set in the sample and hold circuit 6, so the output of the differential amplifier 7 becomes as shown in the figure, and at this output voltage level, the bias current from the constant current source 9 6 is determined. Furthermore, at timings T3 and T4, pulses @32 and 33
A voltage corresponding to is set in sample and hold circuits 4 and 6. Here, the voltage level set in the sample and hold circuit 4 is higher than the previous voltage level, and this increase is higher than the output voltage level of the sample and hold circuit 6, so the differential amplifier 7 The output signal level becomes high.As a result, the laser bias current 1 is suppressed as shown in the figure, and the laser output of the semiconductor laser 1 is controlled to be low.As is clear from the above description, this embodiment For example, changes in the dark current of the photodiode are canceled out per horizontal scan, and a voltage that truly corresponds to the laser beam intensity is detected,
Laser drive control can be performed. Other Embodiments (Fig. 6)> Fig. 6 is a block diagram of a laser control circuit showing a second embodiment of the present invention. Parts common to those in Fig. 2 described above are designated by the same numbers, and their parts are The explanation will be omitted. In the first embodiment described above, a case was explained in which the corrected output voltage of the differential amplifier 7 is fed back to the LED light emitting region current (bias current) of the laser diode l.
In this embodiment, the correction output of the differential amplifier 7 is fed back to the constant current source 11 to vary the operating current of the laser emitting region and control the laser beam intensity. Here, the constant current source 11a also serves as a constant current source for the threshold current generator and the operating current (2). Therefore, the constant current source 11a drives a drive current of 111 + I p+ when the temperature is T, and I $2 + I P2 (I s+ = I sz) when the temperature is T2, and as a result, the same as in the first embodiment is obtained. Similar control is performed. In FIG. 6, the laser beam intensity is detected according to the recording signal 105 of a pulse width 30 at which the operating current of the light emitting region of the laser 1 is the minimum, and a pulse width 31 at which the operating current of the laser light emitting region becomes a predetermined value, Then photodiode 2
The monitor voltage obtained by subtracting the dark current component is fed back to the constant current source 11a. Thereby, the operating current of the light emitting region of the laser I is made variable, and the driving of the semiconductor laser 1 is controlled. As described above, according to this embodiment, the intensity of the laser beam can be detected by canceling out the change in the dark current of the photodiode for detecting the light intensity of the laser beam. Thereby, by controlling the driving current of the laser, the light emission output of the laser can be kept stable.

【Effect of the invention】

As described above, according to the present invention, the intensity of laser light can be accurately detected and the laser can be stably driven.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing the schematic configuration of the laser beam printer of this embodiment, Fig. 2 is a block diagram of the laser control circuit of this embodiment, and Fig. 3 is the waveform of each signal of the laser control circuit of Fig. 2. 4 is a diagram for explaining the temperature characteristics of a laser diode, FIG. FIG. 2 is a block diagram of a laser control circuit showing a second embodiment. In the figure, 1... laser diode, 2... photodiode, 3... current-voltage converter, 4, 5.6...
Sample and hold circuit, 7... Differential amplifier, 8...
Bias setting circuit, 9, 11. , lla... constant current source, 10... operating current setting circuit, 12... switching circuit, 21.22... gate signal, 101... input section, 102... control section, 103... ...Laser control section, 104... Recording section, 105... Recording signal, 110
... Polygon Mira, 111 ... Photosensitive drum, 112
, 113... is a motor.

Claims (2)

[Claims] (1) An image forming apparatus that scans a photoreceptor with a laser beam to form an image using an electrophotographic process, comprising: a beam intensity detection means for detecting the intensity of the laser beam; and a beam intensity detection means for detecting the intensity of the laser beam. first holding means and second holding means for holding outputs; differential amplification means for detecting the difference between the outputs of the first holding means and the second holding means; and the output of the differential amplification means. An image forming apparatus comprising: a control means for controlling a laser drive current according to the following. (2) The image forming apparatus according to claim 1, wherein the control means controls a bias current of the laser.