JP2698099B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an image forming apparatus such as a laser printer and a digital copying machine.

[Related Art] In recent years, digital copiers that read an original by a line sensor or the like including a charge-coupled device and output the image data to a laser printer have become widespread. In this digital copying machine, the target documents include those having gradations such as photographs and pictures.
189574, JP-A-61-189575, JP-A-61-18
It is known from Japanese Patent No. 9577.

Conventionally, when an image is recorded by a laser printer using image data having such gradations, a gradation expression method in which gradation expression is performed using a teaser matrix method has been adopted. In this gradation expression method, for example, one pixel is constituted by a 4 × 4 matrix, and 4 × 4 = 16 gradations are expressed. If it is desired to obtain more gradations, this can be realized by increasing the size of the matrix. For example, in the case of expressing 64 gradations, an 8 × 8 matrix is used.

In a laser printer, a pulse width modulation method for controlling a laser lighting time for the purpose of expressing a laser beam in a multi-valued manner is also used.

However, in the above-described gradation expression method, it is necessary to increase the size of the matrix with an increase in the number of gradations, so that there is a disadvantage that pixels become rough and resolution is deteriorated. In addition, the pulse width modulation method can realize multi-valued expression of a laser beam.However, when the number of desired multi-values increases to 10 or more, the minimum pulse width must be reduced. To perform 10-value modulation at 100 ns, a minimum pulse width of about 10 ns is generally required. Such a short pulse requires sufficient consideration for the design of signal transmission, switching elements, and mounting, and may lead to a complicated device.

Therefore, an electrophotographic image forming apparatus that solves these problems has been proposed. FIG. 2 shows a digital copying machine as an example of the image forming apparatus, and FIG. 3 shows an image input apparatus in the example.

In the image input device, a document 11 is illuminated by a light source 12, and a reflected light image from a reading line 13 is formed on a light receiving surface of a line sensor 15 composed of a charge-coupled device (CCD) by a lens 14 and photoelectrically converted. This results in time-series image data. In this case, the document 11 is sub-scanned in the direction indicated by the arrow. The CCD reading unit 16 processes the image data output from the line sensor 15 and outputs it as an analog pixel signal. This signal is subjected to density conversion by an analog / logarithmic (LOG) conversion circuit 17. The image data from the analog / logarithmic conversion circuit 17 is converted by the analog / digital converter 18 into 6
The digital signal is converted into an 8-bit digital signal, and the shading correction circuit 19 performs shading correction.
The image data from the shading correction circuit 19 is γ-converted by the γ-conversion circuit 20, and the modulation circuit 21 modulates the semiconductor laser 22 composed of a laser diode with the image data from the γ-conversion circuit 20. This semiconductor laser 22 is used as a light source in a laser printer.

 FIG. 4 schematically shows the laser printer.

The modulated light emitted from the laser diode 22 is converted into parallel light by a collimator lens 23, and is repeatedly deflected by a deflector 25 driven to rotate by a motor 24.
The modulated light from the deflector 25 scans the photosensitive drum 27 via the condensing lens 26, and the photosensitive drum 27 is driven to rotate by a motor and is uniformly charged by the charger. As a result, an electrostatic latent image is formed. This electrostatic latent image is developed by a developing device and transferred to a transfer paper by a transfer device. The light from the condenser lens 26 is reflected by the mirror 28 outside the image area and detected by the photodetector 29, and the modulation of the laser diode 22 is started in synchronization with the output signal of the photodetector 29.

As the modulation circuit 21, a pulse width modulation method for modulating the lighting period of the laser diode 22, a power modulation method for modulating the light emission power (amplitude) of the laser diode 22, and the number of times of light emission of the laser diode 22 in one pixel are modulated. Although various methods such as a pulse number modulation method are conceivable, any modulation method may be used. In this example, the modulation circuit 21 is an example of a power modulation method as shown in FIG.

In this modulation circuit 21, the image data Video input from the γ modulation circuit 20 is multi-valued image data. In this example, the image data Video has a 4-bit configuration, and the least significant bit (LSB) is Vin, the most significant 3 bits are PDATA, and the most significant bit (MSB) is P2. In FIG. 9, reference numeral 30 denotes a switch circuit for switching the current of the laser diode 22, which comprises field effect transistors 31, 32 and resistors 33 to 35. Reference numeral 36 denotes a constant current circuit for setting a current when the laser diode 22 is turned on, and includes a transistor 37 and a resistor 38. Reference numeral 39 denotes a voltage generating circuit for determining a set current of the constant current circuit 36. The voltage generating circuit 39 includes a digital / analog converter 40, a buffer 41, and a resistor 42. The output signal Vs includes not only the image signal PDATA but also a power correction circuit described later. Is also controlled by the correction voltage CV according to the detected value of the light emission amount of the laser diode 22. This correction voltage CV
Is applied to the reference voltage terminal ref of the digital / analog converter 40 via the buffer 41. On the other hand, image data Video
The upper three bits PDATA are input to the data conversion circuit 43. This data conversion circuit 43 is a read only memory (RO
M), and a conversion table as shown in FIG. 7 is created in the ROM 43. The selection signal SEL is a signal for selecting one conversion table from a plurality of conversion tables, and the output of the ROM 43 is 8 bits. The image signal PDATA is assigned to the lower three bits of the address signal of the ROM 43, and the selection signal SEL is assigned to the upper bits of the address signal. First, a conversion table is selected by the selection signal SEL, and the image signal PDATA is converted by the conversion table and output from the ROM 43 as data IDATA for current setting. This data IDATA is applied to the digital input terminal of the digital / analog converter 40, and the digital / analog converter 40
It is converted to analog. The digital / analog converter 40 has 8 bits, and its output voltage Vs is given as follows.

As described above, Vs is a voltage determined by the correction voltage CV corresponding to the detected light emission amount of the laser diode 22 and the value IDATA corresponding to the image data, and is given to the constant current circuit 36. The collector current of the transistor 37 in the constant current circuit 36 becomes a current substantially proportional to the output voltage Vs of the digital / analog converter 40, and becomes a current load of the switch circuit 30. Also, image data Video input from the γ conversion circuit 20
Is applied to the gate of the field effect transistor 31 in the switch circuit 30 via the inverters 44 and 45. The switch circuit 30 is a switch circuit of a differential type, and operates such that one of the field effect transistors 31 and 32 is turned on in accordance with “1” and “0” of the input voltage Vin. For example, when the input voltage Vin is “1”, the field effect transistor 31 on the side of the laser diode 22 turns on and the laser diode 22
Is turned on, and the ON current value becomes the current value set by the constant current circuit 36 and the voltage generation circuit 39. That is, the correction voltage CV according to the light emission amount detection value of the laser diode 22,
The current value of the constant current circuit 36 determined by the image data PDATA is turned on / off in accordance with “1” and “0” of the image data Vin, and the drive current of the laser diode 22 is multi-valued. Will be.

The relationship between PDATA and IDATA is not a mere proportional relationship to such multi-valued conversion, and it is often the case that an optimal relationship is provided according to the characteristics of the entire system (such as a light emitting source system or an imaging system). If needed. Data conversion for that is ROM4
In this case, PDATA is 3 bits and IDATA is 3
When the bit is set, an error with respect to a desired data conversion characteristic increases, and a sufficient effect is not exhibited. Therefore, in this example, the output of the ROM 43 is set to 8 bits, as shown in FIG.
The expressible range of DATA was wide enough.

Furthermore a selection signal SEL to have assigned to the upper address A _H of ROM 43, can be selected according to the conversion table on the situation. In this example, the selection signal SEL is obtained by detecting the temperature of the laser diode 22. Generally, the laser diode 22 has a temperature characteristic, and as shown in FIG. 5, the emission power changes from a solid curve to a drive curve as shown by a dashed curve according to the temperature. In FIG. 5, Ith is a threshold current, and the laser diode 22 emits light only at a current equal to or higher than the threshold current Ith. Threshold current I
th has a particularly strong temperature dependency, and the slope of the curve also changes with temperature. It is extremely difficult to detect such a change and control the drive current of the laser diode 22 in an analog manner, which leads to a complicated circuit. In this example, the temperature detecting element 46 composed of a thermistor is connected to the laser diode 22.
Placed near the temperature sensor to detect its temperature.
The output Vt obtained by dividing the power supply voltage by the resistor 46 and the resistor 56 is subjected to analog / digital conversion by a 4-bit analog / digital converter 47 and supplied to the ROM 43 as a selection signal SEL. When the temperature rises, the resistance of the thermistor 46 decreases, so that Vt
Rise and SEL increases. Conversely, when the temperature drops, SEL
Becomes smaller. The analog / digital converter 47 does not need to be dedicated, but may be the one used in the overall control system including the microcomputer. In this case, since the temperature change of the laser diode 22 is relatively gradual, the selection signal SEL from the overall control system to the ROM 43 may be updated at fixed time intervals. As shown in FIG. 6, the relationship between PDATA and IDATA can be changed as a, b, and c according to the detected temperature value by the selection signal SEL. Although the ROM 43 is used, a random access memory may be used instead, and the contents of the conversion table may be rewritten each time according to a detection result such as temperature. In this case, the selection signal SEL can be omitted.

As the power correction circuit, for example, a semiconductor laser output control circuit as shown in FIG. 8 is used.

The light detector 48 receives a light beam emitted backward from the laser diode 22 and a current proportional to the intensity flows,
This current is converted into a voltage by the amplifier 49 and is compared by the comparator 50 with the reference voltage Vref. The output signal of the comparator 50 becomes high level or low level depending on the magnitude relation between the output signal of the amplifier 49 and the reference voltage Vref, and is supplied to the up / down counter 51 as an up / down signal. For example, when the intensity of the output light of the laser diode 22 is lower than the reference value, the output signal of the comparator 50 becomes low level, and the up / down counter 51 enters the up count mode. When the edge detection circuit 52 applies an enable signal to the up / down counter 51 according to the timing signal T, the up / down counter 51 is enabled and counts the clock from the oscillator 53, and the count value is converted to the digital / analog converter 54. Is applied to the reference voltage terminal ref of the digital / analog converter 40 via the buffer 41 as a correction voltage CV. The edge detection circuit 52 detects the rise and fall of the output signal of the comparator 50 and returns the up / down counter 51 to the disabled state.

The light-current characteristic of the laser diode 22 is as shown in FIG. 5, but changes from a solid line curve to a broken line curve due to temperature, aging, and the like. Thus power even by changing the above by the laser diode 22 is turned by the current I ₀ not emit light at a power of P ₀ fluctuates.
I ₁ from I ₀ current in response to environmental changes of the laser diode 22 to prevent this may be changed to I _2, to do this is the power correction circuit. Since such changes are generally very slow, the power correction circuit may have a very slow response. Emission power of the laser diode 22 will be modulated by the image signal determined by the power correction circuit (stabilized) level P ₀ as a reference. For example, the current of the laser diode 22 is 0
When performing four-level modulation from I to I _0, if the set value of each value is set to be proportional to I ₀ , even if I ₀ is controlled to I _{1 to} I ₂ by power correction by the power correction circuit, The set value is set accordingly, and the four-level gradation can be stably maintained.

In this example, the correction voltage CV according to the light emission amount detection value of the laser diode 22 is applied to the reference voltage terminal ref of the digital / analog converter 40 via the buffer 41.
V may be supplied to the ROM 43 as the selection signal SEL similarly to the output signal of the analog / digital converter 47. In this case, the selection signal is set according to the light emission amount detection value of the laser diode 22.
SEL is generated, and IDATA necessary for keeping the light emission amount of the laser diode 22 constant with respect to the selection signal SEL is written in the ROM 43. Then, an optimum conversion table in the ROM 43 is selected by a combination of the selection signal SEL according to the detected light emission amount of the laser diode 22 and the selection signal SEL from the analog / digital converter 47.

In the above example, a circuit for the bias current of the laser diode 22 (corresponding to Ith in FIG. 5) is not particularly necessary.
What is necessary is just to include the value in consideration of Ith in the conversion table. This has the effect of preventing heat generation and shortening of the life of the laser diode due to the fact that a bias current is always supplied to the laser diode.

[Problems to be solved by the invention]

In the above-described example of the digital copying machine, multi-level laser conversion is performed by power modulation. However, as the number of multi-level levels increases, the stability of each level is required and the circuit becomes more severe. Come.

The present invention solves the above-mentioned disadvantages, and can obtain many gradations with a simple configuration, and can perform excellent gradation expression with a simple configuration without deteriorating the resolution of an image. It is an object of the present invention to provide an image forming apparatus capable of greatly increasing the degree of freedom in setting a writing gradation according to image data and securing flexibility in a system.

[Means for solving the problem]

The present invention is directed to an electrophotographic image forming apparatus that drives a semiconductor laser by a drive signal to expose a photosensitive member with light emitted from the semiconductor laser to form an image, wherein a predetermined bit of image data of a plurality of bits is used. Pulse width modulation means for controlling the drive signal pulse width of the semiconductor laser based on the image data, and a laser for controlling the drive signal amplitude of the semiconductor laser based on the other bit image data of the plurality of bits of image data Power modulating means.

(Operation)

The pulse width modulation means controls the drive signal pulse width of the semiconductor laser based on predetermined bit image data of the plurality of bits of image data, and the laser power modulation means controls other bits of the plurality of bits of the image data. The drive signal amplitude of the semiconductor laser is controlled based on the image data.

In the embodiment of the present invention, a modulation circuit as shown in FIG. 1 is used as the modulation circuit 21 in an example of an electrophotographic image forming apparatus including the above-described digital copying machine. In this embodiment, the image data Video input from the gamma conversion circuit 20 is image data represented by 5 bits. This image data Video is lower 2 bits PW
Is input to the pulse width modulation circuit 55, and the 2-bit data PW
Is converted into a signal Vin having a pulse width corresponding to the pulse width, and is input to the switch circuit 30. The upper three bits of the image data Video are input to the data conversion circuit 43 to perform power modulation in the same manner as in the above-described example. In this way, eight kinds of powers (drive currents of the laser diode 22) are set by the PDATA for Vin having three kinds of pulse widths, so that the emission energy of the laser diode 22 is modulated to 24 kinds. be able to. By using pulse width modulation in this way, many gray scales can be obtained with a simple configuration.

In the above-described embodiment, the pulse width and the amplitude of the laser beam can be independently set for each dot, and the laser beam can be multi-valued according to the setting. As a result, halftone expression can be performed without deteriorating the resolution of an image as in the case of conventional grayscale expression using area grayscale. Also, since the relationship between the image data and the multi-level of the laser beam can be set freely,
It is possible to flexibly cope with the characteristics of the entire system, the characteristics of the laser, and the like, and obtain an optimum gradation characteristic.

As described above, in this embodiment, the semiconductor laser 22 is driven by the drive signal by the switch circuit 30 and the semiconductor laser 22 is driven.
In an electrophotographic image forming apparatus that forms an image by exposing the photoconductor 27 with light emitted from the semiconductor laser 22, a drive signal pulse of the semiconductor laser 22 is generated based on predetermined bit image data Vin of a plurality of bits of image data Video. A pulse width modulation circuit 55 as pulse width modulation means for controlling the width; and laser power modulation for controlling the drive signal amplitude of the semiconductor laser 22 based on the image data PDATA of another bit of the multi-bit image data Video. Since the data conversion circuit 43, the voltage generation circuit 39, and the constant current circuit 36 are provided as means, many gradations can be obtained with a simple configuration, and further, the resolution of an image can be degraded with a simple configuration. It is possible to perform excellent gradation expression without any problems and greatly expand the degree of freedom of setting the writing gradation according to the image data, and secure the flexibility for the system. Theft is possible.

(Effect)

As described above, according to the present invention, in an electrophotographic image forming apparatus that drives a semiconductor laser by a drive signal and exposes a photoconductor with light emitted from the semiconductor laser to form an image, Pulse width modulation means for controlling a drive signal pulse width of the semiconductor laser based on predetermined-bit image data of the data; and driving of the semiconductor laser based on another bit of the plurality of bits of image data. Since laser power modulation means for controlling signal amplitude is provided, many gradations can be obtained with a simple configuration, and excellent gradation expression can be achieved with a simple configuration without deteriorating image resolution. In addition, the degree of freedom in setting the write gradation according to the image data can be greatly expanded, and the flexibility for the system can be secured.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a modulation circuit according to an embodiment of the present invention, FIG. 2 is a block diagram showing the same example, FIG. 3 is a perspective view showing an image input device of the same example, and FIG. FIG. 5 is a characteristic diagram showing a light-current characteristic of a laser diode, FIG. 6 is a characteristic diagram for explaining the above example, and FIG. 7 is made in a ROM in the above example. FIG. 8 is a block diagram showing a power correction circuit of the above example, and FIG. 9 is a block diagram showing a modulation circuit in an example of a digital copying machine. 22 ... Laser diode, 30 ... Switch circuit, 36 ...
Constant current circuit, 39… Voltage generation circuit, 43… ROM, 46…
Temperature detection element, 47 …… Analog / digital converter, 55
…… Pulse width modulation circuit.

Claims (1)

(57) [Claims] An electrophotographic image forming apparatus for driving a semiconductor laser according to a drive signal and exposing a photosensitive member with light emitted from the semiconductor laser to form an image, wherein a predetermined number of image data of a plurality of bits is provided. Pulse width modulation means for controlling a drive signal pulse width of the semiconductor laser based on bit image data, and controlling a drive signal amplitude of the semiconductor laser based on image data of another bit of the plurality of bits of image data An image forming apparatus comprising: a laser power modulation unit.