Image forming apparatus, optical writing device, and controlling method thereof

An image forming apparatus is disclosed that is capable of making it simple to initialize a laser system thereof. The image forming apparatus includes a first photo detector that detects a part of a laser beam from each of the lasers and generates respective power adjustment signals for the lasers, a second photo detector that detects another part of the scanning laser beam of each of the lasers and generates a synchronization signal corresponding to each laser, and a power adjustment control unit that changes the output power of each of the lasers to a predetermined value. During the adjustment of output powers of the lasers, the power of a laser is monitored by using the synchronization signal. The power adjustment control unit turns on a laser for power adjustment, and turns off the laser when the scanning synchronization signal is detected twice to complete the power adjustment of the laser, and then starts power adjustment of the next laser.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical writing device that includes a plurality of semiconductor lasers and is capable of adjusting amounts of the laser beams by using a photo sensing element, a controlling method thereof, and an image forming apparatus including the optical writing device.

2. Description of the Related Art

In the related art, a conventional image forming apparatus equipped with an optical writing device has a laser chip including a light emitting element and a photo sensing element (for example, a photo diode), and a semiconductor laser controller including an optical negative feed back loop for control of the laser chip. In order to increase printing speed and image quality of the image forming apparatus, a number of laser chips and the same number of laser controllers are provided in the image forming apparatus.

To increase the printing speed and image quality, a laser chip is proposed which includes a number of light emitting elements arranged in a matrix manner, forming a light emitting element array.

When using the light emitting element array including a number of light emitting elements arranged in a matrix manner in a laser chip, because there is only one photo sensing element commonly used by the light emitting elements, it is difficult to perform an APC (Auto Power Control) operation for the light emitting elements at the same time. As a result, it is difficult to apply the APC pull-in technique of the related art directly to the light emitting element array configuration used in a multi-beam writing device.

In the image forming apparatus of the related art, for example, as disclosed in Japanese Laid Open Patent Application, No. 10-166649, which discloses a method of APC pull-in process suitable for a multi-beam writing device, any one of the light emitting elements can be used to generate laser beams to successively generate synchronization signals. In synchronization with the synchronization signals, the powers (or, the amount of light emitted from a light emitting element) of other light emitting elements are adjusted to a desired value.

However, in the image forming apparatus disclosed in Japanese Laid Open Patent Application, No. 10-166649, when a light emitting element is turned on to generate a synchronization signal, adjustments of other light emitting elements have to be interrupted, and the adjustments have to be monitored constantly.

SUMMARY OF THE INVENTION

It is a general object of the present invention to solve one or more problems of the related art.

According to the present invention, when the output power of one of the lasers changes (for example, increase), the change of the output power of the laser can be monitored by using the synchronization signal. Specifically, the control unit determines that the output power of the laser under adjustment reaches the predetermined value when the synchronization signal is detected. This scheme makes it easy to monitor the output powers of the lasers during adjustment, and still ensures sufficiently reliable power adjustment up to the line APC process.

According to the present invention, the control unit adjusts the output powers of the lasers to be the desired value one by one, that is, after completion of adjustment of one laser, start adjustment of the next laser. Further, during the adjustment of one laser, the control unit monitors the output power of the laser using the synchronization signal generated by the laser under adjustment itself, but not by any other lasers. Specifically, the control unit stops the power adjustment when the synchronization signal is detected twice. Therefore, this scheme makes it simple to initialize the laser array, and makes the time for initializing the laser array short. Further, it is not necessary to stop the power adjustment of any lasers in order to generate a synchronization signal during the adjustment, making the initialization of the image forming apparatus smoothly.

According to the present invention, the number of the lasers that need to be adjusted can be appropriately changed. As a result, it is possible to make the initialization operation suitable to the system configuration.

These and other objects, features, and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments given with reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Below, preferred embodiments of the present invention are explained with reference to the accompanying drawings.

In the following descriptions of embodiments of the present invention, the image forming apparatus includes a photo sensing element for detecting laser beams emitted from a number of semiconductor lasers, a number of semiconductor laser control circuits each of which has an optical negative feed back loop for controlling a forward current of the corresponding semiconductor laser so as to make an electric signal obtained by converting the laser beam emitted from the corresponding semiconductor laser and detected by the photo sensing element equal to that of a light emitting level designating signal, and an APC (Auto Power Control) timing determination unit for determining the timing of the APC operations of the semiconductor laser control circuits.

FIG. 1is a block diagram showing a schematic configuration of an image forming apparatus according to an embodiment of the present invention.

The image forming apparatus shown inFIG. 1includes a scanning unit1for reading in data of an original image, and a writing unit6for printing the image read by the scanning unit1on a medium.

The scanning unit1includes a VPU2for converting the input analogue signals corresponding to the original image into digital signals and for performing black offset correction, shading correction, and image position correction, and an IPU3performing image processing.

The writing unit6includes a laser diode (LD) array21for forming static latent images on a photo-conducting drum, an LD control unit5for controlling the LD array21, and a writing control ASIC (Application Specific Integrated Circuit)4for controlling the writing unit6. Additionally, the image forming apparatus includes a CPU7for controlling the image forming apparatus, a ROM8in which control programs are stored, a RAM9for temporary use of the control programs, an internal system bus10for exchanging data between different parts in the image forming apparatus, an interface (I/F)11between the system bus10and the IPU3, an image memory12for storing the image read by the scanning unit1, and an operational unit13by which an operator inputs commands.

FIG. 2shows the writing control ASIC4and the LD control unit5, and a portion of an optical system of the writing unit6. Shown inFIG. 2are the writing control ASIC4, a number of LD control units5, the LD array21, a deflector22formed by a rotating polygon mirror, an fθ lens23, a photo-conducting drum24, and a photo detector25. The LD array21includes a photo sensing element26and a number of light emitting elements29. Each of the LD control units5includes an LD driver27and an APC control unit28.

The LD array21emits multiple laser beams in a forward direction. The laser beams are collimated by a not-illustrated collimator lens, deflected by the deflector22, and are shaped by the fθ lens23to form image spots on the photo-conducting drum24which is uniformly charged by a charger. Due to the rotation of the deflector22, the image spots on the photo-conducting drum24scan from side to side repeatedly along the axial direction of the photo-conducting drum24(this direction is referred to as the main scan direction). At the same time, the photo-conducting drum24is rotating, making the image spots move on the photo-conducting drum24along the rotating direction of the photo-conducting drum24(this direction is referred to as the sub scan direction).

The photo detector25is arranged out of the laser scanning region to detect the laser beams deflected by the deflector22so as to generate a synchronization signal. The writing control ASIC4applies an image signal to the LD drivers27in the semiconductor laser control units5, and the timing of applying an image signal to the LD drivers27is controlled by the synchronization signal generated by the photo detector25.

The LD drivers27drive the LD array21to form a static latent image on the photo-conducting drum24. The static latent image is developed by a developer and is transferred to a paper by a transferring unit.

Each of the light emitting elements29in the LD array21also emits a laser beam in the backward direction. Each of these laser beams are detected by the photo sensing element26, and the photo sensing element26determines the strength of the incident laser (or power of the corresponding laser). The signal generated by the photo sensing element26corresponding to the power of the laser is input to APC controller28corresponding to the light emitting element29that emits the laser beam.

The APC controller28controls the LD drivers27according to the output signal from the photo sensing element26to make the output power of the light emitting element29in the LD array21equal to a desired value. Specifically, the power supply of the light emitting element29in the LD array21is adjusted according to the signal from the photo sensing element26so as to the output power of the light emitting element29is equal to the desired value. Then the resultant status of the power supply is maintained.

The output powers of the other light emitting elements29are also adjusted in the same way by using the photo sensing element26so that the output powers of all the light emitting elements29equal to the desired value.

FIG. 3is a schematic view showing a configuration of the writing control ASIC4.

As shown inFIG. 3, the writing control ASIC4includes a memory block31for converting the speed and the format of the image data from the IPU3, a unit32for processing the image data from the memory block31, and an output data decoder controller34for performing γ transformation on the image data, assignment of P sensor pattern, and so on. Additionally, the output data decoder controller34includes an pixel counter35for counting the numbers of the light emitting dots of the laser diodes, and a synchronization detection controller36.

FIG. 4is a block diagram showing a configuration of the output data decoder controller34of the writing control ASIC4.

The output data decoder controller34includes a P pattern block41, a γ transformation block42, an APC block43, and an LD ON/OFF block44.

The P pattern block41extracts data determining process conditions from the data input from the image processing unit32, and assigns a P sensor pattern for adding toner at a certain density to the photo-conducting drum24. The y transformation block42changes weight factors of data. The APC block43outputs image data in synchronization with the timing of APC operations for maintaining output powers of the laser diodes to be constant. The LD ON/OFF block44provides light emitting data for synchronization detection.

FIGS. 5A through 5Mare timing charts showing the operations from the synchronization detection to a line APC operation in the present embodiment, illustrating the initialization operations of the laser diode array21.

As shown inFIGS. 5A through 5D, the laser diode of channel1is turned on and initialization of APC (or referred to as “an APC initialization operation” or “an APC pull-in operation”) starts. The laser beam from the laser diode of channel1is deflected by the deflector22and scans the photo-conducting drum24. Each time the laser beam is detected by the photo detector25located in the scanning plane of the laser beam, a synchronization signal is generated.

In the present embodiment, when a synchronization signal is generated twice, the laser diode of channel1is turned off, completing the APC initialization operation of channel1. As shown inFIGS. 5E through 5G, after channel1is turned off, channel2is turned on, and similarly, when a synchronization signal is generated twice, the laser diode of channel2is turned off.

This operation is repeated for all channels (FIGS. 5H through 5Mshow the APC operations for channel3and channel4), and when a synchronization signal for the laser diode of the last channel is generated twice, the line APC operation, which is for maintaining the output powers of the laser diodes starts in synchronization with the synchronization signal generated by channel1.

In the present embodiment, for each main scan operation, the laser diodes of four channels are used for writing. Therefore, in the APC initialization operation, first, after the power of the image forming apparatus is turned on, the deflector22is rotated, and channel1is turned on, and the power of the laser diode of channel1is adjusted. When the power of channel1is greater than a preset level, the photo detector25detects the laser beam and outputs a first synchronization signal. Next, channel1is kept to be turned on, and when a second synchronization signal is generated, channel1is turned off, and the laser diode in channel2is turned on.

Similarly, when the power of channel2is greater than the preset level, the photo detector25detects the laser beam and outputs a synchronization signal. Next, channel2is kept to be turned on compulsorily, and when the second synchronization signal is generated, channel2is turned off, and the laser diode in channel3is turned on.

For channel3and channel4, the same operations are repeated, and the respective synchronization signals are obtained. When a synchronization signal for the last channel is generated twice, the operation for adjusting the output powers of the laser diodes to the aforementioned desired value (that is, the APC initialization operation) is completed. Then the line APC operation for maintaining the output powers of the laser diodes to the desired values starts. Specifically, while the synchronization signal is generated by channel1, the line APC operation starts in synchronization with the synchronization signal.

In the present embodiment, timing of the line APC operation is determined by using a main scan counter that performs counting using a writing clock signal. For example, a value is preset for the main scan counter, and other signals are started or stopped when counts of the main scan counter reach this preset value. By comparing counts of these signals with this preset value, a gate signal is output. The preset value may be modified by using, for example, a SP mode, which is capable of modifying various setting data. By using the SP mode, it is also possible to set how many channels of LDs should be adjusted before transferring to the line APC operation, and as a result, it is possible to make the initiation of the LD array suitable to the configuration of the image forming apparatus.

While the present invention is described with reference to specific embodiments chosen for purpose of illustration, it should be apparent that the invention is not limited to these embodiments, but numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention.

Summarizing the effects of the present invention, during an APC initialization operation for adjusting output powers of lasers in a laser array, the output powers of the lasers are monitored using the synchronization signal. This makes it easy to monitor the output powers of the lasers. Further, the adjustments of changing the output powers of lasers to the desired value are made sequentially, and therefore, it is possible to make the time for initializing the laser array short. In addition, the power adjustment is completed when the synchronization signal is detected twice. Therefore, it is not necessary to stop the power adjustment when a synchronization signal is generated, making the initialization of the image forming apparatus smoothly. Further, the number of lasers that need to be adjusted can be appropriately changed. As a result, it is possible to make the initialization operation suitable to a configuration of the image forming apparatus.

This patent application is based on Japanese Priority Patent Application No. 2002-260518 filed on Sep. 5, 2002, the entire contents of which are hereby incorporated by reference.