Electrophotographic printer

Abstract: An electrophotographic printer, comprising: a photosensitive drum (21) which has an outer circumference longer than a whole length of a recording paper (7) and which can be rotated selectively forward and backword and at a variable speed; an LED head (20) for exposing a surface of the photosensitive drum (21) to light so as to write a latent image at every scanning line; a motor control circuit for stopping the photosensitive drum (21) at a position of the LED head corresponding to a scanning line. as to an input picture signal and a developer (19) which is separated from the surface of thephotosensitive drum (21) when the latent image is written and which abuts to the surface of the photosensitive drum (21) when a toner is adhered to the written latent image to develop.

Description: BACKGROUND OF THE INVENTION The present invention relates to an electrophotographic printer using an electrophotographic process. Fig. 3 is a side view of a main portion of a conventional electrophotographic printer as disclosed, for example, in Japanese Patent Unexamined Publication No. 61-95956, and Fig. 4 is a view of the flow of a picture signal in such a electrophotographic printer. In the drawings, reference numeral 1 designates a photosensitive drum, and 20 designates an LED head constituted by an LED array 2 and a lens array 3. The LED head 20 is attached to a frame (not shown) in such a manner that the light emitted from the LED array 2 is focused by the lens array 3 on an outer circumference of the photosensitive drum 1 and a group of the focuses forming a scanning line becomes parallel to the axis of the photosensitive drum 1. Thus the surface of the photosensitive drum 1 is exposed at its position corresponding to the scanning line so that a latent image is written on the surface of the photosensitive drum 1 in accordance with a video signal supplied from a picture processing unit 12 which will be described later. Reference numeral 4 designates a charger for uniformly charging the surface of the photosensitive drum 1; 5, a developing device for causing toner to adhere to the written latent image; 6, a transfer device for transferring the toner developed on the surface of the photosensitive drum 1 on a recording paper 7; 8, a fixer for heating the transferred toner so as to fix the toner onto the recording paper 7; 9, a cleaning device for cleaning the surface of the photosensitive drum 1 after the toner has been transferred; 10, a discharger; 11, a host computer which is a source for generating a picture signal; and 12, a picture processing unit for conversion-processing the picture signal generated by the host computer 11. The picture processing unit 12 converts the picture. signal by one page into picture data and stores the picture data in an RAM 13. The picture processing unit 12 takes out the stored data from the RAM so as to convert the taken-out data into a video signal, and supplies the video signal to an LED driver 14. Reference numeral 15 designates a motor for rotating the photosensitive drum 1 at a predetermined speed. In the thus configured electrophotographic printer, the picture data stored in the RAM 13 is taken out from the RAM 13 in response to a picture print command successively in accordance with the speed of rotation of the photosensitive drum 1, and the LED driver 14 causes the LED array 2 to emit light by every scanning line so that the surface of the photosensitive drum 1 is exposed to the light and the latent image is written on the surface of the photosensitive drum 1. The portion where the latent image has been written is rotated by the motor 15 in the direction shown by an arrow A, the toner is caused to adhere onto the portion of the latent image by the developing device 5, and the toner is transferred by the transfer device 6 onto the recording paper 7. Next, the recording paper 7 is heated by the fixer 8 so that the toner is fixed onto the recording paper 7 which is taken out as a picture-print completed recording paper. In the conventional electrophotographic printer as described above, although the speed at which the latent image is written by every scanning line is high, the speed of the process (the speed of rotation of the photosensitive drum 1) is restricted by physical conditions such as developing, transferring, fixing, etc. in order to make the quality of the printed picture good. Accordingly, in order to absorb the speed difference, it has been necessary that the picture data of one page is stored in the RAM 13 and the stored data is succeedingly taken out. Accordingly, there has been a problem in that the RAM 13 is required, the RAM 13 requires about 8 megabits per page of an A4 size paper in the case of 300dpi (dots/inch), and a large number of memories make the apparatus expensive. GB-A-2 185 608 describes a printer wherein a RAM is necessary, too. SUMMARY OF THE INVENTION The present invention has been attained in order to solve the problems as described above, and an object thereof is to provide an inexpensive electrophotographic printer using no RAM for temporary storage. According to the present invention, the electrophotographic printer comprises: a photosensitive drum which has an outer circumference longer than the whole length of recording paper, a means to selectively relate said photosensitive drum forward and backward and at a variable speed; an LED head arranged to expose a surface of the photosensitive drum to light so as to write a latent image in every scanning line; a means arranged to assign a rotation stopping position of the photosensitive drum to a position of the LED head corresponding to a scanning line as to an input picture signal; and a developing device arranged to be separated from the surface of the photosensitive drum when a latent image is written and to abut to the surface of the photosensitive drum when toner is to be adhered to develop the written latent image. In the electrophotographic printer according to the present invention, the process of writing the latent image and the process of developing - fixing are separated from each other, the speed and direction of the rotation of the photosensitive drum are changed to the speed and direction necessary for each process, and the latent image is directly written by the LED head on the surface of the photosensitive drum in accordance with the conversion-processes picture signal produced from the host computer. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a side view of a main portion of an embodiment of the electrophotographic printer according to the present invention; Fig. 2 is a view showing the flow of a picture signal in the embodiment; Fig. 3 is a side view of a main portion of a conventional electrophotographic printer; and Fig. 4 is a view showing the flow of a picture signal in the conventional printer. DETAILED DESCRIPTION OF THE INVENTION Fig. 1 is a side view of a main portion of an embodiment of the electrophotographic printer according to the present invention, and Fig. 2 is a view of the flow of a picture signal in the embodiment. In the drawings, reference numerals 2 - 4, 6 - 11, and 20 designate the same portions as those described in the prior art. Reference numeral 21 designates a photosensitive drum having an outer circumference longer than the whole length of the recording paper 7 and reference numeral 21a designates a reference point for explaining a position on the outer circumference of the photosensitive drum 21. Reference numeral 19 designates a developing device having a structure in which a portion thereof for causing toner to adhere can be separably touched on the outer circumferential surface of the photosensitive drum 21. Reference numeral 16 designates a picture processing unit for processing the picture signal supplied from the host computer 11 to convert it into a video signal. The picture processing unit 16 supplies the video signal to an LED driver 14, and supplies positional information of a scanning line corresponding to the video signal to a motor control circuit 18 which will be described later. Reference numeral 17 designates a stepping motor for rotating the photosensitive drum 21. The motor control circuit 18 for controlling the rotation of the stepping motor 17 is arranged to control the direction of rotation, the speed of rotation, and the stop position of the photosensitive drum 21 in accordance with the positional information of the scanning line supplied from the picture processing unit 16 for the respective processes. In the electrophotographic printer arranged as described above, at the start of picture printing, in the state where the recording paper 7 is not yet fed, the transfer device 6, the cleaning device 9, and the discharger 10 are stopped operating, the developing device 19 is separated from the photosensitive drum 21, and the photosensitive drum 21 is rotated in the direction shown by an arrow A so that the whole surface of the photosensitive drum 21 is uniformly charged by means of the charger 4. Then, the charger 4 is stopped operating. The picture processing unit 16 requests that the host computer 11 sends the picture signal, and receives the picture signal corresponding to the first scanning line. Then, the picture processing unit 16 processes the picture signal to convert it into the video signal, supplies the video signal to the LED driver 14, and supplies the scanning line positional information corresponding to the video signal to the motor control circuit 18. The motor control circuit 18 rotates the stepping motor 17 in the direction shown by an arrow B so as to assign and stop the photosensitive drum 21 at the position where the LED head 20 is opposed to the scanning line position from to the reference point 21a, and the LED driver 14 causes the LED array 2 to emit light in accordance with the position-assigning completion signal of the stepping motor 17 so as to expose the surface of the photosensitive drum 21. Upon receiving an exposure completion signal, the picture processing unit 16 supplies a request to the host computer 11 so that the host computer 11 sends out the picture signal corresponding to the succeeding scanning line, and then exposure corresponding to every scanning line is similarly succeedingly performed to thereby form a latent image on the surface of the photosensitive drum 21. At that time, in the case where the scanning-line positions of the picture signals sent from the host computer 11 are not in order but in reverse order, the stepping motor 17 rotates the photosensitive drum 21 fast or reversely so as to assign the scanning line position to the photosensitive drum 21. When the exposure of one page has been completed, the recording paper 7 is fed, the developing device 19 is caused to abut onto the surface of the photosensitive drum 21, the photosensitive drum 21 rotates at a predetermined speed necessary to the development-fixing in the direction shown by the arrow A, the latent image is developed, and the toner is transferred on the recording paper 7 so as to be fixed, thus completing the picture printing. After the toner has been transferred, the surface portion of the photosensitive drum 21 is cleaned by the cleaning device 9 and discharged by the discharger 10 so as to return to the original state for the following picture printing. As described above, according to the present invention, the process of writing the latent image and the process of developing-fixing are separated from each other, the speed of rotation is changed to a required speed for each of the processes, and the latent image is directly written by the LED head on the surface of the photosensitive drum in accordance with the picture signal supplied from the host computer. Accordingly, it is possible to provide an inexpensive electrophotographic printer using no RAM for temporary storage.

Claims: An electrophotographic printer, comprising: a photosensitive drum (21) which has an outer circumference longer than the whole length of a recording paper; a means (17) to rotate said photosensitive drum (21) selectively forward and backward and at a variable speed; an LED head (20) arranged to expose a surface of said photosensitive drum to light so as to write a latent image at every scanning line; a means (18, 21a) arranged to assign a rotation stopping position of said photosensitive drum to a position of said LED head corresponding to a scanning line as to an input picture signal; and a developing device (19) arranged to be separated from the surface of said photosensitive drum when the latent image is written and to abut to the surface of said photosensitive drum when toner is to be adhered to develop the written latent image.

Inventors: NISHIOKA TAKAFUMI C O MITSUBIS, NISHIOKA, TAKAFUMI, C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1990
Main classification: G03G15
IPC codes: G03G15, G03G21