1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a facsimile telegraph, a copying machine, or a composite machine combining and having these functions, etc.
2. Description of the Related Art
For example, the image forming apparatus such as a copying machine, a printer, a facsimile telegraph, etc. widely uses a apparatus for fixing an unfixed toner image formed and carried in accordance with image information of an object in a transfer system (indirect system) or a direct system to a recorded material (a paper sheet member such as a transfer material, photosensitive paper, electrostatic recording paper, printing paper, etc.) as a sheet by a suitable image making process mechanism such as an electro-photographic system, an electrostatic recording system, a magnetic recording system, etc.
Further, an apparatus for directly forming the image on the sheet by a liquid including dyes and a pigment as in an ink jet system, etc. is also used.
In such apparatuses, the sheet fed by a sheet feed section one by one from a sheet support section (paper feed cassette) once hits against the nip of a stopped resist roller pair adjacent to the sheet conveyance upper stream of an image forming section.
In this case, the sheet tip is detected by a sheet detecting sensor arranged in the vicinity of the upper stream of the resist roller pair. After the sheet tip hits against the resist roller pair, the operation of a conveying roller arranged on the upper stream of the resist roller pair is generally stopped in predetermined timing for feeding the sheet by 5 to 15 mm.
Accordingly, an oblique movement correction is made by arranging the sheet tip in parallel with a nip line of the resist roller pair by rigidity of the sheet.
Thereafter, the positions of the sheet and the image are aligned with each other by operating the resist roller pair in synchronization with the image formed in the image forming section, and forming the image on the conveyed sheet. The circumferential speed of the resist roller pair is set to be approximately equal to an image forming speed.
The mainstream of the sheet detecting sensor is constructed by a sensor formed by combining a rotational lever biased by a spring in one direction and a transmission type photo-interrupter constructed by oppositely arranging a light emitting element and a light receiving element. This sensor detects the existence or nonexistence of the sheet by interrupting and uninterrupting the light of the transmission type photo-interrupter by the lever when the lever is thrown down by the sheet. The mainstream of the sheet detecting sensor is also constructed by a reflection type photo-interrupter for irradiating light from a light emitting element to the sheet and directly detecting reflected light and diffused light by a light receiving element.
Further, there is a structure having a striking plate able to be selectively shunted at a position before a conveying roller arranged on this side of the image forming section without arranging the resist roller pair. In this structure, a sheet detecting sensor similar to the above sheet detecting sensor is arranged in the upper stream of the striking plate, and the operation of the conveying roller in the upper stream of the striking plate is stopped in predetermined timing from a detecting signal of this sheet detecting sensor.
In this structure, similar to the resist roller pair, the oblique movement correction is made by arranging the sheet tip in parallel with the striking plate, and the positions of the sheet and the image are aligned with each other by shunting the striking plate in synchronization with the image formed by the image forming section.
In the image forming apparatus represented by the facsimile telegraph, the printer and the copying machine, digitalization is advanced in recent years. For example, the copying machine of the electro-photographic system uses a system in which the reflected light of illumination of a reader section is read by a photoelectric element such as CCD, etc., and is A/D converted, and a laser beam is emitted. In this system, the image is written to a photosensitive member by scanning the laser beam in the direction of a bus line of the photosensitive body by rotating a polygon mirror, and is also written to the photosensitive member by LEDs arranged at a small pitch. An ink jet system for forming the image by directly ejecting an ink droplet to the sheet is also used. Further, a heat-sensitive recording system is further used.
However, when the above digitalization is advanced, the number of technical problems is increased and cost of the solution of these technical problems is increased as e.g., the number of rotations of the polygon mirror, and the driving frequencies of an LED head and an ink jet head are raised.
Further, it is desirable to energetically reduce the image forming speed as much as possible in view of the durability of all parts moved or rotated.
Accordingly, it is also desired to increase productivity by narrowing the sheet interval as much as possible in a structure having the same image forming speed.
However, when the sheet interval is narrowed, the problem of shortening of a back scan time in the reader section is caused in e.g., the copying machine. There is a method for arranging a memory on the entire face of the image able to be read by one reader scan as one of methods for solving such a problem.
In accordance with this construction, the sheet interval can be narrowed since the time of the back scan of the reader section can be omitted by repeatedly discharging the image read once from the memory.
The memory was previously expensive, but its cost is remarkably reduced in recent years. A memory, a hard disk, etc. having a means for storing the image and mounted to a main body are sold.
Accordingly, a factor for determining the sheet interval is determined by sheet conveying performance of the image forming apparatus. This factor is similarly determined by the sheet conveying performance in the printer and the facsimile telegraph.
Accordingly, when it is intended that the conveying interval of the sheet is narrowed, the condition of a minimum sheet conveying interval is that the sheet can be detected by the sheet detecting sensor in the upper stream of the resist roller pair. Accordingly, the amount (since no sheet is advanced at this time) provided by adding an inter-sheet minimum detecting interval of the sheet detecting sensor and a feed-in amount (hereinafter called a loop amount) in making the oblique movement correction of the sheet as mentioned above becomes the minimum sheet conveying interval except for small defects such as a detecting error, etc.
Further, when an electromagnetic clutch is used in e.g., drive coupling in the driving construction of the resist roller pair, and the dispersion of a coupling time of the electromagnetic clutch is set to 30 msec, (30 msec)xc3x97(a sheet interval margin corresponding to a conveying speed) must be added to the above minimum sheet conveying interval. Further, when the resist roller pair is operated by a pulse motor, a starting up time and its loss time must be added to the minimum sheet conveying interval although the dispersion is small.
In consideration of the above problems, an object of the present invention is to provide an image forming apparatus able to improve productivity by minimizing the interval of a conveyed sheet without increasing cost.
The present invention is characterized in an image forming apparatus comprising an image forming section for forming a latent image on the basis of a signal outputted at a constant time interval, and forming the image on a sheet by developing the latent image; a registration section arranged on the upstream side of the image forming section, and able to change a conveying speed for re-feeding the sheet after the sheet is stopped for a predetermined time; and a control section for starting the conveyance of the sheet re-fed by the registration section where a speed higher than an image forming speed in the image forming section, and controlling the conveying speed so as to be equal to the image forming speed at a position before a predetermined distance from the image forming section; wherein the control section sets the conveying speed at a high speed conveying time in the registration section so as to be equal to the image forming speed at the position before the predetermined distance from the image forming section.