Electrophotography – Control of electrophotography process
Reexamination Certificate
2000-02-18
2001-10-09
Braun, Fred L (Department: 2852)
Electrophotography
Control of electrophotography process
C399S301000
Reexamination Certificate
active
06301451
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to an image forming apparatus which uses an electrophotographic method to form an image on paper and output it, like copying machines and printers.
2. Description of the Prior Art
An image forming apparatus using the electrophotographic method generally forms an image on paper by forming an electrostatic latent image corresponding to an image signal on a photosensitive body, developing the latent image into a toner image, transferring the toner image onto the paper, and fusing the toner image on the paper.
In such an image forming apparatus, sheets of paper stacked on a paper tray are usually picked up one by one and transported toward a toner image transfer position. During the paper transporting process, any difference between the timing at which the paper reaches the image transfer position and the timing at which the toner image reaches there will result in a shift of the toner image position on the paper from the desired position. When an image is formed over an entire surface of the paper, for example, edge dimensions become uneven, thus significantly degrading the appearance of the formed image.
For this reason, some image forming apparatuses incorporate into the paper transporting system a mechanism for adjusting the timing of the paper reaching the image transfer position. For example, there is a construction in which, as shown in
FIG. 19
, two paper transporting rollers A, B are arranged along the paper transporting direction of the paper P. In this construction, the paper P fed by the paper transporting roller B on the upstream side is struck against the paper transporting roller A on the downstream side which is at rest and then the timing at which to start rotating the paper transporting roller A is adjusted to match the timing at which the paper arrives at the transfer position to the toner image arrival timing.
In the conventional image forming apparatus described above, however, as shown in
FIGS. 20A and 20B
the way the paper engages the paper transporting roller A at rest may vary depending on the curled state of the lead edge of the paper, causing the paper position relative to the paper transporting roller A to change from one sheet of paper to another, as shown in
FIGS. 21A and 21B
. This may result in a difference in the transfer position arrival timing between the paper and the toner image.
To deal with this problem, a so-called nonstop servo registration control may be adopted which temporarily stops the paper in front of the paper transporting roller A and rotates the paper transporting roller A as the lead edge of the paper reaches the paper transporting roller A. This will eliminate possible variations in the paper engagement condition and thereby improves the precision of the paper position relative to the paper transporting roller A.
In this case, while the precision of the timing of the paper arriving at the transfer position improves, variations in the toner image arrival timing become a serious problem. That is, when variations in the toner image arrival timing occur due to the manufacturing precision and assembly errors of components of the image forming apparatus or their changes with elapse of time, it becomes difficult to match the paper arrival timing with the toner image arrival timing, even with an improved precision of the timing of the paper arriving at the transfer position. Especially in an image forming apparatus that uses an image carrying belt, such as a photosensitive belt and an intermediate transfer belt, variations in the toner image arrival timing increase because the image carrying belt elongates or contracts in response to changes in temperature, humidity or belt tension.
Therefore, as shown in
FIG. 22A
or
22
B, when the nonstop servo registration control is adopted, the amount of variation in the toner image arrival timing needs to be determined by detecting the position of the front end of the toner image on an image carrier
41
, such as a photosensitive drum or an intermediate transfer belt, by an image front end reading sensor
42
provided near the toner image transfer position. That is, for the image front end reading sensor
42
to detect the front end position of the toner image, it is necessary to form a toner patch on the image carrier
41
for reading the timing, or form a predetermined marking (such as a notch) on the image carrier
41
.
The formation of the toner patch on the image carrier
41
in turn requires a pattern generator, control, cleaning, and others, leading to an increase in the apparatus cost and in the development man-hours. Further, if the image front end reading sensor
42
is of a general type, because its reading sensitivity characteristic depends on the color of the toner patch or marking, a satisfactory reading sensitivity characteristic may not be obtained when a single color image (toner patch) is formed. Another problem is that when a marking is formed on the image carrier
41
in advance, the pitch of the marking cannot be changed, thus greatly lowering the productivity of image forming.
The image front end reading sensor
42
is arranged close to the image carrier
41
in order to be able to read the toner patch or marking on the image carrier
41
. This renders the toner on the image carrier
41
more likely to be scattered and the scattered toner may contaminate the marking, resulting in a degraded detection precision or even a failure to detect.
SUMMARY OF THE INVENTION
The present invention provides an image forming apparatus which can match, with high precision, the timing of the paper arriving at the transfer position with the timing of the toner image arriving there, without being influenced by positional misregistrations of paper in the paper transporting direction or variations in the timing of the toner image arriving at the transfer position, and does not require detection by a sensor of a front end position of the toner image.
According to an aspect of the present invention, the image forming apparatus has an image forming unit to make an image on an image carrier, a transfer unit to transfer the image formed on the image carrier by the image forming unit onto paper, a transporting unit to transport the paper toward an image transfer position of the transfer unit, a calculation unit to calculate a time when the image on the image carrier will arrive at the image transfer position or nearby position and correct the calculated result according to a predetermined parameter, and a control unit to control the paper transporting operation of the transporting unit according to the time determined by the calculation unit.
In the image forming apparatus of the above configuration, when the image forming unit forms an image on the image carrier, the calculation unit calculates a point in time at which the image will arrive at the image transfer position of the transfer unit or a nearby position. The calculation unit corrects the calculated result as required according to predetermined parameters, such as an elongation or contraction of the image carrying surface of the image carrier, precisions of constitutional parts of the image forming apparatus and parts mounting errors. Based on the time determined by the calculation unit, the control unit controls the paper transporting operation of the transporting unit by adjusting the speed and timing of transportation of the paper by the transporting unit. As a result, the timing of the image arriving at the image transfer position of the transfer unit and the timing of the paper arriving there are made to coincide with each other, without detecting a variation of the image arrival timing by a sensor.
REFERENCES:
patent: 5875380 (1999-02-01), Iwata et al.
patent: B2-6-93145 (1994-11-01), None
patent: A-11-194561 (1999-07-01), None
Ando Ryo
Kazama Toshiyuki
Mori Hirotaka
Braun Fred L
Fuji 'Xerox Co., Ltd.
Oliff & Berridge, P.L.C.
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