Skew control apparatus for endless-belt-shaped recording material

A skew control apparatus for use in a recording apparatus, capable of detecting the skew of an endless-belt-shaped recording material, and reversing the direction of the skew during a non-recording period, even if such skew is detected during the recording operation period.

BACKGROUND OF THE INVENTION 
The present invention generally relates to a recording apparatus for 
recording on an endless-belt-shaped recording material which is supported 
and driven by a plurality of rollers, in particular to a skew control 
apparatus for an endless-belt-shaped recording material for use in the 
recording apparatus. 
In a recording apparatus of the above-mentioned type, an electrostatic 
recording type apparatus, a magnetic recording type apparatus and an 
electrophotographic recording type apparatus are known. In these 
apparatuses, it is important that such endless-belt-shaped recording 
material is always transported in the same posture. 
The outline of a conventional recording apparatus of an electrophotographic 
type will now be explained by referring to FIG. 1. 
FIG. 1 shows the main portion of the electrophotographic recording 
apparatus, in which, as the recording material, an endless-belt-shaped 
photoconductor 1 (hereinafter referred to as the photoconductor 1) 
comprising a base film made of polyethylene terephthalate and an organic 
or inorganic photoconductor deposited on the base film is employed. The 
photoconductor 1 is supported and driven in rotation in the direction of 
arrow A by a drive roller 2 and a driven roller 3. Around the 
photoconductor 1, there are arranged a charger 4 for applying charges to 
the photoconductor 1; an exposure apparatus 5 for exposing the 
electrically charged photoconductor 1 to a light image L of an original, 
thereby forming a latent electrostatic image on the photoconductor 1; a 
development apparatus 6 for developing the latent electrostatic image to a 
visible image by a developer; an image transfer charger 7 for transferring 
the visible image from the photoconductor 1 to a recording sheet; a 
cleaner 8 for cleaning the surface of the photoconductor 1 after the image 
development process; a quenching charger 9 for eliminating remaining 
charges from the surface of the photoconductor 1 in preparation for reuse 
of the photoconductor 1, followed by charging and exposure; and other 
members. 
In a housing (not shown) of the recording apparatus, there is disposed a 
sheet stacking box (not shown), from which recording sheets are supplied 
and then discharged out of the recording apparatus through a path shown by 
the arrow P. In the course of the transportation, a visible image formed 
on the photoconductor 1 is transferred therefrom to a recording sheet by 
the image transfer charger 7. 
The above-described recording apparatus employing the endless-belt-shaped 
photoconductor has an advantage over a recording apparatus employing a 
drum-shaped photoconductor in that the apparatus can be made small in size 
. On the other hand, the former has a shortcoming in that the belt-shaped 
photoconductor is apt to be skewed in the direction normal to the running 
direction of the belt. For instance, in the recording apparatus as shown 
in FIG. 1, the skewing of the photoconductor 1 occurs due to imperfections 
in the shapes, sizes and attachment positions of the drive roller 2 and 
driven roller 3 or due to the difference in tension applied to the 
photoconductor 1 by those rollers between the opposite ends thereof. 
In case no countermeasures are taken against such skewing, the 
photoconductor 1 will gradually deviate from the correct position for 
latent image formation and visible image transfer. 
In order to prevent such inconvenience, conventionally the skewing of the 
photoconductor 1 is prevented by disposing flanges at the opposite end 
portions of the shaft of the drive roller 2 or of the driven roller 3. 
This method is effective when the skewing of the photoconductor 1 is 
slight, since, in that case, the skewing is stopped by the side portions 
of the photoconductor 1 coming into contact with the flanges. However, 
when the variations in shape and size of the above-mentioned rollers are 
great and accordingly when the skewing force applied to the photoconductor 
1 is great, the skewing cannot be stopped even if the side portions of the 
photoconductor 1 come into contact with those flanges. The result is that 
the side portions of the photoconductor 1 are damaged by the flanges and 
the image formation area in the photoconductor 1 undulates. As a matter of 
course, when this occurs, copy images faithful to the original images 
cannot be obtained. 
The inventors of the present invention previously proposed a skew control 
apparatus capable of eliminating the above-described conventional 
shortcomings, which comprises an inclination mechanism for reversing the 
skewing direction of the endless-belt-shaped recording material by 
inclining one of the two rollers (corresponding to the rollers 2 and 3 in 
FIG. 1), over which the endless-belt-shaped recording material is trained, 
relative to the other roller, within a plane normal to the running 
direction of the recording material or within a horizontal plane; a skew 
detection means for generating a skewing detection signal upon detecting a 
predetermined amount of skewing of the recording material; and a drive 
switching means for receiving the detection signal generated from the skew 
detection means and switching the position of the inclination mechanism to 
its operational position as long as the detection signal is generated. 
By this skew control apparatus, the shortcomings of the conventional skew 
control apparatus, such as the damage to the side portions of the 
belt-shaped recording material and the undulation of the recording 
material caused by its skewing, can be eliminated, since the skew 
detection means and the inclination mechanism are constructed in such a 
manner as not to apply pressure to the side edge portions of the 
belt-shaped recording material. 
In the above-described skew control apparatus, upon detecting the skew, the 
skew-reversing control is done by switching the position of one or the 
other of the rollers to a first inclined position or a parallel position 
to a second oppositely inclined position. The skew-reversing control is 
instantly performed by a magnetic solenoid or the like, and the skewing is 
gradually corrected as the belt runs. As a matter of course, such 
skew-reversing control, that is, switching the inclined position of each 
roller, may be done during the recording operation of the recording 
apparatus. When it is done during the recording operation, the image 
quality may vary before and after such switching of the position of the 
rollers. Referring to FIG. 1, for instance, when the roller 3 is inclined 
during the operation of the image transfer charger 7, the gap between the 
photoconductor 1 and the image transfer charger 7 is abruptly changed, so 
that the image transfer efficiency changes before and after the 
inclination of the roller 3. The result is that the transferred image may 
be blurred. Likewise, since it is considered that the peripheral speed of 
the photoconductor 1 may be temporarily changed when the roller 3 is 
inclined, it is preferable that the roller 3 not be inclined during the 
exposure operation by the exposure apparatus 5. In particular, when the 
exposure is done by laser beams, images may be blurred considerably. This 
is because, in the case of the exposure by laser beams, the recording 
material is moved in the sub-scanning direction by an extremely small 
distance, for instance, a certain fraction of 1 mm, after each line of the 
main scanning. Therefore, so long as the load applied to the drive system 
of the belt is changed, regardless of the switching direction of the 
skewing, the peripheral speed of the photoconductor is temporarily 
changed. The result is that the images are considerably blurred at the 
time of exposure. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a skew 
control apparatus for endless-belt-shaped recording materials, from which 
the shortcomings of conventional recording apparatus have been eliminated. 
According to the present invention, there is provided a skew control 
apparatus for use in a recording apparatus which performs recording on an 
endless-belt-shaped recording material which is supported and driven in 
rotation by a plurality of rollers, the skew control apparatus comprising 
a skew detection means which detects the skewing of the 
endless-belt-shaped recording material and generates a skew detection 
signal when the recording material is skewed beyond a predetermined 
tolerance in the direction normal to the driven direction of the recording 
material; a skewing direction reversing control means which reverses the 
skewing direction of the endless-belt-shaped recording material; a timing 
signal generation circuit which generates a timing signal indicating that 
the recording apparatus is not in the recording operation period; a 
control circuit which generates an output when (i) the skew detection 
signal generated from the skew detection means and (ii) the timing signal 
generated from the timing signal generation circuit are input thereto at 
the same time; and a drive switching means which switches the operation of 
the skewing direction reversing control means, depending upon the presence 
of the output from the control circuit. In the above, the "recording 
operation period" signifies an entire period including various steps 
required for image formation, such as charging, exposure, image transfer, 
image fixing and others, or a period including predetermined steps which 
have significant effects on the image formation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the accompanying drawings, an embodiment of a skew control 
apparatus according to the present invention will now be explained. 
For the convenience of explanation, it is supposed that the skew control 
apparatus according to the present invention is employed in an 
electrophotographic recording apparatus of the same type as that shown in 
FIG. 1, which includes a endless-belt-shaped recording material. 
Referring to FIG. 2, there are shown an endless-belt-shaped photoconductor 
and a skew detection means in the embodiment of a skew control apparatus 
according to the present invention. In the figure, apparatuses arranged 
around the photoconductor 1, such as an exposure apparatus, are omitted 
for simplification of explanation. The skew detection means in this 
embodiment comprises light-reflection type photosensors 10a and 10b 
situated above the endless-belt-shaped photoconductor 1, and detection 
patterns 11a and 11b disposed at the opposite end portions of the 
endless-belt-shaped photoconductor. The other members in FIG. 2 which bear 
the same reference numerals are the same as those employed in FIG. 1. 
The photoconductor 1 is supported by the drive roller 2 and the driven 
roller 3 and is driven in the direction of the arrow A by the drive roller 
2. The detection patterns 11a and 11b are attached to the entire opposite 
peripheral side portions of the photoconductor 1 and are moved in the 
direction of the arrow A, together with the photoconductor 1. The 
detection patterns 11a and 11b have a different reflectance from that of 
the photoconductor 1. The light-reflection-type photosensors 10a and 10b 
are situated above the detection patterns 11a and 11b. The positions of 
the photosensors 10a and 10b in terms of their roller-shaft direction 
(i.e., the direction of the arrow B in FIG. 2) are inside the detection 
patterns 11a and 11b and are not shifted in the directions (i.e., the 
direction of the arrow C or the direction of the arrow D) normal to the 
driving direction A of the photoconductor 1 (in the normal moving state of 
the photoconductor 1), relative to the opposite side portions of the 
photoconductor 1. 
Referring to FIG. 3, there is shown the above-described positional 
relationship between the photosensors 10a and 10b and the detection 
patterns 11a and 11b. The view in FIG. 3 is a front view from the 
direction of the arrow E in FIG. 2. In FIG. 2 and FIG. 3, the same members 
bear the same reference numbers. 
When the photoconductor 1 is being moved in the normal moving state free 
from skewing, the photoconductor 1 is in the detectable area of the 
photosensors 10a and 10b. Therefore, signals corresponding to the 
reflectance of the photoconductor 1 are output from the photosensors 10a 
and 10b. 
In this embodiment, the output signals from the photosensors 10a and 10b, 
corresponding to the reflectance of the photoconductor 1, are pre-set so 
as to be at a level higher than the level of signals corresponding to the 
reflectance of the detection patterns 11a and 11b. Hereinafter the output 
signal corresponding to the reflectance of the photoconductor 1 is 
referred to as the H signal, while the output signal corresponding to the 
reflectance of the detection patterns 11a and 11b is referred to as the L 
signal or the skew detection signal. 
In the course of repeating the copying process including the charging, 
exposure, image transfer, image fixing and cleaning steps, when the 
photoconductor 1 is skewed in the direction of the arrow C due to 
imperfections in the sizes and shapes of the previously described rollers, 
the detection patterns 11a and 11b are shifted in the direction of the 
arrow C at the same time. During the shifting of the detection patterns 
11a and 11b, the detection pattern 11b enters the detectable area of the 
photosensor 10b. At that moment, the output of the photosensor 10b is 
changed from the H signal to the L signal. During the shifting of the two 
detection patterns 11a and 11b, the photoconductor 1 remains in the 
detection area of the photosensor 10a, so that the output from the 
photosensor 10a is the H signal and does not change. In other words, the 
skewing of the photoconductor 1 in the direction of the arrow C can be 
detected by the output of the photosensor 10b being changed from the H 
signal to the L signal. 
When the photoconductor 1 is skewed in the direction of the arrow D, which 
is opposite to the direction of the arrow C, that skew is detected by the 
output signal of the photosensor 10a being changed from the H signal to 
the L signal. 
Referring to FIG. 4, there are shown a roller-inclination mechanism 12 and 
a drive switching means therefor in the embodiment of a skew control 
apparatus according to the present invention, in which the same members or 
apparatuses as those employed in the electrophotographic recording 
apparatus shown in FIG. 1 bear the same reference numbers. 
The roller-inclination mechanism 12 includes a roller moving member 14 of 
which the center point is rotatably supported on a fulcrum 13 fixed to the 
body of the recording apparatus. In the rear end portion of the roller 
moving member 14, there are disposed a solenoid 15 and a return tension 
spring 16 which work as drive switching means. In the front end portion of 
the roller moving member 14, there is formed a slot which extends in the 
longitudinal direction of the roller moving member 14 and opens in the 
extreme front end thereof as shown in FIG. 4. One end portion 3a of a 
driven roller 3 is fitted in the slot 17. Because of the above-described 
construction, the roller end portion 3a is movable in the vertical 
direction in FIG. 4 in accordance with the rotation of the roller moving 
member 14. The other end portion 3b of the driven roller 3 (refer to FIGS. 
2 and 5) is supported by the body of the recording apparatus, so as not to 
be vertically movable. The result is that the driven roller 3 can be 
inclined, with the roller end portion 3b being its fulcrum, by the turning 
of the roller moving member 14. 
Above the roller moving member 14, there is disposed the tension spring 16, 
and below the roller moving member 14, there is disposed the solenoid 15. 
The solenoid 15 is electrically connected to the previously mentioned skew 
detection means through a control circuit which will be described in 
detail later, so that in accordance with the output signal generated from 
the skew detection means, the solenoid 15 is energized (ON) or deenergized 
(OFF). When the solenoid 15 is deenergized, the driven roller 3 is 
inclined in such a manner that its end portion 3a is positioned slightly 
below a horizontal position (indicated by the alternate long and short 
dash line), since the roller moving member 14 is urged to rotate 
counterclockwise by the tension of the spring 16. 
When the photoconductor 1 is driven in rotation in the direction of the 
arrow A (refer to FIG. 4), with the end portion 3a of the roller 3 being 
inclined downward as indicated by the solid line in FIG. 5, the 
photoconductor 1 tends to be skewed in the direction of the arrow D in 
FIG. 5. When the skew exceeds its tolerance in the direction of the arrow 
D, the photosensor 10a in the previously described skew detection means 
detects that skew. Specifically, the output signal of the light reflection 
type photosensor 10a is changed from the H signal to the L signal. When 
the skew in the direction of the arrow D is detected, the solenoid 15 is 
energized, so that the roller moving member 14 is turned clockwise (in 
FIG. 4) about the fulcrum 13. By that turn of the roller moving member 14, 
the end portion 3a of the roller 3 is moved upwards as shown by the broken 
lines in FIG. 5, whereby the skew of the photoconductor 1 in the direction 
of the arrow D is stopped, and the photoconductor 1 is then moved in the 
direction of the arrow C. When the photoconductor 1 is moved in the 
direction of the arrow C and its skew comes within the tolerable range, 
the output of the photosensor 10a returns to the H signal, so that the 
solenoid 15 is deenergized and the roller 3 returns to its initial 
position as shown by the solid line in FIG. 5. By the repetition of the 
above-described steps, the skew of the photoconductor 1 is continuously 
corrected. 
In this embodiment, as the skew detection means, the light-reflection type 
photosensors and the detection patterns are disposed on the opposite end 
portions of the photoconductor. However, the present invention is not 
limited to such structure. For instance, the following skew detection 
means can be employed: A detection pattern is attached to an entire 
peripheral side portion of the photoconductor (not on the two opposite 
side portions of the photoconductor), and two light-reflection type 
photosensors are disposed side by side, with one of the photosensors being 
located in the detectable area of the detection pattern, and the other 
photosensor being located in the detectable area of the photoconductor, 
and the skew of the photoconductor is detected by the change in the level 
of the output signals from those two photosensors. 
Further, it is not always necessary that such a detection pattern be 
disposed integrally with the photoconductor. The detection pattern can be 
disposed separately from the photoconductor, for instance, under the 
photoconductor, with a photosensor disposed so as to detect the detection 
pattern; the only indispensable requirement is that the reflectance of the 
detection area be changed when the skew of the photoconductor takes place. 
The control and correction of the skew of the photoconductor are done as 
described above. However, as mentioned previously, if switching of the 
inclined position of the roller 3 is done during the recording operation, 
such switching may have an adverse effect on the images obtained. 
Therefore, in the present invention, when the recording apparatus is in the 
recording operation period (defined below), the operation of the drive 
switching means for the roller-inclination mechanism is inhibited even if 
the skew detection signal is output from the skew detection means, and 
when the recording operation has been finished, the drive switching means 
is operated. In the above, the "recording operation period" signifies a 
period in which the recording apparatus is in such an operation state that 
an adverse effect would be had on the image formation if the skew of the 
photoconductor were to be then corrected. 
For instance, in the above-described embodiment, when the roller 3 is 
inclined during the operation of the image transfer charger 7, the gap 
between the photoconductor 1 and the image transfer charger 7 is abruptly 
changed, so that the image transfer efficiency changes before and after 
the inclination of the roller 3. The result is that the transferred image 
may be blurred. Likewise, since it is considered that the peripheral speed 
of the photoconductor 1 may be temporarily changed when the roller 3 is 
inclined, it is preferable that the roller 3 not be inclined during the 
exposure operation by the exposure apparatus 5. In particular, when the 
exposure is done by laser beams, images may be blurred considerably. 
Therefore, the "recording operation period" signifies an entire period 
including various steps required for image formation, such as charging, 
exposure, image transfer and image fixing, or a period of one of the 
above-mentioned steps which have significant effects on the image 
formation. In contrast, the non-recording period signifies the periods 
before and after the image formation, which does not include a cleaning 
period. In the case where individual sheets are fed successively for 
continuous printing and the sheet feeding intervals are so short that, 
before image transfer is done on the preceding sheet, image formation is 
done for the following sheet, the non-recording period signifies such 
sheet feeding intervals. 
Thus, even if the skew is detected during the recording operation period, 
the correction of the skew is not initiated during that period, but the 
correction is initiated when the non-recording period begins. In the 
present invention, a control circuit is constructed in order to perform 
the above-described operation. Referring to FIG. 6, there is shown an 
example of such a control circuit. This control circuit includes a NAND 
circuit 18. To one input terminal of the NAND circuit 18, there is input 
an output signal, for example, an output signal S.sub.1 which is output 
from the photosensor 10a in the skew detection means, while to the other 
input terminal of the NAND circuit 18, there is input a timing signal 
S.sub.2, which is output from the recording apparatus, the level of which 
timing signal S.sub.2 is changed to the L level during the non-recording 
operation period. An output signal S.sub.3 of the NAND circuit 18 is 
transmitted to the solenoid 15 which serves as the drive switching means 
for the roller-inclination mechanism 12. As mentioned previously, the 
level of the output signal S.sub.1 of the photosensor 10a is set so as to 
be at the level L at the time of detection of the skew of the 
photoconductor, and at the level H at the time of non-detecting the skew 
of the photoconductor. The timing signal S.sub.2 is set so as to be at the 
level H at the time of image recording, and at the level L at the time of 
non-image recording. 
As shown in the time chart in FIG. 7, the NAND circuit 18 outputs an H 
signal only when both the signals S.sub.1 and S.sub.2 are at the level L 
(hereafter, signals at the level L are referred to as the L signals). 
Specifically, the NAND circuit 18 outputs the H signal only when the 
photoconductor is skewed in the direction of the arrow D and the recording 
apparatus is in the non-recording period, so that the solenoid 15 is 
energized, and the driven roller 3 is moved from a position shown by the 
solid lines to a position shown by the broken lines in FIG. 5. 
Thus, even if the photoconductor is skewed, the drive switching means does 
not work as long as the image recording continues. When the recording 
period is finished, for example, at the completion of the image transfer 
process, the drive switching means is actuated and begins to work. In this 
case, the skew of the photoconductor is corrected with a time lag after 
the detection thereof. However, since the skew of the photoconductor does 
not develop quickly, substantially there is no problem with such a time 
lag. 
The embodiment described is intended to be merely exemplary and those 
skilled in the art will be able to make variations and modifications in it 
without departing from the spirit and scope of the invention. 
For instance, one such variation is as follows. The skew of the 
photoconductor in the direction of the arrow D is detected by the 
photosensor 10a, and the roller 3 is moved from the position shown by the 
solid lines to the position shown by the broken lines in FIG. 5, and that 
position is maintained even if the output signal of the photosensor 10a 
returns to the level H. When the photosensor 10b located opposite the 
photosensor 10a detects the skew in the direction of the arrow C, the 
roller 3 is returned to the position indicated by the solid lines in FIG. 
5. In this case, a second magnetic solenoid is employed instead of the 
tension spring 16 shown in FIG. 4, and that solenoid is controlled by an 
auxiliary control circuit which is similar to the circuit shown in FIG. 6. 
Specifically, the second magnetic solenoid is energized only when the 
photosensor 10b detects the skew in the direction of the arrow C and the 
recording apparatus is in the non-recording operation period. That the 
outputs of the two photosensors 10a and 10b come to the level L at the 
same time in the configuration shown in FIG. 3 does not occur. Therefore, 
it does not occur that the first and second solenoid work simultaneously. 
However, since it is not economical to supply power continuously to both 
magnetic solenoids when the NAND circuit outputs signals, a structure is 
preferred, in which the roller 3 is moved to the position indicated by the 
solid lines or to the position indicated by the broken lines in FIG. 5 by 
supplying power to one of the magnetic solenoids for a short time and 
stopping the roller mechanically by a pawl, that pawl being released when 
the other magnetic solenoid is energized. 
In the above modification, the roller-inclination mechanism 12 is disposed 
at one end portion 3a of the roller 3, and the solenoid is actuated in 
association with one of the photosensors in the skew detection means. A 
further modification can be employed, in which the roller-inclination 
mechanisms are disposed on both opposite end portions of the roller 3 and 
the magnetic solenoids for those roller-inclination mechanisms are 
activated in association with the two photosensors of the skew detection 
means. Specifically, the roller 3 can be maintained at a position parallel 
to the roller 2 when the solenoid is deenergized. When one of the 
photosensors 10a and 10b disposed on the opposite sides of the 
photoconductor 1 in FIG. 2--for example, the photosensor 10b--outputs a 
skew detection output and detects that the skew occurs on the side of the 
photosensor 10a, the side of the roller end portion 3a is pushed in such a 
direction as to reverse the skew, for example, in the upward direction, 
and that position is maintained. When the skew detection signal of the 
photosensor 10b ceases by the skew being corrected in the course of 
repeated copying processes, the roller 3 is returned to its initial 
parallel position. 
In the detection elements for the skew detection means, in the previously 
explained embodiment, light-reflection type photosensors and detection 
patterns are employed. Instead of such detection elements, there can be 
employed a skew detection means comprising a pair of contact members, 
which are supported so as to be rotable about the center of each contact 
member, one end portion of each of the contact members being in contact 
with one or the other side of the photoconductor, and the other end 
portion of each of the contact members having an interrupting member, and 
a pair of photo interrupters. In the case of this skew detection means, 
when the photoconductor is skewed, the contact member is pushed by the 
skewed photoconductor, so that the interrupting member which is integral 
with the contact member is moved to a light interrupting position, and the 
output signal generated from the photo interrupter is changed, whereby the 
skew of the photoconductor is detected. This skew detection means can be 
modified by replacing the photo interrupters and the interrupting members 
by microswitches and operation members therefor. 
In the embodiment of a skew control apparatus according to the present 
invention, an apparatus capable of inclining rollers over which the 
photoconductor belt is trained so as to reverse the skewing direction has 
been explained. However, the present invention is not limited to such an 
embodiment. 
For example, a method of disposing a guide roller for lifting slightly one 
side portion of the photoconductor belt when the photoconductor belt is 
skewed, and bringing about a difference in tension applied to the 
photoconductor belt by the two roller shafts of the photoconductor, 
thereby reversing the skew direction of the photoconductor belt, can be 
applied to the present invention. 
To a skew control system in which variations in load applied to the belt 
drive system occur when the skew direction is controlled, thereby the 
peripheral speed of the photoconductor belt being temporarily changed, the 
present invention can also be applied with all adverse effects on the 
recorded images at the time of skew reversing being eliminated.