Belt slippage correcting device which controls movement of the belt in a direction perpendicular to the belt transporting direction

A belt slippage correcting device and method for correcting slippage of a belt which is transported in a predetermined direction. The belt slippage correcting device has a slippage detecting device for detecting slippage of the belt in a direction perpendicular to the belt transporting direction and a moving device for moving the belt in the direction perpendicular to the belt transporting direction and control device for controlling the moving device based on the detected slippage of the belt by the belt slippage detecting device The slippage of the belt is corrected while the moving velocity of the belt in the direction perpendicular to the belt transporting direction is maintained within a predetermined scope of velocity. The device and method may be used in an image forming apparatus, such as an electrophotographic image forming apparatus.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention pertains to a belt slippage correcting device used to 
correct the slippage of a belt conveyed such that it circulates along a 
prescribed route. Belts that are conveyed such that they circulate along a 
prescribed route include, for example, in the area of electrophotographic 
image forming devices, a belt-shaped photosensitive member, an 
intermediate transfer belt and a fusing belt used in image formation. 
2. Description of the Related Art 
Conventionally, various types of devices have been proposed as belt 
slippage correcting devices to correct the slippage of a belt conveyed 
such that it circulates along a prescribed route. For example, in the area 
of electrophotographic image forming devices, many belt slippage 
correcting devices that correct the position of the belt by changing the 
tilt angle of one of the suspension rollers supporting the belt have been 
proposed, as belt slippage correcting devices to correct the slippage of a 
belt-shaped photosensitive member, intermediate transfer belt, fusion 
belt, etc. However, these belt slippage correcting devices have low 
correction accuracy and in particular, when used on belts used for image 
formation, such as a belt-shaped photosensitive member and intermediate 
transfer belt, there are cases where color discrepancies and image 
discrepancies occur due to said slippage. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a slippage correcting 
device capable of performing highly accurate correction of the slippage of 
a belt. 
Another object of the present invention is to provide a belt slippage 
correcting device that corrects the slippage of a belt used for image 
formation in an electrophotographic image forming device with high 
accuracy such that color discrepancies and image discrepancies may be 
reduced. 
Yet another object of the present invention is to provide a slippage 
correcting device that can perform correction of the slippage of a belt 
with high accuracy such that a prescribed point on the belt in the 
direction perpendicular to the direction of conveyance may approach a 
preset target position at a prescribed belt speed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a cross-sectional view of electrophotographic image forming 
device 100. 
Electrophotographic image forming device 100 is an electrophotographic 
printer that forms an image after receiving data from a host computer, and 
primarily comprises photosensitive unit 1, intermediate transfer unit 2, 
optical system unit 3, developing unit 4, paper feed cassette 5, copy 
paper transfer unit 6 and fusing device 70. 
Photosensitive unit 1 houses photosensitive member 10 and image forming 
elements such as a charging device and a cleaner located around 
photosensitive member 10. Photosensitive member 10 is uniformly charged by 
the charging device after it is cleaned by the cleaner. 
Print head 3 houses a laser diode, scanning optical system, etc. It 
controls the laser diode based on data from the host computer, and forms 
an electrostatic latent image on photosensitive member 10 that is 
uniformly charged. 
Developing unit 4 is located such that it can rotate around developing unit 
shaft 11. Developing unit 4 houses developing devices 4Y, 4M, 4C and 4K. 
The selected developing device faces photosensitive member 10 by means of 
the rotation of developing unit 4. The developing device that comes to 
face photosensitive member 10 develops the electrostatic latent image 
formed on photosensitive member 10 forming a toner image. 
Paper feed cassette 5 feeds printing paper P housed inside it at prescribed 
times and transfers the paper to the area between timing rollers 30 and 
31. 
The construction of intermediate transfer unit 2 will now be explained in 
detail. FIG. 2 is a cross-sectional view showing the construction of 
intermediate transfer unit 2. 
Intermediate transfer unit 2 primarily comprises intermediate transfer belt 
20 drive roller 21, energizing roller 22 second transfer facing roller 23, 
intermediate transfer belt cleaner 25, first transfer pre-roller 27 and 
first transfer roller 28. Intermediate transfer belt 20 is a continuous 
loop belt having a length of 640 mm and a width of 350 mm, and is made of 
polycarbonate. The size of intermediate transfer belt 20 should be at 
least 50 mm larger and preferably at least 100 mm larger than the copying 
paper both in the primary and secondary scanning directions, respectively 
where A4 size paper is used as the largest printing paper. 
Intermediate transfer belt 20 is suspended with tension over drive roller 
21, energizing roller 22, second transfer facing roller 23, first transfer 
pre-roller 27 and first transfer roller 28, and is in contact with 
photosensitive member 10 by means of first transfer roller 28. Drive 
roller 21 is formed using rubber on its surface A drive force is 
transmitted to it from main motor 15 via a drive force transmitting device 
(not shown in the drawing) comprising gears, a timing belt, etc., and 
drive roller 21 rotates in the direction indicated by an arrow in the 
drawing. The rotation of drive roller 21 is transmitted to intermediate 
transfer belt 20, which is conveyed counterclockwise at approximately the 
same speed as the rotation speed of photosensitive member 10. Energizing 
roller 22 is given force in the direction indicated by arrow a, providing 
tension to intermediate transfer belt 20 such that it will not slacken 
between the rollers. Because of this, the rotation of drive roller 21 is 
efficiently transmitted to intermediate transfer belt 20. 
First transfer roller 28 is given force in the direction indicated by arrow 
b in order to put intermediate transfer belt 20 into contact with 
photosensitive member 10, and first transfer bias voltage is applied to 
it. The toner image formed on photosensitive member 10 is transferred onto 
intermediate transfer belt 20 by means of the first transfer bias voltage 
applied to first transfer roller 28. The toner image transferred onto 
intermediate transfer belt 20 is carried to the area facing second 
transfer roller 24 while sticking to intermediate transfer belt 20. 
Grounded second transfer facing roller 23 is located at the area where 
intermediate transfer belt 20 comes to face second transfer roller 24. 
Second transfer bias voltage is applied to second transfer roller 24, 
which can be pressed onto or moved away from intermediate transfer belt 
20. Second transfer roller 24 has drive motor 16 located separately from 
main motor 15. A drive force is transmitted to it from drive motor 16 via 
a drive force transmitting device (not shown in the drawing) comprising 
gears, pulleys, a timing belts etc. Second transfer roller 24 may be 
rotated in the direction indicated by an arrow in the drawing at a speed 
faster than intermediate transfer belt 20 by using for intermediate drive 
motor 16 a drive voltage different from that used for main motor 15 and 
changing the gear ratio, etc., of the drive transmitting device. Second 
transfer roller 24 shifts its position in response to the carriage of 
printing paper P. When said second transfer roller 24 is shifted inward, 
it comes into contact with second transfer facing roller 23 via 
intermediate transfer belt 20. The number and arrangement of suspension 
rollers over which intermediate transfer belt 20 is suspended are not 
limited to the configuration of this embodiment and may be set depending 
on the configuration of the image forming device. 
Intermediate transfer belt cleaner 25 has cleaning blade 26 that can make 
and lose contact with intermediate transfer belt 20. Cleaning blade 26 is 
formed using an elastic material such as silicone rubber. When it is in 
contact with intermediate transfer belt 20, cleaning blade 26 presses 
against intermediate transfer belt 20 at a maximum pressure of 200 g and 
removes residual toner on said belt. 
First transfer pre-roller 27 is constructed such that one end is fixed and 
the other end may be shifted (tilted) in the +s and -s directions. By 
controlling the tilt of first transfer pre-roller 27, the lateral movement 
of intermediate transfer belt 20 may be controlled. In this embodiment, 
shifting is executed with regard to the first transfer pre-roller. 
However, the present invention is not limited to this and another 
suspension roller or rollers may be selected for this purpose. 
FIG. 3 is an enlarged cross-sectional view showing the shifting mechanism 
of first transfer pre-roller 27. FIG. 4 is an enlarged perspective view 
showing important areas of the shifting mechanism of first transfer 
pre-roller 27. The shifting mechanism of first transfer pre-roller 27 is 
explained with reference to FIGS. 3 and 4. 
The shifting mechanism of first transfer pre-roller 27 primarily comprises 
stepping motor 51, motor gear 52, drive gear 53, drive pulley 54, drive 
wire 55, idle pulleys 56 and 57 and bearing holder 58 and bearing 59. 
Stepping motor 51 can rotate in either direction by a specified angle, and 
has motor gear 52 attached to its rotational axis. The rotation of 
stepping motor 51 is transmitted to drive gear 53 via motor gear 52. 
Stepping motor 51 can rotate motor gear 52 fifty steps in the +s direction 
and fifty steps in the -s direction. However, in this embodiment, the 
range of steps is limited to ten steps in the +s direction and ten steps 
in the -s direction. Drive pulley 54 is located on drive gear 53, said 
drive pulley being formed as one unit with said drive gear 53. Drive 
pulley 54 rotates together with drive gear 53 as drive gear 53 rotates. 
Drive wire 55 is wound around drive pulley 54 and idle pulleys 56 and 57 
in a loop fashion. Drive wire 55 is conveyed via the rotation of drive 
pulley 54. Bearing holder 58 is fixed to drive wire 55 such that bearing 
59 may be located between idle pulleys 56 and 57, and it moves in the +s 
and -s directions by means of drive wire 55. 
Bearing holder 58 holds shaft 27a of first transfer pre-roller 27 using 
bearing 59. Shaft 27a of first transfer pre-roller 27 shifts by 16 .mu.m 
via the movement of bearing holder 58 each time stepping motor 51 rotates 
one step. 
Furthermore, as shown in FIG. 4, bearing 60 is located on one shaft 27b of 
first transfer pre-roller 27, and slide bearing 63 and bearing 59 are 
located on the other shaft 27a. Bearing 60, slide bearing 63 and bearing 
59 rotatably hold first transfer pre-roller 27 with shaft 27a and shaft 
27b as the rotation axis. 
Slide bearing 63 is inserted in regulating hole 64 located in shaft 
shifting direction regulating plate 61, and can move in regulating hole 
64. Bearing 60 is loosely held by bearing holding plate 65. By means of 
this construction, when slide bearing 63 has moved in regulating hole 64, 
first transfer pre-roller 27 becomes tilted with bearing 60 as the 
supporting point. Therefore, through the drive of stepping motor 51, shaft 
27a shifts with bearing 60 as the supporting point. The direction and 
range of shifting of first transfer pre-roller 27 may be regulated by 
means of the configuration of regulating hole 64. 
Regarding the direction of shifting of first transfer pre-roller 27, it is 
most efficient to shift first transfer pre-roller 27 in the directions 
perpendicular to the line that equally divides the suspension angle of 
intermediate transfer belt 20 over first transfer pre-roller 27, as shown 
in the drawing in FIG. 5. 
Where first transfer pre-roller 27 is shifted from the parallel position, a 
force that laterally moves intermediate transfer belt 20 is generated. In 
other words, by shifting first transfer pre-roller 27 by means of stepping 
motor 51, intermediate transfer belt 20 may be laterally moved such that 
its position may be adjusted. In this embodiment, first transfer 
pre-roller 27 is tilted by shifting only one end of said roller. However, 
it is also acceptable to use a construction in which the roller is tilted 
by shifting both ends of the roller. 
FIG. 6 is a perspective view of intermediate transfer unit 2. In 
intermediate transfer unit 2, drive roller 21, energizing roller 22, 
second transfer facing roller 23, first transfer pre-roller 27 and first 
transfer roller 28 are rotatably located between side panels 70 and 71. 
Intermediate transfer belt 20 is suspended over said rollers. 
Around intermediate transfer unit 2 are located first position detection 
sensor 40 to detect the lateral position of intermediate transfer belt 20 
and second position detection sensor 41 to read the position of 
intermediate transfer belt 20 in the direction of conveyance First 
position detection sensor 40 comprises light emitting element 40a and 
light receiving element 40b, and is constructed such that the light 
emitted by light emitting element 40a is detected by light receiving 
element 40b. While Omron's laser parallel light linear sensor (set 
resolution 2.2 .mu.m) is used for first position detection sensor 40 in 
this embodiment, as long as the sensor is capable of performing position 
detection, other sensors may be used instead. On the other hand, second 
position detection sensor 41 is a reflection-type photosensor in which 
light emitting element 41a and light receiving element 41b are integrally 
housed together and is constructed such that light emitted by light 
emitting element 41a is projected onto an object and light reflected by 
the object is detected by light receiving element 41b. 
FIG. 7 is a block diagram of the control circuit of image forming device 
100. First CPU 45 controls various elements including the main motor, 
print head 3, intermediate transfer belt cleaner 25, timing rollers 30 and 
31, second transfer roller 24 and first transfer roller 28, based on the 
inputs from the host computer, operation panel, second CPU 46, etc. 
Second position detection sensor 41 is connected to second CPU 46. The 
output value from second position detection sensor 41 is sent to second 
CPU 46. Second CPU 46 detects timing mark M formed on intermediate 
transfer belt 20 on the side of side panel 71, and generates a timing 
signal from the output value from second position detection sensor 41. 
First position detection sensor 40 is connected to second CPU 46 via 
amplifier unit 47 and digital panel meter 48. The output value from first 
position detection sensor 40 is amplified by amplifier unit 47, after 
which it is converted into one of five digital signals by digital panel 
meter 48 and sent to second CPU 46. 
FIG. 8a is a drawing showing the relationship between the light receiving 
width of light receiving member 40c of first position detection sensor 40 
and intermediate transfer belt 20. Light receiving member 40c of first 
position detection sensor 40 outputs a voltage signal proportional to the 
amount of light received after receiving light from light emitting element 
40a. A part of intermediate transfer belt 20 is inserted between light 
emitting element 40a and light receiving member 40c such that a part of 
the light irradiated by light emitting element 40a onto light receiving 
member 40c is blocked. Therefore, light receiving member 40c generates a 
voltage signal corresponding to the amount of light received by the part 
not covered by intermediate transfer belt 20. 
FIG. 8b is a graph showing the relationship between the light receiving 
width of light receiving member 40c and the voltage output from first 
position detection sensor 40. From this graph, it is seen that the output 
voltage from first position detection sensor 40 changes in response to the 
position of intermediate transfer belt 20. In this embodiment, the 
direction in which the output value from first position detection sensor 
40 increases is deemed direction H, and the direction in which said output 
value decreases is deemed direction L. 
Table 1 shows the relationship between the voltage signal from first 
position detection sensor 40 and the determination level. 
TABLE 1 
______________________________________ 
Determination Intermediate transfer 
Reference 
level belt reference position 
potential 
______________________________________ 
HH (second level) 
2.090mm 3.428V 
H (first level) } 67.mu.m 
2.023mm 3.399V 
M (target position) } 46.mu.m 
1.977mm 3.378V 
L (first level) } 67.mu.m 
1.910mm 3.348V 
LL (second level) 
______________________________________ 
Digital panel meter level determination 
The reference potential in Table 1 indicates the border value of the 
voltage signal from position detection sensor 40, and the reference 
position indicates the position of intermediate transfer belt 20 that 
corresponds to the reference potential based on the graph in FIG. 8b. The 
numbers between the reference positions indicate the distances between 
respective reference positions. The voltage signal from first position 
detection sensor 40 is amplified by amplifier unit 47 and input to digital 
panel meter 48. Digital panel meter 48 converts the voltage signal into 
one of five digital signals (HH, H, Mg L or LL) (hereinafter called `the 
determination level`) and outputs the same. 
This determination level is input to second CPU 46. A timing signal and an 
intermediate transfer belt drive signal are also input to second CPU 46 
from second position detection sensor 41 and first CPU 45, respectively. 
On the other hand, motor driver 49 that drives stepping motor 51, as well 
as amplifier unit 47, is connected to the output ports of second CPU 46. 
Stepping motor 51 is driven by a drive signal output from second CPU 46 
and input to motor driver 49, and the light emission by light emitting 
element 41a of first position detection sensor 40 is controlled by ON/OFF 
signals output to amplifier unit 47. Since an element that outputs laser 
light is used for light emitting element 41a in this embodiment, control 
is carried out in order to ensure safety so that laser light is emitted 
only when measurement is performed. 
If intermediate transfer belt 20 slips, it moves in direction H or 
direction L, which are perpendicular to the direction of conveyance. On 
the other hand, where the conveyance of intermediate transfer belt 20 is 
stable and said belt does not slip, it does not move either in direction H 
or direction L. The position of first transfer pre-roller 27 in this 
condition is deemed the stable position. However, in actuality, the 
condition of belt conveyance changes due to various changes over time, 
including changes in the ambient conditions, wear of the suspension 
rollers and deterioration of the belt itself, and therefore the belt 
conveyance position will not become completely stable. Therefore, the 
position of first transfer pre-roller 27 at which intermediate transfer 
belt 20 does not move either in direction H or direction L is deemed the 
provisional stable position. This provisional stable position changes as 
the conveyance condition pertaining to intermediate transfer belt 20 
changes. 
By shifting one end of first transfer pre-roller 27 from the provisional 
stable position in the +s direction or the -s direction, the position of 
intermediate transfer belt 20 can be corrected toward direction H or 
direction L. When one end of first transfer pre-roller 27 is shifted from 
the provisional stable position in the +s direction or the -s direction so 
that it becomes tilted by angle .theta., first transfer pre-roller 27 
becomes tilted by angle .theta. relative to the direction of conveyance of 
intermediate transfer belt 20 as well. Therefore, the direction in which 
first transfer pre-roller 27 conveys intermediate transfer belt 20 also 
becomes tilted by angle .theta.. Through this tilting, intermediate 
transfer belt 20 moves in the direction perpendicular to the direction of 
conveyance Where first transfer pre-roller 27 is shifted in the +s 
direction, intermediate transfer belt 20 moves toward direction H, and 
where first transfer pre-roller 27 is shifted in the -s direction, 
intermediate transfer belt 20 moves toward direction L. 
Amount of movement .DELTA.d of intermediate transfer belt 20 is expressed 
using a general formula shown below, if the length of intermediate 
transfer belt 20 is l and the tilt angle of first transfer pre-roller 27 
is .theta.. 
EQU .DELTA.d&gt;l.times.tan .theta. 
FIG. 9 is a flow chart showing the operation control sequence of 
electrophotographic image forming device 100. The control program shown in 
this operation control flow chart is mainly integrated in first CPU 45 and 
controls the operations of electrophotographic image forming device 100. 
When the main switch of electrophotographic image forming device 100 is 
turned ON, the sequence of the operation control flow chart begins, and 
initialization takes place in step S10. When initialization is completed, 
warmup begins in step S20 and the drive of intermediate transfer belt 20 
begins in step S30. Where the drive of intermediate transfer belt 20 
begins, an intermediate transfer belt drive signal is sent from first CPU 
45 to second CPU 46. When it is determined in step S40 that warmup has 
been completed, drive of intermediate transfer belt 20 stops in step S50, 
whereby the preliminary operation is completed. 
Then in step S60, an internal timer starts and it is determined in step S70 
whether a print instruction has been issued from the host computer. Where 
a print instruction is not present, the process advances to step S90 and 
the process returns to step S60 after the time set in the timer elapses. 
Where a print instruction from the host computer is present in step S70, 
the process advances to step S80, in which the printing operation 
subroutine is executed. When the printing operation subroutine in step S80 
is completed, the process advances to step S90. When the time set in the 
timer has elapsed, the process then returns to step S60. 
FIG. 10 shows the printing operation subroutine (step S80) in the operation 
control flow chart of FIG. 9. 
When the process enters the printing operation subroutine of step S80 in 
the operation control flow chart, the drive of intermediate transfer belt 
20 begins in step S100 and an electrophotographic copying operation is 
carried out in step S110. Here again, where the drive of intermediate 
transfer belt 20 begins, an intermediate transfer belt drive signal is 
sent from first CPU 45 to second CPU 46. Photosensitive unit 1, 
intermediate transfer unit 2, print head 3, developing unit 4, etc., are 
controlled in the electrophotographic copying operation of step S110, and 
the electrophotography process including charging, exposure, developing 
and first transfer is executed. Through this electrophotographic image 
forming operation, a toner image is formed on intermediate transfer belt 
20. 
It is then determined in step S120 whether or not the electrophotographic 
image forming operation has been completed, and the electrophotographic 
image forming operation of step S110 is repeated if necessary. For 
example, where a full-color image is to be formed, the electrophotographic 
image forming operation is repeated until four-color images are formed on 
intermediate transfer belt 20 and image overlapping is completed. When the 
completion of the electrophotographic image forming operation is confirmed 
in step S120 second transfer and fusion are carried out in step S130 and 
an image is output. It is determined in step S140 whether or not one or 
more prescribed printing operations are completed. Where two or more 
printing operations are specified, the operations of steps S110 through 
S130 are repeated as many times as specified, and two or more images are 
output. When all printing operations have been completed the drive of 
intermediate transfer belt 20 is stopped in step S150 and the process 
returns to the main routine flow chart. 
FIG. 11 is a belt slippage correcting operation flow chart. The sequence of 
the belt slippage correcting operation flow chart is stored in second CPU 
46 and is executed in tandem with first CPU 45. When the main switch is 
turned ON and second CPU 46 starts running initialization takes place 
first in step S210 in which 0 is substituted for variables Sn (the current 
step position of the stepping motor), St (the value read from the slippage 
correction table), Sc (the number of steps by which to drive the stepping 
motor), and Ss (the provisional stable position). The initial phase is 
given to stepping motor 510 For example, where a four-phase stepping motor 
is used for stepping motor 51, (HH, H, L, LL) are output respectively for 
the four phases (A+, B+, A-, B-). The output of laser diode, light 
emitting element 40a of first position detection sensor 40, is then turned 
OFF and the delay timer to stabilize the output of the laser diode is 
initialized. Further, central position M is substituted for variable 
State0 that indicates the previous determination level and variable State1 
that indicates the current determination level. 
When initialization is completed, the process advances to step S220, in 
which it is determined whether or not an intermediate transfer belt drive 
signal has been issued from first CPU 45. The process waits in this step 
until an intermediate transfer belt drive signal is sent. When an 
intermediate transfer belt drive signal is sent, the process advances to 
step S230. 
Next, in steps S230 and S240, the rising of a timing signal output from 
second position detection sensor 41, to detect a mark indicating a 
reference position on intermediate transfer belt 20, is detected. 
When the rising of a timing signal is detected in steps S230 and S240, 
light emitting element 40a of first position detection sensor 40 is turned 
ON in step S250 and the process advances to step S260. 
In step S260, a second timer delay is set to stabilize light emitting 
element 40a of first position detection sensor 40. 
When the timer set in step S260 counts up to the set time, the current 
determination level is read from digital panel meter 48 and is substituted 
for variable State1. When the reading from digital panel meter 48 is 
completed, the output of light emitting element 40a of first position 
detection sensor 40 is turned OFF in step S280. 
The stepping motor drive subroutine of step S290 and the provisional stable 
position correction subroutine of step S300 are then carried out. The 
details of the stepping motor drive subroutine of step S290 and the 
provisional stable position correction subroutine of step S300 are 
described below. 
After the value of current determination level variable State1 is 
substituted for previous determination level variable State0 in step S310 
the process waits for the timing signal of second position detection 
sensor 41 to turn OFF in step S320 and then returns to step S220. 
FIG. 12 is the stepping motor drive subroutine of step S290 in the belt 
slippage correcting operation flow chart. 
When the stepping motor drive subroutine of step S290 is called in the belt 
slippage correcting operation flow chart, the value located at the 
intersection of the row indicated by the previous determination level 
(variable State0) and the column indicated by the current determination 
level (variable State1) of the slippage correction table 1 shown in Table 
2 is read and substituted for variable St (the value read from the 
slippage correction table). 
TABLE 2 
______________________________________ 
Slippage correction table 1 
Current determination level 
HH H M L LL 
______________________________________ 
Previous HH -1 0 C C C 
determination 
H -2 -1 C C +6 
level M -6 -1 0 +1 +6 
L -6 C C +1 +2 
LL C C C 0 +1 
______________________________________ 
Slippage correction table 1 shown in Table 2 is explained below. 
The symbols + and - attached to the numbers indicate the direction of 
rotation of stepping motor 51. In other words, the + symbol indicates the 
rotation of stepping motor 51 in the +s directions while the - symbol 
indicates the rotation of stepping motor 51 in the -s direction. 
C means that first transfer pre-roller 27 should be returned to the 
provisional stable position, and varies depending on the direction of 
rotation of stepping motor 51 and the control conditions at that time. 
Slippage correction table 1 shown in Table 2 will now be explained. 
Where the previous determination level is M and the current determination 
level is M, the lateral position of intermediate transfer belt 20 is M, 
which is the target position, and there is no need for correction. 
Therefore, stepping motor 51 is not driven. 
Where the previous determination level is M and the current determination 
level is H, this means that intermediate transfer belt 20 has shifted 
laterally from target position M to the H side, and therefore it is 
necessary to return it to target position M. In order to gradually return 
intermediate transfer belt 20 to target position N, stepping motor 51 is 
rotated in the minus direction by one step. By making the number of steps 
one, sudden lateral movement of intermediate transfer belt 20 can be 
prevented. If the provisional stable position is appropriate, intermediate 
transfer belt 20 should not move from target position M in terms of its 
lateral position. However, the fact that the belt moves from target 
position M to the H side means that the provisional stable position was 
wrong. Therefore, it is necessary to change the provisional stable 
position toward the minus direction. This change is carried out in the 
provisional stable position correction subroutine explained below. 
Where the previous determination level is M and the current determination 
level is HH, this means that intermediate transfer belt 20 suddenly has 
shifted laterally from target position M to the H side. Therefore, it is 
determined that some contingency has taken place and stepping motor 51 is 
immediately rotated by six steps in the minus direction. 
Where the previous determination level is H and the current determination 
level is L, this means that intermediate transfer belt 20 has shifted 
laterally from the first level on the H side to the first level on the L 
side, passing target position M. Therefore, it is determined that 
excessive correction and overshoot have occurred, and first transfer 
pre-roller 27 is returned to the provisional stable position for the time 
being. 
Where the previous determination level is H and the current determination 
level is M, this means that intermediate transfer belt 20 that was on the 
H side has returned to the target position in terms of its lateral 
position. Therefore, first transfer pre-roller 27 is returned to the 
provisional stable position. 
Where the previous determination level is H and the current determination 
level is H, this means that intermediate transfer belt 20 continues to be 
at the first level on the H side in terms of its lateral position. 
Therefore, it is necessary to gradually return intermediate transfer belt 
20 to target position M. For this purpose, stepping motor 51 is rotated by 
one step in the minus direction. Here, the amount of movement of stepping 
motor 51 is limited to one step in order to avoid sudden lateral movement 
of intermediate transfer belt 20. 
Where the previous determination level is H and the current determination 
level is HH, this means that intermediate transfer belt 20 has shifted 
further over to the H side, i.e., from the first level to the second level 
on the H side, in terms of its lateral position. Therefore, it is 
necessary to prevent intermediate transfer belt 20 from moving further 
over to the H side from target position M. For this purpose, stepping 
motor 51 is rotated by two steps in the minus direction. By limiting the 
amount of movement of stepping motor 51 to two steps, sudden lateral 
movement of intermediate transfer belt 20 is avoided here. 
Where the previous determination level is HH and the current determination 
level is LL, this means that intermediate transfer belt 20 that was at the 
second level on the H side has shifted laterally to the second level on 
the L side, passing target position M. Therefore, it is determined that 
excessive correction and overshoot have occurred, and first transfer 
pre-roller 27 is returned to the provisional stable position. 
Where the previous determination level is HH and the current determination 
level is L, this means that intermediate transfer belt 20 that was at the 
second level on the H side has shifted laterally to the first level on the 
L side, passing target position M. Therefore, it is determined that 
excessive correction and overshoot have occurred, and first transfer 
pre-roller 27 is returned to the provisional stable position. 
Where the previous determination level is HH and the current determination 
level is M, this means that intermediate transfer belt 20 has returned to 
target position M in terms of its lateral position. Therefore, first 
transfer pre-roller 27 is returned to the provisional stable position. 
Where the previous determination level is HH and the current determination 
level is H, it is determined that intermediate transfer belt 20 that was 
at the second level on the H side is gradually returning to target 
position M in terms of its lateral position, and in order to make sure 
that said belt is properly returning to said target position, the angle of 
first transfer pre-roller 27 is maintained. 
Where the previous determination level is HH and the current determination 
level is HH, this means either that intermediate transfer belt 20 is 
stable at the second level on the H side or is moving further over to the 
H side from said second level in terms of its lateral position. In either 
case, in order to return intermediate transfer belt 20 to target position 
M, it is necessary to control stepping motor 51 such that said belt may 
move to the L side. When this is done, the amount of movement is made 
small by limiting the number of steps used to rotate stepping motor 51 to 
one. This is done in order to prevent intermediate transfer belt 20 from 
suddenly changing its lateral position when it is stable at the second 
level on the H side. 
Where the previous determination level is L and the current determination 
level is H, this means that intermediate transfer belt 20 that was at the 
first level on the L side has shifted laterally to the first level on the 
H side, passing target position M. Therefore, it is determined that 
excessive correction and overshoot have occurred, and first transfer 
pre-roller 27 is returned to the provisional stable position. 
Where the previous determination level is L and the current determination 
level is HH, this means that intermediate transfer belt 20 that was at the 
first level on the L side has shifted laterally to the second level on the 
H side, passing target position M. Therefore it is determined that 
excessive correction and overshoot have occurred and first transfer 
pre-roller 27 is returned to the provisional stable position. 
Where the previous determination level is LL and the current determination 
level is HH, this means that intermediate transfer belt 20 that was at the 
second level on the L side has shifted laterally to the second level on 
the H side, passing target position M. Therefore it is determined that 
excessive correction and overshoot have occurred, and first transfer 
pre-roller 27 is returned to the provisional stable position. 
A value set in this way is substituted for variable St in step S350, after 
which it is determined in step S360 whether this variable St is C, or in 
other words, whether or not movement to the provisional stable position is 
instructed. 
Where variable St is C, the difference between variable Ss (the provisional 
stable position) and variable Sn (the current step position of the 
stepping motor) is substituted for variable Sc (the number of steps by 
which to drive the stepping motor) in step S370 and the process advances 
to step S380. Where variable St is not C, the process advances to step 
S400 at which the process advances in one of two routes, one being the 
case where variable St is positive and another the case where variable St 
is not positive. 
Where variable St is determined to be positive in step S400, the process 
advances to step S410, in which it is determined whether or not a value 
obtained by adding variable St (the value read from the slippage 
correction table) to variable Sn (the current step position of the 
stepping motor) exceeds 10, which is the upper limit of the stepping motor 
51 movable range. Where this value exceeds 10, the process advances to 
step S420 in which the upper limit is set by substituting (10-Sn) for 
variable Sc (the number of steps by which to drive the stepping motor) 
such that stepping motor 51 may rotate to the upper limit. The process 
then advances to step S380. 
Where the sum of variables Sn and St is determined to be 10 or less in step 
S410, the process advances to step S430 in which variable St (the value 
read from the slippage correction table) is substituted for variable Sc 
(the number of steps by which to drive the stepping motor) The process 
then advances to step S380. 
Where variable St is determined not to be positive in step S400, the 
process advances to step S440, in which it is determined whether or not 
the sum of variables Sn and St is less than -10. 
Where said sum is determined to be less than -10 in step S440, the process 
advances to step S450 in which (-10-Sn) is substituted for variable Sc 
such that stepping motor 51 may rotate to the lower limit. The process 
then advances to step S380. 
Where said sum is determined not to be less than -10 in step S440, the 
process advances to step S460 in which variable St is substituted for 
variable Sc, and the process advances to step S380. 
In step S380, stepping motor 51 is driven in accordance with the value of 
variable Sc. 
While in this embodiment a stepping motor, the upper limit and lower limit 
of whose movable range are +50 and -50, is used and the upper limit and 
lower limit are set at +10 and -10, respectively, it is also acceptable to 
use a stepping motor having different movable range upper and lower limits 
and a different minimum stepping angle where necessary. By having a 
smaller minimum stepping angle, more precise control becomes possible. 
Finally, in step S390, the value of variable Sn (the current step position 
of the stepping motor) is replaced with the sum of variable St (the value 
read from the slippage correction table) and variable Sc (the number of 
steps by which to drive the stepping motor), and the process returns to 
the routine of the belt slippage correcting operation flow chart. 
While target position M is set and the belt is returned to target position 
M in terms of its lateral position in this example, as long as the purpose 
is to prevent slippage, it is not necessary to set a target position, and 
the conveyance of the belt may be stabilized at a prescribed position. 
However, the belt is laterally slipping at all times. As a result, if the 
conveyance is stabilized at a prescribed position, the stabilization 
position gradually shifts and eventually exceeds the limit determined by 
the widths of the suspension rollers and the range in which slippage can 
be detected. In order to prevent the occurrence of this phenomenon, 
control is carried out in this example such that the belt returns to 
target position M in terms of its lateral position. 
FIG. 13 shows the provisional stable position correction subroutine shown 
in the belt slippage correcting operation flow chart. 
When the provisional stable position correction subroutine of step S300 is 
called in the belt slippage correcting operation flow chart, first, in 
step S500, it is determined whether or not State0 (the previous 
determination level) is M. Where the result of this determination is NO, 
no correction is made to variable Ss that indicates the provisional stable 
position and the process returns to the routine of the belt slippage 
correcting operation flow chart. Where the result is YES, the process 
advances to step S510. 
In step S510, the process takes different routes depending on the value of 
State1, the current determination level (LL, L, M, H or HH). 
Where the value of State1 is L in step S510, the process advances to step 
S520, in which variable Ss is increased by 1 and the process returns to 
the routine of the belt slippage correcting operation flow chart. This is 
because intermediate transfer belt 20 that was at target position M has 
shifted laterally to the L side, and therefore it is necessary to correct 
the provisional stable position. 
Where the value of State1 is H in step S510, the process advances to step 
S530 in which variable Ss is reduced by 1 and the process returns to the 
main routine of the belt slippage correcting operation flow chart. This is 
because intermediate transfer belt 20 that was at target position M has 
shifted laterally to the H side, and therefore it is necessary to correct 
the provisional stable position. 
Where the value of State1 is M, LL or HH in step S510, the process returns 
to the routine of the belt slippage correcting operation flow chart 
without any further operation being performed. 
As described above, by carrying out the provisional stable position 
correction subroutine, stable belt conveyance with the belt positioned at 
target position M, in which the belt does not slip, becomes possible. 
FIG. 14 is a graph showing the result of the belt slippage correcting 
operation based on the present invention. 
The conditions of the experiment were set as shown below: 
Length of intermediate transfer belt 20: 640 mm 
Width of intermediate transfer belt 20: 350 mm 
Intermediate transfer belt 20 conveyance speed: 150 mm/sec. 
Tension of intermediate transfer belt 20: 1 Kgf 
Suspension angle of intermediate transfer belt 20 over first transfer 
pre-roller 27: 90 degrees 
Material of first transfer pre-roller 27: Rubber (EPDM) 
Diameter of first transfer pre-roller 27: .o slashed.30 
Minimum tilt amount: 16 .mu.m/step 
Target value M in terms of the lateral position of intermediate transfer 
belt 20 was set to be 2 mm. Since it is set in this experiment such that 
bearing 59 shifts by 16 .mu.m each time stepping motor 51 rotates by one 
step and the distance between bearing 59 and bearing 60 is 380 mm, shift 
angle .theta. of first transfer pre-roller 27 can be expressed as tan 
.theta.=0.016/380. 
The graph shown in FIG. 14 is derived by calculating the changes in the 
lateral position of intermediate transfer belt 20 and four-rotation 
deviations obtained as a result of this experiment and plotting said 
changes and four-rotation deviations for every four rotations of 
intermediate transfer belt 20 (rotations 1 through 4, 2 through 5, 3 
through 6, . . . ) A four-rotation deviation is a deviation of the 
measured value of the lateral position while intermediate transfer belt 20 
rotates four times. If a sudden shifting of the belt takes place, this 
four-rotation deviation increases. 
It is seen from the graph shown in FIG. 14 that intermediate transfer belt 
20 is controlled to be around the 2 mm position and the four-rotation 
deviation is also controlled to be within approximately 60 .mu.m, although 
there are several values that deviate from this range. While the target 
value in terms of the lateral position of intermediate transfer belt 20 
was set at 2 mm in this experiments this value may be set otherwise 
depending on the construction of the belt, which is the target of control, 
and the configuration of the lateral position detection sensors. 
As is clear from this graph, by using the present invention, the slippage 
of the belt can be corrected and at the same time the speed of lateral 
movement of the belt can be held to a certain level or lower. If the speed 
of lateral movement of the belt can be held to a certain level or lower, 
in the case of an image carrying belt such as an intermediate transfer 
belt or a belt-shaped photosensitive member in particular, inconveniences 
such as positional discrepancies of dots among images of different colors, 
straight line distortion and color discrepancies may be reduced. For 
example, where a straight line is to be formed along the direction in 
which the intermediate transfer belt is conveyed, if the intermediate 
transfer belt moves laterally, the formed image of the straight line 
becomes distorted. By holding the lateral movement speed of the 
intermediate transfer belt to a certain level or lower, the distortion of 
straight lines may be minimized. In particular, since the resolution of 
the human eye is only around 8 cycles/mm, which may be converted into 62.5 
.mu.m (1 mm/16 lines) in terms of positional discrepancy, as long as the 
positional discrepancy is within a 60 .mu.m range, a human eye can hardly 
see any difference. Because the length of intermediate transfer belt 20 
used in this experiment is 640 mm, where intermediate transfer belt 20 
rotates once with first transfer pre-roller 27 shifted by angle .theta., 
if the amount of lateral movement of intermediate transfer belt 20 is 
.DELTA.d, tan .theta.=.DELTA.d/640 results and .DELTA.d becomes 27 .mu.m. 
Since 27 .mu.m is a value well below the resolution capabilities of the 
human eye, distortion and color discrepancies in the image due to lateral 
movement of intermediate transfer belt 20 are not recognized. 
Incidentally, if the discrepancy as to one dot of the formed image is to 
be lower than the recognition limit, .DELTA.d should preferably be 60 
.mu.m (one dot for a 400 dpi laser optical system). 
FIG. 15 is a belt slippage correcting operation flow chart showing another 
example of the belt slippage correcting operation. In this belt slippage 
correcting operation, the belt is moved close to a pre-set target position 
using a method in which different controls are used when the main switch 
is turned ON or the machine has returned from trouble, such as jamming or 
a failure of the image forming device, and during the regular copying 
process, such that occurrence of color discrepancies and image 
discrepancies is reduced and accuracy is increased 
The sequence of the slippage correcting operation flow chart shown in FIG. 
15 is different from the slippage correcting operation flow chart shown in 
FIG. 11, in that an operation determination subroutine of step S610 and a 
rough adjustment completion determination subroutine of step S710 are 
added and the sequence of the provisional stable position correction 
subroutine in step S700 is different. Other than that the former flow 
chart is the same as the latter flow chart. Therefore, the slippage 
correcting operation flow chart shown in FIG. 15 will be explained 
focusing on differences from the slippage correcting operation flow chart 
shown in FIG. 11. 
When second CPU 46 operates based on the turning ON of the main switch or 
the machine's return from a trouble state, initialization takes place in 
step S600, following which the process enters the operation mode 
determination subroutine in step S610. Where a slippage correcting 
operation was executed previously and data such as provisional stable 
position Sc has been replaced step S600 may be skipped. However, where 
there is a possibility that the tilt of first transfer pre-roller 27 may 
change in the process of returning from a trouble state such as jamming 
because of the mechanical construction, it is better to carry out step 
S600 because if the tilt of first transfer pre-roller 27 changes, the 
previous provisional stable position is rendered meaningless. 
FIG. 16 shows the sequence of the operation mode determination subroutine 
When the process enters the operation mode determination subroutine the 
copy state is determined in step S800. This copy state is a flag raised to 
identify the copy operation mode when the main switch is turned ON or when 
the device has returned from a trouble state such as jamming or 
maintenance in the progress of the control program shown in the operation 
control flow chart of FIG. 9. 
Where it is determined in step S800 that the copy state is `main switch 
turned ON`, the process advances to step S810 and the slippage correction 
table 2 is set as the slippage correction control table used in step S350 
of the stepping motor drive subroutine Table 3 shows an example of 
slippage correction table 2. 
TABLE 3 
______________________________________ 
Slippage correction table 2 
Current determination level 
HH H M L LL 
______________________________________ 
Previous HH -6 0 C C C 
determination 
H -6 -3 C C +6 
level M -6 -1 0 +1 +6 
L -6 C C +3 +6 
LL C C C 0 +1 
______________________________________ 
This slippage correction table 2 is different from the slippage correction 
table 1 explained above only regarding the underlined numbers. 
Specifically, the number of steps by which to drive the stepping motor is 
set to be large when (i) the previous determination level is H and the 
current determination level is H or HH, (ii) the previous determination 
level is HH and the current determination level is HH, (iii) the previous 
determination level is L and the current determination level is L or LL, 
or (iv) the previous determination level is LL and the current 
determination level is LL. Through this setting even where the 
intermediate transfer belt 20 has shifted laterally to a large extent 
while image forming device 100 is not running positional correction can be 
speedily performed during preliminary running. In such a case, since no 
images are formed on intermediate transfer belt 20, sudden lateral 
movement does not give rise to any defective images. 
Next, in step S820, flag Send is set to 0 in order to indicate that rough 
adjustment is being made to the provisional stable position of first 
transfer pre-roller 27, after which the process returns to the routine of 
the slippage correcting operation flow chart. 
Where it is determined in step S800 that the copy state is `returned from a 
trouble state`, the process also advances to step S810 in which the 
slippage correction table 2 is set as the slippage correction table. After 
setting flag Send to 0 in step S820, the process returns to the routine of 
the slippage correcting operation flow chart. 
Where it is determined in step S800 that the copy state is `regular copy`, 
the process advances to step S830 in which the slippage correction table 1 
is set as the slippage correction table. After setting flag Send to 1 to 
indicate that the provisional stable position of first transfer pre-roller 
27 has been found, the process returns to the routine of the slippage 
correcting operation flow chart. 
When the process returns from the operation mode determination subroutine 
shown in FIG. 16 in this way, steps S620 through S690 are executed. Since 
the details of steps S620 through S690 are the same as those of steps S220 
through S290 of the slippage correcting operation flow chart of FIG. 11, 
their explanations are omitted here. The sequence of the stepping motor 
drive subroutine of step S690 is the same as the sequence of the stepping 
motor drive subroutine of step S290 in FIG. 12 except that either slippage 
correction table 1 or slippage correction table 2 selected in the 
operation mode determination subroutine is used for the slippage 
correction table used in step S350. 
When the steps up to step S690 are carried out, the provisional stable 
position correction subroutine of step S700 is executed. 
FIG. 17 shows another example of the provisional stable position correction 
subroutine. The provisional stable position correction subroutine shown in 
FIG. 17 is a method to calculate the provisional stable position by 
obtaining the average of two or more measurement values. By using this 
method, increased precision can be achieved. When the process enters the 
provisional stable position correction subroutine, determination as to the 
counter Count.Ave is carried out in step S850. When this counter value 
reaches 20, the process advances to the routine to correct the provisional 
stable position (steps S860 through S900). Otherwise, the process advances 
to the routine to change average level variable Sa (steps S910 through 
S960) in accordance with the current determination level (State1). 
Where it is determined in step S850 that the counter value is less than 20, 
the process advances to step S910 and then takes different routes in 
accordance with variable State1 that indicates the current determination 
level. 
Where State1 is M, no correction is made to variable Sa and the process 
advances to step S960 without any operation being performed. 
Where State1 is LL, average level variable Sa is reduced by 2 in step S920 
and the process advances to step S960. 
Where State1 is L, average level variable Sa is reduced by 1 in step S930 
and the process advances to step S960. 
Where State1 is H, average level variable Sa is increased by 1 in step S940 
and the process advances to step S960. 
Where State1 is HH, average level variable Sa is increased by 2 in step 
S950 and the process advances to step S960. 
In step S960, the value of counter Count.Ave is increased by 1 and the 
process returns to the routine of the slippage correcting operation flow 
chart. 
Next, where it is determined in step S850 that the counter Count.Ave count 
value has reached 20, the process advances to step S860. In step S860, it 
is determined whether average level variable Sa changed in accordance with 
State1 (determination level) before the value of counter Count.Ave reached 
20 is positive or negative. 
Where variable Sa is 0, State1 (determination level) is output with an 
equal positive/negative distribution during the twenty measurements, and 
therefore the provisional stable position is determined to be correct, 
whereupon the process advances to step S890 without correcting variable Ss 
that indicates the provisional stable position. 
Where variable Sa is negative in step S860, it is determined that the 
average determination level of the twenty measurements inclined to the 
negative side, whereupon variable Ss that indicates the provisional stable 
position is increased by 1 and the process advances to step S890. 
Where variable Sa is positive in step S860, it is determined that the 
average determination level of the twenty measurements inclined to the 
positive side, whereupon variable Ss that indicates the provisional stable 
position is reduced by 1 and the process advances to step S890. 
After correction is made to variable Sc in accordance with the value of 
variable Sa, 0 is substituted for the value of counter Count.Ave and 
variable Sa in steps S890 and S900, respectively, and the process returns 
to the routine of the slippage correcting operation flow chart. 
If correction is made to the provisional stable position based on an 
average value of two or more measurements, more stable control can be 
carried out. While the number of measurements is set at twenty in this 
embodiment, said number is not limited to this. Any other approximate 
number of measurements may be set. 
When the provisional stable position correction subroutine is completed and 
the process returns to the routine of the slippage correcting operation 
flow chart, the rough adjustment completion determination subroutine is 
then executed 
The provisional stable position correction subroutine of FIG. 13 may be 
used in this control example, and the provisional stable position 
correction subroutine of FIG. 17 may be used in the first control example. 
FIG. 18 shows the sequence of the rough adjustment completion determination 
subroutine. 
When the rough adjustment determination subroutine is called in step S710 
of the slippage correcting operation flow chart shown in FIG. 15, the 
process advances to step S1000 in which it is determined whether the 
provisional stable position of first transfer pre-roller 27 is being 
sought based on the content of flag Send set in the operation mode 
determination subroutine of FIG. 16. 
In other words, where flag Send is not 0, or where the provisional stable 
position has been found, the process returns to the routine of the 
slippage correcting operation flow chart without any further operation 
being performed. 
If flag Send is found to be 0 in step S1000, or in other words, where the 
provisional stable position is being sought, the process advances to step 
S1010. It is determined in step S1010 whether or not the lateral movement 
of intermediate transfer belt 20 is stable. In this embodiment, due to the 
design of the slippage correction table, if the lateral position of 
intermediate transfer belt 20 is at target level M twice in a row, it can 
be determined that the lateral position of intermediate transfer belt 20 
is stable at target position M and first transfer pre-roller 27 is at the 
provisional stable position. Therefore, if State0 is M and State1 is M, it 
is determined that the provisional stable position of first transfer 
pre-roller 27 has been found and the process advances to step S1020. It is 
also acceptable for the process to advance to step S1020 after the lapse 
of a prescribed period of time or after intermediate transfer belt 20 has 
rotated a prescribed number of times. In step S1020, flag Send is set to 1 
and the process returns to the routine of the slippage correcting 
operation flow chart. 
If the result of the determination in step S1010 is NO, the process returns 
to the routine of the slippage correcting operation flow chart. 
When the rough adjustment completion determination subroutine is completed, 
the value of current determination level variable State1 is substituted 
for previous determination level variable State0 in step S720 and the 
process waits for the timing signal of second position detection sensor 41 
to turn OFF in step S730. When it turns OFF, the process returns to step 
S620. 
By using this control example, the belt can be returned to a stable state 
when the main switch of the image forming device is turned ON and when the 
running of the belt is resumed after jamming and other problems are 
resolved, as a result of which more stable images can be obtained. 
Correction in the direction of shifting of the slippage correcting roller 
will now be explained. In the present invention, as shown in FIG. 2, 
correction regarding the slippage of the belt is performed by shifting the 
position of first transfer pre-roller 27, which is one of the rollers over 
which intermediate transfer belt 20 is suspended, thereby having it 
function as a slippage correcting roller. Where the rollers over which 
intermediate transfer belt 20 is suspended are exactly parallel, the 
lateral movement characteristics when the belt slippage correcting roller 
corrects the position of intermediate transfer belt 20 are identical 
regardless of the direction in which the slippage correcting roller is 
shifted. However, it is difficult in actual practice to align the rollers 
over which intermediate transfer belt 20 is suspended so as to be exactly 
parallel to one another. Where the suspension rollers are not exactly 
parallel to one another, each suspension roller generates a force that 
shifts the belt in the direction corresponding to the tilting of the 
roller and causes the belt to move laterally in one direction. The 
slippage correcting roller is shifted to the provisional stable position 
in order to prevent this lateral movement, but because the roller system 
itself has a characteristic to shift the belt in one direction, the amount 
of lateral movement changes depending on the direction of shift of the 
slippage correcting roller. 
FIG. 19 is a graph showing the result of measuring the amount of lateral 
movement of the belt for each rotation when the amount of shift (amount of 
tilt) of the steering roller for slippage correction (first transfer 
pre-roller 27 in this embodiment) is changed, using intermediate transfer 
belt 20 in the embodiment shown in FIG. 2. In the graph shown in FIG. 19, 
the horizontal axis indicates the amount of shift (amount of tilt) of the 
steering roller, while the vertical axis indicates the amount of lateral 
movement for each rotation of the belt. As can be seen from this graph, 
while an ideal movement characteristic line is point-symmetrical with 
respect to the amount of shift of the steering rollers the actual lateral 
movement tends to occur with an inclination toward one direction. If 
slippage correction is performed with the slippage correcting mechanism in 
this condition, the lateral movement of the belt changes depending on the 
direction in which the steering roller is shifted. Further, there are 
cases where overshoot occurs, even where the motor is driven by a small 
number of steps, when the steering roller is shifted in the direction in 
which slippage tends to be greater, as well as cases where the belt is not 
sufficiently moved to the target position, even where the roller is 
shifted by the prescribed number of steps, when the steering roller is 
shifted in the direction in which slippage tends to be less. This 
phenomenon can be prevented by driving the motor using the number of drive 
steps corrected into the ideal movement characteristic, by multiplying 
correction constant A or B set in accordance with the direction of shift 
of the steering roller by the value read from the slippage correction 
table, as shown in the graph of FIG. 19. 
FIG. 20 is a flow chart used in the case where correction as described 
above is carried out. The flow chart of FIG. 20 is a modification of the 
stepping motor drive subroutine shown in FIG. 12. Therefore, an 
explanation will be given focusing on differences from the subroutine of 
FIG. 12 and explanations as to processes common to both subroutines are 
omitted. 
In FIG. 20 the value of variable Sc (the number of steps by which to drive 
the stepping motor) is corrected before the stepping motor is driven in 
accordance with the value of variable Sc in step S380. In other words, it 
is determined in step S330 whether the position of first transfer 
pre-roller 27 is on the plus side or the minus side relative to the 
provisional stable position, or namely, whether or not the value of 
variable Sn (the current step position of the stepping motor) is larger 
than variable Ss (the provisional stable position). Where it is larger, 
the process advances to step S331 in which variable Sc is multiplied by 
correction constant B, and where it is not larger, the process advances to 
step S332 in which variable Sc is multiplied by correction constant A. 
Correction constants A and B are values obtained experimentally The actual 
movement characteristic can be corrected to the ideal movement 
characteristic by multiplying variable Sc by one of said values, as shown 
in the graph of FIG. 19. 
Further, intermediate transfer belt 20 may be moved away from 
photosensitive member 10 and run in order to achieve positioning faster 
when no images are being formed. FIG. 21 is a cross-sectional view showing 
the construction of intermediate transfer unit 2 in the case where 
intermediate transfer belt 20 is moved away from photosensitive member 10. 
Intermediate transfer belt 20 is suspended in the condition shown as the 
first route during a regular image forming process. It is pressed onto 
photosensitive member 10 by first transfer roller 28. Drive roller 21 and 
first transfer roller 28 can be moved to positions 21' and 28', 
respectively. When drive roller 21 and first transfer roller 28 move to 
positions 21' and 28' respectively intermediate transfer belt 20 is 
suspended in the condition shown as the second route and is no longer 
pressed onto photosensitive member 10. If second transfer roller 24 is 
moved to position 24' and the pressing of cleaning blade 26 is eliminated 
in this condition, intermediate transfer belt 20 comes into a condition in 
which it may be conveyed backward. Since nothing is externally pressing 
against intermediate transfer belt 20, intermediate transfer belt 20 may 
be moved at high speed both during forward and backward conveyance 
Therefore, positioning of the belt can be performed quickly. 
FIG. 22 shows a subroutine to drive intermediate transfer belt 20 with or 
without said belt being in contact with the outside rollers. This 
subroutine is included in step S110 of the printing operation subroutine 
shown in FIG. 10 In this subroutine, intermediate transfer belt 20 makes 
or loses contact with photosensitive member 10 and is driven in tandem 
with the image forming operation. 
The belt contact/non-contact drive subroutine described above will now be 
explained with reference to FIG. 22. When the process enters the belt 
contact/non-contact drive subroutine in step S110, state determination 
takes place in step S1100. Since the state is 1 in the initialization 
setting, the process advances to step S1110. 
It is then determined in step S1110 whether or not time period t1 to time 
the belt drive has elapsed. If it is determined in step S1110 that time 
period t1 has elapsed, the process advances to step S1120. If time period 
t1 has not elapsed, the process returns to the main routine. The counting 
of timer value t1 begins when the start of the transfer operation is set 
in the main routine 
Next, in step S1120 intermediate transfer belt 20 is driven in the positive 
direction, and in step S1130, drive roller 21 and first transfer roller 28 
are moved such that intermediate transfer belt 20 may be pressed onto 
photosensitive member 10. 
Where a signal that indicates that intermediate transfer belt 20 has become 
pressed onto photosensitive member 10 is detected in step S1140, the 
process advances to step S1150O In step S1150, a transfer flag is set to 
1. This flag indicates that intermediate transfer belt 20 is at a position 
where image transfer is possible. 
In step S1160, timer value t1 is reset. 
In step S1170, timer value t2 is set and counting starts. The value of 
timer value t2 is equal to the time period in which a transfer operation 
is performed. 
In step S1180 the state flag is set to 2. 
Where it is determined in step S1100 that the state flag is 2, the process 
advances to step S1190. 
It is determined in step S1190 whether or not time period t2 equal to the 
time period in which a transfer operation is performed has elapsed. If it 
is determined in step S1190 that time period t2 has elapsed, the process 
advances to step S1200 and where time period t2 has not elapsed the 
process returns to the main routine. 
Next, in step S1200 drive roller 21 and first transfer roller 28 are moved 
such that intermediate transfer belt 20 is moved away from photosensitive 
member 10. 
When a signal indicating that intermediate transfer belt 20 has been moved 
away from photosensitive member 10 is detected in step S1210, the process 
advances to step S1220. In step S1220, the transfer flag is set to 0. This 
flag indicates that intermediate transfer belt 20 is at a position where 
it is not in contact with photosensitive member 10. 
In step S1230, timer value t2 is reset. 
In step S1240, the state flag is set to 3. 
Where it is determined in step S1100 that the state flag is set to 3 the 
process advances to step S1250. 
It is determined in step S1250 whether image formation has been completed 
up to the fourth color. Where it is determined in step S1250 that image 
formation has been completed up to the fourth color, the process advances 
to step S1260 in which the state flag is set to 1 and the process returns 
to the main routine. 
Where it is determined in step S1250 that image formation has not been 
completed up to the fourth color, the process advances to step S1270 in 
which intermediate transfer belt 20 is conveyed at a high speed. When 
second position detection sensor 41 detects timing mark M on intermediate 
transfer belt 20 in step S1280, the drive of intermediate transfer belt 20 
is stopped in step S1290. Finally, in step S1240, the state flag is set to 
3, and the process returns to the main routine. 
As the conveyance route of intermediate transfer belt 20 changes as it 
makes or loses contact with photosensitive member 10, the positional 
relationship among rollers over which intermediate transfer belt 20 is 
suspended change. Therefore, it is necessary to change the provisional 
stable position of first transfer pre-roller 27 in response to the 
conveyance route. 
FIG. 23 is a belt slippage correcting operation flow chart applicable when 
the route of intermediate transfer belt 20 is changed. 
The sequence of this flow chart is partially different from that of the 
slippage correcting operation flow chart shown in FIG. 15, and is used in 
place of the latter sequence. Therefore, only differences from the 
sequence of the slippage correcting operation flow chart of FIG. 15 will 
be explained here. 
The sequence of FIG. 23 is different from that of the slippage correcting 
operation flow chart of FIG. 15 only in that the route determination 
subroutine of step S681 is added. 
After the output of light emitting element 40a of first position detection 
sensor 40 is turned OFF in step S680, the conveyance route determination 
subroutine to determine the conveyance route of intermediate transfer belt 
20 and correct the provisional stable position is executed in step S681. 
The stepping motor drive subroutine of step S690 is then executed. 
FIG. 24 shows the sequence of the route determination subroutine. 
It is confirmed in step S1400 whether the conveyance route of intermediate 
transfer belt 20 has been switched. Where it has not been switched, the 
process returns to the main routine without any further operation being 
performed. 
Where it is confirmed in step S1400 that the conveyance route of 
intermediate transfer belt 20 has been switched and the switching has been 
from the first route to the second route, the process advances to step 
S1410. 
In step S1410, provisional stable position variable Ss1 in the first route 
is subtracted from variable Sn (the current step position of the stepping 
motor), and the value resulting from the subtraction is substituted for 
variable Sft (the amount of shift) In this way, the degree to which 
variable Sn (the current step position of the stepping motor) has shifted 
relative to variable Ss1 (the provisional stable position in the first 
route) is determined. Provisional stable position variable Ss2 in the 
second route is then substituted for provisional stable position variable 
Ss to change provisional stable position data. 
In step S1420, the number of steps that is needed to make correction to the 
provisional stable position as the route is switched from the first route 
to the second route is obtained. In step S1420, a value obtained by 
subtracting variable Ss1 (the provisional stable position in the first 
route) from variable Ss2 (the provisional stable position in the second 
route) is substituted for variable Sc (the number of steps by which to 
drive the stepping motor), and the process advances to step S1450. 
Where it is confirmed in step S1400 that the conveyance route of 
intermediate transfer belt 20 has been switched and the switching has been 
from the second route to the first route, the process advances to step 
S1430. 
In step S1430, provisional stable position variable Ss2 in the second route 
is subtracted from variable Sn (the current step position of the stepping 
motor), and the value resulting from the subtraction is substituted for 
variable Sft (the amount of shift) In this way, the degree to which 
variable Sn (the current step position of the stepping motor) has shifted 
relative to variable Ss2 (the provisional stable position in the second 
route) is determined. Provisional stable position variable Ss1 in the 
first route is then substituted for provisional stable position variable 
Ss to change provisional stable position data. 
In step S1440, the number of steps that is needed to make correction to the 
provisional stable position as the route is switched from the second route 
to the first route is obtained. In step S1440, a value obtained by 
subtracting variable Ss2 (the provisional stable position in the second 
route) from variable Ss1 (the provisional stable position in the first 
route) is substituted for variable Sc (the number of steps by which to 
drive the stepping motor) and the process advances to step S1450. 
In step S1450, stepping motor 51 is driven in accordance with the value of 
variable Sc (the number of steps by which to drive the stepping motor) 
obtained in step S1420 or step S1440. 
In step S1460, a value obtained by subtracting variable Sft (the amount of 
shift) from variable Ss (the provisional stable position) is substituted 
for variable Sn (the current step position of the stepping motor), and the 
process returns to the main routine. 
By carrying out correction as described above, stable slippage correction 
can take place even if the conveyance route is changed. 
FIG. 25 is a belt contact/non-contact drive subroutine showing another 
example of control when intermediate transfer belt 20 makes or loses 
contact with photosensitive member 10 and is driven in one of said two 
contact conditions. The belt contact/non-contact drive subroutine of FIG. 
25 is different from the belt contact/non-contact drive subroutine shown 
in FIG. 22 only in regard to step S1271. Due to step S1271, when 
intermediate transfer belt 20 moves out of contact with photosensitive 
member 10, the direction of conveyance is reversed and said belt is 
conveyed at a high speed. 
Where intermediate transfer belt 20 is conveyed backward in this way, if it 
is conveyed backward with first transfer pre-roller 27 tilted by angle 
.theta. in order to return intermediate transfer belt 20, which has come 
off the target position, to said target position, intermediate transfer 
belt 20 laterally moves in the direction opposite to the direction of its 
lateral movement during forward conveyance. Therefore, during backward 
conveyance of intermediate transfer belt 20, it is necessary to carry out 
a correcting operation different from that used during forward conveyance. 
For the correcting operation during backward conveyance of intermediate 
transfer belt 20, a method to tilt first transfer pre-roller 27 in the 
direction opposite to the direction of tilt used during forward conveyance 
is possible. 
FIG. 26 is a perspective view of the intermediate transfer unit. As shown 
in FIG. 26, if first transfer pre-roller 27 is tilted by angle -.theta., 
during backward conveyance of intermediate transfer belt 20, the same 
correction that is performed when said roller is tilted by angle 0 during 
forward conveyance can be carried out. 
It is also necessary to change the slippage correction table because during 
backward conveyance of intermediate transfer belt 20 the direction of 
shifting of first transfer pre-roller 27 and the direction of lateral 
movement of intermediate transfer belt 20 are reversed as compared to 
forward conveyance. Table 4 shows a slippage correction table 3 that is 
used during backward conveyance The plus and minus symbols of the values 
in the slippage correction table 3 are reversed from the slippage 
correction table 1 shown in Table 2 which is used during forward 
conveyance. 
TABLE 4 
______________________________________ 
Slippage correction table 3 
Current determination level 
HH H M L LL 
______________________________________ 
Previous HH +1 0 C C C 
determination 
H +2 +1 C C -6 
level M +6 +1 0 -1 -6 
L +6 C C -1 -2 
LL C C C 0 -1 
______________________________________ 
In this slippage correction table 3, the number of steps for shifting are 
the same as in the slippage correction table 1 explained above, but the 
direction of shifting is reversed. 
FIG. 27 is a belt slippage correcting operation flow chart used when 
intermediate transfer belt 20 is conveyed forward and backward. 
The sequence of this flow chart is partially different from the sequence of 
the slippage correcting operation flow chart shown in FIG. 15, and is used 
instead of the latter sequence. Therefore, only differences from the 
sequence of the slippage correcting operation flow chart of FIG. 15 will 
be explained here. 
The sequence of the flow chart shown in FIG. 27 is different from the 
sequence of the slippage correcting operation flow chart of FIG. 15 in 
that a first transfer pre-roller angle setting subroutine of step S682 is 
added and the sequence of the stepping motor drive subroutine of step S690 
is different. 
After the output of light emitting element 40a of first position detection 
sensor 40 is turned OFF in step S680, the direction of conveyance of 
intermediate transfer belt 20 is determined in step S682. The first 
transfer pre-roller angle setting subroutine to change the tilt angle of 
first transfer pre-roller 27 is then executed, after which the stepping 
motor drive subroutine of step S690 is carried out. 
FIG. 28 shows the sequence of the first transfer pre-roller angle setting 
subroutine. 
It is confirmed in step S1500 whether or not the direction of rotation of 
intermediate transfer belt 20 has been switched. Where it has not been 
switched the process returns to the main routine. 
Where it is confirmed in step S1500 that the direction of rotation of 
intermediate transfer belt 20 has been switched, the process advances to 
step S1510. 
In step S1510, variable Ss (the provisional stable position) is subtracted 
from variable Sn (the current step position of the stepping motor), and 
the value resulting from the subtraction is substituted for variable Sft 
(the amount of shift). By this, the degree to which variable Sn (the 
current step position of the stepping motor) has shifted relative to 
variable Ss (the provisional stable position) is determined. 
In step S1520, a value obtained by multiplying the value of variable Sft by 
-2 is substituted for variable Sc (the number of steps by which to drive 
the stepping motor). In this step, the number of steps needed to shift 
first transfer pre-roller 27 by the target angle relative to variable Ss 
(the provisional stable position) is obtained. 
In step S1530, stepping motor 51 is driven in accordance with the value of 
variable Sc (the number of steps by which to drive the stepping motor) 
obtained in step 51520. 
In step S1540, a value obtained by subtracting variable Sft (the amount of 
shift) from variable Ss (the provisional stable position) is substituted 
for variable Sn (the current step position of the stepping motor), and the 
process returns to the main routine. 
FIG. 29 shows the stepping motor drive subroutine of step S690 in the belt 
slippage correcting operation flow chart shown in FIG. 27. 
The sequence of this subroutine is partially different from the sequence of 
the stepping motor drive subroutine shown in FIG. 12 and is used instead 
of the latter sequence Therefore, only differences from the stepping motor 
drive subroutine of FIG. 12 will be explained here, and the same numbers 
will be used for common steps. 
When the stepping motor drive subroutine of step S690 is called in the belt 
slippage correcting operation flow chart shown in FIG. 27, it is 
determined in step S335 whether the direction of conveyance of 
intermediate transfer belt 20 is forward or backward. 
If it is forward, the process advances to step S337 in which the slippage 
correction table 1 shown in Table 2 is selected, and the process advances 
to step S350. Since step S350 and steps thereafter are identical to those 
shown in FIG. 12, their explanations will be omitted. 
Where it is determined in step S330 that the direction of conveyance is 
backward, the process advances to step S336 in which the slippage 
correction table 3 shown in Table 4 is selected. The process then advances 
to step S350. 
Where control is carried out during backward conveyance using the same 
number of steps as during forward conveyance but in the direction opposite 
to that during forward conveyance, one slippage correction table will 
suffice for both directions of conveyance, using the sequence of the 
stepping motor drive subroutine as shown in FIG. 30. 
The sequence of this flow chart is partially different from that of the 
stepping motor drive subroutine shown in FIG. 12 and is used instead of 
the latter sequence, in the same way as the sequence of the flow chart of 
FIG. 29. Therefore, only differences from the sequence of the stepping 
motor drive subroutine shown in FIG. 12 will be explained here, and the 
same numbers will be used for common steps. 
When the stepping motor drive subroutine of step S690 is called in the 
sequence of the belt slippage correcting operation flow chart shown in 
FIG. 27, the set value of the slippage correction table 1 is read as 
variable St (the value read from the slippage correction table) in step 
S340. 
It is then determined in step S341 whether the direction of conveyance of 
intermediate transfer belt 20 is forward or backward. 
In the case of forward conveyance, the process advances to step S360. Since 
step S360 and steps thereafter are identical to those shown in FIG. 12, 
their explanations will be omitted. 
Where it is determined in step S341 that the direction of conveyance is 
backward, the process advances to step S342 in which the plus/minus symbol 
of the value of variable St is reversed, and the process advances to step 
S360. By carrying out this control, one slippage correction table can 
accommodate both directions of conveyance. 
A method to perform slippage correction for the forward and backward 
conveyance of intermediate transfer belt 20, which is different from the 
methods explained above in which different control methods are used for 
forward conveyance and backward conveyance, will be now explained. 
FIG. 31 is a perspective view of a modified version of the intermediate 
transfer unit shown in FIG. 26. Therefore, an explanation will be given 
focusing on differences from FIG. 26, and explanations as to common parts 
will be omitted. 
The intermediate transfer unit shown in FIG. 31 has third position 
detection sensor 75 located at a position point-symmetrical with first 
position detection sensor 40 relative to the center line of intermediate 
transfer belt 20. When intermediate transfer belt 20 moves forward, first 
position detection sensor 40 is used, and when said belt moves backward, 
third position detection sensor 75 is used. When intermediate transfer 
belt 20 moves backward, third position detection sensor 75 outputs a belt 
detection value having a plus/minus symbol opposite from that of the value 
output by first position detection sensor 40 used during forward 
conveyance. If slippage correction is carried out using the value output 
from third position detection sensor 75 and in accordance with the flow 
charts of FIGS. 11 through 13 and FIGS. 15 through 18, the shift angle for 
first transfer pre-roller 27 is set in the direction opposite to the shift 
angle used during forward conveyance. Therefore, the same slippage 
correction as during forward conveyance is carried out during backward 
conveyance of intermediate transfer belt 20 as well. 
Further, a control method to accommodate the belt slippage correcting 
operation during backward conveyance only by using a different circuit 
construction will be explained, said method itself being the same as that 
used during forward conveyance. 
FIG. 32 is a modified version of the control circuit block diagram shown in 
FIG. 7. Therefore, an explanation will be given focusing on differences 
from FIG. 7, and explanations regarding common parts will be omitted. 
The control circuit block diagram of FIG. 32 is the same as the circuit 
block diagram of FIG. 7 except that inverter 80 is added. The signal 
output from first position detection sensor 40 via amplifier unit 47 is 
directly input to the digital panel meter during forward conveyance, but 
during backward conveyance, it is input to the digital panel meter via 
inverter 80. Since inverter 80 outputs the detection value of first 
position detection sensor 40 with the opposite absolute value, when 
intermediate transfer belt 20 rotates backward, the belt detection value 
has the opposite absolute value of that for forward conveyance. When 
slippage correction is performed based on this detection value and in 
accordance with the sequences shown in the flow charts of FIGS. 11 through 
13 and FIGS. 15 through 18, the shift angle of first transfer pre-roller 
27 is set in the direction opposite to that during forward conveyance. 
Therefore, the same slippage correction performed during forward 
conveyance of intermediate transfer belt 20 is carried out during backward 
conveyance as well. 
By making the shift angle of the roller used for the purpose of slippage 
correction different for backward conveyance from that for forward 
conveyance as described above, slippage correction can be performed 
regardless of whether the belt is being conveyed forward or backward. 
While the slippage correction device of the present invention was used in 
connection with an intermediate transfer belt in this embodiment, the 
application is not limited to this, and the present invention may be used 
regarding a belt-like photosensitive member and fusing belt as well. 
Although the present invention has been fully described by way of examples 
with reference to the accompanying drawings, it is to be noted that 
various changes and modification will be apparent to those skilled in the 
art. Therefore, unless otherwise such changes and modifications depart 
from the scope of the present invention, they should be construed as being 
included therein.