Image forming apparatus

This invention relates to an image forming apparatus having a plurality of image bearing members including a first image bearing member and a second image bearing member, a transferring material bearing member for bearing the transferring material to respective transfer positions of the plurality of image bearing members, detecting a detecting device for detecting the density of a toner patch formed on the second image bearing member after the toner patch has passed the transfer position of the second image bearing member. The orientation of the image bearing members are such that the time from when the first image bearing member and the transferring material bearing member are detached from each other to when the toner patch reaches the transfer position of the second image bearing member is larger than L/V (sec) where L (mm) is the distance between the first image bearing member and the second image bearing member in a direction of movement of the transferring material bearing member and V (mm/sec) is the speed of the transferring material bearing member.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image forming apparatus such as an 
electrophotographic apparatus and an electrostatic recording apparatus, 
and more particularly to an image forming apparatus for forming an image 
on a transferring material being born by a transferring material bearing 
member. 
2. Related Background Art 
There is a known image forming apparatus for forming a full color image in 
which a toner image is formed on a photosensitive drum as a single image 
bearing member and transferred to a transferring material being born by a 
transfer drum as a transferring material transporting member. In the 
apparatus, when toner in a developing unit for forming the toner image 
decreases, it is necessary to replenish it depending on the decreased 
amount of toner. For this purpose, there is a known method in which a 
toner patch is formed on the photosensitive drum by means of the 
developing unit and the toner in the developing unit is replenished based 
on the density of the toner patch to keep the toner in the developing unit 
to be a predetermined amount. 
For example, EP-A-519710 discloses that a sensor for detecting the density 
of a toner patch is provided in a direction of moving a photosensitive 
drum on the downstream side of a transfer position and on the upstream 
side of a cleaning position. With this structure, the sensor is prevented 
from being soiled by toner scattered from the developing unit. Also, 
EP-A-519710 discloses that when the toner patch passes the transfer 
position such that it is not conveyed to the surface of a transfer drum, 
the pressing of a pressing member for pressing a transferring material 
bearing sheet of a transfer drum toward the photosensitive drum is 
released. 
Further, there is an image forming apparatus in which in order to form a 
full color image at high speed, a plurality of photoreceptors for 
respective colors, i.e., four photoreceptors for respective cyan, magenta, 
yellow and black are provided and toner images of respective colors are 
transferred to a transferring material being born by a transfer belt as a 
transferring material bearing member. In this apparatus, for example, when 
detecting a toner patch formed on the photoreceptor for black toner in the 
same manner as in EP-A-519710, the following problem occurs even though 
the transfer belt is detached from the photoreceptors while the toner 
patch is passing the transfer position. 
That is, when the pressing of pressing members for pressing the transfer 
belt is released, the transfer belt is vibrated due to the release 
operations of the pressing members in the respective magenta, cyan and 
yellow stations and the vibration reaches the black station located 
downstream as the transfer belt moves. As a result, in the downstream 
black station, the toner patch is brought into contact with the vibrating 
transfer belt, so that the toner patch cannot be detected accurately. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an image forming 
apparatus in which a toner patch is prevented from being brought into 
contact with a transferring material bearing member at a transfer 
position. 
It is another object of the present invention to provide an image forming 
apparatus in which distortion of a toner patch is prevented and the 
density of the toner patch can be detected accurately. 
It is still another object of the present invention to provide an image 
forming apparatus in which after the vibration level of a transferring 
material bearing member is lowered, a toner patch is made to pass a 
transfer position. 
The above and other objects and advantages of the present invention will be 
apparent from the following detailed description of the preferred 
embodiments when read in connection with the following drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An image forming apparatus according to an embodiment of the present 
invention will be described with reference to the accompanying drawings. 
FIG. 1 shows a portion of the image forming apparatus of the present 
invention. 
FIG. 2 shows the whole structure of a full color laser beam printer 9 as 
the image forming apparatus wherein the like elements of FIGS. 1 and 2 are 
labelled in a consistent manner. 
The laser beam printer 9 has light scanning means by the use of a plurality 
of laser beams and four photosensitive drums 1M, 1C, 1Y and 1K. This 
printer is provided with four color image forming stations formed by 
disposing image forming means around respective electrophotographic 
photoreceptors. Toner images formed on the respective photosensitive drums 
in the image forming stations are transferred to a transferring material P 
being transported by a transfer belt 6a as transferring material bearing 
means which are moved in opposition to the photosensitive drums. 
The photosensitive drums 1M, 1C, 1Y and 1K as image bearing members are 
disposed in the magenta, cyan, yellow and black image forming stations Pm, 
Pc, Py and Pk, and rotated in directions indicated by arrows in the 
drawing (clockwise direction). Also, around the respective photosensitive 
drums 1M, 1C, 1Y and 1K are disposed the image forming means having 
primary (corona) electrifiers 2M, 2C, 2Y and 2K as latent image forming 
means, scanning optical units 3M, 3C, 3Y and 3K as the light scanning 
means, developing units 4M, 4C, 4Y and 4K as developing means, and 
cleaning units 5M, 5C, 5Y and 5K. 
Further, a transfer section 6 constituting the image forming means has the 
transfer belt 6a commonly used for the image forming stations and transfer 
chargers 6M, 6C, 6Y and 6K as transferring means for the drums. A full 
color image is formed by transferring toner images of the respective 
colors formed on the photosensitive drums successively to the transferring 
material P being supported by the transfer belt 6a and transported in a 
predetermined direction. Also, in order to obtain nip pressure between the 
photosensitive drums and the belt 6a, polyethylene telephthalate 
sheet-like backup pressing members 10M, 10C, 10Y and 10K are provided 
below the respective photosensitive drums 1M, 1C, 1Y and 1K so as to be 
able to push up the back surface of the transfer belt 6a. Owing to the 
on-off control of the backup members 10M, 10C, 10Y and 10K, the 
photosensitive drums and the transfer belt are brought into contact with 
each other or detached from each other. The backup members are controlled 
by solenoids (not shown). 
Also, density sensors 11M, 11C, 11Y and 11K as density detecting means for 
reading the densities of the toner images on the respective photosensitive 
drums are provided in the respective image forming stations. The density 
sensor 11M is disposed on the opposite side of the developing unit 4M via 
the transfer belt 6a, i.e., in the direction of moving the photosensitive 
drum 1M on the downstream side of a transfer position and on the upstream 
side of a developing position so as to avoid toner scattered from the 
developing unit 4M. The density sensors 11C, 11Y and 11K are disposed in 
the same manner as the density sensor 11M. The transferring material P is 
supplied from a cassette 7. When the transfer process has been completed, 
the transferring material P is ejected to a tray 9 via fixing unit 8. 
In each of the developing unit of the above full color image forming 
apparatus, it is preferable to utilize a two component developer 
consisting of toner and carrier from the standpoint of tone of the image. 
As well known, the toner density of the two component developer is a very 
important factor to stabilize the image quality. 
As the development proceeds, the toner of the developer decreases and the 
toner density changes. Therefore, it is necessary to detect the toner 
density of the developer by the use of a developer density control device 
accurately and to replenish the toner in the developing unit in accordance 
with the density change thereby to keep the toner density constant and to 
maintain the image quality preferably. That is, it is desirable to keep 
the amount of toner in the developing unit always constant. 
Now, the developer density control of the image forming apparatus will be 
described, taking the Pk station as example. First, the surface of the 
photosensitive drum 1K is charged by the primary electrifier 2K uniformly. 
Thereafter, the surface of the photosensitive drum 1 is scanned and 
exposed by the scanning optical unit 3K modulated in accordance with an 
image information signal corresponding to a predetermined reference 
density patch, whereby a static latent image corresponding to the 
reference density patch is formed on the photosensitive drum 1. Next, the 
static latent image on the photosensitive drum 1 is developed by the 
developing unit 4K using the two component developer of the mixture of the 
toner and carrier. As a result, the static latent image becomes a 
reference toner image. If the photosensitive drum 1K were left to be in 
contact with the transfer belt 6a, the reference toner image on the 
photosensitive drum 1K would be in contact with the transfer belt 6a and 
distorted. Then, before the reference toner image passes the transfer 
position, the backup member 10K is switched off to detach the transfer 
belt 6a from the photosensitive drum 1K thereby to provide about a 1 mm 
space between the transfer belt 6a and the photosensitive drum 1K. Then, 
the reference toner image is made to pass the 1 mm space. After the 
reference toner image has passed the transfer section, its density is read 
by the density sensor 11K. 
As shown in FIG. 3, the density sensor 11K has a light emitting device 21, 
an optically transparent window 22, a light receiving device 23 and a 
light receiving device 24 for monitoring direct light from the light 
emitting device 21. The density of the reference toner image is read by 
the density sensor 11K and the read value v.sub.1 is compared with a 
preset initial value v.sub.0 in a CPU 22. As a result, the toner of the 
developing unit is replenished when necessary, whereby the image density 
is kept constant. 
However, in the Pk station located downstream, when the backup member 10K 
is switched off to detach the transfer belt 6a from the photosensitive 
drum 1K and the reference toner image is made to pass between the space 
between the photosensitive drum 1K and the transfer belt 6a, the following 
problem occurs, as mentioned above. 
When the backup members 10M, 10C, and 10Y of the upstream M, C and Y 
stations are switched off, the transfer belt 6a is vibrated with the 
equilibrium position as the center within a certain amplitude. The 
vibration is transmitted to the downstream station as the transfer belt 6a 
moves. As a result, depending on its timing, the reference toner image is 
brought into contact with the vibrating transfer belt 6a to be distorted, 
whereby stable developer density control cannot be performed. 
In order to solve this problem, the black toner patch is formed on the 
photoreceptor in the black station in such timing that the black toner 
patch passes the transfer position of the black station after the backup 
members were detached from the transfer belt and a greatest vibration wave 
has passed the black station. 
In particular, after the transferring material P has passed the final black 
station Pk, the backup members 10M, 10C, 10Y and 10K are detached from the 
transfer belt 6a simultaneously so as to detach the transfer belt 6a from 
the photosensitive drums. At this time, the time at which the backup 
members 10M, 10C, 10Y and 10K are detached from the transfer belt 6a is 0 
sec (start). As shown in FIG. 1, as the distance between adjacent transfer 
position is 200 mm, the distance between the transfer position of the 
black station and the transfer position of the magenta station is 
200.times.3=600 mm. 
When the backup members are detached from the transfer belt 6a, the tension 
of the transfer belt 6a is reduced and the transfer belt 6a starts 
vibrating from points at which the backup members 10M, 10C, 10Y and 10K 
was in contact with the photosensitive drum 1M, 1C, 1Y and 1K via the 
transfer belt 6a. This vibration wave is transmitted at a transfer belt 
advancing speed of 200 mm/sec and it takes 600/200=3 sec until a vibration 
wave generated in the most upstream magenta station Pm reaches the most 
downstream black station Pk. The reference black toner patch starts to be 
formed so as to become timing that the reference black toner patch reaches 
the transfer position of the black station after this vibration wave from 
the magenta station Pm has passed the black station Pk. It is to be noted 
that a largest vibration wave is generated at the moment when the backup 
members are detached from the transfer belt and thereafter small vibration 
waves are generated. However, the small vibration waves do not have an 
amplitude which causes the black toner patch to be distorted. Also, after 
passing the black station, the largest vibration wave damps at a position 
of a roller supporting the transfer belt. 
More specifically, a reference image signal generating circuit for 
generating a reference image signal having a signal level corresponding to 
a predetermined density is provided. The reference image signal from the 
generating circuit is supplied to a pulse width modulating circuit which 
generates a laser drive pulse having a pulse width corresponding to the 
predetermined density. The laser drive pulse is sent to the scanning 
optical unit 3K such as a semiconductor laser, which then emits light for 
the time corresponding to the pulse width of the laser drive pulse so as 
to scan the photosensitive drum 1K. Thereby, a reference static latent 
image corresponding to the predetermined density is formed on the 
photosensitive drum 1K. Thereafter, the reference static latent image is 
developed by the developing unit 4K. The resultant patch-like reference 
toner image is read by the density sensor 11K consisting of a LED and a 
photoelectric transfer device thereby to be an output signal. This output 
signal corresponds to the density of the reference toner image, and 
eventually corresponds to the actual toner density of the two component 
developer in the developing unit 4K, i.e., the amount of toner in the 
developing unit 4K. 
The output signal of the photoelectric transfer device is supplied to one 
input of a comparator. To the other input of the comparator is supplied a 
reference signal corresponding to a prescribed toner density (the toner 
density of the initial set value) of the developer from a reference 
voltage signal source. Namely, the comparator compares the prescribed 
toner density with the actual toner density in the developing unit. As the 
result of the comparison of both input signals, the comparator generates 
an output signal indicating that the actual toner density of the developer 
in the developing unit 4K is higher than the prescribed value or an output 
signal indicating that the toner density is lower than the prescribed 
value. Also, when there is no difference between both input signals, an 
output signal indicating it may be generated. The output signal from the 
comparator is supplied to the CPU. 
The reference toner image passes through the space between the transfer 
belt 6a and the photosensitive drum 1K until it reaches the density sensor 
11K. Also, the reference toner image passes through the space after the 
vibration wave generated in the most upstream magenta station Pm has 
passed the black station. 
Next, a case in which a three color image is formed on a transferring 
material by the use of the three color image forming stations excluding 
the magenta station will be described. In the image forming apparatus of 
FIG. 2, a single color mode, two color mode or three color mode 
(arbitrarily selected colors) other than the above-mentioned four color 
mode for forming a four color image on the transferring material P can be 
specified by operating an operating panel. Therefore, when the three color 
mode of cyan, yellow and black is specified, a three color image of cyan, 
yellow and black is formed on a transferring material. At this time, the 
most upstream magenta station is not utilized and when forming the three 
color image, the backup member 10M is switched off. Therefore, at the time 
of the image formation, among the stations in which the backup members are 
made to be in the on-state, the cyan station Pc is the most upstream 
station and the timing of a reference toner image formation and a density 
detection in the black station is controlled in accordance with the on-off 
operation of the backup member 10C of the cyan station Pc. That is, after 
a vibration wave generated in the most upstream cyan station Pc has passed 
the black station Pk, a black reference toner image passes the transfer 
position of the black station Pk. Since the distance between the cyan 
station and the black station is 200.times.2=400 (mm) and the speed of the 
transfer belt 6a is 200 (mm/sec), the time from when the backup member of 
the cyan station Pc is switched from on to off to when the black toner 
patch reaches the transfer position of the black station Pk is made longer 
than 400/200=2 (sec). 
Also, in the above embodiments, the switching of the backup members of the 
respective stations from on to off is performed simultaneously, but each 
backup member may be switched from on to off after the transferring 
material has passed corresponding one of the transfer positions of the 
respective stations. That is, in the case of a four color image formation, 
the backup members are switched from on to off successively in order of 
the magenta, cyan, yellow and black stations. In particular, it is 
desirable to switch each backup member from on to off as soon as the 
trailing end of the transferring material has passed corresponding one of 
the transfer positions of the stations. Also, in such a case, the black 
toner patch is formed in accordance with the off-timing of the backup 
member located most upstream. 
Also, in the above embodiments, although all the backup members are 
detached from the transfer belt when detecting the black toner patch, the 
purpose of this is to detach the transfer belt 6a from the photosensitive 
drum 1K of the black station completely. Further, when detaching the 
transfer belt 6a from the photosensitive drum 1K, it is preferable to 
detach the backup members from the transfer belt 6a without moving a 
plurality of rollers supporting the transfer belt 6a downward, i.e., with 
the distance between the plurality of rollers and the photosensitive drums 
kept constant. 
Also, instead of the transfer chargers 6M, 6C, 6Y and 6K and the backup 
members 10M, 10C, 10Y and 10K, transfer blades may be provided so as to be 
in contact with the back surface of the transfer belt 6a at the respective 
transfer positions. In this case, transfer voltage is applied to each 
transfer blade. When detaching the transfer belt 6a from the 
photosensitive drums of the respective stations, the transfer blades are 
detached from the transfer belt 6a instead of detaching the backup members 
10M, 10C, 10Y and 10K from the transfer belt 6a. The timing of detaching 
the transfer blades from the transfer belt 6a is the same as the timing of 
detaching the backup members from the transfer belt 6a, as mentioned 
above. 
Furthermore, the image forming apparatus of FIG. 2 is provided with the 
four image forming stations, but the present invention can be applied to 
an image forming apparatus having only two image forming stations. 
In the above embodiments, the transfer belt 6a has a seam. Then, when the 
transferring material is placed on the seam portion, preferable transfer 
cannot be achieved. So, the transferring material is placed on the 
transfer belt 6a other than the same portion. Accordingly, when starting 
image formation, the position of the seam is checked by a seam detecting 
device 25 as shown in FIG. 2. After having finished the image formation, 
in preparation for the following image formation, it is preferable to stop 
the transfer belt 6a when its seam portion reaches the detecting position 
of the sensor 25. Also, when detecting the black toner patch after the 
completion of transfer, it is preferable to move the transfer belt 6a 
until the seam is detected by the sensor 25. 
The present invention is not limited to the above-described embodiments and 
various modifications can be taken within the scope and spirit of the 
invention.