Image Forming Apparatus

1. Field of the Invention

This invention relates to an image forming apparatus such as a copying machine or a printer of the electrophotographic type or the electrostatic recording type in which a developer is caused to adhere to a latent image formed on an image bearing member to thereby make the latent image into a visible image.

2. Related Background Art

There is known an image forming apparatus in which predetermined image forming conditions, for example, a charging voltage, an exposure amount, developing bias, etc. are set to predetermined values and a patch-like image is formed on a photosensitive medium and the density thereof is read and the quantity of toner supplied is controlled on the basis of the result of this reading so as to keep the density of an output image constant.

As an example of the above-described image forming apparatus, a full color image forming apparatus is schematically shown in FIG. 8 of the accompanying drawings. In this image forming apparatus, a photosensitive drum 1 as an image bearing member is supported for rotation in the direction of arrow R1, and around it, there are disposed a corona charger 2, an optical system 3, a developing device 5, a transfer device 6 and a cleaning device 7.

The optical system 3 is a laser beam exposure device which comprises an original scanning portion and a color resolving filter and applies a color-resolved optical image E or an optical image E corresponding thereto to the photosensitive drum 1. The optical image E for each resolved color is applied to the photosensitive drum 1 uniformly charged by the charger 2 to thereby form an electrostatic latent image. The developing device 5 is a rotatable developing device comprising four developing devices, i.e., a black developing device 5BK, a cyan developing device 5C, a magenta developing device 5M and a yellow developing device 5Y, disposed around a center shaft 5a, and a predetermined one of the developing devices is rotated to a developing position opposed to the photosensitive drum 1 to thereby develop an electrostatic latent image on the photosensitive drum 1 and form a toner image on the photosensitive drum 1 using a toner having resin as a base substance.

Further, the toner image on the photosensitive drum 1 is transferred to a recording medium conveyed along a paper path indicated by a dotted line, from a recording medium cassette 30 through a conveying system and the transfer device 6, and supplied to a position opposed to the photosensitive drum 1. In the present example, the transfer device 6 has a transfer drum 6a, a transfer corona charger 6b, an adsorbing corona charger 6c for electrostatically adsorbing the recording medium, an adsorbing roller 6d opposed thereto, an inner corona charger 6e and an outer corona charger 6f, and a recording medium carrying sheet 6g formed of a dielectric material is cylindrically and integrally extended in a peripheral open region of the transfer drum 6a supported so as to be rotatively driven. As the transfer drum 6a is rotated in the direction of arrow R2, the toner image on the photosensitive drum 1 is transferred onto the recording medium carried on the recording medium carrying sheet 6g, by the transfer charger 6b. Toner images of other colors are successively transferred to the recording medium adsorbed and conveyed on the recording medium carrying sheet 6g and finally, a desired number of colored images are transferred to the recording medium, whereby a full color image is formed thereon.

When the transfer of a desired number of toner images is terminated in this manner, the recording medium is separated from the transfer drum 6a by a separating device 9 and is discharged onto a paper discharge tray 31 through a heat roller fixing device 10. On the other hand, any residual toner on the surface of the photosensitive drum 1 after the image transfer may be removed by the cleaning device 7 and the drum again becomes ready for use in a series of image forming processes.

The operation of controlling the density of the developer is performed in parallel with the above-described image forming processes. As shown in FIG. 2 of the accompanying drawings, a patch-like reference electrostatic latent image (hereinafter referred to as the "patch latent image") corresponding to a predetermined density is formed on the photosensitive drum 1 and it is developed by a toner and thereby made into a patch-like reference image (hereinafter referred to as the "patch image") P, and light is applied from an LED 11a of a density detection sensor (density detecting means) of the optical reflected light quantity detection type to the patch image P, and the reflected light therefrom is received by a photodiode 11b to thereby detect the density of the patch image P. Since this density corresponds to the toner density of the two-component developer in the developing device 5, the detected density is compared with a reference value to thereby take the difference therebetween, and on the basis of this difference, the amount of fluctuation of the density of the developer is calculated, and it is converted into a toner supply quantity (supply time), and a predetermined quantity of toner is supplied from a toner supply tank into the developing device 5, whereby control is effected so as to keep the density of the output image constant.

In the above-described prior art, however, the patch latent image formed on the image bearing member is developed into the patch image P, and the reflection density of this patch image P is detected by the density detection sensor and on the basis of the result of this detection, the supply of the toner is effected, but as shown in FIG. 3 of the accompanying drawings, there is eccentricity on the photosensitive drum 1 in each phase, and the gap between the photosensitive drum 1 and the developing sleeve of the developing device (hereinafter referred to as the "S-D gap") fluctuates by the rotation of the photosensitive drum and developing efficiency varies. FIG. 4 of the accompanying drawings shows the characteristic of the output of the density detection sensor for the S-D gap. As can be seen from this figure, the S-D gap fluctuates, whereby the density of the image varies and, as a matter of course, the output of the density detection sensor also greatly fluctuates. Therefore, it has been found that even if the patch image P is formed under the same image forming conditions, when the patch image formation position on the photosensitive drum 1 varies, the output of the density detection sensor includes not only the variation in the density of the developer, but also the variation in developing efficiency by the difference in the eccentric amount of the photosensitive drum, i.e., the difference in the S-D gap. Accordingly, if the output value of the sensor is simply fed back to the density control for the developer, then accurate control of the density of the developer can not be accomplished.

The present invention has been made in view of the above-noted problem and an object thereof is to provide an image forming apparatus in which the image forming conditions of image forming device can be controlled accurately.

Another object of the present invention is to provide an image forming apparatus in which the density of a developer in a developing device can be controlled to a desired value.

Still another object of the present invention is to provide an image forming apparatus in which the irregularity of the density of an output image can be minimized.

Further objects of the present invention will become apparent from the following detailed description when read with reference to the accompanying drawings.

FIG. 1 is a longitudinal cross-sectional view schematically showing an image forming apparatus according to Embodiment 1 of the present invention;

FIG. 2 is a perspective view showing the construction of a density detection sensor;

FIG. 3 shows the eccentric state of a photosensitive drum;

FIG. 4 is a graph showing the relation between the S-D gap and the output of the density detection sensor;

FIG. 5 is a longitudinal cross-sectional view schematically showing an image forming apparatus according to Embodiment 2 of the present invention;

FIG. 6 shows the eccentric state of a photosensitive drum having a large diameter (180 mm) in Embodiment 2;

FIG. 7 shows the charging irregularity caused by the eccentricity of the photosensitive drum;

FIG. 8 is a longitudinal cross-sectional view schematically showing an image forming apparatus according to the prior art; and

FIG. 9 is a graph showing the relation between the S-D gap and the output of a density detection sensor.

Some embodiments of the present invention will hereinafter be described with reference to the drawings.

<Embodiment 1>

As an embodiment of the image forming apparatus according to the present invention, a color image forming apparatus for forming a full color image is schematically shown in FIG. 1. In this apparatus, members functionally similar to those in the afore-described apparatus are given the same reference characters.

In the image forming apparatus of the present embodiment, a photosensitive drum 1 which is an image bearing member, is supported for rotation in the direction of arrow R1. Around this photosensitive drum 1 and along the direction of rotation thereof, there are successively disposed image forming means such as a corona discharger 2, an optical system 3 and a developing device 5, a transfer device 6 and a cleaning device 7.

The optical system 3 is, for example, a laser beam exposure device which comprises an original scanning portion and a color resolving filter and applies a color-resolved optical image E or an optical image E corresponding thereto to the photosensitive drum 1. The optical image E for each resolved color is applied to the photosensitive drum 1 uniformly charged by the charger 2 to thereby form an electrostatic latent image on the photosensitive drum 1. The developing device 5 is a rotatable developing device provided with a rotatable member 5b rotatable about a center shaft 5a, and four developing devices carried thereon, i.e., a black developing device 5BK, a cyan developing device 5C, a magenta developing device 5M and a yellow developing device 5Y, and a predetermined one of the developing devices is rotated to a developing position opposed to the photosensitive drum 1 and a toner having resin as a basic substance is caused to adhere to the electrostatic latent image on the photosensitive drum 1 to thereby develop the latent image, thus forming a toner image on the photosensitive drum 1.

Further, the toner image on the photosensitive drum 1 is transferred to a recording medium supplied from a recording medium cassette (not shown) to a transfer position N opposed to the photosensitive drum 1 (in the direction of arrow K1 along a paper path indicated by a dotted line) through a conveying system and the transfer device 6. The transfer device 6, in the present embodiment, has a transfer drum 6a, a transfer corona charger 6b, an adsorbing corona charger 6c for electrostatically adsorbing the recording medium, an adsorbing roller 6d opposed thereto, an inner corona charger 6e and an outer corona charger 6f, and a recording medium carrying sheet 6g formed of a dielectric material is cylindrically and integrally extended in the peripheral open region of the transfer drum 6a supported so as to be rotatively driven. As the transfer drum 6a is rotated, the toner image on the photosensitive drum 1 is transferred onto the recording medium carried on the recording medium carrying sheet 6g, by the transfer charger 6b. A desired number of colored images are transferred onto the recording medium adsorbed and conveyed on the recording medium carrying sheet 6g, whereby a full color image is formed on the recording medium.

When the transfer of a desired number of toner images is terminated in this manner, the recording medium is separated from the transfer drum 6a by a separating device 9, and an output image is obtained through a heat roller fixing device 10. On the other hand, any residual toner on the surface of the photosensitive drum 1 after the image transfer is removed by the cleaning device 7 and the drum again becomes ready for use in the image forming processes.

The control of the density of the developer is effected in parallel with the above-described series of image forming processes. That is, it is effected each time an image is formed or each time a predetermined number of images are formed. As shown in FIG. 2, a patch-like patch latent image (reference electrostatic latent image) corresponding to predetermined density is formed in front of the image forming area on the photosensitive drum 1 and is developed with the toner to thereby form a patch-like patch image (reference image) P. Light is applied from the LED 11a of the density detection sensor (density detecting means) 11 of the optical reflected light quantity detection type to the patch image P, and the reflected light thereof is received by a photodiode 11b to thereby detect the density of the patch image P. The output of the density detection sensor 11 is then corrected on the basis of the eccentric amount of the photosensitive drum relative to the patch image, and the corrected value is compared with a reference value to thereby take the difference therebetween, and the amount of fluctuation of the density of the developer is calculated from this difference, and it is converted into a quantity of toner supply (supply time) and a predetermined quantity of toner is supplied from a toner supply tank into the developing device 5, whereby control is effected so as to keep the density of the output image constant.

FIG. 3 is a graph illustrating the amount of eccentricity at the developing position of a photosensitive drum (the position at which a developing sleeve in the developing device is opposed to the photosensitive drum), the amount of eccentric of the photosensitive drum 1 being measured for each phase. As shown in FIG. 3, the photosensitive drum delicately differs in the distance between the rotary shaft and peripheral surface thereof, i.e., the radius thereof, depending on the circumferential position thereof. By this eccentricity, the S-D gap (the distance between the surface of the photosensitive drum and the surface of the developing sleeve) fluctuates and the output of the density detection sensor varies as shown in FIG. 4. So, in the present embodiment, a memory device (not shown) pre-stores therein the eccentric amount of the photosensitive drum 1 for each phase with the home position M set on the photosensitive drum 1 as the reference.

The control of the density of the developer in the present embodiment will now be specifically described.

As shown in FIG. 1, a home position sensor (home position detecting means) 12 is provided near the density detection sensor 11 in opposed relationship with the photosensitive drum 1. A fluctuated value of the output of the density detection sensor due to fluctuation of the S-D gap of the position at which the patch image P is formed can be calculated on the basis of the home position M, the eccentricity data of the photosensitive drum 1 and the phase from the home position M to the patch image P. So, description will first be made of a method of correcting the output value of the density detection sensor due to a variation in the S-D gap when the density of the developer in the developing device is free of any variation and is constant at a reference value.

It is to be understood that the density of the developer is constant at 5% by weight. It is also to be understood that the S-D gap at the position whereat the eccentric amount of the photosensitive drum is 0 (zero) is 500 .mu.m. If under such conditions, the home position M of the photosensitive drum 1 is at a position A in FIG. 3 and the patch image P is formed at a position B, then the output I of the density detection sensor will be detected as a value (0.95 V) lower than a proper value by 50 mV, from FIG. 4, by the eccentric amount at the position B (+25 .mu.m, and at this time, the S-D gap is 495 .mu.m). However, the + (plus) direction of the eccentric amount is a direction in which the S-D gap shortens. Also, the density detection sensor used herein detects the density of the patch image by the quantity of reflected light from the patch image and therefore, the higher the density of the patch image becomes, the smaller the detection output becomes, and the lower the density becomes, the greater the detection output becomes.

Accordingly, if the difference 50 mV from the reference output value H=1.0 V of the density detection sensor when the density of the developer in the developing device is constant, i.e., a predetermined value 5% by weight, and the patch image is formed at the position whereat the eccentric amount of the photosensitive drum is zero is added to the output value I, then the correction of a variation in the developing efficiency caused by the eccentricity of the photosensitive drum at the position B could be effected. If likewise, the patch image P is formed at a position C in FIG. 3, the S-D gap will become 550 .mu.m, wider by 50 .mu.m, and the density of the patch image will become lower than when the patch image is formed at a position whereat the S-D gap is 500 .mu.m and, therefore, the output value I' of the sensor will be detected as being higher by about 100 mV (I'=1.1 V).

Consequently, by subtracting 100 mV from the detection output I' and taking the difference from the reference value H, an accurate signal can be obtained. That is, the home position sensor 12 is mechanically or electrically provided on the photosensitive drum 1, the eccentricity data of the photosensitive drum 1 is memorized, the phase from the home position M to the patch forming position is measured, and from the phase difference, the fluctuated value of the S-D gap can be determined. Therefore, if the output value of the sensor is corrected by this amount of fluctuation of the S-D gap, then there could be always obtained the output value of the sensor when the eccentric amount of the photosensitive drum is zero.

While in the present embodiment the output value of the sensor when the patch image is formed at the position whereat the eccentric amount is zero is used as the reference value, this is not restrictive, but the output value of the sensor at any position other than the position at which the eccentric amount is zero may be used as the reference value. However, since the output value of the sensor at the position whereat the eccentric amount is zero is generally approximate to the average value of the sensor output for one round of the photosensitive drum, it is preferable that the output value of the sensor at the position whereat the eccentric amount is zero be used as the reference value.

Also, actually, the variation in the density of the developer in the developing device is added to the output value of the sensor. This state is shown in FIG. 9. In the apparatus of the present embodiment, if the density of the developer varies by 1% by weight, then the output value of the sensor will vary by 200 mV. In FIG. 9, a solid line l represents the relation between the S-D gap and the sensor output when the density of the developer is 4% by weight, and a solid line n represents the relation between the S-D gap and the sensor output when the density of the developer is 6% by weight. As a matter of course, the variations are not limited to these patterns, but are infinite.

So, as previously described, if the output value of the sensor is corrected on the basis of the value of the S-D gap (or the eccentric amount) i.e., the eccentricity data, whereafter on the basis of this corrected output value, the amount of supply of the toner is controlled so that the density of the developer may assume a predetermined value (here, 5% by weight), then proper density control could always be effected even when the patch image is formed anywhere on the surface of the photosensitive drum.

When for example, in FIG. 9, the density of the developer is thinner by 1% by weight than a desired value 5% by weight and the patch image is formed on the surface of a photosensitive drum in which the S-D gap is 475 .mu.m, the output value of the density detection sensor becomes J=1.15 V. So, the corrected value 0.05 V of the eccentric amount of the photosensitive drum is added to J=1.15 V to thereby obtain a corrected output value K=1.2 V. Toner corresponding in quantity to the difference between the reference value H=1.0 V when the density is 5% by weight and K=1.2 V can then be supplied.

In the present embodiment, a black toner containing carbon is used, but a similar effect can be obtained irrespective of a carbon constituent and the color of the toner.

<Embodiment 2>

FIG. 5 shows the general construction of a full color copying apparatus. This copying apparatus has a digital color image reader unit I in the upper portion thereof and a digital color image printer unit II in the lower portion thereof.

In the reader unit I, an original is placed on an original supporting glass table 21 and is exposure-scanned by an exposure lamp 22, whereby the reflected optical image from the original is condensed on a full color sensor 25 by a lens 23 to thereby obtain a color-resolved image signal. The color-resolved image signal is processed by a video processing unit (not shown) via an amplifying circuit (not shown) and is delivered to the printer unit.

In the printer unit II, a photosensitive drum 1 which is an image bearing member is supported for rotation in the direction of arrow R1, and around the photosensitive drum 1, there are disposed a pre-exposure lamp 26, a corona charger 2, a laser exposure optical system 3, a potential sensor 27, four developing devices 5Y, 5C, 5M and 5BK differing in color, means for detecting the quantity of light on the drum (density detection sensor) 11, a transfer device 6 and a cleaning device 7. In the laser exposure optical system 3, the image signal from the reader unit I is converted into an optical signal by a laser output portion (not shown), and the converted laser beam is reflected by a polygon mirror 3a and is projected onto the surface of the photosensitive drum 1 via a lens 3b and a mirror 3c.

During image formation by the printer unit II, the photosensitive drum 1 is rotated in the direction of arrow R1, and the photosensitive drum 1 after being de-electrified by the pre-exposure lamp 26 is uniformly charged by the charger 2, and an optical image E is applied to the photosensitive drum for each resolved color to thereby form an electrostatic latent image thereon.

Subsequently, a predetermined one of the developing devices is operated to develop the electrostatic latent image on the photosensitive drum 1, thus forming a toner image having resin as a base substance on the photosensitive drum 1. The respective developing devices are designed to approach the photosensitive drum 1 alternatively in conformity with the respective resolved colors by eccentric cams 29Y, 29C, 29M and 29BK.

Further, the toner image on the photosensitive drum 1 is transferred to a recording medium supplied from a recording medium cassette 30 to a position opposed to the photosensitive drum 1 through a conveying system and the transfer device 6. In the present embodiment, the transfer device 6 has a transfer drum 6a, a transfer charger 6b, an adsorbing charger 6c for electrostatically adsorbing the recording medium, an adsorbing roller 6d opposed thereto, an inner charger 6e and an outer charger 6f, and a recording medium carrying sheet 6g formed of a dielectric material is cylindrically and integrally extended in the peripheral open region of the transfer drum 6a supported so as to be rotatively driven. The recording medium carrying sheet 6g used is a sheet of a dielectric material such as polycarbonate film.

As the drum-shaped transfer device, i.e., the transfer drum 6a, is rotated, the toner image on the photosensitive drum 1 is transferred onto the recording medium carried on the recording medium carrying sheet 6g, by the transfer charger 6b.

In this manner, a desired number of colored images are transferred onto the recording medium adsorbed and conveyed on the recording medium carrying sheet 6g to thereby form a full color image. In the case of full color image formation, when the transfer of the toner images of four colors is terminated, the recording medium is separated from the transfer drum 6a by the actions of a separating pawl 9a, a separating and pushing-up roller 9b and a separating charger 6h, and is discharged onto a paper discharge tray 31 through a heat roller fixating device 10.

On the other hand, the photosensitive drum 1 after image transfer has any residual toner on its surface removed by the cleaning device 7 and becomes ready for use in another cycle of the image forming process. Also, in order to prevent the scattering and adherence of powder on the recording medium carrying sheet 6g of the transfer drum 6a and the adherence or the like of oil on the recording medium, cleaning is effected by the actions of a fur brush 32 and a backup brush 33 opposed to the fur brush 32 with the recording medium carrying sheet 6g interposed therebetween, and an oil removing roller 35 and a backup brush 36 opposed to the roller 35 with the recording medium carrying sheet 6g interposed therebetween. Such cleaning is effected before or after image formation, and when paperjam has occurred. Such cleaning may be effected at any time.

The operation of controlling the density of the developer is performed in parallel with the above-described image forming processes. The eccentric amount of the photosensitive drum 1 for each phase is measured in advance and its relation with the home position M is memorized. Subsequently, a patch image P is formed and the density is detected by the density detection sensor 11. The density detection output is then corrected on the basis of the eccentric amount by the phase difference between the patch image forming position and the home position M, and the corrected density detection output is compared with a reference value to thereby calculate the amount of fluctuation of the density of the developer, and it is converted into a quantity of toner supply and the supply of the developer is effected, whereby the density of the developer and the density of the output image can be stabilized. FIG. 6 shows the eccentric amount measured when the diameter of the photosensitive drum 1 is 180 mm, and the larger the diameter becomes, the more the eccentric amount increases. When the diameter exceeds 140 mm, the irregularity of the image density due to eccentricity is liable to be conspicuous. The present invention is particularly effective when an attempt is made to obtain a high quality image when the photosensitive drum 1 has a large diameter like this.

<Embodiment 3>

In an image forming apparatus using a photosensitive drum 1 of a large diameter like that of Embodiment 2, the charge potential of the photosensitive drum 1 due to the charger 2 is fluctuated particularly by the eccentricity of the photosensitive drum 1 depending on the frequency of the eccentricity, as indicated by a solid line in FIG. 7. The eccentricity of the photosensitive drum 1 is detected with the home position M given to the photosensitive drum i and the charging voltage of the charger 2 is corrected, whereby as indicated by b (dot-and-dash line) in FIG. 7, any irregularity due to the charging can be decreased and a uniform output image can be obtained.

As described above, the apparatus of the present invention has a memory for storing therein the eccentricity data of a rotatable member having an image bearing portion on the surface thereof, and correcting means for correcting the output from the density detecting means on the basis of said eccentricity data, and the image forming conditions of the image forming means are controlled on the basis of said corrected output value and therefore, even if a patch image is formed at any position on the surface of the rotatable member, an accurate adjustment of the image forming conditions including the density of the developer can be effected.

The present invention is not restricted to the above-described embodiments, but covers any and all modifications within the same technical idea.