Patent Application
20070269228
Various other objects, features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood from the detailed description when considered in connection with the accompanying drawings in which like reference characters designate like corresponding parts throughout and wherein:
In order to solve the above-mentioned problem, the present inventors have proposed an image forming apparatus. Specifically, the image forming apparatus includes an information detecting device, which obtains information (such as image area proportion of produced images) to determine the replacement amount of the toner in the two-component developer contained in the developing device. The image forming apparatus can determine the content of new toner (or old toner) in the developer contained in the developing device (i.e., to determine the developing ability of the developing device) on the basis of the information obtained by the information detecting device. In addition, the image forming apparatus includes a toner concentration target correction device, which corrects the toner concentration target on the basis of the information obtained by the information detecting device to produce images having a constant image density. Since the replacement amount of toner can be determined without using the toner, occurrence of the toner consumption increasing problem can be prevented.
In this image forming apparatus, the toner concentration target is changed on the basis of the information obtained by the information detecting device to control the image density to be constant. In order to increase the toner concentration, the toner is replenished. In this case, the image density of images can be quickly controlled. In order to decrease the toner concentration, replenishing of the toner is suppressed so that the toner concentration decreases with time (i.e., with production of images). Therefore, it takes a relatively long time until the image density becomes to the target.
As mentioned above, it is necessary to determine the content of new toner (or old toner) in the developing device to produce images having a constant image density. In the present application, information is obtained to determine the replacement amount of toner in the developing device within a predetermined period. From this information, the amounts of the consumed toner and added toner (i.e., the content of new toner (or old toner)) can be determined and thereby the developing ability of the developing device can be determined. On the basis of the information, the development potential correcting device corrects the development potential. By adjusting the development potential (i.e., by adjusting the potential difference between an electrostatic image on the image bearing member and the developer bearing member to which a development bias is applied), the amount of toner adhered to an electrostatic image can be changed. Therefore, the image density can be quickly changed. Even when the image area proportion of images is changed and thereby the replacement amount of toner is changed, the image density can be controlled to be constant by adjusting the development potential. In addition, the information can be obtained without consuming the toner, and therefore the toner consumption increasing problem can be avoided.
Next an example (a color laser printer) of the image forming apparatus of the present invention will be explained with reference to drawings.
The laser printer has four image forming devices 1Y, 1C, 1M and 1Bk, which respectively form yellow (Y), cyan (C), magenta (M) and black (Bk) color images and which are arranged from an upstream side to a downstream side relative to the moving direction (indicated by an arrow A) of an intermediate transfer belt 6 serving as an intermediate transfer medium. Each of the four image forming devices 1Y, 1C, 1M and 1Bk has a photoreceptor unit 10Y, 10C, 10M or 10Bk including a photoreceptor drum 11Y, 11C, 11M or 11Bk, which serves as an image bearing member; and a developing device 20Y, 20C, 20M and 20Bk. The image forming devices 1Y, 1C, 1M and 1Bk are arranged at regular intervals in the direction (A) while the photoreceptor drums 11Y, 11C, 11M and 11Bk are set such that the rotation axes thereof are parallel to each other.
The image forming devices 1Y, 1C, 1M and 1Bk form respective color toner images on the respective photoreceptor drums 11Y, 11C, 11M and 11Bk. The color toner images are transferred one by one onto the intermediate transfer belt 6 so as to be overlaid. The color toner images thus overlaid on the intermediate transfer belt 6 are transported to a secondary transfer nip formed by the intermediate transfer belt 6 and a secondary transfer roller 3.
The image forming apparatus illustrated in
A sheet of a receiving material is fed along a chain line. Specifically, an uppermost sheet of the receiving material in the cassette is fed by plural feed rollers while guided by a feeding guide (not shown). The thus fed sheet is stopped once at a pair of registration rollers 5, and is timely fed to the secondary transfer nip such that the color toner images on the intermediate transfer belt 6 are transferred to a proper position of the sheet. The sheet bearing the color toner images thereon is fixed by a fixing device 7. The sheet bearing a fixed color image is discharged to a discharge tray (not shown).
In
Referring to
The image forming operation of the image forming device 1Y is as follows. At first, the surface of the photoreceptor drum 11Y is uniformly charged by the charging roller 15Y to which a voltage is applied. The laser light beam LY emitted by the optical writing unit (not shown) after modulation and deflection irradiates the charged photoreceptor drum 11Y, resulting in formation of an electrostatic image thereon. The thus formed electrostatic image is developed with the developing device 20Y (explained below), resulting in formation of a yellow toner image thereon. The yellow toner image is then transferred to the intermediate transfer belt 6 at a primary transfer nip in which the photoreceptor drum 11Y faces the intermediate transfer belt 6. After the primary transfer operation, the cleaning blade 13Y cleans the surface of the photoreceptor drum 11Y, and the lubricant application brush roller 12Y applies a lubricant to the surface of the photoreceptor drum 11Y while discharging the photoreceptor drum. Thus, the photoreceptor drum 11Y becomes ready for the next image forming operation.
The developing device 20Y uses, as a developer, a two-component developer including a magnetic carrier and a negatively charged yellow toner. The developing device 20Y includes a rotatable developing sleeve 22Y (serving as a developer bearing member, which is made of a non-magnetic material and has therein a fixed magnet roller (not shown, serving as a magnetic field generating member) and which is arranged so as to be partially exposed to the photoreceptor while projected from an opening of a case of the developing device. Further, the developing device 20Y includes agitation screws 23Y and 24Y configured to agitate the developer in the developing device; a development doctor 25Y configured to form a developer layer having a uniform thickness on the developing sleeve 22Y; a magnetic permeability sensor 26Y serving as a toner concentration detecting device; and a powder pump 27Y serving as a toner replenishing device configured to feed the toner.
A development bias, e.g., a negative DC voltage (DC component) overlapped with an AC voltage (AC component), is applied to the developing sleeve 22Y by a development bias power source 28 serving as a development potential forming device so that the developing sleeve 22Y is biased so as to have a predetermined voltage relative to the metal substrate of the photoreceptor drum 11Y. In this regard, only a negative component (a negative DC voltage) may be applied as a development bias.
Referring to
When toner images are formed, the toner in the developer in the case of the developing device is consumed, resulting in decrease of the concentration of toner in the developer. The magnetic permeability sensor 26Y measures the toner concentration and outputs the measurement data (i.e., a voltage Vt, hereinafter referred to as an output Vt). On the basis of the output Vt output from the magnetic permeability sensor 26Y, the toner in the toner cartridge 30Y (in
In addition, it is preferable that at regular intervals (for example, every 10 (a number of from 5 to 200) copies, which is determined depending on the copy speed, etc.), process conditions such as the toner concentration target Vtref, the potential of the charged photoreceptor drum, and the light quantity of the laser beam used for forming electrostatic images are adjusted. Specifically, for example, the optical densities of half tone images or solid images, which are formed on photoreceptor and then transferred to the intermediate transfer belt 6, are measured with a reflection density sensor 62 illustrated in
Among the four photoreceptor drums 11Y, 11C, 11M and 11Bk, only the photoreceptor drum 11Bk, which is located on the downmost stream side is always contacted with the intermediate transfer belt 6. The other photoreceptor drums can be contacted with and separated from the intermediate transfer belt 6. When a multi-color image is formed, all the four photoreceptor drums are contacted with the intermediate transfer belt 6. When a black color image is formed, only the photoreceptor drum 11Bk is contacted with the intermediate transfer belt 6 and the other photoreceptor drums are separated therefrom.
Next, a controller serving as a controlling device for controlling the toner concentration will be explained.
In the controller 100, the CPU 101 executes a predetermined toner concentration control program and sends a control signal to a toner replenishment driving motor 31, which drives the powder pump 27 through the I/O unit 104, to control the toner replenishment operation. Further, the CPU 101 executes a predetermined toner concentration target correction program to correct the toner concentration target Vtref so that the concentration of the toner in the developer is controlled so as to be constant. Furthermore, the CPU 101 executes a predetermined development potential correction program to correct the development potential every image forming operation so that the concentration of the toner in the developer is controlled so as to be constant.
The ROM 102 stores the programs that the CPU 101 executes, such as the toner concentration control program, and the image density control parameter correction programs (e.g., the toner concentration target correction program and the development potential correction program). The RAM 103 includes a Vt register configured to temporarily store the output Vt obtained from the magnetic permeability sensor 26 through the I/O unit 104; a Vtref register configured to store the toner concentration target Vtref, i.e., the reference output value that the magnetic permeability sensor 26 should output when the toner concentration in the developer is equal to the targeted toner concentration; a Vs register configured to store the output Vs output from the reflection density sensor 62.
Next, the development potential correction operation will be explained in detail.
When images with a high image area proportion are produced, the replacement amount of the toner in a predetermined period of time is relatively high. In other words, the amount of old toner particles, which have been staying in the developing device 20 for a long period of time and are excessively charged, is relatively small. Therefore, the developing device has a higher developing ability than in the case where images with a low image area proportion are produced.
Thus, the developing ability of toner changes depending on the replacement amount of the toner in a developing device. When the developing ability changes, the image density of the produced images also changes, resulting in occurrence of an image density problem in that images with a constant image density cannot be produced. In the present invention, the development potential is corrected to prevent occurrence of the image density problem even when the replacement amount of the toner changes.
The replacement amount of toner in a predetermined period of time can be obtained from various information such as the area (in units of cm2) of image portions of the produced images or the image area proportion (%) of the produced images. In this example, the replacement amount of toner is obtained from the information on the image area proportion. In this example, the image area proportion (%) is converted to a toner replacement amount (mg/page). Specifically, when the developer has a normal developing ability, and an A-4 size solid image with an image area proportion of 100% is output, 300 mg of the toner is consumed while 300 mg of new toner is replenished. In this case, the toner replacement amount is 300 mg/page. When the image area proportion is converted to the toner replacement amount in a case where receiving sheets with various sizes are used for forming images, the image area proportion data should be calculated after the various receiving sheets are converted to the standard receiving sheet (for example, an A-4 size sheet in a landscape orientation). In this example, the developing device 20 contains 240 g of the developer.
Next, the development potential correction operation on the basis of the image area proportion will be explained with reference to a flowchart illustrated in
In this example, correction of the development potential is performed by adjusting the development bias. In addition, the development potential correction operation is started after completion of each printing job. At first, the average of the image area proportions of the produced images is calculated (Step S1). In order to calculate the average of the image area proportions, the image area proportion (%) of each of the produced images is calculated. When performing this correction operation, it is acceptable that the image area proportions of the produced images are averaged from a certain time (for example, from a time when the last potential control operation is performed). However, it is preferable to determine the moving average of the image area proportions in a predetermined period of time. By using the moving average, the history of the toner replacement amount can be well determined and thereby the characteristics of the developer at the present time can be properly determined. In the present application, the moving average of the image area proportion is determined by the following equation (1).
M(i)=(1/N)×{(M(i−1)×(N−1)+X(i)} (1)
wherein M(i) represents the present value of the moving average of the image area proportion; M(i−1) represents the last value of the moving average of the image area proportion; N represents the cumulative number of the images produced; and X(i) represents the image area proportion of the present image. In this regard, each of M(i) and X(i) is calculated for each color image.
In this example, only the data of the last moving average of the image area proportion is stored in the RAM 103 (namely, it is not necessary to store therein the data of the image area proportions of all the images (for example, tens of images or more) which have been produced from a time when the last potential control operation is performed). Therefore, the area of the RAM used for storing the data can be dramatically reduced. In addition, it is preferable to change the preset value of the cumulative number (N) of images depending on changes of the environmental conditions and the period of service of the image forming apparatus in order to effectively control the image density.
After the moving average of the image area proportion is calculated, the present value and initial value of the development bias Vb are obtained (Step S2). Next, the development gamma characteristic γ at the last potential controlling operation is obtained (Step S3). In this regard, the potential controlling is performed every 200 copies. Further, the present value of the toner concentration target Vtref is obtained (Step S4) and the last value of the output Vt of the magnetic permeability sensor is obtained (Step S5) to calculate the difference (Vt−Vtref) (Step S6). Next, it is decided whether or not to perform the correction operation depending on the information thus collected and calculated (Step S7). It is preferable that the decision is performed depending on whether the last potential controlling is successful and/or whether the difference (Vt−Vtref) falls in a predetermined range (for example, ±0.2V). When it is decided that any correction operation is not performed, the development potential correction operation is ended.
When it is decided that the correction operation is performed, the correction value ΔVb of the development potential is determined with reference to a look-up table (LUT) (Step S8). One example of the look-up table is as follows.
In this example, the center of the image area proportion is considered to be 5%. When the cumulative average of the image area proportion deviates from the center, the development potential correction operation is performed. However, the correction operation is not limited thereto, and a correction method in which only when the cumulative average is much lower or higher than the center, the development potential correction operation is performed can also be used. Specifically, there is a case where depending on the properties of the developer used, it is not necessary to correct the development potential when the cumulative average is lower the center.
The correction value ΔVb of the development potential is determined as follows. At first, on the basis of the thus determined moving average of the image area proportion, the deviance Δγ of the development gamma characteristic γ can be obtained. Next, the deviance is converted to the correction value ΔVb of the development potential using the following equation (2).
ΔVb={Δγ/(γ1×γ2)}×M (2)
Wherein γ1 is the development gamma characteristic which is measured in the last potential controlling operation; γ2 is the development gamma characteristic which can be predicted on the basis of the cumulative average of the image area proportion; and M represents the target of the amount of toner of a toner image formed on the intermediate transfer belt.
As shown in Table 1, when the image area proportion is not greater than 10%, the image area proportion is changed at intervals of 1% while the image area proportion is changed at intervals of 10% when the image area proportion is greater than 10%. However, the intervals are not limited thereto, and can be set to proper intervals depending on the conditions of the developer and developing device used.
When plural color developers are used, the correction values ΔVb of the development potentials therefor are preferably determined by the following formula (3).
ΔVb (for each color)=ΔVb (above-obtained)×C (3)
wherein C represents the color correction coefficient for the color.
After the correction value ΔVb is determined, the development bias Vb (Vb(current)) is determined by the following equation (4) (Step S9).
Vb(current)=Vb(initial)×ΔVb (4)
Next, the Vb(current) is subjected to an upper and lower limit treatment (Step S10). Specifically, when the Vb(current) is not less than the predetermined upper limit, the Vb(current) is set to the upper limit (i.e., Vb(max)). In contrast, when the Vb (current) is not greater than the predetermined lower limit, the Vb(current) is set to the lower limit (i.e., Vb(min)). After the upper and lower limit treatment, the thus obtained development bias Vb (current) is stored in the RAM 103 (Step S1). In this example, the Vb(max) and Vb(min) are 700V and 350V, respectively.
When the development bias Vb is changed, the charging bias Vc, at which the photoreceptor drum is charged, is changed using the following equation (5).
Vc=Vb+140 [V] (5)
wherein Vc represents the DC component of the charging bias.
The above-mentioned development potential correction operation is preferably performed at a time before the start of the present development operation is performed and after the completion of the last development operation is completed. By performing the development potential correction operation at such a frequency, each image can be produced while the development potential is set to a proper potential. Therefore, the image density of the produced images can be stabilized.
However, when the moving average of the image area proportion is dramatically changed (for example, a case where images with an image area proportion of about 0% are continuously produced and then solid images with an image area proportion of about 100% are produced), the deviance Δγ of the development gamma characteristic γ increases and therefore the correction value ΔVb also increases. In such a case, it is possible that the development potential is gradually changed step by step. For example, when the correction value ΔVb is 100V, the development potential is changed at a ratio of 10 V/10 pages or 20 V/20 pages. In addition, it is also possible to provide a limiter to set an upper limit for the correction value ΔVb.
The correction of the development potential is not necessarily performed by adjusting the development bias, and can be performed by adjusting the potential of surface of the photoreceptor 11 and/or the energy of the laser beam, which is emitted by the optical writing unit for forming electrostatic latent images.
In the present example, similarly to the correction of the development potential, correction of the toner concentration target can also be performed on the basis of the replacement amount of the toner in the developing device 20 within a certain period of time (e.g., on the basis of the image area proportion of the images produced in the certain period of time). By using this method, the toner concentration can be properly controlled, and thereby the image density of the produced images can be controlled even when the replacement amount of the toner is largely changed. By combining the development potential correction with the toner concentration correction, the image density of the produced images can be controlled more stably.
An experiment was performed to compare a case where the development bias correction operation is performed with a case where the development bias correction operation is not performed.
In this experiment, one hundred copies of an image with an image area proportion of 80% were produced using a laser printer at a normal linear speed mode of 138 mm/sec. The image densities of the produced images were measured. In
As mentioned above, the laser printer, which is an example of the image forming apparatus of the present invention, includes at least the following.
(1) the photoreceptor 11 serving as an electrostatic latent image bearing member;
(2) the developing device 20 including the developing sleeve 22 serving as a developer bearing member, which bears thereon a two component developer including a toner and a magnetic carrier and contacts the developer with the electrostatic latent image on the photoreceptor to form a toner image on the photoreceptor;
(3) the development bias power source 28 serving as the development potential forming device configured to form a development potential between the developing sleeve and the electrostatic latent image on the photoreceptor;
(4) the powder pump 27 serving as a toner replenishing device configured to supply the toner to the developing device;
(5) the magnetic permeability sensor 26 serving as a toner concentration detecting device configured to detect and output the concentration of the toner in the two component developer in the developing device; and
(6) the controller 100 serving as a toner concentration controlling device, which compares the output Vt from the magnetic permeability sensor with the target Vtref and controls the powder pump on the basis of the comparison data so that the output Vt approaches the Vtref, i.e., the toner concentration approaches the target of toner concentration.
In addition, the controller 100 serves as an information detecting device which obtains information (such as image area proportion) concerning the replacement amount of toner in the developing device within a predetermined period of time. Further, the controller 100 also serves as a correction device configured to adjust the development potential on the basis of the information (such as detection results of the image area proportion). Thus, the development ability of the developing device (i.e., the amount of toner adhered to an electrostatic images) can be controlled by adjusting the development potential (i.e., the difference in potential between the electrostatic latent image on the photoreceptor and the development bias applied to the developing sleeve). Therefore, the development ability (i.e., the amount of toner adhered to an electrostatic images) can be rapidly adjusted. Therefore, even when the toner replacement amount largely changes (for example, when images with a high image area proportion are produced), the image density of images can be stably controlled because the development potential is adjusted. Since the information concerning the toner replacement amount can be obtained without consuming the toner (i.e., without forming toner images), the development potential correction operation can be performed without consuming the toner.
In this example, the controller 100 serves as the information detecting device and determines the average of the image area proportion of the images produced in a predetermined period of time. Therefore, the information on the toner replacement amount can be easily obtained by a simple device without consuming the toner.
In this example, the controller 100 adjusts the development potential on the basis of the moving average of the image area proportion of the images produced in a predetermined period of time. Therefore, the history of the toner replacement amount for the last several images, which history is useful for determining the current characteristic of the developer, can be obtained. Accordingly, correction of the development potential can be performed more properly. Since the moving average (M(i)) can be calculated using the above-mentioned equation (1), only a small area of the RAM 103 is used for determining the moving average.
In this example, when the toner replacement amount is greater than the target amount, the controller 100 adjusts the development potential so as to decrease. In contrast, when the toner replacement amount is less than the target amount, the controller 100 adjusts the development potential so as to increase. Therefore, even in a case where images with a high image area proportion are produced and therefore the development gamma characteristic γ increases, the development potential is decreased to decrease the development gamma characteristic γ. Accordingly, the image density can be easily controlled so as to fall in a predetermined range.
In addition, the controller 100 adjusts the development potential at an interval between the end of a developing operation of the developing device and the start of the next developing operation. Therefore, each image is produced while controlling the development potential.
Further, the controller 100 adjusts the development potential step by step so that the development potential falls in the predetermined range. Use of this method prevents occurrence of an image density changing problem in that the image density rapidly changes, for example, when images with a high image area proportion are produced after production of images with a low image area proportion (i.e., when the moving average of the image area proportion is largely changed).
This document claims priority and contains subject matter related to Japanese Patent Application No. 2006-141920, filed on May 22, 2006, incorporated herein by reference.
Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit and scope of the invention as set forth therein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-141920 | May 2006 | JP | national |
