An image forming apparatus

Abstract: The image forming apparatus of the invention controls the toner density by any one of the following configurations: by correcting the toner density of the developer in association with the agitation total; by controlling a process parameter so that the density of a toner patch formed on the photoreceptor corresponds to a prescribed density value and determining that developing performance of the developer is improved and canceling the toner density correction when the process parameter reaches the prescribed value; by changing the toner density reference value when the variation as to the charger output is equal to or greater than a first predetermined value and maintaining the changed toner density reference value until the variation of the charger output again becomes equal to or greater than the first predetermined value; or by prohibiting toner supply for a constant duration to prevent excessive toner supply if time from the end of the last operation of the developing unit to the start of a next operation, inclusive of the power-activation is equal to or longer the a predetermined period when the developing unit is activated to commence agitation of the developer.

Description: BACKGROUND OF THE INVENTION (1) Field of the Invention The present invention relates to an image forming apparatus such as copiers, laser printers, PPC facsimiles and the like, having a photoreceptor and using the electrophotographic process wherein a static latent image is formed on the photoreceptor and developed into a visual image by the developer, and in particular relates to a toner density control for creating stabilized images in such an image forming apparatus. More detailedly, the present invention relates to an image forming apparatus including: a developing unit for developing a static latent image with a two-component developer consisting of toner and carriers; a toner density detecting means provided in . the developing unit for measuring the magnetic permeability of carriers to output the measurement as a reference toner density; and an automatic toner control device wherein toner density is controlled by comparing the toner density inside the developing unit with the reference toner density outputted from the toner density detecting means and supplying toner into the developing unit to adjust the toner density so as to correspond to the reference toner density. (2) Description of the Prior Art In an image forming apparatus effecting image forming based on the electrophotography using a two-component developer consisting of toner and carries, in order to maintain the toner density of the developer, the toner density inside the developing vessel in the developing unit is detected by a toner density sensor. The detected level is compared with a predetermined reference toner density and supplying amount of the toner to the developing vessel is controlled based on the comparison so that the output from the toner density sensor is made equal to the reference toner density. The above-mentioned toner density sensor typically uses a magnetic permeability sensor which detects the variation of inductance of the developer and detects the toner density utilizing the fact that the magnetic permeability depends on the ratio of the toner as a non-magnetic material and the carries as a magnetic material. As such a two-component developer is charged by mixing and agitating inside the developing vessel, the apparent volume density of the developer will change depending on the quantity of charge carried on the toner. This physical change of the developer changes the inductance even if the toner density of the developer is unchanged. Therefore, the detection of the toner density involves errors depending on the quantity of charge. Consequently, it is impossible to keep the practical toner density constant. To deal with this, it is disclosed in Japanese Patent Application Laid-Open Sho 62 No.25,778, that the detection error of the toner density sensor due to the variation of the quantity of charge on the developer is compensated by modifying the supply amount of toner as the number of copies increases which has been counted from the replacement of the developer in the developing unit. Fig.2 is a chart showing the change of the quantity of charge versus the number of copies. As seen from this chart the quantity of charge relatively rapidly increases at the initial stage as the number of the copies increases and thereafter the charge quantity gradually goes down to a stabilized state. In the case where the toner density sensor detects the density of toner based on the inductance, the sensor output lowers as the toner density is large as shown in Fig.3 while as seen in Fig.4, the sensor output lowers as the quantity of charge on the developer becomes great. Accordingly, as the charge quantity on the developer becomes greater, the toner density tends to be estimated greater than the actual toner density. As a result of the automatic toner density control of the developer in association with the output from the toner density sensor, the density of the image formed lowers at the initial stage as the number of the copies increases and then recovers to the normal state as shown in Fig.1. As disclosed in the above publication, if the toner density is modified in association with the number of copies, the initial lowering of the image density can anyhow be compensated. The quantity of charge on the developer, however, is affected by the conditions on which the developer is mixed and agitated, that is, the temperature and moisture of the environment or by the operated condition of the developing unit and will not change in a unique manner. The charging performance itself will also lower due to the degradation of the developer. As a result, although it is possible to effect the correction of the toner density properly in the initial stage, the compensation becomes excessive gradually with the increase of the number of copies. As a result, degradation of image could occur such as the image density becomes high and the toner scattering could occur to pollute the machine inside. Further, as stated heretofore, in the case that the two-component developer consisting of toner and carriers is used to effect the development, if the toner density of the developer inside the developing vessel, or the ratio of mixing of carriers and toner is not appropriate, the image density becomes too low, or the image becomes to have too high density and produces too much fog. Alternatively, there are other possibilities such as toner scattering or the like. Used in the conventional copier, laser printer or the like is a toner density sensor which detects toner density by measuring the change of the apparent volume density of the developer, for example, as a change of magnetic permeability. In such a configuration, an output value (output voltage) from the toner density sensor when the developer having an optimal toner density has been well agitated is previously set into the memory as a reference value (reference voltage). Then toner supply control is effected so that the output value from the toner density sensor may meet the reference value to thereby maintain the toner density, appropriately. However, as the developer has been agitated with great stress in the developing vessel over a prolonged period of time, toner particles could stick to the surface of carriers, the coating agent may peel off the carrier surface, or the toner particles may be made small in diameter. The flow property and other factors of the developer vary due to the degradation by such phenomena, whereby the output value from the toner sensor is caused to change despite that the toner density of the developer is unchanged. Therefore, it might be impossible to keep the toner density appropriately by the toner supplying control based on merely the output value from the toner density sensor. To deal with this, a method of the toner density control has been done under the consideration of the used state of the developer. That is, in order to estimate the used state of the developer, the occurrences of copies made are counted. And the reference value in the toner density sensor is corrected by a predetermined constant determined depending on the total number of copies so that the utility state of the developer is taken into account to thereby maintain the toner density appropriately. The output value from the toner density sensor is affected by the quantity of charge which is generated on the toner by friction between toner and carriers when the developer is agitated. For example, when the quantity of charge on the toner increases, the apparent volume density of the developer lowers therefore the output value from the toner density sensor lowers. In contrast, if the quantity of charge on the toner decreases, the apparent volume density of the developer increases therefore the output value from the toner sensor increases. Accordingly, in the conventional configuration, the toner density sensor may present a proper output value reflecting the actual toner density when the developer has been well agitated during copying or right after copying. However, if the developer, not agitated but has been left as it is for a long time, the quantity of charge on the toner lowers due to leak of charge and consequently, the output value from the toner density sensor increases. That is, despite that the actual toner density is unchanged, the output value from the toner density sensor could change. As a result, when a copying operation is done after a prolonged deactivation, the toner density sensor outputs a greater value than the reference value despite that the developer has a correct toner density, whereby the sensor erroneously detects that the toner density is low (or the developer is in the under-toner state) and effects' toner supply. ,This oversupply of toner inhibits sufficient generation of charge and causes excess density, background fog, toner scattering etc. in the copied image. Since the electrification of the developer largely depends on the environmental conditions; for example, the rising performance of charge on toner is poor under a high-humid environment, the output value from the toner density sensor varies greatly. Nevertheless, in the conventional configuration, the toner density sensor effects the toner density detection without regarding the environmental conditions, so that the output value from toner density sensor fluctuates and therefore it was impossible to create images with stabilized toner density at any time. Japanese Patent Publication Sho 60 No.2,661 discloses a way of properly keeping the toner density. In this configuration, generated is a correcting signal which corresponds to the level difference between the detection by the toner density means when an operation of the developing unit was stopped and the detection when a next operation is started. When the operation of the developing unit is activated, the deviation of the output detected by the detecting means right after the operation start of the developing unit from the last detection is compensated by adding the correcting signal to the output signal and attenuating the correction signal as time elapses, whereby proper toner density can be maintained. However, there is a fear that the scheme proposed in Japanese Patent Publication Sho 60 No.2,661 does not work effectively. Consider a case that the developer has not been agitated well because, for example, the developing unit is deactivated right after toner supply. In this case, the detection level detected by the toner density detecting means does not indicate the actual toner density. Then, when the developing unit is activated after the developer has been left inoperative for a while in the above state, the output signal from the toner density detecting means at the start of the operation of the developing unit is corrected by adding the correcting signal which corresponds to the level difference between the detection by the toner density means when the last operation of the developing unit was stopped and the detection when this operation is started. However, the detection level by the toner density detecting means at that time does not indicate the actual toner density. Accordingly, despite that the toner density is correct, the developer is erroneously detected as in the under-toner state until the developer will have been agitated enough, and during this period, toner supply could be continued. Japanese Patent Publication Sho 60 No.2,661 also discloses a method in which correction of toner density is made by comparing the detection level by the toner density detecting means at the start of the operation with a control reference level for the toner density detecting means and adding the correcting signal to the output signal from the toner density detection means at the start of the operation of the developing unit, but this method also involves anxieties over occurrences of the problems described above. Further, the conventional copier, laser printer, PPC facsimiles or the like uses devices and supplies such as a charging device, exposure device, photoreceptor and developer; these devices and supplies have characteristics depending on environmental surroundings (temperature and humidity) and time-dependence characteristics. Since images obtained by charging and exposing the photoreceptor and the development of it must be affected by those factors, the image tends to be unstable. To deal with the above problems, recent copiers, laser printers or PPC facsimiles or the like incorporate an image stabilizing device as disclosed in Japanese Patent Application Laid-Open Hei 6 No.51,551, Japanese Patent Application Laid-Open Hei 6 No.19,259 or Japanese Patent Application Laid-Open Hei 6 No.11,929, in order to stabilize the output image by controlling the process conditions (on charging, exposure and development). Japanese Patent Application Laid-Open Hei 6 No.51,551 proposed a correcting scheme of electrophotographic-process parameters by preparing a toner patch in a predetermined area on the photoreceptor surface and detecting the density of the toner patch and the non-image area, comparing them to each other and determining the process parameters based on the comparison. Japanese Patent Application Laid-Open Hei 6 No.19,259 proposed a correcting scheme in which the copy lamp voltage is changed whenever a certain number of copies have been made and the relation between the output from the original density detection sensor and the developing bias voltage is corrected based on the magnitude of the change of the copy lamp voltage. Japanese Patent Application Laid-Open Hei 6 No.11,929 proposed a process control of the toner patch scheme in which the post-transfer amount of toner adhered in the toner patch portion on the photoreceptor is detected to determine the transfer efficiency and the erasure output is controlled based on the ratio. In the conventional typical image stabilizing apparatuses as described above, the control can be effected to a certain degree of precision, still it is difficult to precisely make corrections for the variations arising due to environmental characteristics (temperature and humidity) or with the passage of time and therefore it is difficult to maintain the same quality of image as in the initial stage up to the end of life. Therefore, it is also important to appropriately keep the toner density in the developer in order to maintain the quality of image at a high level. For example, since the quantity of charge on the toner in the developer as having been exposed to a high temperature and humidity environment and/or left inoperative for a prolonged period becomes low, various problems occur such as lowering of tone reproducing performance due to the image density rise, increase of toner consumption, increase of background fog, toner scattering and the like. Therefore, it is necessary to lower the toner density in the developing unit. On the toner hand, since the quantity of charge on the toner in the developer as having been exposed to a low temperature and humidity environment and/or after a continuous operation of copies becomes high, problems such as lowering of the image density, lowering of transfer performance and the like occurs. Therefore, it is necessary to increase the toner density in the developing unit. SUMMARY OF THE INVENTION The present invention has been achieved to solve the above problems in view of what has been described above. It is therefore an object of the present invention to provide an image forming apparatus for solving the above problems by canceling the corrections when the toner density formed in the actual image forming is recovered to a predetermined density value. Another object of the invention is to provide an image forming apparatus which is able to maintain the toner density of the developer appropriately by estimating the variation of the developer over a prolonged period of being left inoperative as well as estimating environmental conditions and the like. Since, of the process conditions (charging, exposure and development) which are controlled by detecting a toner patch formed on the photoreceptor, the charger output is corrected to be lowered when the apparatus is exposed to a high temperature and high humidity environment or left inoperative over a prolonged period while it is corrected to be increased when the apparatus is exposed to a low temperature and low humidity environment or used for a continuous copying operation, it is a further object of the invention to create stabilized images substantially free from poor density and background foggy by detecting the change of the variation in the charger output and determining whether the toner density correcting reference value is high or low to appropriately control the toner density inside the developing unit. Suppose that the apparatus is constructed such 'that corrections of the toner density is canceled after a certain number of copies have been made, it is not always possible to cancel the toner density corrections at appropriate timing since electrification performances of the developer differ depending on the use conditions or degradation levels of the developer. In order to correct the toner density control by toner supply while actual developing performances of the developer is being detected, an image forming apparatus in accordance with a first feature (embodiment) of the invention includes the features of claim 1. In order to prevent a sharp transition of the image density when the above correction is canceled, in an image forming apparatus in accordance with a second feature of the invention, the toner density correction canceling means is adapted to gradually reduce the correcting quantity by the toner density correcting means with the increase of the detected agitation extend. In order that images formed under the condition right after the cancellation of the above correction is to be made in proper toner density, an image forming apparatus in accordance with a third feature of the invention further includes means for activating the process parameter controlling mean after the toner density correction canceling means has canceled the correction set up by the toner density correcting means. A fourth feature of the invention in an image forming apparatus wherein the image density is controlled regularly by forming a patch of toner on the photoreceptor and varying the charger output as one of image forming conditions based on the density of the patch and control of toner supply to the developing unit is effected so that the toner density in the developing unit corresponds to a toner density reference value and the image forming apparatus is constructed such that when the variation as to the charger output is equal to or greater than a first predetermined value, the toner density reference value is changed, then the changed toner density reference value is maintained until the variation of the charger output again becomes equal to or greater than the first predetermined value. Further, a fifth feature of the invention resides in an image forming apparatus having the fourth configuration wherein after a change of the toner density reference value, another change of the toner density reference value is prohibited for a predetermined period of time. Moreover, a sixth feature of the invention resides in an image forming apparatus having the fourth configuration wherein after a change of the toner density reference value, if the variation of the charger output is equal to or greater than a second predetermined value, it is detected that the apparatus is in an anomalous state. In accordance with a seventh feature of the invention, there is provided an image forming apparatus in which an electrostatic latent image formed on an image carrier is developed using a two-component developer and in which the toner density of said developer is regulated in accordance with the output of a toner density sensor arranged to detect the toner density of said developer in a developer unit, wherein there is also provided means for applying an adjustment to the control operation performed using said toner density sensor output, in accordance with a detected parameter indicative of the extent of agitation of the developer, and means for reducing the adjustment when the value of a process parameter which is used to operate a device performing part of the image formation process and which is controlled so as to regulate toner patch density of a toner patch on the image carrier, satisfies a predetermined condition. In the image forming apparatus in accordance with the first feature of the invention, the toner density sensor detects the toner density of the developer stored in the developing vessel; and the toner density control means compares the output from the toner density sensor with a reference toner density and maintains the toner density of the developer at the reference toner density by controlling the amount of supplying toner to the developing vessel. On the other hand, the agitation total detecting means detects as an agitation total the number of agitation of the developer inside the developing vessel, or the operation time of the image forming apparatus approximately or the number of image forming, either of which is proportional to the number of agitation; and the toner density correcting means corrects the output from the toner density sensor or the reference toner density in association with the augment of the agitation total. Thus, the lowering of the image density at the initial stage of the developer can be corrected. The toner patch density detecting means creates a toner patch on the photoreceptor and detects the density of the toner patch; and the process parameter controlling means controls process parameters including an applied voltage to the main charger so as to adjust the toner patch density detected by the toner patch density detecting means to a predetermined density value. The toner density correction canceling means cancels the correction by the toner density correcting means when the process parameter has reached a predetermined value with improvement of developing performances of the developer. In the above operation, the toner density correcting means corrects the lowering of the image density at the starting stage of the developer such that, for example, as shown in Figs.5 and 6, the apparent toner density is increased as the agitation total of the developer increases and then cancels the toner density correction before the overcorrection occurs so that apparent toner density is restored to the original value at the time the correction is not made. As a result, it is possible to improve the quality of image and prevent the pollution of the machine inside due to toner scattering. Abrupt transition of the toner density in the developer is conceivably attributed to the change of the electrification performances of the developer. In accordance with the image forming apparatus having the second feature, when the toner density correction is canceled as the agitation total of the developer increases, the correcting quantity set up by the toner density correcting means is gradually reduced, for example, as shown in Fig.6. Therefore it is possible to obtain stabilized images before and after the cancellation of the toner density correction. In accordance with the image forming apparatus having the third feature, after the toner density correction canceling means has canceled the correction set up by the toner density correcting means, the process parameter controlling means controls process parameters so that toner patch density corresponds to previously determined density. By this operation, it is possible to effect image forming with appropriate process parameters right after the toner density correction is canceled. Next, in accordance with the fourth feature of the invention, it is possible to prevent lowering of tone reproducing performance, augment of toner consumption, increased background foggy, toner scattering and other defects, all attributed to the elevation of the image density due to the lowering of the quantity of charge on the developer as having been exposed to a high temperature and humidity environment or after a prolonged period of inactive state. It is also possible to prevent occurrences of problems such as lowering of the image density, degradation of transfer performances and the like due to the elevation of the quantity of charge on the developer as having been exposed to a low temperature humidity environment, after a continuous copying operation or the like. Thus, it is possible to effect appropriate toner density corrections and thus it possible to produce markedly stabilized images. In accordance with the fifth feature of the invention, upon the change of the toner density reference value, copying must be continued to a certain degree right after the toner density reference value is changed, in order for the toner density of the developer to reach the modified reference value. Accordingly, it is possible to prevent redundant modification of the toner density reference value by correcting the charger output in the round of the process control which would be activated by erroneous determination that the modification of the toner density has not been performed yet despite that actual change of the toner density reference value has been done. As a result, it is possible to effect more stabilized control of the toner density inside the developing unit. In accordance with the sixth feature of the invention, if the toner density reference value has been modified so as to increase the toner density inside the developing unit, the charger output is adapted to be corrected to lower. In contrast, if the toner density reference value has been modified so as to decrease the toner density inside the developing unit, the charger output is adapted to be corrected to increase. Nevertheless, after the modification of the toner density reference value, the charger output deviates beyond the predetermined maximum or minimum, it is judged that some trouble happens in the image forming apparatus and consequently the apparatus can be determined in an anomalous state. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a chart showing a tendency of initial lowering of image density when a virgin developer is used; Fig.2 is a chart showing changing behavior of the charge quantity of a developer; Fig.3 is a chart showing a relation between toner density of a developer and output from a toner density sensor; Fig.4 is a chart showing a relation between the quantity of charge on a developer and output from a toner density sensor; Fig.5 is a chart showing a relation between agitation total and apparent toner density; Fig.6 is a chart showing a relation between agitation total and apparent toner density; Fig.7 is a schematic sectional view showing an image forming apparatus of an embodiment of the invention; Fig.8 is a block diagram showing a configuration of a controller of a copier; Fig.9 is a table showing a relation between total rotating time and reference voltages determined based on the total rotating time; Fig.10 is a flowchart showing the order of procedures for automatically controlling toner density in a developer; Fig.11 is a flowchart showing the order of procedures for automatically controlling the applied voltage to the main charger; Fig.12 is a flowchart showing the order of procedures for controlling timing for effecting the procedures shown in Fig.11; Fig.13 is a flowchart showing the order of procedures for correcting toner density; Fig.14 is a flowchart showing part of the procedures of toner density correction in accordance with a second embodiment; Fig.15 is a chart showing a relation between total rotational time and output voltages from a toner density sensor; Fig.16 is a chart showing a relation between total rotational time and output voltages from a toner density sensor; Fig. 17 is a main sectional view showing an image forming apparatus of the invention; Fig. 18 is a chart showing a relation between the charger output and variations of toner density reference values in an image forming apparatus of the invention; Fig. 19 is a diagram showing a control block of the invention; and Fig. 20 is a flowchart showing the operation of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig.7 is a schematic sectional view of a copier. Designated at 1 is a photoreceptor which is formed of an aluminum drum with a photoconductive layer formed on the surface thereof. The photoconductive layer is formed by uniformly applying a charge generating layer of 0.5 µm thick and then uniformly applying a charge transfer layer of 34 µm thick over the charge generating layer. A reference numeral 2 designates a main charger made up of a scorotron charger with a screen grid. A reference numeral 3 designates an optical system for illuminating the document placed on the original table and focusing the reflected light on the photoreceptor. A reference numeral 4 designates a developing unit for visualizing the static latent image formed on the photoreceptor with toner. A reference numeral 5 designates a transfer unit which transfers the toner image on the photoreceptor to the copy sheet. The copy paper with the toner image transferred thereto is peeled off from the photoreceptor by means of separator 6 and introduced to a fixing unit 7 where the toner image is fused and fixed to the sheet and then discharged outside the machine. A reference numeral 9 designates a standard white plate having non-reflective portion as a part thereof and 8 designates a blank lamp. A latent image of a toner patch is formed by exposing the image of the standard white plate 9 onto the photoreceptor surface as selectively turning on the blank lamp 8 in accordance with prescribed timing. The thus formed latent image is developed by the developing unit 4 into a toner patch. A reference numeral 10 designates a photosensor 10 which detects the density of the toner patch on the photoreceptor. A reference numerals 11 designates a temperature and humidity sensor for detecting the temperature and humidity inside the copier. A reference numeral 12 designates a toner density sensor which detects the toner density of the developer inside the developing unit 4 based on the inductance. Fig.8 is a block diagram showing a configuration of a controller of the copier. A CPU 21 executes a prescribed program previously written in a ROM 22 to perform a series of processing described hereinbelow. A RAM 23 is used for working areas for the processing. A timer circuit 24 effects time-counting operations independently of the process by the CPU 21. The CPU 21 resets the timer circuit 24 at a desired timing and reads the counted value. An AD converter 26 converts into digital data any of the output signal from the optical sensor 10, the output signal from the toner density sensor 12 or the output signal from the temperature and humidity sensor 11, which is selected by a multiplexer 25. The CPU 21 switches over the multiplexer 25 at a necessary timing and reads the output value from the A/D converter 26. A main motor 29 is a driving source of driving the portions such as the photoreceptor, the original table, the transfer system of copy paper; a developing motor 31 is a driving source of rotary parts in the developing unit; and an agitator clutch 33 is a mechanism of effecting the switching operation of whether the rotation of the developing motor 31 is transmitted to agitator blades. A toner supply motor 35 is a driving source of supplying toner into the developing vessel of the driving unit. The blank lamp 8 and the main charger 2 is as already described with reference to Fig.7. A bias supply circuit 38 supplies a developing bias voltage to the developing unit. The CPU 21 controls these peripheral devices through an I/O port 27 as well as a driver circuit 28, 30, 32 ,34, 36 or 37. Fig.10 is a flowchart showing the order of procedures for controlling the toner density of the developer based on the output from the toner density sensor 12. First, a judgment is made on whether the developer has been agitated for a predetermined period of time from when the last toner density control was made. If the judgment is determined to be positive, the output value V from the toner density sensor is read out. Determined then is whether this value V falls within a predetermined range. If this value belongs to the outside of the range, a treatment for anomalous toner density will be effected. If the output V from the toner density sensor belongs to the predetermined range, another judgment is made as to the state. That is, if the toner density control is effected first time, for example, immediately after the reset when the developer is replaced, the output value V from the toner density sensor is set at V0 (as an initial reference voltage) and this voltage is stored into the memory. If the operation is determined not to be in the initial condition, the output value V is compared with a reference voltage which is set up in the toner density correcting process aforementioned. When the output value V is above the reference voltage, the toner supply motor is activated to supply a predetermined amount of toner to the developing vessel. If V is not more than the reference voltage, toner supply will not be done. The above processing will be repeated whereby the toner density of the developer is controlled so that the output V from the toner density sensor may be equal to the reference voltage. (First embodiment) Now, a toner density correction in accordance with a first embodiment of the invention as well as the order of the procedures will be described. Fig.9 shows an example of reference voltages used upon the above toner density control. In this table, the total rotating time is a total rotating time as to the main motor and indicates the extent of agitation. Although no particular mention is made in the flowchart, the controller measures the total rotating time of the main motor and effects a process to determine a CNT value shown in Fig.9. Also the controller changes the reference voltage in accordance with the total rotating time of the developer for the purpose of toner density correcting process mentioned later. As mentioned above, if, for example, the output voltage from the toner density sensor immediately after the replacement of the developer is 2.375 V, this value is set as the initial reference voltage V0 and stored in the memory. If the total rotating time belongs to a range of 0 to 99 seconds (the CNT value for this range is set at 0), the reference voltage is kept at V0. Then, if the total rotating time belongs to 100 to 199 seconds (CNT = 1), the reference voltage is set at V1 = 2.355 V, which is 0.02 V lower than V0. Similarly, if for example the total rotating time belongs to a range of 2,000 to 19,999 seconds (CNT = 20), the reference voltage is set at 1.975 V (V20), which is 0.02 V lower than V19. Thus, before 2,000 seconds, the reference voltage is reduced by 0.02 V every time the total rotating time increases by 100 seconds. In this way, as the reference voltage is varied in association with the agitating time of the developer, the toner density control shown in Fig.10 is repeatedly done, whereby the output value from the toner density sensor as following the reference voltage, varies stepwise from V0 to V20 as the total rotating time increases. As shown in Fig.3, the apparent toner density can be corrected from about 6 wt.% to about 5 wt.%. Fig.15 shows variations of the output voltage from the density sensor in accordance with the above control. As shown in the figure, the output from the toner density sensor varies stepwise from V0 to V20 with the increase of the total rotating time. By this operation, the apparent toner density gradually increases as shown in Fig.5, whereby the lowering of the image density occurring at the initial stage where the developer is just started to use can be corrected. Fig.11 is a flowchart showing the order of procedures of a process parameter control for setting up process parameters which is practically independent from the above toner density control. Initially, a latent image for creating a toner patch is formed on the photoreceptor surface. This latent image is developed into a toner patch. Subsequently, the digital value outputted from the photosensor 10 is picked up as its toner patch density. Then a grid potential MC of the main charger 2 is set up so that the toner patch density may be equal to a previously determined density value. The process control shown in Fig.11 is repeatedly done at predetermined intervals as will be stated hereinafter. Thus the surface potential of the photoreceptor is determined by the above MC set up as above. Fig.12 is a flowchart showing the order of procedures for controlling the operation timing of the process parameter control shown in Fig.11. In the beginning, the above process parameter control is effected when the apparatus is energized, and a copy counter A for counting the number of copies is reset (n1 → n2). Subsequently, the timer circuit 24 is reset and the timer is started (n3). The start of a copying operation is waited (n4). When a copying operation starts, the copy counter A is incremented by 1 while another copy counter B to be aftermentioned also is incremented by 1 (n5 → n6). Then the value of the timer at the time of start of the copying operation is compared with a reference value, and if the value of the timer does not reach the reference value, a judgment is made on whether the value of the copy counter A is equal to or above a prescribed number. If the sum of the value of the counter A and the number of copies to be made in the current copying operation exceeds the prescribed number, the above process parameter control is effected prior to the actual copying operation (at the time of the pre-rotation) (n7 → n8 → n9). Thereafter the copy counter A is reset and the end of the copying operation is waited (n10 → n11). Thus one round of the process parameter control shown in Fig.11 is performed whenever copies of the prescribed number have been made. The above timer also measures the standby time during which no copying operation has been done. If time longer than a predetermined duration has elapsed without any operation when a next copying operation is made, the process parameter control is effected regardless of whether the number of copies has not reached the prescribed number at that time (n4 → n5 → n6 → n7 → n9). Fig.13 is a flowchart showing the order of the toner density correcting process. Initially, the counter CNT value shown in Fig.9 is detected (n21). If the CNT value is zero, the voltage V0 shown in Fig.9 is set up as the reference voltage (n23 → n24). If CNT = 1, V1 is set up as the reference voltage (n25 → n26). Similarly, if CNT = 20, V20 is set up as the reference voltage (n29). Thereafter, if the total rotating time increases and the counter CNT value reaches 21, a loop counter PC is reset and next activation of the process parameter control is waited (n22 → n30 → n31). The process parameter control is done at the timing shown in Fig.12. When the process control is effected, the grid voltage MC of the main charger is compared with a prescribed value (n32). If developing performance of the developer is still low, the surface potential of the photoreceptor or the MC value is increased to maintain the density of the toner patch at a prescribed value. On the other hand, if MC is still beyond the prescribed value, the loop counter PC is reset and next activation of the process parameter control is waited (n32 → n30). Thereafter, as the agitation total of the developer increases, developing performance of the developer is improved. As a result, the grid voltage MC gradually decreases as some or several rounds of the process parameter control shown in Fig.11 have been done. If the MC determined by the process control take a value equal to or below the predetermined value twice in succession, it is judged that it is no longer necessary to correct the toner density and the toner density correction tends to cause an overcorrected state. Accordingly, the reference voltage is set at V0 after this detection (n32 → n33 → n34 → n31 → n32 → n33 → n35). Subsequently, the copy counter B as a counter for counting the number of copies to be made from this point of time is reset and the operation will be waited until the copy counter B counts up to a predetermined number (n36 → n37). The copy counter B is incremented in the flow shown in Fig.12. When copies of the predetermined number have been made, the process parameter control shown in Fig.11 is forcibly effected, independently of the timing shown in Fig.12 (n38). By the above operation, the reference voltage at the time of T1 is set at V0 as shown in Fig.15 and the toner supply to the developing vessel is stopped or lowered in quantity and the output from toner density sensor as following this setting, rises up to V0. (Second embodiment) Next, Fig.14 shows part of the procedures of a toner density correcting flow in accordance with a second embodiment. The prior process to the procedures shown in Fig.14 is the same with that of steps n21 through n34 in Fig.13. That is, when the toner density correction is canceled after developing performance of the developer has been improved, the copy counter B for counting the number of copies from the point of time is reset and thereafter the reference voltage is determined based on the value of the copy counter B. For example, the reference voltage is set and kept at the voltage V19 shown in Fig.9 until the value of the copy counter B reaches a predetermined number C0 (n46 → n48). When for example, the value of the copy counter B exceeds the predetermined number C0, the reference voltage is set at the voltage V18 shown in Fig.9 (n47). If for example, the value of the copy counter B exceeds the predetermined number C19, the reference voltage is set at the voltage V1 shown in Fig.9 (n44 → n45). Further, if the number of copy increases and the value of the copy counter B becomes equal or above a predetermined number C20, the reference voltage is set at VO (n42 → n43). In this way, since the reference voltage is varied in accordance with the increase of the copy number while the toner density control shown in Fig.10 is repeatedly done in the course of the cancellation of the toner density correction, the output from the toner density sensor, as following the reference voltage, varies stepwise from V20 to V0 with the augment of the number of copies. Fig.16 shows variations of the output voltage from the toner density sensor by the above control. As seen, the output from the toner density sensor varies stepwise from V20 to V0 with the augment of the number of copies. This control allows the apparent toner density to gradually lower as shown in Fig.6, whereby change of the characteristic of electrification of the developer is regulated so that it is possible to create stabilized images before and after the cancellation of the toner density correction. (Third embodiment) Next, a configuration of an image forming apparatus in accordance with a third embodiment will be described. In the above examples, the judgment of either continuation or cancel of the toner density correction is made when the total rotating time reaches a predetermined value as shown in Fig.9. The quantity of charge on the developer changes depending upon the temperature and humidity as shown in Fig.2. To deal with this, in this third embodiment, timing of the judgment of whether the toner density correction is to be continued or canceled is set up as follows. The total rotating time at CNT 20 shown in Fig.9 is set up as a standard value for the standard environment, and the total rotating time at CNT20 at the time of high temperature (30°C or more) or high humidity (70% or more) is set at a half of the standard value, specifically in a range from 2,000 to 9,999 seconds while the total rotating time at CNT20 at the time of low temperature (15°C or less) or low humidity (35% or less) is set at a double of the standard value, specifically in a range from 2,000 to 39,999 seconds. The other control is effected in the same manner. (Fourth embodiment) Next, a configuration of an image forming apparatus in accordance with a fourth embodiment will be described. In the above examples, although the total rotating time of the main motor is associated with the agitation total of the developer, the actual agitation total of the developer varies depending on the use condition of the copier or the average copy number per one operation in the copier. Therefore, in the fourth embodiment, timing of the judgment of whether the toner density correction is to be continued or canceled is changed based on the use condition of the copier. Specifically, the total copy number when CNT = 20 as shown in Fig.9 or when the total rotating time of the main motor has reached 2,000 seconds is assumed to be represented by n, CNT20 and CNT21 are set up on the following conditions: L Setup Time for CNT20 Setup Time for CNT21 L ≥ 0.65 2,000 to 29,999 sec. 30,000 sec. L ≤ 0.25 2,000 to 9,999 sec. 10,000 sec. where L = n/2,000. Other than these are the same as shown in Fig.9. (Fifth embodiment) Next, a configuration of an image forming apparatus in accordance with a fifth embodiment will be described. Although in the second embodiment, the cancellation of the toner density correction is effected by counting the total number of copies from the start of the canceling mode and changing the reference voltage for the toner density control stepwise based on the total number of copies thus counted, the quantity of charge on the developer changes depending upon the temperature and humidity as shown in Fig.2. Therefore, in this fifth embodiment, Tx and Vx shown in Fig.16 will be changed depending on the conditions as follows: Standard Environment Mode (to be abbreviated as S.E. mode): Vx = 0.02V Tx = 100 sec. High Temperature and Humidity Mode : Vx = 2 x (the value in S.E. mode) Tx = 1/2 x (the value in S.E. mode) Low Temperature and Humidity Mode : Vx = 1/2 x (the value in S.E. mode) Tx = 2 x (the value in S.E. mode). (Sixth embodiment) Next, a configuration of an image forming apparatus in accordance with a sixth embodiment will be described. Although in the above examples, the cancellation of the toner density correction is effected by counting the total number of copies from the start of the canceling mode and changing the reference voltage for the toner density control stepwise based on the total number of copies thus counted, the actual agitation total of the developer varies depending on the use condition of the copier or the average copy number per one operation in the copier. Therefore, in this sixth embodiment, Tx and Vx shown in Fig.16 will be changed depending on the conditions as follows: When L ≥ 0.65 Vx = 1/2 x (the value in S.E. mode) Tx = 2 x (the value in S.E. mode) When L ≤ 0.25 Vx = 2 x (the value in S.E. mode) Tx = 1/2 x (the value in S.E. mode), where L = n/2,000 and Vx = 0.02 V and Tx = 100 sec. at the standard environment mode. (Seventh embodiment) Next, a configuration of an image forming apparatus in accordance with a seventh embodiment will be described. In the first embodiment, the correction is made by varying the standard voltage by the step of 0.02v every time the count value CNT of the total rotating time increases by 1. The quantity of charge on the developer, however, changes depending upon the temperature and humidity as shown in Fig.2. As shown in Fig.4, when the deviations of the charge quantity at the high temperature and humidity environment, the normal temperature and humidity environment, and the low temperature and humidity environment are represented by Δ1, Δ2 and Δ3, respectively, the decreasing amounts of the output voltage from the toner density sensor are about 0.33 V at the high temperature and humidity environment, about 0.66 V at the normal temperature and humidity environment, and about 1.0 V at the low temperature and humidity environment. Therefore, in the seventh embodiment the reference- voltage is changed in accordance with the temperature and humidity as follows: when high temperature and humidity is detected: the reference voltage is changed by 0.01 V as CNT increases by 1; when low temperature and humidity is detected: the reference voltage is changed by 0.03 V as CNT increases by 1. Although in the first embodiment, the process parameter control is executed when the apparatus is energized, this execution of the process parameter control is not requisite. It is also possible to effect one round of the process parameter control shown in Fig.11 by temporarily interrupting the copying operation when certain conditions are satisfied during the copying operation and judging developing performance of the developer based on the grid voltage of the main charger set up by the process parameter control. In the above embodiment, the grid potential of the main charger is set up so that the toner patch density may be equal to a previously determined value by the process parameter control while developing performance of the developer is detected based on the variation of the grid potential of the main charger. Similarly, a variation of the above embodiment can be constructed by setting up the bias potential applied to the developing unit so that the toner patch density may be equal to a target value and detecting developing performance of the developer based on the change of the bias potential. In accordance with the image forming apparatuses of the embodiments described heretofore, the lowering of the image density at the starting stage of the developer is corrected while the toner density correction is canceled before the overcorrection occurs so that apparent toner density is restored to the original value at the time the correction is not made. As a result, it is possible to improve the quality of image and prevent the pollution of the machine inside due to toner scattering. In accordance with the foregoing image apparatuses, when the toner density correction is to be canceled as the agitation total of the developer increases, the change of electrification characteristics of the developer is regulated. Therefore it is possible to obtain stabilized images before and after the cancellation of the toner density correction. Further, in accordance with the foregoing image forming apparatuses, it is possible to attain stabilized image forming with appropriate process parameters as soon as the toner density correction is canceled as the agitation total of the developer has been increased. (Eighth embodiment) Next, Fig.17 schematically shows an image forming apparatus to which a toner density correcting method of the invention is applied and the overall configuration and the operation will be described hereinbelow. As shown in the figure, an image forming apparatus in accordance with an eighth embodiment, includes: an original table 201; an exposure lamp 202; a first mirror 203, a second mirror 204, a third mirror 205; a lens 206 ; a fourth mirror 207; a fifth mirror 208; a sixth mirror 209; a photoreceptor drum 210 of OPC; a charger unit 211; a developing unit 212; a transfer unit 213; a cleaning unit 214; a cleaning blade 215; a paper conveyer unit 216; a fixing unit 217; an erasing lamp 218; a blank lamp 219; a photosensor 220 for detecting the image state on the photoreceptor; a toner hopper 221; a toner supply motor 222; a non-magnetic sleeve 223; a magnet 224; an agitating roller 225; and a toner density sensor 226. As the copy start button is pressed with a document set on the original table 201, the exposure lamp 202, first mirror 203, second mirror 204, third mirror 205 are adapted to move in parallel to the original table 201 (in the directions of a bidirectional arrow in the figure). Light emitted from the exposure lamp 202 is reflected on the document placed on the original table 201 and the reflected light is introduced (as indicated by broken lines) through the first mirror 203, second mirror 204, third mirror 205, lens 206, fourth mirror 207, fifth mirror 208, sixth mirror 209 to illuminate the surface of the photoreceptor drum 210 electrified by the charging unit 211. This illumination creates a static latent image on the surface of the photoreceptor drum 210. The blank lamp 219 is selectively turned on so as to illuminate unnecessary part of the static latent image to cancel charges thereon. The thus trimmed latent image is visualized by means of the developing unit 212 into a toner image. The toner image is transferred by means of the transfer unit 213 to the paper supplied from the paper cassette. The paper with the toner image transferred thereon is conveyed by the paper conveyer unit 216 to the fixing unit 217 where the toner image is fixed on the sheet, and then discharged outside. A remaining toner image, which is left on the photoreceptor drum 210 after the toner image has been transferred, is scraped to be cleaned by the cleaning blade 215 of the cleaning unit 214, then all the remaining charges on the photoreceptor are canceled by the erasing lamp 218. The developing unit 212 is composed of the non-magnetic sleeve 223 disposed opposite the photoreceptor 210 and rotationally driven and the agitating roller 225 for agitating the developer. The developer is agitated by the agitating roller 225 and the carriers and toner particles are friction-electrified. The developer is conveyed by the action of the magnet 224 fixed inside the non-magnetic, sleeve 223 and toner particles in the developer are transferred to the static latent image formed on the photoreceptor 210 to form a visualized image. Because only the toner of the developer is consumed in the above operation, a toner density sensor 226 attached inside the developing unit detects the change of the toner density inside the developing unit as shown in Fig.19. The thus detected measurement of the current toner density is compared to a reference value of the initial toner density inside the developing unit which has initially been memorized in a CPU 229. If the output from the toner density sensor 226 is higher than the toner density reference value, the toner density inside the developing unit is determined as to be low and the toner supply motor 222 is activated to rotate to supply the toner stored inside the toner hopper 221 into the developing unit. Independently of the above toner control scheme, in the image forming apparatus having an image stabilizing device, the above copying process is interrupted as necessary and process control will be effected under specific conditions, periodically (whenever a regular number of copies has been made or a regular period of time has elapsed) and at the time the apparatus is activated. As shown in Fig.19, the density of toner patches formed on the photoreceptor 210 is detected by the photosensor 220, whose output is amplified to an appropriate level by an amplifier 227 and then converted into digital quantity by an A/D converter 228 so as to be inputted to the CPU 229. Based on the thus inputted value, a charger output driver circuit 230 and the like are operated to correct parameters for the electrophotographic process. Detailedly, a plurality of static latent images having different surface potentials are created by varying the charger output. These latent images are developed by the developing unit 212 into toner patches having different density levels which are in turned detected by the photosensor 220. If one of these detected values corresponds to a predetermined value P, the charger output which created the static latent image of the toner patch corresponding to the value P is adopted as a charger output for copying operation. Since the number of patches having a plurality of different density created is limited, a toner patch whose density corresponds to the predetermined value P is not always formed. In such a case or if there is no toner patch corresponding, a charger output Vg may be calculated based on detected values P1, P2 (satisfying the relation P1 < P < P2) by the photosensor 220 which are closest to the predetermined value P. Suppose that the detected values P1 and P2 correspond to charger outputs Vg1 and Vg2, respectively, relations Vg1 = aP1 + b and Vg2 = aP2 + b will be expectedly hold. Accordingly, it is possible to calculate a and b from the two relations. From this result, the target charger output Vg is determined based on the relation Vg = aP + b. In this way, the charger output Vg is modified at regular intervals. The change of the charger output Vg is shown in Fig.18. Referring next to a flowchart shown in Fig.20, the operation of the eighth embodiment will be described. First, a toner patch is formed on the photoreceptor and detected on its toner patch density. Based on the data thus obtained, process control is performed to correct parameters related to the electrophotographic process. Upon the control, if the charger output Vg is corrected to a value equal to or greater than (initial Vg + α), it is judged if the value is equal to or lower than a maximum (Vgmax) of the charger output. If the value exceeds the maximum, an anomaly indication will be displayed (S401 through S406). If the value does not exceed Vgmax, it is judged if t ≥ T holds, that is, if a count time t from the previous change of the toner density reference value is equal to or greater than T (T is a period of time required for the toner density in the developing unit to be stabilized after the change of the toner density reference value). If the judgment is affirmative, the toner density correcting reference value is changed by - β (S407 and S408). Unless t ≥ T, the operation returns to the normal copy cycle. After the toner density correcting reference value is changed by - β, the count time t is reset and the operation returns to the normal copy cycle (S409). Next, if the charger output Vg is corrected to a value equal to or smaller than (initial Vg - α), it is judged if the value is equal to or greater than a minimum (Vgmin) of the charger output. If the value is smaller than the minimum, an anomaly indication will be displayed (S410 through S412). If the value is not inferior to Vgmin, it is judged if t ≥ T holds, that is, if the count time t from the previous change of the toner density reference value is equal to or greater than T. If the judgment is affirmative, the toner density correcting reference value is changed by + β (S413 and S414). Unless t ≥ T, the operation returns to the normal copy cycle. After the toner density correcting reference value is changed by + β, the count time t is reset and the operation returns to the normal copy cycle (S415). In the invention, when the process control is effected, as shown in Fig.18, of the corrected parameters, if the charger output Vg is corrected to a value equal to or below (initial Vg - α), it is determined that the developing performance is too high and the toner density reference value is changed by + β. This modification lowers the toner density in the developing unit. Thereafter, the current reference value will be maintained until another correction of the charger output beyond the range of (initial Vg ± α) is to be made. Conversely, if the charger output Vg is corrected to a value equal to or below (initial Vg + α), it is determined that the developing performance is too low and the toner density reference value is changed by - β. This modification increases the toner density in the developing unit. Thereafter, the current reference value will be maintained until another correction of the charger output beyond the range of (initial Vg ± α) is to be made. Thus, it is possible to obtain stabilized quality of image over a prolonged period of time. In Fig.18, the lateral axis is named a rotating time in the developing vessel, which indicates the rotational time of the non-magnetic sleeve 223, that is, total operation time of the developing unit 212 from the power-activation with non-operation time of the developing unit excluded. If the toner density reference value is changed, it takes time after the correction for the toner density value inside the developing vessel to change to the toner density reference value. When the toner density reference value is changed so as to increase the toner density inside the developing unit, toner supply to the developer will be frequently performed immediately after the change. (Ninth embodiment) When the toner density reference value is changed so as to decrease the toner density inside the developing unit, the toner density inside the developing unit will not lower unless a certain amount of copies are taken. Accordingly, as indicated at (1) in Fig.18, even if the charger output Vg is corrected to (initial Vg ± α) within the time T from the change of the toner density reference value until the toner density inside the developing unit is stabilized, the modification of the toner density reference value will be prohibited. This feature enables more stabilized toner density control as compared to the eighth embodiment. Here, the count time t will be reset (t ← 0) when the apparatus is energized. The count time t is counted up during only the operating time of the developing unit, whereby the aforementioned rotating time in the developing vessel is measured. (Tenth embodiment) Further, in the normal state, the charger output must be corrected to increase after the change of the toner density reference value by + β. But if the charger output is corrected lower than the minimum (Vgmin) as indicated at (2) in Fig.18, or conversely, when the charger output is corrected greater than the maximum (Vgmax) as indicated at (2) in Fig. 18 after the toner density reference value is changed by - β, it is judged that some trouble happens in the image forming apparatus and consequently, the apparatus is determined in an anomalous state. In accordance with the image forming apparatus of the foregoing embodiments, it is possible to prevent lowering of tone reproducing performance, increase of toner consumption, increased background foggy, toner scattering and other defects, all attributed to the elevation of the image density due to the lowering of the quantity of charge on the developer as having been exposed to a high temperature and humidity environment or after a prolonged period of inactive state. It is also possible to prevent occurrences of problems such as lowering of the image density, degradation of transfer performances and the like due to the elevation of the quantity of charge on the developer as having been exposed to a low temperature humidity environment, after a continuous copying operation or the like. It is possible to effect appropriate toner density corrections and thus it possible to produce markedly stabilized images. In the above embodiment, upon the change of the toner density reference value, copying must be continued to a certain degree right after the toner density reference value is changed, in order for the toner density of the developer to reach the modified reference value. Accordingly, it is possible to prevent redundant modification of the toner density reference value by correcting the charger output in the round of the process control which would be activated by erroneous determination that the modification of the toner density has not been performed yet despite that actual change of the toner density reference value has been done. As a result, it is possible to effect more stabilized control of the toner density inside the developing unit. Further, in the embodiment, if the toner density reference value has been modified so as to increase the toner density inside the developing unit, the charger output is adapted to be corrected to lower. In contrast, if the toner density reference value has been modified so as to decrease the toner density inside the developing unit, the charger output is adapted to be corrected to increase. Nevertheless, after the modification of the toner density reference value, the charger output deviates beyond the predetermined maximum or minimum, it is judged that some trouble happens in the image forming apparatus and consequently the apparatus can be determined in an anomalous state.

Claims: An image forming apparatus comprising: a toner density sensor for detecting the toner density of a two-component developer stored in a developing vessel; toner density control means for comparing the output from said toner density sensor with a reference toner density and maintaining the toner density of the developer at the reference toner density by controlling the toner supply to the developing vessel; agitation detecting means for detecting the extent of agitation of the developer inside the developing vessel, or the approximate operation time of the image forming apparatus or the number of image forming operations since the apparatus was last inactive, either of which is proportional to said extent of agitation; toner density correcting means for correcting the output from said toner density sensor or the reference toner density in accordance with the detected extent of agitation; toner patch density detecting means for creating a toner patch on a photoreceptor and detecting the density of the toner patch; process parameter controlling means for controlling a process parameter including an applied voltage to a main charger in order to adjust the toner patch density detected by said toner patch density detecting means to a predetermined density value; and toner density correction cancelling means for cancelling the correction by said toner density correcting means when the process parameter has reached a predetermined value. An image forming apparatus according to claim 1 wherein said toner density correction cancelling means is adapted to gradually reduce the degree of correction applied by said toner density correcting means in accordance with the increase of said detected agitation extent. An image forming apparatus according to claim 1 further comprising means for activating said process parameter controlling means after said toner density correction cancelling means has cancelled the correction set up by said toner density correcting means. An image forming apparatus wherein the image density is controlled regularly by forming a patch of toner on a photoreceptor and varying a charger output as one of a set of image forming conditions based on the density of the patch and wherein control of toner supply to a developing unit is effected so that the toner density of two-component developer in the developing unit corresponds to a toner density reference value, characterized in that when the variation in the charger output is equal to or greater than a first predetermined value, the toner density reference value is changed, then the changed toner density reference value is maintained until the variation of the charger output again becomes equal to or greater than the first predetermined value. An image forming apparatus according to claim 4 wherein after a change of the toner density reference value, another change of the toner density reference value is prohibited for a predetermined period of time. An image forming apparatus according to claim 4 wherein after a change of the toner density reference value, if the variation of the charger output is equal to or greater than a second predetermined value, it is detected that the apparatus is in an anomalous state. An image forming apparatus in which an electrostatic latent image formed on an image carrier is developed using a two-component developer and in which the toner density of said developer is regulated in accordance with the output of a toner density sensor arranged to detect the toner density of said developer in a developer unit, wherein there is also provided means for applying an adjustment to the control operation performed using said toner density sensor output, in accordance with a detected parameter indicative of the extent of agitation of the developer, and means for reducing the adjustment when the value of a process parameter which is used to operate a device performing part of the image formation process and which is controlled so as to regulate toner patch density of a toner patch on the image carrier, satisfies a predetermined condition.

Inventors: ASANUMA MASATO, HATANAKA EISAKU, INO TOSHIAKI, ISHIDA TOSHIHISA, ITOYAMA MOTOYUKI, KITABATAKE YASUO, MASUDA JITSUO, NAOI HIROO, NARIMATSU MASAYASU, OKAMOTO KANSHIRO, SAIKO HIDEJI, SUMIDA KATSUAKI, TAKESUE YUICHIRO, TOKUYAMA MITSURU, ASANUMA, MASATO, HATANAKA, EISAKU, INO, TOSHIAKI, ISHIDA, TOSHIHISA, ITOYAMA, MOTOYUKI, KITABATAKE, YASUO, MASUDA, JITSUO, NAOI, HIROO, NARIMATSU, MASAYASU, OKAMOTO, KANSHIRO, SAIKO, HIDEJI, SUMIDA, KATSUAKI, TAKESUE, YUICHIRO, TOKUYAMA, MITSURU
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1996
Main classification: G03G15
IPC codes: G03G15