Image forming apparatus which corrects the image forming factors in response to density sensing means and duration of inactive state

An electrophotographic copier, facsimile machine, laser printer or similar image forming apparatus having a photoconductive element. The apparatus corrects a bias voltage for development, amount of charge, amount of exposure or similar image forming factor on the basis of an inactive state of the photoconductive element. When the duration of the inactive state is short and does not need a correction of the image forming factor, an image forming procedure is executed immediately. When the inactive state has lasted a long time, the image forming factor is corrected before the start of an image forming procedure.

BACKGROUND OF THE INVENTION 
The present invention relates to an image forming apparatus of the type 
using all electrophotographic procedure and, more particularly, to an 
image forming apparatus capable of producing a stable and clear-cut image 
at all times. 
A prerequisite for an electrophotographic copier, facsimile machine, laser 
printer or similar image forming apparatus is that it produces an image 
with stable density and clear-cutness over a long period of time. An 
electrophotographic copier, for example, has a photoconductive element 
which is implemented by OPC or selenium-based organic semiconductor. A 
photoconductive element using OPC in particular has a problem that a 
surface potential thereof, especially in a comparatively low potential 
area, is susceptible to a change in surface temperature and deterioration. 
Hence, an electrophotographic copier with an OPC photoconductive element 
causes the background of a reproduction to be blurred or an image to be 
lost after a long time of use, even though it may be provided with an 
expedient for controlling toner density. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an image 
forming apparatus which produces images with stable density and 
clear-cutness despite a long time of use by correcting an image forming 
factor on the basis of the duration of an inactive state of the apparatus. 
It is another object of the present invention to provide a generally 
improved image forming apparatus. 
An image forming apparatus having an image carrier for carrying a latent 
image of the present invention comprises a state sensor for sensing an 
inactive state of the image carrier, a density sensor for sensing a 
density of a predetermined density sensing portion of the image carrier, 
and a controller for correcting an image forming factor in response to an 
output signal of the state sensor and an output signal of the density 
sensor which are representative of a sensed inactive state and a sensed 
density, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
To better understand the present invention, how the surface potential of a 
photoconductive element installed in an image forming apparatus changes 
will be described. Let the image forming apparatus be an 
electrophotographic copier by way of example. 
As shown in FIG. 1, the surface potential, whether it be associated with a 
black portion or a white portion, differs by about 100 volts from the 
initial state to the aged state, constituting a cause of background 
contamination. This has customarily been coped with by maintenance which 
is performed either periodically or in response to a user's demand. 
On the other hand, with this kind of copier, it is a common practice to 
correct an image forming factor such as a bias voltage for development in 
association with the surface temperature of a photoconductive element, the 
number of copies produced and the duration of an inactive state of the 
machine which are measured on a minutes basis or an hours basis. FIGS. 2A 
and 2B show a conventional method of correcting a bias voltage for 
development in association with the surface temperature of a 
photoconductive element, the number of copies produced and the duration of 
an inactive state, as stated above. Specifically, when the surface 
temperature lies in the range of 10.degree. C. to 20.degree. C., the bias 
voltage is corrected on the basis of the duration of an inactive state in 
the manner shown in FIG. 2A. When the surface temperature lies in the 
range of 20.degree. C. to 40.degree. C., the bias voltage is corrected as 
shown in FIG. 2B. Further, when the surface temperature is lower than 
10.degree. C., the bias voltage is not corrected at all. Another method 
known in the art uses a surface temperature of 30.degree. C. for a 
reference and corrects a reference bias voltage for development by a ratio 
of 14 volts per 1.degree. C. when the temperature is 5.degree. C. to 
15.degree. C. and by a ratio of 6 volts per 1.degree. C. when the 
temperature is 15.degree. C. to 50.degree. C. Japanese Patent Laid-Open 
Publication No. 62-209569 proposes another implementation which uses a 
first and a second reference latent image which are formed in an area of a 
photoconductive element remote from an image forming area, the second 
reference latent image being lower in document density than the first 
reference latent image. Specifically, the supply of a toner is controlled 
in response to a sensed density of a toner image associated with the first 
latent image, while the bias voltage for development is corrected in 
response to a sensed density of a toner image associated with the second 
latent image. 
A problem with the prior art approach shown in FIGS. 2A and 2B is that it 
does not work sufficiently when an inactive state lasts some days, i.e., 
it copes only with suspensions of minutes and hours at most. The method 
using the ratios of 14 volts per 1.degree. C. and 6 volts per 1.degree. C. 
for the temperature ranges of 5.degree. C. to 15.degree. C. and 15.degree. 
C. to 50.degree. C. as stated previously is not satisfactory when it comes 
to such a long duration of an inactive state also, because the reference 
bias voltage for development itself will then become inadequate. In the 
procedure taught in Laid-Open Publication No. 62-209569, the first and 
second latent images are developed, the densities of the two images are 
sensed, and the reference bias voltage for development is corrected, every 
time a copying operation is performed. More specifically, even when the 
copier is restarted after a short time of suspension (e.g. 5 seconds) 
which does not need any correction in practice, the first and second 
latent images are formed and their densities are measured. This results in 
wasteful consumption of parts, toner, and power. While the reference bias 
may be corrected by using a surface potential sensor which constantly 
senses the surface potential of a photoconductive element, this shceme 
brings about another problem in the aspect of production cost. 
Referring to FIG. 3, an image forming apparatus embodying the present 
invention is shown in a schematic block diagram. There are shown in FIG. 3 
an optics controller 1, a magnification changing motor 2, a position 
sensor 3, a main controller 4, a density sensor 5, a charging unit 6, 
various sensors 8, various loads 9, a drum thermistor 10, a fixation 
thermistor 11, an operation board 12, an AC driving unit 13, a display 14, 
keys 15, a heater 16, a main motor 17, and a lamp 18. 
FIG. 4 shows the construction of an essential part of the illustrative 
embodiment. As shown, the construction includes a photoconductive element 
in the form of a drum 20, a charger 21, an eraser 22, a developing unit 
23, the density sensor 5, a developing bias 25, a glass platen 26, a scale 
27, a white reference pattern 28, a lamp 29, mirrors 30a to 30d, and a 
lens 31. 
FIG. 5 is a front view of the white reference pattern 28 of FIG. 4 together 
with its associated parts and elements. The white reference pattern 28 is 
shown as extending over about 10 millimeters from the leading edge LE of 
the glass platen 26. 
The density sensor 5 is made up of a light emitting element and a 
light-sensitive element which are implemented by an LED (Light Emitting 
Diode) and a phototransistor, respectively. Light issuing from the LED and 
then reflected by the drum 20 is incident to the light-sensitive element 
and photoelectrically converted. The control over toner density, toner-end 
detection and the like are effected in response to the result of 
photoelectric conversion. As shown in FIG. 6, the density sensor 5 reads 
not only a developed pattern portion A.sub.1 -A.sub.2 but also the 
portions which precede and succeed it. Hence, both an output associated 
with the background of the drum 20 and an output associated with the 
pattern portion are read. Assume that the background output is VSG and the 
pattern output is VSP, as shown in FIG. 6. The toner density is determined 
on the basis of the ratio of VSG and VSP, i.e., the toner density is 
determined to be low (black toner little reflects light) when the output 
VSP is high. Then, toner supply control and toner-end detection are 
executed on the basis of VSG and VSP. In the illustrative embodiment, the 
pattern portion for correcting the reference bias voltage is implemented 
as a developed pattern of the white reference pattern 28. The reference 
bias voltage is corrected in response to outputs of the density sensor 5 
which are individually associated with the developed pattern and the 
background of the drum 20. 
The measurement of the background density of the drum 20 by the density 
sensor 5 is quite susceptible to the eccentricity of the drum 20 and the 
reflectance of the drum surface. In light of this, the illustrative 
embodiment divides the circumference of the drum 20 into equal parts, 
reads the densities of the individual parts, and then produces an output 
representative of a mean density. Specifically, as shown in FIG. 7, the 
circumference of the drum 20 is divided into six segments each 60 degrees, 
of each segment is read six consecutive times in synchronism with drum 
pulses, mean values of the individual segments are produced, and then an 
average of those average values is produced. Numerical values shown in 
FIG. 7 are representative of actually measured outputs. For the 
measurement, the output VSG associated with the background of the drum 20 
was selected to be 4.0 volts, and the read start timing was random. By so 
reading outputs resulting from one full rotation of the drum 20, it is 
possible to reduce the influence of eccentricity. When VSG is read while 
the drum 20 is in rotation, an output representative of background 
contamination will be corrected in matching relation to VSG (ratio to 
VSG), enhancing accurate detection. 
Since the density condition of a drum differs from one machine to another, 
a density sensor senses the developed white reference pattern density and 
the drum background density on a machine basis and, based on the sensed 
densities, a reference voltage for a developing unit is corrected machine 
by machine. Specifically, the pattern portion provided on the scale 27, 
FIG. 5, is different in optical path length from the document surface. 
This, coupled with the fact that the position of a filament of a lamp 
and/or the position of a reflector differs from one machine to another, 
causes the measured density to vary over a substantial range. 
Referring to FIGS. 8A to 8C, there is shown a relationship between the 
density of the white reference pattern and the amount of light with 
respect to various positions of a filament and by using the potential VL 
of the white reference pattern image as a parameter. Experiments showed 
that the density of the white reference pattern image changes from OD 0.1 
to a range of OD 0.18 to OD 0.35 (in terms of document surface). 
Specifically, the amount of exposure as measured on the drum surface 
changes by a substantial amount even if the combination of an amount of 
exposure (lamp voltage) and a reference pattern is maintained the same. 
For this reason, the reference value (initial reference data) has to be 
set machine by machine. The difference in the amount of exposure between 
machines can be coped with by correcting the reference bias voltage 
machine by machine. 
In the illustrative embodiment, the drum thermistor 10 and fixation 
thermistor 11, FIG. 3, play the role of means for sensing an inactive 
state of the copier. The outputs of the thermistors 11 and 10 are read 
through an AN port when the power switch of the copier is turned on. 
Generally, when the power switch is turned off after a fixing roller has 
been warmed up, the temperature of the fixing roller sequentially lowers 
toward room temperature. Such a temperature drop occurs along a curve 
which is expressed as: 
##EQU1## 
where A and B are the constants particular to a machine, and t is the 
time. 
Assuming that the temperature sensed by the fixation thermistor 11 is TF 
and the temperature sensed by the drum thermistor 10 is TD, their 
difference .DELTA.T is produced by: 
EQU .DELTA.T=TF-TD (2) 
In the equation (2), TD substantially equals room temperature if the 
suspension time is long. Therefore, the duration of an inactive state of 
the whole copier, i.e., the suspension time of the drum can be estimated 
from the level of at least one of .DELTA.T (assuming that TD is nearly 
equal to room temperature) and TF. Since the sensitivity of the drum 
changes with the suspension time as previously discussed, determining a 
suspension time in terms of .DELTA.T or TF and correcting the reference 
bias voltage on the basis of the determined suspension time is successful 
in promoting efficient and accurate correction. For more accurate 
detection of suspension time, use may be made of a backed up timer in 
place of .DELTA.T or TF. 
As stated above, in the illustrative embodiment, a suspension time of the 
copier is determined before the start of background contamination 
detection so that, when .DELTA.T and/or TF does not satisfy the above 
condition, background contamination detection may not be performed (a 
detection start flag is not set (to logical ONE)). 
In operation, a reference value for background contamination detection is 
set. Specifically, after the image adjustment of the copier, the 
background of the drum 20 and the white reference pattern 28 are exposed 
and developed by any suitable developing bias, while the density sensor 5 
senses their densities. The resulting two outputs of the density sensor 5 
are loaded in a non-volatile memory as reference outputs. To hold the 
conditions of that instant, an optical path length (magnification), lamp 
voltage, bias output and other various conditions are also stored in the 
form of data. Specifically, the non-volatile memory is loaded with at 
least a density output VSGS associated with the drum background, a density 
output VSST associated with the white reference pattern 28, and a 
reference bias voltage VBS for developing latent images representative of 
the drum background and white reference pattern 28. In the illustrative 
embodiment, the reference value is set under the following conditions: 
EQU VSGS.gtoreq.3.6 volts (3) 
EQU VSGS.gtoreq.VSST.gtoreq.3.0 volts (4) 
Thereupon, an inactive state of the copier is detected, as stated 
previously. If the suspension condition is satisfied, outputs VSGC and 
VSCK of the density sensor 5 associated with the drum background and the 
developed white reference pattern, respectively, are read and, at the same 
time, a bias voltage VBS+.DELTA.VB is applied to develop the drum 
background and reference white pattern 28. In the illustrative embodiment, 
for the background density output of 4.0 volts, the density outputs VSST 
and VSCK and the potential VL of the white reference pattern image have a 
relationship: 
##EQU2## 
In this embodiment, the bias voltage for development is provided in 30 volt 
steps, .DELTA.L is nearly equal to 30 volts and, therefore, the correction 
value .DELTA.VB is produced by: 
##EQU3## 
Why the bias voltage is selected to be VBS+.DELTA.VB at the time of 
comparison value reading is as follows. When the bias voltage VBS is 
maintained constant, the range which follows .DELTA.VL is (1.0-3.7) 
volts.times.(-57).apprxeq.154 volts which which is not more than 2.5 
notches. In contrast, the bias volage VBS+.DELTA.VB allows the density 
output associated with the background to lie in the range of 1.0 volt to 
3.7 volts at all times, satisfying the equation (5) without fail. 
In this manner, the correction value .DELTA.VE is sequentially added up at 
each time of background contamination detection. Specifically, VSCK is 
corrected by VSGS and VSGC to produce: 
##EQU4## 
By Subsituting it for the equation (5), there is obtained: 
##EQU5## 
In this embodiment, since the output step of the bias voltage is 30 volts, 
subsituting .DELTA.VL=30 volts for the equation (7) produces: 
EQU .vertline.VSST-VSCK'.vertline.=0.526 (8) 
Since the difference in pattern density is produced by 
EQU 0.526.div.(5.0 volts/255 bits).apprxeq.27 bits, 
the equation (6) for correcting (VSST-VSCK') by (.DELTA.VE=) 30 volts every 
twenty-seven bits is obtained in the control aspect. 
As stated above, in the illustrative embodiment, the bias voltage VBS is 
changed until the relationship 3.0.ltoreq.VSST.ltoreq.VSGS holds. However, 
when a BSG error flag or a comparison value error flag is set, the 
correction data is not added during ordinary copying operation. 
Referring to FIG. 9, a specific operation of the illustrative embodiment 
will be described. As shown, the turn-on of the main motor, charging, 
image transfer, paper separation, PCC, PQC/BR, QL, PTL, turn-on of the 
lamp, scanning, erasure, application of the bias voltage for development, 
and light emission and detection by the density sensor are sequentially 
performed in the individual clock pulse ranges. Also performed are the 
detection of VSGS and VSGC, detection of VSST and VSCK, and correction of 
the bias voltage for satisfying the condition 3.0 
volts.ltoreq.VSST.ltoreq.VSGS. 
The background contamination is ascribable to the elevation of the surface 
temperature of the drum (drum potential) and the decrease in the amount of 
exposure which is caused by the contamination of the optics. The decrease 
in the amount of exposure invites an elevation of the potential VL of the 
white reference pattern image. 
FIG. 10 indicates a relationship between the drum potential VO and the 
potential of the white reference pattern image. In the graph, numerical 
values represented by rectangles indicate scattering. 
FIG. 11 shows a relationship between the amount of exposure and the 
potential VL of the white reference pattern image. When this relationship 
was determined, the drum potential VO, thermistor temperature and drum 
temperature were 760 volts, 32.degree. C. to 33.degree. C., and 25.degree. 
C. to 27.degree. C., respectively. 
FIG. 12 shows a relationship between the potential VL of the white 
reference pattern image and the output of the density sensor by using the 
drum potential VO as a parameter. 
FIG. 13 shows a relationship between the potential VL of the white 
reference pattern image and the output of the density sensor with respect 
to various densities. 
The general procedure for correcting the bias voltage particular to the 
illustrative embodiment will be described. 
Referring to FIG. 14, a main routine begins with a step S1 for determining 
whether or not the suspension condition of the copier is satisfied. If the 
answer of the step S1 is YES, a step S2 is executed; if otherwise, a 
copying operation is performed in an ordinary copy mode. The subroutine 
represented by the step S2 is executed as shown in FIG. 15 specifically. 
In FIG. 15, if a bias up-down flag is not set as determined in a step S17, 
whether or not a detection start flag is set is determined in a step S18. 
If the answer of the step S18 is YES, the program advances to a step S19; 
if otherwise, the program enters into an ordinary copy wait routine. If a 
read flag is set as determined in the step S19, the detection start flag 
is set in a step S20 and a contamination check end flag is set in a step 
S21. If the answer of the step S19 is NO, the program directly advances to 
a step S21. If the answer of the step S17 is YES, the program directly 
advances to the step S20. In a step S22, an under contamination check flag 
is set. In the following step S23, a correction end flag is reset. Then, 
in a step S24, whether or not a reference set flag is set is determined. 
If the answer of the step S24 is YES, a reference read flag is reset in a 
step S25 and, in a step S26, a comparison value read flag is reset. This 
is followed by a step S27 in which a first scanner is moved by 10 
millimeters to a standby position. In the next step S28, various counters 
assigned to background contamination checking are cleared. 
A step S3 shown in FIG. 14 is shown in FIG. 16 specifically. In a step S29, 
a power relay is turned on and, in a step S30, a solenoid associated with 
a blade is energized. In a step S31, key inputs on the operation board are 
inhibited. This is followed by a step S32 for starting a 200 milliseconds 
timer assigned to the blade. 
FIG. 17 shows a step S4 of FIG. 14 specifically. In a step S33, whether or 
not the above-mentioned 200 milliseconds timer is incrementing is 
determined. If the answer of the step S33 is YES, a step 34 is executed 
for turning on the main motor, and then a step S35 is executed for turning 
on the eraser. When a VSG read flag is set as decided in a step S36, a 
step S37 is executed for turning on PQC. In this manner, by the step S4 
shown in FIG. 14, the scanner is moved to the position where the white 
reference pattern is located. 
A step S5 of FIG. 14 is shown in FIG. 18 specifically. When the VSG read 
flag is set as decided in a step S38, a step S39 is executed to see if a 
drum clock pulse counter has exceeded "20". If the answer of the step S39 
is YES, the program advances to a step S40. In the step S40, whether or 
not the drum clock pulse counter has exceeded "421" is determined. If the 
answer of the step S40 is YES, the program advances to a step S41 for 
activating the image transferring section, paper separating section, and 
PCC. This is followed by a step S42 for turning on the lamp. If the answer 
of the step S40 is NO, a step S43 is executed to see if the drum clock 
pulse counter has reached "570". If the answer of the step S43 is YES, a 
step S44 is executed for turning off PCC; if otherwise, a step S45 is 
executed for deactivating the transferring and paper separating sections. 
FIG. 19 indicates a step S16 of FIG. 14 specifically. As shown, when the 
VSG read is set as decided in a step S46, a step S47 is executed to see if 
the drum clock pulse counter has reached "100". If the answer of the step 
S47 is YES, whether or not the drum clock pulse counter has reached "380" 
is determined by a step S48. If the answer of the step S48 is YES, a step 
S49 is executed to turn off the charging section; if otherwise, a step S50 
is executed to turn it on. 
FIG. 20 shows a step S7 of FIG. 14 specifically. As shown, in a step S51, 
whether or not the VSG read flag is set is determined. If the answer of 
the step S51 is YES, a step S52 is executed to see if the drum clock pulse 
counter has exceeded "140". If the answer of the step S52 is YES, a step 
S53 is executed to see if the drum clock pulse counter has exceeded "380". 
If the answer of the step S53 is YES, full-face erase processing is 
executed in a step S54; if otherwise, a step S55 is executed for executing 
erase processing except for the density sensor reading section. When the 
answer of the steps S51 and S52 are NO and when the answer of the step S53 
is YES, the program advances to the step S54. 
FIG. 21 shows a step S8 of FIG. 14 specifically. In FIG. 21, whether or not 
the drum clock pulse counter has exceeded "200" is determined in a step 
S56. If the answer of the step S56 is YES, a step S57 is executed to turn 
on the LED of the density sensor. 
FIG. 22 shows a step S9 of FIG. 14 specifically. When the drum clock pulse 
counter has increased beyond a predetermined value as decided in a step 
S58, a step S59 is executed to see if the drum clock pulse counter has not 
exceeded another predetermined value. If the answer of the step S59 is 
YES, a step S60 is executed to save lower three bits of byte data. When 
the lower three bits are associated with a predetermined read processing 
operation as decided in a step S61, a step or subroutine S62 is executed. 
Then, the program advances to a step S63 to see if an output read end flag 
has been set. If the answer of the step S63 is YES, a step S64 is executed 
to memorize mean data in a particular address which differs from one read 
processing to another. In a step S65, the output read end flag is reset, 
and in a step S66 the lower three bits are changed for the next read 
processing. FIG. 23 shows details of steps S58, S59, S61, S64 and S66 of 
FIG. 22 each being associated with a particular angular position. 
FIG. 24 shows a step S10 of FIG. 14 specifically. As shown, when the bias 
up-down flag is set as decided in a step S67, a step S68 is executed to 
see if a bias up flag is set. If the answer of the step S68 is YES, a 
correction for increasing the bias voltage is effected in a step S69, 
followed by a step S70. If the answer of the step S68 is NO, the program 
directly advances to the step S70. In the step S70, whether or not a bias 
down flag is set is determined. If the answer of the step S70 is YES, a 
step S71 is executed to effect a correction for reducing the bias voltage, 
followed by a step S72. If the answer of the step S70 is NO, the program 
directly advances to the step S72. In the step S72, the correction level 
is memorized. In a step S73, a bias change end flag is set, in a step S74 
a correction associated with a drum suspension time is executed. This 
correction associated with a drum suspension time is equivalent to a 
correction which is performed during ordinary copying operation, i.e., a 
degree of recovery of the drum from fatigue is estimated beforehand so 
that a correction based on the estimated recovery is effected by a bias or 
similar output. 
FIG. 25 shows a step S11 of FIG. 14 specifically. In a step S75, whether or 
not the total of correction data is smaller than "7" is determined. If the 
answer of the step S75 is NO, a step S76 is executed to set the total of 
correction data to "7". This is followed by a step S77. If the answer of 
the step S75 is YES, the program directly advances to the step S77. In the 
step S77, the correction data is added to the correction level. Then, a 
step S78 is executed to execute a correction associated with the drum 
suspension time. 
FIG. 26 shows a step S12 of FIG. 14 specifically. Whether or not the drum 
clock pulse counter has reaches "650" is determined in a step S79. If the 
answer of the step S79 is YES, a step S80 is executed to move the first 
scanner to the home position. This is followed by a step S81 for turning 
off various outputs around the drum. Then, a step S82 is executed to turn 
off the main motor, followed by a step S83 for resetting the under 
contamination check flag. 
FIG. 27 shows a step S13 of FIG. 14 specifically. Whether or not the under 
contamination check flag is set is determined in a step S84. If the answer 
is NO, LOOP 1 shown in FIG. 14 is executed; if it is YES, a step S85 or 
PWO80 processing (FIG. 28) is executed. The step S85 is followed by a step 
S86 or PWO90 processing (FIG. 30). Thereafter, if the VSG read flag is set 
as decided in a step S87, a step S89 is executed to see if a bias 
correction upper limit flag is set. If the answer of the step S89 is NO, a 
bias correct counter is incremented in a step S90. This is followed by a 
step S91 for determining whether or not the bias correct counter is equal 
to "15". If the answer is YES, a step S92 is performed to set the bias 
correction upper limit flag. 
FIG. 28 shows the step S85 of FIG. 27 specifically. As shown, whether or 
not the under contamination check flag is set is determined in a step S93. 
If the answer is NO, a step S94 is performed to clear a data totalize 
counter. This is followed by a step S95 for loading the data totalize 
counter with the sum of six output data associated with the individual 
angular positions of the density sensor. Then, in a step S96, the value 
stored in the data totalize counter is divided by 6 to produce a mean 
value. In a step S97, whether or not the VSG read flag is set is 
determined. If the answer of the step S97 is NO, a step S98 is executed to 
see if the reference set flag is set. If the answer of the step S98 is 
YES, a step S99 is executed to memorize the mean value as the reference 
output VSGS; if otherwise, a step S100 is executed to memorize the mean 
value as the comparison output VSGC. If the answer of the step S97 is YES, 
whether or not the reference set flag is set is determined in a step S101. 
If the answer of the step S101 is YES, a step S102 is executed to memorize 
the mean value as a reference white pattern image output. Next, a step 
S103 (FIG. 29) and a step S104 are sequentially executed to see if the 
bias up-down flag is set. If the answer is NO, a step S105 is executed to 
reset the reference set flag. In the following step S106, a reference read 
flag is set. If the answer of the step S104 is YES, the operation is 
transferred to a step S106. If the answer of the step S101 is NO, a step 
S107 is executed to memorize the mean value as the comparison output VSCK 
of the white reference pattern, followed by a step S108 for setting a 
comparison value set flag. 
FIG. 29 shows the step S103 of FIG. 28 specifically. As shown, a VSG error 
flag is reset in a step S107, and whether or not VSGS has exceeded 3.6 
volts is determined in a step S108. If the answer of the step S108 is YES, 
a step S109 is executed to see if whether VSST has exceeded 3.0 volts. If 
the answer of the step S109 is YES, whether or not VSST is greater than 
VSGS is determined in a step S110. If the answer of the step S110 is YES, 
a step S111 is executed to set the bias down flag. This is followed by a 
step S112 for resetting the bias change end flag. If the answer of the 
step S110 is NO, the bias up flag is reset in a step S113 while the bias 
flag is reset in a step S114. If the answer of the step S109 is NO, the 
bias up flag is set in a step S115 while the bias change end flag is reset 
in a step S116. If the answer of the step S108 is NO, the program advances 
to a step S117 for setting the VSG error flag. 
FIG. 30 indicates the step S86 of FIG. 27 specifically. In a step S118, 
whether or not the correction end flag is set is determined. If the answer 
is NO, whether or not a correction value read flag is set is determined in 
a step S119. If the answer of the step S119 is YES, the program executes a 
step S120 for performing the following calculation: 
##EQU6## 
The step S120 is followed by a step S121 to see if VSGC has exceeded 25 
volts. If the answer of the step S121 is YES, whether or not VSCK' has 
exceeded 5.0 volts is determined in a step S122. If the answer of the step 
S122 is NO, PWO91 processing (FIG. 31) is executed in a step S123, 
followed by a step S124 for setting the correction end flag. If the answer 
of the step S121 is NO and the answer of the step S122 is YES, a step S125 
is executed to set a comparison value error flag. 
FIG. 31 shows the step S123 of FIG. 30 specifically. In a step S126, 
whether (VSST-VSCK') is greater than zero is determined. If the answer is 
YES, whether (VSST-VSCK') is greater than 0.52 volt is determined in a 
step S127. If the answer of the step S127 is YES, whether (VSST-VSCK') is 
greater than 1.06 volts is determined in a step S128. If the answer of the 
step S128 is YES, whether or not (VSST-VSCK') is greater than 1.59 volts 
is determined in a step S129. If the answer of the step S129 is YES, a 
step S130 is executed to see if (VSST-VSCK') is greater than 2.0 volts. If 
the answer of the step S130 is YES, the fourth correction data is produced 
in a step S131. If the answers of the steps S126 and 127 are NO, the 
zeroth correction data is produced in a step S132. If the answer of the 
step S128 is NO, the first correction data is produced in a step S133. If 
the answer of the step S129 is NO, the second correction data is produced 
in a step S134. Further, if the answer of the step S130 is NO, the third 
correction data is produced in a step S135. In a step S136, the correction 
data produced by the above procedure is added to a bias voltage for an 
ordinary copying operation. 
FIG. 32 demonstrates output data read processing. When a density sensor 
read start flag is set as decided in a step S137, a step S138 is executed 
to see if a density sensor read flag is set. If the answer of the step 
S138 is YES, density sensor output is added to a data add buffer in a step 
S139. In a step S140, the density sensor read flag is reset. In a step 
S141, a data add counter is incremented. When the data add counter has 
exceeded "8" as determined in a step S142, a step S143 is executed to set 
a density sensor read end flag. In a step S144, the density sensor read 
start flag is reset, followed by a step S145. In the step S145, a value 
produced by dividing the data add buffer by the data add counter is 
determined to be the mean data. In a step S146, the data add buffer and 
data add counter are cleared. If the answer of the step S137 is NO, a step 
S147 is executed to set the density sensor read flag, followed by the step 
S146. 
As stated above, in the illustrative embodiment, the bias voltage for 
development is corrected on the basis of a density output VSGC associated 
with the background of a drum and representative of the fatigue of the 
drum, a density output VSCK associated with a white reference pattern 
image, and their reference values. At the same time, the toner supply is 
controlled on the basis of the density output VSGC, as stated with 
reference to FIG. 6. 
The embodiment shown and described achieves various unprecedented 
advantages, as follows. Since state detecting means determines whether or 
not a bias voltage correction is necessary and allows it to be executed 
only if it is necessary, wasteful power consumption and decrease in the 
life of parts are eliminated. The correction is extremely accurate because 
it is based on a density output associated with the background of a drum 
and accurately representative of the fatigue of the drum and a density 
output associated with a white reference pattern image. 
While the illustrative embodiment has concentrated on a bias voltage for 
development, the present invention is practicable with any other kind of 
image forming factor to be corrected, e.g. an amount of charge or an 
amount of exposure. 
In summary, it will be seen that the present invention provides an image 
forming apparatus which is efficiently operable because an image forming 
factor is corrected only when necessary, as determined by means which is 
responsive to an inactive state of the apparatus. The correction is 
accurate because it is performed on the basis of an output of density 
sensing means which is responsive to the density of a reference density 
sensing portion of a photoconductive element which is an exact 
representation of the fatigue of the apparatus. 
Various modifications will become possible for those skilled in the art 
after receiving the teachings of the present disclosure without departing 
from the scope thereof.