Device for adjusting output of image density sensor incorporated in image forming equipment

A device incorporated in image forming equipment for adjusting the output of an image density sensor which optically senses the density of a toner image formed on a photoconductive element and representative of a reference pattern. The density sensor is made up of a light emitting element and a light-sensitive element. Thresholds divide an output range wherein the output characteristic of the sensor does not change into a plurality of subranges having the optimal output associated with the background of the photoconductive element as the center value. The sensor senses the background of the photoconductive element at predetermined intervals. Whether or not to adjust the output of the sensor is determined on the combination of the subrange where the resulting output of the sensor lies and the number of times that the former lies in the latter. The sensor output, i.e., a PWM (Pulse Width Modulation) duty to be fed to the light emitting element is changed to control the adjusted output associated with the background to the optimal value.

BACKGROUND OF THE INVENTION 
The present invention relates to a copier, facsimile apparatus, printer or 
similar image forming equipment and, more particularly, to a device 
incorporated in such equipment for adjusting the output of an image 
density sensor which optically senses the density of a toner image formed 
on a photoconductive element and representative of a reference pattern. 
A prerequisite with image forming equipment of the type forming a toner 
image on a photoconductive element is that the image density and contrast 
be controlled to desirable levels in order to insure high image quality. 
The image density and contrast are effected by the toner concentration of 
a developer, bias voltage for development, lamp voltage for exposure, etc. 
To meet the above requirement, one conventional image forming apparatus 
forms a toner image representative of a reference pattern on a 
photoconductive element, determines the density of the toner image by use 
of an image density sensor, and controls, for example, the toner 
concentration or the bias voltage, as needed. However, the problem is that 
the output of the sensor is susceptible to the surface configuration and 
eccentricity of the photoconductive element (distance to the sensor), 
toner particles depositing on and smearing the light-sensitive surface of 
the sensor, a voltage saturation range particular to the conversion of the 
output current of the light emitting element (e.g. phototransistor) to a 
voltage, temperature drift, etc. It is likely, therefore, that the output 
of the sensor is deviated from a desirable range of output characteristic, 
resulting in erroneous control. In light of this, it has been customary to 
sense the background of the photoconductive element together with the 
toner image of interest, compare the difference between the resulting two 
outputs and the difference between their reference outputs, control the 
toner concentration of a developer on the basis of the result of 
comparison, and thereby adjust the output of the sensor against the smears 
caused on the light-sensitive surface of the sensor by the toner. This 
kind of implementation is disclosed in, for example, Japanese Patent 
Laid-Open Publication No. 53869/1984. 
However, the above-stated scheme determines whether or not to adjust the 
sensor output by using a single threshold, i.e., the difference between 
reference outputs. This brings about a problem that even an output little 
different from the threshold is immediately corrected, and the correction 
is effected due to the influence of the eccentricity of the 
photoconductive element or noise. To eliminate this problem, Japanese 
Patent Application No. 134674/1989 teaches a procedure consisting of 
repetitively detecting the background of the photoconductive element a 
predetermined number of times while determining whether or not the 
resulting output lies in a predetermined range each time, and adjusting 
the sensor output when it is not found in the predetermined range. Since 
this procedure repeats the detection a predetermined number of times 
without exception, even a sensor output which is far different from the 
target output and almost saturated is apt to continuously hold until the 
detection has been repeated the fixed number of times, obstructing 
accurate control over the toner concentration and other subjects. In 
addition, assume equipment of the kind displaying an error when the 
adjusted output does not fall in the predetermined range, and a sensor 
extremely susceptible to, for example, temperature drift. Then, since the 
sensor output noticeably varies with the temperature inside the equipment, 
the equipment is apt to display an error frequently. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a sensor 
output adjusting device for image forming equipment which is capable of 
adjusting the output of an image density sensor with accuracy despite a 
change in the distance between a photoconductive element and the sensor 
and a change in ambient temperature. 
In accordance with the present invention, a device for adjusting the output 
of an image density sensor responsive to the density of a toner image 
formed on a photoconductive element and representative of a reference 
pattern comprises a counting circuit for counting the number of times that 
the output of the image density sensor exceeds a plurality of thresholds, 
and a deciding circuit for determining whether or not the output of the 
image density sensor should be adjusted on the basis of the combination of 
the thresholds and the number of times. 
Also, in accordance with the present invention, an image forming apparatus 
comprises an image density sensor for sensing the density of a toner image 
formed on a photoconductive element and representative of a reference 
pattern, a comparing circuit for comparing the output of the image density 
sensor with more than three thresholds, a counting circuit responsive to 
the output of the comparing circuit for counting the number of times that 
the output of the image density sensor exceeds the threshold values on a 
threshold value basis, while weighting the number of times on a threshold 
value basis, and an adjusting circuit for adjusting the output of the 
image density sensor in response to a count outputted by the counting 
circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1 of the drawings, image forming equipment to which a 
device embodying the present invention is applied is shown and implemented 
as electrophotographic equipment by way of example. As shown, the 
equipment has a photoconductive drum 1 and an image density sensor 2 which 
faces the drum 1. The image density sensor 2 is made up of an LED (Light 
Emitting Diode) 3 and a phototransistor 4 which serve as a light emitting 
element and a light-sensitive element, respectively. Conventional units 
for effecting an electrophotographic process are arranged around the drum 
1, although not shown in the FIG. The LED 3 has an anode thereof connected 
to a power source line Vcc (+5 V) and has a cathode connected to an LED 
turn-on circuit 5 via a driver 6. The LED turn-on circuit 5 may be 
constituted by a timer IC (Integrated Circuit), for example. A low active 
PWM (Pulse Width Modulation) waveform having a resolution of several ten 
kHz and eight bits is applied to the LED 3. The emitter of the 
phototransistor 4 is connected to ground via a load resistor 7 and 
connected to the input terminal of an analog-to-digital (A/D) converter 
built in a main control board via an analog port 8 of the main control 
board. In this configuration, the voltage across the load resistor 7 is 
converted to a digital signal and fed out as the output of the image 
density sensor. 
Toner density control using the output of the image density sensor 2 will 
be outlined first. A reference pattern having a reference density is 
provided on a glass platen outside of an area where a document is to be 
laid. As the reference pattern is illuminated, the resulting reflection 
electrostatically forms a corresponding latent image on the drum 1. A 
developing unit, not shown, develops the latent image to produce a toner 
image representative of the reference pattern. While the density of such a 
toner image is repetitively sensed, a change in the toner concentration of 
a developer stored in the developing unit is determined in terms of a 
change in the density of the toner image. Control over the toner 
concentration is effected on the basis of the change in the toner 
concentration. Specifically, the image density sensor 2 senses a light 
reflected from the toner image, i.e., reference pattern toner image formed 
on the drum 1 and light reflected from the background of the drum 1 where 
the toner image is not formed, producing an output Vsg associated with the 
background and an output Vsp associated with the toner image. Whether or 
not to supply a toner is determined on the basis of a ratio Vsp/Vsg. Such 
an operation for sensing the image density and determining whether or not 
to supply a toner is performed when the first copy is to be produced after 
the turn-on of the power source and every time ten copies are produced. 
Based on the result of this decision, the operation for supplying a toner 
is effected up to the instant when the reference pattern toner image 
should be detected again. The position of the drum 1 where the reference 
pattern toner image is to be formed is determined randomly by the timing 
for forming the reference pattern toner image. The turn-on of the LED 3 
for detection and the reading of sensor output by the main control board 
begin at such a timing that the sensing operation begins at a background 
portion preceding the reference pattern toner image with respect to the 
direction of rotation of the drum 1. On the start of the sensing 
operation, the resulting data are sequentially stored in a shift register 
at the intervals of, for example, 4.75 msec. At the instant when an output 
whose level clearly distinguishes the background and the toner image (e.g. 
2.5 V) has appeared, a predetermined number of data stored in the shift 
register and corresponding to the outputs preceding and following the 
above-mentioned output (some data adjoining the boundary are neglected) 
are averaged to produce data Vsg and Vsp. Assume that the density of the 
reference density toner image has lowered to an unusual degree due to an 
error in the developing unit or similar process unit, preventing the 
above-mentioned output of the distinguishing level from appearing. Then, 
since neither the data Vsg nor the data Vsp exists, "0" is stored in the 
register. In this case, a toner is supplied in a constant amount up to the 
time for sensing the reference pattern toner image again. 
As stated above, since the embodiment uses a ratio of the output associated 
with the reference pattern toner image and output associated with the 
background, the influence of, for example, the eccentricity of the drum 1 
on the two outputs is cancelled so long as they are adequate. This is 
successful in controlling the toner concentration adequately. However, as 
shown in FIG. 2, the output of the sensor 2 has a saturation range (4.7 V 
in the figure) wherein it does not change despite an increase in the 
amount of incident light. While the abscissa of FIG. 2 indicates PWM 
duties which determine the quantity of light from the LED 3, let the PWM 
duties be regarded as the quantities of light sensed by the 
light-sensitive element. The saturation range prevents the density of the 
reference pattern toner image from being accurately detected and, 
therefore, obstructs adequate toner concentration control if the density 
is sensed at the moment when the distance between the drum 1 and the 
sensor 2 is reduced due to, for example, the eccentricity of the drum 1, 
i.e., if the output associated with the background which is relatively 
high coincides with the saturation range. Further, when the toner smears 
the light-sensitive surface of the sensor 2 to lower the sensor output, 
the resolution of A/D conversion is lowered to in turn reduce the accuracy 
of sensing operation although errors ascribable to the saturation range do 
not occur. In FIG. 2, the range .alpha. is representative of scattering 
among products with respect to the time when the output begins to 
saturate. While such scattering causes each product to have a different 
inclination associated with Vsg and PWM, each product has a certain 
constant inclination. 
In light of the above, the output of the sensor 2 is adjusted such that the 
output Vsg associated with the background which is relatively high does 
not coincide with the saturation range and lies in an advantageous range 
from the resolution standpoint (referred to as a range wherein the sensor 
characteristic does not change hereinafter). 
The embodiment selects the range wherein the sensor characteristic does not 
change by considering the optimal value of Vsg. The optimal value of the 
output Vsg of the sensor 2 at Vcc+5 V is selected to be 4.0 V, for the 
following reasons. First, the output should preferably be as great as 
possible within an allowable range to enhance the resolution of A/D 
conversion. Second, the output fluctuates by about 0.3 V due to the 
eccentricity of the drum 1 and other similar causes. Third, the output of 
the sensor 2 begins to saturate at about 4.5 V and then fully saturates 
(about 4.7 V), as shown in FIG. 2. Fourth, the sensor 2 has a temperature 
drift of about 0.1. Therefore, the optimal value is determined to be 4.1 V 
which remains when the sum of the fluctuations ascribable to the 
eccentricity of the drum 1 and temperature drift (tending to increase the 
output of the sensor 2) is subtracted from 4.5 V at which the sensor 
output begins to saturate, preferably 4.0 V with some margin. The output 
of 4.5 V is selected as the upper limit of the range wherein the sensor 
characteristic does not change. The lower limit of the range of interest 
is determined to be 3.5 V by taking account of the eccentricity of the 
drum 1, temperature drift, and the fluctuation ascribable to the 
contamination of the sensor 2 (tending to reduce the output), and so as 
not to reduce the resolution as far as possible. 
Further, the embodiment subdivides the range wherein the sensor 
characteristic does not change, so that the output Vsg may lie in such a 
range without fail and the adjustment may occur slowly to eliminate 
overcorrection. Specifically, as shown in FIG. 3, the range above 3.5 V 
and below 4.5 V wherein the sensor characteristic does not change is 
subdivided. FIG. 3 indicates the outputs Vsg associated with the 
background of the drum 1 on the ordinate thereof and shows threshold 
levels for the decision on whether or not to effect adjustment. As shown, 
the range of interest is subdivided into subranges D, C, B, A, B', C' and 
D' by six thresholds, i.e., three thresholds above 4.0 V which is optimal 
and three thresholds below 4.0. The subrange A (above 3.85 V and below 
4.15 V) delimited by the thresholds 2.15 V and 3.85 V each being 0.15 V 
distant from 4.0 V is set up in consideration of the fluctuation of 0.3 V 
ascriable to the eccentricity of the drum 1 and which is most probable. 
When the output Vsg lies in the subrange A, no adjustment is effected. The 
subrange B delimited by the threshold 4.15 V and the threshold 4.30 V 
(above 4.15 V and below 4.30 V) is defined in consideration of greater 
eccentricity of the drum 1. The subrange C delimited by the threshold 4.30 
V and the threshold 4.40 V (above 4.30 V and below 4.40 V) is determined 
mainly in consideration of the fluctuation ascribable to temperature 
drift. The subrange D is higher than the threshold 4.40 V. Likewise, the 
subranges B', C' and D' which are lower than the optimal 4.0 V are 
delimited by thresholds 3.70 V and 3.50 V and selected mainly in 
consideration of the fluctuation ascribable to the smears of the sensor 2. 
Regarding the subrange B, adjustment is effected if the output Vsg is 
determined to lie in the subrange B or the subrange C three consecutive 
times. Regarding the range C, adjustment is effected when the output Vsg 
is determined to lie in the subrange D two consecutive times. Further, 
regarding the subrange D, adjustment is effected when the output Vsg is 
determined to lie in the subrange D once. Likewise, regarding the 
subranges B', C' and D', adjustment is performed when the output Vsg is 
determined to lie in the subrange B' or the subrange C' three consecutive 
times, to lie in the subrange D' two consecutive times, or to lie in the 
subrange D' once. 
In the above-described manner, whether or not to perform adjustment is 
determined on the basis of the level and threshold of the output Vsg and 
the number of times that the output Vsg has exceeded the threshold. The 
PWM duty to be fed to the LED 3 is changed by the output Vsg in such a 
manner as to control the output Vsg to 4.0 V which is optimal, whereby the 
output of the image density sensor 2 is adjusted. 
Specific control over the adjustment of the sensor output will be described 
with reference to FIGS. 3-7. First, the control over the decision as to 
whether or not to effect adjustment will be described with reference to 
FIG. 5. In FIG. 5, whether or not the output Vsg has been read is 
determined (step 1). For this purpose, use may be made of a decision 
request flag which is set when the sensor 2 is turned off after having 
been turned on to produce outputs Vsg and Vsp every predetermined number 
of times. If the answer of the step 1 is positive, the decision request 
flag is reset (step 2), and then whether or not output data Vsg is present 
is determined (step 3). Assume that data Vsg is absent (N, step 3) due to 
an error in the developing unit or similar process unit, as stated 
earlier. Then, the operation is transferred to a step 31 shown in FIG. 7 
for resetting an over 1 flag, an over 2 flag, an over 3 flag, a down 1 
flag, a down 2 flag and a down 3 flag. These flags are used to determine 
the number of times that the output Vsg has been continuously found in the 
range B or C. Subsequently, a buffer 3, a buffer 2 and a buffer 1 are 
reset (step 32). These buffers serve to store the outputs Vsg. An 
adjustment request flag is reset (step 33). The adjustment request flag is 
set when adjustment which will be described is necessary. 
If data Vsg is present (Y, step 3), whether or not it lies in the subrange 
A is determined (steps 4 and 5). If the data Vsg lies in the subrange A 
(N, steps 4 and 5), the flags are reset as in the step 31, FIG. 7, (step 
6). Then, the buffer 3 is updated by the output data Vsg stored in the 
buffer 2 (step 7). Likewise, the buffer 2 is updated by the data Vsg 
stored in the buffer 1 (step 8), and then the buffer is updated by the 
latest data Vsg (step 9). If the data Vsg is greater than the subrange A 
(Y, step 4), the operation is transferred to a step 10 of FIG. 6 for 
resetting the down 1 flag, down 2 flag and down 3 flag. Subsequently, 
which of the subranges B, C and D has the data Vsg therein is determined 
(steps 11 and 17). If the data lies in the subrange D (Y, step 17), the 
operation is immediately transferred to a step 20 of FIG. 5 to set an 
adjustment request flag which will be described. This is followed by steps 
6-9 for updating the buffers 1, 2 and 3 in the above-stated manner. 
If the data Vsg lies in the subrange B or C (N, step 11 or 17), the over 1 
flag, over 2 flag and over 3 flag are used to reference the result of 
immediately preceding Vsg detection or, if necessary, the results of two 
preceding Vsg detections to thereby select particular processing. 
Specifically, when the output Vsg lying in the subrange A sequentially 
increases to enter the subrange B for the first time as determined by the 
detection, steps 11-15 are executed since all of the over 1 flag, over 2 
flag and over 3 flag have been reset. In the step 15, the over flag 1 is 
set, and then in steps 7-9 of FIG. 5 the buffers 1, 2 and 3 are updated. 
When the output Vsg is again determined to lie in the subrange B by the 
next detection, steps 14-16 are executed since the over 1 flag has already 
been set. In the step 16, the over 2 flag is set, and then the steps 7-9 
are executed. In the event of the next Vsg detection, since the over 2 
flag has already been set, steps 13-20 are executed to set the adjustment 
request flag (steps 11, 12, 13 and 20 or steps 17, 18, 19, 13 and 20) even 
if the output Vsg lies in the subrange B or C. Further, all of the over 1 
flag, down 1 flag and other similar flags are reset (step 6). Then, all 
the buffers are updated (steps 7-9). 
Assume that the output Vsg following the output Vsg which has been 
determined to lie in the subrange B for the first time lies in the 
subrange C. Then, since the over 1 flag has already been set, the steps 
17-19 are again executed. In the step 19, the over 3 flag is set. 
Subsequently, the over 2 flag is set (step 16), followed by the steps 7-9. 
When the next Vsg detection indicates that Vsg lies in the subrange B, the 
steps 11-13 are executed. At this instant, since the over 2 flag has 
already been set, the adjustment request flag is set (step 20). On the 
other hand, if the output Vsg lies in the subrange C, meaning that the 
over 2 flag has been set, the adjustment request flag is set (steps 17, 18 
and 20). This is again followed by the steps 6-9. 
When the output Vsg lying in the subrange A sharply increases to enter the 
subrange C for the first time, the steps 17-19 are executed. Specifically, 
the over 3 flag is set (step 19). Since the over 1 flag and over 2 flag 
have been reset, the over 1 flag is set (step 15), followed by the steps 
7-9. When the next Vsg detection also indicates that Vsg lies in the 
subrange C, the adjustment request flag is set (steps 17, 18 and 20) since 
the over 3 flag has already been set. This is followed by the steps 6-9. 
When the output Vsg following Vsg having been determined to lie in the 
subrange C for the first time lies in the subrange B, the program advances 
from the step 11 to the step 12 for resetting the over 3 flag, and then 
the steps 13-16 are executed. Specifically, the step 16 sets the over 2 
flag and is followed by the steps 7-9. In the event of the next Vsg 
detection, since the over 2 flag has been set and the over 3 flag has been 
reset, the operation is transferred from the step 13 to the step 20 to set 
the adjustment request flag (steps 1, 12, 13 and 20 or steps 17, 18, 19, 
13 and 20) even if Vsg is determined to lie in the subrange B or C, as as 
been the case with the continuous detection of Vsg in the subrange B. 
Thereafter, the steps 6-9 are executed. 
The step 20 for setting the adjustment request flag is executed only when 
Vsg is determined to lie in the subrange B or C three or two consecutive 
times, as stated above. If Vsg is found in a subrange other than B and C 
during such successive detections, all of the over 1 flag and other 
similar flags are reset (step 6 and step 21, FIG. 8). As a result, all the 
results accumulated by the over 1 flag and other flags are cleared. 
When the data Vsg is smaller than the subrange A (Y, step 5, FIG. 5), the 
operation is transferred to a step 21, FIG. 8, for resetting the over 1 
flag and other similar flags. Thereafter, which of the subranges B', C' 
and D' has the data Vsg therein is determined (steps 22 and 28). If the 
data Vsg lies in the subrange D', the adjustment request flag is set (step 
20, FIG. 5), and then the steps 6-9 are executed, as with the data Vsg 
lying in the subrange D. If the data Vsg lies in the subrange B' or C' (N 
in step 22 or 28), the down 1 flag, down 2 flag and down 3 flag are used 
to reference the result of immediately preceding Vsg detection or, if 
necessary the results of two preceding Vsg detections so as to select 
particular processing, as with the data Vsg lying in the subrange B or C. 
In this case, the down 1 flag and other similar flags correspond to the 
over 1 flag and other similar flags, the steps 21-27 of FIG. 8 correspond 
to the steps 10-16 of FIG. 6, and the steps 28-30 of FIG. 8 correspond to 
the steps 17-19 of FIG. 6. 
How to adjust the sensor output will be described with reference to FIG. 4. 
As shown, whether or not the adjustment request flag is set is determined 
(step 1) and, if it is set, whether or not the buffer 2 stores data is 
determined (step 2). Since the buffer 2 usually stores data, PWM is 
calculated (step 4). Based on the fact that Vsg and PWM have a linear 
relation shown in FIG. 2, the embodiment calculates PWM (new) for 
controlling the output Vsg to 4.0 V which is optimal. To reduce the 
influence of the eccentricity of the drum 1, the embodiment uses a mean 
value of the latest data Vsg stored in the buffer 1 and the immediately 
preceding Vsg data as the current output. If Vsg and PWM do not have such 
a linear relation, PWM which causes Vsg to coincide with 4.0 V may be 
calculated by using the maximum value of .DELTA.Vsg/.DELTA.PWM, i.e., 
inclination of the tangent of the characteristic curve as a reference. 
This is successful in reducing the variation of PWM from the current PWM, 
compared to the case wherein other .DELTA.Vsg/.DELTA.PWM are used, and 
thereby eliminating overadjustment. Subsequently, the buffers 1, 2 and 3 
are cleared (step 5), and then the adjustment request flag is reset (step 
6). When the output Vsg lies in the subrange D or D', the adjustment 
request flag is immediately set while the buffers 1, 2 and 3 are updated. 
Hence, if Vsg is found in the subrange D or D' by the first Sg detection, 
i.e., in the absence of past results, data is absent in the buffer 2. 
Therefore, when the buffer 2 is determined to be empty, data stored in the 
buffer 1 is also written to the buffer 2 (step 3). 
As stated above, the illustrative embodiment clearly defines an output 
range wherein the output characteristic of the image density sensor does 
not change, and subdivides such an output range so that the adjustment may 
occur slowly targeting the center of the output range, thereby eliminating 
overadjustment (excessively high outputs and excessively low outputs). 
Hence, the toner concentration can be controlled to a level which lies in 
a range wherein the sensor output characteristic is stable and which is 
least susceptible to disturbances (surface configuration and eccentricity 
of drum and temperature drift), insuring stable image density. The 
adjustment does not entail any adverse effect (overadjustment). When the 
image forming apparatus has a nonvolatile RAM or similar storage, the PWM 
value of the LED may be stored therein after adjustment. The flags and 
buffers 1-3 may also be stored in such a storage, if possible. This will 
allow the detection and adjustment to be performed without being 
influenced by the turn-on or turn-off of the power source. When the sensor 
is controlled by a plurality of CPUs, the CPU for practicing the present 
invention may send the result of adjustment and other data to the other 
CPUs in a particular format (serial communication). 
In summary, in accordance with the present invention, an image density 
sensor senses the background of a photoconductive element where a toner 
image is absent, and the resultant output of the sensor is compared with 
thresholds which subdivide a predetermined output range including an 
optimal output value into more than four subranges. Such a sensing and 
comparing procedure is repeated. Whether or not to adjust the sensor 
output is determined on the basis of the subrange where the sensor output 
lies and the number of times that the sensor output has been found in such 
a range. The invention, therefore, adequately adjusts the sensor output 
despite a change in the distance between the photoconductive element and 
the sensor or a change in ambient temperature. 
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.