Copying machine having an image density control device

An image density control device for use in an electrophotographic copying machine, employs a scanning light beam having a certain spot size for scanning over a printed original to be copied. Line-width detecting circuitry operates to determine print character line widths on the original according to the output of detecting circuitry on which the light beam reflected from the original impinges. A compensation signal is generated based on the output of the line-width detecting circuitry, to provide a compensation factor based on the ratio of the scanning spot size and the detected line widths. A compensated detected density signal is then produced by compensating circuitry according to the generated compensation signal, and the copy image density is then controlled according to the compensated detected density signal.

BACKGROUND OF THE INVENTION 
This invention relates to a copying machine having an image density control 
device, in which the density of a copy image is controlled by detecting 
the density of an original to be copied. 
To begin with, an electrophotographic copying operation is described 
according to FIG. 1 which shows a general type electrophotographic copying 
machine, schematically. 
In FIG. 1, an original document (not shown) to be copied is placed on a 
copy board 1 such as glass and is pressed by platen cover 2. When a copy 
start button (not shown) is operated, an exposure light source 3 is turned 
on to start scanning in the direction of an arrow A. An image of the 
original is transmitted to a photosensitive drum 5 which serves as an 
electrostatic latent image carrier through an optical system 4 comprising 
a movable first mirror unit 41 having the exposure light source 3, a 
second mirror unit 42 movable synchronously with the first mirror unit 41, 
a fixed lens unit 43 and a fixed mirror 44, as per se well known. 
The photosensitive drum 5 comprises an electrically grounded metal cylinder 
having on its outer peripheral surface a photoconductive layer such as 
selenium or the like, and it rotates in the direction of an arrow 
synchronously and in a link motion with the exposure scanning motion, of 
the exposure light source 3 described above. On the photosensitive drum 5, 
an electrostatic latent image of the original is formed in such a manner 
that the above-mentioned photoconductive layer is uniformly charged into 
positive electricity, for example, by a charge electrode 6 which is 
applied with a DC (Direct Current) high voltage of 5KV; photosensitive 
drum 5 receives a light image corresponding to the original thereon as 
optical system 4 is exposure-scanning the original; at this moment the 
conductivity of a part of the drum exposed to light is increased; the 
charge in this part escapes to the metal cylinder of the drum; positive 
charge remains in the dark parts; and thus, an electrostatic latent image 
corresponding to the original image is formed on the photoconductive 
layer. 
When photosensitive drum 5 further rotates, negative-charged toner is 
electrostatically attracted from developing unit 7 at the developing 
station as a result, electrostatic latent image is changed to a visible 
image or a toner image on the surface, of the drum 5. 
Moreover, copy paper is sent out through a pair of feed rollers or resister 
rollers 10 from a selected cassette stored in paper feed unit 8 with 
proper timing to keep the front edge of the toner image in line on the 
drum 5 with the front edge of the copy paper, and the toner on the surface 
of the drum 5 is then transferred to the copy paper by operation of a 
transfer electrode 9. 
Thereafter, in the case of an electrostatic separation system, for example, 
the copy paper is separated from photosensitive drum 5 by a separation 
electrode 11 to which is applied an AC high voltage. The separated copy 
paper having a toner image is transported to a fixing unit 13, called a 
roller type fixing unit, generally, the toner image is fixed on the paper 
when passing the unit. Thereafter, the copy paper is ejected out to a tray 
(no reference numeral) by rollers 14. There are some instances where, even 
if a toner image is transferred to a copy paper by transfer electrode 9, a 
small amount of the toner of the image remains on the surface of the drum 
5, therefore, the surface thereof is cleaned up by a cleaning unit having 
a blade (no reference numeral) of which the lower edge contacts with the 
surface of the drum, so as to be ready for the next copying process. 
Copying of an original is thus performed in the above-mentioned cycle. 
In a process for controlling the density of a copy image, for example, an 
original density detector 18 (described hereinafter, see FIG. 8) is 
provided onto the first mirror unit 41 or the second mirror unit 42 
disposed in a space wherein the optical system 4 is movably prepared, and 
the density of an original to be copied is detected on the copy board by 
the original density detector when the optical system 4 is preliminarily 
scanning in the direction of an arrow B (see FIG. 8) by operating the copy 
start button. In accordance with the detected density the conditions of 
processes are thereby controlled, such as charging, exposing, developing 
steps and the like, that is, the substantive copying operation, and thus, 
a copy paper onto which pattern of the original is copied with a proper 
density can be obtained. 
There normally used an original density detector 18 in wherein a reading 
spot for reading minute parts of an original is exposed on the original 
and the detector receives the reflected light therefrom. The original 
density detector is moved relatively to the original(see FIG. 8, reference 
numeral 19) so that the reading spot can scan the original. For improving 
the accuracy of detecting an original density, an important factor is how 
small is the reading spot area on the original, that is, the area to be 
detected. 
Namely, the smaller the area to be detected is, the more accurate is the 
density value of the line image density. On the contrary, if the area to 
be detected is broad, the density value including that of the background 
surrounding the line image must be detected, so that the detected value 
becomes lower than the real density value. Therefore the correct line 
image density value cannot be detected. 
Accordingly, such an area to be detected must be as small as the line image 
density can be correctly detected. However, when a detecting means is 
constructed so as to make the detection area thereof small, the 
relationship of the mechanical positions between the detector and an area 
to be detected i.e., an original surface, must be maintained with a high 
accuracy, therefore, it becomes difficult to put into practice. 
For example, in printing type for news-paper, the most lightface is of the 
order of 0.10 to 0.15 mm in size. It is therefore difficult to maintain 
the accuracy of the relationship of the mechanical positions between a 
detector and an area to be detected if the detection area is made to be a 
spot of 0.10 mm in width, because a scanning must be made while 
maintaining the accuracy of the relationship of 0.01 to 0.05 mm. As 
described above, it has been difficult in a copying machine to detect 
accurately the density value of a line image original. 
OBJECT AND SUMMARY OF THE INVENTION 
It is an object of the invention to provide a copying machine having an 
image density control device which is capable of improving the density 
detection accuracy even if an area to be detected is broadened enough to 
the order of the size for practical application. 
To attain the above-mentioned object and others of the invention, a copying 
process is controlled in the manner that a detected density value lowered 
in accuracy by broadening an area to be detected, is compensated according 
to the data of a line width given from a signal of the detected density to 
obtain more accurate density value. The invention is embodied in an 
electrophotographic copying machine having an image density control device 
in which a copy density is controlled by detecting an original density. 
The image density control device comprises 
density detecting means for detecting the original density, 
original line-width detecting means for detecting line widths of the 
original from the output of the density detecting means, 
compensation signal output means for generating a compensation signal 
corresponding to the output of the line-width detecting means, 
compensating means for compensating the output of the density detecting 
means corresponding to the output of the compensation signal output means, 
and 
copy-process controlling means for controlling a copy-image density by the 
output of the compensating means. 
In a preferred embodiment, the copy-process controlling means comprising; 
means for storing a density detection frequency which corresponds to each 
of compensated density values, and 
means for controlling the copy-image density according to the maximum 
density and the minimum density obtained from the density range having 
frequency of not less than a specific frequency selected from the 
frequency distribution of the density obtained by the storing means.

DETAILED DESCRIPTION OF THE INVENTION 
Scanning of a line image of an original operated by a reading spot of the 
density of the original to be copied is shown in FIG. 2. A reading spot 17 
having the diameter d scans a black-line 16 of the line-width W of an 
original in the perpendicular direction to the black-line 16, the diameter 
d of reading spot 17 is fixed and line-width W of black-line 16 is made as 
a parameter, and thus, detected density DD shows the characteristics shown 
in FIG. 3. As is apparent from FIG. 3, when W and d are in the relation of 
W.gtoreq.d, detected density DD becomes about 100 per cent of black-level 
provided that white-level is regarded as 0 per cent. On the other hand, 
when the relation is W&lt;d, the detected density DD lowers as line-width W 
becomes narrower. 
When both diameter d of reading spot 17 and line-width W are fixed, d and W 
are set in the relation of d&lt;&lt;W, and the parameter is image density OD of 
black-line 16, then resulting relation is as shown in FIG. 4. 
Next, when line-width W is fixed and the parameter is diameter d of reading 
spot 17, then resulting relation is as shown in FIG. 5. In FIG. 5, the 
origin at scanning distance x is charged according to diameter d of the 
reading spot, and W and d is in the relation of W&gt;&gt;d. 
As described above, diameter d of the reading spot must be smaller than the 
line-width W if the density of a fine line-width of an original should be 
more accurately detected. 
However, in a reflection type density detector provided with a light 
emission device and a light, receiving device, when diameter d of a 
reading spot from a light beam emitted from the light emission device to 
an original surface is made small, then the distance between the density 
detector and the original surface must be constantly fixed during the 
whole period for scanning the original by the density detector, because 
the diameter d of the reading spot must be kept constant and the distance 
between the light receiving device and the original surface becomes short. 
However, the finest line width of a news paper type or the like is within 
the order of 0.01 to 0.15 mm. If the width is to be detected more 
accurately, the diameter d of the reading spot must be not larger than the 
above-mentioned order. In such cases, accuracy required for parts or 
assembly between an original density detector and an original surface may 
be hardly attained. 
Therefore it is considered that the diameter d of the reading spot be made 
larger. In such cases, the waveform of detected density DD does not reach 
the original black level at the time when line width W is not wider than 
the diameter d, as shown in FIG. 3, therefore, the detected density signal 
is multiplied by a compensation coefficient to increase the density, so 
that a compensation is made to the essential detection level. To attain 
this, the data of the line width must be needed, and this data can be 
obtained from the pulse width of the detected waveform in the case of 
scanning an orignal by the density detector at a constant velocity v. 
Namely, such a pulse width Tw is in the relation of Tw=W/v. 
The pulse width Tw can be measured in such a manner, as shown in FIG. 6, 
that time Twh of a half-width, i.e., the width of a level that is one half 
of the peak-value, of a detected density waveform, or time Tppf from the 
time reaching the high peak-value to the time reaching the low peak-value, 
is measured, and then the value thus obtained is doubled. 
With respect to the relation formula, line-width W is obtained from the 
pulse width Tw thus obtained, and according thereto, as shown in FIG. 7, a 
compensation coefficient K is obtained from a compensation curve made to 
correspond to the diameter of a reading spot to be used, and thereby the 
high peak-value of a detected density signal is multiplied to obtain an 
accurate density value. 
In FIG. 7, when line-width W is not wider than d/5, it may be considered 
that compensation coefficient K may become greater so that an error in 
measurement by a pulse-width Tw detection system affects seriously, and 
that an electric impulse noise or the like also affects susceptibly. For 
example, a compensation coefficient may be allowed to be constant when W 
is narrower than d/5. 
FIG. 8 illustrates an example of the invention, wherein original density 
detector 18 must have a special heat-resistance measure applied when it is 
in the vicinity of first mirror unit 41, therefore, in this embodiment the 
density detector is disposed at the second mirror unit 42 so there is no 
need to apply such measures. The detector is movable in the vertical 
direction through a proper driven mechanism (not shown). But it is 
allowable to move the detector in same direction with the second mirror 
unit, generally. In FIG. 8, an original density is detected during a 
scanning in the returning direction (shown by arrow B) of optical system 
4. 
As shown in FIG. 9, original density detector 18 of the reflection type 
comprises a tungsten lamp 18a used as a light emitting element thereof, 
and a phototransistor 18b used as a light receiving element. Condenser 
lenses 18c and 18d are mounted on both elements described above, 
respectively, as one body. 
In this example, original density detector 18 moves in the horizontal 
direction perpendicular to the direction of an arrow B (normal to the 
plane of the drawing) simultaneously when second unit 42 mirror moves in 
the direction of an arrow B. As shown in FIG. 10, during a scanning made 
by optical system 4 in the direction of an arrow B, reading spot 17 having 
the diameter of about 1 mm scans in the diagonal direction (indicated by 
arrow C) an area of original 19 which is put on copy board 1 so as to read 
a reflected light to a light receiving device from the original surface on 
which the light beam emitted by original density detector 18 impinges. 
Thus, the original density of the area scanned by reading spot 17 is 
detected sequentially. 
In this case, letters are usually written in parallel with or in the 
perpendicular direction to the margin of original 19, therefore, if the 
original 19 is scanned in the diagonal direction to the original as 
mentioned above, anyone of the letters will be scanned without fail, so 
that the image conditions of the original 19 may be accurately detected. 
At this time, original density detector 18 is some distance from exposure 
light source 3, therefore when an original 19 is large in size, the 
scanning covers only a part of the original but the information of the 
image of the original 19 can be satisfactorily obtained. 
The above-mentioned operation is performed by pressing a copy-start button 
(not shown) in a course of the preliminary scanning prior to an 
exposure-scanning for copying an original. (In this example, the 
preliminary scanning is done in a return-scanning by optical system 4.) At 
this time, the density information of original 19 is detected and then the 
exposure-scanning for an innate copy is operated by optical system 4 such 
as exposure light source 3 which scans in the opposite direction of an 
arrow A to the direction of an arrow B. 
When the density of the original 19 is thus detected, the detected density 
signal is, as shown in FIG. 11, amplified by amplifier 20 and is then 
converted into a digital signal by A/D converter 21. Then, every 
peak-value of detected density values are held every time when detected, 
by peak-value hold unit 22. Furthermore, a line-width is detected by 
line-width detector 23 based on the aforementioned principle. When the 
line width data is thus obtained, the data signal is sent to 
date-compensator 24 and a data signal with compensation coefficient K is 
therefrom generated to multiply the peak-value of each detected density by 
the compensation coefficient K, at computing or operating section 25. 
Thus, a compensated and accurately detected density signal is outputted 
from the computing section 25. 
In the meantime, there are two kinds of image control methods, i.e., one is 
a process control method in which, among various densities of an original 
detected, the minimum density thereof is regarded as the background 
density of the image, or the maximum density thereof is regarded as the 
image density; and another method is that a frequency distribution shown 
as a histogram to the density value of an original is obtained and the 
maximum value in the distribution is detected and is then processed in 
some statistical process to obtain the original density value. 
However, in the former, when an electrical noise, a noise caused by a 
mechanical vibration or the like is mixed in a density detection signal, 
the noise is detected as the minimum or maximum density and thereby an 
erroneous operation may possibly to occur; and in the latter, it is 
required to raise the sampling frequency to detect a density value 
accurately. Accordingly, a memory unit for obtaining a frequency 
distribution cannot help being large in size as well as the processing 
means thereof. 
In this example, therefore, a density histogram is obtained, as shown in 
FIG. 12, by storing and memorizing in memory unit 26 every frequency by 
density values one after another corresponding to the respective density 
values each obtained from the above-mentioned density signals. 
In the drawing, the characteristics shown by a broken line is that of the 
actual frequency distribution in which some frequency not less than m is 
saturated by data processing unit 27. Then, the minimum density D.sub.1 
and maximum density D.sub.2 of this frequency m are detected by data 
processing unit 27. D.sub.0 is the standard of a background density. 
When the frequency distribution shown in the histogram of FIG. 12 is 
obtained in which the minimum density D.sub.1 and the maximum density 
D.sub.2 are regarded as the background density and the image (letter or 
the like) density respectively, processing condition control unit 28 
controls copying process conditions such as the exposure conditions by 
controlling the voltage of exposure light source 3, the development bias 
conditions by controlling the voltage applied to a sleeve made of 
non-magnetic and electrically conductive material, as per se well known, 
of magnetic-brush type developing unit 7, the exposure conditions by 
controlling the lens-opening of optical system 4, and the like. 
When doing this, the detection of a frequency corresponding to each density 
value may be obtained by detecting whether or not the frequency is not 
less than a fixed frequency m, therefore, a storage unit necessary for 
obtaining a frequency distribution may be small in scale, and erroneous 
operations caused by noises can be reduced because a prescribed frequency 
or more than that is taken up as a subject. 
Next, referring to a concrete example of the image control methods using 
the maximum density and the minimum density each having frequency of not 
less than that prescribed from the viewpoint of frequency distribution, H 
is some specific value. 
(a) D.sub.0 &lt;D.sub.1 --Dark background original 
(b) D.sub.0 &gt;D.sub.1,.vertline.D.sub.2 -D.sub.1 .vertline.&gt;H--Light 
background and high contrast 
(c) D.sub.0 &gt;D.sub.1,.vertline.D.sub.2 -D.sub.1 .vertline.&lt;H--Light 
backgound and low contrast 
Therefore, in the case of (a), a copy density is lowered by increasing 
exposure and/or by increasing a development bias voltage; in the case of 
(b), an ordinary exposure and development bias voltage are applied; and in 
the case of (c), a copy density is increased by decreasing exposure and/or 
by decreasing a development bias voltage. 
In the above example, peak-value holding unit 22, line-width detector 23, 
compensation data unit 24, computing section 25, storage unit 26 and data 
processing unit 27 may be replaced altogether by a microcomputer or the 
like so as to function with the soft-ware of such microcomputer or the 
like. In this example, the density detector was disposed at second mirror 
unit 42, however, it can also be disposed at first mirror unit 41. Further 
in the example, an original density is detected at the time of scanning in 
the direction of returning to the initial position (i.e., the scanning in 
the direction of arrow B) after completion of innate scanning an original 
to be copied. However, it is also possible to perform a preliminary 
scanning only for detecting an original density over the whole surface of 
the original or a small part of the surface thereof in the direction of an 
arrow A that is the same direction as the copy-scanning direction. 
According to the invention, the line-width of an original is detected, and 
a compensation is applied to the detection output of the original density 
which is read based on the detection signal of the original line-width, 
therefore, the area of the original to be detected can be broadened when 
the density thereof is to be detected. Consequently, it is not necessary 
to control the distance between an original density detecting means and 
the surface of an original, and to strictly control the variation of the 
original area to be detected. Further, in the invention, any image density 
can be detected accurately even if the line-width of such image is fine, 
and a storage unit for obtaining a frequency distribution at the time of 
image control may be small enough in scale, and in addition, any influence 
from noises can also be avoided.