Developer concentration controlling device

A device for controlling the developer concentration in an electrophotographic copier or the like, in which a first detector for detecting the developer concentration and a second detector for detecting the image density are utilized for controlling process devices to maintain a constant optimum image density.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a device for controlling the concentration 
of the developer employed in recording apparatus such as a copier or a 
laser beam printer, thereby maintaining a constant optimum image density 
in such recording apparatus. 
2. Description of the Prior Art 
Developer composed of a mixture of toner particles and carrier particles is 
employed in certain recording apparatus such as dry-process 
electrophotographic copier and electrostatic recording apparatus. As an 
example of such recording apparatus, FIG. 1 schematically shows the 
structure of a conventionally known laser beam printer, in which a laser 
beam 17 generated by a laser device 15 is modulated by an A/0 modulator 16 
in response to video signals 13 corresponding to the information for 
printing stored in a page memory 11. The laser beam 17 thus modulated is 
reflected by a polygonal mirror 25 rotated in a direction a for effecting 
principal scanning, then focused by an f-.theta. lens 27 and guided by a 
deflecting mirror 29 to a photosensitive member 33 essentially composed of 
a conductive substrate, a CdS photoconductive layer and an insulating 
laryer and provided on a photosensitive drum 31. 
Said photosensitive member 33 of the drum 31 rotated in a direction b for 
auxiliary scanning is at first uniformly charged to a potential of several 
thousand volts by a corona discharge from a primary charger 41, then is 
exposed to the laser beam 17 modulated by the A/0 modulator 16 according 
to the video signals 13 simultaneously with the charge elimination with a 
charge eliminater 42 by an AC corona discharge or a DC corona discharge of 
a polarity opposite to that of the primary charging, and is uniformly 
illuminated by an exposure lamp 44 to form an electrostatic latent image 
of an elevated contrast on said photosensitive drum 31. The photosensitive 
member 33 bearing said electrostatic latent image shows locally varied 
electrostatic potential. 
The electrostatic latent image formed on the photosensitive member 33 is 
developed into a visible image in a developing device 43 by depositing 
toner from developer 45 charged to a potential of several hundred volts. 
The toner image thus developed is transferred by a transfer charger 47 
onto a transfer sheet 49 transported in a direction c, thus reproducing 
the information stored in the page memory 11 on said transfer sheet 49. 
After the image transfer, a cleaning blade 51 removes the remaining toner 
and unrepresented means dissipates the remaining potential to prepare the 
photosensitive member for the succeeding image formation. 
The developer 45 contained in the developing device 43 is generally a 
two-component developer composed of a mixture of toner particles and 
carrier particles, and the weight mixing ratio thereof has a significant 
influence on the image development to be achieved by the developing device 
43. 
For example, an excessively low toner concentration in the developer 45 
will provide a low image density, while an excessively high toner 
concentration will give an excessively high image density combined with a 
background fog, eventually forming an unacceptable image on the transfer 
sheet 49. 
Consequently, in order to constantly obtain an image of a desirable 
density, it is essential to maintain the toner concentration in the 
developer 45 at an appropriate and constant level during the image 
development. 
There have been proposed various methods for maintaining a constant toner 
concentration. 
One of such methods is to control the amount of toner replenishment in 
response to the optical detection of the image density after the 
development. Although this method allows control of the amount of toner 
replenishment in direct relation to the image density, it is inevitably 
associated with unstable density control because of the error in the 
detection caused for example by a smear on the optical sensor. 
Other methods control the amount of toner replenishment through the 
detection of a change in the magnetic permeability or dielectric constant 
of the developer or through the detection of a change in the color of the 
developer composed of carrier and toner of different colors. Such methods 
are however unable to maintain the toner concentration at an appropriate 
level over a prolonged period, as they are apt to be influenced by the 
spent toner particles which are present in the developer but do not 
contribute to the developing process. Also such methods, not based on the 
direct measurement of the developed image density, are unable to 
compensate the image density by regulating the toner concentration in case 
the electrostatic potential on the photosensitive member shows a change 
after prolonged use to a level giving inappropriate image density. 
Also there are known improved methods of detecting the change in the volume 
of developer 45 and effecting control to maintain said volume at a 
constant level, as disclosed in Japanese Laid-Open Pat. No. Sho 50-19459 
entitled "Method for detecting and controlling the concentration of 
electrophotographic developer" and in Japanese Laid-Open Pat. No. Sho 
51-78343 entitled "Electrophotographic developing device". Again referring 
to FIG. 1, these methods are based on the fact that the consumption of the 
developer 45 in the developing device 43 is mostly due to consumption of 
the toner and that the consumption of the carrier is zero or very small. 
Thus, assuming that the decrease in the volume of the developer 45 is 
caused solely by the consumption of toner or by the consumption of 
developer in which the consumption of toner is at a fixed level determined 
by the consumption of the carrier, these methods measure the decrease in 
the volume of the developer 45 and feed a replenishing developer in which 
the toner concentration is 100% or said fixed level to maintain the 
developer 45 at a constant volume in the developing device 43, thereby 
maintaining a constant toner concentration in the developer 45. 
In practice, however, the ratio of the consumption of toner and carrier is 
not constant but is dependent on the area of recording. Consequently the 
replenishing developer of a fixed toner concentration causes a change in 
the toner concentration in the developer 45 after a prolonged period. In 
order to resolve such drawback, there has been proposed a method of 
optically detecting the image density after development and replenishing 
the carrier when the image density becomes excessively high due to the 
consumption of the carrier. In such method, however, the image density 
will become excessively low if the carrier is excessively replenished by 
error, and it will become necessary in such case to immediately regulate 
the toner concentration in the developer 45 by an additional replenishment 
of the toner. However, in case the toner replenishment is controlled by 
the detection of decrease in the developer volume as explained above, the 
toner replenishment may not be conducted in such state since the volume of 
the developer 45 is already increased by the replenishment of the carrier. 
Consequently, in such state it becomes necessary to continue to use the 
recording apparatus with the thus lowered image density for a considerable 
period, as the removal of carrier along from the developer is generally 
quite difficult. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a developer concentration 
controlling device capable of maintaining a stable and appropriate 
development in a recording apparatus for a prolonged period. 
Another object of the present invention is to provide a recording apparatus 
capable of detecting the developer concentration and the image density, 
and of controlling at least one of plural process means in response to the 
result of said detection. 
Still another object of the present invention is to provide a developer 
concentration controlling device capable of adjusting the detection level 
for the developer concentration in response to the image density after 
development. 
Still another object of the present invention is to provide a developer 
concentration controlling device capable of detecting the decrease of 
carrier in the developer and replenishing said carrier. 
Still another object of the present invention is to provide a developer 
concentration controlling device capable of regulating the developer 
concentration to a reference value when said developer concentration 
becomes outside an acceptable range. 
The foregoing and still other objects of the present invention will be made 
fully apparent from the following description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now the present invention will be clarified in detail by the following 
description to be taken in conjunction with the attached drawings. 
FIG. 2 shows the structure of a laser beam printer embodying the present 
invention, which is different from that shown in FIG. 1 in having a 
central processing unit 61 (to be hereinafter called a CPU) and a related 
control unit, with components the same as those in FIG. 1 being 
represented by the same numbers. 
In FIG. 2, in order to identify the toner concentration of the developer 45 
in the developing device 43, a developer concentration detector unit 67 
supplies a concentration detection signal 69 to the CPU 61 in response to 
a detection signal from a magnetic permeability sensor 63. 
FIG. 3 shows the structure of said developer concentration detector unit 
67, wherein the magnetic permeability sensor 63 is formed as a flat plate 
of a determined area maintained in constant contact with the developer 45 
in the developing device 43 and changes the equivalent conductance 
corresponding to the change in the magnetic permeability of the developer 
45. An AC power source 101 supplies an AC voltage of a determined angular 
frequency .omega. to said sensor 63, and also to an AC amplifier 103 
through a condenser 102. The output signal 105 from said amplifier 103 is 
supplied to a rectifying-smoothing circuit 107 for example composed of a 
diode and a condenser to obtain a signal 109 of a DC voltage Vd, which is 
supplied, together with a reference voltage Vr from a variable voltage 
source 111, to a comparator 113 to obtain the aforementioned concentration 
detection signal 69 therefrom. 
FIG. 4 shows the details of a part of the structure shown in FIG. 2, 
wherein a reflective density detector unit 143 is positioned close to the 
photosensitive member 33 in order to measure the optical density of an 
image developed from an electrostatic latent image formed, as will be 
explained later, in a reference image area 141 on said photosensitive 
member 33. As shown in FIG. 5, in said reflective density detector unit 
143, a light-emitting diode 203 positioned in a casing 201 emits a light 
beam 205 toward the photosensitive member 33, and the reflected light 207 
is received by a photodiode 209. 
An outer casing 301 of the developing device 43 is provided with an 
aperture 303, and a toner hopper 431 is mounted on the developing device 
43 in such a manner that a toner supply aperture 313 of said hopper 431 
coincides with said aperture 303. 
Now the function of the above-explained apparatus will be explained in the 
following in relation to FIGS. 2 to 4. In case the toner concentration in 
the developer 45 is lowered, thus elevating the proportion of the magnetic 
carrier, the sensor 63 increases the equivalent conductance L 
corresponding to the toner concentration, whereby the output signal from 
the amplifier 113 based on the signal of a determined frequency from the 
AC power source 101 changes, causing a significant change in the DC 
voltage Vd supplied to the comparator 113. When said voltage Vd exceeds 
the reference voltage Vr, the detection signal 69 supplied from said 
comparator 113 energizes a toner hopper drive unit 121 in the CPU 61, 
thereby activating the toner hopper 431 through a drive signal 125 to 
replenish the developing device 43 with a replenishing developer composed 
solely of toner or containing carrier with a determined proportion through 
the aforementioned aperture 313. Upon continued toner replenishment, the 
toner concentration in the developer 45 is gradually elevated whereby the 
DC voltage Vd approaches the reference voltage Vr. When the DC voltage Vd 
becomes equal to the reference voltage Vr, the toner hopper drive unit 121 
is deactivated to terminate the toner replenishment, whereby the toner 
concentration in the developer 45 is maintained at a constant level 
determined by the setting of the variable voltage source 111. 
However the structure explained in the foregoing merely detects the toner 
concentration including the aforementioned spent toner not contributing to 
the image development, and is therefore unable to distinguish a state of 
adequate image density from a state of suspended toner replenishment due 
to erroneously identified image density. Also no compensation is provided 
for the time-dependent change of the photosensitive member. 
Consequently, according to the present invention, the detection level of 
the toner concentration explained above is changed in response to the 
output of detector means for optically measuring the image density. 
FIG. 6 shows the CPU 61, which can be composed for example of a 
microprocessor 8080 supplied by Intel Corp., and related control units. 
There will now be explained the control sequence shown in FIG. 7, while 
making reference to FIGS. 2 to 6. 
In the following it is assumed that the concentration controlling operation 
is conducted during the pre-rotation of the photosensitive drum 31 
immediately prior to the recording operation. In said concentration 
controlling mode, the A/0 modulator 16 is so controlled by a control 
signal 401 from the CPU 61 that the laser beam 17 is not directed toward 
the mirror 25, whereby the photosensitive member 33 is not subjected to 
charge elimination by the laser beam 17, thus obtaining an electrostatic 
potential E.sub.1, which is detected by the potential sensor 405 
positioned close to the photosensitive member 33. The detection signal 407 
thus obtained is supplied, after digitizing in an A/D converter 501, to 
the CPU 61 for storing said potential E.sub.1. Subsequently the CPU 61 
controls the A/0 modulator 16 in such a manner as to direct the laser beam 
17 toward the photosensitive member 33 through the mirror 25, whereby the 
photosensitive member 33 is subjected to charge elimination by the laser 
beam 17, thus obtaining an electrostatic potential E.sub.2 which is 
likewise detected by the potential sensor 405 (Step 701). Then the CPU 61 
releases a charging control signal 411 to regulate the charging voltage of 
the primary charge 41 in such a manner that said potentials E.sub.1 and 
E.sub.2 remain at determined constant levels (Steps 703 and 705). It is to 
be noted that such control of the charging voltage can also be conducted 
in the secondary charger 42 or in the primary and secondary chargers 41 
and 42. 
Subsequently the CPU 16 controls the A/0 modulator 61 to direct the laser 
beam toward the mirror 25, and the principal scanning (direction a) and 
the auxiliary scanning (direction b) are suitably effected with the laser 
beam 17 in this state to form an electrostatic latent image in the 
reference image area 141. Then a developer drive unit 435 for the 
developing device 43 is activated by a signal 437 to develop said latent 
image in the reference image area 141. The density of the thus developed 
image is measured by the reflective density detector unit 143, and the 
obtained detection signal 441 is digitized by an A/D converter 502 to 
store the reference image density D.sub.p in the CPU 61. Then the A/0 
modulator 16 is suitably controlled and the background density D.sub.o on 
the photosensitive member 33 after the image development is similarly 
measured (Step 707). Then it is identified if said densities D.sub.p and 
D.sub.o are at abnormal state outside the determined range (Step 709). If 
such abnormal levels are found, an abnormality signal is generated for 
display for example through a lamp and the detection level is shifted to 
the standard value (Step 711), and the program proceeds to the ordinary 
printing mode (Step 713). 
Also when said densities are identified as normal in the Step 709, it is 
further identified if said densities D.sub.p and D.sub.o satisfy a desired 
relationship, for example a predetermined density ratio or density 
defference (Step 721). If the result of said identification is affirmative 
indicating an appropriate developer concentration, the program proceeds to 
the printing mode (Step 731). On the other hand, if said result is 
negative, it is further identified if the reflective density is 
excessively high (image being too dark) or excessively low (image being 
too light) (Step 723), and the detection level Vr of the developer 
concentration detector unit 67 is respectively lowered (Step 725) or 
elevated (Step 727). Now there will be explained the means for controlling 
said detection level Vr. 
In response to the image densities D.sub.p and D.sub.o stored in the CPU 61 
corresponding to the detection signal 441 from the reflective density 
detector unit 143, the CPU 61 releases a control signal 95 to control the 
reference voltage Vr from the variable voltage source 111 according to a 
known process. For instance said signal 95 is composed of a coded digital 
signal which is converted into an analog reference voltage Vr. As an 
alternative method said variable voltage source 111 can be composed of 
plural constant voltage sources from which a desired voltage is selected. 
It is furthermore possible to obtain the detection signal 69 by selecting 
one of plural comparators having mutually different reference voltages. 
During the printing mode in the Step 713, the replenishment of the 
replenishing developer from the toner hopper 431 is conducted according to 
the thus newly determined detection level Vr. 
The aforementioned influence of the spent toner is thus avoided since the 
toner concentration in the developer is controlled to obtain an optimum 
image density in direct response to the measured density of the developed 
image. 
The reflective density detector unit 143 is however apt to be smeared for 
example by toner, and the normal detecting function is hindered 
particularly when the light-receiving face of the photodiode 209 is 
smeared. Also a functional failure may occur in the detector unit 143 
itself. Such failure may lead to a significant change in the toner 
concentration due to an abnormal setting of the detection level Vr for the 
magnetic permeability. In order to avoid such undesirable situation, in 
the present embodiment, the Step 711 shifts said detection level to a 
standard value in response to the detection of an abnormal image density, 
whereby the toner concentration control is effected with said standard 
detection level to allow continued use of the recording apparatus without 
interruption although the image density to be obtained is not optimum 
until the detector unit 143 is repaired. 
The above-explained operation of the concentration controlling mode can 
also be effected once every day immediately prior to the first recording 
operation after the start of power supply, or every time prior to the 
re-start of the recording operation after it is interrupted, or at a 
regular interval through the use of a timer. 
It is furthermore possible to select one of plural detection levels for 
detecting the developer volume in the developing device in response to the 
optically measured density of a reference image formed on the 
photosensitive member, and to control the developer replenishment 
according to the thus selected detection level. Such procedure is employed 
in another embodiment to be explained in the following. 
FIGS. 8A and 8B show different examples of the developing device 43, 
wherein a container 71 is equipped with three sensors 631, 632 and 633 
constituting the developer volume sensor 63. The laser beam printer in 
this case is structured essentially the same as that shown in FIG. 2. It 
is assumed that the reference detection level is selected at a level 82 
corresponding to the detector 632. A decrease of the developer volume 
below the reference level 82, caused by the consumption of the toner, can 
therefore be detected by said detector 632, and the corresponding 
detection signal 69 activates the toner hopper through the CPU 61 to 
replenish the developing device 43 with a replenishing developer having a 
determined ratio of toner and carrier, whereby the developer volume in the 
developing device 43 is restored to said level 82. A similar function is 
attained also when the reference detection level is selected at another 
level 81 or 83 corresponding to another detector, and a constant developer 
volume can be maintained with respect to either detection level to achieve 
a constant developer concentration. Such process is in practice capable of 
maintaining a constant toner concentration in the developer for a 
relatively short period corresponding to 100 to 200 thousand copies of A4 
size. 
FIG. 9 shows the developer volume detector unit 67, wherein three detectors 
631, 632 and 633 in the developing device 43 have equivalent inductances 
631L, 632L and 633L which vary according to the magnetic property of the 
carrier contained in the developer 45. More specifically said inductance 
631L, 632L or 633L increases when the developer 45 is respectively present 
at the level 81, 82 or 83. A switch 91 is provided to select one of said 
detectors 631, 632 and 633. It is now assumed that the switch 91 is 
connected to a contact f for selecting the detector 632 in response to a 
selecting signal 95 from the CPU 61. The AC power source 101 supplies an 
AC voltage of a determined angular frequency .omega. to said detector 632, 
and also to the amplifier 103 through the condenser 102. The amplified 
output signal 105 from said amplifier 103 is supplied to the 
rectifying-smoothing circuit 107 composed for example of a diode and a 
condenser to obtain a signal 109 of a DC voltage Vd, which is supplied, 
together with a reference voltage Vr from the variable voltage source 111, 
to the comparator 113. 
A resonance circuit is formed by said detector 632 and the condenser 102. 
In the presence of the developer 45 at the level 82, the detector shows an 
increased inductance 632L causing a resonance with the angular frequency 
.omega. of the AC power source 101. 0n the other hand, such resonance does 
not take place if the developer 45 is positioned below the detection level 
82. 
In case the switch 91 is connected to the contact e or g, a similar 
resonance takes place only when the developer volume reaches the level 81 
or 83 respectively. 
Such resonance significantly changes the DC voltage, Vd, so that the 
presence of developer at the level 81, 82 or 83 can be detected through 
comparison with a suitably selected reference voltage Vr. The volume 
detection signal 69 obtained from the comparator 113 is supplied to the 
toner hopper drive circuit 121 in the CPU 61 to generate a drive signal 
125 for the toner hopper. In this manner the toner concentration control 
previously explained in relation to FIGS. 8A and 8B can be achieved with a 
sensitivity improved by said resonance, through the selection of the 
detection level 81, 82 or 83 corresponding to the detector 631, 632 or 
633. 
FIG. 8B shows another embodiment of the developing device 43, which is 
different from the embodiment of FIG. 8A in that a screw 131 is rotated at 
the detection of the developer volume in order to accumulate the developer 
45 against an inner wall of the container 71 where the detectors 631, 632 
and 633 are positioned, thereby enabling more accurate detection of the 
developer volume. 
FIG. 10 shows the control sequence in this embodiment, wherein the density 
measurement of the reference image is conducted in the same manner as in 
the foregoing embodiment shown in FIGS. 4 and 5, and the control unit is 
structured in the same manner as shown in FIG. 6. Now the control sequence 
will be explained in the following while making reference to FIGS. 2, 4 to 
6 and 8 to 10. 
The Steps 801 to 807 are similar to the Steps 701 to 707 shown in FIG. 7 
and are therefore omitted from the following explanation. 
A Step 809 identifies if the densities D.sub.p and D.sub.o satisfy a 
determined relationship, for example a determined density ratio or density 
difference. If the result of said identification is affirmative indicating 
an appropriate developer concentration, the program proceeds to the 
printing mode (Step 811). 
On the other hand, if said result is negative indicating an inappropriate 
image density, the developer concentration is corrected in the following 
procedure, in which provided are three detection levels for the developer 
volume as already explained in relation to FIGS. 8A and 8B. 
In this state the CPU 61 is capable of identifying whether the reference 
detection level is located at the level 81, 82 or 83, from the logic state 
of the selecting signal 95 for the switch 91. 
Subsequent to the above-mentioned Step 809, a Step 813 identifies if the 
density of the developed image is abnormal. If it is identified as 
abnormal, the detection level is shifted to a standard level, for example 
the level 82 (Step 815), then an abnormality signal is produced for 
example for display through a lamp (Step 817), and the program proceeds to 
the printing mode (Step 811). 
If the result of identification in the Step 813 is negative, it is 
identified whether the reflective density is excessively high (Step 821), 
and, if so, the CPU 61 shifts the switch 91 to select the lower detection 
level (Step 823). The program then proceeds to the printing mode (Step 
811). The recording apparatus in such state provides an appropriate image 
density since the replenishing developer is not supplied until the 
developer volume reaches the thus lowered detection level. 
Also in case the identification in the Step 821 indicates an excessively 
low density, the detection level is elevated (Step 825) and the program 
proceeds to the printing mode (Step 811). In such state the replenishing 
developer is supplied from the toner hopper 431 until the developer volume 
reaches the thus elevated detection level, whereby the toner concentration 
in the developer is elevated to an appropriate level capable of providing 
an appropriate image density. 
During the printing operation in the Step 811, the developer volume in the 
developing device 43 is constantly measured according to a principle 
already disclosed in the aforementioned Japanese Laid-Open Pat. Nos. Sho 
50-19459 and Sho 51-78343, and, upon detection of a decrease in the 
developer volume by the detector unit 67, the motor of the toner hopper 
431 is energized, only when said developing device 43 is activated, to 
replenish the developing device 43 with a replenishing developer composed 
solely of toner or having a determined carrier ratio through the aperture 
313 until said detector unit 67 no longer detects said decrease of 
developer. 
As already explained in relation to the foregoing embodiment, such 
concentration controlling operation can be conducted either once every day 
immediately prior to the first printing operation after the start of power 
supply, or every time prior to the re-start of the printing operation 
after it is interrupted for a while, or at a regular interval through the 
use of a timer. 
As already explained in relation to FIG. 10, the CPU 61 selects a standard 
detection level (Step 815) in response to an abnormality signal produced 
in the Step 817. More specifically, in response to a selection signal 95 
from the CPU 61, the switch 91 is connected to the contact, f for 
selecting the detector 632, thus selecting the detection level 82, so that 
the toner concentration control is continued during the printing mode 
(Step 811) under such standard detection level. Such procedure not only 
avoids the danger of significant change in the toner concentration in the 
developer eventually resulting from an erroneous detection in the detector 
unit 143 caused for example by a smear on the photodiode 209 in the 
detector unit 143, but also allows the continued use of the recording 
apparatus under said toner concentration control with such standard 
detection level, thereby eliminating the necessity for the immediate 
repair of the detector unit 143. 
Although the volume sensor 63 in the foregoing embodiment is composed of 
three elements 631, 632 and 633 selectable by a switch, it is also 
possible to constitute said sensor by a single sensing element. 
FIG. 11 shows an embodiment of such single-element sensor 63, which has a 
certain length and is positioned on an inner wall of the container 73 in 
such a manner that the longitudinal direction thereof crosses the volume 
levels of the developer 45. Said sensor 63 shows a variable equivalent 
inductance according to the sensor area covered by the developer 45, and 
provides different inductances for example when the developer volume 
reaches a level 81A and another level 82A, thus accordingly regulating the 
DC voltage Vd from the detector unit 67 shown in FIG. 4. Consequently a 
similar detecting operation is rendered possible by variably selecting the 
reference voltage Vr from the variable voltage source 111 according to the 
required detection level. It is furthermore possible, in such case, to 
select one of plural reference voltage sources or to select one of plural 
comparators having mutually different reference voltages. 
FIGS. 12A and 12B show another embodiment in which the detection level is 
rendered variable by the displacement of the sensor 63. The detection 
level, positioned at the center of the sensor 63, is shifted from a level 
81B to 82B by a upward displacement of the sensor 63 as shown in FIG. 12A, 
or shifted to 83B by a downward displacement of the sensor 63 as shown in 
FIG. 12B. In this manner there can be provided three different detection 
levels in this case. 
FIG. 13 shows still another embodiment in which the sensor 63 is composed 
of plural elements arranged horizontally instead of the vertical 
arrangement shown in FIGS. 8A and 8B. In this embodiment, the sensor 
elements 631, 632 and 633 respectively representing the detection levels 
81, 82 and 83 are selected by a switch as shown in FIG. 9. In the present 
embodiment the volume detection is achieved by accumulating the developer 
by the screw 131 as shown in FIG. 8B. 
The number of detection levels for the developer volume explained in 
relation to FIGS. 8A and 8B is not necessarily limited to two or three but 
can be further increased if necessary or desirable. For example there may 
be employed more than three sensor elements constituting the sensor 63, 
for selection by the CPU 61. If the image density is excessively low, the 
CPU 61 stepwise elevates the detection level until it exceeds the present 
developer volume, and effects the toner replenishment up to said detection 
level. On the other hand, if the image density is excessively high, the 
detection level is lowered by a step and the toner replenishment is 
suspended until the developer volume reaches said detection level. The 
detection level is further lowered if the image density is still 
excessively high. 
It is furthermore possible to select one of plural detection levels for 
detecting the developer volume in the developing device in response to the 
optically measured density of a developed reference image formed on the 
photosensitive member or an image recorded therefrom, and to control the 
developer concentration by the replenishment of toner or carrier to the 
developer. 
Such process is employed in another embodiment of the laser beam printer 
shown in FIG. 14, in which the same components as those in FIG. 2 are 
represented by same numbers. 
In FIG. 14, in response to the detection signal 65 from the developer 
volume sensor 63 representing the developer volume in the developing 
device 43, the developer volume detector unit 67 supplies the volume 
detection signal 69 to the CPU 61, which thus releases a drive signal 437 
for the developing device and a carrier control signal 451 as will be 
explained later. 
FIGS. 15A and 15B show two different examples of the developing device 43, 
wherein the container 71 is internally provided with two sensor elements 
631 and 632 constituting the developer volume sensor 63. It is assumed 
that the volume detection is effected at the lower detection level 81 
corresponding to the sensor element 631. When the developer volume becomes 
lower than the level 81 by the consumption of the toner in the developer 
45, the sensor element 631 releases the detection signal 69, in response 
to which the CPU 61 activates the toner hopper to be explained later to 
replenish the developing device with a replenishing developer composed 
solely of toner or containing carrier in a determined proportion, thereby 
restoring the developer volume in the developing device 43 to the level 
81. A similar function is effected also when the higher detection level 82 
corresponding to the other sensor element 632 is selected for the 
developer volume detection. In this manner it is possible to maintain a 
constant toner concentration in the developer by controlling the developer 
volume to the constant detection level, as already explained, for a 
relatively short period corresponding to 100-200 thousand copies of A4 
size. In practice, however, the consumption in the developer 45 is not 
limited to the toner but also takes place for the carrier. 
As the ratio of consumption of toner and carrier varies depending on the 
recording area, the toner concentration in the developer 45 fluctuates 
during a prolonged period if a replenishing developer composed solely of 
toner or containing a determined proportion of carrier is employed. 
However the toner concentration in the developer always increases if the 
amount of carrier in said replenishing developer is less than the amount 
of carrier consumption in the developer 45. 
FIG. 16 shows the developer volume detector unit 67 employed in the present 
embodiment, wherein the sensor elements 631, 632 positioned in the 
developing device 43 show a change in the equivalent inductances 631L, 
632L according to the magnetic property of the carrier present in the 
developer 45. More specifically, the inductance 631L or 632L respectively 
increases when the developer 45 is present at the level 81 or 82. The 
switch 91 selects either the element 631 or 632. When the switch 95 is 
connected to a contact e in response to the selecting signal 95 from the 
CPU 61, the AC power source 101 supplies an AC voltage of a determined 
angular frequency .omega. to the element 631, and also to the amplifier 
103 through the condenser 102. The output signal 105 from said amplifier 
103 is supplied to the rectifying-smoothing circuit 107 composed for 
example of a diode and a condenser to a DC signal 109 of a voltage Vd, 
which is supplied to the comparator 113 together with the reference 
voltage Vr from the variable voltage source 111. 
A resonance circuit is formed by the element 631 and the condenser 102. 
When the developer 45 is present at the lower detection level 81, the 
sensor element 631 shows an increased inductance 631L, causing a resonance 
with the angular frequency .omega. supplied from the AC power source 111. 
Such resonance does not occur, however, if the developer 45 is positioned 
below the detection level 81. 
In case the switch 91 is shifted to the contact f, a similar resonance 
occurs or not respectively when the developer 45 is present at or below 
the detection level 82. 
Such resonance significantly changes the DC voltage Vd, thus enabling 
detection of the presence of the developer 45 at the detection level 81 or 
82 through comparison with a suitably selected reference voltage Vr. The 
volume detection signal 69 obtained from said comparator 113 is supplied 
to the toner hopper drive unit 121 in the CPU 61 to generate the drive 
signal 125 for the toner hopper 431. In this manner the toner 
concentration control explained in relation to FIGS. 15A and 15B can be 
achieved with a sensitivity improved by the afore-mentioned resonance, by 
selecting the detection level 81 or 82 respectively corresponding to the 
sensor 631 or 632 through the CPU 61. 
FIG. 15B shows another example of the developing device 43, which is 
different from the structure of FIG. 15A in that a screw 131 is rotated at 
the volume detection of the developer 45 in order to accumulate said 
developer 45 against an inner wall of the container 71 where the sensor 
elements 631 and 632 are positioned, thereby enabling a more accurate 
volume detection. 
FIG. 17 shows the details of a part of the structure shown in FIG. 14 
wherein the reflective density detector unit 143 is positioned close to 
the photosensitive member 33 in order to optically measure the image 
density developed from an electrostatic latent image formed as will be 
explained later in the reference image area 141 on the photosensitive 
member 33. As already explained in the foregoing embodiments, said 
reflective density detector unit 143 emits a light beam 205 from the 
light-emitting diode 203 positioned in a casing 201 to the photosensitive 
member 33, and the reflected light 207 is received by the photodiode 209, 
as shown in FIG. 5. 
The developing device 43 of the structure shown in FIG. 15B is provided, in 
the outer casing 301 thereof, with two apertures 303 and 305 communicating 
with the container 71. A toner hopper 431 and a carrier hopper 433 are 
mounted on said casing 301 in such a manner that a toner supply aperture 
313 of said toner hopper 431 coincides with said aperture 303 and that a 
carrier supply aperture 317 of said carrier hopper 433 coincides with said 
aperture 305. 
FIG. 18 shows the CPU 61 and the related control units, and FIG. 19 shows 
the related control sequence which will be explained in the following in 
relation to FIG. 18. 
The Steps 901 to 907 are similar to the Steps 701 to 707 shown in FIG. 7 
and are therefore omitted from the following explanation. 
A Step 909 identifies whether the densities D.sub.p and D.sub.o satisfy a 
desired relationship, for example a determined density ratio or density 
difference. If the result of said identification is affirmative indicating 
an appropriate developer concentration, the program proceeds to the 
printing mode (Step 911). 
On the other hand, if said result is negative indicating an inappropriate 
image density, the developer concentration is corrected in the following 
procedure, in which provided are two detection levels for the developer 
volume as already explained in relation to FIGS. 15A and 15B. 
In this state the CPU 61 is capable of identifying whether the detection 
level is located at the level 81 or 82, from the logic state of the 
selecting signal 95 for the switch 91. 
Subsequent to the above-mentioned Step 909, a Step 921 identifies whether 
the density of the developed image is abnormal. If the density is 
excessively low, the CPU 61 identifies the state of the detection level as 
explained above (Step 923) and shifts said detection level from the lower 
level 81 to the higher level 82 by shifting the switch 91 to the contact f 
in response to the selecting signal 95 (Step 925). Then the program 
proceeds to the printing mode (Step 911), in which the toner is supplied 
from the toner hopper 431 until the developer volume reaches the thus 
elevated level 82, whereby the toner concentration in the developer is 
elevated to an appropriate level capable of providing an appropriate image 
density. 
On the otherhand, if the detection level is positioned at the higher level 
82 in the Step 923, an abnormally signal is produced in the Step 931 since 
such situation is not normally expected. 
Also if an excessively low reflective density is detected in the Step 921, 
a Step 941 identifies, from the state of the selecting signal 95, whether 
the switch 91 is connected to the contact e corresponding to the lower 
detection level. If the result of said identification is negative 
indicating that said switch 91 is connected to the contact f, the CPU 91 
controls the selecting signal 95 to shift the switch 91 to the contact e, 
thus selecting the lower detection level 81 corresponding to the sensor 
element 631 (Step 943), and the program proceeds to the printing mode 
(Step 911). In this state the toner replenishment is suspended until the 
developer volume is reduced by consumption to said detection level 81, so 
that the toner concentration is gradually reduced during the use of the 
recording apparatus to maintain an appropriate image density. 
On the other hand, if the Step 941 identifies that the detection level is 
already set at the lower level 81, the CPU 61 releases a carrier control 
signal 451 to supply a determined amount of carrier from the carrier 
hopper 433 (Step 947), thereby reducing the toner concentration with the 
resulting increase of developer volume, and the program proceeds to the 
printing mode in the Step 911. 
During the printing operation in the Step 911, the developer volume in the 
developing device 43 is constantly measured, as already explained in the 
foregoing embodiment, according to a principle previously explained in 
relation to FIGS. 15A and 15B as disclosed in the aforementioned Japanese 
Laid-Open Pat. Nos. Sho 50-19459 and Sho 51-78343, and, upon detection of 
a decrease in the developer volume by the detector unit 67, the motor of 
the toner hopper 431 is energized, only when said developing device 43 is 
activated, to replenish the developing device 43 with a replenishing 
developer composed solely of toner or having a determined carrier ratio 
through the aperture 313 until said detector unit 67 no longer detects 
said decrease of developer. 
As already explained in relation to the foregoing embodiment, such 
concentration controlling operation can be conducted either once every day 
immediately prior to the first printing operation after the start of power 
supply, or every time prior to the re-start of the printing operation 
after it is interrupted for a while, or a regular interval through the use 
of a timer. 
The CPU 61 does not store the state of the selecting signal 95 indicating 
the detection level in certain situations, for example after the start of 
power supply to the recording apparatus. FIG. 20 shows a control sequence 
for the concentration control in such situation. In the following 
description the sequence up to a Step 909 is omitted as it is same as that 
shown in FIG. 19. 
If the Step 909 identifies that the reflective density of the developed 
image is not appropriate, said density is further identified in a Step 
951. If the density is excessively low, the CPU 61 controls the selecting 
signal 95 to connect the switch 91 to the contact f, thereby selecting the 
element 632 corresponding to the higher detection level 82 (Step 953). In 
this state a Step 955 identifies if the developer volume reaches thus 
elevated detection level 82. If the result of said identification is 
negative, the program proceeds to the ordinary printing mode (Step 961). 
On the other hand, if said result is affirmative, indicating a developer 
volume exceeding the higher detection level 82 combined with an 
excessively low image density, an abnormality signal is generated since 
such situation is not anticipated (Step 971). 
On the other hand, if the Step 951 identifies an excessively high image 
density, the CPU 61 controls the selecting signal 95 to connect the switch 
91 to the contact e thereby selecting the lower detection level 81 
corresponding to the sensor element 631 (Step 981). Then a Step 983 
identifies whether the developer volume reaches the thus lowered detection 
level 81. If the result of said identification is affirmative, the program 
proceeds to the printing mode of the Step 961. On the other hand if said 
result is negative, indicating a developer volume less than the lower 
detection level 81 combined with an excessively high image density, the 
motor of the carrier hopper 433 is activated to replenish the developing 
device 43 with a determined amount of carrier through the carrier supply 
aperture 317, thereby reducing the toner concentration (Step 985), and the 
program proceeds to the printing mode in the Step 961. 
During the printing mode in the Step 961, the developer volume detection 
explained in FIGS. 15A and 15B and the toner replenishment are effected in 
the same manner as explained in Step 911 in FIG. 19. 
If the abnormality signal is generated in the Step 931 in FIG. 19 or in the 
Step 971 in FIG. 20, the CPU 61 selects a standard detection level for the 
developer volume detection, by so controlling the selecting signal 95 as 
to connect the switch 91 for example to the contact e, thus selecting the 
lower detection level 81 corresponding to the sensor element 631, and the 
printing mode is thereafter initiated in the Step 911 or 961, whereby the 
toner concentration control is continued under the thus selected standard 
detection level. Such procedure not only avoids the danger of significant 
change in the toner concentration in the developer eventually resulting 
from an erroneous detection in the detector unit 143 caused for example by 
a smear on the photodiode 209 in said detector unit 143, but also allows 
the continued use of the recording apparatus under said toner 
concentration control with such standard detection level, thereby 
eliminating the necessity for immediate repair of the detector unit 143. 
Although the volume sensor 63 in the foregoing embodiment is composed of 
two elements 631 and 632 selectable by a switch, it is also possible to 
constitute said sensor by a single sensing element. 
The number of detection levels for the developer volume explained in 
relation to FIGS. 15A and 15B is not necessarily limited to two but can be 
increased to three or more if necessary or desirable. For example the 
sensor 63 may be composed of three sensor elements selectable by the CPU 
61. If the image density is excessively low, the CPU 61 stepwise elevates 
the detection level until it exceeds the present developer volume, and 
effects the toner replenishment up to said detection level. On the other 
hand if the image density is excessively high, the detection level is 
lowered by step and the toner replenishment is suspended until the 
developer volume reaches said detection level. The detection level is 
further lowered if the image density is still too high, and a determined 
amount of carrier is supplied if the image density is still too high at 
the lowest detection level. The amount of the carrier replenishment need 
not be accurately controlled by the use of several detection levels since 
the toner concentration in the developer can be re-adjusted by appropriate 
change of the detection level. 
In the foregoing description, the reflective density detector unit 143 
detects the density developed from a latent image formed in a reference 
image area 142 on the photosensitive member 33. It is nevertheless 
possible also to transfer said developed image onto a transfer sheet 49 
and to measure the optical density of the thus recorded image by optical 
means similar to the detector unit 143. Furthermore it is possible to 
deposit toner on a separate probe composed of a transparent electrode or a 
metal electrode and to measure the transmission or reflective density of 
the thus deposited toner by similar optical means. In summary, the density 
of the developed image can be directly or indirectly measured by 
appropriate optical means. 
The present invention is applicable not only to the laser beam printer but 
also to any recording apparatus in which an electrostatic latent image 
formed on a photosensitive member is developed with a developer containing 
toner and carrier. For example in a copier in which the latent image is 
formed by known slit exposure process, the latent image in the reference 
image area 141 can be obtained by scanning a reference density pattern 
positioned in a non-image area on the original platen. 
As explained in detail in the foregoing, the present invention provides a 
developer concentration controlling device capable of providing a constant 
optimum image density by comparing the density of a reference image with a 
predetermined reference value and by regulating the detection level for 
the developer volume detection or replenishing the carrier in response to 
the result of said comparison. According to the present invention the 
developer concentration is so controlled as to provide an optimum image 
density through the direct measurement of the image density instead of the 
measurement of the toner concentration in the developer. 
In addition the present invention allows avoidance of rapid change of the 
toner concentration in the developer caused by a failure in the image 
density detector unit or by an excessive replenishment of the carrier.