Apparatus for measuring developer density

Developer density measuring apparatus comprising a transparent detection window confronting the interior of a developing device, illumination means for illuminating developer comprising a toner and a carrier accommodated in said developing device, and density determining means for determining developer density by the amount of light reflected from said developer measured through said transparent detection window, and wherein the surface of the transparent detection window on the side confronting the interior of the developing device is roughened.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention relates to an apparatus for measuring developer 
density, and specifically relates to an apparatus for measuring developer 
density provided with a transparent detection window confronting the 
interior of a developing device, which measures density of a developer by 
detecting the amount of light reflected from a developer through said 
transparent detection window. 
Description of the Related Art 
In image forming apparatus such as copiers, printers and the like using 
two-component developers comprising a toner and a carrier, the toner and 
carrier accommodated in the developing device are mixed, to as to be 
charged and circulated within the developing device. At a position 
opposite the photosensitive member, charged toner adheres to an 
electrostatic latent image formed on the surface of a photosensitive 
member, thereby developing said latent image. 
At this time, the carrier is maintained in an almost constant amount 
because it is not supplied for developing and only the toner is 
transported. In contrast, toner is gradually depleted due to consumption 
in the developing process, such that the density of the developer 
accommodated in the developing device, i.e., the toner weight mix ratio 
relative to the carrier, is gradually reduced. 
Accordingly, in order to maintain a suitable developer density it becomes 
necessary to measure the density of the developer within the developing 
device and replenish a suitable amount of toner in accordance with the 
measurement results. Thus, various devices have been proposed for 
measuring the density of a developer accommodated in a developing device. 
For example, U.S. Pat. Nos. 5,117,259 and 5,383,007 disclose devices for 
measuring developer density by optical means. 
The aforesaid devices illuminate the developer accommodated in a developing 
device through a transparent detection window confronting the interior of 
the developing device, measure the amount of light reflected from the 
developer through said transparent detection window via an optical sensor 
or the like, and determine the density of the developer via said measured 
amount of reflected light. In contrast to the carrier which absorbs light, 
toner has the characteristics of reflecting light, such that the amount of 
light reflected from the developer is greater as the developer density 
increases (i.e., the toner content is higher), and said amount of 
reflected light is less as the developer density decreases (i.e., the 
toner content is lower). 
In the aforesaid devices, an electrically conductive layer is provided on 
the surface of the transparent detection window on the side confronting 
the interior of the developing device, so as to apply a bias voltage 
having the same polarity as the toner charge polarity to said electrically 
conductive layer. The aforesaid bias voltage is applied in order to 
prevent accurate density measurement from being impossible due to the 
light reflected by the charged toner which electrostatically adheres to 
the surface of the transparent detection window 
In the aforesaid devices, there are times when accurate developer density 
measurement cannot be accomplished due to the condition of the transparent 
detection window. 
Since the transparent detection window is confronting the inside of the 
developing device, the surface of the window is unavoidably subject to 
friction-induced flaws due to rubbing on the window of the toner 
circulating within the developing device. Friction-induced flaws cause 
scattering of the reflected light, and the extent of said flows increases 
as developing is repeated, such that the amount of reflected light 
measured through the transparent detection window is reduced without 
correlation to the actual developer density to the extent that accurate 
density measurement becomes no longer possible. 
On the other hand, when an electrically conductive layer is provided on the 
surface of the transparent detection window such as the aforesaid devices, 
the electrically conductive layer is scraped off by the rubbing of the 
developer, such that the bias voltage is no longer suppliable. Conductive 
layers formed on the surface of the transparent detection window, for 
example, by vapor deposition of indium oxide, titanium oxide or the like, 
are subject to wear via rubbing by the developer. When wear of the 
conductive layer progresses such that the layer is scraped off and the 
bias voltage is no longer suppliable, charged toner electrostatically 
adheres to the surface of the transparent detection window so as to 
prevent accurate density measurement as previously described. Furthermore, 
transparent detection windows provided with a conductive layer require 
periodic replacement, thereby increasing the maintenance cost. 
SUMMARY OF THE INVENTION 
In view of the previously described disadvantages, an object of the present 
invention is to provide a developer density measuring apparatus capable of 
accurate and stable density measurement by reducing the effects of 
friction-induced flaws on the surface of the transparent detection window 
due to rubbing of the developer. 
Another object of the present invention is to provide developer density 
measuring apparatus capable of accurate and stable density measurement by 
preventing wear of the conductive layer on the transparent detection 
window due to rubbing of the developer. The present invention further 
provides a developer density measuring apparatus which, by virtue of the 
aforesaid objects, either extends the replacement period of the 
transparent detection window having a conductive layer or does not require 
replacement. 
The aforesaid first object is achieved by providing a developer density 
measuring apparatus comprising a transparent detection window confronting 
the inside of a developing device, illumination means for illuminating 
developer comprising a toner and a carrier accommodated in said developing 
device, and density determining means for determining developer density by 
the amount of light reflected from said developer measured through said 
transparent detection window, and wherein the surface of the transparent 
detection window on the side confronting the inside of the developing 
device is roughened. 
The aforesaid second object is achieved by providing a developer density 
measuring apparatus comprising a transparent detection window confronting 
the inside of a developing device, illumination means for illuminating 
developer comprising a toner and a carrier accommodated in said developing 
device, density determining means for determining developer density by the 
amount of light reflected from said developer measured through said 
transparent detection window, electrically conductive layer provided on 
the surface of said transparent detection window on the side confronting 
the interior of the developing device, and bias supplying means to supply 
a bias voltage having the same polarity as the toner charge polarity to 
said electrically conductive layer, and wherein a dielectric layer is 
provided on the electrically conductive layer of said transparent 
detection window. 
According to the aforesaid first developer density measuring apparatus, it 
is difficult to produce new flaws in the surface of the transparent 
detection window even when rubbed by developer, and the amount of measured 
reflected light does not change greatly even when flaws are produced on 
said surface. Thus, accurate and stable developer density measurement is 
possible. 
According to the aforesaid second developer density measuring apparatus, 
there is no wearing of the electrically conductive layer even when rubbed 
by the developer. Thus, accurate and stable developer density measurement 
is possible, and the replacement period of the transparent detection 
window is prolonged or such replacement is not required, thereby 
decreasing the maintenance cost.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First Embodiment 
A first embodiment of the present invention is described hereinafter with 
reference to the accompanying drawings. 
1) Construction 
FIGS. 1 and 2 are section views of developing device 8 of a copying machine 
of an electrophotographic type. Developing device 8 comprises the three 
parts of developing unit 20, mixing unit 30, and replenishing unit 60. 
The electrophotographic copying apparatus provided with developing device 8 
has a well known construction such as that disclosed in U.S. Pat. No. 
5,383,007. Developing device 8 is not only suitable for copying apparatus, 
but is also suitable for general image forming apparatus of 
electrophotographic types including electrophotographic printers and the 
like. 
(1) Developing unit 20 
Developing unit 20 is positioned opposite photoconductive member 2, and 
houses sleeve roller 21. Sleeve roller 21 is driven by motor M1 so as to 
rotate in the direction indicated by the arrow in FIG. 2. Magnetic roller 
22 is housed within sleeve roller 21 in a stationary, non-rotating state. 
(2) Mixing unit 30 
Mixing unit 30 comprises first mixing path 31 adjacent to developing unit 
20, and second mixing path positioned behind first mixing path 31. First 
mixing path 31 and second mixing path 32 are separated by partition 33, 
and are connected via communicating paths 34 and 35 provided on both ends 
of said partition 33. 
Bucket roller 36 is accommodated in first mixing path 31. Bucket roller 36 
is driven by motor M2 so as to rotate in the arrow direction in FIG. 2. 
Mixing screw 37 is provided in second mixing path 32. Mixing screw 37 is 
driven by the motor M2 as same as the bucket roller 36, so as to rotate in 
the arrow direction in FIG. 2. 
Magnet support member 41 is fitted at a position near communicating path 35 
of shaft 38 of mixing screw 37. Magnet support member 41 comprises two 
fan-shaped protrusions which protrude in mutually opposite directions 
encircling shaft 38, and magnets 45 and 46 are provide at the protruding 
ends of said two fan-shaped protrusions. Magnet support member 41 rotates 
on shaft 38 in conjunction with the rotation of mixing screw 37. 
Detection plate 39 is mounted on the portion of shaft 38 protruding from 
the developing tank. Detection plate 39 rotates in conjunction with the 
rotation of mixing screw 37, so as to be detected by photointerrupter 40. 
The rotational positions of the aforesaid magnets 45 and 46 are detected 
by photointerrupter 40. 
Developer density measuring sensor 50 (hereinafter referred to as "sensor 
50") is provided at a position opposite magnet support member 41 on the 
back wall (the wall opposite partition 33) of second mixing path 32. 
Sensor 50 is arranged so as to confront transparent detection window 54 at 
a region at which magnets 45 and 46 pass in conjunction with the rotation 
of magnet support member 41. The construction of sensor 50 is described 
later. 
(3) Replenishing unit 60 
Replenishing unit 60 is positioned behind mixing unit 30, and is connected 
to second mixing path 32 via resupply aperture 61. Transport screw 62 is 
provided in replenishing unit 60. Transport screw 62 is driven by motor 
M3, so as to rotate in the arrow direction in FIG. 2. A toner hopper (not 
illustrated) is connected to replenishing unit 60 to supply toner from 
said hopper to replenishing unit 60. 
(4) Sensor 50 
FIG. 3 is a section view of sensor 50. Sensor 50 comprises housing 51, 
light emitter 52 and light receiver 53 anchored within housing 51, and 
transparent detection window 54 (hereinafter referred to as "detection 
window 54") covering light emitter 52 and light receiver 53. Sensor 50 is 
arranged such that detection window 54 is disposed on the rear wall of 
second mixing path 32 confronting the interior of the developing device, 
as previously described. 
The surface of detection window 54 at least on the side confronting the 
interior of the developing device is formed of a material 
triboelectrically charged to the same polarity as the toner charge 
polarity, so as to prevent electrostatic adhesion of charged toner on the 
surface of detection window 54. Examples of such materials having a 
tendency for positive charging include glass, acrylic resin, acetate resin 
and the like; examples of such materials having a tendency for negative 
charging include fluororesin such as PFA, vinyl chloride resin, polyether 
sulfone and the like. 
2) Operation 
(1) Developing 
Mixing unit 30 accommodates developer comprising a toner and a carrier. 
When bucket roller 36 and mixing screw 37 start rotation, the developer in 
mixing unit 30 is mixed as it is transported, so as to circulate within 
mixing unit 30 on the route indicated by the arrow in FIG. 1. The toner 
and carrier are charged to mutually opposite polarities via the aforesaid 
mixing, and are circulated in this charged state. 
A portion of the developer circulating in mixing unit 30 is supplied to the 
exterior surface of sleeve roller 21 via bucket roller 36 in first mixing 
path 31. The developer supplied to sleeve roller 21 is maintained on the 
exterior surface of said sleeve roller 21 via the magnetic force exerted 
by magnet roller 22, and is supplied toward photosensitive member 2 in 
conjunction with the rotation of sleeve roller 21. At a position 
confronting photosensitive member 2, the charged toner adheres to an 
electrostatic latent image formed on the surface of photosensitive member 
2, thereby developing said latent image. At this time, the carrier is 
returned to mixing unit 30 along with the toner not supplied for 
developing, and the circulation is repeated. 
(2) Density measurement 
In the apparatus of the first embodiment, the density of the developer in 
the second mixing path 32 is measure using sensor 50. Sensor 50 
illuminates the developer in second mixing path 32 via light emitter 52, 
the light reflected from the developer is received by light receiver 53, 
and a voltage signal corresponding to the amount of received light is 
output to CPU 100 (refer to FIG. 2). CPU 100 determines the density of the 
developer based on the output signal of sensor 50. 
Density measurement is summarized below but details are omitted from the 
present discussion inasmuch as it is accomplished by well known sequences 
as described in U.S. Pat. No. 5,383,007. 
A portion of the developer circulating in mixing unit 30 forms a magnetic 
brush maintained by magnets 45 and 46 in second mixing path 32. The 
magnetic brush alternatingly sweeps the surface of detection window 54 in 
conjunction with the rotation of magnet support member 41 (refer to FIG. 
2). 
In CPU 100, the output from sensor 50 is readout when the magnetic brush 
maintained by magnet 46 sweeps detection window 54, and developer density 
is determined by comparing said output with a predetermined threshold 
value. The thickness of the magnetic brush maintained by magnet 46 is 
normally constant, such that accurate density measurement is accomplished 
based on the light reflected from a normally constant amount of developer 
regardless of the amount of developer within second mixing path 32. 
Furthermore, the magnetic brush maintained by magnet 45 removes the 
electrostatic attraction of developer adhering to the surface of detection 
window 54, and is not used for developer density measurement. 
CPU 100 determines insufficient density when the output of sensor 50 drops 
below the threshold value, and outputs instructions to resupply toner from 
the toner hopper to replenishment unit 60. 
Toner resupplied for the toner hopper to toner replenishment unit 60 is 
transported by transport screw 62 and supplied from supply aperture 61 to 
second mixing path 32. 
Sensor 50 may be mounted on developing unit 20, so as to measure the 
density of developer in developing unit 20. 
3) Processing of detection window 54 and the other 
Since the surface of detection window 54 is rubbed by developer circulating 
within the developing device, it is unavoidably subjected to damage. When 
the surface of detection window 54 becomes flowed by such rubbing, the 
window surface scatters the light reflected from the developer, and as 
developing is repeated, such flaws increase causing a reduction in the 
amount of reflected light received by the light receiver regardless of the 
actual developer density. As a result, the signal level output from sensor 
50 to CPU 100 drops, and accurate developer density measurement cannot be 
achieved. 
In the apparatus of the first embodiment, the surface of detection window 
54 on the side confronting the interior of second mixing path 32 is 
processed to a roughness having a mean depth (Rp) of 50 .mu.m. The 50 
.mu.m value is equal to the diameter of the carrier particles used in the 
apparatus of the first embodiment. The carrier particle size is rather 
larger than the toner particle size (toner particle size is generally 
3.about.12 .mu.m), and the carrier comprises about 90% of the developer. 
Thus, when the surface of detection window 54 is roughened under the 
aforesaid conditions, it resembles the state of a window surface damaged 
by rubbing of developer. 
FIGS. 4a to 4c are graphs respectively showing the relationship between the 
number of developments and sensor output; the vertical axis expresses 
sensor output, and the horizontal axis expresses the number of 
developments. In the graphs of FIGS. 4a to 4c, developer density is 
normally constant. 
FIG. 4a is a graph showing the change in output of a conventional sensor 
using a smooth flat-surfaced detection window. From this graph it can be 
understood that as the number of developments increases the window surface 
is increasingly damaged, precipitously decreasing sensor output from its 
initial level. The reduction of sensor output is putted on the brakes over 
10,000 developments, after which sensor output stabilizes, because the 
damage to the window surface does not progress over a certain level. 
FIG. 4c is a graph showing the change in output of sensor 50. Since the 
surface of detection window 54 is roughened in sensor 50 as previously 
described, the flawing condition of said surface is initially near that 
condition at which sensor output is stable, such that the change from the 
initial sensor output is slight and remains stable. 
The surface of detection window 54 is randomly roughened to minimize the 
change in sensor output and stabilize the output. When compared to such 
randomly roughened surfaces, a surface roughened at regular spacing has 
room for new damage induced by developer rubbing on the surface of the 
detection window. 
FIG. 4b is a graph showing the change in sensor output when the surface of 
the detection window is roughened at uniform spacing. Comparison of the 
graphs of FIGS. 4b and 4c discloses a slight drop from the initial sensor 
output in the case of a surface regularly roughened at uniform spacing. 
Furthermore, in the case of a window surface regularly roughened at uniform 
spacing, the value of said spacing is about identical as the value of the 
mean depth (Rp) in the roughening, such that new damage is unlikely and 
fluctuation of sensor output is minimized. 
The degree of roughening should be at maximum less than a mean depth of 80 
.mu.m because the common currently used carrier size is 30.about.80 .mu.m. 
FIG. 5 is a graph showing the relationship between developer density and 
sensor output. The vertical axis expresses sensor output, and the 
horizontal axis expresses developer density. 
The solid line in the graph represents the output of a conventional sensor 
using a smooth flat-surfaced detection window, and the dashed line 
represents the output of sensor 50. The diagonal line region A represents 
the threshold value level for determining developer density when a 
conventional sensor is used in a developer density measuring apparatus. 
Diagonal line region B represents the threshold value level of the 
apparatus of the first embodiment using sensor 50. 
As shown in the graphs, sensor 50 has reduced output at equivalent 
developer density compared to a conventional sensor using a smooth 
flat-surface detection window. This reduction is due to scattering of 
light reflected from the developer by the surface of a detection window 
processed by roughening. 
Therefore, in the apparatus of the first embodiment, the threshold value 
for determining developer density is set lower than an apparatus using a 
conventional sensor. As can be understood from FIG. 5, although 
insufficient density is determined when developer density drops below 8% 
regardless of the apparatus, the lower limit of the threshold value is set 
relatively lower at 1.5 V in the apparatus of the first embodiment 
relative to the 2.0 V of an apparatus using a conventional sensor. 
Of course, the level of the threshold value is not limited to the numerical 
values of the present embodiment. 
Second Embodiment 
A second embodiment of the invention is described hereinafter with 
reference to the accompanying drawings. 
(1) Construction 
FIG. 6 is a section view of developer density measuring sensor 70 
(hereinafter referred to as "sensor 70") of the second embodiment. 
Sensor 70 comprises housing 71, light emitter 72 and light receiver 73 
attached within housing 71, and transparent detection window 74 
(hereinafter referred to as "detection window 74") covering said light 
emitter 72 and light receiver 73. 
Detection window 74 comprises a three-layer construction as shown in FIG. 
7, i.e., transparent resin 75, conductive layer 76, and dielectric layer 
77. 
Conductive layer 76 is formed by vapor deposition of indium oxide on the 
surface of transparent resin 75. Bias voltage source 78 is connected to 
conductive layer 76, which supplies thereto a bias voltage Vw having a 
polarity identical to the polarity of the charged toner. The material of 
conductive layer 76 is not limited to indium oxide, and may also be a 
material such as titanium oxide, tin oxide and the like. Alternatively, a 
conductive film may be adhered to the surface of detection window 75 
instead of the aforesaid coating. 
Dielectric layer 77 is provided on conductive layer 76. Dielectric layer 77 
is formed of polyethylene resin (PET), and is adhered to conductive 
surface 76 by, for example, a heat-hardening adhesive agent having light 
transmitting characteristics. Dielectric layer 77 may be formed of 
polyvinylidene fluoride or the like instead of PET, or a transparent glass 
may be used. 
The construction of the developing device, CPU and the like is identical to 
those of the first embodiment. The connections and arrangements of sensor 
70 with the aforesaid components is also identical to that of sensor 50 of 
the first embodiment. 
2) Operation 
The sequence of the developing operation and measuring developer density is 
identical to those of the first embodiment and further discussion is 
omitted. 
When measuring the light reflect from the developer through a detection 
window as in the apparatus of the first and second embodiments, preventive 
measure must be devised relative to electrostatic adhesion of charged 
toner to the surface of the detection window. 
When toner adheres to the detection window, most of the light from the 
light emitter is reflected by the adhered toner. Thus, the amount of 
reflected light received by the light receiver is not reduced even when 
developer density is actually reduced and the absolute amount of toner is 
insufficient, such that developer density cannot be accurately determined, 
and toner replenishment cannot be achieved. 
In the apparatus of the second embodiment, toner adhesion on detection 
window 74 is prevented by a bias voltage application method. That is, when 
a developing operation starts, a bias voltage Vw is supplied from bias 
power source 78 to conductive layer 76 of detection window 74, and 
adhesion of the toner to detection window 74 is prevented by the repulsion 
action between the bias and the toner charge. 
At this time, a disadvantage arises in conventional apparatus insofar as 
the conductive layer is scrapped off and the bias voltage is no longer 
suppliable due to the rubbing of the developer on the window surface 
because the conductive layer is only a coating on the surface of the 
detection window (transparent resin or the like). 
FIGS. 8a and 8b are graphs respectively showing the relationship between 
the number of developments and sensor output, such as the graphs of FIGS. 
4a to 4c. The graph of FIG. 8a shows the change in sensor output of a 
conventional sensor using a detection window the surface of which is 
provided with a conductive layer coating. The graph of FIG. 8b shows the 
change in sensor output of sensor 70. 
As shown in the graph of FIG. 8a, in a conventional sensor, when the number 
of developments exceeds a certain number, sensor output rises 
precipitously. This output rise is due to wear on the conductive layer of 
the detection window by the rubbing of the developer during repeated 
developments, which leads to the scraping off of said layer, and 
preventing the bias voltage from being supplied. 
In contrast, in sensor 70, a dielectric layer 77 having a degree of 
hardness relatively higher than the conductive layer is provided on 
conductive layer 76, such that the conductive layer 76 does not become 
worn by the developer. Thus, stable and accurate density measurement is 
possible over a long term, as shown in the graph of FIG. 8b. 
Even when dielectric layer 77 is provided over conductive layer 76, the 
function of preventing toner adhesion is not adversely affected because a 
charge having a polarity the same as the bias applied conductive layer 76 
is realized by the dielectric polarization on the surface on the side 
opposite the conductive layer 76 of dielectric layer 77, i.e., on the 
surface confronting the interior of the developing device. 
The drop in sensor output early stage number of developments seen in FIG. 
8b is due to the scattering of light reflected from developer by the 
damage to the surface of the detection window due to rubbing of the 
developer (refer to FIG. 1). Thus, it is possible to accurately measure 
density with scant fluctuation of sensor output by subjecting the surface 
of dielectric layer 77 to a roughening process identical to that of 
detection window 54 of sensor 50 in the first embodiment. 
According to the apparatus of the second embodiment, the replacement period 
of the conventional detection window requiring replacement periodically to 
due to wearing of the conductive layer is prolonged, and in some 
circumstances replacement is unnecessary, thereby reducing maintenance 
costs. 
Although the present invention has been described with the preferred 
embodiments thereof with reference to the accompanying drawings, it is to 
be noted that various changes and modifications are apparent to those 
skilled in the art. Such changes and modifications are to be understood as 
included within the scope of the present invention as defined by the 
appended claims, unless they depart therefrom.