Method and apparatus of detecting toner concentration of dry developer

A method and apparatus to detect the toner concentration of two component dry developer comprising toner and carrier includes the use of a toner catcher having a conductive member and which is capacitively charged. The developer is applied to the catcher, which traps the toner to determine the toner content.

BACKGROUND OF THE INVENTION 
The invention relates to a method of detecting the toner concentration of 
two component dry developer which is used in the developing of an 
electrostatic latent image, and an apparatus used to carry out the method. 
An electrostatic latent image is converted into a visual image by using two 
component developer. To this end, toner and carrier are intermixed and 
rubbed against each other to charge the toner to a polarity which is 
opposite to the polarity of charge which forms an electrostatic latent 
image. Coulomb force acting between the toner and carrier causes the toner 
to be attached to the carrier, and the developer under this condition is 
applied to a surface to be developed. Carrier usually comprises glass 
beads, magnetic powder or the like while the toner comprises heat 
sensitive, pressure sensitive or solvent gas sensitive resin powder 
containing a color pigment. The purpose of using the carrier in the 
developer is to prevent a photographic fog in a non-image region by the 
Coulomb force which pulls the toner portion attaching to non-image regions 
back to the carrier. In addition, the application of the carrier to the 
surface to be developed achieves a uniform attachment of a suitable 
concentration of toner to the surface. 
In a known electrophotographic apparatus which utilizes the aforementioned 
dry developer, it is readily apparent that a varying toner concentration 
or content of the developer results in a change in the optical density of 
the image formed on a copy. An electrophotographic process to obtain a 
plurality of copies by repeating the developing of an electrostatic latent 
image and the transfer of the developed image onto a transfer sheet is 
disclosed in U.S. Pat. No. 2,951,443, as well as other patents. To enable 
the number of copies to be produced without noticeable degradation in the 
image quality, it is necessary to avoid the erasure of the latent image 
during the developing step. This can be achieved by increasing the toner 
concentration where a semi-conductive carrier such as magnetic powder is 
used. However, an excessively high toner concentration causes a fog to be 
produced in non-image regions and also causes a reduced flowability of the 
developer. Thus a control of the toner concentration is required. It is 
experimentarily established that for a developer including the iron powder 
carrier having a resistivity on the order of 1.times.10.sup.6 to 
1.times.10.sup.8 .OMEGA. cm, a toner concentration in the range from 4 to 
15% by weight is satisfactory. 
To achieve a proper control of toner concentration, a variety of methods 
and apparatus have been proposed which detect the toner concentration of a 
developer contained in a developer vessel to supply toner or developer 
from time to time in order to maintain a proper value of toner 
concentration. The detection can be accomplished in two ways. A first 
technique utilizes the detection of a change in the physical properties of 
the developer such as color, specific gravity, permeability, dielectric 
constant, resistance or the like. However, a change in these properties is 
minimal as compared with a corresponding change in the toner 
concentration, and hence the resulting signal suffers from a poor S/N 
ratio. This disadvantageously necessitates a complex and expensive 
detection arrangement. 
Another detection technique employs the optical determination of the toner 
quantity on a detecting surface which is developed with the developer. The 
detecting surface may comprise a charged insulating layer or an 
electrically energized conductive member. An example of the former is a 
detecting tape which is subjected to corona charging before the developing 
takes place. The required arrangement is complex and requires an extensive 
space to permit the deployment of the tape. As a further example, an 
apparatus is known which regulates the optical density of an image. With 
this apparatus, a developed image on a photosensitive drum is optically 
scanned, and a signal indicative of the maximum optical density is used to 
control the toner concentration. This requires a complicated 
instrumentation circuit, and it is believed that control parameters must 
be varied in accordance with an original. Hence, this approach leads to a 
complex control of the toner concentration. The latter approach is 
exemplified by U.S. Pat. Nos. 3,376,854, No. 3,430,606, No. 3,526,338, No. 
3,527,387, No. 3,604,939, No. 3,635,373, and No. 3,754,821. However, from 
experiments which will be described later, it is found that when the 
developing is effected with a voltage applied to the conductive member, 
the saturation density rises to a substantially high value independently 
of the toner concentration, rendering it difficult to detect and control 
the toner concentration with an effective accuracy. 
Other examples of the latter approach are given in U.S. Pat. No. 3,777,173 
and No. 3,791,744. Essentially, these patents describe a process in which 
a developer including a glass bead carrier is used to effect a cascade 
developing of a toner catcher plate, and the concentration of toner 
trapper by the plate is determined. No positive means is provided which 
applies an electrical influence upon the catcher plate to trap the toner. 
This approach is inapplicable to an apparatus which incorporates a magnet 
brush developing system since it is directed to a cascade developing of 
the catcher plate. 
SUMMARY OF THE INVENTION 
It is a first object of the invention to provide a method of detecting the 
toner concentration of two component dry developer with a high accuracy 
while eliminating the described disadvantages of the prior art. 
It is a second object of the invention to provide an apparatus which is 
optimally adapted to carry out the method of the invention. 
In accordance with the invention, the toner concentration can be detected 
with a high accuracy by developing a toner catcher which is capacitively 
charged with a developer, and determining the developed density of the 
catcher to provide an indication of the toner concentration. The 
indication can be relied upon in controlling the supply of toner in order 
to maintain a proper toner concentration. The apparatus which is used to 
carry out the method requires no special space since a charged conductive 
member can be disposed in a region adjacent to a developer vessel where it 
is susceptible to developing by the developer.

DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 schematically illustrates the principle of the invention. Conductive 
plate 1 is connected with the ground through capacitor 2, which is adapted 
to be charged from a d.c. source 3 connected therewith through switch 4. 
The developing unit is disposed below the conductive plate 1, and 
typically comprises a magnet roller developing unit including magnet 
roller 5 located opposite to conductive plate 1 and connected with the 
ground. A quantity of dry developer 6 is distributed around the roller 5. 
In operation, switch 4 is turned on for a short interval to charge 
capacitor 2. It is desirable that the charging step be completed when 
conductive plate 1 is located out of the developing effect or within a 
time period which is short enough to prevent any developing effect. Charge 
stored across capacitor 2 is transferred to conductive plate 1, which 
therefore attracts toner contained in the developer 6. Charge continues to 
be supplied from capacitor 2 as toner is attracted to conductive plate 1, 
and when a developing takes place over a prolonged period of time, toner 
is trapped by conductive plate 1 in an amount which corresponds to the 
magnitude of charge stored across capacitor 2. The amount of trapped toner 
is determined by photoelectric means to provide an indication of the toner 
concentration of developer 6. It is believed that as the toner 
concentration of developer 6 increases, there is an increased quantity of 
toner which is freed from the toner carrier, i.e., a force of less 
magnitude is sufficient to separate toner from carrier. Thus when the 
capacitance of capacitor 2, the voltage of source 3 and the developing 
period are fixed, a change in the toner concentration can be detected with 
a degree of high accuracy. 
An experiment has been conducted by developing conductive plate 1 while 
maintaining switch 4 closed. Then a large quantity of toner is rapidly 
deposited on conductive plate 1, resulting in a failure to detect a 
differential toner concentration. The same holds true if the capacitance 
of capacitor 2 or the voltage of source 3 is varied. Thus it is concluded 
that the toner concentration cannot be effectively detected with the 
process disclosed in U.S. Pat. No. 3,376,854 unless the described problem 
is overcome. 
FIG. 2 shows one form of toner catcher 7 used in the method of the 
invention. Toner catcher 7 comprises conductive base 8, insulating layer 9 
and conductive surface layer 10 in laminated form. Catcher 7 functions as 
capacitor 2 shown in FIG. 1 and traps toner on its surface layer 10, which 
constitutes a conductive member. The capacitance of catcher 7 can be 
adjusted by varying the thickness of insulating layer 9 or alternatively 
by connecting additional capacitor 11 in shunt or series with toner 
catcher 7. 
FIGS. 3A, B and C graphically show the results of experiments obtained with 
the toner catcher 7 shown in FIG. 2, illustrating the developed saturation 
density D on the ordinate plotted against the applied voltage V shown on 
the abscissa. Three models of toner catcher have been used having 
capacitance per square centimeter of 25, 50 and 100 pF, respectively. They 
are charged for a given interval to varying voltages, and are then 
manually developed by moving a magnet carrying developer thereon close to 
them and maintaining the magnet in position for a sufficient period of 
time. One developer comprises iron powder carrier and 3% by weight of 
toner (corresponding curves shown in dotted lines), another developer 
comprises 10% by weight of toner (shown in solid line). In these Figures, 
it is noted that the developed density for the 3% toner concentration 
varies with both the applied voltage and the capacitance. Differences 
which occur with a change in the toner concentration are seen from solid 
line and dotted line curves shown in FIGS. 3A and B. It will be noted that 
there is a difference in the fog density which prevails at zero applied 
voltage. Thus a fog density can be utilized where it is desired to detect 
a change in the toner concentration at a relatively high level. To prevent 
a triboelectric charging between developer and toner catcher, it is 
advantageous to develop the surface layer of the toner catcher as 
mentioned. 
As will be evident from FIG. 3, a suitable combination of the capacitance 
of the toner catcher and the applied voltage depends on the level of toner 
concentration to be controlled, but it will be noted that a high 
capacitance is advantageous for a low toner concentration while a low 
capacitance is advantageous for a high toner concentration. A higher value 
is preferred for the applied voltage, which is determined in connection 
with the response of a concentration detector. This is because if a 
photoelectric determination of the density is utilized, it is possible 
that the photoelectric output may be reduced to degrade S/N ratio if there 
is sufficient density differential in a region of higher optical density, 
thus resulting in a failure to detect the density differential. A signal 
indicative of the toner concentration can be used to control a toner 
supply mechanism so that a proper toner concentration can be maintained in 
the developing unit. 
The saturation developed density varies with the toner concentration 
because the quantity of toner which attaches to carrier varies an 
mentioned previously. Specifically, if the charge on the toner catcher is 
held constant, there will be a balance reached between the toner quantity 
which is attracted to the surface layer under the influence of 
electrostatic interaction and the toner quantity which is removed or 
scraped off as a result of rubbing against the carrier, the balance value 
being dependent on the degree of toner attachment to the carrier. In other 
words, a saturation will be reached with a quantity of toner which is less 
than that which attaches under the electrostatic force. As a consequence, 
the saturation developed density varies with the toner concentration. 
FIG. 4 shows an apparatus for detecting the toner concentration which is 
constructed according to one embodiment of the invention. The apparatus is 
used with an electrophotographic apparatus which includes developing drum 
12 comprising a conductive drum which is peripherally provided with a 
photosensitive layer. An electrostatic latent image is formed on the drum 
and developed and thereafter transferred onto a transfer sheet to provide 
a copy during the rotation of the drum. Developing unit 13 is located 
adjacent to the periphery of drum 12, and comprises magnet roller 16 which 
carries and retains thereon developer 15 from vessel 14, agitator blades 
17, and toner supply 18. Toner supply 18 includes hopper 20 which contains 
a quantity of toner 19, and peripherally knurled roller 21 disposed below 
the bottom opening of hopper 20. Roller 21 responds to a toner supply 
command, to be described later, by rotating to supply or replenish toner 
19 from hopper 20 to vessel 14. 
In accordance with the invention, drum-shaped toner catcher 22 is disposed 
adjacent to magnet roller 16. In the example shown, toner catcher 22 
comprises conductive roller 23 which is peripherally coated with 
insulating layer 24, on top of which conductive surface layer 25 is 
applied. Surface layer 25 is divided into three segments which are 
electrically connected with three corresponding slip rings 26 mounted on 
the rotating shaft of conductive roller 23 with suitable insulation 
interposed therebetween. Where an adjustment of the capacitance of catcher 
22 is desired, additonal capacitors may be connected between surface layer 
25 and conductive roller 23 as indicated by phantom lines. Sliding brush 
27 is disposed for contact with the three slip rings 26. A d.c. source 28 
is connected between conductive roller 23 and brush 27 to permit a 
charging of insulating layer 24 in its region located between conductive 
roller 23 and that surface layer segment 25 which is connected with a 
particular slip ring 26 which engages the brush 27. The brush 27 is 
located such that a segment of surface layer 25 which is postioned for 
contact with developer 15 cannot be energized. In the present embodiment, 
conductive roller 23 is electrically connected with magnet roller 16 so as 
to assume an equal potential. The conductive drum of developing drum 12 
may also be at the same potential as these rollers, but a source of 
variable developing bias 29 is connected therebetween in the embodiment 
shown. Optical density detector 30 is disposed in the vicinity of catcher 
22 in order to determine the quantity of toner that is trapped on the 
surface layer 25. It comprises light source 31, condenser lens system 32 
and light receiver 33. 
Conductive roller 23 is driven, by drive means not shown, for rotation in a 
given direction at a uniform rate in integral manner with the slip rings 
26, and hence toner catcher 22 experiences one cycle of charging, 
developing and toner concentration detection during its one revolution. 
Detector 30 produces a signal indicative of the toner concentration, which 
selectively activates toner supply 18. If the detector 30 detects a toner 
concentration below an optimum value, it directs a toner supply command to 
the toner supply 18. In response thereto, roller 21 rotates to supply 
toner 19 from hopper 20 to vessel 14. In this manner, a desired toner 
concentration can be maintained within the vessel 14. 
FIG. 5 shows another embodiment in which surface layer 25 is not divided 
into three segments as in FIG. 4, but is a continuous conductive surface 
layer 25A. In this embodiment, toner catcher 22 is charged by turning 
switch 34 on for a short interval during the developing step. Toner 
catcher 22 may be rotated by friction with developer 15. However, if the 
rate of rotation thus achieved is unsatifactory and a sufficient 
developing period cannot be assured, an arrangement as shown in FIG. 5 may 
be used. As shown, cam disc 35 is mounted on the rotating shaft 23a of 
conductive roller 23 for engagement with stop 36. The stop 36 may be moved 
to the disengaged position by the action of solenoid 37 to permit an 
intermittent rotation of toner catcher 22 so that sufficient developing 
period can be assured. 
FIG. 6 shows a circuit diagram of a charging circuit for toner catcher 22. 
In this instance, the toner catcher 22 shown in FIG. 5 is employed. A 
series combination of normally open switch 38 and resistor 39 is connected 
between conductive roller 23 and surface layer 25A. A single pole double 
throw switch 40 has its movable contact connected for ganged movement with 
switch 38 and is normally thrown to contact 40a which connects charging 
capacitor 41 across roller 23 and layer 25A. Source 28 is connected with 
the other contact 40b of switch 40 and with roller 23. 
In operation, when switch 38 is closed, switch 40 is changed to the other 
contact 40b, whereby capacitor 41 is connected with and charged by source 
28. The closure of switch 38 causes a discharge of toner catcher 22 
through resistor 39. Subsequently when switch 40 is switched again, 
capacitor 41 acts as a secondary power source to charge toner catcher 22, 
thus enabling toner to be trapped by surface layer 25A. The use of 
capacitor 41 avoids an excessive power dissipation. The provision of 
resistor 39 prevents an instantaneous discharge of capacitor 41 which 
might otherwise occur as a result of a mismatched switching of switches 
38,40 such that switch 38 remains closed when the movable contact of 
switch 40 is thrown to contact 40a. Nevertheless, the presence of resistor 
38 does not pose any problem when toner catcher 22 is to be discharged 
since certain time allowance is possible. However, it will be understood 
that resistor 39 can be dispensed with if switches 38, 40 are separately 
controlled. 
FIG. 7 shows a further form of toner catcher. In this instance, toner 
catcher 22 comprises rockable conductive arm 42 having arcuate free and 
42a on which insulating layer 24 and conductive surface layer 25 are 
sequentially laminated. Toner catcher 22 is developed while it is located 
adjacent to magnet roller 16, and is then rocked to position 42A shown in 
phantom lines where optical concentration detector 30 operates to detect 
the quantity of toner trapped on the surface layer 25. This arrangement 
can be used with any one of the charging circuits shown in FIGS. 4 to 6. 
FIG. 8 shows still another form of charging circuit. Here, a separate 
capacitor 43 is connected between conductive roller 23 and surface layer 
25 of toner catcher 22 and is also connected with source 28 through a 
series combination of power switch 34 and polarity reversal switch 44 so 
as to apply a voltage of either polarity across the toner catcher 22. This 
permits the surface layer 25 to repel toner which has attached thereto, 
thus increasing the accuracy and response rate of detection. Where such 
repellent is unavailable, toner is removed from toner catcher 22 by 
carrier through a slow process. 
FIG. 9 shows another form of toner catcher which simply comprises 
conductive roller 23 alone. Since no charge can be stored on catcher 
roller 23 itself, capacitor 45 is connected between conductive roller 23 
and magnet roller 16 to provide the charge storing capability. The 
charging circuit of FIG. 9 is similarly constructed as shown in FIG. 8, 
but may be replaced by any one of the circuits shown in FIGS. 4 to 6. The 
configuration of toner catcher 22 need not be a drum. 
FIG. 10 shows an alternative form of toner catcher. It comprises conductive 
roller 23, insulating layer 24 which is laminated thereon, and an array of 
alternately disposed conductive surface layers 25 and grounded conductive 
layers 46. Surface layer 25 is adapted to be charged while grounded layer 
46 is maintained at the ground potential in order to provide a correction 
of fogging density. With this toner catcher, a developed density 
corresponding to the applied voltage is detected on surface layer 25 while 
a fogging density is detected on the grounded layer 46. If a single 
optical density detector is used with this toner catcher, it will produce 
a signal a shown in FIG. 10B, which enables a change in the toner 
concentration to be detected from a change in the difference between the 
developed density and the fogging density. It is to be understood that a 
pair of toner catchers may be provided, one having a charged surface layer 
and the other having a grounded conductive layer, for cooperation with 
separate optical density detectors which detect the developed density and 
the fogging density, respectively. 
It is to be noted that in the described embodiments, means disclosed in 
connection with a particular embodiment can also be used in another 
embodiment. As a modification, an experiment has been conducted to effect 
the developing while applying an a.c. voltage to toner catcher 22. A 
pattern corresponding to the a.c wave is developed on the catcher. It is 
found that the density of the pattern is not significantly influenced by 
the magnitude of the a.c. voltage applied, but that the pitch of the 
undulating pattern varies with the speed of movement of the magnetic brush 
or the carrier which attaches to the magnet roller 16 in the form of tuft. 
This indicates that the attachment and the separation of toner takes place 
at a relatively high rate. The average value of the toner concentration 
was close to a fogging density, and hence it is found possible to detect 
the toner concentration with this technique since the fogging density 
corresponds to the toner concentration. 
It is to be understood that the invention is not limited to the embodiments 
described above, and that a number of changes and modifications can be 
made therein. By way of example, the optical density can be detected by 
means other than the photoelectric detector described, as by the 
determination of the resistance or charge of trapped toner. In addition, 
the conductive member of the toner catcher on which toner is trapped may 
be coated with a very thin insulating layer for purpose of protection. 
Furthermore, the invention is not limited to its use in an 
electrophotographic apparatus, but is equally applicable to other systems 
such as facsimile system in which an electrostatic latent image formed is 
developed with two component dry developer. Finally, though toner catcher 
is shown in a rotating drum configuration in FIGS. 4 to 10A, it may be 
shaped as a flat plate for cascade developing or shaped into any other 
form combined with a suitable developing process.