US3897748A

US3897748A - Apparatus for controlling toner concentration of developer in electrostatic development

Info

Publication number: US3897748A
Application number: US470362A
Authority: US
Inventors: Masao Hirata; Shigeru Inowa; Tatsuo Ohto; Kiyoshi Kimura
Original assignee: Konica Minolta Inc
Current assignee: Konica Minolta Inc
Priority date: 1973-05-25
Filing date: 1974-05-16
Publication date: 1975-08-05
Anticipated expiration: 1992-08-05
Also published as: DE2425060B2; JPS5010141A; DE2425060C3; DE2425060A1; GB1475547A; JPS5246095B2

Abstract

The invention is directed to an improved apparatus for controlling toner concentration of a developer comprising a developer receptacle; means in the form of a magnetic brush for conveying the developer through a developing state; an electrically conductive rotatable probe placed in contact with said brush whereby toner in the developer is attached to said probe; means for measuring the amount of toner attached to the probe, said measuring means comprising a light source to illuminate said probe and a photoelectric element to detect the light from said probe; and means for controlling a replenishing amount of the toner to said receptacle based on the result of the measurement.

Description

United States Patent Hirata et a].

APPARATUS FOR CONTROLLING TONER CONCENTRATION OF DEVELOPER [N ELECTROSTATIC DEVELOPMENT Inventors: Masao Hirata; Shigeru Inowa;

Tatsuo Ohto; Kiyoshi Kimura, all of Tokyo, Japan Konishiroku Photo Industry Co. Ltd., Tokyo, Japan Filed: May 16, 1974 Appl. No.: 470,362

Assignee:

Foreign Application Priority Data May 25, 1973 Japan 48-57831 US. Cl. 118/7; 117/175; 118/637; ZZZ/DIG. 1; 1l7/17.5

Int. Cl. B05C 11/00; G030 15/00 Field of Search 118/636, 637, 7, 8, 9, 118/10 References Cited UNITED STATES PATENTS 51 Aug. 5, 1975 3,610,205 10/1971 Rarey et a1, 118/637 3.707.134 12/1972 Gawl'on r r 113/7 3,791,744 2/1974 Emy et a]. i, 356/201 3,830,401 8/1974 Benwood et a1. 1. 118/637 OTHER PUBLICATIONS Research Disclosure, Triboelectric Toner Moniton," 53058-0044, No. 103, Nov. 1972, page 35.

Primary Examiner-Mervin Stein Assistant E,\aminerDouglas Salser Attorney, Agent, or Firm-Bierman & Bierman [57] ABSTRACT The invention is directed to an improved apparatus for controlling toner concentration of a developer comprising a developer receptacle; means in the form of a magnetic brush for conveying the developer through a developing state; an electrically conductive rotatable probe placed in contact with said brush whereby toner in the developer is attached to said probe: means for measuring the amount of toner attached to the probe, said measuring means comprising a light source to illuminate said probe and a photoelectric element to detect the light from said probe; and means for controlling a replenishing amount of the toner to said receptacle based on the result of the measurement.

10 Claims, 5 Drawing Figures PATENTED AUB 51975 SHEET FIG.

SHEET PATENTEDAUB 5W5 $29 6 T Emod m5 2 628 M935 058 5 523 6 5528 52B 0 m wo mi? F TONER EBN'TETGT ELECTRIC CHARGE DENSITY TONER DENSITY STIRRING TIME(MINUTES) F I G. 3

ELECTRIC CHARGE DENSITY O2468l0l2l4|6l82 NUMBER OF COPiED SHEET (XIOOO) PATENTEDAUG 51975 SHEET A m o 858mm Ni. 20 outmomwo tmzwo 150.?

DISTANCE BETWEEN THE SLEEVE AND THE PROBE (L) APPARATUS FOR CONTROLLING TONER CONCENTRATION OF DEVELOPER IN ELECTROSTATIC DEVELOPMENT This invention relates to an electrophotographic copying process, and more particularly to an apparatus for controlling the toner content in the developer that consists of two or more components. In the ordinary electrophotographic copying processes, as is generally known, if the toner content in the developer is increased, the toner particles could deposit even in the area that should become the blank portion of the image, and this causes fogging, resulting in deteriorated image quality, while if the toner content is low, the image is faded generally with the result that the fine portion or solidblack portion (portion where the black area is wide) fails to be reproduced faithfully. It is therefore required to maintain the toner content in the developer constant, but since there are various types of originals which vary widely from each other in toner consumption, it is hardly possible to maintain the toner content constant by merely replenishing toner at constant rate irrespoctively to the optical density of the original. In order to solve this problem, various methods have been proposed for controlling the toner concentration by photoelectrically or electrically measuring the toner concentration in the developer and replenishing the toner automatically based on the measurement. Among these methods are the following:

I. A method in which the toner concentration is controlled based on the measurment of variation of color or density of the developer.

2. A method in which the electric conductivity of the developer is measured and thereby the toner con centration is controlled.

3. A method of measuring specific gravity of the de veloper.

4. A method in which by the measurment of variations of characteristics other than the above, such as fluidity of the developers, is caused by differences of toner content.

5. A method in which a probe (a member for collecting thereon the toner to be detected) which has been electrically charged or impressed with a voltage is inserted into the developer and the amount of toner which has deposited on said probe is detected.

All of these controlling methods, however, have respectively their own defects such as mentioned below:

1. As color variation is limited within a small range,

it is difficult to obtain high precision.

2. Electroconductivity of the developer is low and noise is high.

3. Measurement of weight and volume in the apparatus is easily affected by vibration.

4. This method is also suspectible to vibration.

5. A specific charging means is required. Also, as the probe is used repetitively, means is required for removing toner which has deposited on the probe in each use. Hence, the apparatus is complicated in mechanism and enlarged in size. Further, even if the density of electric charge retained by the toner, charged through friction with the carrier, is changed due to relative humidity, fatigue of developer and difference of toner consumption etc, toner is forced to be deposited electrostatically on the probe because the Coulomb force due to the charge impressed on the probe is considerably higher as compared with the force between toner and carrier.

Therefore, although it is possible with the above-said methods to merely keep constant the toner content in the developer, these methods are quite ineffective for excluding the influence of change of relative humidity and other factors to the image.

An object of the present invention is to provide a novel toner content controlling method which is completely free from the defects of the above-mentioned prior art methods. The above object is accomplished by an apparatus which comprises a developer receptacle. means for conveying the developer to and from a developing state, an electrically conductive rotatable probe positioned at the location where said probe contacts the developer being conveyed whereby toner in the developer attaches itself to said probe, means for measuring an amount of the attached toner, and means for controlling a repremishing amount of the toner to said receptacle based on the measurment.

FIG. 1 shows an embodiment of the present invention.

FIG. 2 is a graph showing the relationship among the developer stirring time, electrostatic charge densities retained by toner particles, and an amount of the toner deposited on the electrically conductive probe, as obtaind by use of the apparatns of FIG. 1.

FIG. 3 is a graph showing variation of toner content in the developer and density of electric charge on the toner, as observed while the apparatus of FIG. 1 is operated to obtain 20,000 sheets of copies.

FIG. 4 is a graph showing the variation of amounts of toner deposited on the probe with change of depth of distance between the sleeve and the probe.

FIG. 5 is a control circuit diagram used in the present invention.

Referring to FIG. 1 of the drawings, there is shown a diagrammatic cross-section of the portion including a magnetic brush type developing mechanism and a detection unit. Reference numeral I in the figure indicates a drum carrying electrostatic latent images and arranged to be rotatable counterclockwise, and numeral 2 designates generally the magnetic brush type developing mechanism. It comprises a developer receptacle 20 of which the interior surface is curved to present a substantially circular section 2I to effect smooth circulation of the developer D (composed of toner and carrier) in cooperation with a magnetic brush which is described later. At a side of the drum is provided a bent protuberance 22 adapted to regulate the working amount D of the developer D. Numeral 23 designates a magnetic brush consisting of a mandrel 24 around which the permanent magnets 25 of different polarities are arranged alternately and which itself is secured to a side plate (not shown) of the receptacle 20, and a nonmagnetic sleeve 26 disposed surrounding the mandrel 24 so as to be rotatable counterclockwise. 27 denotes a mixing plate adapted to distribute the developer D uniformly in the axial direction of the rotation of the brush 23 after the developer carried up by the action of the brush 23 has been scraped down from the sleeve 26. Numeral 3 designates generally a toner content detector unit comprising the unit body portion 31 having at its bottom an opening through which a part of an electrically coductive probe is exposed to the developer, a probe 32 formed from an electrically conductive and transparent glass cylinder arranged rotatable counterclockwise and retained at a location where its surface is contacted with the working developer D, a lamp 33 providing a light source, and a photoelectric element (CdS cell) 34 disposed in the probe 32 and adapted to detect the amount of toner deposited on the probe 32 by a transmission method in cooperation with the lamp 33 and send a detection signal to a toner re plenishment control circuit (described after) to which the photoelectric element is connected. Numeral 4 refers to a toner container from which toner 42 is replenished into the developing mechanism in response to an information from the circuit.

In operation, the developer D accumulated at the bottom of the receptacle is attracted to the surface of the sleeve 26 by the attraction of said permanent magnets and urged to move to the right with rotation of the sleeve 26. In its way upward, the developer D is regulated into a suitable volume (working developer D) by the bent protuberance 22 of the receptacle 20, and immediately thereafter, the electrostatic latent image formed on the surface of the rotating drum 1 is developed into a visual image by the action of the working developer D. The thus regulated developer continues to move forward until it contacts the probe 32 which is moving oppositely to the direction of the movement of said developer. The mechanism of attaching toner to the surface of the probe is considered as following at the point where the probe contacts the developer; at this point, the developer gains kinetic energy whereby the toner particles in the developer which cling to the carrier merely by the force of triboelectrical charges are separated from the carrier and suspended in the form of cloud in the space 50. In this space 50, the suspended toner particles carrying triboelectric charges of a certain polarity induce on the probe the electric charges of the opposite polarity, so that the suspended toner is attracted to the probe by an electrostatic force which is generally called image force.

Light from the lamp 33 is applied to the toner deposited on the probe and the quantity of light that has transmitted through the probe 32 is received by the photoelectric element 34 which, in turn, is actuated to energize the control circuit to which the element is connected, whereby when toner in the developer is deficient, the toner container 4 is operated to automatically supply the deficiency. The toner deposited on the probe 32 is removed away by the action of the developer when the probe makes another rotation and is again contacted with said developer D. FIG. 2 shows the relationship between the developer stirring time, electric charge density of the toner and amount of the toner deposited on the probe, which were all determined by using the apparatus of the present invention operated under the following conditions provided that replenishing of the toner and development are not conducted: linear velocity of the probe l mm/sec, and linear velocity of the sleeve =400 mm/sec. As apparent from the graph of FIG. 2, the density of electric charge retained by the toner increases sharply with the developer stirring time till about 13 minutes after the start, and thereafter said density remains substantially constant. On the other hand, the amount of toner deposition on the probe is decreased with lapse of the stirring time in a way almost contrary to the density of electric charge, describing a curve substantially symmetrical with the toner charge density curve. It was found that the relation between the density of electric charge retained by the toner and the amount of toner deposited on the probe by the action of image force is analogous to the relation between the density of electric charge of toner and the density of a developed image. and from this finding, it was confirmed that an always optimal image can be obtained free of any influence of variation of relative humidity and other factors by controlling the amount of toner deposition on the probe. This may be better understood from the following theory. That is, as easily understood from the fact that the electrostatic development is effected by the coulomb force of the medium having electric charge and the triboelectrucally charged toner, the density of electric charge carried by the toner has a relation to the amount of toner which deposits on the electrostatic latent image. Therefore, in order to obtain always constant developing effect, it is important to maintain constant not only the toner content in the developer but also the density of electric charge retained by the toner.

However, the density of electric charge of toner can not stay constant because such electric clra ge of toner is induced through friction with the carrier and also because triboelectricity of toner is greatly affected by relative humidity and further the degree of leakage of electric charge retained by the developer varies ac cording to the various conditions. For instance, the lower a relative humidity is, the more triboelectric charge is induced on the toner, resulting in toner deficiency in the formed image. This is due to the fact that if density of electric charge held by the toner is increased, coulomb force between toner and carrier is also enlarged to retard transfering of toner toward the electrostatic latent image. That is, it becomes hard to obtain required density as the density of electric charge held by the toner is small.

Referring now to FIG. 3, it graphically shows variation of toner content in the developer and veriation of density of electric charge on toner as observed in a test where 20,000 sheets of copies were obtained by using the controlling method of the present invention. The copying operation was performed by using the originals of the B4 sizes which consume toner of 20 to 300 mg per a sheet in an atmosphere kept at 25C with relative humidity of 20 and in the first half and the last half of the operation, respectively.

Even under such conditions, an always excellent de veloping effect is obtained to produce almost constant image density. This is achievable beacuse the method of the present invention is capable of controlling the toner content corresponding to variation of the density of electric charge retained by the toner, regardless of variation of toner consumption in the developer.

FIG. 4 shows variation of the amount of toner deposited on the electroconductive probe as observed when changing the distance between the sleeve and the probe with the developer and as measured at the contact area of the two (showed in FIG. 4 as A, and A and after the probe has been contacted thoroughly with the toner cloud (showed in the figure as B and B The tests were conducted by usin two types of developer with different toner contents one being 6.17% (A. and B and the other 4.29% (A and Bflland under the follow ing conditions: linear velocity of the probe l mm/sec', linear velocity of the sleeve surface 400 mm/sec; working developer layer thickness 9 mm; and the distance between the sleeve and the probe 3 to 8 mm.

in the graph of FIG. 4, the abscissa represents the dis tance 1 between the sleeve surface and the probe surface, and the ordinate represents the amount of toner deposited on the probe, such amount being measured optically by a transmission method and expressed by numerical values in terms of transparent density as unit. As apparent from the graph, in the condition where the probe surface is well contacted with the developer, the amount of toner deposited on said probe is small when said probe is contacted deep in the working developer, but is increased as the depth of contact is reduced, until reaches a maximum value, but it is again sharply decreased at the moment just before the probe is dis-contacted from the developer. lt is also noted that there is a great difference between the amount of toner deposited on the probe at the contact area of the probe and developer and that after they were thoroughly contacted with the toner cloud. This is indicative of the fact that the toner particles which have once deposited on the probe are substantially re moved away from the probe surface as they are again contacted with the developer. And therefore, the density of toner on the probe can be measured continuously by rotating the probe.

Discussed in the following are the optimal conditions of the controlling according to the present invention.

1. Probe configuration and material The probe is used repetitively as it is adapted in a recording device which is operated continuously, so that it is desirable that the probe is of a configuration allowing easy and smooth rotation, such as cylindrical, discshaped or endless belt-shape. The probe is preferably made from electrically conductive glass coated with a film of tin oxide as this material provides relatively high friction resistance when contacted with the developer. It is of course possible to use other metal materials such as aluminium, copper, brass, or iron. [t is also possible to use even a poor-conductive material if such material has a certain degree of electric conductivity sufficient to allow deposition of toner particles by image force. Therefore, the probe may be formed by covering an insulator with an electroconductive material.

The electroconductive probe surface may be coated with an insulator for the purpose of protecting the probe, but the thickness of such coating must be less than 1 micron because if such coating thickness is greater than 1 micron, the image force is excessively weakened to make it unable to achieve the end of the method of the present invention.

2. Direction of movement of the probe and developer It may be considered that the amount of toner deposited on the conductive probe by the image force is proportional to the density of the toner cloud. However, this is changed according to the toner content in the developer, adhesion between toner and carrier, and the force of separating toner from carrier, that is, the revolving speed of the magnetic brush or the way of contacting the moving developer with the probe. In order to enhance precision of toner replenishment in relation to the image density, it is desirable to rotate the probe so as to move oppositely to the direction of movement of the developer at the point where said probe is contacted with the developer. If the probe is rotated so as to move in the same direction as movement of the developer at the contact point, it is found that the density of the toner cloud formed in the other space 60 in FIG. 1 is extremely low. In this case, however, it is possible to obtain substantially same volume of toner cloud as in the space 50 by disposing an obstacle at a suitable location in the other space 60 so as to expedite separation of toner from carrier.

3. Measurement of toner deposition The toner deposition on the probe may be determined either by detecting the change of transmittance or the degree of reflection of the probe by use of photodetector such as photoelectric element (like CdS) or phototube, or by measuring the amount of electric charge of the toner deposited. However, the measuring method using a photoelectric converter such as CdS cell is more convenient for the reason of simplicity. In this case, in order to compensate the influence of transient change with time of the light source or photoelectric converter, it is preferable to use two CdS cells to take advantage of the difference between the signals obtained simultaneously from the toner-deposited probe and from the toner-free probe.

FIG. 5 shows an exemplary circuit diagram where two CdS cells are used. In the diagram, numerals I0] and 102 designate CdS cells, [03 to 107 specific resistances, 108 a variable resistance, adapted for regulating the toner content controlling level, and 109 an operational amplifier adapted such that the replenishing toner container 4 will be operated by the output signal therefrom.

When the toner content controlling method of the present invention is used in a case where a bias voltage of a polarity opposite to that of electric charge retained by the toner is applied to the magnetic brush for preventing the toner from being electrostatically deposited on the undesired portion of a member carrying electrostatic latent images, it is considered that the probe has the same potential as the magnetic brush due to its electric conductivity, and hence change of toner deposition on the probe caused by application of said bias voltage is negligible and poses no problem practically. Also, in case the present method is employed in cas cade development, it is merely required to provide a suitable guide plate between the bucket conveyor and the member carrying electrostatic latent images and to dispose the probe thereabove so that it may contact the developer, whereby it is possible to produce a satisfac tory toner cloud no matter what direction the probe is rotated.

As described above, use of the toner content control according to the present invention can dispense with means for applying voltage to the probe as well as means for cleaning the probe, thus contributing to reduction of the machine size, and further, it is possible to obtain copies with always constant image density regardless of variation of various external factors. Moreover, the present invention can be applied not only in the field of electrophotographic reproduction but also in any sort of powder developing apparatuses such as facsimiles.

What is claimed is:

1. An apparatus for controlling toner concentration of a developer in electrostatic development comprising a developer receptacle; means in the form of a magnetic brush for conveying the developer through a developing state, an electrically conductive rotatable probe placed in contact with said brush whereby toner in the developer is attached to the probe, said measuring means comprising a light source to illuminate said probe and a photoelectric element to detect the light from said probe; and means for controlling a replenishing amount of the toner to said receptacle based on the result of the measurement.

2. An apparatus according to claim I wherein said probe is transparent to the light, said photoelectric element being disposed in the probe.

3. An apparatus according to claim 2 wherein the light source is placed outside the probe.

4. An apparatus according to claim 1 wherein said probe is positioned in contact with the brush after having passed the developing state.

5. An apparatus according to claim 1 wherein said probe is adapted to rotate oppositely to the movement of the brush.

6. An apparatus according to claim 1 wherein said measuring means comprises two photoelectric elements.

7. An apparatus according to claim 6 wherein one of the elements is adapted to measure the light from a first area of the probe, said first area having no toner while the other element is adapted to measure the light from a second area of the probe, said second area having the toner.

8. An apparatus according to claim 7 wherein both of the elements are disposed in the probe, the probe being transparent.

9. An apparatus according to claim 8 wherein said measuring means comprises means for comparing a signal from one of the elements with that of the other.

10. An apparatus according to claim 9 wherein the control means is adapted to control the replenishing amount of the toner based on a signal from the comparison means.

* l i i

Claims

2. An apparatus according to claim 1 wherein said probe is transparent to the light, said photoelectric element being disposed in the probe.