US3928764A - Method and apparatus for measuring and controlling the toner concentration in electrophotographic reproduction machines - Google Patents

Unite States Patent [1 1 Bock et a1.

[ Dec. 23, 1975 METHOD AND APPARATUS FOR MEASURING AND CONTROLLING THE TONER CONCENTRATION IN ELECTROPHOTOGRAPHIC REPRODUCTION MACHINES [75] Inventors: Hartmut Bock, Fischbach, Taunus;

Hans Giitte, Kelkheim, Taunus, both of Germany [73] Assignee: Hoechst Aktiengesellschaft,

Frankfurt, Germany 22 Filed: June 4, 1973 21 Appl. No.: 366,510

[30] Foreign Application Priority Data June 2, 1972 Germany 2226924' [52] US. Cl. 250/272; 250/273; 250/308; 250/359; 118/637 [51] Int. Cl G0ln 23/12; 00111 23/20 [58] Field of Search 250/272, 273, 277, 308, 250/359, 360; 118/637; 222/57, DIG. l

11 Claims, 5 Drawing Figures U.S. Patent Dec. 23, 1975 Sheet 1 of2 3,928,764

ELECTRICAL CIRCUIT US. Patent Dec.23, 1975 Sheet2of2 3,928,764

\ FIG.2

METHOD AND APPARATUS FOR MEASURING AND CONTROLLING THE TONER CONCENTRATION IN ELECTROPHOTOGRAPHIC REPRODUCTION MACHINES The present invention relates to a method of and apparatus for measuring or measuring and controlling the toner concentration of a developer circulating in an electrophotographic reproduction machine. Such apparatus is particularly suitable for use in automatic high-speed reproduction machines.

In electrophotographic reproduction machines the images are developed with developers which generally consist of two components, known as the toner and the carrier. The toner may, for example, be a colored synthetic resin powder and the carrier a granular material consisting of glass, or iron coated with synthetic resin. The carrier is used for carrying the toner and in charging it triboelectrically. The toner is used for inking the copies. In the case of liquid developers, finelydivided toner materials (e.g. droplets of an adhesive liquid) are contained in a liquid carrier. During development, the toner is used up at a rate dependent upon the nature and number of the copies produced. Further amounts of toner must be added in order to maintain the density of the copies at the correct level. For the purpose of controlling the toner concentration in the developer, it is necessary to use measuring apparatus, the signals from which can be used to adjust the rate of replenishment of the toner.

Various methods are known for measuring the toner concentration. U.S. Pat. No. 2,956,487 and German Pat. No. 1,926,198, for example, deal with the determination of the optical density of a toner-treated charge image or of the developer liquid. German Pat. Nos. 1,936,815 and 2,014,430 disclose methods of determining the permeability of the developer mixture in cases where the toner or the carrier is ferromagnetic.

The optical methods are relatively complicated and therefore prone to breakdown.

The determination of the permeability of dry mixtures having a customary composition of ferromagnetic carrier and non-ferromagnetic toner has the disadvantage of falling sensitivity with rising toner concentration. This results from the fact that the first toner particles force the carrier granules relatively widely apart, and further toner is preferentially deposited in the gaps thus created. Furthermore, these methods are limited to very specific developer substances, i.e., those containing ferromagnetic carrier materials.

One object of this invention is to provide a method of measuring toner concentration that possesses a high degree of sensitivity even where high concentrations of toner are concerned, and is relatively simple and thus not veryprone to breakdown.

The present invention accordingly provides a method of measuring or measuring and controlling the toner concentration of a developer circulating in an electrophotographic reproduction machine. In this method, in a sampling step, there is prepared from the circulating developer a sample for measurement, which sample consists either of the said developer itself or of a material composed of or enriched in the toner component of the said developer. The sample is so arranged that the toner therein can be estimated as a proportion of the required toner concentration of the developer. In a measurement step the prepared sample is irradiated by B-radiations from a radioactive preparation which are caused to impinge on the sample and to be back-scattered therefrom and a measurement is made of the back-scattered radiation.

It has been found that this method, known per se in radiation measurement techniques, is particularly suitable, in measuring toner concentration, for detecting small proportions of components which differ from one another as regards atomic number. The sensitivity of this back-scatter method is just as great as the through irradiation method described in copending application Ser. No. 276,307. Compared with through irradiation, however, the use of back-scatter offers the advantage that the determination of the toner concentration is not dependent upon the degree of compacting of the developer mixture prior to carrying out the measurement. Also, the measured intensity is less dependent upon the geometry of the arrangement.

Furthermore, the back-scatter method offers the advantage that considerably lower strengths of preparation (smaller activity quantity of the preparation) can be used. When a suitable measuring arrangement is used, the activity quantity of the radioactive preparation can be kept within the limits permitted by antiradiation regulations, so that in an arrangement of this kind it is possible to dispense entirely with radiation screening even of the simple kind required in the case of B-radiation. Finally, the cost involved in measuring back-scatter radiation is lower, since the B-radiation used in the back-scatter measurement is preferably measured with a Geiger counter, whereas a scintillation counter is necessary for detecting y-radiation. With a Geiger counter the number of counts in the case of 'y-radiation is 100 times less than with B-radiation. The counter and the measuring apparatus occupy less space and are cheaper in the case of a Geiger counter.

According to the present invention, preference is given to a method in which a layer of developer is continuously produced and passed in a uniform manner between and at a constant distance from the radiation source and the detector. It is possible with this continuous method to operate an electrophotographic copying apparatus fully automatically, the measured change in the impulse rate being used to control means for adding toner. It is of course necessary to realize that when the counting rate is above its normal level, the toner concentration is below its normal level. Thus, replenishment of the toner material is always to be carried out when the counting rate has exceeded a certain level.

Instead of using the developer mixture itself as the sample for measurement, it is possible, by the method of the present invention, to use, as this sample, a surface of material which provides strong back-scatter of B-radiation and on which varying amounts of toner, depending upon its concentration in the developer, have been deposited. In this case, as a thin strip of material there can be used a charged photoconductor layer on a thin support of plastic material, or a thin support of plastic material which carries thereon an electrode which is constituted in particular by a vapordeposited metal layer and to which a constant voltage can be applied. The surface may, for example, take the form of an electrode made of copper or iron or a heavy metal (gold in particular), a certain electrical potential being applied to the electrode which is used to assess the developer under investigation. Varying quantities of toner are deposited on the heavy-metal electrode depending upon the toner concentration, and with increase in the quantity of toner there is a reduced intensity of the back-scattered B-radiation. The use of heavy-metal electrodes is preferred, since the change in back-scatter intensity is particularly high with these.

The method has been found particularly reliable when a C14 preparation is used as the B-preparation. Other nuclides such as Sr 90/Y 90, Pm 147 or Tl 204 are suitable radiation sources. The C 14 preparation is, however, preferred for the back-scatter method.

According to the present invention, apparatus for performing this method may include an endless circulating belt, of which at least part of the surface faces upwards, in an electrophotographic reproduction machine, located at a point where the developer circulates, means for forming a layer of developer of constant thickness on the upwardly facing portion of the surface of the belt, a B-preparation, so located as to be presented to the free surface of the layer of developer mixture, and a detector for the B-radiation back-scattered by the developer mixture, located on the same side of the layer of developer mixture as the radiation source but so arranged that it is screened from the direct B-radiation of the radiation source.

Apparatus for performing the method of the present invention in which the back-scatter measurement is made by means of an electrode on which toner has been deposited includes a metal surface so formed and disposed in an electrophotographic reproduction machine that it is developed by the developer circulating in the reproduction machine, and a measuring head, including a B-radiation source and a detector, presented to this metal surface, a certain electrical potential being applied to this metal surface during the developing.

In this arrangement, it is preferred to provide, laterally of a repeatedly usable photoconductor of an electrophotographic reproduction machine, one or more electrodes which can be moved with the photoconductor through the developing and cleaning zones, and to locate a stationary measuring head in a position in which it is presented to the moving electrode or electrodes.

The present invention is illustrated by way of example in the accompanying drawings, in which:

FIG. 1 is a diagrammatic view of an embodiment employing the back-scatter method of measurement of toner concentration,

FIG. 2 is a diagrammatic section through the measuring head of the apparatus of FIG. 1,

FIG. 3 shows a front elevation view of a further embodiment of the device for use in the method of the present invention, and

FIG. 4 is a side elevation view of the device of FIG. 3, and

FIG. 5 is a diagrammatic section through a measuring head the radiation source of which is arranged on a ring around a counting tube.

The distance (h) of the preparation from the surface of the layer under investigation is adjusted depending upon the form of the measuring head. In the arrangement here described this distance is a few millimeters.

With a suitable arrangement, the intensity of backscatter is directly proportional to the atomic number of the substance causing the back-scatter. The determination of the toner concentration by the back-scatter method is thus based upon the fact that the B-rays are back-scattered to a less pronounced extent in the layer of toner, consisting mainly of carbon, hydrogen and oxygen atoms, than in the thin coatings of plastic material and the iron cores of the particles constituting the carrier. The thicker the layer of toner, the lower the intensity of the B-rays back-scattered by the developer mixture becomes.

When use is made of B-rays that are not too hard, back-scatter occurs only on the uppermost layers of carrier particles contained in the developer mixture. The back-scatter method is thus independent of the density of compacting caused by shaking down the material. Conversely, in the through radiation method the change in the density of the developer mixture with change of toner concentration constitutes the basis on which the toner concentration is determined (increase in intensity of the rays passing through, with increasing thickness of the layer of toner and with correspondingly decreasing density).

Back-scatter measurements using carbon 14 as the B-radiation source have indicated a change in backscatter intensity of with a change in the concentration of toner of from 1 to 2% by weight. Since a 2% change in intensity can be measured on a reproducible basis, a change in toner concentration of up to 0.1% by weight (eg a change of from 1.6 to 1.5%) can be determined by the back-scatter method.

It thus becomes possible to replenish the toner in a very accurate manner and to maintain the toner concentration very accurately.

6 What is claimed is: l. A method of measuring the toner concentration of a developer containing toner circulating in an electrophotographic reproduction machine, comprising forming a horizontal layer of developer of constant thickness from the circulating developer for measurement,

passing said layer past a source of B-rays that are effected by the toner, irradiating the layer by B-rays and measuring the B-rays being scattered back from said irradiation to determine the amount of toner present.

2. The method as claimed in claim 1, in which the sample constituted by the developer itself is at a constant distance at its closest point from the radiation source and measuring device, whereby irradiation and back-scatter are from the same side of the sample.

3. The method as claimed in claim 1, in which the radiation source is a C 14 preparation.

4. The method as claimed in claim 1, in which the radiation source is a Sr /Y 90, a Pm 147 or a Tl 204 preparation.

5. An apparatus for measuring the toner concentration of a developer containing toner circulating in an electrophotographic reproduction machine including means for preparing and arranging a sample from the developer for irradiation comprising an endless belt mounted for circulation with part of its outer surface horizontal and facing upwards and means for forming a layer of developer of constant thickness on this part of the belt, a measuring head as a source of B-rays located for irradiation of the sample being located to irradiate developer on this part of the belt and a detector for measuring the effect of the sample on the said radiation.

10. The apparatus as claimed in claim 9, in which the detector is of circular section and the B-ray source is a counting tube positioned in the center of this circle between the detector and the developer, said detector being screened from said source.

11. The apparatus as claimed inclaim 9, in which the detector is a counting tube and the radiation source is located on a ring around it.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,928,764

DATED December 23, 1975 INVENTOR(S) I Hartmut BOCK et a1 It tscertrfied that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

ONAL NE RUTH C. MASON D D w BAN R Arresting Oflr'cer Commissioner of Patents and Trademarks