The present invention relates to a toner concentration control apparatus for an electrophotographic copying machine, electrostatic recording apparatus or the like, for controlling the concentration of toner in the supply of a developer comprising at least toner and carrier, by optically sensing the toner concentration of the developer, and more particularly to a toner concentration control apparatus having photosensing means responsive to light reflected from the developer, and replenishing toner to the developer on the basis that the reflectivity of the developer depends upon the relative proportions of toner and carrier in the developer, the higher the concentration of toner, the lower the reflectivity of the developer, and vice versa, whereby the toner concentration of the developer is controlled accurately.
The development apparatus wherein such a toner concentration control apparatus is located applies toner to a latent electrostatic image-bearing photoconductor, forming a visible image on the photoconductor. In electrophotographic copying machines, for example, the surface of the latent electrostatic image bearing photoconductor is continuously brought very close to, or into actual contact with, the toner held in the development apparatus, which toner has been electrically charged to a polarity opposite to that of the latent electrostatic image. In the case of two-component-type developer consisting essentially of toner and carrier, when the toner is mixed with the carrier in the development apparatus, the toner particles become charged triboelectrically and cling to the surfaces of the carrier particles.
In order to produce satisfactory copies, it is necessary that the proper ratio of toner to carrier be maintained. However, each time a copy is made, some toner is used for imaging and thus depleted from the supply, and the images produced on subsequent copies will be fainter (at first not noticeably so, but increasingly as more copies are made) unless quantities of toner are added to the developer to replace that which has been used.
In order to eliminate this shortcoming, the development apparatuses of conventional electrophotographic copying machines can be provided with toner concentration control means which operate toner replenishing devices for adding toner to the developer when necessary. Generally, a toner concentration control means comprises a toner mixing-ratio detection means for detecting the ratio of toner to carrier in the developer, and a toner replenishment decision means, which compares the output signal from the toner mixing-ratio detection means with a reference signal indicating, for example, the desired concentration, and which actuates the toner replenishing device in accordance with the decision of the decision means.
An example of such toner concentration control apparatus is disclosed in Japanese Patent Publication No. 38-17245. In the toner concentration control apparatus disclosed, light from a light source is projected onto the developer held in the reservoir, and the light reflected from the developer is detected by a photosensor element. Thereafter, in accordance with the intensity of the reflected light, toner is replenished. This toner concentration control apparatus has proved to be entirely unsatisfactory, since, if the intensity of light from the light source varies due to some variation in the power or for other reasons, the output of the light-receiving photosensor element will be affected. Further, if the photosensor element changes in characteristics, for example, due to age or change in the ambient temperature, its output may also vary. Therefore, the change in the concentration of toner in the developer cannot be detected accurately by the toner concentration control apparatus disclosed in Japanese Patent Publication No. 38-17245.
Furthermore, in this type of toner concentration control apparatus, smearing of the photosensor element with developer, which is apt to be scattered out of the developer reservoir, is a serious problem causing inaccurate measurement of the toner concentration. Nothing is mentioned as a countermeasure for eliminating such drawbacks in Japanese Patent Publication No. 38-17245.
Another example of a toner concentration control apparatus, which is an improvement on the above-mentioned Japanese Patent Publication No. 38-17245 to some extent, is disclosed in U.S. patent publication No. 3,756,192. In this apparatus, a chopper wheel, which serves as a calibrated reflector, having a value of reflectance on its blades equal to that of developer of the desired toner concentration, is rotated in front of a single light source. Light is thus alternately reflected from the developer containing toner and from the calibrated reflector, to a photosensor. When an imbalance of toner in the developer results in a difference in reflected light intensity between the developer and reflector, the difference is converted into an electrical signal having an AC component. A phase-sensing circuit detects the position of the peak of the AC signal component with reference to the position of the calibrated reflector and provides a control signal to a toner replenishing device. This toner concentration control apparatus has an advantage over the aforementioned prior-art toner concentration control apparatus in that utilization of a single light source and a calibrated reflector for reference compensates for variations in output of the light source. However, this toner concentration control apparatus requires a device for rotating the chopper wheel, i.e., the calibrated reflector, which makes the toner concentration control apparatus complex in mechanism, over-sized and expensive. Furthermore, since it contains movable portions, those movable portions may be abraded while in use over an extended period of time, with the result that the reliability of the apparatus may be lowered.
Furthermore, in this toner concentration control apparatus disclosed in U.S. patent publication No. 3,756,192, the calibrated reflector and the photosensor are susceptible to smearing by the airborne toner particles, since they are not protected from the toner particles.
A still further type of toner concentration control apparatus, disclosed in U.S. patent publication No. 3,233,781, issued to W. J. Grubbs, utilizes the difference in reflectivity exhibited by toner and carrier particles as a means for monitoring the concentration of toner particles in the developer. According to the Grubbs disclosure, the reflectance of the developer is monitored by directing light from a light source toward the developer and detecting the light reflected from the developer by use of a photosensor. Such photosensor, together with a similar photosensor which is illuminated directly by the same light source and thereby provides a reference signal, is employed as a variable resistance arm of a bridge circuit which is capable of activating a toner replenishing device in response to a predetermined change in the ratio of photosensor outputs, such change being characterized by an imbalance in the circuit.
This toner concentration control apparatus has not proven entirely satisfactory in operation, since, as can be better understood from the disclosure itself, airborne toner particles are free to circulate in parts of the apparatus, while other parts thereof are relatively sealed. This allows accumulation of toner particles on the window shielding the lamp from the portion of the apparatus where the primary photosensor is located (while the reference photosensor is within the same housing as the lamp), and also allows accumulation of toner on the primary photosensor, both of which accumulations of toner serve to cause inaccurate readings as between the two photosensors, and thus inaccurate measurements of the toner concentration.
Further, in this toner concentration control apparatus, fluctuations in the intensity of the output of the lamp itself can be compensated for. However, only the lamp and the reference photosensor are enclosed within a housing so as to be free from ambient light. The reference photosensor is not similarly enclosed and will, therefore, sense ambient light present in the toner concentration control apparatus, causing error uncompensated for in the measurement of the toner concentration.
A further toner concentration control apparatus of the type which senses toner concentration by sensing variations in reflectivity of the developer is disclosed in U.S. patent publication No. 3,830,401. In this toner concentration control apparatus, pulses of radiation (i.e., light), periodically produced by a radiation source at a selected frequency, are directed at the developer mixture and the reflectance thereof is monitored by a photoelectric transducer which produces a first output signal representative of the intensity of such reflectance. A second photoelectric transducer illuminated directly through the airborne toner cloud in the apparatus by the radiation source produces a second output signal representative of the intensity of the radiation emanating from the source as modulated by the surrounding environment. The toner concentration is detected by comparing the first output with the second output and, in accordance with the result of the comparison, toner is replenished to the developer when necessary, whereby the concentration of toner in the developer is controlled. In this apparatus, variations in output of the radiation source can be compensated for, since the radiation source is shared by the two photoelectric transducers. However, this apparatus requires a pulse generator for energizing the radiation source to produce pulses of radiation at a selected frequency, which is very expensive.
Furthermore, in this toner concentration control apparatus, the radiation source and the two photoelectric transducers are faced with the airborne toner particles circulating within the apparatus. Therefore, it has the shortcoming that uncompensated for error will be caused with respect to the radiation source and the two photoelectric transducers if they are smeared differently with the airborne toner particles.