1.  Field of the Invention
The present invention relates to an image forming apparatus utilized for a copying machine, a printer, FAX, etc.  that involve using an electrophotographic system. 
2.  Related Background Art
FIG.  11 shows one example of the image forming apparatus. 
There has been provided the image forming apparatus, wherein a developer supplied onto the surface of a developing sleeve 3 defined as a developer carrying member by developer agitation carrying means 11, 12, is held in a magnetic bead chain state by a magnetic force of a magnet roller 13, and is carried to a developing area of an opposite portion facing a photosensitive drum 40 defined as an image bearing member on the basis of a rotation of the developing sleeve 3, and the magnetic bead chain is scraped off by a return member 1 defined as a developer stay amount regulating member for regulating an amount of developer staying on the developing sleeve and by a blade 2 defined as a magnetic bead chain height regulating member, thereby properly maintaining an amount of developer carried to the developing area. 
To describe it in greater detail, an interior of a developing device 44 is partitioned into a developing chamber 16 and an agitating chamber 17 by a partition wall 6 extending in a vertical direction, wherein the developing chamber 16 and the agitating chamber 17 accommodate a two-component developer containing a non-magnetic toner and a magnetic carrier. 
A screw type of first and second developer agitation carrying means 11, 12 are, as described above, disposed in the developing chamber 16 and in the agitating chamber 17, respectively.  The first agitation carrying means 11 agitates and thus carries the developer in the developing chamber 16.  Further, the second agitation carrying means 12, under control of a developer density control device, agitates and carries the toners supplied to an upstream side of the second agitation carrying means 12 from a toner replenishment tank (unillustrated) and the developer already existing in the agitating chamber 17, and uniformizes the toner density.  The partition wall 6 is formed with developer paths (unillustrated) through which the developing chamber 16 and the agitating chamber 17 communicate with each other at side end portions on this side and on a deep side as viewed in FIG.  11.  Carrying forces of the first and second agitation carrying means 11 and 12 carry the developer in the developing chamber 16, of which a toner density decreases due to the toners being consumed for the development, into the agitating chamber 17 via the other path. 
As shown in FIG.  11, the first screw type agitation carrying means 11 is disposed, substantially in parallel with a line-of-axis direction of the developing sleeve 3, i.e., with a development widthwise direction, in a bottom portion within the developing chamber 16.  The first screw type agitation carrying means 11 takes a screw structure that a blade member is spirally provided around the axis of rotation, wherein the first agitation carrying means 11 rotates and thus carries the developer in the developing chamber 16 in one direction along the line-of-axis direction of the developing sleeve 3 in the bottom portion of the developing chamber 16.  Further, the second screw type agitation carrying means 12 takes the same screw structure (that the blade member is spirally provided around the axis of rotation in a direction opposite to the first agitation carrying means 11) as the first screw type agitation carrying means 11 has.  The second agitation carrying means 12 is disposed substantially in parallel with the first agitation carrying means 11 in a bottom portion within the agitating chamber 17, and rotates in the same direction as the first agitation carrying means 11 rotates and thus carries the developer in the agitating chamber 17 in a direction opposite to the first agitation carrying means 11.  Thus, the developer is circulated between the developing chamber 16 and the agitating chamber 17 by dint of the rotations of the first and second agitation carrying means 11 and 12. 
In this circulation of the developer, a toner density detection means 7 for detecting a toner density in the developer in order to detect a toner-to-carrier ratio is provided on this side (as viewed in FIG.  11) in a thrust direction in the vicinity of the developing sleeve 3 within the developing chamber 16.  The reason why the toner density detection means 7 is provided not on a deep side but on this side is that if provided on the deep side (on an upstream side in the circulation of the developer in the developing chamber 16), it is impossible to detect a decrease in the toner density when the toner is consumed on a downstream side, however, if provided on this side, a detection position exists on the downstream side in the thrust direction in the developing chamber 16, and hence the decrease in the toner density is detected even when the toner is consumed everywhere in the thrust direction. 
Even when the toner-to-carrier ratio of the developer is thus kept fixed, there is still a case where the density of a developed toner image might fluctuate due to deterioration of the carrier, an environmental change, a change in image of an original, etc. 
Such being the case, a contrivance is proposed, wherein a patch image having a predetermined density is formed on a photosensitive drum, the density of the toner image is fixed by changing a target value of the toner-to-carrier ratio on the basis of a result of detecting a density of the patch image by a density detection means.  This contrivance is disclosed in, e.g., Japanese Patent Application Laid-Open No.  H09-022179. 
Another contrivance is proposed, wherein for attaining stable toner density control even when whatever copy mode is set in a way that eliminates an undershoot and an overshoot of the toner density control which occur in a copy mode with a large variation in toner consuming amount, the toner is replenished based not on the signal from the toner density detection means 7 provided in the developing device 44 but on a video counter count if the density of the original changes over a predetermined value as compared with when copied last time on such an occasion that the image to be formed is switched over.  This contrivance is disclosed in, e.g., Japanese Patent Application Laid-Open No.  H09-127780. 
In the contrivance (construction) disclosed in Japanese Patent Application Laid-Open No.  H09-022179, however, it takes much time for the replenished toner to reach the toner density detection means 7 in the developing device, and consequently the toner replenishment amount has a ripple as shown in FIG.  12, with the result that the toner density largely deflects.  In FIG.  12, the axis of ordinates indicates a toner amount that should be replenished according to the result of the detection by the toner density detection means, wherein a toner replenishing operation is not conducted in one direction.  When the image density is switched over, the toner replenishment amount changes, thereby causing a much larger ripple.  Further, even if noises enter the result of the detection by the toner density detection means 7 due to a developer volume density deflection, a contamination on the toner density detection means 7, etc, the toner replenishment amount becomes improper.  In those cases, as shown in FIG.  13, the image density deflects or gets out of a normal state. 
As proposed in Japanese Patent Application Laid-Open No.  H09-127780, the above ripple can be slightly reduced by replenishing the toner on the basis of a result of the video counter count, however, a degree of this reduction was not sufficient.