Image Forming Apparatus Responsive to Ambient Condition Detecting Means

1. Field of the Invention

The present invention relates to a copying machine using an electrophotographic technology, LBP and other image formation system, and the multicolored image formation system with two or more colors.

2. Related Background Art

FIG. 2 is a block diagram for the image forming system, whereby the conventional environmental control and its correction method are explained. Here described is a case where the present invention is used for a laser beam printer, that is, the mechanism of forming images by scanning a laser beam on the photosensitive drum in synchronization with reading of an original will be explained below.

First, an original 9 is read by a CCD 1. The obtained analog image signal is amplified to a given level by an amplifier 2, and then converted into an 8-bit digital image signal (0 to 255 gradation) by a A/D converter 3. Next, the digital image signal passes through a gamma (.gamma.) converter 10 (gradation control table containing a 256 type RAM) for gamma correction, and then enters a D/A converter 14.

The digital signal is converted into an analog signal again. Then, the comparator 16 compares the analog signal with a signal of a specific frequency which is generated by a triangular wave generator 15. Then, the pulse width is modulated. The binary coded image signal whose pulse width has been modulated enters the laser drive circuit 17 as it is, which is used as an emission ON/OFF control signal for the laser diode 18. A laser beam emitted from the laser diode 18 is scanned in main scanning direction by a known polygonal mirror 19. Then, after passing through the f/.theta. lens 20 and reflection mirror 21, the beam is irradiated on the photosensitive drum 22 or an image supporting or bearing material which is rotating in direction shown by arrow to thereby form an electrostatic latent image.

On the other hand, the photosensitive drum 22 is uniformly discharged by the exposure unit 28, and charged uniformly with negative electricity by the electrostatic charger 23. After that, when the photosensitive drum receives a laser beam mentioned above, it forms an electrostatic latent image on its surface according to the image signal. In addition, the so-called image scanning method or a method of exposing the portion to be developed (black pixels) is employed as usually so in the laser beam printer. Therefore, the developing unit 24 uses the known reverse development method to adhere toner with a negative charge characteristic to the portion of the photosensitive drum 22 discharged by laser. Thus, the latent image becomes visible.

FIG. 3 shows the relations between the surface potential of the photosensitive drum and the development contrast when the said reverse development is performed. Here, V.sub.D represents the negative potential charged uniformly by the charger 23 shown in FIG. 2. V.sub.OO represents the potential obtained when the laser diode is driven with the image signal of OO.sub.H (.theta. level) which has been digitized. Potential on the surface of the photosensitive drum V.sub.FF is the potential obtained in the same way as mentioned above for FF.sub.H (256 levels). Therefore, assuming that .vertline.V.sub.DEV -V.sub.FF .vertline. as shown in FIG. 3 is contrast potential Vcont and the development density developed with Vcont is Dmax, the Vcont should be set appropriately to optimize image density (generally, approximately 1.2 to 1.8 in electrophotography). (In general, Vcont may be .vertline.V.sub.D -V.sub.FF .vertline.).

It should be noted that the background removing potential (Vback) in FIG. 3 is used to fully remove the fog or background from a white-ground portion of an image which is irradiated with light quantity OO.sub.H.

The visible image formed on the photosensitive drum 22 according to the procedure mentioned above (a toner image with negative charge) is transferred to a recording material (paper, in general) 26. The remaining toner on the photosensitive drum 22 is scraped off by the cleaner 27 later. Then, said series of processes is repeated again.

For a laser beam printer with the configuration mentioned above, the conventional environmental control varies the contrast potential mentioned previously (Vcont) depending on the environment so that an optimal image density can be output constantly. That is to say, as shown in FIG. 4, if the development characteristic (V-D curve) varies with the environment as A for high humidity, B for normal humidity, and C for low humidity, Vcont is changed to Vcont-a for high humidity, Vcont-b formal humidity, and Vcont-c follow humidity as shown in FIG. 5. Thus, Dmax will be constant at 1.5 regardless of the environment.

To realize the environmental control, a temperature/humidity sensor 11 is provided as shown in FIG. 2. According to the absolute humidity detected and the Vcont table of the solid lines in FIG. 5 that is stored in memory 13, the CPU 12 calculates a proper Vcont value to change the amount of charge in the charger 23.

The environmental dependency of the V-D curve shown in FIG. 4 varies with the humidity adjustment state of developer in the developing unit. The temperature/humidity sensor 11 is, therefore positioned near the developing unit 24 so that the humidity adjustment state of developer can be well reflected on the sensor. However, even with such control, Dmax dependent on the environment is not always constant at 1.5 due to the machine type, the deterioration of the developer or minor difference in the production lot of developer. Namely, Dmax may become constant at 1.6 or 1.4.

To correct these variations, a correction means that adds or subtracts a certain amount of correction to or from the previously mentioned Vcont in all environments has been devised. Namely, a means to shift the Vcont table in FIG. 5 vertically by parallel movement has been made available. This correction has been provided, for instance, as a correction means used when the V-D curve changes in any environment from A, B, or C to A', B', or C' as shown in FIG. 6 along with the deterioration of developer or difference in the production lot number, or as a correction means used when the aimed Dmax cannot be obtained due to the distance between the development sleeve and photosensitive drum, or the difference in the amount of developer on the sleeve.

However, as mentioned above, such a way of correction that a certain amount of correction is added (or subtracted) to the function shown in FIG. 5 has drawback of spoiling the stability of Dmax achieved by the said environmental control.

To be more specific, if the V-D curve indicating development characteristic is changed from A, B, or C to A', B', or C' because of the deterioration of developer or delicate differences in the characteristics of developer for a machine having environmental variations shown in FIG. 4, or if a machine is produced in the way that it will have a characteristic represented with the V-D curve of A', B', or C' in FIG. 6 instead of the standard V-D curves in FIG. 4, constant Dmax of 1.5 cannot be output with the Vcont table in FIG. 5 intact. Therefore, the table must be corrected to return Dmax to 1.5. For instance, Vcont is added by .DELTA.V to characteristic C' in FIG. 6 so as to obtain 1.5 of Dmax, .DELTA.V of Vcont is also added to A' and B'. Then, the Vcont table is modified to be the dotted line in FIG. 5. As a result, Dmax is not the same in three environments any longer. Namely, Dmax-A'>Dmax-B'>Dmax-C'=1.5. This means that the employment of conventional correction will spoil environmental control for stabilization of density.

This is also true when correction is made to return Dmax to 1.5 based on the V-D curve of A'. In this case, 1.5=Dmax-A'>Dmax-B'>Dmax-C'. This is because the same amount of correction is applied for all environments although the amount of contrast potential necessary to change Dmax differs from environment to environment. Consequently, the conventional correction method has drawback that it is poor at following up the environment.

This invention is made to overcome the problem mentioned above. It is an object of the present invention to provide a system that can permit constant image density regardless of variations in the inner environment of the system.

The other objectives will be revealed through detail explanation of this invention.

FIG. 1 is a flow chart indicating the adjusting procedure and image forming procedure in the present invention;

FIG. 2 is a schematic diagram for the image formation system of the present invention;

FIG. 3 is an explanatory drawing for the photosensitive drum surface potential and development contrast;

FIG. 4 is a graph that shows the D-V curve indicating the normal development characteristic;

FIG. 5 is a graph indicating the Vcont table for environmental control;

FIG. 6 is a graph indicating variations in the normal development characteristic; and

FIG. 7 shows the effects of the correction method of the present invention.

The first embodiment of the presnet invention will be described below.

Shown on the left of FIG. 1 is an adjusting procedure to set up parameters which indicate the extent of correction to be added if Dmax differs from the target value. On the right, thereof an image formation procedure is described, which outlines how to obtain images with constant Dmax by changing the amount of correction or development contrast according to the environment using the said correction parameters.

First, Dmax is measured in processing S1 on the left and difference .DELTA.D from the aimed value Dmax is calculated. In processing S2, the correction parameters used to correct .DELTA.D according to the table in FIG. 1 are stored in memory which contains the Vcont table in FIG. 2. The parameters are integers act in units of 0.1 Dmax values and stored together with the signs. After copying has started as shown on the right of FIG. 1, humidity information which the temperature/humidity sensor 11 in the system detects in processing S3, the Vcont table stored in memory 13, and the said correction parameters are read into the CPU 12.

In processing S4, a normal proper Vcont value without correction is calculated based on the said humidity information and Vcont table. After that, the amount of unit correction of Vcont is determined in processing S5. In this example, the amount of unit correction is set to 1/16 of the proper Vcont value calculated in processing S4. Namely, a certain ratio or 1/16 of the proper Vcont value which varies depending on humidity information is set as an amount of unit correction, whereby the amount of correction can be changed according to the environment. Thus, the main purpose of the present invention has been embodied.

Now, the amount of unit correction or 1/16 of Vcont brings about a difference of approximately 0.1 at Dmax due in the environmental dependency of the V-D curve as shown in FIG. 4. That is to say, at a density of 1.5 on the V-D curve of A, the slope of curve A is approximately 10 V for 0.1 of density, that of curve B approximately 15 V, and that of curve C approximately 25. On the other hand, the proper contrast potentials of A, B, C are shown as 180 V, 280 V, 420 V, respectively in the graph. The 1/16 of these values are approximately 11 V, 17 V, 26 V which agree with the values for 0.1 of density. Therefore, a value obtained by multiplying the proper Vcont value by 1/16 is used as the amount of unit correction per 0.1 of density to correct Dmax.

In processing S6, the correction parameters stored according to the adjustment procedure in SP and read in S3 are applied to the amounts of unit correction obtained as mentioned above. Thus, the necessary amounts of correction are determined. Namely, the amount of unit correction is re-converted into the value for 0.1 or 0.2 at Dmax.

Finally, in processing S7, the above amount of correction is added to the proper Vcont value calculated at processing S4. At this time, needless to say, the minus and plus signs must be included in the amount of correction. When the CPU provides the charger 23 shown in FIG. 2 with control information or one of image the necessary Vcont can be obtained. Then, an image is formed according to the Vcont value.

FIG. 7 shows the state of a laser beam printer to which the Vcont correction procedure mentioned above applies.

First, it is seen at step of adjustment that Dmax is lower by 0.1 The correction parameter is 1. The environmental states showing the environmental characteristics of A', B', and C' are corrected. Then, the amounts of correction are added to them by 1/16 Vcont-a, 1/16 Vcont-b, and 1/16 Vcont-c, so that the proper Vcont will be Vcont-a+1/16 Vcont-a, Vcont-b+1/16 Vcont-b, and Vcont-c+1/16 Vcont-c. When the Vcont table indicated with the dot-dash line in FIG. 5 is used, correction can be done so properly that Dmax becomes constant at 1.5 in all environments. As a result, constant images can be supplied.

The processing sequence shown in FIG. 1 until processing S4 when the proper Vcont is calculated, which has been explained in Embodiment 1, is also applicable to Embodiment 2.

Supposing curves A', B', and C' have more sharp slopes than those shown in FIG. 5, approximation using the certain ratio of Vcont a the amount of unit correction, which is performed in Embodiment 1, will be less reliable.

Therefore, a function expressed with the alternate long and two dashes line in FIG. 5 is designed in advance. Then, the function is stored in memory 13 in FIG. 2, converted into the amount of correction with a correction parameter in the same way as processing S6 in FIG. 1, and then added to the normal Vcont table indicated with the solid line in FIG. 5. Thus, correction can yield the same effects as those for normal curves.

As explained heretofore, when the present invention applies to an image formation system which performs environmental control for development contrast to obtain proper development density according to the amount detected by an environmental detection means in the system, if proper development density cannot be obtained, correction is performed in such a way that the amount of correction to correct development contrast is modified according to the result detected by the environmental detection means. This, even if correction applies to normal environmental control, the development density can remain constant regardless of the environment.