Method of and apparatus for forming image using a Non-Contact Charger

A charger charges a body with a voltage in which an AC voltage is superimposed on a DC voltage. The charger and the body have a small gap therebetween. A sensor measures a humidity in the gap between the charger and the body. A magnitude of the AC voltage to be superimposed on the DC voltage is determined based on the humidity detected.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to a technology in which there is a gap between a latent image carrier and a charger, which charges the latent image carrier.

2) Description of the Related Art

In an image forming apparatus like a copying machine, a printer, a facsimile, a printing machine etc., an electrostatic latent image is formed on the latent image carrier according to information from a stepper, a host computer etc. The electrostatic latent image is then visualized and the visualized image is transferred to a recording medium like a paper. The image is then fixed and output as either as a copy or an original.

A surface of a photosensitive layer of the latent image carrier is uniformly charged with a charger, which is generally a roller (hereinafter, “charging roller”), before forming the electrostatic latent image. Two charging methods are known; non-contact charging and contact charging. In the non-contact charging, the charging roller does not make a physical contact with the latent image carrier. The non-contact charging is also called corona charging.

On the other hand, in the contact charging, a charging roller makes a physical contact with the latent image carrier. In the non-contact charging, a minute gap is set between the latent image carrier and the charging roller and a discharge is caused to occur in the minute gap. For example, Japanese Patent Application Laid Open Publication No. 1991-240076 discloses the non-contact charging.

The contact charging has an advantage over the non-contact charging in that less quantity of ozone gas is generated in the charging process. However, in the contact charging, since the charging roller touches the latent image carrier, residual toner, paper dust etc. easily gets stuck to the charging roller and causes uneven charging of the latent image carrier. Uneven charging of the latent image carrier leads to degraded image. In this respect the non-contact charging in preferable over the contact charging.

Two methods are widely used to apply a charging bias to the charging roller, whether it be the contact charging or the non-contact charging; AC-application method and DC-application method. In the DC-application method, a DC voltage, which is constant voltage controlled, is applied to the charging roller. In the AC-application method, an AC voltage, which is constant voltage controlled, is superimposed on a DC voltage, which is constant voltage controlled, and the resultant voltage is applied to the charging roller.

The charging bias applied to the charging roller varies depending on the physical properties, for example, the surface resistance, of the charging roller. The physical properties of the charging roller change with the environmental conditions, for example, humidity, temperature, around the charging roller. Japanese Patent Publication Nos. 3154628, 1997-120199 disclose changing the charging bias applied to the charging roller depending on the environmental conditions around the charging roller.

The non-contact charging has a typical problem that the width of the gap between the charging roller and the latent image carrier changes with the environmental conditions around the gap. The change in the width of the gap is due to expansion or reduction of the charging roller, the latent image carrier, or a spacer, used to maintain the gap, due to a change in the humidity or the temperature around the gap. The change in the gap is difficult to deal with because the charging roller, the latent image carrier, and the spacer expand or reduce differently at different locations. If the width of the gap at different locations is different, the charging becomes uneven and that degrades the quality of the image.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a charger charges a body with a voltage in which an AC voltage is superimposed on a DC voltage. The charger and the body have a small gap therebetween. A sensor measures a humidity in the gap between the charger and the body. A magnitude of the AC voltage to be superimposed on the DC voltage is determined based on the humidity detected.

According to another aspect of the present invention, a charger charges a body with a voltage in which an AC voltage is superimposed on a DC voltage. The charger and the body have a small gap therebetween. A sensor measures a humidity and a temperature in the gap between the charger and the body. A magnitude of the AC voltage to be superimposed on the DC voltage is determined based on at least one of the humidity and the temperature detected.

DETAILED DESCRIPTION

Exemplary embodiments of the image forming method and image forming apparatus according to the present invention are explained below while referring to the accompanying diagrams.

FIG. 1is a schematic of an image forming apparatus according to an embodiment of the present invention. This image forming apparatus may be a copying machine, a printer, a facsimile, or a multifunction device. A multifunction device is the one that includes two or more selected from a group consisting of copying machine, a printer, and a facsimile.

The image forming apparatus includes an image carrier5. The image carrier5includes a substrate6, which is drum-shaped, and a photosensitive layer7on the surface of the substrate6. The image carrier5rotates in a direction of the arrow AR1.

A charging unit1uniformly charges the photosensitive layer7of the image carrier5. The charging unit1includes a roller-shaped charger2. There is a gap G between the image carrier5and the charging unit1.

The charger2includes a core8. The core8is made of electrically conductive material. A layer9is provided on the surface of the core8. The layer9is made of, for example, material having elastic and electrically resistive material, like rubber. The layer9may be made of hard material, like resin. A thin layer10made of highly electrically resistive material may be formed on the surface of the layer9.

The charger2is interlocked with the image carrier5so as to rotate in the direction of an arrow AR2with the image carrier5. The charger2and the image carrier5may be driven with a single driving means or separate driving means. When the charger2and the image carrier5are driven with a single driving means, spacers11(seeFIG. 2) provided at the ends of the charger2convey the driving force from the charger2to the image carrier5or vice-versa.

FIG. 2is a front view of the charger2and the image carrier5. The spacers11maintain the gap G between the charger2and the image carrier5. The spacers11are essentially made of a ring-shaped resin film. The resin film is stuck to the outer surface of the charger2with adhesive. Total thickness of the resin film and the adhesive is about 60 μm. The charger2, and therefore the spacers11, is pressed towards the image carrier5by a spring SP.

A power supply unit3applies voltage of about −750 volts to the core8of the charger2under the control of a control unit (for example, a CPU)4. As a result, an electric discharge is generated in the gap G. This electric discharge charges the photosensitive layer on the image carrier5to a prescribed polarity.

A writing unit12forms an electrostatic latent image on the charged image carrier5based on image data. The writing unit12, for example, irradiates laser light L to form the electrostatic latent image. An electric potential of the portion of the image carrier5on which the laser light L is irradiated falls as compared to the portion on which the laser light L is not irradiated. As a result, the portion of the image carrier5on which the laser light L is irradiated becomes an image and the other portion becomes a non-image.

The electrostatic latent image on the image carrier5is then visualized. A developing unit27carries out this visualization by spraying a negatively charged toner on the image carrier5. A transfer unit13, which is roller, transfers the visualized image on the image carrier5to a recording medium S. A cleaning unit14cleans the image carrier5and removes the toner remained on the image carrier5.

A voltage in which AC voltage is superimposed on DC voltage is applied to core8of the charger2. The AC voltage and the DC voltage are constant voltage controlled. Precisely, a peak-to-peak voltage of the AC voltage is constant voltage controlled. As a result, a uniform electric potential is generated in the gap G irrespective of a variation in the width of the gap G.

Consider, for example, that an AC voltage of which the peak-to-peak voltage is constant voltage controlled is superimposed on a DC voltage of −750 volts and the resultant voltage is applied to the core8.FIG. 3is a graph of the peak-to-peak voltage Vpp and a surface potential of the image carrier5. The curves X1, X2, X3, and X4correspond to cases when the width of the gap G is 80 μm, 60 μm, 40 μm, and zero, respectively. It is assumed here that the frequency of the AC voltage is constant.

It can be seen from the graph inFIG. 3that, when the peak-to-peak voltage Vpp is greater than a certain value, the surface potential of the image carrier5is almost constant and does not depend on the width of the gap G. Moreover, the surface potential is almost same as the DC voltage (that is, −750 volts) applied to the core8.

For example, when the width of the gap G is 80 μm, an electric potential of −750 volt is generated on the surface of the image carrier5if the peak-to-peak voltage increases to a value little higher than Vpp1. Similarly, when the width of the gap G is 60 μm, 40 μm, and zero, an electric potential of −750 volt is generated on the surface of the image carrier5if peak-to-peak voltages increase to values little higher than Vpp2, Vpp3, and Vpp4, respectively. The values of Vpp1, Vpp2, Vpp3, and Vpp4can be easily obtained by experiments.

Thus, there is no effect of the width of the gap G on the surface potential of the image carrier5if the peak-to-peak voltage is such that the surface potential of the image carrier5is equal to the DC voltage. For example, in an image forming apparatus in which the width of the gap G is 80 μm, if an AC voltage in which the peak-to-peak voltage is higher than Vpp1is superimposed on a DC voltage and the resultant voltage is appiled to the core8, then the image carrier will be charged with a constant surface potential (equal to the DC voltage) even if there are small variations in the width of the gap G.

Similarly, in an image forming apparatus in which the width of the gap G is 60 μm, if an AC voltage in which the peak-to-peak voltage is higher than Vpp2is superimposed on a DC voltage and the resultant voltage is appiled to the core8, then the image carrier will be charged with a constant surface potential (equal to the DC voltage) even if there are small variations in the width of the gap G. Similarly, in an image forming apparatus in which the width of the gap G is 40 μm, if an AC voltage in which the peak-to-peak voltage is higher than Vpp3is superimposed on a DC voltage and the resultant voltage is appiled to the core8, then the image carrier will be charged with a constant surface potential (equal to the DC voltage) even if there are small variations in the width of the gap G. And, in an image forming apparatus in which there is almost no gap between the image carrier and the charger, if an AC voltage in which the peak-to-peak voltage is higher than Vpp4is superimposed on a DC voltage and the resultant voltage is appiled to the core8, then the image carrier will be charged with a constant surface potential (equal to the DC voltage) even if a small gap is generated between the image carrier and the charger.

Thus, if the voltage is applied to the core8in this manner, since the image carrier5can be charged with a uniform surface potential irrespective of the width of the gap G between the image carrier and the charger2, the image quality does not change even if the width of the gap G varies.

Moreover, in the present invention the AC voltage to be superimposed on the DC voltage is constant voltage controlled rather than constant current controlled. The constant current control of the voltage is not preferable because, output of the power pack becomes unstable as the output voltage is varied according to the variation in the gap. This tends to give rise to a defective image, which is a problem peculiar to the non-contact charging.

It should be noted that, a voltage Vpp5(seeFIG. 3), which is higher than Vpp1, may be applied to the core8to obtain the same results. However, since the image quality degrades as the voltage becomes high, it is desirable that minimum required voltage is applied.

As already mentioned above, the width of the gap G changes as the environmental conditions around the gap change. For example, the width of the gap G changes as the charger2or the image carrier5expands or contracts due to changes in an ambient temperature or a humidity in the gap G.

When there is a variation in the gap G, the value of the peak-to-peak voltage that is set based on results inFIG. 3, differs from the value that is in accordance with the variation in the gap G. This results in application of excessively high or low voltages, which gives rise to uneven charging.

Table 1 illustrates the experimental results about the width of the gap G against environmental conditions around the gap G. The absolute humidity was calculated from the temperature and the relative humidity.

FIG. 4is a graph of width of the gap G against the absolute humidity. The gap G becomes narrower as the absolute humidity increases. In other words, if the AC voltage to be applied to the core is adjusted based on the absolute humidity around the gap G, then an effect of change in the width of the gap due to change in the absolute humidity on the surface charge of the image carrier can be eliminated.

Referring toFIG. 1, a sensor15measures the environmental conditions, for example, the ambient temperature and the humidity, around the gap G, and passes the data to the control unit4. The control unit4calculates the absolute humidity from the temperature and relative humidity and controls the power supply unit3based on the result of the calculation.

Table 2 indicates results of the measurement of lower limit peak-to-peak voltage Vpp at which an image with defective discharge was formed due to insufficient AC voltage (insufficient AC bias voltage) in various environments caused by combination of a charging roller and an image carrier. A graph plotted with the absolute humidity on the horizontal axis, indicates a relationship as inFIG. 5.

An excess AC voltage is to be avoided to avoid occurrence of filming phenomenon. To avoid the excess AC voltage, it is necessary to apply voltage higher than lower limit peak-to-peak voltage Vpp and peak-to-peak voltage Vpp as close to the lower limit peak-to-peak voltage. The control unit4sets the peak-to-peak voltage Vpp in accordance with the absolute humidity, and the power supply unit3applies that voltage to the core8of the charger.

In an experiment, the inventor of the present invention set the peak-to-peak voltage as indicated by a dotted line inFIG. 5for each of the absolute humidity and observed images on the surface of the image carrier5after passing 20,000 papers. The charging was found to be ideal without occurrence of filming and defective discharge on the image. However, some filming was observed on the photoreceptor.

It is desirable to change the peak-to-peak voltage when there is a change in the environment. However, if the peak-to-peak voltage is changed abruptly, the image carrier is charged unevenly and the image quality degrades.

In the present embodiment, the bias change takes place during charging of the area corresponding to the non-image area on the image carrier. That is, when the image forming is continued, there is a rise in temperature inside the apparatus during passing of a paper and variation in the absolute humidity. A bias change of AC voltage according to the variation in the absolute humidity is necessary. Timing for bias change is a time between the passing of two papers. In other words, bias change is carried out during charging of a position that corresponds to the image forming position except the position of the paper passing section.

Experimental results produced a satisfactory image without any horizontal lines after setting the time between passing of two papers for 200 hundred papers as time for bias change.

Thus, when a correction control is carried out for a set of prescribed number of papers, it is possible to apply a suitable bias all the time. Moreover, it is possible to apply suitable bias according to variations in the charging unit, which cannot be detected only by temperature and humidity detecting unit. For example, it is possible to apply suitable bias even in case of a rise in resistance of a roller due to visible contamination of the roller.

However, the curve of the peak-to-peak voltage Vpp shown in FIG. has an irregularity at point A. This irregularity occurred due to environment having high temperature and low humidity (see Table 2).

An electric resistance of the charger2, which is made of rubber having low resistance, varies with the temperature. Especially, at low temperature, the electric resistance tends to vary considerably.

The relationship between the temperature and the resistance that is illustrated inFIG. 6is known. As a result of this relationship, if the peak-to-peak voltage Vpp is set such that there is no defective charging even in a low temperature high moisture environment, there is a tendency that bias of AC voltage becomes excessive in a high temperature low moisture environment of the absolute moisture rather than the low temperature high moisture environment. This means that it is desirable to carry out correction corresponding to the temperature and along with the correction corresponding to the absolute humidity.

In the image forming apparatus according to the present embodiment, as indicated inFIG. 7, correction corresponding to the temperature is performed based on whether 1) the temperature is less than 20° C., 2) the temperature is in the range of 20° C. to 25° C., and 3) the temperature is above 25° C., in addition to the correction corresponding to the absolute humidity.

In an experiment, 20,000 papers were passed through the image forming apparatus, in the same manner as explained in connection with the explanation ofFIG. 5, and satisfactory charging without occurrence of filming of the photoreceptor was achieved.

It is explained above that the correction corresponding to the temperature is performed depending upon three temperature conditions. However, the correction corresponding to the temperature may be performed depending upon more than three temperature conditions. Correction that is more precise can be performed if the temperature conditions are more.

In the present embodiment, when correction of bias conditions is carried out in accordance with the absolute humidity and temperature, a mechanical error between the charger2and the image carrier5is also taken into consideration. The combination of roller that is used as a charger and the image carrier5is same for a particular lot of image forming apparatuses that is manufactured but it varies from lot to lot. Due to the variation, thickness of the spacers11which is used for setting either of the resistance of the charging roller and the minute gap, is not uniform in all lots. Therefore, if there is a variation in either of the resistance value and the thickness, the relationship between the absolute humidity and the maximum gap that is illustrated graphically inFIG. 4is not uniform for all image forming apparatuses.

FIG. 8is a graph of the absolute humidity against the maximum width of the gap under same conditions as inFIG. 4. As it is seen inFIG. 4, there is some variation in the relationship due to characteristics of material that is used.

For this reason, it is not possible to use the relationship indicated inFIG. 4as a standard for correction of AC voltage in all image forming apparatuses as a common item. When the variation in each lot is such that it cannot be neglected, it is necessary to assess a characteristic value for each apparatus, to find out the relationship illustrated inFIG. 8, and based on this to carry out correction control of AC voltage according to the relationship indicated in either ofFIGS. 5 and 7. However, this sort of measures takes lot of time and hence not practical.

In the present embodiment, in application of AC component, when AC voltage that is subjected to constant current control is superimposed on DC voltage that is subjected to constant voltage control, current that is supplied to the charger2and the charging potential on the surface of the image carrier have almost a uniform relationship irrespective of variation in the gap (indicated by reference numerals G1, G2, and G3inFIG. 9). If the current is greater than or equal to I0, the electric potential on the surface of the image carrier is maintained at a uniform value. The matching of the uniform value with the constant DC voltage that is applied on the charger2is taken into consideration. Peak-to-peak voltage obtained at target current value that is equivalent to a saturation current value greater than or equal to the current value I0, is detected while detecting the current that is supplied to the charger2when a certain peak-to-peak voltage is applied to AC voltage. Assuming a feed back control in which the peak-to-peak voltage is applied on the AC voltage, the target value current that is used for the control is adjusted in accordance with the absolute humidity.

That is, the target value of current is divided in 5 stages according to the absolute humidity as indicated in Table 3.

In the present embodiment, by adjusting the target value of current during detection of the peak-to-peak voltage in accordance with the absolute humidity, insufficient application of bias of DC voltage due to variation in environment like temperature and humidity is eliminated. Further, if there is a difference in either of resistance and gap for different lots, the charging characteristics between two different lots can be made uniform.

The inventor of the present invention carried out image formation by passing 20,000 papers without using correction in accordance with the absolute humidity and with conditions in the present embodiment and compared the two. When the correction was not carried out, the toner filming did not appear on the image but was observed on the image carrier. When correction was carried out, there was no toner filming on the image carrier.

In the present embodiment, it is possible to carry out the correction control not only for the absolute humidity as indicated in Table 3, but also for temperature in addition to the absolute humidity.

Thus, it is possible to maintain the charging characteristics throughout, irrespective of the variation in the environment by adjusting minutely the bias conditions of AC voltage rather than just carrying out correction in accordance with the absolute moisture.

In the present embodiment, it is assumed that the feed back control and the correction of bias conditions for a set of prescribed number of papers is carried out. However, to carry out the correction of the bias conditions in accordance with the variation in the minute gap that occurs due to variation in environment like temperature and humidity, is a necessary condition to maintain uniform charging characteristics. Therefore, in the present embodiment, it is also possible to carry out the correction when the variation in the temperature and the humidity is beyond prescribed level.

Moreover, the correction can be carried out for a period of time during which the temperature and humidity tend to vary considerably. That is, when the image forming apparatus starts operating, units in the apparatus, which are in stopped condition so far, start operating, due to which the temperature and humidity tend to vary. Particularly, the starting of operation of the fixing unit raises the temperature inside the image forming apparatus. Therefore, the relative humidity and temperature vary, thereby dropping the resistance of the charger2as indicated inFIG. 6. Due to the drop in the resistance, the AC voltage on which the peak-to-peak voltage is applied rises excessively thereby hastening the fatigue of the image carrier. Besides, due to initial draft of the power pack, the output to the power pack tend to delay slightly by prescribed time after the power supply is put ON. Therefore, in the present embodiment, till the annulment of this instability, i.e. after passing of prescribed time only after putting the power supply ON, the correction control is carried out once again. Thus, it is possible to improve an accuracy of the correction control by annulling the instability.

According to the present invention, when a charging bias is applied by superimposing an AC voltage on a DC voltage to carry out discharge in a minute gap, the AC voltage is corrected in accordance with an absolute humidity. Therefore, even when the minute gap varies due to a variation in the environment, bias conditions of the AC voltage corresponding to the variation are obtained, thereby enabling to prevent a variation in charging characteristics.

Moreover, even if the electric resistance of a charger varies due to variations in the environment conditions, a correction of bias in accordance with a temperature and a humidity is possible. Therefore, a variation in charging characteristics can be prevented satisfactorily.

Furthermore, a control unit carries out control of AC voltage in accordance with a value of DC current detected in a charging unit. The control unit can vary a target value for control in accordance with a temperature and humidity. Therefore, even if the electric resistance varies due to a difference in accuracy of each product in different lots, it is possible to carry out a feed back control of a target value for control of an electric resistance for each lot. It is also possible to uniform charging characteristics of a latent image carrier in a lot by changing the target value for control in accordance with temperature and humidity. Thus, charging unevenness in the latent image carrier can be prohibited.

Moreover, a bias correction of AC voltage is carried out if the environment conditions vary beyond a prescribed level. Therefore, it is possible to maintain desirable charging characteristics throughout by a correction that is required to be carried out from time to time.

Furthermore, a bias correction is carried out in accordance with a variation in an environment and a variation in an output immediately after a power supply to an apparatus is put ON. Therefore, it is possible to maintain desirable charging characteristics throughout right from a start up, thereby preventing a charging unevenness in a latent image carrier.

Moreover, a correction control of AC voltage is carried out. Therefore, it is possible to prevent variation in an apparatus, which cannot be recognized only by variation in environment conditions. For example, a variation in charging characteristics due to contamination etc. of a charger can be prevented.

Furthermore, a control of a bias change is carried out outside an image forming section (area) of an image. Therefore, there is no bias variation in an image area and it is possible to stabilize charging characteristics while reducing noise (defective image having horizontal lines) in the image.

Moreover, even if the charging roller is made of an elastic material of medium resistance, it is possible to stabilize charging characteristics in accordance with environment conditions by reducing variation in charging characteristics that is caused due to influence of temperature. Thus, it is possible to prevent charging unevenness in the latent image carrier.

The present document incorporates by reference the entire contents of Japanese priority document, 2002-223687 filed in Japan on Jul. 31, 2002.