Image forming apparatus which includes an image bearing body surface potential detection feature

An image forming apparatus includes an image bearing body which can bear an electrostatic image; a bias member to which a predetermined bias is applied from a bias applying device; and a surface potential detection device which detects a surface potential at the image bearing body. The surface potential detection device includes a detector portion which generates a signal corresponding to the surface potential at the image bearing body and a potential detection portion which detects the surface potential by the signal from the detector portion. In the image forming apparatus, the potential detection portion is also used for detection of a bias value which the bias applying device applies to the bias member, the bias applying device is controlled based on the detection result of the bias which the bias applying device applies, and the bias detection result is obtained by the potential detection unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus such as an electrophotographic printer and an electrophotographic copying machine.

2. Related Background Art

FIG. 13shows a development bias circuit and a surface potential measurement circuit as a configuration example of an image producing (image forming) control circuit in the image forming apparatus such as the electrophotographic printer and the electrophotographic copying machine. At this point, the conventional development bias circuit will be described as an example of bias generation circuits. Because a constant-voltage system bias generation circuit such as grid bias has the same configuration and control method, the description of the constant-voltage system bias generation circuit is omitted.

InFIG. 13, the reference numeral11adenotes a photoconductor drum which is rotated in the direction of arrow R1, the reference numeral12adenotes a primary charger which evenly charges a surface of the photoconductor drum11a, the reference numeral18adenotes a surface potential sensor which detects a surface potential at the photoconductor drum11a, and the reference numeral14adenotes a development device which develops an electrostatic latent image on the photoconductor drum11a.

The reference numeral70ashows the configuration of the development bias circuit. The development bias circuit70ahas a direct-current bias generation portion71a, a generation bias detection portion72a, and a direct-current bias control portion73a. The reference numeral90ashows the configuration of the surface potential measurement circuit. The surface potential measurement circuit90ahas a sensor control portion91a, a sensor direct-current bias generation portion92a, a sensor generation bias detection portion93a, and a detection-signal transmission portion94a. The reference numeral95shows an apparatus control portion which controls the image forming apparatus. The apparatus control portion95has a D/A conversion portion96awhose output portion is connected to the development bias circuit70aand an A/D conversion portion97awhose output portion is connected to the surface potential measurement circuit90a.

In the image producing control circuit having the above configuration, the development bias circuit70ais operated according to a control signal from the apparatus control portion95. At first the apparatus control portion95directs the development bias circuit70ato output a desired bias output value by an analog signal level through the D/A conversion portion96a. In the development bias circuit70a, the direct-current bias control portion73areceives the analog signal. In response to the signal from the D/A conversion portion96a, the direct-current bias control portion73aoperates direct-current bias generation portion71ato cause the direct-current bias generation portion71ato generate a direct-current bias which is of a development bias. The direct-current bias generated in the above way is converted into a detection signal by the generation bias detection portion72a, and the detection signal is transmitted to the direct-current bias control portion73a. The direct-current bias control portion73acompares the detection signal to the analog signal from the D/A conversion portion96a, and the direct-current bias control portion73atransmits the control signal to the direct-current bias generation portion71aso that the detection signal and the analog signal agree with each other.

Then, the surface potential measurement circuit90ais also controlled by the apparatus control portion95. The sensor control portion91atransmits a drive signal to the surface potential sensor18a. The surface potential sensor18ais operated according to the drive sensor to send out a measurement signal following the potential difference between the surface potential sensor18aand the photoconductor drum11a. The sensor control portion91areceives the signal to operate the sensor direct-current bias generation portion92aso that the signal is minimized, i.e. the surface potential at the photoconductor drum11abecomes equal to the potential at the surface potential sensor18a.

Thus, the surface potential at the photoconductor drum11aand the generation bias value of the sensor direct-current bias generation portion92ais controlled so as to become the same potential. On the other hand, the sensor generation bias detection portion94aconverts the generation bias of the sensor direct-current bias generation portion92ainto the detection signal to transmit the detection signal to the A/D conversion portion97athrough the detection signal transmission portion94a. The A/D conversion portion97aperforms digital conversion of the detection signal to notify the apparatus control portion95of the detection result.

With reference to a technique of improving detection accuracy of the surface potential sensor, Japanese Patent Application Laid-Open No. H08-201461 discloses a method in which switch means for switching the photoconductor drum to a floating state is provided, a reference voltage is provided to the photoconductor drum in the floating state, and detection properties are corrected by measuring the potential at the photoconductor drum with a potential sensor.

However, according to the above-mentioned image forming apparatus, the surface potential sensor measurement circuit of the photoconductor drum and the bias circuit which performs an image producing process such as the development bias individually have the bias detection circuit. Further, the bias detection circuits are separately attached to different places due to constraints of an apparatus space. Therefore, variations in components constituting the detection circuit, temperature characteristics of the components, variations in temperature environment, and the like affect subtly detection characteristics and detection errors of the components, which generates variations in potential detection result and bias output control result. As a result, there is the problem that image densities differ from one another among the apparatuses, or the problem that difference in image density is generated according to temperature change among the apparatuses even if the image densities agree with one another under a certain condition.

Even in the same apparatus, there is the problem that the image density fluctuates according to the temperature change in the apparatus. In the case of the color image forming apparatus, there is the problem that color tint of the image is changed.

Because the temperature change in the apparatus is largely generated during continuous print in which plural sheets are printed, there is the problem that the initial print sheet differs from the print sheet, which is printed after a certain time elapses, in the image density and the initial color tint during continuous printing.

A surface temperature of the photoconductor drum varies during continuous printing, which changes a surface potential VL (light section potential) of the photoconductor drum in the maximum exposure. Therefore, there is generated the problem that the image density and the color tint are changed.

The temperature change in a bias measurement system in a primary grid changes a dark section potential VD and the light section potential VL, which generates the problem that the image density and the color tint are fluctuated.

When the light section potential VL is measured during the continuous print, sometimes there is the problem that a fog image is generated in the measurement to shorten a life of the cleaning device of the photoconductor drum.

Because the above problems are generated in each photoconductor drum, the same problems including the difference in color tint exist with respect to the fluctuation in image quality.

In the A/D conversion of the potential measurement detection result, or in the bias output detection result and the A/D conversion during the digital control of the bias circuit, since each circuit has a quantization error, and sometimes a mutual shift caused by the quantization error emerges by adding the mutual shift to a measurement error, which generates the problem that the image density is further changed.

According to the method disclosed in Japanese Patent Application Laid-Open No. H08-201461, the measurement accuracy can be increased based on the development bias output by utilizing the development bias generation device which is of the bias generating means for applying the reference voltage. However, in the case where the development bias output itself is changed due to the temperature change, there is the problem that a relationship between a charged potential and a development potential cannot be kept constant. Although the problem can be solved by repeating correction control, it is necessary that the photoconductor drum is in the floating state. Therefore, because it is necessary to stop the image forming process, the correction cannot be realized without interrupting the printing during the continuous print.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of the invention is to provide an image forming apparatus which can stably form an image by detecting potential more stably.

In order to achieve the object, an image forming apparatus according to the invention including:

an image bearing body which can bear an electrostatic image;

an bias member which is provided opposite to the image bearing body and to which a predetermined bias is applied;

bias means which applys the predetermined bias to the bias member;

surface potential detection means which detects a surface potential at the image bearing body, the potential detection means including a detector portion which generates a signal corresponding to the surface potential at the image bearing body and potential detection means which detects the surface potential by the signal from the detector portion,

wherein the potential detection means is also used for detection of a bias value which the bias means applies to the bias member; and

control means which controls the bias means based on the detection result of the bias which the bias means applies to the bias member, the bias detection result being obtained by the potential detection means.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the accompanying drawings, preferred embodiments of the invention will be described. In the drawings, the same constituent having the same configuration or action is indicated by the same reference numeral and sign. A redundant description regarding the same constituent shall be omitted as appropriate.

First Embodiment

FIG. 1is a longitudinal sectional view showing a main part of an image forming apparatus to which the invention can be applied. InFIG. 1, an image forming apparatus1is an electrophotographic image forming apparatus. The image forming apparatus1includes a reader portion (optical system)1R in an upper part of the image forming apparatus1and a printer portion (image output portion)1P in a lower part. The reader portion1R reads an image of a manuscript, and the printer portion1P forms the image (toner image) in a transfer material P based on image information from the reader portion1R. The image forming apparatus1has plural (four) image forming stations (image forming portion in narrow sense)10a,10b,10c, and10dwhich are arranged in parallel in an image forming portion (image forming portion in a broad sense)10. An intermediate transfer body method is used for the image forming apparatus1. Particularly the invention is effectively applied to the image forming apparatus to which the intermediate transfer body method is used.

The printer portion1P mainly includes an image forming portion10, a paper-feed portion20, an intermediate transfer portion30, a fixing portion40, and a control portion80(not shown).

The image forming portion10includes the four image forming stations10a,10b,10c, and10dhaving the substantially same configuration. Yellow (Y), cyan (C), magenta (M), and black (K) toner images are sequentially formed in the four image forming stations10a,10b,10c, and10d. Drum-shaped electrophotographic conductor bodies (hereinafter referred to as hotoconductor drum11a,11b,11c, and11dwhich are of an image bearing body are journaled in the center of the image forming stations10a,10b,10c, and10drespectively. The photoconductor drums are rotated in the direction of their respective arrows (counterclockwise direction inFIG. 1). Primary chargers (charging means)12a,12b,12c, and12d, exposure devices (irradiating means)13a,13b,13c, and13dwhich are of an exposure device, folding mirrors16a,16b,16c, and16, and development devices (bias member)14a,14b,14c, and14dare respectively arranged in a rotating direction of the photoconductor drums11ato11awhile being opposite outer surfaces of the photoconductor drums11ato11d.

As shown in a part of the photoconductor drum11aofFIG. 5, each of the photoconductor drum11ato11dhas an electrically conductive drum substrate (base layer)11A which is grounded and a photoconductor layer11B which is provided so that the outer surface of the drum substrate11A is covered with the photoconductor layer11B.

Each of the primary chargers12ato12dprovides a uniform amount of charge to the surface (hereinafter simply referred to as photoconductor drum surface) of each photoconductor layer11B of the photoconductor drums11ato11d. Then, the exposure devices13ato13dmodulate a light beam (exposure light) such as a laser beam according to a recording image signal to expose the photoconductor drums11ato11dwith the light beams through the folding mirrors16ato16d, which forms the electrostatic latent image on the photoconductor drums11ato11d.

The electrostatic latent image is visualized as a toner image (development image) by the development devices14ato14din which development agents (hereinafter referred to as “toner”) such as yellow, cyan, magenta, and black color development agents are stored respectively. The visualized toner image is transferred (primary transfer) in image transfer areas Ta, Tb, Tc, and Td of an intermediate transfer belt31which is of an intermediate transfer body.

When the photoconductor drums11ato11dare rotated, on the downstream side where the photoconductor drums11ato11dpass through the image transfer areas Ta to Td, cleaning devices15a,15b,15c, and15dclean the photoconductor drum surface by wiping out the toner which is not transferred to intermediate transfer belt31but remains on the photoconductor drums11ato11a. Thus, the image formation performed through the above process with each toner is sequentially performed.

The paper-feed portion20includes cassettes21aand21b, a manual feed tray27, pickup rollers22a,22b, and26, plural pairs of conveying rollers23, plural paper-feed guides24, and registration rollers25aand25b. The sheets of transfer material P are stored in the cassettes21aand21b. Each of the pickup rollers22a,22b, and26delivers the sheet of transfer material P one by one from the cassettes21aand21bor the manual feed tray27. The plural pairs of conveying rollers23and the plural paper-feed guides24convey the transfer material P delivered from each of the pickup rollers22a,22b, and26to the registration rollers25aand25b. The registration rollers25aand25bdeliver the transfer material P to a secondary transfer area Te in synchronization with image forming timing of the image forming portion10.

An endless intermediate transfer belt31is provided in the intermediate transfer portion30. The intermediate transfer belt31is entrained about three rollers, i.e. a drive roller32which transfer drive to the intermediate transfer belt31, a driven roller33which is rotated while following the rotation of the intermediate transfer belt31, and a secondary transfer opposing roller34which is located opposite to the secondary transfer area Te while sandwiching the intermediate transfer belt31. A primary transfer plane A is formed between the drive roller32and the driven roller33. In the drive roller32, the surface of a metal roller is coated with rubber (urethane or chloroprene) having a thickness of several millimeters in order to prevent a slip between the drive roller32and the intermediate transfer belt31. The drive roller32is rotated in the direction of the arrow by a pulse motor (not shown), which rotates the intermediate transfer belt31in the direction of arrow B.

The primary transfer plane A is opposite the image forming portions10ato10d, and the photoconductor drums11ato11dare configured to be opposite to the primary transfer plane A of the intermediate transfer belt31. Accordingly, the primary transfer areas Ta to Td are located in the primary transfer plane A. In the primary transfer areas Ta to Td where the photoconductor drums11ato11aare opposite to the intermediate transfer belt31, primary transfer chargers35a,35b,35c, and35dare arranged on the backside of the intermediate transfer belt31. A secondary transfer roller36is arranged opposite to the secondary transfer opposing roller34, and the secondary transfer area Te is formed by a nip between the secondary transfer roller36and the intermediate transfer belt31. The secondary transfer roller36is pressed against the intermediate transfer belt31with proper pressure. On the downstream of the secondary transfer area Te on the intermediate transfer belt31, a belt cleaner50is provided at a position corresponding to the driven roller33. The belt cleaner50has a cleaning blade51and a waste-toner box52. The cleaning blade51cleans the image forming plane (surface) of the intermediate transfer belt31, and the waste-toner box52which is wiped out by the cleaning blade51.

The fixing portion40includes a fixing device41, a guide43, a pair of inner paper-discharge rollers44, and a pair of outer paper-discharge rollers45. The fixing device41has a fixing roller41awhich includes a heat source such as a halogen lamp heater inside the fixing roller41aand a pressing roller41bwhich is pressed against the fixing roller41a. (In some cases, the pressing roller41bincludes the heat source inside the pressing roller41b.) The guide43guides the transfer material P to the nip portions of the pair of the fixing roller41aand the pressing roller41b. The pair of inner paper-discharge rollers44and the pair of outer paper-discharge rollers45further discharge the transfer material P delivered from the pair of the fixing roller41aand the pressing roller41bto a paper-discharge tray48located outside the image forming apparatus.

Then, the image producing (image forming) process will be described in detail referring toFIG. 2. The image forming station10awill be described here as a representative of the image forming portion10. Needless to say, the image forming stations10b,10c, and10dhave the configuration.

A primary grid17aand a surface potential sensor18aare shown inFIG. 2while the primary grid17aand the surface potential sensor18aare neither described nor shown inFIG. 1. The primary grid17ais an electrode which is set to a predetermined voltage, and the primary grid17ais provided between the primary charger12aand the photoconductor drum11ain parallel with the primary charger12a. The primary grid17aadjusts a current flowing into the photoconductor drum11afrom the primary charger12a, which allows the amount of charge on the surface of the photoconductor drum11ato be controlled. The surface potential sensor18ais provided on the downstream side of the exposure position (position irradiated with the laser beam from the exposure device13a) along the rotating direction of the photoconductor drum11aand on the upstream side of the development device14a. The surface potential sensor18ameasures the charge potential on the surface of the photoconductor drum11a, which enables the stabilization of the image density and the control of the image quality.

FIG. 3shows charging characteristics of the photoconductor drum11a. The charge characteristics indicates the relationship between the surface potential at the photoconductor drum11aand the development bias applied to the development device14a, and the relationship determines the image quality. InFIG. 3, a horizontal axis represents a setting potential (grid potential). Vg in which the primary grid17ais set, and a vertical axis represents the surface potential (potential amount) V. The sign VD denotes the dark section potential (after the photoconductor drum surface is charged, the surface potential at photoconductor drum11awhen the exposure is not performed), the sign VL denotes the light section potential (the surface potential at the photoconductor drum11awhen the exposure is performed at the maximum level), and the sign Vdc denotes the setting potential at the development bias.

The charge amount V of the photoconductor drum11atends to increase as the setting voltage Vg of the primary grid17ais increased. The increase in dark section potential VD inFIG. 3shows the characteristics. The light section potential VL tends to increase as the dark section potential VD is increased, and the light section potential VL inFIG. 3shows the characteristics.

The setting value of the development bias is determined by permissible value of a fog amount in a portion where the image is not formed. The reason why the fog is generated is that the toner having the different charge amount which exists exceptionally in the development device14a(for example, the toner having the exceptionally higher charge amount) possesses enough potential to develop the light section potential VD. Accordingly, the development bias Vdc is set to the level in which the exceptional toner is slightly attracted with respect to the dark section potential so that the fog caused by the exceptional toner is not generated. The potential from the development bias Vdc, which does not attract the exceptional toner, is referred to as fog eliminating potential Vback, and the potential is usually set in the range from about 100V to about 200V. Thus, the development bias Vdc is determined, and the gradation (contrast) expression between the light and the dark is performed by a contrast potential Vcont between the light section potential VL and the development bias Vdc.

Then,FIG. 4shows another gradation characteristic which determines the image quality. InFIG. 4, the horizontal axis represents the image density when the write is performed on photoconductor drum11aby the laser beam, and the vertical axis represents the density of the development image which is developed with the toner. As shown inFIG. 4, in the formed toner image, the density of the development image has saturation areas in the light section and the dark section. Usually the characteristics are refeffed to as gamma (γ) characteristics. The γ characteristics directly show the above engine of the image forming apparatus, and the γ characteristics are determined by the photoconductor drum or the toner used, process speed of the image formation, and the like. Because the γ characteristics are expressed in the contrast potential Vcont, when the contrast potential Vcont becomes narrow, the write density largely affects the change in density of the toner image, i.e. γ is steep. On the contrary, when the contrast potential Vcont becomes broad, γ is gentle. In the case where γ is steep, usually the toner image whose contrast is clear can be formed. In the case where γ is gentle, usually the toner image in which the halftone is amply expressed can be formed.

FIG. 5is a block diagram showing the configuration of the image forming apparatus to which the invention can be applied.

InFIG. 5, the reference numeral11adenotes the photoconductor drum which is rotated in the direction of arrow R1, the reference numeral12adenotes the primary charger which evenly charges the surface of the photoconductor drum11a, the reference numeral17adenotes the primary grid which can adjust the current flowing into the photoconductor drum11afrom the primary charger12ato control the charge amount on the surface of the photoconductor drum11a, the reference numeral18adenotes the surface potential sensor which detects the surface potential at the photoconductor drum11a, and the reference numeral14adenotes the development device which develops the electrostatic latent image on the photoconductor drum11a.

The reference numeral70ashows the configuration of the development bias circuit. The development bias circuit70aincludes a grounded direct-current bias generation portion.

The reference numeral90adenotes the configuration of the surface potential measurement circuit (surface potential measurement means)90a. The surface potential measurement circuit90ahas the sensor control portion91a, the sensor direct-current bias generation portion92a, the sensor generation bias detection portion (first bias detection means)93a, and a detection signal transmission portion94a. The reference numeral95shows the apparatus control portion which controls the image forming apparatus. The apparatus control portion95has the D/A conversion portion96awhose output portion is connected to the development bias circuit70aand the A/D conversion portion97awhose output portion is connected to the surface potential measurement circuit90a. The surface potential measurement circuit90aand the surface potential sensor18aconstitute the surface potential measurement means.

The reference numeral101adenotes a development bias measurement electrode to which the development bias signal for the development device14ais conducted. The reference numeral102adenotes a motor which is of moving means for the surface potential sensor18abetween the measurement position (development bias measurement position M1) of the development bias measurement electrode101aand the measurement position (surface potential measurement position M2) of the photoconductor drum11a.

In the image forming apparatus having the configuration shown inFIG. 5, first the apparatus control portion95moves the surface potential sensor18ato the development bias measurement position M1opposite to the development bias measurement electrode101ausing the motor102a. Then, the apparatus control portion95sets the generation bias to the development bias circuit70athrough the D/A conversion portion96a. The development bias circuit70aperforms the bias generation control according to the setting, and the development bias circuit70agenerates the bias output to the development device14aand the development bias measurement electrode101aaccording to the setting. In the state of things, the surface potential measurement circuit90aperforms the potential measurement to measure the output bias value of the development bias.

Then, the apparatus control portion95causes the development bias circuit70ato change the generating bias value, and the development bias measurement is performed again. Thus, the output change and measurement of the development bias are repeated in plural times, and the characteristics of the generation bias value for the setting of the development bias circuit70aare computed based on the measurement result of the surface potential measurement circuit90a. The computation is performed as follows.

At this point, the case where linear approximation is performed by two-point measurement will de described. It is assumed that the bias value is set to V1at the first point, the measurement result at the first point by the surface potential measurement circuit90ais set to E1. The bias value is set to Vs at the second point, and the measurement result by the surface potential measurement circuit90ais set to E2. Then, the bias output characteristics based on the surface potential measurement circuit90aare expressed by the following equation (1):
Vdc=(E1−E2)·V/(V1−V2)+E1−(E1−E2)·V1/(V1−V2)  (1)

where Vdc is the bias generation value outputted based on the surface potential measurement circuit reference, and V is the bias setting value inputted from the apparatus control portion95in order to generate Vdc.

FIGS. 6A and 6Bshow a mechanism model for realizing the first embodiment. The mechanism model includes the surface potential sensor18aand the development bias measurement electrode101a.FIG. 6Ais a top view, andFIG. 6Bis a side view.FIGS. 6A and 6Bshow the case in which the surface potential sensor18ais attached to the development device14a. A bearing gear201aaround which a gear is formed is attached to the surface potential sensor18a. A shaft205a, a gear202a, and the motor102aare attached to the development device14a. The bearing gear201ais attached to the shaft205a. The gear202atransmits power to the bearing gear201a. The motor102arotates the gear202a. A stopper203aand a stopper203aare also provided. The stopper203asecurely stops the surface potential sensor18aat the surface potential measurement position M2which is located opposite to the surface of photoconductor drum11a. The stopper204asecurely stops the surface potential sensor18aat the development bias measurement position M1which is located opposite to the development bias measurement electrode101a. Namely, the development bias measurement electrode101ais attached at the position opposite to the position (development bias measurement position) where the surface potential sensor18ais stopped by the stopper204a. A switch mechanism202is formed by the bearing gear201athe shaft205a, the gear202a, the motor102a, the stoppers203aand204a, and the like.

Thus, only the apparatus control portion95sets the rotating direction of the motor102ato rotate the motor102a, which allows the apparatus control portion95to switch the measurement objects of the surface potential sensor18a.

As described above, according to the first embodiment, the same surface potential measurement circuit90acan selectively measure the surface potential at the photoconductor drum11aand the generation potential at the development bias by switching the surface potential sensor18a. Therefore, the generation voltage at the development bias circuit70acan be corrected based on the surface potential measurement circuit reference, and all the changes in detection result caused by the variation in components used for the bias detection portion and the temperature change can be corrected based on the surface potential measurement system reference. Namely, the dark section potential VD, the light section potential VL and the development bias Vdc are measured based on the surface potential measurement system reference, which allows the variations in contrast potential Vcont to be eliminated to realize the stable contrast potential Vcont. As a result, the image forming apparatus which reduces the fluctuation in image density and the fluctuation in color tint can be realized.

Further, according to the configuration of the first embodiment, the measurement of surface potential at the photoconductor drum11aand the correction of the generation bias of the development bias circuit70aare performed using the same bias detection portion93aand the same A/D conversion portion97a, so that the shifts caused by the quantization error of the A/D conversion portion97abecome the same characteristics. When compared with the case in which the A/D conversion portions are separately prepared for the measurement of surface potential and the correction of the generation bias, the shifts caused by the quantization error can also be taken in the surface potential measurement system reference. Therefore, the influences caused by the quantization errors on the contrast potentials Vcont can be eliminated, and the stable image density and color tint can be realized.

The development bias is described as an example of the correction object of the surface potential measurement system reference in the first embodiment. However, the invention is not limited to the first embodiment. For example, the invention can also be applied to the bias control circuit for the primary grid17a(seeFIG. 2). In this case, the dark section potential VD can stably set, and the higher-accuracy contrast potential Vcont and fog eliminating potential Vback can be set, so that the image forming apparatus, in which the fog is decreased and the fluctuation in image density is decreased, can be realized.

Second Embodiment

FIG. 7shows a schematic configuration of an image forming apparatus (according to a second embodiment) of the invention.

InFIG. 7, the reference numeral301adenotes high-voltage switch means. The high-voltage switch means301ais configured to connect the development bias generation portion70ato a measurement point of the sensor generation bias detection portion93ain the surface potential measurement circuit90ain response to the direction from the apparatus control portion95.

In the configuration shown inFIG. 7, the apparatus control portion95turns on the high-voltage switch301a, and the apparatus control portion95set a predetermined bias output value in the development bias circuit70a. In response to the direction from the apparatus control portion95, the development bias circuit70aperforms the bias generation control according to the setting value. Therefore, the output according to the set bias value is generated in the development device14a, and the output is applied to the sensor generation bias detection portion93athrough the high-voltage switch301a.

On the other hand, at this point, the apparatus control portion95control the sensor direct-current bias generation portion92ato the stop state. Therefore, the measurement system (sensor bias detection portion93aand A/D conversion portion97a) in the surface potential measurement circuit90abecomes the configuration for measuring the generation output of the development bias circuit70a.

In the configuration described above, the apparatus control portion95performs the control by switching the plural generation bias values of the development bias circuit70a, and the measurement system in the surface potential measurement circuit90ameasures each of the set generation outputs of the development bias circuit. Therefore, as with the first embodiment, the generation bias of the development bias circuit70acan be corrected by the measurement system reference of the surface potential measurement circuit, the same effect as the first embodiment can be obtained.

It is possible that a mechanical relay or a semiconductor relay is used as the high-voltage switch301a. It is also possible to form a switch circuit with a high-voltage transistor and the like.

Third Embodiment

FIG. 8is a flowchart for explaining the apparatus control in an image forming apparatus (according to a third embodiment) of the invention.

In the third embodiment, the predetermined bias is measured by the surface potential measurement system during the continuous print, and the apparatus control portion performs the correction control to the objective bias circuit when the shift from the surface potential measurement system is generated.

First it is determined whether the last print is performed or not (Step S11). When the last print is performed (Yes in Step S11), the control flow is ended. When the last print is not performed (No in Step S11), the objective bias is measured by the surface potential measurement system (Step S12).

Then, it is determined whether the measured bias value is changed or not (Step S13). When the measured bias value is not changed (No in Step S13), it is determined that the difference in detection result does not exist between the surface potential measurement system and the bias control system, and the control flow returns to Step S11. When the measured bias value is changed (Yes in Step S13), it is determined that difference in characteristics of the detection portion is generated between the surface potential measurement system and the bias control system, and the control flow goes to Step S14. In Step S14, the termination of the print for one screen is waited. In Step S15, the objective bias output is changed to the control bias value in which the surface potential measurement system is set to the reference. At this point, the one-time maximum value in the correction is determined so that the setting is not extremely changed before and after the bias output is changed, and the correction is performed based on the maximum value. Therefore, the stable image quality can be realized without extremely changing the print quality.

The correction object is not described in the third embodiment. However, the correction is performed in the development bias, the primary grid bias, the primary charge in the case when the primary charge is formed by a roller charge system, and the like. From a safety standpoint of the circuit, the measurement object of the surface potential measurement system is switched when the bias output is stopped.

Fourth Embodiment

FIG. 9is a flowchart for explaining the apparatus control in an image forming apparatus according to a fourth embodiment of the invention.

In the fourth embodiment, the light section potential VL is measured during the continuous print, and the apparatus control portion performs the correction control to the development bias circuit when the light section potential VL is generated.

First it is determined whether the last print is performed or not (Step S21). When the last print is performed (Yes in Step S21), the control flow is ended. When the last print is not performed, it is determined whether the predetermined number of sheets is reached or not (Step S22). When the predetermined number of sheets is not reached (No in Step S22), a sheet counter is incremented (Step S23), and the control flow returns to Step S21. When the predetermined number of sheets is reached (Yes in Step S22), the light section potentials VL are measured between the images (Step S24). At this point, the development bias output is tuned off so that the fog image is not generated on the photoconductor drum, and then the exposure is performed.

Then, it is determined whether the light section potential VL is changed or not (Step S25). When the light section potential VL is not changed (No in Step S25), the sheet counter is reset, and the control flow returns to Step S21. When the light section potential VL is changed (Yes in Step S25), the generation bias value of the development bias circuit is measured by the surface potential measurement system, and the generation bias setting value of the development bias circuit is changed so that the contrast potential Vcont is kept constant in agreement with the measured light section potential VL (Step S26). Then, the sheet counter is reset (Step S27), and the control flow returns to Step S21.

In the control of the fourth embodiment, in order to measure the light section potential VL, the development bias is turned off, the exposure is performed, and then the light section potential VL is measured. Further, it is necessary to start up the development bias Vdc (sometimes the setting is changed). Therefore, sometimes the control of the fourth embodiment cannot be realized between the images. In this case, the control is performed so that the start of printing the next image is delayed.

As described above, according to the fourth embodiment, while image writing is delayed during the continuous print if necessary, the light section potential VL is measured to correct the development bias Vdc. Therefore, the same effect as the third embodiment can be obtained.

As with the third embodiment, the image forming apparatus of the fourth embodiment is configured to set the upper limit value in the correction of the development bias Vdc so that the rapid change in image density is not generated.

From a safety standpoint of the circuit, it is desirable that the switch between the measurement of the generation bias in the development bias circuit and the measurement of the light section potential VL is performed at timing during which the generation bias of the development bias circuit is turned off when the photoconductor drum surface potential becomes the minimum potential at the light section potential VL.

Fifth Embodiment

FIG. 10is a flowchart for explaining the apparatus control in an image forming apparatus (according to a fifth embodiment) of the invention.

In the fifth embodiment, the dark section potential VD is measured during the continuous print, and the apparatus control portion performs the correction control to the primary grid circuit when the dark section potential VD is generated.

The dark section potential VD is measured (Step S31). The measurement can be performed between the images (sheet interval). It is determined whether the measured dark section potential VD is changed or not (Step S32). When the dark section potential VD is not changed, the flow is ended. When the dark section potential VD is changed, the setting potential Vg of the primary grid is changed (Step S33), and the control from Step S21in the flowchart shown inFIG. 9in the fourth embodiment is performed.

According to the control of the fifth embodiment, when the dark section potential VD measured by the surface potential measurement system is generated by the shift from the measurement system of the primary grid circuit due to the temperature change, the output of the primary grid circuit can instantly be adjusted, which allows the contrast potential Vcont and the fog eliminating potential Vback to be kept constant based on the surface potential measurement system in conjunction with the control shown in the fourth embodiment. Therefore, in addition to the effects shown in the third and fourth embodiments, the image fog can be prevented from generating by the stabilization of the fog eliminating potential Vback.

Sixth Embodiment

FIG. 11is a block diagram for explaining an image forming apparatus (according to a sixth embodiment) of the invention.

InFIG. 11, the reference numerals18a,18b,18c, and18ddenote surface potential sensors corresponding to the photoconductor drums11a,11b,11c, and11d(seeFIG. 1). The reference numerals90a,90b,90c, and90ddenote surface potential measurement circuits. The reference numerals97a,97b,97c, and97ddenote A/D conversion portions which are provided in the apparatus control portion95. The reference numerals701a,701b,701c, and701ddenote measurement electrodes which are fixed at the surface potential measurement positions opposite the surface potential sensors18ato18drespectively. The reference numeral702denotes a reference power supply (reference bias generation means) which is commonly connected to the measurement electrodes701ato701d.

The surface potential sensors18ato18dare configured to be able to switch the measurement positions of the measurement electrodes701ato701dand the surface potential measurement position of the photoconductor drums11ato11drespectively.

In the configuration shown inFIG. 11, the apparatus control portion95causes the reference power supply702to output the predetermined bias. The output bias is commonly applied to the measurement electrodes701ato701d, and the surface potential measurement circuits90ato90dconvert the applied bias into the detection signals through the surface potential sensors18ato18d. The detection signals are transmitted to the A/D conversion portions97ato97dcorresponding to the surface potential sensors18ato18d, and the detection signals are digitalized. Then, the digitalized detection signal is processed by the apparatus control portion95. The above control is repeated in plural times by changing the setting voltage of the reference power supply702, which allows the detection characteristics in each measurement system to be obtained.

Then one of the measurement systems is selected as a representative, and the detection characteristics of other measurement systems are corrected based on the detection characteristics of the selected measurement system. When the above correction sequence is repeated at proper timing, the temperature change and the variation with time of the detection characteristics in each measurement system can be integrated into the same the temperature change and the same variation with time of the detection characteristics in the specific measurement system. Therefore, the density change caused by the variation in characteristics of each measurement system can become equal in the image forming portions, and the variations in color tint of the color images can be suppressed to the minimum level.

Various methods can be cited as the correction method. For example, the correction can be achieved using the linear approximation by the two-point measurement described in the first embodiment.

Seventh Embodiment

FIG. 12is a block diagram of a development bias circuit for explaining an image forming apparatus (according to a seventh embodiment) of the invention.

InFIG. 12, the reference numeral801denotes a development bias generation circuit (first polarity bias generation means) which develops the electrostatic latent image into the toner image, and the reference numeral802denotes a fog removing bias generation circuit (second polarity bias generation means) which generates the bias output different from that of the development bias generation circuit801.

In the configuration shown inFIG. 12, the development bias generation circuit801is used for the development of the electrostatic latent image. On the other hand, the fog removing bias generation circuit802is used during the measurement of the light section potential VL. According to the fourth embodiment in which the light section potential VL is measured during the continuous print to correct the development bias Vdc, in order to measure the light section potential VL during the continuous print, it is desirable that the development device is configured so as not is be detachable due to the print speed of the apparatus. In the configuration in the current status, when the potential at the photoconductor drum surface falls to the light section potential VL without detaching the development device, there is the problem that the fog toner is developed in the photoconductor drum even if the development bias is turned off. The problem should be solved in the invention in which the light section potential VL is frequently measured. Therefore, in the seventh embodiment, the fog removing bias generation circuit802is provided in the development bias circuit801, and the development bias Vdc is set to the reverse polarity during the measurement of the light section potential VL to avoid the adhesion of the fog toner to the photoconductor drum.

In the first embodiment to the seventh embodiment, during the image forming process, the photoconductor drum surface is charged in the positive polarity, and the high density portion of the image is exposed to form the image. However, the invention is not limited to the above embodiments. For example, the invention can be applied to a negative polarity charge system and a background exposure system in which the background of the image is exposed. The same effects can be obtained when the invention is applied to other systems except for the positive polarity charge system.

This application claims priority from Japanese Patent Application No. 2004-085804 filed Mar. 23, 2004, which is hereby incorporated by reference herein.