Image forming apparatus

An Image Forming Apparatus has a controller for controlling a second bias, which is applied to a cleaning member, based on a value of electric current measured by a current sensor. The controller controls the second bias on based on the value of the current measured when a predetermined measurement range of a transport belt is in contact with the cleaning member. The measurement range is a range in which conditions, under which the first bias and the second bias are applied, are identical.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese Patent Application NO. 2008-293592, which was filed on Nov. 17, 2008, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to an image forming apparatus having a belt and a cleaning member for the belt.

Image forming apparatuses are known in which a recording sheet for forming an image thereon is transported by a belt. In such an Image Forming Apparatus, a cleaning member for the belt is provided to remove developer and paper dust attached to the belt. In terms of the configurations adopted for the cleaning members, the cleaning member is adapted to scrape off the developer by coming into contact with the belt, or by applying a bias to the belt to electrically separate the developer from the belt.

For example, in a related art, a bias is applied between the belt and the cleaning member to thereby remove the developer from the belt. Further, by focusing attention on the fact that the state of such as the surface of the belt changes due to the aging deterioration and fouling, the voltage between a back roller disposed on the back side of the belt and a first obverse member disposed on the obverse side of the belt is subjected to constant current control so as to maintain satisfactory cleaning performance irrespective of the state of the belt. Additionally, for the purpose of this constant current control, the magnitude of the electric current flowing between the back roller and the first obverse member is measured, and the magnitude of the voltage between the back roller and the first obverse member is changed on the basis of the measured magnitude of the current.

However, according to studies conducted by the present inventors, it has become clear that even if the current is constant, the cleaning performance does not necessarily become consistent and an optimum cleaning bias changes due to changes in the environment and due to the aging and use of the cleaning member and the belt. As a method for detecting these changes, a method of estimating these changes on the basis of a resistance value at the time of the cleaning as been proposed. However, it is necessary to accurately measure the resistance value of the belt. Specifically, it is essential to accurately detect the current through the belt. However, it has also become clear that if the current is measured while a predetermined voltage is being applied between the cleaning member and the belt, the current varies based on the measuring position of the belt. Specifically, a bias is applied between the belt and the cleaning member, and a bias is also applied between the belt and each of a process unit and a transfer member, each disposed opposite the belt to effect image formation. These biases form a charged state corresponding to the biases applied to the belt surface, and this charged state affects the magnitude of the current flowing between the cleaning member and the belt during cleaning. Additionally, a similar problem occurs not only in the belt for transporting the recording sheet but also in an intermediate transfer belt for temporarily holding a toner image.

SUMMARY OF THE INVENTION

Accordingly, an object of the invention is to provide an image forming apparatus in which the recording sheet is transported by the belt, and which makes it possible to improve the cleaning performance of the belt by accurately reflecting the state of the belt based on the bias of the cleaning member.

To attain the above object, in accordance with an aspect of the invention there is provided an image forming apparatus comprising: an endless belt; image carriers juxtaposed along an outer peripheral surface of the belt; transfer members which are respectively provided in correspondence with the image carriers, which are respectively disposed opposite the image carriers with the belt interposed therebetween, and to which a first bias is applied; a cleaning member disposed in contact with the outer peripheral surface of the belt; a current sensor for measuring a value of electric current flowing across the cleaning member; and a controller for controlling a second bias, which is applied to the cleaning member based on of the value of the electric current measured by the current sensor, wherein the controller controls the second bias based on the value of the electric current measured when a predetermined measurement range of the belt is in contact with the cleaning member, and wherein the measurement range is a range in which conditions under which the first bias and the second bias are applied are identical.

In accordance with another aspect of the invention, there is provided a method of controlling an image forming apparatus which comprises: an endless belt; a plurality of image carriers juxtaposed along an outer peripheral surface of the belt; a plurality of transfer members which are respectively provided in correspondence with the image carriers, which are respectively disposed opposite the image carriers with the belt interposed therebetween, and to which a first bias is applied; and a cleaning member disposed in contact with the outer peripheral surface of the belt, the method comprising:

measuring a value of electric current flowing across the cleaning member; and

controlling a second bias, which is applied to the cleaning member based on the measured value of the electric current when a predetermined measurement range of the belt is in contact with the cleaning member wherein the measurement range is a range in which conditions under which the first bias and the second bias are applied are identical.

DETAIL DESCRIPTION OF EXEMPLARY EMBODIMENTS

Next, a detailed description of exemplary embodiments of the present invention with reference to the accompanying drawings will be given. It should be noted that, in the following description, after the overall configuration of the color printer is first described, the details of the exemplary embodiment of the present invention will be described.

In the following description, the description of directions will be given in terms of directions based on the user at the time of using the color printer. Namely, it is assumed that inFIG. 1, when facing the drawing, the left side is the “front side,” the right side is the “back side,” the farther side is the “left side,” and the nearer side is the “right side.” Further, it is assumed that, facing the drawing, the vertical direction is the “vertical direction.”

As shown inFIG. 1, a color printer1comprises an apparatus body2, a sheet feeding section20for feeding a sheet P; an image forming section30for forming an image on the fed sheet P; a sheet discharging section90for discharging the sheet P with the image formed thereon; and a controller100.

An opening2A is formed in an upper portion of the apparatus body2. This opening2A is adapted to be opened and closed by an upper cover3, which is rotatably supported by the apparatus body2. The upper surface of the upper cover3serves as a sheet discharging tray4for accumulating the sheets P discharged from the apparatus body2, and a plurality of LED mounting members5for holding below-described LED units40, are provided on the lower surface thereof.

The sheet feeding section20is provided in a lower portion inside the apparatus body2, and mainly includes a sheet feeding tray21, which is detachably installed in the apparatus body2, as well as a sheet supplying mechanism22for transporting the sheet P from the sheet feeding tray21to the image forming section30. The sheet supplying mechanism22is provided on a front side of the sheet supplying tray21, and mainly includes a feed roller23, a separation roller24, and a separation pad25.

In the sheet feeding section20, the sheets P in the sheet feeding tray21are separated one by one and are sent upward, and paper dust is removed as the sheet P passes between a paper dust removal roller26and a pinch roller27. The sheet P then passes along a transport path28to undergo a direction change toward the backward direction, and is supplied to the image forming section30.

The image forming section30comprises four LED units40, four process cartridges50, a transfer unit70, a cleaning section10, and a fixing unit80.

Each LED unit40is swingably connected to the LED mounting member5, and is supported by being appropriately positioned by a positioning member provided in the apparatus body2.

The process cartridges50are disposed between the upper cover3and the sheet feeding section20in such a manner as to be arranged in the front-back direction, and each comprise a photoconductor drum53on which an electrostatic latent image is formed, as well as a charger, a development roller, and a toner accommodating chamber, which are shown with reference numerals omitted and are known. A drum cleaner54is in contact with the photoconductor drum53and temporarily holds the toner remaining on the photoconductor drum53and returns it to the photoconductor drum53during a cleaning operation. A voltage is applied to the drum cleaner54by the controller100so as to electrically hold the toner and return the toner to the photoconductor drum53.

The transfer unit70is provided between the sheet feeding section20and the process cartridges50, and mainly includes a drive roller71, a driven roller72, a transport belt73, and transfer rollers74.

The drive roller71and the driven roller72are disposed in parallel in such a manner as to be spaced apart in the front-back direction, and the transport belt73comprises an endless belt stretched therebetween. An outer surface of the transport belt73is in contact with each photoconductor drum53. Additionally, four transfer rollers74, which nip the transport belt73between the transfer rollers74and the photoconductor drums53, are disposed on the inner side of the transport belt73in face-to-face relation with the respective photoconductor drums53. A transfer bias (first bias) is applied to each of these transfer rollers74by constant current control during the transfer.

The cleaning section10is provided on that portion of the transport belt73, which is stretched on the lower side, and includes a waste toner case11, as well as a cleaning roller12, a backup roller13, a second cleaning roller14, and a blade15.

The cleaning roller12is disposed adjacently to an outer peripheral surface of the transport belt73.

The backup roller13is disposed opposite to the cleaning roller12with the transport belt73positioned between the backup roller13and the cleaning roller12, and nips the transport belt73in cooperation with the cleaning roller12.

The second cleaning roller14is disposed in the rear of the cleaning roller12and is contiguous therewith.

The blade15has its leading end in contact with the second cleaning roller14, and is adapted to scrape off the toner attached to the second cleaning roller14.

The waste toner case11is disposed below the cleaning roller12and the second cleaning roller14, and is configured as to receive the toner scraped off by the blade15.

A bias (second bias) for causing the toner on the transport belt73to move toward the cleaning roller12is applied between the backup roller13and the cleaning roller12by the controller100. This second bias is changed, as required, in accordance with the operational mode such as during cleaning and printing. It should be noted the bias (second bias) applied to the cleaning roller will be hereafter referred to as the cleaning bias irrespective of the magnitude of its voltage.

The fixing unit80is disposed on the rear side of the process cartridges50and the transfer unit70, and includes a heat roller81and a pressure roller82, which are disposed in opposing relation to the heat roller81and are adapted to press the heat roller81.

In the image forming section30thus configured, after the surface of each photoconductor drums53is first charged uniformly by the charger, the surface of each photoconductor drums53is exposed by each LED unit40. Consequently, the potential at the exposed portion drops, and an electrostatic latent image is formed on each photoconductor drum53based on image data. Then, as toner is supplied to the electrostatic latent image by the development roller, a toner image is carried on the photoconductor drum53.

Next, as the sheet P fed onto the transport belt73passes between each photoconductor drum53and each transfer roller74disposed on the inner side of the transport belt73, the toner image formed on each photoconductor drum53is transferred onto the sheet P. Then, as the sheet P passes between the heat roller81and the pressure roller82, the toner image transferred onto the sheet P is thermally fixed.

The sheet discharging section90includes a discharged sheet side transport path91, which is formed in such a manner as to extend upward from an outlet of the fixing unit80and then to curve back toward the front side of the color printer, and a plurality of pairs of transport rollers92for transporting the sheet P. The sheet P onto which the toner image has been transferred and thermally fixed is transported along the discharged sheet side transport path91by the transport rollers92, is discharged to the outside of the apparatus body2, and is accumulated on the sheet discharging tray4.

<Bias Control of Cleaning Section>

Next, a description will be given of control of the bias applied to the cleaning roller12.

Among the drawings to which reference is made,FIG. 2is a circuit diagram for applying a voltage to each roller, andFIGS. 3A and 3Bare graphs illustrating the change over time of the current flowing across the cleaning roller. It should be noted that, in this embodiment, a description will be given by describing a positively chargeable toner as an example, but exemplary embodiments of the present invention are similarly applicable to the case of the toner having an opposite polarity. The polarity and magnitude of the transfer bias is appropriately set in correspondence with the charging polarity of the toner.

As shown inFIG. 2, a power supply, which applies a negative voltage to the cleaning roller12, is connected to the cleaning roller12, while the backup roller13is connected to the ground and is provided with an ammeter18to measure the electric current flowing across the backup roller13. An embodiment of the ammeter18may directly measure the voltage. A switch SW0is provided in a circuit that allows the current to flow between the backup roller13and the cleaning roller12.

Meanwhile, the base material of each of the four photoconductor drums53is formed of aluminum, and the aluminum is connected to the ground. A power supply, which applies a negative voltage to the transfer roller74, is connected to each of the transfer rollers74, and switches SW1, SW2, SW3, and SW4are respectively provided in circuits connecting each of the transfer rollers74and their respective power supplies.

With reference toFIGS. 3A and 3B, a description of the change of the measured current accompanying the movement of the transport belt73will be given hereinafter.

It should be noted that the time scales onFIG. 3A and 3Bare different. A time period Tn corresponds betweenFIGS. 3A and 3B. It should also be noted that although the graphs of the time periods T1to T7inFIGS. 3A and 3Bare not based on identical measured data, the graphs are based on data taken during equivalent operations of the printer, so that the time periods are herein denoted by the same reference numerals T1to T7for convenience' sake.

In the graphs ofFIGS. 3A and 3B, the cleaning bias is not applied during the time periods T4, T8, and T12, so that the current is 0. During the time periods T1to T3, T5to T7, T9to T11, and T13to T15, a cleaning bias is applied at a fixed voltage. The measured current values during these periods varies due to the transport belt73being moved with time, resulting in different portions of the transport belt73being nipped by the cleaning roller12and the backup roller13. As a result of the direction and magnitude of the applied electric field differing in the respective portions of the transport belt73, the charged state for the respective portions also differed, and the difference in this charged state appeared as the difference in measured current values.

For example, the time periods T1and T2are periods when the transport belt73idled prior to the start of image formation after the starting of the printer. The time period T1indicates a first revolution of the transport belt73, and the time period T2indicates a second revolution of the transport belt73. Between the first revolution and the second revolution of the transport belt73, charging with a same polarity gradually progressed, resulting in it gradually become difficult for the current to flow (current value became smaller). The time period T3is a period when the portion of the transport belt73which was used in printing, i.e., the portion of the transport belt73to which a transfer bias was applied (hereafter, this portion will be simply referred to as the “portion”), was opposed to the cleaning roller12. The time period T4is a period when the transport belt73was temporarily stopped after printing, and a cleaning bias was not being applied. The time period T5is a period when the portion to which the transfer bias was applied was being measured.

Further, the time period T6is a period when the transfer bias was not being applied, and the time period T7is a portion when the toner was discharged from the photoconductor drum53onto the transport belt73. When the toner is discharged from the photoconductor drum53onto the transport belt73, the toner is caused to move from the drum cleaner54onto the transport belt73through the photoconductor drum53, so that the bias conditions are similar to those at the time of transfer.

As shown inFIG. 3B, during the time periods T9to T15, operational modes similar to those of the time periods T1to T7are repeated. Namely, the time periods T9to T15correspond to the printing of a second sheet, and if a comparison is made with the first sheet, the measured current values are offset slightly.

Thus, the measured value of the current flowing across the cleaning roller12changes greatly in correspondence with the transfer bias and the cleaning bias applied to the transport belt73, and if a comparison is made between the time of operation of the first sheet (first time) and the time of operation of the second sheet (second time), the measured current value changes slightly. Additionally, it can be understood that there are small fluctuations in the current value even during the same operational mode, e.g., even in the range of the time period T1, for example. Thus, even if there is a large difference in the measured current value, it does not follow that the state of the transport belt73changes during a short time period. Therefore, in order to allow the change of the state due to the aging and use of the belt to be reflected on the cleaning bias during cleaning, it is necessary to acquire a current value fixed in the time scale such as the one shown inFIG. 3Aor3B (a time span during which only a few sheets are printed).

For this reason, in this embodiment, the controller100controls the second bias (cleaning bias) on the basis of the value of the current measured when a predetermined measurement range of the transport belt73is in contact with the cleaning members (the cleaning roller12and the backup roller13), the measurement range being set to a range in which the conditions under which the first bias (transfer bias) and the second bias are applied are identical.

For example, the range in which the conditions under which the first bias and the second bias are applied are identical may be a range in which both the first bias and the second bias are not applied. As another example, the range may be a range in which the first bias is applied a predetermined number of times, e.g., one time. As still another example, the range may be a range in which the first bias is not applied and the second bias is applied a predetermined number of times, e.g., one time.

Thus, since the current value is measured in the range in which the conditions under which the first bias and the second bias are applied are identical, the measurement range may be set to a range in which the transport belt73is disposed opposite to the cleaning members after a predetermined time duration has lapsed since the transport belt73was driven and the second bias was applied.

Additionally, the measurement range may be set to a range in which the transport belt73is disposed opposite to the cleaning members after a predetermined time duration has lapsed since the transport belt73was driven and the first bias was applied.

Furthermore, if the measurement range is considered by focusing attention on the position of the transport belt73, the measurement range may be set to a predetermined range in which the position on the transport belt73is offset in a backward direction by a predetermined length of the transport belt73, in the advancing direction, from a reference position on the transport belt73disposed opposite the cleaning members when the transport belt73is driven and the second bias is applied.

Still further, the measurement range may be set to a predetermined range offset in a backward direction by a predetermined length of the transport belt73, in the advancing direction, from a reference position on the transport belt73disposed opposite the transfer roller74when the transport belt73is driven and the first bias is applied. Here, when there are a plurality of the transfer rollers74, as in this embodiment, determining which one of the transfer rollers74to set as a reference can be problem. However, if the reference position is set to be a single one point, the reference may be any one of the transfer rollers74. For example, the reference may be set to be the most downstream transfer roller74. Additionally, since the transfer bias, which is applied to each transfer roller74during printing, is in the same range on the transport belt73, as will be described later, any one (or all) of the plurality of transfer rollers74may be set as the reference as far as the time of printing is concerned.

The bias applied to the transport belt73is not limited to the first and second biases, and a third bias may be applied to the transport belt73by further providing a third bias application member. The measurement range may also be set in such a range that the conditions, under which the first bias, the second bias, and the third bias are applied, are identical.

Further, the third bias application member may, at this time be, for example, a roller is provided in the rear, in the advancing direction of the transport belt73, of the transfer member, and which nips the sheet P in cooperation with the transport belt73.

Additionally, to eliminate the effect of the aforementioned “fluctuations” of the measured current value in a short time period, the controller100may desirably be configured to control the second bias on the basis of a value obtained by averaging a plurality of current values measured in the measurement range.

Further, before the measuring of the current after the image formation has been started, the controller100may control the second bias on the basis of the value of the current previously detected by the ammeter18. By so doing, it is possible to satisfactorily control the second bias even before the current has been measured.

Additionally, the belt may be an intermediate transfer belt onto an outer peripheral surface of which the toner is supplied from the plurality of photoconductor drums53, as will be described later. Still further, the image forming apparatus may further comprise a secondary transfer roller for transferring an image from the intermediate transfer belt onto the sheet P, as a fourth bias is applied while the secondary transfer roller nips the sheet P in cooperation with the intermediate transfer belt, and the measurement range being set in a range in which the conditions, under which the first bias, the second bias, and the fourth bias are applied, are identical.

Referring now to the drawings, a detailed description will be given of the various specific embodiments of the second bias control in accordance with the above described features of the exemplary embodiment of the present invention.

<Embodiment 1: Measurement in a Range in which Biases Are Not Applied>

Among the drawings to which reference is made,FIG. 4is a flowchart explaining the control of the cleaning bias.FIG. 5is a graph illustrating the timings of the operation of the transport belt, application of the cleaning bias, and current measurement.FIGS. 6A and 6Bare diagrams explaining the charged state of the transport belt and the measurement range.

When the measurement is made in the range in which the transfer bias and the cleaning bias are not applied, a case is assumed herein in which the color printer1has not been operated for some time, and the printing operation is initiated from the state in which the transport belt73is not electrically charged. It should be noted that various controls, such as arithmetic operations and storage are thereafter effected by the controller100, though they are not described point by point.

Upon receiving a print job in a sleep state (S101), the color printer1initiates a predetermined starting operation (S102), and starts the driving of the transport belt73(S103). Then, as a cleaning bias is applied (S104), the toner and paper dust attached to the surface of the transport belt73are cleaned.

Then, a determination is made as to whether or not a predetermined time period TP has elapsed after the application of the cleaning bias (S105). After the predetermined time period has lapsed (S105: YES), the current value is measured a number of times at a predetermined interval by the ammeter18, and the measured values are stored. For example, measurement is made 10 times at an interval of 10 msec, and the measured values of these 10 measurements are stored (S106). Furthermore, if the measurement is made 26 times at an interval of 50 ms, the measurement range becomes double the cycle of the cleaning roller, thereby making it possible to average the fluctuations in the cycle of the cleaning roller. In other words, if the current value is measured a plurality of times in a range corresponding to an integral multiple of the circumferential length of the cleaning roller, and these current values are averaged, it is possible to average the fluctuations corresponding to the cycle of the cleaning roller.

Next, the controller100averages the plurality of stored current values (S107) to obtain an average value. The average at this time may be a simple arithmetic average or a weighted average. Then, the cleaning bias is determined from the average value of the current values by making reference to a table (not shown) to convert a current value stored in advance to the cleaning bias (S108). The cleaning bias thus determined is applied to the cleaning roller12(S109).

The current flowing across the cleaning roller12is measured in the range disposed opposite to the cleaning roller12after the lapse of a predetermined time period, i.e., at a timing after the lapse of a predetermined time period from the point in time of application of the cleaning bias through the above-described processing. It should be noted that this timing must be set such that the range, in which the conditions of the biases applied to the transport belt73are identical, can be measured each time. Here, a description will be given of an example in which the measurement range is set to a range in which the biases are not applied. In the graphs shown inFIGS. 3A and 3B, the measurement in the range in which the biases are not applied corresponds to the measurement in the time period T1. Referring toFIG. 5, this is a period in which the transport belt73makes one revolution from the point of time (t1) when the cleaning bias (SW0) was applied, and the current is measured after the lapse of a predetermined time period TP at the timing t1(there is a temporal width since the measurement is made a plurality of times).

With reference toFIGS. 6A and 6B, a description will be given of the charged state of the transport belt73in this measurement. As shown inFIG. 6A, when the transport belt73has started driving, SW0is turned ON to apply the cleaning bias. At this time, the portion of the transport belt73which is nipped (faced) by the cleaning roller12and the backup roller13is set as a reference position R. A measurement range M1is a range which is located rearwardly from the reference position R by a predetermined length in the moving direction of the transport belt73.

As shown inFIG. 6B, after the lapse of several seconds from the application of the cleaning bias, the reference position R moves to a vicinity of the driven roller72, so that the measurement range M1also moves. At this time, the range between the reference position R of the transport belt73and the cleaning roller12is charged such that the inner side becomes positive due to the application of the cleaning bias. Further, the current is measured by the ammeter18from this point in time while the measurement range M1is in contact with the cleaning roller12. Then, at the timing inFIG. 6B(timing t2inFIG. 5), the transport belt73has not made one revolution after application of the cleaning bias, and the transfer bias has not been applied, so that the measurement range M1is a range in which neither the transfer bias nor the cleaning bias is being applied.

As the current is thus measured in the predetermined range in which neither the transfer bias nor the cleaning bias is being applied, the measured current values become stabilized. Additionally, since the current value is measured a plurality of times, and the cleaning bias is determined by using an averaged value, it is possible to improve the cleaning performance in cleaning the transport belt73by causing the states of the cleaning roller12and the transport belt73to be correctly reflected in the cleaning bias.

<Embodiment 2: Measurement in a Range in which Cleaning Bias is applied a Predetermined Number of Times>

Among the drawings to which reference is made,FIG. 7is a graph illustrating the timings of the operation of the transport belt, application of the cleaning bias, and current measurement.FIG. 8is a diagram explaining the charged state of the transport belt and the measurement range.

In this case, the current value is measured in the range in which the cleaning bias is applied a predetermined number of times, and like the above-described measurement in a range in which the biases are not applied, the measurement range is set to a range of the transport belt73disposed opposite the cleaning roller12when a predetermined time period has elapsed since the application of the cleaning bias, as shown inFIG. 4. However, the timing of the measurement is set to a predetermined timing when the transport belt73has made one revolution or more after the start of application of the cleaning bias, as shown inFIGS. 7 and 8. As shown inFIG. 8, the range of the transport belt73, which has been charged by the application of the cleaning bias one time, is disposed opposite the cleaning roller12at the reference position R, i.e., at a point in time when the position on the transport belt73where the cleaning bias was started to be applied has entered a second revolution after having undergone one revolution. Accordingly, as the current value is measured by the ammeter18when a measurement range M2, disposed to the rear from the reference position R by a predetermined length, is disposed opposite the cleaning roller12, it is possible to measure the current in a predetermined range under conditions in which the cleaning bias has been applied one time and the transfer bias has not been applied a single time. For this reason, the measured current values become stabilized, and changes of the cleaning roller12and the transport belt73due to the environment, aging, or use can be correctly reflected on the cleaning bias, thereby making it possible to improve the cleaning performance in cleaning the transport belt73.

<Embodiment 3: Measurement in a Range in which Transfer Bias is Applied a Predetermined Number of Times>

Next, a description will be given of a case in which the current is measured in a range in which the transfer bias is applied a predetermined number of times.

Among the drawings to which reference is made,FIG. 9is a flowchart explaining the control of the cleaning bias.FIG. 10is a graph illustrating the timings of the operation of the transport belt, application of the cleaning bias, and current measurement.FIGS. 11A to 13Bare diagrams explaining the charged state of the transport belt and the measurement range.

In this form, upon receiving a print job (S201), the color printer1starts driving of the transport belt73(S202). A transfer bias is consecutively applied to each transfer roller74corresponding to the photoconductor drum53corresponding to each color to form a toner image on the sheet P. Upon completion of the transfer onto the sheet P, a cleaning bias is applied to the cleaning roller12to effect the cleaning of the transport belt73. A determination is then made as to whether or not the predetermined time period TP has elapsed after the switch SW4has been turned ON for transfer, i.e., after the switch SW4has been turned ON to apply a bias to the transfer roller74most downstream in the moving direction of the transport belt73(here, the moving direction along the four juxtaposed transfer rollers74) among the four transfer rollers74(S205).

After the lapse of the predetermined time period TP (S205: YES), the current value is measured a number of times at a predetermined interval by the ammeter18, and the measured values are stored (S106). Next, the controller100averages the plurality of stored current values (S107) to obtain an average value. Then the cleaning bias is determined from the average value of the current values by making reference to a table (not shown) to convert from a current value stored in advance to the cleaning bias (S108). The cleaning bias thus determined is applied to the cleaning roller12(S109).

The current flowing across the cleaning roller12is measured in the range disposed opposite the cleaning roller12after the lapse of a predetermined time period, i.e., at a time after the lapse of a predetermined time period from the point in time of the application of the transfer bias to the most downstream transfer roller74through the above-described processing. It should be noted that this timing must be set such that the range, in which the conditions of the biases applied to the transport belt73are identical, can be measured each time. To facilitate understanding, a description will be given here of an example in which the measurement range is set to a range in which the durations of the time periods T1and T2are shortened in the graphs ofFIGS. 3A and 3B, the transfer bias is applied four times, and the cleaning bias is applied one time.

As shown inFIG. 10, after the driving of the transport belt73has started (ON) (t0), the cleaning bias (SW0) is applied, and the switches SW1to SW4are consecutively turned ON (t2to t5), thereby transferring a toner image from each photoconductor drum53onto the sheet P. Then, the current value is measured by the ammeter18after the lapse of the predetermined time period TP from the time (t5) at which the application of the finally applied transfer bias was started. It should be noted that although the timings when the respective switches SW1to SW4are ON are offset from each other, the respective switches SW1to SW4are changed between ON and OFF at the same position on the transport belt73and the sheet P because the transport belt73and the sheet P are moving.

The charged state of the transport belt73during this measurement will be described with reference toFIGS. 11A to 13B. As shown inFIG. 11A, after the driving of the transport belt73has started, the switch SW0is turned ON to apply the cleaning bias. Then, as shown inFIG. 11B, the switch SW1is turned ON when the sheet P enters a nip between the most upstream position of photoconductor drum53and the transfer roller74.

As shown inFIG. 12, when the sheet P is transported by the transport belt73, the switches SW1to SW4are consecutively turned ON to thereby apply a transfer bias to each transfer roller74. The portion of the transport belt73disposed opposite the transfer roller74, when the SW4is turned ON, is set as the reference position R. The surface of the transport belt73is gradually charged such that the outer side becomes positively higher due to this transfer bias.

As shown inFIG. 13A, when the transfer of the toner onto the sheet P is completed, the portion of the transport belt73, whose outer side became positive due to the application of the transfer bias, moves toward the cleaning section10. When the reference position R has passed the cleaning roller12, the current is measured by the ammeter18, as shown inFIG. 13B. Here, the range of the transport belt73disposed, by a predetermined length in the advancing direction of the transport belt73, toward the rear of the reference position R is a measurement range M3. As can be seen fromFIGS. 13A and 13B, the measurement range M3is the range in which the cleaning bias has been applied one time, and the transfer bias has been applied four times. It should be noted that the number of times the cleaning bias is applied increases in correspondence with the number times the transfer belt73has been idling.

As the current is measured in the predetermined range in which the transfer bias and the cleaning bias have been applied a predetermined numbers of times, the measured current values become stabilized. In addition, since the current value is measured a plurality of times, and the cleaning bias is determined by using an averaged value, it is possible to improve the cleaning performance in cleaning the transport belt73by causing the surface state of the transport belt73to be correctly reflected in the cleaning bias.

It should be noted that in the case where the cleaning bias is applied again at timings t7and t8inFIG. 10, the cleaning bias may be determined by using a current value measured and previously stored previously. As a result, even in a case where there has been no timing for measuring the current value for some time in view of the operational mode, it is possible to determine an optimum cleaning bias as practically as possible.

<Form 4: Measurement in a Range in Which Attraction Bias (Third Bias) is Applied>

Next, a description will be given of a case in which the current is measured in a range in which an attraction bias is applied.

Among the drawings to which reference is made,FIG. 14is a graph illustrating the timings of the operation of the transport belt, application of the cleaning bias, transfer bias, and attraction bias, and current measurement.FIGS. 15A to 16are diagrams explaining the charged state of the transport belt and the measurement range.

As shown inFIG. 15A, the color printer according to this embodiment is similar to the above-described color printer1except that an attraction roller110is provided as an example of a third bias application member to which a voltage is applied to attract the sheet P to an upper portion of the driven roller72, and a switch SW5for the application of this voltage is thus provided.

As shown inFIG. 14, since the attraction bias is for attracting the sheet P, the attraction bias is applied at timings t1to t6at which the sheet P opposes the transport belt73.

Further, as shown inFIGS. 15A and 15B, as the sheet P approaches the transport belt73, the switch SW5is turned ON to apply an attraction bias (t1). The attraction bias is applied so that the outer side of the transport belt73becomes negative. The transfer of the toner image onto the sheet P is effected in the same way as in Embodiment 3, and the transport belt73is disposed opposite the cleaning roller12in a state in which its outer side is positively charged, as shown inFIG. 16. Then, by using as the reference position R (seeFIG. 12) the position on the transport belt73disposed opposite the most downstream transfer roller74when the transfer bias is started to be applied to that transfer roller74disposed on the most downstream side in the advancing direction, a measurement range M4is disposed in the rear, in the advancing direction of the transport belt73, from the reference position by a predetermined length in the same way as in the form3. After the lapse of the predetermined time period TP (t7) from the time the switch SW4was turned ON (t5), the controller100measures the value of the current flowing across the cleaning roller12with the ammeter18. As a result, it is possible to measure the current value when the measurement range M4is disposed opposite to the cleaning roller12.

As can be understood fromFIGS. 15A to 16, the measurement range M4is a range in which the cleaning bias is applied one time, the attraction bias is applied one time, and the transfer bias is applied four times.

As the current is measured in the predetermined range in which the attraction bias, the transfer bias, and the cleaning bias have each been applied a predetermined numbers of times, the measured current values become stabilized. For this reason, it is possible to improve the cleaning performance in cleaning the transport belt73by causing the states of the cleaning roller12and the transport belt73to be correctly reflected in the cleaning bias.

<Embodiment 5: Measurement in a Range in Which Secondary Transfer Bias (Fourth Bias) is Applied>

Next, a description will be given of a case in which the toner image is not transferred onto the sheet on the transport belt, but the toner image is transferred onto the sheet through an intermediate transfer belt.

Among the drawings to which reference is made,FIG. 17is a schematic diagram of a printer having a secondary transfer roller.FIG. 18is a graph illustrating the timings of the operation of the transport belt, application of the cleaning bias, primary transfer bias, and secondary transfer bias, and current measurement.FIGS. 19A to 20are diagrams explaining the charged state of the intermediate transfer belt and the measurement range.

The color printer1shown inFIG. 17is provided with an intermediate transfer belt173, instead of the transfer belt73, and a secondary transfer roller120is disposed in a face-to-face relation with the drive roller71. In the light of the layout, the process cartridges50are provided below the intermediate transfer belt173, and the cleaning section10is provided above the intermediate transfer belt173. Primary transfer rollers174, similar to the transfer rollers74, are disposed in correspondence with the respective photoconductor drums53.

As shown inFIG. 18, the controller100applies a primary transfer bias to each of the primary transfer rollers (t2to t5) in the same way as the transfer bias was applied in embodiments3and4discussed above. As a result, a color toner image consisting of four color toners is formed on the intermediate transfer belt173. Then, at a predetermined timing (t6to t7) when the sheet P opposes the intermediate transfer belt173, a secondary transfer bias is applied to the secondary transfer roller120to transfer the toner image onto the sheet P. The controller100measures the current value with the ammeter18, after the lapse of the predetermined time period TP since the time the secondary transfer bias started to be applied (t6).

The charged state of the intermediate transfer belt173during this measurement will now be described with reference toFIGS. 19A to 20. As shown inFIG. 19A, after the intermediate transfer belt173has started driving, the switch SW0is turned ON to apply the cleaning bias. Then, as shown inFIG. 19B, a primary transfer bias is consecutively applied to the primary transfer rollers174at appropriate timings. As a result, a toner image is formed on the intermediate transfer belt173, and its outer side is positively charged. When this toner image approaches the secondary transfer roller120, the sheet P is fed, and while the sheet P is nipped between the drive roller71and the secondary transfer roller120, the switch SW6is turned ON to apply a secondary transfer bias (fourth bias) to the secondary transfer roller120(t6and t7), thereby transferring the toner image onto the sheet P. Then, upon completion of the transfer, the inner side of the intermediate transfer belt173is positively charged by the secondary transfer bias. If the position on the intermediate transfer belt173disposed opposite the secondary transfer roller120when the secondary transfer bias is started to be applied (t6) is set as the reference position R, the range of the intermediate transfer belt173located in the rear from the reference position by a predetermined length in the advancing direction of the intermediate transfer belt173becomes a measurement range M5. The current value is measured by the ammeter18when this measurement range M5disposed opposite to the cleaning roller12. As can be understood fromFIGS. 19A to 20, the measurement range M5is a range in which the cleaning bias is applied one time, the primary transfer bias is applied four times, and the secondary transfer bias is applied one time.

As the current is thus measured in the predetermined range in which the cleaning bias, the primary transfer bias, and the secondary transfer bias have been applied a predetermined numbers of times, the measured current values become stabilized. For this reason, it is possible to improve the cleaning performance in cleaning the intermediate transfer belt173by causing the states of the cleaning roller12and the intermediate transfer belt173to be correctly reflected in the cleaning bias.

As has been described in the above-described forms, according to the respective embodiments of the invention, since the current is measured in the range of the belt under the same conditions under which the bias was applied to the belt, the measured current values become stabilized. Hence, it is possible to improve the cleaning performance in cleaning the belt by causing the surface state, such as the deterioration and fouling, of the belt to be correctly reflected in the cleaning bias.

In particular, as the current value is measured a plurality of times and is averaged, it is possible to eliminate high frequency fluctuations of measured values, making it possible to obtain stabilized measured values.

Further, as the cleaning bias is determined by using the current value previously measured and previously stored, even in a case where there has been no timing for measuring the current value for some time (for example, after starting image forming and before measuring the current), it is possible to determine an optimum cleaning bias as practically as possible.

The invention can be utilized in various forms, as illustrated below, without being limited to the above-described embodiments.

Although in the above-described embodiments a description has been given under the premise that the toner is positively charged, the polarity of each bias is appropriately set to be the opposite of the charging quality of the toner, and the charged state of the belt also changes correspondingly. Additionally, not only the polarity but the magnitude of each bias can also be set appropriately.

Although in the above-described embodiments the invention has been described with reference to a color printer of the so-called tandem type as an example, embodiments of the present invention are also applicable to a color printer of the so-called 4-cycle type.

Although in the embodiments the exposure of the photoconductor drum53is effected by LEDs, the exposure may alternatively effected by scanning with a laser. In addition, although an example has been illustrated in which each switch is open in the state in which the bias is in the OFF state, it is possible to use a power supply in which the output becomes 0V when the bias is OFF. Further, the ammeter may be provided on the power supply side.