Image forming apparatus, control method therefor, and storage medium storing control program therefor

An image forming apparatus that is capable of reducing an effect of variation of specific inductive capacity of toner due to environmental variation with a small detection error when the remaining toner amount is detected. The image forming apparatus forms an image with an electrophotographic system. A container unit stores toner. A toner detection unit has sensor modules that are arranged at positions where the toner is stagnated in the container unit, and that show different electrostatic capacities with respect to the same toner thickness. An electrostatic capacity detection unit detects the electrostatic capacities of the sensor modules. A determination unit determines a remaining toner amount in the container unit based on the electrostatic capacities of the sensor modules that are detected by the electrostatic capacity detection unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technique of detecting a remaining toner amount in an image forming apparatus like a copying machine or a printer using an electrophotographic system.

2. Description of the Related Art

There is a known image forming apparatus using the electrophotographic system that is provided with a toner remaining amount sensor for detecting a remaining toner amount or existence of toner. There are many methods of detecting the remaining toner amount. One of them is known as an electrostatic capacity method by which the remaining toner amount is detected as electrostatic capacity between electrodes.

The remaining toner amount sensor of the electrostatic capacity method detects the toner remaining amount on the principle that the electrostatic capacity between the electrodes varies with the amount of toner (dielectrics) between the electrodes. Accordingly, when specific inductive capacity ∈t of the toner changes, the different electrostatic capacities are detected for the same remaining toner amount. For example, the specific inductive capacity ∈t of toner changes with humidity and temperature. Since the specific inductive capacity ∈t increases as the moisture content of toner increases, the toner amount that is determined as little under the dry condition may be determined as enough under the absorbed moisture condition.

In view of such a problem, Japanese Laid-Open Patent Publication (Kokai) No. 2002-132038 (JP 2002-132038A) suggests a method of correcting the detected remaining toner amount by reflecting measured environmental conditions, such as temperature and humidity, that become factors of varying the specific inductive capacity ∈t of toner as a method of raising the detection accuracy of the remaining toner amount by the remaining toner amount sensor.

However, the measured environmental condition is not necessarily coincident with the condition of toner. Accordingly, when the measured environmental condition is different from the condition of toner, the remaining toner amount detected by the remaining toner amount sensor in the electrostatic capacity method has an error.

SUMMARY OF THE INVENTION

The present invention provides an image forming apparatus, a control method therefor, and a storage medium storing a control program therefor, which are capable of reducing an effect of variation of specific inductive capacity of toner due to environmental variation with a small detection error when the remaining toner amount is detected.

Accordingly, a first aspect of the present invention provides an image forming apparatus that forms an image with an electrophotographic system comprising a container unit configured to store toner, a toner detection unit configured to have a plurality of sensor modules that are arranged at positions where the toner is stagnated in the container unit, and that show different electrostatic capacities with respect to the same toner thickness, an electrostatic capacity detection unit configured to detect the electrostatic capacities of the sensor modules, and a determination unit configured to determine a remaining toner amount in the container unit based on the electrostatic capacities of the sensor modules that are detected by the electrostatic capacity detection unit.

Accordingly, a second aspect of the present invention provides a control method for an image forming apparatus that forms an image with an electrophotographic system, and that has a container unit that stores toner, a toner detection unit that has a plurality of sensor modules that are arranged at positions where the toner is stagnated in the container unit and that show different electrostatic capacities with respect to the same toner thickness, the control method comprising an electrostatic capacity detection step of detecting the electrostatic capacities of the sensor modules, and a determination step of determining a remaining toner amount in the container unit based on the electrostatic capacities of the sensor modules that are detected in the electrostatic capacity detection step.

Accordingly, a third aspect of the present invention provides a non-transitory computer-readable storage medium storing a control program causing a computer to execute the control method of the second aspect.

The present invention can reduce an effect of variation of specific inductive capacity of toner due to environmental variation when the remaining toner amount is detected. Accordingly, the remaining toner amount can be detected with a small detection error.

DESCRIPTION OF THE EMBODIMENTS

Hereafter, embodiments according to the present invention will be described in detail with reference to the drawings.

FIG. 1is a general perspective view of an image forming apparatus1according to the embodiment of the present invention. The image forming apparatus1consists of an automatic original feeding device2, an image reading section3, an image forming unit4, and an operation unit5in general.

The automatic original feeding device2conveys an original onto a contact glass automatically. The image reading section3reads the original which the automatic original feeding device2conveys, and outputs image data. The image forming unit4forms an image on a sheet, such as a recording paper, according to image data that is outputted from the automatic original feeding device2or that is inputted from an external apparatus connected via a network. The operation unit5has a GUI (graphical user interface) that allows a user to perform various operations, and a loudspeaker that notifies the user of a reception of an operation and an abnormal condition of the system.

FIG. 2is a sectional view showing a configuration of the image forming unit4. The image forming unit4operates in the electrophotographic system. InFIG. 2, alphabets Y, M, C, and K added to ends of reference numerals represent configurations corresponding to the toners of yellow, magenta, cyan, and black, respectively. In the following description, a common configuration for all the toners is indicated by a reference numeral without adding the alphabet Y, M, C, or K, and an individual configuration for each toner is indicated by a reference numeral with the alphabet Y, M, C, or K.

A controller216controls communication ports (not shown), such as USB and LAN, and the image reading section3of the image forming apparatus1, and generates data that is transmitted to the image forming unit4. Moreover, the controller216performs transmission of image data and communication of control information to an engine control unit217through a printer control I/F215. The engine control unit217performs overall sequence control of the image forming unit4according to the image data and the control information received from the controller216.

A photosensitive drum225that is a photoconductor as an image bearing member on which a full color electrostatic image is formed rotates in a direction of an arrow A by a motor (not shown). Around the photosensitive drum225, a primary charging device221, an exposure device218, a development device223, a transfer device220, a cleaning device222, and a discharging device271are arranged.

The development device223K is used for monochrome development by developing a latent image on the photosensitive drum225K with K toner supplied from a toner bottle224K. The development devices223Y,223M, and223C are used for full color development by developing latent images on the photosensitive drums225Y,225M, and225C with Y toner, M toner, and C toner supplied from toner bottles224Y,224M, and224C, respectively. The toner images of four colors developed on the surfaces of the photosensitive drums225are multi-transferred to a transfer belt226by the transfer devices220so that the toner images are overlapped.

The transfer belt226is looped over rollers227,228, and229while keeping tension. The roller227is combined with a driving source (not shown), and functions as a driving roller that drives the transfer belt226. The roller228functions as a tension roller that adjusts the tension of the transfer belt226. Further, the roller229functions as a backup roller of a transfer roller as a secondary transfer device231. A transfer roller swinging unit250is a drive unit that makes the secondary transfer device231contact with or move away from the transfer belt226. A cleaner blade232is arranged downstream of the secondary transfer device231along the transfer belt226. The cleaner blade232scrapes the remaining toner off the transfer belt226.

Sheets are stored in cassettes240and241or are mounted on a manual feeding unit253. The cassettes240and241, and the manual feeding unit253have sheet detection sensors243,244, and245for detecting the existence of a sheet, respectively. Moreover, the cassettes240and241, and the manual feeding unit253have feeding sensors247,248, and249for detecting a poor pick up of a sheet, respectively.

When an image formation starts, a sheet stored in the cassette240(or241) is picked up by a pickup roller238(or239) one-by-one and is conveyed to a feeding roller pair235through longitudinal pass roller pairs237and236, and a feed path266. Moreover, a sheet mounted on the manual feeding unit253is picked up by a pickup roller254one-by-one and is conveyed to the feeding roller pair235. The feeding roller pair235conveys a sheet to a registration roller255. At this time, a registration sensor256, which is arranged near the registration roller255at the upstream side of the registration roller255, detects the passage of the sheet.

When predetermined time elapses after the registration sensor256detects the passage of the sheet, the conveyance operation is interrupted. Thereby, the sheet bumps against the stopped registration roller255, and stops. In that case, the front end of sheet is fixed so that the sheet end becomes vertical to the conveyance path, and a skew of the sheet to the conveyance path is corrected. Next, the sheet is fed to a contact point between a secondary transfer device231and the transfer belt226by the registration roller255. It should be noted that the registration roller255is combined with a driving source (not shown), and a driving force of the driving source is transferred to the registration roller255via a clutch (not shown) to rotate.

A toner image is formed in synchronization with such a sheet conveyance. That is, the surface of the photosensitive drum225is uniformly charged with predetermined minus electrification potential by impressing voltage to the primary charging device221. Next, the exposure device218that consists of a laser scanner unit exposes an image region on the electrified photosensitive drum225so as to become predetermined exposure potential and to form a latent image. The exposure device218forms the latent image corresponding to the image on the photosensitive drum225by turning on and off a laser beam based on the image data sent from the controller216through the printer control I/F215.

A developing bias predetermined for each color is applied to a developing roller of the development device223beforehand, and the latent image on the photosensitive drum225is developed by the toner so as to be visualized as a toner image when the image passes the position of the developing roller. The toner image is transferred to the transfer belt226by the transfer device220. At this time, the toner remained on the photosensitive drum225is removed and recovered by the cleaning device222. Moreover, the photosensitive drum225is uniformly discharged up to about 0V (zero voltage) by the discharging device271, and gets ready for the following image-formation cycle.

When the toner image transferred to the transfer belt226is transferred to the sheet, the secondary transfer device231is held by the transfer roller swinging unit250so as to contact with the transfer belt226. In this state, the toner image is transferred onto the conveyed sheet by the secondary transfer device231at the contact point between the transfer belt226and the secondary transfer device231. The sheet to which the toner image was transferred passes a post-registration conveyance path268, and is conveyed through a fixing conveying belt230to the fixing device234.

In the fixing device234, the toner image is charged by pre-fixing electrostatic chargers251and252in order to compensate the adsorptive power of toner and to prevent image deterioration, and then, the toner image is fixed by heat with a fixing roller233. When the print process is finished, the sheet after the heat fixing process for the toner image is ejected to an ejection tray242by an ejection roller270through the conveyance path that is switched to an ejection path258by an ejection flapper257.

It should be noted that the image forming unit4can continuously feed the sheets from the cassettes240and241, and the manual feeding unit253. In this case, the sheets are fed from the cassettes240and241, and the manual feeding unit253at the intervals so as not to overlap the sheets in consideration of the length of the preceding sheet. As mentioned above, the sheet is supplied to the secondary transfer device231by starting the registration roller255, and the registration roller255temporally stops when the sheet passed. This is for correcting a position of the following sheet in the same manner as the preceding sheet.

Moreover, the image forming unit4is provided with a mechanism that returns the sheet to the secondary transfer device231for forming an image onto the other side after forming the image onto one side of the sheet as mentioned above. This mechanism is provided with a sensor269, the ejection flapper257, a reverse side path259, a reversal roller260, a double-sided inversion path261, a paper-re-feeding roller264, and a paper-re-feeding sensor265, etc., and a sheet is reversed and conveyed with these elements to the feed path266again.

FIG. 3is a sectional view showing the detailed configuration of the development device223. Since the fundamental configuration is common for each of colors Y, M, C, and K, the elements inFIG. 3are designated by the reference numerals without the alphabets Y, M, C, and K. The development device223can be divided mainly into a buffer unit (a container unit)301and a developing unit305.

After the toner that fills a toner bottle224goes into the buffer unit301that stores toner, the toner is conveyed by a rotating mixing screw302to the developing unit305. A toner sensor304for detecting the amount of toner that stagnates in the buffer unit301is arranged in the buffer unit301. The toner conveyed to the developing unit305is supplied to the photosensitive drum225through a development cylinder303, and the latent image on the photosensitive drum225is visualized.

FIG. 4is a block diagram schematically showing a configuration of the engine control unit217. A CPU401is connected with an external device (not shown) through a bus405. Moreover, the CPU401is connected with a nonvolatile memory402, a ROM403, a RAM404, an exposure control IC406, and an I/O control IC407through the bus405.

The ROM403is a program memory that stores a program for the CPU401. The RAM404functions as a work memory for the CPU401. When receiving an instruction sent from the controller216through the printer control I/F215, the CPU401controls the entire system of the image forming unit4by developing and running the predetermined program stored in the ROM403to the work area of the RAM404.

The nonvolatile memory402stores the data that needs to keep among the control information about the engine control unit217, when the image forming apparatus1is turned off. The exposure control IC406controls the exposure device218according to the command from the CPU401.

The I/O control IC407is equipped with many input-output ports connected to various devices, controls actuators like a motor according to commands from the CPU401, and receives the information inputted from various kinds of sensors. An electrostatic capacity detection circuit408connected to the I/O control IC407detects electrostatic capacities of a first sensor module510, a second sensor module511, and a third sensor module512with which the toner sensor304is provided (they will be described later with reference toFIG. 5). Moreover, a temperature sensor409connected to the I/O control IC407measures internal temperature of the image forming unit4. The CPU401controls the toner sensor304, the electrostatic capacity detection circuit408, and the temperature sensor409through the I/O control IC407, and acquires electrostatic capacity data and thermal data.

FIG. 5A,FIG. 5B, andFIG. 5Care views showing the configuration of the toner sensor304.FIG. 5Ais a perspective view,FIG. 5Bis a side view seen in a direction of arrow A inFIG. 5A, andFIG. 5Cis a plan view (top plan) seen in a direction of arrow B inFIG. 5A.FIG. 6is a view showing the configuration of the electrostatic capacity detection circuit408, and the connection configuration of the electrostatic capacity detection circuit408and the toner sensor304.

As shown inFIG. 5AandFIG. 5B, electrodes501through506that are conductors of predetermined line widths are formed on a substrate507at predetermined spacings. A pair of electrodes501and502comprise the first sensor module510, a pair of electrodes503and504comprise the second sensor module511, and a pair of electrodes505and506comprise the third sensor module512, respectively. Each of these first, second, and third sensor modules510,511, and512detects the electrostatic capacity between the pair of electrodes. It should be noted that the surface of the substrate507is covered with a protection sheet508for protecting the electrodes501through506.

The electrostatic capacities of the first sensor module510, the second sensor module511, and the third sensor module512increase as the thickness of toner on the surface of the toner sensor304increases. Accordingly, the toner thickness on the surface of the toner sensor304can be detected based on variations of the electrostatic capacities of the first sensor module510, the second sensor module511, and the third sensor module512.

As shown inFIG. 5C, the first sensor module510has the electrodes501and502of the line width W1that are formed at the conductor spacing D1in this embodiment. Moreover, the second sensor module511has the electrodes503and504of the line width W2that are formed at the conductor spacing D2. Furthermore, the third sensor module512has the electrodes505and506of the line width W3that are formed at the conductor spacing D3.

The different line widths and the different conductor spacings are given to the electrodes of the first, second, and third sensor modules510,511, and512so that the sensor modules show different capacitance laws with respect to the same variation of toner amount (thickness). Details thereof will be described below. It should be noted that the line widths and the conductor spacings of the electrodes are set so as to satisfy relations of W1<W2<W3and D1<D2<D3in this embodiment. However, the configurations of the sensor modules are not limited to the above mentioned configurations. One of the line width and the conductor spacing may be changed in order to acquire different capacitance laws for the respective sensor modules.

Next, variations in the electrostatic capacities of the first, second, and third sensor modules510,511, and512to a variation of the toner amount (thickness) on the surface of the toner sensor304will be described with reference toFIG. 7. Then, the detail ofFIG. 6is described anew.

FIG. 7A,FIG. 7B, andFIG. 7Care views schematically showing relations between the thickness of a toner layer509(referred to as “toner thickness” in the following description) on the sensor surface of the toner sensor304and the electrostatic capacities detected.

FIG. 7Ashows the state where toner does not exist on the sensor surface of the toner sensor304. In this state, the electrostatic capacities of the first sensor module510, the second sensor module511, and the third sensor module512are dependent on specific inductive capacity ∈b of the substrate507and specific inductive capacity ∈s of the protection sheet508. In addition, since the inter electrode distance between adjacent sensor modules is set to be larger than the spacing of the pair of electrodes in each sensor module, the electrostatic capacity of each sensor module shall not be affected by the electrodes of other sensor modules.

The substrate507and the protection sheet508are made from material with small hygroscopic property. Accordingly, the specific inductive capacities ∈b and ∈s are considered as constants, respectively. The electrostatic capacities of the first sensor module510, the second sensor module511, and the third sensor module512in the state shown inFIG. 7Aare detected under an initial state of the image forming apparatus1, and are stored in the nonvolatile memory402as standard electrostatic capacities. Here, the standard electrostatic capacity of the first sensor module510shall be “C10”, the standard electrostatic capacity of the second sensor module511shall be “C”, and the standard electrostatic capacity of the third sensor module512shall be “C30”.

FIG. 7Bshows the state where the toner layer509accumulated on the sensor surface of the toner sensor304by thickness t1. In this state, since there is the toner layer509that is dielectric with larger specific inductive capacity than 1.0 near the electrodes of the first, second, and third sensor modules510,511, and512, the electrostatic capacities of the first, second, and third sensor modules510,511, and512become larger as compared with the state shown inFIG. 7A. Electrostatic capacities of the first, second, and third sensor modules510,511, and512in the state shown inFIG. 7Bshall be “C11”, “C21”, and “C31”, respectively.

FIG. 7Cshows the state where the toner layer509accumulated on the sensor surface of the toner sensor304by thickness t2. Since the thickness t2is larger than the thickness t1, the electrostatic capacities of the first, second, and third sensor modules510,511, and512become larger as compared with the state shown inFIG. 7B. Electrostatic capacities of the first, second, and third sensor modules510,511, and512in the state shown inFIG. 7Cshall be “C12”, “C22”, and “C32”, respectively.

That is, the electrostatic capacities of the first, second, and third sensor modules510,511, and512increase with the toner thickness accumulated on the sensor surface of the toner sensor304. Moreover, since the respective sensor modules have the different configurations, the first, second, and third sensor modules510,511, and512show the different capacitance laws (the different properties in detection of electrostatic capacity), even if toner thickness is the same.

The electrostatic capacity detection circuit408connected with the toner sensor304is enough to detect the variations in the electrostatic capacities of the first, second, and third sensor modules510,511, and512, and the circuit configuration of the electrostatic capacity detection circuit408is not limited to that shown inFIG. 6.

The electrostatic capacity detection circuit408consists of a reference voltage generation unit601, an ADC602that converts an analog signal into a digital value, a standard capacitor603that generates reference voltage, and a switch604. The reference voltage generation unit601and the ADC602are connected to the switch604via a pair of signal lines610and611.

A pair of signal lines612and613are connected to the pair of electrodes501and502of the first sensor module510, and a pair of signal lines614and615are connected to the pair of electrodes503and504of the second sensor module511. Moreover, a pair of signal lines616and617are connected to the pair of electrodes505and506of the third sensor module512. These three pairs of signal lines612and613,614and615,616and617are connected to the switch604.

The switch604selects the connection target of the pair of signal lines610and611from among the pair of signal lines612and613, the pair of signal lines614and615, and the pair of signal lines616and617. For example, when the electrostatic capacity of the first sensor module510is detected, the switch604connects the pair of signal lines610and611to the pair of signal lines612and613. Thereby, a voltage divider that connects two capacitors in series between the reference voltage generation unit601and GND is configured. Then, the ratio of the electrostatic capacities of the standard capacitor603and the first sensor module510can be acquired by measuring the electric potential of the middle point of the voltage divider. That is, when the generated voltage of the reference voltage generation unit601shall be “V1”, the electrostatic capacity of the standard capacitor603shall be “C1”, and the input voltage of the ADC602shall be “V2”, the relation of the following formula 4 holds.
C1/Ca=(V1−V2)/V2  [Formula 4]

Since the values of “Ca” and “V1” are known, the value of “C1” can be acquired by measuring the value of “V2”. The output value of the ADC602is outputted to the CPU401through the I/O control IC407, and the CPU401calculates the electrostatic capacity of the first sensor module510based on the output value from the ADC602. Similarly, the CPU401controls the I/O control IC407to change the connection target of the switch604, and calculates the electrostatic capacities of the second sensor module511and the third sensor module512.

FIG. 8is a graph showing the capacitance laws of the first, second, and third sensor modules510,511, and512. In this example, the line width W1of the first sensor module510is 0.1 mm and the conductor spacing D1thereof is 0.1 mm. The line width W2of the second sensor module511is 0.5 mm and the conductor spacing D2thereof is 0.5 mm. The line width W3of the third sensor module512is 1.0 mm and the conductor spacing D3thereof is 1.0 mm.

Curves corresponding to the first sensor module510, the second sensor module511, and the third sensor module512shown inFIG. 8indicate the values that are obtained by normalizing the variations of the electrostatic capacities due to the variation of toner thickness (the values after subtracting the values C10, C20, and C30in the case of zero toner thickness) by the electrostatic capacities in the case of infinite toner thickness. That is, the electrostatic capacity indicated by the vertical axis inFIG. 8is a relative value with respect to the case of infinite toner thickness. As shown inFIG. 8, the first sensor module510, the second sensor module511, and the third sensor module512show different capacitance laws to the variation of toner thickness. Accordingly, the toner thickness is presumed using the ratio of the electrostatic capacities of the first sensor module510, the second sensor module511, and the third sensor module512in this embodiment.

FIG. 9is a view showing the ratio of the capacitance laws of the second sensor module511and the third sensor module512to the capacitance law of the first sensor module510, and shows the way of thinking of toner-thickness presumption typically. When the ratio of the electrostatic capacities (relative values) of the third sensor module512and the first sensor module510is 0.8, the toner thickness can be presumed as about 1 mm in view ofFIG. 9. Similarly, the toner thickness can be presumed based on the ratio of the electrostatic capacities (relative values) of the second sensor module511and the first sensor module510. The toner thickness can be detected more correctly by comparing the two estimation values acquired in this way. Details thereof will be described later with reference toFIG. 12.

Moreover, the electrostatic capacities of the first sensor module510, the second sensor module511, and the third sensor module512vary also depending on the specific inductive capacity ∈t of the toner layer509. When the image forming apparatus1is in a working state, the temperature around the apparatus and moisture environment vary. Particularly, the variation of moisture content of toner due to the variation of environmental moisture changes the specific inductive capacity ∈t sharply.FIG. 10is a graph showing variations of the electrostatic capacity of the first sensor module510that are normalized by the toner thickness likeFIG. 8for the toner layers509of which the specific inductive capacities ∈t are equal to “4” and “8”. It is shown that the electrostatic capacity of the first sensor module510differs greatly when the specific inductive capacity ∈t of the toner layer509differs.

FIG. 11is a graph showing a ratio between the electrostatic capacities of the first and second sensor modules510and511for the toner layers509of which the specific inductive capacities ∈t are equal to “4” and “8”. Even if the specific inductive capacity ∈t of the toner layer509varies, the ratio between the electrostatic capacities of the first and second sensor modules510and511does not vary, and two curves corresponding to the cases where the specific inductive capacities ∈t are “4” and “8” overlap.

Since the moisture content of toner gently follows the variation of environmental moisture, the moisture content of the toner layer presumed based on the environmental moisture includes an error with respect to the actual moisture content, and it is difficult to lessen this error with the above-mentioned configuration of JP 2002-132038A. On the other hand, when the toner sensor that includes a plurality of sensor modules of which electrostatic capacities are different to the same toner thickness is used like this embodiment, the remaining toner amount can be detected with a reduced detection error because the detection is less subject to the variation of the specific inductive capacity of toner due to environmental variation.

FIG. 12is a flowchart showing a remaining toner amount detection process executed by the engine control unit217. Each step is executed by running the predetermined program that the CPU401read from the ROM403and developed to the work area of the RAM404. This remaining toner amount detection process is performed at definite time interval or predetermined timing (for example, when the power of the image forming apparatus1is turned ON and when a maintenance work is performed).

First, the electrostatic capacities of the first sensor module510, the second sensor module511, and the third sensor module512with which the toner sensor304is provided are detected (step S1201(an electrostatic capacity detection step)). Then, the CPU401determines whether all the electrostatic capacities of the first sensor module510, the second sensor module511, and the third sensor module512have been detected (step S1202). After the detection of electrostatic capacities is completed (YES in the step S1202), the detected electrostatic capacities are corrected according to a compensation table based on the temperature measured by the temperature sensor409, and also the standard electrostatic capacities (C10, C20, C30) are subtracted from the corrected electrostatic capacities (step S1203(a standard electrostatic capacity subtraction step)). It should be noted that the compensation table and the standard electrostatic capacities are beforehand stored in the nonvolatile memory402(a first storage unit).

If the three electrostatic capacities acquired in the step S1203shall be C1, C2, and C3, three ratios R1, R2, and R3will be calculated according to the following formula 5 (step S1204(an electrostatic capacity ratio calculation step)).
R1=C2/C1,R2=C3/C1, andR3=C3/C2  [Formula 5]

Next, the toner thickness is detected by checking the ratios R1, R2, and R3against a conversion table that is stored beforehand in the ROM403(a second storage unit) (step S1205(a toner thickness detection step)). It should be noted that the conversion table is generated by taking the ratios of the capacitance laws of the first sensor module510, the second sensor module511, and the third sensor module512as described with reference toFIG. 9. In this embodiment, the toner thickness detected in the step S1205is not a final toner thickness. Accordingly, the toner thickness acquired in the step S1205is hereafter referred to as an “estimated value”.

Next, the CPU401determines whether the maximum difference (referred to as a “measurement error”, hereafter) among the three estimated values acquired in the step S1205is below a first threshold value defined beforehand (step S1206). When the measurement error is below the first threshold value (YES in the step S1206), the process proceeds to step S1207. When the measurement error is larger than the first threshold value (NO in the step S1206), the CPU401determines whether the measurement error is below a second threshold value defined beforehand (step S1210).

When the measurement error is below the second threshold value (YES in the step S1210), the CPU401determines an estimated value that is farthest from the average of the three estimated values as an invalid estimated value, excepts this invalid estimated value (step S1211), and proceeds with the process to the step S1207. When the measurement error is larger than the second threshold value (NO in the step S1210), the CPU401selects the smallest estimated value among the three estimated values as a final toner thickness (step S1212), and proceeds with the process to step S1208.

In the step S1207, the CPU401calculates the average of the three estimated values presumed in the step S1205or the average of the two estimated values selected in the step S1211, and determines the average as the final toner thickness. Next, the CPU401determines whether the final toner thickness determined in the step S1207or the step S1212is below a specified value (step S1208). When the final toner thickness is below the specified value (YES in the step S1208), the CPU401determines that the remaining toner amount is little, and notifies the controller216of a remaining-toner-amount alarm (step S1209). Then, the remaining toner amount detection process finishes.

When receiving the notice in the step S1209, the controller sends a notice to a user by displaying a remaining-toner-amount alarm message through the operation unit5or by sounding alarm sound from a loudspeaker in order to urge the user to supply toner. At the same time, the operation of the entire image forming apparatus1is restricted or prohibited if needed.

In the first embodiment, the toner sensor304is configured by forming the electrodes of different line widths at the different conductor spacings on the same surface of the substrate507so that the first sensor module510, the second sensor module511, and the third sensor module512have different sensitivities to the thickness of the toner layer509, respectively. On the other hand, in a second embodiment, two sensor modules are arranged by forming two pairs of electrodes of the same line width at the same conductor spacing so that the sensor modules show different electrostatic capacities with respect to the toner layer509.

FIG. 13Ais a sectional view showing a first example of another configuration of the toner sensor304.FIG. 13Bis a sectional view showing a second example of another configuration of the toner sensor304.

The toner sensor304A shown inFIG. 13Ais configured by arranging one sensor module that consists of a pair of electrodes1301and1302on one surface of the substrate507and by arranging the other sensor module that consists of a pair of electrodes1303and1304on the other surface of the substrate507. The electrodes1301,1302,1303, and1304have the same line width, and the conductor spacing between the electrodes1301and1302is identical to the conductor spacing between the electrodes1303and1304. Since the two sensor modules have different distances to the toner layer509(the distance to a toner detection face) by the thickness of the substrate507, they show different electrostatic capacities with respect to the toner thickness. Accordingly, the second embodiment can determine the toner thickness (remaining toner amount) in the same manner as the first embodiment.

On the other hand, the toner sensor304B shown inFIG. 13Bis configured so that the substrate507is attached with inclination to the toner detection face of the buffer unit301. Two sensor modules that consist of a pair of electrodes1305and1306, and a pair of electrodes1307and1308with the same width and the same conductor spacing are arranged on the same side of the substrate507. However, since the two sensor modules have different distances to the toner layer509, they show different electrostatic capacities. Accordingly, the third embodiment can determine the toner thickness (remaining toner amount) in the same manner as the first embodiment.

It should be noted that only one ratio is acquired as the ratio of the electrostatic capacities of the sensor modules when the toner sensor304A or304B that includes two sensor modules is used. Accordingly, the thickness of the toner layer509will be determined by checking the calculated ratio of the electrostatic capacities against the conversion table in this case, and the comparisons with the first threshold value and the second threshold value that are required in the first embodiment are unnecessary.

Although the embodiments of the invention have been described, the present invention is not limited to the above-mentioned embodiments, the present invention includes various modifications as long as the concept of the invention is not deviated. The embodiments mentioned above show examples of the present invention, and it is possible to combine the embodiments suitably.

For example, although the above-mentioned embodiments described the configuration for detecting the remaining toner amount in the buffer unit301, the toner amount in other units, such as a toner bottle and a recovery toner container, can be detected similarly. Moreover, although the above-mentioned embodiments described the configuration for detecting the amount of toner that is powder ink, the quantity of liquid ink etc. can be detected by an equivalent configuration.

OTHER EMBODIMENTS

This application claims the benefit of Japanese Patent Application No. 2012-086478, filed on Apr. 5, 2012, which is hereby incorporated by reference herein in its entirety.