DEVELOPER CONTAINER UNIT, DEVELOPING UNIT, AND PROCESS CARTRIDGE

A developer container unit includes a container containing developer and a piezoelectric film for detecting an amount of developer in the container. A sensitivity of the piezoelectric film to a stress in a direction parallel to a film surface is greater than a sensitivity of the piezoelectric film to a stress in a direction perpendicular to the film surface, and the piezoelectric film is deformable with a movement thereof relative to the developer.

DESCRIPTION OF THE EMBODIMENTS

First Embodiment

(1) Overview of Structure and Operation of Image Forming Apparatus

First, the overall structure of an electrophotographic image forming apparatus will be described.FIG. 1is a schematic cross-sectional view of an image forming apparatus100according to the first embodiment. The image forming apparatus100is a full-color laser printer using a tandem method and an intermediate transfer method. The image forming apparatus100can form a full-color image on a recording medium (such as a recording sheet, a plastic sheet, or a piece of fabric) on the basis of image information. The image information is input from a host machine that is connected to an image forming apparatus body, such as an image reader or a personal computer that is communicatively connected to the image forming apparatus body.

The image forming apparatus100includes first, second, third, and fourth image forming units SY, SM, SC, and SK, which respectively form yellow (Y), magenta (M), cyan (C), and black (K) images. In the first embodiment, the first to fourth image forming units SY, SM, SC, and SK are arranged along a line that intersects the vertical direction.

In the first embodiment, the first to fourth image forming units have substantially the same structure and are operated in substantially the same way, except that they form images of different colors. Therefore, hereinafter, the characters Y, M, C, and K for denoting the colors will be omitted unless it is necessary to discriminate between the colors.

The image forming apparatus100includes four photoconductor drums1(electrophotographic photoconductors) that are arranged along a line that intersects the vertical direction. Each of the photoconductor drums1corresponds to an image carrier. The photoconductor drum1is rotated by a driving unit (not shown) in the direction of arrow A (clockwise direction) inFIG. 1. A charge roller2and a scanner unit3(exposure device) are arranged around the photoconductor drum1. The charge roller2is a charger that uniformly charges the surface of the photoconductor drum1. The scanner unit3is an exposure unit that irradiates the surface of the photoconductor drum1with a laser beam on the basis of image information so as to form an electrostatic image (electrostatic latent image) on the photoconductor drum1. Moreover, a developing unit4(developer container unit) and a cleaning member6(cleaning unit) are arranged around the photoconductor drum1. The developing unit4develops an electrostatic image into a toner image. The cleaning member6removes toner (residual toner) remaining on the photoconductor drum1after the toner image has been transferred. An intermediate transfer belt5(intermediate transfer body) is disposed so as to face the four photoconductor drums1. The intermediate transfer belt5transfers toner images on the photoconductor drum1to a recording medium12.

The developing unit4uses a non-magnetic single component toner as a developer. In the first embodiment, the developing unit4performs reversal development by making the developing roller (described below), which is a developer carrying member, contact the photoconductor drum1. To be specific, in the first embodiment, the developing unit4develops an electrostatic image by making toner, which has been charged so as to have a polarity the same as that of the photoconductor drum1(in the first embodiment, a negative polarity), adhere to a portion (image portion, exposed portion) of the photoconductor drum1at which the charge has been weakened by irradiation of a laser beam.

The photoconductor drum1, the charge roller2(a process unit acting on the photoconductor drum1), the developing unit4, and the cleaning member6are integrated with each other and constitute a process cartridge7. The process cartridge7is attachable to and removable from the image forming apparatus100by means of attachment members, such as a guide member and a positioning member, which are disposed in the image forming apparatus body. In the first embodiment, all of the process cartridges7have the same shape and respectively contain yellow (Y), magenta (M), cyan (C), and black (K) toners.

The intermediate transfer belt5(intermediate transfer body) is an endless belt that is in contact with all of the photoconductor drums1. The intermediate transfer belt5moves around (rotates) in the direction of arrow B (counterclockwise direction) inFIG. 1. The intermediate transfer belt5is looped over support members, including a drive roller51, a secondary transfer opposing roller52, and a driven roller53.

Four primary transfer rollers8(primary transfer units) are arranged along an inner peripheral surface of the intermediate transfer belt5so as to respectively face the four photoconductor drums1. Each of the primary transfer rollers8presses the intermediate transfer belt5toward a corresponding one of the photoconductor drums1and forms a primary transfer region N1at which the intermediate transfer belt5and the photoconductor drum1contact each other. To each of the primary transfer rollers8, a bias voltage having a polarity opposite to a regular polarity of the charge of toner is applied by a primary transfer bias power source (high-voltage power supply, not shown), which is a primary transfer bias application unit. Thus, toner images on the photoconductor drums1are transferred (primarily transferred) to the intermediate transfer belt5.

A secondary transfer roller9(second transfer unit) is disposed outside of the intermediate transfer belt5at a position facing the secondary transfer opposing roller52. The secondary transfer roller9is in pressed against the secondary transfer opposing roller52with the intermediate transfer belt5therebetween so as to form a secondary transfer region N2at which the intermediate transfer belt5and the secondary transfer roller9contact each other. To the secondary transfer roller9, a bias voltage having a polarity opposite to the regular polarity of the charge of toner is applied by a secondary transfer bias power source (high-voltage power supply, not shown), which is a secondary transfer bias application unit. Thus, toner images on the intermediate transfer belt5are transferred (secondarily transferred) to the recording medium12.

When the image forming apparatus100forms an image, first, the charge roller2uniformly charges the surface of the photoconductor drum1. Next, the scanner unit3emits a laser beam on the basis of image information, the charged surface of the photoconductor drum1is scanned by the laser beam, and thereby an electrostatic image based on the image information is formed on the photoconductor drum1. Next, the developing unit4develops the electrostatic image formed on the photoconductor drum1into a toner image. The primary transfer roller8transfers (primarily transfers) the toner image formed on the photoconductor drum1to the intermediate transfer belt5.

When forming a full-color image, the first to fourth image forming units SY, SM, SC, and SK successively perform the operation described above, and thereby color toner images are primarily transferred to the intermediate transfer belt5in an overlapping manner.

Subsequently, the recording medium12is transported to the secondary transfer region N2in synchronism with the movement of the intermediate transfer belt5. Then, the secondary transfer roller9, which is pressed against the intermediate transfer belt5with the recording medium12therebetween, simultaneously secondarily-transfers the four-color toner images on the intermediate transfer belt5to the recording medium12.

The recording medium12, to which the toner images have been transferred, is transported to a fixing device10(fixing unit). The fixing device10applies heat and pressure to the recording medium12, and thereby the toner images are fixed to the recording medium12.

The cleaning member6removes and recovers toner remaining on the photoconductor drum1after the primary transfer operation has been finished. An intermediate transfer belt cleaning device11removes toner remaining on the intermediate transfer belt5after the secondary transfer operation has been finished.

The image forming apparatus100may form a monochrome or a multi-color image by using one or more (but not all of the) image forming units.

(2) Process Cartridge

The overall structure of the process cartridge7mounted in the image forming apparatus100according to the first embodiment will be described.

FIG. 2is a schematic cross-sectional view of the process cartridge7according to the first embodiment, seen in the longitudinal direction of the photoconductor drum1(the direction of the rotation axis). In the first embodiment, the process cartridges7for the four colors have substantially the same structure and are operated in the substantially same way, except that they contain developers of different types (colors).

The process cartridge7includes a photoconductor unit13, which includes the photoconductor drum1and other components, and the developing unit4, which includes a developing roller17and other components.

The photoconductor unit13includes a cleaning frame body14for supporting various components of the photoconductor unit13. The photoconductor drum1is rotatably mounted on the cleaning frame body14through bearings (not shown). A driving force of a driving motor (not shown) is transmitted to the photoconductor unit13, and the photoconductor drum1is rotated in the direction of arrow A (clockwise direction) in accordance with an image formation operation. The photoconductor drum1is a main component for performing an image formation process. The photoconductor drum1is an organic photoconductor drum including an aluminum cylinder whose peripheral surface is coated with functional layers, including an undercoat layer, a carrier generating layer, and a carrier transport layer in this order.

The photoconductor unit13includes the cleaning member6and the charge roller2, which are in contact with the peripheral surface of the photoconductor drum1. The cleaning member6removes residual toner on the surface of the photoconductor drum1. The residual toner drops into and is contained in the cleaning frame body14.

The developing unit4includes the developing roller17, a developing blade21, a toner supply roller20, a toner80used for development, and a toner container18.

An agitation member25for agitating toner is disposed in the toner container18. The agitation member25includes a rotary shaft22, an agitation sheet23(flexible sheet) one end of which is fixed to the rotary shaft22, and a piezoelectric film24(seeFIGS. 3A and 3B) affixed to the agitation sheet23. When the driving unit (not shown) rotates the rotary shaft22, the agitation sheet23agitates toner contained in the toner container18and transports the toner toward an upper portion of the toner supply roller20in the direction of an arrow G inFIG. 2. In the first embodiment, the agitation member is rotated when the developing unit performs development.

The developing blade21is in contact with the developing roller17in a counter direction to the developing roller17. The developing blade21regulates the amount of toner with which the surface of the developing roller17is coated. The developing blade21also charges the toner. The developing blade21is a thin plate-shaped member that generates an elastic force with which the developing blade21is pressed against the developing roller17. The surface of the developing blade21is in contact with the toner and the developing roller17. The developing roller17rotates in the direction of an arrow D, and the toner is charged with triboelectricity generated by friction between the developing blade21and the developing roller17. At the same time, the developing roller17regulates the thickness of the toner. A blade bias power source (not shown) applies a predetermined voltage to the developing blade21, so that coating with toner can be stably performed.

In a region (contact region) in which the developing roller17and the photoconductor drum1face each other, the surfaces of the developing roller17and the photoconductor drum1move in the same direction (in the first embodiment, upward). In the first embodiment, the developing roller17is in contact with the photoconductor drum1. Alternatively, the developing roller17may be disposed at a predetermined small distance from the photoconductor drum1.

In the first embodiment, the toner is negatively charged with triboelectricity. Because a predetermined DC bias is applied to the developing roller17, an electrostatic latent image is developed into a visible image as the toner is transferred to only exposed portions of the photoconductor drum1that have been irradiated with a laser beam.

The toner supply roller20and the developing roller17are disposed so as to form a nip therebetween. The toner supply roller20rotates in the direction of an arrow E inFIG. 2(counterclockwise direction). The toner supply roller20is an elastic sponge roller including an electroconductive core metal whose peripheral surface is coated with a foam material. The toner supply roller20and the developing roller17are in contact with each other so that the surface of the toner supply roller20is recessed by a predetermined amount. In the nip region, the toner supply roller20and the developing roller17rotate in opposite directions. The toner supply roller20supplies toner to the developing roller17in the nip region and subsequently removes toner from the developing roller17.

The developing roller17and the toner supply roller20each have an outer diameter of φ20, and the developing roller17is pressed against the toner supply roller20so that the surface of the toner supply roller20is recessed by the amount of 1.5 mm.

(3) Structure of Agitation Member and Method of Detecting Remaining Amount of Toner

(3-1) Structure of Agitation Member

FIG. 3Ais a schematic view of the agitation member25,FIG. 3Bis a schematic cross-sectional view of the agitation member25seen in the axial direction, andFIG. 3Cis an enlarged cross-sectional view of the piezoelectric film24. The piezoelectric film24is made by Tokyo Sensor Co., Ltd. and has a thickness of 20 μm. The material of the piezoelectric film24is polyvinylidene fluoride (PVDF). The piezoelectric film24includes a piezopolymer (PVDF) substrate24aand silver-ink electrodes24bformed on both surfaces of the piezopolymer substrate24a.

The sensitivity (piezoelectricity) of the piezoelectric film24to a stress in a direction parallel to a film surface is greater than the sensitivity (piezoelectricity) of the piezoelectric film24to a stress in a direction perpendicular to the film surface. The sensitivity of the piezoelectric film24to a compressive stress is greater than the sensitivity of the piezoelectric film24to a tensile stress. In particular, the sensitivity to a tensile stress in a rolling direction, in which the piezoelectric film24was rolled in a manufacturing process, is the highest. The piezoelectric film24is bonded to the agitation sheet23so that the rolling direction is perpendicular to the axial direction of the agitation member25. The agitation sheet23is electrically insulating. As illustrated inFIG. 3B, in the first embodiment, the piezoelectric film24having a width of 10 mm is bonded to a middle portion of the agitation sheet23in the longitudinal direction so as to be integrated with the agitation sheet23. The agitation sheet23has flexibility to a bending stress and a sufficient elastic resilience to a bending stress. The material of the agitation sheet23is polyphenylene sulfide and the thickness of the sheet is 150 μm. The silver-ink electrodes24bof the piezoelectric film24are connected to metallic films and metallic wires (not shown) extending to the outside and are connected to a voltage detection circuit of the image forming apparatus body through sliding electrodes26. A signal generator90(seeFIG. 1), which is disposed in the image forming apparatus body, generates an alarm signal for raising an alarm about the amount of toner on the basis of an output voltage of the piezoelectric film24. The signal generator90corresponds to an alarm signal generator.

By disposing the piezoelectric film24on the agitation sheet23as described above, a slight change in toner-powder pressure can be detected by using a piezoelectric film having a relatively small area.

The operational effects of the structure of the first embodiment will be described below in comparison with the structure an agitation member of a related-art example, which is described in Japanese Patent Laid-Open No. 3-271785.

The piezoelectric film is thin and flexible. Because the film is thin and has a very small cross-sectional area, a greater stress is generated by a small tension in a direction parallel to the film surface. In particular, the piezoelectric film has the highest sensitivity to a tension in the rolling direction. The ratio of the standard effective sensitivity in the rolling direction to that in the thickness direction is about 1000:1. With the first embodiment, the toner-powder pressure can be detected with a high sensitivity by effectively utilizing such characteristics of the piezoelectric film24.

FIGS. 4A and 4Billustrate a comparison between the structure of the related-art example and the structure of the first embodiment. As illustrated inFIG. 4A, the idea of the related-art example is converting a toner-powder pressure in the thickness direction of a polymer piezoelectric plate28into a deformation amount (strain amount) of the polymer piezoelectric plate28in the thickness direction and then converting the deformation amount into a voltage. Because the piezoelectric plate28of the related-art example receives a toner-powder pressure in the thickness direction, a substantially rigid body is used as an agitation plate27so that the agitation plate27does not deform. With such a structure, when the piezoelectric plate28receives a toner-powder pressure in the thickness direction, the piezoelectric plate28deforms only in such a way that the piezoelectric plate28contracts in the thickness direction. The Young's modulus of a general polymer piezoelectric element is in the range of 2 to 4×109N/m2. Therefore, it is clear that the piezoelectric plate28deforms only slightly in the thickness direction when a very small toner-powder pressure is applied to the piezoelectric plate28. Accordingly, a stress generated in the piezoelectric plate28is very small. As a result, with the structure of the related-art example, only a very low voltage is generated when the toner-powder pressure changes.

In contrast, with the structure of the first embodiment illustrated inFIG. 4B, the piezoelectric film24, which is a thin film, is bonded to a deformable surface of the agitation sheet23, which is a flexible member having elastic resilience, so as to be integrated with the surface. Thus, as illustrated inFIG. 4B, a very small toner-powder pressure can be converted into a large extensional deformation in the rolling direction.

As illustrated inFIG. 3B, the piezoelectric film24is bonded to the agitation sheet23so as to be integrated with the agitation sheet23. The piezoelectric film24is located at a position separated from a neutral axis25aof the agitation member25(which is a neutral axis in a cross section perpendicular to the film surface, along which extension and contraction do not occur when the agitation member25deforms). Thus, a large strain can be generated in the piezoelectric film24when the agitation sheet23deforms.

In the first embodiment, the agitation sheet23has a free end. Therefore, a very small toner-powder pressure can cause a large deformation of the agitation sheet23and a large change in voltage.

In the first embodiment, the piezoelectric film24is disposed so that the amount of deformation of the piezoelectric film24in the rolling direction, in which the piezoelectric film24has the highest sensitivity, is greater than the amount of deformation of the piezoelectric film24in a direction perpendicular to the rolling direction. However, even if the piezoelectric film24is disposed so that the amount of deformation of the piezoelectric film24in the width direction perpendicular to the rolling direction is larger, an advantage of a sensitivity greater than that of the related-art example can be obtained in principle. For example, the piezoelectric film24may be disposed so that the rolling direction of the piezoelectric film24coincides with the axial direction of the rotary shaft22.

In the first embodiment, the piezoelectric film24is affixed to a portion of the agitation sheet23that is in the middle in the longitudinal direction and that extends from one end to the other end of the agitation sheet23in the transversal direction (radial direction from the rotary shaft). However, this is not a limitation. For example, the piezoelectric film24may be affixed only to a portion of the agitation sheet23near the free end or to any appropriate portion of the agitation sheet23in accordance with the structure of the agitation member and the structure of the developer container.

(3-2) Overview of Output Voltage Profile of Piezoelectric Film

FIGS. 5A to 5Fare schematic views illustrating how the amount of deformation of the agitation sheet23changes in one cycle of rotation of the agitation member25according to the first embodiment.FIG. 6is a graph representing a profile of an output voltage generated between the electrodes24bof the piezoelectric film24as the agitation member25rotates.

The relationship between the amount of deformation of the agitation sheet23illustrated inFIGS. 5A to 5Fand the profile illustrated inFIG. 6will be described. The agitation sheet23starts rotation from the position illustrated inFIG. 5A, an end portion of the agitation sheet23enters through the surface of toner inFIGS. 5B and 5C, and thereby deformation of the agitation sheet23occurs. At the same time, the piezoelectric film24generates a voltage in accordance with the amount of deformation. Subsequently, the amount of deformation increases as illustrated inFIGS. 5D to 5Eand becomes the largest inFIG. 5F. InFIG. 5A, the deformation is suddenly released. At this time, the agitation sheet23deforms in a direction such that the agitation sheet23returns to its original shape. As illustrated inFIG. 6, due to the change of the direction of deformation and a sharp change in the amount of deformation, the piezoelectric film24generates a peak voltage in the negative direction.

As illustrated inFIGS. 5D to 5E, in order to efficiently transport the toner, the free end of the agitation sheet23slides over the bottom wall of the container. Because the characteristics of the piezoelectric film24are efficiently used, the structure of the first embodiment has a very high detection sensitivity. Therefore, a slight change in the toner-powder pressure can be detected from an output of the piezoelectric film24even when the output includes an influence of a change in the amount of deformation caused by contact with the bottom wall.

(3-3) Method of Detecting Remaining Amount of Toner

In the profile illustrated inFIG. 6, the values of the following parameters change in accordance with the remaining amount of toner.

Parameters that change in accordance with Remaining Amount of Toner

(i) negative peak voltage occurrence timing Ta, negative peak voltage Va

(ii) toner surface entry timing Tb

(iii) positive peak voltage Vf

(iv) integral value of profile for one cycle of agitation member

examples: integral value α=sum of absolute value of output voltage

integral value β=sum of positive output voltage

integral value γ=sum of negative output voltage

Referring to the profile shown inFIG. 6, how and why the values of the parameters (i) to (iv) change when the amount of toner decreases will be described.

When the amount of toner (developer) decreases, the surface of the toner becomes lower (inFIG. 2and other figures) and the amount of toner agitated by the agitation sheet23decreases.

Because the surface of the toner becomes lower and the amount of toner agitated by the agitation sheet23decreases, the timing at which the amount of deformation of the agitation sheet23starts decreasing is advanced, and therefore the negative peak voltage occurrence timing Ta ((i)) is advanced in one rotation cycle of agitation. For the same reason, the maximum amount of deformation of the agitation sheet decreases, the amount of recovery of the agitation sheet23decreases, and therefore the negative peak voltage Va decreases.

Because the toner surface becomes lower, the toner surface entry timing Tb ((ii)) is delayed. Because the total amount of toner agitated by the agitation sheet23decreases, the maximum amount of deformation of the agitation sheet23decreases, and therefore the positive peak voltage Vf ((iii)) decreases.

The integral value of the profile ((iv)) decreases as the surface of the toner become lower decreases and the amount of toner agitated by the agitation sheet23decreases.

FIG. 7Ais a flowchart of a process of detecting the amount of toner. In step S101, rotation of the agitation member25is started. Immediately after rotation of the agitation member25is started, in step S102, stabilization of the output and detection the rotation phase of the agitation member are performed. In the structure of the first embodiment, after a printing operation is started and the agitation member has rotated twice, it is possible to stabilize the output and detect the rotation phase. In step S103, the output voltage of the piezoelectric film24is analyzed. In step S104, the amount of toner is detected. In step S105, rotation of the agitation member25is stopped.

FIG. 7Billustrates the relationship between the amount of toner and a change ΔTb in the toner surface entry timing Tb ((ii)) when the amount of toner in the developer container decreases as the image forming apparatus according to the first embodiment performs a printing operation. Here, ΔTb is the difference between the value of Tb when the toner container is full (initial) and the value of Tb when the amount of toner decreases (now). As illustrated inFIG. 7B, there is a correlation between ΔTb and the amount of toner, and therefore it is possible to successively detect the amount of toner.

As illustrated inFIG. 7B, ΔTb is used in the first embodiment. Alternatively, any of the aforementioned parameters of the output voltage profile, whose values change in accordance with the amount of toner, and a combination of such parameters may be used. The aforementioned parameters, whose values change in accordance with the amount of toner, may be selectively used in accordance with the structure of the agitation member and the structure of the developer container. The parameters used may be changed in accordance with the amount of toner.

The agitation sheet23is disposed so that the free end of the agitation sheet23does not contact the inner wall of the container at a timing at which the agitation sheet23enters through the toner surface (when the agitation sheet23is near the positions shown inFIGS. 5C and 5D). That is, when the agitation sheet23rotates downward, the agitation sheet23reaches a bottom wall18bof the container without contacting a side wall18aof the developer container. Thus, the accuracy of detection of the toner surface entry timing Tb can be further increased.

In the first embodiment, as illustrated inFIG. 5A, at a timing at which the agitation sheet emerges through the toner surface (when the agitation sheet23is near the position shown inFIG. 5A), the free end of the agitation sheet does not contact the inner wall of the container. Therefore, the amount of extension and the speed of extension of the agitation sheet can be increased, and the accuracy of detection of the amount of toner can be increased further when detecting the amount of toner by using the negative peak voltage occurrence timing Ta and the negative peak voltage Va.

The structure and the advantage of the first embodiment are mainly as follows.

The developing unit4according to the first embodiment includes the toner container18containing toner and the piezoelectric film24for detecting the amount of toner in the developer container. The sensitivity of the piezoelectric film24to a stress in a direction parallel to a film surface is greater than the sensitivity of the piezoelectric film24to a stress in a direction perpendicular to the film surface. Thus, the amount of toner can be detected with high accuracy.

The piezoelectric film24is rotatable in the developer container, and the sensitivity of the piezoelectric film24to a stress in a direction parallel to the film surface and perpendicular to the rotation axis of the piezoelectric film24is greater than the sensitivity of the piezoelectric film to a stress in a direction of the rotation axis. Thus, when the piezoelectric film24rotates, a force that the piezoelectric film24receives from the toner is efficiently converted into a voltage, and thereby the accuracy of detection of the amount of toner can be further increased. The sensitivity of the piezoelectric film24to a stress in the direction parallel to the film surface and perpendicular to the rotation axis of the piezoelectric film24is greater than the sensitivity of the piezoelectric film24to a stress in any other directions parallel to the film surface. Thus, the accuracy of detection of the amount of toner can be further increased.

The piezoelectric film24is integrated with the agitation sheet23having an elastic resilience greater than that of the piezoelectric film24, and the piezoelectric film24and the agitation sheet23constitute the agitation member25that agitates the toner. Thus, high accuracy of detection of the amount of toner and high agitation performance of the agitation member can be both obtained. The piezoelectric film24is located at a position separated from a neutral axis of the agitation member25in a cross section of the agitation member25along a plane perpendicular to the film surface. Thus, the accuracy of detection of the amount of toner can be further increased. The piezoelectric film24is disposed on a surface of the agitation member25on a downstream side in a rotation direction of the agitation member25(seeFIG. 4B). Thus, when the agitation member25rotates, the piezoelectric film24can efficiently deform, and the accuracy of detection of the amount of toner can be further increased.

The length L of the piezoelectric film24in a direction perpendicular to the rotary axis of the piezoelectric film24is greater than the length W of the piezoelectric film24in the direction of the rotation axis. Accordingly, the piezoelectric film24can efficiently deform and the accuracy of detection of the amount of toner can be further increased.

The piezoelectric film24is disposed close to or in contact with the rotary shaft. Accordingly, the piezoelectric film24can be electrically connected to the image forming apparatus body easily.

In the first embodiment, the flexible piezoelectric film24is affixed to the flexible agitation sheet23so as to be integrated with the flexible agitation sheet23. Alternatively, the agitation sheet23and the piezoelectric film24may be disposed so as to be rotatable independently, and the piezoelectric film24may be only used to detect the amount of toner.

For example, an advantage the same as that of the first embodiment can be obtained by using an agitation member illustrated inFIG. 8.FIG. 8illustrates an agitation member29including the piezoelectric film24affixed to a flexible sheet30having elastic resilience. The flexible sheet30is rolled up and is attached to an agitation paddle31that is a substantially rigid body and that does not deform. Also with this structure, the amount of toner can be detected with high accuracy.

In the first embodiment, the piezoelectric film24is disposed on the surface of the agitation member on the downstream side in the rotation direction of the agitation member. Alternatively, the piezoelectric film24may be disposed on the surface of the agitation sheet on the upstream side. Further alternatively, the piezoelectric film24may be sandwiched between a plurality of agitation sheets. As long as the piezoelectric film24is deformable as described above, an output voltage obtained with the first embodiment is greater than that of existing structures, which is dependent on deformation in the thickness direction. Therefore, the amount of toner can be detected with high accuracy.

Second Embodiment

In the first embodiment, the piezoelectric film24is disposed on the agitation sheet23. In the second embodiment, the piezoelectric film24is independent from the agitation sheet23, and the piezoelectric film24is disposed on the inner wall of the developer container. Components of the second embodiment the same as those of the first embodiment will not be described.

FIG. 9Ais a schematic cross-sectional view of a process cartridge according to the second embodiment. As illustrated inFIG. 9A, a toner-amount detection member32is attached to an inner wall (bottom wall) in a lower portion of the developer container. The toner-amount detection member32corresponds to a developer-amount detection member. The toner-amount detection member32includes a flexible sheet35having a thickness of 100 μm and the piezoelectric film24the same as that of the first embodiment. The piezoelectric film24is bonded to the flexible sheet35so as to be integrated with the flexible sheet35. The material of the flexible sheet35is PPS. As in the first embodiment, when an agitation member rotates, the toner-amount detection member32receives a toner-powder pressure and deforms. In order to maximize the sensitivity of the piezoelectric film24, the piezoelectric film24is affixed to the flexible sheet35so that the piezoelectric film24deforms in a direction in which the piezoelectric film24has the highest sensitivity as an agitation member33rotates.

The toner-amount detection member32is affixed to a middle portion of the inner wall of the developer container in the longitudinal direction. The width of the toner-amount detection member32in the longitudinal direction of the developer container is 10 mm. The length of the toner-amount detection member32from a free end to a fixed end that is fixed to the inner wall of the developer container is 20 mm. With such a structure, the accuracy of detection of the amount of toner can be increased while suppressing the influence of decrease in the agitation performance due to the presence of the toner-amount detection member.

As in the first embodiment, electrodes are formed on both surfaces of the piezoelectric film24, and the electrodes are connected to a voltage detection circuit of the image forming apparatus body. As compared with the first embodiment, the second embodiment can be manufactured easily because the electrodes for detecting the voltage generated in the piezoelectric film have a simpler structure. With the first embodiment, it is necessary to electrically connect the piezoelectric film24, which is affixed to the agitation sheet23, to the output voltage detector of the image forming apparatus through sliding electrodes. In contrast, with the second embodiment, it is not necessary to use the sliding electrodes. Instead, it is only necessary to electrically connect the piezoelectric film24to the outside of the container through the inner wall of the container.

In the second embodiment, the toner-amount detection member32is disposed on the bottom surface of the toner container18. Thus, after the amount of toner has decreased to a certain level, the amount of deformation of the toner-amount detection member32changes for every rotation cycles of the agitation member. The amount of toner can be detected with high accuracy from the profile of voltage generated in the piezoelectric film24at this time.

The structure and the advantage of the second embodiment are mainly as follows.

The developing unit4according to the second embodiment includes the toner container18containing toner and the piezoelectric film24for detecting the amount of toner in the developer container. The sensitivity of the piezoelectric film24to a stress in a direction parallel to a film surface is greater than the sensitivity of the piezoelectric film24to a stress in a direction perpendicular to the film surface. Thus, as in the first embodiment, the amount of toner can be detected with high accuracy.

The piezoelectric film24is integrated with the flexible sheet35having an elastic resilience greater than that of the piezoelectric film24, and the piezoelectric film24and the flexible sheet35constitute the toner-amount detection member32. The toner-amount detection member32is attached to the inner wall of the toner container18, and the toner-amount detection member32deforms when the agitation member33agitates the toner. Thus, the piezoelectric film24easily returns to its original shape after deforming as the agitation member33agitates the toner. Therefore, the accuracy of detection of the amount of toner can be increased. As compared with the first embodiment, the structure of electrical contacts connected to the piezoelectric film24can be simplified.

The piezoelectric film24is located at a position separated from the neutral axis of the toner-amount detection member32. Thus, the accuracy of detection of the amount of toner can be further increased. The piezoelectric film24is disposed on the surface of the toner-amount detection member32on the upstream side in the rotation direction of the agitation member33. Thus, when the agitation member33rotates, the piezoelectric film24can efficiently deform, and the accuracy of detection of the amount of toner can be further increased.

In the second embodiment, the agitation member33is disposed so that the agitation member33contacts the toner-amount detection member32while the agitation member33agitates the toner. By doing so, it is possible to detect an output voltage that is specifically generated at the instant at which the agitation member33contacts the toner-amount detection member32. Therefore, as compared with the first embodiment, the rotation phase of the agitation member can be easily detected in principle. Accordingly, the accuracy of analysis the output voltage is increased and the accuracy of detection is increased.

In the second embodiment, the toner-amount detection member32is disposed on the inner wall of the developer container so as to have a free end. Alternatively, the toner-amount detection member32may be rolled up and disposed as illustrated inFIG. 9B. Also with such a structure, the advantage of the present invention can be obtained.

In the second embodiment, the piezoelectric film24and the flexible sheet35are integrated with each other. Alternatively, only the piezoelectric film24may be attached to the inner wall of the developer container. By doing so, as compared with the first embodiment, the structure of electrical contacts connected to the piezoelectric film can be simplified. In this case, in order to facilitate detection of the rotation phase of the agitation member33, the agitation member33may be disposed so as to contact the piezoelectric film when agitating the toner.

In the second embodiment, the piezoelectric film24is disposed on the surface of the toner-amount detection member32on the upstream side in the rotation direction of the agitation member33. Alternatively, the piezoelectric film24may be disposed on the surface of the toner-amount detection member32on the downstream side. Further alternatively, the piezoelectric film24may be sandwiched between a plurality of flexible sheets. As long as the piezoelectric film24is deformable as described above, an output voltage obtained with the second embodiment is higher than that of existing structures, which is dependent on deformation in the thickness direction. Therefore, the amount of toner can be detected with high accuracy.

With the present invention, a developer container unit that can detect the remaining amount of developer with higher accuracy can be provided.

This application claims the benefit of Japanese Patent Application No. 2012-285802, filed Dec. 27, 2012, which is hereby incorporated by reference herein in its entirety.