Water content measuring device, measuring method, and image forming device

A water content measuring device measures water content of an object to be measured. The water content measuring device includes two plate members that are disposed facing each other, a current generating unit, a measuring unit, and an arithmetic processing unit. The current generating unit generates current supplied between the two plate members. The measuring unit measures electrostatic capacitance generated by the current supplied between the two plate members. The arithmetic processing unit is configured to convert the electrostatic capacitance measured by the measuring unit to water content. Two plate members each have a shape or are disposed such that at least two points on a leading edge of the object enter between the two plate members at different timings in a direction perpendicular to a passing direction in which the object passes through the two plate members.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2017-091720, filed on May 2, 2017. The contents of which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a water content measuring device, a measuring method, and an image forming device.

2. Description of the Related Art

In a copier, a printer, a facsimile, a multifunction peripheral including these functions, and the like, which use an electrophotographic process; water content included in a recording medium (sheet) to which toner is attached affects quality of an image formed on the recording medium and conveyance of the recording medium.

In view of the foregoing, there has been proposed a technique for measuring water content of a sheet using an electrostatic capacitance sensor so as to reflect the water content on image forming control (for example, Japanese Patent No. 3133862).

However, in the conventional technique, there is one electrostatic capacitance sensor and only water content of one spot on a sheet can be measured. Thus, a water content distribution on the sheet cannot be obtained.

A plurality of electrostatic capacitance sensors can be used in order to implement image forming control depending on a water content distribution, but there is a problem in that it costs a lot and control becomes complicated.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a water content measuring device measures water content of an object to be measured. The water content measuring device includes two plate members that are disposed facing each other, a current generating unit, a measuring unit, and an arithmetic processing unit. The current generating unit generates current supplied between the two plate members. The measuring unit measures electrostatic capacitance generated by the current supplied between the two plate members. The arithmetic processing unit is configured to convert the electrostatic capacitance measured by the measuring unit to water content. Two plate members each have a shape or are disposed such that at least two points on a leading edge of the object enter between the two plate members at different timings in a direction perpendicular to a passing direction in which the object passes through the two plate members.

DESCRIPTION OF THE EMBODIMENTS

An object of an embodiment is to provide a water content measuring device capable of obtaining a water content distribution of an object to be measured using one device, a measuring method of the water content distribution of an object to be measured, and an image forming device that includes the water content measuring device.

FIG. 1is a view illustrating a configuration example of an image forming device. The image forming device illustrated inFIG. 1is a multifunction peripheral that has functions of a copier, a printer, a facsimile, and a scanner, but the image forming device may be a copier, a printer, a facsimile, a scanner, and the like.

The image forming device is formed of a plurality of devices and units. The image forming device includes an automatic document feeder (ADF)10, an image reading device11, a printer unit12, a sheet feeding unit13, a manual sheet feeding unit14, a paper ejection15, an operation panel16, and an electrostatic capacitance sensor17as a water content measuring device. This is one example, and the image forming device does not necessarily include the ADF10, the manual sheet feeding unit14, and the like and may include other devices and units.

The ADF10includes a document table on which a document is placed, a conveying mechanism that conveys a document on the document table, and a discharge tray that discharges the conveyed document, and moves a document on a contact glass of the image reading device11. The image reading device11includes a light source, a plurality of mirrors, an image forming lens, and an imaging element, and irradiates a document on the contact glass with light from the light source and causes the reflected light to enter the imaging element through the mirrors and the image forming lens. Examples of the imaging element can include a charged coupled device (CCD) image sensor, a complementary metal oxide semiconductor (CMOS) image sensor, and the like, and the imaging element converts entering light to an electrical signal and outputs the electrical signal as image data.

The printer unit12includes a writing unit20, a photoconductor drum21, a developing device22, a conveying belt23, and a fixing device24. The writing unit20receives an instruction from a controller that controls the image forming device, irradiates the photoconductor drum21with light, and forms a latent image on a surface of the photoconductor drum21. The developing device22is filled with toner, discharges the toner to the photoconductor drum21, and develops a latent image formed on a surface of the photoconductor drum21. The conveying belt23conveys a sheet, and transfers an image that is developed by developing on the sheet. The fixing device24applies heat and pressure to a sheet that has an image transferred thereon and is conveyed by the conveying belt23so as to fix the image on the sheet.

The sheet feeding unit13includes a sheet feeding tray in which sheets are housed and a sheet feeding roller that feeds sheets housed in the sheet feeding tray one by one, and receives an instruction from the controller and supplies a sheet. The manual sheet feeding unit14includes a manual sheet feeding tray on which sheets are placed and a sheet feeding roller that feeds sheets on the manual sheet feeding tray one by one, and receives an instruction from the controller and supplies a sheet. The paper ejection15includes a paper ejection tray that ejects a sheet on which an image is fixed by the fixing device24.

The operation panel16receives an operation of a user, and instructs the controller to execute reading, print, and the like, of a document. The operation panel16displays buttons for selecting functions in order to receive operation of a user, start buttons and the like for executing print and the like, and displays the execution state, error, and the like.

The electrostatic capacitance sensor17is provided in a conveying path where a sheet fed from the sheet feeding unit13or the manual sheet feeding unit14is conveyed. The conveyed sheet passes through between parallel plates serving as two plate members forming the electrostatic capacitance sensor17. The parallel plates have conductivity, and are formed of two plates that include surfaces facing each other. The two plates have the same shape, the same size, and the same thickness. A metal plate can be used as the parallel plates, and examples of the metal plate can include a stainless plate, a copper plate, and an aluminum plate. A mounting position of the parallel plates may be any one of the following: in the sheet feeding unit13, in the manual sheet feeding unit14, and between the sheet feeding unit13or the manual sheet feeding unit14and the photoconductor drum21.

FIG. 2is a view illustrating a configuration example of the electrostatic capacitance sensor17. The electrostatic capacitance sensor17includes a current generating device30, parallel plates31, an electrostatic capacitance measuring device32, and a controller33. The parallel plates31have conductivity, and are formed of two plates that are disposed apart in parallel with each other. A sheet34as a recording medium enters between the parallel plates31, and passes through between the parallel plates31.

The current generating device30generates current having a predetermined current value to be supplied between the parallel plates31. When current is supplied between the parallel plates31, the parallel plates31function as a capacitor, and an electrical charge is stored between the parallel plates31. An amount of the stored electrical charge is called electrostatic capacitance. The electrostatic capacitance C(F) can be calculated by the following expression (1) where the area of each of the parallel plates31is defined as S (m2), a gap between the parallel plates31is defined as d (m), and a dielectric constant is defined as ε.

When a current is supplied between the parallel plates31, the current does not flow between the parallel plates due to an insulator such as air existing between the parallel plates31. However, polarization where one plate serves as a positive electrode and the other serves as a negative electrode is generated, so that an electrical charge can be stored. The dielectric constant ε is a value that represents a magnitude of this polarization, and varies due to an insulator existing between the parallel plates31. The dielectric constant ε varies depending on whether the insulator between the parallel plates31is only air or includes a sheet when the sheet passes through between the parallel plates31. Because water conducts electricity, the dielectric constant ε differs between a dry sheet and a sheet containing moisture (water), and varies depending on the water content of a sheet. In addition, when water content is not uniform and water content is distributed in one sheet, the dielectric constant ε varies depending on a position on the sheet.

Examples of the electrostatic capacitance measuring device32can include an inductance-capacitance-resistance (LCR) meter that causes alternating current (AC) to flow between the parallel plates31serving as an object to be measured, detects amplitude ratio and a phase difference between a voltage signal and a current signal, calculates impedance R from the detected amplitude ratio and phase difference, and calculates inductance L and electrostatic capacitance C from frequency of the impedance R and AC. In this embodiment, the LCR meter is used as one example of the electrostatic capacitance measuring device32, but the electrostatic capacitance measuring device is not limited to the LCR meter. The electrostatic capacitance measuring device32can be any device capable of measuring the electrostatic capacitance C.

The controller33controls the current generating device30and the electrostatic capacitance measuring device32, and performs arithmetic processing for converting the electrostatic capacitance C measured by the electrostatic capacitance measuring device32to a water content. The controller33holds, for example, a table that indicates a relation between the electrostatic capacitance C and water content, and can convert the electrostatic capacitance C measured using the table to water content. The conversion to water content is not limited to the conversion executed using the table, and may be conversion executed using a conversion equation and the like.

The controller33can include, in order to execute control of the current generating device30and the like, and arithmetic processing such as conversion to water content, a read only memory (ROM) and a flash memory as a storage device that stores a control program, a conversion program, and the like. The controller33can include a central processing unit (CPU) that reads and executes a computer program from the storage device, a random access memory (RAM) that provides a work area to the CPU, and the like.

The parallel plates31each have a shape or each are disposed such that at least two points on the leading edge of the sheet enter between the parallel plates31at different timings in a direction (sub-scanning direction) perpendicular to a passing direction (main scanning direction) in which the sheet34passes through between the parallel plates31. In the example illustrated inFIG. 2, the parallel plates31each have a stepped shape in which lengths in the main scanning direction are different in three steps, so that three points34a,34b, and34con the leading edge of the sheet34corresponding to the respective steps enter between the parallel plates31not at the same time but shifted in time.

The shape of each of the parallel plates31is not limited to the three-stepped shape illustrated inFIG. 2, and may be a two-stepped shape and a four-stepped shape or more, a tapered shape having different lengths in the main scanning direction, and the like. Alternatively, the parallel plates31may each have a rectangular shape and be disposed not in parallel to the leading edge of the sheet34but inclined at a predetermined angle with respect to the leading edge of the sheet34. The shape or disposition of the parallel plates31is not limited to these examples, and may be any shape or disposition in which at least two points on the sheet34in the sub-scanning direction enters between the parallel plates31at different timings. The following describes a case where the parallel plates31each having a stepped shape are used.

FIG. 3is a view illustrating the parallel plates31and the sheet34when viewed from the top. The sheet34has long sides in the main scanning direction that is a conveying direction as indicated by an arrow A and has short sides in the sub-scanning direction. A conveying destination of the sheet34is the parallel plates31each of which one end has a stepped shape having different lengths in tree steps in the main scanning direction. InFIG. 3, the parallel plates31each have the longest length at the left side in the main scanning direction, and each have the shorter length that reduced stepwise toward the right side. In the parallel plates31illustrated inFIG. 3, the longest length in the main scanning direction is defined as H.

The points34a,34b, and34con the leading edge of the sheet34are at the left side, the center, and the right side, respectively, in a single line in the sub-scanning direction. When the sheet34moves in the conveying direction as indicated by an arrow A and enters between the parallel plates31, the point34aenters between the parallel plates31first, the point34benters between the parallel plates31with delay in time, and the point34centers between the parallel plates31with more delay in time. In other words, the points34a,34b, and34con the sheet34enter between the parallel plates31in sequence as the sheet34is conveyed at a certain distance.

FIG. 4is a view illustrating an example of the distribution of the dielectric constant ε of the sheet34. When the sheet34does not yet enter between the parallel plates31or after the sheet34passes through between the parallel plates31, a dielectric constant ε0of the air is always a contact value without being affected by environmental changes such as a sheet temperature and moisture.

In the example illustrated inFIG. 4, when the sheet34is conveyed between the parallel plates31, the dielectric constant ε differs for each length H in the main scanning direction of the sheet34and for each area35a,35b, and35cinto which the sheet34is divided from the left side, the center, and the right side. In other words, the dielectric constant when the area35aon the left side and having the length H from the point P1(i.e., the leading edge) to the point P2of the sheet34enters between the parallel plates31is defined as εa1, the dielectric constant when the area35bat the center enters between the parallel plates31is defined as εb1, and the constant when the area35con the right side enters between the parallel plates31is defined as εc1. The dielectric constant when an area36aon the left side and having the length H from the point P2to the point P3of the sheet34enters between the parallel plates31is defined as εa2, the dielectric constant when an area36bat the center enters between the parallel plates31is defined as εb2, and the constant when an area36con the right side enters between the parallel plates31is defined as εc2.

The length H can be arbitrarily set, and can be set to a suitable length corresponding to a water content distribution on the sheet34. The number of steps in a stepped shape can also be arbitrarily set to a suitable number of steps corresponding to a water content distribution on the sheet34.

FIG. 5is a view illustrating a case where the leading edge of the sheet34moves to the position X, and a part of the sheet34enters between the parallel plates31. In the example illustrated inFIG. 5, a gap d between the parallel plates31is constant, an area S of the parallel plate31is divided in six, the area35aof the sheet34enters between the parallel plates31, and the other areas35b,35c, and36ato36cdo not enter yet. In this example, one of the three equal areas obtained by dividing each of the areas35ato35cand36ato36cinto three is equal to one of the six areas obtained by dividing the area (hatching region) of the parallel plate31into six.

InFIG. 5, electrostatic capacitance is measured when the sheet34is conveyed by a certain distance, in other words, by a distance of one third of the length H (i.e., H/3). The measured electrostatic capacitance C is composite capacitance of electrostatic capacitance Ca1when the area35aof the sheet34enters between the parallel plates31and electrostatic capacitance C0of the air. The electrostatic capacitance C is based on a volume ratio of the measured area that varies every time the sheet34is conveyed between the parallel plates31by a certain distance, in other words, on the ratio of the volume of a measured area in which the electrostatic capacitance Ca1is measured and the volume of a measured area in which the electrostatic capacitance C0is measured.

In the example illustrated inFIG. 5, the ratio of the volume of a measured area in which the electrostatic capacitance Ca1is measured and the volume of a measured area in which the electrostatic capacitance C0of the air is measured can be, if the gap d is made constant, determined by the area S, and is 1:5. Thus, the electrostatic capacitance Ca1can be measured using the measured electrostatic capacitance C, the volume ratio, and the previously calculated electrostatic capacitance C0by subtracting 5*C0from the electrostatic capacitance C. When the electrostatic capacitance of air is measured between the plates, the total capacitance measured is 6*C0as the space between the plates includes six C0regions.

FIG. 6is a view illustrating a case where the leading edge of the sheet34moves to a position Y, and the sheet34further enters between the parallel plates31. In the example illustrated inFIG. 6, the area35bof the sheet34in addition to the area35aenters between the parallel plates31, but the areas35c,36a,36b, and36cdoes not yet enter between the parallel plates31.

InFIG. 6, electrostatic capacitance is measured when the sheet34is conveyed by a certain distance from the position illustrated inFIG. 5. The measured electrostatic capacitance C is composite capacitance of the electrostatic capacitance Ca1when the area35aenters between the parallel plates31, electrostatic capacitance Cb1when the area35benters between the parallel plates31, and electrostatic capacitance C0of the air. In this case, the ratio of the volume of a measured area in which the electrostatic capacitance Ca1is measured, the volume of a measured area in which the electrostatic capacitance Cb1is measured, and the volume of a measured area in which the electrostatic capacitance C0of the air is measured can be, if the gap d is made constant, determined by the area S, and is 2:1:3. The electrostatic capacitance Ca1is already calculated, and the electrostatic capacitance Cb1can be calculated using the measured electrostatic capacitance C, the volume ratio, and the previously calculated electrostatic capacitance C0and Ca1in the same manner as described above.

FIG. 7is a view illustrating a case where the leading edge of the sheet34moves to a position Z, and the sheet34further enters between the parallel plates31. In the example illustrated inFIG. 7, the area35cof the sheet34in addition to the areas35aand35benters between the parallel plates31.

InFIG. 7, electrostatic capacitance is measured when the sheet34is conveyed by a certain distance from the position illustrated inFIG. 6. The measured electrostatic capacitance C is composite capacitance of the electrostatic capacitance Ca1when the area35aenters between the parallel plates31, the electrostatic capacitance Cb1when the area35benters between the parallel plates31, and electrostatic capacitance Cc1when the area35centers between the parallel plates31. In this case, the ratio of the volume of a measured area in which the electrostatic capacitance Ca1is measured, the volume of a measured area in which the electrostatic capacitance Cb1is measured, and the volume of a measured area in which the electrostatic capacitance Cc1is measured can be, if the gap d is made constant, determined by the area S, and is 3:2:1. The electrostatic capacitances Ca1and Cb1are already calculated, and the electrostatic capacitance Cc1can be calculated using the measured electrostatic capacitance C, the volume ratio, and the previously calculated electrostatic capacitances Ca1and Cb1in the same manner as described above.

The electrostatic capacitance of each area on the sheet34can be calculated by repeating the steps illustrated inFIGS. 5 to 7, and a water content distribution on the sheet can be obtained by converting the electrostatic capacitance of each area to the water content.

FIG. 8is a flowchart illustrating an example of processing for measuring water content. In response to the start of printing, processing starts at step S800. At step S805, the sheet34starts to be conveyed between the parallel plates31, and it is detected that the leading edge of the sheet34has entered between the parallel plates31. Entry of the sheet34between the parallel plates31can be detected by changes in measured electrostatic capacitance. For example, when electrostatic capacitance changes more than a certain amount, it is detected that the sheet34has entered between the parallel plates31.

At step S810, it is determined whether the sheet34is conveyed by a certain distance. The certain distance is a distance from the point at which the leading edge of the sheet34starts to enter between the parallel plates31to the point at which the leading edge reaches the position X. This distance is the same as a distance from the position X to the position Y and as a distance from the position Y to the position Z. Thus, when the length H is used, a certain distance is defined as H/3. When the sheet34is not conveyed by the certain distance, determination at step S810is repeated until the sheet is conveyed by the certain distance. When the sheet34is conveyed by the certain distance, the process goes to step S815.

At step S815, the electrostatic capacitance measuring device32measures electrostatic capacitance at that point. At step S820, the controller33acquires the measured electrostatic capacitance, and converts the electrostatic capacitance to water content using a table and the like. The converted water content can be stored in a storage device. The controller33can store, at the time of storing the converted water content, the converted water content in association with area identification information for identifying an area in the sheet34on which the electrostatic capacitance being a conversion source is measured. In this manner, a water content distribution can be output as a drawing and the like to a display device and the like.

At step S825, it is determined whether the sheet34passes through between the parallel plates31. In other words, it is determined whether the trailing edge of the sheet34has passed through the parallel plates31. Whether the sheet34has passed through the parallel plates31can be determined by measuring electrostatic capacitance and depending on whether the measured electrostatic capacitance indicates almost the same value as the electrostatic capacitance C0of the air. Almost the same value means that a value is within a certain error range.

If the sheet34has not passed through the parallel plates31, the process goes back to step S810, and processing at steps S810to S825is repeated. If the sheet34has passed through the parallel plates31, the process goes to step S830and it is checked whether printing is completed. Because the number of printing sheets is designated at the time of printing, whether printing is completed can be determined by counting the number of sheets34having been passed through the parallel plates31and checking whether the counted count value reaches the number of printing sheets. If printing is not completed, the next sheet34is conveyed, and the process goes back to step S805and the processing at steps S805to S830is repeated. If printing is completed, this processing ends at step S835.

In the electrostatic capacitance sensor17, even when there is no error of the area between the parallel plates31and there is no positional deviation in a horizontal direction between the parallel plates31, there may be an error of a gap between the parallel plates31. In addition, even when there is no error of the gap, there may be an error of the area; or even when there is no error of the area, there may be a positional deviation in a horizontal direction between the parallel plates31.

In this case, an error and a positional deviation can be calculated and corrected by: measuring electrostatic capacitance before the sheet34enters between the parallel plates31, in other words, in a state where nothing exists between the parallel plates31; and comparing the measured electrostatic capacitance with electrostatic capacitance calculated by the expression 1. This processing may be performed at any time before printing is started. This correction processing is not necessarily performed for each printing, and can be performed, for example, after an image forming device is turned on and returns from a power-saving state.

FIG. 9is a flowchart illustrating another example of processing for measuring water content. Processing starts from step S900, and electrostatic capacitance is measured in a state where nothing exists between the parallel plates31at step S905. At step S910, an error in the area and the like of parallel plates31and a positional deviation thereof in the horizontal direction are calculated. Because processing at step S915to S925and after steps S935is the same as that at steps S805to S835illustrated inFIG. 8, the explanation is omitted.

At step S930, the electrostatic capacitance is corrected based on the error and the positional deviation calculated at step S910. For example, the electrostatic capacitance can be corrected by measuring a correction coefficient (correction parameter) from the calculated error and positional deviation and multiplying the electrostatic capacitance by the calculated correction parameter. Herein, the electrostatic capacitance is corrected. Alternatively, water content after conversion may be corrected.

A dielectric constant when the sheet34is conveyed between the parallel plates31varies depending on temperature of a housing unit that houses the sheet34, moisture in the housing unit, thickness of a sheet, a sheet size, and the like, besides the water content of the sheet34. The reasons are as follows. The dielectric constant of the air is not affected by temperature and moisture, but water content of the sheet34varies depending on temperature and moisture, and the dielectric constant varies due to change in water content. When thickness or size of the sheet34is changed, a volume ratio of the air between the parallel plates31and the sheet34varies, and a dielectric constant varies due to the change in volume ratio.

These factors affecting the electrostatic capacitance can be input as a parameter for selecting, for example, a table used in converting the measured electrostatic capacitance to water content. When a table corresponding to a thickness of a sheet is stored, thickness of a sheet used for printing is input so as to select a table corresponding to the input thickness, and the measured electrostatic capacitance can be converted to water content using the selected table. In this manner, precise water content can be obtained.

FIG. 10is a flowchart illustrating still another example of processing for measuring water content. Processing starts from step S1000, and at step S1005, an ambient temperature and the like are measured by a different device such as a thermometer and are input as a parameter affecting electrostatic capacitance. Because processing after step S1010is the same as that at steps S805to S835illustrated inFIG. 8, the explanation is omitted. At step S1025, a table corresponding to an input parameter is selected and electrostatic capacitance is converted to water content using the table.

Correction processing illustrated inFIG. 9may be applied to processing that includes parameter input illustrated inFIG. 10. When there is no corresponding table, water content corresponding to electrostatic capacitance can be calculated using the interpolation method, for example.

By using the electrostatic capacitance sensor17that has the above-described configuration and performs the above-described processing, even one sensor can obtain water content of a plurality of areas on the sheet34and obtain a water content distribution. Thus, an image forming device performs image forming control corresponding to the obtained water content. Examples of image forming control include control of changing a heating condition corresponding to a water content distribution so as to decrease bending (curling) of the sheet34occurring in fixing. This control can reduce deterioration in print quality and a paper jam.

According to the present invention, a water content distribution of an object to be measured can be obtained using one device.