JP7183787B2 - image forming device - Google Patents

Description

The present invention relates to an image forming apparatus such as a printer.

In an image forming apparatus such as a laser printer or an LED printer that transfers toner onto a sheet of paper and then fixes the toner onto the sheet of paper by heat, the information on the basis weight and water absorption rate of the sheet before the toner is transferred depends on the transfer conditions. and fusing conditions. For this reason, some image forming apparatuses estimate the basis weight from information on the paper type (thin paper, thick paper, copy paper) preset by the user, or estimate the water absorption rate from the internal humidity of the apparatus.

However, when using paper that has been placed in a different environment outside the device, the water absorption rate may differ greatly from the estimated one, so it is not always possible to set the optimal operating conditions. I didn't.

In view of this, Japanese Patent Laid-Open No. 2002-200001 proposes a technique of measuring the water absorption rate of paper after feeding using a capacitance sensor.

JP 2018-25724 A

In the technology related to this proposal, even if the water absorption rate of the paper after feeding can be detected correctly, the toner is transferred and fixed to the paper immediately after the detection, so the temporary operating conditions and the detected water absorption rate If there is a large divergence from the optimum operating conditions (especially if the fixing temperature needs to be changed), it may take a long time to change the conditions, resulting in a longer time required for the first print.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an image forming apparatus capable of suppressing the time required from an instruction to form an image to the start of an image forming operation even when the operating conditions are changed to conditions suitable for the water absorption rate of paper. is to provide

To achieve the above object, an image forming apparatus according to the present invention includes: a main body; An apparatus, wherein the main body includes an image forming unit that forms an image on paper supplied from a paper feed cassette, a first electrode provided movably in the main body, and a capacitance detector, The paper feed cassette is provided vertically movable between a lowered position and a raised position above the lowered position, and includes a pressure plate that supports the paper from below, and a second electrode provided on the pressure plate, The first electrode and the second electrode are arranged so as to overlap each other when viewed from the thickness direction of the paper supported by the pressure plate in a state where the paper feed cassette is correctly installed at the installation position set in the main body, The capacitance detector outputs a detection signal corresponding to the capacitance between the first electrode and the second electrode, and the controller controls the capacitance detector when the pressure plate is positioned at the lowered position. Using the value of the detection signal and the value of the detection signal of the capacitance detector when the pressure plate is positioned at the raised position, a value that varies according to the water absorption rate of the paper supported by the pressure plate is obtained.

According to this configuration, when the paper feed cassette is correctly installed in the installation position of the main body and the pressure plate is raised from the lowered position to the raised position, the value that varies according to the water absorption rate of the paper supported by the pressure plate is set. can ask. Therefore, before starting to supply paper from the paper feed cassette to the main body, the operating conditions of the image forming operation by the image forming section can be changed to conditions suitable for values that fluctuate according to the water absorption rate of the paper. Therefore, it is possible to form an image on a sheet under conditions suitable for the water absorption rate of the sheet without taking time from an instruction for image formation until the image forming section starts the image forming operation.

According to the present invention, it is possible to form an image on a sheet under conditions suitable for the water absorption rate of the sheet without taking time from an instruction for image formation until the image forming section starts the image forming operation.

1 is a cross-sectional view schematically showing the configuration of an image forming apparatus according to an embodiment of the present invention, showing a state where a paper feed cassette is installed at an installation position and a pressure plate is positioned at a raised position; FIG. FIG. 2 is a cross-sectional view schematically showing the configuration of the image forming apparatus, showing a state where the paper feed cassette is installed at the installation position and the pressure plate is located at the lowered position; 2 is a block diagram showing the essential parts of the electrical configuration of the image forming apparatus; FIG. 4 is a flowchart showing the flow of capacitance storage processing; 4 is a flowchart showing the flow of image forming processing; FIG. 4 is a diagram schematically showing the relationship between the first electrode, the second electrode, and the paper when the pressure plate is positioned at the lowered position; FIG. 4 is a diagram schematically showing the relationship between the first electrode, the second electrode, and the paper when the pressure plate is positioned at the raised position; FIG. 5 is a diagram showing an example of a characteristic line showing the relationship between the capacitance value of paper and water absorption. FIG. 10 is a schematic side view showing a configuration in which the first electrode is shifted with respect to the second electrode when the pressure plate is positioned at the raised position, and shows the state where the pressure plate is positioned at the lowered position; FIG. 10 is a schematic side view showing a configuration in which the first electrode is displaced from the second electrode with the pressure plate positioned at the raised position, and shows the state where the pressure plate is positioned at the raised position; FIG. 5 is a diagram showing the relationship between sheet height and capacitance value;

BEST MODE FOR CARRYING OUT THE INVENTION Below, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

<Overall composition>
As shown in FIG. 1, the image forming apparatus 1 includes a housing 11 (an example of a main body). The casing 11 forms an outer shell of the image forming apparatus 1 .

A paper feed cassette 12 is attached to the bottom of the housing 11 . Specifically, the mounting position of the paper feed cassette 12 is set at the bottom of the housing 11 . The paper feed cassette 12 is inserted into the housing 11 from one side toward the mounting position, and is mounted at the mounting position. The paper feed cassette 12 can be removed from the housing 11 by pulling it out from the mounting position to one side of the housing 11 .

For use in the following description, one side of the housing 11, that is, the side where the paper feed cassette 12 is pulled out from the mounting position (the left side in FIG. 1) is defined as the "front side", and the opposite side is defined as the "rear side". do. Then, the left and right sides are defined with reference to a state in which the image forming apparatus 1 is viewed from the "front side". A direction orthogonal to both the front-rear direction and the left-right direction is the “vertical direction”, and the “upper side” and the “lower side” are based on the state in which the image forming apparatus 1 is installed on a horizontal plane.

The paper feed cassette 12 is configured to accommodate a plurality of sheets of paper P in a stacked state. A pressure plate 13 that supports the paper P from below is provided in the paper feed cassette 12 . The pressure plate 13 has a pressure plate shaft 14 extending in the left-right direction. The pressure plate shaft 14 is rotatably supported by the paper feed cassette 12 . As a result, the pressure plate 13 is provided swingably between a raised position where the front end is lifted from the bottom surface of the paper feed cassette 12 and a lowered position extending along the bottom surface of the paper feed cassette 12 with the pressure plate shaft 14 as a fulcrum. there is As the pressure plate 13 swings, the front end of the paper P supported by the pressure plate 13 moves up and down.

A paper feed roller 15 is provided in the housing 11 . The paper feed roller 15 is arranged at a position higher than the mounting position at the front end of the housing 11 . When the paper feed cassette 12 is mounted at the mounting position and the pressure plate 13 is positioned at the lowered position, the paper P supported by the pressure plate 13 does not contact the paper feed roller 15, and the pressure plate 13 moves from the lowered position to the raised position. As it rises, the front end of the uppermost paper sheet P comes into contact with the paper feed roller 15 from below.

In this state, the paper feed roller 15 is rotated to feed the uppermost sheet P from the paper feed cassette 12 to the transport path 16 provided in the housing 11 . The upper surface of the housing 11 is formed with a concave portion serving as a paper discharge tray 17 having a bottom surface that sinks downward toward the rear side. The transport path 16 bulges forward and curves upward from the position of the paper feed roller 15 , extends linearly rearward, bulges rearward and curves upward, and reaches the rear end of the paper discharge tray 17 . open above. An image forming section 18 that forms an image on the paper P is provided in the housing 11 , and the transport path 16 passes through the image forming section 18 . The paper P fed from the paper feed cassette 12 to the transport path 16 is transported along the transport path 16 toward the paper discharge tray 17, and passes through the image forming section 18 while being transported. An image is formed on the paper P as it passes through the image forming section 18 . The paper P on which the image is formed is discharged from the transport path 16 to the paper discharge tray 17 and stacked on the paper discharge tray 17 .

The image forming section 18 is configured to form an image on the paper P by electrophotography.

Specifically, the image forming section 18 includes a photosensitive drum 21, a charger 22, an exposure device 23, a developing device 24, a transfer roller 25 (an example of a transfer section), and a fixing device 26 (an example of a fixing section). The photoreceptor drum 21 is rotatable about an axis extending in the left-right direction. The surface of the photoreceptor drum 21 is uniformly charged by the discharge from the charger 22 as the photoreceptor drum 21 rotates, and then selectively exposed to the laser beam from the exposure device 23 . This exposure selectively removes charges from the surface of the photoreceptor drum 21 and forms an electrostatic latent image on the surface of the photoreceptor drum 21 . When the electrostatic latent image faces the developing roller 27 of the developing device 24, toner is supplied from the developing roller 27 to the electrostatic latent image, and the electrostatic latent image is developed into a toner image. When the toner image faces the transfer roller 25, the paper P is interposed between the photosensitive drum 21 and the transfer roller 25 in synchronization with the image forming operation in the image forming unit 18 and the conveyance of the paper P. is performed. A transfer voltage is supplied to the transfer roller 25 . When the paper P passes between the photoreceptor drum 21 and the transfer roller 25, the toner image is transferred from the surface of the photoreceptor drum 21 to the upper surface of the paper P due to the action of the transfer voltage. The paper P onto which the toner image has been transferred is conveyed to the fixing device 26 . In the fixing device 26, the paper P passes between a heat roller 28 and a pressure roller 29, and the toner image is fixed on the paper P by heating and pressing. Thus, image formation on the paper P is achieved.

<Electrode>
A first electrode 31 is movably provided in the housing 11 . The paper feed cassette 12 is provided with a second electrode 32 paired with the first electrode 31 .

The first electrode 31 is held by an electrode holder 33 . The electrode holder 33 is provided at a position spaced rearward from the paper feed roller 15 . The electrode holder 33 is swingable via a first arm 35 swingably supported by a holder shaft 34 extending in the left-right direction, and an arm shaft 36 (an example of an arm fulcrum) extending in the left-right direction on the first arm 35 . and a second arm 37 connected to the The holder shaft 34 and the arm shaft 36 each extend in the left-right direction, so that when the paper feed cassette 12 is mounted at the mounting position, the holder shaft 34 and the arm shaft 36 are parallel to the upper surface of the paper P supported by the paper feed cassette 12. Extend. The first arm 35 hangs down from the holder shaft 34 . A swing range is set for the second arm 37, and the lower limit position of the swing range is set at a position extending forward from the arm shaft 36 along the horizontal direction. That is, the electrode holder 33 is configured to swing within a range whose lower limit is the position where the second arm 37 extends in the horizontal direction, and the second arm 37 does not rotate downward beyond the lower limit position. . The first electrode 31 is a metal plate and attached to the lower surface of the second arm 37 .

The second electrode 32 is a metal plate having the same shape as the first electrode 31 and is attached to the upper surface of the pressure plate 13 . In a state where the paper feed cassette 12 is installed at the installation position in the housing 11 and the pressure plate 13 is located at the lowered position, the first electrode 31 and the second electrode 32 are arranged in parallel and completely overlapped in the vertical direction. , the positions of the first electrode 31 and the second electrode 32 are set. The distance between the second electrode 32 and the pressure plate shaft 14 , which is the swing fulcrum of the pressure plate 13 , is equal to the distance between the arm shaft 36 and the first electrode 31 .

<Electrical configuration>
The image forming apparatus 1 includes a control board 41 as shown in FIG. The control board 41 is arranged inside the housing 11 .

A capacitance detector 42 is mounted on the control board 41 . The first electrode 31 is electrically connected to the capacitance detector 42 . The electrostatic capacitance detector 42 outputs a signal corresponding to the amount of electricity stored in the first electrode 31. Specifically, when the paper feed cassette 12 is mounted at the mounting position in the housing 11, the first electrode 31 and the second In a state in which the electrode 32 faces the second electrode 32 in a direction perpendicular to it (hereinafter, this direction is referred to as the “perpendicular direction”), the electrostatic capacitance between the first electrode 31 and the second electrode 32 Output a signal.

The pressure plate 13 of the paper feed cassette 12 is entirely made of resin and does not have electrical conductivity, or at least the portion supporting the second electrode 32 and the peripheral portion surrounding it are made of resin and are electrically conductive. have no gender.

A CPU 43 (an example of a control section) and a memory 44 (an example of a storage section) are mounted on the control board 41 . Memory 44 includes non-volatile memory and volatile memory. Although not shown, the control board 41 is mounted with electronic components necessary for controlling each part of the image forming apparatus 1 .

The CPU 43 detects the capacitance value between the first electrode 31 and the second electrode 32 from the output signal of the capacitance detector 42, and detects the detected capacitance value from the paper feed cassette 12 (pressing plate 13). ) to calculate the dielectric constant of the paper P supported by . Then, the water absorption rate of the paper P is determined from the dielectric constant of the paper P, and the transfer voltage (transfer current) supplied to the transfer roller 25 of the image forming section 18 is controlled according to the water absorption rate. The output (fixing temperature) of the heater 45 for heating the heat roller 28 of the fixing device 26 of 18 is controlled.

Note that the CPU 43 and the memory 44 may be incorporated in an ASIC (Application Specific Integrated Circuit), and the ASIC may be mounted on the control board 41 .

A communication unit 46 (an example of a reception unit), which is an interface for communication with an external terminal such as a PC (Personal Computer) communicably connected to the image forming apparatus 1, is connected to the control board 41. there is The communication unit 46 may be configured to perform wired communication with the external terminal, or may be configured to communicate wirelessly.

<Capacitance value storage processing>
FIG. 4 is a flowchart showing the flow of capacitance value storage processing.

While the image forming apparatus 1 is powered on, the CPU 43 executes the capacitance value storage process shown in FIG. 4 at a predetermined cycle.

In the capacitance value storage process, the CPU 43 first detects the capacitance value C from the output signal of the capacitance detector 42 (S11).

Next, the CPU 43 determines whether or not the detected capacitance value C is less than a predetermined attachment determination value (S12). As indicated by the phantom lines in FIG. 2, when the paper feed cassette 12 is not mounted at the mounting position within the housing 11, the first electrode 31 and the second electrode 32 do not face each other in the orthogonal direction. Therefore, the capacitance value C detected from the output signal of the capacitance detector 42 is smaller than the attachment determination value. Therefore, when the capacitance value C is less than the attachment determination value (S12: YES), the CPU 43 determines that the paper feed cassette 12 is removed from the housing 11 and that at least the paper feed cassette 12 is not attached to the attachment position P. (S13: paper feed cassette not installed).

When the paper feed cassette 12 is inserted into the housing 11 and mounted at the mounting position, as shown in FIG. A pseudo-capacitor is formed by 31 and the second electrode 32, and charge is stored in the pseudo-capacitor. As a result, the capacitance value C detected from the output signal of the capacitance detector 42 becomes larger than the mounting determination value.

Therefore, when the capacitance value C is equal to or greater than the attachment determination value (S12: NO), the CPU 43 determines that the paper feed cassette 12 is correctly attached to the attachment position in the housing 11 (S14). It is determined whether or not the capacitance value C is less than the raised position determination value (S15). The raised position determination value is set to a value larger than the mounting determination value.

Further, the CPU 43 determines whether or not the capacitance value C is less than the lowered position determination value (S16). The lowered position determination value is set to a value within a range larger than the mounting determination value and smaller than the raised position determination value. This value is set to a value exceeding the capacitance value C, which increases as the second electrode 32 approaches the first electrode 31 while the pressure plate 13 moves from the lowered position to the raised position.

When the capacitance value C is less than the lowered position determination value (S16: YES), the CPU 43 determines that the pressure plate 13 is positioned at the lowered position (S17). On the other hand, if the capacitance value C is equal to or greater than the lowered position determination value (S16: NO), the CPU 38 determines that the pressure plate 13 is positioned at the raised position (S18).

When the CPU 43 determines that the pressure plate 13 is positioned at the lowered position, the CPU 43 detects the capacitance value Cb from the output signal of the capacitance detector 42 and stores the detected capacitance value Cb in the memory 44 . (S19). After that, the CPU 43 permits the pressure plate 13 to rise from the lowered position to the raised position (S20). In response to this lift permission, the pressure plate 13 rises from the lowered position to the raised position.

When the CPU 43 determines that the pressure plate 13 is positioned at the raised position, the CPU 43 detects the capacitance value Ca from the output signal of the capacitance detector 42 and stores the detected capacitance value Ca in the memory 44 . (S21). After that, the CPU 43 terminates the capacitance value storage process.
In the state where the pressure plate 13 is positioned at the raised position, if even one sheet of paper P is supported by the pressure plate 13, the second electrode 32 does not come into direct contact with the first electrode 31. The capacitance value C detected from the output signal of does not exceed the out-of-paper determination value. On the other hand, when no sheet P is supported by the pressure plate 13, that is, when the paper feed cassette 12 is empty, the second electrode 32 is in direct contact with the first electrode 31. The capacitance value C detected from the output signal of the capacitance detector 42 swings out beyond the no-paper determination value. Therefore, when the capacitance value C detected from the output signal of the capacitance detector 42 is equal to or greater than the raised position determination value, the CPU 43 determines whether or not the capacitance value C is less than the out-of-paper determination value. (S22). Then, when the capacitance value C is less than the no-paper determination value (S22: YES), the CPU 43 determines that there is paper P in the paper feed cassette 12 (S23). If the value is equal to or greater than the no determination value (S22: NO), it is determined that there is no paper P in the paper feed cassette 12 (S24), and the capacitance value storage process ends.

<Image forming processing>
When the paper feed cassette 12 is mounted at the mounting position in the housing 11 and the pressure plate 13 of the paper feed cassette 12 is positioned at the raised position, the CPU 43 executes the image forming process shown in FIG. 5 at a predetermined cycle. be.

In the image forming process, the CPU 43 determines whether or not an image forming instruction has been input (S31). When instructing image formation, the user operates the external terminal, selects a file for image formation on the setting screen of the printer driver, and sets the size of the paper P used for image formation. When the execution key is pressed on the screen of the printer driver after the file selection and various settings are completed, the selected file and various setting information are transmitted from the external terminal to the image forming apparatus 1 . When the communication unit 46 receives the data transmitted from the external terminal, the CPU 43 determines that an image formation instruction has been input. While the image forming instruction is not input (S31: NO), the CPU 43 maintains the standby state without performing the image forming process.

When the CPU 43 determines that an image forming instruction has been input (S31: YES), it detects the capacitance value Can from the output signal of the capacitance detector 42 (S32). The electrostatic capacitance value Can is the electrostatic capacitance value detected in response to the input of the image forming instruction, and is the latest value when the pressure plate 13 is positioned at the raised position.

After that, the CPU 43 compares the capacitance value Can and the capacitance value Ca stored in the memory 44, that is, the capacitance value Ca immediately after the pressure plate 13 is raised to the raised position, and determines the capacitance value. It is determined whether or not the difference between Can and the capacitance value Ca is equal to or greater than a predetermined value (S33). For example, when a large number of sheets of paper P are used in the previous image formation, the distance between the first electrode 31 and the second electrode 32 is greatly reduced, so that the capacitance value Can changes to the capacitance It deviates greatly from the value Ca. Further, when the water absorption rate of the paper P fluctuates greatly, the dielectric constant εp of the paper P also fluctuates greatly, so that the capacitance value Can greatly deviates from the capacitance value Ca.

If the difference between the capacitance value Can and the capacitance value Ca is equal to or greater than the predetermined value (S33: YES), the CPU 43 stores the memory so as to match the relationship between the capacitance value Can and the capacitance value Ca. 44, that is, the capacitance value Cb when the pressure plate 13 is in the lowered position is corrected (S34). When the electrostatic capacitance value Cb is corrected, the corrected electrostatic capacitance value Cb is used to calculate the dielectric constant εp described below. If the difference between the capacitance value Can and the capacitance value Ca is less than the predetermined value (S33: NO), the capacitance value Cb is not corrected.

After that, the CPU 43 calculates the dielectric constant εp of the paper P supported by the paper feed cassette 12 from the capacitance value Can and the capacitance value Cb (S35).

When the pressure plate 13 of the paper feed cassette 12 is at the lowered position, the height of the paper P supported by the pressure plate 13 is between the first electrode 31 and the second electrode 32, as shown in FIG. 6A. A gap d is obtained by adding the thickness of the gap between the first electrode 31 and the upper surface of the paper P to dp. When the pressure plate 13 of the paper feed cassette 12 is positioned at the lowered position, the first electrode 31 is not in contact with the upper surface of the paper P, and the first electrode 31 is positioned at the lower limit position of the swing range. A distance d is a prescribed maximum distance that occurs between the first electrode 31 and the second electrode 32 . Therefore, when the electrode area of the first electrode 31 and the second electrode 32 (equal to the facing area of the first electrode 31 and the second electrode 32) is S, the capacitance value Cb is given by the following equation (1): can be represented.
Cb=(εp×S)/{εp×(d−dp)+dp} (1)

On the other hand, when the pressure plate 13 of the paper feed cassette 12 is positioned at the raised position, the first electrode 31 is lifted together with the paper P by the pressure plate 13, so that the first electrode 31 and the second electrode 32 are aligned as shown in FIG. 6B. coincides with the height dp of the paper P supported by the pressure plate 13 . Therefore, the capacitance value Can can be expressed by the following equation (2).
Can=εp×S/dp (2)

Eliminating the height dp of the paper P from the above equations (1) and (2) and solving for the dielectric constant εp of the paper P yields
εp=d×Can+SS×Can/Cb (3)
becomes.

The CPU 43 calculates the dielectric constant εp by substituting the capacitance values Can and Cb, the known distance d between the first electrode 31 and the second electrode 32, and the electrode area S into the above equation (3).

After calculating the dielectric constant εp, the CPU 43 determines the water absorption rate of the paper P from the dielectric constant εp and the capacitance value Can (S36). Specifically, the memory 44 stores the capacitance value between the first electrode 31 and the second electrode 32 when the paper P is sandwiched between the first electrode 31 and the second electrode 32 for each of a plurality of dielectric constants. A characteristic line indicating the relationship between the water absorption rate of the paper P, that is, the relationship between the capacitance value of the paper P and the water absorption rate is stored as characteristic information in the form of a map or an arithmetic expression. FIG. 7 shows characteristic lines corresponding to three dielectric constants .epsilon.1, .epsilon.2 and .epsilon.3. From this characteristic line, it is understood that the higher the water absorption rate of the paper P, the larger the capacitance value Can. The CPU 43 uses the characteristic line stored in the memory 44 to determine the water absorption from the dielectric constant εp and the capacitance value Can.

Then, the CPU 43 determines whether or not the water absorption rate of the paper P is equal to or greater than a predetermined threshold (S37). When the water absorption rate of the paper P is less than the threshold value (S37: NO), that is, when the paper P is relatively dry, the CPU 43 sets the transfer voltage supplied to the transfer roller 25 of the image forming section 18 to the first value. voltage (S38), and the fixing temperature, which is the temperature of the heat roller 28 of the fixing device 26 of the image forming section 18, is set to the first temperature (S39). On the other hand, if the water absorption rate of the paper P is equal to or higher than the threshold (S37: YES), that is, if the paper P is relatively wet, the CPU 43 sets the transfer voltage to a second voltage lower than the first voltage. (S40), the fixing temperature is set to a second temperature lower than the first temperature (S41).

After setting the fixing temperature and the transfer voltage, the CPU 43 controls the image forming section 18 and the like to perform image formation according to the image formation instruction (S42), and ends the image forming process.

<Effect>
As described above, the water absorption rate of the paper P supported by the pressure plate 13 is obtained when the paper feed cassette 12 is correctly mounted in the mounting position of the housing 11 and the pressure plate 13 is raised from the lowered position to the raised position. be able to. Therefore, before the paper P is started to be transported from the paper feed cassette 12, the transfer voltage and the fixing temperature, which are operating conditions for the image forming operation of the image forming section 18, can be changed to conditions suitable for the water absorption rate of the paper P. Therefore, it is possible to form an image on the paper P under conditions suitable for the water absorption rate of the paper P without taking a long time from an instruction for image formation until the image forming section 18 starts the image forming operation.

Specifically, based on the capacitance value C detected from the output signal of the capacitance detector 42, it is determined whether or not the paper feed cassette 12 is mounted at the mounting position, and the paper feed cassette 12 is mounted. After being mounted at the position, the water absorption rate of the paper P supported by the pressure plate 13 is obtained according to the input of the image forming instruction. As a result, the water absorption rate of the paper P can be obtained at the stage when the supply of the paper P from the paper feed cassette 12 to the transport path 16 in the housing 11 is ready. The operating conditions of the image forming operation can be changed by using the time until the start of . Therefore, even if the image forming speed is increased, the time required for the first print (image formation on the first sheet of paper P) is not prolonged, and the image is formed on the paper P under operating conditions suitable for the state of the paper P. Forming can take place.

The memory 44 stores characteristic information that associates the capacitance and water absorption of the paper P for each of a plurality of dielectric constants. It can be determined from the dielectric constant εp and the capacitance value Can when the pressure plate 13 is positioned at the raised position.

Then, when the water absorption rate of the paper P is less than the threshold value, the fixing temperature is set to the first temperature, and when the water absorption rate is equal to or higher than the threshold value, the fixing temperature is set to the second temperature lower than the first temperature. set. As a result, curling of the paper P after fixing can be suppressed when the paper P is wet.

Also, when the paper P is wet, the electric resistance of the paper P is lower than when it is dry. Therefore, when the paper P is sandwiched between the photosensitive drum 21 and the transfer roller 25 , a large amount of current flows due to the transfer voltage supplied to the transfer roller 25 . Therefore, when the water absorption rate of the paper P is less than the threshold value, the transfer voltage supplied to the transfer roller 25 is set to the first voltage, and when the water absorption rate is equal to or higher than the threshold value, the transfer voltage is set to the first voltage. is set to a second voltage lower than As a result, regardless of whether the paper P is dry or wet, the toner image can be satisfactorily transferred from the photosensitive drum 21 to the paper P.

<Other embodiments>
Although one embodiment of the present invention has been described above, the present invention can also be implemented in other forms.

For example, in the above-described embodiment, the first electrode 31 and the second electrode 32 face each other in the orthogonal direction regardless of whether the pressure plate 13 is positioned at the lowered position or the upper position. and a configuration equal to the area S of the second electrode 32 . Without being limited to this configuration, as shown in FIG. 8A, the distance d2 between the pressure plate shaft 14, which is the swing fulcrum of the pressure plate 13, and the second electrode 32 is the distance between the arm shaft 36 and the first electrode 31. When the pressure plate 13 is at the lowered position, the first electrode 31 and the second electrode 32 completely overlap each other when viewed from the orthogonal direction. As shown in 8B, the first electrode 31 deviates from the position where it completely overlaps the second electrode 32 when viewed in the orthogonal direction, and the facing area between the first electrode 31 and the second electrode 32 is smaller than the electrode area S. A configuration that reduces the area to Ss may be employed.

In this configuration, when the sheet P is supported by the pressure plate 13 to the support limit amount and the pressure plate 13 is positioned at the lowered position, the area S where the first electrode 31 and the second electrode 32 overlap in the orthogonal direction is In a state where the pressure plate 13 supports the sheet P to the support limit amount and the pressure plate 13 is located at the raised position, the ratio of the overlapping area Ss of the first electrode 31 and the second electrode 32 in the orthogonal direction is 1. The first electrode 31 and the second electrode 32 are provided such that the ratio is equal to or greater than /10 and less than 1.

With this configuration, the capacitance value detected from the output signal of the capacitance detector 42 differs between the state where the pressure plate 13 is positioned at the lowered position and the state where the pressure plate 13 is positioned at the raised position, as shown in FIG. can make a big difference.

Then, for each of a plurality of dielectric constants of the paper P, the relationship between the capacitance value and the height dp of the paper P when the pressure plate 13 of the paper feed cassette 12 is positioned at each of the lowered position and the raised position is stored in the memory 44. , the height dp of the paper P supported by the paper feed cassette 12 can be estimated from the capacitance value Can when the pressure plate 13 is at the raised position. Furthermore, if the type (thick paper, plain paper, etc.) of the paper P supported by the paper feed cassette 12 is set, the number of sheets can be estimated from the height dp of the paper P.

The height dp of the paper P supported by the paper feed cassette 12 can also be calculated. Eliminating the dielectric constant εp of the paper P from the above equations (1) and (2) and solving for the height dp of the paper P yields
dp=S×d+(S ² /Can)−(S ² /Cb) (4)
becomes. The height dp can be calculated by substituting the capacitance values Can and Cb, the known distance d between the first electrode 31 and the second electrode 32, and the electrode area S into the above equation (4).

In addition, various design changes can be made to the above configuration within the scope of the matters described in the claims.

1: Image forming apparatus 11: Housing 12: Paper feed cassette 13: Pressure plate 14: Pressure plate shaft 18: Image forming unit 25: Transfer roller 26: Fixer 31: First electrode 32: Second electrode 33: Electrode holder 35: First arm 36: Arm shaft 37: Second arm 42: Capacitance detector 43: CPU
44: Memory 46: Communication part P: Paper

Claims (12)

the main body;
a paper feed cassette that is detachably provided in the main body and supports paper to be supplied to the main body;
An image forming apparatus including a control unit,
The body is
an image forming unit that forms an image on paper supplied from the paper feed cassette;
a first electrode movably provided within the body;
a capacitance detector;
The paper feed cassette is
a pressure plate vertically movable between a lowered position and a raised position above the lowered position for supporting the sheet from below;
a second electrode provided on the pressure plate;
The first electrode and the second electrode overlap each other when viewed from the thickness direction of the paper supported by the pressure plate in a state in which the paper feed cassette is properly installed at the installation position set in the main body. is placed,
The capacitance detector outputs a detection signal corresponding to the capacitance between the first electrode and the second electrode,
The controller controls the value of the detection signal of the capacitance detector when the pressure plate is positioned at the lowered position, and the detection of the capacitance detector when the pressure plate is positioned at the raised position. and a value of the signal to obtain a value that varies according to the water absorption rate of the paper supported by the pressure plate. The image forming apparatus according to claim 1,
The image forming apparatus according to claim 1, wherein the value that varies according to the water absorption is the dielectric constant of the paper supported by the pressure plate. The image forming apparatus according to claim 1 or 2,
the body further comprises an electrode holder that holds the first electrode;
The electrode holder includes a first arm and a second arm swingably connected to the first arm via an arm fulcrum, and the axis of the arm fulcrum is supported by the pressure plate. and a lower limit of a swinging range of the second arm is set at a position where the first electrode and the second electrode are parallel to each other. The image forming apparatus according to claim 3,
The pressure plate is swingable between the lowered position and the raised position about a pressure plate shaft extending parallel to the axis of the arm fulcrum,
The arm fulcrum and the pressure plate shaft are at different positions when projected onto a plane perpendicular to the upper surface of the paper supported by the pressure plate and parallel to the paper transport direction. is longer than the distance from the arm fulcrum to the first electrode. The image forming apparatus according to claim 4,
When the sheet is supported by the pressure plate up to the support limit amount and the pressure plate is positioned at the lowered position, the second electrode is the same as the first electrode when viewed from the thickness direction of the sheet supported by the pressure plate. In a state where the paper is supported by the pressure plate up to the support limit amount with respect to the area overlapping the second electrode and the pressure plate is positioned at the raised position, the thickness direction of the paper supported by the pressure plate is An image forming apparatus provided so that the ratio of the overlapping areas of the first electrode and the second electrode is 1/10 or more and less than 1 when viewed. The image forming apparatus according to any one of claims 1 to 5,
The control unit
determining whether the paper feed cassette is correctly mounted at the mounting position from the detection signal of the capacitance detector;
when it is determined that the paper feed cassette has changed from a state in which the paper feed cassette is not properly mounted in the mounting position to a state in which it is properly mounted, obtaining a value that varies according to the water absorption rate of the paper supported by the pressure plate; Image forming device. The image forming apparatus according to claim 6,
The main body further includes a reception unit that receives an instruction for image formation by the image formation unit,
The image forming apparatus according to claim 1, wherein the control section obtains a value that varies according to the water absorption rate of the sheet supported by the pressure plate in response to the receiving section receiving the image forming instruction. The image forming apparatus according to claim 7,
The main body has a storage unit,
The control unit
a value of the detection signal of the capacitance detector in the state where the pressure plate is positioned at the lowered position, which is obtained after it is determined that the paper feed cassette has changed to the state where the paper feed cassette is properly installed at the installation position; storing in the storage unit values of the detection signals of the capacitance detectors when the pressure plate is positioned at the raised position;
In response to the acceptance of the image forming instruction by the reception unit, the value of the detection signal of the capacitance detector is obtained when the pressure plate is positioned at the raised position, and the pressure plate is positioned at the raised position. storing in the storage unit the value of the detection signal of the capacitance detector in the state of
The value stored in the storage section when the paper feed cassette is attached and the pressure plate stored in the storage section when the reception section receives an image forming instruction in a state where the pressure plate is positioned at the raised position. If the difference between the compared values is greater than or equal to a predetermined value, the value stored in the storage unit when the paper feed cassette is attached and the receiving unit receive an image formation instruction. An image forming apparatus, wherein the operating conditions of the image forming operation by the image forming section are changed using the values stored in the storage section. The image forming apparatus according to any one of claims 1 to 8,
The image forming unit
A configuration for forming an image on paper by an electrophotographic method,
a transfer unit that transfers toner onto paper;
and a fixing section that thermally fixes the toner transferred onto the paper by the transfer section. The image forming apparatus according to claim 9,
Equipped with a storage unit,
The storage unit stores a plurality of pieces of characteristic information corresponding to a plurality of dielectric constants, which is characteristic information in which the capacitance and water absorption of the paper are associated with each other,
The controller controls the value of the detection signal of the capacitance detector when the pressure plate is positioned at the lowered position, and the detection of the capacitance detector when the pressure plate is positioned at the raised position. From the value of the signal, the dielectric constant of the paper supported by the pressure plate is calculated, the characteristic information corresponding to the paper supported by the pressure plate is determined, and the characteristic information corresponding to the paper supported by the pressure plate is determined. An image forming apparatus, wherein the water absorption is determined using the characteristic information and the value of the detection signal of the capacitance detector when the pressure plate is positioned at the raised position. The image forming apparatus according to claim 10,
The control unit sets the fixing temperature for heat fixing by the fixing unit to the first temperature when the water absorption rate is less than the threshold value, and sets the fixing temperature to the first temperature when the water absorption rate is equal to or higher than the threshold value. An image forming apparatus that is set to a second temperature that is lower than the temperature. The image forming apparatus according to claim 10 or 11,
The control unit sets the transfer voltage supplied to the transfer unit to the first voltage when the water absorption rate is less than the threshold value, and sets the transfer voltage to the first voltage when the water absorption rate is equal to or higher than the threshold value. An image forming apparatus that is set to a second voltage that is lower than the voltage.

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