Sheet conveying device and image forming apparatus

Abstract

The invention discloses a sheet conveying device and an image forming apparatus. The sheet conveying apparatus includes: a conveying unit configured to convey a sheet; a guide member deployed to define a transport path; a light-emitting element; a light receiving element configured to output a detection signal that changes according to the amount of received light; a first light passing portion disposed to define an optical path from the light emitting element to the transmission path; and a second light passing portion disposed to define an optical path from the transmission path to the light receiving element. The light receiving element is configured such that the detection signal varies depending on whether or not a sheet is present in the transport path. Each of the first and second light-passing portions has a higher reflectivity for light emitted from the light-emitting element than the guide member has a higher reflectivity for light emitted from the light-emitting element.

Description

[0001] This application is a divisional application of a Chinese invention patent application with the application number 202110799745.9, the application date of July 15, 2021, and the title of "Sheet Feeding Device and Image Forming Device". Technical Field

[0002] The present invention relates to a sheet feeding device for feeding a sheet and an image forming device for forming an image on the sheet. Background Art

[0003] Image forming devices such as printers, copiers, or multifunctional printers include a sheet detection mechanism provided in a sheet conveyance path for detecting a sheet to control the conveyance of a sheet used as a recording material or a document. As image forming devices become faster, optical type sheet detection mechanisms that use light to detect the presence or absence of a sheet are increasingly used due to their fast response.

[0004] In addition, an image forming device of an electrophotographic system includes a fixing unit of a heat fixing system that heats a toner image transferred onto a sheet to fix the toner image onto the sheet. In the case where an optical type sheet detection mechanism is deployed near the fixing unit, if water vapor generated from the sheet condenses on a member in the optical path or the amount of light emission decreases due to deterioration of the light emitting unit caused by high temperature, the amount of light incident on the light receiving unit decreases, and the detection accuracy may deteriorate. Japanese Patent Laid-Open No. 2018-47967 discloses deploying an optical sensor near the fixing unit and blowing cooling air to the light emitting unit and the light receiving unit of the optical sensor to cool the light emitting unit and the light receiving unit to suppress condensation and deterioration of the light emitting unit.

[0005] In order to accurately detect the presence or absence of a sheet by an optical type sheet detection mechanism, it is necessary that the amount of light incident on the light receiving unit changes according to the presence or absence of the sheet, and the detection signal from the light receiving unit changes by more than a preset threshold. However, in some cases, the sheet is detected under adverse conditions other than the above-mentioned condensation and deterioration of the light emitting unit (such as deterioration of the light emitting unit due to an increase in the cumulative light emission time or the presence of stray light). In this case, conventionally, the difference in the amount of light incident on the light receiving unit between the state where the sheet is present and the state where the sheet is absent may become small, and thus the detection accuracy decreases. Summary of the Invention

[0006] The present invention provides a sheet feeding device and an image forming device that can stably detect the presence or absence of a sheet with high detection accuracy.

[0007] According to one aspect of the present invention, a sheet conveying device includes: a conveying unit configured to convey a sheet; a guiding member deployed to define a conveying path for the sheet; a light emitting element configured to emit light; a light receiving element configured to output a detection signal that changes according to the amount of received light; a first light passing portion deployed to define an optical path from the light emitting element to the conveying path; and a second light passing portion deployed to define an optical path from the conveying path to the light receiving element, wherein the light receiving element is configured such that the detection signal changes according to whether a sheet exists in the conveying path, and wherein the reflectance of each of the first light passing portion and the second light passing portion for the light emitted from the light emitting element is higher than the reflectance of the guiding member for the light emitted from the light emitting element.

[0008] Other features of the present invention will become clear from the following description of exemplary embodiments with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 is a schematic diagram of an image forming apparatus according to a first embodiment.

[0010] Figure 2A and Figure 2B are perspective views of a sheet detection mechanism according to a first embodiment, respectively.

[0011] Figure 3A is a side view of a sheet detection mechanism according to a first embodiment.

[0012] Figure 3B and Figure 3C are cross-sectional views of a sheet detection mechanism according to a first embodiment, respectively.

[0013] Figure 4 is a sensor substrate according to a first embodiment.

[0014] Figure 5A and Figure 5B are perspective views of a fixing table according to a first embodiment, respectively.

[0015] Figure 6A and Figure 6B are diagrams for describing the operation of a sheet detection mechanism according to a first embodiment, respectively.

[0016] Figure 7 is a diagram for describing the influence of diffused reflection light in a first embodiment.

[0017] Figure 8 is a graph illustrating the relationship between the output voltage of a light receiving portion and a threshold value for a first embodiment and a conventional example.

[0018] Figure 9A and Figure 9BEach is a perspective view of the sheet detection mechanism according to the second embodiment.

[0019] Figure 10A Is a side view of the sheet detection mechanism according to the second embodiment.

[0020] Figure 10B And Figure 10C Each is a cross-sectional view of the sheet detection mechanism according to the second embodiment.

[0021] Figures 11A to 11C Each is a diagram for describing the operation of the sheet detection mechanism according to the second embodiment.

[0022] Figure 12 Is a diagram for describing the influence of transmitted light in the second embodiment.

[0023] Figure 13A And Figure 13B Each is a perspective view of the sheet detection mechanism according to the third embodiment.

[0024] Figure 14A Is a side view of the sheet detection mechanism according to the third embodiment.

[0025] Figure 14B And Figure 14C Each is a cross-sectional view of the sheet detection mechanism according to the third embodiment.

[0026] Figure 15A And Figure 15B Each is a diagram for describing the operation of the sheet detection mechanism according to the third embodiment.

[0027] Figure 16 Is a diagram for describing the influence of diffuse reflected light in the third embodiment.

[0028] Figure 17A And Figure 17B Each is a diagram for describing the method of measuring reflectance. Detailed Description of the Invention

[0029] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings.

[0030] First Embodiment

[0031] Next, the sheet detection mechanism and the image forming apparatus according to the first embodiment will be described. First, the overall configuration of the image forming apparatus 1 and its operation in image formation will be described below with reference to Figure 1 Describe the overall configuration of the image forming apparatus 1 and its operation in image formation.

[0032] Figure 1The image forming apparatus 1 shown in the figure is an intermediate transfer tandem type electrophotographic image forming apparatus, which forms an image by transferring toner images of four colors, yellow (Y), magenta (M), cyan (C), and black (K), to an intermediate transfer belt by primary transfer and then transferring the toner images to a sheet by secondary transfer. The image forming apparatus 1 includes an image forming section 1B that forms a toner image on a sheet, a sheet feeding section 1D that supplies the sheet to the image forming section 1B, and a fixing unit 18 that fixes the toner image to the sheet.

[0033] The sheet feeding section 1D includes a cassette 23, a feeding roller 24, and a separating roller 25. The cassette 23 is a sheet supporting section that supports the sheet S to be fed, and can be attached to and detached from the main body of the image forming apparatus 1. The main body of the image forming apparatus 1 includes a frame member and an outer casing, and will be hereinafter referred to as the apparatus main body 1A. Note that, as the sheet S used as a recording material, various sheets of different materials having different sizes can be used. Examples of the sheets that can be used include paper sheets such as ordinary paper sheets and cardboard, plastic films, cloths, surface-treated sheet materials such as coated paper sheets, and irregularly shaped sheet materials such as envelopes and index sheets.

[0034] The feeding roller 24 is a feeding member that rotates by driving from a driving unit (not shown) fixed to the apparatus main body 1A and abuts against the uppermost sheet among the sheets S supported in the cassette 23 to feed the uppermost sheet S to the conveyance path. For example, the driving unit includes a driving mechanism such as a gear set held by a frame member constituting the apparatus main body 1A and a driving source such as a motor that supplies driving force to the driving mechanism. The separating roller 25 abuts against the feeding roller 24 to form a separating nip portion therebetween, and applies a frictional force to the sheet S in the separating nip portion to separate other sheets S from the uppermost sheet S conveyed by the feeding roller 24, thereby suppressing simultaneous conveyance of a plurality of sheets S.

[0035] The image forming section 1B includes a laser scanner unit 9, an intermediate transfer unit 10, and process cartridges 3Y, 3M, 3C, and 3K. The process cartridges 3Y, 3M, 3C, and 3K have the same configuration except for forming toner images of different colors. That is, the process cartridges 3Y, 3M, 3C, and 3K form toner images by using toners of yellow (Y), magenta (M), cyan (C), and black (K), respectively. The process cartridges 3Y, 3M, 3C, and 3K can be attached to and detached from the apparatus main body 1A, and each cartridge is composed of a photoconductor unit 5 and a developing unit 4.

[0036] Each of the photoconductor units 5 includes a photosensitive drum 1a as a photosensitive member formed in a drum shape and serving as an image-bearing member, a charging roller 2 serving as a charging unit, a cleaning blade 8 serving as a cleaning unit, and the like. In addition, each of the developing units 4 serving as a developing unit includes a developing roller 6 serving as a developer-bearing member, a toner application roller 7, and the like.

[0037] The intermediate transfer unit 10 includes an intermediate transfer belt 12 serving as an intermediate transfer member, four primary transfer rollers 11, a secondary transfer roller 16, a secondary transfer counter roller 13, a cleaning unit 26, and a tension roller 14. The intermediate transfer belt 12 is an endless tubular belt tensioned by the secondary transfer counter roller 13 and the tension roller 14, and is driven by the secondary transfer counter roller 13 to rotate in the Figure 1 counterclockwise direction in. A transfer nip portion serving as a secondary transfer portion is formed between the secondary transfer counter roller 13 and the secondary transfer roller 16 that face each other and hold the intermediate transfer belt 12 therebetween.

[0038] The fixing unit 18 is configured as a heat fixing system that includes a fixing roller pair 19 serving as a pair of rotating members for holding and conveying the sheet S and a heating unit for heating the toner image on the sheet S. In the present embodiment, the fixing roller pair 19 includes a heating roller 19a that abuts against the image surface of the sheet S that is the surface on which the toner image is transferred in the secondary transfer portion and a pressure roller 19b that is in pressure contact with the heating roller 19a. The sheet S is held and conveyed by the holding portion of the fixing roller pair 19 serving as a fixing nip portion. Note that, for example, a guiding member assembled in a tubular film can be used instead of the heating roller 19a and / or the pressure roller 19b. As the heating unit, for example, a halogen lamp, a heater in which a resistance heating element is disposed on a ceramic substrate, an induction heating device that heats a conductive layer provided in a heating roller or a film by electromagnetic induction, or the like can be used.

[0039] When the image forming apparatus 1 performs an image forming operation, when the controller 1C included in the apparatus main body 1A outputs a print signal, the sheet S accommodated in the cassette 23 is fed one by one by the feeding roller 24 and is conveyed to the alignment roller pair 17. The skew and timing of the sheet S are corrected by the alignment roller pair 17, and then the sheet S is delivered to the secondary transfer portion of the image forming portion 1B. Note that the controller 1C includes at least one processor and a memory, and controls the operation of the image forming apparatus by loading a control program from the memory by the processor and executing the control program. In addition to the process of the image forming operation, the control program executed by the controller 1C also defines a method for determining whether a sheet exists based on a detection signal from a sheet detection mechanism 32 described later.

[0040] In the image forming section 1B, first, each surface of the photosensitive drum 1a is uniformly charged to a predetermined polarity by one of the corresponding charging rollers 2. In this embodiment, the predetermined polarity is a negative polarity. Next, the laser scanner unit 9 emits laser light modulated according to signals obtained by decomposing the image information to be printed into component images of respective colors from a light source (not shown), and thus exposes the surface of the photosensitive drum 1a. As a result, an electrostatic latent image corresponding to the component images of respective colors is formed on the surface of the corresponding photosensitive drum 1a. Then, the toner serving as a developer is attached to the electrostatic latent image to develop the electrostatic latent image, and thus a monochromatic toner image is formed on each surface of the photosensitive drums 1a.

[0041] By applying a bias voltage having a polarity opposite to the normal charging polarity of the toner to the primary transfer roller 11 that opposes the photosensitive drum 1a with the intermediate transfer belt 12 therebetween, the toner image formed on the surface of the photosensitive drum 1a is transferred to the intermediate transfer belt 12 by primary transfer, so as to form a full-color toner image. In this embodiment, the polarity opposite to the normal charging polarity of the toner is a positive polarity. As a result of the rotation of the intermediate transfer belt 12, the toner image formed on the intermediate transfer belt 12 reaches the secondary transfer section. Then, in the secondary transfer section, a bias voltage having a polarity opposite to the normal charging polarity of the toner is applied to the secondary transfer roller 16, and thus the toner image is transferred to the sheet S by secondary transfer.

[0042] The sheet S onto which the toner image formed by the image forming section 1B is transferred in the secondary transfer section is conveyed to the fixing unit 18. The fixing unit 18 heats and presses the toner image on the sheet S while sandwiching and conveying the sheet S by the fixing roller pair 19, and thus fixes the toner image to the sheet S. The sheet S that has passed through the fixing unit 18 is further conveyed by the conveying roller pair 27 and discharged onto the discharge tray 22 by the discharge roller pair 21.

[0043] The feed roller 24 and the alignment roller pair 17 arranged along the conveyance path of the sheet S in the above-described image forming apparatus 1 serve as examples of the conveyance unit for conveying the sheet S. Similarly, the secondary transfer roller 16, the secondary transfer opposing roller 13, the fixing roller pair 19, the conveying roller pair 27, and the discharge roller pair 21 arranged along the conveyance path of the sheet S in the image forming apparatus 1 serve as other examples of the conveyance unit for conveying the sheet S.

[0044] Note that, although the image forming unit 1B of the electrophotographic unit of the intermediate transfer type is described as an example of the image forming unit in the present embodiment, a direct transfer type electrophotographic unit that transfers the toner image formed on the image carrier member to the recording material without using an intermediate transfer member may also be used. In addition, the image forming unit is not limited to the electrophotographic system, and for example, an image forming unit of an inkjet system or a lithographic printing system may be used.

[0045] Sheet detection mechanism

[0046] Next, an optical type sheet detection mechanism 32 for detecting the presence or absence of a sheet in the sheet conveyance path for conveying the sheet in the image forming apparatus 1 will be described. As Figure 1 illustrated, the sheet detection mechanism 32 of the present embodiment is disposed downstream of the fixing roller pair 19 and upstream of the conveyance roller pair 27 in the conveyance direction of the sheet S, and outputs a detection signal indicating the presence or absence of the sheet S delivered from the fixing roller pair 19. In addition, the detection signal of the sheet detection mechanism 32 is transmitted to the controller 1C.

[0047] The controller 1C performs conveyance control of the sheet S and jam notification based on the detection signal received from the sheet detection mechanism 32. For example, in a case where the sheet detection mechanism 32 does not output a detection signal indicating the presence of the sheet S even after a predetermined time has elapsed since the start of the image forming operation, the controller 1C may determine that a conveyance failure or a paper jam has occurred at a position before the fixing unit 18. In addition, for example, the controller 1C determines when to start and stop driving the discharge roller pair 21 by using the time points at which the detection result of the sheet detection mechanism 32 is switched as the time points at which the leading end and the trailing end of the sheet S have passed through predetermined positions in the conveyance path, respectively. The leading end and the trailing end of the sheet S are the downstream end and the upstream end in the conveyance direction, respectively. Therefore, it is necessary for the detection signal of the sheet detection mechanism 32 to correctly reflect the presence or absence of the sheet S so that the image forming apparatus 1 can fully exhibit its potential.

[0048] Basic configuration of the sheet detection mechanism

[0049] The basic configuration of the sheet detection mechanism 32 will be described. In the following description, the moving direction of the sheet through the sheet detection mechanism 32 will be referred to as the "transport direction Z". The direction along the surface of the sheet perpendicular to the transport direction Z will be referred to as the "sheet width direction X". The direction perpendicular to the sheet width direction X and the transport direction Z - i.e., the normal direction of the sheet surface at the position opposite to the sheet detection mechanism 32 will be referred to as the "thickness direction Y" of the sheet. In the present embodiment, the sheet width direction X is substantially the same as the rotational axis direction of the heating roller 19a and the pressure roller 19b in the fixing unit 18 - i.e., the longitudinal direction of the fixing nip portion and the main scanning direction during image formation. The transport direction Z is substantially the same as the direction in which the sheet is delivered from the fixing nip portion - i.e., the short side direction of the fixing nip portion. In addition, the shape of the members related to the sheet detection mechanism 32 and the positional relationship between the members related to the sheet detection mechanism 32 will be described based on the state in which these members are installed as a part of the image forming apparatus 1.

[0050] Figure 2A and Figure 2B are perspective views of the sheet detection mechanism 32, respectively. Figure 3A is a side view of the sheet detection mechanism 32 observed in the sheet width direction X. Figure 3B is along the line Figure 3A 3B - 3B in the sheet detection mechanism 32 is a cross-sectional view of the sheet detection mechanism 32 taken along one of the optical paths in the sheet detection mechanism 32, which is perpendicular to the transport direction Z in the present embodiment. Figure 3C is observed in the sheet width direction X along the line Figure 3B 3C - 3C in the sheet detection mechanism 32 is a cross-sectional view of the sheet detection mechanism 32 taken along a virtual plane perpendicular to the sheet width direction X.

[0051] As Figures 2A to 3C illustrated in, the sheet detection mechanism 32 includes a sensor unit 20 and a reflector 44 attached to a guide unit that defines a transport path downstream of the fixing roller pair 19 in the transport direction Z.

[0052] The guiding unit includes a first conveying guide 28 that guides the first surface of the sheet and a second conveying guide 45 that faces the first conveying guide 28 through a conveying path that serves as a space for the sheet to move therebetween and guides the second surface of the sheet. The first conveying guide 28 and the second conveying guide 45 each have guiding surfaces 28a and 45a that extend in the sheet width direction X and the conveying direction Z, respectively, and the conveying path is defined between the guiding surfaces 28a and 45a that face each other. Note that the guiding surfaces 28a and 45a are not limited to flat surfaces. For example, a plurality of ribs may be formed on the guiding surfaces 28a and 45a along the conveying direction Z, or lattice-shaped holes or slits may be defined in the guiding surfaces 28a and 45a as ventilation holes.

[0053] The sensor unit 20 includes a sensor substrate 31 that serves as a sensor board and a fixing base 29, and is attached to the first conveying guide 28. Specifically, the fixing base 29 is fixed to the surface of the first conveying guide 28 that faces the guiding surface 28a, that is, the other side of the first conveying guide 28 as observed from the conveying path, such that the protruding area of the fixing base 29 is within the first conveying guide 28, and the sensor substrate 31 is attached to the fixing base 29.

[0054] Figure 4 is a perspective view of the sensor substrate 31. Positioning holes 65a and 65b and through holes 62 are defined in the sensor substrate 31. The positioning holes 65a and 65b serve as positioning portions 65 for fixing the position of the sensor substrate 31 relative to the fixing base 29, and the through holes 62 serve as held portions held by the fixing base 29. A light emitting portion 33 and a light receiving portion 35 are mounted on the sensor substrate 31. The light emitting portion 33 serves as a light emitting element or a first optical element portion that emits light, and the light receiving portion 35 serves as a light receiving element or a second optical element portion that optically detects the sheet in cooperation with the light emitting portion 33.

[0055] As the light emitting portion 33, a light emitting diode LED as an electroluminescent device is preferably used because of its low power consumption. As the light emitting portion 33 of the present embodiment, for example, an infrared LED having a peak wavelength of 850 nm is used, and its output value is about 40 mA to 25 mA. Note that an LED having a main wavelength of 850 nm can be used as the light emitting portion 33, and an LED having wavelength parameters different from those of any of the above LEDs, such as a visible light LED, can be used as the light emitting portion 33. In the following description, it is assumed that the optical type sheet detecting mechanism 32 for detecting the "light" of the sheet includes any electromagnetic wave that can be blocked by an ordinary paper sheet that can be widely used as a recording material in addition to visible light and infrared light, and the "light quantity" refers to the radiant energy of such light.

[0056] As the light receiving unit 35, a phototransistor can be preferably used. In a state where the light receiving unit 35 does not receive light, the phototransistor of the light receiving unit 35 outputs a voltage of approximately 3.3V. When the light receiving unit 35 receives the light emitted from the light emitting unit 33, current becomes more likely to flow therein, and thus the output voltage serving as a detection signal output from the sensor unit 20 becomes smaller. In addition, a circuit is formed on the sensor substrate 31, and the light emitting unit 33 and the light receiving unit 35 are electrically connected to the controller 1C. Note that, Figures 2A to 3C even when the positions of the light emitting unit 33 and the light receiving unit 35 are opposite to those illustrated in

[0057] In addition, as illustrated in Figures 2A to 3C Figures 2A to 3C , a reflecting plate 44 serving as a reflection member is fixed to the guide surface 45a of the second conveyance guide 45, and reflects the light emitted from the light emitting unit 33 toward the light receiving unit 35 by specular reflection. The light emitting unit 33 and the light receiving unit 35 are arranged in the sheet width direction X at substantially the same position in the conveyance direction Z, and the reflecting plate 44 overlaps the light emitting unit 33 and the light receiving unit 35 in the conveyance direction Z and is positioned between the light emitting unit 33 and the light receiving unit 35 in the sheet width direction X. Note that, Figure 3B a light beam Lb that is the shortest path among the paths from the light emitting unit 33 via the reflecting plate 44 to the light receiving unit 35 is illustrated as a representative example of the light beam from the light emitting unit 33 via the reflecting plate 44 to the light receiving unit 35.

[0058] As the reflection member, a member having a reflectivity at least higher than that of the guide surface 45a with respect to the light emitted from the light emitting unit 33 is used. In the present embodiment, the reflecting plate 44 made of a shiny metal plate, specifically a stainless steel plate, is used as the reflection member. Although the reflection member is not limited thereto, and a resin sheet formed of polyethylene terephthalate: PET or the like and having aluminum deposited thereon or a mirror formed by depositing aluminum or silver on the glass surface can be used as the reflection member, a metal plate is preferred in consideration of the stability of the output of the light receiving unit 35. In particular, stainless steel is preferred, and among stainless steels, ferritic stainless steel containing 18% chromium is preferred. This is because, even near the fixing unit 18 where the temperature changes relatively drastically, its surface is not easily deformed, and thus light can be stably reflected. In addition, even in a high humidity environment, stainless steel is corrosion resistant, and in the case of using stainless steel, a shiny surface can be obtained at low cost, and thus the amount of light reflected toward the light receiving unit 35 can be stably increased. Note that, the "direction toward the light receiving unit 35" includes not only the direction in which the light travels directly toward the light receiving unit 35 as indicated by the arrow in Figure 3B Figure 3B , but also the direction in which the light travels indirectly toward the light receiving unit 35 via a light guiding unit, other reflection members, etc. to be described later.

[0059] FIG. 5 is a perspective view of the fixing table 29. As Figure 5A and Figure 5B illustrated, the fixing table 29 includes a holding portion 61 that holds the sensor substrate 31 and projections 66a and 66b that serve as positioning portions and are respectively engaged with the positioning holes 65a and 65b of the sensor substrate 31 to position the sensor substrate 31. In the case where the sensor substrate 31 is attached to the fixing table 29, the projections 66a and 66b are engaged with the positioning holes 65a and 65b, so that the sensor substrate 31 is positioned in the direction along the main surface of the sensor substrate 31, which is the sheet width direction X and the conveyance direction Z in the present embodiment. In addition, the claws of the holding portion 61 are engaged with the rear surface of the sensor substrate 31 through the through holes 62 of the sensor substrate 31, so that the sensor substrate 31 is positioned in the thickness direction Y and fixed to the fixing table 29. Note that the method for positioning and fixing the sensor substrate 31 and the fixing table 29 is not limited to the above method, and for example, the two may be fixed to each other by using screws.

[0060] Note that, as Figure 3B illustrated, by disposing the positioning projections 66a and 66b between the light emitting portion 33 and the light receiving portion 35, even when there is a minute gap between the fixing table 29 and the sensor substrate 31, the light from the light emitting portion 33 leaking from the gap and directly received by the light receiving portion 35 can be suppressed. That is, this contributes to an improvement in the detection accuracy of the sheet detection mechanism 32. That is, when viewed in the direction from the light emitting portion 33 to the light receiving portion 35, it is sufficient that at least one of the projections 66a and 66b overlaps with the light emitting portion 33 and the light receiving portion 35. Note that the following configuration may be employed: a through hole through which a projection or a screw for positioning is to penetrate is provided in the sensor substrate 31, and a positioning hole engaged with the projection or an internal thread engaged with the screw is provided in the fixing table 29 so that the projection or the screw is positioned between the light emitting portion 33 and the light receiving portion 35.

[0061] Here, as Figure 5A and Figure 5BAs shown in the figure, the fixed stage 29 includes a first light guiding portion 30a having a substantially tubular shape and guiding the light emitted from the light emitting portion 33 to the transfer path, and a second light guiding portion 30b having a substantially tubular shape and guiding the light from the transfer path to the light receiving portion 35. The first light guiding portion 30a serves as a first light passing portion which is provided to define an optical path from the light emitting portion 33 to the transfer path, and the second light guiding portion 30b serves as a second light passing portion which is provided to define an optical path from the transfer path to the light receiving portion 35. The first light guiding portion 30a abuts against the sensor substrate 31 while surrounding the light emitting portion 33 at the first opening end of its tubular shape, and opens at its second opening end toward a first opening portion 41a defined in the guiding surface 28a of the first transfer guiding member 28. The second light guiding portion 30b abuts against the sensor substrate 31 while surrounding the light receiving portion 35 at the first opening end of its tubular shape, and opens at its second opening end toward a second opening portion 41b defined in the guiding surface 28a of the first transfer guiding member 28. Thus, the space inside the first light guiding portion 30a and the second light guiding portion 30b communicates with the transfer path which is the space between the first transfer guiding member 28 and the second transfer guiding member 45 through the opening portions of the first transfer guiding member 28.

[0062] Note that each of the first light guiding portion 30a and the second light guiding portion 30b of the present embodiment is formed as a tubular shape portion having a part of its side surface opened in a cross-sectional view, that is, an angular C shape. Further, as Figures 2A to 3C shown in the figure, the opening sides of each of the first light guiding portion 30a and the second light guiding portion 30b are blocked by the flat surface portion 28b of the first transfer guiding member 28, thus forming a tubular shape surrounding the optical path. The flat surface portion 28b of the first transfer guiding member 28 is a surface extending away from the transfer path from the upstream end of the guiding surface 28a in the transfer direction Z. The flat surface portion 28b of the first transfer guiding member 28 also serves as an attachment surface to which the sensor unit 20 can be attached from above. Note that instead of adopting the configuration of forming a tubular shape surrounding the optical path by blocking the opening portions of the first light guiding portion 30a and the second light guiding portion 30b by a part of the first transfer guiding member 28, the first light guiding portion 30a and / or the second light guiding portion 30b may be formed as a complete tubular shape such as an angular tubular shape or a cylindrical shape. That is, the first light passing portion constitutes at least a part of the inner surface of a tubular shape having a first opening end and a second opening end, the first opening end being opposite to the substrate provided with the light emitting element, and the second opening end communicating with the transfer path. The second light passing portion constitutes at least a part of the inner surface of a tubular shape having a third opening end and a fourth opening end, the third opening end being opposite to the substrate provided with the light receiving element, and the fourth opening end communicating with the transfer path.

[0063] The extending directions of the first light guiding part 30a and the second light guiding part 30b - that is, the directions of the central axes of the tubular shapes are preferably set to be the directions of the V-shaped shortest paths along which the light beam Lb in Figure 3B indicates, from the light emitting part 33 via the reflection plate 44 to the light receiving part 35. As a result, the light emitted from the light emitting part 33 can be effectively guided toward the reflection plate 44, and the light traveling from the reflection plate 44 to the light receiving part 35 can be effectively collected.

[0064] Detection operation

[0065] Next, the sheet detection operation of the sheet detection mechanism 32 will be described. Figure 3B The sectional view of

[0066] illustrates the sheet detection mechanism 32 in a state where no sheet exists in the conveyance path. In a state where no sheet exists, the light emitted from the light emitting part 33 is radiated to the conveyance path through the first light guiding part 30a and is reflected by the reflection plate 44. The light reflected by the reflection plate 44 is incident on the light receiving part 35 through the second light guiding part 30b. Then, it becomes easier for current to flow in the phototransistor of the light receiving part 35, and the output voltage of the light receiving part 35 decreases. Figures 3A to 3C The controller 1C of the present embodiment presets a threshold value for the output voltage of the light receiving part 35, and when the output voltage is lower than the threshold value, it determines that there is no sheet in the conveyance path. That is, in

[0067] Figure 6A and Figure 6B illustrates the sheet detection mechanism 32 in a state where a sheet S exists in the conveyance path. Figure 6A is a side view of the sheet detection mechanism 32 observed in the sheet width direction X. Figure 6B is a sectional view of the sheet detection mechanism 32 taken along a virtual plane indicating one of the optical paths of the sheet detection mechanism 32 along Figure 6A the line 6B - 6B. In the present embodiment, Figure 6B is a sectional view taken along a virtual plane perpendicular to the conveyance direction Z.

[0068] As Figure 6BAs shown in the figure, in the state where the sheet S exists, as indicated by the light beam Lb, the light from the light emitting unit 33 is blocked by the sheet S and does not reach the light receiving unit 35. In this case, no current flows in the phototransistor of the light receiving unit 35, and the output voltage does not decrease. Since the output voltage of the light receiving unit 35 is higher than the threshold value, the controller 1C determines that there is a sheet S in the conveyance path, that is, the presence of the sheet S is detected.

[0069] In other words, the sheet detection mechanism 32 of the present embodiment has the following reflection type configuration: the light emitting element, the first light passage portion, the light receiving element, and the second light passage portion are arranged on the first side of the conveyance path in the thickness direction Y, and the reflection member is arranged on the second side opposite to the first side of the conveyance path in the thickness direction Y. Further, in the case where there is no sheet in the conveyance path, the light emitted from the light emitting element reaches the light receiving element through the optical path from the first light passage portion via the reflection member to the second light passage portion, and in the case where there is a sheet in the conveyance path, the optical path from the first light passage portion via the reflection member to the second light passage portion is blocked by the sheet. That is, the sheet detection mechanism 32 is configured such that the amount of light incident on the light receiving element in the case where there is a sheet in the conveyance path is less than the case where there is no sheet in the conveyance path.

[0070] Factors for degradation of detection accuracy

[0071] However, depending on the actual situation when the sheet detection mechanism 32 detects a sheet, the output voltage of the light receiving unit 35 in the state where there is no sheet may be higher than 0V, and the output voltage in the state where there is a sheet may be lower than the theoretical value, which is 3.3V in the present embodiment. That is, there may be a situation where the change amount of the output voltage corresponding to the presence or absence of the sheet is small, and a sufficient margin to avoid false detection cannot be ensured between the output voltage and the threshold value of the output voltage. Figure 8 The "conventional example" shown in the figure indicates a situation where the output value in the state where there is no sheet and the output value in the state where there is a sheet become closer to the threshold value P due to multiple factors. The reason why the output voltage of the light receiving unit 35 becomes closer to the threshold value P will be described below.

[0072] (1) Reduction in the amount of light of the LED

[0073] As indicated by Figure 8 in (1a) of the figure, it is generally known that the amount of light of the LED, which is usually used as the light emitting unit 33, decreases as the cumulative light emission time increases. Further, as indicated by Figure 8 in (1b) of the figure, it is known that the reduction in the amount of light of the LED corresponding to the cumulative light emission time is larger under the influence of heat. As a result, even in the state where there is no sheet, the amount of light reaching the light receiving unit 35 decreases, and thus the output voltage becomes higher.

[0074] (2) Condensation on the reflector

[0075] When the image forming operation is repeatedly performed and a plurality of sheets are continuously conveyed, water vapor is generated from the sheets due to the heat generated by the fixing unit 18, and sometimes the water vapor condenses on the members in the optical path, particularly on the reflector 44. When condensation occurs in the reflector 44, the ratio of the amount of light reflected on the reflector 44 by specular reflection to the amount of light incident on the reflector 44 (i.e., the reflectance) decreases. As Figure 8 indicated in (2) of [], when the reflectance of the reflector 44 decreases, the amount of light reaching the light receiving unit 35 decreases and the output voltage becomes higher even in a state where there is no sheet in the conveyance path.

[0076] (3) Attenuation of light in the optical path

[0077] In the optical path where the light emitted from the light emitting unit 33 is radiated into the conveyance path through the first light guide unit 30a, then reflected by the reflector 44, and reaches the light receiving unit 35 through the second light guide unit 30b, the light is attenuated due to absorption and scattering by the members constituting the optical path. As a result, as Figure 8 indicated in (3) of [], the amount of light reaching the light receiving unit 35 decreases and the output voltage becomes higher even in a state where there is no sheet.

[0078] Note that in order to suppress the influence of the heat indicated by the above (1b), it is also possible to consider providing a fan or an air passage to blow cooling air to the light emitting unit 33, the light receiving unit 35, and the reflector 44. However, when such an element for blowing cooling air is provided, the manufacturing cost and size of the apparatus increase. In addition, in this case, sometimes the toner scatters in the image forming apparatus 1 and adheres to the reflector 44, the first light guide unit 30a, or the second light guide unit 30b, and the attenuation of the light in the optical path increases.

[0079] (4) Stray light between the sheet and the conveyance guide

[0080] In addition, even when there is a sheet S in the conveyance path, there is not necessarily a situation where no light reaches the light receiving unit 35. In fact, a part of the light reaches the light receiving unit 35 as stray light. As Figure 7 illustrated in [], a part of the light reaches the light receiving unit 35 while being reflected between the sheet S and the first conveyance guide 28. Therefore, as Figure 8 indicated in (4) of [], the output voltage of the light receiving unit 35 decreases. In addition, in recent years, the types of sheets used as recording materials in image forming apparatuses have diversified, and when the sheet S is of a type that reflects more light, more stray light reaches the light receiving unit 35 and the output voltage further decreases.

[0081] (5) Dark current

[0082] As a property of the phototransistor used as the light receiving unit 35, even in a state where no light is incident thereon, a leakage current or a dark current flows therein. As indicated by Figure 8 (5) of, the effect of the dark current is to reduce the output voltage of the light receiving unit 35.

[0083] For the reasons (1a) to (5) exemplified above, as Figure 8 shown on the left side of, in the conventional example, sometimes the output voltage of the light receiving unit 35 in the state where the sheet is not present increases, while the output voltage in the state where the sheet is present decreases, so that the amount of change in the output voltage corresponding to the presence or absence of the sheet becomes small. In order to suppress false detection, it is desirable to set a threshold P that allows ensuring a sufficient margin with respect to the output voltage in the actual usage situation, but it becomes difficult to ensure the margin when the amount of change in the output voltage corresponding to the presence or absence of the sheet is small. Therefore, when relatively adverse conditions such as (1a) to (5) are accumulated, there is a possibility that false detection is more likely to occur.

[0084] When false detection of the sheet detection mechanism 32 occurs, there is a possibility that an abnormality occurs in the control executed by the controller 1C based on the detection signal of the sheet detection mechanism 32. Therefore, in the conventional example, sometimes in consideration of the influence that becomes larger according to the accumulation of the light emission time such as (1a) and (1b) described above, this is solved by setting the lifetime of the sheet detection mechanism 32 or the image forming apparatus 1. Regarding this, as a method for reducing the possibility of false detection, it can be considered to set the light emission amount of the light emitting unit 33 to a sufficiently large value so as to maintain the light amount of the light emitting unit 33 even after the accumulated light emission time increases. However, when the light emission amount of the light emitting unit 33 is set to a large value, there is a possibility that the deterioration of the light emitting unit 33 becomes faster and the lifetime of the sensor unit 20 becomes shorter.

[0085] Details of the sheet detection mechanism of the present embodiment

[0086] Therefore, in the present embodiment, the following configuration is adopted so that more light reaches the light receiving unit 35 in the absence of the sheet, and less light reaches the light receiving unit 35 in the presence of the sheet. First, in order to suppress the attenuation of light in the optical path from the light emitting unit 33 to the conveying path and from the conveying path to the light receiving unit 35, the fixing table 29 is formed of a white resin. In addition, the surfaces of the first light guiding portion 30a and the second light guiding portion 30b that constitute the optical path - that is, the inner surfaces of the tubular shape serving as the light guiding surfaces are mirror-finished. Mirror finishing is a surface processing method that reduces the unevenness of the surface of a member by a method such as buffing polishing, so that the surface of the member has luster. For example, the surface after mirror finishing has an arithmetic mean roughness Ra of 0.2 μm or less.

[0087] In contrast, in order to reduce the stray light reaching the light receiving unit 35 through the gap between the sheet and the conveying guide, the first conveying guide 28 is formed of a black resin. The black resin can be formed by kneading a component having a high light absorption rate such as carbon black into a resin material used as a base material, for example. In addition, the guiding surface 28a of the first conveying guide 28 is matte-finished to further reduce the stray light. Matte finishing is a surface processing method that increases the unevenness of the surface of a member - that is, the surface area by a method such as applying a particulate coating or sandblasting, and the surface after matte finishing has an arithmetic mean roughness Ra of 1.6 μm or more, for example. It should be noted that, for example, the arithmetic mean roughness Ra of the guiding surface 28a is preferably set to 6.3 μm or less so as not to interfere with the conveyance of the sheet.

[0088] As a result of the above configuration, the reflectance of each of the first light guiding portion 30a and the second light guiding portion 30b for the light emitted from the light emitting unit 33 is set to be higher than the reflectance of the first conveying guide 28 for the light emitted from the light emitting unit 33. In other words, the reflectance of each of the first light passing portion and the second light passing portion for the light emitted from the light emitting element is configured to be higher than the reflectance of the guiding member for the light emitted from the light emitting element.

[0089] The reflectance of each of the light guiding surfaces of the first light guiding portion 30a and the second light guiding portion 30b serving as the first reflectance is preferably 3% or more, and more preferably 4% or more. In contrast, the reflectance of the guiding surface 28a of the first conveying guide 28 for the light emitted from the light emitting unit 33 is preferably 1.5% or less, and more preferably 1% or less. In the present embodiment, the reflectance of the mirror-finished white resin for the light emitted from the light emitting unit 33 is about 4%, and the reflectance of the matte-finished black resin for the light emitted from the light emitting unit 33 is about 1%.

[0090] Note that the reflectivity of the first light guiding portion 30a and the reflectivity of the second light guiding portion 30b may be different from each other as long as the above conditions are satisfied.

[0091] In addition, although in this embodiment, the guiding surface 28a of the first conveying guide 28 is entirely formed of a matte-finished black resin, a configuration in which the reflectivity is reduced only in a region that makes a large contribution to the stray light finally reaching the light receiving portion 35 may also be employed. Regions that make a large contribution to the stray light are, for example, Figure 2A the peripheral portions of the first opening 41a and the second opening 41b of the guiding surface 28a illustrated in the figure. Therefore, the reflectivity of each of the first light guiding portion 30a and the second light guiding portion 30b may be any value as long as the reflectivity is at least higher than the reflectivity of the peripheral portions of the first opening 41a and the second opening 41b of the guiding surface 28a in the first conveying guide 28 used as a guiding member.

[0092] Figure 17A and Figure 17B illustrates a method of measuring reflectivity. Figure 17A is a schematic diagram illustrating how the light emitted from the light emitting portion 91 is measured by the light quantity meter 92 deployed on the optical axis. Figure 17B is a schematic diagram illustrating how the light emitted from the light emitting portion 91 is reflected on the measurement surface 93a of the measurement target material 93 and measured by the light quantity meter 92. As the light emitting portion 91, the same light emitting element as the light emitting portion 33 of the sheet detection mechanism 32 is used. In this embodiment, the light emitting element is an infrared LED.

[0093] In Figure 17A , the optical path length from the light emitting portion 91 to the light quantity meter 92 is 2×La. In this case, in Figure 17B , the optical path length from the light emitting portion 91 to the measurement surface 93a of the measurement target material 93 is La, and the distance from the measurement surface 93a of the measurement target material 93 to the light quantity meter 92 is La. In addition, the incident angle θ1 of the light emitted from the light emitting portion 91 and the reflection angle θ2 on the measurement surface 93a of the measurement target material 93 are each set to 45°. Figure 17A the output value of the light quantity meter 92 in is T, and Figure 17B the output value of the light quantity meter 92 in is H. The reflectivity of the measurement surface 93a is represented by H / T.

[0094] In as Figures 3A to 3CIn the case where there is no sheet in the conveyance path as illustrated in the figure, the light emitted from the light emitting unit 33 is reflected by the first light guiding unit 30 having a relatively high reflectance and is guided to the conveyance path. In addition, the light reflected by the reflecting plate 44 and entering the second light guiding unit 30b is reflected by the second light guiding unit 30b having a relatively high reflectance and is guided to the conveyance path. Therefore, by setting the reflectance of the inner surfaces of the first light guiding unit 30a and the second light guiding unit 30b serving as light guiding surfaces to be high, compared with the conventional example, the attenuation of the light in the first light guiding unit 30a and the second light guiding unit 30b indicated by (3) in Figure 8 can be suppressed. In other words, by setting the reflectance of the inner surfaces of the first light guiding unit 30a and the second light guiding unit 30b to be high, the ratio of the light finally reaching the light receiving unit 35 to all the light (i.e., the total radiant flux) emitted from the light emitting unit 33 can be increased.

[0095] In contrast, in the case where there is a sheet S in the conveyance path as illustrated in Figure 6A and Figure 6B , the light reflected by the sheet S is absorbed by the first conveyance guide 28 having a relatively low reflectance. Therefore, by setting the reflectance of the guide surface 28a of the first conveyance guide 28 to be low, compared with the conventional example, the amount of stray light reaching the light receiving unit 35 due to reflection between the sheet S and the first conveyance guide 28 indicated by (4) in Figure 8 can be suppressed. In particular, since the amount of stray light reaching the light receiving unit 35 can be suppressed even in a situation where reflection of light by the sheet S is relatively likely to occur (such as when the sheet S has a glossy surface), high detection accuracy can be obtained for various types of sheets.

[0096] As a result, as illustrated on the right side of Figure 8 , the difference in the output voltage of the light receiving unit 35 between the sheet absent state and the sheet present state becomes larger, and it becomes easier to ensure a sufficient margin between the output voltage and a preset threshold P. In addition, the controller 1C determines the presence or absence of the sheet based on the output voltage of the light receiving unit 35, so the presence or absence of the sheet in the conveyance path can be determined more precisely, and the occurrence of false detection can be suppressed.

[0097] That is, according to the configuration of the present embodiment, even under relatively adverse conditions, the possibility of false detection can be reduced and the detection accuracy can be maintained. In other words, high detection accuracy can be stably achieved. As a result, for example, even in Figure 8When the amount of light emitted from the LED serving as the light emitting unit 33 indicated by (1a) and (1b) decreases due to an increase in the cumulative light emission time, it is possible to suppress the attenuation of light in the optical path indicated by (3) and to suppress a significant decrease in the amount of light received by the light receiving unit 35. Therefore, the lifetimes of the sheet detection mechanism 32 and the image forming apparatus 1 can be set longer than in the conventional example while maintaining the detection accuracy of the sheet.

[0098] As described above, according to the present embodiment, it is possible to extend the lifetimes of the sheet detection mechanism 32 and the image forming apparatus 1 while maintaining the detection accuracy. The sheet detection mechanism 32 of the present embodiment can preferably be used to detect a sheet near the fixing unit 18 of the heat fixing system.

[0099] Modification

[0100] Although in the above-described first embodiment, a mirror-finished white resin is used to increase the reflectivity of each of the first light guide portion 30a and the second light guide portion 30b, the configuration is not limited thereto as long as the reflectivity of each of the first light guide portion 30a and the second light guide portion 30b is higher than the reflectivity of the first conveyance guide 28. For example, a metal tape such as an aluminum tape can be pasted on the light guide surfaces of the first light guide portion 30a and the second light guide portion 30b, a metal-containing coating can be applied to the light guide surfaces, or a metal film or metal layer can be formed on the light guide surfaces by vapor deposition of a metal or the like. In addition, the first light guide portion 30a and the second light guide portion 30b can be formed of a metal material such as stainless steel.

[0101] Meanwhile, although in the above-described first embodiment, a matte-finished black resin is used to suppress the reflectivity of the first conveyance guide 28, the configuration is not limited thereto as long as the reflectivity of the first conveyance guide 28 is lower than the reflectivity of each of the first light guide portion 30a and the second light guide portion 30b. For example, a carbon-based material or coating having a high light absorption rate can be pasted or applied to at least the region a1 of the guide surface 28a of the first conveyance guide 28. For example, a coating having a high light absorption rate is a coating having an absorption rate of 99% or more for light in a wide wavelength band. Specific examples of the coating include Vantablack manufactured by Surrey Nano System and having an absorption rate of 99.965% TM and Musou Black manufactured by Koyo Orient Japan Co., Ltd. and having an absorption rate of 99.3% TM .

[0102] In addition, although in the above-described first embodiment, the V-shaped optical path from the light-emitting unit 33 via the reflector 44 to the light-receiving unit 35 is formed along the X-Y plane perpendicular to the sheet conveyance direction Z, the design of the optical path can be appropriately modified. For example, the V-shaped optical path can be formed along the Y-Z plane perpendicular to the sheet width direction X, as a configuration in which the light-emitting unit 33 and the light-receiving unit 35 are arranged in the conveyance direction Z. In addition, an optical path (e.g., a C-shaped optical path) in which light emitted to the conveyance path travels toward the light-receiving unit 35 through multiple specular reflections can be formed by deploying a plurality of reflecting members on the first conveyance guide 28 and the second conveyance guide 45. In addition, an L-shaped optical path in which light reflected by the reflecting member reaches the light-receiving unit 35 through a path on the back surface of the guide surface 28a or 45a can be formed by deploying the light-emitting unit 33 on the first side of the conveyance path in the thickness direction Y and deploying the light-receiving unit 35 and the reflecting member on the second side opposite to the first side of the conveyance path in the thickness direction Y.

[0103] Note that the first light passage portion and the second light passage portion are not limited to members formed in a tubular shape like the first light guide portion 30a and the second light guide portion 30b of the present embodiment. For example, when a directional LED is used as the light-emitting unit 33 and the light-emitting unit 33 is mounted such that the central axis of the emitted light is directed toward the light-receiving unit 35 via reflection on the reflector 44, the first light passage portion and the second light passage portion are formed in a shape having an opening that does not intersect the central axis. At this time, a surface for guiding light that is offset from the central axis and cannot reach the light-receiving unit 35 without the first light passage portion and the second light passage portion to the light-receiving unit 35 can be provided on at least one side with respect to the central axis and used as the first light passage portion and the second light passage portion.

[0104] Second Embodiment

[0105] Next, a second embodiment will be described with reference to the drawings. In the present embodiment, a configuration in which the light-emitting unit and the light-receiving unit are deployed on opposite sides of the conveyance path is used. In the following description, it is assumed that elements denoted by the same reference numerals as those in the first embodiment have substantially the same configurations and effects as those in the first embodiment, and parts different from the first embodiment will be described.

[0106] Basic Configuration of Sheet Detection Mechanism

[0107] The basic configuration of the sheet detection mechanism 70 according to the present embodiment will be described. Figure 9A and Figure 9B are perspective views of the sheet detection mechanism 70 according to the present embodiment, respectively. Figure 10A is a side view of the sheet detection mechanism 70 observed in the sheet width direction X. Figure 10B is along the one defined by Figure 10AA cross-sectional view of the sheet detection mechanism 70 taken along a virtual plane of one of the optical paths of the sheet detection mechanism 70 indicated by the line 10B-10B in []. In the present embodiment, the virtual plane is perpendicular to the conveyance direction Z. Figure 10C is a cross-sectional view of the sheet detection mechanism 70 taken along a virtual plane perpendicular to the sheet width direction X as observed in the sheet width direction X and indicated by the line 10C-10C in []. Figure 10B

[0108] As Figures 9A to 10B illustrated in [], the sheet detection mechanism 70 includes a sensor unit 71 attached to a guide unit that includes a first conveyance guide 28 and a second conveyance guide 45 and defines a conveyance path downstream of the fixing roller pair 19 in the conveyance direction Z.

[0109] The sensor unit 71 includes a first fixing base 50 attached to the first conveyance guide 28, a first sensor substrate 51 provided with a light emitting portion 33, a second fixing base 52 attached to the second conveyance guide 45, and a second sensor substrate 53 provided with a light receiving portion 35. A first light guide portion 54 that defines an optical path from the light emitting portion 33 to the conveyance path is formed on the first fixing base 50 so as to extend from the first sensor substrate 51 to the guide surface 28a of the first conveyance guide 28. A second light guide portion 55 that defines an optical path from the conveyance path to the light receiving portion 35 is formed on the second fixing base 52 so as to extend from the guide surface 45a of the second conveyance guide 45 to the second sensor substrate 53.

[0110] Each of the first light guide portion 54 and the second light guide portion 55 is formed in a substantially tubular shape surrounding the optical path. It should be noted that each of the first light guide portion 54 and the second light guide portion 55 of the present embodiment is formed in a tubular shape having a part of a side surface open in the cross-sectional view, that is, an angular C shape. In addition, the open sides of the first light guide portion 54 and the second light guide portion 55 are blocked by the flat surface portion 28b of the first conveyance guide 28 and the flat surface portion 45b of the second conveyance guide 45, respectively, and thus a tubular shape surrounding the optical path is formed. It should be noted that the first light guide portion 54 and the second light guide portion 55 may be formed in a complete tubular shape such as an angular tubular shape or a cylindrical shape.

[0111] As Figure 9A and Figure 10B illustrated in [] and [], the first light guide portion 54 abuts on the first sensor substrate 51 while surrounding the light emitting portion 33 at a first end portion of its tubular shape, and opens toward a first opening portion 56 defined in the guide surface 28a of the first conveyance guide 28 at its second open end. As Figure 9B and Figure 10BAs shown in the figure, the second light guide portion 55 abuts against the second sensor substrate 53 while surrounding the light receiving portion 35 at the first end portion of its tubular shape, and opens toward a second opening 57 defined in the guide surface 45a of the second transfer guide 45 at its second end portion.

[0112] It should be noted that, as a method for positioning and fixing the first sensor substrate 51 to the first fixing table 50 and a method for positioning and fixing the second sensor substrate 53 to the second fixing table 52, the method of the first embodiment can be used. That is, holding portions and protrusions for positioning can be provided on each of the first fixing table 50 and the second fixing table 52, and through holes serving as held portions and positioning holes engaged with the protrusions can be provided in each of the first sensor substrate 51 and the second sensor substrate 53.

[0113] Detection operation

[0114] Next, the sheet detection operation of the sheet detection mechanism 70 will be described. Figure 10B The sheet detection mechanism 70 in a state where no sheet exists in the transfer path is shown. In this state, the light from the light emitting portion 33 is radiated into the transfer path through the inside of the first light guide portion 54, then enters the second opening 57, and reaches the light receiving portion 35 through the inside of the second light guide portion 55. That is, the light travels along a linear optical path from the light emitting portion 33 to the light receiving portion 35. Therefore, in a state where no sheet exists, current is more likely to flow in the phototransistor of the light receiving portion 35, and the output voltage decreases. The controller 1C presets a threshold value P for the output voltage of the light receiving portion 35, and determines a state where the output voltage of the light receiving portion 35 is lower than the threshold value P as a state where no sheet exists.

[0115] Figures 11A to 11C The sheet detection mechanism 70 in a state where a sheet S exists in the transfer path is shown. Figure 11A It is a side view of the sheet detection mechanism 70 observed in the sheet width direction X. Figure 11B It is along a virtual plane indicating one of the optical paths of the sheet detection mechanism 70 Figure 11A The cross-sectional view of the sheet detection mechanism 70 taken along the line 11B - 11B in. In the present embodiment, Figure 11B It is a cross-sectional view taken along a virtual plane perpendicular to the transfer direction Z. Figure 11C It is observed in the sheet width direction X and is a cross-sectional view of the sheet detection mechanism 70 taken along a virtual plane perpendicular to the sheet width direction X indicated by the line 11C - 11C in Figure 11B The cross-sectional view of the sheet detection mechanism 70 taken along a virtual plane perpendicular to the sheet width direction X indicated by the line 11C - 11C in.

[0116] As Figure 11BAs shown in the figure, in a state where the sheet S exists, the light from the light-emitting unit 33 is blocked by the sheet S and does not reach the light-receiving unit 35. In this case, no current flows in the phototransistor of the light-receiving unit 35, and the output voltage does not decrease. The controller 1C determines that the state where the output voltage of the light-receiving unit 35 has not decreased and is higher than the threshold P is a state where the sheet S does not exist.

[0117] In other words, the sheet detection mechanism 70 of the present embodiment has the following transmissive configuration: the light-emitting element and the first light passage portion are disposed on the first side of the conveyance path in the thickness direction Y, and the light-receiving element and the second light passage portion are disposed on the second side opposite to the first side of the conveyance path in the thickness direction Y. In addition, when there is no sheet in the conveyance path, the light emitted from the light-emitting element reaches the light-receiving element through the optical path that passes through the conveyance path in the thickness direction from the first light passage portion to the second light passage portion, and when there is a sheet in the conveyance path, the optical path that passes through the conveyance path in the thickness direction from the first light passage portion to the second light passage portion is blocked by the sheet. That is, when there is a sheet in the conveyance path, the amount of light incident on the light-receiving element is configured to be less than when there is no sheet in the conveyance path.

[0118] Factors for deterioration of detection accuracy

[0119] However, in actual situations, sometimes, the amount of light reaching the light-receiving unit 35 in a state where there is no sheet in the conveyance path may decrease due to condensation on the optical path, and depending on the cumulative light-emitting time and heat, the light-emitting amount of the LED serving as the light-emitting unit 33 decreases. In addition, although the attenuation of light is smaller than that in the first embodiment because the optical path length is shorter than that in the first embodiment and the optical path has a linear shape, the light still attenuates in the optical path to the light-receiving unit 35 due to absorption, scattering, etc. of the members constituting the optical path.

[0120] In contrast, when the sheet S is in the conveyance path, as Figure 12As shown in the figure, actually the sheet S slightly transmits light and scatters light. As a result, the light transmitted through the sheet S may directly reach the light receiving part 35, and the light transmitted through the sheet S and scattered in the sheet S may reach the light receiving part 35 as stray light after diffuse reflection between the sheet S and mainly the second conveyance guide 45. That is, even in a state where the sheet S is present, a part of the light may reach the light receiving part 35, so a current may flow in the light receiving part 35, and the output voltage of the phototransistor may decrease. In addition, in the case of the latest type of sheet S that is thin and may transmit light, more light reaches the light receiving part 35, and the output voltage is further decreased. As described above, also in the sheet detection mechanism 70 of the present embodiment, when adverse conditions accumulate, there is a possibility that the margin between the output voltage of the light receiving part 35 corresponding to the presence or absence of the sheet in the conveyance path and the threshold value of the output voltage becomes small, and an erroneous detection is more likely to occur.

[0121] Therefore, also in the present embodiment, the reflectance of each of the first light guide part 54 and the second light guide part 55 for the light emitted from the light emitting part 33 is configured to be higher than the reflectance of the guide unit including the first conveyance guide 28 and the second conveyance guide 45 for the light emitted from the light emitting part 33.

[0122] Specifically, an aluminum foil tape is adhered to the surfaces of the first light guide part 54 and the second light guide part 55 that constitute the optical path - that is, at least a part of the inner surface of the tubular shape that serves as the light guide surface. In addition, the second conveyance guide 45 is formed of a black resin, and a coating material having a high light absorption rate is applied thereto. For example, the coating material having a high light absorption rate is a coating material having an absorption rate of 99% or more for light in a wide wavelength band.

[0123] By setting the reflectance of each of the first light guide part 54 and the second light guide part 55 to be high, attenuation of the light emitted from the light emitting part 33 in the first light guide part 54 and the second light guide part 55 can be suppressed in a case where there is no sheet in the conveyance path. The reflectance of the aluminum foil tape is about 70%, which is higher than the reflectance of 4% of the white resin that has been mirror-finished, so the reduction in the amount of light can be more suppressed compared with the first embodiment. In addition, since the optical path length is small and the optical path is linear, the amount of light reaching the light receiving part 35 can be more easily increased compared with the first embodiment.

[0124] In contrast, by setting the reflectance of the second conveyance guide 45 to be low, the amount of light that is transmitted through the sheet, scattered in the sheet, and reaches the light receiving part 35 as stray light after diffuse reflection between the sheet S and the second conveyance guide 45 can be reduced in a case where there is a sheet in the conveyance path.

[0125] As a result, the change amount of the amount of light reaching the light receiving unit 35 due to the presence or absence of the sheet increases, and the change amount of the output voltage of the light receiving unit 35 increases. Therefore, it becomes easier to ensure a sufficient margin between the output voltage of the light receiving unit 35 corresponding to the presence or absence of the sheet and the threshold value P preset by the controller 1C. Thus, even in relatively adverse conditions, the possibility of false detection can be reduced and the detection accuracy can be maintained. In addition, even when the amount of light emitted from the LED serving as the light emitting unit 33 decreases, a large decrease in the amount of light received by the light receiving unit 35 can be suppressed, and the life of the sheet detection mechanism 70 and the image forming apparatus 1 can be set to be long while maintaining the detection accuracy of the sheet detection mechanism 70.

[0126] Note that, as described in the first embodiment, the configuration for setting the reflectance of the light emitted from the light emitting unit 33 to be high or low can be appropriately modified as long as the reflectance of each of the first light guide unit 54 and the second light guide unit 55 is higher than the reflectance of the second conveyance guide 45. The reflectance of each of the first light guide unit 54 and the second light guide unit 55 is preferably 3% or more, and more preferably 4% or more, and the reflectance of the second conveyance guide 45 is preferably 1.5% or less, and more preferably 1% or less. The value of the reflectance of the first light guide unit 54 and the value of the reflectance of the second light guide unit 55 may be different from each other.

[0127] In addition, although in the present embodiment, a coating having a high light absorption rate is applied to the entire guiding surface 45a of the second conveyance guide 45, a configuration in which the coating is applied only to the region that makes a large contribution to the stray light finally reaching the light receiving unit 35 may be employed. The region that makes a large contribution to the stray light is, for example, Figure 12 the peripheral portion of the second opening 57 of the guiding surface 45a illustrated in. Therefore, the reflectance of each of the first light guide unit 54 and the second light guide unit 55 may be any value as long as the reflectance is at least higher than the reflectance of the peripheral portion of the second opening 57 of the guiding surface 45a in the second conveyance guide 45 serving as the guiding member.

[0128] Third Embodiment

[0129] Next, a third embodiment will be described with reference to the drawings. In the present embodiment, a configuration in which the light receiving unit detects reflected light from a sheet when the sheet exists in the conveyance path is used. In the following description, it is assumed that elements denoted by the same reference numerals as those in the first embodiment have substantially the same configurations and effects as those in the first embodiment, and the parts different from the first embodiment will be described.

[0130] Basic Configuration of Sheet Detection Mechanism

[0131] The basic configuration of the sheet detection mechanism 90 according to this embodiment will be described. Figure 13A and Figure 13B are perspective views of the sheet detection mechanism 90 according to this embodiment, respectively. Figure 14A is a side view of the sheet detection mechanism 90 observed in the sheet width direction X. Figure 14B is Figure 14A a cross-sectional view of the sheet detection mechanism 90 taken along a virtual plane along one of the optical paths of the sheet detection mechanism 90 indicated by the line 14B-14B in Figure 14C is a cross-sectional view of the sheet detection mechanism 90 taken along a virtual plane perpendicular to the sheet width direction X and indicated by the line 14C-14C in Figure 14B when observed in the sheet width direction X.

[0132] In this embodiment, different from the first embodiment, no reflection member is provided on the second transfer guide 45. In addition, different from the first embodiment, the first light guide portion 30a and the second light guide portion 30b are arranged such that a virtual line extending as the central axis of the first light guide portion 30a and a virtual line extending as the central axis of the second light guide portion 30b intersect each other at a point inside the transfer path. Other elements of the sensor unit 20 are substantially the same as those in the first embodiment.

[0133] Detection operation

[0134] Next, the sheet detection operation of the sheet detection mechanism 90 will be described. Figure 14B illustrates the sheet detection mechanism 90 when there is no sheet in the transfer path. In the state where there is no sheet, the light from the light emitting portion 33 reaches the guide surface 45a of the second transfer guide 45 after passing through the first light guide portion 30a. However, since no reflection member is provided on the guide surface 45a, the light is not mainly reflected toward the light receiving portion 35. In addition, since almost no light reaches the light receiving portion 35, current does not flow in the phototransistor of the light receiving portion 35, and the output voltage does not decrease. The controller 1C determines that the state where the output voltage of the light receiving portion 35 has not decreased and is higher than a preset threshold P is the state where there is no sheet S.

[0135] Figure 15A and Figure 15B illustrates the sheet detection mechanism 90 in the state where there is a sheet S in the transfer path. Figure 15A is a side view of the sheet detection mechanism 90 observed in the sheet width direction X. Figure 15B is along the virtual plane indicating one of the optical paths of the sheet detection mechanism 90 Figure 15A a cross-sectional view of the sheet detection mechanism 90 taken along the line 15B-15B inFigure 15B It is a sectional view taken along a virtual plane perpendicular to the conveyance direction Z.

[0136] When the sheet S exists in the conveyance path, the light emitted from the light emitting unit 33 and radiated into the conveyance path through the first light guide unit 30a is reflected by the sheet S and enters the light receiving unit 35 through the second light guide unit 30b. As a result, current becomes easier to flow in the phototransistor of the light receiving unit 35, and the output voltage decreases. In the present embodiment, the controller 1C determines that the state where the output voltage of the light receiving unit 35 is lower than the threshold P is the state where the sheet S exists.

[0137] In other words, the sheet detection mechanism 90 of the present embodiment has the following diffuse reflection type configuration: the light emitting element, the first light passage portion, the light receiving element, and the second light passage portion are arranged on one side of the conveyance path in the thickness direction Y. Further, when the sheet exists in the conveyance path, the light emitted from the light emitting element reaches the light receiving element through the optical path from the first light passage portion via reflection on the surface of the sheet to the second light passage portion. When the sheet does not exist in the conveyance path, the optical path from the first light passage portion via reflection on the surface of the sheet to the second light passage portion is not formed. That is, when the sheet exists in the conveyance path, the amount of light incident on the light receiving element is configured to be larger than when the sheet does not exist in the conveyance path.

[0138] However, in actual situations, as described in the first embodiment, sometimes the amount of light reaching the light receiving unit 35 in the state where the sheet exists in the conveyance path may decrease due to condensation on the members in the optical path, and depending on the cumulative light emission time and heat, the light emission amount of the LED serving as the light emitting unit 33 decreases. Further, in the present embodiment, since the light is reflected by the sheet toward the light receiving unit 35, the amount of light reaching the light receiving unit 35 is smaller than in the first embodiment using the reflector 44 having a higher reflectivity. Further, depending on the material of the sheet, the amount of light reaching the light receiving unit 35 may be small. Further, the amount of light reaching the light receiving unit 35 varies according to the inclination and warping of the surface of the sheet in the conveyance path.

[0139] In addition, when the sheet S does not exist, actually the second conveyance guide 45 slightly reflects light. As Figure 16 illustrated, the light reflected by the second conveyance guide 45 is reflected between the second conveyance guide 45 and the first conveyance guide 28 by diffuse reflection, and when a part of the reflected light reaches the light receiving unit 35 as stray light, it is possible that current flows in the light receiving unit 35 and the output voltage of the light receiving unit 35 decreases.

[0140] Therefore, also in the present embodiment, the reflectance of each of the first light guiding portion 30a and the second light guiding portion 30b with respect to the light emitted from the light emitting portion 33 is configured to be higher than the reflectance of the guiding unit with respect to the light emitted from the light emitting portion 33. Specifically, the surfaces of the first light guiding portion 30a and the second light guiding portion 30b that constitute the optical path - that is, the inner surfaces of the tubular shape serving as the light guiding surfaces - are formed of white resin or mirror-finished, a metal tape is pasted thereon, a metal-containing coating is applied thereon, or a metal layer is formed thereon by vapor deposition or the like. In addition, the guiding surfaces 28a and 45a of the first transfer guiding member 28 and the second transfer guiding member 45 are formed of black resin or matte-finished, or a coating having a high light absorption rate is applied thereon.

[0141] It should be noted that the reflectance of each of the first light guiding portion 30a and the second light guiding portion 30b is preferably 3% or more, and more preferably 4% or more. The reflectance of each of the first transfer guiding member 28 and the second transfer guiding member 45 is preferably 1.5% or less, and more preferably 1% or less. The values of the reflectance of the first light guiding portion 30a and the reflectance of the second light guiding portion 30b may be different from each other.

[0142] In addition, although the overall reflectance of the guiding surfaces 28a and 45a of the first transfer guiding member 28 and the second transfer guiding member 45 is configured to be low in the present embodiment, a configuration in which the reflectance is reduced only in a region that makes a large contribution to the stray light that finally reaches the light receiving portion 35 may be adopted. Regions that make a large contribution to the stray light are, for example, Figure 16 the peripheral portions of the first opening 41a and the second opening 41b of the guiding surface 28a illustrated in the figure and the portions of the guiding surface 45a that face the first opening 41a and the second opening 41b. Therefore, the reflectance of each of the first light guiding portion 30a and the second light guiding portion 30b may be any value as long as the reflectance is at least higher than the peripheral portions of the first opening 41a and the second opening 41b of the guiding surface 28a and the portions of the guiding surface 45a that face the first opening 41a and the second opening 41b among the first transfer guiding member 28 and the second transfer guiding member 45 that serve as guiding members.

[0143] By setting the reflectance of each of the first light guiding portion 30a and the second light guiding portion 30b to be high, attenuation of the light emitted from the light emitting portion 33 in the first light guiding portion 30a and the second light guiding portion 30b can be suppressed in the case where a sheet is present in the transfer path. Therefore, the amount of light reflected by the sheet and reaching the light receiving portion 35 can be increased.

[0144] In contrast, by setting the reflectivities of the first conveyance guide 28 and the second conveyance guide 45 to be low, the amount of light that reaches the light receiving unit 35 as stray light after diffuse reflection between the first conveyance guide 28 and the second conveyance guide 45 in the case where there is no sheet in the conveyance path can be reduced.

[0145] As a result, the variation amount of the amount of light reaching the light receiving unit 35 according to the presence or absence of the sheet increases, and the variation amount of the output voltage of the light receiving unit 35 increases. Therefore, it becomes easier to ensure a sufficient margin between the output voltage of the light receiving unit 35 corresponding to the presence or absence of the sheet and the threshold value P preset by the controller 1C. Thus, even in a relatively adverse situation, the possibility of false detection can be reduced and the detection accuracy can be maintained. In addition, even when the amount of light emitted from the LED serving as the light emitting unit 33 decreases, a large reduction in the amount of light received by the light receiving unit 35 can be suppressed, and while maintaining the detection accuracy of the sheet detection mechanism 90, the lifetimes of the sheet detection mechanism 90 and the image forming apparatus 1 can be set to be long.

[0146] Other embodiments

[0147] Although in the above-described first to third embodiments, the image forming apparatus including the sheet detection mechanism for detecting the sheet delivered from the fixing unit 18 has been described as an example of the sheet conveyance apparatus, the present technology can also be applied to other parts of the image forming apparatus or other sheet conveyance apparatuses. For example, in Figure 1 the image forming apparatus 1 illustrated, a sheet detection mechanism having the same configuration as one of the first to third embodiments can be disposed between the feed roller 24 and the registration roller pair 17. Examples of "other sheet conveyance apparatuses" include a sheet processing apparatus that performs processing such as binding processing on the sheet received from the image forming apparatus, and an automatic document feeder that conveys the sheet serving as a document toward the image sensor to read image information from the sheet.

[0148] Although the present invention has been described with reference to the exemplary embodiments, it is to be understood that the present invention is not limited to the disclosed exemplary embodiments. The scope of the appended claims should be given the broadest interpretation so as to encompass all such modifications as well as equivalent structures and functions.

Claims

1. A sheet conveying device, comprising: A conveying unit configured to convey a sheet; A guiding member deployed to define a conveying path for conveying the sheet; A light-emitting element configured to emit light; A light-receiving element configured to output a detection signal that changes according to the amount of received light; A first light passage portion deployed to define an optical path from the light-emitting element to the conveying path; and A second light passage portion deployed to define an optical path from the conveying path to the light-receiving element, wherein the light-receiving element is configured such that the detection signal changes according to whether a sheet is present in the conveying path, and wherein the reflectance of light emitted from the light-emitting element by each of the first light passage portion and the second light passage portion is higher than the reflectance of light emitted from the light-emitting element by the guiding member, wherein the light-emitting element, the light-receiving element, the first light passage portion, and the second light passage portion are deployed on one side of the conveying path in the thickness direction of the sheet passing through the conveying path, and wherein the amount of light reaching the light-receiving element when a sheet is present in the conveying path is greater than the amount of light reaching the light-receiving element when no sheet is present in the conveying path.

2. The sheet conveying device according to claim 1, wherein, Each of the first light passage portion and the second light passage portion is formed of a white resin.

3. The sheet conveying device according to claim 1, wherein, Each of the first light passage portion and the second light passage portion is formed of a metal material.

4. The sheet conveying device according to claim 1, wherein, The surface of each of the first light passage portion is mirror-finished, and the surface of the second light passage portion is mirror-finished.

5. The sheet conveying device according to claim 1, wherein, A metal layer is formed on the surface of each of the first light passage portion and the second light passage portion.

6. The sheet conveying device according to claim 1, wherein, A metal tape is pasted on the surface of each of the first light passage portion and the second light passage portion.

7. The sheet conveying device according to claim 1, wherein, A metal-containing coating is applied to the surface of each of the first light passage portion and the second light passage portion.

8. The sheet conveying device according to claim 1, wherein, The guiding member is formed of a black resin.

9. The sheet conveying device according to claim 1, wherein, The surface of the guiding member is matte-finished.

10. The sheet conveying device according to claim 1, wherein, A coating having an absorption rate of 99% or higher for light emitted from the light-emitting element is applied to the surface of the guiding member.

11. The sheet conveying device according to claim 1, wherein, The reflectance of light emitted from the light-emitting element by each of the first light passage portion and the second light passage portion is 3% or greater, and wherein the reflectance of light emitted from the light-emitting element by the guiding member is 1.5% or less.

12. The sheet conveying device according to claim 1, wherein, The reflectance of light emitted from the light-emitting element by each of the first light passage portion and the second light passage portion is 4% or greater, and wherein the reflectance of light emitted from the light-emitting element by the guiding member is 1% or less.

13. The sheet conveying device according to claim 1, wherein, The light-emitting element is a light-emitting diode.

14. The sheet conveying device according to claim 1, wherein, The first light passage portion forms at least a part of the inner surface of a first tubular shape having a first open end and a second open end, the first open end facing the substrate provided with the light-emitting element, and the second open end communicating with the conveying path, wherein the second light passage portion forms at least a part of the inner surface of a second tubular shape having a third open end and a fourth open end, the third open end facing the substrate provided with the light-receiving element, and the fourth open end communicating with the conveying path, and wherein the reflectance of light emitted by the light-emitting element by each of the inner surface of the first tubular shape and the inner surface of the second tubular shape is higher than the reflectance of light emitted by the light-emitting element by the guiding member.

15. An image forming apparatus, comprising: An image forming unit configured to form an image on the sheet; and A sheet conveying device according to any one of claims 1 to 14, configured to convey a sheet.

16. The image forming apparatus according to claim 15, wherein, The conveyance unit includes a fixing unit, and the fixing unit includes a pair of rotating members configured to hold and rotate a sheet, and a heating unit configured to heat the holding portion of the pair of rotating members, wherein the fixing unit is configured to heat the toner image while conveying the sheet having the toner image formed thereon by the image forming unit through the pair of rotating members, so as to fix the toner image onto the sheet carrying the toner image, and wherein the light emitting element, the light receiving element, the first light passing portion, and the second light passing portion are arranged such that the detection signal changes according to whether there is a sheet delivered from the holding portion toward the conveyance path.

Images