Image forming apparatus

Abstract

To detect sheets in transit using an inexpensive configuration, regardless of the sheet's transparency, even when transporting sheets with different levels of transparency. [Solution] If the media sensor 70 determines that the type of sheet P is not a transparent sheet, the CPU 26 sets the light intensity of the light-emitting unit 33 to a first light intensity Lgt1 and sets the threshold to a first threshold Vth1. If the media sensor 70 determines that the type of sheet P is a transparent sheet, the CPU 26 sets the light intensity of the light-emitting unit 33 to a second light intensity Lgt2, which is lower than the first light intensity Lgt1, and sets the threshold to a second threshold Vth2.

Description

Technical Field

[0001] The present invention relates to an image forming apparatus that detects the arrival and presence of a sheet on a conveyance path by means of an optical sensor having a light-emitting portion and a light-receiving portion.

Background Art

[0002] Conventionally, in order to detect the arrival and presence of a sheet such as recording paper on a conveyance path in an image forming apparatus, a flag-type sensor that performs detection through mechanical rotation of a flag pressed by a sheet using the flag and a photointerrupter has been employed. In addition, an optical sensor that allows the light-receiving portion to detect that light emitted by the light-emitting portion is blocked by the sheet has been employed. Since this optical sensor does not involve mechanical operation unlike the flag-type sensor, it can detect the arrival of a sheet with good responsiveness even when the interval between a preceding sheet and a succeeding sheet is shortened. However, the detection accuracy of the optical sensor tends to depend on the type of sheet. For example, a transparent sheet such as an OHT (overhead transparency) sheet has a higher light transmittance than plain paper or the like, so the optical sensor cannot accurately detect the arrival of the leading edge or trailing edge of the OHT sheet, which causes paper jams. Therefore, the image forming apparatus described in Patent Document 1 includes both a flag-type sensor and an optical sensor. In this image forming apparatus, control is performed to use both sensors in combination such that the flag-type sensor is used when conveying an OHT sheet, and the optical sensor is used in consideration of printing speed when conveying a sheet such as plain paper other than an OHT sheet.

Prior Art Literature

Patent Literature

[0003]

Patent Document 1

Summary of the Invention

Problem to be Solved by the Invention

[0004] Since incorporating both flag-type and optical sensors increases the cost of the image forming apparatus, there is a need to establish inexpensive configurations and control methods for handling OHT sheet transport.

[0005] This disclosure is made under these circumstances, and one of its objectives is to detect sheets in transit with an inexpensive configuration, regardless of the transparency of the sheets, even when transporting sheets with different levels of transparency. [Means for solving the problem]

[0006] To solve the above-mentioned problems, the present invention has the following configuration.

[0007] (1) An image forming apparatus configured to perform an image forming operation to form an image on a sheet, comprising: a light-emitting unit that irradiates light across a transport path for transporting a sheet; a light-receiving unit that receives the light irradiated from the light-emitting unit and outputs a voltage corresponding to the received light as a detection voltage; a control unit that determines the presence or absence of a sheet, or the arrival of the leading edge or the arrival of the trailing edge, by comparing the detection voltage with a threshold; a drive unit that drives the light-emitting unit based on the amount of light set by the control unit; and a sheet type determination unit that determines the type of sheet being transported on the transport path, wherein the control unit sets the amount of light to a first amount of light and the threshold to a first threshold when the sheet type determination unit determines that the type of sheet is not a transparent sheet, and sets the amount of light to a second amount of light lower than the first amount of light and the threshold to a second threshold when the sheet type determination unit determines that the type of sheet is a transparent sheet. [Effects of the Invention]

[0008] According to this disclosure, even when transporting sheets with different levels of transparency, it is possible to detect the sheet being transported with an inexpensive configuration, regardless of the transparency of the sheet. [Brief explanation of the drawing]

[0009] [Figure 1]Schematic diagram of the image forming apparatus in Example 1 [Figure 2] Perspective view of the sheet sensor in Example 1 [Figure 3] Plan view of the sheet sensor in Example 1 [Figure 4] Circuit diagram of the sheet sensor in Example 1 [Figure 5] Diagram showing the circuit operation of the sheet sensor in Example 1. [Figure 6] Flowchart showing the control of the sheet sensor in Example 1 [Figure 7] Schematic diagram of the image forming apparatus in Example 2 [Figure 8] Diagram showing the circuit operation of the sheet sensor in Example 2. [Figure 9] Cross-sectional view showing the ventilation path of the sheet sensor in Example 2 [Figure 10] Flowchart showing the control of the sheet sensor in Example 2 [Modes for carrying out the invention]

[0010] The embodiments for carrying out the present invention will be described in detail below with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in these embodiments should be appropriately modified depending on the configuration of the device to which the invention is applied and various conditions. In other words, the scope of the present invention is not intended to be limited to the following embodiments.

[0011] Furthermore, in the following description and in each drawing, the vertical direction when the image forming apparatus is installed on a horizontal plane is defined as the Z direction. The direction intersecting the Z direction, which is the direction of the rotation axis of the photosensitive drum (the rotation axis direction of the photosensitive drum) described later, is defined as the Y direction. The direction intersecting both the Z and Y directions is defined as the X direction. The X and Y directions are preferably horizontal. Also, the X, Y, and Z directions are preferably orthogonal to each other. Furthermore, as necessary, the directions of the arrows X, Y, and Z shown in each drawing are represented as the +X side, +Y side, and +Z side, respectively, and the opposite sides are represented as the -X side, -Y side, and -Z side, respectively. [Examples]

[0012] <Image forming apparatus> The image forming apparatus of Embodiment 1 of the present invention will now be described. Figure 1 is a schematic diagram showing the cross-sectional configuration of the image forming apparatus of Embodiment 1. The image forming apparatus 100 shown in Figure 1 is equipped with process cartridges 5Y, 5M, 5C, and 5K that can be attached to the main body. The letters Y, M, C, and K assigned to the reference numerals indicate toner colors corresponding to yellow, magenta, cyan, and black, respectively, and are omitted when explaining matters common to each color. The process cartridge 5 has a toner container 23, a photosensitive drum 1, a charging roller 2, a developing roller 3, a cleaning member 4, and a waste toner container 24. The process cartridge 5, together with the exposure unit 7, forms an image forming unit 101. The toner container 23 contains a developer (hereinafter referred to as toner). The photosensitive drum 1 is an image carrier that carries an electrostatic latent image or toner image. The charging roller 2 uniformly charges the surface of the photosensitive drum 1. The exposure unit 7 outputs laser light according to image information and forms an electrostatic latent image on the surface of the photosensitive drum 1. The developing roller 3 develops the toner supplied from the toner container 23 by adhering it to the electrostatic latent image, thereby forming a toner image.

[0013] An example of a transfer mechanism, the intermediate transfer unit 102, includes an intermediate transfer belt 8, a drive roller 9, an opposing roller 10, and a primary transfer roller 6. The primary transfer roller 6 is positioned opposite the photosensitive drum 1 and transfers the toner image carried on the photosensitive drum 1 to the intermediate transfer belt 8 (primary transfer). The intermediate transfer belt 8 is stretched between the drive roller 9 and the opposing roller 10 and rotates driven by the drive roller 9. The intermediate transfer belt 8 rotates in the direction indicated by arrow A, transporting the toner image to the secondary transfer section. The secondary transfer section is formed by the intermediate transfer belt 8 and the secondary transfer roller 11.

[0014] The sheet feeding cassette 13 accommodates a plurality of sheets P. The sheets P are recording media (recording materials) such as plain paper and overhead transparency (hereinafter referred to as OHT) sheets. The sheet feeding roller 14 picks up the sheet P and feeds it out to the conveyance path. The conveyance roller 15 further conveys the sheet P delivered from the sheet feeding roller 15 to the downstream side in the conveyance direction. The registration roller (hereinafter referred to as resist roller) 16 is a conveyance roller that synchronizes the timing at which the sheet P arrives at the secondary transfer portion with the timing at which the toner image arrives at the secondary transfer portion.

[0015] A media sensor 70 serving as a sheet type determination unit is disposed between the resist roller 16 and the secondary transfer portion, and is a sensor that detects the basis weight and surface property as characteristics of the sheet P to determine the type of the sheet P. The media sensor 70 is an example of a sensor that determines the type of a sheet conveyed to the conveyance path. As the media sensor 70, for example, a mechanism is conceivable (not shown) in which ultrasonic waves generated by an ultrasonic wave generating element are emitted to the sheet P, and the basis weight is detected based on the amplitude value of the ultrasonic wave attenuated through the sheet P received by an ultrasonic wave receiving element. Further, as the media sensor 70, for example, a mechanism is conceivable (not shown) in which the surface of the sheet P is imaged by an imaging element, and the degree of unevenness of the sheet P, that is, the surface property, is detected from the feature amount of the shading difference of the imaging pixels. The toner image is transferred onto the sheet P, the type of which has been determined by the media sensor 70, in the secondary transfer portion (secondary transfer). The belt cleaner 21 removes the toner remaining on the intermediate transfer belt 8 and collects it into the waste toner container 22.

[0016] The sheet P onto which the toner image has been transferred is conveyed to a fixing device 17. The fixing device 17 includes a heating roller 18 and a pressure roller 19 that apply heat and pressure to the toner image and the sheet P. A heating means such as a heater 30 and a temperature sensor 12 that measures the temperature of the heater 30 are provided inside the heating roller 18. The sheet P carrying an unfixed toner image is nipped and conveyed by the heating roller 18 and the pressure roller 19, and at this time, the toner image on the sheet P (on the sheet) is fixed by heat and pressure. The portion where the heating roller 18 and the pressure roller 19 are in contact with each other is hereinafter referred to as a fixing nip portion. A discharge roller 20 discharges the sheet P with the fixed toner image to the outside of the image forming apparatus 100. The discharge roller 20 is an example of a discharge unit that discharges a sheet passing through the fixing device to the outside of the apparatus.

[0017] Inside the fixing device 17, on the downstream side of the heating roller 18 and the pressure roller 19 in the conveyance direction of the sheet P, a sheet sensor 31 that is a reflective optical sensor is provided. The sheet sensor 31 detects the arrival and presence / absence of the sheet P conveyed by the heating roller 18 and the pressure roller 19. The arrival of the sheet P means the timing at which the leading edge of the sheet P is detected by the sheet sensor 31. Although FIG. 1 shows an example in which the sheet sensor 31 is provided inside the fixing device 17 (in the fixing device), the sheet sensor 31 may be provided, for example, in a conveyance path portion between the fixing device 17 and the discharge roller 20. The sheet sensor 31 is an example of a light emitting unit and a light receiving unit provided between the fixing device and the discharge unit. That is, the sheet sensor 31 only needs to be provided downstream of the fixing device 17 in the conveyance direction.

[0018] The control board 25 has electrical circuits that control various parts of the image forming apparatus 100. For example, the control board 25 is equipped with a CPU 26 that controls various parts of the image forming apparatus 100 by executing a control program. The CPU 26 is responsible for controlling the drive source (not shown) and sheet sensor 31 related to the transport of the sheet P, the drive source (not shown) of the process cartridge 5, image forming control, and fault detection control. The switching power supply 28 converts the AC voltage of the AC power input from the power cable 29 connected to the external power supply into a DC voltage and supplies it to the control board 25 and the drive source (not shown). The CPU 26 also receives the result of the sheet type determination by the media sensor 70. Based on the sheet type determination information input from the media sensor 70, the CPU 26 determines the appropriate transport speed according to the characteristics of the sheet P and the target temperature when controlling the temperature of the fixing device 17 (heater 30). Furthermore, the operation panel 90 is a user interface for the user to input various specifications such as the print mode to the image forming apparatus 100, and for example, it can set the type of sheet P stored in the paper feed cassette 13.

[0019] <Seat sensor configuration> Figures 2(A) and 2(B) are perspective views of the sheet sensor 31. Figures 2(A) and 2(B) show different viewpoints of the sheet sensor 31, and each direction is also shown to make it easier to understand the orientation of the sheet sensor 31. The Z direction indicates the height direction of the image forming apparatus 100 and is parallel to the transport direction of the sheet P in the fixing device 17 (see Figure 1).

[0020] The first guide 36 is positioned above the pressure roller 19 and is a guide member that guides the sheet P. The cross-section of the first guide 36 parallel to the ZX plane (a virtual plane perpendicular to the Y direction) is approximately U-shaped. In other words, when viewed in the Y direction, the cross-section of the first guide 36 is approximately U-shaped.

[0021] The first guide 36 is formed from a first member 41, a second member 42, and a third member 43, and can also be said to have a first member 41, a second member 42, and a third member 43. The first member 41 and the third member 43 are members that are parallel to the YZ plane (a virtual plane perpendicular to the X direction) and extend in the Y direction. In the X direction, the first member 41 is located closer to the fixing nip than the third member 43. The second member 42 is a member that is parallel to the XY plane (a virtual plane perpendicular to the Z direction) and extends in the Y direction. The Y direction is also called the longitudinal direction of each member. In addition, the Z direction is also called the transverse direction for the first member 41 and the third member 43, and the X direction is also called the transverse direction for the second member 42.

[0022] The first member 41 is connected to the second member 42, and the second member 42 is connected to the third member 43. In other words, in the short-side direction of each member, one end of the first member 41 is joined to one end of the second member 42. Also, the other end of the second member 42 is joined to one end of the third member 43. As a result, the first guide 36 has the U-shaped cross-section described above. In the short-side direction of the first member 41 and the third member 43, the end that is not joined to the second member 42 is sometimes called the open end. The second member 42 is located closer to the pressure roller 19 than the open ends of the first member 41 and the third member 43. The first member 41 has a guide surface 41a that guides the sheet P.

[0023] The second guide 37 is a guide member that guides the sheet P, provided above the heating roller 18 and facing the first guide 36. The cross-section of the second guide 37 parallel to the ZX plane is approximately L-shaped. In other words, when viewed in the Y direction, the cross-section of the second guide 37 is approximately U-shaped.

[0024] The second guide 37 is formed from a fourth member 44 and a fifth member 45, and can also be said to have a fourth member 44 and a fifth member 45. The fourth member 44 is a member parallel to the YZ plane (a virtual plane perpendicular to the X direction) and extending in the Y direction. The fifth member 45 is a member parallel to the XY plane (a virtual plane perpendicular to the Z direction) and extending in the Y direction. In the fourth member 44, the Z direction is also called the short side direction, and in the fifth member 45, the X direction is also called the short side direction.

[0025] The fourth member 44 is connected to the fifth member 45. In other words, in the short direction of each member, one end of the fourth member 44 is joined to one end of the fifth member 45. As a result, the second guide 37 has the L-shaped cross-section described above. In the fourth member 44, the end that is not joined to the fifth member 45 is sometimes called the non-joined end. The fifth member 45 is located closer to the heating roller 18 than the non-joined end of the fourth member 44. The fourth member 44 has a guide surface 44a that guides the sheet P and is parallel to the first member 41. That is, the guide surface 41a of the first member 41 of the first guide 36 and the guide surface 44a of the fourth member 44 of the second guide 37 face each other with a predetermined distance L1 between them. The sheet P is conveyed between the guide surface 41a and the guide surface 44a.

[0026] A notch 41b is provided in the longitudinal (Y direction) center of the first member 41 of the first guide 36. The second member 42 is provided with a substrate holding member 46 that protrudes upward (+Z side) from the surface 42a and a light-shielding member 47 that protrudes upward (+Z side) from the surface 42a of the second member 42. In the X direction, the light-shielding member 47 is located closer to the second guide 37 than the substrate holding member 46.

[0027] A substrate 35 is fixed to the substrate holding member 46. A light-emitting unit 33 and a light-receiving unit 34 are mounted on the substrate 35. It can also be said that the light-emitting unit 33 and the light-receiving unit 34 are located on the same side with respect to the sheet P being transported. The light-emitting unit 33 emits light across the transport path that transports the sheet P. The light-receiving unit 34 receives the light emitted from the light-emitting unit 33 and outputs a voltage corresponding to the received light as a detection voltage. The detection voltage output from the light-receiving unit 34 is input to the CPU 26. The CPU 26, acting as a control unit, determines the presence or absence of the sheet P, or whether the leading edge or trailing edge has been reached, by comparing the detection voltage with a threshold value.

[0028] The light-emitting unit 33 emits light, and the light-receiving unit 34 receives light. The substrate 35 is fixed to the substrate holding member 46 so that the light-emitting unit 33 and the light-receiving unit 34 are aligned in the Y direction. The light-shielding member 47 is provided between the light-emitting unit 33 and the light-receiving unit 34. The substrate holding member 46 and the light-shielding member 47 are provided on the second member 42 so that the light-emitting unit 33, the light-receiving unit 34 and the light-shielding member 47 are exposed toward the second guide 37 side at the notch 41b of the first member 41.

[0029] A notch 44b is also provided in the center of the fourth member 44 of the second guide 37. The fifth member 45 is provided with a reflector member holding portion 48 that protrudes upward (towards the +Z side) from the surface 45a. A reflector member 38 is fixed to the reflector member holding portion 48. The reflector member 38 is an example of a reflector that is provided opposite the light-emitting portion and reflects the light emitted from the light-emitting portion toward the light-receiving portion. The reflector member holding portion 48 is provided on the fourth member 44 such that the reflector member 38 is exposed toward the first guide 36 side at the notch 44b of the fourth member 44.

[0030] In this example, the reflective member holding portion 48 and the substrate holding portion 46 are parallel to each other. Furthermore, the light-emitting portion 33, the reflective member 38, and the light-receiving portion 34 are positioned such that the light emitted from the light-emitting portion 33 is specularly reflected by the reflective member 38, and the reflected light is incident on the light-receiving portion 34. The reflective member 38 can be any material or reflective film that has the property of reflecting light; for example, a mirror, a glossy metal, or a resin can be used as the reflective member 38.

[0031] Figures 3(A), 3(B), and 3(C) are plan views of the sheet sensor 31. Figure 3(A) shows the state of the sheet sensor 31 when no sheet P is passing through, Figure 3(B) shows the state when a sheet P made of ordinary paper is passing through, and Figure 3(C) shows the state when a sheet P made of transparent sheet material such as an OHT sheet is passing through. Here, the space through which the sheet P passes, formed by the guide surfaces 41a and 44a, is called the transport path 49.

[0032] As shown in Figure 3(A), the light emitted by the light-emitting unit 33 crosses the transport path 49 and reaches the reflective member 38 of the second guide 37. The light emitted from the light-emitting unit 33 is reflected by the surface of the reflective member 38 and crosses the transport path 49 to reach the light-receiving unit 34. On the other hand, as shown in Figure 3(B), when a sheet P made of plain paper is being transported along the transport path 49, the light from the light-emitting unit 33 reaches the surface of the sheet P, but the light is blocked by the surface of the sheet P. In other words, the light does not reach the reflective member 38, and the light-receiving unit 34 does not receive the reflected light from the reflective member 38. Also, as shown in Figure 3(C), when a sheet P made of OHT is being transported along the transport path 49, the amount of transmitted light is higher than that of plain paper, so the light emitted by the light-emitting unit 33 passes through the sheet P and crosses the transport path 49 to reach the reflective member 38 of the second guide 37. The light emitted from the light-emitting unit 33 is reflected by the surface of the reflective member 38, and that light passes through the sheet P again, crosses the transport path 49, and reaches the light-receiving unit 34.

[0033] <Circuit operation and control method of the sheet sensor> Figures 4(A) and 4(B) are circuit diagrams of the sheet sensor 31. Figure 4(A) shows the drive circuit 133 for the light-emitting unit 33. The drive circuit 133 is an example of a drive unit that drives the light-emitting unit based on the light intensity set by the control unit. The CPU 26 outputs a PWM signal (pulse signal) to control the light intensity of the light-emitting unit 33, i.e., the light intensity. Hereinafter, the proportion of low levels in one cycle of the PWM signal will be called the off-duty cycle, and the proportion of high levels will be called the on-duty cycle. The PWM signal is smoothed by a smoothing circuit composed of a resistor R1 and a capacitor C1 and input to the base terminal of transistor Tr1. This turns on transistor Tr1. A current-limiting resistor R2 is provided between the emitter terminal of transistor Tr1 and the reference voltage Vcc to limit the current. The light-emitting diode LED1 constitutes the light-emitting unit 33. The anode terminal of the light-emitting diode LED1 is connected to the collector terminal of transistor Tr1, and the cathode terminal is grounded.

[0034] The CPU 26 switches the light output of the LED 1 by changing the off-duty cycle of the PWM signal. In other words, setting a high off-duty cycle for the PWM signal increases the light output of the LED 1, and setting a low off-duty cycle for the PWM signal decreases the light output of the LED 1. Note that the drive circuit 133 for the light-emitting section 33 is not limited to Figure 4(A), and may also be designed to increase the light output of the LED 1 by setting a high high-duty cycle for the PWM signal.

[0035] On the other hand, Figure 4(B) shows the light receiving detection circuit 134 of the light receiving unit 34. The collector terminal of the phototransistor PTr1, which receives light output from the light-emitting diode LED1, is connected to a reference voltage Vcc via a pull-up resistor R3. The collector terminal of the phototransistor PTr1 is also connected to the CPU 26, and the voltage at the collector terminal is input to the CPU 26 as a detection voltage Vsns. In other words, the phototransistor PTr1 constitutes the light receiving unit 34. The emitter terminal of the phototransistor PTr1 is grounded.

[0036] Because the phototransistor PTr1 operates based on its transistor characteristics in response to the amount of light received, the detected voltage Vsns changes from approximately 0V to Vcc. The light-emitting diode LED1 and phototransistor PTr1 are mounted on the substrate 35 as the light-emitting part 33 and light-receiving part 34, respectively, in the sheet sensor 31, while the resistors R1, R2, R3, capacitor C1, and transistor Tr1 are mounted on the control board 25.

[0037] Figures 5(A), 5(B), and 5(C) show the circuit operation of the sheet sensor 31. In Figure 5(A), the horizontal axis shows the light intensity of the light-emitting diode LED1, and the vertical axis shows the detection voltage Vsns. When sufficient light is received to turn on the phototransistor PTr1, the phototransistor PTr1 operates in the saturation operating region Ra, and the detection voltage Vsns becomes Vsat, which is approximately 0V, as the saturation voltage. On the other hand, if sufficient light is not received to turn on the phototransistor PTr1, the phototransistor PTr1 operates in the dark voltage operating region Rb, and the detection voltage Vsns becomes approximately the reference voltage Vcc. Furthermore, when the phototransistor PTr1 receives a light intensity that corresponds to an intermediate position between the dark voltage operating region Rb and the saturation operating region Ra, the phototransistor PTr1 operates in the linear operating region Rc. Therefore, the detection voltage Vsns becomes a voltage that decreases linearly with a characteristic approximately proportional to the light intensity.

[0038] Figure 5(A) also shows the saturation operating region Ra, the dark voltage operating region Rb, and the linear operating region Rc. The saturation operating region Ra is an example of the first region, where the detection voltage does not change even when the light intensity emitted by the light-emitting unit is increased. The linear operating region Rc is an example of the second region, where the detection voltage changes linearly in response to the light intensity, even at a light intensity lower than that of the first region. The dark voltage operating region Rb is an example of the third region, where the detection voltage does not change even when the light intensity is lower than that of the second region.

[0039] Because the detection voltage Vsns has these characteristics, the input port of the CPU 26 to which the collector terminal of the phototransistor PTr1 is connected is a port with an AD input function that can distinguish analog values. Based on this operation, the CPU 26 detects whether sheet P has been reached or is present by comparing the input detection voltage Vsns with a predetermined threshold.

[0040] Let's consider the case where sheet P is not a transparent sheet like an OHT sheet, but ordinary paper. As mentioned above, when sheet P is not passing through the sheet sensor 31 (hereinafter referred to as "no paper"), the phototransistor PTr1 receives the light output from the light-emitting diode LED1. On the other hand, when sheet P is passing through the sheet sensor 31 (hereinafter referred to as "with paper"), the light from the light-emitting diode LED1 is sufficiently blocked by sheet P.

[0041] (For plain paper) Figure 5(B) shows the operation of the light receiving unit 34 when detecting the arrival or presence of sheet P when sheet P is plain paper, and shows the relationship between the change in the state of the sheet sensor 31 over time and the detected voltage Vsns. In Figure 5(B), time is shown on the horizontal axis and the detected voltage Vsns on the vertical axis. Below the graph, the state of the sheet sensor 31 corresponding to the change in the detected voltage Vsns is also shown. Here, "No paper" indicates the state until the leading edge of the plain paper reaches the sheet sensor 31, or after the trailing edge of the plain paper has passed the sheet sensor 31. Here, "Paper present" indicates the state where the plain paper is passing through the transport path 49 at the installation position of the sheet sensor 31, as shown in Figure 3(B).

[0042] Here, the light intensity of the light-emitting diode LED1 is set so that the detection voltage Vsns when there is no paper is Vsat as the saturation voltage, and the detection voltage Vsns when there is paper is Vnml as the voltage of the reference voltage Vcc. In other words, the CPU 26 sets the light intensity of the light-emitting diode LED1 to the light intensity Lgt1 (hereinafter also referred to as the first light intensity Lgt1) at which the detection voltage Vsns sufficiently saturates when there is no paper. The CPU 26 also sets a threshold of Vth1 = Vcc / 2 (first threshold) for determining whether there is no paper or paper based on the detection voltage Vsns. Since the voltage Vnml is approximately equal to the reference voltage Vcc, the first threshold can also be expressed as Vth1 = Vnml / 2. The CPU 26, as the control unit, sets the first threshold based on the detection voltage (Vnml) obtained when light is irradiated from the light-emitting unit at a light intensity within the third region.

[0043] Here, since the sheet sensor 31 is located inside or around the fixing device 17, if the sheet P that has passed through the fixing device 17 has absorbed moisture, the water vapor generated may adhere to the surface of the reflective member 38, causing condensation and potentially reducing the reflectivity of the reflective member 38. In this case, the amount of light from the light-emitting diode LED1 reflected by the reflective member 38 toward the phototransistor PTr1 tends to decrease. In other words, if the amount of light from the light-emitting diode LED1 is not set sufficiently high when condensation occurs on the reflective member 38, the detection voltage Vsns when there is no paper will increase, making it impossible to reliably detect the arrival or presence of the sheet P based on the first threshold. Therefore, in order to stably detect the arrival or presence of the sheet P even when condensation occurs on the reflective member 38, it is desirable to set the first light intensity and first threshold to predetermined values ​​when the sheet P is ordinary paper. The CPU 26 is an example of a control unit that, when the sheet type determination unit determines that the sheet type is not a transparent sheet, sets the light intensity of the light-emitting unit to the first light intensity within the first region and sets the threshold to the first threshold.

[0044] (In the case of OHT sheets) Next, consider the case where sheet P is a transparent sheet such as an OHT sheet. As shown in Figure 3(C), transparent sheets transmit more light than ordinary paper. Therefore, even when there is paper present as sheet P passes through the sheet sensor 31, the light from the light-emitting diode LED1 is not sufficiently blocked, and the detection voltage Vsns cannot sufficiently exceed the threshold, making it difficult to detect the presence of paper. For example, if the light intensity of the light-emitting diode LED1 is set to the light intensity Lgt1 (first light intensity) at which the detection voltage Vsns saturates when there is no paper, the detection voltage Vsns will remain at the saturation voltage Vsat even when there is paper, and the presence of paper cannot be detected. However, since the light intensity from the light-emitting diode LED1 is reduced to some extent before and after passing through the OHT sheet, it is possible to determine whether there is no paper or there is paper by detecting this degree of light intensity reduction.

[0045] Figure 5(C) shows the operation of the light receiving unit 34 when detecting the arrival or presence of sheet P when sheet P is an OHT sheet, and, similar to Figure 5(B), it shows the relationship between the state change of the sheet sensor 31 over time and the detection voltage Vsns. The light intensity of the light-emitting diode LED1 is set lower than the first light intensity so that the detection voltage Vsns when there is no paper becomes the intermediate voltage Vcalib which operates in the linear operating region Rc of the phototransistor PTr1. That is, the CPU 26 sets the light intensity of the light-emitting diode LED1 to the light intensity Lgt2 (hereinafter also referred to as the second light intensity Lgt2) which operates in the linear operating region Rc when there is no paper. <Lgt1)。

[0046] This intermediate voltage Vcalib is pre-set when there is no sheet P at the position of the sheet sensor 31, such as before or after a print job in the image forming apparatus 100. The CPU 26 determines this by detecting the detection voltage Vsns when the light intensity value of the light-emitting diode LED1 is experimentally varied (changed) and searching for the light intensity setting value at which the phototransistor PTr1 operates linearly. For example, the CPU 26 searches for the light intensity corresponding to the voltage at which the detection voltage Vsns becomes lower than the voltage corresponding to dark voltage operation, and the light intensity corresponding to the voltage at which the detection voltage Vsns becomes higher than the saturation voltage Vsat. The CPU 26 then calculates a second light intensity corresponding to the intermediate voltage Vcalib by using linear interpolation between two points for each light intensity value multiple times. Hereinafter, the acquisition of the second light intensity by the CPU 26 is referred to as light intensity calibration.

[0047] Thus, when the system is operated in the linear operating region Rc where the detection voltage Vsns does not saturate, the amount of light transmitted through the OHT sheet decreases by about 20%, for example, so the detection voltage Vsns when paper is present rises to the detection voltage Voht shown in Figure 5(C). The CPU 26 sets the threshold Vth2 as follows to establish a threshold between the intermediate voltage Vcalib when no paper is present and the detection voltage Voht when paper is present. For example, the threshold Vth2 is set to 10% higher than the intermediate voltage Vcalib, so Vth2 = Vcalib × 1.1 (second threshold). Although 10% was used when setting the second threshold Vth2, this is not limited to this value, and this value may be determined according to the characteristics of the light-emitting part 33 and the light-receiving part 34 of the sheet sensor 31. The CPU 26 is an example of a control unit that, when the sheet type determination unit determines that the sheet type is a transparent sheet, sets the light amount to the second light amount in the second region which is lower than the first light amount, and sets the threshold to the second threshold. Furthermore, the CPU 26, acting as the control unit, sets a second threshold value based on the detection voltage (Vcalib) obtained when light is irradiated from the light-emitting unit at a second light intensity.

[0048] By performing this type of control, the system can handle OHT sheet transport even when the sheet sensor 31 is an optical sensor. In other words, there is no need to provide a separate sensor for detecting OHT sheets in addition to the sensor for detecting ordinary paper. Here, since OHT sheets generally have lower moisture absorption than ordinary paper, the situation of reduced light intensity due to condensation on the reflective member 38 as described above is less likely to occur, thus enabling this control.

[0049] <Control flowchart for seat sensor> Figure 6 is a flowchart showing the control of the sheet sensor 31, and the control of Embodiment 1 will be explained based on this flowchart. In step (hereinafter referred to as S), the CPU 26 performs the above-described light intensity calibration and calculates the second light intensity when there is no sheet P in the sheet sensor 31, such as before or after a print job of the image forming apparatus 100. The CPU 26 is an example of a control unit that performs light intensity calibration to search for the light intensity in the second region and sets the second light intensity when there is no sheet in the optical path from the light-emitting unit to the light-receiving unit, and when the image forming apparatus 100 is not performing an image forming operation. After the image forming apparatus 100 starts printing, the media sensor 70 determines whether the sheet P is plain paper or an OHT sheet.

[0050] In S102, the CPU 26 determines whether sheet P is an OHT sheet based on the result of the sheet type determination by the media sensor 70. If the CPU 26 determines in S102 that sheet P is plain paper, it proceeds to S103. In S103, the CPU 26 sets the light intensity of the light-emitting diode LED1 to the first light intensity Lgt1, sets the threshold for determining the presence or absence of sheet P (hereinafter also called the detection threshold) to the first threshold Vth1, and proceeds to S105. As a result, the CPU 26 causes the light-emitting diode LED1 to emit light at the first light intensity Lgt1, which is the detection voltage Vsns from the phototransistor PTr1 that operates at the saturation voltage. In S105, the CPU 26 detects the presence or absence of sheet P, which is plain paper, in the transport path 49, and detects the arrival of the leading and trailing ends as edge detection, and terminates the process (S105).

[0051] If the CPU 26 determines in S102 that sheet P is an OHT sheet, it proceeds to S104. In S104, the CPU 26 sets the light intensity of the light-emitting diode LED1 to the second light intensity Vth2, sets the detection threshold to the second threshold Vth2, and proceeds to S105. As a result, the CPU 26 causes the light-emitting diode LED1 to emit light at the second light intensity Vth2, which operates with the detection voltage Vsns from the phototransistor PTr1 at the intermediate voltage Vcalib. The CPU 26 detects the presence or absence of sheet P, which is an OHT sheet, in the transport path 49, and also detects the arrival of the leading and trailing ends as edge detection (S105).

[0052] Although an example is described in which the type of sheet P is determined by the media sensor 70, the description is not limited to this. For example, the type of sheet P may be determined in advance by the user via the operation panel 90 before the print job of the image forming apparatus 100. The operation panel 90 is an example of a user interface for inputting various specifications from the user. In this case, the CPU 26, which acts as the sheet type determination unit, can be said to determine the type of sheet based on the information input via the user interface.

[0053] By using the configuration and control method described above, the sheet sensor 31 can be an optical sensor in both cases: when transporting OHT sheets and when transporting sheets other than OHT sheets, such as plain paper. This eliminates the need to separately provide a flag-type sensor. Therefore, it is possible to handle sheet transport with an inexpensive configuration that does not depend on the transparency of the sheet.

[0054] In Example 1, the light-emitting unit 33 and the light-receiving unit 34 were arranged on the same side with respect to the sheet P, and a reflective member 38 was provided on the side opposite to the light-emitting unit 33 with respect to the sheet P, so that the light-receiving unit 34 receives the reflected light. However, the light-receiving unit 34 may be arranged on the opposite side of the sheet P from the light-emitting unit 33. That is, the light-receiving unit 34 may be arranged in the position of the reflective member 38 in Example 1. In this configuration as well, when plain paper is being transported, the light output from the light-emitting unit 33 is blocked, and the light-receiving unit 34 cannot receive the light. Also, when an OHT sheet is being transported, the light output from the light-emitting unit 33 passes through the OHT sheet, and the light-receiving unit 34 can receive the transmitted light. For this reason, in this configuration as well, the amount of light that causes the light-emitting unit 33 to emit light should be set to a first light amount or a second light amount depending on the type of sheet, and the threshold should be set to a first threshold or a second threshold.

[0055] As described above, according to Example 1, even when transporting sheets with different levels of transparency, it is possible to detect the sheet being transported with an inexpensive configuration, without depending on the transparency of the sheet. [Examples]

[0056] Example 2 will now be described. The main parts will be described in the same way as in Example 1, and the same reference numerals will be used for components that are the same as in Example 1, and their descriptions will be omitted. Here, only the parts that differ from Example 1 will be described.

[0057] In Example 2, the difference from Example 1 is the addition of control to detect the condensation state of the reflective member 38 and control to eliminate the condensation. Generally, OHT sheets have lower moisture absorption than plain paper, so in print jobs that continuously print OHT sheets, the degree of condensation on the reflective member 38 (hereinafter referred to as the degree of condensation) is low, and the reflective member 38 is less affected by the reduction in light intensity due to condensation. However, if an attempt is made to print an OHT sheet immediately after a state in which the surface of the reflective member 38 has condensed and its reflectivity has been reduced due to continuous printing of moisture-absorbing plain paper, even an OHT sheet will be affected.

[0058] For example, when the reflective member 38 is condensed (hereinafter referred to as the condensed state), the CPU 26 calculates a second light intensity by light intensity calibration before the print job and sets a second threshold. In this case, as the condensation on the reflective member 38 begins to dissipate over time, the light intensity of the light-emitting diode LED1 reflected by the reflective member 38 toward the phototransistor PTr1 recovers to a higher level. As a result, the intermediate voltage Vcalib when there is no paper and the detection voltage Voht when there is paper move toward a lower level. Therefore, it becomes difficult to determine the detection voltage Vsns, which corresponds to a light intensity that is about 20% lower in the amount of light transmitted through the OHT sheet, and the threshold Vth2 = Vcalib × 1.1 set as the second threshold ceases to function as a way to distinguish between the presence and absence of paper. In other words, a situation may arise where the sheet sensor 31 cannot respond to the OHT sheet.

[0059] Therefore, it is desirable that the absence of condensation on the reflective member 38 be detected in advance, and that the light intensity calibration be performed in that state. Furthermore, if condensation is detected, it is desirable to perform control to eliminate the condensation. The following describes condensation detection control and condensation elimination control.

[0060] <Condensation detection control> Figure 7 is a schematic diagram showing the cross-sectional configuration of the image forming apparatus of Embodiment 2. The difference between Figure 7 and Figure 1 is that Figure 7 has a memory 85 located inside the fixing device 17 (within the fixing device) that can store various setting values ​​related to the fixing device 17, the sheet sensor 31, etc. The memory 85 is an example of a memory unit that stores the second light quantity set by light quantity calibration and the detected voltage when light is irradiated from the light-emitting unit with the second light quantity as the previous value. The memory 85 can communicate with the CPU 26, and can write data to the memory 85 or read data from the memory 85 based on instructions from the CPU 26.

[0061] Figures 8(A) and 8(B) show the circuit operation of the sheet sensor 31. Figure 8(A) shows the relationship between the light intensity of the light-emitting diode LED1 and the detection voltage Vsns when there is no paper and the reflective member 38 is not condensed, which is the same as in Figure 5(A). On the other hand, Figure 8(B) shows the relationship between the light intensity of the light-emitting diode LED1 and the detection voltage Vsns when there is no paper and the reflective member 38 is condensed. In Figure 8(B), the solid line indicates the condensation state. The dashed line in Figure 8(B) is the graph of Figure 8(A).

[0062] (When condensation is present on the reflective material) As shown in Figure 8(B), when the reflective member 38 is condensed, the amount of reflected light reflected by the reflective member 38 toward the phototransistor PTr1 is reduced. Therefore, even if the amount of light output from the light-emitting unit 33 is the same, the detection voltage Vsns is relatively higher compared to the characteristics shown in Figure 8(A). As mentioned above, as the condensation on the reflective member 38 begins to dissipate over time, the amount of reflected light reflected by the reflective member 38 toward the phototransistor PTr1 recovers to a higher level. Therefore, the slope of the detection voltage Vsns operating in the linear operating region Rc recovers from the characteristics shown by the solid line toward the characteristics shown by the dashed line, i.e., toward the characteristics shown in Figure 8(A).

[0063] Here, we consider updating and writing the second light intensity Lgt2, determined during each light intensity calibration, and the intermediate voltage Vcalib at that time to memory 85 using the CPU 26. When the CPU 26 performs light intensity calibration, it causes the light-emitting diode LED1 to emit light at the second light intensity Lgt2 determined in the previous light intensity calibration. Here, the detected voltage Vsns obtained from the reflective material 38, which is in a condensed state, is denoted as Vmoi. As the condensation begins to dissipate, the difference between the detected voltage Vmoi and the intermediate voltage Vcalib determined in the previous light intensity calibration increases. Let the difference be ΔV (=|Vmoi-Vcalib|).

[0064] If the CPU 26 determines that this deviation value ΔV corresponds to, for example, Vmoi ≥ Vcalib × 1.1, it determines that there is a high probability that condensation is present on the reflective member 38. The value "1.1" may be determined according to the second threshold Vth2. The CPU 26 then samples the value of the detected voltage Vsns a predetermined number of times and evaluates its stability. Under these conditions, where the phototransistor PTr1 is operating at low light levels, if condensation is present on the reflective member 38, the condensation will begin to dissipate over time, and the value of the detected voltage Vsns will fluctuate significantly. Therefore, the CPU 26 can determine whether or not condensation is present. For example, if the CPU 26 determines that the reflective member 38 is not condensed if the variation in the value of the detected voltage Vsns sampled 10 times is stable within ±5%, it determines that the reflective member 38 is condensed if it exceeds ±5%. The CPU 26 then performs condensation dissipation control or standby control, as described later, to confirm that there is no condensation on the reflective member 38. Specifically, after condensation elimination control or standby control, the CPU 26 samples the detected voltage Vsns and confirms that the variation in the detected voltage Vsns is within ±5%. Then, the CPU 26 performs light intensity calibration again and updates memory 85 with the second light intensity Lgt2 and the intermediate voltage Vcalib at that time. The number of sampling cycles is not limited to 10. Furthermore, the variation range of the detected voltage Vsns used to determine the presence or absence of condensation is not limited to ±5%.

[0065] CPU26 is an example of a condensation detection unit that detects the condensation state of the reflective surface. CPU26 is also an example of a control unit that, when the sheet type determination unit determines that the sheet type is a transparent sheet, and the condensation detection unit detects the condensation state of the reflective surface, operates a condensation elimination unit to eliminate the condensation state. The condensation elimination unit will be described later. As a condensation detection unit, CPU26 evaluates the time-dependent fluctuation of the detected voltage when light is irradiated from the light-emitting unit with a light intensity in the second region, when there is no sheet in the optical path from the light-emitting unit to the light-receiving unit, and when the image formation operation is not being performed. As a result, CPU26 as a control unit detects the condensation state of the reflective surface.

[0066] The CPU26 also serves as an example of a condensation detection unit that evaluates whether the difference between the detected voltage obtained when light is irradiated from the light-emitting unit with the second light intensity set by this light intensity calibration and the previously stored detected voltage in the memory unit is greater than a predetermined value. In this way, the CPU26, as a condensation detection unit, detects the condensation state of the reflective surface.

[0067] <Condensation control> Furthermore, Figure 7 differs from Figure 1 in that the image forming apparatus 100 has a blower unit 32. The blower unit 32 is an example of a condensation elimination unit that eliminates condensation. The blower unit 32 is located outside the fixing device 17 and has the function of blowing air through the ventilation passage inside the fixing device 17 and blowing air toward the sheet sensor 31. Figure 9 is a diagram showing the ventilation passage of the sheet sensor 31.

[0068] The blower unit 32 has a fan 32a that blows out or sucks out air, and a motor (not shown) that drives the fan 32a. Fan 32a is an example of a blower that blows air toward the reflector. The blower unit 32, as a condensation removal unit, eliminates condensation on the reflector by blowing air from the blower.

[0069] In Figure 9, the white arrows indicate the airflow. The exhaust guide 39 guides the air blown out from the blower unit 32 to the first guide 36. The exhaust guide 39 and the first guide 36 form an air passage 40. As shown in Figure 9, the substrate 35 is placed inside the air passage 40. In addition, a gap Sp1 is provided between the first member 41 of the first guide 36 and the light-emitting part 33 for the air entering from the exhaust guide 39 to pass through. The air that has passed through this gap Sp1 is guided to the reflective member 38 through notches 41b and 44b by a wall 47a that forms part of the light-shielding member 47, which has a trapezoidal cross-sectional shape.

[0070] By blowing air onto the reflective member 38, for example, by controlling the heater 30 in the fixing device 17 to a predetermined temperature and directing the air flowing from the blowing unit 32 to the reflective member 38, the condensation on the reflective member 38 can be eliminated through warm-up control. As an example of condensation elimination control, the following control is also possible. For example, a standby control can be considered in which the image forming apparatus 100 is put into standby mode using the counter function in the CPU 26 until a predetermined time, i.e., the time required for the condensation to be sufficiently eliminated, is counted from the state in which condensation is detected.

[0071] The counter function of CPU 26 is an example of a counting unit that starts counting from the moment the condensation detection unit detects condensation and counts the elapsed time. CPU 26 is an example of a condensation removal unit that waits until the condensation on the reflective part is resolved by making the image forming operation wait until the counting unit has finished counting for a predetermined time, and then resolves the condensation. In this way, when CPU 26 detects the condensation state of the reflective member 38, it executes condensation removal control, such as warm-up control or standby control.

[0072] <Control flowchart for seat sensor> Figure 10 is a flowchart showing the control of the sheet sensor 31, and the control of Embodiment 2 will be explained based on this flowchart. Note that the same steps as those described in Figure 6 are given the same step numbers, and their explanations are omitted.

[0073] In S106, the CPU 26 compares the detected voltage Vsns obtained from the current (S101) light intensity calibration with the detected voltage Vsns obtained from the previous light intensity calibration stored in memory 85. The CPU 26 determines whether the current detected voltage Vsns is 1.1 times or more the previous detected voltage Vsns, or in other words, whether the deviation value ΔV exceeds a predetermined value (also known as a high degree of deviation).

[0074] In S106, if the CPU 26 determines that the current detected voltage Vsns is 1.1 times or more than the previous detected voltage Vsns, i.e., that the deviation is high, it proceeds to S107. In S107, the CPU 26 samples the value of the detected voltage Vsns 10 times and determines whether the variation of these detected voltages Vsns is stable within ±5%. In S107, if the CPU 26 determines that the variation of the value of the detected voltage Vsns is not stable within ±5%, it determines that condensation has occurred on the reflective member 38 and proceeds to S108.

[0075] In S108, the CPU 26 determines whether the print job is in a mode that transports the OHT sheet. If the CPU 26 determines in S108 that it is in a mode that transports the OHT sheet, it proceeds to S109. In this case, the second light quantity Lgt2 and second threshold Vth2 set for the condensation state may reduce the accuracy of detecting the arrival or presence of the OHT sheet. In S109, the CPU 26 performs condensation elimination control to eliminate the condensation on the reflective member 38. Here, as condensation elimination control, the CPU 26 operates the aforementioned blower unit 32 to perform warm-up control or standby control. As a result, the CPU 26 eliminates or avoids condensation and returns to processing S101.

[0076] After performing condensation elimination control in this manner, the CPU 26 again performs light intensity calibration in S101 to calculate the second light intensity. The CPU 26 determines that the variation in the detected voltage Vsns values, which have been sampled 10 times, has stabilized within ±5%, and updates the memory 85 with the second light intensity and detected voltage Vsns obtained from the light intensity calibration (S110).

[0077] In S106, if the CPU 26 determines that the current detection voltage Vsns is not 1.1 times or more the previous detection voltage Vsns, i.e., the degree of deviation is low, it proceeds to S110. In S107, if the CPU 26 determines that the variation in the value of the detection voltage Vsns is stable within ±5%, it determines that the reflective member 38 is not condensed and proceeds to S110. In S110, the CPU 26 stores the second light quantity Lgt2 (light quantity adjustment value) obtained in S101 in memory 85. At this time, the CPU 26 also stores the detection voltage Vsns (in this case, the intermediate voltage Vcalib) corresponding to the second light quantity Lgt2 in memory 85. In S108, if the CPU 26 determines that it is not in a mode to transport the OHT sheet, it proceeds to S103.

[0078] Thus, even when attempting to print an OHT sheet immediately after condensation occurs on the surface of the reflective member 38 due to continuous printing of moisture-absorbing plain paper, the sheet sensor 31 can be made compatible with sheet transport.

[0079] As described above, according to Example 2, even when transporting sheets with different levels of transparency, it is possible to detect the sheet being transported with an inexpensive configuration, without depending on the transparency of the sheet.

[0080] <Other Embodiments> The present invention can also be realized by supplying a program that implements one or more of the functions of the above embodiments to a system or device via a network or storage medium, and by having one or more processors in the computer of that system or device read and execute the program. It can also be realized by a circuit (e.g., an ASIC) that implements one or more functions.

[0081] This embodiment includes the following configuration. (Composition 1) An image forming apparatus configured to perform an image forming operation to form an image on a sheet, A light-emitting unit that shines light across the transport path that carries the sheets, A light receiving unit receives light emitted from the light-emitting unit and outputs a voltage corresponding to the received light as a detection voltage, A control unit that determines the presence or absence of a sheet, or the arrival of the leading edge or the arrival of the trailing edge, by comparing the detected voltage with a threshold value, A drive unit that drives the light-emitting unit based on the light intensity set by the control unit, A sheet type determination unit that determines the type of sheet to be transported along the transport path, Equipped with, When the amount of light emitted by the light-emitting unit is changed to increase the amount of light, the region in which the detection voltage does not change is defined as the first region, and the region in which the amount of light is lower than that of the first region and the detection voltage changes linearly in accordance with the amount of light is defined as the second region, The control unit, If the sheet type determination unit determines that the sheet type is not a transparent sheet, the light intensity is set to the first light intensity within the first region and the threshold is set to the first threshold. If the sheet type determination unit determines that the sheet type is a transparent sheet, the light intensity is set to a second light intensity within the second region that is lower than the first light intensity, and the threshold is set to a second threshold. An image forming apparatus characterized by the following: (Configuration 2) When the amount of light emitted by the light-emitting unit is changed to increase the amount of light, the region in which the detection voltage does not change is defined as the first region, and the region in which the amount of light is lower than that of the first region and the detection voltage changes linearly in accordance with the amount of light is defined as the second region, The first light quantity is the light quantity within the first region, The aforementioned second light quantity is the light quantity within the aforementioned second region. The image forming apparatus according to configuration 1, characterized in that it is a picture forming apparatus. (Composition 3) Equipped with a fixing device that fixes toner images onto a sheet, The light-emitting unit and the light-receiving unit are provided within the fixing device. An image forming apparatus according to configuration 1 or configuration 2, characterized by the above. (Composition 4) A fixing device that fixes toner images onto a sheet, A discharge unit for discharging the sheet that has passed through the fixing device to the outside of the device, Equipped with, The light-emitting unit and the light-receiving unit are provided between the fixing device and the discharge unit. An image forming apparatus according to configuration 1 or configuration 2, characterized by the above. (Composition 5) The light-emitting unit and the light-receiving unit are provided on the same side with respect to the sheet being transported. A reflecting portion is provided opposite the light-emitting portion and reflects the light emitted from the light-emitting portion toward the light-receiving portion. An image forming apparatus according to any one of configurations 1 to 4, characterized by the above. (Composition 6) The sheet type determination unit is a sensor that determines the type of sheet being transported along the transport path. An image forming apparatus according to any one of configurations 1 to 5, characterized by the above. (Composition 7) It features a user interface for inputting various specifications from the user. The sheet type determination unit determines the type of the sheet based on the information input by the user interface. An image forming apparatus according to any one of configurations 1 to 5, characterized by the above. (Composition 8) The control unit performs light intensity calibration, which is an operation to set the second light intensity, at a time other than when the image forming operation is performed, and when there is no sheet in the optical path from the light emitting unit to the light receiving unit. The image forming apparatus according to configuration 5, characterized by the features described herein. (Composition 9) A condensation detection unit for detecting the condensation state of the reflective surface, A condensation-eliminating unit for eliminating the aforementioned condensation state, Equipped with, The control unit operates the condensation elimination unit to eliminate the condensation when the sheet type determination unit determines that the sheet is a transparent sheet, and the condensation detection unit detects the condensation state on the reflective surface. The image forming apparatus according to configuration 8, characterized by the above. (Composition 10) When the amount of light emitted by the light-emitting unit is changed to increase the amount of light, the region in which the detection voltage does not change is defined as the first region, and the region in which the amount of light is lower than that of the first region and the detection voltage changes linearly in accordance with the amount of light is defined as the second region, The condensation detection unit detects the condensation state of the reflecting portion by evaluating the change in the detection voltage over time when light is irradiated from the light-emitting portion with the light intensity in the second region, at a time other than when the image forming operation is performed, and when there is no sheet in the optical path from the light-emitting portion to the light-receiving portion. The image forming apparatus according to configuration 9, characterized by the features described therein. (Composition 11) The system includes a storage unit that stores the second light intensity set by the light intensity calibration and the detection voltage when light is irradiated from the light-emitting unit with the second light intensity as previous values. The condensation detection unit detects the condensation state of the reflecting part by evaluating whether the difference between the detection voltage obtained when light is irradiated from the light-emitting part with the second light intensity set by the current light intensity calibration and the detection voltage of the previous value stored in the memory unit is greater than a predetermined value. The image forming apparatus according to configuration 9, characterized by the features described therein. (Composition 12) Equipped with a fixing device that fixes toner images onto a sheet, The storage unit is provided within the fixing device. The image forming apparatus according to configuration 11, characterized by the features described above. (Composition 13) The condensation elimination unit has a blower that blows air toward the reflecting unit, and the condensation on the reflecting unit is eliminated by the air blown by the blower. An image forming apparatus according to any one of configurations 9 to 12, characterized by the features described herein. (Composition 14) The condensation elimination unit has a counting unit that starts counting from the moment the condensation detection unit detects condensation and counts the elapsed time, and waits until the image forming operation is stopped by the counting unit until the counting of a predetermined time is completed, thereby waiting until the condensation state of the reflective part is eliminated. An image forming apparatus according to any one of configurations 9 to 12, characterized by the features described herein. (Composition 15) When the amount of light emitted by the light-emitting unit is defined as follows: the region in which the detection voltage does not change even when the amount of light is increased is defined as the first region; the region in which the amount of light is lower than that of the first region and the detection voltage changes linearly in accordance with the amount of light is defined as the second region; and the region in which the amount of light is lower than that of the second region and the detection voltage does not change even when the amount of light is decreased is defined as the third region, The control unit, The first threshold is set based on the detection voltage obtained when light is irradiated from the light-emitting unit with the light intensity within the third region. The second threshold is set based on the detection voltage obtained when light is irradiated from the light-emitting unit with the second light intensity. An image forming apparatus according to any one of configurations 1 to 14, characterized by the above. [Explanation of Symbols]

[0082] 26 CPU 33 Light-emitting part 34 Light receiving part 70 Media Sensors 133 Drive Circuit

Claims

1. An image forming apparatus configured to perform an image forming operation to form an image on a sheet, A light-emitting unit that shines light across the transport path that carries the sheets, A light receiving unit receives light emitted from the light-emitting unit and outputs a voltage corresponding to the received light as a detection voltage, A control unit that determines the presence or absence of a sheet, or the arrival of the leading edge or the arrival of the trailing edge, by comparing the detected voltage with a threshold value, A drive unit that drives the light-emitting unit based on the light intensity set by the control unit, A sheet type determination unit that determines the type of sheet to be transported along the transport path, Equipped with, The control unit, If the sheet type determination unit determines that the sheet type is not a transparent sheet, the light intensity is set to the first light intensity and the threshold is set to the first threshold. If the sheet type determination unit determines that the sheet type is a transparent sheet, the light intensity is set to a second light intensity lower than the first light intensity, and the threshold is set to a second threshold. An image forming apparatus characterized by the following:

2. When the amount of light emitted by the light-emitting unit is changed to increase the amount of light, the region in which the detection voltage does not change is defined as the first region, and the region in which the amount of light is lower than that of the first region and the detection voltage changes linearly in accordance with the amount of light is defined as the second region, The first light quantity is the light quantity within the first region, The aforementioned second light quantity is the light quantity within the aforementioned second region. The image forming apparatus according to feature 1.

3. Equipped with a fixing device that fixes toner images onto a sheet, The light-emitting unit and the light-receiving unit are provided within the fixing device. The image forming apparatus according to feature 1.

4. A fixing device that fixes toner images onto a sheet, A discharge unit for discharging the sheet that has passed through the fixing device to the outside of the device, Equipped with, The light-emitting unit and the light-receiving unit are provided between the fixing device and the discharge unit. The image forming apparatus according to feature 1.

5. The light-emitting unit and the light-receiving unit are provided on the same side with respect to the sheet being transported. A reflecting portion is provided opposite the light-emitting portion and reflects the light emitted from the light-emitting portion toward the light-receiving portion. The image forming apparatus according to feature 1.

6. The sheet type determination unit is a sensor that determines the type of sheet being transported along the transport path. The image forming apparatus according to feature 1.

7. It features a user interface for inputting various specifications from the user. The sheet type determination unit determines the type of the sheet based on the information input by the user interface. The image forming apparatus according to feature 1.

8. The control unit performs light intensity calibration, which is an operation to set the second light intensity, at a time other than when the image forming operation is performed, and when there is no sheet in the optical path from the light emitting unit to the light receiving unit. The image forming apparatus according to feature 5.

9. A condensation detection unit for detecting the condensation state of the reflective surface, A condensation-eliminating unit for eliminating the aforementioned condensation state, Equipped with, The control unit operates the condensation elimination unit to eliminate the condensation when the sheet type determination unit determines that the sheet is a transparent sheet, and the condensation detection unit detects the condensation state on the reflective surface. The image forming apparatus according to feature 8.

10. When the amount of light emitted by the light-emitting unit is changed to increase the amount of light, the region in which the detection voltage does not change is defined as the first region, and the region in which the amount of light is lower than that of the first region and the detection voltage changes linearly in accordance with the amount of light is defined as the second region, The condensation detection unit detects the condensation state of the reflecting portion by evaluating the change in the detection voltage over time when light is irradiated from the light-emitting portion with the light intensity in the second region, at a time other than when the image forming operation is performed, and when there is no sheet in the optical path from the light-emitting portion to the light-receiving portion. The image forming apparatus according to feature 9.

11. The system includes a storage unit that stores the second light intensity set by the light intensity calibration and the detection voltage when light is irradiated from the light-emitting unit with the second light intensity as previous values. The condensation detection unit detects the condensation state of the reflecting part by evaluating whether the difference between the detection voltage obtained when light is irradiated from the light-emitting part with the second light intensity set by the current light intensity calibration and the detection voltage of the previous value stored in the memory unit is greater than a predetermined value. The image forming apparatus according to feature 9.

12. Equipped with a fixing device that fixes toner images onto a sheet, The storage unit is provided within the fixing device. The image forming apparatus according to feature 11.

13. The condensation elimination unit has a blower that blows air toward the reflecting unit, and the condensation on the reflecting unit is eliminated by the air blown by the blower. The image forming apparatus according to feature 9.

14. The condensation elimination unit has a counting unit that starts counting from the moment the condensation detection unit detects condensation and counts the elapsed time, and waits until the image forming operation is stopped by the counting unit until the counting of a predetermined time is completed, thereby waiting until the condensation state of the reflective part is eliminated. The image forming apparatus according to feature 9.

15. When the amount of light emitted by the light-emitting unit is defined as follows: the region in which the detection voltage does not change even when the amount of light is increased is defined as the first region; the region in which the amount of light is lower than that of the first region and the detection voltage changes linearly in accordance with the amount of light is defined as the second region; and the region in which the amount of light is lower than that of the second region and the detection voltage does not change even when the amount of light is decreased is defined as the third region, The control unit, The first threshold is set based on the detection voltage obtained when light is irradiated from the light-emitting unit with the light intensity within the third region. The second threshold is set based on the detection voltage obtained when light is irradiated from the light-emitting unit with the second light intensity. The image forming apparatus according to feature 1.

Images