JP2008254919A - Paper conveying apparatus, image forming apparatus, threshold setting method, amplification factor adjusting method, program, recording medium - Google Patents

Abstract

[PROBLEMS] To reduce the labor of a user.
An image forming apparatus includes: a paper transport path; an ultrasonic receiver that outputs a signal having a lower intensity as the paper is thicker when the paper is transported to the paper transport path; and an intensity of the signal. And a comparison circuit 125 that determines whether or not double feeding is occurring in the paper transport path 31 by comparing the value indicating the above and a threshold value. Further, the image forming apparatus 11 includes a first signal output from the comparison circuit 125 when two sheets of plain paper are stacked and simultaneously transported in the paper transport path 31, and is thicker than the plain paper in the paper transport path 31. When the second signal output from the comparison circuit 125 is acquired by conveying the cardboard, the signal value B indicating the intensity of the first signal and the signal value A indicating the intensity of the second signal are between. A threshold value setting circuit 124 is provided for setting an intermediate value as the threshold value.
[Selection] Figure 12

Description

  The present invention relates to a paper transport device provided with a double feed detection device, and an image forming apparatus provided with the paper transport device.

  In a paper conveyance device provided in an image forming apparatus such as a printer, a copier, or a multifunction peripheral, the paper is conveyed one by one, but two or more sheets are overlapped and conveyed simultaneously due to a malfunction. A state (hereinafter, this state is referred to as “double feeding”) may occur. In particular, when double feeding occurs on the upstream side of the printing unit in the image forming apparatus, printing failure occurs.

  In view of this, conventionally, in a sheet conveying device provided in an image forming apparatus, a double feed detection device (double feed detection sensor) for determining whether or not double feed has occurred is provided, and double feed is generated by the double feed detection device. If it is determined that the print processing is determined, control for canceling the print processing is performed in advance. In addition, various devices have been known as the above-described multifeed detection device, but recently, an ultrasonic multifeed detection device is frequently used. In the following, the principle of the ultrasonic multifeed detection device will be described.

  FIG. 1A is a diagram illustrating a state in which a sheet is normally conveyed in the sheet conveyance path, and FIG. 1B is a diagram illustrating a state in which double feeding is occurring in the sheet conveyance path. It is. As shown in FIG. 1 (a) or FIG. 1 (b), an ultrasonic multifeed detection device 500 includes an ultrasonic transmitter 510 and an ultrasonic receiver that are disposed to face each other with the paper transport path S interposed therebetween. And a device 520. The ultrasonic transmitter 510 transmits an ultrasonic wave toward the ultrasonic receiver 520 at the timing when the paper P is conveyed between the ultrasonic transmitter 510 and the ultrasonic receiver 520. The ultrasonic wave thus transmitted is attenuated by passing through the paper P being conveyed, and this attenuated ultrasonic wave is received by the ultrasonic receiver 520, and the ultrasonic receiver 520 responds to the received ultrasonic wave. Output the signal. The intensity of the signal output from the ultrasonic receiver 520 increases as the intensity of the received ultrasonic wave increases, and decreases as the intensity of the received ultrasonic wave decreases.

  Furthermore, since the degree of attenuation increases as the transported paper is thicker, the intensity of the signal output from the ultrasonic receiver 520 decreases as the transported paper is thicker. Therefore, when double feeding occurs in the paper conveyance path S as shown in FIG. 1B, the paper is normally conveyed in the paper conveyance path S as shown in FIG. 1A (one sheet). The attenuation is greatly increased, and the intensity of the ultrasonic wave received by the ultrasonic receiver 520 is greatly reduced, and the output from the ultrasonic receiver 520 is output. The strength of the transmitted signal is also greatly reduced.

Therefore, when the signal value of the output signal of the ultrasonic receiver 520 is larger than the threshold value, it is determined that the paper is normally conveyed in the paper conveyance path S, and the signal value of the output signal of the ultrasonic receiver 520 is the threshold value. If it is less than, it is possible to detect that double feeding has occurred if it is determined that double feeding has occurred in the paper transport path S.
Japanese Patent Laid-Open No. 2-110053 (Publication date: April 23, 1990) JP-A-1-184490 (Publication date: July 24, 1991)

  By the way, regarding the threshold for detecting double feed, conventionally known threshold setting methods include the signal value a of the output signal of the ultrasonic receiver 520 obtained when the test paper is normally conveyed, The signal value b of the output signal of the ultrasonic receiver 520 obtained when two sheets of the same test paper as the test paper are transported at the same time is actually measured, and the intermediate between the signal value a and the signal value b A method of setting a value as a threshold is known. For example, in Patent Document 1, although it is not an ultrasonic double feed detection device but an optical double feed detection device, the output value c of the optical sensor obtained when the test paper is normally conveyed, and this Actually measure the output value d of the optical sensor obtained when two test sheets that are the same as the test sheet are stacked and conveyed simultaneously, and the intermediate value between the output value c and the output value d is double-feeded. A method of setting as a threshold value for detection is disclosed.

  Here, in the image forming apparatus, when only the paper having the same thickness as the test paper is used, the threshold setting method described above is sufficient, but when papers having various thicknesses are used, Each time the thickness is changed, the threshold value must be reset by the threshold value setting method, which causes a problem that the user is troublesome. This problem will be described in detail below.

  As described above, the intensity of the signal output from the ultrasonic receiver 520 not only decreases when double feeding occurs, but also decreases as the sheet conveyed in the sheet conveying path S increases. Therefore, for example, when thick paper thicker than plain paper is used in addition to plain paper in the image forming apparatus, the intensity of the signal output from the ultrasonic receiver 520 is normal, as shown in FIG. It becomes weaker in the order of normal transport of paper, normal transport of thick paper, multi-feed of plain paper, and multi-feed of thick paper.

  When the threshold value is set by the above threshold setting method using plain paper as a test paper, as shown in FIG. 2, the signal strength value during normal paper normal conveyance and the signal strength value during plain paper double feeding are used. Is set as the threshold value α, and according to the threshold value α, it is possible to discriminate between normal conveyance and double feeding of plain paper. However, depending on the thickness of the thick paper, as shown in FIG. 2, the intensity value of the output signal during normal transport of the thick paper may be smaller than the threshold value α. There is a possibility that it will happen.

  On the other hand, when the threshold value is set by the above-described threshold setting method using the thick paper as the test paper, as shown in FIG. Is set as the threshold value β, and according to this threshold value β, it is possible to discriminate between normal conveyance of thick paper and double feeding of thick paper. However, depending on the thickness of plain paper, as shown in FIG. 2, the intensity value of the output signal at the time of plain paper double feeding may be larger than the threshold value β. There is a possibility that the situation will be erroneously determined.

  Therefore, in order to avoid the above situation in the threshold setting method used conventionally, a threshold suitable for the thickness of the used paper is set by the above threshold setting method every time the thickness of the used paper is changed. There was a problem that it had to be set again and was troublesome for the user.

  The present invention has been made in view of the above problems, and an object thereof is to provide a paper transport device that reduces the labor of the user as compared with the prior art, and an image forming apparatus including the paper transport device.

  In order to achieve the above object, the present invention provides a paper transport path, output means for outputting a signal having a strength correlated with the thickness of the paper when the paper is transported to the paper transport path, A sheet including a determination unit that determines whether or not a double feed state in which two or more sheets are overlapped and simultaneously transported in the sheet transport path by comparing a value indicating the strength with a threshold value. In the transport device, a first signal output from the output means by simultaneously transporting two sheets of the first sheet overlapped in the sheet transport path, and a second signal thicker than the first sheet in the sheet transport path. When a second signal output from the output means is acquired by conveying a sheet, a first intensity value indicating the intensity of the first signal and a second intensity value indicating the intensity of the second signal are obtained. Setting value to set the value between Characterized in that it comprises a.

  If the determination unit determines whether or not the double feed is performed using the threshold set by the above configuration, if the first paper is double-fed, it is determined as “double feed” and the second paper is normal. In the case of being transported, it is determined as “non-multiple feeding”.

  Furthermore, the determination of “multiple feeding” when the first paper is double-fed means that “double feeding” is naturally determined even when the second paper thicker than the first paper is generated. Will be. Further, since “non-multiple feeding” is determined when the second sheet is normally conveyed, naturally “non-multifeed” is also obtained when the first sheet thinner than the second sheet is normally conveyed. Will be judged.

  Therefore, according to the threshold set by the setting means, it is possible to determine each of normal conveyance and double feeding of the first sheet and normal conveyance and double feeding of the second sheet thicker than the first sheet. Even if the paper used in the paper transport device is changed from the first paper to the second paper, and the second paper is changed to the first paper, there is no need to reset the threshold value, and the thickness of the paper to be used is changed. Compared to the conventional configuration in which the threshold value needs to be reset every time, the user's labor can be reduced.

  In addition to the above-described configuration, the paper transport device of the present invention is fed in a state in which a pair of paper sheets on which two first paper sheets are stacked and bonded together, and a plurality of paper sheets are stacked from the tray. A separation unit that separates one sheet from the plurality of sheets and supplies the one sheet to the sheet conveyance path; and a conveyance process that conveys the pair of sheets from the tray to the sheet conveyance path. It is preferable that the apparatus includes a conveyance control unit that controls the separation unit, and a prohibition unit that performs a control to prohibit the separation operation of the separation unit during the conveyance process.

  According to this configuration, when the process of transporting the pair of sheets from the tray to the sheet transport path is performed, the separation operation by the separating unit is prohibited, so that the first sheets of the pair of sheets are not separated from each other. In other words, the paper is supplied to the paper conveyance path. As a result, two sheets of the first sheet can be stacked and conveyed simultaneously in the sheet conveyance path.

  Furthermore, in addition to the above-described configuration, the paper transport device of the present invention is configured to place a first tray on which a pair of papers, each of which is a stack of two first papers, and a second paper are placed. The second tray, the conveyance control for performing the process of conveying the pair of sheets from the first tray to the sheet conveyance path, and the process of conveying the second sheet from the second tray to the sheet conveyance path in a predetermined order. And a means including a means. In addition to the above configuration, the sheet transport device of the present invention includes a third tray for stacking and placing a pair of sheets obtained by bonding two sheets of the first sheet and the second sheet, and the third tray. A transport control unit configured to perform a process of transporting the paper pair from the tray to the paper transport path and a process of transporting the second paper from the third tray to the paper transport path in a predetermined order. Also good.

  In the paper conveying apparatus of the present invention, in addition to the above configuration, the setting means acquires a plurality of first signals and a plurality of second signals by acquiring a plurality of first signals and a plurality of second signals. When the second intensity value is obtained, a set of the first intensity value and the second intensity value having the smallest difference is determined, and a value between the determined first intensity value and the second intensity value is determined. Is preferably set as the threshold value.

  According to this configuration, since the first intensity value and the second intensity value that are samples are increased, the reliability is higher than the configuration in which the threshold is set from the single first intensity value and the single second intensity value. A high threshold can be set.

  Furthermore, in order to achieve the above object, the paper transport device of the present invention outputs a signal having a strength correlating with the thickness of the paper when the paper is transported to the paper transport path and the paper transport path. And amplifying means for amplifying the signal output from the output means based on a predetermined amplification factor, and a value indicating the intensity of the signal after being amplified by the amplifying means and a threshold for determination And determining means for determining whether or not a double feed state in which two or more sheets are overlapped and simultaneously transported in the sheet transport path is determined in the sheet transport path. A first signal output from the output unit by transporting two sheets of paper at the same time and a second signal thicker than the first sheet in the paper transport path are output from the output unit. Second second And determining means for determining as a virtual threshold a value between a first intensity value indicating the intensity of the first signal and a second intensity value indicating the intensity of the second signal, and the virtual threshold Adjusting means for adjusting the amplification factor in accordance with the ratio of the threshold for determination to.

  According to the above configuration, by amplifying a signal handled in the multifeed determination according to a ratio of the determination threshold to the virtual threshold while making the determination threshold for determining the multifeed state constant, Even if the determination of double feeding is performed using the determination threshold value, the same determination result as that of performing determination of double feeding using the virtual threshold value is obtained.

  Therefore, even with the above-described configuration, it is possible to discriminate between normal conveyance and multi-feed of the first paper and normal conveyance and multi-feed of the second paper thicker than the first paper, which are used in the paper conveyance device. Even if the paper is changed from the first paper to the second paper, and the second paper is changed to the first paper, it is not necessary to reset the threshold value, and the threshold value is reset every time the paper to be used is changed. Compared to the conventional configuration that has been necessary, it is possible to reduce the labor of the user.

  In addition, it is preferable that the sheet conveying apparatus of the present invention is provided in an image forming apparatus such as a printer, a copier, a multifunction machine, or a facsimile machine.

  Further, the image forming apparatus provided with the sheet conveying device of the present invention further includes a fixing processing unit that thermally fixes an image to the sheet, and the plurality of first intensity values include the one after the heat fixing is performed. A value indicating the intensity of the signal output from the output unit by simultaneously conveying two first sheets to the sheet conveying path at the same time is included, and the plurality of second intensity values include the heat It is preferable that a value indicating the intensity of a signal output from the output unit when the second sheet after fixing is conveyed to the sheet conveyance path is included.

  In the image forming apparatus, when the second side of the duplex printing process is printed, the sheet after the thermal fixing process is conveyed to the sheet conveying path. According to the above configuration, the sheet after the thermal fixing process is conveyed to the sheet conveying path. It is possible to set a threshold in consideration of what is done. Further, when the output unit is configured to change the intensity of the output signal in accordance with the ambient temperature (for example, an ultrasonic sensor), the output unit may change the output unit depending on the paper after the heat fixing process conveyed to the sheet conveyance path. Although it is assumed that the ambient temperature changes and the intensity of the output signal varies, according to the above configuration, it is possible to set a threshold value that takes this variation into account.

  The present invention also provides a sheet conveying path, output means for outputting a signal having an intensity correlated with the thickness of the sheet when the sheet is conveyed to the sheet conveying path, a value indicating the intensity of the signal, and a threshold value. The threshold value is set in a sheet conveying apparatus including a determination unit that determines whether or not a double feeding state in which two or more sheets overlap and are simultaneously conveyed in the sheet conveying path And a first signal output from the output means by simultaneously transporting two first sheets of paper on the paper transport path, and the first signal on the paper transport path. When the second signal output from the output means is acquired by conveying a second sheet thicker than the sheet, the first intensity value indicating the intensity of the first signal and the intensity of the second signal are indicated. A value between a second intensity value and the threshold Characterized in that it comprises a setting step of setting as.

  Furthermore, the present invention provides a paper conveyance path, an output means for outputting a signal having an intensity correlated with the thickness of the paper when the paper is conveyed to the paper conveyance path, and the output based on a predetermined amplification factor. Amplifying means for amplifying the signal output from the means, and comparing the value indicating the intensity of the signal after being amplified by the amplifying means with a threshold value for determination, so that two or more sheets in the paper conveying path In a gain adjustment method for adjusting the gain in a paper transport device including a determination unit that determines whether or not a double feed state in which papers are overlapped and transported at the same time occurs in the paper transport path. A first signal output from the output unit by simultaneously transporting two sheets of paper one on top of the other, and an output from the output unit by transporting a second sheet thicker than the first sheet in the paper transport path. Is When the second signal is acquired, a determination step of determining, as a virtual threshold value, a value of a first intensity value indicating the intensity of the first signal and a second intensity value indicating the intensity of the second signal; And an adjustment step of adjusting the amplification factor in accordance with a ratio of the determination threshold value to the threshold value.

  Further, the above threshold value setting method or amplification factor adjusting method can be realized by causing a computer to execute a program, and the program may be recorded on a computer-readable recording medium.

  In order to achieve the above object, the present invention provides a paper transport path, output means for outputting a signal having a strength correlated with the thickness of the paper when the paper is transported to the paper transport path, A sheet including a determination unit that determines whether or not a double feed state in which two or more sheets are overlapped and simultaneously transported in the sheet transport path by comparing a value indicating the strength with a threshold value. In the transport device, a first signal output from the output means by simultaneously transporting two sheets of the first sheet overlapped in the sheet transport path, and a second signal thicker than the first sheet in the sheet transport path. When a second signal output from the output means is acquired by conveying a sheet, a first intensity value indicating the intensity of the first signal and a second intensity value indicating the intensity of the second signal are obtained. Setting value to set the value between Characterized in that it comprises a.

  Thereby, it is possible to reduce the labor of the user as compared with the conventional configuration in which the threshold value needs to be reset every time the thickness of the paper to be used is changed.

  In order to achieve the above object, the present invention provides a sheet conveying path, an output means for outputting a signal having an intensity correlated with the thickness of the sheet when the sheet is conveyed to the sheet conveying path, The amplification means for amplifying the signal output from the output means based on the amplification factor, and by comparing a threshold value for determination with a value indicating the intensity of the signal after being amplified by the amplification means, And a determination unit that determines whether or not a double-feed state in which two or more sheets are overlapped at the same time in the sheet conveyance path is determined. A first signal output from the output unit by transporting the sheets simultaneously and a second signal output from the output unit by transporting a second sheet thicker than the first sheet in the sheet transport path. Signal and get Determining means for determining a value between a first intensity value indicating the intensity of the first signal and a second intensity value indicating the intensity of the second signal as a virtual threshold; and for determining the virtual threshold Adjusting means for adjusting the amplification factor in accordance with the ratio of the threshold value.

  Therefore, it is possible to reduce the labor of the user as compared with the conventional configuration in which the threshold value needs to be reset every time the thickness of the paper to be used is changed.

  In the following, an embodiment of the sheet conveying apparatus of the present invention will be described with reference to the drawings. FIG. 3 is a diagram illustrating an overall configuration of the image forming apparatus 11 including the sheet conveying device of the present embodiment.

  The image forming apparatus 11 is an electrophotographic printer that forms a monochrome image (single color and black image) on a sheet based on digital image data received from an external terminal or the scanner 1 installed on the image forming apparatus 11. .

  As shown in FIG. 3, the image forming apparatus 11 includes an exposure unit 400 including the exposure unit 13 and a paper transport unit (paper transport device) 500 disposed below the exposure unit. The paper transport unit 500 includes a developing unit 15, a photosensitive drum 17, a charger 19, a cleaner unit 21, a fixing unit 23, paper feed trays 25 a, 25 b, and 25 c, registration rollers 29, a paper transport path 31, and a refeed transport path. 83, a reverse conveyance path 99, a paper discharge tray 33, a pickup roller 70, a separating roller 71, and the like.

  Note that the sheet conveyance path 31 includes a junction C (a connection point between the sheet conveyance path 31 and the refeed conveyance path 83), the image transfer unit 47, and the fixing processing unit 66 from each of the sheet feeding trays 25a, 25b, and 25c. Then, it is formed in a range reaching the paper discharge tray 33. The re-feed conveyance path 83 is a conveyance path branched from a branch point T located between the fixing unit 23 and the paper discharge tray 33 in the paper conveyance path 31, and extends from the branch point T to the junction C. It is the conveyance path currently formed in this.

  The charger 19 is a charging means for uniformly charging the surface of the photosensitive drum 17 to a predetermined potential. In the image forming apparatus 11 shown in FIG. 3, a charger-type charger 19 is used. A type charger or a brush type charger may be used.

  The exposure unit 13 is a laser scanning unit (LSU) including a laser irradiation unit 35 and a reflection mirror 37 as shown in FIG. 3, but is not limited to LSU. For example, the light emitting elements are arranged in an array. The exposure unit may be an EL or LED writing head.

  The LSU may be either a one-beam LSU, a two-beam LSU for high-speed printing, or a four-beam LSU. In this embodiment, two lasers are used as shown in FIG. A two-beam LSU in which the irradiation units 35 and 35 are configured is employed.

  The exposure unit 13 exposes the photosensitive drum 17 uniformly charged by the charger 19 based on the input image data, so that the electrostatic latent image corresponding to the input image data is formed on the surface of the photosensitive drum 17. Is formed.

  The developing device 15 visualizes (develops) the electrostatic latent image formed on the photosensitive drum 17 with toner. The cleaner unit 21 removes and collects toner remaining on the surface of the photosensitive drum 17 after development and image transfer.

  The toner image (image) visualized on the photosensitive drum 17 is transferred onto the paper in the image transfer unit 47. The transfer belt unit 39 for performing the transfer is applied with an electric field having a polarity opposite to that of the charge on the toner on the photosensitive drum 17, and the toner on the photosensitive drum 17 is applied to the sheet by the electric field. Is transcribed. For example, when the toner on the photosensitive drum 17 has a negative polarity charge, the polarity of the electric field applied to the transfer belt unit 39 becomes a positive polarity.

  The transfer belt unit 39 includes a drive roller 41, a driven roller 43, an elastic conductive roller 49, a roller 96, and a transfer belt 45 that spans these rollers 41, 43, 49, and 96.

The transfer belt 45 is a belt member having a volume resistance value in the range of 1 × 10 ⁹ Ω · cm to 1 × 10 ¹³ Ω · cm.

  An elastic conductive roller 49 for applying a transfer electric field is disposed in the vicinity of the image transfer portion 47, which is a region where the photosensitive drum 17 and the transfer belt 45 are in contact with each other. The elastic conductive roller 49 has elasticity. As a result, the photosensitive drum 17 and the transfer belt 45 are not in line contact but in surface contact with each other with a predetermined width (referred to as a transfer nip). As a result, the transfer efficiency to the conveyed paper can be improved.

  Further, on the downstream side of the image transfer section 47 of the transfer belt 45, a charge eliminating roller is used to neutralize the sheet charged by the voltage applied when passing through the image transfer section 47 and smoothly carry it to the next process. 51 is arranged. The neutralizing roller 51 is disposed on the back surface of the transfer belt 45.

  Further, the transfer belt unit 39 is provided with a cleaning unit 53 that removes toner stains on the transfer belt 45 and a static elimination mechanism 55 that neutralizes the transfer belt 45. As a charge removal method by the charge removal mechanism 55, there is a method in which the transfer belt 45 is grounded through an apparatus, or a method in which an electric field having a polarity opposite to the polarity of the transfer electric field is positively applied to the transfer belt 45. The sheet on which the toner image (image) is transferred by the transfer belt unit 39 is conveyed to the fixing unit 23.

  The fixing unit 23 includes a heating roller 57 and a pressure roller 59, and a sheet peeling claw 61, a thermistor 63 (roller surface temperature detection member), and a roller surface cleaning member 65 are disposed on the outer periphery of the heating roller 57. . A heat source 67 for heating the surface of the roller to a predetermined temperature (fixing set temperature: approximately 160 to 200 ° C.) is disposed inside the heating roller 57.

  At both ends of the pressure roller 59, a mechanism such as a load spring is configured, and the pressure roller 59 is pressed against the heating roller 57 with a predetermined load by this mechanism. Further, similarly to the outer periphery of the heating roller 57, a sheet peeling claw and a roller surface cleaning member are disposed on the outer periphery of the pressure roller 59.

  The fixing unit 23 has a fixing processing unit (also referred to as a fixing nip unit) 66 that is a pressure contact portion between the heating roller 57 and the pressure roller 59. The unfixed toner image is thermally fixed on the paper.

  The paper feed trays 25a, 25b, and 25c are trays for storing sheets (recording paper) used for printing. In the image forming apparatus 11 of the present embodiment, the photosensitive drum 17, the transfer belt unit 39, and the like. Is provided below the image forming unit and in the vicinity of the side wall of the apparatus. In the image forming apparatus 11 of the present embodiment, a plurality of paper feed trays 25 capable of storing 500 to 1500 fixed-size sheets are arranged so that continuous printing can be performed even for an extremely large amount of sheets. ing.

  Further, on the side of the apparatus, there is a large-capacity paper feed cassette 73 that can store a plurality of different types of paper in large quantities, and a manual feed tray 75 that is mainly used for printing on irregular-size paper. Has been placed.

  The paper discharge tray 33 is disposed on the side of the apparatus opposite to the manual feed tray 75. However, the paper discharge tray 33 is removed, and a post-processing device (stapling, punching, etc.) of discharged paper or a multi-stage paper discharge is performed. The tray can be arranged as an option.

  Next, a paper conveyance path when printing processing is performed in the image forming apparatus 11 will be described.

  When a print request (print request command) is input to the image forming apparatus 11, the paper is transported by the pick-up roller 70 and the separation roller 71 in the paper feed tray 25 in which the paper conforming to the print request is stored. Supplied to the channel 31. Then, the sheet is conveyed to the registration roller 29 disposed upstream of the image transfer unit 47 in the sheet conveyance path 31 via the junction C, and is temporarily stopped.

  Further, the registration roller 29 is rotated again at a timing such that the leading edge of the sheet coincides with the leading edge of the toner image (image) on the photosensitive drum 17 in the image transfer unit 47, and the sheet is conveyed to the image transfer unit 47. . As a result, the toner image on the photosensitive drum 17 is transferred to the surface (front surface) of the sheet in the image transfer unit 47, and the sheet is guided to the fixing unit 23 and transferred to the sheet in the fixing unit 23. The toner is fixed (thermally fixed) on the paper.

  Thereafter, the paper transport path is switched according to the single-sided printing mode / double-sided mode. More specifically, in the single-sided printing mode, the gate of the transport path switching mechanism 80 in FIG. 3 is turned so that the paper discharged from the fixing unit 23 is reversed before being guided to the paper discharge tray 33. The direction is switched. In the double-sided printing mode, the direction of the gate of the transport path switching mechanism 80 is switched so that the paper discharged from the fixing unit 23 is turned upside down and then the paper is sent to the refeed transport path 83. . Then, the sheet sent to the sheet refeed conveyance path 83 passes through the sheet refeed conveyance path 83 and is sent again to the sheet conveyance path 31 via the junction C. Thereafter, the sheet is conveyed again to the image transfer unit 47, and the image transfer unit 47 transfers the image to the back surface. Thereafter, the sheet is discharged to the discharge tray 33 through the fixing unit 23.

  As described above, in the image forming apparatus 11, the printing process is performed by performing both the image transfer process in the image transfer unit 47 and the fixing process in the fixing process unit 66. In other words, the printing process includes an image transfer process and a fixing process, and the image transfer unit 47 and the fixing processing unit 66 may be collectively referred to as a printing unit.

  Further, in the image forming apparatus 11 of the present embodiment, in addition to the various configurations described above, as illustrated in FIG. 3, a double feed detection sensor DD is configured on the upstream side of the printing unit of the paper transport path 31. Thus, it is possible to detect the double feed occurring on the upstream side, and when this double feed occurs, the printing process is stopped. Hereinafter, the details of the image forming apparatus 11 will be described focusing on the double feed detection sensor DD.

  FIG. 4 is a diagram showing a detailed configuration of the paper transport path 31. As shown in FIGS. 3 and 4, the sheet conveyance path 31 is located between the junction C and the printing unit (the image transfer unit 47 and the fixing processing unit 66) from the upstream side to the downstream side in the sheet conveyance direction. The sheet detection sensor PD1, the double feed detection sensor DD, the sheet detection sensor PD2, and the registration roller 29 are arranged in this order.

  The sheet detection sensor PD1 is a contact sensor provided between the merging point C and the double feed detection sensor DD, and passes through the merging point C to the front end of the sheet before reaching the double feed detection sensor DD. It is a sensor that detects the leading edge of the sheet by contact and transmits a sheet detection signal indicating that this detection has been performed to the control unit 100 (see FIG. 5) described later.

  The double feed detection sensor DD is an ultrasonic double feed detection device that determines whether or not double feed is occurring in the paper transport path 31 and transmits a determination signal indicating the determination result to the control unit 100. It is a device to do. Here, the double feed means a state in which two or more sheets are overlapped and conveyed simultaneously as shown in FIG. Details of the configuration of the double feed detection sensor DD will be described later.

  The paper detection sensor PD2 is a contact type sensor provided between the double feed detection sensor DD and the registration roller 29, and passes through the double feed detection sensor DD to the front end of the paper before reaching the registration roller 29. It is a sensor that detects the leading edge of the sheet by contact and transmits a sheet detection signal indicating that this detection has been performed to the control unit 100. The sheet detection sensors PD1 and PD2 are not limited to contact sensors, and may be transmissive optical sensors or reflective optical sensors.

  Next, a detailed description will be given of a control unit that controls each sensor and each hardware of the image forming apparatus 11. FIG. 5 is a functional block diagram showing various hardware and control units configured in the image forming apparatus 11.

  As shown in FIG. 5, the image forming apparatus 11 includes a gate drive solenoid 101, a conveyance motor 102, an electromagnetic clutch 106, a driver, in addition to the above-described control unit 100, double feed detection sensor DD, and paper detection sensors PD 1 and PD 2. 103, 104, and 105 are provided.

  The gate drive solenoid 101 drives the gates 80a, 80b, 80c, 80d, and 80e (see FIGS. 16 to 19) included in the transport path switching mechanism 80. The driver 103 is a driver circuit for operating the gate drive solenoid 101 and is connected to the control unit 100. In other words, the control unit 100 operates the gate drive solenoid 101 by transmitting a drive signal to the driver 103, thereby driving and controlling the gates 80a, 80b, 80c, 80d, and 80e.

  The transport motor 102 is a motor that applies torque to the rollers provided in the transport paths 31, 83, and 99 and the paper feed trays 25. The driver 104 is a driver circuit for operating the transport motor 102 and is connected to the control unit 100. That is, the control unit 100 rotates each roller configured in each of the transport paths 31, 83, and 99 and the paper feed trays 25 by operating the transport motor 102 by transmitting a drive signal to the driver 104, As a result, control for conveying the sheet inside the image forming apparatus 11 is performed.

  As shown in FIG. 6, the electromagnetic clutch 106 connects or disconnects the conveying motor 102 a that applies driving force to the roller 71 in FIG. 3 and the lower roller 71 b (see FIG. 7) of the roller 71. Is to do. The driver 105 is a driver circuit for driving the electromagnetic clutch 106 and is connected to the control unit 100. That is, the control unit 100 performs control to switch connection / disconnection between the lower roller 71b and the conveyance motor 102a by transmitting a drive signal to the driver 105 to drive the electromagnetic clutch 106.

  Here, the roller 71 will be briefly described. As shown in FIGS. 6 and 7, the winding roller 71 includes an upper winding roller 71a and a lower winding roller 71b.

  When the transport motor 102a rotates, the upper roller 71a receives the torque of the transport motor 102a by the gear 173, and rotates in the clockwise direction by this torque. Torque is transmitted to the lower roller 71b from the transport motor 102a via the electromagnetic clutch 106 and the torque limiter 107.

  Therefore, when the lower roller 71b and the conveyance motor 102a are disconnected by the electromagnetic clutch 106, even if the conveyance motor 102a rotates, the torque of the conveyance motor 102a is not transmitted to the lower roller 71b, and FIG. ), The lower roller 71b receives a driving force from the upper roller 71a, and is driven by the upper roller 71a to rotate in the opposite direction to the upper roller 71a. As a result, as shown in FIG. 7C, even if the multi-feed paper is sent to the separating roller 71, the paper is conveyed as it is. Hereinafter, the state where the lower roller 71b and the conveyance motor 102a are disconnected by the electromagnetic clutch 106 is referred to as a “separation operation OFF state”.

  Even if the lower roller 71b and the conveyance motor 102a are connected by the electromagnetic clutch 106, when only one sheet is fed to the roller 71 as shown in FIG. 7A (in the case of normal conveyance). By the action of the torque limiter 107, the driving force of the upper roller 71a is preferentially transmitted to the lower roller 71b over the torque of the conveying motor 102a, and the lower roller 71b is driven by the upper roller 71a. Thus, the upper roller 71a is driven to rotate in the opposite direction. As a result, as shown in FIG. 7A, when the sheet is normally fed to the roller 71, the sheet is conveyed as it is.

  Further, when the lower winding roller 71b and the transport motor 102a are connected by the electromagnetic clutch 106, and the paper that is being multifed as shown in FIG. The distance from the separating roller 71b is increased, and the driving force of the upper separating roller 71a is not easily transmitted to the lower separating roller 71b, and the torque of the transport motor 102a is preferentially transmitted to the lower separating roller 71b by the action of the torque limiter 107. Thus, the lower roller 71b rotates in the same direction as the upper roller 71a. As a result, as shown in FIG. 7B, when the double fed paper is fed to the separating roller 71, one of the sheets is conveyed in the normal conveying direction by the upper separating roller 71a, and the other is conveyed. The sheet is pushed back in the direction opposite to the normal transport direction by the lower roller 71b. Therefore, the sheets that are being double fed are separated one by one by the separating roller 71, and the sheets are normally conveyed.

  Hereinafter, the state where the lower roller 71b and the transport motor 102a are connected by the electromagnetic clutch 106 (the state shown in FIGS. 7A and 7B) is referred to as a “separation operation ON state”.

  Next, details of the double feed detection sensor DD will be described. As shown in FIGS. 4 and 5, the double feed detection sensor DD includes an ultrasonic transmitter 110 and an ultrasonic receiver 120 that are disposed to face each other with the paper conveyance path 31 interposed therebetween, and a control circuit 121. It is. Note that the double feed detection sensor DD is switched to a driving state in which the power is turned on or a non-driving state in which the power is turned off in accordance with a command from the control unit 100.

  As illustrated in FIG. 5, the ultrasonic transmitter 110 includes an ultrasonic drive circuit 111 and a transmitter 112. As shown in FIG. 5, the control circuit 121 includes an amplifier circuit 122, a smoothing circuit 123, a threshold setting circuit 124, and a comparison circuit 125.

  The ultrasonic drive circuit 111 is a drive circuit that drives the transmitter 112 at the switching timing when the control unit 100 switches the double feed detection sensor DD to the operating state. The transmitter 112 is a device that transmits ultrasonic waves toward the ultrasonic receiver 120 by being driven by the ultrasonic driving circuit 111.

  The ultrasonic receiver 120 is a device that receives the ultrasonic wave transmitted from the transmitter 112, converts the received ultrasonic wave into a signal, and transmits this signal to the amplifier circuit 122. The amplifier circuit (amplifying unit) 122 is a circuit that amplifies the signal input from the ultrasonic receiver 120 based on the amplification factor set in the amplifier circuit 122 and transmits the amplified signal to the smoothing circuit 123. . The smoothing circuit 123 is a circuit that smoothes the signal sent from the amplifier circuit 122 and transmits the smoothed signal to the comparison circuit 125.

  The threshold setting circuit 124 is a circuit that sets a threshold used in the comparison circuit 125 under the control of the control unit 100. The threshold value is a threshold value for determining whether or not double feeding occurs in the paper transport path 31.

  In the image forming apparatus 11 of the present embodiment, the threshold setting mode for setting the threshold is executed by the control unit 100 controlling the threshold setting circuit 124 and the like according to a command from the user. However, this threshold setting mode will be described in detail later.

  The comparison circuit 125 is a comparator that compares the signal transmitted from the smoothing circuit 123 with the threshold set in the threshold setting circuit 124 and transmits a determination signal indicating the comparison result to the control unit 100. Specifically, when the signal sent from the smoothing circuit 123 is equal to or greater than the threshold value, the comparison circuit 125 determines that double feed has not occurred in the paper transport path 31 and indicates that double feed has not occurred. When the signal sent to the control unit 100 and the signal sent from the smoothing circuit 123 falls below the threshold value, it is judged that double feeding has occurred in the paper transport path 31, and a determination signal indicating that double feeding has occurred Is transmitted to the control unit 100. When the control unit 100 receives a determination signal indicating that double feeding has occurred, the control unit 100 outputs a print prohibition command before the paper being double fed in the paper transport path 31 enters the printing unit, The image forming apparatus 11 is controlled to stop printing.

  In the above configuration, the ultrasonic transmitter 110 transmits an ultrasonic wave toward the ultrasonic receiver 120 at the timing when the sheet is conveyed between the ultrasonic transmitter 110 and the ultrasonic receiver 120. . The ultrasonic wave thus transmitted is attenuated by passing through the paper being conveyed, and this attenuated ultrasonic wave is received by the ultrasonic receiver 120. The ultrasonic receiver 120 responds to the received ultrasonic wave. Output a signal. Note that the intensity of the signal output from the ultrasonic receiver 120 increases as the intensity of the received ultrasonic wave increases, and decreases as the intensity of the received ultrasonic wave decreases.

  Further, since the attenuation is increased as the sheet being conveyed is thicker, the intensity of the signal output from the ultrasonic receiver 120 is lower as the sheet being conveyed is thicker. Therefore, when double feeding occurs between the ultrasonic transmitter 110 and the ultrasonic receiver 120, when the paper is normally conveyed (only one sheet or only one sheet is conveyed). The attenuation of the ultrasonic wave received by the ultrasonic receiver 120 is greatly reduced, and the intensity of the signal output from the ultrasonic receiver 120 is also greatly reduced. Therefore, if the comparison circuit 125 compares the signal sent from the ultrasonic receiver 120 with the threshold value, the presence / absence of double feeding in the paper transport path 31 can be determined.

  The control unit 100 controls the overall operation of the image forming apparatus 11, and includes (a) a microcomputer, (b) a ROM that stores a control program that describes a processing procedure executed by the microcomputer, and (c) (1) RAM serving as a data development area during data processing, (d) non-volatile memory for storing data necessary for controlling the image forming apparatus 11, (e) a peripheral device such as a sensor, and the like An input buffer including an A / D conversion circuit for digitizing the signal, (f) an output for outputting a drive signal to the drivers 103, 104, and 105 for driving the motor and solenoid It consists of a circuit.

  Hereinafter, a threshold setting mode, which is a process for setting a threshold used to determine whether or not double feeding has occurred, will be described in detail with reference to FIGS. 8 and 9. FIG. 8 is a flowchart showing the flow of the threshold setting mode executed in the image forming apparatus 11. FIG. 9 is a timing chart showing driving timings of various hardware included in the image forming apparatus 11.

  First, before the image forming apparatus 11 executes the threshold setting mode, the user of the image forming apparatus 11 sets the thick paper A in the paper feed tray 25a as shown in FIG. A paper pair in which two plain papers B thinner than the thick paper A are stacked and pasted together is set in advance. After this setting, the user inputs a threshold setting mode execution command from the operation panel (not shown) of the image forming apparatus 11.

  When this execution command is input, the control unit 100 controls the electromagnetic clutch 106 to turn on the separation operation for the separating roller 71 provided in the paper feed tray 25a (S1).

  Next, the control unit 100 controls the transport motor 102 to drive the pickup roller 70 and the separation roller 71 provided in the paper feed tray 25a to pick up the cardboard A as shown in FIG. The sheet is fed into the sheet conveyance path 31 (S2). At this time, although the separation roller 71 is in the separation operation ON state, only one sheet (thick paper A) is fed to the separation roller 71, so that the lower separation roller 71b is as shown in FIG. The upper roller 71a is driven and rotated in the opposite direction to the upper roller 71a. Therefore, the thick paper A is normally fed into the paper transport path 31 as it is by the rolling roller 71.

  After that, when the leading end of the thick paper A sent to the paper transport path 31 passes through the junction C and reaches the paper detection sensor PD1, the paper detection sensor PD1 detects the leading end of the thick paper A and sends a paper detection signal to the control unit 100. Send to. Thereby, the control unit 100 can recognize the timing when the leading edge of the thick paper A is detected by the paper detection sensor PD1 (YES in S3).

  Then, as shown in pattern 1 in FIG. 9, the control unit 100 drives the double feed detection sensor DD when a predetermined time T1 has elapsed since the timing when the leading edge of the thick paper A was detected by the paper detection sensor PD1. Switch to state. In other words, after recognizing the timing when the leading edge of the thick paper A is detected by the paper detection sensor PD1, the control unit 100 waits for a predetermined time T1 (S4), and switches the double feed detection sensor DD to the drive state after this standby. (S5). The predetermined time T1 is the time required for the leading edge of the paper being conveyed to reach the double feed detection sensor DD from the paper detection sensor PD1. Therefore, the double feed detection sensor DD is switched to the driving state when the leading end of the cardboard A reaches between the ultrasonic transmitter 110 and the ultrasonic receiver 120 in the double feed detection sensor DD.

  As a result, an ultrasonic wave is transmitted from the ultrasonic transmitter 110 to the ultrasonic receiver 120, and the ultrasonic wave passes through the cardboard A conveyed to the paper conveyance path 31 and is transmitted to the ultrasonic receiver 120. Received. Then, the ultrasonic receiver 120 outputs a signal corresponding to the received ultrasonic wave, and this signal is input to the threshold setting circuit 124 via the amplifier circuit 122 and the smoothing circuit 123, and further, a value indicating the intensity of this signal. (Hereinafter referred to as “signal value A”) is written to the memory in the threshold value setting circuit 124 (S6). That is, the signal value A is a signal output from the ultrasonic receiver 120 when the single thick paper A is conveyed between the ultrasonic transmitter 110 and the ultrasonic receiver 120 in the paper conveyance path 31. It is the value which showed the intensity | strength of.

  In the image forming apparatus 11, during normal printing processing, the signal output from the ultrasonic receiver 120 is input to the comparison circuit 125 through the amplifier circuit 122 and the smoothing circuit 123, but in the threshold setting mode. The signal output from the ultrasonic receiver 120 is input to the threshold setting circuit 124 without being input to the comparison circuit 125 via the amplifier circuit 122 and the smoothing circuit 123 (see FIG. 5).

  Then, the control unit 100 conveys the thick paper A to the image transfer unit 47 to transfer the image, and further conveys the thick paper A to the fixing processing unit 66 to perform the fixing process. After that, the control unit 100 performs the SBin process, which is a process for guiding the sheet discharged from the fixing unit 23 to the reverse conveyance path 99, in each of the gates 80a, 80b, 80c, 80d, and 80e of the conveyance path switching mechanism 80. The orientation is switched so as to be the orientation shown in FIG. Accordingly, the cardboard A discharged from the fixing unit 23 is sent to the reverse conveyance path 99, and the front surface and the back surface of the cardboard A are reversed. Further, the control unit 100 performs the SB-DC process, which is a process for guiding the paper in the reverse conveyance path 99 to the paper discharge tray 33, and the gates 80a, 80b, 80c, 80d, and 80e of the conveyance path switching mechanism 80. The direction is switched to the direction as shown in FIG. Then, the control unit 100 reversely rotates the transport roller 97 in the reverse transport path 99, whereby the thick paper A in the reverse transport path 99 is discharged to the paper discharge tray 33 (S7).

  After S7, the control unit 100 controls the electromagnetic clutch 106 to turn off the separating operation with respect to the separating roller 71 provided in the paper feed tray 25b (S8).

  Next, the control unit 100 controls the transport motor 102 to drive the pickup roller 70 and the separation roller 71 provided in the paper feed tray 25b as shown in FIG. Pick up the paper pair B and send it to the paper conveyance path 31 (S9). At this time, the two plain papers B and B are sent to the whirling roller 71 in a state where they are simultaneously and stacked. However, since the whirling roller 71 is in the separation operation OFF state, as shown in FIG. The two plain papers B and B overlap each other and are simultaneously fed into the paper transport path 31 to cause double feed.

  After that, the two plain papers B and B fed into the paper transport path 31 reach the paper detection sensor PD1 when the leading ends of the plain papers B and B pass the junction C and the paper detection sensor PD1 The leading ends of B and B are detected, and a paper detection signal is transmitted to the control unit 100. Thereby, the control unit 100 can recognize the timing when the leading edge of the plain paper B / B is detected by the paper detection sensor PD1 (YES in S10).

  Then, as shown in pattern 1 of FIG. 9, the control unit 100 recognizes the timing at which the leading edge of the plain paper B / B is detected by the paper detection sensor PD1, and when a predetermined time has elapsed, the double feed detection sensor DD. To the driving state. In other words, after recognizing the timing when the leading edge of the plain paper B / B is detected by the paper detection sensor PD1, the control unit 100 waits for a predetermined time T1 (S11), and after this standby, drives the double feed detection sensor DD. Switch to the state (S12).

  As a result, an ultrasonic wave is transmitted from the ultrasonic transmitter 110 to the ultrasonic receiver 120, and the ultrasonic wave passes through the plain paper B / B that is double-fed to the paper conveyance path 31 and is ultrasonic. Received by the receiver 120. Then, the ultrasonic receiver 120 outputs a signal corresponding to the received ultrasonic wave, and this signal is input to the threshold setting circuit 124 via the amplifier circuit 122 and the smoothing circuit 123, and further, a value indicating the intensity of this signal. (Hereinafter referred to as “signal value B”) is written to the memory in the threshold value setting circuit 124 (S13). That is, the signal value B is output from the ultrasonic receiver 120 when two plain papers B are double-fed between the ultrasonic transmitter 110 and the ultrasonic receiver 120 in the paper transport path 31. This is a value indicating the intensity of the signal.

  Then, the control unit 100 conveys the plain paper B / B to the image transfer unit 47 to transfer the image, and further conveys the plain paper B / B to the fixing processing unit 66 to perform the fixing process. After performing the SBin process and the SB-DC process, the plain paper B and B are discharged to the discharge tray 33 (S14).

  When S <b> 14 ends, the control unit 100 transmits a threshold setting command to the threshold setting circuit 124. Upon receiving this command, the threshold setting circuit 124 calculates an intermediate value between the signal value A and the signal value B stored in the memory (S15), and this intermediate value is used by the comparison circuit 125. It is set as a threshold value (threshold value for double feed detection) (S16).

  The intermediate value here means a value corresponding to the median value in the numerical range from the signal value A to the signal value B. For example, when the signal value A is 1 and the signal value B is 7, the intermediate value is 4.

  As described above, the image forming apparatus 11 according to the present embodiment outputs a signal having a strength that correlates with the thickness of the sheet when the sheet is conveyed through the sheet conveyance path 31 and the sheet conveyance path 31. By comparing the receiver (output means) 120 and a signal value indicating the intensity of the signal output from the ultrasonic receiver 120 with a threshold value, it is determined whether or not a double feed state has occurred in the paper transport path 31. And a comparison circuit (determination means) 125. The ultrasonic receiver 120 outputs a signal having a lower strength as the conveyed paper is thicker.

  Further, the image forming apparatus 11 is provided with a threshold setting circuit (setting unit) 124 for setting a threshold used in the comparison circuit 125 (threshold used for determination of presence / absence of double feeding). The threshold setting circuit 124 outputs a first signal output from the ultrasonic receiver 120 when two sheets of plain paper (first paper) B are stacked and simultaneously transported in the paper transport path 31, and the paper transport path 31. When a second signal output from the ultrasonic receiver 120 is acquired by conveying a thick paper (second paper) A that is thicker than plain paper, a signal value B (first intensity value) indicating the intensity of the first signal is obtained. ) And a signal value A (second intensity value) indicating the intensity of the second signal as a setting means (setting process) for setting the intermediate value as the threshold value.

  If the comparison circuit 125 determines the presence or absence of double feed using the threshold value set in this manner, as shown in FIG. 12, if double feed of plain paper B occurs, it is determined as “double feed”. When the cardboard A is normally conveyed, it is determined as “non-multiple feeding”.

  Further, as shown in FIG. 12, when the double feed of the plain paper B occurs, it is determined as “double feed”. Therefore, the double feed of the thick paper A thicker than the plain paper B is naturally “ It will be determined as “double feed”. In addition, as shown in FIG. 12, when the thick paper A is normally conveyed, it is determined as “non-double feeding”, and therefore the plain paper B thinner than the thick paper A is naturally conveyed. It is determined as “non-multifeed”.

  Therefore, according to the threshold set by the threshold setting circuit 124, the comparison circuit 125 can determine whether the normal paper B is normally conveyed and double-fed and the thick paper A is thicker than the normal paper B. Thus, even if the paper used in the image forming apparatus 11 is changed from the plain paper B to the thick paper A, or from the thick paper A to the plain paper B, there is no need to reset the threshold value, and every time the paper to be used is changed. Compared with the conventional image forming apparatus in which the threshold value is reset, the user's trouble can be reduced.

  Further, according to the threshold setting mode of the image forming apparatus 11 of the present embodiment, unlike the method of Patent Document 1 in which the slice level (threshold) must be adjusted based on the display by the display unit, the thick paper A and 2 Since the threshold value is automatically set after the sheets of plain paper B are conveyed in order, there is an advantage that even the user who is not familiar with the operation of the image forming apparatus 11 can easily set the threshold value. Furthermore, the fact that the threshold value can be easily set on the user side means that the threshold value for double feed determination is reset according to the usage conditions of each user (for example, using non-genuine paper). Becomes easier.

  Further, the image forming apparatus 11 described above is in a state in which a plurality of sheets are stacked from the sheet feeding tray 25b by the pickup roller 70 and the sheet feeding tray 25b on which a pair of sheets in which two plain sheets B are stacked and bonded together are placed. And a sheet feeding roller 25 (separating means) 71 that separates one sheet from the plurality of sheets and supplies the one sheet to the sheet conveying path 31, and the sheet conveying path 31 from the sheet feeding tray 25b. And a control unit (conveyance control means) 100 for controlling processing for conveying the paper pair. Then, the control unit 100 functions as a prohibiting unit that prohibits the separation operation with respect to the separating roller 71 during the process of transporting the pair of sheets to the sheet transport path 31 (S8). Therefore, when the process of transporting the paper pair from the paper feed tray 25b to the paper transport path 31 is performed, the separation operation by the separating roller 71 is prohibited, so that the two plain papers B and B in the paper pair are The paper is supplied to the paper transport path 31 without being separated from each other. As a result, two plain papers B and B can be stacked and conveyed simultaneously in the paper conveyance path 31.

  Further, according to the image forming apparatus 11 described above, as shown in FIG. 10, the paper feed tray (first tray) 25 b and the thick paper A on which a pair of paper sheets in which two plain papers B are stacked and bonded together are placed. A sheet feeding tray (second tray) 25a is provided, and the control unit 100 performs a process of feeding the pair of sheets from the sheet feeding tray 25b to the sheet conveying path 31, and a thick sheet A from the sheet feeding tray 25a. The process of feeding into the paper transport path 31 is performed in a predetermined order.

  However, the configuration is not limited to the configuration shown in FIG. 10 in which the thick paper A and the plain paper B are placed on separate trays, and the thick paper A and the plain paper B are placed on the same tray. Also good. That is, in the image forming apparatus 11, a pair of sheets of plain paper B bonded together and a thick sheet A are stacked and placed on the same tray, and the control unit 100 transfers the thick sheet A from the tray to the sheet conveyance path 31. The configuration may be such that the feeding process and the process of feeding the paper pair from the tray to the paper transport path 31 are performed in a predetermined order.

  For example, before causing the image forming apparatus 11 to execute the threshold setting mode, the user of the image forming apparatus 11 stacks two sheets of plain paper B on the paper feed tray (third tray) 25c as shown in FIG. The pair of sheets pasted together is placed, and the cardboard A is placed on the pair of sheets. When the user inputs an execution command for the threshold setting mode, the thick paper A placed on the paper feed tray 25c is supplied to the paper transport path 31, and then the plain paper B placed on the paper feed tray 25c. The procedure may be such that B paper pairs are double-fed in the paper transport path 31.

  A flow chart showing the flow of this procedure is shown in FIG. However, S11 in FIG. 13 is the same as S1 in FIG. 8, S13 to S18 in FIG. 13 are the same as S3 to S8 in FIG. 8, and S20 to S26 in FIG. 13 are the same as S10 to S16 in FIG. is there.

  Further, according to the threshold setting mode of the present embodiment, a single signal value A and a single signal value B are acquired, and an intermediate value between the signal value A and the signal value B is set as a threshold. However, the signal value A or the signal value B used as a sample may not be single but may be plural.

  For example, when the threshold setting mode (S1 to S16) in FIG. 8 is performed a plurality of times, the threshold setting circuit 124 outputs the first signal output from the ultrasonic receiver 120 by simultaneously transporting two sheets of plain paper B. And a plurality of second signals output from the ultrasonic receiver 120 are acquired by conveying the cardboard A, and a plurality of signal values A and a plurality of signal values B are written in the memory. become. In such a case, the threshold value setting circuit 124 determines a set of the signal value A and the signal value B with the smallest difference from each value written in the memory, and the signal value A and the signal value of the determined set. An intermediate value between B and B may be set as the threshold value. In this way, since there are a plurality of sample signal values A and B, the threshold value is more reliable than the configuration in which the threshold value is set from the single signal value A and the single signal value B. Can be set.

  For example, when the threshold value setting mode of FIG. 8 is repeated twice, as shown in FIG. 14, when the signal value A1, the signal value A2, the signal value B1, and the signal value B2 are obtained, the signal value having the smallest difference is obtained. An intermediate value between A1 and signal value B1 is set as the threshold value.

  Further, when a plurality of signal values A or signal values B are used as samples, the signal value obtained by transporting the cardboard A after the fixing process in the fixing processing unit 66 to the sheet transport path 31 or the normal value after the fixing process. The threshold value may be set by including a signal value obtained by double feeding the paper B in the sample. A processing procedure for realizing such setting will be described with reference to the flowchart of FIG.

  First, before the image forming apparatus 11 executes the threshold setting mode, the user of the image forming apparatus 11 sets the thick paper A in the paper feed tray 25a as shown in FIG. A paper pair in which two plain papers B thinner than the thick paper A are stacked and pasted together is set in advance. After this setting, the user inputs a threshold setting mode execution command from the operation panel (not shown) of the image forming apparatus 11.

  When this execution command is input, the processes from S31 to S36 are executed in the image forming apparatus 11 as shown in FIG. Since S31 to S36 are the same as S1 to S6 in FIG. 8, the description thereof is omitted here.

  After S36, the control unit 100 conveys the thick paper A to the image transfer unit 47 to transfer the image onto the thick paper A, and further conveys the thick paper A to the fixing processing unit 66 to perform the fixing process on the thick paper A to perform fixing. The cardboard A is discharged from the unit 23. After that, the control unit 100 performs the SBin process, which is a process for guiding the sheet discharged from the fixing unit 23 to the reverse conveyance path 99, in each of the gates 80a, 80b, 80c, 80d, and 80e of the conveyance path switching mechanism 80. The orientation is switched to the orientation shown in FIG. 16 (S37). As a result, the cardboard A discharged from the fixing unit 23 is fed into the reverse conveyance path 99, and the front surface and the back surface of the cardboard A are reversed.

  Further, the control unit 100 waits for a predetermined time after performing the process of S37 (S38), and then executes the SB-RC process, which is a process of guiding the paper in the reverse conveyance path 99 to the refeed conveyance path 83. Then, the orientations of the gates 80a, 80b, 80c, 80d, and 80e of the transport path switching mechanism 80 are switched to the orientations shown in FIG. 18 (S39). Here, the control unit 100 can guide the thick paper A in the reverse conveyance path 99 to the refeed conveyance path 83 by reversing the conveyance roller 97 in the reverse conveyance path 99 (S40). As a result, the cardboard A subjected to the fixing process is sent again to the paper conveyance path 31 via the refeed conveyance path 83 and the junction C.

  After the thick paper A subjected to the fixing process is sent again to the paper transport path 31 via the junction C, S41 to S44 are performed, but S41 to S44 are the same as S3 to S6. The description is omitted here. However, in S43, the double feed detection sensor DD is driven at a timing when the cardboard A subjected to the fixing process passes between the ultrasonic transmitter 110 and the ultrasonic receiver 120. Therefore, the signal value A2 written in the memory in S44 means that one cardboard A after the fixing process is performed between the ultrasonic transmitter 110 and the ultrasonic receiver 120 in the paper transport path 31. When the signal passes, the value indicates the intensity of the signal output from the ultrasonic receiver 120.

  After S44, the control unit 100 conveys the thick paper A to the image transfer unit 47 to transfer the image, and further conveys the thick paper A to the fixing processing unit 66 to perform the fixing process. Thereafter, the control unit 100 performs the DC process, which is a process of directly discharging the paper from the fixing processing unit 66 to the paper discharge tray 33, and each gate 80a, 80b, 80c, 80d, 80e of the transport path switching mechanism 80. Is switched to the direction shown in FIG. 19, and the thick paper A of the fixing processing unit 66 is discharged to the paper discharge tray 33 (S45).

  After S45, the image forming apparatus 11 performs the processes from S46 to S60. Since S46 to S60 are the same processes as S31 to S45, the description thereof is omitted here. However, in S46 to S60, the two plain papers B and B are double-fed in the image forming apparatus 11. The signal value B1 in S51 is equal to the two plain papers B in the paper transport path 31. This is a value indicating the intensity of the signal output from the ultrasonic receiver 120 when passing between the ultrasonic transmitter 110 and the ultrasonic receiver 120 simultaneously, and the signal value B2 in S59 is the fixing. A signal output from the ultrasonic receiver 120 when the two plain papers B that have been processed pass between the ultrasonic transmitter 110 and the ultrasonic receiver 120 in the paper transport path 31 simultaneously. It is the value which showed the intensity | strength of.

  When S <b> 60 ends, the control unit 100 transmits a threshold setting command to the threshold setting circuit 124. Upon receiving this command, the threshold setting circuit 124 compares the signal value A1 and the signal value A2 stored in the memory to obtain the lower value Amin (S61), and is stored in the memory. The signal value B1 and the signal value B2 that are present are compared to obtain Bmax, which is the higher value (S62). Then, the threshold setting circuit 124 calculates an intermediate value between Amin and Bmax (S63), and sets this intermediate value as a threshold (threshold for detecting double feed) used in the comparison circuit 125 (S64). ).

  In this way, when there are a plurality of signal values A or B as the samples, the signal values obtained by transporting the thick paper A and the plain paper B after the fixing process to the paper transport path 31 are obtained as described above. Can be included in the specimen. In the image forming apparatus 11, the sheet after the fixing process is conveyed to the sheet conveying path 31 when printing the second side of the duplex printing process. According to the procedure of FIG. A threshold value that considers that the paper is transported to the paper transport path is set, and a threshold value that more conforms to actual use conditions can be obtained. In particular, the ultrasonic receiver 120 may vary the intensity of the output signal according to the ambient temperature. When the paper after the fixing process is conveyed to the sheet conveyance path 31, the ambient temperature varies and the output signal is changed. The strength of the paper fluctuates and adversely affects the determination of double feeding, and it is important to set a threshold value that also considers the conveyance of the paper after the fixing process.

  Further, according to the above embodiment, the threshold value setting mode is executed in the image forming apparatus 11 so that the threshold value used for determining the presence or absence of double feeding is reset. A mode in which the threshold value is made constant and the amplification factor of the amplification circuit 122 is changed according to the paper used may be used. Hereinafter, this process will be described as “amplification adjustment mode”.

In this amplification factor adjustment mode, the points S1 to S15 in FIG. 8 are executed in the same manner as the threshold setting mode. However, after S15, S16 is not executed and the control unit 100 calculates the intermediate value calculated in S15. The value is determined as a virtual threshold, and then the corrected amplification factor is calculated by calculating the following equation (a).
Amplification factor after correction = Amplification factor before correction × (Determination threshold value / virtual threshold value) (a)
It should be noted that the pre-correction amplification factor means the amplification factor set in the amplifier circuit 122 up to the present time, and the determination threshold value means a threshold value for multifeed determination used in the comparison circuit 125.

  Then, the control unit 100 sets the correction gain calculated from the equation (a) in the amplifier circuit 122 as the gain used in the subsequent amplification processing. That is, the control unit 100 has a function as a determination unit (determination step) that determines an intermediate value between the signal value A and the signal value B as a virtual threshold, and sets the ratio between the determination threshold and the virtual threshold. Accordingly, it has a function as an adjusting means (adjusting step) for adjusting the amplification factor.

  In such an “amplification rate adjustment mode”, even when the threshold for determination for multi-feed determination is made constant, by changing the amplification rate of the signal to be determined according to the virtual threshold, A determination result similar to the determination of multifeed using a threshold value is obtained. Therefore, when the amplification circuit is adjusted by the “amplification adjustment mode” and the presence / absence of the double feed is determined by the comparison circuit 125, substantially the same effect as that obtained when the threshold setting is performed by the threshold setting mode can be obtained. Can do. Therefore, even when the amplification factor is adjusted by the “amplification factor adjustment mode”, it is possible to discriminate between normal conveyance and double feeding of the first sheet and normal conveyance and double feeding of the second sheet thicker than the first sheet. Even if the paper used in the paper transport device is changed from the first paper to the second paper, and the second paper is changed to the first paper, there is no need to reset the threshold value. Compared to the conventional configuration in which the threshold value needs to be reset every time it is changed, it is possible to reduce the labor of the user.

  Further, in the present embodiment, as shown in FIG. 12, an intermediate value between the signal value A obtained during normal conveyance of thick paper and the signal value B obtained during double feeding of plain paper is set as a threshold value. However, it is not limited to the intermediate value, and any value between the signal value A and the signal value B can be set as the threshold value. This means that if any value between the signal value A and the signal value B is set as a threshold value, it is possible to discriminate between double feeding of the first sheet and normal conveyance of the second sheet thicker than the first sheet. This is because it is possible to determine whether the second sheet is double-fed and the first sheet is normally conveyed.

  Further, the output means in the present invention is configured to output a signal having an intensity correlated with the thickness of the paper when the paper is transported in the paper transport path 31, and the output means is transported. The detecting means α may detect the thickness of the paper and output a signal having a lower strength as the paper is thicker. Alternatively, the detecting means α may detect a thickness of the paper being conveyed and output a signal having a higher strength as the paper is thicker. It may be the detection means β that outputs.

  The detection means α may be a transmission type ultrasonic sensor that outputs a signal corresponding to the intensity of the ultrasonic wave that is transmitted through the conveyed paper, or the light that is transmitted through the conveyed paper. A transmissive optical sensor that outputs a signal corresponding to the intensity can be used. Further, as the detection means β, a reflection type ultrasonic sensor that outputs a signal corresponding to the intensity of ultrasonic waves reflected to the conveyed paper, or a light reflected from the conveyed paper. A reflective optical sensor that outputs a signal corresponding to the intensity can be used. Note that the ultrasonic receiver 120 of the present embodiment corresponds to a transmissive ultrasonic sensor.

  Further, the present invention is not limited to the process of setting a threshold value between the signal value A obtained during normal conveyance of thick paper and the signal value B obtained during normal paper double feeding. For example, the signal value obtained during normal conveyance of normal paper and plain paper A threshold value may be set between the signal value obtained at the time of thin paper multi-feeding. In short, if the processing is such that a threshold value is set between the signal value obtained when the first sheet is double-fed and the signal value obtained when the second sheet thicker than the first sheet is normally conveyed, it is normal. It is not limited to paper, cardboard, thin paper, etc.

  Further, in the image forming apparatus 11 of the present embodiment, the control unit 100 drives the double feed detection sensor DD at the timing when the leading end of the sheet (the leading end in the conveyance direction) reaches between the ultrasonic transmitter 110 and the ultrasonic receiver 120. However, the double feed detection sensor DD may be driven at a timing other than this timing. For example, the control unit 100 may drive the double feed detection sensor DD at a timing when the rear end of the paper (rear end in the transport direction) reaches between the ultrasonic transmitter 110 and the ultrasonic receiver 120. In order to drive the double feed detection sensor DD at the timing when the trailing edge of the paper reaches between the ultrasonic transmitter 110 and the ultrasonic receiver 120, for example, as shown in “Pattern 2” in FIG. In addition, the double feed detection sensor DD may be switched to the operating state at a timing when a predetermined time T2 elapses from when the leading edge of the sheet is detected by the sheet detection sensor PD1. Here, the predetermined time T2 is from the time when the leading edge of the paper being conveyed reaches the paper detection sensor PD1 until the trailing edge of the paper reaches between the ultrasonic wave transmitter 110 and the ultrasonic wave receiver 120. It takes time to complete.

  When the double feed detection sensor DD is driven at the timing when the trailing edge of the paper reaches between the ultrasonic transmitter 110 and the ultrasonic receiver 120 (pattern 2 in FIG. 9), the leading edge of the paper is the ultrasonic transmitter. Compared to the case where the double feed detection sensor DD is driven at a timing reaching between 110 and the ultrasonic receiver 120 (pattern 1 in FIG. 9), the drive period of the double feed detection sensor DD can be shortened, and the double feed detection sensor DD can be shortened. There is an advantage that the amount of self-heating can be reduced.

  Further, as shown in FIG. 4, the image forming apparatus 11 temporarily stops the paper transported toward the image transfer unit 47 in the paper transport path 31 and sends the paper to the image transfer unit 47. A registration roller 29 for re-conveying is disposed. Therefore, the control unit 100 may be configured to drive the double feed detection sensor DD when the paper is temporarily stopped (before the paper is re-conveyed). In order to realize such a configuration, for example, as shown in “Pattern 3” in FIG. 9, the control unit 100 determines that the predetermined time T3 has elapsed from when the leading edge of the sheet is detected by the sheet detection sensor PD1. What is necessary is just to drive the double feed detection sensor DD at the elapse timing. Here, the predetermined time T3 is a time required for the leading edge of the paper to reach the registration roller 29 after the leading edge of the paper being conveyed reaches the paper detection sensor PD1.

  Note that the image forming apparatus 11 of the present embodiment is a monochrome printer, but may be a color printer. Further, the image forming apparatus 11 of the present embodiment may be a printer included in a multifunction peripheral, a printer included in a copying machine, or a printer-dedicated machine configured separately from the scanner 1. It may be a facsimile machine.

  In the present embodiment, an ultrasonic type sensor is used as the double feed detection sensor DD, but the threshold setting mode described above uses an optical double feed detection sensor as disclosed in Patent Document 1, for example. Since the present invention can also be applied to a paper transport device, the double feed detection sensor DD is not limited to the ultrasonic method.

  Further, in the image forming apparatus 11 described above, the threshold setting circuit 124 shown in FIG. 5 is realized only by hardware, but the function realized by the threshold setting circuit 124 is a program for realizing the function. It may be realized in combination with hardware (computer) that executes the program. The control unit 100 is configured by a PC-based computer. Various processes in the control unit 100 are performed by causing a computer to execute a program. Each of the programs described above may be in the form of reading out and using a program recorded on a removable medium (computer-readable recording medium) such as a CD-ROM, or installed in a hard disk or the like. It is also possible to use a form that is read out and used. Further, when the control unit 100 is configured to be connected to a communication network such as the Internet, a mode in which the program is downloaded via the communication network, installed on a hard disk, and the like can be considered.

  The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims, and the embodiments can be obtained by appropriately combining the respective technical means disclosed in the above-described embodiments. The form is also included in the technical scope of the present invention.

  The paper conveying apparatus of the present invention is suitable for an image forming apparatus such as an electrophotographic printer, a copying machine, a multifunction machine, and a facsimile.

(A) is a diagram illustrating a state in which a sheet is normally conveyed in the sheet conveyance path, and (b) is a diagram illustrating a state in which double feeding occurs in the sheet conveyance path. It is the figure which showed the relationship between the intensity | strength of the output signal of an ultrasonic receiver, and the conveyance state of a paper. 1 is a diagram illustrating an image forming apparatus according to an embodiment of the present invention. FIG. 4 is a diagram illustrating a sheet conveyance path configured inside the image forming apparatus illustrated in FIG. 3. FIG. 4 is a functional block diagram illustrating various hardware configured in the image forming apparatus illustrated in FIG. 3. FIG. 4 is a view showing a mechanism for driving the winding roller shown in FIG. 3. (A) is a view showing a state in which one sheet is fed to a separating roller in the separation operation ON state, and (b) is a state in which the separation roller in the separation operation ON state separates the sheet and the sheet. (C) is the figure which showed the separating roller of the separation operation OFF state. 4 is a flowchart showing a flow of processing in a threshold setting mode executed in the image forming apparatus shown in FIG. 3. 4 is a timing chart showing drive timings of various hardware included in the image forming apparatus shown in FIG. 3. It is the figure which showed a mode that the thick paper and plain paper which are used in this embodiment were set to the separate tray. It is the figure which showed a mode that the thick paper and plain paper used in this embodiment were set to the same tray. FIG. 6 is a diagram illustrating a relationship between a signal value A obtained during normal transport of thick paper, a signal value B obtained during plain paper double feed, and a threshold for detecting double feed. It is the flowchart which showed the flow of the process in the threshold value setting mode different from the threshold value setting mode of FIG. FIG. 6 is a diagram illustrating a relationship between signal values A1 and A2 obtained during normal conveyance of thick paper, signal values B1 and B2 obtained during plain paper double feeding, and a threshold for detecting double feeding. It is the flowchart which showed the flow of the process in the threshold value setting mode different from the threshold value setting mode of FIG. 8 and FIG. FIG. 4 is a diagram illustrating a state in which a sheet is sent from a fixing unit to a reverse conveyance path. FIG. 6 is a diagram illustrating a state where a sheet on a reverse conveyance path is discharged from an image forming apparatus. FIG. 6 is a diagram illustrating a state in which a sheet on a reverse conveyance path is sent to a refeed conveyance path. FIG. 4 is a diagram illustrating a state in which a sheet is discharged from the fixing unit to the outside of the image forming apparatus.

Explanation of symbols

11 Image forming apparatus 25a Paper feed tray (second tray)
25b Paper feed tray (first tray)
25c Paper feed tray (third tray)
31 Paper transport path 66 Fixing processing unit 70 Pickup roller 71 Rolling roller (separating means)
71a Upper roller 71b Lower roller 100 Control unit (conveyance control means, prohibition means, determination means, adjustment means)
110 Ultrasonic transmitter 120 Ultrasonic receiver (output means)
122 Amplification circuit (amplification means)
124 threshold setting circuit (setting means)
125 comparison circuit (determination means)
410 Paper transport unit (paper transport device)
DD Double feed detection sensor (output means)

Claims (13)

A paper transport path;
An output means for outputting a signal having an intensity correlated with the thickness of the paper when the paper is transported to the paper transport path;
Determining means for comparing whether a value indicating the intensity of the signal is compared with a threshold value, thereby determining whether or not a double feed state in which two or more sheets overlap and are simultaneously transported in the sheet transport path; In a paper transport device including
A first signal output from the output unit by simultaneously transporting two sheets of the first sheet overlapped on the sheet transport path, and a second sheet thicker than the first sheet transported on the sheet transport path. Thus, when the second signal output from the output means is acquired, a value between a first intensity value indicating the intensity of the first signal and a second intensity value indicating the intensity of the second signal is obtained. A sheet conveying apparatus comprising setting means for setting as the threshold value. A tray for placing a pair of sheets in which the first sheets are stacked and bonded together;
Separation means for separating one sheet from the plurality of sheets and feeding the one sheet to the sheet conveyance path when a plurality of sheets are sent out from the tray in a stacked state;
Transport control means for controlling transport processing for transporting the paper pair from the tray to the paper transport path;
The sheet conveying apparatus according to claim 1, further comprising a prohibiting unit that performs control to prohibit the separation operation from being performed on the separating unit during the execution of the transporting process. A first tray for placing a pair of sheets in which two sheets of the first sheet are stacked and bonded together;
A second tray for placing the second paper;
Conveying control means for performing a process of conveying the sheet pair from the first tray to the sheet conveying path and a process of conveying the second sheet from the second tray to the sheet conveying path in a predetermined order. The paper conveying apparatus according to claim 1, wherein A third tray for stacking and stacking a pair of sheets obtained by bonding two sheets of the first sheet and the second sheet;
A conveyance control unit that performs a process of conveying the pair of sheets from the third tray to the sheet conveyance path and a process of conveying the second sheet from the third tray to the sheet conveyance path in a predetermined order. The sheet conveying apparatus according to claim 1.   The setting means obtains a plurality of first signals and a plurality of second signals to obtain a plurality of first intensity values and a plurality of second intensity values. The set of the 1st intensity value and the 2nd intensity value is determined, and the value between the determined 1st intensity value and the 2nd intensity value is set as the threshold value. Paper transport device. A paper transport path;
An output means for outputting a signal having an intensity correlated with the thickness of the paper when the paper is transported to the paper transport path;
Amplifying means for amplifying a signal output from the output means based on a predetermined amplification factor;
By comparing the value indicating the intensity of the signal after being amplified by the amplifying means with the threshold value for determination, a double feed state in which two or more sheets overlap and are simultaneously transported in the sheet transport path occurs. A sheet conveying apparatus including a determination unit that determines whether or not
A first signal output from the output unit by simultaneously transporting two sheets of the first sheet overlapped on the sheet transport path, and a second sheet thicker than the first sheet transported on the sheet transport path. Thus, when the second signal output from the output means is acquired, a value between a first intensity value indicating the intensity of the first signal and a second intensity value indicating the intensity of the second signal is obtained. Determining means for determining as a virtual threshold;
And a adjusting unit that adjusts the amplification factor in accordance with a ratio of the determination threshold to the virtual threshold.   An image forming apparatus comprising the paper transport device according to claim 1. An image forming apparatus comprising the paper conveying device according to claim 5.
A fixing processing unit that thermally fixes an image on paper;
Among the plurality of first intensity values, the intensity of a signal output from the output unit is shown by overlapping two sheets of the first sheet after the thermal fixing and simultaneously transporting the sheets to the sheet conveying path. Value is included,
Among the plurality of second intensity values, a value indicating the intensity of a signal output from the output unit when the second sheet after the thermal fixing is performed is conveyed to the sheet conveying path. An image forming apparatus. A paper transport path;
An output means for outputting a signal having an intensity correlated with the thickness of the paper when the paper is transported to the paper transport path;
Determining means for comparing whether a value indicating the intensity of the signal is compared with a threshold value, thereby determining whether or not a double feed state in which two or more sheets overlap and are simultaneously transported in the sheet transport path; A threshold value setting method for setting the threshold value in a paper transport device including:
A first signal output from the output unit by simultaneously transporting two sheets of the first sheet overlapped on the sheet transport path, and a second sheet thicker than the first sheet transported on the sheet transport path. Thus, when the second signal output from the output means is acquired, a value between a first intensity value indicating the intensity of the first signal and a second intensity value indicating the intensity of the second signal is obtained. And a setting step for setting as the threshold value. A paper transport path;
An output means for outputting a signal having an intensity correlated with the thickness of the paper when the paper is transported to the paper transport path;
Amplifying means for amplifying a signal output from the output means based on a predetermined amplification factor;
By comparing the value indicating the intensity of the signal after being amplified by the amplifying means with the threshold value for determination, a double feed state in which two or more sheets overlap and are simultaneously transported in the sheet transport path occurs. In a gain adjustment method for adjusting the gain in a paper transport device including a determination unit that determines whether or not
A first signal output from the output unit by simultaneously transporting two sheets of the first sheet overlapped on the sheet transport path, and a second sheet thicker than the first sheet transported on the sheet transport path. Thus, when the second signal output from the output means is acquired, a value between a first intensity value indicating the intensity of the first signal and a second intensity value indicating the intensity of the second signal is obtained. A determining step for determining as a virtual threshold;
And an adjustment step of adjusting the amplification factor according to a ratio of the determination threshold value to the virtual threshold value.   A program for causing a computer to execute the threshold setting method according to claim 9, wherein the program causes the computer to execute the setting step.   A program for causing a computer to execute the amplification factor adjustment method according to claim 10, wherein the program causes the computer to execute the determination step and the adjustment step.   A computer-readable recording medium on which the program according to claim 11 or 12 is recorded.

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