Patent Publication Number: US-2019171149-A1

Title: Recording material discriminating apparatus, image forming apparatus, and method of controlling recording material discriminating apparatus

Description:
The entire disclosure of Japanese patent Application No. 2017-231657, filed on Dec. 1, 2017, is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Technological Field 
     The present invention relates to a recording material discriminating apparatus, an image forming apparatus, and a method of controlling the recording material discriminating apparatus. More specifically, the invention relates to a recording material discriminating apparatus that discriminates the type of a recording material, an image forming apparatus, and a method of controlling the recording material discriminating apparatus. 
     Description of the Related art 
     Electrophotographic image forming apparatuses include multi function peripherals (MPs), facsimile apparatuses, copying machines, and printers. The MFPs have a scanner function, a facsimile function, a copying function, a printer function, a data communication function, and a server function. 
     In a conventional image forming apparatus, a user needs to set the type of a recording material based on, for example, the basis weight of the recording material through an operation panel or a personal computer (PC) when replenishing a cassette of the image forming apparatus with recording materials. The image forming apparatus forms a high-quality image by selecting a transportation speed and a fixing temperature that are optimum for the set type of recording material. Unfortunately, difficulty and troublesomeness in a method of setting the type of the recording material may cause inaccurate setting of the type of the recording material, leading to paper jamming and deterioration in the formed image. 
     Thus, for example, JP 2009-75370 A and JP 2009-46217 A propose recording material discriminating apparatuses (media sensors) that reduce burdens on users who set the type of the recording material by automatically detecting the physical property of the recording material to be printed and accordingly discriminating the type of the recording material. 
     JP 2009-75370 A discloses a technique for detecting, for example, paper thickness, plain paper, thin paper or gloss paper by detecting a reflection amount (surface property of a recording material) and a transmission amount (thickness of the recording material) of light with a plurality of optical sensors. 
     JP 2009-46217 A discloses an image forming apparatus having a function of detecting back paper. The image forming apparatus is provided with image detecting unit in a reverse passage. The image detecting unit detects whether a back surface of a recording paper has already been printed based on an amount of light that is applied onto and reflected by the back surface. 
     Techniques in, for example, JP 2009-75370 A and JP 2009-46217 use only optical sensors, which are detection sensors using light. Unfortunately, the optical sensors cannot detect, for example, back paper and label paper, and the accuracy in discriminating the type of a recording material is low. Users of image forming apparatuses desire printing on back paper and label paper. 
     SUMMARY 
     The invention is to solve the above-described problems, and an object thereof is to provide a recording material discriminating apparatus that can improve the accuracy in discriminating the type of a recording material, an image forming apparatus, and a method of controlling the recording material discriminating apparatus. 
     To achieve the abovementioned object, according to an aspect of the present invention, a recording material discriminating apparatus reflecting one aspect of the present invention comprises: a hardware processor that: detects a physical property of a recording material in different ways; and when transmittance of the recording material is within a first range, discriminates a type of the recording material by using a detection result from the hardware processor, and when the transmittance of the recording material is within a second range different from the first range, discriminates the type of the recording material by using a detection result from the hardware processor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages and features provided by one or more embodiments of the invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention: 
         FIG. 1  is a cross-sectional view schematically illustrating a configuration of an image forming apparatus according to one embodiment of the invention; 
         FIG. 2  is a cross-sectional view illustrating partial configurations of optical, ultrasonic, and paper thickness sensors in one embodiment of the invention, and taken along an xz plane at a first position in a y-axis direction; 
         FIG. 3  is a cross-sectional view illustrating the partial configurations of the optical, ultrasonic, and paper thickness sensors in one embodiment of the invention, and taken along the xz plane at a second position in the y-axis direction; 
         FIG. 4  illustrates the partial configurations of the optical, ultrasonic, and paper thickness sensors in one embodiment of the invention, and the vicinity of a substrate as seen in a direction, from positive to negative, of an x-axis; 
         FIG. 5  illustrates the partial configurations of the optical, ultrasonic, and paper thickness sensors in one embodiment of the invention, and the vicinity of the substrate as seen in a direction, from positive to negative, of a z-axis; 
         FIG. 6  is a perspective view illustrating the partial configurations of the optical, ultrasonic, and paper thickness sensors in one embodiment of the invention, and illustrating the vicinity of a resist roller; 
         FIG. 7  illustrates a method of detecting the basis weight of a recording material with the optical sensor in one embodiment of the invention; 
         FIGS. 8A to 8C  schematically illustrate various printing patterns of back paper; 
         FIG. 9  illustrates a method of detecting the basis weight of the recording material with the ultrasonic sensor in one embodiment of the invention; 
         FIG. 10  illustrates a method of detecting the basis weight of the recording material with the paper thickness sensor in one embodiment of the invention; 
         FIGS. 11A and 11B  schematically illustrate information output by a sensor in one embodiment of the invention; 
         FIG. 12  is a graph illustrating the relation between the transmittance of light that is transmitted through the recording material detected by the optical sensor in one embodiment of the invention and the actual basis weight of the recording material; 
         FIG. 13  illustrates the relation between a voltage value output by the ultrasonic sensor in one embodiment of the invention and the actual basis weight of the recording material; 
         FIG. 14  is a graph illustrating the relation between the basis weight of the recording material detected by the paper thickness sensor in one embodiment of the invention and the actual basis weight of the recording material; 
         FIG. 15  illustrates the detection accuracy of each of the optical, ultrasonic, and paper thickness sensors for various recording materials; 
         FIG. 16  illustrates a combination of the transmittance of the recording material in one embodiment of the invention and the sensors used for basis weight detection; 
         FIG. 17  illustrates a method of discriminating the type of the recording material in one embodiment of the invention; 
         FIG. 18  schematically illustrates a setting table in one embodiment of the invention; 
         FIG. 19  is a first part of a flowchart illustrating an operation of the image forming apparatus in one embodiment of the invention; and 
         FIG. 20  is a second part of the flowchart illustrating the operation of the image forming apparatus in one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, one or more embodiments of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the disclosed embodiments. 
     In the following embodiment, a recording material discriminating apparatus mounted on an image forming apparatus will be described. Examples of the image forming apparatus include, for example, MFPs, printers, copying machines, and facsimiles. In addition, the recording material discriminating apparatus may be mounted on an apparatus other than an image forming apparatus, such as a storage apparatus for recording materials and a postprocessing apparatus. 
     Note that, in the drawings, an x-axis direction corresponds to a width direction of an image forming apparatus  1  as seen from the front, a y-axis direction corresponds to a depth direction, and a z-axis direction indicates a height direction. X-, y-, and z-axes are orthogonal to each other. 
     Configuration of Image Forming Apparatus 
       FIG. 1  is a cross-sectional view schematically illustrating a configuration of the image forming apparatus  1  in one embodiment of the invention. 
     Referring to  FIG. 1 , the image forming apparatus  1  in the embodiment (one example of recording material property detecting apparatuses) discriminates the type of a recording material, and performs printing on the recording material under a printing condition according to the discriminated type of the recording material. The image forming apparatus  1  mainly includes a recording material transporter  10 , a toner image former  20  (one example of printers), a fixing apparatus  21 , a power supply  22 , an operation input  23  (one example of receptors), a controller  40 , an optical sensor  51  (one example of outputters), an ultrasonic sensor  52  and a paper thickness sensor  53 . 
     The recording material transporter  10  transports a recording material M along a transportation route TR. The recording material transporter  10  includes a paper feeding tray  11  (one example of storages), a paper feeding roller  12 , a transportation roller  13 , a resist roller  14 , a paper discharging roller  15 , and a paper discharging tray  16 . The paper feeding tray  11  carries and stores the recording material M used for forming an image. A plurality of paper feeding trays  11  may be used. The paper feeding roller  12  is provided between the paper feeding tray  11  and the transportation route TR. The transportation roller  13  and the resist roller  14  are provided along the transportation route TR. The resist roller  14  includes a metal roller  141  and a rubber roller  142 . The paper discharging roller  15  is provided at the most downstream part of the transportation route TR. The paper discharging tray  16  is provided at the uppermost part of an image forming apparatus body  1   a.    
     The toner image former  20  composes an image of four colors: yellow (Y), magenta (M), cyan (C), and black (K) in a so-called tandem system, and forms a toner image on the recording material M at a printing position ST. The toner image former  20  includes an intermediate transfer belt  20   a.  The toner image former  20  also includes a photoreceptor drum, a charging roller, an exposure apparatus, a development apparatus, a primary transfer roller, and a secondary transfer roller. 
     The fixing apparatus  21  fixes the toner image on the recording material M by gripping and transporting the recording material that carries the toner image along the transportation route TR. 
     The power supply  22  supplies power to each of the members in the image forming apparatus  1  under the control of the controller  40 . 
     The operation input  23  receives operation input about print setting for the recording material M or operation of the image forming apparatus  1 . 
     The controller  40  controls operation of the entire image forming apparatus  1 . The controller  40  includes a central processing unit (CPU), a read only memory (ROM), and a random access memory (RAM). The CPU executes a control program. The ROM stores programs such as the control program. The RAM constitutes a work area of the CPU. 
     Each of the optical, ultrasonic, and paper thickness sensors  51 ,  52 , and  53  is used for detecting a physical property of the recording material in different ways under the control of the controller  40 . Although the physical property of the recording material detected with the optical, ultrasonic, and paper thickness sensors  51 ,  52 , and  53  is the basis weight of the recording material here, other physical properties such as the thickness of the recording material may be detected. Detection positions of the optical, ultrasonic, and paper thickness sensors  51 ,  52 , and  53  are on the more upstream side of the transportation route TR than that of the resist roller  14 . 
     In the image forming apparatus  1 , the paper feeding roller  12  feeds the recording material M stored in the paper feeding tray  11  to the transportation route TR one by one, and the transportation roller  13  and the resist roller  14  guide the recording material M to the printing position ST along the transportation route TR. In the image forming apparatus  1 , the resist roller  14  once stops the recording material M, and transports the recording material M to the printing position ST according to the timing of printing with the toner image former  20 . 
     Meanwhile, in the image forming apparatus  1 , the toner image former  20  generates the toner image on the intermediate transfer belt  20   a  of the toner image former  20  in well-known electrophotographic and tandem systems, and rotation of the intermediate transfer belt  20   a  transports the toner image to the printing position ST. The resist roller  14  sends the recording material M to the printing position ST, and the toner image former  20  transports the toner image also to the printing position ST. The image forming apparatus  1  transfers the toner image from the intermediate transfer belt  20   a  to the recording material M at the printing position ST. 
     In the image forming apparatus  1 , the fixing apparatus  21  heats and presses the recording material M on which the toner image is formed. This operation fixes the toner image on the recording material M. In the image forming apparatus  1 , the paper discharging roller  15  then discharges the recording material M to the paper discharging tray  16 . 
       FIGS. 2 to 6  illustrate partial configurations of the optical, ultrasonic, and paper thickness sensors  51 ,  52 , and  53  in one embodiment of the invention.  FIG. 2  is a cross-sectional view taken along an xz plane at a first position in the y-axis direction.  FIG. 3  is a cross-sectional view taken along the xz plane at a second position in the y-axis direction.  FIG. 4  illustrates the vicinity of a substrate  512  as seen in the direction, from positive to negative, of the x-axis.  FIG. 5  illustrates the vicinity of the substrate  512  as seen in the direction, from positive to negative, of the z-axis.  FIG. 6  is a perspective view of the vicinity of the resist roller  14 . 
     Referring to  FIGS. 2 to 6 , the image forming apparatus  1  further includes two rollers  61  and a guide  62 . The two rollers  61  and the guide  62  face each other across the transportation route TR. The two rollers  61  are placed on the right side of the transportation route TR in  FIG. 2 . The two rollers  61  are arranged in the y-axis direction. The guide  62  is placed on the left side of the transportation route TR in  FIG. 2 . The recording material M fed by the paper feeding roller  12  is transported along the transportation route TR in the gap between the two rollers  61  and the guide  62  while the two rollers  61  and the guide  62  inhibit displacements of the recording material M in the thickness direction during transportation. 
     The optical sensor  51  outputs a voltage value corresponding to an amount of light that is transmitted through the recording material M at a position along the transportation direction of the recording material M. The optical sensor  51  also outputs a voltage value corresponding to an amount of light that is reflected through the recording material M. The optical sensor  51  includes a substrate  511 , the substrate  512 , light emitting elements  513  and  514 , a light receiving element  515 , and light guide plates  516  and  517 . 
     The substrate  511  is placed between the two rollers  61  on the right side of the transportation route TR in  FIG. 2 . The light emitting element  513  is fixed on the surface, facing the transportation route TR, of the substrate  511 . The substrate  512  is placed on the left side of the transportation route TR in  FIG. 2 . The light emitting element  514  and the light receiving element  515  are fixed on the surface, facing the transportation route TR, of the substrate  512 . 
     The light guide plate  516  is provided in an upper part of the substrate  511  on the right side of the transportation route TR in  FIG. 2 . The light guide plate  516  extends in the y-axis direction, and guides light from the light emitting element  513  in the y-axis direction. The light guide plate  517  is provided in an upper part of the substrate  512  on the left side of the transportation route TR in  FIG. 2 . The light guide plate  517  extends in the y-axis direction, and guides light that is transmitted through the recording material M in the y-axis direction. 
     The ultrasonic sensor  52  outputs a voltage value corresponding to an amount of ultrasonic waves that are transmitted through the recording material M. The detection position of the ultrasonic sensor  52  is on the more downstream side of the transportation route TR than that of the optical sensor  51 , and is on the more upstream side of the transportation route TR than that of the resist roller  14 . The ultrasonic sensor  52  includes a transmission substrate  521 , a reception substrate  522 , a transmission sensor  523 , and a reception sensor  524 . The transmission substrate  521  is placed on the left side of the transportation route TR in  FIG. 2 . The substrate  512  and the transmission substrate  521  are arranged in the y-axis direction. The reception substrate  522  is placed on the right side of the transportation route TR in  FIG. 2  obliquely upward from the transmission substrate  521 . The transmission sensor  523  is fixed on the surface, facing the transportation route TR, of the transmission substrate  521 . The reception sensor  524  is fixed on the surface, facing the transportation route TR, of the reception substrate  522 . 
     The paper thickness sensor  53  is in contact with the metal roller  141 . The metal roller  141  is movable in a direction AR 1  perpendicular to the rotational axis of the metal roller  141 . When the recording material M passes between the resist rollers  14 , the metal roller  141  moves in response to the thickness of the recording material M. The paper thickness sensor  53  includes an actuator  55  (one example of displacement members) in contact with the metal roller  141 , and outputs information on the thickness of the recording material M based on a displacement amount of the actuator  55  at the time when the recording material M passes between the resist rollers  14 . 
     Method of Detecting Basis Weight of Recording Material with Each of Sensors 
     A method of detecting the basis weight of the recording material with each of the optical, ultrasonic, and paper thickness sensors  51 ,  52 , and  53  will now be described. 
       FIG. 7  illustrates a method of detecting the basis weight of the recording material M with the optical sensor  51  in one embodiment of the invention. 
     Referring to  FIG. 7 , the controller  40  includes a source controller  411 , an averager  412  (one example of averagers), a physical property detector  413  (one example of detector), a discriminator  414  (one example of discriminators), and a printing condition setter  415 . The optical sensor  51 , the source controller  411 , the averager  412 , and the physical property detector  413  constitute a first physical property detector for detecting a physical property (here the transmittance and basis weight) of the recording material M based on an amount of light that is transmitted through the recording material M. 
     The light emitting element  514  sequentially applies light L 3  to the recording material M under the control of the source controller  411 . The light L 3  for each of color of red (R), green (G), and blue (B), and turns to reflected light. The light receiving element  515  receives reflected light L 4  reflected by the recording material M, and outputs a voltage value corresponding to an amount of the received reflected light to the controller  40 . The physical property detector  413  detects the color tone of the recording material M based on the voltage value corresponding to the amount of the reflected light output by the light receiving element  515 . Note that, to increase the amount of the reflected light, a white back plate may be provided behind a position, to which the light L 3  is applied, of the recording material M. 
     The light emitting element  513  applies light L 1  to the recoding material M under the control of the source controller  411 . The light L 1  has a property based on the color tone of the recording material M detected by the physical property detector  413 , and is transmitted through the recording material M. The light emitting element  513  applies the light L 1  at timing different from a light emitting timing of the light emitting element  514 . The light receiving element  515  receives transmitted light L 2  that is transmitted through the recording material M (mainly scattered light scattered by the recording material M), and outputs a voltage value corresponding to an amount of light, which is an amount of the transmitted light L 2 , at a position along the transportation direction of the recording material M to the controller  40 . 
     The averager  412  calculates an average value obtained by averaging the voltage value output by the light receiving element  515 . The physical property detector  413  detects the transmittance and basis weight of the recording material M based on the average value calculated by the averager  412 . 
     As described later, the discriminator  414  discriminates the type of the recording material M by using one of detection results from the first, a second, and a third physical property detectors. In the embodiment, the discriminator  414  discriminates the type of the recording material M among plain paper, label paper, back paper, envelopes, and overhead projector (OHP) sheets. 
     As described later, the printing condition setter  415  sets a printing condition according to the discriminated type of the recording material M. 
     Plain paper is generally used in image forming apparatuses, and has both surfaces that are not printed. Label paper is a recording material that includes a material to the back of which an adhesive is deposited, and a mount to which the material is attached, and whose material is allowed to be attached to an object by peeling the material off the mount. Back paper is obtained by performing printing on only one surface of plain paper. Envelopes are obtained by stacking paper in a bag shape. OHP sheets are transparent films on which an image is to be drawn. The image serves as the base of an image to be projected with an overhead projector (OHP). 
     Incidentally, it is expected that basis weights of plain paper, back paper, and label paper can be measured. It is expected that envelopes and OHP sheets can be discriminated from other recording materials. 
     To inhibit deterioration of the accuracy in detecting the basis weight of the recording material M, a passing paper width (displacement of the recording material M in the thickness direction during transportation) at the detection position of the optical sensor  51  is preferably set in a range of ±1 mm to ±2 mm based on a position of the transportation route TR. In addition, the ratio of a distance d2 to a distance d1 (d1:d2) is preferably set to be 6:4. The distance d1 is from the light emitting element  513  to the recording material M. The distance d2 is from the light emitting element  514  to the recording material M. The averager  412  preferably averages voltage values obtained at ten or more detection positions, which are provided at 5 mm pitch in the recording material M. The optical sensor  51  requires positional stability of the recording material Mat the detection positions. The optical sensor  51  is required to secure accuracy for a wide range of light amount. The optical sensor  51  is preferably placed near the nip of an intermediate roller. 
       FIGS. 8A to 8C  schematically illustrate various printing patterns of back paper. Note that, in  FIGS. 8A to 8C , the voltage values output by the optical sensor  51  on detection of each sheet of the back paper are also indicated. 
     Referring to  FIGS. 8A to 8C , when the light guide plate  516  is provided, the light L 1  can be applied in a wide range of the recording material M along a width direction (lateral direction in  FIGS. 8A to 8C ). When the light guide plate  517  is provided, the transmitted light L 2  is collected from the wide range of the recording material M along the width direction, and received by the light receiving element  515 . This causes the voltage value output from the optical sensor  51  to correspond to an amount of light that is transmitted through the entire region in a direction perpendicular to the transportation direction of the recording material M (width direction). As a result, in addition to back paper with a printing pattern (symbol (*) or hatching part) on a center line P 1  in the width direction of the recording material M as illustrated in  FIGS. 8A and 8B , the optical sensor  51  can also detect back paper with a printing pattern on a part other than the center line P 1  as illustrated in  FIG. 8C . 
     When the recording material M is back paper, the voltage value of the optical sensor  51  is greatly decreased at a printed part. This causes a variation amount AV of the voltage value output from the optical sensor  51  in the recording material M to be greater than a predetermined amount VA. When the variation amount ΔV is greater than the predetermined amount VA, the discriminator  414  may determine that the recording material M is back paper without calculating the average value at the averager  412 . 
     In addition, when back paper has been stored in the paper feeding tray  11  before the recording material M is stored in the paper feeding tray  11 , the recording material M is also highly likely to be back paper. When back paper has been stored in the paper feeding tray  11  before the recording material M is stored in the paper feeding tray  11 , and the variation amount ΔV of the voltage value of the optical sensor  51  is greater than a predetermined amount VB (the predetermined amount VB may be less than the predetermined amount VA), the discriminator  414  may determine that the recording material M is back paper without calculating the average value at the averager  412 . 
       FIG. 9  illustrates a method of detecting the basis weight of the recording material M with the ultrasonic sensor  52  in one embodiment of the invention. 
     Referring to  FIG. 9 , the ultrasonic sensor  52 , the source controller  411 , and the physical property detector  413  constitute a second physical property detector for detecting the basis weight of the recording material M based on an amount of ultrasonic waves that are transmitted through the recording material M. 
     The transmission sensor  523  applies an ultrasonic wave L 11  to the recording material M under the control of the source controller  411 . Note that the ultrasonic wave L 11  is required to be an electromagnetic wave having a wavelength different from that of the light L 1 . 
     The ultrasonic wave L 11  is divided into an ultrasonic wave L 12  and an ultrasonic wave L 13 . The ultrasonic wave L 12  is transmitted through the recording material M. The ultrasonic wave L 13  is reflected by the recording material M. The reception sensor  524  receives the ultrasonic wave L 12  transmitted through the recording material M, and outputs a voltage value corresponding to an amount of the received ultrasonic waves to the controller  40 . 
     The physical property detector  413  detects the basis weight of the recording material M based on the voltage value received from the reception sensor  524 . The physical property detector  413  detects the basis weight of the recording material M based on the ratio (attenuation factor) of the crest value of the ultrasonic wave amount indicated by the voltage value that is output from the reception sensor  524  to that of the ultrasonic wave amount sent from the transmission sensor  523 . 
     Note that envelopes include stacked paper, and have a property of not easily transmitting ultrasonic waves compared to plain paper. Consequently, when the ultrasonic wave amount indicated by the voltage value that is output from the reception sensor  524  (amount of ultrasonic waves that are transmitted through the recording material M) is lower than a predetermined ultrasonic-wave-amount threshold value TH, the discriminator  414  may determine that the recording material M is an envelope without depending on detection results on the basis weight from other sensors. In particular, an angle θ of a direction in which the ultrasonic wave L 11  is applied by the transmission sensor  523  to the transportation route TR is preferably 38 to 45 degrees. In this angular range, the ultrasonic wave amount is remarkably decreased when the recording material is an envelope. Adopting the angular range thus can improve the accuracy in detecting envelopes. 
     To inhibit deterioration of the accuracy in detecting the basis weight of the recording material M, a passing paper width at the detection position of the ultrasonic sensor  52  is preferably set within 2.4 mm. This is because positional stability of the recording material M is required at the detection position of the ultrasonic sensor  52 . 
       FIG. 10  illustrates a method of detecting the basis weight of the recording material M with the paper thickness sensor  53  in one embodiment of the invention. 
     Referring to  FIG. 10 , the paper thickness sensor  53  and the physical property detector  413  constitute a third physical property detector for detecting the basis weight of the recording material M based on a displacement amount of the actuator  55  at the time when the recording material M passes through the detection position of the actuator  55 . Note that the first, second, and third physical property detectors detect the basis weight of the recording material M in different ways. 
     The paper thickness sensor  53  includes a sensor  54  and the actuator  55 . The sensor  54  detects a movement amount of the actuator  55 , and outputs information on the detected movement amount to the controller  40 . The sensor  54  includes two supports  541 , a base  542 , an irradiator  543 , and a light receiver  544 . The actuator  55  is swingably supported between the two supports  541 . The base  542  fixes the two supports  541 . The irradiator  543  is provided inside one of the supports  541 , and applies light to an encoder  552  of the actuator  55 . The light receiver  544  is provided inside the other support  541 , and receives light transmitted through the encoder  552  from the irradiator  543 . The light receiver  544  outputs information corresponding to the amount of the received light to the controller  40 . 
     The actuator  55  can move in both directions indicated by arrows SA and SB around a rotational shaft  554 . The actuator  55  includes a contact part  551 , the encoder  552 , an arm  553 , and the rotational shaft  554 . 
     The contact part  551  is in contact with the metal roller  141 . The contact part  551  has an arc shape. 
     The encoder  552  includes a plurality of high transmittance parts  552   a  and a plurality of low transmittance parts  552   b.  The transmittance of the high transmittance parts  552   a  is higher than that of the low transmittance parts  552   b.  Each of the plurality of high transmittance parts  552   a  and each of the plurality of low transmittance parts  552   b  are alternately arranged along the swinging direction of the actuator  55 . This causes the transmittance of the encoder  552  at a portion where light is applied from the irradiator  543  to periodically change due to movements of the actuator  55 . The arm  553  connects the contact part  551  and the encoder  552 . This configuration integrally swings the contact part  551  and the encoder  552 . The rotational shaft  554  is fixed near the boundary between the encoder  552  and the arm  553 . The rotational shaft  554  extends in a thickness direction. 
     The physical property detector  413  detects the basis weight of the recording material M based on the information output from the light receiver  544 . 
       FIGS. 11A and 11B  schematically illustrate information output by the sensor  54  in one embodiment of the invention.  FIG. 11A  illustrates an output from the sensor  54  at the time when the actuator  55  is moved in a plus direction (arrow SA direction).  FIG. 11B  illustrates an output from the sensor  54  at the time when the actuator  55  is moved in a minus direction (arrow SB direction). 
     Referring to  FIGS. 11A and 11B , the light receiver  544  outputs two phases of an A phase and a B phase. Each of the A and B phases is a pulse. The phases in the movement direction of the actuator  55  (thickness direction of the recording material M) are shifted by 90 degrees. The thickness thus can be measured with resolution (accuracy) corresponding to a distance PE between the pulse edges. This resolution is, for example, 5 μm. 
     The transmittance of the encoder  552  at a portion where light is applied from the irradiator  543  periodically changes as the movement amount of the actuator  55  in the plus or minus direction increases. The amount of light received by the light receiver  544  of the sensor  54  also periodically changes as the movement amount of the actuator  55  increases. Consequently, as described above, each of the A and B phases turns to a pulse that repeats high (H) and low (L) states at intervals corresponding to twice the distance PE as the movement amount of the actuator  55  in the plus or minus direction increases. 
     Note that, to inhibit wear of the contact part  551 , the paper thickness sensor  53  is required to be placed such that the contact part  551  is in indirect contact with the recording material M and displacements of the metal roller  141  accompanying passage of the recording material M are accurately transmitted to the actuator  55 . 
     The optical, ultrasonic, and paper thickness sensors  51 ,  52 , and  53  may detect the physical property of the recording material M being transported (moved) by a recording material transporter  10 , and may detect the physical property of the recording material M once being stopped by the resist roller  14 . 
     Accuracy in Detecting Physical Property with Each of Sensors 
       FIG. 12  is a graph illustrating the relation between the transmittance of light that is transmitted through the recording material detected by the optical sensor  51  in one embodiment of the invention and the actual basis weight of the recording material. 
     Referring to  FIG. 12 , the physical property detector  413  stores a curve LN 1 . The curve LN 1  is a reference curve on two-axis coordinates. The reference curve indicates a change of a reference value of basis weight corresponding to the transmittance of light transmitted through the recording material (transmitted light L 2 ) detected by the optical sensor  51  (hereinafter may be referred to as the transmittance of the recording material). The curve LN 1  is calculated from a relation obtained in the case where the recording material is plain paper. The physical property detector  413  detects the transmittance of the recording material based on the amount of light that is transmitted through the recording material. The physical property detector  413  also detects the basis weight of the recording material by using the curve LN 1  based on the detected transmittance of the recording material. 
     When the recording material is plain paper, points indicating the relation between the detected transmittance of the recording material and the actual basis weight of the recording material are positioned substantially on the curve LN 1 . In contrast, when the recording material is back paper, an envelope, or label paper, the points indicating the relation between the detected transmittance of the recording material and the actual basis weight of the recording material are alienated from the curve LN 1 . 
     Specifically, when the recording material is back paper or an envelope, the basis weight obtained from the curve LN 1  based on the detected transmittance of the recording material is greater than the actual basis weight. This is because back paper has a printed part and thus light is not easily transmitted compared to plain paper. This is because envelopes include stacked paper and thus light is not easily transmitted compared to plain paper. When the recording material is label paper, the basis weight obtained from the curve LN 1  based on the detected transmittance of the recording material is less than the actual basis weight. This is because label paper has a greater specific gravity than plain paper. 
     In addition, when the focus is on the accuracy in detecting the basis weight detected by using the curve LN 1  based on transmittance, the followings are found. In the range where the transmittance of light transmitted through the recording material is more than 8.0% (range RG 1 ), alienation of the points, which indicate the relation between the transmittance and the actual basis weight of the recording material, from the curve LN 1  is small, and the accuracy in detecting the basis weight detected by using the curve LN 1  is high. In the range where the transmittance of light transmitted through the recording material is 8.0% or less, alienation of the points, which indicate the relation between the transmittance and the actual basis weight of the recording material, from the curve LN 1  is large, and the accuracy in detecting the basis weight detected by using the curve LN 1  is low. 
     Note that, although not illustrated in the graph of  FIG. 12 , the light transmittance of an OHP sheet is extremely higher than those of, for example, plain paper, label paper, back paper, and an envelope (higher than a predetermined transmittance threshold value). This enables the optical sensor  51  to detect that the recording material is an OHP sheet. 
       FIG. 13  illustrates the relation between the voltage value output by the ultrasonic sensor  52  in one embodiment of the invention and the actual basis weight of the recording material. 
     Referring to  FIG. 13 , the physical property detector  413  stores a curve LN 2 . The curve LN 2  is a reference curve indicating the relation between the voltage value output by the ultrasonic sensor  52  and the actual basis weight of the recording material. The curve LN 2  is the reference curve calculated from a relation obtained in the case where the recording material is plain paper. The physical property detector  413  detects the basis weight of the recording material by using the curve LN 2  based on the voltage value output by the ultrasonic sensor  52 . 
     When the recording material is plain paper, label paper, or back paper, points indicating the relation between the voltage value and the actual basis weight of the recording material are positioned substantially on the curve LN 2 . In particular, when the recording material is back paper, printed parts have little influence on ultrasonic waves. In contrast, when the recording material is an envelope, the points indicating the relation between the voltage value and the actual basis weight of the recording material are alienated from the curve LN 2 . When the recording material is an envelope, the basis weight of the recording material detected by using the curve LN 2  based on the voltage value is greater than the actual basis weight. This is because envelopes include stacked paper and ultrasonic waves are not easily transmitted compared to plain paper. 
     In addition, when the focus is on the accuracy in detecting the basis weight obtained by using the curve LN 2  based on the voltage value, the followings are found. In the range where the voltage value is 2.8 V or more (range RG 2 ), alienation of the points, which indicate the relation between the voltage value and the actual basis weight of the recording material, from the curve LN 2  is small, and the detection accuracy for the basis weight detected by using the curve LN 2  is high. In the range where the voltage value is less than 2.8 V, alienation of the points, which indicate the relation between the voltage value and the actual basis weight of the recording material, from the curve LN 2  is large, and the accuracy in detecting the basis weight detected by using the curve LN 2  is low. Here, the range where the voltage value is 2.8 V or more corresponds to the range where the light transmittance is 2.8% or more in the graph of  FIG. 12  (range RG 2 ). 
       FIG. 14  is a graph illustrating the relation between the basis weight of the recording material detected by the paper thickness sensor  53  in one embodiment of the invention and the actual basis weight of the recording material. 
     Referring to  FIG. 14 , it is found that, in the range where the detected basis weight of the recording material is more than 200 g/m 2  (range RG 3 ), alienation of the detected basis weight of the recording material from the actual basis weight of the recording material is small, and the accuracy in detecting the basis weight detected by the paper thickness sensor  53  is high. It is found that, in the range where the detected basis weight of the recording material is 200 g/m 2  or less, alienation of the detected basis weight of the recording material from the actual basis weight of the recording material is large, and the accuracy in detecting the basis weight detected by the paper thickness sensor  53  is low. Here, the range where the detected basis weight of the recording material is more than 200 g/m 2  corresponds to the range where the light transmittance is less than 2.8% in the graph of  FIG. 12  (range RG 3 ). 
       FIG. 15  illustrates the detection accuracy of each of the optical, ultrasonic, and paper thickness sensors  51 ,  52 , and  53  for various recording materials. 
     Referring to  FIG. 15 , the optical sensor  51  has high accuracy in detecting the basis weight of thin plain paper (basis weight: 150 g/m 2  or less), detecting the basis weight of thick plain paper (basis weight: 150 g/m 2  or more), and discriminating OHP sheets. The ultrasonic sensor  52  has high accuracy in detecting the basis weight of thin back paper (basis weight: 150 g/m 2  or less), discriminating label paper, and discriminating envelopes. The paper thickness sensor  53  has high accuracy in detecting the basis weight of thick plain paper (basis weight: 150 g/m 2  or more) and detecting the basis weight of thick back paper (basis weight: 150 g/m 2  or more). 
     Method of Discriminating Type of Recording Material 
     As described above, types of sensors capable of detecting basis weight with high accuracy are different depending on the transmittance of a recording material. For this reason, in the embodiment, when the transmittance of a recording material is within a first range, the discriminator  414  discriminates the type of the recording material by using a detection result from the first physical property detector, and when the transmittance of the recording material is within a second range different from the first range, the discriminator  414  discriminates the type of the recording material by using a detection result from the second physical property detector. 
     In addition, when the transmittance of the recording material is within the first range, the discriminator  414  identifies the physical property of the recording material with the first physical property detector and when the transmittance of the recording material is within the second range, the discriminator  414  identifies the physical property of the recording material with the second physical property detector. The discriminator  414  discriminates the type of a target recording material based on the state of alienation of the identified physical property from a reference value. 
     In addition, when the transmittance of the target recording material is within a third range different from the first and second ranges, the discriminator  414  may discriminate the type of the target recording material by using a detection result from the third physical property detector. 
       FIG. 16  illustrates a combination of the transmittance of the recording material in one embodiment of the invention and the sensors used for basis weight detection. 
     Referring to  FIGS. 12 and 16 , when the transmittance of the recording material detected by the optical sensor  51  is within the range RG 1  (one example of the first range) greater than 8.0% (one example of a first threshold value), the discriminator  414  identifies (detects) the basis weight of the recording material by using only the optical sensor  51  from the optical, ultrasonic, and paper thickness sensors  51 ,  52 , and  53  (by using only the voltage value output by the optical sensor  51 ). 
     When the transmittance of the recording material detected by the optical sensor  51  is within the range RG 2  (one example of the second range) from 2.8% (one example of a second threshold value) to 8.0%, the discriminator  414  identifies (detects) the basis weight of the recording material by using only the ultrasonic sensor  52  from the optical, ultrasonic, and paper thickness sensors  51 ,  52 , and  53  (by using only the voltage value output by the ultrasonic sensor  52 ). 
     When the transmittance of the recording material detected by the optical sensor  51  is within the range RG 3  (one example of the third range) less than 2.8%, the discriminator  414  identifies (detects) the basis weight of the recording material by using only the paper thickness sensor  53  from the optical, ultrasonic, and paper thickness sensors  51 ,  52 , and  53  (by using only the information output by the paper thickness sensor  53 ). 
       FIG. 17  is a graph illustrating a method of discriminating the type of the recording material in one embodiment of the invention. In the graph, the vertical axis represents the detected basis weight, and the horizontal axis represents the transmittance of the recording material. 
     Referring to  FIG. 17 , the discriminator  414  then discriminates the type of a recording material based on the state of alienation of the identified physical property from a reference value. The discriminator  414  plots a point indicating the transmittance of the recording material and the detected basis weight (for example, points PT 1  to PT 3 ) on two-axis coordinates. The discriminator  414  discriminates the type of the recording material among plain paper, label paper, and back paper based on the state of alienation of the plotted point from the curve LN 1 . 
     Specifically, when the plotted point is within a region G 1  that is not alienated from the curve LN 1  (when the plotted point is, for example, the point PT 1 ), the discriminator  414  determines that the type of the recording material is plain paper. This is because, when the recording material is plain paper, the actual basis weight is substantially the same as the basis weight detected by the optical sensor  51 , as indicated in  FIG. 12 . 
     In addition, when the plotted point is within a region G 2  that is alienated downward from the curve LN 1  (when the plotted point is, for example, the point PT 2 ), the discriminator  414  determines that the type of the recording material is back paper. This is because, when the recording material is back paper, the actual basis weight is less than the basis weight detected by the optical sensor  51 , as indicated in  FIG. 12 . 
     Furthermore, when the plotted point is within a region G 3  that is alienated upward from the curve LN 1  (when the plotted point is, for example, the point PT 3 ), the discriminator  414  determines that the type of the recording material is label paper. This is because, when the recording material is label paper, the actual basis weight is greater than the basis weight detected by the optical sensor  51 , as indicated in  FIG. 12 . 
     Method of Determining Printing Condition 
       FIG. 18  schematically illustrates a setting table in one embodiment of the invention. 
     Referring to  FIG. 18 , the printing condition setter  415  sets a printing condition according to the discriminated type of the recording material by using the setting table. 
     Printing conditions are described in the setting table. The printing conditions are set when the discriminated type of the recording material is plain paper, label paper, back paper, an envelope, and an OHP sheet. The printing conditions include items of system speeds (printing speeds), distances between recording materials, transfer currents, fixing temperatures, fixing press-separation (pressure contact state between a fixation member and a pressure member), finisher (FS) prohibition (prohibition relating to postprocessing), duplex prohibition (prohibition of duplex printing), and printing directions. 
     When the recording material is back paper, duplex printing is prohibited since a character has already been printed on the back surface. In addition, when the recording material is label paper, switching the system speed between a middle speed and a low speed (setting a value different from the system speed in the case where the recording material is plain paper (slow-speed value)) efficiently transfers fixing heat to the recording material. 
     Note that the printing condition in  FIG. 18  is one example, and any printing condition is set according to the type of recording material. The printing condition setter  415  preferably sets the system speed in the case where the recording material is determined to be label paper to a value different from the printing speed in the case where the recording material is determined to be plain paper (value lower than the printing speed in the case of being determined to be plain paper). In addition, when the recording material is determined to be back paper, the printing condition setter  415  preferably prohibits duplex printing, and does not receive setting for duplex printing through the operation input  23 . 
     Note that the printing condition setter  415  may set a printing condition according to the basis weight of the recording material in addition to the type of the recording material. 
     Flowchart 
       FIGS. 19 and 20  are flowcharts illustrating an operation of the image forming apparatus  1  in one embodiment of the invention. 
     Referring to  FIG. 19 , when feeding of the recording material is started, the controller  40  acquires outputs from each of the optical, ultrasonic, and paper thickness sensors  51 ,  52 , and  53  (S 1 ). The controller  40  then determines whether the variation amount ΔV of the voltage value output from the optical sensor  51  is greater than a predetermined amount (S 3 ). 
     When the variation amount ΔV of the voltage value output from the optical sensor  51  is determined to be greater than the predetermined amount in Step S 3  (YES in S 3 ), the controller  40  determines that the recording material is back paper (S 5 ). The controller  40  performs printing on the recording material with the toner image former  20  under a printing condition according to the determined type of the recording material (S 17 ), and terminates processing. 
     When the variation amount ΔV of the voltage value output from the optical sensor  51  is determined to be not greater than the predetermined amount in Step S 3  (NO in S 3 ), the controller  40  determines whether the ultrasonic wave amount indicated by the voltage value that is output from the reception sensor  524  is lower than a predetermined ultrasonic-wave-amount threshold value (S 7 ). 
     When the ultrasonic wave amount indicated by the voltage value that is output from the reception sensor  524  is determined to be lower than the predetermined ultrasonic-wave-amount threshold value in Step S 7  (YES in S 7 ), the controller  40  determines that the recording material is an envelope (S 9 ). The controller  40  performs printing on the recording material with the toner image former  20  under a printing condition according to the determined type of the recording material (S 17 ), and terminates processing. 
     When the ultrasonic wave amount indicated by the voltage value that is output from the reception sensor  524  is determined to be not lower than the predetermined ultrasonic-wave-amount threshold value in Step S 7  (NO in S 7 ), the controller  40  detects the transmittance of the recording material with the optical sensor  51  (S 11 ), and determines whether the detected transmittance is higher than a predetermined transmittance threshold value (S 13 ). 
     When the detected transmittance is determined to be higher than the predetermined transmittance threshold value in Step S 13  (YES in S 13 ), the controller  40  determines that the recording material is an OHP sheet (S 15 ). The controller  40  performs printing on the recording material with the toner image former  20  under a printing condition according to the determined type of the recording material (S 17 ), and terminates processing. 
     When the detected transmittance is determined to be not greater than the predetermined transmittance threshold value in Step S 13  (NO in S 13 ), the controller  40  proceeds to the processing of Step S 21  in  FIG. 20 . 
     Referring to  FIG. 20 , in step S 21 , the controller  40  determines a sensor to be used for basis weight detection based on the detected transmittance (S 21 ). The controller  40  then identifies the basis weight from an output value of the determined sensor (S 23 ), and plots a point indicating the transmittance of the recording material and the identified basis weight (S 25 ). The controller  40  then determines whether the position of the point is alienated downward from the curve LN 1  (S 27 ). 
     When the position of the point is determined to be alienated downward from the curve LN 1  in Step S 27  (YES in S 27 ), the controller  40  determines that the recording material is back paper (S 29 ). The controller  40  performs printing on the recording material with the toner image former  20  under a printing condition according to the discriminated type of the recording material (S 37 ), and terminates processing. 
     When the position of the point is determined to be not alienated downward from the curve LN 1  in Step S 27  (NO in S 27 ), the controller  40  determines whether the position of the point is alienated upward from the curve LN 1  (S 31 ). 
     When the position of the point is determined to be alienated upward from the curve LN 1  in Step S 31  (YES in S 31 ), the controller  40  determines that the recording material is label paper (S 33 ). The controller  40  performs printing on the recording material with the toner image former  20  under a printing condition according to the discriminated type of recording material (S 37 ), and terminates processing. 
     When the position of the point is determined to be not alienated upward from the curve LN 1  in Step S 31  (NO in S 31 ), the controller  40  determines that the recording material is plain paper (S 35 ). The controller  40  performs printing on the recording material with the toner image former  20  under a printing condition according to the discriminated type of recording material (S 37 ), and terminates processing. 
     Effect of Embodiment 
     In the embodiment, the type of a target recording material is discriminated by using a detection result from a physical property detector according to the transmittance of the recording material, whereby, for example, back paper and label paper can be detected, and the accuracy in discriminating the type of the recording material can be improved. 
     Others 
     The recording material discriminating apparatus of the invention may be any apparatus that, when the transmittance of the recording material is within the first range, discriminates the type of the recording material by using a detection result from the first physical property detector, and when the transmittance of the material is within the second range different from the first range, discriminates the type of the recording material by using a detection result from the second physical property detector. Each of the first and second physical property detectors can take any method of detection. In addition to methods of detection by using an amount of light or ultrasonic waves that are transmitted through the recording material, methods of detection by using reflectance of light that is reflected by the recording material, and methods of detection by using a displacement amount of a displacement member at the time when the recording material passes through the detection position of the displacement member may be used. The recording material discriminating apparatus of the invention preferably discriminates the type of the recording material among at least a part of options of plain paper, label paper, back paper, envelopes and OHP sheets. 
     Software or a hardware circuit may perform processing in the above-described embodiment. In addition, programs that execute processing in the above-described embodiment can be provided. The programs may also be provided to users after being recorded in recording media such as CD-ROMs, flexible disks, hard disks, ROMs, RAMs, and memory cards. Computers such as CPUs execute the programs. In addition, the programs may be downloaded to apparatuses through communication lines such as the Internet. 
     Although embodiments of the present invention have been described and illustrated in detail, the disclosed embodiments are made for purposes of illustration and example only and not limitation. The scope of the present invention should be interpreted by terms of the appended claims, and all changes which come within the meaning and range of equivalency of the claim are intended to be embraced therein.